Solid-state scrolls from hierarchical self-assembly of T-shaped rod-coil molecules

Article abstract of DOI:10.1002/anie.200804307

(Figure Presented) On a roll: Attachment of flexible coils to the middle of a rigid rod generates T-shaped rod-coil molecules that self-assemble into layers that roll up to form filled cylindrical and hollow tubular scrolls, depending on the coil length, in the solid state (see picture); the rods are arranged parallel to the layer plane.

Full text of DOI:10.1002/anie.200804307

Communications
DOI: 10.1002/anie.200804307
Nanostructure Self-Assembly
Solid-State Scrolls from Hierarchical Self-Assembly of T-Shaped
Rod–Coil Molecules**
Dong-Je Hong, Eunji Lee, Haemi Jeong, Jeong-kyu Lee, Wang-Cheol Zin, Trung Dac Nguyen,
Sharon C. Glotzer, and Myongsoo Lee*
One-dimensional nanostructures such as cylinders and
tubules are of great interest owing to their unique electro-
optical properties and potential applications in areas ranging
from nanotechnology to biotechnology, but they are challeng-
ing to synthesize.[1] Such anisotropic nanostructures with
extremely high aspect ratios can be formed when thin
crystalline layers roll up under disparate surface stresses on
opposite sides of the layers.[2,3] Single-crystal nanotubes have
also been synthesized using nanowire templates,[4] and this
method has been extended to prepare polymeric nanotubules
with controlled diameters.[5] Self-assembly of block copoly-
mers and amphiphilic molecules in solution leads to the
formation of tubular structures in a selective solvent that acts
as a molecular structure-directing template.[6–8] However, the
self-assembly of hollow tubules without any template or
solvent has not yet been realized, with few exceptions.[9,10]
Furthermore, we are not aware of any natural or synthetic
self-assembling layers that spontaneously form scrolls in the
solid state, which is crucial for practical applications in
nanotechnology. Herein we report the formation of hollow
tubules from the self-assembly of T-shaped rod–coil mole-
cules in the solid state.
molecules endow the materials with anisotropic properties
arising from their strong tendency to be aligned perpendicular
to the direction of rod orientation. In addition to conventional
layered structures, the rodlike components of rod–coil build-
ing blocks can be made to self-assemble into a wide variety of
complex geometries, including perforated layers, ribbons, and
bundles, through covalent attachment of long flexible chains
to their distal section.[12] These nanoscale objects are further
organized into two-dimensional (ribbons) and three-dimen-
sional (bundles) structures.[12,13] However, the rod segments
arrange parallel to each other to form only flat local structures
as opposed to radial structures, a prerequisite for nano-
tubules.[14] Herein, we report the formation of filled cylin-
drical and hollow tubular scrolls from the self-assembly of T-
shaped rod–coil molecules in the solid state. The scrolls have a
hierarchical structure comprised of alternating rod and coil
layers with a narrow distribution of outer scroll diameters.
The rod–coil molecules described herein consist of a
penta-p-phenylene conjugated rod connected to a poly(pro-
pylene oxide) (PPO) coil laterally attached through an
imidazole linkage (Figure 1a).[11] Rod–coil molecules based
on lateral chains self-assemble into layered structures in
which the rod segments are aligned parallel to the layer
planes.[15,16] The stiff rod layers are separated by the amor-
phous layers of the lateral coils. Within the layers, the rods are
organized parallel to each other to form sublayers. This
special organization of the rod segments into anisotropic 2D
layers could be envisioned to roll up into tubules under
appropriate conditions.[17–19]
Rod–coil molecules are attractive for generating highly
defined nanostructures that have physical dimensions as small
as a few nanometers.[11] The stiff rodlike components of the
[*] D.-J. Hong, E. Lee, H. Jeong, Prof. M. Lee
Center for Supramolecular Nano-Assembly and Department of
Chemistry, Yonsei University
Shinchon 134, Seoul 120-749 (Korea)
Fax: (+82)2-393-6096
E-mail: mslee@yonsei.ac.kr
Rod–coil molecules 1 and 2, and their analogues 3 and 4
with longer PPO chains, show an ordered state that is retained
up to an isotropization transition (Table 1). The ordered
structure is thermodynamically stable, as evidenced by differ-
ential scanning calorimetry (DSC; Table 1). The solid-state
structure was confirmed by X-ray measurements (Figure 1b).
