Communication pubs.acs.org/cm
2D Covalent Organic Frameworks with Alternating Triangular and Hexagonal Pores Luke A. Baldwin,‡ Jonathan W. Crowe,‡ Matthew D. Shannon, Christopher P. Jaroniec, and Psaras L. McGrier* Department of Chemistry & Biochemistry, The Ohio State University, Columbus, Ohio 43210, United States
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S Supporting Information *
C
ovalent organic frameworks (COFs)1,2 are an incipient class of porous crystalline materials that have attracted considerable attention for applications related to gas storage,3 separations,4 optoelectronics,5−7 and catalysis.8,9 The modular nature of COFs permits the integration of various rigid πconjugated molecular building blocks into highly ordered columnar periodic arrays producing polymeric materials that exhibit low densities, permanent porosity, and high thermal stabilities. Many COFs are generally constructed through a dynamic nucleation-elongation process to produce the desired highly ordered polymeric networks.10,11 Utilizing this process, two-dimensional (2D) and three-dimensional (3D)12 COFs have been synthesized with imine,13 boronate ester,14 borazine,15 and hydrazone16 bond linkages. The predictable design and applicability of various bond linkages has enabled the creation of many distinctive COFs with tunable pore sizes. Although the pore size and shape can be modulated by meticulous selection of the monomer, finding ways to incorporate functional monomers that can bind analytes (i.e., metal cations) or interact selectively with guest molecules without compromising the surface area and pore volume of the materials is still a particular challenge. However, Zhao and co-workers have recently shown that the careful choice of monomers can lead to a 2D COF structure with a mixture of micro- and mesopores utilizing a one step polycondensation reaction.17 However, the micropore of the COF investigated in this report relies upon monomers with D2h and C2 symmetries, which can limit the control over the specific size of the micropore as the lengths of the C2-symmetric monomers are protracted. Having precise control over the size of the micropore is important for designing functional multipore COF architectures that can interact with specific analytes as the pore size is extended to increase the surface area of the material. Jiang and co-workers recently reported utilizing a C3-symmetric phenanthrene cyclotrimer and C2-symmetric monomers to construct star-shaped multipore 2D COFs, which demonstrated the potential of employing a macrocycle to control the micropore of 2D COFs.18 Such investigations are imperative for advancing the structural diversity of this advanced class of polymeric materials. Herein, we report the synthesis of two 2D COFs (Scheme 1) with a mixture of alternating triangular and hexagonal pores utilizing C3-symmetric π-conjugated dehydrobenzoannulenes (DBAs)19 and a C2 symmetric 1,4-benzenediboronic acid (BDBA) monomer. DBAs are planar macrocycles that exhibit the ability to form ordered 2D porous networks,20,21 and the potential to bind low oxidation state transition metals.22 Both © 2015 American Chemical Society
Scheme 1. Synthesis of DBA-COF 1 and DBA-COF 2 Using DBA Monomers and a Diboronic Acid Linker
features make DBAs attractive monomers to utilize for the construction of a distinct class of functional COFs. We demonstrate that the size of the micropore (0.4 to 0.5 nm) and mesopore (3.2 to 3.6 nm) can be tuned by extending the length of the DBA monomer by adding three additional alkynyl units to yield periodic crystalline networks with very high surface areas.23 DBA-COF 1 and DBA-COF 2 were synthesized under solvothermal conditions by reacting DBA[12] and DBA[18] monomers24 with BDBA in a 2:1 (v/v) 1,4-dioxane and mesitylene mixture in flame-sealed glass ampules at 105 °C for 3 days. DBA-COF 1 and DBA-COF 2 were obtained by filtration and washed with acetonitrile to afford green crystalline powders that were insoluble in common organic solvents. The COF materials were purified by immersing them in acetonitrile for 24 h to remove unreacted monomers and dried under vacuum (Supporting Information (SI)). Thermogravimetric analysis (TGA) revealed that DBA-COF 1 maintained more than 97% of its weight up to 465 °C (Figure S10, SI) whereas DBA-COF 2 maintained ∼80% of its weight up to the same temperature (Figure S11, SI). Received: June 1, 2015 Revised: August 22, 2015 Published: August 27, 2015 6169
DOI: 10.1021/acs.chemmater.5b02053 Chem. Mater. 2015, 27, 6169−6172
