Benzoxazole-Linked Ultrastable Covalent Organic Frameworks for Photocatalysis

Article abstract of DOI:10.1021/jacs.8b00571

The structural uniqueness of covalent organic frameworks (COFs) has brought these new materials great potential for advanced applications. One of the key aspects yet to be developed is how to improve the robustness of covalently linked reticular frameworks. In order to make the best use of π-conjugated structures, we develop herein a ";killing two birds with one stone" strategy and construct a series of ultrastable benzoxazole-based COFs (denoted as LZU-190, LZU-191, and LZU-192) as metal-free photocatalysts. Benefiting from the formation of benzoxazole rings through reversible/irreversible cascade reactions, the synthesized COFs exhibit permanent stability in the presence of strong acid (9 M HCl), strong base (9 M NaOH), and sunlight. Meanwhile, reticulation of the benzoxazole moiety into the π-conjugated COF frameworks decreases the optical band gap and therefore increases the capability for visible-light absorption. As a result, the excellent photoactivity and unprecedented recyclability of LZU-190 (for at least 20 catalytic runs, each with a product yield of 99%) have been illustrated in the visible-light-driven oxidative hydroxylation of arylboronic acids to phenols. This contribution represents the first report on the photocatalytic application of benzoxazole-based structures, which not only sheds new light on the exploration of robust organophotocatalysts from small molecules to extended frameworks but also offers in-depth understanding of the structure-activity relationship toward practical applications of COF materials.

Full text of DOI:10.1021/jacs.8b00571

Article
Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
pubs.acs.org/JACS
Benzoxazole-Linked Ultrastable Covalent Organic Frameworks for
Photocatalysis
Pi-Feng Wei, Ming-Zhu Qi, Zhi-Peng Wang, San-Yuan Ding, Wei Yu,† Qiang Liu,† Li-Ke Wang,†
†
†
†
†
†
†
,†,‡
Huai-Zhen Wang, Wan-Kai An, and Wei Wang*
†State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou,
Gansu 730000, China
‡
Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300071, China
*
S Supporting Information
ABSTRACT: The structural uniqueness of covalent organic frame-
works (COFs) has brought these new materials great potential for
advanced applications. One of the key aspects yet to be developed is
how to improve the robustness of covalently linked reticular
frameworks. In order to make the best use of π-conjugated
structures, we develop herein a “killing two birds with one stone”
strategy and construct a series of ultrastable benzoxazole-based COFs
(
denoted as LZU-190, LZU-191, and LZU-192) as metal-free
photocatalysts. Benefiting from the formation of benzoxazole rings
through reversible/irreversible cascade reactions, the synthesized
COFs exhibit permanent stability in the presence of strong acid (9 M
HCl), strong base (9 M NaOH), and sunlight. Meanwhile,
reticulation of the benzoxazole moiety into the π-conjugated COF frameworks decreases the optical band gap and therefore
increases the capability for visible-light absorption. As a result, the excellent photoactivity and unprecedented recyclability of
LZU-190 (for at least 20 catalytic runs, each with a product yield of 99%) have been illustrated in the visible-light-driven
oxidative hydroxylation of arylboronic acids to phenols. This contribution represents the first report on the photocatalytic
application of benzoxazole-based structures, which not only sheds new light on the exploration of robust organophotocatalysts
from small molecules to extended frameworks but also offers in-depth understanding of the structure−activity relationship
toward practical applications of COF materials.
INTRODUCTION
work, regular pore structure, and high surface area of COFs are
all advantageous for enhancing their photocatalytic activi-
■
The past decade has witnessed rapid development in the
research field of covalent organic frameworks (COFs).
2d,17−19,21,22
1
ty.
For example, pioneering work by the Lotsch
2d
group has illustrated the superior photoactivity of hydrazone-
based COFs when incorporated with metal cocatalysts. We
Featuring periodicity, porosity, and covalent bonding in
structure, crystalline COFs have shown great potential for
18
19
2
and the Wu group realized the metal-free photocatalysis of
cross-dehydrogenative coupling reactions with hydrazone-based
COFs. In this aspect, the key bottleneck for utilizing COFs in
photocatalysis is still the stability of the reticular frameworks.
