A Case Study on the Influence of Substitutes on Interlayer Stacking of 2D Covalent Organic Frameworks

Article abstract of DOI:10.1002/chem.201700915

Interlayer stacking of 2D covalent organic frameworks (COFs) plays a crucial role in determining not only the geometry of channels inside COFs but also the mobility of carrier transport between COF layers. However, though topological structures of 2D COFs monolayers can be precisely predicted through the structures of building blocks, factors affecting their interlayer stacking remain poorly understood. In this work, a truxene-based building block on which six methyl groups are introduced was designed. The condensation of it with 1,4-diaminobenzene or benzidine afforded 2D COFs with the methyl groups extending out-of-plane of the layers. A significant influence of the methyl groups on interlayer stacking of the COFs was revealed by the adoption of inclined packing of monolayers, which has never been experimentally observed before. This unprecedented stacking manner was confirmed by powder X-ray diffraction analysis, pore-size distribution analysis, and TEM investigation.

Full text of DOI:10.1002/chem.201700915

A Journal of
Accepted Article
Title: A Case Study on the Influence of Substituents on Interlayer
Stacking of 2D Covalent Organic Frameworks
Authors: Xin Zhao, Yu Fan, Qiang Wen, Tian-Guang Zhan, Qiao-Yan
Qi, and Jia-Qiang Xu
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201700915
Link to VoR: http://dx.doi.org/10.1002/chem.201700915
Supported by

Chemistry - A European Journal
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COMMUNICATION
A Case Study on the Influence of Substitutes on Interlayers
Stacking of 2D Covalent Organic Frameworks
[a,b]
[a]
[a]
[a]
[b]
[a]
Yu Fan,
Qiang Wen, Tian-Guang Zhan, Qiao-Yan Qi, Jia-Qiang Xu, and Xin Zhao*
Abstract: Interlayer stacking of 2D covalent organic frameworks
COFs) plays a crucial role in determining not only geometry of
transport and thus has a profound effect on their optoelectronic
properties.
[
8]
(
channels inside COFs but also mobility of carrier transport between
COF layers. However, while topological structures of 2D COFs
monolayers can be precisely predicted through the structures of
building blocks, factors affecting their interlayer stacking remain
poorly understood. In this work, a truxene-based building block on
which six methyl groups are introduced was designed. The
condensation of it with 1,4-diaminobenzene or benzidine afforded 2D
COFs with the methyl groups extending out-of-plane of the layers. A
significant influence of the methyl groups on interlayer stacking of
the COFs was revealed by the adoption of inclined packing of
monolayers, which has never been experimentally observed before.
Over the past decade a variety of 2D COFs have been
constructed. However, although the geometry and unit size of
2D network of a COF usually can be precisely predicted through
the structures of the monomers used for polymerization,
stacking manner of its layers still cannot be forecasted. Due to
the lack of fundamental understanding, very little is known about
how monomer structure exerts an influence on the stacking
[
9]
behavior of COF layers. Some recent theoretical investigations
suggested that notable differences might exist between
experimental assignments and theoretical predictions for the
[
10]
interlayer stacking of 2D COFs. However, so far this subject
This unprecedented stacking manner was confirmed by PXRD study, has hardly been experimentally studied. In this communication
pore size distribution analysis and TEM investigation.
we demonstrate that substituents on building blocks can make a
significant impact on the interlayer stacking of 2D COFs. A
theoretically predicted stacking mode, that is, inclined
Covalent Organic Frameworks (COFs) are a class of crystalline
porous organic polymers whose organic units are covalently
connected to form two-dimensional (2D) or three-dimensional
[10a,b]
stacking,
has been experimentally observed for the first time.
In this work we mainly focused on the influence of substituents
on interlayer stacking of 2D COFs. While early COFs were
mainly constructed from building blocks without substituents,
more recently COFs with substituted skeletons have been
fabricated to introduce specific functions through direct use of
[
1]
(
3D) periodic networks. They have drawn a great deal of
interest over the past decade due to their important applications
[
2]
in many aspects, including gas storage and separation,
[
3]
[4]
[5]
[6]
catalysis,
sensing,
delivery,
energy storage,
and
[3c,11]
[3d,12]
substituted building blocks
or post-modification of COFs
.
