Glossary

AFM

Atomic Force Microscopy BC

Organic Semiconductors

In the 1950s, the concept of "organic semiconductor" was introduced to describe π-conjugated organic materials possessing the ability to transport charges. Since then, organic semiconductors have been the focus of numerous investigations in several fields, from chemistry to physics, opening simultaneously many new opportunities and potential industrial applications [START_REF] Fincher | Electrical Conductivity in Doped Polyacetylene[END_REF][START_REF] Forrest | Introduction: Organic Electronics and Optoelectronics[END_REF][START_REF] Pope | Electronic processes in organic crystals and polymers[END_REF] .

In addition and since the discovery of the conductivity property of polyacetylene by A. J. Heeger, A.G. MacDiamid and H. Shirakawa in 1977, more and more attention has been attracted in the past 25 years and very impressive improvements in performance were achieved [START_REF] Mishra | Small Molecule Organic Semiconductors on the Move: Promises for Future Solar Energy Technology[END_REF] based on these materials in a wide variety of domains (Organic Light-Emitting Diodes (OLED), Organic Field-Effect Transistors (OFET), Organic Photovoltaics (OPV), Liquid Crystals (LC), sensors, etc.).

π-conjugated systems

The basic electronic configuration of a carbon atom are (1s) 2 (2s) 2 (2p) [START_REF] Forrest | Introduction: Organic Electronics and Optoelectronics[END_REF] . In order to form covalent bonds with other atoms, these orbitals can be combined (or hybridized) into new orbitals (called "hybrid orbitals").

When only a hybridization of orbitals into σ bonds is observed (sp [START_REF] Pope | Electronic processes in organic crystals and polymers[END_REF] ), the whole molecule is insulating as all the electrons are fixed in every bond (no charge transport carriers) (Figure 1.1.a). On the contrary, in the case of a double bond, the 2s orbital and two of the three 2p orbitals (2p x , 2p y ) hybridize into three sp 2 orbitals in a plane with an angle of 120°C to each other and the 2p z orbital is perpendicular to this plane (Figure 1.1.b). Thus, when two carbon atoms approach to each other, two of the sp 2 orbitals overlap to form a σ bond and the two 2p z orbitals overlap as well to generate this time a bonding molecular orbital π of lower energy and an antibonding molecular orbital π* of higher energy (Figure 1.1.c). In this configuration, the π electrons cannot be attributed to a single atom: we can speak about conjugation. By increasing the number of hybridized carbon atoms in a molecule (Figure 1.2), the number of molecular orbitals will increase and the energy gap between each π (or π*) energy levels will decrease.

Then the energy level of bonding orbitals (antibonding orbitals) become so close one to the other that they are no more distinguishable but form the so called conduction band and valence band.

a) b) c)

In this context, the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) play a special role as they are associated respectively to the ionization potential and electronic affinity of the compound, so they can characterize the materials ability to give and/or accept electrons. The energy difference between HOMO and LUMO can also be called as the forbidden band (gap) as in the field of inorganic semi-conductors.

Figure 1.2 Diagram of molecular orbitals

As a consequence, we can now distinguish electron-donor and electron acceptor molecules. In fact, the oxidative process of an electron-donor molecule results in the formation of a hole in the HOMO level after forming a radical cation; on the contrary, the reductive process takes place when an electron is gained in the LUMO level via the formation of a radical anion.

Furthermore, the electron in the LUMO level of a radical anion can be transferred to an adjacent neutral molecule and an adjacent neutral molecule can transfer its electron to the hole in the HOMO level of a radical cation. This is the way a charge transfer (conductivity) can happen and be observed in such π-conjugated materials.

π-conjugated small (single) molecules, oligomers and polymers

In this context, there are several well known examples: anthracene, pentacene, rubrene, etc. as single or small molecules [START_REF] Murphy | Organic Semiconducting Oligomers for Use in Thin Film Transistors[END_REF] ; oligothiophene, benzothiophene, thienothiophene, etc as oligomers [START_REF] Mishra | Functional Oligothiophene-Based Materials: Nanoarchitectures and Applications[END_REF] and polypyrrole, polyacetylene, poly(3-hexylthiophene) (P3HT), or poly(p-phenylene vinylene) (PPV) as polymers [START_REF] Mishra | Small Molecule Organic Semiconductors on the Move: Promises for Future Solar Energy Technology[END_REF] (Figure 1.1). As a remark, the frontier between the definition of single molecules and the one of oligomers is often very vague; in this manuscript we consider the mono-dispersed single molecules or oligomers as "small molecules (with a well-defined molecular weight, even though the weight might be relatively high)", the multiple dispersed polymers being classified as "polymers" [START_REF] Forrest | The path to ubiquitous and low-cost organic electronic appliances on plastic[END_REF] .

Both small molecules and/or polymers present advantages and disadvantages.

On the one hand, small molecules offer well defined chemical structures and then can give very precise and reproducible properties. Moreover, they can be readily evaporated to form, for instance, very good defect free thin films and then show excellent electronic or optical properties. Nevertheless, the synthesis should be carried out step-by-step and can be costly as well as the evaporation process itself, even if in some cases small molecules can be easily processed in solution just as the polymers (after introducing solubilizing long alkyl chains for example). [START_REF] Mishra | Functional Oligothiophene-Based Materials: Nanoarchitectures and Applications[END_REF] On the other hand, polymers can be obtained by relatively much less costly polymerization reactions directly from the monomers, and their main advantage is their easy and cheap processability (spin coating, dip coating, etc.). But they present always a chain length distribution, thus obliging to several purification steps to remove or decrease the number of inevitable defects formed during the polymerization, defects which could at the end alter the quality of thin films, for instance.

This is why we can say that, between small molecules and polymers, between the evaporation and solution process, it is never easy to make a definitive choice: there are always different strategies to be taken into account.

Charge transport in organic semiconductors

It is complicated as well in terms of transport properties, as the performances of the organic semiconductors critically depend not only on the efficiency with which charge carriers (electrons and/or holes) move within the π-conjugated materials, but also the device configuration in which they will be integrated.

Indeed, in the case of OLED or OFET devices, the charge carriers are either injected from metal (Au, Ag, etc.) or conducting oxide electrodes (ITO); meanwhile, for OPV technology, the charge carriers are generated via photon-induced charge separation at the interface between electron-donor and electron-acceptor components.

In addition and as a remind, the most important classical inorganic semiconductor currently used in such devices belongs to the family of silicon based materials, which can be defined as a highly crystalline three-dimensional solid presenting highly delocalized bands, due to the fact that all the atoms are held together with strong covalent or ionic bonds. As a consequence, the charges transport can occur, coming from the strong interactions of overlapping atomic orbitals, and this process might be remarkably limited by defects, lattice vibrations, or phonon scattering in the solid [START_REF] Murphy | Organic Semiconducting Oligomers for Use in Thin Film Transistors[END_REF] . On the contrary, the organic semiconductors are composed by single (macro)molecules which are weakly bound together through van der Waals, hydrogen bonding and/or π-π interactions : this gives usually a much less ordered organization in bulk and in this condition, there are two possibilities for the charges to be transported. One way is along the conjugated backbone (intramolecular transport, mostly in the case of polymers); the other way is through the overlapped adjacent molecules π-orbitals (intermolecular transport, observed in both polymer and/or single molecule cases).

In other words, we must remember that in organic semiconductors, it is a hopping from one localized state to another localized state which occurs, and this can be considered as an electron transfer between a charged moiety and another adjacent neutral moiety [START_REF] Zaumseil | Electron and Ambipolar Transport in Organic Field-Effect Transistors[END_REF] . Of course, for some very ordered molecule single crystals, weak bands could also be observed [START_REF] Wurthner | Perylene bisimide dyes as versatile building blocks for functional supramolecular architectures[END_REF][START_REF] Horowitz | Growth and Characterization of Sexithiophene Single Crystals[END_REF] .

Applications of Organic Semiconductors

As we talked previously, several new "markets" were opened based on the development of these organic semiconductors, in particular in the field of Organic Light-Emitting Diodes (OLED), Organic Photovoltaics (OPV) and/or Organic Field-Effect Transistors (OFET).

1.1.4.1 Organic Light-Emitting Diode (OLED) [START_REF] Shirota | Charge Carrier Transporting Molecular Materials and Their Applications in Devices[END_REF] Electroluminescence (EL) from organic devices was first reported using single crystals of anthracene in the 1960s [START_REF] Pope | Electroluminescence in Organic Crystals[END_REF] : however, the fabricated device required a high drive voltage of 400 V to obtain EL from this blue emitter. Later, the use of vacuum-deposited anthracene thin films led to a significant reduction of this drive voltage [START_REF] Vincett | Electrical conduction and low voltage blue electroluminescence in vacuum-deposited organic films[END_REF] . Then in 1987, a doublelayer OLED using thin films of 1,1-bis{4-[di(p-tolyl)amino]-n phenyl}cyclohexane (TAPC) as a hole-transporting material and tris (8quinolinolato)aluminum (Alq3) as an emitting material sandwiched between transparent indium tin oxide (ITO) and an alloy of magnesium and silver was reported to exhibit a luminance of over 1000 cd m -2 at a drive voltage of ca. 10 V [START_REF] Tang | Organic electroluminescent diodes[END_REF][START_REF] Tang | Electroluminescence of doped organic thin films[END_REF] . Subsequently, a single-layer OLED using a thin film of poly(pphenylene vinylene), ITO/polymer/Ca, was reported in 1990 [START_REF] Burroughes | Light-emitting diodes based on conjugated polymers[END_REF] . These two reports triggered extensive research and development of OLEDs from the standpoints of both fundamental science and potential technological applications for full-color, flat-panel displays and lightening. Furthermore, the recent finding of triplet emitters led to remarkable improvements in the EL quantum efficiency [START_REF] Baldo | High-efficiency fluorescent organic light-emitting devices using a phosphorescent sensitizer[END_REF][START_REF] Baldo | Very high-efficiency green organic light-emitting devices based on electrophosphorescence[END_REF][START_REF] Lamansky | Highly Phosphorescent Bis-Cyclometalated Iridium Complexes: Synthesis, Photophysical Characterization, and Use in Organic Light Emitting Diodes[END_REF] . Today, OLEDs are characterized by low drive voltage, high brightness, full-color emission, rapid response, and easy fabrication of large-area thin-film devices.

Concerning OLEDs functioning, they are current-driven devices that utilize emissions from the electronically excited states of molecules. More precisely, the operation of OLEDs based on an unique emitting layer involves charge injection from the anode and the cathode into the adjacent organic layers, transport of injected charge carriers through the organic layers, exothermic recombination of holes and electrons to generate electronically excited states of molecules (often called excitons), followed by their deactivation via the emission of either fluorescence or phosphorescence light taken out of the device as EL (Figure 1.3.a). which is defined as the number of photons emitted per number of injected charge carriers and the luminous power efficiency (Leff) [lm W -1 ] which is defined as the ratio of luminous flux to power.

a) b)

Moreover, when fluorescent emitters are employed, only 25% of the generated excitons are utilized.

On the contrary, when phosphorescent emitters are used, an internal quantum efficiency up to 100% can be achieved in principle since the phosphorescent emitters, which are usually doped in a host material, can capture both singlet and triplet excitons generated by the recombination of injected holes and electrons [START_REF] Baldo | High-efficiency fluorescent organic light-emitting devices using a phosphorescent sensitizer[END_REF][START_REF] Cleave | Harvesting Singlet and Triplet Energy in Polymer LEDs[END_REF] .

The performance of OLEDs, therefore, depends upon various materials and interfaces having specific roles such as charge-injection and charge-transporting, charge-blocking, and emission. More generally, materials involved in OLEDs should meet the following requirements: (a) suitable ionization potentials and electron affinities, that is to say well-matched energy levels for the injection of charge carriers from the electrodes or the organic layer into the adjacent organic layers; (b) Good ability to form smooth and uniform thin films without pinholes; (c) well controlled morphology and thermal stability. (d) In addition to these general requirements, materials should meet further specialized needs depending upon the roles that they play into the devices, as for example, hole transport, electron transport, charge blocking, and light emission.

Organic Photovoltaics (OPV)

Solar energy presents a great potential as a clean and inexhaustible new energy source. By definition, solar cells are devices where light energy is converted into electrical energy.

Such devices allowing photoelectric conversion and based on organic photoactive materials are mainly classified into two categories: photoelectrochemical cells and photovoltaic devices.

First, photoelectrochemical cells consist of two electrodes immersed in an electrolyte solution containing a redox couple. In this domain, Dye-Sensitized organic Solar Cells (DSSC) using nanocrystalline porous TiO 2 , on which an organic dye is adsorbed, and I 3 -/I -redox species in solution or gels, have been topic of recent intensive research and development because of the high conversion efficiencies reached (10%) [START_REF] O'regan | A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films[END_REF][START_REF] Nazeeruddin | Conversion of light to electricity by cis-X2bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) charge-transfer sensitizers (X = Cl-, Br-, I-, CN-, and SCN-) on nanocrystalline titanium dioxide electrodes[END_REF] .

Second, organic solid-state photovoltaic devices (OPVs) consist of thin films of organic photoactive materials sandwiched between two metal electrodes. Both Schottky-type and pnheterojunction cells have been studied [START_REF] Shirota | Charge Carrier Transporting Molecular Materials and Their Applications in Devices[END_REF][START_REF] Günes | Conjugated Polymer-Based Organic Solar Cells[END_REF] . Generally, higher quantum yields for the photogeneration of charge carriers are reached for pn-heterojunction cells compared to Schottky-type devices because of electron donor-acceptor interactions between the two kinds of organic semiconductors, that is, ptype and n-type organic semiconductors (p-type and n-type organic semiconductors generally meaning electron-donating and -accepting organic materials, respectively). The performance of OPVs is evaluated by comparing the power conversion efficiency (η) and fill factor (FF) parameters.

The mechanisms for OPVs functioning have been interpreted in terms of the energy band model applied for inorganic semiconductor PV devices, which may not be applicable to molecular organic solids with well-localized energy levels. The basic operation processes of a pn-heterojunction OPVs is for example described and divided in five steps (Figure 1.9): (a) light absorption by organic semiconductors to form excitons, (b) diffusion of excitons, (c) charge carrier generation and separation at the organic/organic interface, (d) charge transport through the organic layers, and (e) charge collection at both electrodes. Note that these processes are just the opposite to those of OLEDs, namely, (a) charge injection from the electrodes into the organic layers, (b) charge transport through the organic layers, (c) charge recombination to form excitons, (d) emission from excitons, and (e) light extraction out of devices.

1. 1.4.3 Organic Field-Effect Transistors (OFET) (As most of the new synthesized materials in this work were characterized by the OFET, this kind of application will be more detailed here in introduction.)

In 1930, the principle of the field-effect transistor (FET) was first proposed by Lilienfeld 24 .

However, the first OFETs were not described until 1987. Since their characteristics have undergone spectacular improvements during the last two decades [START_REF] Horowitz | Organic Field-Effect Transistors[END_REF] , both small organic molecules and conjugated polymers attracted the community of industrial and academic research groups interested in using organic semiconductors as the active layer in transistors [START_REF] Shirota | Charge Carrier Transporting Molecular Materials and Their Applications in Devices[END_REF][START_REF] Facchetti | Electroactive Oligothiophenes and Polythiophenes for Organic Field Effect Transistors[END_REF] .

In fact, an OFET acts essentially as an electronic valve by modulating the semiconductor channel conductance via the gate field. Moreover, transistors are nowadays essential for all electronic applications, including integrated circuits for memories and sensors and also to drive individual pixels in active matrix displays. In particular, one of the most exciting applications of organic electronic circuits is probably in the supply chain area, where radiofrequency-powered elements (e.g. RFID tag) may replace 1D barcodes for identification and be applicable as a backplane drive for displays. Looking into details, the OFET consists of a gate electrode, a dielectric insulating layer and an organic semiconducting material sandwiched between the source and drain electrodes. Note that the organic semiconductor used in OFETs should not be intentionally doped.

Consequently, the semiconductor carrier concentration is very low, usually <10 14 cm -3 . When a voltage is applied between the source and drain electrodes (V SD ) and the gate field is zero (source-gate voltage = 0 V), the source-drain current is very low and the device is "off". When the gate field is established by the application of a negative or positive source-gate bias (V SG ), accumulation of charges at the interface between the gate dielectric and the semiconductor (carrier concentration >10 20 cm -3 ) occurs. Thus, upon application of the positive or negative source-gate bias, electrons or holes, respectively, are accumulated at the interface between the semiconductor and the dielectric and the source-drain current (I SD ) increases (Figure 1.5). This is called the 'on' state of the transistor.

The I on :I off ratio is also a useful measure of purity, because high off-currents are indicative of considerable extrinsic doping of the semiconductor. As a consequence, field effect mobilities greater than 0.1 cm 2 V -1 s -1 and I on :I off > 10 6 are needed [START_REF] Facchetti | Gate Dielectrics for Organic Field-Effect Transistors: New Opportunities for Organic Electronics[END_REF] to be useful in optoelectronic devices such as active matrix displays that require sharp turn-on and fast switching.

Note also that organic FETs normally operate in the accumulation mode, hence, as described above, the increase in magnitude of V SG enhances channel conductance, in contrast to conventional Si and GaAs transistors. When an organic semiconductor-based FET is active upon the application of negative V SG and V SD , hence accumulation of holes occurs, the organic material is said to be p-channel (or p-type) since holes are the majority charge carriers. On the other hand, when a (positive) sourcedrain current is observed upon the application of positive V SG and V SD , accumulation of electrons occurs, and the semiconductor is n-channel (or n-type) since electrons are mobile (Figure 1.5). In a few cases, OFETs operate for both V SG and V SD polarities and the semiconductor is said to be ambipolar.

To end, it must be emphasized that the fundamental difference between n-/p-organic and -inorganic semiconductors is the following: the latter are based on the gate voltage sign at which they are active whereas the former are based on the mobile carrier type due to the (chemical) doping process.

Therefore, it is important to stress that the categorization of an organic semiconductor as 'p' or 'n'channel (or type) is not an absolute meaning but is strongly related to the FET device structurematerial combination on which the transport characteristics are measured.

This is why very rapidly, we thought interesting to present below different types of OFETs (Figure 1.6), insisting on each type of device configuration with their own fabrication issues, advantages and disadvantages, due to the fact that we tested as well some of them with our materials. In the top-contact/bottom-gate geometry (Figure 1.6 B), the organic semiconductor is deposited on top of the gate dielectric layer and then the source and drain electrodes are deposited on top by either metal evaporation through a shadow mask or by printing. In this device structure, the contact resistance is usually low due to intimate contact between the semiconductor and the source/drain electrodes and the charge carrier mobilities are usually the highest among different FET structures.

However, in this type of structure, small channel lengths are difficult to achieve by both metal thermal evaporation and printing (usually L>>5 μm channels) and solution-processed metal ink/polymeric contacts may damage the semiconductor.

In the bottom-contact/bottom-gate devices (Figure 1.6 A), the source and drain electrodes are first defined on the insulator with photolithography (allowing very small channel lengths) or other deposition processes and only at the end occurs the organic layer deposition. Bottom-contact devices typically exhibit greater contact resistance than top-contact devices and high carrier mobility and typical I -V plots are usually obtained only upon source/drain contact treatment with thiol-based molecules to form a self-assembled monolayer. However, bottom-contact devices may be more easily integrated into low-cost manufacturing processes. Furthermore, smaller device features can be obtained through photolithographic techniques to enhance circuit speed.

Other technologically important OFET structures are the "top-gate" (Figure 1.6 C and D) configurations, where the dielectric and then the gate contact are deposited on top of the organic semiconductor. In the most common device structure, heavily doped silicon is used as both the substrate and the (bottom-) gate electrode; thermally grown SiO 2 with thicknesses of ~100-300 nm act as the dielectric and 30-50 nm thick Au is used as source/drain (top-) contact material. While these silicon-SiO 2 -Au-based devices are extremely important for comparing results among different research groups synthesizing new organic semiconductors, the use of plastic substrates, polymeric insulators and conductive inks as the source and drain electrodes must be developed to allow lowcost/flexible electronic applications.

Charge carrier mobility characterization methods

We precedently presented quickly the different popular devices incorporating organic semiconductors, but in any case the intrinsic charge transport property of the materials must be determined, one key parameter characterizing the materials involved being more precisely the charge mobility µ.

In fact, without any external potential, the transport is merely a diffusive process and can be described as:

〈 〉

Here, <x 2 > means the mean-square displacement of the charges, D is the diffusion coefficient, t is the time, and n represents an integer number equal to 2, 4, or 6 for one-,two-, and three-dimensional (1D, 2D, and 3D) systems, respectively.

Anyway, the charge mobility µ is related to the diffusion coefficient via the Einstein-Smoluchowski equation:

where k B is the Boltzmann constant and e is the electron charge.

When an external electric field is applied, the charge carriers will start to drift; now, the mobility can be described as well as the ratio between the velocity (ν) of the charges and the amplitude of the applied electric field, F: the unit of the carrier mobility is then expressed in cm 2 /Vs.

ν/F

Importantly, diffusion is the local displacement of the charge around an average position, while drift induces a displacement of the average position. This is the reason why drift is more representative to determine the migration of the charges through the organic semiconductors.

There are several methods to determine experimentally the charge mobility in organic semiconductors [START_REF] Karl | Charge carrier transport in organic semiconductors[END_REF][START_REF] Coropceanu | Charge Transport in Organic Semiconductors[END_REF][START_REF] Horowitz | Charge Transport in Semiconducting Oligothiophenes[END_REF] . Among these methods, Time-of-Flight (TOF), Organic Field-Effect Transistor (OFET), Space-Charge Limited Current (SCLC or "Diode") and Pulse-Radiolysis Time-Resolved

Microwave Conductivity (PR-TRMC) are the most wildly used ones. They are presented below.

1.1.5.1 Time-of-Flight (TOF)

The first charge mobility measurement on organic semiconductors was realized by Kepler [START_REF] Kepler | Charge Carrier Production and Mobility in Anthracene Crystals[END_REF] and

Leblanc [START_REF] Leblanc | Hole and Electron Drift Mobilities in Anthracene[END_REF] with a TOF setup, a very suitable technique for measuring the transport properties of organic semiconductors presenting low mobilities [START_REF] Horowitz | Charge Transport in Semiconducting Oligothiophenes[END_REF] . Here, the sample consists of an organic film or crystal sandwiched between two conducting electrodes. The material is first irradiated by a short laser pulse in the immediate vicinity of one electrode to generate hole-electron pairs. For the above purpose, this electrode is most often constituted by a transparent conductor, such as indium doped tin oxide (ITO), but semitransparent metal electrodes are also often used. Driven by the polarity of the applied bias and the corresponding electric field (in the 10 4 -10 6 V/cm range), the photogenerated charges migrate through the material toward the second electrode (see Figure 1.20 Left). This charge transport gives a current recorded in the external circuit. This current is constant, and then falls down to null at the time t, at which the charge sheet arrives at the rear electrode (see Figure 1.20 Right). The transit time is related to the mobility through:

Where, L is the distance between the electrodes, F is the electric field in the organic layer, and V is the external voltage across the sample.

In ideal conditions, the signal in the Figure 1.20 right shows a step shape, where the falloff of the current corresponds to the arrival of the charged sheet. But in practical cases, the charge transport is much more complicated from the front electrode to the rear one, knowing that diffusion and trapping are two important features which take place in the TOF experiments: this is the reason why this technique requires working with highly pure and flawless samples.

1.1.5.2 Space-Charge-Limited Current (SCLC, also referred as "Diode Configuration") [START_REF] Coropceanu | Charge Transport in Organic Semiconductors[END_REF][START_REF] Horowitz | Charge Transport in Semiconducting Oligothiophenes[END_REF][START_REF] Blom | Electron and hole transport in poly(p-phenylene vinylene) devices[END_REF] SCLC will happen in the materials in which free carrier concentrations are very low. When the tested materials are put into intimate contact with a metal electrode (which means the transport is bulk limited and not contact limited), the metal can be considered as a reservoir of free carriers and these free carriers could diffuse into the material if the Fermi level of the metal is close enough to the allowed levels of this materials. So this mechanism can also be called as "injection". With high work function metals, holes can be injected; on the contrary, using low work function metals, only electrons can be injected.

The experiment is carried on by building diodes where the samples (organic material layers) are sandwiched between two electrodes. The electrodes should be carefully chosen to be sure that only electrodes or holes can be injected at a low voltage. In ideal conditions (traps being negligible), the current density J evaluates quadratically with the applied bias V. In this situation, because of the electrostatic potential preventing the injection of additional charges, the number of injected charges will reach a maximum value, and at this value, the corresponding current is the SCLC:

9 8
with ε 0 ε r the permittivity of the polymer and L the device thickness. This formula becomes inaccurate at high electric fields, needing then to be corrected as it becomes a field-dependent function.

1.1.5.3 Pulse-Radiolysis Time-Resolved Microwave Conductivity (PR-TRMC) [START_REF] Coropceanu | Charge Transport in Organic Semiconductors[END_REF][START_REF] Schouten | Charge carrier dynamics in pulse-irradiated columnar aggregates of mesomorphic porphyrins and phthalocyannines[END_REF][START_REF] Schouten | Effect of accumulated radiation dose on pulse radiolysis conductivity transients in a mesomorphic octa-n-alkoxy-substituted phthalocyanine[END_REF][START_REF] Van De Craats | The Core-Size Effect on the Mobility of Charge in Discotic Liquid Crystalline Materials[END_REF][START_REF] Van De Craats | The mobility of charge carriers in all four phases of the columnar discotic material hexakis(hexylthio)triphenylene: Combined TOF and PR-TRMC results[END_REF] In the PR-TRMC method, a high energetic electrons pulse goes through the sample to create a low density of free carriers. The conductivity change Δσ induced by this pulse will be measured by the change in microwave power reflected by the sample and is therefore frequency dependent:

∆ Σ

Where Δµ is the sum of hole and electron mobilities and Ne-h is the generated electron-hole pair's density. Ne-h can be estimated by dividing the amount of energy density transferred to the material by the energy required to create one electron-hole pair; this ratio is further multiplied by a survival probability that accounts for possible charge recombination mechanisms during the duration of the pulse. In PR-TRMC, the pulse generates the charges in the bulk of the material. The electric property is obtained very locally (for example, it can be just part of a polymer chain) and the size of this "information zone" depends on the frequency of the microwave (the lower the frequency, the larger the region that is explored).

With these characteristics, the PR-TRMC method is not much limited by the material purity (no trapping problems), structure (orientation or deposition defects) and the contact. As the tested zone can be so small, this method even can be used to single polymer chain in solution.

The PR-TRMC gives results of the intrinsic AC mobility values of the material in bulk and these values can be taken as the upper limits at low fields. Normally, for a certain sample, the PR-TRMC result is always higher than that obtained by TOF, because the latter is measured with DC probes; it is a macroscopic range test and the carriers crossing the material can be strongly impeded by impurities and structure defects. However, the two methods sometimes can really give quite similar results, for example, in the test of some discotic liquid crystals based on hexathiohexyl triphenylenes (in which the PR-TRMC technique was evaluated) [START_REF] Warman | Charge Mobilities in Organic Semiconducting Materials Determined by Pulse-Radiolysis Time-Resolved Microwave Conductivity: π-Bond-Conjugated Polymers versus π-π-Stacked Discotics[END_REF] .

OFET

Different types of device configurations were described before (Paragraph 1.1.4.3).

To remind, upon application of the positive or negative source-gate bias, electrons or holes, respectively, are accumulated at the interface between the semiconductor and the dielectric and the source-drain current (I SD ) increases: this is called the "on" state of the transistor. The basic relationships describing the OFET drain currents are the followings 40 :

⁄ 2 ⁄ 2 ⁄
where μ FET is the field effect carrier mobility of the semiconductor (the average drift velocity per unit electric field), W the channel width, L the channel length, C i the capacitance per unit area of the dielectric and V T the threshold voltage [START_REF] Horowitz | Temperature and gate voltage dependence of hole mobility in polycrystalline oligothiophene thin film transistors[END_REF] . By increasing the magnitudes of V SD for a given V SG , a 'linear regime' (Equation 7) is initially observed at low source-drain voltages (V SD < V SG ) where the current increases linearly, followed by a regime where the current saturates ("saturation regime", Equation 8when the drain voltage exceeds the gate voltage (V SD > V SG )). Note that these equations assume that the field effect mobility of the material is constant. However, the field-effect mobility in organic semiconductors is usually dependent on the gate voltage, which suggests that a larger V SG leads to a higher density of free (or mobile) charge carrier at the dielectric-semiconductor interface, resulting in increased field effect mobility [START_REF] Katz | The Physical Chemistry of Organic Field-Effect Transistors[END_REF] .

The temperature dependence of the field effect mobility in some organic semiconductors has been examined as well with mixed results. It has been shown that for sexithiophene, the field effect mobility is thermally activated for temperatures T>25 K but becomes thermally independent for T<25 K 43 .

Many models have been suggested to explain this behavior, including the Multiple Trap and Thermal

Release (MTR) model and the polycrystalline model where individual grains are assumed to be trap free and all the traps are concentrated at the grain boundaries [START_REF] Dimitrakopoulos | Low-Voltage Organic Transistors on Plastic Comprising High-Dielectric Constant Gate Insulators[END_REF] . Another important device parameter is the current on-off ratio (I on :I off ), which is the ratio of the current flow between the source and drain when there is no gate bias and the current flow at maximum gate bias. However, this value is highly dependent on the operating voltages, the device geometry and the dielectric material. Therefore, this parameter provides a qualitative measure of semiconductor performance, but identical device structures and biases ranges must be used to compare the results from different semiconductors.

Ambipolar transport

Recently, an ambipolar charge transport (simultaneous or selective transport of electrons and/or holes) in an organic transistor was highly desirable because it would enable the design of circuits with low power dissipation and good noise margins, similar to those encountered in complementary metaloxide semiconductor (CMOS) logic circuits [START_REF] Anthopoulos | Ambipolar Organic Field-Effect Transistors Based on a Solution-Processed Methanofullerene[END_REF] . Moreover, for several important applications of organic semi-conductors in devices, it would be better that the ambipolar charge injection and transport could be achieved (for example, light-emitting field-effect transistors [START_REF] Rost | Ambipolar organic field-effect transistor based on an organic heterostructure[END_REF][START_REF] Lindner | Operation and properties of ambipolar organic heterostructure field-effect transistors[END_REF] ).

Nevertheless, organic low molecular weight materials and conjugated polymers are usually either electron or hole semi-conductors, or they are unintentionally n-or p-doped. Consequently, organic field-effect transistors (OFETs) based on a single material as active layer can only be operated as p-or as n-channel device. This is the reason why various alternative approaches (more complex OFET configurations compare to the "simple" ones) have been recently developed in order to achieve balanced electron and hole transport as well as efficient injection of both types of carriers, i.e. to obtain such ambipolar devices.

Bilayer approach

The first demonstrated ambipolar OFET was obtained by Dodabalapur and co-workers [START_REF] Dodabalapur | Organic Heterostructure Field-Effect Transistors[END_REF][START_REF] Dodabalapur | Organic field-effect bipolar transistors[END_REF] , who combined two layers of n-channel and p-channel semiconducting materials (here meaning semiconductors with high and low electron affinity, respectively) in the same device. In this case, although the mobilities of each n-or p-channel material are lower than its own at independent state, both electron and hole transport were observed in this single device.

As talked before also, it is important to choose the materials carefully according to the relative position of their HOMO and LUMO levels as well as the deposition order for achieving ambipolar characteristics. Indeed, for a long time the difficulty was to find the adequate n-type material, but as more and more n-channel materials were developed recently, the attentions are paid back on this approach [START_REF] Zaumseil | Electron and Ambipolar Transport in Organic Field-Effect Transistors[END_REF] .

In this field, all examples of bilayer ambipolar transistors use sublimed thin films. In other words and to the best of our knowledge, no ambipolar bilayer transistors based on solution-processed semiconductors have been demonstrated up to now. This seems mainly due to the problem of fabricating well-defined smooth bilayer structures by spinning one polymer on top of another: lamination or spontaneous vertical phase separation could also be big problems.

