Biomimetic and microbial approaches to solar fuel generation

Iron(III) Carbene Complexes with Tunable Excited State Energies for Photoredox and Upconversion

Substituting precious elements in luminophores and photocatalysts by abundant first-row transition metals remains a significant challenge, and iron continues to be particularly attractive owing to its high natural abundance and low cost. Most iron complexes known to date face severe limitations due to undesirably efficient deactivation of luminescent and photoredox-active excited states. Two new iron(III) complexes with structurally simple chelate ligands enable straightforward tuning of ground and excited state properties, contrasting recent examples, in which chemical modification had a minor impact. Crude samples feature two luminescence bands strongly reminiscent of a recent iron(III) complex, in which this observation was attributed to dual luminescence, but in our case, there is clear-cut evidence that the higher-energy luminescence stems from an impurity and only the red photoluminescence from a doublet ligand-to-metal charge transfer (2LMCT) excited state is genuine. Photoinduced oxidative and reductive electron transfer reactions with methyl viologen and 10-methylphenothiazine occur with nearly diffusion-limited kinetics. Photocatalytic reactions not previously reported for this compound class, in particular the C-H arylation of diazonium salts and the aerobic hydroxylation of boronic acids, were achieved with low-energy red light excitation. Doublet-triplet energy transfer (DTET) from the luminescent 2LMCT state to an anthracene annihilator permits the proof of principle for triplet-triplet annihilation upconversion based on a molecular iron photosensitizer. These findings are relevant for the development of iron complexes featuring photophysical and photochemical properties competitive with noble-metal-based compounds.

Site-specific electronic structure of covalently linked bimetallic dyads from nitrogen K-edge x-ray absorption spectroscopy

A nitrogen K-edge x-ray absorption near-edge structure (XANES) survey is presented for tetrapyrido[3,2-a:2',3'-c:3″,2″-h:2‴,3‴-j]phenazine (tpphz)-bridged bimetallic assemblies that couple chromophore and catalyst transition metal complexes for light driven catalysis, as well as their individual molecular constituents. We demonstrate the high N site sensitivity of the N pre-edge XANES features, which are energetically well-separated for the phenazine bridge N atoms and for the individual metal-bound N atoms of the inner coordination sphere ligands. By comparison with the time-dependent density functional theory calculated spectra, we determine the origins of these distinguishable spectral features. We find that metal coordination generates large shifts toward higher energy for the metal-bound N atoms, with increasing shift for 3d < 4d < 5d metal bonding. This is attributed to increasing ligand-to-metal σ donation that increases the effective charge of the bound N atoms and stabilizes the N 1s core electrons. In contrast, the phenazine bridge N pre-edge peak is found at a lower energy due to stabilization of the low energy electron accepting orbital localized on the phenazine motif. While no sensitivity to ground state electronic coupling between the individual molecular subunits was observed, the spectra are sensitive to structural distortions of the tpphz bridge. These results demonstrate N K-edge XANES as a local probe of electronic structure in large bridging ligand motifs, able to distinctly investigate the ligand-centered orbitals involved in metal-to-ligand and ligand-to-ligand electron transfer following light absorption.

Photoredox-active Cr(0) luminophores featuring photophysical properties competitive with Ru(II) and Os(II) complexes

Coordination complexes of precious metals with the d6 valence electron configuration such as Ru(II), Os(II) and Ir(III) are used for lighting applications, solar energy conversion and photocatalysis. Until now, d6 complexes made from abundant first-row transition metals with competitive photophysical and photochemical properties have been elusive. While previous research efforts focused mostly on Fe(II), we disclose that isoelectronic Cr(0) gives access to higher photoluminescence quantum yields and excited-state lifetimes when compared with any other first-row d6 metal complex reported so far. The luminescence behaviour of the metal-to-ligand charge transfer excited states of these Cr(0) complexes is competitive with Os(II) polypyridines. With these Cr(0) complexes, the metal-to-ligand charge transfer states of first-row d6 metal complexes become exploitable in photoredox catalysis, and benchmark chemical reductions proceed efficiently under low-energy red illumination. Here we demonstrate that appropriate molecular design strategies open up new perspectives for photophysics and photochemistry with abundant first-row d6 metals.

Functionalized nickel(II)-iron(II) dithiolates as biomimetic models of [NiFe]-H2ases

To develop the structural and functional modeling chemistry of [NiFe]-H₂ases, a series of new biomimetics for the active site of [NiFe]-H₂ases have been prepared by various synthetic methods. Treatment of the mononuclear Ni complex (pnp)NiCl₂ (pnp = (Ph₂PCH₂)₂NPh) with (dppv)Fe(CO)₂(pdt) (dppv = 1,2-(Ph₂P)₂C₂H₂, pdt = 1,3-propanedithiolate) and KPF₆ gave the dicarbonyl complex [(pnp)Ni(pdt)Fe(CO)₂(dppv)](PF₆)₂ ([1](PF₆)₂). Further treatment of [1](PF₆)₂ and [(dppe)Ni(pdt)Fe(CO)₂(dppv)](BF₄)₂ (dppe = 1,2-(Ph₂P)₂C₂H₄) with the decarbonylation agent Me₃NO and pyridine afforded the novel sp³ C-Fe bond-containing complexes [(pnp)Ni(SCH₂CH₂CHS)Fe(CO)(dppv)]PF₆ ([2]PF₆) and [(dppe)Ni(SCH₂CH₂CHS)Fe(CO)(dppv)]BF₄ ([3]BF₄). More interestingly, the first t-carboxylato complexes [(pnp)Ni(pdt)Fe(CO)(t-O₂CR)(dppv)]PF₆ ([4]PF₆, R = H; [5]PF₆, R = Me; [6]PF₆, R = Ph) could be prepared by reactions of [1]PF₆ with the corresponding carboxylic acids RCO₂H in the presence of Me₃NO, whereas further reactions of [4]PF₆-[6]PF₆ with aqueous HPF₆ and 1.5 MPa H₂ gave rise to the μ-hydride complex [(pnp)Ni(pdt)Fe(CO)(μ-H)(dppv)]PF₆ ([7]PF₆). Except for H₂ activation by t-carboxylato complexes [4]PF₆-[6]PF₆ to give a μ-hydride complex ([7]PF₆), the sp³ C-Fe bond-containing complex [2]PF₆ was found to be a catalyst for proton reduction to H₂ under CV conditions. Furthermore, the chemical reactivity of the μ-hydride complex [7]PF₆ displayed in the e^- transfer reaction with FcPF₆ in the presence of CO, the H₂ evolution reaction with the protonic acid HCl, and the H^- transfer reaction with N-methylacridinium hexafluorophosphate ([NMA]PF₆) was systematically studied. As a result, a series of the expected products such as H₂, ferrocene, the dicarbonyl complex [1](PF₆)₂, the μ-chloro complex [(pnp)Ni(pdt)Fe(CO)(μ-Cl)(dppv)]PF₆ ([8]PF₆), the t-MeCN-coordinated complex [(pnp)Ni(pdt)Fe(CO)(t-MeCN)(dppv)](PF₆)₂ ([9](PF₆)₂) and the H^- transfer product AcrH₂ were produced. While all the newly prepared model complexes were structurally characterized by spectroscopic methods, the molecular structures of some of their representatives were confirmed by X-ray crystallography.

