Photochromic radical states in 3D covalent organic frameworks with zyg topology for enhanced photocatalysis

ABSTRACT Covalent-organic frameworks (COFs) with photoinduced donor-acceptor (D-A) radical pairs show enhanced photocatalytic activity in principle. However, achieving long-lived charge separation in COFs proves challenging due to the rapid charge recombination. Here, we develop a novel strategy by combining [6 + 4] nodes to construct zyg-type 3D COFs, first reported in COF chemistry. This structure type exhibits a fused Olympic-rings-like shape, which provides a platform for stabilizing the photoinduced D-A radical pairs. The zyg-type COFs containing catalytically active moieties such as triphenylamine and phenothiazine (PTZ) show superior photocatalytic production rates of hydrogen peroxide (H2O2). Significantly, the photochromic radical states of these COFs show up to 400% enhancement in photocatalytic activity compared to the parent states, achieving a remarkable H2O2 synthesis rate of 3324 μmol g−1 h−1, which makes the PTZ-COF one of the best crystalline porous photocatalysts in H2O2 production. This work will shed light on the synthesis of efficient 3D COF photocatalysts built on topologies that can facilitate photogenerating D-A radical pairs for enhanced photocatalysis.


INTRODUCTION
The photoinduced electron-transfer (PET) process plays an important role in the field of chemistry and biology, as exemplified by enzymatic catalysis [1 ], artificial photosystems [2 ] and solar energy conversion [3 ], among others [4 ,5 ].Searching for new PET systems to produce longlived charge separation states is crucial for the development of functional materials, especially photocatalysts.The charge separation in PET systems typically involves a redox process within the electron donor-acceptor (D-A) pairs, resulting in separated photoinduced radicals that possess stronger reducing or oxidizing ability and enhanced light absorption, in principle, compared with their neutral parent states [6 -8 ].The generation of photochromic radical states (PRSs) with lifetimes ranging from hours to days has been achieved in hybrid materials [9 ,10 ], supramolecular assemblies [11 ], metal-organic frameworks (MOFs) [12 ], hydrogen-bonded organic frameworks (HOFs) [13 ] and organic polymers [14 ], but is sti l l absent in covalent organic frameworks (COFs).COFs have emerged as a new class of highly designable and tailorable crystalline porous organic skeletons with periodically ordered structures, showing great potential in heterogeneous catalysis on account of their high stability and porosity [15 -18 ].Thus, it is a crucial research direction to construct COF materials with PRSs in order to exploit the full extent of the photocatalytic performance.
In the previously reported COFs, only the sub-second lifetime of the photoinduced charge separation (PCS) state can be reached [19 -21 ].The  [6 + 4] topologies, the zyg net can provide a platform for producing obstructive photoinduced charge separation radial pairs.stabilization of long-lived charge separation states in COFs is confronted with two major challenges: one is to ensure the effective PET within the D-A pairs and the other is the inhibition of reversed charge recombination.In view of Marcus-Hush theory [22 ,23 ], regarding the former, distinct electron donating/accepting properties, suitable molecular orbit arrangement and close-in contact of the D-A pairs are required for effective electron jumping while the latter prefers a rather remote distance between the D-A pairs to prevent recombination.This means that building units with proper lengths are required for constructing such photochromic COFs.In view of the reticular chemistry [24 ,25 ], compared with the layer-by-layer stacking of 2D COF structures, the 3D interconnection of the building units in 3D COFs can afford more pathways for electron transfer and stabilize separated photoinduced charges through stronger electrostatic attraction interactions.In addition, 3D COFs with mesoporosity (with pore diameter larger than 2 nm) [26 ] can usually accommodate more accessible donor and acceptor sites with good catalytic activity [27 ].Thus, 3D COF systems with suitable periodically ordered building blocks would be potential molecular platforms for the photogeneration of metastable charge separation radical states (Fig. 1 a).Since the discovery of COFs by Yaghi in 2005 [15 ], most of the work has focused on the exploration of 2D structures and a small number of 3D COFs have been designed and synthesized [16 ,28 ,29 ].In the 3D COFs database, the 4-connected building blocks with tetrahedral ( T d ) symmetry were mostly used to connect with other building units in [4 + 2], [4 + 3] and [4 + 4] modes to form topologies such as dia , bor and pts in the early studies [30 -32 ].In recent years, 3D COFs based on higher connectivity building units have been synthesized with new topologies featuring [6 + 2], [6 + 3], [6 + 4], [6 + 6], [8 + 2] or [8 + 4] nets [25 ].Among these, 3D COF structures based on [6 + 4] net were constructed showing high surface area, high structural stability, an absence of structural interpenetration and decent catalytic properties.However, for [6 + 4]-connected COFs, only four topologies ( hea , soc , stp and she ) assembled by linking 6-connected building units with T d and square nodes, have been reported, respectively (Fig. 1 b) [33 -36 ].
Triphenylamine (TPA) and phenothiazine (PTZ) are well-known electron-rich functional groups with distinct features, such as strong electrondonating characteristics, tunable redox properties and versatility of functionalization, meaning that they cater well to our purpose, acting as building units for 3D COFs with PCS states [37 -39 ].Herein, we have extended the triangular TPA and PTZ groups into (3,3)-type 6-connected building blocks, forming an irregular hexagon.Combining these designed hexagonal bui lding units ( hexa-CHO) with a planar quadrilateral building block (tetra-NH 2 ), we have synthesized a family of (3,3,4)-connected 3D COFs with zyg topology, which has been previously observed in MOFs but is reported in COFs for the first time [40 ].It is noteworthy that two types of 1D pores and an Olympic-rings-like shape are observed in the crystallographic axis c direction in the zygtype 3D COFs.The rare multi-sized 1D pores have the potential to further increase the specific surface area, as well as to carry out some domain-limited catalytic reactions.This is the first 3D COF with a fused Olympic rings shape synthesized in COF structures.Most importantly, the metastable PCS radical states were obtained through light irradiation in the TPA-and PTZ-based COFs, confirmed by the photochromic phenomenon and electron paramagnetic resonance (EPR) characterization.To further investigate the more inherent impacts PCS brings to the COF materials, various photoelectrochemical characterizations were conducted revealing that light harvest ability and carrier transport capacity were both enhanced for the metastable radical states of the zyg COFs.In addition, TPA-and PTZ-based COFs often show high crystallinity, which can improve the photoelectron generation and transfer within the lattice [41 ].Hydrogen peroxide (H 2 O 2 ) is used as a green, clean and storable oxidant in a wide range of applications such as medical disinfection, paper bleaching and organic synthesis [42 ].Herein, the TPA-and PTZ-based COFs were applied to the photocatalytic synthesis of H 2 O 2 , showing significant catalytic activity improvement for the metastable radical states ( > 200% enhancement for TPA COF and > 400% for PTZ COF compared with the as-prepared samples) and the highest H 2 O 2 photosynthesis rate reaches 3324 μmol g −1 h −1 for the activated PTZ-based COF, making it one of the best porous crystalline photocatalysts in H 2 O 2 photosynthesis.This work has successfully constructed metastable charge separation radical states in COFs of zyg topology for the first time and enhanced the catalytic performance by several orders of magnitude, providing an effective strategy on catalytic activity enhancement for 3D COF materials.

