endo-Dibenzosexithiophene-based thienoquinoid: Impact of fused-ring structure on the open-shell state

Thienoquinoids are attractive π-conjugated systems because of their high electron-accepting abilities, narrow band gaps, and tunable open-shell character. In particular, fused-ring thienoquinoids are of considerable interest as their properties can be modulated by the bridging unit. However, most reported systems employ heteroatom bridges, and alternative bridging units have rarely been explored. In this study, a fused-ring thienoquinoid, endo -DBST-CN , incorporating thiophene rings as a bridging unit, was designed to examine the impact of ring fusion on the electronic structure and carrier-transport characteristics. Compared with the non-fused analogue BTDTTT-CN , endo -DBST-CN exhibits a higher open-shell character with a larger diradical index ( y ₀ = 0.74) and a smaller singlet–triplet energy gap ( ΔE ST = 1.64 kcal mol –1 ). The fused-ring structure also imparts endo -DBST-CN with a red-shifted absorption extending into the near-infrared region. Meanwhile, endo -DBST-CN exhibits deeper highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels than BTDTTT-CN , resulting in unipolar n-type characteristics in endo -DBST-CN -based organic field-effect transistor devices. These results demonstrate that fused-ring engineering is an effective strategy for tuning the electronic structure and device performance of thienoquinoid-based semiconductors. • A fused-type thienoquinoid endo -DBST-CN featuring thienyl bridges was developed. • Ring fusion modulates the open-shell character and the singlet–triplet energy gap. • endo -DBST-CN shows an intense absorption band in the near-infrared region. • Organic field-effect transistors based on endo -DBST-CN exhibit n-type properties.