Article pubs.acs.org/cm Rational Design and Synthesis of Porous Polymer Networks: Toward High Surface Area Weigang Lu,† Zhangwen Wei,† Daqiang Yuan,*,‡ Jian Tian,† Stephen Fordham,† and Hong-Cai Zhou*,† † Department of Chemistry, Texas A&M University, College Station, Texas 77842, United States State Key Laboratory of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, P. R. China ‡ S Supporting Information * ABSTRACT: Head-on polymerization of tetrahedral monomers inherently imparts interconnected diamond cages to the resulting framework with each strut widely exposed. We have designed and synthesized a series of 3,3′,5,5′-tetraethynylbiphenyl monomers, in which the two phenyl rings are progressively locked into a nearly perpendicular position by adding substituents of different size at 2, 2′, 6, and 6′ positions, as evident from single crystal structures. Computational simulation suggests that these monomers, though not perfectly regular tetrahedra, could still be self-polymerized into threedimensional frameworks with the same topology. Indeed, five porous polymer networks (PPNs) have been successfully synthesized with these newly designed monomers through Cu(II)-promoted Eglinton homocoupling reaction. Among them, PPN-13 shows exceptionally high Brunauer−Emmett−Teller (BET) surface area of 3420 m2/g. The total hydrogen uptake is 52 mg/g at 40 bar and 77 K, and the total methane uptake is 179 mg/g at 65 bar and 298 K. ■ INTRODUCTION Recent decades have witnessed a rapid growth in the study of porous polymers due to their potential applications in catalysis and gas storage/separation.1 For instance, metal−organic frameworks (MOFs) have greatly challenged our perception of the surface area limit for solid-state materials; the current record holder is NU-110E with a Brunauer−Emmett−Teller (BET) surface area of 7140 m2/g.2 Nevertheless, the susceptibility to exchange reactions of the coordination bonds utilized to support the MOFs undermines their chemical stability in most cases, deterring practical applications under harsh conditions. On the other hand, porous organic polymers (POPs), though amorphous, add new merits, such as low cost and easy processing, to the adsorbents family. More importantly, the robustness of the covalent bonds used to construct the POPs renders most of them with exceptional chemical stability. Thus, they can survive the vigorous posttreatments either required to thoroughly empty the voids or to introduce functionalities into the frameworks. Compared to traditional porous materials, such as zeolites and activated carbons, POPs have more potential of rational design through control of the architecture and function. For example, in the case of polymers with intrinsic microporosity (PIMs), the voids were formed as a direct consequence of the shape and rigidity of the component macromolecules.3 By using reversible co-condensation reactions, covalent organic frameworks (COFs) were synthesized as porous crystalline solids featured with extended periodicity.4 High microporosity and chemical resistance were observed in conjugated microporous © 2014 American Chemical Society polymers (CMPs), in which transition-metal-catalyzed coupling reactions were adopted to generate polymeric frameworks.1d,5 More recently, the surface area have reached new heights in POPs via Yamamoto homocoupling reaction. By using tetrahedral monomer tetrakis(4-bromophenyl)methane, PAF1 (PAF = porous aromatic framework) was synthesized with a BET surface area of 5600 m2/g;6 by using tetrakis(4bromophenyl)silane instead, PPN-4 (PPN = porous polymer network) was synthesized with over 6000 m2/g BET surface area, which was translated into excellent gas storage capacities.7 Close examination reveals several key factors to achieve high surface area in POPs. Size of Monomer. Ideally, one strategy to maximize surface area is to use long and slim organic strut,8 which, however, likely lead to interpenetrated framework;9 if not, structural collapse upon guest solvent removal.8b Although interpenetration has been shown to experimentally stabilize the structure, it is not suggested to be a viable route for improving the surface area. Investigation of the interpenetrated framework indicates that it usually has much less surface accessible to gas molecules than the corresponding noninterpenetrated one.10 Geometry of Monomer. Most POPs are synthesized through kinetic process. The dimensionality of the final product is largely governed by the geometry of the monomer. Linear monomers tend to form polymeric chains; monomers with Received: May 26, 2014 Revised: July 10, 2014 Published: July 20, 2014 4589 dx.doi.org/10.1021/cm501922h | Chem. Mater. 2014, 26, 4589−4597 Chemistry of Materials Article 2′, 6, and 6′ positions, the two phenyl rings are locked into perpendicular positions and four terminal ethynyl groups locate at the corners of a tetrahedron. Computational simulation suggests these monomers, though not perfectly regular tetrahedra, could still be self-polymerized into the same topological frameworks with diamondoid cages. More importantly, the distance from the center of the monomer to the terminal carbon of alkynes is reduced from ∼7.0 Å in TEPM to ∼5.5 Å in 3,3′,5,5′-tetraethynylbiphenyls (Supporting Information Scheme S1), which leads to rather significant decreases in the simulated unit cell parameters (Supporting Information Table S1). For example, the unit cell volume, which is essentially a diamond cage, is reduced from 21 806 Å3 in PPN-1 to 9926 Å3 in PPN-13, greatly diminishing the risk of framework collapsing upon guest-solvent removal, thus, high porosity could be sustained. This hypothesis has been experimentally corroborated; PPN-13, synthesized from 3,3′,5,5′-tetraethynyl-2,2′,4,4′,6,6′-hexamethylbiphenyl through highly