Dihexylquaterthiophene, a two-dimensional liquid crystal-like organic semiconductor with high transport properties

End-substitution of quaterthiophene with hexyl groups leads to a highly soluble conjugated oligomer, alpha,omega-dihexylquaterthiophene (DH4T), which has been characterized for its thermal, structural, and electrical properties. Differential scanning calorimetry indicates the existence of a 3-dimension (3D)-to-mesophase transition, occurring at 84 degrees C, below the melting temperature of the material (179 degrees C). X-ray diffraction patterns performed on crude and thermally treated films of DH4T confirm the existence of a smectic phase with two spacings that increase with temperature. This result is interpreted by a model involving alkyl chain movements, which result in spacing shrinkage, whereas the thienylene sequence remains faced at typical van der Waals distances. DH4T thus forms a 2-dimensional (2D) semiconductor with a liquid crystal-like structural organization. DH4T can be deposited as active semiconducting layer in thin-film transistors, either by vacuum evaporation or by spin coating on an octylsilane-pretreated surface. A high-field-effect carrier mobility has been obtained for both deposition techniques, mu = 3 x 10(-2) and 1.2 x 10(-2) cm(2) V-1 s(-1) respectively, together with an interesting I-on/I-off ratio of 10(5). These data are discussed together with literature results on unsubstituted quaterthiophene (4T) and the parent sexithiophenes (DH6T and 6T). On one hand, results show that the semiconducting quaterthiophene core in DH4T is large enough to allow high charge transport properties, comparable to those of a sexithiophene core, whereas the core is also short enough to allow its alpha,omega-dialkylated derivative to be highly soluble and solution processable, contrary to the case of DHGT. Results also suggest that the larger band gap of the shorter conjugated quaterthiophenes is responsible for, their lower dopant concentration, and hence of their higher dynamic ratio.