Perfluorophenyl Stacking Interactions Enhance Structural

Subscriber access provided by Universitaetsbibliothek | Johann Christian Senckenberg

Article

Phenyl/Perfluorophenyl Stacking Interactions Enhance Structural Order in Two-Dimensional Covalent Organic Frameworks Wade A. Braunecker, Katherine E. Hurst, Keith G. Ray, Zbyslaw Roman Owczarczyk, Madison Martinez, Noemi Leick, Amy Kuehlen, Alan Sellinger, and Justin C. Johnson Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00630 • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Crystal Growth & Design

Phenyl/Perfluorophenyl Stacking Interactions Enhance Structural Order in Two-Dimensional Covalent Organic Frameworks Wade A. Braunecker,† Katherine E. Hurst,† Keith G. Ray,§ Zbyslaw R. Owczarczyk,† Madison B. Martinez, †,‡ Noemi Leick,† Amy Keuhlen, ‡ Alan Sellinger, ‡ Justin C. Johnson†,* †

§

National Renewable Energy Laboratory, 15013 Denver West Pkwy, Golden, CO 80401

Quantum Simulations Group, Lawrence Livermore National Laboratory, Livermore, CA 94550 ‡

Department of Chemistry, Colorado School of Mines, 1012 14th Street, Golden, CO 80401

KEYWORDS: Covalent organic framework, fluorination, crystallinity, porosity, gas sorption

1

ACS Paragon Plus Environment

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT. A two-dimensional imine-based covalent organic framework (COF) was designed and synthesized such that phenyl and perfluorophenyl structural units can seamlessly alternate between layers of the framework. X-ray diffraction of the COF powders reveals a striking increase in crystallinity for the COF with self-complimentary phenyl/perfluorophenyl interactions (FAStCOF). Whereas measured values of the Brunauer-Emmet-Teller (BET) surface areas for the nonfluorinated Base-COF and the COF employing hydrogen bonding were ~37% and 59%, respectively, of their theoretical Connolly surface areas, the BET value for FASt-COF achieves > 90% its theoretical value (~1700 m2/g). Transmission electron microscopy (TEM) images also revealed unique micron-sized rod-like features in FASt-COF that were not present in the other materials. The results highlight a promising approach for improving surface areas and long-range order in 2D COFs.

2


Page 2 of 26

Page 3 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION The ability to control the pore size, shape, and chemical functionalities of covalent organic frameworks (COFs) and metal-organic frameworks (MOFs) gives these particular nanoporous materials distinct advantages over some of their inorganic counterparts such as zeolites and silicates.12

COF and MOF structures can be easily modified with a wide-range of functionalities in order to

tune the material for a specific application, including gas storage and separations,3-6 energy storage,7-9 optoelectronic devices,10-11 and environmental remediation.12 Depending on the geometry of the building blocks, COFs will generally either crystallize into two-dimensional (2D) layered structures13-37 or three-dimensional (3D) networks, each with its own distinct advantages. The eclipsed structures of 2D COFs tend to give these materials higher intrinsic charge mobilities and higher crystalline densities, while the 3D COFs tend to exhibit higher surface areas.38 Enhancing long range order in 2D COFs is attractive for both improved electronic properties and porosity.39 One effective strategy for improving crystallinity in imine-based 2D COFs has been to introduce hydrogen bonding moieties into the framework to suppress torsion and “lock” the 2D sheets into a planar conformation.40 This configuration in turn results in stronger interlayer interactions that effectively extend π-cloud delocalization and improve long-range order and porosity.18, 40 Following the discovery that a mixture of benzene and hexafluorobenzene forms an alternating complex with an increased melting temperature over either parent compound,41 the strong face-to-face stacking interactions between phenyl and perfluorophenyl groups have been exploited in numerous ways to control local order in crystal packing.42-43 When an aromatic molecule possesses partial fluorination in which the fluorines are segregated on one portion of the structure, the perfluorophenyl groups possess a strong tendency to interact most intimately with non-fluorinated

3



groups on a neighboring molecule, resulting in relative molecular dispositions that are highly predictable and lead to extraordinarily stable crystals. Without such a fluorination strategy, the global stability of one specific crystal structure is lessened, and several polymorphs with similar stabilities are likely to exist.44 The non-covalent interaction provided by fluorination can also influence certain solid-state synthetic reactions45 and is emerging as a powerful tool in the field of crystal engineering to influence chemical and photophysical outcomes.46-47 For frameworks, the strategy was recently employed in a 2D COF based on Cu-porphyrin structural units with rectangular pores to produce a material near 1400 m2g-1,48 though the unmatched reactivity of the constituents made optimal formation of perfectly alternating phenyl-perfluorophenyl interactions unlikely. The fluorinated and non-fluorinated terephthaldehyde precursors were included in equal amounts in the reaction mixture, but our results suggest these components will have very different reactivities (vide infra), and it is unclear whether phenyl and perfluorophenyl groups would alternate regularly throughout such a crystal. Additional examples of partial fluorination in 2D COFs have been observed to improve surface areas, though again, the materials were not designed to specifically promote alternating stacking.49 In the latter work, scanning electron microscopy (SEM) images suggested the fluorinated COFs had more crystallite morphology when compared to the smooth, spherical aggregates of a non-fluorinated COF, despite only employing a relatively small weight fraction of fluorine. Indeed, increasing the average crystallite size of COFs is an incredibly important challenge in the field with respect to improving materials quality.39 We demonstrate in this work with transmission electron microscopy (TEM) images that the phenyl-perfluorophenyl alternating-stacking motif, when harnessed appropriately, can influence micron-size crystal growth.

