A rodlike organogelator: Fibrous aggregation of azobenzene derivatives with a syn-chiral carbonate moiety

Article abstract of DOI:10.1039/b205072b

A chiral azobenzene derivative containing a cyclic syn-carbonate moiety functions as a gelator for various organic solvents; the dipole-dipole interaction drives the fibrous self-assembly of the rodlike gelator.

Full text of DOI:10.1039/b205072b

A rodlike organogelator: fibrous aggregation of azobenzene derivatives
with a syn-chiral carbonate moiety†
a
b
c
a
b
Jun-ichi Mamiya, Kiyoshi Kanie, Tamejiro Hiyama, Tomiki Ikeda* and Takashi Kato*
a
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama
26-8503, Japan. E-mail: tikeda@res.titech.ac.jp
Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, Hongo,
Bunkyo-ku, Tokyo 113-8656, Japan. E-mail: kato@chiral.t.u-tokyo.ac.jp
Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Yoshida, Sakyo-ku,
Kyoto 606-8501, Japan
2
b
c
Received (in Cambridge, UK) 27th May 2002, Accepted 8th July 2002
First published as an Advance Article on the web 24th July 2002
A chiral azobenzene derivative containing a cyclic syn-
carbonate moiety functions as a gelator for various organic
solvents; the dipole-dipole interaction drives the fibrous self-
assembly of the rodlike gelator.
Scanning electron microscope (SEM) observation§ reveals
that (R,R)-(+)-1 forms fibrous aggregates (Fig. 1). The xerogel
was prepared from (R,R)-(+)-1/2-propanol by slow evaporation
of the solvent. The gelation ability of the compound is due to the
formation of one-dimensional fibrous structures. Variable-
temperature infrared (IR) measurements for (R,R)-(+)-1 were
carried out on both the dodecane gel and the sol state from 25 to
100 °C.§ The nCNO band of the carbonate group is observed at
Self-assembly is a powerful tool to prepare functional molecular
materials.1 One example is fibrous molecular assembly in
organic solvents, which leads to the formation of physical
,2
3
21
21
organogels. In these materials, non-covalent interactions such
1708 cm in the gel states, while it is seen at 1720 cm in the
sol states. This result suggests that the CNO stretching is
suppressed by intermolecular interactions between syn-chiral
carbonate moieties in the gel state.
as hydrogen bonding,4 lipophilic, and p–p interactions
–8
9
10
drive such one-dimensional assembly of gelators. These
gelators often have functional groups involved in molecular
interactions positioned near to the chiral groups. For example,
cyclohexane diamide and diurea derivatives having chiral
centres at the amino groups are efficient gelators through the
one-dimensional self-assembly by the formation of hydrogen
bonding.4 In the present study, we have found that an aromatic
mesogenic rodlike molecule functions as a new type of
organogelator. This azobenzene molecule of (R,R)-(+)-111 has a
cyclic syn-carbonate moiety at the chiral position.
No gelation is observed for the corresponding racemate of 1
[(±)-1], which forms no fibrous aggregates. Thus, the chirality
of the carbonate moiety may play a determining role in the fibre
formation and the induction of the gelation ability. Compound
(R,R)-(+)-1 melts at 39 °C and shows a highly ordered smectic
phase up to 118 °C.¶ The racemate of 1 also shows an ordered
smectic phase from 61 to 118 °C on heating. However, the
phase transition enthalpy from smectic to isotropic of (R,R)-
,5
2
1
Table 1 shows the minimum gel concentrations (MGC, g
L2 )‡ of (R,R)-(+)-1. Compound (R,R)-(+)-1 acts as an
excellent gelator for various organic solvents. In particular, it
gelates alcohols, ethers, and hydrocarbons at low concentra-
tions. These gels are stable for more than one year at room
temperature. The gel and sol states are thermoreversible. The
sol–gel phase transition temperatures of 2-propanol gels of
(+)-1 is 30 kJ mol , which is twice as high as that of (±)-1.
1
21
For the gel state of 2-propanol with (R,R)-(+)-1 (6 g L ) at
20 °C, weak diffraction peaks at 61.0, 31.3, 20.0 and 15.4 Å are
observed by small-angle X-ray scattering (SAXS) measure-
ments. These diffractions are attributed to the d100, d200, d300
and d400 lattices of the layer structure, respectively. On the other
hand, the SAXS experiments of (R,R)-(+)-1 on the liquid-
crystalline (LC) state reveal that the layer spacing is 56.6 Å at
90 °C. The layer distance is slightly narrower than that in the
(
R,R)-(+)-1 increase from 47 to 55 °C with an increase of the
2
1
concentration from 8 to 20 g L
.
2
-propanol gel state, suggesting that the self-assembled struc-
tures of both the gel and the LC states are basically the same.
The single-crystal X-ray analysis of an analogous compound
of gelator (R,R)-(+)-1 was carried out to examine the self-
Table 1 Minimum gel concentration (MGC) of (R,R)-(+)-1 at 25 °C
1
MGC/g L21
Sol.b
Sol.
6
6
Solvent
MGC/g L2
Solvent
Methanol
Ethanol
Cryst.a
8
12
6
Chloroform
Toluene
Cyclohexane
Hexane
b
1
2
-Propanol
-Propanol
Diethyl ether
Dibutyl ether
Acetone
a
17
12
Sol.
Dodecane
Hexafluorobenzene 25
Perfluorohexane
11
b
Insol.c
Cryst.: Crystallisation. b Sol.: Solution. c Insol.: Insoluble.
†
Electronic supplementary information (ESI) available: synthetic scheme
for (R,R)-2; FT-IR and SAXS measurements of (R,R)-(+)-1. See http://
www.rsc.org/suppdata/cc/b2/b205072b/
Fig. 1 SEM image of the self-assembled fibres prepared from the (R,R)-
(+)-1/2-propanol gel. Scale bar: 1mm.
1
870
CHEM. COMMUN., 2002, 1870–1871
This journal is © The Royal Society of Chemistry 2002

