A water-soluble hexa-peri-hexabenzocoronene: Synthesis, self-assembly and role as template for porous silica with aligned nanochannels

Article abstract of DOI:10.1039/b511868a

A water-soluble hexa-peri-hexabenzocoronene was prepared and shown to undergo ordered columnar self-assembly either in water solution or bulk and therefore served as template for the fabrication of porous silica with aligned nanochannels. The Royal Societ

Full text of DOI:10.1039/b511868a

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A water-soluble hexa-peri-hexabenzocoronene: synthesis, self-assembly
and role as template for porous silica with aligned nanochannels{
Jishan Wu,a Jixue Li,b Ute Kolbb and Klaus Mu¨llen*a
Received (in Cambridge, UK) 22nd August 2005, Accepted 20th October 2005
First published as an Advance Article on the web 15th November 2005
DOI: 10.1039/b511868a
compound 5 in 75% yield. Sonogashira coupling of 5 with
trimethylsilylacetylene in refluxing triethylamine gave 6 and
subsequent desilylation with K2CO3 in methanol afforded
compound 7 in 82% yield. Coupling reaction between the insoluble
HBC building block 213 and 7 was performed in piperidine at 50 uC
for 48 hours whereupon the insoluble suspension became soluble
during the reaction. After column chromatography with acetyl
acetate/methanol as eluents, pure waxy compound 1 was obtained
in 72% yield. The use of Pd-catalyzed coupling reaction as the last
step avoided the tedious removal of FeCl3 (soluble in methanol) in
the previous synthetic method.11d
A water-soluble hexa-peri-hexabenzocoronene was prepared
and shown to undergo ordered columnar self-assembly either in
water solution or bulk and therefore served as template for the
fabrication of porous silica with aligned nanochannels.
Amphiphilic conjugated oligomers or polymers1 based on
thienylene,2 phenylene,3 phenylenevinylene,4 and phenyleneethy-
nylene5 units self-assemble into nano- or micro- scale ribbons,
fibers and micelles due to strong p-stacking and hydrophobic
interactions. Studies on their self-assembly open the possibility to
fabricate devices from individual assembled nanostructures in
‘‘supramolecular electronics’’.6 Recently, disc-like molecules
such as triphenylene,7 metallophthalocyanines8 and hexa-peri-
hexabenzocoronenes (HBC)9 have attracted interest because of
their high tendency to aggregate into columnar superstructures
which serve as one-dimensional charge transport channels.10 Their
amphiphilic derivatives formed unique nanostructures in meso-
In chlorinated solvents such as 1,1,2,2-tetrachloroethane, the
existence of non-aggregated individual molecules of 1 was
confirmed by the independency of 1H NMR signals on
phase, solution and thin film.11,12 HBC derivatives substituted by
11d
branched oligo(ethylene glycol) chains
showed extremely
ordered columnar superstructures in the bulk state.
The self-assembled amphiphilic conjugated p-molecules were
also used as templates to fabricate inorganic nanostructures such
as mesoporous silica which have potential applications as catalyst
supports, low-k materials and membranes.13 For example, the
synthesis of ordered porous silica using columnar charge-transfer
assemblies based on triphenylenes substituted by oligo(ethylene
glycol) chains has been reported. However, without the existence
of suitable electron acceptors no aligned silicate nanochannels were
formed due to the lack of long range order in the columnar
structure.11a The HBC has a larger rigid core and promises stronger
p-interactions as well as higher ordered columnar structures.
Therefore, we now report how the assembly of an amphiphilic
HBC 1 (without additional acceptors) in water can be used as
template to fabricate porous silica with aligned nanochannels.
Recently, we prepared an insoluble hexaiodo-peri-hexabenzo-
coronene (2, Scheme 1), and found that it showed high reactivity in
Pd-catalyzed coupling reactions.14 Therefore using such a building
block, compound 1 bearing oligo(ethylene glycol) chains was easily
prepared as shown in Scheme 1. 1-Bromo-3,4,5-trihydroxylben-
zene (3)13b was first subjected to etherification with tosylate-
protected oligoethylene glycol 415 by K2CO3 in DMF and afforded
aMax-Planck Institut fu¨r Polymerforschung, Ackermannweg 10,
55128 Mainz, Germany. E-mail: muellen@mpip-mainz.mpg.de
bInstitute of Physical Chemistry, Johannes-Gutenberg Universita¨t,
D-55128 Mainz, Germany
{ Electronic supplementary information (ESI) available: synthetic details
and structural characterizations for all new compounds; X-ray diffraction
data for the HBC-SiOx. See DOI: 10.1039/b511868a
Scheme 1 i) K2CO3, DMF, 60 uC, 75%; ii) trimethylsilylacetylene,
Pd(PPh3)2Cl2–CuI, triethylamine, 80 uC; iii) K2CO3, MeOH, 82% for ii)
and iii) steps; iv) Pd(PPh3)4–CuI, piperidine, 50 uC, 72%.
48 | Chem. Commun., 2006, 48–50
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Fig. 1 Fluorescence spectra of compound 1 in mixed solvents of
