Helical, twisting power of new chiral dopants having a trifluoromethyl group at the chiral center for nematic liquid crystals

Article abstract of DOI:10.1080/15421400390221637

New chiral dopants for nematic liquid crystals were synthesized using optically active 4,4,4-trifluoro-3-phenylbutanaic acid. The magnitude of helical twisting power (HTP) was largely influenced by the linkage between the asymmetric frame and the core moiety. The chiral dopant, 4,4,4-trifluoro-1-(4-hexyloxy-phenyl)-3-phenyl-1-butanone showed extremely large HTP value (21.0 μm^-1).

Full text of DOI:10.1080/15421400390221637

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Helical Twisting Power of
New Chiral Dopants Having
a Trifluoromethyl Group at
the Chiral Center for Nematic
Liquid Crystals
a
a
Yoshio Aoki , Kentarou Matsushima , Tetsu
a
a
a
Taroura , Takuji Hirose & Hiroyuki Nohira
a
Department of Applied Chemistry, Faculty of
Engineering , Saitama University , 255 Shimo-
ohkubo, Saitama, 338-8570, Japan
Published online: 18 Oct 2010.
To cite this article: Yoshio Aoki , Kentarou Matsushima , Tetsu Taroura , Takuji Hirose
&
Hiroyuki Nohira (2003) Helical Twisting Power of New Chiral Dopants Having a
Trifluoromethyl Group at the Chiral Center for Nematic Liquid Crystals, Molecular
Crystals and Liquid Crystals, 398:1, 189-193, DOI: 10.1080/15421400390221637
To link to this article: http://dx.doi.org/10.1080/15421400390221637
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Mol. Cryst. Liq. Cryst., Vol. 398, pp. 189–193, 2003
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421400390221637
HELICAL TWISTING POWER OF NEW CHIRAL
DOPANTS HAVING A TRIFLUOROMETHYL GROUP
AT THE CHIRAL CENTER FOR NEMATIC
LIQUID CRYSTALS
Yoshio Aoki, Kentarou Matsushima, Tetsu Taroura,
Takuji Hirose, and Hiroyuki Nohira*
Department of Applied Chemistry, Faculty of Engineering,
Saitama University, 255 Shimo-ohkubo,
Saitama 338-8570, Japan
New chiral dopants for nematic liquid crystals were synthesized using optically
active 4,4,4-trifluoro-3-phenylbutanoic acid. The magnitude of helical twisting
power (HTP) was largely influenced by the linkage between the asymmetric
frame and the core moiety. The chiral dopant, 4,4,4-trifluoro-1-(4-hexyloxy-
ꢀ
1
phenyl)-3-phenyl-1-butanone showed extremely large HTP value (21.0 mm ).
Keywords: nematic liquid crystal; chiral dopant; helical twisting power; trifluoromethyl
INTRODUCTION
Chirality is one of the most interesting subject in liquid crystals [1,2].
Recently liquid crystals are widely applied to flat panel display devices of
mobile computers and TV sets. A liquid crystal display (LCD) is of grater
advantage than a CRT for size, thickness, weight and power consumption.
Chiral nematic liquid crystals applicable to super twisted nematic (STN)
devices are attractive materials [3]. Generally the chiral nematic materials
consist of achiral host liquid mixtures, which have low viscosities and the
wide range of nematic phase, and a chiral dopant having a large helical
twisting power (HTP). Therefore, chiral dopants having a large HTP value
are required, and many optically active compounds for application in
nematic mixtures have been synthesized [4–6]. We reported simple com-
pounds having a methyl group at the chiral center showed a large HTP
value (See Fig. 1) [7].
*Corresponding author.
189

