Substituent Effects on the Mesophase Stability for Cholesteryl Benzoylbenzoates

Abstract The thermal properties of cholesteryl esters of substituted benzoylbenzoic acids were examined. The substituent effects on the thermal stability of the chloesteric phase of these esters are quite striking. For cholesteryl 4-(4′-substituted benzoyl)benzoates, the Ch-I transition temperatures are slightly influenced by an electronic effect of the substituents, because the bond direction of the substituents is tilted with respect to the long molecular axis. For cholesteryl 4-(3′-substituted benzoyl)benzoates and cholesteryl 3-(4′-substituted benzoyl)benzoates, the Ch-I transition temperatures are influenced by the polarizability anisotropy of the terminal phenyl ring, which probably gives rise to a change in the ratio of the number of more-linear isomers to the number of less-linear isomers. For cholesteryl 4-(2′-substituted benzoyl)benzoates, the substituents increase the angle between the two phenyl rings, i.e., the thickness of the benzophenone moiety, due to the repulsive interaction between the substituent(s) and ortho-hydrogens of the other phenyl ring, and therefore decrease the Ch-I transition temperature largely.