4


Page 4 of 26

Page 5 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Here, we investigate four imine-based 2D COF structures (Scheme 1): the Base-COF has been synthesized previously50 and contains no moieties designed to enhance long-range order; OHCOF contains functionalities capable of intermolecular hydrogen bonding that should planarize the 2D sheet and improve crystallinity;40 the F-COF has 75% of the aromatic rings perfluorinated; finally, a COF was designed whose fluorinated and non-fluorinated components will react to yield a material that will always yield a partially fluorinated product COF that can optimize the phenylperfluorophenyl stacking interactions between layers (Fluorinated Alternating Stacking, or FAStCOF). Besides the promotion of directed stacking and crystal stability, fluorinated aromatic groups represent robust chemical substituents that have been employed in organic framework materials to dramatically improve their affinity and storage capacity for oxygen gas,51 carcinogenic aromatic molecules,52 and perfluorocarbons and CFCs.53

EXPERIMENTAL SECTION General Methods. Schlenk flasks or Schlenk bombs with PTFE caps were used as reaction vessels for the synthesis of all precursors and final COF materials. Methylene chloride, toluene, and THF were purified by passing through alumina in an MBraun solvent purification system. Column chromatography was performed with Fluka Silica Gel 60 (220-440 mesh). 1H NMR (400 MHz), 13C NMR (100 MHz), and 19F NMR (376 MHz) were recorded on a Bruker Avance III HD NanoBay NMR Spectrometer. Residual solvent peaks were used as the internal standards for 1H and 13C NMR spectra. Hexafluorobenzene (-164.9 p.p.m.) was used as the internal standard in all 19

F NMR spectra.

13

C spectra were not included for fluorinated precursors since they were not

informative due to the low intensities of the signals caused by poor solubility of the compounds and extensive coupling between 13C and 19F nuclei.

5



The following starting materials and reagents were obtained from commercial sources and used without further purification: 1,3,5-tribromobenzene, 1,3,5-tris(4-bromophenyl)benzene, nBuLi, boron tribromide, ethylene glycol, trifluoroacetic acid, 2-dicyclohexylphosphino-2,6-dimethoxybiphenyl (SPhos), bis(dibenzylideneacetone) palladium(0) (Pd2(dba)3), palladium(II) acetate, sodium acetate, (Aldrich); 2,3,5,6-tetrafluoroaniline, 2,3,5,6-tetrafluorobenzaldehyde (TCI America); 4-formyl-3-methoxyphenylboronic acid (Combi-Blocks). Synthesis. Scheme S1 in the SI illustrates the synthesis of all the COF precursors employed in this work. 1,3,5-Tris(4-formylphenyl)benzene (1),54 1,3,5-tris(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)benzene (2),55 1,3,5-tris(2,3,5,6-tetrafluoroaniline)benzene (5),56 and 1,2,4,5-tetrafluoro-3-(1,3-dioxol-2-yl)benzene (6)57 were synthesized according to literature procedures. 1,3,5-Tris(4-formyl-3-methoxyphenyl)benzene (3). In a Schlenk flask were added 4-bromo-2-methoxybenzaldehyde (0.78 g, 3.6 mmol) and compound (2) (0.51 g, 1.1 mmol) followed by vacuum and argon refill three times. In a separate container, CsF (1.5 g, 10 mmol) was dissolved in 12 mL of argon-sparged deionized water and this was added to the Schlenk flask followed by PdCl2(dppf) (64 mg, 0.08 mmol) and 12 mL of dioxane. The mixture was stirred at 90°C for 3 h and periodically monitored via TLC (4 hexanes: 1 ethyl acetate). After 3 h, a single new TLC spot was observed, and the mixture was allowed to cool to rt. The reaction mixture was then extracted into chloroform (25 mL) and washed with water (10 mL) three times. The aqueous phase was further extracted three times with chloroform (25 mL), combined with the organic phase, dried over anhydrous magnesium sulfate, and filtered. The solvent was removed with a rotary evaporator. The resulting product was boiled in 50 mL of methanol in an Erlenmeyer flask for fifteen minutes, then allowed to cool to rt,

6


Page 6 of 26

Page 7 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


followed by further cooling in a freezer overnight. The solids were recovered by vacuum filtration and dried in a vacuum oven at 50°C for 4 hours yielding an off-white powder (360 mg, 68%). 1H NMR (400 MHz, CDCl3):  10.51 (s, 3H), 7.95 (d, J = 8 Hz, 3H), 7.80 (s, 3H), 7.33 (d, J = 8 Hz, 3H), 7.21 (s, 3H), 4.02 (s, 9H).

13C

NMR (100 MHz, CDCl3):  189.50, 162.42,

148.33, 142.25, 129.56, 126.67, 124.49, 120.25, 110.89, 56.13. 1,3,5-Tris(4-formyl-3-hydroxyphenyl)benzene (4). Compound (3) (100 mg, 0.21 mmol) was dissolved in 30 mL of anhydrous nitrogen purged dichloromethane and cooled to -78 °C in a dry ice / acetone bath. Approximately 1 mL of 1M boron tribromide solution in dichloromethane was added dropwise. The solution stirred at -78 °C for 1 h before being allowed to stir at r.t. overnight. 20 mL of methanol was added slowly to the solution. The grayish white product was filtered, washed extensively with methanol, and dried under vacuum to yield 82 mg of pure product (90% yield). 1H NMR (400 MHz, DMSO-D6):  10.90 (s, 3H), 10.29 (s, 3H), 8.00 (s, 3H), 7.82 (d, J = 8.4 Hz, 3H), 7.52 (d, J = 8 Hz, 3H), 7.45 (s, 3H). 13C NMR (100 MHz, DMSO-D6):  191.74, 160.96, 147.08, 140.49, 130.30, 125.83, 121.56, 118.69, 115.59. HRMS m/z: [M + H]+ calcd. for C27H19O6 439.1181; found 439.1192. 1,3,5-Tris(1,2,4,5-tetrafluoro-3-(1,3-dioxol-2-yl)benzene)benzene (7). 1,3,5Tribromobenzene (527 mg, 1.67 mmol) was added to a Schlenk flask with 3 eq. of compound (6) (1.12 g, 5.02 mmol), along with 850 mg of NaOAc and 23 mg of Pd(OAc)2. The flask was purged with nitrogen, and 8 mL of anhydrous nitrogen purged NMP was added. The solution was heated to 150 °C and stirred rapidly for 48 h. After cooling to r.t., 100 mL of ethyl acetate was added to the solution, which was then washed with copious amounts of water. Flash chromatography was performed with 1:1 ethyl acetate:hexane as the solvent. The solvent

