Preparation and investigation of temperature-responsive calix[4]arene-based molecular gels

Article abstract of DOI:10.1039/c7ra02076g

A novel calix[4]arene-based tetracholesteryl derivative was synthesized, and its gelation behaviours in 30 organic solvents were studied. It showed that 1 gelled long chain alkane, some of alcohol, kerosene, ethyl acetate benzene and toluene. Interestingly the gel of 1/kerosene possessed special property, which was that the mixture solution of the compound 1/kerosene heating for a while was put on the flat contact heater and formed a gel film at 25 °C and 65 °C. Unlike ordinary film, gel film had higher strength and toughness at 65 °C, by contrast other was fragile and broken easily at 25 °C, this phenomenon was few reported before. In order to explore the phenomenon, the mechanism of the gel of 1/kerosene was discussed by Scanning Electron Microscope (SEM), High Nuclear Magnetic Resonance (H NMR), Fourier Transform Infrared Spectrometer (IR) and micro-rheology. SEM revealed that the xerogel of 1/kerosene assembled network through some long and thick fibers at 65 °C, with concentration reducing fibers become thin and its networks were clearly observed. In contrast the morphology of xerogel was porous at low concentration and was a bulk at high temperature system. According to EI data in micro-rheology, the gelation process could be divided into three stages at 65 °C and they could be that some cholesterol segments in the gelator interacting with each other formed some small aggregation in the first stage, and other cholesterol section of the gelator sufficiently were cross-linked with time and assembled into large assembly in the second plateau. In the final moment the large assembly build gel network and the system of gel tended to stable. But there are two stages at low temperature and could be some cholesterol segments that didn't take part in assembly, which may be the cause that the performance of gel was fragile.

Full text of DOI:10.1039/c7ra02076g

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Preparation and investigation of temperature-
responsive calix[4]arene-based molecular gels†
ab
Cite this: RSC Adv., 2017, 7, 28476
Yunhua Xu,a Rong Yanga and Hui Yangc
*
Xiuqin Cai,
A novel calix[4]arene-based tetracholesteryl derivative was synthesized, and its gelation behaviours in 30
organic solvents were studied. It showed that 1 gelled long chain alkane, some of alcohol, kerosene,
ethyl acetate benzene and toluene. Interestingly the gel of 1/kerosene possessed special property, which
was that the mixture solution of the compound 1/kerosene heating for a while was put on the flat
contact heater and formed a gel film at 25 ꢀC and 65 ꢀC. Unlike ordinary film, gel film had higher
strength and toughness at 65 ꢀC, by contrast other was fragile and broken easily at 25 ꢀC, this
phenomenon was few reported before. In order to explore the phenomenon, the mechanism of the gel
of 1/kerosene was discussed by Scanning Electron Microscope (SEM), High Nuclear Magnetic Resonance
(H NMR), Fourier Transform Infrared Spectrometer (IR) and micro-rheology. SEM revealed that the
xerogel of 1/kerosene assembled network through some long and thick fibers at 65 ꢀC, with
concentration reducing fibers become thin and its networks were clearly observed. In contrast the
morphology of xerogel was porous at low concentration and was a bulk at high temperature system.
According to EI data in micro-rheology, the gelation process could be divided into three stages at 65 ꢀC
and they could be that some cholesterol segments in the gelator interacting with each other formed
some small aggregation in the first stage, and other cholesterol section of the gelator sufficiently were
cross-linked with time and assembled into large assembly in the second plateau. In the final moment the
large assembly build gel network and the system of gel tended to stable. But there are two stages at low
temperature and could be some cholesterol segments that didn't take part in assembly, which may be
the cause that the performance of gel was fragile.
