Direct photoalignment

A vast amount of investigations on photoalignment of LCs has been reported during the past one and a half decades. Irradiation with LPL induces photo-induced anisotropy, in which molecules are generally oriented to an unexcitable direction, namely, perpendicular to the electric vector of the polarization direction. This effect is called photoinduced optical anisotropy (Seki, 2004 Natansohn and Rochon, 2002 Ichimura, 2000). 

Fig ure 8.3. TEM images of photoaligned silica mesochannels. (a) and (b) indicate the directions of observation. Reprinted with permission from the Royal Chemical Society. 

It was also reported that photoreactivity of side-chain liquid-crystalline polymers can align liquid crystals both in a parallel mode or perpendicularly, depending on the degree of the photoreaction of the polymers. Presumably this particular polymer can multi-photoalign the liquid crystal pattern without a change of the direction of the linearly polarized UV light. The chemical structure of such an aligning polymer is depicted as follows ... 

FIGURE 7.2 Schematic illustration of photoalignment of azobenzene-contaiiiing PLCPs. After the trans-cis-trans cycles, the azobenzene mesogens are aligned perpendicularly to the LPL polarization direction. The photoalignment state can be destroyed by CPL irradiation or heating. 

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