Heat-set gels and egg-like vesicles using two component gel system based on chiral calix[4]arenes

Article abstract of DOI:10.1039/b713548c

Chiral calix[4]arenes bearing long tertiary alkyl groups at the upper rim and S-1-phenylethylamine groups at the lower rim can form heat-set gels and egg-like vesicles enantioselectively with d-2,3-dibenzoyltartaric acid in cyclohexane, which is the first

Full text of DOI:10.1039/b713548c

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Heat-set gels and egg-like vesicles using two component gel system
based on chiral calix[4]arenes{
Jin-Lan Zhou,ab Xian-Jie Chena and Yan-Song Zheng*ab
Received (in Cambridge, UK) 4th September 2007, Accepted 28th September 2007
First published as an Advance Article on the web 12th October 2007
DOI: 10.1039/b713548c
Chiral calix[4]arenes bearing long tertiary alkyl groups at the
upper rim and S-1-phenylethylamine groups at the lower rim
can form heat-set gels and egg-like vesicles enantioselectively
with D-2,3-dibenzoyltartaric acid in cyclohexane, which is the
first example of heat-set gels resulting from difference in inter-
actions between two component gelators: in addition, the dia-
meter of vesicles decreased with the increase in length of alkyl
groups, which could be used to control the size of the vesicles.
Two-component gel systems, which are significant conceptually
different1a from their one-component analogues, play an increas-
Fig. 1 Schematic structure of calix[4]arene 1.
ingly important role in self-assembling nano- and micro-struc-
Friedel–Crafts alkylation7 but modified to use a long tertiary alkyl
alcohol as the reagent and concentrated sulfuric acid as the
catalyst. This fulfilled the simplest and shortest route for alkylation
of calixarenes at the upper rim. Using 1H-NMR spectra, the chiral
calix[4]arene diamine 1 is shown to be in the cone conformation,
probably due to intramolecular hydrogen bonding between the
hydroxyl groups and the ether oxygen atoms at the lower rim.
A solid mixture of 1a and L-2 was prepared beforehand by
evaporating a solution of 1a (5 mM) and L-2 (10 mM) in
chloroform. The mixture was dissolved in cyclohexane by heating
and the resultant solution (at the same concentration as that in
chloroform) was cooled to 20 uC to give a gel. Performing the same
procedure using 1a and D-2 under the same conditions led to a
clear solution. To our surprise, when heated to 60 uC, the cyclo-
hexane gel of 1a and L-2 became a solution, but the cyclohexane
solution of 1a and D-2 changed into a gel (Fig. 2). The gel that was
formed at high temperature was cooled to about 20 uC and after
standing for about 5 min the solution regenerated, which was still a
solution after standing it at 10 uC for a long time. This process of
tures.1–3 The biggest advantage of two-component gel systems over
one-component systems is that the structure and properties of gel
nano- and micro-materials can be tuned by changing the molecular
ratio of the two components or by changing one of the two
components. This has been demonstrated with some typical
examples from Smith,3a Huc,3b and Hong’s3c groups. However, up
to now, nano- and micro-vesicles prepared by two-component gel
systems have not been reported, although the vesicles have a
specially attractive application in storing, transferring and pro-
tecting guest molecules. In fact, even using single component gel
systems, the resultant vesicles are also rarely reported.4
Gels or viscous fluids induced by heating have great potential
applications in many areas, such as medical materials,5a–c flow
control and separations,5d enhanced oil recovery,5e thermorespon-
sive materials,5f,5g and food preparation.5h However, heat-set gels
using a low-molecular-mass gelator (LMMG) are very rare.5f,5g
After recently finding a novel self-assembly system based on chiral
calix[4]arenes,6 we report that the interaction of chiral calix[4]arene
1, bearing long tertiary alkyl groups at the upper rim, can, with
2,3-dibenzoyltartaric acid 2 form a heat-set gel, gelatinize
cyclohexane through forming vesicles, and by changing the alkyl
chain length allow the sizes of the vesicles to be tuned.
