Photoinduced Water Oxidation by a Tetraruthenium Polyoxometalate Catalyst: Ion-pairing and Primary Processes with Ru(bpy)₃²⁺ Photosensitizer

The tetraruthenium polyoxometalate [Ru₄(μ-O)₄(μ-OH)₂(H₂O)₄(γ-SiW₁₀O₃₆)₂]^10‑ (<b>1</b>) behaves as a very efficient water oxidation catalyst in photocatalytic cycles using Ru­(bpy)₃²⁺ 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)₃²⁺ 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)₃²⁺ excited state. The quenching takes place in ion-paired species with an average <b>1</b>:Ru­(bpy)₃²⁺ 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)₃³⁺ generated by photoreaction of the excited sensitizer with the sacrificial oxidant undergoes primary bimolecular hole scavenging by <b>1</b> at a remarkably high rate (3.6 ± 0.1 × 10⁹ 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 <b>1</b> is shown to undergo, in 40 ms, ca. 45 turnovers in Ru­(bpy)₃³⁺ 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

Long-Range Charge Separation in a Ferrocene–(Zinc Porphyrin)–Naphthalenediimide Triad. Asymmetric Role of 1,2,3-Triazole Linkers

New dyad and triad systems based on a zinc porphyrin (ZnP), a naphthalenediimide (NDI), and a ferrocene (Fc) as molecular components, linked by 1,2,3-triazole bridges, ZnP-NDI (<b>3</b>) and Fc-ZnP-NDI (<b>4</b>), have been synthesized. Their photophysical behavior has been investigated by both visible excitation of the ZnP chromophore and UV excitation of the NDI unit. Dyad <b>3</b> exhibits relatively inefficient quenching of the ZnP singlet excited state, slow charge separation, and fast charge recombination processes. Excitation of the NDI chromophore, on the other hand, leads to charge separation by both singlet and triplet quenching pathways, with the singlet charge-separated (CS) state recombining in a subnanosecond time scale and the triplet CS state decaying in ca. 90 ns. In the triad system <b>4</b>, primary formation of the Fc-ZnP⁺-NDI^– charge-separated state is followed by a secondary hole shift process from ZnP to Fc. The product of the stepwise charge separation, Fc⁺-ZnP-NDI^–, undergoes recombination to the ground state in 1.9 μs. The charge-separated states are always formed more efficiently upon NDI excitation than upon ZnP excitation. DFT calculations on a bridge–acceptor fragment show that the bridge is expected to mediate a fast donor-to-bridge-to-acceptor electron cascade following excitation of the acceptor. More generally, triazole bridges may behave asymmetrically with respect to photoinduced electron transfer in dyads, kinetically favoring hole-transfer pathways triggered by excitation of the acceptor over electron-transfer pathways promoted by excitation of the donor

The Use of a Vanadium Species As a Catalyst in Photoinduced Water Oxidation

The first water oxidation catalyst containing only vanadium atoms as metal centers is reported. The compound is the mixed-valence [(V^IV₅V^V₁)­O₇(OCH₃)₁₂]⁻ species, <b>1</b>. Photoinduced water oxidation catalyzed by <b>1</b>, in the presence of Ru­(bpy)₃²⁺ (bpy = 2,2′-bipyridine) and Na₂S₂O₈, in acetonitrile/aqueous phosphate buffer takes place with a quantum yield of 0.20. A hole scavenging reaction between the photochemically generated Ru­(bpy)₃³⁺ and <b>1</b> occurs with a bimolecular rate constant of 2.5 × 10⁸ M^–1 s^–1. The time-resolved formation of the oxidized molecular catalyst <b>1</b>⁺ in bimolecular reactions is also evidenced for the first time by transient absorption spectroscopy. This result opens the way to the use of less expensive vanadium clusters as water oxidation catalysts in artificial photosynthesis schemes

Photocatalytic Water Oxidation: Tuning Light-Induced Electron Transfer by Molecular Co₄O₄ Cores

Isostructural cubane-shaped catalysts [Co^III₄(μ-O)₄(μ-CH₃COO)₄(<i>p</i>-NC₅H₄X)₄], <b>1-X</b> (X = H, Me, <i>t</i>-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 O₂ productivity with quantum efficiency up to 80% at λ > 400 nm, thus opening a new perspective for in vitro molecular photosynthesi