Covalent Organic Frameworks Materials Research Articles

Multi-Functional 2D Covalent Organic Frameworks with Diketopyrrolopyrrole as Electron Acceptor.

2D covalent organic framework (COF) materials with extended conjugated structure and periodic columnar π-arrays exhibit promising applications in organic optoelectronics. However, there is a scarcity of reports on optoelectronic COFs, mainly due to the lack of suitable π-skeletons. Here, two multi-functional optoelectronic 2D COFs DPP-TPP-COF and DPP-TBB-COF are constructed with diketopyrrolopyrrole as electron acceptor (A), and 1,3,6,8-tetraphenylpyrene and 1,3,5-triphenylbenzene as electron donor (D) through imine bonds. Both 2D COFs showed good crystallinities and AA stacking with a rhombic framework for DPP-TPP-COF and hexagonal one for DPP-TBB-COF, respectively. The electron D-A and ordered intermolecular packing structures endow the COFs with broad UV-vis absorptions and narrow bandgaps along with suitable HOMO/LUMO energy levels, resulting in multi-functional optoelectronic properties, including photothermal conversion, supercapacitor property, and ambipolar semiconducting behavior. Among them, DPP-TPP-COF exhibits a high photothermal conversion efficiency of 47% under 660nm laser irradiation, while DPP-TBB-COF exhibits superior specific capacitance of 384Fg-1. Moreover, P-type doping and N-type doping are achieved by iodine and tetrakis(dimethylamino)ethylene on a single host COF, resulting in ambipolar semiconducting behavior. These results provide a paradigm for the application of multi-functional optoelectronic COF materials.

Combining computational screening and machine learning to explore MOFs and COFs for methane purification

Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) have great potential to be used as porous adsorbents and membranes to achieve high-performance methane purification. Although the continuous increase in the number and diversity of MOFs and COFs is a great opportunity for the discovery of novel adsorbents and membranes with superior performances, evaluating such a vast number of materials in the quickest and most effective manner requires the development of computational approaches. High-throughput computational screening based on molecular simulations has been extensively used to identify the most promising MOFs and COFs for methane purification. However, the enormous and ever-growing material space necessitates more efficient approaches in terms of time and effort. Combining data science with molecular simulations has recently accelerated the discovery of optimal MOF and COF materials for methane purification and revealed the hidden structure–performance relationships. In this perspective, we highlighted the recent developments in combining high-throughput molecular simulations and machine learning to accurately identify the most promising MOF and COF adsorbents and membranes among thousands of candidates for separating methane from other gases including acetylene, carbon dioxide, helium, hydrogen, and nitrogen. After providing a brief overview of the topic, we reviewed the pioneering contributions in the field and discussed the current opportunities and challenges that we need to direct our efforts for the design and discovery of adsorbent and membrane materials.

Developments of photo‐/electro‐catalysis based on covalent organic frameworks: A review

AbstractPhoto‐/electro‐catalysis has the characteristics of low cost, high performance, and zero pollution, which meet the policies on environment and energy. Covalent organic frameworks (COFs), a type of crystalline organic skeleton polymers, have been widely applied and investigated in the area of photo‐/electro‐catalysis owing to their advantages of large specific surface area, regular pore size, excellent stability, flexible structural design, and massive active sites. This article reviews the structural characteristics of COFs and the strategies for strengthening the photo‐/electro‐catalytic activity of COF materials. Subsequently, deep insights were put into the photo‐/electro‐catalysis application of COF materials. In the end, the development prospects and challenges faced by COF materials in photo‐/electro‐catalysis are discussed.

Pyridine‐Bridged Covalent Organic Frameworks with Adjustable Band Gaps as Intelligent Artificial Enzymes for Light‐Augmented Biocatalytic Sensing

