Showing papers on "Polyoxometalate published in 2022"

Confining and Highly Dispersing Single Polyoxometalate Clusters in Covalent Organic Frameworks by Covalent Linkages for CO2 Photoreduction.

Abstract: Single clusters have attracted extensive research interest in the field of catalysis. However, achieving a highly uniform dispersion of a single-cluster catalyst is challenging. In this work, for the first time, we present a versatile strategy for uniformly dispersed polyoxometalates (POMs) in covalent organic frameworks (COFs) through confining POM cluster into the regular nanopores of COF by a covalent linkage. These COF-POM composites combine the properties of light absorption, electron transfer, and suitable catalytic active sites; as a result, they exhibit outstanding catalytic activity in artificial photosynthesis: that is, CO2 photoreduction with H2O as the electron donor. Among them, TCOF-MnMo6 achieved the highest CO yield (37.25 μmol g-1 h-1 with ca. 100% selectivity) in a gas-solid reaction system. Furthermore, a mechanism study based on density functional theory (DFT) calculations demonstrated that the photoinduced electron transfer (PET) process occurs from the COF to the POM, and then CO2 reduction and H2O oxidation occur on the POM and COF, respectively. This work developed a method for a uniform dispersion of POM single clusters into a COF, which also shows the potential of using COF-POM functional materials in the field of photocatalysis.

Single-dispersed polyoxometalate clusters embedded on multilayer graphene as a bifunctional electrocatalyst for efficient Li-S batteries

Abstract: Abstract The redox reactions occurring in the Li-S battery positive electrode conceal various and critical electrocatalytic processes, which strongly influence the performances of this electrochemical energy storage system. Here, we report the development of a single-dispersed molecular cluster catalyst composite comprising of a polyoxometalate framework ([Co 4 (PW 9 O 34 ) 2 ] 10− ) and multilayer reduced graphene oxide. Due to the interfacial charge transfer and exposure of unsaturated cobalt sites, the composite demonstrates efficient polysulfides adsorption and reduced activation energy for polysulfides conversion, thus serving as a bifunctional electrocatalyst. When tested in full Li-S coin cell configuration, the composite allows for a long-term Li-S battery cycling with a capacity fading of 0.015% per cycle after 1000 cycles at 2 C (i.e., 3.36 A g −1 ). An areal capacity of 4.55 mAh cm −2 is also achieved with a sulfur loading of 5.6 mg cm − 2 and E/S ratio of 4.5 μL mg −1 . Moreover, Li-S single-electrode pouch cells tested with the bifunctional electrocatalyst demonstrate a specific capacity of about 800 mAh g −1 at a sulfur loading of 3.6 mg cm −2 for 100 cycles at 0.2 C (i.e., 336 mA g −1 ) with E/S ratio of 5 μL mg −1 .

Single-dispersed polyoxometalate clusters embedded on multilayer graphene as a bifunctional electrocatalyst for efficient Li-S batteries

Abstract: Abstract The redox reactions occurring in the Li-S battery positive electrode conceal various and critical electrocatalytic processes, which strongly influence the performances of this electrochemical energy storage system. Here, we report the development of a single-dispersed molecular cluster catalyst composite comprising of a polyoxometalate framework ([Co 4 (PW 9 O 34 ) 2 ] 10− ) and multilayer reduced graphene oxide. Due to the interfacial charge transfer and exposure of unsaturated cobalt sites, the composite demonstrates efficient polysulfides adsorption and reduced activation energy for polysulfides conversion, thus serving as a bifunctional electrocatalyst. When tested in full Li-S coin cell configuration, the composite allows for a long-term Li-S battery cycling with a capacity fading of 0.015% per cycle after 1000 cycles at 2 C (i.e., 3.36 A g −1 ). An areal capacity of 4.55 mAh cm −2 is also achieved with a sulfur loading of 5.6 mg cm − 2 and E/S ratio of 4.5 μL mg −1 . Moreover, Li-S single-electrode pouch cells tested with the bifunctional electrocatalyst demonstrate a specific capacity of about 800 mAh g −1 at a sulfur loading of 3.6 mg cm −2 for 100 cycles at 0.2 C (i.e., 336 mA g −1 ) with E/S ratio of 5 μL mg −1 .

Supramolecular assemblies of organo-functionalised hybrid polyoxometalates: from functional building blocks to hierarchical nanomaterials

Abstract: This review provides a comprehensive overview of recent advances in the supramolecular organisation and hierarchical self-assembly of organo-functionalised hybrid polyoxometalates (hereafter referred to as hybrid POMs), and their emerging role as multi-functional building blocks in the construction of new nanomaterials. Polyoxometalates have long been studied as a fascinating outgrowth of traditional metal-oxide chemistry, where the unusual position they occupy between individual metal oxoanions and solid-state bulk oxides imbues them with a range of attractive properties (e.g. solubility, high structural modularity and tuneable properties/reactivity). Specifically, the capacity for POMs to be covalently coupled to an effectively limitless range of organic moieties has opened exciting new avenues in their rational design, while the combination of distinct organic and inorganic components facilitates the formation of complex molecular architectures and the emergence of new, unique functionalities. Here, we present a detailed discussion of the design opportunities afforded by hybrid POMs, where fine control over their size, topology and their covalent and non-covalent interactions with a range of other species and/or substrates makes them ideal building blocks in the assembly of a broad range of supramolecular hybrid nanomaterials. We review both direct self-assembly approaches (encompassing both solution and solid-state approaches) and the non-covalent interactions of hybrid POMs with a range of suitable substrates (including cavitands, carbon nanotubes and biological systems), while giving key consideration to the underlying driving forces in each case. Ultimately, this review aims to demonstrate the enormous potential that the rational assembly of hybrid POM clusters shows for the development of next-generation nanomaterials with applications in areas as diverse as catalysis, energy-storage and molecular biology, while providing our perspective on where the next major developments in the field may emerge.

POM-incorporated ZnIn2S4 Z-scheme Dual-functional Photocatalysts for Cooperative Benzyl Alcohol Oxidation and H2 Evolution in Aqueous Solution

Abstract: Efficient aromatic alcohol oxidation with simultaneous H2 evolution under aqueous conditions is achieved in a polyoxometalate (POM)-incorporated ZnIn2S4 dual-functional photocatalytic system. The synergy between HPM and ZIS contributes to the formation of a Z-type heterojunction structure and thus enhances redox capacity. Moreover, the sub-nanometer size of POM clusters endows molecular-level interfacial contact, which acts as an "electron bridge" ensuring faster interfacial charge transfer kinetics. Therefore, an impressive nearly 100% yield of benzaldehyde and 10.6 mmol·g−1·h−1 H2-evolution rate are observed for POM/ZIS nanocomposites even under an anaerobic atmosphere with water as the solvent. This is the first application of POM-based materials in the anaerobic oxidation of benzyl alcohol with concomitant H2 production. This study expands the possibilities for designing multifunctional POM-based photocatalysts for economic and ecological photoredox applications.

