Synthesis of C?C Bonded Two-Dimensional Conjugated Covalent Organic Framework Films by Suzuki Polymerization on a Liquid–Liquid Interface

Article abstract of DOI:10.1002/anie.201811399

Synthesis of free-standing two-dimensional (2D) conjugated covalent organic framework (COF) films linked by C?C bonds is highly desirable. Now a very simple and mild strategy has been developed to synthesize them by Suzuki polymerization on a water–toluen

Full text of DOI:10.1002/anie.201811399

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Accepted Article
Title: Synthesis of C-C bonded Two-dimensional Conjugated Covalent
Organic Framework Films via Suzuki Polymerization on Liquid/
liquid Interface
Authors: Deng Zhou, Xianyang Tan, Huimin Wu, Lihong Tian, and
Ming Li
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201811399
Angew. Chem. 10.1002/ange.201811399
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Synthesis of C-C Bonded Two-dimensional Conjugated Covalent
Organic Framework Films via Suzuki Polymerization on
Liquid/liquid Interface
1
1
Deng Zhou, Xianyang Tan, Huimin Wu, Lihong Tian and Ming Li*
Abstract: Synthesis of free standing two-dimensional (2D)
conjugated covalent organic framework (COF) films linked by C-C
bonds can be considered as a holy grail of synthetic chemistry.
However, reactions for their synthesis are quite rare. Herein we
developed a very simple and mild strategy to synthesize them via
the Nobel Prize reaction (Suzuki polymerization) on water/toluene
interface in a fridge. The versatility of this strategy was proved by the
successful synthesis of two different 2D-COF films: a porous
graphene and a porphyrin-contained 2D-COF. Both 2D-COF films
have large lateral size and their crystalline domains were visualized
by high resolution TEM. Based on the wide compatibility of Suzuki
reaction, our breakthrough work opened a door for the synthesis of
various 2D conjugated COF films. For application studies, the
porous graphene exhibits a good carrier mobility which is much
higher than -C=N- linked 2D COF films and a good catalytic activity
for hydrogen evolution reaction which is comparable with nitrogen or
phosphorus doped graphene.
been used in the synthesis of tremendous conjugated
[12]
polymers , if it can be applied in 2D COF’s synthesis, a door
may be open for tremendous 2D conjugated COF films.
Moreover, compared to most of reported 2D conjugated COFs,
organic synthesis of which are very complicated, there is a big
advantage for Suzuki reaction based 2D COFs since a lot of
Suzuki-monomers are already commercially available or can be
easily synthesized.
Until now, main methods for 2D-COF films’ synthesis include
[13]
[14]
interface-template
and exfoliation of layered crystals . As
Pd-catalyst and base are needed and big boronic group should
be cleaved, interface-template strategy is mostly feasible for the
2D-COF’s synthesis based on Suzuki-reaction. Fortunately,
Suzuki reaction tolerates water so the reaction system can be
separated to two phases: inorganic base in water and water-
insoluble components including aryl boronic ester, aryl halide
and palladium catalyst in water-insoluble organic solvent (Figure
1b). During reaction, components from the organic phase must
meet with the inorganic base from the water phase at the
[
1]
Since the discovery of graphene , synthesis of free standing
[15]
interface to start the transmetalation step
. Since the
[2]
2
D conjugated COF films (to some extent, which can be
termed as 2D conjugated polymer ) directly linked by C-C
bonds can be considered as a holy grail of synthetic chemistry .
However, lack of basic and compatible reactions for their
synthesis leads their further studies to be castles in the air.
Until now, most of 2D conjugated COFs were linked by
reversible bonding such as -C=N- and –B-O- . There are only
a few examples for directly connecting aromatic blocks by C-C
bond, including coupling between aryl-halogens at high
temperature (> 500 ℃) in single crystals (only one example;
the inherent disadvantage is that growing appropriate crystals
still depends on some luck.), Glaser coupling between
acetylenes at interfaces (only two examples gave crystalline
hydroxide/halogen exchange or hydroxylation of boronic group
[
3]
[16]
in this step are reversible
(Figure 1c), if low reaction-
[4]
temperature was applied, the whole polymerization can be at the
equilibrium state to endow the product with internal order.
[
5]
[6]
[7]
[8]
[9]
films)
and Knoevenagel condensation for growth of COF
crystals (only one example; other monomers were tried but
[10]
unsuccessful)
.
