Linker engineering of covalent organic frameworks for efficient photocatalytic hydrogen evolution

Abstract

Interfacial charge transfer and active sites play important roles in the performance of heterogeneous photocatalysts. Reticular chemistry in covalent organic frameworks (COFs) allows the construction of isomeric architectures made of different donor and acceptor monomers for tuning the charge transfer dynamics and active sites. Herein, five D–A dual-pore COFs were prepared from the reaction of naphthalene-2,6-diamine (electron donor) with different tetraaldehyde electron acceptors. Experimental results disclosed that linker engineering, by changing the conjugation systems using heteroatoms of benzooxadiazole, benzothiadiazole, benzoselenadiazole, naphthothiadiazole, and naphthoselenadiazole, tuned the electron-accepting capacity of the corresponding D–A COFs. Among the five samples, the naphthothiadiazole-derived COF demonstrated optimal charge transfer and active sites, exhibiting the highest hydrogen evolution rate of ca. 35 mmol g⁻¹ h⁻¹ in the presence of 3 wt% Pt under visible-light irradiation (>420 nm). This work illustrates linker engineering as a strategy for the simultaneous adjustment of interfacial charge transfer and active sites to enhance the hydrogen generation efficiency, offering new vigor to develop the COF photocatalysts on the basis of reticular synthesis.

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Introduction

Covalent–organic frameworks (COFs) represent a novel class of porous organic polymers with good crystallinity, well-defined ordered structures, low density, high specific surface area, and uniform pores.^1–5 Since the discovery of COFs in 2005, these materials have attracted considerable attention for their diverse structures/topologies and applications.^6–10 Moreover, the reticular synthesis of COFs has become a useful tool to fine-tune their components, linkages, topologies, pores, and dimensions.^11–17 Therefore, a variety of COFs have been constructed for applications in sensing, separation, gas storage, electronics, and photocatalysis.^18–23 In this regard, donor–acceptor (D–A) COFs and their derivatives, including D–A–A, D–π–A, D–A–π–A, and D–A–π–D species, have emerged as hot platform materials to develop novel photocatalysts with enhanced charge separation efficiency and charge transfer.^24–27 However, building smooth pathways between the donors and acceptors and understanding the dynamics of charge transfer in periodic D–A junctions are difficult due to the complicated and rapid mechanisms involved in these semiconductive COFs.²⁸ In this direction, the reticular synthesis of COFs to finely engineer the functional building blocks of photocatalysis is certainly a promising approach to correlate the relationship between their structure and photocatalytic properties.^29–35 It is envisaged that fine adjustments in the electron-donating and electron-accepting ability of the building blocks would aid the exploration of optimal electronic structures of D–A COFs for developing high-performance photocatalysts.

Photocatalytic hydrogen evolution from water splitting is one of the promising methods to tackle the energy crisis using sustainable light and water sources.^36–38 The hydrogen evolution reaction (HER) was first reported on an n-TiO₂ photocatalyst in 1972,³⁹ which sparked significant interest in the exploration of various HER photocatalysts. Inorganic and organic semiconductors, including CdS, ZnO, and carbon nitrides, organic polymers, metal–organic frameworks (MOFs), COFs, and supramolecular materials have been demonstrated to catalyse the HER under mild conditions.^40–46 In contrast to inorganic species, earth-abundant organic polymeric semiconductors offer tunable optical gaps to achieve enhanced visible-light photocatalytic performance.⁴⁷ Recently, COFs have been used as organic polymeric semiconductors in photocatalytic HER.^48–51 In 2015, a series of azine-linked COFs with varying numbers of nitrogen atoms on the aromatic rings was exploited by Lotsch and co-workers, resulting in an H₂ evolution rate of up to 1703 μmol h⁻¹ g⁻¹.⁵² Moreover, diverse methods have been proposed to enhance the photocatalytic HER performance, including the implantation of photoactive sites into the frameworks,⁵³ fabrication of D–A/D–π–A analogues,^54,55 and construction of fully-conjugated species,⁵⁶ and their composites.⁵⁷

