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DEPARTEMENT CHEMIE

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Aan het einde van mijn doctoraat wil ik een aantal mensen bedanken, zonder hen was dit werk niet gerealiseerd. Bij het afwerken van een onderzoeksproject ziijn er heel wat mensen die hun steentje bijdragen om alles tot een goed einde te brengen.

Allereerst wil ik mijn promotoren, prof. Wim Dehaen en prof. Paul Heremans, bedanken. Zij hebben het project leven ingeblazen en elk de voortgang van hun specialisatie opgevolgd. Ze stonden klaar met raad wanneer het onderzoek minder goed ging en gaven me de kans om nieuwe ervaringen op te doen.

Ik wil ook de leden van de examencommissie bedanken voor het zorgvuldig lezen en voor hun nuttige opmerkingen en suggesties. Ik wil hen ook bedanken voor de interesse die ze toonden in ons werk.

In een labo werk je nooit alleen, daarom wil ik alle collega’s van beide onderzoeksgroepen bedanken voor de fijne samenwerking, de interesse in het onderzoek, het uitwisselen van tips en ervaringen en natuurlijk ook de fijne sfeer. In het bijzonder wil ik Wienand bedanken met wie ik het syntheselabo deelde voor de rust die hij altijd uitstraalt. Hanne wil ik bedanken voor het werk dat ze verricht heeft tijdens haar thesisjaar en voor de vele leuke gesprekjes aan onze trekkast die vaak werd opgevrolijkt door Wim V.R.. Stijn en Mario wil ik bedanken voor de sturende elementen die ze hebben aangebracht, vooral in de beginjaren. Dimitri verdient een dank-je-wel voor de hulp bij de praktische zaken op imec, zijn interesse en het welkomgevoel. Kris en Soeren moet ik bedanken voor hun geduld wanneer ik weer eens in de knoop lag met de elektronische kant van de zaak. Kris, dank je wel voor je hulp met IGOR, het baby-systeem en de parameter-analyser en natuurlijk voor de bevoorrading met

substraatjes. Robert wil ik bedanken voor zijn interesse en behulpzaamheid.

Verder zijn er een heleboel mensen die dit onderzoek mee ondersteund hebben. Mijn dank gaat hiervoor uit naar prof. Suzanne Toppet, Karel, Kris M., Stijn D., Peter, Erwin, Bert D. en Rene. Ook de vrolijke poetsvrouwen en het administratief personeel wil ik bedanken.

Tenslotte wil nog mijn vrienden en (schoon)familie bedanken, zij hebben onrechtstreeks bijgedrage tot het slagen van dit werk. In het bijzonder wil ik Sara bedanken voor onze jarenlange vriendschap, voor de steun, de talloze leuke en ernstige gesprekken en nog veel meer. Ook mijn ouders wil ik bedanken voor het geloof dat ze in me stellen en voor hun goede zorgen ook nu ik al lang op eigen benen sta. En natuurlijk moet ik ook Bert bedanken voor zijn eindeloze geduld, om me recht te houden als ik het niet meer zag zitten, voor alle mooie momenten, zijn vertrouwen en zijn liefde. Dank je wel!

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Samenvatting

5

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De laatste jaren is er veel belangstelling voor het onderzoek naar organische-dunne-film transistors. Deze transistors bieden mogelijkheden tot de ontwikkeling van goedkope en flexibele elektronische componenten met grote oppervlakte. Pentaceen heeft tot nu toe de beste elektrische eigenschappen om als organisch halfgeleidermateriaal te gebruiken in OTFT’s. Het praktische gebruik van pentaceen heeft echter heel wat nadelen omwille van zijn slechte oplosbaarheid en zijn gevoeligheid voor oxidatie door zuurstof. Momenteel wordt er gewerkt aan twee oplossingen voor deze problemen. Enerzijds wordt er onderzoek gedaan naar precursormoleculen van pentaceen die later na afzetten op een substraat terug omgezet worden naar het oorspronkelijke pentaceen. Anderzijds verricht men onderzoek naar gesubstitueerde pentacenen. De invoering van substituenten verhoogt de oplosbaarheid en stabiliteit van de pentaceeneenheid. Tevens zorgen de substituenten voor VHOI�DVVHPEO\ waardoor ze zelfs de ordening van de moleculen in de halfgeleiderfilm kunnen verbeteren.

In een eerste deel hebben we getracht nieuwe pentaceenprecursoren te synthetiseren via Diels-Alder-reactie tussen pentaceen en verschillende dienofielen met heteroatomen die de temperatuur van de uiteindelijke regeneratie kunnen verlagen. Om te beginnen werd een nieuwe reductiemethode geoptimizeerd om op een goedkope en milieuvriendelijkere wijze pentaceen te kunnen synthetiseren uitgaande van pentaceenchinon. Vervolgens zijn we erin geslaagd om een adduct te bekomen met thiofosgeen dat bij verhitting via een retro-Diels-Alderreactie terug omzet tot pentaceen.

In het tweede deel van dit werk hebben we ons geconcentreerd op de synthese van nieuwe gesubstitueerde pentacenen. De aard, het aantal

Samenvatting

6

en de posities van de substituenten werden gevarieerd. 6-Monogesubstitueerd pentaceen bleek niet de gewenste stabiliteit te bezitten. Verder kon de introductie van alkylsubstituenten op de 6- en 13-positie niet gerealiseerd worden via de voorgestelde syntheseweg. We zijn er wel in geslaagd om verschillende 6,13-diarylgesub-stitueerde en 5,7,12,14-tetragesubstitueerde pentacenen te bereiden uitgaande van de overeenkomstige pentacenonen. De isomeren van de intermediaire diolen en tetrolen die ontstonden na additie van organolithium-verbindingen, werden indien mogelijk geïsoleerd en gekarakteriseerd. Bij elektronenrijke substituenten geven deze intermediaire alcoholen aanleiding tot de vorming van een omleggingsproduct. Hoger gesubstitueerde pentacenen met zes of acht substituenten werden niet bekomen omwille van problemen bij de synthese van de startproducten namelijk de gesubstitueerde pentacenonen.

In het laatste deel hebben we de mogelijke integratie van de nieuw gesynthetiseerde pentaceenderivaten in transistoren onderzocht. Het was de bedoeling om de materialen uit oplossing af te zetten op een substraat. We zijn er in geslaagd om werkende transistoren te bekomen via spincoating. Hierbij is gebleken dat het gebruikte solvent een cruciale rol speelt. Verder ondervinden de elektrische karakteristieken een invloed van de concentratie van het materiaal en het additief, de spintijd en de spinsnelheid, de nabehandeling en het oppervlak. Er werden verschillen waargenomen in de elektrische eigenschappen afhankelijk van het type substituent en het aantal substituenten. De beste resultaten werden bekomen voor het tetra-(4-octylfenyl)pentaceen met mobiliteiten tot 10-3 cm2/Vs. Verder waren de bekomen transistoren reproduceerbaar. Ons onderzoek laat ruimte voor verdere verbeteringen en geeft reeds een idee van de mogelijke invloeden van de aard en het aantal substituenten op de pentaceeneenheid.

Abstract

7

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During the last few years, the use of organic compounds in electronic applications has known a spectacular evolution. In particular, the development of organic thin-film transistors (OTFT’s) has attracted a great deal of interest. One of the most important materials in this context is pentacene because of its excellent semiconducting properties. However its practical use in transistors has some drawbacks. Firstly, its poor solubility necessitates the use of vacuum systems for deposition. Secondly pentacene is very sensitive to oxidation by oxygen. One way to solve the solubility problems is given by the synthesis of pentacene precursors which can be deposited from solution. After deposition, the parent pentacene is regenerated by a retro-Diels-Alder reaction. Another approach is the introduction of substituents on the pentacene backbone which will increase both solubility and stability. Moreover, the substituents can induce self-assembly resulting in a closer packing of the pentacene moieties.

In the first part of this thesis we have concentrated on the synthesis of new pentacene precursors via Diels-Alder reactions between pentacene and dienophiles with heteroatoms. The heteroatoms should lower the temperature needed for the regeneration of pentacene after deposition. To start we optimized a new reduction for a cheap and more environmentally friendly synthesis for pentacene out of pentacenequinone. We succeeded to prepare an adduct of pentacene with thiophosgene which could be reconverted into pentacene by heating.

In the second part we have investigated the possibility of the introduction of aryl and alkyl substituents on the pentacene backbone. The type, the number and the position of the substituents were varied to study their influence on the electronic properties of the final

Abstract

8

transistors. 6-Monosubstituted pentacene appeared to be also very sensitive to oxidation. The introduction of alkyl substituents on the inner rings of pentacene failed but we prepared with success different 6,13-diaryl substituted and 5,7,12,14-tetra-aryl substituted pentacenes. We observed that electron rich substituents should be avoided because they give rise to a rearranged product instead of the desired pentacene. The isomers of the intermediate diols and tetrols which were formed after addition of organometallic compound to the corresponding pentacenone, were characterised. Unfortunately it was not possible to obtain higher substituted pentacenes with six or eight substituents due to problems with the preparation of the starting materials.

In the last part, the integration of the new substituted pentacenes from solution into transistors was studied. Working transistors could be obtained via spincoating in a reproducible way. The experiments showed that the solvent is a crucial factor to get working transistors. The electric properties depended not only on the solvent but also on the concentration of the pentacene derivative and the additive, the spin time and rate, the after treatment and the substrate treatment. The ISD-VSD-curves revealed problems with contact resistance. When recording the ISD-VGS-curves, we observed that the type and number of the substituents had an influence on the performance of the transistors. The best results were obtained for 5,7,12,14-tetra(4-octylphenyl)pentacene with mobilities around 10-3 cm2/Vs. The research we accomplished leaves room for further improvement and gives an idea of the influence that the type and number of the substituents have on the pentacene backbone.

List of abbreviations

9

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List of abbreviations

10

�VW\UHQHVXOIRQDWH��PET 3RO\�HWK\OHQH�WHUHSKWKDODWH��PGMEA ��0HWKR[\��SURSDQRO�DFHWDWH� �SURS\OHQH�JO\FRO�

PRQRPHWK\O�HWKHU�DFHWDWH�PPV 3RO\�SKHQ\OHQHYLQ\OHQHV��PTS 3KHQ\OWULFKORURVLODQH�PVP 3RO\�YLQ\OSKHQRO��RFID 5DGLR�IUHTXHQF\�LGHQWLILFDWLRQ�WDJ�TES 7ULHWK\OVLO\O TES-pentacene ��%LV�WULHWK\OVLO\OHWK\Q\O�SHQWDFHQH THF 7HWUDK\GURIXUDQ�TIPS 7ULLVRSURS\OVLO\O�TIPS-pentacene ��%LV�WULLVRSURS\OVLO\OHWK\Q\O�SHQWDFHQH�TLC 7KLQ�OD\HU�FKURPDWRJUDSK\�TMS 7ULPHWK\OVLO\O�TMS-pentacene ��%LV�WULPHWK\OVLO\OHWK\Q\O�SHQWDFHQH�UV 8OWUDYLROHW�

Table of contents

11

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4.1 Pentacene 27 ��4.2 Pentacene precursors 31 4.3 Substituted pentacenes 33

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1.1.1 Synthesis of pentacene 50 1.1.2 Pentacene precursors via thiophosgene 55 1.1.3 Pentacene precursors with electron rich bridge 57

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1.2.1 Study of thermal conversion 61

Table of contents

12

1.2.2 Development of semiconductor films 63

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2.1.1 Synthesis via pentacen-6-one 71 2.1.2 Synthesis via pentacenequinone 73

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78

2.2.1 Synthesis of 6,13-di(alkylsulfanyl)pentacenes 78 2.2.2 Synthesis of 6,13-diarylpentacenes 80 2.2.3 Rearrangement of 6,13-dihydro-6,13-

diarylpentacene-6’,13’-diols 90

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93

2.3.1 Synthesis of pentacen-5,7,12,14-one 93 2.3.2 Synthesis of 5,7,12,14-tetra-alkyl substituted

pentacenes 94

2.3.3 Synthesis of 5,7,12,14-tetra-aryl substituted pentacenes

95

��2.3.4 Rearrangement of 5,7,12,14-tetrahydro-5,7,12,14- tetra-arylpentacene-5’,7’,12’,14’-tetrols

102

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2.5.1 Synthesis of 2,3,9,10-tetrasubstituted pentacene-5,7- and 5,12-one

108

2.5.2 Synthesis of 2,3,5,7,9,10- and 2,3,5,9,10,12-hexasubstituted pentacenes

117

Table of contents

13

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��2.6.1. Synthesis of 2,3,9,10-tetrasubstituted pentacene- 5,7,12,14-tetrone

120

2.6.2 Synthesis of 2,3,5,7,9,10,12,14-octasubstituted pentacenes

127


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3.2.1 Transistors consisting of disubstituted pentacenes 147

3.2.2 Transistors consisting of tetrasubstituted pentacenes 160

3.2.3 Comparison between transistors consisting of disubstituted and tetrasubstituted pentacenes

173


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Introduction

15

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During the last few years, the use of organic compounds in electronic applications has known a spectacular evolution. In particular, the development of organic thin-film transistors (OTFT’ s)1 has attracted a great deal of interest. Furthermore in the field of light-emitting diodes2 and photovoltaic cells3, the use of organic compounds is gaining interest. Organic electronics offer the opportunity of developing cheap and large area electronic components that are compatible with flexible plastic substrates. OTFT‘s are important for applications without requirement for high mobility can lead to a range of innovative tools. In this work we will focus on organic materials for the semiconducting layer in these organic thin-film transistors. Organic

semiconductors� can be arranged in three different groups: fully

conjugated polymers, heterocyclic oligomers and polyaromatic rings. Until now, pentacene (Figure 1) is one of the most promising materials. This pentacyclic molecule has shown good field effect mobilities and has been intensively studied around the world. 4,5

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However, pentacene has some drawbacks. First of all, its poor solubility necessitates the use of vacuum systems for deposition. Secondly, pentacene is very sensitive to oxidation, leading to rapid degradation of the material unless manipulated under rigorous

� organic semiconductors are defined as extrinsic semiconductors were in

charge-carriers are introduced by injections from the electrodes1a

Introduction

16

exclusion of oxygen. In the literature, two solutions are offered to solve these problems. A first approach consists in the synthesis of a soluble pentacene precursor.6 After integration in a transistor structure, pentacene is regenerated. A second possibility is the introduction of substituents on the aromatic core which will increase both stability and solubility.7 In this introduction to our work ‘The synthesis of pentacene derivatives and their application in OTFT’ s’ , we will first briefly discuss the differences and advantages of OTFT’ s versus MOSFET’ s. Subsequently, we take a closer look at pentacene, the problems that may occur during its use and some solutions to these problems. Finally an overview is given about the applications, either existing or in the future.

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The main difference between a MOSFET and an OTFT is the composition of the semiconductor layer and the origin of the conductive channel in this layer. There are two types of MOSFET’ s. A n-type MOSFET is composed out of a p-type semiconducting material that also carries the structure. Integrated in this layer are two n-type regions, namely source and drain. On top of the p-type material, there is successively an isolating dielectric layer and a conductive layer, the gate. (Figure 2)8 In a p-type MOSFET n and p are reversed in the above description.

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Introduction

17

In the off-state of the transistor, no current will flow from the source to the drain because there is a reverse bias between the n- and p- type region at the source. These regions form a pn-junction in which holes are the major charge carriers for the p-type material and electrons for the n-type material. Because the two region consist of opposite charge carriers, a small depletion zone is formed called the space charge region. An electrostatic potential drop is present over this region. (Figure 3)9,10

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When a positive voltage is applied to the gate, the holes in the p-type semiconductor are repelled. On the other hand, electrons, the minority charge carriers, are attracted by the gate and will form an n-type conducting channel. A current can flow from the source to the drain and the transistor is now on. Usually, doped silicon is used as a semiconductor in these transistors. Monocrystalline silicon has a very high mobility (103 cm2/Vs) and is used in computers. Due to the high processing temperatures and vacuum needed to obtain a silicon layer, monocrystalline MOSFET’ s are expensive and incompatible with flexible materials. The thickness of the layers obtained is in the mm-range and the layers are brittle, conflicting with flexibility.

Introduction

18

Thin-film transistors can be made using silicon, in particular amorphous silicon but the mobility is low (0.1 to 1 cm2/Vs). The semiconductor layer is in the range of nanometers, much thinner than in a MOSFET. OTFT’ s can give a good alternative for amorphous silicon and offer a lot of advantages. An OTFT needs a substrate to support the structure. There are several architectures that differ from each other in the sequence of the different layers. All consist of a gate, a dielectric, an organic semiconductor and a source and drain contact. (Figure 4)

The semiconductor layer consists of one type of semiconductor, either a p-type material or an n-type material. In contrast to a MOSFET, there is no pn-junction and current will flow through a channel of majority charge carriers. Most organic semiconductors are p-type materials. This means that charge transport is carried out by holes. In an n-type material electrons take care of the transport. The transistor is switched on in the same way by applying a voltage to the gate. A negative gate voltage will induce a hole-channel through a p-type semiconductor. Through this channel, current can flow from the source to the drain. Pentacene is an example of a p-type semi-conductor.

The semiconductor part of an OTFT is always an organic material. Most of these materials are soluble in common organic solvents. This implies that a whole range of solution-processing deposition techniques can be used. These processes usually do not need high

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source drain

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dielectric

gate

substrate


source drain

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gate

substrate

gate

dielectric


source drain

substrate

Introduction

19

temperature in contrast to vacuum techniques. Due to these low-temperature processing techniques, flexible substrates like PET and polyimides can be used. A second advantage of the use of solution processing is the potential low cost. Moreover, for the dielectric and the contacts one can use flexible materials as well as the classical ones. Silicon oxide as a dielectric can be replaced by non-conductive polymers like polyvinyl alcohols or polyimide. Conductive polymers like Pedot/PSS are possible alternatives of the metal contacts. Everywhere in the world, scientists are looking for new organic materials that can substitute the traditional materials in the different parts of a transistor.

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An important characteristic of a semiconductor material in the transistor is the mobility which quantifies the ease of the carrier movement through the semiconductor layer and which depends on the charge transport through the material. The charge transport for metals and inorganics is well known, but for organic materials the story is much more complex.

In the context of charge transport, metals and inorganic materials are easily arranged in three types based on their electronic structure: insulators, semiconductors and conductors. Their electronic structure can be described by the ‘band theory’ . (Figure 5)

The energy levels of the electrons in the lattice are combined in an energy band due to interaction with other atoms. These energy bands are separated by an energy gap. All electrons are placed in these bands with the lowest possible energy. The size of the energy gap between the highest fully occupied band, the valence band, and the lowest unoccupied band, the conducting band, will determine whether the material is an insulator or a semiconductor. When the energy gap is too large (> 3 eV), the electrons will not overcome the barrier to the

Introduction

20

conduction band by thermal excitation. In this case, the material will be an insulator. Because no electrons will be promoted to the conduction band where they would be freed from the atom and could contribute to a current.

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In a (intrinsic�) semiconductor, the band gap is relatively small. At

high temperatures there will be some electrons that can overcome the energy barrier by thermal excitation. In this way charge transport becomes possible. In the case of a conductor, the highest occupied band will be only half filled. Therefore electrons can be easily promoted to a nearby empty energy level in the same band and in this way give rise to conduction.

However, when charge transport in organic materials is considered, the picture becomes much more complex. Semiconducting organic molecules, like pentacene, are fully conjugated. Due to the strong

covalent bonding, the overlap between the S-orbitals along the

conjugated system induces a continuous cloud over the whole molecule in which the electrons are delocalized. Within a

� extrinsic anorganic semiconductor: doped material so that extra enery levels

are induced in the energy-gap to induce semiconductor behaviour analogous to intrinsic semiconductors.

Introduction

21

semiconducting layer in a transistor, there are many molecules present. In order to obtain conduction through the material, the charge carriers have to hop from one molecule to the other. This is only

possible when there is enough overlap between the S-conjugated

orbitals of the different molecules. The total overlap is depending on the packing of the molecules and on the different intermolecular

interactions namely Van der Waals-interactions and S-S-interactions.

These intermolecular interactions are much weaker than the intramolecular interactions. Therefore, the charge carriers will be more delocalized throughout the molecule than over the whole layer. The influence of intermolecular forces increases with the distance. The molecule possesses also a certain geometry which will lead to anisotropy in the conduction.

Exactly how the charge transport in organic materials takes place is still not completely clear. At the moment the most accepted theory is that of the phonon-assisted polaron hopping mechanism.12 A polaron is a quantum mechanic particle that describes a charge carrier and the polarisation that the charge carrier brings along. With the aid of a phonon the polaron can hop from one molecule to another. (Figure 6)13

)LJXUH��3KRQRQ�DVVLVWHG�SRODURQ�KRSSLQJ�

��

Introduction

22

The probability of the hopping depends on the gate-source field, the temperature, the size and shape of the molecule and the morphology.

However, the measured mobility often depends not only on the intrinsic charge transport through the material and its morphology because of the many different traps that can occur. Charge transport can be hampered by impurities, defects in the crystal structure and grain boundaries. At the boundaries the continuity of the layer is disrupted. Here, the distance between the molecules can increase or the orientation of the molecules can change. This will slow down or stop the hopping of charge carriers because the overlap has decreased too much. Furthermore defects will create non-allowed energy levels between HOMO (highest occupied molecular orbital = highest enery level of a molecule which contains electrons) and LUMO (lowest unoccupied molecular orbital = energy level following on the HOMO) of the molecules. When a charge carrier falls in a verbidden zone, it will get stuck. Impurities can cause three different types of trapping. (Figure 7)

)LJXUH��,QIOXHQFH�RI�LPSXULWLHV�

� �

+

�

+

trapping charge carriers

creating extra charge carriers

slowing down charge carriers

Semiconductor

LUMO materialHOMO material

Impurity

LUMO materialHOMO material

Introduction

23

When the HOMO level of the impurity is higher than that of the p-type material, the charge carriers will be captured in the lower energy level. More free carriers will be created when the LUMO level of the impurities equals the HOMO level of the material. The mobility will rise but there will be undesirable rest conductivity. In the last case the HOMO of the impurity will be lower than the HOMO of the material. This will only slow down the transport.

��7UDQVLVWRU�FKDUDFWHULVWLFV�

When a transistor is made from a new material, its characteristics are tested by measuring its output or/and transfer characterisics. When for different gate voltages, the current between the source and the drain is measured in function of the drain-voltage, a output curve is obtained. These measurements can be performed with a small drain voltage (|VDS| < |VGS|, IDS is increasing linearly with VDS) or with a large drain voltage (|VDS| > |VGS|, IDS is tending to saturation with VDS). In the first case one is measuring in the so-called linear regime, in the second case in the saturation regime. (Figure 8)

-5

-4

-3

-2

-1

0

I DS

[µA

]

-20 -15 -10 -5 0VDS [V]

VGS = 1->-15V, step = -1

)LJXUH��7\SLFDO�WUDQVLVWRU�RXWSXW�FXUYH�The transfer characterisics can be obtained by measuring the current between source and drain as a function of the gate voltage with a

Introduction

24

certain drain-voltage. We will here only discuss IDS-VGS-measurements in the saturation regime (measured with VDS situated in the saturation regime of the output-curve). One can speak of a transistor when a typical transfer-curve as displayed in Figure 9, is observed.

10pA

100pA

1nA

10nA

100nA

1µA

10µA

100µAI D

S [A

]

-6 -4 -2 0 2 4 6VGS [V]

Pentacene TFTw/l = 5000/10

)LJXUH��7\SLFDO�WUDQVLVWRU�WUDQVIHU�FXUYH�

When plotting the square root of the current IDS as a function of the gate voltage VGS, one can deduce a few transistor characteristics. (Figure 10) The mobility in the saturation regime can be calculated from the slope by the equation:

P = (2.L.J2)/(W.Ci)

with L = the channel length between the two contacts W = the width of the channel between the contacts

Ci = H0.Hi/d (d = thickness of the dielectric)

J = �I / (VGS-Vt)

Another parameter is the threshold voltage Vt found as the intercept of the tangent to the curve in the point with the steepest slope with the VGS-axis. In an ideal transistor, the threshold voltage equals 0 V. Due to the presence of traps or dopants it can differ from this ideal value.

Introduction

25

When the transitor is switched off again, there may be no left-over conductivity. The ratio between the on- and off-current Ion/Ioff, which is indicative for the switching performance of the transistor, has to be as large as possible. Finally there is the hysteresis which stands for the difference between curve of the measurement from positive to negative gate voltage and the backward curve. In the ideal case both curves are on top of each other. When the backward curve is situated by lower currents, the hysteresis is caused by impurities in the semiconductor. (Figure 10)

)LJXUH��7UDQVLVWRU�FKDUDFWHULVWLFV�IURP�DQ�, � �9 �� PHDVXUHPHQW��

Two other parameters are the inverse of the subthreshold slope and the onset voltage can be deduced from the curve of the source-drain current. (Figure 10) The subthreshold slope is indicating how fast the current increases in the subthreshold regime with changing VGS. The onset voltage is the voltage at which the transistor switches on.

A disturbance that can occur is the contact resistance. Charge carriers are hindered when entering the semiconductor material from the contacts, which can cause non-ideal transistor behaviour. Often this

subthreshold

slope

hysteresis

mobility

threshold voltage onset voltage

10pA

100pA

1nA

10nA

100nA

1µA

10µA

100µA

I DS [A

]

-6 -4 -2 0 2 4 6

VGS [V]

0.014

0.012

0.010

0.008

0.006

0.004

0.002

0.000

SQR

T(ID

S ) [A1/2]

Introduction

26

results in a less than quadratic increase of IDS with VGS in the output-curve (Figure 8).

��2UJDQLF�VHPLFRQGXFWLQJ�PDWHULDOV�All organic semiconductors are conjugated molecules. They consist of two different types of materials namely conjugated polymers and small molecules. The group of small molecules is mostly divided in two categories namely heterocyclic oligomers and polyaromatic hydrocarbons.14

Conjugated polymers were firstly studied concerning their semiconducting properties. Polymers have the advantage that they can be easily processed because of their solubility. Films of polymers can be deposited in a straightforward manner from solution. Thus, they possess all advantages OTFT’s have over traditional MOSFET’s. However, for most polymers it is difficult to obtain crystalline films. The long chains of the polymer often are hindering crystallisation which is mostly causing relatively low mobilities. The most frequently used and optimized semiconducting polymers are poly(thiophenes) (e.g. P3HT) and poly(phenylenevinylenes) (PPV). (Figure 11) At this moment mobilities up to 0.1 cm2/Vs are achieved for the well orded crystalline regioregular P3HT. 14

**n

S **

(CH2)5CH3

n

P3HT PPV

)LJXUH��6HPLFRQGXFWLQJ�SRO\PHUV�

The disadvantage of the mostly low degree of stacking of polymers can be avoided by using small molecules. In this case heterocyclic oligomers are mostly short versions of analogous semiconducting

Introduction

27

polymers. The best representatives of this group are the alkylthiophene oligomers. For sexithiophene the best mobility is now around 1 cm2/Vs.8a However, in contrast with the polymers, these molecules are generally sparely soluble and consequently need to be deposited from high vacuum. With the use of these techniques, these materials lose the advantages correlated with the solution process techniques.

Small molecules that are in the focus of interest are polyaromatic hydrocarbons. The most intensively studied molecule of this class is pentacene (Figure 1). After many improvements of the manufacturing of transistors consisting of pentacene, recently mobilities of 6 cm2/Vs were obtained.8a Some other small molecules have been studied e.g. phthalocyanine, C60 and tetrathiafulvalene derivatives (mobilities between 10-5-10-1 cm2/Vs)15. These molecules have the same drawbacks as the oligomers. Although there are some efforts to develop techniques that make deposition from solution possible, their application is not straightforward and therefore a breakthrough has not yet been forced.

��3HQWDFHQHPentacene is a polyaromatic hydrocarbon that consists of five linearly fused aromatic rings.16 It is a flat molecule and the carbons can be numbered in the way shown in Figure 12. Pentacene is a conjugated molecule and as already mentioned it is the most promising organic semiconductor because of its excellent transistor characteristics.

��

� � � � ��

��

)LJXUH��1XPEHULQJ�LQ�SHQWDFHQH�

Introduction

28

When grown by evaporation in high vacuum, pentacene forms a film with a favourable crystal geometry and orientation. Due to the morphology of the films, there is a good intermolecular overlap that leads to the high mobilities that are measured. The molecules are arranging themselves into a so-called herringbone structure (Figure 13A)5 with their molecular axes perpendicular to the substrate.

$� � � %� � � &�

)LJXUH��&U\VWDO�JHRPHWU\�DQG�RULHQWDWLRQ�RI�SHQWDFHQH�

When the lattice-structure is observed, two types of intermolecular interactions can be distinguished. There is an interaction between the

S-orbitals of molecules that lay parallel to each other. This interaction

is called ‘face to face’ interaction and is illustrated in Figure 13B. Between the molecules that are not oriented in a parallel way to each

other, there is an ‘edge to face’ interaction. Here the S-system of one

molecule is interacting with the hydrogens of the other molecule. (Figure 13C) These interactions are responsible for the high mobility

and on/off ratio. 11a,5

In spite of these high quality properties, the practical use of pentacene in organic thin-film transistors (OTFT’ s) has some drawbacks caused by its sensitivity to oxygen and low solubility, as mentioned at the beginning of this work.

Introduction

29

Due to the extremely low solubility of pentacene, only deposition by evaporation is possible. Until very recently, this was only possible with ultra-high vacuum equipment. Because of the cost and the difficulties of the production of large-area electronics, this is a severe disadvantage for industrial purposes.

A second problem caused by the lack of solubility, is the difficulty chemists experience while trying to purify pentacene with common techniques. The commercially available pentacene is not pure enough for use in OTFT’ s. This necessitates intensive purification by train sublimation. Also here high vacuum and a high temperature is needed (e.g. > 250°C). A part of the loaded pentacene will be degraded under the influence of the temperature. During the purification, dihydropentacene � and a series of polycondensed aromatic hydrocarbons like peripentacene � are produced.17 (Figure 14)

H H

H H

�

)LJXUH��6LGH�SURGXFWV�SURGXFHG�GXULQJ�WUDLQ�VXEOLPDWLRQ�

Furthermore the purification is made even more difficult by the sensitivity of pentacene to molecular oxygen.18 Contamination of pentacene due to reaction with oxygen is occurring so quickly that it has to be stored and treated very carefully under inert atmosphere. The purity is very important to avoid trapping of charge carriers in the transistor. (See point 2)

Thus, under the influence of air, pentacene is unstable and degrades. Also water and UV-light are harmful. This sensitivity is caused by its structure. The fusion of the five rings diminishes its aromaticity and makes the system more reactive, especially the central ring. The

Introduction

30

double bonds in this ring possess radical character which is increasing the reactivity towards oxygen. When oxygen reacts with pentacene, it forms first an HQGR-peroxide on the central ring. The formation of this oxygen-bridge can occur in two ways. Like in anthracene, there is a diene structure present. This can undergo a Diels-Alder reaction with oxygen in its exited state. However, more important is the radical reaction that can take place, because of the radical ground state nature of molecular oxygen. This reaction occurs spontaneously and very fast. The HQGR-peroxide � that is formed can then further convert to the stable pentacenequinone �.18 (Scheme 1) This degradation is promoted by light and therefore it is important to avoid light and oxygen on storage.

6FKHPH��O

O O

O� �

The last few years, there has been much research carried out to solve these problems of solubility and stability. In this context two methods have been developed.

A first method consists of soluble pentacene precursors that can be easily purified and deposited from solution. Once these precursors are on the substrate, they can be converted to pentacene by a retro-Diels-Alder reaction.1a On the other hand a number of substituted pentacenes have been explored.1a By introducing substituents on the aromatic core, both stability and solubility will increase. Moreover, it has been shown that substituents can affect the self-assembly of pentacene moieties resulting in a closer packing which even can improve the electrical properties.7,19

Introduction

31

Previous studies concerning both solutions will be discussed in more detail.

��3HQWDFHQH�SUHFXUVRUV�The pentacene precursors are Diels-Alder adducts of pentacene. Because of their butterfly-like structure, they are more soluble than the planar pentacene and thus can be purified in a straightforward manner. A second advantage of the improved solubility is the possibility of depositing the precursor from solution which makes them competitive with polymers. After deposition the precursor can be reconverted to the semiconducting pentacene by a simple retro-Diels-Alder reaction. Depending on the precursor, this reaction can be induced by heat or light. The need of converting the precursor back to pentacene demands some requirements of the precursor. It has to be relatively easy to break the bridge and one has to be sure to remove the formed products, both in a quantitative way.

This method was first proposed by Müllen and co-workers in cooperation with Philips in 1996. The synthesis of the precursor was published two years later.6 The pentacene precursor � is shown in Scheme 2.

6FKHPH��

ClCl

Cl

Cl200°C

�

A transistor was made by spincoating the adduct from a dichloromethane solution. After processing, the precursor � was converted into pentacene by heating to 200 °C. The mobility that

Introduction

32

could be obtained was 0.1 cm2/Vs (gate bias -20 V). This adduct � was adopted by Philips for further optimization with the prospect of industrial use.20

This first pentacene precursor � left some room for further improvements. Firstly the synthetic route consisted of five steps not including the synthesis of one of the starting materials. Secondly the conversion temperature needed to break the two relatively strong carbon-carbon bonds is about 200 °C. This temperature is too high to be compatible with most flexible substrates.

Solutions to these disadvantages attracted not only our attention (Part 1 of this work) but also that of Afzali and co-workers. Starting from 2002, this group has published different precursors.21 They prepared precursors in a one step reaction by making use of a Diels-Alder addition.

6FKHPH��

S

NO

O

S

NO

OO

S

NO

O

CH3CON=S=OCH3ReO3

chloroform, reflux

(CH3)3COCON=S=OPd2+

chloroform, reflux

SnCl2dichloromethane, RT

�D

�E

�F

N

O

SO

Introduction

33

Their first adduct �D21a was prepared by the reaction of pentacene with a sulfinylamide in the present of methyltrioxorhenium. (Scheme 3) The adduct can be spincoated and converted to pentacene by heating to 130 °C. The transistor showed mobilities up to 0.1 cm2/Vs. The second precursor �E21b can not only undergo a retro-Diels-Alder reaction by heating but also with UV-light in the presence of a photo-acid. After illumination, the non-converted precursor could be washed away with methanol. (Mobility: 0.1 cm2/Vs). A film consisting of precursor �F21c can be patterned. While shining light through a mask, the precursor will polymerise in the illuminated areas. By washing with alcohol the non-polymerised precursor in the dark areas can be removed. Next the polymer is reconverted to pentacene by heating. (Mobility: 10-2 cm2/Vs) Besides the precursors that are published in the open literature, in 2003 there has been a patent published together with IBM, where many pentacene adducts were reported.22 We had also planned to prepare some of the same adducts, so unfortunately, we had to change plans.

A few other pentacene precursors were synthezised by other groups through the years with the intention to integrate them into transistors, but until now no results have appeared.23,24

��6XEVWLWXWHG�SHQWDFHQHV�A few years ago there were only a few substituted pentacenes known. The synthesis and properties of substituted pentacenes only knew a limited interest in the beginning and the middle of the previous century.25 The last few years however, there has been an increase in the interest in this class of compounds.1d

By introducing substituents on the aromatic core of pentacene, the solubility can be increased. This is leading to a better control of the purity and opens the possibility of solution processing. The substituted pentacenes thus gain again the advantages of the use of polymers. The

Introduction

34

substituents will also improve the stability of the original pentacene against oxygen, especially when they are protecting the reactive central ring.

The pentacene derivative can not be converted into pentacene, like with the precursor pentacenes. The substituents are permanent and this implies an uncertainty. Pentacene is known to be a good semiconductor, but with the substituents the structure is changed. So the big question is if they will maintain the required properties on top of all the advantages discussed here. This uncertainty holds back many scientists involved with electronics, to explore more of these and other new materials. However, the substituents can in some cases affect self-assembly and increase the stacking. This can lead to better intermolecular overlap between the pentacene moieties and in this way improve the charge carrier transport. Perhaps the mobilities could even be increased in such a way that they would be even better than the original pentacene.

In 2000 the first substituted pentacene was published with the aim to solve the solubility problems of polyacenes.26 It was substituted with propyl groups on the first, thirth and fourth ring and ester groups on the 2- and 3-position. However the synthetic paper was never followed with any electronic results. The first substituted pentacenes for OTFT’ s that also showed transistor characteristics carried their substituents on the 6,13-positions for optimal protection against oxidation.7,27 Besides studies on substitution of these positions, research has been carried out on the introduction of substituents on the peripheral rings. Substitution at the other positions (5,7,12 and 14) has rarely been investigated.25b

Most research has been carried out with pentacene bearing alkynyl substituents on the inner ring.7,27,28b Only one other group used aryl substituents for OTFT’ s29 and another one for OLED’ s.28a For

Introduction

35

substitution of the outer rings, mostly halogen or alkyl groups were used.30

In 2001 a study of 6,13-bis(trimethylsilylethynyl)pentacene was published by Anthony and co-workers.27c They suggested the use of this derivative in OTFT’ s based on the result of their study. This was the start of a series of 6,13-dialkynyl substituted pentacenes � (Scheme 4) that has been and still is intensively studied. 6,13-Bis(triisopropylsilylethynyl)pentacene (�F) even recently pulled out of academic research and was adopted by Merck.31a The synthesis of such compounds consists of two steps.7,27 First there is an addition reaction and this reaction is then followed by a reduction leading to the desired pentacene. (Scheme 4)

6FKHPH��

O

O

HO

HO

R =

R

R

R

R

Si

Si

Si

TMS D,

TES E,

TIPS F,

G,

�

H

�

�

Anthony HW�DO.27 have claimed that the ethynyl group was necessary as a spacer to increase the distance between the pentacene backbone and

the bulky silyl group. In their vision, the S-stacked interaction

Introduction

36

between the conjugated system would be disrupted by steric hindrance caused by the large silyl groups. Meanwhile there are indications that a spacer is not always necessary when introducing sizeable groups.

The substituted pentacenes showed an increase in solubility and stability. They could be purified by the usual techniques like crystallisation and column chromatography. Some solution processed transistors have been made. The stability against oxygen has significantly been improved. However, recently it has been proven that UV-light is leading to a degradation of the product also without the presence of oxygen. UV-light is promoting dimerisation on the 5 and 14 positions.32 (Figure 15) There are indications that the same is true for pentacene. It is therefore important to protect the substituted pentacenes against UV-light.

R

R

R

R+

R

RR

R

R = C C Si(iPr)3

)LJXUH��'LPHULVDWLRQ�LQGXFHG�E\�89�OLJKW�

While comparing the different 6,13-bis(silylethynyl)pentacenes � with each other, it has been shown that the substituents have a large influence on the performances of the substituted pentacene in the transistor. From all products it has been tried to achieve a continuous film by evaporation integrated in a transistor and subsequently the devices were measured. Pentacene derivative �H did not show any transistor characteristics. The first experiments on the other derivatives showed mobilities from 10-5 cm2/Vs for TMS-pentacene �D and TES-pentacene �E to 10-4 cm2/Vs for TIPS-pentacene �F and t-

Introduction

37

butyl-pentacene �G.27b The first three derivatives formed amorphous films with a one dimensional ‘slipped stack’ . The t-butyl-pentacene �G grew in small poorly ordered grains, while TIPS-pentacene �F showed tall, long, thin grains with also poor substrate coverage. The performance of the latter could be improved until >10-1 cm2/Vs by adjusting the substrate surface and the film thickness. Merck is now claiming even 1 cm2/Vs with a specific transistor structure and deposition process.31b TIPS-pentacene stacked in a two-dimensional ‘bricklayer’ -structure. Also films deposited from solution gave the same values. Recently TIPS-pentacene has been used in a photovoltaic cell.28b Thus, substituted pentacenes are not only interesting for the use in OTFT’ s but could also be applied in OLED’ s or photovoltaic cells.28b,c

The triisopropylsilylethynyl group was also introduced at the 5- and 14-positions, because it was believed that this unsymmetrical derivative would have a nearly perfect stacking, but this was only partly true.7 The molecule formed pairs that had a nearly perfect stacking but the pairs were then arranged in a herringbone pattern. Moreover the compound seemed extremely sensitive to oxygen and the films decomposed rapidly upon exposure to air.

