Structural and Dimensional Transformations between Covalent

Jan 30, 2019 - ... between Covalent Organic Frameworks via Linker Exchange ... the two aldehyde building units biphenyl-4,4′-dicarbaldehyde (BPDA) a...
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Structural and Dimensional Transformations between Covalent Organic Frameworks via Linker Exchange Zhen Li,†,‡ Xuesong Ding,*,‡ Yiyu Feng,*,† Wei Feng,*,†,∥ and Bao-Hang Han*,‡,§ †

School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China § University of Chinese Academy of Science, Beijing 100049, China ∥ Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China Macromolecules Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/31/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: Covalent organic frameworks (COFs) are porous crystalline materials with well-controlled structures and extensive potential applications. However, construction of new COFs has tremendous challenges including design and synthesis of building block monomers, selection of reaction solvent systems, and investigation of reaction time and temperature. Here, we performed the structural transformation in the three-dimensional (3D) COFs (COF-300 and COF-320) and dimensional transformation from 3D (COF-301) to 2D (TPB-DHTP-COF) via the dynamic covalent chemistry principle, and the successful realization of these exchanges was confirmed by FT-IR, 1H NMR, PXRD, and nitrogen sorption isotherm measurements. Based on the results of PXRD patterns and 1H NMR spectra, the COF-320-to-COF-300 transformation was successfully achieved, and an efficiency as high as 89% was attained. A new network structure rather than COF-320 was obtained after transformation in the opposite direction (from COF-300 to COF-320), which contains the two aldehyde building units biphenyl-4,4′dicarbaldehyde (BPDA) and terephthaldehyde (TA) in one framework. Meanwhile, the dimensional transformation of COFs with 78% exchange efficiency for COF-301-to-TPB-DHTP-COF is also achieved. These results not only reveal the COF-toCOF transformation in 3D-to-3D and 3D-to-2D but also provide a new way for constructing new COFs that is difficult to synthesize in one step.

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INTRODUCTION Covalent organic frameworks (COFs)1−4 are an emerging category of materials with well-defined two- or threedimensional (2D5−10 and 3D11−15) structures developed in the past decade, which are based on the atomically precise organization by constructing organic monomers with dynamic covalent bonds. COFs can be precisely controlled in composition, topology, and porosity. With these advantages, they have been utilized in many application fields, such as gas storage and separation,16−20 catalysis,21−24 drug delivery,25,26 optoelectronics,27,28 and energy storage.29−31 Numerous of 2D-COFs have been constructed by using plenty of available planar building block units and extended to a wide range of applications. In contrast, 3D-COFs have been prepared by using specific tetrahedral or other stereo building blocks. To date, categories of monomers available for construction of 3DCOFs are much less than those for 2D-COFs, and the search for suitable reaction conditions (reaction time, temperature, and solvent systems) to form the well-defined networks still takes time. Therefore, 3D-COFs with various structures have been reported only in a limited number, and the development of a new method for constructing 3D-COFs is highly desirable. © XXXX American Chemical Society

To overcome the difficulty in explosion of reaction conditions in the synthesis of new COFs, some researchers try to employ reported COFs as the templates or raw materials to produce the COFs with new structures. For instance, the Yaghi,32,33 Lotsch,34 and Liu groups35 employed the chemical conversion method to change the as-prepared COF’s skeleton; both the Dichtel 36 and Zhao groups 37 realized the reconstruction of COFs from amorphous state to crystallized state, while Wang’s group focused on modifying the structure by metalation to obtain new frameworks, like the series of Salen-COFs.38 However, this method for constructing new COFs should rely on specific structures in the framework skeleton; hence, the universality is relatively restricted. Recently, Zhao’s group presented a new approach for realizing the COF-to-COF transformation with obvious structural conversion via heterogeneous linker exchange.39 Horike’s group employed this approach to achieve the transformation between COF-Naph and COF-Ph and produce different Received: August 22, 2018 Revised: November 28, 2018

