Electronic structures of linear polysilane radical cations in ground and low-lying excited states: a theoretical study

The electronic structures of linear polysilane radical cations in ground and low-lying excited states have been studied by means of the semi-empirical PM3-CI method. The permethyl-oligosilane radical cation [Sin(Me)2n + 2]+ (n = 4–18) was chosen as a model compound of the polysilane. Molecular dynamics calculations and geometry optimizations suggested that the regular all-trans chain was the most stable for the polysilane radical cations. From the CI calculations, it was concluded that the ground state can be expressed by a single Slater determinant (single configuration of the Hartree-Fock solution) with a hole (or unpaired electron) occupied in the HOMO of the parent neutral polysilane (i.e. SOMO), whereas the first excited state is composed of two configurations: the hole is occupied in (SOMO − 1) and (SOMO + 1) states, where SOMO + i means that a unpaired electron is occupied in the i-th molecular orbital relative to the SOMO. The ratio of the weights, (SOMO − 1)(SOMO + 1), is strongly dependent on the number of chain si atoms n. The electronics states of the oligosilane (n = 4–18) and polysilane radical cations with a large chain (n = ∞) are discussed on the basis of the theoretical results.