Taming the Topology of Calix[4]arene-Based 2D-Covalent Organic Frameworks: Interpenetrated vs Noninterpenetrated Frameworks and Their Selective Removal of Cationic Dyes

Supramolecular-macrocycle-based functional organic cocrystals

Supramolecular macrocycles, renowned for their remarkable capabilities in molecular recognition and complexation, have emerged as pivotal elements driving advancements across various innovative research fields. Cocrystal materials, an important branch within the realm of crystalline organic materials, have garnered considerable attention owing to their simple preparation methods and diverse potential applications, particularly in optics, electronics, chemical sensing and photothermal conversion. In recent years, macrocyclic entitles have been successfully brought into this field, providing an essential and complementary channel to create novel functional materials, especially those with multiple functionalities and smart stimuli-responsiveness. In this Review, we present an overview of the research efforts on functional cocrystals constructed with macrocycles, covering their design principles, preparation strategies, assembly modes, and diverse functions and applications. Finally, the remaining challenges and perspectives are outlined. We anticipate that this review will serve as a valuable and timely reference for researchers interested in supramolecular crystalline materials and beyond, catalyzing the emergence of more original and innovative studies in related fields.

β-Cyclodextrin/calix[4]arene hybrid porous organic polymer membrane for synergistic extraction of fluorescent whitening agents migrating from food contact materials

Acurate and sensitive determination of hazards from food contact materials is important to monitor food safety. It is necessary to excavate efficient adsorbent for simultaneous recognition and adsorption of food hazards of trace level for sample preparation. In this work, β-cyclodextrin and calix[4]arene were employed as hybrid functional monomers to prepare macrocyclic porous organic polymer (β-CD-CX4 POP). It was proved that the supramolecular cavities of β-CD-CX4 POP could form inclusion complexes with fluorescent whitening agents (FWAs) through host-guest recognition, which greatly improved the adsorption performance. The hydrophobic cavities of β-cyclodextrin and calix[4]arene of β-CD-CX4 POP exhibited synergistic effect for simultaneous recognition of FWAs. The high-throughput enrichment of FWAs was realized by β-CD-CX4 POP membranes coupled with a multiple-channel syringe pump. Based on membrane-based solid-phase extraction combined with UHPLC-MS/MS, a sensitive analytical method was established to determine six FWAs. The LODs was in range of 3-50 ng/L with the linear range of 0.02-100 μg/L. The developed method was used to quantify FWAs in bread wrapper and bread, and the spiked recoveries ranged from 78.1%-119% with RSD of 2.3%-9.7%. This work indicated that β-CD-CX4 POP was promising for the simultaneous recognition and adsorption of FWAs migrating from food contact materials.

Calixarene incorporated molecular imprinting on covalent organic framework for supramolecular recognition and specific extraction of citrinin

Specific recognition and selective extraction of mycotoxin in environmental and food matrixes is significant to guarantee public health. Covalent organic frameworks (COFs) are promising adsorbents with tailorable functionality, but their low binding affinity and poor selectivity hamper their wide application for selective extraction of trace mycotoxin from complex matrix. Herein, we report calixarene incorporated molecular imprinting on COF to prepare molecularly imprinted calix[4]arene-containing COF (MICOF-CX4) for supramolecular recognition and specific adsorption of citrinin. Calixarene with host-guest chemistry was used as a functional monomer, while amine units with different topologies and function groups were selected to regulate MICOF-CX4 to match with citrinin. The complementary shape and supramolecular interactions of MICOF-CX4 gave highly selective recognition for citrinin. Moreover, MICOF-CX4 with vast accessible surface and plentiful imprinting sites exhibited faster adsorption kinetics and 4-fold higher adsorption capacity for citrinin adsorption than no-imprinted COF-CX4. Combination of MICOF-CX4 based solid-phase extraction with high-performance liquid chromatography-mass spectrometry allowed interference free determination of trace citrinin in real samples with a low detection limit of 0.03 ng mL-1, good precision of 4.5 % and quantitative recovery of 88.2 %-101.4 %. The cooperative functions of calixarene and molecular imprinting make COF promising adsorbent for specific adsorption of trace targets in complex matrixes.

Novel three-dimensional fibrous covalent organic frameworks constructed via silver amalgam bridging for efficient organic dye adsorption and removal

The construction of covalent organic frameworks (COFs) with unique structures has great significance in exploring the structure-function relationship and extending their potential applications. Fibrous COFs have demonstrated superior performance in specific application scenarios owing to the distinctive three-dimensional (3D) structure. Herein, we report a facile strategy for the fabrication of 3D COF nanofiber by exploiting silver amalgam as a bridging agent to assemble one-dimensional-extended PA-COF modules into a tubular structure. Dimensions of the obtained 3D COF nanofiber were predicted by DFT calculations, and the nanofiber was endowed with the merits of favorable uniformity and high stability. Due to the enhanced exposure of conjugatable binding sites for dye retention offered by the novel 3D architecture, the PA-COF nanofiber exhibits fast adsorption (within 5 min) and superior adsorption capacity to various organic dyes, e.g., 1717 mg g-1 for methylene blue (MB) and 978.3 mg g-1 for methyl orange (MO). Moreover, the PA-COF nanofiber shows excellent reusability in dye adsorption, which makes it a potential medium for removing dye pollutants from wastewater. This work presents an effective strategy to construct COF materials with unique architecture and potential prospects in the fields of separation and wastewater treatment.

Tunable Wettability of a Dual-Faced Covalent Organic Framework Membrane for Enhanced Water Filtration

Membrane technology plays a central role in advancing separation processes, particularly in water treatment. Covalent organic frameworks (COFs) have transformative potential in this field due to their adjustable structures and robustness. However, conventional COF membrane synthesis methods are often associated with challenges, such as time-consuming processes and limited control over surface properties. Our study demonstrates a rapid, microwave-assisted method to synthesize self-standing COF membranes within minutes. This approach allows control over the wettability of the surface and achieves superhydrophilic and near-hydrophobic properties. A thorough characterization of the membrane allows a detailed analysis of the membrane properties and the difference in wettability between its two faces. Microwave activation accelerates the self-assembly of the COF nanosheets, whereby the thickness of the membrane can be controlled by adjusting the time of the reaction. The superhydrophilic vapor side of the membrane results from -NH2 reactions with acetic acid, while the nearly hydrophobic dioxane side has terminal aldehyde groups. Leveraging the superhydrophilic face, water filtration at high water flux, complete oil removal, increased rejection with anionic dye size, and resistance to organic fouling were achieved. The TTA-DFP-COF membrane opens new avenues for research to address the urgent need for water purification, distinguished by its synthesis speed, simplicity, and superior separation capabilities.

