Evidence of Photoinduced Charge Separation in the Metal–Organic Framework MIL‐125(Ti)‐NH2

Herein, we describe the photochemical behavior of the porous metal–organic framework MIL‐125(Ti)‐NH₂, built up from cyclic Ti₈O₈(OH)₄ oxoclusters and 2‐aminoterephthalate ligands. While MIL‐125(Ti)‐NH₂ does not emit upon excitation at 420 nm, laser flash photolyses of dry samples (diffuse reflectance) or aqueous suspensions (transmission) of the solid have allowed detecting a transient characterized by a continuous absorption from 390 to 820 nm decaying in the sub‐millisecond timescale, which is quenched by oxygen. This transient has been attributed to the charge‐separation state. Firm evidence for this assignment was obtained by lamp irradiation of aqueous suspensions of MIL‐125(Ti)‐NH₂ in the presence of electron‐donor (N,N,N′N′‐tetramethyl‐p‐phenylenediamine) or electron‐acceptor (methylviologen) probe molecules, which has allowed the visual detection of the corresponding radical ions, in agreement with the occurrence of photoinduced charge separation in MIL‐125(Ti)‐NH₂.     

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.     

Photocatalyzed Hydrogen Evolution from Water by a Composite Catalyst of NH2‐MIL‐125(Ti) and Surface Nickel(II) Species

A composite of the metal–organic framework (MOF) NH₂‐MIL‐125(Ti) and molecular and ionic nickel(II) species, catalyzed hydrogen evolution from water under UV light. In 95 v/v % aqueous conditions the composite produced hydrogen in quantities two orders of magnitude higher than that of the virgin framework and an order of magnitude greater than that of the molecular catalyst. In a 2 v/v % water and acetonitrile mixture, the composite demonstrated a TOF of 28 mol H₂ g(Ni)^?1 h^?1 and remained active for up to 50 h, sustaining catalysis for three times longer and yielding 20‐fold the amount of hydrogen. Appraisal of physical mixtures of the MOF and each of the nickel species under identical photocatalytic conditions suggest that similar surface localized light sensitization and proton reduction processes operate in the composite catalyst. Both nickel species contribute to catalytic conversion, although different activation behaviors are observed.     

Metal–Organic Frameworks (MOFs) as Safer,Structurally Reinforced Energetics

Second‐generation cobalt and zinc coordination architectures were obtained through efforts to stabilize extremely sensitive and energetic transition‐metal hydrazine perchlorate ionic polymers. Partial ligand substitution by the tridentate hydrazinecarboxylate anion afforded polymeric 2D‐sheet structures never before observed for energetic materials. Carefully balanced reaction conditions allowed the retention of the noncoordinating perchlorate anion in the presence of a strongly chelating hydrazinecarboxylate ligand. High‐quality X‐ray single‐crystal structure determination revealed that the metal coordination preferences lead to different structural motifs and energetic properties, despite the nearly isoformulaic nature of the two compounds. Energetic tests indicate highly decreased sensitivity and DFT calculations suggest a high explosive performance for these remarkable structures.     

Alkali‐Metal Azides Interacting with Metal–Organic Frameworks

Interactions between alkali‐metal azides and metal–organic framework (MOF) derivatives, namely, the first and third members of the isoreticular MOF (IRMOF) family, IRMOF‐1 and IRMOF‐3, are studied within the density functional theory (DFT) paradigm. The investigations take into account different models of the selected IRMOFs. The mutual influence between the alkali‐metal azides and the π rings or Zn centers of the involved MOF derivatives are studied by considering the interactions both of the alkali‐metal cations with model aromatic centers and of the alkali‐metal azides with distinct sites of differently sized models of IRMOF‐1 and IRMOF‐3. Several exchange and correlation functionals are employed to calculate the corresponding interaction energies. Remarkably, it is found that, with increasing alkali‐metal atom size, the latter decrease for cations interacting with the π‐ring systems and increase for the azides interacting with the MOF fragments. The opposite behavior is explained by stabilization effects on the azide moieties and determined by the Zn atoms, which constitute the inorganic vertices of the IRMOF species. Larger cations can, in fact, coordinate more efficiently to both the aromatic center and the azide anion, and thus stabilizing bridging arrangements of the azide between one alkali‐metal and two Zn atoms in an η² coordination mode are more favored.     

