The influence of an alkenyl terminal group on the mesomorphic behaviour and electro-optic properties of fluorinated terphenyl liquid crystals

Article abstract of DOI:10.1039/b914260f

Novel liquid crystal terphenyls with two, three and four lateral fluoro substituents and an alkene unit at the end of a terminal chain are presented in terms of synthesis, mesomorphic behaviour and electro-optic properties. The difluoro analogues were found to exhibit the smectic C phase over a wide temperature range, with short temperature ranges of the smectic A and the nematic phase above. Very low melting points were recorded for the trifluoro analogues, and the shorter chain homologues of these materials exhibit the nematic phase over a wide temperatures with a monotropic smectic C phase and the higher homologues exhibit the smectic C phase over a wide temperature range with the nematic phase above. The tetrafluoroterphenyls exhibit the nematic phase over a wide temperature range with the complete absence of smectic phases. As appropriate, the materials were evaluated for dielectric anisotropy and threshold voltage in nematic phase, and mixed with a chiral dopant to evaluate the spontaneous polarisation, tilt angle and switching times in the chiral smectic C phase. The results show that these compounds are strong potential candidates for application in both vertically aligned nematic (VAN) and ferroelectric liquid crystal (FLC) devices.

Full text of DOI:10.1039/b914260f

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
The influence of an alkenyl terminal group on the mesomorphic behaviour
and electro-optic properties of fluorinated terphenyl liquid crystals†
Julita S. Gasowska,a Stephen J. Cowling,a Martin C. R. Cockett,a Michael Hird,b Robert A. Lewis,b
c
E. Peter Raynes and John W. Goodby
a
*
Received 16th July 2009, Accepted 29th September 2009
First published as an Advance Article on the web 11th November 2009
DOI: 10.1039/b914260f
Novel liquid crystal terphenyls with two, three and four lateral fluoro substituents and an alkene unit at
the end of a terminal chain are presented in terms of synthesis, mesomorphic behaviour and electro-
optic properties. The difluoro analogues were found to exhibit the smectic C phase over a wide
temperature range, with short temperature ranges of the smectic A and the nematic phase above. Very
low melting points were recorded for the trifluoro analogues, and the shorter chain homologues of these
materials exhibit the nematic phase over a wide temperatures with a monotropic smectic C phase and
the higher homologues exhibit the smectic C phase over a wide temperature range with the nematic
phase above. The tetrafluoroterphenyls exhibit the nematic phase over a wide temperature range with
the complete absence of smectic phases. As appropriate, the materials were evaluated for dielectric
anisotropy and threshold voltage in nematic phase, and mixed with a chiral dopant to evaluate the
spontaneous polarisation, tilt angle and switching times in the chiral smectic C phase. The results show
that these compounds are strong potential candidates for application in both vertically aligned nematic
(VAN) and ferroelectric liquid crystal (FLC) devices.
(see structure III), which are cylindrical in shape, and we
1. Introduction
show through dielectric studies that the fluoro substituents can
couple to give relatively large values of the negative dielectric
anisotropy.
Over the last decade liquid crystal displays (LCDs) have become
the standard technology for mobile phones, PC monitors and
televisions. Practically all of these devices are based on nematic
liquid crystals, however, the switching time of milliseconds is still
an issue for video applications. With future LCD TVs projected
to become more complex, possibly moving from 2D to 3D
images and with the refresh rates moving towards the 200 Hz
range, Souk of Samsung challenged the international scientific
community at the International Liquid Crystal Conference in
Korea1 to develop faster LC switching modes.
One of the more common LCD modes in usage today is the
vertically aligned nematic (VAN) cell which utilises materials
with negative dielectric anisotropy.2–5 Response times are closely
related to the magnitude of the dielectric anisotropy, the higher
the negativity the better, and the lower the viscosity the better.
