Donor-acceptor 2D covalent organic frameworks for efficient heterogeneous photocatalytic α-oxyamination

Article abstract of DOI:10.1007/s11426-020-9931-4

Covalent organic frameworks (COFs) have received widespread interest due to their high porosity, excellent crystallinity, tailorable structures, and broad application prospects. It has been demonstrated that proper combination and arrangement of electron donor and acceptor units in 2D conjugated COF lattice could promote efficient charge separation and electron transfer, and thus is beneficial for photocatalysis. In this article, three donor-acceptor (D-A) 2D COFs were prepared by Schiff base reaction of electron acceptor 4,4′,4″,4?-(benzo[1,2-d:4,5-d′]bis(oxazole)-2,4,6,8-tetrayl)tetraaniline (BBO) with different electron donors: thieno[3,2-b]thiophene-2,5-dicarbaldehyde (TT), benzo[1,2-b:4,5-b′]dithiophene-2,6-dicarboxaldehyde (BDT) and terephthalaldehyde (Ph), respectively. These D-A 2D COFs exhibited prominent photocatalytic activity towards α-oxyamination of 1,3-dicarbonyl with 2,2,6,6-tetramethyl-1-piperdinyloxy (TEMPO) upon visible light irradiation. Among these D-A_BBO-COFs, D_TT-A_BBO-COF exhibited the highest photocatalytic rates, which can be ascribed to the more negative highest occupied molecular orbital (HOMO) and narrower bandgap. The excellent stability, high activity and superior recyclability render D_TT-A_BBO-COF as a potential and environmentally friendly heterogeneous catalyst for α-oxyamination. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s11426-020-9931-4

SCIENCE CHINA
Chemistry
•ARTICLES•
May 2021 Vol.64 No.5: 827–833
https://doi.org/10.1007/s11426-020-9931-4
Donor-acceptor 2D covalent organic frameworks for efficient
heterogeneous photocatalytic α-oxyamination
Huan Liu1†, Xiaoli Yan1†, Weiben Chen1, Zhen Xie1, Shen Li1, Weihua Chen2, Ting Zhang1,
Guolong Xing1* & Long Chen1*
1Department of Chemistry, Institute of Molecular Plus, Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University,
Tianjin 300072, China;
2College of Chemistry and Green Catalysis Center, Zhengzhou University, Zhengzhou 450001, China
Received November 4, 2020; accepted December 29, 2020; published online March 12, 2021
Covalent organic frameworks (COFs) have received widespread interest due to their high porosity, excellent crystallinity,
tailorable structures, and broad application prospects. It has been demonstrated that proper combination and arrangement of
electron donor and acceptor units in 2D conjugated COF lattice could promote efficient charge separation and electron transfer,
and thus is beneficial for photocatalysis. In this article, three donor-acceptor (D-A) 2D COFs were prepared by Schiff base
reaction of electron acceptor 4,4′,4′′,4′′′-(benzo[1,2-d:4,5-d′]bis(oxazole)-2,4,6,8-tetrayl)tetraaniline (BBO) with different
electron donors: thieno[3,2-b]thiophene-2,5-dicarbaldehyde (TT), benzo[1,2-b:4,5-b’]dithiophene-2,6-dicarboxaldehyde (BDT)
and terephthalaldehyde (Ph), respectively. These D-A 2D COFs exhibited prominent photocatalytic activity towards α-
oxyamination of 1,3-dicarbonyl with 2,2,6,6-tetramethyl-1-piperdinyloxy (TEMPO) upon visible light irradiation. Among these
D-ABBO-COFs, DTT-ABBO-COF exhibited the highest photocatalytic rates, which can be ascribed to the more negative highest
occupied molecular orbital (HOMO) and narrower bandgap. The excellent stability, high activity and superior recyclability
render DTT-ABBO-COF as a potential and environmentally friendly heterogeneous catalyst for α-oxyamination.
