Characterisation and Structure of Some New Smectic F Phases

A number of new compounds of two series have been shown by miscibility studies and optical microscopy to exhibit the hitherto rare smectic F phase. An example of each series has been chosen for detailed X-ray structural investigation : terephthalylidenebis-4-n-pentylaniline (TBPA) and 4-(2'-methyl butylphenyl 4'-n-octyloxybiphenyl-4-carboxylate) (80SF). The structure of the SF phase has been determined for the first time. Locally it consists of a C centred monoclinic cell with a regular hexagonal packing of the molecules in the layers. The detailed ordering is restricted to about 30 molecular diameters within a layer and to 1 or 2 layers although the gross hexagonal symmetry is preserved throughout the bulk of the sample. The tilt direction of the hexagonal net is different for the two compounds : for TBPA it is towards an edge of the hexagon (as for the S, phase of TBBA) while for 8 OSF it is towards a corner. The S, phases of each compound have essentially the same structure but with longer range order and the two different tilt directions are maintained. Preliminary work on the S , phases indicates that they are similar to the S, phase of TBBA but again with two different tilt directions. .