Reaction mechanisms of catalytic photochemical CO2 reduction using Re(I) and Ru(II) complexes

Asymmetric Atomic Dual-Sites for Photocatalytic CO2 Reduction

Atomically dispersed active sites in a photocatalyst offer unique advantages such as locally tuned electronic structures, quantum size effects, and maximum utilization of atomic species. Among these, asymmetric atomic dual-sites are of particular interest because their asymmetric charge distribution generates a local built-in electric potential to enhance charge separation and transfer. Moreover, the dual sites provide flexibility for tuning complex multielectron and multireaction pathways, such as CO2 reduction reactions. The coordination of dual sites opens new possibilities for engineering the structure-activity-selectivity relationship. This comprehensive overview discusses efficient and sustainable photocatalysis processes in photocatalytic CO2 reduction, focusing on strategic active-site design and future challenges. It serves as a timely reference for the design and development of photocatalytic conversion processes, specifically exploring the utilization of asymmetric atomic dual-sites for complex photocatalytic conversion pathways, here exemplified by the conversion of CO2 into valuable chemicals.

Photocatalytic Conversion of CO2 to Formate/CO by an (η6-para-Cymene)Ru(II) Half-Metallocene Catalyst: Influence of Additives and TiO2 Immobilization on the Catalytic Mechanism and Product Selectivity

The catalytic efficacy of the monobipyridyl (η6-para-Cymene)Ru(II) half-metallocene, [(p-Cym)Ru(bpy)Cl]+ was evaluated in both mixed homogeneous (dye + catalyst) and heterogeneous hybrid systems (dye/TiO2/Catalyst) for photochemical CO2 reduction. A series of homogeneous photolysis experiments revealed that the (p-Cym)Ru(II) catalyst engages in two competitive routes for CO2 reduction (CO2 to formate conversion via RuII-hydride vs CO2 to CO conversion through a RuII-COOH intermediate). The conversion activity and product selectivity were notably impacted by the pKa value and the concentration of the proton source added. When a more acidic TEOA additive was introduced, the half-metallocene Ru(II) catalyst leaned toward producing formate through the RuII-H mechanism, with a formate selectivity of 86%. On the other hand, in homogeneous catalysis with TFE additive, the CO2-to-formate conversion through RuII-H was less effective, yielding a more efficient CO2-to-CO conversion with a selectivity of >80% (TONformate of 140 and TONCO of 626 over 48 h). The preference between the two pathways was elucidated through an electrochemical mechanistic study, monitoring the fate of the metal-hydride intermediate. Compared to the homogeneous system, the TiO2-heterogenized (p-Cym)Ru(II) catalyst demonstrated enhanced and enduring performance, attaining TONs of 1000 for CO2-to-CO and 665 for CO2-to-formate.

Efficient Visible-Light-Driven Carbon Dioxide Reduction using a Bioinspired Nickel Molecular Catalyst

Inspired by natural enzymes, this study presents a nickel-based molecular catalyst, [Ni‖(N2S2)]Cl2 (NiN2S2, N2S2=2,11-dithia[3,3](2,6)pyridinophane), for the photochemical catalytic reduction of CO2 under visible light. The catalyst was synthesized and characterized using various techniques, including liquid chromatography-high resolution mass spectrometry (LC-HRMS), UV-Visible spectroscopy, and X-ray crystallography. The crystallographic analysis revealed a slightly distorted octahedral coordination geometry with a mononuclear Ni2+ cation, two nitrogen atoms and two sulfur atoms. Photocatalytic CO2 reduction experiments were performed in homogeneous conditions using the catalyst in combination with [Ru(bpy)3]Cl2 (bpy=2,2'-bipyridine) as a photosensitizer and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as a sacrificial electron donor. The catalyst achieved a high selectivity of 89 % towards CO and a remarkable turnover number (TON) of 7991 during 8 h of visible light irradiation under CO2 in the presence of phenol as a co-substrate. The turnover frequency (TOF) in the initial 6 h was 1079 h-1, with an apparent quantum yield (AQY) of 1.08 %. Controlled experiments confirmed the dependency on the catalyst, light, and sacrificial electron donor for the CO2 reduction process. These findings demonstrate this bioinspired nickel molecular catalyst could be effective for fast and efficient photochemical catalytic reduction of CO2 to CO.

Factors that Impact Photochemical Cage Escape Yields

The utilization of visible light to mediate chemical reactions in fluid solutions has applications that range from solar fuel production to medicine and organic synthesis. These reactions are typically initiated by electron transfer between a photoexcited dye molecule (a photosensitizer) and a redox-active quencher to yield radical pairs that are intimately associated within a solvent cage. Many of these radicals undergo rapid thermodynamically favored "geminate" recombination and do not diffuse out of the solvent cage that surrounds them. Those that do escape the cage are useful reagents that may undergo subsequent reactions important to the above-mentioned applications. The cage escape process and the factors that determine the yields remain poorly understood despite decades of research motivated by their practical and fundamental importance. Herein, state-of-the-art research on light-induced electron transfer and cage escape that has appeared since the seminal 1972 review by J. P. Lorand entitled "The Cage Effect" is reviewed. This review also provides some background for those new to the field and discusses the cage escape process of both homolytic bond photodissociation and bimolecular light induced electron transfer reactions. The review concludes with some key goals and directions for future research that promise to elevate this very vibrant field to even greater heights.

Photosensitizing metal covalent organic framework with fast charge transfer dynamics for efficient CO2 photoreduction

Designing artificial photocatalysts for CO2 reduction is challenging, mainly due to the intrinsic difficulty of making multiple functional units cooperate efficiently. Herein, three-dimensional metal covalent organic frameworks (3D MCOFs) were employed as an innovative platform to integrate a strong Ru(ii) light-harvesting unit, an active Re(i) catalytic center, and an efficient charge separation configuration for photocatalysis. The photosensitive moiety was precisely stabilized into the covalent skeleton by using a rational-designed Ru(ii) complex as one of the building units, while the Re(i) center was linked via a shared bridging ligand with an Ru(ii) center, opening an effective pathway for their electronic interaction. Remarkably, the as-synthesized MCOF exhibited impressive CO2 photoreduction activity with a CO generation rate as high as 1840 μmol g-1 h-1 and 97.7% selectivity. The femtosecond transient absorption spectroscopy combined with theoretical calculations uncovered the fast charge-transfer dynamics occurring between the photoactive and catalytic centers, providing a comprehensive understanding of the photocatalytic mechanism. This work offers in-depth insight into the design of MCOF-based photocatalysts for solar energy utilization.

