Third-Order Nonlinear Optical Properties of a Series of Systematically Designed Conjugated Rigid-Rod Polyquinolines

Abstract

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 gC⁽³⁾ (−3ω;ω,ω,ω) value of 3.2 x 10^-12 esu and 3.3 x 10^-11 esu in the off-resonant and three-photon resonant regions, respectively. A comparison of the nonresonant gC⁽³⁾ of the series of nine systematically derived polyquinolines showed that a scaling law of the form gC⁽³⁾ ∼ (λ_max)^v∼ E_g^-v does not hold; in fact, the nonresonant gC⁽³⁾ was essentially independent of the optical bandgap. These results suggest that structure-gC⁽³⁾ propeny relationships in polymers cannot be inferred from those of oligomers and model compounds.