Mesogen

Mesogen (or Liquid Crystal) is a compound that displays liquid crystal properties and "mesogenic" is the term given to a substance that induces the liquid crystalline state (also known as the liquid crystal phase).

Mesogens can be described as disordered solids or ordered liquids because they arise from a unique state of matter that exhibits both solid- and liquid-like properties called the liquid crystalline state. This liquid crystalline state (LC) is called the mesophase and occurs between the crystalline solid (Cr) state and the isotropic liquid (Iso) state at distinct temperature ranges.

The liquid crystal properties arise because mesogenic compounds are composed of rigid and flexible parts, which help characterize the order and mobility of its structure. The rigid components align mesogen moieties in one direction and have distinctive shapes that are typically found in the form of rod or disk shapes. The flexible segments provide mesogens with mobility because they are usually made up of alkyl chains, which hinder crystallization to a certain degree. The combination of rigid and flexible chains induce structural alignment and fluidity between liquid crystal moieties.

In doing so, varying degrees of order and mobility within mesogens results in different types of liquid crystal phases, Figure 1. The nematic phase (N) is the least ordered and most fluid liquid crystalline state or mesophase that is based on the rigid core of mesogen moieties. The nematic phase leads to long range orientational order and short range positional order of mesogens. The smectic (Sm) and columnar (CoI) phases are more ordered and less fluid than their nematic phases and demonstrate long range orientational order of rod-shaped and disk-shaped rigid cores, respectively.

Figure 1 – Organization of rod-like and disk-like rigid cores in liquid crystal phases of mesogens, where Iso is the isotropic liquid state; N is the nematic phase of the liquid crystal state; SmA is the smectic A phase; SmC is the smectic C phase; and CoI is the columnar phase.

Thermotropic mesogens are liquid crystals that are induced by temperature and there are two classical types, which include discotic mesogens and calamitic mesogens.

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