Control of the homeotropic order of discotic hexa-peri-hexabenzocoronenes

The thermal properties and self-organization of hexa-peri-hexabenzocoronene (HBC) derivatives with dove-tailed alkyl chains of various lengths have been investigated using polarized optical microscopy and wide-angle X-ray scattering. It is shown that the size-related increase of steric interactions among the peripheral side chains substituted to the aromatic core leads to a dramatically lowered isotropization temperature, allowing thermal processing at practical temperatures. Additionally, the introduction of ether linkages within the side chains enhances the affinity of the discotic molecules towards polar surfaces, resulting in homeotropic self-assembly when the compounds are processed from the isotropic state between two surfaces and, for the first time, as a thin film on a single surface. It is established that the degree of homeotropic order is influenced by the phase behavior, the supramolecular order in the bulk, and the surface affinity of the corresponding derivatives. These results are important for the design of photovoltaic cells based on HBC derivatives.