Controlling the Degree of Interpenetration in Chiral Three-Dimensional Covalent Organic Frameworks via Steric Tuning

Network interpenetration plays a crucial role in functionalizing porous framework materials. However, controlling the degree of interpenetration in covalent organic frameworks (COFs) to influence their pore sizes, shapes, and functionalities still remains a significant challenge. Here, we demonstrate a steric tuning strategy to control the degree of COF interpenetration and modulate their physicochemical properties. By imine condensations of 1,1'-bi-2-naphthol-derived tetraaldehydes bearing different alkyl substituents with the monomer tetra(p-aminophenyl)-methane, we synthesized and characterized a family of two-component and three-component chiral COFs with different interpenetrated dia networks. The alkyl groups are periodically appended on the pore walls, and their types/contents that can be synthetically tuned control the interpenetration degree of COFs by minimizing repulsive interactions between the alkyl groups. Specifically, the COF with -OH groups adopts an interpenetrated dia-c5 topology, those with -OMe/-OEt groups take an interpenetrated dia-c4 topology, whereas those with the bulky -OnPr/-OnBu groups exhibit a noninterpenetrated dia-c1 topology. The multivariate COFs with both -OH and -OnBu groups display either a noninterpenetrated or dia-c5 topology, depending on the proportion of -OnBu groups. The extent of interpenetration in COFs significantly affects their porosity, thermal stability, and chemical stability, resulting in varying selective performances in the adsorption and separation of dyes and asymmetric catalysis. This work highlights the potential of using steric hindrance to tune and control interpenetration, porosity, stability, and functionalities of COFs materials, broadening the range of their applications.

A Type of Ferrocene-Based Derivative FE-1 COF Material for Glycopeptide and Phosphopeptide Selective Enrichment

In this work, a new type of FE-1 COF material is prepared by a reversible imine condensation reaction with diaminoferrocene and diaminodiformaldehyde as materials. The material is connected by imine bonds to form a COF skeleton, and the presence of plenty of nitrogen-containing groups gives the material good hydrophilicity; the presence of metal Fe ions provides the material application potential in the enrichment of phosphopeptides. According to the different binding abilities of N-glycopeptide and phosphopeptide on FE-1 COF, it can simultaneously enrich N-glycopeptide and phosphopeptide through different elution conditions to realize its controllable and selective enrichment. Using the above characteristics, 18 phosphopeptides were detected from α-casein hydrolysate, 8 phosphopeptides were detected from β-casein hydrolysate and 21 glycopeptides were detected from IgG hydrolysate. Finally, the gradual elution strategy was used; 16 phosphopeptides and 19 glycopeptides were detected from the α-casein hydrolysate and IgG hydrolysate. The corresponding glycopeptides and phosphopeptides were identified from the human serum. It proves that the FE-1 COF material has a good enrichment effect on phosphopeptides and glycopeptides.

Synthesis of Calix[4]arene-Based Porous Organic Cages and Their Gas Adsorption

Two crystalline large-sized porous organic cages (POCs) based on conical calix[4]arene (C4A) were designed and synthesized. The four-jaw C4A unit tends to follow the face-directed self-assembly law with the planar triangular building blocks such as tris(4-aminophenyl)amine (TAPA) or 1,3,5-tris(4-aminophenyl)benzene (TAPB) to generate a predictable cage with a stoichiometry of [6+8]. The formation of the large cages is confirmed through their relative molecular mass measured using MALDI-TOF/TOF spectra. The protonated molecular ion peaks of C4A-TAPA and C4A-TAPB were observed at m/z 5109.0 (calculated for C336H240O24N32: m/z 5109.7) and m/z 5594.2 (calculated for C384H264O24N24: m/z 5598.4). C4A-POCs exhibit I-type N2 adsorption-desorption isotherms with the BET surface areas of 1444.9 m2 ⋅ g-1 and 1014.6 m2 ⋅ g-1. The CO2 uptakes at 273 K are 62.1 cm3 ⋅ g-1 and 52.4 cm3 ⋅ g-1 at a pressure of 100 KPa. The saturated iodine vapor static uptakes at 348 K are 3.9 g ⋅ g-1 and 3.5 g ⋅ g-1. The adsorption capacity of C4A-TAPA for SO2 reaches to 124.4 cm3 ⋅ g-1 at 298 K and 1.3 bar. Additionally, the adsorption capacities of C4A-TAPA for C2H2, C2H4, and C2H6 were evaluated.

Fabrication of Carboxylate-Functionalized 2D MOF Nanosheet with Caged Cavity for Efficient and Selective Extraction of Uranium from Aqueous Solution

The efficient removal of radioactive uranium from aqueous solution is of great significance for the safe and sustainable development of nuclear power. An ultrathin 2D metal-organic framework (MOF) nanosheet with cavity structures was elaborately fabricated based on a calix[4]arene ligand. Incorporating the permanent cavity structures on MOF nanosheet can fully utilize its structural characteristics of largely exposed surface area and accessible adsorption sites in pollutant removal, achieving ultrafast adsorption kinetics, and the functionalized cavity structure would endow the MOF nanosheets with the ability to achieve preconcentration and extraction of uranium from aqueous solution, affording ultrahigh removal efficiency even in ultra-low concentrations. Thus, more than 97% uranium can be removed from the concentration range of 50-500 µg L-1 within 5 min. Moreover, the 2D nano-material exhibits ultra-high anti-interference ability, which can efficiently remove uranium from groundwater and seawater. The adsorption mechanism was investigated by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) analysis, and density functional theory (DFT) calculations, which revealed that the cavity structure plays an important role in uranium capture. This study not only realizes highly efficient uranium removal from aqueous solution but also opens the door to achieving ultrathin MOF nanosheets with cavity structures, which will greatly expand the applications of MOF nanosheets.

A sulfonate ligand-defected Zr-based metal-organic framework for the enhanced selective removal of anionic dyes

In this work, we introduce a novel defective analogue of the representative 6-connected zirconium-based metal-organic framework (MOF-808), by employing 5-sulfoisophthalic acid monosodium salt (H2BTC-SO3Na) as a defect inducer via a mixed-linker approach. The structural integrity and different physicochemical properties were investigated by various characterization techniques, including powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and nitrogen physisorption at 77 K. Additionally, proton nuclear magnetic resonance (1H-NMR), energy-dispersive X-ray (EDX), and inductively coupled plasma optical emission spectroscopy (ICP-OES) were employed to confirm the presence of 6.9 mol% of the 5-sulfoisophthalate ligand within the highly crystalline MOF-808 structure. The defective material exhibited significant enhancements in the removal efficiency of various organic dyes, including approximately 64% and 77% for quinoline yellow and sunset yellow, and 56% and 13% for rhodamine B and malachite green, compared to its pristine counterpart. Importantly, the defective MOF-808 showed a remarkable selectivity toward anionic species in binary-component dyes comprising both anionic and cationic dyes.

