A new series of liquid crystalline dimers with exceptionally high apparent tilt

Article abstract of DOI:10.1039/b615541c

A new series of dimeric chiral antiferroelectric materials using a trisiloxane as the central connecting linker and having fluorinated tails has been synthesized together with their corresponding monomers. Spacer lengths of 3-6 and 11 methylene groups were used. As a comparison to see the effect of the siloxane length on the phase behaviour, one dimer with a tetrasiloxane core was also synthesized. Monomers with 3-6 carbons in the spacer showed just orthogonal smectic phases and only the long 11 carbon spacer 7F4PPB11 showed tilted phases in a very similar way to what was found in the unfluorinated monomers previously studied. All trisiloxanes showed a high temperature antiferroelectric phase and those with a spacer length of 4 or more carbons also showed one or two columnar antiferroelectric phases when decreasing the temperature. The spontaneous polarisation P_s was high, especially in the columnar phases where it ranged from 800-1200 nC cm^-2 and upward. The apparent tilt angles were exceptionally high, above 45 degrees for all materials studied. In accordance with the trend described previously, the tetrasiloxane containing dimer showed a ferroelectric phase at high temperature. But in this case the columnar phase was found to be antiferroelectric. Hence, the general idea that trisiloxane cores promote antiferroelectricity and tetrasiloxanes promote the ferroelectric phase is still correct also for these materials in the SmC* phase, but not for the columnar phase. It can also be noted that the phase separation between siloxane, fluorinated tail and mesogenic parts of the molecules was so high that the material became columnar on lowering the temperature. The Royal Society of Chemistry 2007.

Full text of DOI:10.1039/b615541c

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
A new series of liquid crystalline dimers with exceptionally high apparent
tilt{
Nils Olsson,a Martin Schro¨der,b Siegmar Diele,b Gunnar Andersson,c Ingolf Dahl,c Bertil Helgeea and
Lachezar Komitov*c
Received 26th October 2006, Accepted 14th March 2007
First published as an Advance Article on the web 3rd April 2007
DOI: 10.1039/b615541c
A new series of dimeric chiral antiferroelectric materials using a trisiloxane as the central
connecting linker and having fluorinated tails has been synthesized together with their
corresponding monomers. Spacer lengths of 3–6 and 11 methylene groups were used. As a
comparison to see the effect of the siloxane length on the phase behaviour, one dimer with a
tetrasiloxane core was also synthesized. Monomers with 3–6 carbons in the spacer showed just
orthogonal smectic phases and only the long 11 carbon spacer 7F4PPB11 showed tilted phases in
a very similar way to what was found in the unfluorinated monomers previously studied. All
trisiloxanes showed a high temperature antiferroelectric phase and those with a spacer length of 4
or more carbons also showed one or two columnar antiferroelectric phases when decreasing the
temperature. The spontaneous polarisation Ps was high, especially in the columnar phases where
it ranged from 800–1200 nC cm22 and upward. The apparent tilt angles were exceptionally high,
above 45 degrees for all materials studied. In accordance with the trend described previously, the
tetrasiloxane containing dimer showed a ferroelectric phase at high temperature. But in this case
the columnar phase was found to be antiferroelectric. Hence, the general idea that trisiloxane
cores promote antiferroelectricity and tetrasiloxanes promote the ferroelectric phase is still correct
also for these materials in the SmC* phase, but not for the columnar phase. It can also be noted
that the phase separation between siloxane, fluorinated tail and mesogenic parts of the molecules
was so high that the material became columnar on lowering the temperature.
the molecule.3–10 Siloxane chains as well as fluorinated chains
1. Introduction
tend to avoid the normally stiff aromatic core of the liquid
Liquid crystals provide a very rich field for research due to the
crystal and create phase separation that can be used to tailor
variety of different phases that these materials exhibit as a
the phase behaviour of the material. Fluorinated chains
result of the different molecular structures and interactions
promote many different types of self assembly of molecules
between them. Traditionally the greatest emphasis was on the
and can be combined with siloxane linking groups in dimers to
get a combined effect.2,11
form of the liquid crystal molecule when it came to designing
new suitable materials, but also other features like polarity and
For a more thorough review of how much the complexity of
phase separation effects have also been studied. Lately it has
the self organisation of a material can increase due to phase
been shown how useful these effects are for providing a
segregation, we recommend to the reader an excellent article
complex phase flora.1 The phase separation has been used
to alter the liquid crystalline phase less profoundly but
nevertheless in important ways such as for instance in the
synthesis of the antiferroelectric materials carried out by Coles
et al.2 That the phase segregation effect of side chain polymer
liquid crystals can be used to create smectic phases is also a
well established fact.
by Cheng et al.12 containing many relevant references.
