Photoinduced Reorganization of Motor-Doped Chiral Liquid Crystals: Bridging Molecular Isomerization and Texture Rotation

We recently reported that the photoisomerization of molecular motors used as chiral dopants in a cholesteric liquid crystal film induces a rotational reorganization which can be observed by optical microscopy and produces the motion of microscopic objects placed on top of the film (Feringa, B. L.; et al. Nature 2006, 440, 163; J. Am. Chem. Soc. 2006, 128,14397). The mechanism underlying the mesoscopic manifestation of the molecular process was not fully understood, and here we present a joint theoretical and experimental investigation, which provides a detailed insight into the mechanism of texture rotation. This description allows us to identify the interplay between the chemical structure of the chiral dopant and the material properties of the liquid crystal host, and to quantify their role in the observed dynamic phenomenon. We have found that a crucial role is played by the hybrid anchoring of the liquid crystal, with the director parallel to the substrate and perpendicular to the interface with air; in this configuration an almost unperturbed cholesteric helix, with its axis normal to the substrate, is present in most of the film, with strong deformations only close to the free interface. The texture rotation observed in the experiment reflects the rotation of the director during the unwinding of the cholesteric helix, produced by the change in shape of the chiral dopant under photoisomerization. The rotational reorganization is controlled by the photochemical process, via the coupling between the chirality of the dopant and the elastic properties of the liquid crystal host.