Partially stripped insulated nanowires: A lightly substituted hexa-peri-hexabenzocoronene-based columnar liquid crystal

Article abstract of DOI:10.1039/b311651d

We present the facile preparation and characterization of the first "unwrapped" hexa-peri-hexabenzocoronene derivative, which forms a stable columnar liquid crystalline mesophase with a practically accessible isotropization temperature.

Full text of DOI:10.1039/b311651d

Partially stripped insulated nanowires: a lightly substituted
hexa-peri-hexabenzocoronene-based columnar liquid crystal†
Zhaohui Wang, Mark D. Watson, Jishan Wu and Klaus Müllen*
Max-Planck-Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany.
E-mail: muellen@mpip-mainz.mpg.de; Fax: +49 6131 379350; Tel: +49 6131 379150
Received (in Cambridge, UK) 23rd September 2003, Accepted 9th December 2003
First published as an Advance Article on the web 9th January 2004
We present the facile preparation and characterization of the
first “unwrapped” hexa-peri-hexabenzocoronene derivative,
which forms a stable columnar liquid crystalline mesophase
with a practically accessible isotropization temperature.
1,2,3-trihydroxybenzene followed by Williamson etherification
with 1-bromododecane. Hagihara–Sonogashira coupling of 3 with
phenylacetylene gave substituted diphenylacetylene 4 in 85% yield.
Diels–Alder cycloaddition between 4 and commercially available
2,3,4,5-tetraphenylcyclopentadienone in refluxing diphenyl ether
afforded the functionalized hexaphenylbenzene 5 in 90% yield.
Despite strong dependence of its success on heteroatom substitu-
tion patterns,9 the final cyclodehydrogenation step10,11 to 6 was
achieved in 70% yield. Given the vanishing solubility of un-
substituted HBC, “unwrapped” HBC 6 is surprisingly soluble in
common organic solvents such as THF and chloroform.
UV–Vis absorption and fluorescence spectra (CHCl3, 1.0*1026
M) of HBC 6 are shown in Fig. 1. The absorption spectrum is
essentially unchanged with reference to unsubstituted and alkylated
HBCs,12,13 in accord with the relatively weak donor effect of the
alkoxy chains. The profile of the emission spectra is likewise
similar, except that the band corresponding to the 0–0 transition
(470 nm), which is otherwise very weak for the D6h-symmetric
unsubstituted HBC, is “more allowed” due to lower symmetry and
some expected non-planarity caused by the alkoxy substituents in
bay positions.
The thermotropic liquid crystalline behaviour of HBC 6 was
studied by differential scanning calorimetry (DSC), polarized
optical microscopy (POM) and wide-angle X-ray diffraction
(WAXD). The first DSC heating scan (supplementary information)
shows two endothermic transitions, 2nd order and 1st order
respectively, at 50 and 188 °C. Subsequent cooling and heating
scans show two reversible 1st order transitions at 48 °C (Col1–Col2)
(DH = 26.20 kJ mol21) and 190 °C (DH = 218.49 kJ mol21), the
latter corresponding to the isotropization as confirmed by POM.
We attribute the dramatic decrease of the isotropization tem-
perature ( ~ 200 °C lower than alkyl substituted HBCs8b,9) to
reduced symmetry and/or the expected slight nonplanarity of the
HBC mesogenic core. The until recently14 practically inaccessible
isotropization temperatures of hexa-alkylated HBC’s have limited
their optimal processing by thermal means within certain devices or
electrical characterization cell geometries.
Discotic materials are receiving increasing attention in the search
for organic materials which are technologically promising for
electronic devices.1 Disk-shaped molecules carrying multiple
peripheral alkyl substituents spontaneously self-assemble into
columnar structures of face-to-face stacked rigid cores, nanophase-
separated from the side-chains, manifesting manifold unique
superstructures.2 The coaxial mantle of saturated hydrocarbon
chains forms a barrier to charge transport between columnar cores,
leading to their designation as one-dimensional nanowires.3 Device
performance4 might be enhanced by some decrease in the one-
dimensionality of charge-transport, e.g. to provide intercolumnar
bypasses at defects and grain/domain boundaries. To this end, we
propose to partially strip away the insulating wrapping of these
nanowires, leaving residual flexible chains sufficient to ensure
