Unique twisted ribbons generated by self-assembly of oligo(p-phenylene ethylene) bearing dimeric bile acid pendant groups

Article abstract of DOI:10.1002/chem.200900484

A series of novel oligo(pphenylene ethynylene) (OPE) derivatives bearing dimeric cholic acid (OPE1), deoxycholic acid (OPE2) and lithocholic acid (OPE3) was synthesized. The self-assembly behavior of these derivatives were systematically studied and compared in a THF/water solvent mixture. The addition of water to THF can induce the helical stacking of oligo(p-phenylene ethylene) bearing dimeric deoxycholic acid end groups, which leads to highly interesting twisted assemblies. Variation of the bile acid group led to nanostructures with drastically different morphologies. The application of spectroscopy in combination with X-ray diffraction techniques provided a reasonable view of the final self-assembled structures. The observed distinctive aggregate shapes and the preference in the type of molecular packing of these steroid-OPE conjugates are attributed to the subtle differences in their molecular features.

Full text of DOI:10.1002/chem.200900484

FULL PAPER
DOI: 10.1002/chem.200900484
Unique Twisted Ribbons Generated by Self-Assembly of Oligo(p-phenylene
ethylene) Bearing Dimeric Bile Acid Pendant Groups
Yan Li,[a, b] Guangtao Li,*[a] Xinyan Wang,[a, c] Weina Li,[a, b] Zhixiang Su,[a]
Yihe Zhang,[a, c] and Yong Ju*[a, b]
Abstract: A series of novel oligo(p-
phenylene ethynylene) (OPE) deriva-
tives bearing dimeric cholic acid
(OPE1), deoxycholic acid (OPE2) and
lithocholic acid (OPE3) was synthe-
sized. The self-assembly behavior of
these derivatives were systematically
studied and compared in a THF/water
solvent mixture. The addition of water
to THF can induce the helical stacking
of oligo(p-phenylene ethylene) bearing
dimeric deoxycholic acid end groups,
which leads to highly interesting twist-
ed assemblies. Variation of the bile
acid group led to nanostructures with
drastically different morphologies. The
application of spectroscopy in combi-
nation with X-ray diffraction tech-
niques provided a reasonable view of
the final self-assembled structures. The
observed distinctive aggregate shapes
and the preference in the type of mo-
lecular packing of these steroid–OPE
conjugates are attributed to the subtle
differences in their molecular features.
Keywords: bile acids · chirality ·
conjugation · self-assembly · supra-
molecular chemistry
Introduction
compounds (e.g., amino acids and nucleic acids) to p-conju-
gated systems provides one convenient way to create a vari-
ety of fascinating morphologies for the development of ad-
vanced materials.[2]
The controlled organization of p-conjugated molecules into
highly ordered arrays at the supramolecular level is of great
importance, owing to the potential applications of these
arrays in optical devices and supramolecular electronics.[1]
Nature elegantly utilizes the self-assembly of biomolecules
to construct functional superstructures. In a manner that
mimics natural processes, the introduction of biogenetic
Steroids make up a class of naturally occurring com-
pounds containing four fused rings, generally aliphatic rings,
examples of which include the biologically important bile
acids and cholesterol.[3] In addition to their physiological
role, steroidal compounds have been presented as an inex-
pensive source of chirality. Due to the rigid steroidal skele-
ton and its hydrophobic nature, such molecules tend to form
supramolecular aggregates in which the position and orien-
tation of the molecule is well organized.[4] In this respect,
cholesterol is widely used as a pendant group, which can be
attached to p-conjugated systems to induce the chiral pack-
ing of chromophores.[5] However, p-conjugated compounds
bearing dimeric bile acid moieties have been far less system-
atically investigated. In contrast to cholesterol, bile acids are
strikingly diverse in nature; therefore, it is highly interesting
to study and compare the self-assembly behavior of these
structurally closely related compounds.
[a] Y. Li, Prof. Dr. G. Li, X. Wang, W. Li, Z. Su, Prof. Dr. Y. Zhang,
Prof. Dr. Y. Ju
Department of Chemistry
Key Lab of Organic Optoelectronics & Molecular Engineering
Ministry of Education, Tsinghua University
Beijing, 100084 (China)
Fax : (+86)10-6279-2905
E-mail: lgt@mail.tsinghua.edu.cn
[b] Y. Li, W. Li, Prof. Dr. Y. Ju
Department of Chemistry
Key Lab of Bioorganic Phosphorus Chemistry & Chemical Biology
Ministry of Education, Tsinghua University
Beijing, 100084 (China)
Fax : (+86)10-6278-1695
E-mail: juyong@mail.tsinghua.edu.cn
Herein, a new series of compounds consisting of an
oligo(p-phenylene ethylene) (OPE) spacer and a bile acid
end group was designed. Scheme 1 shows the synthesized
OPE derivatives bearing dimeric cholic acid (OPE1), deoxy-
cholic acid (OPE2), and lithocholic acid units (OPE3),
which are differentiated only by the number of hydroxyl
groups. Very interestingly, it was found that such subtle var-
[c] X. Wang, Prof. Dr. Y. Zhang
School of Material Science and Technology
China University of Geosciences
Beijing 100083 (China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900484.
