Development of polar order in liquid crystalline phases of a banana compound with a unique sequence of three orthogonal phases

Article abstract of DOI:10.1039/c001736a

Alkyl substituted 4-cyanoresorcinol 1,3-bisbenzoates with terephthalate based rod-like wings show a series of three orthogonal smectic phases SmA-SmAP_R-SmAP_A.

Full text of DOI:10.1039/c001736a

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Development of polar order in liquid crystalline phases of a banana
compound with a unique sequence of three orthogonal phasesw
Christina Keith,a Marko Prehm,a Yuri. P. Panarin,b Jagdish K. Vijb and Carsten Tschierske*a
Received 27th January 2010, Accepted 19th March 2010
First published as an Advance Article on the web 13th April 2010
DOI: 10.1039/c001736a
Alkyl substituted 4-cyanoresorcinol 1,3-bisbenzoates with
terephthalate based rod-like wings show a series of three ortho-
gonal smectic phases SmA–SmAPR–SmAPA.
For the synthesis 4-cyanoresorcinol was acylated with
appropriate 4-[(4-n-alkylphenoxycarbonyl)benzoyloxy]benzoic
acids,9 which were obtained by esterification of 4-formyl
benzoic acid with 4-alkylphenols followed by oxidation of
CHO to COOH10 (for details see ESIw). Investigation of the
mesomorphic properties is based on polarizing microscopy,
differential scanning calorimetry X-ray diffraction (XRD) of
surface aligned samples as well as electrooptical experiments.
The mesophases and transition temperatures of the two
synthesized homologous compounds 1a (n = 6) and 1b (n = 12)
are collated in Table 1. Compound 1a with short chains has
only two LC phases, a nonpolar SmA phase and an antiferro-
electric switching SmAPA phase. The typical textures observed
for these two mesophases are shown in Fig. 1. Upon cooling
the samples from the isotropic state a highly birefringent
fan-like texture is formed (Fig. 1a) which can be homeotropi-
cally aligned as typical for optically uniaxial SmA phases (see
Fig. 1b). In the regions with fan texture the fans get broken at
the phase transitions at 112 1C (Fig. 1c), and a birefringent
schlieren texture appears in the homeotropically aligned areas,
indicating a transition to a biaxial LC phase (Fig. 1d).
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
Most of the smectic LC phases of bent-core mesogens,
reported so far, represent tilted smectic phases (SmC type)
for which it is difficult to obtain well aligned samples over
large areas. Therefore, smectic phases with an orthogonal
organization of the molecules (SmA type) are more promising
applicants for new technologies. However, orthogonal smectic
phases are extremely rare for bent-core molecules and require
special molecular design. An orthogonal polar smectic phase
with alternating polar direction (SmAPA) was predicted in
19925 and observed about ten years later.6,7 Very recently
another interesting bent-core phase with a random distribu-
tion of polar direction (SmAPR) was found.8
The optical textures observed for compound 1b are identical
with those of compound 1a, characterized by an onset of
biaxiality in the homeotropically aligned regions at T = 111 1C
(Fig. S5, ESIw). However, for this compound an additional
relatively broad DSC peak is observed around T = 158 1C in
the DSC heating and cooling scans within the temperature
range of the optically uniaxial SmA phase region (Fig. 2).
XRD patterns of the LC phases of compound 1b, obtained
from oriented samples, show Bragg reflections on the meridian
and a diffuse outer scattering located on the equator (Fig. 3c–e).
Herein we report a first compound (1b) showing the phase
sequence SmA–SmAPR–SmAPA,z which for the first time
combines all three orthogonal banana phases and allows the
investigation of the stepwise development of polar order. In
addition, this is a new lead structure for design of the extremly
rare SmAPR phase over broad temperature ranges. The syn-
thesis is straight forward requiring only few synthetic steps
starting from commercially available materials. This is impor-
tant for large scale synthesis which was not possible with
previously reported materials.8
a Institute of Chemistry, Organic Chemistry, Martin Luther University
Halle-Wittenberg, Kurt-Mothes Str. 2, 06120 Halle, Germany.
