β-Diketone boron difluoride dye-functionalized conjugated microporous polymers for efficient aerobic oxidative photocatalysis

Article abstract of DOI:10.1039/d1cy00384d

Incorporation of organic chromophores into conjugated micro/mesoporous polymers (CMPs) provides a promising avenue for developing recyclable heterogeneous photocatalysts by overcoming tedious separation and low reusability of homogeneous organic dye-based photocatalysts. However, the design principle and the underlying structure-property relationship for fabricating and selecting various organic dye-embedded CMPs for efficient photocatalysis have not been well-constructed so far. In this study, we described the rational fabrication of two new CMPsviathe one-step Sonogashira coupling using β-diketone boron difluoride dye as the key linker and commonly used building blocks (triphenylamine/triphenylbenzene) as the cores. The resulting boron-dye containing CMPs were efficiently employed as the metal-free photocatalysts in two typical aerobic oxidative organic transformations including coupling of benzylamine and oxidation of aryl boronic acids to corresponding aryl phenols, which have never been explored with other boron-dye-embedded CMPs. They exhibited superior photocatalytic performance compared to their boron-free counterparts due to their wide visible-light absorption, narrow optical bandgaps, and extended π-conjugation due to boron-complexation. The present study establishes β-diketone boron difluoride dyes as efficient building blocks for fabricating new CMP-based photocatalysts.

Full text of DOI:10.1039/d1cy00384d

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β-Diketone boron difluoride dye-functionalized
conjugated microporous polymers for efficient
aerobic oxidative photocatalysis†
Cite this: DOI: 10.1039/d1cy00384d
ab
a
a
c
ab
*
*
Kaixun Dong, Lu Liu, Mehdi Hassan and Guiling Ning
Weitao Gong,
Incorporation of organic chromophores into conjugated micro/mesoporous polymers (CMPs) provides a
promising avenue for developing recyclable heterogeneous photocatalysts by overcoming tedious
separation and low reusability of homogeneous organic dye-based photocatalysts. However, the design
principle and the underlying structure–property relationship for fabricating and selecting various organic
dye-embedded CMPs for efficient photocatalysis have not been well-constructed so far. In this study, we
described the rational fabrication of two new CMPs via the one-step Sonogashira coupling using
β-diketone boron difluoride dye as the key linker and commonly used building blocks (triphenylamine/
triphenylbenzene) as the cores. The resulting boron-dye containing CMPs were efficiently employed as the
metal-free photocatalysts in two typical aerobic oxidative organic transformations including coupling of
benzylamine and oxidation of aryl boronic acids to corresponding aryl phenols, which have never been
explored with other boron-dye-embedded CMPs. They exhibited superior photocatalytic performance
compared to their boron-free counterparts due to their wide visible-light absorption, narrow optical
bandgaps, and extended π-conjugation due to boron-complexation. The present study establishes
β-diketone boron difluoride dyes as efficient building blocks for fabricating new CMP-based
photocatalysts.
Received 3rd March 2021,
Accepted 15th April 2021
DOI: 10.1039/d1cy00384d
rsc.li/catalysis
intrinsically low reusability for molecular organic dyes restrict
their further applications. Therefore, the development of
Introduction
Sunlight is a great source of sustainable clean energy and a
good candidate for alleviating energy and environmental
metal-free, heterogeneous photocatalysts has attracted
considerable attention in recent years for resolving the
disadvantages of homogeneous photocatalytic systems.18–20
Conjugated micro/mesoporous polymers (CMPs), as an
emerging class of porous organic materials, have arisen as an
ideal platform for constructing new heterogeneous
photocatalysts. Featuring their high stability, tailorable
porosity, and surface properties, and diverse structural
designability, CMP-type photocatalysts exhibit promoted
activities and performance compared with conventional
homogeneous molecular organic dyes, due to their good
recyclability, efficient separation and migration of
photogenerated electron–hole pairs, and effective substrate
transformations.21–23 To achieve enhanced photocatalytic
properties, implementation of organic chromophores directly
into the backbone of CMPs has been deemed as an attractive
strategy to fabricate photoactive CMPs. Indeed, commonly
used homogeneous organic photoredox catalysts, such as
phthalocyanines, isoindigo, and eosin-Y, have been recently
explored as prominent building blocks for the construction
of various CMPs to realize strong and broadband light
absorption.24–26 However, as a major constituent of organic
dyes, boron-based fluorophores have been largely ignored
difficulties in the modern world.1–4 As
a
promising
photochemical transformation technology driven by sunlight,
artificial photocatalysis has become a research hotspot due to
its renewable and environmentally benign process.5–8
However, the development of task-specific photocatalysts
remains
highly
challenging.
