Increase and saturation of the third order hyperpolarizabilities in homologous series of symmetric cyanines Original Research Article

The chain length dependencies of the static third order hyperpolarizabilities γSTAT for the homologous series of benzthiacyanine dyes and of simple bis(dimethylamino)methine dyes were extrapolated from nondegenerate four wave mixing dispersion measurements and compared with theoretical values. Up to the heptamethine the π-electron contributions γSTATπ of both homologous series show a similar increase with the growing number of π-electrons (N) of the chain (γSTATπ ∼ −N8 ± 2). However, the absolute values of the benzthiacyanines are considerable higher than of the corresponding bis(dimethylamino)methines. Negative valued hyperpolarizabilities γSTATπ in the homologous series increasing up to γSTATπ = − 850 × 10−36 esu were determined. For the first time a saturation of the nonlinearity could be observed experimentally in the series of benzthiacyanines for the longest chain (benzthicyanine nonamethine).