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CHAPTER 1
INTRODUCTION
This chapter furnishes a brief introduction to liquid crystals. The
study of structures of electrically tunable bent core molecules, spontaneous
symmetrisation, transitions of the mesophases, and optical / electro-optical
responses of bent core materials based on the x-ray diffraction, polarizing
optical microscopy, and electro-optical measurement were explored in detail.
1.1. INTRODUCTION TO LIQUID CRYSTALS
The constituent molecules occupy specific sites in a lattice and
point their axis in fixed directions. Thus, they are orientationally and
positionally ordered in highly structured solids. Conversely, the molecules in
the liquid state have no such orders and hence they are optically isotropic. The
melting of solids to the isotropic liquids at a well-defined temperature is a
familiar phase transition. In contrast, a considerable number of organic or
metal-containing compounds of special kind do not melt directly from solid to
isotropic liquid; instead they pass through an intermediate phase, called a
mesophase. In such case, two phase transitions are involved: transition from
crystalline solid to mesophase at low temperature and transition from
mesophase to isotropic phase at high temperature. The term ‘liquid crystal
(LC)’ has been quite commonly and interchangeably used with mesophase.
Technically, they are partially ordered (anisotropic) fluids,
thermodynamically located between three dimensionally ordered crystalline
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solid and isotropic liquid states. Molecules capable of forming LC phase are
normally referred as liquid crystals or mesogens. Hitherto, a wide variety of
LC phases have been discovered essentially differing in the sense of
arrangement of constituent mesogens. A single compound can exhibit more
than one mesophase. The average packing of the molecules in different
phases is shown in Figure 1.1.
Figure 1.1 Schematic representation of molecular arrangement
1.2 HISTORY OF LIQUID CRYSTALS
In 1888 the Austrian botanical physiologist Reinitzer (1888), when
working at the German University of Prague (1858-1927), extracted
cholesterol from carrots to establish its chemical formula. Reinitzer examined
physico-chemical property of various derivatives of cholesterol.
A number of workers already observed some distinct color effects on cooling
cholesterol derivatives just above the solidification temperature. Reinitzer
himself found the same phenomenon in cholesteryl benzoate, but the colors
3
near the solidification of cholesteryl benzoate were not the most peculiar
feature and finally detected that cholesteryl benzoate does not melt like other
compounds but obviously had two melting points. At 145.5°C it melted into a
cloudy liquid and at 178.5°C it melted again and the cloudy liquid suddenly
became clear. Furthermore, the phenomenon was reversible.
Soon after 1900, Vorlander (1908) started a research group working
on LCs and demonstrated the principles of molecular design that highlight the
field. The heroic period of the liquid crystalline state comes to an end around
1920. By this time, a large amount of data on LCs had been collected but LCs
was not popular among scientists in the early 20th century and the material
remained scientific curiosity. Till 1957 all were quiet on the LC front and
nothing new could be expected in this area. Brown and Shaw (1957)
published an article on the LC phase and subsequently sparked an
international resurgence in LC research which then developed into a mature
field of science (Schadt 1989). Later research efforts yielded various
interesting developments expanded the field of research.
In 1991, when liquid crystal displays were well established in our
day-to-day life, de Gennes (1975, 1984) received the Nobel Price in physics.
He discovered “methods developed for studying order phenomena in simple
systems can be generalized to more complex forms of matter, in particular to
liquid crystals and polymers”.
Chandrasekhar et al (1977) explored that the disc shaped molecules
exhibited the liquid crystalline properties. The chronological development
from conventional rod and disc shaped to non-conventional bent-shaped was
incorporated by Mori et al (1996) which showed polar order. The concept of
supramolecular chemistry has broadened in the design of LCs (Lehn 1988,
Scherman 2009). A new molecular design from nano to macro scale is
important to enlarge the functional capabilities of LCs.
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1.3 CLASSIFICATION OF LIQUID CRYSTALS
Materials exhibiting mesophases can be broadly classified into two
categories: lyotropic and thermotropic liquid crystals. In lyotropics the
mesomorphism is influenced by the action of solvents on amphiphiles, while
in the case of thermotropic liquid crystals, the transitions brought by the
action of heat.
1.3.1 Lyotropic Liquid Crystals
A liquid crystalline material is called lyotropic if phases having
long ranged orientational order are induced by the addition of a solvent.
Historically the term was used to describe materials composed of amphiphilic
molecules. Such molecules comprised of water loving 'hydrophilic' head
group (which may be ionic or non-ionic) attached to a water hating
'hydrophobic' group. Typical hydrophobic groups are saturated or unsaturated
hydrocarbon chains. Examples of amphiphilic compounds are the salts of
fatty acids, phospholipids. Many simple amphiphiles are used as detergents.
The aggregation of amphiphiles into micelles and then into
lyotropic liquid crystalline phases as a function of amphiphile concentration
and of temperature are represented in Figure 1.2. The simplest liquid
crystalline phase is formed by spherical micelles known as 'micellar cubic',
denoted by the symbol I1. This is a highly viscous, optically isotropic phase in
which the micelles are arranged on a cubic lattice. At higher amphiphile
concentrations the micelles fused to form cylindrical aggregates of indefinite
length and these cylinders are arranged on a long ranged hexagonal lattice.
This lyotropic liquid crystalline phase is known as the 'hexagonal phase' or
more specifically the 'normal topology' hexagonal phase and generally
denoted by the symbol HI. At higher concentrations of amphiphile the
'lamellar phase' is formed. This phase is denoted by the symbol L . This phase
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consists of amphiphilic molecules arranged in bilayer sheers separated by
layers of water. Each bilayer is a prototype of the arrangement of lipids in cell
membranes. For most amphiphiles consist of a single hydrocarbon chain, one
or more phases having complex architectures are formed at concentrations
that are intermediate between those required to form a hexagonal phase. It
leads to the formation of a lamellar phase. Often this intermediate phase is a
bicontinuous cubic phase.
Figure 1.2 Schematic representation of the aggregation of amphiphiles
into micelles
1.3.2 Thermotropic Liquid Crystals
The essential requirement for a compound to be a thermotropic
liquid crystal is that it should possess rigid (hard) and soft (flexible) regions.
The hard regions of the molecule are usually derived from aromatic or non-
aromatic cores, while paraffinic chains account for soft region of the
molecule. Further, these two distinct regions are combined to obtain
molecules featuring a pronounced shape anisotropy, which plays a vital role
in stabilization of different types of LC phases. The transition from crystal to
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mesophase is called melting point while that from the mesophase to the
isotropic liquid termed as clearing point. Thermodynamically stable LC
phases occur during both heating and cooling processes are called
enantiotropic phases. While, thermodynamically unstable mesophases appear
during the cooling process due to hysteresis in the crystallization are referred
as monotropic. These thermotropic LCs can be broadly classified into two
categories by its shape of molecules: conventional and non-conventional
shaped liquid crystals.
1.4 SHAPE OF MOLECULES AND THEIR MESOPHASES
1.4.1 Conventional Shaped Liquid Crystal
The conventional liquid crystals are broadly classified into two
types, calamitic LCs and discotic LCs which are further classified by
mesophase as shown in the following schematic representation (Figure 1.3).
Figure 1.3 Schematic representation for classification of thermotropic
liquid crystals and its polarizing optical microscopy image
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1.4.1.1 Calamitic liquid crystals
The most commonly encountered liquid crystals (LCs), often called
calamitic LCs. It consist of rod-like molecules with one molecular axis longer
than the other two axes. Figure 1.4a shows a general template that describes
the structure of calamitic liquid crystal.
Figure 1.4 (a) A general template for molecular structure of calamitic
LC and (b) an example of a calamitic LC
In the above mentioned template RC1 and RC2 are the rigid cores
often aromatic in nature (e.g, 1,4-phenyl, 2,5-pyrimidinyl, 2,6-naphthyl etc.)
or they can also be alicyclic (e.g, trans-4-cyclohexyl, cholesteryl etc.,) cores.
These two cores are generally interconnected through either a covalent bond
or linking groups L such as -COO-, -CH2-CH2-, -CH=N, -N=N- etc. The
terminal substituents R and R' are usually either alkyl or alkoxy chains or the
combination of these two. In many cases one of the terminal unit is a polar
substituent (e.g, CN, F, Cl, etc.). In some special cases lateral substituents X
and Y (e.g, F, Cl, CN, CH3, etc.) are incorporated to account for the special
property. A typical example of achiral rod-like mesogen is 4-methoxy-
benzylidene-4-n-butylaniline (MBBA), as shown in Figure 1.4b (Tschierske
and Dantlgraber 2003, Ros et al 2005). The introduction of molecular
chirality in rod-like molecules furnished optically active LCs and exhibited a
variety of interesting mesophases (Meyer et al 1975).
R L R'
X Y
N C4H9H3CO
(a) (b) MBBA
RC1 RC2
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As pointed out earlier, conventional achiral rod-like molecules, in
general, exhibited nematic (one dimensional) and/or smectic (two
dimensional) phases and cholesteric phase. Whereas, chiral rod-like
molecules organize themselves to form macroscopic helical structures that
results in mesophases such as the chiral nematic (N*) and/or chiral smectic C
(SmC*) phases. They also stabilized highly frustrated structures like blue
phase (BP) and twist grain boundary (TGB) phases.
The nematic mesophase has long range orientational order and no
long range positional order is shown in Figure 1.5a (Wright and Mermin
1989). The molecules are oriented parallel in a certain domain of a sample
(the preferred direction can vary from point to point in a medium). The
nematic phase is optically uniaxial (Nu) and is therefore frequently used in
display device technology. The biaxial nematic phase (Nb) had an additional
correlation of the molecules perpendicular to the director and recognized sub-
class of nematic phase (Garoff and Meyer 1978, Crooker 1989, Lagerwall
1999).
Molecules in smectic phases are ordered in layers. The translation
of molecules from one layer to another is limited. A variety of molecular
arrangements is possible within the layered systems. In the most of cases,
there is no positional order in the smectic A (SmA) phase (Figure 1.5b),
within each layer and the long axes of the molecules are on average
positioned perpendicular to the layers. Despite this partial ordering of the
molecular positions, the substance still flows and is therefore a liquid. In the
smectic C (SmC) phase, the molecules tilted with respect to the layer normal.
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.
Figure 1.5 Representation of (a) nematic phase and (b) smectic A phase
The chiral smectic C phase (SmC*) exhibits a helical structure. In
contrast to the cholesteric phase (Figure 1.6a), the subsequent layers with the
tilted molecules are slightly rotated with respect to each other (Figure 1.6b).
In a typical SmC* material the director rotates on the tilt cone about 1° from
one layer to the next (Figure 1.6c). In these materials, the orientation of the
tilt can be influenced by an electric field and therefore this phase can be used
in displays that in theory can be switched much faster than conventional
Figure 1.6 Representation of (a) cholesteric phase, (b) chiral smectic C
phase (SmC*) and (c) the angle in director in subsequent
layers
a b
(c)(b)(a)
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nematic displays. When compared to a SmC phase which has C2h symmetry,
the symmetry of the SmC* phase is further reduced to C2 and the phase is
therefore polar in nature.
The cholesteric phase is a nematic phase composed of chiral
molecules or induced by the presence of chiral molecules. As a consequence,
the system acquires a helical ordering perpendicular to the long axis of the
molecules. The helix may be right- or left-handed depending on the molecular
chirality. The pitch is the distance after which the molecules have the same
average orientation (Figure 1.6 c) (Sackmann and Demus 1969). A special
property of this phase is the light of wavelength equal to the pitch is
selectively reflected and circularly polarized.
