Photodriven Charge Separation Dynamics in CdSe/ZnS Core/Shell Quantum Dot/Cobaloxime Hybrid for Efficient Hydrogen Production

Photodriven charge-transfer dynamics and catalytic properties have been investigated for a hybrid system containing CdSe/ZnS core/shell quantum dots (QDs) and surface-bound molecular cobaloxime catalysts. The electron transfer from light-excited QDs to cobaloxime, revealed by optical transient absorption spectroscopy, takes place with an average time constant of 105 ps, followed a much slower charge recombination process with a time constant of ≫3 ns. More interestingly, we also observed photocatalytic hydrogen generation by this QD/cobaloxime hybrid system, with >10 000 turnovers of H₂ per QD in 10 h, using triethanolamine as a sacrificial electron donor. These results suggest that QD/cobaloxime hybrids succeed in coupling single-photon events with multielectron redox catalytic reactions, and such systems could have potential applications in long-lived artificial photosynthetic devices for fuel generation from sunlight

Ligand Mediation of Vectorial Charge Transfer in Cu(I)diimine Chromophore–Acceptor Dyads

In this work, we present the photoinduced charge separation dynamics of four molecular dyads composed of heteroleptic Cu­(I)­bis­(phenanthroline) chromophores linked directly to the common electron acceptor naphthalene diimide. The dyads were designed to allow us to (1) detect any kinetic preference for directionality during photoinduced electron transfer across the heteroleptic complex and (2) probe the influence of excited-state flattening on intramolecular charge separation. Singular value decomposition of ultrafast optical transient absorption spectra demonstrates that charge transfer occurs with strong directional preference, and charge separation occurs up to 35 times faster when the acceptor is linked to the sterically blocking ligand. Further, the charge-separated state in these dyads is stabilized by polar solvents, resulting in dramatically longer lifetimes for dyads with minimal substitution about the Cu­(I) center. This unexpected but exciting observation suggests a new approach to the design of Cu­(I)­bis­(phenanthroline) chromophores that can support long-lived vectorial charge separation

The Hydrogen Catalyst Cobaloxime: A Multifrequency EPR and DFT Study of Cobaloxime’s Electronic Structure

Solar fuels research aims to mimic photosynthesis and devise integrated systems that can capture, convert, and store solar energy in the form of high-energy molecular bonds. Molecular hydrogen is generally considered an ideal solar fuel because its combustion is essentially pollution-free. Cobaloximes rank among the most promising earth-abundant catalysts for the reduction of protons to molecular hydrogen. We have used multifrequency EPR spectroscopy at X-band, Q-band, and D-band combined with DFT calculations to reveal electronic structure and establish correlations among the structure, surroundings, and catalytic activity of these complexes. To assess the strength and nature of ligand cobalt interactions, the BF₂-capped cobaloxime, Co­(dmgBF₂)₂, was studied in a variety of different solvents with a range of polarities and stoichiometric amounts of potential ligands to the cobalt ion. This allows the differentiation of labile and strongly coordinating axial ligands for the Co­(II) complex. Labile, or weakly coordinating, ligands such as methanol result in larger <i>g</i>-tensor anisotropy than strongly coordinating ligands such as pyridine. In addition, a coordination number effect is seen for the strongly coordinating ligands with both singly ligated LCo­(dmgBF₂)₂ and doubly ligated L₂Co­(dmgBF₂)₂ . The presence of two strongly coordinating axial ligands leads to the smallest <i>g</i>-tensor anisotropy. The relevance of the strength of the axial ligand(s) to the catalytic efficiency of Co­(dmgBF₂)₂ is discussed. Finally, the influence of molecular oxygen and formation of Co­(III) superoxide radicals LCo­(dmgBF₂)₂O₂^• is studied. The experimental results are compared with a comprehensive set of DFT calculations on Co­(dmgBF₂)₂ model systems with various axial ligands. Comparison with experimental values for the “key” magnetic parameters such as <i>g</i>-tensor and ⁵⁹Co hyperfine coupling tensor allows the determination of the conformation of the axially ligated Co­(dmgBF₂)₂ complexes. The data presented here are vital for understanding the influence of solvent and ligand coordination on the catalytic efficiency of cobaloximes

