Chiral liquid crystalline compounds with a re-entrant SmA* phase

Article abstract of DOI:10.1039/c1jm12131f

Unusual mesomorphic behaviour has been found in a series of rod-like liquid crystalline (LC) compounds with two chiral centres in the molecular chain. A re-entrant paraelectric SmA* (SmA*_RE) phase has been discovered below the ferroelectric SmC* phase on cooling for one homologue. So far this compound is the only one exhibiting the re-entrancy of the smectic A phase in chiral LC system with structural parameters so close to the upper SmA* phase. X-Ray studies, as well as dielectric spectroscopy, spontaneous polarization and tilt angle measurements have been carried out to characterize studied compounds. We compare various homologues and look for tendencies in physical properties behaviour. The Royal Society of Chemistry.

Full text of DOI:10.1039/c1jm12131f

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2011, 21, 14807
www.rsc.org/materials
PAPER
Chiral liquid crystalline compounds with a re-entrant SmA* phase†
a
a
a
Vladimıra Novotna, Vera Hamplova, Natalia Podoliak, Miroslav Kaspar, Milada Glogarova,
ꢀ
ꢀ
ꢁ
ꢀ a
ꢁ
ꢀ a
*
Damian Pociechab and Ewa Goreckab
Received 13th May 2011, Accepted 1st July 2011
DOI: 10.1039/c1jm12131f
Unusual mesomorphic behaviour has been found in a series of rod-like liquid crystalline (LC)
compounds with two chiral centres in the molecular chain. A re-entrant paraelectric SmA* (SmA*RE
)
phase has been discovered below the ferroelectric SmC* phase on cooling for one homologue. So far
this compound is the only one exhibiting the re-entrancy of the smectic A phase in chiral LC system
with structural parameters so close to the upper SmA* phase. X-Ray studies, as well as dielectric
spectroscopy, spontaneous polarization and tilt angle measurements have been carried out to
characterize studied compounds. We compare various homologues and look for tendencies in physical
properties behaviour.
homologue exhibits re-entrancy of a smectic A phase below the
Introduction
ferroelectric SmC* phase.4 We assigned this phase to re-entrant
smectic A phase (SmA*RE) and it is the first example of smectic A
phase re-entrancy in a chiral smectic system. The SmA*RE phase
shows the same structural parameters as the upper SmA* phase
so we can exclude formation of dimers or intercalated layers. The
SmA*RE phase has been described on the basis of mean field
theory4 taking into account non-monotonous temperature
dependence of the lowest term coefficient in thermal expansion.
In this contribution we are presenting synthesis and physical
properties of all homologues from this unique series. We
compare various homologues and look for tendencies in behav-
iour of physical properties when changing the length of the non-
chiral chain.
Liquid crystalline (LC) materials exhibit a rich variety of phases.
One can predict stability of different phases on a scale of
decreasing temperature taking into account that a decrease of
temperature leads to an increase of molecular order. Re-entrancy
means that a less ordered system appears on cooling below
a more ordered one. This phenomenon can be observed in liquid
crystals1 due to a large number of competing interactions (e.g.
competing order parameters and their fluctuations1a) and
temperature dependent molecular conformations. In spite of the
fact that re-entrancy has been observed several times in LC
systems,2 origin of this effect has not been completely understood
and unsolved problems connected with re-entrancy still exist in
condensed state physics.
Compounds with chiral rod-like molecules exhibit LC phases
with structures related to those of non-chiral moieties, but with
different properties. Smectic phases with molecules tilted
towards layer normal show ferroelectricity, spontaneous polari-
zation and piezoelectric properties. Nowadays, lactic acid unit is
frequently used as a basis of a chiral part of rod-like molecules
and these liquid crystalline compounds show a variety of liquid
crystalline phases in a broad temperature range.3
Experimental
Materials
We have synthesized a new homologue series of chiral rod-like
molecules consisting of two biphenyls connected by an ester
group, one biphenyl being laterally substituted by a chlorine
atom. The chiral part contains two chiral centres: methyl-butyl
and a lactate group. General formula of the studied materials
denoted as nZBL is in Scheme 1. Synthesis of studied compounds
was carried out according to the synthesis path described in
Scheme 2. The synthesis procedure and NMR results are
Herein, we are reporting a new series of chiral liquid crystalline
compounds based on a molecular core formed by two biphenyls
connected by an ester linkage group with two chiral centres in the
terminal part. Recently it has been discovered that one
aInstitute of Physics, Academy of Science of the Czech Republic, Na
Slovance 2, CZ-182 21 Prague 8, Czech Republic. E-mail: novotna@fzu.
cz; Fax: +420286890527; Tel: +420266053111
bLaboratory of Dielectrics and Magnetics, Chemistry Department,
Warsaw University, Al. Zwirki i Wigury 101, 02-089 Warsaw, Poland.
