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. 2026 Aug 7;21(15):e70886. doi: 10.1002/asia.70886

Structure–Property Relation of Mechanochemically Synthesized β‐Ketoenamine‐Linked COFs in Photocatalytic Hydrogen Evolution

Kiran Asokan 1,2, Sukumaran Santhosh Babu 1,2,✉
PMCID: PMC13448750  PMID: 42563593

ABSTRACT

Covalent organic frameworks (COFs) are promising photocatalysts for solar‐driven hydrogen evolution due to their tunable structures and intrinsic porosity. In this work, a series of Tp–Pa‐based COFs bearing different electron‐withdrawing groups was synthesized via a scalable mechanochemical route to examine the impact of framework functionalization on their structural, electronic, and photocatalytic properties. Spectroscopic analyses (solid‐state 1 3C CP‐MAS NMR and FT‐IR) confirmed the formation of β‐ketoenamine linkages, while PXRD revealed higher crystallinity for pristine Tp–Pa compared to its functionalized analogues. Electrochemical studies indicated n‐type semiconducting behavior, with pristine Tp–Pa exhibiting more efficient charge separation and lower charge‐transfer resistance. Under visible‐light irradiation, Tp–Pa showed the highest hydrogen evolution rate (25,717 µmol g− 1 h− 1), whereas functionalized derivatives displayed reduced activity, with Tp–Pa–NO2 being nearly inactive. Notably, Tp–Pa also demonstrated excellent performance in simulated seawater (33,331 µmol g− 1 h− 1). These findings highlight the critical role of functionalization and synthesis strategy in governing crystallinity, charge transport, and photocatalytic efficiency, offering design guidelines for scalable COF‐based hydrogen evolution systems.

Keywords: 2D polymer, covalent organic framework, hydrogen generation, photocatalysis, substituent effect

Herein, we report the mechanochemical synthesis of covalent organic frameworks (COFs) bearing diverse functional groups to systematically probe their influence on photocatalytic hydrogen evolution. Strikingly, the unfunctionalized COF outperformed its counterparts, delivering enhanced activity, robust stability, and efficient hydrogen production under simulated seawater conditions, underscoring its promise for real‐world applications.

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1. Introduction

The growing global energy demand, driven by population growth and industrialization, has intensified the need for clean and sustainable energy alternatives to reduce greenhouse gas emissions and dependence on fossil fuels. In this context, hydrogen (H2) has emerged as a key element in achieving net‐zero emissions. Continued technological advancements, coupled with targeted strategic investments, are further strengthening (H2)’s role as a transformative pillar in the future clean energy landscape. At present, the global energy system remains predominantly reliant on fossil fuels‐coal, oil, and natural gas—which collectively account for over 80% of the total energy supply [1]. The transition from carbon‐intensive fossil fuels to sustainable energy sources is propelled by the pressing need to address environmental challenges such as CO2 emissions and global warming. In addition to being finite, fossil fuels are the principal drivers of climate change [2]. In this regard, H2 has gained significant attention as a promising energy carrier due to its distinctive attributes: it is the lightest and most abundant element, can be produced domestically, and produces only water as a byproduct during utilization, thereby offering a zero‐emission energy solution [3]. Photocatalytic splitting of water has been one of the sustainable methods for the production of H2.

