Advanced Fabrication Method for the Preparation of MOF Thin Films: Liquid-Phase Epitaxy Approach Meets Spin Coating Method

Here, we report a new and advanced method for the fabrication of highly oriented/polycrystalline metal-organic framework (MOF) thin films. Building on the attractive features of the liquid-phase epitaxy (LPE) approach, a facile spin coating method was implemented to generate MOF thin films in a high-throughput fashion. Advantageously, this approach offers a great prospective to cost-effectively construct thin-films with a significantly shortened preparation time and a lessened chemicals and solvents consumption, as compared to the conventional LPE-process. Certainly, this new spin-coating approach has been implemented successfully to construct various MOF thin films, ranging in thickness from a few micrometers down to the nanometer scale, spanning 2-D and 3-D benchmark MOF materials including Cu2(bdc)2·xH2O, Zn2(bdc)2·xH2O, HKUST-1, and ZIF-8. This method was appraised and proved effective on a variety of substrates comprising functionalized gold, silicon, glass, porous stainless steel, and aluminum oxide. The facile, high-throughput and cost-effective nature of this approach, coupled with the successful thin film growth and substrate versatility, represents the next generation of methods for MOF thin film fabrication. Therefore, paving the way for these unique MOF materials to address a wide range of challenges in the areas of sensing devices and membrane technology.

Metal-Organic Coordination Network Thin Film by Surface-Induced Assembly

The growth of metal-organic coordination network thin films on surfaces has been pursued extensively and intensively to manipulate the molecular arrangement. For this study, the oriented multilayer thin films based on porphyrinic nanoarchitecture were synthesized toward metal-organic coordination networks using surface-induced assembly (SIA). Nanoscale molecular thin films were prepared at room temperature using cobalt(II) ion and porphyrin building blocks as precursors. Stepwise growth with a highly uniform layer was characterized using UV-vis, AFM, IR, and XPS studies. The grazing incidence small-angle X-ray scattering and X-ray reflectivity results remarkably suggested a periodic structure in in-plane direction with constant and high mass density (ca. 1.5 g/cm(3)) throughout the multilayer formation. We propose that orientation of the porphyrin macrocycle plane with a hexagonal packed model by single anchoring mode was tilted approximately 60° with respect to the surface substrate. It is noteworthy that the well-organized structure of porphyrin-based macrocyclic framework on the amine-terminated surface substrate can be achieved efficiently using a simple SIA approach under mild synthetic conditions. The synthesized thin film provides a different structure from that obtained using bulk synthesis. This result suggests that the SIA technique can control not only the film thickness but also the structural arrangement on the surface. This report of our research provides insight into the ordered porphyrin-based metal-organic coordination network thin films, which opens up opportunities for exploration of unique thin film materials for diverse applications.

Photoluminescent Metal-Organic Frameworks for Gas Sensing

Luminescence of porous coordination polymers (PCPs) or metal-organic frameworks (MOFs) is sensitive to the type and concentration of chemical species in the surrounding environment, because these materials combine the advantages of the highly regular porous structures and various luminescence mechanisms, as well as diversified host-guest interactions. In the past few years, luminescent MOFs have attracted more and more attention for chemical sensing of gas-phase analytes, including common gases and vapors of solids/liquids. While liquid-phase and gas-phase luminescence sensing by MOFs share similar mechanisms such as host-guest electron and/or energy transfer, exiplex formation, and guest-perturbing of excited-state energy level and radiation pathways, via various types of host-guest interactions, gas-phase sensing has its unique advantages and challenges, such as easy utilization of encapsulated guest luminophores and difficulty for accurate measurement of the intensity change. This review summarizes recent progresses by using luminescent MOFs as reusable sensing materials for detection of gases and vapors of solids/liquids especially for O₂, highlighting various strategies for improving the sensitivity, selectivity, stability, and accuracy, reducing the materials cost, and developing related devices.

Structural flexibility in crystallized matter: from history to applications

The large reversible flexibility of hybrid crystallized matter is relatively new. After briefly recalling the history of this discovery, the article will analyze the different parameters influencing this phenomenon. They relate first to the various structural characteristics of the framework, in both its inorganic and organic parts. The influence of the energies of the guest-guest and host-guest interactions is then analyzed. Once the reasons are explained, a third section will be devoted to the various physical properties of these flexible solids. The last section concerns recent industrial applications of this family of solids.

