Bichalcogenophene Imide-Based Homopolymers: Chalcogen-Atom Effects on the Optoelectronic Property and Device Performance in Organic Thin-Film Transistors

Driven by the exceptional success of 2,2′-bithiophene-3,3′-dicarboximide imide (BTI) for enabling high-performance polymer semiconductors, herein two BTI analogues 2,2′-bifuran-3,3′-dicarboximide (BFI) and 2,2′-biselenophene-3,3′-dicarboximide (BSeI) are designed and synthesized. The strong electron-withdrawing imide group enables BFI and BSeI with high electron deficiency, differing from typical furan- and selenophene-based building blocks, which are electron-rich. Hence, n-type polymers can be derived based on these two new imides. To investigate the effects of chalcogen-atom substitution on the physicochemical properties and device performance of these imide-bridged materials, two homopolymers PBFI and PBSeI are synthesized together with the previously reported PBTI as control. Structures, optoelectronic properties, and charge transport characteristics of PBFI and PBSeI are studied and compared to those of the thiophene-based analogue PBTI in depth. The optical band gap (E_g ^opt) of the dibrominated bichalcogenophene imide and corresponding homopolymer becomes narrowed gradually as the chalcogen-atom size increases. Among all polymers, PBSeI shows the smallest E_g ^opt of 1.78 eV. In addition, the lowest unoccupied molecular orbital (LUMO) energy level (E_LUMO) of the monomer and its homopolymer is also lowered. Such lowering of E_g ^opts and E_LUMOs by simple chalcogen substitution should have profound implications for device applications. The organic thin-film transistors based on PBFI, PBTI, and PBSeI show n-type performance with the highest electron mobility of 0.085, 1.53, and 0.82 cm² V^-1 s^-1, respectively, indicating that increasing chalcogen-atom size doesn't necessarily improve electron transport. It was found that chalcogen atoms largely affect the packing of polymer chains, which leads to PBTI and PBSeI with a higher crystallinity compared with PBFI. The results demonstrate that in addition to the well-known BTI, BSeI should also be a highly promising unit for constructing n-type polymers, and this study provides an important foundation for further development of high-performance organic semiconductors considering the significance of imide-functionalized building blocks in the field of organic electronics.