Self-organization of bent rod molecules into hexagonally ordered vesicular columns

Article abstract of DOI:10.1021/ja3060904

Bent-shaped rigid-core molecules with flexible chiral dendrons grafted to the outer side of the bend were synthesized and characterized by circular dichroism, differential scanning calorimetry, X-ray scatterings, and transmission electron microscopy in solution and the solid state. The bent aromatic rods based on hepta- and nonaphenylene with nitrile groups at both ends self-assemble into well-ordered hollow tubular structures in aqueous solution, while the bent rod based on heptaphenylene without nitrile groups showed no apparent aggregations in aqueous solution. In the solid state, the rigid-flexible molecules based on heptaphenylene rod without the nitrile group self-assemble into a 2D oblique columnar structure with the columnar cross-section containing two interlocked molecules. Remarkably, the rigid flexible molecules based on hepta-, nona-, and undecaphenylene with nitrile groups self-assemble into a hexagonal columnar structure with weak 3D order. A model of vesicular channel structure is proposed based on small- and wide-angle X-ray diffraction on oriented fibers, density measurement, reconstruction and simulation of electron density maps, and molecular dynamics simulation. In contrast to the hollow tubular structure found in solution, in the solid both the outside and the interior of the columns are filled by the pendant aliphatic coils. Filling of the interior of these vesicular channels is made possible by some bent rod molecules turning their obtuse apex inward. One in 7, 2 in 8, and 4 in 10 molecules are thus inverted in a column slice in compounds with hepta-, nona-, and undecaphenylene cores, respectively. These are new examples of vesicular double-segregated columnar structures recently discovered in some dendrons.

Full text of DOI:10.1021/ja3060904

Article
pubs.acs.org/JACS
Self-Organization of Bent Rod Molecules into Hexagonally Ordered
Vesicular Columns
Ho-Joong Kim,†,‡ Feng Liu,∥,‡ Ja-Hyoung Ryu,†,‡ Seong-Kyun Kang,† Xiangbing Zeng,∥ Goran Ungar,*,§,∥
Jeong-Kyu Lee,⊥ Wang-Cheol Zin,⊥ and Myongsoo Lee*,†
§
†Center for Supramolecular Nano-Assembly and Department of Chemistry and School of Chemical and Biological Engineering,
WCU Program of Chemical Convergence for Energy and Environment, Seoul National University, Seoul 151-747, Korea
∥Department of Materials Science and Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, United Kingdom
⊥Department of Materials Science and Engineering, Polymer Research Institute, Pohang University of Science and Technology,
Pohang 790-784, Korea
S
* Supporting Information
ABSTRACT: Bent-shaped rigid-core molecules with flexible
chiral dendrons grafted to the outer side of the bend were
synthesized and characterized by circular dichroism, differ-
ential scanning calorimetry, X-ray scatterings, and transmission
electron microscopy in solution and the solid state. The bent
aromatic rods based on hepta- and nonaphenylene with nitrile
groups at both ends self-assemble into well-ordered hollow
tubular structures in aqueous solution, while the bent rod based on heptaphenylene without nitrile groups showed no apparent
aggregations in aqueous solution. In the solid state, the rigid-flexible molecules based on heptaphenylene rod without the nitrile
group self-assemble into a 2D oblique columnar structure with the columnar cross-section containing two interlocked molecules.
Remarkably, the rigid flexible molecules based on hepta-, nona-, and undecaphenylene with nitrile groups self-assemble into a
hexagonal columnar structure with weak 3D order. A model of vesicular channel structure is proposed based on small- and wide-
angle X-ray diffraction on oriented fibers, density measurement, reconstruction and simulation of electron density maps, and
molecular dynamics simulation. In contrast to the hollow tubular structure found in solution, in the solid both the outside and
the interior of the columns are filled by the pendant aliphatic coils. Filling of the interior of these vesicular channels is made
possible by some bent rod molecules turning their obtuse apex inward. One in 7, 2 in 8, and 4 in 10 molecules are thus inverted
in a column slice in compounds with hepta-, nona-, and undecaphenylene cores, respectively. These are new examples of
vesicular double-segregated columnar structures recently discovered in some dendrons.
