Primary photoprocesses in molecules of carbocyanine dyes in binary solvent mixtures

The effect of the composition of the dimethyl sulfoxide (DMSO)-toluene mixture on the photophysical processes of thia-, indo-, and imidacarbocyanine dyes was studied. In the mixtures with the DMSO content of more than 20 vol %, the dyes representing solvated cations are characterized by high efficiency of trans → cis photoisomerization and fluorescence, in contrast to the extremely low efficiency of intersystem crossing to the triplet state. With growing the toluene content in the mixture, ion pairs between the dye cation and Cl−, Br−, I−, or BF4− anion are formed. In the cases when the dye counterions are Br− or I−, a sharp increase in the yield of the triplet molecules and a decrease in their lifetime take place. The results are discussed in terms of “the external heavy atom effect” in ion pairs.