Received 19th February 2017
Accepted 15th May 2017
DOI: 10.1039/c7ra02076g
rsc.li/rsc-advances
Physical gels containing supramolecular hosts have stimuli-
responsive to oxidation/reduction,19 sonication,20 tempera-
ture,21,22 shear,23 pH24,25 and were widely applied to catalyst,26
Introduction
Physical gels based on self-assembly structure of molecule
because of potential application in drug/protein delivery,1,2 oil
spill recovery,3–5 have become one of active frontiers of novel
materials in recent years. Researcher struggled to prepare novel
physical gel by bonding some functional groups and intro-
ducing extraordinary metal ions in gelator molecular or
solvent.6,7 By far some common gelators containing amino
acids, saccharides, cholesterol, azo-and metal-compounds were
synthesized and reported.8–13 Relatives to those functional
groups, supramolecular hosts possessing unique recognized
abilities to some molecular and ions were also introduced to
physical gels and endowed gel some smart properties.14–18
separation,27 extraction,28 drug delivery29 and sensor.30 Crown
ether is the rst generation of supramolecular host and in 1991
was rstly imported to the structure of gelator by Shinkai
professor, who and his cooperators synthesized crown ether
derivatives suspending cholesterol and used them prepare gels
with thermo-, light- and metal-responsive.31 Huang's group
created the quadruple-responsive supramolecular polymer
network gel, which was constructed by the benzo-21-crown-7
host and its guest dialkylammonium and they existed in the
same molecules.32 Cyclodextrins as the second supramolecular
host endowed to physical gel special properties. Leong33
prepared smart-shear-thixotropy gel and explained assembling
mechanism of gel, in which cyclodextrins (CD) acted as physical
crosslinking and induced building supromolecular polymer
network. Harada group34 synthesized four polymer gelators
ending with a-CD, b-CD, 1-naphthyl, 2-naphthyl and used them
preparing four hydrogels in which the gelators containing 1-
naphthyl, 2-naphthyl set up again gel network in DMSO by
exchanging solvent. In their work, four kind of gels dressing up
different colours were put into quartz vessel with a little water
aSchool of Materials Science and Engineering, Xi'an University of Technology, Xi'an,
710048, China
bSchool of Chemistry and Materials Science, Weinan Normal University, Weinan
714099, China
cSchool of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an
710119, China
† Electronic supplementary information (ESI) available: SEM of solvent polar
dependence of the gel-networks of 1/kerosene and rheological properties of an
example gel system of 1/kerosene. See DOI: 10.1039/c7ra02076g
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Table 1 Gelation behaviors of compounds 1 (2.5%, w/v)a
and the result showed three gels containing b-CD, 1-naphthyl, 2-
naphthyl respectively combined with together when the quartz
vessel was shook, other gel possessing a-CD kept in the vessel
lonely. Cucurbit is another supramolecular host but recently
several gels relating to it have reported.35,36 Yang35 created
a thermo-responsive supramolecular hydrogels, which con-
taining butan-1-aminium 4-methylbenzenesulfonate and
cucurbit[6]uril. It uncovered the interesting behaviour of the
tempting action of cucurbit[6]uril to form [2]pseudorotaxane.
Kim36 and his colleagues utilized the any modication of
cucurbit[7]uril to prepare pH-responsive hydrogel and exca-
vated the host–guest interaction of cucurbit[7]uril and water/
hydronium by X-ray.
Solvent
1
Solvent
1
Solvent
1
Methanol
Ethanol
I
P
P
G*
PG
G*
G*
G*
G*
S
n-Hexane
n-Heptane
n-Octane
n-Nonane
n-Decane
Cyclohexane
Acetonitrile
Isopropanol
Kerosene
I
P
Acetone
Benzene
Toluene
THF
n-Pentane
DMSO
DMF
CH2Cl2
CHCl3
CCl4
I
G
G
S
P
I
P
S
S
S
n-Propanol
n-Butanol
n-Pentanol
n-Hexanol
n-Heptanol
n-Octanol
n-Nonanol
n-Decanol
G
G
G
PG
I
P
G
G
Ethyl acetate
a
S ¼ solution; I ¼ insoluble; P ¼ precipitate; G ¼ turbid gel; G* ¼ gels
forming at room temperature.
