Third-order hyperpolarizabilities of a homologous series of meso-nitrogen substituted thiacyanines

Static third-order hyperpolarizabilities γstat of the homologous series of thiacyanines and of their meso-nitrogen substituted derivatives were extrapolated from nondegenerate four wave mixing dispersion measurements. Negative hyperpolarizabilities γstat of the homologous series of thiacyanines growing up to γstat = −700 × 10−36 esu were determined. Meso-nitrogen substitution changes the sign but does not alter the modulus of the hyperpolarizability γstat of the trimethine significantly. Compared to the unsubstituted analogues it results in slightly, but strongly diminished, negative hyperpolarizabilities of penta- and hepta-methine respectively. These experimental findings are reproduced by calculations applying an all-valence electron procedure but are contrary to essential predictions of the simple free-electron model.