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DOI: 10.1021/acs.macromol.8b01814 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis of COF-300-to-320-X and COF-320-to-300-X (X = 5, 10, 15)

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structures and compositions of COFs through building block exchange.40 Yan’s group employed the building block exchange strategy for building new amino-functionalized COFs that have difficulty in one-step synthesis.41 This new method is available for getting a new COF that is difficult to obtain by one-step synthesis, intercalating, or doping functional groups into COF. However, although a few 2D-COF-to-2D-COF transformations through heterogeneous linker exchange were successfully realized, to date, 3D-COF-to-3D-COF transformations and dimensional transformation between 3D-COF and 2D-COF have not been reported yet. In this work, we performed the transformations of 3D-COFto-3D-COF (between COF-30042 and COF-32043) and 3DCOF-to-2D-COF (between COF-30144 and TPB-DHTPCOF23) by using the heterogeneous linker exchange method under solvothermal conditions for the first time. We employed several characterization measurements to evaluate the results of these transformation processes, such as powder X-ray diffraction (PXRD), 1H and solid-state 13C NMR spectra, nitrogen sorption isotherm measurements, Fourier transform infrared spectroscopy (FT-IR), and thermogravimetric analyses (TGA). We confirmed that the transformation from COF320 to COF-300 is successfully realized with the efficiency as high as 89%, while the transformation in the opposite direction that is from COF-300 to COF-320 yields a new 3D network structure containing two dialdehyde units rather than COF320. In addition, the dimensional transformation from 3D COF-301 to 2D TPB-DHTP-COF is well achieved with 78% transformation efficiency. Therefore, this research in COFs’ structural transformation (between 3D-COFs) and dimensional transformation (from 3D- to 2D-COF) can well support that heterogeneous linker exchange is an effective and universal methodology to enrich the synthesis routes and categories of COFs.

EXPERIMENTAL SECTION

Preparation of COF-320-to-300-X. COF-320 was synthesized according to the literature.43 A glass tube was charged with biphenyl4,4′-dicarbaldehyde (BPDA) (40 mg, 0.190 mmol) and tetra-4anilylmethane (TAM) (40 mg, 0.105 mmol), which were dissolved in anhydrous 1,4-dioxane (2 mL) and 3 M aqueous acetic acid (0.4 mL). The tube was flash frozen at 77 K, evacuated, and flame-sealed. The system was reacted under 120 °C for 3 days and then cooled to room temperature, with COF-320 soaking in mother solvent without postprocessing. Afterward, the heterogeneous linker building block monomer terephthaldehyde (TA) was added into the glass tube with different equivalents (calculated by COF-320, 0.52 mmol, 5 equiv, or 1.04 mmol, 10 equiv, or 1.56 mmol, 15 equiv), evacuated, and flamesealed the tube again to keep under 120 °C for 3 days. Then a light yellow solid was collected that was isolated by centrifugation and washed with anhydrous tetrahydrofuran (THF) and acetone four times. The resulting powder was dried in a vacuum oven at 100 °C overnight to obtain a light yellow powder. Preparation of COF-300-to-320-X. COF-300−320-X was obtained as light yellow powder in a similar procedure as COF320−300-X, except that COF-300 was synthesized first and BPDA was used as the exchange building block monomer.42 Preparation of COF-301-to-TPB-DHTP-COF-X. TAM (40 mg, 0.105 mmol) and 2,5-dihydroxyterephthalaldehyde (DHTA) (30.8 mg, 0.185 mmol) were dissolved in a mixture of o-dichloromethane/ N,N-dimethylacetamide (o-DCB/DMAc, 0.29 mL/1.71 mL) and 6 M aqueous acetic acid (0.4 mL) in a glass tube. The tube was evacuated and flame-sealed. The system was reacted under 120 °C for 3 days and then cooled to room temperature, centrifugated, and washed with anhydrous THF and acetone. By putting the exchange building block monomer 1,3,5-tri(4-aminophenyl)benzene (TAPB) into the glass tube with the wet COF-301 powder, a mixture of o-DCB/1-butanol (BuOH) (1 mL/1 mL) and 6 M aqueous acetic acid (0.2 mL) was also taken as solvent, and then the tube was degassed and flame-sealed again. The system was reacted at 120 °C for 3 days to yield a brown solid, which was collected by centrifugation and washed with anhydrous THF and acetone four times. The resulting powder was B