7



was removed with a rotary evaporator, and the crude product was recrystallized from methanol to give 300 mg (25% yield) of pure material. 1H NMR (400 MHz, CDCl3): δ 7.65 (s, 3H), 6.29 (s, 2H), 4.23 (m, 6H), 4.06, (m, 6H). 19F NMR (376 MHz, CDCl3, C6F6): δ 145.91 (m, 6F), 147.07 (m, 6F). 1,3,5-Tris(2,3,5,6-tetrafluorobenzaldehyde)benzene (8). Compound (7) (300 mg, 0.1 mmol) was dissolved in 2 mL of chloroform and 1 mL of trifluoracetic acid in a Schlenk bomb. 0.5 mL of a 5% H2SO4 / H2O solution was then added, after which the solution was purged with nitrogen for 15 minutes and sealed with a PTFE cap. The solution was heated at 60 °C for 6 h, during which time a white precipitate formed. The precipitate was filtered, washed with a NaHCO3 / H2O solution and then with H2O followed by a small amount of acetone. The crude product was stirred rapidly over 8 mL of chloroform, filtered, and dried under vacuum to give 180 mg of pure material (74% yield). 1H NMR (400 MHz, CDCl3): δ 10.25 (s, 3H), 8.08 (s, 2H). 19F NMR (376 MHz, CDCl3, C6F6): δ 145.61 (m, 6F), 148.23 (m, 6F). HRMS m/z: [M - H]+ calcd. for C27H5F12O3 605.0047; found 605.0036. General procedure for COF synthesis. The four COF materials were synthesized using optimized conditions from a modified literature procedure for a similar class of partially fluorinated 2-D COF materials.49 A representative example is described below for the FASt-COF. To a nitrogen purged Schlenk tube bomb containing a solid mixture of 1,3,5-tri-(4-aminophenyl)benzene (TAPB) (0.178 mmol, 62.5 mg) and compound 8 (0.178 mmol, 108 mg) was added 5.7 mL of degassed o-dichlorobenzene (o-DCB), 0.3 mL of degassed n-BuOH, and 0.3 mL of nitrogen purged acetic acid. The mixture was bubbled with nitrogen for an additional 10 minutes before it was sealed and heated at 120 °C for three days. The precipitate was collected by

8


Page 8 of 26

Page 9 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


filtration, washed with THF, and subjected to Soxhlet extraction with acetone overnight. The powder was collected and dried at 120 °C under vacuum for 48 h to produce 158 mg FASt-COF in approximately 98% yield. Elemental analysis calcd (%) for C51H21F12N3: C 67.78, H 2.34, F 25.23, N 4.65; found: C 67.07, H 2.54, F 23.55, N 4.70. Base-COF: Elemental analysis calcd (%) for C51H33N3: C 89.06, H 4.84, N 6.11; found: C 84.65, H 4.91, N 5.67. OH-COF: Elemental analysis calcd (%) for C51H33N3O3: C 83.25, H 4.52, N 5.71, O 6.52; found: C 75.81, H 4.27, N 4.24, O 15.6. F-COF: Elemental analysis calcd (%) for C51H9F24N3: C 54.71, H 0.81, F 40.73, N 3.75; found: C 53.14, H 1.06, F 34.97, N 3.66. Structural Characterization and Modelling. X-ray diffraction patterns were collected on a Rigaku DMAX 2500 diffractometer utilizing Cu K (λ = 1.54 Å) radiation. After a thorough alignment procedure, scans of intensity (counts) as a function of 2θ angle (2 – 30°) were collected. Powder indexing and structural refinement of all COF powder XRD data was performed using the Reflex package within Materials Studio 2017. The starting periodic structures for refinement were built from the Base-COF structure. Bonding and functionalization were added, with subsequent geometry optimization performed in Materials Studio using the Forcite package and Universal Force Field model. The unit cell was constructed to include one full COF ring and one COF layer (except for the FASt-COF alternating structure, which has two layers per unit cell). Pawley refinement was performed with lower symmetry geometries allowed (i.e., deviation from planarity and hexagonal symmetry). Rietveld refinement was also performed, but structures were constrained to remain planar and maintain hexagonal crystal symmetry (a=b,  =  = 90°,  = 120°). Corrections for background, peak asymmetry, and broadening due to crystallite size and strain

