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

Covalent organic frameworks have recently gained increasing attention in photocatalytic hydrogen generation from water. However, their structure-property-activity relationship, which should be beneficial for the structural design, is still far-away explored. Herein, we report the designed synthesis of four isostructural porphyrinic two-dimensional covalent organic frameworks (MPor-DETH-COF, M = H2, Co, Ni, Zn) and their photocatalytic activity in hydrogen generation. Our results clearly show that all four covalent organic frameworks adopt AA stacking structures, with high crystallinity and large surface area. Interestingly, the incorporation of different transition metals into the porphyrin rings can rationally tune the photocatalytic hydrogen evolution rate of corresponding covalent organic frameworks, with the order of CoPor-DETH-COF < H2Por-DETH-COF < NiPor-DETH-COF < ZnPor-DETH-COF. Based on the detailed experiments and calculations, this tunable performance can be mainly explained by their tailored charge-carrier dynamics via molecular engineering. This study not only represents a simple and effective way for efficient tuning of the photocatalytic hydrogen evolution activities of covalent organic frameworks at molecular level, but also provides valuable insight on the structure design of covalent organic frameworks for better photocatalysis.

Among all these tested systems, photocatalytic hydrogen evolution reaction (HER) from water is regarded as one of the most attractive ways to meet the increasing demands of clean and sustainable energy 49 . In 2014, Lotsch and co-workers reported the first example of utilizing 2D COF to produce H 2 in the presence of metallic platinum under visible light irradiation 34 . Since this pioneer work, several 2D COFs bearing different photoelectric units have been successfully constructed and found interesting potential in photocatalytic hydrogen evolution [34][35][36][37][38][39][40][41][42][43] . However, although continuing efforts are going on developing new 2D COFs for photocatalytic HER, the rational tuning of their structures and photophysical properties for maximizing the hydrogen evolution efficiency still needs to be further clarified. In an initial study, Lotsch et al. reported several 2D COFs with different numbers of nitrogen atoms in the central phenyl ring 35 , which showed controllable photocatalytic hydrogen evolution efficiencies. Unfortunately, as the tailoring of their photocatalytic performance lies on a multitude of variables (e.g., crystallinity, optoelectronic factors, etc.), it is very difficult to determine the individual contribution that is required for further modification. Therefore, it is highly demanded to construct isostructural 2D COFs with tunable optoelectronic properties and further explore their structure-property-activity relationship in photocatalytic HER from a molecular level.
Porphyrin and its derivatives, a kind of conjugated π-electron macrocycles with unique photophysical and redox properties 50,51 , have been used to construct 2D COFs for heterogeneous photocatalysis 28,29,[46][47][48] . In principle, the incorporation of different metal ions into porphyrin units may rationally tune their photophysical and electronic properties, which can thus affect the photocatalytic activity of corresponding COFs. With this consideration in mind, we report herein the synthesis and characterization of four isostructural hydrazone-linked 2D porphyrinic COFs (Fig. 1), named as MPor-DETH-COF (M = H 2 , Co, Ni, Zn). Our results clearly demonstrate that these four COFs have high crystallinity and surface area, and the incorporation of different transition metal ions into porphyrin rings apparently influences the charge-carrier dynamics properties of corresponding COFs. When irradiated with visible light in the presence of H 2 PtCl 6 and triethanolamine (TEOA), all MPor-DETH-COFs can continually produce hydrogen from water while retaining the framework. More importantly, these four COFs show rationally tunable activity toward photocatalytic hydrogen evolution with the order of CoPor-DETH-COF (25 μmol g −1 h −1 ) < H 2 Por-DETH-COF (80 μmol g −1 h −1 ) < NiPor-DETH-COF (211 μmol g −1 h −1 ) < ZnPor-DETH-COF (413 μmol g −1 h −1 ), which can be mainly explained by their tailored chargecarrier dynamics via molecular engineering.

