Ferroelectric-like behaviour of the SmCP phase in liquid crystalline compounds with asymmetrical bent-core molecules

Article abstract of DOI:10.1039/b601084a

A new series of asymmetrical bent-shaped mesogens has been synthesised and their mesomorphic properties studied. All compounds exhibit the B₂ (SmCP) phase. For some compounds we found a very low coercive field for switching to the saturated ferroelectric state and only one peak in the switching current. Due to the lack of SHG signal in the zero field state and the behaviour of the relaxation mode we cannot unambiguously assign ferroelectric or antiferroelectric character to the B₂ phase observed. The observation of planar samples revealed a striped texture. Large optically active domains appear on very slow cooling, which are attributed to a non-homogeneous (twisted) in-plane structure imposed by surfaces. The twisted domains are suppressed under a critical electric field. A simple model of such a structure and its behavior in the electric field is proposed. The low temperature phase detected on further cooling is attributed to a crystalline-like smectic phase. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b601084a

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Ferroelectric-like behaviour of the SmCP phase in liquid crystalline
compounds with asymmetrical bent-core molecules
a
V. Novotna´,* M. Kasˇpar, V. Hamplova´, M. Glogarova´, L. Lejcek, J. Kroupa and D. Pociecha
a
a
a
a
a
b
ˇ
Received 23rd January 2006, Accepted 2nd March 2006
First published as an Advance Article on the web 22nd March 2006
DOI: 10.1039/b601084a
A new series of asymmetrical bent-shaped mesogens has been synthesised and their mesomorphic
properties studied. All compounds exhibit the B2 (SmCP) phase. For some compounds we found a
very low coercive field for switching to the saturated ferroelectric state and only one peak in the
switching current. Due to the lack of SHG signal in the zero field state and the behaviour of the
relaxation mode we cannot unambiguously assign ferroelectric or antiferroelectric character to
the B2 phase observed. The observation of planar samples revealed a striped texture. Large
optically active domains appear on very slow cooling, which are attributed to a non-homogeneous
(twisted) in-plane structure imposed by surfaces. The twisted domains are suppressed under a
critical electric field. A simple model of such a structure and its behavior in the electric field is
proposed. The low temperature phase detected on further cooling is attributed to a crystalline-like
smectic phase.
we can speculate about interactions between terminal chains
of adjacent layers.
Introduction
The discovery of polar switching in banana-like mesogens in
1996 opened a new direction in the field of liquid crystals.1 The
spontaneous polarization in such materials results from the
in-layer close packing of bent-shaped molecules. The polar
switching occurs in the B2 phase, the phase with liquid like
smectic layers and molecules tilted with respect to the layer
normal. The combination of molecular tilt and layer polariza-
tion makes each smectic layer structurally chiral.2 In ref. 2
four phase structures of the B2 phase were distinguished using
the nomenclature SmCS,APF,A, by which the synclinic (S) or
anticlinic (A) molecular tilt correlation and the ferroelectric
(F) or antiferroelectric (A) polar correlation between adjacent
layers are described. The chirality of subsequent layers can
alternate producing an overall racemic state, or can be the
same in neighbouring layers forming a homochiral phase. The
antiferroelectric synclinic (SmCSPA) and ferroelectric anticlinic
(SmCAPF) structures are racemic, while the antiferroelectric
anticlinic (SmCAPA) and ferroelectric synclinic (SmCSPF)
structures are chiral.2 Most of the B2 phases known so far
have an antiferroelectric ground (field free) state2–8 and both
types of antiferroelectric domains, SmCSPA and SmCAPA,
have been observed. An attempt to obtain a ferroelectric B2
phase was made by Walba et al.3 introducing a chiral chain,
the same as that in the antiferroelectric compound MHPOBC
with rod-like molecules. The compound exhibited the stable
ferroelectric phase SmCAPF. Further detailed studies9 of
textures and SHG analysis clarified the ferroelectricity of the
observed B2 phase in another compound with a chiral moiety
or with a chiral admixture.10 Nevertheless, the mechanism of
emergence of ferro- or antiferroelectricity is not yet clear, but
In spite of intensive research of symmetrical bent-shaped
mesogens,3–11 questions concerning the relationship between
molecular structure and mesomorphic properties are still open.
