Supramolecular helical columns from the self-assembly of chiral rods

Article abstract of DOI:10.1002/chem.200701080

Chiral-bridged rod molecules (CBRs) that consisted of bis(penta-pphenylene) conjugated to an opened or closed chiral bridging group as a rigid segment and oligoether dendrons as flexible segments were synthesized and characterized. In the bulk state, both molecules self-assemble into a hexagonal columnar structure, as confirmed by X-ray scatterings and transmission electron microscopy (TEM) observations. Interestingly, these structures display opposite Cotton effects in the chromophore of the aromatic unit in spite of the same chirality (R,R) of the chiral bridging groups. The molecules were observed to self-assemble into cylindrical micellar aggregates in aqueous solution, as confirmed by light scattering and TEM investigations, and exhibit intense signals in the circular dichroism (CD) spectra, which are indicative of one-handed helical conformations. The CD spectra of each molecule showed opposite signals to each other, which were similar to those in the bulk. Notably, when the opened CBR was added to a solution of closed CBRs up to a certain concentration, the CD signal of the closed CBR was amplified. This implies that both molecules co-assemble into a one-handed helical structure because the opened chiral bridge is conformationally flexible, which is inverted to co-assemble with the closed CBR. These results demonstrate that small structural modifications of the chiral moiety can transfer the chiral information to a supramolecular assembly in the opposite way.

Full text of DOI:10.1002/chem.200701080

FULL PAPER
DOI: 10.1002/chem.200701080
Supramolecular Helical Columns from the Self-Assembly of Chiral Rods
Ja-Hyoung Ryu, Lijun Tang, Eunji Lee, Ho-Joong Kim, and Myongsoo Lee*[a]
Abstract: Chiral-bridged rod molecules
(CBRs) that consisted of bis(penta-p-
phenylene) conjugated to an opened or
closed chiral bridging group as a rigid
segment and oligoether dendrons as
flexible segments were synthesized and
characterized. In the bulk state, both
molecules self-assemble into a hexago-
nal columnar structure, as confirmed
by X-ray scatterings and transmission
electron microscopy (TEM) observa-
tions. Interestingly, these structures dis-
play opposite Cotton effects in the
chromophore of the aromatic unit in
spite of the same chirality (R,R) of the
chiral bridging groups. The molecules
were observed to self-assemble into cy-
lindrical micellar aggregates in aqueous
solution, as confirmed by light scatter-
ing and TEM investigations, and exhib-
it intense signals in the circular dichro-
ism (CD) spectra, which are indicative
of one-handed helical conformations.
The CD spectra of each molecule
showed opposite signals to each other,
which were similar to those in the bulk.
Notably, when the opened CBR was
added to a solution of closed CBRs up
to a certain concentration, the CD
signal of the closed CBR was ampli-
fied. This implies that both molecules
co-assemble into a one-handed helical
structure because the opened chiral
bridge is conformationally flexible,
which is inverted to co-assemble with
the closed CBR. These results demon-
strate that small structural modifica-
tions of the chiral moiety can transfer
the chiral information to a supramolec-
ular assembly in the opposite way.
Keywords: chirality · helical col-
umns · helical inversion · self-as-
sembly · supramolecular chemistry
Introduction
supramolecular chirality by a variety of strategies, which in-
clude hydrogen bonding,[3,4] p–p stacking,[5–7] solvophobic ef-
fects,[8] and metal–ligand interactions.[9] Moreover, the con-
trol of helical sense induced by transfer of chiral informa-
tion from the molecular to the supramolecular level, molec-
ular recognition, and external stimuli has received increasing
attention in biomimetic and synthetic supramolecular sys-
tems.[10–12] For example, the incorporation of chiral moieties
in self-assembling organic molecules can induce chiral
supramolecular assemblies and the configuration of the
chiral moiety can decide the handedness of supramolecular
helices.[13] Stupp et al. reported the mirror image of supra-
molecular helices.[14] They synthesized enantiomers of den-
dron–rod–coils attached to a chiral chain and showed that
each molecule has the opposite Cotton effects depending on
its chiral configuration, that is, the R or S form. Dendron–
rod–coil nanostructures were imaged by atomic force micro-
scopy, which showed one-dimensional helical nanostructures
that were mirror images of each other. Meijer and co-work-
ers also reported a helical structure obtained by the self-as-
sembly of well-designed molecules.[1c,15] They have studied a
variety of p-conjugated systems in which the individual p-
conjugated molecules that contain a side-chain stereocenter
form a one-handed helical structure that is dependent on
the solvent polarity or temperature. Recently, we have
Controlled self-assembly of elaborately designed molecules
is a challenging topic for interdisciplinary research in the
fields of chemistry, biology, and materials science because it
provides the spontaneous generation of a well-defined, dis-
crete supramolecular architecture from molecular compo-
nents under thermodynamic equilibrium.[1] Precise control
of molecular arrangements at the supramolecular level is es-
sential to get well-defined nanoscopic architectures with spe-
cific shape and novel functionalities.[2] In particular, there is
growing interest in the design of synthetic molecules that
are able to self-assemble into compact helical aggregates,
which are analogous to the DNA double helix and the colla-
gen triple helix. Artificial helical architectures in synthetic
self-assembling systems have been achieved with amplified
[a] Dr. J.-H. Ryu, Dr. L. Tang, E. Lee, H.-J. Kim, Prof. M. Lee
Center for Supramolecular Nano-Assembly
and Department of Chemistry
Yonsei University, Shinchon 134
Seoul 120-749 (Republic of Korea)
Fax : (+82)2-393-6096
E-mail: mslee@yonsei.ac.kr
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
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871

shown that incorporation of a conjugated rod into an amphi-
philic dumbbell-shaped molecular architecture gives rise to
the formation of a helical nanostructure.[16] We have also
shown the reversible transformation of the self-assembled
structures of a dumbbell-shaped molecule between helical
strands and nanocages triggered by the addition of aromatic
guest molecules.[17]
or closed forms. Note that the opened chiral-bridged rod
ꢀ
molecule (O-CBR) is able to freely rotate along the C* C*
bond in the chiral bridge. However, bond rotation in the
closed chiral-bridged rod molecule (C-CBR) is hindered as
a result of ring closure.
