2D Covalent Organic Frameworks Research Articles

Electron diffraction analysis of a 3D covalent organic framework

Electron diffraction analysis of a 3D covalent organic framework

A Fluorescent Three-Dimensional Covalent Organic Framework Formed by the Entanglement of Two-Dimensional Sheets.

Herein, we report the introduction of steric hindrance in molecular building blocks to prevent π···π stacking, thus allowing two-dimensional (2D) covalent organic sheets to form three-dimensional (3D) covalent organic frameworks (COFs) through entanglement. Starting from the rationally designed precursors containing a bulky anthracene unit in the vertical direction, a highly crystalline COF (3D-An-COF) was successfully synthesized. Very interestingly, 3D-An-COF was determined as an entangled 2D square net (sql) structure, and the high-resolution data (1.1 Å) obtained by the continuous rotation electron diffraction technique allowed us to directly locate all non-hydrogen atoms. Structurally, the presence of an anthracene group outside the C2h symmetry plane strongly reduces the π···π interactions and promotes the formation of square entanglements. In addition, 3D-An-COF is fluorescent and can be used as a sensor to detect the trace amount of antibiotics in water. This study provides a new strategy for the structural diversification of 3D COFs and will certainly motivate us to construct more entangled COFs for interesting applications in the future.

Synergistically enhanced interfacial and wave-transparent properties of PBO fiber composites: Constructing self-assembly interphase with different dimensional COF

Naphthalimide-linked covalent organic framework (COF) with two-dimensional (A-COF and B–COF) and three-dimensional structure (M-COF) was designed and applied to self-assembly on PBO fiber surface, and the influences of 2D or 3D COF on interfacial and wave-transparent properties as well as their reinforcing mechanisms of composites were investigated. A-COF and M-COF exhibited higher BET surface area and porosity compared to those of B–COF, and the morphology of 2D and 3D COF respectively presented lamellar nanosheet and random particle aggregation. The dense inner COF layer and numerous nanocrystals were attached on 2D COF@PBO fiber, and 3D COF@PBO fiber surface showed nano-porous structures from M-COF nanoparticle stacking. Compared with pristine PBO fiber composites, transverse fiber bundle tensile (TFBT) strength and interfacial shear strength (IFSS) of A-COF@PBO and B–COF@PBO fiber composites were increased by 38.76% and 51.77%, 37.23% and 43.94% due to lamellar stacking structure of 2D COF, while 15.31% and 19.56% increment for M-COF@PBO fiber composites, which was attributed to the weaker binding force between PBO fiber and 3D COF from interpenetrated structure. The optimal wave transmission efficiency (|T|2) at low frequency was obtained for A-COF@PBO fiber composites, resulting from the highest porosity of A-COF and best interfacial compatibility, while M-COF@PBO fiber composites showed the best |T|2 at high frequency due to the higher porosity and scarce electron-coupling phenomenon of M-COF.

Layered 3D Covalent Organic Framework Films Based on Carbon-Carbon Bonds.

The development of covalent organic frameworks (COFs) during the past decades has led to a variety of promising applications within gas storage, catalysis, drug delivery, and sensing. Even though most described synthesis methods result in powdery COFs with uncontrolled grain size, several approaches to grow COF films have recently been explored. However, in all COFs so far presented, the isolated materials are chemically homogeneous, with all functionalities homogeneously distributed throughout the entire material. Strategies to synthetically manipulate the spatial distribution of functionalities in a single film would be game changing. Specifically, this would allow for the introduction of local functionalities and even consecutive functions in single frameworks, thus broadening their synthetic versatility and application potential. Here, we synthesize two 3D crystalline COF films. The frameworks, the ionic B-based and neutral C-based COFs, have similar unit cell parameters, which enables their epitaxial stacking in a layered 3D COF film. The film growth was monitored in real time using a quartz crystal microbalance, showing linear growth with respect to reaction time. The high degree of polymerization was confirmed by chemical analysis and vibrational spectroscopy. Their polycrystalline and anisotropic natures were confirmed with grazing incidence X-ray diffraction. We further expand the scope of the concept by making layered films from COF-300 and its iodinated derivative. Finally, the work presented here will pave the path for multifunctional COF films where concurrent functionalities are embedded in the same crystalline material.

