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COMMUNICATION

Cite this: DOI: 10.1039/c7ce00598a

Received 28th March 2017,Accepted 26th April 2017

DOI: 10.1039/c7ce00598a

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Experimental and theoretical insight into theeffect of fluorine substituents on the properties ofazine linked covalent organic frameworks†

Sampath B. Alahakoon,‡ Gino Occhialini,‡ Gregory T. McCandless,Arosha A. K. Karunathilake, Steven O. Nielsen and Ronald A. Smaldone *

Herein we report a combined experimental and computational

study on the effect of fluorine atom incorporation on the materials

properties of azine-linked COFs. We found that increasing the ra-

tio of fluorinated to non-fluorinated monomers led to substantial

improvements in both crystallinity and porosity. Computational

models suggest that this improvement might be explained by a

substantial energetic stabilization in the face-to-face stacking

interaction of the fluorinated COF monomers.

Covalent organic frameworks (COFs)1,2 are a class ofcrystalline porous polymer networks that are synthesized withthe aid of dynamic covalent bonds.3 The formation ofboronate esters,1,2 spiroborates,4 imines,5,6 hydrazones,7,8

triazines,9 and azines10,11 have all been successfully utilizedto form COFs. These highly porous materials with welldefined crystalline structures have attracted great attentionover the past decade due to the broad spectrum of potentialapplications in gas storage,12 solar and electrical energystorage,12 and catalysis.8,13 Numerous studies14–20 havedemonstrated that the structure and properties of 2D-COFsare greatly influenced by the quality of the non-covalent inter-actions between the 2D layers. We have previously shown thatthe addition of electron withdrawing fluorine atoms greatlyimproves the COF properties even when using non-planarmonomers.21 In an attempt to further elucidate the nature ofthis observation we have carried out a combined experimentaland theoretical study that aims to clarify the role that aro-matic stacking interactions have on the formation and struc-ture of these COFs. In this report we have synthesized a seriesof mixed COFs using fluorinated (TF) and non-fluorinated(NF) monomers with varied feed ratios. Furthermore, we havecarried out a series of quantum mechanical calculations to

determine the interaction energies between these monomersin the face-to-face orientation that would be expected in aneclipsed 2D COF.

Monomers NF and TF were synthesized as reported previ-ously.21 Mixed TFx-COFs were synthesized by varying themonomer feed ratios of NF and TF (Fig. 1). Each of theseCOFs were prepared solvothermally in glass ampoules usinghydrazine as the co-monomer in a solvent system ofo-dichlorobenzeneIJDCB)/n-butanol/6 M aqueous acetic acid(1.9/0.1/0.1 v/v/v).

Each of the mixed linker COFs was digested in acidifiedd6-DMSO and 1H-NMR studies were carried out to determinewhether the final COF composition matched the monomerfeed ratio. The incorporation of different ratios of the mono-mers in the COFs was confirmed by the integration ratios ofNF (Ha-10.013 ppm) and TF (Hb-10.188 ppm) aldehyde peaks(Fig. 2C). These experiments showed that the incorporationratio (Fig. 2C) of each monomer is consistent with the feedratio (Fig. 1).

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Department of Chemistry and Biochemistry, University of Texas at Dallas, 800 W.Campbell Rd., Richardson, TX 75080, USA.E-mail: ronald.smaldone@utdallas.edu† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ce00598a‡ These authors contributed equally.

Fig. 1 Synthesis of the TFX-COF series by varying the mole% of the TFmonomer with the remaining monomers represented by NF.