Columnar assemblies formed from self-assembly of the or-
ganic molecules that contain rigid aromatic rods have been
explored widely in liquid-crystalline and dilute solution
states. Introduction of chirality into the side chains of the
rod building blocks induces helical stacked arrays and mo-
lecular chirality is transferred to the central aromatic core,
and is subsequently amplified through the formation of
supramolecular helical columns.[13,15] However, incorporation
of chiral segments into the center of rod building blocks,
which have a significant influence on the supramolecular
chirality owing to closer proximity to self-assembling units,
remains challenging.[15e] To this end, we synthesized chiral-
bridged rod molecules (CBRs) that contained the chiral
groups in the center of rod building blocks and investigated
their self-assembling behavior in both bulk and solution
states by a combination of polarized optical microscopy, dif-
ferential scanning calorimetry (DSC), circular dichroism
(CD), transmission electron microscopy (TEM), dynamic
light scattering (DLS), and powder X-ray diffraction meas-
urements. As both molecules are based on an identical
chiral configuration (R,R), the supramolecular chirality can
mainly be attributed to the molecular architecture of the
chiral groups in the center of the molecules, that is, opened
Results and Discussion
Synthesis: The synthesis of CBRs that consist of bis(penta-
p-phenylene) conjugated to a chiral moiety as the rigid seg-
ment and oligoether dendrons as the flexible segment is out-
lined in Scheme 1 and begins with the preparation of aro-
matic scaffolds and an oligoether dendron according to pre-
viously described procedures.[18,19] Compound (R,R)-4, which
is based on an opened chiral group, and (R,R)-5, which is
Scheme 1. Synthesis of the CBRs.
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Supramolecular Helices
FULL PAPER
based on a closed chiral group, were obtained by Sharpless
asymmetric dihydroxylation of trans-4,4’-dibromostilbene by
using AD-mix b followed by subsequent methylation of the
hydroxyl group (4) or formation of a ketal from the hydrox-
yl group (5), and finally by replacing the bromide atoms by
boronic acid using butyl lithium and triisopropylborate.[18]
The basic synthetic methodology to generate such a flexible
dendrimer employed a facile convergent route, as reported
previously.[19] The first step was performed by the etherifica-
tion of 4,4’-bromohydroxy biphenyl with the tosylated den-
dron under basic conditions. The elongated rod building
block, 3a, was obtained by a Suzuki coupling reaction of 2
with 4-trimethylsilylbiphenyl-4’-boronic acid. For the next
Suzuki coupling reaction, the trimethylsilyl group of 3a was
substituted with aryl iodide, which is the most active reagent
in Suzuki type aromatic couplings. The final CBRs were syn-
thesized by following the same sequence of reactions, that is,
by the Suzuki coupling reaction of 3b with chiral boronic
acids 4 or 5.
The resulting CBRs (1a (O-CBR) and 1b (C-CBR)) were
1
characterized by H and 13C NMR spectroscopies, polarime-
try, and MALDI-TOF mass spectrometry. All of the analyti-
cal data were in full agreement with the structures present-
1
ed. As confirmed by H NMR spectroscopy, the ratio of the
aromatic protons of the rod building block to the alkyl pro-
tons was consistent with the calculated ratio, and the specific
rotation values of both molecules obtained by polarimetry
appeared to have positive signs with [a]2D0 values of +101.4
and +180.4o for O-CBR and C-CBR, respectively. As
shown in Figure 1, the MALDI-TOF mass spectra of the
molecules exhibit two signals that can be assigned as the Na
and K adducts of the molecular ions. The mass that corre-
sponds to a representative peak in the spectrum is matched
with the calculated molecular weight of each molecule.
Figure 1. MALDI-TOF mass spectra of a) 1a (O-CBR) and b) 1b (C-
CBR).
Conformation of chiral bridging moieties: The opened chiral
bridging moiety in O-CBR is conformationally flexible and
the diol can be assumed to have two limiting conformations,
as illustrated in Figure 2a, and the determination of the
most stable conformation in a single molecular state is a del-
icate procedure. In the aggregated state, however, the rigid
aromatic building blocks of the O-CBR molecule might
favor packing in a parallel fashion to maximize p-stacking
interactions.[20] Therefore, the linear conformation (structure
B) would be more favorable in the self-assembled state.
Similarly, the closed chiral bridging moiety in C-CBR can
also have two conformations (Figure 2b). As previously re-
ported,[21] the most stable conformation of the dioxolane
ring is structure A’, in which the two aromatic rings have a
Figure 2. Possible conformations of chiral bridged moieties in CBRs.
ꢀ
quasi-gauche relationship when viewed along the C* C*
Bulk-state structure: The self-assembling behavior of CBRs
in the bulk state was investigated by means of DSC, thermal
polarized optical microscopy, TEM, and X-ray scattering
methods. Figure 3a presents the DSC heating traces of the
CBRs. Both molecules showed an ordered bulk-state struc-
ture. O-CBR, which is based on the extended rod, showed a
liquid crystalline–isotropic transition at 115.28C on the heat-
ing scan. C-CBR, which is based on the bent-shaped aromat-
bond. This stability is because the aromatic groups are locat-
ed closely to the 2,2-methyl groups of the dioxolane ring,
which gives rise to steric hindrance in structure B’. Conse-
quently, O-CBR adopts a linear-shape, whereas C-CBR
adopts a bent-shape.
