Rational design of isostructural 2D porphyrin-based covalent organic frameworks for tunable photocatalytic hydrogen evolution

Engineering Covalent Organic Frameworks for Photocatalytic Overall Water Vapor Splitting

Photocatalytic overall water vapor splitting (OWVS) into H2 and O2 not only owns the potential of avoiding the backward reaction of O2 reduction reaction reforming H2O, but also realizes H2 production without available liquid water. However, this attempt is still a blank due to the weak absorption of photocatalysts to water vapor. Herein, we report the first example of visible-light-driven OWVS by combining the water-adsorbing ability and photocatalytic activity of covalent organic frameworks (COFs). The overall water splitting (OWS) activity of Tp-COF skeleton was realized by introducing tripyridyltriazine segment. The Pt@Tp-TAPyT-COF achieves high visible-light-driven H2 and O2 evolution rates (HER and OER) of 148.4 and 74.8 µmol g-1 h-1, respectively. Under water vapor conditions with diverse relative humidities (RHs), the Pt@Tp-TAPyT-COF could drive OWVS even without backward reaction. By further optimizing the structure of β-ketoamine section, it was found that the Pt@DHTA-TAPyT-COF showed optimal OWVS activity, with the H2 and O2 evolution rate of 51.2 and 25.6 µmol g-1 h-1 under RH = 88%, respectively. The advantage of OWVS compared to traditional solid-liquid OWS was further confirmed by a continuous activity test of 45 h. Further experiments and theoretical calculations indicated that carbonyl-O and pyridine-N atoms in COFs serve as water-absorbing sites, and the absorbed water molecules could promote water-splitting activity of active sites in COFs simultaneously.

Novel g-C3N4-x nanosheets/MoS2 binary photocatalyst achieves stable construction of Z-scheme heterojunction via nitrogen defect induced 2D interface C-S bonds for efficient PMS activation and degradation of sulfamethoxazole

The inadequate utilization of visible light and the ineffective separation of electrons and holes in bulk-phase g-C3N4 renders its coupling with persulfate (PMS) inefficient in oxidative degradation of refractory organic pollutants. To address this, a novel binary photocatalyst, g-C3N4 nanosheets@MoS2 (MSG5-500) was developed, via an in-situ hydrothermal synthesis and calcination process. This photocatalyst effectively activates PMS under visible light, promoting the efficient oxidation of refractory organic pollutants in water. Influenced by the defect structure, carbon atoms with lone pairs of electrons near nitrogen defect sites in g-C3N4 nanosheets form C-S bonds with sulfur atoms in MoS2, strengthening of interfacial Z-scheme heterojunctions. MSG5-500 demonstrated superior separation of photogenerated carriers. The corresponding rate constant of the MSG5-500/PMS-visible light system exceeded that of bulk-phase g-C3N4/PMS, 2D g-C3N4-x/PMS, and MoS2/PMS by factors of 4.7, 2.9, and 3.6, respectively, achieving a removal rate of up to 97.7 % for 10 mg/L sulfamethoxazole (SMX) within 84 min. The primary free radicals driving SMX degradation were 1O2, ·O2-, ·OH, and SO4·-, with 1O2 playing a predominant role. Three degradation pathways for SMX were identified through the detection of ten intermediates using LC-MS within the MSG5-500/PMS system. The acute and chronic toxicities of most intermediates were significantly reduced, indicating good ecological safety. The system maintained high degradation efficiency for SMX across various environmental waters, could be widely applicable to different organic pollutants and exhibits good stability. This study presents a novel approach in optimizing g-C3N4-based photocatalysts coupled with the PMS system for the efficient removal of organic pollutants.

Semiconducting Covalent Organic Frameworks

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

Efficient photoresponsive one-dimensional covalent organic framework as oxidase-like enzyme for ultrasensitive detection of antioxidants

Natural polyphenolic antioxidants are widely present in foods such as fruits and vegetables, meanwhile applied in food processing and storage to prevent the formation of harmful compounds. While excessive antioxidants lead to negative impacts on human health. Hence, it is crucial to accurately detect antioxidant levels in order to enhance the overall nutritional content and food safety. Herein, a novel one-dimensional covalent organic framework (COF-Por-DPP) was constructed using 5,10,15,20-tetrakis(4-aminophenyl)-21H,23H-porphyrin and 4,4'-(2,6-pyrazinediyl)bisbenzaldehyde. The unique photoresensitive properties and topological structures endowed COF-Por-DPP excellent oxidase-like activity. The COF-Por-DPP based colorimetric assay was established for three antioxidants (gallic acid, tannic acid and caffeic acid). Moreover, this method was used to analyze real samples and a hydrogel sensor was constructed, which demonstrated good accuracy and practicability.

Electronegative Strategic Positions in Covalent Organic Frameworks: Unlocking High-Efficiency Gold Recovery

Gold (Au) concentrations accumulated from electronic waste (e-waste) and industrial leachates far surpass those found in natural ores, a highly valuable resource if efficient recovery methods can be developed. Despite advancements in covalent organic frameworks (COFs), achieving adsorbents with high selectivity, large capacity, and rapid adsorption kinetics remain challenging because of limitations in partial pore wall sites. Here, we present hexaazatriphenylene-based COFs (HATP-COFs) with an electronegative skeleton, specifically designed for selective Au recovery. The hexaazatriphenylene centers, imine linkages, and pyridine linkers within the COFs introduce electron-rich sites that extend across strategic positions-vertex, linkages, and linkers-thereby enhancing the overall structural integrity. These features facilitate efficient Au capture through electrostatic interactions, achieving an exceptional adsorption capacity exceeding 2366 mg g-1 with rapid kinetics, making HATP-COFs one of the most efficient pure COFs reported to date. Moreover, these HATP-COFs demonstrate remarkable selectivity, stability, and scalability. Theoretical calculations reveal that the electronegative skeleton introduces critical binding sites, promoting strong electrostatic interactions with Au3+ ions and improving adsorption kinetics. This work highlights the potential of charge-interface engineering in COFs as a transformative strategy for developing next-generation materials.

Covalent organic framework nanomaterials: Syntheses, architectures, and applications

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

Synergistic Assembly of Single-Crystal 2D Porphyrin-Based Organic Polymers via Dative B─N Bonds and Halogen Bonds

The rigorous synthetic methodologies have significantly impeded the progress in developing single-crystal extended organic polymers. Notably, the existence of macroscopic single-crystalline 2D porphyrin-based organic polymers has never been documented in the literature until now. In this study, we present a groundbreaking example of single-crystal 2D porphyrin-based organic polymers that are compatible with single-crystal X-ray diffraction (SXRD) for precise structural elucidation. Their formation is fundamentally dependent on the synergistic assembly facilitated by dative B─N bonds and halogen bonds. These crystals exhibit remarkable stability in both air and aqueous environments. Notably, the formation of the B-N Lewis pairs within these crystals significantly enhances the separation of photogenerated carriers, and their single crystals demonstrate exceptional photocatalytic activity for the production of hydrogen peroxide (H2O2) from water and oxygen, without the requirement for sacrificial agents. This pioneering discovery establishes a new approach for crystalline control within the realm of organic polymers.

