Hierarchical Integration of Photosensitizing Metal–Organic Frameworks and Nickel‐Containing Polyoxometalates for Efficient Visible‐Light‐Driven Hydrogen Evolution

Metal–organic frameworks (MOFs) provide a tunable platform for hierarchically integrating multiple components to effect synergistic functions that cannot be achieved in solution. Here we report the encapsulation of a Ni‐containing polyoxometalate (POM) [Ni₄(H₂O)₂(PW₉O₃₄)₂]^10? ( Ni₄P₂ ) into two highly stable and porous phosphorescent MOFs. The proximity of Ni₄P₂ to multiple photosensitizers in Ni₄P₂ @MOF allows for facile multi‐electron transfer to enable efficient visible‐light‐driven hydrogen evolution reaction (HER) with turnover numbers as high as 1476. Photophysical and electrochemical studies established the oxidative quenching of the excited photosensitizer by Ni₄P₂ as the initiating step of HER and explained the drastic catalytic activity difference of the two POM@MOFs. Our work shows that POM@MOF assemblies not only provide a tunable platform for designing highly effective photocatalytic HER catalysts but also facilitate detailed mechanistic understanding of HER processes.     

A Bismuth‐Based Metal–Organic Framework as an Efficient Visible‐Light‐Driven Photocatalyst

A visible‐light‐responsive bismuth‐based metal–organic framework (Bi‐mna) is demonstrated to show good photoelectric and photocatalytic properties. Combining experimental and theoretical results, a ligand‐to‐ligand charge transfer (LLCT) process is found to be responsible for the high performance, which gives rise to a longer lifetime of photogenerated charge carriers. Our results suggest that bismuth‐based MOFs could be promising candidates for the development of efficient visible‐light photocatalysts.     

Structural Effects in Visible‐Light‐Responsive Metal–Organic Frameworks Incorporating ortho‐Fluoroazobenzenes

The ability to control the interplay of materials with low‐energy photons is important as visible light offers several appealing features compared to ultraviolet radiation (less damaging, more selective, predominant in the solar spectrum, possibility to increase the penetration depth). Two different metal–organic frameworks (MOFs) were synthesized from the same linker bearing all‐visible ortho‐fluoroazobenzene photoswitches as pendant groups. The MOFs exhibit different architectures that strongly influence the ability of the azobenzenes to isomerize inside the voids. The framework built with Al‐based nodes has congested 1D channels that preclude efficient isomerization. As a result, local light–heat conversion can be used to alter the CO₂ adsorption capacity of the material on exposure to green light. The second framework, built with Zr nodes, provides enough room for the photoswitches to isomerize, which leads to a unique bistable photochromic MOF that readily responds to blue and green light. The superiority of green over UV irradiation was additionally demonstrated by reflectance spectroscopy and analysis of digested samples. This material offers promising perspectives for liquid‐phase applications such as light‐controlled catalysis and adsorptive separation.     

Zirconium–Porphyrin‐Based Metal–Organic Framework Hollow Nanotubes for Immobilization of Noble‐Metal Single Atoms

Single atoms immobilized on metal–organic frameworks (MOFs) with unique nanostructures have drawn tremendous attention in the application of catalysis but remain a great challenge. Various single noble‐metal atoms have now been successfully anchored on the well‐defined anchoring sites of the zirconium porphyrin MOF hollow nanotubes, which are probed by aberration‐corrected scanning transmission electron microscopy and synchrotron‐radiation‐based X‐ray absorption fine‐structure spectroscopy. Owing to the hollow structure and excellent photoelectrochemical performance, the HNTM‐Ir/Pt exhibits outstanding catalytic activity in the visible‐light photocatalytic H₂ evolution via water splitting. The single atom immobilized on MOFs with hollow structures are expected to pave the way to expand the potential applications of MOFs.     

