2D and 3D Porphyrinic Covalent Organic Frameworks: The Influence of Dimensionality on Functionality

The construction of 2D and 3D covalent organic frameworks (COFs) from functional moieties for desired properties has gained much attention. However, the influence of COFs dimensionality on their functionalities, which can further assist in COF design, has never been explored. Now, by selecting designed precursors and topology diagrams, 2D and 3D porphyrinic COFs (2D‐PdPor‐COF and 3D‐PdPor‐COF) are synthesized. By model building and Rietveld refinement of powder X‐ray diffraction, 2D‐PdPor‐COF crystallizes as 2D sheets while 3D‐PdPor‐COF adopts a five‐fold interpenetrated pts topology. Interestingly, compared with 2D‐PdPor‐COF, 3D‐PdPor‐COF showed interesting properties, including 1) higher CO₂ adsorption capacity; 2) better photocatalytic performance; and 3) size‐selective photocatalysis. Based on this study, we believe that with the incorporation of functional moieties, the dimensionality of COFs can definitely influence their functionalities.     

Undulated 2D Covalent Organic Frameworks Based on Bowl‐Shaped Cyclotricatechylene

Two 2D wavy hexagonal hexahydroxyl cyclotricatechylene (CTC) based COFs, CTC‐COF‐2 and CTC‐COF‐3 were synthesized through solvothermal reaction. The bowl‐shaped conformation caused CTC skeletons packed in a columnar manner with the same oriented units, thus forming an undulated structure. The gas adsorption properties of CTC‐COFs were investigated, which show the potential application abilities in hydrogen storage of CTC‐COFs. The introduction of pyrene into CTC‐COF‐3 makes it a potential semiconducting π‐conjugated material.     

Solvothermal depolymerization and recrystallization of imine-linked two-dimensional covalent organic frameworks

Mechanistic understanding into the formation and growth of imine-linked two-dimensional (2D) covalent organic frameworks (COFs) is needed to improve their materials quality and access larger crystallite sizes, both of which limit the promise of 2D COFs and 2D polymerization techniques. Here we report a previously unknown temperature-dependent depolymerization of colloidal 2D imine-linked COFs, which offers a new means to improve their crystallinity. 2D COF colloids form at room temperature but then depolymerize when their reaction mixtures are heated to 90 °C. As the solutions are cooled back to room temperature, the 2D COFs repolymerize and crystallize with improved crystallinity and porosity, as characterized by X-ray diffraction, infrared spectroscopy and N₂ porosimetry. The evolution of COF crystallinity during the solvothermal depolymerization and repolymerization processes was characterized by in situ wide angle X-ray scattering, and the concentrations of free COF monomers as a function of temperature were quantified by variable temperature ¹H NMR spectroscopy. The ability of a 2D COF to depolymerize under these conditions depends on both the identity of the COF and its initial materials quality. For one network formed at room temperature (TAPB-PDA COF), a first depolymerization process is nearly complete, and the repolymerization yields materials with dramatically enhanced crystallinity and surface area. Already recrystallized materials partially depolymerize upon heating their reaction mixtures a second time. A related 2D COF (TAPB-DMTA COF) forms initially with improved crystallinity compared to TAPB-PDA COF and then partially depolymerizes upon heating. These results suggest that both high materials quality and network-dependent properties, such as interlayer interaction strength, influence the extent to which 2D COFs resist depolymerization. These findings offer a new means to recrystallize or solvent anneal 2D COFs and may ultimately inform crystallization conditions for obtaining large-area imine-linked two-dimensional polymers from solution.

Conditions for which imine-linked 2D COF polymerizations are temperature-sensitive are identified that enable a dissolution/repolymerization process akin to molecular recrystallization.     

