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The maximum quantum yield of 4.5 was evaluated from the kinetic analysis for the Methylene Blue-sensitized oxygenation of 2,5-diphenylfuran, and the contribution of a chain process to produce the furan endoperoxide was suggested. After the photoreaction, the thermolysis of the endoperoxide occurred to regenerate 2,5-diphenylfuran and singlet oxygen and to produce cis-1,2-dibenzoylethylene simultaneously. The activation energy of singlet oxygen recovery was estimated to be 18.5 kcal mol−1. The reversible conversion of singlet oxygen took place efficiently with thiazine and xanthene dyes. A concerted dissociation of singlet oxygen from the endoperoxide was supposed from the negative value of activation enthropy (−9.8 e.u.) for the reaction.

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