Construction of a Hierarchical Architecture of Covalent Organic Frameworks via a Postsynthetic Approach

Computational Protocol for the Spectral Assignment of NMR Resonances in Covalent Organic Frameworks

Solid-state nuclear magnetic resonance spectroscopy is routinely used in the field of covalent organic frameworks to elucidate or confirm the structure of the synthesized samples and to understand dynamic phenomena. Typically this involves the interpretation and simulation of the spectra through the assumption of symmetry elements of the building units, hinging on the correct assignment of each line shape. To avoid misinterpretation resulting from library-based assignment without a theoretical basis incorporating the impact of the framework, this work proposes a first-principles computational protocol for the assignment of experimental spectra, which exploits the symmetry of the underlying building blocks for computational feasibility. In this way, this protocol accommodates the validation of previous experimental assignments and can serve to complement new NMR measurements.

Progress and perspectives of sorption-based atmospheric water harvesting for sustainable water generation: Materials, devices, and systems

Establishing alternative methods for freshwater production is imperative to effectively alleviate global water scarcity, particularly in land-locked arid regions. In this context, extracting water from the ubiquitous atmospheric moisture is an ingenious strategy for decentralized freshwater production. Sorption-based atmospheric water harvesting (SAWH) shows strong potential for supplying liquid water in a portable and sustainable way even in desert environments. Herein, the latest progress in SAWH technology in terms of materials, devices, and systems is reviewed. Recent advances in sorbent materials with improved water uptake capacity and accelerated sorption-desorption kinetics, including physical sorbents, polymeric hydrogels, composite sorbents, and ionic solutions, are discussed. The thermal designs of SAWH devices for improving energy utilization efficiency, heat transfer, and mass transport are evaluated, and the development of representative SAWH prototypes is clarified in a chronological order. Thereafter, state-of-the-art operation patterns of SAWH systems, incorporating intermittent, daytime continuous and 24-hour continuous patterns, are examined. Furthermore, current challenges and future research goals of this cutting-edge field are outlined. This review highlights the irreplaceable role of heat and mass transfer enhancement and facile structural improvement for constructing high-yield water harvesters.

Superhydrophilic Triazine-Based Covalent Organic Frameworks via Post-Modification of FeOOH Clusters for Boosted Photocatalytic Performance

The triazine-based covalent organic frameworks (tCOF), an intriguing subtype of COFs, are expected as highly promising photocatalysts for various photocatalytic applications owing to their fully conjugated structures and nitrogen-rich skeletons. However, the inherent hydrophobicity and fast recombination of photoexcited electron-hole pairs are two main factors hindering the application of tCOF in practical photocatalytic reactions. Here, a post-synthetic modification strategy to fabricate superhydrophilic tCOF-based photocatalysts is demonstrated by in situ growing FeOOH clusters on TaTz COF (TaTz-FeOOH) for efficient photocatalytic oxidation of various organic pollutants. The strong polar FeOOH endows TaTz-FeOOH with good hydrophilic properties. The well-defined heterogeneous interface between FeOOH and TaTz allows the photoelectrons generated by TaTz to be consumed by Fe (III) to transform into Fe (II), synergistically promoting the separation of holes and the generation of free radicals. Compared with the unmodified TaTz, the optimized TaTz-FeOOH (1%) shows excellent photocatalytic performance, where the photocatalytic degrade rate (k) of rhodamine B is increased by about 12 times, and the degradation rate is maintained at 99% after 5 cycles, thus achieving efficient removal of quinolone antibiotics from water. This study provides a new avenue for the development of COF-based hydrophilic functional materials for a wide range of practical applications.

COF@COF: Constructing Core-Shell Structured Covalent-Organic Frameworks from Interpenetration Isomers

Core-shell structured covalent-organic frameworks (COFs) have rarely been reported because of the essentially inevitable crystallographic structural distinctions of different COF species. In this contribution, we outline a novel strategy for constructing core-shell structured COFs from interpenetration isomers and take the classic three-dimensional COF-300 as a proof-of-concept. Core-shell particles with 5-fold interpenetrated COF-300 as the core and 7-fold interpenetrated COF-300 as the shell were prepared via a two-step process, using exactly the same monomers but under different reaction conditions within each step. Moreover, the thicknesses of both the core and shell show adjustable characteristics. This approach may promote the future advancement of hierarchical microstructures with predesigned functions in different hierarchies.

