Crystalline coordination framework endowed with dynamic gate-opening behaviour by being downsized to a thin film

Layered Silicates Exhibiting MOF-Like Gate-Opening Behaviors in Liquid-Phase Adsorptions: Experimental and Theoretical Investigations

Layered silicates, including clay minerals, can be used as liquid-phase adsorbents in many important applications. However, because their two-dimensional interlayer space is narrow and not entirely opened due to the presence of interlayer species, guest species are forced to penetrate while expanding the interlayer space, which limits their adsorption performances compared with microporous materials such as MOFs and zeolites. Herein, as reported for the adsorption of gaseous species on flexible MOFs, we report a layered silicate that exhibits gate-opening adsorption in liquid phases. This layered silicate, synthesized via dilute acid treatment of the parent sodium-type, exhibits an abrupt increase in the basal spacing (layer thickness + interlayer space) to reach a plateau even at an earlier stage of benzoic acid adsorption from acetonitrile, whereas a typical layered silicate, magadiite, exhibits a gradual increase in the basal spacing as adsorption progress under identical conditions. The layered silicate shows an excellent adsorption capacity and rate for benzoic acid uptake from acetonitrile, which is considerably higher than that of magadiite. With comprehensive adsorption tests using different adsorbates and solvents, we propose that the layered silicate has zeolite-like but distorted, flexible open microchannels within each layer, and the intralayer microchannels can effectively and rapidly accommodate the solvent (acetonitrile) molecules, which are capable of expanding the framework to initiate the adsorption of aromatic compounds. The density function theory calculation revealed the adsorption mechanism, where the layered silicate accommodates acetonitrile in the intralayer microchannel followed by the interlayer space, and the former selectively plays a role as the adsorption site of aromatic compounds via exchange with acetonitrile.

Fe-MOFs nanosheets for photo-Fenton degradation of carbamazepine

Iron(II)-based metal organic framework (Fe(II)-MOF) nanosheets have emerged as promising candidates for photo-Fenton catalysis. However, efficiently synthesizing Fe(II)-MOF nanosheets remains a significant challenge. Here, a bottom-up synthesis strategy is proposed to prepare two-dimensional Fe-MOF nanosheets (TFMN) with micrometer lateral dimensions and nanometer thickness, featuring Fe(II) as the metal nodes. The application of TFMN in the photo-Fenton degradation of carbamazepine (CBZ) demonstrates remarkable CBZ degradation performance and excellent efficiency across a wide range of pH values. The electron density and density of states are further calculated by density functional theory. Mechanism analysis identifies h+, •OH and •O2- as the predominant active species contributing to the catalytic oxidation process in the Vis/TFMN/H2O2 system.

Layer-by-layer assembly of a [Fe-(pyrazine){Pd(CN)4}] spin crossover thin film

[Fe-(pyrazine){Pd(CN)4}] (pyrazine = pz) thin films were fabricated using a layer-by-layer assembly approach, a method known to be tunable, versatile, and scalable, since thin films are better-suited for industrial applications. In this study, [Fe-(pz){Pd(CN)4}] powder was synthesized, and the results obtained from a vibrating sample magnetometer verified the presence of an abrupt hysteresis loop with widths of 45 K centered around 300 K, indicating good cooperativity. Super conducting quantum interference device magnetometry results indicated a slow spin transition with temperature but with evidence of hysteresis for thin film samples. X-ray absorption analysis provided further support of the spin crossover behavior but differs from the magnetometry because the spin state transition at the surface differs from the bulk of the thin film. X-ray photoelectron spectroscopy provided some insight into issues with the film deposition process and multiplex fitting was used to further support the claim that the surface of the film is different than the bulk of the film.

Universal Strategy for Metal-Organic Framework Growth: From Cascading-Functional Films to MOF-on-MOFs

Transformation of metal-organic framework (MOF) particles into thin films is urgently needed for the persistent development of well-applicable devices, and recently emerging functional-integrated hybrid frameworks. Although some flexible polymers and exclusive modification approaches have been proposed, the additive-free and widely applicable strategy has not been reported, hampering the deep investigation of the structure-performance relationship. A universal strategy for the in situ growth of large-area and continuous MOF films with controllable microstructures is introduced, through the modification of multi-scale and multi-structure substrates with poly(4-vinylpyridine) as the anchor to capture metal ions via Coulomb attraction. Based on the clarified structure-adsorption-separation mechanisms, the customized devices fabricated by in situ growth can achieve highly selective adsorption and excellently synergetic separation of various industrially relevant isomers. In addition, this strategy is also feasible for the construction of MOF-on-MOFs with varied lattice parameters. This strategy is easy to implement and will be widely applicable to the surface growth of diverse MOFs on desired substrates, and provides a new concept for developing hybrid MOFs integrating with customized functionalities.

Guest-selective shape-memory effect in a switchable metal-organic framework DUT-8(Zn)

Crystal size engineering allows tailoring of flexible metal-organic frameworks (MOFs) to achieve new properties. The gating type flexibility of the DUT-8(Zn) ([Zn2(2,6-ndc)2(dabco)]n, 2,6-ndc = 2,6-naphthalene dicarboxylate, dabco = 1,4-diazabicyclo-[2.2.2]-octane) compound is known to be extremely particle size sensitive. Here, the physisorption of ethanol vapor gives rise to so-called shape-memory effect, leading to rigidification and flexibility suppression. According to powder X-ray diffraction and nitrogen physisorption experiments, the open pore phase is retained selectively after desorption of alcohols, which could be attributed to the nano-structuring and surface deformation of the crystals as a result of exposure to alcohols.