To
date,
numerous
organometallic compounds9–12 and metal-free organic
chromophores13–17 have been investigated as photocatalysts
for visible-light-induced organic transformations due to their
adjustable absorptive capacity for visible-light and redox
potentials in excited states. However, the high cost and
toxicity associated with metal-based photocatalysts and the
a State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian
University of Technology, Dalian 116024, P. R. China. E-mail: ninggl@dlut.edu.cn
b Engineering Laboratory of Boric and Magnesic Functional Material Preparative
and Applied Technology, Dalian, Liaoning Province, 116024, P. R. China
c Department of Chemistry, University of Baltistan, Skardu, Pakistan
† Electronic supplementary information (ESI) available: TGA curves, XRD
patterns, SEM images, Mott–Schottky curves, 1HNMR spectra and the
comparison of catalytic performance with the reported catalysts. See DOI:
10.1039/d1cy00384d
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and only BODIPY-type dyes have been reported as key
building blocks for constructing CMPs. It is well-known that
the main drawback of BODIPY dyes is their narrow Stokes
shifts inducing strong self-absorption and this would restrict
their light-harvesting activity. Thus, the presently reported
organic transformations photocatalyzed by BODIPY-based
CMPs are just limited in the oxidation of sulfides and
α-terpinene.27–31 Therefore, the rational construction of new
boron-dye-based CMP photocatalysts for expanding organic
transformation is highly desired.
Among all boron-based fluorophores, the β-diketone boron
difluoride complex, built around an O^B^O pattern, has
recently gained much interest in fluorescence sensing and
photocatalysis due to its diverse structural modification and
excellent photooptical properties.32–35 Chelating four-
coordinated BIII atoms to configurationally lock O, O ligands is
beneficial for fine-tuning their optical properties by making the
π-system more planar, thereby enhancing the charge-transfer
along the main molecular axis. However, it is a pity that there
is no report regarding the incorporation of β-diketone boron
difluoride dyes as key building blocks into CMPs.
To examine the potential feasibility of new boron-dye-
based CMPs as efficient photocatalysts and realize expanded
photocatalytic organic transformations, in this manuscript,
we designed and prepared two new boron-dye-based
photoactive CMPs, named TPA-B-CMP and TPB-B-CMP via
the one-step Sonogashira coupling using the β-diketone
boron difluoride dye as the key linker and commonly selected
building blocks (triphenylamine and triphenylbenzene) as
the cores (Scheme 1). The as-prepared polymers were then
exploited as efficient metal-free photocatalysts in two typical
aerobic oxidative organic transformations including coupling
of primary amines and oxidation of aryl boronic acids to
corresponding aryl phenols, which have never been explored
with other boron dye-embedded CMPs. TPA-B-CMP with
enhanced electron donor–acceptor characteristics exhibited
better photocatalytic activity than TPB-B-CMP. More
significantly, because of their wide visible-light absorption,
narrow optical bandgaps, and extended π-conjugation due to
boron-complexation, the two boron-containing CMPs
displayed superior photocatalytic performance compared to
boron-free counterparts, simplified as TPB-NB-CMP, implying
the positive role of the β-diketone boron complex, which may
provide new insight into designing new types of organic dye-
based CMPs for photocatalytic organic transformations.
