Three-dimensional Covalent Organic Frameworks Research Articles

Three-Dimensional Covalent Organic Frameworks with hea Topology

Three-dimensional (3D) covalent organic frameworks (COFs) are a new type of crystalline organic porous material, which have great application potential in various fields due to their complex pore s...

Tailoring the Pore Surface of 3D Covalent Organic Frameworks via Post‐Synthetic Click Chemistry

AbstractThree‐dimensional covalent organic frameworks (3D COFs) have gained increasing attention for their attractive features. However, the development of 3D COFs is strongly restricted, mainly due to their synthetic difficulty and complicated structure determination. Post‐synthetic modification, which can avoid these problems by incorporating functional moieties into a predetermined framework, provides an alternative way to construct 3D COFs with specific functions. Herein, we report the designed synthesis and characterization of a series of highly crystalline 3D COFs with different loadings of ethynyl groups. Notably, these alkyne‐tagged 3D COFs provide a platform for targeted anchoring various specific groups onto the pore walls via click reactions. Moreover, the pore surface engineering can accordingly change their properties, for example, the obtained click products exhibited higher CO2/N2 selectivity. We describe a simple but powerful strategy to build functional 3D COFs, which will certainly advance them for a ranging of interesting applications in the future.

Analysis of mycotoxins in grain samples using 3D covalent organic frameworks and stable isotope labeling technique

A new three-dimensional (3D) covalent organic framework (COF) tris (4-formylphenyl) amine-tetra (4-aminophenyl) methane (TFPA-TAM) was designed and synthesized. It was used in the dispersed solid-phase extraction (DSPE) of mycotoxins from grain sample extractant. The adsorption performance of 3D COF (TFPA-TAM) was compared with four kinds of different COFs with various spatial structures and steric hindrances to illustrate the key factors in the adsorption process. Stable isotope labeling (SIL) strategy with dansyl chloride (DNS-Cl) and deuterated dansyl chloride (d6-DNS-Cl) as labeling agents was applied for the first time in the high-performance liquid chromatography-tandem mass spectrometry (HPLC–MS/MS) quantitation of mycotoxins. SIL strategy enhanced the mass spectrometry (MS) sensitivity and minimized matrix effects. The optimized adsorption conditions were sample solution pH 7; adsorbent amount, 20 mg; desorption time, 5 min; elution volume, 1 mL. The limits of detection (LODs) of mycotoxins in grain samples were in the range of 4–7 µg/kg, while limits of quantification (LOQs) were in the range of 13–24 µg/kg. The linearity coefficients for all analytes were higher than 0.9977. The recoveries of this method were in the range of 90.3–94.3% with relative standard deviations of <6.3%, and the matrix effects were in the range of 93.5–98.8%.

Flexible and Robust Three-Dimensional Covalent Organic Framework Membranes for Precise Separations under Extreme Conditions.

Membranes based on covalent organic frameworks (COFs) have demonstrated huge potential to resolve the long-standing bottlenecks in separation fields due to their structural and functional attributes. Herein, a three-dimensional COF featuring interpenetrated apertures, 3D-OH-COF, is rationally synthesized on polyimide supports to generate flexible, robust membranes. The resultant 3D-OH-COF presents excellent crystallinity, prominent porosity, and exceptional solvent resistance, enabling the produced membrane a sharp and durable selectivity to small molecules in water and organic solvents. Impressively, the membrane also exhibits excellent flexibility and robustness as verified by the well-maintained performances after serious bending and solvent soaking under elevated temperatures. We further chemically convert 3D-OH-COF into the carboxyl-decorated 3D-COOH-COF by a postsynthetic strategy. The 3D-COOH-COF retains high crystallinity, and the converted membrane receives a remarkable capture ability for targeted multivalent ions over other competing ions. This study exploits a viable avenue to produce practical 3D COF membranes toward ultimate separations under extreme conditions.

