Synthesis Of Covalent Organic Frameworks Research Articles

Synthesis and Biomedical Applications of Covalent Organic Frameworks for Disease Diagnosis and Therapy.

Covalent organic frameworks (COFs) have emerged as a distinguished class of porous materials. Owing to their ability to be constructed through covalent bonds involving light elements, such as hydrogen, boron, carbon, nitrogen, and oxygen, COFs offer greater stability and lower cytotoxicity than metal organic frameworks do, addressing critical limitations in in vivo applications. Their unique attributes, such as high surface area, customizable pore sizes, and versatile surface functionalities, make them ideal for various biomedical applications. This review aims to provide an overview of the recent advancements in modern COFs for biomedical uses. First, a variety of methods for the synthesis of COFs are outlined, which ensures their suitability for medical use. Next, we delve into innovative biomedical applications, emphasizing their role in disease diagnostics and therapies. Finally, challenges, such as clinical translation, biocompatibility, and controlled drug release, are critically discussed, providing comprehensive insight into the potential of COFs in revolutionizing biomedical technologies. Overall, this review offers a comprehensive overview of COFs' capabilities and future prospects in enhancing biomedical technologies.

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.

Convenient Aqueous‐Phase Synthesis of Covalent Organic Frameworks for Anhydrous Proton Conduction

Covalent organic frameworks (COFs) as a kind of emerging materials has been widely concerned. Developing and recovering new COF materials is of great importance for anhydrous proton conduction, most of the synthesis methods are not organic solvent phase or convenient enough, which would inevitably lead to solvent waste and increase environmental burden. Herein, a method of COF synthesis in aqueous phase is developed, which is used for nonpolluting and sustainable fabrication. Benefits from the suitable channels and stability of HCOF‐1, the composite material H3PO4@HCOF‐1 exhibit high proton conductivity (1.29 × 10−4 S cm−1) at 413 K and the crystallinity of HCOF‐1 is maintained well after measurements. This work broadens the scope of COF synthesis and promotes the development of anhydrous proton conduction.

Mild and Subtle Synthesis of β-Ketoenamine COFs with High Crystallinity and Controllable Solubility Guided by a Monomer Preassembly Strategy.

The stability of covalent organic frameworks (COFs) is crucial for their applications in demanding environments. However, increasing the stability of COFs often comes with challenges such as higher synthesis difficulty, lower crystal quality, and reduced controllability during synthesis, making it difficult to regulate dimensions and morphology, thereby impacting the processing and shaping of stable COFs. Herein, the study presents a novel confined polymerization approach guided by hydrogen bonding preassembly to synthesize a soluble and stable COF featuring β-ketoenamine linkage. The presence of relatively weaker hydrogen bonds accelerates the orderly arrangement of monomers, ensuring appropriate spacing, and orientations among functional groups. This facilitates efficient covalent polymerization, leading to the creation of the framework while minimizing the "self-correction" mechanism during crystal growth, thereby enhancing the efficiency of COF synthesis. Furthermore, this method offers precise control over the size of the synthesized COF. The resulting crystalline COF can be toggled between dissolution and precipitation states, facilitating the fabrication of mixed matrix membranes (MMMs) through leveraging the solubility properties of COF. Overall, this pioneering strategy yields valuable insights for advancing weak bond assembly-mediated confined polymerization approaches, the controlled synthesis of stable COFs, and the preparation and processing of soluble COFs in diverse applications.

Fully sp2 Carbon-Conjugated Covalent Organic Frameworks with Multiple Active Sites for Advanced Lithium-Ion Battery Cathodes.