Isoreticular 3D covalent organic frameworks with non-interpenetrated -derived topology: pore regulation from micropores to mesopores

Three-dimensional (3D) covalent organic frameworks (COFs) offer tremendous potential for a range of applications due to their unique structural and porous features. However, achieving the reticular synthesis of 3D COFs with regulated pores through isoreticular expansion remains a significant challenge, primarily due to the occurrence of interpenetration. In this study, we present a novel strategy that utilizes high-coordinated building blocks, acting as a binodal group of tetrahedral nodes, to synthesize isoreticular 3D COFs (JUC-300 to -302) with tunable pore sizes and uncommon non-interpenetrated pcu-derived dia topology. The pore sizes of these COFs were successfully tuned from 1.6 to 5.2 nm. The mesopores with a size of 5.2 nm in JUC-302 are the largest reported among 3D COFs to date and demonstrated the effective incorporation of a large protein, myoglobin. The strategy provides a new pathway for synthesizing isoreticular 3D COFs with reduced interpenetration, enabling applications that depend on various pore sizes.

Recent advances in photocatalytic H2O2 production: modification strategies of 2D materials and in situ application of H2O2

Environmental pollution and the energy crisis are two major problems that threaten human health and restrict industrial development. Hydrogen peroxide (H2O2) is a green oxidant and clean energy widely used in sterilization, degradation of pollutants and as an energy carrier, which is one of the important strategies to solve these two major problems. In recent years, solar-driven photocatalytic production of H2O2 has gained significant attention and been extensively studied. Two dimensional (2D) material photocatalysts offer promising prospects and distinct advantages for H2O2 production. However, their performance is hindered by challenges such as rapid electron-hole recombination, wide bandgaps, and slow reaction kinetics. Additionally, the high solubility of H2O2 in water and its tendency to decompose easily make it difficult to recover from solutions containing sacrificial agents, thereby restricting its practical applications. To the best of our knowledge, there are few reviews focused on the photocatalytic production of H2O2 using 2D material composite catalysts and its in situ applications. This review provides a detailed discussion of various strategies, including introducing vacancy defects, elemental doping, heterojunction engineering, functionalization and multi-strategy coupling, to improve the photocatalytic performance of 2D material composite photocatalysts. Furthermore, this review highlights the in situ applications of H2O2 produced through photocatalysis in diverse fields, including water purification, sterilization, and pharmaceutical intermediate synthesis. It concludes by outlining the key challenges in the photocatalytic production of H2O2 and proposing practical solutions.

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.

Programming Covalent Organic Frameworks for Photocatalysis: Investigation of Synthesis Methods

Covalent organic frameworks (COFs) are crystalline, porous materials with exceptional potential as high-performance photocatalysts for applications such as hydrogen and hydrogen peroxide production. In this study, we systematically evaluates various synthesis methods to optimize the preparation of COFs, specifically targeting enhanced photocatalytic activity. Four different synthetic strategies including solvothermal synthesis, microwave synthesis, ultrasonic synthesis, and mechanochemical synthesis were explored to prepare TAPT-COF with identical molecular structures. Although all methods produced COFs with comparable crystallinity, porosity, and light absorption capacity, their photocatalytic efficiencies in hydrogen and hydrogen peroxide production varied significantly. Notably, the mechanochemical method, which involves ball milling to disrupt interlayer interactions and produce COF nanosheets, exhibited the highest photocatalytic performance, particularly in the COF-BM-30 min sample. This difference highlights the importance of morphology in photocatalytic activity. Our findings shed light on the impact of synthesis methods on COF properties and provide valuable guidance for molecular strategy development and industrial applications in photocatalysis.

Engineered covalent organic frameworks (COFs) for adsorption-based metal separation technologies: A critical review

Porous covalent organic frameworks (COFs) are promising materials used for separation and purification during environmental remediation. This critical review focuses on two key aspects. First, it critically examines strategies to improve COF design and structure and evaluates their impact on separation performance. Second, engineering approaches for enhancing the interactions between COF-based adsorbents and metals for enhanced separation and capture are elucidated. The latest body of research on separating metals (e.g., Li, K, Sr, Hg, Cd, Pb, Cr, Au, Ag, Pd, and U) using COF-based adsorbents is discussed to understand the factors that influence their performance. However, it is to be noted that COF-based adsorbents are still in their infancy and remain largley unexplored, mainly hindered by synthetic complexities and suboptimal crystalline structures. This highlights the need for further research and development to fully unlock the excellent potential of COFs for metal separation applications, particularly in environmental and energy applications.

Redox-Active Metal-Covalent Organic Frameworks for Dendrite-Free Lithium Metal Batteries

Lithium (Li) metal has gained attention as an anode material for lithium-metal batteries (LMBs) owing to its low electrochemical potential, high specific capacity, and low density. However, the accumulation of Li dendrites and unstable solid electrolyte interphases, caused by sluggish Li+ migration and uneven Li deposition, limit practical LMB applications. This study presents the first report on redox-active metal-covalent organic frameworks (MCOFs) with dual-active centers as functional separators for LMBs. These MCOFs facilitate homogeneous Li nucleation and accelerate Li+ ion transport. The synergistic effects of redox-active diarylamine units and trinuclear copper clusters modulate local electron-cloud density, regulating microenvironment of Li+ ions and ensuring homogeneous Li nucleation. The MCOF-based separator's well-defined 1D channels in MCOF-based separator enable uniform Li+ flux, and promote homogeneous Li deposition, resulting in high Li+ transference number of 0.93 and an ionic conductivity of 2.01 mS cm-1 at room temperature. The Li|Cu cell demonstrates a low Li nucleation barrier of 16 mV, while the Li symmetric cell exhibits stable Li plating/stripping for over 1600 h at 0.5 mA cm-2. When coupled with LiFePO4 cathodes, the assembled LMB exhibits stable capacity retention of ≈98%. This work paves the way for dendrite-free Li metal anodes in high-performance LMBs.

