Thermal switching of the optical anisotropy of a macroscopically aligned film of a discotic liquid crystal

We have studied the temperature dependence of anisotropy in the optical absorption and charge transport properties of an aligned film of hexakis-dodecyl-hexa-peri-hexabenzocoronene (HBC-C12) formed by zone-casting on a quartz substrate. At room temperature the film displays a large anisotropy in (photo)conductivity, as determined using the flash photolysis time-resolved microwave conductivity technique, with charge transport in the casting direction favoured by a factor of at least 10. The anisotropy in the optical absorption is however negligible. At the temperature corresponding to the transition from the crystalline solid to the liquid crystalline mesophase (c. 110degreesC), the optical anisotropy increases abruptly, with absorption of light polarized in the direction perpendicular to the alignment direction favoured by a factor of c. 3. On cooling, the dichroism reverts to its initial very low value with a hysteresis of c. 30degreesC. The results are explained in terms of a reversible change in the orientation of the molecules with respect to the axis of the aligned columnar stacks from tilted (at c. 45degrees) in the crystalline phase to close to orthogonal in the liquid crystalline phase.