Aligned thin films of discotic hexabenzocoronenes: Anisotropy in the optical and charge transport properties

The anisotropy in the optical absorption and photoconductivity of thin layers of mesomorphic derivatives of hexa-pri-hexabenzocoronene (HBC) have been investigated for aligned films prepared via three different methods: deposition on friction-deposited polytetrafluoroethylene (PTFE), zone-casting (ZC), and Langmuir-Blodgett (LB) multilayer dipping. The ratio of the optical density for light polarized perpendicular to the alignment direction, OD₊, to that for light polarized parallel, OD₌, varies from close to 1.0 up to 12.5 depending on whether the HBC cores are tilted at close to 45° or 90° with respect to the axis of the self-assembled columnar stacks. For all aligned films the photoconductivity, determined using the electrode-less flash-photolysis time-resolved microwave conductivity technique (FP-TRMC), was found to be favored in the direction of columnar alignment by up to a factor of 30 for a PTFE-aligned film. The effect of varying the temperature of the films over a range encompassing the temperature at which the transition from the crystalline solid to the columnar mesophase occurs in the bulk materials has been investigated. High-temperature annealing increases the optical and conductivity anisotropy for the LB film significantly, but has little effect for the PTFE and the ZC films. The relative efficacy of the different alignment procedures is discussed.