Although they are relatively stable, the hydrazone linkages in
photocatalytic COFs could not remain unchanged after four
diverse applications. In this regard, further improvement of
3
−11
their structural robustness turns out to be crucial
for
practical applications. Following the basic principles of reticular
1
2
13
chemistry and dynamic covalent chemistry, the rational
construction of COF structures mostly relies on the reversible
formation of covalent bonds between the rigid building blocks.
The Achilles’ heel of COF structures is therefore the mortise-
18,19
catalytic cycles.
As a result, significant loss in crystallinity
1
4
and severe decrease in photocatalytic activity were exclusively
observed. We therefore targeted the construction of more
robust COFs for practical photocatalysis.
and-tenon joints where dynamic covalent chemistry operates.
Elegant efforts have thus been devoted to strengthening the
covalent linkages via different strategies.
3
,6,7,9
However,
The rational design of COFs for applications requires one to
applications have rarely been conducted to further demonstrate
the extra advantages offered by the strengthened frameworks.
We report herein a “killing two birds with one stone” approach
through which the construction of an ultrastable structure and
functionalization for metal-free photocatalysis have been
simultaneously achieved.
14,23
consider both the stability and the functionality.
These two
criteria are governed, in most cases, by different sites in the
COF structure: the stability comes from the covalent joints,
while the functionality comes from the building blocks. This
1
5
Photocatalysis is one of the most promising applications
Received: January 16, 2018
2
d,16−22
tailored for COFs.
The extended π-conjugated frame-
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.8b00571
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Journal of the American Chemical Society
Article
Scheme 1. (a) Classical Formation of the Benzoxazole Structure, such as 2-Phenylbenzoxazole (PBO-1), from Benzaldehyde
and o-Aminophenol via Cascade Imine Formation/Cyclization/Oxidation; (b) One-Pot Construction of Benzoxazole-Linked
a
COFs (LZU-190, LZU-191, and LZU-192) via the Cascade Reactions Shown in (a)
a
As depicted in the mortise-and-tenon model, the structural robustness has been strengthened through the ubiquitous formation of benzoxazole
cyclic joints. The best solvothermal conditions for synthesizing highly crystalline benzoxazole-linked COFs are to use N-methyl-2-pyrrolidone
(
NMP) and mesitylene (1:1 v/v) as the mixed solvent, benzimidazole (4.5 equiv) as the additive, and 185 °C as the reaction temperature.
3
brings much complexity in each step from the designed
synthesis of building blocks to the concise construction of
reticular frameworks. We envisioned offering a straightforward
solution by making the covalent joints both ultrastable and
functional. The benzoxazole linkage (Scheme 1) was therefore
irreversible sequence could be ideal for the construction of
robust COF structures (Scheme 1b). On the other hand,
benzoxazole-related small molecules have shown advantageous
photophysical properties and can undergo photoinduced
2
6
electron transfer in the solid state. Although, like many π-
conjugated analogues, they have not found photocatalytic
applications because of a narrow absorption region and short-
24
selected for the following reasons. The classical synthesis of
the benzoxazole structure (Scheme 1a) applies cascade
2
5
27
reactions involving reversible imine bond formation followed
by irreversible oxazole ring formation. This reversible/
lived photoexcited state, reticulation into COF frameworks
should make a difference.