the honeycomb frames are constructed by walls of parallel
arranged aromatic rods aligned perpendicular to the resulting
channels. Percec et al. pioneered construction of supra-
molecular porous columns with a hollow center ranging from
a few Angstroms to a few nanometers using self-assembly of
conical dendrons.5 Furthermore, precise control of the apex
length of the conical dendrons generates the doubly segregated
supramolecular channel-like columns and vesicles exhibiting
polyhedral shapes.5e Helicenes with pendant alkyl chains were
shown to form 6-strand helical tubular liquid crystal columns
with a nearly 1 nm central void.5b
Rod−coil block molecules consisting of rigid aromatic rod
and flexible coil segments are excellent candidates for creating
well-defined supramolecular structures via a process of
spontaneous organization.6 In contrast to coil−coil block
molecules, aromatic rod−coil molecules can form well-ordered
structures in spite of the low molecular weights because the
anisometric molecular shape and stiff rod-like conformation of
INTRODUCTION
■
Supramolecular assembly of elaborately designed molecules
offers a new strategy for creating novel nanostructures with
desired shape and size, which are important modules in the
development of nanoclusters and biomaterials.1 The diblock or
triblock molecules, including block copolymers, amphiphilic
rods, dendrons, and peptide derivatives, self-assemble into
diverse nanostructures which are manipulated by precise
control of molecular structure such as the type and relative
length of the respective block.2 Among the various supra-
molecular nanostructures, construction of channel-like struc-
tures has received enormous attention due to their application
potentials in electro-optic materials, biological channel, and
nanopatterning process.3 For instance, the unique channel-like
honeycombs have been developed in liquid crystal (LC) states
by Tschierske and co-workers.4 They have shown formation of
a series of honeycomb-like channel structures from triangles via
squares and pentagons to hexagons and giant cylinder
structures through the self-assembly process from polyphilic
molecules composed of a rigid aromatic core, polar end groups,
and one or two hydrophobic lateral chains. In these LC arrays,
Received: June 22, 2012
Published: July 24, 2012
© 2012 American Chemical Society
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a
Scheme 1. Synthesis of the Bent-Shaped Molecules
a
Reagents and conditions: (a) biphenyl-4-ylboronic acid, Pd(0)Ph3P, Na2CO3, H2O, THF, reflux, 24 h; (b) 4′-formylbiphenyl-4-ylboronic acid,
Pd(0)Ph3P, Na2CO3, H2O, THF, reflux, 24 h; (c) I2, NH3−H2O, THF, rt, 1 h; (d) 4-trimethylsilyl-phenyl-boronic acid, Pd(0)Ph3P, Na2CO3, H2O,
THF, reflux, 24 h; (e) 1.0 M ICl, CH2Cl2, −78 °C, 1 h; (d) 4-trimethylsilyl-biphenyl-boronic acid, Pd(0)Ph3P, Na2CO3, H2O, THF, reflux, 24 h.
the rod segment impart orientational organization. It has been
also experimentally proven that systematic variation of the coil/
rod volume fraction manipulates a variety of supramolecular
nanostructures from lamellar, bicontinuous cubic, columnar, to
cubic morphologies.7 Recently, the rigid−flexible combination
in a self-assembly system has been extended to laterally grafted
bent rod molecules which self-assemble into a unique
supramolecular structure, since assembly of bent rods would,
in effect, give rise to a curved assembly as opposed to flat local
structures.8−10 For example, we have shown construction of 2D
and 3D channel-like structures in bulk state from theself-
assembly of laterally grafted bent-shaped molecules with an
oligoether flexible chain at the internal bay position of the bent
rods.11 Six bent-shaped molecules self-assemble into hexameric
macrocycles through a combination of shape complementarity
and phase separation of dissimilar blocks, and then they stack
into channel-like structures where the interiors are filled by the
flexible oligoether chains. In addition, we recently communi-
cated our preliminary findings on a hollow tubular structure
that can be constructed by self-assembly of hexameric
supramolecular macrocycles of the amphiphilic bent-shaped
aromatic rod with an externally attached oligoether chain in
aqueous solution.12 The supramolecular macrocycles helically
stack on top of one another to form water-soluble cylindrical
tubules which self-dissociate into discrete toroidal stacks in
response to salt addition. Since self-assembly of the laterally
grafted bent rods can be used as a novel scaffold for shape-
persistent macrocyclic frameworks in solution, we are interested
in development of a unique channel-like structure in the solid
state using bent rod molecules.
