Photocatalytic Systems for CO2 Reduction: Metal-Complex Photocatalysts and Their Hybrids with Photofunctional Solid Materials.

ConspectusPhotocatalytic CO2 reduction is a critical objective in the field of artificial photosynthesis because it can potentially make a total solution for global warming and shortage of energy and carbon resources. We have successfully developed various highly efficient, stable, and selective photocatalytic systems for CO2 reduction using transition metal complexes as both photosensitizers and catalysts. The molecular architectures for constructing selective and efficient photocatalytic systems for CO2 reduction are discussed herein. As a typical example, a mixed system of a ring-shaped Re(I) trinuclear complex as a photosensitizer and fac-[Re(bpy)(CO)3{OC2H4N(C2H4OH)2}] as a catalyst selectively photocatalyzed CO2 reduction to CO with the highest quantum yield of 82% and a turnover number (TON) of over 600. Not only rare and noble metals but also earth abundant ones, such as Mn(I), Cu(I), and Fe(II) can be used as central metal cations. In the case using a Cu(I) dinuclear complex as a photosensitizer and fac-Mn(bpy)(CO)3Br as a catalyst, the total formation quantum yield of CO and HCOOH from CO2 was 57% and TONCO+HCOOH exceeded 1300.Efficient supramolecular photocatalysts for CO2 reduction, in which photosensitizer and catalyst units are connected through a bridging ligand, were developed for removing a diffusion control on collisions between a photosensitizer and a catalyst. Supramolecular photocatalysts, in which [Ru(N∧N)3]2+-type photosensitizer and Re(I) or Ru(II) catalyst units are connected to each other with an alkyl chain, efficiently and selectively photocatalyzed CO2 reduction in solutions. Mechanistic studies using time-resolved IR and electrochemical measurements provided molecular architecture for constructing efficient supramolecular photocatalysts. A Ru(II)-Re(I) supramolecular photocatalyst constructed according to this molecular architecture efficiently photocatalyzed CO2 reduction even when it was fixed on solid materials. Harnessing this property of the supramolecular photocatalysts, two types of hybrid photocatalytic systems were developed, namely, photocatalysts with light-harvesting capabilities and photoelectrochemical systems for CO2 reduction.Introduction of light-harvesting capabilities into molecular photocatalytic systems should be important because the intensity of solar light shone on the earth's surface is relatively low. Periodic mesoporous organosilica, in which methyl acridone groups are embedded in the silica framework as light harvesters, was combined with a Ru(II)-Re(I) supramolecular photocatalyst with phosphonic acid anchoring groups. In this hybrid, the photons absorbed by approximately 40 methyl acridone groups were transferred to one Ru(II) photosensitizer unit, and then, the photocatalytic CO2 reduction commenced.To use water as an abundant electron donor, we developed hybrid photocatalytic systems combining metal-complex photocatalysts with semiconductor photocatalysts that display high photooxidation powers, in which two photons are sequentially absorbed by the metal-complex photosensitizer and the semiconductor, resulting in both high oxidation and reduction power. Various types of dye-sensitized molecular photocathodes comprising the p-type semiconductor electrodes and the supramolecular photocatalysts were developed. Full photoelectrochemical cells combining these dye-sensitized molecular photocathodes and n-type semiconductor photoanodes achieved CO2 reduction using only visible light as the energy source and water as the reductant. Drastic improvement of dye-sensitized molecular photocathodes is reported.The results presented in this Account clearly indicate that we can construct very efficient, selective, and durable photocatalytic systems constructed with the metal-complex photosensitizers and catalysts. The supramolecular-photocatalyst architecture in which the photosensitizer and the catalyst are connected to each other is useful especially on the surface of solid owing to rapid electron transfer from the photosensitizer to the catalyst. On basis of these findings, we successfully constructed hybrid systems of the supramolecular photocatalysts with photoactive solid materials. These hybridizations can add new functions to the metal-complex photocatalytic systems, such as water oxidation and light harvesting.

Photocatalysis of a Dinuclear Ru(II)-Re(I) Complex for CO2 Reduction on a Solid Surface.