B
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Article
We accordingly developed a facile approach to construct a
series of benzoxazole-linked ultrastable COFs via the cascade
reactions shown in Scheme 1. After extensive screening, we
found that the best conditions involved the use of
benzimidazole as the additive and N-methyl-2-pyrrolidone
polarization/magic-angle spinning (CP/MAS) NMR spectra of
LZU-190 (Figure 1a), LZU-191 (Figure S24a), and LZU-192
(
NMP)/mesitylene as the mixed solvent, through which the
7
benzoxazole-linked COFs LZU-190, LZU-191, and LZU-192
were obtained with high crystallinity. As a result of the
irreversible formation of aromatic benzoxazole joints through-
out the frameworks, the synthesized COFs exhibit superior
stability in structure compared with imine-linked COFs, such as
2
c
COF-LZU1. Moreover, LZU-190 showed excellent activity in
28
visible-light-driven aerobic oxidation, which is a new and
benign process in photocatalysis. Control experiments identi-
fied that the photocatalytic activity of LZU-190 indeed
originates from the benzoxazole-linked π-conjugated crystalline
framework. Meanwhile, benefiting from the strengthened
covalent linkages in its structure, LZU-190 showed unprece-
dented recyclability among all the COF catalysts: its activity
and crystallinity were retained well after at least 20 runs. This
contribution may not only boost the designed synthesis of
COFs as robust photocatalysts but also shed light on the
exploration of unique advantages originating from the COF
structures.
Figure 1. (a) 13C CP/MAS and (b) NQS/MAS spectra of LZU-190.
1
3
Asterisks denote spinning sidebands. The assignments of the
C
chemical shifts of LZU-190 are indicated in the chemical structure.
13
The presence of the signal at 163 ppm in the C NQS/MAS NMR
spectrum (b) indicates the successful formation of the benzoxazole
rings.
(
Figure S38a) exhibited signals at 163, 148, and 140 ppm,
which originate from the carbon atoms of the N-heterocyclic
rings (the assignments are shown in Figure 1). The signal at
1
63 ppm is indicative of the formation of benzoxazole rings.
1
3
The C nonquaternary suppression (NQS)/MAS NMR
experiment on LZU-190 (Figure 1b) further confirmed that
the signal at 163 ppm corresponds to the quaternary carbon
RESULTS AND DISCUSSION
Designed Synthesis and Structural Characterization.
The benzoxazole-linked COFs LZU-190, LZU-191, and LZU-
■
30
atoms of the benzoxazole rings. Similar information was also
obtained from 13C NQS/MAS NMR measurements on LZU-
191 (Figure S24b), LZU-192 (Figure S38b), and PBO-1
(Figures S1 and S2). All of these data provided overwhelming
evidence for the successful formation of benzoxazole rings.
The crystalline structures of LZU-190, LZU-191, and LZU-
192 were verified by powder X-ray diffraction (PXRD)
measurements. As shown in Figure 2, the observed PXRD
pattern of LZU-190 exhibited an intense peak at 4.99° and four
other peaks at 8.55°, 9.83°, 12.98°, and 26.03°, which
correspond to the 100, 110, 200, 210, and 001 reflections,
respectively. Similarly, the experimental PXRD pattern of LZU-
191 showed five peaks at 2.97°, 5.24°, 6.01°, 7.83°, and 26.27°,
and the experimental PXRD pattern of LZU-192 showed three
peaks at 3.95°, 5.67°, and 7.93°. Simulation of their PXRD
patterns (Figures S17, S28, and S42) suggested that all of these
COFs should be preferably assigned as the eclipsed structures,
as shown in the Figure 2 inset. Scherrer analysis revealed that
the domain sizes of LZU-190, LZU-191, and LZU-192 are
approximately 43, 43, and 33 nm, respectively.
1
92 were synthesized under solvothermal conditions in sealed
ampules via the condensation of 2,5-diamino-1,4-benzenediol
dihydrochloride (1) with different aldehyde building blocks,
such as 1,3,5-triformylbenzene (2), 2,4,6-tris(4-formylphenyl)-
1
,3,5-triazine (3), and 1,3,6,8-tetrakis(4-formylphenyl)pyrene
(4). The solvothermal conditions were thoroughly screened,
and the detailed results are shown in the Supporting
Information (SI). The best crystallinity was obtained using
NMP mixed with mesitylene (1:1 v/v) as the solvent, an excess
of benzimidazole (4.5 equiv) as the additive, and 185 °C as the
reaction temperature (Figures S6 and S7). These conditions
have never been applied in COF synthesis and are much
different from those for synthesizing benzoxazole-related small
2
4
7,29
molecules or polymeric materials.
