Barely porous organic cages for hydrogen isotope separation

Etched stainless steel wire modified with conjugated microporous polymers-F6 for jacket-free stir bar sorptive extraction of benzoylureas in juice sample

Benzoylurea (BU) insecticides have been widely used for pest control as third-generation insecticides. Considering that their residues in food may cause adverse effects on human health, the upper limits of BUs remaining in food have been set by the administration. Therefore, it is essential to develop a sensitive and efficient analytical method to determine the residues of BUs in food. Stir bar sorptive extraction (SBSE) is a novel sample preparation technique, and stainless steel wire (SSW) is an ideal substrate for an SBSE device. In this work, a novel SBSE device of SSW jacket-free stir bar with a dumbbell shape was designed and prepared. The conjugated microporous polymer CMP-F6, which possesses a porous structure, high hydrophobicity and rich fluorine-containing functional groups, was immobilized on the surface of SSW by the method of polyacrylonitrile glue adhesion. Compared with previous studies, which used SSW as a substrate, the method of etching partial SSW with hydrochloric acid, on the one hand, made the surface of SSW rough and easy to modify the extraction coating, and on the other hand, converted itself into a dumbbell-shaped structure, which is conducive to improving the extraction efficiency and stability of the SBSE device. The method of SBSE-HPLC-UV was established for determining five BUs. Owing to the hydrophobic interaction and F-F interaction between CMP-F6 and analytes, this method showed good extraction efficiency and had good linearity (R2 ≥ 0.9945) and high sensitivity (LODs in the range of 0.1-0.2 ng mL-1). It was used for the analysis of benzoylurea in an apple juice sample, and the recoveries were 74.3-117.9%.

Stepwise Synthesis of Low-Symmetry Hexacationic Pyridinium Organic Cages

An efficient approach was developed for the synthesis of the well-known BlueCage by pre-bridging two 2,4,6-tris(4-pyridyl)-1,3,5-triazine (TPT) panels with one linker followed by cage formation in a much improved yield and shortened reaction time. Such a stepwise methodology was further applied to synthesize three new pyridinium organic cages, C2, C3, and C4, where the low-symmetry cages C3 and C4 with angled panels demonstrated better recognition properties toward 1,1'-bi-2-naphthol (BINOL) than the high-symmetry analogue C2 featuring parallel platforms.

Synthesis, polymorphism, and shape complementarity-induced co-crystallization of hexanuclear Co(II) clusters capped by a flexible heteroligand shell

Polymorphism and co-crystallization have gradually gained attention as new tools in the development of modern crystalline functional materials. However, the study on the selective self-assembly of metal clusters into multicomponent crystals is still in its infancy. Herein, we present the synthesis and characterization of two new heteroleptic hydroxido-acetato and acetato Co(II) clusters [Co6(OH)2(OAc)4(pyret)6] (1) and [Co6(OAc)6(pyret)6] (2) incorporating auxiliary 2-pyrrolidinoethoxylate (pyret) ligands. On this occasion, we revealed that the commonly used thermal procedure for dehydration of cobalt(II) acetate leads to a reagent comprising substantial contamination by cobalt hydroxido moieties. Comprehensive structural analysis of new compounds demonstrated intriguing crystal structure diversity of hydroxido-acetato cluster 1, which represents a rare example of both conformational and packing polymorphism in one compound, originating from the flexibility of organic O,N-ligands in the secondary coordination sphere. Furthermore, both clusters exhibit an interesting propensity for the selective formation of co-crystals 1·2 driven mainly by van der Waals forces and specific shape complementarity between co-formers.

Multifold Post-Modification of Macrocycles and Cages by Isocyanate-Induced Azadefluorination Cyclisation

We present the multiple post-modification of organic macrocycles and cages, introducing functional groups into two- and three-dimensional supramolecular scaffolds bearing fluorine substituents, which opens up new possibilities in multi-step supramolecular chemistry employing the vast chemical space of readily available isocyanates. The mechanism and scope of the reaction that proceeds after isocyanate addition to the benzylamine motif via an azadefluorination cyclisation (ADFC) were investigated using DFT calculations, and a series of aromatic isocyanates with different electronic properties were tested. The compounds show excellent chemical stability and were fully characterised. They can be used for subsequent cross-coupling reactions, and ADFC can be used directly to generate cross-linked membranes from macrocycles or cages when using ditopic isocyanates. Single-crystal X-ray (SC-XRD) analysis shows the proof of the formation of the desired supramolecular entity together with the connectivity predicted by calculations and from 19F NMR shifts, allowing the late-stage functionalisation of self-assembled macrocycles and cages by ADFC.

Non-statistical assembly of multicomponent [Pd2ABCD] cages

Self-assembled hosts, inspired by biological receptors and catalysts, show application potential in sustainable synthesis, energy conversion and medicine. Implementing multiple functionalities in the form of distinguishable building blocks, however, is difficult without risking narcissistic self-sorting or a statistical mess. Here we report a systematic series of integratively self-assembled heteroleptic cages in which two square-planar PdII cations are bridged by four different bis-pyridyl ligands, A, B, C and D, via synergistic effects to exclusively form a single isomer-the lantern-shaped cage [Pd2ABCD]. This self-sorting goal-forming just one out of 55 possible structures-is reached under full thermodynamic control and can be realized progressively (by combining progenitors, such as [Pd2A2C2] with [Pd2B2D2]), directly from ligands and PdII cations or by mixing all four corresponding homoleptic cages. The rational design of complex multicomponent assemblies that enables the modular incorporation of diverse chemical moieties will advance their applicability in functional nanosystems.

