Helical ribbon aggregate composed of a crown-appended cholesterol derivative which acts as an amphiphilic gelator of organic solvents and as a template for chiral silica transcription

Article abstract of DOI:10.1021/ja010508h

New crown-appended cholesterol-based organogelator 1, which has two cholesterol skeletons as a chiral aggregate-forming site, two amino groups as an acidic proton-binding site, and one crown moiety as a cation-binding site, was synthesized, and the gelati

Full text of DOI:10.1021/ja010508h

J. Am. Chem. Soc. 2001, 123, 8785-8789
8785
Helical Ribbon Aggregate Composed of a Crown-Appended
Cholesterol Derivative Which Acts as an Amphiphilic Gelator of
Organic Solvents and as a Template for Chiral Silica Transcription
Jong Hwa Jung,†,‡ Hedeki Kobayashi,† Mitsutoshi Masuda,§ Toshimi Shimizu,§ and
Seiji Shinkai†,*
Contribution from the Chemotransfiguration Project, Japan Science and Technology Corporation (JST),
2432 Aikawa, Kurume, Fukuoka 839-0861, Japan, and Nanoarchitechtonics Research Center,
National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba Central 5,
1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
ReceiVed February 26, 2001
Abstract: New crown-appended cholesterol-based organogelator 1, which has two cholesterol skeletons as a
chiral aggregate-forming site, two amino groups as an acidic proton-binding site, and one crown moiety as a
cation-binding site, was synthesized, and the gelation ability was evaluated in organic solvents. It can gelate
acetic acid, acetonitrile, acetone, ethanol, 1-butanol, 1-hexanol, DMSO, and DMF under 1.0 wt %, indicating
that 1 acts as a versatile gelator of various organic solvents. To characterize the aggregation mode in the
organogel system, we observed a CD spectrum of the acetic acid gel 1. In the CD spectrum, the λθ)0 value
appears at 353 nm, which is the same as the absorption maximum λmax ) 353 nm. The positive sign for the
first Cotton effect indicates that the dipole moments of azobenzene chromophores tend to orient in a clockwise
direction. Very surprisingly, the TEM images of the 1 + acetic acid gel resulted in the helical ribbon and the
tubular structures. Sol-gel polymerization of tetraethoxysilane was carried out using 1 in the gel phase. The
silica obtained from the 1 + acetic acid gel showed the helical ribbon with 1700-1800-nm pitches and the
tubular structure of the silica with ∼560-nm outer diameter. As far as can be recognized, all the helicity
possesses a right-handed helical motif. Since the exciton-coupling band of the organogel also shows R (right-
handed) helicity, we consider that a microscopic helicity is reflected by a macroscopic helicity.
Introduction
various structures of silica such as linear fibers,8a,b lamellar,8c-e
helical fibers,8f and spherical8g structures by sol-gel polymer-
ization. These results indicate that the superstructures formed
in organogels are useful as templates to create various silica
structures. One particularly interesting class of gelators, crown-
appended cholesterol-based gelators, frequently resulted in
The sol-gel synthesis of well-ordered inorganic materials
offers a new and wide-ranging approach to useful materials with
controlled architecture and porosity across a range of length
scales.1,2 The direct synthesis of discrete inorganic architectures
necessitates the use of dispersed organic supramolecular struc-
tures with commensurate dimensionality; for example, hollow
fibers of silica have been prepared with self-assembled viroid
cylinders,1 organic crystals,2e or phospholipid fibers2f as tem-
plates.
Recently, numerous thermoreversible physical gels formed
with low-molecular-weight organic molecules have been
reported.3-7 The interest shown lies in the numerous potential
applications envisaged for these materials such as hardeners of
solvents, drug delivery systems, membranes, and sensors. Very
recently, organogels were applied as novel media to produce
(3) (a) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.;
Komori, T.; Ohseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994,
116, 6664 and references therein. (b) James, T. D.; Murata, K.; Harada, T.;
Ueda, K.; Shinkai, S. Chem. Lett. 1994, 273. (c) Jeong, S. W.; Murata, K.;
Shinkai, S. Supramol. Sci. 1996, 3, 83. (d) Shinkai, S.; Murata, K. J. Mater.
