Effects of surfactants on the aggregation behaviour of thiacarbocyanine dyes

The properties of monomer, dimer and higher aggregates of thiacarbocyanine (1) and three derivatives with methyl (2) and ethyl (3, 4) meso-substituents were studied in microheterogeneous media by spectroscopic methods. Sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB) and Triton X-100 in aqueous solution were used for anionic, cationic and non-ionic micelles, respectively. At surfactant concentrations higher than the critical micelle concentration (cmc) the amount of solubilized monomer of 1-4 is generally enhanced and that of dimer reduced. At high surfactant concentrations (larger than the cmc) enhanced fluorescence originating from the all-trans monomer bound to the micelle was observed. The SDS concentration, at which the monomer and dimer absorption and the fluorescence intensity of 1 and 3 reached 1/2 of their maximum values (conc(1/2)), coincides with the cmc. For oppositely charged reactants, i.e. 1, 3 or 4 and CTAB or 2 and SDS, the conc(1/2) values for dimer deaggregation are much lower than the cmc. In these cases dimers are converted into H- and/or J-aggregates at surfactant concentrations much below the cmc and the aggregates are split into monomers around the cmc or on further increasing the surfactant concentration. The triplet yield of 3 decreases and the yield of trans --> cis photoisomerization increases with increasing SDS concentration, the conc(1/2) values likewise coincide with the cmc, The higher conc(1/2) values as compared to the Triton X-100 cmc are due to the larger dye concentration relative to that of the micelles. The dependences of the relative amounts of monomer and dimer versus the surfactant concentration for similarly charged reactants are confirmed by model simulations. (C) 1999 Elsevier Science B.V. All rights reserved.