Postsynthetic Approach for the Rational Design of Chiral Ferroelectric Metal–Organic Frameworks

Chiral cadmium-amine complexes for stimulating non-linear optical activity and photoluminescence in solids based on aurophilic stacks

The design of high-performance optical materials can be realized using coordination polymers (CPs) often supported by non-covalent interactions, such as metallophilicity. The challenge is to control two or more optical effects, e.g., non-linear optics (NLO) and photoluminescence (PL). We present a new strategy for the combination of the NLO effect of second-harmonic generation (SHG) and the visible PL achieved by linking dicyanidoaurate(i) ions, which form luminescent metallophilic stacks, with cadmium(ii) complexes bearing chiral amine ligands, used to break the crystal's symmetry. We report a family of NLO- and PL-active materials based on heterometallic Cd(ii)-Au(i) coordination systems incorporating enantiopure propane-1,2-diamine (pda) ligands (1-S, 1-R), their racemate (2), and enantiopure trans-cyclopentane-1,2-diamine (cpda) ligands (3-S, 3-R). Due to acentric space groups, they exhibit the SHG signal, tunable within the range of 11-24% of the KDP reference, which was correlated with the dipole moments of Cd(ii) units. They show efficient blue PL whose energy and quantum yield, the latter ranging from 0.40 to 0.83, are controlled by Cd(ii) complexes affecting the Au-Au distances and vibrational modes. We prove that chiral Cd(ii)-amine complexes play the role of molecular agents for the stimulation of both the NLO and PL of the materials based on aurophilic stacks.

Design of Multivariate Biological Metal-Organic Frameworks: Toward Mimicking Active Sites of Enzymes

Mimicking enzymatic processes carried out by natural enzymes, which are highly efficient biocatalysts with key roles in living organisms, attracts much interest but constitutes a synthetic challenge. Biological metal-organic frameworks (bioMOFs) are potential candidates to be enzyme catalysis mimics, as they offer the possibility to combine biometals and biomolecules into open-framework porous structures capable of simulating the catalytic pockets of enzymes. In this work, we first study the catalase activity of a previously reported bioMOF, derived from the amino acid L-serine, with formula {CaIICuII6[(S,S)-serimox]3(OH)2(H2O)} · 39H2O (1) (serimox = bis[(S)-serine]oxalyl diamide), which is indeed capable to mimic catalase enzymes, in charge of preventing cell oxidative damage by decomposing, efficiently, hydrogen peroxide to water and oxygen (2H2O2 → 2 H2O + O2). With these results in hand, we then prepared a new multivariate bioMOF (MTV-bioMOF) that combines two different types of bioligands derived from L-serine and L-histidine amino acids with formula CaIICuII6[(S,S)-serimox]2[(S,S)-hismox]1(OH)2(H2O)}·27H2O (2) (hismox = bis[(S)-histidine]oxalyl diamide ligand). MTV-bioMOF 2 outperforms 1 degrading hydrogen peroxide, confirming the importance of the amino acid residue from the histidine amino acid acting as a nucleophile in the catalase degradation mechanism. Despite displaying a more modest catalytic behavior than other reported MOF composites, in which the catalase enzyme is immobilized inside the MOF, this work represents the first example of a MOF in which an attempt is made to replicate the active center of the catalase enzyme with its constituent elements and is capable of moderate catalytic activity.

Highly Stable MOF-Type Lead Halide Luminescent Ferroelectrics

Lead halide molecular ferroelectrics represent an important class of luminescent ferroelectrics, distinguished by their high chemical and structural tunability, excellent processability and distinctive luminescent characteristics. However, their inherent instability, prone to decomposition upon exposure to moisture and light, hinders their broader ferroelectric applications. Herein, for the first time, we present a series of isoreticular metal-organic framework (MOF)-type lead halide luminescent ferroelectrics, demonstrating exceptional robustness under ambient conditions for at least 15 months and even when subjected to aqueous boiling conditions. Unlike conventional metal-oxo secondary building units (SBUs) in MOFs adopting highly centrosymmetric structure with limited structural distortion, our lead halide-based MOFs occupy structurally deformable [Pb2X]+ (X=Cl-/Br-/I-) SBUs that facilitate a c-axis-biased displacement of Pb2+ centers and substantially contribute to thermoinducible structural transformation. Importantly, this class of MOF-type lead halide ferroelectrics undergo ferroelectric-to-paraelectric phase transitions with remarkably high Curie temperature of up to 505 K, superior to most of molecular ferroelectrics. Moreover, the covalent bonding between phosphorescent organic component and the light-harvesting inorganic component achieves efficient spin-orbit coupling and intersystem crossing, resulting in long-lived afterglow emission. The compelling combination of high stability, ferroelectricity and afterglow emission exhibited by lead halide MOFs opens up many potential opportunities in energy-conversion applications.