3D Covalent Organic Frameworks from Design, Synthesis to Applications in Optoelectronics

Covalent organic frameworks (COFs), a new class of crystalline materials connected by covalent bonds, have been developed rapidly in the past decades. However, the research on COFs is mainly focused on two-dimensional (2D) COFs, and the research on three-dimensional (3D) COFs is still in the initial stage. In 2D COFs, the covalent bonds exist only in the 2D flakes and can form 1D channels, which hinder the charge transport to some extent. In contrast, 3D COFs have a more complex pore structure and thus exhibit higher specific surface area and richer active sites, which greatly enhance the 3D charge carrier transport. Therefore, compared to 2D COFs, 3D COFs have stronger applicability in energy storage and conversion, sensing, and optoelectronics. In this review, it is first introduced the design principles for 3D COFs, and in particular summarize the development of conjugated building blocks in 3D COFs, with a special focus on their application in optoelectronics. Subsequently, the preparation of 3D COF powders and thin films and methods to improve the stability and functionalization of 3D COFs are summarized. Moreover, the applications of 3D COFs in electronics are outlined. Finally, conclusions and future research directions for 3D COFs are presented.

Size-controllable synthesis of large-size spherical 3D covalent organic frameworks as efficient on-line solid-phase extraction sorbents coupled to HPLC

BACKGROUND

Covalent organic frameworks (COFs) have found promising applications in separation fields due to their large surface area and high adsorption capacity, but the exiting COFs can not be directly used as the packing materials of on-line solid-phase extraction (SPE) coupled to HPLC and HPLC because their nano/submicron size or irregular shapes might cause ultrahigh column back pressure and low column efficiency. To synthesize the large-size spherical COFs larger than 3 μm as sorbents might be able to address these problems, however it is still a great challenge till now.

RESULTS

In this work, two large-size spherical 3D COFs (COF-320 and COF-300) were size-controllably synthesized within 10-90 μm via a two-step strategy. These two spherical COFs showed large surface area, fine crystallinity, good chemical/mechanical stability, and good reproducibility. As an application case, when used as the on-line SPE sorbents coupled to HPLC, the large-size spherical COF-320 displayed high binding capacity for bisphenol F (Qmax of 452.49 mg/g), low column back pressure (6-8 psi at flow rate of 1 mL/min), and good reusability (at least 30 cycles). The developed on-line-SPE-HPLC-UV method presented good analytical performance with enrichment factor of 667 folds, linear range of 1.0-400 ng/mL, limit of detection (LOD, S/N = 3) of 0.3 ng/mL, limit of quantification (LOQ, S/N = 10) of 1.0 ng/mL, and recoveries of 100.3-103.2 % (RSDs of 2.0-3.5 %) and 95.2-97.0 % (RSDs of 4.3-5.6 %) for tap water and lake water samples, respectively.

SIGNIFICANCE

This is the first case to synthesize the large-size spherical COFs within 10-90 μm, and this work made it possible to directly use COFs as the filling materials of on-line SPE coupled to HPLC and HPLC. The developed analytical method can be potentially applied to the rapid and sensitive detection of trace bisphenol F in environmental water samples.

Recent progress in iodine capture by macrocycles and cages

The effective capture of radioiodine is vital to the development of the nuclear industry and ecological environmental protection. There is, therefore, a continuously growing research exploration in various types of solid-state materials for iodine capture. During the last decade, the potential of using macrocycle and cage-based supramolecular materials in effective uptake and separation of radioactive iodine has been demonstrated. Interest in the application of these materials in iodine capture originates from their diversified porous characteristics, abundant host-guest chemistry, high iodine affinity and adsorption capacity, high stability in various environments, facile modification and functionalization, and intrinsic structural flexibility, among other attributes. Herein, recent progress in macrocycle and cage-based solid-state materials, including pure discrete macrocycles and cages, and their polymeric forms, for iodine capture is summarized and discussed with an emphasis on iodine capture capacities, mechanisms, and design strategies.

Azo-Linked Porous Polycalix[n]arenes for the Efficient Removal of Organic Micropollutants from Water

Developing novel porous adsorbents for efficient wastewater treatment is significant to the environment protection. Herein, three porous polycalix[n]arenes (n = 4, 6, and 8) which had varying cavity sizes of the macrocycle (Azo-CX4P, Azo-CX6P, and Azo-CX8P) were prepared under mild conditions and tested for their potential application in water purification. Azo-CX8P with a larger cavity size of the macrocycle outperformed Azo-CX4P and Azo-CX6P in screening studies involving a range of organic micropollutants. It was proved that Azo-CX8P was especially efficient in the removal of cationic dyes because of its high negative surface charge. In terms of the adsorption of Rhodamine B with Azo-CX8P, the pseudo-second-order rate constant reaches 5.025 g·mg-1·min-1 with the maximum adsorption capacity being 1345 mg·g-1. These values are significantly higher compared with those recorded for most adsorbents. In addition, the easily prepared Azo-CX8P can be reused at least six times without a loss of the adsorption efficiency, demonstrating its potential use in water purification.

Flower-like calix[6]arene-based covalent organic framework for membrane extraction of sulfonamides in animal-derived food through host-guest interaction prior to determination with ultra-high performance liquid chromatography-tandem mass spectrometry

Supramolecular macrocycle-based covalent organic frameworks (COFs) are promising adsorbents for adsorption of hazards due to their host-guest recognition property. However, most supramolecular macrocycles are conformationally flexible, making them challenging to introduce into COFs. In this work, a calix[6]arene-based COF (CX6-BD COF) was fabricated with a unique flower-like morphology and high crystallinity. Especially, the cavity of CX6 exhibited host-guest inclusion interaction for sulfonamides (SAs), which was verified by quantum chemistry calculation. The integration of the porosity of COFs with the recognition cavity of CX6 made CX6-BD COF display excellent enrichment performance for SAs, with good enrichment factors (EFs) between 77 and 96. The material was employed as an adsorbent for COF membrane filter extraction, coupled with ultra-high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) to simultaneously enrich and determine seven SAs in animal-derived food. The analytical method showed a wide linear range (0.01-100 µg/L and 0.05-100 µg/L) and low detection limits (3-10 ng/L). The established method was successfully applied to sensitively determine SAs in chicken, pork and beef samples, which achieved satisfactory recoveries (73.8-113%). These results demonstrated CX6-BD COF has good application potential in determination of trace and ultra-trace SAs in complex food matrices as an adsorbent.

