Low‐dimension confinement effect in COF‐based hetero‐photocatalyst for energy‐conversion application

Covalent organic framework (COF) materials have aroused tremendous interest in photocatalytic applications due to their tunable pore structure and photoelectric properties. The regular nanopore of COF itself presents a strongly confinement effect, which provides a unique regulatory effect for photons, electrons, protons, and other quantum‐scale reaction groups. However, due to the weak surface electron coupling and transfer ability between the reactive groups and basic elements of its structural units, the activity of pure COFs photocatalyst is still not satisfactory. Therefore, the confinement modification strategy of confining low‐dimension entities within COFs has been proposed, thus realizing new active sites construction and band structure regulation has been intensively studied, but yet to be summarized systematically. In this paper, the semi‐conductivity of COFs is discussed dialectically based on photocatalytic thermodynamics, and the influence of internal linkage motifs and stacking behaviors on the band structure is collected. Then, the basic understanding of confinement characteristics and their influence on photocatalytic performance in dynamics is further explained according to the spatial dimension classification of low‐dimension entities. And the application and mechanism of these COF‐based confined catalysts in energy conversion reactions are discussed in detail. Lastly, the current challenges and development prospects of COF‐based confined hetero‐photocatalysts are discussed.


| INTRODUCTION
Since Fujishima discovered Honda-Fujishima Effect guided by Honda in 1967, 1 the research related to photocatalysis has been widely carried out.Traditionally, photocatalysis is initiated by inorganic semiconductors such as TiO 2 and CdS. 2,39][20][21] The difference between COFs and traditional inorganic materials is that COFs can be prepared from clearly defined molecular building blocks which is attributed to the dependable molecular synthesis and hierarchical nanopore structure through crystal engineering.][32][33][34] Last but not least, their aperture adjustability, photochemical stability, good crystallinity, and topological structure, as well as their inherent hybrid properties are all directed to applications in photocatalysis. 35,36In 2008, the a priori plain resemblances between COFs and transition metal oxides caused the huge potential of application of COFs in photocatalysis.The best-known example is TP-COF, in which the ordered π-stacking and highly crystalline skeleton of this structure can give a predicted path to boost photo-generated carrier migration and inhibit photo-excited electron-hole pair recombination, which has obvious semiconductor traits. 37Afterwards, Lotsch and collaborators 38 first reported the TFPT-COF with Pt as cocatalyst driven by visible light to produce hydrogen in 2014.
In the past few years, COFs have shown the great potential of photocatalytic application.And it is fairly clear by now that the chemical and electronic properties of these organic materials may be very dissimilar to those of classical inorganic semiconductors, which has brought an unusual opportunity for the development of COFs in the field of photocatalysis.However, most pure COF photocatalysts still show low catalytic efficiency.On the one hand, the weak surface electron coupling and transfer ability between the reactive groups and the basic elements (nonmetallic light elements C, N, H, etc.) of its structural units severely restricts its catalytic efficiency.On the other hand, the probability of photoinduced electron-hole recombination is so high that the overall photocatalytic efficiency is restrained.For the photocatalytic process involving quantum-scale particles conversion such as photons, electrons, protons, and so forth, higher requirements are put forward for regulating the quantum characteristics of the photocatalyst such as electron energy level, and the conventional catalyst regulation strategy is slightly difficult.Therefore, strategies different from those applied in traditional semiconductors to improve photocatalytic performance should be developed for COFs.
For the past few years, the confinement modification strategy has provided abundant opportunities for the development of high-efficient photocatalytic processes.There have been many studies demonstrating that lowdimension entities (nanoparticles [NPs], nanoclusters [NCs], and single atoms) exhibit remarkable performance in catalytic reactions because of their unique microgeometric configurations and electronic structures. 39owever, these elaborate low-dimension entity catalysts may appear undesirable situations such as poor stability and agglomeration during working with harsh reaction conditions, which makes the performance of the catalysts degrade.Surprisingly, when these low-dimension entities are confined and act as active species in COFs, to some extent, they make up for the deficiency of COFs and lowdimension entities in photocatalytic applications due to the interaction between them (confinement effect).The realization of the above-mentioned good results is mainly related to the following reasons: (1) the large surface area, high stability, and inherent hybridization of COF provides an ideal host molecular platform for confining low-dimension entities [40][41][42][43] ; (2) the adjustable pore size of COF makes it suitable for low-dimension entities with different sizes 44-46 ; (3) the unique porous microstructure of COF is advantageous to the selective adsorption (enrichment) of reactants and the rapid transfer of substances 47 ; (4) outspread π-conjugated system of COF can promote carriers separation and migration. 48,49herefore, with this ingenious modification strategy, the aggregation inactivation problem for low-dimension entities can be solved on the basis of cleverly constructing active sites.
In this review, from the point of view of electronic structure, we have discussed the confinement modification strategy for confining low-dimension entities in porous COF-based materials.First, in the terms of photocatalytic thermodynamics, we have discussed the band structure of the COF and gathered essential lights about how the linkage motif and stacking behavior affect the band structure from the semiconductor framework.Then, we describe the influence of confinement on photocatalytic reaction, especially on dynamics.According to the classification of low-dimension entities (NPs, nanoclusters, and single atoms), the superiority of confining low-dimension entities to construct active sites in COF materials and the diverse photocatalytic activities produced by confinement strategies have been further interpreted with various photocatalytic reactions (Figure 1C,D).In the end, the actual application of COF-based confined hetero-photocatalyst is discussed, and the further development of COF-based confined hetero-photocatalyst is also prospected.

| BAND STRUCTURE OF COFS
The electrons in valence band absorb optical energy and transition to conduction band when the incident photon energy is greater than or equal to the band gap of semiconductor material, and simultaneously holes are generated in valence band.Eventually, photo-generated carriers will contribute to the oxidation-reduction reaction (e.g., hydrogen and oxygen evolution reduction, carbon dioxide reduction, and degradation of organic pollutants) ideally. 50n general, the photocatalytic system mainly includes the following basic photochemical processes: The photocatalytic reaction is initiated by light irradiation, and the semiconductor photocatalyst absorbs the incident photon with appropriate wavelength and thus excites electronhole pairs; Then, electrons (e − ) and holes (h + ) are further separated and migrate to the surface of photocatalyst; In an ideal situation, the holes with oxidizing ability and the electrons with reducing ability will eventually undergo redox reaction with the reaction groups adsorbed on the surface of photocatalyst (Figure 1B).Consequently, photocatalysts used in various photocatalytic reactions are highly dependent on their thermodynamic and dynamic properties.Taking the classic photocatalytic water splitting as an example, the standard Gibbs free energy variation (ΔG) of 237 KJ/mol is required for water splitting into H 2 and O 2 .Therefore, the band gap (E g ) of semiconductor photocatalyst ought to be over 1. 23 eV, and the wavelength of incident photon ought to be less than 1000 nm.To promote the reduction and oxidation of H 2 O by light-excited electrons and holes, conduction band minimum (CBM) must be more negative than the reduction potential of H + /H 2 (0 V relative to the normal hydrogen electrode (NHE), at pH = 7.00), while valence band maximum (VBM) must be corrected (1.23 V vs. NHE, at pH = 7.00) than the oxidation potential of O 2 /H 2 O.
Accordingly, COFs used as photocatalysis must have rational band gap and proper conduction band minimum and valence band maximum.In this chapter, we will discuss the semi-conductivity of COFs and collect some valuable insights about the impacts of COF structures on their band structures.