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Figure 1. (a) PXRD patterns of COF-320, COF-300, and COF-320-to-300-X (X = 5, 10, 15). (b) PXRD patterns of COF-300, COF-320, and COF-320-to-COF-300-15 synthesized for different reaction times.

Scheme 2. Proposed Process for the Transformation from COF-320 to COF-300

dried in a vacuum oven at 100 °C overnight to obtain a brown powder.

Preparation of TPB-DHTP-COF-to-COF-301-X. TPB-DHTPCOF-to-COF-301-X was obtained as brown powder in a similar C

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Macromolecules procedure as COF-301-to-TPB-DHTP-COF-X, except that TPBDHTP-COF was synthesized first23 and TAM was used as the exchange building block monomer.

Interestingly, although the PXRD data suggest that the crystallinity of COF-320 already disappears after only 0.5 h of reaction time, the solid state materials can be observed in reaction system during the COF-to-COF transformation course (Figure S1, Supporting Information). It means that this transformation may not be a process of COF’s dissolution into monomeric state and reprecipitation as polymer, and this phenomenon is consistent with a previous report.39 Based on the experimental results analyzed above, a probable mechanism for COF-to-COF transformation is proposed as drawn in Scheme 2. When TA is added into COF-320 suspensions, the highly porous structure of COF-320 makes TA able to be easily diffused into the channels of COF-320 and then nucleophilically attacked by imine bonds in an acidic environment. In this process, BPDA units are gradually replaced with TA units, and finally almost all the BPDA units in COF-320 are removed and substituted by TA to form COF-300. Owing to the existence of the solid state materials (pristine and final COFs, and the intermediates where the linkers were partially replaced) in reaction solution throughout all the process, this COF-to-COF transformation may happen at the solid−liquid interface.39

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RESULTS AND DISCUSSION For investigation of 3D-COF-to-3D-COF transformation by heterogeneous linker exchange, COF-300 and COF-320 were selected as the research materials in consideration of the similarity in their architectures. These two COFs have same tetrahedral building block TAM and similar linear building blocks TA and BPDA, which were connected through [4 + 2] condensation reactions under solvothermal conditions to produce COF-300 and COF-320, respectively. After COF300 or COF-320 was successfully synthesized, different building block monomers were added to the mother solvent that contains COFs, more specifically, TA added to COF-320 and BPDA added to COF-300, respectively (Scheme 1), and then the transformation products are referred as COF-300-to320-X and COF-320-to-300-X (X = (n1/n2) × 100, where n1/ n2 is the molar ratio of the exchanging monomer to initial COF; X = 5, 10, and 15). The PXRD measurement was performed to examine the change in COFs’ crystallinity after structural transformation by adding different equivalents of linker exchange monomers. As shown in Figure 1a, the PXRD patterns of 3D-COFs are in accordance with the reported literature42,43 and exhibit strong peaks corresponding to the (200) diffraction at 6.44° and 5.60° for COF-300 and COF-320, respectively. Compared with previous works of COF-to-COF transformation, the selected COFs in this research present a more obvious difference in the position of the strongest diffraction peaks, which may avoid confusion in judging the transformation degree according to PXRD patterns. From Figure 1a, the position of the strongest peak in sample COF-320-to-300-5 is very closely to COF-320, suggesting that 5 equiv of TA is not enough for COF-to-COF transformation. To drive the equilibrium to the formation of COF-300, more building block monomers need to be added into the reaction system. With the equivalents of added TA increasing to 15, the product shows the best result that the sharp peak corresponding to the (200) diffraction is very closely to COF-300, indicating that COF-320 to COF-300 transformation is successfully realized. Although COF-320-to300-10 shows obviously low crystallinity in PXRD pattern, a weak peak already appears at 6.44° that can be assigned to the (200) facet of COF-300, and no diffraction peak belonging to COF-320 remains. This result is probably because in this reaction condition the COF-320 is already disassembled as well as the network of COF-300 has been formed with only low crystalline structure. To deeply understand this COF-toCOF transformation process, we performed time-dependent experiments that COF-320-to-300-15 was synthesized for different reaction times (0.5, 1, 2, 4, 8, and 16 h) and examined the results by PXRD measurement (Figure 1b). The COF synthesized for only 0.5 h displays no obvious diffraction peaks in PXRD pattern, suggesting that the crystallinity of COF-320 almost disappears in a short exchange period though the formation of COF-300 still has not been detected. When the reaction time was extended to 1 h, a weak diffraction peak can be observed at 6.44° that can be ascribed to the (200) facet of COF-300. The crystallinity of COF-320-to-300-15 is improved strikingly with the reaction time being prolonged (2−16 h), indicating that COF-300 is formed quickly and continuously in this COF-to-COF transformation course.