Bioinspired and biomolecular catalysts for energy conversion and storage

Metalloenzymes are remarkable for facilitating challenging redox transformations with high efficiency and selectivity. In the area of alternative energy, scientists aim to capture these properties in bioinspired and engineered biomolecular catalysts for the efficient and fast production of fuels from low-energy feedstocks such as water and carbon dioxide. In this short review, efforts to mimic biological catalysts for proton reduction and carbon dioxide reduction are highlighted. Two important recurring themes are the importance of the microenvironment of the catalyst active site and the key role of proton delivery to the active site in achieving desired reactivity. Perspectives on ongoing and future challenges are also provided.

High Intrinsic Phosphorescence Efficiency and Density Functional Theory Modeling of Ru(II)-Bipyridine Complexes with π-Aromatic-Rich Cyclometalated Ligands: Attributions of Spin-Orbit Coupling Perturbation and Efficient Configurational Mixing of Singlet Excited States

A series of π-aromatic-rich cyclometalated ruthenium(II)-(2,2'-bipyridine) complexes ([Ru(bpy)₂(π_Ar-CM)]⁺) in which π_Ar-CM is diphenylpyrazine or 1-phenylisoquinoline were prepared. The [Ru(bpy)₂(π_Ar-CM)]⁺ complexes had remarkably high phosphorescence rate constants, k _RAD(p), and the intrinsic phosphorescence efficiencies (ι_em(p) = k _RAD(p)/(ν_em(p))³) of these complexes were found to be twice the magnitudes of simply constructed cyclometalated ruthenium(II) complexes ([Ru(bpy)₂(sc-CM)]⁺), where ν_em(p) is the phosphorescence frequency and sc-CM is 2-phenylpyridine, benzo[h]quinoline, or 2-phenylpyrimidine. Density functional theory (DFT) modeling of the [Ru(bpy)₂(CM)]⁺ complexes indicated numerous singlet metal-to-ligand charge transfers for ¹MLCT-(Ru-bpy) and ¹MLCT-(Ru-CM), excited states in the low-energy absorption band and ¹ππ*-(aromatic ligand) (¹ππ*-L_Ar) excited states in the high-energy band. DFT modeling of these complexes also indicated phosphorescence-emitting state (T_e) configurations with primary MLCT-(Ru-bpy) characteristics. The variation in ι_em(p) for the spin-forbidden T_e (³MLCT-(Ru-bpy)) excited state of the complex system that was examined in this study can be understood through the spin-orbit coupling (SOC)-mediated sum of intensity stealing (∑SOCM-IS) contribution from the primary intensity of the low-energy ¹MLCT states and second-order intensity perturbation from the significant configuration between the low-energy ¹MLCT and high-energy intense ¹ππ*-L_Ar states. In addition, the observation of unusually high ι_em(p) magnitudes for these [Ru(bpy)₂(π_Ar-CM)]⁺ complexes can be attributed to the values for both intensity factors in the ∑SOCM-IS formalism being individually greater than those for [Ru(bpy)₂(sc-CM)]⁺ ions.

Zinc(II) Complexes with Triplet Charge-Transfer Excited States Enabling Energy-Transfer Catalysis, Photoinduced Electron Transfer, and Upconversion

Many Cu^I complexes have luminescent triplet charge-transfer excited states with diverse applications in photophysics and photochemistry, but for isoelectronic Zn^II compounds, this behavior is much less common, and they typically only show ligand-based fluorescence from singlet π-π* states. We report two closely related tetrahedral Zn^II compounds, in which intersystem crossing occurs with appreciable quantum yields and leads to the population of triplet excited states with intraligand charge-transfer (ILCT) character. In addition to showing fluorescence from their initially excited ¹ILCT states, these new compounds therefore undergo triplet-triplet energy transfer (TTET) from their ³ILCT states and consequently can act as sensitizers for photo-isomerization reactions and triplet-triplet annihilation upconversion from the blue to the ultraviolet spectral range. The photoactive ³ILCT state furthermore facilitates photoinduced electron transfer. Collectively, our findings demonstrate that mononuclear Zn^II compounds with photophysical and photochemical properties reminiscent of well-known Cu^I complexes are accessible with suitable ligands and that they are potentially amenable to many different applications. Our insights seem relevant in the greater context of obtaining photoactive compounds based on abundant transition metals, complementing well-known precious-metal-based luminophores and photosensitizers.