Solution-Processable Route for Large-Area Uniform 2D Semiconductor Nanofilms

The semiconductor thin film engineering technique plays a key role in the development of advanced electronics. Printing uniform nanofilms on freeform surfaces with high efficiency and low cost is significant for actual industrialization in electronics. Herein, a high-throughput colloidal printing (HTCP) strategy is reported for fabricating large-area and uniform semiconductor nanofilms on freeform surfaces. High-throughput and uniform printing rely on the balance of atomization and evaporation, as well as the introduced thermal Marangoni flows of colloidal dispersion, that suppresses outward capillary flows. Colloidal printing with in situ heating enables the fast fabrication of large-area semiconductor nanofilms on freeform surfaces, such as SiO2/Si, Al2O3, quartz glass, poly(ethylene terephthalate) (PET), Al foil, plastic tube, and Ni foam, expanding their technological applications where substrates are essential. The printed SnS2 nanofilms are integrated into thin-film semiconductor gas sensors with one of the fastest responses (8 s) while maintaining the highest sensitivity (Rg/Ra = 21) (toward 10 ppm NO2), as well as an ultralow limit of detection (LOD) of 46 ppt. The ability to print uniform semiconductor nanofilms on freeform surfaces with high-throughput promises the development of next-generation electronics with low cost and high efficiency.

Catalysis with Two-Dimensional Metal-Organic Frameworks: Synthesis, Characterization, and Modulation

The demand for the exploration of highly active and durable electro/photocatalysts for renewable energy conversion has experienced a significant surge in recent years. Metal-organic frameworks (MOFs), by virtue of their high porosity, large surface area, and modifiable metal centers and ligands, have gained tremendous attention and demonstrated promising prospects in electro/photocatalytic energy conversion. However, the small pore sizes and limited active sites of 3D bulk MOFs hinder their wide applications. Developing 2D MOFs with tailored thickness and large aspect ratio has emerged as an effective approach to meet these challenges, offering a high density of exposed active sites, better mechanical stability, better assembly flexibility, and shorter charge and photoexcited state transfer distances compared to 3D bulk MOFs. In this review, synthesis methods for the most up-to-date 2D MOFs are first overviewed, highlighting their respective advantages and disadvantages. Subsequently, a systematic analysis is conducted on the identification and electronic structure modulation of catalytic active sites in 2D MOFs and their applications in renewable energy conversion, including electrocatalysis and photocatalysis (electro/photocatalysis). Lastly, the current challenges and future development of 2D MOFs toward highly efficient and practical electro/photocatalysis are proposed.

Chemoselectivity Inversion of Responsive Metal-Organic Frameworks by Particle Size Tuning in the Micrometer Regime

Gated adsorption is one of the unique physical properties of flexible metal-organic frameworks with high application potential in selective adsorption and sensing of molecules. Despite recent studies that have provided some guidelines in understanding and designing structural flexibility for controlling gate opening by chemical modification of the secondary building units, currently, there is no established strategy to design a flexible MOF showing selective gated adsorption for a specific guest molecule. In a present contribution it is demonstrated for the first time, that the selectivity in the gate opening of a particular compound can be tuned, changed, and even reversed using particle size engineering DUT-8(Zn) ([Zn2(2,6-ndc)2(dabco)]n, 2,6-ndc = 2,6-naphthalenedicarboxylate, dabco = 1,4-diazabicyclo-[2.2.2]-octane, DUT = Dresden University of Technology) experiences phase transition from open (op) to closed (cp) pore phase upon removal of solvent from the pores. Microcrystals show selective reopening in the presence of dichloromethane (DCM) over alcohols. Crystal downsizing to micron size unexpectedly reverses the gate opening selectivity, causing DUT-8(Zn) to open its nanosized pores for alcohols but suppressing the responsivity toward DCM.

Effect of Polymorphism on the Sorption Properties of a Flexible Square-Lattice Topology Coordination Network

The stimulus-responsive behavior of coordination networks (CNs), which switch between closed (nonporous) and open (porous) phases, is of interest because of its potential utility in gas storage and separation. Herein, we report two polymorphs of a new square-lattice (sql) topology CN, X-sql-1-Cu, of formula [Cu(Imibz)2]n (HImibz = {[4-(1H-imidazol-1-yl)phenylimino]methyl}benzoic acid), isolated from the as-synthesized CN X-sql-1-Cu-(MeOH)2·2MeOH, which subsequently transformed to a narrow pore solvate, X-sql-1-Cu-A·MeOH, upon mild activation (drying in air or heating at 333 K under nitrogen). X-sql-1-Cu-A·MeOH contains MeOH in cavities, which was removed through exposure to vacuum for 2 h, yielding the nonporous (closed) phase X-sql-1-Cu-A. In contrast, a more dense polymorph, X-sql-1-Cu-B, was obtained by exposing X-sql-1-Cu-(MeOH)2·2MeOH directly to vacuum for 2 h. Gas sorption studies conducted on X-sql-1-Cu-A and X-sql-1-Cu-B revealed different switching behaviors to two open phases (X-sql-1-Cu·CO2 and X-sql-1-Cu·C2H2), with different gate-opening threshold pressures for CO2 at 195 K and C2H2 at 278 K. Coincident CO2 sorption and in situ powder X-ray diffraction studies at 195 K revealed that X-sql-1-Cu-A transformed to X-sql-1-Cu-B after the first sorption cycle and that the CO2-induced switching transformation was thereafter reversible. The results presented herein provide insights into the relationship between two polymorphs of a CN and the effect of polymorphism upon gas sorption properties. To the best of our knowledge, whereas sql networks such as X-sql-1-Cu are widely studied in terms of their structural and sorption properties, this study represents only the second example of an in-depth study of the sorption properties of polymorphic sql networks.