In this work, five two-dimensional (2D) dual-pore D–A COFs (USTB-43, USTB-44, USTB-46, USTB-57, and USTB-58) were prepared via the Schiff-base condensation reaction of naphthalene-2,6-diamine (NDA) as an electron donor with five tetraaldehyde monomers, namely, 5,5′-(benzo[c][1,2,5]oxadiazole-4,7-diyl)diisophthalaldehyde (BODP), 5,5′-(benzo[c][1,2,5]thiadiazole-4,7-diyl)diisophthalaldehyde (BTDP), 5,5′-(naphtho[2,3-c][1,2,5]thiadiazole-4,9-diyl)diisophthalaldehyde (NTDP), 5,5′-(benzo[c][1,2,5]selenadiazole-4,7-diyl)diisophthalaldehyde (BSDP), and 5,5′-(naphtho[2,3-c][1,2,5]selenadiazole-4,9-diyl)diisophthalaldehyde (NSDP) units, as shown in Scheme S1.† The electron-accepting capacity of the D–A COFs was tuned by using heteroatoms of benzooxadiazole (BO), benzothiadiazole (BT), and benzoselenadiazole (BS)in USTB-43, USTB-44, and USTB-57, respectively. Their conjugation systems were further enlarged to naphthothiadiazole (NT) and naphthoselenadiazole (NS) in USTB-46 and USTB-58, respectively. The photocatalytic HER performances of these five COFs were evaluated under visible-light irradiation in the presence of 3 wt% platinum as a cocatalyst and ascorbic acid (Aa) as a hole scavenger. Notably, USTB-46 showed the highest hydrogen evolution rate of ca. 35 mmol g⁻¹ h⁻¹, surpassing most previously reported COF-based photocatalysts.

Results and discussion

Non-nitrogen heteroatoms, such as aromatic BO, BT, and BS, in combination with the enlargement of the conjugation core to NT and NS, are expected to regulate the electronic structures of the acceptors. As a result, the electron donor and the compatible acceptor can enhance charge transfer in the COFs to achieve efficient charge separation for HER. For this, five corresponding tetraaldehydes, including three new compounds (BODP, BSDP, and NSDP) and two reported linkers (BTDP and NTDP), were selected and synthesized using the Suzuki–Miyaura coupling reaction, as depicted in Fig. S1–3.† Additionally, we observed that both BODP and BSDP were insoluble in common organic solvents, such as DMSO and CHCl₃, making it difficult to obtain their NMR spectra. Subsequently, a series of five D–A COFs, including three new compounds USTB-43, USTB-57, and USTB-58, as well as the old species USTB-44 and USTB-46,⁵⁸ based on benzothiadiazole derivatives were prepared under conventional solvothermal conditions, as illustrated in Scheme 1. The employment of three benzothiadiazole analogues led to the formation of USTB-43, USTB-44 and USTB-57 in yellow color (Fig. S4†). The use of naphthothiadiazole analogues resulted in the generation of USTB-46 and USTB-58 in red and dark red color, respectively. The distinct colors of these COFs primarily arise from different chromophores, indicating their variable electronic structures.

Scheme 1 Schematic of the synthesis and structures of USTB-43, USTB-44, USTB-46, USTB-57, and USTB-58.

Fourier-transform infrared (FT-IR) spectra were recorded for the three new COFs, USTB-43, USTB-57, and USTB-58 (Fig. S5–7†). In comparison with the FT-IR spectra of the tetraaldehyde monomers with a strong band at ca. 1700 cm⁻¹, USTB-43, USTB-57, and USTB-58 displayed a tiny band at the same positions due to the presence of unreacted aldehyde groups at the edges of the COFs. The bands at 3390 and 3303 cm⁻¹ derived from the amino group of NDA were also absent in these three compounds. In addition, strong bands at ca. 1632 cm⁻¹ were seen owing to the generation of imine bonds in these COFs. These changes in FT-IR spectra were verified for USTB-44, USTB-46 and their corresponding monomers (Fig. S8 and 9†). For the three new COFs, the solid-state ¹³C NMR spectra revealed a signal at 157 ppm (Fig. S10–12†), corresponding with imine carbon peaks. Moreover, distinct carbon peaks were observed at 146 ppm for the BO moieties in USTB-43, and 157 ppm for the BS and NS moieties of USTB-57 and USTB-58, respectively. These results confirm the integration of BODP, BSDP, and NSDP into the COFs.

