A novel supramolecular polymer gel constructed by crosslinking pillar[5]arene-based supramolecular polymers through metal-ligand interactions

Article abstract of DOI:10.1039/c5cc07252b

A novel heteroditopic A-B monomer was synthesized and used to construct linear supramolecular polymers utilizing pillar[5]arene-based host-guest interactions. Specifically, upon addition of Cu²⁺ ions, the supramolecular polymer chains are crosslinked through metal-ligand interactions, resulting in the formation of a supramolecular polymer gel. Interestingly, this self-organized supramolecular polymer can be used as a novel fluorescent sensor for detecting Cu²⁺ ions.

Full text of DOI:10.1039/c5cc07252b

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A novel supramolecular polymer gel constructed by crosslinking
pillar[5]arene-based supramolecular polymers through metal–ligand
interactions
5
Pi Wang,† Hao Xing,† Danyu Xia and Xiaofan Ji*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/c0xx00000x
widely used in constructing various interesting supramolecular
systems.9 In addition, metal–ligand coordination, a kind of strong,
50 directional, and highly versatile driving force, has been widely
used in the preparation of functional supramolecular systems.10
Moreover, this kind of non-covalent interaction may bring unique
magnetic, redox, optical, and electrochromic properties,
benefitting the applications in the fields of heterocatalysis,
55 electronics, and gas storage.
A novel heteroditopic A-B monomer is synthesized and used
to construct linear supramolecular polymers utilizing
10 pillar[5]arene-based hostguest interactions. Specifically,
upon addition of Cu2+ ion, the supramolecular polymer
chains are crosslinked through metalligand interactions,
resulting in the formation of a supramolecular polymer gel.
Interestingly, this self-organized supramolecular polymer can
15 be used as a novel fluorescent sensor for detecting Cu2+ ion.
Herein, we present the design and synthesis of
a
heteroditopic A-B monomer 1 which further self-organize into
linear supramolecular polymers at high concentration utilizing
pillar[5]arene-based host–guest interactions (Scheme 1), and the
60 synthesis and characterrzation of 1 and G were shown in ESI
(Figs. S1S15, ESI†). Furthermore, due to the presence of
mesogenic bidentate Schiff base group in the middle of monomer
1, the supramolecular polymer chains were crosslinked through
metal–ligand interactions upon the addition of Cu2+ ions,
65 resulting in the formation of a supramolecular polymer gel.
Interestingly, we found that the fluorescence of the solution of
monomer 1 could be quenched after adding Cu2+ ions, so this
self-organized supramolecular polymer can be used as a novel
fluorescent sensor for detecting Cu2+ ions.
Polymer gels are three-dimensional structures of crosslinked
macromolecules, and are usually classified as chemical or
physical gels.1 The chains of chemical gels are crosslinked by
permanent covalent bonds, while those of physical gels are
20 interconnected by non-covalent interactions. Because of the
reversibility and stimuli-responsiveness, physical gels, namely
supramolecular polymer gels, exhibit fascinating properties such
as facile processing, recycling, stimuli-responsiveness, self-
healing, and shape-memory.2 Supramolecular polymer gels are
25 commonly formed by two types of strategies: 1) the side-chain
cross-linking of covalently joint polymers through non-covalent
interactions;3 2) supramolecular bonds serve to construct the gels
totally from non-covalently associating low molecular weight
monomers.4 In the former strategy, the initial design and
30 preparation of traditional polymer backbones require time-
consuming organic synthesis. However, in the later one, the
functional groups in the low molecular weight monomers are
well-organized, and traditional polymerization for the synthesis
of polymer backbones is avoided. Therefore, it is important to
35 construct supramolecular polymer gels from non-covalently
associating low molecular weight monomers.
It is efficient and convenient to combine different types of
non-covalent interactions to construct supramolecular polymer
gels, resulting in the formation of systems with a high degree of
40 complexity.5 Pillararenes,6 born after crown ethers, cyclodextrins,
calixarenes, and cucurbiturils, are a new class of macrocyclic
hosts for supramolecular chemistry.7,8 Their repeating units are
connected by methylene bridges at the para-positions, forming a
special rigid pillar-like architecture. It is the unique structures and
45 easy functionalization property of pillararenes that endow them
with outstanding abilities to selectively bind different kinds of
guests and further make pillararene-based host–guest chemistry
70
Initially, the formation of a high-molecular-weight polymeric
structure utilizing pillar[5]arene-based host-guest interactions
was investigated. At first, 1,4-dimethoxypillar[5]arene (DMP5)
and G were selected as model host and guest to study the binding
strength between monomer
1
during supramolecular
75 polymerization. The ability of DMP5 to bind G was evaluated
by 1H NMR titration of DMP5 into a 1.00 mM solution of G in
CDCl3 (Fig. S16, ESI†). A molar ratio plot confirmed that the
complexation between DMP5 and G was of 1:1 stoichiometry in
CDCl3 (Fig. S17, ESI†). Accordingly, the association constant
80 (Ka) was determined to be 1.25  103 M1 (Fig. S18, ESI†).
