2D Covalent Organic Frameworks: From Synthetic Strategies to Advanced Optical-Electrical-Magnetic Functionalities

Design, Synthesis, and Application of Immobilized Enzymes on Artificial Porous Materials

Enzymes have been recognized as highly efficient biocatalysts, whereas characteristics such as poor stability and single reaction type greatly significantly limit their wide application. Hence, the exploitation of suitable carriers for immobilized enzymes enables the provision of a protective layer for the enzyme, with the capability of chemical and biological cascade catalysis. Among the various immobilization carriers, metal-organic frameworks (MOFs), covalent organic frameworks (COFs) and hydrogen-bonded organic frameworks (HOFs) have been emerging as a promising strategy to surpass the inherent instability and other limitations of free enzymes. Specifically, the integration of such artificial porous materials as carriers improves the stability and reusability of enzymes, while simultaneously affording a platform for multifunctional applications. Herein, this review systematically discusses the various preparation strategies and advantages of artificial porous materials, while elucidating the effects of different immobilization methods on enzyme activity. Furthermore, the innovative applications of artificial porous materials as multifunctional carriers in the field of enzyme immobilization fields such as enzyme carriers, photocatalysts, chemical catalysts and sensing are also comprehensively summarized here, thus demonstrating their multifunctional characteristics and promising applications in addressing complex biotransformation challenges.

Solid-State Photoluminescent Imine-Linked Two-Dimensional Covalent Organic Frameworks

The development of efficient solid-state luminescent covalent organic frameworks (COFs) is crucial for advancing applications in sensing, imaging, and optoelectronics. However, achieving high photoluminescent quantum yields (PLQY) in imine-linked COFs remains challenging due to the presence of complex nonradiative quenching pathways. Here, the design and synthesis of a novel series of solid-state photoluminescent imine-linked 2D covalent organic frameworks (2D COFs) are reported through condensation of rigid building blocks. These COFs display high crystallinity and porosity, and with a remarkable PLQY of up to 39% in the solid state. The high luminescent efficiency is attributed to the donor-acceptor-donor structure within the aldehyde moieties, which facilitates selective charge transfer excitation between the donor moiety, triphenylamine, and the acceptor moiety, benzothiadiazole, bypassing the imine bonds, suppressing nonradiative quenching pathways associated with imine bond rotation in the excited states. Furthermore, the obtained COF shows potential for bioimaging applications.

Topological Control Over Porphyrin-Based Covalent Organic Frameworks for Elucidating Electron Transfer Characteristics

Two-dimensional covalent organic frameworks (2D COFs) have emerged as promising functional materials due to their programmable architectures and tunable functionalities. Nevertheless, the structural diversity of porphyrin-based 2D COFs remains restricted by the prevalent use of sql topology, hindering comprehensive structure-property exploration. Herein, we systematically designed and synthesized porphyrinic 2D COFs featuring distinct sql and bex topological configurations. Comprehensive structural characterization confirmed precise control over lattice geometries, revealing monoporous structure in sql topology versus biporous architecture in bex topology. Electrochemical investigations uncovered topology-governed electron transport characteristics, with the unique coordination geometry of bex topology exhibiting enhanced electron transfer efficiency. Band structure analysis demonstrated that topological configuration and chemical composition collectively modulate electronic structures. Inspired by these findings, we developed nickel-incorporated bex-COFs for electrocatalytic oxygen evolution. The optimized Ni-BBFPP-TAPP-COF with bex topology demonstrated remarkable catalytic performance, achieving a low overpotential of 342 mV at 10 mA cm-2, which surpasses most reported porphyrin-based electrocatalysts. This study not only significantly expands the structural repertoire of porphyrinic COFs but also establishes explicit correlations between topological engineering and electrocatalytic performance, providing fundamental design principles for advanced energy conversion materials.

Hopping-Type Charge Transport in Controllably p-Doped Polaronic Two-Dimensional Polymers

In this work, we find that controllable p-type doping leads to Holstein-type polarons in four electron-rich two-dimensional polymers (2DPs). Substoichometrically injecting holes into these 2DPs leads to small optical bandgaps (<1.0 eV) and electrical conductivities (17 mS m-1) significantly higher than their undoped analogs. Fourier-transform infrared spectroscopy and continuous-wave electron paramagnetic resonance spectroscopy both reveal that this arises from the formation of paramagnetic polarons. We achieve maximal conductivities when 2DPs comprised of electron-rich nodes and electron-rich linkers are combined, which is a consequence of more delocalized polarons as unveiled by diffuse-reflectance UV-vis-NIR spectroscopy. Variable-temperature electrical conductivity measurements reveal two distinct Arrhenius regimes in all 2DPs investigated, which we attribute to the different thermally activated processes inherent to in-plane and cross-plane electronic transport in stacked 2DP multilayers. This resulted in a maximum electronic conductivity of 326 mS m-1 at an elevated temperature. Collectively, this report provides fundamental insight into polaron-based charge-transport in p-type 2D organic layers, which we expect will form the foundation for the eventual deployment of these materials in electronic devices.

Construction of Luminescent Three-Dimensional Covalent Organic Frameworks for Molecular Decoding of Wide Organic Compounds

Constructing highly conjugated three-dimensional covalent organic frameworks (3D COFs), particularly those with luminescent features, remains a significant challenge. In this work, we successfully synthesized a 3D COF, named 3D-Py-SP-COF, using a rigid and orthogonal spirobifluorene building block for the spatial 3D structure construction and planar pyrene as luminescent units. The incorporation of the pyrene and the unique rigid 3D network structure endow 3D-Py-SP-COF with fluorescent properties. The successful formation of this 3D COF was verified by FT-IR, solid-state 13C CP-MAS NMR. Structural simulations based on the experimental powder X-ray diffraction analysis revealed that 3D-Py-SP-COF adopted a two-fold interpenetrated pts topology. The highly conjugated porous framework and fluorescent nature allow precise detection and localization of more than two dozen volatile organic compounds (VOCs), including aromatics, alcohols, and other commonly encountered industrial VOCs.

Applications of covalent organic frameworks (COFs)-based sensors for food safety: Synthetic strategies, characteristics and current state-of-art

Food safety is a pressing global public issue that has garnered significant attention worldwide, especially recent outbreaks of foodborne illnesses. The use of emerging porous materials enables the development of effective and durable detection methods for the detection of food contaminants. Covalent organic frameworks (COFs), as a class of emerging porous crystalline materials, rendered with the advantage of large specific surface area, highly controllable and ordered structures, diverse pore structures, high stability, and controllable surface functionalization. Especially in the development of sensors, COFs exhibit versatile roles as signal amplifiers, molecular recognizers, molecular transfer mediators, carriers, catalysts, and reporters, making them highly valuable in various applications. In the context of food safety, COFs-based sensing platforms have shown great potential. This review aims to provide an in-depth understanding of COFs-based sensors by discussing recent advancements in this field. It begins with a systemic introduction of the synthetic strategies of COFs and the pros and cons, followed by the distinctive characteristics of COFs and their diverse functional roles in sensing strategies, emphasizing their importance in analysing food safety risks. Then the review further presented a comprehensive summary of the applications of COFs in sensing, specifically highlighting significant breakthroughs in the detection of various food contaminants like foodborne pathogens, mycotoxins, pesticides, antibiotics, heavy metals, etc. Additionally, the review addressed the challenges and opportunities associated with COFs-based sensors in the detection of food safety issues. The aim of the review was to contribute to the ongoing development and advancement of COFs for ensuring food safety.