AbstractOne of the biggest challenges in biotechnology and medical diagnostics is finding extremely sensitive and adaptable biosensors. Since metal‐based enzyme‐mimetic biocatalysts may lead to biosafety concerns on accumulative toxicity, it is essential to synthesize metal‐free enzyme‐mimics with optimal biocatalytic activity and superior selectivity. Here, the pyridine‐bridged covalent organic frameworks (COFs) with specific oxidase‐like (OXD‐like) activities as intelligent artificial enzymes for light‐augmented biocatalytic sensing of biomarkers are disclosed. Because of the adjustable bandgaps of pyridine structures on the photocatalytic properties of the pristine COF structures, the pyridine‐bridged COF exhibit efficient, selective, and light‐responsive OXD‐like biocatalytic activity. Moreover, the pyridine‐bridged COF structures show tunable and light‐augmented biocatalytic detection capabilities, which outperform the recently reported state‐of‐the‐art OXD‐mimics regarding biosensing efficiency. Notably, the pyridine‐bridged COF exhibits efficient and multifaceted diagnostic activity, including the extremely low limit of detection (LOD), which enables visual assays for abundant reducibility biomarkers. It is believed that this design will offer unique metal‐free biocatalysts for high‐sensitive and low‐cost colorimetric detection and also provide new insights to create highly efficient enzyme‐like COF materials via linkage‐modulation strategies for future biocatalytic applications.

Bioinspired Water Preorganization in Confined Space for Efficient Water Oxidation Catalysis in Metallosupramolecular Ruthenium Architectures.

ConspectusNature has established a sustainable way to maintain aerobic life on earth by inventing one of the most sophisticated biological processes, namely, natural photosynthesis, which delivers us with organic matter and molecular oxygen derived from the two abundant resources sunlight and water. The thermodynamically demanding photosynthetic water splitting is catalyzed by the oxygen-evolving complex in photosystem II (OEC-PSII), which comprises a distorted tetramanganese-calcium cluster (CaMn4O5) as catalytic core. As an ubiquitous concept for fine-tuning and regulating the reactivity of the active site of metalloenzymes, the surrounding protein domain creates a sophisticated environment that promotes substrate preorganization through secondary, noncovalent interactions such as hydrogen bonding or electrostatic interactions. Based on the high-resolution X-ray structure of PSII, several water channels were identified near the active site, which are filled with extensive hydrogen-bonding networks of preorganized water molecules, connecting the OEC with the protein surface. As an integral part of the outer coordination sphere of natural metalloenzymes, these channels control the substrate and product delivery, carefully regulate the proton flow by promoting pivotal proton-coupled electron transfer processes, and simultaneously stabilize short-lived oxidized intermediates, thus highlighting the importance of an ordered water network for the remarkable efficiency of the natural OEC.Transferring this concept from nature to the engineering of artificial metal catalysts for fuel production has fostered the fascinating field of metallosupramolecular chemistry by generating defined cavities that conceptually mimic enzymatic pockets. However, the application of supramolecular approaches to generate artificial water oxidation catalysts remained scarce prior to our initial reports, since such molecular design strategies for efficient activation of substrate water molecules in confined nanoenvironments were lacking. In this Account, we describe our research efforts on combining the state-of-the art Ru(bda) catalytic framework with structurally programmed ditopic ligands to guide the water oxidation process in defined metallosupramolecular assemblies in spatial proximity. We will elucidate the governing factors that control the quality of hydrogen-bonding water networks in multinuclear cavities of varying sizes and geometries to obtain high-performance, state-of-the-art water oxidation catalysts. Pushing the boundaries of artificial catalyst design, embedding a single catalytic Ru center into a well-defined molecular pocket enabled sophisticated water preorganization in front of the active site through an encoded basic recognition site, resulting in high catalytic rates comparable to those of the natural counterpart OEC-PSII.To fully explore their potential for solar fuel devices, the suitability of our metallosupramolecular assemblies was demonstrated under (electro)chemical and photocatalytic water oxidation conditions. In addition, testing the limits of structural diversity allowed the fabrication of self-assembled linear coordination oligomers as novel photocatalytic materials and long-range ordered covalent organic framework (COF) materials as recyclable and long-term stable solid-state materials for future applications.

