Magnetic uniaxial alignment of the columnar superstructure of discotic metallomesogens over the centimetre length scale

Abstract

The uniaxial alignments of the columnar superstructure of discotic metallomesogens, cobalt octa(n-dodecylthio)porphyrazine (CoS12), over the centimetre length scale have been achieved by spinning samples under a static magnetic field. The orientations of the columnar superstructures were investigated by small angle neutron scattering (SANS) and cryogenic transmission electron microscopy (cryo-TEM). Upon cooling from the isotropic phase to the columnar mesophase in the presence of a static magnetic field (0.4–1.1 T), CoS12 formed oriented columnar superstructures with the columnar directors being isotropically distributed in the plane normal to the external magnetic field. When the samples were continuously spun during cooling in a static applied magnetic field of 1.0 T, CoS12 was observed to form uniaxially aligned columnar superstructures with the columnar domain directors being parallel to the rotation axis which was normal to the external field. The optimal rotation speed for the alignment was found to be as low as 5–10 rpm, where the full width at half maximum of the domain director distribution is minimized. The uniaxial alignments were achieved over a macroscopic length scale (ca. 1 cm). Cryo-TEM measurements revealed that the persistence length of the uniaxially aligned columns was at least 1 µm.