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M. Lee et al.
cooling from the isotropic liquid, the formation of unique
domains that correspond to a hexagonal columnar texture
could be easily observed under a polarized optical micro-
scope (Figure 3b).
To corroborate the bulk-state structure of CBRs, small-
and wide-angle X-ray scattering experiments were per-
formed. The small-angle X-ray diffraction (SAXS) pattern
of O-CBR displayed three sharp reflections with the ratio of
p
1: 3:2 in the low-angle region that can be assigned as a 2D
hexagonal columnar structure with a lattice constant of
6.6 nm (Figure 4a). Only a diffuse halo could be observed in
the wide-angle X-ray diffraction (WAXS) pattern, which is
indicative of liquid-crystalline order with a mean intermo-
lecular distance of 4.5 Š. For further analysis, the sample
was cryomicrotomed to a thickness of approximately 50 to
70 nm and stained with RuO4 vapor and observed by TEM.
Figure 4c shows a hexagonal array of dark spots in a matrix
of light oligoether segments. The interdomain distance ap-
peared to be approximately 6.6 nm from the TEM images.
Considering the lattice constant and extended molecular
length (8.7 nm by Corey–Pauling–Koltun (CPK) molecular
model), this dimension implies that the rodlike rigid seg-
ments arrange axially with their preferred direction within
a cross-sectional slice of the column, in which flexible
dendrons are located in the periphery of the slice. Calcu-
lated from the density (ꢁ1.17 gcmꢀ3) and the lattice
volume, it was estimated that approximately five molecules
were necessary to fill a slice of each column that was 4.5 Š
thick.[22]
Figure 3. a) DSC traces (108Cminꢀ1) recorded during the first heating
scan of A) O-CBR and B) C-CBR. The inset is a magnification of the
box indicated on the graph. b) Representative optical polarized micro-
graph (100”) of the texture exhibited by O-CBR at the transition from
the isotropic state at 1008C.
ic unit, however, showed a sig-
nificant depression of this tran-
sition (84.48C). This depression
is attributed to the difficulty in
packing caused by bent-shaped
rigid segments in the liquid-
crystalline state. From DSC
data, the enthalpy change asso-
ciated with the liquid crystal-
line–isotropic transition can be
related to the relative degree of
liquid-crystalline packing. The
heat of fusion (3.05 kJmolꢀ1) of
O-CBR was much larger than
that of C-CBR (0.12 kJmolꢀ1),
which means that the liquid-
crystalline ordering of O-CBR
is much larger than that of C-
CBR. Between cross polarizers,
these waxy solids showed
strong birefringence. Although
no discernible texture could be
identified from C-CBR, O-
Figure 4. SAXS patterns of a) O-CBR and b) C-CBR at 258C, and TEM images of c) O-CBR and d) C-CBR.
TEM images of ultramicrotomed films of CBR stained with RuO4 revealing columnar array of alternating
light-colored dendritic layer and dark aromatic layers. The inset image of c) at perpendicular beam incidence
CBR showed a characteristic
texture associated with supra-
molecular ordering. On slow shows a hexagonally ordered array of aromatic core of O-CBR.
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Supramolecular Helices
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The SAXS pattern of C-CBR also showed two reflections
in the ratio of 1: 3 in the low-angle region (Figure 4b). This
other with mutual rotation in the same direction to avoid
the steric hindrance between the bulky dendron units, which
leads to a one-handed helical column. This formation pro-
cess is similar to that of other helical structures of self-as-
sembling molecules that are based on a conjugated rod
block.[15] The specific handedness of this helical column
arises from steric constraints imposed by the chiral centers
in the rigid segment. As described above, the aromatic seg-
ment adopts the linear rod-shaped conformation to maxi-
mize p–p interactions between aromatic segments in aggre-
gated states. To pack efficiently, the methoxy group in the
chiral segments moves toward the inside, and subsequently,
bis(penta-p-phenylene) groups twist to reduce steric hin-
drance between the phenyl and methoxy groups, which re-
sults in the propellerlike conformation of the bis(penta-p-
phenylene) groups (Figure 6b). As a result, this propellerlike
structure stacks into a right-handed (P) supramolecular heli-
cal column.
p
means that C-CBR also self-assembles into a 2D hexagonal
columnar structure with a lattice constant of 6.0 nm. As
shown in Figure 4d, the TEM image of C-CBR shows the
columnar array. Calculated from the experimental values of
the intercolumnar distance (6.0 nm) and the density
(ꢁ1.15 gcmꢀ3), the average number of C-CBR molecules in
a single slice of the cylinders with a thickness of 4.5 Š (as
discussed above) was found to be approximately four.
CD in the bulk state: One can easily imagine that the col-
umns within the mesophase adopt a helical organization be-
cause both CBRs are enantiomerically pure. CD experi-
ments were subsequently carried out for the purpose of
studying the optical activity in the liquid-crystalline states.
