3RD-ORDER NONLINEAR OPTICAL-PROPERTIES OF CONJUGATED RIGID-ROD POLYQUINOLINES

We have used picosecond third harmonic generation spectroscopy to investigate the third-order optical nonlinearities of a series of 12 systematically designed homopolymers and random copolymers in the class of conjugated rigid-rod polyquinolines and polyanthrazolines. The chi(3)(-3-omega;omega,omega,omega) spectra of these materials in the 0.9-2.4-mu-m wavelength (1.4-0.5 eV) range showed resonance peaks which were identified as the three-photon resonances. The magnitude of the three-photon resonance enhanced chi(3) varied in the range 0.8 x 10(-11)-3.3 x 10(-11) esu with backbone structure of the polymers. Off-resonance at 2.38-mu-m, the chi(3) values for all the polymers were found to be very close to each other, approximately (2.5 +/- 0.5) x 10(-12) esu within the experimental errors. The magnitude of the resonant or nonresonant chi(3) could not be correlated with the optical bandgap of the series of polymers. A theoretical fit of the chi(3) dispersion data using a three-level model based on essential states mechanism suggested that more than one excited state is responsible for the observed third-order optical nonlinearity of the materials. The nonlinear optical properties of the random copolymers were, within experimental errors, the molar averages of those of the respective constituent homopolymers, suggesting the absence of significant changes in the third-order optical nonlinearities with introduction of disorder in the copolymer backbone.