Small-angle X-ray scattering (SAXS) reveals that 1 and 2
have a layered structure with a primary spacing of 3.5 and
4.6 nm, respectively. These dimensions correspond roughly to
the respective laterally extended molecular lengths, suggest-
ing that the rod segments within the domains are parallel to
the layer planes. Notably, both 1 and 2 exhibit an additional
sharp reflection at an identical q spacing of 3.57 nmÀ1 and
show similar wide-angle X-ray scattering (WAXS) patterns
(Figure S2 in the Supporting Information), indicating that
both compounds have an essentially identical crystalline
structure within the rod layers. This finding is also reflected in
the similar transition temperatures and corresponding
enthalpy changes (Table 1). The existence of a sharp peak at
an intermediate angle suggests that the rod segments in 1 and
Homepage: http://csna.yonsei.ac.kr
J.-K. Lee, W.-C. Zin
Department of Materials Science and Engineering
Polymer Research Institute, Pohang University of Science and
Technology, Pohang 790-784 (Korea)
T. D. Nguyen, S. C. Glotzer
Department of Chemical Engineering and Department of Materials
Science and Engineering, University of Michigan
Ann Arbor, MI 48109-2136 (USA)
[**] This work was supported by the Creative Research Initiative
Program of the Ministry of Education, Science and Technology of
the Korean Government. D.J.H. and E.L. thank a fellowship of the
BK21 program of the Ministry of Education, Science and Technology
of the Korean Government. T.D.N. acknowledges the support of the
Vietnam Education Foundation. T.D.N. and S.C.G. acknowledge the
support of the US Department of Energy, Basic Energy Science
Program and the US Air Force Office of Scientific Resesarch.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200804307.
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cates that the rods are tilted relative to the sublayer normal
with an angle of 378. Recent simulations of laterally grafted
rod–coil molecules predict the existence of in-plane sublayers
with rod tilt.[16]
In contrast to conventional layered structures, these layers
form scrolls with a nearly uniform diameter, as revealed by
the transmission electron microscopy (TEM) images in
Figure 2. The side-view image of cryomicrotomed films of 2
Figure 2. a) TEM image of a layered structure confined in the scrolls,
revealing an ordered array of alternating light-colored PPO layers and
dark aromatic layers. b) Cross-sectional image along the rolled lamellar
sheet axis. Scale bars 50 nm. c) 2D SAXS pattern obtained from 2 at
258C. d) 2D WAXS pattern of 2. The diffraction pattern shows {010},
{110} reflections of the rolled lamellar structure. e) Schematic repre-
sentation of the filled scrolls from 2.
Figure 1. a) Molecular structures of 1–4. b) 1D SAXS patterns of 1–4
plotted against q (=4psinqlÀ1). Black arrows indicate an intermedi-
ate-angle reflection for each molecule.
stained with RuO4 shows a layered structure with a primary
spacing of 4.6 nm, consistent with that obtained from SAXS.
Views of the cross-section, however, reveal the formation of
scrolls with a spiral arrangement of alternating light coil and
dark rod layers with scroll diameters ranging from 80 to
100 nm (Figure 2b). Additional information on the structural
dimensionality was obtained by 2D X-ray diffraction with an
annealed sample of 2 exhibiting macroscopic orientation. As
shown in Figure 2c, the X-ray beam was directed perpendic-
ular to the principle axes of the scrolls. At small angles, two
pairs of strong (100) and (200) reflections are diffracted along
the equator. At an intermediate angle, the additional repeat
distance associated with the sublayer thickness in each rod
layer occurs at the same direction relative to the layer
reflections, demonstrating that the scroll axis is parallel to the
sublayer direction. At wide angles, two pairs of reflections
Table 1: Characterization of 1–4 by X-ray scattering and thermal
transition.
