1

INTRODUCTION

1.CHEMISTRY OF LIQUID CRYSTALS AND THEIR INDUSTRIAL

APPLICATIONS:

Liquid crystals are special substances that exhibit an intermediate state of matter

that exhists between crystal and liquid states; ever since Reinitzer in 1888 reported first

time for a number of Cholesteryl1 esters and later reported by 'Lehmann'2.

Approximately 5% of all organic compounds exhibiting LC behaviour have important

characteristic properties like anisotropy, magnetic and electric susceptibility3.

'Lehmann' first used the term “Fluid Crystal” but later this terminology was changed

and the word liquid crystal is often used in literature. Later Friedel4 proposed the term

“mesophase” an intermediate state of matter and liquid crystals are also referred as

mesomorphic or mesogenic.

The distinguishing characteristic of the liquid crystalline state is the tendency of

the molecules (mesogens) to point along a common axis, called the director. This is in

contrast to molecules in the liquid phase, which have no intrinsic order. In the solid

state, molecules are highly ordered and have little translational freedom. The

characteristic orientational order of the liquid crystal state is between the traditional

solid and liquid phases and this is the origin of the term “mesogenic state” used

synonymously with liquid crystal state. Polymorphism arises between the

transformation of crystal to isotropic fluid due to weak intermolecular forces that

imparts the equilibrium, results in well defined phase changes. This was known as

mesomorphism and the phases known as 'mesophases'.

2

Compounds exhibiting mesomorphic behavior can broadly be categorized into

two systems.

(a) Lyotropic systems of natural and synthetic origin5,6, where the liquid crystalline

order is solvent induced and

(b) Thermotropic system (including polyschiff's base polymers and aromatic

polyesters) where the liquid crystalline order is thermally induced.

The former having lot of biological importance and the later has numerous

industrial applications in display devices, which earned the interest of physicists and

chemists towards its synthesis and characterization.

Characterizing liquid crystals:

The following parameters describe the liquid crystalline structure.

(a) Positional order, (b) Orientational order, (c) Bond orientatinal order

Each of these parameters describe׳s the extent to which the liquid crystal sample

is ordered. Positional order refers to the extent to which an average molecule or group

of molecules shows translational symmetry. Orientational order, as discussed above,

represents a measure of the tendency of molecules to align the director on a long range

basis. Bond orientational order describes a line joining the centre's of nearest neighbor

molecules without requiring a regular spacing along that line.

By virtue of the optical studies, Friedel categorized three different types of

mesophases viz., smectic, nematic and cholesteric (Fig.1). One of these three

geometric anisotropies are necessary but not sufficient for liquid crystallinity. Many

organic compounds7,8 and some polymers were reported to exhibit liquid crystallinity.

The general common molecular feature is an elongated, narrow molecular frame work

3

which usually is depicted as a rod or cigar-shaped entity. Some disc shaped molecules

also exhibit liquid crystalline properties9-12.

.

smectic liquid crystal nematic liquid crystal cholesteric liquid crystal

Fig.1

A schematic representation of the main classification, sub categories and structures that

exhibit liquid crystal properties are given in Fig.2.

4

Liquid crystals

Thermotropic liquid crystalsLyotropic liquid crystals

Nematic liquid crystals (N)

Smectic liquid crystals (S)

Ordinarynematicstructure ( NO)

Cybotacticnematic

Normal (Nnc)

Skewed (Nsc)

Cholestericnematicstructures ( Nch)

SA SB SC SD SESF SG SH SJ SK

One-dimentional periodicity

Two- dimentional periodicity

Three- dementionalperiodicity

Lamellarstructures

Hexagonalstructures

Tetragonalstructures

Regular Inverted

Cubicstructures

Regular Inverted

Cubic structures

Fig.2. Classification of liquid crystals.

5

CHEMISTRY OF SMECTIC LIQUID CRYSTALS

Smectic is the name coined by 'Friedel' for certain mesophases with mechanical

properties reminiscent of soaps. Structurally, the smectic phase consists of a parallel

arrangement of molecules arranged in layers. The arrangements are either grouped

randomly or ordered. Smectic phases when observed exhibit different types of textures

viz., Schlieren texture (Fig.3), focal conic texture (Fig.4) or mosaic texture (Fig.5).

These textures are dependent upon the

Schlieren texture(Fig.3) Focal conic texture (Fig.4)

Mosaic texture (Fig.5).

6

i. Nature of the compound.

ii. The way in which the mesophase is produced i.e. by heating the crystal or by

cooling the isotropic fluid or by cooling the nematic mesophase.

iii. The nature and cleanliness of the supporting material employed to mount the

specimen.

