Effects of Surfactants on the Aggregation of 6,6'-Disubstituted Thiacarbocyanine Dyes in Aqueous Solutions

The aggregation properties of a number of 6,6'-disubstituted thiacarbocyanine dyes are studied by spectral-fluorescent methods: T-304, T-306, T-307, T-336 and, for comparison, thiacarbocyanine Cyan 2, which has no substituents in the 6,6'-positions, in aqueous buffer solutions, or in the presence of various types of surfactants. The method of moments is used to characterize the absorption spectra (band positions, width, shape). Substituents in the 6,6'-positions significantly increase the ability of dyes T-304, T-306, T-307, and T-336 to aggregate (dimerization and the formation of disordered aggregates with broad low-intensity absorption spectra). The introduction of surfactants leads to the rearrangement of the spectra associated with the complex nature of the equilibria between monomers and aggregates of various structures (including surfactant molecules, if present), and in particular, with a decrease in the contribution of disordered aggregates. However, the decomposition of dimeric aggregates of 6,6'-disubstituted cyanines is observed only at very high surfactant concentrations (~20 CMC or higher, where CMC is the critical micelle concentration). At the same time, the passing of surfactant concentrations through CMC does not significantly affect the spectral-fluorescent properties of the dyes, which is probably due to the rather strong interactions of the dyes with individual surfactant molecules and premicellar associates of the surfactants.