E-mail: carsten.tschierske@chemie.uni-halle.de;
Fax: ++49 345 5527346; Tel: ++49 345 5525664
b Department of Electronic and Electrical Engineering,
Trinity College Dublin, Dublin, Ireland. E-mail: jvij@tcd.ie;
Fax: 00 353 1 6772442; Tel: 00 353 1 8961431
w Electronic supplementary information (ESI) available: Synthesis,
analytical data, additional XRD data. See DOI: 10.1039/c001736a
Fig. 1 Changes of the textures of compound 1a at the SmA-
to-SmAPA phase transition (crossed polarizers); (a,b) SmA phase at
T = 130 1C; (c,d) SmAPA phase at T = 100 1C; (a,c) fan-like texture;
(b,d) homeotropically aligned sample.
ꢀc
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Table 1 Phase transitions of compounds 1a
Comp.
n
T/1C [DH/kJ molꢁ1
Cr ꢂ 128 [39.6] (SmAPA 112 [1.1]) ꢂ SmA ꢂ 150 [5.3] ꢂ Iso
]
1a
1b
6
12
Cr ꢂ 112 [38.4] (SmAPA 111 [0.5]) ꢂ SmAPR ꢂ 158 [2.8] ꢂ SmA ꢂ 166 [4.6] ꢂ Iso
a
Cr = crystalline solid; SmA = uniaxial smectic phase without in-plane order; SmAPR = polar switching uniaxial SmA phase with randomized
polar direction; SmAPA = antiferroelectric switching biaxial SmA phase; Iso = isotropic liquid.
(SmAPA, compound 1b, see Fig. 3b), indicating an increasing
packing density. This is in line with the development of the
d-spacing depending on temperature (Fig. 3a) which signifi-
cantly increases from d = 4.6 nm at T = 160 1C (SmA) to
d = 5.0 nm at T = 90 1C (SmAPA).
Electrooptical investigations were carried out in 6 mm ITO
cells. For compound 1a, in the optically uniaxial SmA phase,
no current response could be observed, confirming a non-polar
SmA phase. At the transition to the biaxial phase at 112 1C
two well developed sharp peaks can be detected per half period
of the applied triangular wave field (see Fig. S4, ESIw). This
Fig. 2 DSC heating and cooling curves (10 K minꢁ1) of compound 1b.
is a clear indication for a tristable switching process between
an antiferroelectric ground state and two ferroelectric states,
The XRD pattern does not significantly change in the tempera-
hence, this biaxial SmA phase is assigned as SmAPA. The
ture range of the smectic phases, i.e. at all temperatures the
value of the spontaneous polarization was calculated to
770 nC cmꢁ2
maxima of the diffuse scattering are located on the equator
.
and the layer reflection retains its position on the meridian.
This indicates that also in the biaxial smectic phase the
molecular long axis is on average perpendicular to the layer
planes. This on average orthogonal alignment of the molecules
and the occurrence of a birefringent schlieren texture at the
phase transition at 111 1C, indicates that the low temperature
phase of compounds 1a (for XRD patterns see Fig. S1, ESIw)
and 1b is a biaxial SmA phase. With decreasing temperature
the diffuse outer scattering maximum is slightly shifted from
d = 0.48 nm at 160 1C (SmA) to d = 0.47 nm at 90 1C
Surprisingly, for compound 1b polar switching can already
be observed in the temperature range of the uniaxial SmA
phase. It starts shortly below the phase transition from the
isotropic liquid to the SmA phase at approximately 155–160 1C,
where within the SmA phase region a relatively broad peak
can be seen in the DSC traces (see Fig. 2). This broad peak is
not associated with any change of the texture or XRD pattern
except an continuous increase of the packing density as
discussed above. There is a relatively broad polarization peak
in each half cycle which is typical for the switching process in
SmAPR phases (Fig. 4a).8 The broadness indicates the field-
induced alignment of the dipoles from random orientation.8
The polarization value increases with further decreasing
Fig. 3 XRD data of compound 1b: (a) dependence of layer distance
d and (b) shape of the diffuse wide angle scattering on temperature;
(c–e) diffraction pattern of a surface aligned sample (after subtraction
of the scattering pattern in the Iso phase at T = 178 1C); (c) SmA at
T = 160 1C; (d) SmAPR at T = 130 1C; (e) SmAPA at T = 90 1C (for
original patterns, see Fig. S2, XRD patterns of compound 1a are
shown in Fig. S1, ESIw).
