Channel structures from self-assembled hexameric macrocycles in laterally grafted bent rod molecules

Article abstract of DOI:10.1021/ja907457h

Internally grafted bent rod molecules consisting of a bent-shaped nona-p-phenylene and different lengths of oligoether chains at the bay position were synthesized and characterized. All of the bent-shaped molecules showed ordered bulk-state structures as

Full text of DOI:10.1021/ja907457h

Published on Web 11/04/2009
Channel Structures from Self-Assembled Hexameric
Macrocycles in Laterally Grafted Bent Rod Molecules
Ho-Joong Kim, Young-Hwan Jeong,† Eunji Lee, and Myongsoo Lee*
Center for Supramolecular Nano-Assembly and Department of Chemistry, Seoul National
UniVersity, 599 Kwanak-ro, Seoul 151-747, Republic of Korea
Received September 3, 2009; E-mail: myongslee@snu.ac.kr
Abstract: Internally grafted bent rod molecules consisting of a bent-shaped nona-p-phenylene and different
lengths of oligoether chains at the bay position were synthesized and characterized. All of the bent-shaped
molecules showed ordered bulk-state structures as characterized by differential scanning calorimetry, X-ray
scatterings, and transmission electron microscopy. The bent rod based on a short oligo(propylene oxide)
chain self-assembles into a 2-D channel-like columnar structure, whereas the molecules with an intermediate
length of flexible chains self-assemble into discrete channels that self-organize into honeycomb layers. A
further increase in the length of the flexible chain induces a layered structure. In contrast to the bent-
shaped molecules based on a linear chain, the molecules based on a branched chain self-assemble into
an inverted 2-D columnar structure with an aromatic core surrounded by branched chains. We proposed
the model of the channel structure on the basis of experimental data obtained from X-ray results and density
measurements. Within the channels, six bent rods self-assemble into hexameric macrocycles that stack
on one another to form channel-like columns where the interiors are filled by the flexible oligoether chains.
Remarkably, the elongated channels break up into discrete channels of a well-defined length with increasing
length of the oligoether chain. The resulting discrete channels self-organize into a hexagonally ordered
honeycomb layer. The defined length of a channel is believed to be responsible for the formation of unique
honeycomb layers.
scrolled layers and stepped strips.4 Although this rod-coil
concept has been widely exploited in the assembly of elongated
Introduction
A major challenging task in supramolecular chemistry is the
design of simple molecular components that are capable of
organizing into complex nanostructures, the essence of which
is self-assembly through various types of intermolecular interac-
tions.1 Among their constituting units, aromatic rod building
blocks have proven to be particularly interesting due to their
great potentials as electrical and optical materials.2 Self-
assembled nanostructures of molecular rods can be manipulated
by incorporation of flexible coils into the rod blocks.3 The
supramolecular structures are precisely controlled by systematic
variation of the type and relative length of the respective blocks.
Recently, the rigid-flexible combination in a molecular archi-
tecture has been extended to laterally grafted rod-coil molecules
which organize into a unique solid-state structure such as
rod segments, only a few examples of bent rod systems have
been reported.5,6 Lateral incorporation of a flexible coil into a
bent rod is expected to lead to dramatic changes in self-assembly
behavior, since the assembly of bent rods would, in effect, give
rise to a curved assembly as opposed to flat local structures. In
particular, we envisioned that, when the bent rods are internally
grafted by a relatively short flexible coil, they may form self-
assembled macrocyclic units with internal coil segments as a
consequence of shape complementarity and phase separation
of rigid and flexible blocks. Recently, internally grafted bent-
core molecules have been reported to self-assemble into 2-D
honeycomb structures which are the inverse of the columnar
structures formed from conventional bent-core mesogens.6 The
aromatic cores containing hydroxyl groups of the molecules
assemble to form channel walls through, predominantly, specific
hydrogen bond interactions.
