Formation Of Covalent Organic Frameworks Research Articles

Continuous Synthesis and Processing of Covalent Organic Frameworks in a Flow Reactor.

Covalent organic frameworks (COFs) are typically prepared in the form of insoluble microcrystalline powders using batch solvothermal reactions that are energy-intensive and require long annealing periods (>120 °C, >72 h). Thus, their wide-scale adoption in a variety of potential applications is impeded by complications related to synthesis, upscaling, and processing, which also compromise their commercialization. Here we report a strategy to address both the need for scalable synthesis and processing approaches through the continuous, accelerated synthesis, and processing of imine- and hydrazone-linked COFs using a flow microreactor. The flow microreactor is capable of unprecedented COF productivities, up to 61,111 kg m-3 day-1, and provides control over key stages of COF formation, including nanoparticle growth, self-assembly, and precipitation. Additionally, the technique successfully yields highly crystalline and porous COFs in versatile macroscopic structures such as monoliths, membranes, prints, and packed beds. We also show that a COF synthesized using the flow microreactor acts as an excellent photocatalyst for the photocatalytic degradation of perfluorooctanoic acid (PFOA) outperforming the degradation efficiency of its batch analogue and other classical photocatalysts such as titanium dioxide (TiO2).

Unlocking Synthesis of Polyhedral Oligomeric Silsesquioxane-Based Three-Dimensional Polycubane Covalent Organic Frameworks.

Reticular chemistry effectively yields porous structures with distinct topological lattices for a broad range of applications. Polyhedral oligomeric silsesquioxane (POSS)-based octatopic building blocks with a rare Oh symmetric configuration and attracting inorganic features have great potential for creating three-dimensional (3D) covalent organic frameworks (COFs) with new topologies. However, the intrinsic flexibility and intensive motion of cubane-type POSS molecules make the construction of 3D regular frameworks challenging. Herein, by fastening three or four POSS cores with per aromatic rigid linker from rational steric directions, we successfully developed serial crystalline 3D COFs with unpresented "the" and scu topologies. Both the experimental and theoretical results proved the formation of target 3D POSS-based COFs. The resultant hybrid networks with designable chemical skeletons and high surface areas maintain the superiorities of both the inorganic and organic components, such as their high compatibility with inorganic salts, abundant periodic electroactive sites, excellent thermal stability, and open multilevel nanochannels. Consequently, the polycubane COFs could serve as outstanding solid electrolytes with a high ionic conductivity of 1.23 × 10-4 S cm-1 and a lithium-ion transference number of 0.86 at room temperature. This work offers a pathway to generate ordered lattices with multiconnected flexible cube motifs and enrich the topologies of 3D COFs for potential applications.

Designing of cobalt doped pyridine based covalent organic frameworks for efficient visible light driven CO2 reduction

Covalent organic frameworks (COF) are the class of compounds that impulses the boundaries of organic/polymer chemistry and materials science to achieve the exceptional control of predesigned long-range ordered structures. The present work deals with one-pot reaction of pyridine-2,6-diamine and terephthalaldehyde in pyrex tube at slightly elevated temperature and resulted in the formation of PDT/COF. The obtained PDT/COF was then functionalized using cobalt (Co@PDT/COF). The formation of COF and metal incorporation is confirmed by Fourier transform Infrared spectroscopy (FTIR). The simulated AB staggered stacking model for both PDT/COF and Co@PDT/COF correlate well with the observed PXRD pattern than AA eclipsed stacking. The theoretical findings using materials studio software showed increase in the porosity and interlayer spacing after the addition of Co to PDT/COF. The superior photocatalytic CO2 reduction activity is observed in Co@PDT/COF compared to PDT/COF. Co@PDT/COF was able to produce 13, 24 and 45 μmol g−1h−1 of H2, CH4 and CO, respectively. The obtained experimental optical properties were correlated well with Density functional theory (DFT) studies. Based on the characterization results, mechanism of photocatalysis has been discussed. Good stability and efficiency towards photocatalytic reduction CO2 in presence of the Co@PDT/COF shows its suitability for further research on pyridine based COFs for different applications.

A Collaboration to Create Functional Frameworks from Multiple Components.