| Semi-conductivity of COFs
Part of COFs are classified as semiconductors base on their electro-chemical and photochemical activities and optical transitions. 51,52Nevertheless, such activity does not definitely signify semi-conductivity.The inorganic semiconductor is characteristic of delocalized electronstate band structure, which allows charge carriers to migrate. 52In contrast, the typical feature of organic semiconductors is that charge carriers migrate through extended conjugated π-bonds. 53,54For COFs, a certain degree of π-delocalization is indispensable to display semiconductive behaviors. 55According to the band theory, the existence of filled valanced bands and empty conduction bands with small energy gap is the principal cause of semiconductor conduction.In COFs, the band gap is defined as the energy gap width between the lowest unoccupied molecular orbital (LUMO) energy level and the highest occupied molecular orbital (HOMO) energy level, corresponding to the CBM and VBM in inorganic solid materials, respectively. 56Band gap is a basic parameter of semiconductor, which can be acquired by the experiment of intrinsic light absorption or conductivity changing with temperature.Measuring the electric current flowing through a material or its carrier mobility is a method that can directly ascertain whether the material is (semi-)conductive.Generally speaking, the analysis techniques of conductivity and carrier mobility may be different in terms of the experimental parameters and the space scope of related processes.The commonly used analysis and characterization techniques include hall effect, 57 two/four-contact probe, 58 time-resolved microwave conductivity (TRMC), 59 time-of-flight (ToF), 60 the optical pump terahertz probe spectroscopy (OPTP), 61 and field effect transistors. 62Conductivity is a parameter that gives a description of the difficult degree of charge flow in a material, and it is related to the capability of the material itself to conduct current.
The conductivity has been reported for a small subset of COFs.In 2008, the first example of semiconductor COF was reported by Jiang and his collaborators.It was composed of triphenylene and pyrene functional groups, which were alternately connected in a hexagonal mesostructure (Figure 2A). 37TP-COF can trap photons in the wavelength range from ultraviolet to visible light because of the existence of the triphenylene units.Monocrystals of triphenylene and pyrene both exhibit semiconductivity, 63,64 so researchers have speculated that the π-π stacked TP-COF synthesized by polycondensation of these two π-conjugated components is highly likely to be a semiconductor.Under room temperature and air conditions, a nearly linear I-V curve is obtained by applying bias voltages to the TP-COF sample in contact with the two electrodes of a two-contact probe device.The two-contact probe technology reflects the average conductivity of materials between two contact spots.When the applied bias voltage is 2 V, the current is 4.3 nA (Figure 2B).For contrast, under the same conditions, a monomers' mixture mixed at the ratio of 2:3 showed a low current of 79 pA.After doping TP-COF with iodine, the current increases about 5 times, which indicates that TP-COF is a semiconductor.Hao et al. 65 have used C 3symmetric benzotrithiophene tricarbaldehyde (BTT) to construct BTT-based COFs with various apertures.The results of optical absorption experiments indicate that these COFs have optical band gaps of 2.04-2.08eV.Bredas et al. 66 have calculated the electronic structure of π-conjugated COFs with BTT cores by using tight-binding model.The chemical structures of the BTT-based COFs with the cores linked in anticonfiguration, syn configuration, and through an ethynylene linker are shown in Figure 2C,D, respectively.And the band structures of BBT-COF with anti and syn configurations are illustrated in Figure 2F,G, respectively.The VBM and CBM of the two BBT-COFs both get mild dispersion, which originate from next-nearest-neighbor interactions of the kagome lattice.However, the cases in which BTT cores are connected by ethynylene linkers are different (Figure 2E,H).This structure effectually decreases the electronic couplings among the BTT cores, which leads to negligible next-nearest-neighbor interaction, resulting in flat bands near Fermi level.Flat band usually means infinite effective mass of charge carriers and the related carrier mobilities vanish.In essence, the flat band is caused by the highly symmetrical structure in the perspective of charge-carrier migration.Therefore, conceptually speaking, reducing lattice symmetry may be a feasible way to obtain stronger dispersion band structure.It is an interesting fact that the primitive cell of the Lieb lattice comprises three sites as in the case of the Kagome lattice (Figure 2I).In contrast to Kagome lattice, a 2D COF structure corresponding with Lieb lattice causes high dispersion at the top of valence band under the condition of full site occupation.ZnPor-COF and Pyr-COF are two examples of Lieb lattice, 67,68 both of which are COFs with four-arm cores connected by diacetylene linkers.The band structures of the two COFs show strong dispersion characteristics at CBM and VBM.Density functional theory calculation shows that the widths of conduction band and valence band are between 1 and 2 eV, and both holes and electrons are small cause by strong dispersion of the frontier electronic bands.Based on the above work, Li et al. 68 proceeds extended research, they also considered Por-COF (Figure 2J) with four-arm porphyrin cores connected by diacetylene linkers besides ZnPor-COF and Pyr-COF (Figure 2K).The obtained band structure diagram (Figure 2L) shows that all three COFs are direct bandgap semiconductors, and the band gap of Pyr-, Por-, and ZnPor-COF are 1.63, 0.99, and 1.23 eV, respectively.The author predicted the carrier mobility range of these three COFs to be 65-95 cm 2 /(V•s), and based on this, listed these 2D COFs as members of organic (macromolecular) semiconductors with the highest mobility.Additionally, the researchers have prepared ZnPor-COF experimentally, and the steadystate and TRMC measurements demonstrate the considerable broadband photoconductivity of ZnPor-COF, which is consistent with the band structure calculated theoretically.
There are many theoretical calculations to predict the charge carrier transport in COFs because of the intense dispersion near Fermi level. 69In the experiment, Contini and coworkers 70 have proved the existence of Dirac cone structure and flat band in mesoscopic ordered 2D πconjugated COF, and its kagome lattice holds semiconductivity.However, due to defects, impurities, grain boundaries, or other possible reasons, the experimental values of charge carrier migration rates of most COFs are far lower than the theoretical values (100 cm 2 /(V•s) or higher. 68Typically, the conductivity of some molecular semiconductors will decrease with the increase of temperature due to lattice vibration/phonon scattering of carriers. 71However, the covalently connected framework structure of COFs may inhibit electron-phonon scattering. 72Ghosh and coworkers 73 have synthesized two anthracene-based COFs named AntTTF and AntTTH (Figure 3A), respectively, because they have 9,10-bis(4-aminophenylethynyl)anthracene (AntT) as the π-conjugated structure unit connected with two kinds of distinct organic linkers 1,3,5-triformylbenzene (TF) and triformylphloroglucinol (TH).5][76] Therefore, in the geometric optimization during crystallization of crystals, it is observed that AntTTH appears a totally planar 2D πskeleton with high lattice symmetry, while AntTTF shows low lattice symmetry and nonplanarity, owing to the intramolecular hydrogen bond helps AntTTH keep planarity and realize high symmetry.The optical band gaps of AntTTF and AntTTH are 2.24 and 2.10 eV (Figure 3B), respectively.When it involves photocatalysis, photoconductivity (i.e., the mobility of electrons and holes generated by electromagnetic radiation) is also extremely vital.Researchers estimate the inherent charge carrier mobility of above COFs by flash photolysis timeresolved microwave conductivity (FP-TRMC).This method is not affected by contact resistance and grain boundary, and can be used to detect the charge dynamics of various kinds of materials under the excitation of laser pulses in a rapidly oscillating electric field. 77In such measurements, the product of the sum of charge carrier mobility and the quantum yield of charge carrier generation under light excitation is given. 78,79AntTTH shows a maximal inherent photoconductivity of 7.2 × 10 −5 cm 2 /(V•s) (Figure 3C), but the photoconductivity of AntTTF is extremely low since the nonplanar main chain limits the charge carrier delocalization.It is worth noting that the inherent photoconductivity of AntTTH is almost temperatureindependent (Figure 3D), its activated energy is lower than the thermal energy k B T (∼26 meV) at room temperature, and the mobility of charge carriers increases with the decrease in temperature.
Above all, it seems that COFs called semiconductors only exist in a subset of COFs.The electronic interaction between adjoining layers of COFs is attributed to the πstacked layered structure, which leads to distinctive electronic properties.When COF contains building blocks with π-function, its excited charge carriers will be transported along discrete and periodic columns, and such COF may be a semiconductor.Photocatalysts are a semiconductor in nature, and their band gap width decides the sunlight wavelength range that photocatalysts can absorb and utilize.Therefore, to heighten the utilization capacity of COFs-based photocatalysts for sunlight and apply COFs extensively in photocatalysis, it is necessary to regulate the band structure and optoelectronic properties of COFs at atomic and nanoscopic scales.Next, we will collect and discuss some essential insights about the impact of COFs structure on its band structure.

| Impact of COF structure on band structure
The band structure determines the light absorption and redox capacity of COFs as photocatalysts.CBM and VBM mainly determine the carriers' reduction and oxidation ability of COFs, respectively, and the ideal photocatalyst should have an appropriate band position.In COFs, the electronic transitions from the valence band to the conduction band include σ → σ* and π → π*.Typically, the transition from σ to σ* must be excited by ultraviolet light with higher energy, and visible light is enough for the transition from π to π*.To realize the effective utilization of solar energy, the light absorbed by COFs as photocatalyst should be near the visible range, and the electronic transition mode between the conduction band and valence band is corresponding to π → π*.For extended π conjugation, the energy required for π → π* electronic transition mode is comparatively low, resulting in the red-shifting of maximum absorption wavelength.The band structure of COFs is closely related to its π-delocalization degree and chemical composition, and the extended π-π conjugation is beneficial to πelectron delocalization.

| Linkage motif
It is one of the most effective methods to regulate the band structure of COFs by changing the linkage motifs coupled with the building blocks.Boron-containing bond is the initial linkage motif that results in COFs crystallization.This stable linkage motif provides a suitable tool for the generation of donor-acceptor COFs.Thomas et al. 80 prepared a new photoactive donoracceptor TP-Por COF by cocondensation of 2,3,6,7,10, 11-hexahydroxytriphenylene (HHTP) and 5,15-bis(4boronophenyl)porphyrin (1) (Figure 4A).The obtained TP-Por COFs shows light absorption covering almost the whole visible light region up to 680 nm (Figure 4B).The band structure corresponding to the donor-acceptor structure was obtained by calculating the sum of HOMO energy and the optical band gap measured by UV-Vis spectroscopy (Figure 4C).However, the conjugate loss and poor stability of this linkage motif limit its application in photocatalysis.The polymerization of amine and aldehyde forms layered COFs with imine bonds, which is analogous to borate COFs.Different from boron-containing COFs, COFs connected by imine bonds show higher stability.As amine and aldehyde linkages are available in large quantities and imine bonds generate π-conjugated building blocks, imine bonds have become one of the most appealing linking motifs in COFs.In 2009, Yaghi and collaborators 81 synthesized the first COF using imine bond as linkage motifs (COF-300) via copolymerization of Tetrahedral tetra-(4-anilyl)methane and linear terephthaldehyde building blocks.The existence of imine bond leads to a larger πconjugated system thus affects the band structure of COF.To enhance the charge carrier mobility of COFs, highly conjugated π-electronic porphyrin units and their metal derivatives participated in the construction of functional imine-linked COFs.The COF-66 and COF-366 with extended planar π-conjugated system have been obtained by solvothermal reaction of porphyrin with tetrahydroxy anthracene and TA, respectively. 82Compared with the instability of imine-based COFs in water, covalent triazine-based framework (CTFs) has high chemical and thermal stability which is a subclass of COFs.Thomas and coworkers have prepared CTF-1 by heating the mixture of aromatic nitrile and ZnCl 2 at high temperature (≥400°C). 83However, the CTFs synthesized by this way has almost no electronic band gap.To preserve the band gap, Cooper and collaborators 84 have synthesized four kinds of CTFs (Figure 4E) by condensation reaction between different aldehydes (1,4-benzene-dialdehye, 4,4'-biphenyl-dialdehyde, tris(4formylphenyl)-amine, and tris(4-formylbiphenyl)-amine) and amidine dihydrochloride with relatively mild conditions (≤120°C, without strong acid), and named as CTF-HUST-1, CTF-HUST-2, CTF-HUST-3, and CTF-HUST-4 respectively.The optical photos of these solid CTF-HUST materials are shown in Figure 4F-I.They can absorb light up to 650 nm in the visible region (Figure 4J), and the four samples' colors are in accordance with their absorption characteristics.The band gaps of four CTF-HUST are CTF-HUST-4 (2.13 eV) < CTF-HUST-3 (2.22 eV) < CTF-HUST-1 (2.33 eV) < CTF-HUST-2 (2.42 eV).The CTFs with various components have distinct optical band gaps in the CTF-HUST series, which indicates that the optical band gaps of CTFs can be adjusted by modifying the chemical composition.Changing the number of benzene rings in organic units has a significant effect on the band structure of CTF.Cooper et al. 85 have synthesized CTFs with different phenyl linkage units from phenylene to quarterphenylene, and their band gap values are related to the length of phenyl linkage units between adjacent triazine groups.With the increase in the number of phenyl units, the band gap of CTF decreases from 2.95 to 2.48 eV, and both CBM and VBM move to more negative positions.This phenomenon may be caused by the increase of π delocalization of phenyl linkage units.However, using phenyl as a linkage unit probably brings unfavorable effects.For example, the free-spinning aryl motif causes the reduction of planarity and the weakening of effective π conjugation, which limits the light absorption range due to the larger band gap.The fully π-conjugated COFs with sp 2 carbonconjugated linkages have the characteristics of πconjugated structure and stable C ═ C bond, which can transmit π-conjugation more effectively, thus enhancing π-electron delocalization.Jiang and coworkers 86 have synthesized a fully π-conjugated COF (sp 2 c-COF) via a C ═ C linkage motif formation reaction of 1,4phenylenediacetonitrile and tetrakis(4-formylphenyl) pyrene.In each layered structure of sp 2 c-COF with a band gap of 1.9 eV, π conjugation expands along the directions of x and y axes, forming a two-dimension πelectron conjugation system (Figure 4D).
Compared with imine-based Por-COF, Por-sp 2 c-COF appears red-shift phenomenon in the ultraviolet-visible absorption spectrum (Figure 5B) because the C ═ C connection is more efficient than the C ═ N connection in delivering π conjugate on a two-dimension skeleton.Furthermore, the CBM and VBM of Por-sp 2 c-COF are determined to be 3.5 and 5.21 eV versus vacuum level with a band gap of about 1.75 eV (Figure 5C,D).The fully conjugated three-dimension structure of sp 2 carbon-conjugated linkages can provide extended π conjugation along the x, y, and z directions at the same time, which can realize the transmission of electrons between layers.Cao et al. 88 have constructed the first threedimension COF (BUCT-COF-4) with C ═ C connection and full π-conjugated structure by Knoevenagel condensation polymerization of the saddle-shaped aldehyde-substituted cyclooctatetrathiophene and 1,4-phenylenediacetonitrile (Figure 5E).Solid-state ultraviolet absorption shows that BUCT-COF-4 has a wide absorption band and a narrow band gap (1.85 eV) (Figure 5F).