Since their discovery, antiferroelectric liquid crystals
(AFLCs)13,14 have been considered to be very promising
materials for displays. Compared to ferroelectric liquid
crystals (FLC), AFLCs can be aligned more easily and their
electronic driving is simpler since there is no need for dc
compensation. Moreover, AFLCs have potential for genera-
tion of a grey scale. In particular AFLC materials with a
molecular tilt of 45 degrees, and thus, as described by de
Meyere et al.,15 capable of displaying high contrast images
due to the inherent field free dark state of such a material
when aligned properly, have been of great interest in the last
few years. Several studies of materials with an inherent field
free dark state have been performed on both monomer
blends,2,6,16–19 banana shaped molecules (in this case not
having a dark state for the same reason),20 dimeric single
compound siloxanes2,6,9,10,21,22 and dimers with an alkane as
the connecting linker.23
For low molar mass liquid crystals a favoured way of
causing phase segregation is to use a siloxane chain as a part of
aPolymer Technology, Department of Chemical and Biological
Engineering, Chalmers University of Technology, SE-412 96,
Gothenburg, Sweden
bMartin-Luther-Universita¨t Halle-Wittenberg, Institut fu¨r Physikalische
Chemie, Mu¨hlpforte 1, 06108 Halle, Deutschland
cLiquid Crystal Physics, Department of Physics, Go¨teborg University,
SE-412 96, Gothenburg, Sweden. E-mail: komitov@physics.gu.se
{ Electronic supplementary information (ESI) available: further
synthesis details; some pictures of the antiferroelectric switching
behaviour of di(7F4PPB6)3Si. See DOI: 10.1039/b615541c
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The goal of the present work has been to make very stable
antiferroelectric materials by increasing the phase segregation
of some dimers, previously synthesized by us,9 by fluorinating
the tails of the mesogenic groups. The syntheses and liquid
crystalline properties of 1,5-di(49-alkyloxy)biphenyl-4-car-
boxylic acid 4-[1-(2,2,3,3,4,4,4-heptafluorobutoxycarbonyl)-1-
(R)-ethoxy]phenyl ester) 1,1,3,3,5,5-hexamethyltrisiloxanes
with spacer lengths of 3–6 and 11 carbons, and the
corresponding monomers are reported. We synthesized one
tetrasiloxane dimer in order to differentiate between the
siloxane effect and the effect from the dimeric structure as
shown in Scheme 1. The chiral centres of these liquid
crystalline dimers are located near the periphery of the
molecule to allow the chiral group great freedom of movement.
Characterisations using microscopic observation, powder and
oriented X-ray diffraction, calorimetric and electro-optical
methods were applied to deduce the phase behaviour and
physical characteristics of the substances. All dimers proved to
be antiferroelectric with very high apparent tilt angles, above
45 degrees, as well as showing columnar phases. These dimers
are, to the best of our knowledge, the first dimers with a
siloxane central connector that have an apparent tilt that
exceeds 45 degrees.
2. Experimental
The proposed structures of all the dimers and their inter-
mediates were verified using 300 MHz 1H NMR spectra
obtained using a Varian VXR300 spectrometer. All spectra
were run in CDCl3 or DMSO-d6 solutions. Infrared
spectra were recorded on a Perkin-Elmer 1000 FT-IR spectro-
photometer using a BaF2 cell for liquid phase IR and toluene
as the solvent.
Materials and reagents were of commercial grade quality
and used without further purification unless otherwise noted.
Dry toluene and methylene chloride were obtained by passing
the solvents through a bed of aluminium oxide (ICN Alumina
N-Super I). (+)-2-(4-Hydroxyphenoxy)propionic acid was
generously provided by BASF. The 4-(2-alkenoxy)biphenyl-
4-carboxylic acids (2a–e) were previously prepared by us to
make the analogous non-fluorinated compounds.9
2.1. Synthesis
The synthesis of monomers 3a–3e and dimers 4a–f was carried
out according to the reactions in Scheme 1. All structures were
1
verified with H NMR spectroscopy and NMR data were in
accordance with the structures in all cases.
Scheme 1 Reagents and conditions: (i) heptafluorobutanol, 1,2-diethoxyethane, cyclohexane, PTSA; (ii) C6H6, EtOH; (iii) acetone, alkyl bromide,
K2CO3, followed by EtOH + NaOH; (iv) DCC/DMAP, CH2Cl2; (v) toluene, Pt catalyst, dihydrosiloxane.
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The monomers and dimers described are all based on the
central mesogenic unit derived from the phenoxy propanoic
ester of biphenyl carboxylic acid (PPB) and after the synthetic
part we will use the abbreviated name based on the PPB unit
with numbers depending on the number of carbons in the
tail and spacer, number of fluorine atoms in the tail, and the
number of silicon atoms in the siloxanes used.