Photocatalytic hydrogen evolution (PHE) employing organic catalysts has gained considerable attention in recent years due to several inherent advantages over inorganic catalysts. Covalent organic frameworks (COFs) are a promising class of photocatalysts with wide‐ranging applications in energy storage [4, 5, 6, 7, 8], catalysis [9, 10], biomedical [11], membranes [12], and gas storage [13]. Notably, COFs have also demonstrated superior H2 evolution performance compared to many other catalytic systems. The structural flexibility of COFs, particularly the ability to judiciously select and design organic linkers, allows precise tuning of their electronic and physicochemical properties for photocatalytic applications [4, 14, 15, 16]. Over the years, various strategies have been employed to enhance photocatalyst stability and improve H2 evolution efficiency [17, 18, 19, 20, 21, 22, 23, 24, 25, 26]. Collectively, these advancements highlight the versatility and significant potential of COFs for sustainable H2 production. At the same time, only a limited number of articles addressed the bulk‐scale synthesis of photocatalysts for the same. Our group reported a commercially viable scale‐up synthesis of COFs to achieve in kgs [13]. As a notable achievement, our group developed a hybrid photocatalyst synthesized on a 20 g scale capable of producing H2 from simulated seawater and industrial wastewater [27]. Although different strategies have been utilized to enhance the activity of organic photocatalysts, the structure–property relationship is one of the crucial parameters that define the PHE. Over the past decade, extensive efforts have been devoted to understand the factors governing the photocatalytic performance of organic photocatalysts, including COFs [28, 29]. The influence of structural parameters, including heteroatom positioning [30], imine bond orientation [31], charge‐carrier separation and complexation [32], and so forth, on photocatalytic activity was studied earlier. Notably, the substituents on the COF backbone play a critical role in regulating the separation and recombination of photogenerated electron‐hole pairs, light absorption, and band gap engineering [23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35]. Consequently, substituent effects have been recognized as a key determinant in modulating the catalytic performance of COF‐based photocatalysts. Additionally, many existing reports lack PHE using nonpotable water sources such as seawater and wastewater. Evaluating catalyst performance in these challenging and realistic conditions is key to bridging the gap between laboratory studies and real‐world applications.

In this context, the development of efficient catalysts capable of H2 production from alternative water sources is gaining increasing attention, particularly in light of the growing scarcity of clean and pure water. This approach not only broadens the scope of photocatalytic systems but also aligns with sustainable and resource‐efficient energy generation strategies. In this work, we aim to investigate the influence of electron‐withdrawing substituents on the PHE activity of β‐ketoenamine‐linked COFs. Our study primarily focuses on establishing a substituent–property–activity relationship while addressing the feasibility of bulk‐scale COF synthesis. Furthermore, we extend our investigation to evaluate PHE performance in simulated seawater, enabling a more comprehensive understanding of how substituent effects govern catalytic activity under practical and challenging reaction environments.

2. Results and Discussion

2.1. Synthesis and Characterization

All COFs were synthesized using a previously reported mechanochemical method [13, 22]. In a typical synthesis, the amine monomer and p‐toluenesulfonic acid were first ground thoroughly using a mortar and pestle, followed by the addition of 1,3,5‐triformylphloroglucinol (Scheme 1). The resulting mixture was then transferred to a glass vial and heated at 90°C to facilitate framework formation. The crude product was subsequently purified by Soxhlet extraction. Detailed experimental procedures are provided in the Supporting Information. The synthesized catalysts are denoted as Tp‐Pa‐X, where X represents the functional substituents incorporated into the COF backbone. In this study, amine linkers bearing ─Cl, ─CN, ─SO3H, and ─NO2 groups with different electron‐withdrawing natures were employed, along with the pristine Tp‐Pa COF for comparison. At first, Solid‐state 1 3C cross‐polarization magic‐angle spinning (CP‐MAS) NMR spectroscopy was employed to confirm the formation of the COF frameworks (Figure 1a). For Tp‐Pa and Tp‐Pa‐X samples, chemical shifts observed at approximately 104.6 and 182 ppm are attributed to the ─C═C─ and ─C═O functionalities of the β‐ketoenamine linkage, respectively. These characteristic signals provide clear evidence of framework formation and offer important insights into the chemical composition and structural integrity of the synthesized materials, thereby validating the successful construction of the COFs. Further, this was confirmed by FT‐IR spectroscopy. The stretching frequencies of ─C─N and ─C═C were observed around 1256 and 1570 cm−1, respectively, which correspond to the enamine bond formation between aldehyde and amine (Figure 1b). The crystallinity of samples was analyzed using the powder X‐ray diffraction (PXRD) technique. It is to be noted that Tp‐Pa exhibited high crystallinity compared to other derivatives (Figure 1c). The decrease in crystallinity can be attributed to the presence of an electron‐withdrawing group and disturbing dynamic covalent chemistry due to mechanochemical grinding (Figure 1c). Basically, dynamic covalent chemistry (DCC) enables reversible bond formation, facilitating self‐healing and error‐correction in initially amorphous materials and ultimately leading to the development of highly crystalline structures [36]. During COF synthesis, a kinetically favored amorphous phase forms quickly owing to its low activation barrier and subsequently equilibrates into the thermodynamically stable crystalline phase, accompanied by a significant reduction in standard Gibbs free energy. The extension of DCC depends on several factors, such as reaction method, solvent system, temperature, pressure, and so on. It is to be noted that due to these reasons, DCC works well in the solvothermal method. Since the solvothermal method is not feasible in bulk‐scale synthesis of COFs, other synthesis methods have to be explored. Our objective is to employ a scalable approach for the synthesis of the COF photocatalyst; hence, we proceeded with the mechanochemical method.

