Diversity of Covalent Organic Frameworks (COFs): A 2D COF

Letter pubs.acs.org/macroletters

Diversity of Covalent Organic Frameworks (COFs): A 2D COF Containing Two Kinds of Triangular Micropores of Different Sizes Shun-Qi Xu,‡ Tian-Guang Zhan,‡ Qiang Wen, Zhong-Fu Pang, and Xin Zhao* Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China S Supporting Information *

ABSTRACT: Although a lot of covalent organic frameworks (COFs) have been constructed over the past 10 years, the topologies of COFs are still quite limited. In this work, we report one-step construction of a COF with an unprecedented topology from the combination of C3-symmetrical and C2-symmetrical building blocks. It contains two types of triangular micropores of different sizes and chemical environments: one is 11.3 Å and the other is 15.2 Å. The structure of the dual-pore COF was confirmed by powder X-ray diffraction (PXRD) investigation, nitrogen adsorption−desorption study, and theoretical calculations.

C

distributed in the 2D lattices. The dual-pore COF reported herein represents an unprecedented topology with C3symmetry and exhibits its structural uniqueness. Furthermore, such topology also creates a COF that possesses two different types of channels with distinct chemical environments. The dual-pore COF was designed by the condensation of terephthalaldehyde and a hexaazatriphenylene (HAT) derivative, that is, HAT-6NH2,29 which peripherally bears six amino groups. Heating a mixture of HAT-6NH2 and terephthalaldehyde (1:3) in dimethylacetamide-mesitylene-acetic acid (aq., 6 M) (5/5/1) ternary solvent in a sealed glass ampule at 120 °C for 72 h resulted in a yellow powder (named as HAT-COF) which was insoluble in water and common organic solvents. Fourier transform infrared spectroscopy revealed that the bands at 2755−2805 cm−1 and ∼3345 cm−1 which correspond to the stretching vibrations of C−H of aldehyde and NH2 groups of the starting materials, respectively, were largely attenuated in the IR spectrum of HAT-COF, suggesting a high degree of polymerization (Figure S1 in Supporting Information). Moreover, although the band corresponding to newly formed CN bonds from the condensation of aldehyde and amino groups could not be distinguished from the band of CN of the HAT skeleton (both appear between 1623 and 1602 cm−1), the comparison of the IR spectra of the starting materials, HATCOF, and model compound (see Supporting Information for its structure) did reveal a band corresponding to CO vibration (1697 cm−1). It could be attributed to the terminal aldehyde at the edges of the COF. This result strongly suggests the formation of an extended framework from the condensation

ovalent organic frameworks (COFs) are organic crystalline porous materials with atomically precise integration of building blocks into periodic two-dimensional (2D) and three-dimensional (3D) structures. The most intriguing feature of COFs is the periodical distribution of holey structures, whose shape and size can be well tailored through delicate design of building blocks.1−6 Since Yaghi and co-workers reported the first COF in 2005,7 over the past 10 years more than a hundred COFs have been constructed and found extensive applications in many aspects including gas storage and separation,8−10 catalysis (as active catalysts or supports),11−13 and electronic devices.14−20 However, in spite of the progress achieved so far, compared to their analogues metal organic frameworks (MOFs),21 the topologies of COFs are still quite limited. In this context, to develop COFs with new topologies is highly desired. It not only will enrich the family of COFs but also has great potentials to develop novel features and applications for COF-based materials. Furthermore, the integration of pores of different shapes or sizes into a 2D COF structure can create distinct channels in the stacked COF lattice, which should enable it to exhibit some properties different from the COFs with the same type of pores. However, the construction of COFs that hold such topology still faces great challenges.22−26 In this letter, we report one-step construction of a COF which contains two kinds of triangular micropores of different sizes, in which the large pores (15.2 Å) and the small ones (11.3 Å) are alternately arranged in 2D sheets of the COF (Scheme 1). It should be noted that very recently Jiang et al. reported a practical synthesis of COFs with triangular pores from the combination of C6-symmetrical and C2-symmetrical building blocks,27 following the theoretical prediction of Heine and co-workers.28 However, those COFs contain only one type of triangular pores which symmetrically © XXXX American Chemical Society