Regulating Catalytic Oxidation Enantiomers Behavior by Imparting Chiral Microenvironment in Zr-Based Metal-Organic Frameworks

Chiral inversions of enantiomers have significantly different biological activities, so it is important to develop simple and effective methods to efficiently identify optically pure compounds. Inspired by enzyme catalysis, the construction of chiral microenvironments resembling enzyme pockets in the pore space structure of metal-organic frameworks (MOFs) to achieve asymmetric enantioselective recognition and catalysis has become a new research hotspot. Here, a super-stable porphyrin-containing material PCN-224 is constructed by solvothermal method and a chiral microenvironment around the existing catalytic site of the material is created by post-synthesis modifications of the histidine (His) enantiomers. Experimental and theoretical calculations results show that the modulation of chiral ligands around Zr oxide clusters produces different spatial site resistances, which can greatly affect the adsorption and catalytic level of the enantiomeric molecules of tryptophan guests, resulting in a good enantioselective property of the material. It provides new ideas and possibilities for future chiral recognition and asymmetric catalysis.

The past 10 years of molecular ferroelectrics: structures, design, and properties

Ferroelectricity, which has diverse important applications such as memory elements, capacitors, and sensors, was first discovered in a molecular compound, Rochelle salt, in 1920 by Valasek. Owing to their superiorities of lightweight, biocompatibility, structural tunability, mechanical flexibility, etc., the past decade has witnessed the renaissance of molecular ferroelectrics as promising complementary materials to commercial inorganic ferroelectrics. Thus, on the 100th anniversary of ferroelectricity, it is an opportune time to look into the future, specifically into how to push the boundaries of material design in molecular ferroelectric systems and finally overcome the hurdles to their commercialization. Herein, we present a comprehensive and accessible review of the appealing development of molecular ferroelectrics over the past 10 years, with an emphasis on their structural diversity, chemical design, exceptional properties, and potential applications. We believe that it will inspire intense, combined research efforts to enrich the family of high-performance molecular ferroelectrics and attract widespread interest from physicists and chemists to better understand the structure-function relationships governing improved applied functional device engineering.

The green synthesis and applications of biological metal-organic frameworks for targeted drug delivery and tumor treatments

Biological metal-organic frameworks (bio-MOFs) constitute a growing subclass of MOFs composed of metals and bio-ligands derived from biology, such as nucleobases, peptides, saccharides, and amino acids. Bio-ligands are more abundant than other traditional organic ligands, providing multiple coordination sites for MOFs. However, bio-MOFs are typically prepared using hazardous or harmful solvents or reagents, as well as laborious processes that do not conform to environmentally friendly standards. To improve biocompatibility and biosafety, eco-friendly synthesis and functionalization techniques should be employed with mild conditions and safer materials, aiming to reduce or avoid the use of toxic and hazardous chemical agents. Recently, bio-MOF applications have gained importance in some research areas, including imaging, tumor therapy, and targeted drug delivery, owing to their flexibility, low steric hindrances, low toxicity, remarkable biocompatibility, surface property refining, and degradability. This has led to an exponential increase in research on these materials. This paper provides a comprehensive review of updated strategies for the synthesis of environmentally friendly bio-MOFs, as well as an examination of the current progress and accomplishments in green-synthesized bio-MOFs for drug delivery aims and tumor treatments. In conclusion, we consider the challenges of applying bio-MOFs for biomedical applications and clarify the possible research orientation that can lead to highly efficient therapeutic outcomes.

Two-Dimensional Metal-Organic Frameworks Towards Spintronics

The intrinsic properties of predesignable topologies and tunable electronic structures, coupled with the increase of electrical conductivity, make two-dimensional metal-organic frameworks (2D MOFs) highly prospective candidates for next-generation electronic/spintronic devices. In this Minireview, we present an outline of the design principles of 2D MOF-based spintronics materials. Then, we highlight the spin-transport properties of 2D MOF-based organic spin valves (OSVs) as a notable achievement in the progress of 2D MOFs for spintronics devices. After that, we discuss the potential for spin manipulation in 2D MOFs with bipolar magnetic semiconductor (BMS) properties as a promising field for future research. Finally, we provide a brief summary and outlook to encourage the development of novel 2D MOFs for spintronics applications.

