Photoinduced hydride transfer reaction between methylene blue and leuco crystal violet

The photoinduced hydride transfer reaction between methylene blue (MB⁺) and leuco crystal violet (CVH) was investigated spectrophotometrically under conditions of direct excitation of MB⁺ with steady-illumination of visible light and of photosensitization by benzophenone (BP) and α-nitronaphthalene (NNP) with UV light. It was found that upon the irradiation of the solution of MB⁺ and CVH with visible light, MB⁺ disappeared and crystal violet (CV⁺) formed simultaneously. The ratio of the CV⁺ concentration formed to that of MB⁺ consumed was unity. The photosensitized reactions by NNP gave similar results. From the findings that the fluorescence of MB⁺ was not quenched by CVH and the triplet photosensitizations were observed, it was concluded that the hydride transfer reaction in acetonitrile occurs in the triplet state of MB⁺. The quantum yield for the disappearance of MB⁺ in acetonitrile was estimated to be 1.1×10^-4 and it was independent of CVH concentration. The electron transfer from CVH to triplet methylene blue (³MB^+*) and the back electron transfer within the geminate radical pair must occur effectively. A spin adduct of a radical formed during the reaction was detected by the ESR method with 2-methyl-2-nitrosopropane (MNP) as a radical trap. This radical was attributed to CV·. The results can be explained by a mechanism with stepwise electron-proton-electron transfers. In tert-butyl alcohol, however, it was observed that the fluorescence of MB⁺ was quenched by CVH and the quantum yield of the disappearance of MB⁺ depended on CVH concentration. Different mechanism from that in acetonitrile was proposed for the reaction in tert-butyl alcohol.