Impact of Imine Bond Orientations on the Geometric and Electronic Structures of Imine-based Covalent Organic Frameworks

Many efforts are currently devoted to improving the stability and crystallinity of imine-based two-dimensional (2D) covalent organic frameworks (COFs) given their wide range of potential applications. The variation in the relative orientations of the imine bonds has been found to be a critical factor that impacts the stacking of the 2D COF layers, leads to the formation of isomer structures, and influences the crystallinity of the final product. Most investigations to date have focused only on the structural properties, while the role of the imine orientations on the electronic properties has not been studied systematically. Here, we explore this effect by examining how the electronic band structures, electronic couplings, and effective masses evolve when considering four isomeric structures of an imine-linked tetraphenyl-pyrene naphthalene-diimide COF. Our results provide an understanding of the impact of the imine orientations and how they need to be controlled to realize COF inter-layer stackings that can lead to efficient cross-plane electron transport. They can be used to guide the design and synthesis of imine-based COFs for applications where charge transport needs to be optimized.     

A Photoresponsive Smart Covalent Organic Framework

Ordered π‐columnar structures found in covalent organic frameworks (COFs) render them attractive as smart materials. However, external‐stimuli‐responsive COFs have not been explored. Here we report the design and synthesis of a photoresponsive COF with anthracene units as the photoresponsive π‐building blocks. The COF is switchable upon photoirradiation to yield a concavo‐convex polygon skeleton through the interlayer [4π+4π] cycloaddition of anthracene units stacked in the π‐columns. This cycloaddition reaction is thermally reversible; heating resets the anthracene layers and regenerates the COF. These external‐stimuli‐induced structural transformations are accompanied by profound changes in properties, including gas adsorption, π‐electronic function, and luminescence. The results suggest that COFs are useful for designing smart porous materials with properties that are controllable by external stimuli.     

Nonlinear Optical Switching in Regioregular Porphyrin Covalent Organic Frameworks

Covalent organic frameworks (COFs) have garnered immense scientific interest among porous materials because of their structural tunability and diverse properties. However, the response of such materials toward laser‐induced nonlinear optical (NLO) applications is hardly understood and demands prompt attention. Three novel regioregular porphyrin (Por)‐based porous COFs—Por‐COF‐HH and its dual metalated congeners Por‐COF‐ZnCu and Por‐COF‐ZnNi—have been prepared and present excellent NLO properties. Notably, intensity‐dependent NLO switching behavior was observed for these Por‐COFs, which is highly desirable for optical switching and optical limiting devices. Moreover, the efficient π‐conjugation and charge‐transfer transition in ZnCu‐Por‐COF enabled a high nonlinear absorption coefficient (β=4470 cm/GW) and figure of merit (FOM=σ₁/σ_o, 3565) value compared to other state‐of‐the‐art materials, including molecular porphyrins (β≈100–400 cm/GW), metal–organic frameworks (MOFs; β≈0.3–0.5 cm/GW), and graphene (β=900 cm/GW).     

An overview on covalent organic frameworks: synthetic reactions and miscellaneous applications

Covalent organic frameworks (COFs) are an emerging class of porous crystalline materials which are completely constructed from organic building blocks through robust covalent bonds. High surface areas, compositional and structural tunability, low density, and superior stability have rendered COF candidates in a variety of applications, such as adsorption and separation, catalysis, electronics, chemical sensing, optics, and so forth. To better understand the structures and properties of COFs as well as the design principles, it is of great significance to learn about the linkages formed during synthetic reactions that contribute to the high crystallinity and stability of COFs. In this review, we will first discuss various linkages that have been utilized for COF construction up to date, followed by an outline of their miscellaneous applications, providing a comprehensive and detailed overview in this file.     

Inorganic nanocrystal-dynamic porous polymer assemblies with effective energy transfer for sensitive diagnosis of urine copper

Despite their remarkable mechanical, optical, and electrical properties, inorganic particles and dynamic polymer assemblies encounter difficulties in their compatibility with regards to structural order and complexity. Here, covalent organic frameworks (COFs) constructed through reversible coupling reactions were exploited as dynamic porous polymers to prepare inorganic nanocrystal-polymer assemblies. Under an in situ growth process, carbon quantum dots (CDs) were gradually prepared in the COF cavity, with a narrow size distribution (2 ± 0.5 nm). The well-established assemblies achieve effective energy transfer from the inorganic to the organic part (efficiency > 80%), thus rendering a ∼130% increase in quantum yield compared with the pristine COF network. Notably, the hybrid material realizes a simple, selective, and sensitive diagnostic tool for urine copper, surpassing the detection limit of COF solid by 150 times. Beyond the scientific and fundamental interests, such hybrid assemblies are attractive from technological perspectives as well, for example, in energy storage, electronics, catalysis, and optics.