Methane Formation Induced via Face-to-Face Orientation of Cyclic Fe Porphyrin Dimer in Photocatalytic CO2 Reduction

Iron porphyrins are known to provide CH4 as an eight-electron reduction product of CO2 in a photochemical reaction. However, there are still some aspects of the reaction mechanism that remain unclear. In this study, we synthesized iron porphyrin dimers and carried out the photochemical CO2 reduction reactions in N,N-dimethylacetamide (DMA) containing a photosensitizer in the presence of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as an electron donor. We found that, despite a low catalytic turnover number, CH4 was produced only when these porphyrins were facing each other. The close proximity of the cyclic dimers, distinguishing them from a linear Fe porphyrin dimer and monomers, induced multi-electron CO2 reduction, emphasizing the unique role of their structural arrangement in CH4 formation.

Trinuclear Cu-based covalent organic framework: π-conjugated framework regulating electron delocalization to promote photoreduction CO2

Sunlight-driven CO2 reduction to value-added chemicals is an effective strategy to promote carbon recycling. The exploration of catalysts with efficient charge separation is crucially important for highly efficient CO2 photoreduction. In this work, the preparation of metal-cluster-based covalent organic framework (CuABD) integrated features from both metal organic frameworks (MOFs) and covalent organic frameworks (COFs) through the condensation of diamines and functionalized trinuclear copper clusters demonstrate a thoughtful design strategy. The reported yield of 1.3 mmol g-1 h-1 for formic acid (HCOOH) under simulated solar irradiation is impressive, surpassing the performance of many COF- and MOF-based catalysts previously reported. Compared to its isomorphic metal-free structure (named BDFTD) and bare trinuclear Cu cluster which present extremely poor catalytic activities, CuABD displays remarkably enhanced CO2 reduction activity. Experimental and theoretical investigations reveal that the efficient charge transfer between diamine monomer and cyclic trinuclear copper (I) units, and the electron delocalization of the π-conjugated framework are responsible for the appealing catalytic performance. In summary, the work presents a well-structured and scientifically sound exploration of a metal-cluster-based covalent organic framework for efficient CO2 reduction under sunlight.

Tuning Photoredox Catalysis of Ruthenium with Palladium: Synthesis of Heterobimetallic Ru-Pd Complexes That Enable Efficient Photochemical Reduction of CO2

New Ru-Pd heterobimetallic complexes were synthesized and structurally characterized utilizing 6,6″-bis(phosphino)-2,2':6',2″-terpyridine as a scaffold for the metal-metal bond. The dicationic Ru-Pd complex was found to exhibit high catalytic activity as a photocatalyst for photochemical reduction of CO2 to CO under visible light irradiation. This study established a new design of transition metal catalysts that tune photoredox catalysis with metalloligands.

Modeling and experimental analysis of CO2 methanation reaction using Ni/CeO2 monolithic catalyst

In this study, the effect of the cell density of monolithic catalysts was investigated and further mathematically modeled on cordierite supports used in CO2 methanation. Commercial cordierite monoliths with 200, 400, and 500 cpsi cell densities were coated by immersion into an ethanolic suspension of Ni/CeO2 active phase. SEM-EDS analysis confirmed that, owing to the low porosity of cordierite (surface area < 1 m2 g-1), the Ni/CeO2 diffusion into the walls was limited, especially in the case of low and intermediate cell density monoliths; thus, active phase was predominantly loaded onto the channels' external surface. Nevertheless, despite the larger exposed surface area in the monolith with high cell density, which would allow for better distribution and accessibility of Ni/CeO2, its higher macro-pore volume resulted in some introduction of the active phase into the walls. As a result, the catalytic evaluation showed that it was more influenced by increments in volumetric flow rates. The low cell density monolith displayed diffusional control at flow rates below 500 mL min-1. In contrast, intermediate and high cell density monoliths presented this behavior up to 300 mL min-1. These findings suggest that the interaction reactants-catalyst is considerably more affected by a forced non-uniform flow when increasing the injection rate. This condition reduced the transport of reactants and products within the catalyst channels and, in turn, increased the minimum temperature required for the reaction. Moreover, a slight diminution of selectivity to CH4 was observed and ascribed to the possible formation of hot spots that activate the reverse water-gas shift reaction. Finally, a mathematical model based on fundamental momentum and mass transfer equations coupled with the kinetics of CO2 methanation was successfully derived and solved to analyze the fluid dynamics of the monolithic support. The results showed a radial profile with maximum fluid velocity located at the center of the channel. A reactive zone close to the inlet was obtained, and maximum methane production (4.5 mol m-3) throughout the monolith was attained at 350 °C. Then, linear streamlines of the chemical species were developed along the channel.

Reduction of CO to Methanol with Recyclable Organic Hydrides

The reaction steps for the selective conversion of a transition metal carbonyl complex to a hydroxymethyl complex that releases methanol upon irradiation with visible light have been successfully quantified in acetonitrile solution with dihydrobenzimidazole organic hydride reductants. Dihydrobenzimidazole reductants have been shown to be inactive toward H2 generation in the presence of a wide range of proton sources and have been regenerated electrochemically or photochemically. Specifically, the reaction of cis-[Ru(bpy)2(CO)2]2+ (bpy = 2,2'-bipyridine) with one equivalent of a dihydrobenzimidazole quantitatively yields a formyl complex, cis-[Ru(bpy)2(CO)(CHO)]+, and the corresponding benzimidazolium on a seconds time scale. Kinetic experiments revealed a first-order dependence on the benzimidazole hydride concentration and an unusually large kinetic isotope effect, inconsistent with direct hydride transfer and more likely to occur by an electron transfer-proton-coupled electron transfer (EΤ-PCET) or related mechanism. Further reduction/protonation of cis-[Ru(bpy)2(CO)(CHO)]+ with two equivalents of the organic hydride yields the hydroxymethyl complex cis-[Ru(bpy)2(CO)(CH2OH)]+. Visible light excitation of cis-[Ru(bpy)2(CO)(CH2OH)]+ in the presence of excess organic hydride was shown to yield free methanol. Identification and quantification of methanol as the sole CO reduction product was confirmed by 1H NMR spectroscopy and gas chromatography. The high selectivity and mild reaction conditions suggest a viable approach for methanol production from CO, and from CO2 through cascade catalysis, with renewable organic hydrides that bear similarities to Nature's NADPH/NADP+.

