Marcel Kastler

Porous graphenes: two-dimensional polymer synthesis with atomic precision.

We demonstrate, by surface-assisted coupling of specifically designed molecular building blocks, the fabrication of regular two-dimensional polyphenylene networks with single-atom wide pores and sub-nanometer periodicity.

Rational optimization of benzo[2,1-b;3,4-b']dithiophene-containing polymers for organic field-effect transistors.

Adv. Mater. 2010, 22, 83–86 2010 WILEY-VCH Verlag Gmb Thin-film transistors based on organic semiconductors have attracted a lot of attention in academic and industrial research in recent years. First commercial applications are about to be launched into new products which cannot be realized with conventional, expensive, and energy-demanding silicon-based electronics. Technologies using organic materials promise large area devices, which are light weight, flexible, and come at low cost. Experts predict a huge demand for devices such as radio frequency identification tags (RFIDs) for single item tagging or other integrated circuits. The search for new materials which meet the requirements for these applications is, therefore, of great importance. These crucial requirements are (i) good solution processability especially on flexible polymer substrates, (ii) short annealing times at low temperatures to enable high-speed processing such as mass printing, (iii) high operational stability at ambient conditions to avoid the necessity of costly barrier layers, (iv) high charge-carrier mobilities, and (v) a facile and economically feasible synthesis which can be scaled up for production. Semiconducting polymers show a microcrystalline or amorphous superstructure simplifying the fabrication of homogeneous films over large area, which makes this class very promising. The charge-carrier mobilities are usually not as high as for small molecules but still sufficient for most applications. Thiophene-containing polymers have proven to be most effective. This paper presents the incorporation of benzo[2,1-b; 3,4-b0]dithiophene into a polythiophene as active material in a field-effect transistor (FET). The structure motif of a 2,5 connected thiophene, which is found, for example, in the well-known poly-3-hexylthiophene (P3HT), is modified by a benzo annellation. This reduces the ionization potential of the polymer and is thus expected to improve air stability under operation. The extended p system should, furthermore, help in forming highly ordered films which are needed to transport charge-carriers efficiently. The first approach toward a polymer-containing benzo[2,1-b;3,4-b0]dithiophene is the iron(III) chloride mediated oxidative polymerization of benzodithiophene having solubilizing alkyl chains attached to the benzo unit. In this way polymer P1 (Scheme 1) is obtained with a molecular weight of Mn1⁄4 18 kgmol 1 as determined by size exclusion chromatography in 1,2,4-trichlorobenzene at 135 8C in reference to polystyrene as standard. Good solubility in dichlorobenzene enables the polymer to be tested as the active material in an FET. The polymer is spin cast from 1,2-dichlorobenzene solution onto a silicon wafer and dried at 100 8C for 5min. Finally, top-contact gold electrodes are deposited by thermal evaporation. Good device characteristics can be recorded, showing low charge-carrier mobility in the 10 4 cm V 1 s 1 region. The organization in the thin film is probed with X-ray reflection diffraction revealing a low organization of the macromolecules. Even better insight into the organization behavior is gained by X-ray scattering of an extruded polymer fiber. In addition to the low degree of order, a p–p distance of 0.43 nm is detected, a large value in comparison to highperformance polymers reported in the literature and unfavorable for high mobility. It seems that polymer P1 is too stiff to obtain high order; more structural flexibility along the backbone is needed. For this reason, additional thiophene units bearing alkyl chains are incorporated into the backbone yielding polymer P2, a general concept known in the literature. By optimization of the polymerization conditions, a comparable molecular weight as for polymer P1 could be obtained (Mn1⁄4 22 kg mol ), minimizing field-effect mobility differences due to different polymer chain lengths. FETs are prepared following exactly the same procedure as for P1 to ensure maximum comparability. Average charge-carrier mobility in the low 10 3 cm V 1 s 1 region is measured for polymer P2. In fact, higher order and decreased p–p distance (0.39 nm) in comparison to polymer P1 are determined by fiber X-ray scattering. Obviously, the gain in backbone flexibility proves to be favorable, but the electronic performance is still not satisfactory. As polymer P2 possesses high solubility in organic solvents, fewer alkyl chains seemed possible to still guarantee the necessary solubility for processing. Therefore, polymer P3 is synthesized having no alkyl chains at the additional thiophene units to avoid ‘‘overalkylation’’ which may hinder close contact of adjacent polymer chains. Since using dodecyl chains as for polymer P1 renders an insoluble polymer, longer hexadecyl