Calixarene as the third supramolecular host attracted great
attention because they have a big adjustable hydrophobic cavity
and multiple modify sites. Calixarene has been studies as
a component of gelators.37–44 Our group have reported two gel
works based on calix[4]arene and for the rst time constructed
a novel gelator with calix[4]arene fragment and two cholesterol.
The rst work of our group based on calix[4]arene was that novel
gelator was synthesized and used it prepared smart shear-
thixotropy gel in the mixture solvent of decane and acetoni-
trile.17 Chiral L- and D-phenylalanine residue as a linker con-
necting calix[4]arene unit and cholesteryl structure in the
second work, and gelation tests revealed that isopropanol, only
one of the solvents, could be gelled by the compound con-
necting D-phenylalanine in its linker and mechanical agitation
or sonication not only promotes the gelation, but also enhances
the strength of the gel.43 The gel showed the excellent properties
of shear thixotropy and mechano-responsive, and this could be
owe to the gelator combining with calix[4]arene and two
cholesterols.17,43 Based on our previous work, we rst combined
calix[4]arene unit with four cholesterol groups via hexanedi-
amine moieties in the linkers as a novel gelators (compound 1).
Compared with preceded gelators, new compound 1 increased
two cholesterol units and amine groups, which could drive the
formation of gel network. Gelation tests revealed that 1 could
long chain alkane, some of alcohol, kerosene, ethyl acetate,
benzene, toluene and one interesting property of gel of 1/kero-
sene was transparent high strength and a certain toughness gel
forming immediately at high temperature and become fragile at
the low temperature, which might be correlate with mechanism
of assembly process. Under certain condition, this special
property might have potential application in some special
environment. This paper reports the details in the following.
hydrocarbons, could be suit to the cavity of gel network at
difference temperature and could participate in the formation
progress of gel network. This was proved that the network of gel
collapsed during vacuum freeze-drying because kerosene might
interact with compound 1. In order to explore this phenom-
enon, two hot solutions of 1/kerosene (2.5%, w/v) were put into
two glass model that were placed on two different temperature-
controlled at contact heater, in which the temperature of one
contact heater set at 65 ꢀC, the other was 25 ꢀC. So the different
temperature of supramolecular gel lms were made when gel
forming from above-mentioned solution and glass model
reached the same temperature. When removing the model, gel
lm were picked perfectly by a piece of steel sheet-shape at
65 ꢀC, this may be it has better toughness, in contrast other was
broken rapidly at 25 ꢀC (Fig. 1), it could be deduced that the
cholesterol moieties of compound 1 assembled partially so that
the crosslinking densities of the gelator decreased and its gel
was easily broken in low temperature and the temperature of gel
effected on formation of the gel network, this was a unusual gel
behaviour for recent years.
Morphology studies
In order to studied the morphologies of the molecular assem-
blies of the compounds 1, the xerogel system of 1 were observed
at different temperature, concentration and solvent via SEM.
Result and discussion
Gelation behaviours of the compounds
Gelation behaviours of the compounds 1 was studied in 30
different organic solvents including polar/apolar solvents in
a concentration of 2.5% (w/v), and the results are summarized
in Table 1. It showed that 1 gelled long chain alkane, butanol,
pentanol, hexanol, octanol, heptanol, nonanol, kerosene, ethyl
acetate benzene and toluene, and found that the gel of 1/kero-
sene had high strength at high temperature and easily broken at
Fig. 1 Photograph of the gel film of 1/kerosene system (2.5%, w/v) at
low temperature for
a
while. The kerosene, mixing different temperature.
