Mechano-responsive calix[4]arene-based molecular gels: Agitation induced gelation and hardening

Article abstract of DOI:10.1039/c3sm50577d

Two novel cholesteryl derivatives of calix[4]arene with l- or d-phenylalanine residues in the linkers (1 and 2, respectively) were designed and synthesized. The gelation behaviors of the compounds in 30 organic solvents were tested. It was demonstrated that 1 gels n-butanol and n-pentanol at room temperature, but 2 gels isopropanol only under the treatment of heating-cooling cycling or energy supplementing via sonication, vortex or agitation at room temperature. AFM and SEM measurements revealed that the structures of the gel networks are greatly affected by the spatial configurations of the linkers contained in the calix[4]arene derivatives. Furthermore, mechanical treatment not only promotes the gelation of the system of 2-isopropanol, but also enhances the strength of the gel. Specifically, 18 min of agitation at room temperature makes the storage modulus and the yield stress of the gel (3.5%, w/v) exceed 1 × 10⁶ Pa and 6 × 10³ Pa, respectively, which are second only to the mechanically strongest low molecular mass gelator-based molecular gel reported until now. XRD analysis revealed the hexagonal packing structure of 1 in its n-pentanol gel.

Full text of DOI:10.1039/c3sm50577d

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Mechano-responsive calix[4]arene-based molecular
gels: agitation induced gelation and hardening†
Cite this: Soft Matter, 2013, 9, 5807
a
a
a
a
a
a
Xiuqin Cai,ab Ying Wu, Liya Wang, Ni Yan, Jing Liu, Xiaohua Fang and Yu Fang
*
Two novel cholesteryl derivatives of calix[4]arene with L- or D-phenylalanine residues in the linkers (1 and 2,
respectively) were designed and synthesized. The gelation behaviors of the compounds in 30 organic
solvents were tested. It was demonstrated that 1 gels n-butanol and n-pentanol at room temperature,
but 2 gels isopropanol only under the treatment of heating–cooling cycling or energy supplementing
via sonication, vortex or agitation at room temperature. AFM and SEM measurements revealed that the
structures of the gel networks are greatly affected by the spatial configurations of the linkers contained
in the calix[4]arene derivatives. Furthermore, mechanical treatment not only promotes the gelation of
the system of 2–isopropanol, but also enhances the strength of the gel. Specifically, 18 min of agitation
at room temperature makes the storage modulus and the yield stress of the gel (3.5%, w/v) exceed 1 ꢀ
106 Pa and 6 ꢀ 103 Pa, respectively, which are second only to the mechanically strongest low molecular
mass gelator-based molecular gel reported until now. XRD analysis revealed the hexagonal packing
structure of 1 in its n-pentanol gel.
Received 25th February 2013
Accepted 6th April 2013
DOI: 10.1039/c3sm50577d
www.rsc.org/softmatter
of the LMMGs.10–12 As for the stimulus, it could be light,13,14
Introduction
electric/magnetic eld,15 ultrasound,16,17 shear stress,18 and even
chemical.19,20
As a kind of smart material, low molecular mass gelator
(LMMG)-based stimulus-responsive molecular gels have
received increasing attention during the last few years due to
their potential applications1–3 in a variety of elds such as
controlled release,4 mild separation,5 smart cleaning,6 and new
mediums for preparation7 and purication.8,9 Similar to chem-
ical gels, formation of a gelator network in a gel system is a pre-
requirement for the gelation of the system. It is, however, also
different from a chemical gel, as the gelator network in a
molecular gel is formed through the self-assembly of the
molecules of the gelator, rather than chemical cross-linking.
The driving forces behind the self-assembly could either be one
or a combination of weak interactions, including but not
limited to hydrogen bonding, p–p stacking, cation–p interac-
tions, van der Waals interactions, dipole–dipole interactions,
coordination interactions, electrostatic interactions, hydro-
phobic or hydrophilic interactions etc., among the molecules
Creation of LMMG-based stimulus-responsive molecular
gels is always a challenge. A commonly adopted strategy is to
introduce a specic structural unit, which shows a structural
response when stimulated.21–26 For example, Yi and co-workers
created a photochromic switch in an organogel system by
employing a change between the open and closed structure of
a diarylethane.27 Haldar et al. synthesized a tripodal peptide
and used it as a LMMG to gel benzene, toluene, xylene,
cyclohexane, kerosene and petroleum. However, unlike
commonly found molecular gels, sonication is a necessity for
the gelation of these systems.28 Wei and co-workers designed
and prepared two different metal ligands, which gel water in
the presence of Cd2+. Interestingly, the introduction of EDTA
turns the hydrogels back into solutions. The phase transition
could be reverted by providing more Cd2+. Moreover, acid–
base exchange can also be used to switch the process.29 Zhu's
group reported two LMMGs adopting a redox active tetra-
thiafulvalene unit and a urea structure, one of which gels 2-
propanol under sonication, and the other gels polar solvents.
