Structural Dynamism-Actuated Reversible CO2 Adsorption Switch and Postmetalation-Induced Visible Light Cα-H Photocyanation with Rare Size Selectivity in N-Functionalized 3D Covalent Organic Framework

Silver(I)-Functionalized COF-LZU1 for High-Performance CO2/CH4 Separation

Using porous materials for CO2/CH4 separation is an ecofriendly and energy-efficient method. Covalent organic frameworks are crystalline porous frameworks linked by covalent bonds with precisely adjustable pore size and pore chemistry and are considered potential porous adsorbent materials. At present, various methods/materials for the purification and separation of industrial mixtures have been widely studied. In this study, silver ions (Ag+) were successfully anchored to a classical COF: COF-LZU1. This modification utilized the π-complexation interaction between Ag+ and CO2 to enhance the adsorption and separation efficiency for CO2. The resulting composite, designated as Ag@COF-LZU1 (1.5 equiv, containing 1.5 equiv of Ag+), exhibited the highest CO2 uptake of 34.1 cm3/g and demonstrated a remarkable CO2/CH4 selectivity factor of 32.1 at 298 K and 1 atm. Breakthrough experiments indicated that the modified COF-LZU1 displayed a prolonged retention time. Furthermore, the stability of COF-LZU1 after Ag+ anchoring was confirmed through three consecutive breakthrough experiments, which revealed no significant variation in the penetration time. The selective adsorption mechanisms of COF-LZU1, both prior to and following the anchoring of Ag+, were elucidated by using density functional theory calculations. This study presents a novel approach for developing adsorbents targeting the capture of CO2 in practical industrial applications.

Structural Reassignment of Covalent Organic Framework-Supported Palladium Species: Heterogenized Palladacycles as Efficient Catalysts for Sustainable C-H Activation

Recent decades have witnessed remarkable progress in ligand-promoted C-H activation with palladium catalysts. While a number of transformations have been achieved with a fairly broad substrate scope, the general requirements for high palladium loadings and enormous challenges in catalyst recycling severely limit the practical applications of C-H activation methodologies in organic synthesis. Herein, we incorporate N,C-ligand-chelated palladacycles into rigid, porous, and crystalline covalent organic frameworks for the C-H arylation of indole and pyrrole derivatives. These heterogeneous palladium catalysts exhibit superior stability and recyclability compared to their homogeneous counterparts. We not only produce several highly reactive palladacycles embedded on new framework supports to facilitate C-H activation/C-C bond-forming reactions but also reassign heterogenized palladium species on frameworks containing a benzaldehyde-derived imine moiety as imine-based palladacycles via comprehensive characterization. Our findings provide guidance for the rational design of framework-supported metallacycles in the development of heterogeneous transition-metal catalysis.

Promoting Photocatalytic Direct C-H Difluoromethylation of Heterocycles using Synergistic Dual-Active-Centered Covalent Organic Frameworks

Difluoromethylation reactions are increasingly important for the creation of fluorine-containing heterocycles, which are core groups in a diverse range of biologically and pharmacologically active ingredients. Ideally, this typically challenging reaction could be performed photocatalytically under mild conditions. To achieve this separation of redox processes would be required for the efficient generation of difluoromethyl radicals and the reduction of oxygen. A covalent organic framework photocatalytic material was, therefore, designed with dual reactive centers. Here, anthracene was used as a reduction site and benzothiadiazole was used as an oxidation site, distributed in a tristyryl triazine framework. Efficient charge separation was ensured by the superior electron-donating and -accepting abilities of the dual centers, creating long-lived photogenerated electron-hole pairs. Photocatalytic difluoromethylation of 16 compounds with high yields and remarkable functional group tolerance was demonstrated; compounds included bioactive molecules such as xanthine and uracil. The structure-function relationship of the dual-active-center photocatalyst was investigated through electron spin resonance, femtosecond transient absorption spectroscopy, and density functional theory calculations.

