Photoinduced water oxidation by a tetraruthenium polyoxometalate catalyst: ion-pairing and primary processes with Ru(bpy)3(2+) photosensitizer

Atomically dispersed Ir catalysts exhibit support-dependent water oxidation kinetics during photocatalysis

Heterogeneous water oxidation catalysis is central to the development of renewable energy technologies. Recent research has suggested that the reaction mechanisms are sensitive to the hole density at the active sites. However, these previous results were obtained on catalysts of different materials featuring distinct active sites, making it difficult to discriminate between competing explanations. Here, a comparison study based on heterogenized dinuclear Ir catalysts (Ir-DHC), which feature the same type of active site on different supports, is reported. The prototypical reaction was water oxidation triggered by pulsed irradiation of suspensions containing a light sensitizer, Ru(bpy)₃²⁺, and a sacrificial electron scavenger, S₂O₈^2-. It was found that at relatively low temperatures (288-298 K), the water oxidation activities of Ir-DHC on indium tin oxide (ITO) and CeO₂ supports were comparable within the studied range of fluences (62-151 mW cm^-2). By contrast, at higher temperatures (310-323 K), Ir-DHC on ITO exhibited a ca. 100% higher water oxidation activity than on CeO₂. The divergent activities were attributed to the distinct abilities of the supporting substrates in redistributing holes. The differences were only apparent at relatively high temperatures when hole redistribution to the active site became a limiting factor. These findings highlight the critical role of the supporting substrate in determining the turnover at active sites of heterogeneous catalysts.

Physico- and Electrochemical Properties of First-Row Transition-Metal-Substituted Sandwich Polyoxometalates

The physico- and electrochemical behaviors of a series of [WZn₃(H₂O)₂(ZnW₉O₃₄)₂]^12- (Zn-WZn₃) and its first-row transition-metal-substituted analogues [WZn(TM)₂(H₂O)₂(ZnW₉O₃₄)₂]^12- (Zn-WZn(TM)₂; TM = Mn^II, Co^II, Fe^III, Ni^II and Cu^II) are reported. Various spectroscopic studies, including Fourier transform infrared (FTIR) spectroscopy, UV-visible spectroscopy, electrospray ionization (ESI)-mass spectrometry, and Raman spectroscopy, show similar spectral patterns in all sandwich polyoxometalates (POMs) because of their isostructural geometry and constancy of the overall negative charge (-12). However, the electronic properties highly depend on the transition metals at the "sandwich core" and correlate well with the density functional theory (DFT) study. Further, depending on the substituted TM atoms, there is a decrease in the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) band-gap energy in these transition-metal-substituted POM (TMSP) complexes wrt Zn-WZn₃, as confirmed by diffuse reflectance spectroscopy and DFT study. Cyclic voltammetry reveals that the electrochemistry of these sandwich POMs (Zn-WZn₃ and TMSPs) is highly dependent on the pH of the solution. Moreover, the dioxygen binding/activation studies of these polyoxometalates show that Zn-WZn₃ and Zn-WZnFe₂ have better efficiency toward dioxygen binding, as confirmed by FTIR spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA), which is also reflected in their catalytic activity toward imine synthesis.

Grafting of Anionic Decahydro-Closo-Decaborate Clusters on Keggin and Dawson-Type Polyoxometalates: Syntheses, Studies in Solution, DFT Calculations and Electrochemical Properties

Herein we report the synthesis of a new class of compounds associating Keggin and Dawson-type Polyoxometalates (POMs) with a derivative of the anionic decahydro-closo-decaborate cluster [B₁₀H₁₀]^2- through aminopropylsilyl ligand (APTES) acting as both a linker and a spacer between the two negatively charged species. Three new adducts were isolated and fully characterized by various NMR techniques and MALDI-TOF mass spectrometry, notably revealing the isolation of an unprecedented monofunctionalized SiW₁₀ derivative stabilized through intramolecular H-H dihydrogen contacts. DFT as well as electrochemical studies allowed studying the electronic effect of grafting the decaborate cluster on the POM moiety and its consequences on the hydrogen evolution reaction (HER) properties.

