Third-Order Nonlinear Optical Properties Of A Series Of Systematically Designed Conjugated Rigid-Rod Polyquinolines
The third-order nonlinear optical properties of thin films of two series of conjugated rigid-rod polyquinolines, exemplified by poly(2, 2'-(l, 4-phenylene)-6, 6'-bis(4-phenyl quinoline)) (PPPQ, 2d) and poly(2, 7-(l, 4-phenylene)-4, 9-diphenyl-l, 6-anthrazoline) (PPDA, 3d), were investigated by third harmonic generation spectroscopy. Of the nine polyquinolines with diverse backbone structures, PPPQ has the largest optical nonlinearity with a χ(3) (-3ω; ω, ω, ω) value of 3.2 × 10−12 esu and 3.3 × 10−11 esu in the off-resonant and three-photon resonant regions, respectively. A comparison of the nonresonant χ(3) of the series of nine systematically derived polyquinolines showed that a scaling law of the form χ(3) ∼ (λmax)v ∼ Eg-v does not hold; in fact, the nonresonant χ(3) was essentially independent of the optical bandgap. These results suggest that structure-χ(3) propeny relationships in polymers cannot be inferred from those of oligomers and model compounds.