Chiral calix[4]arene diamine 1 with long tertiary alkyl groups at
the upper rim was synthesized in several steps (Fig. 1). The key
step is the introduction of varying lengths of alkyl groups onto
the calixarene at the upper rim using a previously reported
aDepartment of Chemistry, Huazhong University of Science and
Technology, Wuhan 430074, P. R. China.
E-mail: zyansong@hotmail.com; Fax: +86-27-87543632;
Tel: +86-27-87543232
bHubei Key Laboratory of Materials Chemistry and Service Failure,
Huazhong University of Science and Technology, Wuhan 430074,
P. R. China
{ Electronic supplementary information (ESI) available: Experimental
materials and methods, spectra and more images. See DOI: 10.1039/
b713548c
Fig. 2 Gels changed with temperature.
5200 | Chem. Commun., 2007, 5200–5202
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Fig. 4 Dynamic light scattering (DLS) diagram of solution of 1a (5 mM)
and D-2 (10 mM) in cyclohexane at 20 uC, the solution was diluted ten
times with cyclohexane.
The aggregate behavior of 1a and D-2 in solution at low
temperature was also investigated by dynamic light scattering
(DLS), circular dichroism (CD) spectra and fluorescent spectra, in
which the solvent was not evaporated. DLS showed that the
solution of 1a and D-2 at 20 uC included mainly small particles
(94%) with diameters in the range of 90–250 nm, which were
accordant to the above results obtained by microscopy. However,
larger aggregates (6%) with diameters in the range of 4–6 mm were
also observed (Fig. 4). This confirmed that two kinds of aggregates
coexist in the solution of 1a and D-2 in cyclohexane at 20 uC, the
main ones being vesicles and the minor ones being nanofibers. This
was not surprising because the nanofibers and nanovesicles can be
transformed into each other but the vesicles are more stable at low
temperatures. The circular dichroism (CD) spectra from mixing of
1a and D-2 at 60 uC and 20 uC were completely different (Fig. S3{).
The gel diluted 6 times with cyclohexane at 60 uC showed positive
Cotton effects at 247, 236, 227 and 220 nm, but the diluted
solution at the same times and solvent at 20 uC showed negative
Cotton effects at 239 nm and 224 nm. This confirmed that the
aggregates formed at high temperature and low temperature were
totally different. We also explored the aggregate behavior with a
fluorescent probe, PEB, a very kind of stable coumarine (Fig. S4{).
The intensity of emission band of PEB at 427 nm in solution of 1a
and D-2 was always larger than that in pure cyclohexane at 20 uC.
It is known that a fluorescent probe will exhibit a more intense
fluorescence upon binding to a supramolecular surface due to the
decrease of self-quenching.8 This indicated that the solution of 1a
and D-2 at low temperatures did truly include aggregates although
the solution seemed to be clear and not very viscous.
Fig. 3 (A and B) TEM images of the gel from 1b and D-2 at 60 uC. (C)
TEM image of solution of 1a and D-2 at 20 uC. (D) FE-SEM image of
solution of 1a and D-2 at 20 uC.
the gel and solution regenerating as the temperature changed could
be repeated for many times. In order to avoid mistakes from
impurities included accidentally, several batches of 1a that were
prepared at different dates were used and these all gave the same
results. After being stored for more than one year at room
temperature, 1a still showed thermally induced gelation.
The gels that resulted from mixture of 1a and L-2 at low
temperature as well as mixture of 1a and D-2 at high temperature
were revealed to be fibers by using TEM and FE-SEM (Fig. 3A–B
and Fig. S1), but the fibers from the gels of 1a and D-2
always existed together with vesicles (Fig. 3B). It is normal that a
gel has a structure of nano- or micro-fibers. However, the solution
of 1a and D-2 at low temperature should also be some kind of
aggregate in cyclohexane instead of free molecules or ion pairs. It
was sure that the solution was composed of egg-like nano-vesicles,
which were shown in both TEM images (Fig. 3C) and FE-SEM
images (Fig. 3D).