Spin crossover in {Fe(pyrazine)[M(CN)4]} (M = Ni, Pt) thin films assembled on fused silica substrates

Thin films with thicknesses in the range between ca. 10-50 nm of the spin crossover (SCO) compound {Fe(pyrazine)[μ4-M(CN)4]} (M = Ni, Pt) have been deposited on fused silica substrates using a sequential assembly method and 4-pyridinecarboxylic acid as anchoring layer. Film morphology and crystallinity were assessed by means of atomic force microscopy and grazing incidence X-ray diffraction, respectively. The intensity of the π-π* transition of the pyrazine ligand at 270 nm, being rather insensitive to the spin state of the complex, was used to follow the film growth as a function of different deposition parameters. On the other hand, the spin state changes were inferred from the temperature dependence of absorption bands appearing at 540, 490 and 310 nm in the low spin state. In line with their amorphous nature, each film displays a very gradual thermal spin crossover between ca. 100-300 K, independently of its thickness and deposition conditions. These results are not only interesting to better understand the effects of size reduction and organization on the SCO phenomenon, but the deposition of these SCO compounds on electrically insulating and/or optically transparent oxide surfaces opens also the door for various photonic or electronic applications.

Metal Organic Frameworks Synthesis: The Versatility of Triethylamine

Metal Organic Frameworks (MOFs) are organic-inorganic hybrid materials with exceptionally customizable composition and properties. MOFs intrinsically possess open metal sites, tunable pore size/shape and an ultra-large specific surface area, and have obtained significant attention over the past 30 years. Furthermore, through the integration of functional moieties such as, molecules, functional groups, noble metal clusters and nanocrystals or nanoparticles into MOFs, the resulting composites have greatly enriched the physical and chemical properties of pure MOFs, enabling their application in a wider range of fields. Triethylamine (TEA) as an organic base has consistently played a fundamental role in the development of MOFs. In this Concept, the versatility of triethylamine when involved in the synthesis of MOFs is discussed. Four sections are used to elaborate on the role of TEA including: (1) Single crystal synthesis; (2) Size and morphology control; (3) Counterion of MOFs; (4) MOFs composites synthesis. In the last part, we highlight the potential of TEA for further developments.

Interfacial Self-Assembly of Oriented Semiconductor Monolayer for Chemiresistive Sensing

Semiconductor nanofilm fabrication with advanced technology is of great importance for next-generation electronics/optoelectronics. Fabrication of high-quality and perfectly oriented semiconductor thin films and integration into high-performance electronic devices with low cost and high efficiency are huge challenges. Here we exquisitely utilized the Marangoni effect to perfectly guide tin disulfide (SnS2) nanocoins into an ordered assembly in milliseconds, resulting in an uniaxial-oriented monolayer semiconductor film. Further exploration revealed that the formed "crumple zone" at the interface caused by the Marangoni force endows the nanofilm with a rapid healable capability, which can be easily transferred to arbitrary substrates. As a proof of concept, the nanocoin-monolayer was transferred onto a micro-interdigitated electrode substrate to form a high-performance chemiresistive sensor that can effectively monitor the trace amounts of toxic gases. In addition, the assembled monolayer nanofilms can be conformally printed on freeform surfaces: both flat and nonflat substrates. This efficient and low-cost Marangoni force-assisted surface self-assembly (MFA-SSA) strategy is promising for advanced microelectronics and real industrial applications.

Hofmann-Type Cyanide Bridged Coordination Polymers for Advanced Functional Nanomaterials

Since the discovery of Hofmann clathrates of inorganic cyanide bridged coordination polymers (Hofmann-type CN-CPs), extensive research is done to understand their behavior during spin transitions caused by guest molecules or external stimuli. Lately, research on their nanoscale architectures for sensors and switching devices is of interest. Their potential is reported for producing advanced functional inorganic materials in two-dimensional (2D) morphology using a scalable solid-state thermal treatment method. For instance, but not restricted to, alloys, carbides, chalcogenides, oxides, etc. Simultaneously, their in situ crystallization at graphene oxide (GO) nanosheet surfaces, followed by a subsequent self-assembly to build layered lamellar structures, is reported providing hybrid materials with a variety of uses. Hence, an overview of the most recent developments is presented here in the synthesis of nanoscale structures, including thin films and powders, using Hofmann-type CN-CPs. Also thoroughly demonstrated are the most recent synthetic ideas with the modest control over the size and shape of nanoscale particles. Additionally, in order to create new functional hybrid materials for electrical and energy applications, their thermal decomposition in various environments and hybridization with GO and other guest molecules is examined. This review article also conveyed their spin transition, astounding innovative versatile adhesives, and structure features.

