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Abstract

Well-organized monolayers of quinquethienyl (QT) have been studied by second-harmonic generation (SHG). Due to the properties of the molecular hyperpolarizability tensor α(2) of QT, SHG signals originating from the direction of the surface normal, which corresponds also to the alignment direction of these molecules of rodlike structure, are symmetry-forbidden. The observed intense SHG signals under p-polarized excitation can either be explained in terms of anisotropic averaging, which accounts for orientational irregularities on a microscopic scale due to steric hindrance or, alternatively, by the influence of an external, symmetry-breaking, static electric field acting on the individual molecules and arising from their noncentrosymmetric environment.