Construction of Photoreactive Chiral Metal-Organic Frameworks and Their [2 + 2] Photocycloaddition Reactions

Chiral metal-organic frameworks (CMOFs) and solid-state [2 + 2] photocyclization have been explored as independent areas in crystal engineering. We herein report the photoreactive CMOFs that undergo a [2 + 2] photocycloaddition reaction for the first time. Through the incorporation of a dipyridyl olefin ligand, 1,4-bis[2-(4-pyridyl)ethenyl]benzene, and d-camphoric acid or l-camphoric acid, we constructed a pair of homochiral Zn(II) CMOFs (d-1 or l-1) with a two-dimensional sql topology via a two-step procedure to avoid racemization. Both d-1 and l-1 were photoinert due to the large olefin bond separation. The removal of the solvent molecules between layers enabled them (d-1a and l-1a) to undergo [2 + 2] cycloaddition reactions; d-1a is more reactive (70%) than l-1a (20%) probably due to proper desolvation-induced rearrangement. The photoluminescence properties are also discussed. This work presents a new perspective on photoreactive homochiral network materials with diverse topologies and applications.

The First Enantiomeric Stereogenic Sulfur-Chiral Organic Ferroelectric Crystals

Chiral ferroelectric crystals with intriguing features have attracted great interest and many with point or axial chirality based on the stereocarbon have been successively developed in recent years. However, ferroelectric crystals with stereogenic heteroatomic chirality have never been documented so far. Here, we discover and report a pair of enantiomeric stereogenic sulfur-chiral single-component organic ferroelectric crystals, Rs -tert-butanesulfinamide (Rs -tBuSA) and Ss -tert-butanesulfinamide (Ss -tBuSA) through the deep understanding of the chemical design of molecular ferroelectric crystals. Both enantiomers adopt chiral-polar point group 2 (C2 ) and exhibit mirror-image relationships. They undergo high-temperature 432F2-type plastic ferroelectric phase transition around 348 K. The ferroelectricity has been well confirmed by ferroelectric hysteresis loops and domains. Polarized light microscopy records the evolution of the ferroelastic domains, according with the fact that the 432F2-type phase transition is both ferroelectric and ferroelastic. The very soft characteristics with low elastic modulus and hardness reveals their excellent mechanical flexibility. This finding indicates the first stereosulfur chiral molecular ferroelectric crystals, opening up new fertile ground for exploring molecular ferroelectric crystals with great application prospects.

A Strategy to Enhance the Ferroelectric Behavior of MOF-802(Hf) via Doping Zr4+ Ions

MOF ferroelectrics, as a crucial member of molecular ferroelectrics, have shown intriguing advantages owing to the designability of structures and tunability of physicochemical properties, which make them an appealing group of ferroelectric materials. However, the weak ferroelectric property still is a huge challenge for further development. Here, a series of Zr-doped MOF-802(Hf)s were successfully synthesized through doping Zr⁴⁺ ions into the parent MOF-802(Hf) to improve ferroelectric properties. The well-shaped P-E hysteresis loops of Zr-doped MOF-802(Hf)s illustrate their ferroelectricity, and ferroelectric properties are effectively enhanced compared with the parent MOF-802(Hf). What's more, remanent polarization reaches 0.511 μC/cm² when the concentration of Zr⁴⁺ ions is 5%, which is 5 times higher than that of the parent MOF-802(Hf) and is on par with some perovskite ferroelectrics. The increased ferroelectric performance is attributed to the enhanced polarity of the whole structure triggered by lattice distortion when Hf⁴⁺ ions of the parent MOF-802(Hf) are substituted by Zr⁴⁺ ions. As far as we know, this is the first report on Hf-MOF exhibiting improved ferroelectric behaviors through doping metal ions into lattice nodes. This work demonstrates that introducing the second metal ions into lattice nodes of MOFs is an efficacious approach for exploiting MOF ferroelectrics with superior performance.

Effective Enhancement of the Ferroelectric Performance of Polar Co-Gallate MOF by Doping M2+ Ions (M = Mg, Mn, Ni) into Framework Nodes

MOF ferroelectrics have been demonstrated to be a promising candidate owing to various structures and controllable properties. However, weak ferroelectricity hampers their boom. Herein, a convenient strategy, doping metal ions into the framework nodes of parent MOF, is adopted to enhance ferroelectric performance. A series of M-doped Co-Gallate (M = Mg, Mn, Ni) were synthesized to improve ferroelectric properties. The electrical hysteresis loop demonstrated its ferroelectric behaviors, exhibiting obviously improved ferroelectric properties compared with the parent Co-Gallate. The remanent polarization was enhanced by two times for Mg-doped Co-Gallate, six times for Mn-doped Co-Gallate, and four times for Ni-doped Co-Gallate. The promoted ferroelectric performances are ascribed to the enhanced polarity of the overall structure triggered by framework distortion. Intriguingly, ferroelectric behaviors increase in the order Mg < Ni < Mn, displaying the same tendency as the difference value in the ionic radius between Co²⁺ ions and M²⁺ metal ions (M = Mg, Mn, Ni). These results demonstrate doping of metal ions is a valid strategy to enhance ferroelectric performances, which may serve as a guide in modulating ferroelectric behaviors.

