Mechanically interlocked daisy-chain-like structures as multidimensional molecular muscles

Daisy chains (DCs) are garlands of flowers that can be worn as bracelets and necklaces. As a result of their beautiful interlocked structures and possible muscle-like motions, cyclic molecular DCs ([cn]DCs, where n is the number of repeating units) have long been attractive synthetic targets for supramolecular chemists. Herein we report artificial molecular muscles that-unlike one-dimensional (1D) biological muscles-contract and stretch in 2D or 3D. These systems have the structures of [c3]- and [c4]DCs with subcomponents that operate as molecular switches, powered through the addition or removal of Zn2+ ions to impart muscle-like behaviour. We assembled these [c3]- and [c4] DCs selectively by exploiting structural rigidity, coordination geometries and bond rotational barriers that disfavoured the formation of smaller homologues. The switching phenomena of our [c3]- and [c4] DCs resulted in the contracted molecular muscles stretching by approximately 23 and 36%, respectively, comparable to the value (27%) for linear biological muscles.