SINGLET EXCITATION IN POLYSILANES - ABINITIO CALCULATIONS ON OLIGOSILANE MODELS

All-electron SCF-CI calculations with several basis sets have been performed for excited singlet states of the linear oligosilanes Si(n)H2n+2, n = 2-5, in order to establish their nature as a function of chain length. The results suggest that the lowest sigma-SiSi --> pi*SiH excited state lies below the first sigma-SiSi --> sigma*SiSi state in the shortest few oligosilanes, and that the order of the states changes as the number of silicon atoms in the chain increases. An assignment is proposed for the lowest observed two-photon allowed state of polysilanes. In the gas phase, Rydberg states are expected to dominate the low-energy region of the spectra of oligosilanes shorter than Si6H14. An increase in the SiSiSi valence angle is computed to lower the sigma-SiSi --> sigma*SiSi excitation energy, and torsion around a central SiSi bond from trans towards cis geometry is computed to increase it. The effect of alkyl substitution was studied using 2,2-dimethyltrisilane as a model compound. It lowers the sigma-SiSi --> sigma*SiSi excitation energy, primarily by destabilizing the highest sigma-SiSi orbital by hyperconjugative interaction with the substituents. The results for the sigma-SiSi --> sigma*SiSi state are in agreement with simple, qualitative arguments and prior semiempirical calculations.