In this paper, we present the formation of 2-D and 3-D
channel structures from self-assembly of internally grafted bent
rod blocks with an oligoether flexible chain. Six bent rods self-
assemble into hexameric macrocycles that stack on one another
to form channel-like columns where the interiors are filled by
the flexible oligoether chains. Notably, the long channels break
up into discrete channels of a well-defined length with increasing
† Present address: Department of Chemistry, Yonsei University, Korea.
(1) (a) Elemans, J. A. A. W.; Rowan, A. E.; Nolte, R. J. M. J. Mater.
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Anderson, H. L. Angew. Chem., Int. Ed. 2007, 46, 1028–1064.
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3892. (b) Ryu, J.-H.; Lee, M. Struct. Bonding (Berlin) 2008, 128, 63–
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Mater. 2005, 4, 399–402. (d) Lee, M.; Cho, B.-K.; Kim, H.; Lee, J.-
Y.; Zin, W.-C. J. Am. Chem. Soc. 1998, 120, 9168–9179.
(4) (a) Hong, D.-J.; Lee, E.; Lee, J.-K.; Zin, W.-C.; Han, M.; Sim, E.;
Lee, M. J. Am. Chem. Soc. 2008, 130, 14448–14449. (b) Hong, D.-J.;
Lee, E.; Jeong, H.; Lee, J.-K.; Zin, W.-C.; Nguyen, T. D.; Glotzer,
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A R T I C L E S
Kim et al.
Scheme 1. Molecular Structures of 1-6
Table 1. Thermal Transitions and Corresponding Enthalpy
Changes Determined from the Second Heating DSC Scansa
compd
fcoil
phase transitions (°C) and corresponding enthalpy changes (kJ/mol)
1
2
3
4
5
6
0.32
0.39
0.52
0.59
0.50
0.56
2-D-Chhex 218.7 (31.1) i
3-D-Chhex 88.0 (2.7) 2-D-Chhex 216.2 (20.9) i
3-D-Chhex 78.8 (4.3) 2-D-Chhex 194.1 (13.5) i
Lam 183.6 (14.4) i
2-D-Colob 159.2 (19.3) i
2-D-Colob 110.5 (15.1) i
Figure 1. Small-angle X-ray diffraction patterns of (a) 1, (b) 2, (c) 3, (d)
3 (at 180 °C), (e) 4, (f) 5, and (g) 6 [(a)-(c) and (e)-(g) at room
temperature].
a Key: Chhex
columnar, i ) isotropic.
) hexagonal, Lam ) Lamellar, Colob ) oblique
length of the oligoether chain. The resulting channels self-
organize into a honeycomb layer. The internally grafted bent
rod molecules that form these structures consist of a bent-shaped
nona-p-phenylene and an oligo(propylene oxide) chain at the
bay position.
Results and Discussion
The synthesis of the bent rod block molecules was performed
with the preparation of a bent-shaped aromatic scaffold starting
from a Suzuki coupling reaction with a 2,6-dibromophenol
derivative and trimethylsilyl-substituted biphenylboronic acids.7
The final block molecules were synthesized by an etherification
reaction of an appropriate oligo(propylene oxide) chain and the
phenolic intermediate and then a Suzuki coupling reaction with
a biphenylboronic acid. See Scheme 1 for the structures of bent
rod block molecules 1-6.
The self-assembling behavior of the bent rod block molecules
in the bulk was investigated by means of differential scanning
calorimetry (DSC), X-ray scatterings, and transmission electron
microscopy (TEM). All of the block molecules show an ordered
structure, and the transition temperatures were determined from
DSC scans (Table 1). As confirmed by small-angle X-ray
scatterings, 1 based on a short penta(propylene oxide) chain
self-assembles into a 2-D hexagonal structure with a lattice
constant of 4.90 nm (Figure 1). The wide-angle X-ray diffraction
pattern shows several sharp reflections that can be indexed as
a monoclinic lattice with unit cell dimensions of a ) 0.51 nm,
b ) 0.76 nm, and c ) 1.11 nm together with a characteristic
Figure 2. Wide-angle X-ray diffraction pattern of 1 at room temperature.
angle of 92.6° (Figure 2), indicating the rod building blocks
within the columns are stacked with a π-π stacking distance
of 0.38 nm along the column axis. On the basis of these results
and the measured density, the number of molecules in a single
slice of the column could be calculated to be six. Therefore,
we consider that the six bent rods in a single slice are packed
in a hexameric macrocycle in which the flexible chains are filled
inside the aromatic frameworks. Subsequently, the self-as-
sembled macrocycles are stacked on top of each other to form
channel-like columns that self-organize into a 2-D hexagonal
structure (Figure 3).