Multilayer approach

The Organic Light-Emitting field effect Transistor (OLET) (Figure 1.23) is another optoelectronic device having the structure of a thin-film transistor and the capability of light generation [START_REF] Muccini | A bright future for organic field-effect transistors[END_REF][START_REF] Sirringhaus | Integrated Optoelectronic Devices Based on Conjugated Polymers[END_REF] .

Bright/multicolor OLETs may allow electroluminescent display fabrication with simpler driving circuits. Furthermore, the most advanced OLETs possess a huge technological potential for the realization of intense nanoscale light sources and highly integrated optoelectronic systems, including the long-researched electrically pumped organic laser. However, in the traditional single-layer devices, the charge carrier accumulation and the exciton formation zones largely coincide, which leads to severe excitons charge quenching [START_REF] Raffaella Capelli | Organic light-emitting transistors with an efficiency that outperforms the equivalent light-emitting diodes[END_REF] .

Multilayered structures offer the opportunity to individually optimize the different materials parameters in a single device. In this way, the bilayer approach could be modified and controlled by playing precisely the materials design. Although the bilayer and multilayer approaches can yield some impressive device characteristics and interesting insight into the electronic properties of organic semiconductor heterointerfaces, its main disadvantage remains the need to deposit two or several layers on top of each other. Furthermore, cheaper solution processable options for bilayers are challenging due to the need for finding an orthogonal solvent for the deposition of the second layer. An alternative method is consequently to use blends of n-and p-channel materials and thus realize ambipolar transport in a single layer. For the blend approach, both coevaporated and solution-processed films are feasible [START_REF] Zaumseil | Electron and Ambipolar Transport in Organic Field-Effect Transistors[END_REF] .

For the coevaporation way, Rost et al. showed that coevaporating PTCDI-C 13 H 27 and αquinquethiophene with equal fractions results in good ambipolar characteristics (Figure 14). Here again, the mobilities obtained are smaller than those for the pure materials [START_REF] Rost | Ambipolar light-emitting organic field-effect transistor[END_REF] . This can be understood in terms of interpenetrating networks of n-channel and p-channel materials, which predominantly conduct one or the other carrier due to their different electron affinities and ionization potentials: thus, the effective mobilities of both electrons and holes in a percolating network are lower compared to that of a neat film. In this configuration, by turning the fraction of n-and p channel material, the electron and hole mobilities can even be adjusted [START_REF] Loi | Tuning Optoelectronic Properties of Ambipolar Organic Light-Emitting Transistors Using a Bulk-Heterojunction Approach[END_REF] .

For the solution processable way, thin films of polymer blends can be easily obtained and are thus well suited for application in integrated circuits. Their microstructure can be tuned by the choice of solvents and spin conditions 56 .

Low band gap approach

As already mentioned, injection of both holes and electrons into a particular semiconductor is one major obstacle to overcome in order to fabricate ambipolar OFET. Most common organic semiconductors (small molecules and polymers) show band gaps between 2 and 3 eV. Thus, injection from a metal electrode with a given work function (high or low) will always result in high injection barriers (>1 eV) for either electrons or holes. One solution against this is to use a material having low band gaps, generally less than 1.8 eV [START_REF] Meijer | Solution-processed ambipolar organic field-effect transistors and inverters[END_REF] . Thus, the injection barrier for both charge carriers is lowered and efficient ambipolar transport can take place.

In this context, for several years, more and more organic semiconductors are synthesized and characterized, following different strategies and focusing in particular on chemical modifications: in this way, the band gap can be tuned and designed with precision [START_REF] Tang | Chlorination: A General Route toward Electron Transport in Organic Semiconductors[END_REF] . As examples, the band gap can be designed through adjusting the following parameters: (1) Bond length alternation; (2) Aromaticity; (3) Conjugation length; (4) Substituent effects; ( 5) Intermolecular interactions [START_REF] Winder | Low bandgap polymers for photon harvesting in bulk heterojunction solar cells[END_REF] . The detailed design methods (synthetic) are summarized in several review articles [START_REF] Winder | Low bandgap polymers for photon harvesting in bulk heterojunction solar cells[END_REF][START_REF] Roncali | Synthetic Principles for Bandgap Control in Linear π-Conjugated Systems[END_REF][START_REF] Xu | How to design low bandgap polymers for highly efficient organic solar cells[END_REF] .

1.1.6.5 Other approaches

Fullerene Derivatives

Fullerenes and their derivatives have been known as n-type semiconductors for some time and thus were applied in bilayer and blend ambipolar transistors. Recently, however, they were shown to have ambipolar characteristics as pure materials as well [START_REF] Anthopoulos | Ambipolar Organic Field-Effect Transistors Based on a Solution-Processed Methanofullerene[END_REF][START_REF] Anthopoulos | Organic complementary-like inverters employing methanofullerene-based ambipolar field-effect transistors[END_REF] .

Single Crystals

Organic single crystals are seen as model systems for charge transport in organic semiconductors, as they are free of grain boundaries, have a low concentration of trap states, and thus show very high field-effect charge transport mobilities. Although most single-crystal FETs till recently only showed pchannel behavior [START_REF] De Boer | Organic single-crystal fieldeffect transistors[END_REF] , it was reported an electron mobility of 0.01 cm 2 V -1 s -1 64 for rubrene as a benchmark molecule for single-crystal p-channel transistors.

High/Low Work Function Electrodes

Another way to avoid the problem of unequal charge carrier injection into a semiconductor is the use of source and drain electrodes with unequal work functions. We have already seen that many organic semiconductors that usually show p-type behavior with standard gold electrodes can also show n-type behavior with low work function electrodes such as calcium or aluminum and suitable trap-free gate dielectrics. Thus, using, e.g., a gold electrode for hole injection and a calcium electrode for electron injection in the same device should lead to ambipolar behavior even for wide band gap semiconductors [START_REF] Zaumseil | Electron and Ambipolar Transport in Organic Field-Effect Transistors[END_REF] .

Liquid Crystals

Generalities

Since the discovery of liquid crystals by the botanist Friedrich Reinitzer in the 19th century, this kind of materials has attracted the attention of chemists and physicists alike [START_REF] Gray | Introduction and Historical Development[END_REF] . The liquid crystal state is an authentic state of matter, intermediate between the classical crystalline solid state and isotropic liquid one. The liquid crystal phases, so-called "mesophases", became a main research field of soft matter [START_REF] De Gennes | Soft Matter (Nobel Lecture)[END_REF] . Their macroscopic behavior is defined by the molecular properties of their constituents: the mesogens. The formation of a liquid-crystalline phase is determined generally by the shape anisotropy of these mesogens.

At present, π-conjugated liquid crystal compounds interest many research groups. Indeed, their self-spontaneous organization (intermolecular interactions) property on large areas should allow a good mobility of the charge carriers, thus making them good candidates for the active layer in electronic components.

Liquid crystal families and mesogens

Two types of liquid crystal are distinguished: lyotropic liquid crystals 67 , where the liquid crystal polymorphism results from interactions between one or several solvents and amphiphilic molecules, and thermotropic compounds where the succession of phases is obtained by changing the temperature.

In some cases, these two processes can be also combined to obtain amphotropic liquid crystals [START_REF] Fuller | Amphitropic liquid crystals. Two lamellar phases in a surfactant containing thermotropic and lyotropic mesogenic groups[END_REF] . As our study involves only thermotropic compounds, we will focus only on this family of materials in the following parts.

Among the organic substances, only certain types of molecules display mesomorphic properties; in fact, not all the molecules are likely to have a mesomorphic state. The existence of these phases (mesophases) is due to the anisotropic shape of the molecules and to their chemical constitution. These mesophases correspond to well-defined molecular organizations that are directly related to the molecular structure of the mesogenic compounds. These compounds are organic molecules made of a rigid part (aromatic ring) and a flexible part (alkyl chains), which are characterized by strong geometric anisotropy. These molecules are formed by an amphipathic structure due to two chemically incompatible constituent parts, which induce segregation and the presence of mesophases.

Generally, the different shapes can be classified as "calamitic" (rod-like), "discotic" (disclike) [START_REF] Chandrasekhar | Liquid crystals of disc-like molecules[END_REF] , "sandic" (board-like) [START_REF] Herrmann-Schönherr | Structure of an aromatic polyamide with disc-like mesogens in the main chain[END_REF] and "banana-shaped" 71 mesogens (Figure 1. 24). The nematic phase, which can be obtained for calamitic or discotic mesogens, is the most disordered mesomorphic phase. It has a structure characterized by an orientational order but no positional order. In fact, the molecules are oriented in parallel to each other along a preferred direction defined (director), but are free to move in space 72 (Figure 1.25). If the molecule is a chiral one, the observed mesophases can be a chiral nematic (N*) or cholesterics (as the first discovered N* molecule was a derivative of cholesterol). In this mesophases, the director varies in direction throughout the medium in a regular way. In any plane perpendicular to the twist axis, the long axes of the molecules tend to align along a single preferred direction in this plane, but in a series of equidistant parallel planes, the preferred direction rotates through a fixed angle (Figure 1.27).

Smectic phases

The smectic phase is characterized by an orientation order and positional order as the molecules are organized in layers [START_REF] Brown | The Mesomorphic State -Liquid Crystals[END_REF] . A variety of smectic phases exist, mainly obtained with calamitic molecules.

Organization of the mesogens within the layers gives rise to several varieties of the lamellar phases.

In fact, the molecules are, as in the nematic phase, oriented parallel to a preferred direction, but in addition, they are arranged in parallel, equidistant layers, which can be free to slide over each other depending on the nature of the smectic phase. In the smectic A (SmA) phase (

Liquid crystal polymers

The existence of the liquid crystalline state requires rigid and/or shape-anisometric molecules. With the knowledge of these structural principles, the construction of polymers with potential liquid crystalline properties is immediately obvious. The macromolecule as a whole can either have a rigid rod-like or disk-like structure or contain mesogenic groups as monomer units [START_REF] Finkelmann | Liquid Crystalline Polymers[END_REF] . On the one hand, polymers whose total structures are mesogenic are only achievable when the polymer itself as rod- In comparison to low molar mass liquid crystals, attachment of the mesogenic groups to the polymer generally restricts the translational and rotational motions of the mesogenic groups. Therefore an influence on the temperature range of existence and a change in the physical properties of the liquid crystalline phase could be directly expected.

Semiconducting liquid crystal small molecules

Apart from other non-conjugated organic materials, the semiconducting small molecules have been widely studied through the past 2 decades since they can be readily purified and form well ordered (often crystalline) thin films for the fabrication of high performance devices. In these molecules, in general, there exists one or several π-conjugated systems and these "rigid cores" can bring the liquid crystal property through adding one or more alkyl chains. A direct benefit of this is that the good selforganization can be preserved while the alkyl chains can render these materials more soluble and then better solution-processable.

In addition, there are other advantages in using liquid crystalline semiconductors [START_REF] Bushby | Columnar Liquid Crystalline Semiconductors[END_REF] . Liquid crystals are partially ordered molecular materials whose properties, including their conduction properties, lie between those of (fully ordered) crystalline and (fully disordered) amorphous molecular materials.

They can consequently combine the best properties of both. More precisely, like amorphous molecular materials they are easily processed and they lack grain boundaries that, in their crystalline counterparts, act as deep traps for the charge carriers. Additionally, most are sufficiently well ordered so that they share some of the advantages of crystalline molecular solids: in theory, charge carrier mobilities could even be almost as high as those in the crystal. Furthermore, at least in the most ordered liquid crystalline phases, impurities that could act as charge traps have limited solubility and then are relatively "forced out of the system".

In order to investigate and develop the advantages of liquid crystalline mesophase to increase the performance of semiconducting properties, numerous semiconducting LC small molecules were designed and reported.

As examples, up to now, it has been reported many different types of liquid crystals exhibiting from good to high mobilities ranging from 10 -4 cm 2 /Vs up to ~1 cm 2 /Vs, including triphenylenes [START_REF] Adam | Fast photoconduction in the highly ordered columnar phase of a discotic liquid crystal[END_REF][START_REF] Simmerer | Transient photoconductivity in a discotic hexagonal plastic crystal[END_REF] , phthalocyanines [START_REF] Deibel | Charge transport properties of a metal-free phthalocyanine discotic liquid crystal[END_REF][START_REF] Iino | Very high time-of-flight mobility in the columnar phases of a discotic liquid crystal[END_REF] , porphirines [START_REF] Yuan | Time-of-flight study of electrical charge mobilities in liquidcrystalline zinc octakis(β-octoxyethyl) porphyrin films[END_REF] , perylenes [START_REF] Struijk | Liquid Crystalline Perylene Diimides: Architecture and Charge Carrier Mobilities[END_REF] , phenylbenzothiazoles [START_REF] Hanna | Fast Hole Transport in a New Calamitic Liquid Crystal of 2-(4′-Heptyloxyphenyl)-6-Dodecylthiobenzothiazole[END_REF] , phenylnaphthalenes [START_REF] Funahashi | Fast ambipolar carrier transport in smectic phases of phenylnaphthalene liquid crystal[END_REF][START_REF] Funahashi | Anomalous high carrier mobility in smectic E phase of a 2-phenylnaphthalene derivative[END_REF] , oligothiophenes [START_REF] Funahashi | High ambipolar carrier mobility in self-organizing terthiophene derivative[END_REF][START_REF] Iino | Availability of Liquid Crystallinity in Solution Processing for Polycrystalline Thin Films[END_REF] , and benzothienobenzothiophenes [START_REF] Mery | Bipolar carrier transport in a lamello-columnar mesophase of a sanidic liquid crystal[END_REF] , etc.

To try to better understand these results, it is necessary to detail a little more the way the charges can move in such small molecules liquid crystal semiconductors, before to give a "state of the art" concerning both p-type and n-type materials.

Charge transport in liquid crystal semiconductors

When discussing charge carrier mobility in organic materials, it is essential to recognize that it is not an intrinsic property of organic molecules. It is a material property and requires hopping of charges between the molecules. For this reason, a sufficient interaction of different π-electron systems (π-π stacking) at the local scale is necessary to allow charge transport between neighboring molecules.

With typical stacking distances of several Angstroms, this intermolecular hopping process of charges occurs many times before reaching the opposing electrodes, which have a distance of 100 nm (minimum) to several mm. While closely packed crystalline samples facilitate local charge hopping between organic molecules, grain boundaries in polycrystalline samples strongly affect the charge carrier mobility on a macroscopic scale (Figure 1.35). In contrast, amorphous films from organic materials (µ=10 -5 -10 -4 cm 2 V -1 s -1 ) suffer from poor local order and weak π-orbital overlap between molecules. Therefore, ideally a mono-domain of a properly packed organic material is needed to achieve (relatively) high charge carrier mobilities [START_REF] Fleischmann | Liquid-Crystalline Ordering as a Concept in Materials Science: From Semiconductors to Stimuli-Responsive Devices[END_REF] . In this context, liquid-crystalline ordering (Figure 1.36) offers the possibility to easily orient organic semiconducting molecules [START_REF] Sergeyev | Discotic liquid crystals: a new generation of organic semiconductors[END_REF] and to prepare mono-domain samples. But even if LC phases possess a higher degree of order than the isotropic melt (amorphous order) it is however lower compared to the perfectly regular and most dense crystal. Thus π-conjugated molecules with liquidcrystalline phases are attractive to improve the charge carrier mobility.

In the case of the discotic liquid crystals, columnarly stacked aromatic macrocyclic cores can provide coaxially-insulated, conductive pathways which offer a unique possibility for the onedimensional (1D) transport of charge (Figure 1.37). This is the reason why discotic liquid crystal semiconductors are serious candidates for future applications as organic charge transport layers in a variety of devices such as OFET, OPV and OLED [START_REF] Van De Craats | The Core-Size Effect on the Mobility of Charge in Discotic Liquid Crystalline Materials[END_REF] .

Differently and interestingly as well, in the case of calamitic liquid crystals, the lamellar organization favors the charge transport in the layers namely, i.e. in a two dimensional (2D) way [START_REF] Shimizu | Mesophase semiconductors in field effect transistors[END_REF] (Figure 1.37). Coronene can also form very ordered columnar liquid crystal mesophases. In the early reports, unity mobility could even be obtained from the hexabenzocoronene (Figure 1.38, HBC) [START_REF] Craats | Record Charge Carrier Mobility in a Room-Temperature Discotic Liquid-Crystalline Derivative of Hexabenzocoronene[END_REF] and recent h + or e - h + or e - work further evidenced that the well ordered columns can be magnetically orientated and as a result show anisotropy OFET mobilities [START_REF] Shklyarevskiy | High Anisotropy of the Field-Effect Transistor Mobility in Magnetically Aligned Discotic Liquid-Crystalline Semiconductors[END_REF] .

(

2) Calamitic ones

One of the first reported calamitic p-type liquid crystal molecules is the 2-(4'-heptyloxyphenyl)-6dodecylthiobenzothiazole (7O-PBT-S12) (Figure 1.39) which was described in 1997 by Hanna et al. [START_REF] Hanna | Fast Hole Transport in a New Calamitic Liquid Crystal of 2-(4′-Heptyloxyphenyl)-6-Dodecylthiobenzothiazole[END_REF] .

A hole mobility of 0.003 cm 2 /Vs was obtained by TOF once in its SmA phase. Then later in the same year, Hanna's group reported another calamitic liquid crystal p-type semiconductor, 2-(4'octylphenyl)-6-dodecyloxynaphthalene (8-PNP-O12), which gave a mobility of 1.6x10 -3 cm 2 /Vs in a lower temperature smectic phase [START_REF] Funahashi | Fast ambipolar carrier transport in smectic phases of phenylnaphthalene liquid crystal[END_REF] . Being the simplest calamitic semiconducting liquid crystal material in the oligothiophenes family, the α,ω-dialkylterthiophene was thoroughly studied as well [START_REF] Leroy | Symmetrical and Nonsymmetrical Liquid Crystalline Oligothiophenes: Convenient Synthesis and Transition-Temperature Engineering[END_REF] . In the SmG phase of 8-TTP-8 (Figure 1.39), a 0.01 cm 2 /Vs mobility value was obtained [START_REF] Funahashi | High ambipolar carrier mobility in self-organizing terthiophene derivative[END_REF] . Recently, this 8-TTP-8 was again tested as a model molecule to compare with α-benzothienobenzothiophene (C10-BTBT) [START_REF] Iino | Availability of Liquid Crystallinity in Solution Processing for Polycrystalline Thin Films[END_REF] (Figure 1.39). In the new experiment, by using a "in temperature" spin-coating method, the mobility of 8-TTP-8 was increased to 0.2 cm 2 /Vs and the C10-BTBT showed a higher and remarkable mobility of 3 cm 2 /Vs if measured by OFET [START_REF] Iino | Availability of Liquid Crystallinity in Solution Processing for Polycrystalline Thin Films[END_REF] .

Based on the self-healing property of some liquid crystal molecules, Breemen et al. [START_REF] Van Breemen | Large Area Liquid Crystal Monodomain Field-Effect Transistors[END_REF][START_REF] Lengyel | Solution-Processed Thin Films of Thiophene Mesogens with Single-Crystalline Alignment[END_REF] reported also a terthiophene derivative (Figure 1 Finally with the modern well-established molecular chemistry, more and more "combined" calamitic liquid crystal molecules can be synthesized by coupling or fusing different basic conjugating bricks (benzene, thiophene, and other heterocyclic rings) (Figure 1.40, M3, M4) [START_REF] Sonar | High-Mobility Organic Thin Film Transistors Based on Benzothiadiazole-Sandwiched Dihexylquaterthiophenes[END_REF][START_REF] Meng | High-Performance, Stable Organic Thin-Film Field-Effect Transistors Based on Bis-5'-alkylthiophen-2'-yl-2,6anthracene Semiconductors[END_REF] : thus, we can expect to obtain novel liquid molecules.

(3) Others (glassy-nematics): oligofluorenes Contrary to their polymer analogues (Paragraph 1.2.5), monodisperse oligofluorenes are capable of self-organization into uniaxially aligned, defect-free films for the realization of strongly polarized and highly efficient organic electroluminescence devices 100 . Furthermore, spun-cast glassy nematic films comprising monodisperse oligofluorenes are stable against recrystallization which is essential to the prevention of device failure.

As an example, Yasuda et al. 100 reported a semiconducting LC oligofluorene F(MB)10F(EH)2

(Figure 1.41) fabricated OFET containing a monodomain glassy-nematic film, a field-effect mobility of 0.012 cm 2 /Vs was observed.

Figure 1.41 An example of oligofuorene

There are more glassy nematic liquid crystal oligoflurene derivatives that are reported for OLED 101 and other optoelectronic devices 102, 103 but this is not our main topic.

N-type semiconducting liquid crystal small molecules

In comparison with their numerous p-type homologues, the n-type semiconductors were always and are still not as much developed due to the fact that the transport in n-channel conductors is often degraded easily by air.

Nevertheless, at the moment, rylene and related diimide molecules represent the most important family of n-type semiconducting LC molecules. This is due to the fact that they are a robust, versatile class of polycyclic aromatic electron-transport materials which present excellent thermal and oxidative stability, high electron affinities, and, in many cases, high electron mobilities 104 . In the up-shown family, particularly some perylene diimide molecules can self-assemble into highly ordered one-dimensional columnar stacks, making them promising candidates for use as charge-transport materials in organic electronics.

High electron mobilities were determined by using different characterization methods. Mobilities of 0.11 cm 2 /Vs have been for example measured by pulse radiolysis time-resolved microwave conductivity (PR-TRMC) techniques for M5 in tis liquid crystal phase [START_REF] Struijk | Liquid Crystalline Perylene Diimides: Architecture and Charge Carrier Mobilities[END_REF] ; on the other side, Marder et al. found that at room-temperature, M6 displays a space-charge-limited current (SCLC) mobility as high as 1.3 cm 2 /Vs under ambient conditions 105 while even higher values (up to 6.7 cm 2 /Vs for M7 )

were found for closely related columnar discotic coronene-2,3:8,9-tetracarboxylic diimides 106 .

Recently as well, Bao, Würthner, and co-workers reported that OFETs based on M8 can exhibit mobilities as high as 0.72 cm 2 /Vs, which decreases only slightly after air exposure and remain stable for more than 50 days 107 . Note that as for n-type materials, chemistry is very usefull: many p-type or ambipolar molecules can be chemically modified into n-type molecules by, for example, introducing one or multiple electron withdrawing groups to adjust the LUMO level. This allow us at the same time to remind that we must be careful with this notion of n or p-type material, as it could change depending the counter material used or the device configuration tested.

Semiconducting Liquid Crystal Polymers

As we presented just before a state of the art concerning mobilities in small molecules, we can do the same for both main-chain and side-chain semiconducting liquid crystal polymers.

In fact, the best way to enhance charge carrier mobility in polycrystalline polymer films is the reduction of grain boundaries to increase the crystalline domain sizes. More precisely, the key idea in achieving this goal is to take advantage of the material's ability to self-heal local defects within the film: compounds exhibiting a liquid crystalline state have the potential for such kind of self-healing [START_REF] Sergeyev | Discotic liquid crystals: a new generation of organic semiconductors[END_REF][108][109][110] .

1.2.6.1 Main-chain liquid crystal polymers

(1) Polyfluorenes based.

Polyfluorenes (PFO) are rigid rod polymers that can be rendered soluble in organic solvents by appropriate substitution at the C9 bridging position: some of them exhibit high temperature liquid crystalline phases, which can be thermally exploited to achieve optimal thin film microstructures and good transistor performance (Figure 1.43) 111-114 . Through the addition of liquid crystalline side-chains, the (conjugated) polymers can show different specific properties (birefringence, self-orientation, induced anisotropy, etc.) 109,120 They are typically prospective candidates for elaborating functional materials as well as or even better than main-chain type liquid crystalline conjugated polymers 121 .

On the one hand, their electrical and optical properties can be controlled by the various liquid crystalline side chains and on the other hand, furthermore, with their polymeric properties, macroscopic alignment of the molecules can be progressed by an external perturbation such as shear stress, electric or magnetic field 122 .

Side-chain liquid crystal polymers

Even if main-chain liquid crystal polymers are more numerous, a few side-chain liquid crystal polymers were also synthesized recently, including either calamitic 122 or discotic 123,124 pending groups as in our research group. One reason is that by designing the backbone and the side-groups, it opens a door for a versatile method towards different kinds of LC polymers with various properties.

Ambipolar semiconducting materials

In order to conclude this part on liquid crystals and remembering the high interest actually concerning ambipolar organic semiconductors (Paragraph 1.1.6), it must be said that this is still challenging and, until recently, there were only a few reports showing that a given material can efficiently transport both electrons and holes 125 . Indeed, in order to avoid the phase separation issues such as in multilayer or blend systems, homogenous materials should be developed. Here we present several strategies employed to surpass this challenge.

Combined systems: donor-acceptor dyads and triads

A very logic and also simple idea is to link a donor molecule and an acceptor molecule by a covalent manner in a single molecule. Based on their corresponding size and electronic properties, these kinds of molecules are called dyads or triads.

Oligothiophenes, triphenylenes and phthalocyanine are good candidates as donor moieties and perylene diimides and fullerenes are often chosen as acceptors to form these well self-organized (liquid crystal) dyads and triads [126][127][128][129][130][131] . By following a similar concept, the semiconducting polymer backbone can be modified by attaching "opposite type" pending groups to form a "multiade" macromolecule.

Polythiophene, as a classical self-organized high performance p-type polymer, is often chosen as the donor backbone.

As an example, Thelakkat et al. reported a well self-organized crystalline-crystalline D-A block copolymer, by combing one bloc of regioregular polythiophene and a perylene diimide substituted polyacrylate (Figure 1.47 , M23) 132,133 . On parallel to these "combined systems", notable progress has been made over the past decade in understanding the structure/function relationships governing material performance in the field of organic semiconducting materials: thus, it is possible to adjust the electronic properties (modifying the HOMO/LUMO levels, in particular to decrease the gap) and simultaneously improve the selforganizing characteristics of the materials by designing new aromatic systems based on the existing basic aromatic cores [START_REF] Roncali | Synthetic Principles for Bandgap Control in Linear π-Conjugated Systems[END_REF]135 . Note that this is possible both regarding small molecules and/or polymers. As an example of polymer, by alternatively mixing electron rich and electron poor bricks, the bandgap through the entire all conjugated polymer chain could be reduced. For the polymers presented below, inserting some kind of diimide moieties into the polythiophene chain, the polymers are modified to show ambipolar properties 138 . The symmetric triphenylene derivatives can be easily synthesized through an oxidative cyclization, for example, by FeCl 3 139 or MoCl 5 140 in acidic environment. But in order to modify or to link triphenylene moities to other structures, a functionalization and/or desymmetrization is necessary. One of the common methods is to activate one position of the core by keeping a residual hydroxyl group to form a monohydroxy-functionalized derivative. Generally, it is realized by following one of the three existing different strategies.

(1) Multi-step This way includes six steps of synthesis but the purifications are relatively facile and the yield of each step is also good enough 141 . Inspired from the cyclization reaction of the symmetric triphenylene synthesis, with a 1:2 ratio of the two primary bricks (2B and 2C), the target product can also be oxidatively synthesized in one step 144 . In fact, for several cases, the two basic bricks are co-products in their preparation. Although this method's purification is a little more time consuming but in terms of global yield and number of steps, it seems advantageous to follow this way.

Perylenes and other rylene related diimides

The rylene related diimides are a well known family as organic semiconducting small molecules and the perylene diimides were particularly wildly studied during the last two decades 104,145 . Basically, the chemistry of functionalization of the perylene diimides can be divided into two directions: on the core or on the imide position (Figure 1.54).

Indeed, the strong π-π stacking between two perylene aromatic cores brings it good electronic performances but also several difficulties in synthesis.

Moreover, the first problem comes from the low solubility as the conjugated cores give the molecules a great tendency to aggregate, which makes their purification very difficult. Thus, grafting branched chains onto it is a good way to increase the solubility: the core position branching is more efficient compared with the imide position because it can better decrease the π-π stacking effect. But, this core modification will also get rise to a change of HOMO/LUMO levels. Thus, chemical design can be modified in such a way that we could adjust the final opto-eletronic properties 147 .

Another issue is the functionalization. As the anhydride group can be easily reacted with aliphatic amine, it is a good functional position itself and this kind of modification will not dramatically change the core property (HOMO/LUMO for example). Usually, the Perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) is a common starting material for the perylene diimide synthesis but cannot be used directly to reach the corresponding imideanhydride perylene: in fact, the mono-imidification cannot be controlled but directly go to the diimide derivative. So it is necessary to first isolate this diimide perylene before desymmetrization 148 .

Figure 1.56 Functionalization of smaller rylene imide cores 28

As the "smaller" members in the rylene family, pyromellitic and naphthalene diimides are also two groups of well studied semiconducting materials (usually as n-type) and the functionalization methods are similar as those of perylene, too [START_REF] Wang | Semiconducting π-Conjugated Systems in Field-Effect Transistors: A Material Odyssey of Organic Electronics[END_REF] (Figure 1.56).

Oligothiophenes and regioregular polythiophenes

Short introduction: towards regioregular materials

As seen before (see introduction), the field of organic conjugated materials and organic electronics literally exploded recently and a tremendous development took place. The vision to produce cheap (printable) electronics on a large scale triggered as well extensive researches in academia institutes and even more in industry, expecting huge markets and many emerging companies and divisions worldwide [START_REF] Mishra | Functional Oligothiophene-Based Materials: Nanoarchitectures and Applications[END_REF] .

In this context, thiophene based materials are among the most important conjugated materials encountered in organic electronics. More precisely, oligo-and polythiophenes have being widely investigated and most frequently successfully implemented as active component incorporated in organic devices [START_REF] Mishra | Functional Oligothiophene-Based Materials: Nanoarchitectures and Applications[END_REF][149][150][151][152] such as light-emitting diodes (OLEDs) and lasers, field effect transistors (OFETs), integrated circuits and solar cells (OPVs).

As an example, the most prominent and frequently used materials are doubtless poly (3,4ethylenedioxythiophene) Polythiophene (PT) powders were initially prepared by using protic acid, Lewis acid or electrochemical oxidative polymerizations, leading to non-regioregular polymers 163,164 . Another way to reach PT is to take advantage of electrophilic reactions which favor sites in α position of the sulfur atom, enchaining predominantly 2,5-couplings to form an extended π-system with quasi onedimensional delocalization: but a strong tendency to associate gives some intermolecular overlap and also renders PT insoluble and intractable. As a result, some efforts have been done by enchaining 2,4

defects but caused backbone twisting, electron localization and widening of the optical bandgap. On the other hand, eliminating couplings β to sulfur resulted in the maximization of effective conjugation length but materials with low processability were obtained by this strategy. This is why the solubility was improved by adding ring substituents. This way following different polymerization methods, several soluble 3-alkylthiophenes were obtained with moderate 165 or high 166 molecular weight. Moreover, this increase of solubility is crucial in view of preparing low-cost and large area electronic devices.

Consequently, in the next pages we will focus essentially on the solution-processable thiophene derivatives which mean that, in this case, the problem of regiochemistry is reintroduced as it is essential.

Before to precise the different synthetic tools available to obtain regioregular PT, including the carbon-carbon bond-forming and other cross-coupling protocols 163 , it must be remembered that several regioregular structures can be observed (Figure 1.59).

Among them, the head-to-tail has unique and distinct physicochemical properties which make it the most interesting. Indeed, its architecture is preorganized for three-dimensional self-assembly, generating mesoscopic order and intrinsically large magnitude electronic properties. These include efficient two-dimensional charge transport 167,168 with high mobility (0.1 cm 2 V -1 s -1 ) 169 , current density (28 mC m -2 ) 169 and ambipolar transport 170 . Low-energy (∼2.09 eV) inter-chain excitons indicating two-dimensional delocalizations are also most intense in the structure formed by HT-PT 171 , these excitons depending strongly on inter-chain order and being diminished with head-to-head (HH) coupling defects. To sum up, an improved synthesis to obtain highly regioregular HT-PT results usually in enhancing the transport properties of the final semiconducting material. protocol (~75% yield, Mn = 24000-34000, PDI = 1.4) 172 . These were later joined by Stille (organotin) 173,174 and Suzuki (organoboron) 175,176 methods.

