Macro/Microporous Covalent Organic Frameworks for Efficient Electrocatalysis

Hierarchically Ordered Macro-Microporous Polyoxometalate-Based Metal-Organic Framework Single Crystals

Facile construction of ordered macroporous polyoxometalate-based metal-organic frameworks (POM@MOFs) to break the intrinsic microporous restriction is significant but remains challenging. On one hand, the POMs introduced improve the structural stability and modify the pores of MOFs, e.g., introducing functional catalytic and adsorptive units. Meanwhile, the acidic POMs severely affect the nucleation and growth of the POM@MOFs, resulting in complicated synthesis and difficult assembly control. Herein, a general approach has been developed to fabricate ordered macroporous POM@MOF single crystals, involving close-packed polystyrene (PS) nanosphere templates. The artificially selected polar solvents exerting strong solvent effect with POMs weaken the affinity between POMs and metal ions, thereby effectively stabilizing the precursors from assembly before filling into the PS template interstices. The weak alkaline carboxylate used regulates the in situ nucleation and growth of POM@MOFs through deprotonation of the ligands as well as coordinating modulation, affording a series of hierarchically cuboctahedral POM@MOF single crystals with ordered macropores (ca. 180 nm) and intrinsic micropores after template removal. The ordered macroporous structure and thinned microporous skeleton markedly improve mass diffusion properties, while the integral single-crystal lattice retains superior stability.

Cobalt Coordinated Cyano Covalent-Organic Framework for High-Performance Potassium-Organic Batteries

Potassium ion batteries (PIBs) are expected to become the next large-scale energy storage candidates due to its low cost and abundant resources. And the covalent organic framework (COF), with designable periodic organic structure and ability to organize redox active groups predictably, has been considering as the promising organic electrode candidate for PIB. Herein, we report the facile synthesis of the cyano-COF with Co coordination via a facile microwave digestion reaction and its application in the high-energy potassium ion batteries for the first time. The obtained COF-Co material exhibits the enhanced π-π accumulation and abundant defects originated from the Co interaction with the two-dimensional layered sheet structure of COF, which are beneficial for its energy-storage application. Adopted as the inorganic-metal boosted organic electrode for PIBs, the COF-Co with Co coordination can promote the formation of the π-K⁺ interaction, which could lead to the activation of aromatic rings for potassium-ion storage. Besides, the porous two-dimensional layered structure of COF-Co with abundant defects can also promote the shortened diffusion distance of ion/electron with promoted K⁺ insertion/extraction ability. Enhanced cycling stability with large reversible capacity (371 mAh g^-1 after 400 cycles at 100 mA g^-1) and good rate properties (105 mAh g^-1 at 2000 mA g^-1) have been achieved for the COF-Co electrode.

CoS2 Nanoparticles Embedded in Covalent Organic Polymers as Efficient Electrocatalyst for Oxygen Evolution Reaction with Ultralow Overpotential

Cobalt disulfide (CoS₂ ) has been explored as attractive electrocatalyst for oxygen evolution reaction (OER). However, bulk CoS₂ sheets have limited catalytic activity due to low exposure of active sites. Herein, through an in-situ vulcanization approach, CoS₂ nanoparticles are embedded into bipyridine-containing covalent organic polymer (BP-COP). The as-prepared nanocomposite CoS₂ @BP-COP exhibits high catalytic activity toward OER with an ultra-low overpotential of 270 mV (vs. RHE) at a current density of 10 mA cm^-2 , a small Tafel slope of 36 mV dec^-1 , and an excellent durability for 24 h without decay. The surface of CoS₂ is partially converted into CoOOH to form CoS₂ /CoOOH as active sites under OER conditions. CoS₂ @BP-COP displays superior OER catalytic activity to CoS₂ nanosheets and commercially available RuO₂ under the same conditions. The outstanding OER performance activity of CoS₂ @BP-COP could be attributed to the uniform and small particle sizes of CoS₂ /CoOOH distributed in BP-COP.

Synergistic effects of Co/CoO nanoparticles on imine-based covalent organic frameworks for enhanced OER performance

The development of non-noble metal electrocatalysts toward the oxygen evolution reaction (OER) is a key challenge in advancing electrocatalytic water splitting, which is essential for the commercialization of clean and renewable energy. A covalent organic framework (COF) has a precise and controllable structure, high π-π conjugation, large surface area, and porosity and shows great potential as an OER electrocatalyst. However, the relative conductivity and inherent instability greatly limit the further improvement of its performance. Herein, imine-based COF-supported Co/CoO nanoparticles (Co/CoO@COF) were developed for the high-performance electrocatalytic OER. For the Co/CoO@COF catalyst, Co/CoO could form a conjugation effect with the COF, which can increase the electron cloud density of the delocalized large π bond, then improve the conductivity. The combination of Co/CoO and COF effectively enhances the structural stability of the catalyst and enriches the catalytic active sites. Under alkaline conditions, the Co/CoO@COF shows a very low overpotential of 278 mV at a current density of 10 mA cm^-2, and a Tafel slope of 80.11 mV dec^-1 which is better than that of commercial RuO₂.