Two-Dimensional Covalent Organic Frameworks in Organic Electronics

Two-dimensional covalent organic frameworks (2DCOFs) are a unique class of crystalline porous materials interconnected by covalent bonds, which have attracted significant attention in recent years due to their chemical and structural diversity, as well as their applications in adsorption, separation, catalysis, and drug delivery. However, research on the electrical properties of 2DCOFs remains limited, despite their potential in organic electronics. Early studies recognized the poor electrical conductivity of 2DCOFs as a significant obstacle to their application in this field. To overcome this challenge, various strategies have been proposed to enhance conductivity. This review first introduces the concept of computational screening for 2DCOFs and explores approaches to improve their intrinsic conductivity, with a focus on four key aspects: in-plane and out-of-plane charge transport, topology, bandgap, and morphology. It then examines the application of pristine 2DCOFs in organic electronics, including applications in field-effect transistors, memristors, photodetectors, and chemiresistive gas sensors. We support these strategies with detailed statistical data, providing a comprehensive guide for the design and development of novel 2DCOFs for organic electronics. Finally, we outline future research directions, emphasizing the challenges that remain to be addressed in this emerging area.

Linkage Engineering of Semiconductive Covalent-Organic Frameworks toward Room-Temperature Ppb-Level Selective Ammonia Sensing

Rational design of molecular architectures is crucial for developing advanced materials such as covalent-organic frameworks (COFs) with excellent sensing performance. In this work, two isostructural COFs (β-keto-AnCOF and imine-AnCOF) with the same conjugated linkers but distinct linkages are constructed. Although both COFs have porous structure and semiconductor behavior conferred by the identical conjugated backbones, β-keto-AnCOF with ─C═O side groups exhibits superior room-temperature ammonia (NH3) sensing performance than imine-AnCOF and even the state-of-the-art dynamic and commercial NH3 sensors, i.e., high sensitivity up to 18.94% ppm-1, ultralow experimental detection limit of 1 ppb, outstanding selectivity, and remarkable response stability and reproducibility after 180 days. In situ spectroscopy and theoretical calculation reveal that the additional charge transfer between NH3 and ─C═O sites in β-keto-AnCOF effectively increases the distance between Fermi level and the valence band, enabling highly-sensitive NH3 detection at ppb levels. This work provides novel molecular architectures for next-generation high-performance sensors.

Manipulating Constitutional Isomerism of Imine Linkages in Interfacially Confined Nanofilms toward Enhanced Fluorescence Sensing

Photoluminescence efficiencies of covalent organic frameworks (COFs) are significantly restricted by electron delocalization and charge transfer among the conjugated skeletons. Two nanofilms using tetraphenylethylene and benzo[c][1,2,5]thiadiazole as the building blocks were facilely prepared via an interfacially confined condensation strategy. The distinct dipole moment orientations of imine linkages are involved in the π-delocalization of conjugated donor-acceptor systems diversely. They also played critical roles in affecting the fluorescence turn-on sensing of the obtained nanofilms for gaseous trifluoroacetic acid (TFA). The joint donor-C═N-acceptor sequence in nanofilm #2 resulted in relatively stronger fluorescence originally than that of nanofilm #1, featuring the disturbed donor-N═C-acceptor sequence. While after blowing trace TFA, the latter nanofilm #1 possessed prominent fluorescence enhancement and obvious color visualization. Comparative transient absorption observations and theoretical calculations elucidated the effective manipulation of the intramolecular charge transfer (ICT) efficiencies among the imine-linked functional skeletons. With the help of a laminated fluorescent sensor, a compact sensing platform was further integrated using optimized nanofilm #1. It exhibited good selectivity, excellent reversibility (≥50 recycles), an extraordinary detection limit (∼0.1 ppt), and a rapid recovery process to gaseous TFA. Our findings provide valuable optimizations of π-linkages in COFs and reliable fluorescent film sensors for monitoring toxic and hazardous gases.

2D monolayer electrocatalysts for CO2 electroreduction

The electrocatalytic carbon dioxide reduction reaction (CO2RR) is an attractive method for converting atmospheric CO2 into value-added chemicals and fuels. In order to overcome the low efficiency and durability that hinder its practical application, a significant amount of research has been dedicated to designing novel catalysts at the nanoscale and even the atomic scale. Two-dimensional (2D) monolayer materials inherit the merits of both 2D materials and single-atom materials. Through bridging the gap between heterogeneous and homogeneous catalysis, 2D monolayer materials exhibit great potential in the CO2RR due to their unique structural/electronic properties, high atom utilization, low mass transfer resistance and uniform active sites. Here, we systematically overview the development and application of 2D monolayer catalysts for the electrocatalytic CO2RR. First, an overview of the CO2RR technology is presented. Subsequently, a comprehensive discussion is undertaken on various types of 2D monolayer electrocatalysts, such as 2D graphene-based materials, 2D monolayer metal-organic frameworks (MOFs), 2D monolayer covalent organic frameworks (COFs) and 2D monolayer metal-based materials. Their respective electrocatalytic performances are also systematically analyzed. More importantly, novel perspectives on the primary challenges and opportunities associated with the utilization of 2D monolayer materials in the CO2RR are presented. Achieving high-quality 2D monolayer materials and producing highly selective multi-carbon products remain the two major challenges in the design, synthesis and application of 2D monolayer electrocatalysts. Addressing these synthesis-related and performance-related issues is significant for the progression and practical utilization of 2D monolayer materials in the CO2RR.

Multiple Redox-Active Centers in An Azatriangulenetrione-Based Covalent Organic Framework for High-Capacity, High-Rate and Ultra-Stable Sodium-Ion Batteries

Sodium-ion batteries (SIBs) suffer from sluggish kinetics, large volume change, and limited specific capacity due to the large radius of Na+. These issues can be solved through using covalent organic frameworks (COFs) as electrodes. Herein, an azatriangulenetrione-containing COF (denoted as CityU-33) was designed and synthesized as an electrode material for SIBs. Due to its inherent abundance of multiple redox-active sites and fast intercalation kinetics, CityU-33 delivered a high discharge capacity of 410.4 mAh g-1 at 0.1 A g-1 and showed remarkable long-term cycling stability, where a discharge capacity of 288 mAh g-1 at 0.2 A g-1 with 97 % retention over 2000 cycles was achieved, making it the top COF electrode material for SIBs.

Boron-Containing Organic Two Dimensional Materials: Synthesis and Application

Organic two-dimensional materials have garnered widespread attention due to their well-defined structures, structural diversity, and rich electronic effects, demonstrating significant application potential across various fields. Atomic-level manipulation of the structures of organic two-dimensional materials has been a primary strategy for enriching and optimizing their properties. The introduction of heteroatoms often significantly affects their electronic structure, thereby endowing these materials with novel and unique properties. Boron atoms, due to their electron-deficient nature, have been extensively studied in luminescent materials, semiconductor materials, and chemical sensing materials. Consequently, boron-containing organic two-dimensional materials are also believed to be promising as a new class of materials with excellent optoelectronic and chemical activities. This article collates and summarizes the preparation and property studies of three types of boron-containing organic two-dimensional materials in recent years.

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.