Locking volatile organic molecules by subnanometer inorganic nanowire-based organogels

Abstract: The intermolecular forces among volatile organic molecules are usually weaker than water, making them more difficult to absorb. We prepared alkaline earth cations–bridged polyoxometalate nanoclusters subnanometer nanowires through a facile room-temperature reaction. The nanowires can form three-dimensional networks, trapping more than 10 kinds of volatile organic liquids effectively with the mass fraction of nanowires as low as 0.53%. A series of freestanding, elastic, and stable organogels were obtained. We prepared gels that encapsulate organic liquids at the kilogram scale. Through removing solvents in gels by means of distillation and centrifugation, the nanowires can be recycled more than 10 times. This method could be applied to the effective trapping and recovery of organic liquids. Description Trapping and recovering organics Hydrogels consist of cross-linked organic polymers that can swell to hold up to 90% water, making them useful as absorbents and for tissue engineering. Zhang et al. synthesized inorganic nanowires from polyoxometalates, calcium ions, and oleylamine and found that these nanowires readily formed three-dimensional networks. The networks swell when exposed to a range of volatile organic compounds added at fractions even below 1% to form organogels. The gels are stable to physical squeezing without a substantial loss of liquid. However, the liquids can be recovered using distillation and centrifugation, and the nanowires can be reused, making possible the trapping and recovery of organic solvents. —MSL Volatile organic liquids are trapped by a subnanometer nanowire network to form freestanding and elastic organogels.

Visible-LED-light-driven photocatalytic synthesis of N-heterocycles mediated by a polyoxometalate-containing mesoporous zirconium metal-organic framework

Abstract: A mesoporous metal-organic framework with photothermal properties, namely PCN-222, was solvothermally synthesized from meso-tetra(4-carboxyphenyl)porphyrin and zirconium chloride employing both benzoic acid (BA) and trifluoroacetic acid (TFA) as modifiers. The MOF material subsequently served as a porous support for a polyoxometalate (POM), H3PW12O40, via a facile impregnation method which rendered a novel porous POM@PCN-222 composite. The solid was characterized by FT-IR, PXRD, SEM/EDX, TGA/DSC, ICP-OES, UV–Vis DRS, cyclic voltammetry (CV), and BET surface area. The one-pot synthesis of N-heterocycles (pyridine derivatives) was investigated utilizing the hybrid material via one-pot pseudo four-component reaction between aromatic aldehydes, methyl acetoacetate and ammonium acetate promoted under visible LED light irradiation in the presence of molecular oxygen as green oxidant. Products were selectively formed in good yields in the presence of the recyclable heterogeneous solid. Remarkably, POM@PCN-222 showed a superior performance for this procedure as compared to both unfunctionalized MOF and POM. The photosensitizer and photothermal properties of the porphyrin linkers combined with Lewis acidic sites derived from PW12 and Zr6-nodes were responsible for the observed excelling performance. To understand the mechanism, control investigations, electron paramagnetic resonance (EPR) analysis and FT-IR reaction monitoring were performed. The work discloses, for the first time, a simple and environmentally friendly approach for the direct production of pyridines via one-pot thermo-photocatalytic approach using a novel POM-modified MOF in the absence of any chemical additive.

An excellent multifunctional photocatalyst cooperated by polyoxometalate-viologen framework for CEES oxidation, Cr(VI) reduction and dyes decolorization under different light regimes

Abstract: Photocatalytic detoxification of highly toxic agents is a promising approach to protect ecological environment and human health, and the key problem lies in the development of novel efficient photocatalysts. Herein,...

Visible-LED-light-driven photocatalytic synthesis of N-heterocycles mediated by a polyoxometalate-containing mesoporous zirconium metal-organic framework

Abstract: A mesoporous metal-organic framework with photothermal properties, namely PCN-222, was solvothermally synthesized from meso-tetra(4-carboxyphenyl)porphyrin and zirconium chloride employing both benzoic acid (BA) and trifluoroacetic acid (TFA) as modifiers. The MOF material subsequently served as a porous support for a polyoxometalate (POM), H3PW12O40, via a facile impregnation method which rendered a novel porous [email protected] composite. The solid was characterized by FT-IR, PXRD, SEM/EDX, TGA/DSC, ICP-OES, UV–Vis DRS, cyclic voltammetry (CV), and BET surface area. The one-pot synthesis of N-heterocycles (pyridine derivatives) was investigated utilizing the hybrid material via one-pot pseudo four-component reaction between aromatic aldehydes, methyl acetoacetate and ammonium acetate promoted under visible LED light irradiation in the presence of molecular oxygen as green oxidant. Products were selectively formed in good yields in the presence of the recyclable heterogeneous solid. Remarkably, [email protected] showed a superior performance for this procedure as compared to both unfunctionalized MOF and POM. The photosensitizer and photothermal properties of the porphyrin linkers combined with Lewis acidic sites derived from PW12 and Zr6-nodes were responsible for the observed excelling performance. To understand the mechanism, control investigations, electron paramagnetic resonance (EPR) analysis and FT-IR reaction monitoring were performed. The work discloses, for the first time, a simple and environmentally friendly approach for the direct production of pyridines via one-pot thermo-photocatalytic approach using a novel POM-modified MOF in the absence of any chemical additive.

Coupling Ni-substituted polyoxometalate catalysts with water-soluble CdSe quantum dots for ultraefficient photogeneration of hydrogen under visible light

Abstract: The development of robust and efficient hydrogen-evolving system remains a substantial but promising challenge to convert solar energy into clean fuel. Herein, we report the construction of water-compatible, robust, and ultraefficient hydrogen-evolving system by coupling water-soluble CdSe light-absorbers with Ni-substituted polyoxometalate (Ni-POM) catalysts and AA electron donor. Such facile catalytic system exhibits superior and robust hydrogen production activity to date even among known semiconductor/POM hybrids-based hydrogen production systems. Multiple stability experiments confirm the molecular stability of Ni-POM catalysts under turnover conditions. Various experimental and spectroscopic analyses reveal that the synergistic cooperation between high photostability of CdSe light-absorber, outstanding reversible multi-electron-transferring property of Ni-POM catalyst, and the fast hole-removing ability of AA electron donor account for the exceptional performance of present catalytic system. Our present work provides new research insights into the continued development of effective hydrogen-evolving systems through coupling other QDs-based light-absorbers and earth-abundant transition-metal-substituted POM catalysts.

