An Ethynyl-Linked sp-Carbon-Conjugated Covalent Organic Framework through Sonogashira Cross-Coupling Reactions

We report the synthesis of ethynyl-linked covalent organic frameworks through microinterfacial polymerization with Sonogashira cross-coupling reactions of 5,10,15,20-tetrakis(4-ethynylphenyl)porphyrin (TEPP) and 5,10,15,20-tetrakis(4-bromophenyl)porphyrin (TBPP). This framework consists of extended two-dimensional tetragonal layers with porphyrin knots and ethynyl (-C≡C-) linkers and constitutes layered frameworks with an AB-stacking mode, creating periodically ordered porphyrin arrays and one-dimensional microporous channels. The ethynyl-linked porphyrin framework features a broad absorption spectrum that covers the entire visible region and extends to the infrared zone exceeding 1400 nm, exhibits a low optical bandgap of 1.32 eV, and is photoconductive, generating prominent photocurrent. The micropores formed by the AB stacking are highly accessible, enabling iodine uptake, CO2 capture, and Li+ confinement. Remarkably, the Li+-confined frameworks exhibit Li+ conductivity of 2.1 × 10-5 S cm-1 at 100 °C under anhydrous conditions and achieve an exceptional Li+ conductivity of 3.9 × 10-3 S cm-1 at a relative humidity of 98%. Temperature-dependent experiments revealed that Li+ conduction is facilitated by a low-energy barrier hopping mechanism. We envision that our ethynyl linkage strategy will not only develop novel sp-carbon-conjugated frameworks but also enable unprecedented functions.

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.

Azadiene-Linked Carbon-Organic Frameworks for Enhanced Photocatalytic Efficiency in Organic Synthesis

2D covalent organic frameworks (2D-COFs) have garnered huge attention as a promising platform for solar to chemical energy conversion. In this work, two novel azadiene-linked COFs (COF-JLU236 and COF-JLU237) were designed and successfully synthesized under solvothermal conditions for the first time. And new materials have high crystallinity, large surface area, good durability, and broad absorption range, as well as excellent photoelectric properties. Importantly, they can effectively induce selective oxidation of sulfides and phenylboronic acids under visible light, and show wide substrate adaptability and superior reusability.

Design and structure-function interplay in covalent organic frameworks for photocatalytic CO2 reduction

The escalating global energy demands and the need to alleviate the rapid rise in greenhouse gases have led to colossal interest in designing efficient catalytic systems for photocatalytic CO2 reduction. While inorganic semiconductors have been the frontrunners for a long time, porous photocatalysts, particularly covalent organic frameworks (COFs), are gaining traction due to their atomically precise structures, enabling tuning their structural and chemical properties. Designed using the principles of reticular chemistry, the building units of COFs can be modulated to incorporate catalytically active sites periodically using robust covalent bonds to endow them with high efficiency, selectivity, and stability. Unlike the non-porous congeners, COFs, with their high porosity and precisely defined pore channels, allow for quicker diffusion of substrates and products, enabling the utilization of deeply buried photocatalytic sites. Our approach is to comprehend the significant roadblocks that must be overcome for designing state-of-the-art catalysts for photocatalytic CO2 reduction. Building upon that, we highlight the key strategies devised to design COF-based CO2RR photocatalysts. A fundamental understanding of the structure-property relationship is quintessential for utilizing the precision of COF chemistry for developing next-generation materials combining activity, selectivity, and efficiency in a single system. Throughout this review, we have taken a closer look at how the critical design aspects and molecular engineering reciprocate towards augmenting the bulk photocatalytic properties of efficiency and selectivity. Understanding molecular engineering and structure-property relationships will be conducive to developing sophisticated systems to solve global crises in this burgeoning area of research.

Low-dimensional materials for ammonia synthesis

Ammonia is an essential chemical due to its immense usage in agriculture, energy storage, and transportation. The synthesis of "green" ammonia via carbon-free routes and renewable energy sources is the need of the hour. In this context, photo- and/or electrocatalysis proves to be highly crucial. Low-dimensional materials (LDMs), owing to their unique properties, play a significant role in catalysis. This review presents a vast library of LDMs and broadly categorizes their catalytic performance according to their dimensionality, i.e., zero-, one-, and two-dimensional catalysts. The rational design of LDMs can significantly improve their catalytic performance, particularly in reducing small molecules like dinitrogen, nitrates, nitrites, and nitric oxides to synthesize ammonia via photo- and/or electrocatalysis. Additionally, converting nitrates and nitrites to ammonia can be beneficial in wastewater treatment and be coupled with CO2 co-reduction or oxidative reactions to produce urea and other valuable chemicals, which are also discussed in this review. This review collates the works published in recent years in this field and offers some fresh perspectives on ammonia synthesis. Through this review, we aim to provide a comprehensive insight into the catalytic properties of the LDMs, which are expected to enhance the efficiency of ammonia production and promote the synthesis of value-added products.

A Computational Design of Covalently Bonded Mixed Stacking Cocrystals

In this study, a computational design of a new type of donor-acceptor mixed stacking cocrystals is introduced. Our approach involves functionalizing trisilasumanene frameworks with electron-donating groups (-CH3, -OH, -NH2) and electron-withdrawing groups (-F, -CN), and then stacking donors and acceptors alternatively while connecting them either by sp3- and sp-carbon chains. Using the B3LYP-D3/6-31+G(d) level of theory, we demonstrate that these covalently bonded cocrystals can overcome the issue of thermal and mechanical instabilities observed in the non-covalently mixed stacking. Furthermore, modifying donor and acceptor groups can vary the bandgaps, approximated by the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) gaps, from 1.50 to 3.50 eV. The results also predict the covalently bonded mixed stacking cocrystals having much larger conductance via Yoshizawa model. In addition, variations in bridge lengths were found to have a small effect on the HOMO-LUMO gaps but allow for a new control parameter regarding the porosity of the materials. These results encourage experimental explorations.

Polarization-Induced Quantum Confinement of Negative Charge Carriers by Organic Nanoporous Frameworks

We characterize the attachment of excess-electrons to organic nanoporous systems such as molecular nanohoops and models of covalent organic frameworks (COFs) using many-body methods. All the nanopore systems exhibit diffuse electronic states where the excess-electron is bound to the molecular scaffold via long-range polarization forces, and the excess-electron is predominantly localized in the interior of the nanopore or away from the molecular scaffold. Such "nanopore-bound" states show an enhanced electron-transfer coupling compared to more strongly-bound skeletal-states (or valence-bound states), where the excess-electron is confined to the molecular skeleton. For 1D assemblies of nanohoops, the bands formed from nanopore-bound states have a consistent nearly-free-electron character, indicating an efficient excited-state pathway for charge-carriers, while the bands from skeletal-states have higher effective mass along certain lattice directions. The nanopore-bound states show distinct size-dependent variations in electron affinities compared to skeletal-states and previously observed molecular quantum corral states. We conclude that nanopore-bound states emerge from polarization-induced quantum confinement, forming a distinct common feature of organic nanoporous matter with potential for efficient electron-transport.