Chem. 1998, 8, 485. (e) Yoza, K.; Amanokura, N.; Ono, Y.; Akao, T.;
Shinmori, H.; Takeuchi, M.; Shinkai, S.; Reinhout, D. L. Chem. Eur. J.
1999, 5, 2722.
(4) (a) Wang, R.; Geiger, C.; Chen, L.; Swanson, B.; Whitten, D. G. J.
Am. Chem. Soc. 2000, 122, 2399. (b) Duncan, D. C.; Whitten, D. G.
Langmuir 2000, 16, 6445. (c) Geiger, C.; Stanescu, M.; Chen, L.; Whitten,
D. G. Langmuir 1999, 15, 2241.
(5) (a) For recent comprehensive reviews, see: Terech, P.; Weiss, R. G.
Chem. ReV. 1997, 3313. (b) Otsuni, E.; Kamaras, P.; Weiss, R. G. Angew.
Chem., Int. Ed. Engl. 1996, 35, 1324 and references therein. (c) Terech, P.;
Furman, I.; Weiss, R. G. J. Phy. Chem. 1995, 99, 9558 and references
therein.
(6) (a) Hanabusa, K.; Yamada, M.; Kimura, M.; Shirai, H. Angew. Chem.,
Int. Ed. Engl. 1996, 35, 1949. (b) Loos, M.; Esch, J. v.; Stokroos, I.; Kellogg,
R. M.; Feringa, B. L. J. Am. Chem. Soc. 1997, 119, 12675. (c) Schoonbeek,
F. S.; Esch, J. v.; Hulst, R.; Kellogg, R. M.; Feringa, B. L. Chem. Eur. J.
2000, 6, 2633. (d) Esch, J. v.; Feringa, B. L. Angew. Chem., Int. Ed. 2000,
39, 2263.
* Corresponding author: (fax) +81-942-39-9012; (e-mail) seijitcm@
mbox.nc.kyushu-u.ac.jp.
† Japan Science and Technology Corp.
‡ Present address: CREST, JST, Nanoarchitectonics Research Center,
National Institute of Advanced Industrial Science and Technology (AIST),
Tsukuba Central 4, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japan.
§ National Institute of Advanced Industrial Science and Technology.
(1) Mann, S., Ed. Biomimetic Materials Chemistry; VCH: New York,
1996.
(2) (a) Shenton, W.; Douglas, T.; Young, M.; Stubbs, G.; Mann, S. AdV.
Mater. 1999, 11, 253. (b) Douglas, T.; Young, M. Nature 1998, 393, 152.
(c) Shenton, W.; Pum, D.; Sleytr, U.; Mann, S. Nature 1997, 389, 585. (d)
Davis, S. A.; Burkett, S. L.; Mendelson, N. H.; Mann, S. Nature 1997,
385, 420. (e) Nakamura, H.; Matsui, Y. J. Am. Chem. Soc. 1995, 117, 2651.
(f) Burkett, S. L.; Mann, S. Chem. Commun. 1996, 321. (g) Shenton, W.;
Douglas, T.; Young, M.; Stubbs, G.; Mann, S. AdV. Mater. 1999, 11, 253.
(7) (a) Melendez, R.; Geib, S. J.; Hamilton, A. D. In Molecular Self-
Assembly Organic Versus Inorganic Approaches; Fujita, M., Ed.;
Springer: Berlin, 2000. (b) Carr, A. J.; Melendez, R.; Geib, S. J.; Hamilton,
A. D. Tetrahedron Lett. 1998, 39, 7447. (c) Shi, C.; Kilic, S.; Xu, J.; Enick,
R. M.; Beckman, E. J.; Carr, A. J.; Melendez, R. E.; Hamilton, A. D. Science
1999, 286, 1540.
10.1021/ja010508h CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/18/2001

8786 J. Am. Chem. Soc., Vol. 123, No. 36, 2001
Jung et al.
