Imine-based Covalent Organic Frameworks Research Articles

Totally conjugated and coplanar covalent organic frameworks as photocatalysts for water purification: Reduction of Cr (VI) while oxidizing water borne organic pollutants

Covalent organic frameworks (COFs) have emerged as photocatalytic materials with bandgaps in the visible region. Imine-based COFs, which have been extensively explored, often suffer from limited stability and poor conjugation, hindering their photocatalytic activities. The chemical and hydrolytic stability and the photocatalytic performance of COFs is drastically enhanced by constructing 2D COFs that are fully conjugated in the x,y plane, that have alternating donor–acceptor (D-A) units for better charge separation and that have enhanced conjugation in the z-axis by p-orbital overlap by using highly planar building blocks. In this study, we introduce three highly crystalline sp2 COFs that are able to photocatalyticlly reduce highly toxic Cr (VI) species to much less toxic and easily removable Cr (III) residues, while simultaneously oxidizing water borne organic pollutants. One of them, the TEB-COF, with the integration of the acetylene group, exhibited excellent photocatalytic activity due to its superior planarity and extended conjugation. TEB-COF is able to completely remove the model dye Rhodamine B and Cr (VI) (10 mg/L) in less than 30 min. This research provides valuable insights into the development of recyclable metal-free photocatalysts for wastewater treatment.

Efficient Capture and Release of the Rare-Earth Element Neodymium in Aqueous Solution by Recyclable Covalent Organic Frameworks.

Rare-earth elements (REEs) are present in a broad range of critical materials. The development of solid adsorbents for REE capture could enable the cost-effective recycling of REE-containing magnets and electronics. In this context, covalent organic frameworks (COFs) are promising candidates for REE adsorption due to their exceptionally high surface area. Despite having attractive physical properties, COFs are heavily underutilized for REE capture applications due to their limited lifecycle in aqueous acidic environments, as well as synthetic challenges associated with the incorporation of ligands suitable for REE capture. Here, we show how the Ugi multicomponent reaction can be leveraged to postsynthetically modify imine-based COFs for the introduction of a diglycolic acid (DGA) moiety, an efficient scaffold for REE capture. The adsorption capacity of the DGA-functionalized COF was found to be more than 40 times higher than that of the pristine imine COF precursor and more than four times higher than that of the next-best reported DGA-functionalized solid support. This rationally designed COF has appealing characteristics of high adsorption capacity, fast and efficient capture and release of the REE ions, and reliable recyclability, making it one of the most promising adsorbents for solid-liquid REE ion extractions reported to date.

In-situ charge polarization effect of copper(II) ion-coordination covalent organic framework induced aerobic PET-ATRP in aqueous phase

Covalent organic frameworks (COFs) are ideal heterogeneous photocatalytic platforms offering fascinating spaces for manipulation given their clearly defined structures along with pre-customizable skeletons. Herein, an imine-based COF Im-oMe featuring N and O sites allowing the integration of CuII ions via coordination bonds in local regions of the skeleton is reported for aerobic photoinduced electron/energy transfer atom transfer radical polymerization (PET-ATRP) in the aqueous phase. CuII@Im-oMe of anchored CuII ions is capable of triggering skeleton localized polarization effects, producing greater dipole moment driving force and facilitating the charge separation and transport to enhance its photopolymerization performance. Meanwhile, the system does not require additional addition of transition metal halides, and remains highly active after 3 cycles. The present work expands the feasible design methods and strategies of heterogeneous photocatalysts available for precision production of polymers in open-air environments.

Reticular Ratchets for Directing Electrochemiluminescence.