Figure 1.60 The regioselectivity and their corresponding chemistry

Two important features of these cross-coupling reactions are the selective C-C bond formation and, in some cases, the regioselectivity of the catalyst. Regiospecific conversion of an aryl halide monomer to a metal monofunctional monomer (3-6, Figure 1.60) followed by selective cross-coupling gives head-to-tail coupled poly(3-alkylthiophene)s. Substituents may be limited by the organolithium intermediates required to access the cross-coupling competent monomer. However, designing for post polymerization functionalization provides a partial solution 177,178 . Telechelic synthesis 173,179 or isolable monomers, allowing polymerization in diverse solvents, are as well notable advantages.

On the one hand, with the Rieke's method (Figure 1.61 upper), dibromothiophene 8 quantitatively reacts with "Rieke zinc" (Zn*) to generate a ca 9:1 or higher ratio of isomers 9 and 10 172 . The ratio varies with the reaction conditions, temperature in particular, and in some cases, substituent bulk.

Despite this, polymerization affords exclusively HT polythiophene. Rieke group demonstrated that steric congestion on the catalyst determines selectivity in the coupling reaction 172 .

On the other hand, a Grignard metathesis (GRIM) method was developed by McCullough's group in 1999 180,181 (Figure 1.61,lower). This convenient synthesis uses commercially available preformed Grignard reagents to metallate the monomer quantitatively. Magnesium-halogen exchange But as in our work and study we essentially choose the GRIM strategy, it is necessary to detail the mechanism involved in this specific polymerization.

Mechanism of GRIM, a nickel-mediated cross-coupling polymerization

Comparing the well understood bimolecular cross-coupling, the mechanism of polymerization of thiophenes has some very different characteristics. For example, the HT regioregularity increases with Mn for both regioirregular polythiophene (irPT) and regioregular polythiophene (rrPT) 183 .

In fact, it is agreed that for a nickel-phosphine complex-catalyzed Grignard coupling reaction with thiophene derivatives, the mechanism follows several steps (1) oxidative addition of an aryl halide to a metal-phosphine catalyst, ( 2) metathesis (transmetalation) of the catalyst complex with an organometallic reagent to generate a diorganometallic complex and ( 3) reductive elimination of the coupled product either with regeneration of the metal-phosphine catalyst or concurrent oxidative addition of an aryl halide 184 . More precisely, effective syntheses based on organomagnesium, organozinc, organoboron, organotin or organosilane reagents have been demonstrated 185 .

Moreover, it was evidenced that in a GRIM synthesis of a regioregular poly(3-hexylthiophene) (reP3HT), the polymerization proceeds by a chain growth mechanism. Indeed, the conversion vs Mn and conversion vs Mw/Mn plots indicates that an initiator exists 186 . Furthermore, with the living nature of nickel-mediated cross-coupling polymerization of 3-alkylthiophenes, the degree of polymerization (DPn) increases with monomer conversion and this DPn can be predicted by controlling the molar ratio of the monomer to the nickel initiator 187 .

As a consequence and due to this quasi-living processes, the rrPT can be obtained with low Figure 1.64 Proposed mechanism for the binding of 5,5'-dibromobithienyl to a low-valent nickel moiety Ni(dppp) 0 (formation of the associated pair [3•4] in Figure 1.

63) 163

Beside the McCullough mechanism, Yokozawa and co-workers proposed another possible explanation 177 . In their mechanism, the oxidative addition of the polymer chain occurs selectively, being kinetically slower or thermodynamically less stable for the monomer than for the growing polymer chain. This effect could be due to a decreased electron density of the thiophene ring on the polymer chain 163 .

Polythiophenes chemical modification

Poly(3-alkylthiophene)s are chemically robust, withstanding strong reductants including borane 189 and LiAlH 4 190 . However, the electron-rich backbone can be readily functionalized by oxidative methods. Li and co-workers exploited this to replace the 4-proton with Cl, Br or NO 2 functionality 191,192 , thus providing the possibilities for the chain functionalizations.

For the end functionalization, McCullough's group reported a versatile and simple method for in situ end-group functionalization of regioregular poly(3-alkylthiophene)s using the GRIM method. This method eliminates the need to synthesize and isolate a polymer bearing predominately H/Br end groups. In the mechanism of the polymerization, the nickel catalyst acts as an initiator and therefore at the end of the reaction, the rrPT chain will be still bound to the nickel initiator, along with a Br group and the phosphine ligand (Figure 1.65).

Figure 1.65 Proposed mechanism of end-capping of regioregular poly(3-alkylthiophene) synthesized by GRIM 193

In addition, this in situ chain end functionalization of nickel terminated polymers is perhaps the strongest argument that proves the 'living' nature of GRIM polymerization of 3-alkylthiophenes.

Regioregular poly(3-alkylthiophene)s with various end groups are now readily available 193,194 .

Some GRIM tolerated functional groups can be introduced directly from the monomer and then we can talk about substituent modification. For example, from the halogenated (often terminal brominated) alkyl chain, various groups can be grafted 123,124,177,195 or can be transformed to continue the derivatization 177,196,197 . Another example is that the esters are also tolerated by GRIM and this provides a complementary route to the carboxylic acid 198 (Figure 1.66). More precisely, the crude product of polymerization may contain impurities including inorganic salts, from the catalyst or coupling byproducts, phosphines or other ligands, monomer or low molecular weight oligomers. Bad consequences include alteration of the absorption spectra and the decrease of electronic properties, quenched fluorescence, modification of thermal behavior and ash in the elemental analysis [199][200][201] .

At the same time, it was evidenced that careful purification to a metal content of 10 ppm or less optimizes electronic properties, allowing balanced ambipolar transport in transistors 170 .To this aim, precipitation of crude regioregular polythiophenes from chloroform into methanol, a common purification technique, can occur too rapidly to remove these impurities suitably: dilute solutions and an extended precipitation time are consequently better and crucial to obtaining analytically pure material by this method 201 .

Moreover, soxhlet extraction by increasingly 'good' solvents conveniently separates oligomers and narrows the molecular weight distribution of fractions from crude, polydisperse samples. Yamamoto et al. reported that the solubility of regioregular polythiophenes increased in the following solvent order:

methanol < acetone < hexane < dichloromethane < tetrahydrofuran < xylene < chloroform 202 .
Successive extractions gave also fractions with increased molecular weights and regioregularity and red-shifting visible absorption λ max 203 . This effect was seen regardless of polymer regiochemistry 163 .

Finally, the polythiophenes can also be purified with the preparative GPC [204][205][206] technique. Based on the hydrodynamic radius (or "the size of the polymer chain"), this can be sometimes even more accurate than the Soxhlet fractionation.

1 .3.3.6. Characterizing (regioregular) polythiophenes

In order to characterize these polymers, we use to take advantage of several known experimental techniques devoted to determine the chemical structure and physical-chemical behavior of organic materials, but with some specificities related to this family of regioregular polythiophenes, specificities that we point out and describe quickly below.

NMR spectroscopy

As most of the substituted polythiophenes can be dissolved in common solvents, solution NMR became a very efficient method to characterize the chemical structure and stereochemistry (molar mass, regioregularity, etc). In the NMR spectroscopy, the details of the chain microstructures can hardly be explained by a single coupling motif but a sequence of unities 207 . Commonly studied 1 H and 13 C nuclei proved sensitivity not only to the connectivity within the monomer ring but also to neighboring ring orientation and substitution. The proton NMR chemical shifts of the polythiophene main chain (aromatic rings) are listed in Analysis of chain ends provides also important information on functionality, as well as an accurate determination of molecular weight. In particular in our case, with the GRIM method, we focus on the polymer chain growth from a homo-coupled dimer giving chains terminated by a tail-to-tail (TT) dyad.

The α-CH 2 peak of the TT terminal ring is thus interesting as it shifts upfield by ∼0.2 ppm vs that of internal rings. In addition and because only one defect is enchained, this provides a distinct marker for the determination of the absolute molecular weight of regioregular poly(3-alkylthiophene)s

(comparing the H/Br-and H/H-terminated chains shows that the former shifted upfield by a further 0.05 ppm, allowing end-group discrimination (Figure 1.67).

UV-Vis spectroscopy

The extent of π-electron delocalization within the conjugated backbone is conveniently probed by

Thermal analysis (DSC, TGA)

Substituted polythiophenes are comb-like polymers, with both backbone and side-chains independently capable of crystallization. This allows complex behavior from competing processes and polymorphic behavior has been noted 202,216,221,223,227 . DSC data can be combined with the temperature dependent X-ray diffraction to study the different mesophases (mesophases transitions).

For example, generally, the low-temperature transitions come from the alkyl substitutions (side chains)

and high-temperature transitions are designated to "backbones". Some additional transition (glass temperature) can be observed in rrP3AT, transforming a glassy crystal to a plastic crystal with different inter-ring torsion. This was dubbed a twist-glass transition 228 .

Finally, other characterization methods can be cited but were not used during this work, such as solid-state NMR 163 , solid-state UV-Vis spectroscopy 229,230 , solid-state vibrational spectroscopy (IR, Raman), a few microscopy techniques (AFM, STM) 124,231 and light scattering (for studying aggregates) 232 .

Parameters influencing charge carrier mobility in rrPT

As discussed in the previous paragraphs, the applications of organic semiconductors lay mainly in thin-film transistors, ranging from flat-panel displays to smart cards and electronic identification tags 233 . In particular, solution-processable organic semiconductors could make it easy and cost-effective to fabricate large-area thin films using established printing technologies 234 . Moreover, for organic semiconductors to be valuable in most practical applications, field-effect mobilities of greater than 0.1 cm 2 V -1 s -1 (with on/off ratio greater than 10 6 ) are needed for the driving circuits in liquid crystal displays [START_REF] Katz | The Physical Chemistry of Organic Field-Effect Transistors[END_REF] , and mobilities approaching that of hydrogenated amorphous silicon (0.5-0.1 cm 2 V -1 s -1 ) are desirable. This is the reason why before to present our results, here we conclude our discussion on the importance of controlling several parameters to reach high charge carrier mobility of polythiophenes in OFET configuration (for more about OFET, see Paragraph 1.1.5.4).

About two decades ago, the first relatively high mobility was obtained from rrP3AT as 10 -2 235 , and recently, much higher mobilities were achieved as 0.2 cm 2 V -1 s -1 163 . As expected, regioregularity is critical for good electrical properties and consequently riP3AT has a much lower mobility than rrP3AT, typically ranging between 10 -4 and 10 -5 cm 2 V -1 s -1 236-238 .

In particular, the rrP3ATs are widely used to study the factors which will influence the carrier mobility, such as processing conditions 235,239,240 , surface treatment 241 and molecular weight 242 and the polymer structure and morphology can be correlated with the carrier mobility results. Generally, the best mobilities are obtained using relatively high molecular weight rrP3ATs (Mn >25 kDa by GPC)

under slow evaporating (annealing) conditions on hydrophobic substrates. These conditions promote self-organization of rrP3ATs with the π-π stacking direction perpendicular to the substrate surface, and also good interconnectivity of crystalline domains 242 .

Another important factor that influences mobility in conjugated polymers is charge carrier density: mobility tends to increase with charge carrier density as traps become filled 243 . Furthermore, when the charge density is sufficiently high, conjugated polymers can reach the metal-insulator boundary, where very high conductivities are expected 244,245 . Consequently, the highest field effect mobility ever reported for HT-PHT was obtained using a high-capacitance polymer-electrolyte dielectric that could induce a very large hole density into rrP3ATs.

The influence of the side chain length was studied as well by several groups and the results are summarized in Table 1.3 163 . It is believed that the alkyl side-chains act as a barrier to charge transport between π-conjugated main chains and shorter side-chain length results in a higher π-stacking density in the charge transport region, leading to higher mobility 246 . In different configurations, it is also possible to find that the hexyl side-chain is optimal for charge transport due to better self-organization than other polymers in the series 247 . So when the mobility values are compared, the different configurations must be taken into account.

Polymer Mobility (cm 2 V -1 s -1 )

Kaneto et al Babel and Jenekhe

Park et al.

rrP3BT 1 × 10 -2 1 × 10 -3 1 × 10 -2 rrP3HT 3 × 10 -3 1 × 10 -2 5 × 10 -3 rrP3OT - 3 × 10 -4 1 × 10 -5 rrP3DDT 8 × 10 -4 3 × 10 -5 -

Table 1.3 Summary of previous work on effect of side-chain length on field effect mobility for rrP3ATs 163

Another issue with rrP3HT is its low on/off ratio when exposed to air, which is attributed to its low ionization potential (IP ≈ 4.8 eV). Indeed, although high on/off ratios could be obtained under inert conditions or after "dedoping" the polymer film under vacuum, on/off ratios quickly decrease when the polymer is exposed to ambient air. In fact, both water and oxygen are responsible for the lower on/off ratio, moisture being the dominant factor 248 . Of course, high on/off ratios and good stability can be obtained if rrPHT is kept under an inert atmosphere 234,249 .

Thus, the environmental stability of rrP3ATs is also an issue to be considered. As its ionization potential is relatively low 248 , the rrP3ATs are susceptible to be oxidized in wet air 250 . Furthermore, polythiophenes can undergo photochemical reactions and therefore must be protected from the combination of light and oxygen 251 . But interestingly, the environmental stability is quite dependent on the synthesis methods and the different configurations of devices [252][253][254] . By shortening the conjugated length of the polymer or decreasing the electron density of the backbone, the ionization potential can be increased and in this way, the oxidative stability can be increased (also to reduce the off-state current) 255 .

Eventually, concerning the degradation in OFETs, it is suggested that ozone, not oxygen, is the real cause of polymer degradation 256 . In the absence of light, oxygen was not a strong dopant, instead, ozone was found to be a strong dopant for polythiophene polymers. These results explain the variations in stability reported in the literature and suggest that environmental stability studies should be conducted in controlled environments in order to define realistic limits to stability.

To conclude, developing conjugated polymers with higher mobilities than rrP3HT seems difficult.

To date, poly (2,5-bis(3-hexadecylthiophen-2-yl)thieno [3,2-b]thiophene) (PBTTT) is the only conjugated polymer that has a higher mobility than rrP3HT, its high performance being attributed to its highly organized morphology obtained after annealing on silanized (OTS) substrates 117 . As a consequence, more and more efforts are made to achieve a well controlled modification on both the polymer chain and its substituents: the present work deals with these objectives.

Aim of this work

In this work, we were interested in designing and preparing different self-organized semiconducting architectures in which it could be possible to inject and transport both holes and electrons. Our approach is based on liquid crystalline materials incorporating different kind of electron-donor (p-type) and electron acceptor (n-type) π-conjugated systems in a unique molecular or macromolecular architecture. We expect that these ambipolar materials could present a spontaneous nanosegregation of p-type and n-type entities in bulk, leading to well defined distinct conductive channels for each type of charge carriers.

Concerning the molecular approach, we propose to design two families of small molecular donor- The family of discotic donor-acceptor molecules will be based on conventional discotic semiconductors such as triphenylene derivatives as p-type building blocks and perylene or naphthalene derivatives as n-type building blocks. The chemical structures of these ambipolar discotic dyads and triads are given in Figure 1.72.a, and they boil down to one or two alkoxytriphenylenes linked with a perylene or naphthalene diimide by an alkyl chain spacer. This family will be described in the chapter 2 which will focus on the design, synthesis and characterization of these different materials.

acceptor
The family of calamitic donor-acceptor molecules will be based on usual calamitic semiconductors such as terthiophene derivatives as p-type building blocks and pyromellitic derivative as n-type building blocks. The chemical structures of these ambipolar dyads are given in Figure 1.72 b, and they boil down to an end-capped terthiophene moiety linked with an end-capped pyromellitic diimide moiety by an alkyl chain spacer. This family will be described in the chapter 3 which will focus on the design, synthesis and characterization of these materials. In this context, we will first develop and complete the previous work on the model liquid crystalline side-chain polymer in studying and evaluating the influence of the controlled dilution of the mesogenic side groups (i.e. the degree of polymer lateral substitution) on the supramolecular organizations and on charge transport properties. That is why in the chapter 4, we will prepare and study the self-organization properties in bulk and in thin films of a homopolymer comprising triphenylene side groups on every thiophene units with an alternated copolymer with thiophene/triphenylene ratios of 2:1 (Figure 1.75). In addition, the self-organization properties will be also correlated to the charge transport properties of these materials studied in field effect transistor configuration. In particular, we will describe and discuss the synthesis, the thermal behavior, the self-organization, as well as the optical and electronic properties of this molecules family. This synthetic route for the preparation of compound 2D consists in three steps. The two first ones concern the preparation of building blocks 2B and 2C obtained by classical Williamson etherification of catechol through adjusting the ratio of bromoalkane. The last step is a ferric chloride oxidatively catalyzed statistic cyclization. The key point in this "statistic" method is to precisely maintain the ratio between 2B and 2C as 1:2 to reduce the byproducts (symmetric, dihydroxy, trihydroxy, etc.).

Synthesis

Even though the yield of the last step is quite low and the purification of compound 2D is tricky due to the presence of the multiple byproducts formed by the statistic cyclization (mainly the symmetric one), this method is more rapid than the "multi-step" way and more efficient (in total yield) than the "post-cleavage way" (see Paragraph 1.3.1).

Synthesis of the amino-alkyl-functionalized triphenylene via Gabriel synthesis

In order to functionalize the monohydroxyalkoxy triphenylene 2D, we used the Gabriel synthesis to introduce an amino-terminated alkyl chain.

In this context, Figure 2.3 shows one of the different synthetic routes, compound 2D being involved from the very first step and being trailed in each of the successive three steps.

Figure 2.3 A possible synthetic route towards amino-alkyl-functionalized triphenylene 2H

To avoid this linear synthetic route which will dissipate the monohydroxy triphenylene, we chose finally another strategy to carry out the Gabriel reaction with the aim of economizing the compound 2D (Figure 2.4). In this synthetic route, the molecule 2F bearing a "pre-Gabriel" functional group was first prepared in large quantity by reaction between one equivalent of 1,10-dibromodecane and one equivalent of potassium phthalimide 2E. Then compound 2F was grafted onto the triphenylene derivative 2D by Williamson etherification: here, 2F was used in large excess to optimize the nucleophilic attack on the precious monohydroxyalkoxy triphenylene 2D and to obtain the compound 2G with a high yield. 2G

was finally treated by aqueous hydrazine hydrate to complete the Gabriel synthesis with a quantitative yield and the final compound amine 2H was purified easily by simple recrystallization.

Synthesis of mono-anhydride mono-imide perylene

In order to improve the solubility of perylene diimide building block, a branched alkyl chain substitution was chosen rather than a linear one 148 . Our choice was consequently the aminoalkyl building block 2J which presents two long chains of seven carbons directly connected to the carbon bearing the amine group.

As the amine 2J is not commercially available, it was synthesized from its ketone precursor, i.e. As the mono-imidification on perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) cannot be achieved directly 257 , an alternative route was applied, consisting the symmetric bisimide derivative preparation and then the cleavage of one terminal branched chain to reach the mono-anhydride monoimide perylene 2M (Figure 2.6). More precisely, the branched amine 2J was firstly grafted on commercially available PTCDA 2K through imidification reaction in melted imidazole, catalyzed by zinc acetate (Lewis acid) according to the literature procedure 258 . Then, the symmetric bisimide compound 2L was treated in strong basic medium to open the imide ring and to remove the swallow-tail-amine chains statistically. During this reaction, the progress should be timely followed by TLC in order to stop the alkyl chain cleavage at the optimum moment. The starting material, the over-cleaved (2K) and the target molecule (2M) can be finally separated easily by silica column chromatography.

Synthesis of the triphenylene/perylene and triphenylene/naphthalene based dyads and triads

The synthetic route to prepare the triphenylene/perylene based dyad and triad is given in Figure 2.7.

The synthesis of the dyad TP was carried out by the coupling between the mono-anhydride monoimide perylene 2M and the amino-alkyl functionalized triphenylene 2H by following the same procedure as the synthesis of compound 2L. Similarly, the synthesis of the triad TPT can be achieved from the same triphenylene derivative 2H and the commercially available PTCDA 2K. Concerning the synthesis of the triphenylene/naphthalene based dyad and triad, the previous strategy developed for the perylene based compounds couldn't be applied. Indeed, the attempt to prepare the mono-anhydride mono-imide naphthalene derivative failed. On the one hand, as described for the perylene derivatives, the mono-imidification from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA) is nearly impossible and on the other hand, contrary to the perylene case, the mono-cleavage from the symmetric diimide naphthalene is not possible either. Thus, we were obliged to develop another strategy based on a statistic approach, as described in Figure 2.8.

The imidification of NTCDA was eventually performed in one-pot by adding the two amine derivatives 2H and 2J in a 1:1 ratio, assuming that these two amines possess similar reactivities in these experimental conditions. This way, the dyad TN and triad TNT were obtained in acceptable yields after easy purifications.

Thermal behavior

The thermal behavior of TP, TPT, TN and TNT were investigated by Polarized-light Optical Microscopy (POM) and Differential Scanning Calorimetry (DSC).

Polarized-light optical microscopy

Under POM, the dyad TP appeared as a slightly birefringent dark red powder which melted at around 84°C to give a clearly birefringent viscous phase. On further heating, this viscous phase transformed to a non-birefringent isotropic phase at around 146°C. Then on cooling from previous isotropic state, the birefringence phase reappeared at around 140°C, showing a typical pseudo focalconic characteristic of a columnar hexagonal mesophase (Figure 2.9.a). On further cooling, this pseudo focal-conic texture was clearly maintained until room temperature in a very viscous phase, suggesting the presence of a glass transition at higher temperature and The triad TPT presented a thermal behavior similar to the dyad TP with an isotropization temperature around 149°C on heating. However, the dendritic texture obtained on cooling (Figure 2.10.a) and the large oriented mosaic domains obtained with very slow cooling rate (Figure 2.10.b) seem to indicate the presence of a more ordered mesophase that in the case of the TP dyad. Moreover and similar to the dyad TP, this mesophase can also be preserved on cooling until room temperature. Nevertheless, concerning the triphenylene-naphthalene-triphenylene triad TNT, this compound appeared as a slightly birefringent gray-violet powder when observed under crossed polarizers at room temperature. This powder melted progressively on heating to give a birefringent viscous phase which

20°C

transformed into an isotropic phase at around 173°C. On cooling from the isotropic state, the birefringent phase reappeared at around 170°C, showing a mix of homeotropic and typical pseudo focal-conic textures, which is characteristic of a columnar hexagonal mesophase (Figure 2.12.a).

Moreover, when being cooled at very low rate, large homeotropic domains were obtained and these domains can be maintained until room temperature in a very viscous phase as illustrated in Figure 2.12.b. For instance, the DSC traces of dyad TP show a nice reversible transition at around 145°C, corresponding to the transition between hexagonal columnar mesophase (Col h ) and the isotropic phase.

The glass transition of the compound assumed by POM observations can be also clearly detected at 84.4 °C with a high heating rate (20°C/min).

The DSC traces of dyad TN also confirmed the amorphous state of the compound at room temperature as no exothermic transitions can be observed on heating. In addition, a clear glass transition at around 9.8°C (5°C/min) can also be detected. where λ is the wavelength of incident X-ray and θ is the angle incidence. Thus, in the wide angle, the very broad halo centered at 4.5 Å (h ch ) indicates the molten state of the aliphatic chains and the other broad halo around 3.5 Å (h T +h PDI ) evidences the short range ordered π-π stacking of the segregated discotic mesogens. This X-ray pattern suggests the amorphous characteristic of the material with columnar cybotactic pre-organization in the pristine state. On heating and just above the Tg at 84.4°C (detected by DSC), we can observe a sudden self-organization enhancement of the compound to a hexagonal columnar (Col H ) mesophase, resulting in a strong increasing of the small angle reflection (Figuer 2.13.b). The Col H phase was maintained on cooling until room temperature as proved by the X-ray pattern given in Figure 2.13.c. This pattern presents: i) in the wide angle region, the broad diffuse halo centered at 4.5 Å (h ch ) corresponding to the liquid-like disorder of the aliphatic chains and the broad peak around 3.5 Å (h T +h PDI ) corresponding to the stacking period in the columns;

Self

ii) in the small angle region, a sharp and intense reflection assigned to (10) with a Bragg spacing of 16.5 Å, followed by a set of weak reflections with a d-spacing ratio of 1: √ : √ : √ : √ : √ corresponding to (10), ( 11), ( 20), ( 21), ( 30) and ( 12) reflections of a hexagonal columnar phase. Based on this study, we can propose a packing model for the dyad TP where the two moieties (one triphenylene and one perylene) segregate into two individually separated columns with a correlation distance ξ of ~ 100 Å (calculated from Scherrer formula 126,259 ) isolated by the peripheral aliphatic chains as illustrated in Figure 2.13.d.

However, the nearly same size of columns in the Col H lattice associated with the limited correlation distance (around 30 times of the mesogens stacking distance) induce a macroscopic un-differentiation of columns 260 , implying that the segments of stacked identical mesogens frequently exchange with preserving the segregation of any mesogen from the columnar periphery. As expected, the oblique lattice vanished at the isotropization temperature but is restored by reentering the mesophase and is finally maintained on cooling until room temperature as proved in In comparison with the triad TPT, the crossing from a differentiated to an undifferentiated columnar packing is obviously related to the smaller size of naphthalene core compared to perylene core, translating into weaker interactions within stacks, and thus, into reduced segregation tendency.

Conversely, perylene stacks exhibit strong cohesion that leads to the presence of numerous higher order reflections in the dyad TP, evidencing the well confinement of mesogens in columns separated from the aliphatic periphery by sharp interfaces. The degree of segregation of perylenes and triphenylenes into different stacks is presumably comparable to triad TPT, and the loss of the differentiation in the dyad TP is just the consequence of the different molecular geometry. Whilst in the triad the 1 to 2 ratio allows the optimal segment segregation at the nodes and centers of a honeycomb lattice, the 1 to 1 ratio in the dyad only permits first neighbor segregation into bookshelf layers formed by alternating column rows. Unlike the rigid honeycombs, the direction in parallel to the column row alternation is intrinsically different from the perpendicular in-plane direction, and layers may therefore bend and refold within columnar domains. This mechanism would then mix positions of locally segregated columns to an average lattice apparently constituted of undifferentiated columns.

Atomic force microscopy

Due to lack of time, we choose the dyad TP as example: its morphology of thin films was studied by tapping mode Atomic Force Microscopy (AFM). These measurements were carried out at the Institut des NanoSciences de Paris (INSP) in collaboration with Dr. E. Lacaze.

The TP films were prepared by spin-coating from a chloroform solution of material (1% w/w, 2000 rpm) onto silicon wafer substrates. The thickness is around 85 nm as determined by profilometer. The film samples were studied before and after annealing (120 °C for 2 hours in vacuum). The typical topography images are given in Figure 2.18.

Grazing-incidence X-ray scattering

The Grazing Incidence X-ray Scattering (GIXS) is the method of choice to probe the structure and the morphology of organic thin films. Indeed, the X-ray scattering cross section of organic materials is small in comparison with inorganic materials such as metals because organic materials are mostly consisted of low-Z elements such as C, O and H. Therefore, X-ray scattering signals are week in organic thin layers. To overcome this problem, grazing incidence technique is used to investigate organic thin films systems. GISX technique requires actually small incident angle of X-rays to the surface of the sample, which increases the penetration depth of X-rays in the sample. GIXS is a reflection mode, which provides the information of the surface of a sample. A 2D detector is generally used to simultaneously measure the vertical and the horizontal alignments of atoms or molecules.

Therefore, the GIXS measurements allow to determine the orientation of molecules in thin films as for example the "face-on" or "edge-on" orientations, as illustrated in Paragraph 2.3.2. The vertical diffraction peaks of 2D images correspond to the (0k0) in the face-on crystallites; whereas they correspond to the (h00) in the edge-on crystallites.

Thus the film structure and morphology of dyad TP were studied by GIXS. These measurements were carried out at Pohang Accelerator Laboratory (PAL), in collaboration with Prof. H-J Kim (Pusan National University).

The TP thin film samples were prepared on silicon wafer substrates by spin-coating with the same conditions as for AFM samples (see Paragraph 2.3.2).

The two dimensional GIXS image of annealed TP film is given in Figure 2.20.a. In addition to the very diffuse halo around 1.4 Å -1 (4.5Å), corresponding to the liquid-like disorder of the aliphatic chains (h ch ), this pattern presents one broad and intense in-plane reflection at around 1.82 Å -1 (along q x ), corresponding to the π-π stacking (3.5 Å) of the segregating discotic mesogens (h T +h PDI ); a vast number of sharp reflections in and out of specular plane (along q z ) can be assigned to the usual reflections of the hexagonal columnar phase (Col H ), already described in the bulk X-ray diffraction study (Figure 2.20.b). GIXS measurements. That indicates no preferential alignment of columns and we can consider that there is no macroscopic orientation of the columnar domains in the plane of the thin film (as illustrated in Figure 2.21), which can be a problem for charge transport in an OFET channel as for instance.

Optical properties (absorption and emission)

Optical properties in solution

The absorption and emission properties of this family of discotic dyads and triads and their corresponding building block molecules were studied in diluted chloroform solutions (ca. 10 -6 M). In this condition, these molecules can be considered as adequately isolated, i.e. the aggregation and the intermolecular interactions can be neglected. In the case of the triphenylene derivative T, the absorption maximum occurs at about 280 nm and the fluorescence appears mainly in the region between 360 and 420 nm. Concerning the optical properties of perylene diimide P, three characteristic absorption vibronic peaks can be observed at 459, 490 and 526 while in the fluorescence spectrum the mirror symmetric peaks arise at 537, 575 and 627 nm, respectively. For the naphthalene diimide N, its optical behaviors are similar to those of perylene diimide but strongly blue shifted (342, 361 and 382 nm for the absorption; 393, 410 and 434 nm for the fluorescence). It should be also noted that compound T and compound N present very low fluorescence quantum yields 262, 263 contrary to compound P whose quantum yield is close to unity 146 .

The absorption and emission spectra of dyad TP and triad TPT were carried out and are given in More precisely, the absorption band at wavelengths shorter than 400 nm is due to the triphenylene unit while the absorption in the visible region of the spectrum is due to the perylene moiety 264 . The maximum of the absorption from the triphenylene unit appears at about 280 nm, which can be attributed to the symmetry-allowed S0-S4 electronic transition [265][266][267] . Low intensity peaks at 345 and 360 nm are related to the symmetry-forbidden S0-S1 transition. Regarding the contribution from the perylene moiety, the absorption peak at 525 nm corresponds more particularly to the (0,0) vibronic transition while the peak at 488 nm is attributed to the (0,1) transition. Peak-to-peak ratio between the (0,0) and (0,1) peak intensities is around 1.6, which indicates that there is no aggregation between the perylene moieties in the diluted solutions 267,268 . The absorption spectrum of triad TPT (Figure 2.22.b) is quite similar to that of dyad TP but evidently, as the triphenylene/perylene ratio is of 2/1, the absorption spectrum in the range of triphenylene is doubled.

Concerning now the emission properties, dyad TP and triad TPT are strongly fluorescent in chloroform solutions. The emission spectra are in fact dominated by the emission from perylene as proved by the presence of the three peaks at 537, 575 and 627 nm which can be attributed to isolated perylene moieties, confirming that there is no aggregation 269,270 . When excited at 280 nm, a weak signal around 390 nm from the triphenylene moieties is also visible [265][266][267] . Not surprisingly, the emission signal coming from triphenylene unit in triad TPT is slightly stronger than that in dyad TP.

Concerning these two compounds, it is worth noticing that energy transfer may occur in TP and TPT solutions, due to the overlapping of triphenylene (donor) emission spectra with perylene (acceptor) absorption spectra.