Reticular synthesis based on tetraaldehydes and NDA monomers enabled the formation of dual-pore COFs in an eclipsed (AA) packing mode.⁵⁸ The 2D biporous structural models of these new imine-bonded COFs were found to belong to the trigonal system with the P₃ space group, as displayed in Fig. 1a and S13.† The ordered structures of USTB-44 and USTB-46 were checked using powder X-ray diffraction (PXRD), and the patterns are shown in Fig. 1b and c. The consistency between the experimental and simulated PXRD profiles confirms the successful generation of the frameworks. The crystallinity of USTB-43, USTB-57, and USTB-58 was also examined based on PXRD investigation. For USTB-43, three diffraction peaks (2θ) were observed at ca. 4.61°, 9.29°, and 25.85°, corresponding to the (210), (420), and (001) planes, respectively (Fig. 1d). These reflections indicate that USTB-43 had high periodicity in the three-dimensional space. Furthermore, Pawley refinement was performed on the experimental PXRD data to derive the unit cell parameters as follows: a = b = 37.59 Å, c = 3.45 Å, α = β = 90°, and γ = 120° (R_p = 4.21% and R_wp = 5.13%). The substitution of oxygen atoms in BODP with selenium atoms resulted in a slight shift of the diffraction peaks to ca. 4.58° and 9.17° in the USTB-57 pattern (Fig. 1e), corresponding to the (210) and (420) lattice planes, respectively. This minor displacement can likely be attributed to a slight interlayer slip caused by the large atomic radius of selenium. The refined cell parameters were a = b = 37.85 Å, c = 3.44 Å, α = β = 90°, and γ = 120° (R_p = 3.06% and R_wp = 2.33%). According to the PXRD study, a slight structural difference exists between USTB-46 and USTB-58 made up of NDA and naphthothiadiazole derivatives (Fig. 1f).

Fig. 1 (a) AA stacking mode of USTB-46. Experimental and simulated PXRD patterns of (b) USTB-44, (c) USTB-46, (d) USTB-43, (e) USTB-57, and (f) USTB-58.

The scanning electron microscopy (SEM) images revealed an irregular rod-like morphology composed of nanosheets for USTB-43, USTB-57, and USTB-58, as well as USTB-44 and USTB-46 (Fig. S14†). The transmission electron microscopy (TEM) images further confirmed the layer-like nanosheet structures in these COFs (Fig. S15†). As shown in Fig. S16,† similar lattice fringe spacings were measured: 1.80, 1.80, 1.77, 1.79, and 1.78 nm for USTB-43, USTB-44, USTB-57, USTB-46, and USTB-58, respectively. This is due to their isostructural nature with similar unit cell parameters.

Energy-dispersive X-ray (EDX) mapping images clearly revealed the presence of C, N, and individual O/Se elements in USTB-43, USTB-57, and USTB-58 (Fig. S17–21†). The chemical composition of USTB-43, USTB-57, and USTB-58 was further analyzed using X-ray photoelectron spectroscopy (XPS), which confirmed the presence of C, N, and O/Se elements across the three samples (Fig. S22†). In the deconvoluted N 1s XPS spectrum, USTB-43 exhibited double characteristic peaks at 398.30 and 398.91 eV (Fig. S23†) because of the nitrogen atoms in the imine bond and oxadiazole ring, respectively. The O 1s XPS spectrum displayed a binding energy peak at 532.41 eV owing to the benzooxadiazole oxygen atom. The isostructural COFs USTB-57 and USTB-58 were also analyzed by XPS, and similar N 1s peaks were observed at corresponding binding energy values for N_imine and N_{selenadiazole}: at 398.30 eV and 398.91 eV for USTB-57 and 398.32 and 399.21 eV for USTB-58, respectively. Their Se 3d XPS data was composed of Se 3d_3/2 and Se 3d_5/2 signals at 57.66 and 56.80 eV for USTB-57 and 57.98 and 56.98 eV for USTB-58 (Fig. S24 and 25†).

