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3D Porphyrin-Based Covalent Organic Frameworks Guiqing Lin, Huimin Ding, Rufan Chen, Zhengkang Peng, Baoshan Wang,* and Cheng Wang* Key Laboratory of Biomedical Polymers (Ministry of Education), College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China S Supporting Information *

ABSTRACT: The design and synthesis of three-dimensional covalent organic frameworks (3D COFs) bearing photoelectric units have been considered as a big challenge. Herein, for the first time, we reported the targeted synthesis of two 3D porphyrin-based COFs (3D-Por-COF and 3D-CuPor-COF), starting from tetrahedral (3D-Td) and square (2DC4) building blocks connected through [4 + 4] imine condensation reactions. On the basis of structural characterizations, 3D-Por-COF and 3D-CuPor-COF are microporous materials with high surface areas, and are proposed to adopt a 2-fold interpenetrated pts topology with Pmc21 space group. Interestingly, both 3D COFs are photosensitive and can be used as heterogeneous catalyst for generating singlet oxygen under photoirradiation. However, 3D-Por-COF shows enhanced photocatalytic activity compared with 3D-CuPor-COF, indicating the properties of 3D porphyrin-based COFs can be tuned by metalation of porphyrin rings. The results reported here will greatly inspire us to design and synthesize 3D COFs bearing other metalloporphyrins for interesting applications (e.g., catalysis) in the future.

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for the first time, we reported a novel 3D pyrene-based COF (3D-Py-COF), starting from tetrahedral (3D-Td) and rectangle (2D-C2) building blocks that are connected through [4 + 4] imine condensation reactions.14 From this result, we believe it is possible to synthesize other 3D COFs through cocondensation of tetrahedral and quadrilateral building blocks. Moreover, due to the existence of isolated pyrene units in the 3D framework, 3D-Py-COF is the first fluorescent 3D COF and can be used in explosive detection. Therefore, the incorporation of photoelectric units into 3D COFs can also endow the resulting materials with interesting properties. However, the design and synthesis of 3D COFs bearing photoelectric units have been considered as a big challenge and very few examples have been reported to date.13c,14 Porphyrin and its metal derivatives, a kind of highly conjugated π-electron macrocycles with unique photophysical and redox properties, have been widely used as important molecular building blocks in materials chemistry.15 2D porphyrin-based COFs,16 in which porphyrins align in a precise order via π−π interactions to facilitate charge carrier transport in the stacking direction, have been well investigated and found interesting applications in different areas, e.g., semiconductor.17 However, the incorporation of porphyrins into 3D COFs, which can allow all porphyrin units in the framework accessible and thus making them extremely attractive for catalysis, has never been reported. Considering their interesting properties and promising applications, the design and synthesis of porphyrin-functionalized 3D COFs are highly demanded.

INTRODUCTION Covalent organic frameworks (COFs) are a novel class of crystalline organic polymers with inherent porosity and structural periodicity.1 Since first discovered by Yaghi and coworkers in 2005,2 COFs have gained considerable attention and exhibited great potential in various applications such as gas storage and separation,3 catalysis,4 sensor,5 optoelectronics,6 and energy storage.7 Depending on the geometry of building blocks, COFs can be categorized into either two-dimensional (2D) layered stacking structures or three-dimensional (3D) networks. For the former, due to the existence of extra template-directed polymerization effect in the synthesis,8 the formation of 2D COFs can be facilitated and, as a result, the synthesis of 2D COFs has been relatively well established. By using a topology diagram,1d many 2D COFs with different designed structures have been reported, and their interesting properties and applications have also been intensively investigated.9 3D COFs, in which the molecular building blocks are threedimensionally organized to form the extended networks, can characteristically possess numerous open sites and show promising applications in different areas, especially gas storage10 and catalysis.11 However, as the driving force for construction of 3D COFs relies only on covalent bonds, their design and synthesis have been proven to be a big challenge.1a Until now, since Yaghi and co-workers announced the first example in 2007,12 only a few 3D COFs have been reported.10−14 Among them, it should be noted that, almost all systems were synthesized by the self-condensation of tetrahedral nodes,10b,13e or its co-condensation with linear10a,c,13a,c,d or triangular building blocks.11,12,13b,f Recently, © 2017 American Chemical Society