Communication
Chemistry of Materials
profiles for both COFs, which were indexed using a primitive hexagonal lattice. Because both DBA monomers are C3symmetric, it was anticipated that combining DBA[12] or DBA[18] with the linear C2-symmetric BDBA building block would yield ordered 2D hexagonal layers. We predicted that these layers would pack in eclipsed bnn (P6/mmm) or staggered gra (P63/mmc) conformations. DBA-COF 1 displays intense peaks at 3.01, 5.16, 5.93, 7.83, 10.65, and 26.1° that correspond to the (100), (110), (200), (210), (310), and (001) planes, respectively. The broad reflection peak at 26.1 corresponding to the (001) reflection plane highlights the vertical spacing between the stacked COF layers at a distance of 3.4 Å. The crystal structure of DBA-COF 1 was simulated using the Reflex module of the Materials Studio 7.0 software. Pawley refinement of the observed PXRD data using a bnn net provided unit cell parameters of a = b = 33.929 Å and c = 3.4 Å (residuals Rp = 6.89%, Rwp = 8.83%). The diffraction peaks for DBA-COF 2 are analogous to DBA-COF 1 displaying intense peaks at 2.67, 4.55, 5.23, 6.92, 9.44, and 26.2 corresponding to the (100), (110), (200), (210), (310), and (001) planes, respectively. The unit cell was also refined using a bnn net to provide parameters of a = b = 37.889 Å and c = 3.4 Å (residuals Rp = 3.86%, Rwp = 4.96%). With an interlayer spacing of 3.4 Å, the butadiyne units of DBA[18] are not in close enough proximity to undergo polymerization,25 an observation that is consistent with another previously reported COF using a similar butadiyne building block.26 Although the simulated PXRD patterns were in good agreement with the experimental peak positions for DBA-COF 1 and DBA-COF 2, computational studies have shown28 that the adjacent layers of most COFs are slightly offset from their completely eclipsed bnn packing structure. We also considered gra PXRD patterns in which the a and b planes are offset by half of the unit cell for both materials (Figures S5 and S7, SI). However, the simulated PXRD patterns could not reproduce the experimental peak intensities. The permanent porosity of DBA-COF 1 and DBA-COF 2 were determined by nitrogen gas adsorption measurements at 77 K. DBA-COF 133 exhibits a type IV isotherm displaying a sharp uptake under low relative pressure (P/P0 < 0.01) followed by a sharp step between P/P0= 0.04 and 0.21, which is indicative of a mesoporous material (Figure 2a). The Brunauer−Emmett−Teller (BET) model was applied over the low-pressure region (0.04 < P/P0 < 0.18) of the isotherm
DBA-COF 1 and DBA-COF 2 were characterized by Fourier transform infrared (FT-IR) and 13C cross-polarization magic angle spinning (CP-MAS) spectroscopies. The FT-IR spectra of DBA-COF 1 and DBA-COF 2 showed stretching modes at 1326 and 1329 cm−1, respectively, which is indicative of boronate ester (B−O) formation (Figures S1 and S2, SI). Solid-state 13C CP-MAS NMR was used to establish the connectivity of the COF materials. The presence of alkynyl units was confirmed via the characteristic resonance at 94.0 ppm for DBA-COF 1 and two signals at 78.6 and 83.2 ppm for DBA-COF 2 (Figure S9, SI). For both COF materials the spectra also contain all the expected 13C resonances in the 120−160 ppm region and no other signals. To assign the 13C resonances, in addition to utilizing the general chemical shift trends, we performed R1871 1H−13C dipolar recoupling experiments34 for DBA-COF 1 (Figure S8, SI). These measurements enable the differentiation of 13C types based on their proximity to 1H nuclei. Specifically, for 13C atoms with directly bonded protons, significant dipolar dephasing of the 13C spectral intensity is observed for short (∼100 μs) recoupling times. In contrast, longer dipolar recoupling periods (∼1−2 ms) are required to modulate the resonance intensities for the remaining carbons, with the extent of dephasing correlated with the distance to the nearest proton. Scanning electron microscopy (SEM) images revealed hexagonal crystallites and one bulk phase morphology for both COFs (Figures S20 and S21, SI). Powder X-ray diffraction (PXRD) was used to assess the crystallinity and unit cell parameters of DBA-COF 1 and DBACOF 2. Figure 1 shows the experimental and refined PXRD
Figure 1. Indexed experimental (red) and Pawley refined (blue) PXRD patterns of DBA-COF 1 (top) and DBA-COF 2 (bottom) compared to the bnn simulated unit cell (green) with views along the c and b directions for each hexagonal crystal model.
