xxiii πανελληνιο ÏƒÏ Î½ÎµÎ´ÏÎ¹Î¿ Ï†Ï ÏƒÎ¹ÎºÎ·Ï‚ στερεας καταστασης & επιστημης ...

Structure and Dynamics of Branched Hexaalkyl and Functionalized Hexa-perihexabenzocoronenes
M. M. Elmahdy 1* , G. Floudas 1 , X. Doo 2 and K. Müllen 2
1 Department of Physics, University of Ioannina, P.O. Box. 1186, 451 10 Ioannina, Greece and Foundation for Research and
Technology-Hellas, Biomedical Research Institute (FORTH-BRI)
2 Max-Planck Institute for Polymer Research, 55128 Mainz, Germany
* me01641@cc.uoi.gr
In the past two decades, discotic liquid crystalline (LC) materials have attracted considerable interest because of their unique
self-organization behaviour into columnar superstructures, leading to high charge carrier mobilities along the columnar stacks [1].
This feature resulted in a successful application of such materials in field-effect transistors and photovoltaic devices [2]. Hexaperi-hexabenzocoronenes
(HBCs) are particularly promising because their large aromatic core permits one of the highest values
for the intrinsic charge carrier mobility for mesogenes [3,4].
T=373 K T=383 K
7
6
α
T c
(DSC)=344 K
HBC-4Me
5
Col h
-log(τ max
/sec)
4
3
2
1
0
μ α
C r
μ α
-1
T g
=251K
T g
=235 K
Cooling
Heating
-2
2.4 2.8 3.2 3.6 4.0 4.4
1000/T(K -1 )
(a)
(b)
Figure 1. (a): 2D-WAXS images of the Methoxy-HBC obtained from an oriented fibre (T extr =313 K) at 373 K and 383 K. Notice
the change in the structure from crystalline phase (C r ) (a-left) to columnar hexagonal liquid crystalline phase (Col h ) (a-right) on
heating and cooling. (b): Temperature dependence of the relaxation times of the Methoxy-HBC. Notice the change in dynamics in
the two phases (C r and Col h ) with corresponding glass temperatures of 251 and 235 K, respectively. This suggests a slowing-down
of τ(T) reflecting the reduced mobility within the highly ordered C r phase.
Herein, we report on the self-assembly mechanism, thermal properties, and the associated molecular dynamics of branched
alkyls hexa-peri-hexabenzocoronene (HBC) and functionalized-HBCs, bearing dipoles attached to the core using polarizing
optical microscopy, wide-angle X-ray scattering, differential scanning calorimetry, and dielectric spectroscopy (DS) [5] . This
dipole is a very sensitive probe of the molecular dynamics within the low temperature crystalline phase and the temperature
columnar phase. We show that DS is a sensitive probe of the particular mesophase through the temperature- and pressuredependent
relaxation times associated with the dipole dynamics. In parallel, we employ X-ray scattering from oriented fibre that
provides with the structural organization.
[1] Boden N., Bushby R. J., Clements J., Movaghar B., Donovan K. J., Kreouzis T., Phys. Rev. B 52 (1995) 13274.
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