Mesomorphism of a banana mesogen: influence of a fluoro substituent in the central core

The results of experiments with stable five-ring banana-shaped molecules consisting of laterally 4-fluoro substituted 1,3-phenylene ring as the central unit in Bis-4-(4′-n-alkyloxybenzoyloxy)salicylidene]-phenylene-4-fluoro-1,3-diamines are presented. These compounds are thermally and hydrolytically stable due to intermolecular or intramolecular hydrogen bonding and exhibit SmCP_A and low-temperature crystalline B_X phases. The phases had been characterised by thermal microscopy and differential scanning calorimetry. A representative example has also been characterised by X-ray diffraction studies and electric field effects. The influence of a lateral fluoro substituent in the central core is clearly demonstrated with the induction of the SmCP_A phase which is absent in the parent compound without the substituent. Further these compounds exhibit a large tilt angle. We also describe a comparison of phase behaviour with similar analogous compounds, taking the position of fluoro substitution, changes in the linking groups and the direction of the linking groups into consideration.