Photochemistry Journey to Multielectron and Multiproton Chemical Transformation.

The odyssey of photochemistry is accompanied by the journey to manipulate "electrons" and "protons" in time, in space, and in energy. Over the past decades, single-electron (1e-) photochemical transformations have brought marvelous achievements. However, as each photon absorption typically generates only one exciton pair, it is exponentially challenging to accomplish multielectron and proton photochemical transformations. The multistep differences in thermodynamics and kinetics urgently require us to optimize light harvesting, expedite consecutive electron transfer, manipulate the interaction of catalysts with substrates, and coordinate proton transfer kinetics to furnish selective bond formations. Tandem catalysis enables orchestrating different photochemical events and catalytic transformations from subpicoseconds to seconds, which facilitates multielectron redox chemistries and brings consecutive, value-added reactivities. Joint efforts in molecular and material design, mechanistic understanding, and theoretical modeling will bring multielectron and proton synthetic opportunities for fuels, fertilizers, and chemicals with enhanced versatility, efficiency, selectivity, and scalability, thus taking better advantage of photons (i.e., sunlight) for our sustainable society.

Mechanistic Insights Into Iron(II) Bis(pyridyl)amine-Bipyridine Skeleton for Selective CO2 Photoreduction.

A bis(pyridyl)amine-bipyridine-iron(II) framework (Fe(BPAbipy)) of complexes 1-3 is reported to shed light on the multistep nature of CO2 reduction. Herein, photocatalytic conversion of CO2 to CO even at low CO2 concentration (1 %), together with detailed mechanistic study and DFT calculations, reveal that 1 first undergoes two sequential one-electron transfer affording an intermediate with electron density on both Fe and ligand for CO2 binding over proton. The following 2 H+ -assisted Fe-CO formation is rate-determining for selective CO2 -to-CO reduction. A pendant, proton-shuttling α-OH group (2) initiates PCET for predominant H2 evolution, while an α-OMe group (3) cancels the selectivity control for either CO or H2 . The near-unity selectivity of 1 and 2 enables self-sorting syngas production at flexible CO/H2 ratios. The unprecedented results from one kind of molecular catalyst skeleton encourage insight into the beauty of advanced multi-electron and multi-proton transfer processes for robust CO2 RR by photocatalysis.

Identifying a Real Catalyst of [NiFe]-Hydrogenase Mimic for Exceptional H2 Photogeneration.

Inspired by the natural [NiFe]-H2 ase, we designed mimic 1, (dppe)Ni(µ-pdt)(µ-Cl)Ru(CO)2 Cl to realize effective H2 evolution under photocatalytic conditions. However, a new species 2 was captured in the course of photo-, electro-, and chemo- one-electron reduction. Experimental studies of in situ IR spectroscopy, EPR, NMR, X-ray absorption spectroscopy, and DFT calculations corroborated a dimeric structure of 2 as a closed-shell, symmetric structure with a RuI center. The isolated dimer 2 showed the real catalytic role in photocatalysis with a benchmark turnover frequency (TOF) of 1936 h-1 for H2 evolution, while mimic 1 worked as a pre-catalyst and evolved H2 only after being reduced to 2. The remarkably catalytic activity and unique dimer structure of 2 operated in photocatalysis unveiled a broad research prospect in hydrogenases mimics for advanced H2 evolution.

Concurrent Ammonia Synthesis and Alcohol Oxidation Boosted by Glutathione-Capped Quantum Dots under Visible Light.

Mother nature accomplishes efficient ammonia synthesis via cascade N2 oxidation by lightning strikes followed with enzyme-catalyzed nitrogen oxyanion (NOx -, x = 2,3) reduction. The protein environment of enzymatic centers for NOx --to-NH4 + process greatly inspires the design of glutathione-capped (GSH) quantum dots (QDs) for ammonia synthesis under visible light (440 nm) in tandem with plasma-enabled N2 oxidation. Mechanistic studies reveal that GSH induces positive shift of surface charge to strengthen the interaction between NOx - and QDs. Upon visible light irradiation of QDs, the balanced and rapid hole and electron transfer furnish GS·radicals for 2e-/2H+ alcohol oxidation and H·for 8e-/10H+ NO3 --to-NH4 + reduction simultaneously. For the first time, mmol-scale ammonia synthesis is realized with apparent quantum yields of 5.45% ± 0.64%, and gram-scale synthesis of value-added acetophenone and NH4Cl proceeds with 1:4 stoichiometry and stability, demonstrating promising multielectron and multiproton ammonia synthesis efficiency and sustainability with nature-inspired artificial photocatalysts.

Self-assembled inorganic clusters of semiconducting quantum dots for effective solar hydrogen evolution.

Owing to promoted electron-hole separation, the catalytic activity of semiconducting quantum dots (QDs) towards solar hydrogen (H2) production has been significantly enhanced by forming self-assembled clusters with ZnSe QDs made ex situ. Taking advantage of the favored interparticle hole transfer to ZnSe QDs, the rate of solar H2 evolution of CdSe QDs can be increased to ∼30 000 µmol h-1 g-1 with ascorbic acid as the sacrificial reagent, ∼150-fold higher than that of bare CdSe QDs clusters under the same conditions.