As shown in Fig. 3B, some small vesicles, which did not
transform into fibers at high temperature, existed together with
one ribbon-like nanofiber. The shell thickness of the small vesicles
was about 5 nm, and the thickness of the up-rolled edges of the
ribbon-like nanofiber was also about 5 nm. It indicated that the
shell of the small vesicles and the edges of the ribbon-like
nanofibers were made of one layer of a 1a?22?1a complex, which
has an extended molecular length of about 5 nm, calculated by 3D
ChemDraw software. However, at low temperature, the nanofibers
as well as the small vesicles are transformed into larger egg-like
vesicles with a diameter from about 200 nm to 600 nm as shown in
TEM image (Fig. 3C). From this TEM image it was found that the
thickness of the vesicles was around 10 nm, indicating that it was
composed of two layers of the 1a?22?1a complex. The FE-SEM
image in Fig. 3F also showed that the gel of 1a and D-2 at low
temperature was made of egg-like vesicles that a diameter from
180 nm to 400 nm, which was in accordance with the results
obtained from TEM images. In addition, this kind of egg-like
vesicle was also confirmed by AFM (Fig. S2{).
Compared with 1a, both 1b and 1c could gelatinize cyclohexane
at 20 uC together with either L-2 or D-2 under the same conditions,
and the resultant gels had a similar tendency to change with
temperature. Upon heated from 20 uC to 60 uC, the gel resulting
from a mixture of 1b or 1c and L-2 became a solution, but the gel
from a mixture of 1b or 1c and D-2 did not melt and remained the
same. Standing at 10 uC for more than one hour, the gel of 1b or
1c and L-2 remained a gel, and the gel of 1b or 1c and D-2 became
a suspension. Moreover, all gels obtained from 1b and 1c were
revealed to contain vesicles, as shown in TEM images (Fig. 5 and
Fig. S11–13{). The diameter of the vesicles ranged from 400 nm to
1.0 mm for 1b (Fig. 5B)and from 1.0 mm to 2.5 mm for 1c (Fig. 5C),
compared to a diameter of 200 nm to 600 nm for 1a vesicles
(Fig. 5A and Fig. 3C–D). This means that the diameter of the vesi-
cles decreased with an increase in the length of the alkyl groups, so
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The authors thank National Natural Science Foundation of
China for financial support (No. 20072007 and 20672039) and
thank the Analytical and Testing Centre at Huazhong University
of Science and Technology for measurement.
Notes and references
1 Review for gel using two-component system: (a) A. R. Hirst and
D. K. Smith, Chem.–Eur. J., 2005, 11, 5496–5408; (b) D. K. Smith, Adv.
Mater., 2006, 18, 2773–2778; (c) D. K. Smith, Chem. Commun., 2006,
34–44.
2 Reviews for gel materials using LMMG: (a) A. Brizard, R. Oda and
I. Huc, Top. Curr. Chem., 2005, 256, 167–218; (b) M. A. B. Block,
C. Kaiser, A. Khan and S. Hecht, Top. Curr. Chem., 2005, 245, 89–150;
(c) X. Y. Gao and H. Matsui, Adv. Mater., 2005, 17, 2037–2050; (d)
T. Shimizu, M. Masuda and H. Minamikawa, Chem. Rev., 2005, 105,
1401–1443; (e) D. T. Bong, T. D. Clark, J. R. Granja and M. R. Ghadiri,
Angew. Chem., Int. Ed., 2001, 40, 988–1011; (f) P. Terech and R. G.
