Precision Construction of 2D Heteropore Covalent Organic Frameworks by a Multiple-Linking-Site Strategy

Article abstract of DOI:10.1002/chem.201603043

Integrating different kinds of pores into one covalent organic framework (COF) endows it with hierarchical porosity and thus generates a member of a new class of COFs, namely, heteropore COFs. Whereas the construction of COFs with homoporosity has already been well developed, the fabrication of heteropore COFs still faces great challenges. Although two strategies have recently been developed to successfully construct heteropore COFs from noncyclic building blocks, they suffer from the generation of COF isomers, which decreases the predictability and controllability of construction of this type of reticular materials. In this work, this drawback was overcome by a multiple-linking-site strategy that offers precision construction of heteropore COFs containing two kinds of hexagonal pores with different shapes and sizes. This strategy was developed by designing a building block in which double linking sites are introduced at each branch of a C₃-symmetric skeleton, the most widely used scaffold to construct COFs with homogeneous porosity. This design provides a general way to precisely construct heteropore COFs without formation of isomers. Furthermore, the as-prepared heteropore COFs have hollow-spherical morphology, which has rarely been observed for COFs, and an uncommon staggered AB stacking was observed for the layers of the 2D heteropore COFs.

Full text of DOI:10.1002/chem.201603043

DOI: 10.1002/chem.201603043
Full Paper
&
Covalent Organic Frameworks
Precision Construction of 2D Heteropore Covalent Organic
Frameworks by a Multiple-Linking-Site Strategy
Cheng Qian+,[a, b] Shun-Qi Xu+,[b] Guo-Fang Jiang,[a] Tian-Guang Zhan,[b] and Xin Zhao*[b]
Abstract: Integrating different kinds of pores into one cova-
lent organic framework (COF) endows it with hierarchical po-
rosity and thus generates a member of a new class of COFs,
namely, heteropore COFs. Whereas the construction of COFs
with homoporosity has already been well developed, the
fabrication of heteropore COFs still faces great challenges.
Although two strategies have recently been developed to
successfully construct heteropore COFs from noncyclic build-
ing blocks, they suffer from the generation of COF isomers,
which decreases the predictability and controllability of con-
struction of this type of reticular materials. In this work, this
drawback was overcome by a multiple-linking-site strategy
that offers precision construction of heteropore COFs con-
taining two kinds of hexagonal pores with different shapes
and sizes. This strategy was developed by designing a build-
ing block in which double linking sites are introduced at
each branch of a C3-symmetric skeleton, the most widely
used scaffold to construct COFs with homogeneous porosity.
This design provides a general way to precisely construct
heteropore COFs without formation of isomers. Furthermore,
the as-prepared heteropore COFs have hollow-spherical
morphology, which has rarely been observed for COFs, and
an uncommon staggered AB stacking was observed for the
layers of the 2D heteropore COFs.
Introduction
Covalent organic frameworks (COFs) are a class of POPs but
distinguish themselves from other POPs by exhibiting high
crystallinity.[27–30] Due to their highly ordered framework struc-
tures, COFs are superior platforms for the fabrication of hier-
archically structured porous materials compared to their amor-
phous counterparts, as the size and shape of pores in COFs
can be precisely controlled by means of the symmetry and
length of building blocks. In this context, introducing different
kinds of pores into one COF will lead to the formation of
a COF with a periodic distribution of well-defined but different-
ly shaped or sized pores, which generates a new class of COFs,
that is, heteropore COFs. In principle, heteropore COFs can be
constructed by two strategies. A direct approach is to use
cyclic building blocks that bear intrinsic pores (Scheme 1a).
However, so far the COFs fabricated by this approach only ex-
hibited one kind of detectable pore in each COF, although
structurally they look like heteropores.[31–33] This can be attrib-
uted to the fact that the pores introduced by the cyclic mono-
mers are too small to be detected. In order to make true heter-
opore COFs, the internal cavity of cyclic monomers must be
large enough to be identified and characterized. Furthermore,
a prerequisite for the utilization of this approach is the availa-
bility of cyclic building blocks, synthesis of which, however, is
usually quite challenging, especially for those with large inter-
nal cavities.
