Photochemical upconversion in water.

A major challenge remaining in photochemical upconversion lies in identifying appropriate chromophore combinations that function in pure water in the absence of hydrophobic hosts or surfactant additives. The current investigation achieves this goal using combinations of water soluble Ru(ii) metal-to-ligand charge transfer (MLCT) sensitizers in concert with 9-anthracenecarboxylate (AnCO2-) and 1-pyrenecarboxylate (PyCO2-) in neat H2O.

Homogeneous Photocatalytic H2 Production Using a RuII Bathophenanthroline Metal-to-Ligand Charge-Transfer Photosensitizer.

The prototypical [Ru(bpy)3 ]2+ (bpy=2,2'-bipyridine) photosensitizer has been previously demonstrated to be labile in aqueous photocatalytic solutions, especially in the presence of coordinating electron donors. Here, an alternative RuII molecular sensitizer, [Ru(dpp)3 ]2+ (dpp=4,7-diphenyl-1,10-phenanthroline or bathophenanthroline), is described, which is considerably more stable than its bpy congener, allowing enhanced photocatalysis metrics in conjunction with a cobalt glyoxime ([Co(dmgH)2 pyCl], dmgH=dimethylglyoxime, py=pyridine) water reduction catalyst and N,N-dimethyl-p-toluidine (DMT) as the sacrificial donor in a 1:1 mixture of CH3 CN/H2 O. Photoluminescence studies revealed that DMT reductively quenches the excited state of [Ru(dpp)3 ]2+ with a bimolecular rate constant of kq =4.9×109 m-1 s-1 . The rate constant measured for electron transfer from the reduced sensitizer to the [Co(dmgH)2 pyCl] was found to be near the diffusion limit, kCo =2.4×109 m-1 s-1 . H2 production by photocatalysis was independently monitored by using a high-throughput photochemical reactor equipped with pressure transducers, gas chromatogram, and a mass spectrometer for detection; this illustrated that the composition yields high turnover numbers (TONs), approaching 10 000 (H2 /Ru) with respect to the sensitizer and deuteration studies using D2 O confirmed that H2 is primarily produced from protons derived from water in these systems.

Homogeneous Photocatalytic H2 Production Using a RuII Bathophenanthroline Metal-to-Ligand Charge-Transfer Photosensitizer.

Invited for this month's cover are the research groups of Prof. Rony S. Khnayzer at Lebanese American University and Prof. Felix (Phil) N. Castellano at North Carolina State University. The cover picture illustrates the absorption of visible light by a ruthenium bathophenanthroline MLCT chromophore followed by a sequence of electron transfer steps that ultimately leads to the efficient production of hydrogen gas from water. Read the full text of the article at 10.1002/cplu.201600227.

Bioinspired design of redox-active ligands for multielectron catalysis: effects of positioning pyrazine reservoirs on cobalt for electro- and photocatalytic generation of hydrogen from water.

Mononuclear metalloenzymes in nature can function in cooperation with precisely positioned redox-active organic cofactors in order to carry out multielectron catalysis. Inspired by the finely tuned redox management of these bioinorganic systems, we present the design, synthesis, and experimental and theoretical characterization of a homologous series of cobalt complexes bearing redox-active pyrazines. These donor moieties are locked into key positions within a pentadentate ligand scaffold in order to evaluate the effects of positioning redox non-innocent ligands on hydrogen evolution catalysis. Both metal- and ligand-centered redox features are observed in organic as well as aqueous solutions over a range of pH values, and comparison with analogs bearing redox-inactive zinc(ii) allows for assignments of ligand-based redox events. Varying the geometric placement of redox non-innocent pyrazine donors on isostructural pentadentate ligand platforms results in marked effects on observed cobalt-catalyzed proton reduction activity. Electrocatalytic hydrogen evolution from weak acids in acetonitrile solution, under diffusion-limited conditions, reveals that the pyrazine donor of axial isomer 1-Co behaves as an unproductive electron sink, resulting in high overpotentials for proton reduction, whereas the equatorial pyrazine isomer complex 2-Co is significantly more active for hydrogen generation at lower voltages. Addition of a second equatorial pyrazine in complex 3-Co further minimizes overpotentials required for catalysis. The equatorial derivative 2-Co is also superior to its axial 1-Co congener for electrocatalytic and visible-light photocatalytic hydrogen generation in biologically relevant, neutral pH aqueous media. Density functional theory calculations (B3LYP-D2) indicate that the first reduction of catalyst isomers 1-Co, 2-Co, and 3-Co is largely metal-centered while the second reduction occurs at pyrazine. Taken together, the data establish that proper positioning of non-innocent pyrazine ligands on a single cobalt center is indeed critical for promoting efficient hydrogen catalysis in aqueous media, akin to optimally positioned redox-active cofactors in metalloenzymes. In a broader sense, these findings highlight the significance of electronic structure considerations in the design of effective electron-hole reservoirs for multielectron transformations.

Light-driven hydrogen evolution by BODIPY-sensitized cobaloxime catalysts.

We report four photocatalytically active cobaloxime complexes for light-driven hydrogen evolution. The cobaloxime catalysts are sensitized by different meso-pyridyl boron dipyrromethene (BODIPY) chromophores, bearing either two bromo- or iodo-substituents on the BODIPY core. The pyridine linker between the BODIPY and cobaloxime is further modified by a methyl substituent on the pyridine, influencing the stability and electronic properties of the cobaloxime catalyst and thus the photocatalytic efficiency of each system. Four cobaloxime catalyst complexes and three novel BODIPY chromophores are synthesized and characterized by absorption, fluorescence, infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry, and electrochemistry. Crystal structures for the BODIPY-cobaloxime complexes 2 and 3 are presented. In contrast to the photocatalytically inactive, nonhalogenated reference complex 1, the four newly reported molecules are active for photocatalytic hydrogen evolution, with a maximum turnover number (TON) of 30.9 mol equiv of H2 per catalyst for the meso-methylpyridyl 2,6-diiodo BODIPY-sensitized cobaloxime complex 5. We conclude that accessing the photoexcited triplet state of the BODIPY chromophore by introducing heavy atoms (i.e., bromine or iodine) is necessary for efficient electron transfer in this system, enabling catalytic hydrogen generation. In addition, relatively electron-donating pyridyl linkers improve the stability of the complex, increasing the overall TON for hydrogen production.