Bandgap engineering in benzotrithiophene-based conjugated microporous polymers: a strategy for screening metal-free heterogeneous photocatalysts

Article abstract of DOI:10.1039/d0ta10232f

Metal-free conjugated microporous polymers (CMPs) as visible-light active and recyclable photocatalysts offer a green and sustainable alternative to classical metal-based photosensitizers. However, the strategy for screening CMP-based heterogeneous photocatalysts has not been interpreted up to now. Herein, we present a general strategy for obtaining excellent solid photocatalysts, which is to implement bandgap engineering in the same series of materials. As a proof of concept, three conjugated porous materials containing benzo[1,2-b:3,4-b′:5,6-b′′]trithiophene building blocks (BTT-CMP1, BTT-CMP2 and BTT-CMP3) were successfully constructed. They possess permanent porosity with a large specific surface area and excellent stability. By changing the linker between benzotrithiophene units, the bandgaps, energy levels and photoelectric performances including the absorption, transient photocurrent responses and photocatalytic performances of BTT-CMPs could be handily modulated. Indeed, BTT-CMP2 displayed the best catalytic activity for visible-light-induced synthesis of benzimidazoles among the three CMP materials, even higher than that of small molecule photocatalysts. As a metal-free photocatalyst, interestingly, the screened BTT-CMP2 also showed extensive substrate applicability and outstanding recyclability. Additionally, we have the opinion that this strategy will prove to be a guiding principle for screening superior CMP-based photocatalysts and broaden their application fields.

Full text of DOI:10.1039/d0ta10232f

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ARTICLE
Bandgap engineering in benzotrithiophene-based conjugated
microporous polymers: A strategy for screening metal-free
heterogeneous photocatalyts
Received 00th January 20xx
Accepted 00th January 20xx
Songjie Han,a Ziping Li,a Si Ma,a Yongfeng Zhi,a Hong Xia,b Xiong Chen,c and Xiaoming Liu*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Metal-free conjugated microporous polymers (CMPs) as visible-light active and recyclable photocatalysts offer a green and
sustainable option to classical metal-based photosensitizers. However, the strategy for screening CMP-based heterogeneous
photocatalysts has not been interpreted up to now. Herein, we present a general strategy for obtaining the excellent solid
photocatalysts, which is to implement the bandgap engineering in the same series of materials. As a proof of concept, three
conjugated porous materials containing benzo[1,2-b:3,4-b′:5,6-b″]trithiophene building blocks (BTT-CMP1, BTT-CMP2 and
BTT-CMP3) were successfully constructed. They possess permanent porosity with large specific surface area and excellent
stability. By changing the linker between benzotrithiophene units, the bandgaps, energy levels and photoelectric
performances including absorption, transient photocurrent responses and photocatalytic performances of BTT-CMPs could
be handily modulated. Indeed, BTT-CMP2 displayed the best catalytic activity for visible-light-induced synthesis of
benzimidazoles in three CMP materials, even higher than that of the small molecule photocatalysts. As a metal-free
photocatalyst, interestingly, the screened BTT-CMP2 also showed extensive substrate applicability and outstanding
recyclability. Additionally, we hold the opinion that this strategy will prove to be a guiding principle for screening superior
CMP-based photocatalysts and broaden their application fields.
been extensively researched because of its low cost and
promising photoelectric properties. In order to increase the
visible light absorption and reduce the recombination of
photogenerated carriers, however, some modification
technologies including doping, copolymerization and dye
sensitization are necessary and challenging.7,8
Introduction
The exploration and development of semiconductor
photosensitizers for the conversion solar energy to chemical
energy have turn into one of the most frontier topics of material
science owing to the severe energy and environmental
challenges.1,2 In recent years, a lot of photocatalytic systems
have been intensively researched. Despite huge progress in this
area, most photocatalysts are short of the basic inherent
requirement such as an appropriate optical band gap with
optimal frontier electron orbits, effective generation and
separation of photogenerated carriers, and outstanding
stability. Besides metal-based compounds3,4 and organic dyes5
often suffer from high price, toxicity and inevitable metal
leaching, poor reusability and tedious product purification
process, thereby limiting their application in practical purpose.