Photocatalytic conversion of CO2 to CO by Ru(II) and Os(II) octahedral complexes: a DFT/TDDFT study

The reaction mechanisms of the photocatalytic reduction of CO2 to CO catalyzed by [(en)M(CO)3Cl] complexes (M = Ru, Os, en = ethylenediamine) in the presence of triethanolamine (TEOA), R3N (R = -CH2CH2OH), in DCM and DMF solvents, were studied by means of DFT/TDDFT electronic structure calculations. The geometric and free energy reaction profiles for two possible reaction pathways were calculated. Both reaction pathways studied, start with the 17e-, catalytically active intermediate, [(en)M(CO)3]˙+ generated from the first triplet excited state, T1 upon reductive quenching by TEOA which acts as a sacrificial electron donor. In the first possible pathway, TEOA- anion binds to the metal center of the catalytically active intermediate, [(en)M(CO)3]˙+ followed by CO2 insertion into the M-OCH2CH2NR2 bond. The latter upon successive protonations releases a metal 'free' [R2NCH2CH2OC(O)(OH)] intermediate which starts a new and final catalytic cycle, leading to the formation of CO and H2O while regenarating TEOA. In the second possible pathway, the 17e-, catalytically active intermediate, [(en)M(CO)3]˙+ captures CO2 molecule, forming an η1-CO2 complex. Upon 2H+/2e- successive protonations and reductions, CO product is obtained along with regenarating the catalytically active intermediate [(en)M(CO)3]˙+. The nature of the proton donor affects the reaction profiles of both mechanisms. The nature of the solvent does not affect significantly the reaction mechanisms under study. Finally, since photoexcitation and T1 reductive quenching are common to both pathways, we have srutinized the photophysical properties of the [(en)M(CO)3Cl] complexes along with their T1 excited states reduction potentials, . The [(en)M(CO)3Cl] complexes absorb mainly in the UV region while the absolute are in the range 6.4-0.9 eV.

Structural and Photophysical Trends in Rhenium(I) Carbonyl Complexes with 2,2':6',2″-Terpyridines

This is the first comprehensive review of rhenium(I) carbonyl complexes with 2,2':6',2″-terpyridine-based ligands (R-terpy)-encompassing their synthesis, molecular features, photophysical behavior, and potential applications. Particular attention has been devoted to demonstrating how the coordination mode of 2,2':6',2″-terpyridine (terpy-κ2N and terpy-κ3N), structural modifications of terpy framework (R), and the nature of ancillary ligands (X-mono-negative anion, L-neutral ligand) may tune the photophysical behavior of Re(I) complexes [Re(X/L)(CO)3(R-terpy-κ2N)]0/+ and [Re(X/L)(CO)2(R-terpy-κ3N)]0/+. Our discussion also includes homo- and heteronuclear multicomponent systems with {Re(CO)3(R-terpy-κ2N)} and {Re(CO)2(R-terpy-κ3N)} motifs. The presented structure-property relationships are of high importance for controlling the photoinduced processes in these systems and making further progress in the development of more efficient Re-based luminophores, photosensitizers, and photocatalysts for modern technologies.

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).

Assembling of a Water-Soluble N^C^N-Coordinated Pt(II) Complex Aggregate Assisted by Carbon Dioxide in Basic Aqueous Solution

We report an unprecedented result of self-aggregation of [Pt(L1 )Cl] (HL1 =1,3-di(5-carboxy-2-pyridyl)benzene) triggered by CO2 in basic aqueous solution. The color of basic aqueous solution containing [Pt(L1 )Cl] changes from yellow to blue-green during the aggregation resulted from a reaction with CO2 in air. Upon CO2 gas bubbling, strong and broad absorption bands of aggregate assigned to the metal-metal-to-ligand charge-transfer transition appeared at 701 and 1152 nm. Recrystallization of [Pt(L1 )Cl] from Na2 CO3 aqueous solution afforded polymorphic crystals of red and blue-green forms. A single X-ray crystallography revealed that the red form of crystal consists of a Pt-Pt stacked dimer bridged by CO3 2- ion and one of the carboxy groups of L1 is deprotonated. An elemental analysis provided evidence that the blue-green crystal is constructed by linear array consisting of the [Pt(L2 )(CO3 )]3- (HL2 =1,3-di(5-carboxylate-2-pyridyl)benzene) units. The formation process of blue-green aggregate in aqueous solution was monitored through a transient absorption spectrum, and the absorption of aggregates involved in the spectral change were examined by a global analysis. A singular value decomposition and kinetic analysis provide that there are four species resulted from the self-assembling reaction in the solution and the maximal degree of aggregation is at least 32-mer.

Visible light-driven CO2 photoreduction by a Re(I) complex immobilized onto CuO/Nb2O5 heterojunctions

The immobilization of Re(I) complexes onto metal oxide surfaces presents an elegant strategy to enhance their stability and reusability toward photocatalytic CO2 reduction. In this study, the photocatalytic performance of fac-[ClRe(CO)3(dcbH2)], where dcbH2 = 4,4'-dicarboxylic acid-2,2'-bipyridine, anchored onto the surface of 1%m/m CuO/Nb2O5 was investigated. Following adsorption, the turnover number for CO production (TONCO) in DMF/TEOA increased significantly, from ten in solution to 370 under visible light irradiation, surpassing the TONCO observed for the complex onto pristine Nb2O5 or CuO surfaces. The CuO/Nb2O5 heterostructure allows for efficient electron injection by the Re(I) center, promoting efficient charge separation. At same time CuO clusters introduce a new absorption band above 550 nm that contributes for the photoreduction of the reaction intermediates, leading to a more efficient CO evolution and minimization of side reactions.

Photocatalytic Reduction of CO2 into CO with Cyclometalated Pt(II) Complexes of N^C^N Pincer Dipyridylbenzene Ligands: A DFT Study

In this work, density functional theory (DFT) calculations were employed to study the photocatalytic reduction of CO2 into CO using a series of Pt(II) square planar complexes with the general formula [Pt(5-R-dpb)Cl] (dpb = 1,3-di(2-pyridyl)benzene anion, R = H, N,N-dimethylaniline,T thiophene, diazaborinine). The CO2-into-CO conversion process is thought to proceed via two main steps, namely the photocatalytic/reduction step and the main catalytic step. The simulated absorption spectra exhibit strong bands in the range 280-460 nm of the UV-Vis region. Reductive quenching of the T1 state of the complexes under study is expected to be favorable since the calculated excited state redox potentials for the reaction with sacrificial electron donors are highly positive. The redox potentials reveal that the reductive quenching of the T1 state, important to the overall process, could be modulated by suitable changes in the N^C^N pincer ligands. The CO2 fixation and activation by the three coordinated Pt(II) catalytically active species are predicted to be favorable, with the Pt-CO2 bond dissociation energies D0 in the range of -36.9--10.3 kcal/mol. The nature of the Pt-CO2 bond of the Pt(II) square planar intermediates is complex, with covalent, hyperconjugative and H-bonding interactions prevailing over the repulsive electrostatic interactions. The main catalytic cycle is estimated to be a favorable exergonic process.