After describing the optical properties of dyad and triad incorporating the perylene diimide unit, we realized the same study for the corresponding materials involving the naphthalene diimide unit. Thus, the absorption and emission spectra of dyad TN and triad TNT are given in Figure 2.24. Here also, their absorption spectra are the sum of the isolated triphenylene and naphthalene absorbances. However, the weak naphthalene moiety's absorption band at higher energy (λ max located at 380 nm) is slightly overlapped with the triphenylene's and consequently, it is not very visible especially for triad TNT where the triphenylene band intensity is doubled.

The dyad TN and triad TNT are also fluorescent in chloroform solution (even though much weaker compared to the corresponding perylene based dyad TP and triad TPT). However, this time, the emission spectra are dominated by the emission from triphenylene (fluorescence quantum yield is about 6% 262 for T and between 0.2%-0.6% for N 271,272 ) as proved by the presence of the vibronic band at around 390 nm, characteristic of triphenylene band.

Concentration dependent absorption and emission

To better understand the intermolecular interaction and aggregation phenomena, the dyad TP was taken as an example to further investigate the influence of concentration on absorption and emission properties in a poor solvent such as methylcyclohexane 273 . The result, i.e. the "concentration dependent normalized absorption spectra" is shown in Figure 2.25.

At very low concentration, the absorption spectra are similar to those in diluted chloroform (Figure 2.23.a). When the concentration increases, due to the molecular stacking of both perylene units and triphenylene units, the apparent absorption coefficients of both chromophores decreased and the absorption bands are shifted and broadened. This can be attributed to the electronic coupling between the perylene moieties and the triphenylene moieties. Concerning the contribution from the perylene moiety, it can be seen that an isosbestic point appears at about 530 nm, which suggests the formation of π-π face-to-face aggregates at high concentration.

Finally, in the photoluminescence spectra, at low concentration, we can find the same emission spectra as in the diluted chloroform (Figure 2

Optical properties in solid state

As an extension of the concentrated solution to extreme and to provide some additional information about the thin film properties (including thermal treatment effects), the dyad TP film absorption properties were studied as well. Indeed, the solid state can be considered as an extremely concentrated solution (concentration → 100%).

A thermal treatment (similar to the annealing conditions in AFM and GIXS measurements, 120 °C for 2 hours in vacuum, namely in liquid crystal mesophase) was also carried out to evaluate its influence on the optical properties which are correlated with the intimate molecular organization changes with temperature. Figure 2.27 shows the absorption spectra of dyad TP thin film before and after annealing. The as-prepared film's absorption is consistent with the spectrum observed at high concentration in the previous concentration-dependent absorption study in solution (Figure 2.25). It is obvious that the peak-to-peak ratio between the (0,0) and (0,1) peak approaches to unity.

After annealing, it can be observed that all the vibronic peaks from the perylene moieties are about 5 nm red-shifted with a shape-regression to the isolated molecules case, suggesting a little increase of - intermolecular interactions between perylene units due to small changes in molecular packing upon annealing. Regarding the position of the absorption peaks of the triphenylene moieties, a small red-shift of about 2 nm was observed after annealing. This result, together with a little increase of the absorption tail in the range 350-400 nm, suggests a little increase of the aggregation of the triphenylene units in the annealed dyad TP films which coincides with the temperature dependent Xray diffraction experiments.

It should be noted that, in the solid state, there is almost no fluorescence observed in TP (neither in TPT nor in TNT, TN) and that is the reason why the emission properties were not further studied.

Electrochemical behavior and HOMO/LUMO energy levels

The electrochemical behaviors of TP, TPT, TN and TNT were studied using cyclic voltammetry.

All the experiments were carried out at room temperature in chloroform solutions (concentrations around 10 -3 M, n-Bu 4 NPF 6 0.05 M being added as supporting electrolyte), at a scan rate of 100 mV s -1 , with Pt as the working and counter electrodes and Ag/AgCl electrode (saturated KCl) as the reference electrode. Note that oxidation and reduction potentials were determined as an average value between each anodic and corresponding cathodic potentials: E 1/2 red/ox = ½ (E pa + E pc ). In the case of triad TNT (Figure 2.29.a), the reversible oxidation happened at 1.07 V (vs Ag/AgCl) and this is in agreement with the triphenylene moiety characteristic as seen previously in dyad TP and triad TPT. As the acceptor core was replaced from perylene to naphthalene, the two reductions appeared at -0.65 V and -1.19 V (vs Ag/AgCl), respectively. In the dyad TN (Figure 2 For all the molecules, the HOMO values are mainly issued from the first oxidation potential of the triphenylene moiety, as the LUMO energies are determined mainly from the oxidation potential of the perylene (TP and TPT) or naphthalene (TN and TNT) moiety.

Moreover and as an important remark, these HOMO/LUMO values do not represent any absolute solid-state or gas-phase ionization potentials but can be used to compare different compounds relative to one another. Note as well that the calculated band gap reported in Table 2 In this context, by comparing all these HOMO/LUMO values with the ones of each donor and/or acceptor taken alone (free), we can observe that generally TP and TPT have similar HOMO/LUMO values as in isolated model perylene diimide and triphenylene, respectively. Similarly, TN and TNT show typical HOMO/LUMO values regarding the ones of an isolated model naphthalene diimide and isolated triphenylene as well. These results evidence that donor and acceptor units don't interact in solution, as it was expected, because the covalent linker is long and not conjugated as observed for the other similar systems 126, 279 .

Preliminary results on charge transport properties

The charge transport properties of dyad TP were investigated by temperature-dependent time-of- Unfortunately, even if the sample was carefully orientated into large, homeotropic liquid crystal domains in the TOF cells (as in Figure 2.9.c), the obtained photocurrent stayed very low (range of nA).

Moreover, the deflection point did not shift with applied voltage, indicating that it cannot be considered as the transit time. As a result, the charge carriers mobility in the mesophase can hardly be evaluated by TOF technique.

However, in isotropic state, the TOF experiments gave some exploitable results. The sample was photo-excited by lasers in using two different beam wave lengths (337 nm and 508 nm), close to the maximum absorptions of triphenylene and perylene moieties, respectively.

When the sample was excited by the laser beam at 337 nm with applied negative voltage, untypical transient photocurrent curves were recorded, as displayed Figure 2.30.a. First of all, the UV laser pulse should excite triphenylene core, which behaves like a p-type semiconductor, i.e. is able to conduct only holes. Thus, applying negative voltage should result in no photocurrent curve. In contrast, when the negative voltage was applied, the signal of electrons was detected, corresponding obviously to the hopping between perylene diimide cores.

In the TOF method, the absorption of light pulse leads to creation of excitons. Then, due to the electric field they dissociate into free electrons and holes. Depending on the sign of voltage applied, one kind of carriers flow through the sample and the other ones were absorbed by the first electrode.

Thus, after initial rise, the photocurrent should only decrease, as all the carriers were created immediately after the pulse. However, a clear increase of the TP photocurrent in the 1-20 µs range could be observed, after a fast decay, when excited by 337 nm laser pulse (see inset in Figure 2.30.a).

Moreover, the transit time was clearly visible, even in lin-lin scale, which is highly uncommon for such experimental conditions.

This uncommon increase of transient photocurrent did not occur when the sample was excited at 508 nm which is directly in the perylene absorption band. The electron mobility was determined to be equal to 5.3X10 -5 cm 2 /Vs irrespectively of the excitation wavelength, and remained independent on the electric field within the experimental range of 125-250 kV/cm. The energy of 508 nm laser pulse was significantly lower than the pulse emitted at the wavelength of the pump laser (337 nm), which explains why the photocurrent in Figure 2.30.b is lower than in Figure 2.30.a. Based on these observations, a model was proposed to explain this uncommon behavior of transient photocurrents in the isotropic state of TP. The model is based on the assumption that the 377 nm laser beam creates excitons in triphenylene sites. These excitons can either move to another triphenylene moiety or transfer energy to perylene moiety then move to another perylene site. Here, it should be noticed that the triphenylene excitons are singlets while perylene excitons decay into long-lived triplet states. In case of organic semiconductors, generally, charge separation can occur only at the metalorganic semiconductor interface, after exciton diffusion. However, we postulated that in the case of TP, the exciton dissociation can also occur at the TP-ITO electrode interface, or in the bulk between triphenylene and perylene diimide, which has been previously reported for molecules with similar structure 262,280 . The bulk dissociation would result in the observation of two components in the transient photocurrent, for holes and electrons flow, respectively. In electric field the carriers flow in opposite directions, but their currents sum up.

The electrochemical study (see Paragraph 2.5) on TP confirms that after charge separation, electrons tend to stay on perylene moieties and holes on triphenylene moieties. This is another positive proof for the previous model.

Conclusion

In this chapter, the design and synthesis of a family of discotic liquid crystal molecules were described. Their thermal behaviors, self-organization properties, optical and electronic properties are discussed.

The triphenylene-perylene based dyad TP and triad TPT together with the triphenylenenaphthalene based triad TNT present liquid crystalline properties. All of these three liquid crystalline molecules self-organize into well defined columnar nanostructures and these organizations can be preserved after cooling to room temperature. That means the organization could be self-healed in the liquid crystal phase and then be maintained until room temperature.

Then studies were carried out both in solid state films and in solutions particularly on the triphenylene-perylene dyad TP. AFM and GIXS measurements on TP solid films prove that this dyad can self-organize into hexagonal columnar structures and orientated on parallel to the silicon wafer substrate surface. This is a promising property in the domain of organic transistors where the 2D orientation is a key factor for the transistor performance. Then through the absorption/emission spectra in solution, a photon transfer from triphenylene moiety to perylene moiety was observed. Combining the concentration dependent absorption spectra in a bad solvent and absorption spectra of solid state film, the molecular aggregation properties was further studied.

The electrochemical behaviors of these molecules confirmed their ambipolar properties. As the donor and acceptor moieties are linked by long alkane chains, their initial electrochemical properties can be preserved. With the columnar self-organization, we can then envisage a separate ambipolar charge transport inside of this material.

With a preliminary time of flight study on dyad TP, an untypical charge transport was observed in its isotropic state which could be explained by the same photon transfer in solution absorption.

Unfortunately, in the liquid crystal and solid state, neither TOF nor OFET have revealed any mobility.

The short correlation distance in the columnar and non-optimized orientation could be the two main reasons. To surmount this, chemical modifications can be done onto the individual moieties to prolong the correlation distance and other deposition methods can be tried to have a better control on 3D orientation.

Thus, we can conclude that, via this design strategy, a family of self-organized liquid crystalline ambipolar molecules was prepared. With their self-organization and orientation properties, photon transfer properties, this family of molecules can be interesting candidates in the domains of organic electronics and potonics.

Experimental

TOF configurations

The TOF measurements carried out in this chapter were in the most fundamental configuration, i.e.

no CGL was used. Standard liquid crystal cells with Indium-Tin-Oxide (ITO) as semitransparent electrodes with thickness of 8 μm were purchased from Instec Inc. The compound TP (in powder) was placed in the vicinity of one entrance of the cell and heated to isotropic state. Then, it flowed into the cell by capillary forces.

The samples are then mounted in the TOF setup, using a home-built polarizing microscope and a microscopic hot stage (Instec HCS302). Multiple in-situ transitions between the liquid crystalline and isotropic states are carried out to significantly increase the domain size and to be able to reduce the effects of the grain boundaries on the charge transport. The samples are excited by a nitrogen nanosecond pulsed laser (LTB Lasertechnik Berlin GmbH, model MNL 205-L) emitting at 337 nm with a repetition rate of 1.8 Hz. The photogeneration of a sheet of photocarriers in the samples leads to a transient photocurrent signal, which is amplified and then recorded by a Tektronix TDS 5034 350 MHz digital oscilloscope.

Synthesis

Prepared as 2C except the feeding ratio between catechol and 1-bromopentane which was modified as 1:1 to give the title compound ( 12.8 g, 79%) as a yellow oil; NMR H (ppm) (200 MHz, CDCl3) 6.90 (4H, m, Harom), 5.67 (1H, s, -OH), 4.04 (2H, t, -OCH 2 -) 1.84 (2H, m, Haliph), 1.44 (4H, m, Haliph), 0.94 (3H, t, J 6.9, -CH3).

Catechol (10 g, 0.09 mol), 1-bromopentane (25 mL, 0.20 mol), potassium carbonate (50 g, 0.36 mol), potassium iodide (0.94 g, 6 10 -3 mol) and a catalytic amount of tetrabutylammonium bromide were added to absolute ethanol (200 mL) under an atmosphere of argon. The light pink mixture was heated at reflux for 24h, cooled down to room temperature and extracted with dichloromethane (100 mL). The organic layer was successively washed with water (3 x 50 mL), dried over MgSO 4 , filtered and the solvent removed under reduced pressure. The crude product was filtered through a short plug of silica gel using a 1:1 mixture of DCM/Hexane to yield 2C (18.79 g, 83%) as a yellow oil; NMR  H (ppm) (250 MHz, CDCl 3 ) 6.90 (4H, s, H arom ), 4.01 (4H, t, J 6.6, -OCH 2 -), 1.84 (4H, m, H aliph ), 1.44 (8H, m, H aliph ), 0.94 (6H, t, J 6.9, -CH 3 ).

Acetonitrile is added drop-wisely to a well-stirred suspension of Iron (III) trichloride (3.24 g, 20 mmol) in anhydrous dichloromethane (20 mL) until the initially observed suspension becomes solution. This new solution is cooled by an ice bath to about 0°C. 30 minutes after, a solution of 2B

(1.0 g, 4 mmol) and 2C (0.36 g, 2 mmol) in 15 mL of anhydrous dichloromethane was added carefully, drop by drop at 0°C. The reaction was continued for 2 h at 0°C. MeOH (15 mL) was added to stop the reaction and the mixture was washed twice with HCl (5%, 20 mL) and brine (20 ml). The organic layer was dried with MgSO 4 , and the solvents were removed under reduced pressure. The crude product was purified by column chromatography (silica gel, DCM/hexane 1:1) and afforded 2D (0. CDCl 3 ) 149. 23, 149.08, 148.90, 148.83, 145.94, 145.35, 124.03, 123.78, 123.72, 123.65, 123.32, 123.08, 107.6, 107.42, 107.25, 106.51, 104.39, 69.97, 69.94, 69.68, 69.21, 69.17, 29.30, 19.25, 29.16, 29.12, 28.51, 28.48, 28.45, 28.42, 22.71, 22.68, 22.67, 14.26, 14.23, 14.20.

2E (3.7 g, 20 mmol

) and 1,10-dibromodecane (30 g, 100 mmol) were mixed in DMF (50 mL) and heated at 100 for 12 hours. The condenser was then changed to a set for distillation, and the excess of 1,10-dibromodecane and DMF were distilled off under reduced pressure. The crude product was finally purified by column chromatography (silica gel, ethyl acetate/petroleum ether 1:4) to afford a colorless solid (5.27 g, 72%). NMR δ H (ppm) (200 MHz, CDCl 3 ): 7.84 (m, 2H), 7.71 (m, 2H), 3.68 (t, 2H), 3.40 (t, 2H), 1.87-1.63 (m, 4H), 1.43-1.28 (m, 12H).

A mixture of 2D (2.2 g, 3.26 mmol), 2F (2.39 g, 6.52 mmol), and potassium carbonate (4.5 g, 32.6 mmol), KI (cata), TBAB (cata) in DMF (30 ml) was heated under reflux for 2 days. After cooling to room temperature, 100 ml of dichloromethane was added. The organic solution was washed with water (200mL) twice, and brine (100 ml) and dried over MgSO 4 . After the solvent was removed under reduced pressure, the resulting residue was purified by column chromatography (silica gel, ethyl acetate / petroleum 1:9) to give 2G (2.57 (20 ml) and the mixture was refluxed.

Hydrazine hydrate (80 mg) was added to the mixture, and the reaction mixture was refluxed for 1h.

The reaction mixture was allowed to cool and neutralized with 5 wt% aqueous HCl, and then, refluxed again for 30 min. The reaction mixture was cooled, and the precipitate was filtrated off. The solvent was evaporated, and the obtained residue was poured into 5 wt% aqueous KOH, then the product was extracted with CHCl 3 , dried (MgSO4) and concentrated. 2H was obtained quantitatively as a colorless solid. NMR  H (ppm) (200 MHz, CDCl 3 ) 7.83 (s, 6H), 4.23 (t, 12H), 2.68 (t, 2H), 2.02-1.88 (m, 12H), 1.66-1.32 (m, 36H), 0.97 (t, 15H).

A solution of 2I (4.5 g, 20 mmol) in 60 mL of ethanol and 30 mL of pyridine was added by 2.8 g (40 mmol) of hydroxylamine hydrochloride and then heated to 100°C for 2 h. The resulting mixture was concentrated by rotary evaporation and the residue was extracted between 5% HCl (aq) and hexanes. The organic layer was then dried over MgSO 4 and after evaporating the solvent again, yielded to 4.6 g (95%) of 8-pentadecanone oxime as yellowish viscous liquid.

The oxime was dissolved in 80 mL of toluene. About 10 mL was distilled off to dry the system, and then the system was protected under nitrogen atmosphere. RedAl (Sodium bis(2methoxyethoxy)aluminum hydride, 70% in toluene) were added over 5 min then the reaction mixture was refluxed for 2 h. The resulting mixture was carefully added, under stirring, by 100mL of 5% HCl; then more concentrated HCl was introduced to dissolve aluminum salts and render the mixture acidic to litmus. The solution was extracted by hexane and the organic layer was washed with 5% NaOH, dried, and the solvent was removed by rotary evaporation to yield 4.3 g (99%) of 2J. NMR δ H (ppm) (200 MHz, CDCl 3 ): 2.68 (m, 1H), 2.14 (br, 2H), 1.20 (m, 32H), 0.81 (t, 6H).

A mixture of 2K (3g, 7.56 mmol), Zn(OAc) 2 , (1.0 g, 5.67 mmol), imidazole (15 g) and 2J (4.3g, 18.9 mmol) was stirred at 160 °C for 4 h. After cooling to room temperature, the mixture was dissolved in minimum amount of chloroform and purified directly by column chromatography (silica gel, chloroform) to give 2L (4.94 g, 81%) as a red solid. NMR δ H (ppm) (200 MHz, CDCl 3 ): 8.64 (m, 8H), 5.20 (m, 2H), 2.25 (m, 4H), 1.86 (m, 4H), 1.21 (m, 40H), 0.85 (m, 12H).

To a mixture of 2L (1.1g, 1.36 mmol) and t-BuOH (34 mL) was added 85% KOH powder (0.45 g).

The resulting mixture was refluxed at 90°C and the conversion was monitored via TLC (CHCl 3 /AcOH). After complete disappearance of 2L, the mixture was poured slowly (under stirring) into AcOH (34 mL) and stirred for 2h. Then 2N HCl (14 mL) was added and stirring was continued for additional 30 min. The resulting precipitate was collected by filtration, washed with water until the washings were neutral and dried at 80 °C in vacuum. The crude product was further purified by column chromatography on silica gel with CHCl 3 to remove bisimide and then with CHCl 3 /AcOH 10:1 to elute the product. 2I was obtained as a dark red solid: 0.35 g, 43%. NMR δ H (ppm) (200 MHz, CDCl 3 ): 8.66 (8H, m), 5.20 (m, 1H), 2.25 (m, 2H), 1.86 (m, 2H), 1.21 (m, 20H), 0.85 (m, 6H). 2H (300 mg, 0.36 mmol), 2M (217 mg, 0.36 mmol) zinc acetate (66.3 mg, 0.36 mmol) and imidazole (2.0 g) were added under stirring. The mixture was heated to 160°C for 2 h under Ar before it was cooled to room temperature. The solid mixture was dissolved in chloroform and was purified by column chromatography (silica gel, ethyl acetate/hexane, 1:3-1:4), and recrystallized from chloroform to methanol to give TP (381 mg, 75%) as a dark violet solid. NMR  H (ppm) (500 MHz, CDCl 3 ) 8. 52-8.11 (8H, m, H arom ), 7.54-7.37 (6H, m, H 126, 148.733, 134.178, 134.006, 130.679, 129.333, 128.814, 125.996, 125.943, 123.340, 123.270, 123.231, 123.161, 122.912, 122.627, 122.478, 107.001, 106.894, 106.698, 106.604, 106.573, 106.492, 69.705, 69.660, 69.511, 69.429, 69.347, 54.892, 40.642, 32.545, 32.004, 29.750, 29.440, 29.404, 29.286, 29.005, 28.905, 28.811, 28.657, 28.553, 28.517, 27.795, 27.280, 26.866, 25.874, 22.770, 22.749, 14.293, 14.268, 14.222 78,15; H, 8.55; N, 1.98; O, 11.32. Found: C, 78.58; H, 8.74; N, 1.92; O, 11.06. 2H (300mg, 0.36 mmol), and zinc acetate (27 mg, 0.123 mmol) were dissolved in 2g of imidazole at 160°C under stirring. Then compound 2K (64.4 mg, 0.164 mmol) was added. After 2 hours, the mixture was cooled to room temperature, and extracted with 150 mL of chloroform, washed by 100 mL 20% HCl and then washed by 100 mL brine successively. The organic phase was then dried over anhydrous MgSO 4 and concentrated under reduced pressure. The crude product was submitted to column chromatography on silica gel using DCM/MeOH=10/1 as eluant to yield the TPT (150 mg, 45%). NMR  H (ppm) (200 MHz, CDCl 3 ) 8. 22-8.18 (4H, H arom , perylene), 7.87-8.83 (4H, H arom , perylene), 7.50-7.42 (12H, H arom , triphenylene), 4.16-4.01 (28H, m) : C, 77,42; H, 8.50; N, 1.40; O, 12.70. Found: C, 77.32; H, 8.89; N, 1.71; O, 12.79. A mixture of 2N (269 mg, 1.0 mmol), 2H (830mg, 1.0 mmol), 2J (227 mg, 1.0 mmol), and zinc acetate (147 mg, 0.8 mmol) in 3.0 g of imidazole was stirred at 160°C for 2.5 hours. After being cooled to room temperature, this mixture was extracted with 150 mL of chloroform and the organic phase was washed by 100 mL 20% HCl then, by 100 mL brine successively. The organic phase was dried over anhydrous MgSO 4 and concentrated under reduced pressure. The crude product was submitted to column chromatography over silica gel using dichloromethane as eluant to yield the TNT (373 mg, 20%) and the TN (430 mg, 33%).

Triad TNT

NMR δ H (ppm) (200 MHz, CDCl 3 ) 8.41 (4H, s, H arom ), 7.68-7.71 (12H, m, H (25 MHz, CDCl 3 ) 162.71, 149.05, 149.01, 123.50, 107.11, 69.70, 29.29, 28.53, 22.74, 14.27 : C, 76,15; H, 8.84; N, 1.48; O, 13.53. Found: C, 76.46; H, 9.33; N, 1.43; O, 13.57.

Dyad TN

NMR δ H (ppm) (200 MHz, CDCl 3 ) 8.66 (4H, H arom , naphthalene), 7. 80-7.73 (6H, H arom , triphenylene), 5.15 (1H, m, N-CH), 4.26-4.07 (14H, m), 2.23-2.16 (2H, m, H aliph ), 2.02-1.88 (14H, m, H aliph ), 1.64-1.20 (54H, m, H aliph ), 0.98 (15H, t, H aliph ); 0.82 (6H, t, H aliph ); δ C (ppm) (25 MHz, CDCl 3 ) 162. 91, 149.08 149.05, 130.86, 126.43, 123.63, 107.25, 69.75, 32.43, 31.90, 29.60, 29.33, 29.29, 28.53, 27.10, 22.73, 14.27, 14.19 76,36; H, 9.07; N, 2.17; O, 12.40. Found: C, 77.12; H, 9.59; N, 2.08; O, 12.50.

CHAPTER 3 SYNTHESIS AND CHARACTERISATION OF DONOR AND DONOR-σ-ACCEPTOR SMECTIC LIQUID CRYSTAL DYADS

In this chapter, we will study two liquid crystalline dyads, combining p-type and n-type calamitic derivatives. This strategy could lead to ambipolar lamellar nanostructured materials presenting twodimensional charge transport in the sublayers. These architectures are based on terthiophene derivatives as donor moiety and pyromellitic diimide as acceptor moiety linked by alkyl chain spacers. Thus, two dyads have been designed: the first one is a donor-σ-donor model dyad based on calamitic terthiophene building blocks [START_REF] Iino | Availability of Liquid Crystallinity in Solution Processing for Polycrystalline Thin Films[END_REF]255 in order to demonstrate that we can achieve a lamellolamellar organization; the second one is a donor-σ-acceptor dyad in order to obtain a lamello-lamellar organization exhibiting ambipolar charge transport properties. The chemical structures of these calamitic dyads are given in Figure 3.1.

Figure 3.1 Chemical structures of the target calamitic dyad series

In this work, we will describe and discuss the synthesis, the thermal behavior, the self-organization, the optical, electrochemical and charge transport properties of this molecule family.

Synthesis

Synthesis of model donor-σ-donor calamitic dyad based on terthiophene moieties

The convergent synthetic route of the model calamitic dyad 3H is shown in Figure 3.2. The building block 3E was prepared from the commercially available 2-octylthiophene 3A, which was first converted into its stannane derivative in quantitative yield before a Stille coupling with the 2-bromothiphene 3C [281][282][283][284] , leading to the 2-octylbithiophene 3D. 3D was finally converted into its stannane derivative to lead to the building block 3E.

Concerning the building block 3G, it was prepared from the commercially available 2bromothiophene 3C which was first linked to another 2-bromothiophene by lithiation with nbutyllithium followed by quenching with a long dibromoalkane, to lead to the intermediate 3F. Then, with N-bromosuccinimide (NBS), the building block 3G was obtained in a satisfactory global yield.

Finally, the two previous building blocks 3E and 3G were combined through a Stille coupling reaction, to reach the target dyad 3H based on terthiophene moieties.

Synthesis of donor-σ-acceptor calamitic dyad based on terthiophene and pyromellitic diimide moieties

The synthetic route of the donor-σ-acceptor calamitic dyad 3O is shown in Figure 3.3. The building block 3K was prepared from the commercially available 2-bromothiophene 3C which was first treated with n-butyllithium followed by being quenched with a large excess of a long dibromoalkane, to lead the intermediate 3I. Then, 3I was brominated with NBS to give the next intermediate 3J which was finally combined through a Stille coupling with the building block 3E

previously prepared, to reach the donor building block 3K with a satisfactory global yield.

The building block 3N was simply prepared from the commercially available pyromellitic diimide 3L which was attacked by the 1-decanol 3M via a Mitsunobu reaction.

Finally, the two previous building blocks 3K and 3N were combined in a mild condition to lead to the donor-σ-acceptor calamitic dyad 3O.

Thermal behavior

The thermal behavior of dyads 3H and 3O were investigated by Polarized-light Optical Microscopy (POM) and Differential Scanning Calorimetry (DSC).

Polarized-light Optical Microscopy

Under POM with crossed polarizers, the model dyad 3H appeared as a birefringent yellowish powder which melted at around 121°C to give a birefringent and very viscous phase. On further heating, this viscous phase transformed at around 137°C into an isotropic fluid phase presenting no birefringence. On cooling from the isotropic state, the birefringent phase reappeared at around 134°C

showing torical focal conic textures characteristic of a lamellar mesophase (Figure 3. In the case of the donor-σ-acceptor calamitic dyad 3O, this compound appeared as a birefringent slightly yellow powder under POM with crossed polarizers. On heating, the birefringent powder melted at around 149°C to give an isotropic fluid phase. On cooling from the isotropic state a highly viscous birefringent phase is observed at about 156°C showing a crystal-like texture and indicating the presence of a very ordered mesophase (Figure 3.5). On further cooling, this mesophase seems to be maintained until room temperature.

Differential Scanning Calorimetry

The DSC thermograms obtained for the dyads 3H and 3O coincide the typical thermal behavior of low-molecular weight liquid crystal molecules, containing endothermic transitions at high temperature and corresponding to crystalline phase/mesophase or mesophase/isotropic phase transitions in The DSC thermogram of 3H recorded on heating shows three endothermic peaks (Figure 3.6.a, table 3.1). The first weak and broad peak at 60.7°C (ΔH=1.5 J g -1 ) can be attributed to a crystal/crystal transition (Cr 1 →Cr 2 ). The large second peak at 120.9°C (ΔH=46.0 J g -1 ) corresponds to the transition from the Cr 2 to the Smectic B phase (Cr 2 →SmB, see the following paragraph), as also detected by POM. The last peak at 136.7°C (ΔH=29.2 J g -1 ) can be associated to the transition from the SmB to the isotropic phase (SmB→Iso). On cooling and in agreement with observations on heating, the DSC trace shows also three peaks at 134.1°C (ΔH=29.2 J g -1 ), 109.9°C (ΔH=45.7 J g -1 ) and 44.2°C (ΔH=3.6 J g -1 ) which can be attributed respectively to the Iso→SmB, SmB→Cr 2 and Cr 2 →Cr 1 ' transitions. It is worth noticing that the high enthalpy value of the SmB→Iso transition and the slight delay for the SmB→Cr 2 transition temperature on cooling are fully consistent with the presence of ordered smectic mesophase such as SmB.

The DSC thermograms of 3O recorded on heating shows three endothermic peaks (Figure 3.6.b, Table 3.1). The first weak and very broad peak at 45.0°C (ΔH=10.0 J g -1 ) can be attributed to a progressive crystal/mesophase transition (C r →M Lam1 ). The second weak peak at 119°C (ΔH=1.4 J g -1 ) can be attributed to a mesophase/mesophase transition (M Lam1 →M Lam2 ). The last large peak at 148.9°C (ΔH=45.0 J g -1 ) corresponds to the transition to the isotropic phase.

On cooling, the isotropic-mesophase transition took place at 148.9°C with hysteresis effect. Also, differently from heating, the tiny transition between two M Lam disapeared, and only a very large mesophase-crystalline transition was observed at about 32.3°C.

The thermal behavoirs of both dyads are sammerized in Table 3

Grazing-Incidence X-ray Scattering

The thin film structure and morphology of the calamitic dyads 3H and 3O were also studied by Grazing-Incidence X-ray Scattering (GIXS). These measurements were carried out at Pohang Accelerator Laboratory (PAL, Korea), in collaboration with Prof. H-J Kim (Pusan National University). The 3H and 3O films were prepared by spin coating or thermal evaporation from a chloroform solution of material (0.25% w/w. 3000 rpm) onto bare silicon wafer. The thickness of films varies from 50 to 100 nm as evaluated by profilometer. These samples were studied after annealing (120°C for 2 hours in vacuum).

The typical two dimensional GIXS images of annealed samples of 3H and 3O are given in Figure 3.9.a and Figure 3.9.c., respectively. They both present several peaks in the specular plane (along q z ), corresponding to the (00l) reflections of the lamellar organizations. They also present numerous inplane (along q x ) and out of plane peaks which can be assigned to the (hk0) and (hkl) reflections observed as well in the wide-angle region of the bulk X-ray diffraction patterns of 3H and 3O crystalline phases after annealing (Figure 3.9.b et 3.9.d).

These typical GISX patterns confirm the lamellar organization of both dyads in the polycrystalline annealed thin films and the edge-on orientation of molecules on the substrate with the layers parallel to the surface, as illustrated in schematic models in Figure 3.9.a and Figure 3.9.c.

Figure 3.9 Spin cast film GIXS patterns after annealing at 120°C (2 hours) of a ) 3H and c) 3O on silicon wafer (with different contrast, red and black); grazing angle=0.12°. X-Ray diffraction patterns at 20°C on cooling of b) 3H and d) 3O

GIXS measurements were also performed on a thermal evaporated thin film of the model dyad 3H on silanized (OTS) substrate with the same parameters as in the fabrication of OFET (Experimental 3.5.1). The elaboration of the thin film is described in the OFET experimental part. The two dimensional GIXS image is given in Figure 3.10 and presents lamellar peaks at once in-plane (along q x ) and out-of-plane (along q z ). That confirms the similar 3D crystalline organization which was observed previously with 3H spin cast film, but also the presence of two different preferential lamellar orientations with the layers parallel and perpendicular to the substrate as illustrated in Figure 3.10.

