Introduction and historical overview. What are Liquid Crystals?

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1 Introduction and historical overview Research on liquid crystal has been involved in chemistry, physics, Biology, electric and electronic engineering and many other fields. Most of this research has been reported by the universities and research institutions. The study of liquid crystals began in 1888 by Australian Botanist F. Reinitzer [1]. Liquid crystal materials are unique in their properties and uses. As research into this field continues and as new applications are developed, liquid crystals will play an important role in modern technology. What are Liquid Crystals? The term Liquid Crystals seems to be a self-contradiction as it suggest that a substance is in two quite different state of matter at the same time. The two most common states of condensed matter are the isotropic liquid phase and the crystalline solid phase. In a crystal, the molecules or atoms have both orientational and three-dimensional positional order over a long range. In an isotropic liquid, however, the molecules have neither positional nor orientational order, they are distributed randomly. There is no degree of order, so three degrees of freedom are left. There is no preferred direction in a liquid, thus the name isotropic. The transition from one state to another normally occurs at a very precise temperature. When pure crystalline solid is heated beyond its melting temperature, it undergoes a single transition to isotropic liquid. e.g. ice-water is such a common phase transition. There are, however many organic compound that do not immediately transform to liquid phase when heated beyond the melting temperature but exhibit more than a single transition from solid to liquid showing the existence of one or more intermediate phases, exhibiting the properties of both solids and liquids. For examples p-azoxy anisole when heated does not transform into the liquid state but adopts structure (turbid condition) that is both birefringence and fluid the consistency varying with different compounds that of a paste to that of a freely flowing liquid. 1

2 Transitions are definite and precisely reversible. Materials undergoing such a phase transitions are called Liquid Crystals [2]. History of liquid crystals The discovery of liquid crystals is thought to have occurred nearly 150 years ago although its significance was not fully realized until over a hundred years later. Around the middle of the last century Virchow[3], Mettenheimer et al.[4] have found that the nerve fiber they were studying formed a fluid substance when left in water which exhibited a strange behaviour when viewed using polarized light. They did not realize this was a different phase but they are attributed with the first observation of liquid crystals. Later, in 1877, Further investigations of this phenomenon were carried out by the German physicist O. Lehmann [5] who observed and confirmed, using the first polarized optical microscope designed by himself, the existence of "crystals [which] can exist with a softness that one could call them nearly liquid". He found that one substance would change from a clear liquid to a cloudy liquid before crystallising but thought that this was simply an imperfect phase transition from liquid to crystalline. The first reported 2

3 documentation of the LC state was through an accidental observation by an Austrian botanist, Friedrich Reinitzer [1] in 1888, working in the Institute of Plant Physiology at the University of Prague. He observed double melting" behaviour of cholesteryl benzoate. The crystals of this material melted at oc into a cloudy fluid, which upon further heating to 178.5oC became clear. This discovery represented the first recorded documentation of the LC phase. He was the first to suggest that this cloudy fluid was a new phase of matter. He has consequently been given the credit for the discovery of the liquid crystalline phase. Puzzled by his discovery, Reinitzer turned for help to the German physicist Otto Lehmann, who was an expert in crystal optics. Lehmann became convinced that the cloudy liquid had a unique kind of order. In contrast, the transparent liquid at higher temperature had the characteristic disordered state of all common liquids. Eventually he realized that the cloudy liquid was a new state of matter and coined the name "liquid crystal," illustrating that it was something between a liquid and a solid, sharing important properties of both. In a normal liquid the properties are isotropic, i.e. the same in all directions. In a liquid crystal they are not; they strongly depend on direction even if the substance itself is fluid. That new types of liquid crystalline states of order were discovered. Up till 1890 all the liquid crystalline substances that had been investigated naturally occurring and it was then that the first synthetic liquid crystal, p- azoxyanisole, was produced by Gatterman and Ritschke. Subsequently more liquid crystals were synthesized and it is now possible to produce liquid crystals with specific predetermined material properties. Maier and Saupe [6] formulated a microscopic theory of liquid crystals, Frank and later Leslie and Ericksen developed continuum theories for static and dynamic systems and in 1968 scientists from RCA first demonstrated a liquid crystal display [7]. The interest in liquid crystals has grown ever since, partly due to the great variety of phenomena exhibited by liquid crystals and partly because of the enormous commercial interest and importance of liquid crystal displays. Today, thanks to Reinitzer, Lehmann and their followers, we know that literally thousands of substances have a diversity of other states. Some of them have been found very usable in several technical innovations, among which liquid crystal screens and liquid crystal thermometers may be the best known. In the 1960s, a French theoretical 3

4 physicist, Pierre-Gilles de Gennes, who had been working with magnetism and superconductivity, turned his interest to liquid crystals and soon found fascinating analogies between liquid crystals and superconductors as well as magnetic materials. His work was rewarded with the Nobel Prize in Physics The modern development of liquid crystal science has since been deeply influenced by the work of Pierre-Gilles de Gennes [8]. This new idea was challenged by the scientific community, and some scientists claimed that the newly-discovered state probably was just a mixture of solid and liquid components. But between 1910 and 1930 conclusive experiments and early theories supported the liquid crystal concept at the same time. In 1922 the French scientist G. Friedel produced the first classification scheme of LCs [9], dividing them into three different types of mesogens (materials able to sustain mesophases), based upon the level of order the molecules possessed in the bulk material: 1.nematic (from the Greek word nematos meaning "thread"), 2.Smectic (from the Greek word smectos meaning "soap"), and 3.Cholesteric (better defined as Chiral nematic)[10]. Following these first observations and discoveries, the scientific research turned attention towards a growing number of compounds, which displayed liquid crystalline properties. In order to establish a relationship between the molecular structure and the exhibition of liquid crystalline properties, a series of systematic modifications of the structures of mesogens was undertaken, leading, in 1973 [11], to the discovery of the most technologically and commercially important class of LCs to date: the 4-alkyl-4'- cyanobiphenyl (CB) of which an example, 4-pentyl-4'-cyanobiphenyl (5CB) 1 is illustrated in Figure 1. Figure 1 Figure 1. Molecular structure of 4-pentyl-4'-cyanobiphenyl (5CB) 1. (The transition temperatures are expressed in o C). 4

