Facile synthesis of 3D covalent organic frameworksviaa two-in-one strategy

Article abstract of DOI:10.1039/d0cc08129a

A “two-in-one” strategy was employed to construct 3D-COFs for the first time. Based on this strategy, a 3D-Flu-COF could be readily synthesized in various simplex organic solvents. Benefitting from the non-conjugated structure, the 3D-Flu-COF showcased ex

Full text of DOI:10.1039/d0cc08129a

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Facile synthesis of 3D covalent organic
frameworks via a two-in-one strategy†
Cite this: Chem. Commun., 2021,
57, 2136
Xiaoli Yan,a Hui Li,b Pengna Shang,a Huan Liu,a Jingjuan Liu,a Ting Zhang,a
b
a
Guolong Xing,*a Qianrong Fang
and Long Chen
*
Received 15th December 2020,
Accepted 25th January 2021
DOI: 10.1039/d0cc08129a
rsc.li/chemcomm
A ‘‘two-in-one’’ strategy was employed to construct 3D-COFs for structural illustration.9 In contrast to the strong intra-layer covalent
the first time. Based on this strategy, a 3D-Flu-COF could be readily bonds and interlayer interactions existing in 2D COFs, only covalent
synthesized in various simplex organic solvents. Benefitting from bonds direct the formation of 3D COFs, which make them more
the non-conjugated structure, the 3D-Flu-COF showcased excellent inclined to afford amorphous frameworks.7a,8,9 To address these
acidichromic sensing performance with good sensitivity, reversibility challenges, great efforts have been devoted to optimize the synthesis
and naked eye visibility.
of 3D COFs, and significant progress has been recently made. For
example, a facile synthetic method towards 3D COFs under ambient
Covalent organic frameworks (COFs) represent a new class of conditions has been reported by Fang et al. using the ionothermal
polymers composed of light elements (such as C, H, O, N, and method, which could generate high crystallinity COFs in minutes.12
B) featuring high crystallinity and permanent porosity, which Despite the great advantages of this approach, there are several open
are solely connected by covalent linkages.1 Since the first questions that remain to be addressed. For example, the residual
example reported by Yaghi in 2005,1a COFs have received ionic liquid in the pores of the as-prepared COFs may reduce the
considerable research interests due to their prominent applica- internal surface areas and make the structure–property relationships
tion potential in gas storage and separation,2 energy storage,3 elusive. Therefore, it is still highly desired to develop simple yet
heterogeneous catalysis,4 drug delivery,5 and sensors,6 among effective approaches for 3D COF synthesis for further advancing this
others. Generally, COFs can be categorized into two- promising research field.
dimensional (2D) COFs and three-dimensional (3D) COFs
Recently, our group developed a ‘‘two-in-one’’ strategy that
according to their structural dimensionality. 2D COFs are directly uses bifunctional monomers to construct 2D COFs via
mainly synthesized from relatively planar building blocks, self-polycondensation. Thanks to the stoichiometry of the two
which are beneficial to form layered eclipsed (AA) stacking functional groups which is kept identically equal irrespective of
structures. In contrast, 3D COFs are constructed from non- synthetic conditions, such a ‘‘two-in-one’’ strategy avoids the
planar (typically tetrahedral) building blocks that tend to form tedious screening process of solvents compared to the conven-
highly porous networks.7a However, compared to the large tional co-condensation and thus reduces the difficulties in
family of 2D COFs, only about 40 examples of 3D COFs have obtaining highly crystalline 2D COFs. Accordingly, a series of
been reported to date,7c although their intrinsic characteristics 2D COFs with good solvent adaptability, high crystallinity and
of higher surface area and lower density make them ideal excellent repeatability were obtained by using this ‘‘two-in-one’’
candidates for gas storage, gas separation and catalysis.7 The strategy.4c,10,11d However, this strategy has not yet been
possible reasons and remaining challenges that cause the explored in 3D COF synthesis. We envision that the ‘‘two-in-
development of 3D COFs to persistently lagbehind 2D COFs one’’ strategy is also adapted for the synthesis of 3D COFs
might be the poor synthetic accessibility7a,8 and difficulties in (Scheme 1).
