Tailoring exciton and excimer emission in an exfoliated ultrathin 2D metal-organic framework

Construction of a series of pH stable Ca-based MOFs, their CO2 adsorption and catalytic activity

In this study, three different solvent systems have been employed to investigate the effect of reaction parameters on the synthesis of four alkaline earth metal-based MOFs namely [Ca(0.5 1,4-phenyl diacetic acid)2(H2O)DMF]∞ (Ca-MOF-1), [Ca(1,4-naphthalene dicarboxylate)DMF]∞ (Ca-MOF-2), [Ca2(0.5 1,2,4,5-benzene tetracarboxylate)2(H2O)3DMF]∞ (Ca-MOF-3) and [Ca2(2,6-naphthalene dicarboxylate)2(H2O)6]∞ (Ca-MOF-4). The crystal structures of these four MOFs have been resolved through single crystal X-ray analysis and the bulk phase purity of these MOFs was assessed using PXRD and FT-IR analysis. To check the stability of these MOFs, thermogravimetric analysis (TGA) was carried out. To analyze the robustness of these MOFs, the PXRD of the samples was also collected at different pH levels. These MOFs were further explored as Lewis acid catalysts for the alcoholysis of epoxides and the activity of these catalysts depend on the open metal sites present in the MOFs. The catalytic activity follows the order: Ca-MOF-2 > Ca-MOF-4 > Ca-MOF-1 > Ca-MOF-3. The activity was also checked with various epoxide substrates using Ca-MOF-2. Density functional theory (DFT) calculations also support this trend with the help of the thermodynamic feasibility of epoxide binding, considering model MOF structures. The weak interaction between the epoxide oxygen and the metal centre of the most stable MOF structure has also been clarified by computational studies.

Self-condensation-assisted chemical vapour deposition growth of atomically two-dimensional MOF single-crystals

Two-dimensional metal-organic frameworks (MOFs) have a wide variety of applications in molecular separation and other emerging technologies, including atomically thin electronics. However, due to the inherent fragility and strong interlayer interactions, high-quality MOF crystals of atomic thickness, especially isolated MOF crystal monolayers, have not been easy to prepare. Here, we report the self-condensation-assisted chemical vapour deposition growth of atomically thin MOF single-crystals, yielding monolayer single-crystals of poly[Fe(benzimidazole)2] up to 62 μm in grain sizes. By using transmission electron microscopy and high-resolution atomic force microscopy, high crystallinity and atomic-scale single-crystal structure are verified in the atomically MOF flakes. Moreover, integrating such MOFs with MoS2 to construct ultrathin van der Waals heterostructures is achieved by direct growth of atomically MOF single-crystals onto monolayer MoS2, and enables a highly selective ammonia sensing. These demonstrations signify the great potential of the method in facilitating the development of the fabrication and application of atomically thin MOF crystals.

Exfoliation of Metal-Organic Frameworks to Give 2D MOF Nanosheets for the Electrocatalytic Oxygen Evolution Reaction

The structure and properties of materials are determined by a diverse range of chemical bond formation and breaking mechanisms, which greatly motivates the development of selectively controlling the chemical bonds in order to achieve materials with specific characteristics. Here, an orientational intervening bond-breaking strategy is demonstrated for synthesizing ultrathin metal-organic framework (MOF) nanosheets through balancing the process of thermal decomposition and liquid nitrogen exfoliation. In such approach, proper thermal treatment can weaken the interlayer bond while maintaining the stability of the intralayer bond in the layered MOFs. And the following liquid nitrogen treatment results in significant deformation and stress in the layered MOFs' structure due to the instant temperature drop and drastic expansion of liquid N2, leading to the curling, detachment, and separation of the MOF layers. The produced MOF nanosheets with five cycles of treatment are primarily composed of nanosheets that are less than 10 nm in thickness. The MOF nanosheets exhibit enhanced catalytic performance in oxygen evolution reactions owing to the ultrathin thickness without capping agents which provide improved charge transfer efficiency and dense exposed active sites. This strategy underscores the significance of orientational intervention in chemical bonds to engineer innovative materials.

Square-planar Tetranuclear Cluster-based Alkaline Earth Metal-organic Frameworks with Enhanced Proton Conductivity

