Effect of the position of lateral fluoro substituents on the phase behaviour and ferroelectric properties of chiral 1-methylheptyl 4′-[(2- or 3-fluoro-4-tetradecyloxyphenyl)propioloyloxy]biphenyl-4-carboxylates

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Abstract
The syntheses of four chiral laterally fluoro-substituted propiolate esters are described, along with transition temperatures, ferroelectric properties, phase diagrams and related data. The position of the fluoro-substituent was found to influence dramatically the formation of twist grain boundary (TGB A* and TGB C*) phases as well as the magnitudes of spontaneous polarization and optical tilt angle in the ferroelectric smectic C* mesophases. Differential scanning calorimetric studies revealed the presence of a diffuse liquid-liquid transition above the clearing point in both of the 3-fluoro enantiomers, but not in the racemate. Circular dichroism and optical rotation measurements, carried out over a temperature range in which the diffuse peak occurs, appear to confirm the presence of a degree of chiral organization within the isotropic liquid. It is suggested that this phenomenon may be due to the presence of cybotactic groups or a network of entangled screw dislocations occurring close to the clearing point.