Functional crystalline porous framework materials based on supramolecular macrocycles

Crystalline porous framework materials like metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs) possess periodic extended structures, high porosity, tunability and designability, making them good candidates for sensing, catalysis, gas adsorption, separation, etc. Despite their many advantages, there are still problems affecting their applicability. For example, most of them lack specific recognition sites for guest uptake. Supramolecular macrocycles are typical hosts for guest uptake in solution. Macrocycle-based crystalline porous framework materials, in which macrocycles are incorporated into framework materials, are growing into an emerging area as they combine reticular chemistry and supramolecular chemistry. Organic building blocks which incorporate macrocycles endow the framework materials with guest recognition sites in the solid state through supramolecular interactions. Distinct from solution-state molecular recognition, the complexation in the solid state is ordered and structurally achievable. This allows for determination of the mechanism of molecular recognition through noncovalent interactions while that of the traditional recognition in solution is ambiguous. Furthermore, crystalline porous framework materials in the solid state are well-defined and recyclable, and can realize what is impossible in solution. In this review, we summarize the progress of the incorporation of macrocycles into functional crystalline porous frameworks (i.e., MOFs and COFs) for their solid state applications such as molecular recognition, chiral separation and catalysis. We focus on the design and synthesis of organic building blocks with macrocycles, and then illustrate the applications of framework materials with macrocycles. Finally, we propose the future directions of macrocycle-based framework materials as reliable carriers for specific molecular recognition, as well as guiding the crystalline porous frameworks with their chemistry, applications and commercialization.

Porphyrin-Based Covalent Organic Frameworks for CO2 Photo/Electro-Reduction

Photo/electro-catalytic CO2 reduction into high-value products are promising strategies for addressing both environmental problems and energy crisis. Duo to their advantageous visible light absorption ability, adjustable optic/electronic properties, definite active center, post-modification capability, and excellent stability, porphyrin-based covalent organic frameworks (COFs) have emerged as attractive photo/electro-catalysts towards CO2 reduction. In this review, the research progress of the porphyrin-based COFs for photo/electro-catalytic CO2 reduction is summarized including the design principles, catalytic performance, and reaction mechanism. In addition, this review also presents some challenges and prospects for the application of porphyrin-based COFs in photo/electro-catalytic CO2 reduction, laying the base for both fundamental research and application efforts.

Modulating Coordination Environment of Single Nickel Atom in Covalent Organic Framework to Enhance Photocatalytic Hydrogen Evolution Reaction

Covalent organic frameworks (COFs) have emerged as fascinating platforms for photocatalytic hydrogen evolution reactions in recent years. However, the relationship between the coordination environment of metals in a covalent organic framework and catalytic properties is still rarely studied. In this study, a covalent organic framework (COF-BP), containing a benzothiazole unit with typical electron-withdrawing properties, was successfully synthesized from 4,4'-(benzo-2,1,3-thiadiazole-4,7-diyl)dianiline and 3,3',5,5'-tetraformyl-4,4'-biphenyldiol. The free hydroxyl and imine groups in COF-BP were used for postmodification loading of Ni2+ to form COF-BP-Ni1, and then the coordination environment of Ni was further modulated through ligand exchange, replacing acetate anions by salicylideneaniline to construct COF-BP-Ni2 containing Schiff base-Ni complexes. The experimental results demonstrated that modulating the ligands of metal within the COF to alter its coordination environment enhanced the synergy with the COF framework, thereby promoting photoelectron separation and transfer, further significantly improving its photocatalytic activity. COF-BP-Ni2 showed good photocatalytic hydrogen evolution performance with a rate of 12.21 mmol g-1 h-1 in the presence of Pt as cocatalyst, which was 2.2-fold that of COF-BP-Ni1 at the same condition. Interestingly, the hydrogen evolution rate of COF-BP-Ni2 reaches 1.27 mmol g-1 h-1 even without Pt as a cocatalyst. This work provides new insights into how to improve the catalytic performance of COF-based catalysts.

Deciphering a volcano-shaped relationship between radical stability and reticular electrochemiluminescence

Electrochemiluminescence (ECL) is a light-emitting process in which the stability of electrochemically generated radicals has a crucial impact on the efficiency and durability of excited state generation. Therefore, deciphering a relationship between radical stability and ECL performance is highly appealing. In this work, three sp2 carbon-conjugated covalent organic framework (COF) reticular nanoemitters compositing of same pyrene luminophores but different acrylonitrile linkers are designed with progressive electron affinities, named as CN-COF-1, 2, and 3. By precisely modulating the electron affinity of CN-COFs, a volcano relationship between ECL and radical stability is discovered with 78 folds enhancement in ECL intensity. Density functional theoretical calculations indicate that CN-COF-2 exhibits moderate radical stabilization capacity as well as efficient electron transport between the pyrene cores, facilitating ECL generation. Significantly, the appropriate radical stability of CN-COF-2 not only achieves the self-enhanced cathodic ECL but also promotes durability of the ECL intensity. The rational regulation of radical stability paves the way for developing efficient reticular nanoemitters and decoding the ECL fundamentals.

Precisely Tuning Band Gaps of Hexabenzocoronene-Based MOFs Toward Enhanced Photocatalysis

Precise adjusting the band gaps in metal-organic frameworks (MOFs) is crucial for improving their visible-light absorption capacity during photocatalysis, presenting both a formidable challenge and a charming opportunity. This present study employed a symmetry-reduction strategy to pre-design six novel 4-connected ligands with systematic substituents (-NO2, -H, -tBu, -OCH3, -OH and -NH2) and synthesized the corresponding pillared-layer Zr-MOFs (NKM-668) retaining the hexaphenylbenzene fragment. Subsequently, the NKM-668 MOFs were transformed into large-π-conjugated hexabenzocoronene-based MOFs (pNKM-668) via the Scholl reaction. These twelve MOFs exhibited broad and tunable band gaps over 1.41 eV (ranging from 3.25 eV to 1.84 eV), and the photocatalytic CO2 conversion rate raised by 33.2-fold. This study not only enriches the type of hexaphenylbenzene-based MOFs, but also paves the way for nanographene-containing MOFs in the further application of photocatalysis.

Predesign of Covalent-Organic Frameworks for Efficient Photocatalytic Dehydrogenative Cross-Coupling Reaction

The dehydrogenative cross-coupling reaction is the premier route for synthesizing important 4-quinazolinone drugs. However, it usually requires high reaction temperature and long reaction time, and the yield of the final product is low. Here two stable and photosensitive covalent-organic frameworks (COFs), TAPP-An and TAPP-Cu-An are purposefully designed and constructed to serve as unprecedented heterogeneous tandem catalysts to complete dehydrogenative cross-coupling reactions in a short time and under mild reaction conditions (room temperature and light), leading to the high-efficient photosynthesis of 4-quinazolinones. Particularly, TAPP-Cu-An is the best heterogeneous catalyst currently available for the synthesis of 4-quinazolinones, even surpassing all the catalysts reported so far. It also enables one-step photosynthesis of 4-quinazolinones with higher conversion (>99%) and selectivity (>99%) in a shorter time, and the product can be easily prepared on a gram scale. Extensive experiments combined with theoretical calculations show that the excellent photogenerated charge separation and transport capability, as well as the synergistic An-Cu catalysis in TAPP-Cu-An are the main driving forces for this efficient reaction.