Results
COF synthesis and characterization. In order to construct stable porphyrin-based 2D COFs for photocatalytic hydrogen evolution from water, we designed and synthesized porphyrinic aldehydes p-MPor-CHO (M = H 2 , Co, Ni, and Zn), which could react with 2,5-diethoxyterephthalohydrazide (DETH) to form the designed isostructural MPor-DETH-COF (M = H 2 , Co, Ni, and Zn) through condensation reaction (Fig. 1). It should be mentioned here, as the solubility and reactivity of these porphyrinic aldehydes are different, the reaction conditions need to be optimized to obtain high crystalline COFs. Generally, the condensation reaction was performed in a mixed solvent of 1,2-dichlorobenzene, butanol, and aqueous acetic acid at 120°C, but the ratio of the solvents, the concentration and the amount of acetic acid and the reaction time were optimized for each COF synthesis. The chemical structure of these COFs was then assessed by a combination of Fourier transform infrared (FT-IR) spectroscopy and solid-state nuclear magnetic resonance (ssNMR) spectroscopy. The powder X-ray diffraction (PXRD) experiment was performed to elucidate the crystalline nature of these COFs. As shown in Fig. 2e−h, all MPor-DETH-COFs showed two main diffraction peaks at 3.0°and 6.1°, corresponding to (110) and (220) crystal facets, respectively. Detailed crystal model was then simulated using Materials Studio software package (see Supplementary section 4 for details) and the unit cell parameters were optimized according to density-functional tight-binding (DFTB) calculations. Obviously, the calculated diffraction patterns of AA eclipsed stacking model matched well with the experimental PXRD patterns, suggesting that all MPor-DETH-COFs adopted AA layer stacking structure. Furthermore, the Pawley refinement for these COFs yielded PXRD patterns ( Fig. 2e−h) which were in good agreement with the experimentally observed data, as evidenced by the negligible difference. The detailed crystal structure information, including the unit cell parameters, could be found in Supplementary section 4. We also characterized the porosity of all MPor-DETH-COFs by nitrogen sorption isotherms measurement at 77 K. As shown in Fig. 2i−l, all of these COFs exhibited typical type-IV sorption isotherm curves, which is a characteristic evidence for mesoporous structures. The Brunauer-Emmett-Teller (BET) surface areas were calculated to be 826 m 2 g −1 , 942 m 2 g −1 , 773 m 2 g −1 , and 1020 m 2 g −1 for H 2 Por-DETH-COF, CoPor-DETH-COF, NiPor-DETH-COF and ZnPor-DETH-COF, respectively. Evaluation of the isotherms of all these COFs using quenched solid density functional theory (QSDFT) showed a main peak centered at around 2.4 nm, which agreed well with the calculated pore size (2.4 nm). To gain more insight of the microstructures of these four COFs, scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM) were also performed. SEM images showed that all MPor-DETH-COFs acquired the same morphology of stacked sheets ( Supplementary Fig. 12), where the corresponding HR-TEM images confirmed the layered stacking structure ( Supplementary Fig. 13).
Optical and electronic properties. UV-Vis diffuse reflection absorption spectra of all MPor-DETH-COFs were first recorded (  corresponding COFs. Moreover, cyclic voltammetry (CV) spectroscopy of these four COFs was performed to study their electronic bands ( Supplementary Fig. 21). Accordingly, their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) were calculated. As shown in Fig. 3b, the LUMO energy levels were calculated to be about  Table 5). In addition, the valence band X-ray photoelectron spectroscopy (XPS) analysis of these four COFs exhibited comparable results (Supplementary Table 6). Obviously, these experimental orbitals locate at a very suitable scope for photocatalytic HER from water. We then carry out the time-correlated single-photon counting (TCSPC) of all MPor-DETH-COFs to estimate their excited-state lifetimes in solid state, which are related to the carrier separation dynamics 42,53 arising from the π-π* transitions of the four COFs in solid state 54 . As shown in Fig. 3c, ZnPor-DETH-COF showed the longest emission lifetime while H 2 Por-DETH-COF has the shortest value in the nanosecond range. However, for CoPor-DETH-COF, it is essentially non-emissive ( Supplementary  Fig. 24). According to literature 55  This result is consistent with the trend of emission decays, which confirms again the most efficient charge carrier transport in ZnPor-DETH-COF. Therefore, the incorporation of different metal ions into porphyrin precursors can influence the chargecarrier dynamics of the resulting COFs.
