Edge-Functionalized Graphene Nanoribbon Frameworks for the

Article pubs.acs.org/Macromolecules

Edge-Functionalized Graphene Nanoribbon Frameworks for the Capture and Separation of Greenhouse Gases Yearin Byun,† Moses Cho,† Daeok Kim,† Yousung Jung,*,† and Ali Coskun*,†,‡ †

Graduate School of EEWS, Korea Advanced Institute of Science and Technology (KAIST), 291, Daehak-ro, Yuseong-gu, Daejeon 34141 Republic of Korea ‡ Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea S Supporting Information *

ABSTRACT: We demonstrate a bottom-up synthetic approach for the synthesis of graphene nanoribbon frameworks (GNFs) incorporating edge-functionalized graphene nanoribbons via the Diels−Alder cycloaddition polymerization and a subsequent FeCl3-catalyzed cyclodehydrogenation reactions. This approach not only allowed us to precisely position substituents, namely, −OMe (GNF-0), −H (GNF-1), −CF3 (GNF-2), and −F (GNF-3), but also enabled to tune textural properties and gas affinity of resulting frameworks. GNFs exhibited promising physical properties such as high surface areas (up to 755 m2 g−1) and excellent physicochemical and thermal stabilities (up to 400 °C). Narrow pore size distribution and the presence of large aromatic units led to high affinity toward gases such as CO2 (27.4−30.9 kJ mol−1 at 1 bar), CH4 (21.3−26.0 kJ mol−1 at 1 bar), and H2 (6.5−8.2 kJ mol−1 at 1 bar). Notably, GNFs also showed promising CO2/CH4 breakthrough separation performance for natural gas sweetening and landfill gas separations at 298 K. The edge-functionalization of GNFs with −CF3 and −F significantly improved their affinity toward perfluorocarbons and CFCs, which are classified as potent greenhouse gases. Compared to GNF-3, GNF-2 containing −CF3 moieties showed much higher uptake capacity toward CFC-113 (67 wt % at 298 K).

1. INTRODUCTION Porous organic polymers have drawn a great deal of attention in recent years due to their physicochemical stability, high surface area, tunable chemical compositions, and promising gas storage and separation properties.1−3 Importantly, the textural properties and functions of these polymers can be controlled by simply varying the chemical structure of monomeric units and polymerization routes. As such, several well-known polymerization strategies involving either catalytic or stoichiometric amounts of metal catalysts, namely, Sonogashira−Hagihara and Suzuki−Miyura cross-coupling reactions, Yamamoto-type Ullmann reaction, and Friedel−Crafts reaction as well as metal-mediated trimerization of alkynes and nitriles, have been adopted for the preparation of highly cross-linked network polymers.4−6 While these approaches led to the preparation of some high surface area porous polymers with promising gas storage properties, the need for metal catalysts in their synthesis is still a major drawback for their scalability. Moreover, the presence of residual catalysts within the pores also presents a significant challenge. In this context, there has been a continuous interest in metal catalyst-free polymerization strategies for the synthesis of porous organic polymers, i.e., BILPs,7 azo-COPs,8 and BTAPs,9 that can potentially alleviate environmental and economic concerns. The Diels−Alder reaction,10 which involves [4 + 2] cycloaddition between conjugated diene and dienophile, is a powerful tool for the © XXXX American Chemical Society

catalyst-free, highly efficient formation of C−C bonds. For these reasons, it was the reaction of choice for the bottom-up synthesis of polyaromatic hydrocarbons such as graphene nanoribbons (GNRs). GNRs are exciting class of twodimensional carbon materials (2D) with unique electronic, electrical, thermal, and mechanical properties.11 Irreversible agglomeration of GNRs driven by the van der Waals interactions is an important challenge, which limits the processability of GNRs and eventually nullifies their unique properties such as electrical/thermal conductivity.12 One promising direction to address these challenges could be the introduction of permanent pores between the layers.13−17 In this context, we have recently reported18 (Figure 1) on the synthesis of a graphene nanoribbon framework (GNF) using the Diels−Alder cycloaddition polymerization reaction between a dicyclopentanedienone derivative and three-dimensional (3D) arylacetylene core, which led to the formation of a conjugated six-membered ring between two monomers due to the extrusion of a −CO moiety, and a subsequent FeCl3catalyzed cyclo-dehydrogenation step was employed to in situ form GNRs up to ∼2 nm in length and ∼1.1 nm in width within GNFs. Importantly, this approach allowed us to Received: November 16, 2016 Revised: December 23, 2016

A

DOI: 10.1021/acs.macromol.6b02483 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Synthesis of the Edge-Functionalized Graphene Nanoribbon Frameworks via the Diels−Alder Polymerization and FeCl3-Catalyzed Oxidative Cyclodehydrogenation

Figure 1. Graphical representation of a fragment of pre-GNF (left) and its subsequent aromatization (right) via cyclo-dehydrogenation reaction to form edge-functionalized graphene nanoribbons within GNFs.

incorporate large polycyclic aromatic hydrocarbons into the porous organic polymers, which is rather challenging to achieve using conventional polymerization routes due to the low solubility of GNRs in common organic solvents. Importantly, using this strategy, one can achieve edge-selective functionalization of GNRs starting from prefunctionalized monomeric units. Edge-functionalization of GNRs is a powerful tool to not only tune their intrinsic properties, but it could also allow control over the textural properties and gas affinities of resulting GNFs.19 In addition, the synthesis of porous organic polymers via Diels−Alder cycloaddition polymerization could lead to the formation of two isomeric adducts, which in turn can affect the porosity of the resulting polymers. Thus, one approach to control distribution isomers within the GNFs is to tune (Scheme 1) electronic properties of dicyclopentanedienone by varying its substituents.20,21 Notably, it has already been shown that electronic properties of conjugated dienes in the [4 + 2] cycloaddition plays a major role in controlling the regioselectivity of the Diels−Alder reaction.22,23 Accordingly, in an effort to control textural and gas sorption properties of GNFs, here, we synthesized (Scheme 1) GNFs incorporating GNRs with precisely positioned substituents, namely, −OMe (GNF0), −H (GNF-1), −CF3 (GNF-2), and −F (GNF-3). Importantly, decreasing the π-electron density of dicyclopentanedienone led to an increase in the formation of trans isomer as evidenced from the detailed analysis of model compounds, which, in turn, gave rise to higher surface area GNFs (up to 755 m2 g−1). We have also further verified this trans selectivity using density-functional theory (DFT) calculations. It has also been shown that the edgefunctionalization of GNRs within GNFs could affect their affinity toward small gases such as H2, CO2, and CH4. While the narrow pore size distribution and the presence of high πsurface area GNRs led to high isosteric heat of adsorption (Qst) for H2, GNFs also showed promising CO2/CH4 breakthrough separation performance under the simulated natural and landfill gas conditions. Moreover, the edge-functionalization of GNRs with −CF3 and −F within the GNFs significantly improved their affinity toward perfluorocarbons and CFCs. While both GNF-2 and -3 can adsorb CFCs, GNF-2 with higher fluorine content originating from −CF3 moieties showed higher CFC113 uptake capacity of 67 wt % at 298 K. Notably, GNFs

exhibited high thermal stabilities up to 400 °C under an air atmosphere.