Mechanism of photocatalytic CO2 reduction to HCO2H by a robust multifunctional iridium complex

The tetradentate PNNP-type IrIII complex Mes-IrPCY2 ([Cl-IrIII-H]+) is reported to be an efficient catalyst for the reduction of CO2 to formate with excellent selectivity under visible light irradiation. Density functional calculations have been carried out to elucidate the mechanism and the origin of selectivity in the present work. Calculations suggest that the double-reduced complex 1-H (1[IrI-H]0) demonstrates higher activity than the single-reduced complex 2-H (2[IrIII(L˙-)-H]+), possibly owing to the higher hydride donor ability of the former compared to the latter; thus 1-H functions as the active species in the overall CO2 reduction reaction. In the HCOO- formation pathway, the hydride of 1-H performs a nucleophilic attack on CO2via an outer-sphere fashion to generate species 1-OCHO (1[IrI-OCHO]0), which then releases HCOO- to produce an IrI intermediate. A subsequent protonation and chloride coordination of the Ir center leads to the regeneration of catalyst 1[Cl-IrIII-H]+. For the CO production, a nucleophilic attack on CO2 takes place by the Ir atom of 1-Hvia an inner-sphere manner to afford complex O2C-3-H (1[O2C-IrIII-H]0), followed by a two-proton-one-electron reduction to furnish the OC-2-H complex (2[OC-IrIII(L˙-)-H]+) after liberating a H2O. Ultimately, CO is released to form 2-H. The stronger nucleophilicity as well as smaller steric hindrance of the hydride than the Ir atom of the active species 1-H (1[IrI-H]0) is found to account for the favoring of formate formation over CO formation. Meanwhile, the CO2 reduction reaction is calculated to be preferred over the hydrogen evolution reaction, and this is consistent with the experimental product distributions.

1,2-Azolylamidino ruthenium(II) complexes with DMSO ligands: electro- and photocatalysts for CO2 reduction

New 1,2-azolylamidino complexes fac-[RuCl(DMSO)3(NHC(R)az*-κ2N,N)]OTf [R = Me (2), Ph (3); az* = pz (pyrazolyl, a), indz (indazolyl, b)] are synthesized via chloride abstraction from their corresponding precursors cis,fac-[RuCl2(DMSO)3(az*H)] (1) after subsequent base-catalyzed coupling of the appropriate nitrile with the 1,2-azole previously coordinated. All the compounds are characterized by 1H NMR, 13C NMR and IR spectroscopy. Those derived from MeCN are also characterized by X-ray diffraction. Electrochemical studies showed several reduction waves in the range of -1.5 to -3 V. The electrochemical behavior in CO2 media is consistent with CO2 electrocatalytic reduction. The catalytic activity expressed as [icat(CO2)/ip(Ar)] ranged from 1.7 to 3.7 for the 1,2-azolylamidino complexes at voltages of ca. -2.7 to -3 V vs. ferrocene/ferrocenium. Controlled potential electrolysis showed rapid decomposition of the Ru catalysts. Photocatalytic CO2 reduction experiments using compounds 1b, 2b and 3b carried out in a CO2-saturated MeCN/TEOA (4 : 1 v/v) solution containing a mixture of the catalyst and [Ru(bipy)3]2+ as the photosensitizer under continuous irradiation (light intensity of 150 mW cm-2 at 25 °C, λ > 300 nm) show that compounds 1b, 2b and 3b allowed CO2 reduction catalysis, producing CO and trace amounts of formate. The combined turnover number for the production of formate and CO is ca. 100 after 8 h and follows the order 1b < 2b ≈ 3b.

Black Phosphorus-based Photocatalysts: Synthesis, Properties, and Applications

Photocatalysis is considered as an eco-friendly and sustainable strategy, since it uses abundant light for the advancement of the reaction, which is freely accessible and is devoid of environmental pollution. During the last decades, (nano)photocatalysts have gained broad industrial applications in terms of purification and detoxification of water as well as production of green fuels and hydrogen gas due to their special attributes. The degradation or remediation of toxic and hazardous compounds from the environment or changing them into non-toxic entities is a significant endeavor and necessary for the safety of humans, animals, and the environment. Black phosphorus (BP), a two-dimensional single-element material, has a marvelous structure, tunable bandgap, changeable morphology from bulk to nanosheet/quantum dot, and unique physicochemical properties, which makes it attractive material for photocatalytic applications, especially for sustainable development purposes. Since it can serve as a photocatalyst with or without coupling with other semiconductors, various aspects for multidimensional exploitation of BP are deliberated including their preparation via solvothermal, ball milling, calcination, and sonication methods to obtain BP from red phosphorus. The techniques for improving the photocatalytic and stability of BP-based composites are discussed along with their multifaceted applications for environmental remediation, pollution degradation, water splitting, N2 fixation, CO2 reduction, bacterial disinfection, H2 generation, and photodynamic therapy. Herein, most recent advancements pertaining to the photocatalytic applications of BP-based photocatalyst are cogitated, with a focus on their synthesis and properties as well as crucial challenges and future perspectives.

Self-Photosensitizing Dinuclear Ruthenium Catalyst for CO2 Reduction to CO

The promise of artificial photosynthesis to solve environmental and energy issues such as global warming and the depletion of fossil fuels has inspired intensive research into photocatalytic systems for CO2 reduction to produce value-added chemicals such as CO and CH3OH. Among the photocatalytic systems for CO2 reduction, self-photosensitizing catalysts, bearing the functions of both photosensitization and catalysis, have attracted considerable attention recently, as such catalysts do not depend on the efficiency of electron transfer from the photosensitizer to the catalyst. Here, we have synthesized and characterized a dinuclear RuII complex bearing two molecules of a tripodal hexadentate ligand as chelating and linking ligands by X-ray crystallography to establish the structure explicitly and have used various spectroscopic and electrochemical methods to elucidate the photoredox characteristics. The dinuclear complex has been revealed to act as a self-photosensitizing catalyst, which acts not only as a photosensitizer but also as a catalyst for CO2 reduction. The dinuclear RuII complex is highly durable and performs efficient and selective CO2 reduction to produce CO with a turnover number of 2400 for 26 h. The quantum yield of the CO formation is also very high─19.7%─and the catalysis is efficient, even at a low concentration (∼1.5%) of CO2.

Unexpected Redox Chemistry of P∩N- and As∩N-Rhenium(I) Tricarbonyl Complexes in the Presence of CO2 Acting as an Acid

This study reports on Re tricarbonyl complexes bearing 8-(diphenylphosphanyl)quinoline, P∩N, and 8-(diphenylarsanyl)quinoline, As∩N, as bidendate ligands. We studied the reactivity of these complexes in comparison with fac-Re(N∩N)(CO)3Cl (with N∩N = 2,2'-bipyridine or 4,4'-dimethyl-2,2'-bipyridine). We used a combination of electrochemical and spectroelectrochemical methods with time-resolved spectroscopy over 10 orders of magnitude (100 ps-1 s) to investigate the peculiar reactivity of one-electron-reduced Re(CO)3(P∩N)Cl and Re(CO)3(As∩N)Cl complexes also in the presence of protons.