5 These are the materials, which still constitute the simple common displays found in calculators or mobile phones. However, the numerous and increasingly sophisticated applications, relying upon the use of liquid crystalline materials, require such a complexity of superior properties to achieve improved devices performance, that the quest for ever new LCs has grown enormously over the last three decades. Nowadays, LCs play a dominant role in a large part of the display technology. Liquid crystal is solid or liquid? It is sometimes difficult to determine whether a material is in a crystal or liquid crystal state. The amount of energy required to cause the phase transition is called latent heat of the transition and is useful to measure of how different the two phases are. In the case of cholesteryl myristate, the latent heat of solid to liquid crystal is 65 calories/gram, while the latent heat for liquid crystal to liquid transition is 7 calories/gram. These numbers allow us to answer the question posed earlier. The smallness the latent heat of liquid crystal to liquid phase transition is evidence that liquid crystal are more similar to liquids than they are to solids. when a solid melts to a liquid crystal, it loses most of the order it had and retains only a bit more order than a liquid possesses. This small amount of order is then lost at the liquid crystal to liquid phase transition. The fact that liquid crystals are similar to liquids with only a small amount of additional order, is the key to understanding many physical properties that make them nature s most delicate state of matter [12]. Order Parameter To quantify just how much order is present in a material, an order parameter (S) is defined. Traditionally, the order parameter is given as follows: where theta is the angle between the director and the long axis of each molecule. The brackets denote an average over all of the molecules in the sample. In an isotropic liquid, 5

6 the average of the cosine terms is zero, and therefore the order parameter is equal to zero. For a perfect crystal, the order parameter evaluates to one. Typical values for the order parameter of a liquid crystal range between 0.3 and 0.9, with the exact value a function of temperature, as a result of kinetic molecular motion. This is illustrated below for a nematic liquid crystal material. The tendency of the liquid crystal molecules to point along the director leads to a condition known as anisotropy. This term means that the properties of a material depend on the direction in which they are measured. For example, it is easier to cut a piece of wood along the grain than against it. The anisotropic nature of liquid crystals is responsible for the unique optical properties exploited by scientists and engineers in a variety of applications. Types of LCs Mesomorphic State Liquid Crystals Ordered fluid mesophase (Solid-like liquids) Plastic Crystals Disordered Crystal mesophase (Liquid-like solid) Thermotropic Liquid crystals (Non-amphiphilic) Lyotrophic Liquid crystals (Amphiphilic) Smectic (Two dimensional order) Nemetic (One dimensional order) Cholesteric (Chosterol-derivatives) (Helical structure) 6

7 Different types of molecules can form liquid crystalline phases. The common structural feature is that these molecules are form anisotropic: one molecular axis is much longer or wider than another one. The two major categories are: 1.Thermotropic LCs, whose mesophase formation is temperature (T) dependent, and 2. Lyotropic LCs, whose mesophase formation is concentration and solvent dependent. Lyotropic LCs Lyotropic LCs are two-component systems where an amphiphile is dissolved in a solvent. In blends of different components phase transitions may also depend on concentration and these liquid crystals are called lyotropic. Thus, lyotropic mesophases are concentration and solvent dependent. The amphiphilic compounds are characterised by two distinct moieties, a hydrophilic polar "head" and a hydrophobic "tail". Examples of these kinds of molecules are soaps (Figure 2 a) and various phospholipids like those present in cell membranes [13-15] (Figure 2 b). Figure 2. Chemical structure and cartoon representation of (a) sodium dodecylsulfate (soap) forming micelles, and (b) a phospholipids (lecitine), present in cell membranes, in a bilayer lyotropic liquid crystal arrangement. Today, Lytropic liquid crystalline materials have been widely used as display devices [16] and lytropics are also important for biological systems, e.g. membranes [17-19]. 7

8 Thermotropic LCs Fig. 3: liquid crystalline mesophases between the solid and isotropic liquid phase Thermotropic transition occur in most liquid crystals, and they are defined by the fact that the transitions to the liquid crystalline state are induced thermally. That is, one can arrive at the liquid crystalline state by raising the temperature of a solid and/or lowering the temperature of a liquid. Condensed matter which exhibit intermediate thermodynamic phases between the crystalline solid and simple liquid state are now called liquid crystals or mesophases (Fig.3). This fourth state of matter generally possess orientational or weak positional order and thus reveals several physical properties of crystals but flow like liquids. If transitions between the phases are given by temperature, they are called thermotropic [20-21] While thermotropics are presently mostly used for technical applications [22]. The essential requirement for a molecule to be a thermotropic LC is a structure consisting of a central rigid core (often aromatic) and a flexible peripheral moiety (generally aliphatic groups). This structural requirement leads to two general classes of LCs: 1. Calamitic LCs, and 2. Discotic LCs both of which have other molecular subclasses. Thermotropic liquid crystals can be classified into two types: 8

