Growing single crystals of two-dimensional covalent organic frameworks enabled by intermediate tracing study

Rapid Synthesis of Single-Crystal Covalent Organic Framework with Controllable Crystal Habits

Covalent organic frameworks (COFs) linked by poorly reversible covalent bonds lack dynamic formation and cleavage, so the synthesis of their single-crystal structures necessitates slow crystallization rates to mitigate defect formation. This, however, inherently restricts opportunities for facet-selective engineering beyond traditional compositional and topological controls. To address this fundamental limitation, we developed an acetal/CH3COOH protocol that paradoxically accelerated crystallization while enhancing structural perfection, reducing the synthesis time for 60 μm-sized single-crystal COF-300 to 1 h, while achieving crystal sizes of up to 120 μm within 48 h, and 300 μm after 30 days. Capitalizing on this accelerated synthesis platform, we systematically interrogated crystallization landscapes through multiparameter exploration─modulator chemoselectivity, catalyst dosages, temporal evolution, and reactive temperature─to decode possible growth mechanisms of single-crystal COFs. Based on these, the relationship between reaction conditions and the crystal size, size distribution, shape, and growth dynamics of single-crystal COFs was trained and predicted by a machine learning (ML) model.

Supersaturation-Controlled Single-Crystal Growth of Covalent Organic Frameworks with Binary Solvents

The ability to rapidly produce large single crystals is crucial for advancing the applications of covalent organic frameworks (COFs). Although the modulation strategy provides a straightforward method for growing high-quality single crystals, the slow crystallization process of COFs often limits their practical use. In this study, we combined the principles of crystallization thermodynamics and kinetics with the modulation strategy to develop a binary solvent-supersaturation method, enabling the growth of single-crystal COFs in a significantly shorter time. By systematically investigating the crystal-growth kinetics across different solvent ratios, we established a diffusion-reaction growth model, highlighting the essential role of supersaturation in controlling COF crystal growth. Especially, under this crystallization guidance, elegant single crystals of COFs built with heteroatom or other functionality can also facilely obtained, which spontaneously validate the universality of the protocol. Importantly, the resulting single-crystal COFs, characterized by high structural symmetry, exhibited notable second harmonic generation (SHG) activity, which could open new avenues for future research in this field.

3D Imaging Reveals Widespread Stacking Disorder in Single Crystal 2D Covalent Organic Frameworks

Although tailored porosity is a defining feature of layered, two-dimensional (2D) polymers known as 2D covalent organic frameworks (COFs), understanding the interplanar stacking of 2D COFs and their resulting three-dimensional (3D) pore structure remains challenging. Here, we use scanning transmission electron microscopy and ptychography, an emerging 3D angstrom-scale imaging method, to study single-crystalline particles of the imine-linked 2D COF TAPB-DMPDA. Previously assumed to adopt an average-eclipsed structure with only angstrom-level stacking disorder, we find the crystals contain widespread stacking disorder of larger magnitudes, including interplanar shifts up to a half unit cell and nanoscale inhomogeneities in stacking and tilt. 3D visualizations show pore channels are distorted by this stacking disorder. The extensive stacking disorder found in even high-quality 2D COFs has profound implications for envisioned applications and should motivate the development of design strategies to control their 3D structures.

Moiré two-dimensional covalent organic framework superlattices

The on-surface synthesis of two-dimensional (2D) polymers from monomers represents a useful strategy for designing lattice, orbital and spin symmetries. Like other 2D materials, the ordered stacking of 2D polymers into bilayers may allow developing unique optoelectronic, charge transport and magnetic properties not found in the individual layers. However, controlling layer stacking of 2D polymers remains challenging. Here we describe a method for synthesizing 2D polymer bilayers or bilayer 2D covalent organic frameworks at the liquid-substrate interface through the direct condensation of monomers. More importantly, we also show how factors such as monomer structure and solvent mixture influence the bilayer stacking modes and how, under certain conditions, large-area moiré superlattices emerge from the twisted bilayer stacking. This finding offers new opportunities for the design of bilayer stacked framework materials with tunable electronic and structural properties.

Advances in synthetic strategies for two-dimensional conjugated polymers

Two-dimensional conjugated polymers (2D CPs) are typically represented by 2D conjugated covalent organic frameworks (COFs) that consist of covalently cross-linked linear conjugated polymers, which possess extended in-plane π-conjugation and out-of-plane electronic couplings. The precise incorporation of molecular building blocks into ordered polymer frameworks through (semi)reversible 2D polycondensation methodologies enables the synthesis of novel polymer semiconductors with designable and predictable properties for various (opto)electronic, spintronic, photocatalytic, and electrochemical applications. Linkage chemistry lays the foundation for this class of synthetic materials and provides a library for subsequent investigations. In this review, we summarize recent advances in synthetic strategies for 2D CPs. By exploring synthetic approaches and the intricate interplay between chemical structure, the efficiency of 2D conjugation, and related physicochemical properties, we are expected to guide readers with a general background in synthetic chemistry and those actively involved in electronic device research. Furthermore, the discussion will appeal to researchers intrigued by the prospect of uncovering novel physical phenomena or mechanisms inherent in these emerging polymer semiconductors. Finally, future research directions and perspectives of highly crystalline and processable 2D CPs for electronics and other cutting-edge fields are discussed.

A Phototautomeric 3D Covalent Organic Framework for Ratiometric Fluorescence Humidity Sensing

Photoinduced proton transfer is an essential photochemical process for designing photocatalysts, white light emitters, bioimaging, and fluorescence sensing materials. However, deliberate control of the excited/ground states and meticulous manipulation of the excited state intramolecular proton transfer (ESIPT) pathway constitute a significant challenge in liquids and dense solids. Here, we present the integration of a hydronaphthoquinone fluorophore into a crystalline, porous, phototautomeric dynamic 3D covalent organic framework (COF) to show guest-induced fluorescence turn-on, emission redshift enhancement, and shortened lifetimes for ratiometric fluorescence humidity sensing. Theoretical and spectroscopic studies provide mechanistic insights into the conformational dynamics, charge transfer coupled with local excitation, and ground-state uphill regulation for the multiple tautomers. We illustrate the sensitive, rapid, steady, and self-calibrated ratiometric fluorescence sensing for a wide range of humidity benefiting from the architectural and chemical robustness and crystallinity of such a phototautomeric 3D COF. These findings provide molecular insights into the design of functional porous materials that integrate host-guest mutual recognition and photoelectronic response for multiplex molecular sensing for environmental monitoring and biomedical diagnostics applications.

