(227e) Cyclic and Polymeric Rotaxanes

Among the most important advances in chemistry in the last 60 years is the synthesis of molecules featuring a mechanical bond or the mechanical interlocking of components. A molecule featuring a mechanical bond has additional mechanical degrees of freedom, namely the relative position and orientation of the interlocked components, that can have dramatic and controllable effects upon the shape and size of the molecule. An example of such an interlocked molecule is a linear rotaxane, comprised of a ring threaded with an axle that is stoppered at each end to prevent the ring from falling off. As the ring and axle are not covalently linked, the ring is free to translate the length of the axle and provides a continuum of accessible mechanical conformers, which can be unlocked to do work, as in a drug delivery system, or to dissipate energy, as in a molecular shock absorber.

Here we describe the mechanical conformers of (1) cyclic rotaxanes, and (2) branched polymers. We demonstrate that with only a few mechanical bonds, we can achieve significant fluctuations in molecular size and shape for mechanical bonds that are unbiased (as in freely mobile rings or an entropy-dominated molecule), as well as those biased with attractive stations (as in a molecular switch with added enthalpy control). Our statistical mechanics models suggest that the cyclic rotaxane can be used as a switchable host in an inclusion complex and that a mechanically-branched polymer provides switchable colloidal stabilisation via an adjustable depletion force.