Discotic hexa-peri-hexabenzocoronenes with strong dipole: Synthesis, self-assembly and dynamic studies

Article abstract of DOI:10.1039/c1cc16740e

Strong dipole moments have been built into two hexa-peri-hexabenzocoronene (HBC) derivatives (1 and 2) originating from the push-pull structure of the molecules with one electron-donating and one electron-withdrawing substituent. The influence of dipole m

Full text of DOI:10.1039/c1cc16740e

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COMMUNICATION
Discotic hexa-peri-hexabenzocoronenes with strong dipole: synthesis,
self-assembly and dynamic studiesw
Long Chen,a Xi Dou,a Wojciech Pisula,a Xiaoyin Yang,a Dongqing Wu,a George Floudas,b
Xinliang Feng*a and Klaus Mullen*a
¨
Received 31st October 2011, Accepted 18th November 2011
DOI: 10.1039/c1cc16740e
Strong dipole moments have been built into two hexa-peri-
hexabenzocoronene (HBC) derivatives (1 and 2) originating from
the push–pull structure of the molecules with one electron-
donating and one electron-withdrawing substituent. The influence
of dipole moment on the self-assembly of HBCs in solution and in
bulk has been investigated.
cyanide at 65 1C resulting in 4. The remaining bromo group
was then converted into an amino substituent via a Buchwald
cross-coupling reaction with a diphenyl amine leading to the
target compound (1 or 2). The strong aggregation in solution
does not allow one to measure the dipole moments of
1 and 2. Therefore, these values were calculated as m =
8.25 D for 1 and m = 8.64 D for 2 using the Becke three-
parameter Lee–Yang–Parr (B3LYP) functional and 6-31G(d,p)
basis set in the Gaussian 03 package.8
The 1H-NMR chemical shifts were monitored in d2-1,1,2,2-
tetrachloroethane at different temperatures and concentrations
(see Fig. 1). It has been well established that an upfield shift
of aromatic proton signals is a signature of intermolecular
association involving p-stacking interaction.9 Interestingly,
below 60 1C the peaks for 1 and 2 are broad in sharp contrast
to other hexaalkyl-substituted HBC derivatives, suggesting strong
intermolecular forces of these HBCs at low temperature.10 The
six different aromatic protons only appear as six sharp singlets
above 90 1C indicating the existence of monomeric species at
high temperature (Fig. 1a).11 The signals of the aromatic protons,
which are located next to the cyano group in 1 (Ha) and 2
(Hb), are shifted up-field by 0.20 and 0.11 ppm, respectively, when
the temperature drops from 140 1C to 30 1C. For comparison,
a 0.26 ppm upfield shift of the aromatic proton resonance was
observed for the non-polar hexa-dodecyl substituted HBC
Discotic liquid crystalline (LC) molecules, which are generally
composed of a rigid flat core and a periphery of flexible alkyl
substituents, can self-assemble into well-ordered columnar
superstructures under the influence of p-interactions.1 Typical
examples are triphenylene and hexa-peri-hexabenzocoronene
(HBC) derivatives. Tremendous efforts have been devoted to
achieve highly ordered columnar superstructures of discotic
molecules in bulk.2 To this end, additional intermolecular
forces, such as hydrogen bonds,3 amphiphilic4 and dipole–dipole
interactions,5 are often utilized to improve their supramole-
cular organization. It has been demonstrated that dipole–dipole
forces can effectively stabilize the columnar mesophase of
triphenylene and dibenzophenazine derivatives.5,6 Moreover,
stronger dipole–dipole interplay can even assist half-disc
shaped mesogens to arrange into well-defined hexagonal
columnar LC phases.7
Our previous results encouraged us to introduce even
stronger dipole moments at the HBC core (Scheme 1, 1 and
2) with the aim to further improve the stability of the liquid
crystalline phase, but simultaneously to ensure pronounced
molecular interactions in solution. In this work, we present
HBCs 1 and 2 with dipole moments of 8.25 D and 8.64 D,
respectively, which are significantly higher than those of the
earlier studied HBC derivatives.
The synthesis of 1 and 2 was accomplished through a
stepwise substitution of 3 (Scheme S1, ESIw) as illustrated in
Scheme 1. Based on the different reactivity of iodo- and
bromo- groups, compound 3 was mono-cyanated with cuprous
a Max Planck Institute for Polymer Research, Ackermannweg 10,
55128 Mainz, Germany. E-mail: feng@mpip-mainz.mpg.de,
muellen@mpipmainz.mpg.de; Fax: +49 6131 379350;
Tel: +49 6131 379150
Scheme 1 Synthesis of the dipole functionalized HBCs (1 and 2):
(i) CuCN, THF, Pd(PPh3)4, reflux, 15 h, 81%, (ii) di-tolyl amine,
Pd2(dba)3, t-Bu3P, sodium butoxide, toluene, 80 1C, 16 h, 79%,
(iii) di-4-methoxylphenyl amine, Pd2(dba)3, t-Bu3P, sodium butoxide,
toluene, 80 1C, 16 h, 74%.
b Department of Physics, University of Ioannina, 451 10 Ioannina,
Greece
w Electronic supplementary information (ESI) available: Synthesis
and measurement details. See DOI: 10.1039/c1cc16740e
c
702 Chem. Commun., 2012, 48, 702–704
This journal is The Royal Society of Chemistry 2012