SCHEME 1.

SCHEME 1

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Synthesis procedure of Tp‐Pa derivatives.

FIGURE 1.

FIGURE 1

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Characterization of Tp‐Pa and derivatives. (a) 13C CP MAS, (b) FT‐IR, (c) PXRD, (d) TGA, and (e) XPS of Tp‐Pa.

The stability of the catalyst was analyzed with thermogravimetric analysis (TGA). All COFs except Tp‐Pa‐SO3H exhibited better stability up to 380°C, which corresponds to a robust framework (Figure 1d). X‐ray photoelectron spectroscopy (XPS) revealed the presence of C 1s, N 1s, and O 1s in all COFs, and for Tp‐Pa‐Cl and Tp‐Pa‐SO3H, extra peaks of Cl2p and S2p were respectively observed (Figures 1e and S1). Further, the deconvoluted spectra for N 1s and O 1s confirmed the β‐ketoenamine linkage existing in Tp‐Pa‐X (Figures S2 and S3). The morphology of the samples was examined using field emission scanning electron microscopy (FE‐SEM) and high‐resolution transmission electron microscopy (HR‐TEM) (Figures 2a,b, and S4–S8). All COFs exhibited a sheet‐like morphology, where the sheets were stacked to form a layered structure. HR‐TEM images further confirmed the sheet‐like morphology of all COFs, displaying distinct crystalline fringes. Elemental mapping using scanning transmission electron microscopy (STEM) was also performed, confirming the presence of elements in all COFs (Figures S4–S8).

FIGURE 2.

FIGURE 2

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The characterization of Tp‐Pa and derivatives. (a) FE‐SEM and (b) HR‐TEM images of Tp‐Pa. (c) BET N2 isotherm of Tp‐Pa, and the inset shows the corresponding pore size distribution. (d) DRS spectra and (e) Tauc Plot of Tp‐Pa and derivatives. (f) Mott–Schottky analysis of Tp‐Pa.

The permanent porosity and surface area of the samples were evaluated using the Brunauer–Emmett–Teller (BET) method and density functional theory (DFT) based pore size distribution analysis. Tp‐Pa exhibited a higher BET area of 1171 m2/g, whereas other derivatives showed less surface area (Figures 2c and S9–S12). Pore size analysis revealed that Tp‐Pa possessed a pore size of 1.6 nm (Figure 2c, inset). In contrast, Tp‐Pa‐NO2 and Tp‐Pa‐SO3H exhibited different pore sizes, likely due to the loss of crystallinity and long‐range order. To further assess the hydrophilicity of the samples, water contact angle was measured, and the results indicated that all the derivatives exhibited distinct contact angles, reflecting differences in their surface wettability (Figure S13). Among them, Tp‐Pa‐SO3H showed the lowest contact angle, indicating the highest hydrophilic character. This enhanced hydrophilicity can be attributed to the presence of strongly polar sulfonic acid (─SO3H) groups, which promote stronger interactions with water molecules. Tp‐Pa‐NO2 also displayed relatively high hydrophilicity due to the polar nitro (─NO2) functionality. In comparison, Tp‐Pa‐CN and Tp‐Pa‐Cl exhibited moderate hydrophilic behavior, while pristine Tp‐Pa showed the highest contact angle, indicating comparatively lower affinity toward water.

Ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS) was employed to investigate the absorption features of the synthesized COFs (Figures 2d and S14). All the COFs exhibited broad absorption in the visible region, spanning from 300 to 750 nm, indicating their strong light‐harvesting ability. Such wide absorption suggests that these frameworks can effectively utilize a significant portion of the solar spectrum. The optical band gap energies of the materials were estimated from Tauc plots derived from the UV–vis DRS data. The calculated band gaps were 2.15, 2.08, 2.11, 1.99, and 1.97 eV for Tp‐Pa, Tp‐Pa‐Cl, Tp‐Pa‐CN, Tp‐Pa‐SO3H, and Tp‐Pa‐NO2 , respectively (Figures 2e and S15). These results indicate that functionalization of the parent Tp‐Pa framework slightly modifies the electronic structure, leading to variations in the band gap energies. In particular, Tp‐Pa‐SO3H and Tp‐Pa‐NO2 displayed relatively narrower band gaps compared to the other derivatives, which can be attributed to the presence of strong electron‐withdrawing functional groups that influence the electronic distribution within the framework. Importantly, the obtained band gap values are consistent with those reported in previous studies, confirming the reliability of the synthesized materials and their optical characteristics [33, 34, 37]. Mott–Schottky analysis was performed to investigate the semiconductor properties of the synthesized catalysts (Figures 2f and S16). All the samples exhibited a positive slope in the Mott–Schottky plots, indicating their n‐type semiconductor behavior, where electrons act as the majority charge carriers. From the intercepts of the Mott–Schottky plots, the flat band potentials were estimated to be −0.38, −0.54, −0.44, −0.53, and −0.45 eV for Tp‐Pa, Tp‐Pa‐Cl, Tp‐Pa‐CN, Tp‐Pa‐SO3H, and Tp‐Pa‐NO2 , respectively.

In addition, electrochemical impedance spectroscopy (EIS) measurements were conducted to examine the charge transfer characteristics at the electrode‐electrolyte interface (Figure 3a). The Nyquist plots reveal that Tp‐Pa has a smaller semicircular radius, corresponding to lower charge‐transfer resistance, whereas the functionalized derivatives exhibit larger semicircles, indicating hindered charge transport. Taken together, these electrochemical results demonstrate that incorporation of different electron‐withdrawing functional groups into the COF framework leads to diminished charge separation and slower charge‐transfer kinetics. Consequently, the pristine Tp‐Pa framework facilitates more efficient charge carrier separation and migration, which is reflected in its superior photocurrent response relative to the derivatized COF materials. The photocurrent responses of all samples were systematically evaluated using light‐on/off chronoamperometry measurements under open‐circuit potential conditions to assess their photoactivity (Figure 3b). Among the investigated materials, Tp‐Pa and Tp‐Pa‐CN exhibited significantly higher photocurrent densities compared to the other derivatives, indicating more efficient photoinduced charge generation and separation, as well as superior light‐harvesting capability. This enhanced response reflects their improved electronic properties and charge transport behavior under illumination. In contrast, Tp‐Pa‐SO3H and Tp‐Pa‐NO2 displayed negligible or almost no photocurrent response, suggesting poor photoexcitation efficiency and rapid recombination of charge carriers. This observation is consistent with their inferior performance in H2 evolution studies, further confirming that these functional groups adversely affect the photocatalytic activity (vide infra).

FIGURE 3.

FIGURE 3

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(a) EIS spectra, (b) photocurrent response, (c) experimental valence band‐conduction band position and band gap of Tp‐Pa derivatives.

