Pillar[5]arene based conjugated macrocycle polymers with unique photocatalytic selectivity

Article abstract of DOI:10.1016/j.cclet.2020.04.020

The development of heterogeneous catalysts with substrate shape, size or electronic constitution selectivity is a huge challenge in photocatalysis. Reported herein is a host-guest interaction strategy to endow photocatalysts with special selectivity. By a

Full text of DOI:10.1016/j.cclet.2020.04.020

Journal Pre-proof
Pillar[5]arene based conjugated macrocycle polymers with unique
photocatalytic selectivity
Hui Qiang, Tao Chen, Zhuo Wang, Wenqian Li, Yunzhe Guo, Jie
Yang, Xueshun Jia, Hui Yang, Weibo Hu, Ke Wen
PII:
S1001-8417(20)30238-2
https://doi.org/10.1016/j.cclet.2020.04.020
CCLET 5599
DOI:
Reference:
To appear in:
Chinese Chemical Letters
Received Date:
Revised Date:
Accepted Date:
4 March 2020
27 March 2020
9 April 2020
Please cite this article as: Qiang H, Chen T, Wang Z, Li W, Guo Y, Yang J, Jia X, Yang H, Hu
W, Wen K, Pillar[5]arene based conjugated macrocycle polymers with unique photocatalytic
selectivity, Chinese Chemical Letters (2020), doi: https://doi.org/10.1016/j.cclet.2020.04.020
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Chinese Chemical Letters
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Communication
Pillar[5]arene based conjugated macrocycle polymers with unique photocatalytic
selectivity
Hui Qianga, b, Tao Chenb, c, Zhuo Wangd, Wenqian Lib, c, Yunzhe Guob, c, Jie Yangd, Xueshun Jiaa, Hui
b
b,
b, d,*

Yang , Weibo Hu , Ke Wen
a College of Science, Shanghai University, Shanghai 200444, China
b Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, China
c University of Chinese Academy of Sciences, Beijing 100049, China
d School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China
Graphical Abstract
Enhanced catalytic photo-oxidation efficiency of the pillar[5]arene-contained conjugated organic polymers (CMP-1 and CMP-2)
toward their guest-like substrate S-1 was demonstrated and a ‘host-guest’ interaction related mechanism was proposed. The conversion
yield of S-1 achieved near 18 folds over the non-guest –like substrate S-2 in the case that CMP-2 was used as a photocatalyst.
ARTICLE INFO
ABSTRACT
Article history:
The development of heterogeneous catalysts with substrate shape, size or electronic constitution
selectivity is a huge challenge in photocatalysis. Reported herein is a host-guest interaction
strategy to endow photocatalysts with special selectivity. By adjusting the precursors,
conjugated macrocycle polymers (CMPs) with pillar[5]arene struts (CMP-1 and CMP-2 ) and a
corresponding non-pillar[5]arene-contained conjugated organic polymer (COP-1) were prepared
Received 4 March 2020
Received in revised form 27 March 2020
Accepted 31 March 2020
Available online
 Corresponding authors.
E-mail addresses: huwb@sari.ac.cn (W. Hu), wenk@sari.ac.cn (K. Wen)