Coupling Ni-substituted polyoxometalate catalysts with water-soluble CdSe quantum dots for ultraefficient photogeneration of hydrogen under visible light

Abstract: The development of robust and efficient hydrogen-evolving system remains a substantial but promising challenge to convert solar energy into clean fuel. Herein, we report the construction of water-compatible, robust, and ultraefficient hydrogen-evolving system by coupling water-soluble CdSe light-absorbers with Ni-substituted polyoxometalate (Ni-POM) catalysts and AA electron donor. Such facile catalytic system exhibits superior and robust hydrogen production activity to date even among known semiconductor/POM hybrids-based hydrogen production systems. Multiple stability experiments confirm the molecular stability of Ni-POM catalysts under turnover conditions. Various experimental and spectroscopic analyses reveal that the synergistic cooperation between high photostability of CdSe light-absorber, outstanding reversible multi-electron-transferring property of Ni-POM catalyst, and the fast hole-removing ability of AA electron donor account for the exceptional performance of present catalytic system. Our present work provides new research insights into the continued development of effective hydrogen-evolving systems through coupling other QDs-based light-absorbers and earth-abundant transition-metal-substituted POM catalysts.

Anderson-Type Polyoxometalate-Assisted Synthesis of Defect-Rich Doped 1T/2H-MoSe2 Nanosheets for Efficient Seawater Splitting and Mg/Seawater Batteries.

Abstract: Designing high-performance hydrogen evolution reaction (HER) catalysts is crucial for seawater splitting. Herein, we demonstrate a facile Anderson-type polyoxometalate-assisted synthesis route to prepare defect-rich doped 1T/2H-MoSe2 nanosheets. As demonstrated, the optimized defect-rich doped 1T/2H-MoSe2 nanosheets display low overpotentials of 116 and 274 mV to gain 10 mA cm-2 in acidic and simulated seawater for the HER, respectively. A magnesium (Mg)/seawater battery was fabricated with the defect-rich doped 1T/2H-MoSe2 nanosheet cathode, displaying the highest power density of up to 7.69 mW cm-2 and stable galvanostatic discharging over 24 h. The theoretical and experimental investigations show that the superior HER and battery performances of the heteroatom-doped MoSe2 nanosheets are attributed to both the improved intrinsic catalytic activity (effective activation of water and favorable subsequent hydrogen desorption) and the abundant active sites, benefiting from the favorable catalytic factors of the doped heteroatom, 1T phase, and defects. Our work presents an intriguing structural modulation strategy to design high-performance catalysts toward both HER and Mg/seawater batteries.

Polyoxometalate-based nanostructures for electrocatalytic and photocatalytic CO 2 reduction

Abstract: Electro/photocatalytic carbon dioxide (CO₂) reduction to value-added chemicals and fuels is being actively studied as a promising pathway for renewable energy storage and climate change mitigation. Because of inert molecular properties and competing hydrogen generation reactions, high-performance electrocatalysts with high Faradaic efficiency and product selectivity but low overpotential are urgently needed. Polyoxometalates (POMs) are a class of polynuclear metal oxide clusters with a precise atomic structure, providing an ideal research platform to reveal the relationship between macroscopic properties and microstructures. Moreover, their highly tunable redox properties and abundant transition metal atom composition ensure thriving research for POM-based nanostructures toward CO₂ reduction. In this review, we first introduce the specific roles of POMs in electro/photocatalytic CO₂ reduction. Recent advances in POM-based nanostructures ranging from single clusters, assemblies, organic–inorganic hybrids to derivatives are systematically summarized. In particular, the structure–performance relationship of POM-based nanostructures is discussed at the atomic and molecular levels. Finally, the challenges and opportunities in the design of high-efficiency POM-based nanostructures are discussed to promote electro/photocatalytic CO₂ reduction.

Latest progress of covalently modified polyoxometalates-based molecular assemblies and advanced materials

Abstract: Covalently modified polyoxometalates (POMs), which benefit from the synergistic effect between POMs and covalently grafted moieties, have received increasing attention in various fields. Recent studies on covalently modified POMs mainly focus on function-directed POM assemblies. This review summarizes the latest progress (2017–2022) concerning covalently modified POMs from a functional perspective, which can be classified as assembly chemistry, photochemistry, electrochemistry, homogeneous and heterogeneous catalysis, and biological applications. The roles of POMs and covalently grafted moieties in these hybrids, especially the rational design for specific applications, were considered and emphasized.

A novel core-shell structured hybrid composed of zinc homobenzotrizoate and silver borotungstate with supercapacitor and photocatalytic dye degradation performance

Abstract: Polyoxometalate-based metal-organic frameworks (POMOFs) are challenging as electrode materials for supercapacitors and catalysts for the photocatalytic degradation of organic pollutants. To this end, a novel core-shell structured zinc homobenzotrizoate and silver borotungstate ([Zn4(BTC)2(μ4-O)(H2O)2]@Ag5[BW12O40], abbreviated as [email protected]5[BW12O40], BTC = 1,3,5-benzylcarboxylic acid) was synthesized by wrapping polyoxometalate (POM) on metal-organic framework (MOF) by a simple grinding method. This unique synergy between the core (Zn-BTC) and shell (Ag5[BW12O40]) sets the compound more surface active sites. The composition, structure, and morphology of the title compound were discussed by infrared (IR), powder X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and other methods. In the three-electrode foam nickel system, the [email protected]5[BW12O40] electrode showed a higher specific capacitance (161.7 F g−1) than the Ag5[BW12O40] (97.7 F g−1) and Zn-BTC (118.4 F g−1), and the capacitance retention rate was 92.8% after 5000 cycles. Assembled into a symmetrical supercapacitor with an energy density of 9.3 Wh Kg−1 and a power density of 501.2 W Kg−1. The capacitance retention rate reached 91.4% after 5000 cycles. In addition, The photocatalytic degradation efficiency of [email protected]5[BW12O40] for Methylene Blue (MB), Methyl Orange (MO), and Rhodamine B (RhB) dyes are all greater than 90% in 140 min. The photocatalytic cycle test has been carried out 5 times, and the degradation effect remains the same. This indicates that the title compound has high activity in the direction of photocatalytic degradation of dyes.

Polyoxometalate Modified by Zeolite Imidazole Framework for the pH-Responsive Electrodynamic/Chemodynamic Therapy.

Abstract: Electrodynamic therapy (EDT) and chemodynamic therapy (CDT) have the potential for future tumor treatment; however, their underlying applications are greatly hindered owing to their inherent drawbacks. The combination of EDT and CDT has been considered to be an effective way to maximize the superiorities of these two ROS-based methodologies. However, the development of novel nanomaterials with "one-for-all" functions still remains a big challenge. In this work, the polyoxometalate nanoparticles (NPs) were decorated using the zeolite imidazole framework (POM@ZIF-8) in order to integrate the EDT with CDT. The resulting POM@ZIF-8 NPs can effectively induce the generation of reactive oxygen species (ROS) via a catalytic reaction on the surface of POM NPs induced by an electric field (E). At the same time, POM@ZIF-8 NPs can catalyze the intracellular H2O2 into ROS via a Fenton-like reaction, thereby achieving the combination of EDT and CDT. Besides, since ZIF-8 is acid-responsive, it can protect normal tissues and avoid side effects. Of great note is that the cytotoxicity and the apoptosis rate of the POM@ZIF-8+E group (80%) were found to be significantly higher than that of the E group (55%). As a result, a high tumor inhibition phenomenon can be observed both in vitro and in vivo. The present study thus provides an alternative concept for combinational therapeutic modality with exceptional efficacy.