Photocatalytic detoxification of a sulfur mustard simulant using donor-enhanced porphyrin-based covalent-organic frameworks

Photocatalytic detoxification of sulfur mustards (e.g., bis (2-chloroethyl) sulfide, SM) is an effective approach for protecting the ecological environment and human health. In order to fabricate COFs with high performance for the selective transformation of the SM simulant 2-chloroethyl ethyl sulfide (CEES) to nontoxic 2-chloroethyl ethyl sulfoxide (CEESO), three porphyrin-based COFs with different donor groups (R = H, OH, and OMe) were synthesized. Among these COFs, COF-OMe, which possesses the strongest electron-donating ability, demonstrated a faster and higher detoxification rate of CEES at various concentrations, achieving selective oxidation of CEES to non-toxic CEESO with 99.2% conversion and 100% selectivity using white LED light irradiation within three hours. The facilitated charge transfer and separation as well as efficaciously produced reactive oxygen species (ROS), including singlet oxygen (1O2) and superoxide radical anions (O2˙-) are supposed to contribute to the excellent performance. The results demonstrated that the donor-enhanced porphyrin-based COFs could act as heterogeneous photocatalysts for visible light driven organic transformation and detoxification of sulfur mustards.

Fine-Tune the Structural Components of Porous Frameworks for Photocatalytic Hydrogen Production

With the depletion of fossil fuels and increasing pollution problems, green and sustainable energy supply attracts worldwide attention. Hydrogen is a green and high-density energy substance, and photocatalytic hydrogen generation is an effective and sustainable method. Therefore, developing high-performance photocatalysts plays a crucial role in practical application. Porous frameworks with large surface areas and diversified structures are considered promising candidates for photocatalysts. This review summarizes the recent progress on porous frameworks and their derivatives through fine-tuning structural components in terms of building monomers, functional groups, and metal hybridization for photocatalytic production of hydrogen. A detailed correlation is conducted between the structural features of porous frameworks and photocatalysis capability. In addition, we summarized the advantages of porous materials for photocatalytic hydrogen production and future development.

Vertically Expanded Covalent Organic Frameworks for Photocatalytic Water Oxidation into Oxygen

Covalent organic frameworks with unique π architectures and pores could be developed as photocatalysts for transformations. However, they usually form π-stacking layers, so that only surface layers function in photocatalysis. Here we report a strategy for developing vertically expanded frameworks to expose originally inaccessible active sites hidden in layers to catalysis. We designed covalently linked two-dimensional cobalt(II) porphyrin layers and explored coordination bonds to connect the cobalt(II) porphyrin layers with bidentate ligands via a three-component one-pot polymerization. The resultant frameworks expand the interlayer space greatly, where both the up and down faces of each cobalt(II) porphyrin layer are exposed to reactants. Unexpectedly, the vertically expanded frameworks increase skeleton oxidation potentials, decrease exciton dissociation energy, improve pore hydrophilicity and affinity to water, and facilitate water delivery. Remarkably, these positive effects work collectively in the photocatalysis of water oxidation into oxygen, with an oxygen production rate of 1155 μmol g-1 h-1, a quantum efficiency of 1.24 % at 450 nm, and a turnover frequency of 1.39 h-1, which is even 5.1-fold as high as that of the π-stacked frameworks and ranks them the most effective photocatalysts. This strategy offers a new platform for designing layer frameworks to build various catalytic systems for chemical transformations.

Regulating the Spin-State of Cobalt in Three-Dimensional Covalent Organic Frameworks for High-Performance Sodium-Iodine Rechargeable Batteries

Although the catalytic activity is heavily reliant on the electronic structure of the catalyst, understanding the impact of electron spin regulation on electrocatalytic performance is still rarely investigated. This work presents a novel approach involving the single-atom coordination of cobalt (Co) within metalloporphyrin-based three-dimensional covalent organic frameworks (3D-COFs) to facilitate the catalytic conversion for sodium-iodine batteries. The spin state of Co is modulated by altering the oxidation state of the porphyrin-centered Co, achieving optimal catalysis for iodine reduction. Experimental results demonstrate that CoII and CoIII are incorporated into the 3D-COFs, exhibiting spin ground states of S=1/2 and S=0, respectively. The low spin state of CoIII is favorable to hybridize with the sp 3d orbitals of I3 -, thus facilitating the conversion of I3 - to I-. Density-functional theory (DFT) calculations further reveal that the presence of CoIII enhances iodide adsorption and accelerates the formation of NaI in 3D-COFs-CoIII, thereby promoting its rapid kinetic behaviors. Notably, the I2@3D-COFs-CoIII cathode achieves a high reversible capacity of 227.7 mAh g-1 after 200 cycles at 0.5 C and demonstrates exceptional cyclic stability, exceeding 2000 cycles at 10 C with a minor capacity fading rate of less than one 0.01 % per cycle.

Enhancing photocatalytic hydrogen peroxide generation by tuning hydrazone linkage density in covalent organic frameworks

The conversion of solar energy into chemical energy or high-value chemicals has attracted considerable research interest in the context of the global energy crisis. Hydrogen peroxide (H2O2) is a versatile and powerful oxidizing agent widely used in chemical synthesis and medical disinfection. H2O2 also serves as a clean energy source in fuel cells, generating electricity with zero-carbon emissions. Recently, the sustainable production of H2O2 from water and oxygen using covalent organic frameworks (COFs) as photocatalysts has attracted considerable attention; however, systematic studies highlighting the role of linkages in determining photocatalytic performance are scarce. Under these circumstances, herein, we demonstrate that varying the imine and hydrazone linkages within the framework significantly influences photocatalytic H2O2 production. COFs with high-density hydrazone linkages, providing optimal docking sites for water and oxygen, enhance H2O2 generation activity (1588 μmol g-1 h-1 from pure water in the air), leading to highly efficient solar-to-chemical energy conversion.

Efficient Quasi-Homogenous Photocatalysis Enabled by Molecular Nanophotocatalysts with Donor-Acceptor Motif

Polymer semiconductors have attracted much attention for photocatalytic hydrogen evolution, but they typically exhibit micrometer-sized particles in water-suspension, causing severe loss in light absorption and exciton recombination. Here a molecular nanophotocatalyst featuring a donor-acceptor motif is presented that solution is processed via a facile stirring nanoprecipitation method assisted by hydrophilic surfactants, enabling an efficient quasi-homogenous hydrogen evolution. In contrast to the original bulk powder (heterogeneous system), these quasi-homogenous nanophotocatalysts exhibit significantly improved light-harvesting, water-wettability, and exciton dissociation, resulting in distinctly enhanced (by four-order-of-magnitude) photocatalytic hydrogen evolution rate. The optimized nanophotocatalysts (4CzPN/DDBAB/SDBS) generate an outstanding hydrogen evolution rate of 116.42 mmol g-1 h-1 and apparent quantum yield of 30.2% at 365 nm, which are among the highest reported for single-junction organic photocatalysts. The scalability of the quasi-homogenous photocatalysts is further demonstrated using a flow-based flash nanoprecipitation (FNP) processing.