Fig. 4 Electrooptical investigation of compound 1b: (a,b) Switching
current response in a 6 mm ITO cell (18 Vpp, 10 Hz, 5 kO) on applying a
triangular wave field; (a) in the SmAPR phase at T = 112 1C; (b) in
the SmAPA phase at T = 105 1C; (c) temperature dependence of the
spontaneous polarization (9.5 mm, 20 V, 100 Hz); (d,e) textures of the
SmAPR phase at 153 1C depending on applied voltage.
ꢀc
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temperature to a value of about 150 nC cmꢁ2. Below the phase
transition at 111 1C the polarization peak splits into two sharp
peaks (Fig. 4b) and the polarization value increases to ca.
350 nC cmꢁ2, confirming a transition to a SmAPA phase. The
Ps(T) curve (Fig. 4c) shows soft-mode like behaviour on
approaching the SmAPR–SmAPA transition. In the SmAPA
and SmAPR phases no textural changes take place during
switching, i.e. the position of the dark extinction brushes,
located parallel to the crossed polarizers, does not move
(Fig. 4d and e), indicating a switching by collective rotation
around the molecular long axis.
were obtained in a new class of easily accessable bent-core
mesogens.
This work was supported by the EU within the FP7 funded
Collaborative Project BIND (Grant No 216025); C. K. is
grateful to Alexander von Humboldt Foundation for a Feodor
Lynen-fellowship.
Notes and references
z Compounds related to 1, but with alkoxy chains instead of alkyl
chains show SmA and SmAPA phases, but no SmAPR phase;7a reversal
of the outer ester groups leads to N and (tilted) SmC phases with much
lower transition temperatures.12 The effect of the direction of the
ester linking groups has several origins. An alternation of electron
rich and electron deficit aromatics (electrostatic surface potential)
favours intermolecular interactions, hence stabilizing smectic phases
(in 1 alternation of electron deficit terephthalate and electron rich
phenylether units).13 Moreover, the different rotational barriers of
the Ar–COO and Ar–OOC bonds gives rise to distinct molecular
flexibilities. Also torsional angles, bending angles, dipole moments and
quadrupole moments are affected by reversal of the COO groups and
influence the mesophase stability.14 The prediction of tilt is even more
difficult and presently based on empiric observations.
Whereas the uniaxial-to-biaxial phase transition enthalpy of
1b is very small (0.5 kJ molꢁ1), the SmA-to-SmAPR transition
enthalpy value is much higher (2.8 kJ molꢁ1) and it is relatively
broad compared to the sharp Iso–SmA transition (4.6 kJ molꢁ1).
This indicates that the major structural change takes place at
the SmA–SmAPR transition, whereas polar order develops
nearly continuously. Furthermore, the current peaks are
close to the 0 V line of the applied voltage (Fig. 4b), which
is untypical for antiferroelectric switching. For this type of
polarization current curves a superparaelectric type of switching
was discussed previously.11
1 R. Amaranatha Reddy and C. Tschierske, J. Mater. Chem., 2006,
16, 907; H. Takezoe and Y. Takanishi, Jpn. J. Appl. Phys., 2006,
45, 597.
2 L. E. Hough, M. Spannuth, M. Nakata, D. A. Coleman,
C. D. Jones, G. Dantlgraber, C. Tschierske, J. Watanabe,
E. Korblova, D. M. Walba, J. E. Maclennan, M. A. Glaser and
¨
There are two distinct models for the SmA–SmAPR–SmAPA
transition. (i) In the first model uniform polarity develops at
the transition SmA-to-SmAPR within the layers but without
correlation of the polar direction in adjacent layers (Fig. 5b).
These polar layers register in an antiparallel correlation at the
transition to the SmAPA phase (Fig. 5a).8a (ii) In an alternative
model8b the molecules, which are rotationally disordered
around the long axis in the SmA phase, become organized in
polar clusters with a preferred polar direction at the transition
to SmAPR (Fig. 5c, note that there is randomization of polar
direction in the layers, but correlation between the layers). In
the SmAPR phase these clusters are rotational disordered
around the director n and their size increases with decreasing
temperature. These ferroelectric clusters can be aligned under
a sufficiently strong electric field, leading to a field induced
switching. At a certain size these clusters align spontaneously,
giving rise to macroscopic phase biaxiality and polar order
(SmAPR–SmAPA transition). Further detailed investigations
are in progress to clarify the modes of supramolecular organi-
zation in the SmAPR and SmAPA phases.