Linkage Regulation of β-Ketoamine Covalent Organic Frameworks for Boosting Photocatalytic Overall Water Splitting

Two dimensional β-ketoamine covalent organic frameworks (2D TP-COFs) are one category of promising metal-free catalysts for photocatalytic overall water splitting (OWS) because of their unusual stability and versatile electronic/optical properties. However, none of the currently reported TP-COFs can accomplish the hydrogen evolution (HER) and oxygen evolution reactions (OER) simultaneously without adding any sacrificial agents and cocatalysts. To address this challenging issue, we rationally designed 23 2D TP-COFs by regulating the linkage groups and comprehensively evaluated their OWS activity by using the first-principles method. First, the electronic band structure calculations at the HSE06 level reveal that the band gap can be reasonably adjusted with values ranging from 1.67-3.16 eV. Among these 23 systems, 10 TP-COFs are realized to match well with both the chemical potentials of H2/H+ and O2/H2O, which are capable of visible-light-driven OWS from an electronic perspective. Further thermal activity results on OWS demonstrate that only Hep-BDA (heptazine-aniline) and Bpy-4 (bipyrimidinamine) based COFs can satisfy the completely spontaneous of HER and OER under light irradiation and neutral conditions. Importantly, the calculated small exciton binding energies and high carrier mobility for Hep-BDA and Bpy-4 TP-COFs propose they are potentially applied in photocatalytic OWS. We also achieved the theoretical energy conversion efficiency of Hep-BDA can reach as high as 13.01%. Because there are very few successful applications of TP-COFs on OWS, this theoretical work not only offers valuable insights and innovative ideas for the exploration of novel metal-free photocatalysts for OWS but also supplies a direction for the development of new TP-COF derivatives.

Enhancing Photocatalytic CO2RR by Modulating the Active Sites of COF-Based Catalysts

The catalytic conversion of CO2 into valuable chemicals using metalized covalent organic frameworks (COFs) as catalysts is a promising method for reducing atmospheric CO2 levels. Herein, a aldehyde-amine COF (TAPT-Tp) at room temperature and pressure and their metallized results is synthesized, Ni-TAPT-Tp and Ti-TAPT-Tp. The photocatalytic results indicate that the CO2 to CO reduction rate is 6182.5 µmol g-1 h-1 for Ni-TAPT-Tp, but only 1615.4 µmol g-1 h-1 for Ti-TAPT-Tp. Density functional theory (DFT) simulations further demonstrate that for intermediates *CO2, *COOH, and *CO, the energy of Ni-TAPT-Tp is consistently lower than that of Ti-TAPT-Tp, indicating that Ni-TAPT-Tp exhibits superior photocatalytic performance for CO2RR. This work provides a reference for optimizing the coordination structure of M-COFs to obtain highly active and selective CO2RR.

A Density Functional Benchmark for Dehydrogenation and Dehalogenation Reactions on Coinage Metal Surfaces

The on-surface synthesis of low-dimensional organic nanostructures has been extensively investigated through both experimental and theoretical methods, particularly by density functional theory (DFT). However, the complex mixture of interactions often poses challenges within the DFT framework, and there is a knowledge-gap regarding how the choice of DFT approach affects the computed results. Here, five different approaches including vdW interactions, i. e., PBE+D3, PBE+vdWsurf, rev-vdWDF2, r2SCAN+rVV10 and BEEF-vdW, are employed to describe three prototypical on-surface reactions; dehydrogenation of benzene, debromination of bromobenzene, and deiodination of iodobenzene on the (111) facets of the coinage metals. Overall, rev-vdW-DF2 outperforms the other methods in describing benzene adsorption, whereas BEEF-vdW falls short. For dehydrogenation and debromination on Cu(111), all functionals except BEEF-vdW give reasonable activation energies compared to experiments. A similar trend is observed for Ag(111) and Au(111), with BEEF-vdW yielding significantly higher activation and reaction energies. For dehalogenation, all the five vdW approaches correctly capture the reactivity trend - Cu(111)>Ag(111)>Au(111) - and the expected hierarchy between bromobenzene desorption and carbon-bromine activation. Only BEEF-vdW fails to predict the faster kinetics of deiodination than the iodobenzene desorption. Our work forms a basis for evaluating density functionals in describing chemical reactions on surfaces.

Application of Nanocomposites in Covalent Organic Framework-Based Electrocatalysts

In recent years, the development of high-performance electrocatalysts for energy conversion and environmental remediation has become a topic of great interest. Covalent organic frameworks (COFs), linked by covalent bonds, have emerged as promising materials in the field of electrocatalysis due to their well-defined structures, high specific surface areas, tunable pore structures, and excellent acid-base stability. However, the low conductivity of COF materials often limits their intrinsic electrocatalytic activity. To enhance the catalytic performance of COF-based catalysts, various nanomaterials are integrated into COFs to form composite catalysts. The stable and tunable porous structure of COFs provides an ideal platform for these nanomaterials, leading to improved electrocatalytic activity. Through rational design, COF-based composite electrocatalysts can achieve synergistic effects between nanomaterials and the COF carrier, enabling efficient targeted electrocatalysis. This review summarizes the applications of nanomaterial-incorporated COF-based catalysts in hydrogen evolution, oxygen evolution, oxygen reduction, carbon dioxide reduction, and nitrogen reduction. Additionally, it outlines design principles for COF-based composite electrocatalysis, focusing on structure-activity relationships and synergistic effects in COF composite nanomaterial electrocatalysts, as well as challenges and future perspectives for next-generation composite electrocatalysts.

One-Step Hydrothermal Deposition of AgSbS2-xSex Thin Films for Solar Cell Applications

AgSbS2-xSex is a promising light-harvesting material for thin film solar cells, characterized by nontoxicity, high chemical stability, and excellent optoelectronic properties. However, the complex chemical composition of AgSbS2-xSex poses significant challenges to thin film preparation, giving rise to an intensive dependence on multi-step preparation methods. Herein, a hydrothermal method is developed for depositing AgSbS2-xSex films and achieves one-step preparation of this kind of thin film materials for the first time. This method can provide sufficient energy for atomic nucleation and adsorption on the substrate surface to promote nuclei aggregation and grow into films. Meanwhile, it achieves control of the chemical kinetics of the deposition solution by introducing EDTA-2Na as an additive and suppressing the enrichment of Ag2Se impurities at the substrate interface. As a result, a high-purity AgSbS2-xSex film with compact and flat morphology is prepared and assembled into solar cells. The device delivers a power conversion efficiency of 3.04% under standard illumination, which is currently the highest efficiency for AgSbS2-xSex solar cells fabricated by the one-step method. This study provides a facile and promising method for the controllable preparation of high-quality AgSbS2-xSex thin films and promoting their application in solar cells.

Fine tuning dual active sites in modulating cascade electrocatalytic nitrate reduction over covalent organic framework

Conversion of NO3- to NH3 proceeds stepwise in natural system under two different enzymes involving intermediate NO2-. Artificial electro-driven NO3- reduction also faces the obstacle of low faradaic efficiency due to insufficient utilization of this intermediate. Herein, we demonstrate a bimetallic COF-based electrocatalyst for the cascade catalysis of NO3--to-NO2--to-NH3 for the first time. TpBpy-Cu2Co4 exhibits a significantly improved performance, with an enhancement factor of 1.4-2 compared to monometallic TpBpy-M. The NH3 yield rate achieves 25.6 mg h-1 mgcat.-1 at -0.55 V vs RHE over TpBpy-Cu2Co4, together with excellent faradaic efficiency (93.4 %). This achievement demonstrates cascade catalysis between Co and Cu units, and their distinct roles are investigated through electrochemical experiments and theory calculations. In electrocatalytic process, Cu site facilities *NO3-to-*NO3H step, while the Co site significantly decreases the energy barrier of *NHOH-to-*NH. The present work provides a valuable inspiration in designing efficient catalysts for cascade reaction.

