Conjugated polymers are attractive semiconductors for solar energy conversion to chemical fuels, particularly photocatalytic hydrogen production. Designing polymeric photocatalysts with tailored electronic structures – to enhance visible light absorption, adjust band gaps and orbital levels, and improve charge separation/transport – is crucial for improving hydrogen evolution efficiency. Most reported polymeric photocatalysts are amorphous or semicrystalline, hindering intermolecular charge transfer. Two-dimensional covalent organic frameworks (2D COFs), crystalline organic porous materials, offer delocalized π-electronic systems and layered structures stabilized by π-stacking, leading to red-shifted absorption, enhanced exciton delocalization, and high charge mobility. Molecular design in 2D COFs involves creating chemically stable linkages (β-ketoenamine, sp² C=C, triazine rings) and incorporating electron-donating/withdrawing moieties to optimize redox capability and charge transport. However, the effect of layered structures on photocatalytic H₂ evolution is less explored. Post-photocatalysis X-ray diffraction often shows attenuated signals, suggesting impaired stacking and amorphization. This structural distortion compromises performance because conjugation length is vital for charge carrier photogeneration. Therefore, strategies to preserve layered π-stacking in 2D COFs are needed.

The literature extensively discusses the design of 2D COFs for photocatalysis. Strategies include using chemically stable linkages such as β-ketoenamine, sp² C=C bonds, and triazine rings to ensure structural integrity under irradiation. Incorporating electron-donating/withdrawing moieties, like diacetylene, hydrazone, azine, and sulfone groups, is also crucial for adjusting redox potentials and enhancing charge separation and transport. However, fewer studies focus on the effect of layered structures in 2D COFs on their photocatalytic H₂ evolution. Existing research reveals that the layered structures of many 2D COFs are often disrupted during photocatalysis, leading to reduced efficiency.

The researchers synthesized a β-ketoenamine-linked COF containing a benzothiadiazole (BT) moiety (BT-COF) using 1,3,5-triformylphloroglucinol (Tp) and 4,4′-(benzo-2,1,3-thiadiazole-4,7-diyl)dianiline (BT) in a solvothermal reaction. Pyrrolidine was used as a catalyst for the β-ketoenamine linkage formation. The BT-COF structure was characterized by FT-IR, solid-state ¹³C CP/MAS NMR, high-resolution TEM, and PXRD. Nitrogen sorption measurements determined porosity. PEG (Mw = 20 kDa) was infiltrated into the BT-COF mesopore channels through a low-pressure driven method followed by thermal annealing. The presence of PEG was confirmed by DSC, solid-state NMR, WAXS, and nitrogen sorption. Wide-angle X-ray scattering (WAXS) was employed to analyze the regularity of molecular arrangements. Solid-state 2D ¹H-¹³C HETCOR NMR confirmed PEG confinement within the pores. The crystalline stability was assessed in various solvents. DFT calculations (TDDFT CAM-B3LYP) were used to simulate UV spectra, optimize excited-state geometry, and analyze the electronic structure. Photocatalytic hydrogen evolution was studied using a modified method, involving BT-COF, PEG-filled BT-COF, and control materials (TP-COF and amorphous poly(TpBT)) with 5 wt% Pt as a cocatalyst, 0.1 M ascorbic acid (AA) as a sacrificial electron donor, and visible light irradiation (>420 nm). The apparent quantum efficiency (AQE) was determined at various wavelengths. Long-term photocatalytic activity was assessed over multiple cycles. Dissipative particle dynamics (DPD) simulations investigated PEG chain conformations in water and in the COF nanopores. Transient photocurrent responses, electrochemical impedance spectroscopy, and time-correlated single-photon counting (TCSPC) were used to analyze photophysical properties before and after photocatalysis.

The synthesized BT-COF showed high crystallinity and a high surface area (1471 m² g⁻¹). PEG infiltration effectively filled the mesopore channels, as evidenced by the absence of a PEG melting peak in DSC and changes in nitrogen sorption isotherms. WAXS confirmed the preservation of the crystalline structure in PEG@BT-COF. Solid-state NMR showed close proximity between PEG chains and BT-COF. The photocatalytic hydrogen evolution rate (HER) for 30%PEG@BT-COF (11.14 mmol g⁻¹h⁻¹) was significantly higher than that of BT-COF (7.70 mmol g⁻¹h⁻¹), TP-COF (1.45 mmol g⁻¹h⁻¹), and amorphous poly(TpBT) (0.89 mmol g⁻¹h⁻¹). The AQE of 30%PEG@BT-COF reached 11.2% at 420 nm, which is among the highest reported for COF-based photocatalysts. Long-term photocatalytic tests revealed that 30%PEG@BT-COF maintained its activity over six cycles (only 8% decrease in H₂ production after 48 h), while BT-COF showed a 21% decrease. The improved performance was attributed to the preservation of the layered structure and enhanced charge transport due to PEG stabilization of the π-stacking. DPD simulations showed that PEG chains inside the COF pores are partially elongated, effectively suppressing the interlayer dislocation during photocatalysis. Photophysical measurements (transient photocurrent, electrochemical impedance, TCSPC) confirmed enhanced charge transport and longer exciton lifetimes for 30%PEG@BT-COF after photocatalysis, further supporting the role of PEG in maintaining the ordered π-stacking.

The study successfully demonstrates a general strategy for enhancing the performance of 2D COF photocatalysts by stabilizing their layered structures using PEG infiltration. The enhanced HER and AQE in PEG@BT-COF, compared to BT-COF and control samples, directly support the positive impact of this strategy. The improved stability, as evidenced by the long-term photocatalytic performance, highlights the practical implications of this method. The findings suggest that the preservation of the ordered π-stacking through PEG filling improves charge transport and exciton lifetime, leading to increased photocatalytic efficiency. The use of DFT and DPD simulations provides mechanistic insights into the role of PEG in enhancing the interlayer interactions and stability of the COF structure. The success of this approach for both BT-COF and TP-COF suggests its broad applicability to other 2D COF systems. This work opens new avenues for designing high-performance COF-based photocatalysts for various applications.

This research presents a novel strategy to enhance the photocatalytic hydrogen evolution of 2D COFs by stabilizing their layered structure through PEG infiltration. The significantly improved HER and AQE, along with excellent long-term stability, showcase the effectiveness of this method. This general approach could be extended to other COF systems and configurations for diverse applications. Further research could explore different PEG molecular weights and other polymer types for optimizing performance.

The study primarily focuses on two specific COFs (BT-COF and TP-COF). While the results suggest broad applicability, further investigation is needed to confirm the effectiveness of this approach across a wider range of COF structures and chemistries. The use of ascorbic acid as a sacrificial electron donor might limit the direct applicability to systems without sacrificial reagents. The long-term stability tests were conducted under specific conditions. The performance might vary under different reaction parameters such as temperature and light intensity. The DFT calculations used simplified models, and more comprehensive simulations might be needed for a more thorough understanding of the underlying mechanisms.