Scheme 1
Table 1. Gelation Ability of 1 in Organic Solventsa
various novel structures such as the fibrous, the lamellar,8d the
vesicular,8g and the multilayered tubular8c structures.
entry
solvent
gelationb
Kunitake and co-workers found that certain amphiphiles can
form the tubular structure through the helical ribbon structure
in aqueous solution.9 They possess a polar head and suitable
chiral hydrophobic groups to form stable aggregates. Also, the
formation of twisted ribbon structures of gelators were reported
by a number of research groups.5,10 However, they were not
grown from the helical ribbon or the tubular structures through
a twisted ribbon structure. Helical ribbon structures of the
organogels, on the other hand, were never observed in organic
solvent systems, and it thus occurred to us that judging from
the versatility of crown-appended cholesterol-based gelators,8e,d,g
one might be able to find both structures, growing from the
helical ribbon to the tubule (as suggested by Kunitake et al.9
for the aqueous solution system). In addition, superstructures
created from these gelators might be useful as templates for
transcription into the silica structures.
1
2
3
4
5
6
7
8
9
acetic acid
propionic acid
acetonitrile
acetone
G
G
G
G
G
G
G
G
G
S
ethanol
1-butanol
1-hexanol
DMSO
DMF
10
11
12
chloroform
tetrahydrofurane
n-hexane
S
I
a Gelator concentration, 1.0 wt %. b Gelation of 1: G, stable gel
formed at room temperature; S, soluble; I, insoluble.
gelates various organic solvents under 1.0 wt % but enables us
to observe both the helical ribbon and the tubule. In addition,
it acts as a template for sol-gel polymerization of tetraethoxy-
silane (TEOS) to produce the novel “helical ribbon silica” as
well as the “double-layered tubular silica”. To the best of our
knowledge, this is the first observation not only for the helical
ribbon structure in the organogel system but also for the direct
transcription of the molecular assemblies into the helical ribbon
structure of the silica.
With these objects in mind, we have designed 1, which has
Results and Discussion
Chracterization of Organogel Superstructure by Circular
Dichroism (CD). Compound 1 was synthesized according to
Scheme 1. The gelation ability of 1 was estimated in various
organic solvents. The results are surmmarized in Table 1. The
gelator 1 could gelate 8 out of 12 organic solvents under 1.0
wt %, indicating that 1 acts as a versatile gelator of various
organic solvents. Gelator 1 formed better gel in organic solvents
in comparison to gelators bearing small size crown moiety such
as 24-crown-8- and 18-crown-6-appended gelators.
two cholesterol skeletons as a chiral aggregate-forming site, two
amino groups as an acidic proton-binding site, and one crown
moiety as a cation-binding site. We have found that 1 not only
(8) (a) Ono, Y.; Nakashima, K.; Sano, M.; Kanekiyo, Y.; Inoue, K.; Hojo,
J.; Shinkai, S. Chem. Commun. 1998, 1477. (b) Ono, Y.; Kanekiyo, Y.;
Inoue, K.; Hojo, J.; Shinkai, S. Chem. Lett. 1999, 23. (c) Jung, J. H.; Ono,
Y.; Shinkai, S. Langmuir 2000, 16, 1643. (d) Jung, J. H.; Ono, Y.; Shinkai,
S.; J. Chem. Soc., Perkin Trans. 2 1999, 1289. (e) Jung, J. H.; Ono, Y.;
Shinkai, S. Angew. Chem., Int. Ed. 2000, 39, 1862. (f) Jung, J. H.; Ono,
Y.; Shinkai, S. Chem. Eur. J. 2000, 5, 4552. (g) Jung, J. H.; Ono, Y.;
Sakurai, K.; Sano, M.; Shinkai, S. J. Am. Chem. Soc. 2000, 122, 8648.
(9) (a) Nakashima, N.; Asakuma, S.; Kunitake, T. J. Am. Chem. Soc.
