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Twelve naphthochromenone photocatalysts (PCs) were synthesized on gram scale. They absorb across the UV/Vis range and feature an extremely wide redox window (up to 3.22 eV) that is accessible using simple visible light irradiation sources (CFL or LED). Their excited-state redox potentials, PC*/PC.- (up to 1.65 V) and PC.+ /PC* (up to -1.77 V vs. SCE), are such that these novel PCs can engage in both oxidative and reductive quenching mechanisms with strong thermodynamic requirements. The potential of these bimodal PCs was benchmarked in synthetically relevant photocatalytic processes with extreme thermodynamic requirements. Their ability to efficiently catalyze mechanistically opposite oxidative/reductive photoreactions is a unique feature of these organic photocatalysts, thus representing a decisive advance towards generality, sustainability, and cost efficiency in photocatalysis.

The oxygen in Earth’s atmosphere is there primarily because of water oxidation performed by photosynthetic organisms using solar light and one specialized protein complex, photosystem II (PSII). High-resolution imaging of the PSII ‘core’ complex shows the ideal co-localization of multi-chromophore light-harvesting antennas with the functional reaction centre. Man-made systems are still far from replicating the complexity of PSII, as the majority of PSII mimetics have been limited to photocatalytic dyads based on a 1:1 ratio of a light absorber, generally a Ru–polypyridine complex, with a water oxidation catalyst. Here we report the self-assembly of multi-perylene-bisimide chromophores (PBI) shaped to function by interaction with a polyoxometalate water-oxidation catalyst (Ru4POM). The resulting [PBI]5Ru4POM complex shows a robust amphiphilic structure and dynamic aggregation into large two-dimensional paracrystalline domains, a redshifted light-harvesting efficiency of >40% and favourable exciton accumulation, with a peak quantum efficiency using ‘green’ photons (λ > 500 nm). The modularity of the building blocks and the simplicity of the non-covalent chemistry offer opportunities for innovation in artificial photosynthesis. In native photosystem II (PSII), multi-chromophore antennas surround the reaction centre, capturing light and triggering the quantized (four-flashes) photo-oxidation of water to oxygen. The PSII ‘quantasome’ is the most efficient photo-electrolyser built so far. An artificial quantasome has now been developed; it is specifically designed for oxygen evolution by self-assembling light-harvesting-perylene bisimides with a ruthenium polyoxometalate water-oxidation catalyst.

Inspired by natural photosynthesis, the aim of light-driven water splitting is to produce renewable fuels by exploiting solar radiation. Sustained hydrogen production is desirable in such systems, and the oxidation of water to oxygen is currently recognized as the bottleneck of the entire process. Therefore, solutions for this difficult task retain a fundamental interest. In this paper, we present a bioinspired, three-component system for water oxidation that comprises a tetracationic porphyrin ZnII complex as the photosensitizer, a tetraruthenium water-oxidation catalyst, and sodium persulfate as the electron acceptor. An in-depth photophysical study reveals the photogeneration of a pentacation radical of the porphyrin (quantum yield up to Φ = 1.01) upon oxidative quenching of the triplet excited state by persulfate. Electron transfer from the water-oxidation catalyst to the pentacation radical (hole scavenging) is slow (bimolecular rate constant, k

