Kinetic Properties of the Triplet States of Methylene Blue and Other Photosensitizing Dyes

Abstract— The long‐lived (> 1 μsec) transients formed in the flash excitation of the representative photosensitizers methylene blue, eosin Y and pyrene have been investigated and various criteria have been used to distinguish between triplet state intermediates and chemical intermediates. Previous assignments of the triplet transients of methylene blue appeared less secure in view of the photochemical reactivity of this dye and its lack of phosphorescence. Earlier assignments of monomeric and dimeric triplet transients of methylene blue are substantiated, however, by the observations that the rate constant for quenching by oxygen is approximately 1/9 th diffusion controlled and the formation rates are commensurate with singlet decay rates and by the observation of triplet‐triplet annihilation. Additional evidence in support of monomer triplet assignments for methylene blue and eosin Y is provided by the effect of heavy atom quenchers Cs + , Hg 2+ and T1 + on decay rates. Due to chemical reactivity, quenching by I − appears less suitable as a diagnostic test for triplet state intermediates. The effect of N 3 − , which is known to quench singlet oxygen molecules and to alter the course of photosensitized oxidations, on the triplet decay of methylene blue, eosin Y and pyrene is also investigated.