The development of CO2-reduction photocatalysts is one of the main targets in the field of artificial photosynthesis. Recently, numerous hybrid systems in which supramolecular photocatalysts comprised of a photosensitizer and catalytic-metal-complex units are immobilized on inorganic solid materials, such as semiconductors or mesoporous organosilica, have been reported as CO2-reduction photocatalysts for various functions, including water oxidation and light harvesting. In the present study, we investigated the photocatalytic properties of supramolecular photocatalysts comprised of a Ru(II)-complex photosensitizer and a Re(I)-complex catalyst fixed on the surface of insulating Al2O3 particles: the distance among the supramolecular photocatalyst molecules should be fixed. Visible-light irradiation of the photocatalyst in the presence of an electron donor under a CO2 atmosphere produced CO selectively. Although CO formation was also observed for a 1:1 mixture of mononuclear Ru(II) and Re(I) complexes attached to an Al2O3 surface, the photocatalytic activity was much lower. The activity of the Al2O3-supported photocatalyst was strongly dependent on the adsorption density of the supramolecular moiety, where the initial rate of photocatalytic CO formation was faster at lower density and higher photocatalyst durability was achieved at higher density. One of the main reasons for the former phenomenon is the decreased quenching fraction of the excited state of the photosensitizer unit by the reductant dissolved in the solution phase in the case of higher density. This is due to the self-quenching of the excited photosensitizer unit and steric hindrance between the condensed supramolecular photocatalyst molecules attached to the surface. The higher durability of the more condensed system is caused by intermolecular electron transfer between reduced supramolecular photocatalyst molecules, which accelerates the formation of CO in the photocatalytic CO2 reduction. Coadsorption of a Ru(II) mononuclear complex as a redox photosensitizer could drastically reinforce the photocatalysis of the supramolecular photocatalyst on the surface of the Al2O3 particles: more than 10 times higher turnover number and about 3.4 times higher turnover frequency of CO formation. These investigations provide new architectures for the construction of efficient and durable hybrid photocatalytic systems for CO2 reduction, which are composed of metal-complex photocatalysts and solid materials.

Factors determining formation efficiencies of one-electron-reduced species of redox photosensitizers.

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 -ΔGbet, 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 -ΔGbet; the escape yield in the case of RuH2+ (-ΔGbet = 1.68 eV) was 5-11 times higher compared to those of OsH2+ (-ΔGbet = 1.60 eV) and OsMe2+ (-ΔGbet = 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 -ΔGbet 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.

Kinetics and Mechanism of Intramolecular Electron Transfer in Ru(II)-Re(I) Supramolecular CO2-Reduction Photocatalysts: Effects of Bridging Ligands.

The supramolecular photocatalysts in which a Ru(II) complex as a molecular redox photosensitizer unit and a Re(I) complex as a molecular catalyst unit are connected with a various alkyl or ether chain have attracted attention because they can efficiently photocatalyze CO2 reduction with high durability and high selectivity of CO formation, especially on various solid materials such as semiconductor electrodes and mesoporous organosilica. The intramolecular electron transfer from the one-electron reduced photosensitizer unit to the catalyst unit, which follows excitation of the photosensitizer unit and subsequent reductive quenching of the excited photosensitizer unit by a reductant, is one of the most important processes in the photocatalytic reduction of CO2. We succeeded in determining the rate constants of this intramolecular electron transfer process by using subnanosecond time-resolved IR spectroscopy. The logarithm of rate constants shows a linear relationship with the lengths of the bridging chain in the supramolecular photocatalysts with one bridging alkyl or ether chain. In conformity with the exponential decay of the wave function and the coupling element in the long-distance electron transfer, the apparent decay coefficient factor (ß) in the supramolecular photocatalysts with one bridging chain was determined to be 0.74 Å-1. In the supramolecular photocatalyst with two ethylene chains connecting between the photosensitizer and catalyst units, on the other hand, the intramolecular electron transfer rate is much faster than that with only one ethylene chain. These results strongly indicate that the intramolecular electron transfer from the one-electron reduced species of the Ru photosensitizer unit to the Re catalyst unit proceeds by the through-bond mechanism.

