Enhanced Structural Organization in Covalent Organic Frameworks Through Fluorination

Article abstract of DOI:10.1002/chem.201700412

Here, we report a structure–function study of imine covalent organic frameworks (COFs) comparing a series of novel fluorine-containing monomers to their non-fluorinated analogues. We found that the fluorine-containing monomers produced 2D-COFs with not only greatly improved surface areas (over 2000 m² g^?1 compared to 760 m² g^?1 for the non-fluorinated analogue), but also with improved crystallinity and larger, more defined pore diameters. We then studied the formation of these COFs under varying reaction times and temperatures to obtain a greater insight into their mechanism of formation.

Full text of DOI:10.1002/chem.201700412

A Journal of
Accepted Article
Title: Enhanced Structural Organization in Covalent Organic
Frameworks Through Fluorination
Authors: Sampath B Alahakoon, Gregory T McCandless, Arosha A. K.
Karunathilake, Christina M Thompson, and Ronald Alexander
Smaldone
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To be cited as: Chem. Eur. J. 10.1002/chem.201700412
Link to VoR: http://dx.doi.org/10.1002/chem.201700412
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Enhanced Structural Organization in Covalent Organic
Frameworks Through Fluorination
Sampath B. Alahakoon, Gregory T. McCandless, Arosha A. K. Karunathilake, Christina M. Thompson
and Ronald A. Smaldone*
Abstract: Herein we report a structure-function study of imine COFs
comparing a series of novel fluorine containing monomers to their
non-fluorinated analogue. We found that the fluorine containing
monomers produced 2D-COFs with not only greatly improved
surface areas (over 2000 m2/g compared to 760 m2/g for the non-
fluorinated analogue), but also with improved crystallinity and larger,
better defined pore diameters. We then studied the formation of
these COFs under varying reaction times and temperatures to obtain
a greater insight into their mechanism of formation.
comparison between fluorinated and non-fluorinated monomers
without introducing significant steric effects. We have
synthesized two fluorine containing 1,3,5-triphenylbenzene
derivatives to take advantage of this co-facial stacking
preference, and were gratified to see that the addition a few
fluorine atoms results in a drastic change not only in the
crystallinity, but also the pore size and surface area of the
resulting COFs.
Monomers NF, TF-1, and TF-2 were synthesized through a
Suzuki cross-coupling reaction (see Supporting Information for
details). All three COFs were prepared solvothermally in glass
ampoules by polymerizing monomers NF, TF-1 or TF-2 with
hydrazine (Figure 1).
Covalent organic frameworks (COFs)[1] are a class of crystalline
porous polymers synthesized utilizing dynamic covalent bonds[2]
such as boronate esters,[1a,1b] imines,[3] hydrazones,[1c,4]
triazines,[5] and azines.[6] These materials have attracted
remarkable attention due to their well-defined structures, high
porosity, and potential applications in gas storage,[1a,7]
catalysis,[4, 8] and as materials for electrical energy storage.[3b, 9]
In 2D-COFs, the structure and properties are dictated by both
covalent bond formation under thermodynamic control, and by
non-covalent interactions between aromatic rings that enables
the formation of periodically aligned molecular columns.[6e] There
have been a number of excellent studies[10] that attribute the
efficiency of non-covalent packing in COF formation to forces as
wide ranging as monomer planarity,[6d] and molecular recognition
through templated docking.[9a,11] Given that each of these
hypotheses have proven to be successful in making specific
types of COFs, it is clear that more studies into the role of non-
covalent packing are imperative for the development of general
design rules.
The purpose of the work reported here is to further elucidate the
importance of aromatic interactions between individual sheets
during COF formation. Interactions between aromatic rings are
controlled by a variety of factors including substituent effects that
change the electron density or polarization of the π-cloud. Many
explanations[12] have been posited to explain the nature of
aromatic stacking interactions, but empirically, electron deficient
rings are known to preferentially adopt face-to-face
arrangements. Since this co-facial orientation is representative
of the structure of most 2D-COFs, we hypothesized that we
could bias toward the formation of stacked COF sheets by
incorporating electron withdrawing substituents onto the
periphery of our COF monomers. We chose to use fluorine
substitution for this study as it is not only electron withdrawing,
but similar in steric bulk to hydrogen, thereby allowing a closer
[*]
S.B. Alahakoon, Dr. G.T. McCandless, A.A.K. Karunathilake, Dr.