To verify our hypothesis, we designed a new tetrahedral A2B2
type monomer through the ‘‘two-in-one’’ strategy. 4,40-(3,6-bis(4-
(5,5-dimethyl-1,3-dioxan-2-yl)phenyl)-9H-fluorene-9,9-diyl)dianiline
(Fig. 1a, A2B2-Flu) with two neopentyl acetal and two amine groups
in one fluorene core was synthesized (Scheme S1, ESI†) and further
applied to construct a fluorene-based imine 3D COF (defined
as 3D-Flu-COFsolvent). Notably, to avoid auto-polymerization of
the A2B2-monomer during the synthesis and workup process,
a Department of Chemistry and Tianjin Key Laboratory of Molecular Optoelectronic
Science, Tianjin University, Tianjin 300072, China. E-mail: xinggl@tju.edu.cn,
long.chen@tju.edu.cn
b State Key Laboratory of Inorganic Synthesis and Preparative Chemistry,
Jilin University, Changchun 130012, China
† Electronic supplementary information (ESI) available: Experimental section,
synthesis and characterization, and supplementary data. See DOI: 10.1039/
d0cc08129a
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characterized by Fourier transform infrared (FT-IR) spectroscopy
and solid-state 13C cross-polarization magic-angle-spinning
(13C CP/MAS NMR spectra). FT-IR spectra of 3D-Flu-COFs
prepared in different solvents were nearly identical. The
stretching bands of free amine (N–H, B3300 cmꢀ1) and methyl
(–CH3, B2900 cmꢀ1) ascribed to the monomer nearly disap-
peared, and a new CQN stretching vibration band appeared
at 1629 cmꢀ1, indicating the formation of imine linkages in
3D-Flu-COFs (Fig. S4, ESI†). The existence of imine bonds was
further verified by 13C solid-state NMR spectroscopy with a clear
characteristic peak at 153 ppm (Fig. 1c). In addition, a residual
weak peak of the neopentyl acetal group was observed at 68 ppm,
which indicates that the deprotection and condensation of the
neopentyl acetal group may occur simultaneously.13 Although
with identical structures, 3D-Flu-COFs prepared in different
solvents showcased different morphologies, as observed from
scanning electron microscopy (SEM) images, 3D-Flu-
Scheme 1 Schematic diagrams for 3D COFs constructed using (a) the
conventional co-condensation method and (b) the ‘‘two-in one’’ strategy.
COFbenzylalcohol
,
3D-Flu-COFdioxane and 3D-Flu-COFmesitylene
exhibited a uniform ‘‘coral rod’’-like morphology, while 3D-Flu-
COFn-BuOH exhibited plate-like aggregates (Fig. S7, ESI†). It
has also been previously reported that the differences in the
morphology of COFs caused by solvents may be due to changes
in the polarity of the solvents.10a,14
The structures of 3D-Flu-COFs were resolved by powder
X-ray diffraction (PXRD) analysis combined with structural
simulations (Fig. 1b). After minimizing geometric energy using
the Materials Studio software package based on a 7-fold
interpenetrating dia network, a fitted unit cell parameter of
a = b = 27.3960 Å, c = 7.1325 Å, a = b = g = 901 was obtained. The
powder samples of 3D-Flu-COFs obtained in different solvents
exhibited exactly the same patterns with an intense peak at
6.451 and other five distinct diffractions at 4.601, 10.251, 13.051,
16.701, and 19.851 corresponding to the (200), (110), (310),
(400), (141), and (431) facets of the space group P43, which
were in good agreement with the simulated PXRD patterns. The
refinement result was nearly overlapped with the observed ones
with reasonable Rwp = 4.32% and Rp = 2.94%. Based on the
Fig. 1 (a) Schematic illustration of the molecular design and synthesis of a
3D-Flu-COF, (b) PXRD patterns, and (c) solid-state 13C NMR (75 MHz)
spectrum of 3D-Flu-COFbenzyl alcohol
.
