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^*Department of Chemistry, University of Karachi, Karachi- 75270.

ABSTRACT

The photochemical reduction of thionine with dimethylamine was studied in 50% aqueous methanol al 25±0.1 °C in a specially designed apparatus. The acidities of the solutions were determined using acetate buffers by a spectrophotometric method. The quantum yield (ِ) of the photochemical reactions was determined at different concentrations of thionine and dimethylamine. It was found that the quantum yield is a function of the acidity as well as the concentration of reductant [AH₂]. The results have been interpreted in terms of reaction mechanism. It was found that the quantum yield of the reactions of thionine with dimethylamine is controlled by two equilibria between (i) triplet state of thionine (³TH⁺) with proton and protonated triplet state of thionine [³T.H₂⁺⁺] and (ii) protonated triplet stale of thionine with reductant and associated complex of thionine with reductant [³T.H₂⁺⁺.AH₂].