6.92 (d, 2 H),6.99 (d, 2 H), 7.15 (d, 2 H), 7.56 (d, 2 H), 7.66 (d,
2 H), 8.21 (d, 2 H).
Preparation of 4b–4f follows the same general principle as
the preparation of 4a and is outlined in detail in the ESI.{
2.2. Characterisation
Phase characterisation techniques. The phase behaviour
of the different materials under study was identified by a
combination of optical microscopy, differential scanning
calorimetry (DSC), electrical and electro-optical measure-
ments. DSC measurements were carried out with a Perkin-
Elmer DSC 7 differential scanning calorimeter.
Preparation of 2-(R)-(4-hydroxyphenoxy)propionic acid
2,2,3,3,4,4,4-heptafluorobutyl ester (1). A solution of 2-(R)-
(4-hydroxyphenoxy)propionic acid (27 mmol,
5 g) and
1,1,1,2,2,3,3-heptafluorobutanol (6.5 g) in 1,2-diethoxyethane
(75 ml) and cyclohexane (80 ml) was refluxed in a Dean Stark
apparatus with sulfuric acid (2 g) for 7 days, periodically
following the reaction progress with TLC. The product was
dissolved in diethyl ether and washed with water. The ether
phase was evaporated and the dry product was purified by
column chromatography on silica gel with petrol ether–ethyl
acetate 2 : 1 as an eluent. Yield 4.7 g (48%).
X-Ray diffraction patterns were recorded using a Guinier
film camera or a Guinier goniometer on non-oriented samples;
X-ray investigations on oriented samples were carried out with
a two-dimensional detector (HI-Star, Siemens AG).
Optical microscopy studies were carried out on standard
E.H.C. cells with gaps of 2, 4 and 6 mm, respectively. The cells
were filled with the liquid crystal material in the isotropic
phase by capillary forces. The inner substrates surfaces of the
cells, precoated with ITO transparent electrodes, were covered
by a thin polyimide alignment layer unidirectionally rubbed.
An alignment layer of this type usually imposes a planar
alignment on most liquid crystals. The electro-optic studies
were carried out in a set-up consisting of a polarising micro-
scope connected to a camera and photo-detector, oscilloscope,
function generator with an amplifier and thermo stage with
temperature control within ¡0.1 uC. The apparent tilt angle,
1H NMR in CDCl3: d 1.63 (d, 3 H), 4.65 (m, 2 H), 4.76 (d,
1 H), 5.65 (s broad, 1 H), 6.76 (m, 4 H).
Preparation of 49-allyloxybiphenyl-4-carboxylic acid 4-[1-
(2,2,3,3,4,4,4-heptafluorobutoxycarbonyl)-1-(R)-ethoxy]phenyl
ester (3a) 7F4PPB3. 360 mg (1.35 mmol) of the acid
49-allyloxybiphenyl-4-carboxylic acid (2a), 493 mg (1.35 mmol)
of 1 and 10 mg dimethylaminopyridine (DMAP) in dry methyl-
ene chloride was treated with 280 mg (1.35 mmol) dicyclo-
hexylcarbodiimide (DCC) at 0 uC. The temperature was
allowed to rise to room temperature and stirring was con-
tinued overnight. Dry glassware was used. Column chromato-
graphy on silica gel with a gradient of toluene and 0–5% ethyl
acetate eluent gave 420 mg of pure product. Yield 51%.
1H NMR in CDCl3: d 1.68 (d, 3H), 2.09 (q, 2 H),4.61 (m,
2 H), 4.66 (d, 2 H), 4.87 (q, 1 H), 5.32 (d, 1 H), 5.48 (d, 1 H),
6.09 (m, 1 H),6.93 (d, 2 H), 7.03 (d, 2 H), 7.15 (d, 2 H), 7.60 (d,
2 H), 7.68 (d, 2 H), 8.22 (d, 2 H).
happ, of the molecules was determined from the electro-optical
studies as half of the switching cone angle in the field-induced
ferroelectric state with ¡1u accuracy as described previously
by us.8 Spontaneous polarisation, Ps, of the field-induced
ferroelectric state was measured using the Diamant-Bridge
method.24
3. Results and discussion
Preparation of 3b–3e followed the same general principle
and is outlined in the ESI.{
3.1. Monomers and their liquid crystal properties
As the monomers, with the phase transitions shown in Table 1,
were not in the focus of this study we made only a cursory
characterisation of their behaviour. Only the monomers with
spacer lengths of 4, 6 and 11 had the phase sequence verified
with Guinier film X-ray measurements but the similarities
between their textures and the respective textures of the other
monomers were convincing enough to assign the phase
sequences given in Table 1.
