3RD-ORDER HYPERPOLARIZABILITIES OF A HOMOLOGOUS SERIES OF MESO-NITROGEN SUBSTITUTED THIACYANINES

Static third-order hyperpolarizabilities gamma(stat) of the homologous series of thiacyanines and of their meso-nitrogen substituted derivat ives were extrapolated from nondegenerate four wave mixing dispersion measurements. Negative hyperpolarizabilities gamma(stat) of the homolo gous series of thiacyanines growing up to gamma(stat) = -700 X 10(-36) esu were determined. Meso-nitrogen substitution changes the sign but does not alter the modulus of the hyperpolarizability gamma(stat) of t he trimethine significantly. Compared to the unsubstituted analogues i t results in slightly, but strongly diminished, negative hyperpolariza bilities of penta- and hepta-methine respectively. These experimental findings are reproduced by calculations applying an all-valence electr on procedure but are contrary to essential predictions of the simple f ree-electron model.