A Wavy Two-Dimensional Covalent Organic Framework from Core-Twisted Polycyclic Aromatic Hydrocarbons

Tubular Nanostructures from Large-Pore 2D Covalent Organic Frameworks

The synthesis of a wavy mesoporous 2D covalent organic framework (COF) with a 6-nm hexagonal pore lattice (Joa-COF-1) is reported. This has been achieved by the synthesis of a terpyrenyl linker of approximately 2.7 nm in length and its subsequent condensation with a 3-connected non-planar cata-hexabenzocoronene. Joa-COF-1 exists as non-covalent tubular domains composed of π-stacked 4 to 5 pores in 2D COF sections that can be separated by mild sonication, resulting in a family of tubular COF nanostructures that combine a 1D morphology with accessible mesopores.

Homochiral versus Racemic 2D Covalent Organic Frameworks

The synthesis of homochiral two-dimensional covalent organic frameworks (2D COFs) from chiral π-conjugated building blocks is challenging, as chiral units often lead to misaligned stacking interactions. In this work, we introduce helical chirality into 2D COFs using configurationally stable enantiopure and racemic [5]helicenes as linkers in the backbone of 2D [5]HeliCOFs as powders and films. Through condensation with 1,3,5-triformylbenzene (TFB) or 1,3,5-triformylphloroglucinol (TFP), our approach enables the efficient formation of a set of homochiral and racemic 2D [5]HeliCOFs. The resulting carbon-based crystalline and porous frameworks exhibit distinct structural features and different properties between homochiral and racemic counterparts. Propagation of helical chirality into the backbone of the crystalline frameworks leads to the observation of advanced chiroptical properties in the far-red visible spectrum, along with a less compact structure compared with the racemic frameworks. Homogeneous thin films of [5]HeliCOFs disclosed photoluminescent properties arising from the controlled growth of highly ordered π-conjugated lattices. The present study offers insight into general chiral framework formation and extends the Liebisch-Wallach rule to 2D COFs.

Semiconducting Covalent Organic Frameworks

Semiconductors form the foundational bedrock of modern electronics and numerous cutting-edge technologies. Particularly, semiconductors crafted from organic building blocks hold immense promise as next-generation pioneers, thanks to their vast array of chemical structures, customizable frontier orbital energy levels and bandgap structures, and easily adjustable π electronic properties. Over the past 50 years, advancements in chemistry and materials science have facilitated extensive investigations into small organic π compounds, oligomers, and polymers, resulting in a rich library of organic semiconductors. However, a longstanding challenge persists: how to organize π building units or chains into well-defined π structures, which are crucial for the performance of organic semiconductors. Consequently, the pursuit of methodologies capable of synthesizing and/or fabricating organic semiconductors with ordered structures has emerged as a frontier in organic and polymeric semiconductor research. In this context, covalent organic frameworks (COFs) stand out as unique platforms allowing for the covalent integration of organic π units into periodically ordered π structures, thus facilitating the development of semiconductors with extended yet precisely defined π architectures. Since their initial report in 2008, significant strides have been made in exploring various chemistries to develop semiconducting COFs, resulting in a rich library of structures, properties, functions, and applications. This review provides a comprehensive yet focused exploration of the general structural features of semiconducting COFs, outlining the basic principles of structural design, illustrating the linkage chemistry and synthetic strategies based on typical one-pot polymerization reactions to demonstrate the growth of bulk materials, nanosheets, films, and membranes. By elucidating the interactions between COFs and various entities such as photons, phonons, electrons, holes, ions, molecules, and spins, this review categorizes semiconducting COFs into nine distinct sections: semiconductors, photoconductors, light emitters, sensors, photocatalysts, photothermal conversion materials, electrocatalysts, energy storage electrodes, and radical spin materials, focusing on disclosing structure-originated properties and functions. Furthermore, this review scrutinizes structure-function correlations and highlights the unique features, breakthroughs, and challenges associated with semiconducting COFs. Furnished with foundational knowledges and state-of-the-art insights, this review predicts the fundamental issues to be addressed and outlines future directions for semiconducting COFs, offering a comprehensive overview of this rapidly evolving and remarkable field.

Covalent organic framework nanomaterials: Syntheses, architectures, and applications

Covalent Organic Frameworks (COFs) are characterized by high thermochemical stability, low backbone density, well-controlled physical and chemical properties, large specific surface volume and porosity, permanently open pore structure, and various synthesis strategies. These remarkable attributes confer COFs with significant potential for a myriad of applications ranging from catalysis technology, gas separation and storage, optoelectronic materials, environmental and energy sciences, and biomedical development. There are many synthetic design methods for COF materials, and dynamic covalent chemistry is the scientific basis of COF materials-oriented design, which gives the error correction ability of the covalent assembly process, and is the key to obtaining crystallization and stability at the same time. However, "crystallinity" and "stability" in the synthesis and preparation of COF materials are often like "You can't have your cake and eat it, too": on the one hand, the reversible covalent bonds used in the synthesis of highly crystalline COF framework are easy to decompose under extreme conditions, which greatly limits its application scenarios; On the other hand, although highly stable COF materials can be prepared by using irreversible covalent bonds, it is usually poor crystalline and difficult to have high performance. In addition, the strict deoxygenation operation required for synthesizing COF materials also limits its macro preparation and large-scale application. Therefore, the synthesis strategy and efficient preparation of highly stable and crystalline COF materials are a major obstacle to the practical application of this field. This paper describes the four structures of COF materials, as well as their synthesis methods, electrical energy-storing electrocatalysis, and significant environmental protection applications. The future directions, prospects, and possible barriers to the development of these materials are envisioned in.