Therefore, finding compatible and mild
reactions for the synthesis of C-C bonded 2D conjugated COF
films is of paramount appealing. Suzuki reaction is powerful for
Figure 1. (a) The scheme of Suzuki reaction between aryl groups; (b)
Synthesizing 2D-COF on the interface via Suzuki polymerization (Ar
represents aryl group; X represents halide atom; B represents boronic group;
m, n≥2 and at least one ≥3); (c) Reversible hydroxide/halogen exchange or
2
2
construction of sp -sp carbon bond (Figure 1a), not only
because the reaction condition is mild but also it is compatible
-
hydroxylation of boronic group in the transmetalation step (OH was generated
[11]
2-
3
with different monomers . As this Nobel Prize reaction has
by the reversible hydrolysis of CO
).
As prototype applications of the above Suzuki-strategy, 2D
conjugated COFs named as 2DCCOF1 (Figure 2a) and
2DCCOF2 (Figure 2c) were synthesized. Compared with
traditional polymer, 2DCCOF1 can be termed as 2D-
homopolymer and 2DCCOF2 can be termed as 2D-copolymer.
All monomers of 2DCCOF1 and one monomer (1,4-bis(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)benzene) of 2DCCOF2 were
D. Zhou, X. Tan, Prof. H. Wu, Prof. L. Tian and Prof. M. Li*
Key Laboratory for the Synthesis and Application of Organic
Functional Molecules, Ministry of Education & Hubei Collaborative
Innovation Center for Advanced Organic Chemical Materials &
Hubei Key Laboratory of Polymer Materials & College of Chemistry
and Chemical Engineering
Hubei University
Wuhan 430062, P. R. China
E-mail: limingljy@163.com
D. Zhou and X. Tan have an equal contribution.
commercially available; another monomer of
2DCCOF2
(
porphyrin-monomer) was one-step-synthesized (ESI Figure S1).
Supporting information for this article is given via a link at the end of
the document.
Both polymerizations were carried out at interface under an
argon atmosphere. During reaction, dilute toluene solution of
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3 4
monomers and the catalyst Pd(PPh ) were on the top of the
water solution of K CO . These two-phase systems were stored
2
3
in a fridge (2 ℃). After a month, stable and large-sized sheets
formed at both interfaces. 2DCCOF1 film exhibits strong blue
fluorescence under the UV light, indicating the formation of large
conjugated system (Figure 2b). The 2DCCOF2 film is brown-
yellow (Figure 2d).
Figure 3. (a) OM image of 2DCCOF1 on a glass slide (scale bar: 0.1 mm); (b)
AFM image and the height profile of 2DCCOF1 on a SiO
um); (c) SEM image of 2DCCOF1 on a lacey carbon coated TEM grid (scale
bar: 1 um).
2
/Si-wafer (scale bar:
2
The EDX spectrum indicates that the film is mainly composed
of carbon. Boron and iodine were almost undetected,
manifesting the high conversion of this reaction (ESI Figure S4).
EDX mapping shows that Carbon is homogenously distributed
on the film (inset of ESI Figure S3). In the Raman spectrum,
-
1
2
peaks at 1446 and 1599 cm are attributed to sp -C (H-C=, =C<)
-1
and the peak at 1350 cm is due to structure defects, similar to
D band of graphene (Figure 4a). In the IR spectrum, peaks of
the product can be observed, including 2930 cm (stretching of
C-H of Ph), 1637 and 1416 cm (stretching of C=C). Especially,
meta-substituted phenyl groups can be proved by peaks of 1870
cm (in-plane vibration of C=C of m-Ph), 1008 cm (in-plane
-1
-
1
-1
-1
Figure 2. (a) Interfacial synthesis of 2DCCOF1 and (b) its fluorescence
photograph under UV light (365 nm); (c) Interfacial synthesis of 2DCCOF2 and
-1
bending of C-H of m-Ph), 986, 823 and 703 cm (out-of-plane
bending of C-H of m-Ph) (Figure 4b). The XPS survey spectrum
majorly shows a C1s peak, except for Si and O peaks from
(d) its photograph under normal light (As it is not easy to differentiate the
fluorescence between 2DCCOF2 and porphyrin-monomer, here we only show
its photograph under normal light.).
