ELECTRONIC STATES OF LINEAR TETRASILANE AND POLYSILANES

The standard Sandorfy C model of electronic structure of polysilanes is inherently incapable of providing a physical interpretation of the observed conformational effects on UV spectra. We describe a more complicated ladder C model, which still considers only the two backbone hybrid orbitals on each silicon atom, but includes the transfer integrals between all pairs of orbitals on a pair of neighboring silicons. Already at the Huckel level, this model accounts for the general features of the observed spectra both as a function of chain length and of conformation. Results of our ab initio calculations suggest, however, that for general non-planar conformations the backbone orbitals should not be treated separately from those that carry the substituents, at least not in relatively short chains. The simplest level at which the conformational effects on the spectra of oligosilanes of all lengths can be accounted for simultaneously will probably be the even more complicated ladder H model, in which all four valence orbitals of each silicon and one orbital of each substituent are considered explicitly.