Photoredox Catalysis by Covalent Organic Frameworks

Abstract

In recent years, photocatalysis that uses solar energy for either fuel production, such as hydrogen evolution and hydrocarbon production, or directed organic transformations, has shown great potential to achieve the goal of finding clean and renewable energy sources. Covalent organic frameworks (COFs) are crystalline organic porous materials formed by the covalent bonding of organic building blocks, which features superior structural regularity, robust framework, inherent porosity, and diverse functionality. The introduction of organic monomers with adjustable light absorption ability into COFs can make them show strong potential in photocatalysis. This chapter presents the recent progress of COF-based photocatalysts. The use of COF photocatalysts in a myriad of photoredox catalysts with a range of applications, including photocatalytic water splitting, photocatalytic CO2 reduction, photocatalytic organic transformations, and photocatalytic environmental pollutant degradation will be highlighted. Furthermore, various linkers between COF building blocks such as nitrogen-containing connections and all sp2-carbon connections will be summarized and compared. Finally, a perspective on the opportunities and challenges for the future development of COF and COF-based photocatalysts will be given.

1. Introduction

Covalent organic frameworks (COFs) materials are a new class of crystalline organic porous polymers, which are formed by the expansion of organic monomers through covalent bonds in two or three dimensions [1, 2, 3, 4]. In 2005, Yaghi et al. reported the synthesis of two kinds of crystalline organic porous materials, namely COF-1 and COF-5 for the first time through trimerization of aromatic boric acid compounds, which opens the research of COFs materials [5]. By elaborately designing the organic monomers that constitute COFs, the precise control of material structures and functions can be realized [6]. By virtue of the topology design theory in Reticular Chemistry, COFs have developed rapidly in recent years [7, 8]. So far, the linking mode of COFs has developed from the initial B∙O bond to C∙N bond and then to C∙O bond and C∙C bond, etc. [9]. In terms of spatial topology, COFs can be divided into two-dimensional (2D) COFs and three-dimensional (3D) COFs [10]. The 2D COFs are similar to graphite in structure. The organic monomers in the layers are connected by covalent bonds to form a 2D plane, and the interlayers are stacked by π-π conjugated interaction. 3D COFs are constructed by using non-planar organic monomers that expand in 3D direction through covalent bonds. 3D COFs have various types of topological structures, usually with interpenetrated structures.

COFs materials usually have the characteristics of high structure order, large specific surface area, and good thermal stability, which endow this kind of material with a wide range of applications in gas storage and separation, catalysis, sensing, energy storage, and photoelectric conversion [11]. In recent years, with more in-depth research on COFs, it has been found that COFs can generate and separate charge carriers under the irradiation of light. Combined with their designable pore structure, COFs have been witnessed as potential photocatalysts [12, 13, 14].

Photocatalysis uses light energy to drive chemical reactions, which is a green catalytic method and has great research value. In recent years, a large number of photocatalysts suitable for different systems have been developed [15]. Generally, photocatalysts are divided into homogeneous and heterogeneous photocatalysts. Compared with homogeneous photocatalysts, heterogeneous photocatalysts have the advantages of recyclability and reusability while maintaining high catalytic activity, which makes heterogeneous photocatalysts more potential for industrial application. However, current heterogeneous photocatalysts based on inorganic metal oxides and other systems still face some challenges, such as weak light absorption ability and poor charge separation ability [16]. In this context, recently developed COFs materials have unique advantages in the field of photocatalysis: (1) Structure designability: The light absorption capacity and energy band structure of COFs can be finely adjusted by changing the chemical structure of the monomers. (2) Facile functionalization. The COFs skeleton can be modified in two ways, before and after preparation, so that the catalytic sites can be easily introduced into the COF skeleton. (3) High surface area and good stability. This allows the catalytic sites on the COFs to be effectively dispersed and fully exposed and to remain stable during the catalytic process. In this chapter, the research progress of COFs in photocatalytic hydrogen production, photocatalytic carbon dioxide reduction, photocatalytic organic reactions, and photocatalytic pollutant degradation is summarized, and the future application of COFs in the field of photocatalysis is also prospected.

2. Photocatalytic hydrogen production

Hydrogen energy is recognized as clean energy. Photocatalytic hydrogen production from water splitting by using solar energy is a very promising approach. At present, the main obstacle to photocatalytic hydrogen production is the lack of efficient catalysts. The structural diversity of COFs enables them excellent visible light absorption ability, tunable band position, and good stability. These advantages make COFs expected to become efficient catalysts in the field of photocatalytic hydrogen production. In recent years, COFs as catalysts for photocatalytic hydrogen production have gradually attracted researchers’ attention. Up to now, there are mainly three kinds of COFs photocatalytic hydrogen production systems: pure COFs photocatalysts, COFs-based heterojunction photocatalysts, and COFs composites photocatalysts.

2.1 Pure COFs photocatalysts

In 2014, Lotsch et al. used 1,3,5-tris(4-formyl-phenyl)triazine (TFPT) and 2,5-diethoxy-terephthalohydrazide as building blocks to construct hydrazone-linked TFPT-COF and used this COF to realize the photocatalytic hydrogen production from proton reduction in water (Figure 1) [17]. Under the irradiation of visible light, in the presence of metallic platinum that was used as the cocatalyst, and triethanolamine used as the hole sacrificial agent, the hydrogen production rate of this COF reached 1970 μmol·h⁻¹·g⁻¹ with an apparent quantum efficiency of 2.2%. The authors speculated that the central triazine unit with good planarity in this COF enables enhanced stacking interactions and thus charge transport in the axial direction, rendering TFPT-COF an effective H₂-evolving photocatalytic system.

Figure 1.

(a) Chemical structure of TFPT-COF. (b) High-resolution transmission electron microscope (TEM) images. (c) Photocatalytic H₂ evolution with TFPT-COF using sodium ascorbate donor and Pt cocatalyst under visible light irradiation. Inset: H₂ evolution using TEOA as an electron donor. Adapted with permission from Ref. [17]. Copyright 2014 Royal Society of Chemistry.

Lotsch et al. further explored the effect of the number of nitrogen atoms in the benzene ring at the central node of COF on the photocatalytic activity [18]. A series of stable azine-linked N_x-COFs, named N₀-COF, N₁-COF, N₂-COF, and N₃-COF, were synthesized by condensation of hydrazine and triphenylaryl monomers with 0, 1, 2, or 3 nitrogen atoms in the central benzene ring, respectively. Under visible light irradiation, with platinum as the cocatalyst and triethanolamine as the hole sacrificial agent, the hydrogen production rates of this kind of COF gradually increased with the increase of nitrogen content, and the hydrogen production rates from N₀-COF to N₃-COF were 23, 90, 438, and 1703 μmol·h⁻¹·g⁻¹, respectively. This indicates that N₃-COF has a high photocatalytic activity, which may be attributed to the stabilization of photogenerated radical anions by the electron-deficient triazine center, which enhances the charge separation ability and facilitates the electron migration to the platinum cocatalyst.

