Computational and Experimental Studies on the Effects of Monomer Planarity on Covalent Organic Framework Formation

Article abstract of DOI:10.1021/jacs.7b05555

We report the synthesis of one new boronate ester-based covalent organic framework (COF) and two new covalent organic polymers (COPs) made with fluoranthene-containing monomers and hexahydroxytriphenylene. The structure of the monomer heavily influences w

Full text of DOI:10.1021/jacs.7b05555

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Article
Computational and Experimental Studies on the Effects of
Monomer Planarity on Covalent Organic Framework Formation
Christina M Thompson, Gino Occhialini, Gregory T McCandless, Sampath B.
Alahakoon, Victoria Cameron, Steven O. Nielsen, and Ronald A. Smaldone
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b05555 • Publication Date (Web): 11 Jul 2017
Downloaded from http://pubs.acs.org on July 11, 2017
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Computational and Experimental Studies on the Effects of
Monomer Planarity on Covalent Organic Framework For-
mation
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Christina M. Thompson,† Gino Occhialini,† Gregory T. McCandless, Sampath B. Alahakoon,
Victoria Cameron, Steven O. Nielsen* and Ronald A. Smaldone*
Department of Chemistry and Biochemistry, University of Texas at Dallas, 800 W. Campbell Rd, Richardson, TX,
75080.
ABSTRACT: We report the synthesis of one new boronate ester based covalent organic framework (COF) and two new
covalent organic polymers (COPs) made with fluoranthene containing monomers and hexahydroxytriphenylene. The
structure of the monomer heavily influences whether this material forms a highly ordered mesoporous material (COF)
or an amorphous, microporous material (COP). The synthesis of the fluoranthene monomers was carried out using a
divergent strategy that allows for systematic structural variation and the ability to conduct a careful structureꢀfunction
study. We found that small structural variations in the monomers dramatically affected the crystallinity, surface area,
pore structure, and luminescence properties of the polymers. While each of the monomers contains the same fluoranꢀ
thene core, the resultant pore sizes range from microporous (10 Å) to mesoporous (37 Å) with surface areas ranging
from ~500ꢀ1200 m2/g. To help explain how these small structural differences can have such a large effect, we carried
out a series of molecular dynamics simulations on the polymers to obtain information with atomic scale resolution on
how
the
monomer
structure
affects
nonꢀcovalent
COF
layer
stacking.
individual sheets into a crystalline network has only
been explored using a few limited structure types.
INTRODUCTION
Covalent organic frameworks1 (COFs) are a class of
crystalline porous polymers that boast high surface
areas, as well as tunable pore sizes and functionalizaꢀ
tion. These attributes have led to the rapid expansion of
COFꢀbased applications including gas storage and sepꢀ
aration,2 catalysis,3 capacitive energy storage,2d,4 and
sensing.5 Given the promising versatility of COFs for
practical use, there have been a number of efforts to
develop a clear and concise understanding of the
mechanism of COF formation.6 Generally speaking,
two fundamental processes control the structure and
formation of a COF: 1) the formation of covalent bonds
under thermodynamic control to form twoꢀdimensional
sheets, and 2) the aggregation of those sheets to form
crystalline arrangements that result in porous frameꢀ
works. The first step of COF formation is typically
achieved through the use of reversible covalent linkages
such as boronic esters, imines, azines, hydrazones, and
others whose reversibility can be controlled under
thermal catalytic conditions. However, despite several
thorough mechanistic studies on COF formation, roꢀ
bust design rules for the de novo synthesis of COFs still
require further elucidation. While it has been shown
that nonꢀplanar substituents can affect COF
structure,2f,3b,7 the extent to which outꢀofꢀplane substitꢀ
uents interfere with the aggregation or templation of
Presented here is the synthesis and polymerization of
three fluoranthene (FLT) based diboronic acid monoꢀ
mers containing different substituents around the peꢀ
riphery (Figure 1). FLT derivatives have been used in a
variety of applications including as precursors for
curved polycyclic aromatic hydrocarbons,8 and as solidꢀ
state sensors for explosive compounds.9 FLT derivaꢀ
tives are an ideal scaffold to study COF formation owꢀ
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ing to their flat aromatic core structure
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B
O
O
2
3
4
5
O
B
O
B
R2
O
O
R1
R2
6
R1
O
O
O
O
B
B
B
B
O
O
O
O
7
8
9
O
O
O
O
B
B
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COF/COP R1 R2
R1
R2
H
Ph
Ph Ph
H
H
R1
R2
FLT-COF-1
FLT-COP-2
FLT-COP-3
B
B
O
O
O
B
O
B
O
B
O
B
R2
O
O
O
O
R1
R1
R2
O
O
B
B
O
O
O
O
B
Figure 1. Structure of the FLT polymers synthesized as
part of this study. The FLT materials were synthesized
through a condensation coꢀpolymerization between diboꢀ
ronic acid monomers and hexahydroxytriphenylene reꢀ
sulting in
a
boronate ester based polymer.
