Third-order nonlinear polarizabilities of a homologous series of symmetric 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 δ electrons dominating the negative π electron contributions in the short chains. In the longer chains the hyperpolarizabilities exhibit negative signs and a strong enhancement (≈ −N11) with increasing chain lengths due to the growing number (N) of contributing π electrons. A static hyperpolarizability of ≈ −1 × 10−33 esu which is obtained for the nonamethine, is the largest off-resonant experimental value reported for a polymethine dye.