| Stacking mode
Compared with other amorphous photocatalysts, the π-π conjugated stacking of COFs can broaden the optical absorption range.The π-system stacking in COFs is another method to precisely regulate its band structure.Two typical stacking structures are J-aggregation and H-aggregation (Figure 6A). 89,90The H-or J-aggregations can be formed in COFs when chromophores exist in COFs and are stacked in a specific space.he monomer centers in H-aggregation are aligned on top of each other, while the monomer centers in Jaggregation deviate in a "head-to-tail" manner.2][93] The band gap of H-aggregation is broadened and the optical properties are blue-shifted (Figure 6B); Whereas the band gap of J-aggregation is narrowed, and the optical properties are red-shifted.Consequently, it can bring direct influence on the band structure of materials by controlling the stacking method.
According to the comparison between theory and experiment, Zwijnenburg and collaborators 94 have demonstrated that stacking is probably the main contributor to the absorption spectrum of condensed CTF.This kind of stacking with J-aggregation structures leads to red shift F I G U R E 6 (A) Schematic illustration of H-and J-aggregation structures and their corresponding photophysical processes.Reproduced with permission: Copyright 2020, Wiley-VCH Verlag GmbH & Co. KGaA. 89(B) The TIR-UV-vis spectra of H-and J-aggregation structures, taking the ZnTPPc at the aqueous-organic interface as an example.Reproduced with permission: Copyright 2021, American Chemical Society. 90n the absorption spectrum, the energy required for π → π* transition between conduction band and valence band decreases and the band gap narrows.The TT-Por COF with ordered porphyrin J-aggregation embedded in a solid matrix has been reported for the first time. 95No matter in the ultraviolet and visible light regions, or in the infrared spectrum, TT-Por COF shows unique optical characteristics.The ordered staircase-like stacking motif leads to fierce J-aggregation behavior in the porphyrin stack, which is trapped by the neighboring skeleton structure and dramatically changes the photoelectric properties of porphyrin (Figure 7A).In another research of COFs with porphyrin, 2D TPP COF has been synthesized by Schiff-base chemistry (Figure 7B). 96The strong J-aggregation stacking of porphyrins within the COFs changes the band structure and enhances the optical absorption.The Soret band and Q band in the UV-Vis absorption spectra display a red shift compared Comparison of absorption spectrum between TT-Por COF (blue) and its monomer Por (magenta).Reproduced with permission: Copyright 2018, American Chemical Society. 95 KGaA. 78ith the monomeric TPP (Figure 7C).The COFs show high light stability and wide spectral efficiency.The porphyrin-containing COF-366 synthesized by Yaghi and colleagues shows H-aggregation structure, 82 which can regulate band structure.The specific arrangement and stacking provide great potential for phthalocyanine-based COFs to obtain new functions. 37,97hthalocyanines are planar π-electron loops and superposed stacking endows aromatic-based COFs with tunable band gap. 98TAT-COF and TPB-COF are prepared from 2,4,6-trimethyl-1,3,5-triazine (TMTA) with 2,7,1 2 -trimethyl-5,10,15-triethyltriindole (TAT) and 1,3,5-tris (4-formylphenyl), respectively. 99Due to the charge transfer transition arising from the donor-acceptor pairs via π conjugation in skeleton and/ or interlayer stacking, the diffuse reflection UV-Visible spectra (Figure 7D) of the two COFs exhibit extended light absorption in the visible region compared with their corresponding monomers.There are obviously different aggregation structures between TAT-COF and TPB-COF (Figure 7E).The transition dipole moment in TAT-COF is delocalized, and its direction is opposite to the Reproduced with permission: Copyright 2019, The Royal Society of Chemistry. 103The refined unit cell parameters (E), and UV-Vis-NIR absorption (F) of Py-a4T, Py-4T TPD, and Py-4 T TT COF.Reproduced with permission: Copyright 2017, American Chemical Society. 104irection of intramolecular charge transfer, which narrows the exciton bandwidth between the bright transition and the lowest dark transition.Consequently, TAT-COF belongs to the category of J-aggregation, 100 and it can be explained that the absorption spectrum of TAT-COF is redshifted by the comparative experimental observation.By contrast, TPB-COF, as H-aggregation, shows the inverse experimental results.The intensity of charge transfer and transition oscillation are significantly enhanced, and the exciton bandwidth is expanded obviously.The Pc-PBBA COF has a square lattice composed of phthalocyanine macrocycles connected by phenylene bis (boric acid) linker, in which phthalocyanines are stacked in an overlapping manner, so the COF assumes an H-aggregation structure. 98Comparing with its monomer, Pc-PBBA COF with H-aggregation structure exhibits the induced blue shift of the maximum absorption in the near-infrared and visible light spectral regions, and the broadening of light absorption in the near-infrared region.The NiPc COF has H-aggregation structure due to the ordered stack of phthalocyanine units (Figure 7F). 78In comparison with the monomer precursor [(MeO) 8 PcNi], the B band of NiPc COF is blueshifted in absorption spectrum, while the Q band is redshifted (Figure 7G).The stacked phthalocyanine structure enables NiPc COF to exhibit stronger lightharvesting capabilities in long-wavelength visible light and near-infrared region.
To some degree, stacking distance in COFs is another parameter that can be adjusted, which controls the property and intensity of electron superposition of adjoining layers, thus affecting their band structure and photophysical properties.Dibenzochrysenetetraamine (DBCTA) is considered as a fused 4,4′,4″,4‴-(ethylene-1,1,2,2-tetrayl)-tetraaniline (ETTA) analogy because of its molecular configuration with four benzene rings converged to one naphthyl unit.What's different is that DBCTA can make the connected benzene rings rotate more minutely out of plane, so as to gain more compact components. 101,102TT DBC-COF has synthesized from thieno[3,2-b]thiophene-2,5-dicarboxaldehyde (TT) and dibenzo[g,p]chrysene (DBC) (Figure 8A). 103In contrast to TT ETTA-COF with higher spatial requirements, the stacking distance of adjoining layers of TT DBC-COF decreases sharply, from about 4.6 Å of TT ETTA-COF to about 3.6 Å of TT DBC-COF (Figure 8B).In the case of extended π conjugation within the COF layer, TT DBC-COF possesses a band gap with 2 eV, showing an apparent red shift in light absorption spectrum compared with TT ETTA-COF owing to the influence of tightly stacked molecular aggregations and biggish π-electron delocalization (Figure 8C,D).TT DBC-COF has extensive optical absorption, covering ultraviolet and the majority of visible spectrum.Thomas et al. 104 make asymmetric modification on the symmetric skeleton to minimize the spatial repulsion of tetra-thiophene (4T)-based COFs (Py-a4T, Py-4T TPD , and Py-4 T TT COF) and thus make thiophene layers pile up in close-packed alternating sequence (Figure 8E).It indicates that the band structure of these oligothiophene COFs can be accurately adjusted, resulting in the optical absorption of 4T-based COFs in the majority of visible light regions (Figure 8F).
The band structure of COFs has an important influence on its physical and chemical properties, especially in light absorption and charge transfer.However, appropriate thermodynamic factors (e.g., conduction band, valence band, and band gap) are not the only factors that determine the excellent performance of photocatalyst.In principle, the photocatalytic process involves four basic steps in dynamics, which are: (ⅰ) light absorption, (ⅱ) excitation and separation of photogenerated electrons/holes, (iii) electron/hole migration and bulk and surface recombination, and (iv) electron/ hole induced surface reduction/oxidation reaction.Besides thermodynamic factors, kinetic factors (e.g., photo-excited carrier dynamics, surface carrier utilization dynamics, adsorption/desorption, and diffusion dynamics) are also considered to be the key factors affecting the performance of photocatalyst.Although the structure of COFs has many advantages in photocatalysis, there are some problems in these fine structures composed of light elements, such as insufficient kinetics and lack of available catalyst sites due to the weak surface electron coupling and transfer ability between the reactive groups and the basic elements of its structural units, which usually limit their photocatalytic efficiency.To cope with this challenge, research has been done to design and develop high-efficiency COF photocatalyst by introducing low-dimension active sites into COF materials.It shows that confinement is a potential strategy for new active site construction, band structure regulation, and reaction kinetic optimization.The influence of nanoassembly effect on the performance of COF-based heterophotocatalyst will be discussed next.