Ultrahigh thermal conductivity in three-dimensional covalent organic frameworks

Covalent organic frameworks (COFs) are a new, growing class of crystalline polymers with exceptional characteristics such as low density, mechanical strength, and flexibility. Taking three-dimensional (3D) COF-300 derivatives as an example, we apply molecular dynamics simulations to show that ultrahigh thermal conductivity (>15 W/(mK)) is achieved at a pore size of d = 0.63 nm. This is the first observation of such an outstanding intrinsic thermal conductivity of a 3D polymer in all directions. Further temperature-dependent thermal conductivity calculations and X-ray diffraction (XRD) patterns highlight the importance of maintaining microscopic order for a large thermal conductivity. We also demonstrate that the tunability and anisotropy in COFs provide sufficient room to engineer and obtain desired thermal conductivity. These findings provide a fundamental understanding of structure-thermal conductivity relation in COFs and shine a light on high-thermal-conductivity COFs for thermal management applications.

A Three-Dimensional sp2 Carbon-Conjugated Covalent Organic Framework.

A first example of an sp2 carbon-conjugated three-dimensional (3D) covalent organic framework (COF) (BUCT-COF-4) is synthesized via the Knoevenagel condensation of the saddle-shaped aldehyde-substituted cyclooctatetrathiophene and 1,4-phenylenediacetonitrile. Ascribed to the extended π-conjugation and long-range ordered structures, BUCT-COF-4 displays high Hall electron mobility of 1.97 cm2 V-1 s-1 at room temperature. After it is doped with iodine, the material not only exhibits an enhanced electron mobility up to 2.62 cm2 V-1 s-1 in ambient air but also presents an unexpected metal-free ferromagnetic phase transition arising from the formation of aligned spins unidirectional across the whole sp2 carbon-conjugated 3D framework. This is the first report of a ferromagnetic phenomenon in 3D COF materials, which would broaden promising applications and open a new frontier in COF materials.

Three-Dimensional Covalent Organic Frameworks with Cross-Linked Pores for Efficient Cancer Immunotherapy.

We report the design and synthesis of a series of three-dimensional (3D) covalent organic frameworks (COFs) as immunogenic cell death (ICD) inducers for cancer immunotherapy. Three triple-topic amine building blocks, inactive to inducing ICD, were used to construct three COFs, COF-607, COF-608, and COF-609, with outstanding ICD eliciting efficiency. Mechanism studies revealed that after linking these ICD inert monomers into the COF backbone, the optical properties of these COFs could be systematically tuned to achieve excellent reactive oxygen species (ROS) production performance. This combined with 3D cross-linked pores, mimicking lung structure, favor the exchange and diffusion of oxygen and ROS, leading to excellent inducing ICD efficacy. One member, COF-609, is capable of triggering abscopal and long-lasting immune memory effects in a mouse model of breast cancer with >95% mice survival after being treated with COF-609+αCD47 for 110 days.

A Cubic 3D Covalent Organic Framework with nbo Topology.

The synthesis of three-dimensional (3D) covalent organic frameworks (COFs) requires high-connectivity polyhedral building blocks or the controlled alignment of building blocks. Here, we use the latter strategy to assemble square-planar cobalt(II) phthalocyanine (PcCo) units into the nbo topology by using tetrahedral spiroborate (SPB) linkages that were chosen to provide the necessary 90° dihedral angles between neighboring PcCo units. This yields a porous 3D COF, SPB-COF-DBA, with a noninterpenetrated nbo topology. SPB-COF-DBA shows high crystallinity and long-range order, with 11 resolved diffraction peaks in the experimental powder X-ray diffraction (PXRD) pattern. This well-ordered crystal lattice can also be imaged by using high-resolution transmission electron microscopy (HR-TEM). SPB-COF-DBA has cubic pores and exhibits permanent porosity with a Brunauer-Emmett-Teller (BET) surface area of 1726 m2 g-1.

