Molecular structure and smectic properties. Part 1. The effect of linkages on smectic a thermal stability in three aromatic ring compounds linked by ester groups

The thermal properties of phenyl 4-(4-alkoxybenzoyloxy) benzoates (1), 4-alkoxyphenyl 4-benzoyloxybenzoates (2), 4-(4-alkoxybenzoyloxy)benzoyloxyxylenes (3), and phenyl 4-(4-alkoxyphenoxycarbonyl)benzoates (4) have been examined. In spite of the structural similarity, the smectic nature of the series is quite different. Series (1) shows a stable smectic A phase starting with the hexyloxy homologue. Series (2) and (3) do not show any smectic phases, and the potential smectic stabilities evaluated from the binary phase diagrams are quite low. Although series (4) shows no smectic phase because of higher melting points, this series intrinsically possesses a smectic A nature, where the potential smectic A-nematic transition temperatures are as high as those of series (1). The difference in the smectic nature for these four series is discussed in terms of the geometrical and electrocstatic properties of the molecules, and a new basis for the thermal stability of the smectic A phase has been proposes.