Effects of complexation by cyclodextrins on the photoreactivity of Rose Bengal and Erythrosin B. A laser flash photolysis investigation

The effect of complexation by cyclodextrins on the photoreactivity of the halogenated xanthenes Rose Bengal and Erythrosin B has been studied by detecting the change induced in the yield and reactivity of triplet and radical states. Solutions of Rose Bengal and Erythrosin B in γ-cyclodextrins, containing 90% of complex in the ground state, and solutions of erythrosin B in α-cyclodextrins, containing 60% of complex in the ground state, were studied by laser flash photolysis. Other complexes could not be prepared in useful fractions, either because of the low solubility of the complexing cyclodextrin or because of the low values of the association constants. The spectrum and the yield of triplet and radical intermediates appear to be identical in water and in systems containing the complexed form. The rates of reaction of the triplets and radicals in the complexed system are slower than in water. The exit rate constant of triplet is less than 2 × 105 s–1 and the inclusion complex of triplet and radicals has an equilibrium constant similar to that of the ground state. A stoichiometry ratio of 1 : 1 was confirmed for all complexes, but there is an indication of some contribution from 2 : 1 (host : guest) in the γ cyclodextrin–erythrosin B system.