Furthermore, a state-of-art example, graphitic carbon nitride
(g-C3N4)6 as classical metal-free organic photocatalysts have
Since the pioneering report by Cooper group in 2007,9
conjugated microporous polymers (CMPs) have drawn
considerable scientific attentions owing to their wide range of
applications as new functional scaffolds.10-12 The CMPs combine
π-conjugated skeleton as well as inherent porosity in one
materials, which display fascinating features. The permanent
pores offer space for transport, action and transformation of
guests, and on the another hand the π-conjugated skeletons
ensure effective light absorption, high fluorescence emission,
and facilitate exciton migration over the skeleton. Therefore,
they have been demonstrated as fluorescent sensing
materials.13-16 For the first time, the Jiang group used TCB-CMP
as a fluorescent molecular sensing device for the detection of
explosives with rapid response time and high sensitivity.13
Furthermore, recent research reports have still exhibited the
underlying applications of CMP materials in heterogeneous
photocatalysis such as light-induced hydrogen evolution,17-22
photoinduced degradation of pollutants,23-26 and photocatalytic
carbon dioxide reduction.27 More importantly, the
photoinduced organic transformations by CMPs are also
performance successfully in recent years.28-34 Up to now,
a. College of Chemistry, Jilin University, Changchun, 130012, P.R. China.
E-mail: xm_liu@jlu.edu.cn
b. State Key Laboratory on Integrated Optoelectronics, College of Electronic Science
and Technology, Jilin University, Changchun, 130012, P.R. China.
c. State Key Laboratory of Photocatalysis on Energy and Environment, College of
Chemistry, Fuzhou University, Fuzhou, 350002, P.R. China
†
Electronic Supplementary Information (ESI) available: Syntheses of monomers and
BTT-CMPs, material characterization, general photocatalytic process, and NMR
spectra of catalytic products See DOI: 10.1039/x0xx00000x
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nevertheless, the exploration of CMPs as effective
heterogeneous photocatalysts is in the initial stage, and the
View Article Online
DOI: 10.1039/D0TA10232F
Scheme 1. (a) Syntheses of BTT-CMPs.
varieties of organic reactions studied are still very limited.29 example, Jiang group recently reported the BTT-based π-
Meanwhile, the guiding principles for screening of metal-free conjugated microporous polymer films through
CMP-based heterogeneous photocatalysts with prominent electropolymerization method at the solution-electrode
performances are also rare.
interface.43 Meanwhile, BTT-based porous materials as metal-
Benzimidazole ring exists widely in biologically active free heterogeneous photocatalysts are also worth expectation.
molecules such as many antihypertensive, antiviral, anticancer, Taking these factors into account, herein we contribute
antifungal, and psychotropic Moreover, general strategy for obtaining the superior metal-free
drugs.35,36
a
benzimidazole derivatives are still important intermediates in heterogeneous photocatalysts, which is to implement the bandgap
organic synthesis, auxiliary ligands in transition metal engineering in porous polymers. As a proof of concept, three CMP
complexes, and are utilized in materials science.37 Hence, the materials containing BTT building blocks (Scheme 1, BTT-CMP1, BTT-
synthesis of benzimidazoles and their derivatives has obtained CMP2 and BTT-CMP3) were successfully synthesized. By changing the
extensive attraction of chemists for many years. Generally, the linker between benzotrithiophene units, the bandgap, energy level
currently available methods suffered from the use of hazardous and photoelectric performances including absorption, transient
and costly materials, or the requirement for harsh reaction photocurrent responses as well as photocatalytic activity of BTT-
conditions. By comparison, the light-induced synthesis of CMPs could be expediently tailored. And screened BTT-CMP2
benzimidazole compounds is a simple, preferably selective, and displayed the best catalytic activity for visible-light-induced synthesis
environment-friendly method.38-40 Unluckily, the utilized of benzimidazoles in three CMPs, even higher than that of the small
photocatalysts are usually noble metal-based complexes, that molecule photocatalysts. More important, it also showed broad
set a limit to their application for large-scale experiment in substrate adaptability and outstanding recyclability.