Figure 2. FT-IR spectra of COF-300, COF-320, and COF-320-to300-X (X = 5, 10, 15).

The successful transformation from COF-320 to COF-300 was also confirmed by employing FT-IR spectra (Figure 2). All these COFs exhibit characteristic peaks at 1620 cm −1 corresponding to the −CN− stretching vibration, which means the imine bonds were successfully formed in both frameworks. The characteristic peaks at 1202 and 1198 cm−1 can be assigned to the imine C−CN−C stretching in COF300 and COF-320, respectively. In the spectra of COF-320-to300-X, the strong stretching vibration at 1202 cm−1 can be observed while the peak at 1198 cm−1 completely disappears, which reveals that COF-320 is finely reconstructed into COF300 and the 3D-COF-to-3D-COF transformation is successfully achieved by this heterogeneous linker exchange method. To further explore the 3D-COF-to-3D-COF transformation efficiency, we employed the 1H NMR spectroscopy measurement to calculate the percentage of benzene ring from TA and biphenyl ring from BPDA in COF-320-to-300-X. COFs powder was hydrolyzed by using concentrated hydrochloride acid and examined by 1H NMR spectra. As shown in Figure 3, the characteristic peaks at 10.15 and 10.09 ppm belong to aldehyde groups, and the peaks at 8.13 and 8.03 ppm can be attributed to benzene ring and biphenyl ring in TA and BPDA, respectively. With the molar ratio of TA increases from 5 to 15, D

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Figure 5. Nitrogen sorption isotherm curves of COF-300, COF-320, COF-320-to-300-5, COF-320-to-300-10, and COF-320-to-300-15 measured at 77 K.

Figure 3. 1H NMR spectra of HCl-degraded solutions of COF-300, COF-320, and COF-320-to-300-X (X = 5, 10, 15).

the peak (8.03 ppm) intensity of biphenyl ring strikingly decreases along with the peak (8.13 ppm) intensity of the benzene ring increasing. This reveals that the transformation is effectively promoted with addition of more exchange linker. To evaluate the transformation efficiency (Figure S2), we calculated the ratios of the benzene ring and biphenyl ring, and the transformation efficiencies of COF-320-to-300-X are 40% (X = 5), 65% (X = 10), and 89% (X = 15). Solid-state 13C NMR spectra of COF-300, COF-320, and COF-320-to-300-15 were employed to confirm the structure of the frameworks. The spectra of COF-300 and COF-320 are similar to each other, except two different peaks at 113 and 133 ppm, which