We have synthesized several series of non-chiral mesogens
composed of symmetric bent-shaped molecules with thermally
stable ester linkages and a core laterally substituted by a
methoxy group.7,11 The first studied symmetrical mesogens
exhibited the B2–B7 phase sequence on cooling from the
isotropic phase.11 Recently, attempts to diminish the symmetry
of molecules have appeared12–19 with the expectation that a
new shape of molecules would provide novel features of
mesophases. Following this idea we changed the character of
the end groups asymmetrically using the double bond at the
end of one of the aliphatic chains. This modification resulted in
a shift of phase transition temperatures, the phase sequence
remaining conserved.20 Herein, instead of a symmetrical core
with seven phenyl rings7,11,20 we have used an asymmetrical
bent core with six phenyl rings, laterally substituted by a
methoxy group (Scheme 1).
We have studied the mesomorphic behaviour of a new series
with asymmetrical bent cores and the physical properties
are presented. Texture observation, switching studies in an
electric field, dielectric spectroscopy, X-ray analysis and SHG
aInstitute of Physics, Academy of Sciences of the Czech Republic, Na
Slovance 2, 182 21 Prague 8, Czech Republic. E-mail: novotna@fzu.cz
bLaboratory of Dielectrics and Magnetics, Chemistry Department,
Warsaw University, Al. Zwirki i Wigury 101, 02-089 Warsaw, Poland
Scheme 1 General formula of studied compounds. Alkyl and alkoxy
chains R1, R2 are –OCnH2n+1, –CnH2n+1 and/or –OC9H18CHLCH2.
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measurements have been performed. A model describing the
texture behaviour in an electric field has been proposed.
dichloromethane, filtered and the solution was used for
esterification of mesogenic phenol M1 using standard reaction
conditions.
1H-NMR 3, R1 = OC11H23: 8.98 s (1H, HAr between –COO
and COCl); 8.56 d (1H, HAr ortho to –COCl); 8.40 d (1H, HAr
ortho to –COO); 7.95 d (1H, HAr para to –OCH3); 7.7 t (1H,
HAr meta to –COCl); 7.5 and 7.6 dd (4H, HAr ortho to –Ar);
7.3 m (3H, HAr ortho to –OCO); 7.0 d (2H, HAr ortho to
–OCH2); 4.0 t (2H, CH2OAr); 3.95 s (3H, –OCH3); 1.8 quint
(2H, CH2CH2OAr); 1.2–1.6 m (16H, CH2); 0.9 t (3H, CH3).
Final products were prepared by esterification in a pyridine/
dichloromethane mixture using standard reaction conditions.
The crude product was crystallized from a toluene/acetone
mixture, purified by column chromatography on silica gel
using a mixture of dichloromethane and acetone (99.5 : 0.5) as
eluent and crystallized twice from acetone. The structures of all
final products 4a–i were confirmed by 1H-NMR (200 MHz,
CDCl3, Varian, Gemini 2000). The chemical purity of the
compounds was checked by high pressure liquid chromato-
graphy (HPLC), which was carried out with an Ecom HPLC
chromatograph using a silica gel column (Separon 7 mm, 3 6
150, Tessek) with a mixture of 99.9% of toluene and 0.1% of
methanol as eluent, and detection of the eluting products by a
UV-VIS detector (l = 290 nm). The chemical purity was found
to be more than 99.9% under these conditions.
1H-NMR 4d (R1 = –C11H21, R2 = –OC11H23): 9.05 s (1H,
HAr between –COO in isoph. acid); 8.5d (2H, ortho to –COO
in isoph. acid); 7 95 dd (1H, para to –OCH3); 7.82 s (1H, ortho
to –OCH3); 7.78 t (1H, meta to –COO in isoph. acid); 7.4–7.6 m
(8H, ortho to Ar); 7.25 m (7H, ortho to –OCO, ortho to –R);
7.0 d (2H, HAr ortho to –OR); 4.0 t (2H ,CH2OAr); 3.95 s (3H,
–OCH3); 2.65 t (2H, –CH2Ar); 1.2–1.8 m (36H, CH2); 0.95 m
(6H, CH3).
