Toward azo-linked covalent organic frameworks by developing linkage chemistry via linker exchange

A salen-based dinuclear cobalt(ii) polymer with direct and indirect synergy for electrocatalytic hydrogen evolution

Optimizing the spatial arrangement and geometric configuration of dinuclear metal sites within catalysts to leverage the dinuclear metal synergistic catalysis (DMSC) effect is a promising strategy for enhancing catalytic performance. In this work, we report a salen-based dinuclear cobalt covalent organic polymer (Co2-COP) that exhibits both direct and indirect DMSC synergistic effects, significantly improving catalytic efficiency for the electrocatalytic alkaline hydrogen evolution reaction (HER). Notably, one of the Co atoms in this structural unit features an OH- anion. The OH- anion facilitates both H2O adsorption through p-p orbital overlapping interaction and the subsequent OH* intermediate removal by pre-attracting cations. As a result, Co2-COP exhibits superior HER activity that surpasses its single-atom counterpart by a factor of 36. Control experiments and theoretical calculations revealed that the enhanced catalytic efficiency of Co2-COP is attributed to both the direct DMSC effect between two CoII ions, and the indirect DMSC involving the OH- anion and alkali cations. This synergistic interaction significantly facilitates water activation and accelerates the removal of the OH* intermediate, all of which are intricately linked to the unique dinuclear structure of the material.

Engineering Bipolar Covalent Organic Framework Membranes for Selective Acid Extraction

Nitric acid (HNO3) is a vital industrial chemical, and its recovery from complex waste streams is essential for sustainability and resource optimization. This study demonstrates the effectiveness of bipolar covalent organic framework (COF) membranes with tunable ionic site distributions as a solution for this challenge. The membranes are fabricated by layering anionic COF nanosheets on cationic COF layers, supported by a porous substrate. The resulting membranes exhibit significant rectifying behavior, driven by the asymmetric charge polarity and the intrinsic electric field, which enhances HNO3 transport. The transmembrane diffusion coefficient of 2.74 × 10-5 cm2 s-1 exceeds the self-diffusion rate of NO3 -, leading to increased HNO3 flux and selectivity compared to the individual anionic and cationic COF membranes. The optimized bipolar membrane configuration achieves remarkable separation factors, ranging from 22 to 242,000 for HNO₃, in comparison to other solutes such as HCl, H2SO4, H3PO4, and various metal salts in an eight-component mixed waste stream. This results in a substantial increase in HNO₃ purity, from 12.5% to 94.1% after a single membrane separation. With the broad range of COF materials and the versatility of the proposed membrane design, this work represents a significant advancement in chemical separation technologies.

Imine-Linked Covalent Organic Frameworks Tuned by Quaternary-Ammonium-Alkyl for Highly Effective and Selective Adsorption of Perchlorate from Water

Perchlorate (ClO4 -) contamination in surface water is an escalating issue for drinking water safety. Herein, an imine-linked covalent organic framework (COF) tuned by quaternary ammonium alkyls (R4N+) is developed for ClO4 - adsorption. The hydrophobic and cationic COF adsorbent achieves a record-breaking adsorption capacity of ClO4 - at 912.7 mg g-1, demonstrating remarkable selectivity for ClO4 - over other environmentally relevant anions and exhibiting rapid adsorption kinetics. Furthermore, the adsorbent maintains excellent recycling performance (removal efficiency ≥ 80% after 10 cycles) using tetrachloroferrate for regeneration. In dynamic flow-through experiments with real water samples, the adsorbent effectively treats ≈3200-bed volumes of feed streams (≈500 µg L-1), with an enrichment factor of 15.2. The hydrophobicity of the COF adsorbent is identified as a premise for its interaction with ClO4 -. Molecular dynamic simulations and density functional theory calculations reveal that R4N+ anchored in COF cores enriches ClO4 - via electrostatic attraction and bonds with ClO4 - via unconventional hydrogen bonds (C─H─O). These key insights pave the way for future design and optimization of adsorbents for removing oxo-anion from water, especially for ClO4 -.

Two-Dimensional Heteropore Covalent Organic Frameworks: From Construction to Functions

ConspectusCovalent organic frameworks (COFs) represent a fascinating class of crystalline porous polymers constructed from organic building blocks linked by covalent bonds. Benefiting from their high crystallinity, large surface area, and ease of functionalization, COFs have demonstrated significant potential across various fields, including gas adsorption, luminescence, sensing, catalysis, energy storage, nanomedicine, etc. In the first decade of COF development, only those with homogeneous porosity have been constructed, and thus, their topological structures are quite limited. An exciting progress in the field of COFs is the emergence of two-dimensional (2D) COFs with hierarchical porosity, known as heteropore COFs, which have garnered considerable attention in recent years. Heteropore COFs are deliberately designed to integrate different types of pores into a single framework, resulting in heterogeneous porosity that imparts captivating properties and functions. Compared to their homopore counterparts, heteropore COFs offer a compelling platform for creating hierarchically structured porous materials, thanks to their distinctive multicompartment architectures and different pore environments. Since we achieved the construction of the first heteropore COF featuring both micropores and mesopores in 2014, substantial advancements have been achieved in the realm of heteropre COFs over the past decade, considerably increasing the topological diversity of 2D COFs. In this Account, we summarize our contributions to the development of 2D heteropore COFs. First, we review representative design strategies for the construction of 2D heteropore COFs, including the angle-specific-vertex, heterostructural-mixed-linker, multiple-linking-site, and desymmetrization-design strategies and their combinations as well as the dynamic covalent chemistry-mediated linker exchange strategy. Based on these strategies, heteroporous frameworks with two, three, and four different kinds of pores and different types of linkages have been successfully fabricated. Next, we discuss the properties and applications of heteropore COFs, including those shared with their homopore counterparts and unique ones originating from their hierarchical porous structures. Our research has shown that heteropore COFs have inherited the common features from their homopore counterparts and exhibited application potentials in gas adsorption, chemical sensing, environmental remediation, etc. More importantly, the multicompartment architecture and heterogeneous pore environment of heteropore COFs offer distinct benefits, for which exclusive applications and unique properties of heteropore COFs distinct from those of homopore COFs have been demonstrated. Finally, we highlight the current challenges and future directions of heteropore COFs, with an emphasis on the development of structural design and synthetic methodologies, precise structural characterization, and the exploration of unique properties and advanced applications. We believe that this Account will offer valuable insights into the design and synthesis of COFs with heteroporous structures, thereby accelerating their applications across a wide range of interdisciplinary research areas.

