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 aque ous solutions have been investigated using thermal lens spectrometry i n addition to conventional absorption and fluorescence experiments. Th e results show that, when the compound is highly fluorescent, the phot othermal method is very sensitive to detect the formation of dimers. I t allowed the determination of the absolute fluorescence quantum yield of the monomer and of the dimerization constant. The experiments, car ried out over a wide range of concentrations, show that while the non- active absorption by dimers is the main quenching process at low conce ntrations, the decrease in the fluorescence quantum yield of the dye a t higher concentrations could also result from migrational quenching i nvolving the non-radiative energy transfer from excited monomers to no n-fluorescent dimers. Solubilization of rhodamine 6G into micellar sol utions has a positive effect on the deaggregation of dimers for dye co ncentrations up to at least 4 x 10(-4) M. In solutions containing 2% a nd 10% of anionic SDS or nonionic Brij 35, respectively, dimers are ab sent and the fluorescence efficiency of the rhodamine is completely re stored. 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 El sevier Science B.V.