A High-Valent Metal-Oxo Species Produced by Photoinduced One-Electron, Two-Proton Transfer Reactivity.

Described herein is a photochemical approach to the generation of a high-valent metal-oxo species that utilizes a chromophore or "sensitizer", a semiconducting electron acceptor, and a redox buffer that poises a catalyst's initial protonation and oxidation state. The photoexcited sensitizer injects an electron into the semiconductor and then oxidizes the catalyst whose reactivity occurs in kinetic competition with back electron transfer. Core-shell SnO2/TiO2 semiconductor nanocrystallites inhibited charge recombination relative to TiO2 acceptors. With low sensitizer-catalyst surface coverages, a novel trapping process is exploited that enables catalysis reactivity to be quantified on time scales ranging from nanoseconds to minutes. A proof-of-principle example provides the demonstration of a light-initiated, (1e-, 2H+)-transfer reaction, with an inverse isotope effect of kH/kD = 0.63, to generate a Ru(IV) oxo species.

Supramolecular photocatalysts constructed with a photosensitizer unit with two tridentate ligands for CO2 reduction.

New supramolecular photocatalysts comprising an asymmetric bis-tridentate Ru(ii) complex that functions as a photosensitizer and a Ru(ii) carbonyl complex as the catalyst were designed. The complexes photocatalyzed the reduction of CO2 to CO or formic acid with high selectivity. The product distribution depended on the catalyst unit. CO and formic acid were the main products when using [Ru(BL)(Clbpy)(CO)]2+ (BL = bridging ligand, Clbpy = 4,4'-dichloro-2,2'-bipyridine) and Ru(BL)(CO)2Cl2 catalysts, respectively.

Photocatalytic Reduction of Low Concentration of CO2.

A novel molecular photocatalytic system with not only high reduction ability of CO2 but also high capture ability of CO2 has been developed using a Ru(II)-Re(I) dinuclear complex as a photocatalyst. By using this photocatalytic system, CO2 of 10% concentration could be selectively converted to CO with almost same photocatalysis to that under a pure CO2 atmosphere (TONCO > 1000, ΦCO > 0.4). Even 0.5% concentration of CO2 was reduced with 60% initial efficiency of CO formation by using the same system compared to that using pure CO2 (TONCO > 200). The Re(I) catalyst unit in the photocatalyst can efficiently capture CO2, which proceeds CO2 insertion to the Re-O bond, and then reduce the captured CO2 by using an electron supplied from the photochemically reduced Ru photosensitizer unit.

Design and Synthesis of Heteroleptic Cyclometalated Iridium(III) Complexes Containing Quinoline-Type Ligands that Exhibit Dual Phosphorescence.

The design and synthesis of some cyclometalated iridium(III) complexes containing quinoline-type ligands as ancillary ligands are reported. The emission spectra of Ir(III) complexes containing a quinolinolate (6, 8, 10) moiety exhibit a single emission peak at ca. 590 nm, resulting in a red colored emission. However, Ir(III) complexes containing 8-sulfonamidoquinoline ligands (11, 13-21) exhibit two different emission peaks (dual emission) at ca. 500 nm and ca. 600 nm upon excitation at 366 nm, resulting in a red-colored emission for 11 and a pale yellow-colored emission for 14-18 at 298 K. Especially, a white emission was observed for 19 at 298 and 77 K in dimethyl sulfoxide. The mechanistic studies based on time-dependent density functional theory calculations and time-resolved emission spectroscopy suggest that this dual emission originates from two independent emission states.

Analysis of Homogeneous Water Oxidation Catalysis with Collector-Generator Cells.