Energy transfer dynamics and the mechanism of biohybrid photosynthetic antenna complexes chemically linked with artificial chromophores

A light-harvesting strategy is crucial for the utilisation of solar energy. In this study, we addressed the expanding light-harvesting (LH) wavelength of photosynthetic LH complex 2 (LH2, from Rhodoblastus acidophilus strain 10050) through covalent conjugation with extrinsic chromophores. To further understand the conjugation architecture and mechanism of excitation energy transfer (EET), we examined the effects of the linker length and spectral overlap integral between the emission and absorption spectra of the energy donor and acceptor pigments. In the former case, contrary to the intuition based on the Förster resonance energy transfer (FRET) theory, the observed energy transfer rate was similar regardless of the linker length, and the energy transfer efficiency increased with longer linkers. In the latter case, despite the energy transfer rate increases at higher spectral overlaps, it was quantitatively inconsistent with the FRET theory. The mechanism of EET beyond the FRET theory was discussed in terms of the higher-lying exciton state of B850, which mediates efficient EET despite the small spectral overlap. This systematic investigation provides insights for the development of efficient artificial photosynthetic systems.

Shedding Light on the Oxidizing Properties of Spin-Flip Excited States in a CrIII Polypyridine Complex and Their Use in Photoredox Catalysis

The photoredox activity of well-known Ru^II complexes stems from metal-to-ligand charge transfer (MLCT) excited states, in which a ligand-based electron can initiate chemical reductions and a metal-centered hole can trigger oxidations. Cr^III polypyridines show similar photoredox properties, although they have fundamentally different electronic structures. Their photoactive excited state is of spin-flip nature, differing from the electronic ground state merely by a change of one electron spin, but with otherwise identical d-orbital occupancy. We find that the driving-force dependence for photoinduced electron transfer from 10 different donors to a spin-flip excited state of a Cr^III complex is very similar to that for a Ru^II polypyridine, and thereby validate the concept of estimating the redox potential of d³ spin-flip excited states in analogous manner as for the MLCT states of d⁶ compounds. Building on this insight, we use our Cr^III complex for photocatalytic reactions not previously explored with this compound class, including the aerobic bromination of methoxyaryls, oxygenation of 1,1,2,2-tetraphenylethylene, aerobic hydroxylation of arylboronic acids, and the vinylation of N-phenyl pyrrolidine. This work contributes to understanding the fundamental photochemical properties of first-row transition-metal complexes in comparison to well-explored precious-metal-based photocatalysts.

Different sizes of polystyrene microplastics induced distinct microbial responses of anaerobic granular sludge

Recent investigations confirmed the inhibitory effect of microplastics with single sizes on the anaerobic granular sludge (AGS) wastewater treatment system. However, the differences of toxicity from different sizes of microplastics toward AGS and their underlying mechanism are still unclear. In this work, the responds of AGS exposed to different particle sizes of polystyrene microplastics (PS-MPs) were reported. The results showed that the increasing particle sizes (from 0.5 μm to 150 μm) of PS-MPs induced a gradually increasing and distinct inhibitory (from 6.7% to 16.2%) effect on the cumulative methane production by AGS, accompanied by the similar decreasing organic carbon degradation trends. Correspondingly, the integrity and the cell viability of the AGS granules were damaged and the populations of the key acidogens and methanogens were reduced when exposed to PS-MPs, which was particularly evident in the reactors affected by the larger micron-sized PS-MPs. The zeta potential and contact angle indicated that the larger-sized PS-MPs had the stronger dispersive properties and affinity for AGS, causing the higher oxidative stress and leachates toxicity. Further investigation revealed that the tolerance of AGS to PS-MPs toxicity also exhibited size-dependent trend. Larger particles (e.g., 150 μm) of PS-MPs inhibited extracellular polymeric substance (EPS) secretion, while smaller particles (e.g., 0.5 μm) promoted EPS generation with the release of more humic acid, alleviating their toxicity.

Solvent-Mediated Charge Transfer Dynamics of a Model Brown Carbon Aerosol Chromophore: Photophysics of 1-Phenylpyrrole Induced by Water Solvation

Nitrogen heterocycles are known to be important light-absorbing chromophores in a newly discovered class of aerosols, commonly referred to as "brown carbon" (BrC) aerosols. Due to their significant absorption and spectral overlap with the solar actinic flux, these BrC chromophores steer the physical and optical properties of aerosols. To model the local aqueous solvation environment surrounding BrC chromophores, we generated cold molecular complexes with water and a prototypical BrC chromophore, 1-phenylpyrrole (1PhPy), using supersonic jet-cooling and explored their intermolecular interactions using single-conformation spectroscopy. Herein, we utilized resonant two-photon ionization (R2PI) and UV holeburning (UV HB) double-resonance spectroscopies to obtain a molecular-level understanding of the role of water microsolvation in charge transfer upon photoexcitation of 1PhPy. Quantum chemical calculations and one-dimensional discrete variable representation simulations revealed insights into the charge transfer efficacy of 1PhPy with and without addition of a single water molecule. Taken together, our results indicate that the intermolecular interactions with water guide the geometry of 1PhPy to adopt a more twisted intramolecular charge transfer (TICT) configuration, thus facilitating charge transfer from the pyrrole donor to the phenyl ring acceptor. Furthermore, the water network surrounding 1PhPy reports on the charge transfer such that the H₂O solvent primarily interacts with the pyrrole ring donor in the ground state, whereas it preferentially interacts with the phenyl ring acceptor in the excited state. Large Franck-Condon activity is evident in the 1PhPy + 1H₂O excitation spectrum for the water-migration vibronic bands, supporting H₂O solvent reorganization upon excitation of the 1PhPy chromophore. Fluorescence measurements with increasing H₂O % volume corroborated our gas-phase studies by indicating that a polar water solvation environment stabilizes the TICT configuration of 1PhPy in the excited electronic state, from which emission is observed at a lower energy compared to the locally excited configuration.