efficient Eglinton homocoupling, exhibits an exceptionally high BET surface area of 3420 m2/g. trigonal/square planar geometry form polymeric sheets. In reality, however, they did generate cross-linked polymers, likely due to the contingent bond rotation. Thus, the relationship between monomer’s geometry and polymer’s dimensionality is obscure in most cases. On the other hand, monomers with tetrahedral/octahedral geometry inherently form three-dimensional (3D) frameworks if head-on polymerization is applied. From the architectural point of view, tetrahedral monomers tend to form 3D framework with a diamond-topology, imposed by the monomers themselves, featuring interconnected diamond cages with each strut widely exposed to gas molecules.11 Efficiency of Polymerization. Linear polymers usually can reach high degrees of polymerization because the intermediates are soluble in reaction solutions, which allow active sites to have better chances to continue colliding and reacting. Unlike linear polymers, in which all of the repeating units are surrounded and therefore solvated by solvents, 3D polymers tend to precipitate at much earlier stage possibly due to the less efficient interaction between the highly cross-linked intermediates and the solvents. Once precipitated, the polymeric propagation would be virtually terminated because the chances of effective collision between the active sites on the precipitates and on the monomers in solution would be greatly reduced. To guarantee efficient gas adsorption through accessible surface area, however, degrees of polymerization in thousands or tens of thousands are desired. Thus, it is essential to reach high degrees of polymerization before precipitation. One approach to achieve this is to apply highly efficient reactions, such as Yamamoto homocoupling,6,12 Eglinton homocoupling,11 and azide−alkyne “click chemistry”,13 in which the transition metals are involved in the instantaneous activation of the monomers, leading to the formation of 3D polymers in substantial degrees before breaking apart from the solution. In our previous study, PPN-1 was synthesized with tetrakis(4-ethynylphenyl)methane (TEPM) through Eglinton homocoupling of the terminal alkynes. The as-synthesized PPN-1 was observed dramatic shrinkage upon guest solvent removal (Supporting Information Figure S25), indicating that the framework collapsed due to the magnitude of the formed voids; a consequence of “nature abhors a vacuum”. To prevent framework from collapsing in this case, an intuitive approach is to shorten the arms of the monomer, thus, reduce the size of the subsequently formed diamond-cage. Starting off from TEPM, 3,3-diethynylpenta-1,4-diyne (DEPD) is the only potential monomer (Supporting Information Scheme S1), however, it is extremely air sensitive and synthetically challenging.14 Herein, we designed a series of tetraethynyl monomers with biphenyl as backbone (Scheme 1); By adding substituents at 2, ■ MATERIALS AND METHODS Solvents, reagents, and chemicals were purchased from Sigma-Aldrich and VWR International. Tetrahydrofuran (THF) was distilled from sodium/benzophenone, and triethylamine (TEA) was distilled from calcium hydride under nitrogen prior to use. Solid materials were powdered. All reactions involving moisture sensitive reactants were performed under a nitrogen atmosphere using oven-dried and/or flame-dried glassware. All other solvents, reagents and chemicals were used as purchased unless stated otherwise. 1,3,6,8-tetraethynylpyrene was synthesized according to the literature procedure.15 Fourier transform infrared spectroscopy (FT-IR) data were collected on a SHIMADZU IRAffinity-1 spectrophotometer; the position of an absorption band was given in wave numbers ν in cm−1. Elemental analyses (C and H) were obtained from Canadian Microanalytical Service, Ltd. Melting points were measured on Thomas-Hoover capillary melting point apparatus. Thermogravimetric analyses (TGA) were performed under a nitrogen atmosphere on a SHIMADZU TGA-50 thermogravimetric analyzer; with a heating rate of 3 °C/min. Scanning electron microscopy (SEM) images were taken on a JEOL JSM-6700F SEM. The samples were ground before observation. 1H NMR spectra were recorded on a Mercury (300 MHz) spectrometer as solutions in CDCl3. Chemical shifts are expressed in parts per million (ppm, δ) downfield from tetramethylsilane (TMS) and referenced to CHCl3 (7.26 ppm) as internal standard. All coupling constants (J) are absolute values and expressed in Hertz (Hz). 19F NMR spectra were recorded on a mercury (282 MHz) spectrometer as solutions in CDCl3. Chemical shifts are expressed in parts per million (ppm, δ) without reference. 13C NMR spectra were recorded on a mercury (75 MHz) spectrometer as solutions in CDCl3. Chemical shifts are expressed in parts per million (ppm, δ) downfield from tetramethylsilane (TMS) and referenced to CHCl3 (77.0 ppm) as internal standard. The solid-state NMR spectra were recorded on a Bruker AVANCE 400 spectrometer operating at 100.6 MHz for 13C. The 13C CP/MAS (cross-polarization with magic angle spinning) experiments were carried out at MAS rates of 13 and 10 kHz using densely packed powders of the PPNs in 4 mm ZrO2 rotors. Synthesis of 3,3′,5,5′-Tetraethynylbiphenyl. To a solution of 1,3,5-tribromobenzene (15.7 g, 50 mmol) in 200 mL of anhydrous Scheme 1. Illustration of Formation of a 3D Polymer with Geometrically Constrained Biphenyl Monomer through Head-on Homocoupling Reaction 4590 dx.doi.org/10.1021/cm501922h | Chem. Mater. 