Book Description

In this investigation, photosensitive chiral materials doped in nematic and cholesteric liquid crystals are studied to develop an optically addressed cholesteric liquid crystal display. The photosensitive chiral materials are mostly composed of an azo-binaphthyl core that undergoes cis-trans isomerization when exposed to light, and is reversible. In a cholesteric liquid crystal, the photosensitive cholesteric liquid crystal changes pitch length with irradiation. The helical twisting power, photosensitivity, thermal relaxation, electro-optical response, UV degradation and various other properties are characterized and analyzed. The study of these materials results in a better understanding of how different molecular structures affect photochemical properties and helps identify the properties ideal for the application of optically addressed cholesteric liquid crystal displays. Properties ideal for this application include a material with a structure very similar to a liquid crystal to yield high helical twisting powers and good solubility, a significantly low HTP for the UV photostationary state as well as substantial difference in HTP from the trans-trans configuration to maximize wavelength shift, an absorption spectra where the overlap in absorption of the trans and cis isomers are at a minimum to result in the largest return possible with visible light, an absorption above 300nm so the wavelength to excite is able to transmit through glass and most plastic substrates, and resilient to UV fragmentation. Optically addressed cholesteric liquid crystal devices are also developed that does not require attached electronics or patterned electrodes without compromising resolution. Capitalizing on the change in pitch, the image can be held indefinitely regardless of lighting environment by switching the cholesteric liquid crystal to planar and focal conic textures at a single voltage.