| CONFINEMENT EFFECT ON HETERO-PHOTOCATALYST
6][107][108] The valence electrons of bulk solid materials with plenty atoms are continuous/quasi-continuous "energy bands."According to the quantum size effect, when the material is scaled down to nanoscale, the movement of electrons is bound and confined by space.This change of electron movement characteristics due to the confinement effect will lead to the transformation of the electronic structure of the system, particularly the valence electron structure, which may lead to quantum mutation. 109,110In brief, the confinement characteristics of COF-based photocatalytic system regulated by confinement modification strategy are essentially the changes of atomic arrangement, metal coordination, and electronic structure with confinement effect.
As porous materials, COFs have all kinds of potential coordination bonds, defects, and so forth, which are connected to low-dimension entities. 111,112The interaction between them realizes the stabilization of lowdimension entities.For example, Banerjee et al. 113 have grown Pd NPs in a stable COFs (Pd@TpBpy) via a judicious selection of Pd-confined tailored building units by in situ synthesis.The Pd NPs in Pd@TpBpy are stable through weak nonbonded interaction with pyridinic nitrogen of bipyridine unit.In addition, the pores/ channels in COFs provide internal conditions for space confinement to control the growth of active sites.Lu et al. 114 have obtained Rh/PC-COF by using the interfacial adhesion energy between Rh NPs and PC-COF to confine Rh NPs in the pores of PC-COF.Rh NPs with high dispersion and small particle size provide more active sites.The inherent advantage of COFs is that lowdimension entities can be uniformly and accurately installed on COFs.Taking COFs with permanent porosity as a carrier to anchor catalytic active sites and remain separated during the whole catalytic process will lead to the long-term stability of the catalyst.
One of the characteristics of the confined environment that affects the catalytic reaction is the steric hindrance caused by the transmission and diffusion of guest molecules, which can usually adjust the local concentration of reactants/products, thus leading to the enhancement of catalytic performance.As for photocatalysts with confined active sites in pores/channels of COFs, it is extremely important to guarantee the accessibility of substrates (reagents and products) to the confined active sites.There is no doubt that the limited diffusion of large-volume reaction substrates can supply superb settings for the selectivity of photocatalysts, that is, selectively allowing smaller substrates to pass through pores/channels and approach the confined active sites, while blocking biggish molecules.In photocatalysis, the reaction pathways over the surface of solid catalysts include diffusion and adsorption of reagents, surface reaction, and desorption of products.Any one of the basic steps may be kinetically related to the whole reaction.6][117][118] What's more, the confinement effect is able to regulate the entropy and enthalpy of the system, thus affecting the Gibbs free energy of reactants and transition states, and ultimately affecting the conversion rate. 119Figure 9 is a schematic diagram of Gibbs free energy for photocatalytic reaction in confined system and open system.Confined system has a higher absorption capacity due to the smaller energy barrier than open system.In addition, the formation energy barrier of *R 2 of confined system is smaller than that of open system.The lower formation energy barrier reveals that the introduction of confined space is beneficial to the dynamic charge transfer photocatalysts.Therefore, the active sites where electrons gather in confined photocatalysts can effectively immobilize *R 2 species, thus enhancing the photocatalytic reaction performance.
On some level, the catalytic reaction is determined by two reaction rates, namely, the reaction rate determined by the most abundant surface intermediates and activation energies of reaction.Once the reaction is carried out in a confined space, these parameters will change.It has been found that the molecular adsorption energy in the confined channel is remarkably stronger than that on the plane, which will significantly affect the particle number and energy level of the reaction intermediates.That is, the reaction rate will change in a finite environment. 120,1213][124] Confinement effect can vary the reaction in varying degrees, which is dependent on the shape and size of pores and the affinity of the confined catalyst for reactant molecules.
The physical/chemical properties of molecules/ions will change fundamentally due to the space limitation and the surrounding coordination environment.The atomic interaction in confined space varies greatly as compared with the open system.The electronic modification of the host (e.g., pore structure) to the guest (e.g., low-dimension entities) can be substantially changed with the variation of the interaction between the confined active sites and COFs.As an example, Zou et al. 125 have synthesized Ni-TpBpy by confining a single Ni site in TPBpy through nickel atom and nitrogen atom coordination (Figure 10A).The pyridine coordination unit in Ni-TpBpy stabilizes the catalytic active Ni sites, and there is interaction between Ni and N (Figure 10B).After introducing Ni species into TpBpy, the crystal structure of TpBpy remains in Ni-TpBpy, and there are no obvious Ni species such as NiO and metallic Ni impurities on Ni-TpBpy except single Ni ion at atomic level (≈0.17 nm).It is further found that the Ni site in confined space shortens the diffusion distance of photo-generated carriers and inhibits the recombination of electronhole pairs.In addition, after confining Ni sites to TpBpy, the band gap of photocatalyst is narrowed and the light absorption range is widened due to the increase of π-delocalization.Owing to the synergistic effect of single Ni sites and TpBpy support, Ni-TpBpy exhibits excellent photocatalytic selective reduction of CO 2 to CO in pure CO 2 or dilute CO 2 atmosphere.The pivotal step of catalysis involves the carriers transfer between the reactants and the catalyst surface.From a conceptual point of view, low-dimension entities can be embedded into COFs as electron acceptors/donors with different ionization potential/electron affinities, which can accomplish carrier separation and transmission.he macroscopic catalyst is in a neutral and stable state.But on the microscopic level, the catalyst itself may have a paradoxical energy confinement system.Such as "rich" and "poor" electrons and "strong" and "weak" coordination can form a certain energy confinement field, which acts on the active center, thus endowing the catalyst with high activity.Wang et al. 126 have utilized Pd-N coordination to confine Pd within the imine-linked COF-LZU1(Figure 10C).Due to the strong coordination between Pd and COF-LZU1, the imine group provides electrons to Pd(II), which reduces the electron deficiency of Pd species.Likewise, in the crystalline porous structure of COFs, the difference of coordination intensities of metal ions can also introduce energy confinement.The ultra-fine Pd NPs and Pt NPs confined in the adjustable CTF platform are used as cocatalysts for photocatalytic hydrogen evolution reaction (Figure 11A-E). 127There are differences in coordination strength between Pd/Pt and pyridine nitrogen in CTF.Because of the strong interaction between metal Pd and pyridine nitrogen, metal Pd is more challenging to reoxidize to Pd(II) than metal Pt under photocatalysis, and the size of Pd NPs formed in CTF is smaller than that of Pt NPs.The smaller Pd NPs are more conducive to the separation of photo-generated carriers, and the more potent interaction between Pd NPs and CTF is also more conducive to photo-electron transfer.Therefore, the hydrogen evolution performance of Pt@CTFs is better than that of Pt@CTFs in the same conditions.
On the whole, the performance of these COF-based confined hetero-photocatalysts relies on confinement effect, such as electronic effect, 130 constraint effect, 131 and molecular-sieving effect, 132 which will optimize the photocatalyst's electronic structure and catalytic performance.

| COF-BASED HETERO-PHOTOCATALYST FOR CONFINEMENT MODIFICATION STRATEGY
Catalysis designed by confinement modification strategy has been proved to present all kinds of alluring properties in aspects of reaction activity, selectivity, and stability by experiments and theories. 133,134In this part, we have further explained the superiority of confinement modification strategy based on COF materials and the influence of confinement on different photocatalytic reactions (Table 1) according to the classification of the spatial dimension classification of low-dimension entities.

| Confinement of COFs on NPs
][166] Although NPs catalysts show many excellent properties, they are prone to noticeable morphological or structural changes due to different harsh reaction environments.The former includes the sintering phenomenon that will lead to surface area reduction, which will lower catalyst activity and change selectivity.Generally, there are two ways to sinter NPs: one is Ostwald ripening [167][168][169] ; Another one is the diffusion of the entire NP and the subsequent merger with other NPs. 170,171It is shown that the intense interaction between NPs and COF materials can lessen sintering or spill-over phenomena, and then enhance the dispersibility and stability of NPs.In addition, the availability of coordination sites for uniform nucleation of NPs in the internal microenvironment, chemical adjustability, high porosity, and ordered structure integration make COFs become ideal supports for confining NPs. 172,1735][176] As an example, TpPa-COF with abundant and evenly dispersed imine groups is an ideal candidate for confining metal NPs. 177e existence of functional confining groups within COFs pores will promote the binding of NPs and the formation of nucleation sites in pores, thus realizing the confined growth of NPs and regulating their particle size.Zhang et al. 178 have synthesized Thio-COF linked with imine, and the sulfide groups evenly distributed in its pore channels promote the confined growth of Pt/Pd NPs through metal-sulfur coordination interaction.The covalent triazine frameworks (CTFs) synthesized from triazine bonds (C ═ N) are abundant in pyridinic nitrogen with an unshared localized electron pair, which provides a tremendous advantage for stabilizing metal NPs. 179,180 series of Pd@CTFs and Pt@CTFs catalysts are designed and synthesized on the platform of tunable CTF with different pyridinic nitrogen contents for photocatalytic hydrogen evolution (Figure 11A).127 The distribution of Pd NPs and Pt NPs on CTF is shown in Figure 11B,C.The Pd and Pt NPs are evenly dispersed on CTF platform without any agglomeration.CTF-HC2 and CTF-HC6 are typical CTFs and have identical structures except for the different content of pyridinic nitrogen.The cornucopian nitrogen content is the crucial factor that affects the electronic band structure and optical absorption properties of CTFs.181 The intensity of interaction between pyridine and metal Pd/Pt plays a prominent role in crystal growth and particle size.The interaction between Pd and pyridinic nitrogen is stronger due to the instability of Pd.It is more difficult for Pd metal to reoxidize to Pd(II) under photocatalysis, resulting in smaller Pd NPs embedded in CTF materials.In contrast, the relatively weak interaction between Pt and pyridinic nitrogen of CTFs leads to the formation of relatively large Pt NPs (Figure 11D,E).The size of Pt and Pd NPs loaded on CTF-HC6 is smaller than those loaded on CTF-HC2, because the nitrogen content of CTF-HC6 is higher than that of CTF-HC2. Smaler Pd NPs promote more effective carrier charge separation and prolong carrier lifetime.The more intense interaction between Pd and pyridinic nitrogen is helpful in building a tight interface between Pd NPs and CTF support, thus facilitating more rapid photoelectron transfer.At the same experimental conditions, the hydrogen evolution rate of Pd@CTF-HC6 is 11 times higher than the Pt@CTF-HC6 when the contents of Pd and Pt are both 1 wt%.Likewise, the hydrogen evolution rate of Pd@CTF-N is approximately 10,556 μmol/(h•g) and is five times as high as Pt@CTF-N.
The interaction between the special functional groups in metal NPs and COF materials can effectively disperse and fix NPs in the pore structure.The stable CTFs is a typical COFs composed of triazine units.Wu et al. 128 have loaded Pt NPs on CTF with graphene-like layered morphology as photocatalyst (CTF-T1) for hydrogen evolution reaction.The Pt NPs are well dispersed on T A B L E 1 Summary of COF-based confined hetero-photocatalysts for typical studies on photocatalytic application.Hydrogen evolution rate is 10,556 μmol/(h•g).[127]   Pd nanoparticles Pd 0 /TpPa-1

eV
The interaction between Pt single atoms and TpPa-1-COF via C 3 N-Pt coordination.Hydrogen evolution rate is 719 µmol/(h•g).[140]   Cu single atoms Cu@TpTG-iCON The strong chelated coordination interaction between N and Cu.

eV
The interaction between PdIn NCs and N 3 -COF.

