Imine-linked COFs Research Articles

Vinylene-Linked Donor-π-Acceptor Metal-Covalent Organic Framework for Enhanced Photocatalytic CO2 Reduction.

Intramolecular charge separation driving force and linkage chemistry between building blocks are critical factors for enhancing the photocatalytic performance of metal-covalent organic framework (MCOF) based photocatalyst. However, robust achieving both simultaneously has yet to be challenging despite ongoing efforts. Here we develop a fully π-conjugated vinylene-linked multivariate donor-π-acceptor MCOF (D-π-A, termed UJN-1) by integrating benzyl cyanides linker with multiple functional building blocks of electron-rich triphenylamine and electron-deficient copper-cyclic trinuclear units (Cu-CTUs) moieties, featuring with strong intramolecular charge separation driving force, extended conjugation degree of skeleton, and abundant active sites. The incorporation of Cu-CTUs acceptor with electron-withdrawing ability and concomitantly giant charge separation driving force can efficiently accelerate the photogenerated electrons transfer from triphenylamine to Cu-CTUs, revealing by experiments and theoretical calculations. Benefiting from the synergistically effect of D-π-A configuration and vinylene linkage, the highly-efficient charge spatial separation is achieved. Consequently, UJN-1 exhibits an excellent CO formation rate of 114.8 μmol g-1 in 4 h without any co-catalysts or sacrificial reagents under visible light, outperforming those analogous MCOFs with imine-linked (UJN-2, 28.9 μmol g-1) and vinylene-linked COF without Cu-CTU active sites (UJN-3, 50.0 μmol g-1), emphasizing the role of charge separation driving force and linkage chemistry in designing novel COFs-based photocatalyst.

A pH-dependent and charge selective covalent organic framework for removal of dyes from aqueous solutions

A novel imine-linked COF is synthesized by the condensation of 2,4,6-tris(4-aminophenyl)−1,3,5-triazine (TAPT) and 2-hydroxy-5-methoxyisophthalaldehyde (HMIPA) under solvothermal conditions. This COF adsorbs preferentially the neutral dye Neutral Red (NR) over the positively charged dye Methylene Blue (MB) at pH 7, and the negatively charged Methyl Orange (MO) over the positively charged Methylene Blue (MB) at pH 3. The maximum adsorption capacities (qe) obtained within very short times (11–60 min) under optimized conditions were 108, 185 and 429 mg.g−1 for the MB, MO, and NR dyes, respectively. These adsorptions obey the Langmuir isotherm and pseudo-second-order kinetics. The prepared TAPT-HMIPA-COF is used successfully for the removal of the dyes from real water and treated wastewater samples. The adsorption data, BET, FTIR, and zeta potential measurements show that the electrostatic, π-π stacking and hydrogen bond interactions are responsible for the adsorption of organic dyes on the surface of the prepared COF. Due to recyclability, high capacity and efficiency for the adsorption of positive, negative and neutral organic dyes, this COF can be considered promising for simultaneous removal of various dyes from aqueous solutions at adjusted pHs.

Covalent organic frameworks as dispersive solid-phase extraction adsorbents for β-lactam detection in aquatic environments

Currently, antibiotic residues in the environment pose a serious public health risk; therefore, new sensitive approaches for detecting the presence of antibiotics are urgently needed. Ionic-type covalent organic frameworks (TAPB-(TFIT)+Cl−-COFs) were designed and prepared in this work, and these imine-linked COFs were characterized by several techniques, proving of porous structure and rich of nitrogen. The materials were applied to the extraction of β-lactams from aqueous samples, and the important parameters of the adsorption–desorption conditions were optimized. A dispersive solid-phase extraction method coupled with ultrahigh-performance liquid chromatography-tandem mass spectrometry (TAPB-(TFIT)+Cl−-COFs-d-SPE-UPLC-MS/MS) was proposed and utilized to determine the levels of 11 antibiotics in the β-lactams in the extracted samples. Good linearity (1.0–200.0 μg·L−1, r > 0.9999) was achieved together with limits of detection and quantification of 0.01–0.92 μg·L−1 and 0.05–2.78 μg·L−1, respectively; recoveries ranged from 75.57 to 114.20 %, conferring relative standard deviations < 8.99 % and negligible matrix effects (84.88–109.40 %). The synthesized TAPB-(TFIT)+Cl−-COFs were demonstrated to have good reusability and high stability. Furthermore, the proposed method was clearly shown to be sensitive and separable for the reliable simultaneous detection of 11 β-lactam residues in aquatic environments.