To explore the molecular origin of the intracolumn structure,
molecular modeling was carried out using the COMPASS
empirical force field calculation. Energy minimization of six
molecules reveals that a cyclic arrangement of the molecules is
energitically favorable, as shown in Figure 3. A side of the
hexagon is made up of partial overlap between two adjacent
bent rods. The outer and inner diameters of a hexameric cycle
(7) Miyaura, N.; Yanai, T.; Szuki, A. Synth. Commun. 1981, 11, 513–
519.
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A R T I C L E S
Figure 4. TEM images of ultramicrotomed films of 3 stained with RuO4
revealing (a) the top view of a channel-like structure (inset image; 2-D
Fourier transformation of 6-folded symmetry characteristic of a hexagonal
structure) and (b) the side view of a columnar array of alternating light-
colored aliphatic and dark aromatic layers.
inside the aromatic frameworks. Considering the π-π stacking
distance of 0.38 nm determined from wide-angle X-ray diffrac-
tion, a column long axis for both molecules consists of 12 slices
with a length of 4.56 nm, and thus, the coil layer thicknesses
are estimated to be 0.47 and 1.66 nm for 2 and 3, respectively
(Figure S6 and Table S4, Supporting Information). Upon
heating, the discrete channels hexagonally arranged in a layer
plane are slipped with each other along their long axes, while
the in-plane hexagonal order remains essentially unaltered.
Consequently, the 3-D structure transforms into a 2-D structure
in which the discrete channels are located in a random way along
the c-axis without affecting the in-plane order (Figure 6), similar
to a transition from a smectic B phase to a hexagonal columnar
phase in liquid crystalline polymers.8
It has been reported that the 2-D columnar liquid crystals
formed from bent-shaped mesogens and bolaamphiphilic sys-
tems transform into 3-D discrete columns on cooling.9,10 The
core parts of these columns, however, consist of aromatic
segments, as opposed to the discrete channels. The hexagonally
ordered channel structure has also been observed in other
internally grafted bent-core molecules.6 However, it is only 2-D,
in contrast to our 3-D structure. In addition, the 2-D channel
walls consist of short aromatic segments and hydrogen bonds
which provide strong cohesive forces. On the other hand, the
walls of our discrete channels consist of only aromatic stackings
formed through weak nonspecific aromatic interactions, indicat-
ing that there is strong demixing of the rigid aromatic and
flexible aliphatic segments in the laterally grafted bent rods.
In contrast to the molecules containing short chains, 4 based
on a long flexible chain forms a layered structure, as confirmed
by X-ray diffraction (Figure 1). The layer thickness is 5.59 nm,
that is, smaller than the calculated molecular length (6.5 nm by
CPK models), and suggests that the molecules are packed with
an interdigitation. The formation of the layers is further supported
by TEM experiments, as shown in Figure S4, Supporting Informa-
tion. The layered structure implies that the bent rods assemble into
an unfolded zigzag arrangement instead of a macrocyclic arrange-
ment to maintain homogeneous density without sacrificing π-stack-
ing interactions (Figure 6b).
Figure 3. Schematic representation of a proposed mechanism for the
formation of a 2-D hexagonal channel structure of 1.
are calculated to be 4.9 and 4.2 nm, respectively, consistent
with the experimental results.