assembled structure of (R,R)-(+)-1. Compound (R,R)-2 with a
short alkyl chain formed sharp needle-like single crystals from
hexane solution by slow evaporation of the solvent at room
temperature.∑ Fig. 2 shows a packing structure of (R,R)-2
looking down the a- and b-axes [Fig. 2(a) and (b)] and a
magnification view of the cyclic syn-carbonate moieties in the
crystal structures [Fig. 2(c)]. The crystal has a layered structure
through the ribbon-like aggregation of the carbonate moieties
along the b-axis [Fig. 2(a)]. The dipole moment is compensated
along the a-axis [Fig. 2(b)]. The stacking distances between the
carbonate moieties along the a- and b-axes are 5.8 and 4.2 Å,
respectively. The dipole–dipole interaction of the chiral carbon-
ate moieties having a large dipole moment plays a key role in
the formation of the ribbon-like structure, leading to the fibrous
self-assembly. The large spontaneous polarisation11 of (R,R)-
solvents. The fibrous self-assembled structure is formed by the
ribbon-like aggregation of (R,R)-(+)-1 through dipole–dipole
interactions of the chiral carbonate moieties. This new molec-
ular design of gelators would lead to the development of novel
molecular assemblies with high functionality. Further studies
on photoresponsive sol-gel transition behaviour of (R,R)-(+)-1
are now in progress.
We thank Mr K. Yabuuchi for his help with the SEM
observation and Dr E. Marfo-Owusu for helpful discussion.
Notes and references
‡
Gelation tests were carried out by mixing (R,R)-(+)-1 and organic solvents
in sealed test tubes. The mixtures were heated up to the isotropic liquid
states, then cooled to appropriate temperatures. The minimum gel
concentration was determined as described in ref. 12.
(+)-1 supports the existence of the large dipole moment.** The
§
SEM observation was performed with a Hitachi S-900S. Variable-
length of the c-axis is 40.7 Å, which is 15.9 Å shorter than the
length of the smectic layer spacing of (R,R)-(+)-1. This
difference corresponds to the difference in the alkyl chain
length.
temperature FT-IR measurements were conducted on a JASCO Micro-20
spectrometer equipped with a Mettler FP-90 hot-stage. See ESI† for the IR
spectra.
¶
SAXS measurements were carried out on a Rigaku RINT 2100 system
with a temperature controller using Cu-Ka radiation. See ESI† for the
SAXS profile of (R,R)-(+)-1 in the liquid-crystalline state.
In summary, we have demonstrated that compound (R,R)-
(
+)-1 shows excellent gelation properties for various organic
∑
Crystal data for (R,R)-2: C23
.1 3 1.1 mm, orthorhombic, space group P2 2 2
H
28
N
2
O
4
, M = 396.487, crystal size 0.1 3
(no. 19), a = 5.8530(2),
0
1 1 1
3
b = 9.3810(5), c = 40.728(3) Å, V = 2236.3(2) Å , T = 298 K, Z = 4,
m(Mo-Ka) = 0.087 mm , 2073 reflections measured, 2050 unique, which
were used in all calculations. The final R and wR(F ) were 0.095 and 0.126,
21
2
respectively. All diagrams and calculations were carried out using maXus
(
Bruker Nonius, Delft & MacSciene, Japan). CCDC reference number
86620. See http://www.rsc.org/suppdata/cc/b2/b205072b/ for crystallo-
graphic data in CIF or other electronic format.
* Using CAChe software, the dipole moment of a single molecule of
R,R)-2 under vacuum was calculated to be 12 Debye. The calculation was
performed using the crystalline state conformation of (R,R)-2.
1
*
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Fig. 2 X-Ray single crystal structure of (R,R)-2.
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