Mechanistic Insights into the Charge Transfer Dynamics of Photocatalytic Water Oxidation at the Lipid Bilayer-Water Interface

Photosystem II, the natural water-oxidizing system, is a large protein complex embedded in a phospholipid membrane. A much simpler system for photocatalytic water oxidation consists of liposomes functionalized with amphiphilic ruthenium(II)-tris-bipyridine photosensitizer (PS) and 6,6'-dicarboxylato-2,2'-bipyridine-ruthenium(II) catalysts (Cat) with a water-soluble sacrificial electron acceptor (Na₂S₂O₈). However, the effect of embedding this photocatalytic system in liposome membranes on the mechanism of photocatalytic water oxidation was not well understood. Here, several phenomena have been identified by spectroscopic tools, which explain the drastically different kinetics of water photo-oxidizing liposomes, compared with analogous homogeneous systems. First, the oxidative quenching of photoexcited PS* by S₂O₈^2- at the liposome surface occurs solely via static quenching, while dynamic quenching is observed for the homogeneous system. Moreover, the charge separation efficiency after the quenching reaction is much smaller than unity, in contrast to the quantitative generation of PS⁺ in homogeneous solution. In parallel, the high local concentration of the membrane-bound PS induces self-quenching at 10:1-40:1 molar lipid-PS ratios. Finally, while the hole transfer from PS⁺ to catalyst is rather fast in homogeneous solution (k_obs > 1 × 10⁴ s^-1 at [catalyst] > 50 μM), in liposomes at pH = 4, the reaction is rather slow (k_obs ≈ 17 s^-1 for 5 μM catalyst in 100 μM DMPC lipid). Overall, the better understanding of these productive and unproductive pathways explains what limits the rate of photocatalytic water oxidation in liposomal vs homogeneous systems, which is required for future optimization of light-driven catalysis within self-assembled lipid interfaces.

Structure-Activity Relationship for Di- up to Tetranuclear Macrocyclic Ruthenium Catalysts in Homogeneous Water Oxidation

Two di- and tetranuclear Ru(bda) (bda: 2,2'-bipyridine-6,6'-dicarboxylate) macrocyclic complexes were synthesized and their catalytic activities in chemical and photochemical water oxidation investigated in a comparative manner to our previously reported trinuclear congener. Our studies have shown that the catalytic activities of this homologous series of multinuclear Ru(bda) macrocycles in homogeneous water oxidation are dependent on their size, exhibiting highest efficiencies for the largest tetranuclear catalyst. The turnover frequencies (TOFs) have increased from di- to tetranuclear macrocycles not only per catalyst molecule but more importantly also per Ru unit with TOF of 6 s^-1 to 8.7 s^-1 and 10.5 s^-1 in chemical and 0.6 s^-1 to 3.3 s^-1 and 5.8 s^-1 in photochemical water oxidation per Ru unit, respectively. Thus, for the first time, a clear structure-activity relationship could be established for this novel class of macrocyclic water oxidation catalysts.

Photoinduced Water Oxidation in Chitosan Nanostructures Containing Covalently Linked RuII Chromophores and Encapsulated Iridium Oxide Nanoparticles