Over the past decades porous organic polymers (POPs) have
been growing into an exciting field of materials science be-
cause of their versatile applications in gas storage and separa-
tion,[1–6] catalysis,[7–10] sensors,[11–13] energy storage,[14–16] and op-
toelectronics.[17–19] The most important feature of POPs is their
porosity, on the basis of which a variety of applications can be
developed. Recently, it was found that materials having hier-
archical porosity exhibit intriguing properties such as improved
mass transport and minimized diffusion barriers. These advan-
tages make hierarchically porous materials highly attractive for
catalysis, energy storage and conversion, sensing, and separa-
tion.[20–22] However, so far most porous materials with hierarchi-
cal porosity are derived from inorganics.[23–26] As far as POPs
are concerned, the introduction of hierarchical porosity still
faces a great challenge due to the very limited strategies to
combine different levels of porosity in one POP.
[a] C. Qian,+ Prof. G.-F. Jiang
State Key Lab of Chemo/Biosensing and Chemometrics
College of Chemistry and Chemical Engineering
Hunan University, Changsha 410082 (P.R. China)
[b] C. Qian,+ S.-Q. Xu,+ Dr. T.-G. Zhan, Prof. X. Zhao
Key Laboratory of Materials Science
The second approach is to construct heteropore COFs by
direct polymerization of noncyclic building blocks. The validity
of this approach has recently been demonstrated by the suc-
cessful construction of several heteropore COFs by combina-
tion of linear linkers with D2h-symmetric tetraphenylethene-
based monomers (Scheme 1b)[34–37] or C2v-symmetric building
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences
345 Lingling Road, Shanghai 200032 (P.R. China)
E-mail: xzhao@mail.sioc.ac.cn
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201603043.
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kinds of hexagonal pores can be produced without formation
of any COF isomers (Scheme 2b). This strategy allows precision
construction of heteropore COFs, and conceptually it could be
applicable to any other C3-symmetric building blocks.
Results and Discussion
To prove the validity of this conceptual strategy, building block
TPA-6CHO (Scheme 2c), in which a triphenylamine unit was
chosen as the core and two aldehyde groups were introduced
at each branch of the C3-symmetric skeleton was designed and
synthesized (Scheme 2c). 1,4-Diaminobenzene and benzidine
were selected as linear linkers. The COFs were prepared under
solvothermal conditions, and various solvents such as dioxane,
mesitylene, dimethylacetamide, ortho-dichlorobenzene, and
their combinations were screened. Different temperatures and
acid concentrations were also examined. Polymers with the
highest crystallinity were obtained by heating a 1:3 mixture of
TPA-6CHO and 1,4-diaminobenzene (giving SIOC-COF-3) or
benzidine (giving SIOC-COF-4) in ortho-dichlorobenzene/3m
aqueous acetic acid (10/1 v/v) binary solvent in sealed glass
ampoules at 1508C for 72 h. The materials were obtained as
yellow powders that were insoluble in water and common or-
ganic solvents.
Scheme 1. Previous strategies used for the construction of heteropore COFs.
blocks (Scheme 1c).[38] Flexible adjustment of the
pore size and shape has also been achieved. Howev-
er, each of these two strategies theoretically can lead
to two types of COFs with different topological struc-
tures. This means that exact structures of the COFs
cannot be predicted, and the outcomes of the con-
densation reactions cannot be precisely controlled to
afford COFs with expected frameworks. The previous
studies have indeed revealed that, although the com-
bination of tetraphenylethene-based D2h-symmetrical
monomers and linear linkers resulted in dual-pore
COFs,[34–37] the formation of single-pore COFs was
also observed when building blocks having the same
symmetry but different structures were used.[39–43] In
the case of C2v-symmetrical building blocks, it was re-
ported that monomers with different spacer lengths
led to different COF isomers.[38] This drawback pre-
vents unambiguous structural prediction of hetero-
pore COFs from the structures of their building
blocks and decreases the control over the construc-
tion of heteropore COFs.
To overcome this drawback, new synthetic meth-
ods must be developed to avoid the formation of
COF isomers. Herein, we report a general strategy to
precisely fabricate heteropore COFs by utilization of
a multiple-linking-site building block. As shown in
Scheme 2a, C3-symmetric building blocks have been
extensively used to construct COFs with homogene-
ous hexagonal pores.[44–50] Introducing two linking
sites at each branch of the original skeleton does not
change their symmetry. However, when it is com-
Scheme 2. Design principle of the building blocks for precision construction of hetero-
bined with linear linkers, COFs bearing two different pore COFs.