The Hydrogen Catalyst Cobaloxime: A Multifrequency EPR and DFT Study of Cobaloxime’s Electronic Structure

Solar fuels research aims to mimic photosynthesis and devise integrated systems that can capture, convert, and store solar energy in the form of high-energy molecular bonds. Molecular hydrogen is generally considered an ideal solar fuel because its combustion is essentially pollution-free. Cobaloximes rank among the most promising earth-abundant catalysts for the reduction of protons to molecular hydrogen. We have used multifrequency EPR spectroscopy at X-band, Q-band, and D-band combined with DFT calculations to reveal electronic structure and establish correlations among the structure, surroundings, and catalytic activity of these complexes. To assess the strength and nature of ligand cobalt interactions, the BF₂-capped cobaloxime, Co­(dmgBF₂)₂, was studied in a variety of different solvents with a range of polarities and stoichiometric amounts of potential ligands to the cobalt ion. This allows the differentiation of labile and strongly coordinating axial ligands for the Co­(II) complex. Labile, or weakly coordinating, ligands such as methanol result in larger <i>g</i>-tensor anisotropy than strongly coordinating ligands such as pyridine. In addition, a coordination number effect is seen for the strongly coordinating ligands with both singly ligated LCo­(dmgBF₂)₂ and doubly ligated L₂Co­(dmgBF₂)₂ . The presence of two strongly coordinating axial ligands leads to the smallest <i>g</i>-tensor anisotropy. The relevance of the strength of the axial ligand(s) to the catalytic efficiency of Co­(dmgBF₂)₂ is discussed. Finally, the influence of molecular oxygen and formation of Co­(III) superoxide radicals LCo­(dmgBF₂)₂O₂^• is studied. The experimental results are compared with a comprehensive set of DFT calculations on Co­(dmgBF₂)₂ model systems with various axial ligands. Comparison with experimental values for the “key” magnetic parameters such as <i>g</i>-tensor and ⁵⁹Co hyperfine coupling tensor allows the determination of the conformation of the axially ligated Co­(dmgBF₂)₂ complexes. The data presented here are vital for understanding the influence of solvent and ligand coordination on the catalytic efficiency of cobaloximes

Charge Separation Related to Photocatalytic H₂ Production from a Ru–Apoflavodoxin–Ni Biohybrid

The direct creation of a fuel from sunlight and water via photochemical energy conversion provides a sustainable method for producing a clean source of energy. Here we report the preparation of a solar fuel biohybrid that embeds a nickel diphosphine hydrogen evolution catalyst into the cofactor binding pocket of the electron shuttle protein, flavodoxin (Fld). The system is made photocatalytic by linking a cysteine residue in Fld to a ruthenium photosensitizer. Importantly, the protein environment enables the otherwise insoluble Ni catalyst to perform photocatalysis in aqueous solution over a pH range of 3.5–12.0, with optimal turnover frequency 410 ± 30 h^–1 and turnover number 620 ± 80 mol H₂/mol hybrid observed at pH 6.2. For the first time, a reversible light-induced charge-separated state involving a Ni­(I) intermediate was directly monitored by electron paramagnetic resonance spectroscopy. Transient optical measurements reflect two conformational states, with a Ni­(I) state formed in ∼1.6 or ∼185 μs that persists for several milliseconds as a long-lived charge-separated state facilitated by the protein matrix

Photocatalytic Hydrogen Production from Noncovalent Biohybrid Photosystem I/Pt Nanoparticle Complexes