In contrast to 1, both 2 and 3 based on a longer oligo(pro-
pylene oxide) chain show two phase transitions, as confirmed
by DSC scans (Table 1). For both molecules at higher
temperatures, small-angle X-ray patterns show several sharp
reflections corresponding to a 2-D hexagonal lattice (Figures
1d and S3, Supporting Inforamtion). The lattice constants are
4.94 and 5.05 nm for 2 and 3, respectively, which are values
very similar to that of 1 (4.90 nm). Similarly to 1, 3 reveals a
number of sharp wide-angle X-ray reflections that can be
indexed as a monoclinic unit cell with nearly the same
dimensions as those of 1. Upon cooling to the lower temperature
structure for both molecules, however, an additional strong
reflection next to the primary peak of the columnar lattice
appears without any discernible change in the peak positions
associated with the hexagonal lattice (Figure 1b,c). On the other
hand, the wide-angle X-ray diffraction patterns remain identical
upon cooling, indicating that the local packing of the aromatic
segments within the column does not change at the phase
transition (see the Supporting Information). Therefore, this
additional peak can be considered to arise from long-range order
along the columnar axis with values of 5.1 and 6.2 nm for 2
and 3, respectively (Figure S6 and Table S4, Supporting
Information). To further confirm this structure, we cryomicro-
tomed 3 to a thickness of ca. 50-70 nm; we then stained it
with RuO4 vapor and observed it by TEM. As shown in Figure
4, the image shows in-plane order of a hexagonal symmetry in
which light flexible chain domains are regularly arrayed in a
dark aromatic framework. The interdomain distance is ap-
proximately 5 nm, consistent with that obtained from X-ray
scattering. The inset shows a 2-D Fourier transform of the image
that has a 6-fold symmetry characteristic of a hexagonal
structure.
The results described here demonstrate that, as the chain
length of the oligo(propylene oxide) increases, the self-as-
sembled structure of the hexameric macrocycle formed through
self-assembly of the bent rods changes from long channels to
discrete channels to layered structures. This structural evolution
can be explained by considering the microphase separation
On the basis of these results and density measurements, both
2 and 3 self-assemble into discrete channels that self-organize
into a 3-D lattice based on a hexagonal order (Figure 5). Within
the channels, the six bent rods in a single slice are arranged in
a hexameric macrocycle in which the flexible chains are filled
(8) Ungar, G.; Feijoo, J. L.; Percec, V.; Yourd, R. Macromolecules 1991,
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A R T I C L E S
Kim et al.
Figure 5. Schematic representation of a proposed mechanism for the formation of a honeycomb layer structure of 2 and 3.
Figure 7. TEM image of ultramicrotomed films of 6 stained with RuO4
revealing a columnar array of alternating light-colored dendritic and dark
aromatic layers. The inset image at perpendicular beam incidence shows
an oblique columnar array of aromatic segments. The scale bars indicate
10 nm.
Figure 6. Schematic representation of (a) the transformation of 3-D
honeycomb layers into a 2-D columnar structure of 2 and 3 and (b) the
lamellar structure of 4.
To investigate the role of the cross section of the flexible
chain in the channel-like molecular organization, we have
prepared 5 (fcoil ) 0.50) and 6 (fcoil ) 0.56) based on
tetrabranched oligoether chains. The melting temperature is
shown to be 159.2 and 110.5 °C for 5 and 6, respectively. Small-
angle X-ray scattering of 5 shows several reflections that can
be indexed as a 2-D oblique columnar structure with a lattice
constant of 5.2 nm and a characteristic angle of 72° (Figure 1).
Similar to 5, 6 also organizes into a 2-D oblique columnar
structure with a lattice dimension of 5.6 nm and an angle of
68.4°. This result indicates that the 2-D lattice dimension
increases with increasing volume fraction of the coil segment,
as opposed to the relationship of 2 and 3. When cryomicrotomed
films of 5 stained with RuO4 were characterized by TEM, dark,
more stained 1-D aromatic domains could be observed (Figure
7). In great contrast to 3 shown in Figure 4a, the top view image
in the inset shows a 2-D array of dark rod domains in a light
coil matrix. This result together with the X-ray scatterings
demonstrates that the columns of 5 and 6 consist of aromatic
between the dissimilar parts of the molecule and the resulting
space-filling requirements.11 The molecule based on a short
flexible chain can be arranged with a hexameric cycles that stack
on top of each other to form long channel-like columns in which
the interiors are filled by the flexible chains. On increasing the
coil length, however, the internal cores require more space to
efficiently fill the flexible chains. To allow more volume for
coils to be less confined without sacrificing intercycle π-π
stacking interactions, the long channels will break up into
discrete channels that self-organize into a honeycomb layer.
