An elementary description of nonlinear optical properties of octupolar molecules: Four-state model for guanidinium-type molecules

A theoretical description of the molecular polarizability (alpha) and first and second hyperpolarizabilities (beta and gamma) of a guanidinium-type octupolar molecule is presented. By using a valence bond and three charge-transfer states with nonzero transfer integrals, the electronic states are obtained as linear combinations of these basis states. It is found that a doubly degenerate excited state is only optically coupled to the ground state. Based on the analytic expressions for alpha, beta, and gamma obtained, it is shown that the resultant tensor elements of beta satisfy the symmetry requirements of point group D-3h. Unlike the linear push-pull polyenes, the magnitudes of alpha, beta, and gamma of the guanidinium-type octupolar molecules increase as the charge-transfer character of the electronic pound state increases. Also, their dependences on the distance (d) between the donor and acceptor are briefly discussed. (C) 1998 American Institute of Physics.