NONLINEAR OPTICAL-PROPERTIES OF POLYANILINES AND DERIVATIVES

The third-order optical susceptibility chi(3)(-3-omega;omega,omega,omega) of polyanilines and derivatives has been systematically investigated by picosecond third harmonic generation spectroscopy on spin-coated thin films of the polymers in the wavelength range 0.9-2.4-mu-m (1.4-0.5 eV). It is shown that the magnitude of chi(3)(-3-omega;omega,omega,omega) of this class of polymers is as large as that of other conjugated polymers and that the optical nonlinearity depends on the oxidation level and the derivatization of the p-phenylene rings. The dispersion of the optical nonlinearity is dominated by the three-photon resonance to the dipole allowed transition occuring at approximately 1.8 eV, so that the excitonic transition is the major contributor to the optical nonlinearity of polyanilines. Polyemeraldine base, with an oxidation level of about 50%, has a larger optical nonlinearity than the fully oxidized form pernigraniline or poly(phenylaniline) which we use aa a model compound for the fully reduced form. The effects of derivatization are more complex. Methoxy substitution of the phenyl ring increases the transition moment, which would increase the microscopic nonlinearity. However, these substituents also reduce the number density of the polymer repeat units, which in turn reduces the macroscopic nonlinearity.