Imine-linked Covalent Organic Frameworks Research Articles

Rational Conversion of Imine Linkages to Amide Linkages in Covalent Organic Frameworks for Photocatalytic Oxidation with Enhanced Photostability.

Covalent organic frameworks (COFs) and their applications in photocatalysis have been extensively studied, but the instability of imine-linked COFs is an important factor limiting their performance. In this work, two imine-linked COFs were successfully converted to amide-linked COFs through post synthetic modification (PSM). The oxidized COFs presented lower binding energy to O2, exhibited higher photocatalytic activity for oxidation of thioethers and coupling of benzylamines with excellent stability. The present work can serve as a reliable reference for the development of novel highly active and stable COF-based photocatalysts.

Robust Amide-Linked Fluorinated Covalent Organic Framework for Long-Term Oxygen Reduction Reaction Electrocatalysis.

The high energy demand of the evolving world opens the door to develop more sustainable and environmentally friendly energy sources. Oxygen reduction reaction (ORR) is a promising candidate, being the 2e- pathway of great interest for the green production of hydrogen peroxide. Metal-free covalent organic frameworks (COFs) electrocatalysts present a suitable alternative to substitute the noble-metals more commonly employed in this application. However, the lability of the linkages building up the framework raises an issue for their long-term use and application in aggressive media. Herein, a stable amide-linked COF is reported through post-synthetic modification of a previously reported imine-linked COF proven to be effective as an electrocatalyst, enhancing its chemical stability and electrochemical response. It is found that after the linkage transformation, the new electrocatalyst displays a higher selectivity toward the H2O2 production (98.5%) and an enhanced turnover frequency of 0.155s-1, which is among the bests reported to date for metal-free and COF based electrocatalysts. The results represent a promising step forward for metal-free non pyrolyzed electrocatalysts, improving their properties through post-synthetic linkage modification for long-term operation.

Linkage Engineering in Covalent Organic Frameworks for Metal-Free Electrocatalytic C2H4 Production from CO2.

Electrocatalytic carbon dioxide reduction reaction (CO2RR) to produce ethylene (C2H4) is conducive to sustainable development of energy and environment. At present, most electrocatalysts for C2H4 production are limited to the heavy metal copper, meanwhile, achieving metal-free catalysis remains a challenge. Noted piperazine with sp3 N hybridization is beneficial to CO2 capture, but CO2RR performance and mechanism have been lacking. Herein, based on linkage engineering, we construct a novel high-density sp3 N catalytic array via introducing piperazine into the crystalline and microporous aminal-linked covalent organic frameworks (COFs). Thanks to its high sp3 N density, strong CO2 capture capacity and great hydrophilicity, aminal-linked COF successfully achieves the conversion of CO2 to C2H4 with a Faraday efficiency up to 19.1 %, which is stand out in all reported metal-free COF electrocatalysts. In addition, a series of imine-linked COFs are synthesized and combined with DFT calculations to demonstrate the critical role of sp3 N in enhancing the kinetics of CO2RR. Therefore, this work reveals the extraordinary potential of linkage engineering in COFs to break through some catalytic bottlenecks.

Protonation of an Imine-linked Covalent Organic Framework for Efficient H2O2 Photosynthesis under Visible Light up to 700 nm.

Covalent organic frameworks (COFs) are promising photocatalysts for H2O2 production from water via oxygen reduction reactions (ORR). The design of COFs for efficient H2O2 production indubitably hinges on an in-depth understanding of their ORR mechanisms. In this work, taking an imine-linked COF as an example, we demonstrate that protonation of the functional units such as imine, amine, and triazine, is a highly efficient strategy to upgrade the activity levels for H2O2 synthesis. The protonation not only extends the light absorption of the COF but also provides proton sources that directly participate in H2O2 generation. Notably, the protonation simplifies the reaction pathways of ORR to H2O2, i.e. from an indirect superoxide radical ([[EQUATION]]) mediated route to a direct one-step two-electron route. Theoretical calculations confirm that the protonation favors H2O2 synthesis due to easy access of protons near the reaction sites that removes the energy barrier for generating *OOH intermediate. These findings not only extend the mechanistic insight into H2O2 photosynthesis but also provide a rational guideline for the design and upgradation of efficient COFs.