Study of the Counter Cation Effects on the Supramolecular Structure and Electronic Properties of a Dianionic Oxamate-Based {NiII2} Helicate

Herein, we describe the synthesis, crystal structure, and electronic properties of {[K₂(dmso)(H₂O)₅][Ni₂(H₂mpba)₃]·dmso·2H₂O}_n (1) and [Ni(H₂O)₆][Ni₂(H₂mpba)₃]·3CH₃OH·4H₂O (2) [dmso = dimethyl sulfoxide; CH₃OH = methanol; and H₄mpba = 1,3-phenylenebis(oxamic acid)] bearing the [Ni₂(H₂mpba)₃]^2- helicate, hereafter referred to as {Ni^II₂}. SHAPE software calculations indicate that the coordination geometry of all the Ni^II atoms in 1 and 2 is a distorted octahedron (O_h) whereas the coordination environments for K1 and K2 atoms in 1 are Snub disphenoid J84 (D_2d) and distorted octahedron (O_h), respectively. The {Ni^II₂} helicate in 1 is connected by K⁺ counter cations yielding a 2D coordination network with sql topology. In contrast to 1, the electroneutrality of the triple-stranded [Ni₂(H₂mpba)₃] ^2- dinuclear motif in 2 is achieved by a [Ni(H₂O)₆]²⁺ complex cation, where the three neighboring {Ni^II₂} units interact in a supramolecular fashion through four R₂²(10) homosynthons yielding a 2D array. Voltammetric measurements reveal that both compounds are redox active (with the Ni^II/Ni^I pair being mediated by OH^- ions) but with differences in formal potentials that reflect changes in the energy levels of molecular orbitals. The Ni^II ions from the helicate and the counter-ion (complex cation) in 2 can be reversibly reduced, resulting in the highest faradaic current intensities. The redox reactions in 1 also occur in an alkaline medium but at higher formal potentials. The connection of the helicate with the K⁺ counter cation has an impact on the energy levels of the molecular orbitals; this experimental behavior was further supported by X-ray absorption near-edge spectroscopy (XANES) experiments and computational calculations.

Metal-organic frameworks as chemical nanoreactors for the preparation of catalytically active metal compounds

Since the emergence of metal-organic frameworks (MOFs), a myriad of thrilling properties and applications, in a wide range of fields, have been reported for these materials, which mainly arise from their porous nature and rich host-guest chemistry. However, other important features of MOFs that offer great potential rewards have been only barely explored. For instance, despite the fact that MOFs are suitable candidates to be used as chemical nanoreactors for the preparation, stabilization and characterization of unique functional species, that would be hardly accessible outside the functional constrained space offered by MOF channels, only very few examples have been reported so far. In particular, we outline in this feature recent advances in the use of highly robust and crystalline oxamato- and oxamidato-based MOFs as reactors for the in situ preparation of well-defined catalytically active single atom catalysts (SACS), subnanometer metal nanoclusters (SNMCs) and supramolecular coordination complexes (SCCs). The robustness of selected MOFs permits the post-synthetic (PS) in situ preparation of the desired catalytically active metal species, which can be characterised by single-crystal X-ray diffraction (SC-XRD) taking advantage of its high crystallinity. The strategy highlighted here permits the always challenging large-scale preparation of stable and well-defined SACs, SNMCs and SCCs, exhibiting outstanding catalytic activities.

A review on chiral metal-organic frameworks: synthesis and asymmetric applications

Chiral metal-organic frameworks (CMOFs) have the characteristics of framework structure diversity and functional tunability, and have important applications in the fields of chiral identification, separation of enantiomers and asymmetric catalysis. In recent years, the application of CMOFs has also been extended to other research fields, such as circularly polarized fluorescence and chiral ferroelectrics. Compared with achiral MOFs, the design of CMOFs only considers the modes of introduction of chirality, and also takes into account the crystallization and purification. Therefore, the synthesis and characterization of CMOFs face many difficult challenges. This review discusses three effective strategies for constructing CMOFs, including direct synthesis of chiral ligands, spontaneous resolution of achiral ligands or chiral template-induced synthesis, and post-synthetic chiralization of achiral MOFs. In addition, this review also discusses the recent application progress of CMOFs in chiral molecular recognition, enantiomer separation, asymmetric catalysis, circularly polarized fluorescence, and chiral ferroelectrics.