Alkaline earth (AE) metal complexes have garnered significant interest in various functional fields due to their nontoxicity, low density, and low cost. However, there is a lack of systematic investigation into the structural characteristics and physical properties of AE-metal-organic frameworks (MOFs). In this research, we synthesized isostructural MOFs consisting of AE4(μ4-Cl) clusters bridged by benzo-(1,2;3,4;5,6)-tris(thiophene-2'-carboxylic acid) (BTTC3-) ligands. The resulting structure forms a truncated octahedral cage denoted as [AE4(m4-Cl)]6(BTTC)8, which further linked to a porous three-dimensional framework. Among the investigated AE ions (Ca, Sr, and Ba), the Ca4-MOF demonstrated good chemical stability in water compared to Sr4-MOF and Ba4-MOF. The N2 adsorption and solid-state UV-vis-NIR absorption behaviors were evaluated for all AE4-MOFs, showing similar trends among the different metal ions. Additionally, the proton conduction study revealed that the Ca4-MOF exhibited ultra-high proton conductivity, reaching 3.52×10-2 S cm-1 at 343 K and 98 % RH. Notably, the introduction of LiCl via guest exchange resulted in an improved proton conduction of up to 6.36×10-2 S cm-1 under similar conditions in the modified LiCl@Ca4-MOF. The findings shed light on the regulation of physical properties and proton conductivity of AE-MOFs, providing valuable insights for their potential applications in various fields.

Time-Resolved Spectroscopy for Dynamic Investigation of Photoresponsive Metal-Organic Frameworks

Photoresponsive MOFs with precise and adjustable reticular structures are attractive for light conversion applications. Uncovering the photoinduced carrier dynamics lays the essential foundation for the further development and optimization of the MOF material. With the application of time-resolved spectroscopy, photophysical processes including excimer formation, energy transfer/migration, and charge transfer/separation have been widely investigated. However, the identification of distinct photophysical processes in real experimental MOF spectra still remains difficult due to the spectral and dynamic complexity of MOFs. In this Perspective, we summarize the typical spectral features of these photophysical processes and the related analysis methods for dynamic studies performed by time-resolved photoluminescence (TR-PL) and transient absorption (TA) spectroscopy. Based on the recent understanding of excited-state properties of photoresponsive MOFs and the discussion of challenges and future outlooks, this Perspective aims to provide convenience for MOF kinetic analysis and contribute to the further development of photoresponsive MOF material.

Brine available two-dimensional nano-architectonics of fluorescent probe based on phosphate doped ZIF-L for detection of Fe3

Herein, we propose a simple and effective strategy for designing a zeolitic imidazolate frameworks (ZIFs) fluorescent probe with a two-dimensional leaf-like structure. By doping ZIF-L with phosphate, we developed a fluorescent probe for iron (Fe3+) in systems with high salinity. The fluorescence of P-ZIF-L was quenched effectively with the presence of Fe3+. The physicochemical structure, surface morphology, selectivity, stability and composition of the probe were investigated. Under optimized conditions, the fluorescent probe had a detection limit of 0.5 μM. Furthermore, the results that the probe exhibited desirable salt-tolerance and was suitable for determination of Fe3+ in brine water samples with satisfactory results.

Tailoring Dye Emissions within Metal-Organic Frameworks for Tunable Luminescence and Ratiometric Temperature Sensing

Metal-organic frameworks (MOFs) have emerged as powerful platforms for tuning the luminescence characteristics of guests due to their various structures and functions. Tunable and stimuli-responsive luminescence of guests within MOFs can be achieved through a judicious choice of guests and hosts. Herein, we demonstrate a dramatic change in the luminescence of dye excimers encapsulated in MOFs. A polar dye presented largely red-shifted excimer emissions in MOFs with higher polarities, while a nonpolar dye showed very different excimer emissions. Interestingly, the excimer emissions tailored by the MOFs showed strong thermal quenching. Cz-Ant@ZIF-8, containing two luminescent dyes (carbazole (Cz) and anthracene (Ant)), was prepared, and it presented ratiometric temperature sensing properties (1.55% K^-1) in the temperature range of 278-353 K. This work sheds light on the luminescence tuning of dyes confined in MOFs and the design of sensitive ratiometric thermometers.

Synthesis of 2D Metal-Organic Nanosheets (MONs) by Liquid Phase Exfoliation: Applications in Effective Delivery of Antiulcer Drugs and Selective Adsorption and Removal of Cationic Dyes

Nowadays, the fabrication of 2D metal-organic nanosheets (2D MONs) has entered the research arena fascinating researchers worldwide. However, a lack of efficient and facile methods has remained a bottleneck for the manufacturing of these 2D MONs. Herein, a 2D metal-organic framework (MOF), i.e., 2D Cu-MOF, was synthesized using a facile and convenient stirring method by using 4,4'-trimethylenedipyridine (TMDP) as an organic linker. The as-prepared MOF was characterized in detail and based on single crystal X-ray diffraction analysis, it was established that tangled layers in the 2D Cu-MOF are interconnected to produce thick strands. These tangled layers could be easily separated via ultrasonication-induced liquid phase exfoliation (UILPE) to give the 2D Cu-MON as illustrated through Tyndall light scattering and exhaustive microscopic exploration such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The application of this 2D Cu-MON was assessed in the field of drug delivery revealing exceptional drug loading for the drug lansoprazole (LPZ) by 2D Cu-MONs as well as drug release in the acidic and neutral medium demonstrating that the 2D Cu-MON is an excellent carrier for antiulcer drug delivery. For environmental protection, the application of 2D Cu-MON was also examined toward the removal of various cationic and anionic dyes with excellent selectivity toward cationic dye removal. The plausible mechanism for dye removal indicated the involvement of cation-π and π-π interactions, for the effective adsorption of cationic dyes as well as a increase in the surface area of 2D Cu-MON by UILPE. Remarkably, the high drug loading and dye removal are imputed to the increase in surface area by UILPE. In a nutshell, the developed 2D Cu-MON will prove to be beneficial for application in the field of drug delivery as well as for wastewater treatment.