Ultrahigh-sensitivity vinyl-COF fluorescent sensor for trace organic arsenic detection

Recently, the misuse of organic arsenic feed additives, such as roxarsone (ROX), has increasingly jeopardized both human health and the environment. In response, a unique electron-rich pyrazine-cored fluorescent covalent organic framework (COF) nanosheet, named as COF-TMP, was synthesized using an alkali-catalyzed reaction between 2, 3, 5, 6-tetramethylpyrazine (TMP) and terephthalaldehyde (TPA). Characterization demonstrated that COF-TMP boasted high porosity, pronounced fluorescence, and an abundance of (E)-2-styrylpyrazine (SPA) groups. These attributes render it an exceptional fluorescent sensor for the ultrahigh sensitivity detection of electron-deficient ROX molecules. The limit of detection (LOD) for COF-TMP in detecting ROX was found to be 0.015 ppb through fluorescence-quenching titration experiments-surpassing all previously reported fluorescent sensors. A combination of experimental results and theoretical calculations suggests that the extraordinary detection capability of COF-TMP for ROX arises from a static quenching mechanism. This study paves the way not only for a novel pyrazine-based fluorescent COF nanosheet with high porosity, exceptional fluorescent capabilities, and abundant SPA groups suitable for highly sensitive and selective ROX detection but also hints at its potential application as a fluorescent sensor for environmental pollution management and related domains.

Construction of 2D/2D Pd Metallene/COFs System with Strong Internal Electric Field for Outstanding Solar Energy Photocatalysis

Due to the severe recombination of charge carriers, the photocatalytic activity of covalent organic frameworks (COFs) materials is limited. Herein, through simple ultrasound and stirring processes, the Pd metallene (Pde) is successfully combined with 2D COFs to form Pde/TpPa-1-COF (Pde/TPC) composites. Obviously, a strong internal electric field (IEF) is successfully formed in Pde/TPC hybrid materials, which significantly boosts the separation of photogenerated charges. In addition, the matched 2D structure of the two materials can also lead to electronic coupling effects, plentiful active sites, and shortened carrier migration paths. Thus, the Pde/TPC hybrid materials own extraordinary carrier separation ability with a longer carriers lifetime (3.3 ns for Pde/TPC and 2.7 ns for TPC), which can be proved series of photoelectrochemical and spectroscopic tests. Benefiting from the formation of IEF and the matched 2D structure, the 8% Pde/TPC demonstrates the highest photocatalytic H2 evolution efficiency, with H2 production rate reaching up to 5.85 mmol g-1 h-1, which is over 25 times greater than that of pristine COFs, also exceeding that of many reported COFs-based photocatalysts. This research provides new perspectives and innovative approaches to further research on enhancing the internal electric field of COFs to promote their photocatalytic performance.

What Up with MOFs in Photocatalysis (?): Exploring the Influence of Experimental Conditions on the Reproducibility of Hydrogen Evolution Rates

Metal-organic frameworks (MOFs) are regarded as promising materials for energy applications, particularly in photocatalytic hydrogen (H2) production. This is due to their structural architectures that facilitate charge transfer, and tunable porous and light absorption properties. However, the many characteristics of MOFs including crystal morphology and sizes, surface facets, porosity, light absorption properties, and optical band gaps, can significantly influence their photocatalytic activity, presenting challenges in achieving reproducibility. In this study, we describe the synthesis of five distinct batches of the photoactive MOF, MIL-125-NH2, utilizing different synthetic conditions. Solid-state characterization confirmed the purity, porosity, and light absorption properties of each MOF batch. Each material was then combined with nano sized Ni2P as a cocatalyst, and their photocatalytic activity for H2 evolution was evaluated. We observed variations in their photocatalytic H2 evolution rates, which depended on the batch of MIL-125-NH2 utilized, ranging from the lowest rate of 2980 μmol·h-1·g-1 to the highest of 4327 μmol·h-1·g-1. Notably, different H2 evolution rates were also observed even when MIL-125-NH2 was synthesized under identical synthetic conditions but by different students. Our research highlights the critical relationship between MOF synthesis parameters─such as reaction time, temperature, and precursor concentration─and resulting properties, including particle size, morphology, surface facets, and light absorption characteristics. These factors significantly influence their photocatalytic activity, as evidenced by varying H2 evolution rates. This underscores the importance of optimizing materials synthesis conditions to improve reproducibility and efficiency in photocatalytic applications.

A Covalent Triazine Framework for Photocatalytic Anti-Markovnikov Hydrofunctionalizations

Porous materials-based heterogeneous photocatalysts, performing selective organic transformations, are increasing the applicability of photocatalytic reactions due to their ability to merge traditional photocatalysis with structured pores densely decorated with catalytic moiety for efficient mass and charge transfer, as well as added recyclability. We herein disclose a porous crystalline covalent triazine framework (CTF)-based heterogeneous photocatalyst that exhibits excellent photoredox properties for different hydrofunctionalization reactions such as hydrocarboxylations, hydroamination and hydroazidations. The high oxidizing property of this CTF enables the activation of styrenes, followed by regioselective C-N and C-O bond formation at ambient conditions. A change in the physicochemical and optoelectronic properties of the CTF, upon protonation during catalysis, lies at the basis of its photocatalytic properties. This allows us to obtain hydrocarboxylations, hydroamination, and hydroazidations from a myriad of electron-donating and -withdrawing aromatic and aliphatic substrates. This catalytic approach is further extended to late-stage functionalization of bio-active molecules. Finally, detailed characterizations of the CTF and further mechanistic investigations provide mechanistic insights into these reactions.

An Eu (III)-functionalized covalent organic framework fluorescent probe for specific detection of Flumequine based on pore restriction and antenna effect

The overuse of quinolone antibiotics has led to a series of health and environmental issues. Herein, we combine the distinct luminescence properties of Eu3+ with the unique structure of covalent organic frameworks (COFs) to develop a precise and sensitive fluorescent probe for detecting Flumequine (Flu) in water. Eu3+ is thoroughly anchored into the channels of COFs as recognition sites, while the synthesized probe material still maintains its intact framework structure. The unique structure of COFs provides excellent support and protection for Eu3+. Therefore, COF-Eu can rapidly bind with Flu which can transfer the absorbed energy to Eu3+ through an "antenna effect", resulting in red fluorescence. Moreover, there is a good linear relationship between Flu concentration in the range of 0-30 µM, with a detection limit of 41 nM. Simultaneously, the material maintains remarkable reproducibility, with its performance remaining almost unchanged after five cycles of use. Remarkably, the probe demonstrates excellent Flu recovery rates in real samples. This study provides a viable approach for the recognition of flumequine in the environment through a customized fluorescence detection method.