Molecular Rubies in Photoredox Catalysis

The molecular ruby [Cr(tpe) ₂ ] ³⁺ and the tris(bipyridine) chromium(III) complex [Cr(dmcbpy) ₃ ] ³⁺ as well as the tris(bipyrazine)ruthenium(II) complex [Ru(bpz) ₃ ] ²⁺ were employed in the visible light-induced radical cation [4+2] cycloaddition (tpe = 1,1,1-tris(pyrid-2-yl)ethane, dmcbpy = 4,4'-dimethoxycarbonyl-2,2'-bipyridine, bpz = 2,2'-bipyrazine), while [Cr(ddpd) ₂ ] ³⁺ serves as a control system (ddpd = N,N'-dimethyl-N,N'-dipyridin-2-ylpyridine-2,6-diamine). Along with an updated mechanistic proposal for the Cr^III driven catalytic cycle based on redox chemistry, Stern-Volmer analyses, UV/Vis/NIR spectroscopic and nanosecond laser flash photolysis studies, we demonstrate that the very weakly absorbing photocatalyst [Cr(tpe) ₂ ] ³⁺ outcompetes [Cr(dmcbpy) ₃ ] ³⁺ and even [Ru(bpz) ₃ ] ²⁺ in particular at low catalyst loadings, which appears contradictory at first sight. The high photostability, the reversible redoxchemistry and the very long excited state lifetime account for the exceptional performance and even reusability of [Cr(tpe) ₂ ] ³⁺ in this photoredox catalytic system.

Linker Engineering in Metal-Organic Frameworks for Dark Photocatalysis

Dark reactions featuring continuous activity under light off conditions play a critical role in natural photosynthesis. However, most artificial photocatalysts are inactive upon the removal of the light source, and the artificial photocatalysts with dark photocatalysis abilities have been rarely explored. Herein, we report a Ti-based metal–organic framework (MOF), MIL-125, exhibiting the capability of dark photocatalytic hydrogen production. Remarkably, the introduction of different functional groups onto the linkers enables distinctly different activities of the resulting MOFs (MIL-125-X, X = NH₂, NO₂, Br). Dynamic and thermodynamic investigations indicate that the production and lifetime of the Ti³⁺ intermediate are the key factors, due to the electron-donating/-withdrawing effect of the functional groups. As far as we know, this is the first report on dark photocatalysis over MOFs, providing new insights into the storage of irradiation energy and demonstrating their great potential in dark photocatalysis due to the great MOF diversity.

A Ti-based MOF with long-lived Ti³⁺ can achieve dark photocatalysis. The different groups on the organic linker modulate electron storage ability and the lifetime of Ti³⁺, significantly regulating dark photocatalytic activity in H₂ production.

Electrocatalytic Behavior of Tetrathiafulvalene (TTF) and Extended Tetrathiafulvalene (exTTF) [FeFe] Hydrogenase Mimics

TTF- and exTTF-containing [(μ-S₂)Fe₂(CO)₆] complexes have been prepared by the photochemical reaction of TTF or exTTF and [(μ-S₂)Fe₂(CO)₆]. These complexes are able to interact with PAHs. In the absence of air and in acid media an electrocatalytic dihydrogen evolution reaction (HER) occurs, similarly to analogous [(μ-S₂)Fe₂(CO)₆] complexes. However, in the presence of air, the TTF and exTTF organic moieties strongly influence the electrochemistry of these systems. The reported data may be valuable in the design of [FeFe] hydrogenase mimics able to combine the HER properties of the [FeFe] cores with the unique TTF properties.

Manganese(I) complexes with metal-to-ligand charge transfer luminescence and photoreactivity

Precious metal complexes with the d⁶ valence electron configuration often exhibit luminescent metal-to-ligand charge transfer (MLCT) excited states, which form the basis for many applications in lighting, sensing, solar cells and synthetic photochemistry. Iron(II) has received much attention as a possible Earth-abundant alternative, but to date no iron(II) complex has been reported to show MLCT emission upon continuous-wave excitation. Manganese(I) has the same electron configuration as that of iron(II), but until now has typically been overlooked in the search for cheap MLCT luminophores. Here we report that isocyanide chelate ligands give access to air-stable manganese(I) complexes that exhibit MLCT luminescence in solution at room temperature. These compounds were successfully used as photosensitizers for energy- and electron-transfer reactions and were shown to promote the photoisomerization of trans-stilbene. The observable electron transfer photoreactivity occurred from the emissive MLCT state, whereas the triplet energy transfer photoreactivity originated from a ligand-centred ³π-π* state.

Sensitization-initiated electron transfer via upconversion: mechanism and photocatalytic applications

Sensitization-initiated electron transfer (SenI-ET) describes a recently discovered photoredox strategy that relies on two consecutive light absorption events, triggering a sequence of energy and electron transfer steps. The cumulative energy input from two visible photons gives access to thermodynamically demanding reactions, which would be unattainable by single excitation with visible light. For this reason, SenI-ET has become a very useful strategy in synthetic photochemistry, but the mechanism has been difficult to clarify due to its complexity. We demonstrate that SenI-ET can operate via sensitized triplet-triplet annihilation upconversion, and we provide the first direct spectroscopic evidence for the catalytically active species. In our system comprised of fac-[Ir(ppy)₃] as a light absorber, 2,7-di-tert-butylpyrene as an annihilator, and N,N-dimethylaniline as a sacrificial reductant, all photochemical reaction steps proceed with remarkable rates and efficiencies, and this system is furthermore suitable for photocatalytic aryl dehalogenations, pinacol couplings and detosylation reactions. The insights presented here are relevant for the further rational development of photoredox processes based on multi-photon excitation, and they could have important implications in the greater contexts of synthetic photochemistry and solar energy conversion.