Anomalous Pressure Response of Temperature-Induced Spin Transition and a Pressure-Induced Spin Transition in Two-Dimensional Hofmann Coordination Polymers

Spin transition (ST) compounds have been extensively studied because of the changes in rich physicochemical properties accompanying the ST process. The study of ST mainly focuses on the temperature-induced spin transition (TIST). To further understand the ST, we explore the pressure response behavior of TIST and pressure-induced spin transition (PIST) of the 2D Hofmann-type ST compounds [Fe(Isoq)2M(CN)4] (Isoq-M) (M = Pt, Pd, Isoq = isoquinoline). The TISTs of both Isoq-Pt and Isoq-Pd compounds exhibit anomalous pressure response, where the transition temperature (T1/2) exhibits a nonlinear pressure dependence and the hysteresis width (ΔT1/2) exhibits a nonmonotonic behavior with pressure, by the synergistic influence of the intermolecular interaction and the distortion of the octahedral coordination environment. And the distortion of the octahedra under critical pressures may be the common behavior of 2D Hofmann-type ST compounds. Moreover, ΔT1/2 is increased compared with that before compression because of the partial irreversibility of structural distortion after decompression. At room temperature, both compounds exhibit completely reversible PIST. Because of the greater change in mechanical properties before and after ST, Isoq-Pt exhibits a more abrupt ST than Isoq-Pd. In addition, it is found that the hydrostatic properties of the pressure transfer medium (PTM) significantly affect the PIST due to their influence on spin-domain formation.

Recent Progress in 2D Metal-Organic Framework-Related Materials

2D metal-organic frameworks-based (2D MOF-related) materials benefit from variable topological structures, plentiful open active sites, and high specific surface areas, demonstrating promising applications in gas storage, adsorption and separation, energy conversion, and other domains. In recent years, researchers have innovatively designed multiple strategies to avoid the adverse effects of conventional methods on the synthesis of high-quality 2D MOFs. This review focuses on the latest advances in creative synthesis techniques for 2D MOF-related materials from both the top-down and bottom-up perspectives. Subsequently, the strategies are categorized and summarized for synthesizing 2D MOF-related composites and their derivatives. Finally, the current challenges are highlighted faced by 2D MOF-related materials and some targeted recommendations are put forward to inspire researchers to investigate more effective synthesis methods.

Shape-Memory Effect Enabled by Ligand Substitution and CO2 Affinity in a Flexible SIFSIX Coordination Network

We report that linker ligand substitution involving just one atom induces a shape-memory effect in a flexible coordination network. Specifically, whereas SIFSIX-23-Cu, [Cu(SiF6 )(L)2 ]n , (L=1,4-bis(1-imidazolyl)benzene, SiF6 2- =SIFSIX) has been previously reported to exhibit reversible switching between closed and open phases, the activated phase of SIFSIX-23-CuN , [Cu(SiF6 )(LN )2 ]n (LN =2,5-bis(1-imidazolyl)pyridine), transformed to a kinetically stable porous phase with strong affinity for CO2 . As-synthesized SIFSIX-23-CuN , α, transformed to less open, γ, and closed, β, phases during activation. β did not adsorb N2 (77 K), rather it reverted to α induced by CO2 at 195, 273 and 298 K. CO2 desorption resulted in α', a shape-memory phase which subsequently exhibited type-I isotherms for N2 (77 K) and CO2 as well as strong performance for separation of CO2 /N2 (15/85) at 298 K and 1 bar driven by strong binding (Qst =45-51 kJ/mol) and excellent CO2 /N2 selectivity (up to 700). Interestingly, α' reverted to β after re-solvation/desolvation. Molecular simulations and density functional theory (DFT) calculations provide insight into the properties of SIFSIX-23-CuN .

Growth mechanisms and anisotropic softness-dependent conductivity of orientation-controllable metal-organic framework nanofilms

Conductive metal-organic frameworks (cMOFs) manifest great potential in modern electrical devices due to their porous nature and the ability to conduct charges in a regular network. cMOFs applied in electrical devices normally hybridize with other materials, especially a substrate. Therefore, the precise control of the interface between cMOF and a substrate is particularly crucial. However, the unexplored interface chemistry of cMOFs makes the controlled synthesis and advanced characterization of high-quality thin films, particularly challenging. Herein, we report the development of a simplified synthesis method to grow "face-on" and "edge-on" cMOF nanofilms on substrates, and the establishment of operando characterization methodology using atomic force microscopy and X-ray, thereby demonstrating the relationship between the soft structure of surface-mounted oriented networks and their characteristic conductive functions. As a result, crystallinity of cMOF nanofilms with a thickness down to a few nanometers is obtained, the possible growth mechanisms are proposed, and the interesting anisotropic softness-dependent conducting properties (over 2 orders of magnitude change) of the cMOF are also illustrated.