In this work, we investigate the solution and solid state
structures of diblock molecules consisting of bent-shaped
aromatic rods and oligoethyleneoxide dendritic segments,
which forms a columnar structure with an amorphous channel
from hierarchical assembly. Bent rods form a hexagonal
columnar structure by assembling in a vesicle-like doubly
segregated fashion. Fourier maps and model fitting of X-ray
intensities reveal that while most aromatic cores bend inward
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with their dendrons facing out, some invert having their
dendrons facing inward and filling the hollow interior of the
tubular structure.
RESULTS AND DISCUSSION
■
Synthesis. Synthesis of the bent rod molecules was
performed with preparation of a phenolic intermediate starting
from a Suzuki coupling reaction with a 2,6-dibromophenol
derivative and trimethylsilyl-substituted phenylboronic acids
(Scheme 1). The bent-shaped scaffolds were synthesized by an
etherification reaction of an appropriate oligoether dendritic
chain and a phenolic intermediate and then a Suzuki coupling
reaction with a biphenylboronic acid for 1. Similar to the
synthetic method of that for 1, compound 2 was synthesized by
Suzuki coupling reactions with 4′-formylbiphenyl-4-boronic
acid and subsequent reduction by iodine and ammonia−water.
Design of compounds 2−4 focuses on construction of the bent-
shaped cores with different rod lengths, that is, aromatic
segments based on heptaphenylene, nonaphenylene, and
undecaphenylene units. Suzuki coupling reaction with TMS-
protected phenyl boronic acid or biphenyl boronic acid was
used to increase the length of the bent rods, following
iodination reaction with ICl. The final bent-shaped compounds
3 and 4 were synthesized by Suzuki coupling reactions with 4′-
formylbiphenyl-4-ylboronic acid and subsequent reduction by
iodine and ammonia−water. The resulting compounds were
characterized by 1H NMR spectroscopy and MALDI-TOF
mass spectrometry and shown to be in full agreement with the
expected chemical structures.
Structural Investigation of Aqueous Solutions. The
self-assembling behavior of the bent-shaped molecules in
aqueous solution was investigated using circular dichroism
(CD) and transmission electron microscopy (TEM). Pre-
viously we showed that compound 2 self-assembled into
hexameric macrocycles in aqueous solution by complementary
electrostatic interactions between electron-withdrawing nitrile
groups and electron-donating phenoxy groups.12 The resulting
macrocycles, subsequently, stack on top of each other with
mutual rotations in the same direction to give rise to helical
tubules. For a more systematic study of the hollow tubular
structure in aqueous solutions, we investigated the solution self-
assembling behavior with different bent rod molecules.
Remarkably, compound 1 which has no functional group at
either end of the rod segment is not soluble in aqueous
solution. Note that the difference between 1 and 2 is only the
presence of the nitrile at the ends of the rod segment. This
indicates that the complementary electrostatic interaction
between the rod end and the bay position of rod segments
plays a key role in forming the well-ordered supramolecular
structures. Compounds 2 and 3 which have the nitrile groups at
the ends show the hollow tubular structures in TEM (Figure
1a,b). The hollow interior is clearly visible, and the diameter is
highly monodisperse. The diameter increases from 6.5 to 7.5
nm when rod length increases from hepta- to nonaphenylene,
respectively. Compound 4, which has an undecaphenylene rod
building block, showed poor solubility due to the high
lipophilic volume fraction. Considering that CD spectra of
the aqueous solution of 2 and 3 show a significant Cotton effect
in the spectral region of the aromatic units (Figure 1c), we can
envision that these elongated objects in water are one-handed
helical tubules. This indicates that the molecular chirality is
transferred to the supramolecular assembly.
Figure 1. TEM images of 2 (a) and 3 (b) from 0.02 wt % aqueous
solution (stained with uranyl acetate). (Insets) Tube with dark
interior, indicative of hollow tubular objects. CD spectra of 2 (red line)
and 3 (black line) (0.02 wt % aqueous solution) show a bisignated
Cotton effect in the absorption region of the bent rods segments.
Highly concentrated aqueous solutions of compound 2
showed a birefringent texture which is typical of a liquid
crystalline phase in a polarized optical microscope (POM).