The luminophore Ru(bpy)₂(dcbpy)²⁺ (bpy=2,2’‐bipyridine; dcbpy=4,4’‐dicarboxy‐2,2’‐bipyridine) is covalently linked to a chitosan polymer; crosslinking by tripolyphosphate produced Ru‐decorated chitosan fibers (NS‐RuCh), with a 20 : 1 ratio between chitosan repeating units and Ru^II chromophores. The properties of the Ru^II compound are unperturbed by the chitosan structure, with NS‐RuCh exhibiting the typical metal‐to‐ligand charge‐transfer (MLCT) absorption and emission bands of Ru^II complexes. When crosslinks are made in the presence of IrO₂ nanoparticles, such species are encapsulated within the nanofibers, thus generating the IrO₂⊂NS‐RuCh system, in which both Ru^II photosensitizers and IrO₂ water oxidation catalysts are within the nanofiber structures. NS‐RuCh and IrO₂⊂NS‐RuCh have been characterized by dynamic light scattering, scanning electronic microscopy, and energy‐dispersive X‐ray analysis, which indicated a 2 : 1 ratio between Ru^II chromophores and IrO₂ species. Photochemical water oxidation has been investigated by using IrO₂⊂NS‐RuCh as the chromophore/catalyst assembly and persulfate anions as the sacrificial species: photochemical water oxidation yields O₂ with a quantum yield (Φ) of 0.21, definitely higher than the Φ obtained with a similar solution containing separated Ru(bpy)₃²⁺ and IrO₂ nanoparticles (0.05) or with respect to that obtained when using NS‐RuCh and “free” IrO₂ nanoparticles (0.10). A fast hole‐scavenging process (rate constant, 7×10⁴ s⁻¹) involving the oxidized photosensitizer and the IrO₂ catalyst within the IrO₂⊂NS‐RuCh system is behind the improved photochemical quantum yield of IrO₂⊂NS‐RuCh.

Phosphate-Templated Encapsulation of a {CoII 4 O4 } Cubane in Germanotungstates as Carbon-Free Homogeneous Water Oxidation Photocatalysts

The ever-growing interest in sustainable energy sources leads to a search for an efficient, stable, and inexpensive homogeneous water oxidation catalyst (WOC). Herein, the PO₄ ^3- templated synthesis of three abundant-metal-based germanotungstate (GT) clusters Na₁₅ [Ge₄ PCo₄ (H₂ O)₂ W₂₄ O₉₄ ] ⋅ 38H₂ O (Co₄ ), Na_2.5 K_17.5 [Ge₃ PCo₉ (OH)₅ (H₂ O)₄ W₃₀ O₁₁₅ ] ⋅ 45H₂ O (Co₉ ), Na₆ K₁₆ [Ge₄ P₄ Co₂₀ (OH)₁₄ (H₂ O)₁₈ W₃₆ O₁₅₀ ] ⋅ 61H₂ O (Co₂₀ ) with non-, quasi-, or full cubane motifs structurally strongly reminiscent of the naturally occurring {Mn₄ Ca} oxygen evolving complex (OEC) in photosystem II was achieved. Under the conditions tested, all three GT-scaffolds were active molecular WOCs, with Co₉ and Co₂₀ outperforming the well-known Na₁₀ [Co₄ (H₂ O)₂ (PW₉ O₃₄ )₂ ] {Co₄ P₂ W₁₈ } by a factor of 2 as shown by a direct comparison of their turnover numbers (TONs). With TONs up to 159.9 and a turnover frequency of 0.608 s^-1 Co₉ currently represents the fastest Co-GT-based WOC, and photoluminescence emission spectroscopy provided insights into its photocatalytic WOC mechanism. Cyclic voltammetry, dynamic light scattering, UV/Vis and IR spectroscopy showed recyclability and integrity of the catalysts under the applied conditions. The experimental results were supported by computational studies, which highlighted that the facilitated oxidation of Co₉ was due to the higher energy of its highest occupied molecular orbital electrons as compared to Co₄ .

Water-Assisted Concerted Proton-Electron Transfer at Co(II)-Aquo Sites in Polyoxotungstates With Photogenerated RuIII (bpy)33+ Oxidant

The cobalt substituted polyoxotungstate [Co6 (H2 O)2 (α-B-PW9 O34 )2 (PW6 O26 )]17- (Co6) displays fast electron transfer (ET) kinetics to photogenerated RuIII (bpy)33+ , 4 to 5 orders of magnitude faster than the corresponding ET observed for cobalt oxide nanoparticles. Mechanistic evidence has been acquired indicating that: (i) the one-electron oxidation of Co6 involves Co(II) aquo or Co(II) hydroxo groups (abbreviated as Co6(II)-OH2 and Co6(II)-OH, respectively, whose speciation in aqueous solution is associated to a pKa of 7.6), and generates a Co(III)-OH moiety (Co6(III)-OH), as proven by transient absorption spectroscopy; (ii) at pH>pKa , the Co6(II)-OH→RuIII (bpy)33+ ET occurs via bimolecular kinetics, with a rate constant k close to the diffusion limit and dependent on the ionic strength of the medium, consistent with reaction between charged species; (iii) at pH Collapse

