Bond-length alternation in symmetrical cyanine dyes

As a probe of transient perturbation of symmetrical cyanine chromophores fr om their C-2v molecular symmetry by displacements along the bond-length alt ernation coordinate, we have selected hyper-Rayleigh scattering (HRS) spect roscopy. Three of the four members investigated of a homologous series of d ithiacarbocyanine dyes exhibit HRS signals in methanol, indicative of hyper polarizabilities comparable to those of the reference compounds, m- and p-n itroaniline, The signals disappear in toluene; we infer that the observed h yperpolarizabilities are a property of the solute-solvation shell system. T he results suggest significant, albeit transient, bond-length alternation i n the ground state. Semi-empirical molecular orbital calculations (at the AM1 level) are used t o 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 bo nd. 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. (C) 2001 Elsevier Science B.V. All rights reserved.