The permanent porosity and surface area of these COFs were explored by performing N₂ physisorption measurements at 77 K. USTB-44 and USTB-exhibited high Brunauer–Emmett–Teller (BET) surface areas of 1339 m² g⁻¹ and 1107 m² g⁻¹, respectively (Fig. 2a and b). As illustrated in Fig. 2c, a typical type-I isotherm was observed for USTB-43, indicating its microporous nature. The BET surface area was calculated to be 449 m² g⁻¹. Furthermore, its pore size distribution was fitted using the density functional theory (DFT) based on the cylindrical pore model, revealing dominant double pores with sizes of 1.45 and 1.63 nm (Fig. 2f). These well-consistent values with the theoretical sizes (1.34 and 1.75 nm) favor the above-proposed structure. Replacing the electron-withdrawing units BTDP with BSDP and NSDP led to BET surface areas of 1088 m² g⁻¹ for USTB-57 and 352 m² g⁻¹ for USTB-58 (Fig. 2d and e). The fitted pore sizes were 1.56 and 1.84 nm for USTB-57 and 1.56 and 1.84 nm for USTB-58. Among this series of COFs, USTB-43 and USTB-58 had smaller BET surface areas in comparison with the other three compounds because their O and Se atoms with either small or big atomic radii are harmful to their tight π–π stacking and thus lead to poor crystallinity.

Fig. 2 N₂ adsorption and desorption isotherms of (a) USTB-44, (b) USTB-46, (c) USTB-43, (d) USTB-57, and (e) USTB-58 obtained at 77 K. (f) Pore size distributions of USTB-43, USTB-44, USTB-46, USTB-57, and USTB-58.

Solid-state ultraviolet-visible diffuse reflection absorption spectra (UV-vis DRS) of COFs showed broad optical absorbance in the range of 400–800 nm. In particular, USTB-43, USTB-44, and USTB-57 revealed visible-light absorbance before 450 nm (Fig. 3a). In contrast, USTB-46 and USTB-58 showed a wider absorption band before 550 nm and 580 nm, respectively. The visible-light absorbance peak widths for these five COFs were in the order of USTB-58 > USTB-46 > USTB-43 > USTB-57 > USTB-44, revealing the same π-conjugated order. The optical band-gaps (E_g) of USTB-43, USTB-44, USTB-46, USTB-57, and USTB-58 calculated from the Tauc plots (Fig. 3b) were 2.19, 2.34, 2.05, 2.31, and 1.85 eV, respectively. The Mott–Schottky (MS) plots of these five COFs presented positive slopes (Fig. S26–30†), demonstrating their n-type semiconductor nature. The positions of their conduction bands (CB) were estimated to be −0.30, −0.33, −0.16, −0.38, and −0.30 V (vs. NHE, pH = 7), respectively. The corresponding valence bands (VB) were at 1.89, 2.01, 1.89, 1.93, and 1.55 V, respectively, based on the formula E_VB = E_CB + E_g. The high VB energy levels of all COFs show their potential in photocatalytic HER from the thermodynamic perspective (Fig. 3c).

Fig. 3 (a) UV-vis DRS spectra, (b) Tauc plots, and (c) energy band structures of USTB-43, USTB-44, USTB-46, USTB-57, and USTB-58. Photocatalytic H₂ evolution activity with different concentrations of (d) Aa and (e) different loadings of Pt, (Conditions: 5.0 mg USTB-46 containing different Pt loadings (from H₂PtCl₆), 20.0 mL H₂O, and different concentrations of Aa under λ > 420 nm light irradiation). (f) Time course data of photocatalytic H₂ evolution using USTB-43, USTB-44, USTB-46, USTB-57, and USTB-58. (g) Recyclability test of USTB-46. (h) Photocurrent responses and (i) EIS plots of USTB-43, USTB-44, USTB-46, USTB-57, and USTB-58.