Key Words

• Co-aquo moiety
• cobalt polyoxometalate
• flash photolysis
• hydrogen bonding
• proton coupled electron transfer

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MESH Headings

• Cobalt/chemistry
• Coordination Complexes/chemical synthesis
• Coordination Complexes/chemistry
• Electrons
• Kinetics
• Light
• Organometallic Compounds/chemistry
• Organometallic Compounds/radiation effects
• Oxidants/chemistry
• Oxidants/radiation effects
• Oxidation-Reduction
• Polymers/chemical synthesis
• Polymers/chemistry
• Protons
• Ruthenium/chemistry
• Ruthenium/radiation effects
• Tungsten Compounds/chemical synthesis
• Tungsten Compounds/chemistry
• Water/chemistry

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Grants

• Fondazione Cariparo
• 10BIRD2018-UNIPD Department of Chemical Sciences at University of Padova
• 2017PBXPN4 Department of Chemical Sciences at University of Padova
• FAR 2020 University of Ferrara
• Fondazione Cariparo
• University of Ferrara

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Affiliation(s)

Francesco Rigodanza Department of Chemical SciencesUniversity of Padovavia Marzolo 135131PadovaItaly Consiglio Nazionale delle Ricerche (C.N.R.)Institute on Membrane Technology section of Padovavia Marzolo 135131PadovaItaly
Nadia Marino Department of Chemistry and Chemical TechnologiesUniversity of Calabria87036Arcavacata di Rende (CS)Italy
Alessandro Bonetto Dept. Environmental Sciences, Informatics and StatisticsUniversity Ca' Foscari Venice VegaparkVia delle Industrie 21/830175Marghera, VeniceItaly
Antonio Marcomini Dept. Environmental Sciences, Informatics and StatisticsUniversity Ca' Foscari Venice VegaparkVia delle Industrie 21/830175Marghera, VeniceItaly
Marcella Bonchio Department of Chemical SciencesUniversity of Padovavia Marzolo 135131PadovaItaly Consiglio Nazionale delle Ricerche (C.N.R.)Institute on Membrane Technology section of Padovavia Marzolo 135131PadovaItaly
Mirco Natali Department of Chemical, Pharmaceutical and Agricultural Sciences (DOCPAS)University of Ferrara, and Centro Interuniversitario per la Conversione Chimica dell'Energia Solare (SOLARCHEM) sez. di Ferraravia L. Borsari 4644121FerraraItaly
Andrea Sartorel Department of Chemical SciencesUniversity of Padovavia Marzolo 135131PadovaItaly

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Synthesis, Physical Properties and Application of a Series of New Polyoxometalate-Based Ionic Liquids

This paper deals with the preparation and the characterization of four new ionic liquids resulting from the pairing of various polyoxotungstates or polyoxomolybdates with the cation trihexyltetradecylphosphonium. The physical properties measured by different techniques evidence that the viscosity and the rheological behaviors of such POM-based ionic liquids, POM-ILs, strongly depend on the nature of the POM, especially its charge. Playing on the nature of the POM, we can indeed obtain Newtonian liquids or some much more viscous materials exhibiting characteristics of resins or pseudo-plastics. In a second part of this study, the potentialities of using such materials both as solvent and catalyst for the oxidation of a series of alcohols are presented as proof of concept. This part highlights great differences in strength and selectivity as a function of the POM-IL used. Furthermore, a very simple way to recycle the catalyst is also presented.