E-mail: pociu@chem.uw.edu.pl; Fax: +48228221075; Tel: +48228221075
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1jm12131f
Scheme 1 Chemical formula for the homologues series nZBL.
J. Mater. Chem., 2011, 21, 14807–14814 | 14807
This journal is ª The Royal Society of Chemistry 2011

View Article Online
dispersion data were analyzed using the Cole-Cole formula for
the frequency dependent complex permittivity complemented by
the second and the third terms to eliminate the low frequency
contribution from D.C. conductivity s and the high frequency
contribution due to resistance of the ITO electrodes, respectively:
ꢀ
ꢁ
D3
1 þ ðif =frÞð1ꢀaÞ
s
3ꢃ ꢀ 3N
¼
ꢀ i
þ Af m
;
(1)
2p30 f n
where fr is the relaxation frequency, D3 the dielectric strength,
a the distribution parameter of relaxation, 30 the permittivity of
vacuum, 3N the high frequency permittivity and n, m, A are
parameters of fitting. Measured real, 30, and imaginary, 300, parts
of dielectric permittivity 3*(f) ¼ 30–i300 were simultaneously fitted
to formula (1).
Small angle X-ray diffraction studies were performed using
Bruker D8 Discover system (Cu-Ka radiation) equipped with
Anton Paar DCS-350 heating stage (temperature stability 0.1 K),
working in the reflection mode. Samples were prepared as one-
surface-free droplets on a heated surface. X-Ray studies were
checked using the Bruker Nanostar system (Cu-Ka radiation,
Vantec 2000 area detector, MRI TCPU H heating stage) working
in the transmission mode. The temperature stability was 0.1 K.
Powder samples for Nanostar were prepared in thin-walled glass
capillaries (1.5 mm diameter).
Tilt angle was measured by a method of rotating analyzer,
described in detail in the literature.5 He–Ne laser beam was
circularly polarized by a Glan-Thompson prism and a quarter-
wave plate. Switching voltage with a rectangular profile at 120
Hz was applied to a sample. After passing through the rotating
analyzer (strong synchronous motor was used at frequency 90
Hz) the light was detected by a photodiode and fed to the input of
the oscilloscope. The intensity data waveform was decomposed
into two sinusoidal curves and numerically analyzed to obtain
a phase difference.
Scheme 2 Synthesis route for nZBL series preparation.
presented in ESI.† Both chiral centres have (S) configuration. We
have varied a number of carbon atoms in the non-chiral chain
(n). Molecular structure of all synthesized compounds was
1
checked using standard analytical methods. H NMR spectra
were acquired on spectrometer Varian-Gemini 300 HC, deuter-
ochloroform and DMSO-d6 serving as the solvents and the
signals of the solvents were used as internal standards. Chemical
shifts are given in d-scale (ppm), coupling constant J(H,H) in Hz.
IR spectra were recorded on Nicolet FTIR 740 spectrometer in
chloroform. Column chromatography was carried out under
atmospheric pressure using silica gel (100–200 mesh, Merck).
Measurements of the spontaneous polarization were carried
out by integrating polarization reversal current under a trian-
gular field. The hysteresis loop profile was checked by a Tek-
tronix TDC70 oscilloscope using a modified Sawyer-Tower
bridge method.
Results
Physical studies
Mesomorphic properties
Phase transition temperatures and enthalpies were determined by
DSC studies (Pyris Diamond Perkin-Elmer 7) during cooling and
heating runs at a rate of 5 K minꢀ1. Samples of 2–5 mg were
hermetically sealed in aluminium pans and placed in the calo-
rimeter chamber inflated by nitrogen. Texture observation and
studies of other physical properties were carried out on planar
samples filled by capillary action in the isotropic phase into cells
composed of glasses with ITO transparent electrodes (5 ꢁ
5 mm2). Commercial cells with homogeneous alignment (planar
geometry) were used with a thickness of about 8 mm. Besides,
home-made cells of 12 mm were prepared without any surfactant
or surface treatment and with mylar folia as spacers.
