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 'O,* is approximately five times more efficient in basic than in acidic medium. This was shown to be related to the pK's of the triplet dyes, by evaluating the yields of '02* 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.
Abstract— It is shown that the photoreduction of tin tetraphenylporphyrin using a variety of reducing agents does not stop at the tetraphenylchlorin stage but proceeds to the isobacteriochlorin derivative. However, this latter compound can be photooxidized in the presence of oxygen to yield exclusively tin chlorin which in turn can again be photoreduced to the isobacteriochlorin. The process appears to be photoreversible at this stage. These reactions have been studied in homogeneous and micellar solutions.
Our study of the kinetics of sensitized photooxidation, as a function of pH in anionic micellar systems using cationic dyes as sensitizers, shows that adsorption of the dye slows the protonation step of his triplet state. This observation has important implicaticins in interpretation of results in the so-called photodynamic effect in biological systems: using cationic dyes as sensitizers the rate of photooxidation for the same pH is different depending whether or not the medium allows adsorption of the dye.
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