Control of morphology in efficient photovoltaic diodes from discotic liquid crystals

We report on the role of morphology in photovoltaic diodes with blend active layers composed of perylene tetracarboxdiimide (EPPTC) and hexabenzocoronene (HBC) derivatives as electron and hole acceptors. Controlled annealing of HBC:EPPTC films while in conformal contact with a flat elastomeric stamp improves photovoltaic response, leading to an external quantum efficiency of 29.5% at 460 nm and an open-circuit voltage of 0.70 V. The improved performance is attributed in part to larger crystalline domains following annealing. The elastomeric stamp restricts the top surface of the thin film during annealing, leading to low surface roughness, while also allowing for greater vertical stratification of the components in the bulk. Blended HBC:EPPTC films also exhibit an unique optical absorption feature near 590 nm, which we attribute to an altered crystalline packing of EPPTC in the presence of HBC. The significance of the local structure at molecular heterojunctions in blended active layer photovoltaic diodes is discussed.

Room-temperature nondispersive hole transport in a discotic liquid crystal

The authors report on room-temperature nondispersive hole transport in films of a discotic liquid crystalline hexa-peri-hexabenzocoronene derivative studied by the time-of-flight technique. Hole mobilities in the order of 10−3cm2∕V−1s−1 have been determined and results of field- and temperature-dependent measurements could be well explained within the concept of charge transport by hopping in a one-dimensional manifold of transport sites formed by the columns of the discotic liquid crystal.

A conjugated thiophene-based rotaxane: synthesis, spectroscopy, and modeling.

A dithiophene rotaxane 1 subsetbeta-CD and its shape-persistent corresponding dumbbell 1 were synthesized and fully characterized. 2D NOESY experiments, supported by molecular dynamics calculations, revealed a very mobile macrocycle (beta-CD). Steady-state and time-resolved photoluminescence experiments in solution were employed to elucidate the excited-state dynamics for both systems and to explore the effect of cyclodextrin encapsulation. The photoluminescence (PL) spectrum of 1 subsetbeta-CD was found to be blueshifted with respect to the dumbbell 1 (2.81 and 2.78 eV, respectively). Additionally, in contrast to previous observations, neither PL spectra nor the decay kinetics of both threaded and unthreaded systems showed changes upon increasing the concentration or changing the polarity of the solutions, thereby providing evidence for a lack of tendency toward aggregation of the unthreaded backbone.

A C216-Nanographene Molecule with Defined Cavity as Extended Coronoid.

We describe the first coronoid nanographene C216-molecule. As an extended polycyclic aromatic hydrocarbon containing a defined cavity, our molecule can be seen as a model system to study the influence of holes on the physical and chemical properties of graphene. Along the pathway of an eight-step synthesis including Yamamoto-type cyclization followed by 6-fold Diels-Alder cycloaddition, C216 was obtained by oxidative cyclodehydrogenation in the final step. The defined molecular structure with a cavity was unambiguously validated by MALDI-TOF mass spectrometry and FTIR, Raman, and UV-vis absorption spectroscopy coupled with DFT simulations.

Molecular dynamics of branched hexaalkyl hexa-peri-hexabenzocoronenes

The molecular dynamics of discotic liquid crystalline hexa-peri-hexabenzocoronenes (HBC-C 10,6 and HBC-C 14,10 ) with long alkyl side chains branched in close vicinity to the HBC aromatic core were studied by dielectric spectroscopy. The two compounds display a weak dipolar relaxation (a-process) manifested by a Vogel-Fulcher-Tammann (VFT) dependence and have almost the same glass temperature (∼215 K) in excellent agreement with the value from differential scanning calorimetry. The origin of this process is the freezing of the in-plane disc rotation. Controlled experiments revealed that the weak dipole moment associated with the α-process is due to oxidation.

Curvature as design concept for semiconducting benzodithiophene-containing polymers in organic field-effect transistors

Organic semiconductors are expected to play an important role in low-cost and energy-efficient electronic products, especially transistors. Our concept to improve the active materials is to introduce curvature into the polymer backbone. The beneficial contribution of the curvature is shown by a series of five isomeric polymers which demonstrate that an intermediate degree of curvature is the optimum compromise between good solubility and high order in the film. In this way, a polythiophene is developed that is easily synthesized, well soluble, exhibits high charge-carrier mobility using mild annealing conditions, and is stable under operation. When processed on a flexible substrate, it shows average mobilities as high as 0.5 cm2V-1s-1 .