Pathway Dependence in Redox-Driven Metal–Organic Gels

Pathway dependence is common in self-assembly. Herein, the importance of pathway dependence for redox-driven gels is shown by constructing a Fe^II/Fe^III redox-based metal–organic gel system is shown. In situ oxidation of the Fe^II ions at different rates results in conversion of a Fe^II gel into a Fe^III organic gel, which controls the material properties, such as gel stiffness, gel strength, and an unusual swelling behaviour, is described. The rate of formation of Fe^III ions determines the extent of intermolecular interactions and so whether gelation or precipitation occurs.     

Lanthanide Metal–Organic Framework Microrods: Colored Optical Waveguides and Chiral Polarized Emission

Lanthanide metal–organic frameworks (Ln-MOFs) have received much attention owing to their structural tunability and widely photofunctional applications. However, successful examples of Ln-MOFs with well-defined photonic performances at micro-/nanometer size are still quite limited. Herein, self-assemblies of 1,3,5-benzenetricarboxylic acid (BTC) and lanthanide ions afford isostructural crystalline Ln-MOFs. Tb-BTC, Eu@Tb-BTC, and Eu-BTC have 1D microrod morphologies, high photoluminescence (PL) quantum yields, and different emission colors (green, orange, and red). Spatially PL resolved spectra confirm that Ln-MOF microrods exhibit an optical waveguide effect with low waveguide loss coefficient (0.012≈0.033 dB μm⁻¹) during propagation. Furthermore, these microrods feature both linear and chiral polarized photoemission with high anisotropy.     

Conversion from Heterometallic to Homometallic Metal–Organic Frameworks

Two new heterometallic metal–organic frameworks (MOFs), LnZnTPO 1 and 2 , and two homometallic MOFs, LnTPO 3 and 4 (Ln=Eu for 1 and 3 , and Tb for 2 and 4 ; H₃TPO=tris(4-carboxyphenyl)phosphine oxide) were synthesized, and their structures and properties were analyzed. They were prepared by solvothermal reaction of the C₃-symmetric ligand H₃TPO with the corresponding metal ion(s) (a mixture of Ln³⁺ and Zn²⁺ for 1 and 2 , and Ln³⁺ alone for 3 and 4 ). Single-crystal XRD (SXRD) analysis revealed that 1 and 3 are isostructural to 2 and 4 , respectively. TGA showed that the framework is thermally stable up to about 400 °C for 1 and 2 , and about 450 °C for 3 and 4 . PXRD analysis showed their pore-structure distortions without noticeable framework–structure changes during drying processes. The shapes of gas sorption isotherms for 1 and 3 are almost identical to those for 2 and 4 , respectively. Solvothermal immersion of 1 and 2 in Tb³⁺ and Eu³⁺ solutions resulted in the framework metal-ion exchange affording 4 and 3 , respectively, as confirmed by photoluminescence (PL), PXRD, IR, inductively coupled plasma atomic emission spectroscopy (ICP-AES), and energy-dispersive X-ray (EDX) analyses.     

Heteroatom-Doped Ag25 Nanoclusters Encapsulated in Metal–Organic Frameworks for Photocatalytic Hydrogen Production

Atomically precise metal nanoclusters (NCs) with unique optical properties and abundant catalytic sites are promising in photocatalysis. However, their light-induced instability and the difficulty of utilizing the photogenerated carriers for photocatalysis pose significant challenges. Here, MAg₂₄ (M=Ag, Pd, Pt, and Au) NCs doped with diverse single heteroatoms have been encapsulated in a metal–organic framework (MOF), UiO-66-NH₂, affording MAg₂₄@UiO-66-NH₂. Strikingly, compared with Ag₂₅@UiO-66-NH₂, the MAg₂₄@UiO-66-NH₂ doped with heteroatom exhibits much enhanced activity in photocatalytic hydrogen production, among which AuAg₂₄@UiO-66-NH₂ presents the best activity up to 3.6 mmol g⁻¹ h⁻¹, far superior to all other counterparts. Moreover, they display excellent photocatalytic recyclability and stability. X-ray photoelectron spectroscopy and ultrafast transient absorption spectroscopy demonstrate that MAg₂₄ NCs encapsulated into the MOF create a favorable charge transfer pathway, similar to a Z-scheme heterojunction, when exposed to visible light. This promotes charge separation, along with optimized Ag electronic state, which are responsible for the superior activity in photocatalytic hydrogen production.     