Since these aromatic macrocycles are surrounded by hydro-
philic oligoether dendrons, they display a lyotropic phase in
aqueous solution. Due to the presence of elongated stiff
hexameric columns as shown in the TEM images (Figure 1a), 2
forms nematic liquid crystals in aqueous solution above 2 wt %
(Figure 2a). As concentration increases above 60 wt %, the
nematic fluid transforms into a hexagonal liquid crystal (Figure
2b). This lyotropic behavior in aqueous solution indicates that
hexameric macrocycles are stable and form stiff rod-like
cylinders.
Structural Investigation in the Solid State. The self-
assembling behavior of the bent-shaped molecules in the bulk
was investigated by means of differential scanning calorimetry
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Figure 2. Optical polarized micrographs of the texture exhibited by (a) nematic phase of 2 in 2 wt % aqueous solution and (b) hexagonal columnar
phase of 2 in 60 wt % aqueous solution.
(DSC), X-ray diffractions (XRD), and transmission electron
microscopy (TEM). All of the compounds show an ordered
structure, and the transition temperatures were determined
from DSC scans (Figure 3). As the length of the bent rod
Figure 3. Second heating and first cooling differential scanning
calorimetry traces of bent-shaped compounds 1−4 (10 °C/min). All
samples were prepared from 3 mg of compounds in aluminum cells.
Indium was used as a calibration standard. Phases, transition
temperature (°C), and associated enthalpy change (kcal/mol) are
indicated. Colob = oblique columnar phase; Colh = hexagonal phase; i
= isotropic.
increases, the melting (isotropization) point increases, indicat-
ing that the ordered structure becomes more stable, presumably
due to increasing interaction between rod segments, including
hydrophobic and π−π interactions.
As confirmed by small-angle X-ray scatterings (SAXS), 1
based on a heptaphenylene aromatic segment self-assembles
into an oblique columnar structure with lattice parameters a =
3.90 nm, b = 2.76 nm, and γ = 128.1° (Figure 4a and Table S1,
Supporting Information). The wide-angle X-ray scattering
(WAXS) pattern shows a weak but sharp reflection, indicating
that the bent-shaped building blocks within the columns are
π−π stacked at a 0.46 nm distance along the column axis
(Figure 4a, inset image). On the basis of the XRD results and
measured density, the calculated number of molecules in a
single slice of the column is about two (Table S7, Supporting
Information). Therefore, we consider that the two bent rods in
a single slice are packed in a dimeric associate in which the
flexible chains surround the aromatic segments. This is
supported by the reconstructed electron density map as
Figure 4. Compound 1: (a) powder SAXS diffractogram (inset,
WAXS). (b) Reconstructed electron density map of the oblique
columnar phase (color scale at the bottom). (c) Snapshot after
molecular dynamics annealing; aromatic cores are shown in green.
shown in Figure 4b (for the procedure see the Supporting
Information). The map shows an elliptical column of high
electron density (blue/purple) formed by the aromatic
segments. Rows of these aromatic ribbons are separated by a
layer of low electron density (yellow/red) which contains the
aliphatic dendrons. To test the viability and space filling of this
model further, molecular dynamics annealing was carried out.
Two molecules of compound 1 were arranged in an oblique
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prism box with experimental lattice parameters and a height of
0.46 nm, under the 3-D periodic boundary condition. Thirty
temperature cycles of constant volume (NVT) dynamics were
run between 300 and 700 K, with a total annealing time of 30
ps, using the Forcyte module of Material Studio (Accelrys Inc.).
Figure 4c shows a snapshot of one molecular layer (c = 0.46
nm) after annealing; the simulation is in fair agreement with the
electron density map, confirming the efficient space filling and
phase separation achieved by this structure.
Compounds 2−4 were functionalized at both ends of the
aromatic core with nitrile groups. Addition of nitrile groups in
compound 2, which has otherwise the same heptaphenylene
aromatic core as 1, significantly changes the mode of molecular
self-assembly. The SAXS pattern of 3 exhibits a number of
sharp reflections corresponding to a 2D hexagonal lattice with a
parameter a = 7.43 nm (Figure 5a and Table S3, Supporting
Information). The SAXS patterns of 2, based on a
heptaphenylene aromatic segment, and 4, based on an
undecaphenylene aromatic segment, give similar SAXS patterns
with a = 6.34 and 8.27 nm, respectively, see Figure S1 and
Tables S2 and S4, Supporting Information.
