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• Since their first report in 2005, covalent organic frameworks (COFs) have attracted significant attention due to their designability and highly ordered porous structures [1-5], which enable various applications in heterogeneous catalysis [6,7], energy storage [8,9], separation [10,11], light-emitting [12,13], cancer therapy [14,15], and organic electronics [16,17]. Currently, most COFs are still constructed by -B-O- and -C=N-bridges, which are dynamically reversible, facilitating the formation and self-repairing of ordered structures [2,18,19]. However, reversibility of these bonds also induces relatively poor stability and weak electron delocalization, which hinders their application in broader contexts [20-22].

  Recently, vinylene-bridged covalent organic frameworks (V-COFs) have been developed to address the limitations of traditional linkages [23-25]. These fully π-conjugated structures exhibit high crystallinity and exceptional chemical stability, and they are primarily constructed through carbonyl condensation reactions, such as Knoevenagel condensation, aldol condensation, and Horner-Wadsworth-Emmons reactions [26]. However, due to the limited reversibility of these reactions, the monomers available for constructing V-COFs are often restricted to a few highly symmetrical electron-deficient aromatic units, such as methyl-substituted triazine units and aromatic systems with cyano-activated methylene sites [27]. This poses a significant challenge for the development of the V-COFs family, resulting in relatively few successful examples of incorporating functional units into V-COFs.

  The diketopyrrolopyrrole (DPP) is a classic organic functional building block that features a planar conjugated bicyclic lactam unit, endowing it with strong electron-accepting properties and a rigid conjugated plane [28]. Materials based on DPP have been extensively studied in organic electronics, including field-effect transistors, solar cells, fluorescent sensors, and two-photon absorption materials [29]. Several COFs utilizing DPP have shown intriguing phenomena [30-35] however, they are also bridged by stacking, and are undesirable for π-delocalization. In 2019, Scott et al. developed a straightforward synthesis of DPP frameworks with exposed methyl side groups [36]. The electron deficiency of the DPP core activates the methyl groups, allowing them to react with various aldehydes through Knoevenagel condensation, thus enabling the possibility of DPP-based V-COFs.

  In theory, the construction of highly ordered V-COF structures relies on a certain level of reversibility during the reaction process, which enables the self-assembly and repair of crystalline domains [18]. In this work, a novel V-COF, Ph-DPP-COF based on an N-phenyl substituted DPP unit was synthesized using ammonium acetate, which has been shown to improve the reversibility of the initial steps of the Knoevenagel condensation [37]. The resulting V-COF was confirmed by PXRD, exhibiting a clear AA stacking pattern. Benefiting from the strong electron deficiency of DPP, the constructed donor-acceptor (D-A) structure of the fully conjugated COF demonstrated strong absorption in the near-infrared region and exceptional photothermal conversion capabilities.

  The Ph-DPP precursor was synthesized according to our previous literature (Scheme S1 in Supporting information) [38], and 1,3,5-tris(4-formylphenyl)benzene (TP) unit was selected as the counterpart for the condensation. Since according the original literature, the methyl should be rather active and easily react with aldehyde under medium basic condition with triethylamine and l-proline [36], various basic catalytic systems were screened initially (Table S1 in Supporting information). However, no crystalline products were obtained under these conditions (Table S1). The resulting materials were majorly dark amorphous powder, which implies that the condensation indeed took place in these conditions, but the crystalline structure did not grow as expected. This may largely be due to the over acidic methyl induced by the strong electron withdrawing effect of DPP core. Upon examining the mechanism of the base-reaction, the first step involved the deprotonation to form a carbanion [37]. The poor reversibility of this step hindered crystallization. Furthermore, the excessively activated methyl groups could hasten the reaction rate, thereby preventing the newly formed branches from organizing into an ordered pattern promptly.

  Therefore, the neutral ammonium acetate was selected, which was suggested to initiate with a highly reversible Schiff base reaction as demonstrated in Scheme 1. The feasibility of this catalyst was first verified through a template reaction (Scheme S2 in Supporting information). Ultimately, using a system where dioxane served as the solvent, ammonium acetate acted as the catalyst, and with a slight addition of acetic acid aqueous solution to further reduce the reaction rate, Ph-DPP-COF with a yield of 55% as a pure black crystalline was obtained after reacted at 150 ℃ for three days (Fig. 1a). Interestingly, the methyl-flanked DPP with N-alkyl substitution (C4-DPP, Fig. 1a) as a precursor was not responsive in either the basic or the ammonium acetate catalytic conditions in organic solvents to generate the corresponding crystalline COF structure. This suggests that reaction conditions may not be the sole determinant in the synthesis of our DPP-based V-COFs; the intermolecular π-π interactions stemming from the N-phenyl groups could also be pivotal in promoting regular crystalline alignment.

  Scheme 1

  

  Scheme 1.  Structures of various DPP derivatives, the two linking methods for DPP-based COFs, and proposed mechanism of Knoevenagel condensation catalyzed by AcONH₄.

