Molecular approaches to the photocatalytic reduction of carbon dioxide for solar fuels

Catalysis of CO2 reduction by diazapyridinophane complexes of Fe, Co, and Ni: CO2 binding triggered by combined frontier MO associations involving a SOMO

Our previous study on the photochemical CO₂ reduction into CO catalyzed by the diazapyridinophane complexes of Fe, Co, and Ni revealed that (i) the Co catalyst shows the highest TOF but degrades rapidly, (ii) the Fe catalyst exhibits a lower TOF relative to Co but shows higher robustness, giving a higher TON, and (iii) the Ni complex shows no activity (Sakaguchi et al., Chem. Commun., 2019, 55, 8552). Here we show our DFT results unveiling that the Fe and Co catalysts can utilize multiple sets of frontier MO associations at the CO₂ binding by including one of the SOMOs in a high-spin d⁷ Fe(I) and d⁸ Co(I) center, respectively, giving an increased driving force for these oxidative addition steps. Remarkably, two-electron reduction of CO₂ to CO₂^2- at the binding step is driven by the two electrons transferred from different d-based orbitals. The Co^I species binds CO₂ at the rate-limiting step with an activation barrier of 15.0 kcal mol^-1, rationalizing the high initial TOF observed. However, the Co^I(CO) species is given as a dead-end product, consistent with its relatively rapid deactivation. The Fe catalyst possesses a slightly higher barrier in CO₂ binding (ΔG^‡ = 15.8 kcal mol^-1) but does not stabilize the Fe^I(CO) species which readily releases CO (ΔG = 3.5 kcal mol^-1). The Ni catalyst has the smallest barrier in CO₂ binding (ΔG^‡ = 11.5 kcal mol^-1) but the CO release is largely prohibited by the dead-end Ni^I(CO) species, consistent with its inactive character towards CO₂ reduction. The combined results all satisfactorily explain the observed catalytic behaviors.

Excited-state structural dynamics of nickel complexes probed by optical and X-ray transient absorption spectroscopies: insights and implications

Excited states of nickel complexes undergo a variety of photochemical processes, such as charge transfer, ligation/deligation, and redox reactions, relevant to solar energy conversion and photocatalysis. The efficiencies of the aforementioned processes are closely coupled to the molecular structures in the ground and excited states. The conventional optical transient absorption spectroscopy has revealed important excited-state pathways and kinetics, but information regarding the metal center, in particular transient structural and electronic properties, remains limited. These deficiencies are addressed by X-ray transient absorption (XTA) spectroscopy, a detailed probe of 3d orbital occupancy, oxidation state and coordination geometry. The examples of excited-state structural dynamics of nickel porphyrin and nickel phthalocyanine have been described from our previous studies with highlights on the unique structural information obtained by XTA spectroscopy. We close by surveying prospective applications of XTA spectroscopy to active areas of Ni-based photocatalysis based on the knowledge gained from our previous studies.

Reticular-Chemistry-Inspired Supramolecule Design as a Tool to Achieve Efficient Photocatalysts for CO2 Reduction

Photocatalytic CO₂ reduction into C1 products is one of the most trending research subjects of current times as sustainable energy generation is the utmost need of the hour. In this review, we have tried to comprehensively summarize the potential of supramolecule-based photocatalysts for CO₂ reduction into C1 compounds. At the outset, we have thrown light on the inert nature of gaseous CO₂ and the various challenges researchers are facing in its reduction. The evolution of photocatalysts used for CO₂ reduction, from heterogeneous catalysis to supramolecule-based molecular catalysis, and subsequent semiconductor-supramolecule hybrid catalysis has been thoroughly discussed. Since CO₂ is thermodynamically a very stable molecule, a huge reduction potential is required to undergo its one- or multielectron reduction. For this reason, various supramolecule photocatalysts were designed involving a photosensitizer unit and a catalyst unit connected by a linker. Later on, solid semiconductor support was also introduced in this supramolecule system to achieve enhanced durability, structural compactness, enhanced charge mobility, and extra overpotential for CO₂ reduction. Reticular chemistry is seen to play a pivotal role as it allows bringing all of the positive features together from various components of this hybrid semiconductor-supramolecule photocatalyst system. Thus, here in this review, we have discussed the selection and role of various components, viz. the photosensitizer component, the catalyst component, the linker, the semiconductor support, the anchoring ligands, and the peripheral ligands for the design of highly performing CO₂ reduction photocatalysts. The selection and role of various sacrificial electron donors have also been highlighted. This review is aimed to help researchers reach an understanding that may translate into the development of excellent CO₂ reduction photocatalysts that are operational under visible light and possess superior activity, efficiency, and selectivity.

DFT insights into the photocatalytic reduction of CO2 to CO by Re(I) complexes: the crucial role of the triethanolamine "magic" sacrificial electron donor

The reaction mechanism for the photocatalytic reduction of CO₂ to CO catalyzed by the [Re(en)(CO)₃Cl] complex in the presence of triethanolamine, R₃N (R = CH₂CH₂OH) abbreviated as TEOA, in DMF solution was studied in-depth with the aid of DFT computational protocols by calculating the geometric and free energy reaction profiles for several possible reaction pathways. The reaction pathways studied start with the "real" catalytic species [Re(en)(CO)₃], [Re(en)(CO)₃]^- and/or [Re(en)(CO)₂Cl]^- generated from the excited triplet T1 state upon single and double reductive quenching by a TEOA sacrificial electron donor or photodissociation of a CO ligand. The first step in all the catalytic cycles investigated involves the capture of either CO₂ or the oxidized R₂NCH₂CH₂O˙ radical. In the latter case, the CO₂ molecule is captured (inserted) by the Re-OCH₂CH₂NR₂ bond forming stable intermediates. Next, successive protonations (TEOA also acts as a proton donor) lead to the release of CO either from the energy consuming 2e^- reduction of [Re(en)(CO)₄]⁺ or [Re(en)(CO)₂Cl]⁺ complexes in the CO₂ capture pathways or from the released unstable diprotonated [R₂NCH₂CH₂OC(OH)(OH)]⁺ species regenerating TEOA and the catalyst. The CO₂ insertion reaction pathway is the favorable pathway for the photocatalytic reduction of CO₂ → CO catalyzed by the [Re(en)(CO)₃Cl] complex in the presence of TEOA manifesting its crucial role as an electron and proton donor, capturing CO₂ and releasing CO.