eV
The interaction between Au and COF-S-SH via Au-S bond.The degradation efficiency of Rhodamine B (RhB) is 97.3%.CTF due to the effective constraint of the inner cavities of CTFs and the interactions between triazine group and Pt NPs, which is completely different from the case where aggregated Pt NPs are observed on g-C 3 N 4 .It is shown in Figure 11F that the maximum light absorption of CTF-T1 is observed in the spectral range of 350-380 nm due to the π-π* transition of the conjugated ring system, and the optical band gap corresponding to its absorption edge is about 2.94 eV.Such band gap is sufficient to drive the water-splitting reaction in theory. 182Triazine and phenyl of CTF-1 are coplanar, so they are strongly conjugated.HOMO energy level and LUMO energy level are shared by triazine and phenyl, and the electron distribution on them has good electron separation state and overlapping orbit, which is very favorable to intramolecular charge transfer transition.At the same reaction conditions, the prepared catalyst exhibits an average hydrogen evolution rate (200 μmol/[h•g]) equivalent to block-type g-C 3 N 4 within 20 h.Promoting photo-generated electrons transfer to effective photocatalytic active sites can improve the photocatalytic hydrogen evolution efficiency.Lu et al. 129 have obtained Pd 0 /TpPa-1 catalyst by confining Pd NPs to TpPa-1 (Figure 11G).The crystal structure of TpPa-1 did not change after Pd NPs was confined on TpPa-1, and Pd NPs were uniformly dispersed on COFs.This strategy improves the transfer of photo-generated carriers in photocatalyst, and photo-generated electrons are transferred from TpPa-1 to Pd active sites for hydrogen evolution.The sequential porous structure with evenly dispersive Pd active sites supplies a prerequisite for efficient photocatalytic hydrogen evolution (Figure 11H).The electronic properties of photocatalyst can be changed by confining metal NPs on COFs, which can effectively promote electron-hole separation and transmission, and reduce photo-generated carrier recombination.
It can be said that the strong interaction between single-component NPs and special sites in COFs makes single-component NPs stable and uniformly dispersed on COFs and does not change the structure of COFs.However, most single-component NPs are precious metals, and their industrial application is hindered by their expensive cost, which is not suitable for large-scaled production.Therefore, it is very vital to explore multicomponent NPs that are cost-effective and meet practical applications.Cadmium sulfide (CdS) is a kind of semiconductor material with low price and easy preparation, and its electronic properties have obvious size dependence.Kurungot and collaborators 137 have taken advantage of the existence of heteroatoms in highly stable COF (TpPa-2) to confine CdS NPs.The existence of π-conjugated skeleton, high surface area, and strong interaction between TpPa-2 and CdS NPs make CdS-COF hybrid exhibit enhanced photocatalytic H 2 evolution compared with bulk CdS.Zou et al. 136 also have immobilized the uniformly dispersed CdS NPs on CTF-1 through strong metal-nitrogen interaction to obtain CdS-CTF-1 (Figure 12A).The confinement effect of CTF-1 on CdS NPs inhibits the photocorrosion and aggregation of CdS NPs, and the introduction of CdS NPs does not affect the structure of CTF-1 (Figure 12B).Under visible light irradiation, the excellent photocatalytic activity of CdS-CTF-1 for hydrogen evolution is attributed to the more active centers exposed and the interaction with CTF-1 and CdS NPs.The optical absorption of CdS-CTF-1 in the region of 470-650 nm is enhanced compared with the original CdS, and on the contrary, it shows contraction compared with the pure CTF-1 (Figure 12C).The matched band potential between CTF-1 and CdS (Figure 12D) leads to the effective separation of photo-generated electron-hole pairs.The charge carrier life of CdS-CTF-1 is longer and the efficiency of charge transfer and separation is faster than that of the original CdS NPs.In addition, the optimum CdS-CTF-1 has no obvious activity loss after three cycles of testing (Figure 12E).Thereby, the efficacious transport and separation of photo-generated carriers can enhance the photocatalytic activity of the composites.
Copper and cobalt are relatively inexpensive metals compared to precious metals.CuCo 2 O 4 nanostructure with inverse spinel structure has fine catalytically active property. 183Bi et al. 143  CTF-1 which is favorable for augmenting the CO 2 concentration on the surface (Figure 12F).The multicomponent CuCo 2 O 4 contains abundant partially-filled 3d orbitals (e.g., Cu 2+ (3d 9 ) and Co 3+ (3d 6 )), which makes it easily enable to accept electrons, thus prolonging the carrier lifespan. 184,185Furthermore, the electron affinity is increased due to the presence of cobalt ions, which is conducive to speed up charge separation. 186,187The catalyst prepared by confining CuCo 2 O 4 NPs on CTF-1 is undoubtedly beneficial for photocatalytic reaction.COFs containing abundant heteroatoms can confine most metal atoms through coordination bonds, thus regulating the reaction dynamics of photocatalysts.Cai et al. 156 have confined Pd NPs in a benzothiazole-linked COF (TTT-COF) through the binding interaction between metal Pd atom and S atom to obtain hybrid material Pd NPs@TTT-COF with Mott-Schottky heterojunction (Figure 13A).All the characteristic peaks of TTT-COF are retained in the FT-IR spectrum (Figure 13E) of Pd NPs@TTT-COF.Furthermore, compared with the particle size of Pd NPs in TTT-COF linked with imine, Pd NPs in TTT-COF are finer since S atoms have a more robust interaction with Pd NPs than N atoms (Figure 13B-D).Under the irradiation of visible light, Pd NPs@TTT-COF has shown remarkable conversion rates (82%-99%) for various substrates in the photocatalytic C-C cross-coupling reactions.After Pd NPs@TTT-COF has undergone four times photocatalytic reaction, no significant variations have been observed in PXRD and IR images before and after the reaction (Figure 13F).Pd NPs@TTT-COF has shown exceptional stability and reusability.In Pd NPs@TTT-COF, the more accessible catalytic sites, lower recombination rate of photo-generated carriers, and better separation and mobility of photo-generated carriers play essential roles in these reactions.

| Confinement of COFs on nanoclusters
When metals or semiconductors are scaled down to only 10-100 atoms, they become "a new kind of materials," which are called nanoclusters (NCs). 188NC is an important transition state from single atom to nanoparticle, which represents the initial state of condensed matter.NC is a group consisting of two or more atoms through physical or chemical bonding force, which exceeds the traditional boundaries of physics and chemistry.NCs are not molecules that chemists think, and different from bulk materials, they are too small for atoms to fully enter the periodic crystal structure.The small size of NCs leads to the disorder of surface atoms, and the electrons can only move in the space of a few atom widths which determines the possible electronic energy levels.Compared with isolated single atom, the NC can accommodate more metal atoms, thus further increasing the metal loading.Moreover, due to the minute size of NCs, they can not only maintain the maximum utilization rate of atoms but also provide multiple active sites.The unique electronic and geometric structure of NCs makes them exhibit enhanced catalytic activity and specified selectivity in the photocatalytic process.Affected by quantum size effect, NCs have larger surface energy than NPs which leads to higher activity.0][191] NCs can be confined to COF via confinement modification strategy, which not only solves the issues of uniform dispersion and overall stability but also significantly improves the performance of photocatalyst.
Reducing NPs into NCs can not only enhance the utilization ratio of metal atoms but also regulate the coordination environment and electronic structure of metal atoms, thus impacting the catalyst activity.Confinement modification strategy is a method that can improve the catalyst activity and reduce the total cost by increasing the dispersion/utilization of precious metals.By metalnitrogen coordination, Mei et al. 192 have successfully immobilized Pd NCs in the pores of bipyridine covalent triazine framework (CTF-BPDA-TPDH) (Figure 14A).The porous structure and customized N sites of CTF are conducive to the in-situ formation of monodisperse Pd NCs confined in CTF pores, which helps maximize the utilization of Pd, stabilize Pd NCs, and promote the transfer of electrons from Pd NCs to CTF through a ligand effect.Similarly, Yang and collaborators 193 have employed Cu-N coordination to confine Cu NCs to bipyridinemodified CTF-B and accordingly change the electronic structure of nitrogen sites.Nitrogen-containing ligands may provide isolated electron pairs metallic Cu species and decrease the charge density of coordinated N atoms, which strengthens the interaction between Cu NCs and CTFs.In consequence, COFs with rationally designed organic ligands and porous frameworks can provide a unique space-confined environment for stabilizing metal NCs.To improve the activity of semiconductor in photocatalytic hydrogen evolution, Yang and collaborators 139 have reduced the diameter of metal Pd NPs from 3.3 nm to single atoms/clusters (SAs/Cs) and loaded it on TP-TTA COF (Pd x /TP-TTA/SiO 2 , x is Pd loading in wt%), which has increased the photocatalytic hydrogen evolution activity from 47.7 to 85.5 μmol/h (Figure 14B).Regarding the coordination environment of Pd SAs/Cs, the most stable structure is that Pd species coordinates with two contiguous carbon atoms of TP-TTA, which are respectively derived from the linker neighboring the nitrogen atom and 1,3,5-tricarbonyl cyclohexane.Pd 0.033 / TP-TTA/SiO 2 is the photocatalyst with the optimum performance, and its visible light absorption range is broadened because the band gap of the photocatalyst is narrowed by the presence of Pd SAs/Cs (Figure 14C).Pd SAs/Cs with high electron-accepting ability can be used as charge carrier trap centers, which can not only improve the separation efficiency of photo-generated carriers but also boost the transfer of photo-generated carriers (Figure 14D-F).
The functionalized organic skeleton of COFs can effectively stabilize NCs via a covalent link.To overcome the poor stability of MnMo 6 NCs in photocatalytic applications, Lan et al. 144 have synthesized TTF-TAPT COF-MnMo 6 (TCOF-MnMo 6 for short) by confining subnanometer MnMo 6 clusters in crystalline COFs via covalent bonds (Figure 15A).This confinement modification strategy not only realizes the stable constraint of MnMo 6 NCs in nanopores of COF (Figure 15B) but also realizes the high dispersion of MnMo 6 NCs in the entire COF material (Figure 15C).Uniform MnMo 6 NCs are confined in channels of COF, and there is a direct interaction between COF lattice and MnMo 6 NCs.As shown in Figure 15D, confining MnMo 6 NCs in COFs not only does not make changes to the structure of COFs but also strengthens the stability of MnMo 6 NCs and inhibits the aggregation of MnMo 6 NCs.TCOF-MnMo 6 has a band gap of 1.46 eV, which can effectively broaden the wavelength range of light absorption (Figure 15G) AND elevate the utilization efficiency of light.The HOMO level of TCOF-MnMo 6 is +0.89 eV and the LUMO level is −0.57eV (Figure 15E), which theoretically satisfies the demands of CO 2 -CO reaction.In the atmosphere of CO 2 and steam, TCOF-MnMo 6 as a photocatalyst exhibits great activity (37.25 μmol/[h•g]) and approximately 100% selectivity for the reduction of CO 2 to CO.The highly dispersive MnMo 6 NCs in the TCOF-MnMo 6 structure can be taken as the active site for photocatalytic CO 2 reduction.In principle, semiconductors with broadband gaps need to be irradiated with high-energy light to excite photo-electrons.The photo-induced excited electrons and holes are separated under the irradiation of visible light, and photoelectrons subsequently transfer along the  139 path of the HOMO center-LUMO center-CO 2 reduction sites.Confining MnMo 6 NCs in TCOF obviously inhibits the recombination of photo-generated electron-hole pairs due to the formation of electron transfer path between TCOF and MnMo 6 ; and the existence of covalent bond between TCOF and MnMo 6 greatly enhances electron transfer (Figure 15F).Thanks to the confinement effect, the final photocatalytic CO 2 reduction performance of TCOF-MnMo 6 is determined by the combined action of many thermodynamic and kinetic factors.
Confining NCs in COFs to construct active sites provides an extraordinary chance to enhance the motivation for numerous reactions with the influence of size effect and synergistic effect.Lu et al. 146 have encapsulated PdIn bimetallic NCs with high activity and superfine into photosensitive N 3 -COF, and obtained a series of Pd x In y @N 3 -COF (x:y = 1:0, 0:1, 1:2, 1:1, 2:1) composites (Figure 16A).Furthermore, due to the strong affinity between CO 2 molecules and PdIn NCs, the CO 2 adsorption capability of PdIn@N 3 -COF is strengthened (Figure 16D).PdIn@N 3 -COF complex can be used to photoreduce CO 2 to alcohols with H 2 O as e − and h + sources, whose overall yield of CH 3 OH (74%) and CH 3 CH 2 OH (26%) is 798 μmol/g (Figure 16B,C).The synergistic effect in PdIn@N 3 -COF has dramatically promoted the C-C coupling and interfacial charge transfer (Figure 16E-G), thus implementing superior activity of converting CO 2 into C 1 /C 2 alcohols over noncopper catalysts.
The modified COF materials can generally form strong interaction bond bridges with metal species to improve the catalyst stability and carrier separation  144 efficiency.Lu and collaborators 158 have utilized the Au-S bond to confine the ultra-small Au NCs in COFcontaining thiol chain (COF-S-SH) to obtain Au@-COF.The ultra-small pores of COF and the intense S-Au binding energy increase the dispersion of Au NCs and ensure the photostability of Au NCs.The Au-S-COF bond bridges formed between Au NCs and COF have successfully constructed the Z-scheme photocatalytic system (Figure 17A), which improves the separation efficiency of photo-generated carriers.Compared with COF(COF-V) without thiol modification, the adsorption performance of Au@COF on Rhodamine B (RhB) is relatively low as the pore structure of COF is blocked with Au NCs (Figure 17D).But in the photodegradation experiment of RhB, the degradation efficiency of Au@COF under visible light is 97.3% (Figure 17C), which is twice that of COF-V.In addition, the degradation efficiency of RhB by Au@COF still has remained above 95.4% after 5 cycles (Figure 17E).The band structures of Au NCs and COF-V (Figure 17B) indicate that they are unable to satisfy the conditions of photodegradation of RhB completely.However, confining Au NCs in COF enables the band structure of photocatalyst to be efficiently adjusted and the electron-hole recombination of Au NCs to be inhibited.The enhancement of photocatalytic performance of Au@-COF is mainly due to its large number of accessible active sites, better charge transfer ability, and higher photo-generated carrier separation ability.