dichloromethane (DCM) and methanol ([1] 5 5.0 x 1026 M), lexc
5
380 nm.
temperature (not shown). However, in polar protic solvents such
as methanol and water compound 1 showed a strong tendency to
aggregate by p-stacking and hydrophobic interactions. The
fluorescence spectra of compound 1 in a mixture of dichloro-
methane and methanol with different volume ratios are shown in
Fig. 1. In pure dichloromethane, a well-resolved fluorescence
spectrum was observed between 470 nm and 600 nm. Upon
addition of methanol, the fluorescence intensity decreased and the
emission maximum shifted to longer wavelength. When the
methanol/dichloromethane ratio reached 7 : 3, an unresolved
band centered at 540 nm was observed. The longer wavelength
band typically arises from the excimer emission, thus gradual
addition of methanol enhances the hydrophobic interactions and
leads to an increase of the size of aggregates.
Fig. 2 a) POM images for 1 recorded at 90 uC during slow cooling from
the isotropic liquid; b) 2D WAXD pattern recorded at 120 uC on an
extruded fiber of compound 1.
stacking of the discs in the column due to the steric hindrance
and space-filling limit during columnar structure formation.13b In
addition, some weak reflexes at the middle-angle area were found
which can be correlated to higher order scattering. After cooling to
room temperature (below the phase transition temperature of 56 uC
observed in the second cooling scan), no obvious changes were
observed in the X-ray diffraction pattern, even after long-time
annealing at 120 uC and at slow cooling rate.
Compound 1 at room temperature displayed a thermotropic
columnar liquid crystalline phase as disclosed by differential
scanning calorimetry (DSC), polarized optical microscopy (POM)
and X-ray diffraction. It should be noted that it can be processed
in the melt because the isotropization temperature (Tiso) of 1 has
been dramatically decreased to 150 uC compared with the normal
alkyl substituted HBCs which usually have high Tiso (. 400 uC).
After slow cooling from the isotropic melt, obvious birefringence
was observed under cross POM (see Fig. 2a) at 90 uC. A typical
2D X-ray diffraction pattern recorded at 120 uC on a mechanically
extruded fiber is shown in Fig. 2b. The wide-angle reflexes at
meridional direction can be assigned to the p-stacking between the
The ordered columnar structure in the solid-state and in water
solution suggested the fabrication of nanoscale organic–inorganic
composites. Sol–gel polymerization was thus conducted according
to the reported method:16 compound 1 was dissolved in aqueous
HCl and then tetraethoxylsilane (TEOS) was added; the mixture
was stirred at room temperature overnight. The precipitate was
collected and further calcinated at 500 uC for 6 hours in air to
remove the organic part. Powder X-ray diffraction on the solid
sample before calcination revealed two weak peaks at 3.33 nm and
1.67 nm in the small-angle area, suggesting that the sol–gel
polymerization of oligo(ethylene glycol) chains and TEOS
maintained the columnar structure of HBCs during the reactions
(See supporting information{). It is worthy of note that in our
previous report, sixfold acryloyl-terminated HBC derivatives
underwent thermal polymerization and afforded an insoluble
network in which the columnar superstructure of the liquid
crystalline phase was also preserved.17 Transmission electron
microscopy (TEM) measurements on the calcinated powder
disclosed ordered nanoporous silica with one-dimensionally
aligned nanochannels (Fig. 3). The diameters of the channels are
about 1.0 nm, which is smaller than the size of the rigid part of
molecule 1. This may be explained by the shrinkage during the
calcination, a phenomenon previously observed.13c To our
˚
HBC discs in the columns. The distance of 3.8 A is larger than for
the normal alkyl substituted HBC, probably because of the higher
temperature and the bulky substitution of the HBC core. Two
strong spots were found at low scattering angle along the
equatorial direction resembling the distances 3.54 nm and
2.68 nm which are correlated to the intercolumnar distances.
However, because of the lack of clearer reflexes, an exact 2D unit
cell can not be figured out. Interestingly, at the diagonal directions
(¡20u from the meridian), four strong reflexes with the same
distance of 2.68 nm were observed. This distance is about seven
times the p–p distance between the HBC discs, and the deviation
from the equator and meridion suggests a probably helical
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Financial supports from EU projects NAIMO (NMP4-CT-
2004-500355) and RADSAS (NMP3-CT-2004-001561) are
acknowledged. We also thank Sirma Koynova for her assistance
with the syntheses.
Notes and references
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50 | Chem. Commun., 2006, 48–50
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