Such requirements dictate the design and synthesis of materials
with molecular structures of a cylindrical shape that possess
large lateral dipoles. In this article we describe the synthesis
and properties of di-, tri- and tetra-fluorinated terphenyls
An alternative approach to addressing the issue of response
time is to deploy a different type of liquid crystal phase. For
example, because of the coupling between the electric field and
the permanent polarisation, surface-stabilised ferroelectric liquid
crystal displays (SSFLCDs) offer switching times in the micro-
second regime, one hundred to one thousand times faster than
nematic switching modes. In combination with silicon back
planes, such devices have already been used in projection appli-
cations and spatial light modulators.6
Thus in this article we also report on the development of novel
ferroelectric liquid crystals and their potential for use in fast,
high-resolution, low-voltage ferroelectric liquid crystal on silicon
(LCOS) devices with optimised performance. Such light proces-
sors can be used in amplitude modulation or phase modulation
mode. With amplitude modulation, very high-resolution
professional monitors and near-to-eye 3D imaging can be ach-
ieved. Compared to digital mirror devices, ferroelectric liquid
crystal devices can have considerably higher pixel density,
leading to more pixels on the same chip area and cost
aDepartment of Chemistry, University of York, Heslington, York, UK
YO10 5DD. E-mail: jwg500@york.ac.uk; sc520@york.ac.uk
bDepartment of Chemistry, University of Hull, Cottingham Road, Hull, UK
HU6 7RX. E-mail: m.hird@hull.ac.uk; r.a.lewis@hull.ac.uk
cDepartment of Engineering, University of Oxford, Parks Road, Oxford,
UK OX1 3PJ. E-mail: peter.raynes@eng.ox.ac.uk
† Electronic supplementary information (ESI) available: Full synthetic
details for the preparation of compounds 5–25 are given along with full
description of characterisation and evaluation techniques. See DOI:
10.1039/b914260f
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effectiveness for high pixel numbers. Due to the fast switching
speed of ferroelectric liquid crystals, colour images can be
realised with one single ferroelectric device using sequential
colour illumination.
can markedly affect device configuration, construction and
performance.10 For example, weak interlayer interactions leading
to the collapse of the hysteresis loop result in a linear electro-
optic response to an applied electric field (V-shaped switching),
thereby enabling the device to exhibit a grey-scale response
suitable for video-frame rate applications.
The basic molecular design of ferroelectric host and dopant
liquid crystals usually involves the incorporation of a central
aromatic or heterocyclic core unit which is sandwiched between
two terminal aliphatic chains.7 In this case fluoro-substituted
terphenyls (see structure I) have also shown considerable promise
in ferroelectric systems for which response times have been
achieved in the 10 to 100 microsecond (ms) regime.8,9 Typically,
when molecules with this type of architecture self-organise they
do so with their rigid, aromatic parts tending to pack together
and their flexible/dynamic aliphatic chains orienting together.
Thereby the overall system becomes locally microphase segre-
gated. Consequently, the main target of material design has been,
by default, the variation in the structure of the central core region
of the molecules, e.g., changing the number and location of
fluoro substituents, in the belief that the core is more important
in influencing mesophase incidence, mesophase temperature
range, isotropization point, melting point, mesophase sequence,
and the reorientational viscosity associated with the mesophase.
However, few systematic studies have been reported where the
terminal positions of the aliphatic chains have been manipulated,
e.g., X and Y in structure II for fluoroterphenyls. Through these
limited, and unsystematic, studies there has been a realisation
that small changes to the termini of the molecular structure can
have a very marked effect on liquid crystal phase formation
and related physical properties, particularly for ferroelectric
phases.
Overall, in a situation where there is a need to design materials
with molecular structures that will induce selective mesophase
formation, being able to control mesophase structure simply by
terminal group selection is a powerful weapon. Thus, the wider
aims of our research are to develop a wide range of materials
based on structure II, in particular where the terminal groups
X or Y are silanes, siloxanes, ethers and fluorocarbons. In this
article we examine the intermediary alkenic terminal systems,
which ultimately can be further derivatised through, for example,
hydrosilylation, to give other liquid crystal systems possessing
terminal microphase segregating groups that exhibit stronger
interactions. Thus, in this article we focus on alkenic materials
based on structure III, which are similar to the alkanes, in
order to compare the uses of the same family of materials in
VAN-LCD and SSFLCD modes of operation.
