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

ABSTRACT: The tetraruthenium polyoxometalate [Ru4(μ- O)4(μ-OH)2(H2O)4(γ-SiW10O36)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 ionpaired 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 × 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 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