Diffusion/Modulator Dual-Mediated Solid-Liquid/Vapor Interfacial Synthesis of Crystalline Covalent Organic Framework Membranes

Constructing oriented crystalline covalent organic framework (COF) membranes with controllable thickness for water purification is highly desirable. Herein, we present a simple and universal protocol to prepare high-quality COF membranes on the inner wall of a glass vessel using a diffusion/modulator dual-mediated solid-liquid/vapor interfacial synthesis strategy. By meticulous control of solvent and temperature, a thin supersaturated spreading liquid layer was formed on the glass wall surface and served as a confined microreactor for incubating crystal nuclei. This induced the aniline-modulated solid-liquid/vapor interfacial exchange reaction and upward growth of a highly ordered COF membrane. The experiments and theoretical simulations revealed the underlying mechanisms of solid-liquid/vapor interfacial nucleation, growth and crystallization. Using this strategy, we created 13 types of new, free-standing, imine-linked COF membranes with exceptional performances in crystallinity, porosity, stability, processability and adsorption capacity. As an application demonstration, a COF membrane-filled filter was coupled with high-performance liquid chromatography system for the automated removal of multitarget liquid-crystal monomers in real water samples (removal efficiency≥96 %). This study enriches the synthesis toolboxes of COF membranes and broadens their application scopes.

Synthesis of high quality two dimensional covalent organic frameworks through a self-sacrificing guest strategy

The quality of covalent organic frameworks (COFs) crucially influences their mechanistic research and subsequent practical implementations. However, it has been widely observed that the structure damage induced by the activation procedure could compromise the quality of COFs. Here we develop a self-sacrificing guest method for synthesizing high-quality two-dimensional COFs (SG-COFs) by incorporating salt guests into the pores of the COF structure. These introduced salts play an indispensable role in supporting COF pores and mitigating quality loss during the activation process. Interestingly, due to the unique characteristic of salts decomposing into gases upon heating, this method can ultimately enable the synthesis of highly pure, high-quality COFs without the presence of residual guest molecules. The resulting sixteen COFs display superior crystallinity and porosity compared to those synthesized using conventional routes. Moreover, these high-quality SG-COFs have demonstrated to be highly efficient adsorbents for removal of per- and polyfluoroalkyl substances.

Controlling crystallization in covalent organic frameworks to facilitate photocatalytic hydrogen production

The catalytic performance, depending on the surface nature, is ubiquitous in photocatalysis. However, surface engineering for organic photocatalysts through structural modulation has long been neglected. Here, we propose a zone crystallization strategy for covalent organic frameworks (COFs) that enhances surface ordering through regulator-induced amorphous-to-crystalline transformation. Dynamic simulations show that attaching monofunctional regulators to the surface of spherical amorphous precursor improves surface dynamic reversibility, increasing crystallinity from the inside out. The resulting COF microspheres display surface-enhanced crystallinity and uniform spherical morphology. The visible photocatalytic hydrogen evolution rate reaches 126 mmol g-1 h-1 for the simplest β-ketoenamine-linked COF and 350 mmol gCOF-1 h-1 for SiO2@COF with minimal Pt cocatalysts. Mechanism studies indicate that surface crystalline domains build the surface electrical fields to accumulate photogenerated electrons and diminish electron transfer barriers between the COF and Pt interface. This work bridges the gap between microscopic molecules and macroscopic properties, allowing tailored design of crystalline organic photocatalysts.

From Fundamentals to Synthesis: Covalent Organic Frameworks as Promising Materials for CO2 Adsorption

Covalent organic frameworks (COFs) are highly crystalline polymers that possess exceptional porosity and surface area, making them a subject of significant research interest. COF materials are synthesized by chemically linking organic molecules in a repetitive arrangement, creating a highly effective porous crystalline structure that adsorbs and retains gases. They are highly effective in removing impurities, such as CO2, because of their desirable characteristics, such as durability, high reactivity, stable porosity, and increased surface area. This study offers a background overview, encompassing a concise discussion of the current issue of excessive carbon emissions, and a synopsis of the materials most frequently used for CO2 collection. This review provides a detailed overview of COF materials, particularly emphasizing their synthesis methods and applications in carbon capture. It presents the latest research findings on COFs synthesized using various covalent bond formation techniques. Moreover, it discusses emerging trends and future prospects in this particular field.

The Devil Is in the Details: Pitfalls and Ambiguities in the Analysis of X-ray Powder Diffraction Data of 2D Covalent Organic Frameworks

X-ray powder diffraction (XRPD) data of covalent organic frameworks (COFs) seem to be simple and apparently do not contain a lot of structural information, as these patterns usually do not show more than 3-5 distinguishable Bragg peaks. As COFs are inherently complex materials exhibiting a variety of disorder phenomena like stacking faults, layer curving, or disordered solvent molecules populating the pores, the interpretation of XRPD patterns is far from being trivial. Here we emphasize the critical need for precision and caution in XRPD data acquisition, refinement, and interpretation to avoid common pitfalls and overinterpretations in data analysis. This perspective serves as a comprehensive guide, educating the community on the nuances of refinement processes necessary for advancing COF research with clarity and accuracy.

Hydrogen-Bond-Regulated Mechanochemical Synthesis of Covalent Organic Frameworks: Cocrystal Precursor Strategy for Confined Assembly

Two-dimensional imine covalent organic frameworks (2D imine-COFs) are crystalline porous materials with broad application prospects. Despite the efforts into their design and synthesis, the mechanisms of their formation are still not fully understood. Herein, a one-pot two-step mechanochemical cocrystal precursor synthetic strategy is developed for efficient construction of 2D imine-COFs. The mechanistic investigation demonstrated that the cocrystal precursors of 4,4',4''-(1,3,5-triazine-2,4,6-triyl)trianiline (TAPT) and p-toluenesulphonic acid (PTSA) sufficiently regulate the crystalline structure of COF. Evidenced by characterizations and theoretical studies, a helical hydrogen-bond network was constructed by the N-H⋅⋅⋅O supramolecular synthons between amino and sulfonic groups in TAPT-PTSA, demonstrating the role of cocrystals in promoting the organized stacking of interlayer π-π interactions, layer arrangement, and interlayer spacing, thus facilitating the orderly assembly of COFs. Moreover, the protonation degree of TAPT amines, which tuned nucleophilic directionality, enabled the sequential progression of intra- and interlayer imine condensation reactions, inhibiting the formation of amorphous polymers. The transformation from cocrystal precursors to COFs was achieved through comprehensive control of hydrogen bond and covalent bond sites. This work significantly advances the concept of hydrogen-bond-regulated COF assembly and its mechanochemical method in the design and synthesis of 2D imine-COFs, further elucidating the mechanistic aspects of their mechanochemical synthesis.

Triazine-Based Large-Sized Single-Crystalline Two-Dimensional Covalent Organic Framework for High-Performance Lithium-Ion Batteries

A large-sized single crystalline 2D COFs with excellent crystallinity and stability was prepared through the traditional thermal solvent method. The electrochemical performance can be significantly enhanced using a straightforward hybrid approach that involves in situ growth of the 2D COFs on multi-walled carbon nanotubes (MWCNTs). Both the advantages of COFs and CNTs are mutually enhanced. The single-crystalline feature of the obtained COFs improves the structural integrity and brings excellent chemical and electrochemical stabilities for lithium-ion battery applications. The resultant COF-CNT core-shell hybrids greatly improved the conductivity and demonstrated excellent lithium-ion storage performance with a high capacity of 228 mAh g-1 (0.2 A g-1).