Electrochemiluminescence (ECL) involves charge transfer between electrochemical redox intermediates to produce an excited state for light emission. Ensuring precise control of charge transfer is essential for decoding ECL fundamentals, yet guidelines on how to achieve this for conventional emitters remain unexplored. Molecular ratchets offer a potential solution, as they enable the directional transfer of energy or chemicals while impeding the reverse movement. Herein, we designed 10 pairs of imine-based covalent organic frameworks as reticular ratchets to delicately manipulate the intrareticular charge transfer for directing ECL transduction from electric and chemical energies. Aligning the donor and acceptor (D-A) directions with the imine dipole effectively facilitates charge migration, whereas reversing the D-A direction impedes it. Notably, the ratchet effect of charge transfer directionality intensified with increasing D-A contrast, resulting in a remarkable 680-fold improvement in the ECL efficiency. Furthermore, dipole-controlled exciton binding energy, electron/hole decay kinetics, and femtosecond transient absorption spectra identified the electron transfer tendency from the N-end toward the C-end of reticular ratchets during ECL transduction. An exponential correlation between the ECL efficiency and the dipole difference was discovered. Our work provides a general approach to manipulate charge transfer and design next-generation electrochemical devices.

Structural modulation of covalent organic frameworks based on redox active center of π-conjugated amine unit for enhanced electrochromic properties

The covalent organic frameworks (COFs) possess high designability, which provides a platform for synthesizing novel optoelectric materials, including electrochromic materials. In this work, N,N,N′,N′-tetra-(4-aminophenyl)-1,4-phenylenediamine (TPDA) with π-electron conjugate system and redox active centers was used as the main building block. The 4,4′-biphenyl diformaldehyde (BDA) and the 2,2′-thiophene-5,5′-diformaldehyde (TDA) were used as the aldehyde building units. Two imine-based COF electrochromic active materials (COFTPDA-BDA and COFTPDA-TDA) were prepared by Schiff base reaction. The test results reveal that COFTPDA-TDA has superior electrochromic properties than COFTPDA-BDA. Their contrasts are 0.46 and 0.40, and the coloring/fading times are 3.1/10.6 s and 10.9/14.3 s, respectively. The COFTPDA-TDA material possessing a bifunctional group structure was synthesized by replacing the biphenyl group with the bithiophene group with redox activity, which can enhance the optical properties and inter-layer π-π conjugation. Besides the electrochromic properties, the morphology and crystal lattice parameters are different. Therefore, it is expected to develop COFs-based electrochromic materials with rich colors, excellent performance, and wide application by introducing construction units with different redox active centers.

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

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

Fast detection of tryptamine in meat products with azide-functionalized covalent organic frameworks confined in molecularly imprinted polymers

Tryptamine is a biogenic amine that affects organoleptic quality through the generation of off-odours in foods. Herein, imine-based covalent organic frameworks (COFs) were synthesized via Schiff base reactions and postmodified with click chemistry to generate azide-functionalized COFs with tunable azide units on the walls. The combination of molecular imprinting with COFs enabled the specific recognition of the targets. The resulting optosensing system (azide-functionalized COFs@MIPs) was used as a sample-to-answer analyser for detecting tryptamine (detection time within 10 min). A linear relationship was observed for the fluorescence response to tryptamine concentrations in the range of 3–120 μg L−1, with a limit of detection of 1.74 μg L−1. The recoveries for spiked samples were satisfactory, with relative standard deviations <9.90%. The optosensing system is a potential tool for the quantitative detection of tryptamine in meat products because of its lower cost, shorter processing time, and simpler processing steps compared to conventional chromatographic techniques.

Pyridine-Based Covalent Organic Frameworks with Pyridyl-Imine Structures for Boosting Photocatalytic H2O2 Production via One-Step 2e- Oxygen Reduction.

Bipyridine-based covalent organic frameworks (COFs) have emerged as promising contenders for the photocatalytic generation of hydrogen peroxide (H2O2). However, the presence of imine nitrogen alters the mode of H2O2 generation from an efficient one-step two-electron (2e-) route to a two-step 2e- oxygen reduction pathway. In this work, we introduce 3,3'-bipyridine units into imine-based COF skeletons, creating a pyridyl-imine structure with two adjacent nitrogen atoms between the pyridine ring and imine linkage. This unique bipyridine-like architecture can effectively suppress the two-step 2e- ORR process at the single imine-nitrogen site, facilitating a more efficient one-step 2e- pathway. Consequently, the optimized pyridyl-imine COF (PyIm-COF) exhibits a remarkable H2O2 production rate of up to 5850 μmol h-1 g-1, nearly double that of pristine bipyridine COFs. This work provides valuable insight into the rational design of functionalized COFs for enhanced H2O2 production in photocatalysis.