Chem. Eur. J. 2009, 15, 6399 – 6407
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6399

Hence, OPE1–OPE3 are either
too soluble or too crystalline
for gel formation.[6]
These results and the facially
amphiphilic character of bile
acids prompted us to investi-
gate the self-assembly of
OPE1–OPE3 in a mixed sol-
vent. Recently, we found that
gradually adding water to THF
(which reinforces the hydropho-
bic interactions without block-
ing the formation of hydrogen
bonds) is an ideal methodology
Scheme 1. Chemical structures of the synthesized OPEs bearing dimeric bile acid end groups.
iations could lead to nanostructures with drastically different
morphologies.
to induce the self-assembly of conjugated polymers bearing
bile acid pendant groups.[7] One or two water molecules
might also be present and may act as bridging groups be-
tween cholate hydroxyl groups, as is observed in the crystal
structure of cholic acid.[8] This work inspired us to adopt a
THF/water solvent system to study the self-assembly of
OPE1–OPE3.
As water was gradually added, transparent solutions of
OPE1 and OPE2 in THF both change into cloudy liquids
that exhibit light scattering, which is indicative of the forma-
tion of nano- or micrometer aggregates. However, the re-
sulting solution of OPE2 was much more turbid than that of
OPE1 (Figure S1 in the Supporting Information). The con-
centration of both compounds in THF was identical (2ꢁ
10À4 m), thus this decrease of transparency is likely to result
from the formation of larger aggregates. Indeed, dynamic
light scattering (DLS) experiments revealed that OPE1
forms particles of the size in the range of 155–309 nm, with
peak intensity at 230 nm and a polydispersity of 0.13. In con-
trast, the supramolecular aggregation of OPE2 is larger than
2 mm, which is out of the range for our DLS measurement
(Figure S2 in the Supporting Information).
Results and Discussion
Synthesis: OPE derivatives OPE1, OPE2, or OPE3 were
synthesized by Pd-catalyzed cross-coupling between the cor-
responding steroidal iodobenzene derivatives and 1,4-diethy-
nylbenzene (Scheme 2). The synthetic process was quite
straightforward, and the structures of these compounds were
confirmed by IR and NMR spectroscopies and mass spec-
trometry.
Optical and self-assembly properties: Because many dimeric
cholesterol-based p-conjugated compounds are recognized
as powerful gelling agents,[5] we first focused our attention
on using these OPE derivatives as potential gelators. Vari-
ous nonpolar solvents (such as cyclohexane, benzene, and
alkyl halides) as well as several polar solvents (such as etha-
nol and THF) were tested, but to our disappointment, none
of the bile acid derived OPEs formed a stable gel in these
solvents. In fact, these compounds were either insoluble or
precipitated upon cooling in most nonpolar (e.g., alkyl hal-
ides and aromatic solvents) and protic solvents (e.g., ethanol
and water). However, except for OPE3, the new OPEs have
rather high solubility (>6 mgmLÀ1) in several aprotic polar
solvents, such as THF and DMF, and form a clear light-
yellow solution in these solvents with blue fluorescence.
Optical spectroscopies (UV/Vis, circular dichroism, and
fluorescence spectroscopies) were used to study the forma-
tion of the aggregates and excimers in the solvent/nonsol-
vent mixtures used. Figure 1a and b show that the solvent-
dependent UV/Vis spectra of OPE1 and OPE2 do not differ
significantly. In absolute THF at 208C, absorption maxima
for both compounds were at about 343 nm. Upon addition
Scheme 2. The synthetic route to OPEs bearing dimeric bile acid pendant groups. a) dicyclohexylcarbodiimide (DCC), 1-hydroxybenzotriazole (HOBt),
4-iodoaniline, RT; b) tetramethylsilane (TMS), [Pd(PPh3)4], CuI, Et3N, DMF, 608C; c) CH3OH, K2CO3, RT; d) [Pd(PPh3)4], CuI, (iPr)2NH, THF.
A
ACHTUNGTRENNUNG
6400
www.chemeurj.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 6399 – 6407

Unique Self-Assembled Twisted Ribbons
FULL PAPER
Figure 1. Absorption spectra of a) OPE1 and b) OPE2 in THF with increasing amounts of water; CD spectra in THF with increasing amounts of water
of c) OPE1 and d) OPE2.
of water, at a rate of 10% per minute under vigorous stir-
ring, the absorption intensity decreased considerably. Mean-
while, the appearance of a vibrational fine shoulder struc-
ture at l=366 nm became much more clear in the aggrega-
tion state, which indicates the increase of effective conjuga-
tion length and the conformational order in the assemblies.
Similar phenomena were also observed for other p-conju-
gated systems such as polythiophene and oligo(p-phenylene
vinylene) in a THF/water system, indicating that changing
the solvent polarity can affect the delicate conformation of
the chromophores.[9] Although both of these compounds
show decreases in absorption intensity, the decrease in
OPE1 was relatively less significant than that of OPE2. Fur-
thermore, an 8 nm redshift of the maximum was observed in
the case of OPE2, which indicates that a J-type packing of
OPE moieties exists in the formed assemblies. However, the
wavelength shift of OPE1 was negligible, suggesting that the
packing of the chromophores in OPE1 was less tight than
OPE2. A striking consequence of the different arrange-
ments of chromophores in these two compounds is the dis-
parity in the circular dichroism (CD) spectra (Figure 1c and
d). The CD silence of OPE1 reveals that it is not able to
form helical aggregates in THF/water solution. On the con-
trary, the CD signal of OPE2 shows a positive Cotton effect
in THF/water solution with the q=0 crossing wavelength
near the absorption maximum (340 nm). With the incremen-
tal addition of water, the CD signal becomes remarkably
stronger, indicating that the chromophores of OPE2 self-as-
sembled in a helical sense. The CD spectra in THF with an
increasing percentage of methanol were also measured. Al-
though OPE2 is not soluble in methanol, no apparent
Cotton effect was observed. The hydrophobic interaction in
methanol is less significant than that in water, which may
hinder the extended packing of molecules for the formation
of chiral aggregates.
