Critical Role of Molecular Symmetry for Charge Transport Properties: A Paradigm Learned from Quinoidal Bithieno[3,4-b]thiophenes

Abstract

High molecular symmetry is always observed in high-performance organic semiconductors. However, whether it is an essential factor for molecular design is unclear. In this work, we designed and synthesized three quinoidal isomers, QBTT-o, QBTT-i, and QBTT-s, with different sulfur orientations and a stable E configuration to investigate the relationship between the structure symmetry and organic thin-film transistor performance. We found that QBTT-o and QBTT-i with high C2h symmetry exhibit electron mobilities of 0.02 and 0.15 cm2 V–1 s–1, respectively, while QBTT-s exhibits an unexpectedly high electron mobility of 0.32 cm2 V–1 s–1 with Ion/Ioff ratios of ≤106. The enhanced electron mobilities from QBTT-o and QBTT-i to QBTT-s can be attributed to the different sulfur orientations, especially, molecular symmetry. The thin-film microstructures of three QBTTs were systematically investigated by grazing incidence wide-angle X-ray scattering, near-edge X-ray absorption fine structure spectroscopy, atomic force ...