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FTIR spectroscopy was used to characterize the as-prepared
powders. The bands at 2733 and about 3350 cmꢀ1 correspond-
ing to the CꢀH stretching vibrations of aldehyde groups and
the NꢀH stretching vibrations of amino groups of the starting
materials, respectively, were dramatically attenuated after the
condensation reactions, and this suggests that the degree of
polymerization is high (Supporting Information, Figures S1 and
S2). Furthermore, characteristic C=N stretching bands were ob-
served at 1617 cmꢀ1 for SIOC-COF-3 and 1620 cmꢀ1 for SIOC-
COF-4, which provided compelling evidence for the formation
of imine bonds by condensation of the aldehyde groups of
TPA-6CHO and the amino groups of 1,4-diaminobenzene or
benzidine. The C=O stretching band (1700 cmꢀ1) was still ob-
served in the IR spectra of both of the COFs, albeit with dra-
matically decreased intensity relative to that of TAP-6CHO. It
can be attributed to terminal aldehyde groups at the edges of
the COFs. The formation of imine bonds was further confirmed
by solid-state CP/MAS 13C NMR spectra, which exhibited strong
resonance signals at 156 ppm, corresponding to the chemical
shift of the newly formed C=N bonds (Supporting Information,
Figure S3 and S4). The comparisons of the solid-state 13C NMR
spectra of the COFs and the starting monomers clearly indicate
dramatic attenuation of carbonyl groups and formation of C=N
bonds after the condensation reactions, which are fully consis-
tent with the results of the FTIR studies (Supporting Informa-
tion, Figures S5 and S6). Elemental analyses indicated that the
C, H, and N contents of the as-prepared materials were very
close to the corresponding theoretical values of the expected
COFs (see Supporting Information for details). Thermal stability
of the as-prepared polymers was assessed by thermogravimet-
ric analysis (TGA) under N2 atmosphere. The materials showed
less than 6% weight loss up to 5008C (Supporting Information
Figures S7 and S8), that is, they have excellent thermal stability.
In the case of SIOC-COF-3, 90% weight retention even when
the temperature was increased to around 9508C suggests ex-
ceptionally high thermal stability.
Figure 1. SEM images of: a) SIOC-COF-3, and b) SIOC-COF-4. TEM images of:
c) SIOC-COF-3, and d) SIOC-COF-4. Scale bars: 1 mm.
The as-prepared materials were then subjected to powder
X-ray diffraction (PXRD) to elucidate their crystal structures. As
shown in Figure 2, the (100) diffraction peak in the PXRD pro-
file of SIOC-COF-3 appears as a shoulder at 2q=2.958 due to
the strong background in the low-2q region, while a strong
peak corresponding to (200) diffraction is clearly observed at
2q=6.158. In addition to these two peaks, diffraction peaks at
11.39 and 21.998 are also observed, which are assignable to
the reflections of (400) and (002) facets. The (002) peak is weak
and broad. It could be attributed to stacking faults between
the layers, which should result from the twisted conformation
of the skeleton of TPA-6CHO. Structural simulations were per-
formed with Materials Studio version 7.0, from which two the-
oretical crystal structures (eclipsed AA and staggered AB stack-
ing) of heteropore COF were modeled after geometry optimi-
zation by semiempirical calculations at the PM3 level of theory.
Pawley refinement gave unit-cell parameters of a=35.13, b=
35.14, c=8.16 ꢁ and a=89.95, b=90.24, and g=119.258, with
The morphologies of these imine-based COFs were investi-
gated by SEM and TEM (Figure 1). The SEM images show that
the two COFs exist as spherical particles of different sizes,
some of which are interconnected. Furthermore, cracked shells
of some of the spheres revealed hollow interior cavities. Inter-
estingly, the SEM image of SIOC-COF-4 shows that the surfaces
of the spheres are covered with flakes, which have never been
observed in the morphologies of other COFs before. The
hollow spherical morphology of the as-prepared COFs was fur-
ther confirmed by TEM, as evidenced by the spherical shape
and the clear contrast between the shell and inner part of the
spheres. In the case of SIOC-COF-4, the flakes observed by
SEM are clearly observed by TEM, which also indicates that the
flakes are solid. Hollow spherical COFs have been demonstrat-
ed to be useful containers to entrap and immobilize functional
materials such as enzymes due to their having both macropo-
rous inner cavities and micro- or mesoporous shells.[51] Howev-
er, so far this type of COFs is quite rare and previously there
have been only three reports in the literature.[51–53] To the best
of our knowledge, this is the first example of hollow spherical
COFs having hierarchically porous shells.
