Polarization-induced charge separation in conjugated microporous polymers for efficient visible light-driven C-3 selenocyanation of indoles

Article abstract of DOI:10.1039/d0sc06951e

Conjugated microporous polymers (CMPs) are cost-effective photocatalysts in organic transformations, while they are usually limited by the insufficient separation of photogenerated charges. Here we report a polarization strategy through molecular geometry optimization to promote the charge separation of CMPs. Three CMP photocatalysts with an alternative donor-acceptor skeleton and tunable symmetry were synthesized by the oxidative coupling of bis-carbazoles with electron-deficient bridges (benzene/pyridine/pyrimidine). Simply regulating the polarization of the starting monomers leads to tailorable porosity, photoelectric properties, and photocatalytic activity of the CMPs. They exhibited high efficiency in C-3 selenocyanation of indoles under visible-light and at room temperature, and pyridine-based CMPs with the largest dipole moment gave a yield of up to 94%, superior to their state-of-the-art photocatalyst counterparts. Photo-physical experiments combined with theoretical calculations further supported that the incorporation of the polarized linker introduced an internal electric field, benefitting efficient charge separation. This offered new insight into developing high-performance photocatalysts.

Full text of DOI:10.1039/d0sc06951e

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Polarization-induced charge separation in
conjugated microporous polymers for efficient
visible light-driven C-3 selenocyanation of indoles†
Cite this: Chem. Sci., 2021, 12, 5631
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
a
b
a
a
a
a
Qiujian Xie, Yumin Yang, Weijie Zhang, Zhu Gao, Xiaofeng Li, Juntao Tang,*
a
a
Chunyue Pan and Guipeng Yu
*
Conjugated microporous polymers (CMPs) are cost-effective photocatalysts in organic transformations,
while they are usually limited by the insufficient separation of photogenerated charges. Here we report
a polarization strategy through molecular geometry optimization to promote the charge separation of
CMPs. Three CMP photocatalysts with an alternative donor–acceptor skeleton and tunable symmetry
were synthesized by the oxidative coupling of bis-carbazoles with electron-deficient bridges (benzene/
pyridine/pyrimidine). Simply regulating the polarization of the starting monomers leads to tailorable
porosity, photoelectric properties, and photocatalytic activity of the CMPs. They exhibited high efficiency
in C-3 selenocyanation of indoles under visible-light and at room temperature, and pyridine-based CMPs
with the largest dipole moment gave a yield of up to 94%, superior to their state-of-the-art
photocatalyst counterparts. Photo-physical experiments combined with theoretical calculations further
supported that the incorporation of the polarized linker introduced an internal electric field, benefitting
efficient charge separation. This offered new insight into developing high-performance photocatalysts.
Received 21st December 2020
Accepted 7th March 2021
DOI: 10.1039/d0sc06951e
rsc.li/chemical-science
developing efficient and eco-friendly photocatalysts for seleno-
cyanation of indoles are highly desirable.
Introduction
Indole derivatives play important roles in organic synthesis and
pharmaceutical chemistry.
As a unique class of materials, conjugated microporous
Specically, the selenium- polymers (CMPs), combining an extended p-conjugation skel-
1–3
containing indole derivatives have attracted broad interest as eton with a permanent microporous structure, have recently
10–13
they are not only pharmaceutical drug candidates but also attracted intensive interest targeting various utilities.
Espe-
versatile intermediates for the production of value-added cially, owing to their excellent chemical robustness and tunable
4
,5
14,15
compounds. Although C-3 selenocyanation is a convenient structures,
CMPs offer a valuable platform for developing
16
and efficient route to incorporate selenium moieties into heterogeneous photoredox catalysts. Since the rst CMP was
indoles, rare feasible synthetic protocols were presented and reported by Cooper's group in 2007, task-specied CMP pho-
17
most of them required the use of diverse toxic selenocyanate tocatalysts with functionality-built-in architectures, including
6–9
18
19
species and heavy-metal catalysts.
