p -Sexiphenyl: A Real-Material Candidate for a Hard-Core Spherocylinder Exhibiting a Smectic A Phase

Despite many studies of the smectic A (SmA) phase using molecular simulations on an assembly of hard-core spherocylinders, the corresponding rod-like mesogens have not been identified in actual liquid crystal compounds so far. As a candidate for molecular realizations, we examined p-sexiphenyl, which is a genuine core-only rod-shaped molecule consisting of six rigid phenyl groups. Analysis of the X-ray diffraction intensities reveals that the molecules form well-ordered periodic packing with a narrow distribution of molecular centers along the normal to the layer in the SmA phase. This structural feature resembles the results of molecular simulations on hard spherocylinders. This organization differs from that of the core-chain type 4-octyloxy-4′-cyanobiphenyl (8OCB), which adopts a straight molecular shape on average in the SmA phase but exhibits a sinusoidal distribution along the layer normal. Based on several reports of MD simulations, we conclude that the hardness of intermolecular repulsion is essential in determining the sharpness of the periodic distribution in the SmA phase.