Molecular structure–thermal behaviour relationship of dimers consisting of different terminal substituents and sulphur–sulphur linking group

A new series of dimers 6-(4-oxybenzylidine-4′-substituted-aniline)hexyldisulphides containing various terminal substituents (CH3, C2H5, Cl and Br) have been synthesized. The correlation between the molecular structure and the properties of all the ultimate compounds has been made on the basis of their thermal behaviour. The dimers with CH3 and C2H5 substituents exhibit predominantly the monotropic N phase of which the C2H5-containing derivative possesses more stable nematogenic temperature range in comparison to its methyl-containing analogue. As for the member with Cl substituent, it exhibits enantiotropic N and monotropic smectic A phase. However, the dimer in which the Cl was substituted by Br exhibits purely enantiotropic smectic A phase. Upon cooling all of the final compounds show the transition from isotropic to mesophase prior to the change from mesophase to crystal except the member with Br substituent which displays mosaic B phase even at room temperature. An extensive study to further substantiate the relationship between the stability of the phase and associated transition temperatures due to different terminal substituent is also studied. This study reveals that the terminal Br increases the clearing temperature and polarizability which enhances the formation and stability of SmA and mosaic B phases.