Weiss, Chem. Rev., 1997, 97, 3133–3159; (g) J. M. Schnur, Science, 1993,
262, 1669–1676; (h) M. George and R. G. Weiss, Acc. Chem. Res., 2006,
39, 489–497; (i) A. D. Guerzo and J.-L. Pozzo, Mol. Gels, 2006,
817–855; (j) T. Ishi-i and S. Shinkai, Top. Curr. Chem., 2005, 258,
119–160.
3 Gel materials using two-component system: (a) A. R. Hirst, D. K. Smith,
M. C. Feiters, H. P. M. Geurts and A. C. Wright, J. Am. Chem. Soc.,
2003, 125, 9010–9011; (b) R. Oda, I. Huc, M. Schmutz, S. J. Candau
and F. C. Mackintosh, Nature, 1999, 399, 566–569; (c) Y. L. Ho,
S. R. Nam and J. I. Hong, J. Am. Chem. Soc., 2007, 129, 1041–1042; (d)
H. A. Behanna, K. Rajangam and S. I. Stupp, J. Am. Chem. Soc., 2007,
129, 321–327; (e) Y. Zhou, M. Xu, T. Yi, S. Xiao, Z. Zhou, F. Li and
C. Huang, Langmuir, 2007, 23, 202–208; (f) M. George,
G. P. Funkhouser, P. Terech and R. G. Weiss, Langmuir, 2006, 22,
7885–7893; (g) S. Arai, K. Imazu, S. Kusuda, I. Yoshihama,
M. Tonegawa, Y. Nishimura, K. Kitahara, S. Oishi and T. Takemura,
Chem. Lett., 2006, 35, 634–635; (h) D. R. Trivedi and P. Dastidar,
Chem. Mater., 2006, 18, 1470–1478; (i) T. Zemp, M. Dubois, B. Deme
and I. Gulik-Krzywicki, Science, 1999, 283, 816–819; (j) S. Yagai,
T. Nakajima, K. Kishikawa, S. Kohmoto, T. Karatsu and A. Kitamura,
J. Am. Chem. Soc., 2005, 127, 11134–11139.
4 (a) N. S. S. Kumar, S. Varghese, G. Narayan and S. Das, Angew.
Chem., Int. Ed., 2006, 45, 6317–6321; (b) A. Ajayaghosh, R. Varghese,
V. K. Praveen and S. Mahesh, Angew. Chem., Int. Ed., 2006, 45,
3261–3264; (c) A. Battaglia and A. J. Ryan, Nat. Mater., 2005, 4,
869–876; (d) F. M. Menger and A. V. Peresypkin, J. Am. Chem. Soc.,
2003, 125, 5340–5345; (e) J. H. Jung, Y. Ono, K. Sakurai, M. Sano and
S. Shinkai, J. Am. Chem. Soc., 2000, 122, 8648–8653; (f) P. Shahgaldian,
A. W. Coleman, S. S. Kuduva and M. J. Zaworotko, Chem. Commun.,
2005, 15, 1968–1970.
5 (a) E. R. Wright, R. A. McMillan, A. Cooper, R. P. Apkarian and
V. P. Conticello, Adv. Funct. Mater., 2002, 12, 149–154; (b) B. Jeong,
K. M. Lee, A. Gutowska and Y. H. An, Biomacromolecules, 2002, 3,
865–868; (c) T. S. Davies, A. M. Ketner and S. R. Raghavan, J. Am.
Chem. Soc., 2006, 128, 6669–6675; (d) C.-W. Kan, E. A. S. Doherty and
A. E. Barron, Electrophoresis, 2003, 24, 4161–4169; (e) T. N. C. Dantas,
V. C. Santanna, A. A. D. Neto, E. L. B. Neto and M. Moura, Colloids
Surf., A, 2003, 225, 129–135; (f) K. Kuroiwa, T. Shibata, N. Takada and
N. Kimizuka, J. Am. Chem. Soc., 2004, 126, 2016–2021; (g) K. S. Moon,
H. J. Kim, E. Jee and M. Lee, Angew. Chem., Int. Ed., 2007, 46, 1–5; (h)
P. J. Shand, H. Ya, Z. Pietrasik and P. K. Wanasundara, Food Chem.,
2007, 102, 1119–1130.