mediated selenocyanation, as
green and sustainable (BODIPY), carbazole and benzothiadiazole,
Metal-free reagent- rose bengal, perylene, dipyrrometheneboron diuoride
20 21 22,23
a
have been
process, which has been highlighted as a research trend, recently employed in photocatalytic hydrogen evolution and
unfortunately remained a signicant challenge. To the best of aerobic oxidation reactions. However, within the efforts for
our knowledge, the utilization of visible-light-driven C-3 sele- pursuing satisfactory performance, a limitation still exists in
nocyanation of indoles has not been reported yet. Hence, separating the photo-excited electrons and delivering them into
searching for an alternative photocatalytic process and the target regions for producing active species. Regulating the
24
linking pattern of building blocks, alternating the composi-
25
tion of donor and acceptor moieties, introducing electron-
26
27
output “tentacles” and creating heterojunctions have been
proved to be efficient strategies. For example, by incorporating
a
Hunan Key Laboratory of Micro & Nano Materials Interface Science, College of
Chemistry and Chemical Engineering, Central South University, Lushan South Road electron-rich pyrene and electron-decient dibenzothiophene-
9
32, Changsha 410083, Hunan, P. R. China. E-mail: gilbertyu@csu.edu.cn;
S,S-dioxide into the framework, Zhao et al. found that the
polymer with a 3,7-linking pattern (PyDOBT-1) exhibits superior
photocatalytic performance in hydrogen generation to that of
PyDOBT-2 with a 2,8-linking pattern. We also reported a core-
tailoring protocol through incorporating electron-rich or
Reynardtang@csu.edu.cn
b
Queen Mary University of London Engineering School, Northwestern Polytechnical
University, Youyi West Road 127, Xian 710072, Shaanxi, P. R. China
28
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d0sc06951e
1
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decient units to tailor the photophysical properties of (pyrimidine-2,5-diyl)-bis(9H-carbazole) were synthesized by the
29
carbazole-based CMPs [CMP-CSUs (s ¼ 5, 6, 7)]. Although such nucleophilic displacement chemistry of 1,4-dibromobenzene,
efficient photocatalytic performance regarding classic trans- 2,5-dibromopyridine and 2,5-dibromopyrimidine, respectively.
formations has been documented, the detailed boosting The carbazole CMPs (termed CMP-CSU5, CMP-CSU14, and
mechanism for photocatalytic properties for most CMPs is not CMP-CSU15) were obtained as insoluble solid powders via
well understood.
a FeCl
3
-catalyzed oxidative polymerization in anhydrous chlo-
) under a nitrogen atmosphere in high yields (85–
Herein, we report a polarization strategy to develop CMP pho- roform (CHCl
tocatalysts consisting of alternating electron-rich (carbazole) and 90%) (Fig. 1 and Scheme S1†). The chemical structure of the
decient (benzene, pyridine or pyrimidine) units. The polymers synthesized CMP-CSUs was proved by Fourier transform
were readily prepared through oxidative coupling of precursors infrared (FT-IR) and
with tuned polarity. We envisioned that the strong polarity in spinning solid-state nuclear magnetic resonance ( C CP/MAS
3
30
1
3
C
cross-polarization/magic-angle-
1
3
electron-decient units would offer a built-in electric eld effect in NMR) measurements. Partial FT-IR spectra demonstrated the
ꢀ1
CMPs, which might facilitate the rapid separation of photo- disappearance of absorption at around 721 cm (disubstituted
generated electrons and holes, therefore promoting the photo- phenyl ring of carbazole) as well as the emerged signals in the
ꢀ
1
catalytic activity. To validate this strategy, C-3 selenocyanation of range of 750–810 cm for the C–H vibration of the disubsti-
indoles was employed as a model reaction. Signicantly, among tuted carbazole ring, conrming the successful coupling (Fig. 2a
31
13
the three samples, an attractive yield of up to 94% and extensive and S1–S3†). In a typical C CP/MAS NMR spectrum of CMP-
substrate adaptability were achieved in the reaction catalyzed by CSU14, the peaks at 135 and 133 ppm were attributed to the aryl
pyridine-linked CMP-CSU14 with the largest dipole moment. carbons adjacent to the carbazole and pyridine nitrogen atom,
Compared to the known synthetic protocols, our CMP catalysts and that at 124 ppm to the carbon of substituted phenyl
32–34
function efficiently under mild conditions without the addition of carbons, further revealing the oxidative chemistry
(Fig. S4†).
