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The reaction mechanism of photochemical water oxidation catalyzed by Al(III)-porphyrins (AlTMPyP(OH)2) covalently adsorbed through one axial OH group coordination on the surface of TiO2 particle 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 H2O2 from water is initiated only by one-photon excitation of catalyst followed by stepwise two-electron conversion processes, providing a promising model for photochemical water oxidation. Upon laser flash excitation of AlTMPyP adsorbed on transparent TiO2 film leads to the formation of AlTMPyP(OH) radical cation through one-electron injection from the excited singlet state of catalyst to the conduction band of TiO2. The radical cation is subsequently deprotonated into AlTMPyP(O*), which further reacts with OH-/H2O to form a key intermediate, the radical anion of AlTMPyP(O-OH), having characteristic O-O bond at the axial position within a delay time of 2-3 µs. Thereafter the radical anion efficiently transfers second electron to TiO2 leading to the formation of AlTMPyP(O-OH) (~ 67 µs), which reacts further with OH-/H2O to liberate free hydrogen peroxide as an end-product with regeneration of the starting catalyst, AlTMPyP(OH), (~ 550 µs).

We report herein a new molecular catalyst for efficient water splitting, aluminum porphyrins (tetra-methylpyridiniumylporphyrinatealuminum: AlTMPyP), containing earth's most abundant metal as the central ion. One-electron oxidation of the aluminum porphyrin initiates the two-electron oxidation of water to form hydrogen peroxide as the primary reaction product with the lowest known overpotential (97 mV). The aluminum-peroxo complex was detected by a cold-spray ionization mass-spectrometry in high-resolution MS (HRMS) mode and the structure of the intermediate species was further confirmed using laser Raman spectroscopy, indicating the hydroperoxy complex of AlTMPyP to be the key intermediate in the reaction. The two-electron oxidation of water to form hydrogen peroxide was essentially quantitative, with a Faradaic efficiency of 99 %. The catalytic reaction was found to be highly efficient, with a turnover frequency up to ∼2×104 s-1 . A reaction mechanism is proposed involving oxygen-oxygen bond formation by the attack of a hydroxide ion on the oxyl-radical-like axial ligand oxygen atom in the one-electron-oxidized form of AlTMPyP(O- )2 , followed by a second electron transfer to the electrode.

Aluminium(III)-tetramesitylporphyrin, with the Earth’s the most abundant metal and the third most abundant element as the Al(III) ion, induces the photochemical oxygenation of cyclohexene in deaerated aqueous acetonitrile to form the corresponding epoxide and alcohol with water as both electron and oxygen atom donor upon visible light irradiation. The Al(III) should be the most available and meaningful element to be utilized in the artificial photosynthetic unit. The excited triplet state of the water-coordinated porphyrin is responsible for the photochemical oxygenation.

Silicon as the second most abundant element on Earth was effectively utilized as the central atom in the porphyrin to induce photochemical oxygenation of alkenes as the first example of photocatalytic reaction through activation of water molecule in the presence of K2PtCl6 as an electron acceptor. Oxygen atom of water was confirmed to be incorporated in the oxygenated product by the photoreaction with H218O. The excited triplet state of silicon porphyrin was revealed to be responsible for the photochemical oxygenation. The one-electron oxidized silicon porphyrin was predicted by DFT calculation to have its spin population mostly on the axially ligated hydroxyl oxygen atom. The oxyl radical character of the axial ligand could rationalize the oxygenation reaction.

Novel environment-benign one-pot syntheses of water-soluble porphyrins, trans-dihydroxy-5,10,15,20-tetra(N-methyl-4′-pyridiniumyl)porphyrinate tin(IV) (Sn(IV)TMPyP(OH)2) and water-insoluble trans-dihydroxy-5,10,15,20-tetra(4-pyridyl)porphyrinate tin(IV) (Sn(IV)TPyP(OH)2), were found to proceed in water as solvent at room temperature with high yield (90 and 93%).

A new water-soluble silicon(IV)-tetra(4-carboxyphenyl)porphyrin (SiTCPP) with silicon(iv), the second most abundant element on Earth, in the center of porphyrin was synthesized. Fundamental properties including protolytic behaviour of axially coordinating hydroxy groups, and electrochemical behaviour were characterized. The properties were compared with those of silicon(IV)-tetra(2,4,6-trimethylphenyl)porphyrin (SiTMP) and silicon(IV)-tetra(4-trifluoromethylphenyl)porphyrin (SiTFMPP) and discussed in respect to the electron donating/withdrawing effect of the substituents. Two axially coordinating hydroxy groups of SiTCPP exhibit a four-step protolytic behaviour under the acidic conditions along with a single step protolysis of peripheral carboxyl groups. Though SiTCPP and SiTFMPP did not show any reactivity in the photochemical oxygenation of a substrate with K2PtCl6 as a sacrificial electron acceptor, the first oxidation wave in the electrochemical process of SiTCPP and SiTFMPP showed catalytic behaviour in aqueous acetonitrile solution at any pH condition, in contrast to SiTMP which has only a reversible oxidation wave under neutral and weakly acidic conditions. The criteria for the electrochemical oxidative activation of water and the photooxygenation of the substrate were obtained. The higher oxidation wave of Si-porphyrins than ∼0.86 volt vs. SHE is required for the electrochemical oxidation of water, while suitable protecting groups such as a methyl substituent is a requisite for the photochemical oxygenation with K2PtCl6 as a sacrificial electron acceptor.

Water-soluble cationic 5,10,15,20-tetra(N-methyl-4′-pyridiniumyl)porphyrinatoaluminum(III) (AlTMPyP) has been synthesized by novel one-pot reaction between the parent free-base porphyrin and trimet...

Water-soluble cationic 5,10,15,20-tetra(N-methyl-4′-pyridiniumyl)porphyrinatoaluminum(III) (AlTMPyP) has been synthesized by novel one-pot reaction between the parent free-base porphyrin and trimet...

Hydrogen evolution coupled with the photochemical oxygenation of cyclohexene with water was observed in the system sensitized by Sn(IV)–porphyrin adsorbed on Pt loaded TiO2 nano-particles in aqueous acetonitrile solution upon visible light irradiation.

One of the most promising solutions to the current global problems of energy and the environment would be to develop a clean system for the chemical conversion of solar energy, for example, photochemical hydrogen evolution and carbon dioxide fixation. In both cases, the water molecule should be the electron source, as is the case with natural photosynthesis. The point of research in this area is to discover how the electrons of water can best be utilized. The oxidation of water can be approached through (a) one-electron (b) two-electron and (c) four-electron processes. In spite of remarkable progress of molecular catalysts, the visible light-induced oxidation of water to molecular oxygen via the four-electron process has been one of the bottle-necks in the field, mostly due to the photon-flux density problem of sunlight, with its intrinsically limited intensity. A photochemical method for the four-electron oxidation of water requires four photons to excite sensitizers and catalysts in a stepwise manner in order to reach the higher oxidized states. Under typical sunlight conditions, the time interval between the arrivals of photons to a single molecule is on the order of seconds, which is far longer than the usual timescale for the survival of the highly unstable oxidized states needed to elicit the catalytic activity. Alternative approaches thus are clearly required. In this context, we have been studying the one-electron initiated two-electron oxidation of water. The latest results on the oxidation of water catalyzed by earth-abundant metalloporphyrins will be reported.