Synthesis of Single-Crystal Two-Dimensional Covalent Organic Frameworks and Uncovering Their Hidden Structural Features by Three-Dimensional Electron Diffraction

Two-dimensional covalent organic frameworks (2D COFs) are formed by the polycondensation of geometrically specific monomers to grow covalently connected 2D polygonal polymers over the a-b plane and supramolecular polymerization and/or crystallization of 2D sheets along the c direction to constitute layer architectures. Despite various efforts, the synthesis of single-crystal 2D COFs remains a challenging goal. Here, we report the synthesis of single-crystal 2D COFs, by taking the representative imine-linked TPB-DMTP-COF as an example, to reveal the key synthetic parameters that control the crystallization of 2D COFs. We systematically tune the synthetic conditions including the glassware setup, the degas method, the solvent, the temperature, the modulator, and the reaction time and observed that all these parameters greatly affect the polymerization and crystallization processes, controlling the crystal quality. We found that a homogeneous system with all components dissolved and the presence of a suitable modulator at a temperature of 50-70 °C allows the growth of TPB-DMTP-COF single crystals as isolated individual rods, with tunable diameters of 200 nm to 3 μm and a length of 1-20 μm. The single-crystal structure was characterized by three-dimensional electron diffraction (3DED), which revealed two conformations of trans and cis for the linker in the 2D polymer sheets, which stack in an antiparallel mode to shape the frameworks with double-sized unit cells. These results uncover these hidden structural features which have been overlooked in polycrystalline and single-crystal studies and provide new insights into the synthesis of high-quality single crystals of 2D COFs.

Preserving High Porosity of Covalent Organic Frameworks via Functional Polymer Guest Introduction

Due to their high structural tunability, remarkable internal surface areas, readily accessible pore space, and host of possible applications, covalent organic frameworks (COFs) remain at the forefront of materials science research. Unfortunately, many COFs suffer from structural distortions or pore collapse during activation, which can lead to a substantial loss of crystallinity and functionality. Thus, herein, we demonstrate a facile method to address this issue by introducing polymer guests. The polymer adheres to the COF internal pore wall, acting as a supporting pillar during activation and effectively preserving the COF porosity and crystallinity. In fact, the surface area of one COF/polymer composite, known as TAPB-TA/PDA, was boosted by a factor of 16 when compared to the parent COF, TAPB-TA. More importantly, the now robust COF structure was able to resist layer shifting and order loss during both solvent immersion and removal. The introduction of functional polymer guests not only solidifies the COF structure and preserves its high porosity but is also shown to enhance the transport and separation of photogenerated charge carriers, thereby facilitating hydrogen evolution during photocatalytic water splitting. Molecular dynamics simulations further support experimental observations that the incorporation of PDA within the COF pores reinforces the walls, preventing its collapse. The proposed mechanism is based on the adsorption of PDA oligomers along the c direction of the unit cell, fastening the COF layers in place via van der Waals interactions. This kind of interaction locks -N═CH-Ph-CH═N- units in a trans-configuration in the COF pores.

Photocatalytic applications of covalent organic frameworks: synthesis, characterization, and utility

Photocatalysis has emerged as an energy efficient and safe method to perform organic transformations, and many semiconductors have been studied for use as photocatalysts. Covalent organic frameworks (COFs) are an established class of crystalline, porous materials constructed from organic units that are easily tunable. COFs importantly display semiconductor properties and respectable photoelectric behaviour, making them a strong prospect as photocatalysts. In this review, we summarize the design, synthetic methods, and characterization techniques for COFs. Strategies to boost photocatalytic performance are also discussed. Then the applications of COFs as photocatalysts in a variety of reactions are detailed. Finally, a summary, challenges, and future opportunities for the development of COFs as efficient photocatalysts are entailed.

Conversion of Isolated Voids into Channel Spaces by Modulating the Stacking Manner of Hydrogen-Bonded Ladders

1,2,3,4-Tetrakis(carboxyphenyl)benzene (CPB) forms a predictable ladder-shaped porous motif through intermolecular hydrogen-bonding of the carboxy groups. The stacking manner of the ladder motif, on the other hand, cannot be controlled, yielding a crystal structure with discrete inclusion spaces. To modulate the stacking manner, its derivative CPB(OMe) with methoxy substituent groups at 5,6-positions was synthesized and crystallized to yield a crystalline hydrogen-bonded organic framework (HOF), in which the ladder motifs are stacking with a different manner to form 1D inclusion channels, instead of discrete voids. Because of the channel structure, removal of the included solvent molecules can be easily conducted, and the resultant activated HOF CPB(OMe)-a exhibited micropores with BET surface area of 199 m2 g-1, which is larger than that of the other HOF CPB-a.

Dimensionality and Molecular Packing Control of Covalent Organic Frameworks through Pendant Group Design

Tuning the dimensions and molecular packing geometry of crystalline organic frameworks and polymers represents an important challenge for reticular chemistry. Here we show that for extended structures made of 1,3,6,8-tetrakis(4-aminophenyl)pyrene (PyTTA) linked with methoxy group functionalized terephthalaldehyde aldehydes, simple substituents on the aldehyde linker can have profound structure directing effects due to noncovalent interactions. Specifically, reacting 2,3-dimethoxyterephthalaldehyde with PyTTA gives a 2D covalent organic framework with unique AA-inclined-AA stacking and bilayer pyrene motifs, whereas reacting 2,5-dimethoxyterephthalaldehyde with PyTTA gives a 1D crystalline polymer with AB stacking and isolated pyrene motifs. Both materials show high crystallinity, allowing for atomic resolution structure determination using three-dimensional electron diffraction, and the similarity of their fluorescence properties shows the electronic structures of pyrene-based extended structures mostly depends on the in-plane structures, which is supported by density functional theory calculations. These two pyrene-based extended structures also show different fluorescence responses to organic vapors due to different pore environments. The current work shows the potential of noncovalent interactions in the reticular design of functional organic materials.

Topology Control of Covalent Organic Frameworks with Interlaced Unsaturated 2D and Saturated 3D Units for Boosting Electrocatalytic Hydrogen Peroxide Production

Modulating the electronic state of multicomponent covalent organic framework (COF) electrocatalysts is crucial for enhancing catalytic activity. However, the effect of dimensionality on their physicochemical functionalities is still lacking. Herein, we report an interlaced unsaturated 2D and saturated 3D strategy to develop multicomponent-regulated COFs with tunable gradient dimensionality for high selectivity and activity electrocatalysis. Compared with the two-component 2D and 3D model COFs, the 2D/3D framework interlaced COFs with locally irregular dimensions and electronic structures are more practical in optimizing the intrinsic electrode surface reaction and mass transfer. Remarkably, the unsaturated 2D-inserted 3D TAE-COF regulates the adsorption mode of OOH* species to supply a favorable dynamic pathway for the H2O2 process, thereby achieving an excellent production rate of 8.50 mol gcat -1 h-1. Moreover, utilizing theoretical calculation and in situ ATR-FTIR experiment, we found that the central carbon atom of the tetraphenyl-based unit (site-1 and site-6) are potential active sites. This strategy of operating the adsorption ability of reactants with dimensionality-interconnected building blocks provides an idea for designing durable and efficient electrocatalysts.

