(711d) Quantitative Analysis of Photoalignment of Liquid Crystals and Conjugated Oligomers on Coumarin-Containing Polymer Films

Traditionally mechanical rubbing has been widely practiced for the preparation of electrooptic devices consisting of liquid crystals. As a noncontact alternative to rubbing, photoalignment using photoreactive polymer thin films has been actively pursued in recent years to avoid the problems arising from rubbing, such as dust, electrostatic charges, and damage to alignment coatings. In addition, photoalignment is instrumental to the realization of wide viewing angles in liquid crystal displays and modulation of diffraction gratings. There are three distinct approaches to photoalignment induced by polarized irradiation: photodegradation of polyimides, cis-trans isomerization of azobenzenes, and (2+2) cycloaddition of cinnamates or coumarins. It has been demonstrated that coumarins are advantageous in thermal and photochemical stability, absence of photo-induced isomerization, and accessibility to a wide range of pretilt angle. In principle, photoalignment is accomplished on a coumarin-containing polymer film that has been treated with linearly polarized UV-irradiation to define an axis along which coumarin monomers are preferentially dimerized. As dimerization proceeds, the increasingly populated dimers become less ordered along the polarization axis, while the diminishing monomers become better oriented in the perpendicular direction. In this process, coumarin dimers and monomers compete for liquid crystal orientation, resulting in crossover from a parallel to a perpendicular alignment at an intermediate stage of dimerization.

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.