The system could easily be destroyed by oxidation.15 Very
recently, Shinkai and colleagues reported a few mechano-
responsive molecular gels by employing some specically
designed anthracene derivatives as LMMGs.30 However,
compared with other stimulus-responsive molecular gels,
mechano-responsive gels have been rarely studied until now,
due to the fact that it is hard to nd a key structural unit
sensitive to shear stress.
aKey Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education,
School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an,
Shaanxi 710062, P. R. China. E-mail: yfang@snnu.edu.cn; Fax: +86-29-85307566;
Tel: +86-29-85310081
bCollege of Chemistry and Life Science, Weinan Normal University, Weinan, Shaanxi
714000, P. R. China
† Electronic supplementary information (ESI) available: Synthesis routes and
gelation behaviors of gelators; AFM and SEM of concentration dependence of
the gel-networks of 1–pentanol and sonication time dependence of the
gel-networks of 2–isopropanol; 1H NMR spectra of compounds 1 and 2. See
DOI: 10.1039/c3sm50577d
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Recently, we developed a calix[4]arene-based dimeric-cho- gel. In contrast, isopropanol could be gelled by the compound
lesteryl derivative which gels the mixture of n-decane and containing D-phenylalanine in its linker (2, Fig. 1) via a heat-
acetonitrile, and the gel as produced exhibits unprecedented ing–cooling cycle, and furthermore, the gelation is so slow
and fully reversible thixotropic properties31—a property exhibi- that at least 30 h is needed for the system to lose its
ted by some gels or uids that are generally viscous or thick uidity. However, mechanical agitation or sonication not only
under normal conditions, but turn to a less viscous state when promotes the gelation,45 but also enhances the strength of the
shaken, stirred or agitated, and the gels later take a certain gel. Considering the fact that for almost all of the thixotropic
period of time to return to their original state when allowed to and LMMG-based molecular gels reported, shear stress or
stand without being disturbed.32–34 However, strictly speaking, agitation induces a gel-to-solution phase transition rather than
the classical denition of thixotropy pertains only to those the reverse, hence the 2–isopropanol gel system may be
uids that exhibit reversible structural changes. From this point recognized as the second example of shear stress or agitation
of view, it is only possible for physical gels, such as the molec- induced gelation. Details are reported in the following section.
ular gels under discussion, to possess this extraordinary prop-
erty. For example, Weiss et al. reported a gel system of a
naphthalene-based cholesteryl derivative in dodecane, and
Experimental section
Reagents and materials
studied its thixotropic properties by conducting rheological
measurements.35 Recently, Shinkai and co-workers reported a
naphthalenediimide-based organogel which shows a thixo-
tropic phenomenon at concentrations higher than 0.075 wt%.36
Rowan and co-workers synthesized a compound of 2,6-bis(1-
alkyl-benzimidazolyl) pyridine attached with an open chain
crown ether, and found that its acetonitrile gel possessed
thixotropic properties.37 No doubt, thixotropy is a complex
phenomenon, especially for systems like molecular gels. The
structural details and the nature behind this property are far
from being thoroughly understood, and in fact, almost all of the
molecular gels possessing thixotropic properties have been
Cholesteryl, L-phenylalanine, D-phenylalanine, dicyclohexyl-
carbodiimide (DCC), N,N-dimethylaminopyridine (DMAP)
(Sinopharm Group Co. Ltd) and p-tert-butylphenol (98%,
Aladdin Chemistry Co. Ltd.) were used directly without further
purication. Tetrahydrofuran (THF) was distilled over sodium
in the presence of benzophenone under a nitrogen atmosphere
before use. All other reagents were of analytical grade and used
directly without further purication.
Synthetic procedures
discovered by serendipity rather than by design. Therefore, the The synthetic routes to the target compounds 1 and 2 are
creation of molecular gels with thixotropic properties and schematically shown in Scheme S1 (ESI†).
guring out the structural features of the corresponding gela-
Synthesis of cholesteryl L-(D)-phenylalaninate (intermediates
tors still remains a big challenge.