Harvesting mechanical energy for hydrogen generation by piezoelectric metal-organic frameworks

Piezocatalysis, the process of directly converting mechanical energy into chemical energy, has emerged as a promising alternative strategy for green H₂ production. Nevertheless, conventional inorganic piezoelectric materials suffer from limited structural tailorability and small surface area, which greatly impedes their mechanically driven catalytic efficiency. Herein, we design and fabricate a novel UiO-66(Zr)-F₄ metal-organic framework (MOF) nanosheet for piezocatalytic water splitting, with the highest H₂ evolution rate reaching 178.5 μmol g^-1 within 5 h under ultrasonic vibration excitation (110 W, 40 kHz), far exceeding that of the original UiO-66 host. A reduced bandgap from 2.78 to 2.43 eV is achieved after introducing a fluorinated ligand. Piezoresponse force microscopy measurements demonstrate a much stronger piezoelectric response for UiO-66(Zr)-F₄, which may result from the polarity of the introduced fluorinated ligand. This work highlights the potential of MOF-based porous piezoelectric nanomaterials in harvesting mechanical energy to drive chemical reactions such as water splitting.

Multivariate Metal-Organic Framework/Single-Walled Carbon Nanotube Buckypaper for Selective Lead Decontamination

The search for efficient technologies empowering the selective capture of environmentally harmful heavy metals from wastewater treatment plants, at affordable prices, attracts wide interest but constitutes an important technological challenge. We report here an eco-friendly single-walled carbon nanotube buckypaper (SWCNT-BP) enriched with a multivariate amino acid-based metal-organic framework (MTV-MOF) for the efficient and selective removal of Pb2+ in multicomponent water systems. Pristine MTV-MOF was easily immobilized within the porous network of entangled SWCNTs, thus obtaining a stable self-standing adsorbing membrane filter (MTV-MOF/SWCNT-BP). SWCNT-BP alone shows a moderately good removal performance with a maximum adsorption capacity of 180 mg·g-1 and a considerable selectivity for Pb(II) ions in highly concentrated multi-ion solutions over a wide range of lead concentration (from 200 to 10000 ppb). Remarkably, these features were outperformed with the hybrid membrane filter MTV-MOF/SWCNT-BP, exhibiting enhanced selectivity and adsorption capacity (310 mg·g-1, which is up to 42% higher than that of the neat SWCNT-BP) and consequently enabling a more efficient and selective removal of Pb2+ from aqueous media. MTV-MOF/SWCNT-BP was able to reduce [Pb2+] from the dangerous 1000 ppb level to acceptable limits for drinking water, below 10 ppb, as established by the current EPA and WHO limits. Thus, the eco-friendly composite MTV-MOF/SWCNT-BP shows the potential to be effectively used several times as a reliable adsorbent for Pb2+ removal for household drinking water or in industrial treatment plants for water and wastewater lead decontamination.

Heterometallic chiral [Mn13Cu8] single-molecule magnets

An enantiomeric pair of 3d-3d heterometallic clusters of {Mn^IIMn^III₁₂Cu^II₈} has been synthesized in an aqueous solution without heating. The crystal structures reveal a five-layered core of {Mn^IIMn^III₁₂Cu^II₈}, which is a slightly distorted {Mn^IIMn^III₁₂} tetradecahedron with its eight triangular faces capped by a Cu^II ion respectively. The magnetic measurement results establish that anisotropic Mn^III atom-containing clusters L-1 and D-1 exhibit slow relaxation of their magnetization vectors.

Post-synthetic modification within MOFs: a valuable strategy for modulating their ferroelectric performance

It is a great route designing new MOF ferroelectrics to enrich the scope of ferroelectrics or improving the ferroelectric performance to enhance the opportunity of applications through the strategy of post-synthetic modification (PSM).