C.M. Thompson, Prof. R. A. Smaldone
Department of Chemistry and Biochemistry
The University of Texas at Dallas, 800 W. Campbell Rd
Richardson, TX 75080, USA.
E-mail: ronald.smaldone@utdallas.edu
Supporting information for this article is given via a link at the end of
the document.
Figure 1. Schematic representation of the synthesis of the azine-linked COFs.
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Though several solvent systems were tested, a mixture of o-
dichlorobenzene(DCB)/n-butanol/6M aqueous acetic acid
(1.9/0.1/0.1 by vol.) was found to be optimal.
of NF-COF. Previous studies have attributed this to improved
COF crystallinity owing to more organized interlayer stacking.[13]
The surface area and pore size distributions of each COF were
determined through nitrogen sorption measurements at 77 K
(Figure 3). The NF-COF exhibits a type-I adsorption isotherm.
However, both the fluorinated COFs exhibit type-IV isotherms,
indicating the presence of mesopores (Figure 3e). The
Brunauer-Emmett-Teller (BET) surface areas of TF-COF 1 and
TF-COF 2 are 1820 and 2044 m2/g, respectively, compared to
only 760 m2/g for NF-COF. To the best of our knowledge 2044
m2/g is among the highest reported for 2D-COFs to date and
COF surface areas over 2000 m2/g are rare.[8b] The pore size
distributions for each COF (Figure 3f) were obtained from the
nitrogen isotherms using the density functional theory method
(DFT). The pore size distribution of NF-COF displays not only
the expected pore size of 25 Å, but also has many other smaller
pores. In combination with its type-I isotherm and poorly
resolved diffraction pattern, it seems likely that the 2-D structure
of NF-COF is poorly ordered and is not well represented by the
hexagonal crystalline model often used to describe these COFs.
TF-COFs 1-2, however, display much more well defined pore
structures. Both of these COFs have properties that are
reminiscent of an eclipsed layer arrangement that demonstrate
excellent agreement with the simulated pore diameters.
To help determine the 2-D structure of each COF, powder X-ray
diffraction (PXRD) measurements were performed. TF-COF 1
exhibited diffraction peaks at 3.67, 6.09, 9.28, 12.80 and 24.83°.
TF-COF 2 exhibited diffraction peaks at 3.71, 6.19, 9.43, 12.41
and 23.70°. The peaks of both the COFs were assigned to the
(100), (110), (120), (130), and (001) reflections, respectively.
The only peaks clearly observed for NF-COF appeared at 3.67
and 6.09, indicating a lower level of crystalline order (Figure 2a).
Computational models of each COF were created using
Materials Studio in both staggered and eclipsed arrangements.
The calculated PXRD patterns of the eclipsed TF-COFs (Figure
2b,c) match well with the experimental pattern. Significant
enhancement in the resolution of the diffraction peaks was
observed with both TF-COFs compared to NF-COF.
Interestingly, the (001) reflection, attributed to the interlayer
spacing between COF sheets, is much more prominent for both
the TF-COFs than for NF-COF, indicating a higher degree of
long range order. SEM images of each COF are shown in Figure
2d-f. Notably, the fluorinated COFs display crystallite
morphology as opposed to the smooth, spherical agglomerates
Figure 2. Experimental PXRDs of (A) NF-COF (black), (B) TF-COF 1 (red), (C) and TF-COF 2 (blue) with Pawley refined spectra, simulated eclipsed models and
difference spectra (purple, green and orange, respectively). Insets: Modeled eclipsed structures of COFs (C, grey; O, red; N, blue; F, light green. (H atoms are
omitted for clarity). (D), (E) and (F) are SEM images of NF-COF, TF-COF 1 and TF-COF 2, respectively.
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Figure 3. (A), (C) and (E) N2 adsorption (solid circles) and desorption (open circles) isotherms (77 K) of 1 d RT, 1 d 120 °C and 3 d 120 °C COFs, respectively.
