Bis(1,3-dithiole) polymethine dyes for third-order nonlinear optics - Synthesis, electronic structure, nonlinear optical properties, and structure-property relations

The synthesis of a series of new sulfur-containing polymethine dyes is repo rted. The linear optical properties of these bis(1,3-dithiole) (mono-, tri- , penta-, and hepta-)-methine dyes show intense and narrow optical absorpti ons typical of cyanine dyes. The absorption maximum is increasingly red-shi fted from 489 nm for the monomethine dye to 911 nm for the heptamethine com pound. Based on third-harmonic generation measurements at fundamental wavel ength between 1064 nm and 1907 nm, gamma values were evaluated to Lie in th e range between 1.10(-33) e.s.u. and 14.10(-33) e.s.u, showing that the non linear response of the bis(1,3-dithiole) polymethine dyes compare well with other organic pi-electron systems of similar size. Comparisons of the expe rimental values of gamma to calculated static values obtained by ab initio and semiempirical calculations (AM1, PM3) have been made as well as compari sons to dynamic values estimated from a free-electron model. The computed d ynamic values can be described by a power law; gamma approximate to L-5 whe re L denotes the length of the molecules. Our studies confirm the stabilizi ng effect of a carbocyclic ring in the cyanine backbone making the heptamet hine dye an unusually stable and highly nonlinear polymethine chromophore p ossessing a strong NIR optical transition and very good transparency in the visible region.