To further increase the mobility obtained with 6,13-bis(triisopropyl-silylethynyl)pentacene �F, the group of Anthony carried out research on the further substitution of the 2-,3-,9- and 10-position.27 First they tried to use ether groups and finally they came up with the substitution with dioxolanes attached to the peripheral rings, although until now without electronic results.33a Very recently, these molecules were tested as red-light-emitting materials for OLEDs.28c Subsequently they changed the extra two five-membered rings from dioxolanes into thiophenes.33b It was necessary to use tritertbutylsilylethynyl groups instead of triisopropylsilylethynyl groups, on the central ring to prevent dimerisation. Because the anthradithiophenes (reported a few years earlier by Katz HW� DO.)34 seemed much more promissing, they

Introduction

38

focused their research on these molecules that resemble the pentacene derivatives. The outer rings of the pentacene backbone are replaced by thiophenes. On the central ring the same silylethynyl groups were introduced. (Figure 16) The molecules arranged differently in the layer than the analogous pentacene derivatives. Here the isopropyl groups forced the molecules into an one-dimensional slipped arrangement and the ethyl groups in a two-dimensional ‘bricklayer’ structure. As expected, the derivative with the two-dimensional ‘bricklayer’ structure possessed the highest mobility. In this case it was the triethylsilylethynyl substituted anthradithiophene that gave the highest mobility. (1 cm2/Vs, solution-processed)35

R

R

�

S

S

R = Si

Si

Si

D

E

F

)LJXUH��$QWKUDGLWKLRSKHQHV�

Also another research group substituted the 2-,3-,9- and 10-position of the 6,13-bis(trimethylsilylethynyl)-pentacene �D. They developed a synthetic pathway to get different ethynyl moieties on the peripheral rings.36 The electronic properties are currently being studied.

By using more electron withdrawing groups on the pentacene backbone, one can switch the charge carriers into electrons and change the p-type pentacene into an n-type semiconductor. This was first shown with perfluoropentacene where all hydrogens are replaced by fluorine atoms.37 This material can be easily grown into thin-films with n-type characteristics by evaporation. (electron mobility: 0.1 cm2/Vs, on/off ratio: 10-5) In accordance the group of Anthony

Introduction

39

introduced halogen (Br and F) and cyano groups on the peripheral rings of their 6,13-bis(triisopropylsilylethynyl)pentacene �F.38 For both the derivatives with one or two fluorinated rings, they published the n-type transistor results. (electron mobility: 10-2 cm2/Vs)

Not only ethynyl groups are introduced onto the pentacene backbone. Very recently a Japanese group managed to attach alkylsulfanyl groups on the inner ring.39 The sulphur atoms promote the stacking of the molecules. Electronic measurements are currently under investigation.

During our research, some substituted phenyl groups were introduced at the 6,13-positions of the pentacene backbone by the group of Kafafi.28 These products were studied for their use in OLED’ s.

In the beginning of 2006 a very interesting series of pentacene derivatives was synthezised and tested by Miao HW� DO.29 (Figure 17) They have substituted pentacene with a phenyl group on different positions. Transistors made by evaporation were tested. The results showed that substitution in the 5-, 7-, 12- and 14-positions was most successful. Derivative ��F had a mobility of 10-3 cm2/Vs. This was one magnitude larger than derivatives ��D and ��G. This mobility increased even with two orders of magnitudes compared with derivative ��E. Substituted pentacene ��I showed no transistor characteristics. When Miao HW� DO. replaced the phenyl group of derivative ��E by a 2-thienyl group (��H), the mobility increased in a spectacular way to 10-1 cm2/Vs.29 These results invalidate the claims of the necessity of a spacer between the pentacene core and a bulky substituent made by Anthony HW�DO.27

For substitution of the outer rings many halogens and alkyl groups were used. In this context 3M published their results of a few derivatives.30a These scientists introduced methyl groups and hexyl groups on the peripheral rings and the derivatives were integrated in a transistor by evaporation.

Introduction

40

��D ��E

��F ��G

��I

S

S��H

)LJXUH��6HULHV�RI�FRPSRXQGV�RI�0DLR�HW�DO��

Both compounds showed the same mobilities as pentacene evaporated under exactly the same conditions. (10-1 cm2/Vs) The same results were obtained for an analogous compound by another group.30b Moreover, four trimethylsilyl groups were placed on the 2-, 3- ,9- and 10-position and there were also derivatives made with ethers substituted on the 2- and 3-position, but until now no electric measurements were shown.24a, 40

Introduction

41

��'HSRVLWLRQ�WHFKQLTXHV��

Different kinds of deposition techniques are available for introducing the organic semiconductor on a transistor substrate. One can divide these techniques into two groups: vacuum processing and solution processing techniques. The method that is used is strongly depending on the characteristics of the material.

When the organic semiconductor possesses a low solubility but can be sublimed which is the case for most ‘small molecules’ semi-conductors, evaporation techniques are employed like organic molecular beam deposition (OMBD) and organic vapour phase deposition (OVPD). The use of OMBD is widespread for molecules like pentacene. The material is evaporated under high to ultra-high vacuum (10-6-10-12 Torr). By placing the substrate into the stream of the sublimed molecules, a semiconductor layer is formed onto the substrate. (Figure 18) The advantage of this technique is the efficient control of the growth conditions of the molecules on the substrate which leads to well ordered thin-films. However the method has a lot of disadvantages like the use of high vacuum and difficulties to process large areas.

)LJXUH��20%'�

Introduction

42

A rather new technique is the OVPD. Here only low vacuum is used in combination with a carrier gas. The material is evaporated and the molecules are transported with the carrier gas towards a ‘showerhead’ with underneath it the substrates. The molecules are driven through the showerhead and are deposited onto the substrate. This technique has the advantage that it is possible to combine low vacuum with good growth control and well ordered films.

Solution processing techniques are mainly used for polymers. These methods avoid the use of vacuum, have low process temperatures and are compatible with large areas. The advantages of deposition from solution is stimulating the research of suitable small molecules that can be applied to the substrate by these methods.

Examples of different solution processing techniques are: Solution/dropcasting: a solution is poured onto the substrate. Aerosol spray: a ‘mist’ of fine droplets is homogenously sprayed on a

substrate. Dipcoating: a substrate is dipped into a solution. ‘Doctorblading’ : a blade is spreading the solution over a substrate. Spincoating: a solution is spread out over the substrate by rotating it.

In the meantime the solvent is evaporating and a film is formed. (Figure 19)

Inkjetprinting: small droplets of a solution are ejected in a patterned way on a substrate. Screenprinting: a solution is pressed through a screen on a substrate.

)LJXUH��6SLQFRDWLQJ�

Introduction

43

��$SSOLFDWLRQV��

Organic thin-film transistors could replace in many applications the traditional thin-film, mostly made of amorphous silicon on a glass substrate. OTFT’ s offer different important advantages in comparison with the traditional silicon technology. The technologies for depositing organic materials can be solution based. This reduce the processing temperatures, makes the processing more straightforward and allows techniques for mass production. Instead of glass, flexible substrates can be used and large-area electronics would not be a problem. These advantages open the path to a whole range of new applications. We will discuss some of the possibilities.

- Replacement of the classic barcodes: OTFT’ s can be used in integrated circuits which lead to electronic product identification. (Figure 20) RFID-tags (Radio-frequency identification tags) have already been made. This system is used in seaports to get track of the enormous container flux. In the future, when RFID-tags can be produced sufficiently inexpensively, one could label every product in the supermarket in this way.

��)LJXUH��5),'�WDJ� � ��)LJXUH��)H[LEHOH�VFUHHQ�

- Intelligent cards: Bank cards and identification badges will be even less expensive and possess a more advanced security.

Introduction

44

- Displays: OTFT’ s will offer the possibility to fabricate flexible and ultra-light screens, (Figure 21) even large-area ones.

- Portable electronics: The use of flexible materials will give the possibility to develop light and thin electronics for integration in clothing, accessories and bags.

- Sensors: One of the ideas is a sensor on the package of food that indicates the freshness. Due to the detection of the presence of thiols and amines, the sensor could warn the costumer when the food has gone bad.

- Electronic nose: Some volatile components could be detected. One possibility could be a scanner on airports that would warn for explosives or replacement of drug hounds by scanners.

These are just a few examples. The advantages of OTFT’ s will inspire people and companies to a lot of innovative applications.

Objectives

45

2EMHFWLYHV�

The aim of this thesis was the realisation of a solution processable organic semiconducting layer in an OTFT base on pentacene. To realise this, the problems which are related to the stability and solubility of pentacene, had to be solved. Two solutions for these problems are proposed in the literature namely pentacene precursors and substituted pentacenes. Both pathways were explored. A series of new molecules was synthezised and processed into thin-films and the obtained transistors were tested on their electronic characteristics.

In the first part the investigation of pentacene precursors is discussed. The ideal adduct of pentacene has to satisfy some important requirements that deal with its stability and the nature of the expelled molecule. The substituents on the bridge may not be too voluminous to allow the optimal reorganisation of pentacene after the thermolysis. The conversion temperature of the precursor has to be as low as possible. In this way the pentacene films obtained with the precursor route will be compatible with a large variety of flexible (plastic) substrates. The conversion has to be quantitative or the non-converted adduct must be easily removable. No side reaction may be induced by the expelled product during the thermolysis and the expelled product has again to be easily removable. At the moment we started our investigation, the area of pentacene precursors for the use in OTFT’ s seemed barely explored. Our aim was to improve the adduct of Müllen by the synthesis of hetero-Diels-Alder adducts. The incorporation of the heteroatom could decrease the conversion temperature due to lower bonding energy. Hence, different substituents were placed on the bridge in order to select the optimal precursor.

Objectives

46

All proposed precursors possess a hetero-atom in the bridge to reduce the conversion energy. Bridges containing C-S, N-N and N-O were elaborated. The proposed precursors with a C-S-bridge were divided into four groups: precursors deduced from thiophosgene and precursors with electron rich, electron poor and electron neutral groups on the bridge. The variety of the precursors was chosen to study the influence of the different bridges on the deposition, the conversion and the removal of the expelled molecules.

In the second part of this thesis the focus is on the new field of substituted pentacenes. These molecules could have an additional advantage to the pentacene precursors. The substituents on the pentacene backbone could induce self-assembly and induce in this way a better stacking and in consequence also better charge carrier mobilities in an OTFT. Furthermore the final products would also be more stable than the parent pentacene. When we started our research, the class of substituted pentacenes was barely explored. The interest in substituted pentacenes for application in OTFT’ s, is increasing although with a low variety of substituents and substitution patterns. In our work we focused on the introduction of aryl substituents. The aim was to build up a broad variety of pentacene derivatives by varying not only the type but also the number and the positions of the substituents. In this way, a better understanding of the influence of these parameters on the performance in a semiconductor layer was envisaged. Those insights are necessary to fine-tune the characteristics of pentacene derivatives for different applications. To obtain a broad spectrum of pentacene derivatives, different aryl substituents, both electron rich and electron poor, were used. In addition the possibility of asymmetrical substitution on the inner ring was investigated. Furthermore some attempts were made to introduce alkyl chains on the pentacene backbone. The aim was to vary the number of substituent from one to eight and to place the substituents

Objectives

47

on the different rings. In this way we hoped to realize 6-monoaryl substituted pentacenes, 6,13-diaryl- and di(alkylsulfanyl)pentacenes, 5,7,12,14-tetra-aryl- and tetra-alkylpentacenes, 5,6,7,12,13,14-hexa(alkylsulfan-yl)pentacenes, 2,3,9,10-tetra-alkyl-5,�(��-diaryl-pentacenes and 2,3,9,10-tetra-alkyl-5,7,12,14-tetra-arylpentacenes.

In the last part of the thesis the processing of the materials into thin-films is discussed. The aim was to integrate the materials in a transistor structure via spincoating. Electrical characterisation of the resulting OTFT’ s would give more insight on the structure-property relations within this class of materials.

Part I

49

3DUW�,�3HQWDFHQH�SUHFXUVRUV�

The first part of this thesis deals with our work concerning pentacene precursors. By preparing a precursor of pentacene via a Diels-Alder reaction, one can circumvent the problems associated with pentacene. Because of the solubility and the stability of the precursor, simple methods like spincoating can be used for deposition on a substrate and the product can be conveniently stored and handled as described in the introduction. After deposition, the precursor is reconverted to pentacene by a retro-Diels-Alder reaction in order to obtain a semiconducting layer. (Figure I.1)

XX

RR retro-Diels-Alder

reaction

)LJXUH��,��5HWUR�'LHOV�$OGHU�UHDFWLRQ�

It has been known for a long time that pentacene reacts as a diene. Different adducts were synthesized with classical dienophiles like benzoquinone, chloranil, maleic anhydride, butenes, styrene and oxygen.43 They were all mainly prepared for reactivity studies. Thus, for the synthesis of new pentacene precursors, we based our proposed adducts on anthracene analogues.44-46 In contrast with pentacene, an enormous variation in Diels-Alder adducts of anthracene has been synthezised throughout the years.

Part I Chapter 1.1: Synthesis precursors

50

&KDSWHU��6\QWKHVLV�RI�SHQWDFHQH�SUHFXUVRUV�

All proposed pentacene precursors can be synthesised in one step, namely, a Diels-Alder reaction between pentacene and a dienophile. To obtain the different adducts and to optimize each reaction, a large amount of experiments has to be performed. Consequently pentacene was needed as a starting material in gram amounts. Because commercially available pentacene is very expensive, we first searched for a suitable preparation.47

Subsequently the synthesis of the precursors will be discussed. First the adducts with a C-S bridge were investigated. Thiophosgene was used as a dienophile because the analogous anthracene adduct showed reconversion temperatures below one hundred and fifty degrees. For the precursors with an electron rich group on their bridge, thioamides were used as the dienophile.

The precursors with electron poor substituents, a neutral substituent and those with a N-O-bridges or N-N-bridge were not studied because during our investigation most of them were published by another research group.23

��6\QWKHVLV�RI�SHQWDFHQH�The most widespread method to prepare pentacene was described by Bruckner in 1961.48 First the pentacenequinone �� is synthezised and subsequently pentacene �� is generated by reduction of the carbonyl functions. (Scheme �I.1) In all other methods, mostly from old literature, pentacene was synthezised by dehydrogenation of dihydropentacene.49 However the synthesis via pentacenequinone �� is preferred because of the much easier and shorter synthetic route.


51

6FKHPH��,��O

O ��

reduction

��

Two different methods to prepare pentacenequinone �� were tested and compared. In the first case pentacenequinone �� was prepared

starting from D,D,D’ ,D’ -tetrabromo-o-xylene and 1,4-benzoquinone.50

The formation of pentacenequinone �� starting from these compounds was observed as a side product in the preparation of 1,4-anthraquinone. To promote the formation of the pentacenequinone ��

the equivalents of D,D,D’ ,D’ -tetrabromo-R-xylene were increased. The

reaction was carried out in dimethylformamide at 65 °C in the presence of sodium iodide for twenty-four hours. (Scheme �I.2) Unfortunately the yield was insufficient.

6FKHPH��,��

2CHO

CHO+

O

O

O

O ��

2CHBr2

CHBr2

+

O

O

NaI, DMF60°C, 24h

KOH, EtOHTr, 5h

The second route, used by Bruckner, consisted of a condensation of o-phthalaldehyde and 1,4-cyclohexanedione.48 These starting materials are inexpensive and readily available. The reaction itself was very straightforward in basic conditions. After stirring for five hours, the pure pentacenequinone �� could be easily filtered with a high yield


52

(average 90%). (Scheme �I.2) This reaction delivered us an easy way to all the pentacenequinone �� we needed during our work.

In a second step the pentacenequinone �� had to be reduced to pentacene ��. In the general procedure, the reduction is carried out with Al-amalgam.48,51 Because of the health and environmental risks involved in the use of mercury, we have been looking for an alternative reduction process of the quinone ��, proceeding in a reproducible and clean way. Different reduction methods were tested using Zn, NaBH4 and LiAlH4 as reducing agent.

The reduction with zinc was carried out in acetic acid.52 The resulting residue was boiled in toluene in order to purify the pentacene. Reflux was required in order to get a purple residue after purification. The purity of the resulting purple pentacene was checked with thin-layer-chromatography (TLC) and mass spectroscopy. It was observed that the purity of the residue strongly depended on the reaction time. Reaction times shorter than 1 hour left a large amount of unreacted quinone ��. However when the reaction time was longer than two hours, overreduction to 6,13-dihydropentacene became a problem. When the reaction was scaled up, these problems became more and more pronounced and the purifications failed. Attempts to adjust the purification were more or less successful when only one or two grams of pentacenequinone �� was used. When adding a small amount of cyclohexanol (to dissolve the pentacenequinone better)53 to the reaction mixture and using an intensive washing sequence of the residue, pure pentacene was obtained. However also here all attempts to scale up failed and the reaction appeared to be unreliable. Due to these two problems, the method is too time-consuming, so our attention was shifted to other reduction methods.

The next reducing agent that was studied was NaBH4.54 The reduction of pentacenequinone to pentacene with NaBH4 consists of two consequent similar steps. (Scheme �I.3)


53

6FKHPH��,��

O

O ��

1) NaBH4 THF, reflux

2) HCl reflux

O

OH

��D

��E1) NaBH4 THF, reflux2) HCl reflux

��

In the first step, one of the carbonyl groups of the quinone was reduced with NaBH4 and subsequently treated with acid to form the intermediate 6,13-dihydro-13,13’ -dihydropentacene-6-one ��D. This compound is in equilibrium with its isomer ��E. In a second step, the second carbonyl group was reduced and worked up in a similar way to form the desired pentacene. Because of the low solubility of the pentacenequinone in THF, reflux conditions were necessary. The reaction time of the reduction with NaBH4 was 12 hours. After acidification and stirring for 3 hours under reflux conditions, the residue obtained by filtration needed further purification. In order to improve the efficiency of the reaction and the purity of the obtained pentacene residue, NaBH4 was replaced with LiAlH4.

The optimal reaction time of the reduction with LiAlH4 was 0.5 h. When the reaction time was increased the yield decreased. The amount of reducing agent was also optimized and four equivalents were found to give the highest yield. Less equivalents decreased the yield and resulted in less pure material. Changing the solvent from


54

tetrahydrofuran to ethylene glycol dimethyl ether (or 1,2-dimethoxyethane, DME), gave lower yield. The use of cyclohexanol resulted only in the recovery of starting material. The optimal reaction conditions to obtain pure pentacene in the best yield were the reduction with four equivalents of LiAlH4 in tetrahydrofuran and reflux conditions during half an hour under argon atmosphere. The reaction could be easily scaled up to 5 g without any problems and gave average yields of 55%. The pentacene could be used without further purification and the result was highly reproducible.

During the optimization of the reduction method with NaBH4 and LiAlH4, it turned out that it was possible to isolate all the intermediates that were formed during this reaction sequence. (Scheme I.4) There are two successive steps needed to obtain the pentacene, each consisting of a reduction followed by elimination of water. To isolated the diol �� the reaction was carried out with NaBH4.

6FKHPH�,��O

O ��

LiAlH4THF, reflux

OOH

��D��E

HCl reflux

��

OHHO

��

NaBH4

THF, reflux

HO

��

HCl

reflux


55

On treatment of the pentacenequinone with NaBH4, first the two carbonyl groups of �� are reduced to alcohols. After careful work-up of the reaction mixture with acetic acid, diol �� could be isolated as a mixture of FLV- and WUDQV-isomers in a ratio of 35:65.

Upon acidification with stronger acids, water is expelled from diol ��, affording ��D�E. Ketone ��D is formed together with the corresponding enol 6-hydroxypentacene (��E). The latter is insoluble and could not be characterized with NMR spectroscopy. However, we suspect its existence because in the mass spectra, the same m/z value was observed for the molecular ion of the soluble yellow ketone ��D and a purple, highly insoluble residue from the same reaction mixture.

The second reduction step that was carried out on the crude mixture containing ��D en ��E, gave pentacene. The amount of pentacene obtained is larger than the amount of ��D that can be isolated after the first step, indicating that ��E is present and can be reduced, probably after tautomerisation to ��D. The second reduction proceeded in the same way. Unfortunately the intermediate alcohol �� could not be isolated. After the reduction and treatment with acid, pentacene �� was obtained.47

��3HQWDFHQH�SUHFXUVRUV�YLD�WKLRSKRVJHQH�The expected Diels-Alder adduct from pentacene with thiophosgene was the adduct ��. (Scheme �I.5) However when the reaction was tested, there were serious doubts that adduct �� was obtained although NMR investigation pointed to its formation. However, the mass spectrum did not correspond with the expected product. It appeared that the adduct had already hydrolysed. Indeed after performing a hydrolysis, the adduct did not change. Therefore, due to its sensitivity towards hydrolysis, the primary adduct �� could not be isolated and the reaction gave immediately the adduct ��D. (Scheme �I.6)


56

This excluded the proposed synthetic route for the thio and imine analogous adducts �� E�F by treatment of �� with respectively hydrogen sulphide and amines.

6FKHPH��,��

��

S

Cl Cl S

��

Cl Cl

S

NRS

��

XDEF

H2X X = O

The yield of the reaction was insufficient (7%), so the reaction conditions were changed in order to increase the amount of adduct formed. Firstly, the pentacene was suspended into toluene and two equivalents of thiophosgene were added. The reaction mixture was heated to 65 °C during five hours. The low yield could be assigned to the low solubility of the pentacene which prevented good contact between the reagents.

6FKHPH��,��

�� 65°C6h

S

Cl Cl S

O

��D

Instead of using toluene as a solvent, more thiophosgene was added. Thus, the thiophosgene took over the role of solvent. In this way the yield was increased to 21%.


57

The reaction yield could be further improved to 38% by altering the work-up. First toluene was added after the reaction and subsequently the unreacted pentacene could be filtered. The filtrate was further purified by column chromatography to obtain the pure adduct. When dichloromethane rather than toluene was added after the reaction, more of the adduct could be extracted from the pentacene residue.47

Different attempts were made to obtain ��E and ��F via another synthetic route. For the preparation of adduct ��E carbon disulphide was tested as a possible dienophile. In the reaction of pentacene with carbon disulphide, the latter was used as a solvent as in the preparation of adduct ��D. The reaction mixture was heated in a closed vessel to 50 °C. No reaction occurred. Also when the temperature was raised to 70 °C which caused a little overpressure, no products were formed. The carbon disulphide seemed unreactive under these circumstances.

Finally we tried to prepare adduct ��F starting from the earlier synthezised adduct ��D. First amines were added to the precursor ��D in order to get addition and elimination of water. After adding the primary amine, methylamine, the desired adduct was only detected with mass spectroscopy. There were mainly two other products formed. Despite many efforts the two products remained unidentified. Also hexylamine gave only rise to two unidentified products. We switched over to a secondary amine, dioctylamine. Also this approach afforded unidentifiable reactionproducts.

In a second attempt to obtain adduct ��F, isothiocyanates were used as the dienophile. Two equivalents of methylisothiocyanate were heated at 85 °C with pentacene in toluene during twelve hours. After purification only unidentified products were isolated. The reaction was repeated without solvent, but without result. The same reaction was performed with phenylisothiocyanate. Again the reaction was leading only to unidentified products. No further attempts were made,


58

because the Diels-Alder reaction did not seem to occur at a sufficiently low temperature (< 100 °C).

��3HQWDFHQH�SUHFXUVRUV�ZLWK�HOHFWURQ�ULFK�EULGJH�In order to obtain an electron rich bridge on the pentacene adduct, groups as amines, ethers and thioethers have to be used. The different precursors that were proposed are shown in Scheme �I.7 and Figure �I.2. The electron rich group would stabilise the double bond between carbon and sulphur that is formed when the bridge is expelled from the adduct by a retro-Diels-Alder reaction. This could lower the conversion temperature.

6FKHPH��,��

��

S

N H S

��

N

R1

R2

D R1, R2 = Me

E R1 = COMe, R2 = benzyl

R1

R2

Dimethylthioformamide was used in order to prepare adduct ��D. The starting material was synthezised from dimethylformamide via the Lawessons reagent55 and it was heated together with pentacene in toluene at reflux. No reaction took place, even not after a long time (12h).

SX R

��

D X = O

E X = S

)LJXUH��,��3HQWDFHQH�SUHFXUVRUV�ZLWK�HOHFWURQ�ULFK�EULGJH��


59

Because of the good results with thiophosgene, our attention was shifted to the analogous thiocarbamoyl chlorides. A Diels-Alder reaction was performed with phenylthiocarbamoylchloride (PhOCCl=S). One would expect that the carbon sulphide double bond in this molecule is more reactive, however also this dienophile gave no results. Even after a long time at 100 °C no reaction occurred and the pentacene was recovered together with its oxidation product. These results indicate that the products ��D and ��D�E probably can not be realised at an acceptable temperature in this way.

In order to prepare precursor ��E, N-benzyl-N-acetylthioformamide �� had to be synthezised. (Scheme I.8) In a first attempt ethyl formate was reacted with the Lawessons reagent to form O-ethyl thioformate. The latter was isolated. N-Benzylamide was prepared by reaction of benzylamine and acetylchloride in the presence of a base (yield: 72%). Then the N-benzylamide �� was added in a dropwise manner to a solution of the O-ethyl thioformate.56 After adding triethylamine and heating at reflux for some time, no product was formed, so the strategy was changed.

6FKHPH�,��

H O EtO Lawessons

reagent

THFH O Et

S

HN Me

O

NEt3, EtOH

N

Me

O

HS��

��

The O-ethyl thioformate was not isolated but compound �� was added directly to the reaction mixture. In the presence of triethylamine the N-benzylamide �� reacted with the in situ generated O-ethyl thioformate to form product ��. The total yield of the reaction was


60

18%. Then the N-benzyl-N-acetylthioformamide �� was used to perform a Diels-Alder reaction with pentacene. From the literature of anthracene analogues, it was known that this reaction benefits from the use of a Lewis-acid catalyst.45 In the first test, boron trichloride was added to the reaction mixture without success. The boron trichloride was replaced by titanium tetrachloride. This time the desired adduct ��E was detected by mass spectroscopy. However, it could not be isolated. In order to increase the formation of the product ��E the temperature was increased from room temperature to 75 °C. Unfortunately, the adduct ��E could not be found. Even when the reaction was repeated at 40 °C to avoid decomposition, we were not able to isolate the adduct ��E.

Part I Chapter 1.2: Study of precursor

61

&KDSWHU��6WXG\�RI�SHQWDFHQH�SUHFXUVRU�After the successful synthesis of the pentacene precursor ��D, the thermal conversion of the adduct into pentacene was studied. Also the possibility of processing thin-films of the adduct from solution for

OTFT’ s was investigated.

��6WXG\�RI�WKHUPDO�FRQYHUVLRQ��

In the final semiconducting layer made of a soluble precursor, normal pentacene would take care of the electronic performance of the device. Thus, after deposition by spincoating, the adduct has to be converted back to pentacene. With adduct ��D this is possible via a retro-Diels-Alder reaction induced by heating the sample. (Scheme �I.9) The thermolysis of adduct ��D was first studied in solution to get an idea of the conversion temperature. It is important to know if conversion close to 100% is possible at a relatively low temperature (< 200 °C) because no precursor should be left in the spincoated film after thermolysis. When the conversion temperature is lower than 140°C it is possible to follow the conversion by 1H NMR spectroscopy in a CDCl2CDCl2 solution.

6FKHPH��,��

S

O

��D

Heat+ C OS

��

A few tests were performed in solution at different temperatures. Conversion was observed already at 100 °C by the formation of a precipitate. At 140 °C the formation of pentacene was clearly noticed by the appearance of a purple colour. To quantify the retro-Diels-


62

Alder reaction, the conversion was monitored by 1H NMR spectroscopy. (Figure �I.3)

The adduct ��D was dissolved in tetrachlorodideuteromethane (CDCl2CDCl2) in order to reach high enough temperatures. During the conversion the signals of the protons decrease. The protons H6 and H13 shift from around 5,5 ppm to the aromatic region. Because pentacene precipitates, the appearance of the signals of H6 and H13 in the aromatic region is vague and inaccurate for the amount of pentacene that is formed. To quantify the amount of precursor ��D that is converted during the experiment an internal standard (SDUD-dichlorobenzene) was added to the solution. The internal standard gave only one singlet signal at 7.3 ppm which is equivalent to four protons. The NMR data for the aromatic protons of adduct ��D are displayed in Figure �I.3. Equivalent amounts (number of mol) of both the standard and the precursor were used. Thus, in the 1H NMR spectrum taken at the start at room temperature all signals in the aromatic part were integrated for four protons. Using the integration

)LJXUH�,��6WXG\�RI�FRQYHUVLRQ�

aromatic protons of adduct 17a: H1, H4, H8, H11: m 4H 7.8 ppm H2, H3, H9, H10: m 4H 7.5 ppm H5, H14 and H7, H12: 2xs 4H 7.9 ppm

S

O

H5

H14

H3

H4

H2H1

H7 H8

H9

H10H11H12

relative to the protons of � -dichlorobenzene.


63

of the standard as a reference, the decrease of the integration of the aromatic protons of adduct ��D was followed in time.

In a first experiment the NMR-tube was heated to 100 °C and maintained at this temperature during 1 h 45 min. As one can see in the graph (Figure I.3) the purple curve drops very quickly in the first quarter but then the reaction reaches an equilibrium at a conversion of 25%. When the experiment was repeated at 120 °C, there was a conversion of 75% after 3 h. Also here the curve (blue curve Figure �I.3) seems to go towards an equilibrium. The observation of the equilibrium in solution can be assigned to the fact that the OCS that is expelled is partially soluble in the solvent of the closed NMR-tube.

These experiments showed that the precursor ��D was suitable for further use in the fabrication of thin-film for the use in OTFT's. The precursor could be reconverted into pentacene at a temperature below 200 °C. Thus subsequently the film formation of the precursor was studied.

��'HYHORSPHQW�RI�VHPLFRQGXFWRU�ILOPV�For the development of semiconductor films from adduct ��D spincoating was used as a deposition technique. To have a good transistor, the film that is formed by spincoating, has to be of good quality. The obtained film has to be continuous and the morphology of the film has to promote the closed stacking of the molecules. These characteristics depend on several different factors. The quality of the film can depend on the material, the solvent that is used and the concentration of the solution that is spincoated. Also the spin conditions such as the speed and time of spinning are important. Furthermore, the substrate on which the film is deposited, plays an important role. The way the sample is treated after spinning can also have an influence on the finally obtained film. All factors have to be


64

studied to find the optimal conditions for achieving a homogeneous film with good transistor characteristics.

When a drop from a solution of adduct ��D was placed on a substrate, conversion could be observed already in two minutes at 140 °C. For the first experiments this temperature was used to induce the conversion. For testing the film formation conditions, normal glass plates were used. Different solvents were tested: dichloromethane, tetrahydrofuran (THF), dimethylformamide, toluene, dioxane, N-methylpyrrolidinone (NMP) and 1-methoxy-2-propanol acetate (PGMEA). The adduct was dissolved at a concentration of 1w% to 1.5w%. The substrate was adapted to the polarity of the solvent. For medium apolar solvents such as dichloromethane and toluene the substrate was covered with a monolayer of octadecyltrichlorosilane (OTS). In this way, the polar glass substrate becomes more apolar substrate. This improves the wetting, important to obtain a total coverage of the substrate, of the apolar solvents.41 Films were spincoated using the same spin conditions (1500 cycles per minute, 50 sec). NMP and DMF did not give good results. After spinning the solvent was not evaporated sufficiently and droplets were still laying on the surface. These remaining droplets could also not be removed during the reconversion because of the high boiling point of these two solvents. In all cases the amount of dissolved precursor ��D was not sufficient to form an interconnected film.

Therefore the concentration was increased to 3w% and even 5w%. The increase implied that some of the solvents were ruled out because they could not dissolve this amount of product ��D. The most appropriate solvent seemed to be tetrahydrofuran. However, the film was not yet fully continuous.

In order to improve the coverage of the substrate, another method of spinning was tested. Instead of first covering the surface with the solution before spinning, the solution was put in a dropwise manner


65

on the substrate during spinning. However, this had the opposite result. Large crystals were formed, which were growing vertically instead of horizontally. (Figure �I.4)

)LJXUH�,��0LFURVFRSH�SLFWXUHV��[��9HUWLFDO�JURZWK�RI�SHQWDFHQH�

PROHFXOHV�

Different spinning speeds from 500 up to 2000 cycles per minute were tested to force the formation of a continuous film. None of the speeds gave the wanted improvement. When using a high speed one had the risk of just spinning too much solution off the substrate. On the other hand when the spin speed was too low there was more solvent remaining within the film. When the film was heated to convert the adduct ��D to pentacene, it took langer to evaporated all the solvent. The pentacene molecules had then more time to migrate over the surface and to aggregate into large two-dimensional crystals (Figure �I.5A). This is not favourable for the continuity of the film. A compromise was found by using 1500 cycles per minute. However, the formation of islands of pentacene molecules could not be fully avoided because the diffusion was also promoted by the heat necessary for the conversion.

It is known that the migration of pentacene molecules can be hampered by modifying the substrate.57 By spinning a polymer solution between the glass substrate and the semiconductor layer, homogenety of the film could be improved. As a polymer, PVP (polyvinylphenol) was used to hinder the diffusion. The aim of the use

10 ��


66

of the polymer film was only partially fulfilled. In Figure �I.5, microscope pictures of a film with (Figure I.5B) and without (Figure I.5A) the intermediate polymer layer are shown. In picture I.5A one can see the large two-dimensional crystals. All pentacene molecules in the area around the crystal have diffused towards the nucleus resulting in an empty circle around it.

A B

)LJXUH�,��0LFURVFRSH�SLFWXUHV��[��7HQGHQF\�WR��GLPHQVLRQDO�JURZWK��$��ZLWKRXW�393�OD\HU�%��ZLWK�393�OD\HU�

When there is a polymer film beneath the pentacene molecules, there are less and smaller three-dimensional crystals formed. (Figure �I.5B) Also the empty area around the macrocrystal is less pronounced.

Finally the conversion conditions were investigated because also the temperature and the temperature profile have an influence on the diffusion of the molecules. Slow heating had no satisfactory result. Then the temperature was raised and the heating time decreased. Above 170 °C the film coloured purple but this colour quickly disappeared again. This is probably due to sublimation of the pentacene molecules.

The heating time was varied from a few seconds to five minutes at a constant temperature of 160 °C. The two extremes are displayed in Figure �I.6. In Figure I.6A one can observe the formation of two-dimensional crystals. By decreasing the heating time the formation of

10 ��


67

these crystals could be avoided. (Figure �I.6B) However, this created another problem because the short heating time did not allow a sufficient conversion. There were many precursor molecules left on the surface.

A B

)LJXUH�,��0LFURVFRSH�SLFWXUHV��[��*URZWK�DW�GLIIHUHQW�FRQYHUVLRQ�WLPH��$��PLQ��%��VHF�

Thus, the prepared pentacene precursor ��D could be successfully converted into pentacene. However, despite the efforts that have been done to fabricate a good quality film for the application in OTFT’ s, no satisfactory results could be obtained. Also when pentacene films are made by evaporation techniques, problems with three-dimensional growth can occur and are related to the deposition conditions. This problem was studied more in detail by Verlaak HW�DO.58

100 ��

Part I Conclusion

68

&RQFOXVLRQ�We can conclude that we found a new, environmentally friendlier method to reduce pentacenequinone to pentacene via a two-step reduction step with lithium aluminium hydride.

A Diels-Alder adduct of pentacene was obtained in a one step reaction between pentacene and thiophosgene. After trying several conditions, the best results were obtained using thiophosgene as a solvent while heating at 65 °C. The primary Diels-Alder adduct could not be isolated. All attempts resulted in the formation of the hydrolysed derivative. Despite the efforts that were made to synthezise adduct ��E and ��F via another route, they could not be obtained. Also the synthesis of pentacene precursors with an electron rich bridge (��D�E and ��D�E) stayed without results.

This new pentacene precursor was found to undergo thermolysis to regenerate pentacene at temperatures exceeding 120 °C. The conversion temperature to obtain a thin-film of pentacene was below 200 °C. The conversion was studied by 1H NMR spectroscopy in solution. These experiments showed that at 100 °C, the reaction reaches equilibrium at a conversion of 25%. At 120 °C a 75% conversion was obtained after approximately 3 h.

We were not able to find the optimal conditions to achieve thin-films of good quality although different factors that influence the film formation were investigated. Three-dimensional growth of the pentacene molecules could not be avoided when sufficient conversion was attained.

Part II

69

3DUW�,,�6\QWKHVLV�RI�VXEVWLWXWHG�

SHQWDFHQHV�

The last few years, there has been an increasing interest in the synthesis of new substituted pentacenes for the use in organic electronics. The introduction of substituents on the pentacene backbone improves the solubility of the pentacene. In this way they are easily purified and are suited for solution processing. The substituents also protect the pentacene against oxidation. In contrast to the situation applying the precursor route, the substituted pentacenes improve the stability of the thin-film. Moreover, it has been shown that substituents can affect the self-assembly of pentacene moieties resulting in a closer packing which can improve the electrical properties.7, 27

With the new pentacene derivatives discussed in this part of the thesis, we would like to contribute to the broadening of the knowledge of the field of substituted pentacenes. Only a few pentacene derivatives have been intensively studied so far. With a broader spectrum of derivatives we hope to collect more information to better fine-tune pentacene derivatives towards their applications.

The pentacene derivatives are arranged according to the number of substituents that are introduced. We will discuss the synthesis of 6-monoaryl substituted pentacenes, 6,13-diaryl- and di(alkylsulfanyl)pentacenes, 5,7,12,14-tetra-aryl and tetra-alkyl-pentacenes, 5,6,7,12,13,14-hexa(alkylsulfanyl)pentacenes, 2,3,9,10-

Part II

70

tetra-alkyl-5,�(��-diarylpentacenes and 2,3,9,10-tetra-alkyl-5,7,12,14-tetra-arylpentacenes respectively.

For the introduction of long chains to further improve the solubility, alkylsulfanyl groups were chosen because the sulphur atoms of the alkylsulfanyl groups can improve the stacking by self-assembly via strong sulphur-sulphur interaction. Different aryl substituents were used in order to obtain variation in electron density for studying the influence of electron rich and electron poor substituents on the electronic properties of the pentacene backbone. The variety of substituents can learn us more about their influence on the properties of the original pentacene such as solubility, stacking behaviour and mobility.

Part II Chapter 2.1: Monosubstituted pentacenes

71

&KDSWHU��6\QWKHVLV�RI��PRQRVXEVWLWXWHG�SHQWDFHQHV�

Two synthetic routes were explored for the synthesis of 6-monosubstituted pentacenes. In both cases the final substituent would be introduced by an addition reaction on a carbonyl function on the 6-position of the pentacene backbone. We tried first to use the carbonyl function of pentacen-6-one that was synthesized starting from pentacenequinone. Secondly the pentacenequinone itself was used and treated with the appropriate amount of reagents to selectively obtain addition on only one of the carbonyl functions.

��6\QWKHVLV�YLD�SHQWDFHQ��RQH�Pentacene could be substituted in the 6-position starting from pentacen-6-one using the reactivity of the carbonyl function towards organometallic compounds. This results in the directed addition of the compound and the formation of an alcohol function.

6FKHPH�,,��

��

��

O HO Arorganometal compound

acid

Ar

When starting from pentacen-6-one, the alcohol function would probably spontaneously be expelled by an aromatization step when brought in contact with acid. (Scheme II.1)


72

Two routes could be followed to obtain the starting material ��. Clar59 described in 1949 a synthesis of pentacen-6-one starting from the pentacenequinone. (Scheme �II.2)

6FKHPH��,,��O

O ��

O

��

1) H2SO4

2) Na2S2O4, NaOH H2O, 90°C, 1h

The first step in the reaction seemed crucial. When the pentacenequinone was not first treated with sulphuric acid, it was almost fully (95%) recovered after the reductive treatment. But also when all steps were conducted, we did not obtain the same result as Clar59. Although the pentacen-6-one was formed, the yield of the

reaction was much lower (67% l > 90%). As the main impurity, the

unreacted pentacenequinone was left, causing a major problem of purification. Both products �� and �� are very similar and hence difficult to separate from each other. Their rather low solubility made purification even more difficult. We were not able to obtain the pentacen-6-one in a pure way by this method, thus another route had to be explored. (Scheme �II.3)

Via previous research47 (Part I) another method was known to prepare the pentacen-6-one starting from pentacenequinone. (Scheme �II.3) The first step proceeded without any problems. The pentacenequinone was reduced by NaBH4 and upon acidification with acetic acid the diol �� was obtained with a yield of 51%. The conversion of the diol ��to the desired ketone �� caused some difficulty. By treatment with a stronger acid, hydrolysis occurred leading to the pentacen-6-one. However only small amounts (yield 25%) of product �� were obtained. This is probably due to the existence of the isomeric 6-


73

hydroxypentacene (Part I). We expect that the equilibrium between the two isomers is in favour of the 6-hydroxypentacene.


O �� HClreflux

OHHO

1) NaBH4THF, reflux

2) acetic acid

O

��

S SLi

-78°C, THF��D

��

In the next step it was tried to accomplish the addition of 2-thienyllithium to the carbonyl function of the pentacen-6-one. Thiophene was reacted with n-butyllithium to obtain 2-thienyllithium. Product �� was added to the reaction mixture. During work-up, often a purple colour was observed indicating that the thienylpentacene ��D�was formed. Nevertheless the thienylpentacene could not be isolated due to rapid degradation during the subsequent extraction. Because of the difficulties in the last step and the low yield of the pentacen-6-one, a second route was investigated to obtain 6-monoarylpentacenes.