Evolving Role of Conjugated Polymers in Nanoelectronics and Photonics

Conjugated polymers (CPs) have emerged as an interesting class of materials in modern electronics and photonics, characterized by their unique delocalized π-electron systems that confer high flexibility, tunable electronic properties, and solution processability. These organic polymers present a compelling alternative to traditional inorganic semiconductors, offering the potential for a new generation of optoelectronic devices. This review explores the evolving role of CPs, exploring the molecular design strategies and innovative approaches that enhance their optoelectronic properties. We highlight notable progress toward developing faster, more efficient, and environmentally friendly devices by analyzing recent advancements in CP-based devices, including organic photovoltaics, field-effect transistors, and nonvolatile memories. The integration of CPs in flexible sustainable technologies underscores their potential to revolutionize future electronic and photonic systems. As ongoing research pushes the frontiers of molecular engineering and device architecture, CPs are poised to play an essential role in shaping next-generation technologies that prioritize performance, sustainability, and adaptability.

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.

A 3D four-fold interpenetrated conductive metal-organic framework for fast and robust sodium-ion storage

Two-dimensional conductive metal-organic frameworks (2D c-MOFs) with high electrical conductivity and tunable structures hold significant promise for applications in metal-ion batteries. However, the construction of 3D interpenetrated c-MOFs for applications in metal-ion batteries is rarely reported. Herein, a 3D four-fold interpenetrated c-MOF (Cu-DBC) constructed by conjugated and contorted dibenzo[g,p]chrysene-2,3,6,7,10,11,14,15-octaol (DBC) ligands is explored as an advanced cathode material for sodium-ion batteries (SIBs) for the first time. Notably, the expanded conjugated and four-fold interpenetrating structure endows Cu-DBC with transmission channels for electrons and sufficient spacing for sodium ion diffusion. As expected, the Cu-DBC cathode showcases higher specific capacity (120.6 mA h g-1, 0.05 A g-1) and robust cycling stability (18.1% capacity fade after 4000 cycles, 2 A g-1). Impressively, the Cu-DBC cathode also exhibits good electrochemical properties at extreme temperatures (-20 °C and 50 °C). A series of in/ex situ characterizations and systematic theoretical calculations further reveal the sodium-ion storage mechanism of Cu-DBC, highlighting a three-electron redox process on the redox-active [CuO4] units. This work provides valuable insights for exploring and enriching the applications of 3D interpenetrated c-MOFs in metal-ion batteries.

Synthesis and Biomedical Applications of Covalent Organic Frameworks for Disease Diagnosis and Therapy

Covalent organic frameworks (COFs) have emerged as a distinguished class of porous materials. Owing to their ability to be constructed through covalent bonds involving light elements, such as hydrogen, boron, carbon, nitrogen, and oxygen, COFs offer greater stability and lower cytotoxicity than metal-organic frameworks do, addressing critical limitations in in vivo applications. Their unique attributes, such as high surface area, customizable pore sizes, and versatile surface functionalities, make them ideal for various biomedical applications. This review aims to provide an overview of the recent advancements in modern COFs for biomedical uses. First, a variety of methods for the synthesis of COFs are outlined, which ensures their suitability for medical use. Next, we delve into innovative biomedical applications, emphasizing their roles in disease diagnosis and therapies. Finally, challenges, such as clinical translation, biocompatibility, and controlled drug release, are critically discussed, providing comprehensive insight into the potential of COFs in revolutionizing biomedical technologies. Overall, this review offers a comprehensive overview of COFs' capabilities and future prospects in enhancing biomedical technologies.

Interlocked 2D Covalent Organic Frameworks from Overcrowded Nodes

A challenging aspect in the synthesis of covalent organic frameworks (COFs) that goes beyond the framework's structure and topology is interpenetration, where two or more independent frameworks are mechanically interlocked with each other. Such interpenetrated or interlocked frameworks are commonly found in three-dimensional (3D) COFs with large pores. However, interlocked two-dimensional (2D) COFs are rarely seen in the literature, as 2D COF layers typically crystallize in stacks that maximize stabilization through π-stacking. The few interlocked 2D COFs described to date have been derived from monomers with aryl groups arranged perpendicularly. Herein, we report an interlocked 2D COF derived from a new class of monomers constituted of sterically overcrowded polycyclic aromatic hydrocarbons. The formation of such an interlocked structure is ascribed to the presence and the bulkiness of the substituents that directly interfere with interlayer π-stacking. The microscopy, gas sorption, spectroscopic, and charge transport characterization are consistent with the absence of π-stacking, as imposed by the interlocked architecture. This work evidences how the use of overcrowded aromatic systems as monomers can generate mechanically interlocked 2D COFs, offering new avenues for the design of COFs with unconventional topologies.