O
O
O
Br
Br
R1 R2
COF/COP
OH
DCC, DMAP
56%
KOH
H
Ph
Ph Ph
H
H
O
O
FLT-COF-1
FLT-COP-2
FLT-COP-3
CH2Cl2:MeOH (1:6)
Br
Br
Br
reflux
2 h
90%
1
OH
HO
1) NaIO4
THF:H2O:EtOAc
3:1:1
B2(Pin)2
Pd(dppf)
KOAc
O
O
O
O
OH
OH
(HO)2B
B(OH)2
Br
Br
B
B
100 ºC, 24 h
1,4-dioxane
2) 1M HCl
rt, 24 h
110 ºC
72 h
88%
Ac2
O
140 ºC / 7d
77%
HO
87%
OH
2
4
3
Mesitylene/
Dioxane
1:1
B2(Pin)2
Pd(dppf)
KOAc
1) NaIO4
THF:H2O:EtOAc
3:1:1
R
R
R
R
O
O
90 ºC
72 h
38-40%
1,4-dioxane
100 ºC, 24 h
B
B
Br
Br
(HO)2B
B(OH)2
)))) 300 W
250 ºC / 2h
diphenyl ether
O
O
110 ºC
72 h
2) 1M HCl
rt, 24 h
a) 74%
b) 83%
a) 89%
b) 85%
a) 85%
b) 81%
6a-b
5a-b
7a-b
R
a) H
b) Ph
R
a) H
b) Ph
R
a) H
b) Ph
Scheme 1: Synthesis of FLT monomers and FLTꢀCOF 1 and FLTꢀCOP 2ꢀ3.
and their concise and modular synthesis, which allows
for the systematic evaluation of structureꢀfunction efꢀ
fects based on planarity, steric hindrance, and electronꢀ
ics. The structure of the polymers are also examined
using molecular dynamics (MD) simulations which aim
to explain how small differences in the monomer strucꢀ
ture can have large effects on the microscopic polymer
morphology.
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ter, with pore sizes of ~10 Å and a lower concentration
1
of pores at ~20 Å.
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Powder Xꢀray diffraction (PXRD) measurements of
FLTꢀCOFꢀ1 showed strong diffraction, signifying a
highly ordered COF. Diffraction peaks were observed
at 2.32º (100) 4.02º (110), 4.65º (200), 6.15º (210),
6.98º (300), 8.06º (220) and 26º (001) 2θ (Figure 3,
top). Computational models of both staggered and
eclipsed conformations for FLTꢀCOFꢀ1 were built using
the gra and bnn topologies, respectively, in Materials
Studio. The models were minimized using the univerꢀ
sal force field in their respective space groups (P3,
eclipsed, P63/m, staggered) and simulated PXRD patꢀ
terns were generated (Figure 3, bottom). As expected,
the simulated diffraction pattern generated from the
eclipsed model resulted in the best fit. This simulation
predicts a 37 Å pore size, which matches the NLDFT
model of our experimental isotherm, indicating that
this is likely the most accurate description of the FLTꢀ
COF 1 structure. In contrast, both FLTꢀCOPꢀ2 and
FLTꢀCOPꢀ3 displayed little diffraction, indicating that
despite their moderate surface areas and microporous
structures, they have poor long range order. This is
surprising given the similarity of the chemical strucꢀ
tures of monomers 4 and 7a-b. Previous experimental
and theoretical studies have shown that the layers of
boronate ester COFs are held together through a comꢀ
bination of πꢀstacking and dipolar interactions between
the lone pairs of the boronate ester oxygen atoms and
the boron atoms of the adjacent layer.10 We hypotheꢀ
sized that the boronate ester linkages may still be formꢀ
ing during the polymerization with HHTP for all three
polymers, but that the steric hindrance caused by the
extra phenyl rings could be disrupting either the aroꢀ
matic stacking interactions between the FLT units of
FLTꢀCOPs 2ꢀ3, or the interactions between the boron
Figure 2. (A) Nitrogen sorption isotherms at 77 K for
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FLTꢀCOFꢀ1 (orange line), FLTꢀCOPꢀ2 (black line), and
FLTꢀCOPꢀ3 (green line). (B) NLDFT pore size distribuꢀ
tions for all of the FLT polymers. (C) BET surface areas
for the FLT polymers.
RESULTS AND DISCUSSION
The synthesis of the fluorantheneꢀbased monomers
(Scheme 1) was carried out from a common cyclopenꢀ
tadienone precursor 1. 1 is converted to each of the fluꢀ
oranthene core structures (2, 5a-b) through a Dielsꢀ
Alder reaction with norbornadiene, phenylacetylene, or
diphenylacetylene. Precursors 2 and 5a-b are then
converted to the corresponding diboronic acid monoꢀ
mers over two steps starting with a palladium catalyzed
crossꢀcoupling reaction with bis(pinacolato)diboron.
The pinacol groups are then removed through an oxiꢀ
dative cleavage reaction to provide the desired boronic
acids. The entire synthetic route is carried out in good
yields with minimal purification, and can be performed
on multiꢀgram scales. The coꢀpolymerization of monoꢀ
mers 4 or 7a-b into COFs or covalent organic polymers
(COPs) with hexahydroxytriphenylene (HHTP) was
performed in a 1.5:1 molar ratio (diboronic acid :
HHTP) in a mixed solvent system of mesitylene : dioxꢀ
ane (1:1) and heated to 90 °C for 72 h (Scheme 1). Afꢀ
ter this reaction, the insoluble powders were collected
by filtration and washed with toluene before being
dried under vacuum and characterized. Other reaction
conditions were tested, and the results are shown in the
supporting information (Table S1).
and the oxygen lone pairs.
If these interactions are
sufficiently hindered, the layered structure of the COF
may form in two dimensions, but may aggregate in a
disordered fashion as the structure directing forces that
align the COF sheets with one another are disrupted.
While these simulated crystal structures are useful for
correlating PXRD patterns in COFs, they are far too
restrictive to provide information on how small changꢀ
es in the monomer structure could potentially disrupt
these important intermolecular, nonꢀcovalent interacꢀ
tions. This was the impetus to study these polymers
using MD simulations discussed later on.
The surface areas and pore size distributions were
characterized by nitrogen adsorption measurements at
77 K (Figure 2aꢀc). The BrunauerꢀEmmettꢀTeller
(BET) surface area for the FLTꢀCOFꢀ1 was calculated to
be 1180 m2/g, while both FLTꢀCOPꢀ2 and FLTꢀCOPꢀ3
variants showed lower surface areas of 555 m2/g and
515 m2/g, respectively. Interestingly, of the three strucꢀ
tural analogs tested, FLTꢀCOFꢀ1 was the only one that
displayed a type IV isotherm, which is indicative of a
mesoporous material. In contrast, FLTꢀCOPs 2ꢀ3 have
type I adsorption isotherms, indicative of microporous
materials.