Novel iridium complexes with N-heterocyclic dicarbene ligands in light-driven water oxidation catalysis: photon management, ligand effect and catalyst evolution

Iridium complexes [IrClCp*diNHC]PF6, with N-heterocyclic dicarbene (diNHC) and pentamethylcyclopentadienyl (Cp*) ligands, have been investigated in light driven water oxidation catalysis within the Ru(bpy)32+/S2O82- cycle (bpy = 2,2'-bipyridine). In particular, the effect of different diNHC ligands was evaluated by employing the complex 1a (diNHC = 1,1'-dimethyl-3,3'-ethylenediimidazol-2,2'-diylidene) and the novel and structurally characterised 2 (diNHC = 1,1'-dimethyl-3,3'-ethylene-5,5'-dibromodiimidazol-2,2'-diylidene) and 3 (diNHC = 1,1'-dimethyl-3,3'-ethylene-dibenzimidazol-2,2'-diylidene). The presented results include: (i) a photon management analysis of the 1a/Ru(bpy)32+/S2O82- system, revealing two regimes of O2 evolution rate, being dependent on the light intensity at low photon flux, where the system reaches an overall quantum yield up to 0.17 ± 0.01 (quantum efficiency 34 ± 2%), while being independent of light intensity at high photon flux thus indicating a change of limiting step; (ii) a trend of O2 evolution activity that follows the order 1a > 2 > 3 both under low and high photon flux conditions, with the reactivity that is favoured by the electron donating nature of the diNHC ligand, quantified on the basis of the carbene carbon chemical shift; (iii) an analogous trend also in the bimolecular rate constants of electron transfer kET from the iridium species to photogenerated Ru(bpy)33+, with kET values in the range 4.2-6.1 × 104 M-1 s-1, thus implying a significant reorganisation energy to the iridium sphere; (iv) the evolution of 1a, as the most active Ir species in the series, to mononuclear iridium species with lower molecular weight and originating from oxidative transformation of the organic ligand scaffold, as proven by converging UV-Vis, MALDI-MS and 1H-NMR evidences. These results can be used for the further design and engineering of novel catalysts.

Light-Driven Water Oxidation with the Catalyst and the Ru(bpy)32+/S2O82- Cycle: Photogeneration of Active Dimers, Electron-Transfer Kinetics, and Light Synchronization for Oxygen Evolution with High Quantum Efficiency

Light-driven water oxidation is achieved with the Ru(bpy)₃²⁺/S₂O₈^2- cycle employing the highly active Ir-blue water oxidation catalyst, namely, an Ir^IV,IV₂(pyalc)₂ μ-oxo-dimer [pyalc = 2-(2'-pyridyl)-2-propanoate]. Ir-blue is readily formed by stepwise oxidation of the monomeric Ir(III) precursor 1 by the photogenerated Ru(bpy)₃³⁺, with a quantum yield ϕ of up to 0.10. Transient absorption spectroscopy and kinetic evidence point to a stepwise mechanism, where the primary event occurs via a fast photoinduced electron transfer from 1 to Ru(bpy)₃³⁺, leading to the Ir(IV) monomer I₁ (k₁ ∼ 10⁸ M^-1 s^-1). The competent Ir-blue catalyst is then obtained from I₁ upon photooxidative loss of the Cp* ligand and dimerization. The Ir-blue catalyst is active in the Ru(bpy)₃²⁺/S₂O₈^2- light-driven water oxidation cycle, where it undergoes two fast photoinduced electron transfers to Ru(bpy)₃³⁺ [with k_Ir-blue = (3.00 ± 0.02) × 10⁸ M^-1 s^-1 for the primary event, outperforming iridium oxide nanoparticles by ca. 2 orders of magnitude], leading to a Ir^V,V₂ steady-state intermediate involved in O-O bond formation. The quantum yield for oxygen evolution depends on the photon flux, showing a saturation regime and reaching an impressive value of ϕ(O₂) = 0.32 ± 0.01 (corresponding to a quantum efficiency of 64 ± 2%) at low irradiation intensity. This result highlights the key requirement of orchestrating the rate of the photochemical events with dark catalytic turnover.

