Modeling of configurations and third-order nonlinear optical properties of methyl silsesquioxanes.

Configuration optimizations, excited state properties, and the frequency-dependent third-order nonlinear optical polarizabilities have been investigated on a series of methyl-silsesquioxane (MeT) cages [CH₃SiO_1.5]_n (n=4, 6, 8, and 10) using ab initio quantum mechanical methods coupled with the sum-over-states methods. The obtained electronic absorption spectra show a redshift as the cage size increases, and the absorption spectra are assigned as charge transfers from oxygen p type to silicon s type atomic orbitals. The calculated average third-order polarizabilities of 〈γ〉 in the three optical physical processes (third-harmonic generation, the electric-field-induced second-harmonic generation, and degenerate four-wave mixing) have wide nonresonance regions. For all the considered species, the values of γ decrease in the order of [MeT]₄>[MeT]₈(C_2v)=[MeT]₁₀>[MeT]₆>[MeT]₈(O_h). [ABSTRACT FROM AUTHOR]