and the photocatalytic activities toward sulfide derivatives were investigated. The sulfides
showed similar conversions when COP-1 was used as a photocatalyst, but exhibited significant
differences when it turned to the CMPs. Remarkably, the conversion yield of S-1 achieved near
18 folds over the one of S-2 when CMP-2 was used as a catalyst. Mechanism studies confirmed
that the “host-guest” effect of pillar[5]arene struts in CMPs was the main cause of the
difference. The present work establishes CMPs as novel heterogeneous photocatalysts with
substrate selectivity, and such a method will inspire the researchers concerning preparation of
heterogeneous catalysts with excellent selectivity.
Keywords:
Heterogeneous photocatalysis
Selectivity
Host-guest
Pillar[5]arene
Sulfide
To alleviate energy crisis, design and preparation of photocatalysts which can efficiently convert solar energy into chemical energy
have drawn increasing interests in recent years [1-7]. Various photocatalytic systems, including inorganic semiconductor, organic
semiconductor [8,9], and metal-organic complex [10-12] have been intensely developed. Among them, organic semiconductors [13-18],
such as conjugated organic polymers (COPs) [19-23], possessing advantages of facilely tailored functionality, thermal stability,
recyclability and environmental friendliness, have gained much attention recently. Indeed, excellent photocatalytic activity has been
achieved by some functional COPs [24-31], but substrate selective photocatalysis have rarely been reported. As such, strategies which
enable photocatalysts with substrate shape, size or electronic constitution selectivity are very welcome.
Enzymatic reactions possess highly substrates selectivity, which is partially due to their binding specificity [28, 32-36]. Similar to
enzymatic reactions, interactions between the reactant molecules and the catalysts is an important process for a heterogeneous
photocatalysis. Hence, once selective adsorption occurred, substrates selective photocatalysis could be achieved. It is well-known that
macrocyclic hosts could bind guest molecules selectively [37-42]. For example, pillar[5]arenes tend to include linear chain molecules
into their cavities [43-46], and the binding constants vary with different functional groups of the linear chain molecules [47]. Recent
studies have indicated that solid materials with macrocyclic host motifs could adsorb specific molecules [48-60]. Therefore, developing
conjugated macrocycle polymer (CMPs) [61] photocatalysts, a class of organic semiconductors possessing macrocyclic host motifs,
might be a feasible way to endow photocatalysts with special substrate selectivity.
Herein, we report a type of conjugated macrocycle polymers with pillar[5]arene struts (CMP-1 [61] and CMP-2) for selective
photocatalytic oxidation of sulfides. A corresponding non-pillar[5]arene-containing conjugated organic polymer (COP-1) was also
prepared for comparison. Electron spin resonance (ESR) studies indicated that the CMPs and COP-1 all possessed the abilities to
1
convert O2 or H2O to active oxygen species (•OH or O2) under visible light irradiation to initiate the oxidation of sulfides. COP-1
showed similar photocatalytic activity toward the oxidation of sulfides with different substituents. CMP-1 and CMP-2 were found to
exhibit much higher activity toward a guest-like sulfide than a non-guest-like one. The influence of the host-guest nature of the
pillar[5]arene struts in the CMPs on the photocatalytic oxidation of the guest-like sulfide was confirmed by competitive experiments.
The CMPs and COP-1 were prepared according to literature reports (details are deposited in Supporting information) [61,62]. The
formation of the polymers was first characterized by Fourier transform infrared spectroscopy (FTIR). The disappearance of the C-H
stretching band around 3250 cm-1 of the alkynes and the trifluoromethane sulfonate group at 931 cm-1 of pillar[4]arene[1]1,4-
phenylenebistriflate indicated total consumption of the starting materials (Figs. S1a-c in Supporting information). 13C cross-polarization
magic-angle spinning (CP-MAS) NMR analysis provided further evidence for the formation of the polymers. Both CMP-1 and CMP-2
showed clear signals at ~30, ~55, ~90 and ~170 ppm, which could be assigned to the carbons of methoxy, methylene of the
pillar[5]arene moiety, and carbons of the C≡C and triazine C=N units, respectively (Fig. 1d and Fig. S1d in Supporting information).
Similarly, in the (CP-MAS) NMR spectra of COP-1, the signals at ~170 ppm assigned to triazine C=N units and ~90 ppm assigned to C
≡C units were observable (Fig. 1d).
Scanning electron microscopy (SEM) was used to study the morphologies of the polymers. Irregular particles with different sizes for
the as-synthesized conjugated polymers of CMP-1 and COP-1 could be observed in the SEM images, and the particles were inclined to
be independent with each other (Figs. 1a-b). Interconnected particles could be found in CMP-2 (Fig. S1e in Supporting information).
Brunauer−Emmett−Teller (BET) surface areas were calculated from N2 adsorption/desorption isotherm at 77 K. CMP-1 and CMP-2
exhibited type-II iso-thermal curve, and the BET areas were only 23.67 m2/g and 8.56 m2/g respectively. The low BET areas of CMP-1
and CMP-2 were speculated to be related the non-porous structure of pillar[5]arene, which could also be found in some previous
reports [47]. COP-1 exhibited type-IV isothermal curve with hysteresis loop at high absolute pressure value which indicated complex
micro-mesoporous structure and the BET areas was 482.63 m2/g. The pore volume of COP-1 was 0.12 cm3/g, while the ones of CMP-1
and CMP-2 were only 0.007 cm3/g and 0.0012 cm3/g respectively. The pore size distributions of polymers were calculated by nonlocal
density functional theory (NLDFT) method (Fig. S2c-d in Supporting information). Thermogravimetric analysis (TGA) revealed that
the polymers were stable at 300 °C (Fig. S2b in Supporting information). UV-vis diffuse reflectance (DRS) spectra showed that the
absorption bands of the polymers extended to the visible light region (Fig. 1c), indicating the possible use of the polymers as
photocatalysts under visible light irradiation.