The remarkable different experimental phenomena and
optical observations were also reflected by the distinctive
morphologies of OPE1 and OPE2 observed by using SEM
and TEM. As demonstrated in Figure 2a, in THF/water so-
lution, OPE1 afforded spherical particles with diameters
ranging from 120 to 250 nm. The particles form opening
holes on their surfaces, which indicated that they had a
hollow interior. The majority of these vesicles tended to
stick together and were approximately 10–30 nm larger in
diameter than the typical micelle size derived from common
surfactants.[10] Moreover, many other vesicles self-assembled
Chem. Eur. J. 2009, 15, 6399 – 6407
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6401

G. Li, Y. Ju et al.
We wondered how the pres-
ence of nonsolvent could affect
the morphological transition of
OPE2 that finally leads to such
a unique microstructure. To ad-
dress this question, the self-as-
sembly of OPE2 in THF (2ꢁ
10À4 m) with increasing addition
of water was monitored by
SEM (Figure 5). At the 5%
volume fraction of water,
bundle-like structures with a di-
ameter of less than 1 mm were
found to be the dominant mor-
phology. These bundles have a
low aspect ratio and possess
several branches at their ends,
which is in sharp contrast to the
nanofibers formed by similar
cholesterol
derivatives.[5,13]
Upon increasing the water con-
tent to 20%, the twisted sense
seems to start to form from the
tips of the branches. In the
meantime, the diameter of the
bundles was increased consider-
Figure 2. SEM and TEM images of supramolecular assemblies of OPE1 (a, b) and OPE2 (c, d) in water/THF
(1:2 v/v).
from small amphiphilic molecules immediately collapsed on
solid surfaces,[11] but these OPE vesicles showed remarkable
stability and retained their shape upon drying, presumably
due to the rigid p-conjugated spacer and the steroid nuclei.
The TEM image in Figure 2b further confirmed these
robust capsule-like aggregates with diameters of about
200 nm, which is in agreement with the DLS experimental
results.[12] It is interesting to note that this linear structure
leads to a spherical morphology.
ably and leads to a ribbon-like structure. Further increasing
the volume of water to 30% finally yields helical ribbons
with an almost regular pitch length of 2 mm, in line with the
strong positive cotton effect observed in the CD spectrum.
On the other hand, OPE2, with dimeric deoxycholate
moieties, shows a very interesting assembly behavior in
water/THF (1:2 v/v): remarkably broad ribbons twisted in a
right-handed helical structure were observed (Figure 2c and
d). The ribbons are approximately 1.5 mm wide and a few
hundred micrometers in length, with an average helical
pitch length of 2 mm. Moreover, the dimensions of these
supramolecular aggregations were large enough to be ob-
served by using normal optical microscopy (Figure 3), which
provides a convenient approach to capture the image of
these twisted ribbons in situ. Polarizing microscopy shows
that periodic color strips appear on the self-assembled twist-
ed ribbons, which suggests the existence of a periodic or-
dered packing of molecular structures. The twisted ribbons
derived from OPE2 exhibited strong blue fluorescence that
can be clearly detected by fluorescent optical microscopy
(Figure 4a). The fluorescent spectra of the aggregate state
were slightly broader and exhibited a redshift (~6 nm) com-
pared with those observed in THF, which is indicative of a
J-type packing of p-conjugated chains in the molecular as-
semblies (Figure 4b).
Figure 3. Photographs (1000ꢁ magnification) of the twisted ribbons in
water/THF 1:2 observed by optical microscopy under a) white light and
b) polarized light. The scale bar is 10 mm.
6402
www.chemeurj.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 6399 – 6407

Unique Self-Assembled Twisted Ribbons
FULL PAPER
Figure 4. a) Photographs (1000ꢁ magnification) of the twisted ribbons
observed under illumination with 330–380 nm UV light. The arrow indi-
cates the helical sense. The scale bar is 10 mm. b) Normalized emission
spectra of OPE2 in THF solution (solid line) and in aggregation state
(dashed line).
The progressive changes of the aggregate morphologies with
the addition of water indicate that a relaxation process may
be involved throughout the entire assembly.[14] In fact, the
relaxation process is evidenced by the fact that when OPE2
was directly dissolved in a fixed ratio of THF/water mixture
at elevated temperature, it did not yield any helical structure
upon cooling (Figure S3 in the Supporting Information). In
our experiment, it seems that the incremental addition of
water to THF under ultrasound irradiation or vigorous stir-
ring is necessary for the formation of these twisted ribbons.
Even though bile acids possess more than ten chiral carbon
atoms, the transfer of chirality from the molecular level to
helical structures on a supramolecular scale that can be
clearly visualized using techniques such as SEM, TEM, and
even optical microscopy makes this system really unusual.