[
7]
optoelectronics . One distinct feature of COFs that is different
from amorphous porous organic materials is their well-defined
channels, which play a pivotal role in the realization of their
versatile functions. For 3D COFs, the extending of building
blocks in 3D space spontaneously results in the formation of
permanent channels inside. In the case of 2D COFs, however,
the building units just covalently extend in 2D space, which
brings in periodical distribution of micro/meso-pores in 2D
monolayers. It is the stacking of monolayers into 3D layered
structures that leads to the generation of 1D channels
orthogonal to the layers. In this context, the geometry of
channels in a 2D COF is not only dictated by the original pore
size and shape in monolayer, but also dependent on packing
manner of the layers. On the other hand, the interlayer stacking
also exerts a huge influence on the overlapping of conjugated
skeletons of COFs, which defines interlayer electron or charge
However, in most COFs the substituted groups are flexible and
are almost in-plane of the 2D layers and thus make very little
effect on the interlayer stacking. In a few cases bulky tert-butyl
groups were introduced and they should be partly out-of-plane of
the building blocks. Although the tert-butyl groups can freely
rotate to partly alleviate the steric hindrance, slipped-AA
stacking was observed for the COF, which was attributed to the
[13]
result of avoiding the clash of tert-butyl groups between
[13a]
neighbouring layers.
In order to enhance the role of
substituents in dictating interlayer stacking, in this work a
building block was designed by incorporating methyl groups on a
truxene core, which generates 3,8,11-tri(4-formylphenyl)-
5
,5,10,10,15,15-hexamethyltruxene (TFPHMT) (Scheme 1).
3
Owing to the tetrahedral configuration of sp carbon, the methyl
groups locate out-of-plane of the truxene core. Using TFPHMT
as a monomer, formation of 2D frameworks with hexagonal
micropores can be predicted from condensations of it with 1,4-
diaminobenzene or benzidine. As a result of the rigid structure
of TFPHMT, the methyl groups should also extend out-of-plane
of the COF monolayers and thus are fixed at both sides of the
layers, which leads to formation of the first COFs with inclined
stacking.
[
[
a]
b]
Y. Fan, Q. Wen, Dr. T.-G. Zhan, Q.-Y. Qi, Prof. Dr. X. Zhao
CAS Key Laboratory of Synthetic and Self-assembly Chemistry for
Organic Functional Molecules, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences
345 Lingling Road, Shanghai 200032 (China)
E-mail: xzhao@sioc.ac.cn
Y. Fan, Prof. J.-Q. Xu
The condensation reactions were carried out under
solvothermal conditions, which afforded SIOC-COF-8 and SIOC-
COF-9 as yellow powders (see supporting information for the
detailed procedures). IR spectra of the powder showed that the
NEST Lab, Department of Chemistry
Shanghai University
Shanghai 200444 (China)
peak corresponding to NH
peaks around 1618 cm appeared, suggesting a high degree of
2
groups almost disappeared and
Supporting information for this article is given via a link at the end of
the document.
-1
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Chemistry - A European Journal
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COMMUNICATION
0
0
1
0
experimental
refined
polymerization and the formation of C=N linkages (Figure S1-2).