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values (Pa) for all three compounds indicate that the aromatic
protons in these monomers are situated in a nearly identical
chemical environment. However, the larger KA values for 1 and
HBC-OMe clearly demonstrate a tendency for molecular aggre-
gation caused by the additional intermolecular dipole–dipole
forces. Especially, in the case of 1, the calculated chemical shift
of the molecule within a molecular stack (Px) is upfield shifted
by nearly 1 ppm as compared to either HBC-OMe or HBC-C12.
In a face-to-face aggregate of HBCs, the aromatic protons of
one molecule are localized in the secondary magnetic field of
the neighbouring aromatics which results in a shielding effect.
The strength of this effect depends on the size of aggregates.15
Thus, the Px value of 1 further proves the existence of larger
molecular aggregates as compared to HBC-OMe and HBC-C12.
Additionally, the parameter j, which describes the ability of a
molecule to form a dimer in the AK model,13 is 7-times larger for
1 in comparison to HBC-OMe and HBC-C12. Thereby, the large
KA, j and Px values of 1 unambiguously reveal the effect of a
strong in-plane molecular dipole (8.25 D) on the formation of
larger aggregates.
Fig. 1 Aromatic region of (a) 1H-NMR spectra recorded at different
temperatures of 1 (left) and 2 (right), measured for 2.05 ꢀ 10ꢁ2
M
CDCl2CDCl2 solutions; (b) 1H-NMR spectra of 1 (left) and 2 (right)
recorded at various concentrations, measured for CDCl2CDCl2 solutions
at 60 1C, 500 MHz.
Differential scanning calorimetry (DSC) reveals the absence
of any phase transition in the temperature range from
ꢁ100 1C to 250 1C for both compounds 1 and 2 (Fig. S5,
ESIw). Two-dimensional wide-angle X-ray scattering (2D
WAXS) experiments on oriented fibers point towards an
identical organization for both compounds that does not
change over the whole investigated temperature range.16 In
both cases, the 2D patterns indicate a characteristic hexagonal
columnar liquid crystalline packing (Fig. S6 and S7w). The
(HBC-C12) during cooling from 120 1C to 60 1C.12 Through
the comparison of the temperature dependent H-NMR signals
1
recorded for 1, 2 and HBC-C12, the smaller upfield shifts of 1
(0.20 ppm) and 2 (0.11 ppm) from 140 1C to 30 1C than that of
HBC-C12 (0.26 ppm, from 120 1C to 60 1C) over a narrower
temperature range indicate a remarkable thermal stability
of aggregates formed by 1 and 2. This result undoubtedly
validates the improved intermolecular attractive interplay
based on additional dipole–dipole interactions.
assignment of the hexagonal columnar structure is based on
pffiffi
the ratio of 1 : 3 : 2 for the positions of the strong equatorial
reflections. The distinct meridional wide-angle reflections are
attributed to the p-stacking distance of 0.35 nm of the
aromatic cores along the columnar axis. The weak reflections
at intermediate distances suggest dipole–dipole correlations
with an angle of about 901 as revealed by the azimuthal
intensity distribution. In fact, most dipole functionalized
HBCs show such a pattern reflecting the enhanced dipole–
dipole correlations within the columnar mesophase.17 These
static correlations improve with decreasing temperature
(Fig. S6bw) for reasons that will become clear below through
the dynamic investigation.
Upon increasing the concentration from 1.00 ꢀ 10ꢁ5 to
2.05 ꢀ 10ꢁ2 M at 60 1C, the resonance of Ha in 1 (Fig. 1b left)
is upfield shifted by 0.38 ppm (from 8.89 to 8.51 ppm); and a
comparable upfield shift of 0.39 ppm (from 8.93 to 8.54 ppm)
is recorded for Hb in 2 (Fig. 1b right), however, within a much
narrower concentration range (from 3.00 ꢀ 10ꢁ5 to 1.20 ꢀ
10ꢁ2 M). The upfield shifts of the aromatic proton resonances
at lower temperatures or in concentrated solutions reflect a typical
co-facial molecular stacking of aromatic cores.12 Obviously,
the weaker temperature and stronger concentration dependence
observed for 2 reflects an increased stability of the aggregates
caused by the larger molecular dipole in comparison to 1.
The self-association constant KA of 1 was calculated as a
typical example according to the nearest neighbour attenuated
K (AK) model13 using a non-linear least-squares curve fitting
The molecular dynamics, that completely relax the dipole
moment within the liquid crystal mesophase, was explored by
Dielectric Spectroscopy (DS). In fact, it has been recently shown
that some molecular dynamics can be activated by the dielectric
stimulus even in unsubstituted HBCs.18 Fig. 2 gives representative
dielectric loss curves for the three compounds (4, 2 and 1) at
313 K. The curves display asymmetric broadening towards
lower frequencies that can best be described by a summation of
two Havriliak–Negami functions (ESIw).19 The characteristic
frequencies at maximum loss for the two main processes are
indicated by arrows in Fig. 2. The increase at lower frequencies is
due to the ionic conductivity whereas an additional local process
(b-process) is evident at higher frequencies.
1
method with the H-NMR experimental data of Ha collected
at different concentrations (Tables S1 and S2, ESIw). For
comparison, the values for the C3-symmetric substituted HBC
with three methoxy groups (HBC-OMe) and HBC-C1214
(Scheme S2, ESIw) are also listed in Table S2.w Both compounds
HBC-OMe and HBC-C12 carry less bulky alkyl side chains and
are therefore expected to show strong aggregation in solution.
Although the calculated chemical shifts for the monomers (Pa) of
all three compounds are identical, the self-association constant
(KA) of 1 is more than twice that of HBC-OMe and about three
times that of HBC-C12. The similar monomeric chemical shift
The relaxation times at maximum loss for the main a and
a0 processes are plotted in Fig. 3 in the usual Arrhenius
representation. Both processes exhibit a strong temperature
c
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Chem. Commun., 2012, 48, 702–704 703