A General Synthesis Method for Covalent Organic Framework and Inorganic 2D Materials Hybrids

A General Synthesis Method for Covalent Organic Framework and Inorganic 2D Materials Hybrids

Covalent Organic Framework Interlayer Spacings as Perfectly Selective Artificial Proton Channels

AbstractBiological proton channels have perfect selectivity in aqueous environment against almost all ions and molecules, a property that differs itself from other biological channels and a feature that remains challenging to realize for bulk artificial materials. The biological perfect selectivity originates from the fact that the channel has almost no free space for ion or water transport but generates a hydrogen bonded wire in the presence of protons to allow the proton hopping. Inspired by this, we used the interlayer spacings of covalent organic framework materials consisting of hydrophilic functional groups as perfectly selective artificial proton channels. The interlayer spacings are so narrow that no atoms or molecules can diffuse through. However, protons exhibit a diffusivity in the same order of magnitude as that in bulk water. Density functional theory calculations show that water molecules and the COF material form hydrogen bonded wires, allowing the proton hopping. We further demonstrate that the proton transport rate can be tuned by adjusting the acidity of the functional groups.

Metal-Catalyzed Multi-Component Approach to Quinoline-Linked Covalent Organic Frameworks

The development of new reaction chemistry is highly desirable to construct new structural and functional covalent organic frameworks (COFs). Benefiting from the extremely large database of metal-catalyzed reaction database, we herein develop a new synthetic strategy that can generate quinoline-linked COFs via a silver-catalyzed three-component one-pot reaction and achieve functionalization by the simple replacement of alcohols. This metal-catalyzed approach to the construction of robust COF structures characterized by extended π-conjugation holds the potential to pave a novel pathway in the synthesis of COF materials endowed with both heightened stability and functionality.

Efficient capture of lead ions from aqueous solution by the functional covalent organic framework materials

Lead is one of the most common harmful metals in aqueous solutions. It has become an urgent need to devise a method that can effectively remove Pb(II) from aqueous solutions. In this paper, a novel covalent organic framework adsorbent was synthesized by post-modification method to remove Pb(II) from aqueous solution. This adsorbent analyzed by adsorption isotherms, kinetic and thermodynamic studies shows that the adsorption is chemically monolayer and controlled by intraparticle diffusion. The whole adsorption process is an exothermic and spontaneous reaction process. The maximum adsorption capacity of the adsorbent for Pb(II) can reach 359.94 mg/g at 298 K and the optimum pH 4. The zeta and density functional theory suggest that the electrostatic and coordination interactions between N, O and S functional groups and Pb(II) play a key role in the adsorption process. The selectivity and reproducibility results showed that this adsorbent was capable of targeted removal of Pb(II) from a polyionic environment and maintained a high adsorption efficiency after 5 cycles. In conclusion, this adsorbent has a very good application in the removal of metal ions from aqueous solutions.

Triazine-Carbazole-Based Covalent Organic Frameworks as Efficient Heterogeneous Photocatalysts for the Oxidation of N-aryltetrahydroisoquinolines.

Covalent organic frameworks (COFs) have attracted growing interests as new material platform for a range of applications. In this study, a triazine-carbazole-based covalent organic framework (COF-TCZ) was designed as highly porous material with conjugated donor-acceptor networks, and feasibley synthesized by the Schiff condensation of 4,4',4''-(1,3,5-triazine-2,4,6-triyl)trianiline (TAPB) and 9-(4-formylphenyl)-9H-carbazole-3,6-dicarbaldehyde (CZTA) under the solvothermal condition. Considering the effect of linkage, the imine-linked COF-TCZ was further oxidized to obtain an amide-linked covalent organic framework (COF-TCZ-O). The as-synthesized COFs show high crystallinity, good thermal and chemical stability, and excellent photoactive properties. Two π-conjugated triazine-carbazole-based COFs with tunable linkages are benefical for light-harvesting capacity and charge separation efficiency, which are empolyed as photocatalysts for the oxidation reaction of N-aryltetrahydroisoquinoline. The COFs catalyst systems exhibit the outstanding photocatalytic performance with high conversion, photostability and recyclability. Photoelectrochemical tests were employed to examine the behavior of photogenerated charge carriers in photo-illumination system. The control experiments provide further insights into the nature of photocatalysis. In addition, the current research also provided a valuable approach for developing photofunctional COFs to meet challenge in achieving the great potential of COFs materials in organic conversion.