1H-NMR 4h (R1 = –OC10H21, R2 = –OC9H18CHLCH2):
9.0 s (1H, HAr between –COO in isoph. acid); 8.4–8.6 dd (2H,
ortho to –COO in isoph. acid); 7.95 dd (1H, para to –OCH3);
7.82 s (1H, ortho to –OCH3); 7.78 t (1H, meta to –COO in
isoph. acid); 7.4–7.6 m (8H, HAr ortho to –Ar); 7.3 m (5H,
ortho to –OCO); 7.0 d (4H, ortho to –OR); 5.82 m (1H,
CH2LCH–); 5.0 m (2H, CH2LCH–); 4.0 t (4H, CH2OAr); 3.95 s
(3H, –OCH3); 2.1 q (2H, CH2LCH–CH2); 1.8 quint (4H,
CH2CH2OAr); 1.2–1.6 m (28H, CH2); 0.9 t (3H, CH3).
Experimental
Synthesis
The general synthesis route is outlined in Scheme 2. First,
mesogenic phenols M1 and M2 were prepared, which will
form the arms of the banana molecules. These phenols contain
alkyl or alkoxy chains, or an alkoxy chain with a terminal
double bond. M1 phenols, where R1 = –OCnH2n+1, were
prepared from 49-alkoxybiphenyl-4-ol and 3-methoxy-(4-
methoxycarbonyloxy)benzoyl chloride in a mixture of pyridine
and chloroform as has been described in detail in ref. 7.
Phenols M1 and M2, with –CnH2n+1 chains, were prepared
by Friedel–Crafts acylation of 4-acetyloxybiphenyl and
subsequent reduction of the keto group in a zinc/hydrochloric
acid mixture as described in ref. 21. Phenols M1 and M2 with
–OC9H18CHLCH2 chains were prepared by the same route as
in Ref. 20 using 1-bromo-10-undecene as a starting material.
Mesogenic acids 1 were obtained by esterification of
mesogenic phenols M1 (20 mmol) by 15 g of isophthaloyl
dichloride in dichloromethane/pyridine solution. The reaction
mixture was refluxed for 4 hours and left to stand at room
temperature overnight. Then 50 ml of dioxane and 10 ml of
water were added and the mixture was stirred for one hour at
room temperature. The mixture was washed with dilute HCl
and the excess of unreacted isophthalic acid was separated
by filtration. The filtrate was washed with water and the
solvent was evaporated. The solid residue crystallized from
glacial acetic acid contained a small amount of symmetrical
product 2. Further purification was carried out by recrystalli-
zation from ethanol in which symmetrical product 2 is more
soluble than acid 1.
1H-NMR 1, R1 = OC11H23: 8.97 s (1H, HAr between –COO
in isoph. acid); 8.4 dd (2H, HAr ortho to –COO in isoph. acid);
7.95 d (1H, HAr para to –OCH3); 7.4–7.7m (1H, HAr meta to
–COO in isoph. acid + 4H HAr ortho to –Ar); 7.3 m (3H, HAr
ortho to –OCO); 6.97 d (2H, HAr ortho to –OCH2); 4.0 t (2H,
CH2OAr); 3.95 s (3H, –OCH3); 1.8 quint (2H, CH2CH2OAr);
1.2–1.6 m (16H, CH2); 0.9 t (3H, CH3).
Mesogenic acids 1 were converted into the corresponding
acyl chlorides 3 by reflux of dry acids with the excess of
the oxalyl chloride overnight. Then the oxalyl chloride
was distilled off in vacuo, the residue was dissolved in
Experimental techniques
All synthesised materials were studied using DSC (Perkin–
Elmer Pyris Diamond). The compounds with a mass of about
5 mg were hermetically closed in aluminium pans. The samples
were placed in a nitrogen atmosphere and cooling and heating
rates of 5 K min21 were applied.
