Quasi-one-dimensional enaminone-linked covalent organic frameworks for efficient CO₂ photoreduction

Domain: neuroscience
Published: 2026-02-12
Editorial outcome: major_revision

Abstract
Quasi-one-dimensional covalent organic frameworks have emerged as promising platforms for photochemical energy conversion due to their low density of basal sites, abundant edge sites, and dual-chain-like structure. However, the development of quasi-one-dimensional covalent organic frameworks in photocatalysis is still highly hindered by their limited linkage chemistry. Herein, we report an enaminone-linked quasi-one-dimensional covalent organic framework. The polar enaminone bonds together with dual-chain-like structure endow enaminone-linked quasi-one-dimensional covalent organic framework with broad light adsorption and effective excitonic dissociation abilities. Significantly, a high CO yield of 3045 μmol g-1 with approximately 100% selectivity was achieved in a 24 h reaction under gas-solid conditions. More interestingly, the hydrogen atom on nitrogen site in enaminone bond could assist in the activation of CO2 molecule via hydrogen-bond interaction. This interaction leads to the strongest adsorption ability for CO2 and the lowest energy barrier for the rate-determining step during CO2 reduction over enaminone-linked quasi-one-dimensional covalent organic framework compared to those over mixture-linked quasi-one-dimensional covalent organic framework and imine-linked quasi-one-dimensional covalent organic framework counterparts. All of these factors directly contribute to the enhanced activity of enaminone-linked quasi-one-dimensional covalent organic framework in the photocatalytic CO2 reduction to CO.

Introduction
Achieving carbon peaking and carbon neutrality are regarded as the pivotal objectives for sustainable development1. To realize this target, numerous strategies have been developed to address the issue of carbon dioxide (CO2), especially to capture and convert CO2 into high-value chemical feedstocks. In this regard, artificial photosynthesis, which is capable of mimicking natural leaves for the conversion of CO2 with H2O into carbohydrate and O2, may provide a facile means to avoid the use of any sacrificial agents, precious metals cocatalysts and photosensitizers2. As a class of porous materials, covalent organic frameworks (COFs) with the π-conjugation, especially two-dimensional (2D) or three-dimensional (3D) COFs3,4 have exhibited immense potential in artificial photosynthesis thanks to their highly controllable skeleton, channels, local structures, advantageous photoelectric properties and structural stability during photochemical processes5–10. However, their 2D or 3D orientation extended frameworks provide limited sites on the outside of pores (located at edges), which limit their photocatalytic activity11.

As an alternative, Quasi-one-dimensional covalent organic frameworks (Q1DCOFs) are recently characterized by a low density of basal sites and a high proportion of edge sites on their frameworks, which significantly enhance the exposure of active sites and the photoelectrochemistry active surface areas11. Besides, the distinct dual-chain architecture of Quasi-one-dimensional configurations could also promote the directional electron transfer, thereby minimizing energy dissipation associated with exciton dissociation12–14. Thus, this structural attribute of Q1DCOFs endows their significant capability to enhance the directed transfer of electrons toward active sites, thereby improving photoelectric conversion efficiency and interfacial catalytic performance. Nevertheless, it is an irrefutable fact that the occurrence of regulation of linkers in Q1DCOFs is rare and that research on their photocatalytic applications is scarce. Currently, imine is the most extensively studied linkage in Q1DCOFs15. Unfortunately, the moderate stability of most imine-based COFs, particularly under prolonged light irradiation, significantly restricts the advancement of Q1DCOFs and impedes their application in artificial photosynthesis16. Therefore, exploiting connection modes is urgently important to develop 1D COFs toward structural and functional diversification, but remains a great challenge.

Enaminone, known as an intrinsic dipole, light-harvesting, and photo-switchable organic unit, has shown high potential in photocatalysis17,18. Theoretical and experimental studies have shown that the integration of enaminone structural units into 2D COFs can greatly improve their photocatalytic performance by accelerating exciton dissociation and optical adsorption. For example, Jiang et al. discovered that enaminone-linked COFs possessed broader light absorption and easier exciton dissociation than imine-linked COFs and quinoline-linked COFs, directly contributing to the advantageous photocatalytic activity in hydrogen evolution reaction19. Besides, our group also demonstrated that enaminone-linked COFs were able to promote the directional transport of photogenerated electrons to reduction sites during CO2 photoreduction, thereby generating a very high CO generation rate20. Though these successful applications of enaminone linkage in 2D COFs have been achieved, no studies have been reported on investigation of artificial photosynthesis through integrating enaminone bonds into the Q1DCOF backbone to improve photocatalytic performance.

Herein, we present the design and synthesis of a Q1DCOF of En-Q1DCOF with the enaminone linkages. For comparative analysis, we also prepare Mix-Q1DCOF (with enaminone and imine linkages) and Im-Q1DCOF (solely with imine linkages) (Fig. 1a). The resulting Q1DCOFs exhibit high crystallinity and good physicochemical stability. Under metal-, photosensitizer-, or sacrificial reagent-free conditions, En-Q1DCOF achieves the highest CO yield of 3045 μmol g−1 in 24 h, which is 7 and 12 times of Mix-Q1DCOF and Im-Q1DCOF, respectively (Fig. 1b). Further investigation indicates that En-Q1DCOF exhibit the highest number of active sites, advantageous visible light absorption and the fastest exciton dissociation. More interestingly, the hydrogen atom on nitrogen site in enaminone bond could assist in the activation of CO2 molecule via hydrogen-bond interaction. This interaction leads to the strongest adsorption ability for CO2 and the lowest energy barrier for the rate-determining step during CO2 reduction over En-Q1DCOFs compared to those over Mix-Q1DCOF and Im-Q1DCOF.

Results
Synthesis and characterizations of COFs
Q1DCOFs of En-Q1DCOF, Mix-Q1DCOF, and Im-Q1DCOF were successfully prepared via Michael addition-elimination reaction or Schiff-base polymerization between Tetrakis(4-aminobiphenyl)ethylene (TAE) and 1,1′-(1,3-Phenylene)bis[3-(dimethylamino)-2-propen-1-one (PDP), 1-(3-Benzaldehyde)-3-(dimethylamino)-2-propen-1-one (BDP), and 1,3-Benzenedicarboxaldehyde (BD), respectively (Figure S1–S7). The crystal structures of these Q1DCOFs were theoretically and experimentally characterized by using Materials Studio simulation and powder X-ray diffraction (PXRD). As illustrated in Fig. 2a, b, En-Q1DCOF only presents an eclipsed stacking model (AA), corresponding to a typical 1D 4-c sql topology12. The simulated pore diameter and interlayer spacing for En-Q1DCOF are 15.07 Å and 6.09 Å, respectively. The interlayer spacings for En-Q1DCOF are larger than normal COFs, which might be attributed to the combined effects of interlayer repulsion induced by the highly polar enaminone bond and the non-planar conformation of biphenyl in TAE. The PXRD pattern of En-Q1DCOF exhibits a main peak at around 4.8°, which is assigned to the (110) facet. Besides, the appearance of other diffraction peaks further indicates the high crystallinity of En-Q1DCOF. Pawley refinements of the PXRD patterns provide three unit-cell parameters of a=29.4992 Å, b=27.3954 Å, and c=6.0908 Å, α=β=γ=90°, with factors of Rp=2.96% and Rwp=3.38% for En-Q1DCOF (Table S1 and Supplementary Data 1). As for Mix-Q1DCOF and Im-Q1DCOF, the three unit cell is well fitted according to the Pawley refinements toward the experimental PXRD patterns, where the parameters for Mix-Q1DCOF and Im-Q1DCOF are a=28.7365 Å, b=24.7115 Å, c=6.2289 Å, Rp=2.72%, Rwp=3.85% and a=31.2157 Å, b=21.5565 Å, c=6.0374 Å, Rp=6.32%, Rwp=8.28%, respectively (Figures S8–S14, Tables S2, S3 and Supplementary Data 2, 3).

The chemical structures of the three Q1DCOFs were further characterized through Fourier transform infrared (FTIR), 13C cross-polarization/magic-angle spinning solid-state nuclear magnetic resonance (CP/MAS ssNMR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). As shown in the FTIR spectroscopy, the disappearance of N-Me vibration peak at ~1430 cm−1 and the generation of the N-C vibration peak at ~1175 cm−1 signify the successful formation of the enaminone bond in En-Q1DCOF and Mix-Q1DCOF (Figs. 2c, S15 and S17)21. In the FTIR spectra of Im-Q1DCOF and Mix-Q1DCOF, the emergence of the peak at ~1625 cm−1 attributing to the stretching vibration for -C=N- bond is accompanied by the disappearance of the -NH2 vibration band at 3200–3500 cm−1 for TAE monomers (Figures S15–S17)22. The successful synthesis of the different linkages and skeletons in these Q1DCOFs was further confirmed by the analysis of solid 13C CP/MAS spectra, where all carbon peaks in the benzene units of all the Q1DCOFs are well identified (Figs. 2d, S18 and S19). Furthermore, we also performed XPS measurement to verify the formation of Q1DCOFs (Figs. 2e and S20). Obviously, the N 1s XPS peak in En-Q1DCOF shifted to a higher binding energy compared to that in Im-Q1DCOF, suggesting a stronger interaction between C and N atoms in enaminone bond than that in imine bond. As for Mix-Q1DCOF, the binding energies of the N 1s XPS peaks are located between those of En-Q1DCOF and Im-Q1DCOF, which suggests the coexistence of enaminone and imine bonds in Mix-Q1DCOF.

The N2 adsorption-desorption measurements at 77 K were performed to evaluate their porous structures. The corresponding Brunauer-Emmett-Teller (BET) surface areas of En-Q1DCOF, Mix-Q1DCOF and Im-Q1DCOF are 594, 204, and 487 m2 g−1, respectively (Fig. 2f). The much lower surface area of Mix-Q1DCOF might be related to its more complex connections and more disordered structures compared with those in En-Q1DCOF and Im-Q1DCOF. According to the pore size distribution curves, the corresponding micropore sizes for En-Q1DCOF, Mix-Q1DCOF and Im-Q1DCOF are 1.55, 1.67, and 1.59 nm, respectively (Figure S21). The pore sizes obtained by N2 adsorption isotherms are closely matched with the model-theoretical micropore size, further confirming the successful synthesis of the three Q1DCOFs. The morphologies of the as-synthesized Q1DCOFs were investigated by the scanning electron microscope (SEM) and transmission electron microscope (TEM) characterizations (Figures S22–S29). All Q1DCOFs exhibit nanosheet morphology (20 nm–3 μm). Crystalline nature of En-Q1DCOF is confirmed by the fast Fourier transform (FFT) images showing distinct diffraction spots (Fig. 2h) and the high-resolution transmission electron microscope (HRTEM) revealing 0.41 nm lattice fringes corresponding to (040) planes. The energy-dispersive X-ray (EDX) elemental mapping further demonstrates a homogeneous distribution of all elements throughout the frameworks (Figs. 2i, S26–S29).

