Liquid crystalline behavior of novel dimesogenic compounds

Liquid crystalline behavior of many series of novel dimesogenic compounds having two identical or two different mesogenic units attached to both ends of a central flexible spacer is reviewed in this article. When central spacers are polymethylene units, these compounds reveal a strong odd-even effect not only in melting point (T-m) and isotropization temperature (T-i), but also in entropy (Delta S-i) of isotropization. Variety of mesophases that are formed by dimesogenic compounds having two identical mesogenic units is much less than that shown by the corresponding monomesogenic compounds. The nematic and smectic group efficiency of terminal substituents in dimesogenic liquid crystals is similar as in monomesogenic compounds. It is also learned that mesomorphic behavior of dimesogenic compounds carrying two different mesogenic units is not a simple combination of those of each mesogenic elements. Some of such dimesogenic compounds reveal unexpected, extraordinary thermal transition sequences which have never been observed to occur in the common monomesogenic compounds, and the incommensurate S-A and twisted grain boundary (TGB) phases are formed.