Osamu Ishitani

Efficient interfacial electron transfer from light-harvesting semiconductors to molecular catalysts remains a major challenge in molecular–semiconductor hybrid photocatalytic CO2 reduction, particularly for narrow-bandgap semiconductors, whose reduced thermodynamic driving force often requires assistance from metallic electron-transfer mediators such as Ag. Here, we show that, despite its narrow bandgap of 2.1 eV, the layered perovskite oxynitride K2LaTa2O6N (KLTON) functions as an effective semiconductor support for a supramolecular RuRu′ CO2-reduction catalyst without relying on Ag nanoparticles. The RuRu′/KLTON hybrid selectively converted CO2 to formate under visible-light irradiation with >99% selectivity, a turnover number of 691, and an apparent quantum yield of 6.1 ± 0.6% at 420 nm. Loading a small amount of Ag further increased the turnover number to 1077, but this 1.5-fold enhancement was modest compared with the larger Ag-induced activity enhancements reported for several conventional oxynitride-based hybrids, highlighting the weak Ag dependence of the KLTON-based system. Spatial photodeposition of Ag and MnOx revealed spatially differentiated surface redox reactivity on the terraced layered structure of KLTON, while time-resolved microwave conductivity and transient infrared absorption measurements showed favorable photogenerated-carrier dynamics and demonstrated that Ag loading modifies the electron dynamics of KLTON. Time-resolved emission measurements further revealed an additional nonradiative decay pathway at the Ru(II) photosensitizer/KLTON interface, consistent with interfacial electronic interaction without uniquely assigning its microscopic origin. These results identify favorable carrier dynamics and weak Ag dependence as key features of the KLTON-based molecular hybrid and highlight layered narrow-bandgap mixed-anion semiconductors as promising support materials for visible-light-driven molecular photocatalysis.

CuGaO2 has limited application for dye-sensitized molecular photocathodes because of the large particle size obtained through high-temperature solid-state synthesis. Planetary ball-milling produced fine CuGaO2 particles for semitransparent electrodes while preserving the delafossite structure and Cu(I) electronic state. Photocathodes modified with a Ru tris-bipyridine-type photosensitizer exhibited enhanced photocathodic reduction currents at more positive potentials than pristine CuGaO2 and NiO electrodes despite a lower amount of immobilized dye.

An ultrathin Al ₂ O ₃ interlayer at a p-type NiO/molecular photocatalyst interface suppresses charge recombination and boosts visible-light-driven CO ₂ reduction.

There is immense scope and interest in the development of photocatalytic CO₂ conversion. We have successfully developed a new supramolecular photocatalyst (Ir-Rh) in which the Ir(III) complex [Ir(piq)₂(BL)]⁺ (piq = 1-phenylisoquinoline, BL = 1,2-bis(4'-methyl[2,2'-bipyridin]-4-yl)ethane) as a photosensitizer and the Rh(III) complex [Rh(BL)(Cp*)Cl]⁺ (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl) as the catalyst are connected by a bridging ligand BL. The photophysical properties and the redox behavior of all complexes were investigated. Ir-Rh efficiently photocatalyzed CO₂ reduction in the presence of 1-benzyl-1,4-dihydronicotinamide (BNAH) as the sacrificial electron donor, selectively yielding formic acid as the major product (S_HCOOH > 90%) with minor amounts of CO and H₂. However, the combined system of the corresponding mononuclear complexes produced a smaller amount of formic acid. The mechanism of the photocatalytic reaction was explored through in situ UV-vis absorption studies and electrochemical measurements.

A non-TADF photosensitizer (N-BAP+) is employed for photocatalytic CO ₂ reduction in combination with a Mn catalyst.

Photocatalytic system quantitatively convert CO ₂ captured via amine scrambling into formic acid at room temperature, enabling direct utilization of low-concentration CO ₂ without energy-intensive regeneration.

CO2 capture by Re(I) catalysts bearing deprotonated triethanolamine, a key step in catalytic CO2 reduction, was investigated kinetically and thermodynamically. The rate-determining step is nucleophilic attack of the coordinating oxygen...

For the practical application of photocatalytic CO₂ reduction systems, it should be important that the catalyst directly reduces low concentrations of CO₂ in photocatalytic systems consisting of only Earth-abundant elements. Despite the development of numerous photocatalytic CO₂ reduction systems, CO₂ utilization at low CO₂ concentrations has not been thoroughly investigated. Although fac-[Mn^I(diimine)(CO)₃L]ⁿ⁺-type complexes are among the most abundant transition-metal complexes and have been studied as CO₂ reduction catalysts in electrocatalytic systems, their application in photocatalytic reactions has been limited by the formation of photochemically unstable Mn(0) dimers. Thus, the photocatalytic activities of systems using Mn complexes as catalysts have not been evaluated at low CO₂ concentrations. In this work, we synthesized a novel Mn(I) complex by introducing one sterically bulky mesityl group at the 6-position of the 4,4'-dimethyl-2,2'-bipyridine (dmb) ligand and used it as a catalyst in photocatalytic reactions. In the presence of trifluoroethanol (TFE) and diisopropylethylamine, the Mn complex captures CO₂ to form the corresponding carbonate ester complex (MnMes-CO₂TFE), and the addition of an organic photosensitizer (4DPAIPN) enables the selective reduction of CO₂ to CO. MnMes-CO₂TFE demonstrated excellent catalytic durability, owing to the complete suppression of Mn dimer formation in the photocatalytic reactions. The turnover number (TON) of CO formation reached a maximum of 8770 based on the MnMes-CO₂TFE used, and the quantum yield of CO formation reached 40%. Furthermore, MnMes-CO₂TFE exhibited high selectivity and catalytic rates for CO production, even at low CO₂ concentrations (1-10%), attributed to the efficient CO₂ capture reaction.

Redox photosensitizers exhibiting thermally activated delayed fluorescence (TADF) are widely used in the various research fields. We investigated the roles of the singlet and triplet excited states of such molecules in photocatalytic CO₂ reduction. Two TADF compounds (4DPAIPN and 3DPAFIPN) were used in combination with a manganese(I) complex as a catalyst and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) and triethanolamine (TEOA) as sacrificial electron donors. In the case of 4DPAIPN, the quantum yield of CO₂ reduction (Φ_CO+HCOO-) was negatively correlated with the concentration of BIH, equaling 43.2% and 20.3% at [BIH] = 5 and 200 mM, respectively. The reduction of the singlet excited state by BIH afforded a singlet geminate ion pair, ¹(4DPAIPN•^-···BIH•⁺), which experienced a markedly faster backward electron transfer affording the ground state than the corresponding triplet, ³(4DPAIPN•^-···BIH•⁺). The escape yield of the singlet state was approximately 10 times lower than that of the triplet state. High BIH concentrations favored the quenching of the singlet excited state and disfavored the formation of the triplet excited state, resulting in low photocatalytic efficiencies. Although the system with 3DPAFIPN exhibited a similar tendency, the maximum Φ_CO+HCOO- was lower (11.9% at [BIH] = 10 mM) because of its greater oxidizing power resulting in the efficient quenching of its singlet excited state by TEOA. Based on these results, we extracted the reaction conditions and molecular designs of TADF photosensitizers suitable for constructing efficient photocatalysts, namely those minimizing the quenching of the singlet excited state and maximizing the quenching of the triplet excited state.