Covalent organic frameworks (COFs) hold great promise for rechargeable batteries. However, the synthesis of COFs with abundant active sites, excellent stability, and increased conductivity remains a challenge. Here, chemically stable fully sp2 carbon-conjugated COFs (sp2c-COFs) with multiple active sites are designed by the polymerization of benzo[1,2-b:3,4-b':5,6-b'']trithiophene-2,5,8-tricarbaldehyde) (BTT) and s-indacene-1,3,5,7(2H,6H)-tetrone (ICTO) (denoted as BTT-ICTO). The morphology and structure of the COF are precisely regulated from "butterfly-shaped" to "cable-like" through an in situ controllable growth strategy, significantly promoting the exposure and utilization of active sites. When the unique "cable-like" BTT-ICTO@CNT is employed as lithium-ion batteries (LIBs) cathode, it exhibits exceptional capacity (396 mAh g-1 at 0.1 A g-1 with 97.9 % active sites utilization rate), superb rate capacity (227 mAh g-1 at 5.0 A g-1), and excellent cycling performance (184 mAh g-1 over 8000 cycles at 2.0 A g-1 with 0.00365 % decay rate per cycle). The lithium storage mechanism of BTT-ICTO is exhaustively revealed by in situ Fourier transform infrared, in situ Raman, and density functional theory calculations. This work provides in-depth insights into fully sp2c-COFs with multiple active sites for high-performance LIBs.

Covalent Organic Framework-Involved Sensors for Efficient Enrichment and Monitoring of Food Hazards: A Systematic Review.

The food safety issues caused by environmental pollution have posed great risks to human health that cannot be ignored. Hence, the precise monitoring of hazard factors in food has emerged as a critical concern for the food safety sector. As a novel porous material, covalent organic frameworks (COFs) have garnered significant attention due to their large specific surface area, excellent thermal and chemical stability, modifiability, and abundant recognition sites. This makes it a potential solution for food safety issues. In this research, the synthesis and regulation strategies of COFs were reviewed. The roles of COFs in enriching and detecting food hazards were discussed comprehensively and extensively. Taking representative hazard factors in food as the research object, the expression forms and participation approaches of COFs were explored, along with the effectiveness of corresponding detection methods. Finally, the development directions of COFs in the future as well as the problems existing in practical applications were discussed, which was beneficial to promote the application of COFs in food safety and beyond.

Hydrogen‐bond‐regulated mechanochemical synthesis of covalent organic frameworks: cocrystal precursor strategy for confined assembly

Two‐dimensional imine covalent organic frameworks (2D imine‐COFs) are crystalline porous materials with broad application prospects. Despite the efforts into their design and synthesis, the mechanisms of their formation are still not fully understood. Herein, a one‐pot two‐step mechanochemical cocrystal precursor synthetic strategy is developed for efficient construction of 2D imine‐COFs. The mechanistic investigation demonstrated that the cocrystal precursors of 4,4',4''‐(1,3,5‐triazine‐2,4,6‐triyl)trianiline (TAPT) and p‐toluenesulphonic acid (PTSA) sufficiently regulate the crystalline structure of COF. Evidenced by characterizations and theoretical studies, a helical hydrogen‐bond network was constructed by the N‐H···O supramolecular synthons between amine and sulfate in TAPT‐PTSA, demonstrating the role of cocrystals in promoting the organized stacking of interlayer π‐π interactions, layer arrangement, and interlayer spacing, thus facilitating the orderly assembly of COFs. Moreover, the protonation degree of TAPT amines, which tuned nucleophilic directionality, enabled the sequential progression of intra‐ and interlayer imine condensation reactions, inhibiting the formation of amorphous polymers. The transformation from cocrystal precursors to COFs was achieved through comprehensive control of hydrogen bond and covalent bond sites. This work significantly advances the concept of hydrogen‐bond‐regulated COF assembly and its mechanochemical method in the design and synthesis of 2D imine‐COFs, further elucidating the mechanistic aspects of their mechanochemical synthesis.

Hydrogen-Bond-Regulated Mechanochemical Synthesis of Covalent Organic Frameworks: Cocrystal Precursor Strategy for Confined Assembly.