Many thermotropic LCs pass through more than one mesophase on
heating from the solid to isotropic liquid state. The typical LCs is called
polymesomorphic and the process known as polymesomorphism. Sackmann
and Demus (1969) derived the rule of the phase sequence by systematic
observation of sequences of different phases in polymesomorphic compounds.
This rule in such compounds predicts a stepwise decrease of order with
increase in temperature. According to this general rule smectic
phases are low temperature phases while the nematic phase occurs at higher
temperatures. Considering all structures known in calamitic LCs a
hypothetical sequence which in general may be written as:
I – N – SmA – SmC – SmB – SmI - SmF – Crystal B – J – G – E – K – H-
solid for achiral material and I – BP – N* – TGB - SmA – SmC* – SmI* -
SmF* for chiral LCs. Until there is no single material that shows all these
phases.
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1.4.1.2 Discotic liquid crystals
In 1908, Vorlander framed a rule that for a compound to show
mesomorphism, it must have linear molecular shape (Kuczynski and
Stegemeyer 1995). Conversely, Chandrasekhar et al (1977) showed that disc-
like (called discotic) molecules also formed LC phases. Pramod et al (1997)
prepared a number of hexa-n-alkanoyloxybenzenes (Figure 1.7a) and showed
that they formed a new class of LCs in which flat-molecules were stacked one
on top of the other to form columns, which in turn arrange themselves into
2D-lattices (Figure 1.7b).
Figure 1.7 (a) General molecular structure of hexa-n-alkanoyloxy
benzenes and (b) schematic representation of the columnar
texture of structure
Since a large variety of discotics have been designed and
synthesized from both basic research and application viewpoints.
Conventional discotics consist of a flat aromatic core surrounded by several
paraffinic chains lead to the formation of two different types of mesophases
namely nematic (N) and columnar (Col) phases. The least ordered discotic
nematic was considered to be a better media for display applications
especially with respect to viewing angle problems (Chandrasekhar et al 2003,
Nair et al 2003). The recent commercialization of discotic nematic in the
O
O
O
O
O
O R
O
RO
R
O
R
O
R OR
O
R = n-C4 to C9
(a) (b)
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production of optical compensation films by Fuji Film Company has created
an immense interest in this area (Kawata 2002). In majority of the discotics,
the columnar phase predominates over nematic phase owing to the presence
of a flat –electron rich aromatic rings, which stack on top of the other
because of attractive intermolecular forces resulting in strong inter-core
interactions. This unique structure of columnar phases fascinated to use as
quasi-one dimensional conductors, photo-conducting systems, light emitting
diodes, photovoltaic solar cells, optical storage devices, hybrid computer
chips for molecular electronics etc., (Boden and Movaghar 1998). The
guidelines framed for the formation of mesophases in achiral discotics have
been followed to realize chiral discotic systems by introducing one or several
chiral chains around the periphery of discotic core exhibited either chiral
nematic (N*) or columnar phase (Malthete et al 1981, Cho and Lim 1988,
Langner et al 1995). Rarely chiral discotic nematic phase has been observed
in pure compounds that have the analogous structure to the chiral nematic
(cholesteric) phase exhibited by calamitics. Interestingly, columnar phase
formed by chiral discotics exhibited ferroelectric switching properties,
appeared to possess advantages over their tilted smectic counterparts in
electro-optical displays (Bock and Helfrich 1992).
1.4.2 Non-Conventional Shaped Liquid Crystals
Over the past two decades there has been a revolution in the
synthesis and characterization of new thermotropic LC materials, termed
“non-conventional LCs”. The anisotropic shape of the molecules deviated
from classical rod-like or disk-like molecular motifs (Demus 1998, Tschierske
2001, 2002). These molecular architectures exhibited complex mesophase
morphologies as compared to conventional ones. Some of the examples of
non-conventional LCs are: oligomeric LCs (OLCs), bent-core molecules,
polycatenars, polyhydroxy amphiphiles, octahedral complexes, star shaped
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molecules, rod-coil molecules and dendrimers (Zeng and Swager 1994,
Pegenau et al 1996, Tschierske 1996, Cameron et al 1997, Lee et al 1998,
Pelzl et al 1999, Gharbia et al 2002) are shown in Figure 1.8. Among these,
OLCs and bent-core molecules are attracting a great deal of attention due to
their remarkable mesomorphic behaviour. The OLCs can be subdivided into
two broad groups: (I) linear OLCs have two or several mesogenic segments
are covalently linked in an end-to-end (axial) fashion by means of paraffinic
spacer/s; (II) non-linear OLCs contain several mesogenic segments around a
central mesogenic unit. The examples of OLCs are dimers, trimer, tetramers,
pentamers, etc., in which respective numbers of mesogenic segments are
joined together through flexible spacers.
Among these OLCs, the achiral/chiral dimers comprised either two
chemically identical (symmetrical) or non-identical (unsymmetrical)
mesogenic segments connected by a central flexible spacer. These dimers
have been investigated extensively owing to their noteworthy LC behaviour.
On the other hand, bent-core (banana-shaped or V-shaped) mesogens formed
by connecting two rod-like segments to a central angular core, now represent
a subfield of thermotropic LCs. In the last ten years, hundreds of such
compounds, in particular the banana-shaped mesogens have been designed
and synthesized. This enormous and unprecedented development stems from
their ability to form eight different phases, which are generally termed as
“Banana” (Bn) phases, with no analogues among other LC phases.
Interestingly, some of the fluid banana phases display electrical switching
behaviour (polar order) although the constituent molecules are achiral. In
essence, it is evident from the literature that the design and synthesis of LC
dimers, non-linear OLCs and bent-cores are of current interest for achieving
novel mesophases and possibly understand their structure-property
relationships.
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Figure 1.8 Molecular structures of different types of non-conventional
LCs
15
Indubitably, there is a considerable scope in this research area to derive such
systems with diverse molecular architecture that possibly show interesting
thermal behaviour.
1.5 (ANTI) FERROELECTRICITY AND SWITCHING
MECHANISM
A mesophase with a permanent polarization in the absence of an
electric field is called a ferroelectric mesophase. In order to possess a bulk
polarization, molecules exhibit spontaneous polarization (Ps). Hindered
rotation around the molecular long axis plays an essential role in the
emergence of Ps. The director of molecular assembly with spontaneous
polarization can be changed by the application of an appropriate electric field.
In most ordinary liquid crystalline phases (N, SmA, SmC), the symmetry is
high rotation around the long molecular axis prevents the occurrence of
ferroelectricity. The symmetry has been lowered further to find
ferroelectricity, for example in chiral tilted smectics (SmC*).
Since the director of a SmC* phase rotates from layer to layer, a
helical arrangement is present and therefore the system escapes from
macroscopic polarization. The SmC* phase can be driven towards the
ferroelectric state by applying an external electric field. In that case the helix
unwinds and the molecules in all layers orient in the same direction. By
applying an electric field of opposite sign the polarized phase (ferroelectric)
switch to the other ferroelectric state (Figure 1.9) (Heppke and Moro 1998).
This behaviour is often referred as bistable switching.
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Figure 1.9 Bistable switching under influence of an electric field
The main advantage of these smectic materials is their relatively
fast switching properties compared with conventional nematics (Heppke and
Moro 1998). The reorientation of the molecules did not require much energy
when compared to nematics. This is mainly caused by the fact that the
molecules rotate collectively around a cone. Apart from the ferroelectric layer
organization the direction of polarization (and also tilt) is the same in all
layers, the tilt alternates from layer to layer to form antiferroelectric (tristable)
structure when no field is applied. In the intermediate ferrielectric phases, the
tilts randomly alternate with preference for one direction. The ferroelectric
phase transfers to the antiferroelectric phase via the ferrielectric state. The
symmetry breaking through a combination of chirality and tilt in addition to
the symmetry of the phases may also break by a combination of tilt and a
polar component perpendicular to the director of the molecules. This was
found in columns of bowl-shaped molecules (Budig et al 1994). These
columnar phases were stabilized by the one-directional stacking of molecules
in the column. This can generate a ferroelectric or antiferroelectric packing of
the columns. This is also found for liquid crystalline phases of banana, bent-
core or bow-shaped molecules. The
+ electric f ield - electric field
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classical SmC* compounds and banana-shaped compounds can similarly
order into ferroelectric and antiferroelectric arrangements, as shown in Figure
1.10 (Kishikawa et al 2005).
Figure 1.10 Two types of layer organization for bent-core molecules (a)
ferroelectric and (b) antiferroelectric
The switching process in banana-shaped compounds under the
influence of an electric field has been linked with the stron -
aromatic cores those results in a restricted molecular rotation around long
axes. Therefore, the field-induced reorientation should take place via rotation
of molecules around the tilt cone and chirality in the layer remains same (tilt
and polar direction reversed). Recently, field induced switching of chirality
was also detected by Keith et al (2004), Amaranatha Reddy et al (2005). In
this case the polar switching was not caused by rotation of director around tilt
cone, but by a collective rotation of molecules around their long axes as
depicted in Figure 1.11 Weissflog et al (2005).
(a) (b)
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Figure 1.11 Two types of polar switching (a) around a cone and (b)
around the molecule
The ground state liquid crystalline phase could be changed from
antiferroelectric to ferroelectric upon introduction of fluorine substituents in
the outer rings (ortho to the terminal tails) or by branching of the terminal
tails (Bedel et al 2000, Walba et al 2000, Nakata et al 2001, Nadasi et al 2002,
Amaranatha Reddy and Sadashiva 2002a, Kumazawa et al 2004). The reason
for this behaviour might be related to dipolar interactions, or intermolecular
interactions at the interlayer interfaces.
1.6 CHIRALITY IN LIQUID CRYSTALS
Ferroelectricity or antiferroelectricity in liquid crystals attracted
considerable interest is due to its potential industrial application. The first
ferroelectric liquid crystal as obtained with chiral molecule organized in
SmC* phase. A lot of new chiral ferroelectric and antiferroelectric liquid
crystals have been synthesized during the last decades (Fukuda et al 1994).
Based on these experiments, chirality was essential for liquid crystalline
molecules to exhibit ferroelectric properties (Patel and Goodby 1987). But
this is not the fact for all the molecules, hence the system must be
non-centrosymmetric, this is the basic requirement for a material to be
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ferroelectric. Moreover, a macroscopic polarization can exist in the system
and therefore essentiality of ferroelectricity in liquid crystals is the polar
arrangement, not the chirality.
The rod-like molecules possess sufficiently incompatible subunits,
and the lateral attraction between identical segments of adjacent molecules is
sufficiently strong. Therefore the molecules are possible to form
non-centrosymmetric structures, although these molecules are achiral
(polyphilic molecules). This was already predicted as theory by Tredgold
(1990) and Vanakaras and Photinos (1998), but for long time ferroelectricity
was found with chiral molecules in practice.
In the last decade great efforts have been made to find achiral
molecules forming switchable phases. For example, polyphilic molecules and
bowl-shaped molecules have been designed to obtain non-chiral ferroelectric
fluid. The comparison of calamitic and bent-core molecules and their
organization in the smectic layers is shown in Figure 1.12 (Tournihac et al
1992, Lei 1983). The SmA and SmC phases formed by calamitics (left) and
the directed organization of bent-core molecules in the smectic layers giving
rise to a Ps parallel to the layer planes (right).
Figure 1.12 Comparison of calamitic and bent-core molecules and their
organization in the smectic layers
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1.7 BENT-CORE LIQUID CRYSTALS
The large diversity of LC phases arose due to the reduced
symmetry of these molecules, leading to polar order and supramolecular
chirality. The bent in the rigid cores of liquid crystals leads to a reduction of
the rotational disorder of molecules around their long axes. The molecular
structure facilitates an organisation into layers when segregation of aromatic
cores and aliphatic chains is sufficiently strong. Since the molecules are
closely packed within smectic layers and additionally, the rotation about their
long axes is strongly hindered, the bend directions align parallel in each layer.
