Improved stability of organic field-effect transistor performance in oligothiophenes including β-isomers

Dialkyl quarter- and quinquethiophenes end-capped with β-connected thiophenes are prepared, and the field-effect transistor (FET) properties are investigated. Molecular orbital calculation as well as the redox and optical measurements indicate that the β-isomers have low-lying HOMO levels and large energy gaps compared with the α-isomers. Molecular packing of the dihexylquaterthiophene with β-isomers consists of a typical herringbone motif analogous to the α-isomers. In the single crystal, the alkyl chains are extending in the tilted directions from the core molecular plane, but straightly extending in the thin films, resulting in even more perpendicular molecular arrangement to the substrate than the α-isomers. These β-isomers show p-type FET performance comparable to the corresponding α-isomers, whereas all new oligothiophenes show air stability better than the corresponding α-isomers. In particular, dihexylquinquethiophene with β-isomers has shown significantly improved air stability maintained over 270 days. This stabilization effect is ascribed to the low-lying HOMO level and the dense packing realized by perpendicular molecular arrangement.