A dual-responsive supramolecular polymer gel formed by crown ether based molecular recognition

Article abstract of DOI:10.1002/anie.201006999

A host-guest recognition motif based on a crown ether moiety was employed to construct a supramolecular polymer gel from a low molecular weight A-B monomer. Reversible thermo- and pH-induced gel-sol transitions of the dual-responsive gel are demonstrated for the controlled release of rhodamineB.

Full text of DOI:10.1002/anie.201006999

DOI: 10.1002/anie.201006999
Supramolecular Polymers
A Dual-Responsive Supramolecular Polymer Gel Formed by Crown
Ether Based Molecular Recognition**
Shengyi Dong, Yan Luo, Xuzhou Yan, Bo Zheng, Xia Ding, Yihua Yu, Zhi Ma, Qiaoling Zhao,
and Feihe Huang*
Reversibility and responsiveness are ubiquitous in nature,
whereby biological systems utilize stimuli-responsive aggre-
gates to develop highly complicated and ordered materials.
The realization of reversibility and responsiveness is of
particular importance in the improvement of properties and
the development of new materials. Supramolecular chemistry,
through the use of noncovalent interactions, offers a platform
to construct advanced and sophisticated materials through a
bottom-up approach.[1] Supramolecular gels constructed from
low molecular weight molecules by reversible noncovalent
interactions is a kind of adaptive material that can be sensitive
to the changes of pH value, temperature, solvent, chemical
stimuli, and even the concentration of components.[2] Being
responsive to multiple stimuli, these supramolecular gels are
expected to be highly advantageous over traditional polymer
gels and possess many unique properties that play a signifi-
cant role in drug-delivery systems, molecular devices, or novel
membranes.[3]
small molecules by crown ether based molecular recognition
have not been reported. Although Shinkai and co-workers
have developed gelators comprised of crown ethers,[6] the
driving forces for the gelation are largely attributed to the
appended groups. It is still a big challenge to design and
synthesize novel stimuli-responsive gels completely based on
the host–guest interactions between crown ether and com-
plementary guest moieties.
Stoddart’s and Gibson’s groups have found that A–B
monomers 1 and 2 with the guest salt units directly attached to
Various noncovalent interactions,[4] such as host–guest
interactions, hydrogen bonds, and metal coordination, have
been used to prepare supramolecular gels from low molecular
weight molecules. Crown ethers, serving as the first gener-
ation of supramolecular hosts, have been widely used as
building blocks to construct different functional assemblies
with complementary guest molecules.[5] Although supra-
molecular alternating copolymers prepared from self-sorting
organization of two heteroditopic monomers were reported
by us,[1i] up to now, supramolecular polymer gels formed from
crown ether moieties are favored to form [c2]daisy chains
instead of linear supramolecular polymers.[7] To effectively
construct the supramolecular polymer gels, attention should
be paid to avoiding the formation of the cyclic oligomers. On
the basis of a structural analysis of previously reported
supramolecular polymer monomers, it was found that the long
flexible alkyl linkers between the host and guest moieties not
only favor the formation of linear supramolecular polymers,
but also lead to a low critical suprapolymerization concen-
tration (CPC).[1i,l,8] On the basis of such observations, we
report herein the design and synthesis of a novel dual-
responsive supramolecular polymer gel, which is constructed
from a heteroditopic A–B monomer 3 utilizing the reversible
[*] S. Dong, Y. Luo, X. Yan, B. Zheng, Prof. Dr. F. Huang
Department of Chemistry, Zhejiang University
Hangzhou, Zhejiang 310027 (China)
Fax: (+86)571-8795-3189
E-mail: fhuang@zju.edu.cn
X. Ding, Prof. Dr. Y. Yu
Shanghai Key Laboratory of Magnetic Resonance
Department of Physics, East China Normal University
Shanghai 200062 (China)
Assoc. Prof. Dr. Z. Ma, Q. Zhao
Shanghai Institute of Organic Chemistry
Chinese Academy of Science
Shanghai 200032 (China)
host–guest
interactions
between
dibenzo[24]crown-8
[**] This work was supported by the National Natural Science
Foundation of China (20774086, 20834004), the Fundamental
Research Funds for the Central Universities (2010QNA3008), the
MOE Key Laboratory of Macromolecular Synthesis and Function-
alization of Zhejiang University (2010MSF02), and the Zhejiang