In contrast to OPE1 and OPE2, OPE3 (which possesses
only two hydroxyl groups) yields large microcrystalline rods
under the same conditions (Figure S4 in the Supporting In-
formation). This compound is so insoluble in aqueous media
that the addition of water leads to rapid precipitation.
Figure 5. SEM images of hierarchical superstructures of OPE2 in THF/
water solution with a volume fraction of water of a) 5%, b) 15%, and
c) 30%. The scale bar is 2 mm.
different from conventional surfactants and the mechanism
of their formation is quite complex, often involving compli-
cated “secondary assemblies”.[15] As mentioned above, the
spectroscopy studies (UV/Vis, CD, and fluorescence) dem-
onstrate that OPE-derived steroidal compounds undergo ex-
tensive conformational changes in solvent/nonsolvent (THF/
water) mixtures. It is believed that the cooperation of aro-
matic stacking, hydrogen bonding, and van der Waals inter-
actions contributes significantly to the observed unique
mesoscopic structures, whereas the orientation of the chro-
mophore plays a crucial role in the optical and chiroptical
properties. With reference to the existing literature, possible
molecular packing models for the superstructures formed
from OPE1 and OPE2 are suggested (Scheme 3).
Mechanism of self-assembly: The mechanism for the forma-
tion of the described superstructures is at present not very
clear. The molecular structure of bile acid is considerably
As demonstrated in Scheme 3a, OPE1 possesses a p-con-
jugated spacer (OPE) and two amphiphilic pendant groups
Chem. Eur. J. 2009, 15, 6399 – 6407
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6403

G. Li, Y. Ju et al.
Scheme 3. The proposed molecular packing models for a) OPE1 and b) OPE2.
(cholate). In the process of self-assembly, the hydrophobic
OPE spacer should be arranged inside the membrane of the
vesicles by p–p stacking, whereas the relatively polar chol-
ate groups should be exposed to the solvent. This is similar
to the packing model of p-conjugated, macrocycle-derived
vesicles reported by Tew et al.[16] It is reasonable that the
hydro- and lipophilic sides of cholic acid meet together to
yield a continuous membrane structure. If this is the case,
there would be many hydroxyl groups on the surface of the
membrane, which is consistent with the fact that the vesicles
tend to stick together due to the formation of hydrogen
bonds.
OPE2 displays several characteristics that are common
with many other compounds that yield helical aggregates,
such as chirality, p stacks, and the ability to form intermo-
lecular hydrogen bonds, so we initially speculated that the
superhelix mechanism may be quite general. However, after
we carefully studied the molecular structure of deoxycholic
acid, the self-assembly mechanism is probably a little differ-
ent from its cholesterol analogue. Unlike cholesterol, bile
acid possesses a curved backbone with the thickness of
about 0.6 nm (C3-C18).[17] To make the extended packing of
chromophores possible, the deoxycholate moieties could
favor an interdigital conformation in which the adjacent
molecules point in opposite directions. The exposed deoxy-
cholate units may also interact with deoxycholates from
other strings and form hierarchical secondary assemblies.
Such packing leads to a helical arrangement of the mole-
cules owing to the steric hindrance and the chirality of the
steroid (Scheme 3b). The propensity of OPE2 to form large
ribbon-like assemblies could also be enhanced by the plana-
rization of OPE moieties, which confines the three benzene
groups to one aromatic platform, thereby maximizing the p–
p interaction. This conformation transition was clearly re-
flected on the UV/Vis spectrum by the enhanced absorption
shoulder at approximately l=370 nm.
The proposed mechanisms were supported by the wide-
angle X-ray diffraction (WAXD) pattern of the supramolec-
ular assemblies formed (Figure 6). X-ray diffraction of the
vesicles formed from OPE1 shows peaks at 10.11, 18.94, and
22.56 in the range of 2q=3–308C, which correspond to the
distances of 8.78, 4.67, and 3.96 ꢂ, respectively. Theoretical-
ly, the distance between the p stacking of OPEs is estimated
to be about 0.4 nm[18] and the other d values were very close
to the results obtained by Osada et al. in the investigation of
giant needles formed by bile acid complex.[19] So the ob-
Figure 6. Proposed molecular packing models for a) OPE1 and b) OPE2.
6404
www.chemeurj.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 6399 – 6407

Unique Self-Assembled Twisted Ribbons
FULL PAPER
served d spacings of 8.78 and 4.67 ꢂ are possibly caused by
the ordered packing of the adjacent cholate units, whereas
the d spacing of 0.40 nm is a result of the center-to-center
distance between the OPE spacers. In other words, this self-
assembly aggregation preserved some of the basic packing
patterns of bile acid, combined with the p–p stacking of
OPE moieties. The X-ray pattern of OPE2 was similar to
that of OPE1, and several peaks appeared in the same
range. It is interesting to note that the peaks corresponding
to 1.47 and 0.74 nm (1.47/2) were not observed for OPE1.
This periodicity indicates a lamellar packing structure. Con-
sidering that the space between the amide group (C24
atom) and the end of the steroid nucleus (C3 atom) is about
1.5 nm, based on the assumption that the alkyl spacer takes
a fully extended conformation, the distance of 1.47 nm very
likely corresponds to the distance between the planes
formed by adjacent OPE strips, which gives clear evidence
for the proposed self-assembly pattern of OPE2. The helical
ribbons of OPE2 were solely right-handed, presumably re-
flecting the characteristic asymmetric nature of the deoxy-
cholic acid. In our experiment, we also noticed that the
signal-to-noise ratio of OPE1 in the XRD result was consis-
tently lower than that of OPE2, which suggests that the mol-
ecules packing in the membrane of the vesicle were less or-
ganized than the twisted ribbons.