| Confinement of COFs on single atoms
Up to now, single atom is the terminal small size limit of metal species particles.Unlike NCs and NPs, isolated and dispersed atoms are active substances in some reactions. 194,195The efficiency can be maximized in principle by reducing the size of metal species catalysts to sub-nanometer clusters or even isolated dispersed atoms. 196,197However, the surface free energy of metal species increases as their particle size decreases, which makes them move easily and tend to aggregate into more stable NCs or even NPs under actual reaction conditions.This kind of aggregation can be prevented by confining well-defined single atoms to suitable supporting materials, thus forming stable and fine-dispersed single atoms.
COFs-based single-atom confined photocatalyst shows better performance because of its superior porous structure compared with many single atoms based on other carriers.COFs with special coordination environment and permanent porosity can be used as functional supports to provide enough periodic coordination sites for confined metal single atoms, which is conducive to improving the stability and actual loading of noble metal single atoms.Kurungot and coworkers 198 have utilized Co-N coordination to confine the active Co single atoms in the COF containing bipyridine (Co-TpBpy) as OER catalyst (Figure 18A).COFs with crystalline porous microstructure offer a defined coordination circumstance to hold metal single atoms, but the coordination strength difference between metal and COF will affect the catalyst's performance.Lan and collaborators 152 have confined a series of transition metal single atoms inside anthraquinone-contained COF, and obtained DQTP COFs (DQTP COF-M, M ═ Co, Ni, Zn) (Figure 18B).Anthraquinone groups have strong metal bonding ability, and quinone oxygen atoms in adjoining layers   140 have confined Pt metal atoms within the pore wall of TpPa-1-COF to obtain Pt 1 @TpPa-1.In TpPa-1-COF, the pore wall rich in heteroatoms (C, N, and O) provides a special coordination environment near the isolated Pt center.Pt atom coordinates with one nitrogen, three carbons, and two chlorine atoms to form a special six-coordinated C 3 N-Pt-C l2 specie (Figure 18C), which leads to the formation of atom-level dispersed Pt on TpPa-1-COF support, and these Pt atoms have positive valence state.Compared with pure TpPa-1, Pt 1 @TpPa-1 indicates a narrower band gap and slightly wider light absorption region (Figure 18E,F) due to the hybridization of C 2p and N 2p orbitals of TpPa-1 with d orbitals of single-atom Pt.0][201] The optimal Pt 1 @TpPa-1 catalyst has a photocatalytic activity of 719 µmol/(h•g) (Figure 18G) and a large turnover frequency of 19.5 h -1 under visible light illumination.Its activity is 3.The coordination bond between metal single atom and special site of COFs can effectively stabilize metal atom.When metal single atoms are confined to COFs, these metal single atoms are very sensitive to the electronic and geometric interaction with COFs atoms.The electronic structure of the metal sites in coordination compounds strongly relies on the coordination environment, which will have a dramatic influence on the photocatalytic performance.The synergic coordination of the N sites of CTFs to metal single atoms and the confinement effect of the skeletons can stabilize the single atoms and affect photocatalytic performance.Pt-SA/CTF-1 is a catalyst obtained by confining Pt single atoms on CTF modified by ethylene glycol (EG) (Figure 19A). 147The Pt-SA/CTF-1 maintains a graphene-like layered structure, and no impurities of Pt NPs or other aggregates are observed on the skeleton of CTF except uniformly dispersed Pt single atoms.As shown in the Figure 19B Copyright 2022, Elsevier. 147orm an isolated Pt-N-C 2 coordination structure with one N atom and two C atoms which means that an unoccupied sp 3 orbital of Pt atom is used to interact with carbon dioxide molecules and thus become an active site for photocatalytic CO 2 reduction.At the same reaction conditions, the CO 2 -CH 4 conversion rate of Pt-SA/CTF-1 is about 26 and four times higher than CTF-1 and Pt-NP/CTF-1 (Figure 19C), respectively.In addition, when Pt-SA/CTF-1 is used as photocatalyst, the selectivity of CH 4 precipitation is about 1.74 times that of Pt-NP/ CTF-1.As compared with CTF-1 and Pt-NP/CTF-1, Pt-SA/CTF-1 shows lower interface charge transfer resistance, higher photocurrent density, and larger effective surface area.These phenomena indicate that confining Pt single atom to CTF-1 can effectively improve carrier separation and migration efficiency.In addition, the comprehensive effect of porous structure of CTF-1 and Lewis acid-base interaction between CO 2 and Pt single atom improves the affinity of Pt-SA/CTF-1 for CO 2 adsorption, which facilitates the formation of Pt-CO 2 adducts.Confining Pt single atom on CTF-1 reduces the reaction barrier of CO 2 -CH 4 and improves the photocatalytic activity of Pt-SA/CTF-1.
Introducing single-atom catalysts into porous COFs materials can stabilize the single-atom catalysts and promote the electron transfer between COFs and catalytic sites, thus enhancing the photocatalytic performance.Fan and collaborators 148 have confined singleatom Ru to CTFs with photoresponse characteristics through Ru-N 2 , and the obtained Ru-CTF catalyst has been used for highly selective photoreduction of CO 2 to formic acid (Figure 20A).The conversion rate of Ru-CTF is 2090 μmol/(g cat •h), and the selectivity is 98.5% (Figure 20B,C), which is far superior to most other reported photocatalysts.The light absorption is enhanced after the introduction of Ru, which is due to the metal-ligand charge transfer of Ru units (Figure 20D).In addition, the formation of Ru-N 2 coordination in Ru-CTF can inhibit the rapid recombination of photogenerated electron-hole pairs and boost the electron transfer between the Ru active sites and CTF support, thus obtaining superior photocatalytic performance (Figure 20E,F).In-situ attenuated total reflectioninfrared (ATR-IR) (Figure 20G) and density functional theory calculation results show that the Ru-N 2 can enhance the adsorption and activation of CO 2 , and it is also verified that replacing the Ru-N 2 with H 2 O is more conducive to the formation of crucial intermediate which reacts with CO 2 , and its free energy barrier is much lower than dicarbonyl precursor, thus realizing the better catalytic activity of Ru-CTF for CO 2 RR (Figure 20H).
There are relatively low reserves of precious metals on the planet, which are insufficient to support the mass production of catalysts with noble metals as active sites.To satisfy the production demand, the abundant nonnoble metals on the planet become the candidate materials for developing cost-effective catalysts.For COFs containing vast heteroatoms (e.g., N, S, O), single atoms are often stabilized by coordination bonds.Hou et al. 149 21F).Furthermore, the introduced Fe sites have strong affinity for CO 2 , which effectively inhibits the evolution of competitive H 2 .
In general, the interaction between support and metal determines the stability of the prepared single-atom catalyst.To keep the single atom state of metal copper atoms, it is necessary to choose a proper support.CTF-1 with mesoporous and nitrogen-rich triazine ring structure is a candidate support for confining Cu single atoms.Bi et al. 150 have constructed Cu SA/CTF photocatalyst with confined Cu atoms by a photochemical reduction method and applied it to photocatalytic CO 2 conversion.The Cu atom coordinates with two C atoms and one N atom on CTF-1 support to form an isolated coordination structure of Cu-N-C 2 (Figure 21G).The coordination interaction of Cu-N accelerates the charge transfer on Copyright 2022, Elsevier. 148u-SA/CTF, and then determines the stability of Cu single atom.The main products of Cu-SA/CTF photocatalyst in visible-light-driven CO 2 reduction system are CH 4 and a trace quantity of CO.The single-atom Cu keeps a vacant sp 3 orbital to interact with CO 2 molecules due to the coordination structure of Cu-N-C 2 , which is deemed as active sites in the photo-reduction of carbon dioxide reaction.The introduction of Cu as active center and electron donor into CTF-1 greatly improved the Lewis basicity of CTF-1, thus effectively enhancing the physical and chemical adsorption capacity of Cu-SA/CTF for CO 2 molecules (Figure 21H).The Cu-SA/CTF exhibits enhanced absorption of visible light compared with the original CTF-1, which can be attributed to the band-gap narrowing of the sample after the introduction of Cu species and further improving the ability of capturing visible light and photo-generated electrons.As shown in the Figure 21I, the CH 4 yield of the optimal Cu-SA/CTF can reach 32.56 µmol/(g•h), which is 19.7 times that of the original CTF-1, and the CH 4 selectivity is 98.31%.The Cu-SA/CTF photocatalyst has good stability in CO 2 reduction system and can be reused for many times.To sum up, the confined of Cu single atom on CTF-1 can effectively broaden the visible light response scope and boost the separation efficiency and lifetime of photo-generated carriers, and augment the adsorption capacity of CO 2 molecules, thus substantially elevating the photocatalytic performance of CO 2 reduction to CH 4 .
COFs with periodic and permanent porosity are capable of accommodating quantities of single active sites and effectively foster the transfer of photoinduced carriers to active sites.When the metal atoms become the active centers of photocatalyst, the metal-nitrogen coordination bonds can stabilize the metal atoms.Maji and coworkers 163 have carried out double metallization of iridium and nickel on TpBpy COF containing chelated bipyridine sites (Figure 22A), which confined the single-atom Ni and Ir within the COF pore via metal-nitrogen coordination, thus