covalent organic frameworks, donor-acceptor, benzodioxazole, heterogeneous photocatalysis, α-oxyamination
Citation:
Liu H, Yan X, Chen W, Xie Z, Li S, Chen W, Zhang T, Xing G, Chen L. Donor-acceptor 2D covalent organic frameworks for efficient heterogeneous
photocatalytic α-oxyamination. Sci China Chem, 2021, 64: 827–833, https://doi.org/10.1007/s11426-020-9931-4
1 Introduction
low recovery rate [5,6]. With the continuous global en-
vironmental deterioration, the appeal of sustainable devel-
opment has led to an urgent pursuit for green chemistry in the
past few decades. Compared with homogeneous photo-
catalysts, heterogeneous photocatalysts are able to fulfill the
requirements of green chemistry better due to their ease of
separation and recovery [7,8]. Therefore, the development of
cheap, non-toxic, heavy metal-free, recyclable, and en-
vironmentally friendly heterogeneous catalysts for α-
oxidative amination is highly desired.
α-Oxyamination of carbonyl compounds is an important
chemical transformation in organic synthesis, and the cor-
responding products (i.e., alkoxyamines) can not only be
used to prepare quinoxalines [1] with anti-cancer [2] and
anti-inflammatory [3] effects, but also served as synthetic
intermediates for heterocyclic tertiary alcohols [4]. Many
inorganic photocatalysts have been developed for α-
oxidative amination, unfortunately, they often suffer from
disadvantages such as high toxicity, poor cycle stability and
Covalent organic frameworks (COFs) are a new class of
organic porous crystalline polymers linked by strong cova-
lent bonds [9–12]. Due to numerous advantages over other
†These authors contributed equally to this work.
*Corresponding authors (email: xinggl@tju.edu.cn; long.chen@tju.edu.cn)
© Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021
chem.scichina.com link.springer.com

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Liu et al. Sci China Chem May (2021) Vol.64 No.5
traditional porous materials, such as robustness, easiness to
modification and functionalization, etc., COFs have showed
excellent application potentials in adsorption and separation
[13–15], heterogeneous catalysis [16–24], organic electro-
nics [25–27], energy storage [28–31], sensing [32–34], and
so on. 2D COFs made from planar building blocks tend to
form layered eclipsed structures with 1D open channels,
which are ideal for charge and exciton transport [35–37]. In
principle, when complementary donor-acceptor (D-A) pairs
were integrated into 2D COF skeletons, the intralayer charge
transfer effects between donor and acceptor units can sig-
nificantly promote the charge separation and electron trans-
fer, thereby improving the photocatalytic efficiency [38].
These characteristics and merits make COFs as a promising
platform for efficient and recyclable photocatalysts. For
example, representative D-A type COFs can not only en-
hance the performance in photoinitiators of radical poly-
merization [39], but also improve the efficiency of
photocatalytic reduction of aqueous chromium (VI) [40] and
optoelectronic applications [41].
It is well established that the benzodioxazole group fea-
tures high photocatalytic activity, which have recently been
incorporated porous polymers for heterogeneous photo-
catalysis [42–45]. Considering the excellent performance of
linear D-A conjugated copolymers in charge generation and
migration [46], it is speculated that if benzodioxazole units
are used as electron-deficient modules to construct 2D D-A
type COFs, intriguing photocatalytic properties could be
expected. Herein, we incorporated electron-donating thie-
nothiophene (TT) unit and electron-deficient benzodioxazole
(BBO) unit into crystalline 2D COF skeletons to construct a
novel D-A polymer network (defined as DTT-ABBO-COF).
Similarly, different electron-donating benzodithiophene
(BDT) and benzene (Ph) units were also integrated into such
D-A type 2D COFs (defined as DBDT-ABBO-COF and DPh-
ABBO-COF, respectively). These D-ABBO-COFs not only
exhibit high crystallinity and large porosity, but also main-
tain good chemical stability after treatments in harsh con-
ditions like 9 M NaOH, 9 M HCl and boiling water for 3
days. Upon visible light irradiation, DTT-ABBO-COF can ef-
fectively catalyze the α-oxidative amination reaction of 1,3-
dicarbonyl with 2,2,6,6-tetramethyl-1-piperdinyloxy (TEM-
PO), and still maintain excellent catalytic activity after
multiple cycles. Notably, there are only limited examples of
organic catalysts reported for α-oxidation amination reaction
[47–51].
