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 investigate the hyperpolarizabilities β of a series of heteroaromatic-substituted sym -triazines in the frame of a tensorial formalism. Effects of donor or acceptor substitution and electron correlation on the octupolar triazines have also been explored. Geometric effects have been considered in a set of calculations by using fully optimized and planar structures. It is found that the effects of the deviations from the planarity on β are very small. Octupolar triazines containing pyrrole derivatives have larger β than the corresponding analogues containing furan and thiophene derivatives, and NO 2 or NH 2 substitution strongly affects β of octupolar benzene-substituted triazine. An additive model is introduced to evidence specific contributions to β arising from the multipolar symmetry. The β tensor of a multipolar system is dissected into two separate tensor representation, an additive term β A representing the tensorial sum of the dipole units' β and a term β I representing interactions between the dipole units constituting the multipole. The results indicate that when the acceptor groups (NO 2 ) are introduced into octupolar triazines containing pyrrole and furan derivatives, the effects of the interaction term exceed that of the additive term and greatly enhance the magnitudes of β, contrasted with unsubstitution. It is also seen that incorporating the donor groups (NH 2 ) into the octupolar molecules not only significantly enhances β A but also β I, compared with unsubstitution and NO 2 substitution. The electron correlation has a large influence on the magnitude of the hyperpolarizabilities.