Optical properties (absorption and emission)

The absorption and emission spectra of the terthiophene building block 3K, the model pyromellitic diimide 3P and the two dyads 3H and 3O in diluted solutions are given in Figure 3.11.

Not surprisingly, the absorption and emission spectra of the model dyad 3H (Figure 3.11c) composed of two terthiophene moieties linked by an alkyl spacer chain presents very similar optical properties to the terthiophene moiety alone 3K (Figure 3.11.a). The absorption and emission spectra of the donor-σ-acceptor dyad 3O (Figure 3.11.d) is quite similar to the model dyad 3H. Indeed, the terthiophene moiety presents a stronger absorption than

(002) (002) (hkl) (hkl)
pyromellitic diimide (Figure 3.12), and that is the reason why the absorption spectrum of 3O is dominated by the terthiophene with just a tiny shoulder at around 320 nm which corresponds to the weak pyromellitic diimide absorption. Consequently, the absorption and emission maxima were measured at 378 nm and 447 nm for both dyads, respectively.

Electrochemical behavior and HOMO/LUMO energies

The electrochemical behaviors of the model calamitic dyad 3H and the donor-σ-acceptor calamitic dyad 3O were studied using cyclic voltammetry (CV). The experimental conditions and optimized test Nevertheless, even though different kinds of experimental optimizations were carried out, there was always a difference in terms of integrated areas of oxidative and reductive waves in this voltammogram, as we should have a 1 to 1 ratio.

Thus, we can follow the same strategy and methodology described in Paragraph 2.5 to determine the HOMO and the LUMO energy levels of the donor-σ-acceptor dyad 3O (Table 3.2).

But concerning the model dyad 3H, as there is no acceptor moiety, the band gap cannot be determined by cyclic voltammetry. As a consequence, here we assume that its electric band gap equals to its optical band gap (E g opt ) which can be approximately determined based on the intersection of its absorption and emission spectra, which is around 410 nm. Thus, the E g opt was estimated to be 3.00 eV in using the following equation:

E 4.1 10 2.

10

where h is the Planks constant = 6.626 x 10 -34 J s, c is the speed of light = 3.0 x 10 8 m/s, and λ is given in nanometers (410 nm).

Consequently, its LUMO energy level can only be estimated by assuming that the electrochemistry gap is approximately equal to the optical gap (in this way, E LUMO = E HOMO + E g opt ).

Compound These values determined here are coherent with the corresponding values for terthiophene 255,[281][282][283][284][285] and pyromellitic diimide 286 reported in literature or other previous works 287 .

Charge transport property study (OFET)

The charge transport properties of both model and donor-σ-acceptor dyads were investigated in OFET configurations. These measurements were carried out at CNRS-Ewha International Research Center (CERC, Korea), in collaboration with Prof. J.-W. Wu and Prof. J.-C. Ribierre (Ewha Womans University). The details about the transistor fabrication and measurement parameters are given in Paragraph 3.5.1.

Preliminary results of the model calamitic dyad 3H

As a preliminary measurement, the thin films of model calamitic dyad were spun cast on bare silicon wafer (SiO 2 surface) in bottom contact (BC) configuration. The output and transfer characteristics are shown in Figure 3.14. The results of 3H in this configuration gave a very low hole mobility of 8.3x10 -7 cmV -1 s -1 with very weak source-drain currents, probably due to a small grain size and a large density of boundaries in the solution-processed film 284 . The effect of annealing (at 120°C for 2 hours) on the transistors was not significant and no evident changes in output and transfer characteristics were observed.

0 -2 0 -4 0 -6 0 0 -1x10 -9 -2x10 -9 -3x10 -9 Vg=0V Vg=-20V Vg=-40V Vg=-60V Vg=-80V Vg=-100V I SD (A) V SD (V)
Then thin film OFETs prepared by thermal evaporation on SiO 2 substrates were fabricated.

Vacuum deposition is a film formation known to reduce the defects and thus improve the mobility.

The output and transfer characteristic of evaporated devices are shown in Figure 3.15. The results of evaporated thin film of 3H on SiO 2 substrates gave a significant improvement on mobility with 5 orders of magnitudes to reach 1.9x10 -2 cmV -1 s -1 .

0 -2 0 -4 0 -6 0 0 -1x10 -4 -2x10 -4 Vg=0V Vg=-10V Vg=-20V Vg=-30V Vg=-40V Vg=-50V I SD (A) V SD (V)
Further, with the purpose to study the influence of thermal treatment of the thin film, the same OFET chips were then annealed at 120°C, namely in the SmB phase, during 2 hours (Figure 3.16) under nitrogen. After annealing, the mobility decreased slightly around one order of magnitude to 1.6x10 -3 cmV -1 s -1

0 -2 0 -4 0 -6 0 0 -1x10 -5 -2x10 -5 Vg=0V Vg=-20V Vg=-40V Vg=-60V Vg=-80V Vg=-100V I SD (A) V SD (V)
and the output gives a worse saturation with higher resistance of contact, indicating a reorganization of the thin film at the substrate surface.

Since the surface property of the substrate plays a very important role on the thin film molecular organization quality 288,289 and thus can affect the charge transport properties, so further, OFETs based on silanized (OTS) SiO 2 substrates were fabricated and investigated as well (Figure 3.17). The mobility of evaporated thin film of 3H on OTS surface was increased of one order of magnitude comparing to that on bare SiO 2 surface, to reach 0.1 cmV -1 s -1 . However, as already observed on the bare SiO 2 surface, the annealing decreased slightly the mobility to 5.2x10 -3 cmV -1 s -1 .

Top contact configurations were also attempted on both surfaces by evaporation, but no better OFET performances were obtained.

In order to better understand the thermal treatment influence in detail, more GISX and other measurements should be systematically carried out. This work is still in progress and further results will be discussed and published elsewhere in the soon future.

Mobilities, threshold voltage (V th ) and on/off current ratio (I on / off ) deduced from the device characteristics of 3H in their corresponding configurations are summarized in Table 3

Preliminary results of the donor-σ-acceptor calamitic dyad 3O

Based on the previous results obtained on model dyad 3H, the charge transport properties of donorσ-acceptor dyad 3O thin films were studied in OFET devices directly prepared by thermal evaporation in bottom contact (BC) configuration on a self-assembled monolayer of OTS at the surface of the SiO 2 gate dielectric layer.

The typical output and transfer characteristics of evaporated OTS-treated devices with no annealing are given in Figure 3.18. It can be found that the devices exhibit hole mobilities of around 5.4x10 -5 cmV -1 s -1 (Figure 3.18.a and 3.18.b). However, no electron transport was detected based on the output and transfer results (Figure 3.18.c and 3.18.d).

Contrary to the model dyad 3H, the annealing process at 135°C for 2 hours of 3O thin films have a beneficial effect on the charge transport properties. Indeed, the hole mobility was increased of almost 2 orders of magnitudes to reach 1.0x10 -3 cmV -1 s -1 , probably due to a favorable reorganization in the thin film (Figure 1. 19.a and 3.19.b). In addition, the appearance of n-type conduction can be observed in the electric characteristics and more particularly, in the transfer curve (Figure 1.19.d), which presents a typical V-shape of both the hole and electron transport 290,291 . However, no value of electron mobility could be extracted due to the absence of linear or saturation regimes in the output characteristics (Figure 1.19.c), which could be explained by an important unbalance of hole and electron mobilities. This unbalance could be explained by the fact that the morphology in the thin films is not optimized or by the fact that the LUMO level of acceptor moiety in dyad 3O (around -3.9

eV) is still too high for an optimum electron injection in the nanostructured layer from Au electrodes (work function of -5.1 eV).

To solve the later problem, a stronger acceptor moiety could be introduced in the donor-σ-acceptor dyad in order to present a lower LUMO level. Mobilities, threshold voltage (V th ) and on/off current ratio (I on / off ) deduced from the device characteristics of 3O are summarized in Table 3.4.

Annealing

Hole mobility (cm 2 V -1 s -1 ) Electron mobility (cm 2 V -1 s -

Conclusion

The model calamitic dyad 3H based on terthiophene-terthiophene and donor-σ-acceptor calamitic dyad 3O based on terthiophene-pyromellitic diimide were designed, synthesized and characterized.

The main results confirmed that the target lamello-lamellar organizations were obtained. They consist on alternating sub-layers containing each building block of these dyads, as demonstrated by POM, DSC and XRD. These organizations should be favorable for 2D charge transport of electrons and holes in the case of the donor-σ-acceptor dyad.

After different thermal and surface treatments, a relatively high hole transport mobility of 0.1 cm 2 V -1 s -1 was obtained from the model dyad 3H and a hole-electron transport for the donor-σ-acceptor dyad 3O.

Nevertheless, the electron transport in the donor-σ-acceptor dyad cannot be clearly characterized with the OFET parameters described in this chapter. This limitation could be explained by the mismatching between the electrode (Au) work function and the LUMO level of the acceptor moiety and/or the non-optimized nanostructures in the thin films. As a consequence, the future work on this promising design will be concentrated first on the optimization of the n-type semiconducting building block.

Experimental

OFET sample preparation and configurations

For the spun cast OFETs, the parameters used were identical in the GIXS sample preparations (see Paragraph 3.3.2 for details).

For thermal evaporation, the product was used directly without further purification or treatment.

The deposition rate used was 0.5 Å/s and the thickness was fixed as 50 nm. The annealing was effected in glove-box, for 2 hours: at 120°C for the model calamitic dyad 3H and at 135°C for the donor-σ-acceptor calamitic dyad 3O.

The Bottom-Gate (BG) OFETs in both Bottom Contact (BC) and Top Contact (TC) configurations were fabricated and characterized. The BC OFETs were commercialized Fraunhofer FET chips (thermally grown 230 nm SiO 2 as dielectric layer) and the TC transistors were fabricated using the same Fraunhofer wafers.

The substrates were sonicated in acetone and isopropanol and then placed in a piranha solution to remove the residues of organic materials and to induce OH groups in the vicinity of the surface. After rinsing the substrates again in isopropanol and tried by nitrogen flow. Then they were cleaned in a UV cleaner during 20 minutes. For the preparation of the OTS-treated substrates, the silanization was carried out in a 3 mM OTS (octyltriethoxysilane) solution of dry hexane for 90 min. All solvents were purchased from Sigma Aldrich and were used without further purification.

Gold electrodes were used in all the OFET measurements.

For the fabrication of BC devices, the product was deposited (by spin-coating or evaporation) directly to the commercialized Fraunhofer chips. This test chip gives four types of transistors. The channel lengths are 2.5, 5, 10 and 20µm respectively and the channel width is always 1 cm.

In the case of TC devices, the product was deposited on the substrate (wafer, OTS or bare silica), annealed (in some cases) and then the electrodes were deposited by thermal evaporation of the metal (gold) through a shadow mask: deposition rate, 0.5 Å/s; channel length, 40µm; channel width 1.6 cm.

OFET characteristics were measured using a double channel source meter unit (Keithley 2600) and all the tests were realized in glove-box. All the field effect mobilities were calculated on the saturation regime (see Paragraph 1.1.4.3).

Synthesis

Under argon atmosphere, 80.2 mmol of n-BuLi (2.5 M in hexane) was added drop by drop to a solution of 76.4 mmol of 2-octyl-thiophene in 60 ml dry THF which was cooled to -78°C previously.

At this temperature, the mixture was stirred during one hour. Then, 91.7 mmol of tributyltin chloride was added drop by drop during about 45 min. The temperature was increased slowly to room temperature. After 16 hours, the mixture was filtrated; the filtrate was diluted with DCM (about 150 ml) and washed thoroughly by a saturated aqueous solution of ammonium chloride. After extraction, the organic phase was dried by magnesium sulfate, filtrated and the solvent was evaporated by

Rotavapor. After a distillation, the small molar mass impurities were eliminated and yellowish oil was 65, 135.18, 133.71, 125.25, 31.88, 31.83, 30.93, 29.96, 29.37, 29.29, 29.25, 29.00, 27.32, 22.68, 14.12, 13.70, 10.78.

(Stille Reaction)

Under argon protection, triphenylphosphine (7.12 mmol) and palladium(II) acetate (1.78 mmol) were dissolved in 110ml of dry THF in order to form the tetrakis(triphenylphosphine)palladium(0) catalyst in situ. Then this catalyst was added to a DMF solution (175 ml) of 3B (42.73mmol) and 2bromothiophene (35.61 mmol) before the temperature was increased to 85°C and maintained for 12 hours. The solvent was removed by Rotavapor and the residue was dissolved in DCM (about 200 ml)

and was washed by Mili-Q water. The organic phase was extracted, dried by magnesium sulfate, filtrated and the solvent was evaporated again by Rotavapor. Then the crude was purified by column chromatography (silica gel, cyclohexane/DCM, 9:1) and afforded the title compound (39%) as a green crystal. NMR  H (ppm) (200 MHz, CDCl 3 ) 7.16(dd, 1H), 7.08(dd, 1H), 6.98(dd, 2H), 6.73 (dt, 1H), 2.78 (t, 2H), 1.28-1.42 (m, 10H), 0.88(t, 3H) ;  C (ppm) (50 MHz, CDCl 3 ) 145. 56, 138.13, 134.90, 127.80, 124.85, 123.86, 123.54, 123.14, 32.01, 31.76, 30.30, 29.47, 29.36, 29.24, 22.86, 14.36.

With the same procedure, the stannylation was carried out on 3D to give 3E as a yellow-orange oil NBS (N-bromosuccinimide) (46.93 mmol) was added in one time and this mixture was stirred during all the night. The majority of DMF was removed by Rotavapor and the final residue was dissolved in DCM (150 ml). After being washed by Mili-Q water, the organic phase was dried by magnesium sulfate, filtrated and the solvent was evaporated by Rotavapor. The crude product was purified by column chromatography (silica gel, petroleum ether) and afforded the 3G (70%) as a yellow powder.

NMR  H (ppm) (200MHz, CDCl3) : 6.83(d, 1H), 6.51(dt, 1H), 2.72(t, 2H), 1.66(quin, 4H), 1.26(m, 13H) ;  C (50MHz, CDCl 3 ) : 147. 76, 129.52, 124.49, 108.86, 31.57, 30.47, 29.59, 29.41, 29.08.

(Stille Reaction)

With the same synthesis procedure as 3D, from 3E and 3G, the compound 3H was obtained as a yellow powder with a yield of 47%. NMR  H (ppm) (200MHz, CDCl 3 ) : 6.95-6.96(m, 8H), 6.66(d, 4H), 2.78(t, 8H), 1.67(t, 8H), 1.28(m, 32H), 0.88(t, 6H) ;  C (ppm) (75MHz, o-DCB-d4, at 70°C):

146. 84, 137.95, 136.26, 126.30, 124.96, 124.82, 33.36, 33.02, 31.69, 31.03, 30.86, 30.71, 30.67, 24.13, 15.39 84, 126.80, 124.24, 122.62, 34.22, 32.97, 31.92, 30.05, 29.64, 29.53; 29.43, 29.22, 28.88, 28.30. Under argon atmosphere, 3I (21.33 mmol) was dissolved in 200ml of DMF at room temperature.

NBS (N-bromosuccinimide) (21.33 mmol) was added in one time and this mixture was stirred during all the night. The DMF was removed essentially by Rotavapor and the residue was dissolved in DCM (150 ml) before being washed by Mili-Q water. The organic phase was dried by magnesium sulfate, filtrated and the solvent was evaporated by Rotavapor. The crude product was purified by column chromatography (silica gel, petroleum ether) and afforded the title compound (67%) as colorless oil.

NMR  H (ppm) (200MHz, CDCl 3 ): δ 6.82(d, 1H), 6.51(dt, 1H), 3.40(t, 2H), 2.72(t, 2H), 1.84(quin, 2H), 1.66(quin, 2H), 1.28(m, 13H) ;  C (ppm) (50MHz, CDCl 3 ) : 147. 58, 129.41, 124.40, 108.60, 34.02, 32.90, 31.48, 30.38, 29.45, 29.32, 29.02, 28.81, 28.23.

(Stille Reaction)

With the same synthesis procedure as 3D, from 3E and 3J, the compound 3K was synthesized. The crude was purified by column chromatography (silica gel, petroleum ether/DCM, 7/3) and afforded the title compound (58%) as colorless oil. NMR  H (ppm) (200MHz, CDCl 3 ): 6. 95-6.97(m, 4H) 57, 145.48, 136.30, 134.81, 134.77, 124.94, 124.92, 123.60, 123.31, 123.30, 34.19, 32.99, 32.01, 31.76, 31.69, 30.32, 29.54, 29.47, 29.44, 29.37, 29.24, 29.16, 28.92, 28.33, 22.81, 14.24. Under argon atmosphere, the pyromellitic diimide (3L) (9.26 mmol), 1-decanol (3M) (4.35 mmol) and PPh 3 (5.21 mmol) were mixed in 100 ml DMF at 0°C. Then diisopropyl azodicarboxylate (DIAD, 5.21 mmol) was dissolved in 30 ml THF and this solution was added to the previous mixture drop by drop. The reaction was continued at room temperature for all night. After removing the solvent by Rotavapor, the residue was directly purified by column chromatography (silica gel, EtOAc/DCM, 1/9) and recrystallized in EtOAc to give the title compound (12%) as colorless crystalline powder. NMR  H (ppm) (200MHz, CDCl 3 ):8.30(s, 2H), 7.94(s, 1H), 3.74(t, 2H), 1.70(t, 2H), 1.24-1.32(m, 14H), 0.88(t, 3H);  C (ppm) (50MHz, CDCl 3 ): 166. 28, 137.71, 118.90, 32.05, 29.66, 29.61/,29.42, 29.27, 28.70, 28.58, 26.99, 22.82, 14.35.

Under argon atmosphere, compound 3K (0.54 mmol) was dissolved in DMF (30 ml) at 120°C.

Meanwhile, 3N (1.62 mmol) was dissolved also in DMF (40 ml) at 40°C, then K 2 CO 3 (1.62 mmol) was added. About 10 minutes later, the later solution/suspension was withdrawn by a syringe and then added to the previous 3K solution. The mixture was further maintained at 120°C for one night. The solvent was removed by Rotavapor and the residue was purified directly by column chromatography 166. 43, 145,41, 137.38, 136.18, 134.65, 124.80, 123.54, 123.24, 118.27, 38.86, 32.03, 31.75, 30.38, 29.58, 29.46, 29.41, 29.35, 29.30, 29.21, 29.11, 29.05, 28.49, 26.92, 14.59

(

CHAPTER 4 SYNTHESIS AND STRUCTURE-CHARGE TRANSPORT PROPERTIES RELATIONSHIP STUDY OF SIDE-CHAIN LIQUID-CRYSTALLINE π-CONJUGATED

POLYMERS 4.1 Introduction

The control of the nanoscale morphology of π-conjugated polymeric architectures, providing longrange order, and the establishment of reliable structure-properties relationships remain the major and fundamental issues in the field of organic semiconductors. In this context, recent works were developed in our laboratory 123,292 and relating to the design and elaboration of a side chain liquid crystal semiconducting polymer where: (i) the backbone was a π-conjugated polymer and (ii) the side groups were π-conjugated discotic mesogens. More precisely, this polymer was composed of polythiophene backbone and triphenylene-molecule side groups. A supramolecular self-assembling of this p-type semiconducting polymer was observed where the backbone and columns were parallel to each other in an intertwined lamello-columnar liquid-crystal structure, consisting in the alternation of rows of columns and backbone sub-layers.

In this chapter, we developed and completed the previous work on the liquid crystalline side-chain polymer structure by studying and evaluating the influence of the controlled dilution of the mesogenic side groups (i.e. the degree of polymer lateral substitution) on the supramolecular organizations and on charge transport properties. In this aim, we prepared three polythiophene derivatives, a model poly(3alkylthiophene) 4B, a homopolymer comprising triphenylene side groups on every thiophene units 4G

and an alternating copolymer with thiophene/triphenylene ratios of 2:1 4M, as illustrated in Figure

Synthesis

All the regioregular poly(3-alkylthiophene) backbones were prepared according to the McCullough procedure, namely the Grignard Metathesis (GRIM) 163,180,293 . This method is detailed in Paragraph 1.3.3.3.

Synthesis of the model poly(3-hexylthiophene) 4B

The model short regioregular poly(3-hexylthiophene) 4B, which will be used as reference in charge transport studies was synthesized in from commercially available monomer 4A by GRIM polymerization in one step (Figure 4.2). In this synthetic route, the 3-(ω-bromodecyl)thiophene 4D was first prepared by lithiation with nbutyllithium of commercially available 3-bromothiophene 4C, followed by quenching with an excess of dibromodecane. After purification by distillation to remove the excess of dibromodecane and silica chromatography to remove the α-substituted byproduct inevitably produced, the compound 4D was dibrominated in a mild condition by N-bromosuccinimide (NBS) to lead to the monomer 4E, which was then polymerized via GRIM method to form the precursory homopolymer 4F.

The synthesis of the precursory alternating copolymer 4L which presents one in two alkylchains with a brome at the end, is described in Figure 4.4. In this polymer, the 3-octylthiophene was chosen to play the role of the "diluent" and was first converted in its boronate ester 4I by the selective attack on the 4-position due to the steric hindrance of lithium tetramethylpiperidide (LiTMP) in the step (vi).

The building block 4I reacted then via a Suzuki coupling with the monobrominated 3-(ωbromodecyl)thiophene 4E', obtained by a regioselective bromination of 4D onto the 2-position with a slightly sub-stoichiometric amount of NBS, to lead the key head to tail bithiophene 4J. Finally, the "pre-regioregular" bithiophene was dibrominated by NBS to give the monomer 4K, which was then polymerized via GRIM method to form the precursory alternating copolymer 4L.

Polymers characterizations

The polymers 4B, 4G and 4M were characterized by Size-Exclusion Chromatography (SEC), NMR spectroscopy and elemental analysis. The detailed chemical structure informations such as molecular weights and polydispersity indexes (PDI) are listed in -1 , RU(4F)=310.3 g mol -1 , RU(4G)=895.3 g mol -1 , UR(4L)=495.6 g mol -1 , UR(4M)=1089.7 g mol -1 . *Here we consider each thiophene unity in the backbone is equal.

It can be noticed that as expected, the synthesis used to prepare the polymers led to well-defined polymeric architectures with narrow PDIs (ranging from 1.2 to 1.6) and highly controlled molecularweights (degree of polymerization, DPn around 45).

Concerning the two side-chain polymers 4G and 4M, the quality of the substitution was confirmed by the 1 H NMR (Figure 4.6.a and 4.6.b). In the NMR spectrums, it can be observed that after the grafting, the signal of the methylene groups (δ = 3.40 ppm) in the α-position to the terminal bromine atom has completely vanished. This confirms that the substitution is almost quantitative, considering the sensitivity of the NMR spectroscopy. Also, the composition change from elemental analysis (see experimental part of this chapter) evidences the quasi-complete substitution. It should be taken into account that the polythiophene backbone is much more rigid than the polystyrene one (standard used to determine M n by SEC), so for a polythiophene with a similar repeating unit mass to the polystyrene, the hydrodynamic radius of polythiophene will be much bigger than the polystyrene one. That is why, in our case, the molecular weight of the model short P3HT 4B measured by SEC is much more overestimated 209,294 . Nevertheless, based on the results of Liu et al 209 , the M n from SEC and the true M n (obtained by Maldi-Tof) can be correlated (Paragraph 1.3.3.6). Thus, in the case of the model polymer 4B, the M n (SEC) found of around 13.4 kDa (DPn=80), is overestimated by a factor of around 1.5, explaining the more realistic value of 9 kDa (DPn=54)

determined by NMR [DPn (SEC) ≈ 1.5 x DPn (NMR)].

Thermal behavior

The thermal behavior of 4B, 4L and 4M were investigated by Polarized-light Optical Microscopy (POM) and Differential Scanning Calorimetry (DSC).

Polarized-light optical microscopy

When observed under POM, the two substituted polymers 4L and 4M did not develop characteristic optical textures. Nevertheless, on heating, birefringent and homogeneous textures gradually appeared with the increase of temperature (Figure 4.7). Then with the further temperature 211.7°C, associated with a quite large enthalpy change (ΔH=23.8 Jg -1 ), is observed, which is characteristic of the melting point (T iso ) of the polymer. On cooling, this transition is reversible at 196.5°C with a substantial hysteresis of about 15°C. Such results are typical of regioregular P3HT thermal behaviors except that the values of T g and T iso of 4B are slightly lower than the values generally reported for regioregular P3HT 295,296 , probably due to the short length of the polymer.

In the case of the substituted homopolymer 4G, on heating, the T g appears at a slightly higher temperature (14.5°C) and the isotropization (T iso ) takes place at 107.3 °C with an enthalpy change of 7.5 Jg -1 . On cooling, the endothermic peak of the reversible transition is shifted to 85.6 °C and no T g was observed even at a high cooling rate (20°C min -1 ).

The substituted alternating copolymer 4M presents similar thermal behaviors to the substituted homopolymer 4G. However, the T g appears at higher temperature (46.1°C) as well as for the isotropization (T iso ), which is observed at 116.2°C with an enthalpy change of 4.8 Jg -1 . On cooling, the reversible transition is observed at 71.4°C and no clear T g was detected.

The summary of the polymer thermal behaviors characterized by DSC is given in Table 4 The typical X-ray pattern of the homopolymer 4G in the mesophase at 50°C, obtained on cooling from the isotropic state is given in Figure 4.9. In this pattern, several sharp and intense Bragg reflections in the small-angle region and two scattering halos in the wide-angle region can be observed.

In the wide-angle region, the very broad and diffuse halo centered at 4.5 Å (h ch ) indicates the molten state of the aliphatic chains and the other broad reflection around 3.5 Å (h T ) evidences the short range ordered π-π stacking of piled disk-like aromatic cores of the triphenylene pending groups, indicative of the development of an intermolecular stacking into the columns with a possible very week contribution of the π-conjugated polymer chains.

In the small-angle region, the sharp reflections can be described as a combination of to interlocked systems: on the one hand, a first set of layered structure reflections with a periodicity d of 50.8 Å, and on the other hand, a second set of reflections indexed in a 2D lattice of columns. The sharpness of these small-angle reflections indicates the long-range correlation of the lamellar ordering and the 2D arrangement of the columns. This organization can be described as a lamello-columnar structure (Lamcol mesophase) consisting in the alternation of triple rows of columns and of polymer backbone sublayers as illustrated in Figure 4.9. The detailed structure characterization of 4G was published previously 124 . The X-ray pattern of the alternating, copolymer 4M (Figure 4.10) which presents half of triphenylene side-groups, does not evidence any change for the Lamcol mesophase organization, for which, very similar geometrical parameters were retrieved (the inter-lamellar distance d increase slightly of 2%). The main differences with respect to the homopolymer 4G consist in the appearance, next to the h T scattering at 3.5 Å, of an additional scattering at 3.8 Å, attributed to h PT , and in the extension of the visible lamellar order series up to the eighth order. These results attest now the regular stacking of polythiophene backbones, leading to a much improved segregation in sublayers with sharper interfaces and then to a reinforced stability of the organization, as also confirmed by the extension of the mesophase range to higher temperature (see Paragraph 4.4.2). A schematic representation of the alternating copolymer 4M in the Lamcol mesophase is illustrate in Figure 4.10.

Atomic force microscopy

The thin film morphology on surfaces of the homopolymer 4G was studied by tapping mode Atomic Force Microscopy (AFM). These measurements were carried out at the Institut des NanoSciences de Paris (INSP) in collaboration with Dr. E. Lacaze.

The 4G thin films were prepared by flow casting process from an octane hot solution (75 μg ml -1 ) on silanized (OTS) silicon wafer substrates 297 , and these films were then annealed at 90°C for 20 min.

The typical AFM images of 4G thin films are given in Figure 4.11. of the bulk structure measured from the previous small angle X-ray diffraction study.

Grazing-Incidence X-ray Scattering

The thin film structure and morphology of the homopolymer 4G were studied by Grazing-Incidence X-ray Scattering (GIXS). These measurements were carried out at Pohang Accelerator Laboratory (PAL, Korea), in collaboration with Prof. H-J Kim (Pusan National University).

The 4G films were prepared by spin coating from a chloroform solution of material (1% w/w. 3000 rpm) onto an OTS treated silicon wafer. The thickness of film varies from 60 to 100 nm as evaluated by profilometer. These samples were studied after annealing (90°C for 2 hours in vacuum).

The typical two dimensional GIXS image of annealed samples of 4G is given in Figure 4.12.a. It presents several small-angle peaks in the specular plane (along q z ), corresponding to the (00l) reflections of the lamellar organization, and several out of plane peaks which can be assigned to the (0kl) reflections of the 2D lattice of columns, as observed in the small-angle region of the bulk X-ray diffraction patterns of 4G after annealing at 50°C (Figure 4.12.c). In the wide-angle region, in addition to the very diffuse halo around 1.4 Å -1 (4.5Å), corresponding to the liquid-like disorder of the aliphatic chains (h ch ), this pattern presents one broad and intense in-plane reflection (along q x ) around 1.82 Å -1 corresponding to the π-π stacking (3.5 Å) of the segregating discotic mesogens (h T ), as also observed in the bulk X-ray diffraction study.

Absorption properties in solution and solid state

The UV-Vis absorption characterizations of the homopolymer 4G and the alternating copolymer 4M were carried out both in solution and in solid state. For the solid state, the annealing effect was also studied in film.

The normalized absorption spectra of homopolymer 4G and 4M in chloroform solution at low concentration (≈10 -6 M in repeating units) are given in Figure 4.13. Two characteristic absorption bands can be observed for both polymers at the same wavelength: the sharp peaks at around 280 nm distributed to the triphenylene side-groups 130 and the broad bands at around 450 nm from the polythiophene backbones 208 . These absorption spectra are the sum of the isolated triphenylenes and the polythiophene absorptions taken separately. When the spectrum is normalized by the polythiophene absorption band, it is obvious that the corresponding integration of triphenylene absorption band of 4G was doubled compared with 4M. This is consistent with the "50% dilution" of the triphenylene side-groups in the copolymer 4M in comparison with the homopolymer 4G. Thus, the maximum wavelengths of polythiophene bands can be observed at 507 nm and 531 nm for 4G and 4M, respectively. The annealing of thin films (90°C, 2 hours) induces a strong effect on the absorption spectra, and more particularly on the polythiophene bands which are red-shifted again of several degrees of nanometers, with the appearance of vibronic structures characteristic of the reorganization of polythiophene backbones in an improved packing 298 , as also observed in temperature dependent X-ray diffraction study. Nevertheless, this reorganization appears more pronounced for the alternating copolymer 4M, as proved by the comparison of the two annealed spectra of 4G and 4M in Figure 1.14.c. Indeed, the polythiophene band of 4M presents a clear vibronic structure and is slightly more shifted in comparison with the 4G one, indicating a better aggregation and better π-π stacking interactions of polythiophene backbones. This observation is perfectly consistent with the presence of the additional scattering at 3.81Å (h PT ) already observed with X-ray pattern of 4M (Figure 4.10). In the case of the model short regioregular poly(3-hexylthiophene) 4B, the hole mobility obtained was 6.1x10 -2 cm 2 V -1 s -1 , which is consistent with the mobility values of the classical regioregular P3HT published previously 298,299 . Nevertheless, such high values are generally obtained for high molecular weight P3HTs, and short polymer chains give generally lower mobilities of one or two orders of magnitudes (10 -4 -10 -3 cm 2 V -1 s -1 ) 299 . This indicates that the 4B films of our transistors present probably an optimized structure. This value of 6.1x10 -2 cm 2 V -1 s -1 can be consequently taken as an internal reference in the discussion of the mobility-structure relationship of the following cases. In the case of the alternating copolymer 4M, the hole mobility was measured about 1.6x10 -3 cm 2 V - 1 s -1 , which is two orders of magnitude higher than the previous homopolymer 4G, and only one order of magnitude lower than the model poly(3-hexylthiophene) 4B. This relatively high value can be explained by the different thiophene/triphenylene ratio in the alternating copolymer 4M in comparison with homopolymer 4G. Indeed, as the self-organization of the polymers 4G and 4M are nearly the same, namely a lamello-columnar structure, the density of thiophene backbones is consequently doubled in the layers of 4M. That induces a more regular stacking of backbones with better π-π interactions between polythiophenes, and therefore, an improvement of charge transport properties not so far from the model P3HT 4B. This interpretation is also well consistent with the presence of the scattering at 3.8 Å in the X-ray pattern ( It is worth noticing that these results indicate that in both polymers 4G and 4M, the charge transport is probably operated almost exclusively by the polythiophene sublayers. This result could be explained by the two-dimensional charge transport of the polythiophene sublayers which will be always more efficient than the one-dimensional one of the triphenylene sublayers (columnar organization), as long as the orientation of the columns in the plane of the film could not be mastered.