Despite their remarkable mechanical, optical, and electrical properties, inorganic particles and dynamic polymer assemblies encounter difficulties in their compatibility with regards to structural order and complexity.     

Acid Exfoliation of Imine‐linked Covalent Organic Frameworks Enables Solution Processing into Crystalline Thin Films

Covalent organic frameworks (COFs) are highly modular porous crystalline polymers that are of interest for applications such as charge‐storage devices, nanofiltration membranes, and optoelectronic devices. COFs are typically synthesized as microcrystalline powders, which limits their performance in these applications, and their limited solubility precludes large‐scale processing into more useful morphologies and devices. We report a general, scalable method to exfoliate two‐dimensional imine‐linked COF powders by temporarily protonating their linkages. The resulting suspensions were cast into continuous crystalline COF films up to 10 cm in diameter, with thicknesses ranging from 50 nm to 20 μm depending on the suspension composition, concentration, and casting protocol. Furthermore, we demonstrate that the film fabrication process proceeds through a partial depolymerization/repolymerization mechanism, providing mechanically robust films that can be easily separated from their substrates.     

Construction of novel benzoxazine-linked covalent organic framework with antimicrobial activity via postsynthetic cyclization

Organic framework materials have shown increasingly promising applications in biomedicine, such as drug delivery and release. In this work, we first synthesized a new hydroxyl-containing imine-linked two-dimensional covalent organic framework (COF) through solvothermal synthesis. Then, the imine group was converted into a benzoxazine group using a cyclization reaction. The results show that the postsynthetic modification did not change the basic framework of the original COF and did not affect the basic properties of the original COF. At the same time, the new benzoxazine group obtained by cyclization gave the COF good antibacterial activity against Escherichia coli and Staphylococcus aureus. The COF efficiency after cyclization was improved, and its antibacterial activity against both bacteria was over 90% compared with the imine-linked COF. Moreover, the benzoxazine-linked COF crystal structure and pore structures were retained, leaving the drug delivery and release functions unaffected. A benzoxazine-linked COF has never been reported because it cannot be synthesized by a direct reaction method. The work in this paper shows that the COFs that cannot be directly synthesized can be obtained through specific postsynthetic modification reactions. This means that more functional COFs can be obtained based on existing COFs, and the diversity of COF types and their potential applications can be further enriched and expanded.     

Linkages Make a Difference in the Photoluminescence of Covalent Organic Frameworks

Covalent organic frameworks (COFs) with structural designability and tunability of photophysical properties enable them to be a promising class of organic luminescent materials by incorporating well-designed fluorescent units directly into the periodic skeletons. The photophysical properties of COFs are mainly affected by the structural features, which determine the conjugation degree, charge delocalization ability, and exciton dynamics of COFs. To understand the relationship between COF structures and their photophysical properties, two COFs with the same pyrene chromophore units but different linkages (imine or vinylene) were designed and synthesized. Interestingly, different linkages endow COFs with huge differences in solid-state photoluminescence quantum yield (PLQY) for imine- and vinylene-linked pyrene-based COFs, which possess PLQY values of 0.34 % and 15.43 %, respectively. The femtosecond-transient absorption spectra and time-dependent density functional theory reveal the different charge-transfer pathways in imine- and vinylene-linked COFs, which influence the exciton relaxation way and fluorescence intensity. In addition, an effective white-light device was obtained by coating the vinylene-linked COF on a light-emitting diode strip.     

