Selective orientation of discotic films by interface nucleation

Thin films deposited from solution by the hollow capillary writing technique exhibit macroscopic uniaxial grains with an in-plane arrangement of discotic stacks. We report a method to change the orientation of thin films from in-plane to homeotropic (discotic columns perpendicular to the substrate). It is observed that annealing of open supported films at a temperature 25 °C below the equilibrium LC/isotropic phase boundary induces homeotropic alignment, while rapid cooling leads to predominantly in-plane alignment. A model based on heterogeneous nucleation at the film/substrate interface accounts for the experimental observations.

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► Crystalline films of discotic phthalocyanine have been produced from solution.
► Selective orientation of discotic columns can be achieved by thermal annealing.
► Nucleation theory suggests different supercooling temperatures for each alignment.
► The mobility increased by two orders of magnitude upon switching alignment.
► The nuclei grow much faster along the columnar direction than perpendicular to it because the growth is limited by kinetics.