A Data-Driven Search of Two-Dimensional Covalent Organic Frameworks for Visible-Light-Driven Overall Water Splitting.

Two-dimensional (2D) covalent organic frameworks (COFs) with versatile structural and optoelectronic properties that can be tuned with building blocks and topological structures have received widespread attention for photocatalytic water splitting in recent years. However, few of these have been reported for overall water splitting under visible light. Here, we present a data-driven search of 2D COFs capable of visible-light-driven overall water splitting by combining high-throughput first-principles computations and experimental validations. Seven 2D COFs were identified to be capable of overall water splitting from the CoRE COF database, and their photocatalytic activities were further verified and optimized by our preliminary experiments. The production rates of H2 and O2 reached 80 and 32 μmol g-1 h-1, respectively, without using sacrificial agents. This work represents an attempt to explore 2D COFs for visible-light-driven overall water splitting with a data-driven approach that could accelerate the discovery and design of COFs toward photocatalytic overall water splitting.

Predesigned covalent organic framework with sulfur coordination: Anchoring Au nanoparticles for sensitive colorimetric detection of Hg(II)

Targeted construction of new covalent organic frameworks (COFs) with specific purposes and rationalities to build colorimetric assay platform for environmental pollutant monitoring have attracted increasing interest. However, it is still challenging due to lack of available coordination sites inside COFs pores and only a slight bonding ability for anchoring metal. In this work, a two-dimensional (2D) COFs (termed as Tz-COF) with high crystallinity, excellent chemical stability, and abundant sulfur coordination in its skeletons was synthesized and used for the confined growth of Au NPs. It was found that the Au NPs showed significant dispersibility for the support of Tz-COF. The proposed Tz-COF@Au NPs possessed outstanding Hg2+-activated peroxidase-like activity benefited from physicochemical properties of gold amalgam and synergistic effect between COFs and Au NPs to oxidize chromogenic substrate. Based on highly efficient activity and distinctive color evolution, the strategy for detecting Hg2+ was developed and successfully applied to determine the content of Hg2+ in real environmental samples. This work manifests that a potential strategy to establish a colorimetric assay platform for environmental pollutant monitoring based on the targeted manufacturing of novel COFs with specific functions.

Turning 3D Covalent Organic Frameworks into Luminescent Ratiometric Temperature Sensors.

In this study, we report hybrid crystalline lanthanide-containing 3D covalent organic framework (Ln@3D COF) materials that are suitable for temperature sensing applications. Different routes to obtain these hybrid materials were tested and compared for material quality and thermometric properties. In the first approach, a bipyridine-containing 3D COF (Bipy COF) was grafted with a range of visible emitting lanthanide (Eu3+, Tb3+, Dy3+, and Eu3+/Tb3+) β-diketonate complexes. In the second approach, a novel nanocomposite material was prepared by embedding NaYF4:Er,Yb nanoparticles on the surface of a nonfunctionalized 3D COF (COF-300). To the best of our knowledge, the luminescent materials developed here are the first 3D COFs to be tested as ratiometric temperature sensors. In fact, for the Bipy COF, two different types of thermometers were tested (the Eu3+/Tb3+ system and a rare Dy3+ system), with both showing excellent temperature sensing properties. The reported NaYF4:Er,Yb/COF-300 nanocomposite material combines upconverting nanoparticles with 3D COFs, similar to previously reported metal organic framework (MOF) nanocomposite materials; however, this type of hybrid material has not yet been explored for COFs. As such, our findings open a new pathway toward potential multifunctional materials that can combine thermometry with other modalities, such as catalysis or drug delivery, in just one nanocomposite material.

Enhancing the Iodine Adsorption Capacity of Pyrene-Based Covalent Organic Frameworks by Regulating the Pore Environment.