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on a series of the samples of the gel, which contained different
temperature gels at the same concentration. Firstly, shear stress
scanning was conducted by placing the pretreated solution
containing kerosene and compound 1, and then leaving the
same mixture of solution on different plate of rheometer at
25 ꢀC, 45 ꢀC, 65 ꢀC, 85 ꢀC for 1 h before the scanning in order to
create better balance of gel network. In the studies, the storage
modulus, G0 (associated with energy storage), and the loss
modulus, G00 (associated with loss of energy) of a sample of the
gel were measured as functions of shear stress at a constant
frequency of 1.0 Hz at 25 ꢀC, 45 ꢀC, 65 ꢀC, 85 ꢀC (Fig. S2†). At low
stress value, the value of G0 was more than the G00, which showed
the dominant elastic character of the gel, while the materials
were transformed into rubbery state or become “liquid-like”
when G00 exceeds G0, the value of which was known as yield
stress.45 Fig. S2† showed the value of G0 increased and then
decreased from 16 280 to 29 212 Pa to 11 395 along with
increasing the temperature of the gel from 25 ꢀC to 85 ꢀC, which
accounted stability of the gels were well dependent upon the
temperature. Furthermore, increasing temperature of a gel also
shows positive role in enhancing the mechanical properties of
the gel in a certain scope of temperature. But the yield stress
correspondingly changed from 68.16 Pa to 230.74 Pa to 214.8
Pa, which rose and lowered with increasing temperature. The
result might indicate that the number of crosslinks of the gel
network formed differently at lower (25 ꢀC) temperature and
higher (45 ꢀC 65 ꢀC 85 ꢀC) temperature. The interaction con-
structing gel network at 85 ꢀC reduced on account of fast
molecular motion and leaded to G0, G00, the yield stress
lessening.
In order to explore the inuence of temperature on the
crosslink gelation process of compound 1/kerosene, elasticity
index (EI) and Solid Liquid Balance (SLB) properties of the gel at
different temperature was studied through micro-rheological
measurements.46 The result was showed in Fig. 3 and 4. The
curve of the EI of 1/kerosene system at 45 ꢀC, 65 ꢀC were
decreased rapidly and then reached a short plateau in the
beginning of test, and then increased to another plateau in
a short time, and nally ascended to the constant with time. The
results showed the graph were divided into three stages
according to the EI graph and preliminary suggested the gela-
tion process appeared three platform: in the rst stage some
cholesterol segments in the gelator interacting with each other
formed small aggregation and produced some crosslinking
sites, and other cholesterol section of the gelator sufficiently
Fig.
2 SEM images of 1/kerosene gel networks with different
concentration at different temperature ((a)–(c), at 65 ꢀC; (d)–(f), at
25 ꢀC; (a) and (d), 0.5%, (b) and (c) 1.0%, (b) and (c) 2.0%).
The gel had been prepared through heating–cooling treatment
and placed for a few minute to reach the system balance. The
corresponding xerogel of the system of 1/kerosene used in the
SEM measurement was prepared by replacing the kerosene of
gel with methanol at homologous temperature. For other
systems, the process of gelation was applied with the same
treatment, and the subsequent xerogels were prepared by
freeze-drying in liquid nitrogen, and thenꢀevaporated via using
a vacuum pump at a temperature of ꢁ85 C for 24 h.
Fig. 2 showed the concentration and temperature-depended
xerogel morphology of 1/kerosene gel system with SEM. The
xerogel of 1/kerosene assembled network through some long
and thick bers at 65 ꢀC, with gel concentration reducing bers
become thin at 1%, 0.5% (w/v) and its networks were clearly
observed in the Fig. 2b. In contrast at 25 ꢀC the morphology of
xerogel network was porous and a bulk at high concentration.
Based on previous result, it concluded that the assembly of
gelator were effected more greatly on temperature than
concentration.
The morphologies of the gelator assemblies existing in gels
of 1/different solvents were showed in ESI Fig. 1.† With refer-
ence to the images shown in Fig. S1,† it is seen that the
morphologies of the aggregates of 1 in decane with low polarity
were lamellar (Fig. S1a†), when increasing the polarity of the
solvent, the aggregates of 1/toluene become into bers. At even
higher polarity, the bers of gelators blend to form porous
network structures in 1/butanol (Fig. S1b†). It can be concluded
that by increasing the polarity of solvent, the morphologies of
its aggregates change from lamellar and long bers (Fig. S1†), to
ber-based porous structures, suggesting that the network
structures in the gel may be effected signicantly by the polarity
of solvent.
