The Substituent Effect on the Thermal Stability of the Smectic A Phase in para- Phenylene Systems Incorporating Ester Linkages

Abstract

Summary: The thermal properties of 4-Z-phenyl 4-(4-octyloxybenzoyloxy)benzoates (1), 4-octyloxy- phenyl 4-(4-Z-benzoyloxy) benzoates (2), 4-(4-octyloxybenzoyloxy) phenyl 4-Z-benzoates (3) and 4- octyloxyphenyl 4-(4-Z-phenoxycarbonyl)benzoates (4) have been examined, where the substituents (Z) are halogens, alkoxy, alkyl, formyl, acetyl, cyano, and nitro groups, etc. The smectic properties are strongly dependent on the orientation of the ester linkages. Compounds 1 and 4 having alkoxy and alkyl groups tend to show smectic A and C phases. Compounds 2 and 3 are intrinsically poor in smectic properties, and only a smectic C phase is formed by the long-alkoxy and alkyl derivatives. Throughout the compounds, the thermal stability of the smectic A phase is strongly dependent on the electrostatic nature of the substituents; that is, both electron-donating substituents such as the methoxy and methyl groups and electron-withdrawing ones such as the cyano and nitro groups, steeply lower the smectic A thermal stability, and the halogens, acetyl and formyl groups enhance it. The substituent effect on the smectic A thermal stability is correlated with the charges around the functional groups calculated by the MNDO method. The ratios of the smectic A-nematic transition temperature to the nematic-isotropic one show an interesting correlation with Hammett's constants, σ_p.