This invited Team Profile was created by the teams of Dana D. Medina, LMU Munich, Germany; Laura M. Salonen, CINBIO, University of Vigo, Spain ; Tim Kowalczyk, Western Washington University, Bellingham, USA; João Rocha, CICECO, University of Aveiro, Portugal; and Akshay Rao, University of Cambridge, UK. They recently published an article on a three-component synthesis strategy for the formation of covalent organic frameworks (COFs) containing extended fused aromatics, which enables the synthesis of the building blocks and COF on a similar time scale. The methodology was also successfully applied to produce highly crystalline, preferentially oriented thin films with nanostructured surfaces on various substrates. "Building Blocks and COFs Formed in Concert-Three-Component Synthesis of Pyrene-Fused Azaacene Covalent Organic Framework in the Bulk and as Films", L. Frey, O. Oliveira, A. Sharma, R. Guntermann, S. P. S Fernandes, K. M. Cid-Seara, H. Abbay, H. Thornes, J. Rocha, M. Döblinger, T. Kowalczyk, A. Rao, L. M. Salonen, D. D. Medina, Angew. Chem. Int. Ed. 2023, 62, e202302872.

A Collaboration to Create Functional Frameworks from Multiple Components

AbstractThis invited Team Profile was created by the teams of Dana D. Medina, LMU Munich, Germany; Laura M. Salonen, CINBIO, University of Vigo, Spain ; Tim Kowalczyk, Western Washington University, Bellingham, USA; João Rocha, CICECO, University of Aveiro, Portugal; and Akshay Rao, University of Cambridge, UK. They recently published an article on a three‐component synthesis strategy for the formation of covalent organic frameworks (COFs) containing extended fused aromatics, which enables the synthesis of the building blocks and COF on a similar time scale. The methodology was also successfully applied to produce highly crystalline, preferentially oriented thin films with nanostructured surfaces on various substrates. “Building Blocks and COFs Formed in Concert–Three‐Component Synthesis of Pyrene‐Fused Azaacene Covalent Organic Framework in the Bulk and as Films”, L. Frey, O. Oliveira, A. Sharma, R. Guntermann, S. P. S Fernandes, K. M. Cid‐Seara, H. Abbay, H. Thornes, J. Rocha, M. Döblinger, T. Kowalczyk, A. Rao, L. M. Salonen, D. D. Medina, Angew. Chem. Int. Ed. 2023, 62, e202302872

Building Blocks and COFs Formed in Concert—Three‐Component Synthesis of Pyrene‐Fused Azaacene Covalent Organic Framework in the Bulk and as Films

AbstractA three‐component synthesis methodology is described for the formation of covalent organic frameworks (COFs) containing extended aromatics. Notably, this approach enables synthesis of the building blocks and COF along parallel reaction landscapes, on a similar timeframe. The use of fragmental building block components, namely pyrene dione diboronic acid as aggregation‐inducing COF precursor and the diamines o‐phenylenediamine (Ph), 2,3‐diaminonaphthalene (Naph), or (1R,2R)‐(+)‐1,2‐diphenylethylenediamine (2Ph) as extending functionalization units in conjunction with 2,3,6,7,10,11‐hexahydroxytriphenylene, resulted in the formation of the corresponding pyrene‐fused azaacene, i.e., Aza‐COF series with full conversion of the dione moiety, long‐range order, and high surface area. In addition, the novel three‐component synthesis was successfully applied to produce highly crystalline, oriented thin films of the Aza‐COFs with nanostructured surfaces on various substrates. The Aza‐COFs exhibit light absorption maxima in the blue spectral region, and each Aza‐COF presents a distinct photoluminescence profile. Transient absorption measurements of Aza‐Ph‐ and Aza‐Naph‐COFs suggest ultrafast relaxation dynamics of excited‐states within these COFs.

Building Blocks and COFs Formed in Concert-Three-Component Synthesis of Pyrene-Fused Azaacene Covalent Organic Framework in the Bulk and as Films.