9



effects were included. The Connolly surface of each COF was determined using Materials Studio with a 1.82 Å probe radius for N2. The resulting surface area was converted to a porosity to be compared with BET using the crystal densities and unit cell volumes calculated in Tables 1 and S2. For the Base-COF the stacking distance was taken to be 3.6 Å since no experimental XRD peak was found for the (001) reflection. The relative cohesive energies of alternating and non-alternating stacking in FASt-COF were calculated with density functional theory (DFT) using the Vienna Ab initio Simulation Package (VASP).58 Pseudopotentials were taken from the VASP PBE library under the projector-augmented wave (PAW) scheme.59 To capture dispersion interactions we used the vdW-DF-cx van der Waals corrected density functional.60-61 The plane wave basis for the electronic wavefunctions employed a cutoff of 600 eV. A 1X1X3 k-point mesh was found to be sufficient to sample the Brillouin zone due to the large transverse size of the COF unit cell. In the calculations, the atomic positions were optimized with a convergence criterion for the forces of 0.005 eV/Å. The COFs powder samples were imaged using the FEI Tecnai ST 30 transmission electron microscope (TEM) operated at 300 kV. Physical Measurements. Thermogravimetric analysis (TGA) of these materials was carried out using a TA SDT Q600 thermal analysis system in order to determine stability and decomposition temperatures of the COFs. Each sample was tested in an alumina pan under 100.0 mL min-1 of N2 gas and was heated from room temperature to 600˚C at a rate of 15˚C min-1. The Brunauer-Emmet-Teller (BET) data were collected using a Micromeritics ASAP 2020. The samples were degassed to 120 °C on a separate vacuum system (base pressure = 1x10-7 Torr) then transferred to the BET system without exposing to air. The relative pressure regimes for the

10


Page 10 of 26

Page 11 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


BET analysis were chosen according to the criteria for evaluating BET surface areas for microporous materials.62

RESULTS AND DISCUSSION The synthetic details and characterization of the COF precursors are described in detail in the experimental section. Formation of the final COF materials are completed by reacting aldehyde and amine precursors (electrophiles and nucleophiles, respectively) using well-known and optimized procedures for an acid-catalyzed condensation reaction.49 Precursors for the Base-COF and OH-COF were reacted at 120 °C for 3 days in a mixture of 1.9/0.1/0.1 parts o-dichlorobenzene, 1butanol, and 6M acetic acid. The COF materials precipitate after several minutes at the reaction temperature, but countless studies have demonstrated that the surface areas of imine-based COF materials continue to improve if left to react for ~3 or more days under these conditions. 2 The moisture and alcohol in the reaction mixture are essential to achieving higher degrees of crystallinity and larger surface areas as they promote reversibility in the condensation reaction that enables defects in the COF structure to be “repaired” with time. These two COFs were recovered in >95% yield.

SCHEME 1. Synthesis of imine-based 2-D COFs. 11



Page 12 of 26

After synthesis of the fluorinated tris-amine in Scheme 1 in one step from a literature procedure,56 the compound was reacted with the non-fluorinated tris-aldehyde in an attempt to synthesize an alternating stacking COF. However, the highly fluorinated tris-amine is a very poor nucleophile and no reaction was observed with the tris-aldehyde over the course of 1 week at 120 °C, even when moisture and alcohol were removed to push the reaction towards the product as much as possible. However, by fluorinating the tris-aldehyde component instead, a highly reactive electrophile was generated that was very reactive with a non-fluorinated amine. The reaction proceeded so quickly that FASt-COF product began precipitating as soon as the monomer components could dissolve and before the acetic acid catalyst was injected. The final product was recovered in ~98% yield. The highly fluorinated FCOF was produced from the strongly reactive fluorinated tris-aldehyde and the weakly reactive fluorinated tris-amine, though the reaction was quite slow; F-COF precipitate was not observed during the first several hours of the reaction, and the final material was only recovered in ~40%

Figure 1. (a) BET isotherms for each COF. (b) Comparison of theoretical Connolly surface area values with those measured by BET porosity analysis.

yield.


Page 13 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Structural parameters of COFs. Base-COF

OH-COF

FASt-COF

F-COF

BET (m2/g)

970

1410

1700

1275

Calc. Connolly area (m2/g)

2595

2394

1850

1573

Pore volume (cm3/g)

0.40

0.69

1.09

1.00

Calc. volume (cm3/g)

1.13

1.11

0.90

0.72

3.58

3.60

3.72

0.52

0.66

0.76

Stacking distance (Å) Calc. density (g/cm3)

-0.45

The surface areas of the four materials were determined through N2 sorption measurements at 77K (Figure 1 and Table 1). The BET surface area of the Base-COF was measured as 970 m2g1

using our optimized conditions, which is more than double the literature optimized value,50

though less than 40% of its theoretical Connolly surface area. The introduction of hydrogen bonding groups in the OH-COF improves the BET value to 1410 m2g-1, though this is still only about 60% of its theoretical Connolly value. We note that the Connolly values for the F-COF and FASt-COF decrease from the Base-COF, due to added mass and pore filling, but the BET surface area improves to 1700 m2g-1 for the FASt-COF, which fulfills the theoretical value to within 10%. A significant increase in pore volume is also observed for FASt-COF. The improvement is likely a function of the intrinsic material design that maximizes alternating phenyl/perfluorophenyl stacking interactions. Pore width distributions calculated from the BET data are shown in Figure S17. For the more crystalline COFs the primary peak is centered around 18-22 Å, which is line with expectations given the molecular structure.

13



Page 14 of 26

X-ray diffraction (XRD) of the COF powders, Figure 2, demonstrates a remarkable increase in long-range order through the sample series. All COFs show a prominent (100) peak at about 4°, and the expected series of peaks at higher angles previously observed for a similar COF family.49 A significant narrowing of peak widths is found for

Figure 2. a) Unnormalized powder XRD patterns for all COFs. Inset: Background-subtracted high angle region. Color scheme is as in Figure 1.