The distinctive self-assembly behavior of bile acid derived
OPEs also raised the question of why the helical twist deter-
iorated when the number of hydroxyl groups either in-
creased or decreased. To approach this question, it is essen-
tial to analyze the molecular arrangements in the supra-
molecular assemblies and the structures of these OPE deriv-
atives. According to the molecular packing model proposed,
one significant difference between these two supramolecular
assemblies is that more steroid moieties in the vesicles are
exposed to the solvent media than in the twisted ribbons in
the every unit volume. In other words, many deoxycholic
acid units were buried inside the large ribbon rather than in
direct contact with the solvent. On the other hand, OPE1
has more hydroxyl groups on the cholate unit than OPE2,
which makes the pendant groups of OPE1 more hydrophilic
than their OPE2 counterparts. Considering the fact that the
solvent used (THF/water mixture) is a relatively polar
medium, it is reasonable that the pendant groups in OPE2
tend to be less exposed to the solvents than those in OPE1,
which facilitates the reduction of the interfacial energy.
In the case of OPE3, the lack of hydrophilic functional
groups means that the hydrophobic interactions of the large
hydrophobic steroid nuclei become the dominant force,
leading to either crystallization or precipitation. There is
also a stronger tendency for growth in a one-dimensional
fashion and loss of the curvature of the aggregates.
romolecular dimensions (10–100 ꢂ) was obtained.[21] Com-
pared with our result, we do not know whether the special
behavior of deoxycholic acid is a coincidence. The backbone
of deoxycholic acid presumably possesses an “appropriate”
hydrophilic–hydrophobic balance, which may endow deli-
cate influences on hydrogen-bond formation and hydropho-
bic interactions in aqueous solution. However, understand-
ing and predicting the intermolecular forces in these com-
plex systems and how it determines their self-assembly mor-
phology remains a significant challenge.
Conclusions
The current work employs naturally occurring bile acids as
chirality-inducing moieties and offers a new means to con-
trol the packing and orientation of the chromophore. Very
interesting is that subtle structure variations could drastical-
ly change the morphologies of the nanostructures formed.
In this respect, it is possible to prepare many other interest-
ing bile acid based molecular assemblies for this purpose by
varying the type of bile acids and p-conjugated bridging
groups of the scaffold. Based on the study of the self-assem-
bly behavior of these structurally closely related compounds,
this work provides new insights into the formation of chiral
supramolecular assemblies starting from the molecular chir-
ality of steroids. On the other hand, the microstructures
formed from them may find use in hybrid-material synthesis
or as a useful chiral medium for manipulating chemical sep-
arations. Some of these possibilities are being explored in
our laboratory.
Experimental Section
Instruments: 1H NMR spectra were recorded at 300 MHz on JOEL
JNM-ECA 300 spectrometers. Chemical shifts (d) are given in ppm rela-
tive to TMS (d=0.0). High-resolution electrospray ionization mass spec-
trometry (HRMS) measurements were recorded on a Bruker APEX
spectrometer in positive mode. UV/Vis characterization was carried out
on a Perkin–Elmer Lambda35 spectrometer. The fluorescence emission
measurements were carried out using a Hitachi F-4500 fluorescence spec-
trometer. CD results were obtained on a Jasco-720 spectrometer. IR
spectra were recorded on an AVATAR 360 ESP FTS spectrophotometer
with KBr pellets. Elemental analyses were performed on a Carlo–Erba-
1106 instrument. DLS measurements were carried out by using a Mal-
vern Zetasizer 3000HS instrument, which supplies vertically polarized
light with a wavelength of 633 nm. SEM experiments were performed
with a LEO-1530 scanning electron microscope. TEM was performed on
a MODEL H-800 electron microscope. All of the samples were stained
with 1.5% phosphotungstic acid hydrate and filter paper was used to
drain the excess solution. The polarizing behavior of the self-assembled
aggregations and the fluorescence images were observed by using an
Olympus 146 Tokyo incident-light optical microscope (ꢁ1000 magnifica-
tion).
The number, location, and configuration of the hydroxyl
groups play a crucial role in the formation of aggregates
from bile acid derivatives.[20] In particular, two research
groups found that among seven kinds of bile acid salts, only
sodium deoxycholate aggregated in aqueous solution. In this
case, an elongated right-handed helical structure with mac-
X-ray diffraction: The vesicles and helical assemblies formed by OPE1
and OPE2 in solution were placed directly on the glass plate and dried in
air. Both diffraction patterns were recorded on a Rigaku D/max diffrac-
tometer by using a CuKa X-ray irradiation source (40 kV, 30 mA). Data
were measured at room temperature between 3 and 308 in 2q/q scan
mode and the scanning rate was kept at 18minÀ1
.
