Spacer-Free Ionic Dye-Polyelectrolyte Complexes: Influence of Molecular Structure on Liquid Crystal Order and Photoinduced Motion

A series of spacer-free ionic azobenzene-containing stoichiometric complexes was prepared from monosulfonated azo dyes and cationic polyelectrolytes [methylated poly(4-vinylpyridine) (PVP) and poly(dimethylaminoethyl methacrylate) (PDM)]. Their thermal and structural characteristics and optical responses, particularly photoinduced birefringence (PIB) and surface relief grating (SRG) inscription, were investigated as a function of selected molecular parameters. All of the complexes have high apparent T-g values, 180-210 degrees C, and show liquid crystal (LC) order of the single-layer SmA type from ambient to very high temperature, usually to degradation. The range of LC order is greater in the complexes of chromophores with longer alkyl tails and shorter in the complex of the chromophore with an OH end group and the complex of methyl orange (MO) with PDM. PIB, SRG quality, and diffraction efficiency were all shown to depend in a similar way oil molecular structural features: the more rigid the molecular structure, the higher the PIB, the better its thermal and temporal stability, and the greater the SRG amplitude. Thus, a flexible alkyl unit in the polyelectrolyte component or ill the chromophore tall or spacer reduces the optical performance, with a clear dependence on alkoxy tail length (e.g., no SRG formation was observed for the complex with a hexoxy tail), whereas the most rigid complex, MO/PVP, provides the best performance, contrasting with many previous literature reports that suggest a beneficial role for flexible spacers. The present paper shows that flexible moieties increase relaxation of photoinduced orientation. The hydroxyl end group also provides much improved PIB and SRG performance, attributed to its rigid structure along with a (temperature-dependent) physical cross-linking effect of H-bonding interactions.