Butyl, hexyl, and decyl derivatives of the liquid-crystalline organic semiconductor 5,5' '-bis(5-alkyl-2-thienylethynyl)-2,2':5',2' '-terthiophene were synthesized and studied with respect to their structural, optical, and electrical properties. By means of an optimized thermal annealing scheme the hexyl and decyl compounds could be processed into self-assembled monodomain films of up to 150 mm in diameter. These were investigated with X-ray diffractometry, which revealed a clearly single-crystalline monoclinic morphology with lamellae parallel to the substrate. Within the lamellae the molecules were found to arrange with a tilt of about 50 degrees with the rubbing direction of the polyimide alignment layer. The resulting, close side-to-side packing was confirmed by measurements of the UV/vis absorption, which showed a dichroic ratio of 19 and indicated H-aggregation. AFM analyses revealed self-affinity in the surface roughness of the monodomain. The compounds showed bipolar charge transport in TOF measurements, with hole mobilities reaching up to 0.02 cm(2)/Vs and maximum electron mobilities around 0.002 cm(2)/Vs. The hexyl derivative was processed into large-area monodomain top-gate field-effect transistors, which were stable for months and showed anisotropic hole mobilities of up to 0.02 cm(2)/Vs. Compared to multidomain bottom-gate transistors the monodomain formation allowed for a mobility increase by 1 order of magnitude.

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Nine different a,v-phenyl-endcapped bithiophenes were synthesised, and the effect of the different side chains on the liquid crystalline properties, alignment ability and charge carrier mobility have been studied. An increase in chain length leads to a decrease in the liquid crystalline-isotropic phase (clearing) transition temperature. Remarkably, introduction of an asymmetric carbon centre close to the p-conjugated segment resulted in the loss of the liquid crystalline phase. Alignment on rubbed polyimide was obtained for the liquid crystalline thiophene derivatives lacking heteroatoms in the side chain and for the chiral a,v-phenyl-endcapped bithiophene. Some bithiophenes showed bipolar charge transport in time-of-flight (TOF) measurements, with mobilities up to 3 6 10 23 cm 2 V 21 s 21 in the liquid crystalline state. Field effect transistors revealed mobility for holes up to 1 6 10 23 cm 2 V 21 s 21 (crystalline state). From the data set obtained, it can be concluded that the use of linear hydrocarbon chains as solubilising tails in these types of p-conjugated building blocks gives the best overall electronic performance. Results and discussion SynthesisThe different phenyl-endcapped bithiophene derivatives (Chart 1, 2-10) were synthesised according to Scheme 1. The

The optical and conductive properties of the liquid-crystalline organic semiconductor 5,5′′-bis (5-hexyl-2thienylethynyl)-2,2′:5′,2′′-terthiophene (TR5-C6) spin-coated on rubbed polyimide (PI) were studied. The absorption of light linearly polarized parallel to the rubbing direction largely exceeds that in the perpendicular direction. In contrast, a small anisotropy in the photoconductance was found from time-resolved microwave conductivity (TRMC) measurements. From analysis of the optical data, it is inferred that the terthiophene chains in the TR5-C6 molecules are oriented along the PI rubbing direction with a tilt angle of 53 degrees with respect to the plane of the substrate. The absence of a strong anisotropy in the mobility of charge carriers can be understood on the basis of calculated charge transfer integrals, which were found to be comparable for charge-transfer steps in different directions. This is due to arrangement of the molecules in a herringbone structure. Due to the relatively small values of the charge transfer integrals, it is likely that charge transport in TR5-C6 occurs via polaron hopping between localized states.