Catalyst-free solid-state cross-linking of covalent organic frameworks in confined space
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Abstract
A “confined space” provides a unique environment to regulate the crystallization thermodynamics and kinetics by confining the reactants in the restricted space dimensions. Solid-state crystal-to-crystal transitions in confined space are controlled by the preassembly of molecules in a crystal lattice and occur inside the lattice. Herein, we report the first case of construction of crystalline cross-linked covalent organic frameworks (CL-COFs) through solid-state cross-linking of acetylenic groups-bridged 2D COFs in spatially limited systems. Specifically, this transformation is thermally induced, yielding CL-COFs with superlative properties, including outstanding enhancement in crystallinity, specific surface area, and stability. We further demonstrate the CL-COFs as high conductivity polymers after iodine doping. This work underscores the opportunity to use lattice-constrained solid-state cross-linking to develop more versatile and feature-rich polyacetylene networks.
Keywords
INTRODUCTION
Polyacetylene has attracted extensive attention due to its interesting properties such as electrical, optical, and magnetic properties and has become an important research topic in the field of conjugated organic polymers throughout its long history[1-5]. In fact, early investigations on acetylene polymerization did not result in a linear conjugated polymer but, instead, a three-dimensional cross-linked organic polymer referred to as cuprene[6,7]. However, differences in starting materials and preparation methods resulted in too complex and poorly defined structures and compositions of these materials[8]. Greater efforts have turned to the preparation of similar conjugated polymer systems with well-defined and controllable compositions. In 1958, Natta et al. achieved the first polymerization of acetylene by organometallic catalysis[9]. However, the polyacetylene powder they prepared was completely insoluble and infusible. It was not until 1977 that Shirakawa et al. succeeded in the direct polymerization of acetylene to form thin films; this groundbreaking discovery aroused great interest in synthetic metals[1]. Cyclic polyacetylene, as another form, has also been developed rapidly during the same period. Since Reppe and Schweckendiek reported the transition-metal catalyzed [2 + 2 + 2] cyclotrimerization of alkynes to form aromatic ring structures[10], researchers became eager to challenge the limits of aromatic ring size basic formula (CH)n[11,12]. In 2021, Miao et al. first reported the catalytic synthesis of [∞]annulene with impressively low defects, highly conjugated structures, and low free-electron density [Scheme 1A][13]. However, the polymerizations involved in the previous efforts have often been accompanied by some of the following problems: (i) the reaction requires a large number of expensive and environmentally unfriendly metal catalysts; (ii) the polymerization conditions are harsh and the operation is cumbersome; and (iii) air sensitivity and poor stability have hampered the commercial success of polyacetylene.
Scheme 1. Historical development of acetylenic polymers. (A) Previously reported acetylenic polymers and corresponding structures; (B) This study presents the synthesis of 2D COFs and the proposed mechanism of transforming them into crystalline CL-COFs via solid-state cross-linking in confined spaces. COFs: Covalent organic frameworks; CL-COFs: crystalline cross-linked covalent organic frameworks.
Solid-state polymerization (SSP) has provided a promising solution because it can force chemical reactions to proceed with a minimum amount of atomic and molecular movement through solid-state confinement and preorganization[14-16]. Due to the lack of solvation, the reactants are almost “naked” in a confined space and ready for reaction with lower activation energy[17-19]. Therefore, SSP can occur without additional catalysis[20]. To the best of our knowledge, direct polymerized acetylenic groups to form a crystalline polymer without the addition of a catalyst is unprecedented and inconceivable. It would, thus, be of general interest to develop an efficient method for the polymerization of acetylenic groups in crystals.
As an emerging class of highly crystalline porous materials, covalent organic frameworks (COFs) are an ideal platform for solid-state reactions in the confined space[21-29]. Huang et al. reported, for the first time, the interlayer [4 + 4] cycloaddition reactions in borate-linked anthracene 2D COFs, realizing the smart light response of COFs[30]. Acharjya et al. showed that UV/Vis light irradiation [2 + 2] cycloaddition reaction of vinyl COF realized the transformation from 2D to 3D. However, during this process, the sp2 hybridized vinyl carbon in the skeleton was converted to sp3-cyclobutane species, and the product lost crystallization[31]. A recent study by Jadhav et al. showed that the degree of polymerization and crystallinity of the vinyl [2 + 2] cycloaddition in COFs is affected by the dispersed solvent, low to medium polar aprotic solvents are more likely to form crystalline products[32]. Besides photoinitiation, thermal initiation is also an effective method to induce SSP. In 2021, Zhu et al. reported thermally induced construction of interlayer conjugated links via SSP in diacetylene-contained COFs, which provided an efficient paradigm for conducting SSP[33].