1985, 107, 09. (b) Fuhrhop, J.-H.; Ko¨ning, J. Membranes and Molecular
Assemblies: The Synkinetic Approach; Royal Society of Chemistry: London,
1994; and references therein. (c) Kunitake, T. Angew. Chem., Int. Ed. Engl.
1992, 21, 709. (d) Nakashima, N.; Kunitake, T. Chem. Lett. 1984, 1709.
To characterize the aggregation mode in the organogel phase,
we observed the CD spectrum of acetic acid gel 1. In the CD
spectrum, the λθ)0 value appears at 353 nm, which is consistent
(10) (a) Oda, R.; Huc, I.; Candau, S. J. Angew. Chem., Int. Ed. 1998,
37, 2689. (b) Menger, F. M.; Keiper, J. S. Angew. Chem., Int. Ed. 2000,
41, 1906.

Crown-Appended Cholesterol-Based Organogelator
J. Am. Chem. Soc., Vol. 123, No. 36, 2001 8787
Figure 1. CD spectrum of the 1 + acetic acid gel.
Figure 3. Powder XRD spectrum of xerogel 1 prepared from acetic
acid.
respectively, even in organic solvents without sonication. The
organogel morphology is clearly different between azacrown-
appended gelator and 1; azacrown-appended gelator gave the
vesicular structure8g whereas 1 gave the helical ribbon structure.
This implies that the helical ribbon structure in the organogel 1
grows more efficiently from the vesicular structure than that in
the azacrown-appended gelator. It is not clear yet, however, why
the further growth of the vesicular structure is suppressed in
azacrown-appended gelator whereas it continues to grow to the
“helical ribbon structure” in 1.
Recently, an X-ray crystallographic methodology for ascer-
taining the molecular packing of gelators in the gel phase has
been reported, and this method is being used to clarify the
gelation mechanism of low-molecular-weight gelators.12 How-
ever, the correlation between the molecular packing of gelator
molecules and the physical gelation properties is still unknown.
The xerogel 1 obtained from acetic acid by a freezing method
resulted in the spongelike aggregate but not the typical crystal.
We obtained information about the molecular-packing mode of
gelator molecules in a neat gel from an X-ray diffraction pattern
of the xerogel. The diffraction pattern is characterized by three
sharp reflection peaks of 41.15, 20.59, and 13.80 Å (Figure 3),
the relative intensity of which is almost exactly in the ratio of
1:1/2:1/3. This means that the xerogel maintains a layered
structure with the interlayer distance of 41.15 Å corresponding
to the (100) plane, while that of sample 1 prepared by
conventional synthetic procedures gave only a broad pattern.
This result supports the view that the organogel possesses the
well-ordered aggregate structure in the gel state.
Figure 2. TEM picture of the xerogel 1 prepared from acetic acid
(negatively stained with UO22+).
with the absorption maximum at λmax ) 353 nm. One can thus
assign the CD spectrum to the exciton-coupling band, although
it is somewhat asymmetrical (Figure 1). It is known that
azobenzene-appended cholesterol gelators with natural (S) C-3
configuration tend to give a positive sign for the first Cotton
effect, indicating that the dipole moments of azobenzene
chromophores tend to orient in a clockwise direction.8e,f
However, the macroscopic helicity of the aggregate formed by
self-assembly is not necessarily related to the microscopic
helicity in a Cotton effect. This problem will be discussed later
in more detail. We confirmed that the contribution of linear
dichronism (LD) to the true CD spectrum is negligible by using
the conventional LD mode. The strong CD intensity suggests,
therefore, that 1 in the organogel adopts a folded conformation
to enjoy efficient intramolecular cholesterol-cholesterol and
azobenzene-azobenzene interactions.11
Helical and Tubular Structures of the Organogel As
Observed by Transmission Electron Microscopy (TEM). To
obtain visual insights into the aggregation mode, we observed
the xerogel structure of acetic acid gel 1 by TEM and scanning
electron microscopy (SEM). Figure 2 shows a TEM picture of
the xerogel 1 obtained from acetic acid. Very interestingly, the
xerogel 1 mainly consists of the tubular structures with ∼520-
nm outer diameter but also illustrates the linear ribbon and the
helical ribbon with 1700-1800-nm pitch. The organogel 1