A comparative study of topological entropy characterization and graph energy prediction for Marta variants of covalent organic frameworks

Covalent organic frameworks are a novel class of porous polymers, notable for their crystalline structure, intricate frameworks, defined pore sizes, and capacity for structural design, synthetic control, and functional customization. This paper provides a comprehensive analysis of graph entropies and hybrid topological descriptors, derived from geometric, harmonic, and Zagreb indices. These descriptors are applied to study two variations of Marta covalent organic frameworks based on contorted hexabenzocoronenes. We also conduct a comparative analysis using scaled entropies, offering refined tools for assessing the intrinsic topologies of these networks. Additionally, these hybrid descriptors are used to develop statistical models for predicting graph energy in higher-dimensional Marta-COFs.

Photons, Excitons, and Electrons in Covalent Organic Frameworks

Covalent organic frameworks (COFs) are created by the condensation of molecular building blocks and nodes to form two-dimensional (2D) or three-dimensional (3D) crystalline frameworks. The diversity of molecular building blocks with different properties and functionalities and the large number of possible framework topologies open a vast space of possible well-defined porous architectures. Besides more classical applications of porous materials such as molecular absorption, separation, and catalytic conversions, interest in the optoelectronic properties of COFs has recently increased considerably. The electronic properties of both the molecular building blocks and their linkage chemistry can be controlled to tune photon absorption and emission, to create excitons and charge carriers, and to use these charge carriers in different applications such as photocatalysis, luminescence, chemical sensing, and photovoltaics. In this Perspective, we will discuss the relationship between the structural features of COFs and their optoelectronic properties, starting with the building blocks and their chemical connectivity, layer stacking in 2D COFs, control over defects and morphology including thin film synthesis, exploring the theoretical modeling of structural, electronic, and dynamic features of COFs, and discussing recent intriguing applications with a focus on photocatalysis and photoelectrochemistry. We conclude with some remarks about present challenges and future prospects of this powerful architectural paradigm.

Growing large single crystals of two- or three-dimensional covalent organic polymers through unconventional Te-O-P linkages

Understanding precise structures of two-/three- dimensional (2D/3D) covalent organic polymers (COPs) through single-crystal X-ray diffraction (SCXRD) analysis is important. However, how to grow high-quality single crystals for 2D/3D COPs is of challenge due to poor reversibility and difficult self-correction of covalent bonds. In addition, the success of introducing tellurium into the backbone to construct 2D/3D COPs and obtaining their single crystals is rare. Here, utilizing the strategy that a heavy element (e.g., tellurium) can form dynamic linkages with a self-correction function, we develop a fast and universal method for growing large-sized single crystals (up to 500 µm) for 2D/3D COPs, especially for 2D COPs. Three 2D COPs and one 3D COP are harvested through dynamic -Te-O-P- bonds in two days, with structures clearly uncovered via the SCXRD analysis. These 2D/3D COPs also show promising photocatalytic activities (nearly 100% selectivity and 100% yield) in superoxide anion radical-mediated coupling of (arylmethyl)amines.

Covalent Organic Framework-Structured Raman Probes for Ultrasensitive In Vivo Bioimaging

Organic Raman probes, including polymers and small molecules, have attracted great attention in biomedical imaging owing to their excellent biocompatibility. However, the development of organic Raman probes is usually hindered by a mismatch between their absorption spectra and wavelength-fixed excitation, which makes it difficult to achieve resonance excitation necessary to obtain strong Raman signals. Herein, we introduce a covalent organic framework (COF) into the fine absorption spectrum regulation of organic Raman probes, resulting in their significant Raman signal enhancement. In representative examples, a polymer poly(diketopyrrolopyrrole-p-phenylenediamine) (DPP-PD) and a small molecule azobenzene are transformed into the corresponding COF-structured Raman probes. Their absorption peaks show an accurate match of less than 5 nm with the NIR excitation. As such, the COF-structured Raman probes acquire highly sensitive bioimaging capabilities compared to their precursors with negligible signals. By further mechanism studies, we discover that the crystallinity and size of COFs directly affect the π-conjugation degree of Raman probes, thus changing their bandgaps and absorption spectra. Our study offers a universal and flexible method for improving the signal performance of organic Raman probes without changing their structural units, making it more convenient to obtain the highly sensitive organic Raman probes for in vivo bioimaging.

Peri-Xanthenoxanthene-Based Covalent Organic Frameworks for High-Performance Aqueous Zn-Ion Hybrid Supercapacitors

Aqueous zinc-ion hybrid supercapacitors (Zn-HSCs) are promising devices for sustainable and efficient energy storage. However, they suffer from a limited energy density compared to lithium-ion batteries. This limitation can be overcome by developing novel electrode materials, with covalent organic frameworks (COFs) standing out as a particularly intriguing option. Herein, peri-xanthenoxanthene (PXX) has been integrated for the first time into a COF scaffold to take advantage of its straightforward synthesis, chemical stability, π-conjugated backbone, and heteroatom content endowing reversible redox reactions at low potentials. Two novel hexagonal COFs have been designed and synthesized by tethering of a PXX-diamine unit having a C2 symmetry with two distinct tris-aldehydes acting as C3-symmetric cornerstones, i.e., triformyl benzene (TFB) and triformylphloroglucinol (Tp), ultimately yielding COF PXX(PhNH2)2-TFB and COF PXX(PhNH2)2-Tp, respectively. As cathodes in Zn-HSCs, COF PXX(PhNH2)2-Tp exhibits a remarkable specific capacitance, energy, and power densities (237 F g-1, 106.6 Wh kg-1, and 3.0 kW kg-1, respectively), surpassing those of COF PXX(PhNH2)2-TFB (109 F g-1, 49.1 Wh kg-1, and 0.67 kW kg-1). Importantly, both COFs display outstanding long-term stability, over 5000 charge/discharge cycles, with capacitance retention >92%. These findings underscore the potential of PXX-based COFs as high-performance cathode materials for HSCs, thereby offering a promising new avenue for energy storage technologies.

