共价有机框架材料在光催化领域中的应用

共价有机框架(COFs)材料是有机构筑基元通过共价键连接而形成的晶态有机多孔材料. COFs具有孔道结构规整、及比表面积高等特点,被广泛地应用于气体储存与分离、催化、传感、储能及光电转化等领域.将具有可调吸光能力的有机构筑基元引入到COFs中,可使其展现出强大的光催化潜力.近年来, COFs在光催化领域中发展迅猛.本文总结了COFs在光催化产氢、光催化二氧化碳还原、光催化有机反应以及光催化污染物降解等方面的研究进展,并展望了其在光催化领域的应用前景.     

Porphyrin-based donor–acceptor COFs as efficient and reusable photocatalysts for PET-RAFT polymerization under broad spectrum excitation

Covalent organic frameworks (COFs) are crystalline and porous organic materials attractive for photocatalysis applications due to their structural versatility and tunable optical and electronic properties. The use of photocatalysts (PCs) for polymerizations enables the preparation of well-defined polymeric materials under mild reaction conditions. Herein, we report two porphyrin-based donor–acceptor COFs that are effective heterogeneous PCs for photoinduced electron transfer-reversible addition–fragmentation chain transfer (PET-RAFT). Using density functional theory (DFT) calculations, we designed porphyrin COFs with strong donor–acceptor characteristics and delocalized conduction bands. The COFs were effective PCs for PET-RAFT, successfully polymerizing a variety of monomers in both organic and aqueous media using visible light (λ_max from 460 to 635 nm) to produce polymers with tunable molecular weights (MWs), low molecular weight dispersity, and good chain-end fidelity. The heterogeneous COF PCs could also be reused for PET-RAFT polymerization at least 5 times without losing photocatalytic performance. This work demonstrates porphyrin-based COFs that are effective catalysts for photo-RDRP and establishes design principles for the development of highly active COF PCs for a variety of applications.

Porphyrin-based donor–acceptor COFs are effective heterogeneous photocatalysts for photoinduced electron transfer-reversible addition–fragmentation chain transfer (PET-RAFT), including for aqueous polymerizations and under red-light excitation.     

2D sp2 Carbon-Conjugated Porphyrin Covalent Organic Framework for Cooperative Photocatalysis with TEMPO

2D covalent organic frameworks (COFs) are receiving ongoing attention in semiconductor photocatalysis. Herein, we present a photocatalytic selective chemical transformation by combining sp² carbon-conjugated porphyrin-based covalent organic framework (Por-sp²c-COF) photocatalysis with TEMPO catalysis illuminated by 623 nm red light-emitting diodes (LEDs). Highly selective conversion of amines into imines was swiftly afforded in minutes. Specifically, the π-conjugation of porphyrin linker leads to extensive absorption of red light; the sp² −C=C− double bonds linkage ensures the stability of Por-sp²c-COF under high concentrations of amine. Most importantly, we found that crystalline framework of Por-sp²c-COF is pivotal for cooperative photocatalysis with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO). This work foreshadows that the outstanding hallmarks of COFs, particularly crystallinity, could be exploited to address energy and environmental challenges by cooperative photocatalysis.     

Boosting CO2 Photoreduction via Regulating Charge Transfer Ability in a One-Dimensional Covalent Organic Framework

Two-dimensional (2D) imine-based covalent organic frameworks (COFs) hold potential for photocatalytic CO₂ reduction. However, high energy barrier of imine linkage impede the in-plane photoelectron transfer process, resulting in inadequate efficiency of CO₂ photoreduction. Herein, we present a dimensionality induced local electronic modulation strategy through the construction of one-dimensional (1D) pyrene-based covalent organic frameworks (PyTTA-COF). The dual-chain-like edge architectures of 1D PyTTA-COF enable the stabilization of aromatic backbones, thus reducing energy loss during exciton dissociation and thermal relaxation, which provides energetic photoelectron to traverse the energy barrier of imine linkages. As a result, the 1D PyTTA-COF exhibits significantly enhanced CO₂ photoreduction activity under visible-light irradiation when coordinated with metal cobalt ion, yielding a remarkable CO evolution of 1003 μmol g⁻¹ over an 8-hour period, which surpasses that of the corresponding 2D counterpart by a factor of 59. These findings present a valuable approach to address in-plane charge transfer limitations in imine-based COFs.     