possesses a right-handed helical motif, since the exciton-
coupling band of the organogle 1 also shows R helicity. In
addition, the SEM picture of the xerogel 1 also reveals the
characteristic right-handed helical ribbon structure. These results
indicate that the structure of the organogel process involves
several metastable intermediate structures, viz. the linear ribbon
f the helical ribbon f the tubule. The structure of 1 is quite
different from double-chain ammonium amphiphiles previously
reported by Kunitake et al. which have long aliphatic groups.9
These amphiphiles were transformed from the helical ribbon
structure (its presence is assumed) into the tubular structure with
∼3.2-µm diameter in aqueous solution by sonication, whereas
1 can form both the helical ribbon structure and the tubular
structure with ∼520-nm diameter and 1700-1800-nm pitch,
Sol-Gel Polymerization of TEOS and SEM/TEM Obser-
vation of the Silica Structure. To transcribe the superstructure
formed in the organogel into the silica structure, we carried out
sol-gel polymerization of TEOS using 1 in acetic acid or
1-butanol gel phase according to the method described previ-
ously.8 After sol-gel polymerization, we observed SEM pictures
of the silica obtained from 1 before and after calcination. Figure
4 shows a SEM picture after calcination: the silica obtained
from acetic acid 1 possesses the helical ribbon structure with
1700-1800-nm pitches and the tubular structure of the silica
(11) A similar folded conformation was proposed for an azacrown
derivative bearing two cholesterol groups.8g
(12) (a) Hanabusa, K.; Matsumoto, M.; Kimura, M.; Kakehi, A.; Shirai,
H. J. Colloid Interface Sci. 2000, 224, 231. (b) Abdallah, D. J.; Sirchio, S.;
Weiss, R. G. Langmuir 2000, 16, 7558. (c) Sakurai, K.; Ono, Y.; Jung, J.
H.; Okamoto, S.; Sakurai, S.; Shinkai, S. J. Chem. Soc., Perkin Trans. 2
2001, 108.

8788 J. Am. Chem. Soc., Vol. 123, No. 36, 2001
Jung et al.
Figure 4. SEM picture of the silica obtained from the 1 + acetic acid
gel after calcination.
with ∼560-nm outer diameter. As far as can be recognized, all
the helicity possesses a right-handed helical motif. Since the
exciton-coupling band of the organogel also shows R (right)
helicity, we consider that in the present system a microscopic
helicity is reflected by a macroscopic helicity. These results
indicate that the novel helical ribbon structure and the tubular
structure of the organogel 1 have successfully been transcribed
into the silica structures. This helical ribbon silica structure is
novel and different from the helical fiber structures obtained
from cyclohexanediamine-based organogels.8f
To further corroborate that the organogel structure really acted
as a template for growth of the helical ribbon into the tubule,
TEM pictures were taken after removal of 1 by calcination
(Figure 5). Very interestingly, 1700-1800 nm of the helical
pitches of the helical ribbon structure remains constant with a
gradual change into the tubular structure (FigureS 5a-c). In
addition, the silica illustrates double layers with an interlayer
distance of 8-9 nm (Figure 5d). These results indicate that
TEOS (or oligomeric silica particles) was adsorbed onto both
surfaces of the double-layered tubules with 8-9-nm thickness.
Therefore, the tubular silica possesses two hollow cavities. The
smaller hollow with 8-9-nm layers was created by the
organogel template whereas the larger hollow with 460-480-
nm inner diameters was created by the growth of helical ribbon.
We believe that such a precise transcription becomes possible
for the first time by using the organogel superstructures as a
template featuring the crystal-like characters. In addition, sol-
gel polymerization of TEOS using a large crown-appended
gelator such as 1 in the presence of metal salts and organic
compounds can be conveniently applied to deposition of noble
metals and organic compounds in the 8-9-nm interlayers of
the helical ribbon structure of the silica. In contrast, the
deposition of noble metals on the inner wall in the helical fiber
of the silica prepared from the cyclohexanediamine gels is very
difficult or almost impossible.