2. EXPERIMENTAL SECTION 2.1. General. All chemicals and solvents were purchased from Sigma-Aldrich or SAMCHUN and used without further purification unless otherwise noted. Air-sensitive liquids were transferred via a syringe and were injected into the reaction flask through rubber septa. Melting points were obtained with a TG/DSC Seteram LabSys Evo 1600 in air. 1H NMR and 13C NMR spectra were obtained using Bruker DMX 300 and Bruker Avance 400 NMR instruments, respectively. Mass spectra were obtained on a Bruker autoflex III matrix-assisted laser desorption ionization mass spectrometer. Elemental analyses were performed using a Fisons Instrument EA 1108 CHNS-O elemental analyzer. Single-crystal X-ray diffraction data were collected on a SMART APEX II diffractometer. Solid-state UV− vis spectra of polymers were measured on a Jasco V-570. FT-IR and Raman spectra were recorded on Shimatzu IRTracer-100 and ARAMS dispersive-Raman spectrometers, respectively. X-ray diffraction patterns of the polymers with 2θ ranging from 5° to 80° were obtained using a Rigaku D/MAX-2500 multipurpose high power X-ray diffractometer. All argon, H2, CO2, and CH4 adsorption and desorption isotherms were performed with a Micromeritics Triflex system, and the data were analyzed using the Triflex data analysis software. Gravimetric uptake of CFCs and fluorocarbons was measured using TGA, Shimadzu DTG-60A by passing N2 through CFCs and fluorocarbons. 2.2. Synthesis. Monomers. Dicyclopentanedienone derivatives were obtained by using a previously reported literature report.18 2,8-Di-tert-butyl-4,6,10,12-tetrakis(4-methoxyphenyl)dicyclopenta[e,l]pyrene-5,11-dione, 3a. The crude product was isolated as a dark green powder (64.9 mg, 13%), which was used without further purification: mp (decomposition) >230 °C. 1H NMR (300 MHz, (CD2Cl2, 298 K): δ 7.46 (s, 4H, Ar−H), 7.25 (d, 8H, Ar−H), 6.97 (s, 8H, Ar−H), 3.83 (s, 12H, OCH3), 0.73 (s, 18H, CCH3). FT-IR B


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Macromolecules (powder): 2954, 1694, 1226, 1150 cm−1. δ FD-MS (4.5 kV): calculated for m/z = 842.361 [M+]; found for m/z = 842.271. 2,8-Di-tert-butyl-4,6,10,12-tetraphenyldicyclopentanepyrene5,11-dione, 3b. The crude product was isolated as a dark green powder (136.1 mg, 47%), which was used without further purification: mp (decomposition) >242 °C. 1H NMR (300 MHz, (CD2Cl2, 298 K): δ 7.44 (s, 4H, Ar−H), 7.42 (m, 20H, Ar−H), 0.66 (s, 18H, CCH3). FT-IR (powder): 2957, 2895, 1696, 758, 696 cm−1. δ FD-MS (4.5 kV): calculated for m/z = 745.3083 [M + Na]+; found for m/z = 745.3057. 2,8-Di-tert-butyl-4,6,10,12-tetrakis(4-(trifluoromethyl)phenyl)dicyclopenta[e,l]pyrene-5,11-dione, 3c. The crude product was isolated as a dark green powder (67.2 mg, 17%), which was used without further purification: mp (decomposition) >265 °C. 1H NMR (300 MHz, (CD2Cl2, 298 K): δ 7.74 (d, 8H, Ar−H), 7.51 (d, 8H, Ar− H), 7.26 (s, 4H, Ar−H), 0.65 (s, 18H, CCH3). FT-IR (powder): 2954, 1694, 1312, 1160, 1132, 1125, 1106 cm−1. δ FD-MS (4.5 kV): calculated for m/z = 994.268 [M + Na]+; found for m/z = 994.260. 2,8-Di-tert-butyl-4,6,10,12-tetrakis(4-fluorophenyl)dicyclopenta[e,l]pyrene-5,11-dione, 3d. The crude product was isolated as a dark green powder (72.8 mg, 23%), which was used without further purification: mp (decomposition) >280 °C. 1H NMR (300 MHz, (CD2Cl2, 298 K): δ 7.38 (s, 4H, Ar−H), 7.33 (t, 8H, Ar−H), 7.16 (t, 8H, Ar−H), 0.72 (s, 18H, CCH3). FT-IR (powder): 2954, 1694, 1226, 1150 cm−1. δ FD-MS (4.5 kV): calculated for m/z = 794.281 [M+]; found for m/z = 794.239. Tetrakis(4-ethynylphenyl)tetraphenylmethane, 4. The synthesis was performed using previously reported procedure.24 1H NMR (300 MHz, CDCl3, 298 K): δ 3.05 (s, 4H, CCH); 7.06 (d, 8H, Ar−Hortho, J = 8.5 Hz), 7.32 (d, 8H, Ar−Hmeta, J = 8.5 Hz). FT-IR (powder): 3285, 3031, 1494, 825 cm−1. Model Compounds. General Synthetic Procedure for the Diels− Alder Cycloaddition Reaction of Model Compounds. Dicyclopentanedienone (0.1 mmol) and phenylacetylene (0.2 mmol) were added into a flame-dried Schlenk flask equipped with a magnetic stir bar and a water-cooled reflux condenser. Then 2 mL of o-xylene was added into the flask. The reaction mixture was refluxed in a preheated oil bath. The solvent was evaporated using a rotary evaporator to obtain a crude product as a mixture of isomers. 2,9-Di-tert-butyl-4,7,11,14-tetrakis(4-methoxyphenyl)-5,13diphenyldibenzo[fg,op]tetracene, cis-5, and 2,9-Di-tert-butyl4,7,11,14-tetrakis(4-methoxyphenyl)-5,12-diphenyldibenzo[fg,op]tetracene, trans-5. The crude product was obtained as brown solid and purified via column chromatography (silica, CH2Cl2/hexane, 60/ 40 (v/v)). The product was obtained as a white powder (74% yield): mp (decomposition) >264 °C. 1H NMR (400 MHz, (C6D6, 298 K) for cis-5: δ 8.35 (dd, 4H, Ar−H), 7.83 (s, 2H, Ar−H), 7.35 (t, 4H, Ar− H), 7.08 (m, 14H, Ar−H), 6.85 (t, 4H, Ar−H), 6.64 (t, 4H, Ar−H), 3.36 (s, 3H, OCH3), 3.25 (s, 3H, OCH3 for), 1.06 (s, 9H, CH3), 0.97 (s, 9H, CH3); for trans-5: δ 8.42 (s, 2H, Ar−H), 8.28 (s, 2H, Ar−H), 7.83 (s, 2H, Ar−H), 7.35 (t, 4H, Ar−H), 7.08 (m, 14H, Ar−H), 6.85 (t, 4H, Ar−H), 6.64 (t, 4H, Ar−H), 3.35 (s, 3H, OCH3), 3.24 (s, 3H, OCH3), 1.02 (s, 18H, CH3). FT-IR (powder): 2950, 2903, 2833, 1605, 1510, 1460, 1363, 1284, 1241, 1173, 1107, 1026, 885, 833, 748, 700, 562, 531 cm−1. δ FD-MS (4.5 kV): calculated for m/z = 990.465 [M+]; found for m/z = 990.461. 2,9-Di-tert-butyl-4,5,7,11,13,14-hexaphenyldibenzotetracene, cis-6, and 2,9-Di-tert-butyl-4,5,7,11,12,14-hexaphenyldibenzotetracene, trans-6. The synthesis was performed using previously reported procedure (48% yield):18 mp (decomposition) >320 °C. 1H NMR (400 MHz, C6D6, 298 K) for cis-6: δ 8.27 (dd, 4H, Ar−H), 7.76 (s, 2H, Ar−H), 7.42 (m, 4H, Ar−H), 7.16 (m, 8H, Ar−H), 7.06 (m, 18H Ar−H), 0.99 (s, 9H, CH3), 0.89 (s, 9H, CH3); for trans-6: δ 8.32 (s, 2H, Ar−H), 8.22 (s, 2H, Ar−H), 7.76 (s, 2H, Ar−H), 7.42 (m, 4H, Ar−H), 7.16 (m, 8H, Ar−H), 7.06 (m, 18H Ar−H), 0.94 (s, 18H, CH3). MALDI-TOF MS m/z calculated for m/z = 870.423 [M]+; found m/z = 870.438. 2,9-Di-tert-butyl-5,13-diphenyl-4,7,11,14-tetrakis(4-(trifluoromethyl)phenyl)dibenzo[fg,op]tetracene, cis-7, and 2,9-Di-tertbutyl-5,12-diphenyl-4,7,11,14-tetrakis(4-(trifluoromethyl)phenyl)dibenzo[fg,op]tetracene, trans-7. Purification of the dark red crude

