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.

Transition-Metal-Free, Site-Selective C-F Arylation of Polyfluoroarenes via Electrophotocatalysis.

Direct CAr-F arylation is effective and sustainable for synthesis of polyfluorobiaryls with different degrees of fluorination, which are important motifs in medical and material chemistry. However, with no aid of transition metals, the engagement of CAr-F bond activation has proved difficult. Herein, an unprecedented transition-metal-free strategy is reported for site-selective CAr-F arylation of polyfluoroarenes with simple (het)arenes. By merging N,N-bis(2,6-diisopropylphenyl)perylene-3,4,9,10-bis(dicarboximide)-catalyzed electrophotocatalytic reduction and anodic nitroxyl radical oxidation in an electrophotocatalytic cell, various polyfluoroaromatics (2F-6F and 8F), especially inactive partially fluorinated aromatics, undergo sacrificial-reagents-free C-F bond arylation with high regioselectivity, and the yields are comparable to those for reported transition-metal catalysis. This atom- and step-economic protocol features a paired electrocatalysis with organic mediators in both cathodic and anodic processes. The broad substrate scope and good functional-group compatibility highlight the merits of this operationally simple strategy. Moreover, the easy gram-scale synthesis and late-stage functionalization collectively advocate for the practical value, which would promote the vigorous development of fluorine chemistry.

Direct Construction of Isomeric Benzobisoxazole-Vinylene-Linked Covalent Organic Frameworks with Distinct Photocatalytic Properties.

Vinylene/olefin-linked two-dimensional covalent organic frameworks (v-2D-COFs) have emerged as advanced semiconducting materials with excellent in-plane conjugation, high chemical stabilities, and precisely tunable electronic structures. Exploring new linkage chemistry for the reticular construction of v-2D-COFs remains in infancy and challenging. Herein, we present a solid-state benzobisoxazole-mediated aldol polycondensation reaction for the construction of two novel isomeric benzobisoxazole-bridged v-2D-COFs (v-2D-COF-NO1 and v-2D-COF-NO2) with trans and cis configurations of benzobisoxazole. Interestingly, the isomeric benzobisoxazole linkers endow the two v-2D-COFs with distinct optoelectronic and electrochemical properties, ranging from light absorption and emission to charge-transfer properties. When employed as the photocathode, v-2D-COF-NO1 exhibits a photocurrent of up to ∼18 µA/cm2 under AM 1.5G irradiation at -0.3 V vs reversible hydrogen electrode (RHE), which is twice the value of v-2D-COF-NO2 (∼9.1 µA/cm2). With Pt as a cocatalyst, v-2D-COF-NO1 demonstrates a photocatalytic hydrogen evolution rate of ∼1.97 mmol h-1 g-1, also in clear contrast to that of v-2D-COF-NO2 (∼0.86 mmol h-1 g-1) under identical conditions. This work demonstrates the synthesis of v-2D-COFs via benzobisoxazole-mediated aldol polycondensation with isomeric structures and distinct photocatalytic properties.

Reductive Carbon-Carbon Coupling on Metal Sites Regulates Photocatalytic CO2 Reduction in Water Using ZnSe Quantum Dots.

Colloidal quantum dots (QDs) consisting of precious-metal-free elements show attractive potentials towards solar-driven CO2 reduction. However, the inhibition of hydrogen (H2 ) production in aqueous solution remains a challenge. Here, we describe the first example of a carbon-carbon (C-C) coupling reaction to block the competing H2 evolution in photocatalytic CO2 reduction in water. In a specific system taking ZnSe QDs as photocatalysts, the introduction of furfural can significantly suppress H2 evolution leading to CO evolution with a rate of ≈5.3 mmol g-1 h-1 and a turnover number (TON) of >7500 under 24 h visible light. Meanwhile, furfural is upgraded to the self-coupling product with a yield of 99.8 % based on the consumption of furfural. Mechanistic insights show that the reductive furfural coupling reaction occurs on surface Zn-sites to consume electrons and protons originally used for H2 production, while the CO formation pathway at surface anion vacancies from CO2 remains.

Semiconductor nanocrystals for small molecule activation via artificial photosynthesis.

