3RD-ORDER NONLINEAR POLARIZABILITIES OF A HOMOLOGOUS SERIES OF SYMMETRICAL CYANINES

Third-order nonlinear polarizabilities of the homologous series of bis(dimethylamino)methine dyes (mono-, tri-, penta-, hepta- and non-amethine) were derived in the static frequency limit from dispersion curves measured by four-wave mixing techniques. Positive signs of the hyperpolarizabilities are obtained for mono- and trimethine: They are caused by positive contributions from a electrons dominating the negative rr electron contributions in the short chains. In the longer chains the hyperpolarizabilities exhibit negative signs and a strong enhancement(approximate to -N-11) with increasing chain lengths due to the growing number (N) of contributing pi electrons. A static hyperpolarizability of approximate to -1 x 10(-33) esu which is obtained for the nonamethine, is the largest off-resonant experimental value reported for a polymethine dye.