From Macro- to Nanoscale: Finite Size Effects on Metal–Organic Framework Switchability

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.

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.

Metal-organic framework-based adsorbents for blood purification: progress, challenges, and prospects

Blood purification, such as hemodialysis (HD), plasma exchange (PE), and hemoperfusion (HP), is widely applied in patients with organ failure (such as kidney and liver failure). Among them, HP mainly relies on porous adsorbents to efficiently adsorb accumulated metabolic wastes and toxins, thus improving purification efficiency. Metal-organic frameworks (MOFs), with a high porosity, large surface area, high loading capacity, and tailorable topology, are emerging as some of the most promising materials for HP. Compared with non-metal framework counterparts, the self-built metal centers of MOFs feature the intrinsic advantages of coordination with toxin molecules. However, research on MOFs in blood purification is insufficient, particularly in contrast to materials applied in other biomedical applications. Thus, to broaden this area, this review first discusses the essential characteristics, potential mechanisms, and structure-function relationship between MOFs and toxin adsorption based on porosity, topology, ligand functionalization, metal centers, and toxin types. Moreover, the stability, utilization safety, and hemocompatibility of MOFs are illustrated for adsorbent selection. The current development and progress in MOF composites for HD, HP, and extracorporeal membrane oxygenation (ECMO) are also summarized to highlight their practicability. Finally, we propose future opportunities and challenges from materials design and manufacture to the computational prediction of MOFs in blood purification. It is anticipated that our review will expand the interest of researchers for more impact in this area.

Modulation of the Dynamics of a Two-Dimensional Interweaving Metal-Organic Framework through Induced Hydrogen Bonding

Inducing, understanding, and controlling the flexibility in metal-organic frameworks (MOFs) are of utmost interest due to the potential applications of dynamic materials in gas-related technologies. Herein, we report the synthesis of two isostructural two-dimensional (2D) interweaving zinc(II) MOFs, TMU-27 [Zn(bpipa)(bdc)] and TMU-27-NH2 [Zn(bpipa)(NH2-bdc)], based on N,N'-bis-4-pyridyl-isophthalamide (bpipa) and 1,4-benzenedicarboxylate (bdc) or 2-amino-1,4-benzenedicarboxylate (NH2-bdc), respectively. These frameworks differ only by the substitution at the meta-position of their respective bdc groups: an H atom in TMU-27 vs an NH2 group in TMU-27-NH2. This difference strongly influences their respective responses to external stimuli, since we observed that the structure of TMU-27 changed due to desolvation and adsorption, whereas TMU-27-NH2 remained rigid. Using single-crystal X-ray diffraction and CO2-sorption measurements, we discovered that upon CO2 sorption, TMU-27 undergoes a transition from a closed-pore phase to an open-pore phase. In contrast, we attributed the rigidification in TMU-27-NH2 to intermolecular hydrogen bonding between interweaving layers, namely, between the H atoms from the bdc-amino groups and the O atoms from the bpipa-amide groups within these layers. Additionally, by using scanning electron microscopy to monitor the CO2 adsorption and desorption in TMU-27, we were able to establish a correlation between the crystal size of this MOF and its transformation pressure.

Breathing Metal-Organic Frameworks Supported by an Arsenic-Bridged 4,4'-Bipyridine Ligand

Bent ligands bridged by heteroatoms have drawn significant interest as supramolecular coordination architectures. Traditionally, divalent group 16 elements are preferred over trivalent group 15 elements because of the anticipated steric hindrance. In this study, we explore metal-organic frameworks (MOFs) based on dipyridinoarsoles (DPAs), 4,4'-bipyridines bridged with an arsenic atom. An MOF with methyl-substituted DPA collapsed upon solvent removal, whereas that with phenyl-substituted DPA demonstrated breathing behavior due to guest molecule adsorption/desorption. In contrast, MOFs using the phosphorus analogue dipyridinophosphole exhibit inferior adsorption and lack breathing behavior. This is the first study to investigate the interplay among substituents, bridging elements, and dynamic behavior in MOFs using bent group 15 ligands.

