Synthesis and the effect of 2,3-difluoro substitution on the properties of diarylacetylene terminated by an allyloxy group

A series of 2,3-difluoro tolane allyloxy-based liquid crystals (LCs), which are composed of the tolane unit, lateral fluoro substituents, alkoxy and allyloxy terminal groups, were synthesised with 4-bromo-2,3-difluorophenol as the starting material via a five-step reaction. Their structures were confirmed by infrared spectroscopy (IR), mass spectroscopy (MS) and nuclear magnetic resonance (NMR). The mesomorphic properties were investigated by differential scanning calorimetry (DSC) and polarising optical microscopy (POM). The dielectric measurement was performed using an independent analyser EC-1 and the birefringence was determined using an Abbe refractometer. The results reveal that LCs have large negative dielectric anisotropy and high birefringence, which are potentially important for the fast response displays and liquid crystal photonic devices (LCDs).