Article pubs.acs.org/Macromolecules
N,N′‑Bicarbazole: A Versatile Building Block toward the Construction of Conjugated Porous Polymers for CO2 Capture and Dyes Adsorption Yuan Yuan,†,§ Hongliang Huang,‡ Long Chen,*,†,§ and Yulan Chen*,†,§ †
Tianjin Key Laboratory of Molecular Optoelectronic Science, Department of Chemistry, Tianjin University, Tianjin 300354, P. R. China ‡ State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, P. R. China § Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, P. R. China S Supporting Information *
ABSTRACT: N,N′-Bicarbazole with almost spatially vertical structure was synthesized by a facile method and was demonstrated as a versatile building block for conjugated microporous polymers (CMPs) through transition metal catalyzed coupling polymerizations including Yamamoto, Suzuki, and Sonogashira polymerizations. The porosities and morphologies of four N,N′-bicarbazole-based CMPs were systematically studied and compared. With high surface area, good thermal stability and microporous structure, the CMP synthesized by Yamamoto polymerization exhibited excellent adsorption capacities of CO2 and organic dyes, and the maximum adsorption capacity of methylene blue (MB) in water reached up to 1016 mg g−1. The good adsorption properties of N,N′-bicarbazole CMPs could be a good candidate for potential applications in water purification and treatment.
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INTRODUCTION Wastewater from textile industries, pharmacy, and biochemistry activities raise a serious issue of pollution due to the dyes contained therein. The improper disposal of organic dyes into receiving waters causes unavoidable damage to the environment and human beings, as even small amounts of these dyes are extremely toxic to aquatic life and generally carcinogenic along the biological chain. Currently, adsorption is regarded as the most favorable method for water cleansing, but conventional adsorbents, including activated carbon, zeolites, and natural fibers, usually suffer from low adsorption capacities, poor selective sorption, and unsatisfactory regeneration ability.1−5 In contrast, organic microporous polymers (OMPs),6 as newgeneration porous polymers, are recently receiving increased research interest, owing to their great advantages compared with other porous materials, such as high surface area, low mass density, easy functionality, and high stability, and thus are considered as a promising kind of adsorbents, whereas until now, only a few examples of OMPs have been developed for the purpose of water treatment.7−12 During the past decade, different kinds of OMPs have been developed, such as covalent organic frameworks (COFs),13,14 hyper-cross-linked polymers (HCPs),15 polymers of intrinsic microporosity (PIMs),16 conjugated microporous polymers (CMPs),7,17−20 and porous aromatic frameworks (PAF).21 And in the meantime, various potential applications such as gas storage,10,22−25 separation,11,26,27 catalysis,28−34 and sensor35,36 © 2017 American Chemical Society
were investigated. Typically, CMPs are amorphous polymers with conjugated framework and inherent three-dimensional porous architecture, which exhibited considerable merits, like unlimited choices of building blocks with diverse functions, versatile modification of material compositions and properties at the molecular level, high flexibility, facile scale-up synthesis, high thermostability, etc. Since the report of the first CMP by Cooper,37 chemists and materials scientists have contributed greatly for the rapid growth of the CMPs family, among which enormous efforts have been focused on the development of different topological and functional building blocks for the construction of functional CMPs.18,19,38−40 From the point view of topology, building blocks that have near 90° kink were rarely reported except 9,9′-spirobifluorene,41,42 triptycene,43,44 and tetraphenylmethane.21,45,46 Incorporating such kinds of monomers was expected to prevent the otherwise stiff polymer chains from space efficient packing; the resulting 3D porous polymers thus would have potential of intrinsic microporosity and have the advantages of high accessible free volume and high surface area and stability. And from the point view of functionality, polycarbazoles with excellent electroactivity and photophysical properties are suitable candidates for exploration of porous organic polymers Received: May 10, 2017 Revised: June 14, 2017 Published: June 27, 2017 4993
