Development of polar order in a bent-core liquid crystal with a new sequence of two orthogonal smectic and an adjacent nematic phase

Article abstract of DOI:10.1039/c1jm14653j

A 4-cyanoresorcinol based bent-core liquid crystal shows a new phase sequence of two orthogonal smectic phases with short range (SmAP_R) and long range (SmAP_A) polar order beside a SmA-type cybotactic nematic phase (N_CybA),

Full text of DOI:10.1039/c1jm14653j

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PAPER
Development of polar order in a bent-core liquid crystal with a new sequence
of two orthogonal smectic and an adjacent nematic phase†
a
a
b
b
Govindaswamy Shanker, Marko Prehm, Mamatha Nagaraj, Jagdish K. Vij and Carsten Tschierske
a
*
Received 19th September 2011, Accepted 21st September 2011
DOI: 10.1039/c1jm14653j
A 4-cyanoresorcinol based bent-core liquid crystal shows a new phase sequence of two orthogonal
smectic phases with short range (SmAPR) and long range (SmAPA) polar order beside a SmA-type
cybotactic nematic phase (NCybA), leading to the phase sequence SmAPA–SmAPR–NCybA; this is of
importance in LCD technology and provides a significant step forward towards polar and biaxial
nematic phases.
However, in most cases the molecules are tilted in these clusters
(NcybC phases) and this type of Nb phases would lead to the
monoclinic type of biaxial nematic phases (Nbm),6e,9 where
biaxiality is affected by the tilt and this is difficult to use for
electro-optical applications. In contrast, the orthorhombic type
of Nb phases (Nbo) requires that the molecules be organized
perpendicularly in the smectic clusters.6e,9 NcybA phases occur-
ring adjacent to biaxial or polar SmA phases (SmAb and
SmAP) would be of special interest if at least a local biaxiality
or polar order could be retained at the transition to the nematic
phase. An orthogonal polar smectic phase with alternating
polar direction (SmAPA) was predicted in 199211 and observed
about ten years later.12,13 More recently polar smectic phases
1. Introduction
Liquid crystalline (LC) materials with a bent molecular shape
(banana-molecules)1 represent a fascinating area in soft matter
science, providing new supramolecular structures, phenomena
and properties, unknown for other materials. Among them there
is the formation of macroscopic chirality by spontaneous
symmetry breaking in fluids composed of achiral molecules2 and
a series of new types of polar (ferroelectric, antiferroelectric)
switching LC phases which are of interest for electro-optical3 and
non-linear optical devices.4
Contemporary interest in the field of bent-core mesogens is
focussed on the challenging task to produce a thermotropic
biaxial nematic phase (Nb). This would not only be of funda-
mental importance for general soft-matter physics, but this
phase is also considered as a possible candidate for next
generation LC displays with enhanced switching performance
compared to presently existing uniaxial nematogens.5
Numerous attempts have already been made to realize Nb
phases with different molecular structures, but were unsuc-
cessful in confirming spontaneous biaxiality unambiguously,6
probably with the exception of LC polymers and laterally
connected oligomeric mesogens at temperatures close to the
glass transition.7,8 However, these materials suffer from high
viscosity and therefore short switching times cannot be expec-
ted. Recently it was proposed that local biaxiality can arise in
the so-called cybotactic nematic phases (Ncyb) composed of
small clusters with smectic structure.6e,9 Such cybotactic
nematic phases are typically formed by bent-core mesogens.10
with a random distribution of the polar direction (SmAPR and
14
SmAPAR
)
and also a ferroelectric switching SmA phase
(SmAPF)15 were found.
Herein we report a first compound showing the phase
sequence SmAPA–SmAPR–NcybA with
a direct transition
between a locally polar SmAPR phase and a cybotactic nematic
phase (NcybA). The importance of this observation is twofold.
At first, the additional nematic phase allows an easy alignment
of the SmAPR and SmAPA phases16 as required for preparation
of defect-free displays. Secondly, it provides a significant step
forward to achieve the goal of biaxial nematic phases of the
orthorhombic type6e and possibly also related polar nematic
phases.17
2. Results and discussion
aInstitute of Chemistry, Organic Chemistry, Martin-Luther-University
Halle-Wittenberg, Kurt Mothes Str. 2, D-06120 Halle/Saale, Germany.
2.1 Synthesis
E-mail: carsten.tschierske@chemie.uni-halle.de;
Fax: +49(0)345
5527346; Tel: +49(0)345 5525664
Compound 1 was obtained by esterification of the 4-[4-(undec-
10-enyloxy)phenoxycarbonyl]benzoic acid 2 (ref. 14c) with the
bDepartment of Electronic and Electrical Engineering, Trinity College,
University of Dublin, Dublin, 2, Ireland. E-mail: jvij@tcd.ie
† Electronic supplementary information (ESI) available: Synthetic
procedure, analytical data, and additional XRD data. See DOI:
10.1039/c1jm14653j
4-cyanoresorcinol monoester
3 (ref. 10a) using DCC as
condensation agent (Scheme 1). The synthetic procedure and
analytical data are reported in the ESI†.