Viologen Covalent Organic Framework Mediates Near-Infrared Light-Induced Electron Transfer for Catalytic Oxidative Coupling Reactions

Near-infrared (NIR) light-driven photoreactions are advantageous over visible light-driven ones because NIR photons have lower energy and fewer side reactions, deeper penetration in reaction media, and high abundance in the solar spectrum. However, currently available covalent organic frameworks (COFs) absorb in the UV-vis region and catalyze photoreactions under blue or white light irradiation. Herein, we report a linker-to-linker charge transfer process in a viologen-linked porphyrin COF (Vio-COF), leading to a novel type of hyperporphyrin effect and extending the absorption into the NIR region with an absorption edge at 998 nm. Under NIR irradiation, photoinduced charge separation in Vio-COF generates a viologen radical that efficiently reduces oxygen to form superoxide radicals for catalytic oxidative coupling reactions. The proximity of porphyrin and viologen units within the framework significantly enhances the catalytic performance of Vio-COF, outperforming its homogeneous counterparts in aerobic oxidative amidation and amine coupling reactions. Vio-COF was readily recycled and used in six oxidative coupling reactions.

Recent advances in the synthetic applications of nitrosoarene chemistry

Nitroso groups are widely present in biologically active compounds in medicinal chemistry, and nitroso compounds serve as important building blocks in organic chemistry and materials science. Nitrosoarenes, in particular, showcase remarkable versatility, functioning as both electrophilic and nucleophilic reagents in a broad spectrum of organic reactions, thereby holding significant relevance in organic chemistry. This review aims to provide a comprehensive overview of the latest advancements in nitrosoarene reactions spanning a decade. Special attention is given to the synthesis of products derived from nitrosoarenes and the conditions that promote these reactions, as well as the type of catalysts. The exploration covers various facets of nitrosoarene chemistry, including cyclization, reactions involving attacks at the oxygen or nitrogen terminus, dimerization, rearrangement, coordination, and other significant reactions. By delving into these diverse reaction pathways and mechanisms, this review aspires to serve as a valuable resource for researchers seeking to deepen their understanding of nitrosoarene chemistry and its applications in both fundamental and applied scientific research.

Induced Charge-Compensation Effect for Boosting Photocatalytic Water Splitting in Covalent Organic Frameworks

Imine-based covalent organic frameworks (COFs) are promising for photocatalytic water splitting, but their performance is often constrained by inefficient charge separation due to the high electron localization nature of polar imine bonds. In this study, we have optimized the electron delocalization across the imine linkage within a COF by implementing a charge compensation effect. This effect is achieved when a strong electron-donating thieno[3,2-b]thiophene linker is directly attached to the iminic carbon of a zinc-porphyrinic COF. This modification significantly reduces the electron binding effect within the imine bonds of the COF, facilitating both in-plane charge separation and out-plane charge transfer to the catalytic site. Conversely, the use of strong electron-withdrawing pyrizine linker aggravates the electron localization at the imine linkage in the ZnP-Pz variant. Consequently, ZnP-Tt shows a substantially improved photocatalytic water-splitting activity under visible light irradiation, with a hydrogen evolution of 44288±2280 μmol g-1 in 4 h, which exceeds the ZnP-Pz counterpart by a factor of 10. These results offer fresh perspectives for the design of imine-based COFs to overcome their limitations in charge separation efficiency.

Photocatalytic applications of covalent organic frameworks: synthesis, characterization, and utility

Photocatalysis has emerged as an energy efficient and safe method to perform organic transformations, and many semiconductors have been studied for use as photocatalysts. Covalent organic frameworks (COFs) are an established class of crystalline, porous materials constructed from organic units that are easily tunable. COFs importantly display semiconductor properties and respectable photoelectric behaviour, making them a strong prospect as photocatalysts. In this review, we summarize the design, synthetic methods, and characterization techniques for COFs. Strategies to boost photocatalytic performance are also discussed. Then the applications of COFs as photocatalysts in a variety of reactions are detailed. Finally, a summary, challenges, and future opportunities for the development of COFs as efficient photocatalysts are entailed.

Nanoscale covalent organic framework-mediated pyroelectrocatalytic activation of immunogenic cell death for potent immunotherapy

The conventional molecular immunogenic cell death (ICD) inducers suffer from poor biocompatibility and unsatisfactory efficacy. Here, a biocompatible nanosized covalent organic framework (nCOF)-based pyroelectric catalyst (denoted as TPAD-COF NPs) is designed for pyroelectric catalysis-activated in situ immunotherapy. TPAD-COF NPs confine organic pyroelectric molecules to rigid TPAD-COF NPs to substantially reduce aggregation and enhance biocompatibility, thus improving pyroelectrocatalytic efficiency. After tumor internalization, TPAD-COF NPs facilitate photothermal tumor ablation under near-infrared (NIR) laser exposure, resulting in effective ICD induction. In addition, TPAD-COF NPs effectively catalyze the conversion of temperature changes to pyroelectric changes, which subsequently react with adjacent O2 to generate reactive oxygen species, thus triggering robust ICD activation. In vivo evaluation using mouse models confirmed that TPAD-COF NPs evidently inhibited the proliferation of primary and distant tumors and prevented lung metastasis under NIR laser illumination. Therefore, this study opens an avenue for designing nCOF-based catalysts for pyroelectric catalysis-activated in situ immunotherapy.