product was achieved by column chromatography (silica, CH2Cl2/ hexane, 25/75 (v/v)). The resulting product was obtained as a white powder in 26% yield: mp (decomposition) >327 °C. 1H NMR (400 MHz, C6D6, 298 K) for cis-7: δ 7.98 (dd, 4H, Ar−H), 7.60 (s, 2H, Ar− H), 7.42 (t, 4H, Ar−H), 7.19 (m, 6H, Ar−H), 6.99 (m, 16H, Ar−H), 0.84 (s, 9H, CH3), 0.76 (s, 9H, CH3); for trans-7: δ 7.98 (dd, 4H, Ar− H), 7.60 (s, 2H, Ar−H), 7.42 (t, 4H, Ar−H), 7.19 (m, 6H, Ar−H), 6.99 (m, 16H, Ar−H), 0.80 (s, 18H, CH3). FT-IR (powder): 2956, 2905, 2866, 1614, 1601, 1406, 1322, 1254, 1165, 1124, 1106, 1066, 1016, 887, 845, 700, 606 cm−1. δ FD-MS (4.5 kV): calculated for m/z = 1142.372 [M+]; found for m/z = 1142.364. 2,9-Di-tert-butyl-4,7,11,14-tetrakis(4-fluorophenyl)-5,13diphenyldibenzo[fg,op]tetracene, cis-8, and 2,9-Di-tert-butyl4,7,11,14-tetrakis(4-fluorophenyl)-5,12-diphenyldibenzo[fg,op]tetracene, trans-8. The crude product was obtained as a dark red powder and purified via column chromatography (silica, CH2Cl2/ hexane, 60/40 (v/v)). The product was obtained as a white powder in 63% yield: mp (decomposition) >330 °C. 1H NMR (400 MHz, C6D6, 298 K) for cis-8: δ 8.14 (dd, 4H, Ar−H), 7.65 (s, 2H, Ar−H), 7.04 (m, 14H, Ar−H), 6.89 (m, 8H, Ar−H), 6.97 (m, 4H, Ar−H), 0.97 (s, 9H, CH3), 0.88 (s, 9H, CH3); for trans-8: δ 8.14 (dd, 4H, Ar−H), 7.65 (s, 2H, Ar−H), 7.04 (m, 14H, Ar−H), 6.89 (m, 8H, Ar−H), 6.97 (m, 4H, Ar−H), 0.93 (s, 18H, CH3). FT-IR (powder): 3049, 2957, 2865, 1599, 1507, 1364, 1218, 1155, 888, 835, 816, 766, 698, 671, 634, 531, 510 cm−1. δ FD-MS (4.5 kV): calculated for m/z = 942.385 [M+]; found for m/z = 942.322. Polymers. General Synthetic Procedure for the Syntheses of preGNFs. Tetrakis(4-ethynylphenyl)tetraphenylmethane, 4 (0.069 mmol), dicyclopentanedienone (0.1 g), and o-xylene were added (70 mM) into a flame-dried Schlenk flask equipped with a magnetic stir bar and a water-cooled reflux condenser. The reaction mixture was refluxed for 24 h under Ar. The desired product was filtered and washed with H2O (2 × 100 mL), methanol (2 × 100 mL), chloroform (2 × 100 mL), and acetone (2 × 100 mL). Then, the solid was subjected to Soxhlet extraction with methanol in order to completely remove trapped oxylene. The desired polymer was obtained after drying under vacuum at 140 °C overnight. pre-GNF-0. The polymer was obtained as a light brown powder (101 mg, 58%) by reacting 3a and 4. FT-IR (powder): 2947.88, 2827.89, 2367.93, 2159.32, 2016.07, 1604.41, 1507.05, 1456.98, 1359.63, 1283.14, 1237.24, 1171.88, 1105.12, 1024.46, 881.21, 825.58, 804.72, 733.78, 697.63, 632.26 cm−1. Anal. Calcd for C132H112O8: C, 86.81; H, 6.18; O, 7.01. Found: C, 83.8; H, 6.63; O, 5.76. pre-GNF-1. The polymer was obtained as dark red powder (109.8 mg, 50%). FT-IR (powder): 2951.09, 1597.06, 1494.83, 1440.83, 1394.53, 1361.74, 1251.80, 1176.58, 1153.43, 1070.49, 1018.41, 881.47, 825.53, 752.24, 617.22, 524.64, 405.05 cm−1. Anal. Calcd for C124H96: C, 93.90; H, 6.10. Found: C, 91.35; H, 6.18. pre-GNF-2. The polymer was obtained as dark red powder (82.5 mg, 51%). FT-IR (powder): 2964.08, 1615.71, 1504.31, 1476.82, 1404.48, 1371.21, 1320.57, 1167.20, 1126.87, 1104.34, 1066.11, 1015.82, 844.53, 824.11, 742.59, 606.20 cm−1. Anal. Calcd for C132H88F24: C, 74.43; H, 4.16; F, 21.41. Found: C, 73.7; H, 12.5. pre-GNF-3. The polymer was obtained as light orange powder (109.5 mg, 61%). FT-IR (powder): 2954.10, 1597.15, 1508.77, 1394.62, 1361.48, 1222.19, 1153.66, 1090.75, 1013.81, 882.94, 836.32, 814.41, 743.08, 551.55, 532.46 cm−1. Anal. Calcd for C124H88O8: C, 86.09; H, 5.13; F, 8.79. Found: C, 83.8; H, 6.63. General Procedure for Cyclodehydrogenation Reaction of preGNFs. pre-GNF (60 mg) was added into a flame-dried Schlenk flask. The FeCl3 (8.17 mmol) dissolved in nitromethane (3 mL) and toluene (1 mL) mixture and purged with Ar gas for 10 min and injected via a septum to the flask containing pre-GNFs. The reaction mixture was stirred for 72 h while Ar was continuously bubbled through the solution. The resulting product was filtered and washed with CH3NO2, 10% HCl, H2O (2 × 100 mL), methanol (2 × 100 mL), chloroform (2 × 100 mL), and acetone (2 × 100 mL) and dried under vacuum at 140 °C overnight. C


