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A Study on Constitutional Isomerism in Covalent Organic Frameworks: Controllable Synthesis, Transformation, and Distinct Difference in Properties

2020; Chinese Chemical Society; Volume: 2; Issue: 2 Linguagem: Inglês

10.31635/ccschem.020.201900094

2096-5745

Rong‐Ran Liang, Fu‐Zhi Cui, A. Ruhan, Qiao-Yan Qi, Xin Zhao,

Advanced Photocatalysis Techniques

Open AccessCCS ChemistryCOMMUNICATION1 Apr 2020A Study on Constitutional Isomerism in Covalent Organic Frameworks: Controllable Synthesis, Transformation, and Distinct Difference in Properties Rong-Ran Liang, Fu-Zhi Cui, Ru-Han A, Qiao-Yan Qi and Xin Zhao Rong-Ran Liang Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032 (China) Google Scholar More articles by this author , Fu-Zhi Cui Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032 (China) Google Scholar More articles by this author , Ru-Han A Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032 (China) Google Scholar More articles by this author , Qiao-Yan Qi Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032 (China) Google Scholar More articles by this author and Xin Zhao *Corresponding author: E-mail Address: [email protected] Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032 (China) Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.201900094 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Isomerism is an essential and widespread phenomenon in organic chemistry but rarely observed in covalent organic frameworks (COFs), a novel class of crystalline porous organic polymers with versatile applications. Herein, we give an account of the first example of a controllable synthesis of constitutional isomers of a COF. The two isomers exhibited marked differences in their gas/vapor adsorption behaviors and chemical stability in various solvents. Furthermore, structure transformation from one isomer to the other was realized. This work not only paves the way for rational design and synthesis of COF isomers but also provides a vivid example of structure–property relationship in crystalline porous polymers. Download figure Download PowerPoint Introduction Isomerism is a typical phenomenon in organic chemistry referring to the occurrence of molecules which have the same molecular formula and chemical composition but exhibit different spatial atomic arrangement (isomers). In this context, the isomers usually exhibit different physical and chemical properties. For example, ethyl alcohol and dimethyl ether display quite a different chemical reactivity, boiling point, and many other physical properties. In addition to small molecules, it has been very well documented that polymeric isomers, represented by isotactic, atactic, and syndiotactic polypropylenes, exist for which different crystallinity and thermal properties are observable.1,2 Beyond traditional molecular isomers, isomerism in metal–organic frameworks (MOFs), termed “framework isomers,” defined by Zhou et al.3 to be “MOFs constructed from the same ligand and metal species that display different network structures,” have also drawn considerable interest in recent years. However, although several types of framework isomerism in MOFs have already been identified experimentally and different physical properties have been observed for the isomers, the number of framework isomers is very limited, and their attainment has been a matter of luck.3 As an emerging class of crystalline porous organic materials, covalent organic frameworks (COFs) could be constructed by linking organic building blocks through robust covalent bonds.4–7 Considering the structural similarity between COFs and MOFs, we expected COFs also to exhibit framework isomerism. Indeed, very recently, the first example of interpenetration isomerism of a three-dimensional COF was reported by Wang and co-workers,8 which, so far, happens to be the only example of isolated COF isomers. Since isomerism is a significant and exciting phenomenon in COFs, as well as in basic chemistry, it is highly desirable to look for the other types of COF isomers. In 2014, we found that the assembly of a D2h symmetric building block and a C2 symmetric one, theoretically, could give rise to two different types of frameworks, one bearing a uniform rhombus pores [single-pore (SP)] and the other, composing of triangular and hexagonal pores [dual-pore (DP)], usually termed Kagome lattice.9 These blocks provided a good scaffold for constructing rationally, constitutional framework isomers of COFs, and investigating the structure–property relationship of the isomers. Under this principle, single-layer COFs of isomeric structures were observed very