Results
Preparation and characterization of the CMPs
Selecting the same β-diketone boron difluoride dye (DBF) as
the key linker and different alkynes as the cores, two boron-
dye-based polymers TPA-B-CMP and TPB-B-CMP were
successfully constructed through the well-developed
Sonogashira–Hagihara cross-coupling reaction (Scheme 1). As
for TPA-B-CMP, the introduction of
a triphenylamine
functional group as the core would enhance the donor–
acceptor characteristics in the structure compared with TPB-
B-CMP and induce different photocatalytic activities
consequently. Additionally, to provide an insight into the role
of β-diketone boron difluoride dye, a control polymer TPB-
NB-CMP was also synthesized under similar reaction
conditions. After polymerization, three polymer networks
were obtained as different colorful powders, which are
insoluble in most common organic solvents, such as
dichloromethane (DCM), ethyl acetate (EA), acetonitrile (CH3-
CN), methanol (MeOH), tetrahydrofuran (THF), and dimethyl
sulfoxide (DMSO). Furthermore, they also showed excellent
chemical stability after treatment of H2O, 12 M HCl and 12
M NaOH for 3 days. The chemical structures of the as-
prepared CMPs were well characterized by FT-IR and solid-
state 13C cross-polarized NMR. As shown in Fig. 1a, the
disappearance of the absorption peaks at around 3265 cm−1
and 3270 cm−1 (attributed to the characteristic stretching
vibration of the H–CC bond on the different alkyne
monomers) and the present peaks at 1482, 1367, and 1045
cm−1 (assigned to the characteristic bands of the β-diketone
core) implied the successful polymerization. The further
detained connections were analyzed via solid-state 13C cross-
polarized NMR spectroscopy. As shown in Fig. 1b, the signals
related to tertiary and carbonyl carbons on the β-diketone
group appeared at around 91 ppm and 181–195 ppm,
respectively, according to the 13C NMR spectrum of model
monomer DBFA (Fig. S7†). The broad range of peaks that
appeared at δ115–150 ppm is assigned to the aromatic
Scheme 1 Synthesis of TPA-B-CMP, TPB-B-CMP and TPB-NB-CMP by the Sonogashira–Hagihara cross-coupling reaction.
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theory (Fig. 2b). The Brunauer–Emmett–Teller (BET) surface
areas of TPA-B-CMP, TPB-B-CMP, and TPB-NB-CMP were
calculated to be 84, 207, and 296 m2 g−1, respectively. Among
them, TPA-B-CMP exhibited the lowest specific surface area
and average pore size, possibly due to the strong π–π stacking
effects between the linkers and the smaller molecular size of
the triphenylamine moiety.36 The SEM image of TPA-B-CMP
shows fused sphere-like features, revealing that it will not
have a large surface area (Fig. S3†). TPB-NB-CMP exhibited
the highest specific surface area and average pore size due to
the absence of coordination boron difluoride.
Photophysical properties
A close inspection of the optoelectronic properties of the
three polymers and their corresponding precursors in the
solid-state was initially carried out by using diffuse
reflectance UV-vis spectroscopy. As shown in Fig. 3a, all the
polymers displayed a broader range of absorption bands
compared with their corresponding monomers indicating
that the extended conjugation was realized after successful
polymerization. Conspicuously, TPA-B-CMP and TPB-B-CMP
exhibited an obvious red-shift compared to their control
polymer TPB-NB-CMP indicating the positive role of boron
complexation in their visible light absorption properties.
TPA-B-CMP with an enhanced electron donor–acceptor
structure showed the maximum absorption range and this
result implied that the light absorption capability of the
corresponding polymers can be adjusted by selecting
different electron-rich coupled monomers. UV-vis experiment
combined with the Kubelka–Munk equation is a commonly
used method for estimating the band gap of polymers
(Fig. 3b). After calculation, the optical bandgaps of TPA-B-
CMP, TPB-B-CMP, and TPB-NB-CMP were determined to be
Fig. 1 (a) Fourier transform infrared (FT-IR) spectra of the monomers
and TPA-B-CMP, TPB-B-CMP and TPB-NB-CMP; (b) solid-state 13C
NMR spectra of TPB-B-CMP, TPA-B-CMP, and TPB-NB-CMP; asterisks
denote the spinning sidebands.
carbon of the benzene ring and the sp-hybridized carbon of
triple bonds in the phenylacetylene unit is found at around δ
= 93 ppm further validating the formation of the polymers.
Thermogravimetric analysis (TGA) (Fig. S1†) showed that the
polymers are thermally stable up to around 400 °C with less
than 10% weight loss. The polymers are amorphous in
nature, as indicated by the broad powder X-ray diffraction
(PXRD) patterns (Fig. S2†), which could also be further
proved by the irregular worm-like structure shown in the
SEM images (Fig. S3†).