Light-Activated Biodegradable Covalent Organic Framework-Integrated Heterojunction for Photodynamic, Photothermal, and Gaseous Therapy of Chronic Wound Infection

Chronic wound healing, impeded by bacterial infections and drug resistance, poses a threat to global human health. Antibacterial phototherapy is an effective way to fight microbial infection without causing drug resistance. Covalent organic frameworks (COFs) are a class of highly crystalline functional porous carbon-based materials composed of light atoms (e.g., carbon, nitrogen, oxygen, and borane), showing potential applications in the biomedical field. Herein, we constructed porphyrin-based COF nanosheets (TP-Por CON) for synergizing photodynamic and photothermal therapy under red light irradiation (e.g., 635 nm). Moreover, a nitric oxide (NO) donor molecule, BNN6, was encapsulated into the pore volume of the crystalline porous framework structure to moderately release NO triggered by red light irradiation for realizing gaseous therapy. Therefore, we successfully synthesized a novel TP-Por CON@BNN6-integrated heterojunction for thoroughly killing Gram-negative bacteria Escherichia coli and Gram-positive bacteria Staphylococcus aureus in vitro. Our research identified that TP-Por CON@BNN6 has favorable biocompatibility and biodegradability, low phototoxicity, anti-inflammatory properties, and excellent mice wound healing ability in vivo. This study indicates that the TP-Por CON@BNN6-integrated heterojunction with multifunctional properties provides a potential strategy for COF-based gaseous therapy and microorganism-infected chronic wound healing.

Hotpots and trends of covalent organic frameworks (COFs) in the environmental and energy field: Bibliometric analysis

Covalent organic frameworks (COFs) have attracted extensive attention due to their low density, adjustable structure, functionalization, and good stability. This paper systematically and comprehensively describes to qualitatively and quantitatively the progress, trends, and hotspots of COFs in the environmental and energy fields from the perspective of bibliometrics. Herein, based on the Web of Science database, a total of 2589 articles from 2005 to October 6, 2020, were collected. Thereafter, co-occurrence, co-citation analysis, and cluster analysis were conducted using CiteSpace and VOSviewer software. The results indicated that COFs research shows the characteristics of rapid growth. The active countries were mainly USA, Germany, Japan, China, and India. More than half of the top 20 active institutions were from China. The research hotspots in this field were systematically elaborated, including synthesis, adsorption, catalysis, membrane, sensor, and energy storage. Research has shown that various COFs are reasonably designed, synthesized, and used in different applications. For example, when COFs are used for photocatalysis, groups containing photocatalytic active sites are integrated into COFs to improve photocatalytic activity. Finally, some challenges were proposed, that are beneficial to the rapid and balanced development of the COFs field. For instance, the preparation methods still need to be further improved for mass production and there is an imbalance in environmental applications such as fewer sensor and membrane applications. We believe that this study provides a comprehensive and systematic overview of the environmental and energy applications of COFs for future investigations.

Presenting porous-organic-polymers as next-generation invigorating materials for nanoreactors

Porous organic polymers (POPs) represent an emerging class of porous organic materials which mainly comprise organic building blocks that are interconnected via strong covalent bonds, thereby offering highly cross-linked frameworks with rigid structures and specific void spaces for accommodating guest molecules. In the past few years, POPs have garnered colossal research interest as nanoreactors for heterogeneous catalysis (thermal, photochemical, electrochemical, etc.) because of their intriguing characteristic features, such as high thermal and chemical stabilities, adjustable chemical functionalities, large surface areas, and tunable pore size distributions. This feature article provides an overview of existing research relating to diverse POP synthetic approaches (COFs, CTFs, and some amorphous POPs), the possible modification of the functionality of POPs, and their exciting application as next-generation nanoreactors. These POPs are extremely interesting, as they offer the potential for either metal-free or metalated polymer catalysts allowing photocatalytic CO₂ reduction to solar-fuel, biofuel upgrades, the conversion of waste cooking oil to bio-oil, and clean H₂ production from water, addressing many scientific and technological challenges and providing new opportunities for various specific topics in catalysis. Finally, we emphasize that the integration of various synthetic approaches and the application of POPs as nanoreactors will provide opportunities in the near future for the precision synthesis of functional materials with significant impact in both basic and applied research areas.

Metalloporphyrin and Ionic Liquid-Functionalized Covalent Organic Frameworks for Catalytic CO2 Cycloaddition via Visible-Light-Induced Photothermal Conversion

We report the construction of a porphyrin and imidazolium-ionic liquid (IL)-decorated and quinoline-linked covalent organic framework (COF, abbreviated as COF-P1-1) via a three-component one-pot Povarov reaction. After post-synthetic metallization of COF-P1-1 with Co(II) ions, the metallized COF-PI-2 is generated. COF-PI-2 is chemically stable and displays highly selective CO₂ adsorption and good visible-light-induced photothermal conversion ability (ΔT = 26 °C). Furthermore, the coexistence of Co(II)-porphyrin and imidazolium-IL within COF-PI-2 has guaranteed its highly efficient activity for CO₂ cycloaddition. Of note, the needed thermal energy for the reactions is derived from the photothermal conversion of the Co(II)-porphyrin COF upon visible-light irradiation. More importantly, the CO₂ cycloaddition herein is a "window ledge" reaction, and it can proceed smoothly upon natural sunlight irradiation. In addition, a scaled-up CO₂ cycloaddition can be readily achieved using a COF-PI-2@chitosan aerogel-based fixed-bed model reactor. Our research provides a new avenue for COF-based greenhouse gas disposal in an eco-friendly and energy- and source-saving way.