Photostable Ruthenium(II) Isocyanoborato Luminophores and Their Use in Energy Transfer and Photoredox Catalysis

Ruthenium(II) polypyridine complexes are among the most popular sensitizers in photocatalysis, but they face some severe limitations concerning accessible excited-state energies and photostability that could hamper future applications. In this study, the borylation of heteroleptic ruthenium(II) cyanide complexes with α-diimine ancillary ligands is identified as a useful concept to elevate the energies of photoactive metal-to-ligand charge-transfer (MLCT) states and to obtain unusually photorobust compounds suitable for thermodynamically challenging energy transfer catalysis as well as oxidative and reductive photoredox catalysis. B(C₆F₅)₃ groups attached to the CN ^- ligands stabilize the metal-based t_2g-like orbitals by ∼0.8 eV, leading to high ³MLCT energies (up to 2.50 eV) that are more typical for cyclometalated iridium(III) complexes. Through variation of their α-diimine ligands, nonradiative excited-state relaxation pathways involving higher-lying metal-centered states can be controlled, and their luminescence quantum yields and MLCT lifetimes can be optimized. These combined properties make the respective isocyanoborato complexes amenable to photochemical reactions for which common ruthenium(II)-based sensitizers are unsuited, due to a lack of sufficient triplet energy or excited-state redox power. Specifically, this includes photoisomerization reactions, sensitization of nickel-catalyzed cross-couplings, pinacol couplings, and oxidative decarboxylative C-C couplings. Our work is relevant in the greater context of tailoring photoactive coordination compounds to current challenges in synthetic photochemistry and solar energy conversion.

Design of molecular water oxidation catalysts with earth-abundant metal ions

The four-electron oxidation of water (2H2O → O2 + 4H+ + 4e-) is considered the main bottleneck in artificial photosynthesis. In nature, this reaction is catalysed by a Mn4CaO5 cluster embedded in the oxygen-evolving complex of photosystem II. Ruthenium-based complexes have been successful artificial molecular catalysts for mimicking this reaction. However, for practical and large-scale applications in the future, molecular catalysts that contain earth-abundant first-row transition metal ions are preferred owing to their high natural abundance, low risk of depletion, and low costs. In this review, the frontier of water oxidation reactions mediated by first-row transition metal complexes is described. Special attention is paid towards the design of molecular structures of the catalysts and their reaction mechanisms, and these factors are expected to serve as guiding principles for creating efficient and robust molecular catalysts for water oxidation using ubiquitous elements.

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.

Photodynamics of Asymmetric Di-Iron-Cyano Hydrogenases Examined by Time-Resolved Mid-Infrared Spectroscopy

Two anionic asymmetric Fe-Fe hydrogenase model compounds containing a single cyano (CN) and five carboxyl (CO) ligands, [Et₄N][Fe₂(μ-S₂C₃H₆)(CO)₅(CN)₁] and [Et₄N][Fe₂(μ-S₂C₂H₄)(CO)₅(CN)₁], dissolved in room-temperature acetonitrile, are examined. The molecular asymmetry affects the redox potentials of the central iron atoms, thus changing the photophysics and possible catalytic properties of the compounds. Femtosecond ultraviolet excitation with mid-infrared probe spectroscopy of the model compounds was employed to better understand the ultrafast dynamics of the enzyme-active site. Continuous ultraviolet lamp excitation with Fourier transform infrared (FTIR) spectroscopy was also used to explore stable product formation on the second timescale. For both model compounds, two timescales are observed; a 20-30 ps decay and the formation of a long-lived photoproduct. The picosecond decay is assigned to vibrational cooling and rotational dynamics, while the residual spectra remain for up to 300 ps, suggesting the formation of new photoproducts. Static FTIR spectroscopy yielded a different stable photoproduct than that observed on the ultrafast timescale. Density functional theory calculations simulated photoproducts for CO-loss and CN-loss isomers, and the resulting photoproduct spectra suggest that the picosecond transients arise from a complex mixture of isomerization after CO-loss, while dimerization and formation of a CN-containing Fe-CO-Fe bridged species are also considered.

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.

Pentanuclear Scaffold: A Molecular Platform for Small-Molecule Conversions

Small-molecule conversions involving multielectron transfer processes enable the conversion of earth-abundant materials into valuable chemicals and are regarded as a solution for environmental and energy shortage problems. In this context, the development of artificial catalysts that promote these reactions is an important research target. In nature, metalloenzymes that contain multinuclear metal complexes as active sites are known to efficiently catalyze reactions under mild conditions. Therefore, using multinuclear metal complexes as artificial catalysts can be an attractive strategy for small-molecule conversions involving multielectron transfer processes. However, multinuclear-metal-complex-based catalysts for these reactions have not been well established. In this Account, we describe our recent advances in the development of multinuclear metal complexes as catalysts for small-molecule conversion, mainly focusing on water oxidation. As small-molecule conversions involving multielectron transfer processes consists of two essential processes, (1) the transfer of multiple electrons and (2) the formation/cleavage of covalent bond(s), catalysts for these reactions should facilitate both steps. Therefore, we assumed that the assembly of redox-active metal ions and the cooperative effect of neighboring coordinatively unsaturated metal ions can promote these processes. On the basis of this assumption, we employed a pentanuclear metal complex as a molecular scaffold for the catalyst. The scaffold has a pentanuclear structure with quasi-D₃ symmetry and consists of a [M₃(μ₃-X)] core (X = O^2- or OH^-) wrapped by two [M(μ-bpp)₃] units (Hbpp = 3,5-bis(2-pyridyl)pyrazole). The metal ions in the triangular core are coordinatively unsaturated, whereas the metal ions at the apical positions are coordinatively saturated. In other words, the pentanuclear scaffold possesses multiple redox-active centers and coordinatively unsaturated sites. It should also be noted that the electron transfer ability of the complex changes dramatically depending on the identity of the constituent metal ions. The iron derivative of the pentanuclear scaffold was found to serve as an electrocatalyst for water oxidation (2H₂O → O₂ + 4e^- + 4H⁺) with a high reaction rate and excellent robustness. The substitution of metal ions in the pentanuclear scaffold to cobalt ions resulted in the development of a catalyst for CO₂ reduction. Furthermore, we investigated the effect of substituents on the ligands of the pentanuclear iron complex and succeeded in precisely manipulating the electron transfer possess. These results clearly demonstrate that the pentanuclear scaffold is an attractive platform for catalysts for small-molecule conversions. Additionally, the intrinsic features of the multinuclear catalytic system, which are totally different from those of conventional mononuclear-metal-complex-based catalysts, are disclosed. In reactions mediated by multinuclear complexes, the multinuclear core can initially accumulate the charge required for catalysis to reach the catalytically active state. Subsequently, the catalyst in the active state reacts with the substrate, initiating electron transfer to the substrate and rearrangement of covalent bonds in the substrate to afford the product. In such a mechanism, the desired number of electrons can be transferred to the substrates in an on-demand fashion, and the formation of undesired chemical species in the targeted catalysis may be prevented. This feature of multinuclear-metal-complex-based catalysts will achieve demanding small-molecule conversions with a high reaction rate, selectivity, and durability.