Selective Generation of Reactive Oxygen Species in Photocatalytic Oxidation by Tuning Porphyrin-Based COFs' Dimensionality

Regulating the selective generation of reactive oxygen species (ROS) is a significant challenge in the field of photocatalytic oxidation, with successful approaches still being limited. Herein, we present a strategy to selectively generate singlet oxygen (1O2) and superoxide radicals (O2•-) by tuning the dimensionality of porphyrin-based covalent organic frameworks (COFs). The transformation of COFs from three-dimensional (3D) solids to two-dimensional (2D) sheets was achieved through the reversible protonation of the imine bond. Upon irradiation, both bulk and thin-layer COF-367 can transfer energy to O2 to generate 1O2. However, thin-layer COF-367 exhibited a superior performance compared to its bulk counterpart in activating O2 to form the O2•- radicals via electron transfer. After excluding the influences of the band structure, O2 adsorption energy, and frontier orbital composition attributed to the dimensionality of the COFs, it is reasonably speculated that the variance in ROS generation arises from the differential exposure ratios of the active surfaces, leading to distinct reaction pathways between the carrier and O2. This study is the first to explore the modulation mechanism of COF dimensionality on the activation of the O2 pathway, underscoring the importance of considering COF dimensionality in photocatalytic reactions.

Covalent Organic Frameworks for Electrocatalysis: Design, Applications, and Perspectives

Covalent organic frameworks (COFs) are an innovative class of crystalline porous polymers composed of light elements such as C, N, O, etc., linked by covalent bonds. The distinctive properties of COFs, including designable building blocks, large specific surface area, tunable pore size, abundant active sites, and remarkable stability, have led their widespread applications in electrocatalysis. In recent years, COF-based electrocatalysts have made remarkable progress in various electrocatalytic fields, including the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, nitrogen reduction reaction, nitrate reduction reaction, and carbon dioxide reduction reaction. This review begins with an introduction to the design and synthesis strategies employed for COF-based electrocatalysts. These strategies include heteroatom doping, metalation of COF and building monomers, encapsulation of active sites within COF pores, and the development of COF-based derived materials. Subsequently, a systematic overview of the recent advancements in the application of COF-based catalysts in electrocatalysis is presented. Finally, the review discusses the main challenges and outlines possible avenues for the future development of COF-based electrocatalysts.

Covalent organic frameworks in tribology - A perspective

Two-dimensional covalent organic frameworks (2D COFs) are an emerging class of crystalline porous materials formed through covalent bonds between organic building blocks. COFs uniquely combine a large surface area, an excellent stability, numerous abundant active sites, and tunable functionalities, thus making them highly attractive for numerous applications. Especially, their abundant active sites and weak interlayer interaction make these materials promising candidates for tribological research. Recently, notable attention has been paid to COFs as lubricant additives due to their excellent tribological performance. Our review aims at critically summarizing the state-of-art developments of 2D COFs in tribology. We discuss their structural and functional design principles, as well as synthetic strategies with a special focus on tribology. The generation of COF thin films is also assessed in detail, which can alleviate their most challenging drawbacks for this application. Subsequently, we analyze the existing state-of-the-art regarding the usage of COFs as lubricant additives, self-lubrication composite coatings, and solid lubricants at the nanoscale. Finally, critical challenges and future trends of 2D COFs in tribology are outlined to initiate and boost new research activities in this exciting field.

2D Conjugated Polymer Thin Films for Organic Electronics: Opportunities and Challenges

2D conjugated polymers (2DCPs) possess extended in-plane π-conjugated lattice and out-of-plane π-π stacking, which results in enhanced electronic performance and potentially unique band structures. These properties, along with predesignability, well-defined channels, easy postmodification, and order structure attract extensive attention from material science to organic electronics. In this review, the recent advance in the interfacial synthesis and conductivity tuning strategies of 2DCP thin films, as well as their application in organic electronics is summarized. Furthermore, it is shown that, by combining topology structure design and targeted conductivity adjustment, researchers have fabricated 2DCP thin films with predesigned active groups, highly ordered structures, and enhanced conductivity. These films exhibit great potential for various thin-film organic electronics, such as organic transistors, memristors, electrochromism, chemiresistors, and photodetectors. Finally, the future research directions and perspectives of 2DCPs are discussed in terms of the interfacial synthetic design and structure engineering for the fabrication of fully conjugated 2DCP thin films, as well as the functional manipulation of conductivity to advance their applications in future organic electronics.

Halogen-Bonded Organic Frameworks (XOFs) Based on N⋅⋅⋅Br+⋅⋅⋅N Bonds: Robust Organic Networks Constructed by Fragile Bonds

Organic frameworks face a trade-off between the framework stability and the bond dynamics, which necessitates the development of innovative linkages that can generate stable frameworks without hindering efficient synthesis. Although iodine(I)-based halogen-bonded organic frameworks (XOFs) have been developed, constructing XOFs based on bromine(I) is desirable yet challenging due to the high sensitivity of bromine(I) species. In this work, we present the inaugural construction of stable bromine(I)-bridged two-dimensional (2D) halogen-bonded organic frameworks, XOF(Br)-TPy-BF4/OTf, based on sensitive [N⋅⋅⋅Br⋅⋅⋅N]+ halogen bonds. The formation of XOF(Br)-TPy-BF4/OTf was monitored by 1H NMR, XPS, IR, SEM, TEM, HR-TEM, SEAD. Their framework structures were established by the results from PXRD, theoretical simulations and SAXS. More importantly, XOF(Br) displayed excellent chemical and thermal stabilities. They exhibited stable two-dimensional framework structures in various organic solvents and aqueous media, even over a wide pH range (pH 3-12), while the corresponding model compounds BrPy2BF4/OTf decomposed quickly even in the presence of minimal water. Furthermore, the influence of the counterions were investigated by replacing BF4 with OTf, which improved the stability of XOF(Br). This characteristic enabled XOF(Br) to serve as an efficient oxidizing reagent in aqueous environments, in contrast with the sensitivity of BrPy2BF4/OTf, which performed well only in organic media. This study not only deepens our fundamental understanding of organic frameworks but also opens new avenues for the development and application of multifunctional XOFs.

Covalent Organic Framework Nanoarchitectonics: Recent Advances for Precious Metal Recovery

The recovery of precious metals (PMs) from secondary resources has garnered significant attention due to environmental and economic considerations. Covalent organic frameworks (COFs) have emerged as promising adsorbents for this purpose, owing to their tunable pore size, facile functionalization, exceptional chemical stability, and large specific surface area. This review provides an overview of the latest research progress in utilizing COFs to recover PMs. Firstly, the design and synthesis strategies of chemically stable COF-based materials, including pristine COFs, functionalized COFs, and COF-based composites, are delineated. Furthermore, the application of COFs in the recovery of gold, silver, and platinum group elements is delved into, emphasizing their high adsorption capacity and selectivity as well as recycling ability. Additionally, various interaction mechanisms between COFs and PM ions are analyzed. Finally, the current challenges faced by COFs in the field of PM recovery are discussed, and potential directions for future development are proposed, including enhancing the recyclability and reusability of COF materials and realizing the high recovery of PMs from actual acidic wastewater. With the targeted development of COF-based materials, the recovery of PMs can be realized more economically and efficiently in the future.

