Multistate structures in a hydrogen-bonded polycatenation non-covalent organic framework with diverse resistive switching behaviors.

The inherent structural flexibility and reversibility of non-covalent organic frameworks have enabled them to exhibit switchable multistate structures under external stimuli, providing great potential in the field of resistive switching (RS), but not well explored yet. Herein, we report the 0D+1D hydrogen-bonded polycatenation non-covalent organic framework (HOF-FJU-52), exhibiting diverse and reversible RS behaviors with the high performance. Triggered by the external stimulus of electrical field E at room temperature, HOF-FJU-52 has excellent resistive random-access memory (RRAM) behaviors, comparable to the state-of-the-art materials. When cooling down below 200 K, it was transferred to write-once-read-many-times memory (WORM) behaviors. The two memory behaviors exhibit reversibility on a single crystal device through the temperature changes. The RS mechanism of this non-covalent organic framework has been deciphered at the atomic level by the detailed single-crystal X-ray diffraction analyses, demonstrating that the structural dual-flexibility both in the asymmetric hydrogen bonded dimers within the 0D loops and in the infinite π-π stacking column between the loops and chains contribute to reversible structure transformations between multi-states and thus to its dual RS behaviors.

Tetranuclear CuII Cluster as the Ten Node Building Unit for the Construction of a Metal-Organic Framework for Efficient C2 H2 /CO2 Separation.

Rational design of high nuclear copper cluster-based metal-organic frameworks has not been established yet. Herein, we report a novel MOF (FJU-112) with the ten-connected tetranuclear copper cluster [Cu4 (PO3 )2 (µ2 -H2 O)2 (CO2 )4 ] as the node which was capped by the deprotonated organic ligand of H4 L (3,5-Dicarboxyphenylphosphonic acid). With BPE (1,2-Bis(4-pyridyl)ethane) as the pore partitioner, the pore spaces in the structure of FJU-112 were divided into several smaller cages and smaller windows for efficient gas adsorption and separation. FJU-112 exhibits a high separation performance for the C2 H2 /CO2 separation, which were established by the temperature-dependent sorption isotherms and further confirmed by the lab-scale dynamic breakthrough experiments. The grand canonical Monte Carlo simulations (GCMC) studies show that its high C2 H2 /CO2 separation performance is contributed to the strong π-complexation interactions between the C2 H2 molecules and framework pore surfaces, leading to its more C2 H2 uptakes over CO2 molecules.

Optimized Sieving Effect for Ethanol/Water Separation by Ultramicroporous MOFs.

High-purity ethanol is a promising renewable energy resource, however separating ethanol from trace amount of water is extremely challenging. Herein, two ultramicroporous MOFs (UTSA-280 and Co-squarate) were used as adsorbents. A prominent water adsorption and a negligible ethanol adsorption identify perfect sieving effect on both MOFs. Co-squarate exhibits a surprising water adsorption capacity at low pressure that surpassing the reported MOFs. Single crystal X-ray diffraction and theoretical calculations reveal that such prominent performance of Co-squarate derives from the optimized sieving effect through pore structure adjustment. Co-squarate with larger rhombohedral channel is suitable for zigzag water location, resulting in reinforced guest-guest and guest-framework interactions. Ultrapure ethanol (99.9 %) can be obtained directly by ethanol/water mixed vapor breaking through the columns packed with Co-squarate, contributing to a potential for fuel-grade ethanol purification.

Partial Linker Substitution Strategy to Construct a Quaternary HKUST-like MOF for Efficient Acetylene Storage and Separation.