Herein, we report, to the best of our knowledge, the first case of solid-state reaction in the confined space to construct cross-linked (CL)-COFs in acetylene bridged 2D COFs [Scheme 1B], and this cross-linking reaction does not require any catalyst. Our strategy is to use acetylenic groups (−C≡C−) to construct a series of 2D COFs and use face-to-face π-stacking interaction to make the adjacent acetylenic groups at a suitable distance. This model of stacking provides the possibility for thermally induced layer-to-layer cross-linking, while only negligible changes in the framework occur. In addition, the CL-COFs formed after cross-linking have higher electronic conductivity than the pristine COFs.
EXPERIMENTAL
Materials and chemicals
All chemicals and solvents were purchased from commercial suppliers. Specifically, 4,4′-diaminobiphenyl (DABP) and 4,4″-diamino-p-terphenyl (DATP) were purchased from Aladdin®. Additionally,
Synthesis of 1,3,5-tri(4-formylphenylethynyl)benzene
Under a nitrogen atmosphere, 1,3,5-triethynylbenzene (1.2 g, 8.0 mmol, 1.0 eq.), 4-iodobenzaldehyde (5.6 g, 24.0 mmol, 3.0 eq.), dichlorobis(triphenylphosphine)palladium(II) (0.34 g, 0.48 mmol, 0.02 eq.) were dissolved in 200 mL of anhydrous THF. Dry triethylamine (8.0 mL, 57 mmol) was added, and the mixture was stirred for 10 min; then, the catalyst copper(I) iodide (0.19 g, 1.0 mmol, 0.04 eq.) was added, turning the solution to dark brown. After stirring for 16 h at room temperature, the solvent was evaporated at reduced pressure, and the resulting solid mixture was washed with brine-saturated solution of NH4Cl and extracted with DCM. The purification was made by column chromatography on silica gel using DCM as eluent to afford the desired product as a colorless solid (2.18 g, Yield: 59%); 1H NMR (400 MHz, CDCl3): δH 10.04 (s, 3H), 7.90 (d, J = 8.3 Hz, 6H), 7.74 (s, 3H), 7.69 (d, J = 8.2 Hz, 6H); 13C NMR (101 MHz, CDCl3): δC 191.5, 136.0, 135.0, 132.4, 129.8, 128.9, 123.8, 91.3, 90.1.
Preparation of CPOF-4
A Pyrex tube measuring o.d. × i.d. = 10 × 8 mm2 was charged with 1,3,5-tri(4-formylphenylethynyl)benzene (TFPEB) (13.9 mg, 0.03 mmol), DABP (8.3 mg, 0.045 mmol) in a mixed solution of o-DCB (0.5 mL), DMAc (0.5 mL), and 6.0 M acetic acid (0.1 mL). The Pyrex tube was flash frozen in a liquid nitrogen bath sealed under vacuum. Upon warming to room temperature, the tube was placed in an oven at 120 °C for three days. The yellow solid was isolated by filtration and washed with THF (3 × 15 mL), acetone (3 × 15 mL), and n-hexane (3 × 15 mL). The powder was dried at 80 °C under vacuum overnight to afford the CPOF-4 as a yellow crystalline solid (15.4 mg, Yield: 75%).
