Ultrastable Covalent Organic Frameworks via Self-Polycondensation of an A2B2 Monomer for Heterogeneous Photocatalysis

Article abstract of DOI:10.1021/acs.macromol.9b01600

Easy preparation, high stability, prominent activity, and excellent recyclability are four key elements for high-performance heterogeneous photocatalysts. Developing covalent organic framework (COF)-based heterogeneous photocatalysts possessing all of these traits is highly challenging. In this study, we successfully synthesized an imine-linked BBO-COF by the "two-in-one" strategy featuring the above four merits. Highly crystalline and porous BBO-COF can be easily prepared in at least 11 different simplex solvents independent of their polarity and boiling points and even under an air atmosphere. Moreover, BBO-COF exhibits extraordinary chemical stability and photostability in strong acid (12 M HCl), corrosive base (12 M NaOH), and visible light for 7 days. Furthermore, BBO-COF exhibited prominent photocatalytic activity toward oxidative hydroxylation reaction of arylboronic acids with excellent substrates tolerance and reusability. This "two-in-one" design strategy open a new avenue for facile constructing novel functional COFs with tailor-made properties.

Full text of DOI:10.1021/acs.macromol.9b01600

Article
pubs.acs.org/Macromolecules
Cite This: Macromolecules XXXX, XXX, XXX−XXX
Ultrastable Covalent Organic Frameworks via Self-Polycondensation
of an A B Monomer for Heterogeneous Photocatalysis
2
2
Xiaoli Yan, Huan Liu, Yusen Li, Weiben Chen, Ting Zhang, Ziqiang Zhao, Guolong Xing,
and Long Chen*
Department of Chemistry and Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin 300072,
China
*
S Supporting Information
ABSTRACT: Easy preparation, high stability, prominent
activity, and excellent recyclability are four key elements for
high-performance heterogeneous photocatalysts. Developing
covalent organic framework (COF)-based heterogeneous
photocatalysts possessing all of these traits is highly
challenging. In this study, we successfully synthesized an
imine-linked BBO-COF by the “two-in-one” strategy featuring
the above four merits. Highly crystalline and porous BBO-COF
can be easily prepared in at least 11 different simplex solvents
independent of their polarity and boiling points and even under
an air atmosphere. Moreover, BBO-COF exhibits extraordinary chemical stability and photostability in strong acid (12 M HCl),
corrosive base (12 M NaOH), and visible light for 7 days. Furthermore, BBO-COF exhibited prominent photocatalytic activity
toward oxidative hydroxylation reaction of arylboronic acids with excellent substrates tolerance and reusability. This “two-in-
one” design strategy open a new avenue for facile constructing novel functional COFs with tailor-made properties.
INTRODUCTION
neously for photocatalysis. Recently, our group developed a
new approach based on self-condensation of bifunctional
monomers (i.e., “two-in-one” strategy) for COFs synthesis.
The stoichiometry of the functional groups for condensation
was kept identically equal in either homogeneous or
heterogeneous conditions over various solvent systems
guaranteed by this strategy. Therefore, the synthesis of COFs
was simplified with better solvent adaptability and improved
crystallinity as well as reproducibility.
■
Covalent organic frameworks (COFs) are a novel class of
porous and crystalline polymers connected by strong covalent
53
1
−6
bonds.
Because of their large porosity, predesignable
structure, tunable chemical composition, and tailor-made
properties, COFs are explored as functional materials in
various fields, such as gas sorption and separation,
7
−11
1
2−19
20−24
heterogeneous catalysis,
energy storage,
sens-
and so on.
2
5−31
32−35
36−42
ing,
drug delivery,
optoelectronics,
Herein, we designed and synthesized a new bifunctional
monomer 4,4′-(2,6-bis(4-aminophenyl)benzo[1,2-d:4,5-d′]bis-
Among them, photocatalytic COFs have recently attracted
increasing attention, since their extended π-conjugated frame-
works are beneficial for absorbing light, the large porosity and
open channels are conducive to exposing more active sites, and
(
oxazole)-4,8-diyl)dibenzaldehyde (Figure 1a, BBO) with two
formyl and two amino groups in a single benzoxazole core
which was further applied to construct benzoxazole-based
imine COF (BBO-COF) through intermolecular self-con-
densation. Unlike most reported COFs prepared by the co-
condensation method which are synthesized upon tedious
solvent combinations screening process to achieve high
43−46
the insoluble characteristics facilitate catalysts recycling.