Recent Advances in Interface Engineering for Electrocatalytic CO2 Reduction Reaction

Electrocatalytic CO₂ reduction reaction (CO₂RR) can store and transform the intermittent renewable energy in the form of chemical energy for industrial production of chemicals and fuels, which can dramatically reduce CO₂ emission and contribute to carbon-neutral cycle. Efficient electrocatalytic reduction of chemically inert CO₂ is challenging from thermodynamic and kinetic points of view. Therefore, low-cost, highly efficient, and readily available electrocatalysts have been the focus for promoting the conversion of CO₂. Very recently, interface engineering has been considered as a highly effective strategy to modulate the electrocatalytic performance through electronic and/or structural modulation, regulations of electron/proton/mass/intermediates, and the control of local reactant concentration, thereby achieving desirable reaction pathway, inhibiting competing hydrogen generation, breaking binding-energy scaling relations of intermediates, and promoting CO₂ mass transfer. In this review, we aim to provide a comprehensive overview of current developments in interface engineering for CO₂RR from both a theoretical and experimental standpoint, involving interfaces between metal and metal, metal and metal oxide, metal and nonmetal, metal oxide and metal oxide, organic molecules and inorganic materials, electrode and electrolyte, molecular catalysts and electrode, etc. Finally, the opportunities and challenges of interface engineering for CO₂RR are proposed.

Sensitized Photocatalytic CO2 Reduction With Earth Abundant 3d Metal Complexes Possessing Dipicolyl-Triazacyclononane Derivatives

Complexes based on nitrogen and sulfur containing ligands involving 3d metal centers are known for the electrocatalytic reduction of CO_2. However, photocatalytical activation has rarely been investigated. We herein present results on the light-driven CO₂ reduction using either Ir(dFppy)₃ [Ir, dFppy = 2-(4,6-difluorophenyl)pyridine] or [Cu(xant)(bcp)]⁺, (Cu, xant = xantphos, bcp = bathocuproine) as photosensitizer in combination with TEA (triethylamine) as sacrificial electron donor. The 3d metal catalysts have either dptacn (dipicolyl-triazacyclononane, L^N3) or dpdatcn (dipicolyl-diazathiocyclononane, L^N2S) as ligand framework and Fe³⁺, Co³⁺ or Ni²⁺ as central metal ion. It turned out that the choice of ligand, metal center and solvent composition influences the selectivity for product formation, which means that the gaseous reduction products can be solely CO or H₂ or a mixture of both. The ratio between these two products can be controlled by the right choice of reaction conditions. With using Cu as photosensitizer, we could introduce an intermolecular system that is based solely on 3d metal compounds being able to reduce CO₂.

Energy Band Alignment and Redox-Active Sites in Metalloporphyrin-Spaced Metal-Catechol Frameworks for Enhanced CO2 Photoreduction

Two new chemically stable metalloporphyrin-bridged metal-catechol frameworks, InTCP-Co and FeTCP-Co, were constructed to achieve artificial photosynthesis without additional sacrificial agents and photosensitizers. The CO₂ photoreduction rate over FeTCP-Co considerably exceeds that obtained over InTCP-Co, and the incorporation of uncoordinated hydroxyl groups, associated with catechol, into the network further promotes the photocatalytic activity. The iron-oxo coordination chain assists energy band alignment and provides a redox-active site, and the uncoordinated hydroxyl group contributes to the visible-light absorptance, charge-carrier transfer, and CO₂ -scaffold affinity. With a formic acid selectivity of 97.8 %, FeTCP-OH-Co affords CO₂ photoconversion with a reaction rate 4.3 and 15.7 times higher than those of FeTCP- Co and InTCP-Co, respectively. These findings are also consistent with the spectroscopic study and DFT calculation.

Nickel-mediated N-N bond formation and N2O liberation via nitrogen oxyanion reduction

The syntheses of (DIM)Ni(NO₃)₂ and (DIM)Ni(NO₂)₂, where DIM is a 1,4-diazadiene bidentate donor, are reported to enable testing of bis boryl reduced N-heterocycles for their ability to carry out stepwise deoxygenation of coordinated nitrate and nitrite, forming O(Bpin)₂. Single deoxygenation of (DIM)Ni(NO₂)₂ yields the tetrahedral complex (DIM)Ni(NO)(ONO), with a linear nitrosyl and κ¹-ONO. Further deoxygenation of (DIM)Ni(NO)(ONO) results in the formation of dimeric [(DIM)Ni(NO)]₂, where the dimer is linked through a Ni–Ni bond. The lost reduced nitrogen byproduct is shown to be N₂O, indicating N–N bond formation in the course of the reaction. Isotopic labelling studies establish that the N–N bond of N₂O is formed in a bimetallic Ni₂ intermediate and that the two nitrogen atoms of (DIM)Ni(NO)(ONO) become symmetry equivalent prior to N–N bond formation. The [(DIM)Ni(NO)]₂ dimer is susceptible to oxidation by AgX (X = NO₃⁻, NO₂⁻, and OTf⁻) as well as nitric oxide, the latter of which undergoes nitric oxide disproportionation to yield N₂O and (DIM)Ni(NO)(ONO). We show that the first step in the deoxygenation of (DIM)Ni(NO)(ONO) to liberate N₂O is outer sphere electron transfer, providing insight into the organic reductants employed for deoxygenation. Lastly, we show that at elevated temperatures, deoxygenation is accompanied by loss of DIM to form either pyrazine or bipyridine bridged polymers, with retention of a BpinO⁻ bridging ligand.