2014, 26, 4589−4597 Chemistry of Materials Article diethyl ether at −78 °C under a nitrogen atmosphere, n-butyllithium (2.5 mol/L hexane solution, 22.0 mL, 55 mmol) was added in 15 min. The resulting mixture was stirred at room temperature for another 2 h. After cooled back to −78 °C, anhydrous copper(II) chloride (7.4 g, 55 mmol) was added in portions. The resulting mixture was stirred at room temperature overnight. Then it was filtered; the filtrate was evaporated to dryness. Flash chromatography with hexanes as eluent and recrystallization with ethanol afforded 3,3′,5,5′-tetrabromobiphenyl (4.5 g, 19.2%). 1H NMR (CDCl3, 300 MHz) 7.70 (t, J = 1.8 Hz, 2H), 7.59 (d, J = 1.8 Hz, 4H) ppm. These values are in good agreement with the literature.16 A single crystal was obtained after layering methanol on the top of its dichloromethane solution overnight (see the Supporting Information). Crystal data: C12H6Br4, FW = 469.81, colorless block, monoclinic, space group C2/c, a = 16.691(11), b = 7.314(5), c = 11.161(7) Å, V = 1271.1 (14) Å3, Z = 4, Dc = 2.455 g/cm3, F000 = 872, T = 110(2) K, 6740 reflections collected, 1426 unique (Rint = 0.0479). Final GooF = 1.075, R1 = 0.0289, wR2 = 0.0669. Its cif file can be found in the Supporting Information. To a degassed mixture of anhydrous THF (20 mL) and TEA (20 mL), 3,3′,5,5′-tetrabromobiphenyl (440 mg, 0.94 mmol), tetrakis(triphenylphosphine)palladium(II) (110 mg, 0.095 mmol, 10 mol %), copper(I) iodide (20 mg, 0.11 mmol), and trimethylsilylacetylene (0.60 mL, 4.2 mmol) were added under a nitrogen atmosphere. The resulting mixture was stirred under at 60 °C overnight. After cooled to room temperature, it was filtered; the filtrate was evaporated to dryness under reduced pressure. The residue was then taken up in dichloromethane/methanol/potassium carbonate (20 mL/20 mL/0.80 g). After stirred for 2 h, it was filtered; the filtrate was evaporated to dryness. Flash chromatography with hexanes as eluent afforded 3,3′,5,5′-tetraethynylbiphenyl as a white solid (178 mg, 0.71 mmol, 76% yield over two steps), mp 187−189 °C. 1H NMR (300 MHz, CDCl3) δ: 7.65 (d, J = 1.5 Hz, 2H), 7.61 (t, J = 1.5 Hz, 4H), 3.13 (s, 4H). Anal. calcd for C20H10 (250.29): C, 95.97; H, 4.03. Found: C, 95.91; H, 4.11. Synthesis of 3,3′,5,5′-Tetraethynyl-2,2′,4,4′,6,6′-hexafluorobiphenyl. 2,2′,4,4′,6,6′-Hexafluorobiphenyl was synthesized accord- 8.067(8), b = 23.10(2), c = 4.340(4) Å, V = 808.9(14) Å3, Z = 2, Dc = 3.144 g/cm3, F000 = 676, T = 110(2) K, 7338 reflections collected, 1954 unique (Rint = 0.0427). Final GooF = 1.161, R1 = 0.0261, wR2 = 0.0706. Its cif file can be found in the Supporting Information. To a degassed mixture of anhydrous THF (20 mL) and TEA (20 mL), 2,2′,4,4′,6,6′-hexafluoro-3,3′,5,5′-tetraiodobiphenyl (650 mg, 0.85 mmol), tetrakis(triphenylphosphine)palladium(II) (100 mg, 0.087 mmol, 10 mol %), copper(I) iodide (20 mg, 0.11 mmol), and trimethylsilylacetylene (0.55 mL, 3.8 mmol) were added under a nitrogen atmosphere. The resulting mixture was stirred at 90 °C overnight. After it was cooled down to room temperature, it was filtered; the filtrate was evaporated to dryness under reduced pressure. The residue was then taken up in 10 mL of tetra-n-butylammonium fluoride in THF (1 mol/L). After stirred for one hr, the mixture was filtered; the filtrate was evaporated to dryness. Flash chromatography with 10% dichloromethane/hexanes as eluent afforded 3,3′,5,5′tetraethynyl-2,2′,4,4′,6,6′-hexafluorobiphenyl as a white solid (170 mg, 0.47 mmol, 56% yield over two steps), Temperature for onset of decomposition: 250 °C. 1H NMR (300 MHz, CDCl3) δ: 3.56 (d, J = 0.6 Hz, 4H); 13C NMR (75 MHz, CDCl3) δ: 88.7 (d), 68.4 (s), aromatic carbon signals are embedded in noise. 19F NMR (282 MHz, CDCl3) δ: −98.65 (2F), −101.34 (4F). Anal. calcd for C20H4F6 (358.24): C, 67.05; H, 1.13; F, 31.82. Found: C, 67.24; H, 1.12. Synthesis of 3,3′,5,5′-Tetraethynyl-2,2′,4,4′,6,6′-hexamethylbiphenyl. To a 500 mL three-necked flask containing 1.44 g of magnesium pieces, degassed anhydrous THF (250 mL) and a pinch of iodine were added under a nitrogen atmosphere. The resulting mixture was heated to 80 °C, and then 2-bromomesitylene (7.0 mL) was added dropwise. The reaction mixture was refluxed for another 3 h. After it was cooled down to room temperature, it was transferred into another 500 mL flask containing a mixture of anhydrous FeCl3 (0.22 g), 1,2-dibromoethane (2.4 mL), and anhydrous THF (3.0 mL) under a nitrogen atmosphere. Stirring was continued for another 1 h, and then the reaction was quenched by the addition of 1.0 mol/L aqueous HCl solution (5.0 mL). Organic solvents were evaporated under reduced pressure. The residue was extracted with dichloromethane. The dichloromethane phase was dried over anhydrous MgSO4, and filtered. Most of dichloromethane was removed under reduced pressure, and then methanol was added. Bimesityl was collected as colorless solid. (4.0 g, 73.5% yield). 1H NMR (300 MHz, CDCl3) δ: 6.93 (s, 4H), 2.33 (s, 6H), 1.86 (s, 12H) ppm. To a mixture of bimesityl (2.0 g, 8.4 mmol), solid iodine (3.5 g, 13.5 mmol), HIO4·2H2O (1.55 g, 6.7 mmol) in a 250 mL flask, add CH3COOH/H2O/H2SO4 (120/24/3.6 mL). The resulting mixture was stirred at 90 °C for 3 days. The reaction mixture was diluted with 250 mL of water. The precipitate was filtered, and washed thoroughly with water. The pink solid was collected and dissolved in 100 mL of CHCl3, then washed with saturated Na2S2O3 solution to remove iodine residue. The organic phase was dried over anhydrous MgSO4, filtered, and evaporated under reduced pressure to produce 3,3′,5,5′tetraiodo-2,2′,4,4′,6,6′-hexamethylbiphenyl as a white solid (4.5 g, 72.2% yield). The product was further purified by recrystallization in hexanes/ethyl acetate (5/1) for characterization. 