Synthesis of π-Bridged Dually-Dopable Conjugated Polymers from Benzimidazole and Fluorene: Separating Sterics from Electronics

Abstract

We describe the synthesis and characterization of three new alternating copolymers containing fluorene and a dually dopable benzimidazole moiety. Poly(2-n-heptyl-benzimidazole-alt-9,9-di-n-octylfluorene) (PBIF), poly(2-n-heptyl-benzimidazole-vinylene-9,9-di-n-octylfluorene) (PBIF-VL), and poly(2-n-heptyl-benzimidazole-ethynylene-9,9-di-n-octylfluorene) (PBIF-EL) were synthesized from Suzuki, Heck, and Sonogashira cross-coupling reactions in reasonable yield. The materials were characterized through ultraviolet–visible spectroscopy, photoluminescence, and cyclic voltammetry. The vinyl and ethynyl bridges in PBIF-VL and PBIF-EL were incorporated to separate benzimidazole and fluorene units while retaining conjugation. This allowed us to differentiate between steric and electronic contributions to the band gap (Eg) changes that occur upon acid/base doping of these materials. We demonstrate that the blue shift arising from acid doping PBIF is due to steric torsion, while the red shift found upon base doping P...