Polyoxometalates (POMs) are a class of inorganic compounds that have been extensively considered in the development of robust water oxidation catalysts (WOCs). In particular, cobalt based POMs have shown remarkable activity as WOCs under both dark and light activated cycles, although current studies focus on their possible evolution to different forms of cobalt oxides under operating conditions. In this paper, we investigate the activity in water oxidation catalysis of a heptanuclear cobalt based POM [{(B-α-PW 9 O 34 )Co 3 (OH)(H 2 O) 2 (O 3 PC(O)(C 3 H 6 NH 3 )PO 3 )} 2 Co] 14− , hereafter Co 7 (Ale) 14 − and of 5 nm cobalt oxide nanoparticles, Co 3 O 4 (Ale) , both carrying the same bisphosphonate alendronate ligand. Under the application of an anodic bias in borate buffer pH 8, both species lead to the formation of an active cobalt oxide layer at the working electrode, although in the case of Co 7 (Ale) 14− participation of the POM scaffold in the electrodeposition is evidenced by the presence of W(VI) in the active film. In particular, Co 7 (Ale) 14− and Co 3 O 4 (Ale) were investigated in a photocatalytic cycle with Ru(bpy) 3 2+ (bpy = 2,2′-bipyridine) as the photosensitizer and persulfate as the electron acceptor, where the photogenerated oxidant is Ru(bpy) 3 3+ . Key findings are: (i) ion pairing and aggregate formation of anionic Co 7 (Ale) 14− and Co 3 O 4 (Ale) with cationic Ru(bpy) 3 2+ are limited under photocatalytic regime (20 mM aqueous buffer, pH 8), as confirmed by the solubility product constant K SP = 2.1 ± 0.4 × 10 −15 M 4 for a Co 7 (Ale) 14− :Ru(bpy) 3 2+ 1:3 adduct, emission quenching experiments, and bimolecular electron transfer (ET) kinetics; (ii) fast ET to Ru(bpy) 3 3+ , approaching diffusional rates, occurs from Co 7 (Ale) 14− ( k ET = 9.4 ± 0.3 × 10 8 M − 1 s − 1 ) overarching by four orders of magnitude the Co 3 O 4 (Ale) nanoparticles; (iii) Co 7 (Ale) 14− enables up to 18 photoinduced ET events in 90 ms, while in the same timeframe, a non-complete bleaching recovery (ET events Co 3 O 4 (Ale) nanoparticle analogs; (iv) photocatalytic oxygen evolution is observed with similar rates for both cobalt based species (TOF per cobalt = 1.2–1.7 × 10 − 3 s − 1 ); (v) dark O–O bond formation is the likely rate determining step of water oxidation for both systems, as the O 2 evolution kinetics do not depend on photoinduced ET rates and oxygenic activity, and light irradiation intensity ( λ = 450 nm, 0.4–1.6 × 10 −7 einstein s −1 ); (vi) leaching of Co(II) ions from Co 7 (Ale) 14− in catalytic conditions is estimated below 2% by 31 P NMR line broadening experiments and supported by persistent flash photolysis kinetics; (vii) the ultimate fate of Co 7 (Ale) 14− under oxygenic turnover involves a polyoxometalate structural rearrangement, while no evidence of cobalt oxide nanoparticles formation is provided by TEM images; (viii) conversely, Co 3 O 4 (Ale) nanoparticles preserve their size along the light driven cycle. Therefore, this study supports the involvement of POM based intermediates in water oxidation catalysis, while future perspectives should be directed toward the identification of such main characters, and of the mechanism of O 2 construction.

Artificial photosynthesis aims at efficient water splitting into hydrogen and oxygen, by exploiting solar light. As a priority requirement, this process entails the integration of suitable multi-electron catalysts with light absorbing units, where charge separation is generated in order to drive the catalytic routines. The final goal could be the transposition of such an asset into a photoelectrocatalytic cell, where the two half-reactions, proton reduction to hydrogen and water oxidation to oxygen, take place at two appropriately engineered photoelectrodes. We herein report a covalent approach to anchor a Co(II) water oxidation catalyst to a Ru(II) polypyridine photosensitizer unit; photophysical characterisation and the catalytic activity of such a dyad in a light activated cycle are reported, and implications for the development of regenerative systems are discussed.

Efficient photocatalytic conversion of carbon dioxide into valuable reduction products is a priority goal for artificial photosynthesis. Iridium(III) photocatalysts with a combined 2-phenylpyridine (ppy) and 2,2′:6′,2″-terpyridine (tpy) ligand set have been shown to selectively reduce CO2 to CO. Here, terpyridine modifications have been investigated that yield a turnover number (TON) of up to 265, a quantum yield of 0.10, and a photocatalyst lifespan of over 10 days. The key to success is the combined effect of adding aromatic substituents to the tpy ligand 4′-position and optimizing lighting conditions. Insights into the photocatalyst fate are provided by kinetics analysis and spectroelectrochemistry, which point out the critical role of the reductively quenched catalyst and its evolution to a spent “green” state via a dark deactivation pathway. The stereoelectronic effect of adding a 9-anthryl substituent together with the use of low-energy blue light proves instrumental in the management of excited and...