A photocatalytic hydrogen-evolving system based on intermolecular electron transfer between native Photosystem I (PSI) and electrostatically associated Pt nanoparticles is reported. Using cytochrome c₆ as the soluble mediator and ascorbate as the sacrificial electron donor, visible-light-induced H₂ production occurs for PSI/Pt nanoparticle biohybrids at a rate of 244 μmol H₂ (mg chlorophyll)⁻¹ h⁻¹ or 21 034 mol H₂ (mole PSI)⁻¹ h⁻¹. These results demonstrate that highly efficient photocatalysis of H₂ can be obtained for a self-assembled, noncovalent complex between PSI and Pt nanoparticles; a molecular wire between the terminal acceptor of PSI, the [4Fe−4S] cluster F_B, and the nanoparticle is not required. EPR characterization of the electron-transfer reactions in PSI/Pt nanoparticle biohybrids shows blocked electron transfer to flavodoxin, the native acceptor protein of PSI, and presents evidence of low-temperature photogenerated electron transfer between PSI and the Pt nanoparticle. This work demonstrates a feasible strategy for linking molecular catalysts to PSI that takes advantage of electrostatic-directed assembly to mimic acceptor protein binding

Harnessing Intermolecular Interactions to Promote Long-Lived Photoinduced Charge Separation from Copper Phenanthroline Chromophores

Facilitating photoinduced electron transfer (PET) while minimizing rapid charge-recombination processes to produce a long-lived charge-separated (CS) state represents a primary challenge associated with achieving efficient solar fuel production. Natural photosynthetic systems employ intermolecular interactions to arrange the electron-transfer relay in reaction centers and promote a directional flow of electrons. This work explores a similar tactic through the synthesis and ground- and excited-state characterization of two Cu(I)bis(phenanthroline) chromophores with homoleptic and heteroleptic coordination geometries and which are functionalized with negatively charged sulfonate groups. The addition of sulfonate groups enables solubility in pure water, and it also induces assembly with the dicationic electron acceptor methyl viologen (MV2+) via bimolecular, dynamic electrostatic interactions. The effect of the sulfonate groups on the ground- and excited-state properties was evaluated by comparison with the unsulfonated analogues in 1:1 acetonitrile/water. The excited-state lifetimes for all sulfonated complexes are similar to what we expect from previous literature, with the exception of the sulfonated heteroleptic complex whose metal-to-ligand charge-transfer (MLCT) lifetime in water has two components that are fit to 10 and 77 ns. For the sulfonated complexes, we detected reduced MV+• in both solvent environments following MLCT excitation, but control measurements in 1:1 acetonitrile/water with the unsulfonated analogues showed no PET to MV2+, indicating that electrostatically driven supramolecular assemblies of the sulfonated complexes with MV2+ facilitate the observed PET. Additionally, the strength of the intermolecular interactions driving the formation of these assemblies changes drastically with the solvent environment. In 1:1 acetonitrile/water, PET occurred from both sulfonated complexes with quantum yields (ΦET) of 2–3% but increased to a remarkable 98% for the sulfonated heteroleptic complex with a 3 μs CS-state lifetime in water

Mechanistic Evaluation of a Nickel Proton Reduction Catalyst Using Time-Resolved X‑ray Absorption Spectroscopy

We report the light-induced electronic and geometric changes taking place in “real time” of a multimolecular [Ru­(bpy)₃]²⁺/[Ni­(P^Ph₂N^Ph₂)₂(CH₃CN)]²⁺/ascorbic acid photocatalytic system by time-resolved X-ray absorption spectroscopy (tr-XAS) in the nano- to microsecond time regime. Using tr-XAS allows us to observe the diffusion-governed electron transfer between the excited photosensitizer and the nickel­(II) proton reduction catalyst on the nanosecond time scale followed by formation of a transient distorted tetrahedral Ni­(I) intermediate. A 50-fold increase in the decay lifetime of the Ni­(I) species, in the presence of the electron donor, shows that the favored catalytic pathway occurs through reductive quenching of the excited photosensitizer followed by electron transfer to the catalyst. Lack of protonation of the Ni­(I) amine groups within our experimental tr-XAS time window suggests that proton binding is the rate limiting step for H₂ photocatalysis by this system. This study is supported by molecular orbital density functional theory (DFT-MO) calculations providing relevant information for ongoing synthetic efforts on “DuBois-type” nickel complexes as well as mechanistic time-resolved understanding of the photoevolution processes in the catalytic cycle for the rational design of molecular hydrogen-evolving photocatalysts