Regulating of pore environment is an efficient way to improve the performance of covalent organic frameworks (COFs) for specific application requirements. Herein, the design and synthesis of two pyrene-based 2D COFs with -H or -Me substituents, TFFPy-PPD-COF and TFFPy-TMPD-COF are reported. Both of them show long order structure and high porosity, in which TFFPy-PPD-COF displays a larger pore volume and bigger BET surface area (2587 m2 g-1 , 1.17 cm3 g-1 ). Interestingly, TFPPy-TMPD-COF exhibits a much higher vapor iodine capacity (4.8g g-1 ) than TFPPy-PPD-COF (2.9g g-1 ), in contrast to their pore volume size. By using multiple techniques, the better performance of TFPPy-TMPD-COF in iodine capture is ascribed to the altered pore environment by introducing methyl groups, which contributes to the formation of polyiodide anions and enhances the interactions between the frameworks and iodine. These results will be helpful for understanding the effect of pore environment in COFs for iodine uptake and constructing novel structure with high iodine capture performance.

Band-Edge Prediction of 2D Covalent Organic Frameworks from Molecular Precursor via Machine Learning.

The band-edge positions of two-dimensional (2D) covalent organic frameworks (COFs) play a crucial role in their applications in photocatalysts and nanoelectronics. However, massive amounts of 2D COFs with targeted band-edge positions from high-level first-principles calculations based on their composition are time-consuming due to the diversity and complexity of unit cell structures. Here, we report a strategy to predict the band-edge positions of 2D COFs by combining first-principles calculations with machine learning (ML). The root-mean-square error (RMSE) of the predicted valence band maximum (VBM) and conduction band minimum (CBM) between ML prediction and first-principles calculated values at the Perdew-Burke-Ernzerhof (PBE) level are 0.229 and 0.247 eV in test data set, respectively. In addition, a linear relationship is established between the PBE results and the HSE06 results with RMSE values of 0.089 and 0.042 eV for VBMs and CBMs in the test data set. Finally, a workflow is developed to determine the band-edge positions of the 2D COFs.

Exploring the Charge Storage Dynamics in Donor-Acceptor Covalent Organic Frameworks Based Supercapacitors by Employing Ionic Liquid Electrolyte.

Two donor-acceptor type tetrathiafulvalene (TTF)-based covalent organic frameworks (COFs) are investigated as electrodes for symmetric supercapacitors in different electrolytes, to understand the charge storage and dynamics in 2D COFs. Till-date, most COFs are investigated as Faradic redox pseudocapacitors in aqueous electrolytes. For the first time, it is tried to enhance the electrochemical performance and stability of pristine COF-based supercapacitors by operating them in the non-Faradaic electrochemically double layer capacitance region. It is found that the charge storage mechanism of ionic liquid (IL) electrolyte based supercapacitors is dependent on the micropore size and surface charge density of the donor-acceptor COFs. The surface charge density alters due to the different electron acceptor building blocks, which in turn influences the dense packing of the IL near its pore. The micropores induce pore confinement of IL in the COFs by partial breaking of coulomb ordering and rearranging it. The combination of these two factors enhance the charge storage in the highly microporous COFs. The density functional theory calculations support the same. At 1 A g-1 , TTF-porphyrin COF provides capacitance of 42, 70, and 130 F g-1 in aqueous, organic, and IL electrolyte respectively. TTF-diamine COF shows a similar trend with 100 F g-1 capacitance in IL.

Polyamide⊥TpPa-PVDF membranes for aliphatic-aromatic solvent separations

To achieve net-zero emissions, the petrochemical industry requires energy-efficient processes for solvent recovery, and traditional distillation technologies may not be adequate. Membrane separations with 2D COFs (covalent organic frameworks) have shown potential due to their tunable pore sizes and modifiable properties. However, the large pores of the COFs have provided challenges in fabricating 2D COF membranes for solvent separation. Recent research has been focused on improving the selectivities and permeation fluxes for solvent mixtures, and to achieve long-term operation, the membranes must have high chemical and mechanical stabilities. To address the issue of large pores, our research group cross-linked the Tp (2,4,6-triformylphloroglucinol) and Pa (p-phenylenediamine) monomers of the COF with TMC (trimesoyl chloride), a monomer used traditionally for reverse osmosis interfacial polymerization, which resulted in COF(TpPa) with TMC cross-linking and small pore sizes for solvent separation. Polyamide⊥TpPa-PVDF membranes were fabricated for the first time with this method, and the TMC cross-linked COF membrane successfully rejected salts and separated aromatic/alkane solvents; a toluene/benzene mixture showed a higher separation factor than methanol/isopropanol. Use of the 2D materials for reverse osmosis at 20 bar enhanced the potential of the energy-efficient membranes for use in future petrochemical industrial applications.