1
90
Y. Aoki et al.
FIGURE 1 Chiral dopants derived from 2-phenylpropanoic acid.
In particular, 1-(4-hexyloxyphenyl)-2-phenylpropane (4*) showed the
ꢀ
1
extremely large HTP value (16.5 mm ). In this paper, optically active 4,4,4-
trifluoro-3-phenylbutanoic acid was synthesized and new chiral dopants
were derived from it. A trifluoromethyl group is a strongly polarized group.
The relationship between the HTP values and the dipole at the chiral
center of the chiral dopants is discussed.
EXPERIMENTS
4
[
,4,4-Trifluoro-3-phenylbutanoic acid (5) was synthesized in a usual way
8]. Optically active 1 was prepared by diastereomeric salt formation
method using optically active erythro-2-amino-1,2-diphenylethanol as a
resolving agent. New chiral dopants were synthesized in a usual way (See
Scheme 1).
They did not show any liquid crystalline phase, so that only their HTP
values were determined. The chiral nematic liquid crystalline mixtures
were prepared by adding one weight percent of a chiral dopant to the host
liquid crystal (ZLI-1132, Merck). Helical pitches (p) in chiral nematic
phases were measured using Cano wedge cell (tan y ¼ 0.0083, 0.0140,
0
.0194, 0.0288, E.H.C.) [9]. The HTP values were calculated from the pitch
of the chiral nematic phase in mm (p) and the weight concentration ratio
ꢀ
1
(
c), namely HTP ¼ 1/pc (mm ). In order to discuss a HTP by a molecule,
we suggest molar helical twisting power (MHTP), which is defined as the
following equation [7];
MHTP ¼ ðpWÞꢀ ¼ HTP ꢁ Mw ꢂ 1000ðmmꢀ1 mol KgÞ
1
ꢀ1
where W is the moles of the chiral dopant per 1 Kg of the mixture, and Mw
is molecular weight of the chiral dopant.

Helical Twisting Power of New Chiral Dopants
191
SCHEME 1 Synthesis of new chiral dopants.
TABLE 1 Helical Twisting Power of Chiral Dopants
ꢀ1
ꢀ1
ꢀ1
Chiral dopants
X
HTP /mm
MHTP*/mm mol kg
2
3
4
5
*
*
*
*
COO
6.5
15.2
21.0
5.8
2.9
5.8
7.9
2.1
CH
CO
CH
2
O
2
*MHTP ¼ HTP6Mw ꢂ 1000. Mw shows molecular weight of a chiral dopant.

1
92
Y. Aoki et al.
RESULTS AND DISCUSSION
The helical twisting power of new chiral dopants are summarized in Table
1
. The HTP values and the MHTP values are largely influenced by the
linkage between the asymmetric frame and the core moiety. The order of
HTP values in these chiral dopants was 8* > 7* > 6* > 9*. The chiral
dopant, 4,4,4-trifluoro-1-(4-hexyloxyphenyl)-3-phenyl-1-butanone (8*)
ꢀ
1
having a carbonyl group showed the largest HTP value (21.0 mm ), while
ꢀ
1
the chiral dopant 9* showed the smallest HTP value (5.8 mm ).
The HTP value of 7* is larger than that of 6*. Along with the case of the
derivatives from 2-phenylpropanoic acid (Fig. 1), it is shown that a car-
bonyl group does not always contribute to increase the HTP value. In the
chiral dopant 8*, a carbonyl group is conjugated with a benzene ring, so
that these lie on the same plane. This planar structure probably contributes
to induce a HTP value. Kuball and coworkers reported chiral induction by
optically active aminoanthraquinones [10]. They analyzed the intra- and
intermolecular chirality transfer, and concluded that the chiral induction
depended on the direction of the orientation axis. It is thought that a
structure of 8* effectively induces a helical structure.
On the other hand, the order of HTP values in the chiral dopants having a
methyl group at the chiral center was 4* > 3* > 1* ꢃ 2*. The fact 4* dose
not have any strongly polarized groups in the molecule suggests that a steric
effect is important to induce a helical macrostructure. Nishiyama and
coworkers reported a relationship between chiral compounds and their HTP
values, and a certain chiral dimer was found to induce strong helical mac-
rostructures [11,12]. According to their discussion, the present chiral
dopants are probably classified into ‘a twin molecule’ which may exhibit
special steric effects on the chirality dependent properties. In order to
compare each chiral structure, the MHTP values of the chiral dopants are
discussed. The MHTP values of 6*, 7* and 8* were greater than those of 1*,
2
* and 3*, respectively. From these data, it is shown that the contribution of
a trifluoromethyl group to the MHTP values is larger than that of a methyl
group. However, the MHTP value of 4* was much larger than that of 9*. The
chain length between the chiral center and the core moiety largely influ-
ences the HTP and the MHTP, because the closer the chiral center to the
core, the less the freedom of its movement due to steric hindrance with the
rigid core. A helical macrostructure is probably induced by a steric effect
and an electronic effect of chiral dopant. However, it is difficult to discuss
the effect of steric factor and electronic factor separately. From the present
results, we conclude that the dopants having a trifluoromethyl group at the
chiral center show the relatively large HTP values, however a strongly polar
group at the chiral center does not always induce a large HTP value.

Helical Twisting Power of New Chiral Dopants
193
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[