The influence of counterion on photochemistry of cationic indopolycarbocyanine dyes in ion pairs

It has been shown that the rate constant of cis—trans isomerization of cyanine dyes studied decreases significantly with changing the solvent from a polar to a nonpolar one at the expense of ion pair formation (dye cation with anion in nonpolar solvent). This phenomenon is accounted for by an increase in the bond order about which the rotation occurs during isomerization. It is observed that ion pair formation has practically no influence on the triplet state lifetime; a sharp increase in the decay rate of dyes III and VIII triplet state results from the heavy atom effect (I − anion).