However, most COFs are photounstable upon light irradiation
during the photocatalysis, which limits their practical
applications. Thus, great efforts have been devoted to improve
the stability, especially the photostability of COFs. Up to now,
several strategies including utilizing the reversible/irreversible
cascade reactions have been applied to prepare both chemically
stable and photostable COFs.
majority of COFs are synthesized via the co-condensation of
two or more monomers,
54
crystallinity, the bifunctional monomer (BBO) can be used
to prepare BBO-COF with good crystallinity in more than 11
different simplex organic solvent systems (e.g., n-butanol (n-
BuOH), ethanol, methanol, mesitylene, tetrahydrofuran
(THF), acetone, N,N-dimethylacetamide (DMAC), dichloro-
methane (DCM), benzyl alcohol, dioxane, o-dichlorobenzene
46,52
However, because the
4
7−50
they suffer from the time-
consuming screening process of specific synthetic conditions
including solvent combinations, reaction temperature, vac-
uum/inert atmosphere, and so on to achieve high crystal-
(
o-DCB), etc.) and even under air. Moreover, BBO-COF
5
1,52
linity.
Thus, it is of great significance to explore a new
Received: July 31, 2019
strategy for easy preparation of COFs with high stability,
excellent catalytic performance, and easy recyclability simulta-
Revised: September 30, 2019
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.macromol.9b01600
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Article
RESULTS AND DISCUSSION
■
Synthesis and Structural Characterization. As shown
in Scheme S1, BBO was synthesized in high yield and
unambiguously characterized by NMR and MALDI-TOF MS
(
Figures S2−S4). In a typical synthetic procedure of BBO-
COFs (Scheme S2), BBO (28 mg, 0.05 mmol) was suspended
in a simplex organic solvent (1 mL) and 6 M acetic acid (0.1
mL) followed by refluxing for 72 h to afford brownish-yellow
crystalline solids (denoted as BBO-COFsolvent) in high yields of
9
0−95%. The as-synthesized BBO-COFs were characterized
by various techniques (Supporting Information). As shown in
Figure S6, the FT-IR spectra of BBO-COFs prepared in
different solvents were nearly identical. For example, the
−
1
stretching bands of free amine (N−H, ∼3300 cm ) and
−
1
formyl groups (CO, 1684 cm ) disappeared, and a new
−
1
CN stretching vibration band appeared at 1549 cm ,
confirming the new generation of imine bonds in BBO−
55
13
COFs. Solid-state C cross-polarization/magic angle spin-
ning nuclear magnetic resonance (CP/MAS NMR) spectros-
copy further revealed the existence of imine linkages, which
exhibited an intensive characteristic peak at 153 ppm (Figure
55
S5).
Figure 1. (a) Schematic illustration of the molecular design and
synthesis of BBO-COF. (b) PXRD patterns of BBO-COFs prepared
under different conditions.
The crystalline structures of BBO-COFs prepared under
different conditions were confirmed by powder X-ray
diffraction (PXRD) analysis. As presented in Figure 1b, the
powder samples obtained in all these 11 solvents exhibited
same patterns, indicating identical structures, facile prepara-
tion, and good reproducibility of BBO-COFs prepared in
different solvents. For convenience, BBO-COFBuOH prepared
in n-BuOH was selected as the representative for further
detailed analysis. The computational structural simulations of
BBO-COFBuOH and Pawley refinement of PXRD were
conducted by Materials Studio software. Both eclipsed stacking
exhibits extraordinary stability in strong acid (12 M HCl),
corrosive base (12 M NaOH), and visible light irradiation for
at least 7 days. Furthermore, BBO-COF was verified to exhibit
excellent photocatalytic performances on aerobic oxidative
hydroxylation of arylboronic acids with retained crystallinity
after 10 runs of recycling catalysis.
