Synthesis, Electrochemical, and Optical Properties of Low Band Gap Homo- and Copolymers Based on Squaraine Dyes

A broad series of monomeric and polymeric squaraines was synthesized to investigate the impact of electron-donating bridges, such as unsaturated triarylamine, carbazole, and saturated piperazine groups, on the spectroscopic and redox properties. These bridges were attached to both standard trans-indolenine squaraines and dicyanomethylene-substituted cis-indolenine squaraines. The conjugates were investigated by absorption, steady-state, and time-resolved fluorescence spectroscopy and cyclic voltammetry. While addition of the donors resulted in significant redshift of the absorption of the model compounds, hardly any further shift or broadening was observed for the copolymers. Also the redox properties remained nearly unchanged compared with the model dyes. In contrast, immense broadening and redshift was observed for homopolymers. This behavior is explained by mostly excitonic coupling of localized squaraine transitions. The increasing distance of the chromophores determined by the bridges led to a decrease of the exciton coupling energy. We also performed semiempirical CNDO/S2 calculations on AM1 optimized structures. (c) 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 890-911