Figure 5b shows an optical micrograph of 3 in the hexagonal
phase taken with crossed polarizers and a full-wave plate
retarder. The orientation of the π-conjugated aromatic cores is
deduced from the color of the fans in the “spherulites”. The
directions of yellow and blue fans with respect to the indicatrix
of the retarder plate confirms the orientation of the high-index
axis as radial to the fans. Since the columns are tangential and
the high index axis is parallel to the oligophenylene long axis, it
follows that the rod-like molecular cores are in the plane
perpendicular to the column axis, which supports the column
structure of these self-assemblies as described.
On the basis of our previous findings of hollow tubular
structure in aqueous solution, we hypothesized similar packing
in the solid state, i.e., that six bent-shaped aromatic segments
might be arranged in a single slice with the terminal nitrile
groups juxtaposed with the bay position of the adjacent
molecule to form a hexameric macrocycle. The resulting
macrocycles could stack on top of each other to form a
hexagonal column structure with a hollow center, as depicted in
Figure 6.
Another interesting feature of the SAXS patterns of 2−4 is
the unusually strong high-order diffraction peaks of the
hexagonal lattice. As previously reported, the enhanced
diffraction peaks can be caused by the presence of a hollow
or an electron-deficient center, reflected in the particular shape
of the form factor of such columns. For example, the inverse
hexagonal lyotropic liquid crystalline phases where the water
channel in the core of the columns cause enhanced higher
order peaks.13 The cubic and columnar structures with hollow
or aliphatic low electron density centers also show strong
higher order reflections.5 These qualitative considerations
would seem to lend credence to the hexameric macrocycle
model with a hollow center (Figure 6). However, such a
packing model leaves a large empty space which is likely to be
energetically prohibitive in solvent-free condensed matter. Of
course, in addition to the form factor, another possible reason
for intense higher diffraction orders is a high positional order
parameter (Debye−Waller factor). The order is indeed likely to
be higher than in classic “disordered” columnar liquid crystals
since, as shown below, this hexagonal phase possesses a degree
of 3-D long-range order.
Structural Investigation in Oriented Fiber. To inves-
tigate molecular assembly in the solid in more detail, we
performed the following further studies. We measured the
macroscopic density of the compounds, carried out X-ray
scattering experiments on oriented fibers in the hexagonal
columnar phase, reconstructed 2-D electron density maps from
SAXS Bragg intensities, as well as simulated the diffraction
patterns using geometric models and comparing the best-fit
models with molecular models obtained by molecular dynamics
(MD) simulation. This concerted effort was directed at
answering the question of how the molecules were arranged
in the column and particularly whether the inside of the column
was empty or not.
Figure 5. Compound 3: (a) SAXS diffractiongrams recorded at 90 and
160 °C. Scattering curves are offset vertically for clarity. (b) Optical
micrograph in the hexagonal phase between crossed polarizers and
with a λ-plate retarder; indicatrix of the retarder is shown at bottom
right; scale bar is 20 μm. (c) WAXS of a partially aligned sample at 90
°C (fiber axis vertical). (d) Radial scan of the fiber pattern in the
limited wide-angle region of the π-stacking diffraction peaks (“first
layer line”). (e) Drawn outline of the first layer line reflections (first
quadrant) for a hypothetical fiber with a higher degree of orientation;
figure shows clustering into four groups of unresolved reflections,
observed as four components in d. This fixes the molecular spacing
along the columns precisely at 0.43 nm (see Table S6, Supporting
Information).
It was important to determine the number of molecules in a
single transverse slice of the column, one molecule thick. This
can be calculated from experimental density and the volume of
the slice. The area of the slice is easily obtained from powder X-
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Figure 6. Schematic representation of proposed self-assembly for formation of hexameric macrocycle.
