Photoprocesses in aqueous solutions of 9-ethylthiacarbocyanine dyes in the presence of surfactants

Abstract

The effect of cationic (CTAB), anionic (SDS) and neutral (Triton X-100) surfactants on the spectral and luminescence properties and photoreactions of three sulfopropyl-9-ethylthiacarbocyanine dyes (Dyes 1–3) was studied in aqueous solutions. In the absence of the surfactants, Dyes 1–3 occur in the form of an equilibrium mixture of cis-monomers and dimers. Neither monomers nor dimers are capable of fluorescence and cis-trans photoisomerization, and only the dimers experience intersystem crossing into the triplet state. In the presence of any of the surfactants at a concentration below the critical micelle concentration, the dimers undergo disaggregation; it is only in the presence of CTAB that the formation of a J-aggregate—which dissociates into monomers with an increase in the CTAB concentration—takes place. As the surfactant concentration increases, dye fluorescence appears, which is accompanied by a decrease in the yield of the dimer triplet state, and the amenability of the thiacarbocyanine monomers to cis-trans photoisomerization and transition to the triplet state is observed. The spectral effects observed are related to the conversion of the cis-form of the monomer to the trans-form upon solubilization of the dye molecules.