Synthesis and characterizations
In order to construct new [6 + 4]-connected COFs, as shown in Fig. 2 , hexa-aldehyde precursors containing functionalized central cores acting as strong electron donors (TPA-6CHO, PTZ-6CHO) and weak ones (Ph-6CHO, TPB-6CHO) were used.The crystalline products, named M-COF , N-COF , S-COF and T-COF , were obtained through the Schiff condensation reaction using the above aldehydes and dimethyl functionalized tetra-amine (TAPB-Me).These COF materials were synthesized in yields ranging from 60% to 70% via the solvothermal method, where the reactant monomers were heated at 120°C in a mixture containing o -dichlorobenzene, dichloromethane and trifluoroacetic acid for 3 days (see Supplementary In-formation for more details).The resulting four COFs were insoluble in common organic solvents such as dimethyl sulfoxide (DMSO), acetone and methanol ( Figs S1 and S2, see Supplementary Information).
The structures of these 3D COFs were unambiguously characterized using various analytical methods.Solid-state 13 C cross-polarization magic angle spinning (CP/MAS) nuclear magnetic resonance (NMR) spectra ( Figs S3-S6) verified the presence of the imine moiety showing peaks at ∼160 ppm.Considering that monomers with multiple reaction sites may give sub-stoichiometric COFs, in order to quantitatively determine the ligand ratios, these COFs were decomposed and dissolved using DCl/DMSOd 6 and analyzed by solutionstate 1 H NMR ( Figs S7 and S8).The ratios of amino and aldehyde precursors in both N-COF and S-COF were close to 3:2.This stoichiometry is consistent with that observed in the crystal structure models.The Fourier transform infrared (FT-IR) spectra of M-COF , N-COF , S-COF and T-COF ( Figs S9-S12) show the formation of imine bonds (1623 cm −1 ) and the disappearance of C-O vibration of the aldehyde group (1698 cm −1 ) and the N-H vibration of the amino group (3352 cm −1 ).Thermogravimetric analysis (TGA) ( Fig. S13) was performed under N 2 atmosphere to assess the thermal stability, and these 3D COFs were stable up to 450°C before being decomposed.Field emission scanning electron microscopy (SEM) images (Fig. 3 a and Fig. S14) reveal that all these COFs show defined hexagonal nanocrystals.