An Appraisal for Providing Charge Transfer (CT) Through Synthetic Porous Frameworks for their Semiconductor Applications

In recent years, there has been considerable focus on the development of charge transfer (CT) complex formation as a means to modify the band gaps of organic materials. In particular, CT complexes alternate layers of aromatic molecules with donor (D) and acceptor (A) properties to provide inherent electrical conductivity. In particular, the synthetic porous frameworks as attractive D-A components have been extensively studied in recent years in comparison to existing D-A materials. Therefore, in this work, the synthetic porous frameworks are classified into conjugated microporous polymers (CMPs), covalent organic frameworks (COFs), and metal-organic frameworks (MOFs) and compare high-quality materials for CT in semiconductors. This work updates the overview of the above porous frameworks for CT, starting with their early history regarding their semiconductor applications, and lists CT concepts and selected key developments in their CT complexes and CT composites. In addition, the network formation methods and their functionalization are discussed to provide access to a variety of potential applications. Furthermore, several theoretical investigations, efficiency improvement techniques, and a discussion of the electrical conductivity of the porous frameworks are also highlighted. Finally, a perspective of synthetic porous framework studies on CT performance is provided along with some comparisons.

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.

Synthetic Strategies to Extended Aromatic Covalent Organic Frameworks

π-Conjugated materials are highly attractive owing to their unique optical and electronic properties. Covalent organic frameworks (COFs) offer a great opportunity for precise arrangement of building units in a π-conjugated crystalline matrix and tuning of the properties through choice of functionalities or post-synthetic modification. With this review, we aim at summarizing both the most representative as well as emerging strategies for the synthesis of π-conjugated COFs. We give examples of direct synthesis using large, π-extended building blocks. COFs featuring fully conjugated linkages such as vinylene, pyrazine, and azole are discussed. Then, post-synthetic modification methods that result in the extension of the COF π-system are reviewed. Throughout, mechanistic insights are presented when available. In the context of their utilization as film devices, we conduct a concise survey of the prominent COF layer deposition techniques reported and their aptness for the deposition of fused aromatic systems.

An electrically conducting 3D coronene-based metal-organic framework

A novel cubic mesoporous metal-organic framework (MOF), consisting of hexahydroxy-cata-hexabenzocoronene (c-HBC) and FeIII ions is presented. The highly crystalline and porous MOF features broad optical absorption over the whole visible and near infrared spectral regions. An electrical conductivity of 10-4 S cm-1 was measured on a pressed pellet.

Expanding Olefin-Linked Covalent Organic Frameworks toward Selective Photocatalytic Oxidation of Organic Sulfides

Olefin-linked covalent organic frameworks (COFs) have exhibited great potential in visible-light photocatalysis. In principle, expanding fully conjugated COFs can facilitate light absorption and charge transfer, leading to improved photocatalysis. Herein, three olefin-linked COFs with the same topology are synthesized by combining 2,4,6-trimethyl-1,3,5-triazine (TMT) with 1,3,5-triformylbenzene (TFB), 1,3,5-tris(4-formylphenyl)benzene (TFPB), and 1,3,5-tris(4-formylphenylethynyl)benzene (TFPEB), namely, TMT-TFB-COF, TMT-TFPB-COF, and TMT-TFPEB-COF, respectively. From TMT-TFB-COF to TMT-TFPB-COF, expanding phenyl rings provides only limited expansion for π-conjugation due to the steric effect of structural twisting. However, from TMT-TFPB-COF to TMT-TFPEB-COF, the insertion of acetylenes eliminates the steric effect and provides more delocalized π-electrons. As such, TMT-TFPEB-COF exhibits the best optoelectronic properties among these three olefin-linked COFs. Consequently, the photocatalytic performance of TMT-TFPEB-COF is much better than those of TMT-TFB-COF and TMT-TFPB-COF on the oxidation of organic sulfides into sulfoxides with oxygen. The desirable reusability and substrate compatibility of the TMT-TFPEB-COF photocatalyst are further confirmed. The selective formation of organic sulfoxides over TMT-TFPEB-COF under blue light irradiation proceeds via both electron- and energy-transfer pathways. This work highlights a rational design of expanding the π-conjugation of fully conjugated COFs toward selective visible-light photocatalysis.