1H NMR (300 MHz, CDCl3) δ: 2.05 (s, 12 H), 3.02 (s, 6 H). Single crystal was obtained after layering methanol on the top of its dichloromethane solution overnight (see the Supporting Information). Crystal data: C18H18I4, FW = 741.92, colorless block, monoclinic, space group P21/n, a = 13.334(3), b = 20.948(4), c = 15.971(3) Å, V = 4062.5(14) Å3, Z = 8, Dc = 2.426 g/cm3, F000 = 2704, T = 110(2) K, 48345 reflections collected, 9693 unique (Rint = 0.0448). Final GooF = 1.046, R1 = 0.0247, wR2 = 0.0467. Its cif file can be found in the Supporting Information. ing to the literature procedure with a good yield.17 To confirm the structure, single crystal was obtained by layering methanol on the top of dichloromethane solution. Crystal data: C12H4F6, FW = 262.15, colorless block, monoclinic, space group C2/c, a = 13.044(5), b = 6.284(3), c = 12.147(6) Å, V = 942.6(7) Å3, Z = 4, Dc = 1.847 g/cm3, F000 = 520, T = 110(2) K, 5634 reflections collected, 1247 unique (Rint = 0.0744). Final GooF = 1.097, R1 = 0.0336, wR2 = 0.0948. Its cif file can be found in the Supporting Information. To a mixture of 2,2′,4,4′,6,6′-hexafluorobiphenyl (1.3 g, 5.0 mmol), solid iodine (3.0 g, 11.8 mmol), and HIO4·2H2O (1.4 g, 6.1 mmol) in a 100 mL round-bottom flask, CH3COOH/H2O/H2SO4 (60/12/1.8 mL) was added. The resulting mixture was refluxed for 3 days. After it was cooled down to room temperature, it was diluted with 200 mL of water. The resulting precipitate was filtered, and washed with water. The pink solid was collected and dissolved in 100 mL of CHCl3, then washed with aqueous Na2S2O3 solution to remove iodine residue (color of solution changed from purple to colorless quickly). The organic phase was dried over anhydrous MgSO4, filtered, and evaporated under reduced pressure to ca. 20 mL. Then 80 mL of methanol was added, and the precipitate was collect as a white solid (3.2 g, 4.1 mmol, 84%). Single crystal was obtained after layering methanol on the top of its dichloromethane solution overnight (see the Supporting Information), and it was confirmed as 2,2′,4,4′,6,6′hexafluoro-3,3′,5,5′-tetraiodobiphenyl. Crystal data: C12 F6I4, FW = 765.72, colorless block, orthorhombic, space group P21212, a = 4591 dx.doi.org/10.1021/cm501922h | Chem. Mater. 2014, 26, 4589−4597 Chemistry of Materials Article tetraethynyl-2,2′,4,4′,6,6′-hexamethylbiphenyl (100 mg, 0.30 mmol) in pyridine (2.0 mL) was added in one portion. The resulting mixture was stirred for 30 min. The precipitate was collected, washed lavishly with methanol and water, and dried in vacuo to give PPN-13 as a brown powder (95 mg, 95% yield). 13C CP/MAS (400 MHz) δ: 134.23 (3C), 118.08, 78.25 (2C), 13.66 (2C). X-ray Single-Crystal Diffraction. Data were collected on a Bruker AXS APEX-II CCD (charge-coupled device) diffractometer with a fine-focus sealed-tube X-ray source (Mo−Kα). The structures were resolved by the direct method and refined by full-matrix leastsquares fitting on F2 by the SHELX-97 software package.19 All nonhydrogen atoms were refined with anisotropic thermal parameters. All the hydrogen atoms were added at geometrically calculated positions and refined as riding on their respective carbon atoms, with Uiso(H) = 1.2Ueq(C), a default treatment for hydrogen atom in SHELX. Creation of PPN models. The theoretical noninterpenetrated frameworks of PPNs were created by repeating the unit of the monomer molecule. The unit cell of PPNs was subject to symmetryconstrained geometry optimization runs based on molecular mechanics simulations and their geometrical structures were optimized using the Forcite Plus module with Universal force field in Material Studio 6.0.20 The successive geometry optimization calculations were performed until the difference between the two successive unit cell dimensions was smaller than 0.001 Å. The porosity and pore volume of these simulated PPN structures were calculated with Platon.21 All the data are shown in Tables 1 and S1 (Supporting Information) for comparison. To a degassed mixture of anhydrous THF (50 mL) and TEA (50 mL), 3,3′,5,5′-tetraiodo-2,2′,4,4′,6,6′-hexamethylbiphenyl (2.0 g, 2.7 mmol), bis(triphenylphosphine)palladium(II) chloride (190 mg, 0.27 mmol), copper(I) iodide (50 mg, 0.28 mmol), and trimethylsilylacetylene (2.0 mL, 14.1 mmol) were added under a nitrogen atmosphere. The resulting mixture was stirred at 90 °C for 3 days. After it was cooled down to room temperature, it was filtered; the filtrate was evaporated to dryness under reduced pressure. The residue was then taken up in dichloromethane/methanol/potassium carbonate (50 mL/ 50 mL/2.0 g) for 2 h. It was filtered; the filtrate was evaporated to dryness. Flash chromatography with hexanes as eluent afforded 3,3′,5,5′-tetraethynyl-2,2′,4,4′,6,6′-hexamethylbiphenyl as a white solid (0.46 g, 51% yield over two steps), mp 157−159 °C. 1H NMR (300 MHz, CDCl3) δ: 3.52 (s, 4H), 2.67 (s, 6H), 2.01 (s, 12H) ppm. 13 C NMR (75 MHz, CDCl3) δ: 142.9, 139.1, 137.0, 120.7, 85.4, 81.3, 20.0, 18.7. Anal. calcd for C26H22 (334.45): C, 93.37; H, 6.63. Found: C, 93.56; H, 6.72. Synthesis of 3,3′,5,5′-Tetraethynyl-2,2′,6,6′-tetramethoxy4,4′-dimethylbiphenyl. 2,2′,6,6′-Tetramethoxy-4,4′-dimethylbiphenyl was synthesized as described in the literature.18 To a mixture of 2,2′,6,6′-tetramethoxy-4,4′-dimethylbiphenyl (2.0 g, 6.6 mmol), solid iodine (3.5 g, 13.5 mmol), and HIO4·2H2O (1.55 g, 6.7 mmol) in a 250 mL flask, CH3COOH/H2O/H2SO4 (120/24/3.6 mL) was added. The resulting mixture was stirred at 120 °C for 3 days. After it was cooled down to room temperature, the reaction mixture was diluted with 250 mL of water. The precipitate was filtered and washed with water. The pink solid was dissolved in 100 mL of CHCl3 and washed with saturated Na2S2O3 solution to remove iodine residue. The CHCl3 phase was evaporated to ca. 20 mL under reduced pressure, and then, 100 mL of methanol was added. 3,3′,5,5′Tetraiodo-2,2′,6,6′-tetramethoxy-4,4′-dimethylbiphenyl was collected as a white solid (4.8 g, 90% yield). 1H NMR (300 MHz, CDCl3) δ: 3.58 (s, 12 H), 2.96 (s, 6 H). 