The sample was prepared by sandwiching the material be-
tween two quartz plates. In this system, the values of linear
dichroism (LD), which can be related to the CD values in
an oriented system, are negligibly smaller than those arising
from CD.[23] The CD spectra did not change appreciably
when the sample was rotated in the plane perpendicular to
the light beam to eliminate any effects of linear birefrin-
gence and linear dichroism by averaging CD spectra (mea-
sured at the same positions as samples rotated through suc-
cessive 30o increments).[24] Remarkably, the CD signals of
the mesophase in both molecules (O-CBR and C-CBR)
showed the opposite sign (Figure 5), although the same mo-
lecular chirality (R,R). O-CBR gives a positive exciton CD
spectrum that consists of the positive Cotton effect at high
wavelengths and the negative Cotton effect at low wave-
lengths with the CD signal passing through zero near the ab-
sorption maximum of the chromophore. This positive cou-
pling suggests the presence of a right-handed helical ar-
rangement of the transition dipoles of O-CBR molecules.[25]
On the other hand, C-CBR showed a negative exciton CD
spectrum that consists of the negative first Cotton effect and
the positive second Cotton
In contrast, the C-CBR molecule based on a bent-shaped
aromatic segment is able to form the columnar slice that is
*
:
Figure 5. Averaging CD spectra of thin films of CBRs. &: C-CBR and
O-CBR.
effect, which is indicative of a
left-handed helical arrange-
ment.
On the basis of the results
described above, the schematic
representation can be construct-
ed as shown in Figure 6. In the
case of O-CBR, the inner core
of the cylinder is composed of a
discrete aromatic core with a
rectangular
cross
section,
whereas the outer flexible den-
drons splay to fill the intercylin-
der matrix in a similar way to
well-known rod–coil molecules
previously reported.[20b] When
the individual slices stack along
the cylinder axis, the rectangu-
Figure 6. Schematic representation of the hexagonal columnar structure of O-CBR and C-CBR in the bulk
state. The opened chiral bridging group packs in a right-handed (P) manner and the closed chiral bridging
lar slice stacks on top of each group packs in a left-handed (M) manner.
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M. Lee et al.
arranged in a coplanar geometry, in which four molecules
comprise a single stratum of the cylinder (Figure 6). This
packing structure is similar to that formed from polycatenar
bent-shaped molecules.[26] It was proposed that a few mole-
cules self-assemble to form an overall disk-shaped object
similar to discotic dendrimers.[27] Figure 6 also shows a plau-
sible way in which the closed chiral bridging moieties stack.
This group adopts a quasi-bisected conformation with the
chloroform (0.005 wt%) exhibits a strong emission maxi-
mum at l=400 and 410 nm for O-CBR and C-CBR, respec-
tively. However, the emission maximum in aqueous solution
is redshifted with respect to that observed in chloroform,
and the fluorescence is significantly quenched, which is indi-
cative of aggregation of the conjugated rod segments.[15]
DLS experiments were performed with CBRs in aqueous
solution to further investigate the aggregation behavior. The
CONTIN analysis of the autocorrelation function of CBR
showed a broad peak that corresponds to a hydrodynamic
radius (RH) that ranges from several nanometers to hun-
dreds of nanometers (Figure 7c). The formation of cylindri-
cal micelles was confirmed by the Kratky plot that shows a
linear angular dependence over the scattering light intensity
of the aggregates (Figure 7d).[28]
ꢀ
plane of the phenyl group rotated towards the C* O bond
of the oxalane ring. In this type of conformation, the bis(-
penta-p-phenylene) groups in C-CBR are distorted to result
in the left-handed (M) supramolecular helix.[21] These results
demonstrate that the helical handedness can be manipulated
by tailoring the chiral bridging moiety in CBRs.
Aggregation behavior in aqueous solution: CBRs, when dis-
solved in a selective solvent for one of the blocks, can self-
assemble into an aggregate structure because of its amphi-
philic characteristics. The aggregation behavior of the mole-
cules was subsequently studied in aqueous solution by using
UV-visible and fluorescence spectroscopies. The absorption
spectrum of the CBR molecule in aqueous solution
(0.005 wt%) exhibits a broad transition with a maximum at
l=329 and 325 nm for O-CBR and C-CBR, respectively,
which results from the conjugated rod block (Figure 7). The
fluorescence spectrum of a solution of the CBR molecule in
The dilute aqueous solutions of CBR displayed a signifi-
cant Cotton effect in the chromophore of the aromatic unit
(Figure 8), which indicates the presence of elongated helical
fibrillar aggregates in solution. This is in sharp contrast to
the molecularly dissolved state (a solution in chloroform),
which does not form a micellar structure and has a very
weak Cotton effect. The CD signals of both molecules in so-
lution in chloroform showed the same signals, which is indi-
cative of the same molecular chirality. However, the signifi-
cant Cotton effect observed in aqueous solution is consid-
ered to be the result of the presence of elongated cylindrical
Figure 7. Absorption (left) and emission (right) spectra of the aqueous solution (0.005 wt%, solid line) and solution in chloroform (dashed line) of a) O-
CBR and b) C-CBR. c) The size distribution graph obtained by DLS at scattering angle of 90o (from CONTIN analysis of the autocorrelation function).
d) Kratky plot and linear fit confirmed the formation of a cylindrical micelle in aqueous solution (c=0.05 gLꢀ1).
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Supramolecular Helices
FULL PAPER
Figure 8. CD spectra of CBRs in aqueous solution and in chloroform
(20 mm).
aggregates with a chiral supramolecular structure of O-CBR
or C-CBR, which is similar to the bulk states. This result in-
dicates that the molecular chirality is transferred to the aro-
matic segments and is subsequently amplified through the
formation of a supramolecular helix. As shown in Figure 8,
the CD signals of both molecules in aqueous solution are
identical to those in the bulk states. It should be noted that
both molecules, which have the same chirality (R,R),
showed opposite CD signals. This strongly suggests that the
cylindrical structures in aqueous solution are self-assembled
in an identical way to the bulk-state p–p stacking of the aro-
matic segments, as proposed in Figure 6.
