Self-assembly of tris(phenylisoxazolyl)benzene and its asymmetric induction of supramolecular chirality

Article abstract of DOI:10.1039/b715871h

Tris(phenylisoxazolyl)benzene stacks in a columnar fashion to form helical fibers that act as an organogelator, and the supramolecular chirality is asymmetrically induced in the presence of a tiny amount of a chiral source in solution. The Royal Society o

Full text of DOI:10.1039/b715871h

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COMMUNICATION
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Self-assembly of tris(phenylisoxazolyl)benzene and its asymmetric
induction of supramolecular chirality{
Takeharu Haino,* Masahiro Tanaka and Yoshimasa Fukazawa
Received (in Cambridge, UK) 15th October 2007, Accepted 7th November 2007
First published as an Advance Article on the web 19th November 2007
DOI: 10.1039/b715871h
which was slowly cooled to room temperature. After leaving it for
1 h, a white gel formed and exhibited no gravitational flow, as it
could be safely turned upside down. The gelation was entirely
Tris(phenylisoxazolyl)benzene stacks in a columnar fashion to
form helical fibers that act as an organogelator, and the
supramolecular chirality is asymmetrically induced in the
presence of a tiny amount of a chiral source in solution.
thermoreversible. Both
1 and 2 showed a high gelation
performance in a variety of organic solvents: methylcyclohexane,
ether, acetone, ethyl acetate, dimethyl sulfoxide and dimethylfor-
mamide, whereas 1,3,5-tris(4-decyloxyphenylethynyl)benzene 4,
possessing triple bonds instead of the isoxazole rings, did not
form gels in those solvents. The critical gelation concentration
varied in the range 0.8–3.5% wt/vol, depending on the solvents.
To gain an insight into the aggregation morphology of 1 and 2,
their xerogels were prepared on glass plates, and studied by field
emission scanning electron microscopy (FESEM). The FESEM
images in Fig. 1(a) and (b) show the three-dimensional networks
responsible for the gelation. The aggregation of 1 formed well-
developed cylindrical fibers with a diameter of about 200 nm.
A significant change in the morphology of the assembly was
observed when the saturated alkyl ends of 1 were replaced by
double bonds in 2 (Fig. 1(c) and (d)). The aggregation of 2
generated both right- and left-handed superhelical fibers with a
diameter of about 3 mm. The size of the superhelical fiber is
15 times as thick as that of 1, suggesting that the terminal double
bonds enhanced the further growth of the initial cylindrical
assemblies via entwinement of the side chains. The superhelical
fiber structure might suggest that the initial cylindrical assemblies
of 1 and 2 should adopt a helical conformation.
Low-molecular-mass organic gelators with p-conjugated molecular
structures represent remarkable examples of stacked molecular
self-assemblies, which create one-dimensional nanostructured
fibers.1 The network coming from the one-dimensional nano-
structured fibers can immobilize solvent molecules to increase the
viscosity of organic media, and then an organogel forms. Such
nanostructured morphology has attracted increasing interest,
particularly for applications in nanomaterials and nanodevices.2
The aggregation of the gelator molecules into a fibrous network is
driven by multiple non-covalent interactions: coulombic, dipole–
dipole, van der Waals, hydrogen bonding, and p–p stacking.
Mostly, a hydrogen bonding function and/or huge planar
molecular surfaces need to be incorporated into a gelator molecule
to produce fibrous gels.3,4 There is a limited number of low-
molecular-mass organic gelators that can assemble to form fibrous
gels via weak intermolecular interactions: p–p stacking, dipole–
dipole, and van der Waals.5–7 In this communication, we report a
new class of low-molecular-mass organic gelators, tris(isoxazolyl)-
benzene derivatives 1 and 2, capable of self-assembling in a helical
fashion, and asymmetric induction in the assembled state in
solution. The molecules have three isoxazole rings, creating the
directional arrangement of their local dipoles. Their one-direc-
tional circular arrangement facilitates p–p stacking interaction
along the C3 axis to form nanostructured fibers, which act as an
organogelator.
1
Self-association of 1 was studied by H NMR and absorption
spectroscopies. The 1H NMR signals of 1 are concentration-
dependent in chloroform-d1 (Fig. 2). Most of the signals shifted
upfield upon increasing the concentration in a range of 0.163–
200 mmol L21, whereas the proton signals of 4 showed no
The gelation ability of 1 was assessed in a variety of solvents. A
suspension of 1 in acetone was heated to give a clear solution,
Department of Chemistry, Faculty of Science, Hiroshima University,
1-3-1, Kagamiyama, Higashi–Hiroshima, Japan.
E-mail: haino@sci.hiroshima-u.ac.jp; Fax: +81-82-424-0724;
Tel: +81-82-424-7427
{ Electronic supplementary information (ESI) available: Details of
synthesis and characterization of compounds 1–4, their gelation properties,
pictures and TEM images of gels, and absorption and fluorescence
spectra. See DOI: 10.1039/b715871h
Fig. 1 FESEM images of xerogels of 1 (a and b) and 2 (c and d).
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468 | Chem. Commun., 2008, 468–470

