pH DEPENDENCE OF SINGLET OXYGEN PRODUCTION IN AQUEOUS SOLUTIONS USING THIAZINE DYES AS PHOTOSENSITIZERS

Abstract

Abstract— The production of singlet oxygen by thiazine dye photosensitization, as measured by the rate of photooxidation of tryptophan, was found to be very sensitive to changes of pH in the range 5–9. For methylene blue in aerated solutions, the production of 1 O 2 * is approximately five times more efficient in basic than in acidic medium. This was shown to be related to the p K 's of the triplet dyes, by evaluating the yields of 1 O 2 * from the lifetimes and the quenching rate constants for the two ionic species of sensitizer triplets measured by laser flash photolysis. Changes in the quenching rate constants of the thiazine triplet states can be correlated with the triplet energies.