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 c onditions of direct excitation of MB+ with steady-illumination of visible l ight and of photosensitization by benzophenone (BP) and alpha -nitronaphtha lene (NNP) with UV light. It was found that upon the irradiation of the sol ution 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 s imilar results. From the findings that the fluorescence of MB+ was not quen ched by CVH and the triplet photosensitizations were observed, it was concl uded that the hydride. transfer reaction in acetonitrile occurs in the trip let state of MB+. The quantum yield for the disappearance of MB+ in acetoni trile was estimated to be 1.1 x 10(-4) and it was independent of CVH concen tration. The electron transfer from CVH to triplet methylene blue ((MB+)-M- 3*) and the back electron transfer within the geminate radical pair must oc cur 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 radica l trap. This radical was attributed to CV.. The results can be explained by a mechanism with stepwise electron-proton-electron transfers. In tert-buty l alcohol, however, it was observed that the fluorescence of MB+ was quench ed 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.