Chem. Eur. J. 2009, 15, 6399 – 6407
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6405

G. Li, Y. Ju et al.
Chemicals: Cholic acid, deoxycholic acid, lithocholic acid, and 4-iodoani-
line were purchased from Sigma and used as received. Trimethylsilylace-
tylene was purchased from Shanghai Reagent Coporation. [PdACTHUNTRGNE(UNG PPh3)4]
was obtained from Pingyang Chemical Company and washed with cold
ethanol prior to use to remove the oxidized impurity. Other reagents and
solvents were received from Beijing Chemical Company without further
purification unless otherwise stated. 1,4-Diethynylbenzene was prepared
and characterized according to a previously published procedure.[22]
OPE2: OPE2 was synthesized as described for OPE1 by using 5 instead
of 4. Yield: 68%; 1H NMR (300 MHz, [D6]DMSO): d=10.08 (s, 2H;
NH), 7.65 (d, J=7.2 Hz, 4H; Ar-H), 7.55 (s, 4H; Ar), 7.49 (d, J=7.2 Hz,
4H; Ar-H), 4.46 (d, J=4.2 Hz, 2H; 12-OH), 4.20 (d, J=3.9 Hz, 2H; 3-
OH), 3.80 (s, 2H; 12-CH), 3.33–3.48 (m, 2H; 3-CH, overlap with the
water signal in [D6]DMSO), 0.85–2.37 (m, 52H; alkyl-H), 0.91 (d, J=
6.0 Hz, 6H; 21-CH3), 0.85 (s, 6H; 19-CH3), 0.61 ppm (s, 6H; 18-CH3);
13C NMR (300 MHz, [D6]DMSO): d=172.6, 140.6, 132.7, 132.0, 123.0,
119.4, 116.5, 92.2, 88.7, 71.6, 70.5, 48.0, 46.7, 46.5, 36.8, 36.2, 35.6, 34.4,
34.1, 33.5, 31.9, 30.8, 29.2, 27.7, 27.5, 26.7, 24.1, 23.6, 17.7, 13.0 ppm; IR
(KBr): n˜ =3412, 2934, 2862, 2214, 1670, 1591, 1520, 1041, 838 cmÀ1; MS
(ESI): m/z: 1080 [M+Na]+; elemental analysis calcd (%) for C70H92N2O6:
C 79.50, H 8.77, N 2.65; found: C 79.00, H 8.52, N 2.94.
N-Cholyl-4-iodoaniline 4: Cholic acid (818 mg, 2.0 mmol), HOBt
(297 mg, 2.2 mmol), and 4-iodoaniline (438 mg, 2.0 mmol) were dissolved
in CH2Cl2/pyridine (12 mL, 5:1 v/v). After stirring at 08C for 10 min,
DCC (453 mg, 2.2 mmol) was added and the resulting reaction mixture
was kept at ambient temperatures for a further 20 h. A white precipitate
was filtered from the purple solution and washed with acetone. The
crude product was purified by column chromatography on neutral Al2O3
(5% MeOH/CH2Cl2) to obtain 4 (926 mg, 76%). 1H NMR (300 MHz,
[D6]DMSO): d=9.58 (s, 1H; NH), 7.52 (d, J=7.2 Hz, 2H; Ar-H), 7.42
(d, J=7.2 Hz, 2H; Ar-H), 3.90 (s, 1H; 12-CH), 3.74 (s, 1H; 7-CH), 3.39–
3.55 (m, 1H; 3-CH, overlap with the water signal in [D6]DMSO), 1.01–
2.39 (m, 27H; alkyl-H), 1.01 (d, J=6.0 Hz, 3H; 21-CH3), 0.87 (s, 3H; 19-
CH3), 0.61 ppm (s, 3H; 18-CH3); 13C NMR (300 MHz, [D6]DMSO): d=
172.5, 139.0, 137.1, 121.4, 85.7, 72.1, 71.1, 67.3, 46.4, 46.1, 41.4, 35.2, 35.1,
34.7, 34.5, 33.7, 31.3, 30.3, 28.3, 27.3, 26.1, 24.6, 23.0, 22.4, 17.2, 12.4 ppm;
HRMS (ESI): m/z calcd for [C30H44INO4+Na]+: 632.2213; found:
632.2200.
OPE3: OPE3 was synthesized as described for OPE1 by using 6 instead
of 4. Yield: 55%; 1H NMR (300 MHz, [D6]DMSO): d=10.07 (s, 2H;
NH), 7.65 (d, J=7.2 Hz, 4H; Ar-H), 7.54 (s, 4H; Ar-H), 7.48 (d, J=
7.2 Hz, 4H; Ar-H), 4.41 (s, 2H; 3-OH), 3.20–3.50 (m, 2H; 3-CH), 1.02–
1.93 (m, 60H; alkyl-H), 0.94 (d, J=5.6 Hz, 6H; 18-CH3), 0.87 (s, 6H; 19-
CH3), 0.61 ppm (s, 6H; 21-CH3); 13C NMR (300 MHz, [D6]DMSO): d=
172.4, 141.1, 136.0, 132.7, 119.9, 118.0, 92.2, 71.0, 57.1, 56.2, 43.0, 42.3,
41.3, 40.5, 36.3, 35.9, 34.5, 32.8, 31.1, 29.1, 27.7, 26.5, 23.7, 23.1, 21.4, 17.0,
11.9 ppm; IR (KBr): n˜ =3481, 3426, 2930, 2850, 2214, 1672, 1585,
1527 cmÀ1; MS (ESI): m/z: 1048 [M+Na]+; elemental analysis calcd (%)
for C70H92N2O4: C 81.99, H 9.04, N 2.73; found: C 81.56, H 9.49, N 2.52.
