Photochemical primary processes of xanthene dyes. 7. Xanthene dyes as probes for the characterization of anionic micelles

The cationic xanthene dyes thiopyronine and selenopyronine are solubilized in the Stern regions of anionic micelles. Upon photoexcitation, the excited triplet states of the dyes are observed to decay via unimolecular processes (k₁ ≈ 2 × 10³ s^-1) for the case of one dye per micelle and via bimolecular processes (k₂ ≈ 5 × 10⁶ s^-1) for the case of more than one dye per micelle. If the number of dye triplet states that undergo slow decay is measured, the number of micelles can be obtained and thus the micellar aggregation numbers can be evaluated. The aggregation numbers of several surfactant alkanesulfonates, 2,5-dialkylbenzenesulfonates with a total of 14 carbon atoms in the alkyl chains, and other detergents, which are determined in this manner, are in agreement with corresponding values determined by using other photophysical methods and with literature values. The aggregation numbers of the dialkylbenzenesulfonate surfactants increase from 38 to 56 with increasing effective chain length. The fluorescence lifetime and fluorescence depolarization of thiopyronine solubilized in the dialkylbenzenesulfonate micelles are consistent with a low local polarity and high local viscosity for the interior of these micelles, compared to micelles of aliphatic detergents. Investigations using sodium 11-(3-hexyl-1-indolyl)undecyl sulfate (6-ln-11) as a fluorescence probe suggest that the polarity of the interior of the dialkylbenzenesulfonate micelles is similar to that of liquid hydrocarbons. These results suggest a compact structure for the dialkylbenzenesulfonate micelles with low water penetration into the micelles.