Two-dimensional imine covalent organic frameworks (2D imine-COFs) are crystalline porous materials with broad application prospects. Despite the efforts into their design and synthesis, the mechanisms of their formation are still not fully understood. Herein, a one-pot two-step mechanochemical cocrystal precursor synthetic strategy is developed for efficient construction of 2D imine-COFs. The mechanistic investigation demonstrated that the cocrystal precursors of 4,4',4''-(1,3,5-triazine-2,4,6-triyl)trianiline (TAPT) and p-toluenesulphonic acid (PTSA) sufficiently regulate the crystalline structure of COF. Evidenced by characterizations and theoretical studies, a helical hydrogen-bond network was constructed by the N-H⋅⋅⋅O supramolecular synthons between amino and sulfonic groups in TAPT-PTSA, demonstrating the role of cocrystals in promoting the organized stacking of interlayer π-π interactions, layer arrangement, and interlayer spacing, thus facilitating the orderly assembly of COFs. Moreover, the protonation degree of TAPT amines, which tuned nucleophilic directionality, enabled the sequential progression of intra- and interlayer imine condensation reactions, inhibiting the formation of amorphous polymers. The transformation from cocrystal precursors to COFs was achieved through comprehensive control of hydrogen bond and covalent bond sites. This work significantly advances the concept of hydrogen-bond-regulated COF assembly and its mechanochemical method in the design and synthesis of 2D imine-COFs, further elucidating the mechanistic aspects of their mechanochemical synthesis.

A Cascade Reaction Triggered by H‐H Steric Hindrance: Dimeric Covalent Organic Frameworks on Au(111) and Dimeric Nanoribbons on Ag(111)

Comprehensive SummaryIn on‐surface synthesis, dimers are typically utilized to explore reaction mechanisms or as intermediates in the formation of final products. However, constructing the innovative nanostructures with dimers as building blocks remains challenging. Here, using non‐planar 2,2′,7,7′‐tetrabromo‐9,9′‐biflurenyliden molecules, we have successfully synthesized dimeric covalent organic frameworks (COFs) on the Au(111) surface through a temperature‐controlled cascade reaction. Notably, the H‐H steric hindrance within precursors caused by double bonds leads to selective stepwise debromination during the thermal annealing, which promotes the dimerization through intermolecular Ullmann coupling and cyclodehydrogenation reaction to form COFs primarily constituted by dimer building blocks. Combining scanning tunneling microscopy/spectroscopy and density functional theory calculations, we have precisely confirmed the structural evolution and reaction mechanism. Furthermore, by introducing Ag adatoms to form C−Ag−C intermediates, we have successfully regulated the reaction path and synthesized one‐dimensional nanoribbons with dimers as building blocks. This work not only validates the strategy of synthesizing dimeric nanostructures on different surfaces through cascade reactions induced by precursor design, but also enriches the research field of surface synthesis of COFs and nanoribbons.

Covalent organic framework-derived C@COF core-shell microspheres for electrocatalytic hydrogen evolution

Although hydrogen represents a vital source of clean energy, the development of cost-effective, durable materials for use as catalysts in the hydrogen evolution reaction (HER) remains a challenge. In this work, covalent organic framework (COF) - derived core-shell microspheres were prepared as novel electrocatalysts. The previously prepared COF microspheres were pyrolyzed into nitrogen-doped porous carbon (N–C) as the core for enhanced electrocatalytic conductivity. With the assistance of a dual-ligand, the COF shell grew on the surface of N–C to form a core-shell microsphere, which was named N–C@COF. The surface morphology and core-shell structure of N–C@COF were characterized by SEM and TEM. FT-IR, TGA, and XPS were utilized to ascertain the synthesis of COF and the composition of the core-shell microspheres. The N–C@COF core-shell microspheres exhibited excellent HER performance in 1 M KOH electrolyte, demonstrating a low overpotential of 104 mV @ 10 mA cm−2, significantly outperforming pure COF and N–C. Furthermore, the N–C@COF demonstrated excellent HER stability over a 24 h period and retained its HER activity following 2000 consecutive cycles of electrolysis. The presence of the N–C core in the N–C@COF enhanced the electrical conductivity of the catalyst, while the COF shell provided abundant active sites for the HER, resulting in a synergistic improvement in the catalytic efficiency. This work presents a straightforward strategy for synthesizing COF-based multicomponent core-shell microspheres, which provides a promising pathway for the development of available and cost-effective electrocatalysts.