To further investigate the band structures of the as-acquired Q1DCOFs, we recorded their ultraviolet-visible diffuse reflectance spectra first. As seen in Fig. 3a, En-Q1DCOF, Mix-Q1DCOF and Im-Q1DCOF behave a strong absorption in the visible region, which is mainly caused by the increase in along the plane conjugation and system polarization intensity. As for En-Q1DCOF, its intrinsic UV/vis absorption edge is about 460 nm. Besides, an obvious red shift of absorption edge for En-Q1DCOFs signifies its smaller band gaps (Eg) and enhanced light adsorption compared with those of Mix-Q1DCOF and Im-Q1DCOFs. It is noting that the lack of strongly conjugated and highly symmetrical units (such as pyrene and porphyrin) enables the Q1DCOFs to exhibit the characteristic of interlayer π-π stacking and less conjugated bonding modes (imine linkages and enaminone linkages). This character induces the folding and displacement of their energy bands in k-space, thus transforming them into indirect bandgap semiconductors9. Accordingly, the Eg values of En-Q1DCOF, Mix-Q1DCOF and Im-Q1DCOF are determined to be 2.42, 2.45, and 2.50 eV according to the transformed Kubelka-Munk function, respectively (Fig. 3b). Combining with the Mott-Schottky analysis (Figure S30) and VB-XPS measurements (Figure S31), the conduction band (CB) and valence band (VB) positions of En-Q1DCOF, Mix-Q1DCOF and Im-Q1DCOF were estimated. Obviously, all the COFs exhibit sufficient driving force to trigger CO2 reduction and water oxidation reactions (Fig. 3c). Furthermore, we also carried out the density functional theory (DFT) calculations by using the VASPKIT code to Q1DCOFs23. The spatial separation of CBM-VBM is more pronounced in En-Q1DCOF, which reduces the possibility of electron-hole pair recombination (Fig. 3d–f). The calculated band gaps of En-Q1DCOF, Mix-Q1DCOF and Im-Q1DCOF are 2.08, 2.16, and 2.47 eV, respectively, which is consistent with those obtained from experiment. As a result, the introduction of enaminone bonds could not only modulate the band gap but also induce the local charge delocalization and distribution.

Photocatalytic CO2 reduction reaction performances of COFs
The catalytic activity of the as-acquired Q1DCOFs was evaluated in the photocatalytic CO2 reduction reaction (CO2RR) with gaseous H2O under visible light irradiation in the absence of photosensitizer, sacrificial agents, and cocatalysts (Fig. 4a). As shown in Fig. 4b, the yields of CO are 426 and 263 μmol g−1 in 24 h when the reaction is catalyzed by Mix-Q1DCOF and Im-Q1DCOF, respectively. Interestingly, the highest CO yield of 3045 μmol g−1 is achieved in 24 h over En-Q1DCOF, which is 7 and 12 times of Mix-Q1DCOF and Im-Q1DCOF, respectively. This performance of En-Q1DCOF is also competitive among the reported metal-free COF photocatalysts and photocatalytic systems up to date (Table S4). Besides, we also simultaneously detected the evolution rates of O2 during CO2RR (Figure S32). The ratio of CO to O2 is about 2:1, suggesting that CO is produced by coupling with the oxidation of H2O. To further confirm the origin of O2, we also implemented the reduction of CO2 by utilizing H218O as the water source. After the reaction, we analyzed the gases by gas chromatography and mass spectrometry, where a peak of 18O2 with m/z=36 was detected, confirming that O2 was originated from the water (Figure S33). CO2RR is significantly impeded in the absence of catalyst, CO2, H2O, or light illumination, demonstrating their indispensability for photocatalytic CO2 reduction to CO (Fig. 4c). Moreover, 13CO2 isotopic labeling experiment shows a dominant peak of 13CO (m/z=29) in the gas chromatography-mass spectrometry (GC-MS) plot, indicating that CO is generated from the photoreduction of CO2 instead of the organic species (Fig. 4d).

In addition, we also investigated the effect of the amount of catalyst on the performance of photocatalytic CO2RR (Figure S34). The slight decrease in the CO evolution rate when increasing the amount of catalyst might be caused by light not penetrating the thicker catalyst layer. To evaluate the light utilization efficiency, the apparent quantum yield (AQY) of En-Q1DCOF was measured under the irradiation of monochromatic light (Fig. 4e). En-Q1DCOF exhibited AQY values of 0.66% at 450 nm and 0.14% at 650 nm. Optimizing the photocatalyst loading to 20 mg enhanced its AQY to 1.94% at 450 nm (Figure S35). However, further mass increases failed to produce linear AQY improvements due to light shielding effects.

Recycling tests of the En-Q1DCOF catalyst showed no significant attenuation after three 24-hour photocatalytic cycles (Fig. 4f), confirming its high stability. The sample recovered after catalytic tests was named as En-Q1DCOF-irradiation. In addition, we also analyzed the FTIR spectra of En-Q1DCOF before and after photocatalytic reaction (Figure S36), where not any change in the intensity or wavenumber of the C=O group at 1632 cm−1 was observed. This result further confirms that the product of CO does not originate from the C=O groups within En-Q1DCOF. Besides, we also collected the PXRD patterns of En-Q1DCOF before and after photocatalytic reaction. As shown in Figure S35, the XRD patterns of En-Q1DCOF before and after the reaction are almost identical, demonstrating that En-Q1DCOF did not undergo structural collapse. In other words, all the bonds in En-Q1DCOF including C=O bond are stable. All the above-mentioned results convincingly demonstrate that the CO is generated from the reduction of CO2, not the degradation of the materials. In addition, the high chemical stability of En-Q1DCOF was also demonstrated by treating it with the solution of 1 M HCl, 1 M NaOH, and 1 M DMF, respectively, where the resulting COFs retained the crystallinity and skeletal structure of En-Q1DCOF (Figure S37).

Photophysical and photochemical properties study of COFs
To reveal the reason for the difference in the photoelectric performance of the enaminone and imine bonds in COFs, we performed DFT calculations. Figure 5a–d present the density of states (DOS) for Q1DCOFs. Prominent DOS peaks at the VB and CB edges reflect their molecular origins. Notably in En-Q1DCOF (Fig. 5b), the CB edge is dominated by the EN structure, while the VB edge originates primarily from TBE. Furthermore, EN in En-Q1DCOF contributes more significantly to the CB than IM does in Mix-Q1DCOF and Im-Q1DCOF (Fig. 5c, d), indicating the more electron-deficient nature of enaminone bonds compared to imine bonds, which favors band gap narrowing24. Figure S38 shows the optimized COFs molecular fragment structures. Ground state electrostatic potential (ESP) analysis of COFs (Fig. 5e) shows that the enaminone bond has greater polarity compared to the imine bond. This facilitates the local polarization of the π-system, generating a strongly polarized electric field. Such a strongly polarized electric field promotes the delocalization of electrons within the system (dipole moments, En-COF: 3.4769 Debye, Im-COF: 2.2511 Debye). In addition, the maximum negative charge distribution of COF is located on the ketone group and imine group, indicating that both of these chemical bonds can effectively adsorb CO2. Interestingly, the charge of O-site on the ketone group is more negative than that of the N-site on the imine group, which proves that the ketone group prefers to capture CO2 (Figure S39). In addition, the localized orbital locator (LOL) indicates that En-COF has weaker localization in the O-site region compared to Im-COF in the N-site region, proving that En-COF is more conducive to the extraction and output of hot π electrons (Figs. 5f, g). The electron-hole distributions of the model COFs molecular fragment constructed by Multiwfn25 and Visual Molecular Dynamics (VMD)26 disclose the role of enaminone and imine bonds in the electronic structures. The calculated charge transfer distances are 2.161 Å and 3.173 Å for Im-COF and En-COF, respectively, indicating the remarkably improved electron-hole separation after enaminone polarization (Fig. 5h, i).

To further reveal the essence for the high CO2RR performance of En-Q1DCOF, several photoelectrochemical measurements were conducted, including electrochemical impedance spectra (EIS), photocurrent profiles, and polarization curves. As expected, En-Q1DCOF demonstrates the highest photocurrent response and the smallest semicircle diameter in the EIS plots, followed by Mix-Q1DCOF and Im-Q1DCOF (Figs. 6a and S40). This result suggests the effective electrical conductivity at the interface and high photoreduction capacity of En-Q1DCOF, which is consistent with the result of CO2RR. Notably, the polarization of the π-system could also decrease the overpotential of CO2 evolution in the Q1DCOF-based system, as evidenced by the polarization curves in Figure S41. Furthermore, electron paramagnetic resonance (EPR) analysis was conducted to evaluate the behavior of photoexcited electrons. As shown in Fig. 6b, En-Q1DCOF exhibits a stronger DMPO-•O2- signal under visible light irradiation, indicating its greater photoexcited electron reduction ability compared with Mix-Q1DCOF and Im-Q1DCOF. No obvious DMPO-•O2– signals are observed under dark conditions, further supporting the nature of the photo-induced activity for En-Q1DCOF (Figure S42).

Figure 6c shows the photoluminescence (PL) spectra of these Q1DCOF. Obviously, En-Q1DCOF exhibits a lower photoluminescence (PL) peak intensity than Mix-Q1DCOF and Im-Q1DCOF, indicating that enaminone bonds effectively contribute to the separation and transfer of photogenerated carriers. Furthermore, temperature-dependent PL measurements were also conducted to investigate the exciton dissociation kinetics, which is one of the essential factors affecting the photocatalytic activity of COFs photocatalysts (Fig. 6d–f). The integrated photoluminescence intensities increase gradually with decreasing temperature from 300 K to 85 K. Besides, the corresponding exciton binding energies (Eb) can be calculated based on the Arrhenius equation27. Obviously, the Eb values decrease rapidly with the increase of enaminone bonds, suggesting that the introduction of enaminone bonds in Q1DCOFs could effectively suppress the exciton effect.

Femtosecond time-resolved transient absorption spectra (fs-TAS) probed excited-state kinetics in Q1DCOF photocatalysts. Spectra recorded at 400 nm excitation (Figs. 6g–i) revealed negative signals (480–650 nm) across all time delays. The negative signal can be assigned to the ground state bleaching (GSB) and stimulated emission (SE) according to UV-vis and steady state PL spectra. Notably, En-Q1DCOF exhibited accelerated SE decay relative to Im-Q1DCOF and Mix-Q1DCOF, aligning with the observed exciton effects28. The kinetics fitting results show biexponential decay processes, namely the short lifetime (τ1) and long lifetime (τ2), corresponding to electron trapping and electron transfer kinetics, respectively. As shown in Fig. 6g, i, the decay time constants for Im-Q1DCOF are τ1 = 0.31 ps and τ2 = 25.4 ps. As for En-Q1DCOF, the decay time constants of τ1 and τ2 decrease to 0.05 and 6.8 ps, respectively. This decrease in τ1 and τ2 indicates different interfacial charge transfer efficiency in Q1DCOF28,29. A similar trend was observed in Mix-Q1DCOF (Fig. 6h). These observations from fs-TAS provide compelling evidence that enaminone bonds can effectively regulate the efficiency of interfacial charge transfer.