Excited-state photophysical processes in a molecular system containing perylene bisimide and zinc porphyrin chromophores

Multichromophoric systems with efficient photoinduced excited-state processes are important for the conversion of solar energy in artificial photosynthesis. However, a low molecular absorption coefficient of these multichromophoric systems in the near-infrared region limits their power conversion efficiency in organic solar cells. It is critical to design molecules with a broad absorption range in the whole spectral region, to better harvest solar energy, and to reveal their important multiple-step photophysical processes for the design of organic solar cells. Here, we investigate a novel compound having three chromophores, namely two near-by N,N'-bis(1-pentyl)hexyl-3,4,9,10-perylenebiscarboximide (PDI) units linked to a zinc porphyrin core side by side (in the form of PDI-ZnPor-PDI), which absorbs solar energy ranging from the ultraviolet (UV) to near-infrared regions. The photophysical behavior of PDI-ZnPor-PDI in both film and solution forms, has been investigated using steady-state and transient spectroscopy measurements. Charge-transfer species and triplet excited-state species are observed, the excited-state evolutions of which are monitored using molecular vibrations as probes. These observations support the idea that PDI-ZnPor-PDI on photoexcitation generates the radical anion and triplet species of the PDI unit (PDI˙- and 3PDI*). Our results demonstrate the effect of solid film state on the photophysical properties in such multichromophoric system, and are valuable for guiding the design and utilization of novel near-infrared electron donors or acceptors for use in organic solar cells.

Polystyrene nanoplastics reshape the anaerobic granular sludge for recovering methane from wastewater

Wastewater has been identified as an important carrier for nanoplastics, which could elicit unintended impacts on critical microbial processes. However, the long-term impacts of nanoplastics on anaerobic granular sludge (AGS) for methane recovery from wastewater and the mechanisms involved remains unclear. In this study, we investigated the long term exposure-response relationship between polystyrene nanoplastics (Nano-PS) and AGS. In continuous test over 120 days with 86 days' Nano-PS exposure, feeding wastewater with 10 μg/L of Nano-PS had no significant impacts on the AGS performance. In comparison, higher levels (i.e., 20 and 50 μg/L) of Nano-PS decreased methane production and chemical oxygen demand (COD) removal by 19.0-28.6% and 19.3-30.0%, respectively, along with volatile fatty acids (VFA) accumulation. More extracellular polymeric substance (EPS) was induced by 10 μg/L of Nano-PS as a response to protect microbes, but higher levels (i.e., 20 and 50 μg/L) of Nano-PS decreased EPS generation, causing a decline in granule size and cell viability. Fluorescence tagging found that a large number of Nano-PS agglomerated/accumulated on the outer layer of AGS and even transferred into deeper layers of AGS over exposure time, producing toxic effects to adherent microorganisms, e.g., Longilinea sp., Paludibacter sp. and Methanosaeta sp.. The oxidative stress induced by Nano-PS was revealed to be a key factor for reshaping the AGS, reflected by the increased reactive oxygen species (ROS) generation and lactate dehydrogenase (LDH) release. The sodium dodecyl sulfate (SDS) leached from Nano-PS was also demonstrated to restrain the activities of antioxidant enzymes, thereby further lessening resistance to oxidative stress induced by Nano-PS. This work improves our ability to predict the risks associated with this ubiquitous contaminant in the environment.

The Coordination Behaviour of CuI Photosensitizers Bearing Multidentate Ligands Investigated by X-ray Absorption Spectroscopy

A systematic series of four novel homo‐ and heteroleptic Cu^I photosensitizers based on tetradentate 1,10‐phenanthroline ligands of the type X^N^N^X containing two additional donor moieties in the 2,9‐position (X=SMe or OMe) were designed. Their solid‐state structures were assessed by X‐ray diffraction. Cyclic voltammetry, UV‐vis absorption, emission and X‐ray absorption spectroscopy were then used to determine their electrochemical, photophysical and structural features in solution. Following, time‐resolved X‐ray absorption spectroscopy in the picosecond time scale, coupled with time‐dependent density functional theory calculations, provided in‐depth information on the excited state electron configurations. For the first time, a significant shortening of the Cu−X distance and a change in the coordination mode to a pentacoordinated geometry is shown in the excited states of the two homoleptic complexes. These findings are important with respect to a precise understanding of the excited state structures and a further stabilization of this type of photosensitizers.

Proton-Coupled Electron Transfer from Tyrosine in the Interior of a de novo Protein: Mechanisms and Primary Proton Acceptor

Proton-coupled electron transfer (PCET) from tyrosine produces a neutral tyrosyl radical (Y^•) that is vital to many catalytic redox reactions. To better understand how the protein environment influences the PCET properties of tyrosine, we have studied the radical formation behavior of Y₃₂ in the α₃Y model protein. The previously solved α₃Y solution NMR structure shows that Y₃₂ is sequestered ∼7.7 ± 0.3 Å below the protein surface without any primary proton acceptors nearby. Here we present transient absorption kinetic data and molecular dynamics (MD) simulations to resolve the PCET mechanism associated with Y₃₂ oxidation. Y₃₂^• was generated in a bimolecular reaction with [Ru(bpy)₃]³⁺ formed by flash photolysis. At pH > 8, the rate constant of Y₃₂^• formation (k_PCET) increases by one order of magnitude per pH unit, corresponding to a proton-first mechanism via tyrosinate (PTET). At lower pH < 7.5, the pH dependence is weak and shows a previously measured KIE ≈ 2.5, which best fits a concerted mechanism. k_PCET is independent of phosphate buffer concentration at pH 6.5. This provides clear evidence that phosphate buffer is not the primary proton acceptor. MD simulations show that one to two water molecules can enter the hydrophobic cavity of α₃Y and hydrogen bond to Y₃₂, as well as the possibility of hydrogen-bonding interactions between Y₃₂ and E₁₃, through structural fluctuations that reorient surrounding side chains. Our results illustrate how protein conformational motions can influence the redox reactivity of a tyrosine residue and how PCET mechanisms can be tuned by changing the pH even when the PCET occurs within the interior of a protein.

