CONFORMATIONS AND ELECTRONIC-STRUCTURES OF NEW CONJUGATED POLYMERS BASED ON POLY(CYCLOPENTADIENYLENE) AND POLYSILOLE

The conformations and electronic structures of several five-membered-ring polymers were investigated with the partial retention of diatomic differential overlap (PRDDO) method. Band structures of the polymers were calculated using the modified extended Huckel (MEH) method. The polymers considered in this study are analogous to heterocyclic polymers such as polythiophene, polyfuran, and polypyrrole; however, they have bridging groups of XY(2) (XY(2) = CH2, CF2, SiH2, and SiF2) instead of heteroatoms. The relative stability of the aromatic and quinoid forms of these polymers was examined through an oligomer approach. The evolution of the band gaps of these systems was analyzed in terms of bond-length alternations, changes in the C1-C4 distances, and the effects of pure electronic interactions between the polymeric backbone and the bridging groups. It was found that insertion of the bridging group into the polymeric backbone affects the band gap in two distinct ways. The decrease of the C1-C4 distance relative to that found in cis-polyacetylenes narrows the band gap of the aromatic form and widens the band gap of the quinoid form. On the other hand, electronic interactions tend to increase the band gap of the aromatic form and decrease the band gap of the quinoid form. The electronic effect of a CH2 group on the band gap is small but not negligible (ca. 0.7 eV), and the resultant band gaps of both the aromatic and quinoid forms are comparable to those of polyacetylenes. The electronic interactions of the other bridging groups are so small that the quinoid forms became more stable in the ground state.