any strong oxidants or toxic additives. The theoretical calculations The scanning electron microscopy (SEM) measurements show
also revealed that the natural dipole moments induced an internal that all CMP-CSUs have an irregular and aggregated
electric eld to force the migration of photogenerated electrons morphology with a particle size in the range of 100–300 nm
and holes in an opposite orientation, therefore enhancing the (Fig. S5†). The powder X-ray diffraction (PXRD) analysis sug-
photogenerated charge separation. Moreover, the excellent stability gested that all the collected polymers are amorphous materials
of these metal-free heterogeneous photocatalysts enabled a robust (Fig. S6†). The porosity features of the obtained polymers were
recycling capability with well-retained performance even over ve acquired from nitrogen (N ) sorption isotherms (77 K) (Fig. S7†)
2
consecutive runs, which is not available for the other known and the corresponding results are collected and summarized in
catalysts for selenocyanation.
Table S1.† According to the International Union of Pure and
Applied Chemistry classication, our polymers displayed type I
combined with type II nitrogen isotherm features and showed
a hierarchically porous structure combining mesoporosity with
Results and discussion
35
0
certain microporosity. All three samples gave considerably
Three carbazole monomers named 9,9 -(benzene)-bis(9H-
0
0
high Brunauer–Emmett–Teller (BET) specic surface area
carbazole), 9,9 -(pyridine-2,5-diyl)-bis(9H-carbazole), and 9,9 -
2
ꢀ1
(
exceeding 500 m g ) and the highest BET surface area
Fig. 1 The design strategy of the corresponding monomers and Fig. 2 (a) Partial FT-IR spectra of CMP-CSUs; (b) normalized solid state
oligomers of CMP-CSUs with polarized structures ((I) oligomer for UV/Vis absorption spectra of CMP-CSUs; (c) photoluminescence
CMP-CSU5, (II) oligomer for CMP-CSU15, and (III) oligomer for CMP- spectra (PL) of CMP-CSUs; (d) photoluminescence decay traces of
CSU14).
CMP-CSUs.
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3
ꢀ1
combined with a pore volume of 0.79 cm g was acquired for photoluminescence lifetimes of CMP-CSU5, CMP-CSU14, and
CMP-CSU5 (benzene-linked), which is in line with the previous CMP-CSU15 were measured to be 0.7 ns (s ¼ 0.50 ns, s ¼ 2.11 ns),
1
2
2
9
reports. CMP-CSU15 gave the lowest BET surface area (528.6 2.46 ns (s ¼ 0.39 ns, s ¼ 3.95 ns), and 2.56 ns (s ¼ 0.55 ns, s ¼
1
2
1
2
2
ꢀ1
43
m
g
), possibly due to the low reactivity of its precursor 4.76 ns), respectively (Table S2†). The prolonged decay lifetime of
(
PymDCz) in the oxidative coupling reaction possibly due to the CMP-CSU14 implies its slower charge recombination and faster
36
electrophilicity of pyrimidine. Based upon NL-DFT calcula- electron transfer than the others, and alternately it could provide
tions, the pore-width-distributions of CMP-CSUs were centered more photo-induced carriers, indicative of improved intrinsic
44
at 1.0–1.7 nm and 2.0–5.0 nm (Fig. S8†). The hierarchical porous activity during the photocatalytic processes.
structure with over 71% mesoporosity appears to be an ideal On the basis of the promising photoredox properties of the
feature for organic transformations involving large-sized three CMP polymers, the possibility for their photocatalytic trans-
37
substrates or products in photocatalytic reactions.
formation was investigated in C–H functionalization for indoles.