Stable Immobilization of Nickel Ions on Covalent Organic Frameworks for Panchromatic Photocatalytic Hydrogen Evolution

Post-coordination design on covalent organic frameworks (COFs) is an efficient strategy for elevating the photocatalytic activity of organic moiety. However, the rigid skeletons and densely layered stacking of two-dimensional (2D) COFs cannot be flexibly adapted for specific conformations of metal complexes, thereby impairing the metal-COF cooperation. Here, we adopt a solvothermal method to immobilize nickel(II) ions into a 2,2′-bipyridine-containing 2D COF, forming a stable coordination motif. Such the complex remarkably enhances the photocatalytic performance, giving an optimized H₂ evolution rate of as high as 51 300 μmol h⁻¹ g⁻¹, 2.5 times higher than the pristine COF. The evolved hydrogen gas can also be detected upon 700-nm light irradiation, while its analog synthesized by the traditional coordination method is photo-catalytically inert. This work provides a strategy for optimizing the metal-COF coordination system and strengthening a synergy for electronic regulation in photocatalysis.     

Isomeric Oligo(Phenylenevinylene)-Based Covalent Organic Frameworks with Different Orientation of Imine Bonds and Distinct Photocatalytic Activities

Imine-linked covalent organic frameworks (COFs) have been extensively studied in photocatalysis because of their easy synthesis and excellent crystallinity. The effect of imine-bond orientation on the photocatalytic properties of COFs, however, is still rarely studied. Herein, we report two novel COFs with different orientations of imine bonds using oligo(phenylenevinylene) moieties. The COFs showed similar structures but great differences in their photoelectric properties. COF-932 demonstrated a superior hydrogen evolution performance compared to COF-923 when triethanolamine was used as the sacrificial agent. Interestingly, the use of ascorbic acid led to the protonation of the COFs, further altering the direction of electron transfer. The photocatalytic performances were increased to 23.4 and 0.73 mmol g⁻¹ h⁻¹ for protonated COF-923 and COF-932, respectively. This study provides a clear strategy for the design of imine-linked COF-based photocatalysts and advances the development of COFs.     

2D sp2 Carbon‐Conjugated Porphyrin Covalent Organic Framework for Cooperative Photocatalysis with TEMPO

2D covalent organic frameworks (COFs) are receiving ongoing attention in semiconductor photocatalysis. Herein, we present a photocatalytic selective chemical transformation by combining sp² carbon‐conjugated porphyrin‐based covalent organic framework (Por‐sp²c‐COF) photocatalysis with TEMPO catalysis illuminated by 623 nm red light‐emitting diodes (LEDs). Highly selective conversion of amines into imines was swiftly afforded in minutes. Specifically, the π‐conjugation of porphyrin linker leads to extensive absorption of red light; the sp² ?C=C? double bonds linkage ensures the stability of Por‐sp²c‐COF under high concentrations of amine. Most importantly, we found that crystalline framework of Por‐sp²c‐COF is pivotal for cooperative photocatalysis with (2,2,6,6‐tetramethylpiperidin‐1‐yl)oxyl (TEMPO). This work foreshadows that the outstanding hallmarks of COFs, particularly crystallinity, could be exploited to address energy and environmental challenges by cooperative photocatalysis.     

Covalent organic frameworks with imine proton acceptors for efficient photocatalytic H2 production

Covalent organic frameworks (COFs) are promising crystalline materials for the light-driven hydrogen evolution reaction (HER) due to their tunable chemical structures and energy band gaps. However, deeply understanding corresponding mechanism is still challenging due to the multiple components and complicated electron transfer and reduction paths involved in photocatalytic HER. Here, the photocatalytic HER investigation has been reported based on three COFs catalysts, 1–3, which are prepared by benzo[1,2-b:3,4-b':5,6-b']trithiophene-2,5,8-trialdehyde to react with C₃ symmetric triamines including tris(4-aminophenyl)amine, 1,3,5-tris(4-aminophenyl)benzene, and (1,3,5-tris-(4-aminophenyl)triazine, respectively. As the isostructural hexagonal honeycomb-type COF of 2 and 3 reported previously, the crystal structure of 1 has been carefully correlated through the powder X-ray diffraction study with the help of theoretical simulations. 1 shows highly porous framework with Brunauer-Emmett-Teller surface area of 1249 m²/g. Moreover, the introduction of ascorbic acid into the photocatalytic system of COFs achieves the hydrogen evolution rate of 3.75, 12.16 and 20.2 mmol g^–1 h^–1 for 1–3, respectively. The important role of ascorbic acid in photocatalysis of HER is disclosed to protonate the imine linkages of these COFs, leading to the obvious absorbance red-shift and the improved charge separation efficiency together with reduced resistance in contrast to pristine materials according to the spectroscopic and electronic characterizations. These innovations of chemical and physical properties for these COFs are responsible for their excellent photocatalytic performance. These results elucidate that tiny modifications of COFs structures is able to greatly tune their band structures as well as catalytic properties, therefore providing an available approach for optimizing COFs functionalities.     