Trefoil-like COFs membrane fabricated by interfacial polymerization for dye/salt separation

Covalent organic frameworks (COFs) with β-ketoenamine structure have shown notable effectiveness in water treatment. In general, COFs can be constructed by the interfacial polymerization (IP) between a C2 symmetry monomer with para-position reaction groups and a C3, C4 and C6 symmetry functionalized monomer. The constrained availability of para-position monomers limits the diversity of COFs synthesis options, necessitating exploration into non-para-position alternatives. Herein, 2,4,6-triformylphloroglucinol (Tp) and ortho-position monomer o-phenylenediamine (OPD) are employed to fabricate a composite membrane featuring COFs with a special trefoil-like structure by interfacial polymerization method. The optimal OPD-Tp COFs/Nylon nanofiltration (NF) membrane has 99.8 % rejection for CR and only 3.2 % rejection for NaCl. Meanwhile, the pure water permeation flux is 132 L m−2 h−1 bar−1. This study shows the significant potential for dye/salt separation of the prepared COFs composite NF membrane and broadens the monomer selective range for the COFs membrane fabrication.

Room-temperature synthesis of covalent organic frameworks with dual functional groups for high-performance adsorption of diclofenac sodium from aqueous solution

Diclofenac sodium (DCF) are commonly found in ambient waterways due to its widespread use, and this poses significant risks to both human health and ecosystems. In this study, we have designed and synthesized a series of imine-linked covalent organic frameworks (COFs) with dual functional groups using a triple assembly strategy. Specifically, the platform molecules 2,5-dimethoxyterephthalaldehyde (DMTP), 2,5-dihydroxyterephthalaldehyde (DHTP), and 1,3,5-tris(4-aminophenyl)benzene (TAPB) were employed for the direct fabrication of COFs through Schiff base condensation reactions. As a proof of concept, three room-temperature synthesized COFs (COF TAPB-DMTP-[OH]n, where n represents the molar mass of DHTP, with n values of 0.02, 0.03, and 0.04) were used for DCF adsorption from water. The adsorption isotherms, kinetics, pH effects, ionic strength, co-existing ions, and COF regeneration for DCF were thoroughly investigated. The incorporation of methoxy (-OMe) and hydroxyl (-OH) groups serves as a functional strategy to facilitate hydrogen bonding interactions with DCF molecules. Consequently, the developed COFs, featuring an ultrahigh surface area and mesoporous structure, demonstrated a maximum adsorption capacity of 413.2 mg/g and a rate constant k2 of 0.0435 g/mg/min, which exceeds the performance of the majority of previously described adsorbents. The adsorption capacity toward DCF remained nearly unchanged after five regeneration cycles of the COF. Furthermore, the self-prepared adsorbent demonstrated efficient adsorption of DCF from real environmental water, indicating its potential for practical DCF removal applications.

Ketazine-Linked Crystalline Porous Covalent Organic Frameworks.

Covalent organic frameworks (COFs) are a class of crystalline porous materials with well-defined π arrays and ordered channels, which can be predesigned with a topology diagram and prepared via a polycondensation reaction. Over the past decade, various types of π building units with different functional groups have been developed. Among them, aldehyde is one of the most widely used monomers that form COFs with azine, hydrazine, imine, squaranine, and C═C linkages. In contrast, its closest analogue, i.e., ketone, despite its broad structural diversity, has not yet been investigated for the design and synthesis of COFs. Herein we report the first examples of ketazine-linked COFs by developing ketones as monomers to enable polycondensation with hydrazine under solvothermal conditions. We observed that a careful screening of reaction conditions including solvent, catalyst, concentration, reaction temperature, and reaction time leads to the finding of optimal polymerization systems to produce highly crystalline and porous ketazine-linked COFs. Surprisingly, the ketazine linkage enables π conjugation between knot and linker sites and renders the resultant materials able to emit a strong blue fluorescence, highlighting the π electronic features of this new family of COFs. Our findings of ketones as monomers and ketazine as linkage bring unprecedented structures, functions, and applications to the field of COFs.