1
2
3
4
Tc [8C][a]
113
16.7
3.5
112
12.0
4.6
75
5.4
6.2
61
2.3
7.2
DH [kJmolÀ1 [b]
d [nm][c]
]
[a] Transition temperature estimated from differential scanning calorim-
etry. [b] Enthalpy change (DH) estimated from differential scanning
calorimetry. [c] Layer thicknesses (d) measured from q values of 1D X-ray
profiles.
2 organize into sublayers with a thickness of 1.76 nm within a
layer. Considering the calculated rod length of 2.2 nm by the
Corey–Pauling–Koltun (CPK) model, this dimension indi-
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associated with the inter-rod distance are diffracted along the
meridian (Figure 2d), indicative of crystallization of the rod
segments within the sublayers. This result demonstrates that
the repeat distance associated with p–p stacking interactions
is perpendicular to the rolling axis.
The electron microscopy observations together with the
X-ray results lead to the structural model of the scrolls of 2 as
shown in Figure 2e. The bulk layers are formed parallel to the
molecular long axes with a thickness of 4.6 nm, and the rod
segments within the layers are organized parallel to each
other to form sublayers with rod tilt with respect to the
sublayer normal at an angle of 378. Subsequently, the layers
roll up along the direction of the sublayers to form scrolls with
diameters ranging from 80 to 100 nm. As evidenced in the
simulations described below, the rod tilt with respect to the
sublayer normal leads to helical scrolls, because the rolling
axis is parallel to the sublayer direction.
Rod–coil molecules 3 and 4, based on longer PPO chains,
also self-organize into layered structures. Each small-angle X-
ray diffraction pattern shows three sharp equidistant reflec-
tions, indicative of layered structures with lattice constants of
6.2 and 7.2 nm for 3 and 4, respectively (Figure 1b). These
dimensions suggest that the layer structure arises from
segregation of the lateral PPO chains from the stiff aromatic
segments. Within the layers, the rod segments are again
organized parallel to the layer planes. Furthermore, both
compounds exhibit an identical intermediate angle reflection
at q = 4.12 nmÀ1 and also show similar wide-angle X-ray
diffraction patterns (Figure S2 in the Supporting Informa-
tion), indicating that both molecules have an essentially
identical packing structure within the rod layers. This
similarity is reflected in similar transition temperatures and
corresponding enthalpy changes (Table 1). The lower tran-
sition temperatures and smaller enthalpy changes compared
to those of the analogues with shorter PPO chains indicate
that the rod segments within the layers of 3 and 4 are aligned
with overall less order than is the case for 1 and 2. The sharp
intermediate-angle peak suggests that the rod segments in the
two compounds organize into sublayers with a thickness of
1.52 nm. Considering the calculated rod length of 2.2 nm by
the CPK model, this dimension indicates that the rods are
tilted with an angle of 478 relative to the sublayer normal.
When cryomicrotomed films of 3 stained with RuO4 were
characterized by TEM, stacks of long cylindrical objects with
nearly uniform diameters of (90 Æ 10) nm and lengths up to
several tenths of micrometers were observed (Figure 3). The
magnified images reveal that the elongated objects consist of
an inner core of approximately 30 nm separated by multilayer
walls of about 30 nm, demonstrating that the 1D objects are
multiwall tubules. The image of the cross-section of the
tubules shows that the walls consist of layers rolled into a
spiral in which the layer thickness is 6.2 nm (Figure 3c),
consistent with the lattice constant determined from small-
angle X-ray diffraction. These results suggest that the
formation of a tubular structure arises from a rolling up of
the layers. When thin films of 3 were cast from dilute
methanol solutions and negatively stained with uranyl acetate
solution, which is able to fill the empty space by capillary
action, the image showed a dark interior in the elongated
Figure 3. a) Low-magnification TEM image of a microtomed film of 3
showing long cylindrical aggregates with length up to several micro-
meters. Scale bar 1 mm. b,c) Close examination of the same film
reveals a multilayered tubular structure with an interlayer distance of
the tube wall of about 6.2 nm. The inset in (c) is a high-magnification
image of the cross-section; it shows the rolled layers with a spiral
arrangement, implying that the tubes are composed of rolled-up
sheets in a layered structure. Scale bars 200 (b) and 100 nm (c).
d) TEM image of a solution-cast thin film of 3 negatively stained with
uranyl acetate, also revealing the formation of a well-ordered tubular
structure. The light regions correspond to the tube wall, the dark
regions to the inner cavity of the tube containing the staining agent.