An Efficient,Visible‐Light‐Driven,Hydrogen Evolution Catalyst NiS/ZnxCd1−xS Nanocrystal Derived from a Metal–Organic Framework

Photocatalytic water splitting for hydrogen production using sustainable sunlight is a promising alternative to industrial hydrogen production. However, the scarcity of highly active, recyclable, inexpensive photocatalysts impedes the development of photocatalytic hydrogen evolution reaction (HER) schemes. Herein, a metal–organic framework (MOF)‐template strategy was developed to prepare non‐noble metal co‐catalyst/solid solution heterojunction NiS/Zn_xCd_1?xS with superior photocatalytic HER activity. By adjusting the doping metal concentration in MOFs, the chemical compositions and band gaps of the heterojunctions can be fine‐tuned, and the light absorption capacity and photocatalytic activity were further optimized. NiS/Zn_0.5Cd_0.5S exhibits an optimal HER rate of 16.78 mmol g^?1 h^?1 and high stability and recyclability under visible‐light irradiation (λ>420 nm). Detailed characterizations and in‐depth DFT calculations reveal the relationship between the heterojunction and photocatalytic activity and confirm the importance of NiS in accelerating the water dissociation kinetics, which is a crucial factor for photocatalytic HER.     

Titanium Phosphonate Based Metal–Organic Frameworks with Hierarchical Porosity for Enhanced Photocatalytic Hydrogen Evolution

Photocatalytic hydrogen production is crucial for solar‐to‐chemical conversion process, wherein high‐efficiency photocatalysts lie in the heart of this area. A photocatalyst of hierarchically mesoporous titanium phosphonate based metal–organic frameworks, featuring well‐structured spheres, a periodic mesostructure, and large secondary mesoporosity, are rationally designed with the complex of polyelectrolyte and cathodic surfactant serving as the template. The well‐structured hierarchical porosity and homogeneously incorporated phosphonate groups can favor the mass transfer and strong optical absorption during the photocatalytic reactions. Correspondingly, the titanium phosphonates exhibit significantly improved photocatalytic hydrogen evolution rate along with impressive stability. This work can provide more insights into designing advanced photocatalysts for energy conversion and render a tunable platform in photoelectrochemistry.     

Keggin‐Type Polyoxometalate‐Based Metal–Organic Networks for Photocatalytic Dye Degradation

The reaction of Keggin‐type polyoxometalate (POM) units, transition‐metal (TM) ions, and a rigid bis(imidazole) ligand (1,4‐bis(1‐imidazolyl)benzene (bimb)) in a hydrothermal environment led to the isolation of four new POM‐based metal–organic networks, [H₂L][CuL][SiW₁₂O₄₀]?2 H₂O ( 1 ), [H₂L]₂[Co(H₂O)₃L][SiW₁₁CoO₃₉]?6 H₂O ( 2 ), KH[CuL]₂[SiW₁₁CoO₃₉(H₂O)]?2 H₂O ( 3 ), and [CuL]₄[GeW₁₂O₄₀]?H₂O ( 4 ; L=bimb). All four compounds were characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, powder X‐ray diffraction, and single‐crystal X‐ray diffraction. Compounds 1 and 3 are new 3D networks with 1D channels. Compounds 2 and 4 contain 2D networks, which further stack into 3D supramolecular networks. The contributions of pH value, the negative charge of the POM, and the TM coordination modes to the construction of 3D networks were elucidated by comparing the synthetic conditions and structures of compounds 1 – 4 . The photocatalytic properties of compounds 1 – 4 were investigated using methylene blue (MB) degradation under UV light. All compounds showed good catalytic activity and structural stability. The possible catalytic mechanism was discussed on the basis of active‐species trapping experiments. The different photocatalytic activities of compounds 1 – 4 were explained by comparison of the band gaps of different POM species and different packing modes of POM units in these hybrid compounds.     

Boosting Photocatalytic Hydrogen Production of a Metal–Organic Framework Decorated with Platinum Nanoparticles: The Platinum Location Matters

Improving the efficiency of electron–hole separation and charge‐carrier utilization plays a central role in photocatalysis. Herein, Pt nanoparticles of ca. 3 nm are incorporated inside or supported on a representative metal–organic framework (MOF), UiO‐66‐NH₂, denoted as Pt@UiO‐66‐NH₂ and Pt/UiO‐66‐NH₂, respectively, for photocatalytic hydrogen production via water splitting. Compared with the pristine MOF, both Pt‐decorated MOF nanocomposites exhibit significantly improved yet distinctly different hydrogen‐production activities, highlighting that the photocatalytic efficiency strongly correlates with the Pt location relative to the MOF. The Pt@UiO‐66‐NH₂ greatly shortens the electron‐transport distance, which favors the electron–hole separation and thereby yields much higher efficiency than Pt/UiO‐66‐NH₂. The involved mechanism has been further unveiled by means of ultrafast transient absorption and photoluminescence spectroscopy.     

