Open-framework Materials Research Articles

Multivariate Flexible Metal-Organic Frameworks and Covalent Organic Frameworks.

Precise control of the void environment, achieved through multiple functional groups and enhanced by structural adaptations to guest molecules, stands at the forefront of scientific inquiry. Flexible multivariate open framework materials (OFMs), including covalent organic frameworks and metal-organic frameworks, meet these criteria and are expected to play a crucial role in gas storage and separation, pollutant removal, and catalysis. Nevertheless, there is a notable lack of critical evaluation of achievements in their chemistry and future prospects for their development or implementation. To provide a comprehensive historical context, the initial discussion explores into the realm of "classical" flexible OFMs, where their origin, various modes of flexibility, similarities to proteins, advanced tuning methods, and recent applications are explored. Subsequently, multivariate flexible materials, the methodologies involved in their synthesis, and horizons of their application are focussed. Furthermore, the reader to the concept of spatial distribution is introduced, providing a brief overview of the latest reports that have contributed to its elucidation. In summary, the critical review not only explores the landscape of multivariate flexible materials but also sheds light on the obstacles that the scientific community must overcome to fully unlock the potential of this fascinating field.

Two Energetic Framework Materials Based on DNM-TNBI as Host Molecule: Effectively Coordinated by Different Cations.

With the demand of develop outstanding-performance energetic materials, 1-(dinitromethyl)-4,4',5,5'-tetranitro-1H,1'H-2,2'-biimidazole (DNM-TNBI) emerged as a great contender (D: 9102 m ⋅ s-1; P: 37.6 GPa). However, the relatively poor thermal stability (Td: 142 °C) limits its practical application. In this study, DNM-TNBI as a host molecule to synthesize two new energetic open-framework materials by effectively coordinated with different cations. Their supramolecular structures were investigated and indicated that [DNM-TNBI2 -][2NH4 +] and [DNM-TNBI2 -][2K+] can be classified as a new energetic hydrogen-bonded ammonium framework (EHAF) and an energetic metal organic framework (EMOF). Meanwhile, their thermal stabilities are higher than that of DNM-TNBI and have satisfactory detonation performance ([DNM-TNBI2 -][2NH4 +], D: 8050 m ⋅ s-1, P: 26.4 GPa; [DNM-TNBI2 -][2K+], D: 8301 m ⋅ s-1, P: 30.8 GPa).

Hydrogen-bond transformations of confined water and abnormal mechanical responses of hydrated AlPO4-11 framework under in situ high pressure

The behaviors of water under non-ambient conditions, especially in confinement environment, have long been of special interest due to the important roles in biological and geological processes. Here, we have performed an infrared spectroscopic investigation on the pressure-induced hydrogen-bond transformation of water in one-dimensional, elliptical channels of AlPO4-11. Upon compression, the hydrogen-bonds of confined water strengthen and water molecules transform from tetrahedral coordination configuration to low and intermediate configurations, which may be a high-density disordered state. This transformation is distinct from the commonly accepted liquid–solid and solid–solid phase transition in bulk water. Stronger hydrogen-bonds are formed and a further transformation of hydrogen-bonded water takes place in the hydrostatic environment of Ar than the case without pressure transmitting medium (PTM). In addition, the mechanical compressible behaviors and framework deformation of the host matrix are also discussed. X-ray diffraction study shows that the hydrated AlPO4-11 exhibits an unexpected high compressibility in Ar. Its bulk modulus before framework collapse is 14.1 GPa, 4 ∼ 5 times smaller than that in silicone. The anisotropic axial compressibilities of AlPO4-11 demonstrate that the ellipticity of the channel cross-section increases in silicone oil while it decreases in Ar because the relatively contractive behavior along minor axis direction is slowed down. Such pressure-induced responses are ascribed to the complex Ar-H2O-framework interaction after Ar penetration. This study offers us a new knowledge of crystal-fluid interaction in determining the mechanical behaviors of open-framework materials under high pressure and provides implications for the water evolution and circulation in water-carrying natural zeolites under extreme geological conditions in nature.

Covalent Organic Framework-derived Composite Membranes for Water Treatment.

