Chemically modified covalent organic frameworks for a healthy and sustainable environment: First-principles study

Pure water is a key element for a sustainable and healthy environment of human inhabitation. Since major sources of water contamination are industrially generated heavy metal cations there is great demand for efficient methods of their treatment. Here, using density functional theory, we investigate the covalent organic framework's electronic and optical properties and their interaction with the most dangerous heavy metal pollutants, namely Hg⁺², Pb^+2, and Cd⁺². We consider biphenyl boroxine covalent organic frameworks before and after chemical modification with CN, COOH, NH₂, and NO₂ groups. In addition to the molecular geometries, such parameters as the dipole moment, chemical potential, electronegativity, chemical hardness, and binding energy are calculated. It is found that CN, COOH, and NO₂ functional groups are favorable for intermolecular bonding with harmful transition metals. The functionalization with the mentioned groups reduces the band gap of the pristine covalent organic frameworks and increases their reactivity. As a result, strong complexes with Cd⁺², Hg⁺², and Pb⁺² can form, which, as follows from our calculations, can be detected by the red shift in their optical absorption spectra.

Cu-α-diimine Compounds Encapsulated in Porous Materials as Catalysts for Electrophilic Amination of Aromatic C-H Bonds

Electrophilic amination has emerged as a more environmentally benign approach to construct arene C-N bonds. However, heterogeneous catalysts remain largely unexplored in this area, even though their use could facilitate product purification and catalyst recovery. Here we investigate strategies to heterogenize a Cu(2,2'-bipyridine) catalyst for the amination of arenes lacking a directing group with hydroxylamine-O-sulfonic acid (HOSA). Besides immobilization of Cu on a metal-organic framework (MOF) or covalent organic framework (COF) with embedded 2,2'-bipyridines, a ship-in-a-bottle approach was followed in which the Cu complex is encapsulated in the pores of a zeolite. Recyclability and hot centrifugation tests show that zeolite Beta-entrapped Cu^II(2,2'-bipyridine) is superior in terms of stability. With N-methylmorpholine as a weakly coordinating, weak base, simple arenes, such as mesitylene, could be aminated with yields up to 59%, corresponding to a catalyst TON of 24. The zeolite could be used in three consecutive runs without a decrease in activity. Characterization of the catalyst by EPR and XAS showed that the active catalytic complex consisted of a site-isolated Cu^II species with one 2,2'-bipyridine ligand.

Recent development in hydrophilic interaction liquid chromatography stationary materials for glycopeptide analysis

Protein glycosylation is one of the most important post-translational modifications, and aberrant glycosylation is associated with the occurrence and development of diseases. Deciphering abnormal glycosylation changes can identify disease-specific signatures to facilitate the discovery of potential disease biomarkers. However, glycosylation analysis is challenging due to the diversity of glycans, heterogeneity of glycosites, and poor electrospray ionization of mass spectrometry. To overcome these obstacles, glycosylation is often elucidated using enriched glycopeptides by removing highly abundant non-glycopeptides. Hydrophilic interaction liquid chromatography (HILIC) is widely used for glycopeptide enrichment due to its excellent selectivity and specificity to hydrophilic glycans and compatibility with mass spectrometry. However, the development of HILIC has lagged far behind hydrophobic interaction chromatography, so efforts to further improve the performance of HILIC are beneficial for glycosylation analysis. This review discusses recent developments in HILIC materials and their advanced applications. Based on the physiochemical properties of glycopeptides, the use of amino acids or peptides as stationary phases showed improved enrichment and separation of glycopeptides. We can envision that the use of glycopeptides as stationary phases would definitely further improve the selectivity and specificity of HILIC for glycosylation analysis.

Multi-Component Synthesis of a Buta-1,3-diene-Linked Covalent Organic Framework

We report a multi-component synthetic strategy on a two-dimensional crystalline covalent organic framework (COF) by connecting acetonitrile with aromatic aldehyde and acetaldehyde moieties to form an unprecedented cyano-substituted buta-1,3-diene linkage. Different from most of the COFs that were crystallized from the condensations from two components, the presented COF is generated from two competitive and reversible reactions among three moieties. The buta-1,3-diene COF exhibits remarkable photoactivity with a low exciton binding energy of 44.4 ± 1.5 meV for promoted charge separation, which enables the buta-1,3-diene-linked COF as an efficient photocatalyst for various aerobic oxidation reactions under visible light. Our multi-component synthesis strategy may provide new sights for synthesizing COFs with structural diversity and functional variability that are hard to achieve by traditional COF synthesis.

