GEOMETRICAL AND ELECTRONIC-STRUCTURES OF PI-CONJUGATED SILICON RING POLYMERS

Two alternative structures of the [C4H2SiR2]n polymer have been considered. Full geometry optimization at different levels of theory indicate that the rive-membered-ring structure (1) is more stable, by about 15 kcal/mol, than the structure with a four-membered ring (2). On the other hand, the electronic structure of polymer 1 indicates a rather low band gap of 1.44 eV, in contrast to the experimental value of 2 eV, while 2 has a calculated band gap of 1.99 eV. Therefore, this structure is more consistent with the experimental absorption spectrum of the polymer than structure 1. This conclusion is also consistent with the NMR data of Wei et al.2