Synthesis And Characterization Of Liquid Crystals Containing Azobenzene Mesogens For Application Of Optical Information Storage Device

Azobenzene chromophores are photo-responsive molecules that can undergo reversible isomerization when irradiated with UV or vis light. When used as dopants in commercial liquid crystals (LCs), the reversible trans-cis photoisomerization of azobenzene can exert large effects on the properties of host LCs. The most salient effect is arguably the destabilization of LC phases by the conversion of azobenzene guest molecules from the trans form to the cis form. This property has been largely explored for possible optical device applications as electro-optical light shutters, information processing materials and optical nonlinear materials. In this article, several kinds of photo-responsive azobenzene derivatives dopant in nematic liquid-crystalline matrixes composite materials are designed and fabricated. The photo-responsive behaviors of these guest/host LC systems, tuned by azobenzene chromophores have been systematically investigated. The stability, optically rewritable property and optical recording reproducibility in these composites are improved. The concrete contents of this article as follows:1. A novel photosensitive liquid crystalline azobenzene (AzoMCl) compound, that made possible the dissolution of a large amount of the dopant in the LC without phase separation due to its own LC nature, has been designed and synthesized. The discovery of a new phenomenon in LCs doped with a high concentration of azobenzene dopant (10wt%) is that the reversible photochemical phase transition could be clearly observed at room temperature by exposing the system alternately to UV radiation at365nm and visible light at455nm. With this special phenomenon, an available optical image storage device, in which the stored image can be rapidly written and erased at room temperature, has been constructed. Further the research of the dynamics of the photoisomerization reaction and the relationship between the photoisomerization process and the change in the contrast of the stored image was presented. The result indicated that AzoMCl is a kind of ideal material for rewritable optical information recording and it allowed multiple uses without apparent fatigue.2. Two novel types of azobenzene compounds containing chiral carbon, Azo-CM and Azo-CP, have been designed and synthesized. On the basis of the above work, we report more details on the photo-responsive behaviors of the guest/host LC systems tuned by an azobenzene chromophore in the absence and presence of chiral tail chain. For the nematic LCs E7doped with the5wt%azobenzene chromophore containing chiral carbon, Azo-CM or Azo-CP, the nematic phase could be switched to the cholesteric phase and then to isotropic phase at room temperature upon application of UV irradiation. However, for the nematic LCs E7doped with5wt%achiral azobenzene chromophore, Azo-nCM or Azo-nCP, despite the drop of the birefringence of severely (but not completely) disordered nematic phase could be noticed upon prolonged irradiation with UV light, the photochemical nematic-to-isotropic phase transition did not occur. The UV-vis spectrum results showed that the two compounds have good photochromism properties and the chiral carbon had significant effect on UV-vis spectrum. Research showed that chiral-azobenzene structure of the compound has larger transformation in the absorption spectrum than achiral-azobenzene structure of the compound.3. Nematic liquid crystalline physical gels suitable for light scattering optical displays have been prepared from a nematic liquid crystal (5CB), a super branch dendritic macromolecule gelator (POSS-G1-BOC) and a photo-responsive azobenzene molecule (2Azo2). In this system, the LC gels form micro-phase-separated structures consisting of LCs and self-assembled fibrous solid aggregates of gelators. Such structures effectively induced the formation of LC polydomains, leading to high light scattering turbid states, which can be switched to transparent states by the application of365nm UV irradiation. This is ascribed to nematic-isotropic phase transition due to trans-cis isomerization of the azobenzene molecules in the LC domains within the gels. As no polariser is needed in light scattering optical displays, the brightness of the displays becomes higher. Due to its simple device structure, light scattering electrooptical switching is expected to be useful for large area display.