All compounds exhibit the Ch-TGBA-SmA* phase sequence,
and for materials with n higher than 7 a blue phase (BP) appears
on cooling from the isotropic one. Both the BP and TGBA (twist
grain boundary) phases exist in a very narrow temperature
interval contrary to the SmA* phase existing in an interval up to
120 degrees for 8ZBL. For compounds with n ¼ 9 and 10 the
SmC* phase occurs. For 9ZBL another smectic phase has been
discovered below the SmC* one on cooling. This phase was
attributed to a re-entrant smectic A phase, the phase transition
SmC*-SmA*RE has been theoretically described in ref. 4. A
typical DSC plot is presented in Fig. 1 for 9ZBL exhibiting the
richest polymorphism. The high temperature part is shown in
enlarged view in the inset. One can see that crystallization and
melting peaks are sharp (for 9ZBL see Fig. 1) and all phase
transition temperatures lie above the melting point, which
Frequency dispersion of permittivity was measured on cooling
using Schlumberger 1260 impedance analyzer in a frequency
range of 1 Hz–1 MHz, keeping the temperature of the sample
stable during frequency sweeps within ꢂ0.1 K. The frequency
14808 | J. Mater. Chem., 2011, 21, 14807–14814
This journal is ª The Royal Society of Chemistry 2011

View Article Online
other textural features or defects appeared in the SmC* phase of
ferroelectric compounds 9ZBL and 10ZBL. For 9ZBL there is
a moderate change in colour and slight textural modification at
the transition to the SmA*RE phase. In the SmA*RE a fan-shaped
texture occurs typical for non-tilted (orthogonal) smectic phase
with no in-plane order. Microphotographs of textures are pre-
sented in ESI.†
Free-standing films with smectic layers parallel to the sample
plane have been prepared and observed under polarizing
microscope. In the TGBA phase a typical filament texture has
been found. Both SmA* and SmA*RE phases appeared black in
crossed polarizers, which confirmed an orthogonal structure. In
the SmC* phase a schlieren texture has been observed with
characteristic point defects. Its low contrast suggested a low tilt
of molecules. By a selective reflection the helicity of both
compounds 9ZBL and 10ZBL was established with a short helix
pitch below 0.1mm.
Fig. 1 DSC plot for 9ZBL. The upper and lower curve shows the second
heating and subsequent cooling run, respectively, at a rate of 5 K minꢀ1
.
In the inset, a vicinity of the Iso-BP-Ch-TGBA-SmA* phase transitions is
enlarged. Arrows mark the SmA*-SmC* and SmC*-SmA*RE phase
transitions not recognizable by DSC measurements (temperatures taken
from other experimental techniques).
Layer spacing
Small angle X-ray diffraction (SAXRD) studies have been per-
formed on cooling from the isotropic phase and the layer spacing,
d, has been evaluated from the peak of diffracted intensity.
Temperature dependencies of d shown in Fig. 3 exhibit a gradual
increase of d in the SmA* phase for all studied compounds. For
9ZBL and 10ZBL a decrease of d has been observed at the SmA*-
SmC* phase transition, connected definitely with the tilt of
molecules. Nevertheless, a tendency of d to increase on cooling
dominates within the SmC* phase and thus d again increases after
reaching a minimum. For 9ZBL maximum drop down is less than
0.5% when we compare with the d value at the SmA*-SmC* phase
transition and 1% if we extrapolated d linearly. For 9ZBL the
temperature evolution of d taken on cooling is presented simul-
taneously with the corresponding intensity of X-ray scattering
peak in Fig. 4. Two phase transitions are distinguished at
evidences enantiotropic character of the observed phases. In the
Fig. 2 phase diagram for the whole homologue series nZBL is
presented. Table 1 summarizes phases, phase transition temper-
atures and corresponding enthalpies obtained from DSC
measurements. The SmA*-SmC*-SmA*RE phase transitions are
not recognizable in calorimetric measurements, the transition
temperatures are taken from a microscope observation or from
other experimental techniques.
Texture studies and optical properties
Planar samples (bookshelf geometry) 8 mm thick were studied
under a polarizing microscope. Observed blue phase texture
exhibits plate- like domains characteristic for a blue phase (BP).6
In the cholesteric phase an oily-streak texture was found. In the
TGBA phase typical coloured blurry pseudo-fan texture was
observed.7 On cooling, the TGBA-SmA* phase transition was
manifested as an abrupt sharpening of contrast and a change of
fan colour. Colour of fan-shaped texture gradually changed on
cooling continuously through the SmA* and SmC* phases
showing an increase of birefringence. No dechiralization lines or
ꢄ
ꢄ
temperatures T ¼ 92 C and T ¼ 58 C on temperature depen-
dencies of layer spacing as well as the diffracted intensity. They
correspond to the SmA*-SmC* and SmC*- SmA*RE transitions
observed in the texture study. The minima of diffracted intensity
at the transitions reflect fluctuations.
For 9ZBL X-ray measurement was performed subsequently
during the cooling and heating runs. Both anomalies mentioned
above are observed on cooling as well as on heating with
a temperature hysteresis lower than 1 K. The wide angle X-ray
diffraction did not detect any order in the smectic layers, so that
hexatic or crystalline order is excluded even in the phase below
ꢄ
58 C which was denoted as SmA*RE. Moreover, the d values
found in the SmA*RE phase exclude formation of smectic bila-
yers that occurs in some compounds with the re-entrant A phase
below the nematic phase.1 The thermal expansion coefficient
ꢂ
taken from linear extrapolation of d(T) is ꢀ0.018 A/deg in the
ꢂ
upper SmA* phase and ꢀ0.023 A/deg in the SmA*RE phase.