(BBO, 8.39 mg, 0.016 mmol) and thieno[3,2-b]thiophene-
2,5-dicarbaldehyde (TT, 6.35 mg, 0.032 mmol), and sus-
pended in 1 mL mesitylene-benzyl alcohol (v/v=1:1). After
the mixture was sonicated for 1 min, 0.2 mL of acetic acid
(6 M) were added. The mixture was sonicated for another
1 min and further degassed by three freeze-pump-thaw cy-
cles, purged with N2 and then heated at 120 °C for 3 days. A
red precipitate was formed, which was collected by sucking
filtration
and
sequentially
washed
with
N,N-
dimethylformamide, anhydrous ethanol, and tetrahydrofuran
(THF). The collected sample was dried under vacuum for
24 h to give a red powder DTT-ABBO-COF.
2.2 Synthesis of DBDT-ABBO-COF and DPh-ABBO-COF
The syntheses of DBDT-ABBO-COF and DPh-ABBO-COF were
carried out using the same procedures as that of DTT-ABBO
-
COF except TT was replaced by BDT (7.88 mg,
0.032 mmol) and Ph (4.29 mg, 0.032 mmol), respectively.
With the increase of the electron donating strength, DPh-
ABBO-COF, DBDT-ABBO-COF, and DTT-ABBO-COF were ob-
tained as orange, brown, and red crystalline powder, re-
spectively (Figure 1).
3 Results and discussion
3.1 Structural characterization
The formation of D-ABBO-COFs were ambiguously char-
acterized by various techniques (Supporting Information
online). As for the FTIR spectra, both the significant at-
tenuation of the stretching bands of amino (N–H,
~3,460–3,200 cm−1) and formyl (C=O, 1,650–1,685 cm−1)
groups and new appearance of a strong vibration at
1,582 cm−1 or 1,572 cm−1 corresponding to the stretching
mode of the C=N bonds [52] (Figure S4, Supporting In-
formation online), collectively indicate the formation of
imine linkages in the D-ABBO-COFs. On the other hand, 13
solid-state cross-polarization magic angle spinning nuclear
magnetic resonance (CP/MAS-NMR) spectra of D-ABBO
C
-
COFs further confirmed the presence of imine linkages,
where the intensive peaks around at 148–149 ppm can be
assigned to the carbon signals of C=N bonds (Figure S5)
[52]. In contrast, the sharp peak ascribed to aromatic C=N
bonds in benzodioxazole units appeared at 160 ppm [36].
Scanning electron microscopy (SEM) images showed that
DTT-ABBO-COF (Figure S12(a, b)), DBDT-ABBO-COF (Figure
S13(a, b)) and DPh-ABBO-COF (Figure S14(a, b)) exhibited
fibrillary morphology. The transmission electron microscopy
(TEM) images showed clear lattice stripes corresponding to
the ordered nanochannels, confirming the high crystallinity
of DTT-ABBO-COF (Figure S12(c, d)), DBDT-ABBO-COF
(Figure S13(c, d)) and DPh-ABBO-COF (Figure S14(c, d)).
2 Experimental
2.1 Synthesis of DTT-ABBO-COF
A 10 mL Pyrex tube was charged with 4,4′,4′′,4′′′-(benzo
[1,2-d:4,5-d′]bis(oxazole)-2,4,6,8-tetrayl)-tetraaniline

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Liu et al. Sci China Chem May (2021) Vol.64 No.5
Figure 1 Co-condensation of BBO with the dialdehydes Ph (orange), TT (olive) and BDT (dark yellow) leads to the formation of the respective imine-
linked 2D COFs (color online).
