Critical effects of alkyl chain length on fibril structures in benzene-trans(RR)- or (SS)-N,N′-alkanoyl-1,2-diaminocyclohexane gels

Article abstract of DOI:10.1039/c1ob06460f

Vibrational circular dichroism (VCD) spectra were recorded on benzene-d⁶ gels formed by chiral low molecular mass gelators (LMGs), trans(RR)- or trans(SS)-N,N′-alkanoyl-1,2-diaminocyclohexane (denoted by RR-C_n or SS-C_n, re

Full text of DOI:10.1039/c1ob06460f

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PAPER
Critical effects of alkyl chain length on fibril structures in benzene-trans(RR)-
or (SS)-N,N′-alkanoyl-1,2-diaminocyclohexane gels†
Hisako Sato,*a Takahiro Nakae,a Kazuya Morimotoa and Kenji Tamurab
Received 25th August 2011, Accepted 15th November 2011
DOI: 10.1039/c1ob06460f
Vibrational circular dichroism (VCD) spectra were recorded on benzene-d6 gels formed by chiral low
molecular mass gelators (LMGs), trans(RR)- or trans(SS)-N,N′-alkanoyl-1,2-diaminocyclohexane
(denoted by RR-Cn or SS-Cn, respectively; n = the number of carbon atoms in an introduced alkanoyl
group). Attention was focused on the effects of alkyl chain length on the structures of the gels. When n
was changed from 6 to 12, the signs of the coupled peaks around 1550 cm−1 in the VCD spectra, which
were assigned to the symmetric and asymmetric CvO stretching vibrations from the higher to lower
wavenumber, respectively, critically depended on the alkyl chain length. In the case of RR-Cn, for
example, the signs of the couplet were plus and minus for n = 8, 9, 10 and 12, while the signs of the
same couplet were reversed for n = 6 and 7. The conformations of LMGs in fibrils were determined by
comparing the observed IR and VCD spectra with those calculated for a monomeric molecule. The
observed reversal of signs in the CvO couplet was rationalized in terms of the different modes of
hydrogen bonding. In the case of C8, C9, C10 and C12, gelator molecules were stacked with their
cyclohexyl rings in parallel, forming double anti-parallel chains of intermolecular hydrogen bonds using
two pairs of >NH and >CvO groups. In case of C6 and C7, gelator molecules were stacked through a
single chain of intermolecular hydrogen bonds using a pair of >NH and >CvO groups. The remaining
pair of >NH and >CvO groups formed an intramolecular hydrogen bond.
Low molecular mass gelators based on trans-1,2-diamino-
Introduction
cyclohexane were first reported by Hanabusa et al.5 Since then,
Organogels form a continuous three-dimensional entangled
network in organic solvents.1 Knowledge of the molecular stack-
ing structure in a fibril would be helpful to understand the origin
of the driving force for gelation.1–4 For this purpose, a number
of spectroscopic methods have been applied such as X-ray and
neutron diffraction analyses. One difficulty, however, is that gels
are often lacking in three-dimensional periodicity so that the
molecular images of gels are often difficult to construct from the
diffraction data alone.
these compounds have attracted extensive attention due to their
high gelating capability towards a wide range of organic sol-
vents. One of the outstanding properties of the gelators is that a
fibril adopts a helical structure, where the helicity depended
uniquely on the RR/SS molecular chirality. Based on these prop-
erties, there have been a number of attempts to use the gels as a
template for asymmetric syntheses. In particular, chiral silica
were prepared by the sol–gel reactions of silanols in the presence
of the chiral gels.6,7 The main driving force for the formation of
fibrils is thought to be the intermolecular hydrogen bonding
between >NH and >CvO groups. It remains to be clarified,
however, how the gelator molecules are stacked to achieve
helical structures. Only recently gelators based on the diamino-
cyclohexane with various alkyl chains were reported, revealing
the role of weak van der Waals interactions among the alkyl
chains.8
Vibrational circular dichroism (VCD) is an extension into the
infrared and near-infrared regions of the spectrum where
vibrational transitions occur in the ground electronic state of a
molecule.9–21 One of the advantages of VCD over electronic cir-
cular dichroism (ECD) is a large amount of information concern-
ing 3N − 6 vibrations (N = the number of atoms in a molecule).
Accordingly the method has been proved to be a powerful tool
aDepartment of Chemistry, Graduate School of Science and
Engineering, Ehime University, Matsuyama, 790-8577, Japan.