Plastic Avalanches in Metal-Organic Framework Crystals Due to the Dynamic Phase Mixing

The compressive properties of metal-organic framework (MOF) crystals are not only crucial for their densification process but also key in determining their performance in many applications. We herein investigated the mechanical responses of a classic crystalline MOF, HKUST-1, using in situ compression tests. A serrated flow accompanied by the unique strain avalanches was found in individual and contacting crystals before their final flattening or fracture with splitting cracks. The plastic flow with serrations is ascribed to the dynamic phase mixing due to the progressive and irreversible local phase transition in HKUST-1 crystals, as revealed by molecular dynamics and finite element simulations. Such pressure-induced phase coexistence in HKUST-1 crystals also induces a significant loading-history dependence of their Young's modulus. The observation of plastic avalanches in HKUST-1 crystals here not only expands our current understanding of the plasticity of MOF crystals but also unveils a novel mechanism for the avalanches and plastic flow in crystal plasticity.

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 .

Fast Gas-Adsorption Kinetics in Supraparticle-Based MOF Packings with Hierarchical Porosity

Metal-organic frameworks (MOFs) are microporous adsorbents for high-throughput gas separation. Such materials exhibit distinct adsorption characteristics owing to the flexibility of the crystal framework in a nanoparticle, which can be different from its bulk crystal. However, for practical applications, such particles need to be compacted into macroscopic pellets, creating mass-transport limitations. In this work, this problem is addressed by forming materials with structural hierarchy, using a supraparticle-based approach. Spherical supraparticles composed of nanosized MOF particles are fabricated by emulsion templating and they are used as the structural component forming a macroscopic material. Zeolitic imidazolate framework-8 (ZIF-8) particles are used as a model system and the gas-adsorption kinetics of the hierarchical material are compared with conventional pellets without structural hierarchy. It is demonstrated that a pellet packed with supraparticles exhibits a 30 times faster adsorption rate compared to an unstructured ZIF-8 powder pellet. These results underline the importance of controlling structural hierarchy to maximize the performance of existing materials. In the hierarchical MOFs, large macropores between the supraparticles, smaller macropores between individual ZIF-8 primary particles, and micropores inherent to the ZIF-8 framework collude to combine large surface area, defined adsorption sites, and efficient mass transport to enhance performance.

Simulating the structural phase transitions of metal-organic frameworks with control over the volume of nanocrystallites

Flexible metal-organic frameworks (MOFs) can undergo structural transitions with significant pore volume changes upon guest adsorption or other external triggers while maintaining their porosity. In computational studies of this breathing behavior, molecular dynamics (MD) simulations within periodic boundary conditions (PBCs) are commonly performed. However, to account for the finite size and surface effects affecting the phase transition mechanism, the simulation of non-periodic nanocrystallite (NC) models without the constraint of PBCs is an important alternative. In this study, we present an approach allowing the analysis and control of the volume of finite-size structures during MD simulations by a tetrahedral tessellation of the (deformed) NC's volume. The method allows for defining the current NC's volume during the simulation and manipulating it regarding a particular reference volume to compute free energies for the phase transformation via umbrella sampling. The application on differently sized DMOF-1 and DUT-128 NCs reveals flexible pore closing mechanisms without significant biasing of the transition pathway. The concept provides the theoretical foundation for further research on flexible materials regarding targeted initialization of the structural phase behavior to elucidate the underlying mechanism, which can be used to improve the applications of flexible materials by targeted controlling of the phase transition.

Surface coverage dynamics for reversible dissociative adsorption on finite linear lattices

Dissociative adsorption onto a surface introduces dynamic correlations between neighboring sites not found in non-dissociative absorption. We study surface coverage dynamics where reversible dissociative adsorption of dimers occurs on a finite linear lattice. We derive analytic expressions for the equilibrium surface coverage as a function of the number of reactive sites, N, and the ratio of the adsorption and desorption rates. Using these results, we characterize the finite size effect on the equilibrium surface coverage. For comparable N's, the finite size effect is significantly larger when N is even than when N is odd. Moreover, as N increases, the size effect decays more slowly in the even case than in the odd case. The finite-size effect becomes significant when adsorption and desorption rates are considerably different. These finite-size effects are related to the number of accessible configurations in a finite system where the odd-even dependence arises from the limited number of accessible configurations in the even case. We confirm our analytical results with kinetic Monte Carlo simulations. We also analyze the surface-diffusion case where adsorbed atoms can hop into neighboring sites. As expected, the odd-even dependence disappears because more configurations are accessible in the even case due to surface diffusion.