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  Figure 1

  

  Figure 1.  (a) Structure and synthesis of Ph-DPP-COF. (b) PXRD patterns of Ph-DPP-COF. (c) BET and Pore size distribution of Ph-DPP-COF. (d) Top and side views of the eclipsed AA-stacking model.

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  The composition of Ph-DPP-COF was validated by Fourier transform infrared spectroscopy (FT-IR) and solid-state ¹³C cross-polarization magic angle spinning (CP/MAS) NMR spectroscopy. In the FT-IR spectroscopy (Fig. S7 in Supporting information), revealing a trans vinyl stretching vibration peak at 973 cm⁻¹, indicating the presence of vinyl bonds with a trans configuration in Ph-DPP-COF [39,40]. Further analysis of ¹³C NMR spectra (Fig. S8 in Supporting information) has also been performed that validate the key structure of Ph-DPP-COF. Thermogravimetric analysis (TGA) of Ph-DPP-COF (Fig. S9 in Supporting information) showed good stability with a decomposition temperature over 300 ℃. Scanning electron microscopy (SEM) images revealed that the morphology of Ph-DPP-COF consists of layered structures stacked together (Fig. S10 in Supporting information), while transmission electron microscopy (TEM) captured a thin layered structure (Fig. S11a in Supporting information), with high-resolution transmission electron microscopy (HR-TEM) image showing distinct lattice fringes with a spacing of approximately 3.0 Å, corresponding to the (001) facet (Fig. S11b in Supporting information).

  The crystalline structure of Ph-DPP-COF was evaluated by powder X-ray diffraction (XRD). As shown in Fig. 1b, Ph-DPP-COF displayed six peaks at 2θ = 3.41°, 7.17°, 9.69°, 13.85°, and 26.78°, which closely match the simulated AA stacking pattern. After Pawley refinement of the experimental XRD data, the unit cell parameters were obtained as a = b = 30.21 Å, c = 3.50 Å, α = β = 90°, γ = 120°, with R_wp = 2.93% and R_p = 2.23%. Meanwhile, the AB and ABC stacking modes were considered (Figs. S13 and S14 in Supporting information), and their corresponding simulated XRD data matched poorly with experimental data. The distinct XRD peaks and fine fitting results to the simulation indicating considerable crystallinity for Ph-DPP-COF material. Additionally, the XRD data comparison between the monomers (TP, Ph-DPP) and Ph-DPP-COF showed that there was almost no obvious monomer residue in the final Ph-DPP-COF obtained (Fig. S15 in Supporting information). Consequently, the porosity of Ph-DPP-COF was examined through nitrogen adsorption experiments at 77 K (Fig. 1c). The nitrogen adsorption and desorption isotherms of Ph-DPP-COF displayed a type Ⅱ isotherm, indicating that it is likely a macro-porous material. The Brunauer-Emmett-Teller (BET) surface area of Ph-DPP-COF was measured to be 226 m²/g. The pore size distribution curve indicated that the limiting pore diameter of Ph-DPP-COF is around 3.9 nm (Fig. 1c). As demonstrated in the AA stacking pattern (Fig. 1d). Ph-DPP-COF exhibited broad absorption from UV to near-infrared regions (Fig. 2a). This should be attributed to the intramolecular charge transfer induced by the strong D-A interactions between DPP as a strong electron-accepting unit and TP as the donor [35]. The optical band gap extracted from the Tauc plot was estimated be 1.60 eV (Fig. S16 in Supporting information). The absorption of Ph-DPP-COF is red-shifted compared to the small molecule model compounds (Fig. 2a), suggesting that the extended fine crystal structure enhances π-conjugation. The electrochemical properties of Ph-DPP-COF was further investigated through cyclic voltammetric measurements (CV) (Fig. S17 in Supporting information), showing low LUMO of −3.58 eV, and high HOMO of −5.11 eV, with estimated band gap of 1.53 eV, close to the optical estimation.

  Figure 2

  

  Figure 2.  (a) UV–vis absorption spectra of DPP-Model and Ph-DPP-COF in solid states. (b) Process of pressing COF powder into a COF disc. (c) Photothermal images of Ph-DPP-COF under on–off of the 660 nm light irradiation at 0.5 W/cm². (d) Heating and cooling recycling of Ph-DPP-COF under on–off cycles of the 660 nm light irradiation at 0.5 W/cm². (e) Photothermal heating and cooling curves of Ph-DPP-COF under different power densities of 660 nm laser. (f) Photothermal heating and cooling curves of DPP-Model and Ph-DPP-COF.