Dielectric spectroscopy
Dielectric spectroscopy has been performed for 9ZBL and
10ZBL with the aim to establish polar fluctuations. One distinct
mode has been detected in studied smectic phases and fitted using
the modified Cole-Cole formula (1). Temperature dependencies
Fig. 2 Phase diagram for nZBL compounds. Empty symbols show
melting points.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 14807–14814 | 14809

View Article Online
Table 1 Melting point, Mp, phase transition temperatures, Ttr, and temperature of crystallization, Tcr, in ꢄC and corresponding enthalpies, DH in kJ
molꢀ1, detected on the second temperature run at a rate of 5 K minꢀ1. Star indicates the phase transition not recognisable in the DSC plots, determined
from texture observation for the blue phase, and for other phases also by dielectric spectroscopy and X-ray temperature dependencies. Designation of
compounds nZBL corresponds to Scheme 1
Mp [DH]
Tcr [DH]
SmARE Ttr
SmC* Ttr [DH]
SmA Ttr [DH]
TGBA Ttr [DH]
Ch Ttr [DH]
BP Ttr
5ZBL
7ZBL
8ZBL
9ZBL
77 [+17.5] 75 [ꢀ10.8]
66 [+22.4] 44 [ꢀ27.3]
43 [+10.3] 24 [ꢀ7.4]
47 [+13.6] 31 [ꢀ12.0]
—
—
—
—
—
—
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
153 [ꢀ0.02]
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
160 [ꢀ0.48]
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
190 [ꢀ0.65]
—
—
ꢅ
ꢅ
ꢅ
160 *
166 [ꢀ0.40]
163 [ꢀ0.35]
160 [ꢀ0.41]
163 [ꢀ0.57]
182 [ꢀ0.21]
180 [ꢀ0.64]
174 [ꢀ0.27]
173 [ꢀ0.66]
162 [ꢀ0.01]
158 [ꢀ0.01]
160 [ꢀ0.01]
181 *
175 *
175 *
ꢅ
58 *
ꢅ
96 [ꢀ0.02]
10ZBL 63 [+20.1] 35 [ꢀ12.9]
—
ꢅ
115 *
Fig. 3 Temperature dependencies of the layer spacing taken on cooling
for denoted compounds. For 9ZBL and 10ZBL arrow indicates the
SmA*-SmC* phase transition, for 9ZBL the dotted arrow marks the
SmC*-SmA*RE phase transition.
of the dielectric strength, D3, and relaxation frequency, fr, have
been established. For 9ZBL temperature dependencies of both
quantities are shown in Fig. 5. One can see a linear decrease of
relaxation frequency of the soft mode observed in the SmA*
phase above the SmA*-SmC* phase transition (Fig. 5a). In the
SmC* phase a Goldstone mode (collective azimuthal director
fluctuations) overwhelmed the soft mode contribution and D3
reaches rather high values, a maximum of about 30 has been
found for 9ZBL and about 50 for 10ZBL. In contrast to 10ZBL
with increasing D3 on cooling within the whole temperature
range of the SmC* phase, for 9ZBL D3 starts to decrease when
approaching the SmC*-SmA*RE phase transition. At the SmC*-
SmA*RE phase transition a steep decrease of D3 and a minimum
of fr occurs (see inset of Fig. 5a). For 10ZBL a continuous
decrease of fr(T) has been found on cooling within the SmC*
phase (empty points in the inset of Fig. 5a). Such a decrease of fr
is typical and is usually connected with an obvious increase of
rotational viscosity of the Goldstone mode with decreasing
temperature.
Fig. 4 Temperature dependencies of the layer spacing, d, and intensity
of X-ray peak for 9ZBL. Phases are denoted, dotted lines mark phase
transition temperatures.
could be seen. We have detected a shift of the phase transition
temperatures bringing about significant enlargement of the
SmC* phase. For 10ZBL a 3D graph of the imaginary part of
permittivity under bias 3 V mmꢀ1 is presented in Fig. 8. Addi-
tionally to the SmA*-SmC* peak at 117 ꢄC a very small anomaly
appeared at about 68 ꢄC. Temperature dependencies of fitted
values of D3 and fr under bias 3 V mmꢀ1 are shown in Fig. 9 for
10ZBL. The soft mode is clearlyꢄ visible at the SmA*-SmC*
transition. A slight anomaly at 68 C might indicate pre-transi-
tional effects of a non-realized SmA*RE phase.
We have also performed permittivity measurements under
a bias field. Under the bias field the Goldstone mode in the SmC*
phase is suppressed. In Fig. 6, a 3D graph of the imaginary part
of dielectric permittivity under bias 3 V mmꢀ1 is presented for
9ZBL. Two distinct anomalies corresponding to the SmA*-
SmC* and SmC*-SmA*RE phase transitions are present.
Temperature dependences of fr and D3 without bias and under
the bias of 1/mm and 3V mmꢀ1 are shown in Fig. 7 for 9ZBL.