Bodipy-Based Metal-Organic Frameworks Transformed in Solid States from 1D Chains to 2D Layer Structures as Efficient Visible Light Heterogeneous Photocatalysts for Forging C-B and C-C Bonds

Boron dipyrromethene (also known as bodipy), as a class of versatile and robust fluorophores and a structural analogue of porphyrins, has received a great deal of interests in the field of light-harvesting and energy-transfer processes. However, the fabrication of bodipy monomers into metal-organic frameworks (MOFs) and the exploitation of their potential still lags behind the porphyrin MOFs. In this work, two bodipy-based MOFs, BMOF 1D with 1D chain structure and BMOF 2D with 2D layer structure, were assembled by using dicarboxyl-functionalized bodipy ligands. BMOF 1D can also be converted to BMOF 2D by inserting additional ligands into BMOF 1D to cross-link the adjacent chains into the rhombic grid layer. During this process, spontaneous exfoliation occurred simultaneously and resulted in the formation of several hundred nanometer thickness BMOF 2D (nBMOF 2D), which can be further exfoliated into one-layer MOF nanosheets (BMON 2D) by using the ultrasonic liquid exfoliation method in a high yield. Featuring the distinct bodipy scaffolds in the porous frameworks, both BMOF 2D and BMON 2D displayed high reactivity and recyclability in the photocatalytic inverse hydroboration and cross-dehydrogenative coupling reactions to afford α-amino organoborons and α-amino amides in moderate to high yields. This work not only highlights the cascade utilization of ligand installation and ultrasonic liquid exfoliation methods to provide the single-layer MOF sheets in high yields but also advances the bodipy-based MOFs as a new type of heterogeneous photocatalysts in the forging of C-B and C-C bonds driven by visible light.

Multicomponent Anti-Kasha's Rule Emission from Nanotubular Metal-Organic Frameworks for Selective Detection of Small Molecules

The peak photoluminescence (PL) of conventional fluorophores is independent of the excitation wavelength (called Kasha's rule), while the search of metal-organic framework materials with the so-called anti-Kasha's rule emission remains very limited. Herein, we report the observation of anti-Kasha's rule emission in a multicomponent PL three-dimensional nanotubular metal-organic framework (abbr. MOF-NT), [Zn(μ-L)(μ-bix)]_n·0.33nH₂O [H₂L = biphenyl-3,5-dicarboxylic acid; bix = 1,4-bis(imidazole-1-ylmethyl)benzene]. The MOF-NT crystalline sample represents a notable example of strong excitation-dependent fluorescence from the ultraviolet to the visible spectral region. Moreover, by virtue of electronic flexibility and high PL efficiency, MOF-NT shows a discriminative PL response between isomeric nitroaromatic compounds. The work demonstrated the intrinsic anti-Kasha's rule emission in the crystalline-state MOF materials, providing new visions for the development of advanced solid-state emissive materials.

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.

Multi-Responsive Sensor Based on Porous Hydrogen-Bonded Organic Frameworks for Selective Sensing of Ions and Dopamine Molecules

Hydrogen-bonded organic frameworks (HOFs), as an emerging porous material, have attracted increasing research interest in fluorescence sensing due to their inherent fluorescence emission units with unique physicochemical properties. Herein, based on the organic building block 3,3',5,5'-tetrakis-(4-carboxyphenyl)-1,1'-biphenyl (H₄TCBP), the porous material HOF-TCBP was successfully synthesized using hydrogen bond self-assembly in a DMF solution. The fluorescence properties of the HOF-TCBP solution showed that when the concentration was high, excimers were easily formed, the PL emission was red-shifted, and the fluorescence intensity became weaker. HOF-TCBP showed good sensitivity and selectivity to metal ions Fe³⁺, Cr³⁺, and anion Cr₂O₇^2-. In addition, HOF-TCBP can serve as a label-free fluorescent sensor material for the sensitive and selective detection of dopamine (DA). HOF-based DA sensing is actually easy, low-cost, simple to operate, and highly selective for many potential interfering substances, and it has been successfully applied to the detection of DA in biological samples with satisfactory recoveries (101.1-104.9%). To our knowledge, this is the first report of HOF materials for efficient detection of the neurotransmitter dopamine in biological fluids. In short, this work widely broadens the application of HOF materials as fluorescent sensors for the sensing of ions and biological disease markers.