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the absorption spectra, the value of the red shift being about 10 nm
on increasing the concentration from 0.001 to 1 mmol L21. This
suggests that the spectral changes originate from self-assembling of
1 into larger aggregates composed of p–p stacked species. These
observations were also matched by quantitative evaluation of the
association constant achieved by fitting the data from the UV/Vis
dilution studies to the isodesmic model8 by non-linear least-squares
regression analysis. The association constant was obtained for 1
(Ka = 180 000 ¡ 30 000 L mol21). The value is very large
compared with that in chloroform. It is apparent that solvophobic
effects as well as stacking and dipole–dipole interactions play key
roles in the formation of the larger aggregates.10
Molecular modeling of 1?1, 1 and its partial structure, 3-(4-
methoxyphenyl)-5-phenylisoxazole 5 by MO calculations using the
DFT method (B3LYP/6-31G*)11 was carried out. Compound 5
has a fairly large dipole moment (m = 1.70 D), directed to the
nitrogen atom along the NLC bond. The three isoxazole rings of 1
accept the circular array due to the local dipole interactions12
(Fig. 4(a)); in fact, flipping one of the isoxazole rings to the
opposite direction increased the total energy of 1 (DE =
1.00 kcal mol21). The intramolecular circular array of the three
dipole moments of course regulates the intermolecular arrange-
ment of its self-assembled aggregate; the DFT calculation of 1?1
gave rise to the S6 symmetric structure (Fig. 4(b) and (c)), in which
the six local dipoles of the isoxazole rings adopt a circular array,
and 1 preferentially stacks in a columnar fashion due to rotational
displacement of a neighboring molecule.
Fig. 2 1H NMR spectra of 1 at concentrations of (a) 0.163, (b) 1.98, (c)
16.8, (d) 100 and (e) 200 mmol L21 at 297 K in chloroform-d1.
concentration dependence. Obviously, the isoxazolyl group is a
crucial incorporation for the self-assembly. Plotting the chemical
shift changes of the protons vs. the concentrations of 1 produced
hyperbolic curves. Non-linear curve-fitting analysis of the curves
by applying the isodesmic model8 produced the estimated
complexation-induced shifts (Ha: 21.34; Hb: 21.10; Hc: 20.80;
Hd: 20.59 ppm) and the association constant (Ka = 3.7 ¡
0.3 L mol21). The complexation-induced shifts decreased with
increasing distance of the protons from the C3 axis of 1, indicating
that flat aromatic compound 1 stacks in a columnar fashion along
the C3 axis.9
Absorption spectra of 1 provide supporting evidence for
the formation of columnar aggregates. A diluted solution
(0.001 mmol L21) of 1 displayed a sharp absorption band at
278 nm in methylcyclohexane (Fig. 3). The maximum peak in the
absorption spectra clearly showed a red shift on increasing the
concentration of the solution of 1, and a new broad band
gradually appeared around 310 nm. When the concentration of 1
reached 1 mmol L21, fibers formed, and the solution turned to a
white suspension whose absorption spectrum showed a peak at
310 nm. Temperature-dependent absorption spectra show similar
spectral features to those observed in the concentration-dependent
measurements. Fluorescence spectra of 1 showed similar trends to
Supramolecular ordering of 1 can form helical columnar
assemblies due to the intra- and intermolecular local dipole–dipole
interactions. In the absence of a chiral source in 1, the helical
assemblies that are formed have both possible helical conforma-
tions (P and M), having equal intermolecular interaction energies;
thus, helical assemblies should exist as racemic mixtures. However,
by means of the presence of a chiral source, the chemical
equilibrium between P and M conformations can be biased on the
higher supramolecular level of organization.13,14
Induced chirality of the supramolecular assembly of 1 was
studied by circular dichroism (CD) spectroscopy in the presence
Fig. 3 Electronic absorption spectra of 1 at concentrations of (a) 1.0, (b)
2.5, (c) 5.0, (d) 7.5, (e) 10.0, (f) 15.0, (g) 20.0, (h) 30.0, (i) 40.0, (j) 50.0 and
(k) 1000.0 mmol L21 at 293 K in methylcyclohexane.
Fig. 4 Calculated structures of 1 and stacked dimer 1?1: (a) 1, (b, c) top
and side views of 1?1. The arrows denote the local dipole moment of each
isoxazole ring.
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Chem. Commun., 2008, 468–470 | 469

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Fig. 5 Circular dichroism spectra of 1 (solid black line, c = 0.1 mmol L1),
R-3 (dashed red line, c = 0.1 mmol L1), S-3 (dashed blue line, c =
0.1 mmol L1), 1 (c = 0.1 mmol L1) with 0.01 mol% R-3 (solid red line), 1
(c = 0.1 mmol L1) with 0.01 mol% S-3 (solid blue line), and absorption
spectrum of 1 (dashed black line) at 293 K in cyclohexane. All CD spectra
were recorded in a 1 6 10 6 45-mm3 quartz cell.
and absence of 3. At a concentration of 0.1 mmol L21, the degree
of polymerization can be estimated to be about 13–15-mers based
on the association constant. CD spectra of achiral 1 with
0.01 mol% R-3 and S-3 were recorded at this concentration.
Mirroring strong Cotton effects were observed upon the self-
assembly of 1 whereas 3 gave rise to very weak Cotton effects at
the same concentration as 1 due to its weak binding ability (Fig. 5).
Remarkably, the supramolecular assembly displays optical activity
in the CD spectrum despite only 0.01 mol% of chiral source 3
being present in solution. It is worth noting that the chiral
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In summary, we have demonstrated that organogelators based
on tris(isoxazolyl)benzene assemble through only weak dipole–
dipole and p–p stacking interactions to create helical structures
both in the gel and in solution, and the helical structures are
influenced by an extremely tiny proportion of the chiral monomer
on the overall helical conformation.
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This work was supported by a Grant-in-Aid for Scientific
Researches (B) and in Priority Area ‘‘Super-Hierarchical
Structures’’ from the Ministry of Education, Culture, Sports,
Science, and Technology, Japan. We are grateful to the Iketani
Foundation for financial support.
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Notes and references
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