��6\QWKHVLV�YLD�SHQWDFHQHTXLQRQH�One can also prepare 6-monosubstituted pentacene directly starting from pentacenequinone. The proposed route is displayed in Scheme �II.4 and involves the addition of one equivalent of the organometallic reagent followed by reduction to the desired pentacene ��.


74


O

O

HO Ar

Ar

H H

H Ar

��D�F

��D�F

��D�F

D�=S

E�= OMe F�= Cl

NaBH3CNZnI2

1,2-dichloroethane

1,5 eq ArLiTHF, -78°C

or ArMgBr��

Indeed, when quinone �� was treated with only 1,5 equivalent of 2-thienyllithium, selective monoaddition occurred. In this manner, the 13-hydroxy-13’ -thienylpentacen-6-one (��D) was obtained in moderate yield. (Scheme �II.4/Table �II.1) However when the addition was carried out with lithium reagents derived from S-methoxyphenyl bromide and S-chlorophenyl bromide, exclusive addition on both carbonyl functions was achieved. To avoid this disubstitution the reaction was repeated with the less reactive Grignard derivatives. Both 6-pentacenones ��E� and ��F could be obtained, although the yield was still very low. (Table �II.1)

In a second step the hydroxyarylpentacenones ��D�F�were planned to be reduced to the corresponding pentacenes ��D�F. The reduction method that was used is known for the preparation of anthracenes.60 However, in contrast with the behaviour of the analogous anthracene derivatives, the reduction of ��D with NaBH3CN in the presence of ZnI2 did not afford the desired pentacene ��D.


75

7DEOH��,,��<LHOGV� RI� ��GLK\GUR��K\GUR[\��¶�DU\O�SHQWDFHQ��RQH� �� DQG��GLK\GUR��DU\OSHQWDFHQH��

Substituent Yield of pentacen-6-one ��

Yield of dihydro-pentacene ��

S

D� 39% 92%

OMe E� 8% 50%

Cl

F� 8% 51%

Instead, the product ��D was overreduced to 6,13-dihydro-6-(2-thienyl)pentacene ��D in a very high yield (Scheme �II.4/Table �II.1). Most likely, the aromatization step, in the case of anthracenes60, spontaneously following the reduction (Scheme �II.5) is too slow because of the lower aromatization energy of pentacene.


HO Ar

Ar

��D�F

��D�F

HO

H Ar

Hreduction

aromatization

Also both other hydroxyarylpentacenones, ��E and ��F� were found to give the corresponding overreduced products instead of the desired 6-arylpentacenes ��E�F upon reduction. Remarkably, the yields of the


76

products ��E�F were significantly lower than the yield of the analogous thienyl derivative ��D (Table �II.1). The formation of sideproducts was observed in the case of the phenyl substituted ��E�and ��F. These were characterised as the unstable 6-hydroxy-13-phenyl-pentacene derivatives ��E�F (Figure �II.1).

Ar

OH ��D�F

)LJXUH��,,��K\GUR[\��SKHQ\O�SHQWDFHQH�GHULYDWLYHV�

In case of the 2-thienyl substituted analogue ��D, the formation of the monohydroxypentacene ��D was not observed. However, when the reduction of monopentacenone ��D was carried out in toluene instead of 1,2-dichloroethane, 6-hydroxy-13-(2-thienyl)pentacene (��D) was found besides ��D� albeit in a low yield (12%). On decreasing the amount of ZnI2 (to about 0.8 eq), the yield of 6-hydroxy-13-(2-thienyl)pentacene (��D) was found to increase up to 79%. The products ��D�F could not completely be purified, because of rapid oxidation resulting in a complex mixture of unidentified products.

Because of the overreduction occurring with the method described above, other reduction methods were tested to obtain the pentacenes ��D�F. Thus, the reduction of ketone ��D was also carried out with NaBH4 and LiAlH4, but in both cases pentacene ��D was isolated in low yields (3% and 11% respectively) instead of the desired pentacene ��D.

In a last attempt to prepare the 6-monoarylpentacenes ��D�F, the dihydropentacenes ��D�F were submitted to an oxidation reaction in order to undo the overreduction. Several mild oxidants as DDQ, S-chloranil and triphenylcarbonium tetrafluoroborate were tested to convert the dihydroderivatives ��D�F to pentacenes. Only with DDQ,


77

the appearance of the purple colou,r indicating the formation of pentacene derivative ��D�F, was observed. All other oxidants resulted in a mixture of unidentified products. However, also with the oxidation of the dihydropentacenes ��E�F, we were not able to isolate the monoarylpentacenes ��E�F. The purple colour that appeared, invariably disappeared during work-up. This shows the sensitivity of solutions of these 6-monosubstituted pentacenes towards oxidation. Despite this fact, traces of the 6-(2-thienyl)pentacene could be detected by mass spectroscopy and in NMR spectroscopy. Probably DDQ may stabilize pentacene derivative ��D by formation of a complex in solution.

Recently the synthesis of 6-phenylpentacene was reported also starting from pentacen-6-one by Maio HW� DO.29 The synthesis of the 6-monosubstituted pentacene was divided in two different steps: the addition of the substituent and the elimination of the formed alcohol. In this way it was possible to isolate the 6-monosubstituted pentacene by filtration which diminishes the risk of oxidation.

�

Part II Chapter 2.2: Disubstituted pentacenes

78

&KDSWHU��6\QWKHVLV�RI��GLVXEVWLWXWHG�SHQWDFHQHV�

By substituting both the 6- and 13-position, the inner ring of pentacene is protected towards oxidation. Since this ring is the most reactive towards oxygen, it is expected that the enhancement of the stability of the pentacene backbone is the largest when substituents are placed on these positions. This is probably also an important reason most research has been carried out on 6,13-disubstituted pentacenes. To create a broad variety of derivates, both alkylsulfanyl and aryl substituents were used to prepare 6,13-disubstituted pentacenes. While conducting this research, it was also noticed that 6,13-dihydro-6,13-diarylpentacene-6’ ,13’ -diols can undergo a rearrangement to ketones. This will be discussed in more detail in the last part of this chapter.

��6\QWKHVLV�RI��GL�DON\OVXOIDQ\O�SHQWDFHQHV�The synthetic pathway that was investigated for the synthesis of 6,13-dialkylsulfanylpentacenes started from a pentacene disulphide polymer. (Scheme �II.6) The method for the preparation of the anthracene analogue of the polymer �� was already explored in our laboratory a few years ago in the framework of another project.61

Pentacenequinone �� was treated with the Lawessons reagent. This resulted in the very insoluble polymer ��. The polymer �� was isolated with a yield of 41% and characterised with infra-red spectroscopy. From the presence of the sulphide absorptions and the pentacene fingerprint, one could assume that the reaction had been successful.


79

6FKHPH��,,��

O

O ��

Lawessons reagent

'T

S

S ��

SR

SR

NaBH4, NaOHRI

��

In a next step the sulphur-sulphur bonds would be broken in situ by a reductive method and subsequently the pentacene-6,13-anions would attack the alkyl iodide that is added to the solution to form product ��. This strategy has been found possible for the anthracene analogues. However, for the pentacene polymer �� no reaction was observed. The first time the reaction was conducted in exactly the same way as described for the anthracene polymer.61 After the reaction, only starting materials were isolated. Hence the second time, instead of keeping the reaction mixture at room temperature for 12 h, it was refluxed, though the result remained the same.

In a last attempt a recent method62 to cleave disulphide bonds and subsequently alkylating the sulphur atoms was applied to the pentacene polymer ��. The alkylation is performed by using tosylates as alkylating agents. To break the disulphide bonds the polymer �� was heated with zinc and aluminium trichloride. After two hours an appropriate tosylate, prepared earlier by adding S-toluenesulfonyl chloride to a solution of butanol and a base, was added. Also here mainly starting material was recovered and the product �� could not be isolated.


80

The failure of both methods to prepare product �� is probably mainly due to the insolubility and low reactivity of the starting material ��.

In 2006, a Japanese research group published a totally different pathway to obtain 6,13-bis(alkylsulfanyl)pentacene starting from pentacene-6,13-diol of which the alcohol functions were substituted by thioethers.39 A dehydrogenative aromatization step finally gave them the alkylsulfanylpentacenes. (Scheme �II.7)

6FKHPH��,,��OH

HO RSH, ZnI2

dichloromethane,RT, 2h

SRRS

SR

SR

S-chloranil'T, 2-3days

Indeed the sulphur groups were found to influence the stacking of the

pentacene units. A cofacial S-stacked packing arrangement was

observed, pointing to intermolecular S-S and S-S interactions.

��6\QWKHVLV�RI��GLDU\OSHQWDFHQHV�The introduction of arylsubstituents on the central ring of pentacene could be realised by reaction between organolithiums and pentacene-6,13-quinone. The quinone �� was conveniently prepared by condensation of R-phthalaldehyde and 1,4-cyclohexanedione which is a high yielding reaction.48 (see Part I) The substituents were introduced by addition of their corresponding lithium derivative on the carbonyl functions of the quinone ��. This reaction resulted in a 6,13-


81

diarylpentacene-6,13-diol ��. Subsequently, the diol �� was reduced by sodium iodide and sodium hypophosphite in acetic acid to afford the corresponding 6,13-diarylpentacene ��. (Scheme II.8) This general pathway to generate substituted pentacenes was first published by Allen and co-workers.25a

Via the reaction sequence displayed in Scheme II.8, different aromatic substituents were introduced on the pentacene backbone. In total nine different groups were used. (Scheme II.8) The yield of the different reactions are shown in Table II.2.

6FKHPH�,,��

O

O ��1) ArLi THF, -78°C2) HCl or NH4Cl

HO Ar

HO Ar

NaI, NaH2PO2acetic acid, reflux

Ar

Ar

��D�L

��D�I

OMe

COMe

(CH2)7CH3

S

S

S COMe

S OMe

SS

S CH(CH2)6CH3

H3C

D�=

E�=

F�=

G�=

H�=

I�=

J�=

K�=

L�=


82

7DEOH�,,��<LHOGV� RI� ��GLK\GUR��GLDU\OSHQWDFHQH��¶��¶�GLROV� �� DQG��GLDU\OSHQWDFHQHV��

Substituent � Yield of diol ��

Yield of pentacene ��

OMe

D� 38% 91%

COMe

E� 22% 58%

(CH2)7CH3 F� 42% 86%

S

G� 58% 87%

S

H� 87% 59%

S COMe

I� 45% 86%

S OMe

J� 80% /

SS

K� 74% /

S CH(CH2)6CH3

H3C

L� 55% /

Bromoanisole, protected bromoacetophenone and 1-(4-bromophenyl)-octane were lithiated with n-butyllithium and the corresponding organolithium reagents were used to prepare diols ��D�F which were obtained in moderate to low yields. The glycol acetal was used as the protective group for the carbonyl function of bromoacetophenone.


83

Diols ��G�I were synthesized via a lithiation step starting from thiophene, benzothiophene or 2-acetylthiophene. The latter was protected with 1,2-ethanediol but the protection was much more difficult than in case of acetophenone. More than twice as much ethanediol was needed and the reaction time was much longer.63

In both cases, deprotection of the carbonyl functions was not achieved completely by the normal work-up condition (1M HCl) after the addition step. Therefore, the crude diols ��E and ��I were stirred during two hours in 2M hydrogen chloride before purification.63 When the deprotection was accomplished before purification, the column chromatography was simplified resulting in a higher yield of the pure diols.

The preparation of diols ��J�L required a few adjustments. Normally the reaction was worked up by acidification with 1M hydrogen chloride. However, when this procedure was used in order to prepare diol ��J, a rearrangement was observed, resulting in pentacenone ��J�instead of the desired diol ��J. (Scheme �II.9)


O

O ��

1)

THF, -78°C

2) HCl

��J

SMeOLi O

SS

MeO

OMe

The asymmetric structure of product ��J was confirmed by the characteristic pattern in the 1H NMR spectrum. Two singlets, two doublets and one multiplet for the pentacene unit were observed in contrast to one singlet and two multiplets in case of a symmetrical diol. The 13C NMR spectral data clearly revealed the carbon of the


84

carbonyl group (167.2 ppm). By work-up with ammonium chloride, the rearrangement could be avoided. However diol ��J seemed extremely sensitive to rearrangement as upon purification of the crude diol ��J by column chromatography on silica gel, the rearrangement again took place. This problem could easily be solved by adding half a percent of triethylamine to the eluent.

When the addition was carried out with the lithium derivative of bithiophene followed by the usual work-up conditions, the WUDQV-isomer of the diol ��K was isolated in a low yield (24%). As a sideproduct the rearranged 13,13-bis(5-thien-2-ylthien-2-yl)pentacen-6-one (��K)�was observed. By replacing the hydrogen chloride by ammonium chloride rearrangement of diol ��K was avoided. Both isomers were isolated and the yield doubled.

Similar to diol ��F� also a 2-thienyllithium reagent with a long alkyl chain was added to the pentacenequinone. Initially 2-octylthiophene was used, however there were some problems to attach this thiophene to the ketone ��. No diol was isolated and no clear other product could be identified. It was believed that maybe the methylene next to the thiophene was not stable in the reaction. With the same method64

as for the 2-octylthiophene, a secondary alkyl chain was created on the 2-position of the thiophene. (Scheme �II.10) First the heptyl Grignard reagent was reacted with 2-acetylthiophene.


S

O

MgBrCH2(CH2)5CH3

S

OH

NaI,TMSCl

acetonitrilS


85

The crude 2-thienyl-2-nonanol was further reduced to 2-(2-nonyl)-thiophene. The global yield of the synthesis was 39%.

This time the addition reaction led to the diol ��L after work-up with ammonium chloride. Because a little of the rearranged product ��L was detected, the yield would probably be better if during purification the acidity of the silica gel would be neutralized.

The addition reaction with thienyl substituents had a moderate to high yield. In general one can say that the yield was higher compared to the additions of the phenyl substituents.

Another difference was the tendency to rearrangement. Phenyl substituents, even those with electron rich groups, did not give rise to rearranged products in contrast to their thienyl analogues. This will be discussed in more detail later on.

The diol can adopt two different configurations: FLV and WUDQV. In case of the phenyl substituents, only the WUDQV-isomers were observed. However the thienyl substituents all gave rise to both FLV- and WUDQV-isomers of the diol.

In most cases the isomers could be separated from each other by careful column chromatography. The different isomers were then assigned by NMR spectroscopy. Both isomers show some characteristic differences in their NMR spectra. To obtain a FLV�WUDQV model, 6,13-thienylpentacenediol ��G was intensively studied with NMR spectroscopy. One of the isomers showed a large upfield shift for one of its thienyl protons. This proton was defined as the proton in the 3-position of the thienyl substituent by homonuclear decoupling experiments on the alcohol proton. NOESY-experiments on the fraction showed correlation between the H3 protons of both thiophene residues. Thus, the fraction was characterised as the FLV-isomer.


86

HO

HOS

S HO

HOS

S

��G�FLV ��G�WUDQV5.6 ppm 7.0 ppm

149 ppm 155 ppm

H H

)LJXUH��,,��&KDUDFWHULVWLF�105�GDWD�IRU�FLV��DQG�WUDQV��LVRPHUV�RI�

��WKLHQ\OSHQWDFHQH��¶��¶�GLRO��G�

When one of the isomer pairs had been assigned, a diagnostic tool was available to discriminate between the isomers of all diols ��. (Figure �II.2) The configuration could be determined by two specific signals in the NMR spectra. When the thienyl groups are present in the FLV-configuration, the proton on the 3-position shows a marked upfield shift (5.63 ppm) compared with the more usual thiophene signal (6.96 ppm) of the WUDQV-isomer. This phenomenon is probably due to the proximity of the thiophenes.

In the 13C NMR spectrum, the carbon on the 2-position (C2) of the thiophene gave different shifts for both isomers. In the WUDQV-isomer C2 is located at around 155 ppm while in the FLV-isomer the same carbon is found at 149 ppm.

From the NMR spectra, the ratio of the FLV� and WUDQV-isomers of the diols �� was determined as respectively: G: 4:6, H: 3:7, I: 4:6, J: 2:8, K: 3:7, L: 3:7.�The WUDQV-isomer was in all cases the major product.

In a second step the pentacenediols �� were reduced in order to obtain the corresponding disubstituted pentacenes �� by the method displayed in Scheme II.8. The reduction to pentacenes ��D�H proceeded in moderate to high yield. (Table II.2). The solubility of these substituted pentacenes ��D�H (10-3 M in THF) is two orders of magnitude higher than that of the parent pentacene. Due to the long


87

alkyl chains of substituted pentacene ��F, this compound ��F also shows good solubility in apolar solvents such as toluene. Concerning the stability, we noticed an increase in the stability of the solid products compared to the parent pentacene. We couled easily store them for a few days under ambient atmosphere and they seemed stable a couple of months when stored under nitrogen atmosphere. However, we advice against long term storage because we say a slightly decrease of the performance of the transitors when a transistor was made with the same batch of material but some months later. In solution the disubstituted pentacenes are degradating much faster, especially when kept in the light.

Because of the observed strong tendency for rearrangement, reduction of diol ��J in acetic acid was anticipated to be impossible. However, also for diols ��K�L� acetic acid was found too acidic and the reduction failed. Instead the rearranged analogues ��K�L� were obtained in moderate yields. (Scheme �II.11)

It is known that reduction of the analogous anthracene derivatives can be achieved by treatment with sodium cyanoborohydride and zinc iodide in 1,2-dichloroethane, avoiding the rearrangement.60


NaI, NaH2PO2

acetic acid, refluxHO Ar

HO Ar

Ar Ar

O

��K�L ��K�L

SS

S CH(CH2)6CH3

H3C

K�= L�=

However, this method was not successful with pentacenediols ��J�L. When it was applied on diol ��K, overreduction was observed.


88

Instead of reduction of one alcohol function followed by elimination of water, the overreduced product �� was formed like in case of the 13-hydroxy-13-arylpentacen-6-ones ��. (Scheme II.13) The dihydropentacene �� was obtained in a yield of 73%. All attempts to oxidise product �� back to the pentacene ��K gave a mixture of unidentifiable products.

6FKHPD��,,��

OH

HO S

S

S

S H

H S

S

S

SNaBH3CNZnI2

1,2-dichloroethanereflux

��K ��

After the reduction of diol ��J with this method, the formation of the reduced rearrangement product �� was observed. (Scheme II.13)

6FKHPH�,,��

OH

HO S

S

OMe

MeO

O

SS

MeO

OMe

SS

MeO

OMe

��J

��J

��


89

This is indicating that in case of diol ��J the rate of rearrangement must be even higher than that of reduction. So far, we did not succeed in the synthesis of diarylpentacenes ��J�L because of their tendency to rearrange.

Some experiments were carried out to brominate the five position of the thiophene in pentacene ��G. The bromide would then be substituted by a methoxy group in order to achieve pentacene ��J.�Different methods were tried, though none of them gave selective bromination of the thienyl functions.

Not only symmetrically substituted pentacenes were made, it was also found possible to obtain asymmetric products. From chapter 1, we know that it is possible to add only one substituent on the quinone ��. Starting from product ��D it was possible to introduce a second, different group. The addition of 4-anisyllithium on 13-hydroxy-13-thienylpentacen-6-one afforded the asymmetric diol ��M in a yield of 58%. (Scheme �II.14)


O

HO S ��D

1)

THF, -78°C

2) HCl

LiMeOOH

HO

S

OMe

��MNaI, NaH2PO2acetic acid, reflux

S

OMe

��M


90

Subsequently, diol ��M was reduced to the corresponding asymmetrically substituted pentacene ��M in a yield of 36%. (Scheme �II.14) We also made an attempt to introduce both an electron rich and electron poor substituent on the pentacene backbone, but we did not succeed. (Scheme II.15)


O

HO SOMe

SLi1)

THF, -78°C

2) 2M HCl, 2h

OH

HO

S

OMe

MeOCMeO O

��G ��N

In a first step a 2-methoxythienyl group was introduced on the quinone ��. With the correct purification, the desired monosubstituted pentacenone ��G was isolated in a yield of 80%. 5-Acetylthien-2-yl was introduced as a second substituent. After column chromatography no clear characterisation of the resulting fraction could be obtained. It was not clear if the asymmetric diol ��N had been formed or if a rearrangement had occurred, possibly induced by the deprotection conditions of the acetyl function.

��5HDUUDQJHPHQW�RI��GLK\GUR��GLDU\OSHQWDFHQH��¶��¶�GLROV�During the research of 6,13-disubstituted pentacenes ��, we noticed that some of the diols with rather electron rich substituents were sensitive to rearrangement when brought in contact with Brønsted acids. 2-Methoxythiophene was the most electron rich substituent used and the corresponding diol ��J had the strongest tendency to rearrange to a 6,13-dihydro-13,13’ -di-arylpentacen-6-one ��. Due to


91

these observations, the tendency of rearrangement of the other diols ��D�I was investigated. It was found that analogous rearrangement could be induced by treatment of diols ��D�I with the strong Lewis acid BF3.OEt2. (Scheme �II.16)

Clearly, electron rich substituents favour rearrangement which is in agreement with the behaviour of anthracene analogues.65 Ketone ��G was formed in moderate yield (45%) while ketone ��H was obtained in low yield (18%). In contrast, treatment of the 5-acetylthienyl substituted diol ��I with BF3.OEt2 did not result in rearrangement. However, pentacenone ��I, could be obtained by acylating thienylpentacene ��G. When thienylpentacene ��G was treated with acetic anhydride in the presence of scandium triflate66 as a catalyst, product ��I was isolated in high yield. (88%)


HO Ar

HO Ar

Ar Ar

O

��D�I ��D�I

BF3.OEt2

dichloromethaneTk, 12h

OMe

COMe

(CH2)7CH3

S

S

S COMe

D�=

E�=

F�=

G�=

H�=

I�=

Also the phenyl substituted diols ��D�F were treated with the Lewis acid. No rearrangement was observed with both diols ��E and ��F. In contrast with the extreme tendency to rearrange of the 5-methoxythienyl substituted diol ��J, only trace amounts of the product ��D were detected. Instead, the reaction of diol ��D mainly resulted in the formation of pentacene ��D.


92

All these results were combined in Figure �II.3. Here the substituents were ordered by their increasing tendency to cause rearrangement under influence of acids. A small distinction could be made between substituent K and L. The bithienyl substituted diol ��K seemed more stable because it could be partially isolated after work-up with hydrogen chloride.

S S> > OMe > COMe

S COMe

(CH2)7CH3

S OMe SS

S CH(CH2)6CH3

H3C

>>

�

�

!"#$ %

&

)LJXUH��,,��7HQGHQF\�RI�UHDUUDQJHPHQW�

During the isolation of diol ��K, an indication was found that there is a difference in the tendency to rearrange between the different isomers. When the work-up was done with hydrogen chloride only the WUDQV-isomer of the pentacenediol ��K was isolated. However, when instead of the acid, ammonium chloride was used for the work-up of the reaction, both isomers were found. The same difference was observed when inducing the rearrangement separately on both isomers of the obtained diol ��K��From the reaction with the FLV-isomer, the rearranged product ��K was obtained in yield of 55% instead of 36% starting from the WUDQV-isomer. This experiment was repeated with the isomers from pentacenediol ��H. Also here the FLV-isomer was more sensitive to rearrangement than the WUDQV-isomer. A yield of 16% was observed for the reaction of the FLV-isomer of ��H, though when the rearrangement was induced on the WUDQV-isomer the yield decreased to 9%.

Part II Chapter 2.3: Tetrasubstituted pentacenes

93

&KDSWHU��6\QWKHVLV�RI��WHWUDVXEVWLWXWHG�SHQWDFHQHV�

The next step was to increase the number of substituents to four. Analogous substituents were used as for the already prepared 6,13-disubstituted pentacenes �� (chapter 2.2) in order to allow comparison of the properties as function of the number of substituents introduced.

��6\QWKHVLV�RI�SHQWDFHQH��WHWURQH�5,7,12,14-Tetrasubstituted pentacenes were prepared starting from the corresponding pentacenetetrone ��. The four carbonyl functions on the 5-, 7-, 12- and 14-position, offer the possibility of adding the substituents in an analogous manner as for disubstituted pentacenes ��67.

The pentacenetetrone �� was synthezised starting from 2-methyl-1,4-naphthoquinone (��) as described in the literature68. (Scheme �II.17) Although the reaction has a low yield (16%), this method was preferred above others69 (see also chapter 2.6) because in this manner, the pentacenetetrone �� could be prepared from inexpensive starting materials in a straightforward and reproducible way. The reaction mixture was just stirred in the dark for 16 h. Afterwards the pure product �� was filtered.


O

2

O2 diethylamine

ethanol

O

O

O

O��


94

��6\QWKHVLV�RI��WHWUD�DON\O�VXEVWLWXWHG�SHQWDFHQHV�To obtain tetra-alkyl substituted pentacenes, attempts were made to introduce a heptyl chain on the tetrone ��. (Scheme II.18) Firstly the usual method was used. Thus, heptyllithium was prepared and subsequently the tetrone �� was added. However, after reaction the desired pentacenetetrol �� could not be detected and only unidentified products were formed.

6FKHPH�,,��

O

O

O

O��

OH

OH

OH

OH��

In a second attempt, the organolithium compound was replaced by the milder Grignard analogue. This time, no reaction occurred and mainly starting materials were recovered. The reaction was repeated but now the reaction mixture was heated at reflux to increase the reactivity. After column chromatography, again only unidentified products were found instead of the 5,7,12,14-tetraheptylpentacene-5,7,12,14-tetrol (��). These results indicate that the 5,7,12,14-tetra-alkyl substituted pentacene probably can not be achieved in the aforementioned method.


95

��6\QWKHVLV�RI��WHWUD�DU\O�VXEVWLWXWHG�SHQWDFHQHV�The tetra-aryl substituted pentacenes �� were prepared in the same way as their disubstituted analogues.35a,67 (Schema II.19) The aryllithium derivatives were generated from the (bromo)arene with Q-BuLi. 5,7,12,14-Pentacenetetrone (��) was directly added to the lithium reagent to form 5,7,12,14-tetra-arylpentacene-5’ ,7’ ,12’ ,14’ -tetrol ��D�F�I as a mixture of diastereomers. The resulting tetrols ��D�F�I could be reduced by sodium iodide and sodium hypophosphite in acetic acid to afford the corresponding pentacenes ��D�F�I. The yields of the reactions are displayed in Table �II.3.

6FKHPD�,,��

��1) ArLi THF, -78°C2) HCl


��D�F�I

O(CH2)6CH3

CO(CH2)6CH3

(CH2)7CH3

S

S

D�=

E�=

F�=

G�=

H�=

I�=

O O

OO

HO HO

ArHO

Ar

HOAr

Ar

C(CH3)3

Ar

Ar

Ar

Ar��D�F�I


96

7DEOH��,,��<LHOGV� RI� ��WHWUDK\GUR��WHWUD�DU\OSHQWDFHQH��¶��¶��¶��¶�WHWUROV��DQG��WHWUD�DU\OSHQWDFHQHV��

Substituent Yield of tetrol ��

Yield of pentacene ��

O(CH2)6CH3 D� 68% 85%

(CH2)7CH3 F� 45% 58%

C(CH3)3 G� 71% 46%

S

H� 90% 62%

S

I� 79% 86%

In order to prepare the tetrols ��D and ��E, the corresponding SDUD-substituted bromobenzenes had to be synthezised. (Scheme

�II.20) The S-bromoheptyloxybenzene was prepared by an alkylation

reaction between 1-bromoheptane and S-bromophenol in the presence of a base.70 The reaction was straightforward and had a high yield (83%).

6FKHPH��,,��OH

Br

Br O(CH2)6CH3

Br(CH2)6CH3

K2CO3DMF, 60°C

COCl

Br

BrMg(CH2)6CH3

Fe(acac)3THF,-78°C

Br CO

(CH2)6CH3


97

The S-bromophenyloctan-1-one was obtained by an iron-catalysed reaction.71 The Grignard reagent generated from the bromoalkane was added to a solution of S-bromobenzoyl chloride that was cooled at -78°C. The reaction immediately took place and after 15 min the reaction was quenched. The S-bromophenyloctan-1-one was isolated in a yield of 44%.

All tetrols �� were obtained without any problems except for the addition of the lithium compound of the protected S-bromo-

phenyloctan-1-one, which failed. (Scheme �II.21) The acetyl function

was protected by 1,2-ethanediol. The addition was performed twice without the formation of the tetrol ��E.


LiO

��1) THF, -78 °C

2) HCl


O O

OO

HO

HOOH

O

O

(CH2)6CH3

O

OH3C(H2C)6(CH2)6CH3

O

+ ��

H

(CH2)6CH3O

(CH2)6CH3O(CH2)6CH3

O

��

O


98

After deprotection and purification, it was found that only three substituents were attached to the tetrone ��affording product �� (yield: 43%) instead of the desired tetrol ��E��The triol �� was further reduced to product �� in a yield around 60%�� Thus, this failure resulted in an incomplete series of tetrasubstituted pentacenes ��D�F.

As already mentioned, the addition of the lithium compounds led to the formation of different diastereoisomers of the pentacenetetrol ��. By careful column chromatography on silica gel, the different isomers of tetra-aryl substituted pentacenetetrol ��H could be separated in three fractions ��H ' , ��H ( and ��H ) in an isolated ratio 4:1:5. The different fractions obtained were characterized by NMR spectroscopy in analogy with the study of 6,13-substituted analogue ��G, where the FLV- and WUDQV-isomers showed some characteristic differences (Figure �II.2). These specifics in the NMR spectra that are related to the configuration of the molecule, could be used as a diagnostic tool to determine the configurations of the tetrol isomers.

By using the FLV- and WUDQV-model obtained in the study of the 6,13-thienyl analogue ��G, all fractions could be characterised. We have to remark that there is a difference in shift of the C2 when the spectrum is recorded in either CDCl3 or in DMSO. In CDCl3 an upfield shift was observed. (&LV: around 152 ppm in DMSO versus around 149 ppm CDCl3, WUDQV: around 155 ppm in DMSO versus around 152 ppm CDCl3) The spectra discussed below, were all recorded in deuterated DMSO. The 1H NMR spectrum taken from fraction ��H ' indicated that two very similar, highly symmetrical diastereomers were present and no upfield shift was noticed (6.5/6.6 ppm). In the 13C NMR spectrum C2 was found around 155 ppm. These results pointed out that the thienyl groups on the same rings were in a WUDQV-configuration in respect to each other. Thus, we observed the presence of two configurations WUDQV�WUDQV�WUDQV- and WUDQV�FLV�WUDQV. (Figure �II.4)


99

HO

HO

HO

HOSS

S S HO

HO

HO

HOSS

S S

)LJXUH��,,��7UDQV�WUDQV�WUDQV��DQG�WUDQV�FLV�WUDQV�

WHWUDWKLHQ\OSHQWDFHQHWHWURO��G * �

We were unable to assign the structures and due to the low solubility it was not possible to separate the isomers with HPLC. The ratio of the two isomers was determined to be 7:3 from the NMR data.

In the spectra of fraction ��H ( , there were four different thiophene absorptions, complicating the spectra. Two thiophenes possessed upfield shifted protons 3H (5.7-5.6 ppm) and their C2 was found around 152 ppm (151.4 ppm), the other two showed no special shift (6.9-6.7 ppm) and C2 was found around 155 ppm (155.3 ppm) in the 13C NMR spectrum. NOESY-experiments revealed a correlation between the protons H3 from the FLV-orientated thiophenes. These results proved ��H ( to be the FLV�WUDQV�WUDQV isomer (Figure �II.5)

HO

HO

HO

HOSS

S S

)LJXUH��,,��&LV�WUDQV�WHWUDWKLHQ\OSHQWDFHQHWHWURO��G +

The NMR spectra taken from the last fraction ��H ) indicated that as in the case of ��H ' , it was consisting of two very symmetrical isomers (ratio 3:7). Fraction ��H ) showed an upfield shift for the H3 from the thiophene (5.8/5.9 ppm) and C2 was located at 151.9/151.8


100

ppm. We could conclude that there was two times a FLV-configuration of the thienyls on the same ring. (Figure �II.6)

HO

HO

HO

HOSS

S S HO

HO

HO

HOSS

S S

)LJXUH��,,��&LV�FLV�FLV��DQG�FLV�WUDQV�FLV�

WHWUDWKLHQ\OSHQWDFHQHWHWURO��G ,

The same isomers were found when 5,7,12,14-tetrabenzothienyl-pentacene-5’ ,7’ ,12’ ,14’ -tetrol (��I) was subjected to chromato-graphic purification. The spectra were taken in deuterated DMSO at slightly elevated temperature (50 °C).

A first fraction was obtained after extracting the crude mixture with hot methanol. A part of the material was dissolved with the impurities and a pure insoluble fraction could be filtered from this solution. After examination, the insoluble fraction was characterised as a mixture of FLV�FLV�WUDQV��FLV-isomers (C2: 151.8/ 152.1 ppm, shielding of H3: 6.0/6.1 ppm against benzothiophene 7.4 ppm). LC-MS gave indeed two very similar spectra (m/e 857.4) of the two separated products.

Column chromatography of the methanol extract on silica gel with dichloromethane:ethyl acetate (95:5) as eluent, revealed the two other isomers. The faster moving isomer was characterized as the FLV�WUDQV-isomer based on the NMR spectra taken in deuterated DMSO. Four different benzothiophenes were found, two standing in a FLV-configuration (C2: 151.8/152.0 ppm, shielding of H3: 5.7/5.8 ppm against benzothiophene 7.4 ppm) and two in WUDQV-configuration (C2: 156.9/156.5 ppm, no shielding of H3: 7.0/7.3). The NOESY spectrum showed a correlation of H3 from the FLV-oriented thiophenes with H6


101

and H13 from the pentacene backbone which confirmed the axial position of the FLV-oriented thiophenes.

The last fraction was found to contain the WUDQV�WUDQV�FLV��WUDQV isomers. (NMR spectra in deuterated DMSO at 50 °C: C2: 155.6 ppm and no shielding of H3: 6.8 ppm, mixture of isomers as apparent by LC-MS). The ratio of the isolated fractions was for FLV�WUDQV�FLV��FLV-, FLV�WUDQV�WUDQV- and WUDQV�WUDQV�FLV��WUDQV-isomers 2,5:6:1,5 respectively. The LC-MS measurements also confirmed the possibility to further separate the FLV�FLV�WUDQV��FLV and the WUDQV�FLV�WUDQV��WUDQV isomers with HPLC. However, due to solubility problems quantitative separation could not be achieved.

In contrast to the thienyl substituted tetrols ��H�I,� the phenyl substituted tetrols ��D�F�G were only formed with the symmetric configuration WUDQV�WUDQV�FLV��WUDQV and asymmetric configuration FLV�WUDQV�WUDQV. The different configurations were assigned based on 1H NMR spectra. The protons on the 2- and 6-position of the phenyl ring also showed in the case of a FLV-configuration an upfield shift. In all cases they were located around 6.1-6.2 ppm. The differences in 13C NMR spectra for C2 of the FLV- and WUDQV-configuration were less pronounced than in case of the thienyl substituted tetrols ��H�I. In the FLV-configuration C2 was found around 139 ppm while in the WUDQV-configuration the signal appeared around 141 ppm.

The ratio of the WUDQV�WUDQV�FLV��WUDQV and FLV�WUDQV�WUDQV isomers was deduced from the NMR spectra and was respectively for tetrols ��D and ��E 2:8 and for tetrol ��G 3:7. The asymmetric configuration was always found in major amount which is similar to the ratio of tetrol ��I. Eventually, the pentacenetetrols ��D�F�I were further reduced to the corresponding 5,7,12,14-tetra-arylpentacenes ��D�F�I. (Schema II.19, Table �II.3) It was expected that the solubility of the pentacene backbone would increase by increasing the amount of substituents.


102

However, in spite of our expectations, the solubility of the tetra(2-thienyl)pentacene ��H and the tetra(benzothien-2-yl)pentacene ��I did not improve with respect to the disubstituted analogues (10-3 M in THF). The introduction of more substituents on the backbone of pentacene seemed in fact to be responsible for a decrease of the solubility. Thus, at first sight pentacenes ��H and ��I showed no improvement in comparison with their 6,13-pentacene analogues for the use in solution processed OTFT’s. The low solubility also troubled the characterization of the products. Finally we succeeded in recording 1H NMR spectra in deuterated 1,1’ ,2,2’ -tetrachloroethane (CDCl2-

CDCl2) for ��I and in deuterated chloroform with a special concentric tube for ��H. Also mass spectroscopy confirmed the formation of the pentacenes ��H�I. The decrease of solubility indicated that there must be more aggregation. Possibly, the presence of sulphur atoms in pentacenes ��H�I is promoting the aggregation of the molecules. To circumvent the solubility problems and break the aggregation, pentacene ��G was synthezised with success. As expected, the tert-butyl groups increased the solubility. More remarkably, the long alkyl chains of the substituted pentacenes ��D and ��F caused a large increase of the solubility in a range of different solvents. This is a great advantage because it will broaden the range of different solvents that can be used for deposition of the materials on a substrate. Now not only solvents such as tetrahydrofuran could be used but also solvents like toluene. Concerning the stability, we noticed that the tetrasubstituted pentacenes �� are less stable than there disubstituted analogues ��. This is caused by the unsubstituted central ring which is the most sensitive. The four substituents around it give not sufficient protection. The purification step was more critical and they had to be stored under nitrogen atmosphere. If not clear light coloured spots appeared after a few weeks indicating possible degradation.


103

��5HDUUDQJHPHQW�RI��WHWUDK\GUR��WHWUD��DU\OSHQWDFHQH��¶��¶��¶��¶�WHWUROV�In previous studies of 6,13-diaryl substituted pentacenes ��, we studied the rearrangement of the intermediate 6,13-dihydro-6,13-diarylpentacene-6’ ,13’ -diols ��.67 A spontaneous rearrangement on contact with acid was observed when the diol beared electron rich substituents like 2-methoxythienyl and bithienyl. We were interested in the behaviour of analogous tetra-arylpentacenetetrols. For this purpose the aforementioned substituents were also introduced starting from the pentacenetetrone ��. (Scheme �II.22) The work-up was carefully performed with ammonium chloride in order to be sure to obtain the tetrols ��J and ��K. The purification was difficult and for both tetrols ��J�K the WUDQV�WUDQV�FLV��WUDQV-isomer could not be isolated. In the case of tetrol ��J� mainly the FLV�WUDQV�FLV��FLV-isomer (8:2) was formed, in contrast to the addition of bithiophene which was leading mainly to the FLV�WUDQV-isomer (9:1).


S OMe SS

J�= K�=

��

1) ArLi THF, -78°C

2) NH4Cl

��J�K

O O

OO

HO HO

ArHO

Ar

HOAr

Ar

�

None of the pentacenetetrols ��J�K could be reduced to the corresponding tetra-aryl substituted pentacenes ��. Similarly to their disubstituted analogues, they rearranged when brought in contact with acid. The characterisation of the rearranged products of the tetrols ��J�K was hindered by the complexity of the NMR spectra and the


104

difficult purification. However, mass spectra and infra-red spectroscopy clearly indicated the precence of the rearranged product. To obtain a better view on the rearrangement and the isomers that are formed, rearrangement was induced on the simplest tetrol� ��H. (Scheme �II.23)


OH

OH

OH

OH

S S

SS

BF3.OEt2

dichloromethaneRT

SSSS

OO

S

S

S

S O

O

��H

��D

��E �

From previous work on the disubstituted analogue ��G� it was

known that it is possible to induce rearrangement of this diol ��G by

treating it with BF3.OEt2 affording 6,13-dihydro-13,13’ -di(2-thienyl)pentacene-6-one (��G).67 And indeed, in the same way also the rearrangement of tetrol ��H to tetrathienylpentacenedione �� was induced. Moreover, both isomers ��D and ��E were formed and they could be separated by column chromatography. The ratio of the isomers was 8:2 respectively. Probably due to steric hindrance the formation of ��E was minor to ��D. (Scheme �II.23) The total yield of the reaction was 22% which is half of the yield for rearrangement of the analogous diols ��G (45%). These results indicate that the tendency of rearrangement for tetrols �� is the same as for their disubstituted analogues ��.

Part II Chapter 2.4: Hexasubstituted pentacenes

105

&KDSWHU��6\QWKHVLV�RI��KH[DVXEVWLWXWHG�SHQWDFHQHV�

In this chapter the first of the two types of hexasubstituted pentacenes will be discussed. All positions of ring two, three and four would be substituted by sulfanyl groups. The solubility would probably improve by the alkyl chains while the sulphur atoms could improve the stacking, similar to analogous 6,13-disubstituted pentacenes.

��6\QWKHVLV�RI��KH[D�DON\OVXOIDQ\O��SHQWDFHQHV�The strategy that was followed to prepare the 5,6,7,12,13,14-hexa-alkylsulfanylpentacenes �� was comparable with the route followed in order to obtain 6,13-alkylsulfanylpentacene ��. (Scheme �II.24)

6FKHPH��,,��S

S

SS

S S

SR SR SR

SRSRSR

S8

trichloro-benzene, 'T

1) NaBH42) RX

��

��

Starting from freshly prepared pentacene �� (See also Part I) the hexasulphurpentacene (��) was obtained after reaction with sulphur according to a literature procedure.72 (Scheme �II.24) The yield of the reaction was moderate (46%) and the product �� appeared poorly soluble. The infra-red spectrum corresponded with the data in the


106

literature.72 Very recently electronic properties of this material were reported.72b

In a next step the sulphur-sulphur bonds would be broken in situ by a reduction and subsequently the sulphur anions would be alkylated. The same methods were applied as already described in chapter 2.2. Also in this case, no satisfying results were obtained even after longer reaction times and higher temperatures still only starting materials were recovered.