Programming Tetrathiafulvalene-Based Covalent Organic Frameworks for Promoted Photoinduced Molecular Oxygen Activation.

Despite the pivotal role of molecular oxygen (O2) activation in artificial photosynthesis, the activation efficiency is often restricted by sluggish exciton dissociation and charge transfer kinetics within polymer photocatalysts. Herein, we propose two tetrathiafulvalene (TTF)-based imine-linked covalent organic frameworks (COFs) with tailored donor-acceptor (D-A) structures, TTF-PDI-COF and TTF-TFPP-COF, to promote O2 activation. Because of enhanced electron push-pull interactions that facilitated charge separation and transfer behavior, TTF-PDI-COF exhibited superior photocatalytic activity in electron-induced O2 activation reactions over TTF-TFPP-COF under visible light irradiation, including the photosynthesis of (E)-3-amino-2-thiocyano-α,β-unsaturated compounds and H2O2. These findings highlight the significant potential of the rational design of COFs with D-A configurations as suitable candidates for advanced photocatalytic applications.

Thiazole-linked isomeric covalent organic frameworks for divergent photocatalysis: Selective oxidation of organic sulfides

The photocatalysis of covalent organic frameworks (COFs) is underpinned by their inherent structures in which the linkage plays a vital role in determining the activity. The transformation of imine into thiazole leads to isomeric structures owing to the different orientations of linkages. Here, two thiazole-linked COFs, COF-TZ-1 and COF-TZ-2, are constructed from two imine-linked COFs, COF-IM-1 and COF-IM-2, with the imine N next to the linker triphenylbenzene and triphenyltriazine, respectively. Despite better absorption, COF-TZ-1 possesses inferior optoelectronic properties to COF-TZ-2. Besides, the dipole moment of COF-TZ-2 is twice that of COF-TZ-1. COF-TZ-2 is much better than COF-TZ-1 for blue light photocatalytic selective oxidation of organic sulfides with O2 under blue light irradiation. Furthermore, the facile adsorption sites of O2 are verified by density functional theory calculations, and the generation of superoxide is tracked over COF-TZ-2. This work highlights that subtle variations of COFs can lead to divergent photocatalysis.

Carbazole-Based Imine-Linked Covalent Organic Framework for Efficient Heterogeneous Photocatalysis

Carbazole-Based Imine-Linked Covalent Organic Framework for Efficient Heterogeneous Photocatalysis

A new nitrogen-rich imine-linked neutral covalent organic framework: Synthesis and high-efficient adsorption of organic dyes

Nowadays, water pollution caused by organic dyes has received increasing attentions due to serious harm to public health and ecosystem, and it is of great significance to design new porous materials with high adsorption capacity to remove organic dye pollutants from wastewater. Herein, a new two-dimensional N-rich imine-linked covalent organic framework (termed as BPA-TAPA COF) has been constructed through Schiff-base reaction between 2, 2′-bipyridine-5,5′-dicarboxaldehyde (BPA) and tri(4-aminophenyl)amine (TAPA) under solvothermal conditions, and showed good crystallinity, high thermal stability and certain porosity. Then, it was used as a functional adsorbent for removal of organic dyes, which exhibited good adsorption performances for both cationic and anionic dyes, namely malachite green (MG), Janus green B (JGB), Congo red (CR), and acid orange (AO). Significantly, the adsorption capacities of BPA-TAPA COF for MG and CR reached up to 1070 and 2016 mg·g−1, in which the latter ranked as the highest in comparison to those of the other documented functional COFs. The removal efficiency of BPA-TAPA COF toward CR (100 ppm) in aqueous solution reached to 99.0% within 180 min. Hydrogen bonds and π-π stacking attractions between COF and dye molecules played crucial roles in adsorption of organic dyes. This work uncovers that neutral COFs with nitrogen-rich functional sites can be used as new adsorbents for removal of both cationic and anionic organic dyes and might provide new information for developing effective adsorbents with high adsorption capacity.