SiO /Si-substrate (Figure 4c). The high resolution XPS spectrum
2
of C1s can be split to two closing peaks at 284.2 and 284.6 eV
2
with similar intensity, which are from two different sp -C (H-C=,
Morphologies of both 2D-COF films were fully characterized by
optical microscope (OM), scanning electron microscope (SEM)
and atomic force microscope (AFM). Chemical compositions
were comprehensively determined by energy-dispersive X-ray
spectroscopy (EDX), Raman spectroscopy, infrared (IR)
spectroscopy, X-ray photoelectron spectroscopy (XPS), UV-vis
spectroscopy and fluorescence spectroscopy (FL). Internal
structures were all revealed by the transmission electron
microscope (TEM).
=C<) of phenyl rings (inset of Figure 4c). The film shows a broad
absorption with four peaks at 282, 315, 372 and 476 nm, and a
broad visible fluorescence (FL) with four peaks at 418, 438, 465
and 510 nm, in accordance with its conjugated skeleton (Figure
4d). The optical band gap of 2DCCOF1 is 1.94 eV (ESI Figure
S5), indicating it is a narrow-band gap semiconductor (DFT
calculated value: 1.93 eV; ESI Table S1).
After reaction, 2DCCOF1 was fished out by substrates such as
glass slid, TEM grid and Si-wafer. The film on the substrate was
then washed by chloroform (×3), toluene (×3), water (×3) and
methanol (×3) to remove small molecules, oligomers and salts.
Under the OM, an mm-lateral-sized flat thin sheet with a crack
was observed (Figure 3a). The AFM imaging (Figure 3b) also
demonstrates the flat character of the film with a thickness of 18
nm (~ 50 layers, considering 0.36 nm / layer). Interestingly, the
edge of the film is quite straight and shows internal angles of
120 °, which may suggest hexagon crystalline facet in the film.
Moreover, the film can be delaminated to thinner sheets in NMP
by ultrasonication, indicating that the film has layers (ESI Figure
S2). SEM images reveal that 2DCCOF1 is stable enough to
free-stand over um to 50 um-sized holes (Figure 3c and ESI
Figure S3). Similar to the AFM image, hexagon wrinkles may
also suggest hexagon crystalline facets in the film (Figure 3c).
Figure 4. (a) Raman spectrum (on SiO
survey spectrum (on SiO /Si-wafer) and high-resolution C1s XPS spectrum
inset), (d) UV-vis (solid curve) and fluorescence (dot curve) spectrum of
DCCOF1.
2
/Si-wafer), (b) IR spectrum, (c) XPS
2
(
2
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The high-resolution TEM (HRTEM) demonstrates the 2D
periodicity of 2DCCOF1 and also its layered structure (Figure
produce porous graphene including (1) etching holes in
[18]
[19]
graphene
,
(2) growing on porous template
,
(3)
[20]
5a). The fast Fourier transformation (FFT) revealed a hexagonal
polymerization of porous monomer on silver surface . However,
these reported methods are limited by several disadvantages
including low productivity, difficulty of controlling pore-size-
distributions, small lateral-size or un-freestanding. Our work
solved a long-lasting problem that efficiently producing free-
standing sub-nano-porous graphene with periodicity and large
lateral-size.
pattern (Figure 5b). Both positions and brightness of these spots
match spots in the simulated diffraction pattern of the structure
with A-B-C…stacking (Figure 5c and ESI Table S1), indicating
that in this sheet, layers stack with the A-B-C… fashion (Figure
5d), rather than the A-A…fashion (ESI Figure S6). The higher
magnification revealed distinct crystallinity of the film (Figure 5e;
it was deteriorated quickly during measurement) and the digital
magnification of a small domain overlaps the view of A-B-C…
stacked structure along the arrow in Figure 5d, also manifesting
that layers stack with the A-B-C… fashion (Figure 5f). From this
structure, the pore-size of 2DCCOF1 in a monolayer is ~0.3 nm.
As same as 2DCCOF1, 2DCCOF2 film was fished out and
washed for characterizations. The OM imaging shows large
lateral sized flat films (Figure 6a). The AFM measurement
exhibits flat films with thickness of 7.5 nm and 23 nm,
demonstrating the ultra-thin character of the film. Additionally,
stair-like steps in the AFM image manifests that the film is
composed of layers (Figure 6b). The SEM image indicates the
film is stable enough to free-stand spanning with um to >10 um-
lateral size (Figure 6c and ESI Figure S7 and S8). Similar to the
AFM image, layers can also be visualized by the contrast in the
magnified SEM image (ESI Figure S7). Furthermore, all images
show vertical angles, which can be a clue of square crystal facet
in the film.