The formation of
2
4e−g
benzoxazole rings
under these condition includes three
steps: (1) formation of imine-linked intermediates, (2) ring
closure to form benzoxazoline intermediates, and (3) accept-
orless dehydrogenation of benzoxazoline intermediates. The
determining step is the first one, through which the crystalline
structure can possibly be formed via the reversible formation of
imine linkages. It should be noted that the addition of a base
such as benzimidazole can indeed facilitate the aromatization in
Nitrogen adsorption−desorption experiments at 77 K were
used to assess the porosities of the benzoxazole-linked COFs.
LZU-190 (Figure S9) and LZU-192 (Figure S34) showed type-
I isotherms for microporous structures, while LZU-191 (Figure
S20) displayed a type-IV isotherm for mesoporous structures.
Furthermore, nonlocal density functional theory gave rise to
narrow pore size distributions centered at 1.2, 2.3, and 1.3 nm
for LZU-190 (Figure S10), LZU-191 (Figure S21), and LZU-
192 (Figure S35), respectively, the data of which agree well
with the calculated pore sizes based on the eclipsed stacking
models (Figure 2, top inset). The Brunauer−Emmett−Teller
(BET) surface areas of LZU-190 (Figure S11), LZU-191
(Figure S22), and LZU-192 (Figure S36) were calculated to be
2
4g
the third step. In addition, as evidenced in Figure S7, in
comparison with other bases, benzimidazole can improve the
crystallinity of the COFs to a larger extent. Therefore,
benzimidazole was employed herein not only as the
24g
dehydrogenation agent to form the benzoxazole rings but
2n
also as the templating agent to obtain COF crystals with high
quality.
The key issue in the structural characterization was to verify
the formation of benzoxazole rings in the reticular frameworks.
The FT-IR spectra showed typical N-heterocyclic bands at
−1
2
−1
1
624, 1421, and 1120 cm , respectively (Figures S12, S23, and
1035, 1305, and 706 m g , respectively. The total pore
S37), which are similar to those of the molecular counterpart,
volumes (P/P = 0.99) were estimated as 1.59, 1.13, and 0.45
cm g for LZU-190, LZU-191, and LZU-192, respectively.
0
1
3
3
−1
2-phenylbenzoxazole (PBO-1) (Figure S3). The C cross-
C
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Figure 2. Indexed experimental (black), Pawley-refined (red), and
predicted (blue) PXRD patterns of LZU-190, LZU-191, and LZU-192.
The difference plots are presented in green. Top inset: eclipsed
structures proposed for LZU-190, LZU-191, and LZU-192. C, gray; O,
red; N, blue. H atoms have been omitted for clarity.
Figure 3. (a) PXRD patterns measured after 3 day treatment of LZU-
All of the data acquired from the structural characterization
confirmed that the benzoxazole-linked COFs were successfully
constructed with high crystallinity.
1
90 in boiling water (red), TFA (blue), 9 M HCl (magenta), 9 M
NaOH (purple), and visible light (green). (b) N adsorption (solid
2
symbols) and desorption (open symbols) isotherms measured after 3
day treatment of LZU-190 in boiling water (red), TFA (blue), 9 M
HCl (magenta), 9 M NaOH (purple), and visible light (green). For
the purpose of comparison, the isotherms have been shifted vertically
in the figure; the original data are shown in the SI.