Deoxygenation of nitrogen oxyanions coordinated to nickel using reduced borylated heterocycles leads to N–N bond formation and N₂O liberation. The nickel dimer product facilitates NO disproportionation, leading to a synthetic cycle.

Immobilising molecular Ru complexes on a protective ultrathin oxide layer of p-Si electrodes towards photoelectrochemical CO2 reduction

Photoelectrochemical CO2 reduction is a promising approach for renewable fuel generation and to reduce greenhouse gas emissions. Owing to their synthetic tunability, molecular catalysts for the CO2 reduction reaction can give rise to high product selectivity. In this context, a RuII complex [Ru(HO-tpy)(6-mbpy)(NCCH3)]2+ (HO-tpy = 4'-hydroxy-2,2':6',2''-terpyridine; 6-mbpy = 6-methyl-2,2'-bipyridine) was immobilised on a thin SiOx layer of a p-Si electrode that was decorated with a bromide-terminated molecular layer. Following the characterisation of the assembled photocathodes by X-ray photoelectron spectroscopy and ellipsometry, PEC experiments demonstrate electron transfer from the p-Si to the Ru complex through the native oxide layer under illumination and a cathodic bias. A state-of-the-art photovoltage of 570 mV was determined by comparison with an analogous n-type Si assembly. While the photovoltage of the modified photocathode is promising for future photoelectrochemical CO2 reduction and the p-Si/SiOx junction seems to be unchanged during the PEC experiments, a fast desorption of the molecular Ru complex was observed. An in-depth investigation of the cathode degradation by comparison with reference materials highlights the role of the hydroxyl functionality of the Ru complex to ensure its grafting on the substrate. In contrast, no essential role for the bromide function on the Si substrate designed to engage with the hydroxyl group of the Ru complex in an SN2-type reaction could be established.

Molecular-Scale Chemical Imaging of the Orientation of an On-Surface Coordination Complex by Tip-Enhanced Raman Spectroscopy

Metal-organic coordination structures at interfaces play an essential role in many biological and chemical systems. Understanding the molecular specificity, orientation, and spatial distribution of the coordination complexes at the nanometer scale is of great importance for effective molecular engineering of nanostructures and fabrication of functional devices with controllable properties. However, fundamental properties of such coordination systems are still rarely studied directly. In this work, we present a spectroscopic approach on the basis of tip-enhanced Raman spectroscopy (TERS) to investigate cobalt(II) tetraphenyl-porphyrine coordination species on the scale of a single molecule under ambient conditions. Coordination species anchored on gold surfaces modified with pyridine thiol self-assembled monolayers can be spectroscopically distinguished and mapped with ca. 2 nm resolution. In addition, in combination with density functional theory simulations, the adsorption configuration and molecular orientation of the coordination complexes are also revealed using TERS imaging.

One-step aerosol synthesis of a double perovskite oxide (KBaTeBiO6) as a potential catalyst for CO2 photoreduction

This study presents a comprehensive investigation on the aerosol synthesis of a semiconducting double perovskite oxide with a nominal composition of KBaTeBiO6, which is considered as a potential candidate for CO2 photoreduction. We demonstrate the rapid synthesis of the multispecies compound KBaTeBiO6 with extremely high purity and controllable size through a single-step furnace aerosol reactor (FuAR) process. The formation mechanism of the perovskite through the aerosol route is investigated using thermogravimetric analysis to identify the optimal reference temperature, residence time and other operational parameters in the FuAR synthesis process to obtain highly pure KBaTeBiO6 nanoparticles. It is observed that particle formation in the FuAR is based on a combination of gas-to-particle and liquid-to-particle mechanisms. The phase purity of the perovskite nanoparticles depends on the ratio of the residence time and the reaction time. The particle size is strongly affected by the precursor concentration, residence time and furnace temperature. Finally, the photocatalytic performance of the synthesized KBaTeBiO6 nanoparticles is investigated for CO2 photoreduction under UV-light. The best performing sample exhibits an average CO production rate of 180 μmol g-1 h-1 in the first half hour with a quantum efficiency of 1.19%, demonstrating KBaTeBiO6 as a promising photocatalyst for CO2 photoreduction.

Understanding entrapped molecular photosystem and metal-organic framework synergy for improved solar fuel production

Artificial photosystems assembled from molecular complexes, such as the photocatalyst fac-ReBr(CO)3(4,4'-dcbpy) (dcbpy = dicarboxy-2,2'-bipyridine) and the photosensitiser Ru(bpy)2(5,5'-dcbpy)Cl2 (bpy = 2,2'-bipyridine), are a wide-spread approach for solar fuel production. Recently metal-organic framework (MOF) entrapping of such complexes was demonstrated as a promising concept for catalyst stabilisation and reaction environment optimisation in colloidal-based CO2 reduction. Building on this strategy, here we examined the influence of MIL-101-NH2(Al) MOF particle size, the electron donor source, and the presence of an organic base on the photocatalytic CO2-to-CO reduction performance, and the differences to homogeneous systems. A linear relation between smaller scaffold particle size and higher photocatalytic activity, longer system lifetimes for benign electron donors, and increased turnover numbers (TONs) with certain additive organic bases, were determined. This enabled understanding of key molecular catalysis phenomena and synergies in the nanoreactor-like host-guest assembly, and yielded TONs of ∼4300 over 96 h of photocatalysis under optimised conditions, surpassing homogeneous TON values and lifetimes.