Capillary coated with three-dimensional covalent organic frameworks for separation of fluoroquinolones by open-tubular capillary electrochromatography

The Schiff-base reaction of 1,3,5-triformylphloroglucinol (Tp) and tetra(4-aminophenyl)methane (TAM) was performed for the synthesis of a three-dimensional covalent organic framework named 3D TpTAM, which was obtained by an ultrasound-assisted method for the first time. The morphology and structure of the synthesized TpTAM were characterized through various methods. Then, TpTAM-coated capillary columns were subsequently prepared by a covalent bonding method within a short time and applied for the separation of fluoroquinolones by capillary electrochromatography (CEC) with good resolution and reproducibility. The intraday relative standard deviations (RSDs) of the retention time and peak areas were 0.88%–0.95% and 2.27%–3.81%, respectively. The interday RSDs of retention time and peak areas were 0.71%–0.89% and 0.88%–3.60%, respectively. The column-to-column RSDs of retention time and peak areas were less than 1.90% and 13.56%, respectively. The interbatch RSDs of retention time and peak areas were less than 3.48% and 3.89%, respectively. The TpTAM-coated capillary columns could be used for no less than 100 runs with no observable changes in the separation efficiency. The separation mechanism was also studied, which indicated that π-π stacking effects, hydrophobic interactions and hydrogen bonding were the main factors. The results revealed that 3D TpTAM should have superior potential as the stationary phase in CEC for chromatographic separation.

Three-Dimensional Radical Covalent Organic Frameworks as Highly Efficient and Stable Catalysts for Selective Oxidation of Alcohols.

With excellent designability, large accessible inner surface, and high chemical stability, covalent organic frameworks (COFs) are promising candidates as metal-free heterogeneous catalysts. Here, we report two 3D radical-based COFs (JUC-565 and JUC-566) in which radical moieties (TEMPO) are uniformly decorated on the channel walls via a bottom-up approach. Based on grafted functional groups and suitable regular channels, these materials open up the application of COFs as highly efficient and selective metal-free redox catalysts in aerobic oxidation of alcohols to relevant aldehydes or ketones with outstanding turn over frequency (TOF) up to 132 h-1 , which has exceeded other TEMPO-modified catalytic materials tested under similar conditions. These stable COF-based catalysts could be easily recovered and reused for multiple runs. This study promotes potential applications of 3D functional COFs anchored with stable radicals in organic synthesis and material science.

Postsynthetic modification of three-dimensional covalent organic frameworks

Postsynthetic modification of three-dimensional covalent organic frameworks

A 3D Anionic Metal Covalent Organic Framework with soc Topology Built from an Octahedral TiIV Complex for Photocatalytic Reactions.

The construction of three-dimensional (3D) covalent organic frameworks (COFs) remains challenging due to the limited types of organic building blocks. With octahedral TiIV complex as the building unit, this study reports on the first 3D anionic titanium-based COF (Ti-COF-1) with an edge-transitive (6, 4)-connected soc topology. Ti-COF-1 exhibits high crystallinity, superior stability, and large specific surface area (1000.4 m2 g-1 ). Moreover, Ti-COF-1 has a broad absorption band in the UV spectrum with an optical energy gap of 1.86 eV, and exhibits high photocatalytic activity toward Meerwein addition reactions. This research demonstrates an attractive strategy for the design of 3D functional COFs.

Imparting Ion Selectivity to Covalent Organic Framework Membranes Using de Novo Assembly for Blue Energy Harvesting.

It has long been a challenge to fabricate angstrom-sized functional pores for mimicking the function of biological channels to afford selective transmembrane transport. In this study, we describe a facile strategy to incorporate ionic elements into angstrom-sized channels using de novo encapsulation of charged dye molecules during the interface polymerization of a three-dimensional covalent organic framework (3D COF). We demonstrate that this approach is tailorable as it enables control over both the type and content of the guest and thus allows manipulation of the membrane function. The resulting membranes exhibit excellent permselectivity and low membrane resistance, thereby indicating the potential for harvesting salinity gradient (blue) energy. As a proof-of-concept study, the reverse electrodialysis device coupled with positive and negative dye encapsulated COF membranes afforded a power density of up to 51.4 W m-2 by mixing the simulated seawater and river water, which far exceeds the commercialization benchmark (5 W m-2). We envision that this strategy will pave the way for constructing new multifunctional biomimetic systems.