Crystalline structure and porosity
Measurements of powder X-ray diffraction (PXRD) combined with theoretical structural simulations were used to elucidate the crystal structures of the above COFs.The structural models for all these COFs were established based on zyg topology in the P 6 3 / mcm space group.As shown in Fig. 2 c, the experimental PXRDs for these four COFs exhibit the first strongest peaks at 2.40°, 2.14°, 2.20°a nd 2.04°, respectively, which are attributed to the  values and acceptable profile differences.Also, Rietveld refinements based on the zyg -type structural models for these COFs against the PXRD data revealed a good match between the calculated and experimental patterns ( Figs S15-S22), confirming the model structures.According to the Reticular Chemistry Structure Resource (RCSR) [24 ], combining a potential co-planar 6-connected node with a square building block can also form the rht -network, which has been observed in a series of {Cu 2 (COO) 4 } paddlewheel-based MOFs [43 -45 ].Therefore, as an example, the crystal structure for N-COF was also modeled based on rht topology, where the three branched isophthalaldehydes of the hexa-CHO are on the same plane.However, the calculated PXRD pattern does not match with the experimental one ( Fig. S23), which rules out the possibility for these COFs to adopt rht topology.In addition, the [6 + 4]-connected network with stp topology showing the same hexagonal symmetry as the zyg -net has been tested for the structural modeling of N-COF .The structure of N-COF-stp shows a significantly larger unit cell (space group: P 6 , a = 52.30Å, c = 13.88Å) than the one provided by the indexing of the experimental PXRD pattern, suggesting the network of N-COF adopts zyg topology rather than stp ( Fig. S24).Furthermore, the crystallinity and periodic porous structures of these COFs were also evidenced by high-resolution transmission electron microscopy (TEM) (Fig. 3  [001] direction.The periodic bright spots in these images associate with the larger empty pore channels of these COF structures.The corresponding inverse fast Fourier transform (IFFT) denoised images clearly show lattice fringes with distances of 3.80 nm for M-COF , 3.9 4 nm for N-COF and 4.20 nm for S-COF , matching well with the dspacings of [100] lattice planes in these zyg -type frameworks.Micro electron diffraction (MicroED) experiments were also conducted on these COF nanocrystals in order to obtain the single crystal structures for these COFs.However, these COF samples showed barely any diffraction spots for single crystal structure solution from MicroED ( Fig. S28).
In the construction of the zyg net, the TAPB-Me unit acts as a square building block, in which the four branched phenyl rings are almost perpendicular to the central phenyl ring (Fig. 4 a).The tetra-amine TAPB (1,2,4,5-tetrakis-(4-aminophenyl)benzene) without methyl groups was also tested to react with the same hexa-aldehydes, but no crystalline products were obtained, suggesting the important role of the two methyl groups in TAPB-Me for directing the formation of the zyg -type frameworks.In these zygtype COFs, the three branched isophthalaldehyde rings of the hexa-CHO are not on the same plane, but with a small t wist bet ween the branched ring and the central phenyl ring (i.e.∼14°for M-COF , Fig. 4 c).The zyg -COFs can be seen in the unit cell c direction with two different sizes of pores, both of which can be simplified into hexagons (Channel A and B as shown in Fig. 4 a and c, respectively).The irregular hexagon of Channel A is formed by two hexa-CHO units and two TAPB-Me moieties as the edges.Channel B is composed of six TAPB-Me units as edges connected with six isophthalaldehyde units as vertices to form a regular hexagon.Therefore, changing the size of the hexa-CHO building block affects the size of Channel A. The crystal structure of the expanded M-COF shows that the zyg network exhibits the shape of fused Olympic rings in the unit cell c axis (Fig. 4 b).By varying the size of the hexa-CHO unit, the above four zyg -type COFs show different sizes of f used Oly mpic rings.To investigate the porosity of these 3D COFs, nitrogen adsorption analysis was performed at 77 K after activation of the materials under vacuum (Fig. 2 d  corresponding pore size distribution peaks near 1.0 and 3.4 nm, which are essential ly simi lar to the pore sizes expected from their crystal structures.

