New Insight into Photoalignment of Liquid Crystals on Coumarin-Containing Polymer Films Chunki Kim, ²,§ Anita Trajkovska, ²,§ Jason U. Wallace, ²,§ and Shaw H. Chen* ,²,‡ Department of Chemical Engineering and Laboratory for Laser Energetics, UniVersity of Rochester, 240 East RiVer Road, Rochester, New York 14623-1212 ReceiVed February 3, 2006; ReVised Manuscript ReceiVed March 23, 2006 ABSTRACT: 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 monomer conversion. The experimental results for monomer conversion as a function of fluence were used to validate the first-order kinetics with an exponentially decaying rate constant as the reaction proceeds. A kinetic model was constructed to describe the evolutions of the orientational order on the parts of the reacted and the unreacted coumarin moieties. 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 on the polymer film comprising 7-substituted coumarin moieties was successfully interpreted by considering three factors: the relative abundance of the reacted and the unreacted coumarin moieties, the degrees of their orientational order predicted by the kinetic model, and the energetics of molecular interaction. Introduction A uniaxial orientation of liquid crystals is the foundation of a wide variety of electrooptic devices. Mechanical rubbing of a polyimide film has been widely practiced for its simplicity, but the dust, electrostatic charges, and mechanical damage resulting from rubbing have adverse effects on device perfor- mance and lifetime. 1-3 Photoalignment is a noncontact method intended to avoid the problems confronting rubbing. Photo- alignment also lends itself to patterning, which is instrumental to accomplishing wide viewing angles in liquid crystal displays 4 and tunability in electrooptic devices. 5-7 Three distinct ap- proaches to photoalignment have been explored in recent years: anisotropic degradation of polyimides, 8-12 cis-trans isomerization of azobenzenes, 13-19 and anisotropic (2 + 2) cycloaddition of cinnamates 20-28 or coumarins. 4,28-36 Coumarin- containing polymers are advantageous in terms of thermal and photochemical stability without complication from isomerization in addition to offering a wide range of pretilt angle. 4 For the preparation of a photoalignment layer, a coumarin-containing polymer is normally spin-cast into a thin film in which coumarin moieties are randomly oriented. Irradiation with UV light at wavelengths longer than 300 nm causes dimerization of cou- marin monomers. At a low irradiation energy (or fluence) of linearly polarized irradiation, dimerization is limited to coumarin moieties whose absorption dipoles fall along the polarization axis. These preferentially placed coumarin dimers serve to orient an overlying nematic liquid crystal along the polarization axis of UV irradiation. At a high fluence, unreacted coumarin moieties have their absorption dipoles lie largely perpendicular to the polarization axis, while coumarin dimers are nearly randomly placed. As a result, the overlying nematic liquid crystal is oriented perpendicular to the polarization axis. This crossover in liquid crystal orientation has been extensively reported in the past 29,30,35 without identifying critical parameters for a quantitative analysis. The present study was motivated by the extent of photo- dimerization as a new perspective on liquid crystal orientation as illustrated by polymers containing 6- and 7-substituted coumarin pendants. Specific aims include (1) to characterize the extent of dimerization in a photoalignment film by UV- vis absorption spectroscopy, (2) to calculate the orientational order parameters for both the unreacted and reacted coumarin moieties through kinetic modeling of dimerization in a photo- alignment film, (3) to characterize liquid crystal orientation on the photoalignment film as a function of the extent of dimer- ization, and (4) to interpret the crossover behavior in terms of the relative abundance and the orientational order of reacted and unreacted coumarin moieties in addition to the energetics of molecular interaction. To accomplish these objectives, poly- mers, monomers, and dimers I and II as depicted in Chart 1 were synthesized and characterized. Experimental Section Material Synthesis. The synthesis schemes and purification procedures for polymers, monomers, and dimers I and II are described in the Supporting Information. The analytical and 1 H NMR spectral data are presented in what follows. Poly[6-[[[6-(methacryloyl)oxy]hexyl]oxy]coumarin], Polymer I. Anal. Calcd: C, 69.07; H, 6.71. Found: C, 68.89; H, 6.38. 1 H NMR spectral data (400 MHz, CDCl 3 ): δ 0.90-1.80 (13H, polymer main chain and spacer linkage), 3.79 (4H, -COOCH 2 -, -CH 2 - OAr-), 6.35 (1H, -HCdCHCO-, coumarin), 6.90-7.17 (3H, aromatics), 7.78 (1H, -HCdCHCO-, coumarin). Poly[7-[[[6-(methacryloyl)oxy]hexyl]oxy]coumarin], Polymer II. Anal. Calcd: C, 69.07; H, 6.71. Found: C, 68.70; H, 6.74. 1 H NMR spectral data (400 MHz, CDCl 3 ): δ (ppm) 0.90-1.80 (13H, polymer main chain and spacer linkage), 3.96 (4H, -COOCH 2 -, -CH 2 OAr-), 6.19 (1H, -HCdCHCO-, coumarin), 6.69-6.79 (2H, aromatics), 7.32 (1H, aromatics), 7.61 (1H, -HCdCHCO-, coumarin). 6-(Heptyloxy)coumarin, Monomer I. Anal. Calcd: C, 73.82; H, 7.74. Found: C, 73.70; H, 7.80. 1 H NMR spectral data (400 ² Department of Chemical Engineering. ‡ Laboratory for Laser Energetics. § Three coauthors in alphabetical order for equal contributions to this work. * To whom correspondence should be addressed: e-mail shch@ lle.rochester.edu. 3817 Macromolecules 2006, 39, 3817-3823 10.1021/ma060269o CCC: $33.50 © 2006 American Chemical Society Published on Web 05/06/2006