N-Deoxycholyl-4-iodoaniline 5: Compound 5 was synthesized as de-
scribed for compound 4 by using deoxycholic acid instead of cholic acid.
1
Yield: 77%; H NMR (300 MHz, [D6]DMSO): d=9.58 (s, 1H; NH), 7.52
Acknowledgements
(d, J=7.2 Hz, 2H; Ar-H), 7.42 (d, J=7.2 Hz, 2H; Ar-H), 3.90 (s, 1H; 12-
CH), 3.74 (s, 1H; 7-CH), 3.39–3.55 (m, 1H; 3-CH, overlap with the water
signal in [D6]DMSO), 1.01–2.39 (m, 27H; alkyl-H), 1.01 (d, J=6.0 Hz,
3H; 21-CH3), 0.87 (s, 3H; 19-CH3), 0.61 ppm (s, 3H; 18-CH3); 13C NMR
(300 MHz, [D6]DMSO): d=172.4, 139.8, 137.4, 121.7, 86.7, 71.5, 70.5,
55.4, 48.0, 46.7, 46.5, 42.1, 36.8, 36.2, 35.7, 35.6, 34.3, 34.0, 33.5, 31.8, 30.8,
30.0, 27.7, 27.5, 26.7, 24.1, 23.6, 22.4, 17.6, 13.0 ppm; HRMS (ESI): m/z
calcd for [C30H44INO3+H]+: 594.2444; found: 594.2439.
We gratefully acknowledge the financial support from the National Sci-
ence Foundation of China (20533050, 20772071, and 50673048), 973 Pro-
gram (2006CB806200) and the transregional project (TRR6). We would
also like to thank Prof. Chun Li for help during the circular dichroism ex-
periment.
N-Lithocholyl-4-iodoaniline 6: Compound 6 was synthesized as described
for compound 4 by using lithocholic acid instead of cholic acid. Yield:
80%; H NMR (300 MHz, [D6]DMSO): d=9.95 (s, 1H; NH), 7.60 (d, J=
[1] a) M. Grell, D. D. C. Bradley, Adv. Mater. 1999, 11, 895–905;
b) J. A. A. W. Elemans, R. van Hameren, R. J. M. Nolte, A. E.
Rowan, Adv. Mater. 2006, 18, 1251–1266; c) U. H. F. Bunz, Chem.
Rev. 2000, 100, 1605–1644.
[2] a) M. J. Plater, J. P. Sinclair, S. Aiken, T. Gelbrich, M. B. Hursthouse,
Tetrahedron 2004, 60, 6385–6394; b) B. Francesco, C. Donato, D. B.
Lorenzo, M. F. Gianluca, H. O. Omar, N. Francesco, P. Gennaro,
Macromolecules 2006, 39, 5206–5212.
1
7.2 Hz, 2H; Ar-H), 7.41 (d, J=7.2 Hz, 2H; Ar-H), 4.44 (brs, 1H; OH),
3.39–3.55 (m, 1H; 3-CH, overlap with the signal of water in [D6]DMSO),
1.05–2.35 (m, 28H; alkyl-H), 1.01 (d, J=6.0 Hz, 3H; 21-CH3), 0.87 (s,
3H; 19-CH3), 0.61 ppm (s, 3H; 18-CH3); 13C NMR (300 MHz,
[D6]DMSO): d=172.5, 140.0, 137.9, 121.7, 85.7, 71.9, 71.1, 55.00, 54.96,
41.8, 41.3, 39.5, 39.3, 35.0, 34.6, 33.5, 30.4, 29.5, 27.3, 26.2, 25.4, 23.3, 22.4,
19.7, 17.5, 12.0 ppm; HRMS (ESI): m/z calcd for [C30H44INO2+H]+:
578.2495; found: 578.2489.
[3] Y. Li, J. R. Dias, Chem. Rev. 1997, 97, 283–304.
[4] a) J. Wu, T. Yi, T. Shu, M. Yu, Z. Zhou, M. Xu, Y. Zhou, H. Zhang,
J. Han, F. Li, C. Huang, Angew. Chem. 2008, 120, 1079–1083;
Angew. Chem. Int. Ed. 2008, 47, 1063–1067; b) X. Wang, Y. Lu, Y.
Duan, L. Meng, C. Li, Adv. Mater. 2008, 20, 462–465.
[5] a) S. Kawano, N. Fujita, S. Shinkai, Chem. Eur. J. 2005, 11, 4735–
4742; b) P. Xue, R. Lu, D. Li, M. Jin, C. Bao, Y. Zhao, Z. Wang,
Chem. Mater. 2004, 16, 3702–3707.
[6] M. S. Neralagatta, U. Maitra, Chem. Soc. Rev. 2005, 34, 821–836.
[7] Y. Li, G. Li, X. Wang, C. Lin, Y. Zhang, Y. Ju, Chem. Eur. J. 2008,
14, 10331–10339.
[8] M. Shibakami, M. Tamura, A. Sekiya, J. Am. Chem. Soc. 1995, 117,
4499–4505.
[9] a) F. Brustolin, F. Goldoni, E. W. Meijer, N. A. J. M. Sommerdijk,
Macromolecules 2002, 35, 1054–1059; b) R. Iwaura, F. J. M.