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The periodic arrangement of the COFs was analyzed usingpowder X-ray diffraction (PXRD) analysis. Of the mixed COFseries, only TF50-COF, TF75-COF, and TF100-COF exhibited ob-servable diffraction peaks at 3.67, 6.09, 9.28, 12.80 and 24.83°which were assigned to the (100), (110), (120), (130), and(001) reflections, respectively. Poor crystallinity was observedin the TF0-COF and TF25-COF as they displayed only two dif-fraction peaks at 3.67 and 6.09 with low intensity (Fig. 2B).The simulated PXRD patterns (generated in Materials Studio)of the eclipsed layers of COFs (Fig. S2†) are in good agree-ment with the experimental patterns. Interestingly, the dif-fraction peak intensity of (001) reflection, which representsthe interlayer spacing between COF sheets, improves signifi-cantly with increasing mole fraction of the TF monomer inthe reaction. SEM images (Fig. 3) of each mixed COF indi-cates a morphological change in the crystallites from smoothspherical agglomerates of TF0-COF to long rod-like morphol-ogy in TF100-COF with the variation of X mol% from 0–100.The change in morphology has been attributed to increasedmicroscopic ordering in previous reports.22

The surface area and pore size distributions of each COFwere determined through nitrogen sorption measurements at

77 K (Fig. 4A and B). The TF0-COF and TF25-COF exhibit type-I adsorption isotherms indicating microporous structures.However, when the TF content increases to X = 50 and 100the shape of the isotherms changes to type IV indicating theformation of mesoporous materials (Fig. 4A). The pore sizedistribution also indicates a consistent change from largelymicroporous to mesoporous character with increasing con-centration of TF in the COF (Fig. 4B). The Brunauer–Emmett–Teller (BET) surface areas of mixed-linker COFsexhibited a linear increment from 710–1064 m2 g−1, when Xvaries form 25–75. However, TF100-COF displayed a surfacearea of 1802 m2 g−1 which is a 756 m2 g−1 improvement com-pared to TF75-COF (Fig. 4C).

One potential explanation for these observations involvesthe polarization of the aromatic rings induced by the fluorinesubstituents. This polarization has been shown to improvethe co-facial interactions23 between aromatic rings that arepresent in eclipsed 2D COFs. To gain better insight into therole of the aromatic stacking interactions, we performedquantum mechanical computational studies to calculate theinteraction energy of the face-to-face interaction of eachmonomer type. Quantum mechanical calculations were

Fig. 2 (A) Eclipsed layers of COFs. (B) PXRD spectra of the series of COFs. Black, green, purple, blue and red lines represent the TF0-COF, TF25-COF, TF50-COF, TF75-COF and TF100-COF, respectively. (C) 1H-NMR spectra of digested mixed COFs and the interaction of each aldehyde peaks inthe mixed COFs.

Fig. 3 Morphological change in the SEM images with the variation of TF content in the COFs.

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performed to measure the interaction energy between themonomers of an optimized dimer structure. Initial genera-tion and preparation of the dimer structures was carried outin Avogadro,24 in which the geometry was optimized usingthe MMFF94s25 force field in order to get a good startingpoint for Hartree–Fock26 optimization. Quantum calculationswere then performed to minimize the dimer structures usingGAMESS (US)27,28 via RHF with the 6-31G(d) basis set andGrimme's semi-empirical dispersion correction.29,30 Afterminimization of the dimer, single point energies were calcu-lated for each individual monomer using the same dispersioncorrected HF 6-31G(d) method. The stacking interaction en-ergy was determined by subtracting the energies of the indi-vidual contributing monomers from the total energy of theminimized dimer structure. This procedure was used to cal-culate the energy of the stacking interactions between TF/TF,NF/TF, and NF/NF monomers (Fig. 5). These calculationsfound stacking energies of −132, −128, and −100 kJ mol−1, re-spectively, for these co-facial arrangements. The change fromNF/NF to NF/TF caused a subsequent increase in associativeinteractions by 28 kJ mol−1, while going from NF/TF to TF/TFan additional 4 kJ mol−1 of stabilization energy. These calcu-lations suggest that there is a stronger preference for the TF–

TF co-facial arrangement which could be a significant drivingforce in converting the kinetic polymer products to the ther-modynamically favored COF.