A and A0). Intermediates A and A0 were synthesized as reported
It is well known that chirality plays a crucial role in previously by us.46,47
molecular assembly processes, taking place not only in solu-
Synthesis of calix[4]arene (intermediate B). Intermediate B
tion but also on surfaces and interfaces.38–40 Furthermore, the was synthesized by a slightly modied method reported by
chirality of the molecules of a LMMG could be transferred to Gutsche et al.48 Specically, 20.5 g (140 mmol) of p-tert-butyl-
its assemblies, which frequently gives rise to the assemblies’ phenol, 14.5 mL of 37% formalin (200.2 mmol), and 0.5 mL of
abnormal properties.41–44 Accordingly, we tried to embed a 12.5 mmol mLꢁ1 NaOH solution were mixed together in a 500 mL
chiral phenylalanine residue in the linker connecting a calix- four-necked ask. The mixture was then stirred and heated
[4]-arene unit and a cholesteryl structure. Gelation tests under a nitrogen atmosphere for 2 h in an oil bath at 110–120 ꢂC,
revealed that the compound with L-phenylalanine in the linker resulting in an orange-colored viscous mixture. Finally, a solid-
(1, Fig. 1) gels n-butanol and n-pentanol efficiently, and the like residue was obtained by allowing this mixture to be cooled to
formed gels are stable. When n-propanol or n-hexanol is used, room temperature. The residue was dispersed in 200 mL of
instead of the other two solvents, the system is only a partial diphenyl ether under a nitrogen atmosphere by continuous
stirring. The suspension was reuxed for 2 h, when the color of
the system eventually became dark red. 200 mL of ethyl acetate
was added aer the system reached room temperature, was
stirred for another 30 min, and then was allowed to stand for at
least 30 min. The precipitate formed in the system was collected
via ltration and washed twice with ethyl acetate and acetic acid,
separately. The obtained product was puried by re-crystalliza-
tion from toluene to give a white product (B) in 50% yield. For B:
1H NMR (CDCl3/Me4Si, 300 MHz): d (ppm) 1.21 (s, 36H,
–C(CH2)3), 3.47 (d, J ¼ 12.0, 4H, –ArCH2Ar–), 4.23 (d, J ¼ 12.0, 4H,
–ArCH2Ar–), 7.04 (s, 8H, –ArH), 10.34 (s, 4H, –OH); HRMS: m/z
calcd for [(M + Na)+]: 671.4071, found: 671.4071.
Synthesis of intermediate C. Intermediate B (0.648 g,
1 mmol), potassium carbonate (0.166 g, 1.2 mmol) and
Fig. 1 Structure of the cholesteryl derivatives of calix[4]arene with L- or D-phe-
nylanine in the linkers (1 and 2, respectively).
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potassium iodide (0.017 g, 0.1 mmol) were dissolved in 80 mL of –ArCH2Ar–), 3.15 (d, J ¼ 12.0, 4H, ArCH2CH), 3.43 (d, J ¼ 12.0,
acetone, and then ethyl chloroacetate (0.294 g, 2 mmol) was 2H, –ArCH2Ar–), 3.97 (d, J ¼ 12.0, 2H, –ArCH2Ar–), 4.12 (d, J ¼
slowly added with stirring. The mixture was then reuxed for 12.0, 2H, –ArOCH2), 4.19 (d, J ¼ 12.0, 2H, –ArOCH2), 4.51 (m,
10 h under a nitrogen atmosphere, followed by the evaporation 2H, oxycyclohexyl), 4.86 (d, J ¼ 12.0, 2H, –ArCH2Ar–), 4.99 (m,
of the solvent. The residue was dried and dissolved/suspended 2H, –NHCHCO(CH2)), 5.47 (s, 2H, alkenyl), 8.96 (m, 4H, –ArH),
in chloroform. The organic system was extracted with 10% 7.01 (s, 14H, –ArH), 7.75 (s, 2H, OH) and 9.38 (d, 2H, –NH–);
hydrochloric acid 3 times, and then collected and dried with FTIR, nmax/cmꢁ1: 3437 (NH, OH), 2954 (CH3), 2929 (CH2), 2858
anhydrous magnesium sulfate. The resulting clear solution was (CH), 1715 (C]O, –O), 1653 (C]O, NH), 1534 (NH, bending
evaporated to dryness. The crude product as prepared was vibration), and 1462 (–C–O). HRMS: m/z calcd for [(M + Na)+]:
rstly puried by column chromatography (silicone gel, 200– 1818.2435, found: 1818.2426; anal. calcd for C120H166N2O10
300 mesh; acetone–petroleum ether, v : v ¼ 1 : 9), and then by (%): C 80.22, H 9.31, N 1.56; found: C 79.73, H 9.39, N 1.59. For
re-crystallization from petroleum ether three times to give compound 2: 1H NMR (CDCl3/Me4Si, 300 MHz): d (ppm): 0.68–
intermediate C as white crystals in a yield of 51%. For inter- 2.34 (m, 122H, –C(CH3)3, cholesteryl protons), 3.04 (d, J ¼ 12.0,
1
mediate C: H NMR (CDCl3/Me4Si, 300 MHz): d (ppm) 1.04 (s, 2H, –ArCH2Ar–), 3.15 (d, J ¼ 12.0, 4H, ArCH2CH), 3.43 (d, J ¼
18H, –C(CH2)3), 1.25 (s, 18H, –C(CH2)3), 1.31 (t, 6H, –CH3), 3.29 12.0, 2H, –ArCH2Ar–), 3.97 (d, J ¼ 12.0, 2H, –ArCH2Ar–), 4.12
(d, J ¼ 15.0, 4H, –ArCH2Ar–), 4.26 (q, 4H, –CH2CH3), 4.10 (d, J ¼ (d, J ¼ 12.0, 2H, –ArOCH2), 4.19 (d, J ¼ 12.0, 2H, –ArOCH2),
15.0, 4H, –ArCH2Ar–), 4.72 (s, 4H, –ArOCH2), 6.81 (s, 4H, –ArH), 4.51 (m, 2H, oxycyclohexyl), 4.86 (d, J ¼ 12.0, 2H, –ArCH2Ar–),
7.01 (s, 4H, –ArH), 7.09 (s, 2H, –OH); HRMS: m/z calcd for [(M + 4.99 (m, 2H, –NHCHCO(CH2)), 5.47 (s, 2H, alkenyl), 8.96 (m,
Na)+]: 843.4806, found: 843.4821.