The obvious requirement for the smectogenic behaviour is stronger cohesive

forces along the sides of the molecules rather than the ends of the molecules. A general

observation in the homologous series of Schiff's bases13, that the lower homologues are

either non mesomorphic or nematic while the middle members are “smectic and

nematic” and higher homologues are purely smectic. The presence of strong dipole

moment substituents enhance the lateral attractive forces across the major axis of the

molecules which encourages layered arrangement at the melting stage.

Some times a strong substituent like nitro group (I), likely to be a disturbed

dipole (because of another polarisable group) may result the nematic phase.

N O

O

N

O

H

N

O

O

( I )

7

Also long chain fatty acids fail to produce required dipole moment across the

long major axis because these exist as dimers. In order to eliminate the dimerization,

incorporation of a dipole group like ester linkage, create intermolecular cohesive forces.

Cyano group14-19 a strongly electron withdrawing at the terminal is responsible for L.C.

mesophase, where as nitro, trifluoro-methyl, perfluoromethyls are stable in chemical

reactions and these substituents increase the dielectric anisotropy18. The presence of

nitrogen19 atom in heterocyclic Schiff's bases are credited with enhancement of

smectogenic properties.

A new series of optically active compounds20 have been prepared using 4,4''

terphenyl dicarboxylic acid (II) and S-2-alcoxy propanols-1. These are Smectic* C,

Smectic A and N* chemically stable- especially with regard to racemization and UV.

light compounds with a small PS value, high twisting power.

O

O CH2

CH3

OR

O

O

H3C

RO

(II)

A number of azo-amil dyes21(111)of the type

NNHCC

HN Y

R1

R

( 111 )

With y=CN or NO2 and R, R' = Me, Me; Me, C6H13; Et, Pr and C6H13; C6H13

were synthesized by reactions of the amino substituted nitrosoamine with phenylene

8

diamine followed by condensation of the resulting aniline with appropriate 4-

substituted benzaldehyde. A nematic phase is observed only when R=R'=CH3, while

Smectic A pahses occurred in the other homologues. All phases exhibit colors ranging

from red to orange with UV max from 448-459 nm and :2.20-3.67 E+4. The H

values range from 16.8 to 30.7 KJ/mole and solubilities in a BDH-E7 liquid crystal

mixture were low.

A new series of liquid crystals, benzotriazol22 type (IV) which have high

absorptivities to ultraviolet (UV) light within the wavelength range from 290 to 350 nm

were synthesized. When the terminal group R' is replaced by a strong polar group such

as cyano group, an ideal temperature range of liquid crystal phase appears.

O

O

N

N

NR

R

( IV)

The homologues of bis [4-(5-alkyl-1,3-dioxan-2yl-1-phenyl] terephthalate23(V)

showed wide mesophase temperature ranges, nematic for butyl to dodecyl and Smectic

C for octyl to tetra decyl derivatives. The trans-cis isomerization of the heterocyclic

rings and dissociation of the ester linkages were observed.

O

O

O

O

R

O

O

O

O

R

(V)

9

New family of liquid crystalline compounds24 with high dipole moment of the

general formula (VI), showed Smectic and nematic phases. The temperature and

enthalpies of phase transition׳s are discussed and compared with the analogous Schiff

base viz., 4-chloro benzylidine-4'-alkoxy anilines.

N N ClRO

( VI)

Replacement of benzene by pyridine25 ring causes permanent dipole leading to

high termal stability. Likewise replacement of benzene ring with cyclohexane26 ring

inherits smectic behaviour except when cyclohexane ring is in between aromatic

esters27.

The 2,5-disubstituted pyridine28 derivatives VII are extremely useful materials

for electro optical display devices, because of the suitability of their mesomorphic

ranges for the displays and because of their high H values.

R2R1

N

R2R1

( VII)

But, electronegative elements like chloro and bromo groups29 increase thermal

stabilities because the dipolar and polarisability effects operate whilst breadth

10

increasing nature is suppressed.

A range of ortho-dichloroterphenyls30 (VIII) are expected to provide a greater

dipole and confer of dielectric biaxiality.

RC5H11

ClCl

(VIII)

A homologous series of 3-fluoro-4-octyloxy phenyl 4-(3-alkyl-2-thienyl)

benzoates31(IX) showed Sc phase.

O

O

SRH2C

F

OC8H17

(IX)

Sulfur bridged compounds, ex: 4-alkyl phenyl, 4'-alkyl or alkoxy benzoates show

greater thermal stability. This is due to smaller bond angle in C-S-C than C-O-C angle.

C-S bond is longer than C-O bond. This gives enhanced planarity of the molecule

because of increased lateral forces and favours smectic phase30(a).