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Macromolecules GNF-0. The polymer was obtained in the form of a black powder (27.7 mg). FT-IR (powder): 2950.20, 2827.90, 2363.40, 2159.80, 2027.08, 1972.79, 1707.36, 1601.80, 1508.29, 1452.49, 1365.02, 1221.75, 1165.94, 1083.99, 1022.67, 867.34, 802.49, 736.13 cm−1. Anal. Calcd for C120H58O8: C, 88.55; H, 3.59; O, 7.86. Found: C, 84.1; H, 4.55; O, 8.63. GNF-1. The polymer was obtained in the form of a black powder (40 mg). FT-IR (powder): 2958.80, 2875.86, 1595.13, 1573.91, 1568.13, 1384.89, 1361.74, 1315.45, 1263.37, 1240.23, 1228.66, 1219.01, 1170.79, 1159.22, 759.95, 738.74, 698.23, 455.20 cm−1. Anal. Calcd for C113H43: C, 96.91; H, 3.09. Found: C, 92.96; H, 4.16. GNF-2. The polymer was obtained in the form of a black powder (36.2 mg). FT-IR (powder): 2964.08, 1613.93, 1503.37, 1402.63, 1322.17, 1278.99, 1165.83, 1123.31, 1065.09, 1017.34, 821.09, 730.16, 665.55, 637.00, 606.22, 554.63, 503.05 cm−1. Anal. Calcd for C120H34F24: C, 74.62; H, 1.77; F, 23.61. Found: C, 71.5; H, 3.29. GNF-3. The polymer was obtained in the form of a black powder (30.1 mg). FT-IR (powder): 2958.86, 2361.15, 2161.60, 2014.31, 1977.25, 1601.90, 1504.02, 1463.14, 1367.10, 1222.19, 1194.66, 1153.66, 1063.23, 1016.61, 880.13, 814.41, 753.75, 663.89, 529.65 cm−1. Anal. Calcd for C112H34F8: C, 87.84; H, 2.24; F, 9.92. Found: C, 83.26; H, 3.30.

Scheme 2. Synthesis of the Model Compounds along with the Chemical Structures of Regioisomers, 5−8, Obtained from the Diels−Alder Reaction of Phenylacetylene with Dicyclopentanedienones, 3a−3d

Figure 2. (a) 1H NMR spectra of isomeric mixture of 5 (black) and pure cis-5 (blue). Inset: crystal structure of cis-5. (b) 1H NMR spectra of isomeric mixture of 7 (red) and pure trans-7 (black). Inset: crystal structure of trans-7. 1H NMR spectra of cis-5 and trans-7 were obtained by dissolving their crystals. Red rectangle highlights the chemical shifts for tert-butyl groups.

such, we varied the substituents of dicyclopentadienones in the order of electron donating to electron-withdrawing groups (R = OMe, H, CF3, and F). Then, the Diels−Alder cycloaddition reaction was carried out between these substituted dienes and phenylacetylene at 140 °C in o-xylene for 24 h under an Ar atmosphere. During the course of the reaction, we observed a color change from dark green to orange, which points to the completion of reaction. The crude products, 5−8, were purified and fully characterized (see the Supporting Information). It is important to note that we were unable to separate trans- and cis-isomers of 5−8 due to their similar polarity using silica gel column chromatography. Instead, each compound was subjected to crystallization as a mixture of isomers. We were able to obtain single crystals for cis-5 and trans-7 by employing slow vapor diffusion of isopropyl ether into benzene at room temperature. The chemical structures of cis-5 and trans-7 isomers were unambiguously confirmed (Figure 2) via singlecrystal X-ray analysis. Both structures showed significant bending of pyrene core in order to minimize steric hindrance between tert-butyl groups and neighboring substituted phenyl rings. We also obtained (Figure 2) 1H NMR spectra of both cis5 and trans-7 isomers by dissolving corresponding single crystals. Notably, we have observed significant differences in the chemical shifts of tert-butyl groups. While cis-5 showed two single peaks at 1.06 and 0.97 ppm, trans-5 revealed only one peak at 1.02 ppm. We attributed the presence of two singlets in the case of cis-5 to the different local environments of tert-butyl

3. RESULTS AND DISCUSSION Model Compounds. We have synthesized (Scheme 2) model compounds by reacting dicyclopentadienones (3a−3d) with phenylacetylene to investigate the effect of substituents on the regioselectivity of the Diels−Alder cycloaddition reaction. The reaction between dicyclopentadienones, 3a−3d, and phenylacetylene can lead to the formation of two types of regioisomers, which were identified as cis and trans depending on the relative orientation of phenyl rings highlighted in blue color (Figure 2). It is well established that the presence of the electron-withdrawing substituents on the diene has a strong effect on the regioselectivity of the Diels−Alder reaction.25 As D


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Figure 3. 1H NMR spectra of model compounds as a mixture of regioisomers. Red circle indicates the tert-butyl peak origination from the transisomer, whereas blue and green circles indicate that of the cis-isomer. Integration of these peaks allowed us to precisely determine the trans/cis ratios.

groups as also evidenced from its crystal structure (Figure 2a). These findings also helped us to precisely determine (Figure 3) trans/cis ratios of 5−8 using the splitting patterns of tert-butyl peaks in their 1H NMR spectra. For the cis-isomers, tert-butyl groups located on the pyrene backbone appeared as two singlets: cis-5 (1.06 and 0.97 ppm), cis-6 (0.99 and 0.89 ppm), cis-7 (0.84 and 0.76 ppm), and cis-8 (0.97 and 0.88 ppm). In the case of trans-isomers, tert-butyl groups appeared as a single peak: trans-5 (1.02 ppm), trans-6 (0.94 ppm), trans-7 (0.80 ppm), and trans-8 (0.93 ppm). From the integration of these three peaks, the quantitative determination of cis:trans ratios was achieved. The integration of peak area was only possible for the tert-butyl groups due to the fact that the other peaks have significant overlap which prevents from accurate ratio determination. Interestingly, we observed increasing trans/cis ratio by decreasing the π-electron density of dicyclopentadienones. While compound 5 showed a trans/cis ratio of 45:55, we observed 54:46, 55:45, and 62:38 for the compounds 6, 7, and 8, respectively. We further elaborated on the trans selectivity using DFT calculations, and the calculation details are provided in the Supporting Information. Figure S1 shows the calculation model used to calculate the reaction energies and the transition state (TS) structures for the Diels−Alder cycloaddition reaction (see Supporting Information). In this model, the cycloaddition reaction between dicyclopentadienone and two phenylacetylenes is assumed to be a sequential process. The first reaction is expected to alter the resulting geometry of the second diene for its reaction with phenylacetylene. Therefore, the second reaction is used for the calculation of TS state of Diels−Alder cycloaddition reaction. All of the calculations were performed using the Q-CHEM quantum chemistry package. We summarized the energetic data calculated for cis/trans formation and the corresponding transition state energy in Table 1. The cycloaddition of phenylacetylene as a dienophile determines the type of isomer depending on the orientation of phenyl rings. Importantly, the difference in the calculated reaction energies of cis/trans products for different functional