Significance of the connecting position between Zn(ii) porphyrin and Re(i) bipyridine tricarbonyl complex units in dyads for room-temperature phosphorescence and photocatalytic CO2 reduction: unexpected enhancement by triethanolamine in catalytic activity

We synthesized three new dyads composed of a Zn porphyrin and fac-Re(bpy)(CO)3Br (bpy = 2,2'-bipyridine) units, ZnP-nBpy[double bond, length as m-dash]ReBr (n = 4, 5, and 6), in which the porphyrin is directly connected at the meso-position through the 4-, 5-, or 6-position of the bpy. We investigated the relationships between the connecting positions and the photophysical properties as well as catalytic activity in the CO2 reduction reaction. The dyad connected through the 6-position, ZnP-6Bpy[double bond, length as m-dash]ReBr, showed obvious phosphorescence with a lifetime of 280 μs at room temperature, in N,N-dimethylacetamide (DMA), whereas the other two dyads showed almost no phosphorescence under the same conditions. The photocatalytic CO2 reduction reactions in DMA using 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole as the electron donor and the three dyads ZnP-nBpy[double bond, length as m-dash]ReBr selectively produced CO with similar initial rates, but the durabilities were low. The addition of triethanolamine (TEOA) suppressed the decomposition of dyads, improving their durabilities and reaction efficiencies. In particular, ZnP-5Bpy[double bond, length as m-dash]ReBr was remarkably improved-it gave the highest durability and reaction efficiency among the three dyads; the reaction quantum yield reached 24%. The reason for this significant activity is no accumulation of electrons on the Zn porphyrin in ZnP-5Bpy[double bond, length as m-dash]ReBr, which would be caused by dual interactions of TEOA with the Re and Zn ions in the dyad. As the highest catalytic activity was observed in ZnP-5Bpy[double bond, length as m-dash]ReBr among the three dyads, which had no room-temperature phosphorescence (RTP), the catalytic activities and RTP properties are considered independent, but they are greatly influenced by the connecting positions on the bpy ligand in ZnP-nBpy[double bond, length as m-dash]ReBr.

Confining charge-transfer complex in a metal-organic framework for photocatalytic CO2 reduction in water

In the quest for renewable fuel production, the selective conversion of CO₂ to CH₄ under visible light in water is a leading-edge challenge considering the involvement of kinetically sluggish multiple elementary steps. Herein, 1-pyrenebutyric acid is post-synthetically grafted in a defect-engineered Zr-based metal organic framework by replacing exchangeable formate. Then, methyl viologen is incorporated in the confined space of post-modified MOF to achieve donor-acceptor complex, which acts as an antenna to harvest visible light, and regulates electron transfer to the catalytic center (Zr-oxo cluster) to enable visible-light-driven CO₂ reduction reaction. The proximal presence of the charge transfer complex enhances charge transfer kinetics as realized from transient absorption spectroscopy, and the facile electron transfer helps to produce CH₄ from CO₂. The reported material produces 7.3 mmol g^-1 of CH₄ under light irradiation in aqueous medium using sacrificial agents. Mechanistic information gleans from electron paramagnetic resonance, in situ diffuse reflectance FT-IR and density functional theory calculation.

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.

Accumulative Charge Separation in a Modular Quaterpyridine Bridging Ligand Platform and Multielectron Transfer Photocatalysis of π-Linked Dinuclear Ir(III)-Re(I) Complex for CO2 Reduction

Four sterically distorted quaterpyridyl (qpy) ligand-bridged Ir(III)-Re(I) heterometallic complexes (Ir-qpy_mm-Re, Ir-qpy_mp-Re, Ir-qpy_pm-Re, and Ir-qpy_pp-Re), in which the position of the coupling pyridine unit of the two 2,2'-bipyridine ligands was varied (meta (m)- or para (p)-position), pypy_x-py_xpy (x = m and m, qpy_mm; x = m and p, qpy_mp; x = p and m, qpy_pm; x = p and p, qpy_pp), were prepared, along with the fully π-conjugated Ir(III)-[π linker]-Re(I) complexes (π linker = 2,2'-bipyrimidine (bpm), Ir-bpm-Re; π linker = 2,5-di(pyridin-2-yl)pyrazine (dpp), Ir-dpp-Re) to elucidate the electron mediating and accumulative charge separation properties of the bridging π-linker in a bimetallic system (photosensitizer-π linker-catalytic center). From the photophysical and electrochemical studies, it was found that the quaterpyridyl (qpy) bridging ligand (BL), in which the two planar Ir/Re metalated bipyridine (bpy) ligands were connected but slightly canted relative to each other, linking the heteroleptic Ir(III) photosensitizer, [(^piqC^N)₂Ir^III(bpy)]⁺, and catalytic Re(I) complex, (bpy)Re^I(CO)₃Cl, minimized the energy lowering of the qpy BL, which hampers the forward photoinduced electron transfer (PET) process from [(^piqC^N)₂Ir^III(N^N)]⁺ to (N^N)Re^I(CO)₃Cl (E_red1 = -(0.85-0.93) V and E_red2 = -(1.15-1.30) V vs SCE). This result contrasts with the fully π-delocalized bimetallic systems (Ir-bpm-Re and Ir-dpp-Re) that show a significant energy reduction due to the considerable π-extension and deshielding effect caused by the neighboring Lewis acidic metals (Ir and Re) on the electrochemical scale (E_red1 = -0.37 V and E_red2 = -1.02 and -0.99 V vs SCE). Based on a series of anion absorption studies and spectroelectrochemical (SEC) analyses, all Ir(III)-BL-Re(I) bimetallic complexes were found to exist as dianionic form (Ir(III)-[BL]^2--Re(I)) after a fast reductive-quenching process in the presence of excess electron donor. In the photolysis experiment, the four Ir-qpy-Re complexes displayed the reasonable photochemical CO₂-to-CO conversion activities (TON of 366-588 for 19 h) owing to the moderated electronic coupling between two functional Ir(III) and Re(I) centers through the slightly distorted qpy ligand, whereas Ir-bpm-Re and Ir-dpp-Re displayed negligible performances as a result of the strong electronic coupling via π-conjugation between the two functional components resulting in the energetic constraints for PET and an unwanted side reactions competing with the forward processes. These results confirm that the qpy unit can be utilized as an efficient BL platform in π-linked bimetallic systems.

