Design of novel a sigma*-pi* conjugated polysilanes

Electronic structures of two novel sigma*-pi* 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 sigma-sigma* transition of the polymers are smaller than that of the conventional polysilanes. The sigma-sigma* bandgaps of these two polymers are essentially determined by the lower-lying conduction band derived from the mixing of the delocalized sigma*-band of the silicon skeleton and the localized pi*-band of the side chains. Results suggest that the a sigma*-pi* conjugation may be more effective for the reduction of the bandgap than the sigma-pi conjugation in a polysilane with aryl side chains.