Design and investigation of de Vries liquid crystals based on 5-phenyl-pyrimidine and (R,R)-2,3-epoxyhexoxy backbone

Calamitic liquid crystals based on 5-phenyl-pyrimidine derivatives have been designed, synthesized, and characterized. The 5-phenyl pyrimidine core was functionalized with a chiral (R,R)-2,3-epoxyhexoxy chain on one side and either siloxane or perfluoro terminated chains on the opposite side. The one involving a perfluorinated chain shows SmA^{*} phase over a wide temperature range of 82 °C, whereas the siloxane analog exhibits both SmA^{*} and SmC^{*} phases over a broad range of temperatures, and a weak first-order SmA^{*}-SmC^{*} transition is observed. For the siloxane analog, the reduction factor for the layer shrinkage R (relative to its thickness at the SmA^{*}-SmC^{*} transition temperature, T_{AC}) is ∼0.373, and layer shrinkage is 1.7% at a temperature of 13 °C below the T_{AC}. This compound is considered to have "de Vries smectic" characteristics with the de Vries coefficient C_{deVries} of ∼0.86 on the scale of zero (maximum-layer shrinkage) to 1 (zero-layer shrinkage). A three-parameter mean-field model is introduced for the orientational distribution function (ODF) to reproduce the electro-optic properties. This model explains the experimental results and leads to the ODF, which exhibits a crossover from the sugar-loaf to diffuse-cone ODF some 3 °C above T_{AC}.