In 2017, Lotsch et al. further demonstrated that both the number and position of nitrogen atoms had a great impact on the photocatalytic activity of COFs [19]. PTP-COF has a similar structure to N₃-COF with three nitrogen atoms located on the pyridine group at the periphery of the 1,3,5-tripyridyl benzene monomer. Under the same photocatalytic hydrogen production conditions as N₃-COF, the hydrogen production rate of PTP-COF is only 83.83 μmol·h⁻¹·g⁻¹. The authors further studied the effect of the number of nitrogen atoms in the peripheral aromatic ring of COF monomer on the photocatalytic hydrogen production activity. Three azine-linked COFs were synthesized by condensation of hydrazine with three monomers containing 0, 1, or 2 nitrogen atoms in the peripheral aryl group of 1,3,6,8-tetrakis(4-ethynylbenzaldehyde)-pyrene [20]. The hydrogen production rates of these COFs decreased gradually with the increase of nitrogen content, which were 98, 22, and 6 μmol·h⁻¹·g⁻¹, respectively. It is obvious that the subtle structural changes of monomers have significant effects on photocatalytic hydrogen production of COFs, but the underlying mechanism needs to be further confirmed by theory and experiment.

The photocatalytic hydrogen production activity of COFs can be changed by designing different monomer structures. Therefore, in recent years, the diacetylene- and thiophene-based COF photocatalysts systems have been developed. Thomas et al. synthesized TP-BDDA-COF and TP-EDDA-COF containing acetylene fragments [21]. The diacetylene groups in TP-BDDA-COF possess larger conjugation, which endows TP-BDDA-COF with a narrower band gap and faster charge mobility. Under visible light irradiation, with platinum as cocatalyst and triethanolamine as hole sacrificial agent, the hydrogen production rate of TP-BDDA-COF is 324 μmol·h⁻¹·g⁻¹, which is about 10 times that of TP-EDDA-COF that is linked by only acetylene group (30 μmol·h⁻¹·g⁻¹). Cooper et al. constructed S-COF and FS-COF by using 3,7-diaminodibenzo[b,d]thiophene and 3,9-diamino-benzo[1,2-b:4,5-b’]bis[1]benzothiophene sulfone as monomers, respectively [22]. The key dibenzo[b,d]thiophene sulfone (DBTS) unit is the monomer of photosensitive polymer P10 [23]. Under visible light irradiation, with platinum as cocatalyst and sodium ascorbate as hole sacrificial agent, the hydrogen production rate of FS-COF was 10.1 mmol·h⁻¹·g⁻¹, while that of S-COF was 4.44 mmol·h⁻¹·g⁻¹. Without platinum as a cocatalyst, FS-COF and S-COF could still produce hydrogen efficiently with hydrogen production rates of 1.32 and 0.6 mmol·h⁻¹·g⁻¹, respectively. They claim that the higher performance of FS-COF is due to the characteristics, such as larger pore size, higher specific surface area, wider light absorption range, and more hydrophilic pore structure, which make FS-COF have better charge transfer ability and more abundant photocatalytic sites. They further investigated the effect of crystallinity of COF on photocatalysis, showing that crystalline FS-COF had higher photocatalytic activity compared with amorphous or semi-crystalline FS-P. They also improved the photocatalytic activity of FS-COF by organic dyes sensitization, enhancing the hydrogen production rate to 16.3 mmol·h⁻¹·g⁻¹.

The functional groups on the skeleton of COFs have a strong influence on their photocatalytic activity. Sun et al. obtained TpPa-COF, TpPa-COF-(CH₃)₂, and TpPa-COF-NO₂ by condensation of p-phenylenediamine, 2,5-dimethyl-p-phenylenediamine, 2-nitro-p-phenylenediamine, and 1,3,5-trimethylphloroglucinol, respectively [24]. Under visible light irradiation, with platinum as cocatalyst and sodium ascorbate as hole sacrificial agent, the hydrogen production rates were 1.56, 8.33, 0.22 mmol·h⁻¹·g⁻¹, respectively. Through a series of characterizations, they proved that the electron donor groups in COFs have a strong conjugation effect, which enhanced the light absorption and carrier mobility of the material, thus improving the photocatalytic activity of the material.

In 2019, Zhang et al. reported a series of vinylene-linked (∙HC∙CH∙) 2D COFs through Knoevenagel condensation of 3,5-dicyano-2,4,6-trimethylpyridine (DCTMP) with different aromatic aldehyde monomers (Figure 2) [25]. The polymerization and crystallization processes were performed in N,N-dimethylformamide (DMF) with piperidine as a catalyst. Upon reaction of DCTMP with 4,4-diformyl-p-terphenyl (DFPTP), 4,4-diformyl-1,1-biphenyl (DFBP), and 1,3,5-tri(4-formylphenyl)benzene (TFPB) respectively, three 2D COFs (denoted as g-C_xN_y-COF) with trans-disubstituted C∙C bond linkages (vinylene linkages) are formed in high yields. The fully aromatic structures of the frameworks containing pyridine rings, benzene rings, vinylene bonds, and cyano groups grant them intrinsic π-electron delocalized properties and excellent light-harvesting capabilities. In addition, the COFs showed ordered structures with high crystallinity, high surface area up to 1235 m² g⁻¹, and appropriate band structures, which allowed them to drive two half-reactions of water splitting to generate hydrogen or oxygen separately under visible light irradiation. The sample g-C₄₀N₃-COF exhibited the most prominent performance with a high H₂ production rate of 4120 μmol·h⁻¹·g⁻¹ and a remarkable apparent quantum efficiency (AQY) of 4.84% (at a wavelength of 420 nm). These values exceeded all the other COF-based photocatalysts at that time, demonstrating the superior catalytic activities of this kind of COF.

Figure 2.

(a) Synthesis of the vinylene-linked g-C₄₀N₃-COF. (b) Powder X-ray diffraction (PXRD) analysis and structural simulation of g-C₄₀N₃-COF. (c) High-resolution TEM images of g-C₄₀N₃-COF. (d) Photocatalytic H₂ evolution performance of Pt@g-C₄₀N₃-COF. (e) Apparent quantum yield (AQY) of g-C₄₀N₃-COF along with its UV-vis diffuse reflectance spectra (DRS). Adapted with permission from Ref. [25]. Copyright 2019, Springer Nature.