Further increasing the coil length, the macrocyclic arrangement
of the bent rods eventually transforms into an unfolded zigzag
arrangement to produce a flat interface which allows a greater
volume for the flexible chains to explore compared to that of
the channels, and thus leading to a layered organization.
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Channel Structures in Bent Rod Molecules
A R T I C L E S
complementarity and phase separation of aliphatic and aromatic
segments as an organizing force. This mechanism of macrocyclic
assembly contrasts that of previous supramolecular macrocycles,
which is dominated by attractive specific interactions.12 Fur-
thermore, all of these macrocycles stack to form only 2-D
columnar structures. Although various honeycomb structures
with rigid frameworks have been reported in polyphilic me-
sogens and bent core liquid crystals, they appear to be only
2-D.6,13,14 Several examples of 3-D honeycomb structures have
been reported in rigid-rod-like molecules.15 However, all the
cases are based on rod layers. Thus, it is remarkable that a 3-D
honeycomb structure forms from self-assembly of discrete channels
based on a self-assembled macrocycle. Another interesting point
to be noted is that the hexameric macrocycles self-assembled from
the bent rods are stacked to form discrete structures with a well-
defined length. This is in significant contrast to conventional
columnar structures from dendrimers,16 amphiphilic rods,13
discotic liquid crystals,17 and rigid cycles18 in which the lengths
of the columns are not defined. Although a few examples of
3-D columnar structures have been reported in liquid crystalline
molecules,9,10 these columns consist of aromatic cores, which
are opposed to the channel structure. We anticipate that the
discrete channels with a hydrophilic cavity will provide a novel
strategy to construct transmembrane ion transport channels,19
which will be the subject of further investigation.
Figure 8. Schematic representation of a proposed mechanism for the
formation of an oblique columnar structure of 5 and 6.
domains surrounded by flexible chains. On the basis of these
results and the measured density, the number of molecules in a
single slice of the column could be calculated to be three.
Therefore, we propose that the three bent rods in a single slice
are arranged to form a triangle-shaped aromatic core surrounded
by bulky dendritic chains. Subsequently, the self-assembled
cores are stacked on top of each other to form long columns
that self-organize into a 2-D oblique lattice (Figure 8).
Compared to the molecular organization of 3 with a similar
volume fraction of flexible chains, this result indicates that, as
the cross sectional area of the flexible chain increases, the
channel structures transform into columnar structures through
phase inversion. This structural inversion could be attributed
to larger steric repulsion between branched chains than that
between linear chains. To reduce the repulsive force, the channel
structure changes into a columnar structure that allows more
space for the branched chains to adopt a less strained conforma-
tion. This result implies that the steric hindrance at the aromatic/
aliphatic interface plays a crucial role in the self-assembled
structure of laterally grafted bent rods.
Acknowledgment. This work was supported by the Creative
Research Initiative Program of the Ministry of Science and
Technology, Korea. This work was partly performed at Yonsei
University.
Supporting Information Available: Synthetic and other
experimental details. This material is available free of charge
via the Internet at http://pubs.acs.org.
JA907457H
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We have demonstrated that internally grafted bent rod blocks
with a linear oligoether flexible chain self-assemble into unique
supramolecular structures in the bulk. Six bent rods self-
assemble into hexameric macrocycles that stack on one another
to form 2-D channel-like columns where the interior is filled
by the flexible oligoether chains. As the chain length of the
oligo(propylene oxide) increases, the long channels break up
into discrete channels and finally transform into a layered
structure. In contrast, the bent rods based on a branched chain
self-assemble into an inverted 2-D columnar structure consisting
of an aromatic core surrounded by flexible chains. The most
notable feature of the bent rod building blocks investigated here
is their ability to self-assemble into a 3-D structure based on
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