2. Experimental
Confirmation of the structures of intermediates and products
1
was obtained by H and 13C NMR spectroscopy (JEOL ECX
400 MHz), infrared spectroscopy (Shimadzu IRPrestige-21
Fourier transform infrared spectrophotometer) and mass
spectrometry (EI-MS, AUTOSPEC WATERS-MICROMASS).
The purity of the compounds in Tables 1–3 was checked by
HPLC (Shimadzu Prominence LC-20AT) and all compounds
were >99.9% pure.
Therefore, understanding the interactions at, and between, the
interfaces of the layers in lamellar smectic phases is of practical
importance in the development of ferroelectric devices and
displays. There are a number of interactions to consider in
devices which include the liquid crystal surface interactions, the
penetration of the surface interactions into the bulk of the liquid
crystal phase, the strength of the lateral interactions between the
molecules, and the strength of the interactions between the
layers, as shown in Fig. 1. The strength of the surface interactions
controls the surface anchoring energies and hence the bistability
of the device operation. The strength of the interactions between
the layers controls the shape of the hysteresis loop for the
ferroelectric phase. Weak interlayer (out-of-plane) interactions
can lead to a collapse of the hysteresis loop, and hence these
General synthetic pathway to the u-alkenic materials is shown
in Schemes 1 and 2. Full synthetic details are given in the ESI.†
Compounds 1a, 1b and 2 (Scheme 1) were prepared using
synthetic methods which have been previously published.11
Compounds 3a and 3b were prepared using a Suzuki coupling
reaction and they were subsequently lithiated using n-butyl-
lithium and converted into the boronic acid with trimethyl
borate. The boronic acids were oxidised using hydrogen peroxide
to give phenols 4a and 4b in good yields. A Williamson ether-
ification using potassium carbonate and the appropriate u-bro-
moalkene yielded alkenic difluoroterphenyls 5–8. A Suzuki
coupling reaction between compounds 9 and 10 and between
compounds 12 and 13 gave terphenyl compounds 14a, 14b, 15a
and 15b in excellent yields. These compounds were converted
into boronic acids and oxidised to the phenol compounds 16a,
16b, 17a and 17b in the same way as for the preparation of
compounds 4a and 4b. A Williamson etherification using
potassium carbonate and the appropriate u-bromoalkene gave
tri- and tetrafluoroterphenyls 18–25. All the final compounds
were purified by column chromatography and recrystallisation.
2.1. Evaluation of structural and physical properties
Detailed descriptions of the evaluation and characterisation
techniques for the determination of transition temperatures,
physical properties and structure analysis are provided in the
ESI†.
Fig. 1 A typical device arrangement for smectic liquid crystals, with the
layer planes perpendicular to the cell surface. The important inter-
molecular and surface interactions are shown.
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Scheme 1 General synthetic route for the preparation of alkenic difluoroterphenyls 5–8.
Coupled with a N–A–C phase sequence makes them ideal
candidates as host materials in ferroelectric smectic C* mixtures
for applications in surface stabilised ferroelectric liquid crystal
displays, SSFLCDs.
3. Results and discussion
3.1. Mesophase classification and melting behaviour
The results obtained on the melting behaviour of all of the
compounds are compiled together in Tables 1–3, where Table 1
collects the family of materials possessing two fluoro substitu-
ents, Table 2 three fluoro substituents, and Table 3 four fluoro
substituents. The transition temperatures reported were deter-
mined by thermal polarised light optical microscopy, whereas the
melting points and recrystallisation temperatures were deter-
mined by differential scanning calorimetry.
In contrast to the difluoroterphenyls, the trifluoroterphenyl
analogues (Table 2) show very low melting points, and they
possess less smectic character, with the smectic A phase being
suppressed. Thus the trifluoroterphenyls exhibit just nematic and
smectic C phases, with the latter phase being monotropic for the
shorter chain homologues. Hence the compounds could make
useful components in mixtures for both VAN and FLC appli-
cations.