Sub-8 nm networked cage nanofilm with tunable nanofluidic channels for adaptive sieving

Biological cell membrane featuring smart mass-transport channels and sub-10 nm thickness was viewed as the benchmark inspiring the design of separation membranes; however, constructing highly connective and adaptive pore channels over large-area membranes less than 10 nm in thickness is still a huge challenge. Here, we report the design and fabrication of sub-8 nm networked cage nanofilms that comprise of tunable, responsive organic cage-based water channels via a free-interface-confined self-assembly and crosslinking strategy. These cage-bearing composite membranes display outstanding water permeability at the 10-5 cm2 s-1 scale, which is 1-2 orders of magnitude higher than that of traditional polymeric membranes. Furthermore, the channel microenvironments including hydrophilicity and steric hindrance can be manipulated by a simple anion exchange strategy. In particular, through ionically associating light-responsive anions to cage windows, such 'smart' membrane can even perform graded molecular sieving. The emergence of these networked cage-nanofilms provides an avenue for developing bio-inspired ultrathin membranes toward smart separation.

Modular Strategy for Constructing para-Cage[n]arenes, meta-Cage[n]arenes, and meta-Bimacrocyclic-Arenes

Here, we present a versatile modular strategy for crafting novel covalent organic cages (para-cage[n]arenes and meta-cage[n]arenes, n = 3,4) and bimacrocycles (meta-bimacrocyclic-arenes) with stable backbones and modifiable rims. These structures can be synthesized from commercially available aromatic multialdehydes in a three-step process: quantitative bromination, Suzuki-Miyaura reaction (yielding over 60%), and a rapid one-pot Friedel-Crafts reaction with paraformaldehyde. Notably, the cage[n]arenes exhibit a well-defined prismatic shape, and the bimacrocyclic-arenes display both dimeric and monomeric configurations.

Investigation of the Dynamic Behaviour of H2 and D2 in a Kinetic Quantum Sieving System

Porous organic cages (POCs) are nanoporous materials composed of discrete molecular units that have uniformly distributed functional pores. The intrinsic porosity of these structures can be tuned accurately at the nanoscale by altering the size of the porous molecules, particularly to an optimal size of 3.6 Å, to harness the kinetic quantum sieving effect. Previous research on POCs for isotope separation has predominantly centered on differences in the quantities of adsorbed isotopes. However, nuclear quantum effects also contribute significantly to the dynamics of the sorption process, offering additional opportunities for separating H2 and D2 at practical operational temperatures. In this study, our investigations into H2 and D2 sorption on POC samples revealed a higher uptake of D2 compared to that of H2 under identical conditions. We employed quasi-elastic neutron scattering to study the diffusion processes of D2 and H2 in the POCs across various temperature and pressure ranges. Additionally, neutron Compton scattering was utilized to measure the values of the nuclear zero-point energy of individual isotopic species in D2 and H2. The results indicate that the diffusion coefficient of D2 is approximately one-sixth that of H2 in the POC due to the nuclear quantum effect. Furthermore, the results reveal that at 77 K, D2 has longer residence times compared to H2 when moving from pore to pore. Consequently, using the kinetic difference of H2 and D2 in a porous POC system enables hydrogen isotope separation using a temperature or pressure swing system at around liquid nitrogen temperatures.

A water-soluble Pd4 molecular tweezer for selective encapsulation of isomeric quinones and their recyclable extraction

Quinones (QN) are one of the main components of diesel exhaust particulates that have significant detrimental effects on human health. Their extraction and purification have been challenging tasks because these atmospheric particulates exist as complex matrices consisting of inorganic and organic compounds. In this report, we introduce a new water soluble Pd4L2 molecular architecture (MT) with an unusual tweezer-shaped structure obtained by self-assembly of a newly designed phenothiazine-based tetra-imidazole donor (L) with the acceptor cis-[(tmeda)Pd(NO3)2] (M) [ tmeda = N,N,N',N'-tetramethylethane-1,2-diamine]. The molecular tweezer encapsulates some quinones existing in diesel exhaust particulates (DEPs) leading to the formation of host-guest complexes in 1 : 1 molar ratio. Moreover, MT binds phenanthrenequinone (PQ) more strongly than its isomer anthraquinone (AQ), an aspect that enables extraction of PQ with a purity of 91% from an equimolar mixture of the two isomers. Therefore, MT represents an excellent example of supramolecular receptor capable of selective aqueous extraction of PQ from PQ/AQ with many cycles of reusability.

Fluorinated Nonporous Adaptive Cages for the Efficient Removal of Perfluorooctanoic Acid from Aqueous Source Phases

Per- and polyfluoroalkyl substances (PFAS) accumulate in water resources and pose serious environmental and health threats due to their nonbiodegradable nature and long environmental persistence times. Strategies for the efficient removal of PFAS from contaminated water are needed to address this concern. Here, we report a fluorinated nonporous adaptive crystalline cage (F-Cage 2) that exploits electrostatic interaction, hydrogen bonding, and F-F interactions to achieve the efficient removal of perfluorooctanoic acid (PFOA) from aqueous source phases. F-Cage 2 exhibits a high second-order kobs value of approximately 441,000 g mg-1 h-1 for PFOA and a maximum PFOA adsorption capacity of 45 mg g-1. F-Cage 2 can decrease PFOA concentrations from 1500 to 6 ng L-1 through three rounds of flow-through purification, conducted at a flow rate of 40 mL h-1. Elimination of PFOA from PFOA-loaded F-Cage 2 is readily achieved by rinsing with a mixture of MeOH and saturated NaCl. Heating at 80 °C under vacuum then makes F-Cage 2 ready for reuse, as demonstrated across five successive uptake and release cycles. This work thus highlights the potential utility of suitably designed nonporous adaptive crystals as platforms for PFAS remediation.