| SUMMARY AND PROSPECT
COFs have vast prospects in the development of confined photocatalytic systems.In this paper, we have summarized the band structure of COFs and the application of low-dimension entities (including NPs, NCs, and single atoms) confined in COFs as active sites in various photocatalytic reactions, and further discussed the influence of confinement modification strategy on the performance of the confined photocatalytic system based on COFs.As for COFs themselves, COFs with semiconductor properties are only a subset of COFs, and their linkage motif and stacking behavior have influence on the band structure.No matter in thermodynamics or dynamics, confinement effect can modulate the photocatalytic performance.On the one hand, the lowdimension entities are confined in COF can change the band structure of the photocatalyst from a thermodynamic point of view; on the other hand, the confinement modification strategy is able to address the lack of active sites and insufficient dynamics of COFs to some extent.Low-dimension entities, which are dominated by size effect, are stably and evenly dispersed over the COFs, which may inhibit their agglomeration and sintering in harsh treatment environments or catalytic processes, and achieve the maximum utilization of active sites.In addition, confinement characteristics dominate the molecular structure, atomic arrangement, electron transfer, and coordination of substances in space, which may improve the dynamics of photocatalytic reaction, including photoexcited carrier dynamics, surface electron-hole utilization dynamics, adsorption/desorption, and diffusion dynamics.Although the research on COF-based confined hetero-photocatalysts is burgeoning and a great many innovative works have been published, challenges remain to be addressed.

F I G U R E 1
Schematic diagram of basic structure (A) and catalytic mechanism (B) of covalent organic framework (COF).(C) Summary diagram of band gap of typical COFs and related COF-based confined hetero-photocatalysts.(D) Schematic diagram of confinement modification strategy for COFs materials.YANG ET AL. | 3 of 43

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I G U R E 3 (A) Synthesis of AntTTH and AntTTF.(B) Flash photolysis time-resolved microwave conductivity (FP-TRMC) transient decay curve of AntTTH covalent organic framework (COF).(C) Temperature-dependent FP-TRMC and Arrhenius plot of intrinsic photoconductivity.(D) Wavelength dependent on-off switching of photocurrent.Reproduced with permission: Copyright 2022, American Chemical Society. 73YANG ET AL. | 7 of 43 F I G U R E 4 (See caption on next page)

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I G U R E 8 (A) Synthesis of the dibenzo[g,p]chrysene (DBC)-containing covalent organic framework (COF).(B) The stacking distance of adjoining layers of TT DBC-COF.Optical absorption spectrum of the DBC-COFs (C), and the corresponding analogous 4PE COFs (D).

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I G U R E 10 Schematic diagram photocatalytic selective reduction of CO 2 over Ni-TpBpy.(B) N 1 s XPS spectra of TpBpy and Ni-TpBpy.Reproduced with permission: Copyright 2019, American Chemical Society. 125(C) Schematic diagram of Pd/COF-LZU1 possessing regular microporous channels.Reproduced with permission: Copyright 2011, American Chemical Society.
Photocatalytichydrogen generation Pd nanoparticles with two contiguous carbon atoms of TP-TTA.
Co-N or Cu-N coordination.
The interaction between Cu and COF via Cu-O/N bond.The Cu-COF serves as an efficient bifunctional photocatalyst for visible-light-driven CO 2 reduction to CO with 94% selectivity.The interaction between Ni and TpBpy via Ni-N bond.
The interaction between Co and DQTP COF via Co-O bond.CO production rate is 1020 μmol/(h•g).
The interaction between Mo and TpBpy via Mo-N bond.The photocatalytic products of Mo-COF under optimal conditions are CO (6 -CON The interaction between Co and COF viaCo-N bond.Co-FPy-CON has produced 10.1μmol CO with a selectivity of 76%.
up to 17.93 mmol/g CO with 81.4%

1 (
MO) in aqueous media with a photodegradation rate of about 0.245 min −for 100 ppm).
The interaction between Ni and Ace-COF through Ni-N coordination.The S-C cross-coupling reactions have yields of 79%-96%.
The interaction between Mo and COF via Mo-N bond.Theoretical results demonstrated that thisMoPc-TFPN catalyst has a considerably low onset potential of −0.24 V, which is comparable to or better than those of widely used noble catalysts.
The interaction between Ni/Ir and Tp-Bpy via Ni/Ir-N bond.The developed protocol enables selective and reproducible coupling of a diverse range of amines (aryl, heteroaryl, and alkyl), carbamides, and sulfonamides with electron-rich, neutral, and poor (hetero) aryl iodides up to 94% isolated yield.

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I G U R E 12 (A) Schematic diagram of the formation of CdS-CTF-1 nanocomposites.(B) XRD patterns of the as-synthesized samples.(C) The UV-Vis DRS spectra of the synthesized samples: (I) CTF-1, (II) 5%CdS-CTF-1, (III) 10%CdS-CTF-1, (IV) 20%CdS-CTF-1, (V) 40% CdS-CTF-1, and (VI) CdS.(D) Proposed mechanism for photocatalytic hydrogen evolution over CdS-CTF-1 under visible light irradiation.(E) XRD patterns of 20%CdS-CTF-1 before and after the reaction.Reproduced with permission: Copyright 2020, Elsevier. 136(F) Adsorption curves of CTF-1, CuCo 2 O 4 and CuCo 2 O 4 /CTF-1 5 .(G) Photocatalytic activities of the CuCo 2 O 4 /CTF-1 composites.(H) Schematic diagram of charge transfer mechanism of CuCo 2 O 4 /CTF-1 composites in photocatalytic CO 2 reduction system.Reproduced with permission: Copyright 2020, The Royal Society of Chemistry.143 have coupled CuCo 2 O 4 and CTF-1 for photocatalytic conversion of CO 2 to CO, and CTF-1 is used to confine CuCo 2 O 4 NPs (CuCo 2 O 4 /CTF-1).The strong metal-nitrogen interaction in CuCo 2 O 4 -CTF inhibits the growth of CuCo 2 O 4 NPs during the synthesis process, so it is observed that CuCo 2 O 4 NPs (about 30-50 nm) on CTF-1 is much smaller than pure CuCo 2 O 4 NPs (80-150 nm).Hence, the excellent photocatalytic performance of CuCo 2 O 4 /CTF-1 composite is closely related to the formed smaller CuCo 2 O 4 NPs within the pores of CTFs, which not only maintains the primary crystalline structure of CTFs but also exposes more active sites.As-prepared CuCo 2 O 4 /CTF-1 composite material puts up enhanced photocatalytic activity for CO production.The porous structure of CTF-1 and Lewis acid-base interaction between absorbed CO 2 molecules and Cu or Co sites, the combined effect of the two factors strengthens the CO 2 adsorption capacity of CuCo 2 O 4 / CuCo 2 O 4 is a p-type semiconductor, CTF-1 is an n-type semiconductor, and their corresponding band gaps are 1.51 and 2.94 eV, respectively.CuCo 2 O 4 and CTF-1 are excited to generate electrons and holes under exposure to visible light (Figure 12H).CuCo 2 O 4 is confined on CTF-1, which has effectively enhanced the space separation of electron-hole pairs and the charge transfer, thus maximizing the redox activity and optimizing the redox potential of photocatalytic CO 2 reduction.The CuCo 2 O 4 / CTF-1 with CuCo 2 O 4 loading of 5 wt% exhibits the best CO yield (14.9 mmol/[h•g]), which is 12.7 and 11.6 times as much as the pure CTF-1 and CuCo 2 O 4 respectively (Figure 12G).Introducing CuCo 2 O 4 also strengthens the absorption capacity of the catalyst for visible light absorption migration of photo-generated carriers.Consequently, CuCo 2 O 4 NPs are confined in CTFs, and the synergistic effect of them enhances the photocatalytic CO 2 reduction performance.

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I G U R E 13 (A) Synthesis diagram of Pd NPs@TTT-COF.High-resolution transmission electron microscopy (HR-TEM) images of Pd NPs@TTT-COF (B) and Pd NPs@TTI-COF (C), the insets show the particle size distribution of Pd NPs.(D) S 2p region in the XPS spectra of TTT-COF before and after Pd loading; N 1s region in the XPS spectra of TTI-COF before and after Pd NPs loading.(E) IR spectra comparison of TTI-COF, TTT-COF, and Pd NPs@TTT-COF.(F) PXRD patterns comparison of Pd NPs@TTT-COF before and after 4th cycle.Reproduced with permission: Copyright 2020, The Royal Society of Chemistry.156

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I G U R E 14 (A) Synthesis of Pd-NCs@CTF.(B) Preparation of Pd/SAs/Cs on covalent organic framework (COF) coatings supported by fumed SiO 2 nanoparticles (NPs) for photocatalytic H 2 evolution.(C) The ultraviolet-visible spectrum of Pd 0.033 /TP-TTA/SiO 2 and TP-TTA/ SiO 2 .(D) Steady-state photoluminescence spectrum, (E) photocurrent density response versus time recorded at 0.5 V versus RHE, and (F) electrochemical impedance spectra (EIS) under visible light irradiation of Pd x /TP-TTA/SiO 2 .Reproduced with permission: Copyright 2022, American Chemical Society.

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I G U R E 17 (A) A covalent organic framework (COF) has been embellished with thiol chains as nucleation sites for Au NCs in its pores.(B) Band alignment of Au NCs, COF-V, H 2 O/.OH, and O 2 /.O 2 − .(C) Photodegradation efficiency of RhB on COF-V and Au@COF.(D) Degradation efficiency of RhB on COF-V and Au @COF under light-free conditions.(E) Cyclic degradation efficiency of RhB on Au @COF exposed to visible light.Reproduced with permission: Copyright 2020, Wiley-VCH Verlag GmbH & Co. KGaA.