Unraveling the Impact of Gold(I)–Thiolate Motifs on the Aggregation-Induced Emission of Gold Nanoclusters

Aggregation-induced emission (AIE) provides an efficient strategy to synthesize highly luminescent metal nanoclusters (NCs), however, rational control of emission energy and intensity of metal NCs is still challenging. This communication reveals the impact of surface Au^I-thiolate motifs on the AIE properties of Au NCs, by employing a series of water-soluble glutathione (GSH)-coordinated Au complexes and NCs as a model ([Au₁₀SR₁₀], [Au₁₅SR₁₃], [Au₁₈SR₁₄], and [Au₂₅SR₁₈]⁻, SR=thiolate ligand). Spectroscopic investigations show that the emission wavelength of Au NCs is adjustable from visible to the near-infrared II (NIR-II) region by controlling the length of the Au^I-SR motifs on the NC surface. Decreasing the length of Au^I-SR motifs also changes the origin of cluster luminescence from AIE-type phosphorescence to Au⁰-core-dictated fluorescence. This effect becomes more prominent when the degree of aggregation of Au NCs increases in solution.     

Bivariate Metal–Organic Frameworks with Tunable Spin-Crossover Properties

In this work, pyrazine ( A ), aminopyrazine ( B ), quinoxaline ( C ), and 5,6,7,8-tetrahydroquinoxaline ( D ) have been screened out among a large number of pyrazine derivatives to construct Hofmann-type metal–organic frameworks (MOFs) Fe(L)[M(CN)₄] (M=Pt, Pd) with similar 3D pillared-layer structures. X-ray single-crystal diffraction reveals that the alternate linkage between M and Fe^II ions through cyano bridges forms the 2D extended metal cyanide sheets, and ligands A – D acted as vertical columns to connect the 2D sheets to give 3D pillared-layer structures. Subsequently, a series of bivariate MOFs were constructed by pairwise combination of the four ligands A–D , which were confirmed by ¹H NMR, PXRD, FTIR, and Raman spectroscopy. The results demonstrated that ligand size and crystallization rate play a dominant role in constructing bivariate Hofmann-type MOFs. More importantly, the spin-crossover (SCO) properties of the bivariate MOFs can be finely tuned by adjusting the proportion of the two pillared ligands in the 3D Hofmann-type structures. Remarkably, the spin transition temperatures, T_c↑ and T_c↓ of Fe( A )_x( B )_1−x[Pt(CN)₄] (x=0 to 1) can be adjusted from 239 to 254 K and from 248 to 284 K, respectively. Meanwhile, the width of the hysteresis loops can be widened from 9 to 30 K. Changing Pt to Pd, the hysteresis loops of Fe( A )_x( B )_1−x[Pd(CN)₄] can be tuned from 9 (T_c↑=215 K, T_c↓=206 K) to 24 K (T_c↑=300 K, T_c↓=276 K). This research provides wider implications in the development of advanced bistable materials, especially in precisely regulating SCO properties.     

Metal–Covalent Organic Frameworks (MCOFs): A Bridge Between Metal–Organic Frameworks and Covalent Organic Frameworks

Many sophisticated chemical and physical properties of porous materials strongly rely on the presence of the metal ions within the structures. Whereas homogeneous distribution of metals is conveniently realized in metal–organic frameworks (MOFs), the limited stability potentially restricts their practical implementation. From that perspective, the development of metal–covalent organic frameworks (MCOFs) may address these shortcomings by incorporating active metal species atop highly stable COF backbones. This Minireview highlights examples of MCOFs that tackle important issues from their design, synthesis, characterization to cutting-edge applications.     

A Nanotubular Metal–Organic Framework with a Narrow Bandgap from Extended Conjugation**

A one-dimensional nanotubular metal–organic framework (MOF) [Ni(Cu-H₄TPPA)]⋅2 (CH₃)₂NH₂⁺ (H₈TPPA=5,10,15,20-tetrakis[p-phenylphosphonic acid] porphyrin) constructed by using the arylphosphonic acid H₈TPPA is reported. The structure of this MOF, known as GTUB-4 , was solved by using single-crystal X-ray diffraction and its geometric accessible surface area was calculated to be 1102 m² g⁻¹, making it the phosphonate MOF with the highest reported surface area. Due to the extended conjugation of its porphyrin core, GTUB-4 possesses narrow indirect and direct bandgaps (1.9 eV and 2.16 eV, respectively) in the semiconductor regime. Thermogravimetric analysis suggests that GTUB-4 is thermally stable up to 400 °C. Owing to its high surface area, low bandgap, and high thermal stability, GTUB-4 could find applications as electrodes in supercapacitors.     