Pore size distributions were calculated for all three polꢀ
ymers using the nonꢀlocal density functional theory
(NLDFT) method, confirming that FLTꢀCOFꢀ1 is largely
mesoporous with pore sizes around 37 Å. FLTꢀCOPs
2ꢀ3 were observed to have mostly microporous characꢀ
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Figure 4. FTꢀIR spectra of FLTꢀCOPꢀ1 and FLTꢀCOPs 2ꢀ
3 (solid lines) and their corresponding diboronic acid
monomers (dashed lines). The relevant functional groups
expected to appear (boronate esters) or disappear (boronic
acids) upon COF formation are highlighted.
Previous reports have shown that crystalline COFs
6a
could be delaminated using certain solvent mixtures
or through self exfoliation,11 resulting in disruption of
the nonꢀcovalent 3D stacking structure and loss of crysꢀ
tallinity while leaving the covalently linked structures
intact. To explore this possibility further, we characterꢀ
ized each monomer and polymer using Fourier transꢀ
form infrared spectroscopy (FTIR). Since there are
characteristic differences12 between the boronic acid
starting materials and the boronate ester linked polyꢀ
mers, this technique should be able to probe the extent
of boronic acid to boronic ester conversion. We hyꢀ
pothesized that the lower surface areas and crystallinity
of FLTꢀCOPꢀ2 and FLTꢀCOPꢀ3 could potentially be
explained by either the presence of poorly ordered imꢀ
purities such as oligomeric boronate esters, or unreactꢀ
ed starting materials. Shown in Figure 4 are the FTIR
spectra of all three polymers and their starting materiꢀ
als 4, 7a-b. Surprisingly, all three polymers showed
the CꢀO stretching mode peak characteristic of boroꢀ
nate ester formation (1246 cmꢀ1 for FLTꢀCOFꢀ1, 1244
cmꢀ1 for FLTꢀCOPꢀ2, and 1246 cmꢀ1 for FLTꢀCOPꢀ3),
and lacked the broad OH stretch seen in each of the
boronic acid monomers. This indicates that for all
three polymers the boronate ester formation is occurꢀ
ring as expected, but that the crystallization step, i.e.,
the layerꢀbyꢀlayer stacking is being disrupted in FLTꢀ
COPs 2ꢀ3. If we consider the staggered model as an
option, the diamond shaped pores in the model are
approximately 10 Å between the FLT units and 20 Å
between the HHTP units, which is consistent with the
NLDFT data. However, as there are no discernible difꢀ
fraction peaks (Figure S2), it is unlikely that this model
is representative of the bulk material. It is possible,
Figure 3. (Top) PXRD analysis for FLTꢀCOFꢀ1 including
the experimental diffraction pattern (orange line), Pawley
refined pattern (black dashed line), simulated diffraction
pattern for the eclipsed (red line) and staggered (blue line)
conformations and the difference between the refined and
experimental patterns (grey line). (Bottom) Models of the
eclipsed and staggered conformations of FLTꢀCOFꢀ1 with
annotated pore sizes.
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however that the sheets are randomly oriented or agꢀ S3). These experiments confirmed that the feed ratios
1
2
3
4
gregated resulting in a small pore size distribution.
were representative of the monomer composition in the
final polymer. This indicates that the rate of boronate
ester formation is not affected by the addition of peꢀ
ripheral substituents.
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In order to help elucidate the dramatic change between
FLTꢀCOFꢀ1 and FLTꢀCOPs 2ꢀ3, molecular dynamics
simulations were performed (Figure 6, 8 and Figure
S4ꢀ5). The initial condition for each of these polymers
was modeled as an eclipsed COF. Four layers of each
2D extended COF structure (extended indefinitely in
two dimensions through the use of hexagonal periodic
boundary conditions) were constructed and subjected
to two simulation protocols. First, a two dimensional
free energy landscape was obtained based on the x and
y lateral offset of the middle two layers using the adapꢀ
tive biasing force method (abf)13 as implemented in
NAMD.14ꢀ15 Second, equilibrium simulations were run
starting from the lowest free energy basin for each polꢀ
ymer’s structure.
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Figure 5. (A) Synthesis scheme and reaction condiꢀ
tions for mixed monomer COFs/COPs. (B) Nitrogen
adsorption (filled circles) and desorption (open circles)
isotherms (77 K) and BET surface areas of the mixed
monomer FLT polymers, (C) NLDFT pore size distribuꢀ
tions and (D) PXRD patterns of each polymer.
We explored the use of mixtures of FLTꢀCOFꢀ1 and
FLTꢀCOPꢀ2 monomers to determine where the transiꢀ
tion from crystalline COF to amorphous COP occurs
(Figure 5). The molar ratio between HHTP and the
diboronic acid was maintained at 1:1.5, but the ratio of
the diboronic acid monomers was varied as shown in
Figure 5. Interestingly, there does not appear to be a
gradual shift from polymers with COFꢀlike character to
amorphous COP materials. The nitrogen adsorption
isotherms (Figure 5b) show that the higher molar ratios
of 4 result in a type IV isotherm similar to FLTꢀCOFꢀ1.