Metal organic frameworks as sorption media for volatile and semi-volatile organic compounds at ambient conditions

In this research, we investigated the sorptive behavior of a mixture of 14 volatile and semi-volatile organic compounds (four aromatic hydrocarbons (benzene, toluene, p-xylene, and styrene), six C₂-C₅ volatile fatty acids (VFAs), two phenols, and two indoles) against three metal-organic frameworks (MOFs), i.e., MOF-5, Eu-MOF, and MOF-199 at 5 to 10 mPa VOC partial pressures (25 °C). The selected MOFs exhibited the strongest affinity for semi-volatile (polar) VOC molecules (skatole), whereas the weakest affinity toward was volatile (non-polar) VOC molecules (i.e., benzene). Our experimental results were also supported through simulation analysis in which polar molecules were bound most strongly to MOF-199, reflecting the presence of strong interactions of Cu²⁺ with polar VOCs. In addition, the performance of selected MOFs was compared to three well-known commercial sorbents (Tenax TA, Carbopack X, and Carboxen 1000) under the same conditions. The estimated equilibrium adsorption capacity (mg.g⁻¹) for the all target VOCs was in the order of; MOF-199 (71.7) >Carboxen-1000 (68.4) >Eu-MOF (27.9) >Carbopack X (24.3) >MOF-5 (12.7) >Tenax TA (10.6). Hopefully, outcome of this study are expected to open a new corridor to expand the practical application of MOFs for the treatment diverse VOC mixtures.

Chiral chemistry of metal-camphorate frameworks

This critical review presents the various synthetic approaches and chiral chemistry of metal-camphorate frameworks (MCamFs), which are homochiral metal-organic frameworks (MOFs) constructed from a camphorate ligand. The interest in this unique subset of homochiral MOFs is derived from the many interesting chiral features for both materials and life sciences, such as asymmetrical synthesis or crystallization, homochiral structural design, chiral induction, absolute helical control and ligand handedness. Additionally, we discuss the potential applications of homochiral MCamFs. This review will be of interest to researchers attempting to design other homochiral MOFs and those engaged in the extension of MOFs for applications such as chiral recognition, enantiomer separation, asymmetric catalysis, nonlinear sensors and devices.

Synthesis of Nanoscale Coordination Polymers in Femtoliter Reactors on Surfaces

In the present work, AFM-assisted lithography was used to perform the synthesis of a coordination polymer inside femtoliter droplets deposited on surfaces. For this, solutions of the metal salt and the organic ligand were independently transferred to adjacent tips of the same AFM probe array and were sequentially delivered on the same position of the surface, creating femtoliter-sized reaction vessels where the coordination reaction and particle growth occurred. Alternatively, the two reagents were mixed in the cantilever array by loading an excess of the inks, and transferred to the surface immediately after, before the precipitation of the coordination polymer took place. The in situ synthesis allowed the reproducible obtaining of round-shaped coordination polymer nanostructures with control over their XY positioning on the surface, as characterized by microscopy and spectroscopy techniques.

Metal-Organic Frameworks as Platforms for Functional Materials

Discoveries of novel functional materials have played very important roles to the development of science and technologies and thus to benefit our daily life. Among the diverse materials, metal-organic framework (MOF) materials are rapidly emerging as a unique type of porous and organic/inorganic hybrid materials which can be simply self-assembled from their corresponding inorganic metal ions/clusters with organic linkers, and can be straightforwardly characterized by various analytical methods. In terms of porosity, they are superior to other well-known porous materials such as zeolites and carbon materials; exhibiting extremely high porosity with surface area up to 7000 m(2)/g, tunable pore sizes, and metrics through the interplay of both organic and inorganic components with the pore sizes ranging from 3 to 100 Å, and lowest framework density down to 0.13 g/cm(3). Such unique features have enabled metal-organic frameworks to exhibit great potentials for a broad range of applications in gas storage, gas separations, enantioselective separations, heterogeneous catalysis, chemical sensing and drug delivery. On the other hand, metal-organic frameworks can be also considered as organic/inorganic self-assembled hybrid materials, we can take advantages of the physical and chemical properties of both organic and inorganic components to develop their functional optical, photonic, and magnetic materials. Furthermore, the pores within MOFs can also be utilized to encapsulate a large number of different species of diverse functions, so a variety of functional MOF/composite materials can be readily synthesized. In this Account, we describe our recent research progress on pore and function engineering to develop functional MOF materials. We have been able to tune and optimize pore spaces, immobilize specific functional groups, and introduce chiral pore environments to target MOF materials for methane storage, light hydrocarbon separations, enantioselective recognitions, carbon dioxide capture, and separations. The intrinsic optical and photonic properties of metal ions and organic ligands, and guest molecules and/or ions can be collaboratively assembled and/or encapsulated into their frameworks, so we have realized a series of novel MOF materials as ratiometric luminescent thermometers, O2 sensors, white-light-emitting materials, nonlinear optical materials, two-photon pumped lasing materials, and two-photon responsive materials for 3D patterning and data storage. Thanks to the interplay of the dual functionalities of metal-organic frameworks (the inherent porosity, and the intrinsic physical and chemical properties of inorganic and organic building blocks and encapsulated guest species), our research efforts have led to the development of functional MOF materials beyond our initial imaginations.