Photons, Excitons, and Electrons in Covalent Organic Frameworks

Covalent organic frameworks (COFs) are created by the condensation of molecular building blocks and nodes to form two-dimensional (2D) or three-dimensional (3D) crystalline frameworks. The diversity of molecular building blocks with different properties and functionalities and the large number of possible framework topologies open a vast space of possible well-defined porous architectures. Besides more classical applications of porous materials such as molecular absorption, separation, and catalytic conversions, interest in the optoelectronic properties of COFs has recently increased considerably. The electronic properties of both the molecular building blocks and their linkage chemistry can be controlled to tune photon absorption and emission, to create excitons and charge carriers, and to use these charge carriers in different applications such as photocatalysis, luminescence, chemical sensing, and photovoltaics. In this Perspective, we will discuss the relationship between the structural features of COFs and their optoelectronic properties, starting with the building blocks and their chemical connectivity, layer stacking in 2D COFs, control over defects and morphology including thin film synthesis, exploring the theoretical modeling of structural, electronic, and dynamic features of COFs, and discussing recent intriguing applications with a focus on photocatalysis and photoelectrochemistry. We conclude with some remarks about present challenges and future prospects of this powerful architectural paradigm.

Azobenzene-Bridged Covalent Organic Frameworks Boosting Photocatalytic Hydrogen Peroxide Production from Alkaline Water: One Atom Makes a Significant Improvement

Covalent organic frameworks (COFs) have been demonstrated as promising photocatalysts for hydrogen peroxide (H2O2) production. However, the construction of COFs with new active sites, high photoactivity, and wide-range light absorption for efficient H2O2 production remains challenging. Herein, we present the synthesis of a novel azobenzene-bridged 2D COF (COF-TPT-Azo) with excellent performance on photocatalytic H2O2 production under alkaline conditions. Notably, although COF-TPT-Azo differs by only one atom (-N=N- vs. -C=N-) from its corresponding imine-linked counterpart (COF-TPT-TPA), COF-TPT-Azo exhibits a significantly narrower band gap, enhanced charge transport, and prompted photoactivity. Remarkably, when employed as a metal-free photocatalyst, COF-TPT-Azo achieves a high photocatalytic H2O2 production rate up to 1498 μmol g-1 h-1 at pH = 11, which is 7.9 times higher than that of COF-TPT-TPA. Further density functional theory (DFT) calculations reveal that the -N=N- linkages are the active sites for photocatalysis. This work provides new prospects for developing high-performance COF-based photocatalysts.

Schiff-Base Cross-Linked Hydrogels Based on Properly Synthesized Poly(ether urethane)s as Potential Drug Delivery Vehicles in the Biomedical Field: Design and Characterization

In situ-forming hydrogels based on the Schiff-base chemistry are promising for drug delivery applications, thanks to their stability under physiological conditions, injectability, self-healing properties, and pH-responsiveness. In this work, two water-soluble poly(ethylene glycol)-based poly(ether urethane)s (PEUs) were engineered. A high-molecular-weight PEU (SHE3350, M̅ n 24 kDa, D 1.7), bearing primary amino groups along each polymeric chain, was synthesized using N-Boc serinol and subjected to an acidic treatment to expose primary amines (ca. 1020 units/gSHE3350). In parallel, a low-molecular-weight PEU (AHE1500, M̅ n 4 kDa, D 1.5) with aldehyde end groups was synthesized by end-capping an isocyanate-terminated prepolymer with 4-hydroxybenzaldehyde, and the aldehyde groups were quantified to be around 1020 units/gAHE1500. Hydrogels were prepared by simply mixing SHE3350 and AHE1500 aqueous solutions and characterized to assess their physico-chemical and rheological properties. Schiff-base bond formation was proved through carbon-13 and proton solid-state NMR spectroscopies. Rheological characterization confirmed the formation of gels with high resistance to applied strain (ca. 1000%). Hydrogels exhibited high absorption ability (ca. 270% increase in wet weight) in physiological-like conditions (i.e., 37 °C and pH 7.4) up to 27 days. In contact with buffer at pH 5, enhanced fluid absorption was observed until dissolution occurred starting from 13 days due to Schiff-base hydrolysis in acidic conditions. Conversely, gels showed a reduced absorption ability at pH 9 due to shrinkage phenomena. Furthermore, they exhibited high permeability and controlled, sustained, and pH-triggered release of a model molecule (i.e., fluorescein isothiocyanate dextran) for up to 17 days. Lastly, the hydrogels showed easy injectability and self-healing ability.

Novel Top-Down Synthesis of Covalent Organic Frameworks for Uranyl Ion Capture

Seeking novel synthetic methodology to further promote the preparation of covalent organic frameworks (COFs) has long been our pursuit but remains a challenging task. Herein, we report a new protocol, a top-down approach for facile synthesis of COFs. Interestingly, our top-down route can impressively generate extended COFs by reticular chemistry which cannot be accessed by the commonly used bottom-up synthesis route. Notably, our top-down method also has outstanding advantages in achieving what we are pursuing in COFs, such as heteropores and multiple components. The current findings not only dramatically reduce the difficulty of COF synthesis but also are generally applicable for the synthesis of complicated COFs constructed from different building blocks and linkages.

Layered hybrid superlattices as designable quantum solids

Crystalline solids typically show robust long-range structural ordering, vital for their remarkable electronic properties and use in functional electronics, albeit with limited customization space. By contrast, synthetic molecular systems provide highly tunable structural topologies and versatile functionalities but are often too delicate for scalable electronic integration. Combining these two systems could harness the strengths of both, yet realizing this integration is challenging owing to distinct chemical bonding structures and processing conditions. Two-dimensional atomic crystals comprise crystalline atomic layers separated by non-bonding van der Waals gaps, allowing diverse atomic or molecular intercalants to be inserted without disrupting existing covalent bonds. This enables the creation of a diverse set of layered hybrid superlattices (LHSLs) composed of alternating crystalline atomic layers of variable electronic properties and self-assembled atomic or molecular interlayers featuring customizable chemical compositions and structural motifs. Here we outline strategies to prepare LHSLs and discuss emergent properties. With the versatile molecular design strategies and modular assembly processes, LHSLs offer vast flexibility for weaving distinct chemical constituents and quantum properties into monolithic artificial solids with a designable three-dimensional potential landscape. This opens unprecedented opportunities to tailor charge correlations, quantum properties and topological phases, thereby defining a rich material platform for advancing quantum information science.