Induction of Chiral Hybrid Metal Halides from Achiral Building Blocks

Chiral hybrid organic-inorganic metal halides (HOMHs) with intrinsic noncentrosymmetry have shown great promise for broad applications in chiroptoelectronics, spintronics, and ferroelectronics. However, the construction strategies for chiral HOMHs often involve chiral building blocks in their frameworks, which greatly limit their chemical diversity. Here, we take advantage of a chiral induction approach and have successfully constructed a series of chiral HOMHs, DMA₄MX₇ (DMA = dimethylammonium, M = Sb or Bi, X = Cl or Br), based on achiral precursors. The resulting chiral products demonstrate a clear enantioenrichment, as confirmed by single-crystal X-ray diffraction analysis and solid-state circular dichroism (CD) spectroscopy. The induction of chiral HOMHs enables superior nonlinear optical performances with very high thermal stability and laser resistance. The successful employment of such a chiral induction approach might facilitate the construction of libraries of chiral HOMH crystals from diverse achiral precursors, in particular those into which it is not easy to introduce intrinsic chiral centers, and would thus pave a new way for rational preparation and application of chiral HOMH materials.

Confining enzymes in porous organic frameworks: from synthetic strategy and characterization to healthcare applications

Enzymes are a class of natural catalysts with high efficiency, specificity, and selectivity unmatched by their synthetic counterparts and dictate a myriad of reactions that constitute various cascades in living cells. The development of suitable supports is significant for the immobilization of structurally flexible enzymes, enabling biomimetic transformation in the extracellular environment. Accordingly, porous organic frameworks, including metal organic frameworks (MOFs), covalent organic frameworks (COFs) and hydrogen-bonded organic frameworks (HOFs), have emerged as ideal supports for the immobilization of enzymes because of their structural features including ultrahigh surface area, tailorable porosity, and versatile framework compositions. Specially, organic framework-encased enzymes have shown significant enhancement in stability and reusability, and their tailorable pore opening provides a gatekeeper-like effect for guest sieving, which is beneficial for mimicking intracellular biocatalysis processes. This immobilization technique brings new insight into the development of next-generation enzyme materials and shows huge potential in healthcare applications, such as biomarker diagnosis, biostorage, and cancer and antibacterial therapies. In this review, we describe the state-of-the-art strategies for the structural immobilization of enzymes using the well-explored MOFs and burgeoning COFs and HOFs as scaffolds, with special emphasis on how these porous framework-confined technologies can provide a favorable microenvironment for mimicking natural biocatalysis. Subsequently, advanced characterization techniques for enzyme conformation, the effect of the confined microenvironment on the activity of enzymes, and the emerging healthcare applications will be surveyed.

Metalated covalent organic frameworks: from synthetic strategies to diverse applications

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

Hydrazine-Hydrazide-Linked Covalent Organic Frameworks for Water Harvesting

We report a postsynthetic strategy and its implementation to make covalent organic frameworks (COFs) with irreversible hydrazide linkages. This involved the synthesis of three 2D and 3D hydrazine-linked frameworks and their partial oxidation. The linkage synthesis and functional group transformation-hydrazine and hydrazide-were evidenced by ¹⁵N multi-CP-MAS NMR. In addition, the isothermal water uptake profiles of these frameworks were studied, leading to the discovery of one hydrazine-hydrazide-linked COF suitable for water harvesting from air in arid conditions. This COF displayed characteristic S-shaped water sorption profiles, a steep pore-filling step below 18% relative humidity at 25 °C, and a total uptake capacity of 0.45 g g^-1. We found that even small changes made on the molecular level can lead to major differences in the water isotherm profiles, therefore pointing to the utility of water sorption analysis as a complementary analytical tool to study linkage transformations.

Synthesis of stack plate covalent organic framework nanotubes using a self-assembled acid as a soft template

COF-LZU1 with nanotube-like morphology has been synthesized with high crystallinity and pore volume in the presence of trimesic acid as a template. The as-synthesized COF nanotubes consist of a stack of plates with a diameter of about 100 nm with a hollow channel inside of about 20 nm.