Provincial Natural Science Foundation of China (R4100009).
(DB24C8) and its complementary guest dibenzylammonium
salt (DBA). It is well known that the dibenzo[24]crown-8
(DB24C8) moiety forms 1:1 threaded structure with the
dibenzylammonium salt (DBA) moiety.[9] Our approach is
that 1) the complexation of DB24C8 and DBA units can be
responsive to two stimuli (pH value or temperature change)
and 2) the flexible long alkyl chain favors the formation of
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201006999.
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Communications
linear supramolecular polymers. This monomer can form a
supramolecular polymer gel in acetonitrile, and reversible
sol–gel transitions can be realized by subsequent alteration of
heating and cooling, or acidification and neutralization. The
thermo- and pH-responsive gel–sol transitions were success-
fully employed in the controlled release of rhodamine B.
Linear supramolecular polymers in solution were first
envisioned to be driven by host–guest interactions through
the strong affinity between DB24C8 and DBA, which is the
basis for the further formation of entangled self-assembled
fibrillar networks. Defined as three-dimensional fiber net-
works, supramolecular polymer gels are derived from the self-
organization of one-dimensional fibrils, which aggregate from
those long supramolecular polymer chains; these supramolec-
ular polymers assemble into bundles at first and subsequently
form an entangled sample spanning network, which is capable
of preventing the flow of bulk solvent. Through the entangle-
ment of supramolecular fibrils, three-dimensional fiber net-
works are constructed and macroscopic organogels are finally
formed by incorporating solvent molecules (Scheme 1).[2d,10]
1H NMR studies provided important insights into the
complexation behavior of monomer 3 in solution. The
concentration-dependent 1H NMR spectra of monomer 3
(Figure 1) were complicated by the slow-exchange complex-
ation of the DB24C8 and DBA units on the proton NMR
timescale. At low concentrations, peaks of cyclic oligomers
and uncomplexed monomer were obvious, and no signals of
linear species were observed (Figure 1d–f), indicating a
preference for the cyclic oligomers. As the initial monomer
concentration was increased, the signals of H1,lin, H2,lin, and
H8,lin related to linear supramolecular polymers became
obvious. With the increase of initial monomer concentration,
the cyclic species dominates at the beginning, and then its
concentration decreases. By contrast, the concentration of
linear species increases constantly, whereas the concentration
of uncomplexed species increases, reaches a maximum at
Figure 1. Partial 1H NMR spectra of monomer 3 (400 MHz,
[D3]acetonitrile, 208C) at various concentrations: a) 400 mm;
b) 200 mm; c) 100 mm; d) 10 mm; e) 5 mm; f) 1 mm. Peaks of linear
polymer, cyclic oligomer, and uncomplexed monomer are designated
by lin, cyc, and uc, respectively.[8d]
about 100 mm, and then decreases. The enhanced signals of
the linear species as well as the gradual disappearance of
cyclic species peaks, along with the broadening of all signals,
confirmed the formation of high molecular weight aggregates
driven by host–guest interactions between the DB24C8 host
and DBA guest moieties.[8d] Those changes showed the
competition between linear chain extension and cyclic
oligomerization of monomer 3.[1i,l,11]
Two-dimensional diffusion-ordered NMR (DOSY)
experiments (see Figure S6 in the Supporting Information)
were also performed to investigate the self-assembly process
of monomer 3 to form the supramolecular polymers. As the
monomer concentration increased from 20 mm to 400 mm, the
measured weighted average diffusion coefficient decreased
considerably from 6.92-
(Æ0.35) ꢀ 10À10 m2 sÀ1
to
5.62(Æ0.28) ꢀ 10À11 m2 sÀ1,
indicating the concentra-
tion dependence of supra-
molecular polymerization
of monomer 3. Based on
the previous reports,[1l] it is
known that a high degree
of polymerization value
for the repeating unit is
necessary to result in a 10-
fold decrease in the diffu-
sion coefficient. Hence, the
current
measurements
clearly indicated the for-
mation of an extended,
high molecular weight
polymer structure.
Viscosity is
a direct
index for the formation of
polymers. Therefore, to
further investigate the
Scheme 1. Formation of a supramolecular polymer gel through self-assembly of monomer 3.
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Angew. Chem. Int. Ed. 2011, 50, 1905 –1909