Scale bar 200 nm. e) 2D-SAXS pattern obtained from 3 at 258C. The
diffraction pattern shows {100}, {200}, and {300} reflections. f) 2D
WAXS pattern of 3. The diffraction pattern shows a broad halo in the
wide-angle region.
objects (Figure 3d), clearly indicating a hollow cavity along
the scroll axis.
To investigate the molecular orientation within the rolled
tubules, 2D X-ray scattering experiments were performed
with an annealed sample of 3. The diffraction pattern shows
two pairs of strong (100) and (200) reflections that are
diffracted along the equator at small angles (Figure 3e). At
intermediate angles, two pairs of diffraction arcs correspond-
ing to a sublayer thickness are found in the cross directions at
an angle of approximately 908, implying that the rods are
packed into a smectic C liquid-crystal-like structure with a tilt
angle of approximately 458 relative to the sublayer normal. At
wide angles, a diffuse scattering centered at 0.45 nm is
diffracted in a circular pattern (Figure 3 f), characteristic of
liquid-crystalline order between the rod segments.
To determine whether the tubules form directly from the
melt or by spontaneous rolling up of planar sheets, we
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investigated cryomicrotomed films of 3 with different anneal-
ing times (Figure 4a–d). When the melt was quickly quenched
by blowing cold argon onto it, the TEM images showed
end-to-end interactions. After initialization and subsequent
relaxation of the sheets (see the Supporting Information), we
observed the spontaneous rolling of sheets, regardless of
aspect ratio (Figure 5a, top row and bottom left image). The
Figure 4. TEM micrographs of intermediate structures kinetically trap-
ped by quick quenching from the melt of 3. TEM analysis was
performed on thin films cryomicrotomed at À708C. TEM images of
the cross-sections from the experiment show the intermediate struc-
tures between lamellar sheets and tubular scrolls. The sheets bend
slightly (a), then roll upon themselves (b,c) to form the tubular scrolls
(d). Scale bars 50 nm. e) Schematic representation of the tubular
scrolls of 3.
Figure 5. Simulation results. a) Top-view snapshots of scrolls formed
by sheets for which the rod packing is as proposed in Figure 2e for 1
and 2 with different aspect ratios (measured in pairs of rods), rod tilt
angles, and short and long coil lengths, corresponding to 1 (“short”
coil) and 2 (“long” coil). Upper row: (48:12, 378, short coil), (60:8,
378, long coil), and (48:8, 378, long coil). Lower row: (60:12, 378, short
coil), (48:12, 08, short coil), and (48:12, 08, long coil). b) Side-view
snapshots of scrolls formed by sheets with different rod tilt angles in
which the rod packing is as proposed in Figure 2e for 2 (coils removed
for clarity). Left: (48:12, 378, long coil). Right: (48:12, 08, long coil).
c) Snapshots of scrolls formed by sheets for which the rod packing is
as proposed for 3 and 4 in Figure 4e. The first two images from the
left are the top view and side view (with coils removed) of a 6:80 sheet
with short coils corresponding to compound 3. The third image shows
the top view of a scroll formed from a 6:80 sheet with long coils
corresponding to compound 4.
loosely curved layers together with uncurved layers. How-
ever, when the films were cooled slowly from the melt to
ambient temperature, we observed closely packed scrolls
throughout the image area (Figure 4d). This result implies
that the molecules first assemble into a layered structure that
subsequently forms scrolls. On the basis of these results, we
hypothesize that the hollow tubules are formed by rolling up
along an axis parallel to the rod direction of the bulk layers in
which the laterally grafted rod segments are arranged parallel
to each other to form sublayers with a rod tilt of approx-
imately 458 relative to the sublayer normal (Figure 4e). These
scrolls have external diameters ranging from 80–100 nm and a
hollow interior with a diameter of approximately 30 nm.