Multicomponent metal-organic frameworks (MOFs) have received much attention as emerging materials capable of precisely programing exquisite structures and specific functions. Here, we applied a partial linker substitution strategy to compile an HKUST-1-like quaternary MOF by introducing a bifunctional ligand into the well-known HKUST-1 structure. FUT-1, a new HKUST-like tbo topology MOF, was assembled with paddlewheel [Cu2(COO)4], triangular metallocycle pyrazole cluster Cu3(µ3-OH) (NN)3 building blocks, and two distinct linkers. FUT-1 exhibited good mechanical stability, water stability, and chemical stability (pH = 3-12) in aqueous solutions. Moreover, the porous environments created by this multicomponent primitive endow FUT-1 with high C2H2 storage and significantly selective separation performance of C2H2/CO2. Dynamic breakthrough experiments and ideal adsorbed solution theory calculations further demonstrate that FUT-1 can selectively capture C2H2 from C2H2/CO2 mixtures under ambient conditions. Based on grand canonical Monte Carlo simulations, the high C2H2 separation performance of FUT-1 is attributed to the π-complex formed between the C2H2 molecule and the trinuclear metallocycle clusters on the wall, which provides stronger affinity for C2H2 recognition than the CO2 molecule.

Switched Proton Conduction in Metal-Organic Frameworks.

Stimuli-responsive materials can respond to external effects, and proton transport is widespread and plays a key role in living systems, making stimuli-responsive proton transport in artificial materials of particular interest to researchers due to its desirable application prospects. On the basis of the rapid growth of proton-conducting porous metal-organic frameworks (MOFs), switched proton-conducting MOFs have also begun to attract attention. MOFs have advantages in crystallinity, porosity, functionalization, and structural designability, and they can facilitate the fabrication of novel switchable proton conductors and promote an understanding of the comprehensive mechanisms. In this Perspective, we highlight the current progress in the rational design and fabrication of stimuli-responsive proton-conducting MOFs and their applications. The dynamic structural change of proton transfer pathways and the role of trigger molecules are discussed to elucidate the stimuli-responsive mechanisms. Subsequently, we also discuss the challenges and propose new research opportunities for further development.

Greatness in Simplicity: Efficient Red Room-Temperature Phosphorescence from Simple Halogenated Maleimides with a 2D Layered Structure.

Herein, two maleimide derivatives substituted by Br (DBM) and I (DIM) with a two-dimensional (2D) layered structure are found to have highly efficient red room-temperature phosphorescence (RTP) at 660 nm in solid state, which is independent of their morphology (crystal, powder, and film). The red RTP of DBM and DIM is closely related to the synergism of nπ-ct-π* transitions and the 2D halogen-bonded network. Interestingly, the red RTP can be excited by visible light of 500 nm, which should be ascribed to the forbidden absorption from the ground state to the triplet state activated in the layered halogen-bonded framework. Due to the rich intermolecular interactions in the rigid layered structure, the red RTP of DBM is very stable under water or external force stimulation. Notably, Hg(II) and Cd(II) ions in a pure aqueous solution result in an opposite change in the RTP intensity of the DBM film. The detection limit of Hg(II) ion is as low as 2.5 × 10-5 nM, lesser than all reported values. The above results not only provide a new idea for the design of simple and efficient red RTP materials but also make it possible to develop solid-state phosphorescent probes for toxic heavy metal ions in environmental sewage with high sensitivity and selectivity.

Efficient Separation of Acetylene-Containing Mixtures Using ZIF-8 Membranes.

Metal-organic framework (MOF) membranes show great potential in the separation of acetylene mixtures. In this work, we have prepared ZIF-8 membranes on polyamide (PA) substrates for the highly selective separation of acetylene/methane and acetylene/carbon dioxide mixtures. The C2H2/CH4 and C2H2/CO2 mixtures can be successfully separated using the ZIF-8 membranes, with separation factors of 12.1 and 1.8, respectively. Based on the results of the cross-permeation tests of C2H2/CH4, CO2/CH4, and C2H2/CO2, the separation mechanism of C2H2/CH4 in our ZIF-8 membrane can be attributed to a higher affinity for acetylene and molecular sieving effect, while C2H2/CO2 separation is related to thermodynamic factors. It is worth noting that this is the first example of MOF membranes to successfully separate C2H2 from CH4 and CO2.

Ethylene/ethane separation in a stable hydrogen-bonded organic framework through a gating mechanism.