The permanent porosities of TPA-B-CMP, TPB-B-CMP, and
TPB-NB-CMP were characterized by using N2 adsorption–
desorption measurements at 77 K (Fig. 2a). All the polymers
were observed to show narrow pore size distributions centered
at 2.38, 3.14, and 4.22 nm, respectively, combined with the
pore size distribution profiles and nonlocal density functional
Fig. 3 (a) Normalized UV/vis absorption spectra of TPB-B-CMP, TPA-
B-CMP, and TPB-NB-CMP with the corresponding monomers; (b)
Kubelka–Munk-transformed reflectance spectra; (c) schematic energy
band structures of TPB-B-CMP (black line), TPA-B-CMP (red line) and
TPB-NB-CMP (blue line); (d) transient photocurrent response of TPB-
B-CMP, TPA-B-CMP, and TPB-NB-CMP under visible-light irradiation.
Fig. 2 (a) Nitrogen adsorption–desorption isotherms of TPA-B-CMP,
TPB-B-CMP and TPB-NB-CMP at 77 K; (b) pore size distributions of
TPB-B-CMP, TPA-B-CMP, and TPB-NB-CMP.
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2.13, 2.25, and 2.44 eV, respectively. To further determine the
relative positions of the valence band (VB) and conduction
band (CB), the electrochemical Mott–Schottky technique was
performed. The positive slopes in the Mott–Schottky plots
showed that three polymers are n-type semiconductors (Fig.
S4†), and the corresponding CB potentials are −0.98, −1.18
and −1.28 eV vs. SCE. Therefore, the position of the valence
band (VB), 1.07 eV for TPA-B-CMP, 1.14 eV for TPB-B-CMP
and 1.15 eV for TPB-NB-CMP, can be derived from the optical
band gap (Fig. 3c). To our delight, the oxidation/reduction
potentials of these polymers are sufficient to activate some
molecules into their active states, for example, the oxidation
potential of DIPEA at +0.90 V vs. SCE and the reduction
potential of O2 at −0.86 V vs. SCE, which are comparable with
those of classic catalysts, such as *[Ru(bpy)3]2+ (−0.81 V vs.
SCE) and [Ru(bpy)3]3+ (+1.29 V vs. SCE).37,38 The photocurrent
response signal of photocatalytic materials can effectively
reflect the photogenerated carrier migration ability of the
materials. As shown in Fig. 3d, TPA-B-CMP exhibits the best
photocurrent response signal due to the presence of an
electron-rich triphenylamine group for the enhanced
electron-donating properties. This establishment of efficient
donor–acceptor characteristics in the polymer skeleton may
promote the charge transfer at the interface and effectively
separate the photoinduced electron–hole pairs.
first model transformation.25,39,40 To our delight, for TPA-B-
CMP and TPB-B-CMP, with the presence of a small amount of
catalyst (1% mmol), the reaction could reach complete
conversion within 18 hours, which was highly comparable to
the organic photocatalysts published previously (Table S1†).
However, as a contrast, the transformation enabled by TPB-
NB-CMP only afforded a low yield of 77%, highlighting the
critical role of β-diketone boron difluoride dye in the polymer
structure. The higher photocatalytic activity and shorter
reaction time of TPA-B-CMP compared with those of TPB-B-
CMP might be ascribed to the strengthened donor–acceptor
structure in its skeleton and the promoted charge-transfer
and separation capability. Furthermore, under the same
conditions, the reaction yield using TPA-B-CMP (entry 3,
Table 1) as the catalyst is 1.7 times higher than that of the
molecular model catalyst (entry 6, Table 1). This result
verified that the extended π-conjugated structure of TPA-B-
CMP is significant for realizing efficient photocatalysis. In
addition, no target product was detected in the absence of an
oxygen source (entry 4), light (entry 5), or photocatalyst (entry
7) in the control experiments, implying the essential role of
catalysts, oxygen, and light.
Since TPA-B-CMP exhibited the highest photocatalytic
activity, we selected it as the photocatalyst to test the
substrate scope of aromatic primary amines to validate the
generality of TPA-B-CMP. As shown in Table 2, in the
presence of TPA-B-CMP, a series of amines with different para
substituents could be converted to imines in good to
excellent yields. Generally speaking, electron-withdrawing
groups are more active (>98%, 2d and 2e), and electron-
donating groups are slightly less active (2b and 2c). As for
amines with ortho substituents, a little bit lower yields were
observed (2g and 2h), which might be due to the steric
hindrance effect.