The light harvesting properties and optical band gaps of the as- The C-3 selenocyanation of indoles was selected as the model
synthesized materials were assessed by diffuse reectance spec- reaction due to its great importance in constructing valuable
trum (DRS) measurements. Both CMP-CSU14 and CMP-CSU15 pharmaceutical drug candidates (Fig. S16†). By choosing 1H-indole
exhibited relatively broad absorption from 200 to 800 nm as a model substrate, potassium selenocyanate (KSeCN) as the
ꢀ
(
Fig. 2b), which indicates that the absorbed light of collected SeCN source, and molecular oxygen as a green oxidant, the photo-
38
polymers could cover a wide range of the visible region. Notably, reaction conditions were screened and optimized under visible-
CMP-CSU5 had a maximum absorption at 350 nm, and a distinct light (Tables 1 and S3†). A yield of 94% for the target product
blue-shi of the absorption peak compared with the other two was obtained by using CSU-CMP14 with tetrahydrofuran (THF) as
polymers with a different electron-decient linker. The optical the medium upon visible-light irradiation for at least 24 h (Table 1,
band gaps were derived from the transformed Kubelka–Munk entry 1). However, decreased yields (41% and 87%) were obtained
theory (Fig. S9†). As for CMP-CSU5, CMP-CSU14 and CMP-CSU15, when the catalyst was replaced by CMP-CSU5 and CMP-CSU15,
the energy levels of the lowest unoccupied molecular orbital respectively (Table 1, entries 2 and 3). To illustrate the inuence
(
LUMO) were recorded to be at ꢀ0.845, ꢀ0.834 and ꢀ0.822 V vs. of surface area of CMP-CSU14 and CMP-CSU15, the reaction time
ꢀ
SCE, respectively, and thus can reduce oxygen to generate O
2
c
was extended to 36 hours to fully make sure they reached the
ꢀ0.57 V vs. SCE) (Fig. S10†). The relative production of O2c in equilibrium state. The yields of the target product were measured
CMP-CSUs has been compared by the oxidation experiment with to be 95% and 86% when catalyzed by CMP-CSU14 and CMP-
39
ꢀ
(
0
0
N,N,N ,N -tetramethyl-1,4-phenylenediamine (TMPD) (Fig. S11†). CSU15. To better understand the relevance of surface area on the
The intensity variations in UV/Vis absorption spectra veried the indole C-3 selenocyanation activity, 1H-indole and CMP-CSUs were
ꢀ
superior O
2
c
production rate of CMP-CSU14 compared to CMP- transferred to a transparent reaction tube and the mixture was
CSU5 and CMP-CSU15. Accordingly, the calculated highest occu- stirred in the dark (Fig. S17†). The lowest adsorption rate for CMP-
pied molecular orbital (HOMO) energy levels were far beyond the CSU5 and the fast abdsorption rate for CMP-CSU15 were revealed,
ꢀ
level of E(SeCN
ꢀ
/cSeCN) (where SeCN represents the selenocyanate demonstrating that the surface area is not the key factor for
anion), which appears to be sufficient to trigger the selenocyana-
tion reaction (Fig. S12 and S13†).
Photoluminescence (PL) spectroscopy was employed to eluci-
date the charge carrier migration and separation behavior of the
samples. The PL intensity of CMP-CSU14 was quenched dramati-
cally in comparison to that of CMP-CSU5 and CMP-CSU15, indi-
cating the effective charge transfer and the inhibited charge carrier
Table 1 Selected catalysts and their catalytic efficiency for C-3 sele-
nocyanation of indoles
a
40
recombination (Fig. 2c). The rate of charge transfer was further
investigated by electrochemical impedance spectroscopy (EIS).