ray diffraction, but the thickness was not clear until oriented
fibers of 2 and 3 were prepared by extrusion in a homemade
microextruder. As seen in Figures 5c and S2, Supporting
Information, the fiber patterns contain sharp equatorial
reflections in the small-angle region, matching exactly the
hexagonal hk0 diffraction peaks in the powder patterns; these
correspond to lattice parameters a = 6.34 and 7.43 nm for 2
and 3, respectively (Tables S2 and S3, Supporting Informa-
tion). In the wide-angle region, there is a series of closely
spaced moderately sharp arcs straddling the meridian. While
the degree of orientation of the fiber was insufficient to index
these reflections directly based on their ξ,ζ cylindrical reciprocal
space coordinates, we could retrieve the hkl indices by finding
the best-fit c lattice parameter through matching of their
measured d spacings (Figure 5d) to those of the first layer line
(hk1) reflections of a 3D hexagonal unit cell of known a, see
Table S6, Supporting Information. The positions of the
azimuthally narrowed Bragg arcs corresponding to the matched
reflections are depicted in Figure 5e. From this indexing, the c
parameter is found to be 0.432 0.004 nm for compounds 2
and 3. On the basis of the measured density (1.18 and 1.20 g/
cm3 for 2 and 3−4, respectively), we arrive at the average
number of molecules in a single slice of the column as 7.1, 8.4,
and 9.6 for 2, 3, and 4, respectively (Table S7, Supporting
Information). These numbers invalidate the model of
hexameric macrocycles and suggest a more disordered
arrangement of the aromatic cores.
Reconstruction of Electron Density Maps, and
Figure 7. Electron density maps of the hexagonal phase of compound
2. (a,b) Three-level geometric model of electron density. (c,d) Fourier
approximations of models I (a) and II (b) using 5 Fourier terms, the
same number as the number of X-ray reflections used in
reconstruction of the corresponding experimental maps (e,f). (e,f)
Reconstructed from experimental diffraction intensities using phases
from best-fit models I (c) and II (d). Match between the model and
experiment should be judged by the similarity of the top and bottom
maps. Color scale is at the bottom.
Simulation of Diffraction Patterns. To resolve the question
of what is in the center of the columns, a 3-level geometric
model of electron density (ED) was built on a 2D hexagonal
lattice. The model consisted of a ring of reference electron
density ED = 1, the outside continuum with reference ED = 0,
and an inner circle, the density of which was an adjustable
parameter H (height) or D (depth), respectively, for models I
and II. The cross-section ED profiles are shown in Figure 7,b.
Both models correspond to a core−shell structure. Fourier
coefficients, i.e., diffraction amplitudes, were calculated analyti-
cally. The variables (fitting parameters) were the outer and
inner radii of the ring and the value of H (D). The two models
gave fittings to the experimentally measured intensities with a
similar fitting quality R2 (Table 1 and Tables S8−S10,
Supporting Information). However, while the D values from
model I varied drastically between the three samples, from
−3.29 to −0.77, the H values from model II were rather similar
for all three samples, ranging only from 0.53 to 0.64 (Table 1).
On this basis, it is concluded that model II is the correct
representation of the ED profile across the column. Thus,
simulation of hk0 diffraction intensities supports the model
where the periphery of the column is formed by the aromatic
backbones (high ED), surrounded both outside and inside by
the ethylene-oxide dendrons (lower ED). It should be
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Table 1. Best-Fit Parameters to Electron Density Models I
and II of the Hexagonal Phase of Compounds 2−4
a
Described in Figure 7
b
compd
a, nm
model
Rout, nm
Rin, nm
D or H
R2
2
6.34
I
II
I
2.34
2.56
2.84
3.02
3.37
3.37
1.30
1.25
1.80
1.99
2.15
2.60
−3.29
0.64
0.992
0.999
0.961
0.997
0.988
0.956
3
4
7.43
8.27
−1.26
0.62
II
I
−0.77
0.53
II
a
Five strongest reflections were used. More detailed data and fits to
more reflections are listed in Tables S8−S10, Supporting Information.
b
Variance.
mentioned, incidentally, that gravimetric density measurements
also favor model II over model I (see previous section).