A three-component synthesis methodology is described for the formation of covalent organic frameworks (COFs) containing extended aromatics. Notably, this approach enables synthesis of the building blocks and COF along parallel reaction landscapes, on a similar timeframe. The use of fragmental building block components, namely pyrene dione diboronic acid as aggregation-inducing COF precursor and the diamines o-phenylenediamine (Ph), 2,3-diaminonaphthalene (Naph), or (1R,2R)-(+)-1,2-diphenylethylenediamine (2Ph) as extending functionalization units in conjunction with 2,3,6,7,10,11-hexahydroxytriphenylene, resulted in the formation of the corresponding pyrene-fused azaacene, i.e., Aza-COF series with full conversion of the dione moiety, long-range order, and high surface area. In addition, the novel three-component synthesis was successfully applied to produce highly crystalline, oriented thin films of the Aza-COFs with nanostructured surfaces on various substrates. The Aza-COFs exhibit light absorption maxima in the blue spectral region, and each Aza-COF presents a distinct photoluminescence profile. Transient absorption measurements of Aza-Ph- and Aza-Naph-COFs suggest ultrafast relaxation dynamics of excited-states within these COFs.

Possibilities and Limitations in Monomer Combinations for Ternary Two-Dimensional Covalent Organic Frameworks.

The diversity and complexity of covalent organic frameworks (COFs) can be largely increased by incorporating multiple types of monomers with different topologies or sizes. However, an increase in the number of monomer types significantly complicates the COF formation process. Accordingly, much remains unclear regarding the viability of monomer combinations for ternary or higher-arity COFs. Herein, we show that, through an extensive examination of 12 two-nodes-one-linker ([2 + 1]) combinations, monomer-set viability is determined primarily by the conformational strain originating from disordered monomer arrangements, rather than other factors such as the difference in COF formation kinetics between monomers. When monomers cannot accommodate the strain associated with the formation of a locally disordered, yet crystalline framework, the corresponding [2 + 1] condensation yields a mixture of different COFs or an amorphous polymer. We also demonstrate that a node-linker pair that does not form a binary COF can be integrated to generate a single-phase framework upon addition of a small amount of the third component. These results will clarify the factors behind the successful formation of multicomponent COFs and refine their design by enabling accurate differentiation between allowed and disallowed monomer combinations.

Functionalized modulators in imine-linked covalent organic frameworks (COFs)

Covalent Organic Frameworks (COFs) are porous materials with high surface areas, making them interesting for a large variety of applications including energy storage, chemical sensing, and gas separation. In gas separation and sensing, functionalization beyond the COF linkage can result in selective COF-gas interactions, tailoring the properties for the desired application. However, not all functional groups are compatible with the synthetic conditions needed for COF formation. Modulators, which are typically monovalent building blocks that would terminate the reaction, have been shown to maintain or even improve the COF porosity by slowing down the reaction kinetics. Herein, we report on a series of several para-functionalized (OMe, Me, F, Cl, CF3, NO2) amine modulators to introduce additional functionality into the framework to study the selective CO2/N2 gas separation under flue gas conditions. Thorough characterization of the modulated COFs showed that the modulators are located on the outside of the polymeric sheet and get replaced by benzidine molecules, favoring a regular network formation over a homogeneous modulator distribution. The fractions of eventually incorporated modulator vary per functional group between 2.1% (NO2-modulated COF) and 8.9% (Me-modulated COF) while maintaining high BET surface areas (>1800 m2/g). It was found that all modulated COFs adsorb moderate quantities of CO2 and show comparable CO2/N2 IAST selectivity values under flue gas conditions. A higher number of functional groups in the framework was shown to enhance the IAST selectivity.

Construction of COF–COF heterojunctions for visible-light driven alcohol oxidation

Covalent organic framework (COF) as a novel type of photocatalysts has attracted great attentions in artificial photosynthesis. However, the photocatalytic activity of COFs is still not satisfied, due to the difficulty in exciton dissociation. Herein, we report the promotion of exciton dissociation with a COF–COF heterojunction prepared by deposition of nanoTp-TTA COF colloids on tetrahydroquinoline-linked QH-COF. The COF–COF heterojunction exhibits significantly enhanced transition photocurrent and conductivity as compared with individual COFs, implying the promotion of charge separation efficiency by the formation of COF–COF heterojunction. In the selective oxidation of benzyl alcohol, the turnover frequency (TOF) of Tp-TTA/QH heterojunction is 2.2 and 3.9 times higher than that of QH-COF and Tp-TTA COF, which is superior to most of COFs and MOFs in recent reports. This study provides an efficient strategy to facilitate the charge separation efficiency of COF-based photocatalysts.