Among the three samples, the pyridine-bridged CMP-CSU14
exhibited the smallest radius in the Nyquist plots (Fig. S14†),
indicating the lowest charge transfer resistance as well as the
Conversion
(%)
b
Entry
Catalyst
Yield (%)
41
highest charge mobility. The highest photocurrent under light
irradiation was observed for CMP-CSU14 (Fig. S15†), which
suggests the most effective separation of photo-induced electrons
1
2
3
4
5
6
7
CMP-CSU14
CMP-CSU15
CMP-CSU5
Fluorescein
94
87
41
48
24
5
100
98
55
60
43
15
Trace
—
42
and holes as well as the fastest charge mobility. Moreover, the
production of transient photocurrent observed for our CMP-CSUs
is highly reversible and would repeat over ve straight ON/OFF
irradiation runs, indicative of high photoelectrochemical stability.
To quantify the lifetimes of the carrier behavior during the pho-
tocatalytic process, photoluminescence decay measurements
g-C
3
N
4
TiO
MoS
—
2
2
Trace
c
d
8
NR
a
Reaction conditions: 1H-indole (0.2 mmol), KSeCN (0.4 mmol, 2
ꢂ
equiv.), catalyst (10 mg), tetrahydrofuran (THF 2 mL), RT ¼ 25 ꢁ 2 C,
(Fig. 2d) were conducted, and the average photoluminescence
ꢀ2
24 hours, O
2
(ꢃ0.1 MPa), using a 14 W LED lamp (0.20 W cm ) as
b 1
lifetimes were calculated using the following equation: saverage
the light source.
standard). No catalyst. No reaction.
H NMR yield (using mesitylene as an internal
d
2
2
c
lifetimes
¼
(A
1
s
1
+
A
2
s
2
)/(A
1
s
1
+
2 2
A s ). The average
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determining the photocatalytic activities. We speculated that the generated SeCNc and CMP-CSU14, respectively. Next, the produced
+
ꢀ
1
intense difference was presumably ascribed to the different charge holes (CMP-CSU14 ) and reactive oxygen species (O c and O )
2
2
separation and transfer efficiency of the photocatalysts. The oxidized the selenocyanate anion to the selenocyanate radical, and
performance of the known state-of-the-art photocatalysts was also a radical intermediate (B) would be generated under the attack of
investigated under identical conditions (Table 1). Note that only the selenocyanate radical. Subsequently, the intermediate B was
moderate yields were detected when organic dyes such as uores- further oxidized to intermediate C, and afforded the terminal
cein were applied (Table 1, entry 6). Negligible yields or barely no product (3A) by giving out a proton.
products were found under the catalysis of g-C
Table 1, entries 7–9).
3
N
4
, TiO
2
and MoS
2
Aer establishing the standard conditions for the C-3 sele-
nocyanation of indoles, the general applicability and selectivity
(
Electron paramagnetic resonance (EPR) tests were carried out to of selenocyanation for diverse indole substrates were investi-
further explore the possible mechanism of the selenocyanation of gated. Moderate to good yields were obtained when incorpo-
C-3 indoles. When 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was rating electron-withdrawing indole derivatives such as 6-uoro,
ꢀ
employed as a superoxide radical (O
2
c ) scavenger, the intensity of 6-chloro, and 6-bromo substituted ones (Table 2, 3B–H).
the EPR signal (magnetic eld of 319–326 mT) of CMP-CSU14 was However, when electron-donating groups were anchored, the
obviously enhanced under light-irradiation, indicating the effective indole derivatives furnished the target products in lower yields,
ꢀ
production of O
oxygen ( O
2
c
upon photo-excitation (Fig. S18†). Singlet which may indicate that the electron-donating characteristic
1
2
) was also captured by 2,2,6,6-tetramethylpiperidine has negative effects on the C-3 selenocyanation (Table 2, 3I–M).