A ratio fluorescence sensor based on rhodamine B embedded metal-organic framework for glyphosate detection in agri-food products

Glyphosate, a broad-spectrum and high-efficiency herbicide, could accumulate in the human body through the consumption of agri-food products. Herein, a ratio fluorescence sensor based on rhodamine B-embedded amino-functionalized iron-based metal-organic framework (MOF, NH2-MIL-88(Fe)@RhB) bonded with Cu2+ was developed for rapid detection of glyphosate. The synthesized NH2-MIL-88(Fe) was a biconical prism and had a cavity for the embedding of RhB as a reference compound. In the presence of Cu2+, Lewis interactions with NH2-MIL-88(Fe)@RhB cause the fluorescence signal to be turned off. When glyphosate was added, the signal was turned on due to chelation with Cu2+ and hydrogen bonding interactions with NH2-MIL-88(Fe)@RhB. Under optimal conditions, the developed sensor exhibited a linear range of 0.60-45 μmol L-1 with a response time of less than 1 min. The sensor was applied in the analysis of agri-food products (tea, soybean, wheat, cucumber), with recoveries between 97.93% and 109.06%, indicating its promising application in agri-food safety.

Ultrathin MOF nanosheet-based resistive sensors for highly sensitive detection of methanol

Sensors with high-sensitivity for resistive methanol gas detection are highly desirable. Herein, we report newly designed ultrathin anionic metal-organic framework (MOF) nanosheets (NSs), with an average thickness of 10 nm and an electrical conductivity of 3.77 × 10^-4 S cm^-1. The ultrathin MOF NSs can be used as the active material in an electronic methanol gas sensor, which exhibits high sensitivity toward methanol gas at room temperature, i.e., high R_air/R_gas (363.2 at 100 ppm), fast gas response/recovery speed (6 s/2 s at 20 ppm), long-term stability, and superior cross-selectivity against other interfering gases.

Excimer Formation in the Non-Van-Der-Waals 2D Semiconductor Bi2 O2 Se

The layered semiconductor Bi₂ O₂ Se is a promising new-type 2D material that holds layered structure via electrostatic forces instead of van der Waals (vdW) attractions. Aside from the huge success in device performance, the non-vdW nature in Bi₂ O₂ Se with a built-in interlayer electric field has also provided an appealing platform for investigating unique photoexcited carrier dynamics. Here, experimental evidence for the observation of excimers in multilayer Bi₂ O₂ Se nanosheets via transient absorption spectroscopy is presented. It is found that the excimer formation is the primary decay pathway of photoexcited excitons and three-stage excimer dynamics with corresponding time scales are established. Excitation-fluence-dependent excimer dynamics further suggest that the excimer is diffusive and its formation can be simply described as excitons relaxed to an excimer geometry. This work indicates the outstanding promise of unique excitonic processes in Bi₂ O₂ Se, which may motivate novel device designs.

Multi-Emission from Single Metal-Organic Frameworks under Single Excitation

Multi-emission materials have come to prominent attention ascribed to their extended applications other than single-emission ones. General and robust design strategies of a single matrix with multi-emission under single excitation are urgently required. Metal-organic frameworks (MOFs) are porous materials prepared with organic ligands and metal nodes. The variety of metal nodes and ligands makes MOFs with great superiority as multi-emission matrices. Guest species encapsulated into the channels or pores of MOFs are the additional emission sites for multi-emission. In this review, multi-emission MOFs according to the different excitation sites are summarized and classified. The emission mechanisms are discussed, such as antenna effect, excited-state intramolecular proton transfer (ESIPT) and tautomerism for dual-emission. The factors that affect the emissions are revealed, including ligand-metal energy transfer and host-guest interaction, etc. Multi-emission MOFs could be predictably designed and prepared, once the emissive factors are controlled rationally in combination with the different multi-emission mechanisms. Correspondingly, new and practical applications are realized, including but not limited to ratiometric/multi-target sensing and bioimaging, white light-emitting diodes, and anti-counterfeiting. The design strategies of multi-emission MOFs and their extensive applications are reviewed. The results will shed light on other multi-emission systems to develop the structure-derived functionality and applications.

From Hydrogen Bond to van der Waals Force: Molecular Scalpel Strategy to Exfoliate a Two-Dimensional Metal-Organic Nanosheet

The facile exfoliation of a two-dimensional metal-organic nanosheet of {[Co(HL)(H₂O)(Py)_3/4]·¹/₂H₂O·DMF}_n [1-Py; H₃L = 5-(1H-pyrazol-4-yl)isophthalic acid and Py = pyridine] was achieved, via a molecular scalpel strategy, by weakening intermolecular forces between adjacent layers. The resulting 1-Py/KB40 (KB = Ketjen black) shows an increased oxygen evolution reaction (OER) performance with an overpotential of 370 mV at a current density of 10 mA cm^-2 and a Tafel slope of 58 mV dec^-1. This work sheds light on the structure-morphology-reactivity relationship of such materials in OER.