Rheological and micro-rheological studies
The mechanical property of a gelation was extremely important
for its practical uses, and thereby the rheological properties of
an example gel system of 1/kerosene were studied systemati-
cally. This system was chosen due to its specicity in the
property of gelation at different temperature. In order to explore
the property in detail, rheological measurements were studied Fig. 3 EI versus times at different temperature (2.5%, w/v).
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Fig. 5 Partial 1H NMR spectra of 1 in CDCl3 at different temperatures
2.0%, w/v, (a) and at different concentration ((b), room temperature).
Fig. 4 SLB versus times at different temperature (2.5%, w/v).
assembled into large assembly in the second plateau, in the
might have taken part in the aggregation process, and hydrogen
bonding could be the most possible mode. Further reference to
the data shown in Fig. 5b, it shows that the two signals of the
protons of amide groups affixed to the cholesterol structure
appeared and took place some displacement (0.10 ppm, 0.05
ppm) at 0.5%, 1.0%, which might be hydrogen bond interaction
inner compound 1. With the concentration of systems
increasing, the interaction of hydrogen bond of compound 1
moved from intramolecule to intermolecule and the two signals
of the protons of amide groups mixed together when the
concentration of solution changed from 1.0% to 2.0%. The
merge peak shis 0.03 ppm for the systems concentration
changing from 2.0% to 3.0%, in supporting of the argument
that hydrogen bonding contributes to the self-assembly of the
gelators.
nal stage the large assembly interacted into gel network and
ꢀ
ꢀ
the system of gel tended to stable state. At 25 C, 85 C Fig. 3
appeared only one plateau, and it could be merely two assembly
process: a part cholesterol segments of compound 1 interacted
ꢀ
to chunks and the chunks accumulated into network at 25 C,
and difference from the above mentioned all cholesterol
ꢀ
segments participated in assembly at 85 C.
SLB illuminated the ratio between solid and liquid behav-
iour, reaction rate. SLB ¼ 0.5 expressed that the liquid and solid
parts are equal, known as sol–gel transition point, 0.5 < SLB < 1
expressed the liquid behaviour dominating, and 0 < SLB < 0.5
expressed the solid behaviour dominating, which was gel
behaviour.47 As shown in Fig. 4, SLB curves nally remained
below 0.5 and above 0 at 25 ꢀC, 45 ꢀC, 65 ꢀC, 85 ꢀCꢀ, which
ꢀ
declared the solid behaviour dominated. At 45 C, 65 C, SLB
graphs increased quickly and exceeded 0.5, indicating the
system showed liquid behaviour, then rapidly decreased to 0.6
and 0.5 respectively, that shortly stayed, which was set up
intersection between cholesterols and it was the same with EI.
With time SLB graphs decreases to 0.2 or so and reached
a constant, which proved that viscous dominant solution
changed into a viscoelastic gel. The SLB curves directly reached
to about 0.25, gel phase at 25 ꢀC, 85 ꢀC. As shown in Fig. 3, when
crosslinker concentrations range was 0.2 or so, there was an
obvious decline of the time, which implicated that the time of
sol–gel transition was reduced. Fig. 4b showed the values of
balance SLB were about 0.27, 0.15, 0.2, 0.23, corresponding
temperature 25 ꢀC, 45 ꢀC, 65 ꢀC, 85 ꢀC, which indicated the
degree of crosslinking rstly increased and then decreased with
temperature increasing.