Suppression of the Goldstone mode is evidenced by a decrease of
D3 with bias and thus a critical dependence of the soft mode
Spontaneous tilt angle and polarization
Methods for measurement of both quantities, tilt angle as well as
polarization, require application of an electric field high enough
14810 | J. Mater. Chem., 2011, 21, 14807–14814
This journal is ª The Royal Society of Chemistry 2011

View Article Online
Fig. 5 Temperature dependence of a) the relaxation frequency, fr, and b)
dielectric strength, D3, for 9ZBL (full symbols) and 10ZBL (empty
symbols). The inset shows temperature dependence of fr in enlarged view
in the vicinity of the SmC*-SmA*RE phase transition. Arrows mark
phase transitions. The first arrow from left side is for crystallization and
dotted arrows are for the SmC*-SmA*RE phase transition.
Fig. 7 Temperature dependencies of the relaxation frequency, fr, and the
dielectric strength, D3, for 9ZBL without field and under the indicated
bias field. Empty circles are for measurement without field, empty squares
for 1 V mmꢀ1 and full symbols for 3 V mmꢀ1. Arrows show phase tran-
sition temperatures for zero bias field.
Fig. 8 Three-dimensional graph of the imaginary part of the dielectric
permittivity versus temperature and frequency for 10ZBL measured in
Fig. 6 Three-dimensional graph of the imaginary part of the dielectric
permittivity versus temperature and frequency for 9ZBL measured in bias
bias of 3 V mmꢀ1
.
of 3 V mmꢀ1
.
extending the width of this phase (see also the dielectric prop-
erties). Out of the limits of the SmC* phase only an electroclinic
effect takes place. For 9ZBL the tilt angle, q, increases continu-
ously with the field up to 15 V mmꢀ1, where saturation occurs4
(see Fig. 10a for low bias field). From dielectric spectroscopy
data (Fig. 7) we can estimate the temperatures for both SmA*-
SmC* and SmC*-SmA*RE phase transitions under fields of up to
3 V mmꢀ1, taking them as maxima in D3(T) dependence, and for
higher bias fields the transition temperatures are linearly
extrapolated. In Fig. 10a dotted lines mark the phase transition
for ferroelectric switching, so that zero field values are not
accessible. Typically, in ordinary SmC* phases the measured
values are not significantly affected by measuring in a field higher
than the switching threshold with the exception of when in close
vicinity of the SmA*-SmC* phase transition. In the case of 9ZBL
with a quite unique phase sequence, the measuring field influ-
ences the values of both tilt angle and polarization within the
whole range of the SmC* phase. Besides, the upper and lower
temperature limits of the SmC* phase are shifted under the field
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 14807–14814 | 14811

View Article Online
approaching the tilted–non-tilted phase transition the collective
fluctuations slow down and finally freeze to form the ferroelectric
phase.9
The tilt of molecules, qx, can be also estimated from the layer
spacing temperature dependence using a formula cos qx ¼ dSmC
/
,
dSmA* from the layer spacing measured in the SmC* phase, dSmC
and the linearly extrapolated value of d(T) from the SmA* phase,
dSmA*. The layer spacing data are obtained in zero electric field.
Let us compare values of qx with the tilt measured electro-opti-
cally under low electric field (Fig. 10a). The comparison shown in
Fig. 11a exhibits qualitatively similar temperature behaviour,
only the data measured under the electric field are higher due to
the electroclinic effect. The departure of both curves is gradually
stronger when approaching the phase transitions to both SmA*
phases because of the increasing electroclinic effect and also due
to the shift of the transition temperatures under the electric field.
For 10ZBL the temperature dependencies of both tilt angle
values determined optically and from the layer spacing data are
shown in Fig. 11b. Both curves exhibit a significant decrease on
cooling within about 50 K. Such dependence is quite unusual in
the SmC* phases, where saturation of the tilt angle value typi-
cally occurs on decreasing temperature. One can expect that the
Fig. 9 For 10ZBL temperature dependencies of dielectric strength, D3,
in full symbols and of the relaxation frequency, fr, in empty symbols
measured under bias field of 3 V mmꢀ1
.
Fig. 10 Temperature dependencies of a) tilt for indicated measuring field
and b) reciprocal electroclinic coefficient, 1/ecc, calculated as a slope of
the tilt angle field dependence (upper dependencies) for 9ZBL.
temperatures taken from the dielectric data. The values of q
within the SmC* phase comprise both spontaneous tilt and the
induced tilt under the field, the later is known as an electroclinic
effect.8
For 9ZBL the electroclinic effect contribution is extremely
high, offering angles comparable with the spontaneous tilt. Up to
the bias field of about 5 V mmꢀ1 the tilt angle dependence on the
applied field is practically linear and the slope of the tilt versus
electric field gives values of the electroclinic coefficient, ecc.
Rather high values of ecc have been obtained for 9ZBL reaching
a maxima up to 2 deg Vꢀ1 mm at the SmA*-SmC* and SmC*-
SmA*RE phase transitions. Temperature dependence of the
reciprocal value of ecc (1/ecc) for 9ZBL is presented in Fig. 10b.