As the adsorption and activation of CO2 on the catalyst surface is a prerequisite for its further reduction, we conducted in situ attenuated total reflection Fourier transform infrared (ATR FTIR) to investigate the adsorption of CO2 by exposing En-Q1DCOF to dry CO2 under dark. Interestingly, a new broad band centered at 3630 cm-1 attributing to the H-O bending of N-H-O=C is observed (Figure S43)30–32. In addition, the hydrogen atoms on the nitrogen atoms in the enaminone unit have higher positive charges, indicating their stronger electrostatic attraction than the hydrogen atoms on other carbon atoms, which is more conducive to forming hydrogen bonds with CO2 (Fig. 7a). This formation of the hydrogen bond could weaken the O=C bond in CO2 molecule33,34. Accordingly, the hydrogen atom on nitrogen atom of the enaminone unit could assist in the adsorption of CO2 molecule. To further directly visualize the process of CO2 photoreduction and water photooxidation over En-Q1DCOF, we further measured the in situ ATR FTIR spectroscopy under the simulated reaction conditions, where En-Q1DCOF was exposed to saturated H2O vapor of CO2 under light irradiation. As shown in Fig. 7b, several peaks gradually appeared between 1100 and 2000 cm−1 with their radiation time from 0 to 60 min. A new broad band centered at 1634 cm−1 is attributed to the H-O-H bending of water molecules35. Due to the presence of water molecules in the reaction process, the adsorbed CO2 molecules can be converted into various intermediate substances, such as bidentate carbonate (b-CO32−) at 1596 and 1208 cm−1, and monodentate carbonate (m-CO32−) at 1482 cm−136. The infrared peaks at 1288 cm−1 is attributed to the symmetrical bending of the intermediate *COOH37. Additionally, the *COOH species, a crucial intermediate in reducing CO2 to CO, gradually increased during the photocatalytic process. Besides, we also characterized the intermediates in the H2O oxidation reaction by using in situ ATR FTIR. As depicted in Fig. 7c, the broad band centered at 950 cm−1 is attributed to *OH, a crucial intermediate in the photooxidation of water to O2. The peaks of *OOH intermediates at 1051 cm−1, are also gradually enhanced during the photocatalytic process. Thus, a possible photocatalytic water oxidation process for En-Q1DCOF is proposed as follows: H2O → *OH → *OOH → *O2 → O238. On the basis of the above experiments and analysis, a reasonable mechanism was proposed for CO2 reduction coupled with H2O oxidation process over En-Q1DCOF (Fig. 7d), which was further supported by DFT calculations. As shown in Fig. 7e, the corresponding CO2 adsorption energy for sites 1–3 was calculated. The results show that the enaminone bond is more favorable for CO2 adsorption than the imine bond. Interestingly, the adsorption energy could be further reduced when hydrogen atom in the enaminone unit is involved in the adsorption of CO2, suggesting that the hydrogen atom on nitrogen site could assist in the activation of CO2 molecule. This result is consistent with that of in situ ATR FTIR. Considering the reaction was conducted under the atmosphere of water vapor, the H2O-mediated solvation effect was weak and transient, thus exerting minimal influence on CO2 adsorption/activation. Furthermore, the experimental conditions for CO2 photoreduction were identical for both enaminone COFs and imine COFs, while the enaminone COFs demonstrated advantageous performance. This result could further exclude the H2O-mediated solvation effect on CO2 reduction. Besides, we also calculated the corresponding O2 adsorption energy for 1,1,2,2-tetra(biphenyl-4-yl)ethene sites 4-6. As shown in Figure S44, the vinyl is more favorable for O2 adsorption than the benzene motif. Combining with in situ ATR FTIR analysis, we finally calculated the Gibbs free energy for the reaction processes of CO2 reduction and water oxidation over En-Q1DCOF and Im-Q1DCOF (Fig. 7f, g). Accordingly, the rate-determining step for the reduction of CO2 to CO is determined to be the transformation from *CO2 to *COOH. The energy barrier for the generation of *COOH on En-Q1DCOF is 2.33 eV, which is lower than that of Im-Q1DCOF of 2.54 eV. The H2O oxidation process on the 1,1,2,2-tetra(biphenyl-4-yl)ethene fragment can be divided into four steps: H2O → *OH → *OOH → *O2 → O238.

Methods
Materials
All chemicals and reagents were of analytical grade and used as received without further purification. The following materials were sourced as indicated: BD (1,3-Benzenedicarboxaldehyde, 98%), TAE (Tetrakis(4-aminobiphenyl)ethylene, 98%), 3-acetylbenzaldehyde (98%), and 1,3-diacetylbenzene (99%) from Shanghai Haohong Biomedical Technology Co.; O-DCB (1,2-Dichlorobenzene, 99%), anhydrous n-But (n-Butanol, 99.4%), acetic acid (>99.0%), and N,N’-dimethylformamide dimethyl acetal (>99.0%) from Macklin Chemicals; tetrahydrofuran (THF, 99.5%) and acetone (ACE) from Tianjin Komeo Chemical Reagent Co.

Synthesis of 1,1′-(1,3-phenylene)bis[3-(dimethylamino)-2-propen-1-one (PDP)
In a typical synthesis, 1.0 g of 1,3-diacetylbenzene was dissolved into 20 mL of N,N’-dimethylformamide dimethyl acetal. The reaction mixture was heated to 90°C and stirred under nitrogen for 12 h. After evaporating the solvent and the subsequent addition of Et2O, the crude product was obtained by filtration. The crystals of PDP were acquired after carefully washing with Et2O and pentane as well as the drying under vacuum. 1H NMR (400 MHz, CDCl3): 8.40 (s, H), 8.00 (q, 2H), 7.83 (d, 2H), 7.46 (t, H), 5.78 (d, 2H), 3.04 (d, 12H) (Figure S1). 13C NMR (100 MHz, CDCl3): δ 188.26, 154.49, 140.39, 129.98, 128.03, 126.50, 92.32, 45.09, 37.41 (Figure S2).

Synthesis of 1-(3-benzaldehyde)-3-(dimethylamino)-2-propen-1-one (BDP)
In a typical synthesis, 1.0 g of 3-acetylbenzaldehyde was dissolved into 20 mL of N,N’-dimethylformamide dimethyl acetal. The reaction mixture was heated to 90°C and stirred under nitrogen for 12 h. Filtration removed insoluble impurities precipitated by the reaction. After evaporating the solvent and the subsequent addition of Et2O, the crude product was obtained by cooling filtration at −78°C. The oily liquid of BDP was acquired after carefully washing with Et2O and pentane, and then removing the low-boiling-point solvents under vacuum conditions. 1H NMR (400 MHz, CDCl3): 10.09 (s, H), 8.38 (s, H), 8.20 (d, H), 7.97 (d, H), 7.87 (d, H), 5.76 (d, H), 3.19 (s, 3H), 2.97 (s, 3H). (Figure S3). 13C NMR (100 MHz, CDCl3): δ 192.23, 154.86, 141.33, 136.25, 133.45, 131.67, 129.01, 128.77, 121.80, 91.60, 66.56, 45.24. (Figure S4).

Synthesis of En-Q1DCOF
A typical synthesis involved charging a Pyrex tube with PDP (19.8 mg, 0.07 mmol), TAE (24.5 mg, 0.035 mmol), 1,2-dichlorobenzene (0.5 mL), and 1-butanol (0.5 mL). After 5 min sonication, 0.1 mL of 6 M aqueous acetic acid was added under shaking, followed by another 5 min sonication. Subsequently, the mixture undergoes flash freezing with liquid nitrogen and is then sealed under vacuum. The reaction mixture was heated to 120°C and held undisturbed for 72 h. The resulting precipitate was collected by filtration, washed with THF and ACE, and dried at 120°C to yield the product as a yellow powder.

Synthesis of Mix-Q1DCOF
A typical synthesis involved charging a Pyrex tube with BDP (15.2 mg, 0.07 mmol), TAE (24.5 mg, 0.035 mmol), 1,2-dichlorobenzene (0.5 mL), and 1-butanol (0.5 mL). After 5 min sonication, 0.1 mL of 6 M aqueous acetic acid was added under shaking, followed by another 5 min sonication. Subsequently, the mixture undergoes flash freezing with liquid nitrogen and is then sealed under vacuum. The reaction mixture was heated to 120°C and held undisturbed for 72 h. The resulting precipitate was collected by filtration, washed with THF and ACE, and dried at 120°C to yield the product as a yellow powder.

Synthesis of Im-Q1DCOF
A typical synthesis involved charging a Pyrex tube with BD (9.4 mg, 0.07 mmol), TAE (24.5 mg, 0.035 mmol), 1,2-dichlorobenzene (0.5 mL), and 1-butanol (0.5 mL). After 5 min sonication, 0.1 mL of 6 M aqueous acetic acid was added under shaking, followed by another 5 min sonication. Subsequently, the mixture undergoes flash freezing with liquid nitrogen and is then sealed under vacuum. The reaction mixture was heated to 120°C and held undisturbed for 72 h. The resulting precipitate was collected by filtration, washed with THF and ACE, and dried at 120°C to yield the product as a yellow powder.