13C NMR (75 MHz, CDCl3) δ: 158.5, 145.6, 121.0, 93.1, 60.8, 36.4. A single crystal was obtained after layering methanol on the top of its dichloromethane solution overnight (see the Supporting Information). Crystal data: C18H18I4O4, FW = 805.92, colorless block, monoclinic, space group Cc, a = 12.997(6), b = 13.035(6), c = 13.301(6) Å, V = 2252.7(19) Å3, Z = 4, Dc = 2.376 g/cm3, F000 = 1480, T = 110(2) K, 12911 reflections collected, 5348 unique (Rint = 0.0582). Final GooF = 1.057, R1 = 0.0399, wR2 = 0.0419. Its cif file can be found in the Supporting Information. To a degassed mixture of anhydrous THF (50 mL) and TEA (50 mL), 3,3′,5,5′-tetraiodo-2,2′,6,6′-tetramethoxy-4,4′-dimethylbiphenyl (2.0 g, 2.5 mmol), bis(triphenylphosphine)palladium(II) chloride (190 mg, 0.27 mmol), copper(I) iodide (50 mg, 0.28 mmol), and trimethylsilylacetylene (2.0 mL, 14.1 mmol) were added under a nitrogen atmosphere. The resulting mixture was stirred at 90 °C for 3 days. After it was cooled down to room temperature, it was filtered; the filtrate was evaporated to dryness under reduced pressure. The residue was then taken up in dichloromethane/methanol/potassium carbonate (50 mL/50 mL/2.0 g) for 2 h. It was filtered; the filtrate was evaporated to dryness. Flash chromatography with gradient elution (from hexanes to 10% ethyl acetate in hexanes) afforded 3,3′,5,5′tetraethynyl-2,2′,6,6′-tetramethoxy-4,4′-dimethylbiphenyl as a white solid (0.75 g, 75% yield over two steps), mp 175−177 °C. 1H NMR (300 MHz, CDCl3) δ: 3.76 (s, 12H), 3.50 (s, 4H), 2.64 (s, 6H) ppm. 13 C NMR (75 MHz, CDCl3) δ: 161.2, 146.9, 119.9, 112.4, 85.4, 78.5, 61.1, 19.5. Anal. calcd for C26H22O4 (398.45): C, 78.37; H, 5.57; O, 16.06. Found: C, 78.01; H, 5.62. General Procedure for the Synthesis of PPNs (Take PPN-13 as an Example). To a clear solution of Cu(OAc)2·H2O (1.2 g, 1.9 mmol) in pyridine (20 mL) at 100 °C, a solution of 3,3′,5,5′- Table 1. Surface Areas, Pore Volumes, and Porosities of Synthesized PPNsa material model space group SLang/SBET/SCalc Vp (exp/calc) porosity (calc) PPN-10 PPN-11 PPN-12 PPN-13 PPN-14 Cmmm P4322 P4322 I4̅2d I41/amd 1332/1128/2210 1551/1742/5677 1551/1742/4329 3966/3420/5703 1910/2160/5100 0.99/0.44 0.92/5.63 1.70/3.88 2.05/3.83 1.25/3.17 41.4% 90.6% 89.3% 84.8% 83.9% a SLang and SBET are experimental Langmuir and BET surface areas (m2/g); SCalc is the calculated Connolly surface area (m2/g); Vp (exp/ calc) are the experimental pore volume (cm3/g) and the pore volume (cm3/g) calculated from the simulated structure; porosity (calc) is the calculated porosity from the simulated structure. Low-Pressure Gas Sorption Measurements. Low pressure (<800 Torr) gas sorption isotherms were measured using a Micrometrics ASAP 2020 surface area and pore size analyzer. Pore size distribution data were calculated from the nitrogen sorption isotherms based on the density functional theory (DFT) model in the Micromeritics ASAP 2020 software package (assuming slit pore geometry). Prior to the measurements, the samples were degassed for 10 h at 80 °C. Ultrahigh purity grade N2, He, H2, and CH4 were used for all measurements. Oil-free vacuum pumps and oil-free pressure regulators were used for all measurements to prevent contamination of the samples during the degassing process and isotherm measurement. High-Pressure Gas Sorption Measurements. High pressure excess adsorption of H2 and CH4 were measured using a HTP1-V Sieverts-type volumetric hydrogen storage analyzer (Hiden) at 77 K (liquid nitrogen bath) or 298 K (room temperature). About 200 mg of sample was loaded into the sample holder. Prior to the measurements, the samples were degassed at 80 °C overnight. The free volume was determined by the expansion of low-pressure He (<5 bar) at room temperature. ■ RESULTS AND DISCUSSION Dihedral Angle. A close-to-90° dihedral angle is key for the biphenyl monomer to adopt a nearly perfect tetrahedral geometry, which is essential to impose wide-open and 4592 dx.doi.org/10.1021/cm501922h | Chem. Mater. 2014, 26, 4589−4597 Chemistry of Materials Article The structures of PPN-13 and PPN-14 have been characterized on a molecular level by solid-state CP/MAS 13 C NMR. The signal assignments for the spectra displayed in Figure 1 were made based on compounds with similar structure interconnected diamond cages to efficiently expose each strut to the gas molecules in the resulting polymeric framework. The dihedral angle of biphenyl, however, is largely dependent on the substituents at 2, 2′, 6, and 6′ positions. To study the correlation between the size of the substituents and dihedral angles, we designed and synthesized a series of 3,3′,5,5′tetraethynylbiphenyl monomers 1−5 (Scheme 2). High-quality Scheme 2. Monomers and Their Corresponding Precursorsa Figure 1. Solid-state CP/MAS 13C NMR spectra of PPN-13 (top) and PPN-14 (bottom); 4 mm, bypass, 10 kHz, field = −2000, p15 = 2000. (inset) Corresponding monomers’ chemical shifts in their 13C NMR spectra in CDCl3. In parentheses are the dihedral angles from single crystal structures. * Average value for the two molecules in one asymmetric unit. a elements reported before,6,7,11,23 as well as a comparison with the solution NMR data of their respective monomers in CDCl3. Overall, the solid-state 13C NMR signals shifted upfield compared to their corresponding monomers’ solution 13C NMR in CDCl3. For example, two methyl carbons, C1 and C2, resonance at 19.98 and 18.66 ppm respectively in the spectrum of PPN-13 monomer’s solution NMR, while they appear overlapped at 13.36 ppm in the solid-state NMR spectrum of PPN-13 (Figure 1 top), which could be attributed to the relatively low resolution of solid-state NMR. For the same reason, aromatic carbons (C6, C7, and C8) are recorded as one broad signal in the solid-state NMR spectrum of PPN-13 (134.23 ppm). Another