Scheme 1. The synthetic route of the CMP-1 (a); the chemical structures of CMP-2 and COP-1 (b); the chemical structures of S-1 and
S-2 (c).
Fig. 1. SEM images of COP-1 (a) and CMP-1 (b); UV-vis diffuse reflectance (DRS) spectra of CMPs and COP-1 (c); 13C CP-MAS
NMR of COP-1 and CMP-1 (d).

Photocatalytic performances of the polymers toward the oxidation of sulfides with different substituents were investigated. The
experiments were carried out at room temperature, with a 200 W blue LED lamp (420 - 460 nm) as light source, using open vessels
1
under air, with photo-irradiated time for 10 h, 20 h and 30 h. The conversion yields of the sulfides were calculated by a H NMR
spectroscopic method [63], using the formula of Conv.(%) (SO-1 or SO-2) = Integration b/Integration [(a + b + c)] × 100% and
Conv.(%) (SF-1 or SF-2) = Integration c/Integration [(a + b + c)] × 100%, where “a”, “b”, “c” stand for the integrated peak area of
characteristic H of sulfides, sulfoxides, and sulfones in the corresponding 1H NMR spectra, respectively (details in Supporting
information). The conversion yields of the sulfides with photo-irradiated time of 10 h, 20 h and 30 h were shown in Table 1. With the
photo-irradiated time increased from 10 h to 30 h, the conversion yield of S-2 to the corresponding sulfoxide (SO-2) catalyzed by both
pillar[5]arene-based CMPs kept lower than 10%, while the conversion yield of S-1 to SO-1 increased with the increase of irradiation
time and finally reached 80% in 30 h irradiation (Figs. S26-39 in Supporting information). The conversion yields of S-2 and S-1 to their
corresponding sulfoxides catalyzed by COP-1 increased with the irradiation time and reached 93% and 92%, respectively, in 30 h
irradiation. The substrate selectivity was examined for pillar[5]arene-based CMPs catalysts as well as COP-1. It can be found that
when COP-1 was used as a catalyst, both sulfides (S-1 and S-2) showed similar conversion yields after 30 h irradiation (92% and 93%).
However, when catalyst CMP-1 was used, the conversion yields of S-1 and S-2 exhibited significant difference. The conversion yield
of S-1 to SO-1 was 80%, but S-2 to SO-2 was only 10%. The conversion yield ratio for S-1:S-2 was 8:1. Similar phenomenon could be
found in the case of catalyst CMP-2, with the conversion yields of S-1 to SO-1 and S-2 to SO-2 being 53% and 3%, respectively, and a
conversion yield ratio for S-1:S-2 was nearly 18 : 1. Prolonged irradiation time after 30 h did not increase the conversion yields of the
products. CMP-1 showed a higher photocatalytic activity than CMP-2, which might result from the triazine units of CMP-1 that cause
better electron-hole separation. Interestingly, in the cases of CMP-1 and CMP-2, the conversion yields of S-1 and S-2 seemed to be
