Molecular origin for helical winding of fibrils formed by perfluorinated gelators

Article abstract of DOI:10.1039/c0cc05802e

A chiral low-molecular weight gelator, N,N′-diperfluoroheptanoyl-1, 2(R,R)- or -1,2(S,S)-diaminocyclohexane, was prepared to form a gel of acetonitrile. The conformation of the gelator in fibrils was determined by vibrational circular dichroism spectra, p

Full text of DOI:10.1039/c0cc05802e

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COMMUNICATION
Molecular origin for helical winding of fibrils formed by perfluorinated
gelatorsw
Hisako Sato,*a Tomoko Yajimab and Akihiko Yamagishic
Received 27th December 2010, Accepted 4th February 2011
DOI: 10.1039/c0cc05802e
A chiral low-molecular weight gelator, N,N0-diperfluorohepta-
noyl-1,2(R,R)- or -1,2(S,S)-diaminocyclohexane, was prepared
to form a gel of acetonitrile. The conformation of the gelator in
fibrils was determined by vibrational circular dichroism spectra,
providing a molecular model for self-assembly in a helical fibril.
Self-assembly with low-molecular mass gelators is an attracting
Chart 1 The molecular structure of RR-CF7.
topic as an example for achieving supramolecular architectures
through the spontaneous aggregation of small molecules.1
A
or SS-CF7, respectively) (Chart 1), was synthesized.10,11 For
preparation, a THF solution of trans-(1R,2R)- or (1S,2S)-1,2-
diaminocyclohexane and perfluoroheptanoyl chloride was
stirred overnight at room temperature under a nitrogen
atmosphere in the presence of triethylamine. After filtering
out triethylammonium chloride, the filtrate was evaporated to
dryness. The residue was dried after being washed with water.
The compounds were identified by 1H NMR, mass spectra,
19F NMR and differential scanning calorimetry (ESIw).
The VCD spectra were measured using a spectrometer
PRESTO-S-2007 (JASCO, Japan). Signals were accumulated
for 104 scans in about 2 hours at 4 cmꢀ1 resolution, using a
liquid nitrogen cooled MCT infrared detector equipped with
ZnSe windows. For the preparation of a gel sample, a weighed
amount (ca. 5 mg) of the enantiomeric sample was dissolved
in CD3CN (ca. 200 mL) at 40 1C. Thereafter about 50 mL of
the solution was sandwiched between two CaF2 plates with a
50 mm spacer. The solution was transformed into a transparent
gel at room temperature. For measuring a solution sample, a
CD3CN solution whose concentration was lower than the
critical concentration of sol–gel transition was injected into a
cell with CaF2 windows of 50 mm optical path length. Vibrational
frequencies and VCD intensities were simulated by the use of
Gaussian 09 program at the DFT level (B3LYP functional)
with 6-31G(d,p), respectively.12 The IR and VCD spectra were
calculated on the basis of the magnetic field perturbation
(MFP) theory. Description of calculated modes was made
from the animations of the modes and spectra with Gauss
view 5.08 (Gaussian Inc.).
chiral gelator is of particular interest because gelation often
leads to the helical winding of fibrils. Attention has been focused
on how the molecular chirality of a gelator is amplified to the
helicity of fibrils.1
A vibrational circular dichroism (VCD) spectrum is an
extension to the infrared and near-infrared regions of the
circular dichroism spectrum.2–7 We recently applied the VCD
spectroscopy to the transparent C6D6, gels of 12-hydroxyocta-
decanoic acid.6 As a result, the long-range chiral transfer was
evidenced by the fact that the carboxyl group, which was distant
from the asymmetric carbon at the 12 position, gave strong
VCD signals in a gel. It was rationalized that the dimerized
carboxyl groups took a chiral conformation in fibrils. The VCD
spectroscopy has been extended to the Langmuir–Blodgett (LB)
films of the same compounds, revealing the difference in
hydrogen bonding formation between the gels and LB films.6
The present work reports the VCD application to the gels
formed by chiral perfluorinated gelators. The chains are known
to wind helically due to the steric hindrance among fluorine
atoms.8,9 The role of perfluorinated carbon chains on gelation
was examined. A novel gelator, N,N0-diperfluoroheptanoyl-
1,2(R,R)- or -1,2(S,S)-diaminocyclohexane (denoted by RR-CF7
a Graduated School of Science and Engineering, Ehime University,
Matsuyama, Ehime 790-8577, Japan. E-mail: h-sato@ehime-u.ac.jp;
Fax: +81 89-927-9599; Tel: +81 89-927-9599
b Graduate School of Humanities and Sciences,
Ochanomizu University, Tokyo 112-8610, Japan
c Department of Chemistry, Toho University, Funabashi,
Chiba 274-8510, Japan
w Electronic supplementary information (ESI) available: 19F-NMR,
mass spectra, elementary analysis, ORD measurement, DSC measure-
The gelating ability of chiral CF7 was studied for a number
of solvents in order to see the effect of perfluoroalkyl chains
on gelation (ESIw). As a result, CF7 was found to form a
gel of fluorinated solvents such as perfluorobenzene and
perfluorotoluene. A similar effect of fluorinated chains was
reported recently for achiral perfluoroalkaneamides.13–15 The
ment, transition temperature, DH and T by Schroder–van-Laar
¨
equation, AFM and SEM of RR-CF7, XRD in powder of SS-CF7,
VCD solution sample, comparison of IR and VCD between RR-CF7
and non-fluorinated gel in the experimental and DFT calculations,
model of helix formation, gelation of solvent, complete ref. 12. See
DOI: 10.1039/c0cc05802e
c
3736 Chem. Commun., 2011, 47, 3736–3738
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Fig. 1 The SEM image of a CH3CN gel of SS-CF7.
dependence of the sol–gel transition temperature on the
concentration of a gelator was studied for acetonitrile gels.
By applying Schroder–van-Laar equation to the observed