An exceptionally stable octacobalt-cluster-based metal-organic framework for enhanced water oxidation catalysis

Extensive efforts have been devoted to developing efficient and durable catalysts for water oxidation. Herein, we report a highly stable metal-organic framework that shows high catalytic activity and durability for electrically driven (an overpotential of 430 mV at 10 mA cm-2 in neutral aqueous solution) and photodriven (a turnover frequency of 16 s-1 and 12 000 cycles) water oxidation, representing the best catalyst for water oxidation reported to date. Computational simulation and isotope tracing experiments showed that the μ4-OH group of the {Co8(μ4-OH)6} unit participates in the water oxidation reaction to offer an oxygen vacancy site with near-optimal OH- adsorption energy.

Mono-/Multinuclear Water Oxidation Catalysts

Water splitting, in which water molecules can be transformed into hydrogen and oxygen, is an appealing energy conversion and transformation strategy to address the environmental and energy crisis. The oxygen evolution reaction (OER) is dynamically slow, which limits energy conversion efficiency during the water-splitting process and requires high-efficiency water oxidation catalysts (WOCs) to overcome the OER energy barrier. It is generally accepted that multinuclear WOCs possess superior OER performances, as demonstrated by the CaMn₄ O₅ cluster in photosystem II (PSII), which can catalyze the OER efficiently with a very low overpotential. Inspired by the CaMn₄ O₅ cluster in PSII, some multinuclear WOCs were synthesized that could catalyze water oxidation. In addition, some mononuclear molecular WOCs also show high water oxidation activity. However, it cannot be excluded that the high activity arises from the formation of dimeric species. Recently, some mononuclear heterogeneous WOCs showed a high water oxidation activity, which testified that mononuclear active sites with suitable coordination surroundings could also catalyze water oxidation efficiently. This Review focuses on recent progress in the development of mono-/multinuclear homo- and heterogeneous catalysts for water oxidation. The active sites and possible catalytic mechanisms for water oxidation on the mono-/multinuclear WOCs are provided.

Kinetics and mechanisms of catalytic water oxidation

The kinetics and mechanisms of thermal and photochemical oxidation of water with homogeneous and heterogeneous catalysts, including conversion from homogeneous to heterogeneous catalysts in the course of water oxidation, are discussed in this review article. Molecular and homogeneous catalysts have the advantage to clarify the catalytic mechanisms by detecting active intermediates in catalytic water oxidation. On the other hand, heterogeneous nanoparticle catalysts have advantages for practical applications due to high catalytic activity, robustness and easier separation of catalysts by filtration as compared with molecular homogeneous precursors. Ligand oxidation of homogeneous catalysts sometimes results in the dissociation of ligands to form nanoparticles, which act as much more efficient catalysts for water oxidation. Since it is quite difficult to identify active intermediates on the heterogeneous catalyst surface, the mechanism of water oxidation has hardly been clarified under heterogeneous catalytic conditions. This review focuses on the kinetics and mechanisms of catalytic water oxidation with homogeneous catalysts, which may be converted to heterogeneous nanoparticle catalysts depending on various reaction conditions.

Electrochemical properties of the [SiW10O36(M2O2E2)]6− polyoxometalate series (M = Mo(v) or W(v); E = S or O)

New insights about the electrochemical properties of S-containing polyoxometalates. Electrochemical properties of the compounds γ-[SiW₁₀O₃₆(M₂O₂E₂)]⁶⁻ (M = Mo(v) or W(v); E = O²⁻ or S²⁻) in DMF.

Shining a light on the photo-sensitisation of organic-inorganic hybrid polyoxometalates

Finding new ways of using visible light (or, more specifically, solar irradiation) to drive commercially significant and/or challenging chemical processes is an ongoing research goal. Polyoxometalates (POMs) are discrete, metal-oxide clusters which are cheap, robust and easily synthesised but can also act as versatile molecular building blocks, allowing for astonishing variety in their structures and properties. In particular, the rich redox chemistry and inherent photo-activity of POMs makes them attractive for use in a variety of photochemical applications, however POMs characteristically only absorb strongly in the UV region. In this perspective, we discuss the various strategies which have been employed in order to sensitise POMs to visible light, with a particular focus on hybrid inorganic-organic POM species. We will discuss the two clear photo-activation mechanisms which have been developed to date and provide an outlook on some of the possible future directions of the field.