The evidence for the formation of the helical aggregates
was also provided by TEM experiments (Figure 9). The mi-
crographs of O-CBR that were negatively stained with an
aqueous solution of uranyl acetate (2 wt%) showed left-
handed twisted bundles of cylindrical aggregates, which
were nanometers in diameter and micrometers in length. As
discussed earlier, the CD signal demonstrated that the mole-
cules within an elementary fiber
Figure 9. a) TEM image of O-CBR with negative staining (0.005 wt%),
and b) magnification in THF/water (1:10).
negative (lmax =344 nm) and positive (lmax =306 nm) values,
which is indicative of a left-handed helix, and as expected, a
racemic mixture of O-CBR does not have a CD signal (Fig-
ure 10a). In great contrast to the (R,R) enantiomer, the left-
handed helices further assembled hierarchically to form left-
handed twisted bundles, as shown by TEM images (Fig-
ure 10b). These results indicate that the elementary fibrils
formed initially with a left- or right-handed helix may be
further coiled in the same direction to form a superhelical
structure that is only left-handed (Figure 11). In the case of
are organized into
a right-
handed helix. Therefore, it can
be concluded that the elementa-
ry fibers further self-assemble,
in
form
a
hierarchical fashion, to
supercoiled structure
a
with opposite helicity. It has
been reported that one-handed
helices give rise to a supercoiled
structure with an opposite helic-
ity through a stepwise hierarchi-
cal assembly process.[6,29]
To further investigate this hel-
ical inversion, we synthesized
(S,S)-O-CBR as an enantiomer
of O-CBR. As expected, the
CD spectrum of (S,S)-O-CBR
showed the opposite signal to
O-CBR, which has an exciton-
Figure 10. a) CD spectra of O-CBR, (S,S)-O-CBR, and a racemic mixture in aqueous solution (0.005 wt%).
coupled bisignate signal with b) TEM image of (S,S)-O-CBR with negative staining (0.005 wt%) in THF/water (1:10).
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M. Lee et al.
C-CBR, although the aggregated helical structure could not
be identified, most probably owing to the loss of ordering
during the drying process, C-CBR also self-assembled into
helical objects in aqueous solution, similar to the bulk state,
as given by the results from DLS and CD experiments.
The racemic mixtures are known not to have a Cotton
effect,[13,14] which is consistent with our results shown in Fig-
ure 10a. In sharp contrast, when O-CBR (20 mm) was added
to a solution of C-CBR (20 mm), an unpredictable amplifica-
tion of the CD intensity was observed (Figure 12a). To in-
vestigate this phenomenon in more detail, we carried out
CD experiments of the aqueous solutions with the various
proportions of O-CBR and C-CBR at a constant total con-
centration (0.005 wt%). As the O-CBR ratio increases to
50 mol% relative to C-CBR, the resulting CD spectra
showed no change in either the shape or the intensity (Fig-
ure 12b). Above this molar ratio (C-CBR/(C-CBR+O-
CBR)=50%), the CD intensity at 343 nm, which is the
most intensive peak, gradually increases and finally the in-
version of the CD signal was observed. More importantly,
no change in the CD signal up to an O-CBR concentration
of 50 mol% relative to C-CBR indicates that O-CBR
adopts a left-handed helical conformation triggered by co-
assembly with C-CBR. These results imply that by increas-
ing the molar ratio, O-CBR co-assembles with C-CBR up to
a certain ratio, above which the excess O-CBR might be
separated and self-assemble into helical fibers with their
original handedness.
This phenomenon can be rationalized by considering the
conformational flexibility induced by bond rotation in the
chiral bridging moiety. The conformation of the chiral bridg-
ing group of C-CBR has a bent shape in structure A’, as
shown in Figure 2b. This conformation prevents the free ro-
ꢀ
tation of the chiral C* C* bond by ring closure, whereas O-
CBR is able to freely rotate. Therefore, O-CBR can co-as-
semble with C-CBR through a bent-shaped conformation
into left-handed helical fibers, which means that C-CBR
acts as a chiral template for the conformational change of
O-CBR.
Conclusion
CBRs were synthesized and their self-assembling behavior
in both the bulk and solution states was investigated. In the
bulk state, the CBRs based on an O-CBR was observed to
self-assemble into a right-handed columnar structure, where-
as the molecule based on a C-CBR self-assembles into a
left-handed columnar structure. In aqueous solution, both
amphiphiles self-assemble into well-defined nanofibers with
opposite handedness, as confirmed by CD measurements.
Two enantiomers of O-CBR self-assemble into helical fibers
with opposite handedness. Remarkably, both left- and right-
handed helices were shown to further assemble in a hier-
archical manner to form only a left-handed superhelical
structure. In addition, when O-CBR was added to a solution
of C-CBR, the CD signal was
amplified. This implies that
both molecules co-assemble
into
a one-handed helix be-
cause conformationally flexible
O-CBR is able to adopt a bent-
shaped conformation for homo-
chiral interactions with C-CBR.
These results demonstrate that
the handedness of helical fibers
can be manipulated by small
structural variations in the
chiral bridging unit of the chiral
rod, both in the bulk and solu-
tion states.
Experimental Section
Materials: 4-Bromo-4’-hydroxybiphen-
yl (99%), tetrakis(triphenylphospha-
ne)palladium(0) (99%), and toluene-
p-sulfonyl chloride (98%) from Tokyo
Kasei were used as received. Triethy-
lene glycol and chlorotrimethylsilane
(98%) from Aldrich and the other
conventional reagents were also used
as received. Hexane, dichloromethane,
and ethyl acetate were distilled before
use. Dry THF was obtained by
vacuum transfer from sodium and ben-
Figure 11. Schematic representation of one-handed helices and hierarchically assembled superhelices with the
left-handedness of O-CBR enantiomers.