High-Performance Perovskite Solar Cells via 3D Covalent Organic Frameworks: Enhanced Efficiency Through Precision Interface Engineering

Covalent organic frameworks (COFs), a class of porous polymers with tunable 2D or 3D structures, have drawn significant attention for their exceptional versatility across various applications. Recently, the integration of COFs into perovskite solar cells (PSCs) has emerged as a promising strategy to address critical challenges, such as instability, interfacial recombination losses, and lead-associated environmental risks. Enhanced charge transport channels, passivation of defects, and customizable molecule architectures are some of the special benefits that COFs offer. In this study, we used Schiff base reactions to create two donor-acceptor (D-A) type COFs with an 8+2 connection motif, which were integrated into PSC self-assembled (SAM) layers. According to characterizations and theoretical calculations, COFs not only effectively optimize the energy level of the ITO/SAM substrate but also passivate perovskite defects and suppress defect-assisted recombination in PSC devices. These modifications significantly enhanced carrier transport and extraction, resulting in an increase in power conversion efficiency (PCE) from 22.57% to 25.20% (DP-BE) and 24.21% (DP-DBE). This work highlights the potential of COFs as multifunctional modifiers for interfacial engineering in PSCs, offering a promising route to improve device performance and stability.

Insights into Decoupled Solar Energy Conversion and Charge Storage in a 2D Covalent Organic Framework for Solar Battery Function

Decoupling solar energy conversion and storage in a single material offers a great advantage for off-grid applications. Herein, we disclose a two-dimensional naphthalenediimide (NDI)-based covalent organic framework (COF) exhibiting remarkable solar battery performance when used as a photoanode. Light-induced radicals are stabilized within the framework for several hours, offering on-demand charge extraction for electrical energy production. Our study reveals mechanistic insights into the long-term charge stabilization using optical spectroscopy and (photo)electrochemical measurements, in conjunction with density functional theory (DFT) simulations. Among several solvents, water provides the best dielectric screening and energetically favorable proton exchange to stabilize photoinduced radicals for more than 48 h without the need for additional metal cations. This study provides fundamental insights into the optoionic charge storage mechanism in NDI-COF, while introducing a highly tunable, nanoporous material platform that surpasses related materials, such as carbon nitrides, metal-organic frameworks (MOFs), or metal oxides, in terms of charge storage capacity. This study opens new perspectives for the design of optoionic charge-storing materials and the direct storage of solar energy to overcome the intermittency of solar irradiation.

Terpyridine- and Quarterpyridine-Based Cationic Covalent Organic Frameworks for Visible-Light-Catalytic H2O2 Synthesis

This paper presents multipyridine-containing covalent organic frameworks (COFs) with precisely defined position and number of pyridinium cationic groups. Specifically, three terpyridine- and quarterpyridine-based trialdehydes were synthesized, and utilized as the starting monomers to polymerize with trimethylpyridinium bromide to yield vinylene-linked iTPy-COF, iTPPy-COF and iQPPy-COF, respectively. Thus constructed donor-acceptor cationic COFs exhibit considerably high visible-light catalytic efficiency for hydrogen peroxide (H2O2) synthesis by the dual-channel mechanisms of oxygen reduction reaction (ORR) and water oxidation reaction (WOR). In pure water and O2 atmosphere, the H2O2 production rate (HPR) of iTPPy-COF after 1 h reaction is as high as 7955 μmol g-1 h-1. Even though in air, its HPR value still reaches 6249 μmol g-1 h-1. Moreover, it is found that changing the arm lengths and ratios of pyridine to benzene ring in the frameworks significantly affects the photocatalytic capability. The structure-property relationship is investigated in terms of the variations of electronic structures through the theoretical simulations and measurements of photophysical parameters such as fluorescence lifetimes, photocurrent intensities, and impedances of charge transfer, which offers new insights into the engineering of multipyridine-based cationic COFs for highly efficient H2O2 photosynthesis.

Processable and Recyclable Covalent Organic Framework Gel Electrolytes

Covalent organic framework (COF)-based electrolytes with abundant ordered channels and accessible interaction sites have shown great potential in energy storage and transformation, although their practical applications are strongly impeded by their inherent insolubility and non-melt processability. Developing processable COF gel electrolytes and recycling them remains a formidable challenge. In this study, the processing of COF to gels demonstrated through interlayer interaction manipulation and enable solution-reconstruction of COF gel electrolytes for the first time, inspired by the working principle of wedges. Good solution-processability of the COF powders in strong acid mediums is achieved by inserting oxygen atoms into its framework to promote the interlayer charge repulsion. This modification enabled the COF readily dispersable as colloidal nanosheets in an aqueous solution of trifluoroacetic acid (TFA). Starting from here, this is modulated competitive interactions among TFA, COF, and water molecules, to reconfigure COF materials between their gelified and colloidally dispersed states. The reconfigured COF gel maintains their mechanical properties and long cycle life as an electrolyte in the battery (>800 h). This approach realizes solution processing of COF powders and can recycle COF out of gels for repeated use, offering new insights and strategies for their preparation and sustainable recycling.

High-Connectivity 3D Covalent Organic Frameworks with pdp Net for Efficient C2H2/CO2 Separation

High-connectivity 3D covalent organic frameworks (COFs) have garnered significant attention due to their structural complexity, stability, and potential for functional applications. However, the synthesis of 3D COFs using mixed high-nodal building units remains a substantial challenge. In this work, we introduce two novel 3D COFs, JUC-661 and JUC-662, which are constructed using a combination of D2h-symmetric 8-nodal and D3h-symmetric 6-nodal building blocks. These COFs feature an unprecedented [8+6]-c pdp net with rare mesoporous polyhedral cages (~3.9 nm). Remarkably, JUC-661 and JUC-662 exhibit outstanding separation capabilities, achieving adsorption selectivities of 4.3 and 5.9, respectively, for C2H2/CO2 (1/1, v/v) mixtures. Dynamic breakthrough experiments confirm their excellent separation capability, maintaining this performance even under conditions of 100 % humidity. Monte Carlo simulations and DFT calculations indicate that the exceptional adsorption performance is attributed to the well-defined pore cavities of the COFs, with fluorination of the building unit further enhancing C2H2 selectivity through improved electrostatic and host-guest interactions. This study expands the structural diversity of COFs and highlights their potential for low-energy separation processes.

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.

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.

Benzotrifuran-Based Covalent Organic Frameworks for Artificial Photosynthesis of H2O2 from H2O, O2, and Sunlight

Hydrogen peroxide (H2O2) is an important green chemical and a potential energy carrier. Artificial photosynthesis of H2O2 from only H2O and O2 under sunlight is a fascinating and sustainable tactic. However, deficient proton sources come from the kinetically sluggish H2O oxidation process (∼s) seriously impede half-reaction of O2 reduction (µs∼ms), leading to inferior photocatalytic efficiency. Herein, we prepare a benzotrifuran-based covalent organic framework featuring electron donor-acceptor character with hydrazone linkage. Spatially separated donor-acceptor π-stacking columns offer rich O2 reduction and H2O oxidation active sites, also serve as channels for photoinduced charge separation and transport. Importantly, the 1D pore containing abundant heteroatoms facilitates H2O delivery to oxidation sites and reduces reaction energy barrier, thus improving the kinetic of H2O oxidation. Accordingly, this porous framework achieves efficient H2O2 photocatalysis with an average mass rate of up to 9800 µmol g-1 h-1 from H2O and O2 in the absence of any sacrificial reagent under simulated sunlight by balancing O2 reduction and H2O oxidation reactions.