Water treatment has experienced a surge in the adoption of membrane separation technology. Covalent organic frameworks (COFs), a class of metal-free and open-framework materials, have emerged as potential membrane materials owing to their interconnected periodic porosity, tunability, and chemical stability. However, the challenges associated with processing COF powders into self-standing membranes have spurred the emergence of COF composite membranes. This review article highlights the rationale behind developing COF composite membranes and their categories, including mixed matrix membranes (MMMs) and thin film composite (TFC) membranes. The common fabrication techniques of each category are presented. In addition, the influence of COF additives on the performance of the resultant composite membranes is systematically discussed, with a focus on the recent progress in applying COF composite membranes in the separation of different categories of water pollutants, including organic ions/molecules, toxic solvents, proteins, toxic heavy metals, and radionuclides.

Taming Multiscale Structural Complexity in Porous Skeletons: From Open Framework Materials to Micro/Nanoscaffold Architectures.

Recent developments in the design and synthesis of more and more sophisticated organic building blocks with controlled structures and physical properties, combined with the emergence of novel assembly modes and nanofabrication methods, make it possible to tailor unprecedented structurally complex porous systems with precise multiscale control over their architectures and functions. By tuning their porosity from the nanoscale to microscale, a wide range of functional materials can be assembled, including open frameworks and micro/nanoscaffold architectures. During the last two decades, significant progress is made on the generation and optimization of advanced porous systems, resulting in high-performance multifunctional scaffold materials and novel device configurations. In this perspective, a critical analysis is provided of the most effective methods for imparting controlled physical and chemical properties to multifunctional porous skeletons. The future research directions that underscore the role of skeleton structures with varying physical dimensions, from molecular-level open frameworks (<10nm) to supramolecular scaffolds (10-100nm) and micro/nano scaffolds (>100nm), are discussed. The limitations, challenges, and opportunities for potential applications of these multifunctional and multidimensional material systems are also evaluated in particular by addressing the greatest challenges that the society has to face.

Shape-Memory Effect Triggered by π-π Interactions in a Flexible Terpyridine Metal-Organic Framework.

Shape-memory polymers and alloys are adaptable materials capable of reversing from a deformed, metastable phase to an energetically favored original phase in response to external stimuli. In the context of metal-organic frameworks, the term shape-memory is defined as the property of a switchable framework to stabilize the reopened pore phase after the first switching transition. Herein we describe a novel flexible terpyridine MOF which, upon desolvation, transforms into a nonporous structure that reopens into a shape-memory phase when exposed to CO2 at 195 K. Based on comprehensive in situ experimental studies (SC-XRD and PXRD) and DFT energetic considerations combined with literature reports, we recommend dividing shape-memory MOFs into two categories, viz responsive and nonresponsive, depending on the transformability of the gas-free reopened pore phase into the collapsed phase. Furthermore, considering the methodological gap in discovering and understanding shape-memory porous materials, we emphasize the importance of multicycle physisorption experiments for dynamic open framework materials, including metal-organic and covalent organic frameworks.

The missing link between zeolites and polyoxometalates.

Open framework materials such as zeolites and metalorganic frameworks are garnering tremendous interest because of their intriguing architecture and attractive functionalities. Thus, new types of open framework materials are highly sought after. Here, we present the discovery of completely new inorganic framework materials, where, in contrast to conventional inorganic open frameworks, the scaffold is not based on tetrahedral EO4 (E = main group element) but octahedral MO6 (M = transition metal) building blocks. These structural features place them closer to polyoxometalates than zeolites. The first representatives of this class of materials are [(R)24(NH4)14(PO(OH)2)6]·[M134(PO3(OH,F))96F120] (M = Co, R = C2Py = 1-ethylpyridinium and M = Ni, R = C4C1Py = 1-butyl-3-methylpyridinium) featuring interlinked fullerene-like nanosphere cavities. Having a transition metal building up the framework brings about interesting properties, for example, spin-glass behavior, and, with this particular topology, a hedgehog-like spin orientation.

Synthesis of Type II Ge and Ge-Si Alloyed Clathrates Using Solid-State Electrochemical Oxidation of Zintl Phase Precursors.

