New Low Band Gap Polymers: Control of Optical and Electronic Properties in near Infrared Absorbing π-Conjugated Polysquaraines

A novel approach to design extremely low band gap polysquaraines with intense near-infrared (NIR) absorption and high intrinsic conductivity is described. Feasibility of the new strategy is illustrated by an A-B type copolymerization of squaric acid and 1,4-dialkoxydivinylbenzene-bridged bispyrroles, which resulted in zwitterionic polysquaraines with resonance stabilized quinoid structures. Incorporation of an electron donating conjugated moiety between each squaraine dye repeating unit has a dramatic influence on the optical and electronic properties of the resulting polysquaraines due to an enhanced donor−acceptor−donor interaction. The solution UV−Vis−NIR absorption maxima of the new polymers, between 772 and 1040 nm with ground-state onset absorptions ranging from 1140 to 1300 nm, is unusual for conjugated polymers and is a signature of their low band gaps. The band gaps of these polymers are around 1 eV with the lowest value of 0.79 eV for 6a. The intrinsic conductivities of these polymers could be m...