In the same year, by using another key monomer 2,4,6-trimethyl-1,3,5-triazine (TMTA) to react with aromatic multi-aldehydes, Zhang et al. established two triazine-cored COFs with unsubstituted carbon-carbon double bond linkages [26]. The resulting sp²-carbon-linked 2D COFs showed high crystallinity, extended π-electron delocalization, tunable band energy levels, as well as high surface areas, regular open channels, and superior chemical stabilities. These features endow the COFs with potential as semiconducting photocatalysts. Moreover, these COFs showed fibrous morphologies, which allowed for fabrication of thin films as photoelectrodes by a simple drop-casting performance. The corresponding photoelectrodes exhibited photocurrents up to 45 μA cm⁻² at 0.2 V vs. RHE, which is significantly higher than that of its imine-linked COF analogue, demonstrating the superior semiconducting properties of the vinylene-linked COFs. In the particulate photocatalysis system, a 3% Pt-modified sample exhibited an H₂ production rate of 292 μmol·h⁻¹·g⁻¹ with an AQY of 1.06% at 420 nm upon visible light irradiation, outperforming its imine-linked COF analogue that nearly lost its crystallinity upon 4-h light exposure, indicating the superior photostability of vinylene-linked COFs over their imine-linked COF counterparts.

In a subsequent work, Zhang et al. developed two semiconducting vinylene-linked COFs with aligned octupolar structures and tunable polarity. The polarity of the COFs was tuned by reticulating D_3h-symmetric 2,4,6-tris(4′-formyl-biphenyl-4-yl)-1,3,5-triazine with D_3h-symmetric tricyanomesitylene (g-C₅₄N₆-COF) and C_2v-symmetric 3,5-dicyano-2,4,6-trimethylpyridine (g-C₅₂N₆-COF), respectively [27]. The highly symmetric g-C₅₄N₆-COF exhibited the octupolar conjugated characters with more promising semiconducting behavior as compared with the less-symmetric g-C₅₂N₆-COF. The octupolar π-conjugated g-C₅₄N₆-COF exhibited enhanced light harvesting as well as excellent photo-induced charge generation and separation capabilities. Due to the appropriate band energy levels that are suitable for water splitting, g-C₅₄N₆-COF enabled the two half-reactions of photocatalytic water splitting with an average O₂ production rate of 51.0 μmol·h⁻¹·g⁻¹ and H₂ production rate of 2518.9 μmol·h⁻¹·g⁻¹, indicating the immense prospect of vinylene-linked COFs for photocatalytic overall water splitting. In summary, g-C₅₄N₆-COF showed significantly higher photocatalytic performance than the less-symmetric g-C₅₂N₆-COF, implying that changing the geometric symmetry of monomers within vinylene-linked COFs could exert profound effects on their polarity, semiconducting properties, as well as photocatalytic performance.

The robust solid-state crystalline structure of COFs is fully modular and hence possesses almost unlimited chemical tunability for the different functions fundamental to the photocatalytic process at an atomic level precision. Among the reported COF-based photocatalysts, vinylene-linked COFs showed higher H₂ evolution efficiency than the other COFs, probably due to their superior π-electron conjugation, excellent light-harvesting capability, fast charge isolation, high carrier mobility, and exceptional stability. However, a wide range of structural and optoelectronic factors need to be well orchestrated to maximize the H₂ evolution efficiency of a COF photocatalyst. Engineering the chemistry of COF—sacrificial electron donor and the COF—cocatalyst interfaces, or creating COF-based heterojunction composites would be vital for further improvement of H₂ evolution efficiency.

2.2 COFs-based heterojunction photocatalysts

To further enhance the separation of photogenerated charges, researchers designed COFs heterojunction catalysts, which were formed by covalent bonding. There are two main design strategies for this type of catalyst: (1) introducing donor and acceptor units into the COFs to form heterojunction catalyst; (2) composite COFs with other materials to form heterojunction catalyst. These two strategies can effectively tune the energy band structure of the photocatalyst and increase the separation efficiency of photogenerated electrons and holes.

Jiang et al. reported a newly developed photocatalyst that consists of all sp² carbon frameworks with full π-conjugation (Figure 3) [28]. The COF was synthesized by Knoevenagel condensation between tetrakis(4-formylphenyl)pyrene and 1,4-phenylenediacetonitrile (PDAN). Further, the framework was designed to constitute built-in donor-acceptor heterojunction by integrating the electron deficient 3-ethylrhodanine (ERDN) unit as an end-capping group to the periphery of the sp²c-COF lattice. The resultant COF created dense yet ordered columnar π-arrays, electron push-and-pull effect, and substantial reaction centers in pores or on surface, which could promote exciton migration and offer a narrow band gap to harvest visible and near-infrared light. Consequently, the COFs enabled an immediate yet continuous stable hydrogen production from water upon irradiation. These results demonstrated that the sp² carbon frameworks ensemble a light-driven photocatalytic system in which a chain of photochemical events—including light harvesting, exciton migration, exciton splitting, and electron transfer and collection—is structurally interlocked and seamlessly coupled. One could rationally design the COF photocatalysts with specific semiconducting properties for the continuous and efficient production of hydrogen from water.

Figure 3.

Design and synthesis of the 2D stable sp² carbon-conjugated COFs for H₂ production from water. Schematic representation of (A) sp²c-COF, (B) sp²c-COFERDN with electron-deficient ERDN end groups. Inset: electron donor-acceptor pull-push effects on the 2D skeletons. Adapted with permission from Ref. [28]. Copyright 2019, Elsevier Inc.

Heterojunctions between COFs and other materials can be formed by covalently bonding COFs with semiconductors that have different energy band structures. For example, COF and g-C₃N₄ are connected by the imine bond to form heterojunction CN-COF. CN-COF can produce hydrogen under visible light with high photocatalytic activity [29]. The rate can reach 10.1 mmol·h⁻¹·g⁻¹, which is mainly due to the suitable light absorption range and reasonable energy band structure of the heterojunction material. In addition, covalent bonding can change the photogenerated electron transport path, inhibit carrier recombination, and effectively improve photocatalytic activity.

The formation of heterojunction materials between metal-organic frameworks (MOFs) and COFs is a new way to improve photocatalytic activity. NH₂-UiO-66 was immobilized on the surface of TpPa-1-COF by the covalent bond, and the maximum hydrogen yield rate was 23.41 mmol·h⁻¹·g⁻¹ under visible light [30]. In this photocatalytic process, the photogenerated electrons of TpPa-1-COF transition from the valence band to the conduction band and are rapidly transferred to the conduction band of NH₂-UiO-66 by heterojunction, so as to ensure the effective separation of photogenerated electrons and holes, and thus improve the catalytic efficiency.