Received: April 24, 2017 Published: June 8, 2017 8705

DOI: 10.1021/jacs.7b04141 J. Am. Chem. Soc. 2017, 139, 8705−8709

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Journal of the American Chemical Society

aluminum sample holder and then coated with gold. UV−vis spectra were recorded on a SHIMADZU UV-3600 UV−vis-NIR spectrophotometer. Fluorescence spectra were recorded on a HITACHI F-4600 spectrofluorometer. Molecular modeling was carried out using Materials Studio suite of programs (Accelrys Inc.). High resolution MALDI-TOF data was recorded on a Bruker Solarix mass spectrometer. The nitrogen isotherms were measured at 77 K using an Autosorb-iQ (Quantachrome) surface area size analyzer. Before measurement, the samples were degassed in vacuum at 100 °C for 12 h. Oil-free vacuum pumps and oil-free pressure regulators were used for measurements to prevent contamination of the samples during the degassing process and isotherm measurement. The photoirradiation experiments were performed by using xenon lamp with an optical filter (λ = 500 nm). To evaluate the photochemical activity, 3D-Por-COF or 3D-CuPor-COF (8 mg) was put in a container filled with a CH3CN solution (40 mL) of 9,10dimethylanthracene (DMA, 10−2 M). The system was bubbled with oxygen for 30 min and then irradiated with a visible light at 500 nm from a xenon lamp. To monitor the process, 40 μL solution was taken out and then diluted with 4 mL CH3CN to measure UV−vis spectra in every 10 min (for 3D-Por-COF) or every 60 min (for 3D-CuPorCOF). For the control experiment, the experiment was performed under the same condition except addition of COF powder. For the recycle test, 3D-Por-COF was recovered by the centrifugation and washed with CH3CN (3 × 10 mL). The collecting powder was then reused in the next photoirradiation under the same condition. Synthesis of [5,10,15,20-Tetrakis(4-benzaldehyde)porphyrin]copper (p-CuPor-CHO). A Pyrex tube was charged with p-Por-CHO (100.0 mg, 0.138 mmol), CuCl2·2H2O (46.9 mg, 0.275 mmol), and 20 mL DMF. The tube was degassed by freeze− pump−thaw technique for three times and then sealed under nitrogen. The tube was stirred and heated at 120 °C for 4 h. After cooling to room temperature, about 80 mL water was added to the solution. The resulting precipitate was collected by filtration, then washed with methanol and dried under vacuum to afford dark red powder in 98% yield (105.9 mg). HR-MS (MALDI−TOF): calcd. for C48H28CuN4O4 m/z = 787.14066 [M]+, found: m/z = 787.14011 [M]+. The UV−vis spectra of p-Por-CHO and p-CuPor-CHO are shown in Figure S1 of the Supporting Information (SI). Synthesis of 3D-Por-COF. A Pyrex tube was charged with TAPM (22.8 mg, 0.06 mmol), p-Por-CHO (43.6 mg, 0.06 mmol), 3.6 mL odichlorobenzene, 0.4 mL butanol, and 0.4 mL of 6 M aqueous acetic acid. The tube was degassed by freeze−pump−thaw technique for three times and then sealed under vacuum. The reaction was heated at 120 °C for 7 d yielding solid at the bottom of the tube. The resulting precipitate was filtered off, exhaustively washed by Soxhlet extractions with tetrahydrofuran and dichloromethane for 24 h, dried at 80 °C under vacuum for overnight. The 3D-Por-COF was isolated as a crimson powder in 72% yield. Elemental analysis for the calculated: C, 84.70%; H, 4.48%; N, 10.82%. Found: C, 81.70%; H, 4.60%; N, 10.42%. Synthesis of 3D-CuPor-COF. It was prepared using the same synthetic protocol of 3D-Por-COF, except starting from TAPM (22.8 mg, 0.06 mmol) and p-CuPor-CHO (47.2 mg, 0.06 mmol). After washing and drying, 3D-CuPor-COF was isolated as a bright red powder in 89% yield. Elemental analysis for the calculated: C, 79.95%; H, 4.04%; N, 10.22%. Found: C, 78.60%; H, 4.17%; N, 9.99%.