Water-Mediated Hydrogen Bond Network Drives Highly Crystalline Structure Formation of Crown Ether-Based Covalent Organic Framework for Sr Adsorption

Covalent organic frameworks (COFs) with crown ether units have drawn great attention due to their potential applications in adsorption, catalysis, and sensing. However, employing crown ethers to construct COFs is still challenging in light of the flexible nature of macrocycles. Here, a highly crystalline one-dimensional covalent organic framework (1D-18C6-COF) with crown ether units on the ribbon edge was synthesized. The water-mediated hydrogen bond network and π-π stacking hold the 1D COF ribbons together. The combination of experimental and DFT studies demonstrated that the hydrogen bond network plays a crucial role in the structure crystallinity. The 1D-18C6-COF was applied as an adsorbent for strontium, and it exhibited rapid kinetics with good selectivity. In the competitive adsorption experiment, a separation factor of 1900 was achieved, representing one of the largest values for cesium/strontium separation. This work provides new insights into the design and functional exploration of crystalline COFs with flexible units.

Calix[4]arene-Derived 2D Covalent Organic Framework with an Electron Donor-Acceptor Structure: A Visible-Light-Driven Photocatalyst

The calixarenes are ideal building blocks for constructing photocatalytic covalent organic frameworks (COFs), owing to their electron-rich and bowl-shaped π cavities that endow them with electron-donating and adsorption properties. However, the synthesis and structural confirmation of COFs based on calixarenes are still challenging due to their structural flexibility and conformational diversity. In this study, a calix[4]arene-derived 2D COF is synthesized using 5,11,17,23-tetrakis(p-formyl)-25,26,27,28-tetrahydroxycalix[4]arene (CHO-C4A) as the electron donor and 4,7-bis(4-aminophenyl)-2,1,3-benzothiadiazole (BTD) as the acceptor. The powder X-ray diffraction data and theoretical simulation of crystal structure indicate that COF-C4A-BTD exhibits high crystallinity and features a non-interpenetrating undulating 2D layered structure with AA-stacking. The density functional theory theoretical calculation, transient-state photocurrent tests, and electrochemical impedance spectroscopy confirm the intramolecular charge transfer behavior of COF-C4A-BTD with a donor-acceptor structure, leading to its superior visible-light-driven photocatalytic activity. COF-C4A-BTD exhibits a narrow band gap of 1.99 eV and a conduction band energy of -0.37 V versus normal hydrogen electrode. The appropriate energy band structure can facilitate the participation of ·O2- and h+ . COF-C4A-BTD demonstrates high efficacy in removing organic pollutants, such as bisphenol A, rhodamine B, and methylene blue, with removal rates of 66%, 85%, and 99% respectively.

Synthesizing Interpenetrated Triazine-based Covalent Organic Frameworks from CO2

Crystalline triazine-based covalent organic frameworks (COFs) are aromatic nitrogen-rich porous materials. COFs typically show high thermal/chemical stability, and are promising for energy applications, but often require harsh synthesis conditions and suffer from low crystallinity. In this work, we propose an environmentally friendly route for the synthesis of crystalline COFs from CO2 molecules as a precursor. The mass ratio of CO2 conversion into COFs formula unit reaches 46.3 %. The synthesis consists of two steps; preparation of 1,4-piperazinedicarboxaldehyde from CO2 and piperazine, and condensation of the dicarboxaldehyde and melamine to construct the framework. The CO2 -derived COF has a 3-fold interpenetrated structure of 2D layers determined by powder X-ray diffraction, high-resolution transmission electron microscopy, and select-area electron diffraction. The structure shows a high Brunauer-Emmett-Teller surface area of 945 m2  g-1 and high stability against strong acid (6 M HCl), base (6 M NaOH), and boiling water over 24 hours. Post-modification of the framework with oxone has been demonstrated to modulate hydrophilicity, and it exhibits proton conductivity of 2.5×10-2  S cm-1 at 85 °C, 95 % of relative humidity.

Synthetically Tunable Suprahybrid Nanoparticle Platform for the Efficacious Delivery of Therapeutics

The discovery of lipid-hybrid nanosystems has offered potential solutions to various drug delivery and theranostic challenges. However, in many instances, the commonly used lipids and other components in these systems often pose challenges related to their solubility, physicochemical properties, immune compatibility, and limited synthetic tunability. In this work, we introduce a synthetically tunable supramolecular scaffold with amphiphilic characteristics based on the calix[4]arene macrocyclic system. We designed and synthesized two novel calix[4]arene-polyethylene glycol (PEG) conjugates, termed Cal-P1 and Cal-P2, and these were characterized utilizing a wide range of spectroscopic and analytical methods. The rational design of Cal-P1 and Cal-P2 demonstrates their utility in forming stable blended nanospheres with sustained drug release characteristics. The synergistic blending of PLGA and the calixarene scaffold (Cal-P1 and Cal-P2) in constructing long-lasting and controlled-release nanoparticles (NPs), which are optimized for encapsulating Nile Red dye, and their successful internalization and retention in HeLa cancer cells are demonstrated through in vitro assays. The potential of these NPs as sustained therapeutic carriers is investigated in vivo, showing improved retention compared to free dye with negligible toxicity. The successful design and construction of Cal-P1 and Cal-P2 nanosystems represent a new paradigm for addressing drug loading challenges, opening up opportunities for the development of highly efficient, synthetically tunable alternative adjuvants for drug encapsulation and delivery.

