A MNDO study of the structural and electronic properties of polysilane model compounds

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Abstract
The results of MNDO geometry optimizations on selected H(SiH2)nH polysilane model compounds are presented. Near energetic degeneracy is indicated for all‐trans(T), alternating gauche–trans (GT), and all‐gauche (G+G+) models (n = 10). The most stable (T) and least stable (G+G+) conformations are separated by only ca. 0.11 eV. The existence of low‐energy barriers to moderate structural distortion is also suggested. Orbital localizations and charge density distributions along the “polymer” backbone are found to be sensitive functions of such distortion. The ground‐state electronic distribution of the saturated all‐trans silane chains are calculated to be considerably more polarizable than the fully conjugated H(CH)nH π‐electron framework of comparable length. The one‐electron HOMO → LUMO excitation can be viewed essentially as an in‐plane Si 3p → Si3s + H1s intramolecular charge transfer transition. The qualitatively different atomic orbital character of the HOMO and LUMO levels yields transition moment components for the separate repeat units which are relatively small. In the case of the rigidly trans conformation, the phase relationships of the transition moment terms are such as to constructively sum to a large net value reflecting strong optical absorption, as is observed experimentally.