ABSTRACT
Generally, liquid crystals (LCs) are formed in rod-like organic mol-ecules which interact through strongly anisotropic intermolecular forces. The LC properties are decided by their molecular structures. With the progress in LC researches, it has been becoming a common view that to show LC phases, the materials should meet the require-ment of the following items: (1) Anisotropic geometry, that means the structure should be either elongated (linear) in shape or inhomogeneous. (2) Rigidity along the molecular axis. (3) A big p-electron system and polarizablity of electron density. 2.1 Basic StructuresTypically, molecular structure of rod-like LCs are shown in Fig. 2.1. Four parts are often included, and they are the mesogenic core rings (B, B′), linking group (A), end groups (X, Y), and lateral substituents (Z, Z′). 2.1.1 Mesogenic Core RingMesogenic core ring systems are one of the most important constituents of molecular structures in LC materials. Typical core rings are phenyl, biphenyl, terphenyl, and pheylcyclohexane groups
(Fig. 2.2). The kind of core ring groups strongly influences the LC properties, such as temperature, birefringence (∆n), dielectric constant (∆e), etc.