Atomically Structural Regulations of Carbon-Based Single-Atom Catalysts for Electrochemical CO2 Reduction

The electrochemical carbon dioxide reduction reaction (CO₂ RR) converting CO₂ into value-added chemicals and fuels to realize carbon recycling is a solution to the problem of renewable energy shortage and environmental pollution. Among all the catalysts, the carbon-based single-atom catalysts (SACs) with isolated metal atoms immobilized on conductive carbon substrates have shown significant potential toward CO₂ RR, which intrigues researchers to explore high-performance SACs for fuel and chemical production by CO₂ RR. Especially, regulating the coordination structures of the metal centers and the microenvironments of the substrates in carbon-based SACs has emerged as an effective strategy for the tailoring of their CO₂ RR catalytic performance. In this review, the current in situ/operando study techniques and the fundamental parameters for CO₂ RR performance are first briefly presented. Furthermore, the recent advances in synthetic strategies which regulate the atomic structures of the carbon-based SACs, including heteroatom coordination, coordination numbers, diatomic metal centers, and the microenvironments of substrates are summarized. In particular, the structure-performance relationship of the SACs toward CO₂ RR is highlighted. Finally, the inevitable challenges for SACs are outlined and further research directions toward CO₂ RR are presented from the perspectives.

Strategies for Improving the Catalytic Performance of 2D Covalent Organic Frameworks for Hydrogen Evolution and Oxygen Evolution Reactions

Hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) have been deemed as clean and sustainable strategies to solve the energy crisis and environmental problems. Various catalysts have been developed to promote the process of HER and OER. Among them, two-dimensional covalent organic frameworks (2D COFs) have received great attention due to their diverse and designable structure. In this minireview, we mainly summarize the diverse linkages of 2D COFs and strategies for enhancing the catalytic performance of 2D COFs for HER and OER, such as introducing active building blocks, metal ions and tailored linkages. Furthermore, a brief outlook for the development directions of COFs in the field of HER and OER is provided, expecting to stimulate new opportunities in future research.

Progress and Perspectives on Covalent-organic Frameworks (COFs) and Composites for Various Energy Applications

Covalent-organic frameworks (COFs), being a new member of the crystalline porous materials family, have emerged as important materials for energy storage/conversion/generation devices. They possess high surface areas, ordered micro/mesopores, designable structures and an ability to precisely control electro-active groups in their pores, which broaden their application window. Thanks to their low weight density, long range crystallinity, reticular nature and tunable synthesis approach towards two and three dimensional (2D and 3D) networks, they have been found suitable for a range of challenging electrochemical applications. Our review focuses on the progress made on the design, synthesis and structure of COFs and their composites for various energy applications, such as metal-ion batteries, supercapacitors, water-splitting and solar cells. Additionally, attempts have been made to correlate the structural and mechanistic characteristics of COFs with their applications.

Perspective on intermetallics towards efficient electrocatalytic water-splitting

Intermetallic compounds exhibit attractive electronic, physical, and chemical properties, especially in terms of a high density of active sites and enhanced conductivity, making them an ideal class of materials for electrocatalytic applications. Nevertheless, widespread use of intermetallics for such applications is often limited by the complex energy-intensive processes yielding larger particles with decreased surface areas. In this regard, alternative synthetic strategies are now being explored to realize intermetallics with distinct crystal structures, morphology, and chemical composition to achieve high performance and as robust electrode materials. In this perspective, we focus on the recent advances and progress of intermetallics for the reaction of electrochemical water-splitting. We first introduce fundamental principles and the evaluation parameters of water-splitting. Then, we emphasize the various synthetic methodologies adapted for intermetallics and subsequently, discuss their catalytic activities for water-splitting. In particular, importance has been paid to the chemical stability and the structural transformation of the intermetallics as well as their active structure determination under operating water-splitting conditions. Finally, we describe the challenges and future opportunities to develop novel high-performance and stable intermetallic compounds that can hold the key to more green and sustainable economy and rise beyond the horizon of water-splitting application.

Light-Driven Hydrogen Evolution Assisted by Covalent Organic Frameworks

Covalent organic frameworks (COFs) are crystalline porous organic polymers built from covalent organic blocks that can be photochemically active when incorporating organic semiconducting units, such as triazine rings or diacetylene bridges. The bandgap, charge separation capacity, porosity, wettability, and chemical stability of COFs can be tuned by properly choosing their constitutive building blocks, by extension of conjugation, by adjustment of the size and crystallinity of the pores, and by synthetic post-functionalization. This review focuses on the recent uses of COFs as photoactive platforms for the hydrogen evolution reaction (HER), in which usually metal nanoparticles (NPs) or metallic compounds (generally Pt-based) act as co-catalysts. The most promising COF-based photocatalytic HER systems will be discussed, and special emphasis will be placed on rationalizing their structure and light-harvesting properties in relation to their catalytic activity and stability under turnover conditions. Finally, the aspects that need to be improved in the coming years will be discussed, such as the degree of dispersibility in water, the global photocatalytic efficiency, and the robustness and stability of the hybrid systems, putting emphasis on both the COF and the metal co-catalyst.