However, with equimolar ratios or lower, the polymer
changes to a microporous material with surface areas
similar to FLTꢀCOPs 2 and 3. The pore size distribuꢀ
tions and PXRD patterns (Figure 5cꢀd) also reflect this
sharp transition. We hypothesize that once a critical
concentration of the sterically hindered monomer 7b is
reached, the crystallization of the COF is sufficiently
disrupted, leading to either an amorphous network
polymer or aggregated COFꢀlike sheets with nonꢀ
ordered interlayer orientations. The staggered model
shown in Figure 3 would still result in observable crysꢀ
tallinity by PXRD, which is not seen in FLTꢀCOPs 2 or
3, or in the mixed monomer systems with higher fracꢀ
tions of monomer 7b. Each polymer was digested in
acidic deuterated dimethylsulfoxide and the monomer
Figure 6. Molecular dynamics simulation results for
three 4ꢀlayer FLTꢀCOF structures. (A, D, and E; B, F,
and G; C, H, I represent FLTꢀCOFꢀ1, FLTꢀCOPꢀ2, and
FLTꢀCOPꢀ3, respectively). AꢀC: Equilibrated COF
structure, top view. D, F, H: Equilibrated COF strucꢀ
ture, side view. E, G, I: Two dimensional free energy
surface based on the x and y lateral offset of the middle
two layers; the free energy minimum for each structure
is assigned a value of zero. The equilibrium structures
are sampled from the lowest free energy (black) reꢀ
gions. The bottom layer is drawn in yellow, the lowerꢀ
middle layer in grey, the upperꢀmiddle layer in red, and
the top layer in blue.
From Figure 6, FLTꢀCOFꢀ1’s preference for the eclipsed
conformation is clear, while FLTꢀCOPꢀ2 and FLTꢀCOPꢀ
3 favor deviations from this configuration towards a
staggered state. From the equilibrium structures, it is
apparent that the layers are forced to offset as more
phenyl rings are added to the FLT core, disrupting wellꢀ
ordered, eclipsed stacking. The side view of the FLTꢀ
COFꢀ2 and FLTꢀCOFꢀ3 equilibrated structures shows
the layers offset significantly compared to the canoniꢀ
cally eclipsed FLTꢀCOFꢀ1 in which the FLT units align
on top of each other. Furthermore, a detailed examinaꢀ
tion of the torsion angles around the FLT core (Figure
8) makes it clear how the extra phenyl rings disrupt the
1
incorporation ratios were analyzed by H NMR (Figure
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Page 6 of 21
layer stacking by causing phenyl rings to orient in all units themselves (in the case of FLTꢀCOFꢀ1) or between
1
2
3
4
5
6
7
8
three spatial dimensions, increasing the steric bulk
around the FLT core in a way that cannot be accommoꢀ
dated in the eclipsed structure. As layers are forced to
offset, favorable boronꢀoxygen electrostatic interacꢀ
tions and πꢀstacking interactions are disrupted. Addiꢀ
tionally, from the twoꢀdimensional free energy surfaces
(Figure 6e, g, and i), FLTꢀCOFꢀ1 has a distinct energy
well, leading to a wellꢀordered crystalline structure,
while FLTꢀCOPꢀ2 and FLTꢀCOPꢀ3 have many wells and
local minima, which could be indicative of the presence
of many kinetic traps that inhibit the formation of a
single crystalline morphology. Detailed two and threeꢀ
dimensional representations of the free energy surfaces
for each polymer are included in the Supporting Inforꢀ
mation (Figure S6ꢀS8).
the FLT units and the more electron rich HHTP monꢀ
omers. This is especially apparent in the cases of FLTꢀ
COPꢀ2 and 3 where the starting materials are very fluoꢀ
rescent, but exhibit decreased fluorescence once polꢀ
ymerized into COPs. An analysis of the torsion angles
obtained from the MD simulations shows that the 7 and
10 phenyl rings are affected by the addition of more
phenyl rings at the 8 and 9 positions (Figure 8). The
steric hindrance caused by these rings being pushed
further out of planarity, in combination with the added
steric hindrance of the added phenyl rings may serve to
both enhance the fluorescence of 7a-b in the solid state
by inhibiting selfꢀquenching, and also to disrupt the
formation of crystalline COF structures. The fact that
there is significant interplay between the fluorescence
properties and the nature of the microscopic organizaꢀ
tion of the FLTꢀCOFs and COPs represents important
information useful for the design of fluorescence senꢀ
sors based on porous FLT materials in the future.
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Solidꢀstate fluorescence measurements were also carꢀ
ried out on both FLTꢀCOFꢀ1 and FLTꢀCOPꢀ2ꢀ3 and
compounds 4 and 7a-b (Figure 7). Each polymer is
blue shifted (10 nm for FLTꢀCOFꢀ1 and 15 and 25 nm
respectively for FLTꢀCOPsꢀ2ꢀ3) in comparison with its
corresponding boronic acid monomer. In all three exꢀ
amples, the polymers do not display visible fluoresꢀ
cence under direct irradiation with 365 nm light,
whereas the diboronic acid monomers range from
mildly (4) to extremely (7a-b) luminescent. This type
of phenomenon has been previously reported,16 and is
not completely understood. We hypothesize that the
more
Figure 8. Average torsion angles of the 7,10 phenyl
rings on the FLT units based on the MD simulations
performed in this study. As more substituents are addꢀ
ed, the phenyl rings are increasingly turned out of
plane with the FLT unit limiting its ability to adopt an
eclipsed conformation in the solid state.
CONCLUSIONS
In conclusion, we have reported a novel COF and two
novel COPs containing fluoranthene units with differꢀ
ent sorption and structural properties based on the
monomer’s functionalization patterns. While all three
polymers are porous and display narrow pore size disꢀ
tributions, only FLTꢀCOFꢀ1 possesses a mesoporous
and highly ordered crystalline structure. Despite the
differences in surface area and crystallinity, the boroꢀ
nate ester structure in all three COFs can be confirmed
by IR. While the FLT units in these COFs render them
luminescent, the emission is largely quenched followꢀ
ing polymerization, owing to the enforcement of the
photoactive units into arrangements where nonꢀ
radiative relaxation pathways are more favorable. The
presence of nonplanar aromatic rings has been shown
as both an aid17 and a hindrance3d to the nucleation and
crystallization of COF structures. This study demonꢀ
strates the fine line between these parameters, and
should provide some insight into situations where the
formation of crystalline COF structures fail. Future
studies will be directed towards developing more clear
and generalized design rules for how nonꢀplanarity can
affect the formation of crystalline COFs. Furthermore,
we believe that the MDꢀbased method of exploring COF
structure could be useful for the design and prediction
of when COFs, rather than amorphous materials, are
likely to form. This method is computationally costꢀ
effective, and could be used to screen large numbers of
potential COF materials prior to experimental testing.