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  Photothermal conversion materials can transform light energy into heat energy and have been widely applied in fields such as seawater desalination [41], photothermal catalysis [42,43], and biomedicine [44]. COFs are gaining increasing attention in the photothermal application due to their diverse combination on the D-A structure promoting long range absorption, while also providing porous structure for potential separation or adsorption function, as well as low thermal conductivity nature [45]. The narrow band gap and broad absorption of Ph-DPP-COF indicate its potential for photothermal conversion performance. To facilitate photothermal data measurements, Ph-DPP-COF powder was first pressed into a disc-shaped film with a diameter of 1.3 cm and a thickness of approximately 0.2 cm using a tablet press. The experiments revealed that the film formed easily with a smooth surface, exhibiting structural stability with good mechanical processing capabilities (Fig. 2b). Under irradiation from a 660 nm infrared laser at a power of 0.5 W/cm², the surface temperature rose from room temperature to 107.3 ℃ in just 5 s, reaching 131.1 ℃ after 30 s, as the laser was turned off, the surface temperature rapidly dropped, returning close to room temperature within 20 s (Fig. 2c). Meanwhile, the photothermal conversion efficiency of Ph-DPP-COF was calculated to be 53% (Fig. S20 in Supporting information), performed excellently among the COFs reported in the photothermal field (Table S3 and Fig. S23 in Supporting information). As depicted in Fig. 2d, the photothermal cycling under 660 nm was repeated for several times with little variation on the maximum temperature reached and almost unaltered responding rate. A linear variation in the surface temperature can be observed along the change of the laser power (Fig. 2e). These results demonstrate excellent photothermal conversion ability of Ph-DPP-COF with outstanding cycling stability, sensitive response and fine tunability. Ph-DPP-COF also exhibited notable photothermal conversion effects (Fig. 2f and Fig. S24 in Supporting information) under an 808 nm laser or Xenon lamp (1 kW/m², simulating one sunlight) [41]. For comparison, the DPP-Model compound was also characterized. It had poor formability and required careful handling and gentle parameters to be pressed into shape, with an uneven surface prone to damage. Under irradiation from the 660 nm, 808 nm, and Xenon light sources, its photothermal conversion ability was inferior to that of Ph-DPP-COF (Fig. 2f, Figs. S24 and S25 in Supporting information), especially under 1 W/cm² power from the 808 nm laser, where Ph-DPP-COF reached a maximum temperature of 130.7 ℃, while DPP-Model only reached 92.8 ℃ (Fig. 2f). This indicates that the fully π-conjugated system and long-range ordered crystal structure of Ph-DPP-COF enhance its photothermal conversion capabilities by broadening the range and capacity for light absorption and facilitating low-resistance phonon transport and strong lattice vibrations.

  In summary, this study successfully synthesized a novel vinylene-bridged COF, Ph-DPP-COF, utilizing DPP unit as core function building block through a Knoevenagel condensation reaction, with neutral ammonium acetate as a catalyst to enhance reaction reversibility. Ph-DPP-COF exhibits a highly ordered AA stacking pattern, good thermal stability, a high BET surface area of 226 m²/g, and extensive light absorption capabilities from the UV to near-infrared regions. Consequently, Ph-DPP-COF demonstrated good mechanical processing capabilities and outstanding photothermal conversion performance, with exceptional responses, sensitive tunability and excellent cycling stability under the irradiation of different light sources, making it highly promising for applications in photothermal conversion technologies. Synthesize of more functional V-COFs comprised of such building blocks should be further noted.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Tianwen Chen: Methodology, Investigation, Data curation. Chunqiang Cai: Validation, Data curation. Li Chen: Validation, Software, Data curation. Yanlin Chen: Visualization, Data curation. Lichun Dong: Validation, Resources. Luxi Tan: Writing – review & editing, Writing – original draft, Funding acquisition, Data curation, Conceptualization. Zitong Liu: Writing – review & editing, Validation, Methodology, Funding acquisition, Data curation.

  Acknowledgment

  This work is supported by the National Natural Science Foundation of China (NSFC, No. 22202024).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111028.

  
  
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• 

  Scheme 1  Structures of various DPP derivatives, the two linking methods for DPP-based COFs, and proposed mechanism of Knoevenagel condensation catalyzed by AcONH₄.

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  Figure 1  (a) Structure and synthesis of Ph-DPP-COF. (b) PXRD patterns of Ph-DPP-COF. (c) BET and Pore size distribution of Ph-DPP-COF. (d) Top and side views of the eclipsed AA-stacking model.

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  Figure 2  (a) UV–vis absorption spectra of DPP-Model and Ph-DPP-COF in solid states. (b) Process of pressing COF powder into a COF disc. (c) Photothermal images of Ph-DPP-COF under on–off of the 660 nm light irradiation at 0.5 W/cm². (d) Heating and cooling recycling of Ph-DPP-COF under on–off cycles of the 660 nm light irradiation at 0.5 W/cm². (e) Photothermal heating and cooling curves of Ph-DPP-COF under different power densities of 660 nm laser. (f) Photothermal heating and cooling curves of DPP-Model and Ph-DPP-COF.

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