Calix[4]arene-based covalent organic frameworks with host-guest recognition for selective adsorption of six per- and polyfluoroalkyl substances in food followed by UHPLC-MS/MS detection

Long-term ingestion or exposure to food contaminated with per- and polyfluoroalkyl substances (PFASs) may cause potential harm to human health. Due to the low contents of PFASs in complex food matrices, it is of great significance to develop adsorbents with excellent properties to enrich PFASs before analysis. Herein, calix[4]arene (CX4) was used as building block to prepare ordered crystalline covalent organic frameworks (COFs). The perfect combination of the host-guest recognition ability of CX4 and the porosity of COFs makes the CX4-COFs selective and high adsorption capacity for linear molecular PFASs (261-1055 mg/g). The adsorption behavior and mechanism were verified by isotherm adsorption experiments and simulation calculations. The CX4-COFs were then used as adsorbents for membrane solid-phase extraction (M-SPE), combined with ultra-high performance liquid chromatography tandem mass spectrometry (UHPLC-MS/MS) to determine PFASs in food. The method has low detection limits (0.11-0.28 ng/kg) and good precision (1.3%-9.8%), and has been successfully applied to the simultaneous enrichment and determination of six PFASs in fish, shrimp and shellfish. Satisfactory recoveries (79.9%-118%) were obtained. This study provides a new strategy for preparing CX4-COFs containing macrocyclic molecules with different morphologies and expands the application of COFs as attractive enrichment media for sample pretreatment.

Azophenyl Calix[4]arene Porous Organic Polymer for Extraction and Analysis of Triphenylmethane Dyes from Seafood

Porous organic polymers (POPs) based on calix[4]arene with a hydrophobic π-rich cavity and host-guest recognition properties exhibit a wide application range of molecular extraction and separation. However, it is still a challenge to improve the extraction and separation selectivity by exploring and seeking appropriate building blocks for the functionalization and pore size adjustment of calix[4]arene. Herein, an azophenyl calix[4]arene porous organic polymer (AC-POP) was proposed. By introducing an electron-rich cavity and adjusting the pore sizes of calix[4]arene, the AC-POP showed high selectivity extraction performance in triphenylmethane (TPM) dyes. The extraction mechanism was explored by adsorption thermodynamics study, density functional theory (DFT) calculation, and reduced density gradient (RDG) and electrostatic potential (ESP) analyses, which suggested that the selectivity adsorption of TPM dyes based on AC-POP was mainly the result of entropy driven by the hydrophobic effect. In addition, the noncovalent interactions including π-π stacking, van der Waals force, and electrostatic interaction were also important factors affecting the adsorption capacity of TPM dyes. Under optimal extraction conditions, the AC-POP possessed a maximum extraction amount of 95.3 mg·g-1 for Rhodamine B (RB), high enrichment factor of about 100, and excellent reusability more than 10 times. Then, an analytical method of TPM dyes with AC-POP as a solid-phase extractant combined with high-performance liquid chromatography-ultraviolet (HPLC-UV) was established, which displayed excellent sensitivity with the limits of detection (LODs) and limits of quantitation (LOQs) in the ranges of 0.004-0.35 and 0.016-1.16, respectively. The mean recoveries for TPM dyes ranged from 85.0 to 109.4% with an RSD of 0.48-9.45%. The proposed method was successfully applied to the analysis of the five TPM dyes in seafood matrix samples.

A π-conjugated covalent organic framework enables interlocked nickel/photoredox catalysis for light-harvesting cross-coupling reactions

Covalent organic frameworks (COFs) are an outstanding platform for heterogeneous photocatalysis. Herein, we synthesized a pyrene-based two-dimensional C[double bond, length as m-dash]C linked π-conjugated COF via Knoevenagel condensation and anchored Ni(ii)-centers through bipyridine moieties. Instead of traditional dual metallaphotoredox catalysis, the mono-metal decorated Ni@Bpy-sp2c-COF interlocked the catalysis mediated by light and the transition metal. Under light irradiation, enhanced energy and electron transfer in the COF backbone, as delineated by the photoluminescence, electrochemical, and control experiments, expedited the excitation of Ni centers to efficiently catalyze diverse photocatalytic C-X (X = B, C, N, O, P, S) cross-coupling reactions with efficiencies orders of magnitude higher than the homogeneous controls. The COF catalyst tolerated a diverse range of coupling partners with various steric and electronic properties, delivering the products with up to 99% yields. Some reactions were performed on a gram scale and were applied to diversify pharmaceuticals and complex molecules to demonstrate the synthetic utility.

Synthesis of a pyridine-based covalent organic framework as an efficient adsorbent for rhodamine B removal

Covalent organic frameworks (COFs), featured with crystalline structures, permanent porosity, and designable organic skeletons, are good candidates for serving as adsorbents. Herein, a new pyridine-based two-dimensional COF (TAPP-DBTA-COF) was constructed via the condensation of 2,4,6-tris(4-aminophenyl)pyridine and 2,5-dibromobenzene-1,4-dicarbaldehyde. TAPP-DBTA-COF displayed high-performance for the removal of rhodamine B (Rh B) from water with high capacity, good adaptability and reusability. The maximum adsorption capacity for Rh B can reach up to 1254 mg g-1, and the kinetic constant was determined as k2 = 0.00244 g mg-1 min-1. Moreover, the corresponding amorphous polymer of TAPP-DBTA-COF, termed as TAPP-DBTA-COP, was synthesized from the same starting materials. The lower efficiency of TAPP-DBTA-COP in capture of Rh B revealed that the ordered pore structure, large specific surface area and rich adsorption sites play an important role in adsorption.

Light-driven self-assembly of spiropyran-functionalized covalent organic framework

Controlling the number of molecular switches and their relative positioning within porous materials is critical to their functionality and properties. The proximity of many molecular switches to one another can hinder or completely suppress their response. Herein, a synthetic strategy involving mixed linkers is used to control the distribution of spiropyran-functionalized linkers in a covalent organic framework (COF). The COF contains a spiropyran in each pore which exhibits excellent reversible photoswitching behavior to its merocyanine form in the solid state in response to UV/Vis light. The spiro-COF possesses an urchin-shaped morphology and exhibits a morphological transition to 2D nanosheets and vesicles in solution upon UV light irradiation. The merocyanine-equipped COFs are extremely stable and possess a more ordered structure with enhanced photoluminescence. This approach to modulating structural isomerization in the solid state is used to develop inkless printing media, while the photomediated polarity change is used for water harvesting applications.

Bioinspired Framework Catalysts: From Enzyme Immobilization to Biomimetic Catalysis

Enzymatic catalysis has fueled considerable interest from chemists due to its high efficiency and selectivity. However, the structural complexity and vulnerability hamper the application potentials of enzymes. Driven by the practical demand for chemical conversion, there is a long-sought quest for bioinspired catalysts reproducing and even surpassing the functions of natural enzymes. As nanoporous materials with high surface areas and crystallinity, metal-organic frameworks (MOFs) represent an exquisite case of how natural enzymes and their active sites are integrated into porous solids, affording bioinspired heterogeneous catalysts with superior stability and customizable structures. In this review, we comprehensively summarize the advances of bioinspired MOFs for catalysis, discuss the design principle of various MOF-based catalysts, such as MOF-enzyme composites and MOFs embedded with active sites, and explore the utility of these catalysts in different reactions. The advantages of MOFs as enzyme mimetics are also highlighted, including confinement, templating effects, and functionality, in comparison with homogeneous supramolecular catalysts. A perspective is provided to discuss potential solutions addressing current challenges in MOF catalysis.

