Towards Engineering and Understanding of Guest Host Interaction Between Dopants and Liquid Crystals in Liquid Crystal Displays

Abstract

Liquid crystal displays are intricate devices which consist of many cells that are filled with liquid crystal hosts. The operation of the liquid crystal cell is to modulate the polarisation of light, by varying their birefringence, which in turn can be used to control the intensity of light and colour as a function of time. Many individual cells grouped together can be controlled to give specific intensity of light and colour, to build up images that are viewed on displays, i.e. pictures on TV’s.

The properties of the liquid crystalline material used in a cell dictate the performance of the device which they are used. Commercially used liquid crystal material is typically a multi-component system that exhibits many physical properties such as birefringence, dielectric anisotropy, voltage holding ratio, visco-elastic, guest-host effect and the kinetic switching response time of the cell between the on state and off state. By manipulating the physical properties we can exert specific control over the properties of the cell of particular importance in display applications is the speed with which cells can be turned between the on and off state; these are known as the rise and decay response times respectively.

Introducing guest molecules into the liquid crystal host may alter the dielectric anisotropy which potentially increases the speed of the switching process, making the device faster. Guest molecules must be compatible with the dielectrically positive or negative liquid crystal host allowing good mixing of the components and alignment between the guest molecule and liquid crystal molecule. This compatibility is important as it allows both, guest and host, to align with the applied electric field when turned on giving the on state of the cell and when turned off allowing both to re-align with the alignment layer in the cell bringing to the cell order of the medium back to the off state of the cell. The time taken for the cell to reach the on state and off state is an important part of this study.

Dopants have been designed with a head, tail and linker core moiety that are compatible with dielectrically positive and negative liquid crystals. Head groups will have polar substituents such as heteroarenes, fluorine and bromine, to exert control over the dielectric anisotropy. Alkoxy or alkyl tails were selected to increase solubility and size compatibility with the liquid crystal hosts. The linkers between the two arenes were selected as acetylene (linear, large Raman cross-section) and ether, methylene and propylene (to bring about a bend in the molecule).
The switching times for liquid crystal devices are studied using an electro-optic method developed in conjunction with SONY MSL (Stuttgart). These studies enable analysis of the transmission of light through the cell as it goes from the on/off state as a function of time and applied potential. By comparison with the currently used liquid crystal materials our work shows that the level of doping, the length of the tail and the nature of the linker do affect the switching time significantly. It is shown that a non-linear linker, which introduces a ‘bite angle’ within the guest molecule brings about the best increase in response times.

Time-Resolved Raman spectroscopy studies of a liquid crystal cell during the turn on/off process were made. These demonstrate the capability of this technique to measure the orientation of the molecules as a function of time as well allowing the independent motion of the guest and host molecules during the switching process. Raman spectroscopy gives a useful insight into the behaviour of the guest and host materials in an operating liquid crystal cell.