Synthesis and activation of CPOF-4-265 °C-X h (X = 12, 48 and 84)
For example, a 5 mL vial charged with CPOF-4 (100.0 mg) and diphenyl sulfone (3 g), The vial was then transferred to a tubular furnace and evacuated, filled with N2 by five cycles. Subsequently, the temperature was raised to 265 °C at the rate of 10 °C·min-1 in a N2 flowing atmosphere with a flow rate of 20 mL·min-1. After heating at 265 °C for a certain period of time, the temperature was reduced to room temperature at the rate of 10 °C·min-1. Finally, the obtained product was washed with THF and acetone to remove the residual diphenyl sulfone and dried at 120 °C under vacuum overnight to afford the CPOF-4-265 °C-X h
Characterizations
1H NMR spectra were measured on a Bruker Fourier 400 MHz spectrometer. Solid-state NMR spectra were recorded at ambient pressure on a Bruker Fourier 600 MHz spectrometer using a standard CP pulse sequence probe with 3.2 mm (outside diameter) zirconia rotors. The Fourier-transform infrared (FT-IR) spectra (KBr) were obtained using a SHIMADZU IRAffinity-1 FT-IR spectrophotometer. A SHIMADZU UV-2450 spectrophotometer was used for all absorbance measurements. Powder X-ray diffraction (PXRD) patterns were carried out in a reflection mode on a Bruker D8 advance powder diffractometer with Cu Kα
Electrical conductivity measurements.
Electrochemistry experiments were conducted on a CHI660C Electrochemical Workstation (Shanghai ChenHua Electrochemical Instrument). The obtained powder was ground before being added into a 0.5-cm die. Then, the die pressure was slowly increased to 4.0 MPa and kept for 1 h to prepare pellets
RESULTS AND DISCUSSION
Our strategic preparation of 2D COFs is based on condensation of triangular building block, TFPEB, and amino-based linear linkers of different lengths, DABP or DATP, that resulted in two novel acetylenic group bridged 2D COFs, namely, CPOF-4, and CPOF-5 (CPOF = crystalline porous organic framework), respectively. After this, solid-state cross-linking of COFs was carried out at 265 °C, and the addition of diphenyl sulfone was targeted to make COFs heated uniformly at high temperatures. In order to verify the cross-linking reaction of acetylenic groups between COF layers to form polyacetylene structures, the
Figure 1. (A) FT-IR spectra analyses of CPOF-4-265 °C-X h (X = 12, 48 and 84) compared with the CPOF-4; (B) 13C CP-MAS NMR spectra of CPOF-4 and CPOF-4-265 °C-84 h; (C) High-resolution C 1s XPS peak of CPOF-4 (down) and CPOF-4-265 °C-84 h (top); (D) Solid-state EPR spectrum of CPOF-4 (black) and CPOF-4-265 °C-84 h (red); (E) Solid-state EPR spectrum of CPOF-5 (black) and CPOF-5-265 °C-84 h (red); (F) UV-vis DRS of CPOF-4 and CPOF-4-265 °C-X h (X = 12, 48 and 84). Inset: CPOF-4 (left) and CPOF-4-265 °C-84 h (right) immersed in THF. FT-IR: Fourier-transform infrared; EPR: electron paramagnetic resonance; DRS: diffuse reflection spectroscopy; THF: tetrahydrofuran.
In order to evaluate the composition of the products after the cross-linking reaction of 2D COFs, we performed detailed hydrolysis experiments on CPOF-4-265 °C-84 h samples [Supplementary Figures 10 and 11]. The synthetic details have been described in Sections S13-20 (Supplementary Materials). From the energy dispersive spectrometer (EDS) analysis results, it can be seen that the residual insoluble solid after hydrolysis does not contain elemental N [Supplementary Figures 12 and 13], and the liquid 1H NMR reveals that DABP monomer is produced [Supplementary Figure 14]. Therefore, it can be determined that the
The chemical compositions and elemental chemical states of COFs and CL-COFs were further investigated by XPS. The XPS general survey of the synthesized powder indicated mainly the presence of C, O, and N without any detectable S contaminants. The presence of O 1s peak is ascribed to the adsorption of air in the pores [Supplementary Figures 24 and 25]. The high-resolution C 1s XPS spectra of the CPOF-4 exhibited three characteristic peaks at 284.7, 285.2, and 286.1 eV [Figure 1C, Supplementary Figures 26 and 27], which are attributed to the C=C (sp2), C≡C (sp), and C=N bonds, respectively[42-44]. Based on analysis of the characteristic peak intensity of the CPOF-4 and CPOF-4-265 °C-84 h, the C 1s XPS spectra of
One of the remarkable features of trans-polyacetylene is its intrinsic neutral soliton defects (bond alternation domain walls) with spin properties[45-48], and the magnetic moments of solitons can be detected by EPR. Here, we performed EPR characterization on these samples. As shown in Figure 1D and E, the pristine 2D COFs showed a very weak EPR signal, which may originate from unreacted terminal amino or aldehyde groups at the boundaries of 2D layered crystals[49]. Impressively, the EPR spectral intensities of CPOF-4-265 °C-84 h and CPOF-5-265 °C-84 h are significantly enhanced, and the narrow Lorentzian line shapes signal with g = 2.0027 can be attributed to neutral soliton defects inherent in the trans-polyacetylene structure. Moreover, the EPR signal intensity of the CPOF-4-265 °C-84 h can be further enhanced after iodine doping, which indicated the generation of more charge carriers as pairs of radicals and cations [Supplementary Figures 28-31][50,51]. Thus, these results fully demonstrate the formation of polarons in CPOF-4-265 °C-84 h and CPOF-5-265 °C-84 h.