Crystalline, Porous Helicene Covalent Organic Frameworks

Helicenes are a class of fascinating chiral helical molecules with rich chemistry developed continuously over the past 100 years. Their helical, conjugated, and twisted structures make them attractive for constructing molecular systems. However, studies over the past century are mainly focused on synthesizing helicenes with increased numbers of aromatic rings and complex heterostructures, while research on inorganic, organic, and polymeric helicene materials is still embryonic. Herein, we report the first examples of helicene covalent organic frameworks, i.e., [7]Helicene sp2 c-COF-1, by condensing [7]Helicene dialdehyde with trimethyl triazine via the C=C bond formation reaction under solvothermal conditions. The resultant [7]Helicene sp2 c-COF-1 exhibits prominent X-ray diffraction peaks and assumes a highly ordered 2D lattice structure originated from the twisted configuration of [7]Helicene unit. The C=C linked [7]Helicene sp2 c-COF-1 materials exhibited extended π conjugation and broadly tuned their absorption, emission, redox activity, photoconductivity, and light-emitting activity, demonstrating rich multifunctionalities and great potentials in developing various applications. This work opens a way to a new family of COFs as well as helicene materials, enabling the exploration of unprecedented π architectures and properties.

Recent Progress on Phase Engineering of Nanomaterials

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

Wavy Two-Dimensional Conjugated Metal-Organic Framework with Metallic Charge Transport

Two-dimensional conjugated metal-organic frameworks (2D c-MOFs) have emerged as a new class of crystalline layered conducting materials that hold significant promise for applications in electronics and spintronics. However, current 2D c-MOFs are mainly made from organic planar ligands, whereas layered 2D c-MOFs constructed by curved or twisted ligands featuring novel orbital structures and electronic states remain less developed. Herein, we report a Cu-catecholate wavy 2D c-MOF (Cu3(HFcHBC)2) based on a fluorinated core-twisted contorted hexahydroxy-hexa-cata-hexabenzocoronene (HFcHBC) ligand. We show that the resulting film is composed of rod-like single crystals with lengths up to ∼4 μm. The crystal structure is resolved by high-resolution transmission electron microscopy (HRTEM) and continuous rotation electron diffraction (cRED), indicating a wavy honeycomb lattice with AA-eclipsed stacking. Cu3(HFcHBC)2 is predicted to be metallic based on theoretical calculation, while the crystalline film sample with numerous grain boundaries apparently exhibits semiconducting behavior at the macroscopic scale, characterized by obvious thermally activated conductivity. Temperature-dependent electrical conductivity measurements on the isolated single-crystal devices indeed demonstrate the metallic nature of Cu3(HFcHBC)2, with a very weak thermally activated transport behavior and a room-temperature conductivity of 5.2 S cm-1. Furthermore, the 2D c-MOFs can be utilized as potential electrode materials for energy storage, which display decent capacity (163.3 F g-1) and excellent cyclability in an aqueous 5 M LiCl electrolyte. Our work demonstrates that wavy 2D c-MOF using contorted ligands are capable of intrinsic metallic transport, marking the emergence of new conductive MOFs for electronic and energy applications.

An 'on-off-on' fluorescent switch based on a luminous covalent organic framework for the rapid and selective detection of glyphosate

Glyphosate, the most used herbicide in the world, has a residue problem that cannot be ignored. However, glyphosate itself does not have fluorescence emission and lacks the conditions for fluorescence detection. In this work, a rapid and selective fluorescence detection method of glyphosate was designed by an 'on-off-on' fluorescent switch based on a luminous covalent organic framework (L-COF). Only the fixed concentration of Fe3+ as an intermediate could trigger the fluorescent switch and no incubation step was required. The proposed method showed good accuracy with a correlation coefficient of 0.9978. The method's limits of detection and quantitation were 0.88 and 2.93 μmol/L, which were lower than the maximum allowable residue limits in some regulations. Environmental water samples and tomatoes were selected as actual samples to verify the application in a complex matrix. A satisfactory mean recovery from 87% to 106% was gained. Furthermore, Fe3+ could induce fluorescence quenching of L-COF through the photo-induced electron transfer (PET) effect, while the addition of glyphosate could block the PET effect to achieve detection. These results demonstrated the proposed method had abilities to detect glyphosate and broaden the application of L-COF.

Microenvironment Restruction of Emerging 2D Materials and their Roles in Therapeutic and Diagnostic Nano-Bio-Platforms

Engineering advanced therapeutic and diagnostic nano-bio-platforms (NBPFs) have emerged as rapidly-developed pathways against a wide range of challenges in antitumor, antipathogen, tissue regeneration, bioimaging, and biosensing applications. Emerged 2D materials have attracted extensive scientific interest as fundamental building blocks or nanostructures among material scientists, chemists, biologists, and doctors due to their advantageous physicochemical and biological properties. This timely review provides a comprehensive summary of creating advanced NBPFs via emerging 2D materials (2D-NBPFs) with unique insights into the corresponding molecularly restructured microenvironments and biofunctionalities. First, it is focused on an up-to-date overview of the synthetic strategies for designing 2D-NBPFs with a cross-comparison of their advantages and disadvantages. After that, the recent key achievements are summarized in tuning the biofunctionalities of 2D-NBPFs via molecularly programmed microenvironments, including physiological stability, biocompatibility, bio-adhesiveness, specific binding to pathogens, broad-spectrum pathogen inhibitors, stimuli-responsive systems, and enzyme-mimetics. Moreover, the representative therapeutic and diagnostic applications of 2D-NBPFs are also discussed with detailed disclosure of their critical design principles and parameters. Finally, current challenges and future research directions are also discussed. Overall, this review will provide cutting-edge and multidisciplinary guidance for accelerating future developments and therapeutic/diagnostic applications of 2D-NBPFs.

