C O M M U N I C A T I O N S
to a flat interface, which forces a strong deformation of the flexible
coils. To release this deformation without sacrificing a parallel
arrangement of the rod segments, the flat ribbons would roll up to
form a helical tubular structure.14 This estimation is further
supported by the fact that a helical pitch (4.7 nm) of the tubular
structure is in reasonable agreement with the extended molecular
length. It should be noted that the wall thickness of the tubule (3
nm) is consistent with b/2 (3.15 nm) obtained from SAXS.
In summary, the results described here demonstrate that the
rigid-flexible macrocycle based on a hexa-p-phenylene self-
assembles into well-defined ribbonlike aggregates with a rod tilt
relative to the ribbon normal at the initial stage. These elementary
strands are further coiled with a preferred handedness to form a
helical tubular structure with a uniform diameter of about 20 nm
and a regular pitch of 4.7 nm. The primary driving force responsible
for the formation of the tubular structure with a coiled ribbon is
believed to be the energy balance between repulsive interactions
among the adjacent flexible chains and π-π stacking interactions.
Such a well-defined coiled ribbon arrangement of conjugated rod
building blocks may provide a new strategy for the design of hollow
1-D nanomaterials with biological and electrooptical functions.
Acknowledgment. We gratefully acknowledge the National
Creative Research Initiative Program of the Korean Ministry of
Science and Technology for financial support of this work. E.L.
thanks the Seoul Science Fellowship program.
Figure 3. TEM images of (a) the unstained tubular structure of 1 with
density profile inset. (b) Negatively stained left-helical tubular structure of
1 with density profile inset. (c) Magnification of the left-helical tubular
structure.
Supporting Information Available: Detailed synthetic procedures,
characterization, XRD, TEM, and DLS data. This material is available
and a uniform diameter of about 20 nm (Figure 3a). Notably, there
is obvious contrast between the periphery and center in the
cylindrical object, characteristic of the projection images of tubular
aggregates. The internal diameter and the wall thickness are 14
and 3 nm, respectively, as confirmed by the density profiles taken
normal to the long axis of individual objects. When the samples
are negatively stained with a 2 wt % aqueous solution of uranyl
acetate, the images show the tubular structure with a left-handed
helical arrangement with a regular pitch (Figure 3b). The density
profiles taken parallel to the long axis of a helical aggregate
confirmed the pitch length to be 4.7 nm. The magnified images
with negatively stained samples show the elementary ribbons with
a uniform width of about 4 nm that is smaller than the extended
molecular length (4.8 nm by CPK).13 Therefore, it can be considered
that the rods are tilted with respect to the ribbon normal with an
angle of 33° that is consistent with a pitch angle (30°) obtained
from TEM (Figure 3c). Considering this width of single strain (4
nm) along with the pitch angle of 30°, the helical pitch of 4.7 nm
suggests that the tubular object is formed by the rolling up of an
elementary ribbonlike object.13
On the basis of these results, the rod segments can be considered
to self-assemble into 1-D ribbonlike aggregates with a laterally
stacked bilayer encapsulated by cyclic aliphatic chains in which
the rod building blocks are tilted with respect to the ribbon normal
at the initial stage. This is supported by molecular dimensions,
lattice constants determined from SAXS, and the width of the
elementary strand in TEM. However, space crowding of coil
segments would be larger in flat ribbonlike aggregates. A ribbonlike
ordering of the rod segments would confine flexible coil segments
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