Effect of Tautomerization on Water Desalination by Covalent Organic Frameworks

Covalent organic frameworks (COFs) have gained increasing interest as promising building blocks for desalination membranes due to their inherent porosity. Efforts have been made to prepare COF membranes and explore their transport mechanisms, however the effect of tautomerization has not been reported. Using non-equilibrium molecular dynamics, we discover here that a mere 0.5-Å atomic displacement between two tautomers of an imine-based COF (TpHZ) can surprisingly lead to dramatic changes in the desalination behavior of their multilayers. The tautomeric transformation (from TpHZ-NH to TpHZ-OH) improves the water permeability by about 1.6–3 times. The interior resistance rather than the entrance resistance is found to dominate the water transport. The weaker adhesion and the smoother channel path of TpHZ-OH result in a much lower interior resistance. Furthermore, NaCl rejection increases from ∼ 89 % to 100 % due to the stronger sieving effect resulting from the reduced compensatory effect of TpHZ-OH for ion hydrations.

Donor-Acceptor Interactions Enhanced Colorimetric Sensors for Both Acid and Base Vapor Based on Two-Dimensional Covalent Organic Frameworks.

Due to the high surface area and uniform porosity of covalent organic frameworks (COFs), they exhibit superior properties in capturing and detecting even trace amounts of gases in the air. However, the COFs materials that possess dual detected functionality are still less reported. Here, an imine-based COF containing thiophene as a donor and triazine as an acceptor to form spatial-distribution-defined D-A structures was prepared. D-A system between thiophene and triazine facilitates the charge transfer process during the protonation process of the imine and the triazine units. The obtained COF exhibits simultaneous sensing ability toward both acidic and alkaline vapors with obvious colorimetric sensing functionality.

Symmetric Variation of Monomers for Constructing 1D/2D Imine-Based Covalent Organic Frameworks

Symmetric Variation of Monomers for Constructing 1D/2D Imine-Based Covalent Organic Frameworks

Double-shelled, rattle-architecture covalent organic framework: harnessing morphological manipulation for enhanced synergistic multi-drug chemo-photothermal cancer therapy.

Morphological modulation in covalent organic frameworks (COFs) with particular emphasis on the correlation between structure and target applications in biomedical fields, is currently in its early stage of evolution. Herein, a multifunctional rattle-architecture imine-based COF with a mobile core of gold nanoparticles (Au NPs) and an outer polydopamine (PDA) shell, tailored for cancer treatment, has been developed to effectively integrate dual responsive release capabilities with the potential for multiple therapeutic applications. The engineered COF displays outstanding crystallinity, a suitable size and precisely controlled morphological characteristics. By leveraging COF and PDA attributes, the successful co-delivery of hydrophilic doxorubicin (DOX) and hydrophobic docetaxel (DTX) within discrete compartments is achieved responsive to both pH and near-infrared triggers. Designed nanocarrier outperforms prior COFs with a superior 83.7% DOX loading capacity, thanks to its expansive internal space and porous shell. Taking advantage of the inclusion of Au core and the concurrent presence of COF and PDA outer shells, the nanocarrier exhibits a significant photothermal-conversion capability. The rattle-architecture double-shelled Au@RCOF@PDA were functionalized with poly(ethylene glycol)-folic acid (PEG-FA) to confer the system with active-targeting capability and enhanced biocompatibility. Through in vitro and in vivo evaluations, the designed system demonstrates an exceptional synergistic anti-tumor effect, along with favorable biosafety and histocompatibility. This study not only sheds light on the remarkable merits offered by regulating the morphology of COF-based systems in cancer therapy but also highlights the potential for synergistic therapeutic approaches in advancing cancer treatment strategies.

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

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

Comparative Study on Proton Conductivity and Mechanism Analysis of Two Imidazole Modified Imine-Based Covalent Organic Frameworks.

