Third-Order Optical Nonlinearities of Model Compounds Containing Benzobisthiazole, Benzobisoxazole, and Benzbisimidazole Units

As part of a continuing study, a series of highly conjugated aromatic benzobisthiazole, benzobisoxazole, and N, N-diphenylbenzbisimidazole model compounds were synthesized, and their third-order nonlinear optical properties investigated by using subpicosecond degenerate four-wave mixing. Measurements were made at 602 nm on THF solutions, vacuum-deposited films, or melt-quenched films. In all but one material, the third-order effect was determined to be instantaneous within the employed temporal resolution, suggesting dominance of electronic nonlinearity. From the experimental data it was possible to formulate structure-nonlinear optical property correlations and demonstrate that the molecular second hyperpolarizability can be increased almost 3 orders of magnitude by simple structural modifications.