Photoinduced anisotropy of second-harmonic generation from azobenzene-modified alkylsiloxane monolayers

Noncontact alignment of liquid crystal displays offers the advantage of reduced contamination and minimal surface, charging. This approach also provides a means of reversible alignment after a device has been assembled. With this objective we have synthesized self-assembled monolayers based on dimethylaminoazobenzene units covalently attached to a glass surface by means of a short alkylsiloxane anchor, a derivatized version of methyl red (d-MR). The resulting architecture favors an orientation in which the. axis of the azobenzene group should be nearly parallel to the surface with an isotropic azimuthal distribution. Under illumination with polarized light the trans-cis isomerization and subsequent relaxation serves, to wiggle the molecule into an orientation perpendicular to the treatment polarization. We have tested this scenario using optical second harmonic generation and supporting optical techniques. We are able to identify a surface order, parameter that characterizes the photoalignment of the azobenzene group, and have shown that the treatment illumination promotes the formation of H aggregates of the azobenzene species. We have also demonstrated that the treated d-MR layer successfully aligns a nematic liquid crystal. (C) 2003 American Vacuum Society.