Figure 4. Solid-state COF-320-to-300-15.

respectively. The pore size distributions (PSD) of COFs are evaluated by nonlocal density functional theory (NLDFT). COF-300 shows main peaks at 0.77 and 2.45 nm, while the main peaks of COF-320 are at 0.85 and 3.42 nm (Figure S4 and Table S1). The PSD profiles of all COF-320-to-300-X samples exhibit obvious COF-300 characteristic peaks and weak peaks from COF-320, and these results are consistent with 1H NMR spectra. To investigate the bidirectionality of 3D-COF-to-3D-COF transformation, COF-300-to-320-X (X = 5, 10, 15) were synthesized by utilizing COF-300 as the initial framework with different equivalents of BPDA to transform COF-300 into COF-320. Transformation efficiency of COF-300-to-320-X was examined by 1H NMR spectroscopy (Figures S5 and S6). As shown in Figure S5, with addition of more equivalents of BPDA, the peak intensity of the benzene ring (8.13 ppm) decreases and the signal of the biphenyl ring (8.03 ppm) becomes obviously strong from COF-300-to-320-5 to COF300-to-320-15. Furthermore, the calculated transformation efficiency of COF-300-to-320-X was 36% (X = 5), 48% (X = 10), and 77% (X = 15). FT-IR spectra and solid-state 13C NMR spectra were also employed to confirm the structure of frameworks (Figures S7 and S8). The porosity of COF-300-to320-X was investigated by the nitrogen sorption isotherm measurement at 77 K (Figure S9). The curves of COF-300-to320-X exhibit both type I and IV sorption models with a rapid uptake at a low pressure (P/P0 < 0.10), and the obvious hysteresis loops can be clearly observed. The BET specific surface area values of COF-300-to-320-5, COF-300-to-320-10, and COF-300-to-320-15 were calculated to be 1110, 1110, and 980 m2 g−1, respectively. The PSD of COF-300-to-320-X exhibits both micro- and mesopore sizes from 0.85 to 3.42 nm (Figure S10 and Table S1). In addition, COF-300-to-320 exhibits good thermal stability and shows no obvious difference in morphology toward COF-320-to-300 via scanning electron microscopy (SEM) observation because of the high similarity between the morphologies of pristine COF300 and COF-320 (Figures S11 and S12). From PXRD patterns of COF-300-to-320-X (Figure 6), the strong peak corresponding to the (200) diffraction in COF300-to-320-5 is very close to COF-300, and with increasing the amounts of BPDA (X = 10, 15), the peak’s position moves from COF-300 to COF-320. However, unlike the case of COF-320-to-300-X, the diffraction peaks of COF-300-to-32015 do not match well with COF-320 or COF-300 as well as a

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C NMR spectra of COF-300, COF-320, and

correspond to the characteristic peaks of the biphenyl ring in COF-320. As shown in Figure 4, these two peaks disappear in the sample COF-320-to-300-15, which implies the transformation from COF-320 to COF-300 successfully occurs. The TGA curve shows that COF-320-to-300-15 maintained good thermal stability similar to COF-300 under a nitrogen atmosphere after linker exchange (Figure S3). Nitrogen sorption isotherms were measured at 77 K to study the porosity of COF-320-to-300-X (Figure 5). The isotherms show the steep rise in uptake at low pressure (P/P0 < 0.10) and an obvious hysteresis loop at a high pressure. All the COF-320to-300-X exhibit type I and IV sorption curves, which means they contain both micro- and mesoporous structures. The Brunauer−Emmett−Teller (BET) specific surface area values of COF-320-to-300-5, COF-320-to-300-10, and COF-320-to300-15 are calculated to be 1140, 1080, and 1330 m2 g−1, E

