A series of bis-phenylpyridine, bis-aquo iridium(III) complexes is herein shown to robustly and efficiently catalyze the oxidation of water to dioxygen in the presence of a sacrificial oxidant. Through substitution on the cyclometalating ligands of these complexes, it is shown that a broad range of oxidation potentials can be achieved within this class of catalyst. Parallel, dynamic monitoring of oxygen evolution, made possible by equipping reaction vessels with pressure-voltage transducers, facilitates correlation of these complexes' ionization potentials with their respective activity toward water oxidation. The importance of these catalysts lies in (A) their ability to oxidize water in a purely aqueous medium, (B) their simplicity of design, (C) their durability, and (D) the ease with which they can be tuned to accommodate the electrochemical needs of photosensitizers in hypothetical photochemical water oxidation and full artificial photosynthetic schemes.

Protons from water are reduced by a catalytic system composed of a heteroleptic iridium(III) photosensitizer [Ir(ppy)2(bpy)]+, platinum catalyst, and sacrificial reductant. The hydrogen quantum yield reaches 0.26 in this study, which proceeds via reductive quenching of the excited photosensitizer by triethanolamine. This simplified approach allows the characterization of degradation products that are otherwise obscured in more complex systems. A novel 16-well setup for parallel kinetic analysis of H2 evolution enables high-throughput screening of reaction conditions and quantization of the decaying reaction rate. DFT calculations rationalize the differences between this and previous studies on tris-diimine ruthenium(II) photosensitizers.

A photocatalytic water-reducing system utilizing a bis-cyclometalated bipyridyl iridium(III) photosensitizer (PS) and a platinum or palladium heterogeneous catalyst was used to identify systematic property-activity correlations among a library of structural derivatives of [Ir(ppy)(2)(bpy)](+). A heterogeneous Pd catalyst proved to be more durable than its previously reported Pt-based counterpart, allowing for more reliable photosensitizer study. The deliberate steric and electronic variations of the ppy and bpy moieties resulted in a dramatic decrease of the degradation rates observed with selected photosensitizers when compared to the more substitution-labile [Ir(ppy)(2)(bpy)](+) parent compound. An improved photosensitizer structure with a Pd catalyst in a nonligating solvent exhibited a 150-fold increase in catalyst turnover numbers compared to the system using [Ir(ppy)(2)(bpy)](+) and a Pt catalyst. Furthermore, photocatalytic and photophysical studies at varied temperatures provided information on the rate-limiting step of the photocatalytic process, which is shown to be dependent on both the PS and the Pt or Pd catalytic species.

This highlight covers recent progress towards complete homogeneous water photolysis. Efforts in designing catalysts for water reduction, as well as their interaction with light harvesting complexes, are discussed. There are several successful catalyst archetypes for water oxidation that are reviewed herein; however, light driven oxygen evolution remains an area of limited success.

The improved stability of a photocatalytic proton reduction system is accomplished when a heteroleptic bis-cyclometalated diimine iridium(III) photosensitizer ([Ir(ppy)(2)(bpy)](+), ppy = 2-phenylpyridine and bpy = 2,2'-bipyridine) is replaced with a novel iridium complex, [Ir(phbpy)(2)](+) (phbpy = 6-phenyl-2,2'-bipyridine). The decomposition of [Ir(ppy)(2)(bpy)](+) analogs in photocatalytic systems has been previously shown to result from 2,2'-bipyridine dissociation, which will be hindered by the improved architecture. Although desirable for reasons beyond stability, syntheses of bis-tridentate iridium complexes of 6-phenyl-2,2'-bipyridine are uncommon, with no previous examples having an analogous coordination sphere to the well-studied [Ir(C--N)(2)(N--N)](+) architecture (where C--N = cyclometalating ligand and N--N = neutral diimine ligand). Ligand modification has proven a successful strategy in tuning the photophysical properties of [Ir(C--N)(2)(N--N)](+) complexes and can now be employed for the more robust [Ir(C--N--N)(2)](+) framework (where C--N--N = cyclometalating diimine ligand). Characterization of the novel complex reveals similar electrochemical properties and calculated orbital densities to the parent [Ir(ppy)(2)(bpy)](+) species, while there are notable differences between the absorption and photophysical properties of the two complexes.