proportional to the binding strength of pillar[5]arene toward the sulfides. As reported before, pillar[5]arenes can effectively bind linear
dinitriles with the binding constant (Ka) of 104 L/mol for hexanedinitrile in CDCl3 [46-47, 64]. Phenyl group is too larger to be included
by the pillar[5]arene cavity [43, 47].
Table 1
The photocatalyzed oxidization reaction of sulfides in the air.a
Entry Catalyst
Sulfide
Conv. (%)
10 h
20 h
30 h
SO
31
-
23
-
36
38
-
SF
-
-
1
-
-
1
-
-
SO
74
-
38
-
79
91
-
SF SO
SF
2
-
1
-
4
1
-
1
2
3
4
5
6
CMP-1
CMP-1
CMP-2
CMP-2
COP-1
COP-1
-
S-1
S-2
S-1
S-2
S-1
S-2
S-1
S-2
2
-
2
-
80
10
53
3
4
5
-
92
93
-
7b
8b
a
-
-
-
-
-
-
Reaction conditions: sulfide (1.0 mmol), CMPs/COP (20.0 mg), in a mixed solvent of CH3CN/H2O (6.0 mL, v/v 2:1) under air at room
temperature, irradiated with a 200 W blue LED lamp as the light source.
b Blank tests, irradiation time: 30 h, without catalysts but same reaction conditions, the conversion was hard to detect.
To confirm the presumption that the host-guest interaction induced catalytic difference in the CMPs photocatalytic systems,
1
corresponding oligomers of the CMPs and COP-1 (O-CMP-1, O-CMP-2 and O-COP-1) were prepared and H NMR experiments
were conducted (Fig. 2a and Figs. S3a-b in Supporting information). Upon addition of O-CMP-1 to the CDCl3 solution of S-1, upfield
shift and broadening for the proton signal of S-1 was observed, suggesting the existence of host−guest interaction between S-1 and the
1
pillar[5]arene moieties of O-CMP-1. Such a change was not observed in the H NMR spectrum of S-1 upon addition of O-COP-1,
1
indicating no interaction existed between S-1 and O-COP-1. In the case of S-2, no change could be observed in its H NMR spectrum
upon addition of both O-CMP-1 and O-COP-1 (Fig. S3b), implying no interaction existed between S-2 and O-CMP-1 (O-COP-1).
Similarly, host-guest interaction can be observed in the 1H NMR spectrum of S-1 and O-CMP-2, but not in the 1H NMR spectrum of S-
2 and O-CMP-2.
Competitive experiments were used to evaluate the host-guest effect of pillar[5]arenes on the photocatalytic oxidation of S-1 (Fig.
2b). Butanenitrile (BN), a well-known guest molecule for pillar[5]arenes [65,66], was used as a competitive guest molecule to bind the
pillar[5]arene struts, weakening the interaction of the CMPs and S-1. As expected, by introducing excess BN into the system of CMP-1
and S-1, a remarkable change was observed in the conversion yield of S-1, dropping from 80% to 18%. Similar phenomenon can be
found in CMP-2 involved photocatalytic system. However, under the same reaction conditions, no obvious change was observed in the
conversion yield of S-1 when COP-1 was used as a photocatalyst. Therefore, we could concluded that the host-guest interaction of
pillar[5]arene moieties in the CMPs with the substrate was vital to catalytic performance of the CMPs.