Interlayer Polymerization to Construct a Fully Conjugated Covalent Organic Framework as a Metal-Free Oxygen Reduction Reaction Catalyst for Anion Exchange Membrane Fuel Cells

Two-dimensional (2D) covalent organic frameworks (COFs) have a multilayer skeleton with a periodic π-conjugated molecular array, which can facilitate charge carrier transport within a COF layer. However, the lack of an effective charge carrier transmission pathway between 2D COF layers greatly limits their applications in electrocatalysis. Herein, by employing a side-chain polymerization strategy to form polythiophene along the nanochannels, a conjugated bridge is constructed between the COF layers. The as-synthesized fully conjugated COF (PTh-COF) exhibits high oxygen reduction reaction (ORR) activity with narrowed energy band gaps. Correspondingly, PTh-COF is tested as a metal-free cathode catalyst for anion exchange membrane fuel cells (AEMFCs) which showed a maximum power density of 176 mW cm-2 under a current density of 533 mA cm-2. The density functional theory (DFT) calculation reveals that interlayer conjugated polythiophene optimizes the electron cloud distribution, which therefore enhances the ORR performance. This work not only provides new insight into the construction of a fully conjugated covalent organic framework but also promotes the development of new metal-free ORR catalysts.

Tunable Structured 2D Nanobiocatalysts: Synthesis, Catalytic Properties and New Horizons in Biomedical Applications

2D materials have offered essential contributions to boosting biocatalytic efficiency in diverse biomedical applications due to the intrinsic enzyme-mimetic activity and massive specific surface area for loading metal catalytic centers. Since the difficulty of high-quality synthesis, the varied structure, and the tough choice of efficient surface loading sites with catalytic properties, the artificial building of 2D nanobiocatalysts still faces great challenges. Here, in this review, a timely and comprehensive summarization of the latest progress and future trends in the design and biotherapeutic applications of 2D nanobiocatalysts is provided, which is essential for their development. First, an overview of the synthesis-structure-fundamentals and structure-property relationships of 2D nanobiocatalysts, both metal-free and metal-based is provided. After that, the effective design of the active sites of nanobiocatalysts is discussed. Then, the progress of their applied research in recent years, including biomedical analysis, biomedical therapeutics, pharmacokinetics, and toxicology is systematically highlighted. Finally, future research directions of 2D nanobiocatalysts are prospected. Overall, this review to provide cutting-edge and multidisciplinary guidance for accelerating future developments and biomedical applications of 2D nanobiocatalysts is expected.

Triazine-structured covalent organic framework nanosheets with inherent hydrophilicity for the highly efficient and selective enrichment of glycosylated peptides

Protein glycosylation plays a crucial role in various biological processes and is related to various diseases. Highly specific enrichment of glycopeptides before mass spectrometry detection is crucial for comprehensive glycoproteomic analysis. However, it still remains a great challenge due to the absence of affinity materials with excellent enrichment efficiency. In this work, a triazine structure linked by a -NH- bond of two-dimensional (2-D) covalent organic framework (COF) nanosheets was synthesized as an affinity adsorbent for the selective capture of glycopeptides. In particular, by introducing hydrophilic monomers via a bottom-up approach, the 2-D COF (denoted as NENP-1) nanosheets were provided with abundant amino groups and inherent hydrophilicity. Owing to the specific surface area and excessive accessible sites for hydrophilicity, the resulting NENP-1 nanosheets exhibited an outstanding glycopeptide enrichment efficiency from standard samples with a superior detection sensitivity (1 × 10-10 M), good enrichment selectivity (1 : 800, HRP tryptic digest to BSA protein), excellent binding capacity (100 mg g-1), great reusability, and recovery (60.2%). Furthermore, using the NENP-1 nanosheet adsorbent, twenty-four endogenous glycopeptides in the serum of patients with gastric cancer were successfully identified by LC-MS/MS technology, which illustrates a promising prospective of hydrophilic COF nanosheets in glycoproteomics research.

Tuning the Pyridine Units in Vinylene-Linked Covalent Organic Frameworks Boosting 2e- Oxygen Reduction Reaction

The N-doped carbon materials are supposed to be the efficient oxygen reduction reaction (ORR) catalysts with the undefined N-doped carbon ring groups. It is essential to well define the role of the nitrogen atoms of these carbon structures in active behavior. Even though, the covalent organic frameworks (COFs) with precise structures are well developed, but unable to exclude the polar linkages influence. This study presents a series of pyridine-containing COFs linked via nonpolar carbon-carbon double bonds (C = C). Their catalytic activity and selectivity for 2e- ORR are successfully modulated by locating the embedded pyridine nitrogen in the backbones through the linking modes of pyridine moieties within the frameworks. Such phenomena can be attributed to their different binding abilities toward O2, leading to the different binding strength of the intermediate OH* to the catalytic sites, also verified by the theoretical calculation. This work provides us a new insight to design high-efficiency ORR catalysts through the exact location of pyridine nitrogen.

On-Water Surface Synthesis of Vinylene-Linked Cationic Two-Dimensional Polymer Films as the Anion-Selective Electrode Coating

Vinylene-linked two-dimensional polymers (V-2DPs) and their layer-stacked covalent organic frameworks (V-2D COFs) featuring high in-plane π-conjugation and robust frameworks have emerged as promising candidates for energy-related applications. However, current synthetic approaches are restricted to producing V-2D COF powders that lack processability, impeding their integration into devices, particularly within membrane technologies reliant upon thin films. Herein, we report the novel on-water surface synthesis of vinylene-linked cationic 2DPs films (V-C2DP-1 and V-C2DP-2) via Knoevenagel polycondensation, which serve as the anion-selective electrode coating for highly-reversible and durable zinc-based dual-ion batteries (ZDIBs). Model reactions and theoretical modeling revealed the enhanced reactivity and reversibility of the Knoevenagel reaction on the water surface. On this basis, we demonstrated the on-water surface 2D polycondensation towards V-C2DPs films that show large lateral size, tunable thickness, and high chemical stability. Representatively, V-C2DP-1 presents as a fully crystalline and face-on oriented film with in-plane lattice parameters of a=b≈43.3 Å. Profiting from its well-defined cationic sites, oriented 1D channels, and stable frameworks, V-C2DP-1 film possesses superior bis(trifluoromethanesulfonyl)imide anion (TFSI-)-transport selectivity (transference, t_=0.85) for graphite cathode in high-voltage ZDIBs, thus triggering additional TFSI--intercalation stage and promoting its specific capacity (from ~83 to 124 mAh g-1) and cycling life (>1000 cycles, 95 % capacity retention).

Covalent Organic Frameworks: Linkage Chemistry and Its Critical Role in The Evolution of π Electronic Structures and Functions

Covalent organic frameworks (COFs) provide a molecular platform for designing a novel class of functional materials with well-defined structures. A crucial structural parameter is the linkage, which dictates how knot and linker units are connected to form two-dimensional polymers and layer frameworks, shaping ordered π-array and porous architectures. However, the roles of linkage in the development of ordered π electronic structures and functions remain fundamental yet unresolved issues. Here we report the designed synthesis of COFs featuring four representative linkages: hydrazone, imine, azine, and C=C bonds, to elucidate their impacts on the evolution of π electronic structures and functions. Our observations revealed that the hydrazone linkage provides a non-conjugated connection, while imine and azine allow partial π conjugation, and the C=C bond permits full π-conjugation. Importantly, the linkage profoundly influences the control of π electronic structures and functions, unraveling its pivotal role in determining key electronic properties such as band gap, frontier energy levels, light absorption, luminescence, carrier density and mobility, and magnetic permeability. These findings highlight the significance of linkage chemistry in COFs and offer a general and transformative guidance for designing framework materials to achieve electronic functions.

