Impact of terminal group of organic dopant on liquid crystal-based electro-optic device

Abstract

In this study, the influences of terminal group of organic dopant in nematic liquid crystals (LCs) are investigated. A novel organic molecule, N-(4-Fluoro-2-methylphenyl)benzenemethanamine (BFM) with fluorine terminal group and a similar one, N-benzyl-2-methyl-4-nitroaniline (BNA) with nitro terminal group are doped into the nematic LCs, separately. The electro-optical characteristics of the BFM-doped and BNA-doped LC cells are demonstrated. The LC cells with organic dopings have the lower threshold and operation voltages than the pristine LC cell, due to the enhanced electrical torque of LCs. The fluorine terminal atoms in the BFM-doped LC cell prevents the drastic absorbance at short visible wavelengths and suppresses the free-ions in the BNA-doped LC cell significantly. BFM-doped and BNA-doped LC cells have the fall times that are 5.79X and 5.05X faster than pristine LC cell, respectively, owing to decreased viscosity and threshold voltage and increased local electric field. Furthermore, density functional theory calculation confirms that organic dopant enhances the dipole moment, polarizability, polarizability anisotropy and hence the dielectric anisotropy of LC mixture. The competition between the dipole moment and free-ions of organic molecule-LC composite has a crucial influence on the electro-optical properties of LC cell. BFM doping not only prevents the color shift but also improves the operation voltage and response time of LC cell significantly.