Characterizations
Powder X-ray diffraction (PXRD) patterns were acquired on a X’pert Powder diffractometer in reflection geometry using Cu Kα radiation (λ=1.5 Å). Data collection spanned 2θ=1-40° and 6°/min. Solid-state ¹³C NMR spectra, acquired via cross polarization magic-angle spinning (CP/MAS), were obtained on a JEOL-NM-ECZL G 600 MHz spectrometer. FT-IR spectra were acquired with a Thermo Fisher Nicolet Avatar 6700 spectrometer. UVDRS spectra were acquired by U-3010 UV-V1S in the range of 200-1200 nm. The Brunauer-Emmett-Teller (BET) surface area was measured on an instrument of Micromertics ASAP 2020M. The tube voltage and current used were 40 kV and 40 mA, respectively. XPS measurements were conducted on K-alpha (Binding energies were calibrated by the peak at 284.8 eV for C). TEM and the corresponding HRTEM were conducted on JEM 2100 with an acceleration voltage of 200 kV. SEM images were obtained by using Hitachi SU8200 with an acceleration voltage of 10 kV. ESR measurements were conducted at 293 K using a Bruker EMX CW microspectrometer operating in the X-band (microwave frequency ≈ 9.87 GHz). The instrument was equipped with an ER 4119HS-WI high-sensitivity optical resonator featuring a front-side grid. Samples were illuminated by a 300 W Xe lamp fitted with a 420 nm cut-off filter (LOT Oriel). All measurements used identical parameters: microwave power 6.74 mW, receiver gain 2×104, modulation frequency 100 kHz, modulation amplitude 3 G, and sweep time 45 s. g-values were calculated from the resonance field (B₀) and frequency (ν) using the resonance condition hν=gβB₀. Calibration employed DPPH (2,2-diphenyl-1-picrylhydrazyl) as a standard (g=2.0036±0.00004). The working electrode was fabricated by dispersing 5 mg of photocatalyst powder into 4 mL of ethanol containing 20 μL of Nafion, followed by 1 h of sonication. Subsequently, 0.5 mL of the resulting suspension was drop-cast onto a 1.9 cm × 3.3 cm ITO glass substrate, with an additional drop applied after the ethanol from the first application evaporated. Finally, the coated substrate was calcined at 150°C for 1 h in a tube furnace under N₂ atmosphere, yielding the working electrode. Photoelectrochemical measurements were performed using a CHI660E electrochemical workstation configured with a standard three-electrode system: a catalyst-coated ITO working electrode, a platinum plate counter electrode, and an Ag/AgCl reference electrode. Illumination was provided by a 300 W Xe arc lamp equipped with a cut-off filter (λ > 420 nm), and 0.1 M Na₂SO₄ (aq) served as the electrolyte. The Mott-Schottky plots were harvested at the frequencies of 1000, 1500 and 2000 Hz. Linear sweep voltammetry with a scan rate of 5 mV/s was carried out in a 0.1 M Na2SO4 aqueous solution. The ATR infrared spectrometric measurements were conducted by a Bruker INVENIO S spectrometer (Germany) with a KBr detector in the system. In a typical procedure, the COF membrane was placed in the chamber before sealing and then purging with N2 gas for 30 min. Typical signals of several intermediates were captured after the introduction of a flowing dry CO2 (g) or CO2-H2O (g) mixture under dark for 30 min and then visible light irradiation for 0–60 min. During the experiment of fs-TAS, transient absorption spectroscopy was recorded at the excitation wavelength of 400 nm. Before the measurement, 2 mg of the as-prepared COF was ultrasonically dispersed into 4 mL of glycol or H2O to acquire the dispersion of COF with a concentration of 0.5 mg/mL. Transient absorption spectroscopy was performed using a Helios pump-probe system (Ultrafast Systems LLC) with an amplified femtosecond laser (Coherent; 35 fs, 1 kHz, 800 nm). Probe pulses (430-603 nm) were generated via focusing 10 μJ of the 800 nm output into a 1 mm CaF₂ crystal. Pump pulses (365 nm) originated from a TOPAS-800-fs optical parametric amplifier. The test container was in a 2 mm optical path colorimetric dish. The test was performed for at least three times. The result was the automatic average value of the equipment. Steady-state PL and temperature-resolved PL spectra were acquired by using Edinburgh Instruments, FLS980 spectrometer. The 13CO gas produced from 13CO2 isotope experiments was examined by a gas chromatograph-mass spectrometer (GC-MS, Agilent 7890B-5977B).

Photocatalytic CO2 reduction method and parameters
Photocatalytic CO₂ reduction was conducted in a 250 mL reactor under visible light (>420 nm, 100 mW cm−2) at 35°C with stirring. Catalyst films were prepared by ultrasonically dispersing 2 mg catalyst in 5 mL H₂O (15 min), immobilizing on a 40 mm glass fiber membrane, and vacuum-activating (120°C, 12 h). For reactions, catalyst films and 3 mL H₂O were loaded into a custom quartz vessel. A 300 W Xe lamp (PLS-SXE300C, Perfectlight) with 420 nm cutoff filter provided illumination. Pre-treatment included reactor evacuation, triple CO₂ purging, and 15 min CO₂ bubbling. Gaseous products were periodically sampled (1 mL) and analyzed by GC (FID: CO/CH₄; TCD: O₂/H₂). The apparent quantum yield (AQY) was measured under the same condition (λ=420, 450, 500, 550, 600 and 650 nm, irradiation area: 0.785 cm2, irradiation time: 1 h), and the following equation was used to calculate AQY:

AQYs (%) = 2MNAhc / SPtλ × 100% (1)

Where M is the amount of CO produced (mol), NA is the Avogadro constant (6.022×1023/mol), ℎ is the Planck constant (6.626×10−34 J·s), c is the speed of light (3×108 m/s), S is the irradiation area of the incident light (m2), P is the intensity of the incident light (W/m2), t is the photoreaction time (h), and λ is the wavelength of the monochromatic light (nm). The computational details for AQY are summarized in Figure S45.

