Electrochemical and Photoelectrochemical Investigation of Water Oxidation with Hematite Electrodes

Atomic layer deposition (ALD) was utilized to deposit uniform thin films of hematite (α-Fe2O3) on transparent conductive substrates for photocatalytic water oxidation studies. Comparison of the oxidation of water to the oxidation of a fast redox shuttle allowed for new insight in determining the rate limiting processes of water oxidation at hematite electrodes. It was found that an additional overpotential is needed to initiate water oxidation compared to the fast redox shuttle. A combination of electrochemical impedance spectroscopy, photoelectrochemical and electrochemical measurements were employed to determine the cause of the additional overpotential. It was found that photogenerated holes initially oxidize the electrode surface under water oxidation conditions, which is attributed to the first step in water oxidation. A critical number of these surface intermediates need to be generated in order for the subsequent hole-transfer steps to proceed. At higher applied potentials, the behavior of the electrode is virtually identical while oxidizing either water or the fast redox shuttle; the slight discrepancy is attributed to a shift in potential associated with Fermi level pinning by the surface states in the absence of a redox shuttle. A water oxidation mechanism is proposed to interpret these results

Bidirectional and Unidirectional PCET in a Molecular Model of a Cobalt-Based Oxygen-Evolving Catalyst

The oxidation of water to molecular oxygen is a kinetically demanding reaction that requires efficient coupling of proton and electron transfer. The key proton-coupled electron transfer (PCET) event in water oxidation mediated by a cobalt-phosphate-based heterogeneous catalyst is the one-electron, one-proton conversion of CoIII−OH to CoIV−O. We now isolate the kinetics of this PCET step in a molecular Co4O4 cubane model compound. Detailed electrochemical, stopped-flow, and NMR studies of the CoIII−OH to CoIV−O reaction reveal distinct mechanisms for the unidirectional PCET self-exchange reaction and the corresponding bidirectional PCET. A stepwise mechanism, with rate-limiting electron transfer is observed for the bidirectional PCET at an electrode surface and in solution, whereas a concerted proton−electron transfer displaying a moderate KIE (4.3 ± 0.2), is observed for the unidirectional self-exchange reaction

Halogen Oxidation and Halogen Photoelimination Chemistry of a Platinum–Rhodium Heterobimetallic Core

The heterobimetallic complexes, PtRh­(tfepma)2(CNtBu)­X3 (X = Cl, Br), are assembled by the treatment of Pt­(cod)­X2 (cod =1,5-cyclooctadiene) with {Rh­(cod)­X}2, in the presence of tert-butylisonitrile (CNtBu) and tfepma (tfepma = bis­(trifluoroethoxyl)­phosphinomethylamine). The neutral complexes contain Pt–Rh single bonds with metal–metal separations of 2.6360(3) and 2.6503(7) Å between the square planar Pt and octahedral Rh centers for the Cl and Br complexes, respectively. Oxidation of the XPtIRhIIX2 cores with suitable halide sources (PhICl2 or Br2) furnishes PtRh­(tfepma)2(CNtBu)­X5, which preserves a Pt–Rh bond. For the chloride system, the initial oxidation product orients the platinum-bound chlorides in a meridional geometry, which slowly transforms to a facial arrangement in pentane solution as verified by X-ray crystal analysis. Irradiation of the mer- or fac-Cl3PtIIIRhIICl2 isomers with visible light in the presence of olefin promotes the photoelimination of halogen and regeneration of the reduced ClPtIRhIICl2 core. In addition to exhibiting photochemistry similar to that of the chloride system, the oxidized bromide cores undergo thermal reduction chemistry in the presence of olefin with zeroth-order olefin dependence. Owing to an extremely high photoreaction quantum yield for the fac-ClPtIRhIICl2 isomer, details of the X2 photoelimination have been captured by transient absorption spectroscopy. We now report the first direct observation of the photointermediate that precedes halogen reductive elimination. The intermediate is generated promptly upon excitation (<8 ns), and halogen is eliminated from it with a rate constant of 3.6 × 104 s–1. As M–X photoactivation and elimination is the critical step in HX splitting, these results establish a new guidepost for the design of HX splitting cycles for solar energy storage

Halogen Oxidation and Halogen Photoelimination Chemistry of a Platinum–Rhodium Heterobimetallic Core

The heterobimetallic complexes, PtRh­(tfepma)2(CNtBu)­X3 (X = Cl, Br), are assembled by the treatment of Pt­(cod)­X2 (cod =1,5-cyclooctadiene) with {Rh­(cod)­X}2, in the presence of tert-butylisonitrile (CNtBu) and tfepma (tfepma = bis­(trifluoroethoxyl)­phosphinomethylamine). The neutral complexes contain Pt–Rh single bonds with metal–metal separations of 2.6360(3) and 2.6503(7) Å between the square planar Pt and octahedral Rh centers for the Cl and Br complexes, respectively. Oxidation of the XPtIRhIIX2 cores with suitable halide sources (PhICl2 or Br2) furnishes PtRh­(tfepma)2(CNtBu)­X5, which preserves a Pt–Rh bond. For the chloride system, the initial oxidation product orients the platinum-bound chlorides in a meridional geometry, which slowly transforms to a facial arrangement in pentane solution as verified by X-ray crystal analysis. Irradiation of the mer- or fac-Cl3PtIIIRhIICl2 isomers with visible light in the presence of olefin promotes the photoelimination of halogen and regeneration of the reduced ClPtIRhIICl2 core. In addition to exhibiting photochemistry similar to that of the chloride system, the oxidized bromide cores undergo thermal reduction chemistry in the presence of olefin with zeroth-order olefin dependence. Owing to an extremely high photoreaction quantum yield for the fac-ClPtIRhIICl2 isomer, details of the X2 photoelimination have been captured by transient absorption spectroscopy. We now report the first direct observation of the photointermediate that precedes halogen reductive elimination. The intermediate is generated promptly upon excitation (<8 ns), and halogen is eliminated from it with a rate constant of 3.6 × 104 s–1. As M–X photoactivation and elimination is the critical step in HX splitting, these results establish a new guidepost for the design of HX splitting cycles for solar energy storage