Macrocycle-Based Covalent Organic Frameworks

Macrocycles with well-defined cavities and the ability to undergo supramolecular interactions are classical materials that have played an essential role in materials science. However, one of the most substantial barriers limiting the utilization of macrocycles is their aggregation, which blocks the active regions. Among many attempted strategies to prevent such aggregation, installing macrocycles into covalent organic frameworks (COFs), which are porous and stable reticular networks, has emerged as an ideal solution. The resulting macrocycle-based COFs (M-COFs) preserve the macrocycles' unique activities, enabling applications in various fields such as single-atom catalysis, adsorption/separation, optoelectronics, phototherapy, and structural design of forming single-layered or mechanically interlocked COFs. The resulting properties are unmatchable by any combination of macrocycles with other substrates, opening a new chapter in advanced materials. This review focuses on the latest progress in the concepts, synthesis, properties, and applications of M-COFs, and presents an in-depth outlook on the challenges and opportunities in this emerging field.

Application of Hydrogen-Bonded Organic Frameworks in Environmental Remediation: Recent Advances and Future Trends

The hydrogen-bonded organic frameworks (HOFs) are a class of porous materials with crystalline frame structures, which are self-assembled from organic structures by hydrogen bonding in non-covalent bonds π-π packing and van der Waals force interaction. HOFs are widely used in environmental remediation due to their high specific surface area, ordered pore structure, pore modifiability, and post-synthesis adjustability of various physical and chemical forms. This work summarizes some rules for constructing stable HOFs and the synthesis of HOF-based materials (synthesis of HOFs, metallized HOFs, and HOF-derived materials). In addition, the applications of HOF-based materials in the field of environmental remediation are introduced, including adsorption and separation (NH3, CO2/CH4 and CO2/N2, C2H2/C2He and CeH6, C2H2/CO2, Xe/Kr, etc.), heavy metal and radioactive metal adsorption, organic dye and pesticide adsorption, energy conversion (producing H2 and CO2 reduced to CO), organic dye degradation and pollutant sensing (metal ion, aniline, antibiotic, explosive steam, etc.). Finally, the current challenges and further studies of HOFs (such as functional modification, molecular simulation, application extension as remediation of contaminated soil, and cost assessment) are discussed. It is hoped that this work will help develop widespread applications for HOFs in removing a variety of pollutants from the environment.

Calix[4]arene-Decorated Covalent Organic Framework Conjugates for Lithium Isotope Separation

Lithium isotope separation has attracted extensive interest due to its important role in fusion and fission reactions. Up to now, it is still a great challenge to separate lithium isotopes (⁶Li and ⁷Li) in an efficient manner due to the low capture ability for lithium ions of related materials and highly similar physicochemical properties between lithium isotopes. In this work, three calix[4]arene-decorated crystalline covalent organic frameworks (COFs) with wave-like extension and AA-stacking configuration were designed and utilized for lithium adsorption and its isotope separation. Experimental studies show that these COFs exhibit an outstanding lithium adsorption capacity up to 94.66 mg·g^-1, which is about 2 times beyond that of adsorbents reported in the literature. The high adsorption capacity of COFs could be attributed to the abundant adsorption sites from calix[4]arene unit. More importantly, this study demonstrates for the first time that calixarene groups can separate lithium isotopes with an excellent separation factor up to 1.053 ± 0.002, comparable to the most successful solid-phase lithium separation adsorbent. The calculation based on density functional theory showed that calixarene played an important role in the lithium adsorption. Interestingly, the lithium isotope separation performance is mainly affected by the amine bridging units. This work demonstrated that calixarene COFs are promising adsorbents for lithium isotope separation.

Isomeric Oligo(Phenylenevinylene)-Based Covalent Organic Frameworks with Different Orientation of Imine Bonds and Distinct Photocatalytic Activities

Imine-linked covalent organic frameworks (COFs) have been extensively studied in photocatalysis because of their easy synthesis and excellent crystallinity. The effect of imine-bond orientation on the photocatalytic properties of COFs, however, is still rarely studied. Herein, we report two novel COFs with different orientations of imine bonds using oligo(phenylenevinylene) moieties. The COFs showed similar structures but great differences in their photoelectric properties. COF-932 demonstrated a superior hydrogen evolution performance compared to COF-923 when triethanolamine was used as the sacrificial agent. Interestingly, the use of ascorbic acid led to the protonation of the COFs, further altering the direction of electron transfer. The photocatalytic performances were increased to 23.4 and 0.73 mmol g^-1  h^-1 for protonated COF-923 and COF-932, respectively. This study provides a clear strategy for the design of imine-linked COF-based photocatalysts and advances the development of COFs.

Triangular Heteroporous Covalent Organic Framework via a K-Shaped "Two-in-One" Monomer: Targeted Synthesis and Selective Removal of Organic Pollutants

Covalent organic frameworks (COFs) have attracted increasing research interest due to their intriguing topological structures and fascinating properties. Diverse COFs with different shapes and sizes are developed by the design of appropriate building blocks. However, the heteroporous COFs to date are still in their infancy due to the relatively limited configuration of precursors. Herein, it is ingeniously designed and synthesized a new K-shaped "two-in-one" building unit (3',6'-bis(4-(5,5-dimethyl-1,3-dixoan-2-yl)phenyl)-[1,1':2',1"-terphenyl]-4,4"-diamine, BPTD), thus realizing the construction of triangular dual microporous COF (BPTD-COF) via self-polycondensation of the K-shaped monomer. The super micropore (0.76 nm) of BPTD-COF endows the higher density of amine activity sites, while the other aperture size (1.35 nm) meets the need for accommodating cationic dyes (rhodamine B, methylene blue), thus BPTD-COF displays a distinctive selective adsorption for cationic dyes with good reusability.