Thiophene-based covalent organic frameworks for highly efficient iodine capture

Development of adsorbent materials for highly efficient iodine capture is high demand from the perspective of ecological environment and human health. Herein, the two kinds of thiophene-based covalent organic frameworks (COFs) with different morphologies were synthesized by solvothermal reaction using thieno[3,2-b]thiophene-2,5-dicarbaldehyde (TT) as the aldehyde monomer and tri(4-aminophenyl)benzene (PB) or tris(4-aminophenyl)amine (PA) as the amino monomer (denoted as PB-TT COF and PA-TT COF) and the as-prepared two heteroatoms-rich COFs possessed many excellent properties, including high thermal stability and abundant binding sites. Among them, PB-TT COF exhibited ultra-high iodine uptake up to 5.97 g/g in vapor, surpassing most of adsorbents previously reported, which was ascribed to its high specific surface (1305.3 m²/g). Interestingly, PA-TT COF with low specific surface (48.6 m²/g) showed good adsorption ability for iodine in cyclohexane solution with uptake value of 750 mg/g, which was 2.38 times higher than that obtained with PB-TT COF due to its unique sheet-like morphology. Besides, the two COFs possessed good reusability, high selectivity and iodine retention ability. Based on experimental results, the adsorption mechanisms of both COFs were studied, revealing that iodine was captured by the physical-chemical adsorption. Furthermore, the both COFs showed excellent adsorption ability in real radioactive seawater treated safely, demonstrating their great potential in real environment.     

Ultrathin 2D Covalent Organic Framework Film Fabricated via Langmuir-Blodgett Method with a “Two-in-One” Type Monomer

In recent years, covalent organic frameworks(COFs) are evolving as a novel kind of porous materials for catalysis and molecular separation, gas adsorption, etc. Various functional building blocks have been explored to tune the pore channels, including the pore size and structures. In this article, a new terphenyl(TP) based COF(TP-COF) was developed via a “two-in-one” strategy by using a symmetric A₂B₂monomer, i.e., 4,4'-diamino-2',5'-diformyl-1,1':4',1'-terphenyl(DADFTP). The pore size of TP-COF was only 0.99 nm by shortening the arm length of the DADFTP monomer. Freestanding, continuous and ultrathin COF films could be facilely prepared at the air-liquid interface through the modified Langmuir-Blodgett(LB) method. TP-COF films exhibited high rejection of over 90% for dyes removal.     

Tuning Local Charge Distribution in Multicomponent Covalent Organic Frameworks for Dramatically Enhanced Photocatalytic Uranium Extraction

Optimizing the electronic structure of covalent organic framework (COF) photocatalysts is essential for maximizing photocatalytic activity. Herein, we report an isoreticular family of multivariate COFs containing chromenoquinoline rings in the COF structure and electron-donating or withdrawing groups in the pores. Intramolecular donor-acceptor (D-A) interactions in the COFs allowed tuning of local charge distributions and charge carrier separation under visible light irradiation, resulting in enhanced photocatalytic performance. By optimizing the optoelectronic properties of the COFs, a photocatalytic uranium extraction efficiency of 8.02 mg/g/day was achieved using a nitro-functionalized multicomponent COF in natural seawater, exceeding the performance of all COFs reported to date. Results demonstrate an effective design strategy towards high-activity COF photocatalysts with intramolecular D-A structures not easily accessible using traditional synthetic approaches.     

Direct and Indirect Excitons in Two-Dimensional Covalent Organic Frameworks

Highly luminescent bulk two-dimensional covalent organic frameworks (COFs) attract much attention recently. Origin of their luminescence and their large Stokes shift is an open question. After first-principles calculations on two kinds of COFs using the GW method and Bethe-Salpeter equation, we find that monolayer COF has a direct band gap, while bulk COF is an indirect band-gap material. The calculated optical gap and optical absorption spectrum for the direct excitons of bulk COF agree with the experiment. However, the calculated energy of the indirect exciton, in which the photoelectron and the hole locate at the conduction band minimum and the valence band maximum of bulk COF respectively, is too low compared to the fluorescence spectrum in experiment. This may exclude the possible assistance of phonons in the luminescence of bulk COF. Luminescence of bulk COF might result from exciton recombination at the defects sites. The indirect band-gap character of bulk COF originates from its AA-stacked conformation. If the conformation is changed to the AB-stacked one, the band gap of COF becomes direct which may enhance the luminescence.     