As a result of this directed organisation, each layer has a spontaneous
polarization Ps parallel or antiparallel to direction of the molecular bent.
Many of the mesophases formed by bent-core molecules result from a strong
desire to escape from a parallel alignment of bent directions in adjacent
layers, i.e., desire to escape macroscopic polar order. There are numerous new
mesophases have been detected with these banana-shaped mesogens. Most of
them have no analogues in LC systems formed by conventional calamitic
molecules. Initially, seven phases were designed as B1 to B7 according to the
sequence of their discovery, where B stands for banana shaped molecules.
The structures and characteristic features of these phases were summarised by
Pelzl et al (1999). Another phase was discovered later and designated as B8
(Bedel et al 2001).
1.7.1 B1 Mesophase
The first compound exhibiting a B1 phase was reported by Sekine
et al (1997) who designated the mesophase as SmAb. The authors speculated a
frustrated anti-phase structure with results of XRD analysis. However,
Watanabe et al (1998) exhibited the existence of a two-dimensional (2D)
rectangular lattice structure in B1 phase using microbeam X-ray diffraction of
a monodomain sample. B1 mesophase is commonly observed in bent-core
21
compounds with short terminal alkyl chains. In a homologous series, this
phase occurs between non-polar B6 phase and polar
B2 phase on ascending series. B1 mesophase develops as dendritic pattern
from isotropic liquid at slow cooling condition, which leads to a
mosaic-like texture. Sometimes the mesophase shows spherulitic pattern,
when the isotropic liquid is cooled slowly. The B6 to B1 phase transition has
been reported in a few bent-core compounds (Pelzl et al 1999, Sadashiva et al
2000, Rouillon et al 2001, Weissflog et al 2001a, Mieczkowski et al 2002,
Shreenivasa Murthy and Sadashiva 2002). During the phase transition textural
changes are minimal. The B1 phase obtained on cooling the schlieren texture
of B6 phase exhibits a mosaic texture as sheared. The enthalpy accompanying
B6 to B1 transition is rather low indicating a weak first order transition. The B1
to B2 transition is rather rare and observed only in very few systems (Shen et
al 2000, Dantlgraber et al 2002, Ortega et al 2004, Shreenivasa Murthy and
Sadashiva 2004). Among the four reports of such transitions, in two systems,
the transition has been seen only on cooling whereas in the other two reports
the transition observed both on heating and cooling. Shen et al (2000)
reported that the mosaic texture of B1 phase changed to a schlieren texture
during the transition.
The X-ray diffraction pattern of the B1 mesophase was indicative of
a two-dimensional rectangular lattice as suggested by Watanabe et al (1998).
Two or more reflections were observed in the small angle region besides a
diffuse wide-angle reflection. One of the small angle reflections corresponds
to half the molecular length, which indicates an intercalation in the structure.
The diffuse peak in the wide-angle region indicates a liquid-like in-plane
order.
The 2D modulated structure for the B1 phase proposed is shown in
Figure 1.13. According to this model, the phase is built of columns formed by
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a
b
Figure 1.13 Schematic representation of the frustrated structure of B1
mesophase
layer fragments. The molecules in layer fragments are organized by the
polarization direction in adjacent clusters are antiparallel. The polarization
direction is perpendicular to the column axis and the molecules are non-tilted.
The lattice parameter ‘a’ provides an approximate number of molecules in the
lattice and the parameter ‘b’ corresponds to the length of the molecule. In this
structure, there are overlaps between the aromatic parts of the molecules at the
interfaces of neighbouring domains, which contributes to stabilizing the
phase. However, there also exists an unfavourable overlap of aromatic cores
and aliphatic chains of neighbouring molecules at the boundaries between the
domains. If the chain length is shorter, the unfavourable interaction is reduced
and the molecules can move across the domains. This causes a collapse of 2D
lattice and gives rise to an intercalated smectic B6 phase. If the chain length is
sufficiently long, the unfavourable chain-core interaction increases and
segregation of aromatic cores and aliphatic chains resulting in a monolayer
structure (B2). Thus, in a homologous series, B6, B1 and B2 phases appeared in
the sequence on increasing the chain length (Sadashiva et al 2000, Rouillon
et al 2001, Weissflog et al 2001). The B1 phase does not show any response to
an applied electric field. This is because the rotation of molecules restricted
23
due to the steric hindrance arising from interactions between column
boundaries.
1.7.1.1 Variants of B1 mesophase
Bedel et al (2000, 2002) reported a two dimensional phase with a
rectangular lattice, which is different from the conventional B1 phase. This
phase was observed in a series of compounds, which contain a flouro
substituent ortho to the terminal n-alkoxy chain. The XRD data obtained for
this mesophase could be indexed to a rectangular lattice and the phase did not
respond to an applied electric field. However, miscibility studies of this
mesophase with the B1 phase of the unsubstituted compound showed a strong
non-ideal behaviour. Hence Bedel et al (2000) designated the mesophase as
Bx. However, they have not proposed any structure for this mesophase.
Szydlowska et al (2003) reported two new modulated phases, which were
initially called as Bx and Bx1. These phases are switchable under an electric
field, in contrast to the commonly observed B1 phase. On the basis of XRD
analysis and electro-optical behaviour, the polarization direction is parallel to
the column axis and the density modulation is in the plane perpendicular to
the polarization vector in the Bx phase. Hence, the symbol was assigned as
B1rev for the Bx phase and B1revtilt for Bx1 phase, which is the tilted analogue of
B1rev. Pelz et al (2003) also reported such phases in two new compounds.
Similar columnar phases were also reported by Amaranatha Reddy et al
(2005).
Recently, Takanishi et al (2006) carried out X-ray microbeam
diffraction measurements on the B1 phase of a prototype bent-core compound
to investigate the local layer structure and intra layer molecular orientation is
shown in Figure 1.14. Their results indicated that the molecular bending plane
was normal to the frustrated plane (parallel to the column axis) and this was
24
different from the model proposed earlier for the B1 phase, but the same as
B1rev phase (Watanabe et al 1998).
Figure 1.14 Proposed structures of (a) B1 and (b) B1rev
1.7.2 B2 Mesophase
The B2 mesophase is the most commonly observed and most
extensively studied among all the banana mesophases. This mesophase was
first observed by Niori et al (1996). This mesophase is generally observed in
bent-core compounds with long terminal alkyl chains and exhibits a variety of
textures. A fingerprint or fringe pattern, schlieren and focal conic textures are
quite often observed. In addition, chiral domains of opposite handedness are
also observed on slow cooling from the isotropic phase. The XRD pattern of
the mesophase exhibits layer reflections, up to third or fourth order in the
small angle region and a diffuse peak in the wide angle region. In a well
oriented sample, the layer reflections are situated along the meridian and wide
angle diffuse scattering peak is inclined with respect to the meridian and the
equator, indicating a tilt of the molecules (Diele et al 1998, Weissflog et al
1998, Bedel et al 2002). The measured first order spacing is less than the
calculated full molecular length, which further supports the tilt of the
molecules in the layers. Local layer structure in the circular domains has also
25
been studied using X-ray microbeam and several types of layer structures as
suggested by Takanishi et al (2003).
A clear understanding of the structure of B2 mesophase was given
by Link et al (1997). They carried out careful electro-optical investigations on
freely suspended films and transparent electro-optic cells filled with samples
of compounds. The experimental results revealed that the optic axis was tilted
relative to the layer normal and the layer polarization, in the plane normal to
the tilt direction, alternates from layer to layer. The layer polarization is due
to the steric packing of bent-core molecules in the layers along the bend
direction. Their observations of the mesophase also suggest a strong biaxiality
and ordering of molecular planes normal to the tilt direction of the optic axis.
The mesophase B2 designated as SmCPA. Heppke and Moro (1998) reported
to consider three distinct planes, a tilt plane, a polar plane and a layer plane
associated with a given layer, as shown in Figure 1.15. The three planes are
assumed to be three co-ordinates of a system and then the mirror image is
non-super imposible. Thus, the layer becomes chiral, although the individual
molecules are achiral.
Figure 1.15 Pictorial representation of the geometry of a smectic layer in
the B2 (SmCPA) phase
26
The combination of polar order and tilt direction gives the layer a
chiral structure in the SmCPA phase. Depending on the tilt direction and polar
direction of the molecules in adjacent layers, two ground state structures can
be considered, namely SmCsPA (synclinic antiferroelectric) and SmCaPA
(anticlinic antiferroelectric). The chirality of the layers is identical in SmCaPA
and hence it represents a homochiral structure whereas chirality alternates
from layer to layer in SmCsPA resulting in a racemic structure. On application
of an electric field, a switching from antiferroelectric to ferroelectric state is
observed. A schematic representation of the molecular arrangements in the
chiral and racemic states at zero electric field and after the application of the
field is shown in Figure 1.16 (Schroder et al 2004). The switching process
takes place by a collective rotation of the molecules around a cone. This
switching process reverses the polar direction as well as tilt direction, but
preserves the layer chirality. Thus the SmCsPA and SmCaPA switch to SmCaPF
(racemic) and SmCsPF (chiral) structures.
However, switching between the antiferroelectric and ferroelectric
states takes place via rotation around a long molecular axis results in
inversion of layer chirality. This process is slower than the motion around a
cone and takes place only if the faster switching around a cone is hindered.
Therefore, this type of switching was observed in undulated smectic phases
and in SmCPA phases with a small tilt (< 20 and large bend
angle (Bedel et al 2004, Keith et al 2004, Amaranatha Reddy et al 2005b).
27
Figure 1.16 Schematic representation of the arrangement of BC
molecules in racemic and chiral states of an antiferroelectric
mesophase and the corresponding field induced ferroelectric
states
1.7.2.1 Variants of B2 mesophase
The existence of B2-like phases showed lamellar XRD pattern and
antiferroelectric behaviour under the field. Eremin et al (2002) reported two
mesophases below a B2 phase on cooling from the isotropic phase in a
compound containing -CH3 group in the angular position of the central ring
and a fluorine atom ortho to each of the terminal alkyl chains. The
mesophases were designated as B2 and B2. Svoboda et al (2003) also reported
two B2 like phases, which were labelled as B2 and B2 in compounds derived
from 1-cyanonaphthalene-2,7-diol. The detailed structures of these
mesophases are not yet established.
Racemic Homogeneously chiral
E=0 E<-Eth E>Eth E=0 E<-Eth E>Eth
28
1.7.3 B3 Mesophase
The B3 phase is a low temperature phase with respect to B2 phase
and appears above B4 phase (Pelzl et al 1999). On rapid cooling from the B2
phase, no textural change could be observed in the B3 phase. However, on
slow cooling, a slight change in the form of breaking of domain was
observed. The XRD pattern shows a number of reflections in the small angle
as well as wide angle regions, which suggests a crystalline structure.
However, the dielectric and terahertz spectroscopic results indicated that the
dynamics in the B3 phase was similar to that in the B2 phase and therefore was
not a crystal (Salfetnikova et al 2000, Takanishi et al 2005). On the basis of
these results, B3 phase is characterized as a highly ordered smectic phase.
1.7.4 B4 Mesophase
The B4 mesophase appears below B2 phase or B3 phase. The
mesophase exhibits dark blue coloured domains under crossed polarizers.