Overall, the photocurrent analysis strongly correlates with the catalytic performance, highlighting the crucial role of functional group modulation in tuning the optoelectronic properties of the materials. To understand the effect of different functional groups on the PHE of COF, different studies were conducted. The photoluminescence emission spectra revealed that not all COFs were emissive in nature (Figure S17). Further, the photoluminescence lifetimes of all derivatives were similar on the ns scale (Figure S17, and Table S1). In order to understand the band structure of all derivatives, Valence band spectroscopy (VBS) was used. From VBS spectra, the EVBM was calculated to be 1.26, 1.66, 1.31, 1.33, 1.02 eV, respectively, for Tp‐Pa, Tp‐Pa‐Cl, Tp‐Pa‐CN, Tp‐Pa‐SO3H, and Tp‐Pa‐NO2 (Figure S18). The E CBM of all derivatives was elucidated using E VBM and the Tauc plot (Figure 3c). Similarly, density functional theory (DFT) calculations were performed to gain deeper insight into the electronic structure and the distribution of frontier molecular orbitals in all the synthesized derivatives (Figure 4). The results revealed a clear spatial separation between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Specifically, the HOMO was predominantly localized on the phenylene diamine linker, indicating that this moiety primarily contributes to the framework's electron‐donating character. In contrast, the LUMO was primarily distributed over the triformylphloroglucinol linker, suggesting that this unit acts as the electron‐accepting site. This distinct localization of HOMO and LUMO across different linkers implies an effective donor–acceptor interaction within the framework, which can facilitate charge separation and enhance the electronic communication between the building blocks.

FIGURE 4.

FIGURE 4

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HOMO–LUMO position of Tp‐Pa derivatives obtained by DFT calculations.

PHE of all derivatives was carried out in a 50 mL quartz round‐bottom flask under visible light irradiation with a sacrificial electron donor (SED) and co‐catalyst. In a typical experiment, 5 mg of catalyst was dispersed in an aqueous solution of 0.056 M (200 mg) ascorbic acid (AA) in 20 mL H2O and 200 µL PVP‐Pt NP. PVP‐coated Pt NPs were synthesized according to the reported literature [38, 39] and used as such as a cocatalyst. HER rate of Tp‐Pa was found to be 25,717 µmol g−1 h−1 (Figure 5a). In fact, the HER rates of the other derivatives were 6398, 21837, and 2474 µmol g−1 h−1 for Tp‐Pa‐Cl, Tp‐Pa‐CN, and Tp‐Pa‐SO3H, which are much lower than that of Tp‐Pa. Also, it is to be noted that Tp‐Pa‐NO2 did not show any HER activity under similar conditions. The reduction in H2 evolution can be attributed to various reasons, starting from the synthesis method to the influence of electron‐withdrawing functional groups. As mentioned earlier, the mechanochemical method relies on diminished DCC, resulting in non‐crystalline framework formation. The Tp‐Pa‐NO2 catalyst was not crystalline and hence did not show any HER activity. We also tried PHE with different SEDs like Sodium Ascorbate, Triethanolamine, and Lactic acid, and unfortunately, none of the COFs performed in these SEDs (Table S2). The cyclic stability of the best‐performing catalyst, Tp‐Pa, was studied and found to be stable up to 9 cycles of 45 h in total (Figure 5b). In fact, H2 evolution increased after the first cycle and then stabilized. This could be due to the anchoring of PVP‐Pt Np to the enamine bond on long light irradiation. Further, the structure and morphology of samples were studied after long‐term PHE studies (Figure S19). The FT‐IR study revealed that Tp‐Pa retained the same stretching bands even after multiple PHE cycles. Also, FE‐SEM and HR‐TEM confirmed the sheet‐like morphology of cycled Tp‐Pa COF. The presence of elements was further confirmed with STEM mapping. The reproducibility of the PHE measurements was assessed using three independent batches, which exhibited consistent values. The corresponding error bars represent the variability among these independent measurements (Figure S20a,b).

FIGURE 5.

FIGURE 5

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(a) PHE of Tp‐Pa derivatives from deionized water, (b) Cyclic PHE study of Tp‐Pa, and (c) PHE of Tp‐Pa derivatives from simulated seawater.