Received: November 10, 2015 Accepted: December 23, 2015

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DOI: 10.1021/acsmacrolett.5b00804 ACS Macro Lett. 2016, 5, 99−102

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ACS Macro Letters

morphology was observed (Figure S3 in Supporting Information). HAT-COF displayed good thermal stability, as revealed by thermogravimetric analysis, which indicated that less than 10% weight loss occurred when the temperature increased to 300 °C (Figure S4 in Supporting Information). In the next step the internal structure of HAT-COF was investigated by powder X-ray diffraction (PXRD). As shown in Figure 1a, in the experimental PXRD profile, the peak corresponding to (100) diffraction is observed at 2θ = 4.20°. It exhibits strong diffraction intensity, indicating good crystallinity of the COF and long-range molecular ordering in its structure. In addition to the (100) peak, diffraction peaks at 7.11°, 8.36°, 10.99°, 12.55°, and 16.61° are also observed, which are assignable to (110), (200), (210), (300), and (400) diffractions, respectively. Pawley refinement was performed using the Reflex package in Accelrys’s Materials Studio 7.0 software to determine the lattice parameters, which gave the unit-cell parameters of a = b = 24.55 Å, c = 4.12 Å, α = β = 90°, and γ = 120°, with Rwp = 2.44% and Rp = 1.56%. The difference plot between the experimental and refined diffraction patterns reveals that they match each other very well (Figure 1b). In order to elucidate the stacked configuration of the 2D layers, two possible stacking models, namely, eclipsed packing with P6 space group (AA model, Figure 1e) and staggered packing (AB model, Figure 1f), were modeled after the geometry optimization by semiempirical calculations at the PM3 level. The simulated PXRD pattern of the eclipsed model well reproduced the observed XRD pattern in peak positions and relative intensities (Figure 1c). In contrast, the simulated PXRD pattern of the staggered model was not in good agreement with the experimental data, especially the appearance of the peaks at the 2θ range of 12−15° (Figure 1d). Therefore, an eclipsed packing model was assigned to the asprepared HAT-COF. It should be noted that the (001) peak (corresponding to the stacking of the 2D sheets) was hard to identify in the experimental XRD profile. This peak should be very weak and thus buried in a broad region between 2θ ∼ 18°

Scheme 1. Synthesis and Structure of the COF Possessing Two Kinds of Triangular Micropores of Different Sizes

of the two starting materials. Solid-state cross-polarization with magic angle spinning (CP/MAS) 13C NMR was further conducted for HAT-COF. It also revealed the existence of newly formed CN bonds (Figure S2 in Supporting Information). Scanning electron microscopy (SEM) was used to visualize the as-prepared HAT-COF, and a particle

Figure 1. (a) Experimental (black) and refined (red) PXRD patterns of the as-prepared COF. (b) Difference plot between the experimental and refined PXRD patterns (orange). (c) Simulated PXRD pattern for eclipsed model. (d) Simulated PXRD pattern for staggered model. (e) Eclipsed packing. (f) Staggered packing. 100