Comparing the UV-vis spectra of CPOF-4 and CPOF-5 at different reaction times, it can be seen that the absorption range of the corresponding products is gradually broadened, and the absorption sideband gradually redshifts as the reaction time is prolonged, indicating that the internal conjugate system of the material increases. More interestingly, the analysis of the Tauc plot reveals that this solid-state cross-linking of acetylenic groups in confined space can effectively adjust the optical band gaps (Eg) of the material
We also directly polymerized the monomer containing acetylenic groups by heating, denoted as the polymer-model. FT-IR analysis showed that the absorption peak of acetylene at 2,208 cm-1 was obviously weakened [Supplementary Figures 35 and 36], indicating that the monomer could be successfully polymerized. The nitrogen adsorption isotherm at 77 K revealed the microporous nature of the polymer-model and yielded a BET surface area of 297 m2·g-1 [Supplementary Figures 37-39]. However, experimental PXRD patterns showcased the amorphous nature of the polymer-model [Supplementary Figure 40]. These results further demonstrate that the solid-state cross-linking under framework constraints can effectively preserve the crystal structure of the material.
The crystalline structures of as-synthesized 2D COFs and CL-COFs were assessed by PXRD analyses. The experimental PXRD patterns exhibited strong diffraction peaks at 2θ = 1.9°, 3.3°, 3.9°, and 5.1° for CPOF-4 and 1.7°, 2.9°, 3.4°, and 4.6° for CPOF-5 corresponding to the (100), (110), (200), and (220) facets, respectively. After a geometrical energy minimization by using the Materials Studio software package based on the 2D hcb topology with AA stacking modes, the unit cell parameters were obtained [Supplementary Figures 41 and 42]. Moreover, full profile pattern matching (Pawley) refinements were applied to their experimental PXRD patterns, and the refinement results yielded unit cell parameters nearly equivalent to the simulations with good agreement factors (CPOF-4: a = b = 53.87 Å, c = 3.47 Å, α = β = 90°, γ = 120°,
Figure 2. (A) PXRD patterns of CPOF-4: comparison between the experimental (black) and Pawley refined (red) profiles, the refinement differences (blue), and the Bragg positions (green); (B) PXRD pattern analyses of CPOF-4-265 °C-X h (X = 12, 48, and 84) compared with the CPOF-4; (C) PXRD patterns of CPOF-4 and CPOF-4-265 °C-84 h: experimentally observed (solid line) and simulated based on eclipsed stacking modes (dashed line); (D) PXRD patterns of CPOF-4-265 °C-84 h: comparison between the experimental (black) and Pawley refined (red) profiles, the refinement differences (blue), and the Bragg positions (green); (E) Space-filling models of CPOF-4-265 °C-84 h. Carbon, gray; Nitrogen, blue; Hydrogen, white; (F) N2 adsorption/desorption isotherms of CPOF-4 and CPOF-4-265 °C-X h
The effect of cross-linking reaction on porosity and textural properties was evaluated by N2 adsorption-desorption measurements at 77 K. The adsorption isotherms of these 2D COFs belong to the typical type-IV isotherm that reveals the characteristics of mesoporous materials [Figure 2F, Supplementary Figures 54-67]. Specifically, the desorption branch of CPOF-4 and CPOF-5 isotherms is found to be higher than the adsorption branch at lower relative pressure, which suggests swelling effect of the framework[52,53]. Meanwhile, the desorption branches of CPOF-4-265 °C-84 h and CPOF-5-265 °C-84 h completely overlap with the adsorption branches, indicating that the frameworks become rigid and stable after layer-to-layer cross-linking. The BET surface areas calculated from the N2 adsorption results were found to be 447 and
The morphologies of both 2D COFs are ascertained by SEM and TEM. SEM inspection revealed a sheet-like morphology of the CPOF-4 and CPOF-5 [Supplementary Figures 68 and 69], and the morphology of the corresponding CPOF-4-265 °C-84 h and CPOF-5-265 °C-84 h formed after cross-linking remained nearly identical [Supplementary Figures 70 and 71]. Furthermore, the crystalline structures of these materials were confirmed by high-resolution TEM (HR-TEM) but only for CPOF-4-265 °C-84 h and CPOF-5-265 °C-84 h because CPOF-4 and CPOF-5 were found to be extremely unstable under the electron beam, and thus, their HR-TEM could not be obtained[54,55]. In sharp contrast, the HR-TEM images of CPOF-4-265 °C-84 h and CPOF-5-265 °C-84 h taken along the [001] direction revealed an ordered, honeycomb-like porous structure with a periodicity of 4.1 ± 0.2 nm for CPOF-4-265 °C-84 h and 4.8 ± 0.1 nm for CPOF-5-265 °C-84 h, respectively [Supplementary Figures 72-75]. The observed results are consistent with the pore-to-pore distance between the pores in the simulated structures.
