Untwisting of a cholesteric elastomer by a mechanical field

A mechanical strain field applied to a monodomain cholesteric elastomer will unwind the helical director distribution. There is an analogy with the classical problem of an electric field applied to a cholesteric liquid crystal, but with important differences. Frank elasticity is of minor importance unless the gel is very weak. The interplay is between director anchoring to the rubber elastic matrix and the external mechanical field. Stretching perpendicular to the helix axis induces the uniform unwound state via the elimination of sharp, pinned twist walls above a critical strain. Unwinding through conical director states occurs when the elastomer is stretched along the helical axis.Comment: 4 pages, RevTeX 3 style, 3 EPS figure

Dynamic control of chirality and self-assembly of double-stranded helicates with light

Helicity switching in biological and artificial systems is a fundamental process that allows for the dynamic control of structures and their functions. In contrast to chemical approaches to responsive behaviour in helicates, the use of light as an external stimulus offers unique opportunities to invert the chirality of helical structures in a non-invasive manner with high spatiotemporal precision. Here, we report that unidirectional rotary motors with connecting oligobipyridyl ligands, which can dynamically change their chirality upon irradiation, assemble into metal helicates that are responsive to light. The motor function controls the self-assembly process as well as the helical chirality, allowing switching between oligomers and double-stranded helicates with distinct handedness. The unidirectionality of the light-induced motion governs the sequence of programmable steps, enabling the highly regulated self-assembly of fully responsive helical structures. This discovery paves the way for the future development of new chirality-dependent photoresponsive systems including smart materials, enantioselective catalysts and light-driven molecular machines