Formation of stimuli-responsive supramolecular polymeric assemblies via orthogonal metal-ligand and host-guest interactions

Article abstract of DOI:10.1039/c3cc42511h

Linear supramolecular polymers, assembled via the combination of orthogonal terpyridine-Zn²⁺ and benzo-21-crown-7/secondary ammonium salt recognition motifs, exhibit dynamic properties responsive to various external stimuli.

Full text of DOI:10.1039/c3cc42511h

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Formation of stimuli-responsive supramolecular
polymeric assemblies via orthogonal metal–ligand
and host–guest interactions†
Cite this: Chem. Commun., 2013,
49, 5951
Received 7th April 2013,
Accepted 9th May 2013
Yue Ding, Peng Wang, Yu-Kui Tian, Yu-Jing Tian and Feng Wang*
DOI: 10.1039/c3cc42511h
www.rsc.org/chemcomm
Linear supramolecular polymers, assembled via the combination of
orthogonal terpyridine–Zn2+ and benzo-21-crown-7/secondary
ammonium salt recognition motifs, exhibit dynamic properties
responsive to various external stimuli.
The principle of orthogonal self-assembly, widely adopted by
biological systems to demonstrate dedicated functionality in
catalysis, recognition as well as signaling,1 inspired the development
of sophisticated artificial polymeric analogues.2 As compared
with conventional supramolecular polymers utilizing one type of
non-covalent interaction, orthogonal self-assembly imparts much
superior properties. First, simultaneous incorporation of multiple
non-covalent bonds leads to a hierarchical assembly process with a
high level of complexity. Second, considering each type of non-
covalent interaction is inherently reversible, manipulating non-
Scheme 1 Schematic representation of the formation of supramolecular poly-
mers 5 and the corresponding intermediates (terpyridine–Zn2+ dimeric complex 3
and host–guest paired [3]pseudorotaxane structure 4).
covalent bonds separately addresses adaptive properties of the complex 3. In addition, 1 could also associate with the complemen-
resulting supramolecular assemblies.3 Up to now, a variety of self- tary homoditopic benzo-21-crown-7 (B21C7) monomer 2 to achieve
assembling architectures, such as supramolecular block copoly- [3]pseudorotaxane structure 4. If terpyridine–metal and B21C7/sec-
mers,4 graft copolymers,5 networks6 and single-chain nanoparticles,7 ondary ammonium salt recognition motifs are proved to be com-
have been fabricated using the orthogonal self-assembling approach. pletely orthogonal, mixing three components (monomers 1–2,
We endeavor to explore novel types of orthogonal building Zn(OTf)2) in a specific ratio would guarantee the formation of linear
blocks, and furthermore develop them into highly adaptive supramolecular polymers 5 with stimuli-responsive and adaptive
supramolecular polymeric assemblies. Crown ether based host–guest behaviors.
interaction, derived from the threading property, renders unusual
The synthetic routes to the desired monomers 1–2 are quite
mobility and unique mechanical behavior to the resulting rotaxane- straightforward (Schemes S1 and S2, ESI†). It is worthy of note
type interlocked structures.8 Meanwhile, coordination of 2,20:60,200- that the length mismatch of monomers 1–2, accomplished via
terpyridine (tpy) to a wide range of transition metal ions provides an introducing a flexible aliphatic spacer on monomer 1, would
ideal platform to combine high thermodynamic stability with tun- destabilize the cyclic oligomers and thereby favor the linear
able kinetics.9 Therefore, our strategy involves the integration of extension,10 resulting in a relatively low critical polymerization
these two non-covalent interactions. Specifically, heteroditopic concentration (CPC) for the supramolecular polymerization process.
monomer 1 is designed which bears a secondary ammonium salt
Slow-exchange host–guest complexation between compounds 1
1
guest and a terpyridine ligand on each side (Scheme 1). Upon and 2 was elucidated using the H NMR spectrum (Fig. S8, ESI†),
addition of Zn(OTf)2, it is expected to form a 2 : 1 terpyridine–Zn2+ which exhibits three sets of signals for the protons belonging to the
B21C7/secondary ammonium salt recognition motif.8a For the
resulting host–guest paired dimer 4, aromatic protons H10–11 and
methylene protons H12 on 1, as well as ethyleneoxy protons H19–20
on 2, move downfield significantly, supporting the formation of
Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and
Engineering, University of Science and Technology of China, Hefei, Anhui 230026,
P.R. China. E-mail: drfwang@ustc.edu.cn; Fax: +86551 6360 6095
[3]pseudorotaxane structure. On the other hand, no obvious
chemical shift change occurs for the terpyridine unit, demonstrating
† Electronic supplementary information (ESI) available: Synthesis, characteriza-
tion, UV-Vis titration data and other materials. See DOI: 10.1039/c3cc42511h
c
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the preferential host–guest complexation without the involve-
ment of the terpyridine ligand.