Porous materials are very promising for the development of cost- and energy-efficient separation processes, such as for the purification of ethylene from ethylene/ethane mixture-an important but currently challenging industrial process. Here we report a microporous hydrogen-bonded organic framework that takes up ethylene with very good selectivity over ethane through a gating mechanism. The material consists of tetracyano-bicarbazole building blocks held together through intermolecular CN···H-C hydrogen bonding interactions, and forms as a threefold-interpenetrated framework with pores of suitable size for the selective capture of ethylene. The hydrogen-bonded organic framework exhibits a gating mechanism in which the threshold pressure required for guest uptake varies with the temperature. Ethylene/ethane separation is validated by breakthrough experiments with high purity of ethylene (99.1%) at 333 K. Hydrogen-bonded organic frameworks are usually not robust, yet this material was stable under harsh conditions, including exposure to strong acidity, basicity and a variety of highly polar solvents.

Photoinduced Electron-Transfer (PIET) Strategy for Selective Adsorption of CO2 over C2 H2 in a MOF.

Similarities in sizes, shapes, and physical properties between carbon dioxide (CO2 ) and acetylene (C2 H2 ) make it a great challenge to separate the major impurity CO2 from products in C2 H2 production. The use of porous materials is an appealing path to replace current very costly and energy-consuming technologies, such as solvent extraction and cryogenic distillation; however, high CO2 /C2 H2 uptake ratio with minor adsorption of C2 H2 at standard pressure was only unexpectedly observed in scarce examples in recent years although the related research started early at 1950s, and general design strategies to realize this aim are still absent. This work has successfully developed an efficient PIET strategy and obtained the second highest CO2 /C2 H2 adsorption ratio for porous materials in a proof-of-concept MOF with a photochromism-active bipyridinium zwitterion. An unprecedented photocontrollable gate effect, owing to change of interannular dihedral after photoinduced generation of radical species, was also observed for the first time. These findings will inspire design and synthesis of porous materials for high efficient gas adsorption and separation.

Two Tb-metal organic frameworks with different metal cluster nodes for C2H2/CO2 separation.

Through regulating the reaction solvent and temperature, two Tb-based metal-organic frameworks, ((CH3)2NH2)2[Tb9(µ3-OH)8(µ2-OH)3(PTB)6]·(DMF)14·(H2O)19 (1) and ((CH3)2NH2)3{[Tb9(µ3-O)2(µ3-OH)12(H2O)6][Tb3(µ3-O)(HCO2)3 (PTB)6]}·(DMF)12·(H2O)7 (2) (H3PTB = pyridine-2,4,6-tribenzoic acid), have been synthesized. Structural analysis showed that the cluster node of 1 is a Tb9 cluster, while 2 contains two different nodes of a Tb3 cluster and a Tb9 cluster, which leads to their different pore structures and may potentially separate C2H2/CO2. Gas adsorption demonstrates that both MOFs can separate C2H2 and CO2, but 2 has a more optimized pore environment than 1 and can exhibit better selective separation of C2H2/CO2.

A Microporous Metal-Organic Framework with Channels Constructed from Nonpolar Aromatic Rings for the Selective Separation of Ethane/Ethylene Mixtures.

The separation of ethane and ethylene is an important segment in the purification of chemical raw materials in industrial production. However, due to their similar physical and chemical properties, the separation of C2 H6 /C2 H4 is challenging. Herein, we report the selective adsorption of ethane over ethylene by a microporous metal-organic framework with nonpolar aromatic rings constructed channels, [Co1.5 (TATB)(H2 O)0.5 ] ⋅ 5DMA ⋅ 3H2 O (Co-TATB, H3 TATB=4,4',4''-(s-triazine-2,4,6-triyl) tribenzoic acid). This compound showed a higher ethane capacity than that of ethylene, and a low adsorption enthalpy of ethane only of 19.4 kJ mol-1 . Further, the dynamic breakthrough experimental confirmed that Co-TATB can selectively adsorb ethane from ethane/ethylene separation.

Integrating the Pillared-Layer Strategy and Pore-Space Partition Method to Construct Multicomponent MOFs for C2H2/CO2 Separation.