Ultrastability of Benzoxazole-Linked COFs. As a result
of the formation of robust benzoxazole linkages throughout the
frameworks, the synthesized COFs exhibited extraordinary
stability. Thermogravimetric analysis (TGA) indicated that they
are thermally stable up to 400 °C (Figures S13, S25, and S39),
which is superior to most 2D COFs, whose decomposition
temperatures are normally below 350 °C. Most importantly,
these COFs showed excellent chemical stability under harsh
conditions, such as upon 3 day treatment in boiling water, pure
trifluoroacetic acid (TFA), aqueous HCl (9 M), or aqueous
NaOH (9 M). For example, after the treatment, the PXRD
Photocatalytic Application. As a start for the photo-
catalytic test, we assessed the optical properties of the
benzoxazole-linked COFs. Figure 4 shows their UV/vis spectra,
which indicate that all of them can absorb light in the UV and
visible regions. The optical band gaps of LZU-190 (Figure
S66), LZU-191 (Figure S68), and LZU-192 (Figure S70) were
1
3
patterns (Figure 3a and Table S4) and C CP/MAS NMR
spectra (Figure S48) of LZU-190 remained unchanged. The N2
adsorption and desorption isotherms (Figure 3b) displayed
only very small changes (Figures S49, S51, S53, and S55). The
residual weight percentages were 91, 89, 86, and 93% after 3
day treatments in boiling water, TFA, 9 M HCl, and 9 M
NaOH, respectively (Figure S46), which further verified the
chemical stability of LZU-190. The scanning electron
microscopy (SEM) images revealed that the morphology was
also retained after the treatment (Figure S47). On the contrary,
the structure of imine-linked COF-LZU1 decomposed under
these conditions. All of these benzoxazole-linked COFs also
possess excellent photostability: the crystalline structures
remained intact upon continuous exposure (for at least 3
days) to visible light (irradiation with 20 W white light-emitting
diodes (LEDs)). Again, the structure of imine-linked COF-
LZU1 decomposed upon visible-light irradiation (Figure S60).
With the data from the stability tests in hand, we further
explored the photocatalytic performance of these robust COFs.
Figure 4. UV/vis absorption spectra of LZU-190 (black), LZU-191
(red), LZU-192 (blue), the model compound PBO-2 (green), and the
building blocks (1, magenta; 2, navy; 3, violet; 4, purple).
D
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calculated to be 2.02, 2.38, and 2.10 eV, respectively. In
crystalline LZU-190, amorphous LZU-190 (denoted as AMP-
190) and the molecular counterparts (PBO-1 and PBO-2) were
also tested. AMP-190 was less efficient (entry 5, 78% yield),
and the molecular catalysts PBO-1 and PBO-2 showed
negligible activity only (entries 6 and 7). In addition, the
2
6
comparison with benzoxazole-based small molecules, such as
PBO-1 (3.06 eV; Figure S72) and PBO-2 (2.96 eV; Figure
S73), these π-conjugated COFs exhibit a shift of their lowest-
energy absorption to longer wavelength and enhanced intensity
31
2c
of visible-light absorption. These ultrastable benzoxazole-
imine-linked COF-LZU1 decomposed under these reaction
32
linked COFs may therefore act as new candidates for metal-
free photocatalysts in visible-light-driven reactions.
conditions (entry 8 and Figure S88). Another robust COF,
34
COF-TpPa-(OH)2, showed no catalytic activity either (entry
9). In addition, LZU-191 and LZU-192 exhibited excellent
photoactivity in this reaction (entries 10 and 11). All of these
data demonstrated that the excellent photocatalytic activity
originates from the benzoxazole-linked π-conjugated crystalline
framework.
28
We chose visible-light-driven aerobic oxidation to examine
the photocatalytic activity of benzoxazole-linked metal-free
COFs, as photocatalytic aerobic oxidation is a new, versatile,
28
and environmentally benign strategy for oxidation processes.
Through this strategy, molecular oxygen can be readily excited
to produce superoxide radical anion, which is subjected to
further transformations. For example, the oxidative hydrox-
The superiority of crystalline LZU-190 as the photocatalyst
was further demonstrated by recycling experiments. LZU-190
could be easily separated from the reaction solution and serve
as a recyclable photocatalyst in the next run (see the SI for
details). Being free of any special treatment or reactivation
procedure, crystalline LZU-190 presented excellent and
unchanged activity even after 20 runs in the visible-light-driven
oxidative hydroxylation of 4-carboxyphenylboronic acid (Figure
5). The crystallinity, porosity, covalent bonding, and morphol-
33
ylation of arylboronic acids via photocatalysis in homoge-
33a
neous media has recently been elucidated by the Xiao and
3
3b
33a
Scaiano
groups. [Ru(bpy) Cl ]·6H O
and methylene
3
2
2
3
3b
blue were applied therein as small-molecule photosensitizers
to initiate the activation of oxygen. Here we found that LZU-
190 exhibited high stability, excellent activity, broad universal-
ity, and unprecedented recyclability as a heterogeneous metal-
free photocatalyst in the oxidative hydroxylation of arylboronic
acids to phenols. Specifically, this transformation can still be
efficiently catalyzed by LZU-190 in the absence of a sacrificial
agent such as iPr NEt.