Photocatalytic hydrogen evolution. Encouraging by above results, we then evaluated the photocatalytic performance of all MPor-DETH-COFs toward hydrogen evolution from water under visible light irradiation (Xe-lamp 300 W, λ > 400 nm) in the presence of H 2 PtCl 6 and triethanolamine (TEOA), where H 2 PtCl 6 was employed as the precursor of co-catalysts and TEOA worked as the sacrificial reagent. Control experiments confirmed that visible light, TEOA, H 2 PtCl 6 , and MPor-DETH-COFs were indispensable for effective hydrogen generation ( Supplementary  Fig. 27a-d). Moreover, H 2 Por-DETH-COF could not produce hydrogen gas when the incident wavelength was above 500 nm ( Supplementary Fig. 27e), indicating that B-band absorption of porphyrin ring mainly contributes to the photoredox reactions. Notably, all four COFs could constantly evolve hydrogen gas during 10 h light irradiation ( Fig. 4a and Supplementary Fig. 28). The average rates of hydrogen evolution were quantified as 80 NiPor-DETH-COF and ZnPor-DETH-COF, respectively. It should be emphasized that, this trend of hydrogen evolution efficiency matches well with the transient emission decays and chopped photocurrent tests of the four COFs, indicating the important factor of charge-carrier dynamics. The apparent quantum efficiency (AQE) of ZnPor-DETH-COF towards photocatalytic hydrogen evolution was measured as 0.063% at 450 nm by taking TEOA as the sacrificial reagent (see details in Supplementary Fig. 29). By optimizing the Pt content and the type of sacrificial reagent, the hydrogen evolution amount of ZnPor-DETH-COF could be further improved ( Supplementary  Fig. 30), and AQE was determined to be 0.32%.
From TEM characterization, Pt nanoparticles (NPs) with similar morphology that identified by the characteristic crystal spacing (2. . In addition, these COFs showed excellent durability of photocatalytic hydrogen evolution. For example, the hydrogen production rate of ZnPor-DETH-COF is preserved even after 120 h irradiation (Fig. 4b).
In order to confirm the durability of electronic and chemical structures for the metal ion centers in photocatalysis process, we then performed X-ray absorption spectroscopy (XAS) measurement of three metalloporphyrin-based COFs (CoPor-DETH-COF, NiPor-DETH-COF, and ZnPor-DETH-COF) before and after hydrogen evolution. In the E-space curves, the M K-edge absorption of these three COFs remained unchanged during the reaction ( Supplementary Fig. 38), confirming the invariability of the electronic configuration of metal centers. From Fourier transform R-space spectra (Fig. 5a-c), these three COFs still showed two main coordinated peaks after photocatalysis, which could be attributed to the unaltered primary M-N bond and secondary M-C coordination layer. In addition, their coordination numbers (CNs) of M-N and M-C were determined as~4 and~8 (see fitting details in Supplementary Table 7), strongly indicating the retained metalloporphyrin structures. Moreover, continuous Cauchy wavelet transform (CCWT) analysis displayed two intensity maximums, which were totally different from the standard Co, Ni, and Zn foils (Fig. 5d-l). Based on XAS R-space and CCWT analysis, we believe the single-atom characteristic of metal ion centers in all of these three COFs are well retained, which can clearly rule out the formation of metal clusters that might serve as catalytic center in the process of hydrogen photogeneration.
Kinetic analysis of photocatalytic processes. As the hydrogen evolution was performed in alkaline condition, the reductive halfreaction occurs via following reaction: We calculated the interaction between H 2 O and metalloporphyrin-based COFs (CoPor-DETH-COF, NiPor-DETH-COF, and ZnPor-DETH-COF) by density functional theory (DFT) (see models in Supplementary Fig. 41). Accordingly, the H 2 O absorption energy change (ΔE) at CoPor-DETH-COF, NiPor-DETH-COF, and ZnPor-DETH-COF is 0.554, 0.847, and 0.659 eV, respectively ( Supplementary Fig. 42). The enormously positive values strongly indicate the adsorption and activation of H 2 O molecules at the metal centers is unfavorable 56 , which matches well with the negligible hydrogen production activity in the absence of Pt-cocatalysts ( Supplementary Fig. 27). Therefore, it can be concluded that these COFs are light absorbers and can be photoexcited to produce electron-hole pairs, and the electrons migrate to the photo-deposited Pt NPs for hydrogen evolution while holes are consumed by sacrificial reagents. The concrete changes in the excited state and the whole catalytic processes can be found in Supplementary Figs. 39 and 40.
