Orientational transitions of discotic columnar liquid crystals in cylindrical pores

Confined in a cylindrical pore with homeotropic anchoring condition, the hexagonal columnar phase of discotic liquid crystals can form a "log-pile" configuration, in which the columns are perpendicular to the long axis of the pore. However, the {100} planes of the hexagonal lattice can orient either parallel (termed (100)_‖ orientation) or perpendicular ((100)_⊥) to pore axis. Here we experimentally show that the (100)_‖ orientation is found in narrower cylindrical pores, and the (100)_‖-(100)_⊥ transition can be controlled by engineering the structure of the molecules. The (100)_‖ orientation is destroyed in asymmetric discotics hepta(heptenyloxy)triphenylene (SATO7); replacing the oxygen linkage in hexa(hexyloxy)triphenylene (HATO6) by sulphur (HATS6) improves the (100)_‖ orientation in small pores; adding a perfluorooctyl end to each alkyl chain of HATO6 (HATO6F8) moves the (100)_‖-(100)_⊥ transition to larger pores. We have provided a semi-quantitative explanation of the experimental observations, and discussed them in the context of previous findings on related materials in a wider pore size range from 60 nm to 100 μm. This allows us to produce a comprehensive picture of confined columnar liquid crystals whose applications critically depend on our ability to align them.