Photocatalytic performance studies
As
mentioned
previously,
to
date,
the
organic
transformations photocatalyzed by boron-dye-based CMPs
were just restricted in the oxidation of sulfides and
α-terpinene.27–31 In order to evaluate the potential feasibility
of new boron-dye-based CMPs as efficient photocatalysts and
realize expanded photocatalytic organic transformations, two
types of photocatalytic aerobic oxidation, including the
oxidative coupling of amines and oxidation of aryl boronic
acids to the corresponding aryl phenols were selected to
screen the most efficient photocatalyst from our as-prepared
boron-containing polymers. Initially, we choose the
photocatalytic oxidation of primary amines to imines as the
Phenol is one of the highly economical organic raw
materials and has important applications in chemistry and
medicine.41 Photocatalytic oxidative hydroxylation of aryl
boronic acids is an effective and green synthetic way to
prepare the corresponding phenols. It has been reported that
benzoxazole and benzothiadiazole-based CMPs exhibit high
Table 1 Photocatalytic oxidative coupling of amines into iminesa
Entry
Catalyst
Time (h)
Sel.b (%)
Yieldb (%)
1
2
TPA-B-CMP
TPA-B-CMP
TPA-B-CMP
TPA-B-CMP
TPB-B-CMP
TPB-NB-CMP
TPA-B-CMP
TPA-B-CMP
DBFA
6
12
18
18
18
18
18
18
18
18
>99
>99
>99
>99
>99
>99
Trace
n.d.
>99
n.d.
63
80
99
75
99
77
Trace
n.d.
59
3
4c
2
3
4d
5e
6
7
No
n.d.
a
Reaction conditions: benzylamine (0.5 mmol), photocatalyst (1 mmol%), CH3CN (5 mL), 10 W blue LED lamp (460–465 nm), room
temperature (RT). b Yield was determined by H NMR. c Catalyst: 0.5 mmol%. No O2. e No light.
1
d
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Table 2 Photocatalytic oxidative coupling of arylamines in the presence of TPA-B-CMP and the product selectivities are given in parenthesesa
2a: 99% (>99%)
2c: 94% (>99%)
2b: 91% (>97%)
2e: 98% (>98%)
2d: 99% (>99%)
2f: 93% (>99%)
2g: 90% (>99%)
2h: 92% (>97%)
a
Reaction conditions: respective primary amines (0.5 mmol), photocatalyst (1 mmol%), CH3CN (5 mL), 10 W blue LED lamp (460–465 nm),
room temperature (RT).
catalytic activity for this transformation.16,42–44 However,
there have been no reports regarding this transformation
photocatalyzed by boron-dye-based CMPs yet. Therefore, we
investigated the photocatalytic performance of TPA-B-CMP,
TPB-B-CMP and TPB-NB-CMP in the aerobic oxidation of aryl
boronic acids to phenols (Table 3). In the presence of 2
mmol% TPB-B-CMP, air, and light, phenylboronic acid was
oxidized into the corresponding phenol (entry 1, >96%) with
a significant yield within 48 h. In contrast, TPB-NB-CMP
showed a lower activity and TPA-B-CMP exhibited the most
excellent transformation efficiency among them, which is
highly comparable with the reported organic photocatalysts
(Table S2†). The photocatalysis factors including the
sacrificial agents, oxygen, and light that may influence this
transformation were systematically assessed. As shown in
Table 3, when the reaction is performed in the dark or an N2
atmosphere, it hardly proceeds (entries 4–6). In particular,
the presence of a sacrificial agent is essential for the
oxidation reaction of phenylboronic acid. When TPA-B-CMP
was replaced with a structural monomer, the yield was only
57% after 48 hours of reaction (entry 7). It can be concluded
that the photocatalytic properties of TPA-B-CMP are not only
derived from the β-diketone boron difluoride moiety but also
related to the π-conjugated framework.