One question we sought to answer with this study waswhether COF formation could be improved using a mixtureof electron rich and electron poor monomers rather thanhomogeneous mixtures. This does not appear to be true inthis case. The behaviour observed with this particular mono-mer system appears to be consistent with other aromatic ma-terials, such as m-phenylene ethynylene foldamers,31 whichhave been shown form more stable folded structures usingaromatic monomer units that contain electron poor units.We believe that this could be a potential design strategy forthe synthesis of COFs using monomers that do not crystallizeeasily. To achieve mixed layer COFs that organize based oncharge-transfer, or donor–acceptor interactions will likely re-quire much larger polarization in the monomer units.

In conclusion, we have carried out experimental and com-putational studies on a class of mixed-linker azine COFs.These studies indicate that the quality of the aromatic stack-ing interactions between COF monomers plays an importantrole in the formation and final structural characteristics ofazine COFs. Experimental studies demonstrate that increasedincorporation of the more electron rich NF monomers dis-rupts the COF formation indicating that tuning the aromaticstacking interactions between the monomers is a key compo-nent to the design of highly ordered COFs. Computationalstudies support this hypothesis by showing that the interac-tion between electron poor TF rings is more favourable thanthose between the comparatively electron rich NF monomers,or even mixed dimers of NF and TF. Future work will include

Fig. 4 (A) Nitrogen isotherms (77 K), (B) NLDFT pore size distributionsand (C) BET surface areas of the TFx-COFs.

Fig. 5 Computational models of the energy minimized stacked dimers(top, hydrogen atoms omitted for clarity). Interaction energies of theface-to-face stacking arrangements possible in each of the representa-tive mixed COFs (bottom). The energies are measured relative to thehighest energy NF/NF dimer.

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the use of other monomer systems to attempt to elucidatethe optimal conditions to make COFs.

AcknowledgementsThis research was supported with funds from the Universityof Texas at Dallas and the American Chemical Society Petro-leum Research Fund (52906-DNI10).

Notes and references1 A. P. Cote, A. I. Benin, N. W. Ockwig, M. O'Keeffe, A. J.

Matzger and O. M. Yaghi, Science, 2005, 310, 1166.2 H. El-Kaderi, J. R. Hunt, J. L. Mendoza-Cortes, A. P. Cote,

R. E. Taylor, M. O'Keeffe and O. M. Yaghi, Science, 2007, 316,268.

3 Y. Jin, C. Yu, R. J. Denman and W. Zhang, Chem. Soc. Rev.,2013, 42, 6634–6654.

4 Y. Du, H. Yang, J. M. Whiteley, S. Wan, Y. Jin, S. Lee and W.Zhang, Angew. Chem., Int. Ed., 2016, 55, 1737–1741.

5 F. Uribe-Romo, J. R. Hunt, H. Furukawa, C. Klock, M.O'Keeffe and O. M. Yaghi, J. Am. Chem. Soc., 2009, 131,4570–4571.

6 S. Dalapati, M. Addicoat, S. Jin, T. Sakurai, J. Gao, H. Xu, S.Irle, S. Seki and D. Jiang, Nat. Commun., 2015, 6, 7786.

7 F. Uribe-Romo, C. J. Doonan, H. Furukawa, K. Oisaki andO. M. Yaghi, J. Am. Chem. Soc., 2011, 133, 11478–11481.

8 L. Stegbauer, K. Schwinghammer and B. V. Lotsch, Chem.Sci., 2014, 5, 2789–2793.

9 P. Kuhn, M. Antonietti and A. Thomas, Angew. Chem., Int.Ed., 2008, 47, 3450–3453.

10 S. Dalapati, S. Jin, J. Gao, Y. Xu, A. Nagai and D. Jiang, J. Am.Chem. Soc., 2013, 135, 17310–17313.

11 S. B. Alahakoon, C. M. Thompson, A. X. Nguyen, G.Occhialini, G. T. McCandless and R. A. Smaldone, Chem.Commun., 2016, 52, 2843–2845.

