Two Valence Bond State Model for Molecular Nonlinear Optical Properties. Nonequilibrium Solvation Formulation

The two valence bond state model frequently used to describe the nonlinear optical properties of push−pull polyenes and the effect of the bond length alternation (BLA) coordinate is generalized to include the effects of nonequilibrium solvation. It is shown how a polar solvent modifies, via the diabatic gap between the valence bond free energy surfaces, the character of the electronically adiabatic ground state and consequently the BLA and the (hyper)polarizabilities. The parameters characterizing a molecule in the model can be readily extracted from experimental measurements. The model is applied to the calculation of (hyper)polarizabilities for several molecules as a function of solvent polarity, and comparison is made with both experimental and previous theoretical results. Some directions for an improved description are discussed.