Visible-light-driven HER activity of the five COFs was explored under light irradiation (λ > 420 nm) with the help of Aa as a sacrificial reagent. In addition, the Pt co-catalyst was employed by in situ photo-deposition to reduce the H₂PtCl₆ precursors during the irradiation process. The effects of Aa concentration and Pt loading were screened to identify the optimal conditions for photocatalysis using USTB-46 as the photocatalyst. As illustrated in Fig. 3d, an Aa concentration of 0.06 M resulted in a hydrogen evolution rate of 20.07 mmol g⁻¹ h⁻¹ at 4.0 h of visible-light irradiation when 3 wt% Pt was used as a co-catalyst. Increasing the Aa concentration to 0.08 and 0.10 M further enhanced the hydrogen generation rate to 27.45 and 35.14 mmol g⁻¹ h⁻¹, respectively. However, the increase in Aa concentration to 0.12 and 0.14 M caused a decline in the hydrogen evolution rate to 26.45 and 21.08 mmol g⁻¹ h⁻¹, respectively. The screening of different Pt loadings disclosed that the optimal loading was 3 wt% at an Aa concentration of 0.1 M (Fig. 3e). Consequently, photocatalytic HER was conducted under optimized conditions: an Aa concentration of 0.10 M and Pt loading of 3 wt%. For USTB-43, the hydrogen evolution rate was 0.13 mmol g⁻¹ h⁻¹. The replacement of the BO core in USTB-43 with BT and BS units correspondingly promoted the higher evolution rate to 6.17 and 2.1 mmol g⁻¹ h⁻¹ in USTB-44 and USTB-57, indicating the important role of non-nitrogen heteroatoms in the photocatalytic HER mechanism. The increase in conjugation using BT and BS in USTB-44 and USTB-57, respectively, to form NT and NS chromophores further changed the hydrogen production rate to 35.14 and 0.4 mmol g⁻¹ h⁻¹ in USTB-46 and USTB-58, respectively. The remarkable photocatalytic performance of USTB-46 is superior to most reported excellent COF-based photocatalysts, such as USTB-10 (21.8 mmol g⁻¹ h⁻¹),⁵⁸ RuCOF-TPB (20.3 mmol g⁻¹ h⁻¹),⁵⁹ BTT-NDA (5.22 mmol g⁻¹ h⁻¹),³¹ TpPa-Cu(II) (14.72 mmol g⁻¹ h⁻¹),⁶⁰ BTH-3 (15.1 mmol g⁻¹ h⁻¹),⁶¹ COF-923-AC (23.4 mmol g⁻¹ h⁻¹),⁶² Co₉S₈@COF (23.15 mmol g⁻¹ h⁻¹),⁶³ and PtSA@S-TFPT (11.4 mmol g⁻¹ h⁻¹).⁶⁴ In addition, in comparison with the several studies that explored the effects of oxygen, sulfur, and selenium substitutions on hydrogen evolution performance, new linkers have been used in this work to prepare COFs, enriching the structural diversity in the field of COFs. On the other hand, USTB-46 exhibits remarkable photocatalytic performance, much better than those of thiadiazole-derivative COFs, such as HIAM-0004,⁶⁵ TMT-BO-COF,⁶⁶ Py-BT-COF,⁶⁷ HIAM-0011,⁶⁸ and HIAM-0015,⁶⁹ (Table S1†).

Over 20 hours, the USTB-46 catalyst exhibited almost no decrease in photocatalytic performance after five cycles under visible-light irradiation (Fig. 3g), indicating its long-term photocatalytic stability. The characterization of all five recycled COFs was conducted using PXRD and FT-IR (Fig. S31–35†). There were no obvious changes in the composition of the COF samples after photocatalysis. As shown in the TEM images in Fig. S36 and 37,† USTB-46, the representative of this series of photocatalysts, retained its layered nanosheet structure after the photocatalytic test. In contrast to fresh USTB-46, fine Pt particles were evenly distributed across the surface of the COF. This observation was corroborated by elemental mapping images, showing the distributions of C, N, S, and Pt in used USTB-46 (Fig. S38†).

For these five COFs, the disorder rule of the hydrogen generation rates was not directly related to the values of bandgap, CB, and VB positions, as well as BET surface areas and porosity size. As a result, the main origin of their different hydrogen evolution rates should be associated with the interfacial charge transfer and active sites. Under 420 nm wavelength excitation, a broad emission band was observed for all five COFs. Their maximum emission peaks were at 537, 533, 534, 614, and 623 nm for USTB-43, USTB-44, USTB-57, USTB-46, and USTB-58, respectively. Replacing an O atom in USTB-43 with S and Se atoms in USTB-44 and USTB-57, respectively, led to a similar broad emission peak but with reduced emission intensity. In contrast to USTB-44 and USTB-57, the introduction of bigger conjugated linkers in USTB-46 and USTB-58 induced a significant red-shift in the maximum emission wavelength (Fig. S39†). The fluorescence intensities of the individual maximum emission peaks of the five COFs followed the order of USTB-46 < USTB-58 < USTB-44 < USTB-57 < USTB-43, which correlates with the gradual reduction in charge separation and migration capabilities.⁶⁴ In addition, we conducted transient fluorescence spectroscopy measurements (Fig. S40†). Notably, the fluorescence lifetimes of the maximum peaks of USTB-43, USTB-44, USTB-57, USTB-46, and USTB-58 were estimated to be 0.98, 2.46, 2.10, 2.88, and 1.76 ns, respectively. The longest lifetime observed for USTB-46 suggests suppression of the recombination of photoinduced charge carriers in this material.⁷⁰ Additionally, we calculated the exciton binding energy (E_b) to validate the photophysical processes of the five COFs (Fig. S41†). The E_b values of USTB-43, USTB-44, USTB-57, USTB-46, and USTB-58 were 1.656, 1.656, 1.650 1.667, and 1.723 eV, respectively. Among the five COFs, the highest E_b value of USTB-58 indicates hindered exciton dissociation.³² The above results indicate that USTB-46 has excellent separation of photogenerated charge carriers and suppressed recombination, leading to its superior hydrogen generation capability among the five COFs.