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Scheme 2 Transition temperatures (T/ꢀC) and associated enthalpy
values (DH, kJ molꢁ1, in parentheses) of the bent-core mesogen 1
(obtained by DSC with heating and cooling rates of 10 K minꢁ1, peak
temperatures are shown); abbreviations: Cr ¼ crystalline solid; Iso ¼
isotropic liquid; SmAPA ¼ antiferroelectric SmA phase; SmAPR
¼
random polar SmA phase; NcybA ¼ nematic phase composed of SmA
type cybotactic clusters.
Scheme 1 Synthesis of compound 1.
2.2 Optical investigations
centred on the equator which indicates that the alignment of the
molecules is along the long molecular axis (Fig. 1a). A diffuse
scattering in the small angle region has its maximum at d ¼
4.43 nm, comparable with the molecular length Lmol ¼ 4.48 nm
(in a V-shaped molecular conformation with an angle of 140ꢀ
and stretched alkyl chains in all-trans conformation). The
intensity of the diffuse small angle scattering is much larger than
that of the diffuse wide angle scattering indicating the presence of
On cooling from the isotropic liquid state, compound 1 shows
three distinct phase transitions with relatively small transition
enthalpy values (Scheme 2).
Fig. 1 shows the polarizing optical micrographs observed in
a homeotropic cell and the 2D XRD patterns of a magnetically
aligned sample for the different LC phases.
Upon cooling the samples inꢀa homeotropic cell a schlieren
texture is observed at T ¼ 146 C under the polarizing micro-
scope (Fig. 1b), indicating a nematic phase (N).‡ This texture
a
cybotactic nematic phase composed of small smectic
immediately adopts
a homeotropic alignment which is
completely dark, so that the next phase transition at T ¼ 136 ꢀC
is difficult to identify optically.‡ However in a planar cell, this
transition is clearly visible by a distinct increase of the birefrin-
gence (occurrence of blue stripes, see Fig. 2b). The dark texture
of the homeotropically aligned sample under crossed polarizers
indicates the uniaxiality of both, the nematic and the adjacent
smectic phase, i.e. this smectic phase is an average non-tilted
smectic phase (SmA, see Fig. 1d).
Upon further cooling at T ¼ 86 ꢀC a birefringent schlieren
texture appears, indicating the transition to a biaxial smectic
phase (Fig. 1f). This phase can be cooled down below room
temperature without crystallization or any additional phase
transition. At room temperature compound 1 remains in the LC
state for prolonged periods (several hours) before crystallization
takes plꢀace. The melting point of the crystallized sample is at
T ¼ 98 C (Scheme 2).
In a planar cell the birefringence continuously increases in the
temperature region of the optically uniaxial smectic phase (colour
change from blue to green), indicating a continuous increase of the
order parameter (Fig. 2c and d). In this alignment the transition
between the uniaxial and the biaxial smectic phase appears to be
continuous without any significant change of the texture (Fig. 2d
and e). The birefringence does not further change below the
transition between the two smectic phases (Fig. 2e and f).
2.3 XRD studies
XRD studies were performed with a sample aligned in a magnetic
field of medium strength (B z 1 T). At T ¼ 140 ꢀC the diffraction
pattern shows a diffuse scattering in the wide angle region with
a maximum at d ¼ 0.47 nm corresponding to the mean lateral
distance between the molecules. The wide angle scattering is
Fig. 1 Textures (right, crossed polarizers) and at the left 2D XRD
patterns (complete patterns after subtraction of the scattering in the
isotropic phase and as insets the small angle diffraction patterns) of the
magnetically aligned compound (for the original diffraction patterns see
Fig. S1†) (a) in the NcybA phase at T ¼ 140 ꢀC, (b) in the NcybA phase at
T ¼ 145 ꢀC, (c and d) in the SmAPR phase at T ¼ 110 ꢀC and (e and f) in
the SmAPA phase at T ¼ 70 ꢀC; in the textures the dark areas are
homeotropically aligned regions.
‡ These temperatures correspond to the equilibrium temperatures as
observed under the polarizing microscope at a fixed temperature,
whereas the temperatures given in Scheme 2 were obtained by DSC
during heating/cooling at a constant rate.
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Fig.
3
Electro-optical investigations: (a and b) switching current
response curves in 5 mm cells on applying a triangular wave field, (a) in the
SmAPR phase at T ¼ 125 ꢀC (10 V mmꢁ1, 1.1 Hz, 5 kU) and (b) in the
SmAPA phase at T ¼ 84 ꢀC (8 V mmꢁ1, 0.6 Hz, 5 kU) and (c) dependence of
the spontaneous polarization on temperature (5 mm, 10 V mmꢁ1, 1.1 Hz).