FASt-COF (see Table S1). The (001) peak that reflects the COF interlayer spacing is most clearly distinguished for FASt-COF (Figure 2, inset) at about 25°. XRD patterns were unchanged after several rounds of degassing and after BET analysis, demonstrating stability. Thermogravimetric analysis (TGA, Fig. S18) also showed less than 2% mass loss up to 400°C, with the highest breakdown temperature achieved for FASt-COF. Transmission electron microscopy (TEM) images of the COFs were collected after deposition of the powdered material on holey carbon substrates, Figure 3.

The

Base-COF images (Figure 3a) reveal globular features of a few hundred nm in size with some internal sharp features that may represent crystallites, but primarily Figure 3. Representative TEM images of a) BaseCOF, b), OH-COF, c) F-COF, and d)-e) FAStCOF. Scale bar is 200 nm for all images.

14

isotropic growth structure. F-COF and OHCOF aggregates show more features that


Page 15 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


appear crystalline (Figure 3b-c), but the size of any single crystallites is less than 100 nm and the growth directions appear random. On the other hand, FASt-COF crystallites are clearly rod- or tube-like, with single features extending into the micron scale, Figure 3d. Additional dendritic growth of wires appears to occur from the primary rods, Figure 3e. The directed growth of such crystallites likely reflects the ordering induced by partial fluorination and detected in the XRD and surface area measurements. These TEM images suggest that the alternating stacking motif employed in FASt-COF represents a promising approach for influenceing long-range order in COF crystals. The COF structures were created computationally by taking appropriate fragments (Fig. S5) and building periodic crystals with hexagonal symmetry. Subsequently, the COFs were functionalized with -OH or -F and subjected to geometry optimization. Pawley and Rietveld refinement based upon these starting structures and the experimental powder XRD resulted in unit cell parameters shown in Table S3. Low residuals after refinement were achieved for Base-COF, OH-COF, and F-COF.

Higher residuals were found for

FASt-COF, especially for Rietveld refinement where the structures were constrained to maintain hexagonal symmetry. For FASt-COF, we also must consider the potential stabilization for the rotated stacking structure that produces phenyl-perfluorophenyl interactions for FASt-COF in alternating layers compared with the non-alternating structure. Figure 4 depicts these two

Figure 4. Side-on view of modeled (a) alternating and (b) non-alternating FASt-COF structures. The orange spheres are fluorines while the blue spheres are nitrogen and white are hydrogen.

15



crystal types. The Rietveld refinements for these FASt-COF structures results in a poorer match to experiment than for the other COFs. Indeed, a simple comparison between predicted XRD patterns after geometry optimization and experiment reveal equal residuals on either side of the observed peaks, Figure S19. Although there is uncertainty about how well the geometry optimization captures the real structures, the results suggest that both crystal types might exist to some extent in the sample. As such, it is not surprising that the FASt-COF refinements do not produce clear preference for one type of structure. Van der Waals corrected density functional theory calculations were also performed on the alternating and non-alternating structures for FASt-COF. The alternating structure was found to have a cohesive energy that was 28.2 kJ/(mol hexagonal unit cell of 8 phenyl groups) higher than the non-alternating structure and the interlayer distance was calculated to be 3.51 Å, or 3 % less than the value calculated for the non-alternating structure, 3.62 Å. Although the layer spacing value measured with XRD is 3.60 Å, the vdW-DF-cx functional has been reported to slightly underpredict layer spacing, for example, by 0.08 Å for graphite.60 See Figure S11 for the DFT-relaxed structures. These results suggest significant stabilization does exist for the alternating structure. The observation that some amount of the directly stacked structure still exists may be due to trapped “faults” that occur upon stochastic crystallization. Unlike small molecule crystallization, the COFs condense in large 2D sheets, which may prevent facile formation of the more thermodynamically stable form. We are exploring crystallization conditions to determine if such faults can be reduced in order to produce the alternating structure exclusively.

16


Page 16 of 26

Page 17 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CONCLUSIONS The high surface area observed for the FASt-COF that is near its theoretical maximum is supported by structural characterization and computational results, which reveal a material with enhanced stability and longer-range order than COF analogues. Due to strategic partial fluorination, this COF can access structures that take advantage of strong interlayer interactions not otherwise present. In demonstrating this promising new design approach that can significantly improve surface areas and stability in this class of materials, the results further elevate 2D COFs as a material platform with great promise for gas storage, separations, and catalysis applications where chemical versatility and robustness must be combined with high porosity.

Supporting Information. Synthetic schemes, NMR Spectra, details on structure refinement, additional porosity characterization. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *[email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was authored in part by Alliance for Sustainable Energy, LLC, the manager and operator of the National Renewable Energy Laboratory for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding provided by the Hydrogen Materials - Advanced

17



Research Consortium (HyMARC), established as part of the Energy Materials Network under the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes. Synthesis of some COF precursors was funded by the NREL Laboratory Directed Research and Development program. We thank Dr. Lieve Laurens at the NREL Spin Resonance Facility for assistance collecting of LDI mass spectrometry data and Dr. Henok Yemam for helpful discussions on the synthesis of COF precursors. REFERENCES 1. Feng, X.; Ding, X.; Jiang, D., Covalent Organic Frameworks. Chem. Soc. Rev. 2012, 41, 60106022. 2. Waller, P. J.; Gandara, F.; Yaghi, O. M., Chemistry of Covalent Organic Frameworks. Acc. Chem. Res. 2015, 48, 3053-3063. 3. Furukawa, H.; Yaghi, O. M., Storage of Hydrogen, Methane, and Carbon Dioxide in Highly Porous Covalent Organic Frameworks for Clean Energy Applications. J. Am. Chem. Soc. 2009, 131, 8875-8883. 4. Tong, M.; Yang, Q.; Xiao, Y.; Zhong, C., Revealing the Structure-Property Relationship of Covalent Organic Frameworks for CO2 Capture from Postcombustion Gas: A Multi-Scale Computational Study. Phys. Chem. Chem. Phys. 2014, 16, 15189-15198.