WO3/Conducting Polymer Heterojunction Photoanodes for Efficient and Stable Photoelectrochemical Water Splitting

An efficient and stable heterojunction photoanode for solar water oxidation was fabricated by hybridization of WO₃ and conducting polymers (CPs). Organic/inorganic hybrid photoanodes were readily prepared by the electropolymerization of various CPs and the codeposition of tetraruthenium polyoxometalate (Ru₄POM) water-oxidation catalysts (WOCs) on the surface of WO₃. The deposition of CPs, especially polypyrrole (PPy) doped with Ru₄POM (PPy:Ru₄POM), resulted in a remarkably improved photoelectrochemical performance by the formation of a WO₃/PPy p-n heterojunction and the incorporation of efficient Ru₄POM WOCs. In addition, there was also a significant improvement in the photostability of the WO₃-based photoanode after the deposition of the PPy:Ru₄POM layer due to the suppression of the formation of hydrogen peroxide, which was responsible for corrosion. This study provides insight into the design and fabrication of novel photosynthetic and photocatalytic systems with excellent performance and stability through the hybridization of organic and inorganic materials.

Ruthenium bistridentate complexes with non-symmetrical hexahydro-pyrimidopyrimidine ligands: a structural and theoretical investigation of their optical and electrochemical properties

The optical and electronic properties of six Ru complexes with non-symmetrical tridentate ligands have been investigated and, as corroborated by electrochemical data, the presence of the hpp ligand strongly affects the oxidation potential of the metal ion.

Photocatalytic hydrogen evolution with ruthenium polypyridine sensitizers: unveiling the key factors to improve efficiencies

An alternative benchmark sensitizer for photochemical hydrogen generation is identified within a series of ruthenium complexes, capable of outperforming the standard Ru(bpy)₃²⁺.

Water splitting with polyoxometalate-treated photoanodes: enhancing performance through sensitizer design

Visible light driven water oxidation has been demonstrated at near-neutral pH using photoanodes based on nanoporous films of TiO₂, polyoxometalate (POM) water oxidation catalyst [{Ru₄O₄(OH)₂(H₂O)₄}(γ-SiW₁₀O₃₆)₂]^10- (1), and both known photosensitizer [Ru(bpy)₂(H₄dpbpy)]²⁺ (P2) and the novel crown ether functionalized dye [Ru(5-crownphen)₂(H₂dpbpy)](H₂2). Both triads, containing catalyst 1, and catalyst-free dyads, produce O₂ with high faradaic efficiencies (80 to 94%), but presence of catalyst enhances quantum yield by up to 190% (maximum 0.39%). New sensitizer H₂2 absorbs light more strongly than P2, and increases O₂ quantum yields by up to 270%. TiO₂-2 based photoelectrodes are also more stable to desorption of active species than TiO₂-P2: losses of catalyst 1 are halved when pH > TiO₂ point-of-zero charge (pzc), and losses of sensitizer reduced below the pzc (no catalyst is lost when pH < pzc). For the triads, quantum yields of O₂ are higher at pH 5.8 than at pH 7.2, opposing the trend observed for 1 under homogeneous conditions. This is ascribed to lower stability of the dye oxidized states at higher pH, and less efficient electron transfer to TiO₂, and is also consistent with the 4^th1-to-dye electron transfer limiting performance rather than catalyst TOF_max. Transient absorption reveals that TiO₂-2-1 has similar 1^st electron transfer dynamics to TiO₂-P2-1, with rapid (ps timescale) formation of long-lived TiO₂(e^-)-2-1(h⁺) charge separated states, and demonstrates that metallation of the crown ether groups (Na⁺/Mg²⁺) has little or no effect on electron transfer from 1 to 2. The most widely relevant findings of this study are therefore: (i) increased dye extinction coefficients and binding stability significantly improve performance in dye-sensitized water splitting systems; (ii) binding of POMs to electrode surfaces can be stabilized through use of recognition groups; (iii) the optimal homogeneous and TiO₂-bound operating pHs of a catalyst may not be the same; and (iv) dye-sensitized TiO₂ can oxidize water without a catalyst.

