2018 Volume 76 Issue 10 Pages 1076-1085
High-birefringence liquid crystal materials are used in a wide variety of optical applications such as reflection films and photo storage devices; they are likely to find more applications with increasing research in this area. The design concept is an essential aspect of high-birefringence liquid crystals. We investigated the structure-optical property relationship (liquid crystallinity, refractive index and birefringence) of rod-like mesogens from the viewpoint of physical organic chemistry. We first investigated how the acetylene bond affects nematic transition behavior and refractive index parameters of the conjugated oligoyne or polyyne compounds with two terminal aromatic rings in each single component medium. We synthesized novel, highly birefringent nematic liquid crystal materials: 1,6-diphenyl-1,3,5-hexatriyne derivatives with various alkoxy chains. The derivatives exhibit stable enantiotropic nematic phases. We revealed that a higher number of acetylene units (from 1 to 3), i.e., extended conjugation, led to wider temperature range nematic phases and higher birefringence. The increment in Δn per acetylene unit was estimated to be 0.14. We then studied the use of heteroatoms with high atomic refraction. Sulfur-containing rod-like materials usually have high birefringence values but do not exhibit enantiotropic mesophases because they tend to crystallize or show mesophase instability. We incorporated carboxylic acid end groups into thioether-containing diphenyl-acetylene-based compounds, our hypothesis being that the acid groups would impart mesophase stability through hydrogen-bond driven dimerization. These compounds exhibited highly correlated long-range mesophases and high birefringence compared with their alkyl and alkoxy analogues. We also developed a new and simple process to prepare phenolic-hydroxyl-group substituted diphenyl-acetylene and diphenyl-diyne derivatives. Using these molecules, we prepared transparent and colorless high-birefringence polymer films. Our studies show that this review could be a guide for the design of high-birefringence liquid crystal materials.