The mesophases were characterised from their natural and
paramorphotic defect textures as described by Gray and
Goodby.12 The difluoroterphenyls (Table 1) characteristically
exhibit nematic, smectic A and smectic C phases, where the
smectic A to smectic C phase transition is essentially second
order and the smectic A temperature range is relatively short.
The tetrafluoroterphenyls (Table 3) show that the inclusion of
an extra fluoro substituent completely suppresses the formation
of smectic phases, and thus the compounds are nematogens
melting around 60 ꢀC and clearing around 100 ꢀC. From the
point of view of mesomorphic properties these materials are
Scheme 2 General synthetic route for the preparation of alkenic tri- and tetrafluoroterphenyls 18–25.
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Table 1 Phase classification, transition temperatures (ꢀC) and enthalpies
of transition [DH, J gꢁ1] for compounds 5–8
Table
4
Phase classification and transition temperatures (ꢀC) of
difluoro-, trifluoro- and tetrafluoroterphenyl n-alkyl analoguesa
Transition temperatures/ꢀC
n
m
a
b
c
d
Transition temperatures/ꢀC
5
5
6
8
H
H
H
H
F
F
F
F
Cr 97.5 SmC 145.5 N 166.0 Iso liq13
Cr 93.5 SmC 144.0 SmA 148.0 N
159.0 Iso liq13
Cr 89.5 SmC 148.0 SmA 151.5 N
154.0 Iso liq13
Cpd no. n m Cr
SmC
SmA
N
Iso Liq
5
6
7
8
5 3
5 5
7 3
7 5
ꢂ
ꢂ
ꢂ
ꢂ
101.31 ꢂ
139.37 ꢂ
151.95 ꢂ 169.41 ꢂ
[5.35] [5.57]
146.37 ꢂ 159.94 ꢂ
[4.41] [5.36]
157.76 ꢂ 162.80 ꢂ
[7.62] [4.89]
152.18 ꢂ 155.27 ꢂ
[5.54] [5.02]
7
8
H
H
F
F
[29.91]
89.87
[26.01]
97.53
[23.27]
87.24
[23.65]
[0.26]
136.85 ꢂ
ꢂ
ꢂ
ꢂ
5
5
7
7
5
7
6
8
6
8
6
6
H
H
H
H
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
Cr 43.8 (SmC 38.1) N 112.3 Iso liq11
Cr 47.3 SmC 50.2 N 110.3 Iso liq11
Cr 40.1 SmC 75.7 N 111.8 Iso liq11
Cr 48.0 SmC 81.0 N 108.8 Iso liq11
Cr 61.1 N 107.5 Iso liq11
[0.20]
140.48 ꢂ
[0.14]
141.71 ꢂ
[0.23]
F
Cr 55.1 (SmC 50.7) N 103.1 Iso liq11
a
Values in parenthesis represent a monotropic phase transition.
potentially suitable for applications in vertically aligned nematic
liquid crystal displays, VAN-LCDs.
Table 2 Phase classification, transition temperatures (ꢀC) and enthalpies
of transition [DH, J gꢁ1] for compounds 18–21a
Comparisons, in terms of transition temperatures and meso-
phase morphology, can be made with the standard alkyl chain
analogues shown in Table 4. It can be seen that the transition
sequences are almost identical, and that the transition temper-
atures are very similar for homologues with similar aliphatic
chain lengths. Thus in this case the incorporation of an alkenic
unit at the terminus of the alkoxy chain has little effect on
mesophase/melting behaviour.
Transition temperatures/ꢀC
Cpd no.