2.3 COFs composites photocatalysts

In addition to the advantages of COFs, such as facile structural designability, compositing COFs with other materials to form photocatalysts is also an important strategy that is developed in recent years. COFs that composite with other materials can not only make full use of their own advantages but also greatly improve the separation and transport efficiency of photogenerated carriers.

Banerjee et al. synthesized β-ketoenamine-linked COF (TpPa-2 COF) with extended π-conjugation, high specific surface area, and excellent stability [31]. Subsequently, CdS nanoparticles were uniformly dispersed in the synthesized TpPa-2 COF to form a composite photocatalyst. Two-dimensional TpPa-2 COF can not only stabilize CdS nanoparticles within its pore channels but also promote charge transfer and inhibit photogenerated carrier recombination, thus improving photocatalytic activity. Under visible light irradiation, the hydrogen production rate of the CdS-COF composite catalyst was up to 3678 μmol·h⁻¹·g⁻¹ with platinum as cocatalyst and lactic acid as hole sacrificial agent. However, the hydrogen production rate of CdS nanoparticles was 128 μmol·h⁻¹·g⁻¹, and that of COF alone was only 28 μmol·h⁻¹·g⁻¹.

COFs combined with other noble metal nanoparticles can also effectively inhibit the recombination of photogenerated carriers and improve photocatalytic ability. Long et al. coated Pt nanoparticles with polyvinylpyrrolidone (PVP) and assembled the coated Pt nanoparticles with hydrophilic imine-linked TP-COF by electrostatic assembly method to prepare a metal-insulator-semiconductor (MIS) photocatalytic system [32]. Under visible light irradiation, the hydrogen production rate of the ternary PT-PVP-TP-COF photocatalyst is 8.42 mmol·h⁻¹·g⁻¹, which is 32 times higher than that of the corresponding metal-semiconductor Mott-Schottky photocatalyst. The authors clarify that the ternary system can extract photogenerated electrons from COF with the help of an electrostatic field and transfer them to platinum nanoparticles through an insulator. However, the existence of an insulator prevents the photogenerated holes generated in COF from migrating to platinum nanoparticles, thus effectively improving the separation efficiency of electrons and holes and suppressing the recombination of photogenerated carriers.

In the process of photocatalytic hydrogen production, platinum is usually needed as a cocatalyst to facilitate the proton reduction to hydrogen in the system. However, the use of platinum increases the cost and hinders the industrial application of photolytic water splitting. In order to solve this problem, Lotsch et al. developed a photocatalytic hydrogen production system with chloro(pyridine)cobaloxime as cocatalyst (Figure 4) [33]. With triethanolamine as the hole sacrificial agent, the hydrogen production rate of N2-COF can reach 782 μmol·h⁻¹·g⁻¹. This work represents an economic system for photocatalytic H₂ evolution using COF photosensitizers and earth-abundant cocatalysts. Recently, Lotsch et al. used an earth-abundant, noble-metal-free nickel-thiolate hexameric cluster as a cocatalyst for photocatalytic hydrogen production [34]. Under the condition of visible light irradiation and triethanolamine as hole sacrificial agent, TpDTz COF synthesized from thiazolo[5,4-d]thiazole monomer was used as a photocatalyst, delivering the hydrogen production rate of 941 μmol·h⁻¹·g⁻¹.

Figure 4.

(a) Schematic illustration of photocatalytic H₂ evolution by using N₂-COF photosensitizer and Co-1 cocatalyst. (b) H₂ evolution rates with various COF photosensitizers using Co-1 cocatalyst and TEOA donor. Adapted with permission from Ref. [33]. Copyright 2017, American Chemical Society.

3. Photocatalytic CO₂ reduction

With the development of human society, the CO₂ content in the atmosphere is increasing rapidly, which leads to the greenhouse effect. It is urgent for human beings to find a green and convenient way to convert CO₂ into usable chemicals. The CO₂ molecule has a highly symmetric structure and is chemically relatively inert. Although chemical and electrochemical methods have demonstrated excellent CO₂ conversion capabilities, the direct use of light energy to achieve CO₂ conversion, especially CO₂ reduction, still presents many challenges. Photocatalytic CO₂ reduction requires not only high stability and strong visible light absorption capacity but also strong CO₂ adsorption capacity and suitable photocatalytic sites. On the basis of high crystallinity, high specific surface area, and adjustable light absorption ability, the introduction of suitable CO₂ catalytic sites will make COFs become potential CO₂ heterogeneous photocatalysts.

3.1 Introduction of metal sites into COFs for photocatalytic CO₂ reduction

Rhenium complexes are excellent homogeneous CO₂ reduction photocatalysts. Cao et al. synthesized pyridine-containing covalent triazene framework (CTF) materials and used pyridine nitrogen atoms in the organic skeleton to anchor Re(CO)₃Cl complexes [35]. The obtained photocatalyst Re-CTF-py with CO₂ reduction ability has high stability, strong CO₂ adsorption ability, and good photoactivity. Under visible light irradiation and triethanolamine as a hole sacrificial agent, the rate of CO₂ reduction to CO is 353. 05 μmol·h⁻¹·g⁻¹ with high selectivity. Due to the coordination of Re and pyridine in Re-CTF-py, the leaching of Re(CO)₃Cl can be effectively avoided, exhibiting excellent recyclability. Similarly, Huang et al. synthesized imine-linked COF using 2,2′-bipyridyl-5,5′-dialdehyde (BPDA) and 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)trianiline (TTA), which anchored Re(CO)₅Cl at the adjacent nitrogen atoms in bipyridine units to form COF-based photocatalyst (Figure 5) [36]. The COF can reduce CO₂ to CO with a selectivity of up to 98% and a reaction rate of 750 μmol·h⁻¹·g⁻¹. The reduction mechanism was investigated by transient spectroscopy, X-ray absorption spectroscopy, and in situ diffuse reflectance spectroscopy. It was concluded that photogenerated electrons could be rapidly transferred from COF to Re catalytic sites. In recent work, TpBpy COF chelates Ni(ClO₄)₂ through the bipyridine units to form Ni-TpBpy COF containing a single atomic Ni site [37]. With Ru(bpy)₃Cl₂ as a photosensitizer, triethanolamine as a hole sacrificial agent, and Ni-TpBpy COF as cocatalyst, the photocatalytic reduction rate of CO₂ to CO can reach 811.4 μmol·h⁻¹·g⁻¹ with the selectivity of 96%. In the Ni-TpBpy COF system, COF can not only serve as a carrier of the CO2 catalytic site but also promote the stability of the intermediate and improve the reduction efficiency of CO₂.

Figure 5.