neopentyl glycol groups were introduced to protect the aldehydes,
which could be in situ deprotected by acetic acid during COF
synthesis (Scheme S1, ESI†).11d,13 As expected, 3D-Flu-COFs can
not only be readily synthesized in various simplex organic solvents,
such as benzyl alcohol, dioxane, mesitylene, n-BuOH etc., but also
exhibit high crystallinity and large surface areas.
As shown in Scheme S1, ESI,† A2B2-Flu was prepared
through a facile three-step synthetic route in high yield, and
was unambiguously characterized by nuclear magnetic reso-
nance (NMR) and high-resolution mass spectroscopy (HR MS).
3D-Flu-COFs were synthesized as off-white powders in 90–97%
yields (Table S1, ESI†) upon self-condensation of A2B2-Flu
under solvothermal conditions in different simplex organic
solvents. The chemical structures of 3D-Flu-COFs were
Fig. 2 (a) Nitrogen adsorption (solid) and desorption (open) isotherms at
77 K, and (b) pore size distribution of 3D-Flu-COFs prepared in benzyl
alcohol (green), n-BuOH (pink), dioxane (orange), and mesitylene (violet).
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functional theory (NLDFT) (Fig. 2b), which was in good agree-
ment with the theoretical value from the crystal structures
(1.16 nm) (Fig. 3).
Taking 3D-Flu-COFbenzyl alcohol as an example, the thermal
stability as well as chemical stability of 3D-Flu-COFs were
investigated in detail. Thermogravimetric analysis (TGA) suggested
that 3D-Flu-COFbenzyl alcohol exhibited a high thermal stability with
only 5% weight loss of its initial mass at 561 1C under a nitrogen
atmosphere (Fig. S9, ESI†). Moreover, the chemical stability of
3D-Flu-COFbenzyl alcohol was explored upon treatments in boiling
water, 12 M HCl, and 12 M NaOH, respectively, for 3 days, and
these samples maintained their chemical constitution, crystallinity
and porosity upon treatments under the above conditions
(Fig. S10–S14, ESI†). All these experiments confirmed that 3D-Flu-
COFs feature excellent stability.
Fig. 3 Structural representations of a 3D-Flu-COF.
Consistent with the off-white colour appearance, the UV-vis
above results, the 3D-Flu-COFs most probably adopt a 7-fold spectrum of 3D-Flu-COFs displayed a dominant absorption
interpenetrated structure with dia topology, showing a micro- peak centred at 390 nm, which can be attributed to the 3D
porous cavity of about B1.16 nm (Fig. 3).
non-conjugated structure (green line in Fig. 4a and the solid
The porosity and surface areas of 3D-Flu-COFs prepared in line in Fig. S15, ESI†). Inspired by the application of imine-
different solvents were measured by N2 adsorption and linked 2D COFs as acidichromic sensors,11a–c we speculated
desorption measurements at 77 K. All these 3D-Flu-COFs that both the responsivity and sensitivity would be improved if
displayed the typical type I isotherms with a sharp N2 uptake a 3D-Flu-COF was applied to acid sensing due to the highly
under low relative pressures (P/P0 o 0.05), which is the typical accessible porosity of such 3D open frameworks. As expected,
characteristic of microporous materials (Fig. 2a). The BET all 3D-Flu-COFs prepared in different solvents undergo a rapid
surface areas for these 3D-Flu-COFs were determined to be and distinctive colour change from off-white to yellow when
1270–1590 m2 gꢀ1 from the N2 adsorption isotherms under the exposed to trifluoroacetic acid (TFA) vapor, corresponding to
low pressure region (Fig. S6, ESI†). All of the 3D-Flu-COFs the red-shifted absorption band in UV-vis spectra from 390 nm
showed a narrow pore size distribution in the range of to 470 nm (Fig. S15, ESI†). The UV-Vis absorption spectra of
1.15–1.20 nm as simulated on the basis of nonlocal density 3D-Flu-COFs prepared in different solvents differ slightly, probably
Fig. 4 (a) UV/Vis absorption spectra of 3D-Flu-COFbenzyl alcohol powder before and after treatment of TFA vapor (inset: Optical photographs of 3D-Flu-
COFbenzylalcohol powder before (off-white) and after (yellow) treatment of TFA vapor); (b) absorption spectra of 3D-Flu-COFbenzyl alcohol powder upon
treatment with different concentrations of TFA in 1,4-dioxane; (c) plot of the absorption difference between the protonated and the non-protonated
3D-Flu-COFbenzyl alcohol upon treatment with different concentrations of TFA in 1,4-dioxane; and (d) optical photographs of colour change for the
3D-Flu-COFbenzyl alcohol powders upon treatment with different concentrations of TFA in 1,4-dioxane.
