Mesogen is the fundamental unit of a liquid crystal that induces structural order in the crystals.
Typically, a liquid-crystalline molecule consists of a rigid moiety and one or more flexible parts. The rigid part aligns molecules in one direction, whereas the flexible parts induce fluidity in the liquid crystal. This rigid part is referred to as mesogen, and it plays a crucial role in the molecule. The optimum balance of these two parts is essential to form liquid-crystalline materials.
In a calamitic liquid crystal, the mesogen is a rod-like structure composed of two or more aromatic and aliphatic rings connected in one direction. In a discotic liquid crystal, the flat-shaped aromatic core that makes molecules stack in one direction is defined as the mesogen.
These rod-like and disk-like structures are formed not only by covalent bonds, but also by non-covalent interactions, such as hydrogen bonds, ionic interactions, and metal coordination. In such cases, key structures which define the macromolecular shapes of the assembled molecules are called mesogens or mesogenic parts.