A PROPOSAL OF NEW ORGANIC 3RD-ORDER NONLINEAR OPTICAL-COMPOUNDS - CENTROSYMMETRIC SYSTEMS WITH LARGE NEGATIVE 3RD-ORDER HYPERPOLARIZABILITIES

An approximate formula for virtual excitation processes (three-type approximate formula) describing the third-order hyperpolarizability (gamma) is rewritten by the use of polarizabilities of the ground and excited states (alpha0, alpha(n)), and a criterion for a classification of gamma for organic compounds is drawn from the formula. We suggest that there exist new organic nonlinear optical systems with negative gamma values. The new systems have centrosymmetric structures which exhibit large ground state polarizabilities. As new model compounds, some alternant and condensed-ring conjugated centrosymmetric systems are investigated using the coupled-Hartree-Fock (CHF) and Moller-Plesset second-order perturbation (MP2) methods combined with the semi-empirical INDO approximation. Comparisons of gamma values calculated by the INDO and ab initio methods are carried out. The MP2 results suggest that the ion radical states of the condensed-ring systems such as C60n- possess large negative gamma values.