The Halle group introduced the common identification of different smectic phase

types by capital letters. A, B, C etc. At present the Halle group has identified phases A to

G and H phase32.

11

In the following pages the structures of these phase types, according to current

general understanding, is described.

Smectic A: Smectic A phases are the least ordered of all smectic phases found at the

high temperature end of the smectic range. The molecules are arranged in layers and

have their long axis on the average perpendicular to the layer plane33,34. The viscosity is

rather high and this phase is generally not useful for devices. All smectic A phases

studied so far have been found to be optically uniaxial and frequently they show the

pseudoisotropic or homeotropic texture in which the smectic layers are parallel to the

supporting surface and the optic axis is perpendicular to it. The layers spacing in the

smectic A and C phases was examined by X-ray diffraction35, showing that the ratio of

the layer spacing to calculated molecular length is dependent on the entity of molecular

structure rather than the length of the alkyl chain. Novel perfluoroalkyl 3- or 4-

substituted benzenes (X) exhibit smectic A phase.

O O

0 F

CF3C3H7O

O

C10H21O

(X)

Smectic C: Smectic C phases are very similar to smectic A phase. The molecules are

arranged in a disordered manner and are fairly free to move around. The layers are

flexible and slide easily with respect to each other. Main difference between the two

phases is that in smectic C molecules are filled with respect to the layers where as in

smectic A they are perpendicular. The layer thickness in 'C' phases is considerably less

than the molecular length36 and are optically biaxial.

12

Some authors37,38 have identified two kinds of C phases. Those in which the tilt

angle is independent of temperature and those in which the tilt angle varies with

temperature.

The smectic A to smectic C* transition of a commercial ferroelectric mixture39

(SCE 13 and its racemic mixture) (XI) has been studied within a surface stabilized cell

using X-ray diffraction 'ranking curve' techniques.

C8H17O

C5H11

F F

(XI)

Ex: 4-n-octyloxy phenyl 4,4'-(n-octyloxybenzoyloxy) benzoate (XII).

O O

0O

C8H17 C8H17

(XII).

Smectic B, G and H: These consist of an ordered arrangement of the molecules

observed in layers rather than a disordered one. The ordered arrangement of molecules

appears to make the layers of B and H phases much more rigid than A and C phases40

leading to frequent appearance of mosaic textures for these phases. It seems that smectic

B phase is a soft solid with three dimensional ordering of finite range.

Smectic B: Ex: N,N'-terephthalylidene bis-(4-n-butyl aniline) (XIII)

13

CH

HC NNC4H9 C4H9

(XIII)

Smectic G: ex: 2-(4-n-Pentyl phenyl)-5-(4-n-pentyloxyphenyl) pyrimidine (XIV).

C5H11C5H11O

(XIV)

Smectic D: Only a few compounds exhibit the smectic D phase, on basis of X-ray

diffraction patterns the D phases were found to be optically isotropic, Diele et al41,

proposed a structure model a cubic close packing of spherical units, each unit consists

several molecules. Winsor42 proposes a similar structure, with “rotating units” composed

of globular assemblies of parallel molecules. Such a structure would not have the layers

generally characteristic of smectic phases and it has been questioned whether it should

be called smectic43,44.

ex: 4'-n-Alkoxy-3'-nitrobiphenyl-4-carboxylic acid (XV).

C5H11C5H11O

(XV)

14

Smectic E: X-ray analysis45,46 indicates a high degree of order within the smectic E

layer and a non-hexagonal lattice. Molecules are normal to the layers with three

dimensional ordering and are rigid.

Ex: Diethyl-p-terphenyl-4,4'-carboxylate (XVI).

O

OC2H5

O

C2H5O

(XVI)

Smectic F: Smectic F is similar to smectic C, both have tilted structures but a pseudo

hexagonal packing arrangement47 occurs in the more ordered F phase.

Ex: 2-(4-n-Pentyl phenyl)-5-(4-n-pentyloxy phenyl) pyrimidine (XVII).

C5H11

N

N

C5H11O

(XVII)

15

CHEMISTRY OF NEMATIC LIQUID CRYSTALS

The origin of the word 'Nematic' is taken from the Greek 'nematos' meaning

'thread like' and physically observed as a turbid fluid, but of much lower viscosity. This

phase could be produced by rapid cooling of isotropic melt. The molecules are aligned

along one direction i.e. they are positionally disordered but orientationally ordered.

Nematics are formed by many rod shaped organic molecules frequently carrying dipolar

groups, which predict the type of interactions in the molecules. Gibb's free energy, short

range intermoelcular interaction of the Vanderwalls type, i.e. dipole-dipole, induced

dipole-dipole and induced dipole-induced dipole forces emphasise structural

requirements.

