Photoorientation of azobenzene moieties in self-assembled polyelectrolyte multilayers

Using the alternating layer-by-layer (LBL) assembly technique, ultrathin polyelectrolyte multilayers with azobenzene-containing bolaamphiphiles or ionenes were prepared. In the initial films the orientational distribution of the azobenzene units is azimuthally isotropic, but a small out-of-plane orientation was found. Except for the photo-inert sample of polyethyleneimine/4,4′-disulfatohexyloxyazobenzene 1, a Z-rich steady state was always established upon UV irradiation. The photoorientation process of the photochromic units was studied irradiating with linearly polarized visible light. This procedure lead to the induction of a significant in-plane dichroism due to the photoorientation of the photochromic unit, where the long axes become oriented perpendicular to the electric field vector of the incident light. For a sample of polydiallyldimethylammonium with 1, for example, an in-plane dichroism of about 0.34 was induced. Films of azobenzene-containing ionenes and polystyrenesulfonate showed a dichroism of up to 0.17, which varied with the irradiation conditions. The results demonstrate a restricted mobility of the azobenzene units in the LBL assemblies caused by aggregation, orientational order and/or electrostatic interactions of the charged groups with the polyelectrolyte matrix so that only a rather weak anisotropy can be induced in such systems.