Photoregulation of liquid crystal alignment by a composite polymer film containing retinoic acid

Abstract

The surface-assisted orientational photocontrol of nematic liquid crystal alignment by composite polymer films containing all-trans retinoic acid is described. The visual pigment derivative of retinoic acid was immobilized in poly(4-vinylpyridine) film through intermolecular hydrogen bonding between carboxyl group of the acid and pyridyl residues of the polymer. UV-vis spectral analysis indicated that photoirradiation of the polymer composite film with 365 nm light leads to E/Z photoisomerization of the retinoic acid unless exposure energies do not exceed about 1.0 J/cm2 while further irradiation gives rise to irreversible consumption of the chromophore. Illumination of the films with linearly polarized light at 365 nm brought about photodichroism, whose direction was perpendicular to the electric vector of the polarized light, as a result of the axially selective photochemical reactions of the all-trans-retinoic acid. A composite polymer film irradiated with linearly polarized 365 nm light exhibited the photocontrollability for azimuthal alignment of nematic liquid crystal molecules. The orientational direction of uniaxial liquid crystal alignment was perpendicular to the polarization plane of the actinic light, being consistent with the director of optical anisotropy induced by the polarization photochemistry of all-trans-retinoic acid. These results suggest that the surface-assisted photocontrol of liquid crystal alignment is attributable to the anisotropic orientation of the retinoic acid at an outermost surface of a photoactive composite polymer film. Copyright © 2000 John Wiley & Sons, Ltd.