New low band gap polymers: control of optical and electronic properties in near infrared absorbing π-conjugated polysquaraines

Abstract

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 modulated between 10^-7-10^-4 S/cm by varying the length of the alkyl side chains. This is in agreement with the molecular packing data obtained from the X-ray analysis that revealed an interdigitated arrangement of the polymer chains. The solubility inducing alkyl side chains play a decisive role in the molecular packing, which control the optical band gap and conductivity of the reported polysquaraines. This is one of the simplest strategies for the synthesis of NIR absorbing conjugated polymers with extremely low band gaps that are soluble and intrinsically semiconducting.