n
5
5
7
7
m
3
Cr
SmC
N
ꢂ
Iso Liq
18
19
20
21
ꢂ
48.44
[44.67]
46.09
[51.43]
30.39
[32.71]
28.15
[40.95]
(ꢂ
35.90)
[0.99]
36.29)
[0.71]
69.95
[0.50]
71.18
[1.03]
117.06
[1.48]
109.42
[1.50]
111.38
[2.80]
107.02
[3.15]
ꢂ
ꢂ
ꢂ
ꢂ
3.2. Electrical field studies
5
ꢂ
(ꢂ
ꢂ
3.2.1. Nematic phases under applied electrical fields. Several
materials were investigated in the nematic phase under applied
electric fields. As the difluoro-substituted materials tended to
show strong smectic properties they were not investigated for
physical properties in the nematic phase. Moreover as they
exhibited relatively short temperature range nematic phases,
comparisons across the three series of compounds would be
meaningless. Thus attention was focused on the electrical field
behaviour of the tri- and tetrafluoroterphenyls. In the following
we describe a direct comparison between compounds 18 and 22,
comparison between other pairs of homologues were, however,
found to be similar. Fig. 2 shows the switching behaviour of the
nematic phase of compound 18 as a function of the reduced
temperature. Fig. 2a shows the values of the parallel dielectric
permittivity, 3k, and the perpendicular dielectric permittivity, 3t,
as a function of the reduced temperature from the clearing point.
Similarly, Fig. 2b shows the values on heating and cooling of the
dielectric anisotropy, D3, and Fig. 2c shows the threshold voltage
for switching, both as a function of the reduced temperature
from the clearing point. Fig. 2a shows that both permittivities
rise linearly with reduced temperature. At a reduced temperature
of 60 ꢀC the order parameter, S, determined via the use of Maier–
Meier theory14 was found to be 0.65, showing that the nematic
phase was fairly well-ordered. The dielectric anisotropy, shown
in Fig. 2b, was found to reach a value of ꢁ4.7 at 80 ꢀC below the
clearing point and just before recrystallisation. Interestingly, the
3
ꢂ
ꢂ
ꢂ
5
ꢂ
ꢂ
ꢂ
a
Values in parenthesis represent a monotropic transition.
Table 3 Phase classification, transition temperatures (ꢀC) and enthalpies
of transition [DH, J gꢁ1] for compounds 22–25
Transition temperatures/ꢀC
Cpd no.
n
5
5
7
7
m
3
Cr
N
Iso Liq
22
23
24
25
ꢂ
56.90
[57.63]
63.76
[53.84]
53.05
[37.20]
64.84
[56.38]
ꢂ
108.78
[1.54]
101.08
[1.16]
103.08
[2.11]
97.06
[1.95]
ꢂ
ꢂ
ꢂ
ꢂ
5
ꢂ
ꢂ
3
ꢂ
ꢂ
5
ꢂ
ꢂ
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Fig. 2 Switching properties of the nematic phase of compound 18 as a function of reduced temperature. (a) Measurement of the parallel permittivity, 3k,
and perpendicular permittivity, 3t; (b) measurement of D3 (3k ꢁ3t); and (c) threshold voltage for switching.
threshold voltage remained low, at 1.4 V for 4.6 mm cell thick-
ness, across the whole temperature range indicating that the
increasing negativity in the dielectric anisotropy offsets the
increase in the viscoelastic constants as the temperature falls.
Fig. 3 shows the comparative switching behaviour of the
nematic phase of compound 22 as a function of the reduced
temperature. The results obtained mirror those found for
compound 18, except that the perpendicular permittivity has
a much larger value, 14 versus 10 at a reduced temperature of
60 ꢀC. Concomitantly, the value of the dielectric anisotropy
reaches a value of almost ꢁ7.7 at a reduced temperature of 65 ꢀC
just before recrystallisation occurs. The Maier–Meier theory
predicts an order parameter S of 0.6 at this value. The more
negative value of the dielectric anisotropy reduces the relative
threshold voltage to approximately 0.7 V for a cell thickness of
4.6 mm. Theoretically, this value should only be approximately
25% lower for the tetrafluoro compound i.e., approximately
1.2 V, due to the 50% increase in D3. This difference may be due
to misalignment occurring on cooling or else larger differences
in the elastic constants between the trifluoro and tetrafluoro
systems.
The larger negative dielectric anisotropy for compound 22
in comparison to 18 can be rationalised as follows. The inter-
annular twisting about the 1-10 carbon–carbon bond of the ter-
phenyl core unit suggests that the dipole associated with fluoro
substituents (one or two) located on the centre ring can oppose
the dipole associated with the adjacent ring that carries two
fluoro substituents, thereby minimising the overall lateral dipole.