Photophysical properties
The structure feature of these COFs provides a geometric platform for stabilizing the charge separation states in a 3D lattice.Therefore, PCS in these COFs was investigated.First, the as-prepared COFs were irradiated by Xe lamp to induce the electron transfer within the COF.All the COF samples showed photochromism, and therefore solid-state EPR spectra were collected before and after light irradiation.possesses better visible-light harvesting properties with a 50 nm wavelength red shifting.The change on UV-Vis DRS mainly results from the introduction of the phenothiazine group, increasing the conjugation property in the structure.The DRS also revealed that the metastable states N-COF* and S-COF* possess better visible light absorption ability, resulting from the radical feature of these activated states.By analyzing the T auc plots of ( αh ν) 2 vs .photon energy (Fig. 5  It can be inferred above that the light activation of N-COF and S-COF samples increased the photon capture abilities and meanwhile narrowed the energy band gaps.To further understand the impact the PCS state brought to these two COFs, the transient photocurrent response was measured to evaluate the photogenerated carrier separation efficiency and the charge transport capacity.The transient-photocurrent-response intensity of S-COF was slightly larger than that of N-COF under visible light ( λ > 420 nm).When i l luminating N-COF and S-COF with an Xe lamp without any optical filter, the photocurrent showed intensity enhancement with i l lumination time increasing, meaning that the PCS states for these COFs benefit the photogenerated carrier separation efficiency and the charge transport capacity, and this trend is more significant for S-COF than N-COF (Fig. 5 e,  and Figs S41 and S42).

Photocatalytic synthesis of H 2 O 2
Compared to the industrial anthraquinone oxidation method, the photocatalytic synthesis of H 2 O 2 is safer and more environmentally friendly as it uti-lizes renewable solar energy and clean resources (H 2 O and O 2 ) [42 ,46 ].Currently, organic polymerbased photocatalysts such as graphitic carbon nitride (g-C 3 N 4 ) have been widely applied in the photocatalytic production of H 2 O 2 [47 ,48 ].However, their further development has been hindered by the unsatisfactory catalytic activity due to the low crystallinity and poor photogenerated carrier separation.Since 2020, COFs have arisen as an ideal candidate for H 2 O 2 photosynthesis because of their well-defined structures, excellent stability and desired semiconductor-like behavior [46 ,49 ].Although the light absorption ability of these COFs is quite moderate, the fine photogenerated carrier separation efficiency and the charge transport capacity, as well as the suitable arrangement of the acceptor-donor pairs, make it possible for them to be used as highly efficient photocatalysts.Therefore, N-COF and S-COF containing photocatalytically active groups (TPA and PTZ) were tested for the synthesis of H 2 O 2 ( Figs S43-S48).First, the photocatalytic activity of the reported COFs was evaluated by photocatalytic oxygen reduction reaction (ORR) for H 2 O 2 production under Xe lamp irradiation using isopropanol as a sacrificial reagent.The photosynthetic rates of H 2 O 2 in 10% isopropanol aqueous solution are 560 and 596 μmol g −1 h −1 for N-COF and S-COF , respectively, while the values increased to 794 and 1474 μmol g −1 h −1 for N-COF* and S-COF* (Fig. 6 a).The enhancement of photocatalytic performance might result from the better photon capture ability, photogenerated carrier separation efficiency and the charge transport capacity for the PCS radical samples.The much more dramatic increase in activity for the activated S-COF compared with N-COF can be attributed to the increase in conjugation property because of the phenothiazine group.Subsequently, H 2 O 2 photocatalytic synthesis with an Xe lamp was also conducted in pure water, displaying H 2 O 2 photosynthetic rates of 656, 450, 710 and 1160 μmol g −1 h −1 for N-COF , S-COF , N-COF* and S-COF* , respectively (Fig. 6 a).There was a clear accumulation of the H 2 O 2 yield with an increase in irradiation time, displaying a close linear relationship between H 2 O 2 production and irradiation time for all the above COFs, suggesting that the high photosynthetic rates could remain even with a long irradiation time (Fig. 6 b).This provides proof for the fact that the light-activated states were quite stable even in the catalytic reaction condition.Furthermore, in consideration of the moderate visible light absorption of these COFs, we also conducted photocatalytic H 2 O 2 production using 380 nm of LED light as the monochromatic i l luminant.This showed H 2 O 2 photosynthetic rates of 491, 759, 1140 and 3324 μmol g −1 h −1 for N-COF , S-COF , N-COF* and S-COF* , respectively (Fig. 6 a).It is  worth noting that S-COF* showed dramatic enhancement in photocatalytic activity, up to four times that of the parent S-COF .Although recently, Liao et al. reported py razine-f unctionalized COFs with a highest photocatalytic H 2 O 2 production rate of 7327 μmol g −1 h −1 in COF photocatalysts [50 ], the H 2 O 2 synthesis rates of most reported COFs are quite moderate [46 ].Thanks to the activity en-hancement resulting from the PCS state, the excellent photocatalytic performance of S-COF* makes it one of the best porous crystalline photocatalysts with regard to H 2 O 2 production (as listed in Table S1).Cycling stability tests were also carried out to evaluate the durability of the photocatalyst for N-COF* and S-COF* , showing slightly decreased catalytic performance after five cycles of photocatalytic tests (Fig. 6 c).These results provide evidence that the PCS states in these COFs were very stable, with a substantially long lifetime.