industry.39
Results and discussion
In the context, the exploration and development of CMPs as
metal-free heterogeneous photocatalysts for the synthesis of
new organic reactions are highly expected. We know well that
As manifested in Scheme 1, three BTT-CMPs with the
different linkers can be synthesized in high yield by the
palladium catalyzed cross-coupling reactions, which are
credible processes to prepare CMPs demonstrated by Cooper
group. The obtained polymers are fluffy solids, furthermore, it
is undissolved in water and general organic solvents. They have
good chemical stability, can even resist dilute acid-base
solutions. Further, the chemical composition of BTT-CMPs was
studied by using Fourier transform infrared (FT-IR), solid-state
13C cross-polarized magic angle spin (CP/MAS) nuclear magnetic
resonance (NMR) and other spectroscopy and analysis methods.
benzo[1,2-b:3,4-b′:5,6-b″]trithiophene
(BTT)
is
a
fused
terthiophene with a sulfur-rich, planar, C3h symmetry and
extended π-system, which was widely explored as excellent
electron donor units for solar cell.41 Additionally, the rigid
conjugated framework of BTT is appropriate for constructing a
porous networks that possess immanent microporosity and
prominent physicochemical stability. Indeed, the research of
BTT as building block with excellent electron donor property has
been extended to the field of organic porous materials.42,43 For
2 | J. Name., 2012, 00, 1-3
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FT-IR spectra of BTT-CMPs and their monomers were exhibited contrast of triethynyl-BTT monomer, however, the siVgienwaAlsrtiaclte2O0nl9in2e
in Fig. 1a. We found that all CMP
cm−1 for C and 3285 cm−1 for alkynylDCO–I:H10.1s0tr3e9t/Dch0TAa1l0m2o32sFt
disappeared in FT-IR spectra of BTT-CMP1. These data indicated
that the polymerization process is efficient. Furthermore, the
chemical structure of BTT-CMPs was contrasted, the broad
signals at about 96.5 ppm for BTT-CMP3 can be contributed to
alkyne Car-C≡C-Car units, and weak peaks at about 78.4 ppm may
be from the homocoupling of the alkyne monomer (Fig. S1). The
BTT-CMP2 without acetylene units displayed more discrete
carbon peaks. The signals at about 143.4 and 116.5 ppm are
corresponded to the quaternary and tertiary carbons in
thiophene, respectively (Fig. 2b). And resonances at about
132.6 and 126.7 ppm are due to the carbons in phenyl rings.
BTT-CMPs did not any signals in powder X-ray diffraction (PXRD)
curves, demonstrating that the new materials possess an
amorphous character (Fig. S2). As depicted in the field-emission
scanning electron microscopy (FE-SEM) images, both materials
containing the acetylene units BTT-CMP1 and BTT-CMP3
possessed a band-like morphology at micrometer scale (Fig. S3).
For BTT-CMP2, nevertheless, the images revealed that the solid
samples are composed of bar-like nanoparticles (Fig. 2a and Fig.
S3e). And we can recognize the presence of pores in BTT-CMP2
from high-resolution transmission electron microscopy (HR-
TEM) images. (Fig. 2b). In addition, energy-dispersive X-ray
spectroscopy (EDS) elemental mapping images clearly showed
that the carbon and sulfur elements were distributed uniformly
on its skeleton, respectively (Fig. 2c and 2d). The thermal
stability of BTT-CMPs was investigated by thermogravimetric
Fig. 1 (a) FT-IR profiles of triethynyl-BTT (red line), BTT-CMP1 (gray line),
Tribromo-BTT (green line), BTT-CMP2 (cyan line), and BTT-CMP3 (blue line).
(b) Solid-state 13C NMR spectrum of BTT-CMP2. (c) The chemical structure of analysis (TGA) in a nitrogen atmosphere (Fig. 2e), and 5% mass
BTT-CMP2.
loss was 300 oC for BTT-CMP1 and BTT-CMP3. In sharp contrast,
the thermal decomposition temperature of BTT-CMP2 is as high
materials still maintain some characteristic vibration signals of
as 550 oC, indicating it owns a remarkable thermal stability. As
BTT structure, while the broadening features accord with the
depicted in Fig. 2f, the adsorption curves display a sharp
nature of the polymer. In particular, BTT-CMP1 gives weak
nitrogen uptake at relatively low pressure, which proves an
broad signals at about 2186 cm−1, which ascribe to the vibration
inherent
of carbon-carbon triple-bond with disubstituted groups. By
Fig. 2 (a) FE-SEM image (b) HR-TEM image of BTT-CMP2. EDS mapping images of C (c) and S (d) for BTT-CMP2. (e) TGA curves of BTT-CMPs. (f) Nitrogen
adsorption-desorption isotherms of BTT-CMPs measured at 77 K (adsorption: filled circle; desorption: open circle.
