Fluorescence of aminofluoresceins as an indicative process allowing one to distinguish between micelles of cationic surfactants and micelle-like aggregates

Among the vast set of fluorescein derivatives, the double charged R²⁻ anions of aminofluoresceins are known to exhibit only low quantum yields of fluorescence, $\phi$. The $\phi$ value becomes as high as that of the fluorescein dianion when the lone electron pair of the amino group is involved in a covalent bond. According to Munkholm et al (1990 J. Am. Chem. Soc. 112 2608–12), a much smaller increase in the emission intensity can be observed in the presence of surfactant micelles. However, all these observations refer to aqueous or alcoholic solvents. In this paper, we show that in the non-hydrogen bond donor (or 'aprotic') solvents DMSO and acetone, the quantum yields, φ, of the 4′- (or 5′)-aminofluorescein R²⁻ species amount to 61–67% and approach that of fluorescein (φ  =  87%), whereas in water φ is only 0.6–0.8%. In glycerol, a solvent with an extremely high viscosity, the φ value is only 6–10%.

We report on the enhancement of the fluorescence of the aminofluorescein dianions as an indicative process, which allows us to distinguish between the micelle-like aggregates of cationic dendrimers of low generation, common spherical surfactant micelles, and surfactant bilayers. Some of these colloidal aggregates partly restore the fluorescence of aminofluoresceins in aqueous media. By contrast, other positively charged micellar-like aggregates do not enhance the quantum yield of aminofluorescein R²⁻ species. Results for several related systems, such as CTAB-coated SiO₂ particles and reverse microemulsions, are briefly described, and the possible reasons for the observed phenomena are discussed. (paper)