Efficient Photogeneration of Hydrogen Boosted by Long-Lived Dye-Modified Ir(III) Photosensitizers and Polyoxometalate Catalyst

Abstract: Open AccessCCS ChemistryRESEARCH ARTICLE1 Jan 2022Efficient Photogeneration of Hydrogen Boosted by Long-Lived Dye-Modified Ir(III) Photosensitizers and Polyoxometalate Catalyst Lin Qin, Chongyang Zhao, Liao-Yuan Yao, Hongbin Dou, Mo Zhang, Jing Xie, Tsu-Chien Weng, Hongjin Lv and Guo-Yu Yang Lin Qin MOE Key Laboratory of Cluster Science, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 102488 Google Scholar More articles by this author , Chongyang Zhao MOE Key Laboratory of Cluster Science, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 102488 Google Scholar More articles by this author , Liao-Yuan Yao Department of Chemistry, The University of Hong Kong, Hong Kong 999077 Google Scholar More articles by this author , Hongbin Dou School of Physical Science and Technology, ShanghaiTec University, Shanghai 201210 Google Scholar More articles by this author , Mo Zhang MOE Key Laboratory of Cluster Science, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 102488 Google Scholar More articles by this author , Jing Xie MOE Key Laboratory of Cluster Science, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 102488 Google Scholar More articles by this author , Tsu-Chien Weng School of Physical Science and Technology, ShanghaiTec University, Shanghai 201210 Google Scholar More articles by this author , Hongjin Lv *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] MOE Key Laboratory of Cluster Science, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 102488 Google Scholar More articles by this author and Guo-Yu Yang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] MOE Key Laboratory of Cluster Science, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 102488 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202000741 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Developing efficient catalysts and photosensitizers is crucial for the construction of effective photocatalytic H2-evolving systems. Here, we report the facile preparation of Coumarin-modified Ir(III) complexes ( PS-2 and PS-3) and their utilization as chromophores to drive favorable photocatalytic H2 evolution using Ni-substituted polyoxometalate ( Ni3PW10) catalyst and triethanolamine (TEOA) as an electron donor. Compared with the commercially available unmodified Ir(III) complex ( PS-1), both PS-2 and PS-3 displayed intensive absorption in the range of 400–550 nm with εmax of 110,620 and 91,430 M−1 cm−1, respectively. Varying the substitutes on the bipyridine ligand affected their physicochemical properties and the corresponding photocatalytic activity dramatically. Under photocatalytic conditions, the quantity of H2 molecules generated by PS-2- and PS-3-containing systems were 13.1 and 2.1 times, respectively, that of the PS-1-containing system. When PS-2 was used as a photosensitizer, the highest turnover number (TON) of 19,739 was obtained versus Ni3PW10 catalyst. Various spectroscopic and computational studies have revealed that factors such as strong and broad visible-light-absorbing ability, long-lived triplet state, suitable redox potential, opposed by using polyoxometalate (POM) catalyst, and large highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap of PS-2 attributed to drastically enhanced photocatalytic activity. Download figure Download PowerPoint Introduction Since the Industrial Revolution in the 19th century, the continuous depletion of traditional fossil fuels has caused serious energy shortages and environmental challenges. To solve these problems, great efforts have been devoted to developing cost-effective, clean, and sustainable energy alternatives. As one of the widely studied renewable energy exploration strategies, solar-driven water splitting into H2 and O2 is a promising pathway to convert and store solar energy into renewable chemical bonds.1–4 In the past few decades, considerable attention has been paid to establishing highly efficient photocatalytic water splitting systems.1,3–8 For the catalytic hydrogen evolution half-reaction, the most widely adopted system consists of three components containing photosensitizer, a catalyst, and a sacrificial reagent.4,9–11 Notwithstanding, key issues have been identified regarding the exploration of viable, efficient, and inexpensive water reduction catalysts (WRCs), as well as the design of tunable, stable, and strong visible-light-responsive photosensitizers.12–16 As an emerging type of multielectron-transfer WRCs, transition-metal-substituted polyoxometalates (TMSPOMs) with tunable structural compositions, physicochemical, and photochemical properties, could combine the advantages of both heterogeneous metal oxide catalysts (e.g., stability) and homogeneous molecular catalysts (e.g., high activity, selectivity, and tunability).2,17–21 After years of numerous efforts, a series of Mn, Ni, Co, and Cu-substituted POMs have been synthesized and studied for catalytic H2 evolution when coupled with visible-light-absorbing photosensitizers and sacrificial reagents.19,21–25 Although TMSPOMs have shown great potential applications in various H2-evolving systems, the challenges facing these systems remain substantial. To date, the most widely used photosensitizers in POM-catalyzed H2-evolving systems are limited to noble metal-based organometallic complexes such as [Ru(bpy)3]2+, [Ir(ppy)2(dtbbpy)]+ and their derivatives,21–24,26–29 as well as pure organic chromophores, including eosin Y, fluorescein, rhodamine, and others.30–32 Such noble metal-based organometallic complexes usually exhibit a long-lived triplet state (3MLCT) to promote photocatalytic hydrogen evolution.4,12,33–38 However, they often suffer from low molar absorption coefficients and narrow visible-light-absorbing range.4,12,39 Meanwhile, pure organic chromophores generally show very high molar absorption coefficients. Nonetheless, the short-lived excited states and poor photostability hinder their practical application in photocatalysis considerably. To improve the solar-to-energy conversion efficiency, some pioneering research studies have reported the construction of strong visible-light-absorbing chromophores via covalently decorating organometallic photosensitizers with organic dyes.38,40–47 For example, sensitization of colloidal Pt-TiO2 with Bodipy or Rhodamine-modified platinum diimine dithiolate (PtN2S2) charge-transfer chromophore could enhance the photocatalytic hydrogen evolution efficiency remarkably with substantial longevity and high turnover numbers (TONs).44,46 Also, the modification of [Ir(ppy)2(bpy)]+ with two different organic dyes greatly increased the visible-light-absorbing ability and hydrogen evolution activity in the presence of [CoIII(dmgH)2(py)Cl] catalyst.47 Although many of these works reported extremely high TONs with respect to the photosensitizers, the TONs versus H2-evolving catalysts are usually unsatisfactory. Moreover, among various dye-modified chromophores, none