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W. Wang, Chem. Soc. Rev., 2013, 42, 548–568; (d) X. Feng, X. Ding
and D. Jiang, Chem. Soc. Rev., 2012, 41, 6010–6022.
due to their different porosity morphology. For convenience,
3D-Flu-COFbenzyl alcohol was used to investigate the details of the
responsive behaviour unless otherwise stated. Then, the responsive
behaviour of the 3D-Flu-COF powder was further investigated in
detail. As shown in Fig. 4b–d, the 3D-Flu-COF is still sensitive to a
trace amount of TFA with a low concentration of 0.0002 mol Lꢀ1
(22.8 ppm) as the new peak at 470 nm still appeared in the UV-vis
spectra. With the increase of the TFA concentration, the absorption
intensity at 470 nm gradually increased without any saturation
effects until the concentration of TFA reached 5 mol Lꢀ1. Besides,
the colour of the acid-treated 3D-Flu-COF (defined as 3D-Flu-COF-
TFA) could be recovered to the original white upon exposure to
trimethylamine (TEA) vapor (Fig. 4d). Notably, the crystallinity,
porosity as well as morphology of the 3D-Flu-COF after the sensing
performance are still maintained and comparable with those of the
pristine samples, except for a slight decrease in PXRD peak intensity
(Fig. S16–S20, ESI†). As already discussed above, 3D-Flu-COFs
applied as acidichromic sensors toward trifluoroacetic acid not only
feature a detection limit as low as 22.8 ppm, but also exhibit a wide
response range of 4 orders of magnitude, which outperform most
reported COFs (Table S2, ESI†).11b–d Similar to previous reports,11
the colour change of the 3D-Flu-COF can be attributed to
the protonation of the imine linkages under acidic conditions
(Fig. S21, ESI†), which was verified by the FI-IR spectra. With the
increase of TFA concentration, a new band at 1668 cmꢀ1 ascribed to
the CQNH+ stretching vibration gradually appeared, accompanied
by the attenuation of the imine CQN band around 1629 cmꢀ1 in
the FT-IR spectra. Furthermore, the FT-IR spectrum of 3D-Flu-COF-
TFA could be recovered to the original mode upon exposure to the
TEA vapour (Fig. S22, ESI†).
2 (a) C. J. Doonan, D. J. Tranchemontagne, T. G. Glover, J. R. Hunt and
O. M. Yaghi, Nat. Chem., 2010, 2, 235–238; (b) H. Fan, A. Mundstock,
A. Feldhoff, A. Knebel, J. Gu, H. Meng and J. Caro, J. Am. Chem. Soc.,
2018, 140, 10094–10098; (c) H. Furukawa and O. M. Yaghi, J. Am.
Chem. Soc., 2009, 131, 8875–8883; (d) C. Gao, J. Li, S. Yin, G. Lin,
T. Ma, Y. Meng, J. Sun and C. Wang, Angew. Chem., Int. Ed., 2019, 58,
9770–9775.