Non-noble metals coordinated covalent organic frameworks as oxidase-like biocatalysts for light-responsive biomarker sensing

Metal-based enzyme-mimetic biocatalysts exhibit much better enzyme-like activity than metal-free biocatalysts. At the same time, both non-noble and noble metal-based catalysts have been proven to exhibit good biocatalytic activity. It is a great challenge to find extremely sensitive and adaptable biosensors with biocatalytic activity in biotechnology and medical diagnostics. For performance improvement and cost reduction, the development of non-noble metal-based biocatalysts is still of great significance. Here, we disclose that the covalent organic frameworks (COFs) coordinated by cobalt (Co) or nickel (Ni) with specific oxidase-like (OXD-like) activities can serve as intelligent artificial enzymes for light-augmented biocatalytic sensing of biomarkers. Because of the effects of metallic elements on the band gap adjustment, the Ni–COF exhibited the most efficient, selective, and light-responsive OXD-like biocatalytic activity (Km = 0.91 mM, Vmax = 1.50 μM s−1). Moreover, COFs loading metals show superior light-augmented biocatalytic detection capabilities. Notably, Ni–COF exhibits the most efficient and multifaceted diagnostic activity, including the extremely low limit of detection (LOD): 0.34 μM for glutathione and 0.54 μM for L-cysteine, which enables visual assays for abundant reducibility biomarkers. This study offers a unique and efficient non-noble metal-coordinated COF biocatalyst for high-sensitive and low-cost colorimetric detection. It also provides new insights into the design of non-noble metal elements in COF materials with better enzyme-like activities in biocatalytic and biomedical applications.

A covalent organic framework integrating photocatalytic center and electron reservoir for photocatalytic CO2 reduction to HCOOH

Direct and efficient photocatalytic conversion of CO2 into fuels and high value-added chemicals under sunlight is of great significance in alleviating the energy crisis and environmental issues. Covalent organic frameworks (COFs) have attracted much attention in the photocatalytic field as materials that can be pre-designed on demand. Taking advantage of the pre-designable properties of COFs materials, the integration of organic building blocks with photocatalytic activity and photoelectron-resident properties into frameworks can improve the electron-hole separation efficiency and photocatalytic performance. Here, we report a newly designed COF (COF-TVBT-N) with triazine and alkenyl groups that can be used as a photocatalyst to accomplish the photoreduction of CO2 to HCOOH under simulated sunlight without the assistance of metal sites and photosensitizers. Furthermore, we unambiguously reveal that the photocatalytic active center in COF-TVBT-N is TVBT through the combination of experiments and theoretical calculations. DFT calculations suggest that the adsorption of H near the active center is beneficial for the adsorption and photoreduction of CO2 in this photocatalytic system. The photoexcitation process and photocatalytic CO2 reduction mechanism of COF-TVBT-N are also reasonably deduced based on DFT calculations. This work provides ideas for the design of COFs as photocatalysts for CO2 reduction and gives insights into the mechanism of CO2 conversion to HCOOH.

Covalent Organic Framework Interlayer Spacings as Perfectly Selective Artificial Proton Channels.

Biological proton channels have perfect selectivity in aqueous environment against almost all ions and molecules, a property that differs itself from other biological channels and a feature that remains challenging to realize for bulk artificial materials. The biological perfect selectivity originates from the fact that the channel has almost no free space for ion or water transport but generates a hydrogen bonded wire in the presence of protons to allow the proton hopping. Inspired by this, we used the interlayer spacings of covalent organic framework materials consisting of hydrophilic functional groups as perfectly selective artificial proton channels. The interlayer spacings are so narrow that no atoms or molecules can diffuse through. However, protons exhibit a diffusivity in the same order of magnitude as that in bulk water. Density functional theory calculations show that water molecules and the COF material form hydrogen bonded wires, allowing the proton hopping. We further demonstrate that the proton transport rate can be tuned by adjusting the acidity of the functional groups.

