Use of thermal lens spectrometry for the investigation of dimerization equilibria of rhodamine 6G in water and aqueous micellar solutions

The aggregation and the fluorescent properties of rhodamine 6G in aqueous solutions have been investigated using thermal lens spectrometry in addition to conventional absorption and fluorescence experiments. The results show that, when the compound is highly fluorescent, the photothermal method is very sensitive to detect the formation of dimers. It allowed the determination of the absolute fluorescence quantum yield of the monomer and of the dimerization constant. The experiments, carried out over a wide range of concentrations, show that while the non-active absorption by dimers is the main quenching process at low concentrations, the decrease in the fluorescence quantum yield of the dye at higher concentrations could also result from migrational quenching involving the non-radiative energy transfer from excited monomers to non-fluorescent dimers. Solubilization of rhodamine 6G into micellar solutions has a positive effect on the deaggregation of dimers for dye concentrations up to at least 4 x 10(-4) M. In solutions containing 2% and 10% of anionic SDS or nonionic Brij 35, respectively, dimers are absent and the fluorescence efficiency of the rhodamine is completely restored. On the contrary, in solutions containing 4% of cationic CTABr, the absolute fluorescence quantum yield of monomers is significantly decreased and dimerization still occurs to a small extent. (C) 1997 Elsevier Science B.V.