Symmetry-breaking dynamics in a tautomeric 3D covalent organic framework

The enolimine-ketoenamine tautomerism has been utilised to construct 2D covalent organic frameworks (COFs) with a higher level of chemical robustness and superior photoelectronic activity. However, it remains challenging to fully control the tautomeric states and correlate their tautomeric structure-photoelectronic properties due to the mobile equilibrium of proton transfer between two other atoms. We show that symmetry-asymmetry tautomerisation from diiminol to iminol/cis-ketoenamine can be stabilised and switched in a crystalline, porous, and dynamic 3D COF (dynaCOF-301) through concerted structural transformation and host-guest interactions upon removal and adaptive inclusion of various guest molecules. Specifically, the tautomeric dynaCOF-301 is constructed by linking the hydroquinone with a tetrahedral building block through imine linkages to form 7-fold interwoven diamondoid networks with 1D channels. Reversible framework deformation and ordering-disordering transition are determined from solvated to activated and hydrated phases, accompanied by solvatochromic and hydrochromic effects useful for rapid, steady, and visual naked-eye chemosensing.

Adjusting the Stacking Model of Two-Dimensional Covalent Organic Frameworks for Volatile Acid Sensing via Spatial Effects.

Covalent organic frameworks (COFs) are polymer networks with a precise structure and permanent porosity, making them an attractive platform for the detection of volatile analytes due to their chemical stability and accessible active sites. In this study, based on electron-rich N,N,N',N'-tetrakis(4-aminophenyl)-1,4-benzenediamine moiety, two 2D COFs with different topological structures and stacking models were designed by the strategy of spatial effect. The conductivity of the AB-stacked COF-NUST-20 was an order of magnitude higher than that of the AA-stacked COF-NUST-30. With the protonation of the imine bond, both COFs exhibited a strong, rapid, and reversible visible color change in response to corrosive HCl vapor. In addition, the AB-stacked COF-NUST-20, which facilitates both interlayer and intralayer charge transfer, shows better sensing performance. These findings demonstrate the usefulness of all-aromatic 2D COFs as real-time responsive chemosensors and provide insight into the design of sensing materials with high sensitivity.

3D Covalent Organic Framework as a Metastable Intermediate in the Formation of a Double-Stranded Helical Covalent Polymer.

The design and development of intricate artificial architectures have been pursued for decades. Helical covalent polymer (HCP) was recently reported as an unexpected topology that consists of chiral 1D polymers assembled through weak hydrogen bonds from achiral building blocks. However, many questions remained about the formation, driving force, and the single-handedness observed in each crystal. In this work, we reveal a metastable, racemic, fully covalently cross-linked, 3D covalent organic framework (COF) as an intermediate in the early stage of polymerization, which slowly converts into single-handed HCP double helices through partial fragmentation and self-sorting with the aid of a series of hydrogen bonding. Our work provides an intriguing example where weak noncovalent bonds serve as the determining factor of the overall product structure and facilitate the formation of a sophisticated polymeric architecture.

Blue light photocatalytic oxidation of sulfides to sulfoxides with oxygen over a thiazole-linked 2D covalent organic framework

With permanent porosity and intrinsic crystallinity, covalent organic frameworks (COFs) have great potential for solar energy conversions. However, the crystallinity of COFs necessitates reversible linkages like imine, leading to inadequate chemical stability of COFs as photocatalysts. To overcome this inadequacy, post-synthetic modifications (PSMs) transform an imine-linked COF, TTI-COF, into a thiazole-linked COF, TTT-COF. The morphology and crystallinity of TTT-COF are similar to that of TTI-COF. Compared to TTI-COF, TTT-COF secures evident amelioration of stability and optoelectronic properties supported by both experiments and theoretical calculations. Consequently, these two COFs are exploited for the photocatalytic oxidation of organic sulfides. Under blue light-emitting diode irradiation, TTI-COF marginally impels the reaction. In contrast, TTT-COF emerges with significantly better activity for selective oxidation of organic sulfides with oxygen in ethanol. This work sheds light on the transformative power of PSMs for tailoring COFs in visible light photocatalysis for oxidations.