Figure 1 showed the concentration-dependent 1H NMR spectra of
monomer 1 (500 MHz, CDCl3, 298 K) in the range of 15.0220
mM which provided further insight into the self-organization
behavior of monomer 1. As the monomer concentration increased,
85 the peak splitting of the aromatic protons, such as Hac, and the
alkyl chain protons disappeared along with broadening, which
confirmed the formation of high molecular weight aggregates
driven by the host-guest interactions between the pillar[5]arene
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host units and trimethylammonium units. This process was
increasing concentration (Fig. S26, ESI†). The average degree of
observed to be fast at first (Figs. 1j1g) and then slow (Figs. 40 polymerization, n, is easily derived as being related to the
1f1a), in accordance with the two-dimensional diffusion-ordered
NMR (DOSY) characterization and viscosity measurements
discussed below (Figs. S19S25, ESI†). The assignment and
correlation of the protons at high concentration were further
validated by a 2D-NOESY NMR spectrum of monomer 1 (Fig.
S26, ESI†).
equilibrium constant Ka and the initial monomer concentration
(Table S1, ESI†).
5
1
Fig. 1 Partial H NMR spectra (500 MHz, CDCl3, 298 K) of monomer 1
45 at different concentrations: (a) 227 mM; (b) 182 mM; (c) 151 mM; (d)
130 mM; (e) 113 mM; (f) 91.1 mM; (g) 75.8 mM; (h) 65.0 mM; (i) 30.0
mM; (j) 15.0 mM.
Encouraged by the above-mentioned results, we envisioned
that the mesogenic bidentate Schiff base group in monomer 1
50 might show typical aggregation-caused quenching (ACQ)
phenomenon when increasing the concentration of monomer 1.11
As shown in Fig. 2a, the significant quenching of the
fluorescence intensity was found upon gradually increasing the
concentration of monomer 1. Due to the formation of the
55 extended, high molecular weight polymeric structures, the
collisional interactions between the aromatic molecules in the
excited and ground states were caused which promoted the
formation of sandwich-shaped excimers and exciplexes.12 This
observation proved the formation of supramolecular polymers
60 with the increasing concentration in another way.
10
Scheme 1. Cartoon representation of the self-assembly process of
monomer 1 and the formation of supramolecular polymer gel driven by
metal-ligand interactions.
Two-dimensional
diffusion-ordered
NMR
(DOSY)
15 experiments were also performed to investigate the self-assembly
process of monomer 1 to form the linear supramolecular
polymers. As the monomer concentration increased from 15.0 to
250 mM in CDCl3, the measured weight-average diffusion D
values decreased considerably from 1.90 × 10−5 to 9.20 × 10−7
20 m2s–1, indicating the concentration dependence of supramolecular
polymerization of monomer 1 (Figs. S20S25, ESI†). It can be
explained by the fact that the extended, high molecular weight
polymeric structures have greater obstacles in molecular motion
and further lead to a lower measured weight average diffusion
25 coefficient. Furthermore, viscometry is a convenient method to
test the propensity of monomers to self-assemble into large
aggregates. Therefore, viscosity measurements were carried out
The conversion from linear to cross-linked supramolecular
polymer was then accomplished by forming a disubsituted
copper(II) complex between Cu(OAc)2 and mesogenic bidentate
Schiff base moieties, and this brought about the sol to gel
65 transition. Since the formation of the copper(II) complex between
Cu(OAc)2 and mesogenic bidentate Schiff base moiety may cause
the quenching of the fluorescence of the solution of monomer 1 at
a relative low concentration, a fluorescent experiment was
performed to confirm the formation of the metal-ligand complexs.
70 As shown in Fig. 2b, upon adding successive Cu2+ ions to the
CHCl3 solution of monomer 1 at 60.0 mM, at which the linear
supramolecular polymer still have strong fluorescence intensity, a
significant quenching of the fluorescence intensity was found
in CHCl3 using
a Cannon Ubbelohde semi-microdilution
1
which was accordance with our expect. Besides, H NMR was
viscometer. As presented in Figure S26, the linear supramolecular
30 polymer assembled from monomer 1 exhibited a viscosity
transition that was characterized by a change in the slope in the
double logarithmic plots of specific viscosity versus
concentration. In the low concentration range, the slopes
approximated unity, which is characteristic of cyclic oligomers
35 with constant size. When the concentration exceeded the critical
polymerization concentration (CPC; approximately 57.0 mM), a
sharp increase in the viscosity was observed (slope = 1.69, at 298
K), indicating the formation of supramolecular polymers with the
75 also used to investigate this process. Upon progressive addition of
a saturated CD3CN solution of Cu(OAc)2·H 2O to the CDCl3
solution of monomer 1 at 80.0 mM, it was found that the
resonance for the phenolic hydroxyl Hg proton gradually
disappeared with increasing the amount of Cu2+ ions (Fig. 3),
80 indicating preferential complexation between the mesogenic
bidentate Schiff base moieties and the copper atom and the
formation of a cross-linked supramolecular polymers. The
minimum concentration of Cu(II) ions to be added to obtain a gel
2
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was 20.0 mM when 1 at a concentration of 80.0 mM. DLS
entangling and crosslinking (Fig. 4b). This morphology transition
experiments were then conducted to study the size distributions 25 showed that the metal-coordination interactions between the
of the assemblies (Fig. S27, ESI†).