The temperature range of a thermotropic liquid crystal mesophase may be

observed visually in a melting point capillary or on a hot stage attachment to a polarizing

microscope. Crystalline p-azoxy anisole (XVIII) melts at 119.50C to a nematic phase

which exists until 1350C when the mesophase is transformed into an isotropic liquid48.

N

N OCH3

H3CO

(XVIII)

Guyon et al49 have found that molecular alignment depend on the angle

between the direction of evaporation and the direction normal to the film surface, when

was between 00 and 450 and 800 produced alignment perpendicular to the projection of

the evaporation beam. The molecular alignment of NLCS can also be controlled by

16

surface active agents50,51 such as Lecithin and siloxanes.

In case of benzylidine anilines, the presence of different substituents of nucleus

leads to various thermal stabilities. Presence of -Ome52 group decreases transition

temperature and thiomethyl, (-SCH3), -NO2 and Cl53 groups make the molecule loose

mesomorphic behaviour because of increased dipolar forces acting along the long

molecular axis, by the electronic substituents.

Habes and coworkers54 have found that the presence of an ortho hydroxy group

in benzylidine aniline (XIX) stabilizes the resulting unit due to intramolecular hydrogen

bonding.

C

N

H

C4H9

H3CO

OH

(XIX)

'Verbit' and Tuggey55 reported the synthesis of a series of ester derivatives with

the general structure (XX).

O

O

X

O

ORO OR

X= C C ,

(XX)

17

They concluded that the nematic-isotropic transitions for the acetylene

compounds were more than 1000C lower than those of the corresponding benzenoid

derivatives, while the melting or crystal-nematic transitions of the acetylenic esters

range from 60-1000C lower.

Van Meter and Klanderman56 reported the effect of terminal alkyl and alkoxy

substitution on the mesomorphic properties of 60 phenyl benzoate esters (XXI) of the

following structure.

O

O

R2

R1

(XXI)

Steinstraisser reported57 the synthesis of 19 phenyl esters of the general

structure(XXII) and observed the crystal to nematic transition temperature is above room

temperature.

OC

O

O

O

R

R

(XXII)

18

Young58 reported the lateral methyl group substitution in a series of phenyl esters

of general structure (XXIII) lowers the crystal-to-nematic transition temperatures. The

crystal to nematic transition temperature observed is an low as 430C.

OC

O

O

O

R1

R2

R3

(XXIII)

Van Meter and Klanderman59,60 have reported synthesis of seven low melting

nematic phenyl-4-benzoyl benzoates with melting points as low as 390C and nematic

ranges between 36 and 1010C. The lowering of the crystal nematic temperatures was

produced by lateral substitution, i.e. the replacement of hydrogen by chloro on an

aromatic ring.

In case of polymorphic nematic liquid crystals, the smectic-nematic transition

temperatures will be found to rise as alkyl chain length increases. The thermal stability

could be explained in terms of the lateral cohesive forces which determine the smectic

nematic transition temperatures.

Three diaza-18-crown-6-carrying aryl esters61 (XXIV) were synthesized and

found to exhibit nematic liquid crystalline properties. It has been found that liquid

crystalline properties, depend upon structure of anions. The influence of doping

inorganic salt was also studied.

19

O

N

O

N

O O

O O

OO

O O

R R

( XXIV)

In general nematic-isotropic temeprature fall with increasing alkyl chain length

between the odd and even numbers of carbon atoms in the alkyl chain.

Ex: 4-n-alkoxy biphenyl-4-carboxylic acids

In the case of polymorphic nematic liquid crystals, smectic nematic transition

temperatures will rise as the alkyl chain length increases62. The thermal stability could

be explained in terms of the lateral cohesive forces which determine the nematic

transition temperatures.

The nematic swarms with parallel orientation by the residual/terminal and lateral

attractions and the strength of these cohesive forces determine N-I transition

temperatures. These trends of homologous series could be understood by increasing

polarisability and the presence of permanent dipole moment of lateral substituents as the

methylene chain is lengthened, thus decrease in terminal intermolecular cohesive. So for

polymorphic mesogens, lower homologous are purely nematic, middle of the series are

smectic C. Lateral substituents increase the breadth of the molecule and increased

molecular weight of the molecule will also tend to make more difficult for termal

vibrations which causes sliding of the molecules out of the layers to give the imbricated

orientation of mesogenic fluid. In general nematic-Isotropic temperature fall with

increasing alkyl chain length between the odd and even numbers of carbon atoms in the

alkyl chain.