3.2 Crystalline structure
3.3 Porosity of D-ABBO-COFs
The crystallinities of D-ABBO-COFs were further confirmed
by powder X-ray diffraction (PXRD) measurements. As
shown in Figure 2(d), the PXRD pattern of DTT-ABBO-COF
showcased an intense peak at 3.57°, and four minor peaks at
about 5.12°, 7.19°, 10.81° and 25.2°, which can be reason-
ably assigned to the (110), (200), (220), (330) and (001)
The inherent porosities of D-ABBO-COFs were evaluated by
N2 adsorption-desorption measurements at 77 K. The
Brunauer-Emmett-Teller (BET) surface areas of DTT-ABBO
-
COF, DBDT-ABBO-COF, DPh-ABBO-COF were determined as
1,350, 1,770, and 800 m2 g−1, respectively. As presented in
Figure 3(a), all the desorption isotherms of DTT-ABBO-COF,
DBDT-ABBO-COF, and DPh-ABBO-COF exhibited character-
istic type IV profile, indicating the existence of mesopores.
The pore size of the DTT-ABBO-COF, DBDT-ABBO-COF, DPh-
facets, respectively. Similar results were observed for DBDT
-
ABBO-COF (Figure 2(g)), where a series of diffraction peaks
appeared at 3.29°, 4.60°, 6.58°, 10.01° and 25.6°, corre-
sponding to the (110), (200), (220), (330) and (001) planes.
In addition, DPh-ABBO-COF (Figure 2(a)) exhibited four
prominent diffraction peaks at 3.84°, 7.61°, 11.57° and
26.01°, corresponding to the (110), (220), (330) and (001)
reflections.
ABBO-COF calculated by the nonlocal density functional
(NLDFT) theory was 2.26, 2.37 and 2.03 nm, respectively
(Figure 3(b)), which was in good agreement of the d-spacing
value obtained from the PXRD data.
The unit cell parameters were determined and optimized
by lattice simulation and Pawley refinement using Materials
Studio software. The simulation results of the eclipsed AA-
stacking could be in better agreement with the experimental
results compared to that of the staggered AB stacking modes
(Figure S17). In addition, the space groups of D-ABBO-COFs
were determined to be P4. The Pawley refinement afforded
the cell parameters of a=b=34.38 Å, c=3.62 Å, α=β=γ=90°
for DTT-ABBO-COF with Rwp=6.31% and Rp=5.29%. The
lattice parameters of DBDT-ABBO-COF were a=b=38.27 Å,
c=3.58 Å, α=β=γ=90°, with Rwp=4.50% and Rp=3.44%.
While the lattice parameters were optimized as a=b=32.68
Å, c=3.64 Å, α=β=γ=90°, with Rwp=3.45% and Rp=2.62% for
DPh-ABBO-COF.
3.4 Thermal stability and chemical stability
The chemical stability of all D-ABBO-COFs were tested upon
treatments in harsh conditions like boiling water, 9 M NaOH,
and 9 M HCl for 3 days (Scheme S6, Supporting Information
online). As revealed in the FTIR spectra (Figure S9), the
characteristic peak of C=N stretching bands were well
maintained. Furthermore, to our surprise, the crystallinity
was also well retained in these harsh conditions (Figure S8).
These results collectively indicated that D-ABBO-COFs ex-
hibited excellent chemical stability. Thermogravimetric
analysis (TGA) revealed that D-ABBO-COFs maintained 95%
of the initial mass even at 500 °C (Figure S6), indicating that
all D-ABBO-COFs possess excellent thermal stability. To

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Liu et al. Sci China Chem May (2021) Vol.64 No.5
ABBO-COFs-400 °C and D-ABBO-COFs-500 °C), respec-
tively. Although the crystallinity of D-ABBO-COFs-500 °C
disappeared, the PXRD of D-ABBO-COFs-400 °C were still
maintained, further confirming the excellent structural
maintenance of D-ABBO-COFs (Figure S7).