12 S. B. Alahakoon, C. M. Thompson, G. Occhialini and R. A.Smaldone, ChemSusChem, 2017, DOI: 10.1002/cssc.201700120.

13 P. Pachfule, S. Kandambeth, D. Diaz Diaz and R. Banerjee,Chem. Commun., 2014, 50, 3169–3172.

14 B. J. Smith and W. R. Dichtel, J. Am. Chem. Soc., 2014, 136,8783–8789.

15 B. J. Smith, A. C. Overholts, N. Hwang and W. R. Dichtel,Chem. Commun., 2016, 52, 3690–3693.

16 L. M. Salonen, D. D. Medina, E. Carbo-Argibay, M. G.Goesten, L. Mafra, N. Guldris, J. M. Rotter, D. G. Stroppaand C. Rodriguez-Abreu, Chem. Commun., 2016, 52,7986–7989.

17 V. S. Vyas, F. Haase, L. Stegbauer, G. Savasci, F. Podjaski, C.Ochsenfeld and B. V. Lotsch, Nat. Commun., 2015, 6, 8508.

18 S. Jin, K. Furukawa, M. Addicoat, L. Chen, S. Takahashi, S.Irle, T. Nakamura and D. Jiang, Chem. Sci., 2013, 4,4505–4511.

19 X. Chen, M. Addicoat, S. Irle, A. Nagai and D. Jiang, J. Am.Chem. Soc., 2013, 135, 546–549.

20 L. Ascherl, T. Sick, J. T. Margraf, S. H. Lapidus, M. Calik, C.Hettstedt, K. Karaghiosoff, M. Doblinger, T. Clark, K. W.Chapman, F. Auras and T. Bein, Nat. Chem., 2016, 8,310–316.

21 S. B. Alahakoon, G. T. McCandless, A. A. K. Karunathilake,C. M. Thompson and R. A. Smaldone, Chem. – Eur. J.,2017, 23, 4255–4259.

22 A. Halder, S. Kandambeth, B. P. Biswal, G. Kaur, N. C. Roy,M. Addicoat, J. K. Salunke, S. Banerjee, K. Vanka, T. Heine,S. Verma and R. Banerjee, Angew. Chem., Int. Ed., 2016, 55,7806–7810.

23 C. R. Martinez and B. L. Iverson, Chem. Sci., 2012, 3,2191–2201.

24 M. D. Hanwell, D. E. Curtis, D. C. Lonie, T. Vandermeersch,E. Zurek and G. R. Hutchison, J. Cheminf., 2012, 4, 17.

25 T. A. Halgren, J. Comput. Chem., 1996, 17, 490–519.26 C. C. J. Roothaan, Rev. Mod. Phys., 1951, 23, 69–89.27 M. W. Schmidt, K. K. Baldridge, J. A. Boatz, S. T. Elbert,

M. S. Gordon, J. H. Jensen, S. Koseki, N. Matsunaga, K. A.Nguyen, S. Su, T. L. Windus, M. Dupuis and J. A.Montgomery, J. Comput. Chem., 1993, 14, 1347–1363.

28 M. S. Gordon and M. W. Schmidt, Theory and Applications ofComputational Chemistry, ed. G. Frenking, K. S. Kim and G.E. Scuseria, 2005, pp. 1167–1189.

29 R. Peverati and K. K. Baldridge, J. Chem. Theory Comput.,2008, 4, 2030–2048.

30 S. Grimme, J. Antony, S. Ehrlich and H. Krieg, J. Chem.Phys., 2010, 132, 154104.

31 D. J. Hill, M. J. Mio, R. B. Prince, T. S. Hughes and J. S.Moore, Chem. Rev., 2001, 101, 3893–4012.

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