Tuning the Topology of Three-Dimensional Covalent Organic Frameworks via Steric Control: From pts to Unprecedented ljh.

Whether or not the topology of three-dimensional covalent organic frameworks (3D COFs) can be tuned via steric control remains a big question and has never been reported. Herein, we describe the designed synthesis of two highly crystalline 3D COFs (3D-TPB-COF-OMe and 3D-TPB-COF-Ph), through the polycondensation of tetra(p-aminophenyl)methane and methoxy- or phenyl- substituted 1,2,4,5-tetrakis(4-formylphenyl)benzene on the 3- and 6-positions. Amazingly, by using the continuous rotation electron diffraction technique, 3D-TPB-COF-OMe is determined to have a 5-fold interpenetrated structure with a reported pts net, while 3D-TPB-COF-Ph adopts an unprecedented self-penetrated ljh topology (ljh = Luojia Hill) that does not exist in the database of ToposPro. Therefore, by altering the substituents from methoxy to phenyl groups, the topology of designed 3D COFs changes accordingly, and a rare net is now available. This result clearly demonstrates that such COF structures need to be carefully determined due to its complexity, and moreover, it is promising to design 3D COFs with new topology for interesting application by increasing the steric hindrance of molecular building blocks.

Three-Dimensional Nanoporous Covalent Organic Framework-Incorporated Monolithic Columns for High-Performance Liquid Chromatography

The application of three-dimensional covalent organic frameworks (3D COF) with complex structure and high stability as the stationary phase is promising for high-performance liquid chromatography (HPLC) but is still limited by their irregular shape and wide size distribution. Herein, we show the fabrication of the first 3D COF-based monolithic column via the facile incorporation of nanoporous 3D-IL-COF-1 consisting of tetrakis(4-formylphenyl)methane and p-phenylenediamine into a porous organic monolith for HPLC. The effect of essential factors including temperature, porogen, and 3D-IL-COF-1 on the column permeability and efficiency is systematically investigated via the orthogonal experiment. The uniform structure, good permeability, and high mechanical stability of the prepared 3D-IL-COF-1 monolithic column not only make it promising for broad-spectrum chromatographic separation of neutral, acidic, and basic compounds but also render it better separation of isomers than a C18 column. This work offers a facile strategy for synthesis of a 3D COF stationary phase and will largely expand the high potential of 3D COF in HPLC.

Bipyridine-linked three-dimensional covalent organic frameworks for fluorescence sensing of cobalt(II) at nanomole level.

A novel fluorometric method based on bipyridine-linked three-dimensional covalent organic frameworks (COFs) was developed for the determination of Co2+. The COFs were synthesized by the polyreaction of tetrakis(4-aminophenyl)methane (TAPM), 2,2'-bipyridine-5,5'-diamine (Bpy), and 4,4'-biphenyldicarboxaldehyde (BPDA) under solvothermal conditions. The fluorescence of the COFs, with excitation/emission peaks at 324/406nm, is quenched by Co2+. Under the optimal conditions, the fluorescence quenching degrees (F0-F) of the resulted COFs linearly enhance as the concentrations of Co2+ increase in the range 0.01 to 0.25 μM, and a limit of detection of 2.63 nM is achieved. The fluorescence response mechanism was discussed in detail. This proposed approach has also been successfully employed to determine Co2+ in complex samples (shrimp and tap water), and satisfactory recoveries (88.1 ~ 109.7%) was obtained. The relative standard deviations are below 4.9%.

π-Interpenetrated 3D Covalent Organic Frameworks from Distorted Polycyclic Aromatic Hydrocarbons.

Three-dimensional covalent organic frameworks (3D COFs) with a pcu topology have been obtained from distorted polycyclic aromatic hydrocarbons acting as triangular antiprismatic (D3d ) nodes. Such 3D COFs are six-fold interpenetrated as the result of interframework π-stacking, which enable charge transport properties that are not expected for 3D COFs.