9 Enantiotropic liquid crystals: which can be changed into the liquid crystal state from either lowering the temperature of a liquid or raising of the temperature of a solid or mesomorphic transitions occur on heating the substance and these transition reveres in the opposite direction on cooling. Such a mesophase is called the enantiotropic mesophase. Monotropic liquid crystals: which can only be changed into the liquid crystal state from either an increase in the temperature of a solid or a decrease in the temperature of a liquid, but not both or there are many compounds, which on heating do not exhibit mesophase and directly pass into an isotropic liquid but on cooling, they exhibit a mesophase is termed as monotropic mesophase. This monotropic temperature is also reversible. In general, thermotropic mesophases occur because of anisotropic dispersion forces between the molecules and because of packing interactions. Although the term thermotropic and lyotropic are widely used, Gray and Winsor [23] prefer the terms amphiphillic (for lyotropics) and non-amphiphillic (for thermotropics ). Polymorphism: Many Liquid crystalline substances which have exclusively smectic mesophase (structure) or exclusively nematic mesophase (structure). But some can exist as both types of mesophase, smectic followed by nematic and they have definite transition temperature defining the stability of the different phase, which are always reproducible. There are substances possessing more than one smectic phase having sharp temperature range of stability of different phases. This phenomenon is known as polymorphism. 9

10 Fig.4: Schematical phase sequence of a liquid crystal. From left to right: smectic C phase (tilt angle between layer normal and mean orientation of the molecules), smectic A phase (layered structure, no tilt), nematic phase, isotropic phase. Above the clearing temperature (Tc) the liquid crystal becomes an isotropic liquid. These properties make liquid crystals an interesting object for the application of thermodynamical methods. Calamitic LCs Calamitic or rod-like LCs are those mesomorphic compounds that possess an elongated shape, responsible for the form anisotropy of the molecular structure, as the result of the molecular length (l) being significantly greater than the molecular breadth (b), as depicted in the cartoon representation in Figure 5. Figure 5: Cartoon representation of calamitic LCs, where length(l) >> breadth(b). Calamitic mesogens usually follow the general structural formula shown in Figure.6 10

11 Figure.6 General structure of calamitic LCs. R' and R" are often flexible terminal units such that at least one R group is an alkyl chain, A, B, C, and D are used to generally describe ring systems (phenyl, cyclohexyl, heteroaromatics, and heterocycles) and [L] represents the linking units, such as CH=N, COO or N=N that can increase the length and flexibility of the molecule, whilst preserving a compatible linear shape suitable for mesophase formation. Calamitic LCs can exhibit two common types of mesophases: 1 Nematic, and 2. Smectic. Nematic phase [24-33] Figure 7. Cartoon representation of N phase The word nematic is derived from the greek word Nema meaning thread like. Under the polarsing microscope, the nematic phase is seen as thread schlieren texture. 11

12 This is the most liquid like structure in which, contrary to isotropic liquids, one or two molecular axes are oriented parallel to one another resulting in an orientational longrange order and short positional order. Molecules can rotate by both the axes, the molecules have several possibility of intermolecular mobility. Because of the high mobility, the nematic phases have low viscosities. They are anisotropic with respect to optical properties, viscosity, electrical and magnetic susceptibility, electrical and thermal conductivity. The nematic substance separate as spherical drops form the melt or solution, which coalesce to give the threaded structure. The least ordered mesophase (the closest to the isotropic liquid state) is the nematic (N) phase, where the molecules have only an orientational order. The molecular long axis points on average in one favoured direction referred to as the director (Figure 7). The classical example of LC displaying a nematic mesophase is the 5CB 1 (Figure 1). The molecules are oriented, on average, in the same direction referred to as the director, with no positional ordering with respect to each other. The molecules in the nematic phase are oriented on average along a particular direction. In consequence, there is a macroscopic anisotropy in many material properties, such as dielectric constants and refractive indices. This is the phase which is used in many liquid crystal devices (e.g., the "twisted nematic" cell), because the average orientation may be manipulated with an electric field and the polarization of light will follow the molecular orientation as it changes through a cell. Typical response times are in the millisecond range. Figure 8: (a) Schlieren texture of a nematic film with surface point defects (boojums). (b) Thin nematic film on isotropic surface: 1-dimensional periodicity. Photos courtesy of Oleg Lavrentovich (c) Nematic thread-like texture. After these textures the nematic phase was named, as nematic Photo courtesy of Ingo Dierking. 12

13 On optical examination of a nematic, one rarely sees the idealized equilibrium configuration. Some very prominent structural perturbation appear as threads from which nematics take their name. These threads are analogous to dislocations in solids and have been termed disclinations by Frank. Several typical textures of nematics are shown in Fig. (8). The first one is a schlieren texture of a nematic film. This picture was taken under a polarization microscope with polarizer and analyzer crossed. From every point defect emerge four dark brushes. For these directions the director is parallel either to the polarizer or to the analyzer. The colors are newton colors of thin films and depend on the thickness of the sample. Point defects can only exist in pairs. One can see two types of boojums with opposite sign of topological charge ; one type with yellow and red brushes, the other kind not that colorful. The difference in appearance is due to different core structures for these defects of different charge. The second texture is a thin film on isotropic surface. Here the periodic stripe structure is a spectacular consequence of the confined nature of the film. It is a result of the competition between elastic inner forces and surface anchoring forces. The surface anchoring forces want to align the liquid crystals parallel to the bottom surface and perpendicular to the top surface of the film. The elastic forces work against the resulting vertical distortions of the director field. When the film is sufficiently thin, the lowest energy state is surprisingly archived by horizontal director deformations in the plane of the film. The current picture shows a 1-dimensional periodic pattern. Many compounds are known to form nematic mesophase. A few typical examples are sketched in Fig. (9). From a steric point of view, molecules are rigid rods with the breadth to width ratio from 3:1 to 20:1. 13