Mobilities, threshold voltage (V th ) and on/off current ratio (I on / off ) of these polymers deduced from the device characteristics of these polymers are summarized in

Conclusion

In order to study and evaluate the influence of the controlled dilution of the mesogenic side groups (i.e. the degree of polymer lateral substitution) on the supramolecular organizations and on charge transport properties of side chain liquid crystal semiconducting polymeric architectures, we synthesized three different regioregular polythiophene derivatives: a model poly(3-alkylthiophene) 4B, a homopolymer comprising triphenylene side groups on each thiophene unit 4G and an alternating copolymer with thiophene/triphenylene ratios of 2:1 4M. They were prepared by Grignard metathesisbased methodology in order to control the chain lengths (in number of thiophene unities) at around 50.

The thermal and self-organization behaviors of these polymers were investigated by polarized-light optical microscopy, DSC, temperature-dependent X-ray diffraction, AFM, GIXS and temperaturedependent UV-vis spectroscopy. Both the substituted homo-and copolymer self-organize into mesophases possessing intertwined lamello-columnar structure (Lamcol mesophase), consisting in the alternation of triple rows of columns and of polymer backbone sublayers.

The hole transport mobility of polymers was tested in OFET configuration. The results indicate that first, in the case of homopolymer 4G, the disorder introduced by the triphenylene presence in the sublayers of polythiophene backbones induces low mobility (1.4x10 -5 cm 2 V -1 s -1) , and second, that in the case of the alternating copolymer 4M with thiophene/triphenylene ratios of 2:1, the increasing of the density (double) of thiophene backbones in the layers leads to a better packing in polythiophene sublayers, and consequently to the improvement of the mobility (1.6x10 -3 cm 2 V -1 s -1 ), not so far from the model poly(3-alkylthiophene) 4B (6.1x10-2 cm 2 V -1 s -1 ).

These results underline the important impact of the molecular structure design of semiconducting polymeric architectures based on two chemically different moieties on the charge transport properties of these self-organized complex semi-conducting materials.

Experimental

SEC parameters

Two apparatus were used: 1) an Agilent 1100 Series SEC equipped with a 300 mm x 7.5 PL Gel Mixed-D 5 μm 10 -4 Å, an refractive-index (RI) detector and Diode-Array UV-vis Detector (DAD); 2) a assemblied SEC constituted by three PL Gel Mixte-C 5μm columns (7.5 X 300 mm; separation limits: 0.2 to 2000 kg.mol -1 ) coupled with a solvent and sample delivery module Viscotek GPCmax and 2 modular detectors: a differential refractive index (RI) detector Viscoteck 3580 and a DAD Shimadzu SPD20-AV. In both cases the column temperature was fixed at 40°C and the flow rate at 1ml min -1 in THF (HPLC-Grade). Both SEC were calibrated by polystyrene standards.

OFET sample preparation and configurations

As 4B, 4G and 4M are polymers (macromolecules), the thermal evaporation method could not be applied in the thin film preparation, and thus only the spin-coating was used.

The parameters used during the spin-coating procedure were similar with those used in Chapter 3: spin speed=2000 rpm, ramp time=0.1 s, spin time= 60 s. The concentration used was 1% weight for the homopolymer 4G and copolymer 4M but for the model polymer 4B, as its solubility is limited, the saturated chloroform solution (<0.5%) was used.

The profilometer confirms that the films thicknesses were all below 100 nm (varies from 40 to about 100 nm). The annealing was carried out in glove-box: 150°C, 30 min for the model polymer 4B; 90°C, 20 min for both the substituted homopolymer 4G and substituted copolymer 4M.

The Bottom-Gate (BG) OFETs in Top Contact (TC) configurations were fabricated and characterized. The wafers used were purchased from MicroChemicals (thermally grown 300 nm SiO 2 as dielectric layer) and then silanized (OTS). The gold electrodes were deposited by thermal evaporation through a shadow mask at a slow rate (0.5 Å/s) directly on top of thin films to give a 40µm channel length and 1.6 cm of channel width.

The substrate cleaning method, silanization procedures, probe-station type and transistor performance calculation methods were identical to those described in Paragraph 3.5.1.

Synthesis

Synthesized from the 2,5-dibromo-3-hexylthiophene in using the GRIM method. The procedures are the same as in the synthesis of 4F.

[M] 0 /[Ni(dppp)Cl 2 ] 0 = 56 ; Yield : 64%. M n = 15500, PDI = 1. 143. 31, 128.38, 125.16, 119.88, 34.20, 32.95, 30.66, 30.39, 29.58, 29.53, 29.42, 28.87, 28.83, 28.28. Anal. Calcd for C 14 H 23 BrS: C, 55.44; H, 7.64; Br, 26.34, Found: C, 55.80; H, 7.68; Br: 25.98. 3-(10-Bromodecyl)thiophene (4D) (1.61 g, 5 x 10 -3 mol) was dissolved in 5 ml of DMF under an atmosphere of nitrogen. N-Bromosuccinimide (2.3 g, 1.3 x 10 -2 mol) in 10 ml of DMF was added drop wise to this solution at room temperature. After stirring the reaction mixture overnight, the reaction mixture was poured into cold water (50 ml) and extracted with 3 x 30 ml of diethyl ether. The organic phase was washed with water (3x30 ml), dried over anhydrous MgSO 4 and concentrated under reduced pressure. The crude product was submitted to column chromatography over silica gel using hexane as eluant to yield the title compound (1.98 97, 109.05, 107.64, 34.20, 32.96, 30.56, 29.56, 29.50, 29.48, 29.32, 29.01, 28.87, 28.30 98, 133.80, 130.59, 128.73, 34.16, 34.14, 32.95, 30.64, 29.64, 29.59, 29.56, 28.91, 28.31 9, 128.2, 125.1, 108.8, 34.0, 32.8, 29.7, 29.4, 29.4, 29.4, 29.3, 29.2, 28.7, 28.2. To a solution of 2,2,6,6-tetramethylpiperidine (3.6 g, 25.5 mmol) in 50 ml of dry THF, 10.2 ml n-BuLi (2.5 M in hexane) was added drop wise at -10°C. The mixture was stirred for another 30 min at -10°C and then cooled down to -78°C. A solution of 3-octylthiophene (4H) (5.0 g, 25.5 mmol) in 20 ml of dry THF was then added slowly and the system was kept under -78°C for 1 h. 7, 138.5, 133.7, 127.5, 84.0, 31.9, 30.7, 30.1, 29.4, 29.3, 29.2, 24.7, 22.7, 14.2. To a 35 ml of dimethoxyethane solution containing 4I (1.81 g, 5.6 mmol) and 4E' (2.15 g, 5.6 mmol), catalyst Pd(Pph 3 ) 4 (323 mg, 0.28 mmol) was added followed by a 20 ml of 1M NaHCO 3 . This mixture was heated to reflux for 16 h and cooled. After filtration, the organic layer was separated and the solvents were removed by vacuum evaporation. The residue was charged to a silica column (hexane as eluant) to give the product 4J as light yellow oil: 2. 139.3, 135.9, 131.0, 129.9, 127.4, 123.5, 119.9, 34.0, 32.8, 31.9, 30.6, 30.3, 29.6, 29.4, 29.4, 29.1, 28.7, 28.2, 22.6, 14 5, 140.5, 134.3, 132.5, 131.5, 127.3, 110.7, 109.1, 34.0, 32.9, 31.9, 30.6, 29.7, 29.4, 29.3, 29.3, 29.3, 29.0, 28.8, 28.2, 22.7, 14.2. To a three-necked 50 ml flask was added 4K (880 mg, 1.3 mmol) and 13 ml dry and oxygen-free To a solution of 2,3,6,7,10-pentakis(pentyloxy)-11-hydroxytriphenylene (T) (405 mg, 0.6 mmol) in 90 ml of DMF, K 2 CO 3 (331 mg, 2.4mmol ) was added and the solution was heated up to 80°C for 1 h.

Then a solution of 120 mg 4L dissolved in 5 ml of THF was added and the mixture was heated to 120°C and the reaction was stirring for 3 days before it was poured into 200 ml of water. The organic materials were extracted by chloroform and washed with brine and dried over anhydrous MgSO 4 .

After removal of the solvents, the residue was redissolved in 5 ml chloroform and precipitated in 150 ml of MeOH. The precipitates were washed with acetone (6 h) and then EtOH (2 days) by using soxhlet apparatus, and dried under vacuum, 110mg, yield of 37%. M n = 17800, PDI = 1.6 (SEC-THF,

Introduction

Ambipolar materials with high and balanced electron and hole mobilities would be of interest for a range of applications such as complementary-like logic circuits [START_REF] Anthopoulos | Organic complementary-like inverters employing methanofullerene-based ambipolar field-effect transistors[END_REF]300 and light-emitting field-effect transistor [START_REF] Muccini | A bright future for organic field-effect transistors[END_REF][START_REF] Raffaella Capelli | Organic light-emitting transistors with an efficiency that outperforms the equivalent light-emitting diodes[END_REF][START_REF] Rost | Ambipolar light-emitting organic field-effect transistor[END_REF]234,301,302 . Here, to access this class of materials, we propose an approach taken advantage of the results obtained in the previous chapter.

In Chapter 4,

Synthesis

The synthesis of side-chain polymers can be generally considered either by "direct polymerization" of a monomer bearing already the side-group 303,304 , or by "post-functionalization" of the polymeric backbone 123,195,305 . Considering that the perylene diimide side-groups are bulky and have a strong tendency to aggregate, we first tested the "direct polymerization" strategy to prepare the polymers 5C

and 5D and to ensure the complete substitution.

Synthesis by "direct polymerization"

The "direct polymerization" strategy considered for synthesizing the homopolymer 5C consisting of regioregular (3-alkyl)polythiophene backbones and perylene diimide side groups is given in Figure 5.2. In this synthetic route, the Gabriel synthesis was performed on the 3-(ω-bromodecyl)thiophene 4D

to transform the brome on terminal alky chain to an amine before to undergo an imidification condensation to introduce the perylene diimide derivative 2M onto the thiophene. After a mild bromination with NBS, the perylene diimide-functionalized monomer was prepared. Unfortunately, this later monomer decomposed during the final GRIM polymerization due to the Grignard Metathesis conditions and more particularly the Grignard reagent which seems incompatible with the imide functions. Consequently, no polymer was formed and the perylene diimide was found dissociated from the thiophene moiety.

Synthesis by "post-functionalization"

Since the "direct polymerization" is not compatible with the imide function of the perylene side groupe, we secondly considered the "post-functionalization" strategy for preparing the polymers 5C

and 5D as described in the Figure 5.3. This strategy is based on the use of the perylene diimide building bloc 2M (see Chapter 2) which was first converted into the perylene derivative 5A by treatment in NH 4 OAc/acetic acid, and then grafted onto the precursory homopolymer and the alternating copolymer 4F and 4L developed in Chapter 4, with similar conditions to the triphenylene substitution. It is worth noticing that in order to avoid using an excess of base which might attack the other imide groups, 5A was firstly reacted with the K 2 CO 3 in stoichiometric condition to form its potassium salt. Then this salt was introduced to the precursory polymers 4F or 4L in fairly excessive quantity to reinforce the substitution ratio. As the excess of perylene was difficult to be removed by Soxhlet extraction and normally precipitates with the final polymers themselves, the crude polymers were then directly purified by the semi-preparative SEC (in chloroform). In both cases, the fractions from 21-28 min were collected as the target polymers 5C and 5D (Figure 5.4). Then the fractionated polymers were characterized by 1 H NMR to evaluate the substitution ratio. In Figure 5.5, in the case of the homopolymer 5C, it can be observed that the residual peak of the proton in α position of brome (at 3.37 ppm) persisted after the grafting reaction and based on its integration, the substitution ratio was estimated as 85%. Concerning the alternating copolymer 5D, this signal at 3.37 almost completely vanished, confirming that the substitution is almost quantitative (considering the sensitivity of the NMR spectroscopy). It is worth noticing that, due to the bulky size of the perylene diimide side-groups and the steric hindrance induced around the terminal Br, it is not surprising that the substitution on the homopolymer was not complete while it was complete for the alternating copolymer presenting substitution sites perfectly distributed every tow thiophenes.

Thermal behavior

The thermal behavior of the homopolymer 5C and alternating copolymer 5D were investigated by Polarized-light Optical Microscopy (POM) and Differential Scanning Calorimetry (DSC).

Polarized-light optical microscopy

Under POM, the homopolymer 5C, in its pristine state, appeared as a solid dark "plastic" showing very weak birefringence which disappeared at around 80° on heating (Figure 5.6.a). On further heating, the polymer softened gradually into a very viscous material at very high temperature (250°C). On cooling and on additional thermal treatment cycles, no birefringence was observed again. Concerning the alternating copolymer 5D, no birefringence was observed in this pristine state (Figure 5.6.b) during the heating and cooling cycles. Contrary to homopolymer 5C, the pristine dark solid melted into a very viscous liquid at around 160°C.

Differential scanning calorimetry

The DSC thermograms of the polymers 5C and 5D are given in Figure 5.7. The homopolymer 5C presents, on the first heating (rate 20 °C/min), a clear glass transition (T g) at 53.8°C, associated with a classical volume relaxation peak which is not visible anymore on the other heating. No other evident transitions were observed at higher temperature neither on heating nor on cooling. For the alternating copolymer 5D, a T g was also observed at slightly higher temperature (61.8°C). On further heating, an endothermic peak at 167.7°C, associated with an enthalpy change of 4.8 Jg -1 , is observed, which is characteristic of the melting point (T mel ) of the polymer as also observed in POM. On cooling, the reversible transition is very difficult to observe and only the Tg can be really observed.

The DSC thermal analysis results are listed in Diffractograms revealed that the annealing at higher temperature and cooling down at room temperature doesn't lead to better structured sample as proved in Figure 5.8.c and by the slightly decreasing of small angle lamellar orders and the scattering at 3.80 Å (h PT ). The annealing at 100°C until the T mel during 1 hour does not change the structure as proved by the similar patterns obtained (Figure 5.9.b). However, the annealing at 200°C (above the T mel ) and cooling down at room temperature leads to an amorphous sample as shown in Figure 5.9.c: the isotropic phase seems to be frozen on cooling, considering the cooling rate used in our measurements (5-10°C/min), which is consistent with DSC observations.

Grazing-Incidence X-ray Scattering

The thin film structure and morphology of alternating copolymer 5D were studied by Grazing-Incidence X-ray Scattering (GIXS). These measurements were carried out at Pohang Accelerator Laboratory (PAL), in collaboration with Prof. H-J Kim (Pusan National University). GIXS results are shown in Figure 5.10.

The 5D film was prepared by spin coating from a chloroform solution of material (0.5% w/w. 3000 rpm) onto an OTS treated silicon wafer. The thickness of film varies from 50 to 60 nm as evaluated by profilometer. The samples were studied after annealing (150°C for 2 hours in vacuum). The typical two dimensional GIXS image of annealed samples of 5D is given in Figure 5.10.a. In the small-angle region, the pattern presents several broad peaks in the specular plane (along q z ), corresponding to the (00l) reflections of the lamellar organization, as observed in the small-angle region of the bulk X-ray diffraction patterns (Figure 5.10.b). It presents also one diffuse ring at 0.3 Å -1 which is compatible with the average lateral distance between perylene diimide piles (D PDI). In the wide-angle region, in addition to the very diffuse halo around 1.4 Å -1 (4.5Å), corresponding to the liquid-like disorder of the aliphatic chains (h ch ), this pattern presents two weak broad in-plane reflections (along q x ) around 1.7 Å -1 and 1.82 Å -1 corresponding respectively to the stacking (3.8 Å) of polythiophene backbones (h PT ) and the π-π stacking (3.5 Å) of the segregating perylene diimide (h PDI ).

These results indicate a poor lamellar structure with short-correlated organizations inside sublayers.

The thin film exhibits the coexistence of a disordered and Lamcol organization presenting, for the later, a weak preferential orientation of lamellae in parallel to the substrate.

Preliminary charge transport property study (OFET)

The charge transport properties of the homopolymer 5C and the alternating copolymer 5D were investigated in OFET configurations. These measurements were carried out at CNRS-Ewha The as prepared transistors of homopolymer 5C presented a nice p-channel output and transfer profiles and a hole mobility of 1.0x10 -6 cm 2 V -1 s -1 can be determined Unfortunately, no clear n-type behavior was observed. It can be observed that the annealing has an impact on the charge transport properties of polymers 5C and 5D since that allows to obtain ambipolar behavior with relatively well balanced mobilities, probably due to the slight improvement of the thin film structures. However, the mobilities are obviously low and the characteristic of these ambipolar transistors are quite modest (low I ON /I OFF ratio and high threshold voltage). That could be explained by the difficulty to master and optimize the film preparation (orientation, alignment) as suggested by the first GISX measurements. Nevertheless, these preliminary promising results show that it is possible to inject holes and electrons into a single semiconductor from the same type of electrode, by combining purely p-type and n-type entities in a unique macromolecular architecture

Conclusion

Based on the results of the previous chapter (Chapter 4) focusing on the design and charge transport properties of a p-type model liquid crystalline side-chain polymers, we developed in this last

Chapter a new family of donor--acceptor liquid crystalline side-chain polymers combining a p-type regioregular (3-alkyl)polythiophene backbone and n-type perylene diimide side-groups in a unique self-organized polymeric architecture.

More precisely, we functionalized the precursory homopolymer and alternating copolymer developed in Chapter 4 by perylene diimide derivatives, to reach these two perylene diimides grafted regioregular polythiophenes: a homopolymer comprising perylene diimide side-groups on each thiophene unit 5C (85% substituted) and an alternating copolymer with a thiophene/perylene diimide ratio of 2:1 5D. Both polymers have a chain length around 50 (in number of thiophene units) which was controlled by the GRIM polymerization during the precursory polymer preparation.

The thermal and self-organization behaviors of these polymers were investigated by polarized-light optical microscopy, DSC, temperature-dependent X-ray diffraction and GIXS. Both the substituted homo-and copolymer self-organize into mesophases possessing intertwined lamello-columnar structure (Lamcol mesophase), consisting in the alternation of double rows of side-groups and of polymer backbone sublayers.

The charge carrier mobility of polymers was tested in OFET configuration. The preliminary results

indicate that both polymers present ambipolar charge transport properties with a hole and electron mobility up to 3.3x10 -5 cm 2 V -1 s -1 and 3.3x10 -6 cm 2 V -1 s -1 respectively, for the homopolymer 5C.

However, we can notice that these low mobilities are probably underestimated due to the difficulty to master the orientation and structure during the thin film elaboration, as suggested by the GIXS results.

Nevertheless, these preliminary promising results validate our approach based on two different donor and acceptor moieties combined in a unique liquid-crystalline polymeric architecture. This novel self-organized complex semiconducting materials can propose a simple and versatile strategy for producing distinct conductive channels at the nanoscale, allowing the injection of holes and electrons from the same type of electrode and leading to a new class of supramolecular ambipolar materials, easily processable, potentially suitable for electronic and optoelectronic applications.

Experimental

Semi-preparative SEC parameters

The semi-preparative SEC parameters were as follows: Steric exclusion was carried out on two successive columns at room temperature, PLgel mixted 10 µm 500 Å (300*25mm) and PLgel mixted 10µm 1000 Å (300*25mm), respective separation limits of 500-30000 and 500-60000 g mol -1 . The automative apparatus used was a model of Agilent: system GPC/SEC ChemStation 1100/1200.

GIXS sample preparation

The samples used in the GIXS were prepared by spin coating.

For both polymers, the films were spun cast from a filtrated (0.4µm) 0.5% wt. chloroform solution onto silanized (OTS) wafer. Spin speed=3000 rpm, ramp time=0.5s, spin time= 60s. The thickness of film was around 50±15 nm (tested by profilometer). These films were then annealed at 200°C for 2 hours in vacuum.

OFET sample preparation and configurations

The thin films of polymers were spun cast onto the test chips with the same parameters in GIXS.

The OFET test chips used were fabricated with MicroChemicals wafer (thermally grown 280 nm SiO 2 as dielectric layer) and the interdigitated Au electrodes. The parameters of the interdigitated transistors are as follows: channel length=25 µm, channel width=1 mm, total channel width= 2cm.

The substrate cleaning method, silanization procedures, probe-station type and transistor performance calculation methods were identical to those described in Paragraph 3.5.1.

Synthesis

A mixture of 2M (1.0 g, 1.66 mmol) and NH 4 OAc (13.0 g, 168.6 mmol) were dissolved in 50 ml of acetic acid and heated to reflux for 24 hours. Then the crude product was poured to brine and extracted by DCM, dried over anhydrous MgSO 4 and the solvent was then removed under reduced pressure.

After a purification by column chromatography over silica gel using a 5% acetic acid solution in chloroform as eluant, 5A was obtained as dark red solid (930 mg, quantitative); NMR,  H (ppm) ( 200MHz, CDCl 3 ) 8. 59-8.75 (m, 8H), 8.53 (br. s, 1H), 5.10-5.25 (m, 1H), 2.13-2.37 (m, 2H), 1.77-1.96 (m, 2H), 1.08-1.44 (m, 20H), 0.82 (t, 6H).

The homopolythiophene precursor 4F (133.8 mg, 0.444 mmol in repeating unit) was dissolved in 5 ml of DMF at 100 °C. In another flask, 5A (800 mg, 1.33 mmol) and K 2 CO 3 (184 mg, 1.33 mmol) were added in 5 ml DMF at 100°C and stirred for about 10 minutes. This suspension was added quickly (to keep the temperature) to the previous precursor polymer solution through a filter to remove the excess K 2 CO 3 and stirred overnight at 130°C. The mixture was then extracted between chloroform and water, dried over anhydrous MgSO 4 and the solvent was then removed under reduced pressure. ). (The NMR integrations listed are theoretical, through the residual peak at 3.37 (br s) which comes from the proton at the α position of the un-reacted brome and its integration, the substitution was estimated as 85%)

The copolythiophene precursor 4L (120 mg, 0.242 mmol in repeating unit or in Brome) was dissolved in 5 ml of DMF at 100 °C. In another flask, 5A (290 mg, 0.484 mmol) and K 2 CO 3 (134 mg, 0.968 mmol) were added in 5 ml DMF at 100°C and stirred for about 10 minutes. This suspension was added quickly (to keep the temperature) to the previous precursor polymer solution through a filter to remove the excess K 2 CO 3 and stirred for 4 hours at 130°C. Then another same suspension of 5A was prepared and added in the same way and the reaction continued overnight. The mixture was extracted between chloroform and water, dried over anhydrous MgSO 4 and the solvent was removed under reduced pressure. The crude product was precipitated from chloroform to cold MeOH and filtered through a Soxhlet thimble. After extracted in Soxhlet with MeOH and EtOH, the mixture was fractionated by the preparative SEC and gives a black solid (79 mg, 32%

CONCLUSIONS AND PERSPECTIVES

During this work, we designed, synthesized and characterized several different self-organized semiconducting architectures incorporating different kind of electron-donor (p-type) and electronacceptor (n-type) π-conjugated systems in unique liquid crystalline molecular or macromolecular architectures. We expect these ambipolar materials could present a spontaneous nanosegregation of ptype and n-type entities in bulk, leading to well defined distinct conductive channels for each type of charge carriers.

More precisely, we started this work with the molecular approach, in designing two innovative families of small molecular donor-π-acceptor liquid crystalline dyads and triads incorporating either calamitic or discotic entities (Chapters 2 and 3).

-First, we chose conventional discotic semiconductors such as triphenylene derivatives as p-type building blocks and perylene or naphthalene derivatives as n-type building blocks the linked by alkyl chain spacers (Chapter 2). Then, after developing an efficient chemical strategy, we determined and discussed their thermal behavior, self-organization, as well as the optical and electronic properties of these new molecules. In particular, it must be noticed that most of the molecules obtained, namely the triphenylene-perylene based dyad TP and triad TPT together with the triphenylene-naphthalene based triad TNT, are liquid crystalline, self-organizing into well defined columnar nanostructures which can even be preserved at room temperature, in glassy state. Moreover, the ambipolar properties of these molecules were confirmed by cyclic voltammetry. Among these compounds, we consequently focused essentially on the dyad TP and we performed AFM and GIXS measurements on solid films evidencing that this material can self-organize into hexagonal columnar structures and orientates with the columns in parallel with surface: it is indeed a favorable property when looking for efficient organic transistors where the preparation of lying columns is a key factor. Unfortunately, in the liquid crystal and/or solid state, neither TOF nor OFET charge transport studies have revealed any mobility until now while an untypical charge transport was observed in its isotropic state which could be explained by the same photon transfer in solution absorption: a short correlation distance in the columnar and/or a non-optimized in-plane orientation of columns could be the two main reasons.

-Second, we synthesized and fully characterized two new compounds, based this time on usual calamitic semiconductors such as p-type terthiophene derivatives and n-type pyromellitic derivatives, both being linked by the same type of alkyl chain spacer (Chapter 3). Both of them revealed smectic liquid crystal mesophases, but and more interestingly through XRD, GIXS and then OFET studies, a charge transport in 2D smectic/lamellar sub-layers was confirmed. Finally, after testing and optimizing different thermal and surface treatments, a relatively high hole transport mobility of 0.1 cm 2 V -1 s -1 was obtained from the model terthiophene-terthiophene dyad Ter-Ter. Concerning the donor-σ-acceptor dyad Ter-Pyr, a lower hole mobility (1x10 -3 cm 2 V -1 s -1 ) was recorded and an weak electron transport was also observed. Nevertheless, the electron mobility in this donor-σ-acceptor dyad could not be evaluated from the OFET results till now, maybe due to a too important mismatching between the electrode (Au) work function and the LUMO level of the acceptor moiety and/or the non-optimized structures in the thin films: new attempts are currently in progress by modifying and adapting the transistor elaborations.

Concerning our macromolecular approach, we followed two different objectives:

-First (Chapter 4), based on the previous work initiated in our laboratory on a model p-type liquid crystalline side-chain polymer, consisting of a regioregular (3-alkyl)polythiophene as backbone and triphenylene discotic mesogens as side-groups, we developed and completed it in studying and evaluating the influence of the controlled dilution of the mesogenic discotic side groups (i.e. the degree of polymer lateral substitution) on the supramolecular organizations, and in correlating them with their charge transport properties obtained in field effect transistor configuration. To do so, we prepared by Grignard metathesis-based methodology one regioregular model rr-P3HT, one fully substituted homopolymer and one "diluted" substituted alternating copolymer, the chain length (in number of thiophene unities) being chosen around 50 for every polymers.

Remarkably, both the substituted homo-and copolymer self-organize into mesophases possessing intertwined lamello-columnar morphologies, resulting from the simultaneous coexistence of lamellar and columnar sublattices. Moreover, by tuning the molecular structure and thus the self-organization, the charge carrier mobility could be adjusted; the short model rr-P3HT 4B presenting a good hole mobility value, the substituted homopolymer 4L showing a much lower one and the substituted copolymer 4M having intermediate values, close to model rr-P3HT 4B.

-Second, to access to a class of ambipolar materials and in taking advantages of the precedent study in chapter 4, we propose an approach based on the design and synthesis of two donor-acceptor liquid crystalline side-chain polymers combining a p-type backbone and n-type side-groups in a unique self-organized polymeric architecture (chapter 5). More precisely, these two polymers are formed by a regioregular polythiophene backbone and perylene diimide side groups. The first one is a homopolymer comprising perylene diimide side-groups on each thiophene unit 5C and the second one is an alternating copolymer with a thiophene/perylene diimide ratio of 2:1 5D. Both polymers have a chain length around 50 (in number of thiophene units) which was controlled by the GRIM polymerization during the precursory polymer preparation. The structural characterization showed that both substituted homo-and copolymer self-organize into mesophases present an intertwined lamellocolumnar structure, consisting in the alternation of double rows of side-groups and of polymer backbone sublayers.

The preliminary studies of charge transport in OFET configuration indicate that both polymers present ambipolar charge transport properties with a hole and electron mobility up to 3.3x10 -5 cm 2 V -1 s - 1 and 3.3x10 -6 cm 2 V -1 s -1 respectively, for the homopolymer 5C. However, these low mobilities are probably underestimated due to the difficulty to master the orientation and structure during the thin film elaboration, as suggested by the GIXS results.

All this study and the preliminary results on charge transport properties are promising and validate our approach based on two different donor and acceptor moieties combined in a unique liquidcrystalline molecular or macromolecular architecture. This novel self-organized complex semiconducting materials can propose a simple and versatile strategy for producing distinct conductive channels at the nanoscale, allowing the injection of holes and electrons from the same type of electrode and leading to a new class of supramolecular ambipolar materials, easily processable, potentially suitable for electronic and optoelectronic applications.

Figure 1 . 1

 11 Figure 1.1 Schematic representations of the bonding in a) ethane, b) ethylene and the molecular orbitals in ethylene

Figure 1 . 1

 11 Figure 1.1 Chemical structures of typical small molecule (oligomer) and polymer type organic semiconductors

Figure 1 . 3 .

 13 Figure 1.3. Operation principles of monolayer(a) and multilayer (b) OLEDs

Figure 1 . 9 .

 19 Figure 1.9. Operation processes of pn-heterojunction OPVs.

Figure 1 .

 1 Figure 1.5 shows a typical OFET structure (or more appropriately, a usual thin film transistor).

Figure 1 . 5

 15 Figure 1.5 Organic field effect (thin-film) transistor structure with highlighted accumulation of electrons and holes for n-and p-channel semiconductors, respectively

Figure 1 . 6

 16 Figure 1.6 Schematic representations of various OFET configurations[START_REF] Wang | Semiconducting π-Conjugated Systems in Field-Effect Transistors: A Material Odyssey of Organic Electronics[END_REF]

Figure 1 .

 1 Figure 1.20 Principle of the time of flight measurement. Left: schematic view of the carrier generation and transport. Right: resulting time dependent current 31 .

Figure 1 .

 1 Figure 1.21 Schematic representation of the OFET operation principles: 1) gate voltage applications; 2) application of the voltage between the source and the drain (linear regime,) reaching the saturation regime; 3) I DS -V DS curve at various V Gvalues when V D is applied (liner regime and in the saturation regime)

Figure 1 .

 1 Figure 1.22 Example of a monolayer and a bilayer configuration OFET[START_REF] Lindner | Operation and properties of ambipolar organic heterostructure field-effect transistors[END_REF]

Figure 1 .

 1 Figure 1.23 Schematic representation of device structures and of main optoelectronic processes occurring in an OLET[START_REF] Muccini | A bright future for organic field-effect transistors[END_REF]

Figure 1 .

 1 Figure 1.24. a) Different shapes of liquid-crystalline molecules. b) Examples of calamitic and discotic molecules.

Figure 1 .

 1 Figure 1.25 Schematic representation of the nematic mesophase in the case of the calamitic (a) and discotic (b) mesogens

Figure 1 .

 1 Figure 1.26 Typical schlieren textures observed by polarized optical microscopy for nematic phases

Figure 1 .

 1 Figure 1.27 Schematic representation of the organization of chiral nematic mesogens

Figure 1 .

 1 Figure 1.28 Some examples of cholesteric textures

Figure 1 .

 1 29.a), the molecules are oriented parallel to the normal of the layers and form uniaxial phases, while in the smectic C (SmC) phase (Figure 1.29.b), the molecules are inclined at an average angle of θ relative by the normal of the layers.