Gradient Channel Segmentation in Covalent Organic Framework Membranes with Highly Oriented Nanochannels

Covalent organic frameworks (COFs) offer an exceptional platform for constructing membrane nanochannels with tunable pore sizes and tailored functionalities, making them promising candidates for separation, catalysis, and sensing applications. However, the synthesis of COF membranes with highly oriented nanochannels remains challenging, and there is a lack of systematic studies on the influence of postsynthetic modification reactions on functionality distribution along the nanochannels. Herein, we introduced a "prenucleation and slow growth" approach to synthesize a COF membrane featuring highly oriented mesoporous channels and a high Brunauer-Emmett-Teller surface area of 2230 m2 g-1. Functional moieties were anchored to the pore walls via "click" reactions and coordinated with Cu ions to serve as segmentation functions. This led to a remarkable H2/CO2 separation performance that surpassed the Robeson upper bound. Moreover, we found that the functionalities distributed along the nanochannels could be influenced by functionality flexibility and postsynthetic reaction rate. This strategy paved the way for the accurate design and construction of COF-based artificial solid-state nanochannels with high orientation and precisely controlled channel environments.

Interfacial chemistries in metal-organic framework (MOF)/covalent-organic framework (COF) hybrids

Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) have been attracting tremendous attention in various applications due to their unique structural properties. Recent interest has been focused on their combination as hybrids to enable the engineering of new classes of frameworks with complementary properties. This review gives a comprehensive summary on the interfacial chemistries in MOF/COF hybrids, which play critical roles in their hybridization. The challenges and perspectives in the field of MOF/COF hybrids are also provided to inspire more efforts in diversifying this hybrid family and their cross-disciplinary applications.

Imine-Linked Covalent Organic Frameworks: A Biocompatible and pH-Dependent Carrier for In Vitro Sustained Release of Doxorubicin

Among the novel drug delivery systems (DDSs), covalent organic frameworks (COFs) show promising features in pharmaceutical science. In this paper, an imine-linked COF with hexagonal topology was synthesized using the autoclave condition. Then, the prepared COF (APB-COF) was used as a pH-dependent carrier for in vitro release of doxorubicin (DOX). The intrinsic properties of APB-COF caused reaching an excellent drug encapsulation efficiency. DOX@APB-COF shows an exemplary pH-dependent release in two different pHs. DOX release at pH = 7.4 was 32%, which increased to 54% by changing the pH to the cancer cell pH (pH = 5.4). Moreover, the cytotoxicity of APB-COF and DOX@APB-COF was studied using the standard MTT test against MCF10 (normal breast cell line) and MDAmb231 cells (breast cancer cell line), respectively. It was observed that the APB-COF does not affect cell proliferation, whereas the DOX@APB-COF only limits cancer cell proliferation. Using APB-COF as the drug carrier can pave the way for using COFs in innovative DDSs.

Linker Exchange as Facile Method for Post-Synthetic Modification of β-Ketoenamine-linked Covalent Organic Frameworks

Post-synthetic linker exchange performed on covalent organic frameworks (COFs) has become an important method to introduce functional building blocks into their backbone and thus to tune their chemical and physical properties. However, the linker exchange method has so far only been described for COFs with relatively weak linkages like imines. Herein, it is shown that this method can be also used for a post-synthetic linker exchange reaction on a β-ketoenamine linked COF. The time needed to achieve considerable linker exchange is much prolonged compared to other COFs with less stable linkages, however, this enables to achieve very good control on the ratio of the respective building blocks within the framework.