Herein, on the basis of the geometry of porphyrin, we report the targeted synthesis of two novel 3D porphyrin-based COFs (named as 3D-Por-COF and 3D-CuPor-COF) for the first time (Scheme 1), starting from tetrahedral (3D-Td) and square (2DScheme 1. Representation of the Synthesis of 3D-Por-COF and 3D-CuPor-COF

C4) precursors connected through [4 + 4] condensation reactions. Our results clearly show that 3D-Por-COF and 3DCuPor-COF are microporous materials with high surface areas, and adopt a 2-fold interpenetrated pts topology with Pmc21 space group. Interestingly, both 3D COFs are photosensitive and can be used as a heterogeneous catalyst for generating singlet oxygen under photoirradiation. However, 3D-Por-COF shows enhanced photocatalytic activity compared with the 3DCuPor-COF, indicating the properties of 3D porphyrin-based COFs can be tuned by metalation of porphyrin rings.

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EXPERIMENTAL SECTION

All the reagents and starting materials were purchased from Acros or Adamas, unless otherwise noted, and used without further purification. Dehydrated solvents were obtained after treating solvents with standard procedures. Tetra(p-aminophenyl)methane (TAPM)18 and 5,10,15,20-tetrakis(4-benzaldehyde)porphyrin (p-Por-CHO)19 were synthesized according to the literature. Solid-state NMR spectra were recorded at ambient temperature on a Bruker AVANCE III 400 M spectrometer. Elemental analysis was conducted on a Flash EA 1112. Fourier transform infrared (FT−IR) spectra were recorded on a NICOLET 5700 FTIR Spectrometer. Powder X-ray diffraction (PXRD) patterns were obtained on a Rigaku Smartlab or Rigaku MiniFlex 600 X-ray diffractometer with Cu Kα line (λ = 1.540 56 Å). Thermogravimetric analysis (TGA) from 50 to 800 °C was carried out on a PerkinElmer TG-DTA6300 in nitrogen atmosphere using a 10 °C/min ramp without equilibration delay. Field-emission scanning electron microscopy (FE-SEM) images were performed on a Zeiss ∑IGMA operating at an accelerating voltage ranging from 0.1 to 20 kV. The samples were prepared by dispersing the material onto conductive adhesive tapes attached to a flat

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RESULTS AND DISCUSSION In order to test the possibility of using [4 + 4] condensation reactions to synthesize 3D porphyrin-based COFs, we first tried to synthesize 3D-Por-COF, by choosing the reported TAPM18 and p-Por-CHO19 as the precursors. After solvothermal condensation of TAPM and p-Por-CHO in the mixture of odichlorobenzene/n-butanol/aqueous acetic acid (9:1:1, by vol.) at 120 °C for 7 days (Scheme 1), a highly crystalline COF (3DPor-COF) was isolated as crimson powder in 72% yield. Encouraged by this result, we then started to synthesize 3D8706

