The effects of the bulkiness of terminal chains on the stability of smectics deduced from the crystal structures of isomeric chiral biphenyl esters

Abstract

Single crystal X-ray analyses have been performed on 4'-octyloxyphenyl 4'-[(S)1-methylheptyloxy]biphenyl-4-carboxylate (8*O-O8) and 4-[(S)-1-methylheptyloxy]phenyl 4'-octyloxybipheny-4-carboxylate (8O-O8*), with the phase sequences Cr-SmC*-Ch-I and Cr-SmX-SmC*-SmA-I, respectively. Both crystals have smectic-like layer structures, in which the molecular tilt angles are 10 and 30, respectively. In the crystal phase of 8*O-O8, biphenyl moieties are twisted probably because of steric hindrance of the bulky chiral chains linked to them, whereas in 8O-O8* they are nearly planar due to the conjugation between the alkoxy oxygen atoms and the carbonyl groups. From comparison of the properties with those of other related compounds, it is concluded that the conformation of the biphenyl moieties are closely related to the stability of smectic phases.