Sulfonic acid functionalized hierarchical porous covalent organic frameworks as a SALDI-TOF MS matrix for effective extraction and detection of paraquat and diquat

Design and construction of a matrix with specific adsorption on the target compounds can effectively reduce the detection limit of surface-assisted laser desorption/ionization time-of-flight mass spectrometry (SALDI-TOF MS) analysis. Sulfonic acid functionalized hierarchical porous covalent organic frameworks (H-COF-SO₃H) was synthesized by defect-structure and post-modification method, and then used as matrix and adsorbent for the determination of quaternary ammonium herbicides paraquat (PQ) and diquat (DQ). N₂ adsorption-desorption experiments confirmed that H-COF-SO₃H possesses hierarchical porosity with pore widths concentrated at 1.3,1.5, and 2.8 nm. The strong UV absorption at 200-450 nm and good thermal stability made H-COF-SO₃H being a promising matrix without background interference. H-COF-SO₃H can efficiently enrich PQ and DQ via electrostatic attraction, and the key role of -SO₃H group on specific adsorption was confirmed by density functional theory (DFT) calculations. The limits of detection (LODs) for PQ and DQ with H-COF-SO₃H enrichment were 0.5 and 0.1 ng·mL^-1, respectively, which were 20 and 60 times higher than those without H-COF-SO₃H enrichment, respectively. The spiked recoveries of PQ and DQ for the three food samples were 92.0-113.2% and 80.1-102.6% with RSDs of 2.2-9.2% and 2.0-8.7%, respectively. This work provides an analyte-oriented approach for fabricating SALDI-TOF MS matrix.

Hierarchically Porous Organic Cages

Imparting mesopores to organic cages of an intrinsic microporous nature to build up hierarchically porous cage soft materials is a grand challenge and will reshape the property and application scope of traditional organic cage molecules. Herein, we discovered how to engineer mesopores into microporous organic cages via their host-guest interactions with long chain ionic surfactants. Equally important, the ionic head of surfactants equips the supramolecularly assembled porous structures with charge-selective uptake and release function in solution. Interestingly, such hierarchically porous organic cage can serve as a nanoreactor once trapping enzymes within the cavity, which show 5-fold enhanced activity of enzymatic catalysis when compared with the free enzymes.

Construction of Covalent Organic Frameworks with Crown Ether Struts

Crown ethers are a class of macrocyclic molecules with unique flexible structures but they are rarely integrated in covalent organic frameworks (COFs). To date, employing flexible organic units such as crown ethers to construct COFs with high crystallinity and surface area are still a challenge. In this work, two new COFs with different flexible crown ethers as backbone rather than side chains are synthesized and further employed for alkali metal ions separation. Both of COFs possess high surface areas, good crystallinity, and excellent chemical stability. Interestingly, these two new COFs with 18-crown-6 or 24-crown-8 units showed remarkable binding ability of K⁺ or Cs⁺ owing to the size-fit effect. This work demonstrated that the unique structural features of crown ethers will lead to increase interest in fabricating COFs with crown ethers.

Quantifying the Likelihood of Structural Models through a Dynamically Enhanced Powder X-Ray Diffraction Protocol

Structurally characterizing new materials is tremendously challenging, especially when single crystal structures are hardly available which is often the case for covalent organic frameworks. Yet, knowledge of the atomic structure is key to establish structure-function relations and enable functional material design. Herein, a new protocol is proposed to unambiguously predict the structure of poorly crystalline materials through a likelihood ordering based on the X-ray diffraction (XRD) pattern. Key of the procedure is the broad set of structures generated from a limited number of building blocks and topologies, which is submitted to operando structural characterization. The dynamic averaging in the latter accounts for the operando conditions and inherent temporal character of experimental measurements, yielding unparalleled agreement with experimental powder XRD patterns. The proposed concept can hence unquestionably identify the structure of experimentally synthesized materials, a crucial step to design next generation functional materials.

Emergent electrochemical functions and future opportunities of hierarchically constructed metal-organic frameworks and covalent organic frameworks

Designing spatial and architectural features across from the molecular to bulk scale is one of the most important topics in materials science which has received a lot of attention in recent years. Looking back to the past research, findings on the influences of spatial features denoted as porous structures on the applications related to mass transport phenomena have been widely studied in traditional inorganic materials, such as ceramics over the past two decades. However, due to the difficulties in precise control of the porous structures at the molecular level in this class of materials, the mechanistic understanding of the effects of spatial and architectural features across from the molecular level to meso-/macroscopic scale is still lacking, especially in electrochemical reactions. Further understanding of fundamental electrochemical functions in well-defined architectures is indispensable for the further advancement of key next-generation energy devices. Furthermore, creating periodic porosity in reticular structures is starting to be recognized as an emerging approach to control the electronic structure of materials. In this review, we focus on the investigations on preparing well-defined molecular-level crystalline porous materials known as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) into hierarchically constructed architectures from molecular structures lower than the reticular frameworks to meso-/macroscopic scale structures. By connecting well-defined nanosized porous structures in MOFs/COFs and additional length-scale space or shapes, emergent electrochemical functions towards emerging devices, such as beyond Li-ion batteries including all-solid-state rechargeable batteries, are expected to be obtained. By summarizing recent advancements in synthetic strategies of hierarchically constructed MOF/COF based materials and fundamental investigation of their structural effect in a wide spectrum of electrochemical applications, we highlight the importance and future direction of this developing field of hierarchically constructed MOFs/COFs, while emphasizing the required chemical stability of the MOFs/COFs which meet the use in the game-changing electrochemical devices.