Introducing multiclusters and multiligands (mm) in a well-defined array will greatly increase the diversity of metal-organic frameworks (MOFs). Here, a series of porous mm-MOFs constructed from a pillared-layer and pore-space partition (PL-PSP) have been achieved. FJU-6 with {Co3}-cluster-based sheets and {Co6}-cluster-based pillars exhibits new (3,9,12)-connected llz topology. By using the substituted analogues of the ligands and metal ions, seven isoreticular mm-MOFs (FJU-6-X, X = PTB, TATB, Me-INA, F-INA, NDC, BrBDC, Ni) have been synthesized with the adjustable BET surface areas ranging from 731 to 1306 m2/g as well as the adsorption capacity of CO2 increasing by 77%. The C2H2/CO2 mixture can be effectively separated in the breakthrough experiments in the fixed bed filled with solid FJU-6-TATB at ambient temperature. In all, integrating pillared-layer strategy and pore-space partitioning is effective at constructing mm-MOFs with multivariate environments for the optimization of gas adsorption and separation.

Isostructural MOFs with Higher Proton Conductivity for Improved Oxygen Evolution Reaction Performance.

Here we synthesized two new isostructural MOFs (FJU-82-Co/FJU-82-Zn) and first observed that tuning of the proton conductivity may provide an effective strategy to improve the electrocatalytic OER perfomances of isostructural crystalline MOF materials. The conductivity value for FJU-82-Co is 7.40 × 10-5 S cm-1, which is 127-fold that for FJU-82-Zn with 5.80 × 10-7 S cm-1 at 60 °C and 98% RH, while the overpotential of FJU-82-Co is 0.57 V at 1 mA cm-2, which is better than that of FJU-82-Zn with 1.17 V.

Pure Metal-Organic Framework Microlasers with Controlled Cavity Shapes.

Metal-organic frameworks (MOFs) are an emerging kind of laser material, yet they remain a challenge in the controlled fabrication of crystal nanostructures with desired morphology for tuning their optical microcavities. Herein, the shape-engineering of pure MOF microlasers was demonstrated based on the coordination-mode-tailored method. The one-dimensional (1D) microwires and 2D microplates were selectively fabricated through changing the HCl concentration to tailor the coordination modes. Both the single-crystalline microwires and microplates with strong optical confinement functioned as low-threshold MOF microlasers. Moreover, distinct lasing behaviors of 1D and 2D MOF microcrystals confirm a typical shape-dependent microcavity effect: 1D microwires serve as Fabry-Pérot (FP) resonators, and 2D microplates lead to the whispering-gallery-mode (WGM) microcavities. These results provide a special pathway for the exploitation of MOF-based micro/nanolasers with on-demand functions.

Steric-Hindrance-Controlled Laser Switch Based on Pure Metal-Organic Framework Microcrystals.

Herein, we demonstrated a steric-hindrance-controlled laser switch in pure metal-organic framework (MOF) microcrystals. The well-faceted MOF microwires with aggregation-induced emission (AIE) lumnogens as linkers function as typical Fabry-Pérot microlasers. The steric hindrance around the AIE linkers can be reduced by the loss of guest molecules, which lead to the enhanced rotation of linkers with red-shifted gain behavior. On this basis, the gain region was readily switched through changing the steric hindrance via the desorption/adsorption of guests. As a result, the reversible switching of the dual-wavelength lasing from MOF microwires was achieved. The results provide a promising route to the development of versatile micro-/nanolasers with desired applications.

Simultaneous implementation of resistive switching and rectifying effects in a metal-organic framework with switched hydrogen bond pathway.

Resistive random-access memory (RRAM) has evolved as one of the most promising candidates for the next-generation memory, but bistability for information storage, simultaneous implementation of resistive switching and rectification effects, and a better understanding of switching mechanism are still challenging in this field. Herein, we report a RRAM device based on a chiral metal-organic framework (MOF) FJU-23-H2O with switched hydrogen bond pathway within its channels, exhibiting an ultralow set voltage (~0.2 V), a high ON/OFF ratio (~105), and a high rectification ratio (~105). It is not only the first MOF with voltage-gated proton conduction but also the first single material showing both rectifying and resistive switching effects. By single-crystal x-ray diffraction analyses, the mechanism of the resistive switching has been demonstrated.

