Synthesis and properties of liquid crystalline organogelators with cholesteryl 4-(4′-alkoxybenzoylamino)benzoates

Article abstract of DOI:10.1246/cl.131173

As a new liquid-crystalline organogelator, cholesteryl 4-(4′- alkoxybenzoylamino)benzoates were prepared. Cholesteryl 4-(4′- alkoxybenzoylamino)benzoates had enantiotropic cholesteric and chiral smectic C phases. Furthermore, cholesteryl 4-(4′-alkoxybenzoylamino)benzoate gelled organic liquid such as ndecane, n-hexadecene, 1-butanol, 1-hexanol, 1-decanol, toluene, salad oil, linalool, geraniol, nerol, citronellol, limonene, linalyl acetate, lavender oil, orange oil, and lemon oil. The terpene gels show good release of the volatile components for a long period.

Full text of DOI:10.1246/cl.131173

CL-131173
Received: December 14, 2013 | Accepted: January 6, 2014 | Web Released: April 5, 2014
Synthesis and Properties of Liquid Crystalline Organogelators
with Cholesteryl 4-(4¤-Alkoxybenzoylamino)benzoates
Kanji Kubo,*1 Kazuki Tsuji,2 Seiji Ujiie,3 and Akira Mori4
1Faculty of Engineering, Hokkai-Gakuen University, Asahimachi, Toyohira-ku, Sapporo, Hokkaido 062-8605
2Graduate School of Engineering Sciences, Kyushu University, Kasuga-koen, Kasuga, Fukuoka 816-8580
3Faculty of Engineering, Oita University, Dannoharu, Oita 870-1192
4Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga-koen, Kasuga, Fukuoka 816-8580
(E-mail: kubo@lst.hokkai-s-u.ac.jp)
As a new liquid-crystalline organogelator, cholesteryl 4-(4¤-
alkoxybenzoylamino)benzoates were prepared. Cholesteryl 4-
(4¤-alkoxybenzoylamino)benzoates had enantiotropic cholesteric
and chiral smectic C phases. Furthermore, cholesteryl 4-(4¤-
O
O
alkoxybenzoylamino)benzoate gelled organic liquid such as n-
1_6
(n = 6)
H2n+1CnO
decane, n-hexadecene, 1-butanol, 1-hexanol, 1-decanol, toluene,
salad oil, linalool, geraniol, nerol, citronellol, limonene, linalyl
acetate, lavender oil, orange oil, and lemon oil. The terpene gels
show good release of the volatile components for a long period.
1_10 (n = 10)
1_14 (n = 14)
O
O
O
N
H
2_6
2_10 (n = 10)
(n = 6)
H2n+1CnO
Self-assembled systems are of great importance, particularly
for their potential application to nanomaterials such as liquid
crystals and gelators.1-4 Many studies have been reported on
their structure and mechanisms of molecular aggregation. More
than 110000 liquid-crystalline compounds have been reported to
date.5 To design a liquid-crystalline molecule, a rigid core and
flexible alkyl side chains are required.6 The number of organo-
gelators has rapidly increased over the last 20 years.1-3 In the
past, new organogelators were often discovered accidentally,
and studies were dedicated to understanding the relationship
between the structure of a gelator and its gelation behavior. The
aggregation of organogelators into fibrous networks is driven by
multiple weak interactions such as dipole-dipole, van der Waals,
hydrogen-bonding, and π-stacking interactions. Gelators are
commonly classified by their driving force for molecular
aggregation: non-hydrogen-bond-based and hydrogen-bond-
based gelators. Amide compounds such as amino acid and urea
and hydroxy compounds such as 12-hydroxystearic acid and
