Increasing the negative dielectric anisotropy of liquid crystals with polar substituents parallel to their long molecular axis through adding terminal 2,2-difluoroethyl group

A series of 2,3-difluorophenylene-based negative liquid crystals which own different terminal groups has been synthesised in high yields through many classical named reactions such as Suzuki coupling and Williamson etherification. Thermal properties of these mesogens were tested by differential scanning calorimetry and the effects of the terminal groups on the melting points of molecules were evaluated. Dielectric anisotropies of the synthesised molecules were then measured and found that relatively long alkyl substituents would decrease the & UDelta;& epsilon; value when compared with the ethyl. For the compound with a polar substitute on the long molecular axis, the introduction of 2,2-difluoroethyl would increase the negative & UDelta;& epsilon; when it was put into the opposite direction of the original polar group, making this strategy a new access for the improvement of negative & UDelta;& epsilon;. DFT calculations were carried out to analyse the orientation of the dipole moment in molecules and found that the introduction of 2,2-difluoroethyl had changed the orientation of the dipole moment into nearly perpendicular to the long molecular axis, which benefited the growth of the negative & UDelta;& epsilon; value, revealing the mechanism of this strategy from the view of molecular level to some extent.