Design and assembly of porous organic cages

Porous organic cages (POCs) represent a notable category of porous materials, showing remarkable material properties due to their inherent porosity. Unlike extended frameworks which are constructed by strong covalent or coordination bonds, POCs are composed of discrete molecular units held together by weak intermolecular forces. Their structure and chemical traits can be systematically tailored, making them suitable for a range of applications including gas storage and separation, molecular separation and recognition, catalysis, and proton and ion conduction. This review provides a comprehensive overview of POCs, covering their synthesis methods, structure and properties, computational approaches, and applications, serving as a primer for those who are new to the domain. A special emphasis is placed on the growing role of computational methods, highlighting how advanced data-driven techniques and automation are increasingly aiding the rapid exploration and understanding of POCs. We conclude by addressing the prevailing challenges and future prospects in the field.

Self-similar chiral organic molecular cages

The endeavor to enhance utility of organic molecular cages involves the evolution of them into higher-level chiral superstructures with self-similar, presenting a meaningful yet challenging. In this work, 2D tri-bladed propeller-shaped triphenylbenzene serves as building blocks to synthesize a racemic 3D tri-bladed propeller-shaped helical molecular cage. This cage, in turn, acts as a building block for a pair of higher-level 3D tri-bladed chiral helical molecular cages, featuring multilayer sandwich structures and displaying elegant characteristics with self-similarity in discrete superstructures at different levels. The evolutionary procession of higher-level cages reveals intramolecular self-shielding effects and exclusive chiral narcissistic self-sorting behaviors. Enantiomers higher-level cages can be interconverted by introducing an excess of corresponding chiral cyclohexanediamine. In the solid state, higher-level cages self-assemble into supramolecular architectures of L-helical or D-helical nanofibers, achieving the scale transformation of chiral characteristics from chiral atoms to microscopic and then to mesoscopic levels.

Dynamic covalent synthesis

Dynamic covalent synthesis aims to precisely control the assembly of simple building blocks linked by reversible covalent bonds to generate a single, structurally complex, product. In recent years, considerable progress in the programmability of dynamic covalent systems has enabled easy access to a broad range of assemblies, including macrocycles, shape-persistent cages, unconventional foldamers and mechanically-interlocked species (catenanes, knots, etc.). The reversibility of the covalent linkages can be either switched off to yield stable, isolable products or activated by specific physico-chemical stimuli, allowing the assemblies to adapt and respond to environmental changes in a controlled manner. This activatable dynamic property makes dynamic covalent assemblies particularly attractive for the design of complex matter, smart chemical systems, out-of-equilibrium systems, and molecular devices.

Light-Triggered Enhancement of Fluorescence Efficiency in Organic Cages

The fluorescence efficiency of excited molecules can be enhanced by many external factors. Here, we showcase a surprising phenomenon whereby light is used as a gating source to increase the fluorescence efficiency of organic cages composed of biphenyl subunits. We show that the enhancement of fluorescence is not due to structural changes or ground-state events. Cryo-fluorescence measurements and kinetic studies suggest a restriction of the phenyl-based structures in the excited state, leading to increased fluorescence, which is also supported by time-resolved measurements. Through computational calculations, we propose that the planarization of the biphenyl units within the cages contributes to emission enhancement. This phenomenon offers insights into the design of optoelectronic structures with improved fluorescence properties.

Single Crystals of Insoluble Porous Salicylimine Cages

Porous organic cages (POCs) are meanwhile an established class of porous materials. Most of them are soluble to a certain extend and thus processable in or from solution. However, a few of larger salicylimine cages were reported to be insoluble in any organic solvents and thus characterized as amorphous materials. These cages were now synthesized as single-crystalline materials to get insight into packing motifs and preferred intermolecular interactions. Furthermore, the pairs of crystalline and amorphous materials for each cage allowed to compare their gas-sorption properties in both morphological states.

Recent applications of organic cages in sensing and separation processes in solution

Organic cages are three-dimensional polycyclic compounds of great interest in the scientific community due to their unique features, which generally include simple synthesis based on the dynamic covalent chemistry strategies, structural tunability and high selectivity. In this feature article, we present the advances over the last ten years in the application of organic cages as chemosensors or components in chemosensing devices for the determination of analytes (pollutants, analytes of biological interest) in complex aqueous media including wine, fruit juice, urine. Details on the recent applications of organic cages as selective (back-)extractants or masking agents for potential applications in relevant separation processes, such as the plutonium and uranium recovery by extraction, are also provided. Over the last ten years, organic cages with permanent porosity in the liquid and solid states have been highly appreciated as porous materials able to discriminate molecules of different sizes. These features, combined with good solvent processability and film-forming tendency, have proved useful in the fabrication of membranes for gas separation, solvent nanofiltration and water remediation processes. An overview of the recent applications of organic cages in membrane separation technologies is given.