Synergetic catalysis of a cobalt-based coordination polymer for selective visible-light driven CO2-to-CO conversion

Herein, based on the strategy of synergetic catalysis, we report a cobalt-based coordination polymer PEI₆-Co. As a heterogeneous catalyst, PEI₆-Co shows a selectivity of 95% and a yield of 1170 mmol g⁻¹ for visible-light-driven CO₂-to-CO conversion in a water containing system, which is almost 2.8 times that of the mononuclear cobalt catalyst CoL¹ and is comparable to that of the dinuclear cobalt catalyst Co₂L.

Herein, based on the strategy of synergetic catalysis, we report a cobalt-based coordination polymer PEI₆-Co for the visible-light-driven CO₂-to-CO conversion in water containing system.

Exploiting singlet excited state conformation for rational design of highly efficient photoinduced electron transfer molecules

In spite of the pivotal role of excited state electronic structures as regulation of photoinduced electron transfer (PET) process, the effect of excited state conformation on PET remains elusive. Here we exploit distinguishable emission characters of trans and cis singlet excited states of donor-acceptor-donor ensemble MTPAAZO to reveal that its PET efficiency and rate are closely depended on its singlet excited state conformation. The PET process occurs solely in cis conformation of MTPAAZO singlet excited states. Novel molecule (MTPA)₂Ab as-designed with similar structure of MTPAAZO cis singlet excited states shows high PET efficacy and rate, leading to long-lived CS states. Our findings enable the rational design of the novel molecules with highly efficient PET process suitable for charge separation applications.

Photophysics and Photochemistry of Iron Carbene Complexes for Solar Energy Conversion and Photocatalysis

Earth-abundant first row transition metal complexes are important for the development of large-scale photocatalytic and solar energy conversion applications. Coordination compounds based on iron are especially interesting, as iron is the most common transition metal element in the Earth’s crust. Unfortunately, iron-polypyridyl and related traditional iron-based complexes generally suffer from poor excited state properties, including short excited-state lifetimes, that make them unsuitable for most light-driven applications. Iron carbene complexes have emerged in the last decade as a new class of coordination compounds with significantly improved photophysical and photochemical properties, that make them attractive candidates for a range of light-driven applications. Specific aspects of the photophysics and photochemistry of these iron carbenes discussed here include long-lived excited state lifetimes of charge transfer excited states, capabilities to act as photosensitizers in solar energy conversion applications like dye-sensitized solar cells, as well as recent demonstrations of promising progress towards driving photoredox and photocatalytic processes. Complementary advances towards photofunctional systems with both Fe(II) complexes featuring metal-to-ligand charge transfer excited states, and Fe(III) complexes displaying ligand-to-metal charge transfer excited states are discussed. Finally, we outline emerging opportunities to utilize the improved photochemical properties of iron carbenes and related complexes for photovoltaic, photoelectrochemical and photocatalytic applications.

Redox-Active Ligand Assisted Catalytic Water Oxidation by a RuIV =O Intermediate

Water splitting is one of the most promising solutions for storing solar energy in a chemical bond. Water oxidation is still the bottleneck step because of its inherent difficulty and the limited understanding of the O-O bond formation mechanism. Molecular catalysts provide a platform for understanding this process in depth and have received wide attention since the first Ru-based catalyst was reported in 1982. Ru^V =O is considered a key intermediate to initiate the O-O bond formation through either a water nucleophilic attack (WNA) pathway or a bimolecular coupling (I2M) pathway. Herein, we report a Ru-based catalyst that displays water oxidation reactivity with Ru^IV =(O) with the help of a redox-active ligand at pH 7.0. The results of electrochemical studies and DFT calculations disclose that ligand oxidation could significantly improve the reactivity of Ru^IV =O toward water oxidation. Under these conditions, sustained water oxidation catalysis occurs at reasonable rates with low overpotential (ca. 183 mV).

Giant iron polyoxometalate that works as a catalyst for water oxidation

A polyoxometalate (POM) cluster [Mo₇₂Fe₃₀O₂₅₂(CH₃COO)₁₂{Mo₂O₇(H₂O)}₂ {H₂Mo₂O₈(H₂O)} (H₂O)₉₁]. ca. 150 H₂O (catalyst I) has been explored as a light-driven water oxidation catalyst. The catalyst is stable and could be reused/recycled several times.

Spin-resolved charge displacement analysis as an intuitive tool for the evaluation of cPCET and HAT scenarios

The spin-resolved version of the charge displacement function is introduced as an intuitive tool for differentiating between hydrogen-atom transfer and concerted proton-coupled electron transfer.

Tri-functional molecular relay to fabricate size-controlled CoOx nanoparticles and WO3 photoanode for an efficient photoelectrochemical water oxidation

Heterojunction and element doping to couple light-harvesting semiconductors with catalytic materials have been widely employed for photoelectrochemical (PEC) water splitting.