Calculations details and structure simulations
Molecular geometry optimizations and electronic structure characterizations were performed at the B3LYP/6-31G level using Gaussian 09. TD-DFT calculations (TD-B3LYP/6-31G, scrf) provided excited state energies and oscillator strengths for transient species, incorporating an implicit solvent model and dispersion correction. Multiwfn (version 3.8) was employed to analyze electron-hole distributions during excitation, electrostatic surface potentials (ESP), and localized orbital locators (LOL)25. Multiwfn and Visual Molecular Dynamics (VMD) generated the electron-hole distributions (Fig. 5h, i), electrostatic surface potential (ESP, Fig. 5e), and localized orbital locator (LOL, Fig. 5f, g)26. Structural modeling of COFs utilized the Materials Visualizer module in Materials Studio. First-principles calculations employed the Vienna ab-initio simulation package (VASP)39. The Projector Augmented Wave (PAW) method described the electron-ion interactions40. Exchange-correlation effects were treated with the Perdew-Burke-Ernzerhof (PBE) functional under the Generalized Gradient Approximation (GGA), including DFT-D3 correction for van der Waals interactions. Brillouin zone integration used A 1×1×1 Gamma-centered k-point mesh41. Relaxation convergence criteria comprised: electronic self-consistent cycle energy tolerance of 10⁻⁶ eV, maximum atomic force below 0.02 eV/Å, and a plane wave cutoff energy of 520 eV. Post-processing of CBM-VBM and DOS data utilized VASPKIT23. While DFT simulations provide valuable insights into catalytic mechanisms, all conclusions should be cautiously interpreted due to approximations in modeling real catalytic environments.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript reported three metal-free quasi-one-dimensional covalent organic frameworks (Q1DCOFs) for photocatalytic CO2 reduction into CO without additional sacrificial agents. The enaminone-linked 1D COF facilitate the activation of CO2 molecule via hydrogen-bond, which lead to superior activity compared to traditional imine-linked 1D COF. This work is interesting and give some ideas for the design of COF-based materials on CO2 photoreduction, I suggest that this manuscript might be published after minor revisions. Issues are listed as follows:\n1. On line 97, what’s the reason that the interlayer spacing for En-Q1DCOF are larger than normal COFs (~3-4 Å).\n2. Figure S17, it seems that all three COFs possess imine bond (1625 cm-1). The author should give a clearer figure to distinguish these three COFs.\n3. On line 141, the pore size obtained by N2 adsorption isotherm gives a significant difference from the simulated structure of COFs. the author should interpret the distinction between the experiment results (>3 nm) and theoretic models (<1.5 nm).\n4. On line 149, what is the diffraction angle corresponded to the (311) facet? The author should mark the (311) facet in PXRD pattern and calculate the lattice space of this facet at first.\n5. Figure S30, M-S results gave the CB of En-Q1DCOF is -0.57 V vs Ag/AgCl, but the energy diagram (Figure 3c) gave the CB of En-Q1DCOF is -0.57 V vs NHE. This is an obvious mistake. According to the reduction potential of CO2/CO (0.51 V vs RHE at pH=7), the CB of En-Q1DCOF should be -0.37 V vs RHE at pH=7), which is not a thermodynamically allowance for the reduction of CO2. The author should interpret this result or give some re-measurements.\n6. On line 207, the author should give a detailed calculation equation for AQY in experimental section, including light intensity, reaction time, irradiation area... etc.\n7. On line 240, “Ground state” seems more suitable for photochemistry than “Base state”.\n8. On line 294, the signal of ground state beaching (GSB) is obviously different from the signal of SE, so this statement is not suitable. The wavelength of SE is usually longer than that of GSB.\n9. On line 300, the lifetime of excited state on En-Q1DCOF shorter than Im-Q1DCOF can not directly infer that a faster shallow electron capture and a more efficient exciton dissociation in En-Q1DCOF. There should be another excited state absorption (ESA) signal which possess a longer lifetime appeared in TAS, in other words, a long-lived charge transfer/separation state is observed (Angew. Chem. Int. Ed. 2024, e202418319).\n10. Figure 7b, some signals such as *c-CO32-, *HCO3-, *COOH are hard to be recognized. The author should revise the figure or give a new data with enhanced adsorption signal.", "reviewer_2": "The manuscript reports the design and synthesis of enaminone-linked covalent organic frameworks (COFs) as photocatalysts for CO₂ reduction under gas–solid conditions, achieving a maximum CO yield of 3045 μmol g⁻¹ with 100% selectivity in 24 h. The materials are well-characterized. However, the work does not meet the standards required for publication in Nature Communications:\n1. The materials have poor absorption in visible region which is not a good photocatalyst in generation for solar fuels.\n2. The reduction potential or CB of these materials is around -0.5-0.6 V vs. NHE, which only show a small driving force as compared to the potential of CO2 reduction into CO. It is not possible to do the reaction.\n3. Although the authors have the isotopic experiment, it is very suspicious the CO is from degradation of the materials. Because only the materials having CO group can produce CO.\n4. The catalytic mechanism is unclear how the materials interact with CO2 and reduce it into CO.\n5. Water oxidation has been proposed in the system, but no oxygen has been detected and quantified.\n6. No light intensity was provided for the photocatalytic experiment. The author only mentioned the power of light source.", "reviewer_3": "Recommendation: Major revision.\nReviewer comments: This work reports an innovative enaminone-linked Q1DCOF (En-Q1DCOF) for photocatalytic CO2 reduction, achieving high performance (3045 μmol g-1 CO yield, ~100% selectivity) under additive-free gas-solid conditions. The design of the polar enaminone linkage structure effectively addresses the long-standing limitations of imine-based Q1DCOFs in photocatalysis. The proposed H-bond-assisted CO2 activation mechanism is intriguing and supported by comparative studies with Mix-Q1DCOF and Im-Q1DCOF. These findings advance the field of low-dimensional COFs for energy conversion. However, the following issues require clarification to strengthen the MS:\n1) The low BET area of Mix-Q1DCOF (282 m2/g) is attributed to the linkage disorder, but there are two inconsistencies: i) Mix-Q1DCOF’s pore size (4.57 nm) lies between En-Q1DCOF (3.42 nm) and Im-Q1DCOF (5.49 nm), conflicting with the predicted inverse correlation between pore size and surface area. ii) Experimental pores (3.42-5.49 nm) vastly exceed theoretical predictions (e.g., 1.51 nm for En-Q1DCOF).\n2) The DFT calculations only illustrate the spatial distributions of CBM/VBM but lack mechanistic insights into how enaminone linkages induce charge delocalization/local polarization.\n3) The 2:1 CO/O2 ratio supports H2O-coupled oxidation but lacks detection of byproducts (H2, CH4) and validation of CO selectivity. Additionally, the molar CO/O2 ratios for all catalysts should be explicitly included in SI.\n4) The increased COF loading reduces the overall CO rate under broadband irradiation but monotonicly boosts the AQY at 420 nm. Are the declining CO rates across other spectral regions responsible for the overall rate decrease?\n5) The dual attribution of the negative fs-TAS signal to both ground-state bleaching (GSB) and stimulated emission (SE) contains errors. GSB signals should align with the ground-state absorption (at shorter wavelengths), not the emission band. The negative signal exhibits a mirror symmetry with steady-state PL emission, characteristic of SE-dominated dynamics.\n6) The claimed hydrogen-bonding advantages of enaminone in CO2 adsorption/activation are derived from gas-phase DFT models. However, does this approach neglect the H2O-mediated solvation effects？"}, "author_response": "Point-by-point response to reviewer comments\nManuscript ID: NCOMMS-25-63513\nReviewer #1:\nThis manuscript reported three metal-free quasi-one-dimensional covalent organic frameworks\n(Q1DCOFs) for photocatalytic CO reduction into CO without additional sacrificial agents. The\n2\nenaminone-linked 1D COF facilitate the activation of CO molecule via hydrogen-bond, which\n2\nlead to superior activity compared to traditional imine-linked 1D COF. This work is interesting\nand give some ideas for the design of COF-based materials on CO photoreduction, I suggest that\n2\nthis manuscript might be published after minor revisions. Issues are listed as follows:\nResponse: We sincerely appreciate the reviewer for the insightful feedback on our manuscript\nwhich has helped us to improve the quality of our work significantly.\n1. On line 97, what’s the reason that the interlayer spacing for En-Q1DCOF are larger than\nnormal COFs (~3-4 Å).\nFigure R1. The simple schematic diagram of the interlayer structure for En-Q1DCOF.\nResponse: Thanks for the good comment. We have drawn the simple schematic diagram of the\ninterlayer structure for En-Q1DCOF. As shown in Figure R1, the highly polar enaminone bond\nand the non-planar conformation of biphenyl in TAE in different layers could generate repulsive\nforces. Thus, we attribute the increased interlayer spacing in En-Q1DCOF to the combined effects\nof interlayer repulsion induced by the highly polar enaminone bond and the non-planar\nconformation of biphenyl in TAE. We have added some sentences in the revised manuscript (p. 5,\nlines 99-101 in text.)\n2. Figure S17, it seems that all three COFs possess imine bond (1625 cm-1). The author should\ngive a clearer figure to distinguish these three COFs.\nFigure R2. (a) Comparison of the FTIR spectra of Q1DCOFs. (b) Partial enlargement of the FTIR\nspectra of Q1DCOFs.\nResponse: Thanks for the comments. As suggested, we have enlarged Figure S17 in the range of\n1700-1500 cm-1. As shown in Figure R2, the vibration peaks of -C=N- and C=O are\ndistinguishingly labelled at 1625 cm-1 and 1632 cm-1, respectively. Obviously, only peaks of -\nC=N- and C=O could be observed in Im -Q1DCOF and En-Q1DCOF, respectively. As for Mix-\nQ1DCOF, both the peaks of -C=N- and C=O are observed. We have included these results in Fig.\nS17.\n3. On line 141, the pore size obtained by N adsorption isotherm gives a significant difference from\n2\nthe simulated structure of COFs. the author should interpret the distinction between the experiment\nresults (>3 nm) and theoretic models (<1.5 nm).\nFigure R3. En-Q1DCOF model aperture.\nResponse: Thanks for the good comments. This difference of the pore sizes between the\nexperimental measurement and theoretical simulation might stem from the limitations of the\ntheoretical model, such as the fact that the theoretical model only measures the planar pore\ndiameter of the pore of the COF, whereas the actual COFs have a more complex pore structure,\nsuch as a cylindrical pore or other non-uniform structures (Figure R3). In addition, theoretical\nmodels are usually based on simplified assumptions, which may ignore the effects of structural\ndefects, distortions, or activation processes present in the material on the pore size. In fact, COFs\noften have multilayered pore structures, including micropores, mesopores and macropores, and\nthese complexities may result in experimental measurements reflecting the presence of larger pores,\nwhereas theoretical models mainly focus on smaller micropores. We have added some sentences\nin the revised manuscript (p. 7, lines 145-149 in main text.)\n4. On line 149, what is the diffraction angle corresponded to the (311) facet? The author should\nmark the (311) facet in PXRD pattern and calculate the lattice space of this facet at first.\nFigure R4. Experimental and Pawley refined PXRD of En-Q1DCOF crystal structure on the basis\nof the AA stacking models.\nResponse: Thanks for pointing out this mistake. According to the Bragg equation, the lattice\nspacing of 4.1 Å corresponds more closely to the (040) crystal plane, and performed crystal plane\nannotation in the PXRD profile at 10.84° (Figure R4). We have rectified this mistake in the revised\nmanuscript (Fig. 2a, p. 8, line 156).\n5. Figure S30, M-S results gave the CB of En-Q1DCOF is -0.57 V vs Ag/AgCl, but the energy\ndiagram (Figure 3c) gave the CB of En-Q1DCOF is -0.57 V vs NHE. This is an obvious mistake.