1
Fig. 2. Partial H NMR (CDCl3, 500MHz) spectra of S-1 (5.0 mg), S-1 (1.0 mg) + O-COP-1 (5.0 mg) and S-1 (1.0 mg) + O-CMP-1
(5.0 mg) (a); The conversion yield of S-1 to product SO-1 with(out) the presence of excess butanenitrile (b).
Table 2
The photocatalyzed oxidization reaction of sulfides in the nitrogen with photo-irradiated time for 30 h.a
Entry
Catalyst
Sulfide
Conv. (%)
SO
41
8
-
-
SF
3
-
-
-
1
2
3
4
5
6
CMP-1
CMP-1
CMP-2
CMP-2
COP-1
COP-1
S-1
S-2
S-1
S-2
S-1
S-2
88
90
3
-
a Reaction conditions: sulfide (1.0 mmol), CMPs/COP (20.0 mg), with the solvent of CH3CN/H2O (6.0 mL, v/v 2:1), in the nitrogen at room
temperature, irradiated with a 200 W blue LED lamp as the light source.
Electron spin resonance (ESR) was used to explore the photocatalytic mechanism of the polymers on oxidation of sulfides under
visible light. In an O2 saturated toluene solution, with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the probe, the polymers all
displayed clear four-line spectra of DMPO-•OH adducts after blue light irradiation, and the corresponding signal intensity increased
with the irradiation time, indicating that all the polymers could generate •OH under visible light irradiation. By changing the spin trap
matter from DMPO to 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO), three-line spectra, which were the characteristic signal for the
1
spin adduct formed by TEMPO and O2, were observed in the systems (Figs. S4a-d in Supporting information), indicating that the
polymers could generate 1O2 under visible light irradiation. •OH and 1O2 are all reactive oxygen species which can attack the S group in
the oxidation reaction of sulfides.
As •OH either originated from the solvent H2O or desolved O2, ESR spectra in N2 saturated ACN/H2O were examined. Except the
CMP-2/DMPO system, clear four-line spectra of DMPO–•OH adducts were detected in the systems of COP-1/DMPO and CMP-
1/DMPO after blue light irradiation, indicating that COP-1 and CMP-1 could efficiently convert H2O to •OH under visible light
irradiation (Figs. S4e-g in Supporting information). Photocatalytic performance of all the polymers in the oxidation of S-1 and S-2
under N2 atmosphere were assessed (Table 2). The result showed that except CMP-2, CMP-1 and COP-1 possessed certain catalytic
activity under N2 atmosphere toward the oxidation of sulfides, but the conversion yields were relatively lower than the ones in the air.
The results were consistent with the ESR, indicating COP-1 and CMP-1 could photocatalytically convert H2O to •OH to oxidize the
sulfides to sulfoxides.
Based on the above observations, a mechanism for CMP-1 (COP-1) involved photocatalytic oxidation reaction was proposed. Under
light irradiation, electron-hole pairs are generated in the polymers which activate H2O or solvated O2 to produce active oxygen species,
the active oxygen species can subsequently oxidize the sulfides to their corresponding sulfoxides. Accessibility of the active oxygen
species to the sulfide substrates is crucial to catalytic efficiency. In the system of CMP-1, pillar[5]arene motifs tend to bind guest-like
sulfide substrates and increase the availability of activated oxygen species to the bonded sulfides. Consequently, the oxidation rate of
guest-like sulfide (S-1) is accelerated. In the system of COP-1, no selective binding exists, and therefore similar photocatalytic
activities of COP-1 toward both sulfides were observed.
In summary, we have demonstrated a host-guest interaction strategy to endow photocatalysts with special selectivity. By adopting
conjugated macrocycle polymers with pillar[5]arene struts (CMP-1 and CMP-2), enhanced oxidation efficiency of the guest-like
substrate (S-1) over non-guest-like substrate (S-2) was observed. Mechanism studies confirmed that the “host-guest” nature of

pillar[5]arene struts in the CMPs was the main cause of the difference. The present work will open up new opportunities in design of
highly efficient organic semiconductor catalysts with reactant selectivity. Related work is ongoing in our lab.
Declaration of interests
√The authors declare that they have no known competing financial interests or personal relationships that
could have appeared to influence the work reported in this paper.
Acknowledgments
This work is sponsored by Shanghai Sailing Program (No. 19YF1452900), National Key Research and Development Program of
China (No. 2017YFA0206500), Science and Technology Commission of Shanghai Municipality (No. 18DZ1100403) and the National
Natural Science Foundation of China (No. 21871281).
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