The helical ribbon and the tubular structures of the silica were
also created in 1-butanol where the amino groups are not
protonated. The result again supports the importance of the
intermolecular hydrogen-bonding interaction in the creation of
the helical ribbon and the tubular structures (Figure 6a). When
a trace amount of acid is present in the sol-gel medium, the
amino groups are protonated and the cationic charges thus
formed can be a driving force to adsorb anionic oligomeric silica
particles. To obtain evidence for the contribution of the
hydrogen-bonding interaction to the binding of oligomeric silica
particles, sol-gel polymerization was carried out in the presence
of Et4NOH (tetraethylammonium hydroxide: 2 equiv to 1). As
expected, the silica produced under this condition also shows
the helical ribbon and the tubular structures. This result indicates
that the hydrogen-bonding interaction between TEOS and the
neutral amino groups of 1 can also act as a driving force to
create the novel structure of the silica (Figure 6b).
Figure 5. TEM pictures of the silica obtained from the 1 + acetic
acid gel after calcination.
Conclusions
The present paper has demonstrated that the organogelator 1
bearing dibenzo-30-crown-10 creates the novel helical and
tubular structures in organic solvent. Sol-gel polymerization
of TEOS in the presence of the helical structure of 1 is useful
to create the helical ribbon structure of the silica. This is a very
rare example of chiral inorganic materials. The helical ribbon
structurs of the silica are created with aid of hydrogen-bonding
as well as electrostatic interactions. The results clearly indicate
the versatility of the template method in the gel phase for the
creation of various silica structures.
In addition, noble metals and organic compounds can be not
only conveniently deposited between the layers of the helical
ribbon structure of the silica obtained from the organogel 1 by
the host-guest interaction13 but also readily applicable to the
design of novel solid catalysts featuring a size recognition
ability of 8-9-nm silica layers. We believe that the present
silica with the helical higher-order morphology is useful as
unique catalysts for asymmetric syntheses without chiral organic
ligands.14
(13) The feasibility of this idea (deposition of noble metals during the
sol-gel process) has been examined; see: References 8d,g. Jung, J. H.;
Shinkai, S. J. Chem. Soc., Perkin Trans. 2 2000, 2393.
(14) To test this intriguing idea, we applied this helical silica to the Soai
reaction (see: Soai, K.; Osanai, S.; Kadowaki, K.; Yonekubo, S.; Shibata,
T.; Sato, I. J. Am. Chem. Soc. 1999, 121, 11235) in collaboration with Soai’s
group. Surprisingly, optical yields of 96-98% ee have been attained.

Crown-Appended Cholesterol-Based Organogelator
J. Am. Chem. Soc., Vol. 123, No. 36, 2001 8789
4-n-Monobromobutoxyl-4′-((cholesteryloxy)carbonyl)azoben-
zene (2). 4-((Bromobutyloxyphenyl)azo)benzoic acid 3 (0.7 g, 1.86
mmol) and cholesterol (0.718 g, 2.23 mmol) were dissolved in 20 mL
of dichloromethane under a nitrogen atmosphere. The solution was
maintained at 0 °C by an ice bath. Dicyclohexylcarbodiimide (DCC;
0.383 g, 1.86 mmol) and (dimethylamino)pyridine (DMAP; 0.022 g,
0.186 mmol) were then added, the reaction mixture being stirred for 4
h at 0 °C. The reaction mixture was filtered, and the filtrate was washed
with acidic and basic aqueous solutions (50 mL each). The organic
layer was evaporated to dryness. The residue was purified by a silica
gel column eluting with THF/n-hexane (1:6 v/v) to give 2 in 26% yield
as a yellow solid (mp 141.5 °C). 1H NMR (300 MHz, CDCl3): δ (ppm)
8.17 (2H, d, J ) 9.0 Hz), 7.72 (2H, d, J ) 9.0 Hz), 7.90 (2H, d, J )
9.0 Hz), 7.10 (2H, d, J ) 9.0 Hz), 5.45 (1H, d, J ) 6.3 Hz), 5.02-
4.88 (1H, m), 41 (2H, t, J ) 6.3 Hz), 3.52 (2H, t, J ) 6.2 Hz), 2.49
(2H, d, J ) 6.2 Hz), 2.28-0.94 (35H, m), 0.88 (3H, s). 13C NMR (75
MHz, CDCl3): δ (ppm) 165.1, 161.88, 155.20, 146.98, 139.9, 130.2,
125.18, 122.84, 122.28, 114.72, 67.22, 66.67, 56.67, 56.11, 50.01, 42.30,
39.71, 39.50, 38.20, 37.01, 36.64, 36.17, 35.79, 33.32, 31.92, 31.86,
29.3, 28.32, 28.01, 27.88, 27.78, 24.28, 23.82, 22.83, 22.56, 21.04,
19.38, 19.38, 18.71, 11.86. MS (SIMS): 745 [M + H]+. IR (KBr):
3005, 1722, 1603, 1579, 1500, 1468, 1284, 1116, 1047 cm-1
.