A Titanium Metal–Organic Framework with Visible-Light-Responsive Photocatalytic Activity

The one-step synthesis and characterization of a new and robust titanium-based metal–organic framework, ACM-1 , is reported. In this structure, which is based on infinite Ti−O chains and 4,4′,4′′,4′′′-(pyrene-1,3,6,8-tetrayl) tetrabenzoic acid as a photosensitizer ligand, the combination of highly mobile photogenerated electrons and a strong hole localization at the organic linker results in large charge-separation lifetimes. The suitable energies for band gap and conduction band minimum (CBM) offer great potential for a wide range of photocatalytic reactions, from hydrogen evolution to the selective oxidation of organic substrates.     

An Ultra-Microporous Metal–Organic Framework with Exceptional Xe Capacity

Molecular confinement plays a significant effect on trapped gas and solvent molecules. A fundamental understanding of gas adsorption within the porous confinement provides information necessary to design a material with improved selectivity. In this regard, metal–organic framework (MOF) adsorbents are ideal candidate materials to study confinement effects for weakly interacting gas molecules, such as noble gases. Among the noble gases, xenon (Xe) has practical applications in the medical, automotive and aerospace industries. In this Communication, we report an ultra-microporous nickel-isonicotinate MOF with exceptional Xe uptake and selectivity compared to all benchmark MOF and porous organic cage materials. The selectivity arises because of the near perfect fit of the atomic Xe inside the porous confinement. Notably, at low partial pressure, the Ni–MOF interacts very strongly with Xe compared to the closely related Krypton gas (Kr) and more polarizable CO₂. Further ¹²⁹Xe NMR suggests a broad isotropic chemical shift due to the reduced motion as a result of confinement.     

Advanced Bifunctional Oxygen Reduction and Evolution Electrocatalyst Derived from Surface-Mounted Metal–Organic Frameworks

Metal–organic frameworks (MOFs) and their derivatives are considered as promising catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which are important for many energy provision technologies, such as electrolyzers, fuel cells and some types of advanced batteries. In this work, a “strain modulation” approach has been applied through the use of surface-mounted NiFe-MOFs in order to design an advanced bifunctional ORR/OER electrocatalyst. The material exhibits an excellent OER activity in alkaline media, reaching an industrially relevant current density of 200 mA cm⁻² at an overpotential of only ≈210 mV. It demonstrates operational long-term stability even at a high current density of 500 mA cm⁻² and exhibits the so far narrowest “overpotential window” ΔE_ORR-OER of 0.69 V in 0.1 m KOH with a mass loading being two orders of magnitude lower than that of benchmark electrocatalysts.     

Quencher-Delocalized Emission Strategy of AIEgen-Based Metal–Organic Framework for Profiling of Subcellular Glutathione

Distinguishing glutathione (GSH) level in different subcellular locations is critical for studying its antioxidant function in the signaling system. However, traditional methods for imaging subcellular GSH were achieved in isolated organelles or fixed cells. In this work, we report a quencher-delocalized emission strategy for in situ profiling of GSH at different subcellular locations in living cells. A nonemissive metal–organic framework (MOF) nanoprobe was designed with AIEgen as the linker and Cu^II as the node and quencher. The AIEgen in MOF structure was lightened up with green emission in a neutral environment due to partial Cu^II delocalization by competitive binding to GSH. Meanwhile, along with the protonation of AIEgen ligand under acidic environment, the AIEgen-based MOF could be completely dissociated in the presence of GSH to yield yellow emission. The two-channel ratiometric analysis of dual-colored emission of AIEgen-based MOF allows visualization of GSH in cytoplasm and lysosome in living cells, which is also able to report the drug effects on different subcellular GSH levels.     

A Chemiluminescent Metal–Organic Framework

The synthesis and characterization of a chemiluminescent metal–organic framework with high porosity is reported. It consists of Zr₆O₆(OH)₄ nodes connected by 4,4′-(anthracene-9,10-diyl)dibenzoate as the linker and luminophore. It shows the topology known for UiO-66 and is therefore denoted PAP-UiO. The MOF was not only obtained as bulk material but also as a thin film. Exposure of PAP-UiO as bulk or film to a mixture of bis-(2,4,6-trichlorophenyl) oxalate, hydrogen peroxide, and sodium salicylate in a mixture of dimethyl and dibutyl phthalate evoked strong and long lasting chemiluminescence of the PAP-UiO crystals. Time dependent fluorescence spectroscopy on bulk PAP-UiO and, for comparison, on dimethyl 4,4′-(anthracene-9,10-diyl)dibenzoate provided evidence that the chemiluminescence originates from luminophores being part of the PAP-UiO, including the luminophores inside the crystals.     