One can see the softening of 1/ecc(T) at the SmA*-SmC* and
SmC*-SmA*RE phase transitions. Similarly to the temperature
dependence of dielectric strength and relaxation frequency
measured by dielectric spectroscopy, ecc evidences that on
Fig. 11 Temperature dependencies of the tilt angle, q, measured electro-
optically in electric field E ¼ 4 V mmꢀ1 are compared with the tilt angle
detected from the layer spacing data (empty symbols) using the rela-
tionship cos q ¼ dSmC/dSmA* between the layer spacing measured in the
SmC* phase, dSmC, and the linearly extrapolated value from the SmA*
phase, dSmA*, for a) 9ZBL and b) 10ZBL.
14812 | J. Mater. Chem., 2011, 21, 14807–14814
This journal is ª The Royal Society of Chemistry 2011

View Article Online
behaviour found here is based on the same mechanism as the
appearance of the SmA*RE phase in the next homologue 9ZBL.
The spontaneous polarization has been measured for both
9ZBL and 10ZBL (Fig. 12). For 9ZBL the temperature range of
the SmC* phase is significantly widened under the field, similarly
as it is observed in the optical measurement of the tilt angle. The
techniques of polarization measurements by means of integra-
tion of the switching current or from the hysteresis loop allow to
eliminate the field induced linear effect (permittivity), so that the
measured values of P include the spontaneous polarization and
non-linear induced polarization only. When approaching the
limits of the SmC* phase the hysteresis loop becomes thinner as
the threshold field for switching is decreasing. Out of the limits of
the SmC* phase the loop becomes theoretically infinitely thin,
representing only non-linear polarization. In reality, behind the
zero-field limits of the SmC* phase the width of the loop might be
finite because of the graduate shift of the phase transition under
increasing field during switching. Due to this effect the non-linear
induced polarization cannot be completely distinguished from
the spontaneous polarization. For both 9ZBL and 10ZBL
temperature dependence of the spontaneous polarization shows
continuous decrease on cooling after reaching a maximum at
about 90 ꢄC. In literature such an anomalous decrease of P(T) on
cooling within the SmC* phase is connected with inversion of the
spontaneous polarization sign.10 In the case of both studied
compounds the behaviour of P(T) is most probably connected
with the same mechanism as the decrease of the tilt angle.
Both compounds differ in the value of saturation field, above
which the polarization and the tilt angle do not increase. For
parameters as the upper SmA* phase, which is a difference from
previously studied re-entrancy of smectic A phase in non-chiral
compounds, where dimerization is responsible for the re-
entrancy.1
For 9ZBL the layer spacing data, d, established in the SmA*RE
phase follows a nearly linear continuation of the temperature
dependence of d in the upper SmA* phase (see Fig. 3). This fact
suggests that both phases are built from the same objects, just
simple molecules. The reason for the increase of d on cooling in
the SmA* phase is probably the increase of the orientational
order and gradual extension of terminal chains. The same
ordering effect might be behind the temperature dependence of
d found for 10ZBL, where on cooling, after the decrease of d due
to tilt of molecules, values of d reach the extrapolated line from
the temperature dependence in the SmA* phase. In the temper-
ature dependencies of the layer spacing, d(T), presented in Fig. 3
we can see that the higher the terminal chain length (higher n
value), the higher the thermal expansion coefficient value (higher
slope of d(T) dependence). It indicates that the chains play an
active role in bringing about the re-entrancy.
One can expect that both effects described above, namely the
increase of the orientational order and extension of the terminal
chains become gradually saturated on decreasing temperature
and another mechanism may become active. We propose a model
with sterical hindrance in packing of the molecules within the
smectic layers because of the bulky chlorine atom laterally
substituted in the molecular core as it is shown in Fig. 13. In the
SmC* phase, at first, layer spacing falls down due to the tilt of
molecules (cf. Fig. 13a, b). Simultaneously, density as well as
quadrupolar ordering increases on cooling, which may result in
a shift of molecule mass centres due to the hindrance effect of the
chlorine atom (cf. Fig. 13b, 13c, 13d). Such a mechanism can
explain the slightly higher thermal expansion coefficient in the
SmA*RE phase compared to that in the SmA* phase. It can also
clarify the discrepancy between the molecular length calculated
9ZBL it is 15 V mmꢀ1, while for 10ZBL it is only 5 V mmꢀ1
.
Discussion and conclusions
We have prepared and studied a homologue series of compounds
exhibiting rather strong chirality, which is manifested by occur-
rence of TGB and/or blue phases. High chirality is related to the
presence of two chiral centres in the terminal chiral chain. For all
homologues the SmA* phase occurs. Compounds with longer
non-chiral chain 9ZBL and 10ZBL exhibit the ferroelectric
SmC* phase. For homologue 9ZBL another smectic A phase
occurs below the ferroelectric SmC* phase. We assigned this
phase to a re-entrant smectic A phase, SmA*RE, and it is the first
example of paraelectric re-entrant smectic A phase below the
SmC* phase. The SmA*RE phase has very similar structural
ꢂ
using ChemOffice in a range of 40–41 A for fully extended
terminal chains, and a slightly higher layer spacing detected in
the orthogonal SmA*RE phase. (cf. Fig. 13a, 13d).