One-Dimensional Covalent Organic Framework-Based Multilevel Memristors for Neuromorphic Computing

Memristors are essential components of neuromorphic systems that mimic the synaptic plasticity observed in biological neurons. In this study, a novel approach employing one-dimensional covalent organic framework (1D COF) films was explored to enhance the performance of memristors. The unique structural and electronic properties of two 1D COF films (COF-4,4'-methylenedianiline (MDA) and COF-4,4'-oxydianiline (ODA)) offer advantages for multilevel resistive switching, which is a key feature in neuromorphic computing applications. By further introducing a TiO2 layer on the COF-ODA film, a built-in electric field between the COF-TiO2 interfaces could be generated, demonstrating the feasibility of utilizing COFs as a platform for constructing memristors with tunable resistive states. The 1D nanochannels of these COF structures contributed to the efficient modulation of electrical conductance, enabling precise control over synaptic weights in neuromorphic circuits. This study also investigated the potential of these COF-based memristors to achieve energy-efficient and high-density memory devices.

Molecular machines working at interfaces: physics, chemistry, evolution and nanoarchitectonics

As a post-nanotechnology concept, nanoarchitectonics combines nanotechnology with advanced materials science. Molecular machines made by assembling molecular units and their organizational bodies are also products of nanoarchitectonics. They can be regarded as the smallest functional materials. Originally, studies on molecular machines analyzed the average properties of objects dispersed in solution by spectroscopic methods. Researchers' playgrounds partially shifted to solid interfaces, because high-resolution observation of molecular machines is usually done on solid interfaces under high vacuum and cryogenic conditions. Additionally, to ensure the practical applicability of molecular machines, operation under ambient conditions is necessary. The latter conditions are met in dynamic interfacial environments such as the surface of water at room temperature. According to these backgrounds, this review summarizes the trends of molecular machines that continue to evolve under the concept of nanoarchitectonics in interfacial environments. Some recent examples of molecular machines in solution are briefly introduced first, which is followed by an overview of studies of molecular machines and similar supramolecular structures in various interfacial environments. The interfacial environments are classified into (i) solid interfaces, (ii) liquid interfaces, and (iii) various material and biological interfaces. Molecular machines are expanding their activities from the static environment of a solid interface to the more dynamic environment of a liquid interface. Molecular machines change their field of activity while maintaining their basic functions and induce the accumulation of individual molecular machines into macroscopic physical properties molecular machines through macroscopic mechanical motions can be employed to control molecular machines. Moreover, research on molecular machines is not limited to solid and liquid interfaces; interfaces with living organisms are also crucial.

Status and Prospect of Two-Dimensional Materials in Electrolytes for All-Solid-State Lithium Batteries

Replacing liquid electrolytes and separators in conventional lithium-ion batteries with solid-state electrolytes (SSEs) is an important strategy to ensure both high energy density and high safety. Searching for fast ionic conductors with high electrochemical and chemical stability has been the core of SSE research and applications over the past decades. Based on the atomic-level thickness and infinitely expandable planar structure, numerous two-dimensional materials (2DMs) have been exploited and applied to address the most critical issues of low ionic conductivity of SSEs and lithium dendrite growth in all-solid-state lithium batteries. This review introduces the research process of 2DMs in SSEs, then summarizes the mechanisms and strategies of inert and active 2DMs toward Li+ transport to improve the ionic conductivity and enhance the electrode/SSE interfacial compatibility. More importantly, the main challenges and future directions for the application of 2DMs in SSEs are considered, including the importance of exploring the relationship between the anisotropic structure of 2DMs and Li+ diffusion behavior, the exploitation of more 2DMs, and the significance of in situ characterizations in elucidating the mechanisms of Li+ transport and interfacial reactions. This review aims to provide a comprehensive understanding to facilitate the application of 2DMs in SSEs.

Modified photoanode by in situ growth of covalent organic frameworks on BiVO4 for oxygen evolution reaction

The development of efficient oxygen evolution reaction (OER) catalysts is of great significance because the water oxidation reaction at the photoanode is the rate-determining step in photoelectrocatalytic (PEC) water splitting. Herein, two hybrid photoanodes named BiVO4/COF-Azo and BiVO4/COF-Ben were prepared by in situ solvothermal growth on a modified BiVO4 photoanode. Characterization results revealed that the Azo and Ben COFs could match with BiVO4 well to form heterojunctions, which could effectively enhance the separation efficiency of photogenerated carriers. Also, the smaller impedance of the composite photoanodes and faster kinetics of the water oxidation reaction promoted the charge transmission and enhanced the reaction efficiency of the surface-reaching holes, respectively. As a result, the composite photoanodes exhibited a larger photocurrent and more negative onset potential compared to the pristine BiVO4. This work not only provides a new strategy to construct efficient hybrid photoanodes, but also expands the applications of COFs.

Engineering 0D/2D Architecture of Ni(OH)2 Nanoparticles on Covalent Organic Framework Nanosheets for Selective Visible-Light-Driven CO2 Reduction

Low-dimensional materials serving as photocatalysts favor providing abundant unsaturated active sites and shortening the charge transport distance, but the high surface energy readily causes the aggregation that limits their application. Herein, it is demonstrated that 2D covalent organic framework (COF) TpBD nanosheets are effective in the dispersion and stabilization of 0D Ni(OH)2 . The COF precursor TpBD is synthesized from the Schiff base condensation of 1,3,5-triformylphloroglucinol (Tp) and benzidine (BD) and exfoliated into 2D nanosheets named BDNs via ultrasonication. The formation of highly dispersive 0D Ni(OH)2 on BDNs is reached under a mild weak basic condition, enabling robust active sites for CO2 adsorption/activation and rapid interface cascaded electron transport channels for the accumulation of long-lived photo-generated charges. The champion catalyst 30%Ni-BDNs effectively catalyze the CO2 to CO conversion under visible-light irradiation, offering a high CO evolution rate of 158.4 mmol g-1 h-1 and turnover frequency of 51 h-1 . By contrast, the counterpart photocatalyst, the bulk TpBD stabilized Ni(OH)2 , affords a much lower CO evolution rate and selectivity. This work demonstrates a new avenue to simultaneously construct efficient active sites and electron transport channels by coupling 0D metal hydroxides and 2D COF nanosheets for CO2 photoreduction.

Solid-Liquid Interfacial Engineered Large-Area Two-Dimensional Covalent Organic Framework Films

Constructing uniform covalent organic framework (COF) film on substrates for electronic devices is highly desirable. Here, a simple and mild strategy is developed to prepare them by polymerization on a solid-liquid interface. The universality of the method is confirmed by the successful preparation of five COF films with different microstructures. These films have large lateral size, controllable thickness, and high crystalline quality. And COF patterns can also be directly achieved on substrates via hydrophilic and hydrophobic interface engineering, which is in favor of preparing device array. For application studies, the PyTTA-TPA (PyTTA: 4,4',4'',4'''-(1,3,6,8-Tetrakis(4-aminophenyl)pyrene and TPA: terephthalaldehyde) COF film has a high photoresponsivity of 59.79 μA W-1 at 420 nm for photoelectrochemical (PEC) detection. When employed as an active material for optoelectronic synaptic devices for the first attempt, it shows excellent light-stimulated synaptic plasticity properties such as short-term plasticity (STP), long-term plasticity (LTP), and the conversion of STP to LTP, which can be used to simulate biological synaptic functions.

Recent advances in two-dimensional polymers: synthesis, assembly and energy-related applications

Two-dimensional polymers (2DPs) are a class of 2D crystalline polymer materials with definite structures, which have outstanding physical-chemical and electronic properties. They cleverly link organic building units through strong covalent bonds and can construct functional 2DPs through reasonable design and selection of different monomer units to meet various application requirements. As promising energy materials, 2DPs have developed rapidly in recent years. This review first introduces the basic overview of 2DPs, such as their historical development, inherent 2D characteristics and diversified topological advantages, followed by the summary of the typical 2DP synthesis methods recently (including "top-down" and "bottom-up" methods). The latest research progress in assembly and processing of 2DPs and the energy-related applications in energy storage and conversion are also discussed. Finally, we summarize and prospect the current research status, existing challenges, and future research directions of 2DPs.

