Supramolecular brush polymers prepared from 1,3,4-oxadiazole and cyanobutoxy functionalised pillar[5]arene for detecting Cu2+

Article abstract of DOI:10.1039/d0ob02587a

A supramolecular brush polymerPoly(P5-OXD)was constructed through the self-assembly of an A1/A2 disubstituted pillar[5]arene P5-OXD with a 1,3,4-oxadiazole unit and a cyanobutoxy group, exhibiting external stimuli responsiveness towards Cu²⁺ion

Full text of DOI:10.1039/d0ob02587a

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Biomolecular Chemistry
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Supramolecular brush polymers prepared from
1,3,4-oxadiazole and cyanobutoxy functionalised
pillar[5]arene for detecting Cu2+†
Cite this: Org. Biomol. Chem., 2021,
19, 1287
Received 29th December 2020,
Accepted 19th January 2021
Shuangyan Liu,‡a Qiuxia Wu,‡a Tianze Zhang,a Huacheng Zhang *b and Jie Han
*
a
DOI: 10.1039/d0ob02587a
rsc.li/obc
A supramolecular brush polymer Poly(P5-OXD) was constructed arene via a “head-to-tail” molecular recognition type,14 leading
through the self-assembly of an A1/A2 disubstituted pillar[5]arene to the formation of only one-dimensional (1D) linear supramo-
P5-OXD with a 1,3,4-oxadiazole unit and a cyanobutoxy group, lecular architectures, as well as limited applications in chemi-
exhibiting external stimuli responsiveness towards Cu2+ ions with cal topology. Besides the linear PSPs, other architectural types
an ON/OFF fluorescence signal output.
such as hyperbranched and cross-linked PSPs with three-
dimensional (3D) topological structures have also been
reported recently. This type of PSP often exhibited reversible
and tunable properties, and has presented potential appli-
cations in the fields of bio-medicine and materials chemistry.
However, the 3D PSPs were always constructed with pillarar-
ene-based host monomers and additional guest monomers,
which made the supramolecular system too complicated and
difficult to further manufacture various polymeric devices.15
In recent years, 1,3,4-oxadiazole (OXD) derivatives have
found many practical applications in the areas of organic
light-emitting diodes (OLEDs)16 and liquid crystals17 due to
the high photoluminescence quantum yield and excellent
chemical stability. Furthermore, the OXD unit has potential
metal coordination sites (N and O) towards metal cations, and
thus could be used as a signaling component in fluorescent
chemosensors for various cations such as Cu2+,18 Zn2+,19
Cd2+ 20 and Ag+.21 However, the application of 1,3,4-oxadiazole
derivatives in the fabrication of advanced topological supramo-
lecular self-assemblies is limited up to now.
Here, to build fresh supramolecular brush polymers and
develop fluorescent chemosensors with high selectivity
towards metal ions, a new pillar[5]arene derivative P5-OXD
(Scheme 1) was designed and synthesised. The novel structure
P5-OXD contains a cyanobutoxy group as a guest moiety,
which could recognise the electron-rich columnar pillararene
cavity, as well as a 1,3,4-oxadizole subunit as a fluorescent and
signaling component threading out of supramolecular poly-
mers such as brushes, and the brush structure could enhance
the capacity of coordinating targeted metal ions. The for-
mation of Poly(P5-OXD)-based supramolecular brush polymers
was further fully characterised by 1H NMR, 13C NMR and scan-
ning electronic microscopy (SEM). The fluorescence properties
of the supramolecular brush polymer were investigated, and a
In comparison with the general covalent bond-based polymers,
supramolecular polymers have many interesting capabilities
such as self-reparability, degradability and self-adaptation due
to the nature of the dynamic and reversible noncovalent bonds
involved.1 In addition, the noncovalent bonds provide a possi-
bility of efficiently integrating multiple functions and pro-
perties together via particular design and control at the mole-
cular level.2 Thus, supramolecular polymers are ideal candi-
dates for preparing functional materials and smart devices,
attracting much attention in the fields of sensors,3 gels,4
chemistry topology5 and advanced materials.6
Pillararenes, a special macrocyclic host discovered in 2008,7
have been widely used in molecular recognition,8 separations,9
light-harvesting systems,10 transmembrane channels,11 and
drug delivery.12 Recently, pillararenes have also been used as
building blocks to construct various supramolecular polymers
through the host–guest interactions between the columnar
electron-rich cavity and various electron-accepting cationic or
neutral guest molecules.13 To date, great progress has been
achieved in pillararene-based supramolecular polymers (PSPs).