14 Figure 9: Typical compounds forming nematic mesophases: (PAA) p-azoxyanisole. From a rough steric point of view, this is a rigid rod of length 20 A and width 5 A. The nematic state is found at high temperatures (between C and C at atmospheric pressure). (MMBA) N-(p-methoxybenzylidene)-p-butylaniline. The nematic state is found at room temperatures (between 20 0 C to 47 0 C). Lacks chemical stability. (5CB) 4- pentyl-4 -cyanobiphenyl. The nematic state is found at room temperatures (between 24 C and 35 C). Biaxial nematic A biaxial nematic is a spatially homogeneous liquid crystal with three distinct optical axes. This is to be contrasted to a simple nematic, which has a single preferred axis, around which the system is rotationally symmetric. The symmetry group of a biaxial nematic is D 2 h i.e. that of a rectangular right parallelepiped, having 3 orthogonal C 2 axes and three orthogonal mirror planes. In a frame co-aligned with optical axes the second rank order parameter tensor of a biaxial nematic has the form Where S is the standard nematic scalar order parameter T a measure of the biaxiality. The first report of a biaxial nematic appeared in 2004 [34, 35] based on a boomerang shaped oxadiazole bent-core mesogen. The biaxial nematic phase for this particular compound only occurs at temperatures around 200 C and is preceded by as yet unidentified smectic phases. It is also found that this material can segregate into chiral domains of opposite handedness [36] for this to happen the boomerang shaped molecules adopt a helical superstructure. 14

15 In one azo bent-core mesogen as shown below in which a thermal transition is found from a uniaxial Nu to a biaxial nematic Nb mesophase [37]. This transition is observed on heating from the Nu phase with Polarizing optical microscopy as a change in Schlieren texture and increased light transmittance and from x-ray diffraction as the splitting of the nematic reflection. The transition is a second order with low energy content and therefore not observed in differential scanning calorimetry. The positional order parameter for the uniaxial nematic phase is 0.75 to 1.5 times the mesogen length and for the biaxial nematic phase 2 to 3.3 times the mesogen length. Another strategy towards biaxial nematic is the use of mixtures of classical rod like mesogens and disk like discotic mesogens. The biaxial nematic phase is expected to be located below the minimum in the rod-disk phase diagram. In one study [38] a miscible system of rods and disks is actually found although the biaxial nematic phase remains elusive. Smectic phases[39-43] The word "Smectic" is derived from the Greek word for soap. This seemingly ambiguous origin is explained by the fact that the thick, slippery substance often found at the bottom of a soap dish is actually a type of smectic liquid crystal. Molecules in this phase show a degree of translational order not present in the nematic. Smectic phase (Liquid Crystal) retain a two dimensional order. In the smectic phase the layer of the molecules are quite flexible. Smectic phase gives focal conic texture. It extends all over the specimen and when examined under polarised light it gives a fan-like appearance. It is unaffected by magnetic and electric fields. A number of different type of smectic liquid crystals are known which differ from each other in the way of layer formation. The increased order means that the smectic state is more "solid-like" than the nematic. Smectic A, B, C, D, E, F, G, H, I. A number of different classes of smectics have been recognized. 15

16 Figure 10. Cartoon representation of (a) the SmA phase, and (b) the SmC phase. In Smectic A: It has a layer structure inside the layers, the molecules are parallel their long axes perpendicular to the plane. These are optically uniaxial and hence homeotropic texture extinguishes light between crossed polarizes. It gives focal conic texture (or batonnets). Smectic-C: (Titled) Smectic C phase is closely related to Smectic-A phase. Smectic-C is a tilted (as shown above) from Smectic-A. The major difference between the two is the tilt (inclined) of the molecular long axes with respect to the layers. This phase is optically biaxial. (Monoclinic symmetry) therefore, it is impossible to have homeotropic texture. It exhibit schlieren texture. It can also form focal conic texture. Broken fan shaped texture. In this phase the molecules are tilted with respect to the layers, and the system is now "biaxial" in character An example of a molecular structure displaying a smectic mesophase is given by the quaterphenyl derivative [44] illustrated in Figure.11, where the presence of such an extended aromatic core, characterised by a large phenyl (ph) system, is responsible for the establishment of lateral stacking interactions between adjacent molecules, resulting in a layered organisation (SmA and SmC). 16

17 Cr 1 Cr 2 Cr 3 Cr 4 SmC SmA I Figure.11 4,4"'-Bis-nonyloxy-[1,1';4',1";4",1''']quaterphenyl 2 exhibiting SmA and SmC phases. (The transition temperatures are expressed in o C). In general a smectic, when placed between glass slides, does not assume the simple form. The layers, preserving their thickness, become distorted and can slide over one another in order to adjust to the surface conditions. The optical properties (focal conic texture) of the smectic state arise from these distortions of the layers. Typical textures formed by smectics are shown in Fig. (12) [45]. (a) (b) (c) Figure 12: (a,b) Focal-conic fan texture of a smectic A liquid crystal (courtesy of Chandrasekhar S., Krishna Prasad and Gita Nair) (c) Focal-conic fan texture of a chiral smectic C liquid crystal. Smectic C* (Chiral) - Ferroelectric The nematic and Smectic-A (SmA) liquid crystal phases are too symmetric to allow any vector order, such as ferroelectricity. The tilted smectics, however, do allow ferroelectricity if they are composed of chiral molecules. The pictures below show the original ferroelectric LC[46-53], DOBAMBC. In the simplest case, the Smectic-C (SmC), the average long molecular axis is tilted from the layer normal z by a fixed angle but the molecules are free to rotate on the so-defined tilt cone. The phase has a C 2 symmetry axis perpendicular to both the 17