Single-Crystal X-ray Structures of Homochiral Brønsted Acidic Covalent Organic Frameworks

Determining the crystal structures of covalent organic frameworks (COFs) with atomic precision is pivotal for uncovering their properties and optimizing functionalities. However, the synthesis of high-quality single crystals of COFs suitable for X-ray diffraction analysis, especially chiral COFs (CCOFs), remains a formidable challenge. In this work, we report two three-dimensional (3D) CCOFs synthesized via imine condensation of tetrahedral tetraamine and tetraaldehydes derived from optically active 1,1'-biphenol phosphoryl chloride or thiophosphoryl chloride. Single crystals of varying sizes are obtained through either a low-temperature modulation strategy, yielding large crystals up to 100 μm, or a solvothermal method. The large single crystals are structurally characterized by single-crystal X-ray diffraction, achieving a resolution of 0.90 Å. These two CCOFs are isostructural and each features a 4-fold interpenetrated diamondoid open framework with all phosphoric acid groups periodically aligned within tubular helical channels, displaying enhanced Brønsted acidity compared to non-immobilized acids. The frameworks exhibit permanent porosity, chemical resistance in boiling water, 14 M NaOH, and 0.1 M HCl, and thermal stability up to 400 °C. Notably, these CCOFs serve as efficient and recyclable heterogeneous Brønsted acid catalysts in the asymmetric addition to aromatic aldehydes, enantioselective transfer hydrogenation of ketimines, and three-component direct asymmetric Mannich reactions involving aldimines and cyclic ketones, achieving good to high enantioselectivities (up to 99.5% ee) that surpass those obtained in analogous systems with homogeneous catalysts. This work represents the first successful demonstration of single-crystal structures of homochiral COFs, paving the way for in-depth investigations into structure-property relationships in enantioselective processes and facilitating the design of novel functional chiral organic materials.

Interface preassembly oriented growth strategy towards flexible crystalline covalent organic framework films for OLEDs

The synthesis of flexible crystalline films for optoelectronic applications remains a significant chemical challenge due to the inherent contradiction between flexibility and crystallinity. The delicate balance between flexibility and crystallinity has long constituted a barrier to the development of high-performance optoelectronic materials. Herein, an interface preassembly oriented growth (IPOG) strategy has been explored to fabricate flexible crystalline covalent organic framework (COF) films with controllable thickness. By synergistically modulating hydrophilic and hydrophobic interactions along with interfacial confinement, a set of uniform and flexible crystalline COF films were successfully synthesized. This achievement unlocks the potential of COFs for device applications in organic light-emitting diodes, leading to unprecedented high-efficiency electroluminescence from COFs. This groundbreaking advancement not only lays the foundation for the progress of COF-based OLEDs but also signifies the advent of an era in the synthesis of flexible crystalline materials, wherein exceptional mechanical properties are seamlessly integrated with superior electronic performance, thus heralding a transformative impact on the landscape of flexible electronics.

Reprogramming Lung Redox Homeostasis by NIR Driven Ultra-Small Pd Loaded Covalent Organic Framework Inhibits NF-κB Pathway for Acute Lung Injury Immunotherapy

Acute lung injury (ALI) refers to damage to lung related cells, typically caused by an uncontrollable inflammatory response, and over-generated reactive oxygen species (ROS). Increasing evidence suggests that reprogramming lung redox homeostasis holds significant potentials for the clinical treatment of ALI. Herein, the simple synthesis of ultra-small Pd loaded covalent organic framework (COF) (TP@Pd) is reported, which, when combined with near infrared (NIR) irradiation, exhibits nanozyme functionalities, including multiple enzyme mimicking activities and broad spectrum ROS scavenging, thereby promoting tissue repair for ALI immunotherapy. Mechanistically, through the therapeutic strategy of TP@Pd+NIR, the damaged cells and tissues are ameliorated by decreasing intracellular ROS levels (total ROS, ·OH and ·O2 -), downregulating inflammatory cytokines levels (IL-6, TNF-α and IL-1β), upregulating antioxidant factor level (SOD2), inducing macrophage M2 directional polarization (downregulation of iNOS and CD86, and upregulation of IL-10 and CD206), activating immunoregulation (CD4+/CD8+ ratio increase), promoting tissue repair factor levels (upregulation of HSP70 and CD31), and suppressing the NF-κB signaling pathway (downregulation of phosphorylated p65 and IκBα). Furthermore, following intravenous (IV) injection in rats, TP@Pd accumulated in lung tissue for 6 h, indicating the promising therapeutic efficacy via this administration route. Notably, the TP@Pd+NIR strategy demonstrated the excellent synergistic effects in alleviating lung inflammation storms, reducing diffuse alveolar damage, and accelerating lung tissue repair. Summarily, this work has designed a novel TP@Pd+NIR strategy for the synergistic enhancement of ALI amelioration, which may serve as a promising therapeutic approach for other ROS related diseases.

Advances in synthetic strategies for two-dimensional conjugated polymers

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

A BODIPY-Containing Covalent Organic Framework as a Highly Porous Photosensitizer for Environmental Remediation and Pollutants Adsorption

The direct incorporation of borondipyrromethene (BODIPY) subunits into the structural backbone of covalent organic frameworks (COFs) gives facile access to porous photosensitizers but is still a challenging task. Here, we introduce β-ketoenamine-linked BDP-TFP-COF, which crystallizes in AA-stacking mode with hcb topology. A comprehensive characterization reveals high crystallinity and enhanced stability in a variety of solvents, excellent mesoporosity (SABET=1042 m2 g-1), broad light absorption in the visible region, and red emission upon the exfoliation of few-layer COF nanosheets. The versatility of multifunctional BODIPY-COFs is highlighted in various applications. Pollutants Bisphenol A (BPA, qmax=426 mg g-1) and Methylene Blue (MB, qmax=96 mg g-1) have been efficiently removed from H2O. Fluorescence quenching or enhancement of exfoliated BDP-TFP-COF nanosheets have been utilized for dual-mode sensing of MB or NEt3, respectively. Ultimately, the photosensitizing effect of the BODIPY units is retained in the COF. Thus, BPD-TFP-COF was established as a metal-free triplet photosensitizer, which efficiently oxidized a mustard gas simulant under visible light irradiation.

Olefin-Linked Covalent Organic Frameworks as Prospective Artificial Platforms for Efficient Photocatalysis

The development of semiconducting materials for photoredox catalysis holds great promise for sustainable utilization of solar energy. Olefin-linked covalent organic frameworks (COFs), which are built by linking organic structs into crystalline frameworks through C=C bonds, have attracted tremendous attention in photocatalysis due to their saliant advantages such as extended π-conjugation, permanent porosity, exceptional chemical stability, light-harvesting and charge separation abilities. This review offers a comprehensive overview of recent new advances toward the development of olefin-linked COFs and their uses as artificial platforms for photocatalytic applications, like hydrogen evolution, carbon dioxide reduction and organic transformations. Structural design strategies, preparation methods and structure-function relationships in various photoredox reactions are summarized, which is accompanied by various approaches to boost their catalytic performance. The challenges and future prospectives are further discussed.