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

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

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

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

Trifluoromethyl-Functionalized 2D Covalent Organic Framework for High-Resolution Separation of Isomers.

Development of novel functional materials for effective isomer separation is of great significance in environmental science, chemical industry, and life science due to the different functions of isomers. However, the similar physicochemical properties of isomers make their separation greatly challenging. Here, we report the fabrication of trifluoromethyl-functionalized 2D covalent organic framework (COF) TpTFMB with 2,2'-bis(trifluoromethyl)benzidine (TFMB) and 1,3,5-triformylphloroglucinol (Tp) for the separation of isomers. TpTFMB was in situ-grown on the inner surface of a capillary for the high-resolution separation of isomers. The introduction of hydroxyl and trifluoromethyl functional groups with uniform distribution in 2D COFs is a powerful tactic to endow TpTFMB with various functions such as hydrogen bonding, dipole interaction, and steric effect. The prepared TpTFMB capillary column enabled the baseline separation of positional isomers such as ethylbenzene and xylene, chlorotoluene, carbon chain isomers such as butylbenzene and ethyl butanoate, and cis-trans isomers 1,3-dichloropropene. The hydrogen-bonding, dipole, and π-π interactions as well as the structure of COF significantly contribute to the isomer separation. This work provides a new strategy for designing functional 2D COFs for the efficient separation of isomers.

Constructing a 3D Covalent Organic Framework from 2D hcb Nets through Inclined Interpenetration.

Three-dimensional covalent organic frameworks (3D COFs) have been of great interest due to their inherent numerous open sites and pore confinement effect. However, it has remained challenging to build 3D frameworks via interdigitation (also known as inclined interpenetration) by generating an entangled network formed by multiple 2D layers inclined with respect to each other. Herein, we report the first case of constructing a 3D COF, termed COF-904, through interdigitating 2D hcb nets, which was formed via [3+2] imine condensation reactions by the use of 1,3,5-triformylbenzene and 2,3,5,6-tetramethyl-1,4-phenylenediamine. The single-crystal structure of COF-904 is solved, and the locations of all non-hydrogen atoms are determined by 3D electron diffraction with a resolution up to 0.8 Å. These results not only broaden the strategy for achieving 3D COFs via interdigitation but also demonstrate that structurally complex extended frameworks can arise from simple molecules.

Intermediate Formation of Macrocycles for Efficient Crystallization of 2D Covalent Organic Frameworks with Enhanced Photocatalytic Hydrogen Evolution

AbstractCovalent organic frameworks (COFs) have gained significant attention as key photocatalysts for efficient solar light conversion into hydrogen production. Unfortunately, the harsh synthetic conditions and intricate growth process required to obtain highly crystalline COFs greatly hinder their practical application. Herein, we report a simple strategy for the efficient crystallization of 2D COFs based on the intermediate formation of hexagonal macrocycles. Mechanistic investigation suggests that the use of 2,4,6‐triformyl resorcinol (TFR) as the asymmetrical aldehyde build block allows the equilibration between irreversible enol‐to‐keto tautomerization and dynamic imine bonds to produce the hexagonal β‐ketoenamine‐linked macrocycles, the formation of which could provide COFs with high crystallinity in half hour. We show that COF‐935 with 3 wt % Pt as cocatalyst exhibit a high hydrogen evolution rate of 67.55 mmol g−1 h−1 for water splitting when exposed to visible light. More importantly, COF‐935 exhibits an average hydrogen evolution rate of 19.80 mmol g−1 h−1 even at a low loading of only 0.1 wt % Pt, which is a significant breakthrough in this field. This strategy would provide valuable insights into the design of highly crystalline COFs as efficient organic semiconductor photocatalysts.