878
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 871 – 881

Supramolecular Helices
FULL PAPER
Synthesis of 2: 4’-Bromo-biphenyl-4-ol (2.15 g, 8.64 mmol), TsOR (2.21 g,
2.16 mmol), and excess K2CO3 were dissolved in anhydrous acetonitrile
(100 mL). The mixture was heated at reflux overnight and then cooled to
room temperature. The solvent was removed by using a rotary evapora-
tor and the resulting mixture was poured into water (50 mL) and extract-
ed with CH2Cl2. The aqueous layer was washed with water, dried over
anhydrous MgSO4, and filtered. Purification of the residue by flash
column chromatography on silica gel by using CH2Cl2 and ethyl acetate/
methanol (8:1 v/v) as the eluent to yield 2 as a colorless liquid (2.00 g,
88.6%). 1H NMR (250 MHz, CDCl3): d=7.54–7.33 (m, 6H; Ar-H), 6.96
(d, J=8.7 Hz, 2H; Ar-H,
CH2Ophenyl), 3.63–3.45 (m, 64H; OCH2), 3.38–3.34 (m, 12H; OCH3),
2.39–2.35 (m, 1H; phenylOCH2CH(CH2O)2), 2.17–2.06 ppm (m, 2H; CH-
AHCTRE(UNG CH2O)2).
m to OCH2), 4.03 (d, J=5.7 Hz, 2H;
AHCTREUNG
Synthesis of 3a and 3b: Compound 3 (2.00 g, 1.82 mmol) and 4,4’-(trime-
thylsilyl)biphenylboronic acid (0.846 g, 2.73 mmol) were dissolved in de-
gassed THF (40 mL). Degassed 2m aqueous Na2CO3 (20 mL) was added
to the solution before tetrakis(triphenylphosphine)palladium(0) (10 mg,
0.009 mmol) was added. The mixture was heated at reflux for 48 h with
vigorous stirring under nitrogen. The solution was then cooled to room
temperature, the layers were separated, and the aqueous layer was then
washed with CH2Cl2 (2”). The combined organic layers were dried over
anhydrous MgSO4 and filtered. The solvent was removed by using a
rotary evaporator and the crude product was purified by column chroma-
tography (silica gel) by using ethyl acetate/methanol (8:1 v/v) as the
eluent to yield 3a as a colorless liquid (1.91 g, 84.3%). Subsequently,
compound 3a (0.776 g, 0.625 mmol) was dissolved in CH2Cl2 (300 mL) at
ꢀ788C and a 1.0m solution of ICl in CH2Cl2 (1.87 mL, 1.87 mmol) was
added dropwise. The reaction mixture was stirred for 1 h under nitrogen
before 1m aqueous Na2S2O5 (15 mL) solution was added and the solution
was stirred for 4 h. The layers were separated, and the aqueous layer was
then washed with CH2Cl2 (2”). The combined organic layers were dried
over anhydrous MgSO4 and filtered. The solvent was removed by using a
rotary evaporator and the crude product was purified by column chroma-
tography (silica gel) by using CH2Cl2/methanol (8:1 v/v) as the eluent to
yield 3b as a colorless liquid (0.750 g, 92.6%).
Figure 12. a) CD spectra of CBRs: i) C-CBR (20 mm); ii) CBR mixture so-
lution of C-CBR (20 mm) and O-CBR (20 mm). b) CD spectra CBRs as
the C-CBR ratio increases at a constant total concentration of 0.005 wt%
in aqueous solution. The inset shows the CD intensity at 343 nm (I) as a
function of the molar ratio (r) of O-CBR/(O-CBR+C-CBR).
3a: 1H NMR (250 MHz, CDCl3): d=7.70–7.55 (m, 14H; Ar-H), 6.99 (d,
J=8.4 Hz, 2H; Ar-H, o to OCH2), 4.05 (d, J=5.5 Hz, 2H; CH2Ophenyl),
3.51–3.76 (m, 64H; OCH2), 3.29–3.27 (m, 12H; OCH3), 2.39–2.35 (m,
zophenone. Visualization of TLC plates was accomplished with UV light
or iodine vapor. Compounds were synthesized according to the proce-
dure described in Scheme 1. 4,4’-(Trimethylsilyl)biphenylboronic acid and
dendritic oligoether (TsOR) were prepared according to the same proce-
dures described previously. Compounds 4 and 5 were prepared according
to literature procedures.
1H; phenylOCH2CH
A
ACHTREUNG(CH2O)2),
0.31 ppm (s, 9H; phenylSiAHCTREUNG
3b: 1H NMR (250 MHz, CDCl3): d=7.77 (d, J=8.3 Hz, 2H; Ar-H, o to
I), 7.72–7.54 (m, 10H; Ar-H), 7.37 (d, J=8.4 Hz, 2H; Ar-H, m to I), 6.99
(d, J=8.4 Hz, 2H; Ar-H,
CH2Ophenyl), 3.51–3.76 (m, 64H; OCH2), 3.29–3.27 (m, 12H; OCH3),
2.39–2.35 (m, 1H; phenylOCH2CH(CH2O)2), 2.17–2.06 ppm (m, 2H; CH-
AHCTRE(UNG CH2O)2).