E-mail: sato.hisako.my@ehime-u.ac.jp; Fax: +81-89-927-9599;
Tel: +81-89-927-9599
bNational Institute for Material Science, Tsukuba, 305-0044, Japan
†Electronic supplementary information (ESI) available: 1H NMR,
elemental analyses, mass spectra, DSC and ORD, yields for C6–C12;
critical gel concentration; sol–gel transition temperatures in toluene;
VCD of C7 and C10 in CDCl3; VCD of powder samples of C10, C8 and
C7 in KBr; VCD of RR-C8 gel sample; The VCD spectra dependence on
concentration of RR-C9 and RR-, SS-C7; VCD calculation of dimer
model of RR-C8 and C6; VCD calculation of RR-C8 and C7 with
benzene; optimized structure of RR-C8 and C7 with benzene; SEM
image of SS-, RR-C7 and SS-C8; XRD of powder in C12, toluene-gel
states of C8, C7, toluene solvent and glass. See DOI: 10.1039/
c1ob06460f
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Table 1 Critical gelation concentration (CGC) in benzene for gelators
Cn
CGC/g L−1
CGC/mol L−1
C6
C7
C8
C9
C10
11.5
8.0
13.8
11.4
7.4
0.037
0.024
0.038
0.029
0.018
for determining the absolute configuration and molecular confor-
mation of chiral molecules in solution.9
Over a number of years the VCD method has been applied to
chiral molecular aggregates such as proteins, gels and films.10–21
We measured the VCD spectra of an organogel formed by
perfluorinated gelators in order to analyze the conformation of a
chiral gelator in a fibril.17 Notably such a gel was found to give
more than one order stronger VCD signals than the same gelator
in solution. By comparing the observed and calculated spectra,
the conformation of a gelator, including the helical winding of
perfluorinated chains, was determined to postulate a stacking
model in a fibril.
In the present study, VCD investigation was performed on
gels formed by trans-diaminocyclohexane derivatives with
various alkyl chains. The experimental VCD spectra of the trans-
parent gels of benzene-d6/trans(RR)- or (SS)-N,N′-alkanoyl-1,2-
diaminocyclohexane (denoted by RR-Cn or SS-Cn, respectively;
n = the carbon number of alkyl chains) were compared with the
theoretical spectra calculated under various assumptions. It was
found that there was a critical length of alkyl chain in determin-
ing the stacking modes in fibrils.
Fig. 1 Sol–gel transition temperature in benzene solvent. The inset
figure shows the change of sol–gel transition temperature as a function
of alkyl chain length at the concentration of 0.05 mol L−1
.
of a gelator. As shown in the figure, C8 gave the lowest Tc, or it
formed the least stable gel. This corresponded to the results in
Table 1. Thus the present results suggested that a fibril under-
went some structural change around the critical chain length of
C8. Moreover, the gels formed by gelators with alkyl chains
shorter than C7 readily released the solvent to become white and
turbid in about 2 h. The sol–gel transition temperature was also
measured for toluene (see ESI†). According to the results, Tc
exhibited a minimum at C8 when plotted against Cn.
IR and vibrational circular dichorism (VCD) spectra of Cn
were first recorded in CDCl3 solution. As an example, the results
for C8 are shown in Fig. 2. The couplet peaks around
1524 cm−1, which are assigned to symmetric and asymmetric
N–H bending vibrations from the higher to lower wavenumber,
respectively, showed opposite signs between RR-C8 and SS-C8,
confirming the reliability of the measurements. The observed fre-
quencies suggested that no hydrogen bonds were formed in this
Results and discussion
RR- (or SS-)N,N′-dialkanoyl-1,2-diaminocyclohexane with alkyl
chains of various length (denoted as RR- or SS-Cn; n = the
carbon number of alkyl chains. Chart 1) were synthesized
according to the reported method.5 The purity of the materials
was confirmed by mass spectra, elemental analyses, ORD (see
ESI†) and HPLC measurements. The compounds (denoted by
C6–C12) gelated various solvents as shown in ESI†. Benzene and
toluene gels were transparent, while the gels of other solvents
(acetonitrile, acetone and hexane) were turbid. Methanol did not
form a gel for Cn (n < 9). Table 1 summarizes the critical gel
concentration (CGC) for benzene at 25 °C. It was noted that C8
gave the highest CGC value or the gel capability was minimum
at this length of an alkyl chain. Fig. 1 shows the sol–gel phase
transition temperature (Tc) for benzene at constant concentration
solvent.5 No VCD peaks was observed around 1650 cm−1
,
Fig. 2 The observed IR (lower) and VCD (upper) spectra of chiral C8
Chart 1 The molecular structure of RR-Cn.
in CDCl3 solution.
