Synthesis and Properties of Liquid Crystals for VAN

Synthesis and properties of liquid crystals for vertically aligned nematic (VAN) displays

Introduction

Liquid crystals where first observed in 1853 and 1855 by Rudolph Virchow and C. Mettenheimer respectively, both observed a flowing fluid like compound which was birefringent (brightly coloured) between cross-polarisers much like a crystalline solid, hence the compound was both liquid and crystal thus liquid crystal. [3, 4]. It was not until the late 1980’s that Liquid crystals and their truly fascinating fundamental properties began their tremendous success in commercial applications.1–5 Subsequently giving rise to the vertically aligned nematic (VAN) mode in the early 1990’s. The liquid crystalline phase can be best described as a hybrid of the two most common phases of matter, Liquids and Crystals. LC compounds diffuse about much like the molecules of a liquid giving them a fluid nature, combined with this they manage to maintain a small magnitude of orientational order and sometimes some positional order in a similar manner as a crystalline solid would. Hence, liquid crystals are anisotropic fluids.

Properties of Liquid crystals and the nematic mesophase

The nematic phase of calamitic (rod like) liquid crystals is the simplest liquid crystal phase. In this phase the molecules maintain a preferred orientatioanl direction as they diffuse throughout the sample. There is no positional order in the phase as depicted by figure 1.1.

Synthesis of Liquid Crystals

General Synthesis

Generally, the most common liquid crystals are based on aromatic sub units due to their ease of synthesis and obtainability. The vast majority of LC building blocks are commercially accessible or fairly simple to synthesize via electrophilic substitutions such as Friedel-Crafts acylation, bromonation and nitration. For those functional groups that cannot be directly substituted interconverions usually take place with bromine often being the chosen leaving group (e.g., CO2H, NH2, CN and OH). Due to the individual nature of substituents their specific directing effect and a specific effect on the rate of reaction must be taken into consideration. By taking this into account reactions must be carried out in the appropriate order to arrive at the desired product.

Figure 1 – Electrohpilic Substiutions of Benzene

A key advancement in synthesis arrived with the recognition that a wide range of intermediates could be efficiently prepared from alkyl-bromo-benzenes due to the ease of conversion of the bromo substituent into a previously inaccessible groups. From a range of synthetic methods described in scheme 1 a valuable number of carboxcylic acids and phenols can be prepared. This follows on to the synthesis of multi-aryl LC materials where esterification (see Scheme 2) is employed to couple multiple aryl units. Esterification commonly occurs in two processes firstly, the traditional method (Method A) of converting the carboxcylic acid into the acid chloride derivative with either thionyl chloride or oxalyl chloride. The acid chloride is then reacted with the phenol in the presence of a base to remove the hydrogen chloride as it is formed. The second and more recent method (Method B) involves an in-situ reaction which uses N,N-dicyclohexylcarbodiimide (DCC) to activate the acid towards nucleophilic attack from the phenol and a proton transfer catalyst ( 4-(N,N-dimethylamino)pyridine ) (DMAP).

Scheme 2 – Esterification coupling reaction

LC materials with multiaryl cores (e.g., biphenyls and terphenyls) are somewhat more difficult to produce due to the direct bond between aryl sections. However, the development of palladium-catalysed cross-coupling reactions has created a means in which to form the direct carbon-carbon bonds needed. There are a vast number of methods to facilitate the generation of these carbon-carbon bonds but by far the most prolific involves the use of aryl bromides (4) and arylboronic acids (5).

Figure 3 – Palladium catalysed cross-coupling

Alternative to the use aryl bromides are the aryl iodides, there increased stability as a leaving group provide a reaction pathway with an increased rate of reaction. Chloro and triflate are also other viable leaving groups, where the triflate group is essential in the synthesis of alkenyl-substituted LCs. Perhaps the most important palladium-catalysed cross-coupling reaction is the selective coupling that can occur by using a bromo-fluoro-iodo-substituted system (see Scheme 4)

Figure 4 – Dicouplong reactions of Benzene derivatives

As the iodo group is a better leaving group it can be coupled with an arylboronic acid, following purification a second coupling reaction can occur on the bromo site giving rise to the synthesis of LC materials with more than two aromatic core units. In order to control the mesomorphic and physical properties of LC lateral substitutions are often employed, the fluoro substituent is the most commonly used lateral unit, as it is electron withdrawing in nature it renders adjacent H atoms acidic and thus making them vulnerable to strong basic conditions. By taking advantage of this vulnerability the desired functional groups for example the boronic acids needed for cross-coupling reactions are far more easily obtained. The only consistent approach for introducing a fluoro substituent into an aromatic system is via the diazotisation and successive fluoronation of the chosen aromatic amine, which in turn generated from the reduction of the nitroarene generated from the nitration of the basic aryl unit. Nonetheless, a broad variety of simple fluoro-substituted materials can be easily acquired commercially and thus synthesis often begins with fluro substituents already present (see Scheme 5).