Porphyrin COF and its mechanical pressing-prepared carbon fiber hybrid membrane for ratiometric detection, removal and enrichment of Cd2+

A nitrogen (N), oxygen (O)-rich porphyrin-based covalent organic framework (COF), in which interlayer porphyrin molecules are vertically stacked, is prepared and characterized. As-prepared N,O-rich TpTph COF shows a high adsorption capacity for Cd2+ due to the abundant coordination sites. More interesting, it is found that the formation of COF enlarges the porphyrin ring center space, thus facilitating the Cd2+coordination, and the resulting optical signal changes make the ratiometric detection of Cd2+ possible. Furthermore, using carbon fiber (CF) filaments, which are obtained from low cost and easy-to-obtain actived carbon mask, as support, porphyrin COF-based CF@TpTph membrane is prepared through in-situ growth of COF on the support followed by simple mechanical pressing. The CF@TpTph membrane is demonstrated to work well for both Cd2+ removal and enrichment from soil and water samples, and shows the advantages of ease of handling, robust stability, reduced secondary pollution risk to samples, and good reusability. This work provides a powerful tool for Cd2+ removal and enrichment, exhibits that preparing porphyrin-based COFs is a feasible way to promote the interactions between porphyrin ring and Cd2+, and demonstrates that mechanical pressing is a promising strategy for the design of COF-based monolithic materials to promote the practical applications of COFs.

In-situ fabricated covalent organic frameworks-polyamide hybrid membrane for highly efficient molecular separation

Covalent organic frameworks (COFs) membranes fabricated via interfacial polymerization typically present high permeance but relatively low selectivity for small molecules. To address this issue, COFs-polyamide hybrid membranes were in-situ fabricated onto polyacrylonitrile (PAN) substrate via interfacial polymerization for highly efficient molecular separation. Specifically, P-phenylenediamine (Pa) or benzidine (BD) as the aqueous monomer was reacted with a mixture of 2,4,6-trihydroxybenzene-1,3,5-tricarbaldehyde (Tp) and trimesoyl chloride (TMC) to generate COFs-polyamide hybrid structure, comprising the imide-linked COFs and the amide-linked polyamide. Since the TMC could react with the residual unreacted amine groups after COFs formation, the COFs-polyamide hybrid structure exhibited few defects and excellent stability. The as-prepared TpPaTMC/PAN or TpBDTMC/PAN hybrid membranes presented excellent performance for dye rejection and dye desalination with pure water permeances of 84∼95 L·m-2·h-1·bar-1, congo red rejection of ∼99%, acid red 66 rejection of 85%∼92%, and Na2SO4/CR separation factor of 65∼86. Moreover, the separation performance of COFs-polyamide hybrid membranes could be easily tunable by adjusting the TMC contents in the organic phase during the membrane fabrication, and the optimized membranes could be applied for dye rejection or dye desalination. The COFs-polyamide hybrid membranes also showed excellent anti-fouling performance and chemical stability. Therefore, the convenient and efficient in-situ fabrication of high-performance COFs-polyamide hybrid membrane would provide novel insight into the structure, property and application prospects of COFs membrane.

Covalent Organic Frameworks (COFS): A Review

The search for supramolecular promising porous crystalline materials with diverse applications such as gas storage, catalysis, chemo-sensing, energy storage, and optoelectronic have led to the design and construction of Covalent Organic Frameworks (COFs). COFs are a class of porous crystalline polymers that allow the precise integration of organic building blocks and linkage motifs to create predesigned skeletons and nano-porous materials. In this review article, a historic overview of the chemistry of COFs, survey of the advances in topology design and synthetic reactions, basic design principles that govern the formation of COFs as porous crystalline polymers as well as common synthetic procedures and characterization techniques are discussed. Furthermore some challenges associate with the synthesis of COFs are highlighted. We hope that this review will help researchers, industrialists and academics in no mean feat.