Breaking Dynamic Behavior in 3D Covalent Organic Framework with Pre-Locked Linker Strategy.

Due to their large surface area and pore volume, three-dimensional covalent organic frameworks (3D COFs) have emerged as competitive porous materials. However, structural dynamic behavior, often observed in imine-linked 3D COFs, could potentially unlock their potential application in gas storage. Herein, we showed how a pre-locked linker strategy introduces breaking dynamic behavior in 3D COFs. A predesigned planar linker-based 3,8-diamino-6-phenylphenanthridine (DPP) was prepared to produce non-dynamic 3D JUC-595, as the benzylideneamine moiety in DPP locked the linker flexibility and restricted the molecular bond rotation of the imine linkages. Upon solvent inclusion and release, the PXRD profile of JUC-595 remained intake, while JUC-594 with a flexible benzidine linker experienced crystal transformation due to framework contraction-expansion. As a result, the activated JUC-595 achieved higher surface areas (754 m2 g-1) than that of JUC-594 (548 m2 g-1). Furthermore, improved CO2 and CH4 storages were also seen in JUC-595 compared with JUC-594. Impressively, JUC-595 recorded a high normalized H2 storage capacity that surpassed other reported high-surface area 3D COFs. This works shows important insights on manipulating the structural properties of 3D COF to tune gas storage performance.

Sequential Oxidation/Cyclization of Readily Available Imine Linkages to Access Benzoxazole‐Linked Covalent Organic Frameworks

AbstractBenzoxazole‐linked covalent organic frameworks (BO−COFs), despite their exceptional chemical stability, are still in their infancy. This is primarily because the current prevalent methods require the use of special ortho‐hydroxyl‐substituted aromatic amines as monomers. Herein, we report an innovative strategy to access BO−COFs directly from imine‐linked COFs (Im−COFs) without pre‐embedded OH groups, using a two‐step sequential oxidation/cyclization process. The two‐step process included the oxidation of Im−COFs into amide‐linked COFs, followed by a copper‐catalyzed oxidative cyclization. Five representative BO−COFs were synthesized with retained crystallinity and high oxidization efficiency, offering the potential to convert a significant portion of Im−COFs into BO−COFs. The structural advantages of the newly designed BO−COFs were demonstrated through their application to photocatalytic organic transformations.

Sequential Oxidation/Cyclization of Readily Available Imine Linkages to Access Benzoxazole-Linked Covalent Organic Frameworks.

Benzoxazole-linked covalent organic frameworks (BO-COFs), despite their exceptional chemical stability, are still in their infancy. This is primarily because the current prevalent methods require the use of special ortho-hydroxyl-substituted aromatic amines as monomers. Herein, we report an innovative strategy to access BO-COFs directly from imine-linked COFs (Im-COFs) without pre-embedded OH groups, using a two-step sequential oxidation/cyclization process. The two-step process included the oxidation of Im-COFs into amide-linked COFs, followed by a copper-catalyzed oxidative cyclization. Five representative BO-COFs were synthesized with retained crystallinity and high oxidization efficiency, offering the potential to convert a significant portion of Im-COFs into BO-COFs. The structural advantages of the newly designed BO-COFs were demonstrated through their application to photocatalytic organic transformations.

Enhanced protonation ability of covalent organic frameworks via N,O-bidentate chelation for photocatalytic H2 evolution.

Inspired by the bidentate coordination chemistry of metal ions, we incorporated hydroxyl (OH) and methoxy (OMe) groups into the skeleton of imine-linked COFs to improve their protonation ability via intramolecular hydrogen bonds (O-H⋯NC). In comparison with the pristine COFs possessing monodentate nitrogen coordination sites, OH and OMe functionalized COFs with (N,O)-bidentate chelating sites exhibited up to 13.8 times faster photocatalytic hydrogen evolution rates (HERs).

One-step synthesis of a highly crystalline covalent organic framework with olefin and imine dual linkages for tuning the catalytic activity

Introducing a novel one-step method for synthesizing a highly crystalline COF with olefin and imine dual linkages, resulting in improved photocatalytic degradation of pollutants and unique 1O2 generation capabilities compared to imine-linked COFs.

Structural Regulation of Photocatalyst to Optimize Hydroxyl Radical Production Pathways for Highly Efficient Photocatalytic Oxidation.

