A stimuli-responsive orthogonal supramolecular polymer network formed by metal-ligand and host-guest interactions

Article abstract of DOI:10.1039/c3cc47468b

A stimuli-responsive supramolecular polymer network was constructed by the orthogonal assembly of metal-ligand interactions and host-guest interactions.

Full text of DOI:10.1039/c3cc47468b

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A stimuli-responsive orthogonal supramolecular
polymer network formed by metal–ligand and
host–guest interactions†
Cite this: Chem. Commun., 2014,
50, 722
Received 30th September 2013,
Accepted 8th November 2013
Jiayi Zhan,a Qi Li,a Qiuyue Hu,a Qianqian Wu,a Cunmin Li,a Huayu Qiu,a
Mingming Zhang*b and Shouchun Yin*a
DOI: 10.1039/c3cc47468b
www.rsc.org/chemcomm
A stimuli-responsive supramolecular polymer network was con- ligands and metal ligands again, which have also been used in the
structed by the orthogonal assembly of metal–ligand interactions preparation of molecular motors and supramolecular polymers.5
and host–guest interactions.
The two interactions do not interfere with each other and their
stimuli-responsiveness is independent of each other. Herein, we
Supramolecular polymers and supramolecular polymer networks, in construct a supramolecular polymer network by the orthogonal
which the repeating units are linked by non-covalent interactions, assembly of these two interactions and study its self-assembly and
have attracted much attention due to their fantastic properties such stimuli-responsive properties. The low molecular weight monomer
as stimuli-responsiveness, self-healing and shape memory, showing 1 was synthesized by linking DB24C8 with two terpyridine units,
broad applications in materials science.1 Although various non- which can form linear supramolecular polymer 3 after the addition
covalent interactions such as hydrogen bonding, p–p stacking, of the metal ligand Zn(OTf)2. By the continuous addition of
metal–ligand, charge transfer and host–guest interactions have been bisammonium cross-linker 2, a supramolecular polymer network
used in the construction of advanced supramolecular polymer 4 was obtained. The conversion between the linear supramolecular
assemblies separately,2 the study on orthogonal assembly by two polymer 3 and supramolecular polymer network 4 can be realized
or more kinds of interactions is still in its early stages.3 The by the destruction and reconstruction of the interactions between
combination of various non-covalent interactions not only DB24C8 and DBAS via a change in pH. Moreover, the supra-
benefits the multiple-stimuli responsiveness of the formed molecular polymer 3 can also be directly converted into monomer
supramolecular assemblies, but also leads to a hierarchical 1 by the addition of 1,4,7,10-tetraazacyclododecane (cyclen), which
assembly with a high degree of complexity, making it possible would destroy the interactions between terpyridine and Zn2+
to use them as smart and adaptive materials.
(Scheme 1).
UV/vis measurements were first carried out to study the
Crown ether-based host–guest interactions and terpyridine-
based metal–ligand interactions have been widely used in the complexation between monomer 1 and Zn(OTf)2. A clear isosbestic
construction of supramolecular polymers due to their highly point at 302 nm was observed by titration of Zn(OTf)2 into a
directional and stable characteristics. The recognition between solution of monomer 1 (Fig. S7, ESI†), indicating the existence of
dibenzo-24-crown-8 (DB24C8) and dibenzylammonium salt (DBAS) metal–ligand interactions during the formation of linear supra-
is well-known in supramolecular chemistry and has been used in molecular polymers. As shown in the inset of Fig. S7 (ESI†), the
the construction of molecular muscles, molecular machines and absorbance at 312 nm showed no significant changes when
supramolecular gels due to their pH- and K+-responsiveness.4 The the molar ratio of Zn(OTf)2/1 reached 1.0, suggesting that the
interactions between terpyridines and metal ligands (Zn2+, Fe2+, stoichiometry of Zn2+ and monomer 1 in linear supramolecular
etc.) can be destroyed and recovered by the addition of competitive polymers and supramolecular networks was 1 : 1.6
The formation of supramolecular polymer 3 can be directly
evidenced by concentration-dependent 1H NMR measurements.
a College of Material, Chemistry and Chemical Engineering, Hangzhou Normal
As the concentration of monomer 1 and metal ligand Zn(OTf)2
increased, the peaks for the signals of all protons on monomer 1
broadened, indicating the gradual formation of supramolecular
polymer 3. The pyridyl protons H1, H2 and H4 shifted upfield
after complexation while downfield shifts were observed for
protons H3 and H5 (Fig. 1 and Fig. S8 and S9, ESI†). The aromatic
protons HAr of DB24C8 and benzyl protons H6 shifted downfield.
University, Hangzhou 310036, P. R. China. E-mail: yinsc@hznu.edu.cn;
Fax: +86 57128867899; Tel: +86 57128867899
b Department of Biochemistry and Chemistry, University of Maryland, College Park,
20740, USA. E-mail: mzhang12@umd.edu
† Electronic supplementary information (ESI) available: Experimental procedures
and full characterization of 1, 2 and 6; UV/vis titration between Zn(OTf)2 and
monomer 1; 1H NMR and COSY spectra of supramolecular polymer 3; NOESY
spectra of polymer network 4. See DOI: 10.1039/c3cc47468b
722 | Chem. Commun., 2014, 50, 722--724
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Fig. 2 Partial 1H NMR spectra (400 MHz, 1 : 1 CDCl3–CD3CN, 293 K) of