Among molecular water oxidation catalysts (WOCs), those featuring a reactive set of four multi-redox transition metals can leverage an extraordinary interplay of electronic and structural properties. These are of particular interest, owing to their close structural, and possibly functional, relationship to the oxygen evolving complex of natural photosynthesis. In this review, special attention is given to two classes of tetrametallic molecular WOCs: (i) M(4)O(4) cubane-type structures stabilized by simple organic ligands, and (ii) systems in which a tetranuclear metal core is stabilized by coordination of two polyoxometalate (POM) ligands. Recent work in this rapidly evolving field is reviewed, with particular emphasis on photocatalytic aspects. Special attention is given to studies addressing the mechanistic complexity of these systems, sometimes overlooked in the rush for oxygen evolving performance. The complementary role of molecular WOCs and their relationship with bulk oxides and heterogeneous catalysis are discussed.

The tetraruthenium polyoxometalate [Ru(4)(μ-O)(4)(μ-OH)(2)(H(2)O)(4)(γ-SiW(10)O(36))(2)](10-) (1) behaves as a very efficient water oxidation catalyst in photocatalytic cycles using Ru(bpy)(3)(2+) as sensitizer and persulfate as sacrificial oxidant. Two interrelated issues relevant to this behavior have been examined in detail: (i) the effects of ion pairing between the polyanionic catalyst and the cationic Ru(bpy)(3)(2+) sensitizer, and (ii) the kinetics of hole transfer from the oxidized sensitizer to the catalyst. Complementary charge interactions in aqueous solution leads to an efficient static quenching of the Ru(bpy)(3)(2+) excited state. The quenching takes place in ion-paired species with an average 1:Ru(bpy)(3)(2+) stoichiometry of 1:4. It occurs by very fast (ca. 2 ps) electron transfer from the excited photosensitizer to the catalyst followed by fast (15-150 ps) charge recombination (reversible oxidative quenching mechanism). This process competes appreciably with the primary photoreaction of the excited sensitizer with the sacrificial oxidant, even in high ionic strength media. The Ru(bpy)(3)(3+) generated by photoreaction of the excited sensitizer with the sacrificial oxidant undergoes primary bimolecular hole scavenging by 1 at a remarkably high rate (3.6 ± 0.1 × 10(9) M(-1) s(-1)), emphasizing the kinetic advantages of this molecular species over, e.g., colloidal oxide particles as water oxidation catalysts. The kinetics of the subsequent steps and final oxygen evolution process involved in the full photocatalytic cycle are not known in detail. An indirect indication that all these processes are relatively fast, however, is provided by the flash photolysis experiments, where a single molecule of 1 is shown to undergo, in 40 ms, ca. 45 turnovers in Ru(bpy)(3)(3+) reduction. With the assumption that one molecule of oxygen released after four hole-scavenging events, this translates into a very high average turnover frequency (280 s(-1)) for oxygen production.

Photoinduced water oxidation to molecular oxygen takes place in systems made of [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine) as the photosensitizer, [Co4O4(O2CMe)4(py)4] (py = pyridine) as the molecular catalyst and Na2S2O8 as the sacrificial electron acceptor. The photochemical quantum yield of the process reaches the outstanding value of 30% and depends on pH and catalyst concentration. Transient absorption spectroscopy experiments aimed to clarify the first events of the photocatalytic process are also reported.

A detailed photophysical investigation of the photocatalytic water oxidation system based on the tetraruthenium polyoxometalate [Ru4(μ-O)4(μ–OH)2(H2O)4(γ-SiW10O36)2]10– (1) as the catalyst, the tetranuclear ruthenium dendrimer [Ru{(μ-2,3-dpp)Ru(bpy)2}3]8+ (2) as the light-harvesting photosensitizer, and persulfate (S2O82–) as the sacrificial electron acceptor has shown that the water oxidation mechanism proceeds through an unusual, “anti-biomimetic” pathway: The first photochemical event is indeed a reductive quenching of the excited photosensitizer by the catalyst, followed by electron scavenging by the sacrificial electron acceptor, both occurring on the picosecond time scale within ion-paired species. As an unprecedented photoreaction mechanism for molecular water oxidation systems, these results suggest a new way to combine photosensitizers and catalysts, taking advantage of suitable chemical interactions and alternative photoinduced processes for the construction of efficient water-splitting devices.

The functional core of oxygenic photosynthesis is in charge of catalytic water oxidation by a multi-redox Mn-III/Mn-IV manifold that evolves through five electronic states (S-i, where i=0-4). The synthetic model system of this catalytic cycle and of its S0S4 intermediates is the expected turning point for artificial photosynthesis. The tetramanganese-substituted tungstosilicate [(Mn3MnO3)-Mn-III-O-IV(CH3COO)(3)(A--SiW9O34)](6-) (Mn4POM) offers an unprecedented mimicry of the natural system in its reduced S(0)state; it features a hybrid organic-inorganic coordination sphere and is anchored on a polyoxotungstate. Evidence for its photosynthetic properties when combined with [Ru(bpy)(3)](2+) and S2O82- is obtained by nanosecond laser flash photolysis; its S0S1 transition within milliseconds and multiple-hole-accumulating properties were studied. Photocatalytic oxygen evolution is achieved in a buffered medium (pH5) with a quantum efficiency of 1.7%.