Fig. 5 (A, B and C) TEM images of gel from (A) 1a and D-2, (B) 1b and
D-2, (C) 1c and D-2 (scale bar for A, B and C, 1 mm). (D) AFM images of
gel from 1b and D-2 on freshly broken mica (25 6 25 mm).
that the size of vesicles could be controled by changing the length
of the alkyl groups at the upper rim of the calix[4]arene. This result
is consistent with Jiang et al.’s report in which they found that the
size of the vesicles based on amphiphilic calixarenes decreased with
the increasing length of the hydrophilic chains.9 The structure of
the vesicles resulting from a mixture of 1b and D-2 was confirmed
by AFM images (Fig. 5D) and FE-SEM images (Fig. S9–10{).
AFM imaging showed that the aggregates were spherical entities
with sizes ranging from 1.0 mm to 2.0 mm, which was similar to the
result observed with the TEM images. Probably due to the bigger
size and reduced solubility (shorter alkyl chain) of the vesicles from
1b and 1c compared to 1a, the mixture of 1b or 1c and D-2 is a gel
but that of 1a and D-2 is a solution at room temperature.
In general, with long alkyl group(s), an amphiphile is more
prone to self-assembly due to hydrophobicity and van der Waals
attraction between the long alkyl groups. Therefore not only the
mixture of 1a and L-2 but also that of 1a and D-2 can self-assemble
into nanofibers. However, the fibrous assembles of 1a–D-2 are not
stable due to the unmatched interaction between the chiral centers
of 1a and D-2. The unmatched interaction will increase as the
temperature decreases and the intermolecular distance shortens.
By curving the flat lamellar, the repulsion between the chiral
centers of 1a and D-2 can be reduced, so that fibrous objects
spontaneously transform into vesicles. This is similar to lamellar
gels and spontaneous vesicles in catanionic surfactant mixtures.10
1b and 1c bear slightly shorter alkyl groups and can only form
vesicles because the van der Waals attraction between alkyl groups
decreases and the unmatched interaction of chiral centers of 1 and
D-2 stand out much more. At lower temperatures the vesicle gel
will become more a stable suspension and even precipitate.
In conclusion, for the first time we have demonstrated that
heat-set gels can form through the transformation from vesicles
to nanofibers due to a difference in interaction between two
component chiral gelators. This finding provides a new approach
to heat-set gels, which have potential applications for thermo-
responsive materials.
6 Y.-S. Zheng, A. Ji, X.-J. Chen and J. L. Zhu, Chem. Commun., 2007, 32,
3398–3400.
7 Y.-S. Zheng and Z.-T. Huang, Synth. Commun., 1997, 27, 1237–1245.
8 (a) K. K. Karukstis, C. A. Zieleniuk and M. J. Fox, Langmuir, 2003, 19,
10054–10060; (b) T. Kodama, A. Ohta, K. Toda, T. Asakawa and
S. Miyagishi, Colloids Surf., A, 2006, 277, 20–26; (c) R. M. Minardi,
P. C. Schulz and B. Vuano, Colloids Surf., A, 2002, 197, 167–172; (d)
W. O. McClure and G. M. Edelman, Acc. Chem. Res., 1968, 1, 65–70.
9 M. Lee, S. Lee and L. Jiang, J. Am. Chem. Soc., 2004, 126,
12724–12725.
10 (a) B. A. Coldren, H. Warriner, R. van Zanten and J. A. Zasadzinski,
Langmuir, 2006, 22, 2465–2473; (b) J. I. Escalante, M. Gradzielski,
H. Hoffmann and K. Mortensen, Langmuir, 2000, 16, 8653–8663.
5202 | Chem. Commun., 2007, 5200–5202
This journal is ß The Royal Society of Chemistry 2007