Maneuvering Applications of Covalent Organic Frameworks via Framework-Morphology Modulation

Translating the performance of covalent organic frameworks (COFs) from laboratory to macroscopic reality demands specific morphologies. Thus, the advancement in morphological modulation has recently gained some momentum. A clear understanding of nano- to macroscopic architecture is critical to determine, optimize, and improve performances of this atomically precise porous material. Along with their chemical compositions and molecular frameworks, the prospect of morphology in various applications should be discussed and highlighted. A thorough insight into morphology versus application will help produce better-engineered COFs for practical implications. 2D and 3D frameworks can be transformed into various solids such as nanospheres, thin films, membranes, monoliths, foams, etc., for numerous applications in adsorption, separation photocatalysis, the carbon dioxide reduction, supercapacitors, and fuel cells. However, the research on COF chemistry mainly focuses on correlating structure to property, structure to morphology, and structure to applications. Here, critical insights on various morphological evolution and associated applications are provided. In each case, the underlying role of morphology is unveiled. Toward the end, a correlation between morphology and application is provided for the future development of COFs.

A Covalent Organic Framework for Cooperative Water Oxidation

The future of water-derived hydrogen as the “sustainable energy source” straightaway bets on the success of the sluggish oxygen-generating half-reaction. The endeavor to emulate the natural photosystem II for efficient water oxidation has been extended across the spectrum of organic and inorganic combinations. However, the achievement has so far been restricted to homogeneous catalysts rather than their pristine heterogeneous forms. The poor structural understanding and control over the mechanistic pathway often impede the overall development. Herein, we have synthesized a highly crystalline covalent organic framework (COF) for chemical and photochemical water oxidation. The interpenetrated structure assures the catalyst stability, as the catalyst’s performance remains unaltered after several cycles. This COF exhibits the highest ever accomplished catalytic activity for such an organometallic crystalline solid-state material where the rate of oxygen evolution is as high as ∼26,000 μmol L^–1 s^–1 (second-order rate constant k ≈ 1650 μmol L s^–1 g^–2). The catalyst also proves its exceptional activity (k ≈ 1600 μmol L s^–1 g^–2) during light-driven water oxidation under very dilute conditions. The cooperative interaction between metal centers in the crystalline network offers 20–30-fold superior activity during chemical as well as photocatalytic water oxidation as compared to its amorphous polymeric counterpart.

Covalent Organic Framework Based on Azacalix[4]arene for the Efficient Capture of Dialysis Waste Products

Azacalix[n]arenes (ACAs) are lesser-known cousins of calix[n]arenes that contain amine bridges instead of methylene bridges, so they generally have higher flexibility due to enlarged cavities. Herein, we report a highly substituted cationic azacalix[4]arene-based covalent organic framework (ACA-COF) synthesized by the Zincke reaction under microwave irradiation. The current work is a rare example of a synthetic strategy that utilizes the chemical functionalization of an organic macrocycle to constrain its conformational flexibility and, thereby, produce an ordered material. Considering the ACA cavity dimensions, and the density and diversity of the polar groups in ACA-COF, we used it for adsorption of uric acid and creatinine, two major waste products generated during hemodialysis treatment in patients with renal failure. This type of application, which has the potential to save ∼400 L of water per patient per week, has only been recognized in the last decade, but could effectively address the problem of water scarcity in arid areas of the world. Rapid adsorption rates (up to k = 2191 g mg^-1 min^-1) were observed in our COF, exceeding reported values by several orders of magnitude.

Ultrathin Covalent Organic Framework Anchored on Graphene for Enhanced Organic Pollutant Removal

Covalent organic frameworks (COFs) are of great potential as adsorbents owing to their tailorable functionalities, low density and high porosity. However, their intrinsically stacked two‐dimensional (2D) structure limits the full use of their complete surface for sorption, especially the internal pores. The construction of ultrathin COFs could increase the exposure of active sites to the targeted molecules in a pollutant environment. Herein, an ultrathin COF with a uniform thickness of ca. 2 nm is prepared employing graphene as the surface template. The resulting hybrid aerogel with an ultralow density (7.1 mg cm⁻³) exhibits the ability to remove organic dye molecules of different sizes with high efficiency. The three‐dimensional (3D) macroporous structure and well‐exposed adsorption sites permit rapid diffusion of solution and efficient adsorption of organic pollutants, thereby, greatly contributing to its enhanced uptake capacity. This work highlights the effect of COF layer thickness on adsorption performance.

Fabrication of Protonated Two-Dimensional Metal-Organic Framework Nanosheets for Highly Efficient Iodine Capture from Water

Radioactive iodine (¹²⁹I and ¹³¹I), produced or released from nuclear-related activities, posed severe effects on both human health and environment. The efficient removal of radioiodine from aqueous medium and vapor phase is of paramount importance for the sustainable development of nuclear energy. Herein, a metal-organic framework (MOF) nanosheet with a positive charge was constructed for the capture of iodine for the first time. The as-synthesized ultrathin nanosheets, with a thickness of 4.4 ± 0.1 nm, showed a record-high iodine adsorption capacity (3704.08 mg g^-1) from aqueous solution, which is even higher than that from the vapor phase (3510.05 mg g^-1). It can be ascribed to the fully interactions between the extensive accessible active sites on the largely exposed surface of 2D MOF nanosheets and the target pollutants, which also gave rise to fast adsorption kinetics with relative high removal efficiencies in the low concentrations, even in seawater. Moreover, a facile recyclability with fast desorption kinetics can also be achieved for the MOF nanosheets. The excellent iodine removal performance in aqueous solution demonstrated that the electrostatic attraction between MOF nanosheets with a positive charge and the negatively charged triiodide (I₃^-, the dominant form of iodine in aqueous solution) is the driving force in adsorption, which endows the adsorbents with the characteristics of fast adsorption and desorption kinetics. The adsorption mechanism was systematically verified by the studies of ζ potential, Fourier transform infrared, X-ray photoelectron spectroscopy, and Raman spectra.

"Rim-Differentiated" Pillar[6]arenes

A "rim-differentiated" pillar[6]arene (RD-P[6]) was obtained successfully, with the assistance of a dimeric silver trifluoroacetate template, among eight different constitutional isomers in a direct and regioselective manner. The solid-state conformation of this macrocycle could switch from the 1,3,5-alternate to a truly rim-differentiated one upon guest inclusion. This highly symmetric RD-P[6] not only hosts metal-containing molecules inside its cavity, but also can form a pillar[6]arene-C₆₀ adduct through co-crystallization on account of donor-acceptor interactions. The development of synthetic strategies to desymmetrize pillararenes offers new opportunities for engineering complex molecular architectures and organic electronic materials.