Taming Two-Dimensional Polymerization by a Machine-Learning Discovered Crystallization Model

Rapidly synthesizing high-quality two-dimensional covalent organic frameworks (2D COFs) is crucial for their practical applications. While strategies such as slow monomer addition have been developed based on an empirical understanding of their formation process, quantitative guidance remains absent, which prohibits precise optimizations of the experimental conditions. Here, we use a machine-learning approach that overcomes the challenges associated with bottom-up model derivation for the non-classical 2D COF crystallization processes. The resulting model, referred to as NEgen1, establishes correlations among the induction time, nucleation rate, growth rate, bond-forming rate constants, and common solution synthesis conditions for 2D COFs that grow by a nucleation-elongation mechanism. The results elucidate the detailed competition between the nucleation and growth dynamics in solution, which has been inappropriately described previously by classical, empirical models with assumptions invalid for 2D COF polymerization. By understanding the dynamic processes at play, the NEgen1 model reveals a simple strategy of gradually increasing monomer addition speed for growing large 2D COF crystals. This insight enables us to rapidly synthesize large COF-5 colloids, which could only be achieved previously by prolonged reaction times or by introducing chemical modulators. These results highlight the potential for systematically improving the crystal quality of 2D COFs, which has wide-reaching relevance for many of the applications where 2D COFs are speculated to be valuable.

Single-Crystal Structural Analysis of 2D Metal-Organic Frameworks and Covalent Organic Frameworks by Three-Dimensional Electron Diffraction

ConspectusIn the development of 2D metal-organic frameworks (MOFs) and 2D covalent organic frameworks (COFs), obtaining structural details at the atomic level is crucial to understanding their properties and related mechanisms in potential applications. However, since 2D-MOFs and COFs are composed of layered structures and often exhibit sheet-like morphologies, it is challenging to grow large crystals suitable for single-crystal X-ray diffraction (SCXRD). Therefore, ab initio structure determination, which refers to solving the structure directly from experimental data without using any prior knowledge or computational input, is extremely rare for 2D-MOFs and COFs. In contrast to SCXRD, three-dimensional electron diffraction (3DED) only requires crystals sized in tens or hundreds of nanometers, making it an ideal method for single-crystal analysis of 2D-MOFs and COFs and obtaining their fine structural details.In this Account, we describe our recent development of the 3DED method and its application in structure determination and property studies of 2D-MOFs and COFs. A key development is the establishment of a continuous 3DED data collection protocol. By collecting electron diffraction (ED) patterns continuously while performing crystal tilting, the electron dose applied to the target nanocrystal is greatly reduced. This allows the acquisition of high-resolution 3DED data from 2D-MOFs and COFs by minimizing their damage under the electron beam. We have also developed an approach to couple 3DED with real-space structure solution methods, i.e., simulated annealing (SA), for single-crystal structural analysis of materials that do not have high crystallinity. We successfully determined two 2D-COF structures by combining 3DED with SA.We provide several examples demonstrating the application of 3DED for the ab initio structure determination of 2D-MOFs and COFs, revealing not only their in-plane structures but also their stacking modes at the atomic level. Notably, the obtained structural details serve as the foundation for further understanding the properties of 2D-MOFs and COFs, such as their electronic band structures, charge mobilities, etc. Beyond structure determination, we describe our work on using 3DED as a high-throughput method for the discovery of new materials. Using this approach, we discovered a novel MOF that was present only in trace amounts within a multiphasic mixture. Through this discovery, we were able to tune the synthesis conditions to obtain its pure phase.We detail how 3DED can be used to probe different levels of molecular motions in MOFs through the analysis of anisotropic displacement parameters (ADPs). Additionally, we show that 3DED can provide accurate information about intermolecular weak interactions such as hydrogen bonding and van der Waals (vdW) interactions. Our studies demonstrate that 3DED is a valuable method for the structural analysis of 2D-MOFs and COFs. We envision that 3DED can accelerate research in these fields by providing unambiguous structural models at the atomic level.

Structure Evolution of 2D Covalent Organic Frameworks Unveiled by Single-Crystal X-ray Diffraction

We report 9 crystal structures of a two-dimensional (2D) covalent organic framework (COF), including the parent Py-1P, 5 derivatives formed by chemical reactions, and 3 dynamic states by solvent exchange/loss. Structure details of these porous crystals, including stacking mode, interlayer distance, pore aperture, and incline angle, before, during, and after conversion processes in solution, were unveiled by single-crystal X-ray diffraction with resolutions up to 0.85 Å. The structure evolution is triggered by stepwise conformational transformation of the molecular building blocks in 2D COF, while their long-range ordering remained unsacrificed.

The development of catalysts and auxiliaries for the synthesis of covalent organic frameworks

Covalent organic frameworks (COFs) have recently seen significant advancements. Large quantities of structurally & functionally oriented COFs with a wide range of applications, such as gas adsorption, catalysis, separation, and drug delivery, have been explored. Recent achievements in this field are primarily focused on advancing synthetic methodologies, with catalysts playing a crucial role in achieving highly crystalline COF materials, particularly those featuring novel linkages and chemistry. A series of reviews have already been published over the last decade, covering the fundamentals, synthesis, and applications of COFs. However, despite the pivotal role that catalysts and auxiliaries play in forming COF materials and adjusting their properties (e.g., crystallinity, porosity, stability, and morphology), limited attention has been devoted to these essential components. In this Critical Review, we mainly focus on the state-of-the-art progress of catalysts and auxiliaries applied to the synthesis of COFs. The catalysts include four categories: acid catalysts, base catalysts, transition-metal catalysts, and other catalysts. The auxiliaries, such as modulators, oxygen, and surfactants, are discussed as well. This is then followed by the description of several specific applications derived from the utilization of catalysts and auxiliaries. Lastly, a perspective on the major challenges and opportunities associated with catalysts and auxiliaries is provided.