\nAccording to the reduction potential of CO /CO (0.51 V vs RHE at pH=7), the CB of En-Q1DCOF\n2\nshould be -0.37 V vs RHE at pH=7), which is not a thermodynamically allowance for the reduction\nof CO . The author should interpret this result or give some re-measurements.\nFigure R6. Valence band XPS spectra of (a) En-Q1DCOF, (b) Mix-Q1DCOF and (c) Im-Q1DCOF.\nFigure R7. The optical band gaps of En-Q1DCOF, Mix-Q1DCOF and Im-Q1DCOF determined\nby Tauc plot.\nFigure R8. Schematic energy band structures of En-Q1DCOF, Mix-Q1DCOF and Im-Q1DCOF.\nResponse: We genuinely thank this reviewer for pointing out this issue. Usually, the bandgap\nproperties of COFs typically lie between direct and indirect bandgaps due to their dual molecular\nand crystalline characteristics (Nat. Rev. Mater., 2021, 6, 168-190). As for Q1DCOFs, the lack of\nstrongly conjugated and highly symmetrical units (such as pyrene and porphyrin) enables them to\nexhibit the characteristic of interlayer π-π stacking and less conjugated bonding modes (imine\nlinkages and enaminone linkages). This characteristic induces the folding and displacement of\ntheir energy bands in k-space, thus transforming them into indirect bandgap semiconductors (Chem.\nSoc. Rev., 2022, 51, 1124). Accordingly, sing the indirect model to analyse their band structure of\nQ1DCOFs is more reasonable than the direct model we have used in the previous manuscript. To\ncorrect this mistake, we have remeasured and analysed the band structures of all three Q1DCOFs\nby using the indirect semiconductor method. In the Tauc plot method for bandgap analysis, the\nvalue of n in the formula of (αhv)1/n = A (hv - E ) has been corrected from 1/2 for direct bandgap\ng\nto 2 for indirect bandgap (Figure R7). Additionally, in the processing of Mott-Schottky curve data,\nwe recalculated the tangent to the longest straight section of the M-S curve and corrected it\nusing the formula: E(NHE) = E(Ag/AgCl) + 0.197 (pH=7) (Figure R5). Besides, we have also\nremeasured the VB-XPS to determine the VB positions of En-Q1DCOF, Mix-Q1DCOF and Im-\nQ1DCOF (Figure R6). Based on the above analysis, we have redrawn band structures of En-\nQ1DCOF, Mix-Q1DCOF and Im-Q1DCOF (Figure R8). Obviously, all the COFs exhibit sufficient\ndriving force to trigger CO reduction and water oxidation reactions. We have included these\n2\nresults in the revised manuscript and supporting information (p. 9, lines 179, Fig. 3b and c, Fig.\nS30 and S31).\n6. On line 207, the author should give a detailed calculation equation for AQY in experimental\nsection, including light intensity, reaction time, irradiation area... etc.\nResponse: Thanks for the good suggestion. The catalytic reactions for determining AQY were\ncarried out in a quartz top irradiation-type reaction vessel with the irradiation area of 10.1 cm2.\nDifferent monochromatic lights centred at 420, 450, 500, 550, 600 and 650 nm were used. The\nAQY was calculated through the following equation:\n2𝑀𝑁 ℎ𝑐\n𝐴𝑄𝑌𝑠 % =\n𝐴\n×100%\n𝑆𝑃𝑡𝜆\nWhere M, N , ℎ, c, S, P, t and λ represent the molar number of generated CO, the Avogadro constant\nA\n(6.022×1023/mol), the Planck constant (6.626×10-34 J·s), the speed of light (3×108 m/s), the\nirradiation area of the incident light (cm2), the intensity of the incident light (W/cm2), the\nphotoreaction time (s) and the wavelength of the monochromatic light (nm), respectively. As\nsuggested, we have added this information in the revised manuscript (p. 11, lines 220-221; p. 24,\nlines 530-537).\n7. On line 240, “Ground state” seems more suitable for photochemistry than “Base state”.\nResponse: Thanks for the suggestion. As suggested, we have replaced “Base state” with “Ground\nstate” in the revised manuscript (p. 13, lines 259).\n8. On line 294, the signal of ground state beaching (GSB) is obviously different from the signal of\nSE, so this statement is not suitable. The wavelength of SE is usually longer than that of GSB.\nResponse: We greatly thank the reviewer for pointing out this confusion. In transient absorption\nspectroscopy, though both ground-state bleaching (GSB) and stimulated emission (SE) manifest\nas negative signals, their physical origins are fundamentally distinct. GSB arises from the depletion\nof ground-state molecules by the pump light, leading to the reduced absorption, whereas SE results\nfrom optical amplification generated by the stimulated emission of excited-state molecules upon\ndetection light stimulation. Although both exhibit signals of the same polarity and often overlap\nspectrally, GSB corresponds to the position of the ground-state absorption spectrum, while SE\ncorresponds to the position of the PL spectra. We have revised the expression in the revised\nmanuscript accordingly (p. 15, lines 311-313).\n9. On line 300, the lifetime of excited state on En-Q1DCOF shorter than Im-Q1DCOF cannot\ndirectly infer that a faster shallow electron capture and a more efficient exciton dissociation in En-\nQ1DCOF. There should be another excited state absorption (ESA) signal which possess a longer\nlifetime appeared in TAS, in other words, a long-lived charge transfer/separation state is observed\n(Angew. Chem. Int. Ed. 2024, e202418319).\nResponse: Thanks for pointing out this mistake. In organic semiconductor COFs, shallow electron\ncapture is related to interlayer stacking, molecular orbital energy level regulation (donor-acceptor\nunits) and the polarity of linker bonds (Adv. Mater. 2024, 2413118). The short excited-state\nlifetimes predominantly reflect exciton effects and deep-trap recombination (Chem. Soc. Rev.,\n2025, 54, 6553-6596). Accordingly, the lifetime of excited state on En-Q1DCOF shorter than Im-\nQ1DCOF indeed cannot directly infer that a faster shallow electron capture and a more efficient\nexciton dissociation in En-Q1DCOF. In the revised manuscript, we have modified the description\nof the multi-exponential decay curves to: enaminone bonds can effectively regulate the efficiency\nof interfacial charge transfer.\nAs for the ESA signal the reviewer mentioned, its observation in certain systems alone\nconstitutes a reasonable physical phenomenon, indicating that excited states primarily return to the\nground state via radiative or non-radiative relaxation rather than through energy dissipation via\nabsorption at higher energy levels. In femtosecond transient absorption spectroscopy, the GSB\nsignal and ESA signal represent two core physical phenomena, each corresponding to distinct\nexcited-state dynamic processes. The recovery time of the GSB signal directly reflects the\ndynamics of ground-state recovery (the rate at which the excited state relaxes back to the ground\nstate). The decay time of the ESA signal reflects the excited-state lifetime (S₁→S₀ internal\nconversion and intersystem crossing). The d orbitals of the metalloporphyrin centre in the research\npaper (Angew. Chem. Int. Ed. 2024, e202418319) couple with the extensive π-conjugated system\nof the porphyrin ring, forming a rich array of electronic energy levels (such as S₁, S₂, T₁, etc.). This\nmulti-level structure permits multiple transitions in the excited state (e.g., S₁→Sₙ or T₁→Tₙ),\nsignificantly enhancing the ESA process. However, the metal-free Q1DCOF in our study is\ntypically designed as a strong donor-acceptor unit for efficient charge separation. On one hand, the\nperiodic π-conjugated framework of Q1DCOFs readily forms delocalised excitons. At high\npumping energies, excessive exciton concentration leads to energy dissipation via non-radiative\nannihilation, thereby suppressing ESA. On the other hand, photo-generated electron-hole pairs\nundergo separation within the sub-picosecond range (<1 ps), resulting in extremely short excited-\nstate (exciton) lifetimes. Consequently, the ESA signal proves difficult for detectors to capture.\nWe have added some sentences in the revised manuscript (p. 15-16, lines 316-323).\n10. Figure 7b, some signals such as *c-CO 2-, *HCO -, *COOH are hard to be recognized. The\n3 3\nauthor should revise the figure or give a new data with enhanced adsorption signal.\nFigure R9. In situ ATR FTIR spectra of CO interactions with En-Q1DCOF under subsequent\n2\nlight irradiation.\nResponse: Thanks for the good suggestion. As suggested, we have provided a clearer figure\n(Figure R9) in the revised manuscript, where the signals of *c-CO 2-, *HCO -, *COOH are easy\n3 3\nto be recognized (P. 16, Figure 7b).\nReviewer #2\nThe manuscript reports the design and synthesis of enaminone-linked covalent organic frameworks\n(COFs) as photocatalysts for CO₂ reduction under gas-solid conditions, achieving a maximum CO\nyield of 3045 μmol g⁻¹ with 100% selectivity in 24 h. The materials are well-characterized.\nHowever, the work does not meet the standards required for publication in Nature Communications:\nResponse: We would like to express our sincere thanks to this reviewer for her/his valuable\ncomments and constructive suggestions which have helped us to improve the quality of our\nmanuscript significantly.\n1. The materials have poor absorption in visible region which is not a good photocatalyst in\ngeneration for solar fuels.\nFigure R10. UV/vis spectra of En-Q1DCOF, Mix-Q1DCOF and Im-Q1DCOF (corresponding\nphotographs of the materials are shown in the inset).\nResponse: Thanks for the good comments. Covalent organic frameworks (COFs), initially\ndeveloped by Yaghi and colleagues in 2005, represent a class of crystalline porous polymers\nconstructed through robust covalent bonding between organic monomers (Chem. Rev. 2020, 120,\n16, 8814–8833). The inherent π-conjugated systems within these frameworks facilitate efficient\nwide light absorption, granting superior photoelectric properties (Angew. Chem.2023,135,\ne202303086). Besides, their covalently bonded light element frameworks could ensure structural\nstability during photochemical processes (Adv. Mater.2024, 2413118). Thus, COF-based catalysts\nhave achieved great strides for photocatalytic applications, including photocatalytic CO reduction\n2\n(Chem. Soc. Rev., 2025, 54, 6062-6087). As for En-Q1DCOF, its intrinsic UV/vis absorption edge\nis about 460 nm (Figure R10). Accordingly, the bandgap of En-Q1DCOF has been determined to\nbe 2.42 eV, which is regarded as an excellent photocatalyst in the visible light region (Chem. Rev.\n2020, 120, 2, 919-985). Besides, we have also determined the band structure of En-Q1DCOF,\nwhich aligns with the energy requirements of the targeted photocatalytic reaction. During gas-solid\nphotocatalytic CO₂ reduction without using any metal, photosensitizer, or sacrificial reagent, the\nphotoelectrochemical conversion performance of En-Q1DCOF is superior to most of the metal-\nfree COFs including 2N-COF, 1N-COF, TFPT-BDF-COF, TFB-BDF-COF, BTE-TBD-COF, etc.\n(J. Am. Chem. Soc. 2023, 145, 9520−9529; Nano Lett. 2024, 24, 5444-5452; Small 2023, 19,\n2208118). We have added some sentences and references in the revised manuscript (p. 9, lines171-\n172).\n2. The reduction potential or CB of these materials is around -0.5-0.6 V vs. NHE, which only show\na small driving force as compared to the potential of CO reduction into CO. It is not possible to\n2\ndo the reaction.\nFigure R11. Mott-Schottky plots of (a) En-Q1DCOF, (b) Mix-Q1DCOF and (c) Im-Q1DCOF.\nFigure R12. Valence band XPS spectra of (a) En-Q1DCOF, (b) Mix-Q1DCOF and (c) Im-\nQ1DCOF.\nFigure R13. The optical band gaps of En-Q1DCOF, Mix-Q1DCOF and Im-Q1DCOF determined\nby Tauc plot.\nFigure R14. Schematic energy band structures of En-Q1DCOF, Mix-Q1DCOF and Im-Q1DCOF.