Kazuhide Koike

Improvement in the photochemical formation efficiency of one-electron-reduced species (OERS) of a photoredox photosensitizer (a redox catalyst) is directly linked to the improvement in efficiencies of the various photocatalytic reactions themselves. We investigated the primary processes of a photochemical reduction of two series [Ru(diimine)₃]²⁺ and [Os(diimine)₃]²⁺ as frequently used redox photosensitizers (PS²⁺), by 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as a typical reductant in detail using steady-irradiation and time-resolved spectroscopies. The rate constants of all elementary processes of the photochemical reduction of PS²⁺ by BIH to give the free PS•⁺ were obtained or estimated. The most important process for determining the formation efficiency of the free PS•⁺ was the escape yield from the solvated ion pair [PS•⁺–BIH•⁺], which was strongly dependent on both the central metal ion and the ligands. In cases with the same central metal ion, the system with larger −ΔG_bet, which is the free energy change in the back-electron transfer from the OERS of PS•⁺ to BIH•⁺, tended to lower the escape yield of the free OERS of PS²⁺. On the other hand, different central metal ions drastically affected the escape yield even in cases with similar −ΔG_bet; the escape yield in the case of RuH²⁺ (−ΔG_bet = 1.68 eV) was 5–11 times higher compared to those of OsH²⁺ (−ΔG_bet = 1.60 eV) and OsMe²⁺ (−ΔG_bet = 1.71 eV). The back-electron transfer process from the free PS•⁺ to the free BIH•⁺ could not compete against the further reaction of the free BIH•⁺, which is the deprotonation process giving BI•, in DMA for all examples. The produced BI• gave one electron to PS²⁺ in the ground state to give another PS•⁺, quantitatively. Based on these findings and investigations, it is clarified that the photochemical formation efficiency of the free PS•⁺ should be affected not only by −ΔG_bet but also by the heavy-atom effect of the central metal ion, and/or the oxidation power of the excited PS²⁺, which should determine the distance between the excited PS and BIH at the moment of the electron transfer.

Supramolecular photocatalysts in which Ru(II) photosensitizer and Re(I) catalyst units are connected to each other by an ethylene linker are among the best known, most effective and durable photocatalytic systems for CO₂ reduction. In this paper we report, for the first time, time-resolved infrared (TRIR) spectra of three of these binuclear complexes to uncover why the catalysts function so efficiently. Selective excitation of the Ru unit with a 532 nm laser pulse induces slow intramolecular electron transfer from the ³MLCT excited state of the Ru unit to the Re unit, with rate constants of (1.0–1.1) × 10⁴ s⁻¹ as a major component and (3.5–4.3) × 10⁶ s⁻¹ as a minor component, in acetonitrile. The produced charge-separated state has a long lifetime, with charge recombination rate constants of only (6.5–8.4) × 10⁴ s⁻¹. Thus, although it has a large driving force (−ΔG⁰ _CR ∼ 2.6 eV), this process is in the Marcus inverted region. On the other hand, in the presence of 1-benzyl-1,4-dihydronicotinamide (BNAH), reductive quenching of the excited Ru unit proceeds much faster (k_q[BNAH (0.2 M)] = (3.5–3.8) × 10⁶ s⁻¹) than the abovementioned intramolecular oxidative quenching, producing the one-electron-reduced species (OERS) of the Ru unit. Nanosecond TRIR data clearly show that intramolecular electron transfer from the OERS of the Ru unit to the Re unit (k_ET > 2 × 10⁷ s⁻¹) is much faster than from the excited state of the Ru unit, and that it is also faster than the reductive quenching process of the excited Ru unit by BNAH. To measure the exact value of k_ET, picosecond TRIR spectroscopy and a stronger reductant were used. Thus, in the case of the binuclear complex with tri(p-fluorophenyl)phosphine ligands (RuRe(FPh)), for which intramolecular electron transfer is expected to be the fastest among the three binuclear complexes, in the presence of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH), k_ET was measured as k_ET = (1.4 ± 0.1) × 10⁹ s⁻¹. This clearly shows that intramolecular electron transfer in these RuRe binuclear supramolecular photocatalysts is not the rate-determining process in the photocatalytic reduction of CO₂, which is one of the main reasons why they work so efficiently.

A water-resistant Sr₂MgSi₂O₇:Eu²⁺,Dy³⁺long-lasting phosphor was hybridized with visible-light active Ag₃PO₄to achieve enhanced photocatalysis and energy-harvesting long lasting photocatalysis.

We carried out time-resolved infrared (TR-IR) and emission lifetime measurements on a Re(I) carbonyl complex having an N-heterocyclic carbene ligand, namely, fac-[Re(CO)₃(PyImPh)Br], under photochemically reactive (in solution in acetonitrile) and nonreactive (in solution in dichloromethane) conditions to investigate the mechanism of photochemical ligand substitution reactions. The TR-IR measurements revealed that no reaction occurs on a picosecond time scale and the cationic product, namely, fac-[Re(CO)₃(PyImPh)(MeCN)]⁺, is produced on a nanosecond time scale only in solution in acetonitrile, which indicates that the reaction proceeds thermally from the excited state. Because no other products were observed by TR-IR, we concluded that this cationic product is an intermediate species for further reactions. The measurements of the temperature-dependent emission lifetime and analysis using transition-state theory revealed that the photochemical substitution reaction proceeds from a metal-to-ligand charge transfer excited state, the structure of which allows the potential coordination of a solvent molecule. Thus, the coordinating capacity of the solvent determines whether the reaction proceeds or not. This mechanism is different from those of photochemical reactions of other types of Re(I) carbonyl complexes owing to the unique characteristics of the carbene ligand.

Photocatalytic systems for CO ₂ reduction using a Ru( ii ) photosensitiser and dinuclear Re( i ) diimine tricarbonyl complexes ( Re(n)Re ), in which diimine ligands are connected by alkyl chains of various lengths (–C _n H _2n –: n = 2, 3, 4, 6, 14), as catalysts were investigated.

Photoemission of Ce³⁺doped gehlenite excites the Ag/AgCl and drives the photocatalysis continuously after elimination of the external light source.