1
0
1
2
2
0
2 0
0
2
0
0
experimental
refined
0 0
2
2
0
1
2 -
1
00
-
0
0
3
0
1
Moreover, peaks corresponding to the vibration of CH
3
groups
a
3
1
3
-
3
0 0 1
3
-
0
b
-
1
0
-1
-1
1
1
were also observed around 3000 cm , indicating existence of
methyl units in the two COFs. The formation of polyimine from
the condensation of the starting materials was also supported by
5
10
2
Theta 15
20
4
8
12
Theta
16
20
2
difference
inclined stacking
difference
13
inclined stacking
the appearance of C=N signals (156 ppm) in their solid-state
C
CP-MAS NMR spectra (Figures S3-4). Furthermore, peaks
corresponding to methyl groups were also observed at 21 ppm
in the spectra, again indicating that methyl groups were retained
AB stacking
AB stacking
AA stacking
AA stacking
20
5
10
15
5
10
15
20
in the COFs. Thermogravimetric analysis (TGA) indicated that
2 Theta
2 Theta
o
5
% weight loss occurred at 470 and 364 C for SIOC-COF-8 and
SIOC-COF-9, respectively (Figures S5), demonstrating that they
have high thermal stabilities. Scanning electron microscopy
(
SEM) and atomic force microscopy (ATM) investigations
revealed that the COFs displayed irregular morphology (Figures
S6).
Figure 1. Experimental, refined and simulated PXRD patterns of (a) SIOC-
COF-8 and (b) SIOC-COF-9 with different stacking manners. And (c) views of
SIOC-COF-8 with inclined stacking generally along the z (left) and x (right)
axes, respectively. Methyl groups were highlighted in red. SIOC-COF-9
exhibits the similar stacking (Figure S9).
order to determine their crystal structures, theoretical
simulations were carried out by using Accelrys Materials Studio
7.0 software package. In the simulations, infinite 2D networks
possessing hexagonal pores, as illustrated in Scheme 1, were
constructed. Since in literature nearly all the 2D COFs have
been experimentally reported to adopt eclipsed (AA) stacking,
PXRD patterns of the COFs with AA stacking of monolayers
were firstly simulated and compared with the experimentally
observed PXRD patterns. It should be noted that the common
interlayer distance range of 3.5-4.5 Å in 2D COFs could not be
set for SIOC-COF-8 and SIOC-COF-9, because in AA stacking
steric repulsions between the methyl groups would arise if the
layers maintain a close distance. Therefore, the interlayer
distance was enlarged to 6.0 Å, a value at which the methyl
groups between the layers of eclipsed packing just separate
from each other and make no contact. The comparisons
indicated that their experimental PXRD patterns show dramatic
differences from the simulated PXRD patterns of AA stacking in
the positions of the diffraction peaks, which strongly suggests
that SIOC-COF-8 and SIOC-COF-9 do not adopt AA stacking
manner. Next, PXRD patterns of staggered (AB) stacking were
simulated, which, however, were not consistent with the
experimental PXRD data, either. Since both the two classic
stacking models are inconsistent with the experimental results, a
third stacking manner, that is, slipped-AA stacking (serrated
[
10]
stacking) was simulated.
In this model, four horizontal offsets
by 5, 10, 15 and 20 Å were used. The resulted theoretical PXRD
patterns were compared with the experimental PXRD patterns
again and it turned out that large discrepancies still existed
between them (Figure S7-8).
Scheme 1. Synthesis and structures of SIOC-COF-8 and SIOC-COF-9. Note:
only one pore unit and single layer are presented for clarity.
In 2010 Heine and co-workers theoretically proposed four
possible interlayer stacking forms, that is, eclipsed (AA),
Powder X-ray diffraction (PXRD) profiles of the as-prepared
powders were recorded. Both of them exhibited diffraction peaks
with high intensities (Figure 1), suggesting a high crystallinity. In
staggered (AB), serrated and inclined stackings, for 2D COFs on
[10a,10b]
the basis of DFT calculations.
