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-substituent s 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 micel les, respectively. At surfactant concentrations higher than the critical mi celle concentration (cmc) the amount of solubilized monomer of 1-4 is gener ally 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. Fo r oppositely charged reactants, i.e. 1, 3 or 4 and CTAB or 2 and SDS, the c onc(1/2) values for dimer deaggregation are much lower than the cmc. In the se cases dimers are converted into H- and/or J-aggregates at surfactant con centrations much below the cmc and the aggregates are split into monomers a round the cmc or on further increasing the surfactant concentration. The tr iplet yield of 3 decreases and the yield of trans --> cis photoisomerizatio n increases with increasing SDS concentration, the conc(1/2) values likewis e coincide with the cmc, The higher conc(1/2) values as compared to the Tri ton X-100 cmc are due to the larger dye concentration relative to that of t he micelles. The dependences of the relative amounts of monomer and dimer v ersus the surfactant concentration for similarly charged reactants are conf irmed by model simulations. (C) 1999 Elsevier Science B.V. All rights reser ved.