Preparation of 1,5-di(49-allyloxybiphenyl-4-carboxylic acid
4-[1-(2,2,3,3,4,4,4-heptafluorobutoxycarbonyl)-1-(R)-ethoxy]-
phenyl ester) 1,1,3,3,5,5-hexamethyltrisiloxane (4a)
di(7F4PPB3)3Si. 94 mg (0.154 mmol) of 3a, 14.5 mg
(0.0695 mmol) dihydro-1,1,3,3,5,5-hexamethyltrisiloxane and
0.1% platinum divinyltetramethyldisiloxane as catalyst were
dissolved in 5 ml of dry toluene in a flask fitted with a calcium
carbonate drying tube and was stirred at room temperature
overnight. Using IR spectroscopy the completion of the
reaction was checked by monitoring the disappearance of the
Si–H stretch at 2160 cm21. The product was purified by column
chromatography on silica gel with toluene–ethyl acetate gradient
as an eluent followed by precipitation in petrol ether from a
minimal amount of toluene and collected with centrifugation.
Dust was removed by dissolving in dichloromethane and passing
the solution through a 0.2 mm Teflon filter. Drying in vacuum
at 50 C yielded 90 mg of pure product. Yield 90%.
7F4PPB3 (3a). The monomer with the shortest spacer
showed the highest transition temperatures with an isotropic to
SmA* transition at 223 uC and crystallisation at 100–105 uC.
This monomer is very easily supercooled. Slow cooling in the
DSC below 1 uC min21 gave primarily a very ordered/crystal
phase below 100 uC while faster cooling produced a mono-
tropic phase, the range of which increased with increasing
cooling rate as crystallization was suppressed. In the thin
E.H.C. cells the more ordered/crystal phase formed very slowly
as the cell surfaces stabilised the monotropic phase in
comparison to the crystal phase. The monotropic phase
1H NMR in CDCl3: d 0.1 (m, 18 H), 0.63 (m, 2 H), 1.68 (d,
3H), 1.87 (m, 2 H),3.98 (m, 2 H), 4.69 (t, 2 H), 4.90 (q, 1 H),
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Table 1 Transition temperatures (uC) of the monomers taken at heating, n + 2 equals the number of carbons in the spacer
Label
n + 2
Transition temperatures/uC
7F4PPB3
7F4PPB4
7F4PPB5
7F4PPB6
7F4PPB11
heating
heating
heating
heating
heating
3
4
5
6
11
Cr 105
SmE 96
Cr1 102 Cr2 108
Cr 110
Cr 73 SmF 84
SmA* 223
SmB 117
SmE 115 SmB 125
SmB 116
SmC* 132
Iso
SmA* 207
SmA* 203
SmA* 190
SmA* 146
Iso
Iso
Iso
Iso
showed the characteristic banded focal conic texture where the
bands stayed clearly visible down to room temperature in the
E.H.C. cell and is believed to be the SmE phase.
when it transforms into the SmB phase at 116 uC and
crystallized at 110 uC as was shown by Guiner-film X-ray
measurements.
7F4PPB4 (3b). The second shortest monomer behaved in a
way very similar to 7F4PPB3 but at lower transition tempera-
tures. The SmA* focal conic texture is seen between 117 uC and
207 uC and below that range the banded SmB* or SmE focal
conic texture appeared. Guinier-film measurements showed that
the top banded texture belong to an SmB phase at 96 uC–117 uC
while the low temperature phase is an SmE phase.
7F4PPB11 (3e). The longest monomer in this study is the
first one with tilted smectic phases coming from the larger
rotational volume of the side chain compared to the volume of
the mesogenic core.
It showed a SmA phase from 132–146 uC and a broad SmC*
phase in the temperature interval 84–132 uC with a tilt angle at
117 uC of about 35 degrees exhibiting spontaneous polarisa-
tion Ps of 220 nC cm22 at 85 uC, followed by what was shown
to be a SmF phase at 73–84 uC. In an E.H.C. cell the SmF
phase was found to be monotropic down to room temperature.
The general feature of the monomers with six or fewer
carbons in the alkylene spacer is the formation of orthogonal
phases while the monomer with 11 carbons in the spacer forms
a broad tilted phase below a SmA* phase. It is important to
note that only 7F4PPB11 showed a tilted phase and that none
of these materials are antiferroelectric.
7F4PPB5 (3c). On cooling this monomer showed
a
transition from isotropic to SmA* at 201 uC with focal conic
texture that showed transition bars at lower temperatures. The
similar behaviour to 7F4PPB4 makes us believe that we have a
transition to an SmB phase at 123 uC and to SmE at 113 uC.
In the DSC measurements we got crystallisation at low
temperatures that showed as two extra melting points before
the SmE phase forms but these crystal phases could not be seen
in the E.H.C. cells.
