Reversible transformation of helical coils and straight rods in cylindrical assembly of elliptical macrocycles

Article abstract of DOI:10.1021/ja907462h

(Figure Presented) We have demonstrated that, as the molecular length of elliptical macrocycle is increased, the self-assembled structure changes from spherical micelles to helical coils and finally to monolayered vesicles, in the order of decreasing inte

Full text of DOI:10.1021/ja907462h

Published on Web 11/18/2009
Reversible Transformation of Helical Coils and Straight Rods in Cylindrical
Assembly of Elliptical Macrocycles
Jung-Keun Kim,† Eunji Lee,† Min-Cheol Kim,‡ Eunji Sim,‡ and Myongsoo Lee*,†
Center for Supramolecular Nano-Assembly and Department of Chemistry, Seoul National UniVersity, 599
Kwanak-ro, Seoul 151-747, Korea, and Department of Chemistry, Yonsei UniVersity, Seoul 120-749, Korea
Received September 3, 2009; E-mail: myongslee@snu.ac.kr
The self-assembly of rigid macrocycles into well-defined nano-
structures has been the subject of intense study in recent years,
both for basic research and for potential applications.1 Most rigid
aromatic macrocycles self-assemble into cylindrical micelles,2 while
certain macrocycles self-assemble into spherical structures.3 The
main challenge in assembling aromatic building blocks into these
nanostructures lies in their capability to respond to external stimuli.4
To obtain controlled aggregates that are able to respond to external
stimuli, however, the more elaborate design of corresponding
building blocks is required. Accordingly, we have synthesized
amphiphilic aromatic macrocycles with an elliptical shape contain-
ing oligoether dendrons that can endow aggregates with a thermo-
responsive feature.
Figure 1. Chemical structure of amphiphilic rigid macrocycles 1-4.
We present herein the structural progression in the self-assembly
of macrocyclic amphiphiles in water from spheres, helical coils to
vesicles with an increase in the molecular length of the elliptical
macrocycle, and reversible transformation of the helical coils into
straight rods triggered by temperature. The macrocyclic amphiphiles
were prepared from the cyclization by a Glaser coupling reaction
according to procedures reported previously.5 The resulting mac-
rocycles characterized by NMR spectroscopy, elemental analysis,
and MALDI-TOF mass spectroscopy (Figure S1) were shown to
be in full agreement with the structures presented.
The aggregation behavior of the molecules was investigated in
aqueous solution by using transmission electron microscopy (TEM).
TEM studies showed that all of these macrocyclic amphiphiles self-
assemble into stable nanostructures. Figure 2a shows a micrograph
obtained from a 0.01 wt % aqueous solution of 1 based on a shorter
elliptical macrocycle cast onto a TEM grid. The negatively stained
sample with uranyl acetate shows the formation of spherical micelles
with a diameter of ∼5 nm which is comparable to the molecular
length (6.5 nm). This result suggests that the discrete nanostructure
consists of an aromatic core in which the rigid macrocycles are
stacked on top of each other with mutual rotation and an oligoether
dendron exterior that is exposed to a water environment.
Interestingly, molecule 2 based on an intermediate length of
elliptical macrocycle self-assembles into helical coils. The evidence
for the formation of the nonspherical structure in aqueous solution
was also provided by cryo-TEM experiments performed with 0.01
Figure 2. (a) TEM image of spherical micelles formed from 1. (b,c) Cryo-
TEM images of helical coils formed from 2. (d) Cryo-TEM image of vesicles
formed from 3. (e) TEM and (f) cryo-TEM images of cylindrical micelles
formed from 4.
wt % solution (Figures 2b, S3). Examination of a large area TEM
image shows helical coils to be the predominant species in solution.
Closer examination of the image reveals that the cylindrical micelles
with a cross-sectional diameter of ∼3 nm are curved to form helical
coils with a uniform diameter of ∼30 nm and a regular pitch of
∼10 nm (Figure 2c). The aromatic cores appear to be dark, whereas
the solvated oligoether dendrons are not directly visible.6 Thus,
comparison with the estimated length of the macrocycle of 2.8 nm
suggests that the macrocycle stacks on top of each other with mutual
rotation surrounded by oligoether dendrons. The solution displays
a significant Cotton effect in the aromatic unit, indicating the
presence of one-handed helical aggregates. This is in sharp contrast
to the solution behavior of 1 and 3 which shows a complete lack
of a Cotton effect (Figure S4).
† Seoul National University.
‡ Yonsei University.
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10.1021/ja907462h  2009 American Chemical Society

