Studies of Third-Order Optical Nonlinearities of Model Compounds Containing Benzothiazole, Benzimidazole, and Benzoxazole Units

To improve our understanding of the molecular structure-nonlinear optical properties relationship, a series of model compounds containing benzothiazole, benzimidazole, and benzoxazole units have been synthesized, and their third-order nonlinear optical properties investigated by using femtosecond degenerate four-wave mixing. For soluble compounds measurements were made with solutions of various concentrations, while for insoluble materials vacuum-evaporated films or melt-quenched films were used. The time-response behavior as well as the concentration dependence of the nonlinearity indicates that there is no one-photon or two-photon resonance encountered at 602 nm, the wavelength of nonlinear optical studies. Therefore, the trend of nonlinearity in the systematically varied model compounds can be related to the structural variations. The present study yields the following information on structure–property relationships: (i) microscopic third-order nonlinearity rapidly increases with an increase of the effective conjugation length for a quasi-one-dimensional molecule; (ii) incorporation of a sulfur-containing heteroaromatic linkage in the conjugated structure enhances the nonlinearity; (iii) transition from a quasi-one-dimensional π-electron delocalization to delocalization in two dimensions introduced by using an imidazole N-linkage enhances the third-order optical nonlinearity without reducing the optical transparency window and at the same time improves the solubility. Possible explanations for these structural effects are presented. Also, the observed qualitative trend is compared with that predicted by a recent semiempirical calculation.