Single-Atom Catalysts on Covalent Organic Frameworks for Energy Applications

Single-atom catalysts (SACs) have been investigated and applied to energy conversion devices. However, issues of metal agglomeration, low metal loading, and substrate stability have hindered realization of the SACs' full potential. Recently, covalent organic framework (COF)-based SACs have emerged as promising materials to enable highly efficient catalytic reactions. Here, we summarize the representative COF-based SACs and their wide application in clean energy devices and conversion reactions, such as hydrogen evolution reaction, carbon dioxide reduction reaction, nitrogen reduction reaction, oxygen reduction reaction, and oxygen evolution reaction. Based on their catalysis conditions, these reactions are categorized into photocatalyzed and electrocatalyzed reactions. We also summarize their design strategies, including heteroatom inclusion, donor-acceptor pairs, pore engineering, interface engineering, etc. Although COF-based SACs are promising, more efforts, such as linkage engineering, functional groups, ionization, multifunctional sites for cocatalyzed systems, etc., could improve them to be the ideal SAC materials. At the end, we provide our perspectives on where the field will proceed in the next 5 years.

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

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

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.

Azobenzene-Bridged Covalent Organic Frameworks Boosting Photocatalytic Hydrogen Peroxide Production from Alkaline Water: One Atom Makes a Significant Improvement

Covalent organic frameworks (COFs) have been demonstrated as promising photocatalysts for hydrogen peroxide (H2O2) production. However, the construction of COFs with new active sites, high photoactivity, and wide-range light absorption for efficient H2O2 production remains challenging. Herein, we present the synthesis of a novel azobenzene-bridged 2D COF (COF-TPT-Azo) with excellent performance on photocatalytic H2O2 production under alkaline conditions. Notably, although COF-TPT-Azo differs by only one atom (-N=N- vs. -C=N-) from its corresponding imine-linked counterpart (COF-TPT-TPA), COF-TPT-Azo exhibits a significantly narrower band gap, enhanced charge transport, and prompted photoactivity. Remarkably, when employed as a metal-free photocatalyst, COF-TPT-Azo achieves a high photocatalytic H2O2 production rate up to 1498 μmol g-1 h-1 at pH = 11, which is 7.9 times higher than that of COF-TPT-TPA. Further density functional theory (DFT) calculations reveal that the -N=N- linkages are the active sites for photocatalysis. This work provides new prospects for developing high-performance COF-based photocatalysts.

II-Scheme Heterojunction Frameworks Based on Covalent Organic Frameworks and HKUST-1 for Boosting Photocatalytic Hydrogen Evolution

Covalent organic frameworks (COFs) are one type of promising polymer semiconductors in solar-driven hydrogen production, but majority of COFs-based photocatalytic systems show low photocatalytic efficiency owing to lack of metal active sites. Herein, we reported II-Scheme heterojunction frameworks based on COF (TpPa-1) and metal-organic framework (HKUST-1) for highly efficient hydrogen production. The coordination bonding directed self-assembly of HKUST-1 on the surface of TpPa-1 endows the heterojunction frameworks (HKUST-1/TpPa-1) with strong interface interaction, optimized electronic structures and abundant redox active sites, thus remarkably boosting photocatalytic hydrogen evolution. The hydrogen evolution rate for optimal HKUST-1/TpPa-1 is as high as 10.50 mmol g-1 h-1, which is significantly enhanced when compared with that of their physical mixture (4.13 mmol g-1 h-1), TpPa-1 (0.013 mmol g-1 h-1) and Pt-based counterpart (6.70 mmol g-1 h-1). This work offers a facile approach to the construction of noble-metal-free II-Scheme heterojunctions based on framework materials for efficient solar energy conversion.

How the Most Neglected Residual Species in MOF-Based Catalysts Involved in Catalytic Reactions to Form Toxic Byproducts

In recent years, multifarious new materials have been developed for environmental governance. Thereinto, metal organic framework (MOF)-based catalysts have been widely employed for heterogeneous catalysis because of their high porosity to confine noble metal particles faraway from aggregation. However, the potential reactions between residual species from the material synthesis process and target pollutants, which could form highly toxic byproducts, are often neglected. Herein, we took the widely used Zr-MOF, UiO-66, with highly thermal stability supported Pd catalysts as the example to investigate how the residual species in catalysts are involved in aromatic volatile organic compounds (VOCs) degradation reaction. The results showed that residual Cl species originated from the ZrCl4 metal precursor participated in the VOC degradation reaction, leading to the production of various chlorine-containing byproducts, even the hypertoxicity dioxin precursor, dichlorobenzene. Meanwhile, the chlorination mechanism for the formation of chlorine-containing byproducts was revealed by density functional theory calculation. Furthermore, the highly efficient residual Cl removal approaches are proposed. Importantly, the migration and transformation of residual Cl during the degradation of five benzene series VOCs are comprehensively studied and elucidated. We anticipate that these findings will raise alarm about the neglected issue of residual species in MOF-based catalysts for heterogeneous catalysis, especially environmentally friendly catalysis.

Tris(triazolo)triazine-Based Covalent Organic Frameworks for Efficiently Photocatalytic Hydrogen Peroxide Production

Two-dimensional covalent organic frameworks (2D-COFs) have recently emerged as fascinating scaffolds for solar-to-chemical energy conversion because of their customizable structures and functionalities. Herein, two tris(triazolo)triazine-based COF materials (namely COF-JLU51 and COF-JLU52) featuring large surface area, high crystallinity, excellent stability and photoelectric properties were designed and constructed for the first time. Remarkably, COF-JLU51 gave an outstanding H2O2 production rate of over 4200 μmol g-1 h-1 with excellent reusability in pure water and O2 under one standard sun light, that higher than its isomorphic COF-JLU52 and most of the reported metal-free materials, owing to its superior generation, separation and transport of photogenerated carriers. Experimental and theoretical researches prove that the photocatalytic process undergoes a combination of indirect 2e- O2 reduction reaction (ORR) and 4e- H2O oxidation reaction (WOR). Specifically, an ultrahigh yield of 7624.7 μmol g-1 h-1 with apparent quantum yield of 18.2 % for COF-JLU52 was achieved in a 1 : 1 ratio of benzyl alcohol and water system. This finding contributes novel, nitrogen-rich and high-quality tris(triazolo)triazine-based COF materials, and also designate their bright future in photocatalytic solar transformations.