Orientation-Dependent Photocatalysis of Imine-Linked Covalent Organic Frameworks Based on Thienothiophenes for Oxidation of Amines to Imines

Toward visible light photocatalysis, covalent organic frameworks (COFs) have recently garnered growing attention. The effect of different orientations of imine of imine-linked COFs on photocatalysis should be elucidated. Here, two COFs are developed with 2,5-diphenylthieno[3,2-b]thiophene (DPTT) and 1,3,6,8-tetraphenylpyrene (Py) linked by imine, affording DPTT-Py-COF and Py-DPTT-COF, respectively. Distinctly, DPTT-Py-COF and Py-DPTT-COF have high crystallinity and porosity, paving the way to highly efficient photocatalysis. Theoretical calculations demonstrate that both DPTT-Py-COF and Py-DPTT-COF are of similar bandgaps but of varied energy positions due to the different orientations of imine. Besides, characterizations disclose that DPTT-Py-COF delivers more enhanced charge separation and transfer than Py-DPTT-COF. Probed by the oxidation of amine to imine, DPTT-Py-COF exhibits a blue light photocatalytic performance superior to that of Py-DPTT-COF. DPTT-Py-COF, a highly recyclable photocatalyst, enables the oxidation of various amines to imines with oxygen. This work highlights that tuning the microenvironment of COFs unravels tenable performances in photocatalysis.

Oxygen- and proton-transporting open framework ionomer for medium-temperature fuel cells

Medium-temperature proton exchange membrane fuel cells (MT PEMFCs) operating at 100° to 120°C have improved kinetics, simplified thermal and water management, and broadened fuel tolerance compared with low-temperature PEMFCs. However, high temperatures lead to Nafion ionomer dehydration and exacerbate gas transportation limitations. Inspired by osmolytes found in hyperthermophiles, we developed α-aminoketone-linked covalent organic framework (COF) ionomers, interwoven with Nafion, to act as "breathable" proton conductors. This approach leverages synergistic hydrogen bonding to retain water, enhancing hydration and proton transport while reducing oxygen transport resistance. For commercial Pt/C, the MT PEMFCs achieved peak and rated power densities of 18.1 and 9.5 Watts per milligram of Pt at the cathode at 105°C fueled with H2 and air, marking increases of 101 and 187%, respectively, compared with cells lacking the COF.

Rigid covalent organic frameworks with thiazole linkage to boost oxygen activation for photocatalytic water purification

Owing to their capability to produce reactive oxygen species (ROS) under solar irradiation, covalent organic frameworks (COFs) with pre-designable structure and unique architectures show great potentials for water purification. However, the sluggish charge separation, inefficient oxygen activation and poor structure stability in COFs restrict their practical applications to decontaminate water. Herein, via a facile one-pot synthetic strategy, we show the direct conversion of reversible imine linkage into rigid thiazole linkage can adjust the π-conjugation and local charge polarization of skeleton to boost the exciton dissociation on COFs. The rigid linkage can also improve the robustness of skeleton and the stability of COFs during the consecutive utilization process. More importantly, the thiazole linkage in COFs with optimal C 2p states (COF-S) effectively increases the activities of neighboring benzene unit to directly modulate the O2-adsorption energy barrier and improve the ROS production efficiency, resulting in the excellent photocatalytic degradation efficiency of seven toxic emerging contaminants (e.g. degrading ~99% of 5 mg L-1 paracetamol in only 7 min) and effective bacterial/algal inactivation performance. Besides, COF-S can be immobilized in continuous-flow reactor and in enlarged reactor to efficiently eliminate pollutants under natural sunlight irradiation, demonstrating the feasibility for practical application.

Linkage-Mixed Covalent Organic Frameworks Synthesized by a Liquid-Solid Two-Phase Strategy for Photoenhanced Uranium Extraction

Synthesizing COFs with hybrid linkage coupling with both reversible and irreversible natures remains a challenging issue. Herein, we report the synthesis of two rare COFs constructed by both reversible and irreversible linkages through a liquid-solid two-phase strategy. A systematic study reveals a one-pot, two-step reaction mechanism for the two COFs, the first step being a reversible Schiff base reaction and the second step being an irreversible Knoevenagel reaction. Interestingly, this hybrid linkage COF is found to show an outstanding photoenhanced uranium extraction performance. The results not only provide a general and green approach to develop the linkage chemistry of COFs but also enrich the synthesis toolboxes and application of COFs.

Unraveling Early-Stage Dynamics of Cage-to-Covalent Organic Framework Transformation at Liquid-Liquid Interface

Postsynthetic linker exchange (PLE) has emerged as an emerging synthetic strategy for constructing high-quality covalent organic frameworks (COFs) from preassembled entities such as linear polymers, amorphous networks, COFs, and porous organic cages by using the principles of dynamic covalent chemistry. The PLE strategy has recently been extended at the liquid-liquid interface to fabricate highly crystalline two-dimensional (2D)-COF membranes at a faster time scale (24 h). Examining the early stages of the interfacial PLE dynamics becomes essential to understanding the expedited COF growth process. In this regard, pendant drop tensiometry has been employed to probe the initial reaction dynamics of the imine cage-to-COF transformation through dynamic interfacial tension (IFT) measurements. The contrasting trends in IFT profiles between PLE-mediated (from cage) and direct COF synthesis (from parent monomers) are in qualitative agreement with the kinetics of bulk-scale interfacial polymerizations. Further, the distinct early-stage interfacial behaviors between the diverse synthetic routes have been experimentally demonstrated using tensiometry, optical microscopy, electron microscopy, and powder X-ray diffraction (PXRD) analysis. The pivotal role of in situ generated imine intermediates (ImIs) and the phenomenon of spontaneous emulsification toward accelerated interfacial COF growth process was delineated. The current study on deploying the pendant drop tensiometric technique to examine early-stage interfacial polymerization dynamics opens up a gripping avenue for mechanistic exploration in PLE-based COF synthesis. The generality of the developed methodology to study the initial COF growth kinetics was extended to a new interfacial PLE-mediated cage-to-COF transformation.