Advancements in Inkjet Printing of Metal- and Covalent-Organic Frameworks: Process Design and Ink Optimization

Metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs) are highly versatile materials based on inorganic modes connected via organic linkers or purely via the connection of organic building blocks, respectively. This results in 3-D nanoporous frameworks, which, due to their combination of high porosity and variability of building blocks, can exhibit exceptional properties that make them attractive. Certain applications (e.g., in electronics and as membranes) require a thin film or even a patterned morphology on various substrates. Inkjet printing of MOFs has emerged as a simple and effective technique for the scalable production of a wide range of MOF (gradient) films and patterns on a wide range of substrates according to specific requirements. This review comprehensively reviews the achievements in inkjet printing of both MOFs and COFs. We discuss the different substrates, ink formulation, and hardware intertwined requirements needed to achieve high-resolution printing and obtain desired properties such as porosity, physical-mechanical characteristics, and uniform thickness. Crucial aspects related to ink formulation, such as colloidal stability and size control of MOFs and COFs, are discussed. Additionally, we highlight potential opportunities for furthering the development of inkjet printing of MOFs/COFs and critically assess the reporting of the printing procedures and characterization of the resultant materials. In this manner, this review aims to contribute to the advancements in understanding and optimization of inkjet printing of MOFs and COFs, as this technique holds great potential for diverse applications and functionalization of MOF/COF films and patterns.

Construction of carbazole-conjugated dual-emission fluorescent covalent organic framework for distinguishing p-nitroaniline/p-nitrophenol and adsorbing nitroanilines/nitrophenols

Nitroanilines (NAs) and nitrophenols (NPs), crucial industrial raw materials, are extensively utilized across various sectors. However, the environmental pollution and health hazards stemming from their usage are significant, necessitating urgent monitoring and removal to address environmental and safety concerns. The challenge is further compounded by the presence of NAs/NPs isomers, making the selective analysis of specific isomers crucial. In response, a new post-modified fluorescent covalent organic framework (COF) termed COF@CB, exhibiting dual-emission fluorescence, was synthesized. This synthesis involved coupling a high-crystallinity fluorescent COF (COF-TTDB) with carbazole-9-ethanol (CB) via a "Williamson" reaction. COF@CB featured exceptional dual-emission fluorescence, a high specific surface area (919.4 m2·g-1), superior thermal stability, and abundant active sites. These attributes enabled COF@CB to function as a ratiometric fluorescence sensor capable of simultaneous detection and adsorption. The distinct number and arrangement of hydrogen bond sites in NAs/NPs isomers influenced the intramolecular charge transfer (ICT) effects on COF@CB, thereby enabling the COF@CB-ratiometric fluorescence sensor to distinguish and selectively detect p-NA/p-NP from isomers. Analysis of actual water samples further underscored the sensor's effectiveness in detecting p-NA/p-NP. Furthermore, the presence of multiple active sites on the COF@CB-ratiometric fluorescence sensor facilitated the adsorption of NAs/NPs, promoting the removal of them from actual samples.

Construction and Band Gap-Regulation of Ordered Macro-Microporous Single Crystals of an Amine-Linked Covalent Organic Framework

Heterogeneity engineering provides an effective route to manipulate the chemical and physical properties of covalent organic frameworks (COFs) but is still under development for their single-crystal form. Here, we report the strategy based on a combination of the template-assisted modulated synthesis with a one-pot crystallization-reduction method to directly construct ordered macro-microporous single crystals of an amine-linked three-dimensional (3D) COF (OM-COF-300-SR). In this strategy, the colloidal crystal-templating synthesis not only assists the formation of ordered macropores but also greatly facilitates the in situ conversion of linkages (from imine to amine) in the COF-300 single crystals. The as-synthesized OM-COF-300-SR120 exhibits a reversible symmetry change from a tetragonal I41/a to monoclinic I2/c space group after activation, which was not observed previously. On the other hand, this strategy allows for a flexible control over the degree of amination (from 0 to 100%, as determined by X-ray photoelectron spectroscopy (XPS) analysis) in COF-300 crystals to regulate their band gap (from 2.57 to 2.81 eV) for the optimization of photocatalytic activity. The high degree of amination and the embedded ordered macropores render OM-COF-300-SR120 with superior photocatalytic activity (with a reaction rate constant of 0.9572 min-1) to its nonmacroporous counterpart (NM-COF-300-SR120, 0.2303 min-1) for the degradation of rhodamine B. In addition, the significant contribution of ordered macropores to confront mass transfer resistance in COF single crystals was also confirmed by the much higher catalytic activity of Au/OM-COF-300-SR120 (with an activity parameter of 7.96 × 103 s-1 mol-1) as compared with Au/NM-COF-300-SR120 (1.43 × 103 s-1 mol-1) in the model reduction reaction of 4-nitrophenol by NaBH4.

Induced Charge-Compensation Effect for Boosting Photocatalytic Water Splitting in Covalent Organic Frameworks

Imine-based covalent organic frameworks (COFs) are promising for photocatalytic water splitting, but their performance is often constrained by inefficient charge separation due to the high electron localization nature of polar imine bonds. In this study, we have optimized the electron delocalization across the imine linkage within a COF by implementing a charge compensation effect. This effect is achieved when a strong electron-donating thieno[3,2-b]thiophene linker is directly attached to the iminic carbon of a zinc-porphyrinic COF. This modification significantly reduces the electron binding effect within the imine bonds of the COF, facilitating both in-plane charge separation and out-plane charge transfer to the catalytic site. Conversely, the use of strong electron-withdrawing pyrizine linker aggravates the electron localization at the imine linkage in the ZnP-Pz variant. Consequently, ZnP-Tt shows a substantially improved photocatalytic water-splitting activity under visible light irradiation, with a hydrogen evolution of 44288±2280 μmol g-1 in 4 h, which exceeds the ZnP-Pz counterpart by a factor of 10. These results offer fresh perspectives for the design of imine-based COFs to overcome their limitations in charge separation efficiency.

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 Strongly Reducing sp2 Carbon-Conjugated Covalent Organic Framework Formed by N-Heterocyclic Carbene Dimerization

Covalent organic frameworks linked by carbon-carbon double bonds (C=C COFs) are an emerging class of crystalline, porous, and conjugated polymeric materials with potential applications in organic electronics, photocatalysis, and energy storage. Despite the rapidly growing interest in sp2 carbon-conjugated COFs, only a small number of closely related condensation reactions have been successfully employed for their synthesis to date. Herein, we report the first example of a C=C COF, CORN-COF-1 (CORN=Cornell University), prepared by N-heterocyclic carbene (NHC) dimerization. In-depth characterization reveals that CORN-COF-1 possesses a two-dimensional layered structure and hexagonal guest-accessible pores decorated with a high density of strongly reducing tetraazafulvalene linkages. Exposure of CORN-COF-1 to tetracyanoethylene (TCNE, E1/2=0.13 V and -0.87 V vs. SCE) oxidizes the COF and encapsulates the radical anion TCNE⋅- and the dianion TCNE2- as guest molecules, as confirmed by spectroscopic and magnetic analysis. Notably, the reactive TCNE⋅- radical anion, which generally dimerizes in the solid state, is uniquely stabilized within the pores of CORN-COF-1. Overall, our findings broaden the toolbox of reactions available for the synthesis of redox-active C=C COFs, paving the way for the design of novel materials.