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levels of BTT-CMPs, the valence band X-ray photoelectron
spectroscopy (VB-XPS) was conducted. The VB edge of BTT-
CMP1, BTT-CMP2 and BTT-CMP3 was lied in 1.12 eV, 1.37 eV
and 1.14 eV, respectively, as illustrated in Fig. 3c. Taking the VB
microporous nature of BTT-CMPs. Based on the nitrogen value from the bandgap energy, therefore, their potential of
adsorption data, the Brunauer-Emmett-Teller (BET) surface conduction band (CB) was calculated to be −0.98 eV for BTT-
area was calculated to be 661 m2 g−1, 546 m2 g−1, 404m2 g−1, CMP1, −1.27 eV for BTT-CMP2, and −0.87 eV for BTT-CMP3.
which were equivalent to the reported BTT-based DA From Fig. 3d, we can clearly see that the energy band structures
polymers.42 The total pore volume at p/p0 = 0.99 was also of BTT-CMPs can be easily adjusted by changing the linkers. In
evaluated to be 0.563 cm3 g−1 for BTT-CMP1, 0.599 cm3 g−1 for addition, the potential of CB clearly indicates the reduction of
•−
BTT-CMP2 and 0.359 cm3 g−1 for BTT-CMP3, respectively.
O2 to O2 ( −0.33 eV vs NHE) was realized smoothly through
photoinduced electron transfer from BTT-CMPs to O2 molecule
under light illumination. And the driving force of BTT-CMP2 for
the electron transfer is larger than that of both BTT-CMPs.
Table 1. Optimization of the photocatalytic conditions for the synthesis of
benzimidazole by BTT-CMPsa
H
NH2
N
CMP
O
+
N
Light
O2
NH2
1
2a
3a
Fig.
3 (a) Solid-state absorption profiles of BTT-CMPs, insets: the
corresponding images. (b) Tauc plots of BTT-CMPs. (c) VB-XPS spectra of BTT-
CMPs. (d) Schematic diagram of energy band structure about BTT-CMPs and
•−
O2/O2
.
The influence of various linker on the photophysical
properties was researched. Firstly, UV/vis diffuse reflectance
spectroscopy of all BTT-CMPs were performed. As displayed in
Fig. 3a, BTT-CMP2 exhibited an intensive visible-light harvesting
with maximum peak at 431 nm (cyan line). In contrast, BTT-
CMP1 (gray line) and BTT-CMP3 (blue line) containing the
acetylene units showed a broader absorption and the band
edge extended to 800 nm. This also corresponds to their color
change, from green for BTT-CMP2 to brownish red for BTT-
CMP1 and BTT-CMP3 (Fig. 3a, inset). Accordingly, their optical
bandgap energy was determined from the corresponding Tauc
plot to be 2.10 eV for BTT-CMP1, 2.64 eV for BTT-CMP2 and 2.01
eV for BTT-CMP3, respectively (Fig. 3b). To evaluate the energy
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Catalyst
Solvent
Time (h) Yield (%)b
chloroform (CHCl3) under light illumination with a blue LED lamp
View Article Online
(30 W, 460 nm). However, methanol (MeDOOHI:)1p0r.1o0v3e9d/Da0TgAe10n2e3r2aFl
yield under identical reaction criteria (65%, Table 1, entry 6). In
sharp contrast, a high yield of 84% was acquired in ethanol
(Table 1, entry 7). Besides, the data of control experiments
pointed out that photocatalyst BTT-CMP2, light excitation and
oxygen gas are all essential for the model reaction (Table 1,
entries 8, 9). On the contrary, utilizing a green LED lamp (63%,
Table 1, entry 10) or white LED lamp (47%, Table 1, entry 11) as
light source, the relatively inferior yields were obtained,
respectively. In addition, the photocatalytic process was hardly
carried out in nitrogen gas (8%, Table 1, entry 12),
demonstrating the critical role of oxygen. it is widely shared that
superoxide radical anion (O2•−) as well as singlet oxygen (1O2) as
most significant reactive oxygen species play a critical role in the
oxybiotic photocatalysis. To ulteriorly explore this
photocatalytic mechanism in detail, a sequence of designed
experiments were implemented. For example, a 1O2 scavenger,
sodium azine (NaN3) was added into the catalytic device, a
moderate yield of 3a was gained (70%, Table 1, entry 13). By