Functional Covalent Organic Frameworks' Microspheres Synthesized by Self-Limited Dynamic Linker Exchange for Stationary Phases

Synthesizing uniform functional covalent organic framework (COF) microspheres is the prerequisite of applying COFs as novel stationary phases for liquid chromatography. However, the synthesis of functionalized COF microspheres is challenging due to the difficulty in maintaining microspheric morphology when conferring functions. Here, a facile and universal "self-limited dynamic linker exchange" strategy is developed to achieve surface functionalization of uniform COF microspheres. Six different types of COF microspheres are constructed, showing the universality and superiority of the strategy. The library of COF microspheres' stationary phases can be further enriched on demand by varying different functional building blocks. The "self-limited dynamic linker exchange" is attributed to the result of a delicate balance of reaction thermodynamics and molecular diffusion energy barrier. As a demonstration, the chiral functional COF microspheres are used as stationary phases of chiral chromatography and realized effective enantioseparation.

Fluorescent carbon nanodot-based rapid cell nucleus staining and application for diagnosis

Cell nucleus status decides the activities of corresponding cells, making its rapid and effective staining important for revealing the actual condition of biological environment in life science and related fields. In this study, fast staining of cell nucleus is realized by fluorescent carbon nanodots (CDs). The staining mechanism is due to the positively charged CD surface-induced cell membrane penetration, which facilitates the CD-nucleus binding via electrostatic attraction. The size of cell nucleus is easily measured with fluorescence imaging technique. In addition, the CD-based cell nucleus stain is applied for discriminating the normal and cancer cells by determining the cell-to-nucleus ratio with fluorescence images.

Photo-Induced Geometry and Polarity Gradients in Covalent Organic Frameworks Enabling Fast and Durable Molecular Separations

Azobenzene, which activates its geometric and chemical structure under light stimulation enables noninvasive control of mass transport in many processes including membrane separations. However, producing azobenzene-decorated channels that have precise size tunability and favorable pore wall chemistry allowing fast and durable permeation to solvent molecules, remains a great challenge. Herein, an advanced membrane that comprises geometry and polarity gradients within covalent organic framework (COF) nanochannels utilizing photoisomerization of azobenzene groups is reported. Such functional variations afford reduced interfacial transfer resistance and enhanced solvent-philic pore channels, thus creating a fast solvent transport pathway without compromising selectivity. Moreover, the membrane sets up a densely covered defense layer to prevent foulant adhesion and the accumulation of cake layer, contributing to enhanced antifouling resistance to organic foulants, and a high recovery rate of solvent permeance. More importantly, the solvent permeance displays a negligible decline throughout the long-term filtration for over 40 days. This work reports the geometry and polarity gradients in COF channels induced by the conformation change of branched azobenzene groups and demonstrates the strong capability of this conformation change in realizing fast and durable molecular separations.

Asymmetric Potential Model of Two-Dimensional Imine Covalent Organic Frameworks with Enhanced Quantum Efficiency for Photocatalytic Water Splitting

Two-dimensional (2D) covalent organic frameworks (COFs) assembled using building blocks have been widely employed in photocatalysis due to their customizable optoelectronic characteristics and porous structure, which facilitate charge carrier and mass movement. Nevertheless, the development of COF photocatalysts encounters a continuous obstacle in enhancing the efficiency of photocatalysis, impeded by a limited comprehension of the orbital interaction between molecular fragments and linkers. In this study, we present a model that examines the interaction between molecular fragments in an imine-based COF at the frontier molecular orbital level, enabling us to comprehend the impact of manipulating linkers on light adsorption, exciton efficiency, and catalytic activity. Our findings demonstrate that altering the connecting orientation of 14 R-C=N-R imine linkers in 2D COFs can enhance solar-to-hydrogen (STH) efficiency under visible light from 2.76% to 4.24%. This research has the potential to provide a valuable model for refining photocatalysts with enhanced photocatalytic performance.

Elaborate Modulating Binding Strength of Intermediates via Three-component Covalent Organic Frameworks for CO2 Reduction Reaction

The catalytic performance for electrocatalytic CO2 reduction reaction (CO2RR) depends on the binding strength of the reactants and intermediates. Covalent organic frameworks (COFs) have been adopted to catalyze CO2RR, and their binding abilities are tuned via constructing donor-acceptor (DA) systems. However, most DA COFs have single donor and acceptor units, which caused wide-range but lacking accuracy in modulating the binding strength of intermediates. More elaborate regulation of the interactions with intermediates are necessary and challenge to construct high-efficiency catalysts. Herein, the three-component COF with D-A-A units was first constructed by introducing electron-rich diarylamine unit, electron-deficient benzothiazole and Co-porphyrin units. Compared with two-component COFs, the designed COF exhibit elevated electronic conductivity, enhanced reducibility, high efficiency charge transfer, further improving the electrocatalytic CO2RR performance with the faradic efficiency of 97.2 % at -0.8 V and high activity with the partial current density of 27.85 mA cm-2 at -1.0 V which exceed other two-component COFs. Theoretical calculations demonstrate that catalytic sites in three-component COF have suitable binding ability of the intermediates, which are benefit for formation of *COOH and desorption of *CO. This work offers valuable insights for the advancement of multi-component COFs, enabling modulated charge transfer to improve the CO2RR activity.

A Visible Light Responsive Smart Covalent Organic Framework with a Bridged Azobenzene Backbone

Condensation of 3,3'-diamino-2,2'-ethylene-bridged azobenzene with 1,2,4,5-tetrakis-(4-formylphenyl) benzene produces a visible light responsive porous 2D covalent organic framework, COF-bAzo-TFPB, with a large surface area, good crystallinity, and thermal and chemical stability. The results demonstrate that the elaborated designed linker can make azo unit on the COF-bAzo-TFPB skeleton undergo reversible photoisomerization. This work expands the application scope of covalent organic frameworks in photo-controlled release, uptake of guest molecules, dynamic photoswitching, and UV-sensitive functions.