Modulation of Self-Assembly Enhances the Catalytic Activity of Iron Porphyrin for CO2 Reduction

Electrochemical reduction of CO₂ in aqueous media is an important reaction to produce value-added carbon products in an environmentally and economically friendly manner. Various molecule-based catalytic systems for the reaction have been reported thus far. The key features of state-of-the-art catalytic systems in this field can be summarized as follows: 1) an iron-porphyrin-based scaffold as a catalytic center, 2) a dinuclear active center for the efficient activation of a CO₂ molecule, and 3) a hydrophobic channel for the accumulation of CO₂ . This article reports a novel approach to construct a catalytic system for CO₂ reduction with the aforementioned three key substructures. The self-assembly of a newly designed iron-porphyrin complex bearing bulky substituents with noncovalent interaction ability forms a highly ordered crystalline solid with adjacent catalytically active sites and hydrophobic pores. The obtained crystalline solid serves as an electrocatalyst for CO₂ reduction in aqueous media. Note that a relevant iron-porphyrin complex without bulky substituents cannot form a porous structure with adjacent active sites, and the catalytic performance of the crystals of this relevant iron-porphyrin complex is substantially lower than that of the newly developed catalytic system. The present study provides a novel strategy for constructing porous crystalline solids for small-molecule conversions.

Luminescent cis-Bis(bipyridyl)ruthenium(II) Complexes with 1,2-Azolylamidino Ligands: Photophysical, Electrochemical Studies, and Photocatalytic Oxidation of Thioethers

New 1,2-azolylamidino complexes cis-[Ru(bipy)₂(NH═C(R)az*-κ²N,N)](OTf)₂ (R = Me, Ph; az* = pz, indz, dmpz) are synthesized via chloride abstraction after a subsequent base-catalyzed coupling of a nitrile with the previously coordinated 1,2-azole. The synthetic procedure allows the easy obtainment of complexes having different electronic and steric 1,2-azoylamidino ligands. All of the compounds have been characterized by ¹H, ¹³C, and ¹⁵N NMR and IR spectroscopy and by monocrystal X-ray diffraction. Photophysical studies support their phosphorescence, whereas their electrochemistry reveals reversible Ru^II/Ru^III oxidations between +1.13 and +1.25 V (vs SCE). The complexes have been successfully used as catalysts in the photooxidation of different thioethers, the complex cis-[Ru(bipy)₂(NH═C(Me)dmpz-κ²N,N)]²⁺ showing better catalytic performance in comparison to that of [Ru(bipy)₃]²⁺. Moreover, the significant catalytic performance of the dimethylpyrazolylamidino complex is applied to the preparation of the drug modafinil, which is obtained using ambient oxygen as an oxidant. Finally, mechanistic assays suggest that the oxidation reaction follows a photoredox route via oxygen radical anion formation.

Light-Driven CO2 Reduction by Co-Cytochrome b 562

The current trend in atmospheric carbon dioxide concentrations is causing increasing concerns for its environmental impacts, and spurring the developments of sustainable methods to reduce CO2 to usable molecules. We report the light-driven CO2 reduction in water in mild conditions by artificial protein catalysts based on cytochrome b 562 and incorporating cobalt protoporphyrin IX as cofactor. Incorporation into the protein scaffolds enhances the intrinsic reactivity of the cobalt porphyrin toward proton reduction and CO generation. Mutations around the binding site modulate the activity of the enzyme, pointing to the possibility of further improving catalytic activity through rational design or directed evolution.

Promoting photocatalytic CO2 reduction with a molecular copper purpurin chromophore

CO2 reduction through artificial photosynthesis represents a prominent strategy toward the conversion of solar energy into fuels or useful chemical feedstocks. In such configuration, designing highly efficient chromophores comprising earth-abundant elements is essential for both light harvesting and electron transfer. Herein, we report that a copper purpurin complex bearing an additional redox-active center in natural organic chromophores is capable to shift the reduction potential 540 mV more negative than its organic dye component. When this copper photosensitizer is employed with an iron porphyrin as the catalyst and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole as the sacrificial reductant, the system achieves over 16100 turnover number of CO from CO2 with a 95% selectivity (CO vs H2) under visible-light irradiation, which is among the highest reported for a homogeneous noble metal-free system. This work may open up an effective approach for the rational design of highly efficient chromophores in artificial photosynthesis.

A proton-responsive ligand becomes a dimetal linker for multisubstrate assembly via nitrate deoxygenation

A bidentate pyrazolylpyridine ligand (HL) was installed on divalent nickel to give [(HL)2Ni(NO3)]NO3. This compound reacts with a bis-silylated heterocycle, 1,4-bis-(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene (TMS2Pz) to simultaneously reduce one of the nitrate ligands and deprotonate one of the HL ligands, giving octahedral (HL)(L-)Ni(NO3). The mononitrate species formed is then further reacted with TMS2Pz to doubly deoxygenate nitrate and form [(L-)Ni(NO)]2, dimeric via bridging pyrazolate with bent nitrosyl ligands, representing a two-electron reduction of coordinated nitrate. Independent synthesis of a dimeric species [(L-)Ni(Br)]2 is reported and effectively assembles two metals with better atom economy.