Rwp =3.35% and Rp =2.59%. The difference plot between the
experimental and refined diffraction patterns shows that they
match each other very well (Figure 2b).
Interestingly, the 2D layers in SIOC-COF-3 adopt staggered
stacking. This is quite different from most of the COFs reported
in the literature, which overwhelmingly adopt eclipsed stack-
ing. This unique packing arrangement was confirmed by com-
parison of the experimental and simulated PXRD patterns. As
shown in Figure 2, the difference between AA and AB stacking
lies in the relative intensity of (100) and (200) diffractions. The
experimental PXRD pattern of SIOC-COF-3 exhibits a low-in-
tensity (100) peak but a high-intensity (200) peak, which
strongly suggests that it adopts AB stacking. Since the first
COF reported in 2005,[44] so far more than a hundred COFs
have been fabricated. However, there has been only one exam-
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Figure 3. 2D synchrotron SAXS profiles of: a) SIOC-COF-3, and b) SIOC-COF-
4, and their corresponding 1D SAXS patterns of: c) SIOC-COF-3, and
d) SIOC-COF-4.
Figure 2. a) Experimental (solid) and refined (dotted) PXRD patterns of SIOC-
COF-3, b) difference plot between the experimental and refined PXRD pat-
terns, and simulated PXRD patterns of the heteropore COF with: c) AB, and
d) AA stacking.
vealed a scattering signal with a d spacing of 3.67 nm (Fig-
ure 3b and d). On the basis of this result the a value of its unit
cell was calculated to be 42.39 ꢁ, which is very close to that
derived from the PXRD data. Compared to that of SIOC-COF-3,
the crystallinity of SIOC-COF-4 is relatively low. This may be at-
tributable to the twisted conformation of the biphenyl units,
which resulted in disordered stacking of the COF layers. More-
over, the larger pore size of SIOC-COF-4 may also account for
its low crystallinity.
ple of a COF adopting staggered AB stacking before this
work.[44] The staggered arrangement observed in SIOC-COF-3
(and SIOC-COF-4, see below) might be attributable to the
unique heteropore structures of these two COFs.
To further corroborate the (100) reflection of SIOC-COF-3,
synchrotron small-angle X-ray scattering (SAXS) was carried
out. The SAXS profile shows a scattering signal at q=
2.06 nmꢀ1, corresponding to a d spacing of 3.05 nm, from
which the a value of the unit cell of SIOC-COF-3 can be calcu-
lated to be 35.20 ꢁ (Figure 3a and c).[54] This is consistent with
the a value obtained from the PXRD data, and further confirms
formation of the as-predicted heteropore COF.