Dispersive 2D Triptycene-Based Crystalline Polymers: Influence of Regioisomerism on Crystallinity and Morphology

The merging of good crystallinity and high dispersibility into two-dimensional (2D) layered crystalline polymers (CPs) still represents a challenge because a high crystallinity is often accompanied by intimate interlayer interactions that are detrimental to the material processibility. We herein report a strategy to address this dilemma using rationally designed three-dimensional (3D) monomers and regioisomerism-based morphology control. The as-synthesized CPs possess layered 2D structures, where the assembly of layers is stabilized by relatively weak van der Waals interactions between C-H bonds other than the usual π-π stackings. The morphology and dispersibility of the CPs are finely tuned via regioisomerism. These findings shed light on how to modulate the crystallinity, morphology, and ultimate function of crystalline polymers using the spatial arrangements of linking groups.

Biofiber waste derived zwitterionic and photocatalytic dye adsorbent: Switchable selectivity, in-situ degradation and multi-tasking application

Dye wastewater and discarded biofiber have brought huge pressure to sustainable developments of ecology and economy. By utilizing dopamine chemistry and benzophenone mediated "grafting onto" atom transfer radical polymerization (ATRP), this work reported a biomass adsorbent containing discarded wool substrate, photocatalytic PDA coating and zwitterionic polymer brushes for dyes removal. The grafted zwitterionic polymer brushes impart the material with not only high adsorption capacity and rapid adsorption rate, but also switchable adsorption selectivity and pH-controlled regeneration capability. Benefiting from such outstanding adsorption performance and excellent free-standing property, the adsorbent could fulfill diversified needs of both static and dynamic adsorptions. Under daylight, the constructed photocatalytic PDA coating could in-situ degrade the captured pollutant, thus achieving consecutive adsorption-degradation-regeneration utilization. Furthermore, through simple dip-coating and cleaner UV-irradiation techniques, the preparation process could be scaled up. This work contributes to both the upcycling of discarded biofiber waste and the development of advanced biomass adsorbent.

Dual Metalation in a Two-Dimensional Covalent Organic Framework for Photocatalytic C-N Cross-Coupling Reactions

Covalent organic frameworks (COFs) are promising hosts in heterogeneous catalysis. Herein, we report a dual metalation strategy in a single two-dimensional-COF TpBpy for performing a variety of C-N cross-coupling reactions. [Ir(ppy)₂(CH₃CN)₂]PF₆ [ppy = 2-phenylpyridine], containing two labile CH₃CN groups, and NiCl₂ are used as iridium and nickel-metal precursors, respectively, for postsynthetic decoration of the TpBpy COF. Moving from the traditional approach, we focus on the COF-backbone host for visible-light-mediated nickel-catalyzed C-N coupling reactions. The controlled metalation and recyclability without deactivation of both catalytic centers are unique with respect to previously reported coupling strategies. We performed various photoluminescence, electrochemical, kinetic, and Hammett correlation studies to understand the salient features of the catalyst and reaction mechanism. Furthermore, theoretical calculations delineated the feasibility of electron transfer from the Ir center to the Ni center inside the confined pore of the TpBpy COF. The dual metal anchoring within the COF backbone prevented nickel-black formation. The developed protocol enables selective and reproducible coupling of a diverse range of amines (aryl, heteroaryl, and alkyl), carbamides, and sulfonamides with electron-rich, neutral, and poor (hetero) aryl iodides up to 94% isolated yield. The reaction can also be performed on a gram scale. Furthermore, to establish the practical implementation of this approach, we have applied the synthetic strategy for the late-stage diversification of the derivatives of ibuprofen, naproxen, gemfibrozil, helional, and amino acids. The methodology could also be applied to synthesize pharmacophore N,5-diphenyloxazol-2-amine and Food and Drug Administration-approved drugs, including flufenamic acid, flibanserin, and tripelennamine.

Supramolecular Interactions Lead to Remarkably High Thermal Conductivities in Interpenetrated Two-Dimensional Porous Crystals

The design of innovative porous crystals with high porosities and large surface areas has garnered a great deal of attention over the past few decades due to their remarkable potential for a variety of applications. However, heat dissipation is key to realizing their potential. We use systematic atomistic simulations to reveal that interpenetrated porous crystals formed from two-dimensional (2D) frameworks possess remarkable thermal conductivities at high porosities in comparison to their three-dimensional (3D) single framework and interpenetrated 3D framework counterparts. In contrast to conventional understanding, higher thermal conductivities are associated with lower atomic densities and higher porosities for porous crystals formed from interpenetrating 2D frameworks. We attribute this to lower phonon-phonon scattering and vibrational hardening from the supramolecular interactions that restrict atomic vibrational amplitudes, facilitating heat conduction. This marks a new regime of materials design combining ultralow mass densities and ultrahigh thermal conductivities in 2D interpenetrated porous crystals.

Merging Charge Transfer into Metal-Organic Frameworks to Achieve High Reduction Potentials via Multiphoton Excitation

Utilization of multiphotons to achieve high reduction potentials is a highly demanding but still challenging task for reductive cleavage of inert bonds. Herein, we report a new charge transfer approach that simultaneously excites the electron-rich dye and the radical anionic of the electron-deficient one for photocatalytic activation of aryl chlorides with high reduction potentials (E_red ≈ -1.9 to -2.9 V). Interactions between the tetraphenylbenzene-1,4,-diamine dyes in the large pores of metal-organic frameworks and the adsorbed 9,10-dicyanoanthracene partly endows charge transfer in the ground state. The first photoexcitation led to the formation charge separation pairs containing both radical cation and anion for second photon excitation. The possibility of modifying each absorption band of the two dyes independently innovated the resultant aryl radicals applied in various useful transformations, expanding multiphoton manifolds on both the dye scopes and reaction versions. A comparison of the catalytic performance between different structural patterns of metal-organic frameworks with the same ligand demonstrated that the incorporating of the organic dyes within the pores of the frameworks was essential to form charge-transfer species and accelerate the interesting chemical conversion.

Flexible metal-organic frameworks for gas storage and separation

Flexible metal-organic frameworks (MOFs) have gradually attracted much attention due to their reversible structural changes and flexible structural responses. The basic research of flexible MOFs is to study their dynamic responses under different external stimuli and translate the responses into applications. Most research studies on flexible MOFs focus on gas storage and separation, but lack a systematic summary. Here, we review the development of flexible MOFs, the structural transformation under the external effects of temperature, pressure, and guest molecules, and their applications in gas storage and separation. Microporous MOFs with flexible structures provide unique opportunities for fine-tuning their performance because the pore shape and size can be controlled by external stimuli. The characteristics of breathing phenomena and large specific surface area make flexible MOFs suitable candidates for gas storage and separation. Finally, the application prospects of flexible MOFs are reported.