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

Abstract

The chain length dependencies of the static third order hyperpolarizabilities γ STAT 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 γ STAT π of both homologous series show a similar increase with the growing number of π-electrons ( N) of the chain ( γ STAT π ∼ − N 8 ± 2). However, the absolute values of the benzthiacyanines are considerable higher than of the corresponding bis(dimethylamino)methines. Negative valued hyperpolarizabilities γ STAT π in the homologous series increasing up to γ STAT π = − 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).