However, domains of different brightness were observed on slightly
decreasing the polarizers (Collings et al 1997). A circular dichroism (CD)
spectrum clearly shows domains with opposite sense and the result indicates
that the domains are chiral (Thisayukta et al 2000a). The mesophase is also
named as smectic blue phase because of the characteristic blue colour
exhibited by B4 phase (Heppke et al 1997, Sekine et al 1997). The XRD
pattern shows several reflections in the small angle as well as in the wide
angle regions, suggesting a crystalline order. However, dielectric studies for a
low frequency relaxation suggested that the B4 phase is not crystalline
(Salfetnikova et al 2000, Nadasi et al 2003). Simple harmonic generation was
observed in this mesophase in the absence of electric field, indicated the
existence of a spontaneous non-centrosymmetric order (Choi et al 1998).
29
A TGB-like structure has been proposed for this phase by Sekine et al (1997).
The X-ray microbeam experiments are consistent with the proposed TGB-like
texture.
1.7.5 B5 Mesophase
The B5 mesophase is reported in only a few systems. This phase
was first observed in BC compound derived from 2-methylresorcinol (Diele
et al 1998). Later this phase was observed in derivatives of 2-methyl and
5-fluororesorcinol, which contain a fluoro substituent ortho to the terminal
n-alkoxy chain on both arms (Eremin et al 2002, Nadasi et al 2002).
B5 phase occurs below B2 phase in these compounds. The transition enthalpy
is small and textural changes at transition are also minimal.
XRD pattern of B5 mesophase shows layer reflections up to sixth
order in the small angle region and additional reflections in the wide angle
region. The reflections in the wide angle region from an oriented pattern have
been indexed to a rectangular lattice and in-plane molecular packing in the
B5 phase was proposed, as shown in Figure 1.17 (Eremin et al 2002). An
antiferroelectric behaviour was observed in the B5 phase on application of an
electric field. However, a ferroelectric switching was also reported for the
lower temperature B5 phase of compound.
Figure 1.17 Pictorial representation of in-plane molecular packing in the
B5 phase
30
1.7.5.1 Variants of B5 mesophase
Five new variants of B5 phase are reported by Nadasi et al (2002).
On cooling the isotropic liquid, they observed five B5 sub-phases below a B2
phase with small enthalpy value for each transition. Among these, four
exhibited antiferroelectric behaviour and transition from an antiferroelectric
phase to ferroelectric phase was also observed. The exact structures of these
sub-mesophases have not been determined.
1.7.6 B6 Mesophase
The B6 mesophase was first observed by Bedel et al (2002) and is
designated as SmAc, SmCc or SmCint. This phase exhibits a fan-shaped texture
similar to SmA phase. However, it is difficult to align homeotropically; the
possibility of a SmA like texture is rule out for this phase. Occasionally
schlieren texture could be obtained on shearing the fan shaped texture of B6
phase. A transition from B6 to B1 phase was observed by Pelzl et al (1999),
Sadashiva et al (2000), Rouillon et al (2001), Weissflog et al (2001),
Mieczkowski et al (2002) and Amaranatha Reddy and Sadashiva 2004. The
XRD pattern of the B6 phase shows lamellar reflections in the small angle
region along with a diffuse wide angle reflection. The first order layer spacing
in the small angle region is smaller than half the calculated molecular length.
This indicates an intercalated structure. An oriented pattern of this mesophase
indicates tilt of the molecules and the estimated tilt angle is about 20-30
schematic representation of the molecular arrangement in the B6 phase is
shown in Figure 1.18. In a homologous series, B6, B1 and B2 phases appear in
the sequence on increasing the chain length (Sadashiva et al 2000). Rouillon
et al (2001) carried out Monte-Carlo conformational search to obtain the
lowest energy conformations of bent-core molecules exhibiting B6 phase and
subjected to semi-empirical quantum mechanical charge calculation. The
electrostatic potential maps were drawn and on the basis of analysis, they
31
proposed a molecular arrangement with an alternation of high and low
potentials. In the periodic structure, the electro positive aliphatic chains fill up
the vacant gaps between the aromatic (electronegative) parts. Due to the
constraints in packing of aromatic cores, only short chains can fill the space.
Hence, the B6 phase is observed for lower members of the series with short
alkyl chains.
Figure 1.18 Schematic representation of the arrangement of BC
molecules in the B6 phase
1.7.7 B7 Mesophase
The B7 phase was first observed by Pelzl et al (1999) in a
compound derived from 2-nitroresorcinol. Later this phase was also observed
in a number of compounds derived from 2-cyanoresorcinol (Amaranatha
Reddy and Sadashiva 2002, 2003, Shreenivasa Murthy and Sadashiva 2003,
2003a). There is also a report of the observation of this phase in compounds
derived from 5- fluororesorcinol (Pelzl et al 2004). Among all the mesophases
exhibited by BC compounds, the B7 phase shows the most beautiful and
fascinating textures. The texture is helical nuclei that appear on slow cooling
the isotropic liquid, resembling that of telephone wires. Jakli et al (2000)
showed that left and right-handed helices occurred in equal numbers and these
32
screw-like domains consisted of smectic filaments, formed single, double or
triple coils. The other textural variants observed for the B7 phase include
lancet-like or thread-like germs, circular domains with equidistant concentric
rings, myelinic-like, checker-board-like and banana-leaf-like textures. It was
considered that the helical filaments are indicative of chirality. However,
Coleman et al (2003) showed that polarization modulation was the essential
element stabilizing the filament texture. They pointed out that the helical
winding of polarization modulation within the filament, with the helix sense
established by the nucleation event and remain fixed during growth provides
an explanation for the twist deformation of filaments. Based on the results,
they argued that helical filament formation neither relies on, nor indicative of
supramolecular chirality. It has also been pointed out that instability of any
layer can induce helical filaments during the growth process. As the result the
phase is modulated, undulated or even a simple smectic mesophase
(Amaranatha Reddy and Tschierske 2006).
XRD pattern of B7 mesophase shows several reflections in the
small angle region besides a wide-angle diffuse reflection. One characteristic
feature of all the XRD patterns of B7 phases exhibited by compounds derived
from 2-nitro-, 2-cyano- and 5-fluoro-resorcinol is the presence of a medium
angle reflection at a distance corresponding to 7-8 Å. Coleman et al (2003)
carried out synchrotron X-ray studies proposed an inter-digitated 2D lattice
for the mesophase. A similar 2D lattice has been proposed for the B7 phase
exhibited by a compound derived from 2- cyanoresorcinol, on the basis of
synchrotron XRD studies (Heppke et al 2000). It has been pointed out by
Amaranatha Reddy and Tschierske (2006) that the distance corresponding to
the medium angle reflection is in a range of typical value of face-to-face
packed dimmers. This face-to-face packing takes place in the B7 phase then
the medium angle reflection might correspond to order between these dimers.
33
Earlier reports indicated that no electro-optical switching could be
observed in a B7 phase at least upto 40 Vm-1 (Pelzl et al 1999a). However,
a transition from non-switchable B7 phase to two antiferroelectric sub-phases
(B7AF1 and B7AF2) in higher homologues of nitro-substituted compounds
has been reported by Shreenivasa Murthy and Sadashiva (2003). The
compounds derived from 5-fluororesorcinol exhibited a transition from an
antiferroelectric SmCPA phase to ferroelectrically switchable B7 phase (Pelzl
et al 2004).
1.7.7.1 Variants of B7 mesophase
There are number of compounds of mesophases exhibit spiral
filaments and other textural variants, on slow cooling the isotropic liquid.
These mesophases have also been designated as B7 despite the fact that their
XRD patterns are different from that of the original B7 phase (Pelzl et al
1999a, Heppke et al 2000, 2000a, Walba et al 2000, Lee et al 2001, Shankar
Rao et al 2001, Bedel et al 2002). These mesophases exhibit layer reflections
in the small angle region and a few of them show satellites of weak intensities
behind layer reflections that indicate a modulation. Importantly, the medium
angle reflection is absent in the XRD pattern of these mesophases. Coleman
et al (2003) carried out several experiments on the mesophase which shows
ferroelectric characteristics. They proposed a polarization modulated/
undulated layer stripe structure stabilized by splay of polarization for this
phase, but assigned the symbol B7. A SmCG structure has also been discussed
for these mesophases (Jakli et al 2001, Eremin et al 2003). It has been
suggested by Amaranatha Reddy and Tschierske (2006) to use the symbol B7
for these mesophases, which exhibit the textural variants of B7 phase and their
XRD patterns are different from classical B7 phase as proposed by Pelzl et al
(1999a).
34
1.7.8 B8 Mesophase
Bedel et al (2004) reported a few BC compounds derived from
isophthalic acid and containing terminal n-alkyl carboxylate groups. They
observed a bilayer texture and an antiferroelectric behaviour for the higher
temperature phase (Sm1). The textures exhibited by this phase are also
different from those observed for other B-phases. Since this mesophase has a
new texture, the authors suggested the symbol B8. Later, the phases SmO and
SmI were characterized as SmCsPA (polar SmC phase, subscripts s and A refer
to synclinic and antiferroelectric) and SmCsG2PA (subscript 2 refers to bilayer
texture made of SmCG layers, G stands for general).
1.7.9 SmCG Phase
SmCG phase has a triclinic configuration with chiral C1 symmetry.
The possibility of the smectic phase with lowest possible symmetry was
proposed by de Gennes (1975), who coined the name SmCG. Later, Brand
et al (1998) gave theoretical model for the realization of these phases in a
system composed of BC molecules. In SmCG phase, a leaning of the
molecules in the tilt plane is considered in addition to tilt of the molecules. An
orientation of BC molecules in all three principal axes makes an angle with
smectic layer different from 0 e.
1.8 STRUCTURE PROPERTY RELATIONSHIP IN BENT-
CORE LIQUID CRYSTALS
In order to determine the types of molecular structures form
mesophases, numerous compounds have been synthesized and their
mesomorphic properties determined to establish structure-property
relationships. The accurate prediction of properties for new compound is still
not possible.
35
Structure-property relations in bent-shaped mesogens are less
predictable than those of calamitics. The only criterion certainly need to be
applied is that two mesogenic groups should be connected non-linearly. This
condition is not guaranteed to obtain mesogens, but need to be prevailed for
banana mesophases. In many cases, a bent structure is non-mesogenic or
exhibits common mesophases like in calamitics. Hence, the golden rule in
banana-field is the compounds showing banana mesophases always consist of
bent-shaped molecules, whereas the bent-shape of molecules does not assure
(banana) mesophase at all.
In this section, up to date trends in structure-property relationships
in bent mesogens will be discussed in the following sequence: central ring,
connecting groups, lateral substituents, and terminal chains (Figure 1.19).
However, it should be noted that these structural factors together determine
the physical properties of the mesogens. Contribution of a particular unit of
the molecules cannot be discussed independently. Moreover the influence of
the different structural factors, especially of the lateral substituents, is strongly
dependent on the size of the molecules.
Figure 1.19 General scheme of a bent-shaped molecules
R R
Y
X'X
Y'
Linking groups(polarity, direction,flexibility)
Core
bend angle
Terminal groups(n-alkoxy/n-alkyl/n-alkanoates)
Substitution by small group(position, volumer, number)
Lo'Lo'
Lm' Lm'
Rn
36
(2)(1)
1.8.1 Central Unit
The central unit in banana mesogens plays a key role in the bent. If
the connection between the wings of the molecule through the central ring is
established not in the right angle, there is no chance to obtain banana
mesogens even if the construction of the molecule in all other building stones
corresponds to the structure of bananas (Matsuzaki and Matsunaga 1993,
Prasad 2001). The loss of the bent results in rod-like molecules with an
extensive ring system, most probably with inconvenient, high transition
temperatures and with no chance for macroscopically chiral phase formed by
achiral molecules. At the same time the bent-core mesogens might also
exhibit “classical” mesophases like N, SmA and SmC phases. In the literature,
two aromatic systems were mostly used as central ring in bent-shaped
compounds: 1,3-disubstituted benzene ring (1) and 2,7- disubstituted
naphthalene ring (2) (Niori et al 1996, Pelzl et al 1999, Amaranatha Reddy
and Sadashiva 2000, Heppke et al 2000a, Thisayukta et al 2000 and 2001,
Amaranatha Reddy et al 2001, Walba et al 2001).