recently by Wang and co-workers10 as mixtures on graphite surface using scanning tunneling microscopy (STM). To obtain isolated COF isomers in bulk synthesis in this study, 4′,4″′,4″″′,4″″″′-(ethene-1,1,2,2-tetrayl)tetrakis([1,1′-biphenyl]-4-carbaldehyde]) (ETTBC) was used as the D2h symmetric monomer,11 and 2,5-diaminotoluene (DAT) was selected as the C2 symmetric monomer (Scheme 1). Since previous studies have indicated that solvents could have a significant impact on crystal structures of porous materials such as MOFs12,13 and molecular cages,14,15 we chose to alter the solvothermal reaction solvents as the controlling factor in the fabrication of the COF isomers. To this end, we screened a series of solvent systems and found that crystalline polymers could be generated from two distinct solvent systems for the preparation of the COF isomers. Scheme 1 | Controllable synthesis of SP-COF-ED and DP-COF-ED isomers and the transformation from one isomer to the other. Download figure Download PowerPoint Results and Discussion Beside the D2h symmetric monomer, ETTBC, and the C2 symmetric monomer, DAT (Scheme 1), selected for the bulk synthesis, we chose two different solvent systems, mesitylene-1,4-dioxane-acetic acid (aq., 6 M) (5/15/2, v/v/v) and n-butyl alcohol-o-dichlorobenzene-acetic acid (aq., 6 M) (5/5/1, v/v/v) mixtures for the bulk synthesis. The crystallites obtained from these two types of solvothermal solvent systems displayed almost the same Fourier-transform infrared (FT-IR) and solid-state carbon-13 nuclear magnetic resonance (13C NMR) spectra, but differed in their powder X-ray diffraction (PXRD) patterns, suggesting that they possessed the same chemical composition but different framework structures. Their IR spectra revealed stretching of the vibrational band of C=N at 1618 cm−1 (), indicating the existence of imine bonds in both polymers. Their solid-state 13C NMR spectra confirmed the C=N linkages, attributable to a characteristic resonance signal of imine carbons at 154 ppm (). Thermogravimetric analysis (TGA) indicated good thermal stabilities for both polymers (). Scanning electron microscopy (SEM) showed irregular morphology of both polymers, and transmission electron microscopy (TEM) images revealed high crystallinity of the crystallites, indicated by the ordered lattice fringes (). The crystal structures of the two crystalline polymers were elucidated by comparing their experimentally observed PXRD patterns with their simulated PXRD profiles of the predicted structures. Both the eclipsed stacking (AA) and staggered stacking (AB) models of SP and DP geometrical structures were established and simulated using Materials Studios (BIOVIA Corporate, San Diego, CA, USA) to create and view the crystalline materials. For the crystallites produced from mesitylene-1,4-dioxane-acetic acid (aq., 6 M), diffraction peaks at 3.26°, 6.52°, and 19.13° were visible in its PXRD pattern (Figure 1a). By comparing with the above-simulated PXRD patterns (Figure 1), we confirmed its structure as an SP framework with AA stacking (SP-AA), and thus, this COF was named SP-COF-ED. The peaks were indexed to be (100), (200), and (001) diffractions, respectively. Pawley refinement afforded the unit cell parameters as a = 29.42 Å, b = 28.91 Å, c = 4.62 Å, and α = β = 90°, γ = 120°, with RP = 2.17% and RWP = 3.63% . The difference plot, also known as the Bland–Altman plot, indicated that the refined pattern imitated the experimental result (Figure 1b). Figure 1 | (a) Experimental (black) and refined (red) PXRD patterns of SP-COF-ED (b); the differences in plot between the experimental and refined PXRD patterns; simulated PXRD patterns for (c) SP-AA, (d) SP-AB, (e) DP-AA, and (f) DP-AB structures. Download figure Download PowerPoint On the other hand, the COF synthesized from n-butyl alcohol-o-dichlorobenzene-acetic acid (aq., 6 M), exhibited diffraction peaks at 1.83° (100), 3.24° (110), 3.70° (200), 4.87° (120), 5.58° (300), 6.67° (130), 7.45° (400), and 19.50° (001) (Figure 2a), which matched well with the simulated PXRD pattern of a DP COF with eclipsed packing. Therefore, this material was designated as the isomer with DP structure and named DP-COF-ED (Figure 2). Pawley refinement was also carried out, which converged little differences between simulated and experimental patterns with the unit cell parameters of a = b = 55.48 Å, c = 4.55 Å, α = β = 90°, and γ = 120°, with RP = 1.97% and RWP = 2.98% . Figure 2 | (a) Experimental (black) and refined (red) PXRD patterns of DP-COF-ED, (b) the difference plot between the experimental and refined PXRD patterns, and simulated PXRD patterns for (c) DP-AA, (d) DP-AB, (e) SP-AA, and (f) SP-AB structures. Download figure Download PowerPoint Since both COFs shared the same chemical constitution, we performed a further investigation to determine the transformation between the two isomers by initially conducting solvent system interconversions. However, this design failed to realize the expected transformation. Thus, to promote the transformation process, we added excess DAT to facilitate the process of employing a monomer exchange.16 Under this condition, we realized successfully, an in situ transformation from DP-COF-ED to SP-COF-ED. PXRD analysis of the samples obtained at different transformation stages revealed that DP-COF-ED disappeared after it was heated with DAT for 1 day, with the production of an amorphous material. Extending the reaction time to 3 days gave rise to a product with a PXRD pattern corresponding to SP-COF-ED. Moreover, its crystallinity improved further after heating for an additional 3 days (). Also, the conversion from SP-COF-ED to DP-COF-ED was investigated, but numerous attempts failed, suggesting a unidirectional transformation process had occurred in which the SP-COF-ED might be the thermodynamic product, whereas DP-COF-ED might be the kinetic product. This assumption was supported by molecular mechanical calculations, which indicated a significantly lower total energy (kcal/mol) of SP-COF-ED (241.59 kcal/mol) than that of DP-COF-ED (681.40 kcal/mol). Then we evaluated the porosity of the two COF isomers by nitrogen adsorption–desorption measurements. We found that the sorption isotherm of SP-COF-ED exhibited the type IV model shown in Figure 3a, indicating a mesoporous feature.17 Its calculated Brunauer–Emmett–Teller (BET) surface area was equal to 360 m2/g using data ranging from 0.05 < P/P0 < 0.2 (where P/P0 is the relative pressure) of the adsorption isotherm (), and its estimated total pore volume was 0.71 cm3/g (P/P0 = 0.99). The pore size distribution of SP-COF-ED was generated using quenched solid density functional theory (QSDFT). A narrow peak centered at 21.9 Å was apparent (Figure 3c), which was in proximity with the theoretical aperture size of the rhombus pores (22.0 Å), providing further evidence for the formation of the SP framework. Figure 3 | N2 adsorption–desorption isotherms (77 K) of (a) SP-COF-ED and (b) DP-COF-ED, and pore size distribution profiles of (c) SP-COF-ED and (d) DP-COF-ED. Download figure Download PowerPoint DP-COF-ED exhibited a different adsorption property, compared with SP-COF-ED. As shown in Figure 3b, DP-COF-ED displayed a stepped sorption isotherm. The expansion step occurred at a relative pressure of ∼ 0.4, after which the COF underwent a further uptake of nitrogen. Such stepwise adsorption is characteristic of two factors; (1) hierarchical porous materials18–20 or (2) stimuli-responsive21–24 or possibly both. Due to its hierarchical structure and the unusually large hexagonal mesopores, which resulted in more flexibility of the COF skeleton, we speculated that the stepwise adsorption behavior of DP-COF-ED should be a comprehensive result of both factors. On the basis of the adsorption branch in a range of 0.05 < P/P0 < 0.2, its BET surface area was estimated to be 559 m2/g (), while its calculated total pore volume was 0.90 cm3/g (P/P0 = 0.99). Additionally, its pore size distribution profile displayed a bimodal distribution, centered at 12.0 and 42.2 Å, respectively (Figure 3d). The former was assigned to the triangular micropores (theoretical pore size: 14.0 Å), and the latter belonged to the hexagonal mesopores (theoretical pore size: 40.0 Å) of the Kagome-like DP COF. We were driven by the differences between SP-COF-ED and DP-COF-ED in their crystalline forms and their adsorption behaviors, which led us to investigate further their chemical stability in different solvents (H2O, THF, DMF, CHCl3, and 1,4-dioxane) and their stimuli-responsiveness toward guest molecules. Our results showed that both COFs demonstrated a partial loss in crystallinity after each was immersed in various solvents; however, they exhibited distinct chemical stabilities in the different solvents, judged by comparing their PXRD peak (100) intensities. For SP-COF-ED, a stability order of THF > CHCl3 > 1,4-dioxane > DMF > H2O was observed (), whereas for DP-COF-ED, the order was CHCl3 > THF > DMF > 1,4-dioxane > H2O (). Moreover, a complete loss in crystallinity was observed for SP-COF-ED when immersed in H2O (), an observation which was remarkably different from DP-COF-ED, which exhibited good stability in water (). The higher hydrolytic stability of DP-COF-ED might be attributable to higher local densities of the imine linkages and the