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Fig. 3 Observed IR (lower) and VCD (upper) spectra of chiral C8 and
C7 as solids in KBr pellets.
although the band assigned to the CvO stretching of a carboxyl
group was observed at the same wavenumber region of IR
spectra. The VCD spectra of C7 and C10 in CDCl3 solutions are
shown in ESI†. According to these, the couplet assigned to NH
bending had the same sign as for C8.
Next, the IR and VCD spectra of Cn in the solid state were
measured as their transparent KBr pellets. Fig. 3a and b show
the results for C8 and C7, respectively. In contrast to the VCD
spectra in solution, clear VCD peaks assigned to CvO stretches
were observed around 1640 cm−1 (the IR and VCD spectra of
C10 are shown in ESI†). The couplet signal around 1640 cm−1
gave the same signs for these compounds (plus and minus signs
from the lower to higher wavenumber for RR-C8 and C7). In
order to estimate dichroic effects on the VCD spectra in the solid
state, the samples were rotated perpendicularly to the monitoring
light. The spectra underwent no change, excluding any dichroic
contribution. No signal was observed for solid samples of
racemic compounds (see ESI†).
Fig. 4 Observed IR (lower) and VCD (upper) spectra of C6D6 gels of
RR-Cn (black solid line) or SS-Cn (grey dotted line), respectively: (a)
C12, (b) C10, (c) C9, (d) C8, (e) C7 and (f) C6.
Finally, the IR and VCD spectra were measured for the C6D6
gels formed by chiral C6–C12. The results are shown in Fig. 4a–
f. The main peaks for the RR- and SS-gels were essentially
Table 2 Summary of observed VCD spectra (cm−1) of RR-Cn
CvO stretching
N–H bending
mirror-images in the wavenumber range of 1700–1400 cm−1
.
C6–C12
Solution
Solid
1655a
1644 (−), 1634 (+)
1524 (−), 1510 (+)
1558 (−), 1538 (+)
When the gel samples were rotated vertically to the monitoring
light, no change of the spectra was observed (see ESI†). Com-
paring the peaks around 1550 cm−1 between the gels and the sol-
utions, the VCD signals for the gel states were enhanced at least
ten times in comparison to the solution spectra. Moreover, the
peak assigned to N–H bending showed a blue shift by about
30 cm−1, suggesting the occurrence of hydrogen bonding.
C8–C12
C6, C7
Gel
Gel
1643 (+), 1632 (−)
1639 (−), 1632 (+)
1547 (+), 1538 (−)
1557 (−), 1544 (+)
a IR spectrum; VCD showed no signal.
In the measured VCD spectra of the gels, the strong couplet
was observed around 1640 cm−1, which was assigned to the
stretching vibration of CvO bonds. The appearance of this band
suggested the formation of intermolecular hydrogen bonds. In
case of RR-Cn (n = 8–12), the signs of the couplet were plus at
1643 cm−1 and minus at 1632 cm−1. In contrast, for C6 and C7,
the couplet assigned to the same CvO stretches showed
reversed signs of minus to plus from the higher to lower wave-
number. When the concentration of a gelator was changed, no
change was observed in the VCD spectra for all gelators, as
shown in ESI†. In order to eliminate any linear dichroism
effects, the cell was rotated along the direction of a monitoring
light and also turned back-inside out; reproducible VCD signals
were obtained for several samples independently prepared (see
ESI†). At the near critical gel concentration, the sol–gel tran-
sition temperature indicates the presence of the minimum point
at C8 as shown in the insert figure of Fig.1; the same tendency is
observed in the VCD spectra of gels.
Table 2 summarizes the observed VCD results of RR-Cn (n =
6–12). Comparing the peaks due to CvO stretches in the VCD
spectra between the gels and the solids, it was concluded that the
gelators with the alkyl chains shorter than C7 adopted the same
structures as in solids, while the gelators with alkyl chains
longer than C8 adopted different structures from those in solids.
The difference was thought to lie in the mode of hydrogen bond
as described below.