20

CHEMISTRY OF CHOLESTERIC LIQUID CRYSTALS

In 1888, Reinitzer reported the synthesis and thermal behaviour of a series of

cholesteryl esters63. Cholesteric liquid crystal phase is not only due to the cholesterol

nucleus and reports show the same phase may be observed by the derivatives of Schiff's

bases64 often called as chiral nematics or twisted nematic and it comprises of helical

arrays, arises due to optical activity.

The cholesteric mesophase has been described as a nematic planar structure in

which there is a twist angle from one layer to the next. Because of this, the cholesteric

mesophase is often called a twisted nematic mesophase65-67.

To date there is no satisfactory reasoning regarding the mesophase behaviour of

cholestrogens. A possible explanation may be like broad and flat arrangement of the

molecule, lateral and terminal cohesive forces between the molecules, would suggest

melting of cholestrogens at different stages. The cholesteric mesophases of -sitosteryl

benzoates showed extremely brilliant colors ranging from red to violet when sample

temperature is raised starting from crystal to isotropic. Higher alkyl chain cholestrogens

do not exhibit colored mesophases because, reflected wavelength falls under UV region.

Increase of alkyl chain length tends to increase smectogenic properties but this

could be observed up to cholesteryl laurate68 and decreases gradually from myristate,

palmitate and stearate.

Mesomorphic properties change with change of the steroid skeleton69. Further

supporting results70-72 suggest that triterpene derivatives showed relative decrease in

melting points and as well as mesophase transitions when compared to steroid skeletons.

21

It may be observed from fore cited references that melting points of both sterol and

triterpene derivatives decreases with increase in their chain length.

Wiegand73 demonstrated the lack of mesomorphism in derivatives of

epicholesterol (XXV) in which the 3-hydroxy function occupies the position out of

plane of the A ring.

HO (XXV)

But the behaviour of the 5-cholestan-3-alkyl thiocarbonates (XXVI)

resembles that of cholesteryl analogs with the exception that all transitions are depressed

about 100C74.

H

HX

( XXVII )

22

The mesomorphic properties of the cholestanyl alkanoates (XXVIII)

ROCO

(XXVII)

studied cannot be compared to other cholesteryl analogs but the substituted benzoates

(R=aromatic) have been explained75. They have lower melting cholesteric-isotropic

temperature than the corresponding cholesteryl derivatives.

Introduction of halogen atom in the branched cholesteryl alkanoates75 of -

isomer showed L.C. phase with bright colors, where as -isomer did not exhibit colors

because of increased molecular breadth by the -halogen atom. Increased number of

halogen atoms76 on the branched chain also show mesophase.

Chirality in the branched chain of sterol or triterpene has considerable effect on

both intramolecular steric interactions and intermolecular attractions in the cholesteric

mesophase. Because of these stereo isomeric71 attractions there will be a difference in

their transition temperatures.

Thiocholesteryl-n-alkanoates possess lower melting points and higher smectic-

cholesteric isotropic transitions77 because of the more bulky and less electronegative

atom. Phenyl substitutions at '' position in thiocholesterol-n-alkanoates show large odd-

23

even decrease in melting point with increasing chain length78. Higher transition

temperatures were observed in case of cholesteryl-n-alkoxy naphthoates79.

Chiral nematic compounds show potential interest because of greater dielectric

anisotropies and increased thermal stabilities than the carbonates of cholesterol79.

Incorporation of chiral centre into various positions of alkyl chain of the known 4-n-

alkyl phenyl and 4-n-alkoxy phenyl of 4,4'-substituted Schiff's bases or 4'-(alkoxy)-4-

cyanobiphenyl or 1,4disubstituted bicyclo [2.2.2] octane exhibits cholesteric

mesophases.

24

CHEMISTRY OF DISCOTIC MESOPHASES

The carbonaceous mesophase, was initially discovered by Brooks and Taylor80,81

during the carbonization of graphitizable substances at high temperatures82,83. This

mesophase presents optical texture very similar to nematic one, and the chemical nature

is not yet well known, but could be composed of a complex mixture of flat poly aromatic

plate or disc like molecules, and hence a study to get pure single component model is

desired to study this mesophase.

The first report of disc like molecules was proposed and reported by

Chandrasekhar9 et. al in 1977 and the first investigated compounds are benzene hexa-n-

alkanoates. Based on X-ray data the authors proposed a structure, in which the discs are

staked one on top of the other in columns, constitutes a hexagonal arrangement, but the

spacings between the discs in each column is irregular. Thus the structure has a

translational periodicity in two dimensions and liquid like disorder in the third

dimension.

Since the discovery of the existence of thermotropic mesomorphism in some

“disc-like” compounds9,84 in 1977, 1978 several fundamental results have been

discovered in this new field of research. The first discovered mesomorphic phases were

discotic, Columnar ones9,18, then the evidence of a complex polymorphism has been

shown85-89. The disc-like liquid crystals will really provide a condensed state of matter.