(TEMP) under light irradiation in the presence of CMP-CSU14, and When a methyl or phenyl unit was incorporated at the C-2
the signals showed a moderate enhancement compared to that in position, corresponding product yields decreased to 72% or
the dark (Fig. S19†). To further investigate the effect of the pho- 48%, respectively (Table 2, 3J–K). This may suggest that the
togenerated electron–hole pairs and intermediates during C-3 steric hindrance and the electron-donating effect at the C-2
selenocyanation of indoles, different scavengers were charged position also attenuated the photocatalytic performance. For
into the reaction mixture under light irradiation (Fig. S20†). In the those derivatives with N-substituted groups, like N-methyl-
presence of a hole scavenger (KI), a decreased yield of 81% was indole (Table 2, 3N), the C-3-selenocyanation product was
obtained, which illustrated that the holes participate in the reac- afforded in a yield of 81%. CMP-CSU14 was selected as a repre-
tion but do not play a critical role. The addition of benzoquinone sentative photocatalyst to demonstrate the recycling capability.
that captures superoxide effectively led to a signicantly low yield Our CMP photocatalyst could be re-employed at least ve times
(15%). By adding a singlet oxygen scavenger (sodium azide), the without substantial decline of photocatalytic efficiency
corresponding product was obtained with a moderate yield of 42%. (Fig. S21†). Meanwhile, the structural features of the catalyst
However, a negligible yield (8%) was acquired when 2,2,6,6-tetra-
methylpiperidine 1-oxyl (TEMPO) was utilized to quench the
a
radical. The control experiments indicate that the reactive oxygen Table 2 Substrate scope for C-3 selenocyanation of indoles
ꢀ
1
species, i.e. O c and O play vital roles in the C-3 selenocyanation
2 2
of indoles. Based on these results, a putative reaction mechanism
for indole C-3 selenocyanation was proposed (Fig. 3). Under light
irradiation, CMP-CSU14 was triggered to form CMP-CSU14* and
ꢀ
quenched by oxygen to generate both superoxide radical (O
2
c ) and
+
CMP-CSU14 via a singlet electron transfer (SET) process. Via the
procedure of energy transfer of the triplet electrons, the generated
1
45
singlet oxygen ( O
2
) would participate in the reaction to oxidize
the intermediate B. The photoredox cycle was completed by a single
ꢀ
+
electron transfer (SET) between SeCN and CMP-CSU14 , which
a
Reaction conditions: indoles (0.2 mmol, 1 equiv.), KSeCN (0.4 mmol, 2
ꢂ
equiv.), CMP-CSU14 (10 mg), THF (2 mL), RT ¼ 25 ꢁ 2 C, 24 hours, O
2
ꢀ2
(
ꢃ0.1 MPa), using a 14 W LED lamp (0.20 W cm ) as the light source.
1
Fig. 3 Proposed mechanism for the C-3 selenocyanation of indoles.
H NMR yield (using mesitylene as an internal standard).
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such as primary skeleton connectivity and morphology were shi of emission maxima was up to 68 nm for CMP-CSU14 with the
retained, with limited changes in the FT-IR spectra along with largest dipole moment (Fig. 4f). We speculated that the dipole
the SEM images aer the photocatalytic cycles (Fig. S22 and moment will induce an internal electric eld that was considered to
48
S23†).
be useful for electron–hole separation upon photoexcitation.
To gain deeper insights into the deciding factor for the attractive Considering the precursor (PyDCz) of CMP-CSU14, the orientation
photocatalytic performance, the geometry of the starting mono- of the induced internal electric eld is opposite to that of the
49
mers and oligomeric precursors was optimized by density func- natural dipole moments (Fig. 5a). Therefore, the separation of
tional theory (DFT) calculations (B3LYP functional, 6-31G(d) basis photogenerated electrons and holes in an opposite orientation is
46
set). As a consequence of the selected nitrogen doping, changes in effectively under the force of the internal electric eld.
the dihedral angle between the central aryl plane and the carbazole To further understand the electron–hole pair separation in
unit could be expected. The optimal geometry of these monomers the monomers of CMP-CSUs, electronic transitions among the
exhibited distinct twisted degrees and dipole moments (Fig. 4a–c monomers were calculated based on time-dependent density
ꢂ
ꢂ
and S24†). BzDCz shows a slightly higher value (54.5 and 54.5 ) functional theory (TD-DFT) calculations (CAM-B3LYP func-
ꢂ
ꢂ
50
compared to pyridine-based PyDCz (35.7 and 55.2 ) and PymDCz tional, 6-31G(d) basis set). The electronic transitions of the
0.07 and 59.2 ). It is noted that PyDCz possesses the largest dipole monomers were obtained by using a multifunctional wave-
moment (1.65522 debye), followed by a decreasing trend with function analyzer (Multiwfn).