Figure 7. (Top) Solidꢀstate fluorescence spectra of
FLTꢀCOFꢀ1 and FLTꢀCOPsꢀ2ꢀ3 and boronic acids 4 and
7a-b . (Bottom) Images of each polymer and their corꢀ
responding diboronic acid monomers under long wave
UV irradiation.
rigidly enforced eclipsed conformation in the COF
could be causing self quenching between either FLT
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4: To a round bottom flask containing 3 (500 mg,
0.825 mmol) was added NaIO4 (2.28 g, 10.7 mmol).
This was dissolved in solution of 3:1:1
EXPERIMENTAL PROCEDURES
1
2
3
4
5
6
7
8
All reagents were purchased from commercial suppliers
(SigmaꢀAldrich and Fisher Scientific) and used as reꢀ
ceived. HHTP was synthesized as previously reported.18
Microwave reactions were carried out in a CEM Disꢀ
cover. Fluorescence measurements were carried out on
a Horiba Fluorolog Spectrophotometer. FTꢀIR spectra
were taken on an Agilent Technologies Cary 600 Series
FTIR. The thermogravimetric analyses were performed
using a Mettler Toledo DSC/TGA 1 under nitrogen atꢀ
mosphere with a heating rate of 10 °C h−1 from 25–
800 °C. Lowꢀpressure gas adsorption experiments (up
to 760 torr) were carried out on a Micromeritics ASAP
2020 surface area analyzer. Ultrahighꢀpurityꢀgrade N2
and He gases (obtained from Airgas Corporation) were
used in all adsorption measurements. N2 isotherms
were measured using a liquid nitrogen bath (77 K).
Pore size distributions were determined using an
NLDFT carbon slitꢀpore model in the Micromeritics
Software Package. Powder Xꢀray diffraction of the FLTꢀ
COFs was carried out on a Bruker D8 Advance diffracꢀ
tometer with a sealed tube radiation source (Cu Kα, λ =
1.54184 Å), low background sample holder, and Lynxꢀ
eye XE detector. Data were collected over a 2θ range of
2ꢀ30° in BraggꢀBrentano geometry with a generator
setting of 40 kV and 40 mA, step size of 0.02°, and exꢀ
posure time per step of 2 s.
a
THF:H2O:EtOAc mixture (15 mL) before being heated
to reflux for 24 h. After this time the reaction was
cooled to room temperature and 1M HCl (10 mL) was
added. The reaction was then allowed to stir at room
temperature for a further 24 h before being poured into
water (50 mL). The resulting precipitate was washed
with more water (20 mL) before being air dried on the
filter. This material did not require further purificaꢀ
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1
tion. (302 mg, 83%). H NMR (400 MHz, DMSO,
ppm): δ 8.2 (s, 4H), 8.05 (d, 4H, J=6.7 Hz), 7.9 (d, 2H,
J=7.0 Hz), 7.6 (d, 4H, J=7.0 Hz), 7.48 (t, 2H, J=7.0 Hz),
7.3 (s, 2H), 7.2 (d, 2H, J=6.8 Hz). 13C NMR (500 MHz,
CDCl3, ppm): δ 142.2, 138.2, 136.3, 135.8, 135.1, 132.3,
130.0, 129.6, 128.3, 128.2, 127.6, 123.2. MALDIꢀToFꢀ
MS (m/z) Calculated for [C28H20B2O4]+: 442.15. Found
442.72
5a: To a microwave vial containing 1 (500 mg, 0.97
mmol) and phenyl acetylene (0.11 mL, 0.97 mmol) was
added diphenyl ether (1 mL). This was then microꢀ
waved for 2h at 250 °C and 300 W. After that time an
orange solid was filtered, washed with hexanes (5 mL)
and methanol (10 mL), and dried under vacuum to give
the desired product as an orange solid (515 mg, 89%).
Spectra matched literature values.9a
5b: To a microwave vial containing 1 (700 mg, 1.5
mmol) and diphenyl acetylene (282 mg, 1.4 mmol) was
added diphenyl ether (1 mL). This was then heated usꢀ
ing microwave irradiation for 2h at 250 °C and 300 W.
An orange solid was filtered, washed with hexanes (5
mL) and methanol (20 mL), and dried under vacuum
to give the desired product without further purification
(770 mg, 85%). Spectra matched literature values.9b
MONOMER AND POLYMER SYNTHESIS
1: To a round bottom flask containing 1,3ꢀbis(4ꢀ
bromophenyl)ꢀ2ꢀpropanone (2 g, 5.4 mmol) was added
acenaphthenequinone (1.1 g, 6.04 mmol). This was disꢀ
solved into CH2Cl2 (5 mL) and then MeOH (30 mL)
was added. To this mixture was added KOH (160 mg,
2.7 mmol) and the reaction was heated to reflux for 2 h.