3 (a) S. B. Alahakoon, C. M. Thompson, G. Occhialini and
R. A. Smaldone, ChemSusChem, 2017, 10, 2116–2129; (b) A. Halder,
M. Ghosh, A. Khayum, M. S. Bera, M. Addicoat, H. S. Sasmal,
S. Karak, S. Kurungot and R. Banerjee, J. Am. Chem. Soc., 2018,
140, 10941–10945; (c) J. Lv, Y.-X. Tan, J. Xie, R. Yang, M. Yu, S. Sun,
M.-D. Li, D. Yuan and Y. Wang, Angew. Chem., Int. Ed., 2018, 57,
12716–12720.
4 (a) S. Lu, Y. Hu, S. Wan, R. McCaffrey, Y. Jin, H. Gu and W. Zhang,
J. Am. Chem. Soc., 2017, 139, 17082–17088; (b) S. Yan, X. Guan, H. Li,
D. Li, M. Xue, Y. Yan, V. Valtchev, S. Qiu and Q. Fang, J. Am. Chem.
Soc., 2019, 141, 2920–2924; (c) X. Yan, H. Liu, Y. Li, W. Chen,
T. Zhang, Z. Zhao, G. Xing and L. Chen, Macromolecules, 2019, 52,
7977–7983.
5 (a) Q. Fang, J. Wang, S. Gu, R. B. Kaspar, Z. Zhuang, J. Zheng,
H. Guo, S. Qiu and Y. Yan, J. Am. Chem. Soc., 2015, 137, 8352–8355;
(b) S. Mitra, H. S. Sasmal, T. Kundu, S. Kandambeth, K. Illath,
´
´
D. Dıaz Dıaz and R. Banerjee, J. Am. Chem. Soc., 2017, 139,
4513–4520; (c) V. S. Vyas, M. Vishwakarma, I. Moudrakovski,
F. Haase, G. Savasci, C. Ochsenfeld, J. P. Spatz and B. V. Lotsch,
Adv. Mater., 2016, 28, 8749–8754; (d) G. Zhang, X. Li, Q. Liao, Y. Liu,
K. Xi, W. Huang and X. Jia, Nat. Commun., 2018, 9, 2785.
6 (a) L. Ascherl, E. W. Evans, M. Hennemann, D. Di Nuzzo,
A. G. Hufnagel, M. Beetz, R. H. Friend, T. Clark, T. Bein and
F. Auras, Nat. Commun., 2018, 9, 3802; (b) P. Wang, F. Zhou,
C. Zhang, S.-Y. Yin, L. Teng, L. Chen, X.-X. Hu, H.-W. Liu, X. Yin
and X.-B. Zhang, Chem. Sci., 2018, 9, 8402–8408; (c) S. Jhulki,
A. M. Evans, X.-L. Hao, M. W. Cooper, C. H. Feriante, J. Leisen,
´
H. Li, D. Lam, M. C. Hersam, S. Barlow, J.-L. Bredas, W. R. Dichtel
and S. R. Marder, J. Am. Chem. Soc., 2020, 142, 783–791; (d) X. Wu,
X. Han, Q. Xu, Y. Liu, C. Yuan, S. Yang, Y. Liu, J. Jiang and Y. Cui,
J. Am. Chem. Soc., 2019, 141, 7081–7089.
In summary, we have successfully constructed a new imine-
linked three-dimensional COF (3D-Flu-COF) via a ‘‘two-in-one’’
molecular design strategy for the first time. This approach
greatly simplified the synthetic procedure of 3D COFs, and a
3D-Flu-COF with high crystallinity could be obtained in various
simplex solvents. Benefiting from the non-conjugated structure
as well as the imine linkages, the 3D-Flu-COF exhibits excellent
acidichromic sensing performance with high sensitivity and
good reversibility. This ‘‘two-in-one’’ strategy would open new
opportunities for the facile construction of novel 3D COFs, and
we envision that more functional 3D COFs will be developed in
the near future.