Additionally there are some heterocyclic central ring, e.g. 2,5-
disubstituted 1,3,4-oxadiazole (3) or 2,5-disubstituted thiophene (4), 2,6-
disubstituted-pyridine (5) (Shen et al 1999, Dingemans and Samulski 2000,
Matraszek et al 2000, Szydlowska et al 2003). In fact, only the pyridine
derivative exhibited banana mesophase. The five-ring heterocyclic derivatives
exhibit smectic and nematic mesophases. It is impossible to consider only a
particular element of the bent-shaped molecules as responsible moiety for the
mesophase behaviour.
37
CO
O
N CH N N HC CH
(6) (7) (8) (9)
N
NN
O S
(3) (4) (5)
1.8.2 Connecting Groups
Connecting or linking groups are as essential elements in bent-
shaped molecule as the bent-core. They connect the rigid core system of
bananas together. Connecting groups used for calamitic mesogens are suitable
for bent-core compounds. Some typical examples for connecting groups in
bent-shaped molecules are depicted as ester (6), azomethine (7), azomethane
(8) and stillbene (9). Additionally there are some instances of linking the
aromatic rings with thiocarbonyl connection or extensive acroyloxy group
(Nguyen et al 1999, Sadashiva et al 2001).
Connecting groups exert a powerful effect on the mesophase
behaviour. They establish the flexibility and influence the polarity of the
molecules. The electron withdrawing or donating effect of the connecting
groups determines the electron density and so the partial polarity of the
aromatic rings. Obviously, the direction of a non-symmetric linking group
between two aromatic rings plays an important role in the formation of
mesophase. Some symmetric five-ring bent-shaped compounds with varied
(in direction and/or chemical class) linking groups were compared by Bedel
et al (2000a). Mesomorphic properties were observed in the compound 10
where the donor or acceptor nature of the four linking groups leads to an
alternate sequence of positive and negative charges on the five benzene rings
according to molecular modelling and electrostatic potential map
computation.
38
Linking groups such as azomethine, ethylene together with the
aromatic rings establish an extensive conjugated system, whereas ester group
is not a completely conjugated unit. The role of conjugation in banana
molecules has not been clear. Furthermore, free rotation around the single
bond of the ester linking group is possible (with consideration of spatial
hindrance and electrostatic repulsion or attraction), while the double bond in
imino, ethylene etc group hinders rotation. In other words, segments of
bent-shaped molecules with ester linking group may possess more kinds of
rotational arrangement than with imino connecting group. Regarding
bent-shaped molecules ester group (where X= X’= -O-CO-) rotation around
this bond may strongly influence the bending angle itself (Weissflog et al
1999). Additionally polarized FT-IR measurements of 1,3-phenylene-bis[4-
(4-decylphenyliminomethyl)benzoate] in SmCP (B2) phase indicate that the
ester groups are twisted with respect to the central phenyl ring and on average
only one of the possible twisted conformations exists in B2 phase what could
result in molecular chirality (Zennyoji et al 2001).
The most successful bent-core mesogenic materials exhibiting
several (switchable) mesophases contain the sensitive azomethine group; it is
thermally unstable and sensitive to proton and metal surfaces. Some Schiff
bases decompose above around 150°C, while others are stable even above
200°C. Hence, the thermal instability is dependent on the structure of the
molecule. Moreover, only compounds containing azomethine linking groups
e.g.2-methyl-1,3-phenylenebis[4-(4-n-alkyloxyiminomethyl)benzoate]s
exhibit polymorphism like SmCP-B5 (Diele et al 1998).
+
- -
+
+
39
Shen et al (1998, 2000) and Dantlgraber et al (2002, 2002a)
reported bent-shaped mesogens containing central unit derived from 1,3-
phenylene ring so that there is no connecting group between the central ring
and one of the middle rings. Biphenyl derivatives exhibit either a wide range
SmCPA (B2) phase or a rectangular columnar phase designated as Colr or B1.
They accounted the compounds without any connecting group between the
central and middle rings. 2,6-diphenylpyridine and m-terphenyl derivatives
were synthesized and they reported about tolane derivatives. Actually, only 2,
6-diphenylpyridine derivatives exhibited liquid crystalline (B1, Bx, Bx1) phase.
1.8.3 Lateral Substitution
In case of lateral substitution three major factors are involved: the
size (spatial contribution) the polarity namely inter and intramolecular forces
of attraction or repulsion of the substituents and the position of the
substitution. The representation of substituents on core is shown in
compound 11. Molecular conformation effects on molecular packing and vice
versa. Consequently, lateral substitution has an impact on the liquid
crystalline state, i.e. on the liquid crystalline phase stability. It is not easy to
predict the influence of lateral substitution on bent-shaped molecules. The
structural factors and the lateral substituents not independently influence the
liquid crystalline behaviour. Systematic study of banana substance classes one
by one, may advance understanding on the structure property relationship.
40
In order to investigate the influence of lateral substitution of bent-
core substances, several new banana mesophases were discovered by
Weissflog et al (2001). They investigated substituted resorcinol derivatives
and some of them exhibited new mesophase such as B5 and B7. Moreover, B5
turned out to be switchable. On the other hand, 2-methyl-resorcinol derivative
of bent mesogen exhibited switchable mesophases SmCP (B2) and B5. These
are the main conclusions to which detailed investigation of mesogenic five-
ring resorcinol bananas led:
Substitution of the central core strongly effects on the liquid
crystalline properties: voluminous substituents like ethyl (-
C2H5), acetyl (-COCH3), hexyl (-C6H13) groups adversely no
mesophase, small substituents like methyl (-CH3), nitro (NO2),
chloro (Cl), cyano (CN) groups depending on the position of
the substituent positively influence them.
Substitution in position R5 results in non-mesogenic
compounds, independently of the class of substituents. The
only exception (R5=F) found will be described by Weissflog et
al (2001). Another exception was found in perfluorinated
terminal chain containing bananas, but for a different
substance class (Kovalenko et al 2000).
In the case when R2=NO2 a new mesophase, called B7, with
very unusual texture was discovered by Pelzl et al (1999a),
Jakli et al (2000).
In the case when R2=CH3 a new mesophase (B5) and a first
banana polymorphism (SmCP (B2)-B5) were observed by
Diele et al 1998.
4,6-dichlororesorcinol derivative bananas exhibit SmA and
SmC mesophases, owing to the stretched (rather rod-like)
conformation of the molecules (Matsuzaki and Matsunaga
41
1993, Weissflog et al 1999).A later study of 4-cyanoresorcinol
derivatives (R4= CN) was reported by Wirth et al (2001) about
SmCP-SmC-SmA polymorphism. But in the case of R4= Cl
switchable SmCP (B2) phase was found as well as in the
homologue series of the resorcinol derivative (Pelzl et al
1999a, Weissflog et al 1999, Jakli et al 2000a, 2000b). The
4-chlororesorcinol bananas have lower melting and clearing
points, while the resorcinol bananas exhibit some additional
crystalline B3 and B4 phases.
Substitution of the central ring of bent-shaped molecules turned out
to be a fruitful field in liquid crystal research and thus several research groups
have been working on this area (Matraszek et al 2000, Csorba et al 2002,
Shubashree et al 2002, Amaranatha Reddy and Sadashiva 2003, Szydlowska
et al 2003, Matyus and Csorba 2003). Another 5-methylresorcinol derivative,
5-methyl-1,3-phenylene bis(alkoxycinnamoyloxy)benzoates was declared to
exhibit banana mesophase (B6 and crystalline B3) (Matyus and Csorba 2003).
The 5-substituted resorcinol derivatives show more favourable phase
behaviour than the 2-substituted resorcinol compounds.
1.8.4 Terminal Chains
The molecular organization of bent-shaped molecules depends on
the balance of electrostatic interactions developed by the polar segment of the
molecule and the van der Waals interactions established by the terminal
chains. The most often occurring terminal chains are alkyl- and alkoxy chains
the mesophase character of bananas is exposed to the influence of length of
alkyl/alkyloxy chain. Namely, short-chain homologues of otherwise
chemically equivalent bent-shaped molecules exhibit B1 long chain
homologues possess SmCP (B2) mesophase (Bedel et al 2002, Amaranatha
42
Reddy and Sadashiva 2003a). There are few examples for banana molecules
with alkyloxycarbonyl and alkenyloxy terminal chains (Bedel et al 2001,
Csorba et al 2002, Lee et al 2002). By comparison, replacing alkyloxy chains
with unsaturated alkenyl chains decreases the clearing points by 10-20°C.
Heppke et al (2000) synthesized bent-core compounds with
terminal alkylthio chains. The substances exhibited crystalline B3 and high
temperature switchable phase. They found that the transition temperatures fall
down replacing alkyloxy with alkylthio terminal chains. Walba et al (2000)
prepared a racemic asymmetric bent-core compound with achiral alkyloxy
and chiral alkyloxycarbonyl chains. The polarization microscopy and electro-
optical measurements revealed on freely suspended films with differing
layers.
Dantlgraber et al (2002) synthesized asymmetric bent-core
mesogens with a dodecyloxy chain and a bulky oligosiloxane unit connected
through a flexible alkyloxy chain to the outer ring. The compounds exhibited
switchable SmCP mesophase. X-ray diffraction (XRD) analysis proved that
each moiety (aromatic cores, aliphatic chains, oligosiloxane units) was
organized into sublayers. Furthermore, they found that the mesophase
stability was nearly independent of the size of the siloxane unit.
Bent-shaped materials were synthesized even with
perfluoroalkyloxy terminal chains (Kovalenko et al 2000). The materials
exhibited the individual property possessing banana mesophase despite the
voluminous methyloxycarbonyl substituent on the top of the central ring (R5=
-COOCH3). The rigid perfluoroalkyloxy chains often intercalate and it might
drive on separation of the perfluoroalkyloxy chains from aliphatic and
aromatic parts of the molecules. It results in increased mesophase stability.
Unfortunately, the transition temperatures are strongly elevated, and so the
mesophase behaviour cannot always be completely characterized. Decreasing
43
the number of aromatic rings to three in bent-shaped molecules or
introduction of substituents may reduce the transition temperature of bent-
core materials with perfluoroalkyloxy chains however they exhibit only
conventional smectic phases.
1.9 APPLICATIONS AND IMPORTANCE OF LIQUID CRYSTALS
The fluid nature of thermotropic LCs can be easily processed into
films which retain the properties of the crystalline materials such as the ability
to rotate the plane of polarized light. In addition, the orientation of the
molecules in liquid crystal films can be modulated on a relatively short-time
scale using low electric field. Due to these unique properties of LCs, they
have found wide application in display devices (Blinov 1998, Chigrinov 1998,
Sage 1998). In particular, the nematic and SmC* phases formed by rod-like
mesogens have been used as media for the fabrication of liquid crystal
displays (LCDs). The displays have been dominated over the cathode ray tube
(CRT) display due to their slim shape, low weight, low voltage operation and
low power consumption. In addition the nematic phase composed of disk-like
molecules has been well exploited in solving viewing angle problems of
LCDs. The current applications for liquid crystal displays are summarised in
table 1.1. Cholestric liquid crystals are used as temperature sensors in
disposable thermometers, aerodynamic testing (Brown 1969). Critical
dependence of the cholesteric pitch length with temperature has resulted in
the wide spread use of chiral nematic liquid crystals in thermal sensors. The
thermal sensors are mainly used in the packaging industry, fever
thermometers and other thermal sensors. Films containing cholesteric liquid
crystals are inexpensive and versatile tools for visualizing invisible radiation.