Analysis of COF-300 synthesis: probing degradation processes and 3D electron diffraction structure

Although COF-300 is often used as an example to study the synthesis and structure of (3D) covalent organic frameworks (COFs), knowledge of the underlying synthetic processes is still fragmented. Here, an optimized synthetic procedure based on a combination of linker protection and modulation was applied. Using this approach, the influence of time and temperature on the synthesis of COF-300 was studied. Synthesis times that were too short produced materials with limited crystallinity and porosity, lacking the typical pore flexibility associated with COF-300. On the other hand, synthesis times that were too long could be characterized by loss of crystallinity and pore order by degradation of the tetrakis(4-aminophenyl)methane (TAM) linker used. The presence of the degradation product was confirmed by visual inspection, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). As TAM is by far the most popular linker for the synthesis of 3D COFs, this degradation process might be one of the reasons why the development of 3D COFs is still lagging compared with 2D COFs. However, COF crystals obtained via an optimized procedure could be structurally probed using 3D electron diffraction (3DED). The 3DED analysis resulted in a full structure determination of COF-300 at atomic resolution with satisfying data parameters. Comparison of our 3DED-derived structural model with previously reported single-crystal X-ray diffraction data for this material, as well as parameters derived from the Cambridge Structural Database, demonstrates the high accuracy of the 3DED method for structure determination. This validation might accelerate the exploitation of 3DED as a structure determination technique for COFs and other porous materials.

New Advances in Covalent Network Polymers via Dynamic Covalent Chemistry

Covalent network polymers, as materials composed of atoms interconnected by covalent bonds in a continuous network, are known for their thermal and chemical stability. Over the past two decades, these materials have undergone significant transformations, gaining properties such as malleability, environmental responsiveness, recyclability, crystallinity, and customizable porosity, enabled by the development and integration of dynamic covalent chemistry (DCvC). In this review, we explore the innovative realm of covalent network polymers by focusing on the recent advances achieved through the application of DCvC. We start by examining the history and fundamental principles of DCvC, detailing its inception and core concepts and noting its key role in reversible covalent bond formation. Then the reprocessability of covalent network polymers enabled by DCvC is thoroughly discussed, starting from the significant milestones that marked the evolution of these polymers and progressing to their current trends and applications. The influence of DCvC on the crystallinity of covalent network polymers is then reviewed, covering their bond diversity, synthesis techniques, and functionalities. In the concluding section, we address the current challenges faced in the field of covalent network polymers and speculates on potential future directions.

Early stages of covalent organic framework formation imaged in operando

Covalent organic frameworks (COFs) are a functional material class able to harness, convert and store energy. However, after almost 20 years of research, there are no coherent prediction rules for their synthesis conditions. This is partly because of an incomplete picture of nucleation and growth at the early stages of formation. Here we use the optical technique interferometric scattering microscopy (iSCAT)1-3 for in operando studies of COF polymerization and framework formation. We observe liquid-liquid phase separation, pointing to the existence of structured solvents in the form of surfactant-free (micro)emulsions in conventional COF synthesis. Our findings show that the role of solvents extends beyond solubility to being kinetic modulators by compartmentation of reactants and catalyst. Taking advantage of these observations, we develop a synthesis protocol for COFs using room temperature instead of elevated temperatures. This work connects framework synthesis with liquid phase diagrams and thereby enables an active design of the reaction environment, emphasizing that visualization of chemical reactions by means of light-scattering-based techniques can be a powerful approach for advancing rational materials synthesis.

Continuous flow synthesis and post-synthetic conversion of single-crystalline covalent organic frameworks

The synthesis and scale-up of high quality covalent organic frameworks (COFs) remains a challenge due to slow kinetics of the reversible bond formation and the need for precise control of reaction conditions. Here we report the rapid synthesis of faceted single crystals of two-dimensional (2D) COFs using a continuous flow reaction process. Two imine linked materials were polymerized to the hexagonal CF-TAPB-DMPDA and the rhombic CF-TAPPy-PDA COF, respectively. The reaction conditions were optimized to produce single crystals of micrometer size, which notably formed when the reaction was cooling to room temperature. This indicated a growth mechanism consistent with the fusion of smaller COF particles. The optimized conditions were used to demonstrate the scalability of the continuous approach by synthesizing high quality, faceted COFs at a rate of more than 1 g h-1. The materials showed high crystallinity and porosity with surface areas exceeding 2000 m2 g-1. Additionally, the versatility of the continuous flow reaction approach was demonstrated on a post-synthetic single crystal to single crystal demethylation of CF-TAPB-DMPDA to afford a hydroxyl functionalized COF CF-TAPB-DHPDA. Throughout the modification process, the material maintained its hexagonal morphology, crystallinity, and porosity. This work reports the first example of synthesizing and post-synthetically modifying imine linked COF single crystals in continuous flow and will prove a first step towards scaling high quality COFs to industrial levels.

Topological Analysis and Structural Determination of 3D Covalent Organic Frameworks

3D covalent organic frameworks (3D COFs) constitute a new type of crystalline materials that consist of a range of porous structures with numerous applications in the fields of adsorption, separation, and catalysis. However, because of the complexity of the three-periodic net structure, it is desirable to develop a thorough structural comprehension, along with a means to precisely determine the actual structure. Indeed, such advancements would considerably contribute to the rational design and application of 3D COFs. In this review, the reported topologies of 3D COFs are introduced and categorized according to the configurations of their building blocks, and a comprehensive overview of diffraction-based structural determination methods is provided. The current challenges and future prospects for these materials will also be discussed.

Dynamic Insights into the Growth Mechanisms of 2D Covalent Organic Frameworks on Graphene Surfaces

Producing high-quality two-dimensional (2D) covalent organic frameworks (COFs) is crucial for industrial applications. However, this remains significantly challenging with current synthetic techniques. A deep understanding of the intermolecular interactions, reaction temperature, and oligomers is essential to facilitate the growth of highly crystalline COF films. Herein, molecular dynamics simulations were employed to explore the growth of 2D COFs from monomer assemblies on graphene. Our results showed that chain growth reactions dominated the COF surface growth and that van der Waals (vdW) interactions were important in enhancing the crystallinity through monomer preorganization. Moreover, appropriately tuning the reaction temperature improved the COF crystallinity and minimized the effects of amorphous oligomers. Additionally, the strength of the interface between the COF and the graphene substrate indicated that the adhesion force was proportional to the crystallinity of the COF. This work reveals the mechanisms for nucleation and growth of COFs on surfaces and provides theoretical guidance for fabricating high-quality 2D polymer-based crystalline nanomaterials.

Positional Thiophene Isomerization: A Geometric Strategy for Precisely Regulating the Electronic State of Covalent Organic Frameworks to Boost Oxygen Reduction

With the oxygen conversion efficiency of metal-free carbon-based fuel cells dramatically improved, the building blocks of covalent organic frameworks (COFs) raised principal concerns on the catalytic active sites with indistinct electronic states. Herein, to address this issue, we demonstrate COFs for oxygen reduction reaction (ORR) by regulating the edge-hanging thiophene units, and the molecular geometries are further modulated via positional thiophene isomerization strategy, affording isomeric COF-α with 2-substitution and COF-β with 3-substitution on the frameworks. The electronic states and intermediate adsorption ability are well-regulated through geometric modification, resulting in controllable chemical activity and local density of π-electrons. Notably, the introduction of thiophene units with different substitution positions into a pristine pure carbon-based COF model COF-Ph achieves excellent activity with a half-wave potential of 0.76 V versus the reversible hydrogen electrode, which is higher than most of those metal-free or metal-based electrocatalysts. Utilizing the combination of theoretical prediction and in situ Raman spectra, we show that the isomeric thiophene skeleton (COF-α and COF-β) can induce the dangling unit activation, accurately identifying the pentacyclic-carbon (thiophene α-position) adjacent to sulfur atom as active sites. The results suggest that the isomeric dangling groups in COFs are suitable for the ORR with promising geometry construction.