Germanium clathrates with the type II structure are open-framework materials that show promise for various applications, but the difficulty of achieving phase-pure products via traditional synthesis routes has hindered their development. Herein, we demonstrate the synthesis of type II Ge clathrates in a two-electrode electrochemical cell using Na4Ge4-ySiy (y = 0, 1) Zintl phase precursors as the working electrode, Na metal as the counter/reference electrode, and Na-ion conducting β″-alumina as the solid electrolyte. The galvanostatic oxidation of Na4Ge4 resulted in voltage plateaus around 0.34-0.40 V vs Na/Na+ with the formation of different products depending on the reaction temperature. When using Na4Ge3Si as a precursor, nearly phase-pure, alloyed type II Ge-Si clathrate was obtained at 350 °C. The Na atoms in the large (Ge,Si)28 cages of the clathrate occupied off-centered positions according to Rietveld refinement and density functional theory calculations. The results indicate that electrochemical oxidation of Zintl phase precursors is a promising pathway for synthesizing Ge clathrates with type II structure and that Si alloying of the Zintl phase precursor can promote selective clathrate product formation over other phases.

Understanding the role of guest ions in the control of thermal expansion of FeFe(CN)6

Understanding the role of guest species in framework materials is important to successfully achieve the control of thermal expansion. In this work, the thermal expansion of the Prussian blue analogue FeFe(CN)6 has been effectively tailored from negative, to zero to positive through the insertion of Na+ ions. To shed light on the role of Na+ ions on thermal expansion behavior, a joint study was conducted by means of high-resolution synchrotron X-ray diffraction, EXAFS spectroscopy and lattice dynamics calculations. It has been found out that the insertion of Na+ ions reduces the structural flexibility of the CN linkages and affects the low-energy phonons, thus allowing the tuning of the overall thermal expansion. This work demonstrates the possibility of precisely controlling the thermal expansion of open-framework materials by inserting guest ions and clarifies the physical mechanism underlying this control.

Biomineralization at the Molecular Level: Amino Acid-Assisted Crystallization of Zeotype AlPO4·1.5H2O–H3

Three aminoacids: proline, histidine, and lysine are used as organic additives in the hydrothermal synthesis of the zeotype material AlPO 4 •1.5H 2 O-H3 (framework topology code APC). A detailed analysis of the factors controlling the formation of AlPO 4 •1.5H 2 O-H3 is performed. The role of aminoacids in the formation process of the frameworks is addressed based on comparisons of the crystal phase selection of different synthetic recipes and the insights provided by the FTIR analysis of the synthetic gels and the crystalline products. The phase transition of AlPO 4 •1.5H 2 O-H3 to AlPO 4-C (APC), and subsequently to AlPO 4-D (APD) upon heating to 1000 o C was followed by in situ XRD analysis. The present work has been the first progress to understanding the role of the bioactive molecules in the crystallization of inorganic open-framework microporous materials. An active, non-classical structuredirecting role that aminoacids play during the zeotype crystallization is proposed, i.e., positively charged aminoacids cap the growing surface of microporous AlPO 4 , and attract aluminate and phosphate anions from the solution phase for further growth.

Synthesis and Fluorescence Properties of Novel Ce3+-doped Manganese Phosphite Open-framework Materials

Synthesis and Fluorescence Properties of Novel Ce3+-doped Manganese Phosphite Open-framework Materials

Turning Flexibility into Rigidity: Stepwise Locking of Interpenetrating Networks in a MOF Crystal through Click Reaction

Post-synthetic modifications of metal–organic frameworks (MOFs) enable synthesis of materials with enhanced performance characteristics or those inaccessible by direct synthetic routes. In this work, for the first time, we utilize inverse-electron demand Diels–Alder (iEDDA) modification to control the structural flexibility and porosity of an open framework material. We selected a series of dienophiles with increasing bulkiness including ethyl vinyl ether (eve), cyclohexene (chx), norbornene (nor), and 5-norbornene-2-methanol (noh) to modify a tetrazine-based linker (3,6-dipyridyl-1,2,4,5-tetrazine, dpt) incorporated in a unique doubly interpenetrated 3D hybrid MOF–HOF porous material (HOF, hydrogen-bonded organic framework), {[Cd2(coh)2(dpt)2]·guests}n (JUK-20). Each subnetwork in JUK-20 is built of 2D coordination layers stacked by strong complementary C═O···H–N hydrogen bonds between carbohydrazide dibenzoate linkers (coh). By using the [4 + 2] click reactions of JUK-20, which proceed in a prominent single-crystal-to-single-crystal manner, we obtained a series of JUK-20-dienophile MOFs. The modifications lead to a stepwise decrease in structural flexibility of the JUK-20 platform until the highest rigidity and stability is reached for JUK-20-noh. Consequently, the adsorption capacity in the JUK-20-dienophile series increases, as revealed collectively by single-crystal X-ray diffraction, physisorption isotherms (N2, CO2, and MeOH), and grand canonical Monte Carlo simulations. Our work demonstrates that post-synthetic iEDDA modification is a versatile and efficient tool for systematic functionalization of open framework materials under mild conditions.