Figure 6. (a) OM image of 2DCCOF2 on a glass slide (scale bar: 10 um); (b)
2
AFM image of 2DCCOF2 film on a SiO /Si-wafer and its height profile (scale
bar: 1 um); (c) SEM image of 2DCCOF2 on a lacey carbon coated TEM grid
(scale bar: 2 um).
The EDX spectrum of 2DCCOF2 film indicates that except for
main elements of carbon and nitrogen, other elements including
iodine, phosphorus and palladium were clearly detected (ESI
Figure S9). This difference from 2DCCOF1 film may owe to that
larger pores in 2DCCOF2 film may absorb more guests and lead
to more defects and more un-reacted terminal groups. The EDX
mapping reveals that carbon and nitrogen have the similar
distribution in the film, in accordance with its structure (insets of
ESI Figure S8). Raman spectrum of the polymer film shows
-1
peaks matching phenyl C=C (1560 and 1458 cm ), porphyrin
-1
C=N (1381 and 1332 cm ), porphyrin skeleton (1237 and 1002
-
1
-1
cm ) and porphyrin NH (965 cm ) (Figure 7a). The broad peak
Figure 5. (a) HRTEM image of 2DCCOF1 (scale bar: 10 nm) and (b) the FFT
pattern; (c) simulated diffraction pattern according to the view along the arrow
as shown in (d); (d) side view and top view of the A-B-C… stacked structure;
-1
at 1560 cm , which is derived from the sharper peak at the
same region of porphyrin monomer, indicates the formation of
extended phenyl rings in the polymer film. IR spectra of
porphyrin monomer and the polymer film are almost the same,
(e) the highest HRTEM image (scale bar: 2 nm); (f) comparing the digital
zoomed HRTEM image and the view of A-B-C… stacked structure along the
arrow in (d) that all benzene rings totally stack.
-1
2
indicating the existence of NH (3315 and 965 cm ), sp -C-H
-
1
-1
(
3046 and 2918 cm ), C=C (1560 and 1475 cm ), porphyrin
-1
-1
C=N (1388 cm ), porphyrin skeleton (1005 cm ) and p-
substituted phenyl C-H (799 cm ). The peak-increasing of
phenyl (1560 cm ) and porphyrin skeleton (1388 cm )
demonstrates the enlargement of the conjugated-system after
polymerization (Figure 7b). The XPS survey spectrum of the film
[17]
2DCCOF1 is a type of graphene with homogenous pores
.
-1
Porous graphene is an advanced membrane with important
applications in nano-filtration, energy storage, optoelectronic
devices and so on. There are some pioneer techniques to
-1
-1
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exhibits the C1s and N1s with an intensity ratio of 13:1, close to
the predicted value of 14:1 (Figure 7c; same as the EDX
spectrum, iodine, phosphorus and palladium can also be
observed). The high resolution XPS spectrum of C1s can be
divided to three peaks (284.4, 286.2 and 287.8 eV) with an
intensity ratio of 24:3.2:1, which can be assigned to C=C, C-N
edges which proposes square crystal facet in the film (ESI
Figure S11). Its internal order was observed in small domains,
which exhibits a square lattice with d-spacing of 1.1 nm between
parallel squares (Figure 8a). The FFT pattern also reveals the
same square periodicity (Figure 8b). Compared with the AB-
aggregation structure (Figure 8c) and its simulated diffraction
pattern (Figure 8d, ESI Table S2), layers in this film should have
the AB-aggregation rather than the AA-aggregation (ESI Figure
S12). Moreover, the pore-size of 2DCCOF2 in a monolayer is
~1.8 nm. It should be noted that 1) the FFT pattern only shows
[21]
and C=N, respectively (Figure 7d) . Furthermore, the high
resolution XPS spectrum of N1s is composed of two peaks
(
399.7 and 399.1 eV) with an intensity ratio of 1:1, which can be
[22]
assigned to NH and N=C, respectively (Figure 7e) . Both
intensity ratios accord with their predicted values (24:3:1 and 1:1, some spots of the simulated diffraction pattern; absence of other
respectively). The characteristic S band (435 nm) and Q bands
525, 570 and 669 nm) of porphyrin units in the UV-vis spectrum
spots can be because the size of ordered domain is too small; 2)
weak spots of 1.5 nm may immerge in the background of the
central spot.