ꢂ
ꢂ
(
51–53
To investigate the electron
PymDCz of 0.90407 debye and BzDCz of 0.00002 debye. In addi- and hole distribution, the optimized geometries of the three
tion, the polarization of CMP-CSUs was further revealed by their monomers in the excited state were divided into three frag-
corresponding oligomers (Fig. S25†). With the introduction of ments (Fig. 5b). In BzDCz, the electrons and holes are mainly
nitrogen atoms, the dipole moment of the oligomeric precursors distributed in the carbazole part, indicating the lowest effi-
was gradually increased from 0.006 debye (for CMP-CSU5) to 4.177 ciency of photoexcited separation. However, for the other two
debye (for CSU-CMP14). The polarization of CMP-CSUs would also monomers, the electrons are mainly distributed in the electron-
47
be identied by the uorescence spectroscopic measurements.
decient part (pyridine and pyrimidine) and the holes are
The shis of the maximum emission wavelength were detected for spread over the carbazole units, demonstrating the efficient
the polymers with diverse natural dipole moments. As depicted in electron–hole separation. Transition density matrix heat map
Fig. 4d, a minimum shi of 36 nm of CMP-CSU5 was found in ethyl (TDM-HM) of excited monomer fragments revealed that the
acetate compared to that in hexane. A slight shi could be attrib- electron transferred from carbazole to the electron-decient
uted to the weakest dipole moment of CMP-CSU5 with minimum unit pyridine or pyrimidine (Fig. 5c and S26a†). Notably, the
polarization. In contrast, for CMP-CSU15, a distinct shi of emis- degree of electron–hole separation is higher for PyDCz
sion maxima was demonstrated with an increase over 52 nm in compared to that of PymDCz, which could illustrate the
ethyl acetate relative to that in hexane (Fig. 4e). Signicantly, the
Fig. 5 (a) The mechanism of the internal electric field for the sepa-
Fig. 4 Internal electric field (E , E , E ), configuration and natural ration of photogenerated electrons and holes; (b) electron and hole
1 2 3
dipole moment of (a) BzDCz, (b) PyDCz, and (c) PymDCz; PL emission distribution of excited monomer fragments (green represents elec-
spectra of (d) CMP-CSU5, (e) CMP-CSU14, and (f) CMP-CSU15 in trons; blue represents holes); (c) transition density matrix heat map of
different polar solvents (each suspension contains 0.2 mg sample and electron and hole contribution distribution of excited monomer
4
mL solvent; EA-ethyl acetate; DCM-dichloromethane; Hex-hexane). fragments.
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superior photocatalytic performance of CMP-CSU14 over CMP-
Synthesis of CMP-CSU14. The synthesis method of CMP-
CSU15. Since the nitrogen atom (N5) only distributes elec- CSU14 was almost the same as that for CMP-CSU5 starting
trons in the electron decient part (pyridine) in the excited state from PyDCz and the desired product was obtained as a dark
(Fig. S26b†), we inferred that the nitrogen atom could also act as yellow solid.
an electronic output site of CMP-CSU14 for transferring the
electrons to the substrate.
Synthesis of CMP-CSU15. The synthesis method of CMP-
CSU15 was almost the same as that for CMP-CSU5 starting
from PymDCz and the desired product was obtained as a yellow
solid.
Conclusions
In summary, we have demonstrated a structural design of
conjugated microporous polymer catalysts based on regulating
the dipole moment of the starting monomers or precursors.
