Second-harmonic generation in quinquethienyl monolayers

Well-organized monolayers of quinquethienyl (QT) have been studied by second-harmonic generation (SHG). Due to the properties of the molecular hyperpolarizability tensor ${\alpha }^{\mathrm{}\left(2\right)\mathrm{}}$ 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.