Reversible scrolling of two-dimensional sheets from the self-assembly of laterally grafted amphiphilic rods

Full text of DOI:10.1002/anie.200900079

Angewandte
Chemie
DOI: 10.1002/anie.200900079
Nanotubes
Reversible Scrolling of Two-Dimensional Sheets from the Self-
Assembly of Laterally Grafted Amphiphilic Rods**
Eunji Lee, Jung-Keun Kim, and Myongsoo Lee*
Hollow tubular structures have attracted much attention
owing to their unique optoelectrical properties and potential
applications in biomimetics, nanodevices, catalysis, and
energy storage.[1] Since the discovery of carbon nanotubes,[2]
extensive efforts have been devoted to experimentally
fabricate organic nanotubes in aqueous solutions, and theo-
retically understand the aspects of nanotube formation. For
example, introduction of a chiral moiety into aromatic
amphiphilic molecular architectures leads to the initial
formation of helical ribbons, which in turn, self-assemble
into nanotubes.[3] It is known that ring-shaped, flat cyclic
peptides composed of an even number of alternating d- and l-
amino acids stack on top of each other through intermolecular
hydrogen bonding to form nanotubes.[4] Despite the above
achievements, the development of a novel route for nanotube
formation is still a challenging field of research.
Theoretical studies have predicted that two-dimensional
(2D) anisotropic sheets with long-range orientational order
might exhibit a tubular phase, which is intermediate in its
properties and location on the phase diagram between a flat
low-temperature phase and a crumpled high-temperature
phase.[5] In this tubular phase, the sheets begin to curl in one
Figure 1. Representation of reversible scrolling of anisotropic planar
direction as thermal stress increases the bending of the sheets
in a preferential direction, stabilizing orientational order
against the thermal stresses. Inspired by this theoretical
prediction, we envisioned that the transformation of sheetlike
structure into tubular structure would be experimentally
feasible if the introduction of a laterally extended aromatic
rodlike scaffold into amphiphilic molecules can lead to the
formation of in-plane anisotropic sheets.[6] Therefore, we
designed laterally grafted rod amphiphiles consisting of a
hepta(p-phenylene) rod in which hydrophilic oligoether
dendrons and hydrophobic branched heptyl chains are
grafted opposite to each other at the midpoint of the rodlike
scaffold (1 and 2).
sheets.
rationally designed rod amphiphiles. These sheets undergo
reversible transformation into tubular scrolls upon heating
(Figure 1). The synthesis of the laterally grafted amphiphilic
rod molecules was performed with the preparation of
oligoether dendrons and branched alkyl chains according to
the procedure described previously.[7,8] The ter(p-phenylene)
aromatic scaffold was prepared using the Suzuki coupling
reaction with 1,4-dibromo-2,5-dimethoxybenzene and a bor-
onic acid derivative followed by iodination. The final rod
amphiphiles were synthesized by etherification of the aro-
matic scaffold and tosylation of the oligoether dendrons and
branched alkyl chains, and thereafter the Suzuki coupling
reaction with the biphenyl units. The analytical data from the
resulting molecules 1 and 2 are in full agreement with the
expected chemical structures.[9]
The aggregation behavior of the laterally grafted amphi-
philic rod molecules was studied in aqueous solution (0.01
wt%) using fluorescence spectroscopy. The intensity of the
fluorescence maxima was quenched with respect to that
observed in chloroform, indicating the aggregation of aro-
matic segments (Figure 2a and Supporting Information,
Figure S2).[10] Dynamic light scattering (DLS) experiments
were performed on the amphiphiles in aqueous solution (0.01
wt%) to further investigate the aggregation behavior.[11] The
relaxation times obtained for these amphiphiles in an aqueous
Herein, we report the spontaneous formation of a sheet-
like structure, which is based on the 2D self-assembly of
[*] E. Lee, J.-K. Kim, Prof. M. Lee
Center for Supramolecular Nanoassembly and
Department of Chemistry, Yonsei University
Shinchon 134, Seoul 120-749 (Korea)
Fax: (+82)2-393-6096
E-mail: mslee@yonsei.ac.kr
Homepage: http://csna.yonsei.ac.kr
[**] This work was supported by the National Creative Research
Initiative Program of the Korean Ministry of Science and Technology.
E.L. and J.-K.K acknowledge a fellowship of the BK21 program from
the Ministry of Education and Human Resources Development.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200900079.