2
The transformation of 4-carboxyphenylboronic acid to 4-
hydroxybenzoic acid was initially selected as the model reaction
for the systematic assessment. The factors that influence this
transformation were thoroughly evaluated via control experi-
ments, and the key results are listed in Table 1. The reaction
yield can reach 99% with LZU-190 as the photocatalyst and
iPr NEt as the sacrificial agent under an air or oxygen
2
atmosphere (entry 1). In the absence of either light (entry
2
), photocatalyst (entry 3), or oxygen source (entry 4), this
transformation can hardly occur. For comparison with
a
Table 1. Control Experiments
Figure 5. Assessment of the reusability of LZU-190 in photocatalyzing
the oxidative hydroxylation of 4-carboxyphenylboronic acid to 4-
hydroxybenzoic acid. The reusability tests were carried out under
identical conditions as shown in Table 1, entry 1.
b
entry
visible light
photocatalyst
LZU-190
air
yield (%)
99
ogy of the recycled LZU-190 catalysts are fully comparable to
1
2
3
4
5
6
7
8
9
on
off
on
on
on
on
on
on
on
on
on
+
+
+
−
+
+
+
+
+
+
+
those of pristine LZU-190, as evidenced by the data acquired
c
13
LZU-190
none
N.D.
from PXRD, nitrogen adsorption−desorption isotherm,
C
trace
trace
78
CP/MAS NMR, and SEM measurements (Figures S80−S87).
These data show that the reusability of LZU-190 is
unprecedented among all of the COF catalysts reported to date.
We then evaluated the substrate scope of arylboronic acids to
validate the generality of LZU-190 as a privileged photocatalyst.
As shown in Table 2, in the presence of LZU-190, a series of
arylboronic acids bearing electron-withdrawing substituents
could be effectively converted to the desired phenols in
excellent yields (>98%; entries 1−5). Other substrates with
electron-neutral or electron-rich substituents also experienced
LZU-190
AMP-190
d
e
PBO-1
trace
10
f
PBO-2
g
COF-LZU1
decomp.
trace
99
h
2
COF-TpPa(OH)
i
1
1
0
1
LZU-191
j
LZU-192
99
a
Conditions: 4-Carboxyphenylboronic acid (0.2 mmol), LZU-190
21.2 mg, 0.015 mmol), CH CN (1.6 mL), H O (0.4 mL), iPr NEt
satisfactory conversion with a slightly prolonged reaction time
(
(
35
3
2
2
(
entries 6−8). Furthermore, a heteroarylboronic acid was
b
5.0 equiv), irradiation with 20 W white LEDs, 48 h. Yields of the
c
d
well-tolerated under these conditions and afforded the
corresponding product in 80% isolated yield (entry 9).
Meanwhile, the size-dependence effect was observed with
polycyclic substrates: the reaction yields decreased with
increasing size of the substrate (entries 7, 10, and 11). For
example, the large-sized 1-pyrenylboronic acid (11.5 Å × 9.1 Å)
isolated products. N.D. = not detected. AMP-190 (21.2 mg, 0.015
mmol) is amorphous LZU-190. PBO-1 (19.5 mg, 0.1 mmol) as a
molecular photocatalyst. PBO-2 (31.2 mg, 0.1 mmol) as a molecular
photocatalyst. COF-LZU1 showed decomposition. COF-TpPa-
e
f
g
h
i
(
OH) (21.2 mg, 0.012 mmol). LZU-191 (25.3 mg, 0.01 mmol).
2
j
LZU-192 (25.0 mg, 0.01 mmol).