Next, the capability to form Zn2+–terpyridine complexes was
investigated by UV/Vis measurements. Stepwise titration of
Zn(OTf)2 with monomer 1 shows a clear isosbestic point at
315 nm (Fig. S9, ESI†), indicating the efficient conversion from
free to metal-complexed terpyridine species. The achievement of
maximum absorbance (lmax = 343 nm) at the Zn(OTf)2/1 molar
ratio of 0.5 apparently supports the formation of a 2 : 1 complex for
terpyridine and Zn2+. After validating the two-component model
systems, we turned to examining metal–ligand complexation in
the presence of host–guest counterparts. Therefore, a 2 : 1 mixture
of monomers 1 and 2 was titrated with Zn(OTf)2 in the manner
described above (Fig. S10, ESI†). The resulting titration curves,
resembling the plots depicted in Fig. S9 (ESI†), exhibit a similar
linear increase and a sharp endpoint at a Zn2+/terpyridine ratio of
0.5 : 1 (Fig. 1a), illustrating the specific complexation between Zn2+
and terpyridine without the participation of host–guest units.
It should be noted that UV/Vis experiments could not distinctly
distinguish various types of metal–ligand complexes such as
Fig. 2 1H NMR titration (300 MHz, CDCl3–CD3CN (3/1, v/v), 25 1C) of 2 : 1
mixtures of monomers 1 and 2 (4 mM and 2 mM, respectively) with increasing
equivalents of Zn(OTf)2: (a) 0 equiv.; (b) 0.2 equiv.; (c) 0.5 equiv.; (d) 0.8 equiv.;
(e) 1.5 equiv. of Zn(OTf)2 versus the terpyridine unit.
performed to evaluate the monomer concentration effect on the
formation of linear supramolecular polymers 5 (Fig. S12, ESI†). Upon
mixing a 1:2:1 molar ratio of Zn(OTf)2, 1 and 2 in CDCl3–CD3CN
(3/1, v/v) at low concentration, well-defined signals for B21C7/
2+
Zn(tpy)2 and Zn(tpy)2+. Therefore, deeper insights into the
exchange kinetics of Zn2+–terpyridine complexes were provided by
1H NMR titration measurements, by means of titration of Zn(OTf)2
with a 2 : 1 mixture of monomers 1 and 2. When the feed ratio of
Zn2+/tpy is below 0.5 (Fig. 2a–c), the complexed terpyridine proton H5
shifts downfield, while H1 reveals a remarkable upfield shift from
8.74 to 7.80 ppm, mainly attributing to the shielding effect of the
neighboring terpyridine unit. Once an exact 1 : 2 Zn2+–terpyridine
molar ratio is achieved (Fig. 2c), original uncomplexed signals totally
disappear, representing the formation of dimeric Zn(tpy)22+ structure
3. However, once the Zn2+/terpyridine ratio is further increased to
3:2, a third set of signals evolve and gradually strengthen, along
1
secondary ammonium salt moieties in H NMR spectra indicate
the domination of cyclic oligomers. As the monomer concentration
gradually increases, the original cyclic benzylic protons H15 and
ethyleneoxy protons of the B21C7 unit progressively decrease, whilst
the newly formed linear species gradually strengthen, revealing the
formation of high-molecular-weight aggregates at high concen-
tration. On the other hand, free terpyridine units are completely
converted to metal-complexed states for all the measured concentra-
tions, consistent with UV/Vis titration results. Moreover, their signals
remain almost unchanged between cyclic oligomers and linear
supramolecular polymers, presumably due to the similar chemical
environment for these two species.
2+
with the disappearance of Zn(tpy)2 signals (Fig. 2e), suggesting
the formation of a 1 : 1 metal–ligand complex Zn(tpy)2+ with the
additional coordination sites saturated by solvent or triflate
counterions.11 Kinetically labile property of the Zn2+–terpyridine
complex is also observed for the model system containing
monomer 2 and Zn(OTf)2 without the presence of homoditopic
host 1 (Fig. S11, ESI†), which further demonstrates that terpyridine–
Zn2+ complexation is not interfered by the neighboring B21C7/
secondary ammonium salt recognition motif.
