Oligo(thienylfuran)s with thiophene rings at both ends (SOSOSOS, DE-SOSOS, DH-SOSOS, DE-SOSOSOS, and DH-SOSOSOS; S and O denote thiophene and furan rings, respectively, DE and DH denote diethyl- and dihexyl-substituted, respectively) were newly synthesized by repetitive Stille coupling reactions. The UV/Vis maximum absorptions of the oligomers, SO, SOSO, SOSOS, SOSOSO, and SOSOSOS, exhibited a clear bathochromic shift with increasing number of heterocycles. The value of the oxidation peak potential (E(pa)1) determined by cyclic voltammetry decreased with an increase in the number of heterocycles by 0.06-0.08 V per heterocycle. The crystal-packing structures of DE-SOSOS and DH-SOSOS determined by X-ray crystallography have a herringbone motif and are denser than the reported structures of pentacene and alpha-sexithiophene. The morphologies of thin films prepared by vacuum deposition and spin coating were investigated by atomic force microscopy and X-ray diffraction. Among these films, those of DE-SOSOS and DH-SOSOS exhibited highly ordered arrangements. The devices based on vacuum-deposited and spin-coated films of DE-SOSOS and DH-SOSOS displayed the highest FET mobilities of 10(-2)-10(-3) cm2 V(-1) s(-1) among the oligomers reported in this study.
Organic field-effect transistors are fabricated using oligomers composed of alternating connected thiophene and furan, and those having alkyl substituents at both ends of the molecules. The mobilities are determined by the transfer line method to correct for the effect of source/drain parasitic resistance. A mobility of 0.042cm2∕Vs is achieved for a thienyl-furan oligomer composed of five heterocycles and having hexyl groups at the terminal rings. The mobility of the oligomers strongly depends on the length of the π-conjugated chain and the alkyl chain substituent at both ends, attributable to the specific structural and morphological properties of the vacuum-deposited films.
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