2ND-ORDER HYPERPOLARIZABILITIES OF STILBAZOLIUM CATIONS STUDIED BY SEMIEMPIRICAL CALCULATION

Second-order hyperpolarizabilities (beta), electron distribution, dipo le moment, and excitation energy together with oscillator strength of stilbazolium cations have been investigated by a semiempirical calcula tion with PM3 parametrization of the MNDO Hamiltonian. Since stilbazol iums were found to generally have several times larger beta value than nitrostilbene derivatives with similar experimental absorption maximu m wavelength, such ionic species are concluded to be advantageous to c reate new second-order nonlinear optical materials having large beta a nd short cutoff. Among the stilbazolium derivatives investigated, the beta values increase with increasing electron-donating ability of the substituent at the para position of benzene ring, and exceptionally ch lorinated one was found to have comparatively large beta value withsta nding its short absorption maximum wavelength. The main reason for the large beta values of stilbazolium cations is the strong acceptor prop erty of the pyridinium ring resulting in large difference in dipole mo ment between the ground and excited states, which could be shown using the two-level model. The tendency of calculated beta values of stilba zolium cations shows good agreement with that obtained from the hyper Rayleigh scattering (HRS) measurement.