supramolecular aggregations in solution, a double logarithmic
plot of specific viscosity versus concentration of monomer 3 in
CH3CN (see Figure S6 in the Supporting Information) was
obtained. In the low concentration range, the curve had a
slope of 1.02. As the concentration increased, the slope of the
curve approached 2.32. The linear relationship (slope of 1.02)
indicated the presence of noninteracting assemblies of a
constant size, whereas an exponential relationship (slope > 2)
is consistent with the presence of a supramolecular polymer-
ization process, in which the size of the resulting polymer
increased with concentration. The CPC for monomer 3 in
CH3CN was about 60 mm as evidenced by the clear change of
slope occurring at this concentration, indicating a ring–chain
transition from the formation of cyclic oligomers to highly
ordered polymers; hence, the mobility of the polymeric chains
is restricted.[1i,j,l,11]
Furthermore, rodlike fibers with a regular diameter of
8 mm were drawn from a high concentration solution and
observed by scanning electron microscopy (SEM), providing
direct evidence of the formation of supramolecular polymers
with high molecular weight and a high degree of linear chain
extension (see Figure 3a). Next, the gelation properties of the
synthesized monomer 3 were investigated (Figure 2). The
novel supramolecular polymer gel was prepared by dissolving
monomer 3 in CH3CN at 508C followed by cooling to room
temperature. Upon increasing the concentration of monomer
3 from 10 mm to 35 mm, the solution became a mixture of
solution and gel, and finally formed a gel at a phase-transition
temperature of approximately 408C; the critical gel concen-
tration was calculated to be 4.6 wt%.
acid. This process was also demonstrated by 1H NMR experi-
ments (see the Supporting Information) and SEM experi-
ments (see the Supporting Information). The addition of
20 mL of triethylamine to a solution of monomer 3 in
[D3]acetonitrile (50 mm, 0.5 mL) caused remarkable changes
of the proton chemical shifts (Figure S7a,b). Almost all
complexed signals disappeared, sharp signals of uncomplexed
protons H8, H6, and H1 appeared, and the signals of protons
H3 and H4 exhibited upfield shifts. All these observations
suggested that the host–guest interactions were destroyed
completely and the complexation between DB24C8 and DBA
was totally quenched. After 14.0 mL of trifluoroacetic acid
was added, the host–guest complexation was recovered and
the complicated complexed signals were observed again
(Figure S7c).[6b,12] In addition, this reversible decomplexa-
tion–complexation transition could be repeated (Fig-
ure S7d,e).
Also, the host–guest interaction between the DB24C8 and
DBAunits could be reduced by heating, as shown by 1H NMR
1
experiments. The variable-temperature H NMR spectra of
monomer 3 in [D3]acetonitrile (see Figure S8 in the Support-
ing Information) provided direct evidence for the assembly
process from gel to sol state. At a relatively low temperature
1
(268 K), the H NMR signals of monomer 3 almost disap-
peared, suggesting strong intermolecular aggregation. By
gradually raising the temperature, the original weak and
broad signals became well-dispersed and can be easily
identified. These results showed a remarkable temperature-
dependent behavior and suggested that the formation of the
gel was weakened at elevated temperatures and eventually
disrupted,[13] which was consistent with the above gelation test
(Figure 2).
Thus, by adjusting the pH and temperature of the solution,
the gelation process can be manipulated to control a range of
gel properties, such as the gelation time, gel transparency, gel
morphology, and the gel–sol transition.
It is generally known that the secondary ammonium salt
unit can be deprotonated by adding base, thus destroying the
host–guest recognition between DB24C8 and DBA and
making the complex disassemble. Hence, the reversible
gel–sol transition could be realized by changing the pH
value. As shown in Figure 2, after the addition of several
drops of triethylamine, the gel dissociated into a transparent
solution within a short time. Re-formation of the gel from the
sol state was achieved by adding a little excess trifluoroacetic
Xerogels, prepared by freeze-drying the gels of monomer
3 in CH3CN, were examined by SEM, revealing an extended
and interconnected fibrous gel network. The SEM images
showed long fibers and
well-developed
three-
dimensional network struc-
tures of fibers with diame-
ters of 1–2 mm and lengths
of several hundred micro-
meters (Figure 3b). The
formation of fibers is a
typical characteristic for
the entanglement of line-
arly connected macrosized
aggregates. These self-
assembled fibers physically
cross-linked together to
form a fibrous network as
the matrix of the gel.[10a]
This extended network fur-
ther interweaved and tan-
gled with fibers to form a
very dense gel network
Figure 2. Supramolecular gel, its gel–sol transitions triggered by stimuli (temperature and pH), and
supramolecular aggregates (glue-like viscous liquids and transparent films).
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Communications
transition was successfully employed for the controlled
release of rhodamine B. We demonstrated the reversibility,
versatility, and multiresponsiveness of supramolecular poly-
mer gel constructed by an asymmetric complementary A–B
monomer. The work presented herein also demonstrated that
the complex of DB24C8 and DBA can be used as a building
block to construct multiple supramolecular aggregates from
supramolecular polymer gels, viscous liquids, fibers to trans-
parent films under different conditions. This dual-responsive
supramolecular polymer gel is promising as a unique
advanced material with applications in biomedical fields,
personal-care products, and drug-delivery systems.
Received: November 8, 2010
Published online: January 12, 2011
Keywords: crown compounds · host–guest systems ·
molecular recognition · sol–gel processes ·
supramolecular chemistry
Figure 3. SEM images of aggregates: a) rod-like fibers; b–d) structure
of the three-dimensional network.
.
(Figure 3c,d). These xerogels can be destroyed by adding
triethylamine (see the Supporting Information).
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thermo- and pH-controlled release system.[14] Prior to heating
or addition of triethylamine, the layer of water was colorless
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