An analogy with rod like liquid crystals but the only fundamental difference is in the

director vector parallel to the molecules in rod like, where as it is perpendicular in the

case of flat disc like molecules90.

25

Polymorphism: Among the various mesogenic polyaromatic91 cores hexa substituted

derivatives of benzene, triphenylenes, truxenes and anthraquinones exhibit one or

several D columnarphases or a ND lenticular nematic phase. A simple scheme of

nomenclature suggested for the six different types of columnar phases requires only two

structural parameters. Firstly, the symmetry of the lattice, (hexagonal Dh, rectangular Dr,

oblique Dob) and secondly, the order or disorder within the column (Fig.6) Several

examples of disc-like compounds which form phases Dho, Dhd, Drd(P21/a), Drd P2/a, Drd(C2/m),

Dob, d are described and discussion of the specificity of the corresponding optical

textures given by C. Destrade91 and also the complex polymorphism of these new

mesogenic materials (normal, inverse, reentrant and columnar-nematic sequence) was

reviewed. A simple classification of various types of themotropic, lyotropic phases was

proposed by Destrade et. al based on the shape of the structural element (finite or infinite

cylinder, disc, infinite sheet and globule). As for the classical nematic and smectic liquid

crystals, disc-like mesogens are restricted to certain structural types i.e. flat and more or

less disc shaped central rigid core usually surrounded by 6 lipophilic long chains (Fig.7)

Central cores: Most of the disc like cores have been more or less aromatic usually

discotic mesogens have flat skeletons with highly symmetric axes. For example benzene

hexaalkanoates9,92 and hexa alkyl phenyl benzenes93 have D6 symmetric axes.

Porphyrins94,95, bipyrans96,97 compounds have D4 and D2 axes respectively. Furthermore,

triphenylenes98,99, truxene100-102, anthraquinone103,104 and benzoquinone105 comopunds

also give rise to the discotic phases, although they have low symmetric axes.

Up to date review of discotics has appeared recently106 but the Pd-catalyzed

couplings are again cropping up as a means of providing more accessible and

26

structurally varied discogens. Praefcke et al107 have used such methods to replace all six

bromo-substituents in a hexabromo benzene or triphenylene with the groups

and more recently they have extended108 this activity to 1, 2, 3, 5, 6, 7 hexabromo

naphthalene, thereby to the range of discotic materials available for study.

C. Mertersdorf and H. Ringsdorf109 reported that the columnar phases are fabular

systems. An example is provided by the azo-crown-6 of structure(XXVIII),

N

N

N

N

N

N

R

R

R

R

R

R

structure(XXVIII),

27

in which the discotic core, 18-ane N6 has groups R=4-C12H25O, C6H4.CO- attached to

the nitrogen's. Attempts to extend the number of long chain functions by using 4, 5

trialkoxy benzoyl and 3,5-dialkoxy benzoyl functions resulted only in glasses which

were not liquid crystalline when 18-ane N6 and 14-ane N4 were used as cores.

Lattermann110 achieved success by using 4-dialkoxy benzoyl groups and especially with

14-ane N4, obtained L.C phases. Recently111 further studies were reported with 3,4-di-n-

alkoxy benzoyl functions linked to the nitrogens of 18-ane N6, 12-ane N3 and 9-ane N3.

These are summarized below.

18-ane N6 (12 chains) K 1040 D 1400.I

12-ane N3 (6 chains) K 37.50D 59.50.I ; cooling gives a discotic glass Tg 29.50

9-ane N3 (6 chains) K 400 D1 450 D2 670. I; cooling gives a discotic glass Tg 32.50

Hexasubstituted benzenes: Possess highly symmetric axes. Synthetic route9 was shown

in scheme-1. The transition phases of hexa-n-alkanoates observed was enantiotropic. All

these mesophases were highly viscous, birefringent and highly ordered, probably

smectic like character. Benzene hexa heptanoate, and octanoate are mesomorphic,

however the hexanoate compound shows pressure induced mesomorphism92. This

transition temperature is in excellent agreement with the results of miscibility studies.

28

Scheme-1

CHO

CHO

O2

Na2SO3

OH

OH

ONaHO

OHNaO

HCl

O

O

OHHO

OHHO

HCl/SnCl2

OH

OHHO

OHHO

OH

RCOCl/Py

OCOR

OCORROCO

OCORROCO

OCOR

112 Later, Goozner and Labes113 studied specific solute-solvent interactions by

dissolving the small non-mesogenic molecules such as xylene, toluene etc., in these

columnar series.