Light-driven carbon-carbon bond formation via CO2 reduction catalyzed by complexes of CdS nanorods and a 2-oxoacid oxidoreductase

Redox enzymes are capable of catalyzing a vast array of useful reactions, but they require redox partners that donate or accept electrons. Semiconductor nanocrystals provide a mechanism to convert absorbed photon energy into redox equivalents for enzyme catalysis. Here, we describe a system for photochemical carbon-carbon bond formation to make 2-oxoglutarate by coupling CO₂ with a succinyl group. Photoexcited electrons from cadmium sulfide nanorods (CdS NRs) transfer to 2-oxoglutarate:ferredoxin oxidoreductase from Magnetococcus marinus MC-1 (MmOGOR), which catalyzes a carbon-carbon bond formation reaction. We thereby decouple MmOGOR from its native role in the reductive tricarboxylic acid cycle and drive it directly with light. We examine the dependence of 2-oxoglutarate formation on a variety of factors and, using ultrafast transient absorption spectroscopy, elucidate the critical role of electron transfer (ET) from CdS NRs to MmOGOR. We find that the efficiency of this ET depends strongly on whether the succinyl CoA (SCoA) cosubstrate is bound at the MmOGOR active site. We hypothesize that the conformational changes due to SCoA binding impact the CdS NR-MmOGOR interaction in a manner that decreases ET efficiency compared to the enzyme with no cosubstrate bound. Our work reveals structural considerations for the nano-bio interfaces involved in light-driven enzyme catalysis and points to the competing factors of enzyme catalysis and ET efficiency that may arise when complex enzyme reactions are driven by artificial light absorbers.

Continuous artificial synthesis of glucose precursor using enzyme-immobilized microfluidic reactors

Food production in green crops is severely limited by low activity and poor specificity of D-ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) in natural photosynthesis (NPS). This work presents a scientific solution to overcome this problem by immobilizing RuBisCO into a microfluidic reactor, which demonstrates a continuous production of glucose precursor at 13.8 μmol g⁻¹ RuBisCO min⁻¹ from CO₂ and ribulose-1,5-bisphosphate. Experiments show that the RuBisCO immobilization significantly enhances enzyme stabilities (7.2 folds in storage stability, 6.7 folds in thermal stability), and also improves the reusability (90.4% activity retained after 5 cycles of reuse and 78.5% after 10 cycles). This work mimics the NPS pathway with scalable microreactors for continuous synthesis of glucose precursor using very small amount of RuBisCO. Although still far from industrial production, this work demonstrates artificial synthesis of basic food materials by replicating the light-independent reactions of NPS, which may hold the key to food crisis relief and future space colonization.

Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) is a difficult enzyme to work with. Here, the authors covalently immobilized it in a microfluidic reactor to enhance its storage/thermal stabilities and reusability, which enabled the continuous artificial synthesis of glucose precursor.

Photodynamics of [FeFe]-Hydrogenase Model Compounds with Bidentate Heterocyclic Ligands

Two asymmetrically structured model compounds for the hydrogen-generating [Fe-Fe]-hydrogenase active site were investigated to determine the ultrafast photodynamics, structural intermediates, and photoproducts compared to more common symmetric di-iron species. The bidentate-ligand-containing compounds studied were Fe2(μ-S2C3H6)(CO)4(bipy), 1, and Fe2(μ-S2C3H6)(CO)4(phen), 2, in dilute room temperature acetonitrile solution and low-temperature 2Me-THF matrix isolation using static FTIR difference and time-resolved infrared spectroscopic methods (TRIR). Ultraviolet-visible spectra were also compared to time-dependent density functional theory (TD-DFT) to ascertain the orbital origins of long wavelength electronic absorption features. The spectroscopic evidence supports the conclusions that only a propyl-bridge flip occurs in low-temperature matrix, while early time CO ejection leads to the formation of solvated isomeric species on the 25 ps time scale in room temperature solution.

Mechanistic Study of Oxygen Reduction at Liquid/Liquid Interfaces by Hybrid Ultramicroelectrodes and Mass Spectrometry

Proton-coupled electron transfer (PCET) reactions at various interfaces (liquid/membrane, solid/electrolyte, liquid/liquid) lie at the heart of many processes in biology and chemistry. Mechanistic study can provide profound understanding of PCET and rational design of new systems. However, most mechanisms of PCET reactions at a liquid/liquid interface have been proposed based on electrochemical and spectroscopic data, which lack direct evidence for possible intermediates. Moreover, a liquid/liquid interface as one type of soft interface is dynamic, making the investigation of interfacial reactions very challenging. Herein a novel electrochemistry method coupled to mass spectrometry (EC-MS) was introduced for in situ study of the oxygen reduction reaction (ORR) by ferrocene (Fc) under catalysis from cobalt tetraphenylporphine (CoTPP) at liquid/liquid interfaces. The key units are two types of gel hybrid ultramicroelectrodes (agar-gel/organic hybrid ultramicroelectrodes and water/PVC-gel hybrid ultramicroelectrodes), which were made based on dual micro- or nanopipettes. A solidified liquid/liquid interface can be formed at the tip of these pipettes, and it serves as both an electrochemical cell and a nanospray emitter for mass spectrometry. We demonstrated that the solidified L/L interfaces were very similar to typical L/L interfaces. Key CoTPP intermediates of the ORR at the liquid/liquid interfaces were identified for the first time, and the four-electron oxygen reduction pathway predominated, which provides valuable insights into the mechanism of the ORR. Theoretical simulation has further supported the possibility of formation of intermediates. This type of platform is promising for in situ tracking and identifying intermediates to study complicated reactions at liquid/liquid interfaces or other soft interfaces.

Artificial photosynthesis: opportunities and challenges of molecular catalysts

Molecular catalysis plays an essential role in both natural and artificial photosynthesis (AP). However, the field of molecular catalysis for AP has gradually declined in recent years because of doubt about the long-term stability of molecular-catalyst-based devices. This review summarizes the development history of molecular-catalyst-based AP, including the fundamentals of AP, molecular catalysts for water oxidation, proton reduction and CO2 reduction, and molecular-catalyst-based AP devices, and it provides an analysis of the advantages, challenges, and stability of molecular catalysts. With this review, we aim to highlight the following points: (i) an investigation on molecular catalysis is one of the most promising ways to obtain atom-efficient catalysts with outstanding intrinsic activities; (ii) effective heterogenization of molecular catalysts is currently the primary challenge for the application of molecular catalysis in AP devices; (iii) development of molecular catalysts is a promising way to solve the problems of catalysis involved in practical solar fuel production. In molecular-catalysis-based AP, much has been attained, but more challenges remain with regard to long-term stability and heterogenization techniques.