Liquid crystalline phase transitions of the 2-(4′-alkoxybiphenyl-4-yl)-5-(4-methylphenyl)pyridines

The mesomorphic phase transitions have been studied for a series of 2-(4'-alkoxybipheny1-4-yl)5-(4-methylphenyl)pyridines (4R-Pn; the carbon number, n, of the alkoxy group is 1-10) by use of DSC and polarizing optical microscopy. The 4R-Pn compounds showed only a nematic phase for n = 1 and 2, but underwent characteristic polymesomorphic sequences when n greater than or equal to 3; Cr reversible arrow CrH reversible arrow SmF reversible arrow SmC reversible arrow SmA reversible arrow N reversible arrow I with n = 3 and 4, and Cr reversible arrow CrH reversible arrow SmF reversible arrow SmA reversible arrow N reversible arrow I with n = 5-10. This mesomorphic phase transition behaviour was similar to that of the 2-(4-alkoxyphenyl)-5-(4-methylphenyl)pyridine (3R-Pn) series reported previously and it is proposed that the 4R-Pn and 3R-Pn compounds have a minimum terminal alkoxy length (critical alkoxy length) for the generation of LC polymorphism.