DOI: 10.1021/jacs.7b04141 J. Am. Chem. Soc. 2017, 139, 8705−8709

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Journal of the American Chemical Society CuPor-COF under the same conditions (Scheme 1), starting from TAPM and p-CuPor-CHO. To our delight, 3D-CuPorCOF was also obtained as a bright red powder in 89% yield. The atomic-level formation of 3D-Por-COF and 3D-CuPorCOF was assessed by solid-state NMR spectroscopy and Fourier transform infrared (FT−IR) spectroscopy. From the 13 C cross-polarization with total suppression of sidebands NMR spectroscopy, 3D-Por-COF showed the characteristic signal for the CN group at 158 ppm (Figure S3). In addition, the FT− IR spectrum of 3D-Por-COF exhibited a stretching vibration band at 1627 cm−1 (Figure S4), confirming again the existence of imine bonds. For 3D-CuPor-COF, the FT−IR spectrum also showed a band at 1627 cm−1 (Figure S4), indicating the formation of imine bonds. Moreover, compared with 3D-PorCOF, the stretching vibrational frequency of the N−H bond (3318 cm−1) of the porphyrin ring disappeared, indicating again that porphyrin rings are coordinated with copper ions in 3DCuPor-COF. From the thermogravimetric analysis (Figure S5), both 3D COFs showed high thermal stability up to 500 °C. The crystalline nature of 3D-Por-COF and 3D-CuPor-COF was confirmed by powder X-ray diffraction (PXRD) analysis. As shown in Figure 1, 3D-Por-COF exhibited two intense peaks at 6.82° and 8.17°, where 3D-CuPor-COF shows two intense peaks at 6.80° and 7.96°. The crystal models were then generated using Materials Studio software package. According to Reticular Chemistry Structure Resource,20 only a few nets (e.g., pth, pti, pts, etc.) are reasonable for 3D-Por-COF and 3D-CuPor-COF. After considering these possible nets with different space groups, the detailed simulation (see section S3 in the SI for details) clearly suggested that both 3D-Por-COF and 3D-CuPor-COF are proposed to adopt a 2-fold interpenetrated pts topology with Pmc21 space group (Figure 1). Full profile pattern matching (Pawley) refinements for both 3D COFs were carried out and the refinement results yield unit cell parameters nearly equivalent to the predictions with good agreement factors (a = 43.9032 Å, b = 23.5003 Å, c = 21.3781 Å, α = β = γ = 90°, wRp = 3.83%, and Rp = 2.80% for 3D-PorCOF; a = 43.7788 Å, b = 23.4757 Å, c = 21.3782 Å, α = β = γ = 90°, wRp = 3.11%, and Rp = 2.09% for 3D-CuPor-COF). Nitrogen sorption isotherms were measured at 77 K to evaluate the permanent porosity of 3D-Por-COF and 3DCuPor-COF. Prior to the measurement, the samples were degassed at 100 °C and 1 × 10−5 Torr for 12 h. As shown in Figure 2a,b, both 3D COFs exhibited a type I isotherm displaying a sharp increase under low relative pressures (P/P0 < 0.01), which is a characteristic of microporous materials. The Brunauer−Emmett−Teller (BET) surface areas were calculated to be 1398 m2 g−1 for 3D-Por-COF and 1335 m2 g−1 for 3DCuPor-COF, respectively. By using the model of nonlocal density functional theory (NLDFT), the pore size distributions were also calculated (Figure 2c,d). 3D-Por-COF displayed two major peaks centered at 0.60 and 1.07 nm, where 3D-CuPorCOF showed two major peaks at 0.63 and 1.18 nm. In addition, the scanning electron microscopy (SEM) images showed that both 3D-Por-COF (Figure 2e) and 3D-CuPor-COF (Figure 2f) possessed a uniform granular morphology. Moreover, we have investigated the stability of 3D-Por-COF and 3D-CuPorCOF in common organic solvents, water and alkaline solution for 24 h. From PXRD patterns (Figure S9), both 3D-Por-COF and 3D-CuPor-COF showed high stability in these solvents. It is well-known that porphyrin and its derivatives can be excited into triplet state under photoirradiation, which will allow them for energy transfer to molecular oxygen and then

Figure 1. Powder X-ray diffraction patterns and simulated structures of 3D-Por-COF (a) and 3D-CuPor-COF (b). The experimental patterns are shown in black, the Pawley refined patterns in red, the differences between experimental and refined profiles in blue. Inset: the stick models for 3D-Por-COF and 3D-CuPor-COF viewed from c axis. Purple: carbon atoms from p-Por-CHO or p-CuPor-CHO precursors. Gray: carbon atoms from TAPM precursors. Blue: nitrogen atoms. Orange: copper atoms. Hydrogen atoms are omitted.

generate singlet oxygen.21 Therefore, the possibility of using 3D-Por-COF as a heterogeneous photocatalyst for activation of molecular oxygen was first explored. In order to detect the production of singlet oxygen, the well-established 9,10dimethylanthracene (DMA) was chosen as the label, and the process was monitored by UV−vis spectroscopy. As shown in Figure 3b, upon photoirradiation (λ = 500 nm) of oxygensaturated CH3CN solution (40 mL) of DMA (10−2 mol/L, 82.5 mg) in the presence of 3D-Por-COF (8 mg), the absorption of DMA decreased gradually. According to the intensity of the peak at 375 nm, 99% DMA degraded after photoirradiation for 90 min (Figure 3d).22 In addition, 3D-Por-COF was still highly active with degradation efficiency up to 94% after three cycles (Figure S17), and from the PXRD experiments, the framework was well retained during this process (Figure S18). 8707

