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and 2b which have an additional methylene linkage are highly
soluble in the above solvents.
structures based on discotic molecules are aligned along the
orientation direction, for example, along the shearing direc-
tion, and thus along the filament axis (Figure 1b).[10] The
alignment of (macro)molecules usually depends strongly on
their molecular aspect ratio.[11] The larger the aspect ratio of
the building blocks, the higher the possibility for an orienta-
tion of the molecules with their molecular axis along the
shearing direction, as is the case for conjugated polymers.[11]
To our knowledge, this is the first case of low-molecular-
weight discotic molecules aligned with their planes along the
mechanical alignment direction.[12] This unusual orientation
of the columns highlights not only (p-stacking) interactions
between single building blocks leading to the columnar
structures, but also intercolumnar interactions, as observed
for 1a. The unique property of 1a, which has an alternating
attachment of the polar ester groups and apolar alkyl chains,
can be ascribed to a phase separation of the substituents in the
disc periphery of 1a as well as intermolecular dipole
interactions induced by ester groups, which are responsible
for the observed in-plane interaction and the orientation of
the discs. Neither 1b (see Figure S10a in the Supporting
Information) nor 2a (Figure 1b) reveal this unexpected
orientation, thus indicating the crucial role of the substitution
pattern and molecular symmetry. This special alignment
under shearing has so far only been observed for high-
molecular-weight main-chain discotic polymers consisting of
covalently linked triphenylenes. In these systems the columns
are aligned perpendicularly to the oriented polymer chains, as
schematically illustrated in Figure 2a.[13] Our case utilizes a
more complex supramolecular approach based on noncova-
lent forces between monomers. It can be assumed that the
strong intermolecular dipole interactions between the ester
groups of individual building blocks of 1a lead first to a 2D in-
plane hexagonal network possessing the necessary aspect
ratio to be oriented in the above-described specific way
(Figure 2b); during the further assembly steps under align-
ment, a 3D hexagonal columnar array is thus formed through
p-stacking interactions.
1
Compound 1b reveals a remarkably resolved H NMR
spectrumat 140 8C (see Figure S5 in the Supporting
Information), whereas the signals of 1a are still broadened
at this temperature, and exhibit a significant upfield shift
(HBC core at d = 7.58 ppm, peripheral phenyl protons at d =
6.70 ppm).[7] The solution UV/Vis and fluorescence spectra
(5.0 10À6 m, see Figure S6 in the Supporting Information)
show the absorption bands of 1a (tailing to 490 nm) are
broader than those of 1b, and the emission band at 549 nm for
1a is bathochromically shifted by 26 nm compared to that of
1b (523 nm).[7] Thus, the self-assembly of 1a and 1b in
solution is significantly different, thus emphasizing the role of
the ester groups directly attached to the central aromatic
system.
Two-dimensional wide-angle X-ray scattering (2D
WAXS) experiments on mechanically oriented filaments
reveal the self-assembly of all four investigated compounds
(1a, 2a, 1b, and 2b) into discotic columnar arrays.[8] Within
the hexagonal columnar unit cells, the packing parameters
(3.00 nmfor 1a, 2.90 nmfor 2a, 2.97 nmfor 1b, and 2.95 nm
for 2b) and the p-stacking distance of 0.35 nmare very close
to those of hexaalkyl-substituted hexaphenyl HBCs.[9] How-
ever, 1a shows a unique columnar alignment in the extruded
filaments since its columns are arranged perpendicular to the
alignment direction (Figure 1a). In general, columnar super-
The self-assembly of suitably decorated disc-type mole-
cules does not only lead to columnar arrangements in the bulk
phase, but also to the growth of fibrous nanostructures from
solution.[14] Thus, the polar solvent methanol was added to a
solution of 1a in THF (1.0 10À5 m). Although no gelation or
precipitation were observed upon increasing the THF/MeOH
ratio up to 1:1, the bands of the UV/Vis and fluorescence
spectra show a significant broadening and tailing (see
Figure S7a and S7b in the Supporting Information), which
indicates pronounced formation of aggregates. Precipitated
filaments were formed within several minutes, and then drop-
cast at roomtemperature. Interestingly, fibrous structures
were obtained with a diameter of around 100–200 nm and
several micrometers in length (Figure 3a and see Figure S8a
in the Supporting Information). Taking into account the
molecular size, the submicrometer-sized fibers consist of 50–
100 bundles of stacked molecular wires of 1a. These results
again confirmthe strong aggregation and p-stacking tendency
of 1a to allow growth of the fibrous superstructures. The high-
resolution transmission electron microscopy (HRTEM)
images (Figure 3b) show individual columns along the fiber
Figure 1. 2D WAXS patterns and schematic illustration of the corre-
sponding alignment of the superstructures in the extruded filaments
(the red arrow indicates the extrusion direction) for: a) 1a and b) 2a.
1704
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 1703 –1706
Feng, Xinliang
