Two‐Electron Oxidation of Water Through One‐Photon Excitation of Aluminium Porphyrins: Molecular Mechanism and Detection of Key Intermediates. Issue 3 (15th November 2017)

• Record Type: Journal Article
• Authors: Mathew, Siby; Kuttassery, Fazalurahman; Remello, Sebastian Nybin; Thomas, Arun; Yamamoto, Daisuke; Onuki, Satomi; Nabetani, Yu; Tachibana, Hiroshi; Inoue, Haruo
• Is Part Of: ChemPhotoChem. Volume 2:Issue 3(2018)
• Page Start: 240
• Page End: 248
• Publication Date: 2017-11-15
• Subjects: aluminium porphyrins -- artificial photosynthesis -- hydrogen peroxide -- laser flash photolysis -- water oxidation; Photochemistry -- Periodicals; Periodicals; Electronic journals; 541.35
• DOI: 10.1002/cptc.201700155
• Languages: English
• ISSNs: 2367-0932

Abstract

Abstract: The reaction mechanism of photochemical water oxidation catalyzed by Al III porphyrins (AlT III MPyP(OH)2 ) covalently adsorbed through the coordination of one axial OH group on the surface of TiO2 particles was investigated by time‐resolved nanosecond laser flash photolysis (NLFP). Three types of transient species with completely different absorption and decay characteristics were detected by means of NFLP, demonstrating that the photochemical production of H2 O2 from water is initiated only by one‐photon excitation of the catalyst followed by stepwise two‐electron conversion processes, providing a promising model for photochemical water oxidation. Laser flash excitation of Al III TMPyP adsorbed on a transparent TiO2 film leads to the formation of the AlTMPyP(OH) radical cation through one‐electron injection from the excited singlet state of the catalyst to the conduction band of TiO2 . The radical cation is subsequently deprotonated into AlTMPyP(O . ), which further reacts with OH − /H2 O to form a key intermediate, the radical anion of AlTMPyP(O‐OH), having a characteristic O−O bond at the axial position within a delay time of 2–3 μs. Thereafter the radical anion efficiently transfers a second electron to TiO2 leading to the formation of AlT III MPyP(O‐OH) (≈67 μs), which reacts further with OH − /H2 O to liberate free hydrogen peroxide as an end‐product with regeneration of the starting catalyst, AlT III MPyP(OH), (≈500 μs). Abstract : Electron in water : The mechanism of the photochemical two‐electron oxidation of water catalyzed by Al III porphyrins was revealed by laser flash photolysis. Detection of the key intermediates, including the radical anion and the complex with the two‐electron oxidation product, hydrogen peroxide, demonstrates that the photochemical production of H2 O2 from water is initiated only by one‐photon excitation of Al III porphyrins.