Multi-shelled Hollow Metal–Organic Frameworks

Hollow metal–organic frameworks (MOFs) are promising materials with sophisticated structures, such as multiple shells, that cannot only enhance the properties of MOFs but also endow them with new functions. Herein, we show a rational strategy to fabricate multi-shelled hollow chromium (III) terephthalate MOFs (MIL-101) with single-crystalline shells through step-by-step crystal growth and subsequent etching processes. This strategy relies on the creation of inhomogeneous MOF crystals in which the outer layer is chemically more robust than the inner layer and can be selectively etched by acetic acid. The regulation of MOF nucleation and crystallization allows the tailoring of the cavity size and shell thickness of each layer. The resultant multi-shelled hollow MIL-101 crystals show significantly enhanced catalytic activity during styrene oxidation. The insight gained from this systematic study will aid in the rational design and synthesis of other multi-shelled hollow structures and the further expansion of their applications.     

Rapid,Selective Extraction of Silver from Complex Water Matrices with a Metal–Organic Framework/Oligomer Composite Constructed via Supercritical CO2

Every year vast quantities of silver are lost in various waste streams; this, combined with its limited, diminishing supply and rising demand, makes silver recovery of increasing importance. Thus, herein, we report a controllable, green process to produce a host of highly porous metal–organic framework (MOF)/oligomer composites using supercritical carbon dioxide (ScCO₂) as a medium. One resulting composite, referred to as MIL-127/Poly-o-phenylenediamine (PoPD), has an excellent Ag⁺ adsorption capacity, removal efficiency (>99 %) and provides rapid Ag⁺ extraction in as little as 5 min from complex liquid matrices. Notably, the composite can also reduce sliver concentrations below the levels (<0.1 ppm) established by the United States Environmental Protection Agency. Using theoretical simulations, we find that there are spatially ordered polymeric units inside the MOF that promote the complexation of Ag⁺ over other common competing ions. Moreover, the oligomer is able to reduce silver to its metallic state, also providing antibacterial properties.     

Alkali Phosphonate Metal–Organic Frameworks

A new family of porous metal–organic frameworks (MOFs), namely alkali phosphonate MOFs, is reported. [Na₂Cu(H₄TPPA)] ⋅ (NH₂(CH₃)₂)₂ ( GTUB-1 ) was synthesized using the tetratopic 5,10,15,20-tetrakis[p-phenylphosphonic acid] porphyrin ( H₈-TPPA ) linker with planar X-shaped geometrical core. GTUB-1 is composed of rectangular void channels with BET surface area of 697 m² g⁻¹. GTUB-1 exhibits exceptional thermal stability. The toxicity analysis of the ( H₈-TPPA ) linker indicates that it is well tolerated by an intestinal cell line, suggesting its suitability for creating phosphonate MOFs for biological applications.     

Metal–Organic Frameworks as Electrocatalysts

Transition metal complexes are well-known homogeneous electrocatalysts. In this regard, metal–organic frameworks (MOFs) can be considered as an ensemble of transition metal complexes ordered in a periodic arrangement. In addition, MOFs have several additional positive structural features that make them suitable for electrocatalysis, including large surface area, high porosity, and high content of accessible transition metal with exchangeable coordination positions. The present review describes the current state in the use of MOFs as electrocatalysts, both as host of electroactive guests and their direct electrocatalytic activity, particularly in the case of bimetallic MOFs. The field of MOF-derived materials is purposely not covered, focusing on the direct use of MOFs or its composites as electrocatalysts. Special attention has been paid to present strategies to overcome their poor electrical conductivity and limited stability.     

Efficient Solar-Driven Water Harvesting from Arid Air with Metal–Organic Frameworks Modified by Hygroscopic Salt

Freshwater scarcity is a global challenge threatening human survival, especially for people living in arid regions. Sorption-based atmospheric water harvesting (AWH) is an appealing way to solve this problem. However, the state-of-the-art AWH technologies have poor water harvesting performance in arid climates owing to the low water sorption capacity of common sorbents under low humidity conditions. We report a high-performance composite sorbent for efficient water harvesting from arid air by confining hygroscopic salt in a metal–organic framework matrix (LiCl@MIL-101(Cr)). The composite sorbent shows 0.77 g g⁻¹ water sorption capacity at 1.2 kPa vapor pressure (30 % relative humidity at 30 °C) by integrating the multi-step sorption processes of salt chemisorption, deliquescence, and solution absorption. A highly efficient AWH prototype is demonstrated with LiCl@MIL-101(Cr) that can enable the harvesting of 0.45–0.7 kg water per kilogram of material under laboratory and outdoor ambient conditions powered by natural sunlight without optical concentration and additional energy input.