Utilizing bicyclohexene as a building block: A promising approach to achieve negative liquid crystals with lower melting points

Two series of novel negative mesogens based on bicyclohexene were designed and synthesized. All of the obtained mesogens possessed liquid crystalline phases ranging from 32 to 87 °C and the introduction of 2-fluoroethoxy decreased the mesophase ranges. The representative compounds 7b and 9b had significantly lower melting points than their corresponding reference compounds based on conventional cores, illustrating the effectiveness of bicyclohexene building block in lowering the melting points of negative mesogens. The dielectric anisotropy and birefringence values of the novel compounds were similar with their references, demonstrating that this new core does not significantly alter physical properties. Density functional calculation found that the geometries of compounds 7b and 9b were similar to those of their references, indicating that this new core would not substantially change the geometries of the mesogens. Three viewpoints of calculated ground states of compounds 7b and 9b pointed out that the orientation of the fluorine on the 2-fluoroethoxyl was similar to that of the 2,3-difluoro groups on the main skeleton, which contributes to the increase of negative dielectric anisotropy values.