[Re(CO)3(5-PAN)Cl], a rhenium(I) naphthalimide complex for the visible light photocatalytic reduction of CO2

A rhenium(i) naphthalimide complex [Re(CO)3(5-PAN)Cl] (Re(5-PAN); 5-PAN = 1-(1,10-phenanthroline)-4-nitro-naphthalimide) was synthesized, characterized, and evaluated as a photocatalyst for CO2 reduction. Characterization included use of MALDI-ToF mass spectrometry, FT-IR, RAMAN, 1H and 13C NMR, elemental analysis, electronic absorption and emission spectroscopy, single crystal X-ray diffraction, DFT and cyclic voltammetry. Photocatalytic (406 nm) reduction of 13CO2 to formate (H13COO) in the presence of this catalyst was tracked via13C NMR. Results support Re5-PAN (φ = 0.021) functioning as a catalyst for the reduction of CO2 (maximum turn-over 48-50 at 300 equiv. triethylamine as the sacrificial electron donor).

Dirhodium complexes as electrocatalysts for CO2 reduction to HCOOH: role of steric hindrance on selectivity

A series of Rh₂(ii,ii) complexes were shown to electrocatalytically reduce CO₂ to HCOOH. Electrochemical and spectroelectrochemical studies reveal a correlation between catalytic selectivity and efficiency with the steric bulk at the axial sites afforded by the bridging ligands. Mechanistic studies point to the presence of a Rh₂(ii,i)-H hydride as a key intermediate in the catalytic cycle.

Enhanced CO2 Photocatalysis by Indium Oxide Hydroxide Supported on TiN@TiO2 Nanotubes

Herein is developed a ternary heterostructured catalyst, based on a periodic array of 1D TiN nanotubes, with a TiO₂ nanoparticulate intermediate layer and a In₂O_3-x(OH)_y nanoparticulate shell for improved performance in the photocatalytic reverse water gas shift reaction. It is demonstrated that the ordering of the three components in the heterostructure sensitively determine its activity in CO₂ photocatalysis. Specifically, TiN nanotubes not only provide a photothermal driving force for the photocatalytic reaction, owing to their strong optical absorption properties, but they also serve as a crucial scaffold for minimizing the required quantity of In₂O_3-x(OH)_y nanoparticles, leading to an enhanced CO production rate. Simultaneously, the TiO₂ nanoparticle layer supplies photogenerated electrons and holes that are transferred to active sites on In₂O_3-x(OH)_y nanoparticles and participate in the reactions occurring at the catalyst surface.

Near-IR Charge-Transfer Emission at 77 K and Density Functional Theory Modeling of Ruthenium(II)-Dipyrrinato Chromophores: High Phosphorescence Efficiency of the Emitting State Related to Spin-Orbit Coupling Mediation of Intensity from Numerous Low-Energy Singlet Excited States

Efficient charge-transfer (CT) phosphorescence in the near-IR (NIR) spectral region is reported for four substituted Ru-(R-dipyrrinato) complexes, [Ru(bpy)₂(R-dipy)](PF₆), where bpy is 2,2'-bipyridine and the substituent R is phenyl (ph), 2,4,6-trimethylphenyl, 4-carboxyphenyl (HOOC-ph), or 4-pyridinyl. The experimentally determined phosphorescence efficiency, ι_em(p) = k_RAD(p)/(ν_em(p))³ (where k_RAD(p) and ν_em(p) are the phosphorescence rate constant and the phosphorescence frequency, respectively), of the [Ru(bpy)₂(R-dipy)]⁺ complexes was approximately double that of [Ru(bpy)(Am)₄]²⁺ complexes (Am = ammine ligand) in the NIR region. Density functional theory (DFT) modeling indicated two strikingly different electronic configurations of the triplet emitting state (T_e) in the two types of complexes. The T_e of [Ru(bpy)₂(R-dipy)]⁺ complexes shows a CT-type corresponding to the metal-to-ligand charge transfer (MLCT)-(Ru-(R-dipy)) and the ππ*-(R-dipy) moiety configurations, and the T_e state in the [Ru(bpy)(Am)₄]²⁺ complexes corresponds to an approximately MLCT excited state consisting of mostly MLCT-(Ru-bpy) with a minimal ππ*(bpy) contribution. DFT modeling also indicated that the low-energy singlet excited states in the T_e geometry (S_n(T)) of the [Ru(bpy)₂(ph-dipy)]⁺ complex consist of numerous CT-S_n(T)-type states of the Ru-dipy and Ru-bpy moieties, whereas the [Ru(bpy)(Am)₄]²⁺ ions show quite simple MLCT-S_n(T)-type states of the Ru-bpy moiety. Based on experimental observations, DFT modeling, and the plain spin-orbit coupling (SOC) principle, we conclude that the remarkably high ι_em(p) amplitudes of the [Ru(bpy)₂(R-dipy)]⁺ complexes relative to those of [Ru(bpy)(Am)₄]²⁺ complexes can be attributed to the relatively substantial contribution of intrinsic SOC-mediated intensity stealing from the numerous low-energy CT-type S_n(T) states.

Critical Aspects of Metal-Organic Framework-Based Materials for Solar-Driven CO2 Reduction into Valuable Fuels

Photoreduction of CO2 into value-added fuels is one of the most promising strategies for tackling the energy crisis and mitigating the "greenhouse effect." Recently, metal-organic frameworks (MOFs) have been widely investigated in the field of CO2 photoreduction owing to their high CO2 uptake and adjustable functional groups. The fundamental factors and state-of-the-art advancements in MOFs for photocatalytic CO2 reduction are summarized from the critical perspectives of light absorption, carrier dynamics, adsorption/activation, and reaction on the surface of photocatalysts, which are the three main critical aspects for CO2 photoreduction and determine the overall photocatalytic efficiency. In view of the merits of porous materials, recent progress of three other types of porous materials are also briefly summarized, namely zeolite-based, covalent-organic frameworks based (COFs-based), and porous semiconductor or organic polymer based photocatalysts. The remarkable performance of these porous materials for solar-driven CO2 reduction systems is highlighted. Finally, challenges and opportunities of porous materials for photocatalytic CO2 reduction are presented, aiming to provide a new viewpoint for improving the overall photocatalytic CO2 reduction efficiency with porous materials.

