Theoretical study on the nonlinear optical properties of para-nitroaniline in different solvents

The solvent effects on the nonlinear optical properties of para-nitroaniline (pNA) molecule are studied on the base of time dependent density functional theory. The polarized continuum model is used to simulate the influence of the solvent environment on the solute molecule. In the first place, the geometrical structures of pNA molecule in each solvent are optimized by use of density functional theory and the influence of solvent on the geometry of pNA molecule is thus illustrated. Then, the energies and dipole moments of the excited states with pNA molecule in different solvents are computed on the base of time dependent density functional theory. The dispersion relations of the first-order nonlinear hyperpolarizabilities in second harmonic generation process for pNA molecule in different solvents are given by using two-state model for the first time. It is shown that polar solvents have much influence on the nonlinear optical properties. At low frequency radiation field, the theoretical results of the dispersion relation agree well with the experimental results. While at higher frequency radiation field, other methods need to be developed to compute the dispersion relation of the first order nonlinear hyperpolarizability. At last, possible explanations are given for the results and the validity of the two-state model is discussed.