Dinitro-dibenzo-30-crown-10 (6). To a stirred solution of dibenzo-
30-crown-10 (0.15 g, 0.28 mmol) in acetic acid (5 mL) and water (1
mL) at 5.0 °C was added dropwise 60% HNO3 (1 mL). The reaction
mixture was warmed to room temperature over period of 30 min and
stirred for an additional 2 h at room temperature. Then, 1.0 M NaOHaq
was added to the reaction mixture until the solution became neutral.
The precipitate was filtered off, to give 5 in 85.0% yield as a yellow
powder (mp 133-135 °C). 1H NMR (300 MHz, CDCl3): δ (ppm) 3.69
(8H, br s), 3.77 (8H, br s), 3.93 (8H, br s), 4.22 (8H, br s), 6.87 (2H,
d, J ) 8.4 Hz), 7.73 (2H, s), 7.84 (2H, d, J ) 8.4 Hz). IR (KBr): 3005,
2987, 1508, 1210 cm-1; MS (SIMS): 644.5 [M + 2H]+. Anal. Calcd
for C29H42N2O14: C, 54.20; H, 6.59; N, 4.36. Found: C, 54.50; H,
6.55; N, 4.30.
Figure 6. SEM pictures of the silica obtained from the 1 + 1-butanol
gel in (a) the absence and (b) the presence of tetraethylammonium
hydroxide.
Diaminodibenzo-30-crown-10 (5). Dinitrodibenzo-30-crown-10
(0.134 g, 0.21 mmol) was added to dimethylacetamide anhydrate (20
mL), and the solution was stirred until the solid was dissolved. Then,
10% Pd-C (0.10 g) was added to the solution, and hydrogen gas was
introduced into the solution for 3 h at room temperature. The reaction
mixture was filtered to remove Pd-C, and the filtrate was evaporated
in vacuo to dryness. The residue was purified by a silica gel column
chromatography eluting with methanol/chloroform (1:9 v/v) to give 5
Experimental Section
Apparatus for Spectroscopy Measurements. H and 13C NMR
spectra were measured on a Bruker ARX 300 apparatus. IR spectra
were obtained in KBr pellets using a Shimadzu FT-IR 8100 spectrom-
eter, and MS spectra were obtained by a Hitachi M-250 mass spec-
trometer.
1
TEM and SEM Observations. For TEM, a piece of the gel was
placed on a carbon-coating copper grid (400 mesh) and removed after
1 min, leaving some small patches of the gel on the grid. After the
specimen was dried at low pressure, its was stained with 10-15-µL
drops of uranyl acetate (2.0 wt % aqueous solution). Then, this was
dried for 1 h at low pressure. The specimen was examined with an
Hitachi H-7100 SEM, using an accelerating voltage of 75-100 kv and
a 16-nm working distance. SEM was taken on an Hitachi S-4500. The
silica was coated with palladium-platinum and observed at 5-15 kV
of the accelerating voltage and an emission current of 10 µA.