Pioneering Iodine-125-Labeled Nanoscale Covalent Organic Frameworks for Brachytherapy

Brachytherapy has been clinically used for the treatment of malignant solid tumors. However, the classic therapeutic radioactive ¹²⁵I seed must be surgically implanted directly into tumors. To avoid the surgery and prevent irrational radioactive distribution, radioiodine-loaded nanomaterials are ever-developing for brachytherapy. Hence, it is still a notable challenge to obtain an advanced material that simultaneously incorporates features of high radiolabeling rate, short labeling time, good radiolabeling stability, and long tumor retention time. Covalent organic frameworks (COFs), which are crystalline polymers with ordered pores, are widely applied in guest delivery of drugs based on their high porosity and modifiable skeleton. Herein, we developed a functionalized nanoscale PEG-COF-Ag material, which could rapidly capture radioiodine reaching a 94% radiolabeling yield in 30 s. In addition, more than 95% ¹²⁵I was maintained after 24 h in PBS (phosphate-buffered saline) as well as in serum and over 90% for nearly 1 week. PEG-COF-Ag-¹²⁵I (¹²⁵I-COF) demonstrated excellent cancer cell killing performance in vitro, and further experiments in vivo revealed a long tumor retention time and effective tumor treatment during the radiotherapy. The results indicate that radioiodine-labeled PEG-COF-Ag could be potentially applied in brachytherapy with a promising therapeutic effect.

Covalent Organic Frameworks: Synthesis, Properties and Applications-An Overview

Covalent Organic Frameworks (COFs) are an exciting new class of microporous polymers with unprecedented properties in organic material chemistry. They are generally built from rigid, geometrically defined organic building blocks resulting in robust, covalently bonded crystalline networks that extend in two or three dimensions. By strategically combining monomers with specific structures and properties, synthesized COF materials can be fine-tuned and controlled at the atomic level, with unparalleled precision on intrapore chemical environment; moreover, the unusually high pore accessibility allows for easy post-synthetic pore wall modification after the COF is synthesized. Overall, COFs combine high, permanent porosity and surface area with high thermal and chemical stability, crystallinity and customizability, making them ideal candidates for a myriad of promising new solutions in a vast number of scientific fields, with widely varying applications such as gas adsorption and storage, pollutant removal, degradation and separation, advanced filtration, heterogeneous catalysis, chemical sensing, biomedical applications, energy storage and production and a vast array of optoelectronic solutions. This review attempts to give a brief insight on COF history, the overall strategies and techniques for rational COF synthesis and post-synthetic functionalization, as well as a glance at the exponentially growing field of COF research, summarizing their main properties and introducing the numerous technological and industrial state of the art applications, with noteworthy examples found in the literature.

Precise Design of Covalent Organic Frameworks for Electrocatalytic Hydrogen Peroxide Production

Electrochemical synthesis of H₂ O₂ with high productivity is a significant challenge in electrocatalysis. Herein, we develop Mg-ion contained covalent organic frameworks (MgP-DHTA-COF), comprising stacked 2D layers, well-defined skeletons, and well-ordered monodispersed active sites, for the electrocatalytic production of H₂ O₂ directly from O₂ and H₂ O. The precise-designed MgP-DHTA-COF achieves H₂ O₂ selectivity up to 96%, high Faradaic efficiency of 91% and reliable stability for H₂ O₂ synthesis in 0.10 mol L^-1 KOH aqueous solution. Both experiments and simulations demonstrate that the pyrrolic-N fixed Mg ions in the knots promote the reactivity of COF and enhance the adsorption ability of OOH*. This work provides a valuable example for the design of an efficient electrocatalyst based on COFs for H₂ O₂ production.

Dual-functional ionic porous organic framework for palladium scavenging and heterogeneous catalysis

Porous organic frameworks (POFs) with predesigned structures and tunable porosities have been widely studied in adsorption and heterogeneous catalysis. Introducing ionic structure into the framework endows POFs with new functionalities that may extend their applications. Here, we report new applications for a guanidinium-based ionic POF (IPOF-Cl) in palladium scavenging and heterogeneous catalysis. Due to the ionic framework and the porous structure, the IPOF-Cl displays fast adsorption kinetics and high adsorption capacities (up to 754 mg g-1) of Na2PdCl4 in aqueous solutions via a chemisorption (ion exchange) process. Significantly, it shows excellent scavenging activity towards trace amount of [PdCl4]2- in aqueous solution. More importantly, the loaded [PdCl4]2- species on the IPOF substrate are further reduced into ultrafine Pd nanoparticles with size of ∼2-5 nm. The obtained IPOF-Pd(0) nanocomposite containing uniformly distributed Pd nanoparticles and hierarchical porous structure demonstrates high activity in catalyzing a range of Suzuki coupling reactions. This study provides new routes for the development of ionic porous organic materials for applications in metal scavenging and catalysis.