Breathing porous liquids based on responsive metal-organic framework particles

Responsive metal-organic frameworks (MOFs) that display sigmoidal gas sorption isotherms triggered by discrete gas pressure-induced structural transformations are highly promising materials for energy related applications. However, their lack of transportability via continuous flow hinders their application in systems and designs that rely on liquid agents. We herein present examples of responsive liquid systems which exhibit a breathing behaviour and show step-shaped gas sorption isotherms, akin to the distinct oxygen saturation curve of haemoglobin in blood. Dispersions of flexible MOF nanocrystals in a size-excluded silicone oil form stable porous liquids exhibiting gated uptake for CO₂, propane and propylene, as characterized by sigmoidal gas sorption isotherms with distinct transition steps. In situ X-ray diffraction studies show that the sigmoidal gas sorption curve is caused by a narrow pore to large pore phase transformation of the flexible MOF nanocrystals, which respond to gas pressure despite being dispersed in silicone oil. Given the established flexible nature and tunability of a range of MOFs, these results herald the advent of breathing porous liquids whose sorption properties can be tuned rationally for a variety of technological applications.

Challenges in modelling dynamic processes in realistic nanostructured materials at operating conditions

The question is addressed in how far current modelling strategies are capable of modelling dynamic phenomena in realistic nanostructured materials at operating conditions. Nanostructured materials used in applications are far from perfect; they possess a broad range of heterogeneities in space and time extending over several orders of magnitude. Spatial heterogeneities from the subnanometre to the micrometre scale in crystal particles with a finite size and specific morphology, impact the material's dynamics. Furthermore, the material's functional behaviour is largely determined by the operating conditions. Currently, there exists a huge length-time scale gap between attainable theoretical length-time scales and experimentally relevant scales. Within this perspective, three key challenges are highlighted within the molecular modelling chain to bridge this length-time scale gap. Methods are needed that enable (i) building structural models for realistic crystal particles having mesoscale dimensions with isolated defects, correlated nanoregions, mesoporosity, internal and external surfaces; (ii) the evaluation of interatomic forces with quantum mechanical accuracy albeit at much lower computational cost than the currently used density functional theory methods and (iii) derivation of the kinetics of phenomena taking place in a multi-length-time scale window to obtain an overall view of the dynamics of the process. This article is part of a discussion meeting issue 'Supercomputing simulations of advanced materials'.

On the role of history-dependent adsorbate distribution and metastable states in switchable mesoporous metal-organic frameworks

A unique feature of metal-organic frameworks (MOFs) in contrast to rigid nanoporous materials is their structural switchabilty offering a wide range of functionality for sustainable energy storage, separation and sensing applications. This has initiated a series of experimental and theoretical studies predominantly aiming at understanding the thermodynamic conditions to transform and release gas, but the nature of sorption-induced switching transitions remains poorly understood. Here we report experimental evidence for fluid metastability and history-dependent states during sorption triggering the structural change of the framework and leading to the counterintuitive phenomenon of negative gas adsorption (NGA) in flexible MOFs. Preparation of two isoreticular MOFs differing by structural flexibility and performing direct in situ diffusion studies aided by in situ X-ray diffraction, scanning electron microscopy and computational modelling, allowed assessment of n-butane molecular dynamics, phase state, and the framework response to obtain a microscopic picture for each step of the sorption process.