Recent Progress in Functional Nanomaterials towards the Storage, Separation, and Removal of Tritium

Tritium is a sustainable next-generation prime fuel for generating nuclear energy through fusion reactions to fulfill the increasing global energy demand. Owing to the scarcity-high demand tradeoff, tritium must be bred inside a fusion reactor to ensure sustainability and must therefore be separated from its isotopes (protium and deuterium) in pure form, stored safely, and supplied on demand. Existing multistage isotope separation technologies exhibit low separation efficiency and require intensive energy inputs and large capital investments. Furthermore, tritium-contaminated heavy water constitutes a major fraction of nuclear waste, and accidents like the one at Fukushima Daiichi leave behind thousands of tons of diluted tritiated water, whose removal is beneficial from an environmental point of view. In this review, the recent progress and main research trends in hydrogen isotope storage and separation by focusing on the use of metal hydride (e.g., intermetallic, and high-entropy alloys), porous (e.g., zeolites and metal organic frameworks (MOFs)), and 2-D layered (e.g., graphene, hexagonal boron nitride (h-BN), and MXenes) materials to separate and store tritium based on their diverse functionalities are discussed. Finally, the challenges and future directions for implementing tritium storage and separation are summarized in the reviewed materials.

Exploration and Insights on Topology Adjustment of Giant Heterometallic Cages Featuring Inorganic Skeletons Assisted by Machine Learning

Inorganic molecular cages are emerging multifunctional molecular-based platforms with the unique merits of rigid skeletons and inherited properties from constituent metal ions. However, the sensitive coordination bonds and vast synthetic space have limited their systematic exploration. Herein, two giant cage-like clusters featuring the organic ligand-directed inorganic skeletons of Ni4[La74Ni104(IDA)96(OH)184(C2O4)12(H2O)76]·(NO3)38·(H2O)120 (La74Ni104, 5 × 5 × 3 - C2O4) and [La84Ni132(IDA)108(OH)168(C2O4)24(NO3)12(H2O)116]·(NO3)72·(H2O)296 (La84Ni132, 5 × 5 × 5 - C2O4) were discovered by a high-throughput synthetic search. With the assistance of machine learning analysis of the experimental data, phase diagrams of the two clusters in a four-parameter synthetic space were depicted. The effect of alkali, oxalate, and other parameters on the formation of clusters and the mechanism regulating the size of two n × m × l clusters were elucidated. This work uses high-throughput synthesis and machine learning methods to improve the efficiency of 3d-4f cluster discovery and finds the highest-nuclearity 3d-4f cluster to date by regulating the size of the n × m × l inorganic cages through oxalate ions, which pushes the synthetic methodology study on elusive inorganic giant cages in a significantly systematic way.

Gas Permeation through Mechanically Resistant Self-Standing Membranes of a Neat Amorphous Organic Cage

The synthesis and characterization of a novel film-forming organic cage and of its smaller analogue are here described. While the small cage produced single crystals suitable for X-ray diffraction studies, the large one was isolated as a dense film. Due to its remarkable film-forming properties, this latter cage could be solution processed into transparent thin-layer films and mechanically stable dense self-standing membranes of controllable thickness. Thanks to these peculiar features, the membranes were also successfully tested for gas permeation, reporting a behavior similar to that found with stiff glassy polymers such as polymers of intrinsic microporosity or polyimides. Given the growing interest in the development of molecular-based membranes, for example for separation technologies and functional coatings, the properties of this organic cage were investigated by thorough analysis of their structural, thermal, mechanical and gas transport properties, and by detailed atomistic simulations.

Cofacial porphyrin organic cages. Metals regulating excitation electron transfer and CO2 reduction electrocatalytic properties

Herein, we introduce a comprehensive study of the photophysical behaviors and CO2 reduction electrocatalytic properties of a series of cofacial porphyrin organic cages (CPOC-M, M = H2, Co(ii), Ni(ii), Cu(ii), Zn(ii)), which are constructed by the covalent-bonded self-assembly of 5,10,15,20-tetrakis(4-formylphenyl)porphyrin (TFPP) and chiral (2-aminocyclohexyl)-1,4,5,8-naphthalenetetraformyl diimide (ANDI), followed by post-synthetic metalation. Electronic coupling between the TFPP donor and naphthalene-1,4 : 5,8-bis(dicarboximide) (NDI) acceptor in the metal-free cage is revealed to be very weak by UV-vis spectroscopic, electrochemical, and theoretical investigations. Photoexcitation of CPOC-H2, as well as its post-synthetic Zn and Co counterparts, leads to fast energy transfer from the triplet state porphyrin to the NDI unit according to the femtosecond transient absorption spectroscopic results. In addition, CPOC-Co enables much better electrocatalytic activity for CO2 reduction reaction than the other metallic CPOC-M (M = Ni(ii), Cu(ii), Zn(ii)) and monomeric porphyrin cobalt compartment, supplying a partial current density of 18.0 mA cm-2 at -0.90 V with 90% faradaic efficiency of CO.

Potential of nonporous adaptive crystals for hydrocarbon separation

Hydrocarbon separation is an important process in the field of petrochemical industry, which provides a variety of raw materials for industrial production and a strong support for the development of national economy. However, traditional separation processes involve huge energy consumption. Adsorptive separation based on nonporous adaptive crystal (NAC) materials is considered as an attractive green alternative to traditional energy-intensive separation technologies due to its advantages of low energy consumption, high chemical and thermal stability, excellent selective adsorption and separation performance, and outstanding recyclability. Considering the exceptional potential of NAC materials for hydrocarbon separation, this review comprehensively summarizes recent advances in various supramolecular host-based NACs. Moreover, the current challenges and future directions are illustrated in detail. It is expected that this review will provide useful and timely references for researchers in this area. Based on a large number of state-of-the-art studies, the review will definitely advance the development of NAC materials for hydrocarbon separation and stimulate more interesting studies in related fields.