To evaluate the practical applicability of the catalyst, PHE studies were carried out using simulated seawater as the reaction medium (Figure 5c). The simulated seawater was prepared following a standard protocol [40], and all PHE experiments were conducted under the previously optimized conditions. Notably, Tp‐Pa exhibited a remarkable H2 evolution rate of 33,331 µmol g−1 h−1, whereas Tp‐Pa‐CN and Tp‐Pa‐Cl showed significantly lower H2 production under identical conditions. The substantial enhancement in HER activity observed for Tp‐Pa in simulated seawater can be attributed to in situ polarization of the framework induced by the adsorption of dissolved metal ions present in the medium [41]. This polarization effect is known to increase the effective dielectric constant of the organic semiconductor, thereby lowering the exciton binding energy and facilitating more efficient exciton dissociation. As a result, improved charge separation and charge carrier mobility are achieved, which ultimately promote enhanced photocatalytic H2 generation. This finding highlights the beneficial role of ionic environments, such as seawater, in modulating the optoelectronic properties and catalytic efficiency of the material. In general, the introduction of functional groups was found to have a detrimental effect on PHE. Electrochemical studies, including Mott–Schottky analysis, photocurrent response, and electrochemical impedance spectroscopy, confirmed that all materials exhibit n‐type semiconductor behavior. However, the pristine Tp‐Pa displayed a significantly higher photocurrent response and lower charge transfer resistance compared to the functionalized derivatives. These observations indicate more efficient charge generation, separation, and transport in the unmodified framework. The introduction of electron‐withdrawing substituents, although capable of modulating the electronic structure and band positions, resulted in reduced charge separation efficiency within the COF platform.

3. Conclusion

In summary, a series of Tp‐Pa‐based covalent organic frameworks functionalized with different electron‐withdrawing groups (─Cl, ─CN, ─SO3H, and ─NO2) were successfully synthesized using a scalable mechanochemical approach and systematically investigated to understand the influence of framework functionalization on their structural, electronic, and photocatalytic properties. Spectroscopic analyses, including solid‐state 1 3C CP‐MAS NMR and FT‐IR, confirmed the successful formation of the β‐ketoenamine‐linked COF frameworks, while PXRD analysis revealed that pristine Tp‐Pa possessed higher crystallinity compared to its functionalized derivatives, likely due to disruption of dynamic covalent chemistry during mechanochemical synthesis and the presence of electron‐withdrawing substituents. Electrochemical measurements confirmed that all COFs behave as n‐type semiconductors, whereas photocurrent and EIS studies revealed superior charge separation and lower charge transfer resistance for pristine Tp‐Pa compared to the functionalized derivatives. Photocatalytic hydrogen evolution experiments demonstrated that Tp‐Pa exhibits the highest HER activity, whereas incorporation of electron‐withdrawing functional groups generally resulted in reduced catalytic performance. The decrease in activity can be attributed to reduced crystallinity, lower surface area, and diminished charge separation efficiency within the functionalized frameworks. The catalyst also performed well with simulated seawater, showing its applicability in real‐world scenarios.

Overall, this study demonstrates that although framework functionalization can modulate electronic properties and band structures, excessive incorporation of electron‐withdrawing groups can adversely affect crystallinity, charge transport, and photocatalytic activity in Tp‐Pa‐based COFs synthesized via mechanochemical routes. Our findings provide important insights for the rational design of scalable COF photocatalysts with optimized structural order, charge separation, and catalytic performance for solar‐driven hydrogen evolution.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

The authors have cited additional references within the Supporting Information.

Supporting File 1: asia70886‐sup‐0001‐SuppMat.pdf

ASIA-21-e70886-s001.pdf (7MB, pdf)

Acknowledgments

S.S.B. conceived the research idea. K.A. synthesized and characterized COFs and conducted all experiments. This work is supported by the Science and Engineering Research Board (SERB), Govt. of India, CRG/2023/001029.

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