One Atom Can Make All the Difference: Gas-Induced Phase Transformations in Bisimidazole-Linked Diamondoid Coordination Networks

Coordination networks (CNs) that undergo gas-induced transformation from closed (nonporous) to open (porous) structures are of potential utility in gas storage applications, but their development is hindered by limited control over their switching mechanisms and pressures. In this work, we report two CNs, [Co(bimpy)(bdc)]_n (X-dia-4-Co) and [Co(bimbz)(bdc)]_n (X-dia-5-Co) (H₂bdc = 1,4-benzendicarboxylic acid; bimpy = 2,5-bis(1H-imidazole-1-yl)pyridine; bimbz = 1,4-bis(1H-imidazole-1-yl)benzene), that both undergo transformation from closed to isostructural open phases involving at least a 27% increase in cell volume. Although X-dia-4-Co and X-dia-5-Co only differ from one another by one atom in their N-donor linkers (bimpy = pyridine, and bimbz = benzene), this results in different pore chemistry and switching mechanisms. Specifically, X-dia-4-Co exhibited a gradual phase transformation with a steady increase in the uptake when exposed to CO₂, whereas X-dia-5-Co exhibited a sharp step (type F-IV isotherm) at P/P₀ ≈ 0.008 or P ≈ 3 bar (195 or 298 K, respectively). Single-crystal X-ray diffraction, in situ powder XRD, in situ IR, and modeling (density functional theory calculations, and canonical Monte Carlo simulations) studies provide insights into the nature of the switching mechanisms and enable attribution of pronounced differences in sorption properties to the changed pore chemistry.

Breathing Behaviour Modification of Gallium MIL-53 Metal-Organic Frameworks Induced by the Bridging Framework Inorganic Anion

Controlling aspects of the μ2 -X- bridging anion in the metal-organic framework Ga-MIL-53 [GaX(bdc)] (X- =(OH)- or F- , bdc=1, 4-benzenedicarboxylate) is shown to direct the temperature at which thermally induced breathing transitions of this framework occur. In situ single crystal X-ray diffraction studies reveal that substituting 20 % of (OH)- in [Ga(OH)(bdc)] (1) for F- to produce [Ga(OH)0.8 F0.2 (bdc)] (2) stabilises the large pore (lp) form relative to the narrow pore (np) form, causing a well-defined decrease in the onset of the lp to np transition at higher temperatures, and the adsorption/desorption of nitrogen at lower temperatures through np to lp to intermediate (int) pore transitions. These in situ diffraction studies have also yielded a more plausible crystal structure of the int-[GaX(bdc)] ⋅ H2 O phases and shown that increasing the heating rate to a flash heating regime can enable the int-[GaX(bdc)] ⋅ H2 O to lp-[GaX(bdc)] transition to occur at a lower temperature than np-[GaX(bdc)] via an unreported pathway.

Particle size-dependent flexibility in DUT-8(Cu) pillared layer metal-organic framework

The nature of metal in the isomorphous flexible metal-organic frameworks is often reported to influence flexibility and responsivity. A prominent example of such behaviour is the DUT-8(M) family ([M₂(2,6-ndc)₂(dabco)]_n, 2,6-ndc = 2,6-naphthalene dicarboxylate, dabco = 1,4-diazabicyclo-[2.2.2]-octane), where the isostructural compounds with Ni, Zn, Co, and Cu in the paddle wheel cluster are known. The macro-sized crystals of Ni, Co, and Zn based compounds transform to the closed pore (cp) phase under desolvation and show typical gate opening behaviour upon adsorption. The choice of metal, in this case, allows the adjustment of switching kinetics, selectivity in adsorption, and gate-opening pressures. The submicron-sized crystals of of Ni, Co, and Zn based compounds remain in the open pore (op) phase after desolvation. In this contribution, we demonstrate that the presence of Cu in the paddle wheel leads to fundamentally different flexible behaviour. The DUT-8(Cu) desolvation does not lead to the formation of the cp phase, independent of the particle size regime. However, according to in situ powder diffraction analysis, the desolvated, macro-sized crystals of DUT-8(Cu)_op show breathing upon adsorption of CO₂ at 195 K. The submicron-sized particles show rigid, nonresponsive behaviour.