After that time the reaction was cooled to room temꢀ
perature and filtered to yield a dark purple solid (2.49
g, 90%). Spectra matched that previously reported in
the literature.9b
6a: To a seal tube containing 5a (500 mg, 0.85 mmol)
was added B2Pin2 (476 mg, 1.9 mmol) followed by
KOAc (837 mg, 8.5 mmol). This was dissolved into
dioxane (5 mL), and degassed under N2 for 15 min. Afꢀ
ter that time Pd (dppf) (105 mg, 0.15 mmol) was added
and the tube was sealed with a Teflon cap before being
heated to 110 ºC for 72h. The reaction was then cooled
to rt and poured into MeOH (50 mL). The resulting
precipitate was filtered and washed with MeOH (20
mL) resulting in pure material (430 mg, 74%). 1H NMR
(400 MHz, DMSO, ppm): δ 8.02 (d, 2H, J=7.5 Hz),
7.85 (d, 2H, J= 7.5 Hz), 7.8ꢀ7.7 (m, 4H), 7.45ꢀ7.35 (m,
4H), 7.35ꢀ7.3 (m, 2H), 7.25ꢀ7.15 (m, 4H), 6.7 (d, 2H,
J=7.0 Hz), 1.4 (s, 24H). 13C NMR (500 MHz, CDCl3,
ppm): δ 143.7, 142.4, 140.9, 140.6, 138.1, 137.8, 136.4,
136.2, 135.7, 135.6, 135.1, 134.8, 133.1, 131.0, 129.9,
129.8, 129.7, 128.6, 127.7, 127.6, 127.5, 126.68, 126.66,
126.4, 123.6, 123.1, 84.0, 83.9, 25.00, 24.98. MS (ESI+),
found: 705.3322 [M+Na]+; calcd for C46H44O4B2:
705.3333.
2: To a round bottom flask containing 1 (2.00 g, 3.9
mmol) and norbornadiene (15 mL, 147 mmol) was addꢀ
ed acetic anhydride (20 mL). The solution was refluxed
at 140 °C for 7d. The solution was cooled to room temꢀ
perature. The resulting solid was filtered out and
washed with methanol to give 2 (1.529 g, 77%). Spectra
matched literature values.19
3: To a seal tube containing 2 (500 mg, 0.98 mmol)
was added B2Pin2 (546 mg, 2.2 mmol) followed by
KOAc (963 mg, 9.8 mmol). This was dissolved into
dioxane (5 mL), and degassed under N2 for 15 min. Afꢀ
ter that time Pd (dppf) (129 mg, 0.15 mmol) was added
and the tube was sealed with a Teflon cap before being
heated to 100 ºC for 72h. The reaction was then cooled
to rt and poured into MeOH (50 mL). The resulting
precipitate was filtered and washed with MeOH (20
mL) resulting in pure material (526 mg, 88%). 1H
NMR (400 MHz, DMSO, ppm): δ 8.05 (d, 4H, J=7.6
Hz), 7.75 (d, 2H, J=7.9 Hz), 7.69 (d, 4H, J=7.6 Hz), 7.35
(t, 2H, J=7.5), 7.3 (m, 4H) 1.4 (s, 24H). 13C NMR (500
MHz, CDCl3, ppm): δ 143.9, 137.9, 136.7, 136.1, 135.0,
132.7, 129.8, 129.0, 128.5, 127.6, 126.8, 123.2, 84.0,
25.0. MS (ESI+), found: 607.3183 [M+H]+; calcd for
C40H40O4B2: 607.3199.
6b: To a seal tube containing 5b (500 mg, 0.75 mmol)
was added B2Pin2 (422 mg, 1.7 mmol) followed by
KOAc (741 mg, 7.6 mmol). This was dissolved into diꢀ
oxane (5 mL), and degassed under N2 for 15 min. After
that time Pd (dppf) (93 mg, 0.11 mmol) was added and
the tube was sealed with a Teflon cap before being
heated to 110 ºC for 72 h. The reaction was then cooled
to rt and poured into MeOH (50 mL). The resulting
precipitate was filtered and washed with MeOH (20
7
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Page 8 of 21
1
mL) resulting in pure material (453 mgs, 83%). H
being collected by filtration and dried under dynamic
1
2
3
4
5
6
7
8
NMR (400 MHz, DMSO, ppm): δ 7.75 (d, 4H, J=7.5
Hz), 7.7 (d, 2H, J=8.2 Hz), 7.35 (d, 4H, J=7.5 Hz), 7.3
(t, 2H, J=7.8 Hz), 7.0ꢀ6.8 (m, 10H), 6.6 (d, 2H, J=7.0
Hz), 1.4 (s, 24H). 13C NMR (500 MHz, CDCl3, ppm): δ
142.9, 140.5, 139.6, 137.1, 136.4, 136.2, 134.6, 133.2,
131.2, 129.54, 129.51, 127.7, 126.7, 126.5, 125.4, 123.4,
84.0, 24.8. MS (ESI+), found: 797.3376 [M+K]+; calcd
for C52H48O4B2: 797.3387.
vacuum at 120 ºC for 12 h. (18.0 mg, 38%).
FLT-COP-3: To a vial containing HHTP (11 mg, 0.034
mmol) fully dissolved in dioxane (4 mL) was added 7b
(30 mg, 0.050 mmol). To this was added mesitylene (4
mL), and the vial was capped and heated at 90 ºC for
72 h. After that time, the resulting powder was filtered
and washed with dry toluene (1 mL) before being colꢀ
lected by filtration and dried under dynamic vacuum at
120 ºC for 12 h. (22.6 mg, 38%).
7a: To a round bottom flask containing 6a (147 mg,
0.22 mmol) was added NaIO4 (600 mg, 2.8 mmol).