Photocatalytic mechanism studies
To investigate the mechanism of photocatalytic H 2 O 2 production for the above COFs catalysts, a series of control experiments were carried out on N-COF* and S-COF* .With Ar bubbling for 30 min to eliminate O 2 or in dark, both N-COF* and S-COF* displayed H 2 O 2 photosynthetic rates less than 30 μmol g −1 h −1 , demonstrating the indispensable process of the photocatalysis of O 2 .Then, photocatalytic water oxidation reaction (WOR) for the H 2 O 2 production was studied with Ar in AgNO 3 aqueous solution.N-COF* and S-COF* exhibited photocatalytic activ ity w ith H 2 O 2 photosynthetic rates of 220 and 150 μmol g −1 h −1 , respectively (Fig. 6 d), and almost no dioxygen was detected under this condition.These results revealed that both photocatalytic WOR and ORR processes of H 2 O 2 production existed in these COFs.Moreover, an isotopic experiment to simulate the real system (without AgNO 3 ) was conducted for N-COF* and S-COF* to verify the as-proposed mechanism of ORR by using 18 O 2 (as an electron acceptor) and H 2 16 O (as an electron donor). 18O 2 ( m / z = 36) was detected in the decomposition product of the reaction between photogenerated H 2 O 2 and MnO 2 (Fig. 6 f and Fig. S49), indicating the H 2 O 2 photosynthesis process from ORR.
As for the H 2 O 2 photosynthesis reaction mechanism, there are two possible routes for ORR and WOR from O 2 and water via the 2e − redox process, respectively.One of them is a 2e − two-step process with hydroxyl radicals ( r OH) and superoxide anion radicals (  To further understand the catalytic mechanism and the reason why the catalytic performances are distinct for these COFs, DFT calculations analyzing the contributions of the molecular orbital and the Gibbs free energy of the ORR and WOR processes were conducted.First, the molecular orbital analysis was carried out on the constructed cluster models, revealing that both N-COF and S-COF present an excellent spatial charge separation in which the HOMO is mainly located in the TPA and PTZ moieties for N-COF and S-COF , respectively, and the LUMO is dominantly located at the imine and its adjacent phenyl group in the TAPB-Me site ( Fig. S51).Thus, we can deduce the reaction pathway as follows: first, during the light activation process of forming the corresponding excited species N-COF* and S-COF* from N-COF and S-COF , the electron transfer probably takes place from the TPA or PTZ group to the imine group site and produces the metastable PCS states; second, the photocatalytic reduction reaction occurs around the electron-rich positions (imine sites) and the oxidation reaction at the hole positions (the TPA or the PTZ sites) for both of N-COF and S-COF .Since the excitation process is realized by light irradiation, the metastable radical species after excitation have been captured by experiments.Therefore, the theoretical calculation is based on the N-COF* and S-COF* intermediates after the PCS process.For comparison, N-COF and S-COF catalytic potential energy surfaces before excitation are also considered (Fig. 6 g).In addition, because N-COF and S-COF contain the same building unit TAPB-Me, the mechanism of the ORR catalytic process is the same for both.So, there are only two lines on the potential energy surface of the ORR process (red line represents the ground state species for N-COF / S-COF and blue line represents the excited species for N-COF* / S-COF* in Fig. 6 g).
In general, the first process in ORR and WOR is the potential determination step for the photocatalytic two-step H 2 O 2 production.Based on the calculation results, with regard to the ORR process,  6 g), respectively.These results reveal that the electron-donating status is beneficial to the photocatalytic WOR process for S-COF (oxidized PTZ species) while it has little effect on N-COF (oxidized TPA species).These results explain the catalytic performance differences as below: (i) the photocatalytic H 2 O 2 production performance was comparable for the groundstates N-COF and S-COF ; (ii) the photocatalytic H 2 O 2 production performance was enhanced for the excited-states N-COF* / S-COF* compared with N-COF / S-COF ; (iii) the performance enhancement compared to their initial states was much more significant for S-COF* than N-COF* .