\nResponse: We thank this reviewer for pointing out this issue. Usually, the bandgap properties of\nCOFs typically lie between direct and indirect bandgaps due to their dual molecular and crystalline\ncharacteristics (Nat. Rev. Mater., 2021, 6, 168-190). As for Q1DCOFs, the lack of strongly\nconjugated and highly symmetrical units (such as pyrene and porphyrin) enables them to exhibit\nthe characteristic of interlayer π-π stacking and less conjugated bonding modes (imine linkages\nand enaminone linkages). This character induces the folding and displacement of their energy\nbands in k-space, thus transforming them into indirect bandgap semiconductors (Chem. Soc. Rev.,\n2022, 51, 1124). Accordingly, using the indirect model to analyse their band structure of\nQ1DCOFs is more reasonable than the direct model we have used in the previous manuscript. To\ncorrect this mistake, we have remeasured and analysed the band structures of all three Q1DCOFs\nby using the indirect semiconductor method. In the Tauc plot method for bandgap analysis, the\nvalue of n in the formula of (αhv)1/n = A (hv - E ) has been corrected from 1/2 for direct bandgap\ng\nto 2 for indirect bandgap (Figure R13). Additionally, in the processing of Mott-Schottky curve\ndata, we recalculated the tangent to the longest straight section of the M-S curve and corrected it\nusing the formula: E(NHE) = E(Ag/AgCl) + 0.197 (pH=7) (Figure R11). Besides, we have also\nremeasured the VB-XPS to determine the VB positions of En-Q1DCOF, Mix-Q1DCOF and Im-\nQ1DCOF (Figure R12). Based on the above analysis, we have redrawn band structures of En-\nQ1DCOF, Mix-Q1DCOF and Im-Q1DCOF (Figure R14). Obviously, all the COFs exhibit\nsufficient driving force to trigger CO reduction and water oxidation reactions. We have included\n2\nthe results in the revised manuscript and supporting information (p. 9, lines 180, Fig. 3b and c, Fig.\nS30 and S31).\n3. Although the authors have the isotopic experiment, it is very suspicious the CO is from\ndegradation of the materials. Because only the materials having CO group can produce CO.\nFigure R15. Mass spectrum for photocatalytic reduction of 13CO to 13CO over En-Q1DCOF.\nFigure R17. PXRD patterns of En-Q1DCOF before and after photocatalytic reaction.\nResponse: Thanks for this comment. We have conducted isotope experiments demonstrating that\nthe CO was originated from CO rather than the material itself (Figure R15). In addition, we have\n2\nalso analysed the FTIR spectra of En-Q1DCOF before and after photocatalytic reaction (Figure\nR16), where no change in the intensity or wavenumber of the C=O group at 1632 cm⁻¹ was\nobserved. This result further confirms that the product of CO does not originate from the C=O\ngroups within En-Q1DCOF. Besides, we have also collected the PXRD patterns of En-Q1DCOF\nbefore and after photocatalytic reaction. As shown in Figure R17, the XRD patterns of En-\nQ1DCOF before and after the reaction are almost identical, demonstrating that En-Q1DCOF did\nnot undergo structural collapse. In other words, all the bonds in En-Q1DCOF including C=O bond\nare stable. Furthermore, as a common group in COFs, the C=O group has been demonstrated to be\nan efficient active site for CO₂ photoreduction (Nano Lett. 2024, 24, 18, 5444-5452) (Angew. Chem.\nInt. Ed.2023,62, e202313392) (J. Am. Chem. Soc. 2021, 143, 16284-16292). As a result, all the\nabove-mentioned results convincingly demonstrate that the CO is generated from the reduction of\nCO , not the degradation of the materials. We have added some sentences in the revised manuscript\n2\n(p. 11, lines 227-236).\n4. The catalytic mechanism is unclear how the materials interact with CO and reduce it into CO.\nFigure R19. Kohn-Sham orbitals of CBM and VBM of En-Q1DCOF.\nFigure R20. Charge distribution and centroids diagram for En-COF and in the excited state (hole\nis blue region and electron is green region).\nFigure R21. Gibbs free energy at different CO adsorption sites of Q1DCOFs\nFigure R24. In situ ATR FTIR spectra of CO and interactions with En-Q1DCOF under\n2\nsubsequent light irradiation.\nFigure R25. DFT-calculated Gibbs free energy profiles for CO photoreduction over En-Q1DCOF\n2\nand Im-Q1DCOF.\nResponse: Thanks for the good comments. We have conducted an in-depth investigation into the\nmechanism of CO₂ reduction to CO over En-Q1DCOF from both experimental and theoretical\nperspectives. First, we determined the band structure of En-Q1DCOF through UV/vis\nspectroscopy, Mott-Schottky, and VB-XPS analysis, demonstrating the thermodynamic feasibility\nof En-Q1DCOF for photocatalytic CO₂ reduction (Figure R18).\nTo elaborate how En-Q1DCOF interacts with CO , we have performed DFT calculations to\n2\ndetermine the CBM-VBM of En-Q1DCOF and the distribution of electrons and holes in the\nexcited state (Figure R19 and R20), thereby establishing that the photoreduction reaction occurs\nat the enaminone motif. Furthermore, we have also investigated the CO₂ adsorption by DFT\ncalculations. As shown in Figure R21, CO molecule is readily adsorbed onto the enaminone unit,\n2\nwhich involves the cooperative binding by the polar C=O group and hydrogen atoms on the\nnitrogen atom. This result was consistent with that of in situ ATR FTIR, where a new broad band\ncentered at 3630 cm-1 attributing to the H-O bending of N-H-O=C was observed (Figure R22).\nFurther DFT calculations indicated that the hydrogen atoms on the nitrogen atoms in the\nenaminone unit have higher positive charges, demonstrating that their stronger electrostatic\nattraction than the hydrogen atoms on other carbon atoms, which is more conducive to forming\nhydrogen bonds with CO (Figure R23).\nreaction conditions to directly visualize the process of CO photoreduction and water\n2\nphotooxidation over En-Q1DCOF. As shown in Figure R24, several peaks attributing to the typical\nintermediates such as CO 2-, *COOH, and *OH gradually appeared between 1100 to 2000 cm-1\n3\nwith the irradiation time from 0 to 60 min. Based on the above experiments and analysis, a\nreasonable mechanism was proposed for CO reduction coupled with H O oxidation process over\n2 2\nEn-Q1DCOF, which was also further supported by DFT calculations (Figure R25). We have\nhighlighted the related results in the revised manuscript (p.9, lines 183-184; p.9, lines 186-191;\np.13, lines 271-274; p.17, lines 334-354; p.17-18, lines 361-368).\n5. Water oxidation has been proposed in the system, but no oxygen has been detected and\nquantified.\nFigure R26. CO and O evolution rates of En-Q1DCOF, Mix-Q1DCOF and Im-Q1DCOF.\n2\nemployed. 2 mg of catalyst was dispersed in 5 mL of H O, followed by ultrasound for 15 min. The\n2\nobtained suspension was immobilized on a glass fiber filter membrane with a diameter of 40 mm,\nwhich was then activated in vacuum at 120 °C for 12 h. In the photocatalytic reduction of CO ,\n2\nthe photocatalyst thin films and 3 mL of distilled water were placed in a home-made quartz glass\nvessel, which was then bubbled with pure CO (99.999%) at a flow rate of ca. 30 mL·min−1 for 30\n2\nmin. Then 0.1 mL of pure He was injected to the reactor as the internal standard through gas sample\nconnection, followed by standing for 30 min for gas diffusion. Though we have tried our best to\nremove O from the reactor, it is still difficult to totally avoid the residue of O . Thus, 0.1 mL of\n2 2\nthe gas was extracted from the reactor to determine the amount of residual O before turning on\n2\nthe light. When the reaction was finished, another 0.1 mL of the gas was extracted from the reactor\nto determine the amount of O . The generated amount of O was acquired by subtracting the\n2 2\nresidue of O before reaction from the final amount of O after reaction. As shown in Figure R26,\n2 2\nan approximate CO/O ratio of 2:1 was obtained for En-Q1DCOF, Mix-Q1DCOF and Im-\n2\nrevised supporting information (p.32; Figure S32).\n6. No light intensity was provided for the photocatalytic experiment. The author only mentioned\nthe power of light source.\nResponse: Thanks for this good comment. The irradiance intensity is 100 mW cm⁻². We have\nadded this information in the revised manuscript (p. 23, lines 519).\nReviewer #3\nReviewer comments: This work reports an innovative enaminone-linked Q1DCOF (En-Q1DCOF)\nfor photocatalytic CO reduction, achieving high performance (3045 μmol g-1 CO yield, ~100%\n2\nselectivity) under additive-free gas-solid conditions. The design of the polar enaminone linkage\nstructure effectively addresses the long-standing limitations of imine-based Q1DCOFs in\nphotocatalysis. The proposed H-bond-assisted CO activation mechanism is intriguing and\n2\nsupported by comparative studies with Mix-Q1DCOF and Im-Q1DCOF. These findings advance\nthe field of low-dimensional COFs for energy conversion. However, the following issues require\nclarification to strengthen the MS:\nResponse: We would like to express our sincere thanks to this reviewer for her/his valuable\ncomments and constructive suggestions which have helped us to improve the quality of our\nmanuscript significantly.\n1. The low BET area of Mix-Q1DCOF (282 m2/g) is attributed to the linkage disorder, but there\nare two inconsistencies: i) Mix-Q1DCOF’s pore size (4.57 nm) lies between En-Q1DCOF (3.42\nnm) and Im-Q1DCOF (5.49 nm), conflicting with the predicted inverse correlation between pore\nsize and surface area. ii) Experimental pores (3.42-5.49 nm) vastly exceed theoretical predictions\n(e.g., 1.51 nm for En-Q1DCOF).\nFigure R27. En-Q1DCOF model aperture.\nResponse: Thanks for the good comments. We have re-checked the N adsorption-desorption data\n2\nof Mix-Q1DCOF. The corresponding pore size for Mix-Q1DCOF was 8.41 nm, not 4.57 nm, and\nwe have corrected this data in the revised manuscript (p. 7, lines 145). As for the difference of the\npore sizes between the experimental measurement and theoretical simulation, it might stem from\nthe limitations of the theoretical model, such as the fact that the theoretical model only measures\nthe planar pore diameter of the pore of the COF, whereas the actual COFs have a more complex\npore structure, such as a cylindrical pore or other non-uniform structures (Figure R27). In addition,\ntheoretical models are usually based on simplified assumptions, which may ignore the effects of\nstructural defects, distortions, or activation processes present in the material on the pore size. In\nfact, COFs often have multilayered pore structures, including micropores, mesopores and\nmacropores, and these complexities may result in experimental measurements reflecting the\npresence of larger pores, whereas theoretical models mainly focus on smaller micropores. We have\nadded some sentences in the revised manuscript (p. 7, lines 145-149 of the main text).\n2. The DFT calculations only illustrate the spatial distributions of CBM/VBM but lack mechanistic\ninsights into how enaminone linkages induce charge delocalization/local polarization.\nFigure R28. Surface ESP map of En-COF and Im-COF.\nResponse: Thanks for the good suggestion. To demonstrate how enaminone linkages induce\ncharge delocalization/local polarization, we have analyzed the the ground state electrostatic\npotential of the COFs linked by enaminone and imine bonds. It was speculated that the following\nthree factors contributed to the different spatial distributions of CBM/VBM between En-COF and\nIm-COF: 1) The strong electronegativity of the oxygen atom (3.5) imparts a significant positive\ncharge to the carbonyl carbon; 2) Electron delocalisation within the conjugated system (e.g., N-\nC=C-C=O structure) promotes synergistic interaction between nitrogen's lone pair and the\ncarbonyl group, enhancing charge separation; 3) Resonance contributions (e.g., R₂N⁺=CR-CR-C-\nO⁻ configuration) result in more pronounced charge polarisation. All of these facilitate the local\npolarization of the π-system, generating a strongly polarized electric field. Such a strongly\npolarized electric field promotes the delocalization of electrons within the system (dipole moment:\n3.4769 Debye for En-COF, Im-COF: 2.2511 Debye) (Figure R28). We have added some sentences\nin the revised manuscript (p. 13, lines 260-262).\n3. The 2:1 CO/O ratio supports H O-coupled oxidation but lacks detection of byproducts (H ,\n2 2 2\nCH ) and validation of CO selectivity. Additionally, the molar CO/O ratios for all catalysts should\n4 2\nbe explicitly included in SI.\nFigure R29. CO and O evolution rates of En-Q1DCOF, Mix-Q1DCOF and Im-Q1DCOF.\nas H₂ and CH₄ by gas chromatography, where no signal attributing to H₂ or CH₄ was observed.\nWhen metal-free COFs are employed for the photoreduction of CO₂, the products typically exhibit\nhigh selectivity, which has been widely reported (e.g., J. Am. Chem. Soc. 2023, 145, 9520-9529,\nNano Lett. 2024, 24, 5444-5452).\nTo detect the generated O , a 250-mL-reactor equipped with gas sample connection was\n2\nemployed. 