Water-Stable Nickel Metal-Organic Framework Nanobelts for Cocatalyst-Free Photocatalytic Water Splitting to Produce Hydrogen

Development of water-stable metal-organic frameworks (MOFs) for promising visible-light-driven photocatalytic water splitting is highly desirable but still challenging. Here we report a novel p-type nickel-based MOF single crystal (Ni-TBAPy-SC) and its exfoliated nanobelts (Ni-TBAPy-NB) that can bear a wide range of pH environment in aqueous solution. Both experimental and theoretical results indicate a feasible electron transfer from the H₄TBAPy ligand (light-harvesting center) to the Ni-O cluster node (catalytic center), on which water splitting to produce hydrogen can be efficiently driven free of cocatalyst. Compared to the single crystal, the exfoliated two-dimensional (2D) nanobelts show more efficient charge separation due to its shortened charge transfer distance and remarkably enhanced active surface areas, resulting in 164 times of promoted water reduction activity. The optimal H₂ evolution rate on the nanobelt reaches 98 μmol h^-1 (ca. 5 mmol h^-1 g^-1) showing benchmarked apparent quantum efficiency (AQE) of 8.0% at 420 nm among water-stable MOFs photocatalysts.

Luminescent Two-Dimensional Metal-Organic Framework Nanosheets with Large π-Conjugated System: Design, Synthesis, and Detection of Anti-Inflammatory Drugs and Pesticides

Two-dimensional (2D) metal-organic framework (MOF) nanosheets, with largely exposed surface area and highly accessible active sites, have emerged as a novel kind of sensing material. Here, a luminescent 2D MOF nanosheet was designed and synthesized by a facile top-down strategy based on a three-dimensional (3D) layered MOF {[Zn(H2L)(H2O)2]·H2O}n (Zn-MOF; H4L = 3,5-bis(3',5'-dicarboxyphenyl)-1H-1,2,4-triazole). With a large π-conjugated system and rigid planar structure, ligand H4L was elaborately selected to construct the bulk Zn-MOF, which can be readily exfoliated into 2D nanosheets, owing to the weak interlayer interactions and easy-to-release H2O molecules in the interspaces of 2D layers. Given the great threat posed to the ecological environment by anti-inflammatory drugs and pesticides, the developed luminescent Zn-MOF nanosheets were utilized to determine these organic pollutants, achieving highly selective and sensitive detection of diclofenac sodium (DCF) and tetramethylthiuram disulfide (TMTD). Compared to the detection limits of 3D Zn-MOF (7.72 ppm for DCF, 6.01 ppm for TMTD), the obviously lower detection limits for 2D Zn-MOF nanosheets toward DCF (0.20 ppm) and TMTD (0.18 ppm) further revealed that the largely exposed surface area with rigid planar structure and ultralarge π-conjugated system greatly accelerated electron transfer, which brought about a vast improvement in response sensitivity. The remarkable quenching performance for DCF and TMTD stems from a combined effect of photoinduced electron transfer and competitive energy absorption. The possible sensing mechanism was systematically investigated by the studies of powder X-ray diffraction, UV-vis, luminescence lifetime, and density functional theory calculations.

Au NPs fabricated on biguanidine-modified Zr-UiO-66 MOFs: a competent reusable heterogeneous nanocatalyst in the green synthesis of propargylamines

In this study, we utilized functionalized metal organic frameworks (MOFs) as a host matrix to embed gold (Au) nanoparticles.

The modulation effect of an electron-rich guest on the luminescence of naphthalene diimide-based metal–organic frameworks

A series of host–guest MOF materials were successfully fabricated by virtue of donor–acceptor interactions, which exhibit color-tunable emissions in a wide wavelength range by rational selection of guest molecules.

Design and Synthesis of Coordination Polymers with Cu(II) and Heterocyclic N-Oxides

The relations of coordination network connectivity with coordination properties of heterocyclic N-oxides, Cu(I,II), and co-ligands were discussed based on the comparative analysis of 623 structures extracted from the Cambridge Structural...