4H, –ArH), 7.01 (s, 14H, –ArH), 7.76 (s, 2H, OH) and 9.31 (d,
Synthesis of intermediate D.49 Intermediate C (0.820 g, 1 2H, –NH–); FTIR, nmax/cmꢁ1: 3437 (NH, OH), 2954 (CH3), 2929
mmol) and 15% NaOH (2.8 mL) were dissolved in 60 mL of (CH2), 2858 (CH), 1715 (C]O, –O), 1653 (C]O, NH), 1534
ethanol, and the mixture was stirred and heated under reux for (NH, bending vibration), and 1462 (–C–O). HRMS: m/z calcd for
6 h. The residue was diluted with cold distilled water (50 mL), [(M + Na)+]: 1818.2435, found: 1818.2355; anal. calcd for
then hydrochloric acid (3 M) was added with vigorous stirring
C120H166N2O10 (%): C 80.22, H 9.31, N 1.56; found: C 79.92, H
until the pH value reached 1, and then the formed solid was 9.39, N 1.58.
collected via ltration. Finally, the residue was dried in air and
dissolved in chloroform. The solution was rst washed with
General methods
hydrochloric acid (3 M) and brine, and was then dried and
concentrated to afford intermediate D (yield: 97%). For D: H
1
Gel preparation. In a typical gelation test, a weighed
NMR (CDCl3/Me4Si, 300 MHz): d (ppm): 1.08 (s, 18H, –C(CH2)3), amount of the potential gelator and a measured volume of the
1.25 (s, 18H, –C(CH2)3), 3.43 (d, J ¼ 14.0, 4H, –ArCH2Ar–), 4.13 solvent mixture were placed in a sealed test tube, then shaken
(d, J ¼ 14.0, 4H, –ArCH2Ar–), 4.70 (s, 4H, –ArOCH2), 6.97 (s, 4H, or sonicated. Later, the system was le to equilibrate. Finally,
–ArH), 7.06 (s, 4H, –ArH), and 7.82 (br s, 4H, OH and COOH). the test tube was placed upside-down to observe if the solution
HRMS: m/z calcd for [(M + Na)+]: 787.4180, found: 787.4170.
inside could still ow. The systems which lose their uidity are
Synthesis of intermediate E. Intermediate D (0.764 g, 1 denoted as “G”, those with only solutions le are referred to as
mmol) was dissolved in 50 mL of toluene containing thionyl “S”. Systems with precipitates or crystals are named as “P”,
chloride (290 mL), and the mixture was stirred and reuxed for and the systems, in which the potential gelator could not be
3.5 h. Removal of the solvent and the un-reacted thionyl chlo- dissolved even at the boiling point of the solvent, are assigned
ride via distillation under reduced pressure furnished the with “I”.
product, diacyl chloride (E), as an off-white solid in quantitative
yield. The product as obtained was used directly in subsequent Scanning Electron Microscopy Spectrometer (Philips-FEI). The
preparations without purication.
acceleration voltage was 20 kV and the emission was 10 mA. The
SEM pictures of the xerogels were taken on a Quanta 200
Preparation of compound 1. Intermediate A (0.9 g, 2 mmol) xerogel was prepared by placing a block of gel onto a freshly
and triethylamine (280 mL) were dissolved in 50 mL of toluene, cleaved mica surface then freeze-dried. Prior to examination,
and then 20 mL of toluene containing intermediate E (0.8 g, 1 the mica with the xerogel was attached to a copper holder by
mmol) was added dropwise to the above solution when stir- using conductive adhesive tape and then coated with a thin
ꢂ
ring, which was stirred at 0 C for 12 h. Aer the reaction, the layer of gold.
