Evidence from infrared dichroism, x-ray diffraction, and atomistic computer simulation for a "zigzag" molecular shape...

Infrared dichroism (IR) and atomistic computer simulation are employed to probe 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, CH2) vibrations in the tilted synclinic (smectic-C) phase of a variety of materials show the phenyl (core) IR absorbance symmetry axes to be consistently tilted at larger angle from the layer normal than the alkyl or methylene (tail). This suggests that, on average, the tails are less tilted than the cores. Furthermore, we find that optic axis tilt angle is close to the core tilt angle measured by IR dichroism, as expected, since liquid crystal birefringence arises primarily from the cores. These results are in accord with the "zigzag" model of Bartolino, Doucet, and Durand. However, we find that only a small fraction of the tail, the part nearest the core, is tilted, and only this part contributes significantly to layer contraction upon molecular tilt.