Stabilisation of mesophases formed by V-shaped shape-persistent nematogens

The synthesis and study of V-shaped shape-persistent mesogens are of high interest because such molecules were considered as good candidates for the formation of the biaxial nematic phase due to their inherent molecular biaxiality. Recently, we could synthesize a fluorenone derivative with an apex angle of about 90° which exhibit a transition from the uniaxial to the biaxial nematic phase. However, the metastable (monotropic) character of the phase hamper structural investigations. Examples for enantiotropic biaxial nematics are reported for oxadiazol derivatives but in a temperature range above 100 °C. We are interested in modifying our parent structure in order to obtain enantiotropic nematic phases close to room temperature without an underlying lamellar structure. Our research focuses on shapepersistent molecules 1 with alkoxy chains in 2,5-position of the middle benzene ring and substituents introducing local dipoles at the terminal aromatic units (figure 1), since, these features reduce the crystallisation tendency of the molecules. Here, we present the synthesis and characterization of C2-symmetrical and non-C2-symmetrical V-shaped mesogens with branched and unbranched alkoxy chains peripheral fluor substituents and fluorenone (B) or oxadiazole (A) cores. The thermotropic properties of the title compounds will be investigated by polarised optical microscopy, differential scanning calorimetry and X-ray diffraction.