Fourier approximations of model I and II ED maps using the
5 Fourier terms corresponding to the 5 diffraction peaks used in
the model fitting are shown in Figure 7c,d. One way to judge
the goodness of the fit of the model is to compare the
“smeared” models in Figure 7c,d with the ED maps
reconstructed directly from experimental intensities using the
same combination of structure factor phases (+ or − signs,
Tables S8−S10, Supporting Information). The latter “exper-
imental” maps are shown in Figure 7e,f. For the case of
compound 2, the fit of model II (Figure 7d,f) is somewhat
better than that for model I (Figure 7c,d), as also indicated by
the R2 values in Table 1. However, the strongest evidence in
favor of model II is the consistency of the ED maps for the
three compounds when using the phase combination from
model II, see Figure 8. The above analysis illustrates the
importance of comparing ED maps and models for a
homologous series, rather than for one compound alone.4c
Molecular Model. To gain further insight into the
columnar structure of 2−4, we again performed MD simulation
by placing the calculated number of molecules in a single slice
of the columns with the experimentally determined unit cell
dimensions. Thus, e.g., to simulate compound 2 seven
molecules were arranged in a 60° rhombic prism box with a
side length of 6.34 nm and a height of 0.43 nm, under the 3d
periodic boundary condition (Figure 9a). This simulation
confirms the viability of the structure with seven bent molecules
in a single slice packed into a shell of aromatic moieties, with six
molecules having their dendritic groups facing outward and one
molecule turned around for its dendron to fill the internal pore.
In the case of compound 3, the 8-molecule aggregate with two
dendrons facing inward gives good space-filling after MD
annealing (Figure 9b), as does the 10-molecule model of
compound 4 with four molecules turned inward (not shown).
Notably, the number of molecules with the dendrons facing
outward remains constant at six for all three compounds.
According to the best-fit electron density models, the diameters
of the rings of aromatic cores in compounds 2 and 3 are Rin +
Figure 8. Electron density maps of compounds 2, 3, and 4
reconstructed using the first five hk0 diffraction peaks with the
phase combination +−−++ (+ = 0 rad, − = π rad). This phase
combination corresponds to that in model II. Maps are scaled in size
proportional to the size of the unit cell.
It is worth remarking at this point that as the fiber patterns of
2 and 3 show (Figures 5c and S2a, Supporting Information),
there is in fact weak 3D long-range order in these thermotropic
column-like structures, so that one might be tempted to classify
them as soft crystals. The distinct hkl Bragg reflections indicate
weak long-range longitudinal correlation between one-mole-
cule-thick slices across different columns. However, in view of
the nonuniform structure within the column slices, with
individual molecules not having a uniquely defined preferred
position, we still prefer to describe these structures as liquid
crystals. It should also be mentioned that a very weak 0.6−0.7
nm reflection is consistently observed close to the meridian, so
that the reflections around 0.4 nm may strictly speaking be on
the second layer line. The weak 0.6−0.7 nm reflection is in fact
commonly observed in “ordered” columnar systems.5b In more
highly oriented fibers these are seen to be off-meridian and
coming from a doubling of the fiber repeat. In the present
systems the weak 0.6−0.7 nm feature seems to arise from the
herringbone arrangement of phenylene planes with alternating
positive and negative tilt along the column.
R
out = 3.8 and 5.0 nm, respectively. These diameters are in good
agreement with the size of the aromatic circles in the MD
annealed models in Figure 9a,b. The MD simulations, together
with the circular nature of the aromatic shell of the averaged
columns, suggest a relatively disordered arrangement of the
bent cores around the cylinder. This allows incorporation of
numbers of molecules larger than 6 into a column slice, with
the extra molecules inverted. This mode of self-assembly avoids
the high energy penalty of the hollow tubular structure.
Lamellar Phase of Compound 3. Notably, compound 3
showed small-angle Bragg diffraction even above the melting
temperature at 160 °C. The melting has a comparatively large
enthalpy change, and the high-temperature phase appeared
fluid in POM, indicating isotropic phase. As seen in Figure 5a,
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Figure 9. Snapshots of the structures of (a) 2 and (b) 3 after MD annealing (CN ends are colored blue and the bent aromatic cores green). Added
black circles have the median diameter (Rin + Rout) of the high electron density (aromatic) rings in the best-fit model of type II (see Table 1). (c)
Schematic representation of formation of the supramolecular channels.
bottom, the two Bragg reflections are the two orders of a 2.42
nm diffraction. The phase is therefore designated as layered,
with alternating sublayers of rod segments and aliphatic
dendrons. The relatively strong second-order reflection
suggests a somewhat unconventional structure, and indeed,
the 1D electron density profile has a narrow high-density
maximum, purple in Figure 10. This suggest that the aromatic
vesicles can be regarded as highly curved double layers. The
usual situation in diblock amphiphiles is a choice between a flat
double layer and a column in which all A blocks face inward
and all B blocks face outward. Bicontinuous cubic phases,
ribbon phases, and undulated bilayers have been observed as
alternative solutions to the frustration created by steric
imbalance between the two blocks. The new vesicular phases
clearly present an alternative solution, and their understanding
requires further study.