Covalent organic frameworks based on electroactive naphthalenediimide as active electrocatalysts toward oxygen reduction reaction

Developing organic electrocatalysts toward the oxygen reduction reaction (ORR) that avoid heteroatom doping processes and high-temperature carbonization is of great significance for the maturing of fuel cell applications. Herein, a series of two-dimensional imide-based covalent organic framework (COFs) electrocatalysts toward the ORR is reported. The hydrodynamic electrochemical study reveals that 3.5 electrons are exchanged during the ORR indicating that the process catalyzed by these COFs has a clear preference for the 4-electron reduction pathway. The COFs contain conjugated electroactive napthalenediimide (NDI) moieties that provides the active sites for the electrocatalysis and promotes the formation of COFs with face-to-face π-π stacked structures to provide intrinsic porosity and large surface areas. These COFs can be essentially considered as an organized pattern of active sites embedded in the pore walls of the COF. The choice of suitable comonomers with variable distortions from planarity offers the possibility of obtaining these electroactive COFs with similar redox ability but different degrees of porosity and interlaminar spacing. This work evidences a new insight into developing novel families of electrocatalysts from COFs. Structure and stacking fashion of the COF-systems are investigated on the basis of DFT calculations, as well as the photoabsorption spectra of the representative molecular entities and a proof-of-concept rationalization of the intermediate steps of the ORR mechanism.

Micro/Nano‐Scaled Covalent Organic Frameworks: Polymerization, Crystallization and Self‐Assembly

AbstractIn the past decade, considerable efforts have been extended on covalent organic frameworks (COFs), however, the basic research about the nucleation and crystal growth of COF materials has been greatly ignored. This review aims at providing an overview over the theoretical foundation and experimental observation of the multi‐scale structural engineering of COF materials, which includes complex polymerization, crystallization and self‐assembly processes. In this perspective, we first highlighted the latest advances on basic knowledge about the nucleation and growth of COFs, including classical and non‐classical crystallization mechanism as well as the synthesis of COF single crystals. Subsequently, we surveyed the effects of synthesis conditions, such as catalysts, monomer adding modes, heating methods, modulator and supramolecular interaction on the formation of COF materials. In the last, the research on the synthesis of COF nanoparticles, nanofibers and nanosheets based on the knowledge of crystal growth are reviewed in detail. The understanding related to the polymerization, crystallization and self‐assembly process during COF formation will surely make it easier to realize COFs with designed hierarchical architectures for targeted applications.

Synthesis of Boroxine and Dioxaborole Covalent Organic Frameworks via Transesterification and Metathesis of Pinacol Boronates.

Boroxine and dioxaborole are the first and some of the most studied synthons of covalent organic frameworks (COFs). Despite their wide application in the design of functional COFs over the last 15 years, their synthesis still relies on the original Yaghi's condensation of boronic acids (with itself or with polyfunctional catechols), some of which are difficult to prepare, poorly soluble, or unstable in the presence of water. Here, we propose a new synthetic approach to boroxine COFs (on the basis of the transesterification of pinacol aryl boronates (aryl-Bpins) with methyl boronic acid (MBA) and dioxaborole COFs (through the metathesis of pinacol boronates with MBA-protected catechols). The aryl-Bpin and MBA-protected catechols are easy to purify, highly soluble, and bench-stable. Furthermore, the kinetic analysis of the two model reactions reveals high reversibility (Keq ∼ 1) and facile control over the equilibrium. Unlike the conventional condensation, which forms water as a byproduct, the byproduct of the metathesis (MBA pinacolate) allows for easy kinetic measurements of the COF formation by conventional 1H NMR. We show the generality of this approach by the synthesis of seven known boroxine/dioxaborole COFs whose crystallinity is better or equal to those reported by conventional condensation. We also apply metathesis polymerization to obtain two new COFs, Py4THB and B2HHTP, whose synthesis was previously precluded by the insolubility and hydrolytic instability, respectively, of the boronic acid precursors.