Spatiotemporal Spectroscopy of Fast Excited-State Diffusion in 2D Covalent Organic Framework Thin Films

Covalent organic frameworks (COFs), crystalline and porous conjugated structures, are of great interest for sustainable energy applications. Organic building blocks in COFs with suitable electronic properties can feature strong optical absorption, whereas the extended crystalline network can establish a band structure enabling long-range coherent transport. This peculiar combination of both molecular and solid-state materials properties makes COFs an interesting platform to study and ultimately utilize photoexcited charge carrier diffusion. Herein, we investigated the charge carrier diffusion in a two-dimensional COF thin film generated through condensation of the building blocks benzodithiophene-dialdehyde (BDT) and N,N,N',N'-tetra(4-aminophenyl)benzene-1,4-diamine (W). We visualized the spatiotemporal evolution of photogenerated excited states in the 2D WBDT COF thin film using remote-detected time-resolved PL measurements (RDTR PL). Combined with optical pump terahertz probe (OPTP) studies, we identified two diffusive species dominating the process at different time scales. Initially, short-lived free charge carriers diffuse almost temperature-independently before relaxing into bound states at a rate of 0.7 ps-1. Supported by theoretical simulations, these long-lived bound states were identified as excitons. We directly accessed the lateral exciton diffusion within the oriented and crystalline film, revealing remarkably high diffusion coefficients of up to 4 cm2 s-1 (200 K) and diffusion lengths of several hundreds of nanometers and across grain boundaries. Temperature-dependent exciton transport analysis showed contributions from both incoherent hopping and coherent band-like transport. In the transport model developed based on these findings, we discuss the complex impact of order and disorder on charge carrier diffusion within the WBDT COF thin film.

Thiol-Ene Click Chemistry: A General Strategy for Tuning the Properties of Vinylene-Linked Covalent Organic Frameworks

Vinylene-linked Covalent Organic Frameworks (V-2D-COFs) are a class of promising porous organic materials that feature fully π-conjugated structures, high crystallinity, ultrahigh chemical stability, and extraordinary optoelectronic properties. However, the types of reactions and the availability of monomers for synthesizing sp2-c linked COFs are considerably limited by the irreversibility of the C═C bond, and the complete π-conjugated structure restricts their in-depth research in hydrophilicity, membrane materials, and proton conductivity. Postsynthetic modification (PSM), which can avoid these problems by incorporating functional moieties into the predetermined framework, provides an alternative way to construct diverse V-2D-COFs. Herein, we report a general strategy to introduce C-C, C-S-C, and functional groups into sp2-c-COFs via the thiol-ene click reaction. To demonstrate the universality of this approach, we synthesized two sp2-c COFs (COF-CN and COF-1), and subsequently introduced six different types of thiol compounds at their skeletal C═C sites. The quantitative yield was confirmed by X-ray Photoelectron Spectroscopy (XPS) and cross-polarization magic angle spinning 13C NMR spectroscopy. This thiol-ene click modification of vinylene-linked COFs at skeletal C═C sites allows for flexible structural design, providing these COFs with new linkages (C-C and C-S-C) that are otherwise difficult to produce directly. Thus, it facilitates precise modulation of their properties, such as photophysical properties, hydrophilicity, and proton conductivity, promising a diverse range of compelling applications for the future.

Designed Synthesis of Mesoporous sp2 Carbon-Conjugated Benzothiadiazole Covalent Organic Frameworks for Artificial Photosynthesis of Hydrogen Peroxide

Artificial photosynthesis of hydrogen peroxide (H2O2) from ambient air, water, and sunlight has attracted considerable attention recently. Despite being extremely challenging to synthesis, sp2 carbon-conjugated covalent organic frameworks (COFs) can be powerful and efficient materials for the photosynthesis of H2O2 due to desirable properties. Herein, we report the designed synthesis of an sp2 carbon-conjugated COF, BTD-sp2c-COF, from benzothiadiazole and triazine units with high crystallinity and ultralarge mesopores (∼4 nm). The sp2 carbon-conjugated skeletons guarantee BTD-sp2c-COF superior optoelectronic properties and chemical stability. BTD-sp2c-COF exhibits an exceptional efficiency of 3066 μmol g-1 h-1 from pure water and air, much better than that of BTD-imine-COF. In contrast, the resilience of BTD-imine-COF is compromised due to the participation of imine linkages in the oxygen reduction reaction. Importantly, in situ characterization and theoretical calculation results reveal that both benzothiadiazole and triazine units serve as oxygen reduction reaction centers for H2O2 photosynthesis through a sequential electron transfer pathway, while the vinylene bridged phenyls serve as water oxidation reaction centers. The sp2 carbon-conjugated COFs pave the way for potent artificial photosynthesis.

Solid-Liquid-Gas Three-Phase Indirect Electrolysis Enabled by Affinity Auxiliary Imparted Covalent Organic Frameworks

The design of efficient heterogeneous redox mediators with favorable affinity to substrate and electrolyte are much desired yet still challenging for the development of indirect electrolysis system. Herein, for the first time, we have developed a solid-liquid-gas three-phase indirect electrolysis system based on a covalent organic framework (Dha-COF-Cu) as heterogeneous redox mediator for S-S coupling reaction. Dha-COF-Cu with the integration of high porosity, nanorod morphology, abundant hydroxyl groups and active Cu sites is much beneficial for the adsorption/activation of thiols, uniform dispersion and high wettability in electrolyte, and efficient interfacial electron transfer. Notably, Dha-COF-Cu as solid-phase redox mediator exhibits excellent electrocatalytic efficiency for the formation of value-added liquid-phase S-S bond product (yields up to 99 %) coupling with the generation of gas-phase product of H2 (~1.40 mmol g-1 h-1), resulting in a powerful three-phase indirect electrolysis system. This is the first work about COFs that can be applied in three-phase indirect electrolysis system, which might promote the development of porous crystalline materials in this field.

Boosting Photoconductivity by Increasing the Structural Complexity of Multivariate Covalent Organic Frameworks

The assessment of the photoconductivity of Donor-Acceptor (D-A) ordered bulk heterojunctions is gaining attention for the development of innovative organic semiconductors in optoelectronics. Here, the synthesis of pyrene-based (D) Covalent Organic Frameworks, achieve through a multivariate reaction involving two distinct acceptors is reported (A). The products are characterized using powder x-ray diffraction, N2 sorption isotherms, electronic microscopy, and in silico calculations, among other techniques. These characterizations reveal that the multicomponent synthesis enables the modification of properties (e.g., bandgap) of the framework while preserving its structural features, such as crystallinity and porosity. The ordered D-A arrays position these materials as promising candidates for photoconductive semiconductors, particularly regarding the variation in the composition of isotopological frameworks. Photoconductivity experiments demonstrate a volcano-type correlation with respect to the A moiety content, with the optimal value reaching 7.9 × 10-5 cm2 V-1 s-1 for the bare NIP25%-COF. This study illustrates how introducing diverse acceptor units through multivariate synthesis can enhance the photoconductivity of these materials via "defect" engineering, without sacrificing their crystalline or porous characteristics and avoiding the need for de novo synthesis.