While the former three have
already been experimentally observed, the last one has never
been found in the COFs previously reported. Since the
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COMMUNICATION
experimental PXRD patterns of SIOC-COF-8 and SIOC-COF-9
did not coincide with those of the three former stacking models,
layered structures with inclined stacking were built for the two
COFs and their theoretical PXRD patterns were simulated
(inclined stacking with offsetting by 8.2 Å) was calculated by
[
15]
using Monte Carlo Metropolis method in Materials Studio,
which yielded a theoretical surface areas of 3533.3 and 3900.2
2
-1
m g for SIOC-COF-8 and SIOC-COF-9, respectively (Figures
S20-23). Moreover, theoretical BET surface areas for the COFs
with inclined stacking at different offsets of adjacent two
monolayers were also calculated for comparison. For SIOC-
(
Figure 1). To our delight, the simulated PXRD patterns of the
COFs with an inclined stacking model (adjacent two monolayers
offsetting by 8.2 Å) well reproduced the experimental data. This
result strongly suggests that the layers in the two COFs adopt
inclined stacking. On the basis of the simulations, the diffraction
2
-1
COF-8, theoretical surface areas of 2296.9 and 3219.5 m
g
were obtained for offset values of 3.1 and 5.5 Å, respectively. In
peaks of SIOC-COF-8 observed at 2.69°，3.42°，4.10°, 5.43°， the case of SIOC-COF-9, the simulations gave rise to 2546.9
2
-1
6.48°，6.85°, 8.97°, 10.29° and 19.30° 2 could be attributed to
and 3294.7 m
g
for the models at offsets of 3.3 and 5.5 Å,
(
(
1 0 0), (1 -1 0), (1 1 0), (2 0 0), (2 1 0), (2 -2 0), (3 1 0), (3 -3 0),
0 0 1) facets, respectively. In the case of SIOC-COF-9, the
respectively (Figures S20-23).
a 400
Adsorption
Desorption
b
25.6 Å
peaks at 2 = 2.35°, 2.99°, 4.69°, 5.99°，9.00° and 19.39° are
assignable to (1 0 0), ( 1 -1 0), (2 0 0), (2 -2 0), (3 -3 0) and (0 0
1
350
300
250
8000
6000
2
4.5 Å
) reflections, respectively. The (0 0 1) peaks indicate interlayer
distances of 4.53 Å and 4.50 Å for SIOC-COF-8 and SIOC-
COF-9, respectively, which are larger than those of the COFs
reported in literature. Pawley refinement gave rise to unit cell
parameters of a = b = 51.52 Å，c = 9.60 Å，α = 40.41°，β =
200
4000
1
50
00
1
2000
0
50
0
0.0
0.2
0.4
0.6
0.8
1.0
20
40
60
80
p
40.45°，γ = 60.00° (residuals: R = 4.55 % and Rwp = 6.07 %)
Relative Pressure(P/P
)
Pore Width (Angstroms)
0
for SIOC-COF-8. In the case of SIOC-COF-9, unit cell
parameters of a = b = 59.00 Å，c = 9.30 Å，α = 40.82°，β =
c
600
Adsorption
Desorption
d
30.2 Å
20000
15000
500
400
300
200
100
0
2
8.6 Å
p
40.91°，γ = 60.00° (residuals: R = 4.18% and Rwp = 5.53%)
were produced by Pawley refinement. The difference plots
indicate that their experimental PXRD patterns match with the
refined ones quite well.
1
0000
5000
0
Another key evidence for the inclined stacking of the layers
in the two COFs was provided by pore size distribution (PSD)
analysis. Nitrogen adsorption-desorption experiments were
carried out for SIOC-COF-8 and SIOC-COF-9 at 77 K. Both the
0.0
0.2
0.4
0.6
0.8
1.0
20
40
60
80
Relative Pressure(P/P0)
Pore Width (Angstroms)
Figure 2. N2 adsorption-desorption isotherms (77 K) of (a) SIOC-COF-8, and
(c) SIOC-COF-9, and pore size distribution profiles of (b) SIOC-COF-8, and (d)
SIOC-COF-9.
[
14]
COFs exhibited the type IV absorption isotherm. Curves of
sharp uptake under low relative pressures at P/P <0.01 followed
by a second step in the range of 0.05<P/P <0.2 were observed,
0
0
which is indicative of mesoporous materials (Figure 2).