mixture was ltered, and the solvent in the solution was
AFM measurements were conducted on a SOLVER P47 PRO
evaporated, leading us to the crude product. Purication of the Atomic Force Microscope. The sample was prepared by drop
product was rst conducted by chromatography (silicone gel, casting a solution of the system to be studied onto a freshly
200–300 mesh; THF–petroleum ether, v : v ¼ 10 : 1), and then cleaved mica surface.
by re-crystallization from ethanol. The puried product, 1, a
All FTIR measurements were performed on a Brucher
white powder, was obtained in a yield of 60%. The procedures EQUINX55 spectrometer in an Attenuated Total Reection
used for the preparation of compound 2 are almost the same, (ATR) mode with a ZnSe sample slot. The KBr pellets mixed with
and satisfactory results were also obtained. For compound 1: samples were measured in transmittance mode.
1H NMR (CDCl3/Me4Si, 300 MHz): d (ppm): 0.68–2.34 (m,
Rheological measurements were carried out with a stress-
122H, –C(CH3)3, cholesteryl protons), 3.04 (d, J ¼ 12.0, 2H, controlled rheometer (TA instrument AR-G2) equipped with
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steel-coated parallel-plate geometry (20 mm diameter). The gap
distance was xed at 1000 mm. The following procedure was
used to load the fresh gel sample: (1) the hot solutions of 1 in n-
pentanol were placed between the parallel plates of the
rheometer and heated to 80 ꢂC to ensure that solutions were
present. The samples were cooled to 15 ꢂC at ꢃ20 ꢂC minꢁ1 and
incubated there for 4 h to reform the gels. (2) The hot solutions
of 2 in isopropanol were cooled to room temperature and
agitated on a stirrer at 200 rpm for different time periods. The
obtained viscous solutions were placed between parallel plates
and incubated there at 15 ꢂC for 4 h to form gels. A solvent-
trapping device was placed above the plate to avoid evaporation.
Morphology studies
The morphologies of the molecular assemblies of the
compounds in dilute solutions and in the gels were studied via
AFM and SEM measurements. Samples of the 2–isopropanol gel
had been prepared through stirring the system at a speed of 200
rpm at room temperature for 15 min, and then leaving for
gelation. The corresponding xerogels used in the SEM
measurements were prepared by freeze-drying the gels with
liquid nitrogen, and then evaporating using a vacuum pump
at ꢁ60 ꢂC for 12–24 h. For other systems, gelation was induced
by heating–cooling treatment, and the subsequent xerogel
preparation remained the same as that described for the 2–
isopropanol gel system.
ꢂ
All measurements were conducted at 15 C, where two sweeps
were investigated: stress sweep and frequency sweep. It was
believed that this sweep provides information about the
mechanical strength of the gel sample. The latter was nished
at a constant shear stress of 1 Pa from 0.1 to 100 Hz.
1H NMR data were collected on a Bruker AVANCE 300 MHz
spectrometer. HRMS data were recorded on a Bruker Apex IV
FTMS with ESI source.
Fig. 2 shows the concentration-dependent morphologies of
the molecular assemblies existing in a 2–isopropanol solution
and gel. With reference to the images, it can be seen that the
morphologies of the aggregates of 2 in isopropanol at low
concentrations are granules (cf. Fig. 2a). With increasing
concentration of 2, the granules start to fuse and form rod-
like aggregates. At even higher concentrations, the rods blend
to form network structures (cf. Fig. 2e and f). As for 1, similar
studies were also conducted and the results are shown in
Fig. S2.† It can be concluded that by increasing the concen-
tration of 1, the morphologies of its aggregates change from
particles and particle-based bers (cf. Fig. S2a†) to rod-like
structures, belts of narrow width, and belts of tens of
micrometers width (cf. Fig. S2b–f†), suggesting that the
network structures in the gel may originate from micro/nano-
particles which should be the primary aggregates of the
molecules of 1. Further comparison of the structures of
the aggregates of 1 and 2 in the two systems, with the
aggregate of 1 being hard and crisp as proved by the fractured
surfaces and the aggregate of 2 being so and pliable, reveals
that the chirality change in the linker affects the aggregation
behavior of the cholesteryl derivatives of calix[4]arene
signicantly.
The XRD data of the samples were collected on a D/Max-
2550/PC with Cu KR X-ray radiation generated under a voltage
of 40 kV and a current of 40 mA. The scans were conducted at a
rate of 0.5ꢂ minꢁ1. The xerogels used in the measurements were
prepared by freeze-drying the gels in liquid nitrogen, and then
ꢂ
evaporating using a vacuum pump at a temperature of ꢁ60 C
for 12–24 h. The extended molecular lengths of 1 were calcu-
lated using Materials Studio 5.0 soware. The energy minimi-
zation of the gelator conformations was made using the
Discover module.