3.5 Heterogeneous photocatalysis on α-oxyamination of
1,3-dicarbonyls with TEMPO
The excellent crystallinity, intense light absorption ability, as
well as the good chemical and thermal stability of D-ABBO
-
COFs encouraged us to investigate their capability as het-
erogeneous photocatalysts. α-Oxyamination of 1,3-dicarbo-
nyls with TEMPO was chosen since it affords alkoxyamines
that are important intermediates for quinoxaline drugs and
heterocycle tertiary alcohols. However, only a few hetero-
geneous organic photocatalysts for this transformation have
been developed yet [47–50]. As shown in Figure 4(a), in our
initial exploration, a conversion efficiency of 92% for the
reaction of ethyl 3-oxo-3-phenylpropanoate (1a) with
TEMPO (2) was achieved when using DTT-ABBO-COF as the
photocatalyst under the visible light irradiation of a LED
lamp (18 W) for 24 h.
To systematically figure out the factors that affect this
photocatalytic process, a series of control experiments were
performed and monitored by NMR analysis. The reaction
could not occurred in the absence of DTT-ABBO-COF, which
verified the pivotal role of DTT-ABBO-COF in this transfor-
mation. Similarly, visible light irradiation is also essential
since product 3a could not be detected when the reaction was
performed under darkness. Furthermore, using the small
molecule of benzodioxazole (BBO) as the catalyst was un-
sucessful, which again supports that the photocatalytic ac-
tivity of DTT-ABBO-COF is not only related to benzodi-
oxazole, but also interrelated with the π-conjugated crystal-
line framework (Figure 4(b)). These control experiments
conjunctively confirmed that both DTT-ABBO-COF and visi-
ble light irradiation are essential for this transformation. We
also synthesized a linear polymer composed of thienothio-
phene and benzodioxazole (defined as D-A-linear polymer,
Scheme S5) for comparison. We found that the yield of the α-
oxyammonation reaction catalyzed by D-A-linear polymer is
40%, which is much lower than the yield of DTT-ABBO-COF
as a catalyst (92%). As shown in Figure S11(b), The band
gap of DTT-ABBO-COF (1.96 eV) was smaller than D-A-
linear polymer (2.04 eV). Compared to D-A-linear polymer,
DTT-ABBO-COF exhibited more negative highest occupied
molecular orbital (HOMO) (Figure S11(c, d)). It is obvious
that expanding the dimensions of the conjugated polymers,
from one- to two-dimensional (1D and 2D), is conducive to
the photocatalytic redox reaction processes [53]. The control
experiments again validated the importance of the π-
conjugated crystalline framework. Benefiting from the good
Figure 2 Experimental PXRD profiles of (a) DPh-ABBO-COF (orange), (d)
DTT-ABBO-COF (olive), and (g) DBDT-ABBO-COF (dark yellow), and their
corresponding Pawley refined (red) and difference (light purple), calculated
profiles by the AA (magenta) and AB (violet) stacking modes. Side and top
view of the ideal eclipsed (AA) structure of DPh-ABBO-COF (b, c), DTT
-
ABBO-COF (e, f), and DBDT-ABBO-COF (h, i) (color online).
Figure 3 (a) Nitrogen adsorption (solid) and desorption (open) isotherms
at 77 K and (b) pore size distribution for DPh-ABBO-COF (orange), DTT
-
ABBO-COF (olive) and DBDT-ABBO-COF (dark yellow) (color online).
further verify their thermal stability, the D-ABBO-COFs were
kept at 400 and 500 °C for 1 h under nitrogen (defined as D-

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Liu et al. Sci China Chem May (2021) Vol.64 No.5
Table 1 Substrate scope of visible light irradiated α-oxyamination reac-
tions
Entry
R1
Ph
R2
OEt
OEt
OEt
OEt
OEt
Ph
Yield (%)a)
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
3a
3b
3c
3d
3e
3f
92
85
83
99
98
95
98
98
97
4-MeOPh
3-MePh
4-ClPh
4-FPh
Ph
1g
1h
1i
H
H
3g
3h
3i
2-Thienyl
H
OEt
OEt
a) Isolated yield. Condition: ethyl benzoylacetate 1a–1i (0.05 mmol), 2
TEMPO (15.6 mg, 0.1 mmol), COFs (10 mg), CH3CN (1 mL), irradiation
with an 18 W LED.
