Infrared light emission from π-conjugated polymers: A diagrammatic exciton basis valence bond theory

There is currently a great need for solid-state lasers that emit in the infrared, as this is the operating wavelength regime for applications in telecommunications. Existing π-conjugated polymers all emit in the visible or ultraviolet, and whether or not π-conjugated polymers that emit in the infrared can be designed is an interesting challenge. On the one hand, the excited-state ordering in trans-polyacetylene, the π-conjugated polymer with a relatively small optical gap, is not conducive to light emission because of electron-electron interaction effects. On the other hand, excited-state ordering opposite to that in trans-polyacetylene is usually obtained by chemical modification that increases the effective bond alternation, which in turn increases the optical gap. We develop a theory of electron correlation effects in a model π-conjugated polymer that is obtained by replacing the hydrogen atoms of trans-polyacetylene with transverse conjugated groups and show that the effective on-site correlation in this system is smaller than the bare correlation in the unsubstituted system. An optical gap in the infrared as well as excited-state ordering conducive to light emission is thereby predicted with similar structural features.