Studies On Synthesis And Photochromic Properties Of Chiral Dopants In Liquid Crystal Materials

At digital information age, Liquid Crystal as one kind of important functionalmaterials is widely used in liquid crystal displays, temperature testing, stress testing,nondestructive detection, HPLC, various spectral analysis and many other fields.Dopants play an important role in liquid crystal displays. Chiral dopants with chiralgroup can convert nematic phase to cholesteric phase or chiral nematic phase andeven if the dopants do not have liquid crystal phase. Chiral dopants can also inducethe formation of helical arrangement of liquid crystal molecules so that liquid crystalmaterials have unique optical property and have broad application prospects innonlinear optics, fast optical switches, micro-electronics and other fields. Therefore,chiral dopants have important applications in liquid crystal materials. At present, thestudies on liquid crystal compounds are more than those of chiral dopants.In this paper, two chiral dopants (C1、C2) were designed and synthesized withL-ethyl lactate as chiral group. Three chiral dopants (C3、C4 and C5) were designedand synthesized with isosorbide as chiral group. The chemical structures of fivecompounds are as following. In order to obtain chiral dopants which have better compatibility with the liquidcrystal compounds, C2 and C4 were synthesized by connecting the cationic structureto C1 and C3.The synthesis conditions of five chiral dopants were investigated. Theinfluences of the ratio of reactants, reaction temperature, reaction time and solvent onthe yield of dopants were discussed. Ultimately the optimum synthesis conditionswere determined.The purity and chemical structure of synthesized compounds were proved andanalyzed by IR, NMR and elemental analysis. Examination of optical rotation andcircular dichroism show that all synthesized compounds were optical active and theextinction phenomenon did not occur during the synthesis. The cationic structure inthe molecular had a positive influence on the optical active while did not affect thedirection of rotation. The dopant containing fluorine atoms had the strongest opticalrotation.The dopants would affect the performance of liquid crystal itself when they weredoped into the liquid crystal compounds. If the concentration of dopants was too large,the melting point and clearing point liquid crystal would change a lot. The resultsshow that the liquid crystal compounds could get a good effect of doping whendopants were doped with the concentration of 1% and basically did not affect the performance of liquid crystal itself.Polarized optical microscopy analysis show that when dopants were doped into theliquid crystal compounds, there was no black-cross extinction phenomena after liquidcrystal compounds reached the clearing point. Besides, the liquid crystal compoundspresented a variety of texture change and would show smectic phase texture. Duringthe cooling process, there would be crystallization generating.