Again it is presumed that the failure of both methods to prepare product �� is due to the insolubility and low reactivity of the starting material ��.


107

&KDSWHU��6\QWKHVLV�RI��KH[DVXEVWLWXWHG�SHQWDFHQHV�

In this chapter we have tried to combine substitution of the peripheral rings with the introduction of aryl substituents on the second and fourth ring. In this way it would be possible to obtain two different hexasubstituted pentacenes. The 2-, 3-, 9- and 10-positions were substituted with alkyl chains for extra solubility and to create long rod-like molecules. Aryl substituents would be placed on either the 5- and 7-position or on the 5- and 12-position. It would be interesting to observe the difference in properties between those two different substitution patterns.

6FKHPH��,,��RQO\�RQH�LVRPHU�LV�VKRZQ�LQ�WKH�VFKHPH��R

ROO

O

O

O

O

CO2HHO2C

O O

R

R

R

R

CO2HHO2C R

R

R

R

O OR

R

R

R

R

R

R

R

R

R

R

R

Ar Ar

+ 2

The preparation of these molecules was based on the peripherally substituted pentacenes published by Terrance HW�DO.30a They introduced chains on the outer rings of the pentacene backbone by preparing the


108

pentacene molecule via a Friedel-Crafts reaction between a 1,2-disubstituted benzene and benzene-1,2,4,5-tetracarboxylic acid dianhydride. (Scheme �II.25) After reduction and ring closure, substituted pentacene-5,7- and 5,12-ones were obtained as intermediates and an additional reduction step was performed to obtain their target pentacenes.30a In contrast, we used the intermediate pentacenones to introduce additional substituents on the pentacene backbone via addition to the carbonyl group. (Scheme �II.25)

��6\QWKHVLV�RI��WHWUDVXEVWLWXWHG�SHQWDFHQH��DQG��RQH�In a first stage, the pentacene-5,7 and 5,12-ones were synthezised. The carbonyl functions would then be used in subsequent transformations. After slightly adjusting the procedure described in the patent of 3M30a, the diacids �� were prepared via a Friedel-Crafts reaction. Both isomers of the diacid �� were formed and could be easily separated due to their difference in solubility. (Scheme �II.26) The different isomers are indicated on the basis of their symmetry as the & -/. - and the & -10 -isomer.


OO

O

O

O

O

R

R2+

AlCl3

base, 12h

CO2HHO2C

O O

R

R

R

R

CO2H

HO2CO

R

RR

RO

��

& 243 ��D�G

& 265 ��D�GR = Me

Et nBuS

ClDE

F

G


109

We always started from RUWKR-disubstituted benzenes. In this case substitution would occur in the 2, 3, 9- and 10-position of the target molecules. Four different R-groups were used namely methyl, ethyl, chloride and butylsulfanyl. The yields are displayed in Table �II.4. Via the chloride it would be possible to introduce for instance alkylsulfanylgroups at a later stage by a substitution reaction by alkane thiols of different lengths.

7DEOH��,,��<LHOGV�RI��ELV��GLVXEVWLWXWHG�EHQ]R\O� EHQ]HQHGL�FDUER[\OLF�DFLG�DQG��ELV��VXEVWLWXWHG�EHQ]R\O� EHQ]HQHGLFDUE�R[\OLF�DFLG��

Substituent R Yield of product ��

CH3 D� 78%

CH2CH3 E� 72%

Cl F� 87% *

S(CH2)3CH3 G� 50%

SUHSDUHG�E\�*ULJQDUG�UHDFWLRQ��LQVWHDG�RI�)ULHGHO�&UDIWV�UHDFWLRQ��

The Friedel-Crafts reaction with R-xylene or 1,2-diethylbenzene and the anhydride ��was carried out without any problems when the reaction mixture was sufficiently cooled. The & -10 -isomer had the lowest solubility and was formed as the minor product relative to the & -/. -isomer. The ratio, in which the isomers were isolated, was respectively 3:7 for the methyl substituted ��D and 1:9 for the ethyl substituted diacid ��E.

However, when the Friedel-Crafts reaction was performed with R-dichlorobenzene, the product ��F could not be detected. The reaction was repeated and now R-dichlorobenzene was used as a


110

solvent. Again no product ��F was found even not when also the reaction temperature was increased.

6FKHPH�,,��

OO

O

O

O

O

R

R MgBr

THF, Tr, 12h

CO2HHO2C

O O

R

R

R

R

CO2H

HO2CO

R

RR

RO

& 748 ��F�H

& 7:9 ��F�HR = Cl FHH

Another option was to prepare the diacid ��F via a Grignard reaction starting from 3,4-dichlorophenyl bromide.(Scheme II.27) Therefore a test reaction was performed with the Grignard reagent derived from bromobenzene to test the reactivity of the anhydride ��. Thus, this reagent was added in a dropwise manner to a solution of the anhydride ��, resulting in the formation of the & -/. - and & -10 -isomer of diacid ��H. The isomers were isolated in a ratio 1:1 in a total yield of 58%. These results confirmed that it was possible to synthezise diacid ��F via a Grignard reaction. Indeed the compound ��F was obtained in high yield (Table �II.4) and both the & -10 - and & -/. -isomer were isolated in a ratio 1:4 respectively.

Also the preparation of diacid ��G was not straightforward. First the 1,2-dibutanesulfanylbenzene was prepared via the reaction of R-dichlorobenzene and sodium butane-1-thiolate. In a first attempt, the thiolate was prepared in situ by adding sodium hydroxide. However, this resulted exclusively in monosubstitution. A second attempt was made using sodium hydride resulting in coupling of the thiols instead of substitution. Finally, the thiolate salt was formed by exchange between sodium ethanolate and butylthiol and isolated.73 After drying


111

the salt in vacuum, the reaction with R-dichlorobenzene was carried out successfully. A minor amount of monosubstituted product (25%) was found but could be separated from the disubstituted compound by column chromatography. Fractional distillation of the two products did not result in efficient separation of the two compounds. The yield of the disubstituted compound was 48%.

The first Friedel-Crafts reaction of 1,2-dibutanesulfanylbenzene on the anhydride �� failed as only one side of the anhydride �� had reacted. So the reaction temperature was increased from room temperature to 40 °C, without result. To allow to further increase the temperature, the solvent was changed from dichloromethane to cyclohexane. This time, the diacid ��G was formed in a moderate yield (Table �II.4) with a ratio of 3:7 for the & -10 - and & -/. -isomer respectively.

6FKHPH�,,��CO2HHO2C

O O

R

R

R

R

CO2H

HO2CO

R

RR

RO

Pd/C, H2& ;:< ��& ;>= ��

CO2HHO2C R

R

R

R

CO2H

HO2C

R

RR

R

& ;:< ��& ;>= ��F3CSO3H

R

R

R

R

R

R

R

R

O O O

O& ;:< ��& ;>= ��


112

The next step consisted of the reduction of the carbonyl functions of the substituted diacids ��D�G. This should be possible by hydrogenation according to the mentioned patent. 30a Subsequently, the closure should proceed easily with trifluoromethanesulfonic acid resulting in the pentacenones �� which are the starting materials for the addition of the aryllithium compounds. (Scheme II.28)30a

When the literature was followed to reduce bis(dimethylbenzoyl)- benzenedicarboxylic acid & ?1@ ��D, we observed no reduction, only the starting material was recovered. From previous research in our laboratory it was known that reduction with hydrogen could be promoted by adding a trace of acid to the reaction mixture.74 However, this had only a positive result when also the solvent was changed from tetrahydrofuran to ethanol. On small scale the bis(dimethyl-benzyl)benzenedicarboxylic acid & ?1@ ��D was obtained in a yield of 77%. Unfortunately, when the reaction was scaled up the yield decreased dramatically to an average of 29%. Neither the increase of the reaction time nor the addition of cyclohexene improved the yield. The & -10 -isomer did not even undergo reduction with hydrogen. Thus,

other methods were explored to reduce the & A�B -isomer and increase

the yield of the & -/. -isomer for reducing it on gram scale.

One of the options was a reduction with zinc in the presence of copper chloride in a basic environment. Unexpectedly, this reaction did not give the reduced bis(dimethylbenzyl)benzenedicarboxylic acids �� but a two-fold cyclisation under reductive conditions was leading to

the formation of bisfuranones ��. (Scheme �II.29) This observation is

in contrast with that for anthracene analogues where under these circumstances the reduced product is obtained.75 The bisfuranones ��D and ��E were collected as a mixture of diastereomers. The yield for reduction of the & -/. -isomer ��D was more than 80% when 200 equivalents of zinc were used. When reducing the amount of zinc also the yield decreased linearly. (100 equivalents zinc gave a yield of 34%; 50 equivalents zinc gave a yield of 19%) The & -10 -isomer ��D


113

gave under optimal conditions a slightly lower yield than the & -/. -isomer ��D (64%).


CO2HHO2C

O O

CO2H

HO2CO

O

& C>D ��D

& C6E ��D

Zn

KOH, CuCl2H2O, reflux, 24h

OO

O O

OO

O

O

��D

��E

In order to obtain the desired bis(dimethylbenzyl)benzenedicarboxylic acids ��,� the five-membered rings of the bisfuranones ��D�E should be cleaved again. This was achieved for compound ��D in a reductive way via hydrogenation. It turned out to be important to use ethyl acetate as a solvent and in this case the yield of the reaction was 42%. The lower yield of the second step brought the overall yield for the preparation of bis(dimethylbenzyl)benzenedicarboxylic acid & ?1@ ��D� to only about 34% which was no real improvement in comparison to the reduction with hydrogen. Moreover the & -10 -isomer of compound �� could not be obtained in this manner.

A second alternative for the reduction with hydrogen was more successful. The carbonyl functions of the bis(dimethylbenzoyl)-benzenedicarboxylic acids ��D�were reduced in a one step reaction


114

with hypophosphorous acid76 in the presence of a trace amount of iodine. (Scheme �II.30)

6FKHPH��,,��CO2HHO2C

O O

R

R

R

R

& F>G �& F�H ��D�G

CO2HHO2C R

R

R

R

& F4G �& F6H ��D�G

H3PO2, I2

acetic acidreflux, 12h

With this method, the yield of the reduction of the & -/. -isomer ��D could be almost doubled compared to the original method. This reduction method was also able to reduce the & -10 -isomer of compound ��D in a comparable yield as its & -/. -isomer. In this way it was possible to reduce all other bis(3,4-disubstituted benzoyl)benzenedi-carboxylic acids ��.

7DEOH��,,��<LHOGV� RI� ��ELV��GLVXEVWLWXWHG� EHQ]\O�EHQ]HQH�GLFDUER[\OLF� DFLG�DQG� ��ELV��GLVXEVWLWXWHG� EHQ]\O�EHQ]HQHGL�FDUER[\OLF� DFLG� ��E\�UHGXFWLRQ�ZLWK��K\SRSKRVSKRULF�DFLG�DQG�LRGLGH�


CH3 D�& A�I��& A�B �

52% 49%

CH2CH3 E�& A�I��& A�B �

66% 64%

Cl F�& A�I��& A�B �

69% 62%

S(CH2)3CH3 G�& A�I��& A�B �

13% 5%


115

The yield of the reactions are displayed in Table �II.5. The yields of the reduction of the different diacids �� are moderate and comparable to each other. Only the reduction of compound ��G had a very low yield. Unfortunately, due to the lack of time the reaction was only performed once. It is probably possible to further optimize the reaction. We believe that maybe the starting material had to be more intensively purified and dried because we had to start from a sticky material which was hard to handle. On the other hand, it could be that the solubility of the compound ��G in acetic acid was less than for the other diacids ��D�F due to the longer alkyl chains. More investigation of the compounds ��G and ��G is necessary if one would like to use these products.

In the last step, the bis(3,4-substituted benzyl)benzenedicarboxylic acids ��D�G had to undergo a ring closure to afford the pentacene-5,7 and 5,12-ones ��D�G. (Scheme �II.31)31a Although this may seen an easy reaction, we encountered severe problems, mainly caused by the unexpected extremely low solubility of the final products ��.


R

R

R

R

O OCO2HHO2C R

R

R

R

& J4K �& J6L ��D�G & J4K �& J6L ��D�G

F3CSO3H

The bis(3,4-methylbenzyl)benzenedicarboxylic acids ��D� served as test materials. In a first experiment, the LVR-isomer of product ��D was stirred in F3CSO3H containing a trace of fluorinated acetic anhydride. At the end an insoluble residue and a filtrate was obtained. Because the solid seemed extremely insoluble, it was believed that the product ��D should be in the filtrate. However, the product ��D could not be detected in any of the fractions obtained after column chromatography. Because the starting material was not recovered, the


116

reaction conditions were made softer in order to prevent possibly degradation of the compounds. Dichloromethane was added as a co-solvent and the reaction was cooled below room temperature but also in this case no product was isolated.

After trying some other methods (treatment with H2SO4, polyphosphoric acid or POCl3)77, known for anthracene analogues of product ��, without any result, we took another look at the first reaction. (Scheme �II.31) The attention was now focused on the residue which was formed and the correct mass in the mass spectrum was found. (m/e 367) However, the NMR spectra in both CDCl3 and DMSO showed no signals. An infra-red spectrum indicated that carbonyl functions were present. Thus, a closer look was taken at the options of recording a NMR spectrum. Finally, we managed to take a 1H spectrum in D2SO4 which confirmed that the insoluble residue was the closed product ��D.

We observed that the signals of the protons on the 12- and 14-position disappeared during time. The protons were exchanged by deuterium which led to the presumption that there is an equilibrium between the product ��D and its enol-isomer. (Figure �II.7) This could partially explain the low solubility and the red colour of the residue. The yield of the reaction is displayed in Table �II.6.

O O

& M>N ��D

OHOH

)LJXUH��,,��(TXLOLEULXP�EHWZHHQ�SURGXFW��D�DQG�LWV�HQRO�LVRPHU�

The ring closure of the & -10 -isomer was achieved with the same

method namely treatment with a mixture of trifluoromethane-sulfonic acid and trifluoroacetic acid30a and slightly longer reaction

time. An insoluble blue residue was obtained and the 1H NMR


117

spectrum recorded in D2SO4, confirmed the formation of 2,3,9,10-tetramethylpentacene-5,12-one. Also the other bis(3,4-substituted benzyl)benzenedicarboxylic acids ��E�G underwent ring closure to the corresponding pentacenones ��E�G. All yield are shown in Table �II.6.

7DEOH��,,��<LHOGV�RI��WHWUDVXEVWLWXWHG�SHQWDFHQH��DQG��RQHV��


CH3 D�& A�I��& A�B �

73% 83%

CH2CH3 ��E� 41%

Cl F�& A�I��& A�B �

40% 46%

S(CH2)3CH3 ��G�� 8%

Due to the low amount of the isomers of product ��E and ��G available, a mixture of the isomers was used. The pentacenones ��D were further used to study the addition of organolithium compounds on the carbonyl functions.

��6\QWKHVLV�RI��DQG��KH[D��VXEVWLWXWHG�SHQWDFHQHV�Up to now, already four of the six desired substituents were introduced on the pentacene framework. In the next step, the two remaining aryl substituents would be introduced via an addition reaction on the carbonyl functions. In a second step the desired hexasubstituted pentacenes would be generated in the same way as for the earlier discussed 6,13- and 5,7,12,14-substituted pentacenes ��67 and ��78. (see chapter 2.2 and 2.3)


118

2,3,9,10-Tetramethylpentacene-5,7-one ��D was reacted with 2-thienyllithium. A product was isolated but could not be characterised as the 2,3,9,10-tetramethyl-5,7-bis(2-thienyl)pentacene-5’ ,7’ -diol. We supposed that only one thienyl group was attached to the compound & ?1@ ��D� because in the 1H NMR spectrum of one of the isolated fractions, three different signals for the methyl protons were observed instead of two signals. Also all other signals seemed to point in this direction, but we were not able to confirm the formation of ��D due to the low quality of the spectrum. (Scheme �II.32)


O O

& O4P ��D

SLi

THF, -78°C

OHOS

OHHOS

��D

S

��

��E

�

The integration gave an indication for the existence of the isomer ��E�and also the upfield shift of the proton on position 6 seemed to point in this direction�� Upon further reduction the mass of 5-thienylpentacene �� was detected by mass spectroscopy.


119

In order to obtain disubstitution, tetramethylethylenediamine (TMEDA) was added to the reaction mixture to increase the reactivity. However, no disubstituted product could be isolated, only the yield of the monosubstituted product �� was increased (47%) For some unknown reason it now looked like only the product ��D was formed and nothing of the isomer ��E. When the product ��D was further reduced, not the pentacene was formed but product �� similar to the formation of the product �� in chapter 2.3. (Scheme �II.33) The protons on position 14 were typically situated between 4 and 5 ppm and the protons next to the carbonyl functions showed a downfield shift. Also the mass spectrum confirmed the formation of product ��.


NaI, NaH2PO2

acetic acid reflux

OHOS

OS

��D ��

In a last attempt a Grignard reagent was used for the addition reaction but mostly starting material was recovered and no other products could be identified.

Thus, it seems that it is not possible to obtain the 2,3,9,10-tetramethyl-5,7-diarylpentacenes. With 2,3,9,10-tetramethylpentacene-5,12-one, the addition reactions were even less successful, as neither with an organolithium compound nor with a Grignard reagent, addition occurred.

Part II Chapter 2.6: Octasubstituted pentacenes

120

&KDSWHU��RFWDVXEVWLWXWHG�SHQWDFHQHV�

The 4,6-bis(3,4-substituted benzoyl)benzenedicarboxylic acid and 2,5-bis(3,4-substituted benzoyl)benzenedicarboxylic acid that are discussed in the previous chapter seemed to give us the opportunity to explore 2,3,5,7,9,10,12,14-octasubstituted pentacenes. Upon ring closure the benzenedicarboxylic acids would afford the 2,3,9,10-tetrasubstituted pentacene-5,7,12,14-tetrone which then could be further substituted with aryllithium compounds to give octasubstituted pentacenes. (Figure �II.8)

Ar

Ar Ar

ArR

R

R

R

)LJXUH��,,��2FWDVXEVWLWXWHG�SHQWDFHQHV�

��6\QWKHVLV�RI��WHWUDVXEVWLWXWHG�SHQWDFHQH��WHWURQH�Although the strategy was easy and it should take only two steps to obtain the 2,3,9,10-tetrasubstituted pentacene-5,7,12,14-tetrone, the ring closure caused a lot of problems. Finally we did not succeed in isolating the desired pentacenetetrones ��, (Scheme II.34) although different methods were used to force a double ring closure on the diacids ��. (Synthesis see chapter 2.5)

Firstly, the method suggested by Terrance HW�DO in one of their patents, was used.30a (Scheme II.34) The diacid ��D was heated in trifluoromethanesulfonic acid with a trace of trifluoroacetic anhydride at 150 °C for 12 hours. A black residue was obtained but could not be identified. We believed that the reaction conditions were too


121

aggressive and hence decreased the reaction time. Again a black residue was found and the investigation of the filtrate only resulted in a few unidentified products.

6FKHPH�,,��

OO

O

O

O

O

R

R2+

AlCl3

base, 12h

CO2HHO2C

O O

R

R

R

R

CO2H

HO2CO

R

RR

RO

��

& Q4R ��D�G

& Q:S ��D�GR = Me

Et

Sbut

Cl

DEFG

R

R

R

R

O

OO

O

F3CSO3H

��D�G �

Subsequently, the reaction temperature was decreased. Different experiments were completed at temperatures of 105, 80, 60 and 40 °C. None of them resulted in the isolation and clear characterisation of ��D��but�only unidentifiable fractions were obtained. In a few cases, the correct mass was observed in the mass spectrum. For one set of conditions at 40 °C, we managed to record in addition to the mass spectrum a 1H NMR spectrum in deuterated sulphuric acid. Though the spectrum showed clear signals and the integration seemed to match the product ��D, there were still some doubts. The presumed signals of the protons on positions 6 and 13 were situated at 9.0 ppm, but in comparison with the signal of the proton on position 6 in


122

pentacen-5,7-one ��D (9.9 ppm), this is rather low. The same method was tested on diacids ��E�G�� Product ��E could be detected via mass spectroscopy when low temperature was used although the product could not be isolated and fully characterised. Neither product ��F nor product ��G could be detected with this method. The difficulties which we experienced, suggested that it may be impossible to obtain the products ��D�G with this method. Also the detour which Terrance HW� DO always made to obtain their substituted pentacenes, seems to point in this direction.30a Instead of just close and reduce the benzoylbenzenedicarboxylic acids, they always first reduced them to the corresponding benzylbenzene-dicarboxylic acids and then carried out a ring closure and reduction to the desired pentacene.

Another literature method is a cyclisation promoted by sulphuric acid described for unsubstituted benzenedicarboxylic acids like ��.69e However when the procedure was followed for product ��D only a black residue was found instead of the reported yellow/light brown crystals. (80 °C, 4.5 h) When milder reaction conditions were used, only starting material was recovered. A shorter reaction time was applied like for anthracene analogues 68c, but mainly the singly closed product was detected. It seems that the closure of substituted bisbenzoyl-benzenedicarboxylic acids is much more difficult than that of the unsubstituted ones which may be due to sidereactions caused by the substituents.

Ring closure of analogous monobenzoylbenzenedicarboxylic acids can also be obtained by polyphosphoric acid and POCl3

69a In both cases, either unidentified products or starting material were obtained after reaction, depending on the different temperatures and reaction times used.

Treatment of the diacid ��D with trifluoroacetic anhydride and H3PO4 in the presence of scandium triflate79 gave a residue that


123

showed the right mass in the mass spectrum together with the mass of a singly closed product but the NMR spectrum was unclear. The same reaction was performed with diacid ��F although in this case no positive results were obtained.

A second identical Friedel-Crafts reaction gave no results. Even after heating at high temperature for a long time only starting material was recovered. When a solventless reaction was carried out with aluminium trichloride in a melt of NaCl 77c, 80 at a temperature of 160 °C for 12 hours, in the mass spectrum the singly closed product ��D was detected. However the product ��D could not be isolated and characterised with other techniques. 1H NMR in deuterated sulphuric acid gave only signals with an inverse integration to what is expected (7.9 ppm, 4H; 7.5 ppm, 2H) Higher temperatures only led to unidentified products. Also this method was applied on diacid ��F�though no products could be properly identified.

A few methods were tried to modify the acid functions to make them more reactive. An attempt was made to prepare an acid chloride with thionyl chloride. Although the acid chloride of ��D could not be characterised, the next step was carried out with aluminium trichloride. However the reaction only resulted in unidentified products. Also an attempt to prepare the acid chloride with oxalyl chloride gave no clear product.

By reaction of the diacid ��D with zinc chloride in a mixture of acetic anhydride and acetic acid, a cyclisation should occur via an anhydride of the diacid. However the method77a,b was not successful for the double closure that was necessary in this case.

A last modification that we made, was the preparation of a di-ester of the diacid which seemed for analogous monoacids in some cases more reactive in ring closure.70c The synthesis of the di-ester ��occurred via thionyl chloride in methanol with moderate yield. (46%, Scheme �II.35)


124

6FKHPH��,,��CO2HHO2C

O O

Me

Me

Me

Me

CO2MeMeO2C

O O

Me

Me

Me

Me

SOCl2

MeOHreflux, 5h��D ��D

Again the ring closure failed and only starting material was recovered when applying the described method for analogous monobenzoyl esters70c with sulphuric acid. A more aggressive treatment with trifluoromethanesulfonic acid resulted in an unidentified precipitate.

An attempt was made to characterise the possibly formed pentacenetetrone �� in an indirect manner because in some of the above described methods the correct product could be detected in the mass spectrum of the insoluble residue that was obtained. To render purification and characterisation more straightforward, it was tried to make compound ��F more soluble via substitution of the chloride atoms with 4-tertbutylphenoxy groups. (Scheme II.36)

6FKHPH�,,��CO2HHO2C Cl

Cl

Cl

ClOO

?

O

O

O

O

O O

OO

NaCl/AlCl3

150°C, 4hHO

K2CO2, DMF3 days,80°C

��F

��

Thus, first a ring closure method was applied which gave the correct product ��F in the mass spectrum. The dark powder was treated then


125

with S-tertbutylphenol in the presence of a base.81 However even after three days at 80 °C, the product �� which should be much more soluble, could not be detected.

Finally we tried to obtain the pentacenetetrones �� indirectly. It is possible to reduce the bisbenzoylbenzenedicarboxylic acids �� to the bisbenzyldicarboxylic acids �� which smoothly undergo ring closure. The pentacenedione �� should only be oxidised in order to obtain the pentacenetetrone ��. (Scheme �II.37) Again, at first sight, it looked like a simple way to solve the ring closure problems of the bisbenzoylbenzenedicarboxylic acids ��. However, also here a lot of problems hindered the isolation of product ��.


R

R

R

R

O Ooxidation

& T>U �& T6V ��

R

R

R

R

O O

O O��

First, DDQ was used as an oxidant without success. Although the correct mass was detected together with the mass of the starting material, product ��D could not be isolated.

Secondly the possibility of the use of an inorganic oxidant such as KMnO4 was investigated.82a The problem with most of them was that there are insoluble inorganic residues of the oxidation process.82 These residues make it impossible to isolate the product �� which apparently is also insoluble. So, the product ��D (together with starting material) was detected by mass spectroscopy of the residue that was collected after the oxidation with KMnO4. However it could not be isolated because it was mixed with inorganic solids and could not be washed out.


126

Finally, a method83 was found where no solid residues had to be expected. The pentacenedione ��D was treated with diluted hydrogen peroxide in a mixture of acetic acid and sulphuric acid. After stirring for two days both the final product and the starting material were found in the mass spectrum. The 1H NMR spectrum showed clearly the starting material. After five days the reaction seemed complete as only the mass 399 was found in the spectrum, though the difference of 4 with the real mass seems to indicate that instead of four carbonyl functions there were four phenol functions present. The oxidation with hydrogen peroxide was also done with pentacenedione ��F. The precipitate was filtered and a different mass was found. (m/e 476, 481) It was difficult to draw conclusion of the data we obtained.

In order to avoid the oxidation step but to keep the advantage of the disappearance of the carbonyl functions to obtain an easy ring closure, a second Friedel-Crafts reaction was performed on the diacid ��D.84


��D

CO2H

O

1) SOCl2reflux

2) R-xylene,AlCl3

dichloromethanereflux 12h

OO

O

��D

CO2HO

��D��D

HO2C

O

O

HO2CO


127

The reaction resulted in the formation of the bisfuranone �� in moderate yield (53%). If the five membered rings were cleaved again, it would be a lot easier to get a ring closure for the formation of a pentacenedione ��. Then, only two extra substituents had to be introduced. (Scheme �II.38) The two aryl substituents already present would increase the solubility in comparison with the pentacenediones ��. The only disadvantage would be that it would be more difficult to vary the substituents on the 5-, 7-, 12- and 14-positions.

The ring opening reaction of product ��D so far could not be brought to success. We tried to cleave the five membered rings in a reductive way with hydrogen and palladium on carbon as a catalyst but despite the different reaction conditions that were tried, only starting material was recovered, although this method has been used successfully to cleave the five membered ring of monofuranones.85 Also treatment with zinc in acidic conditions could not force the ring to open.

��6\QWKHVLV�RI��RFWDVXEVWLWXWHG��SHQWDFHQHV�Due to the difficult characterisation of the insoluble residue, we were never sure that the product �� was formed and if it was pure. If we succeeded in the addition of a lithium compound and were able to characterise the formed tetrol ��, the formation of tetrone �� in reasonable amounts would be indirectly proven. (Scheme �II.39)

A test reaction was done with 2-thienyllithium on the residue obtained from the reaction of diacid & W1X ��D and trifluormethanesulfonic acid (in trifluoroacetic acid, 40 °C, 12 h). The crude mixture was examined with LC-MS. The tetrol �� was detected as the corresponding rearranged product, however also starting material was found. When the reaction was repeated on larger scale (0.5 g) and the crude mixture was submitted to normal column chromatography the tetrol �� was not found.


128


?

SLi

OH HO

HOOH S

S

S

S

��

CO2HHO2C

O O& Y/Z ��D

�

Also the residue obtained upon the oxidation with hydrogen peroxide was reacted with a lithium compound. This time, the lithium derivative of 1-(4-bromophenyl)octane was used to prevent possible solubility problems. Despite the indication in mass spectroscopy that the tetrone ��D was formed in the oxidation reaction, no corresponding tetrol was found.

At this stage we stopped our research, because the addition of lithium compounds could not indirectly prove the formation of the substituted pentacene-5,7,12,14-tetrone in the previous reaction. Thus, it appeared that we did not succeed in the synthesis of the tetrones �� starting from benzoylbenzenedicarboxylic acids ��. Even if these tetrones �� had been obtained the subsequent plans we had for them, namely organolithium addition, were failing. We believe that in order to obtain the planned octasubstituted pentacenes the synthesis should be approached in a completely different way.

Part II Conclusion

129

&RQFOXVLRQ�From our investigation, it can be concluded that we did not find a proper way to prepare 6-monosubstituted pentacenes due to their facile oxidative degradation. However, it could be possible to obtain them by adjusting the synthetic work-up according to recent knowledge23. It seems to be possible to isolate the 6-monosubstituted pentacenes if they can be precipitated from the reaction mixture. However, from our work it is clear that the 6-monosubstituted pentacenes will not meet the most important requirement, that of stability, for industrial use in transistors.

We can conclude that 6,13-diarylpentacenes are relatively easy to obtain and are interesting for further investigation with respect to their potential use in transistors. Different substituents were introduced. Unfortunately, we did not succeed in the synthesis of 6,13-bis(alkylsulfanyl)pentacenes from the disulphidepentacene polymer. However, it is possible to obtain them starting from the pentacene-6,13-diol which was recently reported in literature40. During the research of 6,13-diaryl substituted pentacenes, we observed that rearrangement can occur when electron rich substituents are used. This can prevent the synthesis of the corresponding pentacenes. We could arrange the substituents we used by their sensitivity towards rearrangement induced by acids (Brønsted and Lewis acids). It was also noticed that the FLV-isomers are more sensitive to rearrangement than the corresponding WUDQV-isomers.

We can also conclude that 5,7,12,14-tetra-arylpentacenes are relatively easy to obtain and are interesting for further investigation with respect to their potential use in transistors, in particular 5,7,12,14-tetra(4-alkylphenyl substituted)pentacenes. We have observed that the choice of the substituent which is introduced on the pentacene backbone, is of great importance with respect to the solubility of the resulting pentacene derivative. An increase of the

Part II Conclusion

130

solubility was observed when a long chain was present in the substituent.

We did not succeed in the synthesis of 5,6,7,12,13,14-hexasulfanylpentacenes via 5,6,7,12,13,14-hexasulphurpentacene due to the insolubility and low reactivity of the starting material.

We managed to prepare different 2,3,9,10-tetrasubstituted pentacene-5,7- and 5,12-ones. The addition of aryllithium compounds on the carbonyl functions failed. In the case of the pentacene-5,12-one no addition occurred. The pentacene-5,7-one gave only monosubstitution. Probably the synthesis of 2,3,5,(7),9,10,(12)-hexasubstituted pentacene can not be achieved via this pathway.

Due to problems with the ring closure of bis(3,4-substituted benzoyl)benzenedicarboxylic acids, we did not succeed in preparing the 2,3,5,7,9,10,12,14-octasubstituted pentacenes. The difficulties are caused by the failure of the double ring closure. We doubt that an efficient synthesis of 2,3,5,7,9,10,12,14-octasubstituted pentacenes is possible in this way.

Part III

131

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The new substituted pentacene derivatives were integrated in transistors and subsequently ISD-VGS-curves were measured. We have successfully synthezised 6,13-diaryl substituted pentacenes ��67 and 5,7,12,14-tetra-aryl substituted pentacenes ��78 (see Part II, Chapter 2.2 and 2.3). Both showed a better stability against oxidation by oxygen and were much better soluble than the parent pentacene. These molecules gave us the opportunity to study the influence of the type of the substituents on the film formation and transistor characteristics. It was also possible to get an idea of the influence of the number of substituents by comparing analogous di- and tetrasubstituted pentacenes.

The first challenge was to find a method that allowed preparing thin-films of good quality on a substrate. A high quality film must be continuous and highly ordered. To get a rough idea of the continuity an optical microscope was used. Different solution processing techniques and different conditions were tested. For one material both solution and vacuum processing methods were applied for comparison.

Finally, current-voltage measurements could be performed on transistors which were prepared via spincoating from a solution of

Part III

132

substituted pentacene with a polymer additive. From these results, a preliminary idea of the electronic properties could be obtained.

Part III Chapter 3.1: Formation of films

133

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The substituted pentacenes (Figure �III.1) fulfil the role of semiconductor in the transistor. To integrate the pentacene derivatives into the transistor structure, the material has to be deposited onto a substrate which is the dielectric for a top-configuration or a combination of dielectric and contacts for a bottom-configuration (see introduction). In the first case the surface is smoother but contacts have to be deposited on top of the semiconductor. In contrast, for a bottom-configuration the contacts are already on the substrate prior to the deposition of the semiconducting film, but they introduce some roughness to the surface. The main goal was to deposit the organic semiconductors from solution to have an easy and inexpensive processing technique that is compatible with flexible substrates. For the substituted pentacene ��G also OMBD (see introduction) was used to exclude conclusions of failure due to the material while in reality the failure being related to the deposition technique.

For deposition from solution, the solubility of the materials is crucial

and often the restricting factor. The solubility of all the substituted

pentacenes was compared. In general they mostly ranged in the order

of 10-3 M and by using an ultrasonic bath we could dissolve up to 10-2

M[. Only the substituted pentacenes bearing an acetyl function ��E�I

seemed not to reach 10-3 M.

We observed some difference in the solubility depending on the

different substituents that were used. As already mentioned, the

presence of an acetyl function on the substituent ��E�I seemed to

hinder the solubility of the molecules. The best solvents were

\ (probably oversaturated)


134

tetrahydrofuran (THF) and N-methylpyrrolidone (NMP) while toluene

and dichloromethane seemed only be able to dissolve half of the

amount of substituted pentacenes ��D�E�G�M. However, when a long

alkyl chain was present, as in the substituted pentacene ��F� the

solubility in the first two solvents was comparable but the solubility in

toluene increased to at least the same level. This is an advantage as we

will see later on. These differences between the substituents were also

observed between the tetrasubstituted pentacenes ��D�F�I.

Ar

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In spite of our expectations, the solubility of the tetrasubstituted pentacenes ��was not better than that of the disubstituted analogues


135

��. The introduction of more substituents on the backbone of pentacene seemed in fact to be responsible for a slight decrease of the solubility in the case of the pentacene derivatives ��H and ��I. The low solubility troubled the characterization of the products with NMR spectroscopy. (See Part II, Chapter 2.3) Possibly the presence of sulphur atoms in these pentacene derivatives is promoting aggregation of the molecules. All other pentacenes ��D�E�G showed a solubility comparable to that of their disubstituted analogues. Thus, at first sight the tetrasubstituted pentacenes �� showed no improvements in comparison with their 6,13-pentacene analogues for the use in solution processed OTFT’ s.

For the deposition of the materials onto a substrate, different solution processing methods were tested, namely dipping, bladecoating, dropcasting and spincoating. The first experiments were made on cleaned glass substrates to study the continuity of the resulting film and to slightly optimize the conditions.

The first two techniques seemed not suitable for deposition of the materials. When the substrate was dipped into a solution of a pentacene derivative and pulled out again, the solvent just slipped off quickly and almost no material was left on the substrate. Furthermore, this technique demands a rather large amount of material for testing, so that no great efforts were made to improve the conditions. Also the use of the bladecoating method was hindered by the type of the solution. The blade was not able to spread the solution over the substrate, presumably due to the low viscosity of the solution or the lack of affinity of the solvent for the substrate. The solution was pulled over the substrate or stuck onto the blade.

Dropcasting is one of the easiest techniques and at first sight it gave good results. Dropcasting resulted mostly in thick semiconductor films. These thick films hindered proper characterisation of the transistor properties. In order to obtain nice continuous crystalline


136

thin-films, different solvents, temperatures and surface treatments were tested. In spite of these efforts, we did not manage to obtain thinner films and cover the whole substrate by dropcasting. Only for 6,13-bis(4-methoxyphenyl)pentacene ��D� it was possible to measure a very low mobility (10-8-10-7 cm2/Vs) for a THF solution with a low concentration of material. All other materials gave too thick films or poorly covered substrates due to three-dimensional growth. However the experiments with dropcasting revealed some interesting observations of different behaviour between the pentacene derivatives.

A B

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They pointed out that the type of substituent is playing an important role on the morphology of the films. When THF solutions were dropcasted onto a glass substrate at 40 °C, phenyl substituents as in

10 ]�^


137

the pentacene ��D�E favour a surface covered with needle-like crystals (Figure �III.2 A). Pentacene derivative ��F formed amorphous films, that we attribute to the long alkyl chains (Figure �III.2 B). In contrast, pentacenes with thienyl substituents tended to form cubic-like crystals (Figure �III.2 C).

Not only the type of the substituent has an influence on the morphology, also the number of substituents is important in this respect. When comparing a dropcasted film of pentacene ��G and ��H, we observed that under the same conditions the disubstituted pentacene ��G was forming crystals on the surface while the tetrasubstituted analogue ��H�gave a more amorphous film. (Figure �III.2 C versus Figure III.3)

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While depositing 6,13-thienylpentacene ��G, we observed that depending on the temperature the molecules formed crystals or an amorphous film. By changing the solvent to NMP, the temperature of the substrate could be increased to 150 °C. The result was similar to that of the earlier made films from THF solution dropcasted on a substrate at 40 °C. (Figure �III.4 A versus Figure �III.2 C) However when increasing the temperature of the substrate, the morphology changed from an intermediate stage at 100 °C (Figure �III.4 B) to an amorphous film at 150 °C (Figure �III.4 C). Unfortunately, at that high temperature also cracks appeared in the film.

10 ]�^


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Most substituted pentacenes �� and �� have a high melting point, even exceeding 300 °C. However when an alkyl chain as in the pentacenes ��F and ��D�E, is introduced the melting point is decreasing significantly to below 200 °C. Due to this low temperature it was possible to melt some crystals of pentacene ��F on different substrates. Five generally used surfaces were tested to assign the influence of the surface on the morphology of the cooled melt. (Figure III.5) Like on the dropcasted glass substrates, the melt on silicon dioxide gave an amorphous drop. (Figure III.5 A) It is known that for pentacene the crystallinity can be improved by treatment of the substrate with OTS (octadecyltrichlorosilane).86 In contrast, we observed for pentacene ��F that the morphology is basically identical for a blank substrate as compared to a substrate treated with

10 ]�^


139

PTS (phenyltrichlorosilane). However when the surface was modified with HMDS (hexamethyldisilazane) the melt formed needles while cooling down. (Figure III.5 B) Recently, also polystyrene has been used as a surface modifier to get better performance of the pentacene transistors.87 A morphology with very small crystallites was observed on the polystyrene surface. (Figure III.5 C) This observation indicates the importance of the surface on which the semiconductor is deposited.

A B

C

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The main problem with the deposition via dropcasting was the thickness of the films. To obtain thinner films, spincoating can be used. This technique is better controllable but also more complex. Different factors can play a role in the quality of the final film. Not

100 `�a


140

only the material, solvent and surface treatment but also the rate and time of spinning, viscosity and the after treatment are important.

The first experiments carried out with THF solutions of the materials �� were disappointing. Only a few crystals were found scattered over the surface. (Figure �III.6) Only pentacene ��F did not form crystals and seemed to give rise to an amorphous film. Due to the low viscosity, a large part of the solution is spun off the substrate when it starts spinning. Another problem was the tendency of the molecules to form three-dimensional crystals that grow more perpendicular to the surface instead of covering it.

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Different surface treatments, introduced in the hope to increase the affinity of the material for the surface, could not improve the result. The spin rate was varied to find a rate at which not too much solution was thrown off the surface still being fast enough to prevent three-dimensional growth. However, no adequate rate was found.

Another attempt to cover the whole surface was to successively spin a number of times. The surface was now nicely covered, but no electrical measurements were obtained probably because the first layers were not continuous enough to transport the charge carriers in a proper way. Also the way of applying the solvent on the substrate was changed. Most experiments were carried out by first covering the substrate with solution followed by spinning. A little improvement

10 `�a


141

was found when a solution of pentacene ��D was applied on the substrate while already spinning. Although the film now seemed continuous, the electrical measurements showed only a very low mobility (< 10-8 cm2/Vs). We infer that probably the first layers were still rather discontinuous.