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reaction times (0.5, 1, 2, 4, 8, and 16 h) (Figure S13). Compared with the case of COF-320-to-300-15, although the crystallinity of original COF-300 disappears rapidly, the transformation shows an unsmooth process with a weak diffraction peak, slow crystallization course, and unexpected formation of COF-300n-320 rather than COF-320. It may be ascribed to the issue that BPDA possesses relatively lower reactivity than TA in imine bond formation, which makes BPDA be behind the competition to TA in formation of COF’s skeleton in transformation course. Taking into account the above-mentioned discussion, the transformation from COF300 to COF-320 does not produce COF-320 but reaches the formation of an intermediate network, which contains two dialdehyde building units (TA and BPDA) in the framework like the structure of COF-300n-320. To date, COFs’ transformation between 2D-COF and 3DCOF has not been reported yet and still remains a big challenge. In this work, we also explored COF-to-COF transformation between different dimensions via the heterogeneous linker exchange method. Both 3D-COF-to-2D-COF

Figure 6. PXRD patterns of COF-300, COF-320, and COF-300-to320-X (X = 5, 10, 15).

new peak appearing at 6.00° that is located between the (200) facet crystalline peaks of COF-300 (6.44°) and COF-320 (5.60°). To explain this phenomenon, we employed the threecomponent condensation strategy using the mixture of TA/ BPDA as edge units to synthesize intermediate COF-300n-320 (n is the percentage of TA, n = 25, 50, and 75%), which should

Scheme 3. Synthesis and Structure of COF-301-to-TPBDHTP-COF-X (X = 5, 10, 15)

Figure 7. Synthesis and PXRD patterns of COF-300n-320, where n = 25, 50, and 75%.

and 2D-COF-to-3D-COF were taken into consideration (Scheme 3). We prepared 3D COF-301 and 2D TPBDHTP-COF as the initial materials for COF-to-COF dimensional transformation, which have same aldehyde building block DHTA but with different architectures of amino building block (TAM for COF-301 and TAPB for TPB-DHTP-COF). We realize this transformation in different dimensions by adding different ratios of the exchange linker TAPB to COF301 or TAM to TPB-DHTP-COF, and the procedure is similar to the 3D-COF-to-3D-COF transformation except that the COFs are synthesized by using different solvent systems as the reaction conditions. The structure and transformation efficiency of COF-301-to-TPB-DHTP-COF-X (X = 5, 10, 15) were confirmed by FT-IR, PXRD, 1H NMR, 13C NMR, and TGA measurements. As can be seen from the FT-IR spectra of COF-301-to-TPB-DHTP-COF-X (Figure S14), the obvious difference is that the peaks at 1538 cm −1 corresponding to C−C stretching vibration for phenyl rings in TAM are absent compared with the initial COF-301. From

be constructed as the 3D network with specific proportions of TA and BPDA in the frameworks (Figure 7). PXRD patterns of COF-300n-320 exhibit the most intensive crystalline peaks at 5.98°, 6.06°, and 6.32° (for n = 25, 50, and 75%), respectively (Figure 7). Interestingly, these peaks of COF-300n-320 are also located in the range 5.60°−6.44° that is between the (200) facet crystalline peaks of COF-300 (considered as COF300100%-320) and COF-320 (considered as COF-3000%-320), which means that the greater percentage of TA are contained in COF-300n-320, the closer the peaks are located to that of COF-300, or vice versa. According to these experimental results, we propose that the transformation process from COF300 to COF-320 does occur but terminates in the stage of forming COF-300n-320 rather than that of COF-320. We also employed time-dependent experiments to examine the formation process of COF-300-to-320-15 using different F

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structure of COF-300n-320. The successful transformation in 3D-COFs-to-3D-COFs may provide a promising way for constructing new 3D-COFs for which may be difficulties in direct synthesis by one step. In addition, we also have successfully achieved the transformation from 3D-COF to 2DCOF with 78% exchange efficiency, indicating that the dimensional transformation between COFs is realizable. In the follow-up, we will try to improve 2D-COFs to 3D-COFs transformation efficiency by using heterogeneous linker exchange methodology to enrich synthesis routes and enlarge the categories of 3D-COFs.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 8. PXRD patterns of COF-301, TPB-DHTP-COF, and COF301-to-TPB-DHTP-COF-X (X = 5, 10, 15).