In situ monitoring of mechanochemical covalent organic framework formation reveals templating effect of liquid additive

<h2>Summary</h2> Covalent organic frameworks (COFs) have emerged as a new class of molecularly precise, porous functional materials characterized by broad structural and chemical versatility, with a diverse range of applications. Despite their increasing popularity, fundamental aspects of COF formation are poorly understood, lacking profound experimental insights into their assembly. Here, we use a combination of <i>in situ</i> X-ray powder diffraction and Raman spectroscopy to elucidate the reaction mechanism of mechanochemical synthesis of imine COFs, leading to the observation of key reaction intermediates that offer direct experimental evidence of framework templating through liquid additives. Moreover, the solid-state catalyst scandium triflate is instrumental in directing the reaction kinetics and mechanism, yielding COFs with crystallinity and porosity on par with solvothermal products. This work provides the first experimental evidence of solvent-based COF templating and is a significant advancement in mechanistic understanding of mechanochemistry as a green route for COF synthesis.

Diltiazem-imprinted porphyrinic covalent organic frameworks as solid-phase extractants and fluorescent sensors

Diltiazem, which is a calcium channel blocker, is involved in the formation of covalent organic frameworks (COFs) through the Schiff base reaction of tetrakis (4-aminophenyl)-porphine (TAPP) and dihydroxynaphthalene-dicarbaldehyde (DHNDC) and the next enol-to-keto tautomerization. The diltiazem-imprinted COFs (DICOFs) were optimally formed using Sc(OTf)3 as the catalyst, TAPP/DHNDC/diltiazem in a molar ratio of 2/3/4, N-methylpyrrolidone/mesitylene (v/v = 3/5) as the porogen, and a 1-h reaction with a high imprinting factor of 10.5 compared to the nonimprinted counterparts (NICOFs). The optimized DICOF exhibited a more amorphous XRD pattern, a larger surface area (1650 vs. 930 m2/g), a larger pore volume (1.33 vs. 0.75 cm3/g), and a finer porous SEM feature than NICOF. The selectivity of NICOF toward diltiazem and diazepam at 250 nM (α = 1.03, RSD = 1.3%) was smaller than the selectivity of DICOF (α = 2.94, RSD = 1.6%). The diltiazem samples (5.0–300 ng mL−1) dynamically quenched the fluorescence of 15 μg/mL DICOF in 50 mM phosphate buffer at pH 6.5 at 8.0 min equilibrium; thus, Stern−Volmer plots were linearly constructed for sensing diltiazem with an LOD of 3.4 ng mL−1 and an LOQ of 10.2 ng mL−1. According to the plots, 30 ng mL−1 diltiazem solutions that were diluted from 30 mg-specified tablets had an average measured concentration of 29.5 ng mL−1 (σ = 1.3% and n = 5). In addition to application as fluorescent sensors, DICOFs (30 mg) could be used as dispersive extractants to recover 95.2% of 0.6 ng mL−1 diltiazem from 25 mL phosphate buffer with quadruplicate uses of 0.5 mL methanol/acetic acid (v/v = 9/1) as the eluent. Langmuir and pseudo-second-order models were fitted to the isothermal and kinetic sorption mechanisms, respectively. The maximum sorption capacity of DICOF was ten times larger than that of NICOF (156 vs. 15.2 mg/g). The interday recoveries of 0.6 ng mL−1 spiked in 20-fold diluted human urine, and 60-fold diluted human serum were 93.2% and 90.6%, respectively.

Electron Beam Irradiation as a General Approach for the Rapid Synthesis of Covalent Organic Frameworks under Ambient Conditions.

Crystalline porous materials such as covalent organic frameworks (COFs) are advanced materials to tackle challenges of catalysis and separation in industrial processes. Their synthetic routes often require elevated temperatures, closed systems with high pressure, and long reaction times, hampering their industrial applications. Here we use a traditionally unperceived strategy to assemble highly crystalline COFs by electron beam irradiation with controlled received dosage, contrasting sharply with the previous observation that radiation damages the crystallinity of solids. Such synthesis by electron beam irradiation can be achieved under ambient conditions within minutes, and the process is amendable for large-scale production. The intense and targeted energy input to the reactants leads to new reaction pathways that favor COF formation in nearly quantitative yield. This strategy is applicable not only to known COFs but also to new series of flexible COFs that are difficult to obtain using traditional methods.

COFs Meet Graphene Nanoribbons

2D covalent organic frameworks (COFs) have received considerable attention because of their crystallinity, permanent porosity, and tunability, yet they exhibit low in-plane conductivity. In this issue of Chem, Fischer and co-workers report an approach for growing 2D COF thin films featuring high in-plane conductivity for potential use in organic electronics.