Fast growth of single-crystal covalent organic frameworks for laboratory x-ray diffraction

The imine-exchange strategy makes single-crystal growth of covalent organic frameworks (COFs) with large size (>15 microns) possible but is a time-consuming process (15 to 80 days) that has had limited success (six examples) and restricts structural characterization to synchrotron-radiation sources for x-ray diffraction studies. We developed a CF3COOH/CF3CH2NH2 protocol to harvest single-crystal COFs within 1 to 2 days with crystal sizes of up to 150 microns. The generality was exemplified by the feasible growth of 16 high-quality single-crystal COFs that were structurally determined by laboratory single-crystal x-ray diffraction with resolutions of up to 0.79 angstroms. The structures obtained included uncommon interpenetration of networks, and the details of the structural evolution of conformational isomers and host-guest interaction could be determined at the atomic level.

Control over Charge Separation by Imine Structural Isomerization in Covalent Organic Frameworks with Implications on CO2 Photoreduction

Two-dimensional covalent organic frameworks (COFs) are an emerging class of photocatalytic materials for solar energy conversion. In this work, we report a pair of structurally isomeric COFs with reversed imine bond directions, which leads to drastic differences in their physical properties, photophysical behaviors, and photocatalytic CO2 reduction performance after incorporating a Re(bpy)(CO)3Cl molecular catalyst through bipyridyl units on the COF backbone (Re-COF). Using the combination of ultrafast spectroscopy and theory, we attributed these differences to the polarized nature of the imine bond that imparts a preferential direction to intramolecular charge transfer (ICT) upon photoexcitation, where the bipyridyl unit acts as an electron acceptor in the forward imine case (f-COF) and as an electron donor in the reverse imine case (r-COF). These interactions ultimately lead the Re-f-COF isomer to function as an efficient CO2 reduction photocatalyst, while the Re-r-COF isomer shows minimal photocatalytic activity. These findings not only reveal the essential role linker chemistry plays in COF photophysical and photocatalytic properties but also offer a unique opportunity to design photosensitizers that can selectively direct charges.

Linker-Guided Growth of Single-Crystal Covalent Organic Frameworks

The core features of covalent organic frameworks (COFs) are crystallinity and porosity. However, the synthesis of single-crystal COFs with monomers of diverse reactivity and adjustment of their pore structures remain challenging. Here, we show that linkers that can react with a node to form single-crystal COFs can guide other linkers that form either COFs or amorphous polymers with the node to gain single-crystal COFs with mixed components, which are homogeneous on the unit cell scale with controlled ratios. With the linker-guided crystal growth method, we created nine types of single-crystal COFs with up to nine different components, which are more complex than any known crystal. The structure of the crystal adapted approximately to that of the main component, and its pore volume could be expanded up to 8.8%. Different components lead to complex and diverse pore structures and offer the possibilities to gain positive synergies, as exemplified by a bicomponent COF with 2200 and 733% SO2 uptake capacity of that of the two pure-component counterparts at 298 K and 0.002 bar. The selectivity for separation of SO2/CO2 ranges from 1230 to 4247 for flue gas based on ideal adsorbed solution theory, recording porous crystals. The bicomponent COF also exhibits a 1300% retention time of its pure-component counterparts for SO2 in a dynamic column breakthrough experiment for deep desulfurization.

Ultra-fast supercritically solvothermal polymerization for large single-crystalline covalent organic frameworks

Crystalline polymer materials, e.g., hyper-crosslinked polystyrene, conjugate microporous polymers and covalent organic frameworks, are used as catalyst carriers, organic electronic devices and molecular sieves. Their properties and applications are highly dependent on their crystallinity. An efficient polymerization strategy for the rapid preparation of highly or single-crystalline materials is beneficial not only to structure-property studies but also to practical applications. However, polymerization usually leads to the formation of amorphous or poorly crystalline products with small grain sizes. It has been a challenging task to efficiently and precisely assemble organic molecules into a single crystal through polymerization. To address this issue, we developed a supercritically solvothermal method that uses supercritical carbon dioxide (sc-CO2) as the reaction medium for polymerization. Sc-CO2 accelerates crystal growth due to its high diffusivity and low viscosity compared with traditional organic solvents. Six covalent organic frameworks with different topologies, linkages and crystal structures are synthesized by this method. The as-synthesized products feature polarized photoluminescence and second-harmonic generation, indicating their high-quality single-crystal nature. This method holds advantages such as rapid growth rate, high productivity, easy accessibility, industrial compatibility and environmental friendliness. In this protocol, we provide a step-by-step procedure including preparation of monomer dispersion, polymerization in sc-CO2, purification and characterization of the single crystals. By following this protocol, it takes 1-5 min to grow sub-mm-sized single crystals by polymerization. The procedure takes ~4 h from preparation of monomer dispersion and polymerization in sc-CO2 to purification and drying of the product.

A Review on Covalent Organic Frameworks as Artificial Interface Layers for Li and Zn Metal Anodes in Rechargeable Batteries

Li and Zn metals are considered promising negative electrode materials for the next generation of rechargeable metal batteries because of their non-toxicity and high theoretical capacity. However, the uneven deposition of metal ions (Li+ , Zn2+ ) and the uncontrolled growth of dendrites result in poor electrochemical stability, unsatisfactory cycle life, and rapid capacity decay of batteries assembled with Li and Zn electrodes. Owing to the unique internal directional channels and abundant redox active sites of covalent organic frameworks (COFs), they can be used to promote uniform deposition of metal ions during stripping/electroplating through interface modification strategies, thereby inhibiting dendrite growth. COFs provide a new perspective in addressing the challenges faced by the anodes of Li metal batteries and Zn ion batteries. This article discusses the stability and types of COFs, and summarizes some novel COF synthesis methods. Additionally, it reviews the latest progress and optimization methods of using COFs for metal anodes to improve battery performance. Finally, the main challenges faced in these areas are discussed. This review will inspire future research on metal anodes in rechargeable batteries.

