Stepped strips from self-organization of oligo(p-phenylene) rods with lateral dendritic chains

Article abstract of DOI:10.1021/ja807035j

We have demonstrated that the rod segments with lateral dendritic chains self-assemble into unique stepped strips in which the rods are aligned parallel to the strip long axis. This unique organization of the rod segments arises from a balance between the energetic gain of a parallel arrangement of the rods and the resulting entropic penalty associated with stretching of the lateral flexible chains. Copyright

Full text of DOI:10.1021/ja807035j

Published on Web 10/09/2008
Stepped Strips from Self-Organization of Oligo(p-phenylene) Rods with
Lateral Dendritic Chains
Dong-Je Hong,†,‡ Eunji Lee,†,‡ Jeong-Kyu Lee,§ Wang-Cheol Zin,§ Minwoo Han,‡ Eunji Sim,‡ and
Myongsoo Lee*,†,‡
Center for Supramolecular Nano-Assembly and Department of Chemistry, Yonsei UniVersity, Seoul 120-749, Korea,
and Department of Materials Science and Engineering, Polymer Research Institute, Pohang UniVersity of Science
and Technology, Pohang 790-784, Korea
Received September 5, 2008; E-mail: mslee@yonsei.ac.kr
The design of novel self-assembling molecules offers an op-
portunity to extend our understanding how to create unique
supramolecular structures with desired functions.1 Diverse self-
assembling molecules including dendrimers,2 liquid crystals,3 and
block copolymers4 are being created as a means of manipulating
aggregation structure. Incorporation of a stiff rodlike building block
into a block molecular architecture is particularly attractive for
generating highly defined self-assembled structures that have
Figure 1. Molecular structure of 1 and 2.
physical dimensions as small as a few nanometers.5 In addition to
the conventional layered structures, the rod building blocks self-
assemble into a wide variety of complex geometries, including
perforated layers, columns, and bundles, through covalent attach-
ment of long flexible chains to their distal part.6 Increasing volume
fraction of coil segments drives the layers to break up into smaller
domains including columns in which the rods are arranged
perpendicular to the column axis. Previously, we have shown that
the macrocyclic attachment of a flexible chain into both ends of a
rod frustrates the formation of two-dimensional layers commonly
observed for linear rod-coil molecules.7 Instead, the rod segments
are driven to aggregate in one dimension to form flat ribbons in
which the rod segments are arranged again perpendicular to the
ribbon axis.
We present here the formation of stepped strips from the self-
assembly of molecular rods with laterally attached flexible chains.
The rod segments within the strips are aligned parallel to each other
Figure 2. (a) SAXS pattern of 1, (b) TEM image of ultramicrotomed film
to form discrete sheets which subsequently stack together with a
partial overlap to form a stepped 1D structure. The self-assembling
rod molecules that form these aggregates consist of an oligo(p-
phenylene) rod and penta(ethylene oxide) branched chains that are
grafted together at the mid part of the rod (Figure 1).8,9
The melting temperature of 1 based on an octa(p-phenylene) rod
determined from differential scanning calorimetry (DSC) heating
was observed to be 55 °C, whereas that of 2 based on a dodeca(p-
phenylene) rod is 207 °C. The small-angle X-ray scattering (SAXS)
of 1 in the solid state shows a number of well-resolved reflections
that correspond to a 2-D oblique structure with lattice constants of
a ) 2.8 nm and b ) 2.0 nm and a characteristic angle of 73° (Figure
2a).
The wide-angle X-ray scattering (WAXS) shows a reflection
centered at a q spacing of 14.3 nm-1, which is due to crystal
packing of the rod segments within the aromatic domains with an
inter-rod distance of 4.4 Å (See Supporting Information, Figure
S2). When microtomed films of 1 stained with RuO4 were
characterized by transmission electron microscopy (TEM), the
images showed a 2-dimensional array of dark aromatic domains in
of 1 revealing 2-D oblique columnar array of aromatic stepped core, and
(c) schematic representation of the stepped strip from 1.
a matrix of light-colored dendritic chains as well as views in the
direction of the strip (Figure 2b). The interdomain distances are
approximately 2.5 nm which is well matched with those obtained
from the SAXS data.
Considering the calculated rod length of 3.8 nm, these dimensions
are too small to be interpreted as a perpendicular arrangement of
the rods to strip axis. Therefore, parallel arrangements of the rods
along the strip axis would be an appropriate model. On the basis
of the lattice constants and measured densities, the average number
of rod units per cross section of the strip is calculated to be ∼7.
Considering that the ratio of the lattice parameters a and b is
deviated from unity (a/b ) 1.4), this number of molecules suggests
that the rods are packed parallel to each other into stacks of four
rods to produce monolayer sheets with a width of 1.43 nm (Figure
2c). The individual monolayer sheets subsequently stack together
with an overlap of half of the rod length to form stepped strips
that self-organize into a 2-D oblique lattice.
Molecule 2 based on a dodeca(p-phenylene) rod also self-
organizes into a 2D structure. The small-angle X-ray diffraction
shows a number of reflections which can be indexed as a 2D oblique
† Center for Supramolecular Nano-Assembly, Yonsei University.
‡ Department of Chemistry, Yonsei University.
§ Pohang University of Science and Technology.
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14448 J. AM. CHEM. SOC. 2008, 130, 14448–14449
10.1021/ja807035j CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
a parallel arrangement of the rods and the resulting entropic penalty
associated with stretching of the lateral flexible chains.12
Another interesting point to be noted is that 1 molecules based
on a short rod self-assemble into monolayer sheets, whereas 2
molecules based on a long rod self-assemble into bilayer sheets.
This notable contrast in sheet structure with small variation in rod
length of the molecule could be understood by considering the
similar way monolayers are formed in preference to a bilayer
lamellar structure to reduce chain stretching in smectic liquid
crystals.14 Such packing arrangements of conjugated rods may offer
intriguing potentials for nanowires and organic semiconductors.
Acknowledgment. This work was supported by the Creative
Research Initiative Program of the Ministry of Education, Science
and Technology of the Korean Government and the Korea Science
and Engineering Foundation (KOSEF-R01-2007-000-11831-0, ES).
D.H, E.L, and M.H. thank a fellowship of the BK21 program of
the Ministry of Education, Science and Technology of the Korean
Government.
Figure 3. (a) X-ray diffraction pattern of 2, (b) TEM image of ultrami-
crotomed film of 2 revealing 2-D oblique columnar array of aromatic stepped
core, and (c) schematic representation of stepped ribbon from 2.
structure with lattice constants of a ) 2.8 nm and b ) 4.4 nm and
a characteristic angle of 54° (Figure 3a). The TEM image stained
with RuO4 shows an oblique array of dark spots in a matrix of
light-colored branched chains (Figure 3b). The image reveals that
the interdomain distances are roughly 3 nm, which is comparable
to that the lattice parameters obtained from SAXS. Considering
the rod length (5.6 nm by CPK), these dimensions suggest again
that the rods are arranged parallel to the strip axis.
Supporting Information Available: Details of the synthesis,
analysis, structural characterizations, and property measurements. This
material is available free of charge via the Internet at http://pubs.acs.org.
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