Facile activation and conversion of small molecules (e.g., H2O, CO2, N2, CH4, and C6H6) into solar fuels or value-added chemicals under mild conditions is an attractive pathway in dealing with the worldwide appeal of energy consumption and the growing demand of industrial feedstocks. Compared with conventional thermo- or electro-catalytic approaches, the protocol of photocatalysis shines light on green and low-cost storage of sunlight in chemical bonds. For instance, artificial photosynthesis is an effective way to split H2O into molecular O2 and H2, thereby storing solar energy in the form of hydrogen fuel. Because of rational tunability in band gaps, charge-carrier dynamics, exposed active sites and catalytic redox activities by tailoring size, composition, morphology, surface, and/or interface property, semiconductor nanocrystals (NCs) emerge as very promising candidates for photo-induced small molecule activation, including H2O splitting, CO2 reduction, N2 fixation, CH4 conversion and chemical bond formation (e.g., S-S, C-C, C-N, C-P, C-O). In this review, we summarize the recent advances in small molecule activation via artificial photosynthesis using semiconductor NCs, especially those consisting of II-VI and III-V elements. Moreover, we highlight the intrinsic advantages of semiconductor NCs in this field and look into the fabrication of prototype devices for large-scale and sustainable small molecule activation to store solar energy in chemical bonds.

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.

Unveiling Catalytic Sites in a Typical Hydrogen Photogeneration System Consisting of Semiconductor Quantum Dots and 3d-Metal Ions.

Semiconductor quantum dots (QDs) in conjunction with non-noble 3d-metal ions (e.g., Fe3+, Co2+, and Ni2+) have emerged as an extremely efficient, facile, and cost-effective means of solar-driven hydrogen (H2) evolution. However, the exact structural change of the active sites under realistic conditions remains elusive, and the mechanism of H2 evolution behind the remarkable activity is poorly understood. Here, we successfully track the structural variation of the catalytic sites in the typical H2 photogeneration system consisting of CdSe/CdS QDs and 3d-metal ions (i.e., Ni2+ used here). That is, the nickel precursor of Ni(OAc)2 changes to Ni(H2O)62+ in neutral H2O and eventually transforms to Ni(OH)2 nanosheets in alkaline media. Furthermore, the in operando spectroscopic techniques of electron paramagnetic resonance and X-ray absorption spectroscopy reveal the photoinduced transformation of Ni(OH)2 to a defective structure [Nix0/Ni1-x(OH)2], which acts as the real catalytic species of H2 photogeneration. Density functional theory (DFT) calculations further indicate that the surface Ni-vacancies (VNi) on the Ni(OH)2 nanosheets enhance the adsorption and dissociation of H2O molecules to enhance the local proton concentration, while the Ni0 clusters behave as H2-evolution sites, thereby synergistically promoting the activity of H2 photogeneration in alkaline media.

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.

Quantum Dot Assembly for Light-Driven Multielectron Redox Reactions, such as Hydrogen Evolution and CO2 Reduction.

Light-driven multielectron redox reactions (e.g., hydrogen (H2 ) evolution, CO2 reduction) have recently appeared at the front of solar-to-fuel conversion. In this Minireview, we focus on the recent advances in establishing semiconductor quantum dot (QD) assemblies to enhance the efficiencies of these light-driven multielectron reduction reactions. Four models of QD assembly are established to promote the sluggish kinetics of multielectron transfer from QDs to cocatalysts, thus leading to an enhanced activity of solar H2 evolution or CO2 reduction. We also forecast the potential applications of QD assemblies in other multielectron redox reactions, such as nitrogen (N2 ) fixation and oxygen (O2 ) evolution from H2 O.

Photocatalysis with Quantum Dots and Visible Light for Effective Organic Synthesis.

Colloidal semiconductor quantum dots (QDs) have recently attracted widespread interest for diverse applications. Owing to their quantum confinement effects, rich surface binding properties, high surface-to-volume ratios, broad and intense absorption spectra in the visible region, and low cost as well, QDs offer new and versatile ways to serve as photocatalysts for organic synthesis. Most recently, the use of QDs photocatalysts is springing up in organic synthesis. Herein, we highlight the recent advances of QDs used for smart and clean organic transformations.

Three-Dimensional Graphene Networks with Abundant Sharp Edge Sites for Efficient Electrocatalytic Hydrogen Evolution.