The separation efficiency of photogenerated electrons and holes was examined through transient photocurrent measurements and electrochemical impedance spectroscopy (EIS) tests. As depicted in Fig. 3h, the photocurrent density of the five COFs followed this order: USTB-46 > USTB-44 > USTB-57 > USTB-58 > USTB-43, indicating the same order for the charge separation efficiency. This trend aligns with their respective hydrogen production rates, indicating the superior charge transfer ability of USTB-46, followed by USTB-44 > USTB-57 > USTB-58 > USTB-43. The EIS measurements revealed that the radii for all five COFs followed an inverse order in comparison with the order of photocurrent density (Fig. 3i), disclosing their electronic conductivity was also in the order of USTB-46 > USTB-44 > USTB-57 > USTB-58 > USTB-43. This is beneficial to their charge transfer order. These findings suggest that linker engineering guarantees the adjustment of interfacial charge transfer to obtain high charge separation efficiency.⁷¹ Additionally, Fig. S42† compiles the water contact angles of USTB-43, USTB-44, USTB-46, USTB-57, and USTB-58 in a very narrow range of 112°–125°, indicating their similar hydrophobic nature. This precludes the vital role of hydrophilicity in photocatalytic HER. It is worth noting that the shorter lifetime and bigger exciton binding energy of USTB-58 (1.76 ns & 1.723 eV) than those of USTB-57 (2.10 ns & 1.650 eV) are responsible for its worse capacity in catalyzing photocatalytic hydrogen evolution among the tested COFs.

The charge transfer capacity of the five COFs was calculated based on the density functional theory (DFT). The electrostatic potential (ESP) distribution of the cutout models, namely, USTB-43′, USTB-44′, USTB-46′, USTB-57′, and USTB-58′, are presented in Fig. 4a. For USTB-43′, electron-rich sites were observed around the oxygen and nitrogen atoms in the BO moiety and naphthalen-2-amine moiety, which facilitates the adsorption of water and protons. The substitution of the oxygen atom in BODP with sulfur or selenium led to alterations in the electronic distribution around the nitrogen atoms in USTB-44′, USTB-46′, USTB-57′, and USTB-58′. Notably, USTB-46′ exhibits a pronounced electron-rich nature around the nitrogen atom and thereby displays an enhanced affinity for water and protons. In addition, USTB-46′ demonstrated a dipole moment of 2.29 D, which is larger than those of the other three COFs USTB-44′ (2.07 D), USTB-57′ (1.87 d), and USTB-58′ (2.02 D), indicating that the NT moiety has an excellent capacity to promote charge transfer in USTB-46′ compared with the other COFs.⁷² This would be helpful in the improvement of electron–hole separation in this COF.^73,74 Surprisingly, given the biggest dipole moment of 3.50 D for USTB-43′, it seems that USTB-43 has the most efficient charge transfer capability. However, among the five COFs, USTB-43 showed the greatest emission intensity and shortest lifetime, implying the worst charge separation efficiency. This was confirmed by its smallest photocurrent intensity among these COFs, leading to the worst photocatalytic hydrogen evolution performance. Additionally, we conducted frontier orbital analysis on the cluster models of these five COFs. The highest occupied molecular orbitals (HOMO) were distributed at the naphthalene (donor) and imine (linkage) moieties in USTB-43′, USTB-44′, and USTB-57′, while they were mainly located over the whole model (D–A junction) in USTB-46′, and the BS unit (acceptor) and imine moiety in USTB-58′ (Fig. 4b and c). Conversely, the lowest unoccupied molecular orbitals (LUMO) resided at the acceptor sites, namely, BO, BT, NT, BS, and NS, for these five COFs, respectively. In contrast to BT ad BS in USTB-44′ and USTB-57′, increasing the conjugation system of the corresponding acceptor resulted in an increase in HOMO energy value and a decrease in the LUMO energy values of USTB-46′ and USTB-58′. As a result, their HOMO–LUMO gaps were reduced, facilitating exciton dissociation and charge separation.⁷⁵ Notably, the biggest HOMO–LUMO gap of 3.36 eV in USTB-43′ is harmful in terms of the light captured for excitation and exciton dissociation to obtain the high charge separation efficiency, although it would have efficient charge transfer according to the above theoretical calculation.