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Table 2. Photocatalytic Activity Test of LZU-190 in
Oxidative Hydroxylation of Arylboronic Acids to Phenols
Figure 6, the key mechanistic steps should include generation
of excited LZU-190 via irradiation, formation of superoxide
a
b
entry
Ar
t (h)
yield (%)
1
2
3
4
5
6
7
8
9
4-HO CC H
4
48
48
30
48
48
48
72
96
72
96
96
99
99
99
99
98
86
88
58
80
81
55
2
6
4-CHOC H
6
4
4-NO C H
4
2
6
4-MeO CC H
2
6
4
4-CNC H
6
4
4-BrC H
6
4
Figure 6. Proposed mechanism for the photocatalytic transformation
of arylboronic acids to phenols in the presence of LZU-190. SET
stands for single-electron transfer.
C H
6
5
4-MeOC H
6
4
3-quinolinyl
2-naphthyl
1-pyrenyl
1
1
0
1
radical anion (O2•−)36,37 via single electron transfer (SET)
from excited LZU-190 to oxygen, rearrangement, and
hydrolysis. The photoredox cycle is completed by the
regeneration of LZU-190 in the presence of electron donors,
a
Conditions: Arylboronic acid (0.2 mmol), LZU-190 (21.2 mg, 0.015
mmol), CH CN (1.6 mL), H O (0.4 mL), iPr NEt (5.0 equiv),
irradiation with 20 W white LEDs. Yields of the isolated products.
3
2
2
b
such as iPr NEt. In the absence of the sacrificial agent, the
2
terminal amino groups of LZU-190 might work as the electron
cannot be smoothly accommodated within the pores (∼1.2
nm) of LZU-190, resulting in a lower yield even with doubled
reaction time (entry 11). This size-selective phenomenon could
not exist in homogeneous systems, which highlights again
the unique advantage of COF photocatalysts.
Moreover, we found that LZU-190 could still work as an
efficient photocatalyst in the absence of the sacrificial agent
33a
donor,
although the detailed pathway is not yet well-
understood.
Discussion of the Structure−Activity Relationship.
33a
38
Although COF materials possess many structural advantages,
the linkages connecting the building blocks are generally weak1
because of the intrinsic nature of dynamic covalent bonds.
Therefore, one of the bottlenecks for the practical applications
of COFs is how to further improve their structural robustness.
In particular, as new candidates for use as visible-light
sensitizers, the π-conjugated COFs must be stable enough to
survive repeated photoredox cycles. Although the most
developed, imine-linked COFs can act as robust catalysts in
many thermal reactions, they tend to decompose upon visible-
iPr NEt (Table S7). It should be noted that the sacrificial agent
2
is considered necessary in homogeneous systems because it
33a
works as the electron donor to complete the catalytic cycle.
Indeed, molecular photocatalysts, such as methylene blue,
Ru(bpy) Cl , PBO-1, PBO-2, and benzimidazole, showed only
3
2
negligible activity in the absence of iPr NEt (Table S7). On the
2
contrary, without the sacrificial agent, 4-carboxyphenylboronic
acid could still be photocatalyzed by LZU-190 alone, affording
the desired phenol product in 86% yield (Table S7). However,
in a slight difference from the data previously obtained in the
2c
light irradiation. This contribution focuses on the con-
struction of ultrastable COFs as robust photocatalysts.
3−11
Only a handful of ingenious approaches
have been
reported to strengthen the covalent linkages of COF structures.
For example, in 2012 the Banerjee group developed a
tautomerization strategy to further tautomerize the enol−
presence of iPr NEt (Figure 5), the photocatalytic activity
2
LZU-190 was gradually reduced in the recycling experiments
(
Table S9).
3
imine linkages to stable keto−enamine linkages. In 2016, the
In order to understand the photocatalytic role of LZU-190 in
Yaghi group oxidized the imine-linked COFs to afford more
the visible-light-driven aerobic oxidation, we performed spin-
6
stable amide-linked COFs via postmodification. Closely
36
trapping experiments and examined the trapped adduct by
electron spin resonance (ESR) spectroscopy. When 5,5-
dimethyl-1-pyrroline N-oxide (DMPO) was added to the air-
saturated methanol solution containing LZU-190, the adduct of
superoxide radical anion with DMPO was detected after visible-
light irradiation (Figure S98). To confirm the source of oxygen,
relevant to this contribution, the McGrier group very recently
applied a two-step strategy to construct benzobisoxazole-linked
7
COFs. Although the applications of these robust COFs have
not been further explored, these pioneering works did shed new
light on the stability issue.