Recent Progress in Plasmonic Hybrid Photocatalysis for CO2 Photoreduction and C–C Coupling Reactions

Plasmonic hybrid nanostructures have been investigated as attractive heterogeneous photocatalysts that can utilize sunlight to produce valuable chemicals. In particular, the efficient photoconversion of CO2 into a stable hydrocarbon with sunlight can be a promising strategy to achieve a sustainable human life on Earth. The next step for hydrocarbons once obtained from CO2 is the carbon–carbon coupling reactions to produce a valuable chemical for energy storage or fine chemicals. For these purposes, plasmonic nanomaterials have been widely investigated as a visible-light-induced photocatalyst to achieve increased efficiency of photochemical reactions with sunlight. In this review, we discuss recent achievements involving plasmonic hybrid photocatalysts that have been investigated for CO and CO2 photoreductions to form multi-carbon products and for C–C coupling reactions, such as the Suzuki–Miyaura coupling reactions.

A Photorobust Mo(0) Complex Mimicking [Os(2,2'-bipyridine)3]2+ and Its Application in Red-to-Blue Upconversion

Osmium(II) polypyridines are a well-known class of complexes with luminescent metal-to-ligand charge-transfer (MLCT) excited states that are currently experiencing a revival due to their application potential in organic photoredox catalysis, triplet-triplet annihilation upconversion, and phototherapy. At the same time, there is increased interest in the development of photoactive complexes made from Earth-abundant rather than precious metals. Against this background, we present a homoleptic Mo(0) complex with a new diisocyanide ligand exhibiting different bite angles and a greater extent of π-conjugation than previously reported related chelates. This new design leads to deep red emission, which is unprecedented for homoleptic arylisocyanide complexes of group 6 metals. With a ³MLCT lifetime of 56 ns, an emission band maximum at 720 nm, and a photoluminescence quantum yield of 1.5% in deaerated toluene at room temperature, the photophysical properties are reminiscent of the prototypical [Os(2,2'-bipyridine)₃]²⁺ complex. Under 635 nm irradiation with a cw-laser, the new Mo(0) complex sensitizes triplet-triplet annihilation upconversion of 9,10-diphenylanthracene (DPA), resulting in delayed blue fluorescence with an anti-Stokes shift of 0.93 eV. The photorobustness of the Mo(0) complex and the upconversion quantum yield are high enough to generate a flux of upconverted light that can serve as a sufficiently potent irradiation source for a blue-light-driven photoisomerization reaction. These findings are relevant in the greater contexts of designing new luminophores and photosensitizers for use in red-light-driven photocatalysis, photochemical upconversion, light-harvesting, and phototherapy.

Iron(iii)-bipyridine incorporated metal-organic frameworks for photocatalytic reduction of CO2 with improved performance

Metal-organic frameworks (MOFs) represent an emerging class of platforms to assemble single site photocatalysts for artificial photosynthesis. In this work, we report a new CO₂ reduction photocatalyst (UiO-68-Fe-bpy) based on a robust Zr(iv)-MOF platform with incorporated Fe(bpy)Cl₃ (bpy refers to the 4'-methyl-[2,2'-bipyridine] moiety) via amine-aldehyde condensation. We show that this hybrid catalyst can reduce CO₂ to form CO under visible light illumination with excellent selectivity and enhanced activity with respect to its parent MOF and corresponding homogeneous counterpart. Using steady state and transient absorption (TA) spectroscopy, we show that the enhanced photocatalytic activity of UiO-68-Fe-bpy is attributed to the elongated excited state lifetime of Fe(bpy)Cl₃ after being incorporated to the UiO-68-NH₂ platform. This work demonstrates the great potential of MOFs as a next generation platform for solar fuel conversion.

Porous organic polymer enriched in Re functional units and Lewis base sites for efficient CO2 photoreduction

A porous organic polymer, BTN-Re, which contains two functional units, exhibits outstanding ability for CO2 photoreduction.

Recent advances in pincer-nickel catalyzed reactions

Organometallic catalysts have played a key role in accomplishing numerous synthetically valuable organic transformations that are either otherwise not possible or inefficient. The use of precious, sparse and toxic 4d and 5d metals are an apparent downside of several such catalytic systems despite their immense success over the last several decades. The use of complexes containing Earth-abundant, inexpensive and less hazardous 3d metals, such as nickel, as catalysts for organic transformations has been an emerging field in recent times. In particular, the versatile nature of the corresponding pincer-metal complexes, which offers great control of their reactivity via countless variations, has garnered great interest among organometallic chemists who are looking for greener and cheaper alternatives. In this context, the current review attempts to provide a glimpse of recent developments in the chemistry of pincer-nickel catalyzed reactions. Notably, there have been examples of pincer-nickel catalyzed reactions involving two electron changes via purely organometallic mechanisms that are strikingly similar to those observed with heavier Pd and Pt analogues. On the other hand, there have been distinct differences where the pincer-nickel complexes catalyze single-electron radical reactions. The applicability of pincer-nickel complexes in catalyzing cross-coupling reactions, oxidation reactions, (de)hydrogenation reactions, dehydrogenative coupling, hydrosilylation, hydroboration, C-H activation and carbon dioxide functionalization has been reviewed here from synthesis and mechanistic points of view. The flurry of global pincer-nickel related activities offer promising avenues in catalyzing synthetically valuable organic transformations.