As a consequence, the opposed dipoles would reduce the value of
the perpendicular permittivity. The fact that the perpendicular
permittivity has a larger value for compound 22 in comparison to
18 indicates that dipolar effects of the fluoro substituents are
additive rather than subtractive. Geometry optimisations con-
ducted on a series of di-substituted tetrafluoroterphenyls and
trifluoroterphenyl at the DFT/B3-lyp/def2-TZVPP and RIMP2/
def2-TZVPP levels of theory suggest that the fluoro substituents
preferentially lie on the same side of the molecule. The aromatic
rings are not equiplanar but the fluoro substituents are close
Fig. 3 Switching properties of the nematic phase of compound 22 as a function of reduced temperature. (a) Measurement of the parallel permittivity, 3k,
and perpendicular permittivity, 3t; (b) measurement of D3; and (c) threshold voltage for switching.
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Fig. 4 Minimum energy geometries of methyl compounds related to 18 and 22 computed at the DFT/b3-lyp/def2-TZVPP level of theory.
enough to interact (see Fig. 4). For each model terphenyl, two
stable conformers were identified, one in which the fluorine
atoms lie on the same side of the molecule in essentially a cis
conformation and the other in which the fluorine atoms lie on
opposite sides of the molecule in a trans conformation.
Regardless of the substituents chosen at the ipso position of the
first ring, the cis conformer was consistently found to be the most
stable, before and after zero point energy correction (see Table
5). At the DFT level, the difference in minimum energies varied
from as low as 0.209 kJ molꢁ1 in the methoxy-substituted tetra-
fluoroterphenyl to 0.627 kJ molꢁ1 in the methyl-substituted
terphenyl. Correction for zero point energy increased these
values by between 60 and 80%. For the two lighter tetra-
fluoroterphenyls, optimisations were also completed at the MP2
level of theory. In both cases, this resulted in a considerable
further stabilisation of the cis conformer compared to the trans
with differences in minimum energies before correction for ZPE
being between about 1.6 and 1.9 kJ molꢁ1. At the DFT level, the
difference in stabilities between the cis and trans conformers of
compounds 18 and 22 is essentially the same with the cis
conformer more stable by 0.23 kJ molꢁ1 in each case (see Table 5
and Fig. 4). The results of ZPE corrected DFT calculations and
MP2 calculations performed on the methyl and methoxy-
substituted variants of 22 strongly suggest that the inherent
stabilty of the cis conformers of 18 and 22 would be maintained
at higher levels of theory. Thus, it appears that the preferred
conformation adopted by these compounds is the result of
through-space coupling between the fluoro substituents located
on adjacent rings. This observation would seem to be reinforced
in the electrical field experiments, even though the molecules are
in dynamic motion about their long axes in the smectic state.
3.2.2. Ferroelectric phases under applied electric fields. In
order to compare the properties of the ferroelectric smectic C*
phases of the materials the materials were first doped with the
chiral compound BE8OF2N, structure 26, 5 and 10% by weight,
Table
6
Phase classification and transition temperatures (ꢀC) of
BE8OF2N-doped mixtures of difluoro- and trifluoroterphenyl n-alkyl
analogues
Cpd no.
% BE8OF2N
Transition temperatures/ꢀC
5
5
6
6
7
7
8
8
18
18
19
19
20
20
21
21
5
10
5
10
5
10
5
10
5
10
5
10
5
10
5
10
SmC* 122.7 SmA* 146.7 N* 159.9 Iso liq
SmC* 107.3 SmA* 141.6 N* 152.1 Iso liq
SmC* 123.9 SmA* 140.5 N* 151.7 Iso liq
SmC* 106.7 SmA* 135.7 N* 144.3 Iso liq
SmC* 127.3 SmA* 152.1 N* 155.1 Iso liq
SmC* 109.5 SmA* 146.9 N* 148.0 Iso liq
SmC* 129.6 SmA* 146.1 N* 147.6 Iso liq
SmC* 118.7 SmA* 145.6 Iso liq
SmC* 60.5 N* 107.9 Iso liq
SmC* 40.1 N* 101.5 Iso liq
SmC* 48.9 N* 100.8 Iso liq
SmC* 47.5 N* 94.1 Iso liq
SmC* 64.7 SmA* 72.3 N* 104.3 Iso liq
SmC* 47.2 SmA* 73.4 N* 97.5 Iso liq
SmC* 61.8 SmA* 65.7 N* 95.5 Iso liq
SmC* 33.7 SmA* 65.5 N* 89.6 Iso liq
Table 5 DFT and MP2 absolute and zero point energy corrected minimum energies for a series of disubstituted tetrafluoroterphenyls and tri-
fluoroterphenyls at the DFT/B3-lyp/def2-TZVPP and RIMP2/def2-TZVPP levels of theory
DFT/B3-lyp/def2-TZVPP
RIMP2/def2-TZVPP
Cis/a.u.
