Design of a Dichroic Dye Molecule
A nematic liquid crystal (LC) molecule, in most cases, has a rod-like structure, and so do the vast majority of the investigated dyes. In this case the dye molecules when dissolved in a LC are packed and aligned similar to the LC molecules. For a long time, the design of dichroic dyes (DD) molecules was based on a structural modification of the rod-like dyes by incorporating different substituents into certain positions of the basic molecules. The order parameter of a dichroic azo dye depends greatly on the geometric anisotropy of its molecule, as well as on the availability of terminal or lateral substituents. Azo dyes containing an alkoxy group as one of the terminal substituents have higher values of S as compared to the corresponding azo dyes containing an alkylamino or dialkylamino group as a terminal substituem. The lateral alkyl and alkoxy substituents in a DD serve to increase the solubility in LC.