Robust links in photoactive covalent organic frameworks enable effective photocatalytic reactions under harsh conditions

Developing heterogeneous photocatalysts for the applications in harsh conditions is of high importance but challenging. Herein, by converting the imine linkages into quinoline groups of triphenylamine incorporated covalent organic frameworks (COFs), two photosensitive COFs, namely TFPA-TAPT-COF-Q and TFPA-TPB-COF-Q, are successfully constructed. The obtained quinoline-linked COFs display improved stability and photocatalytic activity, making them suitable photocatalysts for photocatalytic reactions under harsh conditions, as verified by the recyclable photocatalytic reactions of organic acid involving oxidative decarboxylation and organic base involving benzylamine coupling. Under strong oxidative condition, the quinoline-linked COFs show a high efficiency up to 11831.6 μmol·g−1·h−1 and a long-term recyclable usability for photocatalytic production of H2O2, while the pristine imine-linked COFs are less catalytically active and easily decomposed in these harsh conditions. The results demonstrate that enhancing the linkage robustness of photoactive COFs is a promising strategy to construct heterogeneous catalysts for photocatalytic reactions under harsh conditions.

Strategic design of a rare trigonal symmetric luminescent covalent organic framework by linker modification

We designed a trigonal symmetric imine-linked covalent organic framework (COF), TFPC-DAB, with control over the angularity of the building units, where a bent C2-symmetric diamine, such as 1,3-diaminobenzene (1,3-DAB or DAB), with an exo-angle of 120° was used instead of those with an exo-angle of 180°, in combination with a C3-symmetric trialdehyde, such as tri(4-formylphenoxy)cyanurate (TFPC). Its synthesis was accomplished by reacting the building units in a mixture of mesitylene/dioxane/6 M acetic acid under solvothermal conditions. The phase purity, thermal stability, and porosity of TFPC-DAB were established by various analytical techniques. Utilizing the Density Functional Tight Binding (DFTB+) simulation and Pawley refinement, the best fit of the small angle x-ray pattern was found to have an AA stacking of TFPC-DAB in the trigonal space group P3 with low refinement parameters. Such smart materials are in huge demand to detect hazardous corrosive chemicals, such as HCl and NH3. The dual features of electron deficient π-acidic triazine moiety and heteroatoms (N/O) from TFPC and electron rich phenyl units from DAB embodied in the framework enhance its luminescent property and thereby make it suitable for solvent-based HCl and NH3 sensing. The detection limits for HCl and NH3 in methanol were found to be 14 and 82 ppb, respectively. The effect of solvent polarity on the sensing studies was observed with much lower detection limits in dioxane: 2.5 and 11 ppb for HCl and NH3, respectively. A detailed theoretical calculation using density functional theory and configurational bias Monte Carlo modules was conducted for understanding interactions between the COF and HCl or NH3 analytes.

Construction of Layer-Blocked Covalent Organic Framework Heterogenous Films via Surface-Initiated Polycondensations with Strongly Enhanced Photocatalytic Properties.

Imine-linked covalent organic frameworks (COFs) usually show high crystallinity and porosity, while vinyl-linked COFs have excellent semiconducting properties and stability. Therefore, achieving the advantages of imine- and vinyl-linkages in a single COF material is highly interesting but remains challenging. Herein, we demonstrate the fabrication of a layer-blocked COF (LB-COF) heterogeneous film that is composed of imine- and vinyl-linkages through two successive surface-initiated polycondensations. In brief, the bottom layer of imine-linked COF film was constructed on an amino-functionalized substrate via Schiff-base polycondensation, in which the unreacted aldehyde edges could be utilized for initiating aldol polycondensation to prepare the second layer of vinyl-linked COF film. The resultant LB-COF film displays excellent ordering due to the crystalline templating effect from the bottom imine-linked COF layer; meanwhile, the upper vinyl-linked COF layer could strongly enhance its stability and photocatalytic properties. The LB COF also presents superior performance in photocatalytic uranium extraction (320 mg g-1), which is higher than the imine-linked (35 mg g-1) and the vinyl-linked (295 mg g-1) counterpart. This study provides a novel surface-initiated strategy to synthesize layer-blocked COF heterogeneous films that combine the advantages of each building block.