Nitrogen adsorption/desorption measurements revealed that
the isotherms of both SIOC-COF-3 and SIOC-COF-4 fit the typ-
ical type I sorption model[55] and show steep nitrogen uptake
in the low-pressure range (P/P0 =0–0.01), which suggests that
the two COFs have permanent microporosity (Figure 5). Com-
pared to SIOC-COF-4, the N2 isotherm of SIOC-COF-3 exhibited
relatively good reversibility. On the basis of N2 isotherms (P/
P0 =0.05–0.2), the BET surface areas were calculated to be
671.87 m2 gꢀ1 for SIOC-COF-3 and 425.60 m2 gꢀ1 for SIOC-COF-
4 (Supporting Information, Figures S9 and S10). Their total
pore volumes were calculated to be 0.41 cm3 gꢀ1 (SIOC-COF-3)
and 0.42 cm3 gꢀ1 (SIOC-COF-4) at P/P0 =0.99. The pore size dis-
tributions of the two COFs were estimated by using nonlocal
density functional theory (NLDFT), which revealed relatively
wide pore size distributions ranging from 5 to 13 ꢁ for SIOC-
COF-3 and SIOC-COF-4. They could be attributed to the
unique AB stacking of the 2D layers, for such a phenomenon
was also observed in COF-1, which has a staggered AB ar-
rangement, too.[44] In contrast the COFs with eclipsed AA stack-
ing, in which the pores stack on top of one another and are
thus aligned to form 1D channels throughout the 3D packing
lattices, in staggered AB stacking the pores in one layer are
partially blocked by the skeletons of its neighboring layers. As
a result, one pore is divided into several smaller pores that
Similar to that of SIOC-COF-3, the crystal structure of SIOC-
COF-4 was also determined by a combination of experimental
PXRD and structural simulations, through which the expected
structure of the heteropore COF as illustrated in Figure 4 was
confirmed. Peaks at 2.55, 7.21, 13.36, 15.76, and 21.678 are as-
signable to (100), (300), (510), (610), and (002) reflections, re-
spectively (Figure 4). The refined PXRD profile matches the ex-
perimental pattern quite well, which gives unit-cell parameters
of a=b=42.55, c=8.16 ꢁ; a=89.92, b=89.79, and g=
120.068; Rwp =3.67%; and Rp =2.98%. The comparison of the
PXRD patterns of the two simulated structures with the experi-
mental one reveals that SIOC-COF-4 adopts staggered AB
stacking, too. This is evidenced by the diffraction peaks in the
range of 12–178, which are absent in the eclipsed AA stacking
model but appear in the staggered AB stacking pattern. Syn-
chrotron SAXS was also conducted for SIOC-COF-4, which re-
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may have different sizes and shapes. In the case of heteropore
COFs, they bear different kinds of pores in 2D sheets. The divi-
sion of the pores becomes more complicated (see Figure S11
in the Supporting Information for an illustration), and results in
quite broad pore size distributions.
Conclusions
We have developed a new strategy to construct heteropore
COFs, a newly emerging class of COFs which bear different
kinds of well-ordered and periodically distributed pores in an
infinite 2D sheet. Compared to single-pore COFs, these materi-
als exhibit ordered hierarchical porosity, which endows them
with great potential in fabricating functional materials. While
C3-symmetric building blocks have been extensively used to
construct COFs with homogeneous hexagonal pores, introduc-
ing two linking sites on each branch of a C3-symmetric skele-
ton does not change its symmetry but creates multiple linking
sites. Such building blocks lead to heteropore COFs bearing
two kinds of hexagonal pores of different sizes and shapes,
which exhibit unique hollow spherical morphology, uncom-
mon staggered AB stacking of 2D sheets, and exceptionally
high thermal stability. In contrast to previous strategies, which
suffer from the formation of COF isomers, this multiple-linking-
site strategy produces only one type of COF, and no other iso-
mers can be expected. In this way, precision construction of
heteropore COFs has been realized. This new synthetic
method not only guarantees predictability in the structure of
heteropore COFs, but also improves the control over the con-
struction of this type of framework materials. In principle, this
Figure 4. a) Experimental (solid) and refined (dotted) PXRD patterns of SIOC-
COF-4, b) difference plot between the experimental and refined PXRD pat-
terns, and simulated PXRD patterns of the heteropore COF with: c) AB, and
d) AA stacking.
Figure 5. N2 adsorption/desorption isotherms (77 K) of: a) SIOC-COF-3, and c) SIOC-COF-4. Pore size distribution profiles of: b) SIOC-COF-3, and d) SIOC-COF-
4.
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Received: June 26, 2016
Published online on && &&, 0000
&
&
Chem. Eur. J. 2016, 22, 1 – 7
www.chemeurj.org
6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
FULL PAPER
Framework control: A general strategy
to precisely construct two-dimensional
covalent organic frameworks (COFs)
with hierarchical porosity (see figure)
without formation of isomers has been
developed. The as-prepared heteropore
COFs exhibit unique hollow-spherical
morphology, uncommon staggered
stacking, and extremely high thermal
stability.
&
Covalent Organic Frameworks
C. Qian, S.-Q. Xu, G.-F. Jiang, T.-G. Zhan,
X. Zhao*
&& – &&
Precision Construction of 2D
Heteropore Covalent Organic
Frameworks by a Multiple-Linking-Site
Strategy
Chem. Eur. J. 2016, 22, 1 – 7
www.chemeurj.org
7
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