(
AA) and staggered stacking (AB) models were considered
(Figure 2). BBO-COFBuOH exhibited an intense peak at 5.35°
Figure 2. (a) Experimentally observed PXRD pattern of BBO-COF prepared in n-BuOH (black), Pawley-refined pattern (magenta), difference
between the experimental and calculated data (blue), and calculated patterns for AA (red) and AB (green) stacking. (b) Top and (c) side views of
the space-filling AA eclipsed stacking models of BBO-COF and the unit cells. (d) Top and (e) side views of the space-filling AB eclipsed stacking
models of BBO-COF and the unit cells.
B
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and three distinct diffractions at 10.75°, 16.20°, and 25.35°,
which correspond to the 110, 220, 330, and 001 facets,
respectively. The simulated PXRD patterns of the eclipsed AA
stacking were in better agreement with the experimental ones
compared to those of the staggered AB stacking (Figure 2a).
Furthermore, the space group of BBO-COFBuOH was
determined to be P4, and the Pawley refinement afforded
cell parameters of a = b = 23.82 Å, c = 3.49 Å, and α = β = γ =
9
0° with Rwp = 4.33% and R = 3.08%. Scanning electron
p
microscopy (SEM) images disclosed that the BBO-COFBuOH
exhibited uniform “coral cluster”-like morphology (Figure
S9a,b). Transmission electron microscopy (TEM) images
revealed clear lattice stripes corresponding to the ordered
nanochannel, which further confirmed the high crystallinity of
the BBO-COFBuOH (Figure S9c,d).
Nitrogen sorption experiments at 77 K were performed to
evaluate and compare the porosities of BBO-COFs prepared
under different conditions (e.g., n-BuOH, THF, THF under air
atmosphere, o-DCB). All these BBO-COFs displayed type I
reversible isotherms, which are characteristics of microporous
structures (Figure S7a). Surprisingly, the BET surface areas for
BBO-COFs prepared using different conditions were nearly
Figure 3. (a) PXRD profiles of BBO-COFs before (black) and after
treatments in visible light (blue), 12 M HCl (olive), TFA (magenta),
1
2 M NaOH (navy), and boiling water (red) for 7 days. (b) N2
sorption isotherms of BBO-COFs before (black) and after treatments
in visible light (blue), 12 M HCl (olive), TFA (magenta), 12 M
NaOH (navy), and boiling water (red) for 7 days.
2
−1
the same (ca. 1070−1170 m g , Figure S8). The pore size
distribution calculated by the nonlocal density functional
(
NLDFT) method of these BBO-COFs suggested a narrow
pore size distribution at 1.58 nm (Figure S7b), which is in
good agreement with the value of d-spacing observed in PXRD
analysis at 2θ = 5.35° (1.68 nm).
Thermal Stability and Chemical Stability. To our
surprise, BBO-COF exhibited extraordinarily high stability.
Thermogravimetric analysis (TGA) suggested that BBO-COF
retained 95% of its initial mass at 560 °C (Figure S11), and the
long-range ordered framework structure could be maintained
at least 400 °C (Figure S12). In addition, the BBO-COF
showed excellent chemical stability. For example, the
crystallinity and porosity of BBO-COF still remained after
treatments of boiling water, TFA, 12 M HCl, and 12 M NaOH
for 7 days. More interestingly, unlike most reported imine-
linked COFs which are decomposed upon long-term light
4
6
Figure 4. Photocatalytic oxidative hydroxylation. (a) Photocatalytic
reaction for oxidative hydroxylation of 4-formylphenylboronic acid.
irradiation, BBO-COF displayed a good tolerance to visible
light irradiation. The porosity as well as the crystallinity was
retained after irradiating with an 18 W white-light-emitting
diodes (LED) for 7 days (Figures 3, Figures S13 and S14).
The high chemical stability and photostability render BBO-
COF as a good candidate for heterogeneous photocatalysis.
Heterogeneous Photocatalysis on Oxidative Hydrox-
ylation of Arylboronic Acids. Phenols are well-known and
important intermediates for natural products, polymers, and
(b) Control experiments for oxidative hydroxylation of 4-formyl-
phenylboronic acid under different conditions. (c) Assessment of the
reusability of BBO-COF on the oxidative hydroxylation of 4-
formylphenylboronic acid.
donor under oxygen or air atmosphere. In sharp contrast, no
reaction took place in the absence of BBO-COF, which
indicated that BBO-COF plays a critical role as photocatalyst
in this chemical transformation. Similarly, when the reaction
was conducted under a dark or N2 atmosphere, this
transformation was also hard to realize. In addition, no target
product was formed when BBO monomer was used instead of
BBO-COF, which is basically consistent with the literature
56−58
pharmaceutical medicines.