Table 1. Calculated Reaction Energies and Activation Barriers for the Diels−Alder Reaction between Various Substituted Dienes and Phenylacetylenea functional group

exptl yield (%) cis:trans

−OMe

55:45

−H

46:54

−F

38:62

ΔE‡cis vs ΔE‡trans (ΔΔE‡) (kJ mol−1) 43.5 vs 45.9 (−2.4) 59.5 vs 44.8 (14.7) 75.9 vs 43.9 (32.0)

ΔEcis vs ΔEtrans (ΔΔE) (kJ mol−1) −244.2 vs −245.3 (−1.1) −243.5 vs −246.4 (−2.9) −246.7 vs −246.9 (−0.2)

a The selectivity of cis/trans transition state energies is defined as ΔΔE‡ = ΔE‡cis − ΔE‡trans. The differences of cis/trans product energies are calculated as ΔΔE = ΔEcis − ΔEtrans and reported in parenthesis.

groups were found to be negligible (0−2 kJ mol−1). This result indicates that the observed selectivity of cis/trans isomers arise from the transition state, in which phenylacetylene during the reaction plays an important role in determining regioselectivity. The trend in computed barriers (Table 1) for different functional groups during cycloaddition of phenyl ring is −OMe < −H < −F, which agrees well with the experimental polymerization reaction rates. The [4 + 2] cycloaddition reaction occurs, as widely known, between the HOMO of diene and LUMO of dienophile, and the gap between the latter HOMO and LUMO is one of the main factors to determine the reactivity. In this regard, the increase of activation barrier from −OMe to −F functional could be ascribed (Figure S2) to different extents of HOMO− LUMO interactions between dicyclopentadienone (diene) and phenylacetylene (dienophile). The larger gap between HOMO−LUMO corresponds to a higher activation energy, and the trend shows that HOMO−LUMO gap increases from −OMe to −F, following the order of electron-donating to electron-withdrawing groups, thus suggesting that the substituents of dicyclopentadienone play a decisive role in determining TS energies by controlling the level of HOMO E


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backward bending of side benzene group relative to the pyrene core which results in the steric repulsion between freely rotating methoxy functional group (two methoxy groups bended relative to the pyrene core at the same time), the left side, and the tert-butyl group. Thus, the Diels−Alder cycloaddition reaction is kinetically regulated, and the selectivity of isomers is ultimately determined by steric interactions between dicyclopentadienone and phenylacetylene. Synthesis and Characterization of GNFs. GNFs were synthesized by the Diels−Alder cycloaddition polymerization reaction of dicyclopentadienones, 3a−3d, and tetrakis(4ethynylphenyl)methane and subsequent FeCl3-catalyzed intramolecular cyclo-dehydrogenation and de-tert-butylation reactions. The polymerization reaction was carried out in o-xylene at 140 °C. The color change from dark green to dark red during the course of the reactions indicated the completion of the Diels−Alder polymerization to form corresponding pre-GNFs. The rate of polymerization reaction decreased with rising electron-withdrawing strength of substituents as well as their bulkiness (R = OMe > H > F). For example, pre-GNF-0 is formed in less than 5 min, whereas pre-GNF-1 and -3 took 1 and 24 h, respectively. For the pre-GNF-2, however, the reaction ended after 3 days, presumably due to the bulkiness of −CF3 moieties along with their electron-withdrawing nature. Subsequently, pre-GNFs were graphitized to form GNFs through FeCl3-catalyzed cyclo-dehydrogenation (Scholl reaction) and de-tert-butylation reactions. After the thorough washing using 10% HCl until the clear solution comes out, the absence of iron was confirmed by ICP-MS analysis, which showed negligible iron content within GNFs. The formation of GNFs incorporating edge-functionalized GNRs was confirmed by various analytical techniques, including Fourier transform infrared spectroscopy (FT-IR), solid-state cross-polarization magic-angle spinning (CP-MAS) 13C NMR spectroscopy, powder X-ray diffraction (PXRD), and Raman spectroscopy. We also carried out solid-state UV−vis spectroscopy and thermogravimetric analysis (TGA) for GNFs (see Supporting

of diene. The computed geometries at the transition states show that the symmetry allowed [4 + 2] cycloaddition of the present reactions occurs in a concerted manner (i.e., in a single step) as expected from the Woodward−Hoffmann rules, yet in an asymmetric pathway (i.e., one bond forming first with the other bond formation followed).26 Further examinations indicate that the selectivity toward the trans-isomer for −H and −F substituents is attributed to the steric hindrance of phenyl rings between phenylacetylene and dicyclopentadienone core (Figure 4). On the other hand, −OMe is shown to be cis

Figure 4. Geometry of transition state structures for cis/trans-isomers for the substituted dienes. The TS structure from different view for −OMe is given in Figure S3 of the Supporting Information.

selective, which could be due to the higher crowding of methoxy groups with its neighboring tert-butyl groups in trans isomer. The higher crowding of methoxy groups is due to the

Figure 5. Spectroscopic characterization of GNFs: (a) FT-IR spectra, (b) solid-state CP-MAS patterns, and (d) Raman spectra measured at 785 nm (1.58 eV) on a powder sample. F

13

C NMR spectra, (c) powder X-ray diffraction DOI: 10.1021/acs.macromol.6b02483 Macromolecules XXXX, XXX, XXX−XXX

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levels of GNFs were investigated based on the peak intensity of the D band relative to the G band (ID/IG). The minimum value of ID/IG was found from GNF-1 (ID/IG = 0.5). This value is comparable with high quality reduced GO28 (ID/IG ≥ 0.5), which indicates that GNF-1 possess the highest quality of GNRs. In the case of GNF-2 and GNF-3, the ID/IG ratio increased to 0.6. The slight increase in the D band intensity was expected since the edge of GNRs are also introduced into defects by the substitution of functional groups. For GNF-0, there was a substantial increase in the ratio (ID/IG = 1.2). The observed large intensity of the D band as compared to the G band is attributed to the defects created by the edgefunctionalization as well as the higher ratio of cis-isomer, which could favor the defect formation within the network. The extent of graphitization of GNFs was also probed by using solid-state UV−vis spectroscopy in the range between 200 and 800 nm (Figure S8). While the UV−vis spectra of pre-GNFs showed absorption maximum at ∼410 nm, upon graphitization, GNFs showed a very strong absorbance in the entire UV−vis region, thus proving successful formation of GNRs.29 Furthermore, the color change from orange/red to black also indicates the successful formation of GNRs within the GNFs. We also investigated thermal stability of GNFs using TGA analysis (Figure S9), wherein all the GNFs showed exceptional thermal stabilities up to 400 °C under air atmosphere. Gas Sorption Studies of GNFs. Low-pressure Ar sorption analysis was performed at 87 K in order to investigate the textural properties of GNFs (Figure 6). All the samples were