Postsynthetic Modification Strategy for Constructing Electrochemiluminescence-Active Chiral Covalent Organic Frameworks Performing Efficient Enantioselective Sensing

Electrochemiluminescence (ECL), integrating the characteristics of electrochemistry and fluorescence, has the advantages of high sensitivity and low background. However, only a few studies have been reported for enantioselective sensing based on the ECL-active platform because of the huge challenges in constructing tunable chiral ECL luminophores. Here, we developed a facile strategy to design and prepare ECL-active chiral covalent organic frameworks (COFs) Ph-triPy+-(R)-Ru(II) for enantioselective sensing. In such an artificial structure, the ionic skeleton of COFs was beneficial to the electron transfer on the working electrode surface and the chiral Ru-ligand was used as the chiral ECL-active luminophore. It was found that Ph-triPy+-(R)-Ru(II) coupled with sodium persulfate (Na2S2O8) as the coreactant exhibited obvious ECL signals. More importantly, a clear difference toward l- and d-enantiomers was observed in the response of the ECL intensity, resulting in a uniform recognition law. That is, for amino alcohols, d-enantiomers (1 mM) measured by Ph-triPy+-(R)-Ru(II) showed a higher ECL intensity compared with l-enantiomers. Differently, amino acids (1 mM) gave an inverse recognition phenomenon. The ECL intensity ratios between l- and d-enantiomers (1 mM) are in the range of 1.25-1.94 for serine, aspartic acid, glutamic acid, valine, leucine, leucinol, and valinol. What is more interesting is that the ECL intensity was closely related to the concentration of l-amino alcohols and d-amino acids, whereas their inverse configurations remained unchanged. In a word, the present concept demonstrates a feasible direction toward chiral ECL-active COFs and their potential for efficient enantioselective sensing.

Magnetic two-dimensional nanocomposites for multimodal antitumor therapy: a recent review

Magnetic two-dimensional nanocomposites (M2D NCs) that synergistically combine magnetic nanomedicine and 2D nanomaterials have emerged in multimodal antitumor therapy, attracting great interest in materials science and biomedical engineering. This review provides a summary of the recent advances of M2D NCs and their multimodal antitumor applications. We first introduce the design and fabrication of M2D NCs, followed by discussing new types of M2D NCs that have been recently reported. Then, a detailed analysis and discussions about the different types of M2D NCs are presented based on the structural categories of 2D NMs, including 2D graphene, transition metal dichalcogenides (TMDs), transition metal carbides/nitrides/carbonitrides (MXenes), black phosphorus (BP), layered double hydroxides (LDHs), metal organic frameworks (MOFs), covalent organic frameworks (COFs) and other 2D nanomaterials. In particular, we focus on the synthesis strategies, magnetic or optical responsive performance, and the versatile antitumor applications, which include magnetic hyperthermia therapy (MHT), photothermal therapy (PTT), photodynamic therapy (PDT), drug delivery, immunotherapy and multimodal imaging. We conclude the review by proposing future developments with an emphasis on the mass production and biodegradation mechanism of the M2D NCs. This work is expected to provide a comprehensive overview to researchers and engineers who are interested in such a research field and promote the clinical translation of M2D NCs in practical applications.

Double-Walled Covalent Organic Frameworks with High Stability

Double-walled covalent organic frameworks, consisting of two same building blocks parallel to each other forming ladder-shape linkers, could enhance the stability of the frameworks and increase the density of functional sites, thus making them suitable for various applications. In this study, two double-walled covalent organic frameworks, namely DW-COF-1 and DW-COF-2, were successfully synthesized via imine condensation. The resulting DW-COFs exhibited a honeycomb topology, high crystallinity and stability. Particularly, DW-COF-2 showed excellent resistance toward boiling water, strong acid, and strong base, due to its double-walled structure, which limits the exposure of labile imine bonds to external chemical environments. The DW-COFs showed high porosity near 900 m2 /g, making them suitable for gas storage/separation. The selective gas adsorption experiments showed that at 273 K and 1 atm pressure, DW-COF-1 and DW-COF-2 exhibited a good IAST selectivity towards CO2 /N2 (15/85) adsorption, with selectivity values of 121.3 and 56.4 for CO2 over N2 , respectively.