Figure 1 .

 1 Figure 1.29 Schematic representation of the organizations of the mesogens in (a) Smectic A and (b) Smectic C mesophases and (c) some examples of mesogens which can form smectic phases

Figure 1 .

 1 Figure 1.30 Schematic representation of the mesogenic organization within the layer in Smectic B (a) Smectic I (b) and Smectic F (c)

Figure 1 .

 1 Figure 1.31 Typical focal conic textures of Smectic A phase

Figure 1 .Figure 1 .

 11 Figure 1.32 Schematic representations of the (a) columnar mesophase; (b) hexagonal (CoI h ) columnar arrangement; (c) rectangular (Col r ), and oblique (Col o ) columnar arrangement

Figure 1 .

 1 Figure 1.33 Typical optical textures of columnar mesophase

Figure 1 .

 1 Figure 1.34 Types of liquid crystalline polymers derived from rod-shaped and from disk-shaped mesogenic molecules

Figure 1 . 35 .

 135 Figure 1.35. Charge transport in different orientations. A) crystalline: good charge transport, B) amorphous: no charge transport, C) polycrystalline: grain boundaries pin the charges on larger length scales.

Figure 1 .

 1 Figure 1.36. A) Hexagonal assembly of discotic columns (Colh). B) Columnar rectangular mesophases (Colr). C)Charge transport in columnar stacked discotics. The aromatic cores (black) form conductive 1D channels, while the surrounding alkyl-chains (gray) act as an insulating layer around the cores.

Figure 1 .

 1 Figure 1.37 Charge transport model in discotic columnar and calamitic smectic liquid crystals, the arrows indicate the direction of electronic charge hopping.

Figure 1 .

 1 Figure 1.38 Discotic p-type semiconducting liquid crystal small molecules

Figure 1 .

 1 Figure 1.39 Calamitic p-type semiconducting liquid crystal small molecules

  .40 M1) forming easily large-area ordered films after thermal treatment in the liquid crystal mesophase, reaching a mobility of 0.02 cm 2 /Vs by both TOF and OFET measurements. Another case we can notice is an α-chain modified C10-BTBT(Figure 1.39). Méry et al. reported as well a [1]benzothieno[3,2-b][1]benzothiophene-2,7-dicarboxylate (Figure 1.40 M2) in which both hole and electron transports were observed 87 .

Figure 1 .

 1 Figure 1.40 Other calamitic p-type semiconducting liquid crystal small molecules

Figure 1 .

 1 Figure 1.42 some rylene diimide small molecules which give n-type transport properties

Figure 1 .

 1 Figure 1.42 Some perylene diimide based n-type semiconducting liquid crystal small molecules

Figure 1 .( 2 )

 12 Figure 1.43 Some polyfluorenes based Main-chain liquid crystal polymers

Figure 1 .

 1 Figure 1.44 Some cases of polythiophene based semiconducting side chain liquid crystal polymers

Figure 1 .

 1 Figure 1.45 Some cases of polythiophene based semiconducting side chain liquid crystal polymers

Figure 1 .

 1 Figure 1.46 Liquid crystal donor-acceptor dyads and triads 1.2.7.2 Combined systems: donor-acceptor polymers

Figure 1 .

 1 Figure 1.47 Donor-acceptor polymers

Figure 1 .

 1 Figure 1.48 Low band-gap liquid crystal small molecules

Figure 1 .

 1 Figure 1.49 Low band-gap self-organized polymers

Figure 1 .

 1 Figure 1.50 Triphenylene based discotic liquid crystal molecules

Figure 1 .

 1 Figure 1.51 The "multi-step" method for the synthesis of monohydroxy-functionalized triphenylene

Figure 1 .

 1 Figure 1.52 The "post-cleavage" method for the synthesis of monohydroxy-functionalized tryphenylene

Figure 1 .

 1 Figure 1.53 The "Statistic direct cyclization" method for the synthesis of monohydroxy-functionalized tryphenylene

Figure 1 .

 1 Figure 1.54 Perylene related diimides (Figure extracted from reference 146

Figure 1 .

 1 Figure 1.55 Functionalization of perylene imide position

  poly(styrenesulfonate) (PEDOT-PSS) 153-155 in conducting and hole-transport layers of OLEDs and OPVs and also the so-called regioregular or head-to-tail coupled poly(3hexylthiophene) (P3HT) 156-158 which acts as a semiconductor in OFETs and OPVs. Both are even rather rare examples of commercially produced conjugated polymers today (Figure 1.57).

Figure 1 .

 1 Figure 1.57 Commercially produced thiophene based conjugated polymers

Figure 1 .

 1 Figure 1.58 Some examples of oligothiophenes based materials widely studied in organic electronics.

Figure 1 .

 1 Figure 1.59 Traditional syntheses incorporate multiple regioisomers 163

  of a Grignard reagent (alkyl, aryl) with the thienyl halide 8 generates a ca 85:15 mixture of 11 and 12 and an inert alkyl halide. Polymerization with a congested nickel catalyst and purification recovers >99% HT PT in high yields (Mn = 20 000 35000, PDI = 1.2-1.4).

Figure 1 .

 1 Figure 1.61 The Rieke (upper) and Grignard metathesis (GRIM) (lower) syntheses of regioregular polythiophenes

  polydispersities 188 and furthermore, by adding a new portion of a different monomer at the end of the first polymerization, a further increase in the molecular weight is observed, allowing this way a nice opportunity to obtain block copolymers 189 (Figures 1.62).

Figure 1 .

 1 Figure 1.62 Left Conversion (filled symbols) and logarithm of monomer concentration (open symbols) vs time plots for 2bromo-3-hexylthiophene polymerization at different concentration of Ni(dppp)Cl 2 initiator (23-25 °C); [M] 0 = 0.075 mol L -1 : (■, □) [M] 0 :[Ni(dppp)Cl 2 ] = 136:1; (•, ○) [M] 0 :[Ni(dppp)Cl 2 ] = 57:1; (▲, Δ) [M] 0 :[Ni(dppp)Cl 2 ]=49:1. Right GPC traces for 2-bromo-3-hexylthiophene polymerization (23-25 °C); [M] 0 = 0.075 mol L -1 ; [M] 0 :[Ni(dppp)Cl 2 ] =57:1. The McCullough method was used for polymerization of 2-bromo-3-hexylthiophene 187

Figure 1 .

 1 Figure 1.63 Mechanism of Grignard metathesis method for the synthesis of regioregular poly(3-alkylthiophene) 188 .

Figure 1 .

 1 Figure 1.66 Post-polymerization Functionalization of Poly(3-(6-bromohexyl)thiophene)177

Figure 1 .

 1 Figure 1.67 1 H NMR (500 MHz) spectra of (a) rr-poly(3-hexylthiophene) H/Br terminated; (b) expansion of rr-poly(3hexylthiophene) H/H terminated; and (c) expansion of rr-poly(3-hexylthiophene) H/Br terminated. 188

  UV-Vis spectroscopy. Intensely absorbing π →π* transitions, correlating with the HOMO-LUMO gap, provide a qualitative measure of conjugation length in both neutral and oxidized species 208 . Defects, including HH couplings, that limit accessible low-energy conformers reduce the effective conjugation length and blue shift the absorption maxima. The degree of solvation ranging from extended chains through collapsed compact (coiled) structures, or aggregates, to the solid-state likewise has profound spectroscopic consequences. Study in each regime reveals behavior enhancement in regioregular polythiophenes while it is attenuated or absent in regioirregular samples163 .MALDI-TOF mass spectrometryMatrix-Assisted Laser Desorption-Ionization Time-Of-Flight Mass Spectrometry (MALDI-TOF-MS) is a powerful tool for the analysis of molecular weight distributions and end-group analysis of polythiophenes209 . Absolute molecular masses can be determined (Figure1.68), independently of the polymer structure, whereas gel permeation chromatography (GPC) gives molecular weights relative to a calibration standard. GPC correlates the hydrodynamic volume of the randomly coiled polymer chains with polymer molecular weight and is commonly referenced to polystyrene. However, conjugated polymer systems such as poly(3-alkylthiophene)s, adopt a more rod-like conformation in solution, leading to an overestimation of the molecular weight (Figure1.69) 210 .Narrow polydispersity samples obtained by fractionation of regioregular poly(3-hexylthiophene)s were compared by MALDI-TOF-MS and GPC, with the latter giving Mw values 1.5-2.0 times higher.Polydispersities were often identical; though occasionally the GPC value was lower 209 .

Figure 1 .Figure 1 . 69 .

 1169 Figure 1.68 MALDI-TOF mass spectrum of rrP3HT synthesized by the GRIM method Ǻ(Mn = 5476 g mol -1 ; Mw = 5930 g/mol -1 ; PDI = 1.08) 209

Figure 1 .

 1 Figure 1.70 Structure of regioregular poly(3-hexylthiophene): (a) orientation of lamellae; (b) common (hightemperature) form; (c) metastable interdigitated polymorph

  liquid crystalline dyads or triads in combining either p-type and n-type discotic derivatives or p-type and n-type calamitic derivatives. This strategy should lead to ambipolar columnar or lamellar nanostructured materials presenting respectively one-dimensional or two-dimensional charge transport as illustrated in Figure 1.71.

Figure 1 .

 1 Figure 1.71: a) schematic representation of ambipolar discotic and calamitic dyads. b) Schematic representation of selforganization obtained with discotic and calamitic dyads and expected ambipolar charge transport properties.

Figure 1 .

 1 Figure 1.72: a) Chemical structures of discotic donor-acceptor molecule family studied in chapter 2. b) Chemical structures of calamitic donor-acceptor molecule family studied in chapter 3.

Figure 1 .

 1 Figure 1.73: a) schematic representation of donor-acceptor liquid crystalline side-chain polymer combining a p-type backbone and n-type side-groups in a unique polymeric architecture. b) Schematic representation of expected selforganization with donor-acceptor liquid crystalline side-chain polymer and of expected ambipolar charge transport properties.

Figure 1 .

 1 Figure 1.74: a) schematic representation of the model liquid crystalline side-chain polymer combining a p-type backbone and p-type discotic side-groups in a unique polymeric architecture. b) Schematic representation of expected self-organization with this liquid crystalline side-chain polymer and of expected charge transport properties.

Figure 1 . 75 :

 175 Figure 1.75: Chemical structures of the homopolymer and the alternated copolymer with thiophene/triphenylene ratios of 2:1 studied in chapter 4.

Figure 1 .

 1 Figure 1.76: Chemical structures of donor-acceptor liquid crystalline side-chain polymers combining a p-type backbone and n-type side-groups in a unique polymeric architecture.

Figure 2 . 1

 21 Figure 2.1 Chemical structures of the discotic target donor-σ-acceptor dyads and triads family

  8pentadecanone 2I. The compound 2I was first reacted with hydroxylamine to form the oxime intermediate, and then was reduced by the mild hydrogenation reagent RedAl to give the target branched amine 2J in quantitative yield (Figure 2.5).

Figure 2 . 5

 25 Figure 2.5 Synthesis of branched amine 2J. Conditions : vi) NH 2 OH, EtOH, Pyridine, 100°C, 2h / RedAl, Toluene, Reflux, 2h, quantitative.

Figure 2 . 6

 26 Figure 2.6 Synthesis of mono-anhydride-functionalized perylene 2M. Conditions : vii) Zn(OAc) 2 , imidazole, 160°C, 4h, 81% ; viii) KOH, t-BuOH, Reflux, 1.5h / AcOH / HCl, 43%.

Figure 2 . 7

 27 Figure 2.7 Synthesis of triphenylene/perylene based dyad and triad. Conditions: ix) Zn(OAc) 2 , imidazole, 160°C, 2h, 75% ; x) same as the previous, 45%.

Figure 2 . 8

 28 Figure 2.8 Synthesis of naphthalene based dyad and triad. Condition: ix) same as in ix), 34% for TN, 20% for TNT.

Figure 2 . 9 .

 29 Figure 2.9. POM microphotographs (left) and DSC thermograms (right) of TP. POM microphotographs obtained on cooling: (a) typical pseudo-focal conic texture at 140 °C; (b) preserved pseudo-focal conic texture from the hexagonal columnar mesophase at 30°C (below the T g ); (c) large domain of homeotropic texture at 140°C (slow cooling)

  of the columnar mesophase (Figure2.9.b). It is worth noting that, at a very slow cooling rate (0.1 °C/min), it is possible to obtain large (millimeter size) homeotropic domains (Figure2.9.c).

Figure 2 .

 2 Figure 2.10. POM microphotographs (left) and DSC thermograms (right) of TPT. POM microphotographs obtained on cooling: (a) typical pseudo-focal conic texture at 150 °C. (b) large oriented domains at 147°C

Figure 2 .

 2 Figure 2.11. POM microphotographs (left) and DSC thermograms (right) of TN. POM microphotograph obtained on room temperature, TN is in viscous liquid state and no birefringence observed.

Figure 2 . 2 . 2 . 2

 2222 Figure 2.12. POM microphotographs (left) and DSC thermograms (right) of TNT. POM microphotographs obtained on cooling: (a) large domain of homeotropic orientation at liquid crystal mesophase (b) Preserved homeotropic texture of the hexagonal columnar mesophase at 30°C

-organization property study 2 . 3 . 1

 231 Temperature dependent X-ray diffractionThe temperature dependent X-ray diffraction (TDXRD) is the method of choice to assess the mesophases' formation and identity and to probe the effects of the intimate structural modifications onto the mesophase temperature ranges. As consequences, all the compounds of the discotic dyad and triad family (TP, TPT, TN and TNT) were studied by TDXRD in the temperature ranges determined by DSC and POM studies. The measurements were carried out at the Institut de Physique et de Chimie des Matériaux de Strasbourg (IPCMS) in collaboration with Dr. B. Heinrich and Dr. B. Donnio.2.3.1.1 TPTo begin, the Figure2.13 shows the typical X-ray patterns of the dyad TP recorded at room temperature (pristine state), at 140°C and at room temperature after annealing. The pristine state pattern (Figure2.13.a) presents one simple slightly broadened reflection in the small-angle region and two diffuse halos in the wide-angle region. The repeat distances d associated with these reflections can be calculated in using the Bragg's law:2 sin

Figure 2 .

 2 Figure 2.13 X-ray diffraction patterns of dyad TP: (a) in pristine state at 20°C; (b) in liquid crystal state at 140°C; (c) in glass state of liquid crystal phase at 20°C on cooling. d) schematic representation of the Col H mesophase and the lattice parameters. PDI=perylene diimide; T=triphenylene; ch=aliphatic chains, a=lattice constant, S H =area of the bidimentianl lattice, Z=number of molecules per lattice, ξ=correlation length

Figure 2 .

 2 Figure 2.14 X-ray diffraction patterns of dyad TPT: (a) in pristine state at 20°C; (b) in liquid crystal state at 140°C; (c) in glass state of liquid crystal phase at 20°C on cooling. (d) enlargement of small angle region of X-ray patterns at 20°C (pristine), 20°C (annealed) and 140°C. PDI=perylene diimide; T=triphenylene; ch=aliphatic chains

Figure 2 .

 2 Figure 2.15 Schematic representations of the Col R (a) and Col O (b) phases observed for the triad TPT and lattice parameters

Figure 2 .Figure 2 .Figure 2 .

 222 Figure 2.14.c. This oblique columnar mesophase (Col O ) is characterized by a slightly deformed lattice ( =1.85-2, γ'≈93.8°) compared to the pristine rectangular pseudo-hexagonal mesophase ( √3), γ=90°). However, in comparison to the pristine state, the lattice doubles along an axis and thus includes 4 triad molecules corresponding to 4 perylene and 8 triphenylene columns. Nevertheless, the a/b ratio of oblique lattice close to pseudo-hexagonal suggests that triphenylene columns are distributed all around the perylene columns as well, whilst the parameter is close but different to right angle likely results from different and partially compensating in-plane tilt directions of discotic in the lattice, as illustrated in the schematic packing model (Figure 2.15.b). This model considers also an alternation along b of intercalated rows of triphenylenes and of perylenes, with this time, opposite inplane tilts within successive triphenylene rows. The lattice doubling may then be attributed to clusters of 4 triphenylene columns connected to one side of neighboring perylene columns: their distribution in the lattice and maybe the common orientation of their out-of-plane tilts would differ from the next cluster of triphenylenes.

Figure 2 .

 2 Figure 2.17 X-ray diffraction patterns of dyad TNT: (a) in pristine state at 20°C; (b) in liquid crystal state at 150°C; (c) in glass state of liquid crystal phase at 20°C on cooling. (d) schematic representation of the Col H mesophase and the lattice parameters. NDI=naphthalene diimide; T=triphenylene; ch=aliphatic chains, a=lattice constant, S H =area of the bidimentianl lattice, Z=number of molecules stacking forming the bidimentional lattice

Figure 2 . 2

 22 Figure 2.18 Topography AFM images and corresponding height profiles of TP thin films on silicon wafer before (a, b) and after (c, d) annealing

Figure 2 .

 2 Figure 2.19 Schematic representation of the dyad TP film surface morphology (a) "terrace type" nanostructure; (b) homeotropic orientation (face-on orientation of molecules); c) planar orientation (edge-on orientation of molecules)

Figure 2 .

 2 Figure 2.20 (a) GIXS patterns (with different contrast, red and black) of TP film (spun cast on silicon wafer from 1% w/w chloroform solution, annealed at 120°C for 2 hours in vacuum); grazing angle=0.12°.(b) bulk XRD pattern of TP (cooled to 20°C from liquid crystal mesophase)

Figure 2 .

 2 Figure 2.21 Schematic representation of in plane random orientation (top-view) of dyad TP (columns in parallel with the substrate)

First, Figure 2 .

 2 22 shows the absorption and emission spectra of the three isolated building blocks: triphenylene derivative T (Figure 2.22.a), perylene diimide P (Figure 2.22.b) and naphthalene diimide N (Figure 2.22.c).

Figure 2 .

 2 Figure 2.22 UV-vis and fluorescence spectra of three building blocks triphenylene derivative T (a), perylene diimide P (b) and naphthalene diimide N (c) in chloroform (ca. 10 -6 M)

Figure 2 . 23 .

 223 Figure 2.23. In the case of dyad TP (Figure 2.23.a), the absorption spectrum is the sum of the isolated triphenylene and perylene absorbances taken separately.

Figure 2 .

 2 Figure 2.23 UV-vis and fluorescence spectra of dyad TP (a) and triad TPT (b) in chloroform (ca. 10 -6 M)

Figure 2 .

 2 Figure 2.24 UV-vis and fluorescence spectra of dyad TN (a) and triad TNT (b) in chloroform (ca. 10 -6 M)

Figure 2 .

 2 Figure 2.27 Absorption spectra measured in dyad TP thin film, before (black) and after (red) annealing

Figure 2 . 28 .Figure 2 .

 2282 Figure 2.28. Cyclic voltammetry of TP (a) and TPT (b) in chloroform solution of n-Bu 4 NPF 6 (c (n-Bu 4 NPF 6 = 0.05 M, c (TP and TPT) = ca. 10 -3 M, 0.05 V/s, room temperature)

Figure 2 . 29 .

 229 Figure 2.29. Cyclic voltammetry of TN (a) and TNT (b), in chloroform solution of n-Bu 4 NPF 6 (c (n-Bu 4 NPF 6 = 0.05 M, c (TN and TNT) = ca. 10 -3 M, 0.05 V/s, room temperature)

Figure 2 .

 2 Figure 2.30. Dyad TP photocurrent curves excited at a) 337 nm and b) 508 nm wavelength, recorded at 200°C in the isotropic phase. The inset in a) displays the curves in log-log scale.

  3 g, 22%); NMR  H (ppm) (200 MHz, CDCl 3 ) 7.98-7.75 (6H, m, H arom ), 5.91 (1H, s, OH), 4.23 (10H, m, -O-CH 2 -), 1.95 (10H, m, H aliph ), 1.65-1.35 (20H, m, H aliph ), 0.97 (15H, t, J 6.9, -CH 3 );  C (50 MHz,

Figure 3 . 2

 32 Figure 3.2 Synthesis of the model calamitic dyad 3H based on terthiophene moieties. Conditions: i) n-BuLi, THF, -78°C; ClSnBu 3 , 81%. ii) Pd(OAc) 2 , PPh 3 , THF; DMF, 85°C, 39%. iii) n-BuLi, THF, -78°C; ClSnBu 3 , 83%. iv) n-BuLi, 0°C, 1,10dibromodecane, 29%. v) NBS, DMF, 70%. vi) Pd(OAc) 2 , PPh 3 , THF; DMF, 85°C, 47%.

Figure 3 . 3

 33 Figure 3.3 Synthesis of the donor-σ-acceptor calamitic dyad 3O based on terthiophene and pyromellitic diimide moieties. Conditions: v) n-BuLi, 0°C; 1,10-dibromodecane, 66%. vi) NBS, DMF, 67%. vii) Pd(OAc) 2 , PPh 3 , THF; DMF, 85°C, 58%.viii) 1-Decanol, DIAD, PPh 3 , THF, 0°C; DMF, RT, 12%. ix) K 2 CO 3 , DMF, 120°C, 54%.

  4.a). On further cooling, the transition from the lamellar mesophase to crystal phase was clearly observed at around 110°C (Figure 3.4.b) and the crystal phase texture was then maintained unchanged until room temperature (Figure 3.4.c).

Figure 3 . 4

 34 Figure 3.4 Optical textures of the model calamitic dyad 3H obtained on cooling from the isotropic state

Figure 3 . 5

 35 Figure 3.5 Poptical texture of the donor-σ-acceptor calamitic dyad 3O obtained at 150°C, cooling from the isotropic state.

  POM observations. The DSC traces of 3H and 3O are given in Figure3.6 and the phase transition temperatures and enthalpy changes data are summarized in Table3.1.

Figure 3 . 6

 36 Figure 3.6 DSC thermograms of dyads 3H (a) and 3O (b)

1

 1 Temperature dependent X-ray diffractionTo assess the unambiguous mesophases formation and probe the effects of the intimate structural modifications onto the mesophase temperatures ranges, the model and donor-σ-acceptor calamitic dyads 3H and 3O were studied by temperature dependent X-ray diffraction (TDXRD). The measurements were carried out in collaboration with Dr. B. Heinrich and Dr. B. Donnio at Institut de Physique et de Chimie des Matériaux de Strasbourg (IPCMS).

3. 3 . 1 . 1

 311 Model dyad 3HThe Figure3.7 shows the typical X-ray patterns of the model calamitic dyad 3H recorded at room temperature (pristine, Cr 1 ), at 100°C (Cr 2 ) and at 125°C (SmB). The pristine state pattern(Figure 3.7.a) presents a great number of very sharp reflections indicative of a well-developed three-dimensional crystal lattice (Cr 1 ). The two reflections located at the small-angle region with a d-spacing ratio of 1:2 indicate that the molecules are arranged in a lamellar fashion in the pristine crystalline phase. The corresponding layer spacing at around 56 Å calculated with the Bragg's law from the first reflection, is compatible with an untilted parallel packing of mesogens in the layers. On heating above the transition detected by DSC at 51.4°C, we can also observe a clear reorganization of the pristine crystalline phase into a new crystalline phase Cr 2 as shown by the structure spacing (d≈41 Å) and by the appearance of a second small-angle periodicity (d'=15 Å) in the X-ray pattern recorded at 100°C (Figure3.7.b). On further heating, the crystalline phase Cr 2 melted at 120°C to give a typical Smetic B phase (SmB), as shown by the X-ray pattern at 125°C (Figure3.7.c). Indeed, this pattern contains in the small-angle region two sharp reflections related to the smectic layering of molecules, and in the wide-angle region, a diffuse halo and a broad peak both centered at 4.5 Å corresponding respectively to the liquid-like conformation of the alkyl chains and to the existence of a short-range hexagonal order within mesogen sublayers. The inter-lamellar spacing d of 54 Å, calculated from the first small-angle reflection, is compatible with untilted or nearly untilted lamellae, confirming the SmB nature of the mesophase. A schematic representation of the organization of 3H in SmB mesophase is given inFigure 3.7.d.

Figure 3 . 7 XFigure 3 . 8 X

 3738 Figure 3.7 X-ray diffraction patterns of model calamitic dyad 3H: (a) Cr1 phase (pristine) at 20°C; (b) Cr2 phase at 100°C; (c) SmB phase at 125°C; (d) schematic representation of the SmB mesophase and the lattice parameters. ch=aliphatic chains, a=lattice constant.

Figure 3 .

 3 Figure 3.10 GIXS patterns (with different contrast, red and black) of a thermal evaporated 3H film on silanized(OTS) surface; grazing angle=0.12°

Figure 3 .

 3 Figure 3.11 Normalized absorption and emission spectrum of a) 3K (terthiophene), b) 3P (pyromellitic diimide),c) 3H and d) 3O in chloroform (c.a. 10 -6 mol L -1 )

Figure 3 .

 3 Figure 3.12 Absorption spectrum of 3K (terthiophene) and 3P (pyromellitic diimide) in chloroform (1.1x10 4 mol L -1 )

Figure 3 .

 3  M) in 0.05 M Bu 4 NPF 6 chloroform solution (scan rate: 0.05 V/s. Ferrocene as internal standard)

Figure 3 .

 3 Figure 3.14 Output (left) and transfer (right) characteristics (BC, SiO 2 , spun cast) of 3H

Figure 3 .

 3 Figure 3.15 Output (left) and transfer (right) characteristics (BC), SiO 2 , thermal evaporated) of 3H

Figure 3 .

 3 Figure 3.16 Output (left) and transfer (right) characteristics (BC, SiO 2 , thermal evaporated, annealed) of 3H

Figure 3 .

 3 Figure 3.17 Output (left) and transfer (right) characteristics (BC, OTS, thermal evaporated) of 3H

Figure 3 .

 3 Figure 3.18 (a) hole output, (b) hole transfer, (c) electron output and (d) electron transfer characteristics (BC, OTS, thermal evaporated) of 3O

Figure 3 .

 3 Figure 3.19 (a) hole output, (b) hole transfer, (c) electron output and (d) electron transfer characteristics (BC, OTS, thermal evaporated, annealed) of 3O

4 .

 4 1a. The chain lengths (in number of thiophene unity) are chosen around 50 and the spacer length between the polymer backbone and the pending groups is fixed as 10 carbons. Their self-organization properties in bulk and in thin films were investigated and correlated to the charge transport properties, studied in field effect transistor configuration(Figure 4.1.b).

Figure 4 .

 4 Figure 4.1: a) Chemical structures of the target polymers. b) Influence of the degree of polymer lateral substitution on the self-organization and the charge transport properties

Figure 4 . 2 : 4 . 2 . 2

 42422 Figure 4.2: Synthesis of the model short regioregular poly(3-hexylthiophene) 4B. Conditions: (i) (a) iPrMgCl, THF, 0 °C; (b) Ni(dppp)Cl 2 , 0 °C/room temperature, HCl; 64%

Figure 4 . 3 :

 43 Figure 4.3: Synthesis of the precursory homopolymer 4F. Conditions: (ii) (a) n-BuLi, hexane/THF, -40°C; (b) Br(CH 2 ) 10 Br, -10°C/room temperature; 37%; (iii) NBS, DMF, room temperature, 81%; (iv) Same as "(i)", 50%

Figure 4 . 4 : 4 . 2 . 3

 44423 Figure 4.4: Synthesis of the precursory alternating copolymer 4L . Conditions: (iii') NBS, DMF, room temperature, 96%; (vi) LiTMP, dioxaborolane, THF, -78 °C, 68.2%; (vii) Pd(PPh 3 ) 4 , 1 M NaHCO 3 , DME, reflux, 82%; (viii) NBS, DMF, room temperature, 67%; ix) Same as "(iv)", 67%

Figure 4 . 5 :

 45 Figure 4.5: Synthesis of the side chain homopolymer 4G and alternating copolymer 4M. Conditions: (v) (a) K 2 CO 3 , DMF, T, 80°C; (b) 4F, 120 °C; 73%; x) (a) K 2 CO 3 , DMF, T, 80°C; (b) 4L, 120 °C; 37%

Figure 4 . 6 .a 1 H

 461 Figure 4.6.a 1 H NMR spectra of precursory and grafted homopolymer.

Figure 4 . 6 .b 1 H

 461 Figure 4.6.b 1 H NMR spectra of precursory and grafted alternating copolymer.

  birefringence disappeared. This indicates the existence of mesophases for both substituted polymers.

Figure 4 . 7 4 . 4 . 2

 47442 Figure 4.7 POM images of 4G (a) and 4M (b) at 50 °C by heating

Figure 4 . 8

 48 Figure 4.8 DSC thermograms of polymers 4B (a), 4G (b) and 4M (c)

1

 1 Temperature dependent X-ray diffraction The substituted polymers 4L and 4G were studied by Temperature Dependent X-ray Diffraction (TDXRD) in the temperature ranges determined by DSC and POM studies. The measurements were carried out at Institut de Physique et de Chimie des Matériaux de Strasbourg (IPCMS) in cooperation with Dr. B. Heinrich and Dr. Bertrand Donnio.

Figure 4 . 9 X

 49 Figure 4.9 X-ray powder diffraction patterns and schematic representation of self-organization of homopolymer 4G in the lamello-columnar mesophase at 50°C (on cooling from the isotropic state)

Figure 4 .

 4 Figure 4.10 X-ray powder diffraction patterns and schematic representation of self-organization of copolymer 4M in the lamello-columnar mesophase at 50°C (on cooling from the isotropic state)

Figure 4 .

 4 Figure 4.11 AFM a) topography and b) phase images of substituted homopolymer 4G thin film on OTS silanized silicon wafer. c) 3D image of terrace structure of the film on OTS-treated wafer. d) Terrace step profile of the 4G film on OTS treated wafer determined from topography of c)

Figure 4 .

 4 Figure 4.12 Spin cast film GIXS patterns after annealing at 90°C (2 hours) of the homopolymer 4G a), schematic representation of the structure and orientation of the Lamcol organization on OTS treated surface b), and bulk X-Ray diffraction pattern at 50°C on cooling of 4G.

Figure 4 .

 4 Figure 4.13 Comparison of absorption properties of 4G (dashed line) and 4M (solid line) in dilute chloroform solution (≈10-6 M in repeating units)

Figure 4 .

 4 Figure 4.14 Absorption properties in solid state of the homopolymer 4G and the alternating copolymer 4M and the annealing (90°C, 2 hours) effect on film absorptions

Figure 4 .

 4 Figure 4.15 Output and transfer characteristics of the model short poly(3-hexylthiophene) 4B

Figure 4 .

 4 Figure 4.16 Output and transfer characteristics of the homopolymer 4G

Figure 4 .

 4 10) and the red-shifted solid state absorption (

Figure 4 .

 4 Figure 4.17 Output and transfer characteristics of the alternating copolymer 4M

  Figure 5.1.a. The expected self-organized structure is shown in Figure 5.1.b. This organization should lead to two distinct pathways for both charge carriers: holes in the polythiophene lamellae and electrons in perylene lamellae of columns. The self-organization properties in bulk and in thin films have been studied and preliminary results on charge transport properties, investigated in field effect transistor configuration (Figure 5.1.b), are reported.

Figure 5 .

 5 Figure 5.1: a) Chemical structures of the target donor--acceptor polymers. b) Schematic representation of the expected self-organized structure and charge transport properties

Figure 5 . 2

 52 Figure 5.2 The "direct polymerization" route which did not work at the GRIM step

Figure 5 . 3

 53 Figure 5.3 Synthesis of the perylene diimide substituted homopolymers and alternating copolymer Conditions: (i) NH 4 OAc, acetic acid, reflux, quantitative; (ii) (a) 5A, K 2 CO 3 , DMF, 100°C; (b) 4F, 130 °C, 17%. (iii) (a) 5A, K 2 CO 3 , DMF, 100°C; (b) 4L, 130 °C, 32%.