Crystalline Nanoparticles of Water-Soluble Covalent Basket Cages (CBCs) for Encapsulation of Anticancer Drugs

We herein describe the preparation, assembly, recognition characteristics, and biocompatibility of novel covalent basket cage CBC-11, composed of four molecular baskets linked to four trivalent aromatic amines through amide groups. The cage is tetrahedral in shape and similar in size to small proteins (Mw =8637 g/mol) with a spacious nonpolar interior for accommodating multiple guests. While 24 carboxylates at the outer surface of CBC-11 render it soluble in aqueous phosphate buffer (PBS) at pH=7.0, the amphiphilic nature prompts its assembly into nanoparticles (d=250 nm, DLS). Cryo-TEM examination of nanoparticles revealed their crystalline nature with wafer-like shapes and hexagonally arranged cages. Nanoparticulate CBC-11 traps anticancer drugs irinotecan and doxorubicin, with each cage binding up to four drug molecules in a non-cooperative manner. The inclusion complexation resulted in nanoparticles growing in size and precipitating. In media containing mammalian cells (HCT 116, human colon carcinoma), the IC50 value of CBC-11 was above 100 μM. While this work presents the first example of a large covalent organic cage operating in water at the physiological pH and forming crystalline nanoparticles, it also demonstrates its biocompatibility and potential to act as a polyvalent binder of drugs for their sequestration or delivery.

A squarate-pillared titanium oxide quantum sieve towards practical hydrogen isotope separation

Separating deuterium from hydrogen isotope mixtures is of vital importance to develop nuclear energy industry, as well as other isotope-related advanced technologies. As one of the most promising alternatives to conventional techniques for deuterium purification, kinetic quantum sieving using porous materials has shown a great potential to address this challenging objective. From the knowledge gained in this field; it becomes clear that a quantum sieve encompassing a wide range of practical features in addition to its separation performance is highly demanded to approach the industrial level. Here, the rational design of an ultra-microporous squarate pillared titanium oxide hybrid framework has been achieved, of which we report the comprehensive assessment towards practical deuterium separation. The material not only displays a good performance combining high selectivity and volumetric uptake, reversible adsorption-desorption cycles, and facile regeneration in adsorptive sieving of deuterium, but also features a cost-effective green scalable synthesis using chemical feedstock, and a good stability (thermal, chemical, mechanical and radiolytic) under various working conditions. Our findings provide an overall assessment of the material for hydrogen isotope purification and the results represent a step forward towards next generation practical materials for quantum sieving of important gas isotopes.

Polycage membranes for precise molecular separation and catalysis

The evolution of the chemical and pharmaceutical industry requires effective and less energy-intensive separation technologies. Engineering smart materials at a large scale with tunable properties for molecular separation is a challenging step to materialize this goal. Herein, we report thin film composite membranes prepared by the interfacial polymerization of porous organic cages (POCs) (RCC3 and tren cages). Ultrathin crosslinked polycage selective layers (thickness as low as 9.5 nm) are obtained with high permeance and strict molecular sieving for nanofiltration. A dual function is achieved by combining molecular separation and catalysis. This is demonstrated by impregnating the cages with highly catalytically active Pd nanoclusters ( ~ 0.7 nm). While the membrane promotes a precise molecular separation, its catalytic activity enables surface self-cleaning, by reacting with any potentially adsorbed dye and recovering the original performance. This strategy opens opportunities for the development of other smart membranes combining different functions and well-tailored abilities.

Porous Organic Cages

Porous organic cages (POCs) are a relatively new class of low-density crystalline materials that have emerged as a versatile platform for investigating molecular recognition, gas storage and separation, and proton conduction, with potential applications in the fields of porous liquids, highly permeable membranes, heterogeneous catalysis, and microreactors. In common with highly extended porous structures, such as metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and porous organic polymers (POPs), POCs possess all of the advantages of highly specific surface areas, porosities, open pore channels, and tunable structures. In addition, they have discrete molecular structures and exhibit good to excellent solubilities in common solvents, enabling their solution dispersibility and processability─properties that are not readily available in the case of the well-established, insoluble, extended porous frameworks. Here, we present a critical review summarizing in detail recent progress and breakthroughs─especially during the past five years─of all the POCs while taking a close look at their strategic design, precise synthesis, including both irreversible bond-forming chemistry and dynamic covalent chemistry, advanced characterization, and diverse applications. We highlight representative POC examples in an attempt to gain some understanding of their structure-function relationships. We also discuss future challenges and opportunities in the design, synthesis, characterization, and application of POCs. We anticipate that this review will be useful to researchers working in this field when it comes to designing and developing new POCs with desired functions.