Trans/a.u.
D/kJ molꢁ1
Cis/a.u.
Trans/a.u.
D/kJ molꢁ1
2,20,3,30-Tetrafluoro-4,400-dimethyl-1,10:40,100-terphenyl
Energy
ZPE corrected energy
ꢁ1169.75899
ꢁ1169.75875
0.627
1.121
ꢁ1168.28376
ꢁ1168.28304
1.891
—
ꢁ1170.04271
ꢁ1170.04266
—
—
2,20,3,30-Tetrafluoro-4-methoxy-400-methyl-1,10:40,100-terphenyl
Energy
ZPE corrected energy
ꢁ1244.96110
ꢁ1244.96102
0.209
0.341
ꢁ1243.40218
ꢁ1243.40158
1.579
—
ꢁ1244.67242
ꢁ1244.67229a
—
—
2,20,3,30-Tetrafluoro-400-methyl-4-(pent-4-enyloxy)-1,10:40,100-terphenyl
Energy
2,20,3-Trifluoro-400-methyl-4-(pent-4-enyloxy)-1,10:40,100-terphenyl
ꢁ1400.93591
ꢁ1400.93582
0.239
0.231
—
—
—
—
—
—
Energy
ꢁ1301.69465
ꢁ1301.69456
a
First order saddle point.
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and then subjected to electrical field studies using various
waveforms. Materials 22–25 were not investigated using this
methodology because they do not exhibit the smectic C phase.
Compounds 5–8 all exhibit N–SmA–SmC phase sequences, with
second order smectic A to smectic C phase transitions. Thus
typical ferroelectric behaviour was predicted for these materials.
The second family of materials, 18–21, was found to exhibit
nematic to smectic C phase transitions, which were determined to
be weakly first order in nature. The transition temperatures for
the mixtures are shown in Table 6.
molecular volumes and thereby reduce the charge per unit
volume, which, respectively, reduces the relative spontaneous
polarisations. The longer methylene spacer chains also have the
effect of inducing relatively high tilt angles, hence compounds
6 and 8 have higher tilts, but lower polarisations, even though
the higher tilt angles should produce higher spontaneous polar-
isations. Switching studies at higher concentrations of the dopant
show that the relative tilt angles are lower, whereas the sponta-
neous polarisations are much higher as expected.
Fig. 6(a–d) shows the results obtained for the electrical field
studies of the trifluoroterphenyls. The pentyl homologues 18 and
19 exhibit monotropic nematic to smectic C phase transitions,
which are at considerably lower temperatures than the enantio-
tropic transitions obtained for the heptyl homologues 20 and 21.
The transitions in all cases appear to be weakly first order.
However, for both the 5 and the 10 wt% of the dopant the tilt
angle for the pentyl homologues shows a classical first order
jump at the N* to SmC* transition, whereas the heptyl homo-
logues show second order growth in the tilt which is due to the
injection of a smectic A phase in the mixtures upon addition of
the chiral dopant. For both dopant concentrations the sponta-
neous polarisation for the heptyl homologues 20 and 21 is lower
as expected in comparison to the pentyl materials 18 and 19.
Again, like the tilt angle, the spontaneous polarisation shows
a first order jump at the N* to SmC* for the shorter pentyl
homologues in comparison with the heptyl materials.