Among molecular water oxidation catalysts (WOCs), those featuring a reactive set of four multi-redox transition metals can leverage an extraordinary interplay of electronic and structural properties. These are of particular interest, owing to their close structural, and possibly functional, relationship to the oxygen evolving complex of natural photosynthesis. In this review, special attention is given to two classes of tetrametallic molecular WOCs: (i) M4O4 cubane-type structures stabilized by simple organic ligands, and (ii) systems in which a tetranuclear metal core is stabilized by coordination of two polyoxometalate (POM) ligands. Recent work in this rapidly evolving field is reviewed, with particular emphasis on photocatalytic aspects. Special attention is given to studies addressing the mechanistic complexity of these systems, sometimes overlooked in the rush for oxygen evolving performance. The complementary role of molecular WOCs and their relationship with bulk oxides and heterogeneous catalysis are discussed.

New progress towards artificial photosynthetic methods and solar fuels will depend on the discovery of highly robust multi-electron catalysts and materials enabling light-activated water splitting with high quantum efficiency and low overpotential, thus mimicking the natural process.

Isostructural cubane-shaped catalysts [Co(III)(4)(μ-O)(4)(μ-CH(3)COO)(4)(p-NC(5)H(4)X)(4)], 1-X (X = H, Me, t-Bu, OMe, Br, COOMe, CN), enable water oxidation under dark and illuminated conditions, where the primary step of photoinduced electron transfer obeys to Hammett linear free energy relationship behavior. Ligand design and catalyst optimization are instrumental for sustained O(2) productivity with quantum efficiency up to 80% at λ400 nm, thus opening a new perspective for in vitro molecular photosynthesis.

Water oxidation catalysts: evolution of [Co(4)(H(2)O)(2)(α-PW(9)O(34))(2)](10-) to catalytically active species is assessed by laser flash photolysis in sacrificial photocatalytic cycles with Ru(bpy)(3)(2+) as a photosensitizer.

Innovative oxygen evolving catalysts, taken from the pool of nanosized, water soluble, molecular metal oxides, the so-called polyoxometalates (POMs), represent an extraordinary opportunity in the field of artificial photosynthesis. These catalysts possess a highly robust, totally inorganic structure, and can provide a unique mimicry of the oxygen evolving center in photosynthetic II enzymes. As a result POMs can effect H₂O oxidation to O₂ with unprecedented efficiency. In particular, the tetra-ruthenium based POM [Ru(IV) ₄(μ-OH)₂(μ-O)₄(H₂O)₄(γ-SiW(10)O(36))₂](10-), Ru₄(POM), displays fast kinetics, electrocatalytic activity powered by carbon nanotubes and exceptionally light-driven performance. A broad perspective is presented herein by addressing the recent progress in the field of metal-oxide nano-clusters as water oxidation catalysts, including colloidal species. Moreover, the shaping of the catalyst environment plays a fundamental role by alleviating the catalyst fatigue and stabilizing competent intermediates, thus responding to what are the formidable thermodynamic and kinetic challenges of water splitting. The design of nano-interfaces with specifically tailored carbon nanostructures and/or polymeric scaffolds opens a vast scenario for tuning electron/proton transfer mechanisms. Therefore innovation is envisaged based on the molecular modification of the hybrid photocatalytic center and of its environment.

Invited for the for the front cover of the Cluster Issue on Water Oxidation Chemistry is the Helios group at the University of Padova, fostering a new research impetus on solar fuels by a collaborative synergy among catalysis, organometallic, mechanistic, and materials science experts. The cover image shows the novel dicarbene iridium(III) complex and oxygen evolution as a result of its catalytic action under visible light irradiation. This is one key goal of the HELIOS strategic project, targeting new molecules and nanomaterials for solar energy schemes, as represented by the fullerene sunflower logo. Dr. Rami Al-Oweini (ITM-CNR Postdoctoral Fellow) is acknowledged for his creative design and for putting together the cover illustration.