Hoeben, M. Masuda, A. P. H. J. Schenning, E. W. Meijer, T. Shimizu,
J. Am. Chem. Soc. 2006, 128, 13298–13304.
OPE1: [PdACHTUNGTRENNUNG(Ph3P)4] (46.0 mg, 0.04 mmol) and CuI (7.6 mg, 0.04 mmol)
were added to a stirred solution of 4 (632.2 mg, 1 mmol) and 1,4-diethy-
nylbenzene (64.3 mg, 0.51 mmol) in DMF (10 mL) under nitrogen. The
reaction mixture was stirred under nitrogen at 408C for 20 h, then fil-
tered. The filtrate was evaporated under vacuum and further purified by
chromatography on neutral Al2O3 (CHCl3/CH3OH 15:1). The crude
product was dissolved in a small amount of THF and added dropwise
into acetone. OPE1 was obtained as a light yellow solid by filtration and
was dried under vacuum for 24 h (337.6 mg, 62%). 1H NMR (300 MHz,
[D6]DMSO): d=10.08 (s, 2H; NH), 7.65 (d, J=7.2 Hz, 4H; Ar-H), 7.55
(s, 4H; Ar-H), 7.49 (d, J=7.2 Hz, 4H; Ar-H), 4.32 (d, J=4.2 Hz, 2H; 12-
OH), 4.12 (d, J=3.9 Hz, 2H; 7-OH), 4.05 (d, J=3.8 Hz, 2H; 3-OH), 3.80
(s, 2H; 12-CH), 3.62 (s, 2H; 7-CH), 3.19–3.26 (m, 2H; 3-CH), 1.00–2.38
(m, 48H; alkyl-H), 0.98 (d, J=5.6 Hz, 6H; 18-CH3), 0.81 (s, 6H; 19-
CH3), 0.60 ppm (s, 6H; 21-CH3); 13C NMR (300 MHz, [D6]DMSO): d=
172.6, 140.6, 132.7, 132.0, 120.0, 119.0, 118.7, 92.2, 88.8, 71.5, 71.0, 66.8,
46.6, 46.3, 41.9, 36.3, 35.8, 35.7, 35.5, 34.1, 32.1, 31.9, 29.1, 27.8, 26.8, 23.4,
23.2, 17.0, 12.9 ppm; IR (KBr): n˜ =3428, 2923, 2862, 2215, 1669, 1593,
1520, 1046 cmÀ1; MS (ESI): m/z: 1112 [M+Na]+; elemental analysis calcd
(%) for C70H92N2O8: C 77.17, H 8.51, N 2.57; found: C 76.93, H 8.90, N
2.41.
[10] S. Verma, T. Hauck, M. E. El-Khouly, P. A. Padmawar, T. Canteen-
wala, K. Pritzker, O. Ito, L. Y. Chiang, Langmuir 2005, 21, 3267–
3272.
[11] J. H. Fuhrhop, H. H. David, J. Mathieu, U. Liman, H. J. Winter, E.
Boekema, J. Am. Chem. Soc. 1986, 108, 1785–1791.
[12] DLS measurements may slightly overestimate the size of the parti-
cles due to agglomeration in aqueous solution.
6406
www.chemeurj.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 6399 – 6407

Unique Self-Assembled Twisted Ribbons
FULL PAPER
[13] A. Ajayaghosh, C. Vijayakumar, R. Varghese, S. J. George, Angew.
Chem. 2006, 118, 470–474; Angew. Chem. Int. Ed. 2006, 45, 456–
460.
[14] L. Li, H. Jiang, B. W. Messmore, S. R. Bull, S. I. Stupp, Angew.
Chem. 2007, 119, 5977–5980; Angew. Chem. Int. Ed. 2007, 46, 5873–
5876.
[15] a) Y. Zhao, Curr. Top. Mol. Simul. Curr. Opin. Colloid. Interface Sci.
2007, 12, 92–97; b) V. H. S. Tellini, A. Jover, F. Meijide, J. V. Tato,
L. Galantini, N. V. Pavel, Adv. Mater. 2007, 19, 1752–1756.
[16] S. H. Seo, J. Y. Chang, G. N. Tew, Angew. Chem. 2006, 118, 7688–
7692; Angew. Chem. Int. Ed. 2006, 45, 7526–7530.
[18] Y. Wang, B. Erdogan, J. N. Wilson, U. H. F. Bunz, Chem. Commun.
2003, 1624–1625.
[19] T. Kaneko, A. Ichikawa, J. P. Gong, Y. Osada, Macromol. Rapid
Commun. 2003, 24, 789–792.
[20] a) Nonappaa, U. Maitra, Org. Biomol. Chem. 2008, 6, 657–669;
b) L. B. Partay, M. Sega, P. Jedlovszky, Langmuir 2008, 24, 10729–
10736.
[21] a) A. Rich, D. M. Blow, Nature 1958, 182, 423–426; b) N. Ramana-
than, A. L. Currie, J. R. Colvin, Nature 1961, 190, 779–781.
[22] M. J. Plater, J. P. Sinclair, S. Aiken, T. Gelbrich, M. B. Hursthouse,
Tetrahedron 2004, 60, 6385–6394.
[17] M. Alvarez Alcalde, A. Jover, F. Meijide, L. Galantini, N. V. Pavel,
A. Antelo, J. Tato, Langmuir 2008, 24, 6060–6066.
Received: February 22, 2009
Published online: May 26, 2009
Chem. Eur. J. 2009, 15, 6399 – 6407
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6407