Facile Heck coupling synthesis and characterization of a novel tris(4-(pyridine-4-vinyl)phenyl)methylsilane tridentate core

Aromatic rigid ligands with carboxylate, phosphate, or pyridyl terminals are highly important today for application in the manufacturing of metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and other supramolecular structures. Aromatic rigid ligands give rigidity to MOFs and COFs materials. In addition, building units are important in that their judicious selection can result in a 2-D or 3-D framework with moderate or high surface area. Most aromatic linkers are based on carbon centres which are associated with a negative impact on the environment. However, in contrast, silicon-based centres are scarce and benign to the environment, even though they can be prepared facilely via metathesis. Here, we report the facile preparation of a new tris (4-(pyridine-4-vinyl)phenyl) methylsilane using the classical Heck coupling reaction. The bridging ligand was synthesized via the standard Heck coupling of 4-vinylpyridine with tris(4-bromophenyl)(methyl)silane.

Synthesis of nanoflower-shaped covalent organic framework fluorescent probe for sensitive detection of aluminum ions

The widespread application of aluminum has led to numerous adverse effects on organisms due to aluminum pollution. In this study, we synthesized a two-dimensional fluorescent covalent organic framework (TAPB-DMTP-COF) material with nanoflower morphology for fluorescence detection of aluminum ions. The coordination binding of Al3+ with the nitrogen atoms on the imine bonds and the oxygen atoms on the methoxy groups restricts intramolecular rotation in TAPB-DMTP-COF, which inhibits the photo-induced electron transfer (PET) in the COF molecule and triggers the fluorescence emission "turn on" phenomena. A fluorescence method for Al3+ detection was established, and it was found that the method exhibits high sensitivity, selectivity, a wide detection range from 4.0 to 100.0 μM, and a low limit of detection (LOD) of 38.5 nM. This work provided new ideas for the convenient and rapid detection of aluminum ions in environmental analysis and expanded the application of fluorescence COFs in analytical sensing.

Research Progress on Iodine Capture by Covalent Organic Framework Materials

With the development of nuclear power, the removal of radionuclides is an important responsibility and task. Radioactive iodine, as one of the most radionuclide in nuclear wastes, its safe disposal is essential to ensure the sustainable development of the nuclear industry. Covalent organic framework materials are crystalline organic porous materials, which were constructed by covalent bonds. Because of their regular pore structure, large surface area and high chemical stability, covalent organic framework materials are selected as an ideal iodine capturing materials due to their structural characteristics and the adsorption sites of covalent organic framework materials. In this paper, the research progress of covalent organic framework materials in iodine capture was briefly reviewed. Meanwhile, the prospect of high efficiency iodine-capture covalent organic framework materials on industrialized application is predicted.

Metal-Coordinated Covalent Organic Frameworks as Advanced Bifunctional Hosts for Both Sulfur Cathodes and Lithium Anodes in Lithium-Sulfur Batteries.

The shuttling of polysulfides on the cathode and the uncontrollable growth of lithium dendrites on the anode have restricted the practical application of lithium-sulfur (Li-S) batteries. In this study, a metal-coordinated 3D covalent organic framework (COF) with a homogeneous distribution of nickel-bis(dithiolene) and N-rich triazine centers (namely, NiS4-TAPT) was designed and synthesized, which can serve as bifunctional hosts for both sulfur cathodes and lithium anodes in Li-S batteries. The abundant Ni centers and N-sites in NiS4-TAPT can greatly enhance the adsorption and conversion of the polysulfides. Meanwhile, the presence of Ni-bis(dithiolene) centers enables uniform Li nucleation at the Li anode, thereby suppressing the growth of Li dendrites. This work demonstrated the effectiveness of integrating catalytic and adsorption sites to optimize the chemical interactions between host materials and redox-active intermediates, potentially facilitating the rational design of metal-coordinated COF materials for high-performance secondary batteries.

Constructing Heptazine-COF@TiO2 Heterojunction Photocatalysts for Efficient Photodegradation of Acetaminophen under Visible Light.

Constructing heterojunction photocatalysts are widely applied to boost the photocatalytic activity of materials. Here, a novel covalent organic framework (COF) material with heptazine units was developed and hybridized with TiO2 nano particles (NPs) to fabricate the Heptazine-COF@TiO2 photocatalysts for acetaminophen (AAP) photodegradation. The successfully assembled heptazine unit endows the Heptazine-COF with outstanding semiconductor property (optical bandgap is 2.53 eV). The synthesized Heptazine-COF@TiO2 hybrids is proved to have the heterojunction structure with high visible light activity and fast charge-carrier mobility, and exhibits better performance in photodegradation of AAP under visible light. The excellent photodegradation efficiency (rate constant: 0.758 min-1 ) and high reusability (rate constant: 0.452 min-1 in the 6th cycles) of the optimized sample outperform the traditional inorganic photocatalysts and other heterojunction photocatalysts. In addition, these photocatalysts present universal degradation activity for other dyes and antibiotics.

