OPTICAL NONLINEARITIES IN AZOARENES

Calculations are reported on the structure of donor-acceptor phenylazobenzenes, phenylazonaphthalenes, phenylazoanthracenes, and phenylazoheterocyclics using the AM1 molecular orbital method. The transition energies of these systems have been calculated using a modified CNDO method and a CI treatment of both singly and doubly excited configurations generated from an active space of 10 molecular orbitals. The results obtained show good agreement in many cases with the experimentally reported absorption maximum (lambda(max)) values recorded in cyclohexane. The calculated dipole moment, pi-electron charge densities, and bond orders in the ground and lowest dipole allowed excited state have been related to the relative efficacy of the donors and accepters present. The frequency dependent polarizability, the second harmonic generation (SHG), and the third harmonic generation (THG) coefficients of the azoarenes have been computed using the correction vector method. An analysis of the coefficients shows that the replacement of a phenyl ring of the phenylazobenzenes by naphthalene or anthracene results in a modest increase in the polarizability and hyperpolarizability values, but the values obtained are highly dependent on the position of the donor group and the tautomeric form. Enhanced values are obtained when a heterocyclic ring is incorporated into the azoarene with the largest value obtained for a donor-acceptor naphthylazothiophene. Substantial resonance enhancement effects occur for those derivatives which absorb in the red or infrared region of the spectrum.