o to OCH2), 4.05 (d, J=5.5 Hz, 2H;
Techniques: 1H NMR and 13H NMR spectra were recorded as solutions
in CDCl3 by using a Bruker AM 250 spectrometer. The purity of the
products was determined by TLC (Merck, silica gel 60). A Perkin–Elmer
Diamond DSC differential scanning calorimeter was used to determine
the thermal transitions, which were reported as the maxima and minima
of their endothermic or exothermic peaks. In all cases, the heating and
cooling rates were 108Cminꢀ1. X-ray scattering measurements were per-
formed in transmission mode with synchrotron radiation at the 10C1 X-
ray beam line at the Pohang Accelerator Laboratory, Korea. MALDI-
TOF MS was performed by using a Perseptive Biosystems Voyager-DE
STR instrument with a 2,5-dihydroxy benzoic acid matrix. DLS measure-
ments were performed by using an ALV/CGS-3 Compact Goniometer
System instrument. UV/Vis absorption spectra were obtained by using a
Shimadzu 1601 UV spectrometer. The fluorescence spectra were ob-
tained by using a Hitachi F-4500 fluorescence spectrometer. CD spectra
were obtained by using a JASCO J-810 spectropolarimeter. TEM was
performed at 120 kV by using a JEOL-JEM 2010 microscope.
AHCTREUNG
Synthesis of 4: A solution of n-butyllithium (9.5 mL of 1.6m solution in
hexane, 15.1 mmol) was added slowly to a solution of (1R,2R)-1,2-bis(4-
bromophenyl)-1,2-dimethoxyethane (1.5 g, 3.78 mmol) in dry THF
(50 mL) at ꢀ788C under a nitrogen atmosphere. After stirring for 30 min
at ꢀ788C, triisopropyl borate (6.9 mL, 30 mmol) was added, and the re-
action mixture was stirred for 12 h. The reaction was quenched with a 1m
aqueous solution of HCl (50 mL) and extracted with ethyl acetate. The
crude organic extract was treated with a 1m aqueous solution of NaOH
(3”100 mL). The combined aqueous extracts were acidified with concd
HCl and extracted with ethyl acetate. The organic extracts were com-
bined, washed with brine, dried over anhydrous MgSO4, and filtered. Sol-
vent was removed by using a rotary evaporator and the crude product
was purified by column chromatography (silica gel) by using CH2Cl2/
methanol (10:1 v/v) as the eluent to yield 4 as a white solid (0.68 g,
54.9%). 1H NMR (250 MHz, CD3OD): d=7.40 (d, J=7.6 Hz, 4H; o to
B(OH)2), 7.00 (d, J=7.6 Hz, 4H; m to B(OH)2), 4.33 (s, 2H; (CH3O)-
TEM measurements: A drop of a solution of O-CBR (0.005 wt%) in
THF/water (1:10) was placed onto a carbon-coated copper grid and dried
at room temperature before it was subsequently negatively stained with
an aqueous solution of uranyl acetate (2 wt%).
AHCTRENU(G phenyl)CH), 3.30 ppm (s, 6H; CHOCH3).
Synthesis of compound 5: A solution of n-butyllithium (5.0 mL of 1.6m
solution in hexane, 8.0 mmol) was added slowly to a solution of (4R,5R)-
4,5-bis(4-bromophenyl)-2,2-dimethyl-1,3-dioxolane (0.82 g, 1.99 mmol) in
dry THF (50 mL) at ꢀ788C under a nitrogen atmosphere. After stirring
Synthesis: The synthetic procedures used in the preparation of the CBRs
are described in Scheme 1.
Chem. Eur. J. 2008, 14, 871 – 881
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
879

M. Lee et al.
for 30 min at ꢀ788C, triisopropyl borate (5 mL, 20 mmol) was added and
the reaction mixture was stirred for 12 h. The reaction was quenched
with a 1m aqueous solution of HCl (50 mL) and extracted with ethyl ace-
tate. The crude organic extract was treated with a 1m aqueous solution of
NaOH (3”100 mL). The combined aqueous extracts were acidified with
concd HCl and extracted with ethyl acetate. The organic extracts were
combined, washed with brine, dried on anhydrous MgSO4, and filtered.
The solvent was removed by using a rotary evaporator and the crude
product was purified by column chromatography (silica gel) by using
CH2Cl2/methanol (10:1 v/v) as the eluent to yield 5 as a white solid
(0.31 g, 45.1%). 1H NMR (250 MHz, DMSO): d=8.06 (s, 4H; phenyl-
B(OH)2), 7.75 (d, J=7.6 Hz, 4H; o to B(OH)2), 7.18 (d, J=7.6 Hz, 4H;
Acknowledgements
This work was supported by the Creative Research Initiative Program of
the Ministry of Science and Technology, Korea. E.L. thanks the Seoul
Science Fellowship Program and we acknowledge a fellowship of the
BK21 program from the Ministry of Education and Human Resources
Development.
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m to B(OH)2), 4.74 (s, 2H; ((CH3)2CO)
CHOC(CH3)2).
ACHTRE(UNG phenyl)CH), 1.59 ppm (s, 6H;
ACHTREUNG
Synthesis of 1a (O-CBR) and 1b (C-CBR): These compounds were syn-
thesized by using the same procedure. A representative example is de-
scribed for 1a. Compound 3b (0.300 g, 0.231 mmol) and 4 (0.038 g,
0.077 mmol) were dissolved in degassed THF (20 mL). Degassed 2m
aqueous Na2CO3 (10 mL) was added to the solution before tetrakis(tri-
phenylphosphine)palladium(0) (10 mg, 0.009 mmol) was added. The mix-
ture was heated at reflux for 48 h with vigorous stirring under nitrogen.
The solution was cooled to room temperature, the layers were separated,
and the aqueous layer was then washed with CH2Cl2 (2”). The combined
organic layers were dried over anhydrous MgSO4 and filtered. The sol-
vent was removed by using a rotary evaporator and the crude product
was purified by column chromatography (silica gel) by using CH2Cl2/
methanol (8:1 v/v) as the eluent to yield 1a as a white waxy solid
(100 mg, 50.5%).