3.2. Dimers and their liquid crystal properties (Table 2)
7F4PPB6 (3d). The second longest monomer showed a
transition from isotropic to SmA* at 190 uC with focal conic
texture patterned by transition bars at lower temperatures
di(7F4PPB3)3Si (4a). The dimer with the shortest spacer
has the most narrow liquid crystalline temperature range. On
Table 2 Phase transition temperatures (uC) of the dimers taken on heating
Label
n
Transition temperatures/uC
di(7F4PPB3)3Si
di(7F4PPB4)3Si
di(7F4PPB5)3Si
di(7F4PPB6)3Si
di(7F4PPB11)3Si
di(7F4PPB6)4Si
a
3
4
5
6
11
6
Cr 79 Cr2
X 85 X2
X 98 X2
CR 81 X2
79 Col1 102
X 76
134 SmCA
97 Col
106 Col
88 Col
Col2 133.5 SmCA
Col 111
140 Iso
106 SmCA
120 SmCA
120 SmCA
145 SmA*
SmC*
138 Iso
143 Iso
140 Iso
150 Iso
128 Iso
n stands for the number of carbons in the spacer. di(7F4PPB6)4Si has a tetrasiloxane central unit while the other compounds have a
trisiloxane centre.
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heating it shows just a typical SmCA phase between 134 uC
to 140 uC. On cooling the material is easily supercooled and
what seems to be a columnar phase like in the longer members
of this series appears between 118–122 uC when cooling at
5 uC min21 in the DSC and just below the SmCA phase in the
E.H.C. cell. Only a limited study was performed with this
material due to its monotropic behaviour.
a SmA phase which means that the material had a much
greater temperature dependence of the tilt angle then the
previously described substances. The molecular tilt also
exceeds 45u in the columnar phase.
di(7F4PPB6)4Si (4f). This dimer has four siloxane units
instead of the three that di(7F4PPB6)3Si contains and this
change proved to alter the characteristics more then the
variation of the carbon spacers had done. It has an antiferro-
electric columnar phase at 76–111 uC and a SmC* phase
between 111–126 uC. Notice that in this case the phase is a
ferroelectric SmC* and not an antiferroelectric one as in the
previous compounds. This is in agreement with what Coles
et al. described as the effect of 3 and 4 silicons in the bridging
group of the dimers.6,10,11 The columnar phase still has
antiferroelectric behaviour, however, and this can be explained
by the fact that the columnar phases are non-interdigitating
phases, the siloxane parts are assumed to be in the middle of
each layer and the fluorinated end tails are phase separating.
Thus the layer–layer interaction is governed by the fluorinated
tails in all the columnar phases and the siloxane bridging
group in the dimer structure is not as much of an issue.
The siloxane group, however, still has an influence on the
switching process. Although the antiferroelectric phase of the
di(7F4PPB6)4Si is a columnar phase, it is very easy to switch
from the antiferroelectric to the ferroelectric state. The field-
induced antiferro- to ferroelectric transition is of first order
manifested by generation and propagation of field induced
solitary waves invading the anticlinic liquid crystal phase
along the smectic layers. The time for relaxation back to the
antiferroelectric state in the lower temperature region, when
the applied electric field is switched off, is on the order of
minutes. This means that the antiferroelectric structure in this
compound is greatly destabilized compared to the correspond-
ing phase in the analogous dimer with 3 siloxane units. Due to
the very long relaxation time, the double current peak, typical
characteristic of the antiferro- to ferroelectric switching, was
not observed and the X-ray measurements only showed that
the phase of this compound in the temperature range at
76–111 uC is a tilted columnar phase, which could be
either ferro- or antiferro-electric. However, the generation
and propagation of the quasi-one dimensional finger-like
solitary waves observed in di(7F4PPB6)4Si under an applied
electric field is an indication of the antiferroelectric character
of the columnar phase.
di(7F4PPB4)3Si (4b). The dimer with four carbons in the
spacer exhibited a rectangular columnar phase between 97–
108 uC and a SmCA up to 138 uC where it underwent a transi-
tion to the isotropic phase. The field-induced ferroelectric
phase had a very large apparent tilt angle in the entire tem-
perature range 97–138 uC, between 44–46 degrees. The slow
vibration axis changes direction with 90 degrees at 106 uC which
indicates that the tilt angle reaches 45 degrees at 106 uC.8
Below 97 uC the material was still switchable in the E.H.C.
cell, but it was very difficult to measure the apparent tilt due to
the changes in texture taking place under the application of an
electric field.
di(7F4PPB5)3Si (4c). As the spacer is extended to five
carbons the properties stay similar to those of the material
with a four carbon spacer. di(7F4PPB5)3Si exhibited a
rectangular antiferroelectric columnar phase between 106–
120 uC and an antiferroelectric SmCA phase up to 143 uC, after
that transforming to the isotropic phase. The tilt angle is very
large in the entire antiferroelectric phase and varied between
44–47u in the field-induced ferroelectric phase. The 90 degree
switch of the slow axis that indicates the field-off state with 45u
molecular tilt was measured with a l first-order red plate in the
same way as in the study of di(7F4PPB11)3Si.8 In that way the
tilt angle was found to go above 45u at 124 uC and back below
45u at approximately 100 uC. At 124 uC this matches the
measured induced tilt angles well (at 100 uC the induced tilt
angle could not be measured). It should be noted that in the
columnar phase we could not apply enough field strength to
switch the sample without destroying the cell and the switch is
also to a very large extent a continuous transition without a
clear switching threshold.