Uranyl phosphonates: crystalline materials and nanosheets for temperature sensing

Ultrathin nanosheets of luminescent metal-organic frameworks or coordination polymers have been widely used for sensing ions, solvents and biomolecules but, as far as we are aware, not yet used for temperature sensing. Herein we report two luminescent uranyl phosphonates based on 2-(phosphonomethyl)benzoic acid (2-pmbH₃), namely (UO₂)(2-pmbH₂)₂ (1) and (H₃O)[(UO₂)₂(2-pmb)(2-pmbH)] (2). The former has a supramolecular layer structure, composed of chains of corner-sharing {UO₆} octahedra and {PO₃C} tetrahedra which are connected by hydrogen bonds between phosphonate and carboxylic groups. Compound 2 possesses a unique 2D anionic framework structure, where the inorganic uranyl phosphonate chains made up of {UO₇} and {PO₃C} polyhedra are cross-linked by 2-pmb^3- ligands. The carboxylic groups of 2-pmbH^2- ligands are pendant on the two sides of the layers and form hydrogen bonds between the layers. Both compounds can be exfoliated in acetone via a top-down freeze-thaw method, resulting in nanosheets of two-layer thickness. Interestingly, the photoluminescence (PL) of 1 and 2 is highly temperature sensitive. Variable temperature PL studies revealed that compounds 1 and 2 can be used as thermometers in the temperature ranges 120-300 K and 100-280 K, respectively. By doping the nanosheets into polymer matrix, 1-ns@PMMA and 2-ns@PMMA were prepared. The PL intensity of 1-ns@PMMA is insensitive to temperature, unlike that of the bulk sample. While 2-ns@PMMA exhibits similar temperature-dependent luminescence behaviour to its bulk counterpart, thereby enabling its potential application as a thermometer in the temperature range 100-280 K.

Pd immobilization biguanidine modified Zr-UiO-66 MOF as a reusable heterogeneous catalyst in Suzuki-Miyaura coupling

In recent days, nanohybrid metal organic frameworks (MOF) have been considered as next generation catalysts due to their unique features like large surface to volume ratio, tailorable geometry, uniform pore sizes and homogeneous distribution of active sites. In this report, we address the biguanidine modified 3D Zr-centred MOF UiO-66-NH2 following a post synthetic modification approach. Utilizing the excellent chelating ability of biguanidine, Pd ions are immobilized over the host matrix MOF. The as-synthesized material was physicochemically characterized using a broad range of analytical techniques like FT-IR, electron microscopy, EDS, elemental mapping, XRD and ICP-OES. Subsequently the material has been catalytically employed in the classical Suzuki-Miyaura coupling towards the synthesis of diverse biphenyl derivatives at sustainable conditions. There are very few reports on the covalently modified MOFs towards the organic coupling reactions. The catalyst has been isolated by centrifugation and recycled in 9 consecutive runs with almost insignificant leaching and minute decrease in reactivity.

Atomically Thin Materials for Next-Generation Rechargeable Batteries

Atomically thin materials (ATMs) with thicknesses in the atomic scale (typically <5 nm) offer inherent advantages of large specific surface areas, proper crystal lattice distortion, abundant surface dangling bonds, and strong in-plane chemical bonds, making them ideal 2D platforms to construct high-performance electrode materials for rechargeable metal-ion batteries, metal-sulfur batteries, and metal-air batteries. This work reviews the synthesis and electronic property tuning of state-of-the-art ATMs, including graphene and graphene derivatives (GE/GO/rGO), graphitic carbon nitride (g-C₃N₄), phosphorene, covalent organic frameworks (COFs), layered transition metal dichalcogenides (TMDs), transition metal carbides, carbonitrides, and nitrides (MXenes), transition metal oxides (TMOs), and metal-organic frameworks (MOFs) for constructing next-generation high-energy-density and high-power-density rechargeable batteries to meet the needs of the rapid developments in portable electronics, electric vehicles, and smart electricity grids. We also present our viewpoints on future challenges and opportunities of constructing efficient ATMs for next-generation rechargeable batteries.

Incorporation of Chromophores into Metal-Organic Frameworks for Boosting CO2 Conversion

The exploitation of highly stable and active catalysts for the conversion of CO₂ into valuable fuels is desirable but is a great challenge. Herein, we report that the incorporation of chromophores into metal-organic frameworks (MOFs) could afford robust catalysts for efficient CO₂ conversion. Specifically, a porous Nd(III) MOF (Nd-TTCA; TTCA^3- = triphenylene-2,6,10-tricarboxylate) was constructed by incorporating one-dimensional Nd(CO₂)_n chains and TTCA^3- ligands, which exhibits a very high stability, retaining its framework not only in the air at 300 °C for 2 h but also in boiling aqueous solutions at pH 1-12 for 7 days. More importantly, Nd-TTCA has achieved a 5-fold improvement in photocatalytic activity for reducing CO₂ to HCOOH and a 10-fold improvement in catalytic activity for the cycloaddition of CO₂ into cyclic carbonate in comparison to those of H₃TTCA itself. This work gives a new strategy to design efficient artificial crystalline catalysts for CO₂ conversion.