Photocatalytic oxidative hy-
droxylation of arylboronic acids is an effective way to prepare
46,59−62
the corresponding phenols.
Prior to our work, several
porous polymers containing a benzoxazole core have already
shown photocatalytic activities toward the oxidative hydrox-
46
ylation of aryboronic acids. Thus, the transformation of 4-
formylphenylboronic acid (1a) to 4-formylphenol (2a) was
first chosen to evaluate the photocatalytic performance of
BBO-COF. The photocatalysis factors including the photo-
catalyst, sacrificial agents, light source, and oxygen source that
may influence this transformation were systematically assessed
46
reports. Therefore, it can be concluded that the photo-
catalytic properties of BBO-COF are not only derived from the
benzoxazole moiety but also related to the π-conjugated
crystalline framework. All these control experiments validated
the essential roles of the photocatalyst, visible light irradiation,
and oxygen in this reaction. Furthermore, to give more insights
into the kinetics, the time-dependent conversions were
monitored by NMR analysis. As shown in Figures S20 and
S21 as well as Table S1, the conversions of 4-hydroxy-
(
Figure 4). As shown in Figure 4b, the reaction occurred
smoothly to afford the desired 4-formylphenol (2a) in 99%
yield after 48 h with BBO-COF as the photocatalyst and N,N-
diisopropylethylamine (DIPEA) as the sacrificial electron
C
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benzaldehyde gradually increased with reaction time and
reached a full conversion after nearly 48 h.
process in detail. First, the UV/vis absorption spectra of BBO-
COF and BBO were compared (Figure 5a and Figure S10a).
Given the insolubility of BBO-COF in common organic
solvents, it can be easily separated from the reaction mixture by
centrifugation. Thus, the recyclability of BBO-COF as
photocatalyst was evaluated by the cycling experiments on
oxidative hydroxylation of 4-formylphenylboronic acid. As
shown in Figure 4c, after ten cycles, the conversion efficiency
of 4-formylphenylboronic acid to 4-formylphenol still reach up
to 99% with BBO-COF as the photocatalyst. Notably, the
crystallinity and porosity of BBO-COF after 10 runs of cyclic
reactions are still maintained and comparable with that of the
46
pristine samples only with slight decrease on PXRD peak
intensity and BET surface area (Figures S16−S19).
To verify the adaptability of BBO-COFs as photocatalysts
toward the conversion of arylboronic acids to the correspond-
ing phenols, a series of arylboronic acid derivatives were tested
(Table 1). As shown in Table 1, all the substrates can be
Figure 5. (a) Absorbance spectrum of BBO-COF (inset: photograph
of BBO-COF). (b) Plot of Kubelka−Munk function to determine the
band gap of BBO-COF. (c) Cyclic voltammogram of BBO-COF
electrode in nonaqueous electrolyte. (d) Calculated alignment
between the valence band and the conduction band of BBO-COF,
BBO, the oxidation potential of DIPEA, and the reduction potential of
Table 1. Substrate Tolerance Tests of BBO-COF toward
Oxidative Hydroxylation Reaction of Arylboronic Acids
a
O .
2
Both BBO monomer and BBO-COF showed broad absorption
in the region from UV to visible light. According to the Tauc
b
entry
Ar
time (h)
yield (%)
1
2
3
4
5
6
7
8
9
1-pyrenyl
4-MeOC H
2-naphthyl
C H
96
96
96
96
48
48
48
30
28
28
49
56
71
99
99
99
99
99
99
99
plot analysis, the optical band gaps (E ) of BBO-COF (Figure
g
6
4
5b) and BBO (Figure S10b) were estimated to be 2.24 and
2
.14 eV, respectively. Cyclic voltammetry (CV) was further
6
5
measured to figure out the oxidation potentials of BBO-COF
and BBO. The CV of both BBO-COF and BBO showed an
oxidative peak with an onset at +0.93 V (Figure 5c) and +0.62
V versus the saturated calomel electrode (SCE) (Figure S8c)
4-BrC H
6
4
4-CHOC H
6
4
4-NO C H
4
2
6
4-CNC H
6
4
corresponding to the valence band (VB) potentials (E ) of
VB
4-MeO CC H
4
2
6
0
.93 and 0.62 V, respectively. According to the optical band
1
0
4-HO CC H
2 6 4
gaps (E ), the approximate conduction band (CB) potentials
g
a
Conditions: arylboronic acid (0.2 mmol), BBO-COF (21.2 mg,
.010 mmol), CH CN (1.6 mL), H O (0.4 mL), DIPEA (1.0 mmol),
(E ) can be calculated by the following equation:
CB
0
3
2
b
ECB = EVB − Eg
irradiation with an 18 W white LED. Isolated yields.