2D Organic Materials: Status and Challenges

In the past few decades, 2D layer materials have gradually become a central focus in materials science owing to their uniquely layered structural qualities and good optoelectronic properties. However, in the development of 2D materials, several disadvantages, such as limited types of materials and the inability to synthesize large-scale materials, severely confine their application. Therefore, further exploration of new materials and preparation methods is necessary to meet technological developmental needs. Organic molecular materials have the advantage of being customizable. Therefore, if organic molecular and 2D materials are combined, the resulting 2D organic materials would have excellent optical and electrical properties. In addition, through this combination, the free design and large-scale synthesis of 2D materials can be realized in principle. Furthermore, 2D organic materials exhibit excellent properties and unique functionalities along with great potential for developing sensors, biomedicine, and electronics. In this review, 2D organic materials are divided into five categories. The preparation methods and material properties of each class of materials are also described in detail. Notably, to comprehensively understand each material's advantages, the latest research applications for each material are presented in detail and summarized. Finally, the future development and application prospects of 2D organic materials are briefly discussed.

Interfacial Synthesis of an Ultrathin Two-Dimensional Polymer Film via [2 + 2] Photocycloaddition

A carbon-carbon-linked, ultrathin, two-dimensional (2D) polymer film was prepared at the air/water interface through photochemically triggered [2 + 2] cycloaddition. The preorganization of the monomers on the water surface and the subsequent photo-polymerization led to the successful preparation of the ultrathin 2D polymer film. The obtained film is continuous, free standing, and has a large area (over 50 μm²). Transmission electron microscopy (TEM) and atomic force microscopy (AFM) give clear evidence of the ultrathin film morphology. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) indicate successful photo-induced [2 + 2] polymerization.

Self-Assembly of Helical Nanofibrous Chiral Covalent Organic Frameworks

Despite significant progress on the design and synthesis of covalent organic frameworks (COFs), precise control over microstructures of such materials remains challenging. Herein, two chiral COFs with well-defined one-handed double-helical nanofibrous morphologies were constructed via an unprecedented template-free method, capitalizing on the diastereoselective formation of aminal linkages. Detailed time-dependent experiments reveal the spontaneous transformation of initial rod-like aggregates into the double-helical microstructures. We have further demonstrated that the helical chirality and circular dichroism signal can be facilely inversed by simply adjusting the amount of acetic acid during synthesis. Moreover, by transferring chirality to achiral fluorescent molecular adsorbents, the helical COF nanostructures can effectively induce circularly polarized luminescence with the highest luminescent asymmetric factor (g_lum ) up to ≈0.01.

Covalent Organic Frameworks (COFs) as Multi-Target Multifunctional Frameworks

Covalent organic frameworks (COFs), synthesized from organic monomers, are porous crystalline polymers. Monomers get attached through strong covalent bonds to form 2D and 3D structures. The adjustable pore size, high stability (chemical and thermal), and metal-free nature of COFs make their applications wider. This review article briefly elaborates the synthesis, types, and applications (catalysis, environmental Remediation, sensors) of COFs. Furthermore, the applications of COFs as biomaterials are comprehensively discussed. There are several reported COFs having good results in anti-cancer and anti-bacterial treatments. At the end, some newly reported COFs having anti-viral and wound healing properties are also discussed.

COF-Based Electrodes with Vertically Supported Tentacle Array for Ultrahigh Stability Flexible Energy Storage

As an emerging porous crystal polymer, covalent organic frameworks (COFs) possess unique characteristics, such as high porosity, excellent stability, diverse topologies, designable open channels, and functional tunability. However, limited by the solid powder form, most COFs display low active site utilization and weak binding force with the current collector. In this pioneering research, we integrate redox-active COFs onto carbon fiber surfaces (AC-COFs) via strong covalent bridging. The 2,6-diaminoanthraquinone (DAAQ) pillars embedded on the carbon fiber surface are the key to precisely controlling the growth direction of COFs. The obtained tentacle-like array vertically supported on the surface of the carbon fiber can effectively induce charge transfer and prevent COFs from aggregating/collapsing. The strong covalent coupling and increase of accessible active sites contributed to the high specific capacitance of AC-COFs electrode (1034 mF cm^-2). In addition, the COF-based flexible electrode retains an initial capacitance of 98% after 20000 charge-discharge cycles. The flexible all-solid-state symmetric supercapacitor is assembled by PVA/H₂SO₄ gel electrolyte with an areal capacitance of 715 mF cm^-2. Besides, a red LED can be easily powered by three-bending AC-COFs//AC-COFs devices. The innovative synthesis strategy opens up new opportunities to develop high-performance flexible energy storage devices based on COFs.