DOI: 10.1021/acs.macromol.7b00971 Macromolecules 2017, 50, 4993−5003
Article
Macromolecules possessing advanced functions and properties. For example, the nitrogen-containing conjugated structure makes the polymers electron-rich, which may enhance the interactions between specific sorbate molecule and adsorbent. Carbazole-based CMPs with rigid conjugated backbones developed fast in recent years, since they were beneficial for the formation of porous materials with permanent porosity and high physicochemical stability.22,23,35,36,47−50 Until now, most of the carbazole-based CMPs are built up with rigid planar monomers, and CMPs with almost spatially vertical carbazole-containing structures have never been investigated. In this work, we synthesized 3,3′,6,6′-tetrabromo-N,N′bicarbazole (Br-BC) monomer whose single crystal structure showed a large dihedral angle (87.4°) between two carbazole planes. Based on this almost vertical kink bicarbazole-based monomer, metal-catalyzed coupling polymerizations including Yamamoto, Suzuki, and Sonogashira polymerizations were conducted, and four different kinds of CMPs (CMP-YA, CMPSU, CMP-SO-1B2, and CMP-SO-1B3, Scheme 1) were readily obtained. All the CMPs exhibited high surface area and good stability. The structural features render them well suited to adsorb gas and other small molecules. For instance, the
maximum adsorption capacity for methylene blue, a typical dye in wastewater, reached to 1016 mg g−1 by CMP-YA, higher than most of the reported sorbents.9,10,51−57 To this end, systematical investigations on these porous materials allow us to probe into the correlations between chemical structures and their pore structures, photophysical properties, and adsorption behaviors, etc., and demonstrate their promising applications on gas storage and water purification.
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EXPERIMENTAL SECTION
Materials. Carbazole and bis(1,5-cyclooctadiene)nickel(0) [Ni(cod)2] were purchased from Alfa Aesar, 2,2′-bipyridyl was purchased from J&K Scientific, 1,5-cyclooctadiene, 1,4-benzenediboronic acid, tri(o-tolyl)phosphine (p(o-tolyl)3), and CuI were purchased from Energy Chemical. Methylene blue (MB), rhodamine B (Rh), methyl orange (MO), and congo red (CR) were purchased from Tianjin Jiangtian Chemical Company. Solvents were purified according to standard procedures. All other chemicals and reagents were used directly without further purification. The bicarbazole monomers 3,3′,6,6′-tetrabromo-N,N′-bicarbazole (Br-BC) and N,N′-bicarbazole (BC)58 and alkynyl monomers 1,4-diethynylbenzene and 1,3,5triethynylbenzene59 were synthesized according to the literature via modified procedures (see Supporting Information). Characterization Methods. The 1H and 13C NMR spectra of all organic compounds were recorded on a Bruker AVANCE III-400 NMR spectrometer under 25 °C by using CDCl3 as the solvent in all cases. The matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectroscopy measurements were carried out with Bruker Autoflex speed TOF/TOF mass spectrometer. Highresolution mass spectra (HRMS) were obtained with a micro mass GCT-TOF mass spectrometer. Single crystals were obtained in CHCl3 by a slow solvent diffusion method, and the single crystal X-ray diffraction was recorded on a Rigaku SCX-mini diffractometer with graphite monochromatic Mo Kα radiation (λ = 0.7173 Å) by ω scan mode. The UV−vis absorption and diffuse reflectance spectra were obtained on a PerkinElmer Lambda 750 spectrophotometer with standard procedure. Fluorescence