Information). The extent of polymerization and the presence of functional groups were verified (Figures S3−S7) using FT-IR spectroscopy. The complete disappearance of both alkyne stretching band at 3285 cm−1 and −CO stretching band at 1686 cm−1 indicates clearly the consumption of monomers during the polymerization reaction. Notably, after the graphitization of pre-GNFs, the peaks arising from the C−H stretching band at 3024 and 3053 cm−1 decreased. Unlike GNF-1, all the other GNFs exhibited sharp peaks below 1500 cm−1, which points to the presence of substituents. For GNF-0, we observed C−H stretching of methoxy group at 2827 cm−1 and the C−O stretching band in the range of 1238−1023 cm−1. GNF-3 exhibited asymmetric and symmetric CF3 stretching vibrations in the range of 1317−1100 cm −1 , and CF 3 deformation vibrations were observed at 666, 637, and 606 cm−1, which clearly proves the presence of −CF3 functional groups. The FT-IR spectrum of GNF-3 showed aromatic C−F stretching bands at 1218−1154 cm−1 that clearly indicate the presence of −F groups. Furthermore, the bands observed in the fingerprint region at 880−802 cm−1 are likely to be originating from the C−H bonds located at the edges of GNRs. Solid-state 13C NMR spectroscopy (Figure 5b) analysis was employed to probe molecular connectivity and integrity of GNFs. The solid-state CP-MAS 13C NMR spectrum of GNF-0 revealed chemical shifts located at 154.2 and 110.5 ppm originating from the Cα and Cβ carbon atoms of methoxysubstituted ring. In addition, the carbon peaks of methoxy group located at 51.1 ppm also further proves successful formation of −OMe-substituted GNRs. The CF3 carbon peak of GNF-2, which was expected to appear at ∼120 ppm, was found to overlap with the aromatic carbon peaks located at about 124 ppm. In the case of GNF-3, the peaks at 157.2 and 112.5 ppm represent aromatic Cα and Cβ carbon atoms attached to the fluoro group. Furthermore, for all the GNFs, the presence of a broad carbon peak located at ∼125 ppm indicates the formation of GNRs. The carbon peak located at ∼60 ppm represents the quaternary carbon core of tetraphenylmethane. More importantly, the complete disappearance tertbutyl carbon peaks indicates the removal of tert-butyl groups via retro-Friedel−Crafts reaction. In order to assess the crystallinity and characterize the local environment of GNRs within GNFs, PXRD measurements were performed (Figure 5c). The PXRD patterns of GNF-1, GNF-2, and GNF-3 revealed strong broad features at 2Θ = ∼19°, which could be attributed to the π−π stacking interactions between GNRs with the interlayer separations of 4.5, 4.9, and 4.7 Å for GNF-1, GNF-2, and GNF-3, respectively. It is, however, important to note that all the GNFs were found to be amorphous presumably due to the −CO extrusion step, which acts as an irreversible kinetic trap for the Diels−Alder cycloaddition reaction and promotes amorphous nature of the GNFs. Evidently, GNF-0 did not show any features due to its much faster kinetics compared to all the other GNFs. These results point to the fact that the slower reaction kinetics especially for the Diels−Alder cycloaddition reaction promotes the crystallinity of frameworks. Raman spectroscopy (Figure 5d) is a powerful tool to study the formation of GNRs within the GNFs by probing the graphitization of GNRs with the first order D (defect) and G peaks.27 The Raman spectra of GNF-0−3 showed the D band at 1332, 1338, 1327, and 1324 cm−1, respectively, originating from in-plane breathing mode of A1g symmetry caused by 6fold aromatic rings. The G bands of GNF-0−3 were observed at 1600, 1606, 1561, and 1589 cm−1, respectively. The defect

Figure 6. Argon uptake measurements of GNFs at 87 K. Filled and empty symbols represent gas adsorption and desorption, respectively. The surface areas were calculated based on the data points in the relative pressure range of 0.01−0.10, obtained from the Rouquerol plots. Inset: NLDFT pore-size distributions of GNFs.

activated at 140 °C for 5 h prior to the measurements. Ar isotherms of GNFs showed typical type I reversible sorption profile, indicating that the materials predominantly contain micropores. The hysteresis was observed upon desorption for all GNFs in the entire pressure range, which could be attributed to the swelling of sorbent.24,30,31 The surface areas of pre-GNFs (Figure S10, Table S1) were calculated to be 139, 457, 600, and 679 m2 g−1 for pre-GNF-0, pre-GNF-1, pre-GNF-2, and preGNF-3, respectively. Upon graphitization reaction, we observed G


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Table 2. BET Surface Areas and H2, CO2, and CH4 Uptake Capacities along with Their Isosteric Heats of Adsorption (Qst) Values at Zero Coverage for the GNFs H2 adsorption (wt %)

CH4 adsorption (mg g−1)

CO2 adsorption (mg g−1)

polymer

BET, Ar (m2 g−1)

micropore area (m2 g−1)

micropore vol (cm3 g−1)

average pore diam (nm)

77 K

87 K

Qst for H2 (kJ mol−1)

273 K

298 K

Qst for CO2 (kJ mol−1)

273 K

298 K

Qst for CH4 (kJ mol−1)

GNF-0 GNF-1 GNF-2 GNF-3

492 721 731 755

348 430 395 420

0.12 0.16 0.12 0.13

0.59 0.58 0.59 0.59

0.47 1.03 0.73 0.86

0.33 0.77 0.48 0.62

7.4 6.5 8.2 7.5

84.5 94.6 86.0 105.9

43.1 53.3 41.1 65.8

30.9 28.7 27.9 27.4

9.7 11.5 8.4 10.7

5.1 5.4 4.6 6.0

26.0 24.1 21.9 21.3

Figure 7. Gas adsorption isotherms of GNFs: (a) CO2/CH4 adsorption isotherms at 273 K and (b) at 298 K. (c) Hydrogen adsorption isotherms at 77 K.

those of GNF-2 and -3. Interestingly, GNF-1 showed the highest micropore surface area of 430 m2 g−1 among all the GNFs, thus indicating that efficient packing of GNRs could also increase (Table 2) the micropore surface area of GNFs. The second reason could be the electron-deficient nature of GNRs within GNF-2 and -3, which could decrease their affinity to undergo π−π stacking interactions.33 Moreover, in addition to the above-mentioned factors, we also believe that increasing trans/cis ratio of GNRs contributes positively to the resulting surface areas of GNFs.34 Evidently, the BET surface areas calculated for GNFs showed the direct correlation with trans/cis ratio. As shown for the model reaction, the Diels−Alder reaction was found to be regioselective, favoring the sterically less demanding trans isomer as the electron-withdrawing strength of substituents increase, thus resulting in higher surface areas. Although the difference for trans selectivity between model compounds 5 and 6 was only 1%, the difference in the surface areas of GNF-1 and -2 was found to be ∼10 m2 g−1, thus indicating that the trans:cis ratio is increased during the polymerization reaction. These results collectively suggest that the trans/cis selectivity of the Diels−Alder polymerization reaction is a significant factor affecting the textural properties of GNFs. Promising physical properties of GNFs such as high surface areas (up to 755 m2 g−1) and good physicochemical and thermal stabilities prompted us to investigate their gas adsorption properties toward CO2, CH4, H2, and CFCs. We first investigated (Figure 7a,b) CO2 uptake properties of GNFs at 273 and 298 K. The CO2 uptake capacities of GNFs were found to be in the range of 84.5−105.9 mg g−1 at 273 K and 43.1−65.8 mg g−1 at 298 K. At both temperatures, GNF-3 showed the highest CO2 uptake capacity, possibly due to its higher surface area as well as microporosity. The CO2 uptake capacity of GNF-3 was found to be higher than some of the high surface area carbon materials such as AC (SA = 1762 m2