To achieve sustainable production of hydrogen (H2 ) through water splitting, establishing efficient and earth-abundant electrocatalysts is of great necessity. Morphology engineering of graphene is now shown to modulate the electronic structure of carbon skeleton and in turn endow it with excellent ability of proton reduction. Three-dimensional (3D) graphene networks with a high density of sharp edge sites are synthesized. Electrocatalytic measurements indicate that the obtained 3D graphene networks can electrocatalyze H2 evolution with an extremely low onset potential of about 18 mV in 0.5 m H2 SO4 solution, together with good stability. A combination of control experiments and density functional theory (DFT) investigations indicates that the exceptional H2 evolution performance is attributed to the abundant sharp edge sites of the advanced frameworks, which are responsible for promoting the adsorption and reduction of protons.

Self-Assembled Framework Enhances Electronic Communication of Ultrasmall-Sized Nanoparticles for Exceptional Solar Hydrogen Evolution.

Colloidal quantum dots (QDs) have demonstrated great promise in artificial photosynthesis. However, the ultrasmall size hinders its controllable and effective interaction with cocatalysts. To improve the poor interparticle electronic communication between free QD and cocatalyst, we design here a self-assembled architecture of nanoparticles, QDs and Pt nanoparticles, simply jointed together by molecular polyacrylate to greatly enhance the rate and efficiency of interfacial electron transfer (ET). The enhanced interparticle electronic communication is confirmed by femtosecond transient absorption spectroscopy and X-ray transient absorption. Taking advantage of the enhanced interparticle ET with a time scale of ∼65 ps, 5.0 mL of assembled CdSe/CdS QDs/cocatalysts solution produces 94 ± 1.5 mL (4183 ± 67 µmol) of molecular H2 in 8 h, giving rise to an internal quantum yield of ∼65% in the first 30 min and a total turnover number of >1.64â€¯× 107 per Pt nanoparticle. This study demonstrates that self-assembly is a promising way to improve the sluggish kinetics of the interparticle ET process, which is the key step for advanced H2 photosynthesis.

Photocatalysis with Quantum Dots and Visible Light: Selective and Efficient Oxidation of Alcohols to Carbonyl Compounds through a Radical Relay Process in Water.

Selective oxidation of alcohols to aldehydes/ketones has been achieved with the help of 3-mercaptopropionic acid (MPA)-capped CdSe quantum dot (MPA-CdSe QD) and visible light. Visible-light-prompted electron-transfer reaction initiates the oxidation. The thiyl radical generated from the thiolate anion adsorbed on a CdSe QD plays a key role by abstracting the hydrogen atom from the C-H bond of the alcohol (R1 CH(OH)R2 ). The reaction shows high efficiency, good functional group tolerance, and high site-selectivity in polyhydroxy compounds. The generality and selectivity reported here offer a new opportunity for further applications of QDs in organic transformations.

Graphdiyne: A Metal-Free Material as Hole Transfer Layer To Fabricate Quantum Dot-Sensitized Photocathodes for Hydrogen Production.

Graphdiyne (GDY), a novel large π-conjugated carbon material, for the first time, is introduced as the hole transfer layer into a photoelectrochemical water splitting cell (PEC). Raman and ultraviolet photoelectron spectroscopic studies indicate the existence of relatively strong π-π interactions between GDY and 4-mercaptopyridine surface-functionalized CdSe quantum dots, beneficial to the hole transportation and enhancement of the photocurrent performance. Upon exposure to a Xe lamp, the integrated photocathode produces a current density of nearly -70 µA cm(-2) at a potential of 0 V vs NHE in neutral aqueous solution. Simultaneously, the photocathode evolves H2 with 90 ± 5% faradic efficiency over three times and exhibits good stability within 12 h. All of the results indicate that GDY is a promising hole transfer material to fabricate a PEC device for water splitting by solar energy.

Visible light catalysis-assisted assembly of Ni(h)-QD hollow nanospheres in situ via hydrogen bubbles.