Ring-opening of phenol in wastewater is the pivotal step in photocatalytic degradation. The highly selective generation of catalytical active species (•OH) to facilitate this process presents a significant scientific challenge. Therefore, a novel approach for designing photocatalysts with single-atom containment in metal-covalent organic frameworks (M-COFs) is proposed. The selection of imine-linked COFs containing abundant N and O-chelate sites provides a solid foundation for anchoring metal atom. These dispersed metal atom possess rapid accumulation and transfer capabilities for photogenerated electrons, while the periodic π-conjugated structure in 2D-COFs establishes an effective platform. Additionally, the Lewis acid properties of imine bonds in COFs can enhance the adsorption capacity toward gases with Lewis base properties, such as O2 and N2 . It is demonstrated that the Pd2+ @Tp-TAPT, designed based on this concept, exhibits efficient oxygen adsorption and follows the reaction pathway of O2 →•O2 - →H2 O2 →•OH with high selectivity, thereby achieving completely degradation of refractory phenol through photocatalysis within 10 min. It is anticipated that the selective generation of catalytic active species via advanced material design concepts will serve as a significant reference for achieving precise material catalysis in the future.

In situ synthesis of bi-functional NH2-Ti2C3Tx@COF Schottky heterojunction through interfacial electron effects for enhancing U(VI) photoreduction

Reduction of soluble U(VI) to insoluble U(IV) is considered as a promising strategy to avoid uranium contamination. Herein, a bi-functional covalent organic framework based heterojunction (NH2-Ti3C2Tx@COF) was constructed on the amino-modified Ti3C2Tx MXene for efficient photoreduction U(VI) without additional sacrificial agents. Schottky interface with electronic unidirectional channel is established by the formation of local electrophilic/nucleophilic region due to the introduction of reductive COF with alkynyl group, resulting in efficient photogenerated carrier separation and solar energy utilization. More importantly, NH2-Ti3C2Tx with negative charge served as electron collector and transfer channel can effectively surmount the low photocatalytic activity caused by high polarization of imine-linked COFs. Thus, the tailored-made design of NH2-Ti3C2Tx@COF exhibits excellent photocatalytic activity, fast reaction kinetics (removal of 97.8 % in 60 min) and high adsorptive capability (1557.2 mg/g) under the light condition. This multiple-in-one advantage makes well-designed NH2-Ti3C2Tx@COF as a promising capture platform. This work will provide some new insight and inspiration to COF based heterojunctions for enhancing U(VI) capture.

Tuning Crystallinity and Stacking of Two-Dimensional Covalent Organic Frameworks through Side-Chain Interactions.

Two-dimensional covalent organic frameworks (2D COFs) form as layered 2D polymers whose sheets stack through high-surface-area, noncovalent interactions that can give rise to different interlayer arrangements. Manipulating the stacking of 2D COFs is crucial since it dictates the effective size and shape of the pores as well as the specific interactions between functional aromatic systems in adjacent layers, both of which will strongly influence the emergent properties of 2D COFs. However, principles for tuning layer stacking are not yet well understood, and many 2D COFs are disordered in the stacking direction. Here, we investigate effects of pendant chain length through a series of 2D imine-linked COFs functionalized with n-alkyloxy chains varying in length from one carbon (C1 COF) to 11 carbons (C11 COF). This series reveals previously unrecognized and unanticipated trends in both the stacking geometry and crystallinity. C1 COF adopts an averaged eclipsed geometry with no apparent offset between layers. In contrast, all subsequent chain lengths lead to some degree of unidirectional slip stacking. As pendant chain length is increased, trends show average layer offset increasing to a maximum of 2.07 Å in C5 COF and then decreasing as chain length is extended through C11 COF. Counterintuitively, shorter chains (C2-C4) give rise to lower yields of weakly crystalline materials, while longer chains (C6-C9) produce greater yields of highly crystalline materials, as confirmed by powder X-ray diffraction and scanning electron microscopy. Molecular dynamics simulations corroborate these observations, suggesting that long alkyl chains can interact favorably to promote the self-assembly of sheets. In situ proton NMR spectroscopy provides insights into the reaction equilibrium as well as the relationship between low COF yields and low crystallinity. These results provide fundamental insights into principles of supramolecular assembly in 2D COFs, demonstrating an opportunity for harnessing favorable side-chain interactions to produce highly crystalline materials.

