Research Projects

Azo-benzene compounds and their photo-isomerism is a well-studied topic for their applications and possibilities to different technological devices. With the recent advent of nanotechnologies of different materials with applications from nano-machines to bio-applications such as drug carriers. Azo-benzene compounds, having two photoisomers, can be removed reversibly with exposure of UV and visible light. In some cases they lose the reversibility. In industrial application of photo-aligning agent of liquid crystal display devices, azo-benzene side chains are manoeuvred by the polarization of UV and it leads to overall uniform alignment of liquid crystal molecules as that uniform alignment is a pre-requisite of a LCD [8-12]. Yet not much study has been performed to know the details of the irreversibility of the azo-benzene side chains. Some recent works also support the fact that in special situation with prolonged UV irradiation cis-isomers aggregate and phase separate from the homogenous mixture [13,14]. In one such work, it has been shown that an azo?dye has been dissolved in a liquid crystal mixture. The mixture has been loaded in to a glass cell of thickness around 10 micron. UV has been exposed to the cell with a mask. After irradiation, it can be seen that anchoring condition of LC in the irradiated has been changed from planar to homeotropic. Change in anchoring condition directly indicates change in the surface condition, physically or chemically. The cell has been washed to remove the LC and refilled with a fresh pristine LC mixture. The refilled cell confirmed the anchoring change of exposed area. After washing the cell surfaces of the substrates have been examined under AFM and FE-SEM. Both the experiments confirmed the deposition of a new material on to the exposed area. Later the deposition has been examined with different UV intensity and different duration of exposure. It has been found the texture of the deposition has some relationship with UV intensity and duration of exposure. Other azo-dyes have been used for similar experiments and no such deposition has been observed. It also says the fact that not all the azo?compounds are likely to exhibit this type of behavior. Their explanation says with UV exposure, number of cis- isomer increases in the mixture. Cis- isomers have higher dipole moment. So highly populated cis- isomer interacts among themselves and stack together due to dipole-dipole interaction and grows in size. Once achieved a certain size they get phase separated from the main mixture and migrate to the interface and settle down. Gradually they form a layer on the substrate. This layer has been found to be thermally stable up to few hundreds of degrees C and chemically stable against most of the common polar and non-polar solvents. However, the detailed study of mechanism of this deposition has not been performed. After deposition, the chemical and structural properties are also unknown.