Ratiometric Fluorescent pH Sensor Based on a Tunable Multivariate Covalent Organic Framework

Ratiometric detection of pH is always significant in environmental regulation, medical diagnosis, synthetic chemistry, and beyond. The construction of practical ratiometric pH sensors with reusability is still challenging. Herein, by exploiting a multivariate strategy, we first synthesized and reported a series of novel three-component covalent organic frameworks (COF-COOH_X, X = 33, 50, and 67) through Schiff base reaction between 2-hydroxybenzene-1,3,5-tricarbaldehyde (HTA), 4,4'-diamino-3,3'-biphenyldicarboxylic acid (DBA), and 5,5'-diamino-2,2'-bipyridine (BPY) at various molar ratios (X = [DBA]/([BPY] + [DBA]) × 100 = 33, 50, and 67). COF-COOH_X (X = 33, 50, and 67) displayed ratiometric pH sensing performance in acidic conditions with selectivity and repeatability. By tuning the molar ratio of DBA and BPY, the fluorescent properties, linear pH responsive ranges, and pK_a values of COF-COOH_X (X = 33, 50, and 67) can be regulated. Meanwhile, the two-component COF-COOH₀ and COF-COOH₁₀₀ did not exhibit ratiometric pH detection ability. Moreover, the constructed three ratiometric sensors can be applied to detect pH in drug solutions and carbonated drinks with satisfactory results. This work sheds new light on the design and fabrication of innovative ratiometric fluorescent sensors using COFs.

Selective Detection of Nucleotides in Infant Formula Using an N-Rich Covalent Triazine Porous Polymer

The aromatic structure and the rich nitrogen content of polymers based on covalent triazine-based frameworks (CTF) and their unique hydrophilic-lipophilic-balanced adsorption properties make them promising candidates for an adsorbent that can be used for sample pretreatment. Herein, a new covalent triazine-based framework (CTF-DBF) synthesized by a Friedel−Crafts reaction was used for the determination of the content of nucleotides in commercial infant formula. It was shown that the synthetic materials had an amorphous microporous structure, a BET surface area of up to 595.59 m2/g, and 0.39 nm and 0.54 nm micropores. The versatile adsorption properties of this material were evaluated by quantum chemistry theory calculations and batch adsorption experiments using five nucleotides as probes. The quantum chemistry results demonstrated that CTF-DBF can participate in multiple interactions with nucleotides. All the analyses performed present good linearity with R2 > 0.9993. The detection limits of targets ranged from 0.3 to 0.5 mg/kg, the spiked recoveries were between 85.8 and 105.3% and the relative standard deviations (RSD, n = 6) were between 1.1 and 4.5%. All these results suggest that this versatile CTF-DBF has great potential for sample pretreatment.

Electrochemical (Bio)Sensors Based on Covalent Organic Frameworks (COFs)

Covalent organic frameworks (COFs) are defined as crystalline organic polymers with programmable topological architectures using properly predesigned building blocks precursors. Since the development of the first COF in 2005, many works are emerging using this kind of material for different applications, such as the development of electrochemical sensors and biosensors. COF shows superb characteristics, such as tuneable pore size and structure, permanent porosity, high surface area, thermal stability, and low density. Apart from these special properties, COF’s electrochemical behaviour can be modulated using electroactive building blocks. Furthermore, the great variety of functional groups that can be inserted in their structures makes them interesting materials to be conjugated with biological recognition elements, such as antibodies, enzymes, DNA probe, aptamer, etc. Moreover, the possibility of linking them with other special nanomaterials opens a wide range of possibilities to develop new electrochemical sensors and biosensors.

Timing matters: pre-assembly versus post-assembly functionalization of a polyoxovanadate-organic cuboid

The pre-assembly and post-assembly approaches in the functionalization of a polyoxovanadate-organic cuboid, [{V6S}8(QPTC)8{V3}2]10-, are discussed. We have shown that the two pathways have led to distinctly different systems, with either an expanded or contracted interior void space, when phenylphosphonate is introduced at different stages of the self-assembly. One leaves the cuboid framework largely intact, whereas the other results in a compact, twisted cuboid. Kinetic factors will have to be considered in the equilibrium of these complex processes. Furthermore, the exceptional stability of these polyoxometalate-organic systems facilitates mass spectrometric characterization, which confirms the composition of the complexes and also indicates that the methoxide groups on the vanadium cluster nodes are labile. The results will help deepen the mechanistic understanding of the formation mechanisms of polyoxovanadate-based metal-organic cages and other functionalized polyoxovanadate clusters in general.