was applied as light-absorbers to drive catalytic reactions in POM-catalyzed H2-evolving systems. We considered that employing the strong visible-light-absorbing ability of Coumarin 6 chromophore (ε < 105 M−1 cm−1) and the good coordinating ability as the C^N ligand to iridium metal center would be a promising strategy to enhance the photocatalytic performance of POM-catalyzed H2-evolving system, as this approach would combine the advantages of high visible-light-absorbing ability of Coumarin 6 and long-lasting triplet states of potential Ir(III) complexes. Herein, in this context, we prepared different Coumarin-modified Ir(III) complexes (denoted as PS-2 and PS-3) and investigated them as chromophores to build a highly efficient photocatalytic H2-evolving system using a combined Ni3PW10 as catalyst and triethanolamine (TEOA) as the sacrificial electron donor. Various spectroscopic and computational studies revealed that the factors of strong and broad visible-light-absorbing ability, long-lived triplet state, suitable redox potential versus POM catalyst, and large highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap of PS-2 accounted for considerably enhanced photocatalytic activity. Experimental Methods Materials 2-Phenylpyridine, 4,4′-di-tert-butyl-2,2′-bipyridine, 4,4′-dibromo-2,2′-bipyridine, 3-(2-benzothiazolyl)-7-(diethylamino)coumarin (Coumarin 6), 2-tributylstannylthiophene ligands, and Iridium(III) chloride trihydrate were used as received without further purification. Catalyst K6Na[Ni3(H2O)3PW10O39H2O]•12H2O (K6Na- Ni3PW10)48 was prepared according to the modified literature methods and confirmed by single-crystal X-ray diffraction, as well as Fourier transform infrared (FT-IR) spectroscopy. TBA- Ni3PW10 was obtained by the exchange of counter cations with tetrabutylammonium (TBA+) salt from K6Na- Ni3PW10. The spy ligand was prepared using reported methods.49 Iridium complexes were prepared according to a modified method described by Takizawa et al.50 All solvents and other reagents were used as received. Synthesis of 4,4′-di(thien-2-yl)-2,2′-bipyridine (spy) 4,4′-Dibromo-2,2′-bipyridine (0.5 g, 1.6 mmol, 1.0 equiv), 2-tributylstannylthiophene (1.5 g, 4.0 mmol, 2.5 equiv), [Pd (PPh)3Cl2] (67 mg, 95.5 μmol, 0.06 equiv), and CsF (1.0 g, 6.4 μmol, 4.0 equiv) were suspended in a 12 mL degassed solution mixture of toluene/tetrahydrofuran (THF) (2/1). After several degassing and argon bubbling times, the mixture was stirred and heated at 110 °C for 6 h. The resulting mixture was poured into MeOH that afforded a dark grey precipitate, which was filtered off, washed with MeOH, a small quantity of dichloromethane (DCM), and diethyl ether to produce a pure light grey solid product. Yield: 90%; proton nuclear magnetic resonance (1H NMR; 400 MHz, CDCl3, 298 K, δ): 8.70–8.67 (m, 2H), 7.67 (dd, 1H, J = 3.64, 0.84 Hz), 7.53 (dd, 1H, J = 5.04, 1.96 Hz), 7.44 (dd, 1H, J = 5.04, 0.88 Hz), 7.16 (dd, 1H, J = 5.00, 3.72 Hz). High-resolution mass spectrometry (HRMS) (electrospray ionization mass spectrometry [ESI-MS]) (m/z): [M]+ calcd for [C18H13N2S2], 321.0520; found, 321.0500. Synthetic procedure for iridium complexes Synthesis of PS-1 Iridium(III) chloride (450 mg, 1.28 mmol, 1.0 equiv) and 2-phenylpyridine (440 mg, 2.82 mmol, 2.2 equiv) were suspended in 30 mL 2-methoxyethanol/water (3:1 v/v) solution, and the mixture was heated at 110 °C for 48 h under argon conditions. After completion, the solution was cooled to room temperature, then the precipitate was filtered off, washed with water and EtOH to afford a solid orange Ir(III) μ-chloro-bridged dimer product. The resulting Ir(III) μ-chloro-bridged dimer complex (200 mg, 0.19 mmol, 1.0 equiv) and 4,4′-di-tert-butyl-2,2′-bipyridine (110 mg, 0.41 mmol, 2.2 equiv) were suspended in 10 mL of 2-methoxyethanol solution, and the mixture was heated at 110 °C for 12 h under argon conditions. After cooling to room temperature, an excess NH4PF6 aqueous solution was added to the above solution and stirred for 4 h at room temperature. The resulting precipitate was filtered off, washed with water and EtOH, and the crude product was purified by silica gel column chromatography using dichloromethane/MeOH (v/v = 1/0 to 200/1) as the eluent, affording the pure product PS-1 as an orange solid. Yield: 60%. 1H NMR (400 MHz, CDCl3, 298 K, δ): 8.37 (d, 2H, J = 1.56 Hz), 7.88 (d, 2H, J = 8.08 Hz), 7.82 (d, 2H, J = 5.84 Hz), 7.74 (m, 2H), 7.66 (d, 2H, J = 7.68 Hz), 7.60 (d, 2H, J = 5.72 Hz), 7.38 (dd, 2H, J = 5.84, 1.80 Hz), 7.08 (m, 2H), 7.00 (m, 2H), 6.89 (m, 2H), 6.29 (d, 2H, J = 7.52 Hz), 1.42 (s, 18 H). HRMS (ESI-MS) (m/z): [M-PF6]+ calcd for [C40H40IrN4]+, 769.2879; found, 769.2877. Synthesis of PS-2 Iridium(III) chloride (450 mg, 1.28 mmol, 1.0 equiv) and Coumarin 6 (990 mg, 2.82 mmol, 2.2 equiv) were suspended in 30 mL 2-methoxyethanol/water (3:1 v/v) solution; the mixture was heated at 110 °C for 48 h under argon conditions. After completion, the solution was cooled to room temperature, then the precipitate was filtered off, washed with water and EtOH, affording Ir(III) μ-chloro-bridged dimer product as orange solid. Subsequently, the resulting Ir(III) μ-chloro-bridged dimer complex (200 mg, 0.11 mmol, 1.0 equiv) and 4,4′-di-tert-butyl-2,2′-bipyridine (65 mg, 0.24 mmol, 2.2 equiv) were suspended in 10 mL of 2-methoxyethanol solution, and the mixture was heated at 110 °C for 12 h under argon conditions. After cooling to room temperature, an excess NH4PF6 aqueous solution was added to the above solution and stirred for 4 h at room temperature. The resulting precipitate was filtered off, washed with water and EtOH, the crude product was purified by silica gel column chromatography using dichloromethane/MeOH (v/v = 1/0 to 200/1) as the eluent, affording the pure product PS-2 as an orange solid. Yield: 56%. 1H NMR (400 MHz, CDCl3, 298 K, δ): 8.41 (d, 2H, J = 5.96 Hz), 8.24 (d, 2H, J = 1.52 Hz), 7.82 (d, 2H, J = 7.84 Hz), 7.63 (dd, 2H, J = 6.00, 1.68 Hz), 7.29 (m, 2H), 6.91 (m, 2H), 6.38 (d, 2H, J = 2,60 Hz), 6.03 (d, 2H, J = 9.40 Hz), 5.86 (dd, 2H, J = 9.44, 2,60 Hz), 5.72 (d, 2H, J = 8.52 Hz), 3.29 (m, 8H), 1.41 (s, 18H), 1.09 (t, 12H, J = 7.00 Hz). HRMS (ESI-MS) (m/z): [M-PF6]+ calcd for [C58H58IrN6O4S2]+, 1159.3590; found, 1159.3598. Synthesis of PS-3 To synthesize PS-3, the 4,4′-di(thien-2-yl)-2,2′-bipyridine (spy) (77 mg, 0.24 mmol, 2.2 equiv) ligand was used to react with the above Ir(III) μ-chloro-bridged dimer complex (200 mg, 0.11 mmol, 1.0 equiv) in 10 mL of 2-methoxyethanol solution, the mixture was heated at 110 °C for 12 h under argon condition. After cooling to room temperature, an excessive NH4PF6 aqueous solution was added to the above solution and stirred for 4 h at room temperature. The resulting precipitate was filtered off, washed with water and EtOH, and the crude product was purified by silica gel column chromatography using dichloromethane/Hexane (v/v = 4/1 to 1/0) as the eluent, affording the pure product PS-3 as an orange solid. Yield: 40%. 