CONCLUSION
In summary, we have demonstrated a design strategy of utilizing inequilateral hexagonal building units to prepare 3D COFs ( M-COF , N-COF , S-COF and T-COF ) based on a [6 + 4] network with a previously unreported zyg topology in COF struc-tures.It is noteworthy that these zyg -type COFs exhibit two types of 1D channel and a fascinating f used Oly mpic-rings-like pore shape in the unit cell c axis.Both N-COF and S-COF , containing photocatalytically active groups, showed excellent activity in the photosynthesis of H 2 O 2 compared to other COF photocatalysts.Subsequently, the photochromic radical states for the above two COFs ( N-COF* and S-COF* ) were successfully obtained, showing dramatic enhancement of photoactivity for H 2 O 2 synthesis compared to the parent materials.Remarkably, S-COF* showed a H 2 O 2 photosynthetic rate of 3324 μmol g −1 h −1 , > 400% enhancement in activity compared to S-COF , making S-COF* one of the best porous crystalline photocatalysts in H 2 O 2 photosynthesis.This work provides an effective strategy for constructing PRSs in zygtype 3D COFs, which facilitate significant photocatalytic activity enhancement for H 2 O 2 synthesis.This work wi l l shed light on the development of efficient 3D COF photocatalysts, building on the functional organic building blocks assembled in topologies to facilitate photoinduced charge separation.

Figure 2 .
Figure 2. Structures and characterizations of M-COF , N-COF , S-COF and T-COF .(Columns b, c, d from top to bottom are in the order of M-COF , N-COF , S-COF and T-COF ).(a) Synthesis of COFs using the hexa-aldehyde and tetra-amine precursors.(b) Extended structures of COFs viewed along the crystallographic c axis.(c) PXRD patterns and Pawley refinements of the unit cell parameters based on the crystal structure models.(d) N 2 adsorption-desorption isotherms at 77 K and pore size distributions.

Figure 3 .
Figure 3. Electron microscopy images of the as-synthesized COFs.(a) SEM image of N-COF .A hexagonal crystal is marked with the red hexagon.(b) TEM image of N-COF and (c) its IFFT.The magnified region in the area marked with the red box in the IFFT image for N-COF (c) and overlay of the structure model along the [001] direction (d).TEM images of (e) M-COF and (f) S-COF .

Figure 4 .
Figure 4. Structure of the fused-Olympic-rings-shaped framework and the two different channels in the zyg -type COFs exemplified by M-COF .(a) Single and multilayer aperture structures for the smaller hexagon (channel A).(b) Extended structure of M-COF viewed along the c axis and fused five-member rings derived from Olympic rings.(c) Single and multilayer aperture structures for the larger hexagon (channel B).