contraries, 2,2,6,6-tetramethylpiperidine-1-oxyl (TMPO) and p-
benzoquinone (BQ) were injected respectively as radical and
O2•− scavenger in the normative conditions, the yield of product
sharply reduced to 15% (Table 1, entry 14) and 7% (Table 1,
entry 15). Therefore, these findings manifested that the
reactive oxygen species O2•− is an important intermediate in this
photocatalytic reaction. Indeed, the BTT-CMP2 can effectively
1
2
BTT-CMP2
BTT-CMP2
BTT-CMP2
BTT-CMP2
BTT-CMP2
BTT-CMP2
BTT-CMP2
~
DMF
1
1
12
16
10
14
20
65
84
25
22
63
47
8
CH3CN
THF
3
1
4
Toluene
CHCl3
1
5
1
6
CH3OH
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
1
7
1
8
1
9c
BTT-CMP2
BTT-CMP2
BTT-CMP2
BTT-CMP2
BTT-CMP2
BTT-CMP2
BTT-CMP2
BTT-CMP1
BTT-CMP3
g-C3N4
1
10d
11e
12f
13g
14h
15i
16
17
18
19
20
21
1
1
1
1
70
15
7
1
1
1
52
45
42
38
91
93
•−
promote the formation of blue-colored cationic radical and O2
upon light illumination, which was mediated through the
1
electron
transfer
from
N,N,N',N'-Tetramethyl-p-
phenylenediamine (TMPD) to molecular oxygen (Fig. 4a).44,45 In
addition, the electron paramagnetic resonance (EPR)
experiments were conducted in which 5,5-dimethyl-1-
pyrroline-N-oxide (DMPO) was used as O2•− scavenger. We can
see clearly from Fig. 4b, the four characteristic peaks
1
BTT-CMP2
BTT-CMP2
BTT-CMP2
0.5
2
•−
3
corresponding to O2 were also observed under light
illumination. Based on above observations and previous
reports38,39,46 for the photosynthesis of benzimidazoles, a
possible reaction mechanism is proposed (Fig. 5). First, 1 and
2a are condensed to form the imine intermediate I. Then
intramolecular cyclization occurs in I to give intermediate II.
Under light illumination, meanwhile, the photocatalyst
generates the excited species BTT-CMP2*, which is reductively
quenched by intermediate II via a single electron transfer
process to obtain intermediate III and radical anion BTT-CMP2•−.
Subsequently, BTT-CMP2•− is oxidized by molecule oxygen to
form O2•− and regenerate BTT-CMP2. Finally, the target product
3a was generated through deprotonation and dehydrogenation
processes by O2•− and hydroperoxyl radical HOO•, respectively.
aReaction conditions: photocatalyst (3.0 mg), benzaldehyde (0.26 mmol), 1,2-
diaminobenzene (0.26 mmol), Oxygen atmosphere (~1.0 atm), 460 nm blue
LED lamp with 30 W, 298 K, 60 min. bIsolated yield. cDark. d520 nm green LED
lamp with 30 W. eWhite LED lamp with 8 W. fIn N2. gNaN3 as the singlet oxygen
scavenger. hTEMPO as the radical scavenger. iBQ as the superoxide scavenger.
The BTT-CMPs have inherent porosity, excellent stability,
moreover, widespread harvesting in visible-light region and
good photoredox properties. From the basic principle, these
properties satisfied the needs for heterogeneous
photocatalysts. Therefore, we utilized the photosynthesis of
benzimidazole as a typical reaction to determine the optimal
photocatalyst from three BTT-CMPs. And the photocatalytic
performence of new CMPs for the synthesis of benzimidazole
was investigated, in which o-phenylenediamine (1),
benzaldehyde (2a), molecular oxygen were choosed separately
as reaction substrates, and eco-friendly oxidant. The effect of
different solvents on the reaction yield was first researched. A
low yield (10−20%, Table 1, entries 1−5) was procured by BTT-
CMP2 in aprotic solvents such as dimethylformamide (DMF),
acetonitrile (CH3CN), tetrahydrofuran (THF), toluene and
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obvious testimony for the photoinduced carries transfer in BTT-
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DOI: 10.1039/D0TA10232F
CMPs (Fig. 4c). In particular, BTT-CMP2 gave the strongest
transient photocurrent in BTT-CMPs, indicating the more
effective separation of the photogenerated electron/hole pairs.