Some attempts were made to change the solvent. Most of the solvents which dissolved the pentacene derivatives in reasonable amounts (> 10 mg/ml like NMP and indane), have high boiling points (> 140 °C) which hampered the spincoating. The high boiling point prevented the slow evaporation during spinning, resulting in solution drops remaining on the surface after spinning. Increasing the spin time did not improve the results. Spinning at higher temperature could possibly solve this problem, but we had no access to an efficient heating system for the spinner.

Because a part of the problems involving spincoating is due to the viscosity of the solution, experiments were made with adding a polymer to the solution to increase the viscosity. Polystyrene was used as an inert additive to improve the spincoating properties of our solutions.31 The first experiments were made with the pentacene derivatives ��D. Both the molecular mass and the amount of the additive were varied but all attempts remained without the expected improvements.

The three-dimensional growth seemed to dominate the film formation. A breakthrough was found when pentacene ��F was successfully spincoated affording working transistor from a toluene solution instead of a THF solution. (see chapter 3.2) Hence, this film appeared to be more homogeneous. (Figure �III.7)

The addition of a polymer to the solution improved the film formation and gave rise to a working transistor. However, the conditions had still to be further studied.


142

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All materials were tested again with the knowledge that was collected by experiments with pentacene derivative ��F. In the same context, also the tetrasubstituted pentacenes �� were further studied. These results are discussed in detail in the next chapter.

How important the morphology is for a working device was shown when 6,13-di(2-thienyl)pentacene was deposited in two different ways leading to different results. All attempts of solution processing of the pentacene ��G failed and no working transistor could be made. However, when films were grown with an OMBD-system, working transistors were obtained with a mobility up to 10-5 cm2/Vs. Simultaneously, analogous experiments were published with even higher mobilities (0.1 cm2/Vs).29

This indicates that the material itself has semiconducting properties. So the failure of the transistors processed from solution was caused by the deposition technique, hampering the correct packing of the molecules in the film. The films obtained with the vacuum technique are smoother and seems to cover the whole substrate. More information about the morphology and to know if they also grow two-dimensionally, the samples have to be studied more profoundly by AFM measurements. (Figure �III.8)

10 `�a


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A B

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The morphology of the vacuum deposited films showed some variability as a function of different parameters such as surface treatment, evaporation rate and substrate temperature.

The morphology of films deposited with different evaporation rates (1 Å/s, 0.5 Å/s, and 0.3 Å/s) on a substrate at room temperature, varied in crystallinity. The difference in morphology for a polystyrene surface was especially noticed between the deposition rate of 1 Å/s and 0.5 Å/s. (Figure �III.8A 1 Å/s versus Figure III.9A 0.5 Å/s)

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While the first showed a smooth surface, there was clearly a crystalline structure present in the second. However, the measured mobility showed only a slight variation. (Table �III.1) For a surface covered with OTS it was the other way around and there was less crystallinity for the slower deposition rates. Here the difference in the measured mobilities was much more pronounced. It seemed that for the smoother films which were grown at slow deposition rates, the mobility increased compared with films where larger structures (spherulites) were visible. (Table �III.1)

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Surface treatment polystyrene OTS isopropanol

Evaporation rate cm2/Vs cm2/Vs cm2/Vs

1 Å/s 9.7 10-6 1.2 10-6 6.1 10-6

0.5 Å/s 1.1 10-5 9.7 10-6

0.3 Å/s 7.6 10-6 2.3 10-4

The same trend was observed when different surface treatments were compared. The films were deposited at 1 Å/s on a substrate at room temperature. Under these conditions, the surface covered with polystyrene delivered the smoothest film while on OTS and IPA spherilutes were growing. (Figure �III.8A versus Figure �III.10) As a consequence, the transistor with the polystyrene interface showed the best mobility. (Table �III.1)


145

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When the substrate temperature was increased, the surface treatment with OTS showed a large improvement. (Table �III.2) When the substrate was heated at 40 °C during deposition, the molecules did not form spherulites anymore on OTS but the film became smoother like on the polystyrene surface at room temperature (Figure �III.8 A). In contrast for a polystyrene interface the increase of the temperature to 80 °C promoted the formation of spherulites and thus the mobility decreased. (Table �III.2, Figure �III.11)

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OTS 1.2 10-6 1.5 10-5

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Part III Chapter 3.2: Electronic properties

147

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In the previous chapter, it is shown that the deposition of the pentacene derivatives from solution is not straightforward. Besides the intrinsic properties of the semiconductors, the stacking in the film is the most important factor to get a working device. The stacking of the molecules is not only affected by the intrinsic properties of the material but also by the deposition technique and the deposition conditions. Following on the first successful measurement of a transistor of pentacene ��F, we focussed on these conditions and tried to optimize them by studying the different variables. Furthermore, the other disubstituted pentacene derivatives were subjected to this method. In a second step the tetrasubstituted pentacenes were studied based on the knowledge obtained.

��7UDQVLVWRUV�FRQVLVWLQJ�RI�GLVXEVWLWXWHG�SHQWDFHQHV��The semiconductor layer of pentacenes �� was deposited via spincoating onto a substrate. In all cases an additive (polystyrene, Mw: 1,000,000 g/mol) was used to increase the viscosity of the solutions and to simplify the spinning. Spincoating is a rather complex technique and a lot of parameters can have an influence on the quality of the final film and its performances. Different parameters were studied in order to optimize the conditions. Like already shown in the previous chapter, one of the crucial factors to get a working transistor is the solvent. We also noticed that variation in the concentration of the derivative and the additive led to small changes in the mobility. The substrate can be modified to better support ideal arrangement of the molecules. Therefore, also the surface of the substrate was changed by applying a self-assembled monolayer. Furthermore, the spincoating conditions like the spin rate and the spin time, have an influence on the result as well as the treatment of the sample after


148

spinning the semiconductor layer. We also took a closer look at the reproducibility of the process and the stability of the solutions.

Ar

Ar ��D�I��MOMe

COMe

(CH2)7CH3

D�=

E�=

F�=

S

S

S COMe

G�=

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OMe

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The research performed is not covering all possible improvements that can be made. Hence, the results presented here are not the final ones for these pentacene derivatives, they only give an indication of the influence of the different parameters and leave room for further improvements.

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The semiconductor layer was applied on SiO2-substrates with Au-contacts in a bottom configuration. The measurements were carried out for a gate voltage between 10 V and -10 V and a drain voltage of

100 `�a


149

-10 V. The mobilities were calculated for a transistor structure with a

width of 5000 Pm and a length of 10 Pm in the saturated regime and

displayed as an average of the measurements from at least two different transistors on the same substrate. (Figure �III.13)

To start, we evaluated the reproducibility of the solution processed semiconductor layer. Firstly, three identical transistors were prepared under the same conditions from the same solution of pentacene ��F�in toluene. After spincoating the layer, the transistors were measured and mobilities were calculated. We noticed that the mobilities and also the threshold voltages were in the same range. (Table III.3/Figure �III.14) The standard deviation for the mobilities was 0.7 10-6 cm2/Vs.

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Sample A Sample B Sample C

Mobility 2.5 10-6 cm2/Vs 2.4 10-6 cm2/Vs 3.6 10-6 cm2/Vs

Vt -0.5 V -0.7 V -0.5 V

100fA

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150

Secondly two identical solutions were prepared and subsequently used to spincoat a semiconductor layer under the same conditions. When both samples were measured, only a small difference could be observed. The difference can be assigned to the spincoating process and the error on the preparation of the solutions. (Table III.4)

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Solution 1 Solution 2

Mobility 8.2 10-7 cm2/Vs 7.6 10-7 cm2/Vs

Vt -1.1 V -1.0 V

�In order to exclude underestimation of results due to degradation of the used solution of the pentacene derivative, two transistors were made of the same solution. One was made just after the solution was prepared and the other after storage of the solution for 12 days in nitrogen atmosphere.

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We observed that only freshly prepared solutions should be used for making the semiconductor layer because even upon storing the bottle


151

under nitrogen atmosphere, a decrease in performance of the transistor was observed. (Figure III.15)

The solution that is used in solution processing techniques, is important for the final result of the device. In our case the solution consists of a solvent, the pentacene derivative and an additive, here polystyrene. We tried to study the influence of the type of solvent that was used and the amount of semiconductor and additive that was dissolved, on the performance of the final semiconductor layer.

The solvent is one of the crucial factors for a working device. Because most of the pentacenes ��showed the best solubility in THF� at first this solvent had our preference. Nevertheless, only three pentacene derivatives ��D�� I and ��M showed eventually working transistors although with a low mobility. (Table III.5) All other pentacenes did not behave as a semiconductor layer when deposited from a THF solution.

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Pentacene ��D� ��E� ��F� ��G�toluene 1.5 10-8 2.7 10-7 1.3 10-6 / Mobility

cm2/Vs THF 3.0 10-8 / / /

Pentacene ��H� ��I� ��M�

toluene / / / Mobility cm2/Vs THF / 7.4 10-9 3.0 10-7

However, when pentacene ��F which is the only derivative that is better soluble in toluene than in THF, was spincoated from a toluene solution suddenly the transistor did work and showed even a higher mobility than all others. (Table III.5) Subsequently, also the other pentacene derivatives were deposited from a toluene solution. In this


152

way also a working transistor was obtained with pentacenes��D�and ��E, although the concentration of the semiconductor was much lower. (Table III.5) Unfortunately, the change of solvent had no effect on the disubstituted pentacenes ��G and ��H.

It is clear that the solvent has a large influence on the formation of a working transistor and the use of a wrong solvent can lead to negative conclusions on the properties of a potential semiconductor. We observed that only one of the seven derivatives tested, showed transistor characteristics in both THF and toluene. Four of them only worked when the correct solvent was used. What ‘correct’ means, is unclear, two of them worked when deposited from a THF solution and two only when toluene was used. For pentacene ��F it was clear from the microscope picture that there was a difference between the morphology of the film deposited from a toluene or from a THF solution. The first gave rise to a more amorphous but continuous looking film. In contrast, the film from THF consisted out of large needle-like crystals that formed clusters but missed connection between each other. (Figure �III.7, Chapter 3.1) The same phenomenon was observed for pentacene ��M. More continuous areas that appeared more amorphous were detected in the working film although the film was deposited from a THF solution. Between the films made from different solvents for pentacene ��E and ��I no real difference was noticed at microscopic scale.

After the appropriate solvent was found, the composition of the solution was investigated. Due to the use of an additive, we had to deal with a two-component system, thus the concentrations of both materials were changed. First, the concentration of the pentacene derivative was varied while the polymer concentration was kept constant. For pentacene ��F we observed first an increase in the mobility with increasing concentration. However it seemed that there was a limit due to solubility because when a 4% solution was made the mobility


153

decreased again. (Table �III.6) The optimal amount for pentacene ��F in toluene is probably situated somewhere between 25-30 mg for 1g of solvent. More experiments are necessary to define the optimum.

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35 mg 25 mg 15 mg

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VROXWLRQ�ZLWK�D�GLIIHUHQW�FRQFHQWUDWLRQ�RI�SHQWDFHQH��F��[�PJ�LQ��J�RI�VROYHQW��

The same trend was observed for pentacene ��D in THF. Only there the optimal amount is probably less and may be situated around 8 mg of pentacene ��D in 1g of solvent. No mobility was measured for 15 mg but for 25 mg again a working transistor was found. However due to the small mobility it was probably a lucky shot. (Table III.6)

7DEOH� �,,,��0RELOLWLHV� RI� VDPSOHV� SUHSDUHG� IURP� VROXWLRQV� �� J� RI�VROYHQW�� ZLWK� D� GLIIHUHQW� FRQFHQWUDWLRQ� RI� SHQWDFHQHV� �� DQG� WKH�VDPH�FRQFHQWUDWLRQ�RI�DGGLWLYH��PJ��

Mass ��D� Mass ��F�8 mg 2.8 10-8 15 mg 4.5 10-7

15 mg / 25 mg 2.4 10-6 Mobility cm2/Vs

25 mg� 3.9 10-9� 35 mg 1.7 10-6�


154

Secondly the polymer concentration was varied to obtain solutions with a different viscosity and films with different amounts of inert material. It is believed that maybe the polystyrene is concentrating at the bottom of the film and so is forming an isolating layer. When this is true, the thickness of this polymer layer can become important especially when a bottom-contact configuration is used. The polymer could hinder the injection of charge carriers from the contacts to the semiconductor layer and consequently increase the contact resistance and decrease the performance of the transistor. The increase of the polymer concentration had only a rather small influence on the mobility. (Table III.7) The best performance seemed to be situated around 7 mg of polymer dissolved in 1 g of solvent.

7DEOH� ,,,��0RELOLWLHV� RI� VDPSOHV� SUHSDUHG� IURP�D� VROXWLRQ� �� J� RI�VROYHQW�� ZLWK� D� GLIIHUHQW� FRQFHQWUDWLRQ� RI� DGGLWLYH� DQG� WKH� VDPH�FRQFHQWUDWLRQ�RI�SHQWDFHQH��F��PJ��

Mass PS 3 mg 5 mg� 7 mg� 10 mg� 13 mg

Mobility cm2/Vs 1.6 10-6 2.8 10-6 4.0 10-6 3.2 10-6 2.4 10-6

Except by optimizing the solution, the performance of the transistor can also be improved via the adjustment of the surface energy of the substrate. In this way it is possible to increase the affinity of the semiconductor and/or the solvent for the substrate. This can improve the stacking of the molecules and therefore the performance of the transistor. We observed that the affinity to a certain surface is depending on the pentacene substituents. For toluene solutions an OTS treatment is improving the result for pentacene ��D and ��F however for pentacene ��E the mobility decreased dramatically by two orders of magnitude. (Table III.8)


155

7DEOH�,,,��'LIIHUHQFH�LQ�PRELOLWLHV�IRU�WUDQVLVWRUV�PDGH�IURP�WROXHQH�VROXWLRQV�RQ�HLWKHU�D�FOHDQHG�6L2 _ �RU�DQ�276�VXUIDFH��

Surface ��D� ��E� ��F�OTS 8.9 10-9 1.3 10-9 7.7 10-6 Mobility

cm2/Vs SiO2 3.2 10-10 2.7 10-7 1.7 10-6

100fA

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The same comparison was made for THF solutions of the pentacene derivatives ��D, H and M. For pentacene ��D no real improvement was observed for an OTS surface. The difference is so small that it is believed that in this case the type of surface makes no difference. (Table III.9) This is in contrast to the observation made with a toluene solution of compound ��D.

7DEOH�,,,��'LIIHUHQFH�LQ�PRELOLWLHV�IRU�WUDQVLVWRUV�PDGH�IURP�7+)�VROXWLRQV�RQ�HLWKHU�D�FOHDQHG�6L2 _ �RU��DQ�276�VXUIDFH��

Surface ��D� ��I� ��M�OTS 3.0 10-8 / 7.9 10-7 Mobility

cm2/Vs SiO2 2.2 10-8 7.4 10-9 3.0 10-7


156

For pentacene ��M, a small improvement was noticed when using OTS. In contrast, no mobility was measured for derivative ��I. Because also for a cleaned silicon dioxide surface only a very small mobility was observed, it is difficult to make any conclusion about this substituted pentacene.

Furthermore, also the conditions of the deposition technique have an influence on the performance of the final film. Both the spin rate and the spin time can be varied. The optimal spin conditions depend on the solvent and the viscosity of the solution. When the spin time was considered, it was noticed that THF, because it is more volatile, needs shorter spin times to reach an optimal film than toluene (70s, see Table III.18). A solution of pentacene ��D was spincoated for 70, 40 and 20 seconds under the same conditions. The difference in mobility was remarkable. The transistor spun for 40 seconds, possessed a mobility of one order of magnitude larger than the two others. (Table �III.10)

7DEOH� �,,,��0RELOLWLHV� RI� WUDQVLVWRUV� ZKLFK� KDG� D� VHPLFRQGXFWRU�OD\HU� PDGH� RI� D� 7+)� VROXWLRQ� RI� SHQWDFHQH� ��D� ZLWK� GLIIHUHQW�VSLQQLQJ�WLPHV��

Spin time 70 s 40 s 20 s


The influence was much less pronounced, when the spin rate was studied for both solvents. Nevertheless, it appeared that for toluene and THF respectively, 2000 rpm and 3000 rpm gave slightly better results. (7DEOH��,,,�11) We have to remark that the optimal spin rate is also dependent on the amount of polymer within the solution. When a much lower concentration was used a lower spin speed seemed more appropriate.


157

7DEOH� �,,,�� 0RELOLWLHV� RI� WUDQVLVWRUV� PDGH� ZLWK� GLIIHUHQW� VSLQQLQJ�UDWHV� ZKLFK� KDG� D� VHPLFRQGXFWRU� OD\HU� PDGH� RI� D� 7+)� VROXWLRQ� RI�SHQWDFHQH��M��PJ�36��RU�RI�D�WROXHQH�VROXWLRQ�RI�SHQWDFHQH��F��PJ�36��

Spin rate Pentacene 1000 rpm 2000 rpm 3000 rpm

��M� 4.0 10-7 3.0 10-7 5.9 10-7 Mobility cm2/Vs ��F� 2.8 10-6 4.0 10-6 2.9 10-6

After a solution was applied on the substrate and spincoated, the obtained film was not completely dry yet. Therefore the final results can also be influenced by the manner the sample is treated after spincoating, as during drying, the molecules can still rearrange in a more or less favourable stacking. From different pentacene derivatives ��D, ��E and ��F in toluene, a few identical samples were made by spincoating. Afterwards, one sample was heated for two minutes above the boiling point of toluene under nitrogen atmosphere. One was dried at room temperature in a nitrogen environment to prevent oxidation and for pentacene ��F a last one was dried in vacuum at room temperature. It was noticed that the best result were obtained for all three pentacenes �� without heating. (Table �III.12)

7DEOH��,,,��0RELOLWLHV�RI�WUDQVLVWRUV�ZKLFK�KDG�D�GLIIHUHQW�WUHDWPHQW�DIWHU�VSLQFRDWLQJ�WKH�VHPLFRQGXFWRU��

Treatment ��D� ��E� ��F�120 °C / 7.2 10-10 7.6 10-7

RT, N2 1.5 10-8 2.7 10-7 1.1 10-6 Mobility cm2/Vs

RT, vacuum � 1.8 10-6� When the other transistor characteristics of the transistors containing pentacene ��F that were dried in nitrogen atmosphere or in vacuum


158

were compared, it was noticed that the sample dried in vacuum had a somewhat larger current and more hysteresis. (Figure �III.18)

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From all these results it is clear that it is very difficult to define a general procedure to obtain the best possible results for all disubstituted pentacenes ��. The influence of the substituents on the film morphology is relevant and varies for the different derivatives. This makes it also difficult to compare the electronic properties of the different pentacenes ��. (Figure �III.19) We arranged the best result for each product in Table III.13 together with the conditions that were used. The best results until now were observed with pentacene ��F, although the mobility is still low. The injection of charge carriers is also hampered by a resistance at the contact interface. We observed a increase of the mobility (1.1 10-5 cm2/Vs) when a transistor was measured with W/L = 1000/100. The larger distance between the contacts decreases the influence of the contact resistance.


159

7DEOH� �,,,�� %HVW� UHVXOWV� DQG� FRQGLWLRQV� WLOO� QRZ� IRU� SHQWDFHQH�GHULYDWLYHV��

�� Mobility cm2/Vs Solvent��

Conc 2.11/PS

(1g solvent)

After treatment Surface

D 3.0 10-8 THF 25/5 50 °C OTS

E 2.7 10-7 toluene 15/5 Tr,, N2 SiO2

F 7.7 10-6 toluene 35/5 120 °C OTS

G / / / / /

H� / / / / /

I� 1.7 10-6* THF 15/5 50 °C SiO2

M 7.9 10-7 THF 15/5 50°C OTS 6SLQFRDWLQJ�FRQGLWLRQV��

IRU�WROXHQH��V��USP��IRU�7+)��V��USP�h �RQO\�RQH�WUDQVLVWRU��RWKHUV�� i j ��

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SHQWDFHQHV��, kl �9 m�k �PHDVXUHPHQWV

A remarkable observation was made for pentacene ��M. The combination of the thiophene used in pentacene ��G and the 4-methoxyphenyl group of pentacene ��D� improved the performance of the transistor significantly. We also have to stress that these are


160

preliminary results and that one must be careful in excluding certain derivatives when at first sight they are not working. From our own results (chapter 3.1) and a recent publication29 we know that despite the fact that pentacene ��G showed no transistor characteristics when deposited from solution, it is possible to make working transistors with this material even with mobilities as high as 0.1 cm2/Vs by evaporation.29

��7UDQVLVWRUV�FRQVLVWLQJ�RI�WHWUDVXEVWLWXWHG�SHQWDFHQHV��

The same parameters as for the disubstituted pentacenes �� were investigated because it was noticed that although the two types of materials �� and �� possess identical substituents, the difference in number could give the molecules a quite different behaviour. (chapter 3.1) Therefore, the optimal conditions found for the disubstituted analogues �� could not be transferred as such to the tetrasubstituted derivatives ��. The influence of the number of substituents will be discussed in the next point, where we will try to compare the results for the di- and tetrasubstituted pentacenes.

Ar

Ar

Ar

Ar ��D�F�IO(CH2)6CH3D�=

(CH2)7CH3F�=

G�= C(CH3)3

S

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H�=

I�=

)LJXUH�,,,��2YHUYLHZ�RI�WKH�WHWUDVXEVWLWXWHG�SHQWDFHQHV��

The same remark as for the study of the disubstituted pentacenes �� is true for the tetrasubstituted pentacenes ��. Also here, our research is limited and there is room for further improvements.


161

Similar to the disubstituted pentacenes, the semiconductor layer was applied on SiO2-substrates with Au-contacts in a bottom-configuration unless mentioned otherwise. The measurements were carried out for a gate voltage between 10 V and -10 V and a drain voltage of -10 V. The mobilities were calculated for a transistor structure with a width

of 5000 Pm and a length of 10 Pm, unless mentioned otherwise, in the

saturated regime and displayed as an average of the measurements from at least two different transistors on the substrate.

First, the solvent was studied and again we observed that it is crucial for obtaining a working transistor. Toluene worked for most pentacenes ��. Only for the thienylsubstituted pentacene ��H, no transistor characteristics could be observed. (Table �III.14) At first sight, the THF solution of ��D gave a better result however the value displayed in the table is a mean value. We observed a large difference in mobilities between the different transistors on the substrate. Some of them had much lower values than the ones obtained for the transistors made from the toluene solution.

7DEOH� �,,,�� 0RELOLWLHV� RI� VDPSOHV� SUHSDUHG� IURP� VROXWLRQV� RI�SHQWDFHQHV��IRU�VHPLFRQGXFWRU�OD\HU�GHSRVLWHG�IURP�VROXWLRQ�ZLWK�D�GLIIHUHQW�VROYHQW�

Pentacene ��D� ��F� ��G� ��H�toluene 7.8 10-6 7.3 10-4 9.5 10-7 / Mobility

cm2/Vs THF 1.1 10-5 / / /

Pentacene ��I� � �

toluene 1.5 10-9 Mobility cm2/Vs THF /

When the samples were examined under the microscope, it was noticed that for pentacene ��G the film from the toluene solution


162

was more continuous. This was also the case for pentacene ��D.�The film from the THF solution of pentacene ��F looked very strange and more rough than the one from the toluene solution. (Figure �III.21)

)LJXUH��,,,��0LFURVFRSH�SLFWXUH��[��6SLQFRDWHG�VROXWLRQ�RI�

SHQWDFHQH��F�ZLWK�SRO\VW\UHQH�ZLWK�D�GLIIHUHQW�VROYHQW�$��7+)��%�WROXHQH��

Subsequently, a closer look was taken at the concentration of the solution. The concentration of both the pentacene ��F and the additive was varied. From the result of the variation of the pentacene concentration with a constant additive concentration, it appeared that the optimal amount to dissolve should be found somewhere around 26 mg of pentacene ��F in 1 g of solvent. More experiments are necessary to define the optimum. (Table �III.15)

7DEOH� �,,,�� 0RELOLWLHV� RI� VDPSOHV� SUHSDUHG� IURP� VROXWLRQV� RI�VROXWLRQV��J�RI�VROYHQW��ZLWK�D�GLIIHUHQW�FRQFHQWUDWLRQ�RI�SHQWDFHQHV��F�DQG�WKH�VDPH�FRQFHQWUDWLRQ�RI�DGGLWLYH��PJ��

Mass 15 mg 26 mg 35 mg�Mobility cm2/Vs 1.6 10-7 7.3 10-4 3.3 10-5

Subsequently, a fixed pentacene concentration was used to make four solutions with different polystyrene concentrations. The mobility of the film made from the solution containing 13 mg of polystyrene was remarkable larger than the mobilities of the others. The optimum has

10 n�o


163

to be defined around this value. (Table �III.16/Figure �III.22) When much more polystyrene was added (26 mg), the mobility decreased but there was less hysteresis. (Figure �III.22) It is possible that the decrease of the mobility is due to the thicker layer of polystyrene that is formed on the contacts hampering the charge carrier injection to the pentacene ��F. This would mean that there is a segregration between polystyrene and the pentacene derivative in the deposited film.

7DEOH��,,,��0RELOLWLHV�RI�VDPSOHV�SUHSDUHG�IURP�VROXWLRQV��J�RI�VROYHQW��ZLWK�D�GLIIHUHQW�FRQFHQWUDWLRQ�RI�DGGLWLYH�DQG�WKH�VDPH�FRQFHQWUDWLRQ�RI�SHQWDFHQHV��PJ��

Mass PS 4 mg 7 mg� 13 mg 26 mg�Mobility cm2/Vs 9.6 10-5 0.7 10-5 7.3 10-4 8.4 10-5

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However, this theory that is suggested by a number of people in the field, has not been proven yet. The decrease of the mobility could also be explained by the large amount of polymer present through the entire film. When there is no segregation, the polystyrene is mixed between the small molecules. This could hinder the charge transport of the carriers from one substituted pentacene molecule to another.


164

After the study of the solution itself, the surface of the substrate was modified with an OTS layer. However, in all cases this was leading to a decrease of the mobility instead of an improvement (Table �III.17/Figure III.23).

7DEOH��,,,��'LIIHUHQFH�LQ�PRELOLWLHV�IRU�WUDQVLVWRUV�PDGH�IURP�WROXHQH�VROXWLRQV�RQ�HLWKHU�D�FOHDQHG�6L2 p �RU�DQ�276�VXUIDFH��

Surface ��D� ��F� ��G�OTS 1.8 10-6 4.1 10-7 / Mobility

cm2/Vs SiO2 7.8 10-6 1.4 10-3 9.5 10-7

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The reason is related to the film morphology which differs with the surface treatment. On an OTS surface, the molecules were less homogeneously spread on the surface. It also seemed that less material was present. (Figure �III.24) This could point to a smaller affinity of the solvent and/or molecules for the surface with as a result that more material was spun off the substrate.


165

A B

C D

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Also the spincoating conditions were varied to investigate their influence on the performance of the obtained transistors. For this purpose a toluene solution of pentacene ��F�was spincoated with different spin times. For toluene, it was found that 70 s or 40 s made no major difference in mobility while shorter times led to a decrease in the performance of the final transistors (Table �III.1/Figure �III.25).

7DEOH��,,,��0RELOLWLHV�RI�WUDQVLVWRUV�ZKLFK�KDG�D�VHPLFRQGXFWRU�OD\HU�PDGH�RI�D�WROXHQH�VROXWLRQ�RI�SHQWDFHQH��F�ZLWK�GLIIHUHQW�VSLQQLQJ�WLPHV�

Spin time 70 s 40 s 20 s


100 s�t

10 n�o


166

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Also the spin rate was tested for a toluene solution with a certain amount of additive (4 mg). The results were the same as for previous experiments with the disubstituted pentacenes ��. (Table �III.19) We can conclude that for this type of toluene solution, a spin rate of 2000 rpm is optimal.

7DEOH��,,,��0RELOLWLHV�RI�WUDQVLVWRUV�ZKLFK�KDG�D�VHPLFRQGXFWRU�OD\HU�PDGH�RI�D�WROXHQH�VROXWLRQ�RI�SHQWDFHQH��F�ZLWK�GLIIHUHQW�VSLQQLQJ�UDWHV�

Spin rate 1000 rpm 2000 rpm 3000 rpm


After the substituted pentacene solution is spincoated on the substrate, the sample has to dry further which can be achieved by different manners that can have an influence on the performance of the transistors. Therefore pentacene ��F was deposited from a toluene solution. Three identical samples were made and each of them was treated differently after spincoating. One sample was heated above the boiling point of toluene, one below and the last sample was further dried at room temperature in nitrogen atmosphere. When the


167

transistors were measured, it was noticed that the mobility upon drying at room temperature was almost one order of magnitude lower than upon drying by heating. (Table �III.20/Figure �III.26)

7DEOH��,,,��0RELOLWLHV�RI�WUDQVLVWRUV�ZKLFK�KDG�D�GLIIHUHQW�WUHDWPHQW�DIWHU�VSLQFRDWLQJ�DQG�WKHLU�WLPH�VWDELOLW\��

Treatment ��F� ��F�remeasured�120 °C 1.0 10-3 1.4 10-3

90 °C 1.7 10-3 3.4 10-4 Mobility cm2/Vs

RT, N2 4.5 10-4 2.4 10-5

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This difference in performance is due to the morphology of the film. The microscope picture of the film dried at room temperature showed a more heterogeneous surface while the film dried at 120 °C was smoother. (Figure �III.27) The film which was heated at 90 °C, looked like something between the two others although the mobility did not differ from the film which was heated at 120 °C.


168

A B

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However, when the samples were remeasured after one month, the sample which had been heated at 90 °C, was degraded while the one which had been heated above the boiling point of toluene, had not changed significantly. (Table �III.20/Figure �III.28)

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Both samples were stored under the same conditions under nitrogen atmosphere. The film dried at room temperature was less stable in time compared with the film which was heated at 120 °C. Therefore it

10 n�o


169

is maybe better to heat the samples at 120 °C after spinning. Although they may have a slightly lower mobility, the transistors will be more stable in time.

Finally we summarized the best result obtained for each tetrasubstituted pentacene that we prepared, in Table �III.21. (Figure III.29)

7DEOH� �,,,�� %HVW� UHVXOWV� DQG� FRQGLWLRQV� WLOO� QRZ� IRU� SHQWDFHQH�GHULYDWLYHV��

�� Mobility cm2/Vs Solvent��

Conc ��/PS

(1g solvent)

After treatment Surface

D 7.8 10-6 toluene 26/4 120 °C SiO2

F 1.4 10-3 toluene 26/4 120 °C SiO2

G 9.5 10-7 toluene 26/13 90 °C SiO2

H� / / / / /

I� 1.5 10-9 toluene 26/13 90 °C SiO2 6SLQFRDWLQJ�FRQGLWLRQV��

IRU�WROXHQH��V��USP��

Although we tried on the basis all previous observation to further optimize the results, this did not always lead to an improvement the obtained transistors. For example it could be expected that for pentacene ��D and ��F the mobility would improve slightly by adding more additive to the solution. However when 13 mg of polystyrene was used at the end of our work, the mobility decreased (��D: 1.4 10-5 cm2/Vs and ��F: 4.9 10-5 cm2/Vs). Possibly this was due to degradation of the material because there were a few months between the two experiments. Although the materials were stored under nitrogen, it could be that the reactive inner ring was not sufficiently protected by the sterical hindrance of the substituents on the neighbouring rings. The decrease was less pronounced when the


170

influence of the contact resistance was diminished by measuring transistors with a larger length (W/L: 1000/100; ��F: 3.0 10-3 cm2/Vs versus 6.3 10-3 cm2/Vs; see also p.172). So we observed an increase of the contact resistance for the sample made with the older material.

��

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When the different materials are compared to each other, it is noticed that the tetrasubstituted pentacenes �� which contain a long alkyl chain showed the best results. We observed a decrease of the mobility when a more electron rich substituent was used as in pentacene ��D compared with derivative ��F. It would be interesting to see if the mobility could be further improved when a more electron poor substituent could be introduced.

In Figure III.30 we separately displayed the curves of pentacene derivative ��F. For the use in circuits, transistors have to possess two important characteristics besides a good mobility. Preferably, the subthreshold slope must be as steep as possible and the transistor has to possess a threshold voltage close to zero. Here we see that with pentacene derivative ��F we can realize both requirements. The transistor switches on at almost zero volt and subsequently the current is increasing very fast. The square-root curve is showing a threshold


171

voltage close to zero. Because the onset voltage is close to zero and the difference with the threshold voltage is very small, we can assume that there are few electric active impurities in the film.

10pA

100pA

1nA

10nA

100nA

I DS [A

]

-10 -5 0 5 10VGS [V]

6

5

4

3

2

1

0

SQ

RT(ID

S ) [x10-4 A

1/2]

u =1,4 10-3 cm2/Vs Vt = 0,1 V Von = 0,2 V

Ion/Ioff = 1,8 105

hysteresis = - 0,49 V

)LJXUH��,,,��, kl �9 m�k �PHDVXUHPHQW�IRU��WUDQVLVWRU�RI�VXEVWLWXWHG�

SHQWDFHQH��F�

When we took a closer look at the shape of IDS-VDS curves of pentacene derivative ��F, we observed that the curves measured at different gate voltages did not show the ideal behaviour. (see introduction, Figure 8)

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

I DS

[nA]

-12 -8 -4 0VDS [V]

VG = 5 V v -15 V step = -2 V

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SHQWDFHQH��F�


172

The different curves appear to have an equal pathway at some point. It is clear that in this graph there is no quadratic increase of the source-drain current with decreasing gate voltage. (Figure III.31) These observations give an indication that there are problems with the charge carrier injection from the contacts to the material.

The influence of the contact resistance on the transistor characteristics can be reduced by measuring another transistor structure. Instead of

the structure with a W/L of 5000/10 Pm, one can measure a transistor

with a larger channel length L. When the length of the channel between the contacts is becoming larger, the contact resistance becomes less important compared with the resistance over the channel. To get an idea of the effect of the contact resistance we

measured a transistor structure with a W/L of 1000/100 Pm for

pentacene ��F and compared it with the structures we normally measure. The mobilities are displayed in Table �III.22 and indeed there are problems with the injection of charge carriers because the apparent mobility is increasing when the contact resistance is becoming less important.

7DEOH��,,,��0RELOLWLHV�RI�GLIIHUHQW�WUDQVLVWRU�VWUXFWXUHV�RQ�WKH�VDPH�VDPSOH�ZLWK�D�VHPLFRQGXFWRU�OD\HU�RI�SHQWDFHQH��F��

W/L 5000/10 Pm 1000/100 Pm

Mobility 1.4 10-3 cm2/Vs 6.3 10-3 cm2/Vs

To better visualise the increase obtained by measuring a transistor with a longer channel length, the ISD-VGS-curves of the two different transistor structures are displayed in Figure III.32. The measurements

were normalized (by multiple the IDS and �IDS with their

corresponding L/W) in order to be able to compare the curves. The normalized square-root current of the transistor with the longest channel length is clearly higher. When one extrapolated the mobility


173

for a length going to infinity, an estimation is made for the mobility without the influence of the contacts. In this case a mobility of 9.1 10-3 cm2/Vs could be calculated. The charge carrier injection could possibly be improved by modifying the metal-organic interface by a self-assembled monolayer42a.

10fA

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100pA

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I DS .

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[A

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4

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0

SQ

RT(ID

S . L/W) [x10

-5 A1/2]

W/L = 5000/10 W/L = 1000/100

)LJXUH��,,,��1RUPDOL]HG�, kl �9 m�k �FXUYHV�IRU�WUDQVLVWRUV�RI�VXEVWLWXWHG�SHQWDFHQH��F�RQ�WKH�VDPH�VDPSOH�EXW�ZLWK�GLIIHUHQW�OHQJWK�EHWZHHQ�

FRQWDFWV

��&RPSDULVRQ�EHWZHHQ�WUDQVLVWRUV�FRQVLVWLQJ�RI ��GLVXEVWLWXWHG��DQG�WHWUDVXEVWLWXWHG�SHQWDFHQHV��When we tried to make a comparison between the results obtained for transistors made of disubstituted pentacenes �� and tetrasubstituted pentacenes ��, we noticed a few differences and similarities between them.

The solvent was important for both types of materials. It was noticed that only for disubstituted pentacene ��D and tetrasubstituted pentacene ��D with both solvents, THF and toluene, working transistors were obtained. All others worked only in one or none of the solvents. We also observed that the thienyl substituted pentacenes ��G and ��H did not give rise to working transistors from solution.


174

The region were the optimal concentration situated for the pentacene derivatives ��F� in toluene was more or less the same. It was difficult to get an idea of the region of the optimal additive concentration that had to be added in both cases in order to get the best results.

More remarkable was the difference in the preferred surface. Here, the number of substituents is playing a role. While for the disubstituted pentacenes ��D�F an OTS surface gave the best results, the tetrasubstituted pentacenes ��D�F ‘preferred’ a cleaned SiO2 substrate.�Also for the treatment after spincoating the behaviour was opposite for the two different types of materials. We observed an increase of the performance for the disubstituted pentacenes �� when they were further dried without heating under nitrogen atmosphere. For the tetrasubstituted pentacenes �� a heat treatment led to better mobilities and, moreover, even to better stability of the transistors when the temperature was exceeding the boiling point of the solvent. When comparing the films of pentacene ��F without heating and compound ��F with heating, they behave more or less the same. (Figure III.33)

A B

)LJXUH�,,,�� 0LFURVFRSH�SLFWXUH��[��$��VSLQFRDWHG�VROXWLRQ�RI�SHQWDFHQH��F�GULHG�DW�7 x ��%��VSLQFRDWHG�VROXWLRQ�RI�SHQWDFHQH��F�

GULHG�DW��&�

10 y�z


175

To get an idea of the influence on the mobility of the number of substituents on the pentacene backbone, we can compare the results obtained for pentacenes ��F and ��F which contain exactly the same substituents. The best results for each pentacene showed that the introduction of four substituents was leading to an increase of the mobility by almost three orders of magnitude (10-6 cm2/Vs versus 10-3 cm2/Vs). However, it also seemed that the tetrasubstituted pentacenes were less stable when stored for a long time and were degrading more easly in solution making them also harder to purify. These results for pentacenes ��F and ��F are comparable with the results that have been published in the beginning of 2006 for the analogous pentacenes substituted with a phenyl group.29 Also here the mobility of the disubstituted compound was lower than its tetrasubstituted analogue. The group of Maio only used transistors with evaporated films whereas our samples were prepared from solution. Nevertheless, the mobility we measured for the solution processed pentacene ��F is in the same order of magnitude as the corresponding published evaporated tetraphenylpentacene and the value for pentacene ��F is only one order of magnitude lower than its corresponding phenyl substituted pentacene.29

Further work can be done to improve the results we made. Other solvents and surfaces can be investigated. The compounds could be further intensively purified and the film morphology could be studied more profoundly. Moreover this research could be interesting to further fine-tune the substituted pentacenes in order to better meet the requirements of the application in OTFT’ s.

Part III Conclusion

176

&RQFOXVLRQ�During the study of the different substituted pentacenes �� and �� in OTFT’ s, we experienced that it is not straightforward to process a high quality semiconductor film. The different substituents which are used, as well as the number of substituents on the pentacene backbone have an influence on the film formation. This is preventing the simple transfer of optimal conditions of one material to another. No general procedure can be deduced from the experiments. However, interesting observations were made and some trends were discovered.

Fabrication of the films by dropcasting failed because of the thickness of the obtained films. However, we observed that for disubstituted pentacenes, the type of substituent had an influence on the morphology of the film and on the type of crystals formed. Phenyl substituted pentacenes intented to form needle-like crystals while the crystals of thienyl substituted pentacenes had a more cubic shape. The presence of a long alkyl chain on the substituent gave rise to a more amorphous arrangement of the molecules. Furthermore, the deposition temperature could lead to a different morphology of dithienyl-pentacene ��G from crystalline to amorphous or from three-dimensional to two-dimensional growth.

The deposition technique is playing an important role in the quality of the final film. We observed that pentacene ��G did not give rise to working transistors when spincoated from a solution. However when this compound ��G was deposited via vacuum deposition, the transistor did work.

By investigating the possibility of making solution processed transistors via spincoating of the disubstituted �� and tetra-substituted pentacenes ��, we observed that the solvent is a crucial factor to obtain a working transistor. Unfortunately no general trend could be found.

Part III Conclusion

177

From our experiments, it appeared that the spincoating process was reproducible. However, the solutions of the substituted pentacenes could not be stored in nitrogen atmosphere because of degradation of the materials in solution.

The mobility of the transistors increased when the amount of material was raised. However, we must take into account the limit due to the solubility in the solvent. Also the amount of additive had a small influence on the performance of the transistors. However, it was difficult to find a general optimum.

The surface (SiO2 or OTS) that was used to deposit the pentacene derivatives had an influence on the mobility. In some cases the difference in mobility was even more than one order of magnitude. For disubstituted pentacenes ��, there was no general trend but most of them ‘preferred’ an OTS surface. Silicon dioxide was in all cases the best surface for the tetrasubstituted pentacenes ��.

