Liquid crystalline properties of unsymmetrical hexa-alkoxy
triphenylene discotics
Ioan Paraschiv *, Paul Delforterie, Marcel Giesbers, Maarten A. Posthumus,
Antonius T. M. Marcelis, Han Zuilhof and Ernst J. R. Sudhölter
*
E-mail: [email protected]
Laboratory of Organic Chemistry, Wageningen University,
Dreijenplein 8, 6703 HB Wageningen, The Netherlands
Abstract
The synthesis and phase behavior of a new series of unsymmetrical substituted
hexaalkoxy triphenylene liquid crystals are reported. One of the hexyloxy moieties of
hexa-hexyloxy triphenylene (HAT6) was replaced by either a shorter or a longer tail,
HAT-(OC6H13)5(OCnH2n+1). Tail lengths n of 2 -14, 16 and 18 were obtained.
Compounds with n 13 were not liquid crystalline. For all compounds with n ≤ 12
Colh textures were observed by polarization microscopy (POM). X-ray investigation
showed that the intercolumnar distance gradually increased with n from n = 2 to n =
12, while the interdisk distance remained constant (3.6 Å).
Introduction
Hexaalkoxytriphenylenes form columnar liquid crystalline phases. These materials
can act as “molecular wires” with relatively high charge carrier mobility. Stabilization
of the columns is expected to lead to better mobility properties. One of the factors
that determines the stability of the columns is the length of the alkyl chains. In this
study we report the influence of asymmetry in the alkyl chain lengths on the stability
of the liquid crystalline phase of hexahexyloxytriphenylene (HAT6) molecules, by
replacing one of the six hexyloxy tails of HAT6 with a different alkyl tail that is either
longer or shorter.
Results and discussions
The synthetic procedure and the structures of the investigated compounds are given
in Figure 1.
OH
a
OC6H13
OH
OH
OCnH2n+1
OH
2-n series
n = 2 -14, 16, 18
1
OH
OC6H13
b
c
O(CH2)5CH3
O(CH2)5CH3
O(CH2)5CH3
OCnH2n+1
O(CH2)5CH3
CH3(CH2)5O
CH3(CH2)5O
O(CH2)5CH3
O(CH2)5CH3
d
O(CH2)5CH3
O(CH2)5CH3
O(CH2)5CH3
2-6
e
4-n series
n = 2 -14, 16, 18
3
Figure 1. General procedure for the synthesis of HAT-(OC6H13)5(OCnH2n+1) derivatives 4-n.
(a) C6H13Br, ethanol, K2CO3; (b) CnH2n+1Br, 2-butanone (CH3CN), K2CO3 (Cs2CO3);
(c) C6H13Br, CH3CN, Cs2CO3; (d) ICl, CH3Cl; Cu powder, heat; (e) FeCl3, CH2Cl2, CH3OH.
The compounds were characterized using differential scanning calorimetry (DSC),
optical polarization microscopy and X-ray diffraction techniques, in order to
investigate the liquid-crystalline properties of the new unsymmetrical triphenylenes.
A representative optical polarizing microscopy picture is shown in Figure 2. The fanlike texture is typical for a Colh mesophase [1].
Figure 2. Optical texture of 4-2 at 78 ºC indicative of a Colh phase, obtained upon cooling from the
isotropic state.
The mesomorphic properties of the unsymmetrical triphenylene derivatives in the
HAT-(OC6H13)5(OCnH2n+1) series are depicted in Figure 3. All compounds in the
series with n ≤ 12 show liquid-crystalline behavior; those with n 13 are not liquid
crystalline. Both the melting and isotropization temperatures decrease with
increasing asymmetry of the molecule. A clear maximum in the melting points is
observed for the symmetrical compound 4-6. A maximum in the isotropization
temperatures is seen for 4-5. The enthalpies of the isotropization transition (not
shown) also showed a maximum for 4-6, and decreased strongly upon going to
higher n values.
Transition temperature / ºC
110
100
90
80
70
60
50
40
30
20
0
5
10
15
20
n
Figure 3. Dependence of the melting (-- --) and isotropization temperatures (-- --) of series 4-n on
the number of carbon atoms n in the alkyl chains.
X-ray reflectometry of the liquid crystalline phases was used to determine the
intercolumn and interdisk distances. For all liquid crystalline compounds the interdisk
distance was the same (3.6 Å), whereas the intercolumn distances increase with n,
as can be seen in Figure 4. A small “odd-even” effect can be observed for molecules
with n > 6. For an even number of C-atoms the increase of the intercolumnar
Intercolumnar distance (Å)
distance is bigger than in the case of molecules with an odd number of C-atoms.
This effect can be correlated with the all-trans conformation of the alkyl tails around
the triphenylene core. The further the alkyl tails stick out of the “HAT6”-disk, the
more the liquid crystalline phase is destabilized.
22
21
20
0
2
4
6
8
10
12
n
Figure 4. The intercolumnar distances for the 4-n series as a function of n.
This can also be seen in Figure 3, where a small, but distinct odd-even effect is
present in the isotropization temperatures for n > 6. Upon going from n = odd to n =
even the isotropization temperatures decrease stronger than upon going from n =
even to n = odd. A similar effect was found for the isotropization enthalpies, where
the enthalpies also decrease stronger upon going from n = odd to n = even.
These findings suggest that at least for compounds with alkyl tails sticking out of the
“HAT6”-disk (n > 6), a strong destabilization of the columnar phase occurs. This is
directly related to the length and parity of the alkyl chain. Probably, the long alkyl tail
has a strong adverse effect on the stacking of the triphenylene compounds and thus
on their column-forming properties.
Conclusions
This study has shown that introduction of asymmetry in the alkoxy tails of
triphenylene compounds strongly lowers their melting and Colh-I transition
temperatures. When the alkoxy tail becomes too long, n 13, the liquid crystalline
behavior is lost, due to the strong interference of the alkyl tail with the stacking of the
polyaromatic molecules.
References
[1] Allen M. T., Diele S., Harris K. D. M., Hegmann T., Kariuki B. M., Lose D.,
Preece J. A., Tschierske C, J. Mater. Chem., 2001, 11, 302-311.