Abstract
As a probe of transient perturbation of symmetrical cyanine chromophores from their C 2v molecular symmetry by displacements along the bond-length alternation coordinate, we have selected hyper-Rayleigh scattering (HRS) spectroscopy. Three of the four members investigated of a homologous series of dithiacarbocyanine dyes exhibit HRS signals in methanol, indicative of hyperpolarizabilities comparable to those of the reference compounds, m- and p-nitroaniline. The signals disappear in toluene; we infer that the observed hyperpolarizabilities are a property of the solute–solvation shell system. The results suggest significant, albeit transient, bond-length alternation in the ground state. Semi-empirical molecular orbital calculations (at the AM1 level) are used to model ground state cis– trans isomerization in a representative dye. From the calculated energetics and geometries we infer that isomerization locks in the bond-length alternation, enabling isomerization about a low-order bond. According to the generally accepted relaxation scheme for S 1 cyanines, bond-length alternation must likewise be pre-requisite to S 1–S 0 internal conversion.
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