Result and discussion
Gelation behaviors of the compounds
Gelation behaviors of 1 and 2 were studied in 30 different
organic solvents including protic/aprotic and polar/apolar
solvents at a concentration of 2.5% (w/v), and the results are
summarized in Table S1 (ESI†). It was found that 1 gels n-
butanol and n-pentanol, and partially gels n-propanol and n-
hexanol. In contrast, 2 gels isopropanol only. The effect of
chirality upon the gelation behavior of the two compounds is
not only reected in the number of solvents they can gel, but
also reected in the conditions required by the gelation
processes.
To gather further information on the effect of mechanical
agitation on the self-assembly behavior of 2, the evolution
process of its aggregate morphology was monitored as a func-
tion of agitation time, and some typical results are shown in
Fig. 3 (2.5%, w/v). Similar to the results in Fig. 2, with the
In the examination of the gelation behaviors of the
compounds, it was observed that the gels of 1–n-butanol and 1–
n-pentanol formed at room temperature spontaneously without
agitation, sonication, or heating–cooling treatment. Yet for 2,
gelation of isopropanol is not so easy. The gel formed 30 h later
aer dissolution of 2 in the solvent (2.5%, w/v) via heating.
However, mechanical agitation or sonication is able to accel-
erate the gelation process signicantly. To be precise, it only
took 12 min for the system to become a gel aer vigorous
agitation or sonication, indicating that the chiral structure of
the phenylalanine residue connecting the cholesteryl units and
the calix[4]arene structure show a pronounced effect upon the
gelling performances of the compounds.
Fig. 2 AFM image (a) and SEM images (b–f) of 2–isopropanol (a, 0.05%; b, 1%;
c, 1.5%; d, 2%; e, 2.5%; f, 3%, w/v).
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Fig. 4 Evolution of G0 as a function of the applied shear stress at different
concentrations of 2–isopropanol gels (a, stirred at a speed of 200 rpm for 15 min),
and 1–pentanol gels (b).
Fig. 3 SEM images of 2–isopropanol gel networks present in the systems
agitated for different lengths of time (a, 3 min; b, 6 min; c, 9 min; d, 12 min; e, 15
min; 30 min).
n-pentanol with heating and cooling (see Experimental
section) were also conducted, and the results are shown in
Fig. 4b. It is seen that the value of G0 increased from 58 050 to
227 640 along with increasing the concentration of 1 from
1.5% to 3.0% (w/v). Correspondingly, the yield stress increased
from 10 to 30 Pa. These results indicate that both the stability
and the elasticity of the gels are well dependent upon the
structures of the gelators. Furthermore, increasing the
concentration of the gelator in a gel also shows a positive role
in enhancing the mechanical properties of the gel, but the only
obvious effect is found when the concentration is below a
certain value (cf. Fig. 4a and 5b).
As for the system of 2–isopropanol, agitation not only
promotes gelation and formation of the gel networks, but also
enhances the mechanical strength of the gels. To understand
the effect, the solutions of 2 in isopropanol with 3.5% (w/v)
were agitated on the stirrer at a speed of 200 rpm for different
time periods, followed by a stress sweep measurement (cf.
Fig. 5a). Obviously, stronger agitation is needed for gels that
were found aer 18 and 24 min compared with that of 12
min. In further reference to the gure, it is seen that the value
of G0 of the system (3.5%, w/v) increased from 11 830 to
1 058 900 Pa along with increasing the agitation time from 12
to 24 min, and correspondingly, the yield stress increased
from 110 to 6310 Pa. To the best of our knowledge, for
LMMG-based molecular gels, the yield stress of 6310 Pa is
only a little lower than the highest yield stress reported very
recently.50 Initially, these results seem to be counter-intuitive
since ultrasound, agitation and other perturbation-like stimuli
are reported to break network assemblies and eventually
increase in the agitation time, the morphologies of the aggre-
gates of 2 in the system change from globules, to globules and
short rods then to short rods alone, and nally to rod-based
networks. It is to be noted that there is a sharp transition
between 12 min and 15 min of treatment time (cf. Fig. 3d and e).
The aggregates appearing aer 15 min of agitation look much
thinner, and the networks are more homogeneous. A further
increase in the agitation time, however, holds little effect on the
morphologies of the gel networks since the image of the gel
networks obtained aer 15 min of agitation is almost the same
as that obtained aer 30 min (cf. Fig. 3f). Similar measurements
through sonication treatment were also conducted for the gel
system of 2–isopropanol, and the received results are exhibited
in Fig. S3.† It is obvious that a similar morphology evolution
trend was observed. The morphological changes of the aggre-
gates may explain why external energy is needed to make the
systems change into gels.