A cationic thorium-organic framework with triple single-crystal-to-single-crystal transformation peculiarities for ultrasensitive anion recognition

Single-crystal-to-single-crystal transformation of metal-organic frameworks has been met with great interest, as it allows for the creation of new materials in a stepwise manner and direct visualization of structural transitions when subjected to external stimuli. However, it remains a peculiarity among numerous metal-organic frameworks, particularly for the ones constructed from tetravalent metal cations. Herein, we present a cationic thorium-organic framework displaying unprecedented triple single-crystal-to-single-crystal transformations in organic solvents, water, and NaIO3 solution. Notably, both the interpenetration conversion and topological change driven by the SC-SC transformation have remained elusive for thorium-organic frameworks. Moreover, the single-crystal-to-single-crystal transition in NaIO3 solution can efficiently and selectively turn the ligand-based emission off, leading to the lowest limit of detection (0.107 μg kg-1) of iodate, one of the primary species of long-lived fission product 129I in aqueous medium, among all luminescent sensors.

Soft Ferroelectrics Enabling High-Performance Intelligent Photo Electronics

Soft ferroelectrics based on organic and organic-inorganic hybrid materials have gained much interest among researchers owing to their electrically programmable and remnant polarization. This allows for the development of numerous flexible, foldable, and stretchable nonvolatile memories, when combined with various crystal engineering approaches to optimize their performance. Soft ferroelectrics have been recently considered to have an important role in the emerging human-connected electronics that involve diverse photoelectronic elements, particularly those requiring precise programmable electric fields, such as tactile sensors, synaptic devices, displays, photodetectors, and solar cells for facile human-machine interaction, human safety, and sustainability. This paper provides a comprehensive review of the recent developments in soft ferroelectric materials with an emphasis on their ferroelectric switching principles and their potential application in human-connected intelligent electronics. Based on the origins of ferroelectric atomic and/or molecular switching, the soft ferroelectrics are categorized into seven subgroups. In this review, the efficiency of soft ferroelectrics with their distinct ferroelectric characteristics utilized in various human-connected electronic devices with programmable electric field is demonstrated. This review inspires further research to utilize the remarkable functionality of soft electronics.

Chiral UiO-MOFs based QCM sensors for enantioselective discrimination of hazardous biomolecule

Developments of enantioselective devices for discriminating bio-enantiomers is of significant importance. Due to the vital role of Cysteine (Cys) in biological processes and the hazardous effect of its D-enantiomer, discriminating Cys enantiomers without auxiliary enzyme is highly wanted. In this work, a pair of UiO-MOF enantiomers (UiO-tart) have been fabricated through post-modification, which could be further fabricated into enantiomeric sensing devices (UiO-tart@Au). By employing the Quartz Crystal Microbalance (QCM) technology, gravimetric discrimination of Cys enantiomers could be achieved. UiO-tart@Au is highly enantioselective, and the afforded enantioselective factor (5.97 ± 0.54) represents the best performance reported ever. In the fabricated device, MOF layer acts as the chiral selector for specific Cys enantiomer, and the reaction between the captured Cys enantiomer and Au results in the mass growth of the system. Solid-phase extraction (SPE) gives an e.e. value of 71.6 ± 3.8%, substantially confirming the chiral-selector role of UiO-tart. DFT calculations indicate that enantiomeric H-bonding effect and greater reaction enthalpy should be the reason. To the best of our knowledge, this work represents the first example of chiral tartaric acid derived MOF sensors for enantioselective discrimination of Cys, suggesting a promising potential of developing chiral MOFs based devices for enhanced enantioselective application.

Magneto-Electric Directional Anisotropy in Polar Soft Ferromagnets of Two-Dimensional Organic-Inorganic Hybrid Perovskites

Two-dimensional organic-inorganic hybrid perovskites (2D-OIHPs) are attracting interest due to their structural tunability and rich functional characteristics, such as ferroelectricity and ferromagnetism. Here, we report the chiral-polar ferromagnetic 2D-OIHP copper chlorides with discernable electric polarization in the inorganic layers. In these systems, the magneto-electric (ME) correlation has been clearly observed by measuring a magneto-electric directional anisotropy (MEA), in which an optical absorption coefficient changes with reversal of the light propagating direction. We have found that the MEA can be induced by a low magnetic field of about 50 mT, reflecting soft magnetic nature. The present results suggest a new paradigm for designing functional ME multiferroics, which effectively couples magnetic and electric properties.