The treatment of the samples after spincoating also had an influence on the performance of the transistor. Here, the optimal treatment was opposite for both types of materials. The disubstituted pentacenes gave better results when the sample was further dried at room temperature while the tetrasubstituted derivatives ‘preferred’ heating above the boiling point of the solvent. Furthermore, this also increased the stability of the transistors in time.

The number of substituents had an influence on the performance of the final transistors. In agreement with the results of Maio HW�DO�29, we observed a higher mobility for the tetrasubstituted analogue. The mobility measured for the solution processed pentacene ��F was the same as for the evaporated 5,7,12,14-tetraphenylpentacene.29 Pentacene ��F did also have a steep subthreshold slope and an onset and threshold voltage close to zero. These characteristics make it a possible candidate for further research and application in circuits.

Conclusion

179

*HQHUDO�FRQFOXVLRQ�

In the first part of this thesis we explored the possibility of solving the problems concerning pentacene by the synthesis of soluble pentacene precursors via Diels-Alder reactions between pentacene and a dienophile containing a hetero-atom. Due to the large amount of pentacene needed for our investigation, we first searched for a new, environmentally friendlier method to reduce pentacenequinone to pentacene. We succeeded in the reduction via lithium aluminium hydride which gave us the pentacene in a reproducible way in moderate yields. Subsequently we managed to synthezise an adduct of pentacene and thiophosgene. This pentacene precursor was more soluble and could be easily purified. We were able to reconvert the adduct back to pentacene by thermolysis at temperatures lower than 200 °C. The expelled compound is volatile which make it easy to remove. Unfortunately the product could not be spincoated to a continuous thin-film due to three-dimensional growth.

In the second part of this thesis, the synthesis of new substituted pentacenes is described. Different types and numbers of substituents were used and we tried to introduce the substituents on different positions on the pentacene backbone. We did not find a proper way to prepare 6-aryl substituted pentacenes due to their facile oxidative degradation which will also hamper their incorporation in OTFT’ s by solution process techniques. The 2,3,9,10-tetrasubstituted pentacene-5,7- and 5,12-ones were successfully obtained, unfortunately the addition of the aryllithium compounds on the carbonyl functions failed. 6,13-Diaryl substituted pentacenes were obtained with success and different types of substituents could be introduced. We observed

Conclusion

180

rearrangement of the intermediate diols induced by acids which was preventing the formation of the pentacene derivative in some cases. The tendency to rearrange was depending on the substituents and increased when the substituents were more electron rich. We were also able to prepare an asymmetrically substituted pentacene. The number of substituents on the pentacene backbone was varied by the preparation of 5,7,12,14-tetra-aryl substituted pentacenes. Also here, different substituents were introduced. With the synthesis of the different 6,13-diaryl and 5,7,12,14-tetra-aryl substituted pentacene, we managed to obtain a variation in the type and in the number of substituents on the pentacene backbone. These pentacene derivatives were further investigated in order to get an idea of their electronic properties for the use in OTFT’ s.

Finally, the synthezised new substituted pentacenes were integrated in a transistor structure by spincoating in order to test the semi-conducting properties of the obtained thin-films. In the first place, we tried different deposition techniques in order to obtain continuous thin-films from solution. During these experiments, we observed that the film morphology depends on the type of substituent and on the process temperature. Moreover, it was noticed that the deposition technique can be important to obtain working transistors. The spincoating experiments with the addition of polystyrene to the solution, revealed the importance of the use of the correct solvent. The amount of pentacene derivative and additive had a small influence on the performance of the final transistor. Also the type of surface on which the substituted pentacene was applied, was important for the characteristics of the transistors. The samples of disubstituted pentacenes showed better results when they were further dried at room temperature after spincoating. On the other hand the tetrasubstituted pentacenes preferred a heat treatment. Their stability in time was

Conclusion

181

improving when they were heated above the boiling point of the solvent. We observed that the increase of the number of analogous substituents present on the pentacene backbone, gave rise to an increase in the mobility which is in agreement with the results of Maio HW�DO.29 The presence of a long alkyl chain on the substituent improved the solubility in solvents like toluene and increased the mobility of the transistors. The best mobility was obtained for 5,7,12,14-tetra(4-octylphenyl)pentacene (1,4 10-3 cm2/Vs). Our results correspond with the results obtained for evaporated 5,7,12,14-tetraphenylpentacene (10-3 cm2/Vs). The measured mobility was underestimated due to contact resistance and can even increase to 10-2 cm2/Vs when the resistance would be rulled out. Moreover the transistor possessed a steep subthreshold slope and an onset and threshold voltage close to zero.

We can conclude that we managed to synthezise a pentacene precursor and aryl substituted pentacenes which vary in type and number of substituents. For a few of the prepared substituted pentacenes, we could measure semiconducting properties of transistors prepared with them. We can conclude from these measurements that the increase of two to four substituents caused an increase in the mobility and that the presence of an alkyl chain on the substituent made it possible to obtain solution processed transistors which were comparable to an evaporated equivalent29. The transistor characteristics of 5,7,12,14-tetra-(4-octylphenyl)pentacene are promising for further research and potential use in circuits.

Experimental Part

183

([SHULPHQWDO�3DUW�

(TXLSPHQW�DQG�PDWHULDOV�{ �0HOWLQJ�SRLQW��Reichert-Thermovar of Electrothermal 9200. The values are not corrected.

{ �0DVV�VSHFWUD��Kratos MS50TC (ionisation energy 70 eV) coupled to a Kratos Mach 3 processing system and Hewlett-Packard MS-engine 5989A (chemical ionisation-CI- and electron impact-EI) coupled to a HP Apollo 900 Series 400 processing system. High resolution electron-impact-spectra were executed with a resolution of 10000. Micromass Quatro II device (electronspray ionisation –ESI-, solvent mixture: CH2Cl2/Methanol + NH4OAc).

{ � | +�105��Bruker Avance 300 (working at 300 MHz) and Bruker AMX 400 (working at 400 MHz) with tetramethylsilane (TMS) as an internal standard. The chemical shift is expressed in ppm.

{ � |r} &�105��Bruker Avance 300 (working at 75 MHz) and Bruker AMX 400 (working at 100 MHz) with deuterated solvent as an internal standard. The chemical shift is expressed in ppm.

{ 89�9,6�VSHFWUD�Perkin-Elmer Lambda 20 spectrometer

{ ,5�VSHFWUD�Perkin-Elmer 1600 infrared fourier transform spectrometer. Materials were measured as a KBr-pill.

Experimental Part

184

{ &KURPDWRJUDSK\�TLC: Alugram Sil G/UV245 (Macherey-Navel) Column: Silica gel MN-Kiezelgel 60 (70-23 Mesh) (Macherey-Navel and Fluka)

{ 2SWLFDO�PLFURVFRSH�Olympus AX 70 (Provis) with camera Moticom 480. The image software was Motic Image 2000;

{ 3DUDPHWHU�DQDO\VHU�Agilent 4156C with probe table Karl Suss situated in a nitrogen glove box.

{ %DE\�V\VWHP� IRU� 20%'� DQG� 276�GHSRVLWLRQWRRO�� 89�R]RQHWRRO�Internal assembled equiment

{ 6SLQFRDWHQ�Ultra-sonic bath: Manual Branson 2510 Photo-resist spinner of Headway Research Garland Texas Heating plate: PMC dataplate multicontroller 740 Substrate: Highly doped Silicon wafers on aluminium with 100nm thermally growth SiO2 on top. Au-contacts were sputtered for bottom configuration transistors and evaporated for top configuration transistors.

{ 6ROYHQWV�Solvents were purchased from Adrich and Acros Organics. THF, TMEDA and dietyl ether was further dried over Na or moleculair sieve when neccesary. The solvent used for filmpreparation were 99,8% pure and anhydrous.

{ 5HDJHQWV�Reagents were purchased from Aldrich, Acros Organics, Avocado and other specialised companies and used without further purification or prepared by methods described here. BuLi was used as a 2 M solution in hexane.

Experimental Part Part I

185

3DUW�,��3HQWDFHQH�3UHFXUVRUV�

��3URFHGXUHV�RI�FRPSRXQGV�RI�FKDSWHU��6\QWKHVLV�RI�SHQWDFHQHTXLQRQH��A solution of 1,4-cyclohexanedione (11 g, 94 mmol) in ethanol (300 ml) was added in a dropwise manner to a solution of R-phthal-aldehyde (25 g, 0,19 mol) and KOH (5,0g, 89 mmol in 20 ml water) in ethanol (350 ml). The reaction mixture was stirred for 5 h at room temperature. The pentacenequinone was filtered, washed with ethanol and dried in vacuum. The yield of the reaction was 91%.

O

O

Melting point: >300 °C; 1H NMR (300 MHz, CDCl3) 7.9 (m 4H), 8.0 (m 4H), 9.0 (s 4H); 13C NMR (75 MHz, CDCl3�� 127.1, 128.6, 129.4, 129.9, 130.3, 131.9, 135.6, 185.4; Mass Spectrum (MH+): m/z 309.

6\QWKHVLV�RI�SHQWDFHQH��LiAlH4 (0,98 g, 25 mmol) was added to a ice-cooled suspension of pentacenequinone (2.0 g, 6.5 mmol) in dry THF (100 ml) under argon atmosphere. The suspension was heated at reflux for 30 min. The mixture was cooled to room temperature and HCl (6 M, 60 ml) was added while cooling with ice. The mixture was then heated at reflux for another 3h. The residue was filtered, washed with water (2 x 30 ml), dichloromethane (2 x 30 ml), methanol (2 x 30 ml) and diethyl ether (2 x 30 ml) and after drying again treated with LiAlH4 (0.98 g, 25 mmol). The same procedure was repeated. The pure pentacene was filtered and washed with water (2 x 30 ml), dichloromethane (2 x 30 ml), Methanol (2 x 30 ml) and diethyl ether (2 x 30 ml). After drying in vacuum, pentacene was obtained in 54 % yield.


186

Melting point: > 300 °C (dec.); Mass Spectrum (MH+): m/z 279. The spectral characteristics (UV/Vis) are in agreement with the literature.48,49

6\QWKHVLV�RI��GLK\GURSHQWDFHQH��GLRO��To a suspension of pentacenequinone (2.0 g, 6.5 mmol) in dry THF (100 ml) was added NaBH4 (1.06 g, 29 mmol). After heating the mixture at reflux for 12 h, acidifying with acetic acid while cooling with ice, extraction with dichloromethane (200 ml), drying over MgSO4 and evaporation in vacuum, the product was isolated by column chromatography of the residue on silica gel using CH2Cl2:ethyl acetate (95:5). The diol was obtained in 51% yield as a mixture of the FLV- and WUDQV-isomers (35:65).

OHHO

Melting point: 171 °C ; 1H NMR (300 MHz, CDCl3�� WUDQV-isomer 5.8 (d 1H), 6.6 (d 1H), �� P� �+�� P� �+�� V� �+�� FLV-isomer 6.0, 6.1 (d,d 2H) 7.5 (m 4H), 7.9 (m 4H), 8.1 (s 4H); 13C NMR (75 MHz, CDCl3�� 0DVV�Spectrum (MH+): m/z 313.

6\QWKHVLV�RI��GLK\GURSHQWDFHQ��RQH��NaBH4 (0.5 g, 14.5 mmol) was added to an ice-cooled suspension of pentacenequinone (1.0 g, 3.3 mmol) in dry THF (50 ml) under an argon atmosphere. The suspension was heated at reflux for 12 h. The mixture was cooled to room temperature and HCl (6 M, 30 ml) was added under cooling with ice. The mixture was then heated at reflux for another 3 h. The residue was filtered, washed with water (2 x 30 ml) and dichloromethane (2 x 50 ml). The filtrate was taken aside and the two layers were separated. The organic layer was washed with water (3 x 50 ml), dried over MgSO4 and evaporated in vacuum. After


187

column chromatography on silica gel using petroleum ether:CH2Cl2: ethyl acetate (55:40:5), 6,13-dihydropentacen-6-one was isolated in 11% yield.

O

Melting point: 274 °C48; 1H NMR (300 MHz, CDCl3�� V��+��7.5 (t 2H), 7.6 (t 2H), 7.9 (d 2H), 8.0 (s 2H), 8.1 (d 2H), 9.1 (s 2H); 13C NMR (75 MHz, CDCl3�� 129.9, 130.3, 131.9, 135.6, 185.4; Mass Spectrum (MH+): m/z 295.

6\QWKHVLV�RI�DGGXFW��D�A suspension of pentacene (0.46 g, 1.6 mmol) in thiophosgene (2 ml) was heated at 65 °C for 6 h. After cooling to room temperature, dichloromethane (2 ml) was added to the reaction mixture and the unreacted pentacene was removed by filtration. The filtrate was evaporated. Then toluene (2 x 40 ml) was added and again evaporated in order to remove all thiophosgene. The product was purified by column chromatography on silica gel using CH2Cl2:petroleum ether (5:5). The white adduct was obtained in 40% yield.

S

O

Melting point: / decomposition starting at 100 °C; 1H NMR (300 MHz, CDCl3�� V��+��V��+��P��+��P��+��(2xs 2x2H); 13C NMR (75 MHz, CDCl3�� 127.8, 127.9, 132.3, 132.6, 134.4, 137.7; Mass Spectrum (MH+): m/z 339.

6\QWKHVLV�RI�1�EHQ]\ODFHWDPLGH��To a solution of n-benzylamine (0.33 ml, 3.0 mmol) and triethylamine (0.44 ml, 3.2 mmol) in dichloromethane (25 ml) acetyl chloride (0.21 ml, 3.0 mmol) was added in a dropwise manner at 0 °C. The product could be filtered and was washed with a HCl-solution (1M, 3 x 25


188

ml). The product ��was obtained with a yield of 72%. The product was directly used in the next reaction.

HN Me

O

1H NMR (300 MHz, CDCl3�� V��+��G��+��V��+��(m 5H); 13C NMR (75 MHz, CDCl3�� 128.8, 138.6, 170.6; Mass Spectrum (MH+): m/z 150.

6\QWKHVLV�RI�1�EHQ]\O�1�DFHW\OWKLRIRUPDPLGH��To a solution of ethylformate (0.10 ml, 1.2 mmol) in tetrahydrofuran (15 ml) Lawessons reagent (0.97 g, 2.4 ml) was added. After stirring for 1.5 h, the surplus of Lawessons reagent was filtered off. To the filtrate the amide �� (0.18 g, 1.2 mmol) was added. Subsequently triethylamine (0.4 ml, 3.8 mmol) was added to the reaction mixture. After stirring for 1.5 h ethyl acetate was added and the reaction mixture was washed with water (3 x 20 ml), dried over MgSO4 and evaporated in vacuum. After column chromatography on silica gel using CH2Cl2:ethyl acetate (9:1) product �� was isolated in 18% yield.

N

Me

O

HS

1H NMR (300 MHz, CDCl3�� V��+��G��+�� 7.3 (m 5H), 7.9 (s 1H); 13C NMR (75 MHz, CDCl3�� 136.6, 171.6, 201.2; Mass Spectrum (MH+): m/z 194.

Experimental Part Part II

189

3DUW�,,�6\QWKHVLV�RI�VXEVWLWXWHG�SHQWDFHQHV�

��3URFHGXUHV�IRU�FRPSRXQGV�RI�FKDSWHU��6\QWKHVLV�RI��GLK\GURSHQWDFHQ��RQH��The diol �� (0.30 g, 0.94 mmol) was heated together with HCl (10 ml, 1M) at reflux in tetrahydrofuran (50 ml) during 2 h. After cooling dichloromethane (100 ml) was added and the reaction mixture was washed with water (3x150 ml). The organic phase was dried over MgSO4 and evaporated in vacuum. After column chromatography on silica gel using CH2Cl2:petroleum ether (7:3) product �� was isolated in 25% yield.

O

Melting point: 274 °C48; 1H NMR (300 MHz, CDCl3�� V��+��7.5 (t 2H), 7.6 (t 2H), 7.9 (d 2H), 8.0 (s 2H), 8.1 (d 2H), 9.1 (s 2H); 13C NMR (75 MHz, CDCl3�� 126.7, 127.1, 128.6, 129.4, 129.9, 130.3, 131.9, 135.6, 185.4; Mass Spectrum (MH+): m/z 295.

6\QWKHVLV�RI��WKLHQ��\OSHQWDFHQH��D�Dihydropentacene ��D (0.09 g, 0.25 mmol) was heated at reflux for 3 h with DDQ (0.064 g, 0.28 mmol) in dioxane (20 ml). After cooling and evaporating in vacuum, the product was purified by column chromatography on silica gel using dichloromethane. The yield of the impure product was 31%.

S

1H NMR (300 MHz, CDCl3�� P�1H) 6.8 (m 1H) 7.3 (m 1H) 7.5 (m 4H) 7.9 (d+s 2+2H) 8.0 (d 2H) 8.2 (s 1H) 8.4 (s 2H); Mass Spectrum (MH+): m/z 361.


190

6\QWKHVLV�RI�SHQWDFHQHTXLQRQH��See part I chapter 1.1.

O

O

6\QWKHVLV�RI��GLK\GURSHQWDFHQH��GLRO��See part I chapter 1.1.

OHHO

6\QWKHVLV�RI��K\GUR[\��¶�WKLHQ��\OSHQWDFHQ��RQH��D�n-BuLi (2.0 ml, 4.9 mmol) was added to a solution of thiophene (0.39 ml, 4.9 mmol) in THF (20 ml) at -78 °C under argon atmosphere. After 10 minutes, this mixture was added in a dropwise manner to a suspension of pentacenequinone (2 g, 6.5 mmol) in THF (120 ml) which was cooled at -78 °C. After complete addition, the mixture was allowed to warm up to room temperature and stirred for 12 h. The reaction was worked up with HCl (1 M, 25 ml). The precipitate was filtered off, washed with water (3 x 30 ml) and methanol (2 x 20ml) and dried in vacuum. The filtrate was extracted with dichloromethane (75 ml), washed with water (3 x 50 ml), dried over MgSO4 and evaporated in vacuum. After column chromatography on silica gel using petroleum ether:dichloromethane (3:7) of both fractions, the title compound was obtained in 39% yield.

O

HO S

Melting point: 248 °C; 1H NMR (300 MHz, CDCl3�� V��+��-6.4 (d 1H) 6.7 (t 1H) 7.1 (d 1H) 7.5-7.6 (m 4H) 7.9 (d 2H) 8.0 (d 2H) 8.5 (s 2H) 8.8 (s 2H); Mass Spectrum (MH+): m/z 393.


191

6\QWKHVLV�RI��K\GUR[\��¶��PHWKR[\SKHQ\O�SHQWDFHQ��RQH��E�Bromoanisole (2.8 ml, 22 mmol) in dry THF (10 ml) was added in a dropwise manner over a period of 30 min to a suspension of Mg (0.56 g, 23 mmol) in dry THF (10 ml) in the presence of a catalytic amount of iodine and bromoanisole. This mixture was then added in a dropwise manner to a suspension of pentacenequinone (6.0 g, 20 mmol) in THF (120 ml). The reaction mixture was stirred for 12 h and acidified with HCl (1 M). The mixture was extracted with dichloromethane, washed with water and dried over MgSO4. After evaporation in vacuum, the crude product was purified by column chromatography on silica gel using petroleum ether:dichloromethane (4:6). The desired pentacenequinone ��E was obtained in 8% yield.

O

HO

OMe

Melting point: 289°-291 °C; 1H NMR (300 MHz, CDCl3�� V��1H) 3.7 (s, 3H) 6.7 (d, 2H) 7.2 (d, 2H) 7.5-7.6 (m, 4H) 7.9 (d, 2H) 8.0 (d, 2H) 8.3 (s, 2H) 8.9 (s, 2H); 13C NMR (75 MHz, CDCl3�� 113.7, 126.5, 126.9, 128.1, 128.8, 129.0, 129.2, 129.8, 132.3, 136.0, 151.8, 155.6; Mass spectrum (CI) (MH+): m/z 417.

6\QWKHVLV�RI��K\GUR[\��¶��FKORURSKHQ\O�SHQWDFHQ��RQH��F�The procedure was the same as for ��D except purified by column chromatography on silica gel using petroleum ether:dichloromethane (2:8). Yield: 8%.

O

HO

Cl


192

1H NMR (300 MHz, CDCl3�� V��+��V��+��V��+��(d, 5H) 7.8 (d, 2H) 8.0 (m, 2H) 8.2 (2H) 8.9 (m, 2H); Mass spectrum (CI) (MH+): m/z 421 (no melting point and 13C NMR because there was still a little pentacenequinone left).

6\QWKHVLV�RI��GLK\GUR��¶�WKLHQ��\OSHQWDFHQH��D�A suspension of ketone ��D (0.1 g, 0.25 mmol), ZnI2 (0.24 g, 0.75 mmol) and NaBH3CN (0.16 g, 2.5 mmol) in 1,2-dichloroethane (40 ml) was heated at reflux for 12 h. The salts were filtered off and the filtrate was acidified with HCl (3 M, 20 ml). After extraction with dichloromethane (20 ml) and washing with water (3 x 50 ml), the organic layer was dried over MgSO4 and evaporated in vacuum. The product was purified by column chromatography on silica gel using dichloromethane:petroleum ether (5:5). The title compound was obtained as a white solid in 92% yield.

H H

HS

Melting point: 228-230 °C; 1H NMR (300 MHz, CDCl3�� V��+��5.8 (s 1H) 6.5 (d 1H) 6.8 (t 1H) 7.1 (d 1H) 7.5 (4H) 7.8 (s 2H) 7.8-7.9 (m 4H) 8.0 (s 2H) ;13C NMR (75 MHz, CDCl3�� 125.1 125.5 125.9 126.0 126.2 126.6 127.3 127.6 129.5 132.8 135.5 137.8; Mass Spectrum (MH+): m/z 363.

The same reaction in toluene gave a lower yield (44%) and 6-hydroxy-13-thienylpentacene (12%) as a side-product.

6\QWKHVLV�RI��GLK\GUR��¶��PHWKR[\SKHQ\O�SHQWDFHQH��E�The procedure was the same as for product ��D. Yield: 51%.

H H

H

MeO


193

Melting point : 155 °C; 1H NMR (300 MHz, CDCl3�� V��+��(s, 2H) 5.5 (s, 1H) 6.7 (d, 2H) 7.0 (d, 2H) 7.4 (m, 4H) 7.8 (m, 8H) ; 13C NMR (75 MHz, CDCl3�� 127.6, 129.7, 132.9, 133.0, 134.1, 136.3, 139.1, 158.5 ; Mass spectrum (CI) (MH+): m/z 387.

6\QWKHVLV�RI��GLK\GUR��¶��FKORURSKHQ\O�SHQWDFHQH��F�The procedure was the same as for product ��D. Yield: 50%.

H H

H

Cl

Melting point : 174 °C; 1H NMR (300 MHz, CDCl3�� V��+��(s, 1H) 5.5 (s, 1H) 7.0 (d, 2H) 7.1 (d, 2H) 7.4 (m, 4H) 7.8 (m, 8H) ; 13C NMR (75 MHz, CDCl3�� 126.8, 127.4, 127.7, 128.5, 129.7, 132.5, 132.8, 135.8, 136.0, 138.0, 140.4; Mass spectrum (CI) (MH+): m/z 391.

6\QWKHVLV�RI��K\GUR[\��WKLHQ��\OSHQWDFHQH��D�Ketone ��D (0.20 g, 0.50 mmol) was heated at reflux for 12 h in toluene (30 ml) with ZnI2 (0.20 g, 0.63 mmol) and NaBH3CN (0.32 g, 5.1 mmol). The salts were filtered and the filtrate was acidified with HCl (3 M, 15 ml). After extraction with dichloromethane (30 ml) and washing with water (3 x 30 ml), the product was dried on MgSO4 and evaporated in vacuum. The crude product was purified by column chromatography on silica gel using dichloromethane:petroleum ether (50:50). 6-Hydroxy-13-thienylpentacene was obtained in 79% yield.

OH

S


194

1H NMR (300 MHz, CDCl3�� V��+��G��+��W��+��G�1H) 7.4-7.5 (m 4H) 7.8 (d 2H) 7.9 (s 2H) 8.0 (d 2H), 8.8 (s 2H); Mass Spectrum (MH+): m/z 377.


195

��3URFHGXUHV�IRU�FRPSRXQGV�RI�FKDSWHU��6\QWKHVLV�RI�SRO\GLVXOSKLGHSHQWDFHQH��Pentacenequinone �� (1.0 g, 3.2 mmol) was heated at reflux in chlorobenzene under argon atmosphere. To this boiling solution Lawessons reagent (1.2 g, 3.9 mmol) was added through a solid dispenser. After 3 h of heating at reflux, the mixture was cooled down and the precipitate was filtered and washed with chlorobenzene and dichloromethane. The residue was dried in vacuum. The polymer was obtained with a yield of 41%.

S

S �IR-spectrum: (KBr, cm-1) 1600; 1400; 751.

6\QWKHVLV�RI��GLK\GUR��ELV�WKLHQ��\O�SHQWDFHQH��GLRO��G�n-BuLi (4.0 ml, 9.8mmol) was added to a solution of thiophene (0.78 ml, 9.8 mmol) in THF (75 ml) at -78 °C under argon atmosphere. After 10 minutes pentacenequinone (1.0 g, 3.4 mmol) was added and the mixture was allowed to warm up to room temperature and stirred for 12 h. The reaction was worked up with 1M HCl (50 ml), extracted with dichloromethane (150 ml) and washed with water (3 x 50 ml). After drying over MgSO4 and evaporation in vacuum, the crude product was purified by column chromatography on silica gel using petroleum ether:dichloromethane (15:85). The product was isolated in 58% yield as a mixture of the FLV- and WUDQV-isomers (4:6).

HO

HOS

S


196

Melting point : 134 °C; 1H NMR (300 MHz, CDCl3�� WUDQV-isomer 3.3 (s 2H) 6.8 (d 2H) 7.0 (t 2H) 7.3 (d 2H) 7.5 (m 4H) 7.9 (m 4H) 8.0 �P��+��V��+��V��+�� FLV-isomer 3.1 (s 2H) 5.9 (d 2H) 6.3 (t 2H) 6.9 (d 2H) 7.6 (m 4H); 13C NMR (75 MHz, CDCl3�� 125.5 126.2 126.4 127.2 128.3 132.8 138.4 149.0; Mass Spectrum (CI) (MH+): m/z 477 (and 459: rearranged product).

6\QWKHVLV�RI��GLK\GUR��ELV��PHWKR[\SKHQ\O��SHQWDFHQH��GLRO��D�The procedure was the same as for product ��G except purification was done by washing the crude product with methanol (30 ml). The pure product was collected by filtration. Yield: 38%.

HO

HO

OMe

OMe

Melting point: 232 °C; 1H NMR (300 MHz, CDCl3�� V��+��(s 6H) 6.1 (d 4H) 6.5 (d 4H) 7.5 (m 4H) 7.9 (m 4H) 8.4 (s 4H); 13C NMR (75 MHz, CDCl3�� 132.7 134.7 139.6 158.2; Mass Spectrum (CI) (MH+): m/z 525.

6\QWKHVLV�RI��GLK\GUR[\��ELV��DFHW\OSKHQ\O��SHQWDFHQH��GLRO��E�The procedure was the same as for product ��G. The 4-bromo-acetophenone was protected with 1,2-ethanediol. Before purification the carbonyl function was deprotected by stirring the crude product for 2 h in a 2M HCl-solution. After extraction with ethyl acetate (150 ml), the organic phase was washed with water (3 x 100 ml), dried over MgSO4 and evaporated in vacuum. The crude product was boiled in methanol and subsequently the pure product was filtered with a yield of 23%.


197

HO

HO

COMe

COMe

Melting point: 229°-232 °C; 1H NMR (300 MHz, CDCl3�� V��+��2.5 (s 2H) 6.9 (d 4H) 7.3 (d 4H) 7.6 (m 4H) 8.0 (m 4H) 8.4 (s 4H); 13C NMR (75 MHz, CDCl3�� 135.7 139.0 147.4 197.8; Mass spectrum (CI) (MH+): m/z 549 (and 531: rearranged product).

6\QWKHVLV�RI��GLK\GUR[\��ELV��RFW\OSKHQ\O��SHQWDFHQH��GLRO��F�The procedure was the same as for product ��F except the product was purified by column chromatography on silica gel using dichloromethane. Yield: 42%.

HO

HO

(CH2)7CH3

(CH2)7CH3 �Melting point: 59 °C; 1H NMR (300 MHz, CDCl3�� 0.9 (t 6H) 1.2 (m 16H) 1.6 (m 4H) 2.3 (m 4H) 2.6 (t 4H) 2.9 (s 2H) 6.5 (d 4H) 6.6 (d 4H) 7.5 (m 4H) 8.0 (m 4H) 8.5 (s 4H); 13C NMR (75 MHz, CDCl3�� 14.2 22.8 29.4 29.6 31.6 32.0 35.5 76.2 125.0 126.3 127.3 127.7 128.3 132.8 139.6 140.0 141.6; Mass spectrum (CI) (MH+): m/z 675.

6\QWKHVLV�RI��GLK\GUR��¶��¶�ELV�EHQ]RWKLHQ��\O��SHQWDFHQH��GLRO��H�The procedure was the same as for product ��G except the purification by column chromatography on silica gel was carried out


198

with petroleum ether:dichloromethane (1:9). Yield: 87% (mixture of the FLV- and WUDQV-isomer 3:7).

HO

HOS

S

Melting point: WUDQV: 258°-260 °C FLV: 296°-300 °C; 1H NMR (300 MHz, CDCl3�� WUDQV-isomer 3.0 (s 2H) 7.1 (s 2H) 7.3 (m 4H) 7.5 (m 4H) 7.6-7.7 (m 2H) 7.7-7.8 (m 2H) 7.9 (m 4H) 8.3 (s 4H)�� FLV-isomer 3.2 (s 2H) 5.9 (s 1H) 6.6 (d 2H) 6.7 (t 2H) 6.9 (t 2H) 7.3 (d 2H) 7.6 (m 4H) 8.0-8.1 (m 4H) 8.6 (s 4H); 13C NMR (75 MHz, CDCl3):� � WUDQV-isomer 75.4 122.9 124.5 125.0 125.1 127.5 127.6 �� FLV-isomer 53.7 121.2 123.6 124.5 126.9 128.4 133.1 138.5 139.9; Mass Spectrum (CI) (MH+): m/z 559 rearranged product.

6\QWKHVLV�RI��GLK\GUR��¶��¶�ELV��DFHW\OWKLHQ��\O��SHQWDFHQH��GLRO��I�The procedure was the same as for ��G� The acetylthiophene was protected with 1,2-ethanediol (5 eq, 2 days). Before purification the carbonyl function was deprotected by stirring the crude product for 2 h in a 2M HCl-solution. The precipitate was filtered, washed with water (3 x 100 ml) and dried in vacuum. The crude product was boiled in methanol and subsequently the pure product was filtered with a yield of 45%. (mixture of the FLV- and WUDQV-isomer 4:6).

HO

HOS

S

COMe

COMe �Melting point: WUDQV: 233 °C FLV: 250 °C; 1H NMR (300 MHz, DMSO�� WUDQV-isomer 2.5 (s 6H) 6.8 (d 2H) 7.3 (s 2H) 7.5 (m 4H)


199

7.6 (d 2H) 7.9 (m 4H) 8.1��V��+�� FLV-isomer 2.2 (s 6H) 5.8 (d 2H) 7.1 (d 2H) 7.2 (s 2H) 7.6 (m 4H) 8.1 (m 4H) 8.6 (s 4H); 13C NMR (75 0+]�� '062�� WUDQV-isomer 26.3 73.0 126.8 127.2 127.8 128.1 �� FLV-isomer 26.0 71.7 124.5 126.7 127.7 128.1 131.9 132.4 138.9 143.2 159.4 190.5; Mass Spectrum (MH+): m/z 561.

6\QWKHVLV�RI��GLK\GUR��¶��¶�ELV��PHWKR[\WKLHQ��\O�SHQWDFHQH��GLRO��J�The procedure was the same as for product ��G except work-up was performed with NH4Cl and purification by column chromatography on silica gel was carried out with petroleum ether:dichloromethane (1:9) and 0.5% Et3N. Yield: 80% (mixture of the FLV- and WUDQV-isomer 2:8).

HO

HOS

S

OMe

OMe

Melting point: 85°-95 °C; 1H NMR (300 MHz, CDCl3�� V��+��6.0 (d 2H) 6.4 (d 2H) 7.5 (m 4H) 7.9 (m 4H) 8.2 (s 4H); 13C NMR (75 MHz, CDCl3�� 139.0 167.3; Mass Spectrum (CI) (MH+): m/z 519.2 (rearranged product) 559.2 (Na-adduct).

6\QWKHVLV�RI��GLK\GUR��¶��¶�ELV��¶�ELWKLHQ��\O�SHQWDFHQH��GLRO��K�The procedure was the same as for product ��G except work-up was performed with NH4Cl and purification by column chromatography on silica gel was carried out with petroleum ether:dichloromethane (1:9). Yield: 74% (mixture of the FLV- and WUDQV-isomer 3:7).


200

OH

HO S

S

S

S

Melting point: 138°-140 °C; 1H NMR (300 MHz, CDCl3�� WUDQV-isomer 2.9 (s 2H) 6.7 (d 2H) 6.9 (t 2H) 7.0 (d 2H) 7.1 (d 2H) 7.2 (d 2H) 7.5 (m 4H) 7.9 (m 4H) 8.3��V��+�� FLV-isomer 3.2 (s 2H) 5.7 (d 2H) 6.2 (d 2H) 6.7 (d 2H) 6.8 (t 2H) 7.0 (d 2H) 7.6 (m 4H) 7.9 (m 4H) 8.5 (s 4H); 13C NMR (75 MHz, CDCl3): � WUDQV-isomer 75.0 123.6 124.2 124.9 127.1 127.3 128.3.128.3 128.7 133.4 137.6 138.6 139.2 �� FLV-isomer 73.1 122.3 123.7 124.3 124.5 126.8 127.6 128.5 128.5 133.1 133.2 137.2 138.5 138.6; Mass Spectrum (MH+): m/z 623 (rearranged product).

6\QWKHVLV�RI��GLK\GUR��¶��¶�ELV��QRQ\O�WKLHQ��\O�SHQWDFHQH��GLRO��L�First 2-(2-nonyl)thiophene was prepared. To a suspension of Mg (1.3 g, 55 mmol) and a trace of iodine in dry diethyl ether (20 ml) 1-bromoheptane (6.5 ml, 41 mmol) was added in a dropwise manner and stirred for 10 min. Then the reaction mixture was added in a dropwise manner to a solution of 2-acetylthiophene (3.0 ml, 28 mmol) in dry diethyl ether (30 ml). After stirring for 12 h at room temperature the mixture was acidified with 1M HCl (30 ml). The reaction mixture was extracted with diethyl ether (50 ml), washed with water (3x50 ml), dried over MgSO4 and the solvent was evaporated in vacuum. Subsequently the alcohol dissolved in acetonitrile (15 ml) was added to a cooled (0 °C) solution of NaI (30 g, 0.21 mol) and trimethylsilyl chloride (26 ml, 0.21 mol) in acetonitrile (50 ml) which had stirred for 15 min at room temperature. The reaction mixture was allowed to warm up to room temperature and stirred for 15 min. Subsequently the mixture was cooled to 5 °C and a 20% NaOH-solution (50 ml) was added. The reaction mixture was allowed to warm up to room temperature. Ethyl acetate (50 ml) was added and the mixture was stirred for 10 min. Finally the mixture was extracted with ethyl acetate


201

(3x75 ml). The organic phase was washed with Na2S2O3.5H2O (2x 200ml), water (2x 200ml) and brine (2x 200ml), dried over MgSO4 and evaporated in vacuum. Column chromatography on silica gel was carried out with dichloromethane leading to the pure product in a yield of 39%. 1H NMR (300 MHz, CDCl3): 0.9 (t 3H) 1.3 (m 13H) 1.5 (t 2H) 1.6 (s 1H) 6.7 (d 1H) 6.9 (t 1H) 7.1 (d 1H); 13C NMR (75 MHz, CDCl3�� 14.0 22.6 23.1 27.3 29.5 29.6 31.8 35.3 39.3 122.1 122.3 126.3 152.3; Massa Spectrum (MH+): m/z 211.

The procedure was the same as for product ��G except work-up with NH4Cl and column chromatography on silica gel was carried out with petroleum ether:dichloromethane (5:5). Yield: 55% (mixture of the FLV- and WUDQV-isomer 3:7).

HO

HOS

S

CH(CH2)6CH3

CH(CH2)6CH3

H3C

CH3

Melting point: 74 °C; 1H NMR (300 MHz, CDCl3�� WUDQV-isomer 0.9 (t 6H) 1.2 (m 13H) 1.4 (t 4H) 1.9 (s 2H) 3.5 (s 2H) 6.0 (d 2H) 6.2 (d 2H) 7.6 (m 4H) 7.9 (m 4H) 8.6�V��+�� FLV-isomer 1.0 (t 6H) 1.2 (m 13H) 1.6 (t 4H) 1.9 (s 2H) 3.6 (s 2H) 7.5 (d 2H) 7.6 (m 4H) 7.8 (m 4H) 8.0 (d 2H) 8.2 (s 4H); 13C NMR (75 MHz, CDCl3�� WUDQV-isomer 14.9 23.4 24.4 29.9 30.3 30.4 32.6 35.0 45.1 127.3 128.0 128.6 128.7 128.9 129.5 130.2 130.5 132.6 135.7�� FLV-isomer 14.4 23.0 24.2 29.7 29.9 30.2 32.2 36.9 43.2 126.9 128.1 128.2 128.3 128.4 129.5 129.9 132.4 136.0; Mass Spectrum (MH+): m/z 711.9.

6\QWKHVLV�RI��GLK\GUR��WKLHQ��\O��PHWKR[\�SKHQ\O�SHQWDFHQH��GLRO��M�n-BuLi (0.60 ml, 1.4 mmol) was added to a solution of 4-bromo-anisole (0.27 g, 1.5 mmol) in THF (30 ml) under argon atmosphere at


202

-78 °C. After 10 minutes, ketone ��D (0.38 g, 0.97 mmol) was added. The mixture was allowed to warm up to room temperature and stirred for 12 h. The reaction mixture was worked up with HCl (1 M, 10 ml), was extracted with dichloromethane (50 ml), washed with water (3 x 50 ml), dried over MgSO4 and evaporated in vacuum. After purification by column chromatography on silica gel using petroleum ether:dichloromethane (2:8), diol ��M was used in the next reaction. Yield after column chromatography was 58%.

OH

HO

S

OMe 1H NMR (300 MHz, CDCl3�� WUDQV-isomer 3.6 (s 2H) 3.7 (s 3H) 5.8 (d 1H) 6.2 (t 1H) 6.3 (d 2H) 6.7 (d 2H) 6.9 (d 1H) 7.6 (m 4H) 8.0 (m 4H) 8.5 (s+s 2+2+�� FLV-isomer 3.7 (s 2H) 3.8 (s 3H) 5.6 (d 1H) 6.1 (m 3H) 6.5 (d 2H) 6.8 (d 1H) 7.5 (m 4H) 7.7 (m 4H) 8.4 (s+s 2+2H);Mass Spectrum (MH+): m/z 501.

6\QWKHVLV�RI��ELV��WKLHQ��\O�SHQWDFHQH��G�A suspension of diol ��G (2.0 g, 4.2 mmol), NaI (4.3 g, 29 mmol) and NaH2PO2 (4.8 g, 40 mmol) in acetic acid (100 ml) was heated at reflux for 1 h. The product ��G was isolated by filtration and washed with water (3 x 50 ml) and methanol (2 x 25 ml). After drying in vacuum, 6,13-di(thien-2-yl)pentacene was obtained in 87% yield. (same result by using 7eq of NaI and 3 h of heating at reflux).

S

S

Melting point: 300°-305 °C; 1H NMR (300 MHz, CDCl3�� P�4H) 7.4 (d 2H) 7.5 (t 2H) 7.8 (m 6H) 8.5 (s 4H); 13C NMR (75 MHz, CDCl3��


203

Mass Spectrum (CI) (MH+): m/z 443; UV-VSHFWUXP�� max ��QP�� 9.8 103 l/(mol.cm).

6\QWKHVLV�RI��ELV��PHWKR[\SKHQ\O�SHQWDFHQH��D�The procedure was the same as for product ��G. Yield: 91%.

OMe

OMe

Melting point: 303 °C; 1H NMR (300 MHz, CDCl3�� V��+��(m 4H) 7.3 (d 4H) 7.6 (d 4H) 7.8 (m 4H) 8.4 (s 4H); Mass Spectrum (MH+): m/z 491; UV-sSHFWUXP�� max ��QP�� 3 l/(mol.cm).

6\QWKHVLV�RI��ELV��DFHW\OSKHQ\O�SHQWDFHQH��E�The procedure was the same as for product ��G. Yield: 58%.