Rheological studies
The mechanical properties of a material are extremely impor-
tant for its practical uses, thereby the rheological properties of
an example gel system of 2–isopropanol were studied system-
atically. This system was chosen due to its specicity in prepa-
ration and in properties as afore-discussed. To explore these
properties in detail, rheological measurements were conducted
on a series of samples of the gel, which contained different
amounts of the gelator and were agitated for various lengths of
time.
Firstly, shear stress sweeps were conducted on the gels of 2
in isopropanol with an agitation time of 15 min (see Experi-
mental section). In the measurement, the storage modulus, G0,
associated with energy storage, and the loss modulus, G00,
associated with loss of energy, of the gel were measured as
functions of shear stress at a constant frequency of 1.0 Hz at
ꢂ
15 C (cf. Fig. 4a, to be clear, only the values of G0 are shown).
The value of G0 increased from 9080 to 227 680 Pa along with
increasing the concentration of 2 from 2.5 to 4.0% (w/v), and
correspondingly, the yield stress changed from 40 to 1410 Pa.
Fig. 5 Evolution of G0 as a function of the applied shear stress for the 2–iso-
propanol gels (3.5%, w/v) with different agitation times (a), and the traces from
In contrast, the shear stress sweeps of the gels of 1 in frequency scanning of the gel of 2–isopropanol agitated for 15 min (3.5%, w/v).
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result in dissolution of the gels in the vast majority of cases. partial cone conformation possessed by the calix[4]arene
However, there are an increasing number of reports high- residue of 1,56 which may provide a positive environment for
lighting the emergence of these stimuli as triggers of supra- host–guest interactions among the molecules of 1, of
molecular gelation.51–54
which the guest might be the tert-butyl group connected to
Frequency sweep is an important method to examine the the benzene structure in the partial cone of a ipped
ability of a material to tolerate external forces.55 Accordingly, a calixarene.
gel sample of 2–isopropanol (3.5%, w/v) was employed to
conduct the test at a shear stress of 1 Pa, which is well within
the liner region of the gel sample (cf. Fig. 5a), and the result is
FTIR spectroscopy studies
To further investigate the role of hydrogen bonding in the
shown in Fig. 5b. Reference to the gure reveals that the
gelation process, FTIR spectra of the xerogels of 1–n-pentanol,
values of G0 are always greater than those of the corresponding
2–isopropanol, and those of their THF solutions were recor-
G00 of the gel within the frequency range from 0.1 rad sꢁ1 to
ded. The results are shown in Fig. 7. It is seen that the
500 rad sꢁ1. Further increasing the frequency, however, results
characteristic bands of 1 in its THF solution at 3430, 1739
in an abrupt decrease of the value of G0 and a dramatic
and 1682 cmꢁ1 can be assigned to the stretching vibrations of
increase of G00, indicating break-down of the gel. Furthermore,
OH(NH) and CO, as well as the bending vibration of NH,
within the frequency limit of 500 rad sꢁ1, both moduli exhibit
respectively.57 Upon gelation, the band at 3430 cmꢁ1 splits
weak dependence on frequency change, indicating typical
into two sharp bands, which appear at 3481 and 3306 cmꢁ1
,
visco-elastic behavior. All these results strongly demonstrate
that the 2–isopropanol gel shows good tolerance to external
forces.
respectively. Furthermore, the other two bands shi to 1733
and 1676 cmꢁ1, respectively. Fig. 7b shows the FTIR spectra of
the THF solution of 2, and the xerogel of 2–isopropanol. By
comparing the two spectra, it is seen that the characteristic
bands of 2 shi from 1689 and 1652 cmꢁ1 in the solution
state to 1741 and 1689 cmꢁ1 in the xerogel state, respectively.
Similar to the system of 1–n-pentanol, the broad signal at
3436 cmꢁ1 for this system splits into two bands appearing at
3430 and 3345 cmꢁ1, respectively, along with gelation of this
system. Change in the FTIR absorption positions of the
carbonyl groups, the hydroxyl groups and the amide groups
conrm the existence of hydrogen bonds in the gel networks,
which should be one of the driving forces to promote
gelation.
1H NMR spectroscopy studies
To obtain further information on the structural nature of
the gel networks of the gels, the aggregation behaviors of 1
and 2 in CDCl3 were studied in detail by conducting temper-
ature-dependent 1H NMR measurements. The results are
shown in Fig. 6. It is seen that the chemical shis of the
amide protons (>9 ppm) of 1 and 2 shied remarkably up-eld
when the temperature was increased from 298 to 318 K.
