Azobenzene Sulphonic Dye Photoalignment as a Means to Fabricate Liquid Crystalline Conjugated Polymer Chain-Orientation-Based Optical Structures

The use of a noncontact photoalignment method to fabricate in-plane optical structures, defined by the local uniaxial ordering of liquid crystalline conjugated polymer chains, is reported. Molecular orientation is demonstrated for both green-light-emitting fluorene-benzothiadiazole alternating copolymer F8BT and F8BT/red light emitting complex copolymer Red-F binary blend films deposited on a well-known azobenzene sulphonic dye photoalignment material SD1. Absorption anisotropy ratios of up to 9.7 are readily achieved for 150 nm thickness F8BT films. Spatial pattern definition, afforded by masking the UV polarized light exposure of the photoalignment layer, allows the fabrication of optical structures with a resolution down to the micron scale. The alignment process is further extended to enable the serial, independent orientation of films deposited on top of each other and to permit the molecular orientation to follow curvilinear patterns. In the former case, this allows F8BT bilayer structures to be fabricated that show even higher absorption anisotropy ratios, up to ≈12, close to the theoretical limit for the previously deduced ≈22° optical transition dipole moment angle relative to the chain axis.