For assigning the observed peaks in Fig. 2, vibrational proper-
ties were theoretically studied for a single RR-molecule. The
optimized structures of monomeric species are shown for RR-C8
and RR-C7 in Fig. 5a and b, respectively. According to the calcu-
lated results, the alkyl chains adopted an all trans-configuration
for both cases. Fig. 6 shows the IR and VCD spectra calculated
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For RR-C7 the optimized structure is shown in Fig. 5b. The
couplet assigned to CvO stretching was predicted to appear at
1760 (−) and 1739 cm−1 (+) as shown in Fig. 6b. The signs
agreed with the measured spectra and the observed reversal of
the signs was reproduced by the theoretical calculation based on
the monomer model. The couplet assigned to NH bending was
predicted to appear at 1567 (−) and 1543 cm−1 (+). This predic-
tion was the same as those for in solution. When the optimized
structure was examined in detail, a pair of >NH and >CvO
groups on different alkyl chains were positioned closely enough
to form an intramolecular hydrogen bond. In this conformation,
the molecules would find it difficult to form double infinite
chains as found for RR-C8. Instead, they probably form an
aggregate through a single chain of hydrogen bonds by using a
remaining pair of >NH and >CvO groups. In such an aggregate,
the cyclohexyl rings were expected to be arranged on a plane in
an alternatively opposite direction. Thus the aggregate would
take a tape-like shape instead of a column-like aggregate
expected for RR-C8.
Fig. 5 Optimized structures of gelators: (a) RR-C8 and (b) RR-C7.
In order to investigate the effects of molecular aggregation in
more detail, the dimer model was constructed by connecting two
RR-C8 molecules through the hydrogen bond between the
>CvO (upper) and >NH (lower) groups. This was regarded as a
part of a column-like aggregate. The optimized structure as well
as the calculated IR and VCD spectra are shown in ESI†. The
predicted couplets assigned to N–H bending and CvO stretch-
ing were both in accord with the experimental observation.
As a possible model for the stacking of Cn with shorter alkyl
chains, the optimum structure of a RR-C6 dimer was calculated
theoretically. This was regarded as a part of tape-like aggregate
and calculated IR and VCD spectra are shown in ESI†. The
signs of the main CvO stretching peaks and N–H bending
agreed with the observed ones.
As mentioned previously, the gel formed by C7 maintained
solvent molecules so weakly to release them in hours from gel
networks. Based on the present models, such low ability of gela-
tion might be related to the weak binding in the molecular aggre-
gation of C7 through a single chain of hydrogen bonding. In
order to study the interaction of solvent molecules with a gelator,
calculation was made for the systems containing a gelator and a
single benzene molecule. The results are shown in ESI† for the
cases of RR-C8 and RR-C7. Comparing these two structures, it
was deduced that the angle between two alkyl chains in RR-C7
was larger than that in RR-C8 in order to include a benzene mol-
ecule. This was thought to be a main cause for the situation that
one of the >N–H and >CvO pairs was close enough to form an
intramolecular hydrogen bond.
Fig. 6 Calculated IR (lower) and VCD (upper) of the monomer model
of (a) RR-C8 and (b) RR-C7, respectively.
for this molecular conformation. For RR-C8, the calculated
spectra reproduced the observed one (Fig. 4d) to a satisfactory
extent. For example, the signs of the couplet of N–H bending
were calculated to be 1550 cm−1 (−) and 1544 cm−1 (+) for
monomeric RR-C8. When this was compared with the observed
spectra of the solution samples, the calculated signs agreed with
the experimental ones (1524 (−) and 1510 cm−1 (+)). The
couplet due to CvO stretching was predicted to be at 1770 (+)
and 1756 cm−1 (−). As described in the preceding section, no
peak due to CvO stretching appeared in the observed spectra of
the solution samples. When the calculated results were compared
with the observed ones for gels, the predicted signs for N–H
bending were opposite to the observed ones. This discrepancy
might be caused by the effects of the aggregation of gelators in
gel states. In fact, the calculation based on the dimer model suc-
ceeded in better predicting the observed spectra as described
later. As for the peaks due to CvO stretching also, the predicted
signs well reproduced the measured VCD spectra (1643 (+) and
1632 cm−1 (−)).