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I G U R E 18 (A) The Co-N coordination diagram of Co-TpBpy.Reproduced with permission: Copyright 2016, American Chemical Society. 198(B) Synthesis and metallization of DQTP COF.Reproduced with permission: Copyright 2019, Elsevier. 152(C) Pt single-atoms are confined on TpPa-1-COF through C 3 N-Pt coordination.(D) Gibbs free energy drawing for photocatalytic H 2 evolution.(E) UV-Vis DRS spectra of TpPa-1, 3% Pt NPs/TpPa-1, and Pt 1 @TpPa-1 (1%, 3%, and 5%).(F) Tauc plots of TpPa-1 and 3% Pt 1 @TpPa-1.(G) Photocatalytic H 2 evolution bar chart of all samples with error bars.Reproduced with permission: Copyright 2021, American Chemical Society.140 photocatalytic efficiency of H 2 production is attributed to the well-distributed photocatalysis active sites of Pt single atoms and the efficient separation and migration of photogenerated carriers.The coordination bond between metal single atom and special site of COFs can effectively stabilize metal atom.When metal single atoms are confined to COFs, these metal single atoms are very sensitive to the electronic and geometric interaction with COFs atoms.The electronic structure of the metal sites in coordination compounds strongly relies on the coordination environment, which will have a dramatic influence on the photocatalytic performance.The synergic coordination of the N sites of CTFs to metal single atoms and the confinement effect of the skeletons can stabilize the single atoms and affect photocatalytic performance.Pt-SA/CTF-1 is a catalyst obtained by confining Pt single atoms on CTF modified by ethylene glycol (EG) (Figure19A).147The Pt-SA/CTF-1 maintains a graphene-like layered structure, and no impurities of Pt NPs or other aggregates are observed on the skeleton of CTF except uniformly dispersed Pt single atoms.As shown in the Figure19B, Pt atom confined in CTF-1 photocatalytic efficiency of H 2 production is attributed to the well-distributed photocatalysis active sites of Pt single atoms and the efficient separation and migration of photogenerated carriers.The coordination bond between metal single atom and special site of COFs can effectively stabilize metal atom.When metal single atoms are confined to COFs, these metal single atoms are very sensitive to the electronic and geometric interaction with COFs atoms.The electronic structure of the metal sites in coordination compounds strongly relies on the coordination environment, which will have a dramatic influence on the photocatalytic performance.The synergic coordination of the N sites of CTFs to metal single atoms and the confinement effect of the skeletons can stabilize the single atoms and affect photocatalytic performance.Pt-SA/CTF-1 is a catalyst obtained by confining Pt single atoms on CTF modified by ethylene glycol (EG) (Figure19A).147The Pt-SA/CTF-1 maintains a graphene-like layered structure, and no impurities of Pt NPs or other aggregates are observed on the skeleton of CTF except uniformly dispersed Pt single atoms.As shown in the Figure19B, Pt atom confined in CTF-1 have obtained Fe SAS/Tr-COFs by confining the monatomic Fe site to triazinyl COF (Tr-COFs) through the bridge structure of Fe-N-Cl (Figure 21A).The PXRD pattern (Figure 21B,C) shows that the introduction of metal Fe single atom into Tr-COFs does not change the structure of COFs.Also, the internal hollow spaces and pores of COFs are partially occupied by Fe atoms, which not only maintains the permanent open structure but also provides plentiful active sites and effective areas (Figure 21D).Atomic dispersed Fe ions coordinate with N atoms and Cl atoms in COFs to form bonds. Chelation not only improves the stability of Fe SAS/Tr-COFs but also increases the delocalization.With the increase of delocalization, Fe SAS/Tr-COF (1.78 eV) has a smaller band gap than Tr-COFs (1.82 eV).The band structure analysis (Figure 21E) shows that the CB position of COF becomes more negative after the introduction of Fe single atoms.The band structure consistent with the theory shows that Fe SAS/Tr-COFs has strong photocatalytic activity for CO 2 reduction.With visible light irradiation, Fe SAS/Tr-COFs as photocatalysts can achieve 980.3 μmol/(g•h) CO production rate and 96.4% selectivity, which is about 26 times higher than the original Tr-COFs catalyst.It is advantageous for photocatalytic CO 2 reduction to construct a single atomic metal center.Introducing Fe single atom into Tr-COFs enhances the electron transfer ability, shortens the electron transfer distance, and realizes long-life carrier separation.In addition, the synergistic effect of Fe atom and Tr-COF not only promotes CO 2 molecule and activation, but also reduces the reaction energy barrier formed by *COOH intermediate (Figure

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I G U R E 20 (A) Schematic diagram of synthesis of Ru-CTF.(B) The formic acid production of Ru-CTF-2, Ru(dcbpy)(CO) 2 Cl 2 , and physically mixed Ru(dcbpy)(CO) 2 Cl 2 with dcbph-CTF in CO 2 reduction system.(C) Selectivity of HCOO − and yield distribution of various reduction products.(D) UV-Vis DRS, (E) transient photocurrent responses, (F) steady-state photoluminescence spectrum of all samples.(G) In situ ATR-IR of Ru-CTF samples in specific analysis.(H) Gibbs free energy pathway for CO 2 -HCOOH.Reproduced with permission:

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I G U R E 21 (A) Synthesis process of Fe SAS/Tr-COFs.(B) PXRD pattern of experimental Fe SAS/Tr-COFs (green) and (C) Tr-COFs (blue), Pawley-refined (red), difference (faint yellow), the simulated PXRD pattern of eclipsed AA stacking (purple), staggered AB stacking (orange), and the Bragg positions (green).(D) N 2 adsorption-desorption isotherms of 1.1 wt% Fe SAS/Tr-COFs and Tr-COFs.(E) Band structure of Fe SAS/Tr-COFs and Tr-COFs.(F) Gibbs free energy diagram of CO 2 photoreduction on SAS/Tr-COFs and TR-COFs.Reproduced with permission: Copyright 2022, American Chemical Society. 149(G) Coordination structure of Cu-N-C 2 .(H) CO 2 physisorption isotherms of Cu 0.10 -SA/CTF, Cu-NP/CTF, and CTF-1.(I) CO 2 photoreduction performance of Cu x -SA/CTF, Cu-NP/CTF, and CTF-1.Reproduced with permission: Copyright 2022, Springer.150creating a dual photo-redox catalyst (Ni-Ir@TpBpy COF) for C-N bond formation reaction mediated by visible light.The proximity between two different catalytic centers increases the redox activity of C-N cross-coupling reactions.Confining metal within COF prevents nickel-black formation, and electrons may transfer from the Ir site to the Ni site, which makes it possible to selective and reproducible coupling of a diverse range of carbamides, amines (heteroaryl, alkyl, aryl), and sulfonamides (hetero)aryl iodides.The catalytic sites can be kept stable in the skeleton by the confinement effects and coordination interactions between the confined metal single atoms and the binding groups in COFs.Van Der Voort et al.160 have taken a triazine-based COF (Ace-COF) as a light collector, and fixed single-atom Ni on COF by Ni-N coordination, thus obtaining a two-component synergistic photocatalyst (Ace-COF-Ni) (Figure 22B).After the introduction of Ni single atom, the crystal structure of COF has been retained, and it shows a permanent open structure which ensures the accessibility of Ni active sites (Figure 22C,D).Ace-COF-Ni and Ace-COF F I G U R E 22 (A) Catalyst design is carried out by controllable metal devices in TpBpy COF.Reproduced with permission: Copyright 2022, American Chemical Society, 2022. 163(B) The coordination structure of Ace-COF-Ni.(C) Argon adsorption and desorption isotherms of Ace-COF-Ni and Ace-COF measured at 87 K. (D) The pore size distribution of Ace-COF-Ni and Ace-COF.(E)Assessment of the reusability of Ace-COF-Ni.Ultraviolet-visible absorption spectrum and band gap energy (inset) of (F) Ace-COF, and (G) Ace-COF-Ni.(H) PXRD pattern of Ace-COF-Ni pristine and after five cycles run the catalytic reaction.Reproduced with permission: Copyright 2021, Wiley-VCH Verlag GmbH & Co. KGaA. 160possess band gaps of about 1.83 and 1.74 eV, respectively, which can capture light in ultraviolet and visible regions (Figure 22F,G).The porous crystalline Ace-COF-Ni catalyst has excellent catalytic performance in sulfur-carbon cross-coupling reaction under visible light catalysis.The interaction between COF and metal Ni promotes electron transfer from COF to Ni catalytic active sites.Ace-COF-Ni catalyst has a wide range of applicability in S-C cross-coupling reaction, and its corresponding coupling products have high yields (79%-96%) in the reaction of different substrates.In addition, the Ace-COF-Ni catalyst has recycleability, which can be retrieved and reused at least five times without losing its catalytic structure and performance (Figure 22E,H).

( 1 )
Morphology and structure of COF-based confined hetero-photocatalyst constructed by confinement modification strategy.It is well known that the performance of photocatalyst is vitally interrelated to its morphology and structure.It is a key challenge to directionally confine low-dimension entities in the inner space of COFs, instead of depositing on the outer surface or even blocking the surface of channels.In COF-based confined heterophotocatalyst, the precise position, the fine structure, and interaction between the two structural units need to be ascertained by uniting a whole variety of advanced identification and measurement technologies.It provides us with a profounder comprehending of the adjustment mechanism about the catalyst structure, thereby optimizing the morphology and structure of COF-based confined heterophotocatalysts.(2) Photocatalytic mechanism (including carrier separation mechanism and catalytic reaction mechanism) of COF-based confined hetero-photocatalyst constructed by confinement modification strategy.First, the carrier separation mechanism of COF-based confined hetero-photocatalysts involves a complicated calculation process, and numerous ambiguous factors possibly result in a big deviation, making it difficult to deeply study the carrier separation mechanism.It is of great significance for the design of COF-based confined hetero-photocatalysts to analyze the carriers' separation ability accurately by calculating the relationship between photo-generated carrier concentration and photo-generated carrier energy and the relationship between the parameters of the catalyst itself.Second, the catalytic reaction mechanism on COF-based confined heterophotocatalyst is still difficult to discuss because the theoretical calculation results of the active site model have a big deviation, which probably causes the misconception about the structure of COF-based confined hetero-photocatalyst and the difference between the experiment and the reaction mechanism.Moreover, there must be diffusion resistance in the ultra-small pores in COFs, and the interaction between reactants/solvents and active centers fails to be explained reasonably.There are only a few studies on catalytic reaction dynamics by monitoring the preliminary formation rate of products.Therefore, we should carry out model-building and theoretical calculations based on experiments and verify the materials' nature and reaction mechanism through new calculations and dynamics analysis.In this way, we can efficiently interpret the experimental phenomena and supply some reference data for further studies to better utilize the superior catalytic performance of COF-based confined heterophotocatalysts.