Ligand-Structure Effects on N-Heterocyclic Carbene Rhenium Photo- and Electrocatalysts of CO2 Reduction

Three novel rhenium N-heterocyclic carbene complexes, [Re]-NHC-1-3 ([Re] = fac-Re(CO)₃Br), were synthesized and characterized using a range of spectroscopic techniques. Photophysical, electrochemical and spectroelectrochemical studies were carried out to probe the properties of these organometallic compounds. Re-NHC-1 and Re-NHC-2 bear a phenanthrene backbone on an imidazole (NHC) ring, coordinating to Re by both the carbene C and a pyridyl group attached to one of the imidazole nitrogen atoms. Re-NHC-2 differs from Re-NHC-1 by replacing N-H with an N-benzyl group as the second substituent on imidazole. The replacement of the phenanthrene backbone in Re-NHC-2 with the larger pyrene gives Re-NHC-3. The two-electron electrochemical reductions of Re-NHC-2 and Re-NHC-3 result in the formation of the five-coordinate anions that are capable of electrocatalytic CO₂ reduction. These catalysts are formed first at the initial cathodic wave R1, and then, ultimately, via the reduction of Re-Re bound dimer intermediates at the second cathodic wave R2. All three Re-NHC-1-3 complexes are active photocatalysts for the transformation of CO₂ to CO, with the most photostable complex, Re-NHC-3, being the most effective for this conversion. Re-NHC-1 and Re-NHC-2 afforded modest CO turnover numbers (TONs), following irradiation at 355 nm, but were inactive at the longer irradiation wavelength of 470 nm. In contrast, Re-NHC-3, when photoexcited at 470 nm, yielded the highest TON in this study, but remained inactive at 355 nm. The luminescence spectrum of Re-NHC-3 is red-shifted compared to those of Re-NHC-1 and Re-NHC-2, and previously reported similar [Re]-NHC complexes. This observation, together with TD-DFT calculations, suggests that the nature of the lowest-energy optical excitation for Re-NHC-3 has π→π*(NHC-pyrene) and d_π(Re)→π*(pyridine) (IL/MLCT) character. The stability and superior photocatalytic performance of Re-NHC-3 are attributed to the extended conjugation of the π-electron system, leading to the beneficial modulation of the strongly electron-donating tendency of the NHC group.

A Review of Phosphorus Structures as CO2 Reduction Photocatalysts

Effective photocatalytic carbon dioxide (CO₂ ) reduction into high-value-added chemicals is promising to mitigate current energy crisis and global warming issues. Finding effective photocatalysts is crucial for photocatalytic CO₂ reduction. Currently, metal-based semiconductors for photocatalytic CO₂ reduction have been well reviewed, while review of nonmetal-based semiconductors is almost limited to carbon nitrides. Phosphorus is a promising nonmetal photocatalysts with various allotropes and tunable band gaps, which has been demonstrated to be promising non-metallic photocatalysts. However, no systematic review about phosphorus structures for photocatalytic CO₂ reduction reactions has been reported. Herein, the progresses of phosphorus structures as photocatalysts for CO₂ reduction are reviewed. The fundamentals of photocatalytic CO₂ reduction, corresponding properties of phosphorus allotropes, photocatalysts with phosphorus doping or phosphorus-containing ligands, research progress of phosphorus allotropes as photocatalysts for CO₂ reduction have been reviewed in this paper. The future research and perspective of phosphorus structures for photocatalytic CO₂ reduction are also presented.

Manipulating the Reaction Pathway of CO2 Photoreduction via the Microenvironment of a Re Molecular Catalyst

Re molecular complexes incorporated into two metal-organic frameworks were investigated to disclose the host-guest interaction by infrared and ¹H nuclear magnetic resonance and to explore the microenvironment around the Re complex by absorption and photoluminescence spectra. ZIF-8 provides a confined space to isolated Re via an electrostatic interaction, while UiO-66 exerts a relaxed space to accessible Re via a coordination interaction. For CO₂ two-electron photoreduction to CO, the turnover number of 28.6 in Re@ZIF-8 is 10-fold that of 2.7 in Re@UiO-66. The electron transfer is promoted in Re@ZIF-8 by a local electrostatic field with a cross-space pathway, whereas it is retarded in Re@UiO-66 as the solvation shell surrounding Re. In the following CO₂ activation, the charged intermediate species could be stabilized in Re@ZIF-8 by spatial confinement, while Re-triethanolamine adducts prevailed in Re@UiO-66 with the accessibility of the Re complex. This work demonstrates a feasibility of diverting the CO₂ activation pathway by the microenvironment of a molecular catalyst in the field of artificial photosynthesis.

Photoelectrocatalytic Dioxygen Reduction Based on a Novel Thiophene-Functionalized Tricarbonylchloro(1,10-phenanthroline)rhenium(I)

A novel Re (I) complex of [Re(CO)3Cl(L)], {L = 2-([2,2’-bithiophen]-5-yl)-1-phenyl-1H-imidazo [4,5-f][1,10]phenanthroline}, was synthesized, and its optical (UV–Visible absorption and emission spectroscopy), cyclovoltammetric and photoelectrochemical oxygen reduction properties were studied. The geometric and electronic properties were also investigated by density functional theory calculations. It was found that the ITO electrode coated with drop-casted [Re(CO)3Cl(L)] film exhibited cathodic photocurrent generation characteristics. The illuminated film exhibited a maximum cathodic photocurrent up to 30.4 μA/cm2 with an illumination intensity of 100 mW/cm2 white light at a bias potential of −0.4 V vs. SCE in O2-saturated electrolyte solution, which was reduced by 5.1-fold when thoroughly deoxygenated electrolyte solution was used, signaling that the electrode performed well on the photoelectrochemical oxygen reduction. The photo-electrocatalytic hydrogen peroxide production was proved with a maximum H2O2 concentration of 6.39 μM during 5 h of the photoelectrocatalytic process. This work would guide the construction of more efficient rhenium-based photo(electro)catalytic molecular systems for O2 sensing, hydrogen peroxide production and other types of photoelectrochemical energy conversion and storage.

Recent Advances in Electrochemical, Photochemical, and Photoelectrochemical Reduction of CO2 to C2+ Products

Environmental problems such as global warming are one of the most prominent global challenges. Researchers are investigating various methods for decreasing CO₂ emissions. The CO₂ reduction reaction via electrochemical, photochemical, and photoelectrochemical processes has been a popular research topic because the energy it requires can be sourced from renewable sources. The CO₂ reduction reaction converts stable CO₂ molecules into useful products such as CO, CH₄ , C₂ H₄ , and C₂ H₅ OH. To obtain economic benefits from these products, it is important to convert them into hydrocarbons above C₂ . Numerous investigations have demonstrated the uniqueness of the CC coupling reaction of Cu-based catalysts for the conversion of CO₂ into useful hydrocarbons above C₂ for electrocatalysis. Herein, the principle of semiconductors for photocatalysis is briefly introduced, followed by a description of the obstacles for C₂₊ production. This review presents an overview of the mechanism of hydrocarbon formation above C₂ , along with advances in the improvement, direction, and comprehension of the CO₂ reduction reaction via electrochemical, photochemical, and photoelectrochemical processes.

Photoinduced Catalytic Organic-Hydride Transfer to CO2 Mediated with Ruthenium Complexes as NAD+ /NADH Redox Couple Models

The catalytic organic-hydride transfer to CO₂ was first achieved through the photoinduced two-electron reduction of the [Ru(bpy)₂ (pbn)]²⁺ /[Ru(bpy)₂ (pbnHH)]²⁺ (bpy=2,2'-bipyridine, pbn=2-(pyridin-2-yl)benzo[b]-1,5-naphthyridine, and pbnHH=2-(pyridin-2-yl)-5,10-dihydrobenzo[b]-1,5-naphthyridine) redox couple in the presence of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH). The active species for the catalytic hydride transfer to carbon dioxide giving formate is [Ru(bpy)(bpy⋅^- )(pbnHH)]⁺ formed by one-electron reduction of [Ru(bpy)₂ (pbnHH)]²⁺ with BI⋅.