This was dissolved in a 3:1:1 THF:H2O:EtOAc mixture
(5 mL) before being heated to reflux for 24 hrs. After
this time the reaction was cooled to room temperature
and 1M HCl (10 mL) was added. The reaction was then
allowed to stir at room temperature for a further 24 h
before being poured into water (50 mL). The resulting
precipitate was washed with a more water (20 mL) beꢀ
fore being air dried on the filter, resulting in pure maꢀ
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COMPUTATIONAL METHODS
The molecular dynamics simulations used a fully atomꢀ
istic force field which was obtained from a combination
of cgenff20 and prior literature on similar COFs21 (see
ESI, Figure S4 for full details). A fragment of the 2D
structure shown in Figure 1 was generated and boroꢀ
nate ester bonds were applied across the unit cell
boundary to produce an infinitely extended 2D strucꢀ
ture. Then four of these 2D structures were stacked in
the zꢀdirection to generate a fourꢀlayer stack. The hexꢀ
agonal unit cell used for all three COF/COP types was
as follows: cellbasisvector1 47.0 0.0 0.0, cellbasisvecꢀ
tor2 23.5 40.7 0.0, and cellbasisvector3 0.0 0.0 200.0.
The number of total atoms in the unit cell is as follows:
FLTꢀCOFꢀ1: 792; FLTꢀCOPꢀ2: 912; FLTꢀCOPꢀ3: 1032
(for the fourꢀlayer stack). This small system size makes
the MD simulations very efficient; in fact, they can all
be run to completion on a laptop computer within a few
days. We used the NAMD software package15 with the
following parameter choices: temperature 363 K (enꢀ
forced with a Langevin thermostat with damping paꢀ
rameter 1.0 psꢀ1 ; cutoff distance 12 Å (for the van der
Waals interactions and the changeover from real space
to inverse space for the electrostatic interactions);
timestep 1.0 fs; particle mesh Ewald grid spacing of 1.0
Å. For the free energy simulations, we used the adapꢀ
tive biasing force method13 in two dimensions based on
the x and y lateral offset of the middle two stacked layꢀ
ers. We used a grid spacing of 0.4 Å in each direction.
In addition the outer two layers were not allowed to
separate by more than 12 Å to keep the four layer stack
intact. The free energy converges after about 2 x 108
timesteps. Structures corresponding to the minimum
free energy basin for each polymer were extracted from
the free energy runs and used to initiate equilibrium
simulations free from any biasing forces. These simuꢀ
lations were used to generate representative snapshots
for Figure 6. Images were generated using the VMD
software package.22
1
terial. (95 mg, 85%). H NMR (400 MHz, DMSO,
ppm): δ 8.2 (s, 2H), 8.15 (s, 2H), 8.0 (d, 2H, J=7.9 Hz),
7.86ꢀ7.82 (m, 4H), 7.65 (d, 2H, J=7.9 Hz), 7.45 (t, 1H,
J=7.6 Hz), 7.38 (t, 1H, J=7.6 Hz), 7.35 (d, 2H, J=7.9
Hz), 7.3ꢀ7.15 (m, 7H), 6.5 (d, 1H, J=7.1 Hz). 13C NMR
(400 MHz, CDCl3, ppm): δ 141.9, 140.90, 140.86,
140.82, 138.2, 138.0, 135.6, 136.2, 135.5, 135.1, 135.0,
134.9, 132.7, 131.2, 130.1, 130.0, 129.4, 128.3, 128.2,
127.6, 127.1, 123.4, 123.1. MS (m/z): MALDIꢀToFꢀMS
(m/z) Calculated for [C34H24B2O4]+: 518.19. Found
518.72.
7b: To a round bottom flask containing 6b (493 mg,
0.65 mmol) was added NaIO4 (1.8 g, 8.4 mmol). This
was dissolved in a THF:H2O:EtOAc mixture (3:1:1 by
vol, 15 mL) before being heated to reflux for 24 h. After
this time the reaction was cooled to room temperature
and 1M HCl (10 mL) was added. The reaction was then
allowed to stir at room temperature for a further 24 h
before being poured into water (50 mL). The resulting
precipitate was washed with a more water (20 mL) beꢀ
fore being air dried on the filter, resulting in pure maꢀ
terial. (313 mg, 81% yield). 1H NMR (400 MHz, DMSO,
ppm): δ 8.1 (s, 4H), 7.85 (d, 2H, J=8.2 Hz), 7.75 (d, 4H,
J=7.9 Hz), 7.35 (t, 2H, J=7.5 Hz), 7.3 (d, 4H, J=7.9 Hz),
7.0 (m, 4H), 6.9 (t, 4H, J=7.3 Hz), 6.85 (t, 2H, J=7.2
Hz), 6.4 (d, 2H, J=7.1 Hz). 13C NMR (500 MHz, CDCl3,
ppm): δ 141.5, 140.8, 139.8, 137.4, 136.1, 136.0, 134.6,
132.8, 131.3, 129.8, 129.1, 128.3, 127.4, 127.0, 126.1,
123.2. MS (m/z): MALDIꢀToFꢀMS (m/z) Calculated for
[C40H28B2O4]+: 594.22. Found 594.74.
FLT-COF-1: To a vial containing HHTP (14.7 mg,
0.045 mmol) fully dissolved in dioxane (4 mL) was
added 4 (30 mg, 0.068 mmol). To this was added meꢀ
sitylene (4 mL), and the vial was capped and heated at
90 ºC for 72 h. After that time, the resulting powder
was filtered and washed with dry toluene (1 mL) before
being collected by filtration and dried under dynamic
vacuum at 120 ºC for 12 h. (27.9 mg, 40%).
ASSOCIATED CONTENT
Supporting Information
FLT-COP-2: To a vial containing HHTP (12.5 mg,
0.038 mmol) fully dissolved in dioxane (4 mL) was
added 7a (30 mg, 0.058 mmol). To this was added meꢀ
sitylene (4 mL), and the vial was capped and heated at
90 ºC for 72 h. After that time, the resulting powder
was filtered and washed with dry toluene (1 mL) before
The Supporting Information including thermogravimetric
analysis, and the fractional coordinates for the simulated
COF models and MD simulations is available free of
charge on the ACS Publications website.