di(7F4PPB6)3Si (4d). When the spacer has six carbon atoms
the properties stay very much the same. The material exhibits
an antiferroelectric columnar phase between 88–120 uC and
antiferroelectric SmCA phase up to 140 uC becoming isotropic
at higher temperatures. Also in this material the apparent tilt
in the field-induced ferroelectric phase is very large and above
45u in the entire columnar phase. By using a l first-order red
plate, a switch in the vibration axis at 119 uC was shown. In
this case, in contrast to the others, it was easier to measure the
induced tilt angle in the columnar phase than in the SmCA
phase as there seemed to be a rotation of the texture in the
antiferroelectric top phase when an electric field was applied,
which made measurements of tilt angles uncertain.
X-Ray characterisation. X-Ray studies were performed using
the Guinier method and a small angle camera for powder-like
samples as well as a 2-D detector for oriented samples. The
alignment was achieved by slowly cooling a drop on a clean
glass plate from the isotropic state. It can be summarised that
the patterns of the different compounds in the corresponding
phases exhibit a very similar feature. Therefore the general
structural properties from a X-ray viewpoint, that gives the
phase diagram shown in Fig. 1, shall be discussed for the
example of the compound di(7F4PPB11)3Si.
di(7F4PPB11)3Si (4e). The material with the longest spacer
was extensively studied earlier,8 the highlights are that it has
two different columnar phases at 79–103 uC and 103–133 uC as
well as a SmCA phase at 133–145 uC. At 145–150 uC there was
Cooling the sample from the isotropic phase a layer
structure without order in the layers appears. This is indicated
by sharp reflections in the low angle region as well as by a
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Fig. 3 Temperature dependence of the d-value of di(7F4PPB11)3Si.
Fig. 1 Phase diagram of the dimers as deduced from just the X-ray
measurements.
high temperature phase; schlieren texture in the following
phase) and switching behaviour a SmA*–SmCA polymorphism
is proved. At further cooling the pattern is changed at about
133 uC (Fig. 2c). In the small angle region of a pattern of a well
oriented sample (Fig. 2d) several reflections can be seen and
they can be indexed on the basis of a rectangular centred cell
with a c2mm symmetry (only reflections with h + k = 2n are
observed). The pattern is changed once more at about 104 uC
(Fig. 2e). The reflections exhibit a different extinction, which
ruled out a centred cell. Reflections with h + k = 2n + 1 are
observed, too, where the 0k reflections were found only with
k = 2n. Since h0 reflections (on the equator of the pattern)
diffuse scattering in the wide angle region of the pattern. In
the high temperature phase (145–150 uC) the outer diffuse
scattering is more or less smeared out on an arc (Fig. 2a), this
means that a tilt of the molecules within the layer cannot
be ruled out definitely. But in the next phase (133–145 uC),
following with decreased temperature, the diffuse scattering is
concentrated more and more out of the equator of the pattern
(Fig. 2b), from which an increasing tilt with decreasing
temperature can be deduced. This is in agreement with the
temperature dependence of the d-values (Fig. 3). Together with
information about the textures (homeotropic texture in the
Fig. 2 X-Ray diffraction pictures of di(7F4PPB11)3Si as described in the text
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Table 3 Smectic layer thickness of the dimers near the centre of the
SmCA temperature region
Label
d-Value/nm
n + 2
7F4PPB4
7F4PPB5
7F4PPB6
7F4PPB11
2.76
2.85
2.94
3.41
4
5
6
11
could not be detected, a difference between the possible plane
groups p2mg and p2gg can not be distinguished.
If we speculate about a molecular model, then the length of
the molecules (L was estimated by CPK models) has to be
compared with the layer thickness (d-value). In Table 3, the
d-values as a function of the spacer length are shown. The
function displays a linear behaviour, but the increase per
methylene group is much smaller than expected for an
elongated molecular form (0.05 nm instead of 0.125 nm). It
suggests a large tilt and/or a deviation of the alkyl spacers from
the all-trans conformation. Despite this deviation the com-
parison of the d-value with the length of the molecule (L)
Fig. 4 Apparent tilt angle happ of the dimers as a function of tem-
perature. di(7F4PPB4)3Si (r); di(7F4PPB5)3Si (%); di(7F4PPB6)3Si
(m); di(7F4PPB11)3Si (6).
the transition into the low-temperature Colrect phase at 104 uC.
This transition is connected with a change of the symmetry. In
the high temperature phase the symmetry demands identical
building groups at the positions 0, 0 and K, K of the lattice.