One-Dimensional van der Waals Heterostructures as Efficient Metal-Free Oxygen Electrocatalysts

Two-dimensional covalent organic frameworks (2D-COFs) may serve as an emerging family of catalysts with well-defined atomic structures. However, the severe stacking of 2D nanosheets and large intrinsic bandgaps significantly impair their catalytic performance. Here, we report coaxial one-dimensional van der Waals heterostructures (1D vdWHs) comprised of a carbon nanotube (CNT) core and a thickness tunable thienothiophene-pyrene COF shell using a solution-based in situ wrapping method. Density functional theory calculations and operando and ex situ spectroscopic analysis indicate that carbon-sulfur regions in thienothiophene groups in the COF serve as an active catalytic site for oxygen reduction and evolution reactions. The coaxial structure enables n-doping from the CNT core to the COF shell, which is controllable by varying COF shell thickness. The charge transfer from CNTs lowers COF's bandgap and work function, which reduces the charge transfer barrier between the active catalytic sites and adsorbed oxygen intermediates, resulting in dramatically enhanced catalytic activity. The 1D vdWHs were applied as a bifunctional oxygen electrocatalyst in rechargeable zinc-air batteries, delivering a high specific capacity of 696 mAh g_Zn^-1 under a high current density of 40 mA cm^-2 and excellent cycling stability. The 1D vdWHs based on the coaxial structure of 2D COF wrapped around CNT cores may be further used as versatile building units to create multidimensional vdWHs for exploring fundamental physics and chemistry as well as practical applications in electrochemistry, electronics, photonics, and beyond.

Synchronous Construction of Hierarchical Porosity and Thiol Functionalization in COFs for Selective Extraction of Cationic Dyes in Water Samples

Pore size and functionalization are two critical factors for covalent organic frameworks (COFs) as effective adsorbents. However, due to the low crystallinity of COFs, it is a grand challenge to accomplish pore diameter adjustment and functionalization at the same time. In this work, we developed a simple and ingenious strategy, cutting off linkage, to synchronously construct hierarchical porosity and modify thiol groups in COFs under mild conditions. The hybrid COFs containing disulfide bonds were designed and synthesized, and then the disulfide bonds were cleaved by glutathione, resulting in the formation of thiol groups as well as the increase in pore size caused by skeleton defects. The pore diameter of thiol-functionalized hierarchical porous COFs (denoted as HP-TpEDA-SH) was concentrated at 2.6 and 3.5 nm. Thanks to the electrostatic attraction of thiol groups to cationic dyes and the higher number of available adsorption sites, the maximum extraction amounts of methylene blue (MB), malachite green (MG), and crystal violet (CV) by HP-TpEDA-SH were 2.6, 2.1, and 3.3 times those of microporous COFs under optimal extraction conditions, respectively. The proposed analytical method (solid-phase extraction-high-performance liquid chromatography/ultraviolet (SPE-HPLC/UV)) with HP-TpEDA-SH as the adsorbent showed low detection limits of 1.3, 0.13, and 0.12 μg·L^-1 for MB, MG, and CV, respectively. The recoveries of three spiked water samples ranged from 81.5 to 113.8%, with relative standard deviations (RSDs) less than 9.7%. This work not only opened a new avenue for the preparation of functionalized hierarchical porous COFs but also established an effective method for detecting trace cationic dyes in fishery water.

Novel One-Dimensional Covalent Organic Framework as a H+ Fluorescent Sensor in Acidic Aqueous Solution

Covalent organic frameworks (COFs) represent an emerging class of two- or three-dimensional crystalline porous materials with delicate control over topology, composition, and porosity. Here, we develop a new COF made up of 1,3,6,8-tetrakis(p-formylphenyl)pyrene (TFPPy) and 4,4'-diaminobenzophenone (DABP) that exhibits a rare one-dimensional (1D) structure. The resulting frameworks possess good crystallinity, comparatively high Brunauer-Emmett-Teller (BET) surface area (426 m²/g), and good thermal stability (360 °C). Impressively, this 1D COF shows strong fluorescence and can be used as an excellent H⁺ sensor in an acidic aqueous solution.

Covalent organic frameworks: an ideal platform for designing ordered materials and advanced applications

Covalent organic frameworks offer a molecular platform for integrating organic units into periodically ordered yet extended 2D and 3D polymers to create topologically well-defined polygonal lattices and built-in discrete micropores and/or mesopores.

Covalent organic frameworks (COFs) for electrochemical applications

Covalent organic frameworks are a class of extended crystalline organic materials that possess unique architectures with high surface areas and tuneable pore sizes. Since the first discovery of the topological frameworks in 2005, COFs have been applied as promising materials in diverse areas such as separation and purification, sensing or catalysis. Considering the need for renewable and clean energy production, many research efforts have recently focused on the application of porous materials for electrochemical energy storage and conversion. In this respect, considerable efforts have been devoted to the design and synthesis of COF-based materials for electrochemical applications, including electrodes and membranes for fuel cells, supercapacitors and batteries. This review article highlights the design principles and strategies for the synthesis of COFs with a special focus on their potential for electrochemical applications. Recently suggested hybrid COF materials or COFs with hierarchical porosity will be discussed, which can alleviate the most challenging drawback of COFs for these applications. Finally, the major challenges and future trends of COF materials in electrochemical applications are outlined.