DOI: 10.1021/acsmacrolett.5b00804 ACS Macro Lett. 2016, 5, 99−102

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ACS Macro Letters and 25°. A similar phenomenon was also observed in other COF materials.30,31 In the case of HAT-COF, it might be attributed to the twisted conformation of the phenyl units of HAT-6NH2, which resulted from the steric repulsion between the aromatic hydrogens adjacent to the carbon atoms that connect the HAT core. It weakened the interlayer interactions between the 2D COF sheets and thus resulted in disorder in stacking. To shed light on the phenomenon, total energies of both eclipsed and staggered models against stacking distances of COF sheets were generated by force-field-based molecular mechanics calculations (Table S1 and Figure S5 in Supporting Information). It was found that the eclipsed AA model reached low-energy minima at the layer distance of 4.1 Å, which is very close to the c value derived from Pawley refinement (4.12 Å). It should also be pointed out that the total energy just displayed a small change when the layer distance varied in the range of 3.5−5.0 Å. The total energy fluctuated a little bit after the layer distance was larger than 4.1 Å. It might be attributed to the rotation of the twisted phenyl units when effective stacking between COF layers was weakened. In the case of the staggered AB model, it gave much higher total energy than that of the AA model. The large difference in the total energy of AA and AB models theoretically explained the preference of eclipsed stacking over staggered fashion. On the basis of the above result, a crystalline structure as illustrated in Figure 1e could be generated for the as-prepared HAT-COF. It bears two different kinds of triangular micropores in 2D sheets, and the sheets further stacked into a 3D layered lattice in eclipsed fashion to give a porous material possessing two types of aligned triangular channels of different sizes. In order to further confirm the formation of COF containing two kinds of triangular micropores of different sizes, nitrogen adsorption−desorption measurement was carried out for the asprepared HAT-COF at 77 K. As can be seen in Figure 2a, the isotherm of HAT-COF displays a steep nitrogen uptake in the low-pressure range (P/P0 = 0−0.01), indicating permanent microporosity. The application of the Brunauer−Emmett− Teller (BET) model to the isotherm in the range of P/P0 = 0.01−0.3 resulted in a BET surface area of 486.15 m2/g for HAT-COF (Figure S6 in Supporting Information). Compared with other COFs having mixed pores,22−26 the surface area of HAT-COF is low. It might be attributed to the twisted conformation of the phenyl units of HAT-6NH2, which resulted in disordered stacking of the COF layers.32 The total pore volume of the COF was calculated to be 0.708 cm3/g (P/ P0 = 0.99). The pore size distribution of the COF was calculated by using nonlocal density functional theory (NLDFT). It revealed two main pore size distributions at 11.3 and 15.2 Å, respectively (Figure 2b). These two values are very close to the theoretical pore sizes of the triangular pores, which were predicted by PM3 calculations to be 11.64 and 14.01 Å, respectively. This result corroborated again the formation of the dual-pore COF which holds the microstructure as illustrated in Scheme 1. In conclusion, by one-step condensation of a C3-symmetrical hexaamine and C2-symmetrical dialdehyde, a 2D COF containing two kinds of triangular micropores of different sizes (11.3 and 15.2 Å) has been constructed. It represents an unprecedented topology which endows the pores in the COF with different chemical environments: while the large pores possess aromatic CN units of the HAT skeleton, the small ones just share CN linkages of the edges with the former. This leads to the formation of two types of triangular channels

Figure 2. (a) N2 adsorption−desorption isotherm curve at 77 K and (b) pore size distribution profile of HAT-COF.

with different polarity in the layered lattice, which should display different properties and functions. In this context, we believe that this work not only extends the diversity of the COF family but also lays the foundation for further study on applications of COF-based materials with different types of channels.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.5b00804. Procedure for the preparation of HAT-COF and the starting materials, FT-IR spectra, solid-state 13C CP/ MAS NMR spectrum, SEM image, TGA trace, and total energies of the two stacking models of HAT-COF vs stacking distance (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions ‡

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (Nos. 21172249, 21472225). We thank 101

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(32) Chandra, S.; Kandambeth, S.; Biswal, B. P.; Lukose, B.; Kunjir, S. M.; Chaudhary, M.; Babarao, R.; Heine, T.; Banerjee, R. J. Am. Chem. Soc. 2013, 135, 17853.

Prof. Guang-Yu Li and Dr. Jian Wu for their help in collecting the solid-state 13C CP/MAS spectrum.

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REFERENCES

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DOI: 10.1021/acsmacrolett.5b00804 ACS Macro Lett. 2016, 5, 99−102