aromatic carbon C5, however, is well resolved at a relative upfield shift (118.08 ppm) due to the direct shield effect from the alkyne group. Interestingly, two distinctive alkyne-carbon signals, one at 85.42 ppm for the terminal carbon C3 and the other at 81.27 ppm for C4, are observed in monomer’s solution NMR; these two carbons’ signals emerge as one broad peak (78.25 ppm) in the solid-state NMR spectrum of PPN-13, which could be due partly to relatively low resolution of solid-state NMR, and partly to C4 in PPN-13 polymer not being a terminal carbon any more, and therefore its signal shifted upfield. The same assumption can be applied to PPN-14, all the carbon signals in the solid-state spectrum shifted upfield about 4−6 ppm compared to their corresponding carbon signals in monomer’s solution NMR spectrum (Figure 1 bottom). Two distinctive alkyne carbon signals in monomer’s solution NMR, 78.46 and 85.36 ppm, end up as one broad peak (78.68 ppm) in the spectrum of PPN-14’s solid-state NMR. The characteristic resonance at 55.82 ppm in the spectrum of PPN-14 corresponds to the methoxyl carbons at 2, 2′, 6, and 6′ positions, which proves that this functional group stays intact during both the coupling reaction and following activation single crystals were successfully obtained for their precursors (either iodide or bromide; heavy atoms facilitate the single crystal growth and structure solution) by using a liquid/liquid diffusion method. As we can see in Scheme 2, the dihedral angles increase from 37.3° to 57.1° when hydrogens were replaced by fluorines at 2, 2′, 6, and 6′ positions, to 83.5° when replaced by methyl groups, and to almost 90° when replaced by methoxyl groups. Although the dihedral angle from single crystal structure does not necessarily reflect the real conformation of the monomer in the reaction solution, or the real configuration in the polymeric framework, it is correlated to the torsional barrier in principle; the closer-to-90° dihedral angle, the higher torsional barrier. Thus, monomers with a perpendicular position would be dominating in the reaction solution for the framework construction and the probability of deformation of the resulting diamond-cage could be minimized. Chemical Composition and Physical Properties of the PPNs. PPN-10, PPN-11, PPN-12, PPN-13, and PPN-14 (Scheme 2) were synthesized from their corresponding monomers through Eglinton homocoupling reaction. They are brown powders that are insoluble in any common solvents. Unlike PPN-1, which undergoes dramatic shrinkage upon drying and ends up being rock hard solid (Supporting Information Figure S25), these new PPNs remain fluffy powders with a limited shrinkage. SEM images reveal that PPN-13 is comprised of solid spheres with submicrometer dimensions, which is typical for highly cross-linked polymers (Supporting Information Figure S26).22 TGA analyses indicate that these PPNs have lower thermal stability (<300 °C) than PAF-1 (520 °C),6 which is probably due to the intrinsically weak polyyne unit (Supporting Information Figure S20). 4593 dx.doi.org/10.1021/cm501922h | Chem. Mater. 2014, 26, 4589−4597 Chemistry of Materials Article and all of the PPNs show the CC stretch of the disubstituted alkyne at ∼2200 cm−1 (highlighted in gray, Figure 2 right) instead of the CC stretch of the terminal alkyne at ∼2120 cm−1. Porosity of the PPNs. Framework models were built for these PPNs based on the default diamond-topology without taking interpenetration into consideration (Scheme 1). The Connolly surface areas,26 pore volumes, and porosities were calculated using these models (Tables 1 and S1 (Supporting Information). Based on the calculation, PPN-1, PPN-13, and PPN-13′ (built from 3,3-diethynylpenta-1,4-diyne, see Scheme S1 and Table S1 in the Supporting Information for details) show comparable surface areas (over 5000 m2/g). The somewhat smaller surface area of PPN-1 compared to PPN13 reveals the benefit of longer strut for higher surface area is not necessarily valid, especially for the extremely elongated strut. On the other hand, the volumetric surface area of PPN-13 is almost doubled compared to that of PPN-1 based on the calculated densities; low density usually comes as a liability of using the long strut. The porosities of these PPNs were experimentally studied via nitrogen sorption at 77 K (Figure 3). PPN-12 shows type I procedure. Overall, the spectra of PPN-13 and PPN-14 suggest the homogeneity of the materials and the efficiency of the Eglinton coupling. Solid-sate CP/MAS 13C NMR of PPN-11 (Supporting Information Figure S16) is not as informative probably because of its much disordered structure. In the case of nonsubstituted biphenyl, the equilibrium dihedral angle is 44.4° (37.3° measured herein) and the torsional barrier is ca. 1.4 kcal/mol at 0 °C.24 The value is much lower than the energy barrier (ca. 23 to 33 kcal/mol)25 required to prohibit spontaneous conformational isomerization of the substituted biphenyls at room temperature, not to mention the polymerization reactions applied here took place at raised temperatures. Therefore, frameworks constructed from nonsubstituted biphenyls could easily collapse upon solvent removal because of the “free” rotation around the C−C bond connecting the phenyl rings. On the other hand, frameworks constructed from 2,2′,6,6′substituted biphenyls would more likely sustain under the same conditions if the substituents are large enough to prevent “free” rotation, as in the cases of PPN-13 and -14. Solid-sate CP/MAS 13C NMR of PPN-12 (Supporting Information Figure S17) is also not as informative due partly to its disordered structure, as its dihedral angle (57.1°) suggests a low torsional barrier, and partly to the carbon signals being deeply split by two different sets of fluorine groups