1H NMR (400 MHz, d6-DMSO, 298 K, δ): 8.93 (d, 2H, J = 1.76 Hz), 8.68 (d, 2H, J = 6.08 Hz), 8.20 (dd, 2H, J = 4.04, 0.80 Hz), 8.11 (d, 2H, J = 8.00 Hz), 8.03 (dd, 2H, J = 6.00, 1.80 Hz), 7.97 (dd, 2H, J = 4.96, 0.84 Hz), 7.34 (dd, 2H, J = 4.84, 3.84 Hz), 7.25 (t, 2H, J = 7.40 Hz), 7.05 (m, 2H), 6.49 (s, 2H), 6.12 (d, 2H, J = 8.44 Hz), 6.03 (s, 4H,), 3.30 (overlap by solvent peak, m, 8H), 0.97 (t, 12H, J = 6.88 Hz). HRMS (ESI-MS) (m/z): [M-PF6]+ calcd for [C58H46IrN6O4S4]+, 1211.2093; found, 1211.2097. Instrumentation 1H NMR spectra were recorded on a Bruker Ascend 400M (Avance IIIHD 400 MHz) Fourier transform NMR spectrometer (Bruker, Germany) with chemical shifts (δ, ppm) relative to tetramethylsilane (Me4Si). High-resolution ESI-MS was performed using AGILENT Q-TOF 6520 mass spectrometer (Agilent, United States). Infrared spectra were acquired on a Bruker TENSOR II FT-IR spectrometer by preparing dry solid samples mixed with KBr pellets. The X-ray diffraction data were obtained using a Bruker APEXII DUO CMOS detector with monochromated Mo-Kα radiation. Cell refinement and data reduction were processed with the Protenum2 program package.51 Using olex2, the molecular structure was solved with the ShelXS structure solution program using direct methods and refined using the XL refinement package with least-squares minimization.52 Then all non-H atoms of the complexes were refined with anisotropic thermal parameters. Subsequently, the hydrogen atoms were included in idealized positions and refined using fixed geometry relative to their carrier atoms. Crystal structure graphics and their corresponding packing diagrams were acquired using software Mercury 2.4.5 (Cambridge Crystallographic Data Centre, Cambridge). UV–vis absorption spectra following recording on a UV 2600 UV–vis spectrophotometer and all emission spectra were performed on spectrofluorometer FS5 (Edinburgh Instruments Ltd., Edinburgh); errors for λ values (±1 nm) were estimated. The emission lifetime was measured using an EPL-450 picosecond pulsed diode (Edinburgh Instruments Ltd.) laser system (pulse output 450 nm). These long-lived triplet decay kinetic curves were fitted by a single-exponential decay function. Cyclic voltammetry was recorded on a CHI660E Electrochemical Workstation (Shanghai Chenhua Company, Shanghai, China). The experimental data were collected at a scan rate of 100 mV s−1. The working electrode was polished with 0.05 μm alumina, sonicated with deionized water, rinsed with EtOH, and then with deionized water before use. The reference and the counter electrodes were Ag/Ag+ (0.1 M TBAPF6 and 0.01M AgNO3 in acetonitrile) and a platinum wire, respectively. Solutions were degassed for 10 min by N2 before experiments. The E1/2 of the ferrocenium/ferrocene couple (Fc+/Fc) was measured in the same solution as samples and was used as an internal reference. Photocatalytic hydrogen production experiments The photocatalytic hydrogen production was performed in deaerated CH3CN-DMF (v/v = 3/2) solution with TEOA (0.25 M) as a sacrificial electron donor, H2O (2 M) as a proton source, iridium complexes (200 μM) as photosensitizers, and Ni-substituted polyoxometalate ( Ni3PW10; 20 μM) as the catalyst. The reaction solution was degassed with mixed argon/methane gases for 15 min before irritation. Xe lamp (λ < 400 nm, 300 W, Beijing Perfectlight Technology Co. Ltd., Beijing, China) was employed as the irradiation light. All TONs were calculated based on the Ni3PW10 catalyst. Density functional theory calculations Density functional theory (DFT) calculations were performed using B3LYP functional with Grimme’s D3 dispersion correction. The 6-31G(d) basis set was applied on nonmetal atoms, and the Lanl2DZ basis set was used on the Ir atom. The continuum solvation model based on density (SMD) with solvent CH3CN was used to address the solvent effect. The calculations were carried out using the Gaussian09 program (refer to the Supporting Information for details). Results and Discussion Syntheses and characterization The structures of bis-cyclometalated Ir(III) complexes (denoted as PS-1, PS-2, and PS-3) and Ni3PW10 catalyst are illustrated in Figures 1a and 1b. The polyoxoanion Ni3PW10 catalyst was synthesized and confirmed by single-crystal X-ray diffraction and FT-IR spectrum (see Supporting Information). To achieve good solubility in the photocatalytic system, counter cations Na+ and K+ of polyoxoanion Ni3PW10 were exchanged by TBA+ salt; the exchange of counter cations did not affect the geometrical structure of the polyoxoanion Ni3PW10 ( Supporting Information Figure S1). The TBA- Ni3PW10 complex was used for all subsequent experiments unless otherwise stated. The bis-cyclometalated Iridium(III) complexes were prepared using a classical two-step method (Scheme 1). Typically, treatment of IrCl3 with 2-phenylpyridine or Coumarin 6 (2.2 equiv) gave Ir(III) μ-chloro-bridged dimers, which further reacted with a coordinated bipyridine ligand (1.5 equiv) to afford ensembles PS-1, PS-2, and PS-3, respectively (40−60% yields). The 1H NMR spectra of the complexes ( Supporting Information Figures S2–S4) agreed well with their structures and the diamagnetic nature of Ir(III) species. The high-resolution ESI mass spectra of all three complexes are in agreement with the simulated patterns ( Supporting Information Figures S5–S7). Detailed preparation and characterization of these complexes are included in the Supporting Information. Figure 1 | Structures of (a) photosensitizers PS-1, PS-2, and PS-3, and (b) Ni3PW10. (c) Top and (d) side views of single-crystal structures of Ir(III) complexed PS-2. Download figure Download PowerPoint Needle-shaped single crystals of PS-2 considered suitable for X-ray diffraction were obtained by diffusion of diethyl ether into acetonitrile solution of the ensemble. After multiple attempts, crystals of PS-3 were afforded using diverse solvent systems (ether/acetonitrile or dichloromethane/hexane); however, the quality of these crystals was too low for suitable X-ray diffraction. PS-2 crystallized in the monoclinic space group P21 (Figures 1c and 1d). Ir(III) metal ions adopted an octahedral six-coordinated geometry with two C^N (Coumarin 6) and one closely surrounded N^N (bipyridine) ligands. The Ir–N (on Coumarin 6) distances of PS-2 complex are 2.056(8) and 2.063(8) Å were slightly shorter than Ir–N (on bipyridine) distances [2.147(9) and 2.130(7) Å]. The Ir–C distance ranged from 2.045(9) to 2.053(9) Å, with the N1–Ir1–C50, N3–Ir1–C50, and C30–Ir1–C50 bond angles being 169.8(3)°, 101.5(3)°, and 91.8(4)°, respectively. All the Ir center involved bond distances and angles were consistent with other bis-cyclometalated Iridium(III) complexes. Scheme 1 | Synthetic route for Coumarin-modified photosensitizers PS-2 and PS-3. Download figure Download PowerPoint Photophysical properties The photophysical properties of the PS-1, PS-2, and PS-3 were examined in acetonitrile solutions at 298 K. The data of UV–vis absorption maxima (λmax) with molar extinction coefficients, emission maxima (λem), and triplet emission lifetimes (τ) are