In-situ confined preparation of COF@GO nanofiltration membranes for high-efficiency dye removal

Graphene oxide membrane (GOM) possesses ultrafast and tunable two-dimensional water transport pathways, making it an ideal candidate for nanofiltration separation of dye wastewater. However, GOM also faces great challenges such as poor aqueous stability, collapse of non-oxygenated regions under high pressure, and limitations imposed by trade-off effects. In this study, covalent organic frameworks (COFs) were in-situ formed within GOM interlayers and on membrane surface to enlarge interlayer spacings and enhance hydrophilicity of GOM simultaneously via a two-step method including monomer retention in GOM and in-situ confined interfacial polymerization of COFs. Fourier transformation infrared spectroscopy (FT-IR) and X-ray photoelectron spectrometry (XPS) results confirmed the successful synthesis of COF on GOM surface, which enhanced membrane surface hydrophilicity and bridged the split defects of GOM. According to X-ray diffraction measurements (XRD) and scanning electron microscope (SEM) results, COF@GO membranes possessed enlarged interlayer spacings and increased thickness of selective layer, which indicated the successful intercalation of COF into GO interlayer. COF@GO-50 membrane exhibited an ultrahigh pure water permeability (PWP) of 148.1 L/(m2·h·bar), with 99.0 % rejection for direct red 80 (DR80) aqueous solution and 9.6 % NaCl rejection. The ultrahigh water permeance might be ascribed to the enlarged interlayer pathways, the shortened vertical pathways provided by in-plane pores of COF nanosheets, and enhanced water adsorption capacity due to enhanced membrane hydrophilicity. Moreover, COF@GO nanofiltration membrane demonstrated excellent anti-fouling properties and long-term stability in 30h continuous operation, maintaining considerable separation efficiency under harsh conditions. This work provides a straightforward and applicable strategy to simultaneously enlarge interlayer spacings and enhance surface hydrophilicity of GOM via in-situ confined preparation of COFs, achieving ultrahigh water permeance and overcoming the trade-off effect between permeability and selectivity of GOM.

Polyphosphonate covalent organic frameworks

Herein, we report polyphosphonate covalent organic frameworks (COFs) constructed via P-O-P linkages. The materials are synthesized via a single-step condensation reaction of the charge-assisted hydrogen-bonded organic framework, which is constructed from phenylphosphonic acid and 5,10,15,20‐tetrakis[p‐phenylphosphonic acid]porphyrin and is formed by simply heating its hydrogen-bonded precursor without using chemical reagents. Above 210 °C, it becomes an amorphous microporous polymeric structure due to the oligomerization of P-O-P bonds, which could be shown by constant-time solid-state double-quantum 31P nuclear magnetic resonance experiments. The polyphosphonate COF exhibits good water and water vapor stability during the gas sorption measurements, and electrochemical stability in 0.5 M Na2SO4 electrolyte in water. The reported family of COFs fills a significant gap in the literature by providing stable microporous COFs suitable for use in water and electrolytes. Additionally, we provide a sustainable synthesis route for the COF synthesis. The narrow pores of the COF effectively capture CO2.

Covalent-Organic Frameworks for Selective and Sensitive Detection of Antibiotics from Water.

As the consumption of antibiotics rises, they have generated some negative impacts on organisms and the environment because they are often unable to be effectively degraded, and seeking effective detection methods is currently a challenge. Covalent-organic frameworks (COFs) are new types of crystalline porous crystals created based on the strong covalent interactions between blocked monomers, and COFs demonstrate great potential in the detection of antibiotics from aqueous solutions because of their large surface area, adjustable porosity, recyclability, and predictable structure. This review aims to present state-of-the-art insights into COFs (properties, classification, synthesis methods, and functionalization). The key mechanisms for the detection of antibiotics and the application performance of COFs in the detection of antibiotics from water are also discussed, followed by the challenges and opportunities for COFs in future research.

Unraveling the Mechanism of Covalent Organic Frameworks-Based Functional Separators in High-Energy Batteries.

