Second-order hyperpolarizabilities of stilbazolium cations studied by semiempirical calculation

Second-order hyperpolarizabilities (beta), electron distribution, dipole moment, and excitation energy together with oscillator strength of stilbazolium cations have been investigated by a semiempirical calculation with PM3 parametrization of the MNDO Hamiltonian. Since stilbazoliums were found to generally have several times larger beta value than nitrostilbene derivatives with similar experimental absorption maximum wavelength, such ionic species are concluded to be advantageous to create new second-order nonlinear optical materials having large beta and 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 chlorinated one was found to have comparatively large beta value withstanding its short absorption maximum wavelength. The main reason for the large beta values of stilbazolium cations is the strong acceptor property of the pyridinium ring resulting in large difference in dipole moment between the ground and excited states, which could be shown using the two-level model. The tendency of calculated beta values of stilbazolium cations shows good agreement with that obtained from the hyper Rayleigh scattering (HRS) measurement.