Stimulus‐Responsive Metal–Organic Frameworks

Materials that can recognize the changes in their local environment and respond by altering their inherent physical and/or chemical properties are strong candidates for future “smart” technology materials. Metal–organic frameworks (MOFs) have attracted a great deal of attention in recent years owing to their designable architecture, host–guest chemistry, and softness as porous materials. Despite this fact, studies on the tuning of the properties of MOFs by external stimuli are still rare. This review highlights the recent developments in the field of stimulus‐responsive MOFs or so‐called smart MOFs. In particular, the various stimuli used and the utility of stimulus‐responsive smart MOFs for various applications such as gas storage and separation, sensing, clean energy, catalysis, molecular motors, and biomedical applications are highlighted by using representative examples. Future directions in the developments of stimulus‐responsive smart MOFs and their applications are proposed from a personal perspective.     

Efficient Gating of Ion Transport in Three‐Dimensional Metal–Organic Framework Sub‐Nanochannels with Confined Light‐Responsive Azobenzene Molecules

1D nanochannels modified with responsive molecules are fabricated to replicate gating functionalities of biological ion channels, but gating effects are usually weak because small molecular gates cannot efficiently block the large channels in the closed states. Now, 3D metal–organic framework (MOF) sub‐nanochannels (SNCs) confined with azobenzene (AZO) molecules achieve efficient light‐gating functionalities. The 3D MOFSNCs consisting of a MOF UiO66 with ca. 9–12 Å cavities connected by ca. 6 Å triangular windows work as angstrom‐scale ion channels, while confined AZO within the MOF cavities function as light‐driven molecular gates to efficiently regulate the ion flux. The AZO‐MOFSNCs show good cyclic gating performance and high on–off ratios up to 17.8, an order of magnitude higher than ratios observed in conventional 1D AZO‐modified nanochannels (1.3–1.5). This work provides a strategy to develop highly efficient switchable ion channels based on 3D porous MOFs and small responsive molecules.     

Integration of Plasmonic Effects and Schottky Junctions into Metal–Organic Framework Composites: Steering Charge Flow for Enhanced Visible‐Light Photocatalysis

A wide range of light absorption and rapid electron–hole separation are desired for efficient photocatalysis. Herein, on the basis of a semiconductor‐like metal–organic framework (MOF), a Pt@MOF/Au catalyst with two types of metal–MOF interfaces integrates the surface plasmon resonance excitation of Au nanorods with a Pt‐MOF Schottky junction, which not only extends the light absorption of the MOF from the UV to the visible region but also greatly accelerates charge transfer. The spatial separation of Pt and Au particles by the MOF further steers the formation of charge flow and expedites the charge migration. As a result, the Pt@MOF/Au presents an exceptionally high photocatalytic H₂ production rate by water splitting under visible light irradiation, far superior to Pt/MOF/Au, MOF/Au and other counterparts with similar Pt or Au contents, highlighting the important role of each component and the Pt location in the catalyst.     

A Robust Metal–Organic Framework for Dynamic Light‐Induced Swing Adsorption of Carbon Dioxide

Adsorbents for CO₂ capture need to demonstrate efficient release. Light‐induced swing adsorption (LISA) is an attractive new method to release captured CO₂ that utilizes solar energy rather than electricity. MOFs, which can be tailored for use in LISA owing to their chemical functionality, are often unstable in moist atmospheres, precluding their use. A MOF is used that can release large quantities of CO₂ via LISA and is resistant to moisture across a large pH range. PCN‐250 undergoes LISA, with UV flux regulating the CO₂ desorption capacity. Furthermore, under UV light, the azo residues within PCN‐250 have constrained, local, structural flexibility. This is dynamic, rapidly switching back to the native state. Reusability tests demonstrate a 7.3 % and 4.9 % loss in both adsorption and LISA capacity after exposure to water for five cycles. These minimal changes confirm the structural robustness of PCN‐250 and its great potential for triggered release applications.     