Homochiral metal-organic frameworks for enantioseparation

Obtaining homochiral compounds is of high importance to human health and environmental sustainability. Currently, enantioseparation is one of the most effective approaches to obtain homochiral compounds. Thanks to their controlled synthesis and high efficiency, homochiral metal-organic frameworks (HMOFs) are one of the most widely studied porous materials to enable enantioseparation. In this review, we discuss the chiral pocket model in depth as the key to unlock enantioselective separation mechanisms in HMOFs. In particular, we classify our discussion of these chiral pockets (also regarded as "molecular traps") into: (a) achiral/chiral linker based helical channels as a result of packing modality; and (b) chiral pores inherited from chiral ligands. Driven by a number of mechanisms of enantioseparation, conceptual advances have been recently made in the design of HMOFs for achieving high enantioseparation performances. Herein, these are systematically categorised and discussed. Further we elucidate various applications of HMOFs as regards enantioseparation, systematically classifying them into their use for purification and related analytical utility according to the reported examples. Last but not the least, we discuss the challenges and perspectives concerning the rational design of HMOFs and their corresponding enantioseparations.

Soluble/MOF-Supported Palladium Single Atoms Catalyze the Ligand-, Additive-, and Solvent-Free Aerobic Oxidation of Benzyl Alcohols to Benzoic Acids

Metal single-atom catalysts (SACs) promise great rewards in terms of metal atom efficiency. However, the requirement of particular conditions and supports for their synthesis, together with the need of solvents and additives for catalytic implementation, often precludes their use under industrially viable conditions. Here, we show that palladium single atoms are spontaneously formed after dissolving tiny amounts of palladium salts in neat benzyl alcohols, to catalyze their direct aerobic oxidation to benzoic acids without ligands, additives, or solvents. With this result in hand, the gram-scale preparation and stabilization of Pd SACs within the functional channels of a novel methyl-cysteine-based metal-organic framework (MOF) was accomplished, to give a robust and crystalline solid catalyst fully characterized with the help of single-crystal X-ray diffraction (SCXRD). These results illustrate the advantages of metal speciation in ligand-free homogeneous organic reactions and the translation into solid catalysts for potential industrial implementation.

SC-SC Anion-Assisted Linker Exchange within a Three-Dimensional Cu(II)-Triazole Framework: A Luminescent Probe for S2

A three-dimensional (3D) binodal 3,5-connected net, {[Cu(MTP)(H2O)](NO3)}n (1) with the Schläfli symbol of {3·72}{32·75·83} can be transformed into a two-dimensional (2D) kagóme network with the Schlafli symbol of {32·62·72} in an irreversible single crystal-single crystal (SC-SC) guest-assisted linker exchange process. The product of this SC-SC represents the first luminescent probe for S2- based on triazole ligand.

Crystal structures and the ferroelectric properties of homochiral metal-organic frameworks constructed from a single chiral ligand

MOFs have proven to be promising candidates for designing ferroelectric materials. Herein, two new homochiral MOFs, Co-MOF-1 and Co-MOF-2, have been synthesized using the chiral ligand, HL (HL = phenyl-((pyridin-4-ylmethyl)-amino)-acetic acid), and Co(NO₃)₂·6H₂O. Co-MOF-1 was obtained via a two-step synthetic route involving a hydrogel to Zn-MOF conversion and a dissolution-recrystallization process. Co-MOF-2 was directly synthesized by a coordination reaction between chiral ligand, HL, and Co(NO₃)₂·6H₂O under hydrothermal conditions. We investigate the correlation between the ferroelectric properties of the samples and their crystal structures. The ferroelectric properties of Co-MOF-1 and Co-MOF-2 are drastically different. Indeed, Co-MOF-2 shows an obvious hysteretic behavior, while a clear electric hysteresis loop was not observed for Co-MOF-1. These significant disparities may be attributed to the different molecular dipole moments in Co-MOF-1 and Co-MOF-2. The different octahedral coordination units in the molecular structures of the Co-MOFs may alter the dipole moments of the molecules, resulting in the absence of a hysteresis loop for Co-MOF-1.

An unusual high-frequency ferroelectric obtained via the post-synthetic modification of a metal-organic framework

Ferroelectrics as crucial functional materials have attracted much interest since ferroelectricity was discovered in 1920. Herein, an unusual high-frequency ferroelectric, (CH3)2NH·HCl@Cd-MOF, was successfully obtained through a dual-step synthetic methodology. A chiral porous Cd-MOF with a channel size of 6.8 × 6.8 Å was synthesized via self-assembly of chiral Schiff-base ligands and Cd2+ ions. Subsequently, polarizable (CH3)2NH·HCl was introduced into the channels of the Cd-MOF and hence the host-guest system (CH3)2NH·HCl@Cd-MOF was formed. The as-synthesized (CH3)2NH·HCl@Cd-MOF displays obvious ferroelectricity at a high frequency of 1 kHz. Such a high-frequency ferroelectric is extremely rare among MOF-based ferroelectric materials, and the high-frequency ferroelectricity means that (CH3)2NH·HCl@Cd-MOF has potential for use in ferroelectric memories. The results again demonstrate that post-synthetic modification is a promising approach for achieving rational and precise design of ferroelectric materials.