卟啉框架材料在光催化领域的应用

卟啉分子对可见光具有较强吸收能力,被广泛应用于光催化和光敏化材料的设计开发。 基于卟啉单元设计构筑框架结构材料,可以借助框架结构的大比表面积和可调控孔结构,实现对卟啉单元光物理化学性质的修饰和调控,达到提高材料光催化量子产率和光催化选择性的目的。 本文综述了卟啉基金属有机框架材料(MOFs)、卟啉基共价有机框架材料(COFs)、以及卟啉基多孔共价有机聚合物(COPs)在光催化领域的研究进展,通过归纳需要解决的关键问题,对卟啉框架材料的未来发展进行了展望。     

Enaminone-Linked Covalent Organic Frameworks for Boosting Photocatalytic Hydrogen Production

Covalent organic frameworks (COFs), possessing pre-designable structures and tailorable functionalities, are promising candidates for photocatalysis. Nevertheless, the most studied imine-linked COFs (Im-COFs) usually suffer from unsatisfactory stability and photocatalytic performance. To meet this challenge, a series of highly stable enaminone-linked COFs (En-COFs) have been synthesized and afford much improved visible-light-driven hydrogen production activities, ranging from 44 to 1078 times that of isoreticular Im-COFs, with the only difference being the linkages (enaminone vs. imine) in their structures. The enhanced light-harvesting ability, facilitated exciton dissociation and improved chemical stability account for the superior activity. Furthermore, quinoline-linked COFs (Qu-COFs) have been further obtained via the post-modification of Im-COFs. Compared with Im-COFs, the photocatalytic activities of Qu-COFs are significantly improved after modification, but still below those of the corresponding En-COFs (3–107 times). The facile synthesis, excellent activity, and high chemical stability demonstrate that En-COFs are a promising platform for photocatalysis.     

Covalent Organic Frameworks for Energy Conversion in Photocatalysis

Intensifying energy crises and severe environmental issues have led to the discovery of renewable energy sources, sustainable energy conversion, and storage technologies. Photocatalysis is a green technology that converts eco-friendly solar energy into high-energy chemicals. Covalent organic frameworks (COFs) are porous materials constructed by covalent bonds that show promising potential for converting solar energy into chemicals owing to their pre-designable structures, high crystallinity, and porosity. Herein, we highlight recent progress in the synthesis of COF-based photocatalysts and their applications in water splitting, CO₂ reduction, and H₂O₂ production. The challenges and future opportunities for the rational design of COFs for advanced photocatalysts are discussed. This Review is expected to promote further development of COFs toward photocatalysis.     

2D and 3D Porphyrinic Covalent Organic Frameworks: The Influence of Dimensionality on Functionality

The construction of 2D and 3D covalent organic frameworks (COFs) from functional moieties for desired properties has gained much attention. However, the influence of COFs dimensionality on their functionalities, which can further assist in COF design, has never been explored. Now, by selecting designed precursors and topology diagrams, 2D and 3D porphyrinic COFs (2D‐PdPor‐COF and 3D‐PdPor‐COF) are synthesized. By model building and Rietveld refinement of powder X‐ray diffraction, 2D‐PdPor‐COF crystallizes as 2D sheets while 3D‐PdPor‐COF adopts a five‐fold interpenetrated pts topology. Interestingly, compared with 2D‐PdPor‐COF, 3D‐PdPor‐COF showed interesting properties, including 1) higher CO₂ adsorption capacity; 2) better photocatalytic performance; and 3) size‐selective photocatalysis. Based on this study, we believe that with the incorporation of functional moieties, the dimensionality of COFs can definitely influence their functionalities.     