Hydrothermal Synthesis,Structure, and Optical Properties of Two Nanosized Ln26@CO3 (Ln=Dy and Tb) Cluster‐Based Lanthanide–Transition‐Metal–Organic Frameworks (Ln MOFs)

Two Ln₂₆@CO₃ (Ln=Dy and Tb) cluster‐based lanthanide–transition‐metal–organic frameworks (Ln MOFs) formulated as [Dy₂₆Cu₃(Nic)₂₄(CH₃COO)₈(CO₃)₁₁(OH)₂₆(H₂O)₁₄]Cl ? 3 H₂O ( 1 ; HNic=nicotinic acid) and [Tb₂₆NaAg₃(Nic)₂₇(CH₃COO)₆(CO₃)₁₁(OH)₂₆Cl(H₂O)₁₅] ? 7.5 H₂O ( 2 ) have been successfully synthesized by hydrothermal methods and characterized by IR, thermogravimetric analysis (TGA), elemental analysis, and single X‐ray diffraction. Compound 1 crystallizes in the monoclinic space group Cc with a=35.775(12) Å, b=33.346(11) Å, c=24.424(8) Å, β=93.993(5)°, V=29065(16) ų, whereas 2 crystallizes in the triclinic space group P with a=20.4929(19) Å, b=24.671(2) Å, c=29.727(3) Å, α=81.9990(10)°, β=88.0830(10)°, γ=89.9940(10)°, V=14875(2) ų. Structural analysis indicates the framework of 1 is a 3D perovskite‐like structure constructed out of CO₃@Dy₂₆ building units and Cu⁺ centers by means of nicotinic acid ligand bridging. In 2 , however, nanosized CO₃@Tb₂₆ units and [Ag₃Cl]²⁺ centers are connected by Nic^? bridges to give rise to a 2D structure. It is worth mentioning that this kind of 4d–4f cluster‐based MOF is quite rare as most of the reported analogous compounds are 3d–4f ones. Additionally, the solid‐state emission spectra of pure compound 2 at room temperature suggest an efficient energy transfer from the ligand Nic^? to Tb³⁺ ions, which we called the “antenna effect”. Compound 2 shows a good two‐photon absorption (TPA) with a TPA coefficient of 0.06947 cm GM^?1 (1 GM=10^?50 cm⁴ s photon^?1), which indicates that compound 2 might be a good choice for third‐order nonlinear optical materials.     

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.     

Iron Metal–Organic Frameworks MIL‐88B and NH2‐MIL‐88B for the Loading and Delivery of the Gasotransmitter Carbon Monoxide

Crystals of MIL‐88B‐Fe and NH₂‐MIL‐88B‐Fe were prepared by a new rapid microwave‐assisted solvothermal method. High‐purity, spindle‐shaped crystals of MIL‐88B‐Fe with a length of about 2 μm and a diameter of 1 μm and needle‐shaped crystals of NH₂‐MIL‐88B‐Fe with a length of about 1.5 μm and a diameter of 300 nm were produced with uniform size and excellent crystallinity. The possibility to reduce the as‐prepared frameworks and the chemical capture of carbon monoxide in these materials was studied by in situ ultrahigh vacuum Fourier‐transform infrared (UHV‐FTIR) spectroscopy and Mössbauer spectroscopy. CO binding occurs to unsaturated coordination sites (CUS). The release of CO from the as‐prepared materials was studied by a myoglobin assay in physiological buffer. The release of CO from crystals of MIL‐88B‐Fe with t_1/2=38 min and from crystals of NH₂‐MIL‐88B‐Fe with t_1/2=76 min were found to be controlled by the degradation of the MIL materials under physiological conditions. These MIL‐88B‐Fe and NH₂‐MIL‐88B‐Fe materials show good biocompatibility and have the potential to be used in pharmacological and therapeutic applications as carriers and delivery vehicles for the gasotransmitter carbon monoxide.     

Rabies Virus‐Inspired Metal–Organic Frameworks (MOFs) for Targeted Imaging and Chemotherapy of Glioma

The blood–brain barrier (BBB) restricts access to the brain of more than 98 % of therapeutic agents and is largely responsible for treatment failure of glioblastoma multiforme (GBM). Therefore, it is of great importance to develop a safe and efficient strategy for more effective drug delivery across the BBB into the brain. Inspired by the extraordinary capability of rabies virus (RABV) to enter the central nervous system, we report the development and evaluation of the metal–organic framework‐based nanocarrier MILB@LR, which closely mimicked both the bullet‐shape structure and surface functions of natural RABV. MILB@LR benefited from a more comprehensive RABV‐mimic strategy than mimicking individual features of RABV and exhibited significantly enhanced BBB penetration and brain tumor targeting. MILB@LR also displayed superior inhibition of tumor growth when loaded with oxaliplatin. The results demonstrated that MILB@LR may be valuable for GBM targeting and treatment.     

Metal‐Ion Metathesis in Metal–Organic Frameworks: A Synthetic Route to New Metal–Organic Frameworks

A porous metal–organic framework, Mn(H₃O)[(Mn₄Cl)₃(hmtt)₈] (POST‐65), was prepared by the reaction of 5,5′,10,10′,15,15′‐hexamethyltruxene‐2,7,12‐tricarboxylic acid (H₃hmtt) with MnCl₂ under solvothermal conditions. POST‐65(Mn) was subjected to post‐synthetic modification with Fe, Co, Ni, and Cu according to an ion‐exchange method that resulted in the formation of three isomorphous frameworks, POST‐65(Co/Ni/Cu), as well as a new framework, POST‐65(Fe). The ion‐exchanged samples could not be prepared by regular solvothermal reactions. The complete exchange of the metal ions and retention of the framework structure were verified by inductively coupled plasma–atomic emission spectrometry (ICP‐AES), powder X‐ray diffraction (PXRD), and Brunauer–Emmett–Teller (BET) surface‐area analysis. Single‐crystal X‐ray diffractions studies revealed a single‐crystal‐to‐single‐crystal (SCSC)‐transformation nature of the ion‐exchange process. Hydrogen‐sorption and magnetization measurements showed metal‐specific properties of POST‐65.     

