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 alpha -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 x 10(-4) and it was independent of CVH concentration. The electron transfer from CVH to triplet methylene blue ((MB+)-M-3*) 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. (C) 2001 Elsevier Science B.V. All rights reserved.