C O M M U N I C A T I O N S
Figure 4. (a) UV-vis absorption, (b) circular dichroism and (c) fluores-
cence spectra of 2 in aqueous solution (0.01 wt %), with temperature
variation (°C).
Figure 3. (a) Schematic representation of the self-assembled structures of
1-3. (b) Simulated structures of the macrocycle molecule 2. Each molecule
is colored differently for clarity. Top view (left) and side view (right).
Figure 5. Cryo-TEM images of (a) straight rods at 50 °C; (b) undulated
rods on cooling to room temperature, then 6 h annealing; and (c) helical
coils after 3 days annealing of 2 in aqueous solution (0.01 wt %).
An aqueous solution of 3 based on an elongated macrocycle with
a higher aspect ratio is characterized by the presence of vesicular
aggregates. When the aqueous solution (0.01 wt %) is allowed to
be stabilized and then cast onto a TEM grid, the image stained
with uranyl acetate reveals spherical objects with diameters ranging
from several tens to few hundreds of nanometers. DLS studies
(Figure S2) revealed that the spherical object has an average
diameter of ∼160 nm, which is in good agreement with the size
observed by TEM (Figure S5). A cryo-TEM image confirms the
presence of spherical aggregates with a dark outer circle expected
from the 2D projection of vesicular structures with a uniform
thickness of ∼4 nm. Considering the calculated length of the
macrocycle is 3.7 nm, the rigid macrocycle packs into a monolayer
structure with the polar oligoether dendritic side chains exposed to
water (Figure 2d).
in which an absorption band maximum at 323 nm in THF solution
was shown to be red-shifted by 20 nm in aqueous solution (Figure
4a).10
To investigate the role of the elliptical shape of the macrocycle
for the helical coil structure, we have modified the macrocycle from
elliptical to more circular in shape to form an analogue (4) to
molecule 2 with the aim of interference of anisotropic interactions
along the stacking axis. Since the dendritic building blocks are based
on being identical, the shape of the aggregates may mainly be
attributed to the variation in the aspect ratio of the elliptical
macrocycle. As shown in Figure 2e-f, 4 self-assembles into
cylindrical micelles without any regular curvature with a diameter
of ∼6 nm, indicating that the rigid macrocycles are stacked on top
of each other with mutual rotation along the stacking axis. This
result implies that the formation of helical coils originates from
the unique slipped stacking arrangement of the elliptical macro-
cycles with a high aspect ratio.
Remarkably, the self-assembled helical coils undergo a reversible
phase transition at 40 °C. Upon heating to 50 °C, cryo-TEM of the
solution revealed straight rod-like micelles with a uniform diameter
of ∼3 nm and lengths of several hundreds of nanometers (Figures
5a, S9). This result indicates that the helical coils spontaneously
unfold into straight rods without any noticeable changes in cross-
sectional diameter. On cooling to room temperature, the straight
rods started to be undulated (Figure 5b). Complete recovery to the
original helical coils was observed over a period of 3 days of resting
(Figure 5c). This indicates that the structural transformation between
helical coils and straight rods are accompanied by considerable
hysteresis. This was further confirmed by CD measurements with
annealing times at room temperature (Figure S10). On heating, the
phase transition leads to a fast spectral change. However, complete
recovery to the original CD profile requires 3 days. This hysteretic
behavior in the coiling motion of the straight rods seems to arise
from the strong kinetic effect related to the slow breakup of π-π
stacking interactions between the rigid macrocycles when the
straight rods are coiled.
On the basis of the results described above, the possible models
responsible for the generation of the self-assembled structures
depending on the length of the aromatic core can be presented as
shown in Figure 3a. As the molecular length of the elliptical
aromatic unit is increased, structural transformation takes place from
sphere to helical coils to monolayered vesicles, in the order of
decreasing interfacial curvature. This structural progression is most
probably due to increasing π-π stacking and hydrophobic interac-
tions. Qualitatively, the structural change with the variation in the
molecular length of aromatic units resembles that reported for other
amphiphilic systems.7,8 Compared to reported intermediate cylindri-
cal structures, however, a unique feature of our systems is that the
cylindrical micelles are curved in a regular way to form well-defined
helical coils.
To gain insight into the origin of the self-assembly into helical
coils, ab initio simulations were performed using the Gaussian 03
package.9 Using various conformations of a single molecule
including a stable boat conformation, we optimized the structure
by increasing the number of molecules until four molecules are
stacked. Relative energies of stacked structures indicate that the
helical secondary structure becomes stabilized as more molecules
are assembled (Table S1). As the molecules stacked on top of each
other, the molecule with a boat conformation is somewhat slipped
and staggered with respect to its neighbors to reduce steric hindrance
between the nonplanar macrocycles (Figure 3b). The slipped
stacking was also confirmed experimentally by UV-vis spectra,
This transition can be explained by considering the lower critical
solution temperature (LCST) behavior of the dendritic ethylene
oxide chains in aqueous media.11 The LCST temperature was
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C O M M U N I C A T I O N S
Science and Technology and was partially funded by U.S. Air Force
(Air Force Office of Scientific Research) under Grant FA2386-09-
1-4116. E.S. acknowledges the KOSEF grant funded by the Korea
government (MEST) (No. 2009-0054064). J.K.K., E.L., and M.C.K.
acknowledge a fellowship of the BK21 program.
Supporting Information Available: Detailed synthetic procedures,
1H NMR and 13C NMR data. This material is available free of charge
via the Internet at http://pubs.acs.org.
Figure 6. Schematic representation of reversible transformation of helical
coils and straight rods of elliptical macrocycles.
References
subsequently determined to be 40 °C by turbidity measurements.
(Figure S8). At room temperature, the ethylene oxide chains are
fully hydrated and thus adopt a random coil conformation. Above
the LCST, they are dehydrated to be hydrophobic. As a result, the
aromatic cycles are more closely packed due to enhanced hydro-
phobic environments. This packing consideration is reflected in the
increased extent of fluorescence quenching upon heating (Figure
4c).12 The strengthened π-π interactions would lead to a more
planar conformation of the elliptical macrocycles. This is supported
by temperature-dependent UV-vis (Figure 4a) and CD measure-
ments (Figure 4b). Upon heating to the LCST, an absorption
maximum is red-shifted by ∼10 nm and the shape of the CD
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helical structure through a molecular reorganization within the
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Acknowledgment. This work was supported by the National
Creative Research Initiative Program of the Korean Ministry of
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