Halogen Oxidation and Halogen Photoelimination Chemistry of a Platinum–Rhodium Heterobimetallic Core

The heterobimetallic complexes, PtRh­(tfepma)₂(CN^<i>t</i>Bu)­X₃ (X = Cl, Br), are assembled by the treatment of Pt­(cod)­X₂ (cod =1,5-cyclooctadiene) with {Rh­(cod)­X}₂, in the presence of <i>tert</i>-butylisonitrile (CN^<i>t</i>Bu) and tfepma (tfepma = bis­(trifluoroethoxyl)­phosphinomethylamine). The neutral complexes contain Pt–Rh single bonds with metal–metal separations of 2.6360(3) and 2.6503(7) Å between the square planar Pt and octahedral Rh centers for the Cl and Br complexes, respectively. Oxidation of the XPt^IRh^IIX₂ cores with suitable halide sources (PhICl₂ or Br₂) furnishes PtRh­(tfepma)₂(CN^<i>t</i>Bu)­X₅, which preserves a Pt–Rh bond. For the chloride system, the initial oxidation product orients the platinum-bound chlorides in a <i>meridional</i> geometry, which slowly transforms to a <i>facial</i> arrangement in pentane solution as verified by X-ray crystal analysis. Irradiation of the <i>mer</i>- or <i>fac</i>-Cl₃Pt^IIIRh^IICl₂ isomers with visible light in the presence of olefin promotes the photoelimination of halogen and regeneration of the reduced ClPt^IRh^IICl₂ core. In addition to exhibiting photochemistry similar to that of the chloride system, the oxidized bromide cores undergo thermal reduction chemistry in the presence of olefin with zeroth-order olefin dependence. Owing to an extremely high photoreaction quantum yield for the <i>fac</i>-ClPt^IRh^IICl₂ isomer, details of the X₂ photoelimination have been captured by transient absorption spectroscopy. We now report the first direct observation of the photointermediate that precedes halogen reductive elimination. The intermediate is generated promptly upon excitation (<8 ns), and halogen is eliminated from it with a rate constant of 3.6 × 10⁴ s^–1. As M–X photoactivation and elimination is the critical step in HX splitting, these results establish a new guidepost for the design of HX splitting cycles for solar energy storage

Photo-active cobalt cubane model of an oxygen-evolving catalyst

A dyad complex has been constructed as a soluble molecular model of a heterogeneous cobalt-based oxygen-evolving catalyst (Co-OEC). To this end, the Co4O4 core of a cobalt-oxo cubane was covalently appended to ReI photosensitisers. The resulting adduct was characterised both in the solid state (by X-ray diffraction) and in solution using a variety of techniques. In particular, the covalent attachment of the ReI moieties to the Co4O4 core promotes emission quenching of the ReI photocentres, with implications for the energy and electron transduction process of Co-OEC-like catalysts

Intercalation Is Not Required for DNA Light-Switch Behavior

The DNA light-switch complex [Ru(bpy)2(tpphz)]2+ (1, bpy = 2,2‘-bipyridine, tpphz = tetrapyrido[3,2-a:2‘,3‘-c:3‘ ‘,2‘ ‘-h:2‘ ‘‘,3‘ ‘‘-j]phenazine) is luminescent when bound to DNA and in organic solvents and weakly emissive in water. To date, light-switch behavior by transition metal complexes has generally been regarded as confirmation of DNA intercalation. In contrast, the present work demonstrates that the nonintercalating bimetallic complex [(bpy)2Ru(tpphz)Ru(bpy)2]4+ (2) behaves as a DNA light-switch. Weak emission from the 3MLCT excited state of 2 is observed in water with λem = 623 nm (Φem = 1.4 × 10-4), and a red shift (λem = 702 nm) and 40-fold increase in intensity are observed upon addition of 100 μM calf thymus DNA (ct-DNA). Addition of increasing concentrations of 2 to 1 mM herring sperm DNA does not result in an increase in the viscosity of the solution, indicating that the complex is not an intercalator. Additionally, experiments were conducted to ensure that the emission enhancement did not arise from threading intercalation of the complex. The in situ generation of 2 intercalated between the base pairs of ct-DNA in a threading fashion, however, exhibits emission maximum at 685 nm, which is blue-shifted from that of surface-bound 2. DFT calculations show low-lying orbitals in 2 that are expected to exhibit nonemissive character when contributing to the MLCT state, in accord with the lower emission intensity observed for 2 relative to that for 1. To our knowledge, the present work is the first example of a nonintercalating light-switch metal complex, thus showing that light-switch behavior cannot be used exclusively as confirmation of intercalation