Valence-bond charge-transfer solvation model for nonlinear optical properties of organic molecules in polar solvents

A simple model is developed for predicting solvation effects on the nonlinear optical properties of charge transfer organic materials such as 1,1 dicyano,6-(di-butyl amine) hexatriene. This model is based on the valence-bond charge-transfer (VB-CT) framework, using a continuum description of the solvent. The resulting VB-CT solvation model leads to analytic formulas for the absorption frequency (E_g), the polarizability (α), the hyperpolarizabilities (β,γ,δ), and the bond length alternation with only one solvent dependent parameter (ε, the dielectric constant of the solution). The theory involves just four solvent-independent parameters, V₀, t, S_F, and Q which are related to the band gap, bandwidth, geometry, and dipole moment of the CT molecule [plus a length (R _DA) and force constant (k) derivable from standard force fields]. The results are in good agreement with experiment.