(B), (D) and (F) PSD plots of 1 d R, 1 d 120 °C and 3 d 120 °C COFs, respectively. (G) BET surface areas and (H) and mass recovery (%) of COFs.
A previous report[10c] on the synthesis of imine COFs found that
the COF polymers initially formed porous materials that would
equilibrate to more crystalline materials over time. We performed
several polymerizations of our COFs at different temperatures
and reaction times to observe their formation behavior. Each
COF was prepared with reaction times of 1d at either room
temperature or 120 °C and 3d at 120 °C. After each experiment,
the BET surface area, pore size distribution, and PXRD
measurements (Figure S1) were carried out. These experiments
revealed several interesting properties. Surprisingly, TF-COFs 1-
2 display type-IV isotherms after only 1d at RT (Figure 3a) with a
narrow pore size distribution (Figure 3b). As the temperature
and reaction times increased, the surface areas for both
fluorinated COFs increased significantly with the largest
increase coming after heating to 120 ºC for one day. However,
NF-COF did not display the same improvement in crystallinity or
display pore size distribution narrowing over time. The surface
area for NF-COF does increase with increasing reaction time
and temperature, but nowhere near as significantly as the TF-
COFs 1-2, particularly after heating for only one day. Fourier
transform infrared (FTIR) spectra of the COFs confirmed the
formation of the azine linkage through disappearance of the
carbonyl (1690 cm-1) and the appearance of the azine (1622 cm-
1) stretches (Figure S2). These spectra reflect that the formation
of the azine linkages form rapidly and are the major product
(compared to the starting aldehydes) within 1d at 120 °C for all
three COFs. In addition to this, the mass balances of each
polymerization were recorded (Figure 3h) and it was found that
more than 70% of the expected yield was recovered for each
reaction regardless of the time or temperature. Longer reaction
times at high temperatures resulted in somewhat higher yields
(TF-COF 2 was ~90% for example), but overall the indication is
that, consistent with previous work,[10c] a kinetic product appears
to form first, followed by slow reorganization into a crystalline
product with well-defined 2-D structure in the cases of TF-COFs
1-2. This work is novel in that, rather than modifying the
reaction conditions, we have changed the chemical structure of
the monomers. In other words, the electronic structure of the
monomers can also favorably drive the conversion of the
polymeric material from the non-crystalline kinetic product, to the
thermodynamically favorable crystalline COF product. The
presence of electron withdrawing fluorine substituents could
serve to make the azine linkages more reactive and avoid the
formation of kinetic traps. The fluorine atoms also could cause
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Addicoat, S. Jin, T. Sakurai, J. Gao, H. Xu, S. Irle, S. Seki, D. Jiang,
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polarization in the aromatic rings which would lead to stronger
cofacial interactions.[12b] This effect should be especially
pronounced in the case of TF-COF 2 as all three of the fluorine
atoms are on the same ring, and indeed, TF-COF 2 has highest
surface area observed in this study. A previous report[6d] on a
related series of azine-linked COFs showed that crystallinity and
surface area were improved by planarizing the monomers. In
this case, we do not observe a correlation between torsion angle
and surface area, and in fact, the highest surface area materials
(TF-COF 2) has the largest torsion angle (Figure S20). We
believe that these observations show that COF polymerization
and crystallization can be controlled effectively through the
careful design of both the steric and electronic structure of the
monomers.
In conclusion, we have synthesized two novel fluorinated COFs
that have significantly improved physical properties compared
with an isostructural non-fluorinated variant. This work
demonstrates that modifications to the electronic structure of
COF monomers can result in an immense improvement in the
structural properties of the COFs. We believe that these design
principles can be applied to other types of COFs and future work
will be directed towards further elucidating these principles.
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Improved layer stacking: A strategy
to improve the crystallinity and the
surface area of 2D-COFs significantly
by simple variation in the electronic
structure of non-planar precursors by
fluorination.
Sampath B. Alahakoon, Gregory T.
McCandless, Arosha A. K.
Karunathilake, Christina M. Thompson
and Ronald A. Smaldone*
Enhanced Structural Organization in
Covalent Organic Frameworks
Through Fluorination
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