On the quality of three-dimensional electron diffraction data for direct location of guest molecules in open-framework materials

On the quality of three-dimensional electron diffraction data for direct location of guest molecules in open-framework materials

Microwave-assisted reflux synthesis of spherical Co2Fe(CN)6 nanoparticles for high-performance asymmetric supercapacitors

The open framework materials with the Prussian blue structure facilitate the insertion of monovalent K+ ions and exhibit high capacitive performance. In this regard, the spherical Co2Fe(CN)6 (Co-HCF) nanoparticles were synthesized by the microwave-assisted reflux method. The X-ray diffraction pattern confirms the cubic structure, and the transmission electron microscopy images reveal the formation of spherical sized particles. The cyclic voltammetry analysis corroborated the reversibility of the electrode and delivered the specific capacitance of 1131 F g−1 at 2 mV s−1 in 1 M KNO3 electrolyte. Further, the 2 V asymmetric supercapacitor (Co-HCF//AC) was fabricated with activated carbon, which delivers the specific capacitance of 50 F g−1 at 10 mV s−1 and the energy density of 24.06 Wh kg−1 at the power density of 103.7 W kg−1 at 1 A g−1 with cycling stability of nearly 84% over 1000 cycles.

Three-step nucleation of metal–organic framework nanocrystals

Metal-organic frameworks (MOFs) are crystalline nanoporous materials with great potential for a wide range of industrial applications. Understanding the nucleation and early growth stages of these materials from a solution is critical for their design and synthesis. Despite their importance, the pathways through which MOFs nucleate are largely unknown. Using a combination of in situ liquid-phase and cryogenic transmission electron microscopy, we show that zeolitic imidazolate framework-8 MOF nanocrystals nucleate from precursor solution via three distinct steps: 1) liquid-liquid phase separation into solute-rich and solute-poor regions, followed by 2) direct condensation of the solute-rich region into an amorphous aggregate and 3) crystallization of the aggregate into a MOF. The three-step pathway for MOF nucleation shown here cannot be accounted for by conventional nucleation models and provides direct evidence for the nonclassical nucleation pathways in open-framework materials, suggesting that a solute-rich phase is a common precursor for crystallization from a solution.

Uniting Form and Function, Stability and Reactivity in Open Framework Materials

Abstract Functions and performance metrics are much sought after by materials researchers and editors; not to be sidelined, however, are fundamental design works directed at general methodological breakthrough and wide-scope applicability. For a close union of form and function is often crucial in catalysing paradigm shift. We use two forms of contrast to illustrate: the hard/soft design in carboxyl-thiol molecules, and the starburst/backfolded alkyne motifs. The former affords thiol-decked Zr4+-carboxyl frameworks, the latter, domino cyclization for crosslinked nanographene components. The versatile reactivity of the thiol and alkyne functions (e.g., for metal uptake), deployed in symmetrical dendritic forms, serves to bridge the worlds of coordination and covalent solids.

A Photoconductive X-ray Detector with a High Figure of Merit Based on an Open-Framework Chalcogenide Semiconductor.

A wide range of tunability in the physical parameters of a semiconductor used for X-ray detection is desirable to achieve targeted performance optimization. However, in a dense-phase semiconductor, fine-tuning one parameter often leads to unwanted changes in other parameters. Herein, the intrinsic openness in an open-framework semiconductor has been confirmed, for the first time, to be a key structural factor that weakens the mutual exclusivity of the adjustable physical parameters owing to a non-linear control mechanism. The controllable doping of S in a zeolitic In-Se host results in an optimal balance between resistivity, band gap, and carrier mobility, which finally results in an excellent X-ray detector with a high figure of merit for the mobility-lifetime product (7.12×10-4 cm2 V-1 ); this value is superior to that of a commercial α-Se detector. The current strategy of choosing open-framework semiconductor materials opens a new window for targeting high-performance X-ray detection.