Fig. 4 (a) Electrostatic potential distribution (blue and red represent electron depletion and accumulation), (b) HOMO–LUMO distribution and (c) energy levels of HOMO and LUMO of USTB-43, USTB-44, USTB-46, USTB-57, and USTB-58. (d) Proposed H₂ evolution reaction pathway at the naphthothiadiazole nitrogen in USTB-46. C: gray; N: blue; H: white; S: yellow. (e) Free energy diagrams of the H₂ evolution reactions of USTB-43, USTB-44, USTB-46, USTB-57, and USTB-58.

To gain a deeper understanding of the excellent catalytic activity of USTB-46 among the five COFs in photocatalytic HER, the one-step catalytic mechanism of proton-adsorption–reduction-hydrogen-desorption was comparatively studied over fragments of USTB-43, USTB-44, USTB-46, USTB-57, and USTB-58, as shown in Fig. 4d. For example, the various hydrogen adsorption sites across the sulfur, nitrogen, carbon atoms (site 1 to site 13) within USTB-46 were calculated to identify the optimal adsorption sites for this photocatalyst.^76,77 As illustrated in Fig. S43,† the nitrogen atom within the 1,2,5-thiadiazole unit of NT chromophores was found to possess the lowest binding energy, and was thus identified as the optimal catalytic site. The same catalytic site in the thiadiazole derivatives was regarded for the model molecules of USTB-43, USTB-44, USTB-57, and USTB-58, as shown in Fig. 4e. The energy barrier associated with *H intermediate formation at the active site of USTB-46′ was found to be 0.01 eV. In contrast to the models of the other four thiadiazole-derivative COFs, USTB-46′ has a remarkably low absolute value for the *H intermediate formation barrier, implying that USTB-46 is the most suitable catalyst for photocatalytic hydrogen evolution from the thermodynamic perspective. Notably, USTB-58′ exhibited a negative *H intermediate formation energy of −0.25 eV, illustrating that its hydrogen adsorption step is the most thermodynamically favorable among the five COFs. However, it has very poor photocatalytic hydrogen evolution performance, which may be associated with its kinetic photophysical behaviors. The short lifetime and big exciton binding energy of USTB-58 (1.76 ns & 1.723 eV) play crucial roles in its poor charge separation efficiency and thus poor photocatalytic hydrogen generation rate. In a word, USTB-46 has outstanding photocatalytic hydrogen generation performance, as determined by both thermodynamic and kinetic analyses.

Conclusions

In summary, we have designed and synthesized five 2D D–A COFs containing thiadiazole derivatives as both the linker and electron acceptor. The findings exemplify that linker engineering by tuning the non-nitrogen heteroatom and conjugation core enables optimization of the electronic structure of the electron acceptor to match with naphthalene-2,6-diamine as the electron donor, promoting interfacial charge transfer. In addition, this strategy also provides active functional sites for photocatalytic HER. This two-pronged approach has been demonstrated to successfully obtain a high-performance photocatalyst, with a remarkable hydrogen generation rate of 35 mmol g⁻¹ h⁻¹. This work illustrates the powerful advantage of the reticular synthesis toolkit to develop COF photocatalysts. The design and preparation of new D–A COFs and analogues based on functional molecular modules and linkages are in progress to fabricate excellent photocatalysts.

Author contributions

J. Jiang, H. Wang, and X. Ding conceived and supervised the project, X. Ding performed experiments, J. Jiang, H. Wang, and X. Ding linked the experiments and analysis, and all the authors discussed and wrote the manuscript.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Natural Science Foundation of China (No. 22261132512, 22235001, 22175020, and 22131005), Xiaomi Young Scholar Program, and University of Science and Technology Beijing.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5qi00417a

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