summarized in Supporting Information Table S1. As shown in Figure 2a, PS-1 displayed absorption maxima at 263 nm with shoulders around 310 and 340 nm, whereas PS-2 and PS-3 exhibited intense absorption in the range of 400–550 nm with a maximum molar absorption coefficient up to 110,620 and 91,430 M−1 cm−1, respectively. These results demonstrated that the visible-light absorption ability of cyclometalated Iridium(III) complexes were remarkably enhanced by modification of the Coumarin 6 dye groups. Upon varying the groups on bipyridine ligand from tert-butyl to thiophene, the absorption spectra of PS-3 showed a slight change in the range of 400–550 nm with respect to PS-2, indicating there was negligible electronic interaction between bipyridine and Coumarin 6 units. Therefore, the absorption bands of these Ir(III) complexes in the range of 400–550 nm corresponded to intraligand charge transfer (ILCT), while those in the range of 250–400 nm were dominated by metal-perturbed intraligand transitions.41 Figure 2 | (a) UV–vis absorption, (b) emission spectra of PS-1 (λex = 400 nm), PS-2 (λex = 440 nm), and PS-3 (λex = 440 nm) in CH3CN (C = 10 μM) under air. (c) Comparison of PS-2 emission under N2 and air conditions. The decay of (d) PS-1 (λem = 570 nm), (e) PS-2 (λem = 587 nm), and (f) PS-3 (λem = 575 nm) in CH3CN (C = 10 μM) excited at 450 nm under N2. Download figure Download PowerPoint The emission spectra of PS-2 and PS-3 under air (Figure 2b) and N2 (Figure 2c and Supporting Information Figure S8) conditions showed vibronic structured bands, indicating that the emissive excited states were ligand-centered. Under N2 condition, the emission maxima appeared at 570, 506/587, and 506/575 nm for PS-1, PS-2, and PS-3, respectively. The emission lifetimes of the three complexes in deaerated CH3CN solution (Figures 2d–2f) were determined as 259 ns (λem = 570 m), 1559 ns (λem = 587 nm), and 1703 ns (λem = 575 nm) for PS-1, PS-2, and PS-3, respectively, suggesting the phosphorescence nature of emission that originates from triplet-excited states. Besides, the emission spectra of PS-2 (Figure 2c) and PS-3 ( Supporting Information Figure S8b) at longer wavelengths (550–700 nm) showed a noticeable quenching effect by air, which was in agreement with their long phosphorescent lifetimes, while their emission spectra at shorter wavelengths (450–530 nm) presented negligible or a very slight quenching effect by air. By considering their absorption spectra (lower energy level absorption assigned as S0 → S1 transition), the shorter wavelength bands (emission maxima at 506 nm) of PS-2 and PS-3 could be assigned as fluorescence (S1 → S0 transition), which was further verified by the corresponding extremely short-lived fluorescent lifetimes ( Supporting Information Figure S9). In addition, the Coumarin 6 ligand displayed intense fluorescence in the range of 460–650 nm ( Supporting Information Figure S8c); however, its emission was dramatically quenched after coordination to an iridium center, indicating the existence of efficient intersystem crossing (ISC) process47 (singlet to triplet excited-states transition) in PS-2 and PS-3. Visible-light-driven hydrogen evolution The photocatalytic hydrogen production was conducted in deaerated CH3CN-DMF (v/v = 3/2) solution with TEOA as sacrificial electron donor, H2O as proton source, Ir(III) complexes as photosensitizers, and Ni3PW10 as H2-evolving catalyst. In view of the broad and intense absorption spectra of PS-2 and PS-3, Xe lamp (λ < 400 nm, 300 W) was employed as the irradiation light source. All TONs were calculated relative to the Ni3PW10 catalyst. After an identical photocatalytic reaction had proceeded for 6 h, the TONs of PS-2- and PS-3-containing catalytic systems reached up to 1295 (∼155 μmol H2 gas) and 204.4 (∼24.5 μmol H2 gas), which corresponded to 13.1 and 2.1 times, respectively, relative to the PS-1-containing system (TON ∼ 98.9) (Figure 3a). These experimental results illustrated that the modification of Ir(III) complex with Coumarin 6 dyes accelerated the photocatalytic process significantly, thereby leading to efficient generation of H2 gas. The long-term catalytic activity of the system containing PS-2 or PS-3 photosensitizer and Ni3PW10 catalyst was also evaluated. We observed that TONs of 4026 or 807 were achieved for PS-2 or PS-3-containing catalytic systems after 120 h irradiation, respectively (Figure 3b), indicating the robustness of the catalytic system and high photostability of Coumarin-modified Ir(III) chromophores under catalytic conditions. Given the best performance of complex PS-2, additional experiments were conducted to optimize the PS-2-containing catalytic system, as described below. Figure 3 | Photocatalytic H2 generation of PS-2 and PS-3 in (a) 6 h and (b) 120 h under the condition of catalyst Ni3PW10 (20 μM), PS (0.2 mM), TEOA (0.25 M), H2O (2 M), 6 mL CH3CN-DMF (v/v = 3/2); Control experiments (c) based on different components (catalyst Ni3PW10 20 μM, PS-2 0.2 mM, TEOA 0.25 M) as well as by varying the concentration of (d) Ni3PW10 (0–40 μM), (e) PS-2 (0–0.3 mM), and (f) TEOA (0–0.25 M). Conditions: Xe lamp (λ < 400 nm, 300 W, 20 °C), H2O (2 M), 6 mL CH3CN-DMF (v/v = 3/2), 6 h. Download figure Download PowerPoint Various control experiments have been performed to study the effect of each component in the PS-2-containing photocatalytic system. Figure 3c shows that in the absence of any essential components such as the catalyst Ni3PW10, PS-2, or TEOA, no yield or negligible H2 generation was noted. Moreover, replacing Ni3PW10 with three molar equivalents of NiCl2 resulted in an obvious decrease of TON from 1295 to 106, revealing an essential role of an intact skeleton of the catalyst Ni3PW10. To verify the molecular integrity of the Ni-POM catalyst, two distinct sets of experiments were conducted: First, the mercury-poison test (Figure 3c) using up to 200 mg Hg in the photocatalytic solution showed no apparent decrease of H2 production, which excluded the formation of Ni-based nanoparticles and proved the molecular stability of Ni3PW10 during photocatalysis. Second, we collected the FT-IR spectra of the isolated Ni3PW10 catalyst before and after a photocatalytic reaction had proceeded for 12 h. Given the anionic nature of the Ni3PW10 polyoxoanion, it is easy to precipitate the Ni3PW10 catalyst with cationic PS-2 after removing the reaction solvent using rotary evaporation. As shown in Supporting Information Figure S10, the FT-IR spectra of the isolated PS-2- Ni3PW10 adducts remained largely unchanged before and after 12 h of photocatalysis, further confirming the molecular integrity of Ni3PW10 catalyst after the photocatalytic reaction. In addition, varying the concentration of the three reaction components mentioned above affected the catalytic rate considerably. Under optimized concentration of the catalyst Ni3PW10, a TON as high as 19,739 was obtained (Figure 3d and Supporting Information Figure S11), which, to the best of our knowledge, was the highest value recorded, thus far, among all kno

