Molecular design for octupolar nonlinear optical systems: An ab initio, study of first hyperpolarizabilities of symmetrically heteroaromatic-substituted triazines

An ab initio coupled perturbed Hartree-Fock study has been performed to inv estigate the hyperpolarizabilities beta of a series of heteroaromatic-subst ituted sym-triazines in the frame of a tensorial formalism. Effects of dono r or acceptor substitution and electron correlation on the octupolar triazi nes have also been explored. Geometric effects have been considered in a se t of calculations by using fully optimized and planar structures. It is fou nd that the effects of the deviations from the planarity on beta are very s mall. Octupolar triazines containing pyrrole. derivatives have larger beta than the corresponding analogues containing furan and thiophene. derivative s, and NO2 or NH2 substitution strongly affects beta of octupolar benzene-s ubstituted triazine. An additive model is introduced to evidence specific c ontributions to beta arising from the multipolar symmetry. The beta tensor of a multipolar system is dissected into two separate tensor representation , an additive term beta (A) representing the tensorial sum of the dipole un its' beta and a term beta (I) representing interactions between the dipole units constituting the multipole. The results indicate that when the accept or groups (NO2) are introduced into octupolar triazines containing pyrrole and furan derivatives, the effects of the interaction term exceed that of t he additive term and greatly enhance the magnitudes of beta, contrasted wit h unsubstitution. It is also seen that incorporating the donor groups (NH2) into the octupolar molecules not only significantly enhances beta (A) but also beta (I), compared with unsubstitution and NO2 substitution. The elect ron correlation has a large influence on the magnitude of the hyperpolariza bilities.