Artificial molecular muscles might someday be core components of artificial limbs. In the near term they will be useful to power mechanical movements in nanoscale devices. Switchable rotaxanes composed of polyether threads containing pi-donors encircled by a macrocyclic pi-acceptor have already been successfully incorporated in functional molecular electronic devices. To develop a rotaxane-based molecular muscle, it is intuitively desirable to replace the polyether threads with more rigid linkers. However, the ether oxygens are necessary for a successful template-directed synthesis, which relies on weak noncovalent interactions, particularly [CH-O] hydrogen bonds, in organic solvents. In an aqueous self-assembly process, the hydrophobic effect is expected to dominate, a feature which may allow for the efficient synthesis of rotaxanes with increased rigidity. For potential therapeutic and diagnostic applications of interlocked molecules, it is advantageous to study their switching behavior in aqueous solution. This proposal describes the self-assembly in aqueous solution of rigid-thread [2]pseudorotaxanes, [2]rotaxanes, and palindromic [3]rotaxanes which have been designed to promote contraction and extension, as a first step in the development of an artificial molecular muscle, and an examination of the molecular shuttling behavior of these rigid-thread rotaxanes in aqueous and organic solutions.
