ALTERNATION AND SATURATION EFFECTS IN THE CHAIN-LENGTH DEPENDENCES OFTHE 3RD-ORDER HYPERPOLARIZABILITIES OF CYANINES

The chain-length dependences of static third-order hyperpolarizabiliti es gamma(STAT) Of the homologous series of thiacyanines, of their meso -nitrogen-substituted derivatives, and of simple bis(dimethylamino)met hine dyes are determined from nondegenerate four-wave mixing dispersio n curves. We determined nearly the same very rapid increase of gamma(S TAT) with growing numbers of pi electrons of the chain (gamma(STAT) pr oportional to - N8+/-2) for both the thiacyanine and the bis(dimethyla mino)methine series, an alternating influence of meso-nitrogen substit ution for even and for odd double bonds of the chain, and a saturation of the hyperpolarizability for the longest thiacyanine (thiatricarboc yanine). We discuss our results, taking into account semiempirical qua ntum chemical calculations. (C) 1998 Optical Society of America.