A cage-based covalent organic framework for drug delivery

A novel cage-based crystalline covalent organic framework, i.e. Cage-COF-TT (TT = triammonia–terephthalaldehyde), was prepared from a prism-like triammonia-containing molecular cage and terephthalaldehyde.

High-Mobility Semiconducting Two-Dimensional Conjugated Covalent Organic Frameworks with p-Type Doping

Two-dimensional conjugated covalent organic frameworks (2D c-COFs) are emerging as a unique class of semiconducting 2D conjugated polymers for (opto)electronics and energy storage. Doping is one of the common, reliable strategies to control the charge carrier transport properties, but the precise mechanism underlying COF doping has remained largely unexplored. Here we demonstrate molecular iodine doping of a metal-phthalocyanine-based pyrazine-linked 2D c-COF. The resultant 2D c-COF ZnPc-pz-I₂ maintains its structural integrity and displays enhanced conductivity by 3 orders of magnitude, which is the result of elevated carrier concentrations. Remarkably, Hall effect measurements reveal enhanced carrier mobility reaching ∼22 cm² V^-1 s^-1 for ZnPc-pz-I₂, which represents a record value for 2D c-COFs in both the direct-current and alternating-current limits. This unique transport phenomenon with largely increased mobility upon doping can be traced to increased scattering time for free charge carriers, indicating that scattering mechanisms limiting the mobility are mitigated by doping. Our work provides a guideline on how to assess doping effects in COFs and highlights the potential of 2D c-COFs to display high conductivities and mobilities toward novel (opto)electronic devices.

Nanocomposite Based on Organic Framework-Loading Transition-Metal Co Ion and Cationic Pillar[6]arene and Its Application for Electrochemical Sensing of l-Ascorbic Acid

In this study, we constructed a highly sensitive and selective electrochemical sensing strategy for l-ascorbic acid (AA) based on a covalent organic framework (COF)-loading non-noble transition metal Co ion and macrocyclic cationic pillar[6]arene (CP6) nanocomposite (CP6-COF-Co). The COF plays a crucial role in anchoring the Co ion according to its crystalline porous and multiple coordination sites and has an outstanding performance for building an electrochemical sensing platform based on a unique two-dimensional structure. Accordingly, the transition-metal Co ion can be successfully anchored on the framework of COF and shows strong catalytic activity for the determination of AA. Moreover, introduction of host-guest recognition based on CP6 and AA can bring new properties for enhancing selectivity, sensitivity, and practical application in real environment. Host-guest interactions between CP6 and AA were evaluated by the ¹H NMR spectrum. When compared with other literatures, our method displayed a lower determination limit and broader linear range. To the best of our knowledge, this is the first study carried out for the non-noble transition-metal Co ion, COF, and pillar[6]arene hybrid material in sensing field, which has a potential value in sensing, catalysis, and preparation of advanced multifunction materials.

Beyond Frameworks: Structuring Reticular Materials across Nano-, Meso-, and Bulk Regimes

Reticular materials are of high interest for diverse applications, ranging from catalysis and separation to gas storage and drug delivery. These open, extended frameworks can be tailored to the intended application through crystal-structure design. Implementing these materials in application settings, however, requires structuring beyond their lattices, to interface the functionality at the molecular level effectively with the macroscopic world. To overcome this barrier, efforts in expressing structural control across molecular, nano-, meso-, and bulk regimes is the essential next step. In this Review, we give an overview of recent advances in using self-assembly as well as externally controlled tools to manufacture reticular materials over all the length scales. We predict that major research advances in deploying these two approaches will facilitate the use of reticular materials in addressing major needs of society.

Electroactive Covalent Organic Frameworks: Design, Synthesis, and Applications

Covalent organic frameworks (COFs) are an emerging class of crystalline porous polymers with tailorable compositions, porosities, functionalities, and intrinsic chemical stability. The incorporation of electroactive moieties in the structure transforms COFs into electroactive materials with great potential for energy-related applications. Herein, the recent advances in the design and use of electroactive COFs as capacitors, batteries, conductors, fuel cells, water-splitting, and electrocatalysis are addressed. Their remarkable performance is discussed and compared with other porous materials; hence, perspectives in the development of electroactive COFs are presented.

Ultralight covalent organic framework/graphene aerogels with hierarchical porosity

The fabrication of macroscopic objects from covalent organic frameworks (COFs) is challenging but of great significance to fully exploit their chemical functionality and porosity. Herein, COF/reduced graphene oxide (rGO) aerogels synthesized by a hydrothermal approach are presented. The COFs grow in situ along the surface of the 2D graphene sheets, which are stacked in a 3D fashion, forming an ultralight aerogel with a hierarchical porous structure after freeze-drying, which can be compressed and expanded several times without breaking. The COF/rGO aerogels show excellent absorption capacity (uptake of >200 g organic solvent/g aerogel), which can be used for removal of various organic liquids from water. Moreover, as active material of supercapacitor devices, the aerogel delivers a high capacitance of 269 F g⁻¹ at 0.5 A g⁻¹ and cycling stability over 5000 cycles.

Macroscopic architectures of covalent organic frameworks (COF) allow to fully exploit their chemical functionality and porosity but achieving three-dimensional hierarchical porous COF architectures remains challenging. Here, the authors present a COF/reduced graphene oxide aerogel which is synthesized by growing COF during a hydrothermal process along the surface of graphene sheets.