This work was financially supported by the National Natural
Science Foundation of China (51973153), the National Key
Research and Development Program of China (2017YFA02
07500), and the Natural Science Foundation of Tianjin City
(17JCJQJC44600).
7 (a) X. Ma and T. F. Scott, Commun. Chem., 2018, 1, 98;
(b) L. A. Baldwin, J. W. Crowe, D. A. Pyles and P. L. McGrier,
J. Am. Chem. Soc., 2016, 138, 15134–15137; (c) C. Gao, J. Li, S. Yin,
J. Sun and C. Wang, J. Am. Chem. Soc., 2020, 142,
3718–3723; (d) Y. Wang, Y. Liu, H. Li, X. Guan, M. Xue, Y. Yan,
V. Valtchev, S. Qiu and Q. Fang, J. Am. Chem. Soc., 2020, 142,
3736–3741.
´
8 P. J. Waller, F. Gandara and O. M. Yaghi, Acc. Chem. Res., 2015, 48,
3053–3063.
9 X. Guan, F. Chen, Q. Fang and S. Qiu, Chem. Soc. Rev., 2020, 49,
1357–1384.
10 (a) Y. Li, Q. Chen, T. Xu, Z. Xie, J. Liu, X. Yu, S. Ma, T. Qin and
L. Chen, J. Am. Chem. Soc., 2019, 141, 13822–13828; (b) W. Hao,
D. Chen, Y. Li, Z. Yang, G. Xing, J. Li and L. Chen, Chem. Mater.,
2019, 31, 8100–8105.
11 (a) F.-Z. Cui, J.-J. Xie, S.-Y. Jiang, S.-X. Gan, D.-L. Ma, R.-R. Liang,
G.-F. Jiang and X. Zhao, Chem. Commun., 2019, 55, 4550–4553;
(b) L. Ascherl, E. W. Evans, J. Gorman, S. Orsborne, D. Bessinger,
T. Bein, R. H. Friend and F. Auras, J. Am. Chem. Soc., 2019, 141,
15693–15699; (c) B. Zhang, X. Song, Y. Li, Y. Li, Z. Peng, L. Ye and
L. Chen, Chem. Commun., 2020, 56, 3253–3256; (d) D. Chen,
W. Chen, G. Xing, T. Zhang and L. Chen, Chem. – Eur. J., 2020, 26,
8377–8381; (e) Y.-Z. Xie, G.-G. Shan, Z.-Y. Zhou and Z.-M. Su, Sens.
Actuators, B, 2013, 177, 41–49.
12 X. Guan, Y. Ma, H. Li, Y. Yusran, M. Xue, Q. Fang, Y. Yan, V. Valtchev
and S. Qiu, J. Am. Chem. Soc., 2018, 140, 4494–4498.
13 Z.-J. Li, S.-Y. Ding, H.-D. Xue, W. Cao and W. Wang, Chem. Commun.,
2016, 52, 7217–7220.
Conflicts of interest
There are no conflicts to declare.
´
14 (a) G. Das, T. Skorjanc, S. K. Sharma, F. Gandara, M. Lusi,
D. S. Shankar Rao, S. Vimala, S. Krishna Prasad, J. Raya,
D. S. Han, R. Jagannathan, J.-C. Olsen and A. Trabolsi, J. Am. Chem.
Soc., 2017, 139, 9558–9565; (b) W. Ma, Q. Zheng, Y. He, G. Li,
W. Guo, Z. Lin and L. Zhang, J. Am. Chem. Soc., 2019, 141,
18271–18277.
Notes and references
ˆ ´
1 (a) A. P. Cote, A. I. Benin, N. W. Ockwig, M. Keeffe, A. J. Matzger
and O. M. Yaghi, Science, 2005, 310, 1166–1170; (b) C. S. Diercks and
O. M. Yaghi, Science, 2017, 355, eaal1585; (c) S.-Y. Ding and
This journal is The Royal Society of Chemistry 2021
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