Hexasubstituted triphenylenes: Triphenylene derivatives are among the most

extensively studied discotic liquid crystals, because they are known to form an ordered

hexagonal (Dho) mesophase which is ideal for one-dimensional energy114 and electron

transport115. These triphenylene hexaethers and hexaesters were synthesized via the route

outlined in scheme II116,117.

29

Scheme-II

OMe

OMe

ChlorinillConcH2SO4

OMe

MeO

OMe

OMe

MeO

OMe

BBr3

OH

HO

OH

OH

OH

HO

RBr/DMH

RCOCl/Py

OR

RO

OR

OR

OR

RO

OCOR

ROCO

OCOR

OCOR

OCOR

ROCO

But recently an improved procedure118 for the preparation of hexa alkoxy

triphenylenes was reported where by a 1-2 dialkoxybenzene is trimerised directly using

Iron III chloride, followed by reduction119. Destrade et al., concluded that hexahexyloxy

30

benzoates of triphenylenes exhibit new kind of columnar arrangement where the discs

appear to be tilted with respect to the columnar axis. The same authors120 observed a

complex polymorphism in HAT compounds and two of three transitions are practically

second order ones.

Hexa octanoate of Rufigallol (RHO): The synthesis of these compounds was presented

in scheme III. Billard et al., presented a new thermotropic mesogenic disc-like molecule

admitting only two fold symmetry axis viz., Rufigallol hexa octanoate (RHO).

Queguiner et al., concluded that RHO exhibits thermodynamically stable discotic

mesophase having a three fold or six fold symmetry axis and a second monotropic

mesophase. This is a new example of discotic dimorphism. Levelut121 et al., performed

X-ray diffraction experiments on the two mesophases of RHO and concluded that the

enantiotropic phase is similar to that of hexa-octanoate of triphenylene where as the

structure of monotropic mesophase is a new one, which is having three different

orientations of the molecular core.

31

Scheme III

OH

OH

HO

COOH

H2SO4

-2H2O

O

O

HO

HO

OH

OH

OH

OH

Rufigallol

RCOCl/Py

O

O

OCOR

OCOR

OCOR

ROCO

ROCO

OCOR

Hexasubstituted truxenes: The synthetic route of these compounds was shown in

Scheme-IV. The first case of reentrant phenomenon122 in disc like mesogens was

observed in hexa alkanoyloxy truxenes (CnHATX) in this series, out of 10 compounds

synthesized (n=6 to 15) a few of the long chain compounds displayed a reentrant

columnar phase.

Scheme-IV

32

HO

HOCOOH

RCOCl/Py

ROCO

ROCOCOOH

PPA

ROCO

ROCO

O

Trimerisation

OCORROCO

OCOR

OCOR

ROCO

ROCO

33

J.P. Morand123 et al synthesized alkyl thiobenzene(XXIX) with C3 symmetry.

S

S

S

S

S

S

S

S

S

S

S

S

O

S

O

S

O

S

S

S

S

S

S

S

(XXIX)

These molecules are donars and good for the preparation of both charge

transfer complexes and cation radical salts exhibit new interesting electrical properties.

The first alicyclic saturated discotic liquid crystal, viz., hexa-O-alkanoyl-scyllo

inositols ethers (XXX) were synthesized and perhydrogenated analogues of known

benzene derivatives were characterised by Klaus Praefcke et al124.

34

OR

OR

ORRO

OR

RO

(XXX)

The chemical nature of the aliphatic chain has a decisive influence on the

appearance of mesomorphic properties for ex: some hexa substituted benzenes with poly

hetero atomic chains seems to be devoid of mesomorphism. The chains can also be

linked to the central part by means of benzyloxy groups. These bulky substituents are

probably responsible for the appearance of the ND nematic Phase125.

35

CHEMISTRY OF POLYMER LIQUID CRYSTALS

Although liquid crystals offer wide range of applications, the escalating

technological importance sought to study liquid crystalline properties, in particular low

molecular weight polymer systems. Various thermotropic liquid crystalline polymers

were synthesized. The synthetic strategy being, by linking together suitable monomers

through a condensation reaction, gives a rigid type (or) of a flexible nature of polymer

backbone with alternating rigid (mesogenic) and flexible segments. Both types are called

L.C. main chain polymers and each preserves the original L.C. moities126,127.

In the low molecular weight polymer system, the length of the terminal alkyl

chain tend to increase smectic nature. By use of a chiral co-monomers gives

cholesterogens.

Low molecular weight polymers are classified according to structural

considerations into aromatic, acylic, cholesteric and discotic systems. Aromatic

mesogens being considered for different mesogenic groups (CH=N, N=N - CO.O-)

sandwiched between aromatic rings.