Fig. 2 Textures of compound 1 at different temperatures in an 8.9 mm
planar cell as observed between crossed polarizers (a) nematic phase at
140 ꢀC; (b) nematic to SmAPR transition at 136 ꢀC (c) SmAPR phase at
130 ꢀC and (d) at 100 ꢀC; (e) SmAPA phase at 85 ꢀC and (f) at 81 ꢀC.
temperature range of the NcybA phase, but after the transition to
the uniaxial SmA phase at T ¼ 135 ꢀC, immediately a relatively
broad polarization peak in each half cycle of the applied trian-
gular wave voltage is observed which is typical for the switching
process in SmAPR phases14 (Fig. 3a). On cooling the sponta-
clusters.10,18 The cluster size was estimated according to Lk,t z 3
(2/Dq) from the full width at half maximum (Dq)19 as L z 26 nm
k
ꢀ
and Lt z 8 nm i.e. the clusters are comparatively large, incor-
porating about 5–6 layers and about 16 molecules in the diam-
eter. The maxima of the small angle scattering are located at the
meridian of the diffraction pattern (see Fig. 1a, c and e), which
indicates that in the smectic clusters the molecules are non-tilted
and this confirms a cybotactic nematic phase composed of SmA-
like clusters (NcybA).
neous polarization (Ps) rises (see Fig. 3c) and at 84 C two well
developed sharp peaks appear (Fig. 3b), indicating the transition
from a Langevin-type switching as typical for SmAPR phases to
a tristable switching process between an antiferroelectric ground
state and two ferroelectric states as characteristic for SmAPA
phases. Hence, this biaxial SmA phase is a SmAPA phase and the
maximum Ps value in this phase was calculated to ꢂ260 nC cmꢁ2
.
On reducing the temperature, the position, shape and intensity
of the wide angle scattering do not change, confirming that a LC
phase is retained and also that the alignment of the molecules is
retained (Fig. 1a–c and S1†). Only the diffuse scattering in the
small angle region transforms into sharp Bragg peaks with
maxima at the meridian, consistent with a transition to a SmA
phase (Fig. 1c). This diffraction pattern is retained down to T ¼
Overall, the temperature dependence of the measured polariza-
tion Ps(T) in the SmAPR and SmAPA phases (Fig. 3c) shows soft-
mode like behaviour on approaching the SmAPR–SmPA transi-
tion and then it reaches to a saturation value. The decrease in the
polarization at lower temperature is due to the stronger anti-
ferro packing which would require a higher saturation voltage.16b
ꢀ
70 C (Fig. 1e), only the maxima of the diffuse scattering in the
wide angle region change the d value slightly from d ¼ 0.47 nm at
140 ꢀC (NcybA) to d ¼ 0.45 nm at 70 ꢀC (SmAPA), indicating an
increasing packing density (see Fig. S3†). This is in line with the
development of the d-spacing which slightly increases from d ¼
4.4 nm at T ¼ 140 ꢀC (NcybA) to d ¼ 4.6 nm at T ¼ 70 ꢀC
(SmAPA). The orthogonal alignment of the molecules proven by
the relative position of the XRD small angle and wide angle
scatterings (see also Fig. S2†), as well as by the increase of d with
decreasing T and the occurrence of a birefringent schlieren
texture of both 1/2 and 1 desclination strength indicate a biaxial
SmA phase at T < 86 ꢀC.
3. Conclusions
In conclusion, the phase sequence NCybA–SmAPR–SmAPA with
broad enantiotropic SmAPR and monotropic SmAPA phase
ranges was observed for the first time. The SmAPA phase can be
overcooled below room temperature and does not crystallize for
a prolonged time. SmAPA phases are of special interest as
recently a new and extremely fast switching process (<0.5 ms)
was discovered for these phases.16 Such fast rates would be
required for improving LC displays with sequential colour
switching and for 3D TV applications.20 Compound 1 can be
used as a component for room temperature SmAPA mixtures.
More importantly, for display production a nematic phase is
usually required, in which the LC is first aligned and then cooled
to the smectic phase to achieve a defect-free structure of the
SmAPA phase. For this reason the observation of polar smectic
2.4 Electrooptical studies
Electro-optical investigation was carried out in a 5 mm polyimide
coated ITO cell. No current response could be measured in the
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€
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A second important point concerns the nematic phase itself as
the rare NcybA phases are promising candidates for the realiza-
tion of the illusive biaxial nematic phase of the orthorhombic
type. As local polar order, and hence local biaxiality, are proven
for the SmAPR phase and it is known for such phases that
macroscopic biaxiality and polar order can be induced under an
external electric field the question arises if this could probably
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phases.x At present we have no proof of such a behaviour, but
optimization of the properties by further molecular design could
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€
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This work was supported by the EU within the FP7 funded
collaborative project BIND (grant no. 216025).
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x There are only few examples of SmAPA–N transitions and these
transitions were observed only at very high temperatures.21
{ Evidence for polar switching was recently reported for a skewed
cybotactic nematic phase composed of tilted SmC clusters (NcybC).22
18714 | J. Mater. Chem., 2011, 21, 18711–18714
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