Effect of steric hindrance on the interfacial connection of MOF-on-MOF architectures

MOF-on-MOF is attracting great attention due to its improved and/or synergistic properties not exhibited in a single MOF. In particular, the non-isostructural pairs of MOF-on-MOFs can have great potential induced by large heterogeneity, which enables diverse applications in a wide range of fields. HKUST-1@IRMOF is a fascinating platform because the alteration of the IRMOF pores with bulkier substituent groups on the ligands can provide a more microporous environment. However, the sterically hindered linker can affect the seamless growth at the interface, an important issue in practical research fields. Despite many efforts to reveal the growth of a MOF-on-MOF, there is still a lack of studies on a MOF-on-MOF consisting of a sterically hindered interface. Indeed, the effect of a bulky linker at an interface of HKUST-1@IRMOF, a non-isostructural MOF-on-MOF system, has not yet been reported, and thus, how the interfacial strain affects the interfacial growth remains unknown. In this study, we investigate the effect of an interfacial strain on a chemical connection point in an MOF-on-MOF system through a series of theoretical and synthetic experiments using a HKUST-1@IRMOF system. Our results reveal the importance of the proximity of each coordinating site at a MOF-on-MOF interface as well as lattice parameter matching for an effective secondary growth to achieve a well-connected MOF-on-MOF.

Fluids and Electrolytes under Confinement in Single-Digit Nanopores

Confined fluids and electrolyte solutions in nanopores exhibit rich and surprising physics and chemistry that impact the mass transport and energy efficiency in many important natural systems and industrial applications. Existing theories often fail to predict the exotic effects observed in the narrowest of such pores, called single-digit nanopores (SDNs), which have diameters or conduit widths of less than 10 nm, and have only recently become accessible for experimental measurements. What SDNs reveal has been surprising, including a rapidly increasing number of examples such as extraordinarily fast water transport, distorted fluid-phase boundaries, strong ion-correlation and quantum effects, and dielectric anomalies that are not observed in larger pores. Exploiting these effects presents myriad opportunities in both basic and applied research that stand to impact a host of new technologies at the water-energy nexus, from new membranes for precise separations and water purification to new gas permeable materials for water electrolyzers and energy-storage devices. SDNs also present unique opportunities to achieve ultrasensitive and selective chemical sensing at the single-ion and single-molecule limit. In this review article, we summarize the progress on nanofluidics of SDNs, with a focus on the confinement effects that arise in these extremely narrow nanopores. The recent development of precision model systems, transformative experimental tools, and multiscale theories that have played enabling roles in advancing this frontier are reviewed. We also identify new knowledge gaps in our understanding of nanofluidic transport and provide an outlook for the future challenges and opportunities at this rapidly advancing frontier.

Direct observation of highly effective hydrogen isotope separation at active metal sites by in situ DRIFT spectroscopy

In situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy was developed for the first time to observe the hydrogen isotope separation behavior at active Cu^I sites within Cu^I-MFU-4l, and clear evidence of the preferential adsorption of D₂ over H₂ was directly captured. More importantly, our results show direct spectral proof to clarify the chemical affinity quantum sieving mechanism of hydrogen isotope separation within porous adsorbents.

Identifying Anthropogenic Sources of Heavy Metals in Alpine Peatlands over the Past 150 Years: Examples from Typical Peatlands in Altay Mountains, Northwest China

Alpine mountain peatlands are valuable archives of climatic and anthropogenic impact. However, the impacts of human activities on the Altay peatlands are poorly documented. Therefore, studying heavy metal (HM) concentrations, evaluating HM pollution levels, and identifying the sources in the Altay Mountain peatlands are crucial for revealing the intensity of human activities. The present study was performed on two peatland profiles: Jiadengyu (JDY) and Heihu (HH). The contents of HM and ²¹⁰Pb and ¹³⁷Cs dating technologies were used to construct a profile of anthropogenic pollutant distributions in the peatlands. Furthermore, the enrichment factor (EF) and geo-accumulation index (Igeo) of selected HMs were used to evaluate the risk assessment of HMs. The association of metals and assignment of their probable sources were examined using principal component analysis (PCA) and a positive matrix factorization model (PMF). The results showed that the concentrations of elements Cu, Zn, Cr, Pb, Ni, and As were at high levels in the two peatlands of the Altay Mountains, while the elements Hg and Cd were in low concentrations. Moreover, the concentrations of Cu, Cd, Hg, and Sb were higher than the background values of local element and posed a high environmental risk to the ecosystem. Combined with the results of the chronology, the peatland records indicated considerable growth in HMs concentrations from 1970 to 1990 related to recent anthropogenic activities. Additionally, the main sources of HMs are mining activities, domestic waste, and traffic sources in the two peatlands. Due to the environmental protection policies implemented since 2010, the natural processes have been the primary origin of HMs in peatlands, while emissions of industrial, agricultural, and domestic waste were still fundamental sources. The results of this study describe the sedimentary features of HMs in alpine mountains, and the data provide an essential theoretical basis for the evolutionary process through the characteristics of HM deposition.

Highly Effective H2/D2 Separation within the Stable Cu(I)Cu(II)-BTC: The Effect of Cu(I) Structure on Quantum Sieving

Realizing ideal deuterium separation from isotopic mixtures remains a daunting challenge because of their almost identical sizes, shapes, and physicochemical properties. Using the quantum sieving effect in porous materials with suitable pore size and open metal sites (OMSs) enables efficient hydrogen isotope separation. Herein, synthetic HKUST-1-derived microporous mixed-valence Cu(I)Cu(II)-BTC (BTC = benzene-1,3,5-tricarboxylate), featuring a unique network of distinct Cu(I) and Cu(II) coordination sites, can remarkably boost the D₂/H₂ isotope separation, which has a high selectivity (S_D2/H2) of 37.9 at 30 K, in comparison with HKUST-1 and other porous materials. Density functional theory (DFT) calculations indicate that the introduction of Cu(I) macrocycles in the framework decreases the pore size and further leads to relatively enhanced interaction of H₂/D₂ molecules on Cu(II) sites. The significantly enhanced selectivity of Cu(I)Cu(II)-BTC at 30 K can be mainly attributed to the synergistic effect of kinetic quantum sieving (KQS) and chemical affinity quantum sieving (CAQS). The results reveal that Cu(I) OMSs exhibit counterintuitive behaviors and play a crucial role in tuning quantum sieving without a complex structural design, which provides a deeper insight into quantum sieving mechanisms and a new strategy for the intelligent design of highly efficient isotope systems.