In situ Construction of Single-Atom Electronic Bridge on COF to Enhance Photocatalytic H2 Production

Photocatalytic hydrogen production is one of the most valuable technologies in the future energy system. Here, we designed a metal-covalent organic frameworks (MCOFs) with both small-sized metal clusters and nitrogen-rich ligands, named COF-Cu3TG. Based on our design, small-sized metal clusters were selected to increase the density of active sites and shorten the distance of electron transport to active sites. While another building block containing nitrogen-rich organic ligands acted as a node that could in situ anchor metal atoms during photocatalysis and form interlayer single-atom electron bridges (SAEB) to accelerate electron transport. Together, they promoted photocatalytic performance. This represented the further utilization of Ru atoms and was an additional application of the photosensitizer. N2-Ru-N2 electron bridge (Ru-SAEB) was created in situ between the layers, resulting in a considerable enhancement in the hydrogen production rate of the photocatalyst to 10.47 mmol g-1 h-1. Through theoretical calculation and EXAFS, the existence position and action mechanism of Ru-SAEB were reasonably inferred, further confirming the rationality of the Ru-SAEB configuration. A sufficiently proximity between the small-sized Cu3 cluster and the Ru-SAEB was found to expedite electron transfer. This work demonstrated the synergistic impact of small molecular clusters with Ru-SAEB for efficient photocatalytic hydrogen production.

Advancements in Atomically Precise Nanocluster Protected by Thiacalix[4]arene

Coinage metal nanoclusters (NCs), comprising a few to several hundred atoms, are prized for their size-dependent properties crucial in catalysis, sensing, and biomedicine. However, their practical application is often hindered by stability and reactivity challenges. Thiacalixarene, a macrocyclic ligand, shows promise in stabilizing silver, copper, and bimetallic NCs, enhancing their structural integrity and chemical stability. This investigation delves into the unique properties of thiacalix[4]arene and their role in bolstering NC stability, catalytic efficiency, and sensing capabilities. The current challenges and future prospects are critically evaluated, underscoring the transformative impact of thiacalix[4]arene in nanoscience. This review aims to broaden the utilization of atomically precise coinage metal NCs, unlocking new avenues across scientific and industrial applications.

Synergistic Linker and Linkage of Covalent Organic Frameworks for Enhancing Gold Capture

The tunable pore walls and skeletons render covalent organic frameworks (COFs) as promising absorbents for gold (Au) ion. However, most of these COFs suffered from low surface areas hindering binding sites exposed and weak binding interaction resulting in sluggish kinetic performance. In this study, COFs have been constructed with synergistic linker and linkage for high-efficiency Au capture. The designed COFs (PYTA-PZDH-COF and PYTA-BPDH-COF) with pyrazine or bipyridine as linkers showed high surface areas of 1692 and 2076 m2 g‒1, providing high exposed surface areas for Au capture. In addition, the Lewis basic nitrogen atoms from the linkers and linkages are easily hydronium, which enabled to fast trap Au via coulomb force. The PYTA-PZDH-COF and PYTA-BPDH-COF showed maximum Au capture capacities of 2314 and 1810 mg g-1, higher than other reported COFs. More importantly, PYTA-PZDH-COF are capable of rapid adsorption kinetics with achieving 95% of maximum binding capacity in 10 min. The theoretical calculation revealed that the nitrogen atoms in linkers and linkages from both COFs are simultaneously hydronium, and then the protonated PYTA-PZDH-COF are more easily binding the AuCl4 ‒, further accelerating the binding process. This study gives the a new insight to design COFs for ion capture.

Facilitating Charge Transfer via Ti-Knot Pathway in Electrochromic Three-Dimensional Metalated Covalent Organic Frameworks

Due to the ordered one-dimensional channel as well as accessible redox sites, two-dimensional covalent organic frameworks (2D COFs) have garnered extensive attention in the field of electrochromism. However, organic 2D frameworks impose limitations on charge transfer and the weak interlayer interactions in 2D COFs, adversely affecting the stability during switching processes. Herein, we introduced Ti knots to construct three-dimensional metalated covalent organic frameworks (3D MCOFs), denoted as Ti-DHTA-Py. The Ti knots not only serve as templates for organizing organic units into unique 3D topological structures in a controlled manner but also establish charge transfer pathways conducive to electron delocalization and transmission within the framework. As a result, the 3D Ti-DHTA-Py MCOFs electrode exhibited a reduced band gap and remarkable electrochromic (EC) performances: electrochemical cyclic stability of 93.6% retention after 500 cycles, switching times (2.5 s/0.5 s), and a high coloration efficiency (423 cm2 C-1). This research underscores the potential of 3D MCOFs as promising candidates for advancing EC technologies, surmounting the limitations associated with traditional 2D COFs.

Electronic, optical and charge transport properties of Zn-porphyrin-C60 MOFs: a combined periodic and cluster modeling

Density functional theory (DFT) calculations were performed on the 5,15 meso-positions of nine porphyrin-containing MOFs; Zn2(TCPB)-(NMe2-ZnP); (H4TCPB = 1,2,4,5-tetrakis(4-carboxyphenyl)benzene), (NMe2-ZnP = [5,15-bis[(4-pyridyl)-ethynyl]-10,20-bis-(dimethylamine) porphinato]zinc(II)) functionalized with nitrogen-, oxygen-, and sulfur-containing groups to study their effects on the electronic, optical and transport properties of the materials. The properties of these materials have also been investigated by encapsulating fullerene (C60) in their pores (C60@MOFs). The results indicate that the guest C60 in the MOF generates high photoconductivity through efficient porphyrin/fullerene donor-acceptor (D-A) interactions, which are facilitated by oxygen and sulfur functionalities. DFT calculations show that C60 interacts favorably in MOFs due to negative Eint values. Encapsulated C60 molecules modify the electronic band structure, affecting the conduction band and unoccupied states of MOFs corresponding to C60 p orbitals. TD-DFT calculations show that incorporating C60 promotes D-A interactions in MOFs, leading to charge transfer in the near-infrared and visible photoinduced electron transfer (PET) from porphyrins to C60. Nonequilibrium Green's function-based calculations for MOFs with sulfur group, with and without C60, performed using molecular junctions with Au(111)-based electrodes show increased charge transport for the doped MOF. These insights into tuning electronic/optical properties and controlling charge transfer can aid in the design of new visible/near-infrared MOF-based optoelectronic devices.

Anchoring Single-Atomic Metal Sites in Metalloporphyrin-Based Covalent Organic Frameworks for Enhanced Photocatalytic Hydrogen Evolution

A photoactive covalent organic framework (COF) was built from metalloporphyrin and bipyridine monomers and single-atomic Pt sites were subsequently installed. Integrating photosensitizing metalloporphyrin and substrate-activating Pt(bpy) moieties in a single solid facilitates multielectron transfer and accelerates photocatalytic hydrogen evolution with a maximum production rate of 80.4 mmol h-1 gPt -1 and turnover frequency (TOF) of 15.7 h-1 observed. This work demonstrates that incorporation of single-atomic metal sites with photoactive COFs greatly enhances photocatalytic activity and provides an effective strategy for the design and construction of novel photocatalysts.

