Self-exfoliation of 2D covalent organic frameworks: Morphology transformation induced by solvent polarity

Article abstract of DOI:10.1039/c7ra09647j

Recently, covalent organic nanosheets (CONs) have emerged as functional two-dimensional (2D) materials for versatile applications. Strong interaction among layers and the instability of borate ester in moisture are the major hurdles to obtain few layered boron-containing CONs by exfoliation of their bulk counterparts. In this paper, we report a facile approach for preparation of few layered borate ester-containing CONs based on electrostatic repulsion of ions. We incorporated organic ionic groups into porous covalent organic frameworks (COFs) and it has been proved that the COFs with quaternary ammonium group could self-exfoliate into few layered ionic covalent organic nanosheets (iCONs) in polar organic solvents. Interestingly, the morphology of the iCOFs-A could be changed from a multilayered aggregation to nanocapsules, or 2D sheets when solvents with different polarity were used. In contrast, non-ionic covalent organic frameworks COFs-B could not self-exfoliate in various solvents. In addition, the self-exfoliated nanosheets could be used to fabricate uniform thin films on SiO₂ wafer and the film exhibited explicit optical and electrical properties.

Full text of DOI:10.1039/c7ra09647j

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Self-exfoliation of 2D covalent organic frameworks:
morphology transformation induced by solvent
polarity†
Cite this: RSC Adv., 2018, 8, 3803
a
a
a
a
a
b
*
Na Zhang, Taisheng Wang, Xing Wu, Chen Jiang, Fang Chen, Wei Bai
a
*
and Ruke Bai
Recently, covalent organic nanosheets (CONs) have emerged as functional two-dimensional (2D) materials
for versatile applications. Strong interaction among layers and the instability of borate ester in moisture are
the major hurdles to obtain few layered boron-containing CONs by exfoliation of their bulk counterparts. In
this paper, we report a facile approach for preparation of few layered borate ester-containing CONs based
on electrostatic repulsion of ions. We incorporated organic ionic groups into porous covalent organic
frameworks (COFs) and it has been proved that the COFs with quaternary ammonium group could self-
exfoliate into few layered ionic covalent organic nanosheets (iCONs) in polar organic solvents.
Interestingly, the morphology of the iCOFs-A could be changed from a multilayered aggregation to
nanocapsules, or 2D sheets when solvents with different polarity were used. In contrast, non-ionic
covalent organic frameworks COFs-B could not self-exfoliate in various solvents. In addition, the self-
exfoliated nanosheets could be used to fabricate uniform thin films on SiO2 wafer and the film exhibited
explicit optical and electrical properties.
Received 30th August 2017
Accepted 9th January 2018
DOI: 10.1039/c7ra09647j
rsc.li/rsc-advances
delamination35 and chemically delamination.36 However, these
exfoliation approaches unavoidably cause some defects in
structure of the materials, which obviously affect their proper-
ties and limit their applications. Therefore, it is highly desired
to develop a facile approach for preparation of 2D CONs in
solution.
Introduction
Inspired by the discovery of graphene,1 efforts to fabricate two-
dimensional (2D) polymers have never stopped.2–9 By far, one of
the most investigated class of 2D polymers is covalent organic
frameworks (COFs) due to their advantages, such as large
surface areas, tunable properties and functionality. In general,
COFs can be classied into three categories: boron-contain-
ing,10–18 triazine-based,19–23 and imine-based COFs.24–27 Among
these COFs, the aromatic boron-containing COFs are particu-
larly promising materials for many applications, such as
organic electronics,28–32 in part because they incorporate two
distinct molecular components that allow their composition to
be varied independently. However, the insoluble characteristics
seriously hinder their characterization and processing for
practical applications.
It is well known that the interaction of the polymer chains
with inbuilt ionic character will be decreased due to electro-
static repulsion. On this account, it is possible to develop a self-
exfoliating approach for preparation of 2D CONs. For example,
self-exfoliation of imine-based COFs with positive charges was
reported in water without external stimuli.37 However, it should
be noticed that borate ester is not stable in moisture38 or in
water,39 thus, exfoliation in water or aqueous solution is not
feasible for the boron-based COFs. Actually, it is quite a chal-
lenge to achieve self-exfoliation of 2D boron-containing COFs in
solution.