Observation of textures in liquid crystalline phases repre-
sents an essential tool for phase identification. A Nikon
Eclipse E-600 polarizing microscope was used. The tempera-
ture was changed and stabilized with an accuracy of ¡0.1 uC
in the hot stage (Linkam). The samples were filled in the
isotropic phase into cells composed of glass plates provided
with ITO electrodes. The sample thickness was 3.5 mm. Free-
standing films (FSF) were prepared by spreading a melted
compound over the hole in the metal slab.
Scheme 2 Synthesis route.
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Switching studies were performed using the driving voltage
from a Phillips generator PM 5191 accompanied by a linear
amplifier, which allowed a maximum amplitude of ¡120 V to
be reached. A LeCroy 9304 memory oscilloscope provided
information about the switching current profile vs. time.
For dielectric measurements, we prepared cells from glass
plates with non-transparent gold electrodes prepared by
evaporation. Due to the high conductivity of the electrodes
the high frequency mode, that is present in the frequency range
of about 1 MHz if ITO electrodes are used, was shifted to
higher frequency by one order of magnitude. With cells with
gold electrodes weak high frequency modes could be studied.
Dielectric properties were studied using a Schlumberger 1260
impedance analyzer. The frequency dispersions were measured
on cooling at a rate of about 0.2 K min21, keeping the
temperature of the sample stable during frequency sweeps in
the range of 10 Hz–1 MHz. The frequency dispersion data
were analyzed using the Cole–Cole formula for the frequency
dependent complex permittivity
e*(f) = e9 2 ie0:
De
1zðif =frÞð1{aÞ
s
eꢀ{e?~
{i
zAf m
(1)
2pe0f n
where fr is the relaxation frequency, De is dielectric strength,
a is the distribution parameter of the relaxation, e0 is the
permittivity of a vacuum, e‘ is the high frequency permittivity
and m, A are fitting parameters. The second and third terms
in the equation are used to eliminate a low frequency
contribution from the dc conductivity s and a high frequency
contribution due to the resistance of the ITO electrodes,
respectively.
Fig. 1 DSC plots for compounds (a) 4a, (b) 4h and (c) 4i. The upper
and lower curves show the second heating and cooling runs,
respectively. The slopes are adjusted for convenience.
SHG measurements were performed using a Q-switched
Nd–YAG laser (6 ns pulses at 20 Hz, l = 1064 nm). The
sample was turned 45 degrees with respect to the beam
direction and the fundamental wave had p-polarization. The
SH signal at 532 nm was detected by a photomultiplier and
boxcar averager. To avoid sample damage very weak pulses
(,0.1 mJ) were used, without any focusing. The spot size 1 mm
allowed investigation of the average signal from the observed
texture. SHG hysteresis loops were measured using a very low
frequency (0.01–0.003 Hz) triangular voltage.
of the B2 phase (see the melting point in Table 1). The
transition temperatures and associated enthalpy changes are
summarised in Table 1. Other peaks seen at low temperatures
(Fig. 1) represent the phase transitions in the solid state.
The high temperature phase observed for all materials
exhibits a fan-shaped texture, typical for the B2 phase. On
fast cooling from the isotropic phase the fans appear with
stripes parallel to the smectic layers. Under a dc electric field of
0.5–1 V mm21 the stripes disappear.
On very slow cooling from the isotropic phase (rates lower
than 0.1 K min21) two types of domain could be obtained
instead of narrow stripes (see Fig. 2a for 4f). Using mono-
chromatic light we could distinguish different behaviour of
domains when decrossing the analyzer. One type of domain
(green domains in Fig. 2a) exhibits optical extinction in crossed
polarizers along the smectic layer normal. The other type of
domain (yellow domains in Fig. 2a) exhibits extinction at
decrossed polarizers, which indicates the rotation of the
polarized light plane. On the basis of this fact, we consider
that the director is twisted within the smectic layers along the
sample plane normal and call these domains twisted. The
birefringence, Dn, measured by a calibrated Babinet–Soleil
compensator, has been evaluated for the first type of domain,
which has ordinary optical properties, i.e. four extinction
brushes for crossed polarizer and analyzer. Compensation was
obtained in the second and third order of interference light
spectra and the birefringence reaches relatively high values
X-Ray studies were performed using a DRON system
equipped with a graphite monochromator. Experiments were
conducted on non-oriented samples in the reflection mode.