Integrating Multipolar Structures and Carboxyl Groups in sp2-Carbon Conjugated Covalent Organic Frameworks for Overall Photocatalytic Hydrogen Peroxide Production

The direct production of hydrogen peroxide (H2O2) through photocatalytic reaction via H2O and O2 is considered as an ideal approach. However, the efficiency of H2O2 generation is generally limited by insufficient charge and mass transfer. Covalent organic framework (COFs) offer a promising platform as metal-free photocatalyst for H2O2 production due to their potential for rational design at the molecular level. Herein, we integrated the multipolar structures and carboxyl groups into COFs to enhance the efficiency of photocatalytic H2O2 production in pure water without any sacrificial agents. The introduction of octupolar and quadrupolar structures, along with an increase of molecular planarity, created efficient oxygen reduction reaction (ORR) sites. Meanwhile, carboxyl groups could not only boost O2 and H2O2 movement via enhancement of pore hydrophilicity, but also promote proton conduction, enabling the conversion to H2O2 from ⋅O2 -, which is the crucial intermediate product in H2O2 photocatalysis. Overall, we demonstrate that TACOF-1-COOH, consisting of optimal octupolar and quadrupolar structures, along with enrichment sites (carboxyl groups), exhibited a H2O2 yield rate of 3542 μmol h- 1 g-1 and a solar-to-chemical (SCC) efficiency of 0.55 %. This work provides valuable insights for designing metal-free photocatalysts for efficient H2O2 production.

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.

2D Porphyrin-Based Covalent-Organic Framework/PEG Composites: A Rational Strategy for Photocatalytic Hydrogen Evolution

Two-dimensional porphyrin-based covalent-organic frameworks (2D-por-COFs) have gained significant attention as attractive platforms for efficient solar light conversion into hydrogen production. Herein, it is found that introducing transition metal zinc and polyethylene glycol (PEG) into 2D-por-COFs can effectively improve the photocatalytic hydrogen evolution performance. The photocatalytic hydrogen evolution rate of ZnPor-COF is 2.82 times higher than that of H2Por-COF. Moreover, ZnPor-COF@PEG has the highest photocatalytic hydrogen evolution efficiency, which is 1.31 and 3.7 times that of pristine ZnPor-COF and H2Por-COF, respectively. The filling of PEG makes the layered structure of COFs more stable. PEG reduces the distortion and deformation of the carbon skeleton after the experiment of photocatalytic hydrogen evolution. The layered stacking and crystallization of 2D-por-COFs are also enhanced. Meanwhile, the presence of PEG also accelerates the transfer of excited electrons and enhances the photocatalytic hydrogen evolution activity. This strategy will provide valuable insights into the design of 2D-por-COFs as efficient solid photocatalysts for solar-driven hydrogen production.

Cationic covalent organic framework nanosheets as the coating layer of commercial separator for high-efficiency lithium-sulfur batteries

Ion-selective separators, are crucial and in high demand for maximizing the performance of lithium-sulfur (Li-S) batteries, which can conduct lithium ions while efficiently blocking polysulfides. However, commercial separators cannot effectively block the shuttle of polysulfides after multiple cycles due to their large porosity and easy dissolution, resulting in a reduced battery life. Herein, a covalent organic framework nanosheets (CON) ion-coated separator is prepared on the commercial separator. Due to the smaller pore size of CON-TFSI compared to polysulfides, the CON-TFSI modified separator can effectively block the polysulfide from migrating across the separator. By incorporating this innovative functional layer, Li-S batteries demonstrate outstanding performance. In a Li-S battery featuring a sulfur loading of 0.6 mg/cm2, it attains an initial discharge specific capacity of up to 891.9 mA h g-1, and exhibits the capacity retention of 54.6 % after 500 cycles at a current density of 0.2 C. This work offers a fresh perspective on the advancement of high-performance membranes for Li-S batteries.

Cairo pentagon tessellated covalent organic frameworks with mcm topology for near-infrared phototherapy

Despite the prevalent of hexagonal, tetragonal, and triangular pore structures in two-dimensional covalent organic frameworks (2D COFs), the pentagonal pores remain conspicuously absent. We herein present the Cairo pentagonal tessellated COFs, achieved through precisely chosen geometry and metrics of the linkers, resulting in unprecedented mcm topology. In each pentagonal structure, porphyrin units create four uniform sides around 15.5 Å with 90° angles, while tetrabiphenyl unit establish a bottom edge about 11.6 Å with 120° angles, aligning precisely with the criteria of Cairo Pentagon. According to the narrow bandgap and strong near-infrared (NIR) absorbance, as-synthesized COFs exhibit the efficient singlet oxygen (1O2) generation and photothermal conversion, resulting in NIR photothermal combined photodynamic therapy to guide cancer cell apoptosis. Mechanistic studies reveal that the good 1O2 production capability upregulates intracellular lipid peroxidation, leading to glutathione depletion, low expression of glutathione peroxidase 4, and induction of ferroptosis. The implementation of pentagonal Cairo tessellations in this work provides a promising strategy for diversifying COFs with new topologies, along with multimodal NIR phototherapy.

An n-type semiconducting diazaporphyrin-based hydrogen-bonded organic framework

Significant effort has been devoted to the development of materials that combine high electrical conductivity and permanent porosity. This paper discloses a diazaporphyrin-based hydrogen-bonded organic framework (HOF) with porosity and n-type semiconductivity. A 5,15-diazaporphyrin Ni(ii) complex with carboxyphenyl groups at the meso positions afforded a HOF due to hydrogen-bonding interactions between the carboxy groups and meso-nitrogen atoms. The thermal and chemical stabilities of the HOF were examined using powder X-ray diffraction analysis, and the charge-carrier mobility was determined to be 2.0 × 10-7 m2 V-1 s-1 using the flash-photolysis time-resolved microwave conductivity (FP-TRMC) method. An analogous diazaporphyrin, which does not form a HOF, exhibited mobility that was 20 times lower. The results presented herein highlight the crucial role of hydrogen-bonding networks in achieving conductive pathways that can tolerate thermal perturbation.

Processing polymer photocatalysts for photocatalytic hydrogen evolution

Conjugated materials have emerged as competitive photocatalysts for the production of sustainable hydrogen from water over the last decade. Interest in these polymer photocatalysts stems from the relative ease to tune their electronic properties through molecular engineering, and their potentially low cost. However, most polymer photocatalysts have only been utilised in rudimentary suspension-based photocatalytic reactors, which are not scalable as these systems can suffer from significant optical losses and often require constant agitation to maintain the suspension. Here, we will explore research performed to utilise polymeric photocatalysts in more sophisticated systems, such as films or as nanoparticulate suspensions, which can enhance photocatalytic performance or act as a demonstration of how the polymer can be scaled for real-world applications. We will also discuss how the systems were prepared and consider both the benefits and drawbacks of each system before concluding with an outlook on the field of processable polymer photocatalysts.