Artificial light-harvesting 2D photosynthetic systems with iron phthalocyanine/graphitic carbon nitride composites for highly efficient CO2 reduction

Fabricating high-efficient 2D artificial photosynthetic systems for CO2 reduction based on phthalocyanine/graphitic carbon nitride composites.

Luminescence properties of [Ir(C^N)2(N^N)]+ complexes: relations between DFT computation results and emission band-shape analysis data

Luminescence properties of two series of [Ir(C^N)₂(N^N)]⁺ complexes bearing deprotonated 1-phenyl-1H-pyrazole or 1-(2,4-difluorophenyl)-1H-pyrazole as cyclometalating C^N ligands and different α-diimines (2,2′-bipyridine, 1,10-phenanthroline and their derivatives) as ancillary N^N ligands have been studied in acetonitrile solutions at room temperature and in 77 K methanol/ethanol (1 : 1) matrices. Ligand and temperature induced changes in the nature of the emissive ³*[Ir(C^N)₂(N^N)]⁺ species result in well-pronounced changes in their emission properties like emission wavelength, emission quantum yields and emission lifetimes. Depending on the nature of the coordinated C^N and N^N ligands and/or the measurement temperature, the investigated luminophores exhibit emissions arising from the intraligand transitions localized within the N^N ligand or from the metal-to-ligand charge-transfer transitions involving the Ir(C^N)₂⁺ and N^N moieties as confirmed by means of the DFT computations. The computed DFT energies of the excited ³*[Ir(C^N)₂(N^N)]⁺ states and outer/inner reorganization energies associated with the S₀ ← ³*[Ir(C^N)₂(N^N)]⁺ transitions remain in nice agreement with those available from the performed emission band-shape analyses. The observed agreement implies ordinary DFT computations at the B3LYP/LANL2DZ/6-31G(d,p) level of theory, even performed neglecting the spin–orbit phenomena, as enough accurate in the quantitative prediction of the most important parameters characterizing the investigated [Ir(C^N)₂(N^N)]⁺ luminophores.

For two series of [Ir(C^N)₂(N^N)]⁺ luminophores, the computed DFT quantities remain in nice agreement with those available from the emission band-shape analyses.

Nickel is a Different Pickle: Trends in Water Oxidation Catalysis for Molecular Nickel Complexes

The development of novel water oxidation catalysts is important in the context of renewable fuels production. Ligand design is one of the key tools to improve the activity and stability of molecular catalysts. The establishment of ligand design rules can facilitate the development of improved molecular catalysts. In this paper it is shown that chemical oxidants can be used to probe oxygen evolution activity for nickel-based systems, and trends are reported that can improve future ligand design. Interestingly, different ligand effects were observed in comparison to other first-row transition metal complexes. For example, nickel complexes with secondary amine donors were more active than with tertiary amine donors, which is the opposite for iron complexes. The incorporation of imine donor groups in a cyclam ligand resulted in the fastest and most durable nickel catalyst of our series, achieving oxygen evolution turnover numbers up to 380 and turnover frequencies up to 68 min-1 in a pH 5.0 acetate buffer using Oxone as oxidant. Initial kinetic experiments with this catalyst revealed a first order in chemical oxidant and a half order in catalyst. This implies a rate-determining oxidation step from a dimeric species that needs to break up to generate the active catalyst. These findings lay the foundation for the rational design of molecular nickel catalysts for water oxidation and highlight that catalyst design rules are not generally applicable for different metals.

Immobilization of a Molecular Re Complex on MOF-derived Hierarchical Porous Carbon for CO2 Electroreduction in Water/Ionic Liquid Electrolyte

The development of molecular catalysts for CO₂ electroreduction within electrolyzers requests their immobilization on the electrodes. While a variety of methods have been explored for the heterogenization of homogeneous complexes, a novel approach using a hierarchical porous carbon material, derived from a metal-organic framework, is reported as a support for the well-known molecular catalyst [Re(bpy)(CO)₃ Cl] (bpy=2,2'-bipyridine). This cathodic hybrid material, named Re@HPC (HPC=hierarchical porous carbon), has been tested for CO₂ electroreduction using a mixture of an ionic liquid (1-ethyl-3-methylimidazolium tetrafluoroborate, EMIM) and water as the electrolyte. Interestingly, it catalyzes the conversion of CO₂ into a mixture of carbon monoxide and formic acid, with a selectivity that depends on the applied potential. The present study thus reveals that Re@HPC is a remarkable catalyst, enjoying excellent activity (turnover numbers for CO₂ reduction of 7835 after 2 h at -1.95 V vs. Fc/Fc⁺ with a current density of 6 mA cm^-2 ) and good stability. These results emphasize the advantages of integrating molecular catalysts onto such porous carbon materials for developing novel, stable and efficient, catalysts for CO₂ reduction.