2D Covalent Organic Framework Membranes for Liquid-Phase Molecular Separations: State of the Field, Common Pitfalls, and Future Opportunities

2D covalent organic frameworks (2D COFs) are attractive candidates for next-generation membranes due to their robust linkages and uniform, tunable pores. Many publications have claimed to achieve selective molecular transport through COF pores, but reported performance metrics for similar networks vary dramatically, and in several cases the reported experiments are inadequate to support such conclusions. These issues require a reevaluation of the literature. Published examples of 2D COF membranes for liquid-phase separations can be broadly divided into two categories, each with common performance characteristics: polycrystalline COF films (most >1 µm thick) and weakly crystalline or amorphous films (most <500 nm thick). Neither category has demonstrated consistent relationships between the designed COF pore structure and separation performance, suggesting that these imperfect materials do not sieve molecules through uniform pores. In this perspective, rigorous practices for evaluating COF membrane structures and separation performance are described, which will facilitate their development toward molecularly precise membranes capable of performing previously unrealized chemical separations. In the absence of this more rigorous standard of proof, reports of COF-based membranes should be treated with skepticism. As methods to control 2D polymerization improve, precise 2D polymer membranes may exhibit exquisite and energy efficient performance relevant for contemporary separation challenges.

Single-crystal polymers (SCPs): from 1D to 3D architectures

Single-crystal polymers (SCPs) with unambiguous chemical structures at atomic-level resolutions have attracted great attention. Obtaining precise structural information of these materials is critical as it enables a deeper understanding of the potential driving forces for specific packing and long-range order, secondary interactions, and kinetic and thermodynamic factors. Such information can ultimately lead to success in controlling the synthesis or engineering of their crystal structures for targeted applications, which could have far-reaching impact. Successful synthesis of SCPs with atomic level control of the structures, especially for those with 2D and 3D architectures, is rare. In this review, we summarize the recent progress in the synthesis of SCPs, including 1D, 2D, and 3D architectures. Solution synthesis, topochemical synthesis, and extreme condition synthesis are summarized and compared. Around 70 examples of SCPs with unambiguous structure information are presented, and their synthesis methods and structural analysis are discussed. This review offers critical insights into the structure-property relationships, providing guidance for the future rational design and bottom-up synthesis of a variety of highly ordered polymers with unprecedented functions and properties.

Scalable Synthesis and Electrocatalytic Performance of Highly Fluorinated Covalent Organic Frameworks for Oxygen Reduction

In this study, we present a novel approach for the synthesis of covalent organic frameworks (COFs) that overcomes the common limitations of non-scalable solvothermal procedures. Our method allows for the room-temperature and scalable synthesis of a highly fluorinated DFTAPB-TFTA-COF, which exhibits intrinsic hydrophobicity. We used DFT-based calculations to elucidate the role of the fluorine atoms in enhancing the crystallinity of the material through corrugation effects, resulting in maximized interlayer interactions, as disclosed both from PXRD structural resolution and theoretical simulations. We further investigated the electrocatalytic properties of this material towards the oxygen reduction reaction (ORR). Our results show that the fluorinated COF produces hydrogen peroxide selectively with low overpotential (0.062 V) and high turnover frequency (0.0757 s-1 ) without the addition of any conductive additives. These values are among the best reported for non-pyrolyzed and metal-free electrocatalysts. Finally, we employed DFT-based calculations to analyse the reaction mechanism, highlighting the crucial role of the fluorine atom in the active site assembly. Our findings shed light on the potential of fluorinated COFs as promising electrocatalysts for the ORR, as well as their potential applications in other fields.

Reticular Synthesis of Highly Crystalline Three-Dimensional Mesoporous Covalent-Organic Frameworks for Lipase Inclusion

The synthesis and application of three-dimensional (3D) mesoporous covalent-organic frameworks (COFs) are still to be developed. Herein, two mesoporous 3D COFs with an stp topology were synthesized in a highly crystalline form with aniline as the modulator. The chemical composition of these COFs was confirmed by Fourier transform infrared (FT-IR) and 13C cross-polarization magic angle spinning nuclear magnetic resonance (NMR) spectroscopies. These 3D mesoporous COFs were highly crystalline and exhibited permanent porosity and good chemical stability in both aqueous and organic media. The space group and unit cell parameters of COF HFPTP-TAE were verified by powder X-ray diffraction (PXRD), small-angle X-ray scattering, and three-dimensional electron diffraction (3D ED). The appropriate pore size of the COF HFPTP-TAE facilitated the inclusion of enzyme lipase PS with a loading amount of 0.28 g g-1. The lipase⊂HFPTP-TAE (⊂ refers to "include in") composite exhibited high catalytic activity, good thermal stability, and a wide range of solvent tolerance. Specifically, it could catalyze the alcoholysis of aspirin methyl ester (AME) with high catalytic efficiency. Oriented one-dimensional (1D) channel mesopores in HFPTP-TAE accommodated lipase, meanwhile preventing them from aggregation, while windows on the wall of the 1D channel favored molecular diffusion; thus, this COF-enzyme design outperformed its amorphous isomer, two-dimensional (2D) mesoporous COF, 3D mesoporous COF with limited crystallinity, and mesoporous silica as an enzyme host.

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.

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.

Covalent organic frameworks for CO2 capture: from laboratory curiosity to industry implementation

CO2 concentration in the atmosphere has increased by about 40% since the 1960s. Among various technologies available for carbon capture, adsorption and membrane processes have been receiving tremendous attention due to their potential to capture CO2 at low costs. The kernel for such processes is the sorbent and membrane materials, and tremendous progress has been made in designing and fabricating novel porous materials for carbon capture. Covalent organic frameworks (COFs), a class of porous crystalline materials, are promising sorbents for CO2 capture due to their high surface area, low density, controllable pore size and structure, and preferable stabilities. However, the absence of synergistic developments between materials and engineering processes hinders achieving the qualitative leap for net-zero emissions. Considering the lack of a timely review on the combination of state-of-the-art COFs and engineering processes, in this Tutorial Review, we emphasize the developments of COFs for meeting the challenges of carbon capture and disclose the strategies of fabricating COFs for realizing industrial implementation. Moreover, this review presents a detailed and basic description of the engineering processes and industrial status of carbon capture. It highlights the importance of machine learning in integrating simulations of molecular and engineering levels. We aim to stimulate both academia and industry communities for joined efforts in bringing COFs to practical carbon capture.

Beyond Solvothermal: Alternative Synthetic Methods for Covalent Organic Frameworks

Covalent organic frameworks (COFs) are crystalline porous organic materials that hold a wealth of potential applications across various fields. The development of COFs, however, is significantly impeded by the dearth of efficient synthetic methods. The traditional solvothermal approach, while prevalent, is fraught with challenges such as complicated processes, excessive energy consumption, long reaction times, and limited scalability, rendering it unsuitable for practical applications. The quest for simpler, quicker, more energy-efficient, and environmentally benign synthetic strategies is thus paramount for bridging the gap between academic COF chemistry and industrial application. This Review provides an overview of the recent advances in alternative COF synthetic methods, with a particular emphasis on energy input. We discuss representative examples of COF synthesis facilitated by microwave, ultrasound, mechanic force, light, plasma, electric field, and electron beam. Perspectives on the advantages and limitations of these methods against the traditional solvothermal approach are highlighted.