Nanosized Silk-Magnesium Complexes for Tissue Regeneration

Metal ions provide multifunctional signals for cell and tissue functions, including regeneration. Inspired by metal-organic frameworks (MOFs), nanosized silk protein aggregates with a high negative charge density are used to form stable silk-magnesium ion complexes. Magnesium ions (Mg ions) are added directly to silk nanoparticle solutions, inducing gelation through the formation of silk-Mg coordination complexes. The Mg ions are released slowly from the nanoparticles through diffusion, with sustained release via tuning the degradation or dissolution of the nanosized silk aggregates. Studies in vitro reveal a dose-dependent influence of Mg ions on angiogenic and anti-inflammatory functions. Silk-Mg ion complexes in the form of hydrogels also stimulate tissue regeneration with a reduced formation of scar tissue in vivo, suggesting potential utility in tissue regeneration.

Advanced stimuli-responsive membranes for smart separation

Membranes have been extensively studied and applied in various fields owing to their high energy efficiency and small environmental impact. Further conferring membranes with stimuli responsiveness can allow them to dynamically tune their pore structure and/or surface properties for efficient separation performance. This review summarizes and discusses important developments and achievements in stimuli-responsive membranes. The most commonly utilized stimuli, including light, pH, temperature, ions, and electric and magnetic fields, are discussed in detail. Special attention is given to stimuli-responsive control of membrane pore structure (pore size and porosity/connectivity) and surface properties (wettability, surface topology, and surface charge), from the perspective of determining the appropriate membrane properties and microstructures. This review also focuses on strategies to prepare stimuli-responsive membranes, including blending, casting, polymerization, self-assembly, and electrospinning. Smart applications for separations are also reviewed as well as a discussion of remaining challenges and future prospects in this exciting field. This review offers critical insights for the membrane and broader materials science communities regarding the on-demand and dynamic control of membrane structures and properties. We hope that this review will inspire the design of novel stimuli-responsive membranes to promote sustainable development and make progress toward commercialization.

Reversible transformations between the non-porous phases of a flexible coordination network enabled by transient porosity

Flexible metal-organic materials that exhibit stimulus-responsive switching between closed (non-porous) and open (porous) structures induced by gas molecules are of potential utility in gas storage and separation. Such behaviour is currently limited to a few dozen physisorbents that typically switch through a breathing mechanism requiring structural contortions. Here we show a clathrate (non-porous) coordination network that undergoes gas-induced switching between multiple non-porous phases through transient porosity, which involves the diffusion of guests between discrete voids through intra-network distortions. This material is synthesized as a clathrate phase with solvent-filled cavities; evacuation affords a single-crystal to single-crystal transformation to a phase with smaller cavities. At 298 K, carbon dioxide, acetylene, ethylene and ethane induce reversible switching between guest-free and gas-loaded clathrate phases. For carbon dioxide and acetylene at cryogenic temperatures, phases showing progressively higher loadings were observed and characterized using in situ X-ray diffraction, and the mechanism of diffusion was computationally elucidated.

In Silico High-Throughput Design and Prediction of Structural and Electronic Properties of Low-Dimensional Metal-Organic Frameworks

The advent of π-stacked layered metal-organic frameworks (MOFs), which offer electrical conductivity on top of permanent porosity and high surface area, opened up new horizons for designing compact MOF-based devices such as battery electrodes, supercapacitors, and spintronics. Permutation of structural building blocks, including metal nodes and organic linkers, in these electrically conductive (EC) materials, results in new systems with unprecedented and unexplored physical and chemical properties. With the ultimate goal of providing a platform for accelerated material design and discovery, here we lay the foundations for the creation of the first comprehensive database of EC-MOFs with an experimentally guided approach. The first phase of this database, coined EC-MOF/Phase-I, is composed of 1,057 bulk and monolayer structures built by all possible combinations of experimentally reported organic linkers, functional groups, and metal nodes. A high-throughput screening (HTS) workflow is constructed to implement density functional theory calculations with periodic boundary conditions to optimize the structures and calculate some of their most relevant properties. Because research and development in the area of EC-MOFs has long been suffering from the lack of appropriate initial crystal structures, all of the geometries and property data have been made available for the use of the community through an online platform that was developed during the course of this work. This database provides comprehensive physical and chemical data of EC-MOFs as well as the convenience of selecting appropriate materials for specific applications, thus accelerating the design and discovery of EC-MOF-based compact devices.

Mechanical and Thermal ON-OFF Switching of the Vapochromic Behavior of a Luminescent Polymorphic Pt(II) Complex

Vapochromic materials that exhibit color/luminescence changes induced by vapor exposure have attracted considerable attention. Herein, we report the grinding- and heating-induced ON-OFF switching of the vapochromic behavior of [Pt(ppyCl₂)(Clacac)] (1; ppyCl₂ = 2-(3-chlorophenyl)-4-chloropyridinato, Clacac = 3-chloroacetylacetonato). 1 formed yellow and orange polymorphs (1-Y and 1-O), and 1-Y could be converted to 1-Og, which showed a very similar crystal structure but with a broadened X-ray diffraction pattern compared with that of 1-O. Moreover, 1-Og can be reversibly transformed into 1-O via heating and grinding. Notably, 1-Og underwent a N,N-dimethylacetamide vapor-induced transformation to 1-Y, whereas 1-O did not undergo such a transformation. These results indicate the ON-OFF switching of vapochromic behavior induced via grinding and heating. This finding will be beneficial for developing intelligent molecular devices.