The COFs of boronate esters, the most investigated mate-
rials among the family of COFs, are prepared by condensation
of boronic acids and catechols. However, it is well known that
catechols are prone to oxidation and have a poor solubility in
most organic solvents. We presumed that the in situ forma-
tion of catechols from protected catechols could prevent
oxidation of catechols, and moreover, the protected catechols
would possess higher solubility than the catechols in organic
solvents. On the other hand, it is not easy to introduce ionic
groups into the COFs of boronate esters because borate ester
is not stable towards water or moisture. Fortunately, we found
In recent years, considerable efforts have been made to
synthesize covalent organic nanosheets (CONs) by exfoliating
their bulk counterparts through conventional exfoliation
methods
such
as
ultrasonication,33,34
mechanical
aCAS Key Laboratory of So Matter Chemistry, Department of Polymer Science and
Engineering, University of Science and Technology of China, Hefei 230026, P. R.
China. E-mail: bairk@ustc.edu.cn
bDepartment of Chemistry, University of Tennessee, Knoxville, TN 37996, USA. E-mail:
baiwei81@gmail.com
† Electronic supplementary information (ESI) available: 1H NMR, 13C NMR, FT-IR,
PXRD, TGA, SEM, TEM, AFM and element analysis. See DOI: 10.1039/c7ra09647j
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that BF3$OEt2 as a Lewis acid catalyst could not only catalyze
the formation of boron-containing COFs,40,41 but also coor-
dinate with nitrogen atoms to form organic ion pairs.
Therefore, we successfully prepared the iCOFs of boronate
esters by one-pot method using BF3$OEt2 as catalyst. This
one-pot process is a more convenient way for preparation of
boronate iCOFs. The results demonstrated that few-layered
ionic covalent organic nanosheets (iCONs) were obtained via
two steps, formation of the iCOFs in low polar organic
solvents and their self-exfoliation in high polar organic
solvents. Moreover, the iCONs could be well dispersed in
solution and a uniform thin lm of the exfoliated iCONs was
easily fabricated on SiO2 wafer, which exhibited explicit
optical and electrical properties. To the best of our knowl-
edge, this is the rst report on the self-exfoliation of boron-
containing COFs in solution.
Synthesis of compound C2
1 g (1.8 mmol) of compound 2 was dissolved in 75 mL of
benzene. To this solution was added 15 mL of acetone and
300 mg of p-toluenesulfonic acid. The solution was heated to
reux overnight in a Dean-Stark apparatus. Aer the collection
of water ceased, the reaction solution was diluted with 200 mL
of CH2Cl2 and washed with water (2 ꢁ 100 mL). The organic
layers were dried over Na2SO4 and the solvent was removed by
rotary evaporation. The residue was puried by column chro-
matography using silica gel with a mixture of petroleum ether
and dichloromethane (1 : 1, v/v) as eluent to give C2. Compound
C2 was obtained in a yield of 88% (1.0 g). 1H NMR (CDCl3, 300
MHz): d 7.70 (d, 4H), 7.28 (d, 4H), 6.66 (s, 4H), 1.65 (s, 12H). 13
NMR (100 MHz, CDCl3): d 147.3, 138.8, 132.9, 131.8, 128.3,
C
126.7, 121.5, 118.1, 100.8, 26.0.
Synthesis of compound A2
Experimental
General
A Schlenk ask was charged with compound C2 (1.0 mmol), N-
methyl piperazine (2.4 mmol), sodium tertbutoxide (2.8 mmol),
tris(dibenzylideneacetone)dipalladium-(0)
(0.02
mmol,
All solvents were dried following the standard procedures
before use. Unless otherwise indicated, all starting materials
were obtained from commercial suppliers and used without
further purication.
2 mol%), BINAP (0.06 mmol), and toluene (9 mL) under argon.
The ask was immersed in an oil bath and heated to 80 ꢀC with
stirring until the starting material had been completely
consumed as judged by GC analysis. The mixture was allowed to
cool to room temperature, taken up in CHCl3 (150 mL), ltered,
and concentrated. The crude product was then puried further
by ash chromatography on silica gel with a mixture of CHCl3
and CH3OH (10 : 1, v/v) as eluent to give A2. H NMR (CDCl3,
300 MHz): d 7.31 (d, 4H), 7.11 (d, 4H), 6.78 (s, 4H), 3.46 (m, 8H),
2.87 (m, 8H), 2.56 (s, 6H), 1.64 (s, 12H).