Ru(N^N)3-docked cationic covalent organic frameworks for enhanced sulfide and amine photooxidation

Covalent organic frameworks (COFs) have emerged as significant candidates for visible-light photocatalysis due to their ability to regulate performance which is achieved through the careful selection of building modules, framework conjugation, and post-modification. This report focused on the efficient transformation of an imine-linked I-COF into a π-conjugated quinoline-based Q-COF, which enhanced both the chemical stability and conjugation of the network. By methylating the pyridyl groups in the Q-COF, an N+-COF was obtained. Subsequently, the Ru(N^N)3-photosensitizer ([Ru(dcbpy)3]4-) was incorporated into the channels of the cationic N+-COF through electrostatic interactions, resulting in the formation of [Ru(dcbpy)3]4-⊂N+-COF. This composite exhibited exceptional photocatalytic activity, demonstrating high yields and selectivity in the oxidation of sulfides or amines to their respective products.

Modulating Uranium Extraction Performance of Multivariate Covalent Organic Frameworks through Donor-Acceptor Linkers and Amidoxime Nanotraps

Covalent organic frameworks (COFs) can be designed to allow uranium extraction from seawater by incorporating photocatalytic linkers. However, often sacrificial reagents are required for separating photogenerated charges which limits their practical applications. Herein, we present a COF-based adsorption-photocatalysis strategy for selective removal of uranyl from seawater in the absence of sacrificial reagents. A series of ternary and quaternary COFs were synthesized containing the electron-rich linker 2,4,6-triformylphloroglucinol as the electron donor, the electron-deficient linker 4,4'-(thiazolo[5,4-d]thiazole-2,5-diyl)dibenzaldehyde as the acceptor, and amidoxime nanotraps for selective uranyl capture (with the quaternary COFs incorporating [2,2'-bipyridine-5,5'-diamine-Ru(Bp)₂]Cl₂ as a secondary photosensitizer). The ordered porous structure of the quaternary COFs ensured efficient mass transfer during the adsorption-photocatalysis capture of uranium from seawater samples, with photocatalytically generated electrons resulting in the reduction of adsorbed U(VI) to U(IV) in the form of UO₂. A quaternary COF, denoted as COF 2-Ru-AO, possessed a high uranium uptake capacity of 2.45 mg/g/day in natural seawater and good anti-biofouling abilities, surpassing most adsorbents thus far. This work shows that multivariate COF adsorption-photocatalysts can be rationally engineered to work efficiently and stably without sacrificial electron donors, thus opening the pathway for the economic and efficient extraction of uranium from the earth's oceans.

Three-Dimensional Covalent Organic Frameworks: From Synthesis to Applications

Three-dimensional covalent organic frameworks (3D COFs) with spatially periodic networks demonstrate significant advantages over their 2D counterparts, including enhanced specific surface areas, interconnected channels, and more sufficiently exposed active sites. Nevertheless, research on these materials has met an impasse due to serious problems in crystallization and stability, which must be solved for practical applications. In this Minireview, we first summarize some strategies for preparing functional 3D COFs, including crystallization techniques and functionalization methods. Hereafter, applications of these functional materials are presented, covering adsorption, separation, catalysis, fluorescence, sensing, and batteries. Finally, the future challenges and perspectives for the development of 3D COFs are discussed.

Metalated covalent organic frameworks: from synthetic strategies to diverse applications

Covalent organic frameworks (COFs) are a class of organic crystalline porous materials discovered in the early 21st century that have become an attractive class of emerging materials due to their high crystallinity, intrinsic porosity, structural regularity, diverse functionality, design flexibility, and outstanding stability. However, many chemical and physical properties strongly depend on the presence of metal ions in materials for advanced applications, but metal-free COFs do not have these properties and are therefore excluded from such applications. Metalated COFs formed by combining COFs with metal ions, while retaining the advantages of COFs, have additional intriguing properties and applications, and have attracted considerable attention over the past decade. This review presents all aspects of metalated COFs, from synthetic strategies to various applications, in the hope of promoting the continued development of this young field.