Highly Efficient and Selective Visible-Light Driven CO2 Reduction by Two Co-Based Catalysts in Aqueous Solution

Photocatalytic CO₂ reduction has been considered as a promising approach to solve energy and environmental problems. Nevertheless, developing inexpensive photocatalysts with high efficiency and selectivity remains a big challenge. In this study, two Co-based complexes [Co₂(L¹)Cl₂] (1-Co) and [Co(L²)Cl] (2-Co) were synthesized by treating two DPA-based (DPA: dipicolylamine) ligands with Co²⁺, respectively. Under visible-light irradiation, the performance of 1-Co as a homogeneous photocatalyst for CO₂ reduction in aqueous media has been explored by using [Ru(phen)₃]²⁺ as a photosensitizer, and triethylolamine (TEOA) as a sacrificial reductant. 1-Co shows high photocatalytic activity for CO₂-to-CO conversion, corresponding to the high TON_CO of 2600 and TOF_CO of 260 h^-1 (TON_CO = turnover number for CO; TOF_CO = turnover frequency for CO). High selectivity of 97% for CO formation is also achieved. The control experiments catalyzed by 2-Co demonstrated that two Co(II) centers in 1-Co may operate independently and activate one CO₂ molecule each. Furthermore, the proposed mechanism of 1-Co for photocatalytic CO₂ reduction has been investigated via electrochemical analysis, a series of quenching experiments, and density functional theory calculations.

Photons to Formate-A Review on Photocatalytic Reduction of CO2 to Formic Acid

Rising levels of atmospheric carbon dioxide due to the burning and depletion of fossil fuels is continuously raising environmental concerns about global warming and the future of our energy supply. Renewable energy, especially better utilization of solar energy, is a promising method for CO2 conversion and chemical storage. Research in the solar fuels area is focused on designing novel catalysts and developing new conversion pathways. In this review, we focus on the photocatalytic reduction of CO2 primarily in its neutral pH species of carbonate to formate. The first two-electron photoproduct of carbon dioxide, a case for formate (or formic acid) is made in this review based on its value as; an important chemical feedstock, a hydrogen storage material, an intermediate to methanol, a high-octane fuel and broad application in fuel cells. This review focuses specifically on the following photocatalysts: semiconductors, phthalocyanines as photosensitizers and membrane devices and metal-organic frameworks.

Tailored self-assembled photocatalytic nanofibres for visible-light-driven hydrogen production

The creation of efficient artificial systems that mimic natural photosynthesis represents a key current challenge. Here, we describe a high-performance recyclable photocatalytic core-shell nanofibre system that integrates a cobalt catalyst and a photosensitizer in close proximity for hydrogen production from water using visible light. The composition, microstructure and dimensions-and thereby the catalytic activity-of the nanofibres were controlled through living crystallization-driven self-assembly. In this seeded growth strategy, block copolymers with crystallizable core-forming blocks and functional coronal segments were coassembled into low-dispersity, one-dimensional architectures. Under optimized conditions, the nanofibres promote the photocatalytic production of hydrogen from water with an overall quantum yield for solar energy conversion to hydrogen gas of ~4.0% (with a turnover number of >7,000 over 5 h, a frequency of >1,400 h^-1 and a H₂ production rate of >0.327 μmol h^-1 with 1.34 μg of catalytic polymer (that is, >244,300 μmol h^-1 g^-1 of catalytic polymer)).

Nanoporous Ta3N5via electrochemical anodization followed by nitridation for solar water oxidation

Nanoporous tantalum nitride (Ta3N5) is a promising visible-light-driven photoanode for photoelectrochemical (PEC) water splitting with a narrow band gap of approximately 2.0 eV. It can utilize a large portion of the solar spectrum up to 600 nm to improve the activity of photooxidation reactions because of enhanced light scattering and an overall increase of the surface area with high light absorption and carrier collection. Herein, we synthesized a new n-type nanoporous tantalum nitride film on Ta foil by electrochemical anodization with a fluorinated electrolyte. Post-annealing in a nitrogen/ammonia mixture gas environment then transformed amorphous TaOx to crystalline Ta3N5. Effects of annealing temperature on the microstructure, optical properties, and PEC properties of samples were then investigated under changeable stoichiometry of Ta and N elements in the Ta-based nitride film. Results showed that the film annealed at 1000 °C showed high crystallinity, high visible light absorption, and a highly conductive interlayer between the substrates, resulting in the highest photocurrent density (JSC) of ∼0.25 mA cm-2 at 1.23 VRHE in PEC water splitting. In addition, depending on the annealing temperature, it is possible to engineer band alignment in the nanoporous Ta3N5 layer, allowing a beneficial charge transfer process.

CO2 and H2O Coadsorption and Reaction on the Low-Index Surfaces of Tantalum Nitride: A First-Principles DFT-D3 Investigation

A comprehensive mechanistic insight into the photocatalytic reduction of CO2 by H2O is indispensable for the development of highly efficient and robust photocatalysts for artificial photosynthesis. This work presents first-principles mechanistic insights into the adsorption and activation of CO2 in the absence and presence of H2O on the (001), (010), and (110) surfaces of tantalum nitride (Ta3N5), a photocatalysts of significant technological interest. The stability of the different Ta3N surfaces is shown to dictate the strength of adsorption and the extent of activation of CO2 and H2O species, which bind strongest to the least stable Ta3N5(001) surface and weakest to the most stable Ta3N5(110) surface. The adsorption of the CO2 on the Ta3N5(001), (010), and (110) surfaces is demonstrated to be characterized by charge transfer from surface species to the CO2 molecule, resulting in its activation (i.e., forming negatively charged bent CO2−δ species, with elongated C–O bonds confirmed via vibrational frequency analyses). Compared to direct CO2 dissociation, H2O dissociates spontaneously on the Ta3N5 surfaces, providing the necessary hydrogen source for CO2 reduction reactions. The coadsorption reactions of CO2 and H2O are demonstrated to exhibit the strongest attractive interactions on the (010) surface, giving rise to proton transfer to the CO2 molecule, which causes its spontaneous dissociation to form CO and 2OH− species. These results demonstrate that Ta3N5, a narrow bandgap photocatalyst able to absorb visible light, can efficiently activate the CO2 molecule and photocatalytically reduce it with water to produce value-added fuels.