Figure 5 .
Figure 5. (a) EPR spectra for the irradiated samples of as-prepared zyg -type COFs in the solid state.(b) EPR spectra for the irradiated samples of as-prepared zyg -type COFs under the catalytic reaction condition.(c) UV-Vis DRS for N-COF and S-COF and their activated states (inset: photographs of the COF powders).(d) T auc plots for band gap calculations.(e) Transient photocurrent response and (f) band-structure diagram for N-COF , S-COF , N-COF* and S-COF* .NHE stands for normal hydrogen electrode.
When irradiating solid samples or samples dispersed in solution, only a subtle increase in the EPR signals around g -value of 2.0 was observed for M-COF and T-COF after light activation, indicating inefficient electron transfer or rapid charge recombination in them (Fig. 5 a and b, and Figs S33 and S34).The EPR signals obviously emerged for the photo-activated samples of N-COF and S-COF , namely N-COF* and S-COF* , suggesting that the effective electron transfer process occurred and metastable PCS states formed inside N-COF and S-COF (Fig. 5 a and b, and Figs S35 and S36).The electronic properties of N-COF and S-COF , as well as their activated samples, were subsequently investigated via various measurements.The yellow-colored samples of N-COF and S-COF showed slightly different photon capture ability, as revealed by their UV-Vis diffuse reflectance spectra (DRS).As seen in Fig. 5 c, both N-COF and S-COF show broad absorption bands with maximum peaks at 380 nm and S-COF

Figure 6 .
Figure 6.(a) Photocatalytic H 2 O 2 production rates of N-COF, S-COF, N-COF* and S-COF* in pure water and 10% isopropanol aqueous solution.(b) Time-dependent photocatalytic activity of N-COF, S-COF, N-COF* and S-COF* for H 2 O 2 production in pure water.(c) Cycling tests over N-COF* and S-COF* for the photocatalytic H 2 O 2 production from O 2 and H 2 O.(d) Single-variable control experimental tests of the photocatalytic H 2 O 2 production performance for N-COF* and S-COF* under different conditions.(e) EPR spectra of the reaction solution under dark and visible light illumination for S-COF* in the presence of DMPO as the spin-trapping reagent.(f) 18 O 2 isotope experiment to identify the source of O in H 2 O 2 .(g) Free energy diagram of H 2 O 2 photosynthesis through the ORR and WOR pathways for N-COF, S-COF, N-COF* and S-COF* and the key steps of the H 2 O 2 photosynthesis mechanism.

r O 2 −
) as the intermediate species for indirect H 2 O 2 generation, and the other is a direct 2e − one-step process.In order to verify the mechanism of the full reaction of H 2 O 2 photosynthesis for the above two COFs, in situ EPR was conducted using 5,5-dimethyl-pyrroline N -oxide (DMPO) as the free-radical spin-trapping agent to detect r OH and r O 2 − .As depicted in Fig. 6 e and Fig. S50, typical characteristic signals of DMPO-r OOH and DMPOr OH species were observed in O 2 -saturated water for N-COF* and S-COF* under Xe lamp irradiation and were both absent in the dark condition.

the O 2
molecules are adsorbed at the imine N sites and then experience one electron reduction and protonation to form r OOH intermediate.The Gibbs free energy change ( G o ) values for this process are −2.23 eV for N-COF / S-COF and −4.63 eV for N-COF* / S-COF* , meaning that the ORR is thermodynamically favorable for all these COFs and more energy-favorable for the excited species N-COF* / S-COF* .Then one more photogenerated electron and proton transfer into the r OOH intermediate, fulfilling the two-step ORR process from O 2 to H 2 O 2 product with G o values of −3.23 eV and −5.72 eV for N-COF / S-COF and N-COF* / S-COF* relative to the initial point, respectively.This reveals that the electron-w ithdraw ing status (reductive TAPB-Me species) for N-COF and S-COF is more beneficial to the photocatalytic ORR process.With regard to the WOR process, the H 2 O molecules are adsorbed at the N atom of TPA for N-COF / N-COF* and at the phenothiazine ring for S -COF / S -COF* , and one electron oxidation as well as deprotonation subsequently occurs, resulting in the generation of r OH intermediate.The G o values of this step are 2.69 eV and 2.63 eV for N-COF and N-COF* , respectively, which are comparable before and after light activation.By contrast, the free energies of forming r OH intermediate show the distinct energy difference for S-COF and S-COF* ( G o = 2.32 eV and 0.88 eV), respectively.Two r OH intermediates would desorb and combine to generate H 2 O 2 , achieving the twostep WOR process with G o values of 1.19, 1.14, 1.57 and 0.14 eV for N-COF , N-COF* , S-COF and S-COF* (Fig.