Results
The phase transition temperatures and enthalpies were deter-
mined from DSC studies. Typical DSC plots are presented in
Fig. 1a, b, and c for the second heating and cooling runs for
substances 4a, 4b and 4c, respectively. The phase transition
from the isotropic to the high temperature mesophase is
accompanied by a high transition enthalpy. This phase was
identified as the B2 phase from texture observation in a
polarising microscope and characterized under an applied
electric field. Another phase transition was detected on cooling
(see Fig. 1). X-Ray studies indicated that this phase had
crystalline character and we denoted it as CrX (see below).
This assumption is supported by relatively broad overcooling
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Table 1 Melting point, mp, indicated on second heating, phase transition temperatures, Ttr, and corresponding enthalpies, DH, detected on
second cooling are in square brackets. All thermograms were taken at a rate of 5 K min21. R1, R2 are alkyl or alkoxy chains corresponding to
Scheme 1
R1
R2
mp/uC [DH/J g21
]
Tcr/uC [DH/J g21
]
CrX T/uC [DH/J g21
]
B2 T/uC [DH/J g21
]
Iso
4a –OC8H17
4b –OC11H23
4c –OC11H23
4d –C11H23
4e –C10H21
4f –OC9H18CHLCH2 –OC11H23
4g –C8H17
4h –OC10H21
–OC8H17
–OC11H23
–C11H23
–OC11H23
–C10H21
185 [+34.7]
168 [+28.8]
171 [+24.5]
172 [+25.3]
171 [+25.1]
168 [+26.6]
76 [221.1]
94 [232.2]
62 [224.7]
114 [24.7]
66 [28.1]
91 [25.2]
95 [24.2]
73 [218.9]
86 [218.9]
N
N
N
N
N
N
N
N
N
170 [235.5]
159 [234.4]
146 [223.6]
164 [224.7]
155 [28.0]
157 [225.1]
162 [227.7]
156 [225.9]
156 [225.7]
N
N
N
N
N
N
N
N
N
191 [230.9]
189 [228.4]
179 [224.3]
185 [223.4]
171 [225.2]
182 [223.1]
183 [225.6]
181 [225.0]
184 [225.3]
N
N
N
N
N
N
N
N
N
–OC9H18CHLCH2 173 [+29.5]
–OC9H18CHLCH2 166 [+24.7]
–OC9H18CHLCH2 167 [+27.7]
4i
–OC11H23
Fig. 2 Planar texture of compound 4f in the B2 phase at T = 170 uC. (a) Domains obtained on slow cooling from the isotropic phase. The yellow
domains (twisted) rotate the polarized light plane. Also shown is the same part of the sample under a dc electric field of (b) 1 V mm21, (c) 10 V mm21
(d) 15 V mm21. The width of the figures corresponds to 200 mm.
,
within 0.20 and 0.28 for the studied materials (0.26 for green
domains of 4f, see Fig. 2a). Applying a dc electric field of
about 0.5 V mm21 (threshold field E1) causes the twisted
domains to gradually disappear and the sample becomes
almost homogeneously coloured (green fans in Fig. 2b). By
increasing the value of the applied dc field to the threshold
field E2 (1–10 V mm21, varying for studied compounds) we
induce a phase transition accompanied by a birefringence
change. The transition is seen in Fig. 2c, where the red
domains correspond to the high field structure. The saturated
field state texture with homogeneously coloured fans is shown
in Fig. 2d. The birefringence of domains in an electric field is
even higher than that without a field (0.32 for red domains of
4f, see Fig. 2d). After switching off the dc field the twisted
domains usually reappear in the form of narrow stripes so that
the texture resembles the virgin texture reached on fast cooling.