Bottom-Up Synthesis of Covalent Organic Frameworks with Quasi-Three-Dimensional Integrated Architecture via Interlayer Cross-Linking

Developing strategies to enhance the structural robustness of covalent organic frameworks (COFs) is of great importance. Here, we rationally design and synthesize a class of cross-linked COFs (^CCOFs), in which the two-dimensional (2D) COF layers are anchored and connected by polyethylene glycol (PEG) or alkyl chains through covalent bonds. The bottom-up fabrication of these ^CCOFs is achieved by the condensation of cross-linked aldehyde monomers and tritopic amino monomers. All the synthesized ^CCOFs possess high crystallinity and porosity, and enhanced structural robustness surpassing the typical 2D COFs, which means that they cannot be exfoliated under ultrasonication and grinding due to the cross-linking effect. Furthermore, the cross-linked patterns of PEG units are uncovered by experimental results and Monte Carlo molecular dynamics simulations. It is found that all ^CCOFs are dominated by vertical cross-layer (interlayer) connections (clearly observed in high-resolution transmission electron microscopy images), allowing them to form quasi-three-dimensional (quasi-3D) structures. This work bridges the gap between 2D COFs and 3D COFs and provides an efficient way to improve the interlayered stability of COFs.

Conductive and Ultrastable Covalent Organic Framework/Carbon Hybrid as an Ideal Electrocatalytic Platform

Developing covalent organic frameworks (COFs) with good electrical conductivity is essential to widen their range of practical applications. Thermal annealing is known to be a facile approach for enhancing conductivity. However, at higher temperatures, most COFs undergo amorphization and/or thermal degradation because of the lack of linker rigidity and physicochemical stability. Here, we report the synthesis of a conductive benzoxazole-linked COF/carbon hybrid material (BCOF-600C) by simple thermal annealing. The fused-aromatic benzoxazole and biphenyl building units endow the resulting COF with excellent physicochemical stability against high temperatures and strong acids/bases. This allows heat treatment to further enhance electrical conductivity with minimal structural alteration. The robust crystalline structure with periodically incorporated nitrogen atoms allowed platinum (Pt) atoms to be atomically integrated into the channel walls of BCOF-600C. The resulting electrocatalyst with well-defined active sites exhibited superior catalytic performance toward hydrogen evolution in acidic media.

A rigidity-flexibility balance strategy enabling highly photoluminescent two-dimensional covalent organic framework nanosheets

The fluorescence quenching phenomenon commonly found in two-dimensional COFs is due to either the strong interlayer π-π stacking or the non-radiative decay caused by intramolecular rotation. Here, we report a rigidity-flexibility balance strategy for constructing highly photoluminescent 2D COF nanosheets via the integration of rigid fluorescent molecular nodes with flexible non-planar building blocks. The prepared COF nanosheets, termed TPE-DBC-COF, achieve extremely high PLQY in common organic solvents, especially in tetrahydrofuran (43.5%). Besides, the prepared TPE-DBC-COF exhibits excellent sensitivity and selectivity to tetracycline hydrochloride and high cycling stability, so it can be used as a high-efficiency fluorescent sensor for the detection of tetracycline hydrochloride down to the ppm level.

A Crystalline 1D Dynamic Covalent Polymer

The synthesis of crystalline one-dimensional polymers provides a fundamental understanding about the structure-property relationship in polymeric materials and allows the preparation of materials with enhanced thermal, mechanical, and conducting properties. However, the synthesis of crystalline one-dimensional polymers remains a challenge because polymers tend to adopt amorphous or semicrystalline phases. Herein, we report the synthesis of a crystalline one-dimensional polymer in solution by dynamic covalent chemistry. The structure of the polymer has been unambiguously confirmed by microcrystal electron diffraction that together with charge transport studies and theoretical calculations show how the π-stacked chains of the polymer generate optimal channels for charge transport.

Fused-Ring-Linked Covalent Organic Frameworks

The development of linkage chemistry in the research area of covalent organic frameworks (COFs) is fundamentally important for creating robust structures with high crystallinity and diversified functionality. We reach herein a new level of complexity and controllability in linkage chemistry by achieving the first synthesis of fused-ring-linked COFs. A series of bicyclic pyrano[4,3-b]pyridine COFs have been constructed via a cascade protocol involving Schiff-base condensation, intramolecular [4 + 2] cycloaddition, and dehydroaromatization. With a broad scope of Brønsted or Lewis acids as the catalyst, the designed monomers, that is, O-propargylic salicylaldehydes and multitopic anilines, were converted into the fused-ring-linked frameworks in a one-pot fashion. The obtained COFs exhibited excellence in terms of purity, stability, and crystallinity, as comprehensively characterized by solid-state nuclear magnetic resonance (NMR) spectroscopy, powder X-ray diffraction, high-resolution transmission electron microscopy, and so on. Specifically, the highly selective formation (>94%) of pyrano[4,3-b]pyridine linkage was verified by quantitative NMR measurements combined with ¹³C-labeling synthesis. Moreover, the fused-ring linkage possesses fully locked conformation, which benefits to the high crystallinity observed for these COFs. Advancing the linkage chemistry from the formation of solo bonds or single rings to that of fused rings, this study has opened up new possibilities for the concise construction of sophisticated COF structures with high controllability.

