Evidence from infrared dichroism, x-ray diffraction, and atomistic computer simulation for a "zigzag" molecular shape in tilted smectic liquid crystal phases - art. no. 051712

Infrared dichroism (IR) and atomistic computer simulation are employed to p robe molecular shape in smectic liquid crystal phases where the optic axis is tilted relative to the layer normal, Polar plots of absorption profiles due to core (phenyl, C-C) and tail (alkyl or methylene, CH,) vibrations in the tilted synclinic (smectic-C) phase of a variety of materials show the p henyl (core) IR absorbance symmetry axes to be consistently tilted at large r angle from the layer normal than the alkyl or methylene (tail). This sugg ests that, oil average, the tails are less tilted than the cores. Furthermo re, we find that optic axis tilt angle is close to the core tilt angle meas ured by IR dichroism, as expected, since liquid crystal birefringence arise s primarily from the cores. These results are in accord with the "zigzag" m odel of Bartolino, Doucet, and Durand. However, we find that only a small f raction of the tail, the part nearest the core, is tilted. and only this pa rt contributes significantly to layer contraction upon molecular tilt.