Effects of Branching of the Ester Alkyl Chain on the Liquid Crystalline Properties of Alkyl 4-[4-(4-Substituted benzylideneamino)benzoyloxy]benzoates

The liquid crystalline properties of four homologous series of alkyl 4-[4-(4-X-substituted benzylideneamino) benzoyloxy]benzoates, where X = NO2, CI, CH3, and CH3O, have been compared with each other. The ester alkyl groups employed were the following twelve; propyl, butyl, pentyl, and their monomethyi-substituted ones. While all the members of the nitro derivatives are purely nematogenic, the other three series exhibit both nematic and smectic A phases but the former mesophase is mostly eliminated by 1-methylation. The nematic-isotropic transition temperature varies alternately when the branching methyl group shifts from the odd to even position in the alkyl chain. The change in the thermal stability of the smectic A phase by chain branching depends considerably upon the alkyl chain length and also upon the nature of the substituent X located at the other end of the molecule. Some branched alkyl esters attain a smectic A-nematic transition temperature higher than that of the corresponding unbranched alkyl ester.