Azimuthal Photoregulation of a Liquid Crystal with an Azobenzene-Modified Polyimide Langmuir-Blodgett Monolayer

A photochemical method to generate a homogeneous alignment of a nematic liquid crystal by means of a polyimide monolayer is presented. A Langmuir-Blodgett monolayer of a polyimide bearing p-cyanoazobenzene residues transferred onto a silica substrate was irradiated with linearly polarized light and was then used in the assembly of a cell filled with a nematic liquid crystal. The cell showed a uniaxial alignment and good storage stability. In contrast to this, no homogeneous alignment was induced at all when a photoirradiated monolayer was kept standing for 2 days before the cell fabrication, suggesting that the azimuthally orientated state of the azo chromophore is considerably stabilized by the presence of liquid crystal molecules. The direction of the alignment determined by the photodichroism of a doped dichroic dye was perpendicular to the electric vector of the actinic light and could be regulated by changing the polarization plane angle of the actinic light. Contrary to this, monolayers of the polyamic salt used as the precursor of the polyimide displayed no photoregulation of the liquid crystal alignment. The reversible reorientation of the liquid crystal was achieved by exposing the cell to linearly polarized light of a different polarization plane angle.