18 molecular director and the layer normal. The molecules exhibit a net spontaneous polarization along this axis. The magnitude of the polarization depends on temperature, generally decreasing as the tilt angle goes to zero at the SmC-SmA phase transition. The following figure shows the geometry of the chiral SmC phase. Figure 13: Chiral SmC phase: Ferroelectric liquid crystals (FLCs) also exhibit a sponteneous helixing of the polarization, so that over macroscopic distances (a few microns, say) the polarization averages to zero. Since the coupling of the polarization to applied fields is linear in the field, this means that FLCs can be made to switch quickly (typically within a few microseconds) and in a bipolar manner. This makes FLCs ideally suited to electrooptic applications. FLCs are now included in several display technologies [52, 54-57], the most popular of which use the surface stabilized (SSFLC) geometry. Surface-Stabilized Ferroelectric Liquid Crystals Although the molecular director in bulk ferroelectric liquid crystals (FLCs) adopts a helical structure, Noel Clark and Sven Lagerwall found in 1980 that by confining the LC material between closely-spaced glass plates (spaced closer than the ferroelectric helix pitch), the natural helix could be suppressed. This principle is illustrated in the polarized micrograph above, where helix lines are largely absent in the thinner (upper right) part of the cell. Clark and Lagerwall found that the smectic layers were oriented approximately perpendicular to the glass. Furthermore, they discovered that such cells could be switched rapidly between two optically distinct, stable states simply by alternating the sign of an applied electric field. The electro-optic 18

19 properties of an SSFLC depend strongly on the layer geometry as well as on the nature of the orienting properties of the bounding glass plates [53,54]. SSFLCs are being studied in many research laboratories throughout the world. They form the basis for the development of optical shutters, phase plates, and of high-resolution color displays. Antiferroelectric LCs Antiferroelectric liquid crystals are similar to ferroelectric liquid crystals, although the molecules tilt in an opposite sense in alternating layers as show in figure. In consequence, the layer-by-layer polarization points in opposite directions. These materials are just beginning to find their way into devices, as they are fast, and devices can be made "bistable"[58-64]. Cholesteric Phases (Chiral nematic) The cholesteric (or chiral nematic) liquid crystal phase is typically composed of nematic mesogenic molecules containing a chiral center which produces intermolecular forces that favour alignment between molecules at a slight angle to one another[65-72]. This leads to the formation of a structure which can be visualized as a stack of very thin 2-D nematic-like layers with the director in each layer twisted with respect to those above and below. In this structure, the directors actually form in a continuous helical pattern about the layer normal as illustrated by the black arrow in the following figure and animation. The black arrow in the animation represents director orientation in the succession of layers along the stack. 19

20 Nematic Chiral Nematic Fig.14 The molecules shown are merely representations of the many chiral nematic mesogens lying in the slabs of infinitesimal thickness with a distribution of orientation around the director. The phase was first observed in cholesterol derivatives, hence it is known as cholesteric phase. Various colour changes can be observed by winding or unwinding the helix. This can be done by means of changing temperature, mechanical disturbance like pressure or shear. Liquid Crystals of this type is mostly optically active. The cholesteric liquid crystals are optically uniaxial with negative character, it can scatter the light to give bright colour and it shows strong rotalory power. Three type of texture are generally observed in cholesteric phases. 1. Focal conic texture 2. Planar texture and 3. Blue phase (N*-Phase). Pitch: An important characteristic of the cholesteric mesophase is the pitch. The pitch, p, is defined as the distance it takes for the director to rotate one full turn in the helix as illustrated in the above animation. A byproduct of the helical structure of the chiral nematic phase, is its ability to selectively reflect light of wavelengths equal to the pitch length, so that a color will be reflected when the pitch is equal to the corresponding wavelength of light in the visible spectrum. The effect is based on the temperature dependence of the gradual change in director orientation between successive layers (illustrated above), which modifies the pitch length resulting in an alteration of the wavelength of reflected light according to the temperature. The angle at which the 20

21 director changes can be made larger and thus tighten the pitch, by increasing the temperature of the molecules, hence giving them more thermal energy. Similarly, decreasing the temperature of the molecules increases the pitch length of the chiral nematic liquid crystal. This makes it possible to build a liquid crystal thermometer that displays the temperature of its environment by the reflected color. Mixtures of various types of these liquid crystals are often used to create sensors with a wide variety of responses to temperature change. Such sensors are used for thermometers often in the form of heat sensitive films to detect flaws in circuit board connections, fluid flow patterns, condition of batteries, the presence of radiation or in novelties such as "mood" rings. Figure 15: (a) Cholesteric fingerprint texture. The line pattern is due to the helical structure of the cholesteric phase, with the helical axis in the plane of the substrate. Photo courtesy of Ingo Dierking. (b) A short-pitch cholesteric liquid crystal in Grandjean or standing helix texture, viewed between crossed polarizers. The bright colors are due to the difference in rotatory power arising from domains with different cholesteric pitch occuring on rapid cooling close to the smectic A* phase where the pitch strongly diverges with decreasing temperature. Photo courtesy of Per Rudqvist. (c) Long-range orientation 21

22 of cholesteric liquid crystalline DNA mesophases occurs at magnetic field strengths exceeding 2 Tesla. The image presented above illustrates this long-range order in DNA solutions approaching 300 milligrams per milliliter. Parallel lines denoting the periodicity of the cholesteric mesophase appear at approximately 45-degrees from the axis of the image boundaries. Discotic LCs In 1977, a second type of mesogenic structure, based on discotic (disc-shaped) molecular structures was discovered. The first series of discotic compounds to exhibit mesophase belonged to the hexa-substituted benzene derivatives 1 (Figure 16) synthesised by S. Chandrasekhar et al. [73-76] Figure 16. Molecular structure of the first series of discotic LCs discovered: the benzenehexa-n-alkanoate derivatives. Similarly to the calamitic LCs, discotic LCs possess a general structure comprising a planar (usually aromatic) central rigid core surrounded by a flexible periphery, represented mostly by pendant chains (usually four, six, or eight), as illustrated in the cartoon representation in Figure 17. As can be seen, the molecular diameter (d) is much greater than the disc thickness (t), imparting the form anisotropy to the molecular structure[77-85]. Figure 17. Cartoon representation of the general shape of discotic LCs, where d>>t. 22

