N–B ← N Bridged Bithiophene: A Building Block with Reduced Band Gap to Design n-Type Conjugated Polymers
Xingxin Shao, Jiahui Wang, Todd B. Marder, Zhiyuan Xie, Jun Liu, Lixiang Wang
Abstract
The development of conjugated polymers with excellent performance of optoelectronic device depends to a large extent on the molecular design of electron-accepting (A) building blocks. Previously reported A building blocks based on a bithiophene skeleton always show lower lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energies compared to those of bithiophene itself. Herein, we report a new A building block (BNTzT), in which the thiophene–thiazole skeleton is bridged by a N–B←N moiety. Compared to bithiophene, BNTzT exhibits a LUMO level, which is 0.85 eV lower, and a nearly unchanged HOMO level, leading to a reduced band gap. This is due to the electron-withdrawing property and the increased quinodial character of the bithiophene skeleton resulting from the incorporation of the N–B←N moiety. The LUMO energy of the homopolymer of BNTzT is estimated to be −3.68 eV, the band gap as narrow as 1.71 eV, and a near-infrared fluorescence peaked at 700 nm. Owing to their interesting optoelectronic properties, conjugated polymers constructed from the BNTzT unit can function as electron acceptors in all-polymer solar cells (all-PSCs). This work not only provides a novel A building block for conjugated polymers but also demonstrates a new strategy for designing narrow-band-gap polymers by introducing an N–B←N moiety into a conjugated skeleton.