Specically for 1, the shis are 0.15 ppm (cf. Fig. 6a), and 0.16
ppm for 2, respectively (cf. Fig. 6b), suggesting that the amide
groups in both compounds may have taken part in the
aggregation process, and hydrogen bonding may be the
most probable mode. Further reference to the data shows that
the signals of the hydroxyl group protons (7.75 ppm) affixed to
the calix[4]arene structure (cf. Fig. 1) also shied to higher
eld with increasing temperature of the systems. The shis
for the two systems are as high as 0.22 and 0.24 ppm for 1 and
2, respectively, supporting the argument that hydrogen
bonding contributes to the self-assembly of the gelators. In
X-ray diffraction measurements
XRD analysis can be used to elucidate the molecular packing
mode in the crystalline state and clarify the gelation mechanism
of a LMMG in its gel phase.58 Accordingly, the packing mode of
1 in the xerogel of its n-pentanol gel was studied via this tech-
nique, and the XRD trace is shown in Fig. 8. It is seen that the
XRD pattern of the xerogel is characterized by four sharp
reection peaks at 2q ¼ 4.06, 5.18, 6.84 and 8.84ꢂ, of which the
corresponding d values are 2.17, 1.72, 1.29 and 1.00 nm,
respectively. Further interrogation of the data reveals that the
1
addition, Fig. S5† shows the H NMR spectrum of 1 in CDCl3.
The presence of four pairs of doublets between 3.0 and
5.0 ppm in the gure stands as strong evidence for the
pffiffiffi
pffiffiffi
pffiffiffiffiffi
ratio of the d values equals (1= 3) : (1/2) : (1= 7) : (1= 12),
Fig. 6 Partial 1H NMR spectra of 1 (a) and 2 (b) recorded in CDCl3 at different
Fig. 7 FTIR spectra of 1 (a) and 2 (b) in their THF solutions (red) and xerogels
(green) from 1–n-pentanol and 2–isopropanol.
temperatures at a concentration of 2% (w/v).
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Soft Matter
Conclusion
Two novel cholesteryl derivatives of calix[4]arene with L- or
D-phenylalanine residues in the linkers were designed and
synthesized. Gelation tests in 30 organic solvents demonstrated
that 1 gels n-butanol and n-pentanol at room temperature, but 2
gels isopropanol only. Moreover, the gelation for 2 occurs more
than 30 h later, aer heating–cooling treatment or in no more
than 12 min aer vigorous agitation or sonication. It was also
revealed that shear force or sonication treatment accelerates
the gelation of the system of 2–isopropanol and enhances the
strength of the gel. The value of the storage modulus, G0, of the
system (3.5%, w/v) increased to more than 1 ꢀ 106 Pa when
agitated for 24 min, and correspondingly, the yield stress
exceeded 6 ꢀ 103 Pa, which is second only to the mechanically
strongest LMMG-based molecular gel reported by us very
recently. XRD analysis revealed that 1 self-assembles into a
hexagonal structure in its pentanol gel.
Fig. 8 XRD pattern of the xerogel of 1 from its n-pentanol gel.
pffiffiffi
pffiffiffi
which mainly follows the ratio of 1 : (1= 3) : (1/2) : (1= 7) :
pffiffiffiffiffi
(1/3) : (1= 12), characteristic of
a
hexagonal structure.59
Therefore, the molecules of the gelator in the gel are probably
assembled in a hexagonal mode, and accordingly the diffrac-
tions can be indexed in sequence of (110), (200), (300) and (220),
which belongs to a hexagonal columnar lattice. As 2.17 nm is
pffiffiffi
only 1= 3 of the d value of the lattice, the rst peak of the
diffractions should appear at 3.76 nm, which is very close to the
length (3.80 nm) of the molecules of 1 as calculated from
molecular dynamics modeling (Fig. 9). Furthermore, it is to
be noted that in the aggregate, one of the benzene units of the
calix[4]arene residue of a component molecule functions as a
guest, and is hosted by another calix[4]arene residue belonging
to another component molecule. In other words, the compo-
nent molecules within the aggregate are interlocked via host–
guest interaction (cf. Fig. 9), which is different from the hexag-
onal packing of other calix[4]arene derivatives.60,61 As the
spacing is much smaller than the smallest size of the micro-
structure of the gel networks of 1 in its n-pentanol gel
(cf. Fig. S1a†), it is supposed that the XRD data came from a
ber of the aggregates of 1 in the gel. The brous features of the
primary structure is supported by the results that the aggregates
do adopt a ber-like structure as revealed by the AFM and SEM
measurements (cf. Fig. S1†).
Acknowledgements
We thank the Natural Science Foundation of China (91027017
and 21273141) and the 13115 Project of Shaanxi Province
(2010ZDKG-89) for nancial support. This work is also sup-
ported by the “Program for Changjiang Scholars and Innovative
Research Team in University” of China (IRT1070).
Notes and references
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