DIHEXYLQUATERTHIOPHENE, A 2-DIMENSIONAL LIQUID CRYSTAL-LIKE ORGANIC SEMICONDUCTOR WITH HIGH TRANSPORT-PROPERTIES

End-substitution of quaterthiophene with hexyl groups leads to a highl y soluble conjugated oligomer, alpha,omega-dihexylquaterthiophene (DH4 T), which has been characterized for its thermal, structural, and elec trical properties. Differential scanning calorimetry indicates the exi stence 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 treate d films of DH4T confirm the existence of a smectic phase with two spac ings that increase with temperature. This result is interpreted by a m odel involving alkyl chain movements, which result in spacing shrinkag e, whereas the thienylene sequence remains faced at typical van der Wa als 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 vac uum evaporation or by spin coating on an octylsilane-pretreated surfac e. A high-field-effect carrier mobility has been obtained for both dep osition techniques, mu = 3 x 10(-2) and 1.2 x 10(-2) cm(2) V-1 s(-1) r espectively, together with an interesting I-on/I-off ratio of 10(5). T hese data are discussed together with literature results on unsubstitu ted 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, co mparable to those of a sexithiophene core, whereas the core is also sh ort enough to allow its alpha,omega-dialkylated derivative to be highl y soluble and solution processable, contrary to the case of DHGT. Resu lts also suggest that the larger band gap of the shorter conjugated qu aterthiophenes is responsible for, their lower dopant concentration, a nd hence of their higher dynamic ratio.