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were prepared by suspending 100–200 nm thickness film of
the 0.01 wt% solutions on a lacey TEM grid, which was then
plunged into liquid ethane to freeze the solution containing
the aggregates. Thus, the large wrinkled sheets in the
observed TEM image were assumed to be induced by
specimen preparation. The formation of flat sheetlike struc-
tures was further confirmed by atomic force microscopy
(AFM). An AFM investigation of 2 showed planar sheets
with a uniform thickness of about 5.4 nm, which indicates that
the amphiphiles of 2 are also packed in a bilayer arrangement
(Supporting Information, Figure S5). These results demon-
strate that a decrease of hydrophilic chains in length enhances
the aggregation of the laterally extended aromatic amphiphi-
lies and thereby transformation of the ribbons into 2D sheets.
Notably, the aqueous solutions of 1 and 2 were shown to
reversibly transform from transparent to translucent states at
about 458C owing to lower critical solution temperature
(LCST) of the oligoether dendrons (Supporting Information,
Figure S6).[8,14] To corroborate thermoresponsive structural
change at the LCST, cryo-TEM was carried out with both
samples as a function of temperature. When the temperature
was raised to 608C, cryo-TEM images of 1 showed a sheetlike
structure, indicating that the ribbons transform into a 2D
sheet structure upon heating (Figure 3a). An AFM image of
the same sample showed that the thickness of the sheets is
circa 5.9 nm, which is indicative of a bilayer (Figure 3b).
Figure 2. a) Fluorescence emission spectra of 1 (0.01 wt%) in water
(red) and chloroform (black). The excitation wavelength is
lex =293 nm. b) Size distribution graphs of 1 and 2 in aqueous
solution at a scattering angle of 908. Cryo-TEM images showing c) the
flat ribbons of 1 (arrows indicate folded edge of ribbons) and d) the
wrinkled sheets of 2 in aqueous solution (inset: fluorescence micro-
graph in aqueous solution, scale bar: 2 mm).
solution of 1 using the correlation function is shorter than that
of 2 (Supporting Information, Figure S3). The CONTIN
analysis of autocorrelation function for both solutions showed
that 2 formed large aggregates with much broader distribu-
tions compared to 1 (Figure 2b).
Cryogenic transmission electron microscope (cryo-TEM)
has been performed with the 0.01 wt% aqueous solutions of 1
and 2 to further confirm the structure of the aggregates. The
images of 1, which is based on tri(ethylene oxide) chains,
showed dark ribbonlike aggregates with various widths
against the vitrified solution background (Figure 2c). Close
examination of the objects revealed folded edges of individual
aggregates with a thickness of approximately 4 nm. The
hydrophobic cores have a darker appearance; however, the
solvated oligoether dendrons remained invisible.[8a,12] There-
fore, the dimension of the individual aggregates is in
reasonable agreement with twice the length of the hydro-
phobic segments including the aromatic segments and alkyl
chains (ca. 2.3 nm by CPK modeling), thus confirming the
bilayer packing.[13] Based on these results, it can be concluded
that 1 self-assembles into a flat ribbon structure in which the
rods are arranged parallel to the ribbon plane.
The formation of a ribbonlike structure of 1 led us to
investigate whether decreasing the oligoether chain length
leads to the system assuming larger 2D aggregates to reduce
the exposure of the ribbon edges to water molecules. With this
in mind, we have prepared rod amphiphile 2 based on short
di(ethylene oxide) chains. As expected, the fluorescence
microscopy image of 2 (0.01 wt% aqueous solution) revealed
the presence of large sheets (Figure 2d, inset). Also, the
magnification of the 2D objects by cryo-TEM showed the
crumpled sheets ranging in size from several hundreds to a
few micrometers (Figure 2d). However, in the some cases, flat
sheets that are not wrinkled were also observed (Supporting
Information, Figure S4). For cryo-TEM experiments, samples
Figure 3. a) Cryo-TEM image, and b) AFM image (inset: height profile
along black line) of an aqueous solution of 1 (0.01 wt%) at 608C.
Above the LCST, the oligoether dendritic chains are
dehydrated and can assemble into molecular globules, which
can lead to a decrease in the effective hydrophilic volume,
resulting in the exposure of the hydrophobic side faces of the
ribbons to water. To reduce this unfavorable interaction, the
ribbons were associated through side-by-side hydrophobic
interactions to form a 2D sheet. This finding is reflected in the
increased florescence quenching with increasing temperature,
which implies that the aromatic segments are packed more
closely within the core because of the increase in hydrophobic
environments caused by dehydration of oligoether dendritic
chains (Supporting Information, Figure S7).