an increase in surface area as well as micropore surface area, which is attributed to the successful graphitization, de-tertbutylation, and the consequent increase in the accessibility of pores. The surface areas of GNFs were calculated from the Brunauer−Emmett−Teller (BET) model (Table 2), in which the pressure ranges were determined from the corresponding Rouquerol plots (Figure S11). As shown previously, the BET surface areas of porous organic polymers directly affected by the size of substituents as the bulky substituents are expected to fill the void space and decrease the resulting surface area.32,14 GNF-0 incorporating bulky −OMe substituents and the highest cis ratio displayed the lowest surface area of 492 m2 g−1 among all the GNFs. Interestingly, the graphitization of pre-GNF-0 to GNF-0 resulted in a significant increase in its surface area from 139 to 492 m2 g−1 and also micropore surface area from 19 to 348 m2 g−1. We attribute this result to the extremely fast reaction kinetics of pre-GNF-0, which could lead to the formation of an amorphous, highly interpenetrated network structure with reduced void space, which limits the accessibility of the pores. In addition, the higher cis-content of pre-GNF-0 compared to other pre-GNFs along with the presence bulky methoxy groups could also contribute to the blocking of the pores. Compared to methoxy groups, CF3 has a smaller van der Waals radius (Vw of CF3 = 2.00 Å) and the resulting GNF-2 presented higher surface area of 731 m2 g−1. Additionally, the fluoro substituent with van der Waals radius of 1.35 Å displayed much increased surface area of 755 m2 g−1. While GNF-0, GNF-2, and GNF-3 exhibited a correlation between the functional group size and surface area, this trend was not observed for GNF-1, which is substituted with hydrogen. In order to explain this interesting phenomenon, we hypothesized two main reasons. The first reason is the strong π−π stacking interactions between GNRs, which could limit the accessibility of gas molecules. From the PXRD analysis, the interlayer distance of GNF-1 was actually calculated to be lower than H


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Macromolecules g−1, 73.1 mg g−1), meso-carbon (SA = 798 m2 g−1, 66.0 mg g−1), and MWCNT (SA = 407 m2 g−1, 76.1 mg g−1).35 We also calculated the isosteric heats of adsorption (Qst) of CO2 for GNFs (Figure S13 and Table S2) from the dual-site Langmuir parameters at 273 and 298 K by using the Clausius−Clapeyron equation. The Qst values of GNFs were found to be in the range of 27.4−30.9 kJ mol−1, which are below the values expected for a chemisorption process (>40 kJ mol−1). Therefore, the affinity of CO2 toward GNFs is mainly attributed to the inherent surface area, microporosity created by large π-surface area upon graphitization, and also the functional groups.36 CO2/CH4 separation from large landfill or natural gas sources is highly attractive since methane is considered as a cleaner energy source compared to the other fossil fuels. Therefore, we also investigated (Figure 7a,b) the affinity of GNFs toward CH4 at 273 and 298 K. The Qst values of CH4 adsorption in GNFs were in the range of 21.3−26.0 kJ mol−1, calculated based on single-site Langmuir parameters at 273 and 298 K (Figure S14 and Table S3). The high Qst of CH4 is attributed to the C−H/π interactions between methane and GNRs.37 The CH4 uptake capacity of GNFs was found to be in the range of 8.4−10.7 mg g−1 at 273 K and 4.6−6.0 mg g−1 at 298 K. The CH4 uptake capacity of GNFs was found to be significantly lower compared to CO2, which prompted us to investigate the CO2/CH4 adsorption selectivities of GNFs. The ideal adsorbed solution theory (IAST) is a powerful tool to predict selectivities for gas mixtures from single-component gas isotherms. We used (Figures S16 and 17, Table S5) IAST to calculate the selectivity of CO2 over CH4 at 273 and 298 K for the gas mixture compositions similar to those of landfill gas (CO2/CH4 = 50:50) and natural gas (CO2/CH4 = 5:95). The IAST CO2/CH4 selectivity for a landfill gas was found to be 5.3−7.1 at 273 K and 1 bar (Figure S16). With the rising temperature, the selectivity decreased to 4.3−5.8 at 298 K and 1 bar. At both temperatures, GNF-3 displayed the highest selectivity of 7.1 at 273 K and 5.8 at 298 K. For the natural gas sweetening (CO2/CH4 = 5:95), the IAST selectivities of GNFs were found to be 4.7−7.6 at 273 K and 4.1−5.5 at 298 K (Figure S17). These values are higher than the activated carbon (3.7)38 and also comparable to the nitrogen-containing nanoporous polymers such as ALP (4−8).39 In order to elucidate the real CO2/CH4 separation performance of GNFs, binary breakthrough experiments were conducted (for a schematic diagram of our experimental breakthrough setup, see Figure S18). All the GNFs showed complete removal of CO2 under the simulated landfill gas conditions with breakthrough storage capacities of 0.17, 0.25, 0.10, and 1.1 mmol g−1 for GNF-0, GNF-1, GNF-2, and GNF-3, respectively (Figure S19). Under the natural gas conditions, GNFs exhibited CO2 storage capacities of 0.9, 0.2, and 0.6 mmol g−1 for GNF-1, GNF-2, and GNF-3, respectively (Figure S20). The high π-surface area and narrow pore size distribution of GNFs prompted us to investigate their affinity toward H2 gas. The volumetric measurements of H2 sorption isotherms of GNFs at 77 K is represented in Figure 7c. The H2 uptake capacities of 0.47, 1.03, 0.73, and 0.86 wt % was observed at 77 K for GNF-0, GNF-1, GNF-2, and GNF-3, respectively. At 87 K, the uptake capacities decreased to 0.33, 0.77, 0.48, and 0.62 wt %. While the most previous studies showed higher H2 uptake with increasing surface areas,40 a recent study by Nijkamp et al.41 suggested that rather than total porosity, micropore surface area is the dominant factor for high affinity. Accordingly, we found that H2 uptake capacity of GNFs