18


Page 18 of 26

Page 19 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5. Huang, N.; Chen, X.; Krishna, R.; Jiang, D., Two-Dimensional Covalent Organic Frameworks for Carbon Dioxide Capture through Channel-Wall Functionalization. Angew. Chem. Int. Ed. 2015, 54, 2986-2990. 6. Huang, N.; Krishna, R.; Jiang, D., Tailor-Made Pore Surface Engineering in Covalent Organic Frameworks: Systematic Functionalization for Performance Screening. J. Am. Chem. Soc. 2015, 137, 7079-7082. 7. Yang, H., et al., High Conductive Two-Dimensional Covalent Organic Framework for Lithium Storage with Large Capacity. ACS Appl. Mater. Interfaces 2016, 8, 5366-5375. 8. Bai, L.; Gao, Q.; Zhao, Y., Two Fully Conjugated Covalent Organic Frameworks as Anode Materials for Lithium Ion Batteries. J. Mater. Chem. A 2016, 4, 14106-14110. 9. Alahakoon, S. B.; Thompson, C. M.; Occhialini, G.; Smaldone, R. A., Design Principles for Covalent Organic Frameworks in Energy Storage Applications. ChemSusChem 2017, 10, 21162129. 10. Ding, X., et al., An N-Channel Two-Dimensional Covalent Organic Framework. J. Am. Chem. Soc. 2011, 133, 14510-14513. 11. Guo, J., et al., Conjugated Organic Framework with Three-Dimensionally Ordered Stable Structure and Delocalized Pi Clouds. Nat. Commun. 2013, 4, 2736. 12. Sun, Q.; Aguila, B.; Perman, J.; Earl, L. D.; Abney, C. W.; Cheng, Y.; Wei, H.; Nguyen, N.; Wojtas, L.; Ma, S., Postsynthetically Modified Covalent Organic Frameworks for Efficient and Effective Mercury Removal. J. Am. Chem. Soc. 2017, 139, 2786-2793. 13. Ding, S.-Y.; Gao, J.; Wang, Q.; Zhang, Y.; Song, W.-G.; Su, C.-Y.; Wang, W., Construction of Covalent Organic Framework for Catalysis: Pd/COF-LZU1 in Suzuki–Miyaura Coupling Reaction. J. Am. Chem. Soc. 2011, 133, 19816-19822.

19



14. DeBlase, C. R.; Silberstein, K. E.; Truong, T.-T.; Abruña, H. D.; Dichtel, W. R., Ketoenamine-Linked Covalent Organic Frameworks Capable of Pseudocapacitive Energy Storage. J. Am. Chem. Soc. 2013, 135, 16821-16824. 15. Rabbani, M. G.; Sekizkardes, A. K.; Kahveci, Z.; Reich, T. E.; Ding, R.; El‐Kaderi, H. M., A 2D Mesoporous Imine‐Linked Covalent Organic Framework for High Pressure Gas Storage Applications. Chem. Eur. J. 2013, 19, 3324-3328. 16. Zhou, T.-Y.; Xu, S.-Q.; Wen, Q.; Pang, Z.-F.; Zhao, X., One-Step Construction of Two Different Kinds of Pores in a 2D Covalent Organic Framework. Journal of the American Chemical Society 2014, 136, 15885-15888. 17.

Chen, X.; Addicoat, M.; Jin, E.; Zhai, L.; Xu, H.; Huang, N.; Guo, Z.; Liu, L.; Irle, S.;

Jiang, D., Locking Covalent Organic Frameworks with Hydrogen Bonds: General and Remarkable Effects on Crystalline Structure, Physical Properties, and Photochemical Activity. J. Am. Chem. Soc. 2015, 137, 3241-3247. 18. Kandambeth, S.; Venkatesh, V.; Shinde, D. B.; Kumari, S.; Halder, A.; Verma, S.; Banerjee, R., Self-Templated Chemically Stable Hollow Spherical Covalent Organic Framework. Nat. Commun. 2015, 6, 6786. 19. Smith, B. J.; Overholts, A. C.; Hwang, N.; Dichtel, W. R., Insight into the Crystallization of Amorphous Imine-Linked Polymer Networks to 2D Covalent Organic Frameworks. Chem. Commun. 2016, 52, 3690-3693. 20. Leng, W.; Peng, Y.; Zhang, J.; Lu, H.; Feng, X.; Ge, R.; Dong, B.; Wang, B.; Hu, X.; Gao, Y., Sophisticated Design of Covalent Organic Frameworks with Controllable Bimetallic Docking for a Cascade Reaction. Chem. Eur. J. 2016, 22, 9087-9091.