FTIR spectroscopy studies
As well-known, hydrogen bonding was the main driving force
for the self-assembly of cholesterol-based gelator.48 In order to
further probe the function of hydrogen bonding in the gelation
process, FTIR spectra of the xerogels of 1/kerosene and the
CDCl3 solutions of the compound 1 were recorded. Fig. 6
showed that the bands of 1 in its CDCl3 solution at 3416, 1689
and 1680 cmꢁ1 could be assigned to the stretching vibrations of
NH and CO, as well as the bending vibration of NH. The band of
3416 cmꢁ1 moved to 3423 cmꢁ1, the other two bands (1689
1H NMR spectroscopy studies
To obtain further information on the structural nature of the gel
networks under study, the aggregation behaviours of the gela-
tors/1 in CDCl3 were studied in detail by conducting
temperature-dependent and concentration-dependent 1H NMR
measurements. The results showed in Fig. 5. It was seen that the
chemical shis of the amide protons (>9 ppm) of the
compounds moved remarkably to up-eld along with increasing
the temperature of the systems from 298 K to 308 K and then to
318 K. Specically for compound 1, the shis are 0.06 and
0.03 ppm, respectively (Fig. 5a), 0.04 and 0.06 ppm, respectively
Fig. 6 FTIR spectra of compound 1 in its CDCl3 solutions and xerogels
(Fig. 5a), suggesting that both amide groups in compound 1 of kerosene/1.
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cmꢁ1 and 1680 cmꢁ1 in solution) fused into one peak, which –CH2CH3), 3.17 (d, J ¼ 14.0, 4H, –ArCH2Ar–), 4.18 (q, 8H,
was 1685 cmꢁ1 in gel. FTIR absorption positions of the carbonyl –CH2CH3), 4.80 (s, 8H, –ArOCH2), 4.83 (d, J ¼ 14.0, 4H,
groups, the hydroxyl groups and the amide groups moved, –ArCH2Ar–), 6.78 (s, 8H, –ArH), HRMS: m/z calcd for [(M + Na)+]:
which conrmed the existence of hydrogen bonds in the gel 1015.4180, found: 1015.4170.
networks and might drive molecular assembly.
Synthesis of intermediate c.50 b (1.000 g, 1 mmol) and 15% of
NaOH (30 mL) were dissolved in 75 mL of ethanol, and the
mixture was stirred and heated under reux for 24 h. The
residue was diluted with cold water (50 mL), then hydrochloric
acid (3 mol mLꢁ1) was added with vigorous stirring until pH
equals 1, and then abundant white solid as produced was
collected via ltration. Finally, the residue was dried in oven
Experimental section
Reagents and materials
Cholesteryl chloroformate, hexanediamine (Sinopharm group
Co. Ltd) and p-tert-butylphenol (Aladdin Chemistry Co. Ltd.,
98%) used directly without further purication. Toluene was
distilled over sodium in the presence of benzophenone under
N2 atmosphere and collected fraction at 108–110 ꢀC before use.
All other reagents were of analytical grade and used directly
without further purication.
1
(yield: 90%). For c: H NMR (CDCl3/Me4Si, 300 MHz): d (ppm):
1.15 (s, 36H, –C(CH2)3), 3.37 (d, J ¼ 14.0, 4H, –ArCH2Ar–), 4.34
(d, J ¼ 14.0, 4H, –ArCH2Ar–), 4.48 (s, 8H, –ArOCH2), 7.11 (s, 8H,
–ArH), and 7.82 (br s, COOH). HRMS: m/z calcd for [(M + Na)+]:
903.4180, found: 903.4170.
Synthesis of intermediate d.51 c (0.500 g, 0.56 mmol) was
dissolved in 80 mL of toluene containing thionyl chloride (1250
mL), and the mixture was stirred and heated under reux for
3.5 h. Removal of the solvent and the un-reacted thionyl chlo-
ride via distillation under reduced pressure furnished the
product, tetracyl chloride (d), as an off-white solid in quantita-
tive yield. The product as obtained was used directly in subse-
quent preparations without purication.