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.

Single-Crystalline Imine-Linked Two-Dimensional Covalent Organic Frameworks Separate Benzene and Cyclohexane Efficiently

Two-dimensional (2D) covalent organic frameworks (COFs) are composed of structurally precise, permanently porous, layered macromolecular sheets, which are traditionally synthesized as polycrystalline solids with crystalline domain lengths smaller than 100 nm. Here, we polymerize imine-linked 2D COFs as suspensions of faceted single crystals in as little as 5 min at moderate temperature and ambient pressure. Single crystals of two imine-linked 2D COFs were prepared, consisting of a rhombic 2D COF (TAPPy-PDA) and a hexagonal 2D COF (TAPB-DMPDA). The sizes of TAPPy-PDA and TAPB-DMPDA crystals were tuned from 720 nm to 4 μm and 450 nm to 20 μm in width, respectively. High-resolution transmission electron microscopy revealed that the COF crystals consist of layered, 2D polymers comprising single-crystalline domains. Continuous rotation electron diffraction resolved the unit cell and crystal structure of both COFs, which are single-crystalline in the a-b plane but disordered in the stacking c dimension. Single crystals of both COFs were incorporated into gas chromatography separation columns and exhibited unusual selective retention of cyclohexane over benzene, with single-crystalline TAPPy-PDA significantly outperforming single-crystalline TAPB-DMPDA. Polycrystalline TAPPy-PDA exhibited no separation, while polycrystalline TAPB-DMPDA exhibited poor separation and the opposite order of elution, retaining benzene more than cyclohexane, indicating the importance of improved material quality for COFs to exhibit properties that derive from their precise, crystalline structures. This work represents the first example of synthesizing imine-linked 2D COF single crystals at ambient pressure and short reaction times and demonstrates the promise of high-quality COFs for molecular separations.

Two-Dimensional Polymers and Polymerizations

Synthetic chemists have developed robust methods to synthesize discrete molecules, linear and branched polymers, and disordered cross-linked networks. However, two-dimensional polymers (2DPs) prepared from designed monomers have been long missing from these capabilities, both as objects of chemical synthesis and in nature. Recently, new polymerization strategies and characterization methods have enabled the unambiguous realization of covalently linked macromolecular sheets. Here we review 2DPs and 2D polymerization methods. Three predominant 2D polymerization strategies have emerged to date, which produce 2DPs either as monolayers or multilayer assemblies. We discuss the fundamental understanding and scope of each of these approaches, including: the bond-forming reactions used, the synthetic diversity of 2DPs prepared, their multilayer stacking behaviors, nanoscale and mesoscale structures, and macroscale morphologies. Additionally, we describe the analytical tools currently available to characterize 2DPs in their various isolated forms. Finally, we review emergent 2DP properties and the potential applications of planar macromolecules. Throughout, we highlight achievements in 2D polymerization and identify opportunities for continued study.

Scalable Fabrication of Crystalline COF Membranes from Amorphous Polymeric Membranes

Covalent organic framework (COF) membranes hold potential for widespread applicability, but scalable fabrication is challenging. Here, we demonstrate the disorder-to-order transformation from amorphous polymeric membrane to crystalline COF membrane via monomer exchange. Solution processing is used to prepare amorphous membrane and the replacing monomer is selected based on the chemical and thermodynamical stability of the final framework. Reversible imine bonds allow the extraneous monomers to replace the pristine monomers within amorphous membrane, driving the transformation from disordered network to ordered framework. Incorporation of intramolecular hydrogen bonds enables the crystalline COF to imprint the amorphous membrane morphology. The COF membranes harvest proton conductivity up to 0.53 S cm^-1 at 80 °C. Our strategy bridges amorphous polymeric and crystalline COF membranes for large-scale fabrication of COF membranes and affords guidance on materials processing.

