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.

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.

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.

Electroluminescent devices from ionic transition metal complexes (iTMCs) are attractive candidates for display and lighting applications. A major limitation of application of iTMC devices is their turn-on times, which range from minutes to hours at 3 V. We report novel ruthenium and iridium complexes with pendant triethylammonium groups bonded to the ligands with methylene units of various lengths. These materials lead to devices with turn-on times at 3 V as short as 2.5 min for the iridium complexes and as low as 5 s for the ruthenium complexes.

An efficient homogeneous catalytic system for the visible-light-induced production of hydrogen from water utilizing cyclometalated iridium(III) and tris-2,2′-bipyridyl rhodium(III) complexes is described. Synthetic modification of the photosensitizer Ir(C∧N)2(N∧N)+ and water reduction catalyst Rh(N∧N)33+ creates a family of catalysts with diverse photophysical and electrochemical properties. Parallel screening of the various catalyst combinations and photoreaction conditions allows the rapid development of an optimized photocatalytic system that achieves over 5000 turnovers with quantum yields (1/2 H2 per photon absorbed) greater than 34%. Photophysical and electrochemical characterization of the optimized system reveals that the reductive quenching pathway provides the necessary driving force for the formation of [Rh(N∧N)2]0, the active catalytic species for the reduction of water to produce hydrogen. Tests for system poisoning with mercury or CS2 provide strong evidence that the system is a true homogeneous system for photocatalytic hydrogen production.

A family of heteroleptic (C^N)2Ir(acac) and homoleptic fac-Ir(C^N)3 complexes have been synthesized and their photophysical properties studied (where C^N = a substituted 2-phenylpyridine and acac = acetylacetonate). The neutral Δ and Λ complexes were separated with greater than 95% enantiomeric purity by chiral supercritical fluid chromatography, and the solution circular dichroism and circularly polarized luminescence spectra for each of the enantio-enriched iridium complexes were obtained. The experimentally measured emission dissymmetries (gem) for this series compared well with predicted values provided by time-dependent density functional theory calculations. The discovered trend further showed a correlation with the dissymmetries of ionic, enantiopure hemicage compounds of Ru(II) and Zn(II), thus demonstrating the applicability of the model for predicting emission dissymmetry values across a wide range of complexes.

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.