Artificial photosynthesis: a molecular approach to photo-induced water oxidation

By the use of a molecular approach we performed photo-induced water oxidation by combining different photosensitizers and catalysts in order to obtain an efficient system that pave the way to the construction of an artificial photosynthetic system. Different types of molecular catalysts, such as ruthenium and vanadium polyoxometalates or cobalt core stabilized by different organic ligands were combined with ruthenium (II) polypyridine complexes of different nuclearity, mononuclear species like [Ru(bpy)3]2+ or a tetranuclear dendrimer.

Water oxidation catalysis upon evolution of molecular Co(iii) cubanes in aqueous media

The increasing global energy demand has stimulated great recent efforts in investigating new solutions for artificial photosynthesis, a potential source of clean and renewable solar fuel. In particular, according to the generally accepted modular approach aimed at optimising separately the different compartments of the entire process, many studies have focused on the development of catalytic systems for water oxidation to oxygen. While in recent years there have been many reports on new catalytic systems, the mechanism and the active intermediates operating the catalysis have been less investigated. Well-defined, molecular catalysts, constituted by transition metals stabilised by a suitable ligand pool, could help in solving this aspect. However, in some cases molecular species have been shown to evolve to active metal oxides that constitute the other side of this catalysis dichotomy. In this paper, we address the evolution of tetracobalt(iii) cubanes, stabilised by a pyridine/acetate ligand pool, to active species that perform water oxidation to oxygen. Primary evolution of the cubane in aqueous solution is likely initiated by removal of an acetate bridge, opening the coordination sphere of the cobalt centres. This cobalt derivative, where the pristine ligands still impact on the reactivity, shows enhanced electron transfer rates to Ru(bpy)₃³⁺(hole scavenging) within a photocatalytic cycle with Ru(bpy)₃²⁺as the photosensitiser and S₂O₈²⁻as the electron sink. A more accentuated evolution occurs under continuous irradiation, where Electron Paramagnetic Resonance (EPR) spectroscopy reveals the formation of Co(ii) intermediates, likely contributing to the catalytic process that evolves oxygen. All together, these results confirm the relevant effect of molecular species, in particular in fostering the rate of the electron transfer processes involved in light activated cycles, pivotal in the design of a photoactive device.

Photocatalytic water oxidation by a mixed-valent Mn(III)₃Mn(IV)O₃ manganese oxo core that mimics the natural oxygen-evolving center

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 S0→S4 intermediates is the expected turning point for artificial photosynthesis. The tetramanganese-substituted tungstosilicate [Mn(III)3Mn(IV)O3(CH3COO)3(A-α-SiW9O34)](6-)(Mn4POM) offers an unprecedented mimicry of the natural system in its reduced S0 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 S2O8(2-) is obtained by nanosecond laser flash photolysis; its S0→S1 transition within milliseconds and multiple-hole-accumulating properties were studied. Photocatalytic oxygen evolution is achieved in a buffered medium (pH 5) with a quantum efficiency of 1.7%.

Molecule-based water-oxidation catalysts (WOCs): cluster-size-dependent dye-sensitized polyoxometalates for visible-light-driven O2 evolution

From atomic level to understand the cluster-size-dependant behavior of dye-sensitized photocatalysts is very important and helpful to design new photocatalytic materials. Although the relationship between the photocatalytic behaviors and particles' size/shape has been widely investigated by theoretical scientists, the experimental evidences are much less. In this manuscript, we successfully synthesized three new ruthenium dye-sensitized polyoxometalates (POM-n, n relate to different size clusters) with different-sized POM clusters. Under visible-light illumination, all three complexes show the stable O2 evolution with the efficient order POM-3 > POM-2 > POM-1. This cluster-size-dependent catalytic behavior could be explained by the different numbers of M = Ot (terminal oxygen) bonds in each individual cluster because it is well-known that Mo = Ot groups are the catalytically active sites for photooxidation reaction. The proposed mechanism of water oxidation for the dye-sensitized POMs is radical reaction process. This research could open up new perspectives for developing new POM-based WOCs.