Figure 4 (a) The reaction of photocatalytic α-oxyamination; (b) control
experiments for α-oxyamination of β-ketoeseter under different conditions;
(c) assessment of the reusability of COFs on the α-oxyamination of β-
ketoeseter; (d) D-ABBO-COFs catalytic performance comparison (color
online).
3.6 Photocatalytic mechanism
To provide more insight into the reaction mechanism and the
origin of the best catalytic efficiency for DTT-ABBO-COF, a
number of comparative experiments and measurements were
conducted. As displayed in Figure 5(a), the UV/Vis ab-
sorption spectra revealed that the absorption D-ABBO-COFs
almost covered the entire visible light region, which were
conducive for photocatalysis. The optical band gaps of D-
ABBO-COFs were calculated by Kubelka-Munk-transformed
chemical stability and insolubility of these D-ABBO-COF
based catalyst, the recylcling catalysis tests were further
performed. After 5 cycles, DTT-ABBO-COF can still maintain
good crystallinity (Figure S15) and the reaction conversion is
as high as 90% (Figure 4(c)). While the catalytic activity of
DTT-ABBO-COF is obviously higher than or comparable to
that of many other porous materials (Table S1, Supporting
Information online).To evaluate the universality of DTT-
ABBO-COF for catalyzing α-oxidative amination of 1,3-
dicarbonyls and TEMPO, a series of 1,3-dicarbonyls with
different substituents were tested as substrates. As shown in
Table 1, all 1,3-dicarbonyl compounds can be converted to
the corresponding alkylamine products with good isolated
yields. Notably, the substrates with strong electron-donating
group on the aromatic ring led to a lower yield (Table 1,
entries 2–3), while the strong electron deficient groups re-
sulted in a higher yield (Table 1, entries 4–5) within the same
reaction time. Besides, the photocatalytic activities of D-
ABBO-COFs were systematically compared over several
substrates. As shown in Figure 4(d), the catalytic efficiency
of DTT-ABBO-COF was better than those of DBDT-ABBO-COF,
DPh-ABBO-COF. For example, the conversion rate of DTT-
ABBO-COF toward ethyl-3-oxo-3-phenylpropanoate sub-
reflectance spectra (Figure 5(b)). The band gap of DTT-ABBO
-
COF (1.96 eV) was smaller than that of DBDT-ABBO-COF
(2.05 eV) and DPh-ABBO-COF (2.20 eV), which was probably
due to the more prominent intralayer charge transfer effect in
DTT-ABBO-COF originated from the larger electron density in
TT units than that of BDT and Ph motifs. Furthermore, the
photoelectrochemical experiments were performed in aqu-
eous KCl solution (0.5 M) under Xe lamp. By analyzing the
transient photocurrent intensity of the D-ABBO-COFs upon
visible light switching at several time intervals, it was found
that DTT-ABBO-COF exhibited the largest photocurrent re-
sponse (Figure 5(c)). This is probably due to the smaller band
gap, which facilitates to capture more photons and thus
generate more excited state carriers [53]. Cyclic voltammetry
(CV, Figure 5(d)) was performed to determine the reduction
potentials of D-ABBO-COFs. The CV of all D-ABBO-COFs
showed a prominent reductive peak with onset potentials at
−0.45 V (DTT-ABBO-COF), −0.75 V (DBDT-ABBO-COF) and
−0.92 V (DPh-ABBO-COF) versus the Ag/AgCl electrode,
which corresponds to the lowest unoccupied molecular or-
bital (LUMO, Figure 5(e)) levels of −4.01, −3.72 and
strate was 92%, while under identical conditions, DBDT
-
ABBO-COF, DPh-ABBO-COF were 79% and 68%, respectively.
Similarly, DTT-ABBO-COF exhibited the highest catalytic
efficiency in the conversion of other substrates (Figure 4(d)).

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Liu et al. Sci China Chem May (2021) Vol.64 No.5
Figure 6 The proposed mechanism for the α-oxyamination of 1,3-di-
carbonyls (color online).