In the optimized structure calculated for RR-C8, two equatorial
N–H and CvO groups are oriented anti-parallel to each other
and perpendicular to the cyclohexyl ring. The molecules in
fibrils are thought to aggregate under this conformation. Most
probably they form intermolecular hydrogen bonds simul-
taneously at the positions of two pairs of >NH and >CvO
groups. Accordingly two anti-parallel hydrogen-bonding chains
were formed along the linear molecular aggregates. The
enhancement of the VCD signals in the gels might be related to
the infinite zigzag chains of hydrogen bonds. The formation of
such molecular aggregates is expected to increase VCD intensity
particularly when the vibration involves the chemical bonds par-
ticipating in aggregate formation.
The morphology of the gel samples of SS-, RR-C7 and SS-C8
was studied by SEM measurements and results are shown in
ESI†. In the case of SS-C7 and RR-C7, fibrils adopted a planar
form with no helical winding. In contrast, in case of SS-C8,
fibrils adopted a string form with helical winding as already
reported for C12 gels.5 Thus the structural change on a molecular
stacking level was reflected on the fibril properties on a micro-
metre scale. No helical sense was determined for the fibrils of
SS-C8 gels by the SEM images. Thus the helicity could not be
related to the chirality of a gelator at the present stage.22
The XRD patterns of RR-C7 and RR-C8 gel states in toluene
showed a broad diffraction peak around 2θ ∼ 18.5° (ca.
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Powder XRD measurements
0.48 nm) in contrast to the sharp peaks observed for the crystal-
line states of the RR-C12 compounds. This indicates that the gels
of RR-C7 and RR-C8 were composed of less ordered domains
with no crystalline phase (see ESI†).
No odd–even effect was observed for benzene gels. It was
concluded that intermolecular hydrogen bonding was dominant
for the alkyl chain length longer than C8, while the intramolecu-
lar hydrogen bonding was dominant for the alkyl chain length
shorter than C7. In the latter cases, the gel structures were
thought to be similar to those in the solid states.
X-Ray diffraction measurements were performed with a XRD
(Ultima IV, Rigaku) using Cu-Kα radiation (λ = 0.15406 nm) at
the conditions of 40 kV, 30 mA, and 2θ ° min−1 scanning;
results are shown in ESI†.
SEM observation
The morphology of the gel samples was investigated with a
SEM (JSM-6700FT, JEOL) at 7 kV.
Experimental
VCD measurements
Syntheses of RR- and SS-Cn (n = 6–12)
The VCD spectra were measured using a PRESTO-S-2007 spec-
trometer (JASCO, Japan).14,17–18 The absorption signals were
detected using a liquid-nitrogen cooled MCT infrared detector
equipped with ZnSe windows. Spectra were recorded at 4 cm−1
resolution. Signals were accumulated for 3000–10000 scans in
about 0.5–2 h. The FT-IR absorbance was adjusted below 1.0 in
order to attain the optimal signal-to-noise ratio in VCD measure-
ments. The gel concentration was varied from ca. 0.02 to ca.
0.1 M. A gel sample for VCD measurements was prepared in the
following manner: first a weighed amount (ca. 10 mg) of the
enantiomeric sample was dissolved in C6D6 (ca. 200–500 μL) at
40 °C. Thereafter about 50 μL of the solution was sandwiched
between two CaF2 plates with a 50 μm spacer and a transparent
film was formed between the two CaF2 plates. After cooling to
room temperature, the stock solution was transformed into a
transparent gel. A solution sample was prepared by dissolving a
weighed amount of the compound in CDCl3 and injected in a
assembled cell with BaF2 windows of 50 μm optical path. The
viscosity of the sample solutions was so high that a solution cell
was not used. The powder sample was ground in an agate mortar
with KBr. The VCD was measured on the compressed pellets. In
order to eliminate linear dichroism effects, the cell was rotated
along the direction of a monitoring light and also turned back-
inside out in gel and KBr samples. Reproducible VCD signals
were obtained for several samples prepared independently.
N,N′-dialkanoyl-trans-1,2-diaminocyclohexanes (denoted as RR-
or SS-Cn) were synthesized according to the reported method by
Hanabusa et al.5 One equivalent of (1R,2R)- or (1S,2S)-1,2-
diaminocyclohexane was coupled with 2 equiv. of an alkyl acid
chloride using excess triethylamine in THF. The product was
purified by washing with an aqueous 1 M HCl solution (yields
1
60–90%). The molecules were identified by H NMR, optical
rotation and mass spectra. The results of 1H NMR, elemental
analyses, mass spectra and optical rotation are shown in ESI†.