However, there are still some limitations in this field. For
example, such types of supramolecular polymers were mainly
constructed by the self-assembly of mono-functionalised pillar-
aKey Laboratory of Advanced Energy Materials Chemistry (Ministry of Energy),
College of Chemistry, Nankai University, Tianjin 300071, China.
E-mail: hanjie@nankai.edu.cn
bSchool of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an,
Shaanxi 710049, China. E-mail: zhanghuacheng@xjtu.edu.cn
†Electronic supplementary information (ESI) available: Full experimental details
and characterization data including copies of NMR and HRMS spectroscopy of
the products. See DOI: 10.1039/d0ob02587a
‡These authors have contributed equally to this work.
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As seen in Scheme 1, N,N-dimethyl-4-(5-(3-(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolan-2-yl)phenyl)-1,3,4-oxadiazol-2-yl)
aniline 4 is a key intermediate compound to the synthesis of
the target molecule. Initially, compound 4 was designed and
synthesised through three steps of reactions as shown in
Scheme 2. In this synthetic route, 4-(5-(3-bromophenyl)-1,3,4-
oxadiazol-2-yl)-N,N-dimethylaniline 9 was obtained smoothly
in good yield from the economically available 3-bromobenzoic
acid as the starting material. However, in the Suzuki-coupling
reaction between
9 and 4,4,4′,4′,5,5,5′,5′-octamethyl-2,2′-bi
(1,3,2-dioxaborolane), (Bpin)2, only about 60% of 9 could trans-
form into the intermediate 4. Furthermore, it is difficult to
separate the reaction mixture by means of recrystallisation or
column chromatography due to the similar solubility and
polarity of the reagents and product. In contrast, the inter-
mediate 4 was successfully obtained in a total yield of 34.8%
via three steps of reactions as shown in Scheme 2. Then, the
product P5-OXD was prepared in high yield through Suzuki-
coupling reaction between the intermediates 4 and 5, de-
protection of the triflate ester of 6 and introduction of a cyano-
butoxy group to 7 by substitution reaction.
Scheme 1 Synthetic route to P5-OXD and the supramolecular polymer
Poly(P5-OXD).
The formation of the supramolecular polymer was firstly
investigated by means of proton nuclear magnetic resonance
(NMR). The 1H NMR spectroscopy results of P5-OXD and 5-bro-
mopentanenitrile are shown in Fig. 1. All proton signals of P5-
OXD were broad, and the signals derived from the cyanobutoxy
group (Hb, Hc and Hd) shifted upfield remarkably due to the
shielding effect of the electron-rich cavities of pillar[5]arene
suggesting that the neutral cyanobutoxy group was threaded in
the electron-rich cyclic cavity of P5-OXD via host–guest inter-
actions to form a supramolecular polymer with a bigger-sized
and electron-rich 1,3,4-oxadizole subunit threading out of
supramolecular polymers like brushes.22 Moreover, 1H–1H
NOESY experiments were also performed (Fig. 2), and corre-
lation signals were observed between protons on cyanobutoxy
and aromatic protons as well as the bridging methylene
protons on the pillar[5]arene skeleton, suggesting that the cya-
nobutoxy group was threaded in the cyclic cavity of P5-OXD to
form a linear supramolecular polymer.
Scheme 2 Alternative synthetic route for the intermediate compound
4.
typical aggregation-induced quenching (AIQ) phenomenon
was observed accompanied by a red-shift of the emission wave-
length from 411 nm to 426 nm. Upon the addition of Cu2+
ions to the chloroform solution of Poly(P5-OXD), a significant
decrease of the fluorescence intensity was found.
Fig. 1 1H NMR spectra (400 MHz, CDCl3, 298 K) of (a) P5-OXD (20 mM) and (b) 5-bromopentanenitrile (*solvent peaks).
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Fig. 4 SEM image of the supramolecular assembly prepared from P5-
OXD in chloroform.