COMe

COMe

Melting point: > 300 °C; 1H NMR (300 MHz, CDCl3�� V��+��7.3 (m 4H) 7.7 (m 4H) 7.8 (d 4H) 8.2 (s 4H) 8.3 (d 4H); Mass Spectrum (MH+): m/z 515; UV-VSHFWUXP�� max �� QP�� 3 l/(mol.cm).

6\QWKHVLV�RI��ELV��RFW\OSKHQ\O�SHQWDFHQH��F�The procedure was the same as for product ��G. Yield: 58%.


204

(CH2)7CH3

(CH2)7CH3 �Melting point: 133 °C; 1H NMR (300 MHz, CDCl3�� 0.9 (t 6H) 1.4-1.5 (m 20H) 1.8 (m 4H) 2.8 (t 4H) 7.2 (m 4H) 7.5 (m 8H) 7.7 (m 4H) 8.3 (s 4H); 13C NMR (75 MHz, CDCl3�� 14.3 22.9 29.5 29.8 31.7 32.1 36.2 125.1 125.8 128.7 128.9 131.1 131.8 136.9 137.1 142.5; Mass spectrum (CI) (MH+): m/z 641; UV-VSHFWUXP�� max ��QP�� 12 103 l/(mol.cm).

6\QWKHVLV�RI��ELV�EHQ]RWKLHQ��\O�SHQWDFHQH��H�The procedure was the same as for product ��G. Yield: 59%.

S

S

Melting point: > 300 °C; 1H NMR (300 MHz, CDCl3�� P��+��7.6 (m 6H) 7.7 (m 2H) 7.9 (m 4H) 8.0 (m 4H) 8.6 (m 4H); Mass Spectrum (MH+): m/z 542; UV-VSHFWUXP�� max �� QP�� 3 l/(mol.cm).

6\QWKHVLV�RI��ELV��DFHW\OWKLHQ��\O�SHQWDFHQH��I�The procedure was the same as for product ��G. Yield: 86%.


205

S

S

MeOC

COMe �Melting point: > 300 °C; 1H NMR (300 MHz, CDCl3�� 2.8 (s 6H) 7.3 (m 4H) 7.4 (d 2H) 7.8 (m 4H) 8.0 (d 2H) 8.4 (s 4H); Mass Spectrum (MH+): m/z 527.2; UV-VSHFWUXP�� max ��QP�� 3 l/(mol.cm).

6\QWKHVLV�RI��WKLHQ��\O��PHWKR[\SKHQ\O�SHQWDFHQH��M�The procedure was the same as for product ��G. Yield: 36%.

S

OMe

Melting point: 272-274 °C; 1H NMR (300 MHz, CDCl3�� V��+��7.2,7.3 (m 6H) 7.4 (d 1H) 7.5 (t 1H) 7.55 (d 2H) 7.7,7.8 (m 5H) 8.3 (s 2H) 8.5 (s 2H); 13C NMR (75 MHz, CDCl3�� .4 125.8 127.0 127.4 128.2 128.5 128.9 130.0 130.2 132.7 138.3 159.3; Mass Spectrum (MH+): m/z 467; UV-VSHFWUXP�� max ��QP�� 103 l/(mol.cm).

6\QWKHVLV�RI��GLK\GUR��¶�ELV�WKLHQ��\O�SHQWDFHQ��RQH��G�A solution of diol ��G (0.26 g, 0.55 mmol) and BF3.OEt2 (0.10 ml, 8.2 mmol) in dichloromethane (20 ml) was stirred at room temperature for 12 h. After adding NaOH (1 M, 20 ml), the reaction mixture was extracted with dichloromethane. The organic layer was washed with water (3 x 100 ml), dried over MgSO4 and evaporated in


206

vacuum. The product ��G could be obtained after column chromatography on silica gel using dichloromethane, with a yield of 45%.

O

SS

Melting point: > 300 °C; 1H NMR (300 MHz, CDCl3�� G��+��6.9 (t 2H) 7.3 (d 2H) 7.6 (m 4H) 7.8 (s+d 4H) 8.1 (d 2H) 8.9 (s 2H) ; 13C NMR (75 MHz, CDCl3�� 128.4 128.8 128.9 129.7 130.0 132.2 135.2 143.6 152.0 184.8; Mass Spectrum (CI) (MH+): m/z 459.

6\QWKHVLV�RI��GLK\GUR��¶�ELV��PHWKR[\SKHQ\��SHQWDFHQ��RQH��D�The procedure was the same as for product ��G except column chromatography on silica gel was carried out with using dichloro-methane:petroleum ether (5:5). Yield: 1%.

O

OMeMeO 1H NMR (300 MHz, CDCl3)�� V��+��G�G��+��V�P�6H) 7.7 (d 2H) 8.0 (d 2H) 8.8 (s 2H); 13C NMR (75 MHz, CDCl3�� 55.2 113.2 126.7 128.1 128.2 128.5 129.0 129.1 129.3 129.5 131.5 131.6 131.7 134.9 138.6 144.9 158.1 186.0; Mass Spectrum (CI) (MH+): m/z 507.

6\QWKHVLV�RI��GLK\GUR��¶�ELV�EHQ]RWKLHQ��\O��SHQWDFHQ��RQH��H�The procedure was the same as for product ��G except column chromatography on silica gel was carried out with dichloro-methane:petroleum ether (5:5). Yield: 18%


207

O

SS

Melting point: > 300 °C; 1H NMR (300 MHz, CDCl3�� V��+��7.3 (m 4H) 7.6 (m 6H) 7.7 (d 2H) 7.8 (d 2H) 8.0 (s 2H) 8.1 (d 2H) 8.9 (s 2H) ; 13C NMR (75 MHz, CDCl3�� 120.5 121.4 122.3 122.8 123.5 123.6 125.8 128.3 131.4 133.2 134.3 142.7 171.7; Mass Spectrum (CI) (MH+): m/z 559.

6\QWKHVLV�RI��GLK\GUR��¶�ELV��DFHW\OWKLHQ��\O��SHQWDFHQ��RQH��I�Diol ��G (0.25 g, 0.52 mmol) was heated in nitromethane (10 ml) for 4 h at 50 °C together with acetic anhydride (0.20 ml, 2.1 mmol) in the presence of scandium triflate (50 mg, 0.10 mmol). Water (10 ml) was added and the reaction mixture was extracted with dichloromethane (3x20 ml). The organic layer was dried over MgSO4 and evaporated in vacuum. The product ��I could be obtained after column chromatography on silica gel using ethyl acetate:dichloro-methane (5:95), with a yield of 88%.

O

SS

MeOC

COMe

�Melting point: 154 °C; 1H NMR (300 MHz, CDCl3�� 2.5 (s 6H) 6.7 (d 2H) 7.5 (d 2H) 7.6 (t 4H) 7.6 (s 2H) 7.8 (d 2H) 8.0 (s 2H) 8.9 (s 2H); 13C NMR (75 MHz, DMSO�� 26.3 53.0 127.8 128.4 128.9 129.6 129.7 130.3 131.8 133.6 134.8 141.5 144.0 158.9 183.2 190.9; Mass Spectrum (ESI) (MH): m/z 543.2.


208

6\QWKHVLV�RI��GLK\GUR��¶�ELV��PHWKR[\WKLHQ��\O��SHQWDFHQ��RQH��J�The procedure was the same as for product ��G except purification by column chromatography on silica gel was carried out with petroleum ether:dichloromethane (1:9). Yield: 55%. The yield increased to 77% when the work-up was done with NH4Cl.

O

SS

MeO

OMe

Melting point: 117 °C-120 °C; 1H NMR (300 MHz, CDCl3�� V�6H) 5.9 (d 2H) 6.2 (d 2H) 7.5-7.6 (m 4H) 7.9 (d+s 4H) 8.0-8.1 (d 2H) 8.9 (s 2H); 13C NMR (75 MHz, CDCl3�� 128.5 128.8 129.8 130.2 132.3 135.3 136.8 143.0 167.2; Mass Spectrum (CI) (MH+): m/z 519.

6\QWKHVLV�RI��GLK\GUR��¶�ELV��¶�ELWKLHQ��\O��SHQWDFHQ��RQH��K�This compound was obtained when we tried to prepare the corresponding substituted pentacene. The procedure was the same as for product ��G except� the reaction mixture was extracted with dichloromethane, washed with water, dried over MgSO4 and the solvent was evaporated in vacuum. After purification by column chromatography on silica gel using dichloromethane the rearranged product ��K was obtained in 60% yield.

Or

The procedure was the same as for product ��G��Yield: starting for the FLV-isomer: 55%, starting for the WUDQV-isomer: 36%.


209

O

SS

S

S

Melting point: 138°-140 °C; 1H NMR (300 MHz, CDCl3�� G�2H) 6.9 (t 2H) 7.0 (d 2H) 7.0 (d 2H) 7.2 (d 2H) 7.6 (m 4H) 7.9 (d 2H) 7.9 (s 2H) 8.1 (d 2H) 8.9 (s 2H); 13C NMR (75 MHz, CDCl3�� 122.6 123.8 124.6 127.2 127.8 128.4 128.9 129.7 129.8 129.9 132.3 135.2 137.0 138.5 142.8 150.3 184.7; Mass Spectrum (MH+): m/z 623.

6\QWKHVLV�RI��GLK\GUR��¶�ELV��QRQ\O�WKLHQ��\O��SHQWDFHQ��RQH��L�This compound was obtained when we tried to prepare the corresponding substituted pentacene: procedure was the same as for product ��G except� the reaction mixture was extracted with dichloromethane, washed with water, dried over MgSO4 and the solvent was evaporated in vacuum. After purification by column chromatography on silica gel using dichloromethane the rearranged product ��L was obtained in 30% yield.

O

SS

CH(CH2)6CH3

H3C(H2C)6HC

Melting point: 78 °C; 1H NMR (300 MHz, CDCl3�� 0.9 (t 6H) 1.2 (m 10H) 1.3 (t 6H) 1.5 (t 4H) 1.9 (s 2H) 7.5 (m 6H) 7.8 (m 6H) 8.0(d 2H) 8.9 (s 2H); 13C NMR (75 MHz, CDCl3�� 14.9 23.4 30.0 30.2 30.4 30.5 32.6 36.3 42.0 122.7 129.1 129.3 130.2 130.5 130.9 134.0 135.9 136.0 176.5; Mass Spectrum (MH+): m/z 711.9.

�


210

6\QWKHVLV�RI��GLK\GUR��¶��¶�ELV��¶�ELWKLHQ��\O��SHQWDFHQH��The procedure was the same as for product �� except purification by column chromatography on silica gel was carried out with petroleum ether:dichloromethane (5:5). Yield: 73%.

H

H S

S

S

S

1H NMR (300 MHz, CDCl3�� 5.8 (s 2H) 5.9 (d 2H) 6.5 (d 2H) 6.8 (m 4H) 7.0 (d 2H) 7.5 (m 4H) 7.9 (m 4H) 8.0 (s 4H); 13C NMR (75 MHz, CDCl3�� 133.0 136.5 136.8 137.7 147.9; Mass Spectrum (MH+): m/z 609.

6\QWKHVLV�RI��GLK\GUR��¶�ELV��PHWKR[\WKLHQ��\O��¶�GLK\GURSHQWDFHQH��A solution of diol 2.12g (0.50 g, 0.90 mmol), ZnI2 (0.86 g, 2.7 mmol) and NaCNBH3 (0.57 g, 9.0 mmol) in 1,2-dichloroethane (50 ml), was heating at reflux for 12 h. After filtration of the salts, the filtrate was acidified, extracted with dichloromethane (50 ml) and washed with water (2 x 30 ml). After drying over MgSO4 and evaporation in vacuum, the crude product was purified by two successive column chromatographies on silica gel using dichloromethane and di-chloromethane:petroleum ether (5:5) respectively. The product was characterized to be 6,13-dihydro-6,6’ -bis-(5-methoxythien-2-yl)-13,13’ -dihydropentacene �� in a yield of 33%.

SS

MeO

OMe

Melting point: >300 °C; 1H NMR (300 MHz, CDCl3�� V��+�� (s 2H) 6.0 (d 2H) 6.2 (d 2H) 7.4-7.5 (m 6H) 7.8 (m 6H); 13C NMR (75


211

MHz, CDCl3�� 127.4 128.5 132.1 132.8 135.2 135.3 142.1 166.6; Mass Spectrum (CI) (MH+): m/z 505.


212

��3URFHGXUHV�IRU�FRPSRXQGV�RI�&KDSWHU��6\QWKHVLV�RI��SHQWDFHQHWHWURQH��To a solution of 2-methyl-1,4-naphtoquinone (7,8 g, 0.45 mol) in ethanol (600 ml) diethyl amine (12 ml, 0.11 mol) was added. The reaction mixture was stirred in the dark for 12 h with a CaCl2-tube. The pure tetrone �� was filtered and washed with ethanol. The residue was dried in vacuum. The yield of the reaction was 16%.68

O O

OO

Melting point: > 300 °C; 1H NMR (300 MHz, D2SO4�� P��+��8.4 (m 4H) 9.2 (s 2H); 13C NMR (75 MHz, D2SO4 in double tube with '062�� 06��&,��0+��P�]�339; IR-spectrum: 1673 cm-1.

6\QWKHVLV�RI��WHWUDK\GUR��WHWUD�WKLHQ��\O��SHQWDFHQH��¶��¶��¶��¶�WHWURO��H�n-BuLi (10 ml, 27 mmol) was added to a solution of thiophene (2,1 ml, 27 mmol) in dry THF (100 ml) at –78 °C under argon atmosphere. After 10 minutes pentacen-5,7,12,14-tetrone (1.5 g, 4.4 mmol) was added and the mixture was allowed to warm up to room temperature and stirred overnight. The reaction was worked up with 1M HCl and extracted with ethyl acetate. After drying over MgSO4 and evaporation in vacuum, the crude product was purified by column chromatography on silica gel using ethyl acetate:dichloromethane (1:9). The different isomers were separated. The product was isolated in a total yield of 97%.

HO HO

HO HO

S

SS

S


213

Melting point : 239 °C (WUDQV�FLV�WUDQV��WUDQV-isomer), 163 °C (FLV�WUDQV�WUDQV-isomer), > 300 °C (FLV�FLV�WUDQV��FLV-isomer); 1H NMR (300 MHz, CDCl3�� WUDQV�FLV�WUDQV��WUDQV-isomer 6.5 (d 4H) 6.5 (s 4H) 6.7 (t 4H) 7.2 (m 8H) 7.4 (m 4H) 7.7 (s 2H); (400 MHz, DMSO): �FLV�WUDQV-isomer thiophene 1: 5.63 (d 1H) 6.27 (t 1H) 6.95 (d 1H)

6.46 (s OH) thiophene 2: 5.71 (d 1H) 6.28 (t 1H) 7.00 (d 1H) 6.51 (s OH) thiophene 3: 6.67 (d 1H) 6.74 (t 1H) 7.21 (d 1H) 6.72 (s OH) thiophene 4: 6.90 (d 1H) 6.95 (t 1H) 7.34 (d 1H) 6.48 (s OH), 7.24 (m 1H) 7.27 (m 1H) 7.40 (m 2H) 7.41 (m 1H) 7.68 (d 1H) 7.93 (m 2H) 8.11 (s 1H) 8.30 (s 1H); (400 0+]�� '062�� FLV�FLV�WUDQV��FLV-isomer 5.83/5.85 (d 4H) 6.39/6.37 (t 4H) 6.67/6.69 (s 4H) 7.14/7.11 (d 4H) 7.48 (m 4H) 8.03 (m 4H) 8.66 (s 2H); 13C NMR (100 MHz, CDCl3): WUDQV�FLV�WUDQV��WUDQV-LVRPHU� ��128.9 129.8 130.3 134.4 140.5 141.1 151.8 (100 MHz,� '062�� FLV�WUDQV�WUDQV-isomer 70.4 70.6 71.7 72.4 123.4 124.0 124.8 124.9 125.1 125.3 125.4 125.5 125.9 126.0 126.2 126.9 127.5 139.7 139.9 �� 0+]�� '062�� FLV��FLV�WUDQV�FLV-isomer 70.8 70.9 123.1 123.9 124.9 125.0 125.2 125.4 125.5 125.8 127.1 127.3 139.8 140.1 141.2 151.8 155.9; Mass Spectrum (ESI+) (MH+): m/z 657.5 (1x rearrangement).

6\QWKHVLV�RI��WHWUDK\GUR��WHWUD��KHSWR[\�IHQ\O�SHQWDFHQH��¶��¶��¶��¶�WHWURO��D�First the 1-bromo-4-(heptoxy)benzene was prepared. 1-Bromoheptane (7.7 ml, 49 mmol) was added to a solution of 4-bromophenol (7.7 g, 44 mmol) and K2CO3 (8.0 g, 58 mmol) in dimethylformamide (300 ml). The reaction mixture was heated at 60 °C during 50 min. Then the mixture was poured into water (400 ml) and extracted with diethyl ether (3x400 ml), dried over MgSO4 and evaporated in vacuum. The crude product was purified by column chromatography on silica gel using petroleum ether:ethyl acetate (85:15). Yield: 83% 1H NMR (300 MHz, CDCl3): 0.9 (t 3H) 1.4 (m 8H) 1.7 (q 2H) 3.9 (t 2H) 6.7 (d 2H) 7.3 (d 2H); 13C NMR (75 MHz, CDCl3�� 14.0 22.5


214

25.9 28.0 31.6 31.7 34.0 69.0 112.5 116.2 132.0 158.2; Mass Spectrum (MH+): m/z 272.

The same procedure as for product ��H� was followed except purification was done by column chromatography on silica gel using dichloromethane with slow change to ethyl acetate. Yield: 68%.

HO

HOOH

OH

H3C(H2C)6O O(CH2)6CH3

O(CH2)6CH3H3C(H2C)6O

Melting point: 93 °C (WUDQV�FLV�WUDQV��WUDQV-isomer), 85 °C (FLV�WUDQV��WUDQV-isomer); 1H NMR (300 MHz, CDCl3�� WUDQV�FLV�WUDQV��WUDQV-isomer 0.9 (t 12H) 1.2-1.3 (m 40H) 1.7 (m 8H) 3.9 (t 8H) 6.3 (s 4H) 6.4 (d 8H) 7.5 (d 8H) 7.2 (m 4H) 7.4 (m 4H) 7.7 (s 2H); (300 MHz, '062�� FLV�WUDQV-isomer 0.9 (t 12H) -1.3 (m 40H) 1.7 (m 4H) 3.9 (t 4H) 6.2 (s+d 3H) 6.3 (s+d 4H) 6.4 (s 1H) 6.5 (d 4H) 6.8 (d 2H) 7.1 (d 2H) 7.2 (m 8H) 7.3 (d 1H) 7.5 (d 1H) 7.6 (d 1H) 7.7 (d 1H) 7.8 (s 1H) 8.0 (s 1H); 13C NMR (75 MHz, CDCl3�� WUDQV�FLV�WUDQV��WUDQV-isomer 14.5 23.0 26.3 29.4 29.7 32.1 60.7 68.2 113.5 125.4 127.3 127.8 128.8 136.7 140.1 140.8 141.2 157.9, (75 MHz, DMSO) FLV�WUDQV�WUDQV-isomer 14.0 22.5 26.0 29.2 29.3 31.7 67.8 113.0 113.2 133.6 115.4 115.9 123.6 125.9 127.5 128.6 135.5 139.0 139.5 140.3 140.6 140.9 141.3 149.3 157.7 157.8; Mass Spectrum (ESI+) (MH+): m/z not detected.

6\QWKHVLV�RI��WHWUDK\GUR��WHWUD��RFW\O�IHQ\O��SHQWDFHQH��¶��¶��¶��¶�WHWURO��F�The same procedure was followed as for product ��H except purification was done by column chromatography on silica gel using dichloromethane with slow change to ethyl acetate. Yield: 45%.


215

HO

HOOH

OH

H3C(H2C)7 (CH2)7CH3

(CH2)7CH3H3C(H2C)7

Melting point: 86 °C (WUDQV�FLV�WUDQV��WUDQV-isomer), 76 °C (FLV�WUDQV��WUDQV-isomer); 1H NMR (300 MHz, CDCl3�� WUDQV�FLV�WUDQV��WUDQV-isomer 0.9 (t 12H) 1.2-1.3 (m 40H) 1.6 (m 8H) 2.5 (t 8H) 6.9 (d 8H) 7.0 (d 8H) 7.2 (m 4H) 7.3 (m 4H) 7.5 (s 2H); (300 0+]��'062�� FLV�WUDQV�WUDQV-isomer 0.8 (t 12H) 1.1-1.2 (m 44H) 1.5 (m 4H) 2.2 (m 4H) 2.4 (m 4H) 6.0 (s 1H) 6.0 (s 1H) 6.1 (d 2H) 6.2 (s 1H) 6.3 (d 2H) 6.4 (m 5H) 6.9 (d 2H) 7.0 (d 2H) 7.1 (d 2H) 7.3 (m 4H) 7.4 (t 4H) 7.7 (s 1H) 7.8 (d 2H) 7.9 (s 1H); 13C NMR (75 MHz, CDCl3�� WUDQV�FLV�WUDQV��WUDQV-isomer 14.3 22.8 29.5 19.7 31.7 31.9 32.1 35.8 115.2 126.8 128.0 128.1 128.3 129.6 141.1 141.2 141.6 144.5, (75 0+]��'062�� FLV�FLV�WUDQV��WUDQV-isomer 13.9 22.1 28.7 28.9 29.0 31.3 31.9 34.8 125.4 125.8 126.1 126.3 126.6 127.1 127.3 127.6 128.4 128.9 139.4 139.5 139.8 140.1 140.8 141.4 142.0 142.1 142.4 147.9 148.7 ; Mass Spectrum (CI) (MH+): m/z not detected.

6\QWKHVLV�RI��WHWUDK\GUR��WHWUD��WHUEXW\O�SKHQ\O�SHQWDFHQH��¶��¶��¶��¶�WHWURO��G�The same procedure was followed as for product ��H except purification was done by column chromatography on silica gel using dichloromethane with slow change to ethyl acetate. Yield: 71%.

HO

HOOH

OH

1H NMR (300 0+]��'062�� WUDQV�FLV�WUDQV��WUDQV-isomer 1.1-1.2 (36H) 6.6 (d 2H) 7.0 (m 4H) 7.3 (m 18H) 7.6 (m 2H) 7.8 (m 2H) 8.0


216

�P��+�� FLV�WUDQV�WUDQV-isomer 1.1-1.2 (36H) 6.1 (s 1H) 6.1 (m 3H) 6.2 (s 1H) 6.5 (m 3H) 6.6 (d 2H) 6.7 (d 2H) 7.2 (m 6H) 7.3 (m 4H) 7.4 (m 4H) 7.8 (s 1H) 7.9 (s 1H).

6\QWKHVLV�RI��WHWUDK\GUR��WHWUD�EHQ]R�WKLHQ��\O�SHQWDFHQH��¶��¶��¶��¶�WHWURO��I�Same procedure as for product ��H except for the separation of the isomers: the crude mixture was boiled in methanol. The residue consisted of the FLV�FLV�WUDQV��FLV isomer. The filtrate was purified by column chromatography on silica gel using ethyl acetate: dichloromethane (5:95). Yield: 79%.

HO HO

HO HO

S

SS

S

Melting point: 280 °C (FLV�FLV�WUDQV��FLV-isomer), 170 °C (FLV�WUDQV�WUDQV-isomer), 185 °C (WUDQV�FLV�WUDQV��WUDQV-isomer); 1H NMR (400 MHz, DMSO 50 °C) WUDQV�FLV�WUDQV��WUDQV-isomer 6.7 (s 4H) 6.8 (s 4H) 7.2 (t 8H) 7.3 (m 4H) 7.4 (d 8H) 7.6 (m 4H) 8.0 (s H); (400 0+]��'062�� FLV�WUDQV�WUDQV-isomer 5.7 (s 1H) 5.8 (s 1H) 6.7 (d 2H) 6.8 (s 1H) 6.9-7.2 (m 8H) 7.3 (s 1H) 7.4 (m 10H) 7.5-7.6 (m 6H) 7.8 (d 2H) 7.9-8.0 (m 4H) 8.3 (s 1H) 8.4 (s 1H); (400 MHz, '062�� FLV�FLV�WUDQV�FLV-isomer 6.0/6.1 (s 4H) 6.7/6.8 (d, 4H) 6.9 (t 4H) 7.0 (t 4H) 7.5/7.6 (d 4H) 7.6 (m 4H) 8.1 (m 4H) 8.7/8.8 (s 2H); 13C NMR (100 MHz, DMSO 50 °C): WUDQV�FLV�WUDQV�WUDQV-LVRPHU� ��71.9 121.3 121.6 122.7 123.2 123.5 127.4 127.8 128.3 138.7 138.8 139.1 139.9 155.6 (100 0+]��'062�� FLV�WUDQV�WUDQV-isomer 71.0 71.4 71.9 72.7 121.3 121.4 121.6 121.9 122.3 122.4 123.5 123.6 123.7 123.8 124.2 125.5 125.9 127.6 127.8 127.9 128.7 137.9 138.1 138.8 138.9 139.0 139.2 139.3 139.5 139.6 140.0 140.2 140.8 141.2 151.8 �� 0+]��'062�� FLV�FLV�WUDQV��FLV-isomer


217

71.6/71.5 121.6/121.5 122.2/122.1 122.6 123.5 123.7 124.2/124.1 126.0 127.8/127.7 138.1/138.0 139.2/139.1 140.0/139.9 140.8/140.6 152.1/151.8; MS (ESI+) (MH+): m/z 857.4 (1x rearrangement).

6\QWKHVLV�RI��WHWUDK\GUR��WHWUD��PHWK�R[\WKLHQ��\O�SHQWDFHQH��¶��¶��¶��¶�WHWURO��K�The same procedure was followed as for product ��H except the work-up was carried out with 10%-NH4Cl solution instead of HCl and a trace of Et3N was added to the eluent. Yield: 70%.

HO

HO

OHS

S

OMe

OMe

S

S

MeO

MeO

OH

�Melting point: 239 °C (FLV�FLV�WUDQV��FLV-isomer); 1H NMR (300 MHz, '062�� FLV�WUDQV�WUDQV-isomer 3.7 (s 12H) 5.4 (d 1H) 5.6 (d 1H) 5.9 (d 2H) 6.2 (d 2H) 6.3 (s 2H) 6.6 (s 2H) 7.3 (m 3H) 7.5 (m 4H) 7.7 (m 1H) 7.8 (s 1H) 7.9 (m 2H) 8.5 (s 1H); (300 0+]�� '062�� FLV�FLV�WUDQV�FLV-isomer 3.8 (s 12H) 5.9 (d 4H) 6.2 (d, 4H) 6.3 (s 4H) 7.3 (m 4H) 7.7 (m 4H) 7.6 (m 4H) 8.1 (s 4H); 13C NMR (75 MHz, '062�� FLV�WUDQV�WUDQV-isomer 71.0 71.8 102.1 122.3 123.3 125.7 127.3 127.8 137.0 139.4 139.5 139.7 140.8 141.0 165.0 165.4; (75 MHz, DMSO): FLV�FLV�WUDQV��FLV-isomer to low solubility: 71.7 102.1 122.3 127.3 127.8 139.5 139.9 140.7 165.0; MS (ESI+) (MH+): m/z not detected.

6\QWKHVLV�RI��WHWUDK\GUR��WHWUD��¶�EL�WKLHQ��\O�SHQWDFHQH��¶��¶��¶��¶�WHWURO��L�The same procedure was followed as for product ��H except the work-up was carried out with 10%-NH4Cl solution instead of HCl and purification was done by column chromatography on silica gel using dichloromethane:petroleum ether (5:5) with slow change to dichloro-methane. Yield: 88%.


218

HO

HO

OHS

S

S

SOH

S

S

S

S

Melting point: 209 °C (FLV�FLV�WUDQV��FLV-isomer and FLV�WUDQV�WUDQV-isomer); 1H NMR (300 0+]��'062�� FLV�WUDQV�WUDQV-isomer 5.6 (d 2H) 5.7 (d 2H) 5.8 (s 1H) 5.8 (d 1H) 5.9 (d 1H) 6.4 (d 2H) 6.5 (d 2H) 6.6 (d 2H) 6.7 (d 2H) 7.0 (s 2H) 7.1 (d 3H) 7.2 (m 4H) 7.5 (m 6H) 7.9 (m 2H) 8.2 (s 1H) 8.3 (s 1H); (300 0+]��'062�� FLV�FLV�WUDQV�FLV-isomer 6.4 (d 4H) 6.7 (s 4H) 6.8 (d 4H) 6.9 (m 8H) 7.3 (m 4H) 7.4 (d 4H) 7.5 (m 4H) 7.9 (s 2H); 13C NMR (75 0+]�� '062�� FLV�WUDQV�WUDQV-isomer 71.0 71.7 71.8 72.3 122.5 123.1 123.3 123.4 124.7 124.9 125.0 127.7 128.0 128.2 135.1 136.3 136.7 139.5 140.3 150.7 154.3; (75 MHz, DMSO): FLV�FLV�WUDQV��FLV-isomer: 71.4 122.2 122.7 123.0 123.3 124.3 125.6 127.3 127.7 127.9 134.7 136.6 139.2 140.0 150.2 153.7; MS (ESI+) (MH+): m/z 985.1 (1x rearrangement) 967.1 (2x rearrangement)

6\QWKHVLV�RI��WHWUD�WKLHQ��\O�SHQWDFHQH��H�A suspension of tetrol ��H (1.8 g, 2.7 mmol), NaI (6.2 g, 38 mmol) and NaH2PO2 (6.2 g) in acetic acid (200 ml) was heated at reflux for 3 h. The product ��H was isolated by filtration and washed with water (3x75 ml), methanol (2x50 ml) diethyl ether (2x25 ml). After drying in vacuum, 5,7,12,14-tetrathienylpentacene was obtained in 62% yield.

S

SS

S


219

Melting point: > 300 °C; 1H (400 MHz, CDCl3�LQ�FRQFHQWULF�WXEH�� 6.7 (d 4H) 6.9 (t 4H) 7.0 (d 2H) 7.9 (m 4H) 8.0 (m 4H) 8.9 (s 2H); Mass Spectrum (APCI+) (MH+): m/z 607.2; UV-VSHFWUXP�� max : 624 QP�� 3 l/(mol.cm).

6\QWKHVLV�RI��WHWUD��KHSW\OR[\SKHQ\O�SHQWDFHQH��D�The same procedure was followed as for product ��H. Yield: 76%.

O

O

O

O

Melting point: 220 °C; 1H NMR (300 MHz, CDCl3�� W��+��(m 32H) 1.6 (m 8H) 1.9 (m 8H) 4.1 (t 8H) 6.9 (d 8H) 7.1 (m 4H) 7.3 (d 8H) 7.7 (m 4H) 8.3 (s 2H); 13C NMR (75 MHz, CDCl3�� 0.7 24.2 27.3 27.6 29.9 66.2 112.7 122.8 125.7 127.9 128.7 129.0 131.0 141.9 148.8 157.0; Mass Spectrum (ESI+) (MH+): m/z .1039.7; UV-VSHFWUXP�� max ��QP�� 3 l/(mol.cm).

6\QWKHVLV�RI��WHWUD��RFW\OSKHQ\O�SHQWDFHQH��F�The same procedure was followed as for product ��H. Yield: 58%.

Melting point: 188 °C; 1H NMR (300 MHz, CDCl3�� W��+��(m 32H) 1.5 (m 8H) 1.7 (m 8H) 2.7 (t 8H) 7.1 (m 4H) 7.3 (m 16H) 7.7 (m 4H) 8.4 (s 2H); 13C NMR (75 MHz, CDCl3��


220

30.3 32.1 32.4 36.4 124.7 127.7 128.3 129.2 129.6 131.6 136.3 137.5 142.0 146.8; Mass Spectrum (ESI+) (MH+): m/z .1030.8; UV-VSHFWUXP�� max ��QP�� 3 l/(mol.cm).

6\QWKHVLV�RI��WHWUD��WHUWEXW\OIHQ\O�SHQWDFHQH��G�The same procedure was followed as for product ��H. Yield: 46%.

Melting point: > 300 °C; 1H NMR (300 MHz, CDCl3): 3.1 (s 2H) 5.9 (d 2H) 6.3 (t 2H) 6.9 (d 2H) 7.6 (m 4H); 13C NMR (75 MHz, CDCl3): ��0DVV�

Spectrum (APCI+) (MH+): m/z 807.7; UV-VSHFWUXP�� max ��QP�� 12 103 l/(mol.cm).

6\QWKHVLV�RI��WHWUD�EHQ]RWKLHQ��\O�SHQWDFHQH��I�The same procedure was followed as for product ��H. Yield: 86%.

S

SS

S

Melting point: > 300 °C; 1H NMR (400 MHz, CDCl2CDCl2�� .3 (m 8H) 7.35 (s 4H) 7.5 (m 4H) 7.6 (m 4H) 7.7 (m 4H) 8.0 (m 4H) 8.8 (s 2H); Mass Spectrum (APCI+) (MH+): m/z 806.3; UV-VSHFWUXP�� max : ��QP�� 3 l/(mol.cm).


221

6\QWKHVLV�RI��WHWUDK\GUR��WULK\GUR[\��¶��¶��¶�WUL��RFWDQR\OSKHQ\O�SHQWDFHQ��RQH��First 4-bromophenyloctan-1-one was prepared. 1-Bromoheptane (9.0 ml, 58 mmol) was added in a dropwise manner to a suspension of Mg (1.5 g, 63 mmol) and a trace of iodine in diethyl ether. After 10 min., this reaction mixture was then added in a dropwise manner to a solution of 4-bromobenzoylchloride (9.7 g, 44 mmol) and Fe(acac)3 (0.46 g, 1.3 mmol) in dry tetrahydrofuran (150 ml) cooled at -78 °C and under argon atmosphere. The mixture was stirred for 15 min, quenched with NH4Cl (10%, 50 ml) and extracted with diethyl ether (3x75 ml). The organic layer was dried over MgSO4 and evaporated in vacuum. The crude product was purified by column chromatography on silica gel using dichloromethane:petroleum ether (5:5). The yield of the reaction was 44%.

Melting point: 69 °C; 1H NMR (300 MHz, CDCl3�� W��+��P�8H) 1.7 (m 2H) 2.6 (t 2H) 7.6 (d 2H) 7.8 (d 2H); 13C NMR (75 MHz, CDCl3�� 200.0; Mass Spectrum (MH+): m/z 283.

The procedure was the same as for product ��G� The 4-bromophenyloctan-1-one was protected with 1,2-ethanediol (3eq, 2 days). Before purification the carbonyl function was deprotected by stirring the crude product for 2 h in a 2M HCl solution. The precipitate was filtered, washed with water (3 x 100 ml) and dried in vacuum. The crude product was boiled in methanol and subsequently the pure product was filtered with a yield of 43%.

HO

HOOH

O

(CH2)6CH3

O

OH3C(H2C)6(CH2)6CH3

O

�1H NMR (300 MHz, CDCl3):� ��W��+��P��+��P��+��(t 6H) 4.6 (s 1H) 4.7 (s 1H) 4.9 (s 1H) 6.9 (d 2H) 7.1 (m 3H) 7.2 (m


222

3H) 7.3 (d 2H) 7.4 (m 3H) 7.5 (m 5H) 7.6 (m 2H) 7.9 (s 1H) 8.4 (s 1H); 13C NMR (75 MHz, CDCl3�� 74.3 126.6 126.8 126.9 127.3 127.5 127.8 128.0 128.4 128.7 129.4 129.9 135.0 135.1 135.2 140.3 145.7 146.7 150.8 152.8 183.2 200.2 201.0; Mass Spectrum (MH+): m/z not detected.

6\QWKHVLV�RI��GLK\GUR��ELV��RFWDQR\OSKHQ\O��¶��RFWDQR\OSKHQ\O�SHQWDFHQ��RQH��The procedure was the same as for product ��G��Yield: 65%.

O

H

(CH2)6CH3O

(CH2)6CH3O(CH2)6CH3

O

1H NMR (300 MHz, CDCl3):� ��W��+��P��+��P��+��(t 6H) 5.5 (s 1H) 7.1 (d 2H) 7.2 (d 2H) 7.3 (m 2H) 7.4 (d 2H) 7.5 (m 4H) 7.6 (m 3H) 7.7 (d 2H) 8.1 (d 1H) 8.2 (m 3H) 8.8 (s 1H); 13C NMR (75 MHz, CDCl3�� 128.5 128.8 129.4 130.5 131.3 131.6 131.8 132.1 135.6 136.7 137.4 139.7 142.9 143.3 150.0 184.7 200.4; Mass Spectrum (MH+): m/z 901.6.

6\QWKHVLV�RI��WHWUDK\GUR��¶��¶�WHWUD�WKLHQ��\O�SHQWDFHQ��RQH�DQG��WHWUDK\GUR��¶��¶�WHWUD�WKLHQ��\O�SHQWDFHQ��RQH��A solution of diol ��H (1.2 g, 1.8 mmol) and BF3.OEt2 (0.34 ml, 2.7 mmol) in dichloromethane (100 ml) was stirred at room temperature for 12 h. After adding NaOH (1 M), the reaction mixture was extracted with dichloromethane. The organic layer was washed with water (3x100 ml), dried over MgSO4 and evaporated in vacuum. The product �� could be obtained after purification by column


223

chromatography on silica gel using petroleum ether:dichloromethane (5:95) in a total yield of 22%. The isomers were separated with a ratio of 2:8 (5,7-one:5,12-one).

SSSS

OO S

S

S

S O

O

�Melting point: 124 129 °C; 1H NMR (300 MHz, CDCl3�� -one) 6.8 (d 4H) 6.9 (t 4H) 7.2 (d 4H) 7.5 (m 4H) 7.6 (d 2H) 8.2 (d 2H) 8.4 (s 2H); (300 MHz, CDCl3�� -one) 6.6 (d 4H) 6.8 (t 4H) 7.2 (d 4H) 7.4-7.6 (m 7H) 8.3 (d 2H) 9.2 (s 1H); 13C NMR (75 MHz, CDCl3�� -one) 52.0 126.5 127.8 129.3 130.1 133.6 133.7 148.3 150.7; (75 MHz, CDCl3): � ��-one) 52.3 125.0 126.2 127.2 127.7 128.4 128.7 130.0 130.2 130.4 131.0 133.4 147.6 150.4 182.8; Mass spectrum (CI) (MH+): m/z 639.


224

��3URFHGXUHV�IRU�FRPSRXQGV�RI�FKDSWHU��6\QWKHVLV�RI�SHQWDFHQH��See Part I, Chapter 1.1.

6\QWKHVLV�RI��KH[DVXOSKXUSHQWDFHQH��A suspension of pentacene (1.0 g, 3.6 mmol) and sulphur (1.0 g 3.9 mmol) in 1,2,4-trichlorobenzene (150 ml) was heated at reflux for 3 h. After cooling down, the precipitate was filtered and washed with hot solvent (2x30 ml) and ethanol (2x30 ml). After drying in vacuum, the product �� was obtained in a yield of 46%.72

S

S

SS

S S

IR-spectrum: (KBr, cm-1) 752 1025 1254 1317 1398 1465 1598 (in agreement with the literature)72.


225

��3URFHGXUHV�IRU�FRPSRXQGV�RI�FKDSWHU��6\QWKHVLV�RI��ELV��GLPHWK\OEHQ]R\O��DQG��ELV��GLPHWK\OEHQ]R\O��EHQ]HQHGLFDUER[\OLF�DFLG��D�A suspension of benzene-1,2,4,5-tetracarboxylic acid dianhydride (5.0 g, 23 mmol) and AlCl3 (13 g, 98 mmol) in dichloromethane (30 ml) was cooled in an ice/NaCl-bath (-10 °C). To this mixture a solution of o-xylene (7.1 ml, 45 mmol) and diisopropylethylamine (4.1 ml, 25 mmol) in dichloromethane (15 ml) was added in a dropwise manner over a period of 3 h. The reaction mixture was then stirred at room temperature for 12 h. Subsequently the mixture was poured into a beaker with ice (30 g) and HCl (25 ml) and stirred for 1h. Ethyl acetate (100 ml) was added and the mixture was again stirred for a while. Then the mixture was extracted with ethyl acetate and the organic phase was washed with water (3x100 ml), dried over MgSO4 and evaporated in vacuum. The crude mixture was washed with a small amount of ethyl acetate. The residue was filtered and dried. The pale powder was the & ~1� -isomer. The filtrate was evaporated in vacuum. The residue was dried and was characterised as the & ~/� -isomer. The ratio of the isomers was 3:7 respectively and the total yield was 81%.

