Solvation-Shell Effect on the Cyanine-Dye Fluorescence in Binary Liquid Mixtures

Abstract

The steady-state fluorescence anisotropy and the relative quantum yield of fluorescence of cyanine dyes, 3,3′-diethyl-thiacarbocyanine iodide (DTCI) and chloride (DTCC), were measured in toluene/dimethyl sulfoxide (DMSO) liquid mixtures. It has been found that in toluene-rich mixtures the fluorescence quantum yield of DTCC is larger by about one order of magnitude than that of DTCI, the fluorescence quantum yields being the same for both cyanines in neat DMSO. Estimations based on the Perrin formula show that on decreasing DMSO volume fraction (less than 20 vol.%) the size of the preferential solvation shell around the DTCI increases up to 23 DMSO molecules at 3 vol.% DMSO as the solvation shell of DTCC does not vary. The influence of nearby solvent environment on the cyanine dye photophysics can be explained in terms of the Onsager model by taking into account the effect of the dynamic properties of DMSO solvation shell on the dye fluorescent rate constant.