Origin of the nonlinear second-order optical susceptibilities of organic systems

An all-valence-electron self-consistent-field linear-combination-of-atomic-orbitals molecular-orbital procedure including configuration interactions for calculating the magnitude and sign of the nonlinear second-order molecular susceptibility components (hyperpolarizability) for substituted dipolar aromatic molecular systems is reported. Three fundamentally important examples, aniline, nitrobenzene, and $p$-nitroaniline, are considered. Analysis of the microscopic origin of their molecular second-order susceptibilities provides a direct means for understanding the macroscopic nonlinear optical response of organic molecular solids which have already been observed to possess exceptional nonlinear optical properties. The important excited states of aniline, nitrobenzene, and $p$-nitroaniline have been identified and examined in their relationship with the molecular second-order susceptibility-tensor components ${\beta }_{\mathrm{ijk}}$. The detailed nature of the charge separation accompanying these states has been discussed in terms of both the configurations composing the excited states, and also the one-electron molecular orbitals which determine those configurations. These results demonstrate how the bond-additivity approximation is inappropriate for $p$-nitroaniline. Finally, the frequency dependence of the ${\beta }_{\mathrm{ijk}}$ components in each case shows that the Kleinman relations are valid approximations only at relatively low frequencies.