20


Page 20 of 26

Page 21 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


21. Haase, F.; Gottschling, K.; Stegbauer, L.; Germann, L. S.; Gutzler, R.; Duppel, V.; Vyas, V. S.; Kern, K.; Dinnebier, R. E.; Lotsch, B. V., Tuning the Stacking Behaviour of a 2D Covalent Organic Framework through Non-Covalent Interactions. Mater. Chem. Front. 2017, 1, 1354-1361. 22. Xu, S.-Q.; Liang, R.-R.; Zhan, T.-G.; Qi, Q.-Y.; Zhao, X., Construction of 2D Covalent Organic Frameworks by Taking Advantage of the Variable Orientation of Imine Bonds. Chem. Commun. 2017, 53, 2431-2434. 23. Janica, I.; Patroniak, V.; Samorì, P.; Ciesielski, A., Imine‐Based Architectures at Surfaces and Interfaces: From Self‐Assembly to Dynamic Covalent Chemistry in 2D. Chem. Asian J. 2018, 13, 465-481. 24. Liu, H.; Li, Q.; Zhu, Y.; Zhang, M.; Liu, R.; Li, X.; Kang, X.; Li, Z.; Qiao, S., Synthesis and Mechanical Exfoliation of Imine-Linked Two-Dimensional Conjugated Polymers. J. Mater. Chem. C 2018, 6, 722-725. 25. Auras, F., et al., Synchronized Offset Stacking: A Concept for Growing Large-Domain and Highly Crystalline 2D Covalent Organic Frameworks. J. Am. Chem. Soc. 2016, 138, 16703-16710. 26. Cai, S.-L.; Zhang, K.; Tan, J.-B.; Wang, S.; Zheng, S.-R.; Fan, J.; Yu, Y.; Zhang, W.-G.; Liu, Y., Rationally Designed 2D Covalent Organic Framework with a Brick-Wall Topology. ACS Macro Lett. 2016, 5, 1348-1352. 27. Peng, Y.; Wong, W. K.; Hu, Z.; Cheng, Y.; Yuan, D.; Khan, S. A.; Zhao, D., Room Temperature Batch and Continuous Flow Synthesis of Water-Stable Covalent Organic Frameworks (COFs). Chem. Mater. 2016, 28, 5095-5101. 28. Wang, X.; Han, X.; Zhang, J.; Wu, X.; Liu, Y.; Cui, Y., Homochiral 2D Porous Covalent Organic Frameworks for Heterogeneous Asymmetric Catalysis. J. Am. Chem. Soc. 2016, 138, 12332-12335.

21



29. Bhunia, S.; Das, S. K.; Jana, R.; Peter, S. C.; Bhattacharya, S.; Addicoat, M.; Bhaumik, A.; Pradhan, A., Electrochemical Stimuli-Driven Facile Metal-Free Hydrogen Evolution from PyrenePorphyrin-Based Crystalline Covalent Organic Framework. ACS Appl. Mater. Interfaces 2017, 9, 23843-23851. 30. Han, X.; Xia, Q.; Huang, J.; Liu, Y.; Tan, C.; Cui, Y., Chiral Covalent Organic Frameworks with High Chemical Stability for Heterogeneous Asymmetric Catalysis. J. Am. Chem. Soc. 2017, 139, 8693-8697. 31. Liao, Y.; Li, J.; Thomas, A., General Route to High Surface Area Covalent Organic Frameworks and Their Metal Oxide Composites as Magnetically Recoverable Adsorbents and for Energy Storage. ACS Macro Lett. 2017, 6, 1444-1450. 32. Matsumoto, M.; Dasari, R. R.; Ji, W.; Feriante, C. H.; Parker, T. C.; Marder, S. R.; Dichtel, W. R., Rapid, Low Temperature Formation of Imine-Linked Covalent Organic Frameworks Catalyzed by Metal Triflates. J. Am. Chem. Soc. 2017, 139, 4999-5002. 33. Mu, M.; Wang, Y.; Qin, Y.; Yan, X.; Li, Y.; Chen, L., Two-Dimensional Imine-Linked Covalent Organic Frameworks as a Platform for Selective Oxidation of Olefins. ACS Appl. Mater. Interfaces 2017, 9, 22856-22863. 34. Peng, Y., et al., Ultrathin Two-Dimensional Covalent Organic Framework Nanosheets: Preparation and Application in Highly Sensitive and Selective DNA Detection. J. Am. Chem. Soc. 2017, 139, 8698-8704. 35. Rao, M. R.; Fang, Y.; De Feyter, S.; Perepichka, D. F., Conjugated Covalent Organic Frameworks Via Michael Addition–Elimination. J. Am. Chem. Soc. 2017, 139, 2421-2427. 36. Vitaku, E.; Dichtel, W. R., Synthesis of 2d Imine-Linked Covalent Organic Frameworks through Formal Transimination Reactions. J. Am. Chem. Soc. 2017, 139, 12911-12914.

22


Page 22 of 26

Page 23 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


37. Zhang, G.; Tsujimoto, M.; Packwood, D.; Duong, N. T.; Nishiyama, Y.; Kadota, K.; Kitagawa, S.; Horike, S., Construction of a Hierarchical Architecture of Covalent Organic Frameworks Via a Postsynthetic Approach. J. Am. Chem. Soc. 2018, 140, 2602-2609. 38. Bunck, D. N.; Dichtel, W. R., Internal Functionalization of Three-Dimensional Covalent Organic Frameworks. Angew. Chem., Int. Ed. 2012, 51, 1885-1889, S1885/1-S1885/42. 39. Bisbey, R. P.; Dichtel, W. R., Covalent Organic Frameworks as a Platform for Multidimensional Polymerization. ACS Cent. Sci. 2017, 3, 533-543. 40. Chen, X.; Addicoat, M.; Jin, E.; Zhai, L.; Xu, H.; Huang, N.; Guo, Z.; Liu, L.; Irle, S.; Jiang, D., Locking Covalent Organic Frameworks with Hydrogen Bonds: General and Remarkable Effects on Crystalline Structure, Physical Properties, and Photochemical Activity. J. Am. Chem. Soc. 2015, 137, 3241-7. 41. Patrick, C. R.; Prosser, G. S., A Molecular Complex of Benzene and Hexafluorobenzene Nature 1960, 187, 1021. 42. Hori, A.; Arii, T., Cation-Π and Arene-Perfluoroarene Interactions between Cu(II) FluorineSubstituted Β-Diketonate Complex and Benzenes. Cryst. Eng. Comm. 2007, 9, 215-217. 43. Cho, D. M.; Parkin, S. R.; Watson, M. D., Partial Fluorination Overcomes Herringbone Crystal Packing in Small Polycyclic Aromatics. Org. Lett. 2005, 7, 1067-1068. 44. Ryerson, J. L.; Schrauben, J. N.; Ferguson, A. J.; Sahoo, S. C.; Naumov, P.; Havlas, Z.; Michl, J.; Nozik, A. J.; Johnson, J. C., Two Thin Film Polymorphs of the Singlet Fission Compound 1,3Diphenylisobenzofuran. J. Phys. Chem. C 2014, 118, 12121-12132. 45. Coates, G. W.; Dunn, A. R.; Henling, L. M.; Ziller, J. W.; Lobkovsky, E. B.; Grubbs, R. H., Phenyl-Perfluorophenyl Stacking Interactions: Topochemical [2+2] Photodimerization and Photopolymerization of Olefinic Compounds. J. Am. Chem. Soc. 1998, 120, 3641-3649.