Spatiotemporal Design of the Metal-Organic Framework DUT-8(M)

Switchable metal-organic frameworks (MOFs) change their structure in time and selectively open their pores adsorbing guest molecules, leading to highly selective separation, pressure amplification, sensing, and actuation applications. The 3D engineering of MOFs has reached a high level of maturity, but spatiotemporal evolution opens a new perspective toward engineering materials in the 4th dimension (time) by t-axis design, in essence exploiting the deliberate tuning of activation barriers. This work demonstrates the first example in which an explicit temporal engineering of a switchable MOF (DUT-8, [M₁ M₂ (2,6-ndc)₂ dabco]_n , 2,6-ndc = 2,6-naphthalene dicarboxylate, dabco = 1,4diazabicyclo[2.2.2]octane, M₁  = Ni, M₂  = Co) is presented. The temporal response is deliberately tuned by variations in cobalt content. A spectrum of advanced analytical methods is presented for analyzing the switching kinetics stimulated by vapor adsorption using in situ time-resolved techniques ranging from ensemble adsorption and advanced synchrotron X-ray diffraction experiments to individual crystal analysis. A novel analysis technique based on microscopic observation of individual crystals in a microfluidic channel reveals the lowest limit for adsorption switching reported so far. Differences in the spatiotemporal response of crystal ensembles originate from an induction time that varies statistically and widens characteristically with increasing cobalt content reflecting increasing activation barriers.

Unraveling the timescale of the structural photo-response within oriented metal-organic framework films

Fundamental knowledge on the intrinsic timescale of structural transformations in photo-switchable metal-organic framework films is crucial to tune their switching performance and to facilitate their applicability as stimuli-responsive materials. In this work, for the first time, an integrated approach to study and quantify the temporal evolution of structural transformations is demonstrated on an epitaxially oriented DMOF-1-on-MOF film system comprising azobenzene in the DMOF-1 pores (DMOF-1/AB). We employed time-resolved Grazing Incidence Wide-Angle X-Ray Scattering measurements to track the structural response of the DMOF-1/AB film upon altering the length of the azobenzene molecule by photo-isomerization (trans-to-cis, 343 nm; cis-to-trans, 450 nm). Within seconds, the DMOF-1/AB response occurred fully reversible and over several switching cycles by cooperative photo-switching of the oriented DMOF-1/AB crystallites as confirmed further by infrared measurements. Our work thereby suggests a new avenue to elucidate the timescales and photo-switching characteristics in structurally responsive MOF film systems.

The importance of crystal size for breathing kinetics in MIL-53(Al)

Herein we analyze the switching kinetics of a breathing framework MIL-53(Al) with respect to different crystallite size regimes. Synchrotron time-resolved powder X-ray diffraction (PXRD) and adsorption rate analysis of n-butane physisorption at 298 K demonstrate the decisive role of crystal size affecting the time domain of breathing transitions in MIL-53(Al).

Isotope-selective pore opening in a flexible metal-organic framework

Flexible metal-organic frameworks that show reversible guest-induced phase transitions between closed and open pore phases have enormous potential for highly selective, energy-efficient gas separations. Here, we present the gate-opening process of DUT-8(Ni) that selectively responds to D₂, whereas no response is observed for H₂ and HD. In situ neutron diffraction directly reveals this pressure-dependent phase transition. Low-temperature thermal desorption spectroscopy measurements indicate an outstanding D₂-over-H₂ selectivity of 11.6 at 23.3 K, with high D₂ uptake. First-principles calculations coupled with statistical thermodynamics predict the isotope-selective gate opening, rationalized by pronounced nuclear quantum effects. Simulations suggest DUT-8(Ni) to remain closed in the presence of HT, while it also opens for DT and T₂, demonstrating gate opening as a highly effective approach for isotopolog separation.

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.