Light-driven H2O2 production over redox-active imine-linked covalent organic frameworks

Imine-linked covalent organic frameworks (COFs) provide distinctive prospects for promoting photocatalytic H2O2 production, yet the intricate involvement of imine-derived linkages chemistry under light irradiation remains incompletely elucidated. Here, imine-linked COF-LZU1 demonstrates a visible light-driven H2O2 evolution rate of 387 ​μmol ​g−1 ​h−1, particularly, negligible dependence on the sacrificial agents. By virtue of electron paramagnetic spectroscopy and solid-state nuclear magnetic resonance, we experimentally tracked the structure evolution and identified its autooxidation to Wurster's salt mimics under the photocatalytic conditioning. This finding coincides with the radical anion ·O2− governed reaction pathway and is further rationalized with additional theoretical calculations. This study provides insights into both photophysical/photochemical aspects over the imine-linkage structure.

A facile creation of hydrophilic-lipophilic-balanced covalent organic frameworks with mixed-mode retention mechanisms for multiple applications in liquid chromatography

Currently, high-performance liquid chromatographic separation materials that can provide multiple retention mechanisms are highly favored. Covalent organic frameworks (COFs) have undoubtedly demonstrated promising application prospects in chromatographic separation on account of excellent stability and porosity. Nevertheless, the utilization of COFs as liquid chromatographic fillers is still in the early stages of development. Especially, developing powerful COF@SiO2 with versatile separation capabilities still poses certain difficulties. Herein, a hydrophilic-lipophilic-balanced imine-linked COF was synthesized and in-situ growth on the amino-modified silica microsphere to obtain a multifunctional TAPT-TP-COF@SiO2 composite material. The synergistic effect of multiple functional groups including hydroxyl, amino, aldehyde, triazine and aromatic ring in the TAPT-TP-COF provided multiple retention mechanisms. The selection of 2,4,6-triformylphloroglucinol (TP) as the aldehyde counterpart improved the hydrophilic effect of the developed COF-based separation material, which enabled the effective separation of hydrophilic analytes (nucleosides, sulfonamides and vitamins). The novel TAPT-TP-COF@SiO2 column displayed typical reverse-phase and hydrophilic interaction chromatographic retention behavior. The successful applications demonstrated that TAPT-TP-COF@SiO2 had enormous potential for efficient liquid chromatographic analysis.

Imine-linked covalent organic framework with high crystallinity for constructing sensitive purine bases electrochemical sensor

In this work, a covalent organic framework (TADM-COF) with high crystallinity and large specific surface area (2597 m2 g−1) has been successfully synthesized using 1,3,5-(4-aminophenyl) benzene (TAPB) and 2,5-dimethoxy-p-phenyldiformaldehyde (DMTP). The COF was grown in situ on oxide particles to form core–shell nanocomposites (SiO2@TADM COF, Fe3O4@TADM COF and Co3O4@TADM COF) to realize its function as a shell material. Among them, the Co3O4@TADM COF with the highest electrochemical response to purine bases was further cross-linked with multi-walled carbon nanotubes (MWCNT) to construct a novel electrochemical sensor (Co3O4@TADM COF/MWCNT/GCE) for detection of purine bases. In this nanocomposite, Co3O4 possesses rich catalytic active sites, MWCNT ensures superior electrical conductivity and COF provides a stable environment for electrocatalytic reactions as the shell. At the same time, regular pore structure of the COFs also offers smooth channels for the transfer of analytes to the catalytic site. The synergistic effect among the three components showed remarkable sensing performance for the simultaneous detection of guanine (G) and adenine (A) with a wide linear range of 0.6–180 μM and low limits of detection (LODs) of 0.020 μM for G and 0.024 μM for A (S/N = 3), respectively. The developed sensor platform was also successfully applied in the detection of purine bases in thermally denatured herring DNA extract. The work provided a general strategy for amplifying signal of COF and its composite in the electrochemical sensing.

Ambient Aqueous Synthesis of Imine-Linked Covalent Organic Frameworks (COFs) and Fabrication of Freestanding Cellulose Nanofiber@COF Nanopapers.