Synthesis and characterization of COF and Re-COF. Proposed catalytic mechanism for CO₂ reduction. Adapted with permission from Ref. [36]. Copyright 2018, American Chemical Society.

Oxygen atoms in COFs can also act as metal anchoring sites. For example, Lan et al. synthesized DQTP COF with 2,6-diaminoanthraquinone as the monomer, in which the oxygen atoms of quinone between the adjacent layers have suitable distances, which provides a feasible coordination environment for anchoring transition metal catalysts [38]. Interestingly, CO₂ can be selectively reduced to different products after introducing different metal catalytic sites. DQTP COF coordinates with Zn(II), Co(II), and Ni(II) to form DQTP COF-M photocatalyst, respectively. CO₂ was reduced by using Ru(bpy)₃Cl₂ as a photosensitizer and triethanolamine as a hole sacrificial agent under visible light. The reduction product of DQTP COF-Co was CO at a production rate of 1020 μmol·h⁻¹·g⁻¹. When the coordinated metal ion is changed to Zn, the reduction product of DQTP COF-Zn is HCO₂H with a rate of 152.5 μmol·h⁻¹·g⁻¹, and the selectivity is greater than 90%. The authors concluded that the electron-rich coordination environment tends to form CO in the reduction process, while the electron-deficient coordination environment tends to form HCO₂H. CO(II), as a good π-bond donor, is conducive to the conversion of CO₂ to CO. However, Zn(II) is a poor donor of the π-bond, so its reaction product is mainly HCO₂H. The photocatalytic CO₂ reduction products of DQTP COF-Ni with a moderate electron coordination environment are almost equal amounts of CO and HCO₂H.

Metal porphyrins are excellent catalytic sites for CO₂ reduction. Introducing metal porphyrins into the COF skeleton is also an important strategy for the design of CO₂ photoreduction catalysts. Lan et al. synthesized TTCOF-Zn from electron-deficient Zn metal porphyrin and electron-rich tetrathiafulvalene [39]. In TTCOF-Zn, the donor-acceptor structure between tetrathiafulvalene and porphyrin was formed, which could effectively improve the separation ability of photogenerated charge. Under visible light irradiation, without adding any sacrificial agent or photosensitizer, TTCOF-Zn can reduce CO₂ to CO in an aqueous solution with a yield of 0. 2055 μmol·h⁻¹·g⁻¹, while the selectivity reaches 100%. The catalyst can still maintain good catalytic activity after five cycles.

The morphology of the catalyst also has a significant influence on the performance of the catalyst. The bulk catalyst can be nano-sized to expose more catalytic sites and thus improve catalytic activity [40]. Jiang et al. synthesized COF-367-Co nanosheets from 5,10,15,20-tetra(p-aminophenyl)porphyrin (H₂TAPP) and 4,4′-biphenyldialdehyde (BPDA) as monomers [41]. 2,4,6-Trimethylbenzaldehyde (TBA) can be introduced to the edge of COF-367-Co nanosheets by imine exchange reaction. TBA with large steric hindrance obstructs the axial packing of COF 2D lamellae and promotes the growth of COF nanosheets along the plane direction, resulting in ultra-thin 2D COF nanosheets. Ultrathin 2D COF nanosheets can effectively expose the catalytic site of cobalt metalloporphyrin and shorten the distance of photogenerated charge carriers migrating to the catalytic site. Under visible light irradiation, with Ru(bpy)₃Cl₂ as a photosensitizer and ascorbic acid as a hole sacrificial agent, COF-367-Co ultrathin 2D nanosheet can reduce CO₂ to CO at a rate of 10,162 μmol·h⁻¹·g⁻¹ with the selectivity of 78%.

3.2 Introduction of metal-free sites into COFs for photocatalytic CO₂ reduction

In addition to using metal as the catalytic site for CO₂ reduction, nitrogen atoms in the COFs skeleton can be directly used as the catalytic site for CO₂ reduction, which can achieve a totally metal-free heterogeneous photocatalytic system. Zhu et al. synthesized ACOF-1 and N₃-COF by condensation of 1,3,5-triformylbenzene (TFB) and 2,4,6-tris(4-formylphenyl)-1,3,5-triazine (TFPT) with hydrazine, respectively [42]. Through control experiments, it was found that the triazine units in COFs not only improved the light absorption capacity but also contributed to the adsorption of CO₂ and served as catalytic sites. The electron-deficient triazine in N₃-COF can effectively stabilize the negatively charged intermediate in the catalytic process. Without any sacrificial agent, N₃-COF can reduce CO₂ to methanol in an aqueous solution under visible light irradiation at a rate of 0.57 μmol·h⁻¹·g⁻¹. The catalytic activity of N₃-COF is higher than that of ACOF-1 without the triazine ring.

The COF heterojunction based on the donor-acceptor structure can further improve the catalytic activity. To this end, Kong et al. designed an intramolecular donor-acceptor heterojunction CT-COF containing an electron-rich carbazole group as the donor and an electron-deficient triazine group as the acceptor [43]. CT-COF has a wide optical absorption range and narrow band gap due to the push-pull electron effect on the donor-acceptor structure, which effectively improves the separation and transportation efficiency of photogenerated carriers. They proved that the nitrogen atom in the triazine group was the catalytic site of CO₂ reduction by density functional calculation. Under visible light irradiation, the rate of CO₂ reduction to CO in an aqueous solution is 102.7 μmol·h⁻¹·g⁻¹ catalyzed by CT-COF without any co-catalyst or sacrificial agent.

The heterojunction structure formed by COF and inorganic semiconductors can also improve photocatalytic activity effectively. Lan et al. synthesized COF-318 from 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) and 2,3,5,6-tetrafluoro-4-pyridinecarbonitrile (TFPC) as precursors, which then formed a Z-scheme heterojunction catalyst by covalently bonding with inorganic semiconductor TiO₂, Bi₂WO₆, and α-Fe₂O₃, respectively (Figure 6) [44]. Both pyridine nitrogen and nitrile nitrogen in monomers can be used as CO₂ catalytic sites. Meanwhile, the Z-scheme heterojunction formed by COF and inorganic semiconductors can effectively improve the separation efficiency of photogenerated electrons and holes and inhibit carrier recombination. Under visible light irradiation, the reduction rate of CO₂ to CO in an aqueous solution can reach 69.67 μmol·h⁻¹·g⁻¹ without adding other cocatalysts.

Figure 6.

Schematic representation of the preparation of COF-318-SCs via the condensation of COF-318 and semiconductor materials. Adapted with permission from Ref. [44]. Copyright 2020, Wiley-VCH.