CONCLUSION
■
We demonstrated that bent molecules externally grafted with a
dendritic oligoether chain self-assemble into unique supra-
molecular channel structures in the bulk. The results described
here demonstrate that bent rod molecules 2−4 self-assemble
into supramolecular channels with both the interior and the
exterior filled with the amorphous dendritic side groups. The
channels, consisting of between 7 and 10 molecules in cross-
section, are arranged on a hexagonal lattice with high in-plane
order and weak long-range 3D correlations. Formation of such
a channel structure is attributed to the combination of
noncovalent interactions within the supramolecular macro-
cycles and other factors including electrostatic interactions and
space filling. Although compound 1 has the same molecular
structure as 2 except for the absence of nitrile groups, 1 forms
thin columns based on dimers, i.e., with only two molecules in
cross-section. Such core−shell columnar structures are well
known in rod−coil systems. However, since the bent aromatic
core of molecules 2−4 were functionalized with terminal nitrile
groups, they self-assemble into macrocycles driven by
complementary electrostatic interactions between electron-
withdrawing nitrile groups and electron-donating phenoxy
groups. The resulting cycles are stabilized by additional π−π
interactions. Importantly, the aromatic cylinders have large
interior spaces, between 2.5 and 5.2 nm in diameter, depending
on the length of the bent segments. Although the flexible
dendrons are attached to the outer apex of the bent rod, the
Figure 10. Electron density map of the lamellar phase of compound 3.
Schematic representations of molecules are superimposed. Note that
there is no experimental evidence of in-plane periodicity.
rods are oriented parallel to the layer plane (see schematic
molecules in Figure 10), similar to the organization in the
lamellar phases of thermotropic T-shaped amphiphilic liquid
crystals.4a,14 In fact, the latter have also been observed above
the temperature range of honeycomb structures.4a
The obvious question posed by the present work is why do
compounds 2−4 exhibit the vesicular cylinder structure instead
of the lamellar structure observed in 3 at high temperatures.
Thermotropic mesophases with cylinders or spheres in which
most molecules’ wide ends face out but some face in have been
reported recently in low-taper mesogens and dendrons.5e These
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A.; Liu, F.; Prehm, M.; Kieffer, R.; Zeng, X. B.; Walker, M.; Glettner,
B.; Zywocinski, A. Adv. Funct. Mater. 2011, 21, 1296−1323.
inside of the cylinders is filled by such dendrons since some
molecules are turned with their convex apex facing inside.
Although a few channel-like structures have been reported
previously,4,5 self-assembly of channel-like structures with
controllable diameter is rare in material science and macro-
molecular engineering. We believe that this unique strategy for
constructing channels with defined shape and size will result in
novel ion transport channels and electro-optic nanomaterials.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Synthetic and other experimental details. This material is
available free of charge via the Internet at http://pubs.acs.org.
Spiess, H. W.; Hansen, M. R.; Hoger, S. Angew. Chem., Int. Ed. 2011,
50, 3030−3033.
̈
AUTHOR INFORMATION
Corresponding Author
g.ungar@sheffield.ac.uk; myongslee@snu.ac.kr
■
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Tschierske, C. Angew. Chem., Int. Ed. 2007, 46, 7972−7975.
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Author Contributions
‡These authors contributed equally.
R.; Glettner, B.; Nurnberger, C.; Liu, F.; Pelz, K.; Prehm, M.;
̈
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korean government
(MEST) (nos. 2012-0001240 and 2011-35B-C00024) and by
the European Union FP7 project Nanogold (grant no.
228455). We acknowledge the Pohang Accelerator Laboratory,
Korea, and Diamond Light Source, U.K., for allowing us to use
the synchrotron source. We acknowledge a fellowship of the
BK21 program from the Ministry of Education and Human
Resources Development of Korea. G.U. acknowledges support
from the WCU program through the National Research
Foundation of Korea (R31-10013). For help with the
synchrotron experiments we thank Drs. Nick Terrill and Jen
Hiller (beamline I22) at Diamond Light Source. We also thank
Dr. P. Baker and Prof. D. Rice at the Biochemistry Department,
Sheffield, for allowing use of their wide-angle diffraction setup.
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