The extinction position parallel to the layer normal remains up
to the maximum of applied voltage 40 V mm21, when a short
cut usually takes place.
fans slightly changes in the planar texture, and cracks and
additional defects appear. The low temperature phase does
not switch in the applied electric field. The B2 and the low
temperature CrX phases, which are hardly recognizable in the
planar texture without an applied field could be recognized
under an applied dc voltage because of the different
birefringence colors. The coexistence of both phases (Fig. 3)
was observed within a temperature interval of several degrees.
For compounds 4c, 4d and 4g even under lower dc field
(0.2–0.5 V mm21) a direct phase transition to the ferroelectric
phase takes place, without forming the transient homogeneous
texture as described in the previous case and shown in Fig. 2b.
Such compounds exhibit only one peak in the polarization
current profile (see the next paragraph).
Fig. 3 Fan-shaped texture at the B2–CrX phase transition under a dc
electric field for 4b in the planar cell. Orange fans represent the B2
phase. The width of the figure corresponds to 300 mm.
On further cooling the phase transition from the B2 phase to
the low temperature phase takes place. The birefringence of
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Free standing films (FSF) have been successfully prepared
and studied for all compounds. The phase transition from the
isotropic to the B2 phase in FSF is accompanied with the
formation of bubbles and defects (see Fig. 4a). The typical
schlieren texture of the B2 phase transforms into a crystalline-
like texture (Fig. 4). On further cooling the FSF usually breaks
through.
Switching was studied under a triangular wave field of
25 Vpp mm21, 8–11 Hz. Two different types of polarization
current profile were observed. Most of the compounds exhibit
two distinct peaks per half period in the polarization current
in the B2 phase. For three compounds mentioned previously
(4c, 4d and 4g) only one sharp peak was found. In Fig. 5a–d,
we can compare the polarization current profiles for com-
pounds 4b, 4c, 4d and 4f, respectively.
SHG measurements were performed in the B2 phase at
temperatures of about 10 K below the isotropic–B2 phase
transition in a quasi-static electric field. For all studied
compounds zero SHG signal was detected up to the electric
field corresponding to the threshold value E2. A steep increase
of SHG signal with increasing field occurred for electric field
E > E2. For compounds 4b and 4f, which may exhibit twisted
domains under very slow cooling and subsequent two stage
switching and also single peak switching current in an ac field,
E2 is relatively high (see Fig. 6a), as is typical for antiferro-
electric banana liquid crystals.9,8,18 For compounds 4c, 4d and
4g the steep increase of the SHG signal occurs at an electric
field one order of magnitude smaller (see Fig. 6b), indicating
ferroelectric behavior. This is consistent with the low threshold
Fig. 5 Switching current profile for compounds (a) 4b, (b) 4c, (c) 4d
and (d) 4f in the B2 phase at a temperature of 10 degrees below the
phase transition from the isotropic phase, at a frequency of 8–11 Hz.
field for transformation into the ferroelectric state detected in
the texture observation.
In the dielectric spectroscopy measurements, a weak single
relaxation process in the range of hundreds of kHz appeared in
the B2 phase for all studied compounds. Three-dimensional
plots of the imaginary part of the dielectric permittivity versus
temperature are presented in Fig. 7. By fitting the data to
eqn (1) we obtained the temperature dependences of the
dielectric strength, De(T), and relaxation frequency, fr(T). Due
to the high contribution to the low frequency conductivity we
cannot analyze the dielectric spectroscopy data at frequencies
below 1 kHz. Two typical temperature dependences are shown
in Fig. 8a, b. On cooling from isotropic phase De steeply
increases at the isotropic–B2 phase transition and within the B2
phase only decreases slightly when the temperature decreases.
The mode is exponentially quenched in the CrX phase. The low
values of De and high relaxation frequency fr (100–600 kHz)
indicate the antiferroelectric character of the B2 phase.
In the B2 phase, application of a bias electric field leads
firstly to the increase of De and decrease of fr (see Fig. 9) and
the mode is suppressed at the value of threshold field E2, when
the phase transition to the ferroelectric phase takes place. In
planar textures it corresponds to a step change of birefringence
in the electric field E > E2 (see Fig. 2c and d).