linear supramolecular polymers and Cu2+ ions not only achieved
the conversion from linear supramolecular polymers to the 3D
supramolecular polymeric network, but also made an impact on
the microscopic nature of the resultant supramolecular polymer
30 gel.
Fig. 4c showed the sol-glue-gel transition of this system: the
solution of monomer 1 at low concentration exhibited good
fluidity (Fig. 4c, left); when the concentration of monomer 1 was
increased, the high concentration supramolecular polymers turned
35 to glue-like viscous liquids (Fig. 4c, middle), but not gel; after the
addition of Cu2+ ions, a supramolecular polymer gel was finally
obtained (Fig. 4c, right).
5
Fig. 2 (a) The changes of the fluorescence of a CHCl3 solution of 1 upon
increasing its concentration (20.0-250 mM). (b) The changes of the
fluorescene of a CHCl3 solution of monomer 1 (60.0 mM) upon the
addition of Cu2+ ions (01.10 equiv).
40 Fig. 4 (a) SEM image of the polymeric aggregates prepared from self-
organization of 80.0 mM 1 in CHCl3. (b) SEM image of the polymeric
aggregates prepared from self-assembly of 80.0 mM 1 and 40.0 mM Cu2+
in CHCl3. (c) Photos to show the sol-glue-gel transition of this system.
Considering the fluorescence quenching phenomenon after
45 adding Cu2+ ions to the 60.0 mM CHCl3 solution of monomer 1,
we wondered whether our system could be used in detecting Cu2+
ions. In order to investigate this, a thin film of linear
supramolecular polymer formed by self-organization of monomer
1 at 60.0 mM in CHCl3 was prepared by spin-casting solution
50 onto glass slides. Fig. S28 showed that the thin film displayed a
visually clear fluorescence quenching after adding one drop of
Cu(OAc)2·H 2O solution in CH3CN. Therefore, this self-organized
supramolecular polymer can be used as a novel fluorescent sensor
for detecting Cu2+ ions, which showed excellent stability,
55 reversibility, as well as repeatability (Fig. S29, ESI†).
10
In conclusion, we have synthesized a novel heteroditopic A-B
monomer which could self-organize into linear supramolecular
polymers at high concentration utilizing pillar[5]arene-based
host–guest interactions. Due to the presence of mesogenic
60 bidentate Schiff base group in the middle of the monomer, the
supramolecular polymer chains could be crosslinked through
metal–ligand interactions upon the addition of Cu2+ ions,
resulting in the formation of a supramolecular polymer gel.
Interestingly, since this Schiff base group has special recognition
65 to Cu2+ ion, this self-organized supramolecular polymer can be
used as a novel fluorescent sensor for detecting Cu2+ ions. This
new type of pillar[5]arene- and copper-based supramolecular
polymer gel is of high importance for developing novel
functional fluorescent materials and molecular devices in the
Fig. 3 Partial 1H NMR spectra (500 MHz, CDCl3, 298 K) of 1 at a
concentration of 80.0 mM with successive addition of Cu(OAc)2·H 2O: (a)
0.50 equiv; (b) 0.40 equiv; (c) 0.30 equiv; (d) 0.20 equiv; (e) 0.10 equiv;
15 (f) 0.00 equiv.
Furthermore, the morphology transition from linear
supramolecular polymers to the cross-linked supramolecular gel
was investigated. As shown in Fig. 4a, a rod-like extended fiber
was observed by scanning electron microscopy (SEM) from a
20 high concentration solution (150 mM) of monomer 1, which
provided direct evidence for the formation of linear
supramolecular polymers. After an equimolar Cu2+ ions was
added, a denser, three-dimensional network was obtained by
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This work was supported by the Fundamental Research
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75
80
85
90
Funds for the Central Universities.
5
Notes and references
Department of Chemistry, Zhejiang University, Hangzhou 310027, P. R.
China; Fax: +86-571-8795-3189; Tel: +86-571-8795-3189; E-mail:
xiaofanji@zju.edu.cn
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†
Electronic Supplementary Information (ESI) available: Synthetic
10 procedures, characterizations, association constant determination, and
other materials. See DOI: 10.1039/c0xx00000x.
†
These authors contributed equally to this work.
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Text:
45 6
A novel heteroditopic A-B monomer is synthesized and used to
100 construct linear supramolecular polymers utilizing pillar[5]arene-
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metalligand interactions, resulting in the formation of a
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for detecting Cu2+ ion.
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