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Fredrich Reinitzer Cholesteryl benzoate

Liquid crystals are used in panel displaysin laptops, television teleconferencingsystems, portable and high-definitiontelevision (HDTV), and video games

Liquid Crystals: Crystals in Liquid Phase or Crystalline Liquid?

By Clare Teo and Tay Li Kim

You often come across liquid crystal displays (LCDs): the numbers displayed on a digital watch, theHigh definition LCD televisions or even laptop. LCDs are very prevalent but they have only been aroundfor around thirty years. The Britannica Concise Encyclopedia defines a liquid crystal as a substance

that flows like a liquid but maintains some of the ordered structure characteristic ofa crystal. Liquidcrystals constitute an interesting state of matter with properties intermediate between those of trueliquids and those of crystals. Unlike glasses, liquid-crystal states are thermodynamically stable.

Although this is a relatively rare phenomenon, hundreds of organic substances possess anintermediate phase or even a succession of distinct phases. These are called mesophases ormesomorphic phases, and the substances themselves are usually called liquid crystals.

A Brief History of Liquid CrystalsAn experiment in 1888 by an Austrian botanist, Friedrich Reinitzer on cholesteryl benzoate crystals ledto the discovery of liquid crystals. When cholesteryl benzoate crystals were heated to 145.5 oC,

Reinitzer observed that a viscous and cloudy liquid. However, on raising the temperature to 178.5o

C,the liquid became transparent. On cooling, the exact opposite transformation took place. Thesechanges were accompanied by an abrupt change in volume and by an absorption or emission of heat.It was only in the later half of the 20 th century that the first stable, room temperature liquid crystal wasmade and there was no looking back.

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Applications of Liquid CrystalsLiquid crystals have many applications. They are used as displays in digital wristwatches, calculators,and panel meters. They can be used to record, store, and display images which can be projected ontoa large screen. Direct and active-matrix liquid-crystal displays (LCDs) are used in several areas from

laptop computers to communication equipment such as television teleconferencing systems, portableand high-definition television (HDTV), and video games.

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Liquid crystal thermography can be used as a safe and effective means for evaluation of complex painstates associated with arthritis, soft tissue injuries and back-pain diseases. It facilitates identification ofthe damage to the nervous system. In case of nerve injuries, a temperature increase is observed in thearea of the nerve distribution, which becomes colder during later stages of the injury. The techniquehas proved useful in studying hemophilia, tumors of the extremities, arthritis and vascular conditions,including the complications of diabetes.

Thermochromic Liquid Crystals (TLC's) are highly sensitive materials that respond to temperature.Liquid crystals change from black to a rainbow of colors and then back to black again upon heating.Upon cooling the reverse color change occurs. They became popular with the well-known mood ringand are now available in a wide variety of useful and exciting products.

There are some limitations to using liquid crystals. Unfortunately, liquid crystals are very difficult towork with and require highly specialized printing and handling techniques. Liquid crystals are alsomore expensive than other thermochromic inks. They are adversely effected by high temperatures,ultra violet light, and strong solvents or chemicals.

Structure of liquid crystals

Molecules in condensed states of matter are packed closely together and thus they tend to alignthemselves parallel to one another. This is what happens in liquid crystals due to specificintermolecular interactions that favour this tendency. Thermal agitation opposes this, resulting in theactual structure of liquid crystals being a delicate balance of the two effects. Hence the structure canchange with a variation of only a few degrees.

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Clinical thermography, otherwise known as infrared imaging, is based ona careful analysis of skin surface temperatures as a reflection of normalor abnormal human physiology. The procedure is commonly performedeither by the direct application of liquid crystal plates to a part of the body,or via an ultra-sensitive medical infrared camera through a sophisticatedcomputer interface.