Covalent Organic Frameworks: Opportunities for Rational Materials Design in Cancer Therapy

Nanomedicines are extensively used in cancer therapy. Covalent organic frameworks (COFs) are crystalline organic porous materials with several benefits for cancer therapy, including porosity, design flexibility, functionalizability, and biocompatibility. This review examines the use of COFs in cancer therapy from the perspective of reticular chemistry and function-oriented materials design. First, the modification sites and functionalization methods of COFs are discussed, followed by their potential as multifunctional nanoplatforms for tumor targeting, imaging, and therapy by integrating functional components. Finally, some challenges in the clinical translation of COFs are presented with the hope of promoting the development of COF-based anticancer nanomedicines and bringing COFs closer to clinical trials.

Ambient Aqueous Synthesis of Imine-Linked Covalent Organic Frameworks (COFs) and Fabrication of Freestanding Cellulose Nanofiber@COF Nanopapers

Covalent organic frameworks (COFs) are usually synthesized under solvothermal conditions that require the use of toxic organic solvents, high reaction temperatures, and complicated procedures. Additionally, their insolubility and infusibility present substantial challenges in the processing of COFs. Herein, we report a facile, green approach for the synthesis of imine-linked COFs in an aqueous solution at room temperature. The key behind the synthesis is the regulation of the reaction rate. The preactivation of aldehyde monomers using acetic acid significantly enhances their reactivity in aqueous solutions. Meanwhile, the still somewhat lower imine formation rate and higher imine breaking rates in aqueous solution, in contrast to conventional solvothermal synthesis, allow for the modulation of the reaction equilibrium and the crystallization of the products. As a result, highly crystalline COFs with large surface areas can be formed in relatively high yields in a few minutes. In total, 16 COFs are successfully synthesized from monomers with different molecular sizes, geometries, pendant groups, and core structures, demonstrating the versatility of this approach. Notably, this method works well on the gram scale synthesis of COFs. Furthermore, the aqueous synthesis facilitates the interfacial growth of COF nanolayers on the surface of cellulose nanofibers (CNFs). The resulting CNF@COF hybrid nanofibers can be easily processed into freestanding nanopapers, demonstrating high efficiency in removing trace amounts of antibiotics from wastewater. This study provides a route to the green synthesis and processing of various COFs, paving the way for practical applications in diverse fields.

2D Covalent Organic Frameworks with Kagome Lattice: Synthesis and Applications

2D covalent organic frameworks with Kagome (kgm) topology are a promising class of crystalline frameworks that possess both triangular and hexagonal pores. These dual-pore structures enable kgm COFs to exhibit unique advantages in selective separation, mass transfer, and targeted drug release. However, the synthesis of 2D kgm COFs has been hindered by the reliance on empirical methods. This review systematically summarizes the conventional macrocycle-to-framework strategy, typical [4+2] co-polymerization synthetic strategy, and bifunctional molecules self-condensation approach for constructing 2D kgm COFs. Factors influencing the formation of kgm lattice are surveyed, such as monomer type, solvent polarity, substrate concentration, etc., and highlight the representative examples on targeted synthesis. Additionally, applications of 2D kgm COFs and relationships between structure and performances are summarized. Finally, key fundamental perspectives are proposed to accelerate the further development of this intriguing material.

Isomeric Dual-Pore Two-Dimensional Covalent Organic Frameworks

Two-dimensional (2D) covalent organic frameworks (COFs) with hierarchical porosity have been increasingly recognized as promising materials in various fields. Besides, the 2D COFs with kagome (kgm) topology can exhibit unique optoelectronic features and have extensive applications. However, rational synthesis of the COFs with kgm topology remains challenging because of competition with a square-lattice topology. Herein, we report two isomeric dual-pore 2D COFs with kgm topology using a novel geometric strategy to reduce the symmetry of their building blocks, which are four-armed naphthalene-based and azulene-based isomeric monomers. Owing to the large dipole moment of azulene, as-prepared azulene-based COF (COF-Az) possesses a considerably narrow band gap of down to 1.37 eV, which is much narrower than the naphthalene-based 2D COF (COF-Nap: 2.28 eV) and is the lowest band gap among reported imine-linked dual-pore 2D COFs. Moreover, COF-Az was used as electrode material in a gas sensor and exhibits high selectivity for NO2, including a high response rate (58.7%) to NO2 (10 ppm), fast recovery (72 s), up to 10 weeks of stability, and resistance to 80% relative humidity, which are superior to those of reported COF-based NO2 gas sensors. The calculation and in situ experimental results indicate that the large dipole moment of azulene boosts the sensitivity of the imine linkages. The usage of isomeric building blocks not only enables the synthesis of 2D COFs with isometric kgm topology but also provides an azulene-based 2D platform for studying the structure-property correlations of COFs.

Boosting CO2 Photoreduction via Regulating Charge Transfer Ability in a One-Dimensional Covalent Organic Framework

Two-dimensional (2D) imine-based covalent organic frameworks (COFs) hold potential for photocatalytic CO2 reduction. However, high energy barrier of imine linkage impede the in-plane photoelectron transfer process, resulting in inadequate efficiency of CO2 photoreduction. Herein, we present a dimensionality induced local electronic modulation strategy through the construction of one-dimensional (1D) pyrene-based covalent organic frameworks (PyTTA-COF). The dual-chain-like edge architectures of 1D PyTTA-COF enable the stabilization of aromatic backbones, thus reducing energy loss during exciton dissociation and thermal relaxation, which provides energetic photoelectron to traverse the energy barrier of imine linkages. As a result, the 1D PyTTA-COF exhibits significantly enhanced CO2 photoreduction activity under visible-light irradiation when coordinated with metal cobalt ion, yielding a remarkable CO evolution of 1003 μmol g-1 over an 8-hour period, which surpasses that of the corresponding 2D counterpart by a factor of 59. These findings present a valuable approach to address in-plane charge transfer limitations in imine-based COFs.

