10.1002/ejoc.201701650
European Journal of Organic Chemistry
COMMUNICATION
Enhanced Mechanical and Helical Properties with Achiral
Calix[4]arene in Co-Assembled Hydrogel which Exhibited the
Helical Structure
Heekyoung Choi,[a][‡] Hyowon Seo,[a][‡] Misun Go, [a] Shim Sung Lee [a] and Jong Hwa Jung*[a]
Abstract: A mixture of the building blocks 1A and 2G having D-
alanine moieties and glycine-functionalized calix[4]arene moieties,
respectively, formed co-assembled hydrogel. In particular, the
remarkable enhancement of helical intensity of co-assembled gel was
controlled by achiral calix[4]arene 2G, which was attributed the bridge
effect helically between 1A and 2G like helical structure. Furthermore,
the improvements of mechanical strength (G´ and G" values) of co-
assembled hydrogel prepared with 1.0 equiv. of 2G were ca. 7000%
and ca. 4400% as compared to the gel prepared from the 0.25 equiv.
of 2G, respectively. The improved mechanical strength was attributed
to the formation of network structure with the H-bonding interaction
between 1A and 2G. The results indicate that the helical and
mechanical strength enhanced of co-assembled gel could be
controlled by achiral component 2G.
Furthermore, the mechanical property of helical structure gives
higher than that linear structure.[22] Therefore, the well-defined
helical nanofibrous in gels might be exhibited more strong
mechanical property in compared to the linear fiber structures in
gels. In this study, we have improved the viscoelastic and the
helical properties of co-assembled gel composed of D-alanine
appended bipyridine gelator and achiral glycine-appended
calix[4]arene gelator. The mechanical property of co-assembled
gel was remarkably improved upon addition of achiral component.
Upon addition of achiral calix[4]arene in gel system showed
remarkable a well-organized helical property. Here we report on
remarkable improvement of both the mechanical and the helical
properties of co-assembled hydrogel with D-alanine appended
bipyridine gelator and achiral glycine-functionalized calix[4]arene
gelator as a core building block by bridge effect with the
intermolecular hydrogen-bonding interaction.
Soft material development has seen a wealth of applications for
viscoelastic systems such as organogels and hydrogels,
particularly for areas including controlled release[1-3] and soft
tissue reconstruction.[4,5] In biomedical applications, gel systems
have been implemented in energy capture and storage[6,7] as well
as sensing areas[8,9] by the discovery of their exciting responsive
properties toward external stimuli such as temperature, pH, light
and electric field.[10-12]
In most of cases, gels are formed using one gelator. Mixing
different gelators (where each form gels independently) is also
interesting.[13-15] Depending on how these gelators assemble, a
mixed gel systems could be controlled the various physical
properties of the final gels. For instance, Nandi group has been
reported the mechanical and electrical properties of co-
assembled gels.[16] In contrast, a number of systems have been
reported where two components are required to interact to form a
gel,[17-20] however, the individual components do not form gels by
themselves.
Although several groups have demonstrated the control of the
mechanical properties of co-assembled gels,[16,21] the
development of co-assembled gels remains important for the
improvement and control of the mechanical and helical properties.
In particular, the improvement of helicity with achiral components
in co-assembled gels is difficult, with relatively few examples
reported. Because, generally, upon addition of achiral gelators in
co-assembled gels disrupts the helical molecular arrangement.
The bipyridine and 1,3-alternated calix[4]arene moieties were
used as a core building block, which enables formation of three-
dimensional network and metal ion binding. The peptide moieties
were introduced to the control of helicity and the intermolecular
hydrogen-bonding interaction in co-assembly system. For
instance, compound 1A possesses bipyridine moiety as metal
bind site and peptide moiety to the intermolecular hydrogen-
bonding interaction. Compound 2G consists of each two glycine
moieties at upper and lower parts of 1,3-alternated calix[4]arene
as a core group, which each two glycine attached at calix[4]arene
moiety enable to introduction
a well-organized molecular
arrangement of 1A with π-π stacking.
Since compounds 1A or 2G did not form gel in organic solvents
and water, co-assembled hydrogel of 1A (co-hydrogel) was
prepared by the addition of 2G. Gel formation occurred instantly
and was characterized by the cessation of flow in the test tube
inversion experiments (Figure S1). The co-hydrogel (1A-2G) was
stable for two months at room temperature.
[a]
H. Choi, H. Seo, M. Go, Dr. Prof. S. S. Lee, Dr. Prof. J. H. Jung
Department of Chemistry and Research Institute of Natural
Sciences
Gyeongsang National University
Jinju 900 (Republic of Korea)
Absorption spectra of 1A, 2G and co-hydrogel (1A-2G) in a
water showed in Figure S2. Compound 1A exhibited two
absorption bands at 249 nm and 297 nm, which may be ascribed
to be the π-π* and n-π* transition, respectively. In the co-hydrogel
E-mail: jonghwa@gnu.ac.kr
These authors contributed equally in this work.
[‡]
Supporting information for this article is given via a link at the end
of the document.
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