A collector-generator (C-G) technique has been applied to determine the Faradaic efficiencies for electrocatalytic O2 production by the homogeneous water oxidation catalysts Ru(bda)(isoq)2 (1; bda = 2,2'-bipyridine and isoq = isoquinoline) and [Ru(tpy)(bpz)(OH2)](2+) (2; tpy = 2,2':6',2″-terpyridine and bpz = 2,2'-bipyrazine). This technique uses a custom-fabricated cell consisting of two fluorine-doped tin oxide (FTO) working electrodes separated by 1 mm with the conductive sides facing each other. With a catalyst in solution, water oxidation occurs at one FTO electrode under a sufficient bias to drive O2 formation by the catalyst; the O2 formed then diffuses to the second FTO electrode poised at a potential sufficiently negative to drive O2 reduction. A comparison of the current versus time response at each electrode enables determination of the Faradaic efficiency for O2 production with high concentrations of supporting electrolyte important for avoiding capacitance effects between the electrodes. The C-G technique was applied to electrocatalytic water oxidation by 1 in the presence of the electron-transfer mediator Ru(bpy)3(2+) in both unbuffered aqueous solutions and with the added buffer bases HCO3(-), HPO4(2-), imidazole, 1-methylimidazole, and 4-methoxypyridine. HCO3(-) and HPO4(2-) facilitate water oxidation by atom-proton transfer (APT), which gave Faradaic yields of 100%. With imidazole as the buffer base, coordination to the catalyst inhibited water oxidation. 1-Methylimidazole and 4-methoxypyridine gave O2 yields of 55% and 76%, respectively, with the lower Faradaic efficiencies possibly due to competitive C-H oxidation of the bases. O2 evolution by catalyst 2 was evaluated at pH 12 with 0.1 M PO4(3-) and at pH 7 in a 0.1 M H2PO4(-)/HPO4(2-) buffer. At pH 12, at an applied potential of 0.8 V vs SCE, water oxidation by the Ru(IV)(O)(2+) form of the catalyst gave O2 in 73% yield. In a pH 7 solution, water oxidation at 1.4 V vs SCE, which is dominated by Ru(V)(O)(3+), gave O2 with an efficiency of 100%. The lower efficiency for Ru(IV)(O)(2+) at pH 12 may be due to competitive oxidation of a polypyridyl ligand.

Photocatalytic CO2 reduction with high turnover frequency and selectivity of formic acid formation using Ru(II) multinuclear complexes.

Previously undescribed supramolecules constructed with various ratios of two kinds of Ru(II) complexes-a photosensitizer and a catalyst-were synthesized. These complexes can photocatalyze the reduction of CO(2) to formic acid with high selectivity and durability using a wide range of wavelengths of visible light and NADH model compounds as electron donors in a mixed solution of dimethylformamide-triethanolamine. Using a higher ratio of the photosensitizer unit to the catalyst unit led to a higher yield of formic acid. In particular, of the reported photocatalysts, a trinuclear complex with two photosensitizer units and one catalyst unit photocatalyzed CO(2) reduction (Φ(HCOOH) = 0.061, TON(HCOOH) = 671) with the fastest reaction rate (TOF(HCOOH) = 11.6 min(-1)). On the other hand, photocatalyses of a mixed system containing two kinds of model mononuclear Ru(II) complexes, and supramolecules with a higher ratio of the catalyst unit were much less efficient, and black oligomers and polymers were produced from the Ru complexes during photocatalytic reactions, which reduced the yield of formic acid. The photocatalytic formation of formic acid using the supramolecules described herein proceeds via two sequential processes: the photochemical reduction of the photosensitizer unit by NADH model compounds and intramolecular electron transfer to the catalyst unit.

One-electron activation of water oxidation catalysis.

Rapid water oxidation catalysis is observed following electrochemical oxidation of [Ru(II)(tpy)(bpz)(OH)](+) to [Ru(V)(tpy)(bpz)(O)](3+) in basic solutions with added buffers. Under these conditions, water oxidation is dominated by base-assisted Atom Proton Transfer (APT) and direct reaction with OH(-). More importantly, we report here that the Ru(IV)═O(2+) form of the catalyst, produced by 1e(-) oxidation of [Ru(II)(tpy)(bpz)(OH2)](2+) to Ru(III) followed by disproportionation to [Ru(IV)(tpy)(bpz)(O)](2+) and [Ru(II)(tpy)(bpz)(OH2)](2+), is also a competent water oxidation catalyst. The rate of water oxidation by [Ru(IV)(tpy)(bpz)(O)](2+) is greatly accelerated with added PO4(3-) with a turnover frequency of 5.4 s(-1) reached at pH 11.6 with 1 M PO4(3-) at an overpotential of only 180 mV.