Another important use of cholestric liquid crystals is in the medical field
(Kallard 1973). Heilmeier and Golrnacher (1973) have also been used to
produce panels that exhibit an optical memory effect.
44
The discotic materials exhibiting columnar phases, as quasi-one
dimensional conductors, photoconductors, molecular wires and fibres, light
emitting diodes, photovoltaic cell etc., are attracting considerable attention.
Liquid crystals are employed as anisotropic solvents for the study of various
physicochemical properties. Nematics provide the bulk molecular orientation
necessary for the observation of spectroscopic details analogous to a solid
state experiment and thus they can be used as solvents in spectroscopy. Liquid
crystals are used as solvents for reactions to alter the rates of uni- or
bi-molecular thermal photochemical reactions. The usage of LCs in the
various fields is shown in Figure 1.20.
Figure 1.20 Schematic representations on usage of liquid crystal in
various fields
45
Table 1.1 Current applications for liquid crystal displays
Analytical Instruments
Auto Dashboards
Auto Radio & Clocks
Battlefield Computers
Blood Pressure Indicators
Calculators
Cameras
Cash Registers
Clock Radios
Digital Pyrometers
Digital Multimeters
Digital Thermometers
Electric Shavers
Electronic Billboards
Exercise Equipment
Gasoline Pump Indicators
Hand-Held TV
Hand-Held Terminals
Head-Held Data Collection
Heart Monitoring Devices
Highway Signs
Jewellery - Assorted
Marine Engine Indicators
Marine Speedometers
Marine Depth Finders
Overhead Projector Plates
Pens
pH Meters
Photocopy Machines
Portable Radios
Portable Computers
Portable Word Processors
Portable Oscilloscopes
Telephones
Toys & Games
TV Chanel Indicators
Typewriters, Editing
Vacuum Cleaners
Channel Indicators
Windometers
Wrist Watches
Household Appliances
Chromatography is of great importance in modern chemical
analysis and physicochemical investigations. Liquid crystals are among the
materials used in chromatography. The use of liquid crystals as stationary
phases in gas chromatography was described for the first time by Kelker
(1963) and later by Dewar and Schroeder (1964). Ever since, liquid crystal
stationary phase have been applied successfully for separation of polycyclic
hydrocarbons.
46
Possible device applications for metallomesogens, which may
presently be anticipated, are as new thermal or nonlinear optical materials in
electrochemistry as a sensor. Copper laurate has been melt spun into fibres in
its mesophase (Godquin et al 1989). Electron microscopy and XRD
measurements indicate that, the fibres have high degrees of orientation both in
the crystalline and columnar phases.
Liquid crystals are used to control or alter the chemical reactivity of
dissolved solutes (Kelker and Hatz 1980, Weiss 1988). Liquid crystals do
alter reactivity as a result of their ability to control solute diffusion, orient
reactants and discriminate between stable solutes and transition states
according to their sizes and shapes. This ability depends on the degree of
order present in the liquid crystal and the structure of the reactants in relation
to the mesogen. Thus, the electric field induced antiferroelectric - ferroelectric
phase transition is applicable to display device. Two main characteristics are
the sharp threshold behaviour under a direct current (DC) field between
antiferroelectric (AF) and ferroelectric (F) states and hysteresis in switching,
bringing about a bistable device. The switching current peaks are observed at
voltages of the F-AF and AF-F switching and are equivalent to double
hysteresis in a D-E loop, which is characteristic to antiferroelectricity.
The antiferroelectric liquid crystal display (AFLCD) utilises both F
states, as clearly noticed by two driving frames. Two states give the same
transmittance, so that one can use them alternatively. The AFLCD is not
affected by ghost effect which is a serious problem in Ferroelectric liquid
crystals (FLCD). Another problem in FLCD is the weakness against
mechanical shock because of the smectic layer structure. AFLC cells have a
remarkable feature of self alignment recovery during an operation from
damage caused by mechanical and thermal shocks. Also the non-chevron
47
structure gives rise to an additional advantage and relatively high contrast
ratio.
The AFLC-SLM may not be suited for image storage devices, since
a constant voltage has to be applied to the cell to memorise addressing
images. This may be applicable for real time holography. Work is going on in
many laboratories around the world at the present time not only to have a
clear understanding of the complex nature of these mesophases but also to use
them advantageously in many devices.
Lyotropic liquid crystals are exploited for applications in
commercial detergents and for simulation of bio-membranes. Another
important application as a media for controlled drug release is also envisaged
for LCs. Lyotropic LCs is the features of all living organisms. Thus, life itself
is critically based on lyotropic LCs. The living cell, in particular, the cell
membrane, possesses LC property which is formed by the aggregation of
lipids in presence of water thereby generating a fluid two-dimensional matrix.
The essential cellular functions such as recognition, fusion, endocytosis,
exocytosis, transport and osmosis are all membrane mediated processes.
These functions occur readily due to LC property of the cell membranes. In
addition, essential components of the living systems such as DNA, proteins,
tissues etc., also possess LC feature. Lyotropic LC phases are frequently
encountered in everyday life, in household products and foods items. For
example, most surface active reagents (surfactants) viz., detergents, soaps,
liquid soaps, shampoos (toiletries) etc., in water form liquid crystal phases.
However, the significance of lyotropic LCs has been completely surpassed by
the thermotropic LCs, because they are relatively simple to realize and
handle. Overall, lyotropic LCs certainly have a very significant future.
48
O
OO
O
NN
XCnH2n+1H2n+1CnX
2
4
5
6
2'
2''
3'
3''
(12)
Parent compound
O
OO
O
NNN
N
OC2H5C2H5O
(13)
O O
1.10 BENT-CORE LIQUID CRYSTALS – A SURVEY
A new fascinating subfield of LC came into light when Niori et al
(1996) revealed the switching behaviour in an achiral bent-core molecule 12
(X= O, n = 6-10 and 12) that was initially synthesized by Akutagawa et al
(1994). In fact, Volander and Apel (1932) first synthesized banana-shaped
molecule of the type 13, although its LC behaviour (exhibition of B6 phase)
revealed by Pelzl (2001). Since 1996, a large number of different types of
compounds have been prepared to understand the structure-property
relationships.
1.10.1 Effect of Central Unit Variation in Mesophase
The central unit (CU) is the key fragment of banana-shaped
molecules as it accounts for the bending angle between the two rigid cores
attached to it and hence the bent conformation. The commonly used CU are
1,3-phenylene, 2,7-naphthyl, five and six membered heterocyclic aromatics
like oxadiazole, thiophene and pyridine etc., The odd-membered alkylene
spacers is also used as one of the central cores because it gives bent
confirmation. Majority of banana-shaped compounds comprising of five ring
systems contain 1,3 -phenylene as the CU are reported in the literature (Pelzl
et al 1999, Tschierske 2001a, Tschierske and Dantlgraber 2003, Ros et al
2005, Amaranatha Reddy and Tschierske 2006, Takezoe and Takanishi 2006).
In fact the parent compound 12 also contains 1,3-phenylene as CU exhibiting
B1, B2 phases and soft crystal B3 and B4 phases (Pelzl et al 1999). The
49
structural modification of the parent compound 12 was achieved by
introducing various lateral substituent/s to the CU 1,3- phenylene, found to be
a relatively proper way to understand structure-property relationships. At first
chlorine was introduced at position 4 of the central phenyl rings which
exhibited B1 and B2 phase was comparable with parent series (Pelzl et al
1999b). The introduction of second chlorine atom at position 6 leads to the
formation of N and smectic phases instead of banana phases (Weissflog et al
1999a). While the corresponding nitro group at position-2 exhibited the B7
phase (Pelzl et al 1999a). When cyano group was introduced at position 4, it
showed some interesting polymorphism in which B2 phase was observed
below SmC phase (Iso-N-SmA-SmC-B2) (Weissflog et al 2000). The methyl
group was introduced at position 2 showed interesting properties i.e., it
exhibited B2 and B5 phases (Diele et al 1998). The substitution of small
groups like cyano, methyl and methoxy at position 5 did not support the
mesomorphism (Amaranatha Reddy and Tschierske 2006). The molecules
with bulky groups like ethyl, methoxy and ethoxy at various position of this
CU were found to be crystalline obviously indicated that bulky group
substitutions disfavour the mesomorphism.
Another way of varying the nature of CU has been reported,
wherein the 1,3- phenylene ring of 12 is replaced with 2,7-naphthalene core
to yield new mesogens of the type compound 14 (X = H, CH3, Cl and CN).
This core preserves the bending angle (120oC) and thus molecules derived
from it exhibit banana phases. For example, compound 12 without any lateral
substitution (X = H) show an unknown phase (Bx) at higher temperature and
B4 phase at lower temperature (Svoboda et al 2003). While compounds 14
50
with lateral substitutions, X = CH3, Cl, CN, bring down the transition
temperature, as expected and also change the mesomorphic properties when
compared with the non-substituted analogues. The compound 14 with methyl
group (X= CH3) substitution at position 1 exhibits B2 phase featuring racemic
synclinic structure SmCSPA; whereas chloro (X= Cl) analogue stabilizes the
B2 phase having homogeneously chiral anticlinic structure SmCAPA. The
cyano derivative 14, (X= CN) exhibits the phase sequence: Cr-N- SmCSPA
(B2) - SmCAPA (B2)–I; wherein the phase transition between the chiral B2 and
racemic B2 phases was observed at higher temperature. In view of the fact that
the chiral and racemic B2 phases have D2 and C2h symmetry respectively, they
represent different phases. It is also remarkable that the lateral substituents
CH3 or CN lead to a substantial decrease of the layer periodicity of the
smectic phase indicating the change of bending angle and/or the tilt angle in
mesogens of compound 14. Apart from rigid aromatic cores, odd-parity
alkylene spacers give an overall bent molecular conformation are used as the
central unit. For example, switchable banana mesophases are reported for
compound 15 formed by connecting two Schiff’s base mesogenic units to an
odd-parity alkylene spacer through ester linkages (Niori et al 1995, Choi et al
1999, Takanishi et al 1999, Watanabe et al 2000). Later, observations of tilted
and non-tilted B1 mesophases were reported for compounds with semi-flexible
methleneoxycarbonyl moiety as the central unit for the compound (16) (Pelz
et al 2003).
51
Novel series of laterally 1-substitued (H, Cl, CH3, CN, NO2)
naphthalene-2,7-diol (17) based liquid crystals were synthesized by Svoboda
et al (2003) and their mesomorphic properties identified using differential
scanning calorimetry (DSC) studies, texture observation, XRD analysis and
electro-optical measurements. Depending on the chain length and type of
lateral substitution, the compounds exhibit a variety of mesophases. In
materials with short alkyl chains, the rectangular columnar B1 phase was
found. Increasing the alkyl chain length for the non-substituted core causes
the appearance of the so-called B phase. In -CH3 and -Cl substituted
compounds, the antiferroelectric B2 phase (SmCAPA) was found and
introduction of the -CN and -NO2 substituents led to the formation of the B7
phase.