MOFs in the time domain

Many of the proposed applications of metal-organic framework (MOF) materials may fail to materialize if the community does not fully address the difficult fundamental work needed to map out the 'time gap' in the literature - that is, the lack of investigation into the time-dependent behaviours of MOFs as opposed to equilibrium or steady-state properties. Although there are a range of excellent investigations into MOF dynamics and time-dependent phenomena, these works represent only a tiny fraction of the vast number of MOF studies. This Review provides an overview of current research into the temporal evolution of MOF structures and properties by analysing the time-resolved experimental techniques that can be used to monitor such behaviours. We focus on innovative techniques, while also discussing older methods often used in other chemical systems. Four areas are examined: MOF formation, guest motion, electron motion and framework motion. In each area, we highlight the disparity between the relatively small amount of (published) research on key time-dependent phenomena and the enormous scope for acquiring the wider and deeper understanding that is essential for the future of the field.

Molecular Dynamics Simulations of the Breathing Phase Transition of MOF Nanocrystallites II: Explicitly Modeling the Pressure Medium

One of the most investigated properties of porous crystalline metal-organic frameworks (MOFs) is their potential flexibility to undergo large changes in unit cell size upon guest adsorption or other stimuli, referred to as "breathing". Computationally, such phase transitions are usually investigated using periodic boundary conditions, where the system's volume can be controlled directly. However, we have recently shown that important aspects like the formation of a moving interface between the open and the closed pore form or the free energy barrier of the first-order phase transition and its size effects can best be investigated using non-periodic nanocrystallite (NC) models [Keupp et al. (Adv. Theory Simul., 2019, 2, 1900117)]. In this case, the application of pressure is not straightforward, and a distance constraint was used to mimic a mechanical strain enforcing the reaction coordinate. In contrast to this prior work, a mediating particle bath is used here to exert an isotropic hydrostatic pressure on the MOF nanocrystallites. The approach is inspired by the mercury nanoporosimetry used to compress flexible MOF powders. For such a mediating medium, parameters are presented that require a reasonable additional numerical effort and avoid unwanted diffusion of bath particles into the MOF pores. As a proof-of-concept, NCs of pillared-layer MOFs with different linkers and sizes are studied concerning their response to external pressure exerted by the bath. By this approach, an isotropic pressure on the NC can be applied in analogy to corresponding periodic simulations, without any bias for a specific mechanism. This allows a more realistic investigation of the breathing phase transformation of a MOF NC and further bridges the gap between experiment and simulation.

Large-Scale Molecular Dynamics Simulations Reveal New Insights Into the Phase Transition Mechanisms in MIL-53(Al)

Soft porous crystals have the ability to undergo large structural transformations upon exposure to external stimuli while maintaining their long-range structural order, and the size of the crystal plays an important role in this flexible behavior. Computational modeling has the potential to unravel mechanistic details of these phase transitions, provided that the models are representative for experimental crystal sizes and allow for spatially disordered phenomena to occur. Here, we take a major step forward and enable simulations of metal-organic frameworks containing more than a million atoms. This is achieved by exploiting the massive parallelism of state-of-the-art GPUs using the OpenMM software package, for which we developed a new pressure control algorithm that allows for fully anisotropic unit cell fluctuations. As a proof of concept, we study the transition mechanism in MIL-53(Al) under various external pressures. In the lower pressure regime, a layer-by-layer mechanism is observed, while at higher pressures, the transition is initiated at discrete nucleation points and temporarily induces various domains in both the open and closed pore phases. The presented workflow opens the possibility to deduce transition mechanism diagrams for soft porous crystals in terms of the crystal size and the strength of the external stimulus.

MIL-53 Metal-Organic Framework as a Flexible Cathode for Lithium-Oxygen Batteries

Li-air batteries possess higher specific energies than the current Li-ion batteries. Major drawbacks of the air cathode include the sluggish kinetics of the oxygen reduction (OER), high overpotentials and pore clogging during discharge processes. Metal-Organic Frameworks (MOFs) appear as promising materials because of their high surface areas, tailorable pore sizes and catalytic centers. In this work, we propose to use, for the first time, aluminum terephthalate (well known as MIL-53) as a flexible air cathode for Li-O₂ batteries. This compound was synthetized through hydrothermal and microwave-assisted routes, leading to different particle sizes with different aspect ratios. The electrochemical properties of both materials seem to be equivalent. Several behaviors are observed depending on the initial value of the first discharge capacity. When the first discharge capacity is higher, no OER occurs, leading to a fast decrease in the capacity during cycling. The nature and the morphology of the discharge products are investigated using ex situ analysis (XRD, SEM and XPS). For both MIL-53 materials, lithium peroxide Li₂O₂ is found as the main discharge product. A morphological evolution of the Li₂O₂ particles occurs upon cycling (stacked thin plates, toroids or pseudo-spheres).