A Porous Crystalline Nitrone-Linked Covalent Organic Framework.

Herein, we report the synthesis of a nitrone-linked covalent organic framework, COF-115, by combining N, N', N', N'''-(ethene-1, 1, 2, 2-tetrayltetrakis(benzene-4, 1-diyl))tetrakis(hydroxylamine) and terephthaladehyde via a polycondensation reaction. The formation of the nitrone functionality was confirmed by solid-state 13 C multi cross-polarization magic angle spinning NMR spectroscopy of the 13 C-isotope-labeled COF-115 and Fourier-transform infrared spectroscopy. The permanent porosity of COF-115 was evaluated through low-pressure N2 , CO2 , and H2 sorption experiments. Water vapor and carbon dioxide sorption analysis of COF-115 and the isoreticular imine-linked COF indicated a superior potential of N-oxide-based porous materials for atmospheric water harvesting and CO2 capture applications. Density functional theory calculations provided valuable insights into the difference between the adsorption properties of these COFs. Lastly, photoinduced rearrangement of COF-115 to the associated amide-linked material was successfully demonstrated.

Enaminone‐Linked Covalent Organic Frameworks for Boosting Photocatalytic Hydrogen Production

AbstractCovalent organic frameworks (COFs), possessing pre‐designable structures and tailorable functionalities, are promising candidates for photocatalysis. Nevertheless, the most studied imine‐linked COFs (Im‐COFs) usually suffer from unsatisfactory stability and photocatalytic performance. To meet this challenge, a series of highly stable enaminone‐linked COFs (En‐COFs) have been synthesized and afford much improved visible‐light‐driven hydrogen production activities, ranging from 44 to 1078 times that of isoreticular Im‐COFs, with the only difference being the linkages (enaminone vs. imine) in their structures. The enhanced light‐harvesting ability, facilitated exciton dissociation and improved chemical stability account for the superior activity. Furthermore, quinoline‐linked COFs (Qu‐COFs) have been further obtained via the post‐modification of Im‐COFs. Compared with Im‐COFs, the photocatalytic activities of Qu‐COFs are significantly improved after modification, but still below those of the corresponding En‐COFs (3–107 times). The facile synthesis, excellent activity, and high chemical stability demonstrate that En‐COFs are a promising platform for photocatalysis.

Synthesis, Hole Doping, and Electrical Properties of a Semiconducting Azatriangulene-Based Covalent Organic Framework.

Two-dimensional covalent organic frameworks (2D COFs) containing heterotriangulenes have been theoretically identified as semiconductors with tunable, Dirac-cone-like band structures, which are expected to afford high charge-carrier mobilities ideal for next-generation flexible electronics. However, few bulk syntheses of these materials have been reported, and existing synthetic methods provide limited control of network purity and morphology. Here, we report transimination reactions between benzophenone-imine-protected azatriangulenes (OTPA) and benzodithiophene dialdehydes (BDT), which afforded a new semiconducting COF network, OTPA-BDT. The COFs were prepared as both polycrystalline powders and thin films with controlled crystallite orientation. The azatriangulene nodes are readily oxidized to stable radical cations upon exposure to an appropriate p-type dopant, tris(4-bromophenyl)ammoniumyl hexachloroantimonate, after which the network's crystallinity and orientation are maintained. Oriented, hole-doped OTPA-BDT COF films exhibit electrical conductivities of up to 1.2 × 10-1 S cm-1, which are among the highest reported for imine-linked 2D COFs to date.

Covalent organic frameworks based membranes for separation of azeotropic solvent mixtures by pervaporation

Pervaporation of organic-organic mixtures has shown great potential as a replacement for conventional distillation processes, particularly when dealing with azeotropes and liquids with similar boiling points that are difficult to separate. However, the lack of high-performing membranes that can consistently provide reliable selectivity and high production rates has been a major hurdle in this field. Here, we used an in-situ growth method to prepare two prototypes of imine-linked COFs, i.e., COF-LZU1 and COF-300 membranes for separating azeotropic mixtures of polar/non-polar solvent, i.e., acetone/n-hexane, acetone/cyclohexane and methanol/benzene. The composite membranes comprising a continuous and defect-free COF layer on a ceramic hollow fibre substrate demonstrate high solvent fluxes of acetone and methanol, up to 20.0 and 4.9 kg m−2h−1, respectively. The preferential permeation of polar solvents through polar imine-linked nanochannels leads to a high separation factor of the studied mixtures: 30 for acetone/n-hexane, 13.4 for acetone/cyclohexane and 4.3 for methanol/benzene systems. Combined with good stability in solvent systems, our results have proved a technical feasibility of using COF-based membranes for separation of azeotropic solvent mixtures.