2 mg of catalyst was dispersed in 5 mL of H O, followed by ultrasound for 15 min. The\n2\nobtained suspension was immobilized on a glass fiber filter membrane with a diameter of 40 mm,\nwhich was then activated in vacuum at 120 °C for 12 h. In the photocatalytic reduction of CO ,\n2\nthe photocatalyst thin films and 3 mL of distilled water were placed in a home-made quartz glass\nvessel, which was then bubbled with pure CO (99.999%) at a flow rate of ca. 30 mL·min−1 for 30\n2\nmin. Then 0.1 mL of pure He was injected to the reactor as the internal standard through gas sample\nconnection, followed by standing for 30 min for gas diffusion. Though we have tried our best to\nremove O from the reactor, it is still difficult to totally avoid the residue of O . Thus, 0.1 mL of\n2 2\nthe gas was extracted from the reactor to determine the amount of residual O before turning on\n2\nthe light. When the reaction was finished, another 0.1 mL of the gas was extracted from the reactor\nto determine the amount of O . The generated amount of O was acquired by subtracting the\n2 2\nresidue of O before reaction from the final amount of O after reaction. As shown in Figure R30,\n2 2\nan approximate CO/O ratio of 2:1 was obtained for En-Q1DCOF, Mix-Q1DCOF and Im-\n2\n4. The increased COF loading reduces the overall CO rate under broadband irradiation but\nmonotonicly boosts the AQY at 420 nm. Are the declining CO rates across other spectral regions\nresponsible for the overall rate decrease?\nFigure R30. Dependence of the CO evolution rate at 450 nm for En-Q1DCOF with different\nmasses.\nResponse: Thanks for the reviewer’s good comments. As the COF loading increases, the overall\nCO production rate continues to rise under broadband irradiation. However, when the overall CO\nproduction rate is averaged per gram of COF, the CO production rate per gram of COF decreases.\nThis phenomenon might be caused by light not penetrating the thicker catalyst layer (for similar\nresults, see Nature Energy 2023, 8, 1013-1022, Angew. Chem. 2024, 136, e202318562). This light-\nshielding effect also applies to the reaction under the irradiation of monochromatic light with a\nwavelength of 450 nm. As shown in Figure R30, the decrease in the formation rate of CO was also\nobserved when increasing the amounts of catalysts.\nAs for the monotonical boosting the AQY at 450 nm with the increasing amounts of catalysts,\nit was related to the calculation of AQY. The AQY value was calculated based on the followed\nformula:\n2𝑀𝑁 ℎ𝑐\n𝐴𝑄𝑌𝑠 % =\n𝐴\n×100%\n𝑆𝑃𝑡𝜆\nWhere M, N , ℎ, c, S, P, t and λ represent the molar number of generated CO, the Avogadro constant\nA\n(6.022×1023/mol), the Planck constant (6.626×10-34 J·s), the speed of light (3×108 m/s), the\nirradiation area of the incident light (cm2), the intensity of the incident light (W/cm2), the\nphotoreaction time (s) and the wavelength of the monochromatic light (nm), respectively.\nAccordingly, apart from some optical conditions, the value of AQY is related to the total amount\nof CO and is independent of the amount of catalyst. (p. 24, lines 530-537)\nAs for the CO production rate per gram of COF, we calculated its value according to the formula:\nCO reduction rate (μmol/g/h) =n/(m×t), where n, m and t represent the number of moles (μmol) of\ntarget substance produced or consumed during the reaction, the mass (g) of photocatalyst used, the\nduration (h) of the photoreaction. Obviously, the value of CO production rate per gram of COF is\nrelated to the quality of catalyst.\n5. The dual attribution of the negative fs-TAS signal to both ground-state bleaching (GSB) and\nstimulated emission (SE) contains errors. GSB signals should align with the ground-state\nabsorption (at shorter wavelengths), not the emission band. The negative signal exhibits a mirror\nsymmetry with steady-state PL emission, characteristic of SE-dominated dynamics.\nResponse: We greatly thank the reviewer for pointing out this confusion. In transient absorption\nspectroscopy, though both ground-state bleaching (GSB) and stimulated emission (SE) manifest\nas negative signals, their physical origins are fundamentally distinct. GSB arises from the depletion\nof ground-state molecules by the pump light, leading to the reduced absorption, whereas SE results\nfrom optical amplification generated by the stimulated emission of excited-state molecules upon\ndetection light stimulation. Although both GSB and SE exhibit signals of the same polarity and\noften overlap spectrally, GSB corresponds to the position of the ground-state absorption spectrum,\nwhile SE corresponds to the position of the PL spectra. We have revised the expression in the\nrevised manuscript accordingly (p. 15, lines 311-313).\n6. The claimed hydrogen-bonding advantages of enaminone in CO adsorption/activation are\n2\nderived from gas-phase DFT models. However, does this approach neglect the H O-mediated\n2\nsolvation effects？\nResponse: Thanks for the insightful comments. The CO reduction reaction was conducted under\n2\nthe atmosphere of water vapor. Thus, the H O-mediated solvation effect was weak and transient\n2\nunder our reaction conditions, which exerted minimal influence on CO adsorption/activation.\n2\nenaminone COFs and imine COFs, while the enaminone COFs demonstrated superior performance.\nThis result could further exclude the H O-mediated solvation effect on CO reduction.\n2 2\nBased on\nthe above analysis, water-mediated solvation effects should play a negligible role in the catalytic\nprocess and are therefore not discussed as a primary focus of this study. We have added some\nsentences in the revised manuscript (p. 18, lines 368-373)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors addressed the previous issues, now, this revised manuscript can be accepted.", "reviewer_2": "The authors have corrected the driving force by using an indirect bandgap estimation and XPS measurement. The CO2 reduction mechanism has been strengthened by DFT calculation. O2 was also measured in the newly performed experiment and the ratio between CO and O2 matches well. How does the material avoid O2 reduction in this case as compared to CO2 reduction? Also, it is also good to have isotopic experiment to show that O2 is not from leakage.", "reviewer_3": "Recommendation: Minor revision.\nThank you for addressing most of the revisions thoroughly. However, I must highlight a critical scientific inconsistency in your response to the first issue that requires immediate clarification.\nYour revision claims an exceptionally large pore size of 8.41 nm for Mix-Q1DCOF to align with the inverse pore-size/surface-area correlation. However, this claim directly conflicts with the N2 adsorption-desorption data provided in Figure S21, which explicitly shows three distinct pore size distributions peaking below 2 nm across all samples (En-Q1DCOF, Im-Q1DCOF, and Mix-Q1DCOF). In fact, pores exceeding 5 nm are exceptionally rare in crystalline COFs due to synthetic constraints on long-range order."}, "author_response": "Point-by-point response to reviewers’ comments\nManuscript ID: NCOMMS-25-63513A\nReviewer #2\nThe authors have corrected the driving force by using an indirect bandgap estimation and XPS\nmeasurement. The CO reduction mechanism has been strengthened by DFT calculation. O was\n2 2\nalso measured in the newly performed experiment and the ratio between CO and O matches well.\n2 2\nalso good to have isotopic experiment to show that O is not from leakage.\n2\n28\n)\n.\nu\n.\na\n(\ny\nt\ni\ns\nn\ne\nt\nn O\nI 2\n32\n18O Ar\n2\n16O18O 36 40\n34\n25 30 35 40\nm/z\nFigure R1. Mass-spectra of evolved 18O using H 18O as water source for En-Q1DCOF. A certain\n2 2\namount of H 16O exists in H 18O solution (95% purity).\n2 2\nResponse: We genuinely thank the reviewer for the positive feedback on our first revision and the\nconstructive comment about the O reduction. En-Q1DCOF contains active sites for oxygen\n2\nadsorption and oxygen reduction is thermodynamically feasible in this case. From this\nthermodynamic perspective, it is difficult to avoid the reduction of O , as the reviewer concerned.\nreduction of CO , reactants are present at high concentrations while conversion rates remain low.\neffectively reduces the probability of the contact of O₂ with the photocatalyst, thereby suppressing\nits further reduction. Besides, the reviewer also suggested to have isotopic experiment to show that\nO is not from leakage. As suggested, we have implemented the reduction of CO by utilizing\n2 2\nH 18O as the water source. After the reaction, we analyzed the gases by gas chromatography and\n2\nmass spectrometry, where a peak of 18O with m/z=36 was detected, confirming that O was\n2 2\noriginated from the water (Figure R1). Noting that the strong peaks of CO and CO have been\n2\nremoved from Figure R1 to clearly observe the peak of O . Besides, a peak of 16O with m/z=32\n2 2\nwas also detected, which was the residue of O . As we mentioned in the previously revised\n2\nsupporting information, though we have tried our best to remove O from the reactor, it is still\n2\ndifficult to totally avoid the residue of O . Thus, 0.1 mL of the gas was extracted from the reactor\n2\nto determine the amount of residual O before turning on the light. When the reaction was finished,\n2\nanother 0.1 mL of the gas was extracted from the reactor to determine the amount of O . The\n2\ngenerated amount of O was acquired by subtracting the residue of O before reaction from the\n2 2\nfinal amount of O after reaction. We have included these results in the revised manuscript and\n2\nsupporting information (p. 10-11, lines 208-211, Fig. S33).\nReviewer #3\nThank you for addressing most of the revisions thoroughly. However, I must highlight a critical\nscientific inconsistency in your response to the first issue that requires immediate clarification.\nYour revision claims an exceptionally large pore size of 8.41 nm for Mix-Q1DCOF to align with\nthe inverse pore-size/surface-area correlation. However, this claim directly conflicts with the N\n2\nadsorption-desorption data provided in Figure S21, which explicitly shows three distinct pore size\ndistributions peaking below 2 nm across all samples (En-Q1DCOF, Im-Q1DCOF, and Mix-\nQ1DCOF). In fact, pores exceeding 5 nm are exceptionally rare in crystalline COFs due to\nsynthetic constraints on long-range order.\nFigure R2. N adsorption-desorption isotherms of Q1DCOFs.\nResponse: We genuinely thank the reviewer for the positive feedback on our first revision and the\nconstructive comment about the pore size of Mix-Q1DCOF. As the reviewer mentioned, the pores\nexceeding 5 nm are exceptionally rare in crystalline COFs due to synthetic constraints on long-\nrange order. To further analyze the pore sizes of these Q1DCOFs, we have re-conducted the N\n2\nadsorption-desorption tests for three Q1DCOFs. These new results shown in Figure R2 are similar\nto that we have obtained previously. In the previous test, the BET areas of En-Q1DCOF,\nMix-Q1DCOF and Im-Q1DCOF are 667, 282, and 558 m2 g-1 with the average pore diameters of 3.42,\n8.41, and 5.49 nm, respectively. In the re-measurement, the BET areas of En-Q1DCOF,\nMix-Q1DCOF and Im-Q1DCOF are 594, 204, and 487 m2 g-1 with the average pore diameters of 4.17,\n9.31, and 6.33 nm, respectively. Thus, the experimental results of the measurement about the BET\nand the pore diameters for Q1DCOFs are genuine and reproducible. To investigate the reasons for\nthe significant discrepancy between the experimental average pore sizes and the theoretical\nmicropore sizes of Q1DCOFs, we conducted a detailed analysis of the pore size distribution from\nthe second test. As shown in Figure R3, we discovered the presence of numerous mesopores\nwithin Q1DCOFs. These mesopores might be pseudo-mesopores, which are caused by the unique\ndouble-chain structure of Q1DCOFs. This is also the main reason why the average pore diameter\nmeasured by the experiment for Q1DCOFs significantly differs from the model-theoretical\nmicropore size. Thus, it is more reasonable to analyze the pore size distribution of the micropore\nfraction within Q1DCOFs. Accordingly, the corresponding micropores pore sizes for En-Q1DCOF,\nMix-Q1DCOF and Im-Q1DCOF are 1.55, 1.67, and 1.59 nm, respectively, which are closely\nmatched with the model-theoretical micropore sizes, further confirming the successful synthesis\nof the three Q1DCOFs. We have included these results in the revised manuscript and supporting\ninformation (p. 7, lines 141, lines 143-145, Fig. 2f, Fig. S21)."}, {"round": 3, "reviewer_comments": {"reviewer_2": "The authors have well addressed the comments by providing new experimental data. The paper can be accepted as is.", "reviewer_3": "The revised manuscript can be accepted."}, "author_response": ""}]

Decision letter