Thermochromic Liquid Crystals (TLC's) respond to

temperature changes by changing colours fromblack to a rainbow of colors and then back to blackagain upon heating. Thus they became popularwith the well-known mood ring and key chains

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In order for a molecule to display the characteristics of a liquid crystal, it must be rigid and rod-shaped.This is accomplished by the interconnection of two rigid cyclic units. The interconnecting group shouldcause the resulting compound to have a linear planar conformation. Linking units containing multiplebonds such as -(CH=N)-, -N=N-, -(CH=CH)n-, -CH=N-N=CH-, etc. are used since they restrict thefreedom of rotation. These groups can conjugate with benzene rings, enhancing the anisotropic(directionally dependent) polarisability. This increases the molecular length and maintains the rigidity.An example is terephthal-bis-(4-n-butylaniline), called TBBA, whose molecular structure can berepresented as

When melting occurs, the orderly structure in a substance normally disappears at the melting point asthermal motion of particles begins to predominate. When a mesophase exists, the orderliness onlypartiallydisappears. Several intermediate structures are observed.

In the first type, the layers of molecules persist since the centers of the molecules tend to lie in layers.However, these layers of molecules can slide over one another, thus the order between any twosuccessive layers is destroyed. Within each layer, these centers are distributed randomly as in anordinary liquid. This type of mesophase is described as smectic, derived from the Greek word meaningsoap because the consistency of these phases is reminiscent of that of a soft soap.

The second type of mesophase is called nematic where the degree of order here is less than that ofsmectic phase. The molecules are no longer in arranged in layers but they display a preferredorientation. The centers of the molecules are in completely random positions as do those of a normalliquid.

Some substances pass from the crystalline state through the smectic phase to the normal isotropic

liquid state; others pass through the nematic phase instead. Other liquids follow the sequencecrystalsmecticnematicisotropic liquid. A variety of nematic liquid phase is known as cholestericwhere the molecules have a special property: chirality. This means that the molecules are asymmetricain such a way that they cannot be made to conincide with their image in a plane mirror by simplemovements like displacement or rotation. It is analogous to a right hand which can be considered asthe image of a left-hand but will not fit into a left-hand glove.

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In a sheet of perfectly oriented nematic phase, the molecules have their axes parallel to the walls of theglass slides and thus have the same direction. However, in a cholesteric, the asymmetry of themolecules causes those in one layer to make a small angle with those in the adjacent one. The processis repeated from one layer to another, so that in the whole specimen, the molecules are turned throughan angle proportional to the distance from the solid surface. The structure is therefore helical and ischaracterized by the distance between two layers that have the same direction, between which the

molecules have rotated through 180.

The helical structure gives cholesterics very special optical properties-enormous optical rotatory power

which is several thousand degrees per millimeter or one hundred times greater than that of commonoptically active solids and liquids. For a given angle of incidence, there is a wavelength which isreflected much more than the others, so that when illuminated by white light, the sheet appearedcoloured. For instance, a sheet appears blue under grazing incidence but green or yellow when theangle of incidence is closer to the normal. In addition, the colour that is observed depends on theperiodicity and hence on the temperature with which it therefore varies. The colour change is quite

appreciable with a variation of only 0.1C.

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Twist of the liquid crystal director. Chiralliquid crystals spontaneously form twist inthe nematic phase and a precession of tiltin the Smectic C phase.

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Operations of LCDs

In summary, liquid crystals provide remarkable examples of the advances made in our knowledge ofthe structure of matter only very recently, but achieved using relatively simple and well-knownexperimental techniques. Due to its developed technological applications, liquid crystals have beennoted to be the phase of the future.

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Light coming from the right of the assembly is polarized bfilter, it then passes through glass plates enclosing a liqucrystal cell, a second polarizer rotated at 90 degrees, andfinally a color filter. Depending on the orientation of the liqcrystal, more or less light can get through the pixel.

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Review Questions

1 Which of the following chemical compounds can form liquid crystals?

A B

C D

2 Which of the following is an example of a chiralmolecule that occurs in the cholesteric phase?

A CH3CH(CH2CH3)CH3 B CH3C(CH3)2CH3C CH3CH2CH(CH3)CH2CH2CH3 D CH3CH2CH(CH2CH3)CH2CH2CH3

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Referenceplc.cwru.edu/tutorial/enhanced/files/lc/apps/Apps.htmwww.colorchange.com/Liquid%20Crystal-APP.htmwww.pubmedcentral.nih.gov/plc.cwru.edu/tutorial/enhanced/files/lc/chem/chem.htmPrinciples of Modern Chemistry by Oxtoby & MachtriebThe Structure of Matter by Andre Guinier

Ans : 1A 2C

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