Biaxial smectic a liquid crystal in a pure compound

Article abstract of DOI:10.1039/b106084h

The synthesis and characterisation is reported of a low molecular weight organic compound to exhibit the biaxial smectic A (SmA_b) phase, which shows a transition from the partial bilayer uniaxial SmA_d phase to the SmA_db ph

Full text of DOI:10.1039/b106084h

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Biaxial smectic A liquid crystal in a pure compound
B. K. Sadashiva,* R. Amaranatha Reddy, R. Pratibha and N. V. Madhusudana
Raman Research Institute, C.V. Raman Avenue, Bangalore 560 080, India. E-mail: sadashiv@rri.res.in
Received (in Cambridge, UK) 10th July 2001, Accepted 31st August 2001
First published as an Advance Article on the web 26th September 2001
The synthesis and characterisation is reported of a low
molecular weight organic compound to exhibit the biaxial
smectic A (SmAb) phase, which shows a transition from the
partial bilayer uniaxial SmAd phase to the SmAdb phase as
the temperature is lowered.
The liquid crystalline properties were investigated by
differential scanning calorimetry (DSC, Pyris 1D, 5 °C min21
)
and polarised optical microscopy (Leitz Laborlux 12 POL).
Observation using a polarising microscope of a sample taken
from between the untreated slide and coverslip shows the
following sequence of textures as the temperature is lowered.
The dark field of view, corresponding to the isotropic phase,
goes over at a transition point to an SmA texture in which most
of the sample has a homeotropic alignment, but some parts
clearly show focal-conic domains. As the temperature is
lowered further, there is another phase transition in which the
homeotropic regions exhibit a schlieren texture (Fig. 1), with
both two-brush and four-brush defects. Strong fluctuations in
intensity are seen as in the SmA2b phase.10 The presence of two-
brush defects indicates that the low temperature phase has one
of the following structures: (i) a biaxial smectic A phase in
which the two orthogonal directors in the plane of smectic
layers are also apolar in nature; (ii) a packing of the BC
molecules of compound 1 such that each layer is polarised, but
with a mutually antiferroelectric arrangement in neighbouring
layers (the two-brush defects in such a case are dispirations,
which are combinations of half strength disclinations which are
defect patterns in the in-plane director, and screw disloca-
tions;17 this structure can also be described as a biaxial SmA);
and (iii) another possibility would be for the molecules to tilt
with an anticlinic arrangement in successive layers, in which
The discovery of liquid crystalline phases in compounds with
bent-core (BC) or banana-shaped molecules1 has led to a very
intense research activity in the field.2 In the past couple of years
several new compounds having this molecular architecture have
been reported3–8 in an effort to find the relationship between the
molecular structure and the mesomorphic properties associated
with it. At least five different B-(banana) types of liquid crystal
have been clearly identified in pure compounds, though recently
some new variants have been found.8 The most fluid of the B-
type of liquid crystals, which is also electrically switchable is
the B2 phase in which the bent-cores pack along a common axis
to give rise to an electric polarisation of the layer. The molecular
plane tilts with respect to the layer-normal to produce a chiral
layer, though the molecules themselves are usually achiral.9 In
an earlier study, we found that binary mixtures of a compound
exhibiting the B2 phase with one made of rod-like molecules
exhibiting the bilayer smectic A2 phase showed a very
interesting phase diagram.10,11 If the concentration of the rods is
between 4 and 13 mol% the mixtures exhibit a biaxial smectic
A2 (SmA2b) phase in which the BC molecules undergo an
orientational transition as the temperature is lowered from the
uniaxial SmA2 phase. The other known systems exhibiting the
SmAb phase were found (i) in a binary mixture of polymeric and
monomeric species,12 (ii) in an oxadiazole derivative13 at high
temperatures (above 235 °C), and (iii) in a binary mixture
involving a metallomesogen.14
It is of obvious interest to find the biaxial SmAb phase in a
low molecular weight pure compound at a relatively low
temperature. Compounds whose molecules have a strong
biaxiality of shape are necessary for this purpose and the best
systems appear to be those made of BC molecules. Indeed,
several compounds with BC molecules are known to exhibit the
smectic A (SmA) phase but NMR studies have shown that in
such molecules the angle between the two arms of the bent-core
is about 160°, which means that the deviation from a rod-like
structure (in which this angle would be 180°) is quite small.2 On
the other hand, if the molecules have highly bent cores the
compounds are known to exhibit only the B-type of liquid
crystal.
Here, we report the synthesis of a compound made of BC
molecules with a new architecture. The unsymmetrical mole-
cule has an alkoxy chain attached to only one of the arms of the
bent-core, while the other arm ends with the highly polar cyano
group. The synthetic pathway used to obtain this compound is
shown in Scheme 1.‡ Our studies show that the compound
exhibits a partial bilayer smectic Ad phase in which the bent-
cores of two neighbouring molecules overlap in an antiparallel
orientation, as in the case of highly polar rod-like mole-
cules.15,16 Indeed, this is the first example of a single-
component, low molecular weight compound to exhibit the
biaxial smectic A to uniaxial smectic A transition.
Scheme 1 Reagents and conditions: i, DCC, cat. DMAP, dry CHCl3, rt, 15
h, 81%; ii, cat. 5%Pd–C, H2, 1,4-dioxane, 55 °C, 75%; iii, 3-benzylox-
ybenzoic acid, DCC, cat. DMAP, dry CHCl3, rt, 15 h, 72%; iv, cat. 5% Pd–
C, H2, 1,4-dioxane, 55 °C, 73%; v, 3-fluoro-4-benzyloxybenzoic acid,
DCC, cat. DMAP, dry CHCl3, rt, 20 h, 68%; vi, cat. 5% Pd-C, H2,
1,4-dioxane, 55 °C, 80%; vii, 4-n-tetradecyloxybenzoic acid, DCC, cat.
DMAP, dry CHCl3 rt, 20 h, 61%.
† Electronic supplementary information (ESI) available: spectroscopic data
for 1. See http://www.rsc.org/suppdata/cc/b1/b106084h/
2140
Chem. Commun., 2001, 2140–2141
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b106084h