The purification was confirmed compared in the previous paper.8
Measurements of sol–gel transition temperature
CGC values and melting temperatures were determined by the
method using a magnetic stirrer according to the previous
reports.5,8 As a pre-treatment, the gel was heated to dissolve and
subsequently cooled to room temperature. A stirrer was placed
onto a cooled gel and the gel was heated slowly. At the melting
temperature, the stirrer went down from a surface into the inside
of the gel. The test was repeated at least three times. Melting
temperatures were obtained reproducibly within 1 °C. These
melting tests were performed for benzene and toluene gels. The
stability of a toluene-C7 gel was found to change with time.
After three days, the gel became turbid with release of solvent.
The sol–gel temperature increased during this change. Finally
the material became solid and was not dissolved in toluene after
10 days.
Computational details
Monomeric species under C2 or C1 symmetry of RR-C8 and C7
were optimized by the Gaussian 09 program at the DFT level
(B3LYP functional) with 6-31G(d,p).23 A monomer with one
benzene molecule was also optimized to study the capture of the
benzene molecule. The dimer models of RR-C8 and RR-C6 were
optimized for aggregation states. The IR and VCD spectra were
calculated on the basis of magnetic field perturbation (MFP)
theory. Description of calculated modes was made from anima-
tions of the modes and spectra with Gaussview 5.08 (Gaussian
Inc.).
Differential scanning calorimetry (DSC)
DSC measurements were carried out with a DSC6200 (SEIKO,
Japan). Heating and cooling were performed at a scan rate of
10 °C min−1. The results are shown in ESI†.
Optical rotatory dispersion (ORD) measurements
ORD measurements were carried out a DIP-1000 (JASCO,
Japan). The gelator was dissolved in methanol at about
0.16–0.20 mol L−1. The ORD of a methanol solution of a
gelator was measured at 589 nm by using a 10 cm cell and
results are given in ESI†. The molar optical rotation ([α]589) was
obtained to be +18.0 and −18.0 deg/(mol L−1cm) for RR-Cn and
SS-Cn, respectively.
Conclusions
The present paper studied the effects of alkyl chain length on the
structures of gels of benzene/trans-1,2-diaminocyclohexane
derivatives with various alkyl chain lengths. When the carbon
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8 N. Zweep, A. Hopkunson, A. Meetsma, W. R. Browne, B. L. Feringa and
J. H. van Esch, Langmuir, 2009, 25, 8802.
number of an attached alkyl chain (n) was changed from 6 to 12,
the coupled VCD peaks around 1550 cm−1 assigned to the sym-
metric and asymmetric CvO stretching vibrations changed signs
at n = 8. From the analyses of VCD results, it was concluded
that, in the gels by C8, C9, C10 and C12, the gelators formed a
column-like aggregate by double anti-parallel chains of inter-
molecular hydrogen bonds. On the other hand, for the gels by C6
and C7, the gelators formed a tape-like aggregate through a
single chain of intermolecular hydrogen bonds.
9 P. J. Stephens, J. Phys. Chem., 1985, 89, 748; L. A. Nafie,
T. A. Keiderling and P. J. Stephens, J. Am. Chem. Soc., 1976, 98, 2715;
M. Bieri, C. Gautier and T. Bürgi, Phys. Chem. Chem. Phys., 2007, 9,
671; J. Sadlej, J. X. Dobrowolski and J. E. Rode, Chem. Soc. Rev., 2010,
39, 1478; G. Yang and Y. Xu, Top. Curr. Chem., 2011, 298, 189.
10 S. Ma, X. Cao, M. Mak, A. Sadik, C. Walkner, T. B. Freedman,
U. K. Lednev, R. K. Dukor and L. A. Nafie, J. Am. Chem. Soc., 2007,
129, 12364; D. Kurouski, R. A. Lombardi, R. K. Dukor, I. K. Lednev
and L. A. Nafie, Chem. Commun., 2010, 46, 7154.
11 T. J. Measey and R. Schweitzer-Stenner, J. Am. Chem. Soc., 2011, 133,
1066.
12 N. Jiang, R. X. Tan and J. Ma, J. Phys. Chem. B., 2011, 115, 2801.
13 A. G. Petrovic and P. L. Polavarapu, J. Phys. Chem. B, 2005, 109, 23698.
14 T. Kawauchi, J. Kumaki, A. Kitaura, K. Okoshi, H. Kusanagi,
K. Kobayashi, T. Sugai, H. Shinohara and E. Yashima, Angew. Chem.,
Int. Ed., 2008, 47, 515.