The substrate scope was also explored by using TPA-B-
CMP as the photocatalyst. Excellent reaction yields were
obtained for different derivatives of boronic acid validating
the generality of TPA-B-CMP (Table 4). Various aryl boronic
acids, bearing either electron-donating groups such as
methoxy (4e) and phenanthrene (4g) or electron-withdrawing
groups such as bromide (4b) and carboxyl (4c), were
effectively oxidized into the corresponding products in good
to excellent yields. Notably, in general, substrates with
electron-withdrawing groups have better reaction efficiencies
than those with electron-donating groups.
Photocatalytic mechanism
The elucidation of the plausible photocatalytic mechanism
for both reactions was then conducted. It is acceptable that
reactive oxygen species (ROS), like superoxide radicals (O2˙−)
and singlet oxygen (1O2), are indispensable in photocatalytic
aerobic oxidative transformations.45,46 Therefore, the
competence of TPA-B-CMP, TPB-B-CMP and TPB-NB-CMP to
produce 1O2 and O2˙− was examined initially. As shown in
Fig. 4a, all the excited photocatalysts can effectively promote
the generation of a purple colored 1,4-bis(dimethylamino)
benzene-cationic radical and O2˙−. The highest absorption
Table 3 Photocatalytic oxidation of phenylboronic acid to phenola
Entry
Catalyst
Time (h)
Yieldb (%)
1
2
3
TPB-B-CMP
TPA-B-CMP
TPB-NB-CMP
TPA-B-CMP
TPA-B-CMP
TPA-B-CMP
DBFA
48
48
48
48
48
48
48
48
96
99
72
8
Trace
n.d.
57
4c
5d
6e
7
8
No
n.d.
a
Reaction conditions: phenylboronic acid (0.5 mmol), photocatalyst (2 mmol%), DMF (5 mL), air, 10 W blue LED lamp (460–465 nm), room
temperature (RT), 48 h. b Yield was determined by 1H NMR. c No iPr2EtN. d No O2. e No light.
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Table 4 Photocatalytic oxidative arylboronic acid hydroxylation in the
presence of TPA-B-CMPa
4a: 99%
4d: 94%
4b: 98%
4e: 92%
4c: 99%
4f: 98%
Fig. 5 Proposed mechanism for the photocatalytic transformation of
primary amine by oxidative coupling (red line) and aryl boronic acids
(blue line) to phenols in the presence of TPA-B-CMP.
4g: 95%
With all the above results in hand, the plausible reaction
mechanisms for both transformations were proposed,
respectively (Fig. 4, Table S3†). Firstly, TPA-B-CMP generates
TPA-B-CMP* in a photocatalytic cycle under visible light
irradiation. Subsequently, excited TPA-B-CMP* can activate
O2 via a single electron transfer (SET) process and yielding
O2˙−. The imines and sacrificial agents are oxidized by TPA-B-
CMP˙+ to form free radical cations while TPA-B-CMP is
regenerated in a photocatalytic cycle. Superoxide radicals
then extract protons from the primary amine cations, thereby
producing amine intermediates and hydrogen peroxide. The
imine is further reacted with the free amine to complete the
coupling reaction after the removal of ammonia. The
oxidation mechanism of aryl boronic acid is similar to that of
primary amine oxidation. DIPEA is oxidized by losing an
electron to the photogenerated hole in the excited state of
TPA-B-CMP. Simultaneously, aryl boronic acid reacts with
O2˙− to form the peroxide radical as an intermediate. These
intermediates obtain a proton from DIPEA˙+ and lose –OH− to
form intermediate peroxides, which are further rearranged
and hydrolyzed into phenols (Fig. 5).
a
Reaction conditions: respective arylboronic acids (0.5 mmol),
photocatalyst (2 mmol%), DMF (5 mL), air, 10 W blue LED lamp
(460–465 nm), room temperature (RT), 48 h.
peak intensity and the deepest solution color implied that
TPA-B-CMP has the superior photocatalytic activity to
produce O2˙− among the three polymers. Then, the 1O2
production capacity of the photocatalysts was experimentally
validated by electron paramagnetic resonance (EPR)
spectroscopy using 2,2,6,6-tetramethylpiperidine (TEMP) as a
spin-trapping agent for 1O2. As shown in Fig. 4b, upon
exposure to light, all the photocatalysts produce the
characteristic 1 : 1 : 1 triplet signal due to the formation of the
2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) radical by the
1
reaction of TEMP with O2. The highest strength of the EPR
signal indicated that TPA-B-CMP has the best ability to
activate oxygen to produce O2.