Therefore it is assumed that in the high-temperature Colrect
phase the bent direction of the molecules within a column
(perpendicular to the drawing plane) is statistically distributed
from left to right. At cooling into the low temperature Colrect
phase the bent direction is more ordered, which results in a
primitive lattice with the symmetry p2gg.
shows
a great difference. For example, for compound
di(7F4PPB11)3Si: L # 8.5 nm, d = 3.41 nm; for compound
di(7F4PPB4)3Si: L # 7.0 nm, d = 2.74 nm. Such a difference
demands additional assumptions about the conformation and
the packing of the molecules. It is known from the literature that
the siloxane spacer (especially with 3 siloxane groups25) favours
a bent conformation leading to an antiferroelectric structure.
We also see from the apparent tilt measurements (Fig. 4) that
the tilt of the mesogenic part varies between 0–53u. However,
even if a bending angle average of about 120u is assumed it
leads to a length L of 7.5 nm. That is greater than twice the
d-value in the SmCA phase (=3.41 nm) and demands an
intercalation of the molecules by one leg. This packing (Fig. 5)
in connection with the measured tilt angle results in a very good
agreement between the measured and calculated values and
proves the existence of an interdigitated SmCA phase.
The cooling process described above can be illustrated by
Fig. 5 and 6. The highest temperature liquid crystal phase is a
SmA phase which experiments indicate is a de Vries type of
SmA phase. The similar materials studied previously9,21 have
been shown to have such a SmA phase. Each layer in a de
Vries SmA is tilted as shown in Fig. 5b but the orientation of
the tilt angle changes randomly from one layer to another.
Upon cooling the tilt angle increases and the molecular order
transforms into anticlinic, i.e. SmCA, with alternating tilt
direction in every layer (Fig. 5a). The anticlinic molecular
order in SmCA could be changed to synclinic, i.e. SmC
(Fig. 5b), by applying an electric field. The energy required to
rotate the very flexible siloxane/alkane chains in the middle of
The lattice parameters for compound di(7F4PPB11)3Si in
the Colrect phase amount to a long side of 8.5 ¡ 0.3 nm and
short of 6.5 ¡ 0.2 nm. These parameters remain unchanged at
Fig. 5 The interdigitated nature of the dimer ordering with the fluorinated outer parts and the siloxane centre being mixed in the SmCA phase (a)
and in the induced ferroelectric phase (b); d represents the smectic layer thickness, h represents the tilt angle.
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Fig. 7 The spontaneous polarisation of the dimers as a function
of temperature. di(7F4PPB4)3Si (r); di(7F4PPB5)3Si (%);
di(7F4PPB6)3Si (m); di(7F4PPB11)3Si (6).
effect of the siloxane length, one dimer with a tetrasiloxane
core was also made and studied. Monomers with 3–6 carbons
in the spacer showed orthogonal smectic phases and only the
long 11 carbon spacer 7F4PPB11 showed tilted phases in a
very similar way to what we obtained in the unfluorinated
monomers previously studied.9 All trisiloxanes showed a high
temperature antiferroelectric phase followed by one or two
columnar antiferroelectric phases when decreasing the tem-
perature. The polarisation was high especially in the columnar
phases where it ranged from 800–1200 nC cm22 (Fig. 7) but
was also difficult to study as very high fields was required to
induce a transition to the ferroelectric state. The apparent tilt
angles were also very high, above 45 degrees for all materials
studied (Fig. 4). In comparison, the tetrasiloxane containing
dimer showed a ferroelectric phase at high temperature, in
accordance with the trend described by Coles et al.3,6,10,11 for
tetrasiloxanes. But also in this compound the columnar phase
was found to be antiferroelectric even though it was much
easier to switch into a ferroelectric state. It relaxed, however,
very slowly back to the antiferroelectric state. Hence, the
general idea that trisiloxane cores promote antiferroelectricity
is still correct also for this material. It can also be noted that
the phase separation between siloxane, fluorinated and
mesogenic parts of the molecules and the difference in size of
these parts in the series of molecules studied here have a similar
effect as in some fluorinated dimers previously studied11 and in
this case with an even greater effect as the phases became
columnar when lowering the temperature while in the earlier
study it was found that the separation gave rise to modulated
tilted smectic phases. It is of interest to note that these dimers
do show columnar phases with some resemblance to banana
phases, but it has to be kept in mind that although without an
electric field the molecules resemble bananas the siloxane and
alkane spacers ensure that the freedom of movement for each
mesogen is very high. The fact that the mesogens can move
relatively independently of each other under an electric field
means that the switching characteristics to a large degree
resembles those found in normal monomeric liquid crystals,
especially in the SmCA and SmA* phases
Fig. 6 Possible ordering of the molecules that produces a columnar
phase. The box depicts the column with the long and short lattice
parameters a and b.
the dimers is very low, and therefore will not greatly influence
the energy needed to switch the molecules from anticlinic to
synclinic order. At lower temperatures an additional increase
of the molecular order takes place, which results in the
formation of two different columnar phases, with structure
schematically depicted in Fig. 6. However, the exact models of
these phases are still not known. Upon very high electric fields,
close to the dielectric breakdown of conventional cells, a
ferroelectric state can be induced. Due to the high field that is
needed considerable changes in the phase structure may take
place during the switching process.
