Self-assembled fibers of a discotic phthalocyanine derivative: Internal structure, tailoring of geometry, and alignment by a direct current electric field

The self-assembly of discotic molecules of a metal-free alkoxy-substituted phthalocyanine derivative, 2,3,9,10,16,17,23,24-octakis(octyloxy)-29H, 31H-phthalocyanine, in organic solvents was studied by optical microscopy, UV-vis spectroscopy, and scanning electron microscopy. These molecules self-assemble into fibers with well-defined internal structure and controlled external geometry on submicron length scales in dodecane solution. Spectroscopic evidence suggests a tilted molecular arrangement at low temperatures and a cofacial arrangement upon heating, which is consistent with the orthorhombic and ordered columnar hexagonal lattices for the solid or liquid crystalline mesophases of the neat material, respectively. At temperatures above the phase transition, molten side chains of the alkoxy-substituted phthalocyanine in a solvent with similar chemical structure facilitates the splitting of the fibers along their long axis into molecular fibers with characteristic diameters at the nanoscale. Strong interaction of the self-assembled fibers with a direct current electric field allows for their homeotropic or planar alignment on the conductive substrates for controlling the orientation needed for various optoelectronic device architectures. Our studies are consistent with a model describing the self-assembly of single molecular stacks due to pi-pi interaction of the phthalocyanine cores (tilted or cofacial), as well as additional stack aggregation due to side chains interdigitations as a result of van der Waals interaction.