Acknowledgements
The authors thank Mr Yuji Maekawa and Mr Kazuhiro Kohno
(Ehime University) for the syntheses of gelators and measure-
ments of transition temperature. We appreciate Prof. Akihiko
Yamagishi (Toho University) for his suggestions. We thank the
Integrated Center for Science, Ehime University for elementary
analyses and mass spectra.
15 B. Berthier, T. Buffeteau, J.-M. Léger, R. Oda and I. Huc, J. Am. Chem.
Soc., 2002, 124, 13486.
16 V. Setnička, J. Nový, S. Böhm, N. Sreenivasachary, M. Urbanová and
K. Volka, Langmuir, 2008, 24, 7520.
17 H. Sato, T. Yajima and A. Yamagishi, Chem. Commun., 2011, 47, 3736.
18 E. Scwartz, S. R. Domingos, A. Vdovin, M. Doepf, W. J. Buma,
J. J. L. M. Cornelissen, A. E. Rowan, R. J. M. Nolte and S. Woutersen,
Macromolecules, 2010, 43, 7931.
19 C. Merten and A. Hartwig, Macromolecules, 2010, 43, 8373.
20 H. Sato, K. Hori, T. Sakurai and A. Yamagishi, Chem. Phys. Lett., 2008,
467, 140; H. Sato, T. Sakurai and A. Yamagishi, Chem. Lett., 2011, 40,
25.
Notes and references
1 Low Molecular Mass Gelators Design, Self-Assembly, Function, ed.
F. Fages, Springer, Berlin, Heidelberg, 2005; Molecular Gels, ed. R. G.
Weiss and P. Terech, Springer,Netherlands, 2006.
21 M. Kudo, T. Hanashima, A. Muranaka, H. Sato, M. Uchiyama,
I. Azumaya, T. Hirano, H. Kagechika and A. Tanatani, J. Org. Chem.,
2009, 74, 8154.
22 K. Miyamoto, H. Jintoku, T. Sawada, M. Takafuji, T. Sagawa and
H. Ihara, Tetrahedron Lett., 2011, 52, 4030.
2 M.-O. M. Piepenbrock, G. O. Lyoyd, N. Clarke and J. W. Steed, Chem.
Rev., 2010, 110, 1960; L. Frkanex and M. Žinić, Chem. Commun., 2010,
46, 522; A. Ajayaghosh and V. K. Praveen, Acc. Chem. Res., 2007, 40,
644; M. George and R. G. Weiss, Acc. Chem. Res., 2006, 39, 489;
L. A. Estroff and A. D. Hamilton, Chem. Rev., 2004, 104, 1201; P. Terech
and R. G. Weiss, Chem. Rev., 1997, 97, 3133.
23 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson,
H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov,
J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro,
M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov,
R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant,
S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene,
J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts,
R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli,
J. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth,
P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas,
J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, GAUSSIAN 09
(Revision A.2), Gaussian, Inc., Wallingford, CT, 2009
3 J. H. van Esch and B. L. Feringa, Angew. Chem., Int. Ed., 2000, 39,
2263; J. H. van Esch, Langmuir, 2009, 25, 8392.
4 T. Tachibana, T. Mori and K. Hori, Nature, 1979, 278, 578; K. Yabuuchi,
E. Marfo-Owusu and T. Kato, Org. Biomol. Chem., 2003, 1, 3464; Y. Li,
T. Wang and M. Liu, Soft Matter, 2007, 3, 1312; A. R. J. Hirst,
I. A. Coates, T. R. Boucheteau, J. F. Miravet, B. Escuder, V. Castelletto,
I. W. Hamley and D. K. Smith, J. Am. Chem. Soc., 2008, 130, 9113.
5 K. Hanabusa, M. Yamada, M. Kimura and H. Shirai, Angew. Chem., Int.
Ed. Engl., 1996, 35, 1949.
6 J. H. Jung, Y. Ono, K. Hanabusa and S. Shinkai, J. Am. Chem. Soc.,
2000, 122, 5008; K. Hanabusa, K. Maesaka, Y. Suzuki, M. Kimura and
H. Shirai, Chem. Lett., 2000, 1168.
7 J. J. E. Moreau, L. Vellutini, M. W. C. Man and C. Bied, Chem.–Eur. J.,
2003, 9, 1594.
1586 | Org. Biomol. Chem., 2012, 10, 1581–1586
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