New bent-core liquid crystals have been prepared by coupling two
rod like substituents to the roof-shaped pyrazabole ring (18) using a
Sonogashira cross-coupling reaction (Choi et al 2010). The compounds
possess a transverse dipole moment and negative dielectric anisotropy. It was
found that a low viscosity, easy to orient nematic mesophase is obtained for
the shorter pyrazaboles BHCn. The nematic phase displays unusual behaviour
under electric fields and this is related to the bent molecular shape and could
be flexoelectric in origin. Increases in the length of the aliphatic terminal
chains gave rise to the appearance of a new crystal-like phase with low
birefringence and a very well-defined lamellar structure below the nematic
phase, for which a tilted lamellar structure is proposed.
52
Pyrazaboles with extended aromatic cores, BHArC14 and
BHArCox, give rise to intercalated lamellar mesophases of B6 type in addition
to nematic mesophases, with description for the first time of a transition from
a tilted B6 to a nontilted B6 phase. The oxyethylenic terminal chain lowers the
transition temperatures in comparison to tetradecyloxy chain but displays
poor thermal stability. The bend angles in these molecules are in upper range
reported for bent-core liquid crystals, which may promote the formation of
nematic phases, as observed with 2,5- oxadiazole derivatives and other
halogen-substituted resorcinol derivatives. An increase in the size of the
aromatic part leads to a partial micro-segregation between the aromatic rings
and the aliphatic chains, as intercalated layered mesophases are obtained. This
effect is not sufficient to generate mono-layers in the fluid state. However,
well defined layered structures are formed in the crystal-like phases of the low
temperature phases formed on cooling the nematic or the B6 mesophases. The
fact that these compounds tend to inter-digitate in an anti-parallel way could
make them of interest as dopants to stabilize antiferroelectric phases.
Moreover, the compounds readily give rise to spontaneous or field induced
twisting that may be frozen in the low temperature phase.
Four achiral main chain polymers containing V-shaped bent-core
mesogens (19) with acute angled central cores (Ar = 1, 2-phenylene or 2, 3-
N
N
O
O
RO
N
N
O
O
OR
BH H
BHH
m m
BHC10 R= C10H12, m=1
BHC12 R= C12H25, m=1
BHC14 R= C14H29, m=1
BHC16 R= C16H33, m=1
BHC18 R= C18H37, m=1
BHArC14 R= C14H29, m=2
BHArCox R= (C2H4O)4CH3, m=2
(18)
53
naphthylene) and lateral halogen substituents (X=F or Cl) have been
synthesized and characterized (Choi et al 2010). In spite of existence of the
acute-angled central cores, only one polymer with Ar/X = 1, 2-phenylene/Cl
was almost amorphous (Tg=53-61°C) and the remaining three polymers were
semicrystalline (Tm=109-202°C). Although polymers contain V-shaped
mesogens with a lateral halogen substituent could form tilted smectic phases
(d= 2.42-3.05 °-47°), i.e., SmC phases. Moreover, polymer with
Ar/X= 2,3-naphthalene/F could form a ferroelectric SmC phase; on ground
state the spontaneous polarization of smectic phase was not zero (Ps=140
nCcm-2) and on applying a triangular voltage the switching occurred by a
collective rotation of the mesogens around main chain axis. Based on the
observed optical textures, the polar SmC phase with the ferroelectric ground
state (SmCPF) may be regarded as B7 phase.
Supramolecular side-chain liquid-crystalline poly(acrylate)s (20)
have been prepared by Saravanan et al (2010). They studied self-assembly of
H-bond donor and acceptor complexes through intermolecular
complementary hydrogen bond formation. The polymers were employed as
side-chain components in the hydrogen bonding system. The spacer unit
present at the terminal position of all the derivatives played a major role in the
formation of all the complexes. Smectic A and columnar phases that appeared
O O
O O
CH HCN N
O X X O (CH2)12n
X F Cl F Cl
3a 3b 3c 3d
(19)
54
for the nicotinic acid derivatives completely disappeared in the H-bonded
complexes to afford nematic phases.
Kannan et al (2002) synthesized new series of ferrocene containing
aromatic poly-esters (21) with methylene spacers has been synthesized. The
even number of flexible spacers has been varied from two to ten. All the
polymers were found to possess a liquid crystalline property. The glass
transition temperature (Tg) of the polymer was found to low. The liquid
crystalline phase duration of the polymers was decreased with increasing
methylene spacers. The transparency of the liquid crystalline phases was
increased with an increase in spacer length.
Banana-shaped molecules with two side wings attached to the bent-
core may exhibit liquid crystallinity. The most studied material is compound
(20) that comprises 1,3-dihydroxybenzene as a central core, Schiff’s base
moieties as the wing groups and octyloxy tail groups. To clarify the effect of
chemical structure on the liquid crystallinity of the molecule, Thisayukta et al
Fe
C
C
O
O
O
O
m(H2C)
(CH2)m
O
OO
OO
O
n
(20)
(21)
55
(2000) prepared several banana-shaped molecules with side wings and central
cores different from compound 22 and examined their liquid crystallinity,
which is sensitive to change in chemical structure. Especially, changing the
position of the carbonyl group of ester function linking the central core to the
wing and position of nitrogen atom in the Schiff’s base moiety caused a loss
of liquid crystallinity. On the other hand, smectic liquid crystallinity was
maintained for five new types of banana-shaped molecule with different
central core. Although these smectic phases have liquid-like association of the
molecules within the smectic layers and showed unconventional smectic
textures through the separation of spiral, fractal and germ textures from the
isotropic melt. Moreover, a frustrated smectic phase and chiral smectic phases
were found.
1.10.2 Effect of Linking or Connecting Groups (X & Y) Variation in
Mesophase
In banana-shaped mesogens influence the chemical nature and the
sequence of connectivity of linking groups that combine the different rings
appear to be very important for clear understanding the structure-property
relationship. The commonly used connecting groups are ester, azo, imine,
ethylene and acetylene. Inversion of linking groups, resulting in the formation
of structural isomers can exhibit LC behaviour with different mesophase
(22)
56
morphologies or can lead to loss of liquid crystallinity. For instance,
compounds 23 comprising inverted azomethine linkages of parent mesogens
(12), exhibit fluid smectic (n = 1-5, 13-16) or crystalline (n = 6-8) behaviours
(Akutagawa et al 1994). While the isomeric compounds 24 with interchanged
ester and azomethine linkage of series 23 exhibit mesomorphism
convincingly; lower homologues show B1 phase, while higher members
stabilize the B2 phase (Bedel et al 2000).
Two different linking functional groups are employed covalently to
join the linear mesogenic rigid cores (RC1 and RC2) and central unit (CU). In
this way the symmetry of bent-core mesogens has been effectively reduced
and termed as unsymmetrical banana-shaped compounds. Due to molecular
asymmetry, the materials possess lower transition temperatures. For example,
the unsymmetrical bent-core compounds 25 show banana phases with lower
transition temperature compared to symmetrical five ring banana shaped
compounds (Weissflog et al 1998).
The nature and sense of the linking group that connects two
aromatic rings of the arms are also known to vary LC behaviour of banana-
57
shaped compounds. For instance, isophthalic acid based bent-core compounds
26 possessing azomethine (Y= -N=CH-) and carboxylic acid (Y=-OCO)
linking groups each present on the two different arms prevent the formation of
mesophase (Nguyen et al 2003). However, inversion of the sequence of
connectivity of the carboxylic linkage (Y= -COO) induces a metastable SmC
phase.
Shen et al (1998, 1999) synthesized bent-shaped molecules with
either ester or acetylenic linkage group instead of imine linkage. These
banana-shaped molecules (27) with ester linkage stabilized banana phases.
Whereas compounds 28 with acetylenic linkage exhibited conventional
smectic phases or were crystalline with high clearing temperature.
Salicylaldimine-based symmetrical and unsymmetrical bent-core
mesogens are mainly focused due to its stability towards heat and moisture
owing to the presence of H-bonding between H-atom of hydroxyl group and
N-atom of imine group (Shankar Rao et al 2001, Yelamaggad et al 2001,
Walba et al 2001, Achten et al 2004). These systems have stabilized banana
phases and few of them like (29a) and (29b) have stabilized B2 phase well
above and below the room temperatures (Yelamaggad et al 2002).
58
(29a) : R = C8H17; Phase sequence: Iso 131.1ºC (15 J/g) B2 < 30ºC
(29b) : R = C10H21; Phase sequence: Iso 138.2ºC (21 J/g) B2 < 30ºC
1.10.3 Effect of Lateral Substitution in Mesophase
The introduction of lateral substituents either at CU or to the inner
and outer rings of the arms of banana-shaped compounds, as discussed earlier,
is an important structural modification that effectively alters the LC behaviour
of parent systems. In general, studies have shown that the LC behaviour of
bent-core molecules depends on the position, volume and electronic
properties of the lateral substituents (-Br, -Cl, -F, -CN, -CH3, -CF3 and -NO2)
wherever it is present.
The phase structure and electro-optical properties of a new bent-
core mesogen derived from 2-methylresorcinol (30) were studied by Schroder
et al (2008). XRD analysis proves the presence of an oblique columnar phase.
Application of high electric field leads to a transition of Colob phase into
SmCPF phase and the process is reversible. The mechanism of polar switching
depends on frequency of applied field that means a collective rotation around
molecular long axis is observed at very low frequencies and rotation around
the tilt cone at higher frequencies. Furthermore, an enhancement of the
clearing temperature of 3K was found on applying an electric field of 30
Vµm-1 to the isotropic liquid.
O
C10H21O
O O
O
O
N
O
HO OR
59
The molecular structure consists of a thermally and hydrolytically
stable salicylaldimine unit as a linear rigid segment attached to an angular
central 1,3-disubstituted benzene nucleus. The compound expected to be
promising with regard to its stability to heat and moisture owing to the
presence of intramolecular hydrogen bonding. It is well known that mesogens
consisting of a salicylaldimine segment will have higher clearing temperature,
whereas the compound 31 has a clearing temperature that is comparable to
any other related bent-core systems. Interestingly, it exhibits a switchable
banana-mesophase (B2) over 60°C temperature range. Systematic
investigation was focusing on molecular design and synthesis leading to
stable bent-core compounds. It is very much essential to stabilize switchable
banana-mesophases existing over wide and convenient operating temperature
ranges required for many practical applications.
1.10.4 Effect of Terminal Chains (R1 & R2) Variation in Mesophase
The most commonly used terminal chains are alkoxy and alkyl
tails. The short chain homologues usually exhibit N, SmA, SmC or B6 phases.
Whereas the medium chain length homologues exhibit B1 or B2 phase and
further increase in the chain length leads to the formation of B7 mesophase.
The terminal chains can also be linked by sulphur, carbonyl or carboxylic
O O
O O
CH3O O
OO
C16H32O OC16H32(30)
60
groups to the rigid outer rings. The influence of the electronic properties of
these linking units between alkyl chains and outer rings on the mesomorphic
behaviour is witnessed by Takezoe and Takanishi (2006). To reduce the
symmetry of bent-core molecules as well as to understand the structure-
property relationships, incompatible terminal ends such as perfluorinated
chains 32 or oligosiloxane units (33) have been used (Shen et al 2000,
Dantlgraber et al 2002). The presence of perfluoro alkyl chain in compounds
32 supports the formation of antiferroelectric B2 (SmCPA) phase; while the
siloxane tail in 33 supports the ferroelectric B2 (SmCPF) phase. Further,
terminal chains with some special chemical environment can be used as an
important structural parameter for obtaining ferroelectric switching.