Postsynthetic Functionalization of Three‐Dimensional Covalent Organic Frameworks for Selective Extraction of Lanthanide Ions

Chemical functionalization of covalent organic frameworks (COFs) is critical for tuning their properties and broadening their potential applications. However, the introduction of functional groups, especially to three‐dimensional (3D) COFs, still remains largely unexplored. Reported here is a general strategy for generating a 3D carboxy‐functionalized COF through postsynthetic modification of a hydroxy‐functionalized COF, and for the first time exploration of the 3D carboxy‐functionalized COF in the selective extraction of lanthanide ions. The obtained COF shows high crystallinity, good chemical stability, and large specific surface area. Furthermore, the carboxy‐functionalized COF displays high metal loading capacities together with excellent adsorption selectivity for Nd³⁺ over Sr²⁺ and Fe³⁺ as confirmed by the Langmuir adsorption isotherms and ideal adsorbed solution theory (IAST) calculations. This study not only provides a strategy for versatile functionalization of 3D COFs, but also opens a way to their use in environmentally related applications.     

Designing Thiophene-Enriched Fully Conjugated 3D Covalent Organic Framework as Metal-Free Oxygen Reduction Catalyst for Hydrogen Fuel Cells

The application of three-dimensional (3D) covalent organic frameworks (COFs) in renewable energy fields is greatly limited due to their non-conjugated skeletons. Here, we design and successfully synthesize a thiophene-enriched fully conjugated 3D COF (BUCT-COF-11) through an all-thiophene-linked saddle-shaped building block (COThTh-CHO). The BUCT-COF-11 exhibits excellent semiconducting property with intrinsic metal-free oxygen reduction reaction (ORR) activity. Using the COF as cathode catalyst, the assembled anion-exchange membrane fuel cells (AEMFCs) exhibited a high peak power density up to 493 mW cm⁻². DFT calculations reveal that thiophene introduction in the COF not only improves the conductivity but also optimizes the electronic structure of the sample, which therefore boosts the ORR performance. This is the first report on the application of COFs as metal-free catalysts in fuel cells, demonstrating the great potential of fully conjugated 3D COFs as promising semiconductors in energy fields.     

The structure of layered covalent-organic frameworks

Covalent‐Organic Frameworks (COFs) are a new family of 2D and 3D highly porous and crystalline materials built of light elements, such as boron, oxygen and carbon. For all 2D COFs, an AA stacking arrangement has been reported on the basis of experimental powder XRD patterns, with the exception of COF‐1 (AB stacking). In this work, we show that the stacking of 2D COFs is different as originally suggested: COF‐1, COF‐5, COF‐6 and COF‐8 are considerably more stable if their stacking arrangement is either serrated or inclined, and layers are shifted with respect to each other by ～1.4 Å compared with perfect AA stacking. These structures are in agreement with to date experimental data, including the XRD patterns, and lead to a larger surface area and stronger polarisation of the pore surface.     

Stable 2D Heteroporous Covalent Organic Frameworks for Efficient Ionic Conduction

Two‐dimensional (2D) covalent organic frameworks (COFs) feature open and ordered one‐dimensional column nanochannels which offer immense possibilities for incorporation of various guests for specific functions. However, the relatively low chemical stability of most COFs originating from the dynamic covalent linkages hinders their practical application. In this work, a highly crystalline and heteroporous dibenzo[g,p]chrysene‐based COF (DBC‐2P) was synthesized and served as a host material for ionic conduction. DBC‐2P exhibits excellent stability both in strong acid and base due to the large conjugated DBC‐based knot that reinforces the interlayer interactions. Subsequent encapsulation of linear polyethylene glycol (PEG) and PEG‐LiBF₄ salt into the nanochannels of DBC‐2P affords a hybrid material with a high ionic conductivity of 2.31×10^?3 S cm^?1. This work demonstrates an efficient post‐synthetic strategy for the development of new COF–polymer composites with intriguing properties.