For compound 10ZBL a regular phase sequence SmA*-SmC*
occurs; nevertheless specific anomalies have been observed
within the SmC* phase. It is a significant decrease of the tilt and
polarization values on cooling and a small anomaly in dielectric
data when the Goldstone mode is suppressed by a bias electric
field. It allows us to consider that homologue 10ZBL reveals
similar tendencies that bring about the paraelectric SmA*RE
phase in 9ZBL. From a phenomenological point of view the re-
entrancy in 9ZBL was explained by non-monotonous tempera-
ture dependence of the lowest term coefficient in the expansion of
Fig. 12 Temperature dependencies of polarization measured at electric
field of 15 V mmꢀ1 for 9ZBL (full symbols) and 10ZBL (empty symbols).
Fig. 13 Model for the molecular arrangement in the layer for the phase
sequence SmA*-SmC*-SmA*RE
.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 14807–14814 | 14813

View Article Online
the free energy with the tilt angle as a transition parameter.4 It
was supposed that this term changes sign twice, at the SmA*-
SmC* and SmC*-SmA*RE phase transitions. Along this line, we
consider that in 10ZBL the value of the lowest term just
approaches zero without reaching full softening and transition to
the SmA*RE phase.
Notes and references
1 (a) P. E. Cladis, Physical Properties of Liquid Crystals, ed. D. Demus,
J. Goodby, G. W. Gray, H.-W. Spiess, V. Vill, Wiley-VCH, 1999, p.
288–303; (b) P. E. Cladis, R. K. Bogardus and D. Aadsen, Phys. Rev.
A: At., Mol., Opt. Phys., 1978, 18, 2292–2306; (c) S. Singh, Phase
Transitions, 2000, 72, 183–200; (d) P. E. Cladis, Phys. Rev. Lett.,
1975, 35, 48–51; (e) F. Harduin and A. M. Levelut, J. Phys., 1980,
41, 41–46; (f) V. N. Raja, R. Shasidar, B. R. Ratna, G. Hepke and
Ch. Bahr, Phys. Rev. A: At., Mol., Opt. Phys., 1988, 37, 303–305;
(g) Ch. Y. Yelamaggat, I. S. Shashikala, D. S. Shankar Rao,
G. G. Nair and S. Krishna Prasad, J. Mater. Chem., 2006, 16,
4099–4102; (h) S. Abraham, V. A. Mallia, K. V. Ratheesh,
N. Tamaoki and S. Das, J. Am. Chem. Soc., 2006, 128, 7692–7698.
We can find several similarities in molecular properties of
studied ferroelectric homologues 9ZBL and 10ZBL and
compounds exhibiting inversion of polarization and/or tilt
angle.10 Contrary to obvious temperature dependencies in the
SmC* phase, values of polarization and tilt fall down. For
compounds with inversion these quantities change their sign at
an inversion temperature and we can imagine that at this
temperature the SmA* structure occurs. This rare anomaly has
been observed for fluorine-laterally substituted terphenyl deriv-
atives with two chiral centers.10a We can expect that the presence
of multiple chiral centers is important also in our case. Styring
et al.10a explained the inversion effect taking into account
competition of various molecular conformations. We suppose
that in 9ZBL, due to a similar mechanism the SmA*RE phase
appears on cooling after the SmC* phase and then persists within
a certain temperature interval till crystallization.
ꢀ
ꢀ
2 (a) U. Pietrasik, J. Szyd1owska, A. Krowczynski, D. Pociecha,
ꢀ
E. Gorecka and D. Guillon, J. Am. Chem. Soc., 2002, 124(30),
8884–8890; (b) D. Pociecha, E. Gorecka, M. Cepic, N. Vaupotic,
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
B. Zeks, D. Kardas and J. Mieczkowski, Phys. Rev. Lett., 2000, 86,
3048–3051; (c) V. Novotna, M. Glogarova, V. Hamplova and
ꢀ
ꢀ
ꢀ
ꢁ
M. Kaspar, J. Chem. Phys., 2001, 115, 9036–9041; (d)
D. S. Shankar Rao, S. Krishna Prasad, V. N. Raja,
C. V. Yelamaggad and S. Anitha Nagamani, Phys. Rev. Lett., 2001,
87, 085504-1–4; (e) C. V. Yelamaggad, V. Tamilenthi,
D. S. Shankar Rao, G. G. Nair and S. Krishna Prasad, J. Mater.
Chem., 2009, 19, 2906–2908.