Although the use of COFs photocatalysts to reduce CO₂ has been widely reported, the quantum efficiency of COF-based photocatalysis is still difficult to compare with other systems, and it is difficult to obtain more valuable hydrocarbon products such as ethane or ethylene through COFs photocatalysis. However, it still showed great promise, and one could rationally expect that there would be more and more research focusing on the development of more efficient and promising COF-based photocatalysts for photocatalytic CO₂ reduction.

4. Photocatalytic organic reactions

Photocatalytic organic reactions can achieve the green synthesis of organic intermediates and can obtain compounds that cannot be obtained by other methods, which plays an important role in fine organic synthesis. In recent years, a series of homogeneous photocatalysts, such as ruthenium complexes and small-molecule dyes, have greatly improved organic photocatalytic reactions in terms of both reaction types and reaction efficiency. However, the high cost and high toxicity of a series of noble metal catalysts represented by ruthenium complexes limit their application in the pharmaceutical industry. Therefore, it is particularly important to develop nontoxic and recyclable heterogeneous photocatalysts. Among them, porous materials, especially COFs, are used as organic photocatalysts, which have attracted extensive attention from researchers.

Cross-dehydrogenating coupling (CDC) is a highly efficient C∙C bond coupling reaction, which can be used to synthesize a variety of important organic intermediates. For example, Liu et al. designed and synthesized COF-JLU5, which contains an electron-deficient triazine group and electron-rich 2,5-dimethoxyphenyl group to form a donor-acceptor structure [45]. The COF has the characteristics of large crystallinity, high specific surface area, high stability, and photoredox activity. The cross-coupling reaction of N-aryl-tetrahydroisoquinoline can be catalyzed under blue LED irradiation, with a yield up to 99%, a wide substrate range, and excellent recyclability.

Liu et al. further synthesized COF-JLU22 based on electron-rich 1,3,6,8-tetrakis(4-aminophenyl)pyrene and electron-deficient 4,4′-(benzothiadiazole-4,7-diyl)dibenzaldehyde [46]. The pyrene segment can not only enhance the crystallization of COF-JLU22 but also connect with the acceptor structure of the electron-deficient benzothiadiazole group. The resulting COF-JLU22 has a wide optical absorption range, and its band structure is very close to that of Ru(bpy)₃Cl₂ complexes that are widely used in photocatalysis. Conclusively, COF-JLU22 exhibited excellent catalytic performance in the reductive dehalogenation of phenacyl bromide derivatives and α-alkylation of aldehydes under visible-light irradiation and retained its original catalytic activity after three cycles.

Wu et al. synthesized a hydrazone-linked COF with high specific surface area and high crystallization, which could catalyze the CDC reaction of tetrahydroisoquinoline with nitromethane, acetone, acetophenone, and other nucleophiles under visible light, with the yield up to 87% [47]. The COF has high stability, and its catalytic activity is not reduced after three cycles.

Although imine bond-linked COFs photocatalysts show excellent catalytic performance in the field of photocatalysis, such kinds of COFs are unstable in acidic systems due to the reversibility of imine bonds. Therefore, it is very important to design and synthesize COFs with high stability for photocatalytic organic conversion.

Banerjee et al. designed and synthesized β-ketoenamine-linked COF, which has high stability [48]. TpTt COF was synthesized from 1,3,5-triazine-2,4,6-triamine and 2,4,6-triformylphloroglucinol. This COF is isomerized into the β-ketoenamine-based structure, which makes the bond of COF irreversible and thus has high chemical stability. The COF can initiate a cis-trans isomerization reaction of olefin under blue LED light irradiation. Due to the stability of the β-ketoenamine linkage fragment, the COF could still maintain the original photocatalytic activity after four cycles of catalytic reactions.

COFs connected by carbon-carbon double bonds possess higher chemical stability and full π-electron conjugation, representing a kind of exceptional photocatalysts. Wang et al. synthesized 2D porphyrin-based sp²-carbon conjugated COF [49], which could maintain stability in 9 mol l⁻¹ HCl and 9 mol l⁻¹ NaOH. Due to the unique photophysical and redox properties of porphyrin, as well as the high stability and π-conjugation properties of carbon-carbon double bond-linked COF, the photocatalytic yield of Por-sp² COF in the process of catalyzing the oxidation of amines to imines can reach 99% within 30 min. Compared with the COF of the imine bond, it shows obviously higher photocatalytic activity and stability. Subsequently, the fully conjugated sp²-carbon-linked Py-BSZ-COF with donor-acceptor structure was synthesized by the reaction of 1,3,6,8-tetrakis(4-formylphenyl)pyrene and 4,4′-(benzothiadiazole-4,7-diyl)diacetonitrile [50]. The resultant COF has high stability and can maintain the original skeleton structure after 3 days in 12 mol l⁻¹ HCl, 12 mol l⁻¹ NaOH, boiling water, and under 15 W white LED light, respectively. The donor-acceptor structure improves the separation ability of photogenerated electrons and holes, making the COF usable for mediating the photocatalytic oxidative amine coupling and cyclization of thioamide to 1,2,4-thiadiazole, with a broad substrate scope and good cyclability.

In 2020, Zhang et al. developed a variety of vinylene-linked COFs with fully sp²-carbon-connected skeletons by reacting tricyanomesitylene with multi-topic aromatic aldehydes through Knoevenagel condensation (Figure 7) [51]. With the use of appropriate secondary amines as catalysts, which could generate highly reactive iminium cation intermediates by reacting with aldehyde groups, the reversibility of the Knoevenagel condensation was greatly enhanced and highly crystalline vinylene-bridged COFs were achieved, which exhibited long-range ordered skeletons, well-defined nanopores, high surface areas (up to 1231 m² g⁻¹), efficient light-harvesting capabilities, and facilitated exciton migration and charge transport. Upon visible-light irradiation, these COFs accelerated oxidative hydroxylation of arylboronic acids to phenols with a low loading amount and short reaction time, comparable to homogeneous catalysts (e.g., Ru(bpy)₃Cl₂, methylene blue), but recyclable.

Figure 7.

(a) Synthetic scheme of ivCOF-O and vCOF-N. (b) Photocatalytic organic reactions catalyzed by ivCOF-O and vCOF-N. Adapted with permission from Ref. [44]. Copyright 2021, Wiley-VCH.

Very recently, Müllen et al. developed a 2D vinylene-linked COF comprising nanographene units by employing a dibenzo[hi,st]ovalene-based building block to react with 3,5-dicyano-2,4,6-trimethylpyridine (DCTMP) [52]. The resultant DBOV-COF formed unique ABC-stacked sp²-carbon lattices with robust vinylene linkages. By virtue of the narrow-energy-gap nanographene cores as active sites, DBOV-COF demonstrated high photoconductivity, enhanced charge-carrier mobility, and remarkable photocatalytic activity in hydroxylation of boronic acid to phenols with stable cycling performance. This work illustrated a facile strategy of introducing various functional building blocks (e.g., nanographenes) to the library of COFs to novel structures and promising properties (e.g., enhanced photocatalytic activity).