Metal Chalcogenide Supertetrahedral Clusters: Synthetic Control over Assembly, Dispersibility, and Their Functional Applications.

ConspectusMetal chalcogenide supertetrahedral clusters (MCSCs) bear the closest structural resemblance to II-VI or I-III-VI semiconductor nanocrystals and can be considered as well-defined ultrasmall "quantum dots" (QDs). Compared to traditional colloidal QDs that are typically associated with size dispersity, irregular surface atomic structures, poorly defined core-ligand interfaces, and random defect/dopant sites, the nano- or subnano-sized MCSCs feature precise structural properties such as atomically uniform size, precise structure, and ordered dopant distribution, all of which offer ample opportunities for a broad and in-depth understanding of the correlation between the precise local structure and site- or size-dependent properties, which are critical to the exploitation of their functional applications. Our previous Account in 2005 provided a narrative on the efforts to expand the structural diversity of open-framework materials using different-sized and compositionally tunable clusters as building blocks with a primary objective of integrating the semiconducting properties with porosity in zeolite-type solids. Over the past 15 years, significant progress has been made, particularly in the synthetic control of discrete clusters, allowing the establishment of the composition-structure-property correlation of the MCSCs to guide the optimization of their properties for various applications. In the present Account, the recent progress in MCSC-based chemistry is reviewed from three aspects: (1) controllable synthesis of new members and types of MCSC models and the development of organic-ligand-directed hybrid assembly modes for MCSC-based open frameworks; (2) new synthetic strategies for the discretization of MCSCs in crystal lattice and their dispersibility in solvents, affording practical applications of pure inorganic MCSCs as nanomaterials; and (3) functionality of MCSC-based materials including photochemical and electrochemical properties triggered by precise dopant/defect sites, open-framework-related functional expansion via host-guest chemistry, and dispersed cluster-based composite materials with synergy from functional multimetallic components. All these advances show that MCSCs with well-defined structures and atomically precise dopant/defect sites are powerful model systems for establishing the precise structure-composition-property correlation and understanding the photophysical dynamic behaviors, both of which are difficult or impossible to achieve in the traditional QD system. Perspectives on their potential applications are presented in terms of the amorphous assemblies of monodispersed MCSCs, MCSC-based two-dimensional layered materials, and optical/electronic devices.

Effect of bond on negative thermal expansion of Prussian blue analogues MCo(CN)6 (M = Fe, Ti and Sc): a first-principles study

Negative thermal expansion (NTE) is an abnormal physical behavior that has promising applications for high precision thermal control. Since Prussian blue analogues have the two central linking atoms of –C≡N–, they have large structure flexibility and are suitable to explore new NTE materials. However, understanding the nature of structure flexibility from the point of view of chemical bonding is important and urgent. Here, we adopt for the first time first-principles calculations to predict that the cubic TiCo(CN)6 and ScCo(CN)6 have NTE behavior. The calculated results for M in MCo(CN)6 (M = Fe, Ti and Co) indicated that the Sc–N bond is the strongest, but with the weakest direction dependence among the M–N bonds in the three systems. The lattice dynamics calculations results revealed that the low-frequency phonon vibration modes for NTE in MCo(CN)6 have much stronger relationship with the M–N bond feature. The present work reveals the important role of the related bond in the NTE open-framework materials.

Electronic Devices Using Open Framework Materials.

Open framework materials (OFM) constitute a large and growing class of nanoporous crystalline structures that is attracting considerable attention for electronic device applications. This review summarizes the most recent reports concerning electronic devices enabled by either of the two primary categories of OFM, metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs). Devices in which the OFM plays an active role (as opposed to acting only as a selective sorbent or filter) are the principal focus, with examples cited that include field-effect transistors, capacitors, memristors, and a wide variety of sensing architectures. As a brief tutorial, we also provide a concise summary of various methods of depositing or growing OFM on surfaces, as these are of crucial importance to the deployment of electronic OFM. Finally, we offer our perspective concerning future research directions, particularly regarding what in our view are the biggest challenges remaining to be addressed. On the basis of the literature discussed here, we conclude that OFM constitute a unique class of electronic materials with characteristics and advantages that are distinct from either conventional inorganic semiconductors or organic conductors. This suggests a bright future for these materials in applications such as edge computing, resistive switching, and mechanically flexible sensing and electronics.