Probing structure-property relationships in third-order nonlinear optical polymers: Third harmonic generation spectroscopy and theoretical modeling of systematically derivatized conjugated aromatic polyimines

Picosecond third harmonic generation (THG) spectroscopy in the range 0.9-2. 4 mu m (0.52-1.38 eV) and sum-over-states (SOS) theoretical modeling have b een used to investigate the third-order nonlinear optical properties of a s eries of nine conjugated aromatic polyimines designed to elucidate structur e-chi((3)) relationships in conjugated polymers. The THG-measured frequency -dispersed third-order susceptibility (chi((3))) of the series of polymers has off-resonant and three-photon resonance-enhanced values in the ranges o f 1.1 x 10(-12) to 2.0 x 10(-11) and 6.4 x 10(-12) to 7.2 x 10(-11) esu, re spectively, demonstrating a large modulation of nonlinear optical response by simple structural variations. A random copolymer was found to have enhan ced chi((3)) compared to either of its parent homopolymers. Asymmetric dono r side-group substitutions were found to result in 7-10-fold enhancement of off-resonant chi((3)) compared to either symmetric substitutions or no sub stitutions. Theoretical three- and four-level SOS models, justified by site -selective fluorescence spectroscopy, were found to describe the chi((3)) d ispersion data and the observed multiphoton resonances very well. The zero- frequency chi((3)) was predicted from the SOS models to be negative for eig ht of the nine polymers and to be in the range of -0.43 to -8.12 x 10(-12) esu, the magnitudes of which are very close to the observed off-resonance v alues at 2.4 mu m (0.52 eV). A negative real part of chi((3))(-omega;omega, -omega,omega) over a wide wavelength range of lambda > 1.38 mu m was also p redicted by the SOS models for the same eight polymers with strong one-phot on and two-photon resonances, suggesting that they would exhibit self-defoc using phenomena. Similarly predicted chi((3))(-omega;omega,-omega,omega) sp ectra of the poly(azomethines) suggest that third-order susceptibility meas ured by degenerate four-wave mixing would be up to 2 orders of magnitude la rger than the values measured by THG spectroscopy.