Brunauer-Emmett-Teller (BET) model was applied to the
0
isotherms in the range of P/P between 0.05 and 0.2, which
2
-1
gave BET surface areas of 825.17 and 1676.47 m g for SIOC-
COF-8 and SIOC-COF-9, respectively (Figures S10-11). Their
pore size distributions were estimated using nonlocal density
functional theory (NLDFT), which showed narrow pore size
distributions around 25.6 Å for SIOC-COF-8 and 30.2 Å for
SIOC-COF-9 (Figures 2b and 2d). The values are fully
consistent with theoretical diameters of the channels predicted
for the COFs with inclined stacking, which are 24.5 Å (SIOC-
COF-8) and 28.6 Å (SIOC-COF-9) (Figures S12-13). If the COFs
should adopt AA stacking, pore size distributions would be
observed around 51.5 Å for SIOC-COF-8 and 59.0 Å for SIOC-
COF-9 (Figures S14-15). On the other hand, AB stacking of the
layers would give rise to pore size distributions at 20.2 and 25.6
Å for SIOC-COF-8 and SIOC-COF-9, respectively (Figures S16-
17). For serrated stacking, the pore sizes should be close to that
of AA stacking or AB stacking, or exhibit multiple distributions,
depending on the value of the offset (Figures S18-19). The PSD
results corroborate again that SIOC-COF-8 and SIOC-COF-9
adopt inclined stacking. Furthermore, the results also clearly
demonstrate that the size of channels in 2D COFs changes with
the alteration of interlayer stacking. The theoretical maximum
BET surface areas of the COFs experimentally obtained
Figure 3. HRTEM images and fast Fourier transform patterns (top right inset)
of (a) SIOC-COF-8 and (b) SIOC-COF-9. And (c) illustrations for the
projections of the channels in SIOC-COF-8 (left) and SIOC-COF-9 (right)
along (100) facet.
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Transmission electron microscopy (TEM) investigation
provided the third evidence for the inclined stacking of the layers
in SIOC-COF-8 and SIOC-COF-9. The stacked layers resulted in
the formation of 1D channels through the overlapping of pores in
monolayers of the COFs, which were clearly observed in
theirTEM images, as revealed by the highly ordered straight
black-and-white stripes (Figure 3a and 3b). Fast Fourier
transform of the marked areas (enclosed in the red squares) in
the TEM images gave rise to ordered diffraction spots (insets of
Figure 3a and 3b), suggesting extremely high crystallinity of the
COFs. The spacings of the straight stripes can be acquired
through direct measurement in the TEM images, or more
accurately, from the spacings of the diffraction spots in the fast
Fourier transform patterns, which gave the spacings of the
stripes to be 3.02 nm for SIOC-COF-8 and 3.56 nm for SIOC-
COF-9. On the basis of the comparison of the values with the
PXRD data (Table S1-2), they are identified to be the projections
of the channels along (1 0 0) plane, as illustrated in Figure 3c.
In summary, we have demonstrated that substituents can
have a significant influence on interlayer stacking of 2D COFs.
Through introducing steric substituents between the layers of 2D
COFs, inclined stacking of the layers, which was theoretically
proposed several years ago, has been experimentally realized
for the first time. While currently most studies have mainly
focused on constructing COFs with novel structures and
exploiting their applications, fundamental principles of COFs,
such as the formation mechanism of COF networks and
interlayer stacking, are less understood. Better understanding of
these principles is undoubtedly very important for the design of
COFs and development of COF-based materials. We believe
this study should be helpful in analyzing 3D layered lattices of
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endowing COFs with novel properties and functions through
tuning their stacking model can be expected, which may provide
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2
1632004, 21472225, 61371021, 61527818) and the Strategic
Priority Research Program of the Chinese Academy of Sciences
Grant No. XDB20020000) for financial support. The authors
(
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A Case Study on the Influence of
Substitutes on Interlayer Stacking of
2D Covalent Organic Frameworks
Inclined stacking of covalent organic framework monolayers has been
experimentally confirmed for the first time. The unprecedented stacking manner
was realized through the condensation of 1,4-diaminobenzene or benzidine with a
truxene-based building block in which six methyl groups were fixed out-of-plane of
the truxene core.
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