on the benzene ring. In the 1H NMR spectrum of PPN-12’s monomer, the proton on the alkyne carbon gives a signal at 3.56 ppm, split by fluorine(s) with a coupling constant of 0.6 Hz (Supporting Information Figure S3). In its 13C NMR spectrum, the alkyne carbon directly attached to benzene ring gives a signal at 88.67 ppm, split by adjacent fluorine(s) into multiple peaks; and the other alkyne carbon gives a signal at 68.35 ppm with no obvious splitting (Supporting Information Figure S4). In its 19F NMR spectrum, two sets of fluorine signals were observed with a ratio of 1:2 (Supporting Information Figure S5). FT-IR spectra (Figure 2) also suggest the completion and efficiency of the Eglinton homocoupling reaction. All of the PPNs do not show the terminal alkyne C−H vibrations of the monomers at ∼3300 cm−1 (highlighted in gray, Figure 2 left), Figure 3. Nitrogen sorption isotherms for PPN-10 (red), PPN-11 (black), PPN-12 (magenta), PPN-13 (blue), and PPN-14 (olive) at 77 K; adsorption (filled circle)/desorption (open circle). nitrogen sorption isotherms based on the IUPAC classification, indicating extensive microporosity within the frameworks,27 while other four PPNs display a type I isotherm with type IV character at higher relative pressures, which were confirmed by their respective pore size distributions calculated by using DFT models (Supporting Information Figure S27). It is worth pointing out that the small amount of mesoporosity could be caused by the voids between submicrometer agglomerates in these PPNs, which is rather common in porous organic polymers.28 The surface area data obtained from nitrogen sorption isotherms reveal that PPN-13 has the highest surface area (SBET: 3420 m2/g), followed by PPN-14 (SBET: 2160 m2/ g). BET surface area calculations were carried out by using ASAP 2020 software. Isotherm points chosen to calculate the BET surface area were subject to the three consistency criteria detailed by Walton and Snurr.29 First, the pressure range selected has values of V(Po − P) increasing with P/Po (Supporting Information Figure S28). Second, the points used to calculate the BET surface area are linear with an upward slope (Supporting Information Figure S29). Third, the line has a positive y-intercept (Supporting Information Figure S29). Figure 2. FT-IR spectra of monomers (left) and PPNs (right); PPN10 and its monomer (red), PPN-11 and its monomer (black), PPN-12 and its monomer (magenta), PPN-13 and its monomer (blue), and PPN-14 and its monomer (olive). 4594 dx.doi.org/10.1021/cm501922h | Chem. Mater. 2014, 26, 4589−4597 Chemistry of Materials Article typical for organic polymers lacking special adsorption sites. However, it is larger than those of PPN-4 (4.1 kJ/mol)7 and PAF-1 (4.6 kJ/mol)6 probably because of its relatively small pore size, which allow stronger overall interactions of the guest gas molecuels due to the additional interaction with the opposite wall.34 In addition, the polyyne motifs may help increase the gas affinity as well.35 The heat of adsorption of PPN-10 at zero loading is surprisingly large (8.4 kJ/mol), which could be attributed to the uneven distribution of electron density in the pyrene system, but it drops quickly over the whole gas loading range, indicating only limited amount of hydrogen molecules could interact with the pyrene system. The heat of adsorption of PPN-12 at zero loading is aslo large (8.1 kJ/mol), and it remains almost constant over the whole gas loading range, which could possibly be attributed the high density of fluorine; fluorine is believed to have strong affinity toward gas molecules because of its high electronegativity. The hydrogen affinity difference is more reflected in the hydrogen sorption isotherms at a lower pressure range (less than 0.5 bar) (Supporting Information Figure S21), where the hydrogen adsorption amounts in PPN-10 and PPN-12 rise more steeply. In the higher pressure range, where the surface area and pore volume become dominant, PPN-13 has the highest hydrogenuptake. High-pressure gas sorption was performed on PPN-13 using volumetric measurement method. At 77 K, the total hydrogenuptake of PPN-13 was 52 mg/g (4.9 wt %) at 40 bar (Figure 4). The experimental surface areas for PPN-13 and PPN-14 exhibit the same trend compared to the calculated values (Table 1), but the disparity between experimental values is much larger than that of calculated ones. This could be attributed partly to the larger methoxyl group posing as an obstruction to the polymerization reaction and partly to its the negative electronic effect; the first step of Eglinton homocoupling is deprotonation of the terminal alkyne by pyridine, which is relatively less efficient in the PPN-14’s case because of methoxyl group’s electron donating effect. All cavities in PPN-13 are larger than 1 nm, while the other PPNs have considerable amount of cavities with size less than 1 nm. In the cases of PPN-11 and -12, the relative high ratio of small cavities is presumably due to the framework “cave-in” from the “free” rotation of biphenyl. Applying supramolecular building block (SBB) strategy, MOFs have been demonstrated to achieve noninterpenetration with extended ligands.2,8b,30 Conversely, PPNs tend to interpenetrate because they do not have bulky metal clusters as building blocks; monomers usually are connected through single σ bonds. By reducing the size of the monomer, as in the cases of PPNs reported here, one would expect noninterpenetration. Although we cannot completely rule out the possibility of interpenetration, the comparatively short strut makes it less likely to undergo extensive interpenetration; therefore, large voids and high surface areas can be expected. Many POPs with high surface areas have been synthesized recently.4c,6,7,31 Until now, the highest