Microporous Metal–Organic Frameworks for Gas Separation

Microporous metal–organic frameworks (MOFs) are comparatively new porous materials. Because the pores within such MOFs can be readily tuned through the interplay of both metal‐containing clusters and organic linkers to induce their size‐selective sieving effects, while the pore surfaces can be straightforwardly functionalized to enforce their different interactions with gas molecules, MOF materials are very promising for gas separation. Furthermore, the high porosities of such materials can enable microporous MOFs with optimized gas separation selectivity and capacity to be targeted. This Focus Review highlights recent significant advances in microporous MOFs for gas separation.     

Water Stable Zr–Benzenedicarboxylate Metal–Organic Frameworks as Photocatalysts for Hydrogen Generation

The Zr‐containing metal–organic frameworks (MOFs) formed by terephthalate (UiO‐66) and 2‐aminoterephthalate ligands [UiO‐66(NH₂)] are two notably water‐resistant MOFs that exhibit photocatalytic activity for hydrogen generation in methanol or water/methanol upon irradiation at wavelength longer than 300 nm. The apparent quantum yield for H₂ generation using monochromatic light at 370 nm in water/methanol 3:1 was of 3.5 % for UiO‐66(NH₂). Laser‐flash photolysis has allowed detecting for UiO‐66 and UiO‐66(NH₂) the photochemical generation of a long lived charge separated state whose decay is not complete 300 μs after the laser flash. Our finding and particularly the influence of the amino group producing a bathochromic shift in the optical spectrum without altering the photochemistry shows promises for the development of more efficient MOFs for water splitting.     

Defect‐Engineered Metal–Organic Frameworks

Defect engineering in metal–organic frameworks (MOFs) is an exciting concept for tailoring material properties, which opens up novel opportunities not only in sorption and catalysis, but also in controlling more challenging physical characteristics such as band gap as well as magnetic and electrical/conductive properties. It is challenging to structurally characterize the inherent or intentionally created defects of various types, and there have so far been few efforts to comprehensively discuss these issues. Based on selected reports spanning the last decades, this Review closes that gap by providing both a concise overview of defects in MOFs, or more broadly coordination network compounds (CNCs), including their classification and characterization, together with the (potential) applications of defective CNCs/MOFs. Moreover, we will highlight important aspects of “defect‐engineering” concepts applied for CNCs, also in comparison with relevant solid materials such as zeolites or COFs. Finally, we discuss the future potential of defect‐engineered CNCs.     

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.     

Well‐Defined Metal–Organic Framework Hollow Nanocages

Metal–organic frameworks (MOFs) have demonstrated great potentials in a variety of important applications. To enhance the inherent properties and endow materials with multifunctionality, the rational design and synthesis of MOFs with nanoscale porosity and hollow feature is highly desired and remains a great challenge. In this work, the formation of a series of well‐defined MOF (MOF‐5, Fe^II‐MOF‐5, Fe^III‐MOF‐5) hollow nanocages by a facile solvothermal method, without any additional supporting template is reported. A surface‐energy‐driven mechanism may be responsible for the formation of hollow nanocages. The addition of pre‐synthesized poly(vinylpyrrolidone)‐ (PVP) capped noble‐metal nanoparticles into the synthetic system of MOF hollow nanocages yields the yolk–shell noble metal@MOF nanostructures. The present strategy to fabricate hollow and yolk–shell nanostructures is expected to open up exciting opportunities for developing a novel class of inorganic–organic hybrid functional nanomaterials.     

Metal–Organic Frameworks for Oxygen Storage

We present a systematic study of metal–organic frameworks (MOFs) for the storage of oxygen. The study starts with grand canonical Monte Carlo simulations on a suite of 10 000 MOFs for the adsorption of oxygen. From these data, the MOFs were down selected to the prime candidates of HKUST‐1 (Cu‐BTC) and NU‐125, both with coordinatively unsaturated Cu sites. Oxygen isotherms up to 30 bar were measured at multiple temperatures to determine the isosteric heat of adsorption for oxygen on each MOF by fitting to a Toth isotherm model. High pressure (up to 140 bar) oxygen isotherms were measured for HKUST‐1 and NU‐125 to determine the working capacity of each MOF. Compared to the zeolite NaX and Norit activated carbon, NU‐125 has an increased excess capacity for oxygen of 237 % and 98 %, respectively. These materials could ultimately prove useful for oxygen storage in medical, military, and aerospace applications.