The synthesis and mesomorphic behaviour of the 1,2,4,5-tetra-substituted benzenes with (4-tridecyloxyphenyl)ethynyl and (4-tridecyloxyphenyl)carbonyloxy substituents

The synthesis, mesophase morphology and transition temperatures of a series of 1,2,4,5-tetra-substituted benzenes are described. The compounds have (4-tridecyloxyphenyl)carbonyloxy or (4-tridecyloxyphenyl)ethynyl as the peripheral substituents in all possible combinations ranging from the tetra-ester to the tetra-alkyne. Both the substituents used have similar structural dimensions but the greater rigidity of the alkynyl unit ensures that it projects more directly from the central core than does the more flexible ester unit. All of the seven compounds are nematogenic and five show a smectic C phase; the differences observed in the transition temperatures of the compounds are discussed in terms of molecular shape and the relationship of the compounds to appropriate calamitic and discotic systems. Molecular modelling indicates that pi-pi interactions of peripheral phenyl groups can lead to molecules with calamitic shapes.