Figure 2. Nitrogen adsorption isotherms for DBA-COF 1 (a) and DBA-COF 2 (b) measured at 77 K followed by NLDFT pore size distributions of DBA-COF 1 (c) and DBA-COF 2 (d). 6170
DOI: 10.1021/acs.chemmater.5b02053 Chem. Mater. 2015, 27, 6169−6172
Communication
Chemistry of Materials providing a surface area of 1952 m2 g−1. This value is very close to the predicted Connolly surface area value of 2085 m2 g−1 allowing us to activate up to 90% of its maximum nitrogen uptake. It should also be noted that the BET surface area of DBA-COF 1 is larger than COF-5 (1590 m2 g−1),1 NTU-COF2 (1619 m2 g−1),27 COF-10 (1760 m2 g−1),3 and TT-COF (1810 m2 g−1).7 Its total pore volume calculated at P/P0 = 0.993 was 1.27 cm3/g, which is also close to the theoretical value of 1.32 cm3/g. DBA-COF 2 also exhibits a type IV isotherm indicative of a mesoporous material. Application of the BET model over the low pressure 0.08 < P/P0 < 0.21 range provided a surface area of 984 m2 g−1, which is significantly lower than the predicted Connolly surface area of 2166 m2 g−1. We attribute the lower surface area to the presence of unreacted monomers trapped within the 1D hexagonal channels of the material. However, further optimization to attain its theoretical value is ongoing. The total pore volume of DBA-COF 2 calculated at P/P0 = 0.997 was 0.741 cm3 g−1. The nonlocal density functional theory (NLDFT) was used to estimate the pore size distributions of DBA-COF 1 and DBA-COF 2 yielding average pore sizes of 3.2 and 3.6 nm, respectively. The observed pore sizes for DBA-COF 1 and DBA-COF 2 are very close to the predicted values of 3.4 and 3.8 nm, respectively, utilizing the bnn crystal models. However, we were not able to confirm the presence of the predicted micropore values of 0.4 and 0.5 nm for DBA[12] and DBA[18], respectively. We attribute this to possible defects and offsets between the stacked layers of the materials, which is a typical phenomenon that has been observed with other COFs.28 Interestingly, DBA-COF 1 powders are highly fluorescent in the solid-state on account of the coplanar arrangement of the DBA[12] vertices (Figure 3c). UV−vis diffuse reflectance
sensory-based applications,29 we also believe the proof-ofprinciple is important as theoretical studies have shown that DBA monomers exhibit the potential to strongly bind not only alkali and alkaline earth metals, but also low oxidation state transition metals.30 Incorporating metals at the vertices of 2D DBA-based COFs raises the possibility of developing novel functional COF materials with enhanced binding sites for separations31 and gas storage applications.32 Such investigations are currently underway in our laboratory and will be reported in the near future.
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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/acs.chemmater.5b02053. Synthetic procedures, FT-IR, PXRD, solid state 13C NMR, TGA, and SEM (PDF).
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AUTHOR INFORMATION
Corresponding Author
*P. L. McGrier. E-mail:
[email protected]. Author Contributions ‡
L.A.B. and J.W.C. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS P.L.M acknowledges the National Science Foundation (NSF) and Georgia Tech Facilitating Academic Careers in Engineering and Science (GT-FACES) for a Career Initiation Grant, and funding from The Ohio State University.
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REFERENCES
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Figure 3. Kubelka−Munk function diffuse reflectance (a), emission spectrum (b), and photograph (c) of DBA-COF 1 powder (λexc = 365 nm), respectively. A photograph of the fluorescent powder was taken using a hand-held UV-lamp at 365 nm.
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