Hydrogen-Bonded Organic Framework to Upgrade Cycling Stability and Rate Capability of Li-CO2 Batteries

Elaborately designed multifunctional electrocatalysts capable of promoting Li+ and CO2 transport are essential for upgrading the cycling stability and rate capability of Li-CO2 batteries. Hydrogen-bonded organic frameworks (HOFs) with open channels and easily functionalized surfaces hold great potential for applications in efficient cathodes of Li-CO2 batteries. Herein, a robust HOFS (HOF-FJU-1) is introduced for the first time as a co-catalyst in the cathode material of Li-CO2 batteries. HOF-FJU-1 with cyano groups located periodically in the pore can induce homogeneous deposition of discharge products and accommodate volumetric expansion of discharge products during cycling. Besides, HOF-FJU-1 enables effective interaction between Ru0 nanoparticles and cyano groups, thus forming efficient and uniform catalytic sites for CRR/CER. Moreover, HOF-FJU-1 with regularly arranged open channels are beneficial for CO2 and Li+ transport, enabling rapid redox kinetic conversion of CO2 . Therefore, the HOF-based Li-CO2 batteries are capable of stable operation at 400 mA g-1 for 1800 h and maintain a low overpotential of 1.96 V even at high current densities up to 5 A g-1 . This work provides valuable guidance for developing multifunctional HOF-based catalysts to upgrade the longevity and rate capability of Li-CO2 batteries.

Two-step fabrication of COF membranes for efficient carbon capture

Covalent organic framework (COF) materials have been considered as disruptive membrane materials for gas separation. The dominant one-step method for COF nanosheet synthesis often suffers from coupling among polymerization, assembly and crystallization processes. Herein, we propose a two-step method comprising a framework assembly step and functional group switching step to synthesize COF nanosheets and the corresponding COF membranes. In the first step, the pristine COF-316 nanosheets bearing cyano groups are prepared via interfacial polymerization. In the second step, the cyano groups in COF-316 nanosheets were switched into amidoxime groups or carboxyl groups. Through the vacuum-assisted self-assembly method, the COF nanosheets were fabricated into membranes with a thickness below 100 nm. Featuring numerous mass transport channels and homogeneous distribution of functional groups, the amidoxime-modified COF-316 membrane demonstrated excellent separation performance, with a permeance above 500 GPU and a CO2/N2 selectivity above 50. The two-step method may inspire the rational design and fabrication of organic framework membranes.

Covalent organic frameworks for CO2 capture: from laboratory curiosity to industry implementation

CO2 concentration in the atmosphere has increased by about 40% since the 1960s. Among various technologies available for carbon capture, adsorption and membrane processes have been receiving tremendous attention due to their potential to capture CO2 at low costs. The kernel for such processes is the sorbent and membrane materials, and tremendous progress has been made in designing and fabricating novel porous materials for carbon capture. Covalent organic frameworks (COFs), a class of porous crystalline materials, are promising sorbents for CO2 capture due to their high surface area, low density, controllable pore size and structure, and preferable stabilities. However, the absence of synergistic developments between materials and engineering processes hinders achieving the qualitative leap for net-zero emissions. Considering the lack of a timely review on the combination of state-of-the-art COFs and engineering processes, in this Tutorial Review, we emphasize the developments of COFs for meeting the challenges of carbon capture and disclose the strategies of fabricating COFs for realizing industrial implementation. Moreover, this review presents a detailed and basic description of the engineering processes and industrial status of carbon capture. It highlights the importance of machine learning in integrating simulations of molecular and engineering levels. We aim to stimulate both academia and industry communities for joined efforts in bringing COFs to practical carbon capture.

Tuning Local Charge Distribution in Multicomponent Covalent Organic Frameworks for Dramatically Enhanced Photocatalytic Uranium Extraction

Optimizing the electronic structure of covalent organic framework (COF) photocatalysts is essential for maximizing photocatalytic activity. Herein, we report an isoreticular family of multivariate COFs containing chromenoquinoline rings in the COF structure and electron‐donating or withdrawing groups in the pores. Intramolecular donor‐acceptor (D‐A) interactions in the COFs allowed tuning of local charge distributions and charge carrier separation under visible light irradiation, resulting in enhanced photocatalytic performance. By optimizing the optoelectronic properties of the COFs, a photocatalytic uranium extraction efficiency of 8.02 mg/g/day was achieved using a nitro‐functionalized multicomponent COF in natural seawater, exceeding the performance of all COFs reported to date. Results demonstrate an effective design strategy towards high‐activity COF photocatalysts with intramolecular D‐A structures not easily accessible using traditional synthetic approaches.

Recent Advances in the Use of Covalent Organic Frameworks as Heterogenous Photocatalysts in Organic Synthesis

Organic photochemistry is intensely developed in the 1980s, in which the nature of excited electronic states and the energy and electron transfer processes are thoroughly studied and finally well-understood. This knowledge from molecular organic photochemistry can be transferred to the design of covalent organic frameworks (COFs) as active visible-light photocatalysts. COFs constitute a new class of crystalline porous materials with substantial application potentials. Featured with outstanding structural tunability, large porosity, high surface area, excellent stability, and unique photoelectronic properties, COFs are studied as potential candidates in various research areas (e.g., photocatalysis). This review aims to provide the state-of-the-art insights into the design of COF photocatalysts (pristine, functionalized, and hybrid COFs) for organic transformations. The catalytic reaction mechanism of COF-based photocatalysts and the influence of dimensionality and crystallinity on heterogenous photocatalysis performance are also discussed, followed by perspectives and prospects on the main challenges and opportunities in future research of COFs and COF-based photocatalysts.