Photocatalytic Activity of Supported Metal Nanoparticles and Single Atoms

Photocatalysis has been known as one of the promising technologies due to its eco-friendly nature. However, the potential application of many photocatalysts is limited owing to their large bandgaps and inefficient use of the solar spectrum. One strategy to overcome this problem is to combine the advantages of heteroatom-containing supports with active metal centers to accurately adjust the structural parameters. Metal nanoparticles (MNPs) and single atom catalysts (SACs) are excellent candidates due to their distinctive coordination environment which enhances photocatalytic activity. Metal-organic frameworks (MOFs), covalent organic frameworks (COFs) and carbon nitride (g-C₃N₄) have shown great potential as catalyst support for SACs and MNPs. The numerous combinations of organic linkers with various heteroatoms and metal ions provide unique structural characteristics to achieve advanced materials. This review describes the recent advancement of the modified MOFs, COFs and g-C₃N₄ with SACs and NPs for enhanced photocatalytic applications with emphasis on environmental remediation.     

Linkage Microenvironment of Azoles-Related Covalent Organic Frameworks Precisely Regulates Photocatalytic Generation of Hydrogen Peroxide

Artificial H₂O₂ photosynthesis by covalent organic frameworks (COFs) photocatalysts is promising for wastewater treatment. The effect of linkage chemistry of COFs as functional basis to photoelectrochemical properties and photocatalysis remains a significant challenge. In this study, three kinds of azoles-linked COFs including thiazole-linked TZ-COF, oxazole-linked OZ-COF and imidazole-linked IZ-COF were successfully synthesized. More accessible channels of charge transfer were constructed in TZ-COF via the donor-π-acceptor structure between thiazole linkage and pyrene linker, leading to efficient suppression of photoexcited charge recombination. Density functional theory calculations support the experimental studies, demonstrating that the thiazole linkage is more favorable for the formation of *O₂ intermediate in H₂O₂ production than that of the oxazole and imidazole linkages. The real active sites in COFs located at the benzene ring fragment between pyrene unit and azole linkage.     

Porphyrinic covalent organic framework designed by molecular engineering as heterogeneous photocatalyst for aerobic mediated RAFT polymerization

Covalent organic frameworks (COFs) have gained increasing attention as heterogeneous materials for their prominent applications in photocatalytic processes. The already tailored structure endows COFs with ordered dimensional channels for the separation and migration of the electro-hole pairs and improves their photocatalytic properties. In this contribution, oxygen-mediated RAFT polymerization was achieved by using M-TCPP-DHTA-COFs (M = H₂ or Zn) as photocatalysts with the assistance of TEA as co-catalyst producing polymers with accurate molecular weight and narrow molecular weight distribution under visible light irradiation. The control experiments revealed excellent dual control behavior of light and gas toward polymerization processes. Notably, porphyrinic COFs can be straightforwardly separated and recycled for recycling experiments and exhibit remarkable compatibility features of controllable polymerization for functional monomers under aerobic conditions. This study offers a promising pathway for the construction of an efficient heterogeneous catalyst of oxygen-mediated RAFT polymerization and extends the novel applications of porphyrin-based COF materials.     

sp2 carbon-conjugated covalent organic frameworks for efficient photocatalytic degradation and visualized pH detection

The design and fabrication of versatile covalent organic frameworks (COFs) with multiple properties for diverse applications is highly desirable. Here, the difunctional COFs material g–C₁₈N₃–COF was prepared and modified to be applied for efficient photocatalytic degradation of Rhodamine B (RhB) and pH detection, respectively. Owing to the triazine unit which was suitable for photocatalyst construction, g–C₁₈N₃–COF was sensitive to visible light and exhibited excellent photocatalysis capability toward RhB. Specially, the photocatalytic degradation of RhB with a high concentration of 300 ppm using g–C₁₈N₃–COF reached equilibrium within 6 h. Moreover, g–C₁₈N₃–COF was further grown in-situ onto the filter paper to generate a novel composite material g–C₁₈N₃–COF@Paper with bright yellow fluorescence. g–C₁₈N₃–COF@Paper could visualize the pH detection by remarkable changes in its fluorescent intensity and color in the range of pH value from 1 to 5, on account of the protonation of the nitrogen atoms from the triazine ring in g–C₁₈N₃–COF. The triazine-based sp² carbon-conjugated g–C₁₈N₃–COF, respectively, used as photocatalyst and sensor in this work offers a new strategy to construct the versatile COFs material, facilitating the application of functional COFs in the environmental protection field.     