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Fig. 3 Intensity profile of the X-ray diffraction pattern in the SmAdb phase
of compound 1 at 123 °C as a function of the Bragg angle, q. The sharp
reflection corresponds to a layer spacing of 54 Å. A very weak second order
reflection can also be seen. The diffuse maximum corresponds to 4.6 Å,
which is a typical intermolecular separation in the liquid layers.
Fig. 1 Photograph of the schlieren texture of a homeotropic sample of the
biaxial smectic liquid crystal of compound 1. Note several half strength
disclinations with two dark brushes which indicate an apolar in-layer
director field (crossed polars, 3250).
Å is significantly larger than the calculated molecular length l ≈
49 Å in its most extended form with an all-trans configuration
of the alkoxy chain. This clearly implies a partial bilayer
structure of the smectic layers in both SmA and SmAb phases,
which we have to designate as SmAd and SmAdb re-
spectively.
Thus, we believe that the occurrence of both the uniaxial and
biaxial smectic A phases in compound 1 is mainly a con-
sequence of the partial bilayer structure in which there is a dense
region of biaxial aromatic cores and relatively sparsely
populated aliphatic regions, which allow for considerable
orientational freedom of the chains. The arrangement of the BC
molecules in the SmAdb phase in the pure compound 1 is hence
quite different from that proposed for the SmA2b phase
occurring in binary mixtures of rod-like and bent-core mole-
cules. In the latter the BC molecules orient with their arrow axes
along the layer-normal, whereas in the SmAdb phase the arrows
lie in the layer-plane.
Fig. 2 Photographs of the conoscopic patterns from a homeotropically
aligned sample of (a) the uniaxial smectic Ad phase at 128 °C, and (b) the
biaxial smectic Adb phase at 124 °C.
case again dispirations can produce two-brush defects. We rule
out the last possibility in view of the X-ray results to be
described later.
DSC of compound 1 shows the following sequence of
transitions. The numbers above the arrows are transition
temperatures in °C and the numbers in parentheses are
The authors wish to thank Dr V. A. Raghunathan for help in
the X-ray experiments and Mr P. N. Ramachandran and Ms K.
N. Vasudha for technical support.
(1)
enthalpies in kJ mol21. In order to confirm the biaxiality of the
low temperature phase, samples contained in a cell and well
aligned using a lateral electric field are employed. The thickness
of the sample is 37.3 mm which is controlled using appropriate
spacers and measured using an interference technique before
filling the sample. As the temperature of the cell is lowered
below the SmA to SmAb transition point (Tbu) again the
schlieren texture is seen. The application of an electric voltage
of ca. 200 V at 10 kHz between the two ITO regions aligns the
sample uniformly in the gap. Conoscopic observations between
crossed polarisers, which are set at 45° to the direction of the
electric field, clearly show that the uniaxial cross in the SmA
phase splits to give a biaxial pattern below the relevant
transition temperature (Fig. 2). As the temperature is lowered,
the separation between the hyperbolic isogyres [the two dark
arcs seen in Fig. 2(b)] continuously increases.
Notes and references
‡ Compound 1 was synthesised according to a procedure which will be
described in a publication under preparation. The intermediate compounds
have spectral data consistent with their molecular structure.
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X-Ray scattering studies have been carried out by taking the
sample in a Lindemann capillary tube which is slowly cooled to
the desired temperature under a magnetic field of strength ca. 2
kG applied perpendicular to the axis of the tube. The alignment
is only partial as the isotropic to SmA transition has a strong
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