Gelation Test of Organic Fluids. The gelator and the solvent were
put in a septum-capped test tube and heated in an oil bath until the
solid was dissolved. The solution was cooled at room temperature. If
the stable gel was observed at this stage, it was classified as G in
Table 1.
Sol-Gel Polymerization of TEOS. In a typical preparation,
(1.0-2.5) × 10-3 M gelator was dissolved in acetic acid or 1-butanol.
The organogel sample was added to TEOS (15.0-20.0 mg)/water
(2.0-6.0 mg) or TEOS (15.0-20.0 mg)/water (2.0-6.0 mg)/benzyl-
amine (2.0-6.0 mg) as a catalyst and warmed until a transparent
solution was obtained. Then, the reaction mixture was placed at room
temperature under static conditions for 7-14 days. The product was
dried by a vacuum pump at room temperature. Finally, the gelator was
removed by calcination at 200 °C for 2 h, 500 °C for 2 h under a
nitrogen atmosphere, and 500 °C for 4 h under aerobic conditions.
4-(Hydroxyphenyl)azobenzoic Acid (4) and 4-(n-Monobromobu-
toxyl- phenyl)azoazobenzene (3). These compounds were prepared
as described previously.8a,c
1
as a brown solid (0.10 g, 84% yield; mp 68-71 °C). H NMR (300
MHz, CDCl3): δ (ppm) 3.48 (4H, br s), 3.68 (8H, br s), 3.74 (8H, br
s), 3.84 (8H, br d, J ) 12.4 Hz), 4.08 (8H, br s), 6.21 (2H, dd, J ) 8.4,
2.5 Hz), 6.29 (2H, br s), 6.88 (2H, d, J ) 8.4 Hz). IR (KBr): 3350,
3007, 2985, 1521, 1504, 1340, 1205 cm-1. MS (SIMS): 557.2 [M +
2H]+. Anal. Calcd for C29H42N2O10: C, 59.78; H, 7.96; N, 4.81.
Found: C, 60.02; H,7.90; N, 4.53.
4-(Bis(diaminodibenzo-30-crown-10-butoxy)-4′-((cholesteyloxy)-
carbonyl)azobenzene (1). A mixture of 2 (0.24 g, 0.32 mmol), 5 (0.085
g, 0.15 mmol), and sodium carbonate (0.317 g, 3.00 mmol) in dry
butylnitrile (30 mL) was refluxed for 72 h. The solution was filtered
after cooling, and the filtrate was evaporated in vacuo to dryness. The
residue was purified by an aluminum oxide column chromatography
eluting with ethanol/chloroform (1:30 v/v) to give the desired product
1
as a yellow solid (56 mg, 20% yield; mp 168.2-168.7 °C). H NMR
(300 MHz, CDCl3): δ (ppm) 0.70 (6H, s), 0.87 (12H, d, J ) 6.6 Hz),
0.92-2.06 (70H, m), 2.48 (4H, d, J ) 6.4 Hz), 3.16 (4H, t, J ) 6.4
Hz), 3.43 (2H, br s), 3.51-3.79 (16H, m), 3.83-3.92 (8H, m), 4.08
(8H, br s), 4.78-4.93 (2H, m), 5.44 (2H, d, J ) 3.9 Hz), 6.17 (4H, dd,
J ) 18.0, 8.4, 2.4 Hz), 6.22 (2H, br s), 6.27 (2H, d, J ) 2.4 Hz), 6.75
(4H, dd, J ) 15.4, 9.1), 7.01 (4H, d, J ) 8.4 Hz), 7.90 (4H, d, J ) 8.4
Hz), 7.94 (4H, d, J ) 8.7 Hz),8.17 (4H, d, J ) 8.4 Hz). IR (KBr):
3350, 3007, 2987, 1600, 1585, 1500, 1220 cm-1. MS (SIMS): 1898.6
[M + 2H]+. Anal. Calcd for C116H162N6O16: C, 73.46; H, 8.61; N,
4.43. Found: C, 72.71; H, 8.73; N, 4.45.
JA010508H