Selective and Multicyclic CO2 Adsorption with Visible Light-Driven Photodegradation of Organic Dyes in a Robust Metal-Organic Framework Embracing Heteroatom-Affixed Pores

Pore environment modulation with polarizing groups is one of the essential prerequisites for selective carbon dioxide (CO₂) adsorption in metal-organic frameworks (MOFs), wherein judicious installation of the photocatalytic feature can promise visible light-triggered degradation of toxic organic dye molecules. However, astute amalgamation of both these attributes over a single MOF is rather rare, yet much anticipated in view of sustainable applications. Pore engineering is effectively harnessed in a Zn(II)-based three-dimensional (3D) MOF, CSMCRI-16 (CSMCRI = Central Salt and Marine Chemicals Research Institute), through mixed-ligand assembly of a N-rich linker (L), 4,4'-oxybis(benzoic acid) (H₂oba) ligand, and [Zn₂(CO₂)₄N₂] paddle-wheel secondary building units (SBUs). The noninterpenetrated structure contains unbound nitrogen and accessible oxygen atom-decorated porous channels and exhibits admirable stability in diverse organic solvents, open air, and at elevated temperatures. The heteroatom-decorated porous channels facilitated excellent CO₂ uptake in the activated MOF (16a) with high selectivity over N₂ (CO₂/N₂: 155.3) at 273 K. The framework further exhibits reasonable CO₂ affinity and multicyclic CO₂ sorption recurrence without a significant loss in the uptake capacity. Benefitting from the presence of the [Zn₂(CO₂)₄N₂] cluster in conjugation with π-conjugated organic ligands, the extended 3D network revealed an optical band gap energy of 2.55 eV, which makes the MOF an efficient photocatalyst toward the degradation of the cationic dyes crystal violet (CV) and methylene blue (MB) in the presence of a simple 40 W visible light lamp without any assistance of external oxidants. The catalyst exhibits multicyclic performance and short reaction time in addition to the fact that catalytic efficiencies (CV: 97.2%, MB: 97.8%) are comparable to those of contemporary materials.

A luminescent Zn-MOF for the detection of explosives and development of fingerprints

A luminescent 3D metal-organic framework [Zn(NDA)(AMP)] = PUC1 (where, NDA = naphthalene-2,6-dicarboxylic acid and AMP = 4-aminomethyl pyridine) was synthesized under solvothermal conditions. The synthesized 3D framework was fully characterized with the help of different analytical techniques such as SCXRD, FTIR, TGA, PXRD, SEM, BET, etc. PUC1 exhibited a strong emission peak at 371 nm when excited at 290 nm and the resulting emission was efficiently quenched in the presence of various organic explosive substances like pentaerythritol tetranitrate (PETN), 2,4,6-trinitrophenyl-N-methylnitramine (Tetryl), trinitrotoluene (TNT), 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), and 1,3,5,7-tetranitro-1,3,5,7-tetrazoctane (HMX). PUC1 revealed highly sensitive and selective detection of PETN and Tetryl with high quenching constant values of 0.1 × 10⁶ and 0.12 × 10⁵ M^-1 and low detection limits of 0.315 and 0.404 μM respectively. The strong luminescent properties of PUC1 lead to its successful application in the development of latent fingermarks on different non-porous surfaces using the powder dusting method. The accuracy and applicability of the synthesized material were determined by developing fingerprints by using secretions from eccrine and apocrine glands on a glass slide and various other surfaces, followed by dusting the surfaces. The results so obtained were found to be very accurate and promising.

Urea-engineering mediated hydrogen-bond donating Friedel−Crafts alkylation of indoles and nitroalkenes in dual-functionalized and microporous metal-organic framework with high recyclability and pore-fitting-induced size-selectivity

As an effective alternative to Lewis acid activation, hydrogen-bond donating (HBD) organo-catalysis denotes a powerful construction tool to important classes of carbon–carbon bonds, wherein metal-organic frameworks (MOFs) alleviate issues like...