Three-Dimensional Triptycene-Based Covalent Organic Frameworks with ceq or acs Topology.

The growth of three-dimensional covalent organic frameworks (3D COFs) with new topologies is still considered as a great challenge due to limited availability of high-connectivity building units. Here we report the design and synthesis of 3D triptycene-based COFs, termed JUC-568 and JUC-569, following the deliberate symmetry-guided design principle. By combining a triangular prism (6-connected) node with a planar triangle (3-connected) or another triangular prism node, the targeted COFs adopt non-interpenetrated ceq or acs topology, respectively. Both materials show permanent porosity and impressive performance in the adsorption of CO2 (∼98 cm3/g at 273 K and 1 bar), CH4 (∼48 cm3/g at 273 K and 1 bar), and especially H2 (up to 274 cm3/g or 2.45 wt % at 77 K and 1 bar), which is highest among porous organic materials reported to date. This research thus provides a promising strategy for diversifying 3D COFs based on complex building blocks and promotes their potential applications in energy storage and environment-related fields.

Dendrons containing boric acid and 1,3,5-tris(2-hydroxyethyl)isocyanurate covalently attached to silica-coated magnetite for the expeditious synthesis of Hantzsch esters

A new multifunctional dendritic nanocatalyst containing boric acid and 1,3,5-tris(2-hydroxyethyl)isocyanurate covalently attached to core–shell silica-coated magnetite (Fe3O4@SiO2@PTS-THEIC-(CH2)3OB(OH)2) was designed and properly characterized by different spectroscopic or microscopic methods as well as analytical techniques used for mesoporous materials. It was found that the combination of both aromatic π–π stacking and boron–oxygen ligand interactions affords supramolecular arrays of dendrons. Furthermore, the use of boric acid makes this dendritic catalyst a good choice, from corrosion, recyclability and cost points of view. The catalytic activity of Fe3O4@SiO2@PTS-THEIC-(CH2)3OB(OH)2, as an efficient magnetically recoverable catalyst, was investigated for the synthesis of polyhydroacridines (PHAs) as well as polyhydroquinolines (PHQs) via one-pot multicomponent reactions of dimedone and/or ethyl acetoacetate, different aldehydes and ammonium acetate in EtOH under reflux conditions. Very low loading of the catalyst, high to quantitative yields of the desired PHAs or PHQs products, short reaction times, wide scope of the substrates, eliminating any toxic heavy metals or corrosive reagents for the modification of the catalyst, and simple work-up procedure are remarkable advantages of this green protocol. An additional advantage of this magnetic nanoparticles catalyst is its ability to be separated and recycled easily from the reaction mixture with minimal efforts in six subsequent runs without significant loss of its catalytic activity. This magnetic and dendritic catalyst can be extended to new two- and three-dimensional covalent organic frameworks with different applications.

A Crystalline Three-Dimensional Covalent Organic Framework with Flexible Building Blocks.

The construction of three-dimensional covalent organic frameworks (3D COFs) has proven to be very challenging, as their synthetic driving force mainly comes from the formation of covalent bonds. To facilitate the synthesis, rigid building blocks are always the first choice for designing 3D COFs. In principle, it should be very appealing to construct 3D COFs from flexible building blocks, but there are some obstacles blocking the development of such systems, especially for the designed synthesis and structure determination. Herein, we reported a novel highly crystalline 3D COF (FCOF-5) with flexible C-O single bonds in the building block backbone. By merging 17 continuous rotation electron diffraction data sets, we successfully determined the crystal structure of FCOF-5 to be a 6-fold interpenetrated pts topology. Interestingly, FCOF-5 is flexible and can undergo reversible expansion/contraction upon vapor adsorption/desorption, indicating a breathing motion. Moreover, a smart soft polymer composite film with FCOF-5 was fabricated, which can show a reversible vapor-triggered shape transformation. Therefore, 3D COFs constructed from flexible building blocks can exhibit interesting breathing behavior, and finally, a totally new type of soft porous crystals made of pure organic framework was announced.