Chiral emissive porous organic cages

A pair of chiral, emissive and porous tubular multi-functional organic molecular cages were synthesized easily by imine chemistry of 4,4',4'',4'''-(ethene-1,1,2,2-tetrayl)-tetrabenzaldehyde (ETTBA) with (R,R)- or (S,S)-diaminocyclohexane (CHDA). It was found that the chirality of CHDA was transferred and amplified to tetraphenylethylene (TPE) in the process of formation of cages, which further endowed the cages with circularly polarized luminescence (CPL) characteristics. As a result of the synergy of the chirality and porous structure in the solid state, both cages exhibited a good chiral adsorption enantioselectivity to a series of aromatic racemates.

Zeolites in Adsorption Processes: State of the Art and Future Prospects

Zeolites have been widely used as catalysts, ion exchangers, and adsorbents since their industrial breakthrough in the 1950s and continue to be state-of the-art adsorbents in many separation processes. Furthermore, their properties make them materials of choice for developing and emerging separation applications. The aim of this review is to put into context the relevance of zeolites and their use and prospects in adsorption technology. It has been divided into three different sections, i.e., zeolites, adsorption on nanoporous materials, and chemical separations by zeolites. In the first section, zeolites are explained in terms of their structure, composition, preparation, and properties, and a brief review of their applications is given. In the second section, the fundamentals of adsorption science are presented, with special attention to its industrial application and our case of interest, which is adsorption on zeolites. Finally, the state-of-the-art relevant separations related to chemical and energy production, in which zeolites have a practical or potential applicability, are presented. The replacement of some of the current separation methods by optimized adsorption processes using zeolites could mean an improvement in terms of sustainability and energy savings. Different separation mechanisms and the underlying adsorption properties that make zeolites interesting for these applications are discussed.

Tunable Construction of Sandwich-Type Double-[1 + 1] and Half-Folded [2 + 2] Schiff-Base Complexes Controlled by the Combination of Primary and Secondary Template Effects

The first-row transition-metal ions Mn²⁺-Cu²⁺ could serve as effective templates to construct three types of double-[1 + 1], [2 + 2], and [1 + 1] Schiff-base dinuclear macrocyclic complexes in the presence of dialdehydes with different pendant arms and a common 1,8-diamine. The extremely flexible nature of macrocyclic ligands allows for the multiple template-directed syntheses, but the final products could be finely tuned by the subtle variations of Mn²⁺-Cu²⁺ ions in a 3d-electronic configuration, radius, and coordination number/geometry as well as the auxiliary (pendant-armed and anionic) template effect at the same time. Two borderlines are observed at the Co²⁺ ion for forming double-[1 + 1] and [2 + 2] metallacycles involving the H₂pdd precursor and the [1 + 1] Cu²⁺ complex for double-[1 + 1] and [2 + 2] macrocycles containing the H₂hpdd unit, respectively. The structural diversity is originated from the non-perfect match between [1 + 1]/[2 + 2] Schiff-base macrocycles and dinuclear metal centers; hence, a compromise between the metal coordination modes and alterations of the ligand conformation takes place.

Soluble porous organic cages as homogenizers and electron-acceptors for homogenization of heterogeneous alloy nanoparticle catalysts with enhanced catalytic activity

The creation of ultrafine alloy nanoparticles (<5 nm) that can maintain surface activity and avoid aggregation for heterogeneous catalysis has received much attention and is extremely challenging. Here, ultrafine PtRh alloy nanoparticles imprisoned by the cavities of reduced chiral covalent imine cage (PtRh@RCC3) are prepared successfully by an organic molecular cage (OMC) confinement strategy, while the soluble RCC3 can act as a homogenizer to homogenize the heterogeneous PtRh alloy in solution. Moreover, the X-ray absorption near-edge structure (XANES) results show that the RCC3 can act as an electron-acceptor to withdraw electrons from Pt, leading to the formation of higher valence Pt atoms, which is beneficial to improving the catalytic activity for the reduction of 4-nitrophenol. Attributed to the synergistic effect of Pt/Rh atoms and the unique function of the RCC3, the reaction rate constants of Pt₁Rh₁₆@RCC3 are 49.6, 8.2, and 5.5 times than those of the Pt₁Rh₁₆ bulk, Pt@RCC3 and Rh@RCC3, respectively. This work provides a feasible strategy to homogenize heterogeneous alloy nanoparticle catalysts in solution, showing huge potential for advanced catalytic application.

