Some remarks on the evolution of research into the structures of mesogens and mesophases

Research on mesogens and mesophases began with the surprising observation of a fluid and birefringent phase formed by a pure compound having a calamitic (rod-like) molecule. The structures of the smectic A, nematic and cholesteric phases which emerged after this discovery were elucidated essentially by observations with a polarizing microscope. Smectic polymorphism was also established by microscopic observations. The main historical results on the structures of the different smectic mesophases are listed here. The discotic mesophase, previously predicted theoretically, was then obtained and its polymorphism observed. A further type of purposely designed mesophase was the pyramidic type, also exhibiting a rich polymorphism. Research on mesogens and mesophases is a field in continuous expansion.