23



46. Ponzini, F.; Zagha, R.; Hardcastle, K.; Siegel, J. S., Phenyl/Pentafluorophenyl Interactions and the Generation of Ordered Mixed Crystals: Sym-Triphenethynylbenzene and SymTris(Perfluorophenethynyl)Benzene. Angew. Chem. Int. Ed. 2000, 39, 2323-2325. 47. Johnson, J. C.; Michl, J., 1,3-Diphenylisobenzofuran: A Model Chromophore for Singlet Fission. Top. Curr. Chem. 2017, 375, 80. 48. Chen, X.; Addicoat, M.; Irle, S.; Nagai, A.; Jiang, D., Control of Crystallinity and Porosity of Covalent Organic Frameworks by Managing Interlayer Interactions Based on SelfComplementary Pi-Electronic Force. J. Am. Chem. Soc. 2013, 135, 546-549. 49. Alahakoon, S. B.; McCandless, G. T.; Karunathilake, A. A.; Thompson, C. M.; Smaldone, R. A., Enhanced Structural Organization in Covalent Organic Frameworks through Fluorination. Chem. Eur. J. 2017, 23, 4255-4259. 50. Kaleeswaran, D.; Vishnoi, P.; Murugavel, R., [3+3] Imine and -Ketoenamine Tethered Fluorescent Covalent-Organic Frameworks for CO2 Uptake and Nitroaromatic Sensing. J. Mater. Chem. C 2015, 3, 7159-7171. 51. Piscopo, C. G.; Trapani, F.; Polyzoidis, A.; Schwarzer, M.; Pace, A.; Loebbecke, S., Positive Effect of the Fluorine Moiety on the Oxygen Storage Capacity of UiO-66 Metal-Organic Frameworks. New J. Chem. 2016, 40, 8220-8224. 52. Liu, D.-P.; Chen, Q.; Zhao, Y.-C.; Zhang, L.-M.; Qi, A.-D.; Han, B.-H., Fluorinated Porous Organic Polymers Via Direct C-H Arylation Polycondensation. ACS Macro Lett. 2013, 2, 522526. 53. 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.

24


Page 24 of 26

Page 25 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


54. Lange, R. Z.; Hofer, G.; Weber, T.; Schluter, A. D., A Two-Dimensional Polymer Synthesized through Topochemical [2 + 2]-Cycloaddition on the Multigram Scale. J. Am. Chem. Soc. 2017, 139, 2053-2059. 55. Yemam, H. A.; Mahl, A.; Koldemir, U.; Remedes, T.; Parkin, S.; Greife, U.; Sellinger, A., Boron-Rich Benzene and Pyrene Derivatives for the Detection of Thermal Neutrons. Sci. Rep. 2015, 5, 13401. 56. Takfaoui, A.; Touzani, R.; Soule, J.-F.; Dixneuf, P. H.; Doucet, H., Palladium-Catalyzed Direct Arylation Using Free-Amine-Substituted Polyfluoroanilines with Inhibition of Amination-Type Reaction. Asian J. Org. Chem. 2015, 4, 1085-1095. 57. Krebs, F. C.; Jensen, T., Fluorinated Molecules Relevant to Conducting Polymer Research. J. Fluorine Chem. 2003, 120, 77-84. 58. Kresse, G.; Hafner, J., Ab Initio Molecular-Dynamics Simulation of the Liquid-MetalAmorphous-Semiconductor Transition in Germanium. Phys. Rev. B 1994, 49, 14251-14269. 59. Blochl, P. E., Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953-17979. 60. Dion, M.; Rydberg, H.; Schroder, E.; Langreth, D. C.; Lundqvist, B. I., Van Der Waals Density Functional for General Geometries. Phys. Rev. Lett. 2004, 92, 246401. 61. Berland, K.; Hyldgaard, P., Exchange Functional That Tests the Robustness of the Plasmon Description of the Van Der Waals Density Functional. Phys. Rev. B 2014, 89, 035412. 62. Walton, K. S.; Snurr, R. Q., Applicability of the Bet Method for Determining Surface Areas of Microporous Metal-Organic Frameworks. J. Am. Chem. Soc. 2007, 129, 8552-6.

25



For Table of Contents Use Only

Synopsis: Self-complimentary phenyl/perfluorophenyl interactions lead to a striking improvement in both surface area and long-range order in a 2D covalent organic framework.

ACS Paragon Plus26 Environment

Page 26 of 26

Perfluorophenyl Stacking Interactions Enhance Structural

Recommend Documents