The salt effect on riboflavin-photosensitized substitution of the sulfo group for bromine in 1-bromo-2-hydroxynaphthalene

It was shown that the substitution reaction of the sulfo group for bromine in 1-bromo-2-hydroxynaphthalene photosensitized with riboflavin occurred via the riboflavin triplet state. Electron-donor anions quench the fluorescence of riboflavin. Halide (iodide and bromide) ions increase the quantum yield of the substitution reaction at low concentrations because of an increase in the spin-orbital coupling in the radical ion pair generated in riboflavin fluorescence quenching. As a result of the spin-orbital conversion, the triplet-state radical ion pair is formed, which separates into the riboflavin radical anion and the halogen atom. The halogen atom is reduced by the sulfite ions with the simultaneous formation of the sulfite radical anions, which participate in the propagation of the substitution reaction chain. All salts increase the quantum yield of the substitution reaction at high concentrations (>0.5 mol l−1).