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 [C H3 Si O1.5] 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 (C2v) = [MeT]10>[MeT]6>[MeT]8 (Oh).
Bibliographical noteFunding Information:
This investigation was based on work supported by the National Natural Science Foundation of China under Project No. 20373073, the Science Foundation of the Fujian Province (Grant Nos. E0210028 and 2002F010), and the Foundation of State Key Laboratory of Structural Chemistry (Grant No. 030060).
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