AUTHOR INFORMATION
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Page 9 of 21
Journal of the American Chemical Society
†These authors contributed equally
(6) a) Bunck, D. N.; Dichtel, W. R. J. Am. Chem. Soc. 2013,
135, 14952; b) Chen, X.; Addicoat, M.; Irle, S.; Nagai, A.;
Jiang, D. J. Am. Chem. Soc. 2013, 135, 546; c) Smith, B. J.;
1
2
3
4
Corresponding Author
Dichtel, W. R. J. Am. Chem. Soc. 2014, 136, 8783;
d)
* steven.nielsen@utdallas.edu
*ronald.smaldone@utdallas.edu
Smith, B. J.; Hwang, N.; Chavez, A. D.; Novotney, J. L.;
Dichtel, W. R. Chem. Commun. 2015, 51, 7532; e) Smith, B.
J.; Overholts, A. C.; Hwang, N.; Dichtel, W. R. Chem.
Commun. 2016, 52, 3690.
5
6
7
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(7) Chen, X.; Addicoat, M.; Jin, E.; Zhai, L.; Xu, H.; Huang,
N.; Guo, Z.; Liu, L.; Irle, S.; Jiang, D. J. Am. Chem. Soc. 2015,
137, 3241.
(8) Sygula, A.; Rabideau, P. W. J. Am. Chem. Soc 1999,
121, 7800.
Author Contributions
The manuscript was written through contributions of all
authors. / All authors have given approval to the final verꢀ
sion of the manuscript.
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(9) a) Venkatramaiah, N.; Kumar, S.; Patil, S. Chem.
Commun. 2012, 48, 5007; b) Kumar, S.; Venkatramaiah,
N.; Patil, S. J. Phys. Chem. C 2013, 117, 7236; c) Kumar, S.;
Kumar, D.; Patil, Y.; Patil, S. J. Mater. Chem. C 2016, 4, 193.
(10) Spitler, E. L.; Koo, B. T.; Novotney, J. L.; Colson, J. W.;
UribeꢀRomo, F. J.; Gutierrez, G. D.; Clancy, P.; Dichtel, W. R.
J. Am. Chem. Soc 2011, 133, 19416.
(11) a) Das, G.; Biswal, B. P.; Kandambeth, S.; Venkatesh,
V.; Kaur, G.; Addicoat, M.; Heine, T.; Verma, S.; Banerjee, R.
Chem. Sci. 2015, 6, 3931; b) Mitra, S.; Kandambeth, S.;
Biswal, B. P.; Khayum M, A.; Choudhury, C. K.; Mehta, M.;
Kaur, G.; Banerjee, S.; Prabhune, A.; Verma, S.; Roy, S.;
Kharul, U. K.; Banerjee, R. J. Am. Chem. Soc. 2016, 138,
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FUNDING SOURCES
This research was supported with funds from the Uniꢀ
versity of Texas at Dallas and the American Chemical
Society Petroleum Research Fund (52906ꢀDNI10).
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TOC Graphic
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Figure 1. Structure of the FLT polymers synthesized as part of this study. The FLT materials were
synthesized through a condensation co-polymerization between diboronic acid monomers and
hexahydroxytriphenylene resulting in a boronate ester based polymer.
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Scheme 1: Synthesis of FLT monomers and FLT-COF 1 and FLT-COP 2-3.
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Figure 2. (A) Nitrogen sorption isotherms at 77 K for FLT-COF-1 (orange line), FLT-COP-2 (black line), and
FLT-COP-3 (green line). (B) NLDFT pore size distributions for all of the FLT polymers. (C) BET surface areas
for the FLT polymers.
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(Top) PXRD analysis for FLT-COF-1 including the experimental diffraction pattern (orange line), Pawley
refined pattern (black dashed line), simulated diffraction pattern for the eclipsed (red line) and staggered
(blue line) conformations and the difference between the refined and experimental patterns (grey line).
(Bottom) Models of the eclipsed and staggered conformations of FLT-COF 1 with annotated pore sizes.
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Figure 4. FT-IR spectra of FLT-COP-1 and FLT-COPs 2-3 (solid lines) and their corresponding diboronic acid
monomers (dashed lines). The relevant functional groups expected to appear (boronate esters) or
disappear (boronic acids) upon COF formation are high-lighted.
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Figure 5. (A) Synthesis scheme and reaction conditions for mixed monomer COFs/COPs. (B) Nitrogen
adsorption (filled circles) and desorption (open circles) isotherms (77 K) and BET surface areas of the mixed
monomer FLT polymers, (C) NLDFT pore size distributions and (D) PXRD patterns of each polymer.
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Figure 6. Molecular dynamics simulation results for three 4-layer FLT-COF structures. (A, D, and E; B, F, and
G; C, H, I represent FLT-COF-1, FLT-COP-2, and FLT-COP-3, respectively). A-C: Equilibrated COF structure,
top view. D, F, H: Equilibrated COF structure, side view. E, G, I: Two dimensional free energy surface based
on the x and y lateral offset of the middle two layers; the free energy minimum for each structure is
assigned a value of zero. The equilibrium structures are sampled from the lowest free energy (black)
regions. The bottom layer is drawn in yellow, the lower-middle layer in grey, the upper-middle layer in red,
and the top layer in blue.
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Figure 7. (Top) Solid-state fluorescence spectra of FLT-COF-1 and FLT-COPs-2-3 and boronic acids 4 and 7a-
b . (Bottom) Images of each polymer and their corresponding diboronic acid monomers under long wave UV
irradiation.
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Figure 8. Average torsion angles of the 7,10 phenyl rings on the FLT units based on the MD simulations
performed in this study. As more substituents are added, the phenyl rings are increasingly turned out of
plane with the FLT unit limiting its ability to adopt an eclipsed conformation in the solid state.
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