Nonlinear optical properties of nitro-aniline and methyl-aniline compounds - an exact correction vector INDO-SDCI study

We have applied the correction vector (CV) method coupled with the intermediate neglect of differential overlap-single and double configuration interaction semiempirical approach to calculate the nonlinear optical coefficients of donor-acceptor conjugated compounds consisting in ortho-, meta-, and para-substituted benzene derivatives as well as the corresponding octupolar molecules. The CV method has been shown to be both exact and computationally effective. We find that for weak donors such as a methyl group, the NLO response efficiency orders as: octu > para > meta > ortho; however, if the donor becomes stronger and consists in an amino group, then: para > ortho > meta > octu. The results are in agreement with experimental findings and can be rationalized in terms of a competition between the effects related to structural resonance and charge separation length.