Enhancement of Intrinsic Proton Conductivity and Aniline Sensitivity by Introducing Dye Molecules into the MOF Channel.

The encapsulation of dyes into metal-organic frameworks (MOFs) has generated a variety of platforms for luminescence, but little attention has been paid to their application in proton conduction. Here, a cationic MOF {{[In3OL1.5(H2O)3](NO3)}·(DMA)3·(CH3CN)6·(H2O)30} n (FJU-10, H4L = 4,4',4″,4‴-(1,4-phenylenbis(pyridine-4,2,6-triyl))-tetrabenzoic acid, DMA = N, N-dimethylacetamide) was synthesized, and the dye molecule 8-hydroxy-1,3,6-pyrenetrisulfonic acid trisodium salt (HPTS) was further added to the MOF growth solution, but during the reaction, HPTS was nitrated and nitrated HPTS was encapsulated into the FJU-10 to obtain dye@FJU-10. As a result, the intrinsic proton conductivity of dye@FJU-10 is nearly 5 times higher than that of FJU-10 at 90 °C. Dye@FJU-10 exhibits more sensitive fluorescence quenching toward aniline than FJU-10 in DMF solution (the detection limits of FJU-10 and dye@FJU-10 are as low as 0.58 and 0.62 µM, respectively). Here, it is demonstrated for the first time that intrinsic proton conductivity can be effectively improved by encapsulating a nitrated HPTS dye into an MOF.

Two water-stable lanthanide metal-organic frameworks with oxygen-rich channels for fluorescence sensing of Fe(iii) ions in aqueous solution.

The development of metal-organic framework sensors with excellent stability under harsh physical and chemical conditions is of great significance in practical applications. Two isostructural lanthanide-organic frameworks, Ln-MOF ({Ln(L)(H2O)(DMA)}n, {FJU-13-Ln, Ln = Eu, Tb } (H3L = 3,5-(4-carboxybenzyloxy)benzoic acid)), were found to display exceptional stability, not only being stable in both acidic and basic aqueous solutions (pH 3-11), but also maintaining good robustness in boiling water and after activation. Furthermore, the plentiful oxygen atoms in the oxygen-rich channels of FJU-13-Eu and FJU-13-Tb acted as Lewis base sites for sensing metal ions, exhibiting high sensitivity (Stern-Volmer constant, KSV = 2.03 × 104 M-1 for FJU-13a-Eu and KSV = 2.11 × 104 M-1 for FJU-13a-Tb) and low detection limits (1.41 µM for FJU-13a-Eu and 1.01 µM for FJU-13a-Tb) for Fe3+ ions. This suggests that the two Ln-MOFs are promising fluorescent sensors for Fe3+ ion detection.

Robustness, Selective Gas Separation, and Nitrobenzene Sensing on Two Isomers of Cadmium Metal-Organic Frameworks Containing Various Metal-O-Metal Chains.

Poor stability has been one of the major difficulties affecting to the practical application of metal-organic frameworks (MOFs). In this work, we obtained two 3D structurally isomeric Cd-MOFs, {[Cd6(NH2Me2)2(PTB)4(HCOO)2(H2O)]·(DMF)13·(H2O)4} n (FJU-35) and {[Cd6(NH2Me2)2(PTB)4(HCOO)2]·(DMF)6·(H2O)2} n (FJU-36) (H3PTB = pyridine-2,4,6-tribenzoic acid) containing different CdII-O-CdII chains by varying the addition agents. FJU-35 with coordinated solvent and formate in asymmetric µ3-η1:η2 coordination mode within the CdII-O-CdII chains is vulnerable to external attacks and is apt to collapse after activation, while FJU-36 with no coordinated solvent in the CdII-O-CdII chains but fully protected by the carboxylates from the ligands and the symmetric formate in the coordination mode µ3-η2:η2 is stable, and its activated sample shows efficient separation of C2H2/CH4 and C2H2/CO2 mixtures. Conversely, FJU-35 with more vulnerability is more sensitive to the detection of nitrobenzene than FJU-36.