Based on the above experiments and literature reports [47–
50], a plausible mechanism for the photocatalytic α-
oxyamination of 1,3-dicarbonyls using these D-ABBO-COFs
was proposed in Figure 6. Upon visible light irradiation, the
excited state of COF photocatalyst (COF*) was formed.
Afterwards, an electron was extracted from TEMPO through
a single-electron transfer (SET) procedure to afford the ra-
dical anion DTT-ABBO-COF•− together with cation TEMPO+.
Then TEMPO oxidized DTT-ABBO-COF•− to regenerate DTT-
ABBO-COF which again participated in the next catalytic
cycle, while TEMPO itself was converted to TEMPO−. Next,
TEMPO− formed TEMPOH by extracting a proton from 1a,
and 1a was converted to an enolate of the dicarbonyl com-
pound. Finally, the enolate reacted with TEMPO+ to give the
final product 3a.
Figure 5 (a) UV-visible diffuse reflectance spectra; (b) Kubelka-Munk-
transformed reflectance spectra upon visible light irradiation; (c) photo-
current intensity comparisons in aq. KCl (0.5 M); (d) cyclic voltammo-
grams and (e) calculated alignment between the HOMO and LUMO of
DTT-ABBO-COF (olive), DBDT-ABBO-COF (dark yellow) and DPh-ABBO-COF
(orange) (color online).
−3.54 eV, respectively. According to the optical band gaps
(Eg), HOMO levels can be calculated by the equation:
EHOMO=ELUMO−Eg. Thus, the EHOMO values of D-ABBO-COFs
were estimated to be −5.97 eV (DTT-ABBO-COF), −5.77 eV
(DBDT-ABBO-COF) and −5.74 eV (DPh-ABBO-COF), respec-
4 Conclusions
In summary, three new D-ABBO-COFs consisting of electron-
deficient BBO unit were synthesized under solvothermal
conditions. All D-ABBO-COFs featured high crystallinity,
large porosity, excellent chemical and thermal stability. DTT-
tively. These results showed that DTT-ABBO-COF (EHOMO
=
−5.97 eV, ELUMO=−4.01 eV) cannot be reduced by benzoy-
lacetate (1a, EHOMO=−6.3 eV, ELUMO=−2.54 eV) but can be
reduced by TEMPO (2, EHOMO=−5.02 eV, ELUMO=−1.17 eV)
[47] (Figure 5(e)). On the other hand, the chemical reduction
ABBO-COF showed the superior performance toward the
photocatalysis of α-oxidative amination, which may be as-
cribed to the more negative HOMO (−5.97 eV) and lower
bandgap (1.96 eV). The current work represents the first
example to use COFs as the heterogeneous photocatalyst to
catalyze the α-oxyamination, and might open new possibi-
lities to develop novel donor-acceptor based 2D COFs for
efficient photocatalysis.
ability of DBDT-ABBO-COF (EHOMO=−5.77 eV, ELUMO
−3.72 eV) and DPh-ABBO-COF (EHOMO=−5.74 eV, ELUMO
=
=
−3.54 eV) decreased in turn, which has a significant effect on
the catalytic effectiveness. In addition, high crystallinity of
DTT-ABBO-COF makes it features better chemical and phy-
sical properties as well as photocatalytic performance [54].
This results are consistent with the experimental data. In
addition, the reaction mixture of 1a and TEMPO catalyzed
by DTT-ABBO-COF was monitored by GC-MS, and the pre-
sence of TEMPOH (m/z=157.1) was clearly observed (Fig-
ure S16).
Acknowledgements This work was supported by the National Key Re-
search and Development Program of China (2017YFA0207500) and the
Natural Science Foundation of Tianjin City (17JCJQJC44600).
Conflict of interest The authors declare no conflict of interest.

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Liu et al. Sci China Chem May (2021) Vol.64 No.5
Supporting information The supporting information is available online at
http://chem.scichina.com and http://link.springer.com/journal/11426. The
supporting materials are published as submitted, without typesetting or
editing. The responsibility for scientific accuracy and content remains en-
tirely with the authors.
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