Regulated Charge Transfer in Donor-Acceptor Metal-Organic Frameworks for Highly-Sensitive Photodetectors

In photo-induced charge separation, organic thin films with donor and acceptor chromophores are vital for uses such as artificial photosynthesis and photodetection. The main challenges include optimizing charge separation efficiency and identifying the ideal acceptor/donor ratio. Achieving this is difficult due to the variability in molecular configurations within these typically amorphous organic aggregates. Metal-organic frameworks (MOFs) provide a structured solution by enabling systematic design of donor/acceptor blends with adjustable ratios within a crystalline lattice. We demonstrate this approach by incorporating donor and acceptor naphthalenediimide (NDI) chromophores as linkers in a highly oriented, monolithic MOF thin film. By adjusting the NDI acceptor linker concentration during the layer-by-layer assembly of surface-anchored MOF thin films (SURMOFs), we significantly enhanced charge separation efficiency. Surprisingly, the optimum acceptor concentration was only 3 %, achieving a forty-fold increase in photodetection efficiency compared to baseline NDI donor-based SURMOFs. This unexpected behaviour was clarified through theoretical analysis enabled by the well-defined crystalline structure of the SURMOFs. Using density functional theory and kinetic Monte Carlo simulations, we identified two opposing effects from acceptors: the positive effect of suppressing undesirable charge carrier recombination is offset at high concentrations by a reduction in charge-carrier mobility.

Tunable Isometric Donor-Acceptor Wurster-Type Covalent Organic Framework Photocathodes

Covalent organic frameworks (COFs) offer remarkable versatility, combining ordered structures, high porosity, and tailorable functionalities in nanoscale reaction spaces. Herein, we report the synthesis of a series of isostructural, photoactive Wurster-type COFs achieved by manipulating the chemical and electronic nature of the Wurster aromatic amine building blocks. A series of donor-acceptor-donor (D-A-D) Wurster building block molecules was synthesized by incorporating heteroaromatic acceptors with varying strengths between triphenylamine donor groups. These tailored building blocks were integrated into a 2D COF scaffold, resulting in highly crystalline structures and similar morphologies across all COFs. Remarkably, this structural uniformity was also achieved in the synthesis of homogeneous and oriented thin films. Steady-state photoluminescence revealed a tunable red-shift in film emission exceeding 100 nm, demonstrating effective manipulation of their optical properties. Furthermore, photoelectrochemical (PEC) water splitting studies exhibited a doubled current density (8.1 μA cm-2 at 0.2 VRHE) for the COF with the strongest acceptor unit. These findings highlight the potential of Wurster D-A-D COFs in photoelectrochemical water splitting devices and pave the way for further exploration of chemical functionality-reactivity-property relationships in this promising class of photoactive materials.

Double enhancement of protonation and conjugation in donor-imine-donor covalent organic frameworks for photocatalytic hydrogen evolution

Covalent organic frameworks (COFs) have emerged as highly promising platforms for photocatalytic water splitting. However, exploring the structure-activity relationships in different COF systems remains challenging. In this study, three donor-imine-donor (D-I-D) COFs as relatively pure model materials were carefully selected to investigate the effect of protonation and conjugation on the mechanism of photocatalytic H2 evolution. Unlike widely reported donor-acceptor (D-A) COF systems, these three ideal COFs have short electronic channels and lack chemical bond isomerism and heteroatoms in building blocks. These aspects are beneficial for a comprehensive investigation of the underlying mechanisms at the active sites of the imine bond. Both the calculation and experimental results indicate that increasing the conjugation intensity can enhance the efficiency of exciton dissociation and charge transfer rates. Protonation can also dominantly enhance the light absorption capacity and electron transport efficiency of D-I-D COFs. After protonation, the Py-hCOF with optimal conjugation intensity exhibits a remarkable H2 evolution rate of 44.2 mmol g-1 h-1 under visible light, which is 88.4 times higher than that of Tpe-hCOF. This result highlights the crucial roles of simultaneous enhancement of the protonation and conjugation in improving photocatalytic hydrogen evolution of COFs, providing valuable insights for the design of COF materials to achieve the superior electronic functions in photocatalysis.

A Thiol Branched 3D Network Quasi Solid-State Polymer Electrolyte Reinforced by Covalent Organic Frameworks for Lithium Metal Batteries

Quasi solid-state polymer electrolytes (QSPEs) are particularly attractive due to their high ionic conductivity and excellent safety for lithium metal batteries (LMBs). However, it is still a great challenge for QSPEs to achieve strong mechanical strength and high electrochemical performance simultaneously. Herein, a QSPE (SCOF-PEP-PEA) using a covalent organic framework (COF) containing abundant allyl groups (SCOF) as a rigid porous filler as well as a cross-linker to reinforce the polymer network is reported. Benefitting from the unique 3D nanonetwork structure and abundant lithiophilic functional groups, SCOF-PEP-PEA QSPE exhibits high ionic conductivity (4.0 × 10-4 S cm-1) and high lithium-ion transference number (0.82) at room temperature. Moreover, SCOF-PEP-PEA QSPE displays much improved mechanical strength compared to PEP-PEA QSPE (AFM Young's modulus: 453 vs 36 MPa). As a result, the Li/LFP full cell with SCOF-PEP-PEA QSPE shows great rate performance of 141 mAh g-1 at 1C and delivers a high specific capacity retention of 92% after 220 cycles at 0.5 C (60 °C). This work provides a new strategy to design and prepare high-performance QSPEs with COFs as porous organic filler, and further expand the application of COFs for energy storage applications.

A Concise Review on Porous Adsorbents for Benzene and Other Volatile Organic Compounds

Emissions of volatile organic compounds (VOCs) such as benzene, toluene, xylene, styrene, hexane, tetrachloroethylene, acetone, acetaldehyde, formaldehyde, isopropanol, etc., increase dramatically with accelerated industrialization and economic growth. Most VOCs cause serious environmental pollution and threaten human health due to their toxic and carcinogenic nature. Adsorption on porous materials is considered one of the most promising technologies for VOC removal due to its cost-effectiveness, operational flexibility, and low energy consumption. This review aims to provide a comprehensive understanding of VOC adsorption on various porous adsorbents and indicate future research directions in this field. It is focused on (i) the molecular characterization of structures, polarity, and boiling points of VOCs, (ii) the adsorption mechanisms and adsorption interactions in the physical, chemical, and competitive adsorption of VOCs on adsorbents, and (iii) the favorable characteristics of materials for VOCs adsorption. Porous adsorbents that would play an important role in the removal of benzene and other VOCs are presented in detail, including carbon-based materials (activated carbons, active carbon fibers, ordered mesoporous carbons, and graphene-based materials), metal-organic frameworks, covalent organic frameworks, zeolites, and siliceous adsorbents. Finally, the challenges and prospects related to the removal of VOCs via adsorption are pointed out.