Subsequently, two-dimensional diffusion-ordered NMR
(DOSY) experiments were also performed. As the monomer
concentration increased from 4.00 mM to 80.0 mM, the measured
diffusion coefficients decrease considerably from 3.57 ꢀ 10ꢁ10 to
6.37 ꢀ 10ꢁ11 m2 sꢁ1 (Fig. S13, ESI†). The result, in line with
1
the above concentration-dependent H NMR result, suggests that
monomer concentration exerts a significant impact on the reversible
supramolecular polymerization process.
After confirming orthogonal self-assembling properties of
terpyridine–Zn2+ and B21C7/secondary ammonium salt recognition
motifs, concentration-dependent 1H NMR measurements were
Macroscopic properties of the resulting supramolecular
assemblies were further investigated by capillary viscosity measure-
ments. All of the experiments were performed in CHCl3–CH3CN (3/1,
v/v) containing 0.05 M tetrabutylammonium hexafluorophosphate to
exclude the polyelectrolyte effect. Specific viscosities of the dimeric
2+
Zn(tpy)2 complex 3, as well as host–guest paired dimer 4, give
comparably shallow curves (Fig. 3a). In sharp contrast, the specific
viscosity of a 1:2:1 mixture of Zn(OTf)2, 1 and 2 changes exponen-
tially as a function of monomer concentration, characteristic of the
formation of high-molecular-weight polymers 5. Such phenomena
clearly support that both of the orthogonal metal–ligand and host–
guest self-assembling motifs are indispensable for the chain exten-
sion. Attributing to the length mismatch effect of the two monomers,
the propensity to form cyclic species is efficiently suppressed, as
reflected by the relatively low CPC value (9 mM, Fig. S14, ESI†).
Fig. 1 Development of a UV/Vis absorbance band at 343 nm in CHCl3–CH3CN
(3/1, v/v). (a) Titration of a 2 : 1 mixture of monomers 1 and 2 (0.02 mM and 0.01 mM,
respectively) with Zn(OTf)2; (b) stepwise addition of a competitive ligand cyclen
(&) and Zn(OTf)2 (K) to a 1 : 2 : 1 mixture of Zn(OTf)2, 1 and 2.
c
5952 Chem. Commun., 2013, 49, 5951--5953
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In conclusion, we demonstrated that terpyridine–Zn2+ and
B21C7/secondary ammonium salt motifs recognize each other
in an orthogonal manner. Novel linear supramolecular poly-
mers 5 could be successfully constructed via the combination
of these two non-covalent recognition motifs, which were
confirmed by 1H NMR, UV/Vis, DOSY and viscosity measurements.
The resulting supramolecular polymers exhibit multi-stimuli
responsiveness capabilities, as triggered by heat, pH or a competi-
tive ligand, cyclen. Such adaptive assemblies would be an appealing
choice for further fabrication of intelligent supramolecular materials
with tailored properties.
Fig. 3 (a) Specific viscosities of the linear supramolecular polymers 5 (’),
2+
dimeric Zn(tpy)2 complex 3 ( ), and host–guest paired [3]pseudorotaxane 4
(
) (CHCl3–CH3CN (3/1, v/v), 293 K) as a function of terpyridine unit concen-
tration; (b) temperature-dependent specific viscosity of 5 at terpyridine unit
This work was supported by the National Natural Science
Foundation of China (21274139, 91227119), the Fundamental
Research Funds for the Central Universities, and the Anhui
Provincial Natural Science Foundation.
concentration of 90 mM.
Dynamic properties of the resulting supramolecular poly-
mers 5 were further evaluated with a variety of external stimuli.
Specific viscosity of the supramolecular polymers 5 reduces
significantly above 50 1C, indicating the disassembly of supra-
molecular polymers at elevated temperature (Fig. 3b). Detailed
1H NMR measurement reveals that host–guest interactions are
more susceptible to the temperature variation (Fig. 4a). More-
over, adding 1.5 equiv. of Et3N deprotonates the secondary
ammonium salt units, leading to the considerable reduction of
characteristic [3]pseudorotaxane signals and thereby breaking
up the supramolecular polymers 5 (Fig. 4c). Subsequent addition
of 2.8 equiv. of CF3COOH could almost restore the polymeric
assemblies (Fig. 4d), as reflected by the enhancement of com-
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Fig. 4 Partial 1H NMR spectra (300 MHz, CDCl3–CD3CN (3/1, v/v)) of a 1 : 2 : 1
mixture of Zn(OTf)2, 1 and 2 at terpyridine unit concentration of 36 mM: (a) 60 1C;
(b) 25 1C; (c) treating the mixture with 1.5 equiv. of Et3N at 25 1C; (d) subsequent
addition of 2.8 equiv. of CF3COOH.
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 5951--5953 5953