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.

Dynamics of Photoinduced Intramolecular and Intermolecular Electron Transfers in Ligand-Conjugated Ir(III)-Re(I) Photocatalysts

We report the electron transfer (ET) dynamics in a series of Ir(III)-Re(I) photocatalysts where two bipyridyl ligands of Ir and Re moieties are conjugated at the meta (m)- or para (p)-position of each side. Femtosecond transient absorption (TA) measurements identify the intramolecular ET (IET) dynamics from the Ir to Re moiety, followed by the formation of one-electron-reduced species (OERS) via the intermolecular ET with a sacrificial electron donor (SED). The IET rate depends on the bridging ligand (BL) structures (∼25 ps for BL_mm/mp vs ∼68 ps for BL_pm/pp), while the OERS formation happens on an even slower time scale (∼1.4 ns). Connecting the Re moiety at the meta-position of the bipyridyl of the Ir moiety can restrict the rotation around a covalent bond between two bipyridyl ligands by steric hindrances and facilitate the IET process. This highlights the importance of BL structures on the ET dynamics in photocatalysts.

Selective and high-rate CO2 electroreduction by metal-doped covalent triazine frameworks: a computational and experimental hybrid approach

The electrochemical CO₂ reduction reaction (CO₂RR) has attracted intensive attention as a technology to achieve a carbon-neutral society. The use of gas diffusion electrodes (GDEs) enables the realization of high-rate CO₂RRs, which is one of the critical requirements for social implementation. Although both a high reaction rate and good selectivity are simultaneously required for electrocatalysts on GDEs, no systematic study of the relationship among active metal centers in electrocatalysts, reaction rate, and selectivity under high-rate CO₂RR conditions has been reported. In the present study, we employed various metal-doped covalent triazine frameworks (M-CTFs) as platforms for CO₂ reduction reaction (CO₂RR) electrocatalysts on GDEs and systematically investigated them to deduce sophisticated design principles using a combined computational and experimental approach. The Ni-CTF showed both high selectivity (faradaic efficiency (FE) > 98% at -0.5 to -0.9 V vs. reversible hydrogen electrode) and a high reaction rate (current density < -200 mA cm^-2) for CO production. By contrast, the Sn-CTF exhibited selective formic acid production, and the FE and partial current density reached 85% and 150 mA cm^-2, respectively. These results for the CO₂RR activity and selectivity at high current density with respect to metal centers correspond well with predictions based on first-principles calculations. This work is the first demonstration of a clear relationship between the computational adsorption energy of intermediates depending on metal species and the experimental high-rate gaseous CO₂RR.

Multiple C-C bond formation upon electrocatalytic reduction of CO2 by an iron-based molecular macrocycle

Molecular macrocycles are very promising electrocatalysts for the reduction of carbon dioxide into value-added chemicals. Up to now, most of these catalysts produced only C₁ products. We report here that iron phthalocyanine, a commercially available molecule based on earth-abundant elements, can produce light hydrocarbons upon electrocatalytic reduction of CO₂ in aqueous conditions and neutral pH. Under applied electrochemical potential, C₁ to C₄ saturated and unsaturated products are evolved. Isotopic labelling experiments unambiguously show that these products stem from CO₂. Control experiments and in situ X-ray spectroscopic analysis show that the molecular catalyst remains intact during catalysis and is responsible for the reaction. On the basis of experiments with alternate substrates, a mechanism is proposed for the C-C bond formation step.

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.

Significant Promotion of Light Absorption Ability and Formation of Triplet Organics and Reactive Oxygen Species in Atmospheric HULIS by Fe(III) Ions

Metal ions are key components in atmosphere that potentially affect the optical properties and photochemical reactivity of atmospheric humic-like substances (HULIS), while this mechanism is still unclear. In this study, we demonstrated that atmospheric HULIS coupled with Fe³⁺, Cu²⁺, Zn²⁺, and Al³⁺ exhibited distinct optical properties and reactive intermediates from that of HULIS utilizing three-dimensional fluorescence spectroscopy and electron paramagnetic resonance spectroscopy. The HULIS components showed light absorption that increased by 56% for the HULIS-Fe³⁺ system, fluorescence blue shift, and fluorescence quenching, showing a certain dose-effect relationship. These are mainly attributed to the fact that the highly oxidative HULIS chromophores have a stronger complexing ability with Fe³⁺ ions than the other metal ions. In addition, triplet organics (promoting ratio: 53%) and reactive oxygen species (promoting ratio: 82.6%) in the HULIS-Fe³⁺ system showed obvious generation promotion. Therefore, the main assumption of the photochemical mechanisms of atmospheric HULIS in the HULIS-Fe³⁺ system is that Fe³⁺ ions can form ³HULIS*-Fe³⁺ complexation with photoexcited ³HULIS* and then transition to the ground state through energy transfer, electron transfer, or nonradiative transition, accompanied by the formation of singlet oxygen and hydroxyl radicals. Our results provide references for evaluating the radiative forcing and aging effect of metal ions on atmospheric aerosols.

Advancements and Challenges in Reductive Conversion of Carbon Dioxide via Thermo-/Photocatalysis

Carbon dioxide (CO₂) is the major greenhouse gas and also an abundant and renewable carbon resource. Therefore, its chemical conversion and utilization are of great attraction for sustainable development. Especially, reductive conversion of CO₂ with energy input has become a current hotspot due to its ability to access fuels and various important chemicals. Nowadays, the controllable CO₂ hydrogenation to formic acid and alcohols using sustainable H₂ resources has been regarded as an appealing solution to hydrogen storage and CO₂ accumulation. In addition, photocatalytic CO₂ reduction to CO also provides a potential way to utilize this greenhouse gas efficiently. Besides direct CO₂ hydrogenation, CO₂ reductive functionalization integrates CO₂ reduction with subsequent C-X (X = N, S, C, O) bond formation and indirect transformation strategies, enlarging the diverse products derived from CO₂ and promoting CO₂ reductive conversion into a new stage. In this Perspective, the progress and challenges of CO₂ reductive conversion, including hydrogenation, reductive functionalization, photocatalytic reduction, and photocatalytic reductive functionalization are summarized and discussed along with the key issues and future trends/directions in this field. We hope this Perspective can evoke intense interest and inspire much innovation in the promise of CO₂ valorization.