In addition to the photocatalytic oxidative reaction, vinylene-linked COFs have also been explored as efficient photocatalysts for much more complicated photoredox reaction systems. In 2021, Zhang et al. reported a two-dimensional pyrylium-based COF with vinylene linkages through aldol condensation of a trimethylsubstituted pyrylium salt with a tritopic aromatic aldehyde [53]. The resultant oxonium-embedded 2D vinylene-linked COF, namely ivCOF-O, was further converted to a neutral pyridine-cored COF (vCOF-N) by in situ replacement of oxonium ions with nitrogen atoms under a post-synthetic ammonia treatment. Both COFs are conceptually isoelectronic with each other, which means that they have similar geometric structures but significantly different electronic structures. The band structures of the two COFs were evaluated by UV-vis DRS and UPS analyses, showing the ivCOF-O with high oxidative potential and vCOF-N with low reductive potential. Consequently, ivCOF-O enabled photocatalytic [2 + 2 + 2] cyclization of alkynes with nitriles to pyridines upon visible light irradiation due to its high oxidative potential that induced the single electron transfer from alkyne to pyrilium. On the other hand, the lower reductive potential of vCOF-N allows it to reduce molecular oxygen to superoxide radical anion under the visible light excitation. Then the generated active oxygen species induced the coupling of amine with ketone to produce 2,4,6-triarylpyridine derivatives with good substrate tolerance and recyclability. This work provides a method for the precise construction of heteroatom-embedded COFs with highly tailorable band structures and energy levels, thus prospectively expanding the practical applications of COF materials, especially in photocatalysis.

Chen et al. constructed BBO-COF by self-condensation of monomer containing aldehyde and amino functional groups [54]. This “two-in-one” strategy can facilitate the construction of imine-bonded COF with the benzoxazole linkage. The COF can maintain stability in 12 mol l⁻¹ HCl and 12 mol l⁻¹ NaOH. BBO-COF showed high activity, good substrate tolerance, and reusability in the process of photocatalytic oxidative hydroxylation of aryl boric acid. After 10 cycles, COF still maintains the original structure and catalytic activity.

Using the post-synthetic modification (PSM) strategy to transform reversible bonds into irreversible bonds in COFs is also an effective way to enhance the stability of COFs. Wang et al. constructed a series of benzoxazole-linked COFs with the strategy of “killing two birds with one stone” [55]. After 3 days of storage in boiling water, 9 mol l⁻¹ HCl, and 9 mol l⁻¹ NaOH, the COFs’ crystallinity is still retained, indicating the ultrahigh stability of these COFs. The benzoxazole units introduced by means of PSM into these conjugated COF skeletons can not only effectively improve the light absorption capacity but also provide significant catalytic sites. The yield of this kind of COF in the photocatalytic oxidative hydroxylation of aryl boric acid can reach 99%. Meanwhile, it has high recyclability, which was confirmed by that the skeletal structure and catalytic activity are still maintained after 20 cycles.

The morphology of COFs also has a significant influence on their photocatalytic activity. Aleman et al. synthesized a series of imine-linked COFs with spherical, layered, and 3D structures, respectively [56]. They used these COFs to study the effects of molecular structure, crystallinity, and microstructure on photocatalytic performance. In the selective oxidation of thioether to sulfoxide, crystallinity has a great influence on the photocatalytic activity of 2D COF with spherical and layered morphology, while it has a small influence on 3D COF.

In addition to the abovementioned reactions in photoredox organic transformations catalyzed by COFs, several other reactions, such as the polymerization of methyl methacrylate, Alder–Ene reaction, and trans (E) to cis (Z) isomerization of olefins, etc. have also been exploited by employing COFs as the photocatalysts. It showcased that COFs hold great potential in photoredox organic transformation yet need to be further explored.

5. Photocatalytic degradation of pollutants

In recent years, with the development of industrialization, the natural environment has been greatly affected, and a variety of different types of pollutants need to be dealt with. Photocatalytic degradation of pollutants is an effective way to solve environmental pollution. In this process, singlet oxygen ¹O₂ is usually induced by photocatalysts to kill pathogenic bacteria in the environment. In addition, photo-excited catalysts can be used to generate electrons and holes. The photogenerated electrons reduce O₂ to ·O₂⁻ and oxidize OH⁻ to ·OH⁻ in the environment. Both active oxygen species can be used to degrade the pollutants in the environment.

5.1 Singlet oxygen generated by COFs for catalysis

Porphyrins and their derivatives can generate excited triplet states under light irradiation and then interact with oxygen molecules to form singlet oxygen. Heterogeneous catalytic singlet oxygen formation can be achieved by introducing porphyrin derivatives into COFs. CuP-SQ-COF was synthesized by Jiang et al. using copper porphyrin derivatives and squaric acid [57]. The squaraine-linked COFs have a zigzagged conformation that protects the layered structure from sideslip, are highly stable in solvents, provide an extended π-conjugation over the 2D sheets, and have lower band gap energy and greatly enhanced absorbance capability. Using 1,3-diphenylisobenzofuran (DPBF) as a label, CUP-SQ-COF can effectively produce singlet oxygen under visible light irradiation.

Hydrogen-bonding interactions in the COF intralayer can effectively enhance the stability and photocatalytic activity. Jiang et al. reported the condensation between 5,10,15,20-tetrakis(4′-tetraphenylamino) porphyrin derivatives with terephthaladehyde and dihydroxyterephthalaldehyde in different proportions to obtain a series of COF with different numbers of intramolecular hydrogen bonds, which were used in photocatalysis to produce singlet oxygen (Figure 8) [58]. The intramolecular hydrogen bonds formed between imine bonds and hydroxyl groups lock the conformation of 2D in-plane fragments, thereby enhancing the interlayer interaction of COF and enabling it to generate excited triplet states faster and to trigger the transformation of oxygen molecules. The results show that with the increase of hydrogen bonds in COFs, the photocatalytic singlet oxygen generation rate increases gradually. In addition to porphyrin-based COF, imine-linked COFs containing triazine units can also be used for photocatalytic production of singlet oxygen. Zhang et al. synthesized COFs-Trif-Benz and COF-SDU1 by condensation of 2,4,6-tris(4-formylphenyl)-1,3,5-triazine (TFPT) with benzidine and p-phenylenediamine, respectively [59]. These two π-conjugated imine COFs can be used to produce singlet oxygen, and then their bactericidal effect has been explored. After 60–90 min of visible light irradiation, 90% of grape mold and Escherichia coli were killed.