To gain further insight into the rolling behavior of the
layers of T-shaped rod–coil molecules, we performed Brow-
nian dynamics simulations (for details, see the Supporting
Information) of individual preformed bilayer sheets of
molecules 1 or 2 and 3 or 4. Our previous simulations[16]
demonstrated the formation of these sheets from the self-
assembly of T-shaped rod–coil building blocks under melt
conditions, but our simulation approach did not permit their
subsequent assembly into higher-order structures such as
scrolls. We used experimentally determined values to set the
length and spacing of the rods and the coils relative to the
layer thickness. Flat sheets of 1 and 2 with various aspect
ratios (see the Supporting Information) were used in the
simulations with the rods organized into crystalline, in-plane
sublayers with a rod tilt angle of 378 (P2 symmetry[16]), as
shown in Figure 2e. The packing of rods within the sheet was
maintained by a bonding scheme as illustrated in Figure S4a
(in the Supporting Information) to mimic the expected strong
p–p stacking between neighboring aligned rods and weaker
simulations confirm that the rolling axis is parallel to the
sublayer direction for all aspect ratios studied (Figure 2e).
Top-view snapshots of the simulated structures show excellent
agreement with the TEM images of compound 2 (Figure 2b),
in which cylindrical scrolls are formed with alternating layers
of rods and coils and have a coil-filled center. Simulations of
bilayers with zero rod tilt angle also yield cylindrical scrolls
(Figure 5a, bottom middle and right). The side-view snap-
shots of the simulated structures (Figure 5b) indicate that the
scrolls are helical for nonzero tilt angles. We see that the P2
crystalline packing of the rods in the sheet is well maintained
during rolling (Figure 5b) owing to the bonding scheme, thus
demonstrating the importance of the strong p–p interactions
between the rods in the sublayers to achieving the observed
scrolls.
To elucidate the formation of the tubular scrolls, we
simulated bilayer sheets of molecules of 3 and 4, assuming
that the rods are arranged in loosely packed, liquid crystalline
sublayers with a rod tilt angle of 458 (similar to Cmm
packing[16]), as shown in Figure 4e. Again, sheets of various
aspect ratios (Figure S4b in the Supporting Information) were
initialized, relaxed, and allowed to fluctuate dynamically,
which led to their spontaneous rolling. Our simulations
(Figure 5c) confirm that the rolling axis is parallel to the
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rod direction in every case, consistent with the model
The most notable feature of the structurally simple rod–
coil molecules investigated herein is their ability to hierarchi-
cally and reversibly self-assemble into scrolls in the solid state.
It is also remarkable that the core structure of the scrolls, from
filled cylindrical to hollow tubular, can be controlled by only a
small variation of coil length of the T-shaped rod–coil
molecule. Such solid-state scrolls with controlled interior
hold outstanding potential for a broad range of applications in
nanoscience.
proposed in Figure 4e. The simulated structures (Figure 5c,
left) show excellent agreement with the hollow tubules
formed by compounds 3 and 4. We observe that the loose
packing of the rods in interdigitated sublayers is maintained
under rolling (Figure 5c, middle). Furthermore, our simula-
tions reveal that the radius of the hollow core increases with
the coil length (Figure 5c, right), presumably because of the
limited compression of the coils and the increasing thickness
(and thus decreasing bending elasticity) of the bilayer sheets.
The results described herein demonstrate that T-shaped
rod–coil molecules self-assemble into layered structures in
which the long axis of the rod segment is parallel to the layer
plane. The layers, in turn, unexpectedly roll up to form scrolls
in the solid state. Rolling of the layers can be understood by
considering the grafting density at the rod–coil interface and
the resulting space-filling requirements. Lamellar ordering of
rods would confine rod–coil junctions to a flat interface.
However, the large mismatch between the cross-sectional
areas of the coil segments and the side face of the rod results
in a strong energetic penalty associated with low grafting
density at the planar rod–coil interface. As a result, the flat 2D
rod layer curves along the direction of the sublayer (1 and 2)
or the rod axis (3 and 4) and ultimately rolls with the coil layer
inside to release the surface stresses.[20]
Received: September 1, 2008
Published online: December 12, 2008
Keywords: amphiphiles · liquid crystals · nanostructures ·
.
self-assembly
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