Ultrathin Metal-Organic Framework Nanosheets Exhibiting Exceptional Catalytic Activity

Two-dimensional (2D) metal-organic framework nanosheets (MONs) or membranes are classes of periodic, crystalline polymeric materials that may show unprecedented physicochemical properties due to their modular structures, high surface areas, and high aspect ratios. Yet preparing 2D MONs from multiple components and two different types of polymerization reaction remains challenging and less explored. Here, we report the synthesis of MOF films via interfacial polymerization, which involves three active monomers for simultaneous polycondensation and polycoordination taking place in a confined interface. The well-defined lamellar structure of the MOF films allowed feasible and scalable exfoliation to produce free-standing 2D MONs with high aspect ratio up to 2000:1 and ultrathin thickness (∼1.7 nm). The pore structure was revealed by high-resolution TEM images with near-atomic precision. The imide-linkage of MONs provided superior thermal (up to 530 °C) and good chemical stability in the pH range from 3 to 12. More importantly, the MONs exhibited exceptional catalytic activity and superior reusability for the hydroboration reactions of alkynes, in which the turnover frequency (TOF) reached 41734 h^-1, which is 2-4 orders of magnitude greater than that reported for homogeneous and heterogeneous catalysts.

Emerging Synthesis Strategies of 2D MOFs for Electrical Devices and Integrated Circuits

The development of advanced electronic devices is boosting many aspects of modern technology and industry. The ever-increasing demand for advanced electrical devices and integrated circuits calls for the design of novel materials, with superior properties for the improvement of working performance. In this review, a detailed overview of the synthesis strategies of 2D metal organic frameworks (MOFs) acquiring growing attention is presented, as a basis for expansion of novel key materials in electrical devices and integrated circuits. A framework of controllable synthesis routes to be implanted in the synthesis strategies of 2D materials and MOFs is described. In short, the synthesis methods of 2D MOFs are summarized and discussed in depth followed by the illustrations of promising applications relating to various electrical devices and integrated circuits. It is concluded by outlining how 2D MOFs can be synthesized in a simpler, highly efficient, low-cost, and more environmentally friendly way which can open up their applicable opportunities as key materials in advanced electrical devices and integrated circuits, enabling their use in broad aspects of the society.

Thermodynamic Separation of Hydrogen Isotopes Using Hofmann-Type Metal-Organic Frameworks with High-Density Open Metal Sites

Hydrogen isotope separation with nanoporous materials is a very challenging yet promising approach. To overcome the limitation of the conventional isotope separation strategy, quantum sieving-based separation using nanoporous materials has been investigated recently. In this study, to see the thermodynamic deuterium separation phenomena attributed to the chemical affinity quantum sieving effect, we examine Hofmann-type metal-organic frameworks (MOFs), Co(pyz)[M(CN)₄] (pyz = pyrazine, M = Pd²⁺, Pt²⁺, and Ni²⁺), which have microporosity (4.0 × 3.9 Ų) and an extraordinarily high density of open metal sites (∼9 mmol/cm³). Owing to the preferential adsorption of D₂ over H₂ at strongly binding open metal sites, the Hofmann-type MOF, Co(pyz)[Pd(CN)₄] exhibited a high selectivity (S_D2/H2) of 21.7 as well as a large D₂ uptake of 10 mmol/g at 25 K. This is the first study of Hofmann-type MOFs to report high selectivity and capacity, both of which are important parameters for the practical application of porous materials toward isotope separation.

Spin-State Modulation in FeII -Based Hofmann-Type Coordination Polymers: From Molecules to Materials

Spin crossover complexes that reversibly interconvert between two stable states imitate a binary state of 0 and 1, delivering a promising possibility to address the data processing concept in smart materials. Thus, a comprehensive understanding of the modulation of magnetic transition between high spin and low spin and the factors responsible for stabilizing the spin states is an essential theme in modern materials design. In this context, the present review attempts to provide a concise outline of the design strategy employed at the molecular level for fine-tuning the spin-state switching in Fe^II -based Hofmann-type coordination polymers and their effects on the optical and magnetic response. In addition, development towards the nanoscale architectures of HCPs, i. e., in terms of nanoparticles and thin films, are emphasized to bridge the gap between the laboratory and reality.

A spin-crossover framework endowed with pore-adjustable behavior by slow structural dynamics

Host-guest interactions play critical roles in achieving switchable structures and functionalities in porous materials, but design and control remain challenging. Here, we report a two-dimensional porous magnetic compound, [Fe^II(prentrz)₂Pd^II(CN)₄] (prentrz = (1E,2E)−3-phenyl-N-(4H-1,2,4-triazol-4-yl)prop-2-en-1-imine), which exhibits an atypical pore transformation that directly entangles with a spin state transition in response to water adsorption. In this material, the adsorption-induced, non-uniform pedal motion of the axial prentrz ligands and the crumpling/unfolding of the layer structure actuate a reversible narrow quasi-discrete pore (nqp) to large channel-type pore (lcp) change that leads to a pore rearrangement associated with simultaneous pore opening and closing. The unusual pore transformation results in programmable adsorption in which the lcp structure type must be achieved first by the long-time exposure of the nqp structure type in a steam-saturated atmosphere to accomplish the gate-opening adsorption. The structural transformation is accompanied by a variation in the spin-crossover (SCO) property of Fe^II, i.e., two-step SCO with a large plateau for the lcp phase and two-step SCO with no plateau for the nqp phase. The unusual adsorption-induced pore rearrangement and the related SCO property offer a way to design and control the pore structure and physical properties of dynamic frameworks.