Calcium-Based Metal-Organic Frameworks and Their Potential Applications

Metal-organic frameworks (MOFs) built on calcium metal (Ca-MOFs) represent a unique subclass of MOFs featuring high stability, low toxicity, and relatively low density. Ca-MOFs show considerable potential for molecular separations, electronic, magnetic, and biomedical applications, although they are not investigated as extensively as transition metal-based MOFs. Compared to MOFs made of other groups of metals, Ca-MOFs may be particularly advantageous for certain applications such as adsorption and storage of light molecules because of their gravimetric benefit, and drug delivery due to their high biocompatibility. This review intends to provide an overview on the recent development of Ca-MOFs, including their synthesis, crystal structures, important properties, and related applications. Various synthetic methods and techniques, types of building blocks, structure and porosity features, selected physical properties, and potential uses will be discussed and summarized. Representative examples will be illustrated for each type of important applications with a focus on their structure-property relations.

Single-Crystalline Ultrathin 2D Porous Nanosheets of Chiral Metal-Organic Frameworks

Two-dimensional (2D) materials with highly ordered in-plane nanopores are crucial for numerous applications, but their rational synthesis and local structural characterization remain two grand challenges. We illustrate here that single-crystalline ultrathin 2D MOF nanosheets (MONs) with intrinsic porosity can be prepared by exfoliating layered metal-organic frameworks (MOFs), whose layers are stabilized by sterically bulky groups. As a result, three three-dimensional (3D) isostructural lanthanide MOFs possessing porous layer structures are constructed by coordinating metal ions with an angular dicarboxylate linker derived from chiral 1,1'-biphenyl phosphoric acid with pendant mesityl groups. The Eu-MOF is readily ultrasonic exfoliated into single-crystalline nanosheets with a thickness of ca. 6.0 nm (2 layers) and a lateral size of 1.5 × 3.0 μm². The detailed structural information, i.e., the pore channels and individual organic and inorganic building units in the framework, is clearly visualized by a low-dose high-resolution transmission electron microscopy (HRTEM) technique. Benefiting from their ultrathin feature, the nanosheets are well embedded into the polymer matrix to form free-standing mixed-matrix membranes. In both the solution and membrane phase, the fluorescence of the MONs can be effectively quenched by a total of 17 chiral terpenes and terpenoids through supramolecular interactions with uncoordinated chiral phosphoric acids, leading to a chiral optical sensor for detecting vapor enantiomers, which is among the most challenging molecular recognition tasks.

An excellent thermostable dual-functionalized 3D fsx-type Cd(ii) MOF for the highly selective detection of Fe3+ ions and ten nitroaromatic explosives

A 3D fsx-type thermostable MOF was designed and synthesized for the highly selective detection of Fe3+ ions and ten nitroaromatic explosives.

Metal-Organic Layers for Electrocatalysis and Photocatalysis

Metal-organic layers (MOLs) are two-dimensional analogues of metal-organic frameworks (MOFs) with a high aspect ratio and thickness down to a monolayer. Active sites on MOLs are more accessible than those on MOFs thanks to the two-dimensional feature of MOLs, which allows easier chemical modification around the catalytic center. MOLs can also be assembled with other functional materials through surface anchoring sites that can facilitate charge/energy transport through the hybrid material. MOLs are thus quite suitable for interfacial catalysis like electrocatalysis and photocatalysis. In this outlook, we focus on representative progress of constructing unique interfacial sites on MOLs with designer paths for charge separation and energy transfer, as well as cooperative cavities for superior substrate adsorption and activation. We also discuss challenges and potentials in the future development of MOL catalysts and catalysts beyond MOLs.

Destruction of Metal-Organic Frameworks: Positive and Negative Aspects of Stability and Lability

Metal-organic frameworks (MOFs), constructed from organic linkers and inorganic building blocks, are well-known for their high crystallinity, high surface areas, and high component tunability. The stability of MOFs is a key prerequisite for their potential practical applications in areas including storage, separation, catalysis, and biomedicine since it is essential to guarantee the framework integrity during utilization. However, MOFs are prone to destruction under external stimuli, considerably hampering their commercialization. In this Review, we provide an overview of the situations where MOFs undergo destruction due to external stimuli such as chemical, thermal, photolytic, radiolytic, electronic, and mechanical factors and offer guidelines to avoid unwanted degradation happened to the framework. Furthermore, we discuss possible destruction mechanisms and their varying derived products. In particular, we highlight cases that utilize MOF instability to fabricate varying materials including hierarchically porous MOFs, monolayer MOF nanosheets, amorphous MOF liquids and glasses, polymers, metal nanoparticles, metal carbide nanoparticles, and carbon materials. Finally, we provide a perspective on the utilization of MOF destruction to develop advanced materials with a superior hierarchy for various applications.

Color tuning of intrinsic white-light emission in anthracene-linker coordination networks

White light-emitting diodes (WLEDs) have aroused great attention due to their potential technological applications. In this work, we present two new Zn(ii) anthracene-linker-driven coordination polymers that exhibit intrinsic white-light emission. The emission covers the whole visible spectrum at room temperature. The chromaticity coordinates of the broadband emission can be tuned under external stimuli, including thermal and mechanical grinding. The obtained coordination polymer materials emit a "warm" white light at room temperature suitable for indoor lighting applications as well as a "cold" white light at the cryogenic temperature. Hence, the well-defined structures and mechanically tunable emission provide an excellent opportunity for realizing their potential as white emitters in optoelectronics.