spectra were recorded on a Hitachi F-7000 fluorescence spectrophotometer. Solid-state NMR experiments were performed on a Bruker WB AVANCE II 400 MHz NMR spectrometer. The 13C cross-polarization magic angle spinning (CP/ MAS) spectra were recorded with a 4 mm double-resonance MAS probe and at a MAS rate of 10.0 kHz with a contact time of 2 ms (ramp 100) and a pulse delay of 3 s. FT-IR spectra were collected in transmission mode on a Bruker Alpha spectrometer using KBr pellets with a scan range of 400−4000 cm−1. Elemental analyses were measured by Elementar model Vario EL CUBE. The thermal properties of monomer and all the CMPs were evaluated using a thermogravimetric analysis (TGA) with a differential thermal analysis instrument (TA Instruments Q-50) over the temperature range from 20 to 800 °C under a nitrogen atmosphere with a heating rate of 10 °C min−1. Powder X-ray diffraction experiment was carried out on Rigaku SmartLab (9 kW) X-ray diffractometer. Field emission scanning electron microscopies (FE-SEM) were performed on a Hitachi Limited model S-4800 microscope operating at an accelerating voltage of 3.0 kV. High-resolution transmission electron microscopies (HRTEM) were performed on a JEOL model JEM-2100F microscope. The nitrogen adsorption and desorption isotherms were measured at 77 K using a Bel Japan Inc. model BELSOPR-mini II analyzer, and the samples were degassed at 100 °C for 3 h under vacuum (10−5 bar) before analysis. The pore size distribution was calculated from the adsorption branch with the nonlocal density functional theory (NLDFT). Gas uptake experiments were performed on AutosorbIQ-MP (Quantachrome Instruments). All the gases used are high purity. Cyclic voltammetric experiments were carried out using a CHI 660E electrochemical workstation (CH Instruments, ChenHua, Shanghai, China). The voltammogram was acquired at room temperature at the scan rate of 100 mV s−1. The potentials are reported vs the Fc/Fc+ redox couple as a standard. Contact angles
Scheme 1. Synthetic Routes of the Four Bicarbazole-Based CMPs
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DOI: 10.1021/acs.macromol.7b00971 Macromolecules 2017, 50, 4993−5003
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Macromolecules
Figure 1. (a) Molecular structure, (b) single-crystal structure, and (c) packing mode of Br-BC. dried at 100 °C under vacuum for 24 h to afford a dark brown solid (75 mg, 111% yield). Elemental analysis (wt %) calcd for (C56H24N2)n: C 92.80, H 3.34, N 3.86; found: C 73.27, H 4.06, N 3.61. Dye Removal Tests. Four kinds of organic dyesmethylene blue (MB), rhodamine B (Rh), methyl orange (MO), and congo red (CR)were used in this study. In a typical adsorption experiment, 2 mg of CMP was added to 10 mL of dye (MB, Rh, MO, and CR) aqueous solution (25 mg L−1) and magnetically stirred with the rate of 300 rpm. The suspension was separated by centrifugation at different time points, and the centrifugation time to each time point was 8 min at 14 000 rpm; then UV−vis spectra were recorded with 1 cm cuvettes. On the other hand, 1 mg of CMP was added to 10 mL of dye solution, and the initial dye concentration was varied from 25 to 200 mg L−1; the mixture was stirred for 12 h to obtain the adsorption isotherm by UV−vis spectra with 1 mm cuvettes. The reusability experiment was conducted with 2 mg of CMP-YA in 10 mL of MB solution (25 mg L−1) and magnetically stirred with the rate of 300 rpm for 30 min; then centrifugation and UV−vis spectra were recorded with 1 cm cuvettes. Then the CMP-YA washed with water, ethanol, and acetone to desorb MB from CMP surface and dried in a vacuum for reusability tests. The adsorption isotherms were fitted by using the Langmuir isotherm model and the Freundlich isotherm model. The linear form of Langmuir isotherm model can be expressed as follows:
were measured by a contact angle measuring device (Shanghai Zhongchen Digtal Technology Apparatus Co., Ltd.). Synthesis of CMP-YA by Yamamoto Polymerization. 1,5Cyclooctadiene (cod, 0.1 mL, 0.806 mmol, dried over CaH2) was added to a solution of bis(1,5-cyclooctadiene)nickel(0) ([Ni(cod)2], 222 mg, 0.806 mmol) and 2,2′-bipyridyl (126 mg, 0.806 mmol) in dehydrated DMF (10 mL) under an argon atmosphere, and the mixture was heated at 80 °C for 1 h. To the resulting purple solution was added Br-BC (100 mg, 0.155 mmol), and the mixture was stirred at 80 °C for 72 h to afford deep purple suspensions. After cooling to room temperature, concentrated HCl (4 mL) was added to the mixture. After filtration, the residue was washed with H2O, MeOH, CHCl3, THF, and acetone, extracted by Soxhlet with H2O, methanol, CHCl3, acetone, and THF for 1 day, respectively, and dried at 100 °C under vacuum for 24 h, to afford off white powder (51 mg, 64% yield). Elemental analysis (wt %) calcd for (C24H12N2)n: C 87.79, H 3.68, N 8.53; found: C 80.19, H 4.71, N 7.49. Synthesis of CMP-SU by Suzuki Polymerization. A mixture of Br-BC (80 mg, 0.123 mmol) and 1,4-benzenediboronic acid (41 mg, 0.246 mmol) in DMF (10 mL) was added an aqueous solution of K2CO3 (2.0 M, 1 mL) and tetrakis(triphenylphosphine)palladium(0) (14.2 mg, 10 μmol) under argon and stirred at 120 °C for 24 h. The mixture was allowed to cool at room temperature and poured into water. The precipitate was collected by filtration, thoroughly washed with H2O, MeOH, CHCl3, THF, and acetone, extracted by Soxhlet with H2O, methanol, CHCl3, acetone, and THF for 1 day, respectively, and dried at 100 °C under vacuum for 24 h to afford gray solid (66 mg, 84% yield). Elemental analysis (wt %) calcd for (C48H28N2)n: C 91.11, H 4.46, N 4.43; found: C 79.55, H 4.99, N 4.29. Synthesis of CMP-SO-1B2 by Sonogashira Polymerization. PdCl2(PhCN)2 (4 mg, 0.0093 mmol) and p(o-tolyl)3 (5 mg, 0.0186 mmol) were added to DMF (10 mL) in a flame-dried 50 mL Schlenk flask under argon. The reaction mixture was stirred for 5 min at room temperature under argon. 1,4-Diethynylbenzene (35.2 mg, 0.279 mmol), Br-BC (60 mg, 0.093 mmol), and CuI (4 mg, 0.0186 mmol) were added. After diisopropylamine (5 mL) was added, the reaction mixture was heated at 100 °C with stirring for 48 h under argon. After the reaction mixture was cooled to room temperature, MeOH (4 mL) was added to the mixture. After filtration, the residue was washed with H2O, MeOH, CHCl3, THF, and acetone, extracted by Soxhlet with H2O, methanol, CHCl3, acetone, and THF for 1 day, respectively, and dried at 100 °C under vacuum for 24 h to afford a dark brown solid (73 mg, 108% yield). Elemental analysis (wt %) calcd for (C56H28N2)n: C 92.28, H 3.87, N 3.84; found: C 75.71, H 4.03, N 3.24. Synthesis of CMP-SO-1B3 by Sonogashira Polymerization. PdCl2(PhCN)2 (4 mg, 0.0093 mmol) and p(o-tolyl)3 (5 mg, 0.0186 mmol) were added to DMF (10 mL) in a flame-dried 50 mL Schlenk flask under argon. The reaction mixture was stirred for 5 min at room temperature under argon. 1,3,5-Triethynylbenzene (27.9 mg, 0.186 mmol), Br-BC (60 mg, 0.093 mmol), and CuI (4 mg, 0.0186 mmol) were added. After diisopropylamine (5 mL) was added, the reaction mixture was heated at 100 °C with stirring for 48 h under argon. After the reaction mixture was cooled to room temperature, MeOH (4 mL) was added to the mixture. After filtration, the residue was washed with H2O, MeOH, CHCl3, THF, and acetone, extracted by Soxhlet with H2O, methanol, CHCl3, acetone, and THF for 1 day, respectively, and
Ce C 1 = + e Qe KLQ m Qm The linear form of Freundlich isotherm model can be expressed as follows:
ln Q e = ln KF +
1 ln Ce n
where Qe (mg g−1) is the equilibrium adsorption capacity, Ce (mg L−1) is the equilibrium dye concentration, Qm (mg g−1) is maximum adsorption capacity, KL is the Langmuir constant, and KF and n are the Freundlich constant related to the sorption capacity and sorption intensity, respectively. Contact Angle Measurements. 30 mg of CMPs powder was pressed at 10 t of pressure. 10 μL of water was placed onto the surface of the CMPs. Photographs were taken of this droplet, and the contact angle was measured.