3.3. Comparison of liquid crystal properties
In this study all the dimers show complex phase behaviour in
comparison to the non-fluorinated counterparts that only had
the SmA* and SmC*/SmCA* phases.9 It is well known that
fluorination induces greater phase separation and we believe
that in this case we get three different layers, siloxane rich,
fluorine rich and mesogen rich, at temperatures below the
SmCA phase. However, due to the different size of the three
different parts the final structure is not a simple smectic
structure and instead we get alternating columnar blocks to
make up for the size incompatibility of the different layers. All
the dimers also show high apparent tilt angles that exceed
45 degrees (Fig. 4) and very high polarisation in the columnar
phase (Fig. 7). The smectic layer thickness, shown in Table 3,
are consistent with each layer being formed by one tilted
mesogen, being less then half the calculated length of the
total dimer and thus V-shaped and probably interdigitated as
suggested by Coles et al.2,3,10,25
All in all a new series of dimeric chiral antiferroelectric
materials using a trisiloxane as the central connecting linker
and fluorinated tails has been synthesized together with
their corresponding monomers. Spacer lengths of 3–6 and 11
methylene groups were used. As a comparison to evaluate the
4. Conclusions
The main goal of this work was to study the effects of
increased phase separation in siloxane dimeric liquid crystals
2524 | J. Mater. Chem., 2007, 17, 2517–2525
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7 B. Musgrave, P. Lehmann and H. J. Coles, Liq. Cryst., 1999, 26,
1235–1249.
8 N. Olsson, I. Dahl, B. Helgee and L. Komitov, Liq. Cryst., 2004,
31, 1555–1568.
9 N. Olsson, B. Helgee, G. Andersson and L. Komitov, Liq. Cryst.,
2005, 32, 1139–1150.
10 W. K. Robinson, P. S. Kloess, C. Carboni and H. J. Coles, Liq.
Cryst., 1997, 23, 309–312.
11 P. Lehmann, W. K. Robinson and H. J. Coles, Mol. Cryst. Liq.
Cryst. Sci. Technol., Sect. A, 1999, 328, 221–228.
12 X. Cheng, M. K. Das, U. Baumeister, S. Diele and C. Tschierske,
J. Am. Chem. Soc., 2004, 126, 12930–12940.
13 A. D. L. Chandani, T. Hagiwara, Y. Suzuki, Y. Ouchi, H. Takezoe
and A. Fukuda, Jpn. J. Appl. Phys., Part 2, 1988, 27, L729–L732.
14 A. D. L. Chandani, E. Gorecka, Y. Ouchi, H. Takezoe and
A. Fukuda, Jpn. J. Appl. Phys., Part 2, 1989, 28, L1265–L1268.
15 A. De Meyere, J. Fornier and H. Pauwels, Ferroelectrics, 1996, 181,
1–10.
with fluorinated tails. New dimeric materials of this kind were
synthesized and characterized. These compounds possess
higher polarisation and tilt angles than their unfluorinated
counterpart previously made.9 One goal in making these new
materials was to study the properties of antiferroelectric liquid
crystal materials with very high tilt close to 45 degrees. This
goal was achieved and the optical ramifications of passing
45 degrees tilt angle in antiferroelectric materials were studied
by us in detail8 on di(7F4PPB11)3Si, while the present work
contains all other structure–property relationships studied as
well as their synthesis. For practical use further work on design
and mixing of several fluorinated and unfluorinated com-
pounds is needed to obtain AFLC materials possessing a large
molecular tilt, preferable with 45 degrees molecular tilt over a
broad temperature range centred at room temperature, as is
required for display applications.
16 S. Lagerwall, A. Dahlgren, P. Jagemalm, P. Rudquist, K. D’Have,
H. Pauwels, R. Dabrowski and W. Drzewinski, Adv. Funct. Mater.,
2001, 11, 87–94.
17 K. D’Have, A. Dahlgren, P. Rudquist, J. P. F. Lagerwall,
G. Andersson, M. Matuszczyk, S. T. Lagerwall, R. Dabrowski
and W. Drzewinski, Ferroelectrics, 2000, 244, 115–128.
18 K. D’Have, P. Rudquist, S. T. Lagerwall, H. Pauwels,
W. Drzewinski and R. Dabrowski, Appl. Phys. Lett., 2000, 76,
3528–3530.
Acknowledgements
This work was supported by the Swedish Research Council
and the Swedish Foundation for Strategical Research (SSF).
19 R. S. Dabrowski, W. J. Drzewinski, H. Pauwels, A. Dahlgren,
S. T. Lagerwall, P. Rudquist, K. D’Have, M. Matuszcyk and
P. Jagemalm, US Pat. 20020075445, 2002.
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