1a (O-CBR): Yield=50.5%; [a]2D0 =+ 101.4o (c=0.00053 gmLꢀ1
CHCl3); H NMR (250 MHz, CDCl3): d=7.73–7.50 (m, 32H; Ar-H), 7.16
(d, J=8.1 Hz, 2H; Ar-H, m to CHOCH3), 7.00 (d, J=8.6 Hz, 2H; Ar-H,
,
1
o
to OCH2), 4.43 (s, 2H; ArCHOCH3), 4.06 (d, J=5.2 Hz, 4H;
CH2Ophenyl), 3.64–3.46 (m, 128H; OCH2), 3.39–3.36 (m, 24H; OCH3),
2.39–2.35 (m, 2H; phenylOCH2CH(CH2O)2), 2.20-2.17 ppm (m, 4H; CH-
ACHTREUNG
(CH2O)2); 13C NMR (63 MHz, CDCl3): d=158.9, 140.0, 139.8, 139.7,
AHCTREUNG
139.6, 138.9, 137.5, 133.1, 128.6, 128.1, 127.5, 127.4, 127.2, 126.5, 115.0,
88.1, 72.0, 70.7, 70.6, 70.5, 69.8, 69.6, 59.1, 57.4, 40.2, 40.1 ppm; MALDI-
TOF MS: m/z: 2602.48 [M+Na]+, 2618.41 [M+K]+.
1b(C-CBR)
CHCl3); 1H NMR (250 MHz, CDCl3) d=7.74–7.63 (m, 32H; Ar-H), 7.38
(d, J=8.3 Hz, 2H; Ar-H, m to CHOC(CH3)2), 7.00 (d, J=8.6 Hz, 2H;
Ar-H, o to OCH2), 4.88 (s, 2H; ArCHOC(CH3)2), 4.06 (d, J=5.4 Hz,
4H; CH2Ophenyl), 3.65–3.47 (m, 128H; OCH2), 3.39–3.36 (m, 24H;
OCH3), 2.39–2.35 (m, 2H; phenylOCH2CH(CH2O)2), 2.20–2.17 (m, 4H;
CH(CH2O)2) 1.74 ppm (s, 6H; ArCHOC
(CH3)2); 13C NMR (63 MHz,
:
Yield=42.1%; [a]2D0 =+ 180.4o (c=0.00058 gmLꢀ1
,
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AHCTREUNG
AHCTREUNG
AHCTREUNG
A
ACHTREUNG
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7014; b) K. Kishikawa, S. Furusawa, T. Yamaki, S. Kohmoto, M. Ya-
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128.1, 127.6, 127.5, 127.4, 127.2, 126.5, 114.9, 110.2, 85.6, 72.0, 70.7, 70.6,
70.5, 69.8, 69.6, 69.4, 59.2, 40.1, 26.1 ppm; MALDI-TOF MS: m/z:
2614.45 [M+Na]+, 2630.41 [M+K]+.
Synthesis of (S,S)-O-CBR: (S,S)-O-CBR was prepared by the same pro-
cedure as O-CBR by means of a Suzuki-type aromatic coupling of 3b
and (S,S)-1,2-bis(4-dihydroxyboranylphenyl)-1,2-dimethoxyethane (ob-
tained by Sharpless asymmetric dihydroxylation of trans-4,4’-dibromostil-
bene by using AD-mix a). [a]2D0 =ꢀ102.3o (c=0.00057 gmLꢀ1, CHCl3);
1H NMR (250 MHz, CDCl3): d=7.72–7.49 (m, 32H; Ar-H), 7.15 (d, J=
8.1 Hz, 2H; Ar-H, m to CHOCH3), 6.99 (d, J=8.7 Hz, 2H; Ar-H, o to
OCH2), 4.42 (s, 2H; ArCHOCH3), 4.05 (d, J=5.2 Hz, 4H; CH2Ophenyl),
3.64–3.46 (m, 128H; OCH2), 3.37 (m, 24H; OCH3), 2.39–2.35 (m, 2H;
ˇ
´
´
´
37, 2689–2691; c) J. Makarevic, M. Jokic, Z. Raza, Z. Stefanic, B.
ˇ
´
´
´
Kojic-Prodic, M. Zinic, Chem. Eur. J. 2003, 9, 5567–5580; d) M. S.
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phenylOCH2CHACHTREUNG(CH2O)2), 2.20–2.17 ppm (m, 4H; CHACHTRE(UGN CH2O)2);
13C NMR (63 MHz, CDCl3): d=158.9, 140.0, 139.8, 139.7, 139.6, 138.9,
137.5, 133.1, 128.6, 128.1, 127.5, 127.4, 127.2, 126.5, 115.0, 88.1, 72.0, 70.7,
70.6, 70.5, 69.8, 69.6, 59.1, 57.4, 40.2, 40.1 ppm; MALDI-TOF MS: m/z:
2602.48 [M+Na]+, 2618.41 [M+K]+.
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 871 – 881

Supramolecular Helices
FULL PAPER
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[22] From the experimental values of the intercolumnar distance (a) and
the densities (1), the average number (n) of molecules arranged side
by side in a single slice of the cylinders with a thickness (h) of 4.5 Š
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can be calculated by using to following equation, in which M is the
p
3
molecular mass and NA is Avogadroꢄs number: n= 2 a2 hNMA 1.
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6664–6676; b) See the Supporting Information.
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Received: July 14, 2007
Published online: October 1, 2007
Chem. Eur. J. 2008, 14, 871 – 881
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