Figure 5 . 4

 54 Figure 5.4 Preparative SEC profiles of a) 5C and b) 5D

Figure 5 .5 1 H

 51 Figure 5.5 1 H NMR spectra of precursory and grafted homopolymer and alternating copolymers

Figure 5 . 6

 56 Figure 5.6 POM images of a) 5C and b) 5D at 30°C

Figure 5 . 7

 57 Figure 5.7 DSC thermograms of homopolymer 5C and alternating copolymer 5D

  The substituted homopolymer 5C and alternating copolymer 5D were studied by Temperature-Dependent X-Ray Diffraction (TDXRD) in the temperature ranges determined by DSC and POM studies. The measurements were carried out at Institut de Physique et de Chimie des Matériaux de Strasbourg (IPCMS) in cooperation with Dr. B. Heinrich and Dr. Bertrand Donnio. The XRD patterns of the homopolymer 5C are shown in Figure 5.8. In pristine state, the XRD pattern contains only broad scattering signals in the small and wide-angle regions which are due to the first neighbor distances between polythiophene backbones (PT), perylene diimides (PDI) and molten aliphatic segments (Figure 5.8.a). While the presence of a characteristic wide-angle maximum at 3.5 Å evidences the face-to-face piling of the PDI segments (h PDI ), no scattering maximum is visible at 3.80 Å (h PT ), what indicates that polythiophene backbones are not regularly stacked. The unique smallangle scattering around 20 Å with a correlation distance of   40 Å (evaluated from the Scherrer formula 126, 259 ) is compatible with the average distance between PDI piles (D PDI) evaluated from segment partial volumes. These observations indicate the amorphous character of homopolymer 5C in the pristine state. The annealing at 100°C (above the Tg) during 1 hour then leads to a modified pattern at room temperature (Figure 5.8.b), presenting the simultaneous appearance of numerous lamellar orders in the small-angle region and a new scattering visible at 3.80 Å (h PT ). That indicates now a much better segregation in sublayers with sharper interfaces and with a regular stacking of polythiophene backbones. The layer spacing around 58 Å and the intensity modulation in the small-angle harmonic series accord with two PDI and one PT electron-rich sublayers, separated by aliphatic spacers and tails which evidence a frozen lamello-columnar (Lamcol) organization as illustrated in the schematic packing model (Figure 5.8.d).

Figure 5 . 8 Figure 5 . 9

 5859 Figure 5.8 XRD patterns of homopolymer 5C at room temperature (RT): (a) in the pristine state; (b) after annealing at 100°C for 1 h; (c) after annealing at 150°C for 1 h; d) schematic representation of the frozen lamello-columnar organization

Figure 5 .

 5 Figure 5.10 (a) GIXS patterns (with different contras, red and black) of alternating copolymer 5D (spun cast on silicon wafer; annealed at 150°C for 2 h); grazing angle=0.12°; (b) bulk X-ray diffraction pattern of 5D after annealing at 100°C

  Research Center (CERC, Korea), in collaboration with Prof. J.-W. Wu and Prof. J.-C. Ribierre (Ewha Womans University). All the transistors have been prepared with Bottom-Gate (BG) and Bottom-Contact (BC) configurations, and silanized (OTS) treated substrates and they have been measured in as-prepared state and annealed state. The details about the transistor fabrication and measurement parameters are given in Paragraph 5.7.3.

  shaped, for example, via a rigid, helical secondary structure. On the other hand, numerous structures are conceivable for polymers with mesogenic groups as monomer units, whereby one can differentiate between two fundamentally different types(Figure 1.34).

	Calamitic	Discotic
		Rigid main chain liquid
		crystal polymers
		Flexible main chain liquid
		crystal polymers
		Side chain liquid crystal
		polymers
		Combined liquid crystal
		polymers

Table 1

 1

	Regioregular types
	H (δ ppm)	6.98	7.00	7.02	7.05

.1. Alkyl protons are less sensitive to perturbations of the πsystem and give broad, overlapping signals 158 .

Table 1 .

 1

1

[START_REF] Fincher | Electrical Conductivity in Doped Polyacetylene[END_REF]

H chemical shifts (ppm, CDCl 3 ) of configurational triads in poly

(3-alkyl)

thiophenes

163

Table 1 .

 1

	H (δ ppm)	6.90	7.16	7.3	-

2 shows the thiophene proton chemical shifts of different terminal configurations.

Table 1 . 2

 12

[START_REF] Fincher | Electrical Conductivity in Doped Polyacetylene[END_REF]

H chemical shifts (ppm, CDCl 3 ) of terminal rings in poly

(3-alkyl)

thiophenes

163

20 TP OC 5 H 11 OC 10 H 20 C 5 H 11 O C 5 H 11 O C 5 H 11 O OC 5 H 11

	a)	C 5 H 11 O	OC 5 H 11
					O		O						O	O
								C 7 H 15
		C 5 H 11 O C 5 H 11 O		OC 5 H 11 O C 10 H N N O N O O	N O C 7 H 15	C 7 H 15					N O	N O	C 5 H
							C 7 H 15
					O	O
					TN
	b)														O	O
		C 8 H 17	S	S	S		S	S	S	C 8 H 17	C 8 H 17	S	S	S	N	N	C 10 H 21
					Ter-Ter									Ter-Pyr	O	O

11 O C 10 H 20 O OC 5 H 11 OC 5 H 11 OC 5 H 11 C5H11O OC 5 H 11 OC 10 H 20 C 5 H 11 O C 5 H 11 O C 5 H 11 O OC 5 H 11 TPT C 5 H 11 O C 10 H 20 O OC 5 H 11 OC 5 H 11 OC 5 H 11 C 5 H 11 O OC 5 H 11 OC 10 H 20

Table 2

 2

	Compound	T g	a (

.1. The DSC thermograms obtained for this family of dyads and triads are quite typical of high-molecular weight compound, containing characteristic signals of glass transitions. At higher temperature, exothermic transitions corresponding to the isotropic liquid phase are in agreement with the results measured by POM.

°C) Transition b T c (°C) ΔH (J g -1 )

	Dyad TP	84.4	Col h → I	145.8	4.0
		-	I→Col h	142.9	4.3
	Triad TPT	-	Col o → I	148.9	5.9
		-	I→Col o	142.8	6.0
	Dyad TN	-9.8	-	-	-
		-23.7	-	-	-
	Triad TNT	-	Col h →I	172.9	4.8
		-	I→Col h	172.2	5.2

Table 2 .

 2 1. Phase transition temperatures and enthalpy of TP, TPT, TN and TNT . a T g = glass transition determined during heating (rate 20°C/min). b Col h = hexagonal columnar mesophase, Col o = oblique columnar mesophase. I = Isotropic phase.

c Indicated temperatures are the onsets of the peaks on heating and on cooling (rate 5 °C/min)

  .29.b) the oxidation wave from the triphenylene decreased by a half. All the potential values are listed in Table2.2.Based on these redox potential values, the HOMO and LUMO energies of TP, TPT, TN and TNT were estimated (see Table2.2) on the basis of the reference energy level of ferrocene (4.8 eV below the vacuum level 274 ) and taking into account the empirical relationships which has been proposed by Brédas et al.275 and widely used by others274, 276 :

	Compound	E 1/2	red2	E 1/2	red2	E 1/2	red1	E 1/2	red1		E 1/2	ox1	E 1/2	ox1
		(V/Ag/AgCl)	(V/Fc/Fc + ))	(V/Ag/AgCl)	(V/Fc/Fc + ))	(V/Ag/AgCl)	(V/Fc/Fc + )
	TP	-0.86	-1.35	-0.67	-1.16		1.03	0.54
	TPT	-0.81	-1.26	-0.63	-1.09		0.93	0.47
	TN	-0.66*	-1.19*	-0.66*	-1.19*		1.05	0.52
	TNT	-1.19	-1.68	-0.65	-1.15		1.07	0.68
	Table 2.2. Redox potentials of TP, TPT, TN and TNT measured in Solution Cyclic Voltammetry: E 1/2 Fc = 0,49 V vs Ag/AgCl
	E HOMO = -4.8 -E 1/2 ox1 (vs Fc)				E LUMO = -4.8 + E 1/2	red1 (vs Fc)
	Where E 1/2 ox (vs Fc) and E 1/2

red (vs Fc) are, respectively the first potentials values for oxidation and reduction of the studied material relative to the Fc/Fc + internal standard.

Table 2 .

 2

	.3 is not an optical band

3. HOMO/LUMO values of TP, TPT, TN and TNT *The HOMO/LUMO values of the corresponding triphenylene moiety 277 , naphthalene moiety 278 , and the perylene moiety 105 are published in previous reports.

  Flight technique (Experimental 2.7.1 for more details). The measurements were carried out at the

Institut des NanoSciences de Paris (INSP) by M.Sc. Leszek Mazur in cooperation with Dr. Jean-Louis Fave (for more details about the sample preparation and measurements conditions, see Experimental 2.7.1)

The dyad TP was characterized in bulk by conventional TOF configurations: 8 μm thick TP films (sandwiched between two ITO electrodes) were excited by nanosecond pulses at 337 nm, where the sample strongly absorbs light. The measurements were first carried out in the Col h mesophase.

  g, 82%) as a pale yellow solid. NMR  H (ppm)(200 MHz,

	CDCl 3 ) 7.83 (m, 8H), 7.71-7.67 (m, 2H), 4.23 (t, 12H), 3.67 (t, 2H), 2.02-1.88 (m, 12H), 1.60-1.39 (m,
	34H), 0.97 (t, 15H).
	2G (1.3 g, 1.35 mmol) was dissolved in hot ethanol

  arom ), 5.19 (1H, m, N-CH), 4.16-4.00 (14H, m, -O-CH 2 -, N-CH 2 ), 2.27 (2H, m, H aliph ), 1.94 (14H, m, H aliph ), 1.73 (2H, m, H aliph ), 1.64-1.24 (54H, m, H aliph ), 1.01-0.96 (15H, m, H aliph ), 0.96-0.84 (6H, m, H aliph );  C (ppm) (125 MHz, CDCl 3 ) 163.

  . HRMS calcd. for C 92 H 120 N 2 O 10 Na + : 1435.884; found 1435.882. Elemental Analysis: calcd.for C 92 H 120 N 2 O 10 : C,

  , 1.93 (28H, m, H aliph ), 1.49 (62H, m, H aliph ), 0.97 (30H, t, H aliph ); MALDI-TOF: calcd. for C130H170N2O16+Na + 2039.734; Found 2039.545. Elemental Analysis: calcd. for C 130 H 170 N 2 O 16

.

  MALDI-TOF: calcd. for C 120 H 166 N 2 O 16 H + 1892.232; Found 1892.057. Elemental Analysis: calcd. for C 120 H 166 N 2 O 16

  . MALDI-TOF: calcd. for C 82 H 166 N 2 O 10 H + 1289.871; Found 1289.371. Elemental Analysis: calcd. for C 82 H 116 N 2 O 10 : C,

Table 3 .

 3 .1. 1. Phase-transition temperatures and enthalpy changes of dyads 3H and 3O

			T (°C)	ΔH LC (J g -1 )	Transition	T (°C)	ΔH LC (J g -1 )	Transition	T (°C)	ΔH iso (J g -1 )	Transition
	Dyad	Heating	60.7	1.5	Cr 1 →Cr 2	120.9	46.0	Cr 2 →SmB	136.7	29.2	SmB→Iso
	p-p
	3H	Cooling	44.2	3.6	Cr 2 →Cr 1 '	109.9	45.7	SmB→Cr 2	134.1	29.2	Iso→SmB
	Dyad	Heating	45.0	10.0	Cr→M Lam1	119.0	1.4	M Lam1 →M Lam2	148.9	45.0	M Lam2 →Iso
	p-n
	3O	Cooling	32.3	10.1	M Lam →Cr	-	-	-	145.9	42.8	Iso→M Lam

Table 3 .

 3

		E 1/2 ox	E 1/2 red	E HOMO (eV)	E LUMO (eV)	E g CV (eV)	E g opt (eV)
		(V/Ag/AgCl)	(V/Ag/AgCl)
	3H (p-p)	1.01	-	-5.32	-2.32*	-	3.00
	3O (p-n)	1.00	-0.86	-5.31	-3.94	1.37	-

2 Redox potentials and HOMO/LUMO values of the model and D-A dyads (*estimated)

  .3.

		Configuration	Annealing	Gate dielectric surface	Hole mobility (cm 2 V -1 s -1 )	Threshold voltage (V)	I ON /I OFF ratio
	SP*	BC	N	SiO 2	8.3x10 -7	-57.4	1.2x10 4
		BC	N	SiO 2	1.9x10 -2	-5.2	5.4x10 5
		BC	Y	SiO 2	1.6x10 -3	-31.0	7.2x10 6
	Eva*	BC BC	N Y	OTS OTS	1.0x10 -1 5.2x10 -3	-4.4 -5.3	1.3x10 6 3.0x10 4
		TC	N	SiO 2	1.9x10 -2	-1.4	1.3x10 6
		TC	N	OTS	2.1x10 -3	-51.8	5.3x10 5

Table 3 .

 3 3 OFET performances of model calamitic dyad 3H * SP=spun cast; Eva=evaporated

Table 3 .

 3 4 OFET performances of donor-σ-acceptor calamitic dyad 3O (BC, OTS, thermal evaporated) * for hole transport

  BuLi (2.5M in hexane) (0.137 mol) was added drop by drop. After 10 min, about 50 ml of dry THF was added drop by drop until a precipitation appeared. Then the temperature was slowly increased to room temperature and 1 hour later, 1,10-dibromo-decane solution (0.0715 mol in 55 ml dry hexane) was added drop by drop. The reaction continued for another 2 hours and then was quenched by Mili-Q water. This mixture was extracted by ether, washed by water and the organic phase was dried by magnesium sulfate, filtrated and the solvent was evaporated by Rotavapor. The crude product was purified by column chromatography (silica gel, chloroform) and afforded the title

	(83%). NMR  H (ppm) (200MHz, CDCl 3 ): 7.13(d, 1H), 7.04(d, 1H), 6.97(d, 1H), 6.66(d, 1H), 2.79(t,
	2H), 1.15-1.72 (m, 33H), 0.86-0.99 (m, 14H)
	At 0°C under argon atmosphere, 2-bromothiophene (0.143 mol) was dissolved in 280 ml dry
	hexane. The n-

compound (29%) as a yellowish power. NMR  H (ppm) (200 MHz, CDCl 3 ) 7.10(d, 2H), 6.91(t, 2H), 6.77(d, 2H), 2.82(t, 4H), 1.66(t, 4H), 1.28(m, 16H). Under argon atmosphere, 3F (21.33 mmol) was dissolved in 200ml of DMF at room temperature.

  ; MALDI-TOF: calcd. for C 50 H 66 S 6 H + 859.3568; Found 859.0650. At 0°C under argon atmosphere, 2-bromothiophene (0.143 mol) was dissolved in 280 ml dry hexane. The n-BuLi (2.5M in hexane) (0.137 mol) was added drop by drop. After 10 min, about 50 ml of dry THF was added drop by drop until a precipitation appeared. Then the temperature was slowly increased to room temperature and 1 hour later, the mixture was added in one portion to a solution of 1,10-dibromo-decane solution (0.572 mol in 55 ml dry hexane). The reaction continued for another 2 hours and was quenched by adding Mili-Q water. This mixture was then extracted between ether and water. The organic phase was dried by magnesium sulfate, filtrated and the solvent was removed by Rotavapor. The crude product was purified by column chromatography (silica gel, chloroform) and

afforded the title compound (66%) as yellow-brown oil. NMR  H (ppm) (200MHz, CDCl 3 ): 7.10(d, 1H), 6.92(t, 1H), 6.77(d, 1H), 3.41(t, 2H), 2.82(t, 2H), 1.84(quin, 2H), 1.66(quin, 2H), 1.28(m, 13H) ;  C (ppm) (50MHz, CDCl3) : 145.

  silica gel, DCM) and afterwards recrystallized in EtOH to give the title compound (54%) as yellow-

	brown powder. NMR  H (ppm) (200MHz, CDCl 3 ): 8.25(s, 2H), 6.95(s, 2H), 6.93(s, 2H), 6.66(s, 2H),

3.71(t, 4H), 2.78(t, 4H), 1.66(t, 8H), 1.24-1.27(m, 36H), 0.87(t, 6H);  C (ppm) (50MHz, CDCl 3 ):

  ; MALDI-TOF: calcd. for C 50 H 66 N 2 O 4 S 3 H + 855.4265; Found 855.5660.

Table 4 .

 4 1.

	Polymer	type	M n (SEC) (KDa)	DP n (SEC)	DPI (SEC)	M n (NMR) (kDa)	DP n (NMR)
	4B	Model	13.4	80	1.2	9.0	54
	4F	Homopolymer precursor	14.7	47	1.2	14.0	45
	4G	Homopolymer grafted	31.9	36	1.2	40.3	45
	4L	Copolymer precursor	10.6	43*	1.4	10.4	42*
	4M	Copolymer grafted	17.8	33*	1.6	23.1	42*
	Table 4.1 Chemical structure informations of the polymers repeating unit of 4B, RU(4B)=166.3 g mol

  .2.

	Polymer	T g	a (°C)	Transition b	T iso (°C)	ΔH (J g -1 )
	Model polymer 4B		0.0 -	Cr → I I → Cr	211.7 196.5	23.8 29.5
	Homopolymer 4G	14.5 -	Lamcol → I I → Lamcol	107.3 85.6	7.5 7.2
	Alternating copolymer 4M	46.1 -	Lamcol → I I → Lamcol	116.2 71.4	4.8 2.9

Table 4 .

 4

	2. Phase transition temperatures and enthalpies of 4B, 4G and 4M. a T g = glass transition determined during heating (20°C min -1 ; b Transition temperatures are the onsets of the peaks on heating and on cooling, Cr=Crystalline phase,
	I=Isotropic phase, Lamcol=lamello-columnar phase.

Table 4 .

 4 3.

	Polymers	Hole mobility (cm 2 V -1 s -1 )	Threshold voltage (V)	I ON /I OFF ratio
	Model polymer 4B	6.1x10 -2	8.5	2.3x10 5
	Homopolymer 4G	1.4x10 -5	-24.1	7.3x10 3
	Copolymer 4M	1.6x10 -3	-0.7	5.3x10 2

Table 4 .

 4

3 OFET performances of the model polymer 4B, homopolymer 4G and alternating copolymer 4M

  BuLi (19.4 ml, 3.1 x 10 -2 mol, 1.6 M in hexane) was added drop wise via syringe at this temperature. 5 ml of THF was added to the pale yellow solution which was further left stirring for 1h then the resulting white suspension was allowed to warm to -10°C. 1,10-Dibromodecane (28.7 ml, 0.12 mol) was added in one portion at -10°C and the mixture was warmed to room temperature and stirred for another 2 h. The reaction was quenched by pouring 15 ml of water into the flask and

	16 (SEC-THF, PS Standard);
	DP n (NMR)=54. NMR,  H (ppm) (500 MHz, CDCl 3 ): 6.98 (s, 54H), 2.80 (t, 104H), 2.60 (m, 4H), 1.73
	(m, 108H), 1.41 (m, 324H). 0.94 (m, 162H). Anal. Calcd for (C 10 H 14 S) 54 Br: C 71.59 %; H 8.42 %; Br
	0.88 %; S 19.11 %; found: C 71.32 %; H 8.32 %; Br 0.71 %; S 16.69 %.
	3-Bromothiophene (4C) (3ml, 3.1 x 10 -2 mol) was dissolved in 50 ml of freshly distilled hexane
	under an argon atmosphere. The resulting solution was cooled down to -40°C and stirred for 10
	minutes. n-extracted with diethyl ether (2 x 30 ml). The organic phase was washed with water (3 x 30 mL) and
	dried over anhydrous MgSO 4 . A pale yellow oil was obtained as a crude product after removing the
	solvent. The crude was purified by fractional distillation under reduced pressure. After the first
	distillate (the excess dibromodecane) was removed, the residue was purified by the column
	chromatography (silica gel, petroleum ether) and afforded a colorless oil (3.52 g, 37%); NMR,  H (200
	MHz, CDCl 3 ) 7.24 (1 H arom , dd, J 2.9 and 4.9), 6.93 (2H arom , m), 3.41 (2H, t, J 6.8, -CH 2 -Br), 2.63 (2H,
	t, J 7.6, -CH 2 -Ar), 1.86 (2H aliph , m), 1.62 (2H aliph , m), 1.40-1.25 (12H aliph , m);  C (50 MHz, CDCl 3 )

  g, 81%); NMR,  H (ppm) (200 MHz, CDCl 3 ) 6.77 (1 H arom , s), 3.41 (2H, t, J 6.8, -CH 2 -Br), 2.50 (2H, t, J 7.5, -CH 2 -Ar), 1.86 (2H aliph , m), 1.65-1.25 (14H aliph , m);  C (ppm) (75 MHz, CDCl 3 ) 142.00, 131.

  . Anal. Calcd for C 14 H 21 Br 3 S: C,36.47; H, 4.59; Br, 51.99,

	Found: C, 36.70; H, 4.42; Br: 51.15.
	All glasswares and solvents were carefully dried before use. 2,5-dibromo-3-(10-
	bromodecyl)thiophene (4E) (4 g, 8.7 x 10 -3 mol) was poured into 100 ml of dry THF at 0°C under
	argon atmosphere. A solution of isopropyl magnesium chloride (4.3 ml, 8.7 x 10 -3 mol, 2.0 M in THF)
	The title compound was obtained (1.31 g, 50%) after successive extraction with methanol; M n = 14200,
	PDI = 1.15 (SEC-THF, PS Standard); NMR,  H (ppm) (300 MHz, CDCl 3 ): 6.98 (1H, s, Harom), 3.39
	(2H, t, J 6.8, -CH 2 -Br), 2.80 (2H, m, -CH 2 -Ar), 1.90-1.60 (4H, m, H aliph ), 1.50-1.31 (12H, m, H aliph );  C
	(ppm) (75 MHz, CDCl 3 ) 139.

was added drop by drop through a syringe and the resulting pale yellow solution was stirred for 1h at 0°C. A suspension of Ni(dppp)Cl 2

(75 mg, 1.48

x 10 -4 mol, 1.6 mol %) in THF (5 ml) was quickly injected into the flask. The resulting red solution was gently warmed to room temperature and left overnight under constant stirring. The polymerization was quenched by pouring a 6 M solution of HCl (100 ml) followed by extraction with chloroform (100 ml). The organic layer was successively washed by water (3 x 100 ml), dried over MgSO 4 and concentrated under reduced pressure. The crude product was precipitated using methanol and the resulting purple solid was filtered through a Soxhlet thimble.

  . Anal. Calcd for C 14 H 21 BrS: C,55.81; H, 7.03; Br, 26.52, Found: C, 55.43; H, 6.94;

	2,3,6,7,10-Pentakis(pentyloxy)-11-hydroxytriphenylene (T) (0.45 g, 6.6 x 10 -4 mol), potassium
	carbonate (0.18 g, 1.3 x 10 -3 mol) in DMF (20 ml) was heated at 80°C under an atmosphere of
	nitrogen. One hour later, poly[3-(10-bromodecyl)thiophene] (4F) (0.1 g, 3.3 x 10 -4 mol) in THF (15 ml)
	was added slowly to the green mixture. The resulting orange solution was left under heating at 120°C
	for 3 days. The reaction mixture was then cooled down and filtered to remove the excess of base
	followed by extraction with chloroform (100 ml). The organic layer was successively washed with
	water (3 x 100 ml), dried over MgSO 4 and concentrated under reduced pressure. The crude product
	was precipitated from chloroform to an abundant volume of methanol and the resulting purple solid
	was filtered through a Soxhlet thimble. The title compound was obtained (0.22 g, 73%) as a dark red
	powder after successive extraction with methanol; M.p. 107 °C (Lcol -Iso), NMR,  H (ppm) (250
	MHz, CDCl 3 ) 7.73 (6H arom , s), 6.94 (1 H arom , s), 4.16 (12H, br. s, OCH 2 ), 2.76 (2H, br. s, CH 2 -Ar), 1.90
	(12H aliph , br. s), 1.50-1.30 (34H aliph , m), 0.93 (15H, br. s, -CH 3 ). Anal. Calcd for C 57 H 82 O 6 S: C, 76.29;
	H, 9.23. Found: C, 75.75; H, 9.13.
	To a solution of 3-(10-bromodecyl)thiophene (4D) (1.8 g, 5.9 mmol) in 20 ml of DMF, NBS ( 0.95
	g, 5.3 mmol) in 15 ml DMF was added slowly. The mixture was slightly heated up to 35°C for 5 h and
	then poured into 200 ml of water. The organic materials were extracted by dichloromethane and
	washed with brine and dried over anhydrous MgSO 4 . After removal of the solvents, the crude product
	was purified by column chromatography on silica gel (5% dichloromethane in hexane) to give 4E' as
	colorless oil: 1.95 g, yield of 96%. NMR δ H (ppm) (200 MHz, CDCl 3 ): 7.19 (1H, d, J = 5.6 Hz ), 6.79
	(1H, d, J = 5.8 Hz), 3.41 (2H, t, J = 6.7 Hz), 2.56 (2H, t, J = 7.5 Hz), 1.86 (2H, mbr), 1.61 (2H, mbr),
	1.27 (12H, mbr); δ
	Br: 26.21.

C (ppm) (50 MHz, CDCl 3 ): 141.

  wise at -78°C, and then the mixture was allowed to warm to room temperature and stirred overnight. Then the mixture was poured into 200 ml of water and the organic materials were extracted by dichloromethane and washed with brine and dried over anhydrous MgSO 4 . After removal of the solvents, the crude product was purified by column chromatography on silica gel (5% ethyl acetate in

	hexane) to give 4I as colorless oil: 5.6 g, yield of 68.2%. NMR δ H (ppm) (200 MHz, CDCl 3 ): 7.48 (1H,
	s), 7.21 (1H, s), 2.62 (2H, t, J = 7.7 Hz), 1.59 (2H, mbr), 1.24-1.27 (22H, sbr), 0.88 (3H, t, J = 6.5Hz);
	δ C (ppm) (125 MHz, CDCl 3 ): 144.

After stirring at -40°C for another 2.5 h, 2-isopropoxy

-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (5.2g, 28.

0 mmol) was added drop

  28 g, yield of 82.1%. NMR δ H (ppm)

	(200 MHz, CDCl 3 ): 7.15 (1H, d, J = 5.2 Hz), 6.94 (1H, d, J = 1.4 Hz), 6.93 (1H, d, J = 5.2 Hz), 6.89
	(1H, d, J = 1.4 Hz), 3.41 (2H, t, J = 6.9 Hz), 2.74 (2H, t, J = 7.7 Hz), 2.60 (2H, t, J = 7.6 Hz), 1.85 (2H,
	mbr), 1.63 (4H, mbr), 1.30 (22H, mbr), 0.88 (3H, t, J = 6.5Hz); δ C (ppm) (50 MHz, CDCl3): 143.6,

Br 10 S C 8 H 17 Br Br 4K

	Chemical Formula: C 26 H 39 Br 3 S 2
	Molecular Weight: 655.4299
	To a solution of 4J (2.28 g, 4.6 mmol) in 20 ml of DMF, NBS (1.63 g, 9.2 mmol) in 15 ml of DMF
	was added slowly. The mixture was stirred at room temperature overnight and then poured into 200 ml
	of water. The organic materials were extracted by dichloromethane and washed with brine and dried
	over anhydrous MgSO 4 . After removal of the solvents, the crude product was purified by column

.1.

S

chromatography on silica gel (hexane as eluant) to give 4K as light yellow oil: 2.02 g, yield of 67%.

NMR δ H (ppm) (200 MHz, CDCl 3 ): 6.88 (1H, s), 6.74 (1H, s), 3.42 (2H, t, J = 6.9 Hz), 2.64 (2H, t, J = 7.7 Hz), 2.55 (2H, t, J = 7.7 Hz), 1.87 (2H, mbr),

1.57 (4H, mbr), 1.28 (22H, mbr)

, 0.89 (3H, t, J = 6.5Hz); δ C (ppm) (50 MHz, CDCl 3 ): 142.

  THF. i-PrMgCl (0.67 ml, 2M in THF) was added then via syringe at 0°C. After stirring for 1 hour, Ni(dppp)Cl 2 (23 mg, 0.032 mmol ) was added quickly with 1 ml of dry THF in one portion and the ice-bath was then removed. The reaction was stirring for another 3 hours and quenched by 6M HCl solution. The organic layer was extracted by chloroform and washed with brine and dried over anhydrous MgSO 4 . After removal of the solvents, the residue was redissolved in 3 ml chloroform and precipitated in 150 ml of MeOH. The solid polymer was washed by using the Soxhlet extractor with successive solvents, first with MeOH, and then EtOH, and finally chloroform. After removal of chloroform, the polymer 4L was dried under vacuum to give a dark-red solid. 320 mg, yield of 67%.

M n = 10 600, PDI = 1.34 (SEC-THF, polystyrene standard); DP n (NMR) = 22; NMR δ H (ppm) (

200

MHz, CDCl 3 ): 6.97 (2H, sbr), 3.38 (2H, t), 2.79 (2H, mbr), 1.83 (2H, mbr), 1.66 (4H, mbr),

1.28 (22H, mbr)

, 0.89 (3H, t, J = 6.5Hz).

Table 5

 5 .1.

	Polymer	T g	a (°C)	Transition b	T mel	c (°C)	ΔH (J g -1 )
	Homopolymer 5C	53.8	Lamcol → I		-	-
	Alternating Copolymer 5D	61.8	Lamcol → I	167.7	4.8

Table 5 .

 5 1. DSC results of 5C and 5D on heating . a T g = glass transition (20°C min -1 ); b Transition temperatures, Lamcol=lamello-columnar phase, I=Isotropic phase; c T mel : melting temperature (maximum of the large peak).

Table 5 .

 5 2 Field-effect mobility, threshold voltage and on/off current ratio of 5C and 5D with different conditions

  The crude product was precipitated from chloroform to cold MeOH and filtered through a Soxhlet thimble. After extracted in Soxhlet with MeOH and EtOH, the mixture was fractionated by the preparative SEC and gives a black solid (60 mg, 17%). NMR,  H (ppm) (200 MHz, CDCl 3 ): 8.40 (br m, 8H), 6.95 (br. s, 1H), 5.16 (br s, 1H), 4.03 (br s, 2H), 2.77 (br s, 2H), 2.24 (br m, 2H), 1.92-1.25 (br m, 38H), 0.82 (br m, 6H

  ). NMR,  H (ppm) (300 MHz, CDCl 3 ): 8.25 (br m, 8H), 6.90 (br. s, 2H), 5.15 (br s, 1H), 4.08 (br s, 2H), 2.73 (br s, 2H), 2.25 (br m, 2H), 1.92-1.25 (br m, 52H), 0.82 (br m, 9H).

PhD Thesis of Yiming Xiao (UPMC, 2014)

After the annealing treatment (150°C, 1 hour in glove-box), the homopolymer 5C OFET showed clearly an ambipolar behavior since n-type and p-type output and transfer profiles can be recorded (Figure 5.11). An electron mobility of 3.9x10 -6 cm 2 V -1 s -1 was determined and the hole mobility increased more than one order of magnitude, to give 3.3x10 -5 cm 2 V -1 s -1 . The detailed OFET performances are listed in Table 5.2. The transistors of alternating copolymer 5D gave a hole mobility of 4.1x10 -6 cm 2 V -1 s -1.

Unfortunately, as for the previous polymer 5C, no electron mobility could be determined. In addition, similarly to 5C, after the annealing treatment (200°C, 1 hour in glove-box), 5D OFET showed also an ambipolar behavior. The hole mobility was slightly decreased of one order of magnitude from 4.1x10 -6 cm 2 V -1 s -1 to 4.5x10 -7 cm 2 V -1 s -1 after annealing, but more interestingly an electron mobility appeared of around 3.5x10 -8 cm 2 V -1 s -1 . It is worth noticing that the annealing temperature (200°C) was chosen arbitrarily before having the complete thermal behavior characterizations of the polymer 5D, and doesn't fulfill the best conditions.

The thermal treatment conditions, charge carrier mobilities, threshold voltage and on/off current ratio about the 5C and 5D OFETs are listed in Table 5.2.