Preparation of compound 1. a (1.340 g, 2.24 mmol), trie-
thylamine (380 mL) were dissolved in 80 mL of toluene, and then
30 mL of toluene containing d (d is the previous products) was
added drop-wise to the above solution with stirring. The
mixture was stirred at 0 ꢀC for 12 h. Aer the reaction, the
mixture was ltered, and the solvent in the solution was evap-
orated. The residue as obtained is the crude product. Purica-
tion of the product was conducted by recrystallization from
ethanol to give puried product 1, a white power, and obtained
in a yield of 50%. For compound 1: 1H NMR (CDCl3/Me4Si, 300
MHz): d (ppm): 0.68–2.39 (m, 256H, –C(CH3)3, cholesteryl
protons), 3.23 (d, J ¼ 12.0, 4H, –ArCH2Ar–), 4.61 (m, 16H,
ArCH2CH, –ArOCH2, oxycyclohexyl), ArCH2Ar–, 4.98 m, 2H,
–NHCHCO(CH2), 5.33 (S, 4H, alkenyl), 6.82 (m, 8H, –ArH), 7.75
(m, 2H, –NHCHCO(CH2)); HRMS: m/z calcd for [(M + Na)+]:
2945.25, found: 2945.18. Compound 1: C 77.21, H 10.20, N 3.83;
found: C 77.57, H 10.22, N 3.73.
Synthetic procedures
The synthetic routes of the target compound 1 are schematically
shown in Scheme 1. Calix[4]arene and intermediate a are known
compounds, and it was synthesized and characterized in the
way as reported by us a few years ago.17,43
Synthesis of intermediate b.49 Calix[4]arene (0.648 g, 1
mmol), potassium carbonate (0.825 g, 6 mmol) and potassium
iodide (0.650 g, 4 mmol) were dissolved in 20 mL of acetone,
and then ethyl chloroacetate (0.742 g, 6 mmol) was added slowly
and stirred. When the mixture was reuxed for 24 h under
nitrogen atmosphere, ethyl chloroacetate (0.367 g, 3 mmol) was
added again and was reuxed for 7 day under the same condi-
tion. Most of solvent was evaporated at reduced pressure when
the previous reaction nished. The residue as obtained was
dried, and then dissolved/suspended in chloroform. The
organic system was extracted or washed with diluted hydro-
chloric acid for three times, and then collected and dried with
anhydrous magnesium sulfate. Aer that, the clear solution was
evaporated to dryness. The crude product as prepared was rstly
puried by re-crystallization from ethanol to give compound
1
b as white crystals in a yield of 80%. For b: H NMR (CDCl3/
Me4Si, 300 MHz): d (ppm): 1.07 (s, 36H, –C(CH2)3), 1.26 (t, 12H,
General methods
Preparation of gels. In a typical gelation test, the mixture of
amount of the possible gelator and a measured volume of the
solvent were placed in a sealed test vial, then heating for a while.
And then, the system was le to equilibrate for a few minutes.
Finally, the test vial was inverted to observe if the solution inside
could still ow. The systems which lose uidity are denoted as
“G” and which lose partially are denoted as “PG”. Those only
solution remains are denoted as “S”. However, the systems with
precipitates or crystals are denoted as “P”. As for the systems, in
which the potential gelator could not be dissolved even at the
boiling point of the solvent, they are denoted as “I”.
SEM pictures of xerogels were taken on a Quanta 200 scan-
ning electron microscopy spectrometer (Philips-FEI). The
Scheme 1 Synthesis route of compounds 1.
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accelerating voltage was 20 kV, and the emission was 10 mA. Foundation of Shaanxi Province of China (2014JQ2-2008,
The xerogel was prepared by putting a block of gel of 1/kerosene 15jk1235) and this work is also supported by Foundation of
in a beaker containing methanol. The kerosene in the gel Weinan Normal University (14YK001, 15ZRRC11).
diffused into methanol and the previous gel only le the
network structure constructed by compound 1. Prior to exami-
nation, the xerogel was attached to a copper holder by using
conductive adhesive tape, and then it was coated with a thin
Notes and references
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