DOI: 10.1021/jacs.7b04141 J. Am. Chem. Soc. 2017, 139, 8705−8709

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We then studied the photocatalytic performance of 3DCuPor-COF for singlet oxygen evolution. As shown in Figure 3c, 45% DMA was still left after photoirridiation for 12 h under the same condition. Therefore, compared with 3D-Por-COF, 3D-CuPor-COF exhibited much weaker ability to generate singlet oxygen under photoirridiation. This is in agreement with previous results,23 as porphyrins containing paramagnetic metal ions are poor photosensitizers. On the basis of these results, the incorporation of porphyrin units into 3D COF can allow the resulting material with interesting photocatalytic properties, and moreover, the properties of 3D porphyrin-based COFs can be tuned by metalation of porphyrin rings.

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CONCLUSIONS

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ASSOCIATED CONTENT

In summary, we have demonstrated the targeted synthesis of two 3D porphyrin-based COFs for the first time, starting from tetrahedral and square building blocks connected through [4 + 4] imine condensation reactions. Both 3D COFs were characterized by a variety of techniques, and from PXRD patterns and computer modeling, they are proposed to adopt a 2-fold interpenetrated pts net with Pmc21 space group. On the basis of this result, together with the previously reported 3DPy-COF, we believe it is now reasonable to design and synthesize 3D COFs from tetrahedral and quadrilateral building blocks through [4 + 4] condensation reactions and more 3D COFs can be expected. Moreover, 3D-Por-COF and 3DCuPor-COF are photosensitive and can be used as a heterogeneous photocatalyst to generate highly reactive singlet oxygen under photoirradiation. Therefore, the incorporation of porphyrin units into 3D COF can allow the resulting material with interesting properties. However, 3D-Por-COF shows enhanced photocatalytic activity compared with the 3DCuPor-COF, indicating that the properties of 3D porphyrinbased COFs can be tuned by metalation of porphyrin rings. As a variety of metals can be inserted into the porphyrin macrocycle, thus tuning their photophysical and redox properties,24 the incorporation of other metalloporphyrins into 3D COFs should allow the resulting materials with interesting properties and promising applications (e.g., catalysis) in different areas, can be expected. Currently the design and synthesis of other metalloporphyrin-based 3D COFs are ongoing in our lab.

Figure 2. N2 sorption isotherms of 3D-Por-COF (a) and 3D-CuPorCOF (b) at 77 K. Pore size distributions of 3D-Por-COF (c) and 3DCuPor-COF (d). SEM images of 3D-Por-COF (e) and 3D-CuPorCOF (f).

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b04141.

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Detailed synthesis, characterization, structure simulation, and spectroscopic measurements (PDF)

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Figure 3. (a) Schematic diagram of 3D-Por-COF and 3D-CuPor-COF generating singlet oxygen. (b and c) UV−vis spectra of DMA solution with 3D-Por-COF or 3D-CuPor-COF upon light irradiation at 500 nm with a xenon lamp. (d) Plots of the DMA content versus irradiation time for 3D-Por-COF and 3D-CuPor-COF based on the absorbance of the peak at 375 nm.

Baoshan Wang: 0000-0003-3417-9283 Cheng Wang: 0000-0003-0326-2674 Notes

The authors declare no competing financial interest. 8708

DOI: 10.1021/jacs.7b04141 J. Am. Chem. Soc. 2017, 139, 8705−8709

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Natural Science Foundation of China (21572170, 21573165), the Outstanding Youth Foundation of Hubei Province (2015CFA045), and the Beijing National Laboratory for Molecular Sciences. We thank professor Daqiang Yuan (from Fujian Institute of Research on the Structure of Matter, Chinese of Academy of Sciences) for kind discussion on crystal structure.

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DOI: 10.1021/jacs.7b04141 J. Am. Chem. Soc. 2017, 139, 8705−8709