Because the supramolecular brush polymer Poly(P5-OXD)
can emit strong fluorescence by controlling the concentration,
and the “brush-like” 1,3,4-oxadiazole subunits attached to the
supramolecular polymers may enhance the metal coordination
with Cu2+ ions, self-assemblies of Poly(P5-OXD) were further
used as a fluorescent chemosensor for Cu2+. The fluorescence
change of Poly(P5-OXD) was investigated briefly by the
addition of increasing amounts of Cu2+ to its solution in
chloroform. The fluorescence intensity (Fig. 5) at the position
of the emission maxima decreased gradually with the increase
of the loading amount of Cu2+. About 1 equiv. of Cu2+ makes
the quenched fluorescence reach a minimum plateau. It was
supposed that upon the addition of Cu2+ ions, the supramole-
cular brush polymer might coordinate with metal cations via
the 1,3,4-oxadiazole-based brushes, and further form a cross-
linked supramolecular network accompanied by the quench-
ing of the fluorescence.23,18a,b Thus, Poly(P5-OXD) shows the
possibility of application in a potential fluorescent chemo-
sensor for Cu2+ ions. The specific recognition of Cu2+ might be
rationalised by Irving–Williams order stability in terms of
complex formation.24 Cu2+ is a well-known paramagnetic ion
with an unfilled d shell and could strongly quench the fluo-
rescence of a fluorophore through electron or energy transfer
between the metal cation and the fluorophore.18a
Fig. 2 1H–1H NOESY analysis of P5-OXD (20 nM) (400 MHz, CDCl3,
298 K).
Fig. 3 Changes in the fluorescence emission spectra of P5-OXD with
different concentrations in chloroform (λex = 310 nm), and the fluor-
escence intensity versus P5-OXD concentration (inset).
The changes in the fluorescence emission spectra of P5-
OXD under different concentrations in chloroform were inves-
tigated, also indicating the formation of the supramolecular
brush polymer. As shown in Fig. 3, under irradiation with UV-
light (310 nm), the solution of P5-OXD in chloroform (5 mM)
displayed a strong emission (λmax = 411 nm). With the increase
of the concentration of P5-OXD, the intensity of the emission
decreased dramatically with a slight red-shift from 411 nm to
426 nm, indicating the gradual formation of supramolecular
brush polymers due to the aggregation-caused quenching.
Notably, as the concentration was increased up to 60 mM, the
fluorescence intensity gradually tended to remain unchanged,
indicating that the critical aggregation concentration of P5-
OXD is achieved.
Furthermore, the morphology of the supramolecular brush
polymer was investigated by SEM. By rapid drying of an equi-
molar mixture of P5-OXD in chloroform (60 mM), bundles of a
rod-like crystalline precipitate was observed, providing direct
evidence for the formation of supramolecular brush architec-
tures (Fig. 4).
Fig. 5 Fluorescence spectra (λex = 310 nm) of Poly(P5-OXD) in chloro-
form (the concentration of P5-OXD is 60 mM) with various equivalents
of Cu2+ (0–1.3 equiv.).
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In summary, an A1/A2 disubstituted pillar[5]arene (P5-OXD)
was first synthesised by modifying the pillar[5]arene skeleton
with a 1,3,4-oxadiazole subunit and a cyanobutoxy moiety, and
P5-OXD was used to fabricate the supramolecular brush poly-
mers by host–guest interactions between electron-rich pillarar-
ene cavities and proper-sized neutral cyanobutoxy moieties.
Particularly, the bigger-sized electron-deficient 1,3,4-oxadia-
zole subunit stays outside the pillararene cavities after the for-
mation of host–guest inclusions, functioning as “brushes” to
enhance the capacity of self-assembled supramolecular
materials in coordinating with metal ions such as Cu2+. As
confirmed by NMR, SEM and spectral studies, after the
addition of Cu2+ ions, the supramolecular brush polymer exhi-
bits different structural information in accordance with fluo-
rescence quenching, indicating the possibility of changing
into cross-linked supramolecular networks. Thus, this supra-
molecular brush polymer has potential applications in fluo-
rescent chemosensors for metal cations.
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Acknowledgements
This work was financially supported by the Natural Science
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