Ferroelectric switching was reported in B7-like phase formed by
bent-core molecules (34), wherein both the alkoxy tails possess vicinal
electronegative fluorine atoms (Bedel et al 2000). In another instance, the
achiral or chiral terminal tail in the form of 2-octyloxycarbonyl group was
O
O
O
C4F9(H2C)6O
O
O
OC12H25
O
O
O
(32)
O
O
O
O
O
O
OC12H25
O
O
O(33)
(H2C)11SiO
O
Si
Si
H3C
H3C CH3
H3CCH3
H3C
NN
OO
OC12H25C12H25O
(34)
O O
F F
O
O
O
O
NN
O
O
C6H13C9H19O
(35)
O
O
O
O
NN
OO
(36)
(S) (S)
61
employed to promote the formation of anticlinic interlayer interfaces based on
the fact that ferroelectric structures require an anticlinic interlayer correlation
in bent-core systems. Indeed, the B7 like phase of compound 35 was reported
to show ferroelectric switching characteristics (Walba et al 2000). Another
approach for obtaining the similar features, chiral tails are used with branches
at the terminus (36) (Achten et al 2004). Furthermore, it has been found that
one of the arms of bent-core compound possesses the cyano group then polar
biaxial smectic A phase is formed. Hence, the chiral tails have been employed
to realize the chiral bent-core materials to understand the effect on the thermal
behaviour, resulting from the interplay between molecular chirality and bent
conformation.
Two series of asymmetrical ester-like banana-shaped compounds
with different terminal chain lengths have been synthesized and studied by
Achten et al (2004). The symmetrical series of compounds 37 with a central
phenyl group to the symmetrically substituted series of compounds 38 with a
central biphenyl group, the liquid crystalline range were increases
dramatically with retention of the liquid crystalline B-phases. For the
asymmetrical compounds 37 the melting points are lower than the parent
compounds. To effectively reduce the melting points for this series of
compounds a difference in terminal chain length of C-atoms (11-P-16) is
required.
62
For the compounds 38 it is very difficult to reduce the melting
points, since all compounds 38 (also compounds with k=l) have an
asymmetric central part, it is difficult to introduce more asymmetry to lower
the melting points. A small difference in terminal chain length (11-BP-12) can
result in a small reduction of the melting point. Surprisingly, the
asymmetrically substituted compounds 38 k=l (shortest terminal chain
attached to the para-position of the central biphenyl group) give the lowest
melting points. This study has confirmed that the introduction of two different
terminal alkyl chains can lower the melting points of banana-shaped
compounds that exhibit the switchable B2 phase. This method is effective
when the central part of the molecule itself a symmetric.
Novel bent-shaped molecules based on 4,4'-diphenyl-methane and
3,4'-biphenyl moieties as bent-cores and bearing different terminal chains (39)
have been synthesized and characterized by Gimeno et al (2009). The liquid
crystalline properties of the bent-shaped molecules can be attractively
modulated and a variety of effects induced by changing the terminal chain.
When using long alkoxylic chains (from 10 to 18 carbon atoms), both
columnar mesomorphism and SmCaPA phases promoted. Interestingly, bent-
core mesophase polymorphism obtained with the longest chain in the
compound 39e. By using unsaturated chains, the supramolecular arrangement
could also be modified with different effects depending on the central core
selected. Thus, 3,4'-biphenyl derivates allows the verification of SmCP
mesophase around 30°C, or appealingly, the ferroelectric order of SmCP
mesophase can be stabilized in compound 39e. In contrast, these tails on
methylene based structures leads to the mesophase sequence Colr-SmCP or
their coexistence.
63
A room temperature SmCP mesophase was achieved by
incorporating oxyethylene chains in the biphenyl derivatives, whereas it was
not enough for the methylene derivative, even though a short range of
columnar phases close to room temperature was obtained. Sulphur based tails
were not induced any B7 phase; on the contrary columnar self-assembling was
favoured. Thus, the Colr-SmCP polymorphism was detected again for sulphur
containing biphenyl derivative while the methylene core promoted short-
range columnar mesophase. Finally, to induce bent-core liquid crystal order
by using chiral tails, the incorporation of stereogenic centers far from the
central core was recommended. Furthermore, a columnar mesophase could be
also promoted by the appropriate selection of the bent-core.
Bent-core compounds containing terminal n-alkyl carboxylate
groups (40) have been prepared by Umadevi et al (2006). These compounds
showed two mesophases with unusual optical textures as well as electro-
(39)
Series I Series II
64
optical behaviour. On the basis of experimental investigations the higher
temperature mesophase has been designated as B7 phase exhibited bistable but
analogous linear electro-optical switching without any observable polarization
peak. They proposed two theoretical models and found that a local triclinic
symmetry might be responsible for the observed unusual electro-optical
switching.
Dantlgraber et al (2002b) reported new mesophase materials with
interesting properties. The materials are stable and have a low conductivity.
The liquid crystalline phase occurs over a broad temperature range for these
materials. Furthermore, it can be frozen into the glassy state and the dendritic
molecular structure was important for the special organization of the reported
bent-core compounds. At first, the bent-core units fixed to each other which
stabilized the liquid crystalline phases and led to the formation of a glassy LC
state. Second, the materials decoupled the layers of the aromatic bent-cores to
certain extent that the entropically favoured AF (synclinic) interlayer
correlation became disfavoured. On the other hand, intermolecular
interactions were responsible for the anticlinic (FE) correlation which was not
strong enough to dominate the mesophase structure. Therefore, in the ground
state the macroscopic polar order is cancelled, but the switching into the
ferroelectric organization can be achieved by applying external electric fields.
After formation, the ferroelectric states are very stable under the applied
experimental conditions and can be switched between the different
polarization states.
O O
OO
N N
H2n+1CnOOC COOCnH2n+1
(40)
65
The combination of steric layer frustration and interlayer
segregation by incorporating linear or branched carbosilane termini into
bent-core mesogens led to new functional liquid crystalline materials (41)
with interesting characteristics and glass transitions (Zhang et al 2010). They
also highlighted the importance of the structure-property relationships in the
designing of LC materials. The carbosilane units could be incorporated into
other materials such as discotic materials and materials for field-effect
transistors and photovoltaics, leading to new types of chemically stable
stimulated responsive functional materials with useful properties.
Thus, the mesomorphism of banana-shaped molecules depends on
the molecular subunits such as rigid aromatic cores, linking groups, lateral
substitutions and terminal chains. The available data is not sufficient to
establish straightforward structure-property relationships of the banana-
shaped mesogens and thus requires modification in the molecular architecture.
Thus in the present investigation several series of achiral/chiral bent-core
mesogens with variations in the molecular fragments like linking groups,
lateral substitution in the central unit as well as in the arms and the terminal
chain have been synthesized and to study structure-property relationship.
(41)
66
1.11 SCOPE AND OBJECTIVES
The aim of the research described in this thesis is to synthesis and
characterise new molecules with a bent or banana shape. Investigation of
structure – property relations will contribute to a better understanding of this
very fascinating state of matter and the factors that determine their potential
application in switchable cells. Although these molecules are not chiral, and
expected that they can form chiral liquid crystalline phases. In view of any
future applications, e.g. in electro-optical switches for display or
communication applications, the banana-shaped molecules should possess
liquid crystalline behaviour at temperatures as close to room temperature as
possible.
The development of banana shaped liquid crystalline materials is
attracted by the most of the researchers owing to their potential applications in
various fields. Considering the chemical investigations, most of the banana
shaped compounds is Schiff bases with five aromatic rings, but other linking
groups have also been used. Substituents connected to either the central ring
or to the outer rings alter the dipole moment of the molecules and have very
strong influence on the formation of banana phases. In general, enhanced core
rigidity and increased number of phenyl rings tend to increase the phase
transition temperatures of the molecules, whereas asymmetric structure
lowers the phase transition temperatures. Little attention has been devoted so
far to understanding the role of peripheral alkyl tails in effecting the formation
of the B phases. The main focus of the present investigation is to explore an
entirely new concept of banana shaped liquid crystalline materials with
different lateral attachments (Series I), asymmetric in mesogen (Series II) and
asymmetry in mesogen and terminal chains (Series III & IV).
In chapter 2, the main experimental techniques employed for the
present work will be briefly described. At the beginning of the chapter the
67
complementarities of these techniques and their relevance to the study of LC
systems will be specifically addressed.
In chapters 3, among four series, the series I compounds of five-
ring banana-shaped molecules with a central 1,3-substituted phenyl group,
esters as linking groups between the rings and the effect of substituent in
central core are investigated. In series II, the liquid crystalline properties of
compounds with constant terminal chain with vinyl group at one end and
different terminal chain were studied. By introducing more asymmetry in the
molecules and thereby lower the melting points while retaining the liquid
crystalline behaviour and to improve the switchable B2 mesophase formation.
Series III compounds deals with two terminal alkoxy tails of same length and
different linking groups in both arms were prepared and studied. Series IV
deals with two terminal alkyloxy tails of different length and different linking
groups in both arms were prepared and studied. The synthesized compounds
where investigated by spectral technique and their mesomorphic properties
were analysed. The liquid crystalline properties of the long and short chain
are compared with four series.
The chapter IV the liquid crystalline property of all four series of
compounds were compared and highlighted the summary and conclusions of
the present investigations.
To achieve the following objectives, the present investigation is
comprised in Figure 1.21.
68
The objectives are:
Figure 1.21 Molecular structure of target compounds series I-IV
i) Synthesis of precursors
Synthesis of 4-(10-undecenoyloxy)-1-biphenyl-4-carboxylic
acid(1)
Synthesis of 4-(alkyloxy)benzoic acids (2a-2g)
Synthesis of 4-(4-n-alkyloxybenzoyloxy)benzaldehydes (3a-3g)
Synthesis of 4-(4-n-alkyloxybenzoyloxy)benzoic acids (4a-4g)
Synthesis of resorcinolmonobenzylether (5)
Synthesis of 4-((3-(benzyloxy)phenoxy)carbonyl)phenyl
4-(alkyloxy)benzoates (6a-6g)
69
Synthesis of 4-((3-hydroxyphenoxy)carbonyl)phenyl
4-(alkyloxy)benzoates (7a-7g)
Synthesis of 3-(4-(4-(alkyloxy)benzoyloxy)benzoyloxy)
phenyl-4-formylbenzoates (8a-8g)
Synthesis of 4-(alkyloxy)acetanilides (9a-9g)
Synthesis of 4-(alkyloxy)anilines (10a-10g)
ii) Synthesis of bent-core compounds
Synthesis of 1,3-substituted phenylene bis(4’-(10-
undecenoyloxy)-1,1’-biphenyl-4-carboxylate)s (Ia-Ie).
Synthesis of 3-(4-(4-alkyloxy)benzoyloxy)benzoyloxy) phenyl
4'-(10-undecenoyloxy)biphenyl-4-carboxylates (IIa-IIg).
Synthesis of 3-(4-(4-alkyloxy)benzoyloxy)benzoyloxy)
phenyl-4-((4-(alkyloxy)phenylimino)methyl)benzoates
(IIIa-IIIg).
Synthesis of 3-(4-(4-(alkyloxy)benzoyloxy)
benzoyloxy)phenyl-4-((4-(decyloxy)phenylimino)
methyl)benzoates (IVa-IVg).
iii) Characterization of all the precursors and bent-core compounds
were carried out by FT-IR, 1H and 13C NMR spectral techniques.
iv) Liquid crystalline properties of bent-core compounds were
investigated by DSC and Polarized Optical Microscopy (POM).
v) The mesomorphic properties of the compounds were investigated
by the XRD measurement.
vi) Electro-optical property of the compounds were analysed by
triangular wave guide generator.