3 (a) S. L. Wu and C. Y. Lin, Liq. Cryst., 2004, 31, 1613–1617; (b)
S. L. Wu and C. Y. Lin, Liq. Cryst., 2005, 32, 1243–1249; (c)
E. Dzik, J. Mieczkowski, E. Gorecka and D. Pociecha, J. Mater.
Chem., 2005, 15, 1255–1262; (d) S.-L. Wu and H.-N. Hsu, Liq.
Generally, for anomalous phase behaviour occurrence of
multiple chiral centres may be important as well as various
conformations with respect to the chiral chain and their concen-
trations change with temperature. Lots of effects such as dipolar
interactions, packing arrangements, rotational distribution of
various conformers play a significant role in the phase stability.
For conclusion let us mention that the ferroelectric compound
with the SmA*RE phase represents a new chance for technical
applications. The SmA*RE phase is very sensitive to an electric
field which can easily extend the SmC* phase to lower temper-
atures at the expense of the SmA*RE phase. Strong electroclinic
effect was found not only near the SmA*-SmC* phase transition,
which is typical, but within a wide temperature interval of the
SmA*, SmC* and SmA*RE phases. These properties are prom-
ising for electrooptic applications especially when grey-scale
capability and fast response is required.
ꢀ
Cryst., 2007, 34, 1159–1165; (e) V. Novotna, M. Kaspar,
V. Hamplova, M. Glogarova, I. Rychetsky and D. Pociecha, Liq.
ꢁ
ꢀ
ꢀ
ꢀ
ꢀ
Cryst., 2004, 31, 1131–1141; (f) A. Bubnov, V. Hamplova,
M. Kaspar, M. Glogarova and P. Vanek, Ferroelectrics, 2000, 243,
ꢁ
ꢀ
ꢁ
ꢀ
27–35; (g) A. Bubnov, V. Novotna, V. Hamplova, M. Kaspar and
M. Glogarova, J. Mol. Struct., 2008, 892, 151–157.
ꢀ
ꢁ
ꢀ
ꢀ
ꢀ
ꢁ
ꢀ
4 V. Novotna, M. Glogarova, M. Kaspar, V. Hamplova, E. Gorecka,
D. Pociecha and M. Cepic, Phys. Rev. E: Stat., Nonlinear, Soft
Matter Phys., 2011, 83, 020701(R).
5 A. Langhoff and F. Giesselmann, ChemPhysChem, 2002, 3, 424–432.
6 (a) H.-S. Kitzerow, H. Schmid, A. Ranft, G. Heppke,
R. A. M. Hikmet and J. Lub, Liq. Cryst., 1993, 14, 911–916; (b)
H.-S. Kitzerow, P. P. Crooker, S. L. Kwok, J. Xu and G. Heppke,
Phys. Rev. A: At., Mol., Opt. Phys., 1990, 42, 3442–3448; (c)
J. Rokunohe and A. Yoshizawa, J. Mater. Chem., 2005, 15, 275–
279; (d) S. Meiboom and M. Sammon, Phys. Rev. Lett., 1980, 44,
882–885.
7 A. Dierking, Textures of Liquid Crystals, WILEY-VCH Verlag
GmbH, 2003.
8 (a) J. V. Selinger, P. J. Colins and R. Shashidhar, Phys. Rev. E: Stat.
Phys., Plasmas, Fluids, Relat. Interdiscip. Top., 2001, 64, 061705 1–9;
(b) M. S. Spector, P. A. Heiney, J. Naciri, B. T. Weslowski, D. B. Holt
and R. Shashidhar, Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat.
Interdiscip. Top., 2000, 61, 1579–1584; (c) N. Kapernaum,
D. M. Walba, E. Korblova, C. Zhu, C. Jones, Y. Shen, N. A. Clark
and F. Giesselmann, ChemPhysChem, 2009, 10, 890–892.
Acknowledgements
This work is supported by grant P204/11/0723 from the Grant
Agency of the Czech Republic and by projects IAA100100911
(Grant Agency of ASCR) and SVV-2011-263303. The X-ray
diffraction measurements were performed at the Structural
Research Laboratory, Chemistry Department, University of
Warsaw, Poland, which has been established with financial
support from European Regional Development Fund, project
No: WKP 1/1.4.3./1/2004/72/72/165/2005/U.
ꢀ
9 M. Glogarova, Ch. Destrade, J. P. Marcerou, J. J. Bonvent and
H. T. Nguyen, Ferroelectrics, 1991, 121, 285–294.
10 (a) P. Styring, J. D. Vuijk, I. Nishiyama, A. J. Slaney and
J. W. Goodby, J. Mater. Chem., 1993, 3(4), 399–405; (b)
K. Yoshino, H. Taniguchi and M. Ozaki, Ferroelectrics, 1989, 91,
267–276; (c) K. Nakao, M. Ozaki and K. Yoshino, Japanese
Journal of Appl. Physics, 1987, 26–2, 104–106.
14814 | J. Mater. Chem., 2011, 21, 14807–14814
This journal is ª The Royal Society of Chemistry 2011