Engineering Single-Atom Active Sites on Covalent Organic Frameworks for Boosting CO2 Photoreduction

Solar carbon dioxide (CO₂) conversion is an emerging solution to meet the challenges of sustainable energy systems and environmental/climate concerns. However, the construction of isolated active sites not only influences catalytic activity but also limits the understanding of the structure-catalyst relationship of CO₂ reduction. Herein, we develop a universal synthetic protocol to fabricate different single-atom metal sites (e.g., Fe, Co, Ni, Zn, Cu, Mn, and Ru) anchored on the triazine-based covalent organic framework (SAS/Tr-COF) backbone with the bridging structure of metal-nitrogen-chlorine for high-performance catalytic CO₂ reduction. Remarkably, the as-synthesized Fe SAS/Tr-COF as a representative catalyst achieved an impressive CO generation rate as high as 980.3 μmol g^-1 h^-1 and a selectivity of 96.4%, over approximately 26 times higher than that of the pristine Tr-COF under visible light irradiation. From X-ray absorption fine structure analysis and density functional theory calculations, the superior photocatalytic performance is attributed to the synergic effect of atomically dispersed metal sites and Tr-COF host, decreasing the reaction energy barriers for the formation of *COOH intermediates and promoting CO₂ adsorption and activation as well as CO desorption. This work not only affords rational design of state-of-the-art catalysts at the molecular level but also provides in-depth insights for efficient CO₂ conversion.

C2 product formation in the CO2 electroreduction on boron-doped graphene anchored copper clusters

A possible remedy for the increasing atmospheric CO₂ concentration is capturing and reducing it into valuable chemicals like methane, methanol, ethylene, and ethanol. However, a suitable catalyst for this process is still under extensive research. Small sized copper clusters have gained attention in recent years due to their catalytic activity in the CO₂ reduction reaction. Although C₂₊ products have a higher economic value, the formation of C₁ products was investigated most thoroughly. Graphene is a promising support for small copper clusters in the electrochemical reduction of CO₂. It exhibits good mechanical and electrical properties, but the weak interaction between copper and graphene is an issue. Our DFT computations reveal that small Cu clusters on the boron-doped graphene (BDG) support are promising catalysts for the electrochemical reduction of CO₂. We found facile reaction pathways towards various C₁ (carbon-monoxide, formic acid, formaldehyde, methanol or methane) and C₂ (ethanol or ethylene) products on Cu₄ and Cu₇ clusters on BDG. The reactivity is cluster-size tunable with Cu₄ being the more reactive agent, while Cu₇ shows a higher selectivity towards C₂ products.

Unexpected Boost in Activity of a Cu(I) Photosensitizer by Stabilizing a Transient Excited State

In this study, we present a slight but surprisingly successful structural modification of the previously reported heteroleptic Cu(I) photosensitizer Cubiipo ([(xantphos)Cu(biipo)]PF₆; biipo = 16H-benzo-[4',5']-isoquinolino-[2',1':1,2]-imidazo-[4,5-f]-[1,10]-phenanthrolin-16-one). As a key feature, biipo bears a naphthalimide unit at the back, which is directly fused to a phenanthroline moiety to extend the conjugated π-system. This ligand was now altered to include two additional methyl groups at the 2,9-positions at the phenanthroline scaffold. Comparing the novel Cudmbiipo complex to its predecessor, ultrafast transient absorption spectroscopy reveals the efficient suppression of a major deactivation pathway by stabilization of a transient triplet state. Furthermore, quantitative measurements of singlet oxygen evolution in solution confirmed that a larger fraction of the excited-state population is transferred to the photocatalytically active ligand-centered triplet ³LC state with a much longer lifetime of ∼30 μs compared to Cubiipo (2.6 μs). In addition, Cudmbiipo was compared with the well-established reference complex Cubcp ([(xantphos)Cu(bathocuproine)]PF₆) in terms of its photophysical and photocatalytic properties by applying time-resolved femto- and nanosecond absorption, step-scan Fourier transform infrared (FTIR), and emission spectroscopies. Superior light-harvesting properties and a greatly enhanced excited-state lifetime with respect to Cubcp enable Cudmbiipo to be more active in exemplary photocatalytic applications, i.e., in the formation of singlet oxygen and the isomerization of (E)-stilbene.

Highly Robust Rhenium(I) Bipyridyl Complexes Containing Dipyrromethene-BF2 Chromophores for Visible Light-Driven CO2 Reduction

New rhenium bipyridyl complexes with dipyrromethene-BF₂ chromophores (A-ReBDP-CZ, A-ReBDP₂ , ReBDP-CZ, and ReBDP₂ ) were developed for highly efficient photocatalytic carbon dioxide (CO₂ ) reduction to carbon monoxide (CO). These catalysts consisted of two moderate electron-deficient groups (dipyrromethene-BF₂ , BDP) as the visible-light-harvesting antenna as well as both electron donor (N-phenylcarbazole, CZ) and acceptor (BDP) on Re bipyridyl framework. Among ReBDP-CZ and ReBDP₂ complexes, the ReBDP₂ incorporating two electron-deficient BDP chromophores had a longer-lived photoexcited state (182.4 μs) and a twofold enhanced molar absorption coefficient (ϵ=157000 m^-1  cm^-1 ) compared with ReBDP-CZ. Thus, ReBDP₂ achieved the superior photocatalytic reactivity and stability with a CO turnover number (TON_CO ) value as high as 1323 and quantum yield (Φ_CO ) up to 55 %, which was the most excellent photocatalysis efficiency among the single-active-site Re catalysts without additional photosensitizer. Furthermore, the acetylene-bridged linker was detrimental to the photoactivity and durability of the catalyst. In brief, two BDP-based Re bipyridyl systems with outstanding catalytic performance and significant visible-light-harvesting capabilities in the solar spectrum offer a promising strategy for solar-to-fuel conversion schemes.

Photocatalytic CO2 Conversion Using Metal-Containing Coordination Polymers and Networks: Recent Developments in Material Design and Mechanistic Details

International guidelines have progressively addressed global warming which is caused by the greenhouse effect. The greenhouse effect originates from the atmosphere’s gases which trap sunlight which, as a consequence, causes an increase in global surface temperature. Carbon dioxide is one of these greenhouse gases and is mainly produced by anthropogenic emissions. The urgency of removing atmospheric carbon dioxide from the atmosphere to reduce the greenhouse effect has initiated the development of methods to covert carbon dioxide into valuable products. One approach that was developed is the photocatalytic transformation of CO2. Photocatalysis addresses environmental issues by transferring CO2 into value added chemicals by mimicking the natural photosynthesis process. During this process, the photocatalytic system is excited by light energy. CO2 is adsorbed at the catalytic metal centers where it is subsequently reduced. To overcome several obstacles for achieving an efficient photocatalytic reduction process, the use of metal-containing polymers as photocatalysts for carbon dioxide reduction is highlighted in this review. The attention of this manuscript is directed towards recent advances in material design and mechanistic details of the process using different polymeric materials and photocatalysts.