Covalent organic frameworks (COFs) are usually synthesized under solvothermal conditions that require the use of toxic organic solvents, high reaction temperatures, and complicated procedures. Additionally, their insolubility and infusibility present substantial challenges in the processing of COFs. Herein, we report a facile, green approach for the synthesis of imine-linked COFs in an aqueous solution at room temperature. The key behind the synthesis is the regulation of the reaction rate. The preactivation of aldehyde monomers using acetic acid significantly enhances their reactivity in aqueous solutions. Meanwhile, the still somewhat lower imine formation rate and higher imine breaking rates in aqueous solution, in contrast to conventional solvothermal synthesis, allow for the modulation of the reaction equilibrium and the crystallization of the products. As a result, highly crystalline COFs with large surface areas can be formed in relatively high yields in a few minutes. In total, 16 COFs are successfully synthesized from monomers with different molecular sizes, geometries, pendant groups, and core structures, demonstrating the versatility of this approach. Notably, this method works well on the gram scale synthesis of COFs. Furthermore, the aqueous synthesis facilitates the interfacial growth of COF nanolayers on the surface of cellulose nanofibers (CNFs). The resulting CNF@COF hybrid nanofibers can be easily processed into freestanding nanopapers, demonstrating high efficiency in removing trace amounts of antibiotics from wastewater. This study provides a route to the green synthesis and processing of various COFs, paving the way for practical applications in diverse fields.

Ratiometric Fluorescent Detection of Chromium(III) Based on One-Dimensional Imine-Linked Covalent Organic Framework.

Chromium (Cr3+) as a highly toxic pollutant has aroused much attention due to its wide industrial applications. Covalent organic frameworks (COFs) have been considered as one of the most promising metal ion sensors due to their open pore channels and abundant adsorption sites. Herein, a novel luminescent one-dimensional COF (Py-An COF) was constructed by the condensation of 5'-(anthracen-9-yl)-[1,1':3',1″-terphenyl]-4,4″-dicarbaldehyde (An-2CHO) and 1,3,6,8-tetrakis(p-aminophenyl) pyrene (PyTTA). The resulting COF showed high crystallinity, comparative high surface area, and good thermal stability, which can be utilized for Cr3+ fluorescent sensors with high sensitivity and selectivity. Furthermore, the coordination between Schiff-base N atoms and Cr3+ inhibits the photoinduced electron transfer (PET) process, resulting in the enhanced fluorescence intensity of chromophores (436 nm) and decreased the fluorescence intensity of Py-An COF (512 nm) simultaneously, which eventually realizes a highly efficient ratiometric fluorescent sensor for trace Cr3+. Compared with other porous fluorescent materials, Py-An COF possesses a lower detection limit, higher sensitivity, and better selectivity. This work provides strategic guidance for the design of COFs as ratiometric fluorescence sensors.

Cu2+ coordination-induced in situ photo-to-heat on catalytic sites to hydrolyze β-lactam antibiotics pollutants in waters.

For degradation of β-lactam antibiotics pollution in waters, the strained β-lactam ring is the most toxic and resistant moiety to biodegrade and redox-chemically treat among their functional groups. Hydrolytically opening β-lactam ring with Lewis acid catalysts has long been recognized as a shortcut, but at room temperature, such hydrolysis is too slow to be deployed. Here, we found when Cu2+ was immobilized on imine-linked COF (covalent organic framework) (Cu2+/Py-Bpy-COF, Cu2+ load is 1.43 wt%), as-prepared composite can utilize the light irradiation (wavelength range simulated sunlight) to in situ heat anchored Cu2+ Lewis acid sites through an excellent photothermal conversion to open the β-lactam ring followed by a desired full-decarboxylation of hydrolysates. Under 1 W/cm2 simulated sunlight, Cu2+/Py-Bpy-COF powders placed in a microfiltration membrane rapidly cause a temperature rising even to ~211.7 °C in 1 min. It can effectively hydrolyze common β-lactam antibiotics in waters and even antibiotics concentration is as high as 1 mM and it takes less than 10 min. Such photo-heating hydrolysis rate is ~24 times as high as under dark and ~2 times as high as Cu2+ homogenous catalysis. Our strategy significantly decreases the interference from generally coexisting common organics in waters and potential toxicity concerns of residual carboxyl groups in hydrolysates and opens up an accessible way for the settlement of β-lactam antibiotics pollutants by the only energy source available, the sunlight.