Covalent organic frameworks (COFs) are promising porous materials due to their high specific surface area, adjustable structure, highly ordered nanochannels, and abundant functional groups, which brings about wide applications in the field of gas adsorption, hydrogen storage, optics, and so forth. In recent years, COFs have attracted considerable attention in electrochemical energy storage and conversion. Specifically, COF-based functional separators are ideal candidates for addressing the ionic transport-related issues in high-energy batteries, such as dendritic formation and shuttle effect. Therefore, it is necessary to make a comprehensive understanding of the mechanism of COFs in functional separators. In this review, the advantages, applications as well as synthesis of COFs are firstly presented. Then, the mechanism of COFs in functional separators for high-energy batteries is summarized in detail, including pore channels regulating ionic transport, functional groups regulating ionic transport, adsorption effect, and catalytic effect. Finally, the application prospect of COFs-based separators in high-energy batteries is proposed. This review may provide new insights into the design of functional separators for advanced electrochemical energy storage and conversion systems.

Ultrafine Spatial Modulation of Diazapyrene-Based Two-Dimensional Conjugated Covalent Organic Frameworks.

Tailormade bottom-up synthesis of covalent organic frameworks (COFs) from various functional building blocks offer not only tunable topology and pore size but also multidimensional properties. High crystallinity is one of the prerequisites for their structures and associated physicochemical properties. Among different π-conjugated motifs for constructing COFs, pyrene-based tetragonal structures are effective in achieving highly ordered and crystalline states. In the present research, we demonstrated that the substitution of pyrene with 2,7-diazapyrene produces nearly "flat" structures of two-dimensional (2D) COF layers by controlling the torsional angle of linker molecules. Featuring finite pore diameters and excellent thermodynamic stability of ∼500 °C, ordered face-to-face (slipped AA) stacking arrangements were produced. Extended electrical conjugation spanning 2D frames with modest optical bandgaps (Eg) of ∼2.1 eV shows the planar character of diazapyrene-based COFs. The stacking of the conjugated 2D frames with small Eg values is also beneficial for the formation of highly stable conducting pathways in the crystalline state, which was confirmed by the results of the microwave conductivity measurements. Nitrogen centers in diazapyrene units also play a key role as the active sites for proton transfer, and the maximum proton conductivity of σ = 10-2 S cm-1 was achieved along the cocontinuous nanopore structures surrounded by the active sites. Results show that tetragonal COFs based on diazapyrene can be used as a highly crystalline two-dimensional material with special electrical and proton-conducting capabilities.

Covalent Organic Frameworks for Photocatalytic Reduction of Carbon Dioxide: A Review.

Covalent organic frameworks (COFs) are crystalline networks with extended backbones cross-linked by covalent bonds. Due to the semiconductive properties and variable metal coordinating sites, along with the rapid development in linkage chemistry, the utilization of COFs in photocatalytic CO2RR has attracted many scientists' interests. In this Review, we summarize the latest research progress on variable COFs for photocatalytic CO2 reduction. In the first part, we present the development of COF linkages that have been used in CO2RR, and we discuss four mechanisms including COFs as intrinsic photocatalysts, COFs with photosensitive motifs as photocatalysts, metalated COF photocatalysts, and COFs with semiconductors as heterojunction photocatalysts. Then, we summarize the principles of structural designs including functional building units and stacking mode exchange. Finally, the outlook and challenges have been provided. This Review is intended to give some guidance on the design and synthesis of diverse COFs with different linkages, various structures, and divergent stacking modes for the efficient photoreduction of CO2.

Synthesis of Three‐Dimensional Chiral Covalent Organic Frameworks Using Surfactant as Inducer in Water

Comprehensive SummaryChiral covalent organic frameworks (COFs) have shown promising applications in asymmetric catalysis, enantiomer separation and chiral recognition due to their tunable structures and permanent porosity. Currently, synthesis of chiral COFs mainly relies on direct‐ synthesis method which requires asymmetric monomers to polymerize and crystallize with symmetric monomers and post‐synthesis method which has been greatly limited to having complete reactions with the micro‐/meso‐sized pores of COFs. Recently, the synthesis of two‐dimensional COFs by covalent replacement of chiral competitor has been reported. Herein, we present the synthesis of three types of 3D COFs with tunable chirality using chiral amino acid derivative surfactant as inducer in water under ambient conditions. The hydrogen bonds and electrostatic interactions between amino acid derivative surfactant and monomers as well as their precursors facilitate the transfer of the chirality.