Unfortunately this gives rise to complications when trying to introduce terminal alkyl chains to the fluorinated compounds. Accordingly, a different approach is required and thus bromo-fluoro-iodo-benzene units are needed for successful synthesis of fluoro-substituted LC materials.

Scheme 4 shows some reactions of these units to synthesis some adavance LC materials.

The finishing touches

Liquid crystals for VAN mode displays must have one vital property in order to be considered for this application, negative dielectric anisotropy. Negative dielectric anisotropy can be introduced by creating a strong lateral dipole within the LC material this is done by introducing lateral groups with high electronegativity such as fluorine as explained previously in this section, lateral chloro substitutents have also been considered in order to create negative dielectric anisotropy as they create a greater dipole than fluorine. However, the greater size of the chloro substituent renders it of little use as this subsequently gives the material low liquid crystal phase stability and high viscosity making it useless in VAN mode displays.

Figure 5 – Subsitution reactions of difluroaryl compounds

Vertically aligned nematic (VAN) liquid crystal displays

About the VAN displays

The vertically aligned nematic (VAN) mode first came into development in the early 1990’s, first generation LC materials were based on rod like molecular structures and managed to achieve fast switching times of around 25ms. Unfortunately, the early attempts to introduce displays of this kind failed. This was for two major reasons, a switching time of <16 ms needed to be achieve otherwise the moving picture was blured. Secondly the contrast of such displays was far too low for use in LCD televisions and PC monitor displays. It was not until 1998 that a monitor display was successfully introduced. This new found success hinged on the synthesis of two new classes of LC materials based on the use of lateral fluoro substituents to produce large negative dielectric anisotropies within the molecules. Along with this the use of lateral fluoro substituents brought with it high stability of the neamtic phase at temperature of less than -20c, optical anisotropy, viscosity and very crucially reliability properties such as lifetime and stability.

What makes up a VAN display?

VAN devices are made up of two parallel glass plates separated by a small gap of 3-10µm containing the nematic liquid crystal phase, on the top piece of glass sit a thin film of material which polarises a light that passes through it. On the inside of the top piece of glass there is a indium oxide (ITO) layer which acts as a conductor, this layer is linked to a surfactant. The inner layer of the bottom piece of glass is also coated with the ITO layer and the surfactant. The surfactant enables the liquid crystal to be connected with the conductor thus enabling the flow of a current. The display can be designed to be either passive or active. When passive the display does not generate any light itself it instead uses ambient light from surroundings which is reflected by a mirror like surface below the bottom piece of glass. When designed to be active the display is built with a light source behind the display which passes directly through the display rather than being reflected

Working principle of VAN displays

The average molecular orientation (director orientation) without the electric field is perpendicular to the substrate of the display. With this homeotropic orientation and crossed polarizers, the VA mode is working in the so called normally black mode. For the incident light the liquid crystal in the off state behaves like an isotropic medium (the light sees only the ordinary refractive index). As a consequence very good black states can be achieved independent of the wavelength of the light and the operating temperature. Pixel and electrode design of VA displays allow for a high aperture ratio resulting in a high brightness of the display. These two points are the main reason for the good contrast of VA LCDs.. Since the directors are oriented homeotropically in the off state, they can be tilted randomly in any direction by the electric field. This leads to disclination lines between domains of equal orientation, thus deteriorating the optical performance.

Figure 6 – VA Mode working display

As VAN displays use LC materials with negative dielectric anisotropy, application of a voltage to the ITO films cause the director to tilt away from the normal to the glass surfaces as show in figure 2. This introduces a birefringence because the index of refraction for light polarised parallel to the director is different from the index of refraction for light polarised perpendicular to the director. Some of the resultant elliptically polarised light (all of it if the retardation is 180) passes through the crossed polariser and the display appears bright. In fact, since the retardation depends on the magnitude of the voltage applied to the display, this type of display can be used to produce a range of intensities of light. This is called a grey scale. For VA you have perfect black in the off-state and if apply a voltage the VA materials moves into the parallel position and this is bright. Therefore, you get a better contrast ration in VA displays. The second advantage is the switching process. It’s intrinsically faster to move the molecules this way.