Judicious design functionalized 3D-COF to enhance CO2 adsorption and separation

The effects of functional groups (including OH, OCH₃ , NH₂ , CH₂ NH₂ , COOH, SO₃ H, OCO(CH₂ )₂ COOH(E-COOH), and (CH₂ )₄ COOH(c-COOH)) in 3D covalent organic frameworks (3D-COFs) on CO₂ adsorption and separation are investigated by grand canonical Monte Carlo (GCMC) simulations and density functional theory calculations. The results indicate that interaction between CO₂ and the framework is the main factor for determining CO₂ uptakes at low pressure, while pore size becomes the decisive factor at high pressure. The binding energy of CO₂ with functionalized linker is correlated to CO₂ uptake at 0.3 bar and 298 K on 3D-COF-1, suggesting functional groups play a key role in CO₂ capture in microporous 3D-COFs. Moreover, CO₂ selectivity over CH₄ , N₂ , and H₂ can be significantly enhanced by functionalization, where CH₂ NH₂ , COOH, SO₃ H, and E-COOH enhance CO₂ adsorption more effectively at 1 bar. Among them, SO₃ H is the most promising functional group in 3D-COFs for CO₂ separation.

In situ room-temperature preparation of a covalent organic framework as stationary phase for high-efficiency capillary electrochromatographic separation

Covalent organic frameworks (COFs), considered as a series of newly emerging porous organic materials, have been widely utilized in separation fields. Herein, a novel COF (TFPB-BD) was first employed as stationary phase for high-efficiency capillary electrochromatographic separation. Benzidine (BD) and 1,3,5-Tris-(4-formylphenyl)benzene (TFPB) were selected as organic linkers and then introduced into the aldehyde group modified capillary for the in situ growth of TFPB-BD onto the capillary inner wall at room temperature. The morphology and formation of TFPB coated capillary column were confirmed by a variety of tools including Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS). It's interesting that the TFPB globular crystals with nanoscale were uniformly and densely modified on the capillary inner surface. Hence, the prepared column exhibited prominent separation performance for the test analytes including alkylbenzenes, chlorobenzenes and phenolic compounds with high efficiency and high resolution. The maximum column efficiency can reach about 1.8 × 10⁵ plates•m^-1. Additionally, the high resolutions of anilines, amino acids and parabens were also achieved on the TFPB modified capillary. The precisions (RSDs) of the retention times of alkylbenzenes of intra-day runs (n = 3), inter-day runs (n = 3) and parallel columns (n = 3) were all less than 2.83%. This innovative COF-based stationary phase gives great promise for the chromatographic separation field.

Macroscopic Ultralight Aerogel Monoliths of Imine-based Covalent Organic Frameworks

The use of covalent organic frameworks (COFs) in practical applications demands shaping them into macroscopic objects, which remains challenging. Herein, we report a simple three-step method to produce COF aerogels, based on sol-gel transition, solvent-exchange, and supercritical CO₂ drying, in which 2D imine-based COF sheets link together to form hierarchical porous structures. The resultant COF aerogel monoliths have extremely low densities (ca. 0.02 g cm^-3 ), high porosity (total porosity values of ca. 99 %), and mechanically behave as elastic materials under a moderate strain (<25-35 %) but become plastic under greater strain. Moreover, these COF aerogels maintain the micro- and meso-porosity of their constituent COFs, and show excellent absorption capacity (e.g. toluene uptake: 32 g g^-1 ), with high removal efficiency (ca. 99 %). The same three-step method can be used to create functional composites of these COF aerogels with nanomaterials.

Dynamic Transformation between Covalent Organic Frameworks and Discrete Organic Cages

We propose a dynamic covalent chemistry (DCC)-induced linker exchange strategy for the structural transformation between covalent organic frameworks (COFs) and cages for the first time. Studies have shown that the COF-to-cage and cage-to-COF transformations were realized by using borate bonds and imine bonds, respectively, as linkages. Self-sorting experiments suggested that borate cages and imine COFs are thermodynamic minimum compounds. This research builds a bridge between discrete and polymeric organic scaffolds and broadens the knowledge of chemistry and materials for porous materials science.

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.