23 Discotic LCs, as well as calamitic LCs, can show several types of mesophases, with varying degree of organisation. The two principle mesophases are: 1. Nematic discotic and 2. Columnar. Nematic discotic phase Nematic discotic (ND) is the least ordered mesophase [77], where the molecules have only orientational order being aligned on average with the director as illustrated in figure 18. There is no positional order. Figure 18. Cartoon representation of the ND phase, where the molecules are aligned in the same orientation, with no additional positional ordering. Columnar phases Disk-shaped mesogens can orient themselves in a layer-like fashion known as the discotic nematic phase. If the disks pack into stacks, the phase is called a discotic columnar. The columns themselves may be organized into rectangular or hexagonal arrays [86], see Fig. (21). 23

24 Chiral discotic phases, similar to the chiral nematic phase, are also known. The columnar phase is a class of liquid-crystalline phases in which molecules assemble into cylindrical structures to act as mesogens. Figure 19: (a) hexagonal columnar phase Colh (with typical spherulitic texture); (b) Rectangular phase of a discotic liquid crystal (c) hexagonal columnar liquid-crystalline phase. Figure 20: Typical discotics: derivative of a hexabenzocoronene and 2,3,6,7,10,11- hexakishexyloxytriphenylene. K(70K) Colh(100K) I. Originally, these kinds of liquid crystals were called discotic liquid crystals because the columnar structures are composed of flat-shaped discotic molecules stacked one-dimensionally. Since recent findings provide a number of columnar liquid crystals consisting of non-discoid mesogens, it is more common now to classify this state of matter and compounds with these properties as columnar liquid crystals. Figure 21: (1) Columnar phase formed by the disc-shaped molecules and the most common arrangements of columns in two-dimensional lattices: (a) hexagonal, (b) 24

25 rectangular, and (c) herringbone. (2,3) MD simulation results: snapshot of the hexabenzocoronene system with the C12 side chains. Aromatic cores are highlighted. Both top and side views are shown. T = 400 K, P = 0.1MPa. [78] Columnar liquid crystals are grouped by their structural order and the ways of packing of the columns. Nematic columnar liquid crystals have no long-range order and are less organized than other columnar liquid crystals. Other columnar phases with longrange order are classified by their two-dimensional lattices: hexagonal, tetragonal, rectangular, and oblique phases. The discotic nematic phase includes nematic liquid crystals composed of flat-shaped discotic molecules without long-range order. In this phase, molecules do not form specific columnar assemblies but only float with their short axes in parallel to the director (a unit vector which defines the liquid-crystalline alignment and order). In the years following the discovery of the first discotic mesogens, further investigations lead to the synthesis of a vast number of new discotic LCs [87-98] Figure 22. Molecular structure of some discotic mesogens: 2,3,6,7,10,11- hexakishexyloxytriphenylene 3 [87-93], 3,10-dipentylperylene discogen derivative 4 [94], 2,3,7,8,12,13-hexakispentyloxy-10,15-dihydro- 5H-tribenzo [a,d,g] cyclononene 25

26 (bowl-shaped discotic) 5 [95-97], porphyrin metallomesogen 6 [98]. (The transition temperatures are expressed in o C, and the mesophase in brackets represents a monotropic transition). Polycatenar LC's Polycatenar mesogens [99-102] represent a hybrid class of thermotropic LCs, which can be described with intermediate molecular features between classical rod-like and disc-like mesogens. Schematically the central core of polycatenar LCs comprises a calamitic region, with half-discs on the extremities (Figure 23). This hybrid molecular structure allows both calamitic and columnar phases to be generated, depending on the specific molecular structure of the components. Figure 23. Cartoon representation of the architectural molecular structure of polycatenar LCs. Polycatenar molecules possess a number of flexible alkyl chain substituents, which varies from two to six (bi- to hexa-catenar compounds). Bi- catenar LCs are in most of the cases classical rod- like molecules, like compound 7 (Figure 24). Examples of bi-, tri-, tetra- and hexa-catenar LCs are shown in Figures [ ]. Figure 24. Molecular structure of two bi-catenar mesogens 4-pentyl-4'-pentyl biphenyl 7 and 4'-[(3'', 4 -bis-hexyloxy-benzylidene)-amino]-4-carbonitrile 8. 26

27 Figure 25. Molecular structure of a tri-catenar mesogen. Figure 26. Molecular structure of tetra-catenar mesogens 2, 2 -bipyridine derivative 10 and liquid crystalline 3, 4-dioctyloxystilbazole silver complex 11. Figure 27. Molecular structure of a hexa-catenar mesogen 12. Compound 8 and 9 show close similarity to the class of LCs named swallow-tailed LCs and compounds 10 and 11 show similarity to the bi-swallow-tailed LCs Twist-Grain boundary phase (TGB) The TGB phase was Proposed by Renn and Lubensky [107] and discovered by Goodby et al. [108,109]. These are smectic phase where arrays of defects from part of the ordered structure. The nature of the SmC and SmC* TGB structures and their relationship to larger scale superstructures are still open issues[110,111]. 27

28 Banana-shaped LCs Banana-shaped LCs are similar to calamitic LCs [ ], but contain a molecular kink.. They have an elongated shape, with the molecular length being significantly larger than the molecular breadth. These LCs as well as generating the mesophases associated with calamitic LCs, generate a set of their own mesophases. These mesophases have been given the nomenclature of B 1 -B 7 depending on the order of discovery. They are closely related to the smectic phases, for instance in the B 2 mesophase the molecules are tilted as in the SmC mesophase but also resemble to the SmA phase. In smectic liquid crystals Banana-shaped molecules on smectic layers (smectic C ) have a spontaneous polarization, P. Liquid crystal 'blue phases' recent advances Liquid crystal 'blue phases' are highly fluid selfassembled three-dimensional cubic defect structures that exist over narrow temperature ranges in highly chiral liquid crystals[ ]. The characteristic period of these defects is of the order of the wavelength of visible light, and they 28