Covalent-Organic Framework-Based Materials in Theranostic Applications: Insights into Their Advantages and Challenges

Nanomedicine has been essential in bioimaging and cancer therapy in recent years. Nanoscale covalent-organic frameworks (COFs) have been growing as an adequate classification of biomedical nanomaterials with practical application prospects because of their increased porosity, functionality, and biocompatibility. The high sponginess of COFs enables the incorporation of distinct imaging and therapeutic mechanisms with a better loading efficiency. Nevertheless, preliminary biocompatibility limits their possibility for clinical translation. Thus, cutting-edge nanomaterials with high biocompatibility and improved therapeutic efficiency are highly expected to fast-track the clinical translation of nanomedicines. The inherent effects of nanoscale COFs, such as proper size, modular pore geometry and porosity, and specific postsynthetic transformation through simple organic changes, make them particularly appealing for prospective nanomedicines. The organic building blocks of COFs may also be postmodified for particular binding to biomarkers. The exceptional features of COFs cause them to be an encouraging nanocarrier for bioimaging and therapeutic applications. In this review, we have systematically discussed the advances of COFs in the field of theranostics by providing essential features of COFs along with their synthetic methods. Further, the applications of COFs in the field of theranostics (such as drug delivery systems, photothermal, and photodynamic therapy) are discussed in detail with the help of available literature to date. Furthermore, the advantages of COFs over other materials for therapeutics and drug delivery are discussed. Finally, the review concludes with potential future COF applications in the theranostic field.

On-Surface Synthesis of a Radical 2D Supramolecular Organic Framework

The design of supramolecular organic radical cages and frameworks is one of the main challenges in supramolecular chemistry. Their interesting material properties and wide applications make them very promising for (photo)redox catalysis, sensors, or host-guest spin-spin interactions. However, the high reactivity of radical organic systems makes the design of such supramolecular radical assemblies challenging. Here, we report the on-surface synthesis of a purely organic supramolecular radical framework on Au(111), by combining supramolecular and on-surface chemistry. We employ a tripodal precursor, functionalized with 7-azaindole groups that, catalyzed by a single gold atom on the surface, forms a radical molecular product constituted by a π-extended fluoradene-based radical core. The radical products self-assemble through hydrogen bonding, leading to extended 2D domains ordered in a Kagome-honeycomb lattice. This approach demonstrates the potential of on-surface synthesis for developing 2D supramolecular radical organic chemistry.

Recent progress in the synthesis and applications of covalent organic framework-based composites

Covalent organic frameworks (COFs) have historically been of interest to researchers in different areas due to their distinctive characteristics, including well-ordered pores, large specific surface area, and structural tunability. In the past few years, as COF synthesis techniques developed, COF-based composites fabricated by integrating COFs and other functional materials including various kinds of metal or metal oxide nanoparticles, ionic liquids, metal-organic frameworks, silica, polymers, enzymes and carbon nanomaterials have emerged as a novel kind of porous hybrid material. Herein, we first provide a thorough summary of advanced strategies for preparing COF-based composites; then, the emerging applications of COF-based composites in diverse fields due to their synergistic effects are systematically highlighted, including analytical chemistry (sensing, extraction, membrane separation, and chromatographic separation) and catalysis. Finally, the current challenges associated with future perspectives of COF-based composites are also briefly discussed to inspire the advancement of more COF-based composites with excellent properties.

Construction of Thiadiazole-Bridged sp2-Carbon-Conjugated Covalent Organic Frameworks with Diminished Excitation Binding Energy Toward Superior Photocatalysis

Sp2-carbon-conjugated covalent organic frameworks (sp2c-COFs) have emerged as promising platforms for phototo-chemical energy conversion due to their tailorable optoelectronic properties, in-plane π-conjugations, and robust structures. However, the development of sp2c-COFs in photocatalysis is still highly hindered by their limited linkage chemistry. Herein, we report a novel thiadiazole-bridged sp2c-COF (sp2c-COF-ST) synthesized by thiadiazole-mediated aldol-type polycondensation. The resultant sp2c-COF-ST demonstrates high chemical stability under strong acids and bases (12 M HCl or 12 M NaOH). The electro-deficient thiadiazole together with fully conjugated and planar skeleton endows sp2c-COF-ST with superior photoelectrochemical performance and charge-carrier separation and migration ability. As a result, when employed as a photocathode, sp2c-COF-ST exhibits a significant photocurrent up to ∼14.5 μA cm-2 at 0.3 V vs reversible hydrogen electrode (RHE) under visible-light irradiation (>420 nm), which is much higher than those analogous COFs with partial imine linkages (mix-COF-SNT ∼ 9.5 μA cm-2) and full imine linkages (imi-COF-SNNT ∼ 4.9 μA cm-2), emphasizing the importance of the structure-property relationships. Further temperature-dependent photoluminescence spectra and density functional theory calculations demonstrate that the sp2c-COF-ST has smaller exciton binding energy as well as effective mass in comparison to mix-COF-SNT and imi-COF-SNNT, which suggests that the sp2c-conjugated skeleton enhances the exciton dissociation and carrier migration under light irradiation. This work highlights the design and preparation of thiadiazole-bridged sp2c-COFs with promising photocatalytic performance.

Covalent Organic Frameworks: Their Composites and Derivatives for Rechargeable Metal-Ion Batteries

Covalent organic frameworks (COFs) have attracted considerable interest in the field of rechargeable batteries owing to their three-dimensional (3D) varied pore sizes, inerratic porous structures, abundant redox-active sites, and customizable structure-adjustable frameworks. In the context of metal-ion batteries, these materials play a vital role in electrode materials, effectively addressing critical issues such as low ionic conductivity, limited specific capacity, and unstable structural integrity. However, the electrochemical characteristics of the developed COFs still fall short of practical battery requirements due to inherent issues such as low electronic conductivity, the tradeoff between capacity and redox potential, and unfavorable micromorphology. This review provides a comprehensive overview of the recent advancements in the application of COFs, COF-based composites, and their derivatives in rechargeable metal-ion batteries, including lithium-ion, lithium-sulfur, sodium-ion, sodium-sulfur, potassium-ion, zinc-ion, and other multivalent metal-ion batteries. The operational mechanisms of COFs, COF-based composites, and their derivatives in rechargeable batteries are elucidated, along with the strategies implemented to enhance the electrochemical properties and broaden the range of their applications.

Two-Dimensional Conjugated Polymers: a New Choice For Organic Thin-Film Transistors

Organic thin-film transistors (OTFTs) as a vital component among transistors have shown great potential in smart sensing, flexible displays, and bionics due to their flexibility, biocompatibility and customizable chemical structures. Even though linear conjugated polymer semiconductors are common for constructing channel materials of OTFTs, advanced materials with high charge carrier mobility, tunable band structure, robust stability, and clear structure-property relationship are indispensable for propelling the evolution of OTFTs. Two-dimensional conjugated polymers (2DCPs), featured with conjugated lattice, tailorable skeletons, and functional porous structures, match aforementioned criteria closely. In this review, we firstly introduce the synthesis of 2DCP thin films, focusing on their characteristics compatible with the channels of OTFTs. Subsequently, the physics and operating mechanisms of OTFTs and the applications of 2DCPs in OTFTs are summarized in detail. Finally, the outlook and perspective in the field of OTFTs using 2DCPs are provided as well.