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RESULTS AND DISCUSSION The model monomer N,N′-bicarbazole (BC) and the tetrabromo-substituted monomer 3,3′,6,6′-tetrabromo-N,N′bicarbazole (Br-BC) could be readily synthesized according to literature procedures.58 Three observations regarding the monomer provided initial inspiration for further work toward the corresponding CMPs. First, single-crystal structure of BrBC analyzed by X-ray diffraction is depicted in Figure 1. Different from the planar carbazole, the biscabazole has a rigid skeleton with 87.4° kink within one molecule. The threedimensional structure could prevent efficient packing between the stiff polymer chains and therefore facilitate the formation of 4995
DOI: 10.1021/acs.macromol.7b00971 Macromolecules 2017, 50, 4993−5003
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Figure 2. Solid-state 1H−13C CP/MAS NMR spectra of (a) CMP-YA, (b) CMP-SU, (c) CMP-SO-1B2, and (d) CMP-SO-1B3 recorded at a MAS rate of 10 kHz and a contact time of 2 ms. Asterisks denote spinning side bands.
Figure 3. FE-SEM images of (a) CMP-YA, (b) CMP-SU, (c) CMP-SO-1B2, and (d) CMP-SO-1B3 (all the scale bars are 500 nm) and highresolution TEM of (e) CMP-YA, (f) CMP-SU, (g) CMP-SO-1B2, and (h) CMP-SO-1B3 (all the scale bars are 100 nm).
N,N-Bicarbazole-based CMPs (Car-CMPs) were then synthesized according to the routes shown in Scheme 1. Transition-metal-catalyzed coupling polymerizations, including Yamamoto polymerization allowing for the direct coupling of the tetrabrominated monomer, A2B4 type Suzuki crosscoupling polymerization with phenyldibronic acid, A2B4 type Sonogashira polymerization with 1,4-diethynylbenzene, and
porous nanostructures in polymer architectures. Second, the four C−Br bonds are present in the monomer because of the prevalence of coupling polymerizations in their preparation. Third, the cyclic voltammetric measurement confirms that BC is an electron-rich chromophore (Figure S1), which is expected to be an ideal building block to create electroactive functional polymers. 4996
DOI: 10.1021/acs.macromol.7b00971 Macromolecules 2017, 50, 4993−5003
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Figure 4. (a) UV−vis diffuse reflectance spectra of BC (dot), CMP-YA (black), CMP-SU (red), CMP-SO-1B2 (navy), and CMP-SO-1B3 (cyan) and (b) the pictures of bicarbazole CMPs.
sizes. For example, CMP-YA exhibited spherical particle morphology with particle size around 150 nm. Particles obtained from CMP-SU were smaller and less regular with size around 100 nm. In contrast, CMP-SO-1B2 and CMP-SO1B3 showed irregular morphologies and much smaller particle sizes with diameter less than 50 nm. Furthermore, all the microporous textures of these four CMPs could be directly visible by high resolution transmission electron microscopy (HR-TEM), demonstrating their intrinsic microporosity. As shown in Figure 3 and Figure S5, the CMPs networks showed black nanoparticles uniformly dispersed in the polymer networks, which mainly consisted of catalyst residue of palladium. UV−vis diffuse reflectance spectra of the four Car-CMPs as well as the model monomer N,N′-bicarbazole (BC) were recorded to disclose the conjugation effect on their optophysical property. Unlike the absorption and emission properties of BC in solution which absorbed ultraviolet light between 200 and 340 nm and emitted bright blue fluorescence at 358 nm (Figure S6), the solid state BC displayed a broad reflectance band in the range of 200−400 nm and red-shift of fluorescence emission at 398 and 417 nm, resulting from the aggregation effect in solid state. The UV−vis diffuse reflectance spectra of CMP-YA and CMP-SU resemble that of BC due to the limited conjugation of the corresponding repeating units. In contrast, CMP-SO-1B2 and CMP-SO-1B3 with their dark brownish appearance exhibited a distinguished red-shift in diffuse reflectance band by more than 100 nm compared with those of CMP-YA and CMP-SU, which was reasonable since the introduction of alkynyl group in the repeating unit gave rise to the efficient conjugation extension of the polymer network skeletons (Figure 4). To verify the design principle of building up porous architectures using the perpendicularly arranged dimeric structure as a repeating unit, the porosity properties were systematically characterized. According to the nitrogen adsorption/desorption measurements, the isotherms of all CMPs exhibited typical type I physisorption isotherms with a steep increase of gas uptake in lower P/P0 regions (