Modulating the Interlayer Stacking of Covalent Organic Frameworks for Efficient Acetylene Separation

Controllable modulation of the stacking modes of 2D (two-dimensional) materials can significantly influence their properties and functionalities but remains a formidable synthetic challenge. Here, an effective strategy is proposed to control the layer stacking of imide-linked 2D covalent organic frameworks (COFs) by altering the synthetic methods. Specifically, a modulator-assisted method can afford a COF with rare ABC stacking without the need for any additives, while solvothermal synthesis leads to AA stacking. The variation of interlayer stacking significantly influences their chemical and physical properties, including morphology, porosity, and gas adsorption performance. The resultant COF with ABC stacking shows much higher C2 H2 capacity and selectivity over CO2 and C2 H4 than the COF with AA stacking, which is not demonstrated in the COF field yet. Furthermore, the outstanding practical separation ability of ABC stacking COF is confirmed by breakthrough experiments of C2 H2 /CO2 (50/50, v/v) and C2 H2 /C2 H4 (1/99, v/v), which can selectively remove C2 H2 with good recyclability. This work provides a new direction to produce COFs with controllable interlayer stacking modes.

Catalytic Linkage Engineering of Covalent Organic Frameworks for the Oxygen Reduction Reaction

Metal-free covalent organic frameworks (COFs) have been employed to catalyze the oxygen reduction reaction (ORR). To achieve high activity and selectivity, various building blocks containing heteroatoms and groups linked by imine bonds were used to create catalytic COFs. However, the roles of linkages of COFs in ORR have not been investigated. In this work, the catalytic linkage engineering has been employed to modulate the catalytic behaviors. To create single catalytic sites while avoiding other possible catalytic sites, we synthesized COFs from benzene units linked by various bonds, such as imine, amide, azine, and oxazole bonds. Among these COFs, the oxazole-linkage in COFs enables to catalyze the ORR with the highest activity, which achieved a half-wave potential of 0.75 V and a limited current density of 5.5 mA cm-2 . Moreover, the oxazole-linked COF achieved a conversion frequency (TOF) value of 0.0133 S-1 , which were 1.9, 1.3, and 7.4-times that of azine-, amide- and imine-COFs, respectively. The theoretical calculation showed that the carbon atoms in oxazole linkages facilitated the formation of OOH* and promoted protonation of O* to form the OH*, thus advancing the catalytic activity. This work guides us on which linkages in COFs are suitable for ORR.

Exceptionally high charge mobility in phthalocyanine-based poly(benzimidazobenzophenanthroline)-ladder-type two-dimensional conjugated polymers

Two-dimensional conjugated polymers (2DCPs), composed of multiple strands of linear conjugated polymers with extended in-plane π-conjugation, are emerging crystalline semiconducting polymers for organic (opto)electronics. They are represented by two-dimensional π-conjugated covalent organic frameworks, which typically suffer from poor π-conjugation and thus low charge carrier mobilities. Here we overcome this limitation by demonstrating two semiconducting phthalocyanine-based poly(benzimidazobenzophenanthroline)-ladder-type 2DCPs (2DCP-MPc, with M = Cu or Ni), which are constructed from octaaminophthalocyaninato metal(II) and naphthalenetetracarboxylic dianhydride by polycondensation under solvothermal conditions. The 2DCP-MPcs exhibit optical bandgaps of ~1.3 eV with highly delocalized π-electrons. Density functional theory calculations unveil strongly dispersive energy bands with small electron-hole reduced effective masses of ~0.15m0 for the layer-stacked 2DCP-MPcs. Terahertz spectroscopy reveals the band transport of Drude-type free carriers in 2DCP-MPcs with exceptionally high sum mobility of electrons and holes of ~970 cm2 V-1 s-1 at room temperature, surpassing that of the reported linear conjugated polymers and 2DCPs. This work highlights the critical role of effective conjugation in enhancing the charge transport properties of 2DCPs and the great potential of high-mobility 2DCPs for future (opto)electronics.

Minimalist Design for Solar Energy Conversion: Revamping the π-Grid of an Organic Framework into Open-Shell Superabsorbers

We apply a versatile reaction to a versatile solid: the former involves the electron-deficient alkene tetracyanoethylene (TCNE) as the guest reactant; the latter consists of stacked 2D honeycomb covalent networks based on the electron-rich β-ketoenamine hinges that also activate the conjugated, connecting alkyne units. The TCNE/alkyne reaction is a [2 + 2] cycloaddition-retroelectrocyclization (CA-RE) that forms strong push-pull units directly into the backbone of the framework-i.e., using only the minimalist "bare-bones" scaffold, without the need for additional side groups of alkynes or other functions. The ability of the stacked alkyne units (i.e., as part of the honeycomb mass) to undergo such extensive rearrangement highlights the structural flexibility of these covalent organic framework (COF) hosts. The COF solids remain porous, crystalline, and air-/water-stable after the CA-RE modification, while the resulting push-pull units feature distinct open-shell/free-radical character, are strongly light-absorbing, and shift the absorption ends from 590 nm to around 1900 nm (band gaps from 2.17-2.23 to 0.87-0.95 eV), so as to better capture sunlight (especially the infrared region which takes up 52% of the solar energy). As a result, the modified COF materials achieve the highest photothermal conversion performances, holding promise in thermoelectric power generation and solar steam generation (e.g., with solar-vapor conversion efficiencies >96%).

Covalent Organic Frameworks as Model Materials for Fundamental and Mechanistic Understanding of Organic Battery Design Principles

Redox-active covalent organic frameworks (COFs) have recently emerged as advanced electrodes in polymer batteries. COFs provide ideal molecular precision for understanding redox mechanisms and increasing the theoretical charge-storage capacities. Furthermore, the functional groups on the pore surface of COFs provide highly ordered and easily accessible interaction sites, which can be modeled to establish a synergy between ex situ/in situ mechanism studies and computational methods, permitting the creation of predesigned structure-property relationships. This perspective integrates and categorizes the redox functionalities of COFs, providing a deeper understanding of the mechanistic investigation of guest ion interactions in batteries. Additionally, it highlights the tunable electronic and structural properties that influence the activation of redox reactions in this promising organic electrode material.

Structure determination of a low-crystallinity covalent organic framework by three-dimensional electron diffraction

Covalent organic frameworks (COFs) have been attracting intense research due to their permanent porosity, designable architecture, and high stability. However, COFs are challenging to crystallize and their synthesis often results in tiny crystal sizes and low crystallinities, which hinders an unambiguous structure determination. Herein, we demonstrate that the structure of low-crystallinity COF Py-1P nanocrystals can be solved by coupling three-dimensional electron diffraction (3DED) with simulated annealing (SA). The resulting model is comparable to that obtained from high-crystallinity samples by dual-space method. Moreover, for low-resolution 3DED data, the model obtained by SA shows a better framework than those provided by classic direct method, dual-space method, and charge flipping. We further simulate data with different resolutions to understand the reliability of SA under different crystal quality conditions. The successful determination of Py-1P structure by SA compared to other methods provides new knowledge for using 3DED to analyze low-crystallinity and nanosized materials.

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.