The Synthesis and Evaluation of Novel Bent-core Liquid Crystals Designed to Generate a Biaxial Nematic Phase

It is commonly recognised that matter can exist in three states, solid, liquid and gas, however, some materials, notably those that consist of rod-shaped molecules, can additionally show one or more 'liquid crystal' phase(s). Liquid crystal phases are distinct phases of matter that have molecular orderings intermediate between those of the common liquid state and those of the common crystalline solid state. The smectic phases (several are known) are liquids that are quite viscous because the degree of molecular ordering is close that of solids. The nematic phase is very fluid because the only molecular ordering is a statistically parallel arrangement of molecules. It is this fluid nematic liquid crystal that is used in the widely available flat-panel liquid crystal displays that are now used for watches, calculators, mobile telephones, cameras, lap-top computers, desk-top monitors and more recently large area flat-panel televisions. In these devices, the rod-like liquid crystal molecules have their orientation switched by the application of a small electric field (a few volts), and although switching times are quite fast, around 50 microseconds, faster switching would be advantageous, and open up a whole new area of fast switching applications in displays, telecommunications and optical devices.In the nematic phase, the rod-like molecules have a statistically parallel arrangement of the molecules and in this arrangement there is only one axis (the optic axis) through which polarised light travels through with equal velocity, hence the phase is termed uniaxial. Bent-core (but not too bent) molecules can also exhibit the nematic liquid crystal phase, however, the bent-core molecular structure can give rise to a biaxial nematic phase, in which, as the name implies, there are two optic axes. A biaxial nematic phase is of fundamental scientific interest, and additionally it is of technological interest because switching in such a biaxial nematic phase would be much faster enabling the advantageous devices mentioned above, and possibly facilitate the development of novel high technology applications.So far, very few compounds have been proven to generate a biaxial nematic phase, and these are only exhibited at temperatures of around 200 C, and over a relatively short temperature range; which obviously makes evaluation difficult, and use in applications impossible. This proposed research is designed to thoroughly investigate the structure-property relationships of bent-core molecular structures in the generation of the biaxial nematic phase, through the synthesis and evaluation of novel material designs, with a view to providing the phase over a wider temperature range, at or close to room temperature; thus facilitating the evaluation of physical properties and enabling use in applications discussed above.