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on functionalized HBCs with weaker dipoles showed that the
disc axial dynamics are dictated largely by the phase state.18
The present series indicates a strong t(T) dependence of the
disc axial motions within the mesophase. As expected, the
dynamics of 4 are the fastest and give rise to the lower freezing
temperatures in Table S3 (ESIw). The disc axial motion in 1
and 2 is substantially slower giving rise to higher freezing
temperatures for the disc rotational dynamics.
In summary, two novel dipole functionalized HBC derivatives
1 and 2 were synthesized based on the C2v-symmetric building
block 3. The strong molecular dipole moment in both HBCs
1 and 2 provides additional intermolecular dipole–dipole
interactions. The strong dipole was revealed to exert sig-
nificant effect on the self-association of 1 and 2 in solution
as well as the dynamic processes within the liquid crystalline
mesophase.
Fig. 2 Dielectric loss curves as a function of frequency for 1 (triangles),
2 (squares) and 4 (circles) at 313 K. The arrows indicate the approxi-
mate positions of the a- and a0-processes associated with the disc axial
motion.
This work was financially supported by the DFG Priority
Program SPP 1355, SPP 1459, ESF Project GOSPEL (Ref Nr:
09-EuroGRAPHENE-FP-001), EU Project GENIUS and ERC
grant on NANOGRAPH. L. Chen gratefully acknowledges
funding by the Alexander von Humboldt Foundation.
dependence that is distinctly different from an Arrhenius law
and can best be fitted by the Vogel–Fulcher–Tammann (VFT)
equation (ESIw). Interestingly, there exist two glass tempera-
tures for each compound. Both processes are associated with a
dipolar relaxation within the liquid crystalline phase. The a-
process reflects the fast axial motion that leaves an uncompen-
sated residual dipole moment.17 Responsible for this residual
dipole moment and the presence of the slower process are the
dipolar correlations within the columns as determined by
WAXS. Subsequently, this residual dipole moment relaxes
completely through the a0-process at a longer time scale. The
distribution of relaxation times reveals that both processes are
of collective nature (they involve several discs) but the slower
process, that completely randomizes the dipole, is more
collective. Thus, the two glass temperatures in these dipole
functionalized HBCs reflect the freezing of the partial (a)
and complete (a0) dipole randomization. Earlier DS studies
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