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

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 C 2 h symmetry exhibit electron mobilities of 0.02 and 0.15 cm 2 V –1 s –1, respectively, while QBTT- s exhibits an unexpectedly high electron mobility of 0.32 cm 2 V –1 s –1 with I on / I off ratios of ≤10 6 . 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 QBTT s were systematically investigated by grazing incidence wide-angle X-ray scattering, near-edge X-ray absorption fine structure spectroscopy, atomic force microscopy, and molecular dynamic simulations. The crystallinities are critically dependent on sulfur orientations and increase from QBTT- o to QBTT- i to QBTT- s, which agrees well with the organic thin-film transistor (OTFT) performance. The poor OTFT performance of QBTT- o compared to that of QBTT- i with the same C 2 h symmetry can be attributed to the different sulfur orientations; meanwhile, we speculate that the strongest crystallinity of QBTT- s might be attributed to the weak dipole moment that originated from the asymmetric molecular structure. Therefore, molecular symmetry is an important issue that needs to be carefully considered for the design of high-performance organic semiconductors.