surface area in POPs came from Yamamoto homocoupling, such as PPN-4 (SBET: 6461 m2/g)7 and PAF-1 (SBET: 5600 m2/g).6 Not only does Yamamoto homocoupling require stoichiometric amount of Ni(COD)2, but also strict anaerobic reaction condition for efficient polymerization in order to achieve high surface area. In Eglinton homocoupling, on the other hand, low-cost Cu(OAc)2 was used, and the reaction was carried out in open air. POPs synthesized from reversible organic condensation reactions also achieved very high surface area, such as COF-102 (SBET: 3472 m2/g)4c and COF-104 (SBET: 4210 m2/g).4c More importantly, high crystallinity observed by PXRD patterns demonstrates a balance can be struck between two competing factors, thermodynamics and kinetics. These COF materials exhibit high thermal stabilities (400−500 °C) because they are entirely constructed from strong covalent bonds. However, the use of relatively susceptible B−O bond could possibly compromise their chemical stabilities. The nitrogen-rich carbonaceous polymer materials (CTF-1) were synthesized under ionothermal conditions at over 400 °C via the formation of polyaryltriazine networks, which present surface areas and porosities up to 3300 m2/g and 2.4 cm3/g, respectively.31g Hydrogen Storage. Hydrogen storage based on physisorption using adsorbents is an immensely important topic in the clean energy area.32 The U.S. Department of Energy (DOE) reset the gravimetric and volumetric storage targets for on-board hydrogen storage for year 2017 (0.055 kg H2/kg system, 0.040 kg H2/L system) and ultimate (0.075 kg H2/kg system, 0.070 kg H2/L system).33 The high porosity of these synthesized PPNs makes them good candidates for this purpose. In the low pressure region, the hydrogen-adsorption capacity is largely controlled by the hydrogen affinity toward the framework, which can be quantified by isosteric heat of adsorption. Variable-temperature measurements reveal an isosteric heat of adsorption of 5.8 kJ/mol for PPN-13 at zero loading (Supporting Information Figure S22); this value is Figure 4. Hydrogen adsorption isotherms of PPN-13 at 77 K. Excess adsorption is the amount of gas molecules stored on surface interacting with the adsorbent. Total adsorption is excess adsorption plus the amount of gas molecules compressed within the pores. Although the value is lower than those of PPN-4 (total uptake: 158 mg/g, 13.6 wt %, 80 bar)7 and PAF-1 (total uptake: 116 mg/g, 10.4 wt %, 48 bar),6 it can be anticipated that the amount of hydrogen trapped in PPN-13 will be much increased at higher pressure. Methane Storage. The need for alternative fuels is greater now than ever before due to the rapid rising levels of atmospheric carbon dioxide produced from burning of fossil fuels. With considerable sources available and low pollution factor, methane is a natural choice as petroleum replacement in the clean energy applications.36 However, efficient storage methods are still lacking to implement the application of methane in the automotive industry. Advanced porous materials, MOFs and POPs, have received considerable attention in sorptive storage applications owing to their 4595 dx.doi.org/10.1021/cm501922h | Chem. Mater. 2014, 26, 4589−4597 Chemistry of Materials Article Among them, PPN-13 exhibits an exceptionally high BET surface area of 3420 m2/g. exceptionally high surface areas and chemically tunable structures.37 Isosteric heats of adsorption of PPNs for methane, calculated with three sets of isotherm data at different temperatures (273, 286, and 295 K), are around 15−20 kJ/mol (Supporting Information Figure S23), which is deemed to be in the appropriate range for methane storage at close to ambient temperatures. The optimal value of heat of adsorption for methane was calculated to be 18.8 kJ/mol.38 Therefore, we tested methane storage capacity for PPN-13. As with hydrogen storage, the gravimetric methane-uptake is directly proportional to the surface area. The excess methaneuptake of PPN-13 is 81 mg/g at 298 K and 65 bar, while the total methane-uptake is 179 mg/g based on the free volume obtained from nitrogen isotherms at 77 K (Figure 5), the value is lower than that of PPN-4 (389 mg/g at 55 bar)7 but is still among the highest reported for organic porous materials. ■ ASSOCIATED CONTENT S Supporting Information * 1 H NMR, 13C NMR, 19F NMR, solid-state NMR, gas sorption isotherm, TGA, SEM, and crystallographic data (CIF). This material is available free of charge via the Internet at http:// pubs.acs.org. ■ AUTHOR INFORMATION Corresponding Authors *E-mail: ydq@fjirsm.ac.cn (D.Y.). *E-mail: zhou@chem.tamu.edu (H.-C.Z.). Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS This work has been financially funded by Office of Naval Research (N00014-13-1-0753). We acknowledge Dr. Vladimir Bakhmoutov for his help with solid-state NMR and the Supercomputing Facility at Texas A&M for Molecular Simulation and other support. D.Y. thanks the National Science Foundation of China (No. 21271172) and the “One Hundred Talent Project” from the Chinese Academy of Sciences. ■ REFERENCES (1) (a) Long, J. R.; Yaghi, O. M. Chem. Soc. Rev. 2009, 38, 1213. (b) Zhou, H.-C.; Long, J. R.; Yaghi, O. M. Chem. Rev. 2012, 112, 673. (c) Wu, D.; Xu, F.; Sun, B.; Fu, R.; He, H.; Matyjaszewski, K. Chem. Rev. 2012, 112, 3959. (d) Dawson, R.; Cooper, A. I.; Adams, D. J. Prog. Polym. Sci. 2012, 37, 530. (e) Ding, S.-Y.; Wang, W. Chem. Soc. Rev. 2013, 42, 548. (2) Farha, O. K.; Eryazici, I.; Jeong, N. C.; Hauser, B. G.; Wilmer, C. E.; Sarjeant, A. A.; Snurr, R. Q.; Nguyen, S. T.; Yazaydın, A. Ö .; Hupp, J. T. J. Am. Chem. Soc. 2012, 134, 15016. (3) (a) McKeown, N. B. ISRN Mater. Sci. 2012, 2012, 16. (b) McKeown, N. B.; Budd, P. M. Macromolecules 2010, 43, 5163. 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