2-VALENCE-BOND STATE MODEL FOR MOLECULAR NONLINEAR-OPTICAL PROPERTIES- NONEQUILIBRIUM SOLVATION FORMULATION

The two valence bond state model frequently used to describe the nonli near optical properties of push-pull polyenes and the effect of the bo nd length alternation (BLA) coordinate is generalized to include the e ffects of nonequilibrium solvation. It is shown how a polar solvent mo difies, via the diabatic gap between the valence bond free energy surf aces, the character of the electronically adiabatic ground state and c onsequently the BLA and the (hyper)polarizabilities. The parameters ch aracterizing a molecule in the model can be readily extracted from exp erimental measurements. The model is applied to the calculation of (hy per)polarizabilities for several molecules as a function of solvent po larity, and comparison is made with both experimental and previous the oretical results. Some directions for an improved description are disc ussed.