Porous organic polymers in solar cells

Owing to their unique porosity and large surface area, porous organic polymers (POPs) have shown their presence in numerous novel applications. The tunability and functionality of both the pores and backbone of the material enable its suitability in photovoltaic devices. The porosity induced host-guest configurations as well as periodic donor-acceptor structures benefit the charge separation and charge transfer in photophysical processes. The role of POPS in other critical device components, such as hole transporting layers and electrodes, has also been demonstrated. Herein, this review will primarily focus on the recent progress made in applying POPs for solar cell device performance enhancement, covering organic solar cells, perovskite solar cells, and dye-sensitized solar cells. Based on the efforts in recent years in unraveling POP's photophysical process and its relevance with device performances, an in-depth analysis will be provided to address the gradual shift of attention from an entirely POP-based active layer to other device functional components. Combining the insights from device physics, material synthesis, and microfabrication, we aim to unfold the fundamental limitations and challenges of POPs and shed light on future research directions.

Porous Dithiine-Linked Covalent Organic Framework as a Dynamic Platform for Covalent Polysulfide Anchoring in Lithium-Sulfur Battery Cathodes

Dithiine linkage formation via a dynamic and self-correcting nucleophilic aromatic substitution reaction enables the de novo synthesis of a porous thianthrene-based two-dimensional covalent organic framework (COF). For the first time, this organo-sulfur moiety is integrated as a structural building block into a crystalline layered COF. The structure of the new material deviates from the typical planar interlayer π-stacking of the COF to form undulated layers caused by bending along the C-S-C bridge, without loss of aromaticity and crystallinity of the overall COF structure. Comprehensive experimental and theoretical investigations of the COF and a model compound, featuring the thianthrene moiety, suggest partial delocalization of sulfur lone pair electrons over the aromatic backbone of the COF decreasing the band gap and promoting redox activity. Postsynthetic sulfurization allows for direct covalent attachment of polysulfides to the carbon backbone of the framework to afford a molecular-designed cathode material for lithium-sulfur (Li-S) batteries with a minimized polysulfide shuttle. The fabricated coin cell delivers nearly 77% of the initial capacity even after 500 charge-discharge cycles at 500 mA/g current density. This novel sulfur linkage in COF chemistry is an ideal structural motif for designing model materials for studying advanced electrode materials for Li-S batteries on a molecular level.

Metal@COFs Possess High Proton Conductivity with Mixed Conducting Mechanisms

The inherent porous structures and aligned functional units inside the skeleton of covalent organic frameworks (COFs) provide an extraordinary promise for post-modification and deservedly expand their application in the field of proton conduction. Herein, we tactfully introduced copper ions into a two-dimensional COF (TpTta) furnished with ample N,O-chelating sites by a post-modification strategy to achieve two copper(II)-modified products, namely, CuCl₂@TpTta-3 and CuCl₂@TpTta-10. Inspiringly, the two modified COFs demonstrated the higher conductivities of 1.77 × 10^-3 and 8.81 × 10^-3 S cm^-1 under 100 °C and 98% relative humidity, respectively, among the previously reported COFs with higher σ values. In comparison to the pristine COFs, the σ values of CuCl₂@TpTta-3 and CuCl₂@TpTta-10 are boosted by 2 orders of magnitude. On the basis of structural analyses, nitrogen and water vapor adsorption tests, and proton conduction mechanism analysis, we deeply analyzed the reason why the conductivity of the modified COFs was significantly increased. To the best of our knowledge, it is the first time to employ the CuCl₂-modified strategy to boost the conductivity of COFs, which offers a wise idea for the fabrication of highly conductive materials in the field of fuel cells.