The thermal stabilities of the 2D COFs were evaluated by TGA. The two 2D COFs showed barely noticeable skeleton weight loss before 440 °C [Supplementary Figures 76 and 77]. In order to evaluate the effect of solid-state cross-linking on the chemical stability of the materials, the 2D COFs and CL-COFs were immersed in different solvents, including n-pentane, n-hexane, DCM, acetone, THF, MeOH, DMF, boiling water, concentrated NaOH (10 M), and concentrated HCl (1 M), for 24 h. PXRD results showed that both 2D COFs lost their crystallinity after being activated in DCM, acetone, THF, MeOH, and DMF but maintained high crystallinity in n-hexane and n-pentane [Figure 3 and Supplementary Figures 78-81]. This might originate from the acetylenic groups (−C≡C−) into a 2D COF framework, resulting in very weak van der Waals interactions, which do not contribute to interlayer packing of the 2D polymers and lead to very low crystallinity[56,57]. The weak π−π interaction between 2D COF adjacent layers fails to counteract the strong surface tension and capillary forces generated during liquid-gas phase transition of the guest molecules with high surface tension, thereby resulting in framework collapse[58,59]. In addition, both 2D COFs are also found to be unstable in the harsh environments of concentrated acid/base/boiling water. N2 adsorption isotherm tests showed consistent results, such as the activation of CPOF-4 when performed using THF, resulted in a very low BET surface area, whereas activation with ultra-low surface tension solvent n-hexane yielded a high surface area. In sharp contrast, CPOF-4-265 °C-84 h and
Figure 3. Stability test of 2D COFs and CPOF-4-265 °C-84 h. Effect of solvent activation on (A-C) 2D COF structure and (D and E) CL-COF structure; (F) PXRD patterns of CPOF-4 after solvent treatments under different chemical environments for 24 h; (G) PXRD patterns of CPOF-4-265 °C-84 h after treatments under different chemical environments for 24 h. COFs: Covalent organic frameworks; CL-COF: crystalline cross-linked covalent organic framework; PXRD: powder X-ray diffraction.
The high surface areas and stability of CL-COFs inspired us to investigate the solid-state cross-linking reaction characteristics of acetylenic groups in 2D COFs, which will help to identify more suitable reaction conditions to construct CL-COFs. Since reaction temperature can strongly influence the morphology of the polyacetylene with consequences in its quality, isomeric content, crystallinity, and conductivity[60,61], we performed in-situ FT-IR studies of 2D COFs at 215, 265, and 295 °C, respectively
Doped polyacetylene, as an emerging kind of conjugated conducting polymer, demonstrates a substantial increase in conductivity upon doping with halogens[64,65]. Inspired by this, we evaluated the I-V characteristics (I-V curves) of CPOF-4 and CPOF-4-265 °C-84 h after iodine doping to understand the influence of solid-state cross-linking on the conductivity of the material. The I2@CPOF-4 and I2@CPOF-4-265 °C-84 h were synthesized by exposing the powdered samples to I2 vapor at 75 °C for 24 h. As shown in Figure 4, both CPOF-4 and CPOF-4-265 °C-84 h showed insulating behavior before I2 doping. The conductivity of the I2@CPOF-4 was measured to be 1.3 × 10-5 S·cm-1. Notably, I2@CPOF-4-265 °C-84 h showed a marked ten-fold enhancement in conductivity, reaching 1.2 × 10-4 S·cm-1. The significant enhancement in conductivity of CPOF-4-265 °C-84 h from that of 2D COFs is mainly attributed to the conjugated polyacetylene structure of the former, which exhibits high conductivity after doping with charge carriers (details in Section S57, Supplementary Materials). Nevertheless, the conductivity of I2@CPOF-4-265 °C-84 h cannot be compared with classical polyacetylene. We speculate that this may be due to the incomplete cross-linking reaction of acetylene groups leading to low conjugation of the structure of CPOF-4-265 °C-84 h, which has a relatively large band gap and low electrical conductivity[66].