Efficient Identification for Alcohol Homologues and Hyperthermy Based on Coordination Polymer Multiple Structural Transformations

Recently, coordination polymer materials are of high interest due to the potential applications for chemical sensing and luminescent materials. In this work, we designed a photofluorescence coordination polymer material based on a donor-metal-acceptor structure. The donor-metal-acceptor architecture showed unusual multiple environmental responsiveness accompanied by a great change of fluorescence behaviors. Generally, organic homologue molecules are not easily distinguished by coordination polymer sensors because they have similar molecular structures and interaction sites. However, using the feature of multiple structural transformations, the accurate identification for organic homologue molecules can be realized, especially in a short time to quickly identify MeOH in other alcohol homologues (even in mixed atmospheres with only 10% MeOH). The visualization transformation of fluorescence can also be realized by single crystal to single crystal thermal-induced coordination bond ON/OFF behavior. The reversible structure conversion strategy provides new ideas for the accurate identification of organic homolog molecules or external environmental stimuli.

Gas Transport in Mixed Matrix Membranes: Two Methods for Time Lag Determination

The most widely used method to measure the transport properties of dense polymeric membranes is the time lag method in a constant volume/pressure increase instrument. Although simple and quick, this method provides only relatively superficial, averaged data of the permeability, diffusivity, and solubility of gas or vapor species in the membrane. The present manuscript discusses a more sophisticated computational method to determine the transport properties on the basis of a fit of the entire permeation curve, including the transient period. The traditional tangent method and the fitting procedure were compared for the transport of six light gases (H2, He, O2, N2, CH4, and CO2) and ethane and ethylene in mixed matrix membranes (MMM) based on Pebax®1657 and the metal–organic framework (MOF) CuII2(S,S)-hismox·5H2O. Deviations of the experimental data from the theoretical curve could be attributed to the particular MOF structure, with cavities of different sizes. The fitting procedure revealed two different effective diffusion coefficients for the same gas in the case of methane and ethylene, due to the unusual void morphology in the MOFs. The method was furthermore applied to mixed gas permeation in an innovative constant-pressure/variable-volume setup with continuous analysis of the permeate composition by an on-line mass-spectrometric residual gas analyzer. This method can provide the diffusion coefficient of individual gas species in a mixture, during mixed gas permeation experiments. Such information was previously inaccessible, and it will greatly enhance insight into the mixed gas transport in polymeric or mixed matrix membranes.

Overcoming Crystallinity Limitations of Aluminium Metal-Organic Frameworks by Oxalic Acid Modulated Synthesis

A modulated synthesis approach based on the chelating properties of oxalic acid (H2 C2 O4 ) is presented as a robust and versatile method to achieve highly crystalline Al-based metal-organic frameworks. A comparative study on this method and the already established modulation by hydrofluoric acid was conducted using MIL-53 as test system. The superior performance of oxalic acid modulation in terms of crystallinity and absence of undesired impurities is explained by assessing the coordination modes of the two modulators and the structural features of the product. The validity of our approach was confirmed for a diverse set of Al-MOFs, namely X-MIL-53 (X=OH, CH3 O, Br, NO2 ), CAU-10, MIL-69, and Al(OH)ndc (ndc=1,4-naphtalenedicarboxylate), highlighting the potential benefits of extending the use of this modulator to other coordination materials.

Revelation of the chiral recognition of alanine and leucine in an l-phenylalanine-based metal–organic framework

An l-phenylalanine-based Zn-MOF, namely [Zn₂(l-Phe)₂(bpe)₂]_n, was designed for experimentally revealing the chiral recognition mechanism to alanine and leucine by means of ¹³C CP MAS NMR spectroscopy, XPS and control experiment.

Functional metal–organic frameworks as effective sensors of gases and volatile compounds

This review summarizes the recent advances of metal organic framework (MOF) based sensing of gases and volatile compounds.

Metal-dependent photocatalytic activity and magnetic behaviour of a series of 3D Co–Ni metal organic frameworks

Mixed-metal systems in MOFs modulate their photocatalytic activity and magnetic behavior, which may open a new avenue for the preparation of functional materials with adjustable magnetic behaviors.

A pair of homochiral trinuclear Zn(ii) clusters exhibiting unusual ferroelectric behaviour at high temperature

A pair of homochiral trinuclear Zn(ii) clusters exhibit unusual ferroelectric behaviour at high temperature.