Metallodrugs in cancer nanomedicine

Metal complexes are extensively used for cancer therapy. The multiple variables available for tuning (metal, ligand, and metal-ligand interaction) offer unique opportunities for drug design, and have led to a vast portfolio of metallodrugs that can display a higher diversity of functions and mechanisms of action with respect to pure organic structures. Clinically approved metallodrugs, such as cisplatin, carboplatin and oxaliplatin, are used to treat many types of cancer and play prominent roles in combination regimens, including with immunotherapy. However, metallodrugs generally suffer from poor pharmacokinetics, low levels of target site accumulation, metal-mediated off-target reactivity and development of drug resistance, which can all limit their efficacy and clinical translation. Nanomedicine has arisen as a powerful tool to help overcome these shortcomings. Several nanoformulations have already significantly improved the efficacy and reduced the toxicity of (chemo-)therapeutic drugs, including some promising metallodrug-containing nanomedicines currently in clinical trials. In this critical review, we analyse the opportunities and clinical challenges of metallodrugs, and we assess the advantages and limitations of metallodrug delivery, both from a nanocarrier and from a metal-nano interaction perspective. We describe the latest and most relevant nanomedicine formulations developed for metal complexes, and we discuss how the rational combination of coordination chemistry with nanomedicine technology can assist in promoting the clinical translation of metallodrugs.

Covalent organic framework-based porous materials for harmful gas purification

Covalent organic frameworks (COFs) with 2D or 3D networks are a class of novel porous crystalline materials, and have attracted more and more attention in the field of gas purification owing to their attractive physicochemical properties, such as high surface area, adjustable functionality and structure, low density, and high stability. However, few systematic reviews about the application statuses of COFs in gas purification are available, especially about non-CO₂ harmful gases. In this review, the recent progress of COFs about the capture, catalysis, and detection of common harmful gases (such as CO₂, NO_x, SO₂, H₂S, NH₃ and volatile pollutants) were comprehensively discussed. The design strategies of COF functional materials from porosity adjustment to surface functionalization (including bottom-up approach, post-synthetic approach, and blending with other materials) for certain application were summarized in detail. Furthermore, the faced challenges and future research directions of COFs in the harmful gas treatment were clearly proposed to inspire the development of COFs.

Confinement synthesis in porous molecule-based materials: a new opportunity for ultrafine nanostructures

A balance between activity and stability is greatly challenging in designing efficient metal nanoparticles (MNPs) for heterogeneous catalysis. Generally, reducing the size of MNPs to the atomic scale can provide high atom utilization, abundant active sites, and special electronic/band structures, for vastly enhancing their catalytic activity. Nevertheless, due to the dramatically increased surface free energy, such ultrafine nanostructures often suffer from severe aggregation and/or structural degradation during synthesis and catalysis, greatly weakening their reactivities, selectivities and stabilities. Porous molecule-based materials (PMMs), mainly including metal-organic frameworks (MOFs), covalent organic frameworks (COFs) and porous organic polymers (POPs) or cages (POCs), exhibit high specific surface areas, high porosity, and tunable molecular confined space, being promising carriers or precursors to construct ultrafine nanostructures. The confinement effects of their nano/sub-nanopores or specific binding sites can not only effectively limit the agglomeration and growth of MNPs during reduction or pyrolysis processes, but also stabilize the resultant ultrafine nanostructures and modulate their electronic structures and stereochemistry in catalysis. In this review, we highlight the latest advancements in the confinement synthesis in PMMs for constructing atomic-scale nanostructures, such as ultrafine MNPs, nanoclusters, and single atoms. Firstly, we illustrated the typical confinement methods for synthesis. Secondly, we discussed different confinement strategies, including PMM-confinement strategy and PMM-confinement pyrolysis strategy, for synthesizing ultrafine nanostructures. Finally, we put forward the challenges and new opportunities for further applications of confinement synthesis in PMMs.

Porous organic polymers for light-driven organic transformations

As a new generation of porous materials, porous organic polymers (POPs), have recently emerged as a powerful platform of heterogeneous photocatalysis. POPs are constructed using extensive organic synthesis methodologies, with various functional organic units being connected via high-energy covalent bonds. This review systematically presents the recent advances in POPs for visible-light driven organic transformations. Herein, we firstly summarize the common construction strategies for POP-based photocatalysts based on two major approaches: pre-design and post-modification; secondly, we categorize and summarize the synthesis methods and organic reaction types for constructing various types of POPs. We then classify and introduce the specific reactions of current light-driven POP-mediated organic transformations. Finally, we outline the current state of development and the problems faced in light-driven organic transformations by POPs, and we present some perspectives to motivate the reader to explore solutions to these problems and confront the present challenges in the development process.