Additionally, the EIS Nyquist diagram shows that BTT-CMP2 has
a smaller radius of arc than the other two BTT-CMPs,
demonstrating the charge transfer barrier was decreased with
increased charge density in BTT-CMP2 (Fig. 4d). The control
experiments indicated that the absorption intensity of blue
cationic radical solvents decreased gradually with changing
polymer from BTT-CMP2, BTT-CMP1 to BTT-CMP3 (Fig. S4),
indicating their similar trend for photogenerated O2•−ability. To
our delight, it is in good agreement with the photocatalytic
activity of BTT-CMPs. Furthermore, the state of art
heterogeneous photocatalyst g-C3N4 for this model reaction
was also investigated as a comparison with BTT-CMP2.
Surprisingly, an inferior yield (42%, Table 1, entry 18) for the
target product was gained, indicating the importance of BTT
network for photocatalytic activity. Finally, increasing reaction
time to 2 h for the model reaction by BTT-CMP2, a superior yield
of 91% was gained (Table 1, entry 20). However, the yield
increased only slightly when the reaction time was prolonged
(Table 1, entry 21). Accordingly, the optimized conditions were
determined as: 3.0 mg BTT-CMP2 as the photocatalyst, ethanol
as the reaction solvent, oxygen gas as the oxidant, a 30 W blue
LED lamp as the light source at 25 oC for 2 h.
Fig. 4 (a) Absorption profiles of the cationic radical of TMPD generated by
BTT-CMP2 under oxygen and light, insets: their corresponding photographs.
(b) ESR profiles of BTT-CMP2 (2.0 mg mL−1) and DMPO (0.1 M) in MeOH under
light irradiation and dark. (c) Transient photocurrent responses of BTT-CMPs
under light irradiation. (d) The EIS Nyquist plots of BTT-CMPs.
With screening the optimal photocatalytic conditions, we
next researched the photocatalytic benzimidazoles reactions of
multifarious aldehydes, such as aromatic and heterocyclic
aldehydes as well as cyclic ketone in the presence of BTT-CMP2.
As displayed in Table 2, a series of aromatic aldehydes
containing diverse electron-donating and electron-withdrawing
substituents were effectively transformed to give the
corresponding benzimidazole with fine to prominent yields.
When benzaldehyde possessed -F, -Cl, -Br, -CH3, -OCH3 and -OH in
the p-position (Table 2, 3a−3g), we can find that the yield of product
with electron-withdrawing groups is slightly higher than that with
electron-rich groups. Similarly, the same trend for the yield of target
products was obtained, when the substituent is in the o- and m-
positions (Table 2, 3h−3n). In particular, benzimidazole having a
nitro group in p-position needs longer reaction time to
Fig. 5 Proposed mechanism for photosynthesis of benzimidazole in the
presence of BTT-CMP2 under visible-light illumination.
compared with p-substituted substrates (Table 2, 3p). 2-
In addition, the activity of two polymers BTT-CMP1 and BTT-
CMP3 for photosynthesis of benzimidazole was still researched.
Interestingly, the reaction gave more moderate yield of 52% for
BTT-CMP1 (Table 1, entry 16) and 45% for BTT-CMP3 (Table 1,
entry 17) than that of BTT-CMP2 under same conditions, which
indicating the significance of linker structure in BTT-CMPs. It is
well known that the high-efficent production, separation and
transmission of photoinduced charge carrier in photocatalyst
network are also essential for enhancing photocatalytic activity.