¨
results,16 the main cause for the stabilization by CF7 was
concluded to lie in the larger heat of fusion (ESIw).
Fig. 1 shows the SEM image of a CH3CN gel by SS-CF7.
Fibrils wound helically over a wide range of pitch lengths
(0.1–10 mm). Helicity was always left-handed irrespective of
the scale of helices. The fibrils of RR-CF7 gels of the same
solvent were right-handed, indicating that helicity was deter-
mined uniquely by the chirality of a gelator. The AFM images
indicated that the fibrils were the aggregates of thinner fibers
of ca. 20 nm in radius (ESIw).
The VCD spectra were measured on the CD3CN gels of CF7
in the wavenumber range of 1000–1800 cmꢀ1 (Fig. 2(a)). The
measurements were also performed for the CD3CN solutions
of CF7 at the lower concentration. In both cases, main peaks
showed mirror-imaged relation between RR-CF7 and SS-CF7.
When the intensity of the VCD peak around 1680 cmꢀ1 was
compared, the gel samples gave a g-value (DA/A) 18 times
higher than the solution samples. Thus the formation of a gel
resulted in the enhancement of the VCD intensity.
Fig. 2 (a) The observed IR (lower) and VCD (upper) spectra of the
CD3CN gels of RR-CF7 (black) and SS-CF7 (gray), respectively.
(b) The calculated IR and VCD spectra of a single molecular state
of RR-CF7.
In the measured VCD spectra of the gels, a strong couplet
was observed around 1688 cmꢀ1, which was assigned to the
stretching vibration of CQO bonds (indicated by 1 in the
figure). There was a small couplet around 1539 cmꢀ1 in VCD
spectra, which was assigned to the bending vibration of N–H
bonds (indicated by 2). The multiple peaks in the region of
1238–1170 cmꢀ1 were assigned to the stretching vibrations of
C–F bonds (indicated by 3, 4 and 5).
conformation in the aggregated states. The multiple VCD
peaks assigned to the C–F stretches are well predicted by
the present model. Thus the perfluorinated carbon chains in
RR-CF7 are concluded to take a right-handed helix along the
C–C skeletons.
In a fibril, CF7 molecules might be aggregated through the
intermolecular hydrogen bonding between CQO and N–H
groups.7 When one molecule is placed over the other in a
parallel fashion, they stack closely, forming two intermolecular
hydrogen bonds between CQO and N–H groups (ESIw). In
the case of a pair of RR-CF7 molecules, the upper molecule
should rotate in the right-handed way with respect to the
lower molecule in order to attain the simultaneous formation
of two hydrogen bonds. If the gelator molecules are stacked
sequentially under this rotation, the resultant tape-like aggre-
gate would wind helically in a right-handed fashion. The
above intermolecular interaction is regarded as an origin for
hierarchical development attaining the helical fibril on a
micrometre scale. The outer region of the aggregate would
be the layer of perfluorinated groups. This might explain the
tendency of CF7 for gelating fluorinated solvents. As an
additional aspect, the frequency of CQO stretching showed
a red-shift (B40 cmꢀ1) in comparison to the non-fluorinated
gel (ESIw). This might reflect the presence of the interchain
interaction between carbonyl groups and fluorinated chains.
For assigning the observed peaks in Fig. 2(a), vibrational
properties were studied theoretically for a trans-(1R,2R)-
molecule. The molecular structure of a monomeric form was
optimized under C1 symmetry (ESIw). It was noted that two
CQO or N–H groups orient in an opposite direction with
respect to the molecular plane of a cyclohexyl ring. All trans-
perfluorinated chains ejected outwards with the C–F bonds
wounded helically in a right handed direction. The FT-IR and
VCD spectra were calculated under this molecular conforma-
tion. According to the calculated spectrum (Fig. 2(b)), the
negative and positive VCD peaks at the lower and higher
wavenumbers around 1811 cmꢀ1 correspond to the couplet
around 1688 cmꢀ1 in the observed spectra. These peaks were
assigned to the symmetric and asymmetric stretches of CQO
groups, respectively. The calculated peaks corresponding to
the N–H bending vibration have much higher intensity than
the observed peaks. Thus the VCD behavior of N–H bonds is
thought to be delicately dependent on the molecular
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Such weak non-covalent interactions are previously postulated
for hierarchical structures in the case of fluorinated polysilane
gelators.14
4 K. Monde, N. Miura, M. Hashimoto, T. Taniguchi and T. Inabe,
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The X-ray diffraction measurements on the powder sample
of SS-CF7 showed the sharp peaks at 1.87 nm and 0.47 nm,
which were assigned to the molecular length of CF7 and the
intermolecular distance of stacked molecules, respectively
(ESIw). The X-ray diffraction measurements on the gel samples
are now under progress. In summary, the application of the
VCD method gave an insight into the molecular arrangement
in a fiber by determining the precise conformation of a gelator.
VCD spectroscopy has revealed the molecular conformation
of a chiral perfluorinated gelator in fibrils. A molecular mode
was constructed to explain the molecular origin for the helical
winding of fibrils.
8 M. Yamanaka, Y. Miyake, S. Akita and K. Nakano, Chem.
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The authors thank Prof. Hiroaki Igarashi (Toho University)
for SEM measurements, Prof. Akiko Hori (Kitasato University)
for mass spectra measurements, and Prof. Kayako Hori and
Ms. Marie Inokuma (Ochanomizu University) for their support
in syntheses.
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3738 Chem. Commun., 2011, 47, 3736–3738
This journal is The Royal Society of Chemistry 2011