Rapid electron transfer reinforced by interfacial Co-O bonding in MOF/COF hybrids for highly efficient degrade tetracycline by activating peroxymonosulfate

Due to the low electron transfer rate of zeolitic imidazolate framework-67 (ZIF-67)/ peroxymonosulfate (PMS) system, it suffers from a weak degradation ability for refractory pollutants and a risk of leaching of Co2+. In order to overcome these, covalent organic framework (COF) materials with excellent electron excitation and transfer ability are formed into a core-shell structure with ZIF-67. Thanks to the tight contact between the two materials, electrons could migrate rapidly between materials after being excited, helping to stimulate PMS to produce more reactive oxygen species (ROS) to participate in the degradation process. The abundant N and O elements in COF are also attracted to Co2+ in ZIF-67, forming more stable Co-O and Co-N sites. The formation of the Co-N site contributes to the release of CO, which is previously inhibited by N in COF. In this way, the catalytic activity of PMS is further enhanced. According to the experimental results, COF@ZIF-2/PMS system shows excellent TC degradation performance, with a degradation rate of up to 90.20% at 30 min, which is 15.03 times that of COF (6.00%) and 2.24 times that of ZIF-67 (40.12%). In order to be closer to the experimental environment of natural water, the influence of anions is explored. Through liquid chromatography-mass spectrometry (LC-MS) experiment, the intermediate products existing in TC degradation are analyzed and their toxicity is explored. To further explore the free radical pollution that plays a role in the degradation process, a free radical trapping test is carried out. Single oxygen (1O2) and sulfate radicals (SO4•-) act as the major ones in the system. Generally, this work provides a new idea for improving the catalytic activity of PMS by accelerating the electron transfer of the material through the construction of core-shell structure and the formation of Co-O/Co-N sites.

Flexible Linker-Based Triazine-Functionalized 2D Covalent Organic Frameworks for Supercapacitor and Gas Sorption Applications.

Covalent organic frameworks (COFs) having a large surface area, porosity, and substantial amounts of heteroatom content are recognized as the ideal class of materials for energy storage and gas sorption applications. In this work, we have synthesized four different porous COF materials by the polycondensation of a heteroatom-rich flexible triazine-based trialdehyde linker, namely 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine (TPT-CHO), with four different triamine linkers. Triamine linkers were chosen based on differences in size, symmetry, planarity, and heteroatom content, leading to the synthesis of four different COF materials named IITR-COF-1, IITR-COF-2, IITR-COF-3, and IITR-COF-4. IITR-COF-1, synthesized within 24 h from the most planar and largest amine monomer, exhibited the largest Brunauer-Emmett-Teller (BET) surface area of 2830 m2 g-1, superior crystallinity, and remarkable reproducibility compared to the other COFs. All of the synthesized COFs were explored for energy and gas storage applications. It is shown that the surface area and redox-active triazene rings in the materials have a profound effect on energy and gas storage enhancement. In a three-electrode setup, IITR-COF-1 achieved an electrochemical stability potential window (ESPW) of 2.0 V, demonstrating a high specific capacitance of 182.6 F g-1 with energy and power densities of 101.5 Wh kg-1 and 298.3 W kg-1, respectively, at a current density of 0.3 A g-1 in 0.5 M K2SO4 (aq) with long-term durability. The symmetric supercapacitor of IITR-COF-1//IITR-COF-1 exhibited a notable specific capacitance of 30.5 F g-1 and an energy density of 17.0 Wh kg-1 at a current density of 0.12 A g-1. At the same time, it demonstrated 111.3% retention of its initial specific capacitance after 10k charge-discharge cycles. Moreover, it exhibited exceptional CO2 capture capacity of 25.90 and 10.10 wt % at 273 and 298 K, respectively, with 2.1 wt % of H2 storage capacity at 77 K and 1 bar.