(a) monomer 1 and 25.0 mM linear supramolecular polymer 3 with increasing
equivalents of cross-linker 2: (b) 0 equiv.; (c) 0.1 equiv.; (d) 0.3 equiv.;
(e) 0.5 equiv.; (f) 0.7 equiv.; (g) 1.0 equiv.; (h) cross-linker 2. Here ‘‘c’’ and
‘‘u’’ denote the complexed and uncomplexed species, respectively.
We noticed that protons on terpyridine did not shift much after the
addition of bisammonium cross-linker 2 (Fig. 2), suggesting that
the metal–ligand interactions between Zn(OTf)2 and terpyridines
and the host–guest interactions between DB24C8 and DBAS are
independent, which is crucial for orthogonal self-assembly. Due to
the slow-exchanging system of DB24C8 and DBAS,4 the complexa-
tion spectra became very complicated and each of the protons of
DB24C8 and DBAS split into two signals after the addition of cross-
linker 2, corresponding to the complexed and uncomplexed species.
As shown in Fig. 2, upfield chemical shifts were found for aromatic
protons HAr of 1 and benzyloxymethylene protons Hg of 2, while the
benzyl protons Hd of 2 shifted downfield after complexation. This is
consistent with the chemical shift changes of DB24C8–DBAS
systems,4 indicating the formation of host–guest assemblies
between the supramolecular polymer 3 and the cross-linker 2.
This result can also be evidenced by the NOE correlation signals of
aromatic signals HAr of 1 and benzyl protons Hd and benzyloxy-
methylene protons Hg of 2 (Fig. S10, ESI†). Furthermore, with the
increase of the amount of cross-linker 2, the proton signals for
uncomplexed species became more and more weak. When the
ratio of cross-linker 2 to linear supramolecular polymer 3 reached
0.5 (Fig. 2e), the intensities of the proton signals for complexed
species reached their highest value and no obvious changes were
seen even if the amount of cross-linker 2 was further increased.
All these results indicate the formation of highly dense supra-
molecular network 4 by host–guest interactions between linear
supramolecular polymer 3 and the cross-linker 2.
Scheme 1 Schematic representation of the formation of linear supramolecular
polymer 3 and its reversible conversion with supramolecular network 4.
Fig. 1 Partial 1H NMR spectra (500 MHz, 1 : 1 CDCl3–CD3CN, 293 K) of
(a) monomer 1 and supramolecular polymer 3 constructed by mixing equimolar
Zn(OTf)2 and 1 at different concentrations: (b) 1.00; (c) 3.00; (d) 5.00;
(e) 8.00; (f) 10.0; (g) 15.0; (h) 20.0; (i) 25.0 mM. Here ‘‘l’’ and ‘‘c’’ denote the
linear and cyclic species, respectively.
Viscometry gave direct evidence for the formation of supra-
All of these chemical shifts indicated the existence of metal– molecular polymers and supramolecular polymer networks. All of
ligand interactions in supramolecular polymer 3. Furthermore, the viscosity studies were performed in 1 : 1 CHCl3–CH3CN contain-
conversion from cyclic oligomers to linear polymers was also ing 0.05 M tetrabutylammonium hexafluorophosphate to exclude
observed with the increase of the monomer concentrations, the polyelectrolyte effect.8 A double logarithmic plot of specific
indicating a ring-chain mechanism for the formation of this viscosity versus the concentration of 1ÁZn2+ was obtained (Fig. 3a).
kind of supramolecular polymers.7
In the low concentration range, the slope of the curve was 1.13,
We then turned to study the formation of supramolecular net- while in the high concentration range, it reached 2.85, indicat-
works by the continuous addition of bisammonium cross-linker 2. ing the formation of supramolecular polymers. The critical
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and only the uncomplexed signal could be observed (Fig. 4b). When
trifluoroacetic acid was continuously added, the complexed peaks
could be seen again (Fig. 4c). Moreover, the linear supramolecular
polymer 3 can also been directly converted into monomer 1 and then
recovered by the addition of competitive ligands such as cyclen and
then Zn2+ again. This process was evidenced by UV/vis spectroscopy
(Fig. S12 and S13, ESI†), as the UV/vis absorbance intensity at 312 nm
was first quenched and then recovered by the continuous addition of
cyclen and Zn(OTf)2.
In summary, we constructed a stimuli-responsive supramolecular
polymer network by the orthogonal assembly of metal–ligand inter-
actions and host–guest interactions. The structure of this supra-
molecular network can be reversibly converted either into a linear
supramolecular polymer or into its monomer by destruction and
reconstruction of the host–guest interactions and metal–ligand
interactions, respectively. This work can expand the topological
control over supramolecular polymers, which is important for
application of such structures in smart and adaptive materials.
This work was financially supported by the National Natural
Science Foundation of China (91127032, 21174035, 21274034), the
Program for Changjiang Scholars and Innovative Research Team
in Chinese University (IRT 1231), Excellent Young Teachers in
Zhejiang Province and Hangzhou Normal University (HNUEYT
2011-01-019) and National Training Programs of Innovation and
Entrepreneurship for Undergraduates (2012103460010).
Fig. 3 (a) Specific viscosity of the linear supramolecular polymer 3 (1 : 1
CHCl3–CH3CN, 298 K); (b) reduced viscosity of linear supramolecular
polymer 3 (’) and supramolecular polymer network 4 (the molar ratio
of 3 : 2 was kept at 2 : 1) (J) as a function of monomer concentration (1 : 1
CHCl3–CH3CN, 298 K).
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724 | Chem. Commun., 2014, 50, 722--724
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