CO2HHO2C

O O

CO2H

HO2CO

R

RO

�Melting point: 215-219 °C (& ~/� -isomer), 243 °C (& ~1� -isomer); 1H NMR (300 0+]��'062�� & ~/� -isomer 2.2 (s 6H) 2.3 (s 6H) 7.2 (d 2H) 7.4 (m 3H) 7.5 (s 2H) 8.5 (s 1H); (300 0+]��'062�� & ~1� -isomer 2.3 (2xs 2x6H) 7.3 (d 2H) 7.4 (d 2H) 7.6 (s 2H) 7.9 (s 2H); 13C NMR (75 0+]��'062�� & ~/� -isomer 19.6 19.9 127.5 129.3 129.8 130.4 133.9 137.0 143.5 146.5 167.6 195.4; 0.0 130.2 130.4 131.0 133.4 147.6 150.4 182.8; Mass spectrum (ES+) (MH+): m/z 431.2.


226

6\QWKHVLV�RI��ELV��GLHWK\OEHQ]R\O��DQG��ELV��GLHWK\OEHQ]R\O��EHQ]HQHGLFDUER[\OLF�DFLG��E�The procedure was the same as for product ��D� except a small amount of tetrahydrofuran was added during extraction to promote the solubility in the organic layer.�The isomers were separated for each other by washing with a mixture of ethyl acetate:heptane 3:1. Ratio of the & ~1� - and & ~/� -isomer: 1:9; Yield: 72%.

CO2HHO2C

O O

CO2H

HO2CO

O

�Melting point: 162 °C (& ~/� -isomer), 245 °C (& ~1� -isomer); 1H NMR (300 0+]��'062�� & ~/� -isomer 1.2 (m 12H) 2.7 (m 8H) 7.3 (d 2H) 7.4 (m 3H) 7.6 (s 2H) 8.5 (s 1H) 13.5 (acid); (300 0+]��'062�� & ~1� -isomer 1.2 (m 12H) 2.7 (m 8H) 7.3 (d 2H) 7.4 (d 2H) 7.6 (s 2H) 7.9 (s 2H) 13.6 (acid); 13C NMR (75 0+]��'062�� & ~/� -isomer 14.9 24.6 24.9 127.3 128.6 130.7 134.2 141.8 144.9 147.8 165.7 194.7; 13C NMR (75 0+]�� '062�� & ~1� -isomer 15.4 25.2 25.4 127.9 128.8 129.0 129.1 133.7 134.6 142.2 142.5 148.2 166.2 195.1 Mass spectrum (CI) (MH+): m/z 487.3.

6\QWKHVLV�RI��ELV��GLFKORUREHQ]R\O��DQG��ELV��GLFKORUREHQ]R\O��EHQ]HQHGLFDUER[\OLF�DFLG��F�3,4-Dichlorophenylbromide (16 g, 69 mmol) was added in a dropwise manner to a suspension of Mg (2.5 g, 0.1 mol) in dry diethyl ether (2 ml) in the presence of a trace iodine. Subsequently the fresh Grignard reagent was added in a dropwise manner to a solution of benzene-1,2,4,5-tetracarboxylic acid dianhydride (5.0 g, 23 mmol) in dry tetrahydrofuran (200 ml). The solution was slightly cooled to control the vigorous reaction. The reaction mixture was stirred for 12 h, acidified with HCl (1M) and extracted with ethyl acetate (400 ml). The organic layer was washed with water (3x300 ml) and saturated Na2CO3-solution. The basic water layer was again acidified and extracted with ethyl acetate. The organic layer was dried over MgSO4


227

and evaporated in vacuum. The crude mixture was washed with ethyl acetate:heptane 3:1. The residue was filtered and dried. The pale powder was the & ~1� -isomer. The filtrate was evaporated in vacuum. The residue was dried and was characterised as the & ~/� -isomer. The ratio of the isomers was 2:8 respectively and the total yield was 87%. (A part of the & ~/� -isomer was recovered out of the first organic layer).

CO2HHO2C

O O

Cl

Cl

Cl

Cl

CO2H

HO2CO

Cl

ClCl

OCl

�Melting point: 151 °C (& ~/� -isomer), 265 °C (& ~1� -isomer); 1H NMR (300 0+]��'062�� & ~/� -isomer 7.6 (m 4H) 7.8 (d 2H) 7.9 (s 1H) 8.5 (s 1H); (300 0+]��'062�� & ~1� -isomer 7.7 (d 2H) 7.8 (d 2H) 7.9 (s 2H) 8.0 (s 2H); 13C NMR (75 MHz, DMSO�� & ~/� -isomer 126.0 129.4 129.9 132.4 133.4 133.6 135.9 137.9 139.3 168.4 (75 MHz, DMSO):� � & ~1� -isomer 127.3 128.8 130.7 131.1 131.8 132.3 139.8 166.1; Mass spectrum (CI) (MH+): m/z 513.

6\QWKHVLV�RI��ELV��GLEXW\OVXOIDQ\OEHQ]R\O��DQG��ELV��GLEXW\OVXOIDQ\OEHQ]R\O��EHQ]HQHGLFDUER[\OLF�DFLG��G�First 1,2-dibutylsulfanylbenzene was prepared: Sodium (25 g, 1,1 mol) was dissolved into ethanol (350 ml) under cooling to control the exothermic dissolution. 1-Butanethiol (0.11, 1.0 mol) was added to the solution in a dropwise manner. The reaction mixture was stirred for 1 h and the solvent was evaporated in vacuum. The residue was washed with diethyl ether and filtered. Subsequently the residue was washed twice more with diethyl ether (2x50 ml) and dried in vacuum. Yield: 35%.

1,2-Dichlorobenzene (8.0 ml, 70 mmol) was added in a dropwise manner to a solution of sodium butane-1-thiolate (40 g, 0.35 mol) in dimethylformamide (375 ml). The reaction mixture was heated at 100 °C for 3 days. Then the mixture was poured into water (600 ml) and extracted with ethyl acetate (3x400 ml). The organic phase was dried over MgSO4 and evaporated in vacuum. The crude product was


228

purified by column chromatography on silica gel using petroleum ether. Yield 24%.

Melting point: oil; 1H NMR (300 MHz, CDCl3):� ��W��+��P�4H) 1.6 (m 4H) 2.9 (t 4H) 7.1 (t 2H) 7.2 (d 2H); 13C NMR (75 MHz, CDCl3�� 0DVV�6SHFWUXP�(CI) (MH+): m/z 254. 0RQRVXEVWLWXWHG�SURGXFW��PHOWLQJ�SRLQW��RLO�� +�105��0+]��&'&O � �� W��+��P��+��P��+��W��+��W��+��G��+��W��+�� G��+�� &�105��0+]��&'&O � �� 0DVV�6SHFWUXP��&,� �0+��P�]��A suspension of benzene-1,2,4,5-tetracarboxylic acid dianhydride (0.80 g, 4.0 mmol) and AlCl3 (2.0 g, 15 mmol) in cyclohexane (25 ml) was cooled in an ice/NaCl-bath (-10 °C). To this mixture a solution of 1,2-butylsulfanylbenzene (2.0 g, 5.0 mmol) and diisopropylethylamine (0.70 ml, 5.0 mmol) in cyclohexane (20 ml) was added in a dropwise manner over a period of 3 h. The reaction mixture was then heated at reflux 12 h. Subsequently the mixture was poured into a beaker with ice (18 g) and HCl (15 ml) and stirred for 1 h. The mixture was extracted with ethyl acetate:tetrahydrofuran and the organic phase was washed with water (3x100 ml), dried over MgSO4 and evaporated in vacuum. The crude mixture was washed with a small amount of ethyl acetate:heptane. The residue was filtered and dried. The filtrate was evaporated in vacuum. The residue was dried and was characterised as the & ~/� -isomer. The ratio of the isomers was 3:7 respectively and the total yield was 50%��

CO2HHO2C

O O

S

S

S

S

CO2H

HO2CO

S

S

OS

S �Melting point: 106 °C; 1H NMR (300 0+]��'062�� & ~/� -isomer 1.5 (t 12H) 1.7 (m 16H) 2.9 (m 8H) 7.2 (d 2H) 7.3 (s 2H) 7.6 (m 3H) 8.6 (s 1H); (300 0+]��'062�� & ~1� -isomer 1.5 (t 12H) 1.7 (m 16H) 2.9 (m 8H) 7.2 (d 2H) 7.5 (d 2H) 7.7 (s 2H) 7.8 (s 2H); 13C NMR (75 0+]��'062�� 11.3 13.5 18.7 21.1 124.3 126.0 127.8 128.9 129.9 136.1 165.7; Mass spectrum (CI) (MH+): m/z 727.


229

6\QWKHVLV�RI��ELV��GLPHWK\OEHQ]\O��DQG��ELV��GLPHWK\OEHQ]\O��EHQ]HQHGLFDUER[\OLF�DFLG��D�& ~/� -isomer: Iodine (0.78 g, 3.1mmol) and H3PO4 (0.80 ml, 15 mmol) were heated at reflux for 10 min in acetic acid (40 ml) under argon atmosphere. After the colour disappeared a suspension of ��D�(2.0 g, 4.6 mmol) in acetic acid (30 ml) was added in a dropwise manner. Subsequently the heating was kept at reflux during 24 h. The reaction mixture was poured onto ice and the precipitate was filtered and washed with water (2x 40 ml) and diethyl ether (2x40 ml). The residue was dried in vacuum. Yield: 52% Sometimes in was neccesary to perform an extraction with ethyl acetate, followed by washing with Na2S2O3.H2O and brine, drying over MgSO4 and evaporation in vacuum. Purification: careful boiling in dichloromethane. & ~1� -isomer: the procedure was the same as for the & ~/� -isomer. Yield: 49%.

CO2HHO2C CO2H

HO2CR

R

�Melting point: 248 °C (& ~/� -isomer), 290 °C (& ~1� -isomer); 1H NMR (300 0+]��'062�� & ~/� -isomer 2.1 (s 6H) 2.2 (s 6H) 4.3 (s 4H) 6.8 (d 2H) 6.9 (s 2H) 7.0 (d 2H) 7.3 (s 1H) 8.3 (s 1H); (300 MHz, '062�� & ~1� -isomer 2.1 (s 6H) 2.2 (s 6H) 4.2 (s 4H) 6.8 (d 2H) 6.9 (s 2H) 7.0 (d 2H) 7.6 (s 2H) 13.2 (acid); (75 0+]��'062�� & ~1� -isomer 18.9 19.4 37.3 126.0 129.5 129.9 132.4 133.5 133.6 135.9 138.0 139.3 168.4; Mass spectrum (CI) (MH+): m/z 367.

6\QWKHVLV�RI��ELV��GLHWK\OEHQ]\O��DQG��ELV��GLHWK\OEHQ]\O��EHQ]HQHGLFDUER[\OLF�DFLG��E�The procedure was the same as for product ��D�except it washing the & ~/� -isomer with diethyl ether, must be prevented. & ~/� -isomer: Yield: 66%; & ~1� -isomer: Yield: 64%.

CO2HHO2C CO2H

HO2C �


230

Melting point: 181 °C (& ~/� -isomer), 259 °C (& ~1� -isomer); 1H NMR (300 0+]��'062�� & ~/� -isomer 1.1 (m 12H) 2.5 (m 8H) 4.2 (s 4H) 6.8 (d 2H) 6.9 (s 2H) 7.0 (d 2H) 7.3 (s 1H) 8.3 (s 1H); (300 MHz, '062�� & ~1� -isomer 1.1 (m 12H) 2.5 (m 8H) 4.2 (s 4H) 6.8 (d 2H) 6.9 (s 2H) 7.0 (d 2H) 7.6 (s 2H) 13.1 (acid); (300 0+]��'062�� & ~/� -isomer 15.1 24.6 24.8 40.8 124.5 126.1 127.0 128.1 133.5 137.9 138.7 167.9 13C NMR (75 0+]��'062�� & ~1� -isomer 15.4 24.4 24.8 126.1 128.2 128.5 133.5 138.1 138.7 139.2 141.0 141.6 168.4; Mass spectrum (CI) (MH+): m/z not detected.

6\QWKHVLV�RI��ELV��GLFKORUREHQ]\O��DQG��ELV��GLFKORUREHQ]\O��EHQ]HQHGLFDUER[\OLF�DFLG��F�The procedure was the same as for product ��D�except purification of the & ~/� -isomer was done by boiling the residue in heptane.�& ~/� -isomer: Yield: 69%; & ~1� -isomer: Yield: 62%.

CO2HHO2C Cl

Cl

Cl

Cl

CO2H

HO2C

Cl

ClCl

Cl

�Melting point: 143 °C (& ~/� -isomer), 285 °C (& ~1� -isomer); 1H NMR (300 0+]��'062�� & ~/� -isomer 4.3 (s 4H) 7.1 (d 2H) 7.4 (s 2H) 7.5 (d 2H) 7.8 (s 1H) 7.9 (s 1H); (300 0+]��'062�� & ~1� -isomer 4.3 (s 4H) 7.5 (d 2H) 7.8 (s 2H) 7.9 (d 2H) 8.0 (s 2H); 13C NMR (75 MHz, '062�� & ~/� -isomer 30.7 128.7 129.6 130.5 130.8 131.2 143.1 145.8 167.4; (75 0+]��'062�� & ~1� -isomer 34.9 127.3 128.8 130.7 131.1 131.8 132.3 139.7 139.9 166.1; Mass spectrum (CI) (MH+): m/z not detected.

6\QWKHVLV�RI��ELV��GLEXW\OVXOIDQ\OEHQ]\O��DQG��ELV��GLEXW\OVXOIDQ\OEHQ]\O��EHQ]HQHGLFDUER[\OLF�DFLG��G�The procedure was the same as for product ��D�except the products were extracted with ethyl acetate, washed with Na2S2O3.H2O and brine, dried over MgSO4 and evaporated in vacuum. & ~/� -isomer: Yield: 13%; & ~1� -isomer: Yield: 5%.


231

CO2HHO2C S

S

S

S

CO2H

HO2C

S

S

S

S 1H NMR (300 MHz, '062�� & ~/� -isomer 0.9 (m 12H) 1.3 (m 8H) 1.5 (m 8H) 2.8 (m 8H) 4.4 (s 4H) 7.1 (d 2H) 7.2 (m 4H) 7.6 (s 1H) 8.1 (s 1H); (300 0+]��'062�� & ~1� -isomer 0.9 (m 12H) 1.3 (m 8H) 1.5 (m 8H) 2.8 (m 8H) 4.4 (s 4H) 6.9 (d 2H) 7.1 (s 2H) 7.4 (d 2H) 7.6 (s 2H); Mass spectrum (MH+): m/z 700.6.

6\QWKHVLV�RI��WHWUDPHWK\OSHQWDFHQH��GLRQH�DQG��WHWUDPHWK\OSHQWDFHQH��GLRQH��D�CF3SO3H (4 ml) was added to ��D�(0.80 g, 2.0 mmol) at 0 °C. The reaction mixture was stirred at room temperature for 12 h. Subsequently the mixture was poured into ice (30 g) and neutralised with a saturated Na2CO3-solution. The precipitate was filtered and washed with water (2x25 ml), methanol (3x25 ml) and diethyl ether (3x25 ml). The residue was dried in vacuum. & ~/� -isomer: Yield 70%; & ~1� -isomer: procedure is the same as for the & ~/� -isomer expect CF3SO3H (3.5 ml) was added to ��D� (0.90 g, 2.2 mmol) in trifluoroacetic acid (2.8 ml) at 0 °C and stirred for 24 h. Yield: 86%.

O O O

O �Melting point: > 300 °C (& ~/� -isomer), 295 °C (& ~1� -isomer); 1H NMR (300 MHz, D2SO4�� & ~/� -isomer 2.5 (s 6H) 2.6 (s 6H) 4.9 (s 4H) 7.8 (s 2H) 8.2 (s 1H) 8.4 (s 2H) 9.9 (s 1H); (300 MHz, D2SO4�� & ~1� -isomer 2.5 (s 6H) 2.6 (s 6H) 4.9 (s 4H) 7.8 (s 2H) 8.4 (s 2H) 9.0 (s 2H); 13C NMR (75 MHz, D2SO4, in concentric tube with DMSO�� & ~/� -isomer 17.8 20.0 120.0 124.0 128.4 130.5 132.0 140.3 147.9 151.4 158.4 170.9 185.3; Mass spectrum (CI) (MH+): m/z 367; IR-spectrum: 1649 cm-1.


232

6\QWKHVLV�RI��WHWUD�HWK\OSHQWDFHQH��GLRQH�DQG��WHWUD�HWK\OSHQWDFHQH��GLRQH��E�The procedure was the same as for the & ~1� -isomer of product ��D��Yield: 41%.

O O O

O �Melting point: 283 °C ; 1H NMR (300 MHz, D2SO4�� & ~/� -isomer 1.4 (m 12H) 2.9 (m 8H) 4.9 (s 4H) 7.8 (m 2H) 8.3 (s 1H) 8.4 (m 2H) 9.9 (s 1H); (300 MHz, D2SO4�� & ~1� -isomer 1.4 (m 12H) 2.9 (m 8H) 4.9 (s 4H) 7.9 (m 2H) 8.4 (m 2H) 9.0 (s 2H); Mass spectrum (CI) (MH+): m/z not detected.

6\QWKHVLV�RI��WHWUDFKORURSHQWDFHQH��GLRQH�DQG��WHWUDFKORURSHQWDFHQH��GLRQH��F�The procedure was the same as for product ��D� except the & ~/� -isomer should not be washed with diethyl ether��& ~/� -isomer : Yield: 40%; & ~1� -isomer: Yield: 46%.

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

O O O

O �Melting point: > 300 °C (& ~/� -isomer/� & ~1� -isomer); 1H NMR (300 MHz, D2SO4�� & ~/� -isomer 4.8 (s 4H) 7.4 (m 2H) 7.7 (m 1H) 7.8 (m 2H) 9.1 (s 1H); (300 MHz, D2SO4�� & ~1� -isomer 4.5 (m 2H) 8.3 (m 2H) 8.8 (m 2H) 9.5 (m 2H); Mass spectrum (CI) (MH+): m/z 449.

6\QWKHVLV�RI��WHWUD�EXW\OVXOIDQ\O�SHQWDFHQH��GLRQH�DQG��WHWUD�EXW\OVXOIDQ\O�SHQWDFHQH��GLRQH��G �The procedure was the same as for the & ~1� -isomer of product ��D��Yield: 8%.


233

S

S

S

S

S

S

S

S

O O O

O �Mass spectrum (CI) (MH+): m/z 663.5.

6\QWKHVLV�RI��ELV��GLPHWK\OSKHQ\O�ELV��IXUDQRQH��D

Product ��D (0.25 g, 0.58 mmol), Zn (7.5 g, 0.12 mmol), KOH (7.5 g) and a few crystals of CuCl2 were heated at reflux for 24 h in water (30 ml). The reaction was acidified with HCl (10 M). The water was removed and the zinc was washed with ethyl acetate (2x30 ml). The organic phases were collected and dried over MgSO4 and evaporated in vacuum. The crude product was purified by column chromatography over silica gel using petroleum ether:dichloromethane:ethyl acetate (65:20:15). Both isomers were formed in a 1:1 ratio and the total yield was 82%.

OO

O O

�Melting point: 264 °C; 1H NMR (300 0+]��'062�� 1 (s 6H) 2.2 (s 6H) 6.4 (s 2H) 6.9 (m 4H) 7.1 (d 2H) 7.2 (s 1H) 8.5 (s 1H); (300 0+]��'062�� 2.2 (s+s 6+6H) 6.3 (s 2H) 6.9 (m 4H) 7.1 (m 3H) 8.5 (s 1H); 13C NMR (75 0+]��'062�� 7 19.9 82.8 118.1 124.0 124.6 127.7 128.1 130.5 132.8 137.9 138.7 155.8 168.6. 13C NMR (75 0+]��'062�� 7 19.8 83.0 118.3 124.0 125.1 128.2 128.6 130.4 132.5 137.8 138.7 155.3 168.5; Mass spectrum (CI) (MH+): m/z 399.

6\QWKHVLV�RI��ELV��GLPHWK\OSKHQ\O�ELV��IXUDQRQH��E �The procedure was the same as for product ��D�� Ratio of the isomers: 3:7, Yield: 64%.


234

OO

O

O

�Melting point: 233 °C; 1H NMR (300 0+]��'062�� 2 (s 6H) 2.3 (s 6H) 6.4 (s 2H) 7.0 (m 4H) 7.2 (d 2H) 7.9 (s 2H); (300 MHz, '062�� 2.2 (s 6H) 2.3 (s 6H) 6.5 (s 2H) 7.0 (d 2H) 7.2 (d 2H) 7.3 (s 2H) 7.9 (s 2H); 13C NMR (75 0+]��'062�� 7 19.9 82.9 120.6 124.2 127.8 130.5 131.2 132.8 137.8 138.6 150.9 168.9; Mass spectrum (CI) (MH+): m/z 399.

6\QWKHVLV�RI��WHWUDPHWK\O��K\GUR[\��¶��WKLHQ��\O��SHQWDFHQ��RQH��n-BuLi (1.6 ml, 4.1 mmol) was added to a solution of thiophene (0.33 ml, 4.1 mmol) in THF (75 ml) at -78 °C under argon atmosphere. After 10 minutes 2,3,9,10-tetramethylpentacene-5,7-dione (0.50 g, 1.4 mmol) was added and the mixture was allowed to warm up to room temperature and stirred for 12 h. The reaction was worked up with 1 M HCl (50 ml), extracted with dichloromethane (150 ml) and washed with water (3 x 50 ml). After drying over MgSO4 and evaporation in vacuum, the crude product was purified by column chromatography on silica gel using petroleum ether:dichloromethane (5:95). The product was isolated in 25% yield. (addition of TMEDA (0.63 ml, 4.1 ml)).

OHOS

�1H NMR (300 MHz, CDCl3�� 2.3 (s+s 6+3H) 2.4 (s 3H) 5.2 (s 6H) 5.4 (s 1H) 6.4 (d 1H) 6.7 (t 1H) 6.9 (t 1H) 7.1 (d 1H) 7.2 (m 3H) 7.3 (s 3H) 7.4 (s 1H) 7.5 (s 1H) 7.6 (s 3H) 7.7 (s 1H) 7.9 (s 1H) 8.1 (s 1H) 8.7 (s 1H); Mass spectrum (CI) (MH+): m/z not detected.


235

6\QWKHVLV�RI��WHWUDPHWK\O��WKLHQ��\O�SHQWDFHQH��A suspension of product �� (0.16 g, 0.36 mmol), NaI (0.75 g, 5.0 mmol) and NaH2PO2 (0.75 g) in acetic acid (50 ml) was heated at reflux for 3 h. The product �� was isolated by filtration and washed with water (3 x 75 ml), methanol (2 x 50 ml) diethyl ether (2x25 ml). After drying in vacuum, 2,3,9,10-tetramethyl-7-(thien-2-yl)pentacene was obtained in 33% yield.

S

Melting point: 300 °C (VXEOLPDWLRQ); 1H NMR (300 MHz, CDCl3�� 2.3 (s 3H) 2.4 (s 3H) 2.5 (s 6H) 6.1 (m 1H) 7.2 (m 1H) 7.3 (m 1H) 7.6 (s 3H) 7.7 (s 2H) 8.1 (m 2H) 8.3 (s 1H) 8.5 (s 1H); Mass spectrum (CI) (MH+): m/z 417.

6\QWKHVLV�RI��GLK\GUR��WKLHQ��\O�SHQWDFHQ��RQH��

A suspension of product �� (0.16 g, 0.36 mmol), NaI (0.75 g, 5.0 mmol) and NaH2PO2 (0.75 g) in acetic acid (50 ml) was heated at reflux for 3 h. The product �� was isolated by filtration and washed with water (3 x 75 ml), methanol (2 x 50 ml) diethyl ether (2x25 ml). After drying in vacuum, 2,3,9,10-tetramethyl-7-(thien-2-yl)pentacene was obtained.�

OS

1H NMR (300 MHz, CDCl3)� 2.3 (m 12H) 4.4 (s 2H) 7.2 (m 3H) 7.3 (s 1H) 7.6 (d 2H) 7.9 (s 1H) 8.0 (s 1H) 8.3 (s 1H) 8.9 (s 1H); Mass spectrum (CI) (MH+): m/z 433. �


236

��3URFHGXUHV�IRU�FRPSRXQGV�RI�FKDSWHU��6\QWKHVLV�RI��ELV��GLPHWK\OEHQ]R\O�EHQ]HQHGL�PHWK\O��EHQ]RDWH��D

Thionyl chloride (0.26 ml, 3.6 mmol) was added dropwise to a solution of bis(3,4-dimethylbenzoyl)benzenedicarboxylic acid ��D�(0.50 g, 1.2 mmol) in methanol (50 ml). The reaction mixture was heated at reflux for 5 h. The solvent was evaporated in vacuum, the residue was dissolved in diethyl ether (50 ml) and washed with saturated NaHCO3 (50 ml). The organic phase was dried over MgSO4 and evaporated in vacuum. The pure product ��D was obtained in a yield of 46%.

CO2MeMeO2C

O O

Melting point: 126 °C; 1H NMR (300 MHz, CDCl3�� 3 (s+s 6+6H) 3.7 (s 6H) 7.1 (d 2H) 7.2 (s 1H) 7.3 (d 2H) 7.4 (s 2H) 8.7 (s 1H); 13C NMR (75 MHz, CDCl3�� 9 20.2 52.8 127.1 127.5 130.1 130.4 132.3 134.2 137.3 143.7 145.7 165.2 195.4; Mass spectrum (CI) (MH+): m/z 459.

6\QWKHVLV�RI��WHWUD��GLPHWK\OSKHQ\O�ELV��IXUDQRQH��D

Bis-(3,4-dimethylbenzoyl)benzenedicarboxylic acid ��D� (2.0 g, 4.6 mmol) was heated at reflux for 3h in thionyl chloride (20 ml). Subsequently the thionyl chloride was evaporated in vacuum. Aluminium trichloride (4.3 g, 32 mmol) and o-xylene (2.3 ml, 18 mmol) was added to the residue dissolved in dichloromethane (50 ml). The reaction mixture was heated at reflux for 12 h. The mixture was poured into a mixture of ice:HCl (24 g: 20 ml) and stirred for 1 h. After extraction with dichloromethane (50 ml), the organic phase was washed with water (3x100 ml), dried over MgSO4 and evaporated in vacuum. The crude product was purified by column chromatography


237

on silica gel using dichloromethane (gradient). The product was isolated in 53% yield.�

OO

OO

Melting point: 228 °C; 1H NMR (300 0+]��'062�� V�12H) 2.2 (s 12H) 7.0 (m 8H) 7.1 (d 4H) 7.7 (s 1H) 8.4 (s 1H); 13C NMR (75 0+]��'062�� 20.0 91.9 120.7 124.6 124.8 127.5 128.2 129.9 137.1 137.6 157.8 167.9; Mass spectrum (MH+): m/z 607.

Experimental Part Part III

238

3DUW�,,,��7UDQVLVWRUV�RI�VXEVWLWXWHG�SHQWDFHQHV�2QO\� WKH�ZRUNLQJ� FRQGLWLRQV� DUH� GHVFULEHG�� 7KH� VDPSOHV� ZHUH�VSLQFRDWHG� LQ� D� QLWURJHQ� JORYH� ER[�� $OVR� WKH� GU\LQJ� RQ� D�KRWSODWH� DQG� WKH�PHDVXUHPHQWV�ZHUH� FDUULHG� RXW� LQ� D� QLWURJHQ�JORYH� ER[�� 7KH� VXEVWLWXWHG� SHQWDFHQHV� DQG� WKH� VDPSOHV� ZHUH�VWRUHG� LQ�D�QLWURJHQ�FXSERDUG��7KH�VROXWLRQV�ZHUH�SUHSDUHG� LQ�GDUN�ERWWOHV�VFUHZ�KHDG�WR�SUHYHQW�UDSLG�GHJUDGDWLRQ� ��3URFHGXUHV�RI�GLVXEVWLWXWHG�SHQWDFHQHV��

&RQGLWLRQV�RI�H[SHULPHQWV�FRQFHUQLQJ�UHSURGXFLELOLW\��7DEOH�,,,��

A solution of pentacene ��F (20 mg) and polystyrene (5 mg, Mw: 1.000.000) in toluene (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol and cleaned with UV/ozone. The solution was spun at a rate of 2000 rpm for 70 s and dried on a hotplate at 90 °C for 2 min.

&RQGLWLRQV�RI�H[SHULPHQWV�FRQFHUQLQJ�UHSURGXFLELOLW\��7DEOH�,,,��A solution of pentacene ��F (25 mg) and polystyrene (5 mg, Mw: 1.000.000) in toluene (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol and cleaned with UV/ozone. The solution was spun at a rate of 2000 rpm for 70 s and dried on a hotplate at 90 °C for 2 min.


239

&RQGLWLRQV�RI�H[SHULPHQWV�FRQFHUQLQJ�WKH�VROYHQW��7DEOH�,,,��A solution of pentacene ��D (15 mg) and polystyrene (5 mg, Mw: 1.000.000) in toluene (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol, cleaned with UV/ozone and pretreated with OTS in vacuum for 30 min at 140 °C. The solution was spun at a rate of 2000 rpm for 70 s and dried in nitrogen for 12 h.

A solution of pentacene ��D (25 mg) and polystyrene (5 mg, Mw: 1.000.000) in THF (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol, cleaned with UV/ozone and pretreated with OTS in vacuum for 30 min at 140 °C. The solution was spun at a rate of 2000 rpm for 40 s and dried at 50 °C for 2 min.

A solution of pentacene ��E (15 mg) and polystyrene (5 mg, Mw: 1.000.000) in toluene (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol and cleaned with UV/ozone. The solution was spun at a rate of 2000 rpm for 70 s and dried in nitrogen for 12 h.

A solution of pentacene ��F (17 mg) and polystyrene (3 mg, Mw: 1.000.000) in toluene (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol, cleaned with UV/ozone and pretreated with OTS in vacuum for 30 min at 140 °C. The solution was spun at a rate of 2000 rpm for 70 s and dried at 120 °C for 2 min.

A solution of pentacene ��I (15 mg) and polystyrene (5 mg, Mw: 1.000.000) in THF (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The


240

substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol and cleaned with UV/ozone. The solution was spun at a rate of 2000 rpm for 40 s and dried at 50 °C for 2 min.

A solution of pentacene ��J (15 mg) and polystyrene (5 mg, Mw: 1.000.000) in THF (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol and cleaned with UV/ozone. The solution was spun at a rate of 2000 rpm for 40 s and dried at 50 °C for 2 min.

&RQGLWLRQV�RI�H[SHULPHQWV�FRQFHUQLQJ�WKH�FRQFHQWUDWLRQ��7DEOH�,,,��A solution of pentacene ��D (8, 15, 25 mg) and polystyrene (5 mg, Mw: 1.000.000) in THF (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol and cleaned with UV/ozone. The solution was spun at a rate of 2000 rpm for 20 s and dried at 50 °C for 2 min.

A solution of pentacene ��F (15, 25, 35 mg) and polystyrene (5 mg, Mw: 1.000.000) in toluene (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol and cleaned with UV/ozone. The solution was spun at a rate of 2000 rpm for 70 s and dried at 120 °C for 2 min.

&RQGLWLRQV�RI�H[SHULPHQWV�FRQFHUQLQJ�WKH�FRQFHQWUDWLRQ��7DEOH�,,,��A solution of pentacene ��D (8 mg) and polystyrene (1.5, 5, 7, 10 mg, Mw: 1.000.000) in THF (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom


241

configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol and cleaned with UV/ozone. The solution was spun at a rate of 2000 rpm for 20 s and dried at 50 °C for 2 min.

A solution of pentacene ��F (20 mg) and polystyrene (3, 5, 7, 10 mg, Mw: 1.000.000) in toluene (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol and cleaned with UV/ozone. The solution was spun at a rate of 2000 rpm for 70 s and dried at 90 °C for 2 min.

&RQGLWLRQV�RI�H[SHULPHQWV�FRQFHUQLQJ�WKH�VXUIDFH��7DEOH�,,,��A solution of pentacene ��D (15 mg) and polystyrene (5 mg, Mw: 1.000.000) in toluene (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol, cleaned with just UV/ozone and cleaned with UV/ozone and pretreated with OTS in vacuum for 30 min at 140 °C. The solution was spun at a rate of 2000 rpm for 70 s and dried at room temperature under nitrogen for 12h.

A solution of pentacene ��E (15 mg) and polystyrene (5 mg, Mw: 1.000.000) in toluene (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol, cleaned with UV/ozone and cleaned with just UV/ozone and pretreated with OTS in vacuum for 30 min at 140 °C. The solution was spun at a rate of 2000 rpm for 70 s and dried at room temperature under nitrogen for 12h.

A solution of pentacene ��F (35 mg) and polystyrene (5 mg, Mw: 1.000.000) in toluene (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with


242

water, acetone and isopropanol, cleaned with UV/ozone and cleaned with just UV/ozone and pretreated with OTS in vacuum for 30 min at 140 °C. The solution was spun at a rate of 2000 rpm for 70 s and dried at 120 °C for 2 min.

&RQGLWLRQV�RI�H[SHULPHQWV�FRQFHUQLQJ�WKH�VXUIDFH��7DEOH�,,,��A solution of pentacene ��D (25 mg) and polystyrene (5 mg, Mw: 1.000.000) in THF (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol, cleaned with just UV/ozone and cleaned with just UV/ozone and pretreated with OTS in vacuum for 30 min at 140 °C. The solution was spun at a rate of 2000 rpm for 40 s and dried at 50 °C for 2 min.

A solution of pentacene ��I (15 mg) and polystyrene (5 mg, Mw: 1.000.000) in THF (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol, cleaned with just UV/ozone and cleaned with just UV/ozone and pretreated with OTS in vacuum for 30 min at 140 °C. The solution was spun at a rate of 2000 rpm for 40 s and dried at 50 °C for 2 min.

A solution of pentacene ��J (15 mg) and polystyrene (5 mg, Mw: 1.000.000) in THF (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol, cleaned with just UV/ozone and cleaned with just UV/ozone and pretreated with OTS in vacuum for 30 min at 140 °C. The solution was spun at a rate of 2000 rpm for 40 s and dried at 50 °C for 2 min.


243

&RQGLWLRQV�RI�H[SHULPHQWV�FRQFHUQLQJ�WKH�VSLQFRQGLWLRQV� 7DEOH�,,,��A solution of pentacene ��D (25 mg) and polystyrene (5 mg, Mw: 1.000.000) in THF (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol and cleaned with UV/ozone. The solution was spun at a rate of 2000 rpm for 20, 40 and 70 s and dried at 50 °C for 2 min. �&RQGLWLRQV�RI�H[SHULPHQWV�FRQFHUQLQJ�WKH�VSLQFRQGLWLRQV��7DEOH�,,,��A solution of pentacene ��F (20 mg) and polystyrene (7 mg, Mw: 1.000.000) in toluene (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol and cleaned with UV/ozone. The solution was spun at a rate of 1000, 2000 and 3000 rpm for 70 s and dried at 90 °C for 2 min.

A solution of pentacene ��J (15 mg) and polystyrene (5 mg, Mw: 1.000.000) in THF (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol and cleaned with UV/ozone. The solution was spun at a rate of 1000, 2000 and 3000 rpm for 40 s and dried at 50 °C for 2 min.

&RQGLWLRQV�RI�H[SHULPHQWV�FRQFHUQLQJ�WKH�DIWHU�WUHDWPHQW��7DEOH�,,,��A solution of pentacene ��D (15 mg) and polystyrene (5 mg, Mw: 1.000.000) in toluene (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol, cleaned with UV/ozone and pretreated


244

with OTS in vacuum for 30 min at 140 °C. The solution was spun at a rate of 2000 rpm for 70 s and dried at 120 °C for 2 min, room temperature under nitrogen for 12h.

A solution of pentacene ��E (15 mg) and polystyrene (5 mg, Mw: 1.000.000) in toluene (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol and cleaned with UV/ozone. The solution was spun at a rate of 2000 rpm for 70 s and dried at 120 °C for 2 min, room temperature under nitrogen for 12h.

A solution of pentacene ��F (17 mg) and polystyrene (3 mg, Mw: 1.000.000) in toluene (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol and cleaned with UV/ozone. The solution was spun at a rate of 2000 rpm for 70 s and dried at 120 °C for 2 min, room temperature under nitrogen for 12h and room temperature in vacuum for 45 min.


245

��3URFHGXUHV�RI�WHWUDVXEVWLWXWHG�SHQWDFHQHV��&RQGLWLRQV�RI�H[SHULPHQWV�FRQFHUQLQJ�VROYHQW��7DEOH�,,,��A solution of pentacene ��D (26 mg) and polystyrene (4 mg, Mw: 1.000.000) in toluene (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol and cleaned with UV/ozone. The solution was spun at a rate of 2000 rpm for 70 s and dried at 120 °C for 2 min.

A solution of pentacene ��F (26 mg batch 2) and polystyrene (13 mg, Mw: 1.000.000) in toluene (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol and cleaned with UV/ozone. The solution was spun at a rate of 2000 rpm for 70 s and dried at 120 °C for 2 min.

A solution of pentacene ��G (26 mg) and polystyrene (13 mg, Mw: 1.000.000) in toluene (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol and cleaned with UV/ozone. The solution was spun at a rate of 2000 rpm for 70 s and dried at 90 °C for 2 min.

A solution of pentacene ��I (26 mg) and polystyrene (13 mg, Mw: 1.000.000) in toluene (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol and cleaned with UV/ozone. The solution was spun at a rate of 2000 rpm for 70 s and dried at 90 °C for 2 min.


246

&RQGLWLRQV�RI�H[SHULPHQWV�FRQFHUQLQJ�FRQFHQWUDWLRQ�� 7DEOH�,,,��

A solution of pentacene ��F (15, 26, 35 mg batch 2) and polystyrene (7 mg, Mw: 1.000.000) in toluene (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol and cleaned with UV/ozone. The solution was spun at a rate of 2000 rpm for 70 s and dried at 120 °C for 2 min.

&RQGLWLRQV�RI�H[SHULPHQWV�FRQFHUQLQJ�FRQFHQWUDWLRQ��7DEOH�,,,��A solution of pentacene ��F (26 mg batch 2) and polystyrene (4, 7, 13, 26 mg, Mw: 1.000.000) in toluene (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol and cleaned with UV/ozone. The solution was spun at a rate of 2000 rpm for 70 s and dried at 120 °C for 2 min.

&RQGLWLRQV�RI�H[SHULPHQWV�FRQFHUQLQJ�WKH�VXUIDFH��7DEOH�,,,��

A solution of pentacene ��D (26 mg) and polystyrene (4 mg, Mw: 1.000.000) in toluene (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol and cleaned with UV/ozone and cleaned with just UV/ozone and pretreated with OTS in vacuum for 30 min at 140 °C. The solution was spun at a rate of 2000 rpm for 70 s and dried at 120 °C for 2 min.

A solution of pentacene ��F (26 mg) and polystyrene (4 mg, Mw: 1.000.000) in toluene (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration.


247

The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol and cleaned with UV/ozone and cleaned with just UV/ozone and pretreated with OTS in vacuum for 30 min at 140 °C. The solution was spun at a rate of 2000 rpm for 70 s and dried at 120 °C.

&RQGLWLRQV�RI�H[SHULPHQWV�FRQFHUQLQJ�VSLQFRQGLWLRQV��7DEOH�,,,��

A solution of pentacene ��F (26 mg) and polystyrene (4 mg, Mw: 1.000.000) in toluene (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol and cleaned with UV/ozone. The solution was spun at a rate of 2000 rpm for 70, 40 and 20 s and dried at room temperature in nitrogen atmosphere for 12 h.

&RQGLWLRQV�RI�H[SHULPHQWV�FRQFHUQLQJ�VSLQFRQGLWLRQV��7DEOH�,,,��A solution of pentacene ��F (26 mg batch 2) and polystyrene (4 mg, Mw: 1.000.000) in toluene (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol and cleaned with UV/ozone. The solution was spun at a rate of 1000, 2000 and 4000 rpm for 70 s and dried at 120 °C for 2 min.

&RQGLWLRQV�RI�H[SHULPHQWV�FRQFHUQLQJ�DIWHU�WUHDWPHQW��7DEOH�,,,��A solution of pentacene ��F (26 mg) and polystyrene (4 mg, Mw: 1.000.000) in toluene (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol and cleaned with UV/ozone. The solution was spun at a rate of 2000 rpm for 70 s and dried at 120 °C


248

for 2 min, 90 °C for 2 min and room temperature in nitrogen atmosphere for 12 h. Samples were remeasured after a month.

&RQGLWLRQV�RI�H[SHULPHQWV�FRQFHUQLQJ�FRQWDFW�UHVLVWDQFH��7DEOH�,,,��A solution of pentacene ��F (26 mg) and polystyrene (4 mg, Mw: 1.000.000) in toluene (1 g) was put in an ultra-sonic bath for 15 min and afterwards spincoated on a substrate with a bottom configuration. The substrate, consisting of SiO2 with Au-contacts, was washed with water, acetone and isopropanol and cleaned with UV/ozone. The solution was spun at a rate of 2000 rpm for 70 s and dried at 120 °C for 2 min.

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