Hollow spheres are one of the most promising micro-/nanostructures because of their unique performance in diverse applications. Templates, surfactants, and structure-directing agents are often used to control the sizes and morphologies of hollow spheres. In this Article, we describe a simple method based on visible light catalysis for preparing hollow nanospheres from CdE (E = Te, Se, and S) quantum dots (QDs) and nickel (Ni(2+)) salts in aqueous media. In contrast to the well-developed traditional approaches, the hollow nanospheres of QDs are formed in situ by the photogeneration of hydrogen (H2) gas bubbles at room temperature. Each component, that is, the QDs, metal ions, ascorbic acid (H2A), and visible light, is essential for the formation of hollow nanospheres. The quality of the hollow nanospheres depends on the pH, metal ions, and wavelength and intensity of visible light used. Of the various metal ions investigated, including Cu(+), Cu(2+), Fe(2+), Fe(3+), Ni(2+), Mn(2+), RuCl5(2-), Ag(+), and PtCl4(2-), Ni(2+) ions showed the best ability to generate H2 and hollow-structured nanospheres under visible light irradiation. The average diameter and shell thickness of the nanospheres ranged from 10 to 20 nm and from 3 to 6 nm, respectively, which are values rarely reported in the literature. Studies using high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), inductively coupled plasma-mass spectroscopy (ICP-AES), and steady-state and time-resolved spectroscopy revealed the chemical nature of the hollow nanospheres. Additionally, the hollow-structured nanospheres exhibit excellent photocatalytic activity and stability for the generation of H2 with a rate constant of 21 µmol h(-1) mg(-1) and a turnover number (TON) of 137,500 or 30,250 for CdTe QDs or nickel, respectively, under visible light irradiation for 42 h.

Mechanistic insights into the interface-directed transformation of thiols into disulfides and molecular hydrogen by visible-light irradiation of quantum dots.

Quantum dots (QDs) offer new and versatile ways to harvest light energy. However, there are few examples involving the utilization of QDs in organic synthesis. Visible-light irradiation of CdSe QDs was found to result in virtually quantitative coupling of a variety of thiols to give disulfides and H2 without the need for sacrificial reagents or external oxidants. The addition of small amounts of nickel(II) salts dramatically improved the efficiency and conversion through facilitating the formation of hydrogen atoms, thereby leading to faster regeneration of the ground-state QDs. Mechanistic studies reveal that the coupling reaction occurs on the QD surfaces rather than in solution and offer a blueprint for how these QDs may be used in other photocatalytic applications. Because no sacrificial agent or oxidant is necessary and the catalyst is reusable, this method may be useful for the formation of disulfide bonds in proteins as well as in other systems sensitive to the presence of oxidants.

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.

Rational Design of Dot-on-Rod Nano-Heterostructure for Photocatalytic CO2 Reduction: Pivotal Role of Hole Transfer and Utilization.

Inspired by green plants, artificial photosynthesis has become one of the most attractive approaches toward carbon dioxide (CO2 ) valorization. Semiconductor quantum dots (QDs) or dot-in-rod (DIR) nano-heterostructures have gained substantial research interest in multielectron photoredox reactions. However, fast electron-hole recombination or sluggish hole transfer and utilization remains unsatisfactory for their potential applications. Here, the first application of a well-designed ZnSe/CdS dot-on-rods (DORs) nano-heterostructure for efficient and selective CO2 photoreduction with H2 O as an electron donor is presented. In-depth spectroscopic studies reveal that surface-anchored ZnSe QDs not only assist ultrafast (≈2 ps) electron and hole separation, but also promote interfacial hole transfer participating in oxidative half-reactions. Surface photovoltage (SPV) spectroscopy provides a direct image of spatially separated electrons in CdS and holes in ZnSe. Therefore, ZnSe/CdS DORs photocatalyze CO2 to CO with a rate of ≈11.3 µmol g-1 h-1 and ≥85% selectivity, much higher than that of ZnSe/CdS DIRs or pristine CdS nanorods under identical conditions. Obviously, favored energy-level alignment and unique morphology balance the utilization of electrons and holes in this nano-heterostructure, thus enhancing the performance of artificial photosynthetic solar-to-chemical conversion.

Organogelators based on TTF supramolecular assemblies: synthesis, characterization, and conductive property.

A closely related family of organogelators 1-2 appended one or two electroactive tetrathiafulvalene (TTF) residues, has been designed and readily synthesized by Sonogashira reactions. These compounds can gelate a variety of organic solvents in view of multiple intermolecular interactions, and compounds 2 with two TTF subunits exhibit higher gelation ability than their corresponding 1. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigation of the xerogels from toluene gave a visual image showing that fibrillar aggregates are entangled in three-dimensional network structures. The columnar TTF cores stacking in the fiber, evidenced by the mixed-valence states absorption at around 2000 nm in ultraviolet-visible-near-infrared (UV-vis-NIR) spectra, provide an efficient pathway for the electron conduction. Upon oxidized by iodine, these xerogels exhibit semiconductive behaviors with moderate levels of conductivity. Additionally, the electrical conductivity of doped-xerogels 2 is 1 order of magnitude higher than that of 1 under identical conditions.