Design of novel σ*-π* conjugated polysilanes

Electronic structures of two novel σ*-π* conjugated polysilanes, poly(1,1-silole) and poly(1,1-benzo[c] silole), have been studied theoretically using the one-dimensional tight-binding self-consistent field-crystal orbital (SCF-CO) method at the modified neglect of diatomic overlap, Austin Model 1 (MNDO-AM1) level of approximation. It is found that the bandgaps corresponding to the σ-σ* transition of the polymers are smaller than that of the conventional polysilanes. The σ-σ* bandgaps of these two polymers are essentially determined by the lower-lying conduction band derived from the mixing of the delocalized σ*-band of the silicon skeleton and the localized π*-band of the side chains. Results suggest that the σ*-π* conjugation may be more effective for the reduction of the bandgap than the σ-π conjugation in a polysilane with aryl side chains.