Host-guest interactions can play a critical role in achieving switchable porous materials, but controlling them remains challenging. Here the authors report an atypical pore rearrangement in a magnetic 2D porous framework upon water adsorption; the structural transformation affects the magnetic properties of the material.

Old Materials for New Functions: Recent Progress on Metal Cyanide Based Porous Materials

Cyanide is the simplest ligand with strong basicity to construct open frameworks including some of the oldest compounds reported in the history of coordination chemistry. Cyanide can form numerous cyanometallates with different transition metal ions showing diverse geometries. Rational design of robust extended networks is enabled by the strong bonding nature and high directionality of cyanide ligand. By virtue of a combination of cyanometallates and/or organic linkers, multifunctional framework materials can be targeted and readily synthesized for various applications, ranging from molecular adsorptions/separations to energy conversion and storage, and spin-crossover materials. External guest- and stimuli-responsive behaviors in cyanide-based materials are also highlighted for the development of the next-generation smart materials. In this review, an overview of the recent progress of cyanide-based multifunctional materials is presented to demonstrate the great potential of cyanide ligands in the development of modern coordination chemistry and material science.

Emerging Two-Dimensional Covalent and Coordination Polymers for Stable Lithium Metal Batteries: From Liquid to Solid

Lithium metal anodes (LMAs) have attracted much attention in recent years because of their high theoretical capacity (3860 mAh g^-1) and low electrochemical potential (-3.040 V vs standard hydrogen electrode). Lithium metal can be coupled with various cathodes to construct high-energy-density lithium metal batteries (LMBs) which hold great promise for next-generation batteries. However, the unstable solid electrolyte interphases (SEIs) and the uncontrollable lithium dendrite growth severely hinder the commercial development of LMAs. The emerging 2D polymers (2DPs), which possess high mechanical flexibility, high specific surface area, abundant surface chemistry, and rich chemical modification characteristics, have shown great advantages in addressing the inherent issues of LMAs. Herein, the current progress of 2DPs for stable and dendrite-free LMAs in liquid- and solid-based batteries is comprehensively reviewed. Some perspectives for the application of 2DPs in LMBs are also discussed. It is believed that the emerging 2DPs will provide insights into developing high-energy-density LMBs and beyond.

Flexibility Control of Two-Dimensional Coordination Polymers by Crystal Morphology: Water Adsorption and Thermal Expansion

Layer flexibility in two-dimensional coordination polymers (2D-CPs) contributes to several functional materials as it results in anisotropic structural response to external stimuli. Chemical modification is a common technique for modifying layer structures. This study demonstrates that crystal morphology of a cyanide-bridged 2D-CP of type [Mn(salen)]₂ [ReN(CN)₄ ] (1) consisting of flexible undulating layers significantly impacts the layer configuration and assembly. Nanoplates of 1 showed an in-plane contraction of layers with a longer interlayer distance compared to the micrometer-sized rod-type particles. These effects by crystal morphology on the structure of the 2D-CP impacted the structural flexibility, resulting in dual-functional changes: the enhancement of the sensitivity of structural transformation to water adsorption and modification of anisotropic thermal expansion of 1. Moreover, the nanoplates incorporated new adsorption sites within the layers, resulting in the uptake of an additional water molecule compared to the micrometer-sized rods.

Perspectives on the Influence of Crystal Size and Morphology on the Properties of Porous Framework Materials

Miniaturization is a key aspect of materials science. Owing to the increase in quality experimental and computational tools available to researchers, it has become clear that the crystal size and morphology of porous framework materials, including metal-organic frameworks and covalent organic frameworks, play a vital role in defining the physicochemical behaviour of these materials. However, given the multiscale and multidisciplinary challenges associated with establishing how crystal size and morphology affect the structure and behaviour of a material–from local to global structural modifications and from static to dynamic effects–a comprehensive mechanistic understanding of size and morphology effects is missing. Herein, we provide our perspective on the current state-of-the-art of this topic, drawn from various complementary disciplines. From a fundamental point of view, we discuss how controlling the crystal size and morphology can alter the mechanical and adsorption properties of porous framework materials and how this can impact phase stability. Special attention is also given to the quest to develop new computational tools capable of modelling these multiscale effects. From a more applied point of view, given the recent progress in this research field, we highlight the importance of crystal size and morphology control in drug delivery. Moreover, we provide an outlook on how to advance each discussed field by size and morphology control, which would open new design opportunities for functional porous framework materials.

Layer-by-Layer Growth of Preferred-Oriented MOF Thin Film on Nanowire Array for High-Performance Chemiresistive Sensing

High-quality MOF thin films with high orientation and controlled thickness are extremely desired for applications. However, they have been only successfully fabricated on flat substrates. Those MOF 2D thin films are limited by low exposed area and slow mass transport. To overcome these issues, MOF 3D thin films with good crystallinity, preferred orientation, and precisely controllable thickness in nanoscale were successfully prepared in a controllable layer-by-layer manner on nanowire array substrate for the first time. The as-prepared Cu-HHTP 3D thin film is superior to corresponding 2D thin films and showed one of the highest sensitivity, lowest LOD, and fastest response among all reported chemiresistive NH₃ sensing materials at RT. This work provides a feasible approach to grow preferred-oriented 3D MOF thin film, offering new perspectives for constructing MOF-based heterostructures for advanced applications.