solution processability and phase-forming properties. With few
exceptions, stable columnar phases require more than 3 alkyl
substituents on the mesogenic core,5 and exposure of edges can lead
to other arrangements (e.g. edge-to-face).6 Larger polycyclic
aromatic hydrocarbons favour face-to-face stacking even when
unsubstituted,7 in particular hexa-peri-hexabenzocoronenes
(HBC’s) are especially insensitive in this regard.8 Here we present
the synthesis and characterization of the first “unwrapped”
mesogenic HBC derivative, and report on its bulk self-assembly to
a columnar mesophase which can be mechanically aligned.
The synthetic route towards 1,2,3-tris-dodecyloxy-hexa-peri-
hexabenzocoronene (6) is shown in Scheme 1. Commercially
available 1 was demethylated with BBr3 to give 5-bromo-
Variable temperature 2D-WAXD data collected in transmission
through an extruded fibre15 of 6 clearly indicate a columnar
mesophase, which is sufficiently cohesive that the columns are
Scheme 1 Synthesis of “unwrapped” HBC 6.
† Electronic supplementary information (ESI) available: full experimental
details, differential scanning calorimetry (DSC) scans and MALDI-TOF
spectrum of 6. See http://www.rsc.org/suppdata/cc/b3/b311651d/
Fig. 1 UV-Vis absorption and fluorescence spectra of 6.
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C h e m . C o m m u n . , 2 0 0 4 , 3 3 6 – 3 3 7
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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aligned in the shear direction (inset Fig. 2). The number and relative
intensity of reflexes are increased after annealing at 150 °C, in
accord with the 2nd order endothermic transition seen in the first
DSC heating scan. A hyperbolic set of reflexes with vertices on the
meridian (d = 0.48 nm) indicates columns of face-to-face disks
with their molecular planes inclined 45° relative to the columnar
axes. (Tilt angle calculated from q = arccos(d/d0), d0 = 0.34 nm –
the typical p–p stacking distance).16 The limited diffraction data do
not allow determination of the relative direction of the tilt within
adjacent columns.
Reflexes along the equator, which are better resolved in the
reflection measurement (Fig. 2), arise from positional correlation in
the lateral packing of columns. These are indexed as shown to fit a
two-dimensional rectangular unit cell, the corners of which are
formed by the columns. The large spacing of 4.01 nm could
reasonably be explained by a lamellar arrangement of single rows
of columns, with interstitial layers filled by nanophase-separated
alkyloxy chains. Figure 3 shows a single column segment having
approximately the width matching the lamellar spacing. The short
side of the unit cell corresponds well to double the van der Waals
cross-section of the HBC core, i.e. adjacent columns not separated
by alkyl chains.
focal point, which still preserves the ordered columnar stacking.
Unlike the case of rod-like mesogens with substituents at one or
both ends, for HBC 6 with an aspect ratio ~ 1 and the strong
tendency7 of large area p-systems to form columnar stacks,
cohesive columns rather than molecule planes are aligned in the
direction of shear.
To conclude, the combination of facile synthesis, low isotropiza-
tion temperature, good solution processability, and long-range
order make this new kind of “unwrapped” columnar material a
potential active component in organic electronic devices. Charge-
carrier mobilities and performance of devices prepared from 6 are
the subject of future investigations.
This work was financially supported in part by the Zentrum für
Multifunktionelle Werkstoffe und Miniaturisierte Funktionseinhei-
ten (BMBF 03N 6500), EU-TMR project SISITOMAS, the
Deutsche Forschungsgemeinschaft (Schwerpunkt Feldeffecttran-
sistoren), and the EU project DISCEL (G5RD-CT-2000-00321).
Notes and references
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Fig. 2 Powder WAXD (reflection) of HBC 6. Insets: proposed 2D
rectangular unit cell describing lateral positional correlation of columns (a
= 4.01 nm and b = 2.23 nm) and transmission WAXD pattern of extruded
fibre. Measured at RT after annealing at 150 °C.
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Fig. 3 Stacking model of HBC 6.
C h e m . C o m m u n . , 2 0 0 4 , 3 3 6 – 3 3 7
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