Photoinduced anisotropy parallel and perpendicular to the electric field vector

Linearly polarized light induces alternatively an optical axis perpendicula r or parallel to the electric field vector of the incident light in films o f photochromic polymers caused by the orientation of the 4-alkoxy-4'-cyanoa zobenzene side groups. The orientational direction is exclusively governed by the wavelength of the linearly polarized irradiation. Light, which estab lishs a steady state between both azobenzene isomers, induces an orientatio n perpendicular to (e) over right arrow caused by the conventional mechanis m of photoorientation. Irradiating the same polymer film with red light an orientational direction parallel to (e) over right arrow is generated by th e photoselective formation of rod-like E isomers. The last orientation proc ess occurs exclusively after a photochemical pretreatment with a shorter wa velength generating Z isomers and destroying any initial order of the film.