hydrophobic methyl groups in the DP structure than the SP isomer (). Our study on stimuli-responsiveness toward n-hexane vapor indicated strikingly different behaviors of the two isomers. For SP-COF-ED, the adsorption of n-hexane caused a complete loss of the crystallinity of COF (Figure 4a). By contrast, we, instead, observed a stimuli-responsive phenomenon for DP-COF-ED. Compared with that of pristine DP-COF-ED, the diffraction peaks of the n-hexane-absorbed COF shifted to smaller 2 theta region, and the (110) peak disappeared (Figure 4b), suggesting that a guest-inclusion-induced structural deformation had occurred.12–15 Since a smaller 2 theta value usually corresponds to a higher unit cell volume of a crystal structure, this result indicated the dynamically expandable framework of the DP isomer, albeit the expanded structure was hard to quantify, as the displacement was too small. Additionally, after removal of n-hexane from the n-hexane-absorbed DP-COF-ED, the PXRD diffraction peaks of the recovered sample shifted back to the original positions of the pristine COF, and the (110) peak re-appeared, indicating that the guest-inclusion-induced structural deformation was reversible. To collect more evidence for the stimuli-responsive characteristics, we conducted an n-hexane vapor adsorption–desorption measurement for DP-COF-ED. A typical stepwise uptake with an expansion step at P/P0 = 0.3 was observed (Figure 4d), indicating the dynamic response of DP-COF-ED toward n-hexane vapor. Further, for comparison, we performed an n-hexane sorption experiment for SP-COF-ED. Interestingly, no stepwise uptake was observed (Figure 4c), again indicating the different adsorption properties of the two COF isomers. Figure 4 | PXRD profiles of (a) SP-COF-ED and (b) DP-COF-ED, as synthesized (navy), after being exposed in an atmosphere of n-hexane vapor for 12 h (magenta) and recovered after removal of n-hexane (olive). And n-hexane vapor adsorption–desorption isotherms (298 K) of (c) SP-COF-ED and (d) DP-COF-ED. Note: The backgrounds in the PXRD profiles of DP-COF-ED in (b) were subtracted for clarity. Download figure Download PowerPoint Conclusion While, in the literature, framework isomerism has been observed typically, by serendipity, this work has demonstrated the first example of a controllable synthesis of constitutional COF isomers. Although they have the same chemical composition, the SP-COF-ED and DP-COF-ED isomers exhibited distinct differences in their gas/vapor adsorption behaviors and chemical stability, which demonstrates the significant effect of isomeric structures on the properties of coupled COF polymers, and provides a good example for revealing structure–property relationship in COF materials. Furthermore, we have realized, for the first time, a unidirectional transformation from one COF isomer to the other by which the thermodynamic and the kinetic isomers products could be designated with the aid of energy calculations. 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Google Scholar Previous articleNext article FiguresReferencesRelatedDetailsCited ByLi Y, Yang T, Li N, Bai S, Li X, Ma L, Wang K, Zhang Y and Han Y (2021) Multistimuli-Responsive Fluorescent Organometallic Assemblies Based on Mesoionic Carbene-Decorated Tetraphenylethene Ligands and Their Applications in Cell Imaging, CCS Chemistry, 4:2, (732-743), Online publication date: 1-Feb-2022.Li M, Liu J, Li Y, Xing G, Yu X, Peng C and Chen L (2020) Skeleton Engineering of Isostructural 2D Covalent Organic Frameworks: Orthoquinone Redox-Active Sites Enhanced Energy Storage, CCS Chemistry, 3:2, (696-706), Online publication date: 1-Feb-2021.Liu G, Zhou M, Su K, Babarao R, Yuan D and Hong M (2020) Stabilizing the Extrinsic Porosity in Metal–Organic Cages-Based Supramolecular Framework by In Situ Catalytic Polymerization, CCS Chemistry, 3:5, (1382-1390), Online publication date: 1-May-2021.Tang X, Chen Z, Xu Q, Su Y, Xu H, Horike S, Zhang H, Li Y and Gu C (2021) Design of Photothermal Covalent Organic Frameworks by Radical Immobilization, CCS Chemistry, , (2926-2937) Issue AssignmentVolume 2Issue 2Page: 139-145Supporting Information Copyright & Permissions© 2020 Chinese Chemical SocietyKeywordsadsorption behaviorcovalent organic frameworkconstitutional isomercontrollable synthesisisomeric transformationAcknowledgmentsThe authors thank the National Science Fund for Distinguished Young Scholars of China (no. 21725404), Shanghai Scientific and Technological Innovation Project (18JC1410600), and the Strategic Priority Research Program of the Chinese Academy of Sciences (grant no. XDB20000000) for financial support. Downloaded 3,460 times Loading ...