As shown in Scheme 1, we envisioned further making the
covalent linkages both ultrastable and functional. For the first
time, benzimidazole was applied as the dehydrogenation agent,
which converts the dihydrobenzoxazole intermediates to
ultrastable benzoxazole rings in a one-pot fashion. This is the
crucial step to make COF structures robust enough for
persistent photocatalysis. In addition, using the mixed solvent
of NMP and mesitylene resulted in the best crystallinity for
benzoxazole-linked COFs. The ubiquitous formation of
benzoxazole linkages not only brings structural robustness but
also renders the COF frameworks with superior photocatalytic
activity. Although benzoxazole-based structures have been
widely utilized for optoelectricity and sensing, to the best of
our knowledge they have never been applied in photocatalysis.
1
8
18
we further carried out two O-labeling experiments with O2
and H218O (eqs 1 and 2, respectively). High-resolution mass
spectrometry (HRMS) analysis (Figures S89 and S90) showed
that the oxygen atom in the product originated from the
molecular oxygen instead of water. Accordingly, as shown in
F
DOI: 10.1021/jacs.8b00571
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
This contribution has identified that reticulation of benzoxazole
moieties into the COF framework decreases the optical band
gap and increases the capability for visible-light absorption. The
most important findings from the photocatalytic assessment
that revealed the structure−activity relationship are summarized
in the following: (i) In contrasts to benzoxazole-linked COFs,
Detailed synthetic procedures, general procedure for the
oxidative hydroxylation reaction, 13C CP/MAS NMR
spectra, FT-IR spectra, PXRD patterns, modeling details
and atomic coordinates, gas adsorption data, TGA traces,
SEM images, TEM images, UV/vis spectra, CV data, and
liquid NMR data (PDF)
another robust COF, COF-TpPa-(OH) , showed no activity.
2
This indicates that the photocatalytic activity indeed originates
from the benzoxazole moieties. (ii) In comparison with LZU-
AUTHOR INFORMATION
■
190, the amorphous catalyst AMP-190 was less active, which
Corresponding Author
*wang_wei@lzu.edu.cn
ORCID
San-Yuan Ding: 0000-0003-2160-4092
Wei Yu: 0000-0002-3131-3080
Qiang Liu: 0000-0001-8845-1874
Wei Wang: 0000-0002-9263-7927
highlights the contribution from structural regularity. (iii) The
fact that molecular counterparts PBO-1 and PBO-2 showed
negligible activity verifies the importance of extended π-
conjugated frameworks. (iv) Often-used small-molecule photo-
sensitizers such as methylene blue and Ru(bpy) Cl did not
3
2
work in the absence of the sacrificial agent. This implies that the
COF structure itself may act as the electron donor to complete
the photoredox cycle. (v) The reusability of LZU-190 is
superior among all of the COF catalysts reported to date, which
provides overwhelming evidence for the unique advantage of
the ultrastable structure.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Last but not least, this research highlights the difference in
the design of COF catalysts for thermal and photoinduced
catalysis. The catalyst design for thermal reactions relies on the
established knowledge of molecular catalysts; the key
consideration is how to concisely reticulate them into the
COF framework. In contrast, this reference is absent for the
rational design of COF photocatalysts: the molecular counter-
parts may have no photoactivity at all. For example, the
benzoxazole-based small molecules showed negligible photo-
activity only, while reticulation of benzoxazole moieties into π-
conjugated crystalline frameworks made a significant difference.
Accordingly, this contribution not only realizes for the first time
the photocatalytic application of benzoxazole-based structures
but also provides further insight for the exploration of
organophotocatalysts from small molecules to extended
frameworks.
This work was supported by the National Natural Science
Foundation of China (21425206 and 21632004).
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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/jacs.8b00571.
■
*
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Journal of the American Chemical Society
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