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Volume 155, 2012
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How is the water molecule activated on metalloporphyrins? Oxygenation of substrates induced through one-photon/two-electron conversion in artificial photosynthesis by visible light

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Abstract

The reaction mechanism of the highly efficient (Φ = 0.60), selective photochemical epoxidation of alkenes sensitized by CO-coordinated tetra(2,4,6-trimethyl)phenylporphyrinatoruthenium(II) (RuIITMP(CO)), with water acting both as an electron and oxygen atom donor, was investigated. The steady-state light irradiation of the reaction mixture indicated the formation of the RuIITMP(CO) cation radical under neutral conditions, which was effectively trapped by an hydroxide ion to regenerate the starting sensitizer. By means of a laser flash photolysis experiment, the formation of the cation radical as the primary process from the triplet excited state of RuIITMP(CO) was clearly observed. Four kinds of transients were detected in completely different ranges of the delay time: the excited triplet state of RuIITMP(CO) [delay time region <20 μs], the cation radical of RuIITMP(CO)(CH3CN) [20–50 μs], the hydroxyl-coordinated Intermediate [I] [50–200 μs], and the cyclohexane-attached Intermediate [II] [200 μs–8 ms]. A reaction mechanism was revealed that involves RuTMP(CO) cation radical formation from the triplet excited state of the sensitizer, followed by attack of an hydroxide ion to form an hydroxyl-coordinated Ru–porphyrin (Intermediate [I]) and subsequent reaction with cyclohexene to form Intermediate [II]. The kinetics for each step of the successive processes was carefully analyzed and their rate constants were determined. The two-electron oxidation of water by one-photon irradiation, as revealed in the photochemical epoxidation, is proposed to be one of the more promising candidates to get through the bottleneck of water oxidation in artificial photosynthesis.

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Publication details

The article was received on 18 Apr 2011, accepted on 09 May 2011 and first published on 06 Jul 2011


Article type: Paper
DOI: 10.1039/C1FD00069A
Faraday Discuss., 2012,155, 145-163

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    How is the water molecule activated on metalloporphyrins? Oxygenation of substrates induced through one-photon/two-electron conversion in artificial photosynthesis by visible light

    T. Shimada, A. Kumagai, S. Funyu, S. Takagi, D. Masui, Y. Nabetani, H. Tachibana, D. A. Tryk and H. Inoue, Faraday Discuss., 2012, 155, 145
    DOI: 10.1039/C1FD00069A

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