Terephthalaldehyde-based covalent organic frameworks as photocatalysts for CO2 reduction under visible light

As a new type of porous organic materials, two-dimensional covalent organic frameworks (2D COFs) show superior visible-light harvesting and effective separation and transport of the photogenerated charges, which ensures COFs to be promising platforms for photocatalysis. In the current work, two 2D imine-linked COFs (PDA-TAB and PDA-TTA), having the same framework with benzene and/or triazine units, were synthesized to investigate the effects of the introduced triazine units in the skeleton of COFs on photocatalytic CO2 reduction. Compared to PDA-TAB, PDA-TTA exhibits a much higher photocatalytic activity with a HCOOH yield of 65.7 μmol gcat-1h−1. Further studies indicate the introduction of triazine units in the skeleton of PDA-TTA not only can enhance the visible-light absorption due to the high conjugation degree, but also can promote the charge separation and migration, thus improving the photocatalytic performance. The current work facilitates to understand the roles of the key structural units in 2D COFs on photocatalysis and provides some insights to develop efficient photocatalysts for the conversion of CO2.

Extending the π-conjugation system of covalent organic frameworks for more efficient photocatalytic H2O2production

Three highly crystalline and porous COFs with promising photoredox properties have been prepared.Viaa post-sulfurization process, the imine-linked COFs were converted into thiazole-linked COFs, generating a highly conjugated and ultrastable photocatalyst for H2O2 production.

Effective strategies toward imine-linked cationic covalent organic frameworks for rapid and selective removal of 99TcO4− from water: insights from DFT and MD calculations

The dynamic behavior and anion exchange mechanism of 99TcO4− on imine-linked cationic COFs are elucidated to provide theoretical basis for developing novel cationic COFs materials for environmental remediation.

Gold nanoparticle-decorated covalent organic frameworks as amplified light-scattering probes for highly sensitive immunodetection of Salmonella in milk.

Traditional immunoassays exhibit insufficient screening sensitivity for foodborne pathogens due to their low colorimetric signal intensities. Herein, we propose an ultrasensitive dynamic light scattering (DLS) immunosensor for Salmonella based on a "cargo release-seed growth" strategy enabled by a probe, namely gold nanoparticle-decorated covalent organic frameworks (COF@AuNP). Large amounts of AuNPs in COF@AuNP can be released by acid treatment-induced decomposition of the imine-linked COF, and then they are enlarged via gold growth to generate a dramatically enhanced light-scattering signal, leading to a vast improvement in detection sensitivity. Based on an immunomagnetic microbead carrier, the proposed DLS immunosensor is capable of detecting trace Salmonella in milk in the range of 2.0 × 102-2.0 × 105 CFU mL-1, with a limit of detection of 60 CFU mL-1. The immunosensor also demonstrated excellent selectivity, good accuracy and precision, and high reliability for detecting Salmonella in milk.

Rational regulating pore structures of covalent organic frameworks for sulfur hexafluoride capture and separation

Sulfur hexafluoride (SF6) is the most potent greenhouse gas whose emission is in great need of reduction during industrial processes. Here, a variety of imine-linked covalent organic frameworks with varying surface areas, pore size distributions, and topologies are rationally designed and regulated for systematically studying structure–property relationships of COF-based SF6 adsorbents. In terms of imine-linked COFs without further modification, the surface area was found to be a prerequisite for achieving high SF6 uptakes, and a small pore size at ca. 0.9 nm could effectively strengthen the adsorbate-adsorbent interaction and hence enhance the SF6/N2 selectivity. With a large specific surface area, suitable pore size, and strong covalent bonds, RCOF-1 showed superior SF6 adsorption capacities up to 4.13 mmol g−1 and large SF6/N2 selectivity up to 125 (273 K, 100 kPa) accompanied by excellent stability, which can be considered one of the most high-performance porous materials for SF6 capture and separation. This work not only provides a series of high-performance SF6 adsorbents but also broadens the horizon of applications of the emerging COFs, pointing out their development direction in the field of SF6 capture and separation.