A B YX

a b

a,b = simple integers -A=B- = -CH=N, -N=N-

x and y = CN, alkoxy, alkyl

O

N N̄- =

O

N N , CO.O¯

- = - -

36

Central linkage containing a multiple bond maintains rigidity and linearity of the

molecules resulting high Tn-1 values. For these mesogens terminal cyano, alkoxy groups

are favourable than others such as alkyl or halogen in promoting high Tn-1 values.

Low molecular weight smectic liquid crystal systems are valuable for increasing

the sensitivity of the color response of cholesteric phases to temperature and two certain

display devices128 require room temperature.

Liquid crystals in biological systems: Many biological systems129 exhibit the

properties of liquid crystals. Considerable concentration of mesomorphic compounds

have been found in many parts of the body, often as sterol or lipid derivatives. A fluid

crystal phase has been implicate in atleast two degenerative diseases ex: atherosclerosis

and sickle-cell anemia, living tissue, such as muscle, tendon, ovary, adrenal cortex and

nerves show the optical birefringence properties that are characteristic of liquid crystals.

There are two important classes of fibres that are characterized by nematic like

organization and are in the cytoplasm of many plant and animal cells. These are

microfilaments and microtubules that play a central part in determining the cell shape,

either as dynamic element in the contractile mechanism or as the basic cytoskeleton.

There is also a correlation between the type of subsurface organisation of these

fibres and gross cell shape in tissue of other muscle.

37

APPLICATIONS OF LIQUID CRYSTALS

Research on optical & electrical properties of these unique compounds attracted

very much by scientific and industrial community. Later, research at a number of

industries, universities and government laboratories began to focus on their applications,

which exploited the electro-magneto-optic characteristics and photoelectric properties of

nematic and cholesteric type liquid crystals.

The temperature-dependent variation in the color of cholesteric liquid crystals

has led to the use of these substances in the measurement of temperature gradients130-132.

Cholesteric liquid crystal substances, when applied to the surface of the skin, have been

used to locate veins, arteries, infections, tumors and the fetal placenta which are warmer

than the surrounding tissues.

Nematic liquid crystal are useful research tools in the application of magnetic

resonance133-137. Molecules that are dissolved in nematic liquid crystal solvents give a

very highly resolved NMR spectrum exhibiting intermolecular dipole-dipole fine

structures. Analysis of the spectra of molecules in liquid crystal solvents yield

information regarding the anisotropy of chemical shifts, direct magnetic dipole-dipole

interaction, indirect spin-spin couplings, bond angles, bond lengths, molecular order and

relaxation process.

Liquid crystals have been used in chromatographic separations138 as solvents to

direct the course of chemical reactions and to study molecular arrangements and kinetics

139,140 and as anisotropic host fluid for visible, UV and IR spectroscopy of organic

molecules141,142.

38

Now a days liquid crystals are widely used in cosmetic industry in manufacturing

of liquid crystal makeup removers143, lipsticks and lip glasses144 containing cholesteric

liquid crystals and also used in the manufacture of vaginal fluids. Liquid crystals are

using extensively in pharmaceutical industries.

Liquid crystal displays are common in calculators, digital watches,

oscillaographic systems, television displays145 using L.C. screens has also been

developed. Cholesteric liquid crystals have also been used for novelty items such as toys

and decorative materials.

Liquid crystal polymers also gained much interest on industrial applications. The

synthesis of commercially important 4-alkyl-4'-cyano biphenyls and 4-alkoxy-4'-cyano

biphenyls in the early 1970's was taken because of the demand in display system. A large

number of nematic liquid crystalline polymer systems were developed because they do

exhibit a sharper threshold and a lower temperature dependence of threshold voltage. Its

importance in electro optical displays is also high. Later, polyester liquid crystals were

developed for fire resistant, and are used as coating for multifibre146, optical cables due

to good surface roughness, low coefficient of friction. Polyesters are used for molding

with improved elastic modulus. Ferroelectric liquid crystals, mesomorphic free radicals

are used for EPR study and colorless large pitch cholesterics has been developed.

Polysiloxanes, a liquid crystalline elastic form can be converted to rubber.

In addition to this development, L.C's were found to be useful laboratory tools

for GLC and absorption spectroscopy. Films of microencapsulated147. L.C's can be

prepared by spraying, draw-bar coatings or silk screening. After evaporation of the

water, the films are dry to touch and are relatively free from damage by abrasion.

39

A novel family of cholesteric liquid crystals148 have been developed by Dupont

(USA) for the decorative market. These materials, which are still in the experimental

stage, may be used to provide attractive decorative effects on opaque, translucent and

transparent surfaces. In contrast to most commercial liquid crystals these materials exist

as solid glasses at ambient temperature.

40

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