Then, the different photocalytical H 2 generation efficiency of MPor-DETH-COFs was studied by DFT calculations. As shown in Fig. 6 and Supplementary Figs. 43 and 44, the electron density is trapped within the porphyrin cores, thus lowering the in-plane charge transport. In consideration of their AA stacking structures, the out-of-plane charge carrier migration of these four COFs might become the main pathway. To validate this point, the projected density of states (PDOS) for their monolayer and bilayer structures are calculated (Supplementary Fig. 45). Compared with monolayer structures, the band gaps of bilayer counterparties are narrowed~0.2-0.4 eV due to the interlayer ππ interaction (Supplementary Table 8), indicating the favored interlayer charge carrier migration 57 .
In principle, considering the AA stacking structures of MPor-DETH-COFs, the macrocycle-on-macrocycle and metal-on-metal channels within porphyrin columns would play vital roles in the kinetics of charge-carrier separation and migration 21 . Usually, upon light excitation of the porphyrin π-ring, the photogenerated electron migration relies on metal-on-metal channel [M n 2n+ + e − → M n (2n−1)+ (n >> 1, M represents the metal ion)] rather than localized on a specific metal center, while photogenerated holes mainly transfer through macrocycle-onmacrocycle pathway. For H 2 Por-DETH-COF, as no metal exists, both electron and hole migration proceed via macrocycle-onmacrocycle pathway (Fig. 6a), which will increase the possibility of charge recombination and thus lead to a short emission lifetime. However, for CoPor-DETH-COF and NiPor-DETH-COF, ligandto-metal charge transfer (LMCT) process can be taken into consideration, since it significantly restrains the hole migration via macrocycle-on-macrocycle channel. Specifically, for CoPor-DETH-COF, LMCT process is preeminent owing to the 3d 7 configuration of Co 2+ , which suppresses holes migration ( Fig. 6 and Supplementary Figs. 43 and 44). As a result, CoPor-DETH-COF showed the worst activity of hydrogen evolution. With the increase of d-electrons (3d 8 for Ni 2+ ), the LMCT process is partially suppressed, and hole transfer ability through macrocycle-onmacrocycle channel will be improved. Finally, in the case of Zn 2+ ion with 3d 10 configuration, the LMCT process is strictly forbidden (the variation of center metal electrons density from Co 2+ to Zn 2+ can be clearly seen in Fig. 6). Therefore, the holes of ZnPor-DETH-COF can freely migrate via macrocycle-on-macrocycle channel to the surface and the electrons transfer via Zn⋯Zn chain, which will result in the long-time charge-separation state. Accordingly, ZnPor-DETH-COF demonstrates the highest activity toward photocatalytic hydrogen evolution under the identical conditions.

Discussion
In summary, in order to explore the structure-property-activity relationship in photocatalytic HER from a molecular level, we have reported the designed synthesis and characterization of four isostructural porphyrinic 2D COFs, which have high crystallinity and large pore surface. Interestingly, by incorporating different transition metals into the porphyrin rings, the photophysical and electronic properties of the porphyrinic COFs are adjusted. More importantly, these COFs showed tunable photocatalytic hydrogen production rate, mainly ascribed to their tailored charge-carrier dynamics via molecular engineering. Consequently, we believe the charge-carrier dynamics of COFs play a very important role in the photocatalytic HER from water. This study not only represents a simple example to efficiently tune the photocatalytic hydrogen evolution activities of COFs at molecular level, but also provides valuable insight on the structure design COFs for better photocatalytic performance in future. The construction of efficient COF-based photocatalysts (e.g., CO 2 reduction 46 ) is undergoing in the lab.
Photocatalysis experiment. The H 2 photogeneration test was performed with a 20 mL pyrex tube holding MPor-DETH-COF (2.5 mg), 5 mL phosphate buffer solution (0.1 M, pH = 7.0). The suspension was ultrasonicated for 30 min before adding 2.5 μL 8 wt% H 2 PtCl 6 and 50 μL triethanolamine (TEOA), and then degassing by Ar bubbling for 30 min. Six hundred microliter of CH 4 was injected into the system and functioned as the internal standard for quantitative analysis. Xe lamps (300 W) as light source for testing H 2 evolution performance, and using air fan to keep room temperature of the sample. The generated H 2 gas in the headspace of reactor was taken with a gas-tight syringe and measured by using a gas chromatograph (Shimadzu GC2014CAFC/APC) equipped with a thermal conductivity detector and a 5 Å molecular sieves GC column. Ar was used as a carrier gas.

Data availability
The data that support the plots within this paper and other findings of this study are available from the corresponding author on request.