Ab initio time-dependent coupled perturbed Hartree-Fock studies of optical nonlinearities of organic molecules: Alkyl derivatives of 4-amino-β-nitrostyrene

The dipole moment, μ, linear polarizability, α, first hyperpolarizability, β, and second hyperpolarizability, γ, of intramolecular charge transfer (ICT) molecules derived from the alkyl substitution of 4-amino-β-nitrostyrene (ANST) have been calculated by ab initio self-consistent-field method using a double-zeta valence Gaussian basis set. The calculated results for α,β, and γ show strong dependence on μ and the energy difference Δε between the lowest unoccupied and highest occupied molecular orbitals. For the (dimethylamino)-β-nitrostyrene and (diethylamino)-β-nitrostyrene, the calculated results for β(-2ω;ω,ω) are in qualitative agreement with the experimental values. The changes in the values of γ for various third-order effects among the various alkyl-substituted ANST are parallel to the corresponding changes in the values of β. The results indicate that the mechanism which leads to the enhancement of β is also responsible for the enhancement of γ values of ICT molecules and that these molecules may provide an alternative to conjugated polymers as a third-order nonlinear optical material.