Destruction of Metal-Organic Frameworks: Positive and Negative Aspects of Stability and Lability

Metal-organic frameworks (MOFs), constructed from organic linkers and inorganic building blocks, are well-known for their high crystallinity, high surface areas, and high component tunability. The stability of MOFs is a key prerequisite for their potential practical applications in areas including storage, separation, catalysis, and biomedicine since it is essential to guarantee the framework integrity during utilization. However, MOFs are prone to destruction under external stimuli, considerably hampering their commercialization. In this Review, we provide an overview of the situations where MOFs undergo destruction due to external stimuli such as chemical, thermal, photolytic, radiolytic, electronic, and mechanical factors and offer guidelines to avoid unwanted degradation happened to the framework. Furthermore, we discuss possible destruction mechanisms and their varying derived products. In particular, we highlight cases that utilize MOF instability to fabricate varying materials including hierarchically porous MOFs, monolayer MOF nanosheets, amorphous MOF liquids and glasses, polymers, metal nanoparticles, metal carbide nanoparticles, and carbon materials. Finally, we provide a perspective on the utilization of MOF destruction to develop advanced materials with a superior hierarchy for various applications.

Small-angle X-ray scattering as a multifaceted tool for structural characterization of covalent organic frameworks

Small-angle X-ray scattering (SAXS) is an accurate nondestructive method that requires a minimum of sample preparation and is employed to study porosity, morphology and hierarchical structures. Zeolites and silica are among the porous materials that are widely investigated by SAXS. However, studies of covalent organic frameworks (COFs) are still scarce. In the present study, SAXS was employed to investigate meso- and microporous COFs, affording insightful information about their nanostructure textural properties. SAXS is especially useful when combined with other characterization techniques, such as powder X-ray diffraction and N2 adsorption isotherms, emerging as an efficient tool to further characterize COFs. For microporous COFs, SAXS was used mainly to obtain quantitative values of surface roughness as a function of fractal parameters, in all cases indicating surface fractals of the large-scale scattering object, namely the `grain'. Mesoporous COF studies allowed elucidation of their hexagonal structure on the basis of their structure peaks; however, the main result lies in the distinction between the pore and the grain, which are described as a hierarchical structure by the Beaucage model and evaluated according to their fractality. These COFs generally exhibit pores with mass fractal features and grains with surface fractal features when they are submitted to post-functionalization, which may be due to the poor diffusivity of the functionalizing agents into the pores. In addition, a proposed aggregation description of the porous scattering objects was envisioned, based on small-angle scattering premises, which was confirmed for a microporous COF by high-resolution transmission electron microscopy.

Triphenylphosphine-Based Covalent Organic Frameworks and Heterogeneous Rh-P-COFs Catalysts

The synthesis of phosphine-based functional covalent organic frameworks (COFs) has attracted great attention recently. Herein, we present two examples of triphenylphosphine-based COFs (termed P-COFs) with well-defined crystalline structures, high specific surface areas, and good thermal stability. Furthermore, rhodium catalysts with these P-COFs as support material show high turnover frequency for the hydroformylation of olefins, as well as excellent recycling performance. This work not only extends the phosphine-based COF family, but also demonstrates their application in immobilizing homogeneous metal-based (e.g., Rh-phosphine) catalysts for application in heterogeneous catalysis.

Effective enrichment and detection of trace polybrominated diphenyl ethers in water samples based on magnetic covalent organic framework nanospheres coupled with chromatography-mass spectrometry

Novel core shell structured magnetic covalent organic frameworks were synthesized at room temperature and first applied in water samples for the enrichment of trace polybrominated diphenyl ethers (PBDEs) through magnetic solid-phase extraction. The prepared materials were characterized through transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, vibrating sample magnetometer and X-ray photoelectron spectroscopy. During adsorption, the parameters affecting extraction and desorption efficiency were further optimized. Combined gas chromatography and mass spectrometry (GC/MS) revealed that high enrichment factors (275-292), low limits of detection (0.12-0.38 ng·L-1), wide linear ranges (0.5-1000 ng·L-1), and good reproducibility (intra-day 1.40%-4.31% and inter-day 5.14%-9.12%) were obtained under optimal conditions. The method successfully detected PBDEs in different water samples.

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.