The X-ray pattern exhibits sharp small angle reflections and
a diffuse scattering maximum in the large angle region. This
indicates a layered structure with liquid-like molecular packing
within the layers. From the inner reflections the layer spacing,
Fig. 4 Texture of free-standing film of compound 4b (a) in the B2
phase below the phase transition from the isotropic phase, at a
temperature of T = 188 uC, (b) in the CrX phase, at about T = 148 uC.
The width of the figures corresponds to about 200 mm.
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Fig. 6 SHG signal versus dc electric field for compounds (a) 4b and
(b) 4d.
Fig. 7 Temperature–frequency plots of the imaginary part of the
dielectric permittivity taken on cooling from the isotropic phase for
compounds (a) 4c and (b) 4f.
˚
d, is determined. The d values of 49.6 A for 4b, 50.4 A for
˚
˚
4f and 50.8 A for 4i were determined at the temperature
15 degrees below the isotropic–B2 phase transition. For all
studied compounds we found the layer spacing to be smaller
than their calculated molecular length, l, which points to a
tilted arrangement of molecules.
On cooling there is a strong first order transition to the CrX
phase (jump in d and coexistence of two signals). The X-ray
patterns indicate a high degree of ordering. So we conclude the
CrX phase is probably a crystalline smectic phase. On cooling
to still lower temperatures another anomaly is seen on layer
spacing and on intensity. The phase below CrX, which is
observed also on DSC plots (see Fig. 1), exhibits higher order
typical for the solid state.
Discussion and conclusions
We synthesized thermally stable asymmetrical bent-core
mesogens. On cooling from the isotropic phase the B2 phase
appears in all studied compounds. The texture observations in
both planar samples and free-standing films are compatible
with the B2 phase structure. The planar samples revealed a
typical striped texture. On very slow cooling two types of
domain appear instead of stripes. One type rotates the plane
of polarized light, indicating a non-homogeneous (twisted)
in-plane structure, which is assumed to be imposed by surfaces.
Fig. 8 Temperature dependences of relaxation frequency, fr, and
dielectric strength, De, for compounds (a) 4c and (b) 4f.
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Fig. 10 Model of switching of the twisted domains in the electric
field. (a) Twisted structure without field in the sample cross-section.
The glasses are depicted by shading. Bent-shaped molecules rotating
on the surface of the cone are represented by triangles where thicker
lines show molecular arms inclined toward the observer. Dashed lines
indicate the director projected onto the plane of the figure. The arrows
show the molecular spontaneous polarizations the length of which
corresponds to the projection of polarization onto the plane of the
figure. The bottom part illustrates the orientation of polarizations
from a side view. Numbers 1–5 correspond to the same molecular
positions in the upper part of the figure. (b) Under dc field E > E1 the
antiferroelectric structure is restored at the upper surface resulting
in partial unwinding of the twisted structure in every second layer. (c)
Complete switching at E > E2 leading to uniform structures in
both layers.
Fig. 9 Dielectric strength, De, and relaxation frequency, fr, plotted as
a function of an applied dc bias field in the B2 phase at the temperature
T = 176 uC for compound 4d.
The domains rotating the plane of polarized light disappear
under an electric threshold field, E1. For higher electric field
E2 > E1 switching to the ferroelectric structure takes place. The
texture of non-twisted domains exhibits optical extinction in
the position when the layer normal is parallel to the crossed
polarizers for both zero and high field states, which implies
anticlinic structures for both states, and thus the transition
between chiral (SmCAPA) and racemic (SmCAPF) phases
under the field, when considering the simple model from
ref. 2. This model involves the same birefringence in both
field-on and field-off structures in contrast to the experimental
results, and a change of chirality at the transition. Moreover,
the change of chirality can be hardly expected especially
for compounds 4c, 4d and 4g, exhibiting extremely low
coercive fields. For these reason another model was proposed
in refs. 22 and 23, supposing that the structure is composed of
mesoscopic chiral domains SmCAPA of opposite handedness,
which are transformed under the field to the SmCSPF structure
with the opposite tilt in each domain. Then also the overall
structure under the field has the same point symmetry as
the anticlinic one, which is in accordance with our observation
of switching without change of extinction position parallel
to the polarizers. The change of birefringence under the field
is expected because of different averaging of optical polariz-
ability on the scale of two layers and/or on the scale of
mesoscopic domains.
structure and the uniform ferroelectric structure is realized at
the threshold field E2 (Fig. 10b). The detail description of
the twisted structure and its behaviour in the electric field is
under study.