This work elucidates the potential impact of intramolecular H-bonds within the pore walls of covalent organic frameworks (COFs) on proton conductivity. Employing DaTta and TaTta as representative hosts, it was observed that their innate proton conductivities (σ) are both unsatisfactory and σ(DaTta)<σ(TaTta). Intriguingly, the performance of both imidazole-loaded products, Im@DaTta and Im@TaTta is greatly improved, and the σ of Im@DaTta (0.91×10-2 S cm-1 ) even surpasses that of Im@TaTta (3.73×10-3 S cm-1 ) under 100 °C and 98 % relative humidity. The structural analysis, gas adsorption tests, and activation energy calculations forecast the influence of imidazole on the H-bonded system within the framework, leading to observed changes in proton conductivity. It is hypothesized that intramolecular H-bonds within the COF framework impede efficient proton transmission. Nevertheless, the inclusion of an imidazole group disrupts these intramolecular bonds, leading to the formation of an abundance of intermolecular H-bonds within the pore channels, thus contributing to a dramatic increase in proton conductivity. The related calculation of Density Functional Theory (DFT) provides further evidence for this inference.

Combinatorial Synthesis of Covalent Organic Framework Particles with Hierarchical Pores and Their Catalytic Application.

Precise tailoring of the aggregation state of covalent organic frameworks (COFs) to form a hierarchical porous structure is critical to their performance and applications. Here, we report a one-pot and one-step strategy of using dynamic combinatorial chemistry to construct imine-based hollow COFs containing meso- and macropores. It relies on a direct copolymerization of three or more monomers in the presence of two monofunctional competitors. The resulting particle products possess high crystallinity and hierarchical pores, including micropores around 0.93 nm, mesopores widely distributed in the range of 3.1-32 nm, and macropores at about 500 nm, while the specific surface area could be up to 748 m2·g-1, with non-micropores accounting for 60% of the specific surface area. The particles demonstrate unique advantages in the application as nanocarriers for in situ loading of Pd catalysts at 93.8% loading efficiency in the copolymerization of ethylene and carbon monoxide. The growth and assembly of the copolymer could thus be regulated to form flower-shaped particles, efficiently suppressing the fouling of the reactor. The copolymer's weight-average molecular weight and the melting temperature are also highly improved. Our method provides a facile way of fabricating COFs with hierarchical pores for advanced applications in catalysis.

Site-Selective Synthesis and Concurrent Immobilization of Imine-Based Covalent Organic Frameworks on Electrodes Using an Electrogenerated Acid.

Imine-based covalent organic frameworks (COFs) are crystalline porous materials with prospective uses in various devices. However, general bulk synthetic methods usually produce COFs as powders that are insoluble in most of the common organic solvents, arising challenges for the subsequent molding and fixing of these materials on substrates.Here, we report a novel synthetic methodology that utilizes an electrogenerated acid (EGA), which is produced at an electrode surface by electrochemical oxidation of a suitable precursor, acting as an effective Brønsted acid catalyst for imine bond formation from the corresponding amine and aldehyde monomers. Simultaneously, it provides the corresponding COF film deposited on the electrode surface. The COF structures obtained with this method exhibited high crystallinities and porosities, and the film thickness could be controlled. Furthermore, such process was applied for the synthesis of various imine-based COFs, including a three-dimensional (3D) COF structure.

Cycloadditions and Cyclization Reactions via Post-Synthetic Modification and/or One-Pot Methodologies for the Stabilization of Imine-Based Covalent Organic Frameworks

Interest in covalent organic frameworks as high-value materials has grown steadily since their development in the 2000s. However, the great advantage that allows us to obtain these crystalline materials—the reversibility of the bonds that form the network—supposes a drawback in terms of thermal and chemical stability. Among the different strategies employed for the stabilization of imine-based Covalent Organic Frameworks (COFs), cycloaddition and other related cyclization reactions are especially significant to obtain highly stable networks with extended π-delocalization and new functionalities, expanding even further the potential application of these materials. Therefore, this entry gathered the most recent research strategies for obtaining stable COFs by means of cyclization reactions, including the Povarov reaction and intramolecular oxidative cyclization reactions as well as some other recent innovative approaches.

