A theoretical study of polysquaraines

Squaraine dye molecules present a route to semiconducting polymers with small band gaps. We use ab initio calculations to study polysquaraines systematically, and find band gaps that range from 2.3 down to 0.2 eV. Simple arguments based upon a Hückel analysis of the ab initio results enable us to identify the factors that control the size of the band gap. General design rules can be based on polymer topology and symmetry. The band gap can be tuned by chemical substitutions.