(1)
Thus, the ECB values of BBO-COF and BBO were estimated to
be −1.31 and −1.52 V versus SCE, respectively. Because the
smoothly converted to the corresponding phenols with BBO-
COF as photocatalyst. Generally, the reaction rates are faster
for the arylboronic acids with electron-withdrawing substitu-
ents (entries 6−10) than that of arylboronic acids bearing
electron-donating groups (entries 1−3). According to the
reported reaction mechanism, this phenomenon could be
ox
oxidation potential (E ) of DIPEA is +0.90 V and the
red
reduction potential (E ) of O is −0.86 V (versus SCE), the
2
photocatalysts with E higher than +0.90 V and E lower
VB
CB
than −0.86 V are capable to oxidize DIPEA, which leads to the
59
subsequent reduction of molecular O . It is obvious that the
2
•
−
reasonably explained by the fact that the O2 radical anion is
more accessible to the vacant p-orbital of boron atoms in
EVB and ECB of BBO-COF fully meet the above criteria while
the EVB of BBO is less than that of DIPEA (Figure 5d).
Therefore, it is reasonable that the BBO-COF features
prominent photocatalytic performance, while BBO exhibits
no photocatalytic activity even with similar optical band gap. In
addition, the generation of O2• in the aid of BBO-COF was
verified by electron paramagnetic resonance (EPR) spectros-
copy upon adding the superoxide radical scavengers 5,5-
dimethyl-1-pyrroline N-oxide (DMPO) (Figure S15). Based
59
arylboronic acids with electron-deficient substituents.
Photocatalytic Mechanism. To gain more insight into
the photocatalysis process, the essential roles of the
components used in the oxidative hydroxylation reaction of
arylboronic acids were further investigated by a number of
comparative experiments. In the absence of sacrificial agents
−
(
DIPEA), phenylboronic acid is hardly converted to the
59−62
corresponding phenol with BBO-COF as the photocatalyst.
Therefore, the sacrificial agent (DIPEA) is indispensable for
the photocatalytic process. In addition, as discussed above, the
benzoxazole-based (BBO) monomer itself exhibited no
photocatalytic activity (Figure 4b). All these results motivate
us to explore the reaction mechanism of the photocatalytic
on the above experiments as well as the literature reports,
a probable photocatalytic mechanism was proposed (Figure 6).
The excited intermediate BBO−COF* was first generated
upon visible light irradiation, and then an electron was
extracted from the sacrificial electron donor (DIPEA) via a
single-electron transfer (SET) process to afford the radical
D
DOI: 10.1021/acs.macromol.9b01600
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ACKNOWLEDGMENTS
■
This work was financially supported by National Key Research
and Development Program of China (2017YFA0207500),
National Natural Science Foundation of China (51973153),
and the Natural Science Foundation of Tianjin City
(
17JCJQJC44600).
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.9b01600.
■
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AUTHOR INFORMATION
Corresponding Author
E-mail: long.chen@tju.edu.cn (L.C.).
■
*
ORCID
2
(
019, 141, 2920−2924.
Long Chen: 0000-0001-5908-266X
18) Yang, S.; Hu, W.; Zhang, X.; He, P.; Pattengale, B.; Liu, C.;
Notes
Cendejas, M.; Hermans, I.; Zhang, X.; Zhang, J.; Huang, J. 2D
Covalent Organic Frameworks as Intrinsic Photocatalysts for Visible
The authors declare no competing financial interest.
E
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