Incorporating imine-based covalent organic frameworks nanosheet as an active filler for long-term nanofiltration desalination

Covalent organic frameworks nanosheets (CONs) are two-dimensional porous nanomaterials exfoliated from covalent organic frameworks (COFs). The ordered permanent microchannels and excellent water stability of CONs make them attractive active fillers for the preparation of thin-film nanocomposite (TFN) nanofiltration (NF) membranes. Herein, we report the fabrication of TFN membranes by incorporating covalent organic framework nanosheets TpPa-1 CONs as an active filler in the organic phase during the interfacial polymerization (IP) reaction. The addition of TpPa-1 CONs promotes the formation of extra water channels (pores and interfacial cavities) in the PA layer, rendering the TFN membranes with a higher pure water permeance of 8.54 L m−2 h−1 bar−1, which is about 1.4 times compared to TFC membrane, and excellent Na2SO4 rejection of about 98.1%. Owing to the hydrophilic nature of TpPa-1 CONs, the synthesized TFN membranes also exhibited higher hydrophilicity and superior anti-fouling performance. Meanwhile, the 35-days stability experiment verified that the TpPa-1 CONs TFN membrane has superior long-term stability. As a 2D porous nanosheets, TpPa-1 CONs provided some guidance for the preparation of long-term stability NF membranes with excellent separation performance.

Substituted benzophenone imines for COF synthesis via formal transimination.

Covalent organic frameworks (COFs) are a prominent class of organic materials constructed from versatile building blocks via reversible reactions. The quality of imine-linked COFs can be improved by using amine monomers protected with benzophenone forming benzophenone imines. Here, we present a study on substituted benzophenones in COF synthesis via formal transimination. 12 para-substituted N-aryl benzophenone imines, with a range of electron-rich to electron-poor substituents, were prepared and their hydrolysis kinetics were studied spectroscopically. All substituted benzophenone imines can be employed in COF synthesis and lead to COFs with high crystallinity and high porosity. The substituents act innocent to COF formation as the substituted benzophenones are cleaved off. Imines can be tailored to their synthetic demands and utilized in COF formation. This concept can make access to previously unattainable, synthetically complex COF monomers feasible.

De novo synthesis of enzyme-embedded covalent organic frameworks (COFs) using deep eutectic solvent: Pushing the COF limits

Covalent organic frameworks (COFs), highly stable and porous crystalline materials, are an attractive host for embedding enzymes to protect them from harsh ex vivo conditions. However, because COF pore size must match enzyme size, larger enzyme molecules cannot be absorbed into COF matrices via COF pores. Furthermore, the COF synthesis using high temperatures, acid catalysts, and organic solvents is ineffective for the de novo construction of enzyme-embedded COFs. Herein, enzyme embedding within imine-based COFs regardless of pore dimensions is presented by pre-functionalizing the enzyme with COF aldehyde precursor and facilitating “armor-like” COF exoskeleton formation from the enzyme surface via condensation of COF aldehyde and amine precursors at ambient conditions using choline chloride/urea deep eutectic solvent as both solvent and catalyst. This de novo approach affords glucose oxidase (GOx) embedding within two kinds of COFs with pores smaller than GOx size and activity recovery of 55–62% is achieved, whereas GOx embedding into pre-synthesized COFs was not possible. According to time-dependent TEM analysis, GOx@COF morphology is dependent on enzyme and COF precursor geometry which was governed by reversible Schiff-base and Ostwald ripening. Enzyme confinement within the COF architecture, as well as chemical crosslinking of the enzyme surface with the COF exoskeleton, endows GOx@COF with excellent pH and thermo-stability, tolerance to denaturation by urea, trypsin, and organic solvent, and stable bactericidal activity over 5 recycles. The morphological structure of GOx@COF and extent of GOx cross-linking with COF precursor influence its kinetic performance. This work could open a new avenue for utilizing COFs to create a suite of stable enzyme-COF composites for various applications.