Unraveling the Guest-Induced Switchability in the Metal-Organic Framework DUT-13(Zn)*

The switching mechanism of the flexible framework Zn4 O(benztb)1.5 (benztb=N,N,N',N'-benzidine tetrabenzoate), also known as DUT-13, was studied by advanced powder X-ray diffraction (PXRD) and gas physisorption techniques. In situ synchrotron PXRD experiments upon physisorption of nitrogen (77 K) and n-butane (273 K) shed light on the hitherto unnoticed guest-induced breathing in the MOF. The mechanism of contraction is based on the conformationally labile benztb ligand and accompanied by a reduction in specific pore volume from 2.03 cm3  g-1 in the open-pore phase to 0.91 cm3  g-1 in the contracted-pore phase. The high temperature limit for adsorption-induced contraction of 170 K, determined by systematic temperature variation of methane adsorption isotherms, indicates that the DUT-13 framework is softer than other mesoporous MOFs like DUT-49 and does not support the formation of overloaded metastable states required for negative gas-adsorption transitions.

Tailoring metal-organic frameworks-based nanozymes for bacterial theranostics

Nanozymes are next-generation artificial enzymes having distinguished features such as cost-effective, enhanced surface area, and high stability. However, limited selectivity and moderate activity of nanozymes in the biochemical environment hindered their usage and encouraged researchers to seek alternative catalytic materials. Recently, metal-organic frameworks (MOFs) characterized by distinct crystalline porous structures with large surface area, tunable pores, and uniformly dispersed active sites emerged, that filled the gap between natural enzymes and nanozymes. Moreover, by selecting suitable metal ions and organic linkers, MOFs can be designed for effective bacterial theranostics. In this review, we briefly presented the design and fabrication of MOFs. Then, we demonstrated the applications of MOFs in bacterial theranostics and their safety considerations. Finally, we proposed the major obstacles and opportunities for further development in research on the interface of nanozymes and MOFs. We expect that MOFs based nanozymes with unique physicochemical and intrinsic enzyme-mimicking properties will gain broad interest in both fundamental research and biomedical applications.

Impact of Crystal Size and Morphology on Switchability Characteristics in Pillared-Layer Metal-Organic Framework DUT-8(Ni)

Variation of the crystallite size in flexible porous coordination polymers can significantly influence or even drastically change the flexibility characteristics. The impact of crystal morphology, however, on the dynamic properties of flexible metal-organic frameworks (MOFs) is poorly investigated so far. In the present work, we systematically modulated the particle size of a model gate pressure MOF (DUT-8(Ni), Ni2(2,6-ndc)2(dabco), 2,6-ndc-2,6-naphthalenedicarboxylate, dabco-1,4-diazabicyclo[2.2.2]octane) and investigated the influence of the aspect ratio, length, and width of anisotropically shaped crystals on the gate opening characteristics. DUT-8 is a member of the pillared-layer MOF family, showing reversible structural transition, i.e., upon nitrogen physisorption at 77 K. The framework crystalizes as rod-like shaped crystals in conventional synthesis. To understand which particular crystal surfaces dominate the phenomena observed, crystals similar in size and differing in morphology were involved in a systematic study. The analysis of the data shows that the width of the rods (corresponding to the crystallographic directions along the layer) represents a critical parameter governing the dynamic properties upon adsorption of nitrogen at 77 K. This observation is related to the anisotropy of the channel-like pore system and the nucleation mechanism of the solid-solid phase transition triggered by gas adsorption.