Composite materials based on covalent organic frameworks for multiple advanced applications

Covalent organic frameworks (COFs) stand for a class of emerging crystalline porous organic materials, which are ingeniously constructed with organic units through strong covalent bonds. Their excellent design capabilities, and uniform and tunable pore structure make them potential materials for various applications. With the continuous development of synthesis technique and nanoscience, COFs have been successfully combined with a variety of functional materials to form COFs-based composites with superior performance than individual components. This paper offers an overview of the development of different types of COFs-based composites reported so far, with particular focus on the applications of COFs-based composites. Moreover, the challenges and future development prospects of COFs-based composites are presented. We anticipate that the review will provide some inspiration for the further development of COFs-based composites.

General Strategy for Incorporation of Functional Group Handles into Covalent Organic Frameworks via the Ugi Reaction

The library of imine-linked covalent organic frameworks (COFs) has grown significantly over the last two decades, featuring a variety of morphologies, pore sizes, and applications. An array of synthetic methods has been developed to expand the scope of the COF functionalities; however, most of these methods were designed to introduce functional scaffolds tailored to a specific application. Having a general approach to diversify COFs via late-stage incorporation of functional group handles would greatly facilitate the transformation of these materials into platforms for a variety of useful applications. Herein, we report a general strategy to introduce functional group handles in COFs via the Ugi multicomponent reaction. To demonstrate the versatility of this approach, we have synthesized two COFs with hexagonal and kagome morphologies. We then introduced azide, alkyne, and vinyl functional groups, which could be readily utilized for a variety of post-synthetic modifications. This facile approach enables the functionalization of any COFs containing imine linkages.

Aromatic Amine-Functionalized Covalent Organic Frameworks (COFs) for CO2/N2 Separation

CO₂ is a prominent example for an exhaust gas, and it is known for its high impact on global warming. Therefore, carbon capture from CO₂ emissions of industrial processes is increasingly important to halt and prevent the disruptive consequences of global warming. Covalent organic frameworks (COFs) as porous nanomaterials have been shown to selectively adsorb CO₂ in high quantities and with high CO₂/N₂ selectivity. Interactions with amines are recognized to selectively adsorb CO₂ and help capture it from exhaust emissions. Herein, a novel COF (Me₃TFB-(NH₂)₂BD), which was not accessible via a direct condensation reaction, was synthetized by dynamic linker exchange starting with Me₃TFB-BD. Despite the linker exchange, the porosity of the COF was largely maintained, resulting in a high BET surface area of 1624 ± 89 m²/g. The CO₂ and N₂ adsorption isotherms at 273 and 295 K were studied to determine the performance in carbon capture at flue gas conditions. Me₃TFB-(NH₂)₂BD adsorbs 1.12 ± 0.26 and 0.72 ± 0.07 mmol/g of CO₂ at 1 bar and 273 and 295 K, respectively. The COF shows a high CO₂/N₂ IAST selectivity under flue gas conditions (273 K:83 ± 11, 295 K: 47 ± 11). The interaction of the aromatic amine groups with CO₂ is based on physisorption, which is expected to make the regeneration of the material energy efficient.

Reticular Synthesis of One-Dimensional Covalent Organic Frameworks with 4-c sql Topology for Enhanced Fluorescence Emission

Covalent organic frameworks (COFs) have been extensively investigated due to their unique structure, porosity, and functionality. However, at the topological level, COFs remain as two-dimensional (2D) or three-dimensional (3D) structures, while COFs with one-dimensional (1D) topology have not been systematically explored. In this work, we proposed a synthetic strategy for the construction of 1D-COFs based on non-linear edges and suitable high-symmetry vertices. Compared with their 2D-COFs counterparts, the 1D-COFs with AIEgens located at the vertex of the frame exhibited enhanced fluorescence. The density functional theory (DFT) calculations revealed that the dimensional-induced rotation restriction (DIRR) effect could spontaneously introduce additional non-covalent interactions between the strip frames, which could substantially diminish non-radiative transitions. This work also provides protocols for the design of 1D-COFs and a guidance scheme for the synthesis of emitting COFs.

Distributed Li-Ion Flux Enabled by Sulfonated Covalent Organic Frameworks for High-Performance Lithium Metal Anodes

Metallic Li is considered the most promising anode material for high-energy-density batteries owing to its high theoretical capacity and low electrochemical potential. However, inhomogeneous lithium deposition and uncontrollable growth of lithium dendrites result in low lithium utilization, rapid capacity fading, and poor cycling performance. Herein, two sulfonated covalent organic frameworks (COFs) with different sulfonated group contents are synthesized as the multifunctional interlayers in lithium metal batteries. The sulfonic acid groups in the pore channels can serve as Li-anchoring sites that effectively coordinate Li ions. These periodically arranged subunits significantly guide uniform Li-ion flux distribution, guarantee smooth Li deposition, and reduce lithium dendrite formation. Consequently, these characteristics afford an excellent quasi-solid-state electrolyte with a high ionic conductivity of 1.9 × 10^-3  S  cm^-1 at room temperature and a superior Li⁺⁺ transference number of 0.91. A Li/LiFePO₄ battery with the COF-based electrolyte exhibited dendrite-free Li deposition during the charge process, accompanied by no capacity decay after 100 cycles at 0.1 C.

Imine and imine-derived linkages in two-dimensional covalent organic frameworks

Covalent organic frameworks (COFs) are porous crystalline polymers that result from the formation of covalent bonds between precisely assembled organic units. Linkage chemistry is a crucial factor in the controllable synthesis and resulting physicochemical properties of COFs. Imine linkages are popular in the formation of polyfunctional two-dimensional (2D) COFs because they are formed easily with structural and functional diversity. There has been much recent interest in expanding beyond this to COFs with imine-derived linkages. This review highlights the development of chemistry to modify and prepare derivatives of imines within 2D COFs. We discuss the derivation of imine bonds via covalent and noncovalent bonding and the properties and potential applications of the resulting materials in order to provide a better understanding of the relationship between covalent linkages and overall performance for 2D COF materials.

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.

Covalent organic frameworks and multicomponent reactions: an endearing give-and-take relationship

Covalent organic frameworks (COFs) are porous and crystalline materials which are assembled by dynamic covalent bonds with two- or three-dimensional (2D or 3D) features. Unlike other polymers, COFs have significant...