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MECHANISM OF PHOTOREDUCTION OF THIAZINE DYES BY EDTA STUDIED BY FLASH PHOTOLYSIS—III CONSEQUENCES OF A NEWLY FOUND pK T OF THIONINE ON THE MECHANISM IN BASIC SOLUTIONS
Author(s) -
Bonneau Roland,
Pereyre Josette
Publication year - 1975
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1975.tb06648.x
Subject(s) - thionine , chemistry , quantum yield , flash photolysis , methylene blue , thiazine , ethylene diamine , photochemistry , yield (engineering) , reactivity (psychology) , base (topology) , triplet state , inorganic chemistry , medicinal chemistry , kinetics , fluorescence , reaction rate constant , nuclear chemistry , organic chemistry , photocatalysis , thermodynamics , molecule , alternative medicine , mathematics , mathematical analysis , pathology , quantum mechanics , electrochemistry , catalysis , medicine , physics , electrode
— Evidence for an acid‐base equilibrium between the neutral and monocationic forms of the triplet state of thionine, 3 T and 3 TH + (p K T # 8–9) has been found. This equilibrium, which has no counterpart in the methylene blue system can explain the different behaviours of these two similar dyes concerning their photoreduction by ethylene diamine tetracetic acid (EDTA) in basic solutions. With reasonable assumptions about the reactivity of the various species present in solution, it is now possible to calculate over the whole pH range the quantum yield of semireduced dye formation φs, which, according to the previously proposed mechanism, must be proportional to the overall photoreduction quantum yield φR. The very good fit between the calculated curve φs = f (pH) and the experimental one φR = f (pH) for both dyes strongly supports the proposed mechanism.