Figure 4. Study of electron conduction. (A) I-V curves of CPOF-4 and CPOF-4-265 °C-84 h upon I2 oxidation; (B) Schematic representation of electron conduction in CPOF-4-265 °C-84 h. I-V: Current-voltage.
CONCLUSIONS
In conclusion, we have succeeded in developing the first case of exploiting acetylene (−C≡C−) as a reaction site to achieve solid-state cross-linking of acetylene (−C≡C−) within the confined space of the framework under thermal induction, thereby constructing crystalline CL-COFs. This strategy realizes the synthesis of polyacetylene without a catalyst and improves the crystallinity, specific surface area, and stability of the material. Furthermore, the conductivity of the material was markedly improved by doping I2. This study will open new vistas for exploiting nanoconfined spaces of reticular frameworks, facilitating a paradigm shift from solution polymerizations into solid-state cross-linking.
DECLARATIONS
Authors’ contributions
Conducted the preparation and characterization of COFs and prepared the draft manuscript: Zhao Y,
Performed the in-situ Fourier-transform infrared (FT-IR) characterization: Fu J
Collected and analyzed the nuclear magnetic resonance spectroscopy data: Das S
Performed the transmission electron microscopy (TEM) characterization: Wang Y, Zhao Z
Conducted the density functional theory (DFT) calculation: Qiao Z, Gao Y, Cao D, Peng D
Planned the study, analyzed the data, and wrote the manuscript: Zhu W, Ben T
Availability of data and materials
Not applicable.
Financial support and sponsorship
This work was supported by the National Key R&D Program of China (2021YFA1200400), the National Natural Science Foundation of China (Nos. 91956108 and 22201256), and the Natural Science Foundation of Zhejiang Province (No. LZ22B010001).
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2024.
Supplementary Materials
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Wen D, Das S, Zhao Y, Fu J, Qiao Z, Gao Y, Wang Y, Zhao Z, Cao D, Peng D, Zhu W, Ben T. Catalyst-free solid-state cross-linking of covalent organic frameworks in confined space. Chem Synth 2024;4:9. http://dx.doi.org/10.20517/cs.2023.45
AMA Style
Wen D, Das S, Zhao Y, Fu J, Qiao Z, Gao Y, Wang Y, Zhao Z, Cao D, Peng D, Zhu W, Ben T. Catalyst-free solid-state cross-linking of covalent organic frameworks in confined space. Chemical Synthesis. 2024; 4(1): 9. http://dx.doi.org/10.20517/cs.2023.45
Chicago/Turabian Style
Wen, Dan, Saikat Das, Yu Zhao, Jingru Fu, Zelong Qiao, Yijing Gao, Yuxia Wang, Ziqiang Zhao, Dapeng Cao, Daoling Peng, Weidong Zhu, Teng Ben. 2024. "Catalyst-free solid-state cross-linking of covalent organic frameworks in confined space" Chemical Synthesis. 4, no.1: 9. http://dx.doi.org/10.20517/cs.2023.45
ACS Style
Wen, D.; Das S.; Zhao Y.; Fu J.; Qiao Z.; Gao Y.; Wang Y.; Zhao Z.; Cao D.; Peng D.; Zhu W.; Ben T. Catalyst-free solid-state cross-linking of covalent organic frameworks in confined space. Chem. Synth. 2024, 4, 9. http://dx.doi.org/10.20517/cs.2023.45
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