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there is currently much interest in polycyclic
aromatic materials, which form
columnar mesophases, 70 on account
of, for example, their potential applications
as charge transport layers in
devices such as fi eld-effect transistors,
photovoltaic cells, and light-emitting diodes.
71 One such class of materials is
comprised of hexa-peri-hexabenzocoronenes
(HBCs), which have high one dimensional
charge carrier mobility along
the columns. 72 In order to control and
improve supramolecular organization,
one dimensional (1D) p-stacking can
be combined with the high selectivity
and directionality of hydrogen bonds.
Such synergetic combination has been
demonstrated using HBC derivatives
with carboxylic acid functions attached
to the core. Here, combined results
from solid-state-NMR, X-ray scattering
and scanning tunneling microscopy
provide a clear and consistent picture
of the effect of these two interactions
on supramolecular organization. 73
In the solid-state 1 H MAS NMR spectra, the chemical shift of
the acid proton (COOH) refl ects stable hydrogen bonds or the
exchange between the free acid and its corresponding hydrogen-bonded
dimer. It was found that the stability of the underlying
hydrogen-bond crucially depends on the length of the
corresponding tether. In case of the HBC-C 3 H 6 -COOH derivatives,
a diagonal peak at 25 ppm (= 12.5 ppm + 12.5 ppm) is
observed in the corresponding 2D 1 H- 1 H DQMAS spectra thus
confi rming dynamically stable dimers on the timescale of the
experiment (100 μs). In contrast, HBC-C 10 H 20 -COOH does not
form such stable dimeric structures even though the 1 H chemical
shift implied the presence of hydrogen bonds at most of the
70
BUNSEN-MAGAZIN · 11. JAHRGANG · 2/2009
Figure 17: 1 H- 1 H double-quantum filtered spectra (top) and the respective 2D DQ MAS spectra of selected
HBC derivatives. 73
time. An improvement of supramolecular order due to strong
synergetic effects of columnar p-stacking with the hydrogenbonding
motifs was only achieved for relatively short tethers. It
should be noted that the different stacking motifs of the HBCcores
(either tilted herringbone or planar arrangement) lead
to characteristic 1 H- 1 H double quantum NMR spectra that can
be easily distinguished. 74 The highest order was obtained for a
system with two carboxyl groups attached via short tethers in
‘para’ -position to each core.
Another interesting example of supramolecular organisation
comprises the self-aggregation of fl uorinated dendrons and
aromatic p-systems into hexagonal-columnar liquid crystalline
phases (Fig. 18)
Figure 18: Representative spinning sideband pattern for extraction of the respective effective dipolar order parameter: top) 1 H, 13 C-REPT-HDOR pattern of
aromatic CH units at 30 kHz MAS; bottom) 1 H, 1 H DQ sideband pattern of the polycyclic aromatic core at 30 kHz MAS; (right) simplified molecular model of
self-aggregation. 75