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01814.

PXRD patterns of COF-301-to-TPB-DHTP-COF-X (Figure 8), with the amounts of amino monomer linker increasing, the sharp peak at 2.76° corresponding to (100) diffraction appears and the intensity gradually increases, indicating that the chemical equilibrium is moving to the formation of TPBDHTP-COF in this procedure. A time-dependent experiment was also employed for checking the crystallinity to confirm the transformation process (Figure S15). Furthermore, the 3DCOF-to-2D-COF transformation degree was examined by 1H NMR spectroscopy measurement, and the exchange efficiency of COF-301-to-TPB-DHTP-COF-X was calculated to be 42% (X = 5), 58% (X = 10), and 76% (X = 15) (Figures S16 and S17). TGA measurement reveals that COF-301-to-TPBDHTP-COF-15 retained good thermal stability under a nitrogen atmosphere after linker exchange (Figure S18), and solid-state 13C NMR spectra confirm that the structure of the frameworks already transformed from COF-301 into TPBDHTP-COF (Figure S19). Overall, COF-301-to-TPB-DHTPCOF-X has been successfully obtained by the heterogeneous linker exchange method, and this is the first example to realize dimensional transformation between 3D-COF and 2D-COF. Notably, the transformation in the opposite direction, that is, from this 2D to 3D COF, presents an unsuccessful process with much lower exchange efficiency and unexpected crystalline peaks emerging compared with the 3D-to-2D COF’s transformation process (Figures S20 and S21), probably because compared with 3D COF-301, 2D TPBDHTP-COF containing strong layered π−π stacking structures exhibits much better thermal and chemical stability to prevent the 2D-COF-to-3D-COF transformation from occurring.

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Details of characterization methods, FT-IR spectra, corresponding BET specific surface area, PXRD patterns, TGA curves, 1H NMR, and solid-state 13C/CP-MAS NMR spectra (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Tel *Tel *Tel *Tel

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86 86 86 86

10 10 22 22

8254 8254 8535 8740

5576; 5708; 6404; 2059;

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[email protected]. [email protected]. [email protected]. [email protected].

ORCID

Yiyu Feng: 0000-0002-1071-1995 Wei Feng: 0000-0002-5816-7343 Bao-Hang Han: 0000-0003-1116-1259 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of the National Natural Science Foundation of China (Grants 21674026, 21574032, 51573125, and 51573147), the Sino-German Center for Research Promotion (Grant GZ1286), National Key R&D Program of China (No. 2016YFA0202302), National Natural Science Funds for Distinguished Young Scholars (No. 51425306), and the State Key Program of National Natural Science Foundation of China (No. 51633007) is acknowledged.

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CONCLUSIONS In summary, we have reported the COF transformation from 3D-COFs to 3D-COFs and from 3D-COFs to 2D-COFs for the first time by using heterogeneous linker exchange methodology under solvothermal conditions. By controlling the equivalents of the exchanging monomers, the transformation efficiency in COF-to-COF can be precisely controlled. We confirmed that the COF-320 to COF-300 transformation has been successfully achieved by 1H NMR, FT-IR, PXRD, 13C NMR, and nitrogen sorption isotherm measurements, and COF-320-to-300-15 shows the best results in transformation efficiency as high as 89%. Meanwhile, the COF-300 to COF-320 transformation has produced a new network containing two dialdehyde monomers rather than COF-320, which can be verified indirectly by studying the

REFERENCES

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DOI: 10.1021/acs.macromol.8b01814 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b01814 Macromolecules XXXX, XXX, XXX−XXX