Sculpting Mesoscopic Helical Chirality into Covalent Organic Framework Nanotubes from Entirely Achiral Building Blocks.

The diversification of chirality in covalent organic frameworks (COFs) holds immense promise for expanding their properties and functionality. Herein, we introduce an innovative approach for imparting helical chirality to COFs and fabricating a family of chiral COF nanotubes with mesoscopic helicity from entirely achiral building blocks for the first time. We present an effective 2,3-diaminopyridine-mediated supramolecular templating method, which facilitates the prefabrication of helical imine-linked polymer nanotubes using unprecedented achiral symmetric monomers. Through meticulous optimization of crystallization conditions, these helical polymer nanotubes are adeptly converted into imine-linked COF nanotubes boasting impressive surface areas, while well preserving their helical morphology and chiroptical properties. Furthermore, these helical imine-linked polymers or COFs could be subtly transformed into corresponding more stable and functional helical β-ketoenamine-linked and hydrazone-linked COF nanotubes with transferred circular dichroism via monomer exchange. Notably, despite the involvement of covalent bonding breakage and reorganization, these exchange processes overcome thermodynamic disadvantages, allowing mesoscopic helical chirality to be perfectly preserved. This research highlights the potential of mesoscopic helicity in conferring COFs with favourable chiral properties, providing novel insights into the development of multifunctional COFs in the field of chiral materials chemistry.

Fully Conjugated 2D sp2 Carbon-Linked Covalent Organic Frameworks for Photocatalytic Overall Water Splitting.

Covalent organic frameworks (COFs) hold great promise for solar-driven hydrogen production. However, metal-free COFs for photocatalytic overall water splitting remain elusive, primarily due to challenges in simultaneously regulating their band structures and catalytic sites to enable concurrent half-reactions. Herein, two types of π-conjugated COFs containing the same donor-acceptor structure are constructed via Knoevenagel condensation and Schiff base reaction to afford cyanovinylene- and imine-bridged COFs, respectively. The difference in the linkage leads to a remarkable difference in their photocatalytic activity toward water splitting. The 2D sp2 carbon-linked COF exhibits notable activity for photocatalytic overall water splitting, which can reach an apparent quantum efficiency of 2.53% at 420nm. In contrast, the 2D imine-linked COF cannot catalyze the overall water-splitting reaction. Mechanistic investigations reveal that the cyanovinylene linkage is essential in modulating the band structure and promoting charge separation in COFs, thereby enabling overall water splitting. Moreover, it is further shown that crystallinity substantially impacts the photocatalytic performance of COFs. This study represents the first successful example of developing metal-free COFs with high crystallinity for photocatalytic overall water splitting.

Nanostructurally Engineering Covalent Organic Frameworks for Boosting CO2 Photoreduction.

Herein, a series of imine-linked covalent organic frameworks (COFs) are developed with advanced ordered mesoporous hollow spherical nanomorphology and ultra-large mesopores (4.6nm in size), named OMHS-COF-M (M = H, Co, and Ni). The ordered mesoporous hollow spherical nanomorphology is revealed to be formed via an Ostwald ripening mechanism based on a one-step self-templated strategy. Encouraged by its unique structural features and outstanding photoelectrical property, the OMHS-COF-Co material is applied as the photocatalyst for CO2-to-CO reduction. Remarkably, it delivers an impressive CO production rate as high as 15 874µmol g-1 h-1, a large selectivity of 92.4%, and a preeminent cycling stability. From in/ex situ experiments and density functional theory (DFT) calculations, the excellent CO2 photoreduction performance is ascribed to the desirable cooperation of unique ordered mesoporous hollow spherical host and abundant isolated Co active sites, enhancing CO2 activation, and improving electron transfer kinetics as well as reducing the energy barriers for intermediates *COOH generation and CO desorption.