Second harmonic generation microscopy reveals hidden polar organization in fluoride doped MIL-53(Fe)

Polar metal-organic frameworks have potential applications as functional non-linear optical, piezoelectric, pyroelectric and ferroelectric materials. Using second harmonic generation microscopy we found that fluoride doping of the microporous iron(iii) terephthalate MOF MIL-53(Fe) induces a polar organization in its structure, which was not previously detected with XRD. The polar order is only observed when both fluoride and guest molecules are present, and may be related to a complex interplay between the adsorbates and the framework, leading to a modification of the positioning of fluoride in the inorganic Fe-chains. Combined polarized second harmonic generation microscopy and scanning pyroelectric microscopy show that the polar axis is unidirectional and of the same sense over the whole crystal, extending up to 100 micrometers. This finding shows how MOF materials can be endowed with useful properties by doping MOFs with fluoride.

Magnetic Metal-Organic Frameworks for Efficient Carbon Dioxide Capture and Remote Trigger Release

Magnetic metal-organic framework (MOF) composites show highly efficient CO2 desorption capacities upon their exposure to an alternating magnetic field, demonstrating a magnetic induction swing strategy for potentially low-energy regeneration of MOF adsorbents.

Chemical vapour deposition of zeolitic imidazolate framework thin films

Integrating metal-organic frameworks (MOFs) in microelectronics has disruptive potential because of the unique properties of these microporous crystalline materials. Suitable film deposition methods are crucial to leverage MOFs in this field. Conventional solvent-based procedures, typically adapted from powder preparation routes, are incompatible with nanofabrication because of corrosion and contamination risks. We demonstrate a chemical vapour deposition process (MOF-CVD) that enables high-quality films of ZIF-8, a prototypical MOF material, with a uniform and controlled thickness, even on high-aspect-ratio features. Furthermore, we demonstrate how MOF-CVD enables previously inaccessible routes such as lift-off patterning and depositing MOF films on fragile features. The compatibility of MOF-CVD with existing infrastructure, both in research and production facilities, will greatly facilitate MOF integration in microelectronics. MOF-CVD is the first vapour-phase deposition method for any type of microporous crystalline network solid and marks a milestone in processing such materials.

Self-Assembly of Discrete Metallocycles versus Coordination Polymers Based on Cu(I) and Ag(I) Ions and Flexible Ligands: Structural Diversification and Luminescent Properties

Three new Ag(I) and one Cu(I) coordination compounds with two different positional isomers, propane-1,3-diyl bis(pyridine-4-carboxylate) (L1) and propane-1,3-diyl bis(pyridine-3-carboxylate) (L2), of a bis-(pyridyl-carboxylate) ligand have been synthesized. X-ray diffraction analysis revealed that the self-assembly of L1 with AgCF₃SO₃ and AgClO₄ salts leads to the formation of discrete binuclear metallocycles {Ag(L1)CF₃SO₃}₂ (1) and {Ag(L1)ClO₄}₂ (2), respectively. However, self-assembly of the other ligand, L2, with AgCF₃SO₃ and CuCl salts, results in a 1-D zig-zag chain {Ag(L2)CF₃SO₃}_∞ (3) and a 1-D double-stranded helical chain {Cu₂Cl₂(L2)₂}_∞ (4) coordination polymers, respectively. Solid emission spectra recorded at room temperature show interesting luminescence properties for all four compounds in the range from 438 to 550 nm, especially for compound 4 that was found to change its emission color when the wavelength of the excitation radiation is switched from 332 to 436 nm.

A thermally activated manganese 1,4-benzenedicarboxylate metal organic framework with high anodic capability for Li-ion batteries

Mn-1,4-BDC@200 was synthesized, with a capacity of 974 mA h g⁻¹after 100 cycles at a rate of 100 mA g⁻¹.

Conversion of Cu2O nanowires into Cu2O/HKUST-1 core/sheath nanostructures and hierarchical HKUST-1 nanotubes

Hierarchical HKUST-1 nanotubes obtained from the conversion of Cu₂O nanowires followed by core removal showed a fast uptake of dyes.

Highly efficient simultaneous ultrasonic-assisted adsorption of methylene blue and rhodamine B onto metal organic framework MIL-68(Al): central composite design optimization

In this work, metal organic framework (MIL-68(Al)), was synthesized by a simple, fast and low-cost process for simultaneous removal of methylene blue and Rhodamine B, regarded to be toxic and even carcinogenic, from aqueous solution.

Novel nanoporous carbon derived from metal–organic frameworks with tunable electromagnetic wave absorption capabilities

Core–shell ZIF-8@ZIF-67 crystals, which integrate the properties of single ZIF-8 and ZIF-67, are elaborately designed for the first time by applying a seed-mediated growth technique.

Emerging applications of metal–organic frameworks

Metal–organic frameworks are highly crystalline porous materials which present emerging opportunities in biotechnology, catalysis, microelectronics and photonics.

Exploiting redox activity in metal–organic frameworks: concepts, trends and perspectives

This feature article highlights latest developments in experimental, theoretical and computational concepts relevant to redox-active metal–organic Frameworks.

Facile stabilization of cyclodextrin metal–organic frameworks under aqueous conditions via the incorporation of C60 in their matrices

A facile method to improve the stability of γ-cyclodextrin metal–organic frameworks in an aqueous environment has been developed through the incorporation of hydrophobic C₆₀ in their matrices.