Building a robust 3D Ca-MOF by a new square Ca4O SBU for purification of natural gas

For the first time, a new square Ca4O SBU is introduced into a 3D Ca-MOF, ([MeNH2]2[Ca4O(MTB)2(EtOH)4])·(solvent)n (1), to generate a (4,8)-connected flu-topology structure. Compound 1 exhibits selective adsorption of C3 and C2 hydrocarbons and CO2 over CH4 with especially high IAST selectivities for C3 hydrocarbons over CH4 (at 15/85 and 50/50 ratio) at 298K and 1 bar.

Establishing charge-transfer excitons in 2D perovskite heterostructures

Charge-transfer excitons (CTEs) immensely enrich property-tuning capabilities of semiconducting materials. However, such concept has been remaining as unexplored topic within halide perovskite structures. Here, we report that CTEs can be effectively formed in heterostructured 2D perovskites prepared by mixing PEA₂PbI₄:PEA₂SnI₄, functioning as host and guest components. Remarkably, a broad emission can be demonstrated with quick formation of 3 ps but prolonged lifetime of ~0.5 μs. This broad PL presents the hypothesis of CTEs, verified by the exclusion of lattice distortion and doping effects through demonstrating double-layered PEA₂PbI₄/PEA₂SnI₄ heterostructure when shearing-away PEA₂SnI₄ film onto the surface of PEA₂PbI₄ film by using hand-finger pressing method. The below-bandgap photocurrent indicates that CTEs are vital states formed at PEA₂PbI₄:PEA₂SnI₄ interfaces in 2D perovskite heterostructures. Electroluminescence shows that CTEs can be directly formed with electrically injected carriers in perovskite LEDs. Clearly, the CTEs presents a new mechanism to advance the multifunctionalities in 2D perovskites.

Forming charge transfer excitons (CTEs) exclusively within perovskite structures remains as an unexplored issue. Here, the authors report the establishment of CTEs for demonstrating broad light emission within quasi-2D perovskite heterostructures, presenting “intermolecular-type” excited states.

Ultrathin two-dimensional conjugated metal-organic framework single-crystalline nanosheets enabled by surfactant-assisted synthesis

Two-dimensional conjugated metal-organic frameworks (2D c-MOFs) have recently emerged for potential applications in (opto-)electronics, chemiresistive sensing, and energy storage and conversion, due to their excellent electrical conductivity, abundant active sites, and intrinsic porous structures. However, developing ultrathin 2D c-MOF nanosheets (NSs) for facile solution processing and integration into devices remains a great challenge, mostly due to unscalable synthesis, low yield, limited lateral size and low crystallinity. Here, we report a surfactant-assisted solution synthesis toward ultrathin 2D c-MOF NSs, including HHB-Cu (HHB = hexahydroxybenzene), HHB-Ni and HHTP-Cu (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene). For the first time, we achieve single-crystalline HHB-Cu(Ni) NSs featured with a thickness of 4-5 nm (∼8-10 layers) and a lateral size of 0.25-0.65 μm2, as well as single-crystalline HHTP-Cu NSs with a thickness of ∼5.1 ± 2.6 nm (∼10 layers) and a lateral size of 0.002-0.02 μm2. Benefiting from the ultrathin feature, the synthetic NSs allow fast ion diffusion and high utilization of active sites. As a proof of concept, when serving as a cathode material for Li-ion storage, HHB-Cu NSs deliver a remarkable rate capability (charge within 3 min) and long-term cycling stability (90% capacity retention after 1000 cycles), superior to the corresponding bulk materials and other reported MOF cathodes.

CO2 controls the oriented growth of metal-organic framework with highly accessible active sites

The production of 2D metal-organic frameworks (MOFs) with highly exposed active surfaces is of great importance for catalysis. Here we demonstrate the formation of MOF nanosheets by utilizing CO₂ as a capping agent to control the oriented growth of MOF. This strategy has many advantages over the conventional methods. For example, it is template-free and proceeds at mild temperature (35 °C), CO₂ can be easily removed by depressurization, and the properties of the MOF nanosheets can be well adjusted by changing CO₂ pressure. Such a simple, rapid, efficient and adjustable route produces MOF nanosheets with ultrathin thickness (∼10 nm), small lateral size (∼100 nm) and abundant unsaturated coordination metal sites on surfaces. Owing to these unique features, the as-synthesized MOF nanosheets exhibit superior activity for catalyzing the oxidation reactions of alcohols.

Producing two-dimensional metal-organic frameworks (MOFs) with highly exposed active surfaces is important for catalysis. Here, authors prepare MOF nanosheets by using CO₂ as a capping agent to control the oriented growth of MOFs.