29 give rise to vivid specular reflections that are vivid specular reflections that are controllable with external fields. Blue phases may be considered as examples of tuneable photonic crystals with many potential applications. The disadvantage of these materials, as predicted theoretically and proved experimentally, is that they have limited thermal stability: they exist over a small temperature range (0.5 2 C) between isotropic and chiral nematic (N*) thermotropic phases, which limits their practical applicability. Effect of Chemical Constitution on Mesomorphism: Most of the rod-like liquid crystalline compounds consist of two or more rings, which are directly bonded to one another or connected by linking groups. The chemical structure of many mesogens can be represented by the general formula-i R 1 O L 1 O L 2 O R 2 Z 1 Z 2 Z 3 Here L is linking group. The molecule may have terminal substituent (R) and lateral substituent (Z). O-Aromatic/Alicyclic/Heterocyclic rings/ cores. The core is usually a relatively stiff unit, compared to the terminal lateral substituents in most cases are small units such as halogens, methyl, methoxy, hydroxy, cyano groups etc. however, now liquid crystals with long lateral substituents are known. Effect of Core: The major anisotropy of molecules, which is necessary for their mesogenity, results from the cores, which are also responsible for relatively high melting temperatures. The core consists of rings that are connected to one another either directly or by linking groups. Any ring that allows a stretched configuration of the molecules can be used. More complex ring systems cholesterol is also used. The oldest known liquid crystal have benzene rings as core. The increase in number of benzene rings generally results in the increase of melting temperatures. Also, the mesogenity of the compound increases with the number of linearly connected rings. Due to the large conjugated aromatic- systems, the intermolecular attractions of the molecules are very large giving rise to high melting temperatures. 29

30 N N > > N > N N > N N > N N N > N N N N Decrease in mesogenity Influence of nitrogen substituents on mesogenity. The cyclohexane ring is non-aromatic and flexible compared to benzene ring. The flexibility of the central ring has some negative influence on mesogenity. Bicyclooctane derivatives have much stronger nematogenity as compared to the cyclohexane core. Effect of Linking Groups: Small chemical groups between the rings of liquid crystal molecule can increase the length of the molecule while preserving the linear shape. However, when the linking groups produce a bent molecular shape, the mesogenic potential of the molecule is diminished. Besides the geometry of the molecules, additional effects such as conjugative interaction of the linking groups with aromatic groups, effects due to polarity of the linking groups etc. also play an important role in liquid crystalline of a molecule. The effects of linking groups can be quite different in aromatic and non-aromatic compounds, as in the case of non- aromatic compounds, there are no conjugative effects, however, the effect of terminal substituents may sometimes overcome this effect. Effect of Terminal Substituents: Terminally substituted compounds exhibit more stable mesophases compared to unsubstituted mesogenic compounds. The most common terminal substituents are the alkyl and alkoxy groups. The behavior within the homologous series shows that in general there is an alteration of T N-I temperatures. This can be explained by the alteration of the length to breadth ratio. Fig.28 shows a typical six-member ring, with an attached alkyl chain. 30

31 Fig.28: Alteration effect in a terminal alkyl chain The attachment of an odd numbered carbon atom substituent increases the length to breadth ratio more than does the attachment of an even numbered carbon atom substituent. This principle behavior seen in alkyl chain can also be found in other flexible chains. X-rays and other methods have been used to show that compounds containing strongly polar groups like -CN and -NO 2 from double molecules that exist in equilibrium with single molecules [ ]. Due to such dimerization, the breadth increases by the factor of 2 and length only by a factor of 1.1~1.4. Hence, the effective L/B ratio should be reduced. But, highly polar compounds have a much higher density than low polar compounds [130,131]. This accounts for the increase in clearing temperature. The halogens and isothio-cyanato groups introduce relatively large positive dielectric anisotropy into the molecules however, there is no association [132]. Branched terminal substituent also affects mesomorphism. The effect of a branch depends substantially on its position in a chain. When the branch is nearer the centre of the molecules the clearing temperature is lowered. When -CH 2 group in the terminal chains are replaced by an oxygen atom, clearing temperature decreases. Oxygen atom seems to reduce the stiffness of the chain. The terminal group efficiency order which has been compiled for Smectic phase in rod-like aromatic system is: -Ph > -Br > -Cl > -F > -NMe 2 > -Me > -H >-NO 2 > -OMe > -CN and the nematic group efficiency order is, -Ph > -NHCOCH 3 >-CN > -OCH 3 >-NO 2 > - Cl > - Br > -N (CH 3 ) 2 > -CH 3 > - F. Intermolecular Hydrogen Bonding Intermolecular hydrogen bonding interactions have shown great potential in the preparation of new liquid crystalline systems especially thermotropic LCs [133, 134]. They have been used as links, connecting two independent molecular components. These 31

32 form anisotropic molecules, which complies with the main characteristic of liquid crystal molecules. Most of these systems are based on pyridine and acid derivatives [135]. The hydrogen bond in the liquid crystal field enables molecular components that do not themselves exhibit the property, form supramolecular species, which show the liquid crystal behaviour. Also these liquid crystal moieties have greatly enhanced mesomorphic range [136]. Physical properties of liquid crystals Fig. 29: physical properties of liquid crystals As a result of orientational order, most physical properties of liquid crystals are anisotropic[ ] and must be described by second rank tensors. Examples are the heat diffusion, the magnetic susceptibility, the dielectric permittivity or optical birefringence[140]. Additionally, there are new physical qualities, which do not appear in simple liquids as e.g. elastic or frictional torques (rotational viscosity) acting on static or dynamic director deformations, respectively. The most remarkable features of liquid crystals with respect to applications are due to their anisotropic optical properties. Nematics, and SmAs are uniaxial, SmCs weakly biaxial. Cholesterics give rise to Bragg reflections if the helix pitch is in the 32