Overall reaction mechanism of photocatalytic CO2 reduction on a Re(i)-complex catalyst unit of a Ru(ii)-Re(i) supramolecular photocatalyst.

Rhenium(i) complexes fac-[ReI(diimine)(CO)3(L)]n+ are mostly used and evaluated as photocatalysts and catalysts in both photochemical and electrochemical systems for CO2 reduction. However, the selective reduction mechanism of CO2 to CO is unclear, although numerous mechanistic studies have been reported. A Ru(ii)-Re(i) supramolecular photocatalyst with fac-[ReI(diimine)(CO)3{OC(O)OCH2CH2NR2}] (R = C2H4OH) as a catalyst unit (RuC2Re) exhibits very high efficiency, selectivity, and durability of CO formation in photocatalytic CO2 reduction reactions. In this work, the reaction mechanism of photocatalytic CO2 reduction using RuC2Re is fully clarified. Time-resolved IR (TR-IR) measurements using rapid-scan FT-IR spectroscopy with laser flash photolysis verify the formation of RuC2Re(COOH) with a carboxylic acid unit, i.e., fac-[ReI(diimine)(CO)3(COOH)], in the photocatalytic reaction solution. Additionally, this important intermediate is detected in an actual photocatalytic reaction using steady state irradiation. Kinetics analysis of the TR-IR spectra and DFT calculations demonstrated the reaction mechanism of the conversion of the one-electron reduced species of RuC2Re with a fac-[ReI(diimine˙-)(CO)3{OC(O)OCH2CH2NR2}]- unit, which was produced via the photochemical reduction of RuC2Re by 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH), to RuC2Re(COOH). The kinetics of the recovery processes of the starting complex RuC2Re from RuC2Re(COOH) accompanying the release of CO and OH- was also clarified. As a side reaction of RuC2Re(COOH), a long-lived carboxylate-ester complex with a fac-[ReI(diimine)(CO)3(COOC2H4NR2)] unit, which was produced by the nucleophilic attack of TEOA to one of the carbonyl ligands of RuC2Re(CO) with a fac-[ReI(diimine)(CO)4]+ unit, was formed during the photocatalytic reaction. This complex works not only as a precursor in another minor CO formation process but also as an external photosensitiser that photochemically reduces the other complexes i.e., RuC2Re, RuC2Re(COOH), and the intermediate that is reductively converted to RuC2Re(COOH).

Sodium-Glucose Cotransporter-2 Inhibitors Stabilize Coronary Plaques in Acute Coronary Syndrome With Diabetes Mellitus.

Sodium-glucose cotransporter-2 inhibitors (SGLT2is) are widely used in cardiology and are effective in treating acute coronary syndrome (ACS). Their effects on unstable plaque in patients with ACS remains unclear. This study aimed to examine the effectiveness of SGLT2is in coronary plaque based on optical coherence tomography (OCT) images and the prognosis of ACS with type 2 diabetes mellitus. This retrospective study included 109 patients in the total cohort and 29 patients in the OCT cohort. Based on SGLT2i administration after ACS, the total cohort was categorized into non-SGLT2i (n = 69) and SGLT2i (n = 40) groups. The OCT cohort had 15 and 14 patients in the non-SGLT2i and SGLT2i groups, respectively. The OCT images of unstable plaque were analyzed in nonstented lesions during ACS catheterization and at the 6-month follow-up. The total cohort was assessed after 1 year for major adverse cardiovascular events, including all-cause mortality, revascularization, cerebrovascular disease, and heart failure hospitalization. SGLT2is improved unstable lesions with a significantly thicker fibrous cap (48 ± 15 µm vs 26 ± 24 µm, p = 0.005), reduced lipid arc (-29 ± 12° vs -18 ± 14°, p = 0.028), higher % decrease in total lipid arc (-35 ± 13% vs -19 ± 18%, p = 0.01), and lower major adverse cardiovascular event incidence (log-rank p = 0.023, hazard ratio 4.72 [1.08 to 20.63]) and revascularization rate (adjusted hazard ratio 6.77 [1.08 to 42.52]) than the non-SGLT2i group. In conclusion, SGLT2is can improve the markers of plaque stability and may improve the prognosis in patients with type 2 diabetes mellitus.