Photocatalytic Multielectron Reduction of Nitroarenes to Anilines by Utilizing an Electron-Storable Polyoxometalate-Based Metal-Organic Framework.

Abstract: A powerful approach to generate photocatalysts for the highly selective reduction of nitrobenzene using light as the driving force is a combination of photosensitizers and electron-storable components in a cooperative photocatalysis fashion. Herein, a new precious metal-free photocatalyst, {ZnW-TPT}, was prepared by incorporating a Zn-substituted monovacant Keggin polyanion [SiZnW11O39]6- and a photoactive organic bridging link 2,4,6-tri(4-pyridyl)-1,3,5-triazine (TPT) into a framework. In this structure, the direct coordination bond between [SiZnW11O39]6- and the TPT ligand and the π-π interactions between TPT molecules help separate and migrate photogenerated carriers, which improves the photocatalytic activity of {ZnW-TPT}. The photoelectrochemical properties of {ZnW-TPT} were well studied by solid UV-vis absorption, fluorescence, transient photocurrent response, and electrochemical impedance spectroscopy tests. {ZnW-TPT} efficiently converts using hydrazine hydrate with 99% conversion and 99% selectivity for anilines under mild conditions.

Polyoxometalate-based yolk@shell dual Z-scheme superstructure tandem heterojunction nanoreactors: encapsulation and confinement effects

Abstract: Polyoxometalate-based yolk@shell dual Z-scheme superstructure tandem heterojunction nanoreactors exhibit excellent photocatalytic performance, which is ascribed to the dual Z-scheme heterojunction and encapsulation and confinement effects.

Chaotropic Effect as an Assembly Motif to Construct Supramolecular Cyclodextrin-Polyoxometalate-Based Frameworks.

Abstract: In aqueous solution, low-charged polyoxometalates (POMs) exhibit remarkable self-assembly properties with nonionic organic matter that have been recently used to develop groundbreaking advances in host-guest chemistry, as well as in soft matter science. Herein, we exploit the affinity between a chaotropic POM and native cyclodextrins (α-, β-, and γ-CD) to enhance the structural and functional diversity of cyclodextrin-based open frameworks. First, we reveal that the Anderson-Evans type polyoxometalate [AlMo6O18(OH)6]3- represents an efficient inorganic scaffold to design open hybrid frameworks built from infinite cyclodextrin channels connected through the disk-shaped POM. A single-crystal X-ray analysis demonstrates that the resulting supramolecular architectures contain large cavities (up to 2 nm) where the topologies are dictated by the rotational symmetry of the organic macrocycle, generating honeycomb (bnn net) and checkerboard-like (pcu net) networks for α-CD (C6) and γ-CD (C8), respectively. On the other hand, the use of β-CD, a macrocycle with C7 ideal symmetry, led to a distorted-checkerboard-like network. The cyclodextrin-based frameworks built from an Anderson-Evans type POM are easily functionalizable using the molecular recognition properties of the macrocycle building units. As a proof of concept, we successfully isolated a series of compartmentalized functional frameworks by the entrapment of polyiodides or superchaotropic redox-active polyanions within the macrocyclic host matrix. This set of results paves the way for designing multifunctional supramolecular frameworks whose pore dimensions are controlled by the size of inorganic entities.

Oxygen Evolution Reaction Driven by Charge Transfer from a Cr Complex to Co-Containing Polyoxometalate in a Porous Ionic Crystal.

Abstract: Considerable efforts have been devoted to developing oxygen evolution reaction (OER) catalysts based on transition metal oxides. Polyoxometalates (POMs) can be regarded as model compounds of transition metal oxides, and cobalt-containing POMs (Co-POMs) have received significant interest as candidates. Nanocomposites based on Co-POMs have been reported to show high OER activities due to synergistic effects among the components; however, the role of each component is unclear due to its complex structure. Herein, we utilize porous ionic crystals (PICs) based on Co-POMs, which enable a composition-structure-function relationship to be established to understand the origin of the synergistic catalysis. Specifically, a Keggin-type POM [α-CoW12O40]6- and a Cr complex [Cr3O(OOCCH2CN)6(H2O)3]+ are implemented as PIC building blocks for the OER under nonbasic conditions. The potentially OER-active but highly soluble [α-CoW12O40]6- was successfully anchored in the crystalline PIC matrix via Coulomb interactions and hydrogen bonding induced by polar cyano groups of the Cr complex. The PIC exhibits efficient and sustained OER catalytic activity, while each building block is inactive. The Tafel slope of the linear sweep voltammetry curve and the relatively large kinetic isotope effect value suggest that elementary steps closely related to the OER rate involve single-electron and proton transfer reactions. Electrochemical and spectroscopic studies clearly show that the synergistic catalysis originates from the charge transfer from the Cr complex to [α-CoW12O40]6-; the increased electron density of [α-CoW12O40]6- may increase its basicity and accelerate proton abstraction as well as enhance electron transfer to stabilize the reaction intermediates adsorbed on [α-CoW12O40]6-.