2D Materials for All-Solid-State Lithium Batteries

Although one of the most mature battery technologies, lithium-ion batteries still have many aspects that have not reached the desired requirements, such as energy density, current density, safety, environmental compatibility, and price. To solve these problems, all-solid-state lithium batteries (ASSLB) based on lithium metal anodes with high energy density and safety have been proposed and become a research hotpot in recent years. Due to the advanced electrochemical properties of 2D materials (2DM), they have been applied to mitigate some of the current problems of ASSLBs, such as high interface impedance and low electrolyte ionic conductivity. In this work, the background and fabrication method of 2DMs are reviewed initially. The improvement strategies of 2DMs are categorized based on their application in the three main components of ASSLBs: The anode, cathode, and electrolyte. Finally, to elucidate the mechanisms of 2DMs in ASSLBs, the role of in situ characterization, synchrotron X-ray techniques, and other advanced characterization are discussed.

Nonplanar Rhombus and Kagome 2D Covalent Organic Frameworks from Distorted Aromatics for Electrical Conduction

Two-dimensional (2D) covalent organic frameworks (COFs) are an emerging class of promising 2D materials with high crystallinity and tunable structures. However, the low electrical conductivity impedes their applications in electronics and optoelectronics. Integrating large π-conjugated building blocks into 2D lattices to enhance efficient π-stacking and chemical doping is an effective way to improve the conductivity of 2D COFs. Herein, two nonplanar 2D COFs with kagome (DHP-COF) and rhombus (c-HBC-COF) lattices have been designed and synthesized from distorted aromatics with different π-conjugated structures (flexible and rigid structure, respectively). DHP-COF shows a highly distorted 2D lattice that hampers stacking, consequently limiting its charge carrier transport properties. Conversely, c-HBC-COF, with distorted although concave-convex self-complementary nodes, shows a less distorted 2D lattice that does not interfere with interlayer π-stacking. Employing time- and frequency-resolved terahertz spectroscopy, we unveil a high charge-carrier mobility up to 44 cm² V^-1 s^-1, among the highest reported for 2D COFs.

Two-Dimensional Polymers and Polymerizations

Synthetic chemists have developed robust methods to synthesize discrete molecules, linear and branched polymers, and disordered cross-linked networks. However, two-dimensional polymers (2DPs) prepared from designed monomers have been long missing from these capabilities, both as objects of chemical synthesis and in nature. Recently, new polymerization strategies and characterization methods have enabled the unambiguous realization of covalently linked macromolecular sheets. Here we review 2DPs and 2D polymerization methods. Three predominant 2D polymerization strategies have emerged to date, which produce 2DPs either as monolayers or multilayer assemblies. We discuss the fundamental understanding and scope of each of these approaches, including: the bond-forming reactions used, the synthetic diversity of 2DPs prepared, their multilayer stacking behaviors, nanoscale and mesoscale structures, and macroscale morphologies. Additionally, we describe the analytical tools currently available to characterize 2DPs in their various isolated forms. Finally, we review emergent 2DP properties and the potential applications of planar macromolecules. Throughout, we highlight achievements in 2D polymerization and identify opportunities for continued study.

Controllable Synthesis and Performance Modulation of 2D Covalent-Organic Frameworks

Covalent-organic frameworks (COFs) are especially interesting and unique as their highly ordered topological structures entirely built from plentiful π-conjugated units through covalent bonds. Arranging tailorable organic building blocks into periodically reticular skeleton bestows predictable lattices and various properties upon COFs in respect of topology diagrams, pore size, properties of channel wall interfaces, etc. Indeed, these peculiar features in terms of crystallinity, conjugation degree, and topology diagrams fundamentally decide the applications of COFs including heterogeneous catalysis, energy conversion, proton conduction, light emission, and optoelectronic devices. Additionally, this research field has attracted widespread attention and is of importance with a major breakthrough in recent year. However, this research field is running with the lack of summaries about tailorable construction of 2D COFs for targeted functionalities. This review first covers some crucial polymeric strategies of preparing COFs, containing boron ester condensation, amine-aldehyde condensation, Knoevenagel condensation, trimerization reaction, Suzuki CC coupling reaction, and hybrid polycondensation. Subsequently, a summary is made of some representative building blocks, and then underlines how the electronic and molecular structures of building blocks can strongly influence the functional performance of COFs. Finally, conclusion and perspectives on 2D COFs for further study are proposed.

A non-planar 2D covalent organic framework derived from a Z-shaped building unit

Herein, a novel non-planar 2D COF with a stair-stepped structure was constructed from a Z-shaped building block for the first time. Compared with its similar planar COF, the unique stair-stepped non-planar COF possesses larger surface area and stronger fluorescence, which was further applied for specific explosive detection through a fluorescence quenching mechanism. This work not only extends the traditional planar 2D COF structures to unique non-planar structures based on the bottom-up design principle, but also expands the potential applications of COF materials.