X-Ray studies confirmed the layer structure and provided
the information about the layer spacing in the B2 phase. The
measured values of layer spacing of the studied compounds,
˚
˚
49.6 A for 4b, 50.4 A for 4f and 50.9 A for 4i, are very similar
˚
compared to those of compounds with a symmetrical 7-ring
˚
core (see ref. 19, where a value of 50.5 A was presented for a
compound which differs from 4i only in the number of phenyl
rings in the core and has the same ending chains). This
indicates effective packing of the studied molecules in a way
depicted schematically in Fig. 11.
In the studied series we observed two different types of
switching properties. Either one or two distinct peaks were
The behaviour of the twisted domains under the electric field
could be described in the schematic model presented in Fig. 10.
It is supposed that the equilibrium SmCAPA structure occurs
only in the sample bulk, but at the surfaces it is modified to
the synclinic ferroelectric SmCSPF structure due to strong
polar surface anchoring. This anchoring also ensures that the
preferred molecular polarisations at upper and lower surfaces
are opposite. Surface SmCSPF and bulk SmCAPA structures
are mediated by a twist of molecules in the smectic layers along
the sample plane normal. The twist is realized by a rotation
of bent-shaped molecules along the surface of a cone with
polarisations tangential to the cone surfaces (Fig. 10a). When
the applied dc electric field reaches the first critical threshold
field E1, which compensates the polarity of one surface, the
SmCAPA structure is restored at this surface (Fig. 10b). A
further increase of the field leads to unwinding of the twisted
Fig. 11 Scheme of the molecular packing in layers.
J. Mater. Chem., 2006, 16, 2031–2038 | 2037
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observed in the polarization current profiles (Fig. 5). For
compounds 4c, 4d and 4g with only one sharp peak,
SHG study shows that all compounds have no SHG signal
without electric field (Fig. 6). This indicates a centro-
symmetrical ground state. Nevertheless, for compounds 4c,
4d and 4g SHG measurements reveal extremely low field at
which the signal starts to grow. Macroscopic polarization at
zero field can be cancelled not only in the antiferroelectric
structure, but also in the ferroelectric one due to domains or
polar surface interactions, which is consistent with our
model. In such a case a lower threshold field to reach the
saturated ferroelectric state can be expected. From the
switching and SHG experiments one cannot conclude if
the ground state in 4c, 4d and 4g compounds is ferroelectric
or antiferroelectric with a low energetic barrier for transition
to the ferroelectric state.
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For all studied materials dielectric dispersion data exhibit a
weak mode in the frequency range of 100–600 kHz. Such a
mode is typical for the antiferroelectric B2 phase. An
anomalous increase of the dielectric strength of this mode
with an applied bias lower than the critical threshold value
E2 is in agreement with theoretical predictions for the
antiphase phason behaviour24 in an electric field. We cannot
exclude the existence of a relaxation below 1 kHz, which
cannot be detected due to the high conductivity of the studied
materials and which would be consistent with the ferroelectric
phase. But still the detected high frequency relaxation and
its behaviour in the bias field can hardly occur in the
ferroelectric phase.
The different behaviour of studied ferroelectric-like and
typical antiferroelectric compounds is difficult to explain on
the basis of their molecular structure. We would like to point
out that the common feature of the structures of these three
compounds is the dissimilar linkage connecting the aliphatic
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Acknowledgements
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This work was supported by Research Plan AV0Z10100520
and grant No. 202/05/0431 from the Grant Agency of the
Czech Republic and KBN grant No. 4T09A 00425.
ˇ
24 D. Pociecha, E. Gorecka, M. Cepic, N. Vaupotic, K. Gomola and
ˇ
J. Mieczkowski, Phys. Rev. E, 2005, 72, 060701R.
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This journal is ß The Royal Society of Chemistry 2006