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

A theoretical description of the molecular polarizability (α) and first and second hyperpolarizabilities (β and γ) 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 α, β, and γ obtained, it is shown that the resultant tensor elements of β satisfy the symmetry requirements of point group D_3h. Unlike the linear push-pull polyenes, the magnitudes of α, β, and γ of the guanidinium-type octupolar molecules increase as the charge-transfer character of the electronic ground state increases. Also, their dependences on the distance (d) between the donor and acceptor are briefly discussed.