Macroscopic Heterostructure Membrane of Graphene Oxide/Porous Graphene/Graphene Oxide for Selective Separation of Deuterium Water from Natural Water

Deuterium water (D₂ O) is a strategic material that is widely used in and scientific research and has applications in fields such as nuclear energy generation. However, its content in natural water is extremely low. Therefore, the development of a room-temperature technology for achieving simple, efficient, and low-cost separation of D₂ O from natural water is challenging. In this study, porous graphene (PG) nanosheets with "crater-like" pores are sandwiched between two layers of graphene oxide (GO) membranes to prepare a GO/PG/GO membrane with a macroscopic heterostructure, which can be used to separate D₂ O and H₂ O by pressure-driven filtration. At 25 °C, the rejection rate of D₂ O is ≈97%, the selectivity of H₂ O/D₂ O is ≈35.2, and the excellent performance can be attributed to the difference of transmembrane resistance and flow state of H₂ O and D₂ O in the confinement state. In addition, the D₂ O concentration in natural water is successfully enriched from 0.013% to 0.059% using only one stage, and the membrane exhibits excellent structural and cycling stability. Therefore, this method does not require ultralow temperatures, high energy supplies, complex separation equipment, or the introduction of toxic chemicals. Thus, it can be directly applied to the large-scale industrial production and removal of D₂ O.

High Efficiency of Na- and Ca-Exchanged Chabazites in D2/H2 Separation by Quantum Sieving

Quantum sieving is a promising approach for separation of hydrogen isotopes using porous solids as sorbents at cryogenic temperatures (<77 K). In the present work, we characterized the properties of two aluminum-rich chabazites: Na-CHA and Ca-CHA (Si/Al = 2.1). The single-gas D₂ and H₂ adsorption isotherms were measured, and the thermodynamic selectivities were determined through coadsorption experiments in the temperature range 38-77 K. We found that at 38 K, Na-CHA shows a selectivity of 25.8 at a loading of 10.6 mmol·g^-1. At the same temperature, Ca-CHA has slightly lower selectivity (18.3), but its uptake (12.9 mmol·g^-1) is higher than that for Na-CHA. Comparison with the literature shows that the obtained values of selectivity are among the highest reported so far. This property combined with robustness and availability on the industrial scale of Al-rich chabazites makes them very promising materials for separation of hydrogen isotopes by quantum sieving.

Efficient Hydrogen Isotope Separation by Tunneling Effect Using Graphene-Based Heterogeneous Electrocatalysts in Electrochemical Hydrogen Isotope Pumping

The fabrication of a hydrogen isotope enrichment system is essential for the development of industrial, medical, life science, and nuclear fusion fields, and therefore, efficient enrichment techniques with a high separation factor and economic feasibility are still being explored. Herein, we report a hydrogen/deuterium (H/D) separation ability with polymer electrolyte membrane electrochemical hydrogen pumping (PEM-ECHP) using a heterogeneous electrode consisting of palladium and graphene layers (PdGr). By mass spectroscopic analysis, we demonstrate significant bias voltage dependence of the H/D separation factor with a maximum of ∼25 at 0.15 V and room temperature, which is superior to those of conventional separation methods. Theoretical analysis demonstrated that the observed high H/D factor stems from tunneling of hydrogen isotopes through atomically thick graphene during the electrochemical reaction and that the bias dependence of H/D results from a transition from the quantum tunneling regime to the classical overbarrier regime for hydrogen isotopes transfer through the graphene. These findings will help us understand the origin of the isotope separation ability of graphene discussed so far and contribute to developing an economical hydrogen isotope enrichment system using two-dimensional materials.

[2+3] Amide Cages by Oxidation of [2+3] Imine Cages – Revisiting Molecular Hosts for Highly Efficient Nitrate Binding

The pollution of groundwater with nitrate is a serious issue because nitrate can cause several diseases such as methemoglobinemia or cancer. Therefore, selective removal of nitrate by efficient binding to supramolecular hosts is highly desired. Here we describe how to make [2+3] amide cages in very high to quantitative yields by applying an optimized Pinnick oxidation protocol for the conversion of corresponding imine cages. By NMR titration experiments of the eight different [2+3] amide cages with nitrate, chloride and hydrogen sulfate we identified one cage with an unprecedented high selectivity towards nitrate binding vs. chloride (S=705) or hydrogensulfate (S>13500) in CD₂Cl₂/CD₃CN (1 : 3). NMR experiments as well as single‐crystal structure comparison of host‐guest complexes give insight into structure‐property‐relationships.

CO 2 Separation by Imide/Imine Organic Cages

Two novel imide/imine‐based organic cages have been prepared and studied as materials for the selective separation of CO₂ from N₂ and CH₄ under vacuum swing adsorption conditions. Gas adsorption on the new compounds showed selectivity for CO₂ over N₂ and CH₄. The cages were also tested as fillers in mixed‐matrix membranes for gas separation. Dense and robust membranes were obtained by loading the cages in either Matrimid® or PEEK‐WC polymers. Improved gas‐transport properties and selectivity for CO₂ were achieved compared to the neat polymer membranes.

Purely Covalent Molecular Cages and Containers for Guest Encapsulation

Cage compounds offer unique binding pockets similar to enzyme-binding sites, which can be customized in terms of size, shape, and functional groups to point toward the cavity and many other parameters. Different synthetic strategies have been developed to create a toolkit of methods that allow preparing tailor-made organic cages for a number of distinct applications, such as gas separation, molecular recognition, molecular encapsulation, hosts for catalysis, etc. These examples show the versatility and high selectivity that can be achieved using cages, which is impossible by employing other molecular systems. This review explores the progress made in the field of fully organic molecular cages and containers by focusing on the properties of the cavity and their application to encapsulate guests.