Synthesis of compound 1
A solution of 4-bromobenzaldehyde (5.55 g, 30 mmol) and
veratrole (2.8 g, 20 mmol) in chloroform (20 mL) was added
dropwise to a freshly prepared Eaton's reagent (7.7 wt% phos-
phorus pentoxide solution in methanesulfonic acid) in an ice-
bath. Aer addition, the suspension was stirred for 2 h under
reux and then cooled to room temperature. Then DDQ (1
equiv.) was added to the mixture and the reaction was heated at
40 ꢀC for 2 h. Subsequently, the organic mixture was poured into
iced water and neutralized with 2 M NaOH. Aer extraction with
chloroform, the solvent was concentrated under reduced pres-
sure and the residue was recrystallized from a methanol–chlo-
roform mixture to give 1 with a yield of 25%. 1H NMR (300 MHz,
1
Synthesis of compound 3
A ask charged with palladium chloride (0.16 g, 0.9 mmol), 1,1-
bis(diphenylphosphino)ferrocene (DPPF) (0.50 g, 0.9 mmol),
potassium acetate (8.83 g, 90 mmol), and 4,4,5,5,4,4,5,5-
octamethyl-[2,2]bi[[1,3,2]dioxaborolanyl] (8.38 g, 33 mmol) was
ushed with nitrogen. DMSO (120 mL) and 1,3,5-tri-
bromobenzene (3.15 g, 10 mmol) were then added. Aer being
stirred at 80 ꢀC for an appropriate period, the product was
extracted with benzene, washed with water, and dried over
anhydrous magnesium sulfate. Benzene was removed and the
residue was puried by column chromatography using silica gel
with a mixture of petroleum ether and dichloromethane (3 : 2,
v/v) as eluent to give 3, yield: 87% (colorless needle crystal). 1H
CDCl3): d 7.74 (d, 4H), 7.37 (d, 4H), 6.75 (s, 4H), 3.76 (s, 12H). 13
C
NMR (100 MHz, CDCl3): d 149.2, 138.7, 132.8, 132.1, 131.9,
125.7, 121.8, 103.6, 55.7.
Synthesis of compound 2
Compound 1 (1.5 g, 2.47 mmol) was suspended in 47% HBr (aq)
(200 mL) and HOAc (200 mL) and the suspension was deoxy-
genated by passing a stream of Ar through the suspension for
5 min. Then, the mixture was reuxed overnight under Ar. Aer
24 h, the dark solution was allowed to cool slowly under Ar. The
NMR (CDCl3, 300 MHz), d (ppm): 1.33 (s, 36H), 8.36 (s, 3H). 13
NMR (100 MHz, CDCl3): d 144.1, 83.7, 24.9.
C
Synthesis of compound B3
product as a crystalline solid was ltered, washed with water The synthesis of 1,3,5-benzenetriboronic acid was carried out by
and dichloromethane in sequence. Since it is somewhat sensi- a modied method of a published procedure. To the solution of
tive to air, the solid should be stored under Ar. Compound 2 was pinacol 1,3,5-benzenetriboronic ester S7 (1.0 g, 2.19 mmol) in
1
obtained in a yield of 83.3% (1.15 g). H NMR (300 MHz, d6- THF (24 mL) and water (6 mL) sodium periodate (4.22 g,
DMSO): d 7.80 (d, 4H), 7.34 (d, 4H), 6.62 (s, 4H). 13C NMR (100 19.71 mmol) was added. The cloudy white suspension was
MHz, d6-DMSO): d 146.4, 139.3, 133.2, 131.7, 129.1, 124.9, 120.5, stirred overnight at room temperature. Hydrochloric acid (2 M,
106.1.
0.5 mL) was added and the mixture stirred for another 24 hours.
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Methanol (100 mL) was added and the mixture was ltered to
remove salts. The ltrate was concentrated by rotator evapora-
tion and hydrochloric acid (1 M, 30 mL) was added. The mixture
was stirred at room temperature for 2 hours and the resulting
precipitate was collected by ltration and dried in vacuum to
give triboronic acid compound B3 (0.44 g, 95%) as white solid.