Remarkably, the planar sheets of 2 roll up into tubular
scrolls upon heating to the LCST. In contrast to the image
taken at room temperature, the fluorescence micrograph
taken at 608C revealed the formation of elongated rodlike
assemblies (Figure 4a). Some of the objects do not look like
elongated rods owing to their continuous Brownian motion,
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Spontaneous scrolling of the planar sheets upon heating is
attributed again to a LCST behavior of the oligoether chains
located at the planar surface. Above the LCST, the planar
sheets, where the rod segments are aligned parallel to the
sheet plane, would be unstable owing to the enhanced surface
energies arising from the increase in hydrophobicity of the
oligoether chains as a result of dehydration. To reduce
unfavorable contact between hydrophobic surfaces and
water, the planar sheets roll up along the direction of the
rod to form tubular scrolls (Figure 1).
The preservation of the large planar sheets at room
temperature can be attributed to a strong tendency of the rod
segments to efficiently pack with an anisotropic arrangement
through strong p–p stacking interactions. Thus, we envisioned
that the intercalation of the hydrophobic dye molecules
between the aromatic segments in the sheets would frustrate
the aromatic interactions and subsequently drive the sheets to
roll up into tubular scrolls to reduce the penalty in energy
associated with the packing frustration of the rod segments
within the aromatic cores. To show this frustrated interaction,
the intercalation experiments of 2 have been performed with
hydrophobic dye, Nile Red. The intercalation of Nile Red
within the aromatic segments was confirmed by fluorescence
spectroscopy (Figure 5a). When the solution 2 containing the
Figure 4. a) Fluorescence, b) cryo-TEM, and c) SEM micrographs in
aqueous solution of 2 at 608C; scale bar in the inset: 500 nm. d) TEM
images of aqueous solution of 2 (0.01 wt%) at 458C. After heating to
608C, the solution of 2 was slowly cooled to room temperature and
allowed to age for e) 12 h and f) 2 weeks as the tubular scrolls
regenerate into unfolded sheets.
although they are tubular in shape. For direct visualization of
the rodlike objects formed in aqueous solution, cryo-TEM
experiments were performed with a 0.01 wt% aqueous
solution of 2. The images show a hollow tubular structure,
demonstrating that the tubular objects are formed in bulk
solution (Figure 4b and Supporting Information, Figure S8).
However, the presence of large tubular particles of sizes
greater than the thickness of the freely suspended film is not
enough to provide the obvious differences in electron density
that are required to distinguish the structures. Thus, to obtain
the structural information, the frozen specimens were slightly
defrosted, although the transformation into tubular objects
was confirmed by scanning electron microscopy (SEM). The
SEM images clearly showed short tubular objects with a
hollow interior (Figure 4c and Supporting Information,
Figure S8), which is consistent with the results obtained
from fluorescence microscopy and TEM. To understand the
formation mechanism of the tubular structure, TEM experi-
ments were performed at different temperatures. Upon
heating to 458C, TEM micrographs of the solution show the
loosely rolled sheets (Figure 4d). On further heating to 608C,
we observed closed tubular scrolls (Figure 4b and Supporting
Information, Figure S8). After annealing at room temper-
ature for 12 h, the tubular scrolls opened into sheets
(Figure 4e). Complete recovery to the original sheets took
place over a period of two weeks upon resting at room
temperature (Figure 4 f), indicating that the structural trans-
formation between tubular scrolls and planar sheets occurs in
a reversible way.
Figure 5. a) Emission spectra of a solution (black) of 2 (0.01 wt%)
and a mixture (gray) of 2 (0.01 wt%) and Nile Red (0.001 wt%). The
excitation wavelength is lex =293 nm. b) TEM image showing the
scroll-type tubes of 2 containing 10 mol% Nile red without staining
(0.01 wt%).
Nile Red was excited at 293 nm, the fluorescence intensity of
2 was suppressed while exhibiting strong emission at 620 nm
corresponding to the energy-transfer band of the Nile Red
dye. These emission bands clearly indicate the intercalation of
Nile Red within the rod segments.[15] Indeed, TEM image
showed that 2 containing Nile Red forms tubular scrolls
(Figure 5b). The fluorescence and the TEM results indicate
that the in-plane anisotropic sheets have a strong tendency to
form scroll-type tubes by reducing the surface energy against
external stress.
In most of the cases of self-assembling systems including
block copolymers and surfactant molecules, the 2D sheets
fold into hollow spheres.[16] Thus, a remarkable feature of our
system is the ability of the 2D sheets to roll up into hollow
tubules. This unique structural transformation arises from in-
plane orientational order of the rod segments in which the
rods are arranged parallel to the sheet plane.[5] The resulting
anisotropic sheets, upon heating, spontaneously roll up along
the direction of the rod axis to form tubular scrolls. Thus the
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Received: January 6, 2009
Published online: April 16, 2009
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Keywords: amphiphiles · anisotropic sheets · nanotubes ·
rod–coil assemblies · self-assembly
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