Red-light-driven photocatalytic reduction of CO2 using Os(II)-Re(I) supramolecular complexes.

The novel supramolecular complexes, which are composed of an [Os(5dmb)2(BL)](2+)-type complex (5dmb = 5,5'-dimethyl-2,2'-bipyridine; BL = 1,2-bis(4'-methyl-[2,2'-bipyridin]-4-yl)ethane) as a photosensitizer and cis,trans-[Re(BL)(CO)2{P(p-X-C6H4)3}2](+)-type complexes (X = F, Cl) as a catalyst, have been synthesized. They photocatalyzed selective reduction of CO2 to CO under red-light irradiation (λ > 620 nm). The photocatalytic abilities were affected by the phosphine ligands on the Re unit, and the supramolecule with P(p-Cl-C6H4)3 ligands exhibited better photocatalysis (ΦCO = 0.12, TONCO = 1138, TOFCO = 3.3 min(-1)). The detailed studies clarified the electron balance and material balance; i.e., one molecule of the sacrificial electron donor (1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH)) donated two electrons, one molecule of CO2 accepted the two electrons, and another CO2 molecule served as an "O(2-)" acceptor to give each molecule of the two-electron oxidized compound of BIH, CO, and HCO3(-).

A case of chronic total occlusion in popliteal artery recanalized by double snare piercing technique.

BACKGROUND: Although majority of cases with chronic total occlusion (CTO) in femoro-popliteal lesion were treated with antegrade approach only, some lesions require alternative approach due to its complexity. Bi-directional approach is useful on endovascular therapy (EVT) for CTO; however guidewire passage through the lesion is impossible in some challenging cases. The present case shows a successful re-entry technique utilizing two snare catheters from an antegrade and retrograde access site (double snare piecing technique). CASE PRESENTATION: A 79-year-old woman with right leg intermittent claudication (Rutherford category IV), who had undergone unsuccessful EVT for popliteal CTO, required another EVT for the worsening symptom. Following the failed conventional crossing technique (wire knuckle technique, intravascular-ultrasound-guided wiring, and controlled antegrade and retrograde subintimal tracking technique), two snare catheters were placed and the snare loops were pierced by a puncture needle percutaneously. After an 0.014 wire was inserted into the needle, the needle was withdrawn. The wire was pulled from the retrograde side and was externalized. Then, the antegrade snare catheter was pulled and externalized, to make the wire across the lesion. After that, a microcatheter was advanced along the externalized wire from the retrograde side and cross the lesion. The wire was replaced with a new wire, which completely created pull-through system. After the hemostasis by balloon inflation and lesion preparation, this procedure was completed with an endoluminal-covered stent and two inter-woven stents. The re-entry site was covered by the inter-woven stent. Her symptoms improved after the procedure, and the lesion has not developed restenosis at 2-years follow-up. CONCLUSIONS: This re-entry technique of puncturing two snare loops (double snare piercing technique) might be effective for achieving successful passage through challenging femoropopliteal CTO cases.

Supramolecular multi-electron redox photosensitisers comprising a ring-shaped Re(i) tetranuclear complex and a polyoxometalate.

Redox photosensitisers (PSs) play essential roles in various photocatalytic reactions. Herein, we synthesised new redox PSs of 1 : 1 supramolecules that comprise a ring-shaped Re(i) tetranuclear complex with 4+ charges and a Keggin-type heteropolyoxometalate with 4- charges. These PSs photochemically accumulate multi-electrons in one molecule (three or four electrons) in the presence of an electron donor and can supply electrons with different reduction potentials. PSs were successfully applied in the photocatalytic reduction of CO2 using catalysts (Ru(ii) and Re(i) complexes) and triethanolamine as a reductant. In photocatalytic reactions, these supramolecular PSs supply a different number of electrons to the catalyst depending on the redox potential of the intermediate, which is made from the one-electron-reduced species of the catalyst and CO2. Based on these data, information on the reduction potentials of the intermediates was obtained.