Fabrication of Ultrathin Membranes Using 2D-MOF Nanosheets for Tunable Gas Separation

Two-dimensional (2D) metal-organic frameworks (MOF) nanosheets have emerged as novel membrane materials for gas separation. However, the development of ultrathin MOF membranes with tunable separation performances is still a challenge. Herein, we developed a facile GO-assisted restacking method to fabricate defect-free membranes with monolayer Zr-BTB nanosheets. Obtained ultrathin membranes ranging from 130 nm to 320 nm show tunable separation performances and exceed the 2008 Robeson upper bound by changing the amount of nanolayers in vertical stacking direction. Furthermore, a heating filtration method was used to change the restacking process of nanosheets in the horizontal direction. As a result, H₂ /CO₂ selectivity can be enhanced by two times with the same membrane thickness (130 nm) and H₂ permeance is almost maintained to be 7.0×10^-7  mol m^-2  s^-1  pa^-1 . This method may provide a possible way to efficiently tune the gas separation performances of MOF membranes.

Two-dimensional MOFs: Design & Synthesis and Applications

For the past few years, two-dimensional materials have attracted widespread attention owing to their special properties and potential applications. It is well-known that graphene, transition metal disulfide compounds (TMDC), carbon nitride, transition metal carbonitrides (Mxenes), silene and hexagonal boron nitride are typical two-dimensional materials. Compared with these traditional two-dimensional materials, two-dimensional MOF is favored by numerous researchers because of its unique structure. Based on the unique metal ion and organic ligand coordination of MOF and two-dimensional layered structure, the applications of two-dimensional MOF were getting serious, including catalysis, supercapacitor, gas adsorption/separation, sensors and so on. This review presents a relatively comprehensive summary of the design & synthesis and applications of two-dimensional MOF over the past few years. Furthermore, the opportunities and challenges have been discussed to supply a promising prospect to this field.

Concluding remarks: current and next generation MOFs

This paper describes the content of my "Concluding remarks" talk at the Faraday Discussion meeting on "MOFs for energy and the environment" (online, 23-25 June 2021). The panel consisted of sessions on the design of MOFs and MOF hybrids (synthetic chemistry), their applications (e.g., capture, storage, separation, electrical devices, photocatalysis), advanced characterization (e.g., transmission electron microscopy, solid-state nuclear magnetic resonance), theory and modeling, and commercialization. MOF chemistry is undergoing a significant evolution from simply network chemistry to the chemistry of synergistic integration with heterogeneous materials involving other disciplines (we call this the fourth generation type). As reflected in the papers of the invited speakers and discussions with the participants, the present and future of this field will be described in detail.

Strain-Controlled Spin Transition in Heterostructured Metal-Organic Framework Thin Film

Metal-organic framework (MOF) thin films have recently attracted much attention as a new platform for surface/interface research, where unconventional structural and physical properties emerge. Among the many MOFs as candidates for fabrication of thin films, Hofmann-type MOFs {Fe(pz)[M(CN)₄]} [pz = pyrazine; M = Ni (Nipz), M = Pt (Ptpz)] are attractive, because they undergo spin transitions with concomitant structural changes. Here, we demonstrate the first example of a strain-controlled spin transition in heterostructured MOF thin films. The spin transition temperature of Ptpz can be controlled in the temperature range of 300-380 K by fabricating a nanometer-sized heterostructured thin film with a Nipz buffer layer, where the smaller lattice of Nipz causes epitaxial compressive strain to the Ptpz layer. The fabricated heterostructured thin film exhibited a remarkable increase in spin transition temperature with a dynamic structural transformation, confirmed by variable-temperature (VT) X-ray diffraction and VT Raman spectroscopy. By verifying interfacial strain in a heterostructured thin film, we can rationally control the characteristics of MOFs-not only spin transition but also various physical properties such as gas storage, catalysis, sensing, proton conductivity, and electrical properties, among others.

Organic guest molecule induced ultrafast breathing of an epitaxially grown metal-organic framework on a self-assembled monolayer

We report epitaxially grown new two-dimensional metal-organic framework (MOF) thin films on a self-assembled monolayer (SAM). We fabricated these epitaxial thin-films using stepwise layer-by-layer seeding followed by solvothermal treatment. The MOF thin films exhibit ultrafast structural flexibility (through breathing) compared to their bulk samples upon uptake of organic guest molecules.

A Covalent Organic-Inorganic Hybrid Superlattice Covered with Organic Functional Groups for Highly Sensitive and Selective Gas Sensing

Organic-inorganic hybrid superlattices (OIHSLs) hold attractive physical and chemical properties, while the construction of single-crystal covalent OIHSLs has not been achieved. Herein a coordination assembly strategy was proposed to create a single-crystal covalent OIHSL PbBDT (BDT=1,4-benzenedithiolate), where layered [PbS₂ ] sublattice covalently connects with benzene sublattice. The covalent bonding offers better thermo-/chemi-stability, inter-sublattice electron transport, and unique organic-group-functionalized surface, which may enable better performances in chemical applications than non-covalent OIHSL. These features endow PbBDT with the highest sensitivity, the lowest detection limit and excellent selectivity towards NO₂ at room temperature among all chemiresistive gas-sensing materials with reported response time less than 2 min without the need of light assistance.