Design and synthesis of high susceptibility photoactive self-assembled monolayers for the photoalignment of liquid crystals

Alignment of liquid crystals (LCs) on photoactive surfaces is interesting and potentially useful, obviating the need for problematical mechanical buffing. Many approaches for photo buffing involve irreversible photochemistry, providing a permanently altered surface for alignment. However, in some applications, reversible modification of the alignment properties of the substrates of an LC cell "in situ" would be advantageous.; In order to explore ultra-thin photo-addressable surfaces for such an application, a method for preparation of high quality functionalized siloxane monolayers using trialkoxysilane precursors with butylamine catalysis has been developed. A group of new siloxane precursors were prepared using the "push-pull" azobenzene dye methyl red (2-(4-dimethylamino-phenylazo)-benzoic acid) and related derivatives. High quality monolayers on glass were prepared using these siloxane dyes, and in collaboration were characterized by several techniques, including SHG and polarized light microscopy. Irradiation of these surfaces with plane-polarized visible laser light at normal incidence caused a rapid (about a microsecond at the highest intensities studied) net orientation of the dye molecules such that the transition moment ends up preferentially perpendicular to the plane of polarization of the incident light (the Todorov "optical ratchet" effect). This, and anisotropization of the surface was readily apparent in simple LC devices, causing a relatively rapid (several seconds) and reversible re-orientation of the nematic LC director in parallel-aligned cells in situ, upon irradiation of the cells with low-intensity polarized light from an overhead projector or microscope illuminator. In addition to being an order of magnitude faster at the alignment of LCs than any systems reported to date, these surfaces also required an order of magnitude less energy to do so. Details of the preparation of these photoactive monolayers, and their physical properties, are discussed herein.