Therefore, we investigated ulteriorly the transient
photocurrent response and electrochemical impedance
spectroscopy (EIS) of new polymer BTT-CMPs to gain insight into
them. Upon visible-light irradiation, all BTT-CMPs displayed
rapid photocurrent responses with quite a few reduplicative
cycles of intermittent on-off illumination, which gave an
thiophenaldehyde as a representative of heterocyclic aldehydes,
which transformed into the corresponding benzimidazole with
a good yield (Table 2, 3q), suggesting that the photocatalytic
system is still suitable for heteroaromatic aldehydes. In addition,
this system is also effective for the photosynthesis of
benzimidazole with a cycloalkyl group (Table 2, 3r). To our
delight, the photocatalytic activity of metal-free BTT-CMP2 is
compared with that of the previously reported organic dye and
noble metal based polymer.38,39
Table 2. Photocatalytic synthesis of benzimidazoles by BTT-CMP2a
6 | J. Name., 2012, 00, 1-3
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aReaction conditions: photocatalyst BTT-CMP2 (3.0 mg), 1,2-diaminobenzene
(0.26 mmol), aldehydes (0.26 mmol), ethanol (2.0 mL), oxygen (~1.0 atm), 460
nm blue LED lamp with 30 W, 298.15 K, bIsolated yield.
In the heterogeneous reactions, the reusing of the
photocatalyst is a prime merit for industrial applications.
Therefore, we explored the recovery and recycling of screened
BTT-CMP2 as a heterogeneous photocatalyst in synthesis of
benzimidazole under visible-light illumination using 1,2-
diaminobenzene and benzaldehyde as substrates. The
photocatalyst of BTT-CMP2 was facilely separated from the
reaction compounds via centrifugation. And the separated BTT-
CMP2 was washed with THF and H2O, dried in reduced pressure,
and then utilized the next photocatalytic reaction. We can
discover that the BTT-CMP2 can be reused and recycled for 10
cycles without distinct loss of photocatalytic activity (Fig. 6a). It
is satisfied that the recovered BTT-CMP2 samples also
maintained the original network connectivity (Fig. 5b),
aggregated morphology (Fig. 6c and 6d) and large surface area
(Fig. 6e, SABET = 463 m2 g−1), which is majorly due to its remarkable
stability. Finally, we also explored the large scale photosynthesis
of BTT-CMP2 using benzaldehyde and 1,2-diaminobenzene as
substrates. A marvelous yield of 96% was achieved in the large-
scale reaction process, implying its potential application in
industrial preparation (Scheme S1).
Fig. 6 (a) Reusability of BTT-CMP2 in the photocatalyzing synthesis of
benzimidazole of benzaldehyde and 1,2-diaminobenzene. (b) Photocatalyst
of BTT-CMP2 first and tenth cycles in FT-IR spectra. FE-SEM images of BTT-
CMP2 before (c) and after (d) ten cycles. (e) nitrogen adsorption-desorption
choroisotherm (77 K) of BTT-CMP2 after ten cycles (SABET = 463 m2 g−1).
Conclusions
In summary, we have donated a strategy for screening CMP-
based photocatalysts through the implementation of bandgap
engineering. Three benzotrithiophene-based CMPs with various
linkers were triumphantly designed and synthesized. The
porosity, photoelectric property and energy levels, moreover,
bandgaps of obtained CMPs can be easily adjusted by tuning the
structure of linkers. Thanks to more effective generation,
transfer, separation of the photoinduced carrier and stronger
production capacity of O2•−, BTT-CMP2 has the best catalytic
activity for visible-light-induced synthesis of benzimidazoles in
three CMP materials, even higher than that of the small
molecule photocatalysts and most progressive metal-free
photocatalyst g-C3N4. BTT-CMP2, regarding as a metal-free
photocatalyst, not only displayed satisfactory substrate
universality, but also showed appreciable recyclability under
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Journal Name
24. B. Wang, Z. Xie, Y. Li, Z. Yang, L. Chen, Macromolecules. 2018,
visible-light illumination and molecular O2. We highlight that
this work apart from contributes a fresh strategy for screening
CMP-based photocatalysts, also broadens the application of
CMP materials for sustainable development.
View Article Online
51, 3443−3449.
DOI: 10.1039/D0TA10232F
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Conflicts of interest
There are no conflicts to declare
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Journal of Materials Chemistry A
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A metal-free photocatalyst was screened from the same series of CMP materialVsiew Article Online
DOI: 10.1039/D0TA10232F
through the bandgap engineering strategy. It exhibited excellent photocatalytic
performances for synthesis of benzimidazoles with high activity and recyclability.