1H NMR (300 MHz, d6-DMSO), d (ppm): 8.20 (s, 3H), 7.86 (s, 6H).
13C NMR (100 MHz, CD3OD): d (ppm) ¼ 142.5.
Synthesis of iCOFs-A
Fig. 1 Synthesis of iCOFs-A and COFs-B.
Compound A2 (144 mg, 0.214 mmol) and compound B3 (30 mg,
0.143 mmol) were loaded into a 1 dram screw-cap vial and
suspended in a mixture of mesitylene and dioxane (1 : 1,
40 mL). The dark-blue mixture was sonicated for 15 minutes.
Boron triuoride etherate (15 mL, 0.12 mmol) was added, and
the mixture was sonicated for another 15 minutes. The dark,
heterogeneous mixture was ash frozen in a liquid nitrogen
bath. The ampoule neck was ame-sealed in air using a propane
torch, which reduced the total length by 20–30 mm. Aer
warming to room temperature the suspension was placed in
a gravity convection oven at 120 ꢀC for six days. The reaction was
cooled to room temperature, the ampoule was broken at the
scored neck and the mixture was poured onto qualitative lter
paper and ltered under vacuum. The resulting dark solid was
washed with anhydrous mesitylene and dioxane, and dried in
air.
the appearance of the characteristic B–O and C–O stretching
vibrations of the borate ester at 1375 cmꢂ1 and 1231 cmꢂ1
indicated the formation of condensation products (Fig. S2†). All
the stretching and bending vibrations of the synthesized COFs
were summarized in Table S1 and S2.† Moreover, compared 13
C
CP-MAS solid-state NMR spectra of iCOFs-A and COFs-B with
that of A2 and C2, a very sharp peak at 20 ppm of the dimethyl
group disappeared, which is a strong evidence for the formation
of iCOFs-A and COFs-B (Fig. S3†). Since A2 contains N atoms
and B3 does not, the C/N ratio of iCOFs-A can provide the
information for the composition and the structure of the 2D
polymer. Elemental analysis showed that the C/N ratio of iCOFs-
A was 10 : 1 which was consistent with the theoretical value
(Fig. S4†). In addition, TGA proles of the iꢀCOFs-A showed that
the decomposition temperature was ꢃ237 C (Fig. S5†).
Synthesis of COFs-B
The PXRD pattern of iCOFs-A displayed two intense peaks at
2q ¼ 7.27ꢀ and 7.79ꢀ, which correspond to the pore diameters of
iCOFs-A (Fig. 2a and c). The broad peaks at 2q ¼ 17.6ꢀ and 20.95ꢀ
were assigned to the interlayer p–p stacking distance in iCOFs-A
Compound C2 (144 mg, 0.214 mmol) and compound B3 (30 mg,
0.143 mmol) were loaded into a 1 dram screw-cap vial and
suspended in a mixture of mesitylene and dioxane (1 : 1,
40 mL). The dark-blue mixture was sonicated for 15 minutes.
Boron triuoride etherate (15 mL, 0.12 mmol) was added, and
the mixture was sonicated for another 15 minutes. The dark,
heterogeneous mixture was ash frozen in a liquid nitrogen
bath. The ampoule neck was ame-sealed in air using a propane
torch, which reduced the total length by 20–30 mm. Aer
warming to room temperature the suspension was placed in
a gravity convection oven at 120 ꢀC for six days. The reaction was
cooled to room temperature, the ampoule was broken at the
scored neck and the mixture was poured onto qualitative lter
paper and ltered under vacuum. The resulting dark solid was
washed with anhydrous CH3CN and dried in air.
˚
measured to be ꢃ4.3–5.0 A (Fig. 2b and d), which was larger than
˚
that (3.7 A) of COFs-B (Fig. S6†). This could be attributed to the
incorporation of the positively charged nitrogen-atoms, which
decreased the p–p interaction and increased the layer spacing
due to electrostatic repulsion. This result provides a positive
feedback that it may be possible to achieve self-exfoliation of
iCOFs in solution. Inspired by this result, we have successfully
prepared ionic covalent organic nanosheets iCONs-A by self-
exfoliation of iCOFs-A in polar solvents.