Rational Design of FeNi Bimetal Modified Covalent Organic Frameworks for Photoconversion of Anthropogenic CO2 into Widely Tunable Syngas

Direct photoconversion of low-concentration CO₂ into a widely tunable syngas (i.e., CO/H₂ mixture) provides a feasible outlet for the high value-added utilization of anthropogenic CO₂ . However, in the low-concentration CO₂ photoreduction system, it remains a huge challenge to screen appropriate catalysts for efficient CO and H₂ production, respectively, and provide a facile parameter to tune the CO/H₂ ratio in a wide range. Herein, by engineering the metal sites on the covalent organic frameworks matrix, low-concentration CO₂ can be efficiently photoconverted into tunable syngas, whose CO/H₂ ratio (1:19-9:1) is obviously wider than reported systems. Experiments and density functional theory calculations indicate that Fe sites serve as the H₂ evolution sites due to the much stronger binding affinity to H₂ O, while Ni sites act as the CO production sites for the higher affinity to CO₂ . Notably, the widely tunable syngas can also be produced over other Fe/Ni-based bimetal catalysts, regardless of their structures and supporting materials, confirming the significant role of the metal sites in regulating the selectivity of CO₂ photoreduction and providing a modular design strategy for syngas production.

Assembling well-arranged covalent organic frameworks on MOF-derived graphitic carbon for remarkable formaldehyde sensing

Constructing heterostructures with advanced architectures is an effective strategy for enhancing the crystallinity and functional performance of covalent organic frameworks (COFs). Herein, a novel core-shell heterostructure integrating a metal-organic framework (MOF)-derived graphitic carbon core (GC) and a well-arranged COF shell, termed MOF-GC@COF, is reported. ZIF-67 dodecahedra are first chemically etched with a weak organic acid and further converted to MOF-GC via thermal pyrolysis. In the subsequent step, β-ketoenamine-linked COF nanofibers are vertically assembled on the surface of the MOF-GC cores to generate the MOF-GC@COF heterostructure. As a proof-of-concept application, the as-prepared MOF-GC@COF heterostructure is used as an effective quartz crystal microbalance (QCM) sensor for the adsorption of formaldehyde. Benefiting from the synergistic effect of the hybrid composition and the advantages of the core-shell heterostructure, the newly prepared MOF-GC@COF heterostructure exhibits excellent sensing performance toward formaldehyde with rapid adsorption kinetics, high sensitivity, and superior selectivity.

Tuning the electronic energy level of covalent organic frameworks for crafting high-rate Na-ion battery anode

Crystalline Covalent Organic Frameworks (COFs) possess ordered accessible nano-channels. When these channels are decorated with redox-active functional groups, they can serve as the anode in metal ion batteries (LIB and SIB). Though sodium's superior relative abundance makes it a better choice over lithium, the energetically unfavourable intercalation of the larger sodium ion makes it incompatible with the commercial graphite anodes used in Li-ion batteries. Also, their sluggish movement inside the electrodes restricts the fast sodiation of SIB. Creating an electronic driving force at the electrodes via chemical manipulation can be a versatile approach to overcome this issue. Herein, we present anodes for SIB drawn on three isostructural COFs with nearly the same Highest Occupied Molecular Orbitals (HOMO) levels but with varying Lowest Unoccupied Molecular Orbitals (LUMO) energy levels. This variation in the LUMO levels has been deliberately obtained by the inclusion of electron-deficient centers (phenyl vs. tetrazine vs. bispyridine-tetrazine) substituents into the modules that make up the COF. With the reduction in the cell-potential, the electrons accumulate in the anti-bonding LUMO. Now, these electron-dosed LUMO levels become efficient anodes for attracting the otherwise sluggish sodium ions from the electrolyte. Also, the intrinsic porosity of the COF favors the lodging and diffusion of the Na⁺ ions. Cells made with these COFs achieve a high specific capacity (energy density) and rate performance (rapid charging-discharging), something that is not as easy for Na⁺ compared to the much smaller sized Li⁺. The bispyridine-tetrazine COF with the lowest LUMO energy shows a specific capacity of 340 mA h g^-1 at 1 A g^-1 and 128 mA h g^-1 at a high current density of 15 A g^-1. Only a 24% drop appears on increasing the current density from 0.1 to 1 A g^-1, which is the lowest among all the top-performing COF derived Na-ion battery anodes.

Integrating single Ni sites into biomimetic networks of covalent organic frameworks for selective photoreduction of CO2

Selective photoreduction of CO2 into a given product is a great challenge but desirable. Inspired by natural photosynthesis occurring in hierarchical networks over non-precious molecular metal catalysts, we demonstrate an integration of single Ni sites into the hexagonal pores of polyimide covalent organic frameworks (PI-COFs) for selective photoreduction of CO2 to CO. The single Ni sites in the hexagonal pores of the COFs serve as active sites for CO2 activation and conversion, while the PI-COFs not only act as a photosensitizer to generate charge carriers but also exert a promoting effect on the selectivity. The optimized PI-COF with a triazine ring exhibits excellent activity and selectivity. A possible intra- and inter-molecular charge-transfer mechanism was proposed, in which the photogenerated electrons in PI-COFs are efficiently separated from the central ring to the diimide linkage, and then transferred to the single Ni active sites, as evidenced by theoretical calculations.