Photophysical characterization as well as DFT calculations
revealed that the polarization induced internal electric eld has
a crucial impact on the photocatalytic activity of the obtained
networks. Remarkably, the pyridine-bridged CMP-CSU14 was
found to be a promising heterogeneous photocatalyst with the
highest catalytic performance for the C-3 selenocyanation of
indoles, being an attractive alternative to traditional metal-
based catalysts. The high photocatalytic efficiency of CMP-
CSU14 could be attributed to the effective separation of the
photo-induced electron–hole pairs and the stronger generation
ability of active oxygen species. This study also provides
a possible strategy to precisely tailor the optoelectronic prop-
erties of CMPs, which not only offers new insight into devel-
oping high performance organic photocatalysts, but also opens
General procedure for C-3 selenocyanation of indoles based
on CMP-CSU14
Indoles (0.2 mmol), potassium selenocyanate (KSeCN)
(
(
0.4 mmol, 2 equiv.), tetrahydrofuran (2 mL) and CMP-CSU14
10 mg) were transferred to a transparent reaction tube. The
reaction mixture was degassed and protected with an O
2
balloon (ꢃ0.1 MPa), and then stirred under irradiation with
ꢀ
2
a light source (a 14 W blue LED, 0.20 W cm , distance app. 8
cm) for 24 hours. Aer completion of the reactions, the corre-
sponding yields were determined with an internal standard
(
mesitylene). The obtained residue was puried by column
chromatography on silica gel to obtain the target product
PE : DCM ¼ 10 : 1–1 : 2).
(
Author contributions
the gate for extending the ideas to other photocatalytic organic Guipeng Yu, Chunyue Pan, and Juntao Tang proposed the idea
transformations.
and designed the experiments. Qiujian Xie performed the
experiments and carried out the structural characterization.
Weijie Zhang helped to analyse the results of photocatalytic
experiments. Zhu Gao helped with the structural characteriza-
tion analysis. Xiaofeng Li helped to synthesize the materials.
Experimental
Chemicals
2,5-Dibromopyridine, 2,5-dibromopyrimidine, 1,4-dibromo- Yumin Yang performed the DFT calculations. Qiujian Xie,
benzene, 9H-carbazole, copper(I) iodide and 1,10-phenanthro- Juntao Tang, and Guipeng Yu wrote the manuscript with
line were purchased from Aldrich and used without contributions from all the authors.
purication. Nitrobenzene (PhNO
2
) and anhydrous ferric chlo-
ride (FeCl ) were obtained from Aladdin Industrial Corporation
3
Conflicts of interest
(
(
Shanghai, China). Other solvents such as dichloromethane
DCM), petroleum ether (PE), ammonia solution and tetrahy-
There are no conicts to declare.
drofuran were obtained from commercial sources and used
without further purication. The purication of chloroform
(
(
CHCl
CaH ) and stored over 4A molecular sieves prior to utilization.
Synthesis of CMP-CSU5. Under an nitrogen atmosphere,
BzDCz (0.5 g, 1.2 mmol) was dissolved in anhydrous chloroform
30 mL) and then added dropwise to a suspension of ferric
3
) was conducted by distillation over calcium hydride Acknowledgements
2
We acknowledge the nancial support from the National
Natural Science Foundation of China (No. 21674129, 51873232
and 21636010). We also acknowledge the NMR measurements
by the Modern Analysis and Testing Centre of Central South
University.
(
chloride (1.15 g, 6.9 mmol) in anhydrous chloroform (25 mL).
Aer the solution mixture was stirred at room temperature for
48 hours, methanol (100 mL) was added to the reaction mixture
and kept stirring for 1 hour to quench the reaction. The
precipitate was collected and washed with 250 mL methanol,
and stirred vigorously in hydrochloric acid solution (37%) for 12
hours to eliminate the ferric ions. The obtained materials were
further puried by Soxhlet extraction with methanol (24 h),
Notes and references
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20 C in a vacuum for 12 hours. The desired product (CMP-
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5636 | Chem. Sci., 2021, 12, 5631–5637
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©
2021 The Author(s). Published by the Royal Society of Chemistry
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