A Metal–Organic Framework with Exceptional Activity for C−H Bond Amination

The development of catalysts capable of fast, robust C?H bond amination under mild conditions is an unrealized goal despite substantial progress in the field of C?H activation in recent years. A Mn‐based metal–organic framework (CPF‐5) is described that promotes the direct amination of C?H bonds with exceptional activity. CPF‐5 is capable of functionalizing C?H bonds in an intermolecular fashion with unrivaled catalytic stability producing >10⁵ turnovers.     

A Versatile AlIII‐Based Metal–Organic Framework with High Physicochemical Stability

A unique Al^III‐based metal–organic framework (467‐MOF) with two types of square channels has been designed and synthesized by using a flexible tricarboxylate ligand under solvothermal conditions. 467‐MOF exhibits superior thermal and chemical stability and, moreover, shows high CO₂ sorption selectivity over H₂, with a selectivity, based on the ideal adsorbed solution theory (IAST) of approximately 45 at 273 or 293 K. Furthermore, its solvent‐dependent photoluminescence makes it an applicable sensor in the detection of nitrobenzene explosives through fluorescence quenching.     

Controllable Synthesis of Porphyrin‐Based 2D Lanthanide Metal–Organic Frameworks with Thickness‐ and Metal‐Node‐Dependent Photocatalytic Performance

Synthesizing 2D metal–organic frameworks (2D MOFs) in high yields and rational tailoring of the properties in a predictable manner for specific applications is extremely challenging. Now, a series of porphyrin‐based 2D lanthanide MOFs (Ln‐TCPP, Ln=Ce, Sm, Eu, Tb, Yb, TCPP=tetrakis(4‐carboxyphenyl) porphyrin) with different thickness were successfully prepared in a household microwave oven. The as‐prepared 2D Ln‐TCPP nanosheets showed thickness‐dependent photocatalytic performances towards photooxidation of 1,5‐dihydroxynaphthalene (1,5‐DHN) to synthesize juglone. Particularly, the Yb‐TCPP displayed outstanding photodynamic activity to generate O₂^? and ¹O₂. This work not only provides fundamental insights into structure designing and property tailoring of 2D MOFs nanosheets, but also pave a new way to improve the photocatalytic performance.     

Metal–Organic Framework Disintegrants: Enzyme Preparation Platforms with Boosted Activity

An enzyme formulation using customized enzyme activators (metal ions) to directly construct metal–organic frameworks (MOFs) as enzyme protective carriers is presented. These MOF carriers can also serve as the disintegrating agents to simultaneously release enzymes and their activators during biocatalysis with boosted activities. This highly efficient enzyme preparation combines enzyme immobilization (enhanced stability, easy operation) and homogeneous biocatalysis (fast diffusion, high activity). The MOF serves as an ion pump that continuously provides metal ion activators that greatly promote the enzymatic activities (up to 251 %). This MOF–enzyme composite demonstrated an excellent protective effect against various perturbation environments. A mechanistic investigation revealed that the spontaneous activator/enzyme release and ion pumping enable enzymes to sufficiently interact with their activators owing to the proximity effects, leading to a boost in biocatalytic performance.     

Water‐Stable Zirconium‐Based Metal–Organic Framework Material with High‐Surface Area and Gas‐Storage Capacities

We designed, synthesized, and characterized a new Zr‐based metal–organic framework material, NU‐1100 , with a pore volume of 1.53 ccg^?1 and Brunauer–Emmett–Teller (BET) surface area of 4020 m²g^?1; to our knowledge, currently the highest published for Zr‐based MOFs. CH₄/CO₂/H₂ adsorption isotherms were obtained over a broad range of pressures and temperatures and are in excellent agreement with the computational predictions. The total hydrogen adsorption at 65 bar and 77 K is 0.092 g g^?1, which corresponds to 43 g L^?1. The volumetric and gravimetric methane‐storage capacities at 65 bar and 298 K are approximately 180 v_STP/v and 0.27 g g^?1, respectively.