An azine-linked 2D porphyrinic covalent organic framework for red light photocatalytic oxidative coupling of amines

The construction of two-dimensional covalent organic frameworks (2D COFs) with robust stability for photocatalysis has gained intensive attention recently. Herein, we report the design and synthesis of a highly crystalline azine-linked porphyrinic 2D COF (Por-HZ-COF). Our results clearly show that Por-HZ-COF adopts an eclipsed AA stacking structure with a high Brunauer-Emmett-Teller (BET) specific surface area of 1586 m²/g. In addition, Por-HZ-COF is chemically stable under various conditions, even in 12 M sodium hydroxide aqueous solution or 9 M hydrochloric acid. Moreover, Por-HZ-COF can be used for the photocatalytic aerobic oxidative coupling of benzylamines under red light irradiation with high activity and good reusability. This study demonstrates a novel robust 2D COF with azine linkage that has promising applications in photocatalysis.     

Pyrene-Based Covalent Organic Frameworks for Photocatalytic Hydrogen Peroxide Production

Four highly porous covalent organic frameworks (COFs) containing pyrene units were prepared and explored for photocatalytic H₂O₂ production. The experimental studies are complemented by density functional theory calculations, proving that the pyrene unit is more active for H₂O₂ production than the bipyridine and (diarylamino)benzene units reported previously. H₂O₂ decomposition experiments verified that the distribution of pyrene units over a large surface area of COFs plays an important role in catalytic performance. The Py-Py-COF though contains more pyrene units than other COFs which induces a high H₂O₂ decomposition due to a dense concentration of pyrene in close proximity over a limited surface area. Therefore, a two-phase reaction system (water-benzyl alcohol) was employed to inhibit H₂O₂ decomposition. This is the first report on applying pyrene-based COFs in a two-phase system for photocatalytic H₂O₂ generation.     

Solar-to-H2O2 Catalyzed by Covalent Organic Frameworks

Benefiting from the excellent structural tunability, robust framework, ultrahigh porosity, and rich active sites, covalent organic frameworks (COFs) are widely recognized as promising photocatalysts in chemical conversions, and emerged in the hydrogen peroxide (H₂O₂) photosynthesis in 2020. H₂O₂, serving as an environmental-friendly oxidant and a promising liquid fuel, has attracted increasing researchers to explore its potential. Over the past few years, numerous COFs-based photocatalysts are developed with encouraging achievements in H₂O₂ production, whereas no comprehensive review articles exist to summarize this specific and significant area. Herein we provide a systematic overview of the advances and challenges of COFs in photocatalytic H₂O₂ production. We first introduce the priorities of COFs in H₂O₂ photosynthesis. Then, various strategies to improve COFs photocatalytic efficiency are discussed. The perspective and outlook for future advances of COFs in this emerging field are finally offered. This timely review will pave the way for the development of highly efficient COFs photocatalysts for practical production of value-added chemicals not limited to H₂O₂.     

Regulating Relative Nitrogen Locations of Diazine Functionalized Covalent Organic Frameworks for Overall H2O2 Photosynthesis

Nitrogen-heterocycle-based covalent organic frameworks (COFs) are considered promising candidates for the overall photosynthesis of hydrogen peroxide (H₂O₂). However, the effects of the relative nitrogen locations remain obscured and photocatalytic performances of COFs need to be further improved. Herein, a collection of COFs functionalized by various diazines including pyridazine, pyrimidine, and pyrazine have been judiciously designed and synthesized for photogeneration of H₂O₂ without sacrificial agents. Compared with pyrimidine and pyrazine, pyridazine embedded in TpDz tends to stabilize endoperoxide intermediate species, leading toward the more efficient direct 2e^- oxygen reduction reaction (ORR) pathway. Benefiting from the effective electron-hole separation, low charge transfer resistance, and high-efficiency ORR pathway, an excellent production rate of 7327 μmol g⁻¹ h⁻¹ and a solar-to-chemical conversion (SCC) value of 0.62 % has been achieved by TpDz, which ranks one of the best COF-based photocatalysts. This work might shed fresh light on the rational design of functional COFs targeting photocatalysts in H₂O₂ production.