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.

Covalent Organic Framework Controls the Aggregation of Metal Porphyrins for Enhanced Photocatalytic H2 Evolution

Although different post modifications of covalent organic frameworks (COFs) have been developed for achieving hierarchical nanostructures and improved photocatalytic performance, the co-assemblies of COFs with small organic molecules were still rarely studied. Herein, COF/porphyrin composites, which were fabricated at room temperature, reveal that COFs surface can modulate the aggregation of metal porphyrins, which subsequently enhance the photocatalytic properties of COFs assemblies. Thus, the surface of COFs was decorated by porphyrins aggregations with varied thickness, dependent on the metal ions of porphyrins. Ni(II) meso-Tetra (4-carboxyphenyl) porphine (NiTCPP) formed discontinuous monolayer covering on COFs surface, while Pt(II) meso-Tetra (4-carboxyphenyl) porphine (PtTCPP) or Co(II) meso-Tetra (4-carboxyphenyl) porphine (CoTCPP) aggregated into multilayer coverage. Notably, even though NiTCPP did not show any advantages in terms of light absorption or HOMO/LUMO energy levels, COF/NiTCPP with the lowest porphyrin loading still exhibited the highest photocatalytic H2 evolution (29.71 mmol g-1 h-1), which is 2.5 times higher than that of COF/PtTCPP or COF/CoTCPP. These results open new possibilities for making highly efficient photocatalysts upon the co-assemblies of COFs with small organic molecules.

A Fully Saturated Covalent Organic Framework

We report the design and synthesis of the first aliphatic covalent organic framework (COF), NUS-119, and its subsequent conversion to NUS-120, marking the first fully saturated COF. NUS-119 is built by imine-linkages exhibiting high crystallinity and porosity, achieved by using a Lewis acid as a reaction modulator to circumvent compatibility issues between the Brønsted acid and the strong basic monomer. The structure was successfully solved using 3D microelectron diffraction (microED) techniques. NUS-119 and NUS-120 demonstrated remarkable catalytic performance in base-catalyzed Knoevenagel condensation reactions, exhibiting high conversion rates, excellent size selectivity, and good recyclability. This work advances the understanding of COF materials and paves the way for future research and applications.

Perfluoroalkyl Functionalized Superhydrophobic Covalent Organic Frameworks for Excellent Oil-Water Membrane Separation and Anhydrous Proton Conduction

Rapid economic development has led to oil pollution and energy shortage. Membrane separation has attracted much attention due to its simplicity and efficiency in oil-water-separation. The development of membrane materials with enhanced separation properties is essential to improve the separation-efficiency. Proton exchange membrane fuel cells (PEMFCs) are expected to replace conventional engines due to their high-power-conversion rates and other favorable properties. Anhydrous-proton-conducting materials are vital components of PEMFCs. However, developing stable proton-conducting materials that exhibit high conductivity at varying temperatures remains challenging. Herein, two covalent organic frameworks (COFs) with long-side-chains are synthesized, and their corresponding COF@SSN membranes. Both membranes can effectively separate oil-water mixtures and water-in-oil emulsions. The TFPT-AF membrane achieves a maximum oil-flux of 6.05 × 105 g h-1 m-2 with an oil-water separation efficiency of above 99%, which is almost unchanged after 20 consecutive uses. COF@H3PO4 doped with different ratios of H3PO4 is prepared, the results show that the perfluorocarbon-chain system has  excellent anhydrous proton conductivity , achieving an ultra-high proton-conductivity of 3.98 × 10-1 S cm-1 at 125 °C. This study lays the foundation for tailor-made-functionalization of COF through pre-engineering and surface-modification, highlighting the great potential of COFs for oil-water separation and anhydrous-proton-conductivity.

Porous Bimetallic Ti-MOFs for Photocatalytic Oxidation of Amines in Air

A family of microporous titanium-containing metal-organic frameworks (denoted as M2Ti-CPCDC, M = Mn, Co, Ni) has been synthesized by using a bimetallic [M2Ti(μ3-O)(COO)6] cluster and a tritopic carbazole-based organic ligand H3CPCDC. M2Ti-CPCDC are stable and display permanent porosity for N2 and CO2 uptake, ranking among the most porous titanium-based metal-organic frameworks. M2Ti-CPCDC crystals exhibit n-type semiconductor behavior. Further catalytic studies demonstrate that all M2Ti-CPCDC materials are applicable for triggering photo-oxidative reactions of amines in air. More specifically, amines with electron-donating groups afford the aldehydes as the main products, while amines bearing electron-withdrawing groups give rise to imines as the main product. Among them, Mn2Ti-CPCDC exhibit the best photocatalytic activity, with conversion of benzylamine up to 99% and selectivity of 99%. Mn2Ti-CPCDC could be recycled in at least three runs while retaining crystallinity and catalytic activity. The reaction mechanism indicates that photoinduced hole (h+), superoxide radical anion (O2·-), and singlet oxygen (1O2) are the main active species involved in the photo-oxidation process.

Growth of two-dimensional covalent organic frameworks on substrates: insight from microsecond atomistic simulations

While growing two-dimensional covalent organic frameworks (2D COFs) on substrates holds promise for producing functional monolayers, the presence of many defects in the resulting crystals often hinders their practical applications. Achieving structural order while suppressing defect formation necessitates a detailed atomic-level understanding. The key lies in understanding the polymerization process with high nano-scale accuracy, which presents significant challenges. Here, we perform microsecond atomistic molecular dynamics simulations to describe the deposition and polymerization of cyclohexa-m-phenylene on metal substrates, closely mimicking experimental conditions. Our improved approach highlights that 2D polymerization occurs through monomer addition and island coalescence, with a pre-bonding stage allowing monomers/oligomers to dynamically adjust their configurations to the expanding island structures. Our results elucidate the mechanisms underlying the formation of vacancy and dislocation defects during 2D polymerization as well as their healing processes. Overall, our findings underscore the significant roles that high surface mobility, effective monomer-substrate anchoring, high framework rigidity, moderate monomer coordination, and low bonding rate play in forming large, extended 2D crystals while suppressing vacancy and dislocation defects. We demonstrate how these factors can be tuned through substrate selection, deposition rate modulation, and temperature control, thereby offering valuable insight for strategically optimizing on-surface 2D polymerizations.

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.