(
confirm their presence in the film (solid curve in Figure 7f). The
fluorescence shows broad visible peaks (441, 473 and 528 nm)
tailed to red light region, demonstrating large conjugated
[23]
systems in the film (dot curve in Figure 7f) . The optical band
gap of the film is 1.78 eV (ESI Figure S10), matching the DFT
calculation value of 1.75 eV (ESI Table S2).
Figure 8. (a) HRTEM image of 2DCCOF2 (scale bar: 2 nm) and (b) the FFT
pattern; (c) the top-view of the proposed structure with AB-aggregation and (d)
its simulated diffraction pattern.
In order to explore potential applications of our 2D conjugated
COF film in electronics, a field-effect transistor (FET) was
fabricated with 2DCCOF1 as the channel material. Both transfer
and out-put curves of this device demonstrate that carrier
concentration in the channel increases with decreasing the gate
-
3
voltage from 20 to -60 V, revealing its carrier mobility of 3.2×10
2
cm /V.s (Figure 9a, b). This mobility is three orders of magnitude
larger than recently reported -C=N- linked 2D conjugated COF
[24]
films.
Furthermore, the hydrogen evolution reaction (HER)
Figure 7. (a) Raman spectra of porphyrin monomer and 2DCCOF2 on Si-
wafer; (b) IR spectra of porphyrin monomer and 2DCCOF2; (c) XPS survey
spectrum of 2DCCOF2 on SiO /Si-wafer; (d) high resolution XPS spectrum of
2
C1s and (e) N1s of 2DCCOF2; (f) UV-vis (solid curve) and fluorescence (FL,
dot curve, excited at 430 nm) spectrum of 2DCCOF2.
activity of 2DCCOF1 was checked by horizontal deposition of
the film on a copper electrode. As shown in Figure 9c, the bare
copper electrode exhibits very poor HER activity. In contrast, the
2
DCCOF1 coated electrode exhibits a much better HER activity.
2
The overpotential is 541 mV at a current density of 10 mA/cm
and the Tafel slope is 130 mV/decade (Figure 9d). These HER
[25]
values are comparable with N or P doped graphene.
The low magnification TEM image clearly shows that
2DCCOF2 is composed of layers and orthogonal features at
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In conclusion, this work opened the door of using Suzuki
reaction for the synthesis of C-C bonded 2D conjugated COF on
liquid/liquid interface under very mild condition. Two 2D
conjugated COFs were successfully synthesized and both of
them exhibit very large lateral size, naked-eye visibility and
some internal order. This result definitely demonstrates the
power of our strategy for the production of C-C bonded 2D
conjugated COFs. Since Suzuki reaction is a versatile method to
create Csp -Csp bond and has wide compatibility, it is
reasonable to claim that a variety of new 2D conjugated COFs
will be developed by our strategy. Furthermore, our strategy may
be extended to other similar reactions such as Sonogashira
reaction and Heck reaction, which is currently being researched
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domain will be researched in our group as soon as possible.
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Acknowledgements
Support of this work by national natural science foundation of
China (21873027 and 21504023) is gratefully acknowledged.
Authors thank to Jun Su for HRTEM measurement; Lei Liao for
FET measurement; Wei Hu for HER measurement; Yunbin He,
Keyu Zheng and Lei Zhang for sharing labs. M. Li is the project
leader, conceived completely this project, did part experiments
and wrote the paper. D. Zhou did experiments on 2DCCOF1 and
X. Tan on 2DCCOF2. H. Wu and L. Tian provided a quota of
master student to M. Li, respectively.
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Keywords: 2D conjugated COF • Suzuki polymerization •
interface • porous graphene • porphyrin
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Angewandte Chemie International Edition
10.1002/anie.201811399
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
1
1
A very simple and mild strategy to
synthesize 2D conjugated COF films
were developed via Suzuki
Deng Zhou, Xianyang Tan, Huimin
Wu, Lihong Tian and Ming Li*
polymerization on liquid/liquid
interface at low temperature.
Page No. – Page No.
Synthesis of C-C bonded Two-
dimensional Conjugated Covalent
Organic Framework Films via Suzuki
Polymerization on Liquid/liquid
Interface
This article is protected by copyright. All rights reserved.