33 magnitude of the light wavelength. As mentioned above these properties are carried by a fluid, soft material, and therefore are extremely sensitive against external perturbations. Orientational order and hence birefringence can be manipulated easily e.g. with the help of rather weak magnetic, electric or optical fields, leading to huge magneto-optical, electro-optical and opto-optical effects [141,142]. The most successful application are liquid crystal displays well-known from wrist watches, pocket calculators or flat screens of laptop computer which take advantage of electro-optical effects. More recently, it turned out that orientational order can be also affected by optical fields leading to rather sensitive opto-optical effects and nonlinear optical properties, which are important e.g. for all-optical switching and other photonic devices in future optical information technologies [143, 144]. Birefringence in Liquid Crystals Liquid crystals are found to be birefringent, due to their anisotropic nature. That is, they demonstrate double refraction (having two indices of refraction). Light polarized parallel to the director has a different index of refraction (that is to say it travels at a different velocity) than light polarized perpendicular to the director. In the following diagram, the blue lines represent the director field and the arrows show the polarization vector. Thus, when light enters a birefringent material, such as a nematic liquid crystal sample, the process is modeled in terms of the light being broken up into the fast (called the ordinary ray) and slow (called the extraordinary ray) components. Because the two components travel at different velocities, the waves get out of phase. When the rays are recombined as they exit the birefringent material, the polarization state has changed because of this phase difference. 33

34 Light traveling through a birefringent medium will take one of two paths depending on its polarization. Liquid Crystal Textures The term texture refers to the orientation of liquid crystal molecules in the vicinity of a surface. Each liquid crystal mesophase can form its own characteristic textures, which are useful in identification. We consider the nematic textures here. If mesogenic materials are confined between closely spaced plates with rubbed surfaces (as described above) and oriented with rubbing directions parallel, the entire liquid crystal sample can be oriented in a planar texture, as shown in the following diagram. Mesogens can also be oriented normal to a surface with the use of appropriate polymer films or in the presence of an electric field applied normal to the surface, giving rise to the homeotropic texture, as illustrated below. Experimental Identification of Liquid Crystals Liquid crystal phases can be identified by a variety of techniques [145] like optical polarizing microscope, differential scanning calorimetry, X-ray analysis, miscibility studies, neutron scattering studies, cryo-transmission electron microscopy [146], nuclear magnetic resonance[147] and fabry-perot Scattering studies [148]. A few of these are described here. 34

35 Differential Scanning Calorimetry (DSC) Heat is needed to melt a crystalline solid to a liquid crystalline phase. The heat is measured using a DSC instrument. Although DSC cannot identify the type of phase, it provides valuable information like the exact transition temperatures and the enthalpies of the different phases [149]. Polarizing Microscope In a polarising microscope, the light is polarized by passing it through a polarizing filter. It then passes through the sample, and then through a second polarizing filter called the analyzer. When a liquid crystal material is placed on a microscope slide with a cover slip and the slide is heated and viewed using a polarizing microscope, textures characteristic of each type of liquid crystal can be seen. Cooling the liquid can also yield these textures when liquid crystal phases are present [44]. X-ray Crystallography This can be used to study the extent of translational or positional order, and thus infer the type of liquid crystal phase [150]. Extended X-ray absorption fine structure spectroscopy(exafs) EXAFS was used to investigate the local structure of the polar spines of metal ion soaps in the columnar liquid crystalline state [151]. Applications of liquid crystals Display application of liquid crystals The most common application of liquid crystal technology is liquid crystal displays (LCDs.)[ ] This field has grown into a multi-billion dollar industry, and many significant scientific and engineering discoveries have been made. Liquid crystal display devices consisting of digital readouts are used in watches, calculators, and several other instruments like mobile and many household electric appliances [158]. Some liquid crystal substances could be useful in computer industry, for making new computer elements with high memory capacity. 35

36 Liquid crystals displays (LCDS) [159] had a humble beginning with wrist watches in the seventies.continued research and development in this multidisciplinary field have resulted in display with increased size and complexity.after three decades of growth in performance, LCDs now offer a formidable challenge to cathod ray tubes (CRT). Liquid crystal display (LCDs) have many adventages over other display types. They are flate and compact, posses extremely low power consumption (Microwats per square centimeter in the case of the twisted Nematic display), their colour and contrast does not fade with an increase in the illumination intensity. They work both in transmitive and reflective modes in a wide operating temperature range and with a long life time. Because that, LCDs are the most economically produced display systems. LCDs have a brilliant future in high defination TV system, personal computer, measuring devices etc. The most widely used electro optics effects in display are the twist, super twist and guest host modes. There are many types of liquid crystal displays, each with unique properties. The most common LCD that is used for everyday items like watches and calculators is called the twisted nematic (TN) display. This device consists of a nematic liquid crystal sandwiched between two plates of glass. A special surface treatment is given to the glass so that the director at the top of the sample is perpendicular to the director at the bottom. This configuration sets up a 90 degree twist into the bulk of the liquid crystal, hence the name of the display. The underlying principle in a TN display (shown below) is the manipulation of polarised light. The left image shows that when light enters the TN cell, the polarisation state twists with the director of the liquid crystal material. For example, consider light polarised parallel to the director at the top of the sample. As it travels through the cell, its polarisation rotates with the molecules. When the light emerges, its polarisation has rotated 90 degrees from when it entered. 36