Results and discussion
The anthracene-based ionic covalent organic frameworks
iCOFs-A was solvothermally synthesized via condensation
reaction between A2 (0.3 mmol) and B3 (0.2 mmol) (Fig. 1a) in
a sealed Pyrex tube using dioxane/mesitylene in the ratio 1 : 1.
To demonstrate the effect of ion pairs, we also synthesized the
anthracene-based non-ionic covalent organic frameworks
COFs-B via condensation reaction between C2 (0.3 mmol) and
B3 (0.2 mmol) (Fig. 1a). Synthetic route of the A2, B3 and C2 was
presented in Fig. S1.† Synthetic route of the A2, B3 and C2 was
Fig. 2 (a, b) Experimental PXRD patterns of iCOFs-A. (c, d). Simulated
PXRD pattern of iCOFs-A in eclipsed mode. (e) Pore size distribution of
presented in Fig. S1.† In FTIR spectra of iCOFs-A and COFs-B, iCOFs-A. (f) Nitrogen sorption isotherms for iCOFs-A at 77 K.
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Permanent porosity of iCOFs-A and COFs-B were veried by with an average diameter of 20 ꢄ 5 mm (Fig. 3c, f and i) and an
N2 adsorption isotherms of the activated samples at 77 K, the average thickness of 3.5 ꢄ 0.5 nm (Fig. S8†). According to the
results indicated that both of them followed type-II reversible recent reports,43–45 polymers tend to form nanocapsules in poor
adsorption isotherm. The Brunauer–Emmett–Teller (BET) solvents, whereas they prefer forming 2D lms in good solvents.
surface area of iCOFs-A and COFs-B were calculated to be In our case (Fig. 4), since the mixture of dioxane/mesitylene (1/
8.6 m2 gꢂ1 (Fig. 2f) and 16 m2 gꢂ1 (Fig. S7†), respectively. The 1) with low solvation power is not a good solvent for iCOFs-A,
lower surface area of iCOFs-A could be ascribed to poor layer the multilayered iCOFs-A was obtained. When dioxane/
stacking, small pore diameter, and pore blocking by the counter mesitylene was replaced by dioxane, solvation of the iCOFs-A
anions.42 Pore size distribution was calculated on the basis of with dioxane led to a decrease in the interaction of two layers
nonlocal density function theory (NLDFT). The pore diameters and an increase in the layer spacing, and nally, the bulk iCOFs-
of iCOFs-A and COFs-B were calculated to be 1.25 nm (Fig. 2e) A were exfoliated into thin iCOFs-A sheets with a few layers or
and 1.71 nm (Fig. S7†). Due to the lack of proper channelled even a monolayer. However, the thin iCOFs-A sheets tend to curl
pore structure and the pore blocking by counter anions, the size into a ball to reduce the Gibbs-energy,43 this can be attributed to
distribution of iCOFs-A was not as sharp as that in COFs-B.
the solvation power and polarity of dioxane, both of which are
It is well known that ionic organic molecules prefer to not large enough to stabilize the peeled layers. Compared with
dissolve in polar solvents rather than nonpolar solvents due to dioxane, acetonitrile possesses stronger solvation ability, so not
the difference in solvation ability of the ionic molecules in the only the bulk iCOFs-A can be exfoliated by acetonitrile, but also
solvents. To explore self-exfoliation of iCOFs-A in solution, we the stable few layered iCONs-A sheets can be obtained in
performed dialysis of iCOFs-A in organic solvents with different acetonitrile.
polarities.
The self-exfoliation experiments of iCOFs-A were performed in
The effect of solvent polarity on the self-exfoliation of the various organic solvents, but only a few of organic solvents with
iCOFs-A was tracked by a combination of scanning electron relatively strong polarity are effective for the self-exfoliation of
microscopy (SEM), transmission electron microscopy (TEM), iCOFs-A, including DMF, DMSO, CH3CN, and CH3OH (Fig. S11†).