Surface-Specific Modification of Graphitic Carbon Nitride by Plasma for Enhanced Durability and Selectivity of Photocatalytic CO2 Reduction with a Supramolecular Photocatalyst.

Photocatalytic CO2 reduction is in high demand for sustainable energy management. Hybrid photocatalysts combining semiconductors with supramolecular photocatalysts represent a powerful strategy for constructing visible-light-driven CO2 reduction systems with strong oxidation power. Here, we demonstrate the novel effects of plasma surface modification of graphitic carbon nitride (C3N4), which is an organic semiconductor, to achieve better affinity and electron transfer at the interface of a hybrid photocatalyst consisting of C3N4 and a Ru(II)-Ru(II) binuclear complex (RuRu'). This plasma treatment enabled the "surface-specific" introduction of oxygen functional groups via the formation of a carbon layer, which worked as active sites for adsorbing metal-complex molecules with methyl phosphonic-acid anchoring groups onto the plasma-modified surface of C3N4. Upon photocatalytic CO2 reduction with the hybrid under visible-light irradiation, the plasma-surface-modified C3N4 with RuRu' enhanced the durability of HCOOH production by three times compared to that achieved when using a nonmodified system. The high selectivity of HCOOH production against byproduct evolution (H2 and CO) was improved, and the turnover number of HCOOH production based on the RuRu' used reached 50 000, which is the highest among the metal-complex/semiconductor hybrid systems reported thus far. The improved activity is mainly attributed to the promotion of electron transfer from C3N4 to RuRu' under light irradiation via the accumulation of electrons trapped in deep defect sites on the plasma-modified surface of C3N4.

Photocatalyzed CO2 reduction to CO by supramolecular photocatalysts made of Ru(II) photosensitizers and Re(I) catalytic subunits containing preformed CO2TEOA adducts.

Two new supramolecular photocatalysts containing Ru(II) polypyridine units as light-harvesting photosensitizers and Re(I) polypyridine subunits as catalytic centers have been prepared. The new species, RuRe2A and Ru2ReA, contain catalytic Re(I) subunits coordinated by the preformed CO2TEOA adduct (known to be the effective catalytic subunits; TEOA is triethanolamine) and exhibit quite efficient and selective photoreduction of CO2 to CO, with outstanding TONs of 2368 and 2695 and a selectivity of 99.9% and 98.9%, respectively. Such photocatalytic properties are significantly improved with respect to those of previously studied RuRe2 and Ru2Re parent compounds, containing chloride ligands instead of the CO2TEOA adduct. Comparison between photocatalytic performance of the new species and their parent compounds allows to investigate the effect of the CO2TEOA insertion process as well as the eventual effect of the presence of chloride ions in solution on the photocatalytic processes. The improved photocatalytic properties of RuRe2A and Ru2ReA compared with their parent species are attributed to a combined effect of different distribution of the one-electron reduced form of the supramolecular photocatalysts on the Ru-subunit(s) (leading to decreased CO formation due to a poisoning ligand loss process) and on the Re-subunit(s) and to the presence of chloride ions in solution for RuRe2 and Ru2Re, which could interfere with the CO2TEOA adduct formation, a needed requisite for CO forming catalysis. These results strongly indicate the utility of preparing supramolecular photocatalysts containing preformed adducts.

Correction to "Highly Efficient Supramolecular Photocatalyst for CO2 Reduction with Eight Carbon-Carbon Bonds between Ru(II) Photosensitizer and Re(I) Catalyst Unit".

[This corrects the article DOI: 10.1021/acscatal.3c01407.].

Selective electrochemical CO2 conversion with a hybrid polyoxometalate.

A multi-component coordination compound, in which ruthenium antenna complexes are connected to a polyoxotungstate core is presented. This hybrid cluster effectively promotes the electrochemical conversion of CO2 to C1 feedstocks, the selectivity of which can be controlled by the acidity of the media.