Recent Progress in Nanoscale Covalent Organic Frameworks for Cancer Diagnosis and Therapy

Covalent organic frameworks (COFs) as a type of porous and crystalline covalent organic polymer are built up from covalently linked and periodically arranged organic molecules. Their precise assembly, well-defined coordination network, and tunable porosity endow COFs with diverse characteristics such as low density, high crystallinity, porous structure, and large specific-surface area, as well as versatile functions and active sites that can be tuned at molecular and atomic level. These unique properties make them excellent candidate materials for biomedical applications, such as drug delivery, diagnostic imaging, and disease therapy. To realize these functions, the components, dimensions, and guest molecule loading into COFs have a great influence on their performance in various applications. In this review, we first introduce the influence of dimensions, building blocks, and synthetic conditions on the chemical stability, pore structure, and chemical interaction with guest molecules of COFs. Next, the applications of COFs in cancer diagnosis and therapy are summarized. Finally, some challenges for COFs in cancer therapy are noted and the problems to be solved in the future are proposed.

Understanding charge transport in wavy 2D covalent organic frameworks

Understanding charge transport in 2D covalent organic frameworks is crucial to increase their performance. Herein a new wavy 2D covalent organic framework has been designed, synthesized and studied to shine light on the structural factors that dominate charge transport.

Imaging and analysis of covalent organic framework crystallites on a carbon surface: a nanocrystalline scaly COF/nanotube hybrid

Synthesis of covalent organic frameworks (COFs) is well-advanced but understanding their nanoscale structure and interaction with other materials remains a significant challenge. Here, we have developed a methodology for the detailed imaging and analysis of COF crystallites using carbon nanotube substrates for COF characterisation. Detailed investigation using powder X-ray diffraction, infrared spectroscopy, mass spectrometry and scanning electron microscopy in conjunction with a local probe method, transmission electron microscopy (TEM), revealed details of COF growth and nucleation at the nanoscale. A boronate ester COF undergoes preferential growth in the a-b crystallographic plane under solvothermal conditions. Carbon nanotubes were found to not impact the mode of COF growth, but the crystallites on nanotubes were smaller than COF crystallites not on supports. COF crystalline regions with sizes of tens of nanometres exhibited preferred orientation on nanotube surfaces, where the c-axis is oriented between 50 and 90° relative to the carbon surface. The COF/nanotube hybrid structure was found to be more complex than the previously suggested concentric core-shell model and can be better described as a nanocrystalline scaly COF/nanotube hybrid.

Covalent organic frameworks: an ideal platform for designing ordered materials and advanced applications

Covalent organic frameworks offer a molecular platform for integrating organic units into periodically ordered yet extended 2D and 3D polymers to create topologically well-defined polygonal lattices and built-in discrete micropores and/or mesopores.

Boronic-acid-derived covalent organic frameworks: from synthesis to applications

Modular, well-defined, and robust hierarchical functional materials are targets of numerous synthesis endeavors.

Optoelectronic processes in covalent organic frameworks

Covalent organic frameworks (COFs) are crystalline porous materials constructed from molecular building blocks using diverse linkage chemistries. The image illustrates electron transfer in a COF-based donor–acceptor system. Image by Nanosystems Initiative Munich.

Dynamic Transformation between Covalent Organic Frameworks and Discrete Organic Cages

We propose a dynamic covalent chemistry (DCC)-induced linker exchange strategy for the structural transformation between covalent organic frameworks (COFs) and cages for the first time. Studies have shown that the COF-to-cage and cage-to-COF transformations were realized by using borate bonds and imine bonds, respectively, as linkages. Self-sorting experiments suggested that borate cages and imine COFs are thermodynamic minimum compounds. This research builds a bridge between discrete and polymeric organic scaffolds and broadens the knowledge of chemistry and materials for porous materials science.

Structural Approaches to Control Interlayer Interactions in 2D Covalent Organic Frameworks

The ability to design and synthesize monomers can affect fundamental aspects in 2D covalent organic frameworks, such as dimensionality, topology, and pore size. Besides this, the structure of the monomers can also affect interlayer interactions, which provide an additional means to influence crystallinity, layer arrangement, interlayer distances, and exfoliability. Herein, some of the effects that the structure of monomers can have on the interlayer interactions in 2D covalent organic frameworks and related materials are illustrated.

Evaluation-oriented exploration of photo energy conversion systems: from fundamental optoelectronics and material screening to the combination with data science

Light is a form of energy that can be converted to electric and chemical energies. Thus, organic photovoltaics (OPVs), perovskite solar cells (PSCs), photocatalysts, and photodetectors have evolved as scientific and commercial enterprises. However, the complex photochemical reactions and multicomponent materials involved in these systems have hampered rapid progress in their fundamental understanding and material design. This review showcases the evaluation-oriented exploration of photo energy conversion materials by using electrodeless time-resolved microwave conductivity (TRMC) and materials informatics (MI). TRMC with its unique options (excitation sources, environmental control, frequency modulation, etc.) provides not only accelerated experimental screening of OPV and PSC materials but also a versatile route toward shedding light on their charge carrier dynamics. Furthermore, MI powered by machine learning is shown to allow extremely high-throughput exploration in the large molecular space, which is compatible with experimental screening and combinatorial synthesis.