Synthesis and structural, electronic, optical and FET properties of thiophene–pyrrole mixed hexamers end-capped with phenyl and pentafluorophenyl groups

Abstract

α-Linked thiophene–pyrrole mixed hexamers 3a–5a end-capped with two phenyl, two pentafluorophenyl and both phenyl and pentafluorophenyl units were newly synthesized, and their structural, electronic, optical and FET properties were investigated. Structural analyses by means of X-ray crystallography and theoretical calculations showed that the π-system of the oligomer chains is twisted due to the steric repulsion between the hexyl group of the pyrrole unit and the adjacent thiophene moiety. From comparison of the absorption maxima and redox potentials, end-capping with phenyl and pentafluorophenyl units did not affect the HOMO–LUMO gap for the thiophene–pyrrole hexamer, and the multiple fluorine substituents slightly decreased both HOMO and LUMO levels by the inductive effects of the electronegative fluorine atoms. Although the diphenyl derivative 3a with a herringbone-type packing structure did not serve as an active layer for FET devices, a unique crystal-to-crystal phase transition with a color change from yellow to red was observed upon heating. In contrast to the herringbone-type packing structure of 3a, the unsymmetrical phenyl–pentafluorophenyl derivative 5a showed a partially π-stacked structure, most likely due to dipole–dipole interactions. The π-stacked structure of 5a caused a good hole mobility of 4.1 × 10⁻² cm² V⁻¹s⁻¹ in a single crystal FET device.