Orientation-Dependent Photocatalysis of Imine-Linked Covalent Organic Frameworks Based on Thienothiophenes for Oxidation of Amines to Imines

Toward visible light photocatalysis, covalent organic frameworks (COFs) have recently garnered growing attention. The effect of different orientations of imine of imine-linked COFs on photocatalysis should be elucidated. Here, two COFs are developed with 2,5-diphenylthieno[3,2-b]thiophene (DPTT) and 1,3,6,8-tetraphenylpyrene (Py) linked by imine, affording DPTT-Py-COF and Py-DPTT-COF, respectively. Distinctly, DPTT-Py-COF and Py-DPTT-COF have high crystallinity and porosity, paving the way to highly efficient photocatalysis. Theoretical calculations demonstrate that both DPTT-Py-COF and Py-DPTT-COF are of similar bandgaps but of varied energy positions due to the different orientations of imine. Besides, characterizations disclose that DPTT-Py-COF delivers more enhanced charge separation and transfer than Py-DPTT-COF. Probed by the oxidation of amine to imine, DPTT-Py-COF exhibits a blue light photocatalytic performance superior to that of Py-DPTT-COF. DPTT-Py-COF, a highly recyclable photocatalyst, enables the oxidation of various amines to imines with oxygen. This work highlights that tuning the microenvironment of COFs unravels tenable performances in photocatalysis.

Electrostatically Designing Materials and Interfaces

Collective electrostatic effects arise from the superposition of electrostatic potentials of periodically arranged (di)polar entities and are known to crucially impact the electronic structures of hybrid interfaces. Here, it is discussed, how they can be used outside the beaten paths of materials design for realizing systems with advanced and sometimes unprecedented properties. The versatility of the approach is demonstrated by applying electrostatic design not only to metal-organic interfaces and adsorbed (complex) monolayers, but also to inter-layer interfaces in van der Waals heterostructures, to polar metal-organic frameworks (MOFs), and to the cylindrical pores of covalent organic frameworks (COFs). The presented design ideas are straightforward to simulate and especially for metal-organic interfaces also their experimental implementation has been amply demonstrated. For van der Waals heterostructures, the needed building blocks are available, while the required assembly approaches are just being developed. Conversely, for MOFs the necessary growth techniques exist, but more work on advanced linker molecules is required. Finally, COF structures exist that contain pores decorated with polar groups, but the electrostatic impact of these groups has been largely ignored so far. All this suggest that the dawn of the age of electrostatic design is currently experienced with potential breakthroughs lying ahead.

Self-Assembly of Super-Uniform Covalent Organic Framework Colloidal Particles into Multi-Dimensional Ordered Superstructures

Precise self-assembly of colloidal particles is crucial for understanding their aggregation properties and preparing macroscopic functional devices. It is currently very challenging to synthesize and self-assemble super-uniform covalent organic framework (COF) colloidal particles into well-organized multidimensional superstructures. Here, simple and versatile strategies are proposed for synthesis of super-uniform COF colloidal particles and self-assembly of them into 1D supraparticles, 2D ordered mono/multilayers, and 3D COF films. For this purpose, several self-assembly techniques are developed, including emulsion solvent evaporation, air-liquid interfacial self-assembly, and drop-casting. These strategies enable the superstructural self-assembly of particles of varying sizes and species without any additional surfactants or chemical modifications. The assembled superstructures maintain the porosity and high specific surface area of their building blocks. The feasibility of the strategies is examined with different types of COFs. This research provides a new approach for the controllable synthesis of super-uniform COF colloidal particles capable of self-assembling into multidimensional superstructures with long-range order. These discoveries hold great promise for the design of emerging multifunctional COF superstructures.

3D Covalent Organic Frameworks with 16-Connectivity for Photocatalytic C(sp3)-C(sp2) Cross-Coupling

The connectivity (valency) of building blocks for constructing 3D covalent organic frameworks (COFs) has long been limited to 4, 6, 8, and 12. Developing a higher connectivity remains a great challenge in the field of COF structural design. Herein, this work reports a hierarchical expansion strategy for making 16-connected building blocks to construct 3D COFs with sqc topology. The [16 + 2] construction achieved by condensation between a 16-connected carbazolyl dicyanobenzene-based building block (CzTPN) and linear diamino linkers (BD or Bpy) affords two 3D COFs (named CzBD COF and CzBpy COF). Furthermore, attributed to the well-organized donor-acceptor (D-A) heterojunction, the Ni chelated CzBpy COF (Ni@CzBpy COF) exhibits excellent performance for photoredox/Ni dual catalytic C(sp3)-C(sp2) cross-coupling of alkyltrifluoroborates with aryl halides, achieving a maximum 98% conversion and 94% yield for various substrates. This work developed the first case of high-connectivity COFs bearing 16-connected units, which is the highest connectivity reported until now, and achieved efficient photocatalysis applications, thus greatly enriching the possibilities of COFs.

Benzodiazole-Based Covalent Organic Frameworks for Enhanced Photocatalytic Dehalogenation of Phenacyl Bromide Derivatives

Covalent organic frameworks (COFs) have garnered significant interest within the scientific community due to their distinctive ability to act as organic semiconductors responsive to visible light. This unique attribute makes them up-and-coming candidates for facilitating photocatalytic organic reactions. Herein, two donor-acceptor COFs, TPE-BSD-COF and TPE-BD-COF, have been designed and synthesized by incorporating electron-rich tetraphenylethylene and electron-deficient benzoselenadiazole and benzothiadiazole units into the framework through a Schiff-base polycondensation reaction. Both COFs exhibit exceptional crystallinity and enduring porosity. TPE-BSD-COF and TPE-BD-COF exhibit broad light absorption capabilities, a narrow optical band gap, and low electrochemical impedance spectrum (EIS) levels, indicating that the two COFs are effective heterogeneous photocatalysts for the reductive dehalogenation of phenacyl bromide derivatives under blue LED irradiation. A high photocatalytic yield of 98% and 95% was achieved by TPE-BSD-COF and TPE-BD-COF catalysts, respectively, within only one hour.

Interpenetrated Donor-Acceptor Heterojunctions in 2D Conjugated Dibenzo[g,p]chrysene-Based Kagome Covalent Organic Frameworks

Dibenzo[g,p]chrysene can be viewed as a constrained propeller-shaped tetraphenylethylene with reduced curvature and has been utilized to construct dual-pore kagome covalent organic frameworks (COFs) with tightly packed two-dimensional (2D) layers owing to its rigid and more planar structural characteristics. Here, we introduce 2D COFs based on the node 4,4',4″,4‴-(dibenzo[g,p]chrysene-2,7,10,15-tetraphenyl)tetraamine (DBCTPTA) featuring extended conjugation compared to the dibenzo[g,p]chrysene-3,6,11,14-tetraamine (DBCTA) node. We establish two exceptionally crystalline imine-linked 2D COFs with a hexagonal dual-pore kagome structure based on the DBCTPTA core. The newly synthesized thienothiophene (TT) and benzodithiophene (BDT)-based DBCTPTA COFs show a tight stacking behavior between adjacent layers. Furthermore, we obtained an unprecedented, interpenetrated electron-donor/acceptor host-guest system with an electron-donating BDT DBCTPTA COF synthesized in situ with the soluble fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) serving as molecular acceptor. The BDT DBCTPTA COF@PCBM film shows a much shorter amplitude-averaged PL lifetime of 7 ± 2 ps compared to 30 ± 4 ps of the BDT DBCTPTA COF film, indicating the light-induced charge transfer process. The successful in situ formation of interpenetrated donor-acceptor heterojunctions within 2D COFs offers a promising strategy for establishing D-A heterojunctions in diverse framework materials with open channel systems.