Figure 8.

Schematic of the synthesis of 2D porphyrin COFs with tunable content of hydrogen-bonding structures. Adapted with permission from Ref. [58]. Copyright 2015, American Chemical Society.

5.2 Electrons and holes generated by COFs for catalysis

Organic dyes are usually toxic and carcinogenic pollutants. Photocatalytic degradation of organic dyes into harmless substances such as CO₂ and water is of great value. Cai et al. used 1,3,5-triformylphloroglucinol to condense with melamine to form imine-bonded COF, and then the imine bonds irreversibly tautomerize to obtain TpMA COF, which is stable in acid, base, and under light [60]. Upon visible light irradiation, TpMA COF can generate electrons and holes to reduce O₂ to ·O₂⁻ and oxidize OH⁻ to ·OH⁻, respectively. Since ·O₂⁻ and ·OH⁻ have strong oxidation ability, the COF can effectively catalyze photodegradation of methyl orange dye with apparent kinetic constant of 0.102 min⁻¹, which is 59 times that of g-C₃N₄.

In order to further enhance the photocatalytic degradation ability of COFs, it is an effective strategy to construct Z-scheme heterojunction catalysts based on COFs. Z-scheme heterojunctions can effectively inhibit the recombination of photogenerated carriers and thus improve the ability of photocatalytic degradation of pollutants. Cai et al. combined NH₂-MIL-(Ti) with 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde (TTB) and 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)trianiline (TTA) to synthesize MOF/COF composites by one-pot method [61]. The Z-scheme heterojunction formed by NH₂-MIL-(Ti) and TTB-TTA COF can enhance the separation and migration ability of photogenerated carriers and improve the photocatalytic degradation performance of organic pollutants. Under visible light irradiation, photogenerated electrons migrate from the conduction band of NH₂-MIL-(Ti) to the valence band of TTB-TTA COF, generating ·OH⁻ at the valence band of NH₂-MIL-(Ti) and ·O₂⁻ at the conduction band of TTB-TTA COF, which are used to degrade methyl orange dye. The photodegradation kinetics of methyl orange by NH₂-MIL-(Ti)/TTB-TTA COF heterojunction were nine times and two times that of pure NH₂-MIL-(Ti) and pure TTB-TTA COF, respectively. Noble metal nanoparticles can also form heterojunctions with COF for photocatalytic pollutant degradation. Lu et al. anchored gold nanoparticles with thiol-modified imine-bonded COF to form Z-scheme heterojunction, thus improving the efficiency of charge separation (Figure 9) [62]. Under visible light irradiation, the degradation rate of rhodamine dye and bisphenol A by Au-S-COF within 30 min could reach 93.7% and 90%, respectively. In addition, the degradation ability of Au-S-COF was tested in a continuous flow system contaminated by rhodamine dye. In the flow device, Au-S-COF was used as the filter membrane, the clear solution could directly flow out from the end of the filter. The used Au-S-COF filter paper changed from red to yellow after 30 min of light irradiation, indicating that the pollutant could be completely degraded by light.

Figure 9.

Principles of photogenerated electron transport between Au NCs and the COF support. Adapted with permission from Ref. [62]. Copyright 2020, Wiley-VCH.

In addition to the degradation of organic pollutants, the photogenerated electrons of COF can also be used to reduce heavy metal ions in the environment. Chen et al. used tris(4-aminophenyl)benzene (TPB) or tris-(4-aminophenyl)triazine (TAPT) as donor and electron-deficient benzothiadiazole (BT) as acceptor to construct two COFs with donor-acceptor structure, which were denoted as TPB-BT-COF and TAPT-BT-COF, respectively [63]. The Mott-Schottky spectra of TPB-BT-COF and TAPT-BT-COF show that their conduction band positions are −0.44 and −0.30 V (vs. NHE), respectively. Both are lower than the reduction potential of hexavalent chromium to less-toxic trivalent chromium (1.33 V vs. NHE). This indicated that the COFs can be used to reduce chromium heavy metal ions by photocatalysis. The reduction rate of hexavalent chromium can reach 99% without adding any sacrificial agent or adjusting the pH value. Such high performance was attributable to the stronger photoactivity of TPB-BT-COF due to its more negative band position and narrower band gap.

COFs as purely organic materials might have the smallest impact on the natural environment and have great promise in the practical application of environmental remediation. Thus, one could expect that massive efforts would be made to advance the COF photocatalysts in photocatalytic pollutant degradation.

6. Summary and outlook

As a new type of crystalline organic porous materials, COFs have attracted the attention of researchers due to their unique structural and property advantages and have been rapidly developed in the application of photocatalysis. The designability of the building block makes COFs predictable in structure and adjustable in function, so as to achieve strong light absorption ability and superior stability. The ordered structure and uniform channel can improve the separation and migration ability of photogenerated carriers. High specific surface area can expose more catalytic sites and enhance catalytic activity. All these make COFs become advanced heterogeneous photocatalysts with relatively high conversion rates and excellent recyclability.

In recent years, the research on COF photocatalysts has made some achievements, but it is still in the stage of rapid development. At the same time, there are still some challenges in the field of photocatalytic application by using COFs:(1) rapid and scalable synthesis of COFs photocatalysts. At present, the methods of synthesizing high-quality COFs are cumbersome and time-consuming, which restrict the development of their photocatalytic research and industrial application. (2) Since the photocatalytic mechanism of COFs needs to be further explored, it is a great challenge to design the specific COFs photocatalysts according to the requirements of the specific catalytic reactions. (3) Currently, the number of photocatalytic cycles catalyzed by COFs is limited, and the development of highly stable COFs is an important issue to expand the application scope of COFs photocatalysts. (4) The structure of photoactive COFs is simple. Light absorptive monomers can effectively increase light absorption through covalent bonding to the π-conjugated COFs skeleton. However, suitable photoactive monomers are limited. The design of reasonable photoactive monomers is an effective method to enhance the photosensitivity of COFs. (5) When COFs are used in photocatalysis, most of the reaction systems need additional noble metal cocatalysts and sacrificial agents. It is an important development direction to design COFs photocatalytic systems that are independent of cocatalysts and sacrificial agents. (6) The preparation of high-efficiency and low-cost COFs photocatalytic devices will provide an important way to address the environmental pollution issue and energy crisis.

Acknowledgments

The author is profoundly thankful to the National Natural Science Foundation of China for financial support within the project (22005189) and the China Postdoctoral Science Foundation (2020M681277).

Conflict of interest

The author declares no conflict of interest.