1

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

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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

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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.