Fig. 5(a–d) shows the electrical field studies for compounds
5–8 with either 5 (a and b) or 10 wt% (c and d) of the chiral
dopant 26 in cells with 4 mm spacers. The results for the tilt angle
and of the spontaneous polarisation as function of the reduced
temperature are compared for all of the homologues of the
difluoroterphenyls.
It can be seen from the results that below a reduced temper-
ature of 10 ꢀC the spontaneous polarisation increases linearly,
whereas the tilt angles rise at first and then level off to a value
around 25ꢀ, with the homologues possessing the longer methy-
lene chain between the terphenyl core and the alkenic group
having higher values. Conversely, these homologues also have
the lower spontaneous polarisations. This can be rationalised as
follows: the longer methylene spacer lengths increase the relative
In addition to the evaluation of the tilt angle and the sponta-
neous polarisation, the response time was also determined for
a number of materials. A typical example is shown in Fig. 7 for
compound 8. The response time is shown for two mixtures of
Fig. 5 Spontaneous polarisation and tilt angle measurements for difluoro compounds 5–8 as a function of reduced temperature: (a) and (b) 5 wt%
dopant and (c) and (d) 10 wt% dopant.
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Fig. 6 Spontaneous polarisation and tilt angle measurements for trifluoro compounds 18–21 as a function of reduced temperature: (a) and (b) 5 wt%
dopant and (c) and (d) 10 wt% dopant.
compound 8 with dopant 26 Mix1 and Mix2 (with 5 and 10 wt%
of dopant, respectively) as function of the reduced temperature
and at three voltage levels (10, 20 and 30 V over a cell with
a 1.6 mm cell gap). Clearly the response time is much shorter for
mixtures containing a larger proportion of the chiral dopant,
with response times of below 50 ms at reduced temperatures of up
to 30 ꢀC. Obviously the higher the applied field the faster the
response, in combination with a high proportion of the dopant
response times of less than 10 ms can be achieved.
when the voltage is doubled from 10 V to 20 V but there is only
minimal difference between the 20 and 30 V measurements.
Unfortunately, these compounds tended to exhibit a large
number of chevron defects in the cell upon the transition from
the smectic A to smectic C phase. This behaviour is associated to
the layer shrinkage that occurs as the molecules begin to tilt.
4. Conclusions
It was found that the difluoro compounds exhibited good
bistability and it was possible to examine these using applied
current pulses to calculate the s–Vmin curves.15 These results were
in strong agreement with the observed switching times which
were determined by applying an ac field of equivalent voltage to
the cell at a frequency of 50 Hz. It is clear from the results
obtained that there is a significant increase in switching speed
The materials reported in this article give a multifaceted
approach to the development of liquid crystal display device
materials research. The greater the number of fluoro substituents
(2–4) located on the p-terphenyl core unit the more likely it was
found that the material would be nematogenic. The greater the
number of fluoro substituents the larger the value of the negative
dielectric anisotropy, and the lower the threshold voltage for
switching in homeotropically aligned cells. Thus these materials
are of interest in vertically aligned nematic liquid crystal display
(VAN-LCD) devices. Conversely the fewer the number of fluoro
substituents the more likely smectic phases were to be found. For
such materials possessing longer aliphatic chains resulted in the
stabilisation of smectic C phases, such that the materials exhibit
nematic, smectic A and smectic C phases, which is an ideal phase
sequence for materials of practical interest as host systems in
surface stabilised ferroelectric liquid crystal display (SSFLCD)
devices. The incorporation of the chiral dopant BE8OF2N into
mixtures with the difluoro-substituted hosts resulted in materials
with sub-fifty microsecond response times. Overall this work
demonstrates that materials can be developed in tandem for
uses in different and diverse LCDs.
Fig. 7 Switching times for the ferroelectric phase of compound 8 in a
1.6 mm planar aligned cell.
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€
7 J. W. Goodby, I. M. Saez, S. J. Cowling, V. Gortz, M. Draper,
A. W. Hall, S. Sia, G. Cosquer, S.-E. Lee and E. P. Raynes, Angew.
Chem., Int. Ed., 2008, 47, 2754–2787.
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