With the assumption of no orientational relaxation involving coumarin dimers or monomers, we have constructed a kinetic model for the prediction of the orientational order parameters governing coumarin dimers and monomers as functions of the extent of dimerization. Three parameters are relevant to liquid crystal orientation including the crossover behavior: relative abundance and orientation order of coumarin dimers and monomers in addition to the energetics of their interactions with liquid crystalline molecules. Based on this kinetic model, we have also demonstrated that it is the interplay between these three parameters that is responsible for crossover in liquid crystal orientation. This presentation introduces a methodology for the experimental determination of coumarin dimers' and monomers' orientational order parameters at the early stage of polarized UV-irradiation, where dimers are responsible for aligning liquid crystals. Key findings are summarized as follows:
(1) Through polarized absorption spectroscopy and computational chemistry, the absorption dipoles of coumarin monomer and dimer are located as parallel to their long molecular axes. In addition, the orientational order parameters of coumarin monomers and dimers are characterized by UV-Vis absorption dichroism as functions of the extent of photodimerization.
(2) The greater extent of coumrain dimerization and the better orientational order of dimers with a longer flexible spacer originate from (i) the higher rotational mobility of coumarin monomers, and (ii) the lack of mobility of coumarin dimers because of their molecular size and anchoring between polymer chains.
(3) The photoalignment film's ability to orient a conjugated oligomer is quantifiable by the product of dimers' mole fraction and their orientational parameter, reflecting the demand on the photoalignment film because of the extended molecular length. In contrast, nematic liquid crystal is less demanding because of the relatively short molecular length.