1
Recyclability and stability
Finally, reusability is a key factor to evaluate the potential
application of a photocatalyst. Therefore, TPA-B-CMP was
selected as a representative to test the recyclability in the two
reactions. In each reaction, the photocatalyst TPA-B-CMP was
Fig. 4 (a) UV-vis absorption spectra and photographs of NTPD in
acetonitrile in the presence and the absence of TPB-B-CMP, TPA-B-
CMP, and TPB-NB-CMP upon irradiation by visible light (10 W blue
LED); (b) the EPR spectra of TPA-B-CMP in the presence of air and
TEMP after visible light irradiation.
Fig. 6 Recyclability of TPA-B-CMP in primary amine coupling and aryl
boronic acid oxidation. The reaction time was fixed for each cycle.
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simply isolated from the reaction mixtures through
centrifugation then washed, dried, and utilized in the next
run of the model reaction. Five repeated experiments were
performed by using the same TPA-B-CMP catalyst mostly
retaining high catalytic efficiency and selectivity. The
oxidative coupling of benzylamine and the oxidative
conversion of phenylboronic acid can still reach 95% and
94%, respectively (Fig. 6). The chemical stability of TPA-B-
CMP was also investigated by FT-IR spectroscopy (Fig. S6†).
The recovered TPA-B-CMP did not show significant change
compared with the original sample, indicating that the
basic structure and morphology of TPA-B-CMP are still
retained after photocatalysis, except the particle profile
became smaller. The average 1% and 1.2% reduction in the
conversion per run can be attributed to the photobleaching
of the polymer or partial deactivation of superoxide
radicals.
ethynylphenyl)amine were synthesized according to the
reported synthetic methods.47
General procedure for the preparation of CMPs
The one-step Sonogashira coupling reactions were employed
to fabricate TPA-B-CMP, TPB-B-CMP and TPB-NB-CMP.48 In
a
typical procedure, monomer DBF or DBH with a
dibromide group (0.75 mmol), respective alkynes (0.5
mmol), Pd(PPh3)4 (60 mg, 0.052 mmol), and CuI (30 mg,
0.16 mmol) were added to a 50 mL flame-dried Schlenk
tube with mixed dry N,N-dimethyl formamide (DMF) (5 mL)
and triethylamine (5 mL). The reaction suspension was
degassed and then stirred at 100 °C for 3 days in an inert
nitrogen atmosphere. After cooling to room temperature,
the solid was obtained by filtration and washed with DMF,
water, trichloromethane, methanol, and acetone. Further
purification was carried out by Soxhlet extraction with
methanol and trichloromethane successively for 24 h each.
Finally, the solid was stirred in a tetrahydrofuran (THF)/
water (1 : 1 v/v) solution with excess KCN to remove Pd(0).
The product was then dried under a vacuum for 24 h at 60
°C to obtain the polymers as solids with different colors.
The synthetic details for all compounds are given in the
ESI.†
Conclusions
Two new boron-containing CMPs, TPA-B-CMP and TPB-B-
CMP with β-diketone-boron difluoride dye as the key building
block, were successfully prepared and exploited as efficient
metal-free heterogeneous photocatalysts for aerobic oxidation
of amines to imines and aryl boronic acids to phenols for the
first time. They were found to be well capable of generating
both singlet oxygen and superoxide anion radicals under
visible-light irradiation compared to their constituent
precursors and reference polymer (TPB-NB-CMP) without
boron complexation. The bandgaps, energy levels, and the
photocurrent response of both boron-containing CMPs were
conveniently tuned simply by the implementation of
enhanced donor-acceptor properties in the framework. The
introduction of an electron-rich triphenylamine moiety into
TPA-B-CMP can induce better visible-light adsorption and
accelerated charge transfer at the interface giving rise to the
best photocatalytic activity. Furthermore, as a representative
photocatalyst, TPA-B-CMP displayed extensive substrate
adaptability and excellent recyclability for both photocatalytic
aerobic oxidation reactions. The current work provides a new
insight on developing various boron-based dye incorporated
CMPs as efficient photocatalysts and relative work is still
ongoing in our lab.
