Quantitative analysis of photoalignment of liquid crystals on coumarin-containing polymer films

Polymers containing 6- and 7-substituted coumarin moieties were prepared as photoalignment films through linearly polarized UV-irradiation to a varying fluence for an investigation of liquid crystal orientation. Model coumarin monomers and dimers were also synthesized and characterized as part of a novel approach to the interpretation of liquid crystal orientation in terms of the extent of dimerization, X. The experimental data for X as a function of fluence were used to validate the first-order kinetics with an exponentially decaying rate constant as the reaction proceeds. The kinetic model was employed to describe the evolutions of coumarin dimer's and monomer's orientational order. The model was instrumental to the visualization of liquid crystal orientation on photoalignment films at the early and the late stages of dimerization. Furthermore, the observed crossover in liquid crystal orientation was successfully interpreted by considering three factors: the relative abundance of coumarin dimers to monomers, their orientational order parameters, and the energetics of molecular interaction.