Bipolar Molecular Torque Wrench Modulates the Stacking of Two-Dimensional Metal-Organic Framework Nanosheets

The precise modulation of nanosheet stacking modes introduces unforeseen properties and creates momentous applications but remains a challenge. Herein, we proposed a strategy using bipolar molecules as torque wrenches to control the stacking modes of 2-D Zr-1,3,5-(4-carboxylphenyl)-benzene metal-organic framework (2-D Zr-BTB MOF) nanosheets. The bipolar phenyl-alkanes, phenylmethane (P-C1) and phenyl ethane (P-C2), predominantly instigated the rotational stacking of Zr-BTB-P-C1 and Zr-BTB-P-C2, displaying a wide angular distribution. This included Zr-BTB-P-C1 orientations at 0, 12, 18, and 24° and Zr-BTB-P-C2 orientations at 0, 6, 12, 15, 24, and 30°. With reduced polarity, phenyl propane (P-C3) and phenyl pentane (P-C5) introduced steric hindrance and facilitated alkyl hydrophobic interactions with the nanosheets, primarily resulting in the modulation of eclipsed stacking for Zr-BTB-P-C3 (64.8%) and Zr-BTB-P-C5 (93.3%) nanosheets. The precise angle distributions of four Zr-BTB-P species were in agreement with theoretical calculations. The alkyl induction mechanism was confirmed by the sequential guest replacement and 2-D 13C-1H heteronuclear correlation (HETCOR). In addition, at the single-particle level, we first observed that rotational stacked pores exhibited similar desorption rates for xylene isomers, while eclipsed stacked pores showed significant discrepancy for xylenes. Moreover, the eclipsed nanosheets as stationary phases exhibited high resolution, selectivity, repeatability, and durability for isomer separation. The universality was proven by another series of bipolar acetate-alkanes. This bipolar molecular torque wrench strategy provides an opportunity to precisely control the stacking modes of porous nanosheets.

Magnetoresistance in Organic Spin Valves Based on Acid-Exfoliated 2D Covalent Organic Frameworks Thin Films

Covalent organic frameworks (COFs), as a burgeoning class of crystalline porous materials, have made significant progress in their application to optoelectronic devices such as field-effect transistors, memristors, and photodetectors. However, the insoluble features of microcrystalline two-dimensional (2D) COF powders limit development of their thin film devices. Additionally, the exploration of spin transport properties in this category of π-conjugated skeleton materials remains vacant thus far. Herein, an imine-linked 2D Py-Np COF nanocrystalline powder was synthesized by Schiff base condensation of 4,4',4'',4'''-(pyrene-1,3,6,8-tetrayl)tetraaniline and naphthalene-2,6-dicarbaldehyde. Then, we prepared a large-scale free-standing Py-Np COF film via a top-down strategy of chemically assisted acid exfoliation. Moreover, high-quality COF films acted as active layers were transferred onto ferromagnetic La0.67 Sr0.33 MnO3 (LSMO) electrodes for the first attempt to fabricate organic spin valves (OSVs) based on 2D COF materials. This COF-based OSV device with a configuration of LSMO/Py-Np COF/Co/Au demonstrated a remarkable magnetoresistance (MR) value up to -26.5 % at 30 K. Meanwhile, the MR behavior of the COF-based OSVs exhibited a highly temperature dependence and operational stability. This work highlights the enormous application prospects of 2D COFs in organic spintronics and provides a promising approach for developing electronic and spintronic devices based on acid-exfoliated COF thin films.

Porous crystalline materials for memories and neuromorphic computing systems

Porous crystalline materials usually include metal-organic frameworks (MOFs), covalent organic frameworks (COFs), hydrogen-bonded organic frameworks (HOFs) and zeolites, which exhibit exceptional porosity and structural/composition designability, promoting the increasing attention in memory and neuromorphic computing systems in the last decade. From both the perspective of materials and devices, it is crucial to provide a comprehensive and timely summary of the applications of porous crystalline materials in memory and neuromorphic computing systems to guide future research endeavors. Moreover, the utilization of porous crystalline materials in electronics necessitates a shift from powder synthesis to high-quality film preparation to ensure high device performance. This review highlights the strategies for preparing porous crystalline materials films and discusses their advancements in memory and neuromorphic electronics. It also provides a detailed comparative analysis and presents the existing challenges and future research directions, which can attract the experts from various fields (e.g., materials scientists, chemists, and engineers) with the aim of promoting the applications of porous crystalline materials in memory and neuromorphic computing systems.

Recent advancements of covalent organic frameworks (COFs) as proton conductors under anhydrous conditions for fuel cell applications

Recent electrochemical energy conversion devices require more advanced proton conductors for their broad applications, especially, proton exchange membrane fuel cell (PEMFC) construction. Covalent organic frameworks (COFs) are an emerging class of organic porous crystalline materials that are composed of organic linkers and connected by strong covalent bonds. The unique characteristics including well-ordered and tailorable pore channels, permanent porosity, high degree of crystallinity, excellent chemical and thermal stability, enable COFs to be the potential proton conductors in fuel cell devices. Generally, proton conduction of COFs is dependent on the amount of water (extent of humidity). So, the constructed fuel cells accompanied complex water management system which requires large radiators and airflow for their operation at around 80 °C to avoid overheating and efficiency roll-off. To overcome such limitations, heavy-duty fuel cells require robust proton exchange membranes with stable proton conduction at elevated temperatures. Thus, proton conducting COFs under anhydrous conditions are in high demand. This review summarizes the recent progress in emerging COFs that exhibit proton conduction under anhydrous conditions, which may be prospective candidates for solid electrolytes in fuel cells.

A simple and efficient process for the synthesis of 2D carbon nitrides and related materials

We describe here a new process for the synthesis of very high quality 2D Covalent Organic Frameworks (COFs), such a C2N and CN carbon nitrides. This process relies on the use of a metallic surface as both a reagent and a support for the coupling of small halogenated building blocks. The conditions of the assembly reaction are chosen so as to leave the inorganic salts by-products on the surface, to further confine the assembly reaction on the surface and increase the quality of the 2D layers. We found that under these conditions, the process directly returns few layers material. The structure/quality of these materials is demonstrated by extensive cross-characterizations at different scales, combining optical microscopy, Scanning Electron Microscopy (SEM)/Transmission Electron Microscopy (TEM) and Energy Dispersive Spectroscopy (EDS). The availability of such very large, high-quality layers of these materials opens interesting perspectives, for example in photochemistry and electronics (intrinsic transport properties, high gap substrate for graphene, etc...).

Design and Synthesis of a Triazine-Based sp2 Carbon-Conjugated Covalent Organic Framework for Blue Light Photocatalysis

Fully conjugated covalent organic frameworks (COFs) can exhibit great potential in semiconductor photocatalysis. But their syntheses remain elusive due to the low reversibility of vinylene linkage. Herein, by tuning the amount of base and temperature, a novel triazine-based sp2  carbon-conjugated COF (TA-sp2 c-COF) is successfully constructed over Cs2 CO3 . Besides, the influence of modulating factors on the chemical and optoelectronic properties of TA-sp2 c-COF is thoroughly investigated. TA-sp2 c-COF adopts an eclipsed AA stacking structure with uniform micropores (1.4 nm). The blue light photocatalysis of the highly crystalline TA-sp2 c-COF is established for the selective oxidative coupling of amines with oxygen, and the predominant role of superoxide is identified in forming imines. This work foretells that meticulous modulation of reaction conditions is the key to constructing sp2  carbon-conjugated COFs toward solar photocatalysis.