FOURIER-TRANSFORM RAMAN-SPECTRA AND THE STRUCTURE OF ADSORBED 2,2'-CYANINE

Near-infrared excited FT Raman spectra of 2,2'-cyanine adsorbed onto a smooth silver electrode and of polycrystalline 2,2'-cyanine are compared with dispersive Raman spectra obtained by using visible excitation to determine adsorbate structural characteristics. Raman spectra are interpreted as indicating that 2,2'-cyanine exists as a structural composite of polycrystalline and two intermolecular arrangements of cyanine monomers forming the J-aggregate. This structural model is shown to explain Raman intensity variations that depend upon dye concentration, supporting electrolyte, pH, and electrode potential. Variations of the FT Raman relative band intensities over a surface potential range in which the surface concentration increases indicate that one of the structural conformers in the J-aggregate is thermodynamically favored at more negative potentials.