sugars are hydrogen-bond-based gelators, whereas anthracene,
cholesterol, and tropone derivatives are non-hydrogen-bond-
based gelators. Recently, we have reported several liquid-
crystalline organogelators called “organogelling liquid crys-
tal.”7-10 Several organogelling liquid crystals, octaalkoxy-sub-
stituted α-diketonatocopper complex,11 octa(dodecyl)tetra-
pyrazinoporphyrazine,12 3,4,5-trialkoxybenzoylamine deriva-
tives,9,13-24 3,4,5-trialkoxybenzyl derivatives,25,26 2,3,6,7,10,11-
hexaalkoxytriphenylene derivatives,10,27-29 dibenzophenazine
derivatives,30,31 hydrazine derivatives,32-34 4-cyanophenyl 4-n-
alkoxybenzoates,10 4-cyano-4¤-alkoxybiphenyls,10 4,4¤-dialka-
noyloxybiphenyls,10 azoxybenzene derivatives,10 coumarin de-
rivatives,35,36 cholesteryl alkanoate derivatives,10,37-39 and cho-
lesteryl 4-alkoxybenzoates 1 (Figure 1)10 have been reported.
Compound 1 exhibits enantiotropic cholesteric and smectic A
phases and gels organic solvents such as 1-decanol and n-
hexadecane. However, 1 could not be used for gel preparation at
low concentration. In this paper, we report the mesomorphic and
gelation properties of cholesteryl 4-(4¤-alkoxybenzoylamino)-
2_14 (n = 14)
Figure 1. Chemical structures of 1 and 2.
benzoates 2 as new liquid-crystalline organogelators with
cholesteryl benzoate and hydrogen-bonding groups and its
application to terpene and perfume gels.
Cholesteryl 4-(alkanoylamino)benzoates 2 were prepared by
the esterification of cholesterol with the corresponding 4-(4¤-
alkoxybenzoylamino)benzoic acid in the presence of EDC¢HCl
(1-ethyl-3-(3-dimethylaminopropyl)carbodiimide
hydrochlo-
ride) and DMAP (4-(dimethylamino)pyridine). The structures
and purities of the compounds 2 were ascertained by NMR
spectroscopy and elemental analysis.40
The transition temperatures and thermal behaviors of 2 were
determined using a differential scanning calorimeter, a polariz-
ing microscope equipped with a hot stage, and an X-ray
diffractometer. The optical micrographs of chiral smectic C
(SmC*) phase and cholesteric (N*) phase of 2_14 are shown in
Figure 2.
Chiral smectic C and cholesteric phases were determined
from a following observation and an X-ray diffraction study, i.e.,
broken focal-conic fan, banded focal-conic fan, and schlieren
textures for chiral smectic C phase;41 Grandjean steps at the
border and oily streak textures for the cholesteric phase. The
thermal behaviors of the cholesteryl benzoates 1 and 2 are
summarized in Table 1. Compounds 2 had a phase sequence of
Cr-SmC*-N*-Isotropic (Iso), whereas 1 had a phase sequence
of Cr-SmA-N*-Iso.7 The melting and clearing points of 2 were
higher than those of the corresponding derivatives 1. This means
that the hydrogen bonds of benzoylamide group strengthen the
intermolecular interactions.
We measured the X-ray diffraction pattern of the meso-
phases. The smectic layer spacings (d) and molecular lengths (l)
of 2 are listed in Table 2. The molecular lengths (l) of 2 were
calculated by the MM2 method. The d/l ratios of chiral smectic
C (SmC*) phase of 2 are 0.73 for 2_10 and 0.69 for 2_14. This
568 | Chem. Lett. 2014, 43, 568–570 | doi:10.1246/cl.131173
© 2014 The Chemical Society of Japan

