Effect of lateral fluoro substituents of rodlike tolane cyano mesogens on blue phase temperature ranges

Several kinds of tolane cyano derivatives with different lateral fluoro substituents are doped with chiral additives to induce the blue phase (BP). Fluoro substituents are found to exert a great effect on the BP ranges for biphenyl mesogens. The laterally difluorinated compounds exhibit wider BP ranges than their corresponding mono-fluorinated homologues when equal chiral dopant is added, while the non-fluorinated compounds show direct isotropic (Iso) to chiral nematic (N-star) transitions, even in highly chiral systems. By adjusting the concentrations of the chiral dopant ISO(6OBA)(2) in the fluorinated liquid crystals, the BP ranges can be broadened to more than 12 degrees C. Competition between the steric repulsions caused by the lateral fluoro substituents and pi-pi interactions of the aromatic cores is supposed to be the main factor that leads to the above results. Apart from this, a weak C-H center dot center dot center dot F interaction may have also been conducive to the BP stabilization.