Instruments and methods
1H NMR spectra were recorded using a Bruker Avance II 400
instrument with CDCl3, DMSO-d6, and CD3CN as solvents,
where chemical shifts (δ in ppm) were determined with a
residual proton of the solvent as
a standard. Fourier
transform infrared (FTIR) analysis was performed by using a
JASCO IR-4100 spectrometer from 400 to 4000 cm−1. Solid-
state 13C CP/MAS (cross-polarization with magic angle
spinning) spectra were obtained on an Agilent DD2-500 MHz
nuclear magnetic resonance spectrometer. UV-vis studies
were conducted on a Hitachi UV-4100 spectrometer at room
temperature. X-ray diffraction (XRD) measurements were
carried out on
a Rigaku D/max-2400 diffractometer by
depositing the powders on a glass substrate, from 2θ = 2° to
40° with a scanning rate of 2° min−1 at 25 °C. Field emission
scanning electron microscopy (FE-SEM) studies were
conducted by using an FEI Nova NanoSEM 450 scanning
electron microscope at 20 kV. Nitrogen sorption isotherms
were measured at 77 K with a Quantachrome Autosorb iQ
analyzer. Before measurement, the samples were degassed in
a vacuum at 120 °C overnight. Thermogravimetric analysis
(TGA) was performed by using a Mettler Toledo TGA/DSC 3+
thermal analyzer in an atmosphere of flowing N2. All the
samples were heated at room temperature to 800 °C at a
heating rate of 10 °C min−1. The EPR spectra were recorded
on a Bruker A200-9.5/12 EPR spectrometer. The samples were
quantitatively injected into 0.5 mm quartz capillaries for EPR
analysis. The data of the TEMP solution with a concentration
of 0.1 M was collected.
Experimental section
Materials
All required chemicals were obtained from commercial
suppliers unless otherwise specified. 4-Bromoacetophenone,
methyl 4-bromobenzoate, and deuterated solvents for NMR
measurement were purchased from Aladdin Chemical
Reagent. Boron(III) fluoride ethylether complex (98%) and
sodium hydride 60% dispersion in mineral oil were obtained
from Energy Chemical. Organic solvents for reactions were
purified by published standard methods. The monomers
1,3,5-tris-(4-ethynylphenyl)benzene
and
1,3,5-tris(4-
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Photoelectrochemical measurement
Author contributions
The photoelectrochemical properties of the photocatalysts
were studied on a CHI 760E electrochemical workstation
W. Gong conceived and designed the experiments. K. Dong
completed the synthesis. K. Dong, M. Hassan, L. Liu, and W.
Gong analyzed the data and wrote the manuscript. All the
authors discussed the results and commented on and
proofread the manuscript. All the authors have given
approval to the final version of the manuscript.
using
a standard three-electrode system. The working
electrode was prepared with the following procedure: 5 mg
photocatalyst powder and 5% Nafion were mixed in 1 mL of
ethanol and sonicated for 1 h. The mixture was dripped onto
a fluorine-doped tin oxide (FTO) glass (10 × 10 mm2), and the
solvent was evaporated in a vacuum chamber for 1 h to
obtain the working electrode. The Mott–Schottky analysis was
performed using a homemade sample electrode on FTO
conductive glass as the working electrode, a platinum wire
electrode as the counter electrode, and a saturated calomel
electrode (SCE) as the reference electrode, measured at
different frequencies. The photocurrent responses of the
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We are grateful for financial support from the Natural Science
Foundation of Liaoning Province (No. 2019-MS-046), the Open
Funds of the State Key Laboratory of Rare Earth Resource
Utilization (No. RERU2020004) and the National Natural
Science Foundation of China (No. U1808210).
polymers were measured in 0.1
M Na2SO4 under the
irradiation of a 300 W Xe lamp with 10s light on–off cycles.
Typical procedure for photocatalytic primary amine coupling
Notes and references
The photocatalysts (0.005 mmol), corresponding amine (0.5
mmol), and acetonitrile (5 ml) were mixed in a quartz
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of a WP-TEC-1020HSL photochemical reaction system with a
blue LED lamp (10 W) at room temperature. Thin layer
chromatography (TLC) was used to monitor the progress of
the reaction. After the reactants were consumed as indicated
by TLC, the reaction solid was removed by centrifugation and
washed several times with CH3CN. The supernatants were
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