Nanoengineered Light-Activatable Polybubbles for On-Demand Therapeutic Delivery

Vaccine coverage is severely limited in developing countries due to inefficient protection of vaccine functionality as well as lack of patient compliance to receive the additional booster doses. Thus, there is an urgent need to design a thermostable vaccine delivery platform that also enables release of the bolus after predetermined time. Here, the formation of injectable and light-activatable polybubbles for vaccine delivery is reported. In vitro studies show that polybubbles enable delayed burst release, irrespective of cargo types, namely small molecule and antigen. The extracorporeal activation of polybubbles is achieved by incorporating near-infrared (NIR)-sensitive gold nanorods (AuNRs). Interestingly, light-activatable polybubbles can be used for on-demand burst release of cargo. In vitro, ex vivo, and in vivo studies demonstrate successful activation of AuNR-loaded polybubbles. Overall, the light-activatable polybubble technology can be used for on-demand delivery of various therapeutics including small molecule drugs, immunologically relevant protein, peptide antigens, and nucleic acids.

Applications of Dynamic Covalent Chemistry Concept towards Tailored Covalent Organic Framework Nanomaterials: A Review

Covalent organic frameworks (COFs) are a rapidly developing class of materials that has been of immense research interest during the last ten years. Numerous reviews have been devoted to summarizing the synthesis and applications of COFs. However, the underlying dynamic covalent chemistry (DCC), which is the foundation of COFs synthesis, has never been systematically reviewed in this context. Dynamic covalent chemistry is the practice of using thermodynamic equilibriums to molecular assemblies. This Critical Review will cover the state-of-the-art use of DCC to both synthesize COFs and expand the applications of COFs. Five synthetic strategies for COF synthesis are rationalized, namely: modulation, mixed linker/linkage, sub-stoichiometric reaction, framework isomerism, and linker exchange, which highlight the dynamic covalent chemistry to regulate the growth and to modify the properties of COFs. Furthermore, the challenges in these approaches and potential future perspectives in the field of COF chemistry are also provided.

Metalation of Catechol-Functionalized Defective Covalent Organic Frameworks for Lewis Acid Catalysis

Covalent organic frameworks (COFs) have emerged as a fascinating crystalline porous material and are widely used in the field of catalysis. However, developing simple approaches to fabricate conjugated COFs with specific functional groups remains a significant challenge. In this study, the construction of defective COF-LZU1 with Lewis acid sites embedded into the frameworks is fulfilled by a facile solvent-assisted ligand exchange method. A monodentate ligand, protocatechualdehyde, is successfully introduced into the skeleton of COF-LZU1, which endows the defects in the structure of COF-LZU1 via replacement of the original coordinated benzene-1,3,5-tricarbaldehyde ligand. As-synthesized defective COF-LZU1 decorated with protocatechualdehyde is rich of free hydroxy groups for chelating with active metal ions. Specifically, after combining with Fe³⁺ , the defective COF-LZU1 shows excellent activity in catalytic alcoholysis of epoxides under mild conditions. The method reported here will open up the opportunity to incorporate different functional groups into COFs and enrich the strategies for creating new types of porous catalysts.

Covalent Organic Frameworks: Design, Synthesis, and Functions

Covalent organic frameworks (COFs) are a class of crystalline porous organic polymers with permanent porosity and highly ordered structures. Unlike other polymers, a significant feature of COFs is that they are structurally predesignable, synthetically controllable, and functionally manageable. In principle, the topological design diagram offers geometric guidance for the structural tiling of extended porous polygons, and the polycondensation reactions provide synthetic ways to construct the predesigned primary and high-order structures. Progress over the past decade in the chemistry of these two aspects undoubtedly established the base of the COF field. By virtue of the availability of organic units and the diversity of topologies and linkages, COFs have emerged as a new field of organic materials that offer a powerful molecular platform for complex structural design and tailor-made functional development. Here we target a comprehensive review of the COF field, provide a historic overview of the chemistry of the COF field, survey the advances in the topology design and synthetic reactions, illustrate the structural features and diversities, scrutinize the development and potential of various functions through elucidating structure-function correlations based on interactions with photons, electrons, holes, spins, ions, and molecules, discuss the key fundamental and challenging issues that need to be addressed, and predict the future directions from chemistry, physics, and materials perspectives.

Two-dimensional covalent organic frameworks with hierarchical porosity

This review highlights the state-of-the-art progress achieved in two-dimensional covalent organic frameworks (COFs) with hierarchical porosity, an emerging class of COFs constructed by integrating different types of pores into one framework.