atomic force microscopy (AFM) and light-scattering (Fig. 3). The weight and volume ratio between iCOFs-A and organic
First, iCOFs-A was dispersed in dioxane/mesitylene (1/1) and solvent for the self-exfoliation was o.5 mg mLꢂ1. As a control
characterized by electron microscopy. TEM images (Fig. 3a) and experiment, we also performed dialysis of the COFs-B in solvents
SEM images (Fig. 3d) showed that iCOFs-A possessed a layer-by- with different polarities, the results demonstrated that no change
layer structure with an average diameter of 3.5 ꢄ 0.5 mm, which in morphology was observed (Fig. S9†), indicating no self-
consistent with the results of dynamic light scattering (Fig. 3g). exfoliation occurred. Obviously, both electrostatic repulsion and
Then, the iCOFs-A dispersed in dioxane/mesitylene was dia- solvation are important for the self-exfoliation of iCOFs-A. First,
lyzed respectively in dioxane and acetonitrile through a dialysis the electrostatic repulsion led to the weaker p–p interaction
membrane with a molecular weight cut-off (MWCO) of 1000. between the layers and the larger layer spacing, which are
Interestingly, the results revealed that hollow nanospheres were favorable for the intercalating of solvent. Then, the solvation of
formed in dioxane with an average diameter of 250 ꢄ 50 nm polar solvents with iCOFs-A resulted in their self-exfoliation.
(Fig. 3b, e and h) and an average thickness of 7.5 ꢄ 0.5 nm Therefore, the incorporation of ionic groups is of great signi-
(Fig. S8†), whereas nanosheets were obtained in acetonitrile cance for self-exfoliation of the COFs in solution.
Since few-layered nanosheets of the iCONs-A can easily be
obtained from self-exfoliation of the iCOFs-A in polar solvents,
we have successfully prepared uniform, free-standing CON thin
lms of iCONs-A (Fig. S10a and b†), indicating a signicant
impact on processing and applications of COFs. Moreover, we
Fig. 3 Monitoring the morphological transformation from iCOFs-A to
iCONs-A by TEM (a–c), SEM (d–f) and DLS studies (g–i). Scale bars: (a) Fig. 4 Schematic illustration of the formation of 2D ionic covalent
0.5 mm, (b) 0.5 mm, (c) 0.2 mm (inlet image: 2 mm), (d) 2 mm, (e) 1 mm, (f) organic nanosheets. Scale bars: 0.5 mm (left), 0.5 mm (middle), 2 mm
20 mm (inlet image: 10 mm).
(right).
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investigated the electrical conductivity and photoconductivity of
the self-exfoliated nanosheets by casting a thin lm of iCONs-A
on SiO2 wafer. The micro-gap electrodes were fabricated by
photolithography silicon wafer covered with a 300 nm thick SiO2
dielectric layer. The gold electrode pair was 50 mm long and 5 mm
wide and the lm thickness was about 50 mm. Based on the ob-
tained data, smaller current was observed with iCONs-A46 which
may be ascribed to the weaker stacking (Fig. S10d†). Then, on
irradiation with visible light from a xenon lamp, the character-
istics curve showed larger current, which illustrated the photo-
response of the lm. In order to investigate the photocurrent
response, we measured the transient photocurrent of the device
at a bias voltage of 2 V (Fig. S10c†). The photosensitivity was
found to be 3.86. Transient photocurrents were steady and
reproducible during on–off cycles of the visible light irradiation.
The photoresponse of iCONs-A was likely due to their p–p overlap
among re-accumulated nanosheets (Fig. S10d†).47 We believe that
it is possible to obtain nanosheets of iCOFs with excellent elec-
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ˆ
´
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¨
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Conflicts of interest
There are no conicts to declare.
23 C. E. Chan-Thaw, A. Villa, L. Prati and A. Thomas, Chem.–
Eur. J., 2011, 17, 1052–1057.
24 F. J. Uribe-Romo, J. R. Hunt, H. Furukawa, C. Klock,
M. O'Keeffe and O. M. Yaghi, J. Am. Chem. Soc., 2009, 131,
4570–4571.
Acknowledgements
The authors are grateful for nancial support from the National
Natural Science Foundation of China (No. 21674101).
25 F. J. Uribe-Romo, C. J. Doonan, H. Furukawa, K. Oisaki and
O. M. Yaghi, J. Am. Chem. Soc., 2011, 133, 11478–11481.
26 S.-Y. Ding, J. Gao, Q. Wang, Y. Zhang, W.-G. Song, C.-Y. Su
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