(a)
(b)
Table 3. Gelation ability of 2a
2_6
2_10
2_14
n-Hexane
n-Decane
n-Hexadecane
Cyclohexane
Methanol
Ethanol
1-Butanol
1-Hexanol
1-Decanol
Ethyl acetate
Acetonitrile
THF
Insol
Gel
Gel
Insol
Insol
Insol
Gel
Insol
Gel
Gel
Insol
Insol
Insol
Gel
Insol
Gel
Gel
Insol
Insol
Insol
Gel
Figure 2. Optical cross-polarizing micrographs of 2_14 at
(a) 225 (SmC*) and (b) 245 °C (N*). All pictures taken on
cooling from the isotropic phase.
Gel
Gel
Gel
Gel
Gel
Gel
Insol
Cryst
Cryst
Cryst
Cryst
Gel
Insol
Cryst
Cryst
Cryst
Gel
Insol
Cryst
Gel
Cryst
Gel
Table 1. Transition temperatures (°C) of 1 and 2
Benzene
Toluene
Salad oil
Compounds
n
Transition temperatures/°C
1_6
6
Cr1¢113.5¢Cr2¢150.9¢N*¢229.9¢Iso
Cr¢110.4¢SmA¢177.7¢N*¢208.1¢Iso
Cr¢117.6¢SmA¢175.9¢N*¢190.2¢Iso
Cr¢231.4¢N*¢301.7¢Dec
Gel
Gel
1_10
1_14
2_6
10
14
6
aGel: gelation, Cryst: crystallization, and Insol: insoluble.
2_10
2_14
10
14
Cr¢216.1¢SmC*¢254.1¢N*¢264.4¢Iso
Cr¢201.4¢SmC*¢239.4¢N*¢246.6¢Iso
Table 2. Layer spacings (¡) and molecular lengths (¡) of 2
Temperatures Layer spacings (d) Molecular lengths (l)
Compounds
/°C
/¡
/¡
2_10
2_14
220
230
30.5
32.3
41.7
46.5
Figure 3. (a) Photograph and (b) micrograph of 2_10-1-
decanol gel.
result suggests that the molecular long axes of 2 are tilted at 47°
for 2_10 and 44° for 2_14, with respect to the normal to the layer
in the SmC* phase and form the layer.
hydrogen bonds of the amide group play an important role in
the crystal and gel states.
We carried out the gelation test of 2 in various organic
solvents (n-hexane, n-decane, n-hexadecane, cyclohexane, meth-
anol, ethanol, 1-butanol, 1-hexanol, 1-decanol, ethyl acetate,
acetonitrile, THF, benzene, toluene, and salad oil). The result of
the gelation test with 2 is shown in Table 3. Compounds 2 gelled
n-decane, n-hexadecene, 1-butanol, 1-hexanol, 1-decanol, tol-
uene, and salad oil, while 1 gelled 1-decanol and n-hexadecane.
Figure 3 shows a photograph of 2_10-decanol gel and an
optical micrograph of the gel phase of 2_10 in 1-decanol (0.9
w/v%) recorded at 25 °C on cooling from an isotropic liquid
state. Highly intertwined, rod-like fibers aligned to form the
network structures by making bundles with diameters of ca. 1-4
¯m. The minimum gel concentrations (MGC, g dm¹3) of 2 are
40 for 2_6-n-hexadecene, 4 for 2_10-n-hexadecene, 49 for 2_14-
n-hexadecene, 4 for 2_6-1-decanol, 3 for 2_10-1-decanol, and 4
for 2_14-1-decanol. The gelation ability of 2_10 for 1-decanol
and n-hexadecane is superior than that of the other cholesteryl
benzoates.
To study the application of organogelator 2 to new
materials, terpene and perfume gels were prepared. In perfume,
fragrance, and deodorant materials, hydrogelators such as
carrageenan, agar, collagen, gellan gum, gelatin, etc. have been
used for gelations of water-containing terpenes, essential oils,
and perfumes. Therefore, the purity of the oils in the gels is low
and most of the components of these materials comprise water.
Fortunately, under the gel-organic liquid conditions (5.5-
6.8 mg/1.0 cm3), 2_10 could gel the terpenoids such as linalool,
geraniol, nerol, citronellol, limonene, linalyl acetate, and
essential oils such as lavender oil, lemon oil, and orange oil as
shown in Figure 4.
The release tests of the volatile components from 2_10-
limonene, -linalool, and -linallyl acetate gels were carried out.
The caps of the glass tubes containing the fragrance gel were
opened. They were left to stand at room temperature, and the
weight changes of the gels and neat liquids were investigated, as
shown in Table 4. The weights of the gels and neat liquids
decreased with time. The rates of decrease in the weight of the
gels were slower than those of neat liquids, showing that the gel
state controlled the release of the volatile components. After
100 days, each gel maintained the gel state with a characteristic
scent of terpenoids.
In order to evaluate the hydrogen bonding in the gels, the IR
spectra of crystalline, 1-decanol gel (5.0 w/v%), and a chloro-
form solution of 2_10 were measured. The absorption band at
¹1
1655 cm¹1 of 2_10-decanol gel was lower than that (1672 cm
)
of the chloroform solution and close to that (1655 cm¹1) of
crystalline compound. This means that the intermolecular
Chem. Lett. 2014, 43, 568–570 | doi:10.1246/cl.131173
© 2014 The Chemical Society of Japan | 569

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570 | Chem. Lett. 2014, 43, 568–570 | doi:10.1246/cl.131173
© 2014 The Chemical Society of Japan