N. A. Clark, Science, 2009, 325, 452.
3 (a) T. Niori, T. Sekine, J. Watanabe, T. Furukawa and
H. Takezoe, J. Mater. Chem., 1996, 6, 1231; (b) D. R. Link,
G. Natale, R. Shao, J. E. Maclennan, N. A. Clark, E. Korblova
and D. M. Walba, Science, 1997, 278, 1924.
4 J. Etxebarria and M. B. Ros, J. Mater. Chem., 2008, 18, 2919.
5 H. R. Brand, P. E. Cladis and H. Pleiner, Macromolecules, 1992,
25, 7223.
6 A. Eremin, S. Diele, G. Pelzl, H. Nadasi, W. Weissflog,
J. Salfetnikova and H. Kresse, Phys. Rev. E: Stat., Nonlinear,
Soft Matter Phys., 2001, 64, 051707.
7 (a) L. Kovalenko, M. W. Schroder, R. A. Reddy, S. Diele, G. Pelzl
¨
and W. Weissflog, Liq. Cryst., 2005, 32(7), 857–865;
(b) R. Amaranatha Reddy and B. K. Sadashiva, J. Mater. Chem.,
2004, 14, 310; B. Glettner, S. Hein, R. Amaranatha Reddy,
U. Baumeister and C. Tschierske, Chem. Commun., 2007, 2596.
8 (a) D. Pociecha, M. Cepic, E. Gorecka and J. Mieczkowski, Phys.
Rev. Lett., 2003, 91, 185501; (b) Y. Shimbo, E. Gorecka,
D. Pociecha, F. Araoka, M. Goto, Y. Tankanishi, K. Ishikawa,
J. Mieczkowski, K. Gomola and H. Takezoe, Phys. Rev. Lett.,
2006, 97, 113901; (c) K. Gomola, L. Guo, E. Gorecka,
D. Pociecha, J. Mieczkowski, K. Ishikawa and H. Takezoe, Chem.
Commun., 2009, 6592.
In summary, the unique phase sequence SmA–SmAPR–SmAPA
was observed for the first time and broad SmAPR regions
9 (a) G. Pelzl, H. N. Shreenivasa Murthy, M. W. Schroder, S. Diele,
¨
Z. Vakhovskaya, H. Kresse and W. Weissflog, J. Mater. Chem.,
2006, 16, 1702; (b) R. A. Reddy, U. Baumeister and C. Tschierske,
Chem. Commun., 2009, 4236.
10 G. S. Lee, Y.-J. Lee, S. Y. Choi, Y. S. Park and K. B. Yoon, J. Am.
Chem. Soc., 2000, 122, 12151.
11 M. F. Achard, J. Ph. Bedel, J. P. Marcerou, H. T. Nguyen and
J. C. Rouillon, Eur. Phys. J. E, 2003, 10, 129.
12 C. Keith, A. Lehmann, U. Baumeister, M. Prehm and
C. Tschierske, Soft Matter, 2010, DOI: 10.1039/b923262a.
13 (a) P. Bedel, J. C. Rouillon, J. P. Marcerou, M. Laguerre,
H. T. Nguyen and M. F. Achard, Liq. Cryst., 2001, 28, 1285;
(b) E. Terazzi, L. Guenee, P.-Y. Morgantini, G. Bernardinelli,
B. Donnio, D. Guillon and C. Piguet, Chem.–Eur. J., 2007, 13,
1674.
14 S. A. Rama Krishnan, W. Weissflog, G. Pelzl, S. Diele, H. Kresse,
Z. Vakhovskaya and R. Friedemann, Phys. Chem. Chem. Phys.,
2006, 8, 1170.
Fig. 5 Models of molecular arrangements (green arrows = polar
direction, skewed arrows point out of or into the projection plane):
(a) SmAPA; (b,c) SmAPR; (b) randomization of polar direction
between the layers, (c) randomization of polar direction within the
layers (ferroelectric clusters).
ꢀc
This journal is The Royal Society of Chemistry 2010
3704 | Chem. Commun., 2010, 46, 3702–3704