increased in the order of increasing micropore surface area. GNF-3 with the highest surface area exhibited H2 uptake capacities of 0.86 and 0.62 wt % while GNF-1 with the highest micropore area displayed the highest H2 uptake of 1.03 and 0.77 wt % at 77 and 87 K, respectively. We also calculated the Qst of H2 from the adsorption data at 77 and 87 K by using the Clausius−Clapeyron equation (Figure S15). Highly fluorinated GNF-2 exhibited highest adsorption affinity at 8.2 kJ mol−1 toward hydrogen, whereas GNF-3 showed slightly lower affinity (Qst = 7.5 kJ mol−1). We attribute the increase in the Qst value to the increased fluorine content as the electronegative fluorine atoms can create a highly polarized surfaces, which could lead to stronger interaction with H2 molecules.42 GNFs exhibited high H2 adsorption enthalpy of 6.5−8.2 kJ mol−1, with GNF-2 being the highest. Remarkably, the Qst value of GNF-2 was found to be higher than some of the representative MOFs such as ZIF-8 (4.5 kJ mol−1), MOF-5 (5.3 kJ mol−1), and MIL-100 (6.3 kJ mol−1)43 and also partially fluorinated coordination polymers such as PCP (6.5−8 kJ mol−1)44 and Zn(bpe) (tftpa)· cyclohexanone (6.2 kJ mol−1).45 Moreover, these Qst values were higher compared to other carbon-based materials such as activated carbon AX-21 (4.66 kJ mol−1), BCMBPs (6−7.5 kJ mol−1),40 and PAFs (4.6−6.6 kJ mol−1).46 These results point to the positive impact of fluorine atoms and GNRs on the H2 uptake capacity of porous organic polymers. We also note that increasing the surface area of GNFs while retaining their microporosity could further boost their H2 uptake capacities. As porous fluorinated hydrophobic materials, GNF-2 and GNF-3 could possess high adsorption ability for CFCs and fluorocarbons. The capture of CFCs and fluorocarbons is particularly important (Table 3) since they are considered as Table 3. Boiling Points and 100 year Global Warming Potentials of Halogenated Guests and Sorption Capacities for GNF-2 and GNF-3 toward the Guest Molecules uptake (wt %) halogenated guests Cl2FC−CClF2 (CFC113) perfluorohexane CF3CH2F (R-134a)

bp (°C)

100 year GWP (vs CO2)

GNF-2

GNF-3

48

6130

67

55

56 −26.3

9300 1430

46 13

32 11

the vital cause of increased stratospheric halogens in the ozone layer with high global warming potential and long lifetime.47 The uptake of liquid and gas guests was measured using TGA as depicted in Figure S21. We investigated the gravimetric uptake performance of GNF-2 and -3 for CFC-113 (Figure 8), perfluorohexane (Figure S22), and R134a (Figure S23). Since CFC-113 and perfluorohexane are in liquid form at room temperature, we used N2 as a carrier gas, whereas R134a was used directly from a gas tank. The activation of the samples was achieved by keeping the samples at 140 °C for 1 h under the continuous flow of nitrogen. The temperature was then lowered to 25 °C, and the gas inlet was switched to the guest of interest for the gravimetric uptake measurements. While GNF-2 showed the uptake capacity of 67 wt % for CFC113, GNF-3 exhibited slightly lower uptake capacity of 55 wt %. Notably, these values are comparable to the recently reported porous noncovalent organic frameworks (64.9−65.6 wt %) with higher surface areas and fluorine contents, thus showing the high affinity of GNFs toward CFCs. The perfluorohexane I


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selectivity from CO2/CH4 mixture. GNFs with −CF3 or −F moieties also showed high affinities toward ozone-depleting substances such as perfluorocarbons and CFCs. We believe that this approach can be extended for the preparation of wide range of GNFs with catalytic activities as well as physical and chemical properties, for which porosity, hydrophilic/hydrophobicity, and thermal/electrical conductivity can be tuned by simply varying the monomers for the desired application.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02483. Experimental details, FT-IR spectra, thermogravimetric analysis, isosteric heats of adsorption, IAST selectivity graphs, breakthrough graphs and fluorocarbon uptake experiments, and crystallographic data (PDF) Crystallographic data for cis-5 (CIF) Crystallographic data for trans-7 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (Y.J.). *E-mail [email protected] (A.C.). ORCID

Ali Coskun: 0000-0002-4760-1546 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MEST) (NRF-2014R1A4A1003712, NRF2015R1A2A1A15055539, and BK21 PLUS program).

Figure 8. (a) Temperature program used for degassing and CFC-113 uptake experiments of GNFs. (b) Adsorption profile of CFC-113 for (b) GNF-2 and (c) GNF-3. Black and blue lines indicate N2 atmosphere and CFC-113 vapor for which N2 gas used as a carrier, respectively.

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REFERENCES

(1) Dawson, R.; Cooper, A. I.; Adams, D. J. Nanoporous organic polymer networks. Prog. Polym. Sci. 2012, 37, 530−563. (2) Chen, T. H.; Popov, I.; Kaveevivitchai, W.; Chuang, Y. C.; Chen, Y. S.; Daugulis, O.; Jacobson, A. J.; Miljanic, O. S. Thermally robust and porous noncovalent organic framework with high affinity for fluorocarbons and CFCs. Nat. Commun. 2014, 5, 5131. (3) El-Kaderi, H. M.; Hunt, J. R.; Mendoza-Cortes, J. L.; Cote, A. P.; Taylor, R. E.; O’Keeffe, M.; Yaghi, O. M. Designed synthesis of 3D covalent organic frameworks. Science 2007, 316, 268−272. (4) Ben, T.; Ren, H.; Ma, S. Q.; Cao, D. P.; Lan, J. H.; Jing, X. F.; Wang, W. C.; Xu, J.; Deng, F.; Simmons, J. M.; Qiu, S. L.; Zhu, G. S. Targeted synthesis of a porous aromatic framework with high stability and exceptionally high surface area. Angew. Chem., Int. Ed. 2009, 48, 9457−9460. (5) Islamoglu, T.; Kim, T.; Kahveci, Z.; El-Kadri, O. M.; El-Kaderi, H. M. Systematic Postsynthetic Modification of Nanoporous Organic Frameworks for Enhanced CO2 Capture from Flue Gas and Landfill Gas. J. Phys. Chem. C 2016, 120, 2592−2599. (6) Xu, Y. H.; Jin, S. B.; Xu, H.; Nagai, A.; Jiang, D. L. Conjugated microporous polymers: design, synthesis and application. Chem. Soc. Rev. 2013, 42, 8012−8031. (7) Rabbani, M. G.; El-Kaderi, H. M. Template-free synthesis of a highly porous benzimidazole-linked polymer for CO2 capture and H2 storage. Chem. Mater. 2011, 23, 1650−1653. (8) Patel, H. A.; Je, S. H.; Park, J.; Chen, D. P.; Jung, Y.; Yavuz, C. T.; Coskun, A. Unprecedented high-temperature CO2 selectivity in N2-

uptake capacities for GNF-2 and GNF-3 were found to be 46 and 32 wt %, respectively. For R134a, GNF-2 and GNF-3 presented 13 and 11 wt %, respectively. Overall, GNF-2 substituted with CF3 groups presents higher uptake capacity when compared to GNF-3 with F substituents. We suggest that fluorine atoms not only increase hydrophobicity but also create polarized, electron-deficient π-surfaces for efficient binding of guests both in liquid and gas forms. The good sorption capacity of GNFs for CFCs render them as promising candidates for the elimination of harmful CFCs from the atmosphere.

4. CONCLUSIONS We have successfully synthesized a series of edge-functionalized graphene nanoribbon frameworks through the Diels−Alder cycloaddition polymerization and FeCl3-catalyzed cyclo-dehydrogenation reactions. The textural and gas sorption properties of GNFs were tuned simply by varying the electronic properties of dicyclopentanedienone. The edge-functionalized GNFs showed high thermal and physicochemical stabilities, which suggests that GNFs can be alternatives to various engineering materials under a wide range of conditions. GNFs showed high affinity toward H2 gas and remarkable CO2 breakthrough J


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