Metal–organic framework (MOF)-, covalent-organic framework (COF)-, and porous-organic polymers (POP)-catalyzed selective C–H bond activation and functionalization reactions

The review summarizes the state-of-the-art of C–H active transformations over crystalline and amorphous porous materials as new emerging heterogeneous (photo)catalysts.

Chemically Robust and Bifunctional Co(II)-Framework for Trace Detection of Assorted Organo-toxins and Highly Cooperative Deacetalization-Knoevenagel Condensation with Pore-Fitting-Induced Size-Selectivity

Acute detection of assorted classes of organo-toxins in a practical environment is an important sustainable agenda, whereas cooperative and recyclable catalysis can mitigate hazards by minimizing energy requirements and reducing waste generation. We constructed an acid-/base-stable Co(II)-framework with a unique network topology, wherein unidirectional porous channels are decorated by anionic [Co₂(μ₂-OH)(COO)₄(H₂O)₃] secondary building units and neutral [CoN₂(COO)₂] nodes. An intense luminescent signature of the hydrolytically robust framework is harnessed for the selective, fast-responsive, and regenerable detection of two detrimental organo-aromatics, 4-aminophenol (4-AP) and 2,4,6-trinitrophenol (TNP). Alongside remarkable quenching, their nanomolar detection limits (4-AP: 99.5 nM; TNP: 67.2 nM) rank among the lowest reported values in water and corroborate their ultra-sensitivity. Density functional theory (DFT) calculations verify the electron-transfer route of sensing through portraying redistribution of energy levels of molecular orbitals in a three-dimensional network by each analyte and further envisages non-covalent host-guest interactions. Benefiting from the concurrent existence of an open-metal site and a triphenylamine-moiety-functionalized ligand, the activated framework acts as an outstandingly cooperative heterogeneous catalyst in deacetalization-Knoevenagel condensation under mild conditions. The acid-base dual catalysis is detailed for the first time from combined inputs of control experiments and DFT validations. To the best of tandem reaction, larger-sized substrate exhibits insignificant conversion, and certifies rarest pore-fitting induced size-selectivity.

Multichannel Electron Transmission and Fluorescence Resonance Energy Transfer in In2S3/Au/rGO Composite for CO2 Photoreduction

Efficient electron transmission is an important step in the process of CO₂ photoreduction. In this paper, a multi-interface-contacted In₂S₃/Au/reduced graphene oxide (rGO) photocatalyst with the fluorescence resonance energy transfer (FRET) mechanism has been successfully prepared by the solvothermal, self-assembly, and hydrothermal reduction processes. Photocatalytic CO₂ reduction experiments showed that the In₂S₃/Au/rGO (IAr-3) composite exhibited excellent photoreduction performance and photocatalytic stability. The yields of CO and CH₄ obtained after the photoreduction process with IAr-3 as the catalyst were around 4 and 6 times higher than those of pure In₂S₃, respectively. Photoelectrochemical analysis showed that the multi-interface contact and FRET mechanism greatly improved the generation, transmission, and separation efficiency of carriers photogenerated within the photocatalyst. In situ FTIR test was applied to analyze the photocatalytic CO₂ reduction process. ¹³C isotope tracer test confirmed that the carbon source of CO and CH₄ was the CO₂ molecules in the photoreduction process rather than the decomposition of catalyst or TEOA. A potential enhanced photocatalytic mechanism has been discussed in total.

Solid-state behaviors of imines: colossal biaxial positive thermal expansion, motion capability, and phase transitions

The torsional flexibility of imines affects solid-state packing and properties. Behaviors including colossal thermal expansion, pedal motion, and phase transitions in imine-containing solids are described.

Emerging new-generation hybrids based on covalent organic frameworks for industrial applications

This review highlights the advancement of COF hybrid-based materials for diverse industrial applications.