Computational Analysis of Switchable Rotaxanes
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A rotaxane is an assembly of interlocked molecules in which a dumbbell-shaped component is encycled by one (or more) chemically independent ring component(s). For example, a rotaxane is created when a cyclodextrin or another ring molecule such as a crown ether (called the “shuttle” or “ring”) is threaded with a linear molecule (called the “shaft” or “chain”). The shaft terminates with a bulky substituent on each end to prevent its unthreading, and is therefore sometimes referred to as a “dumbbell” because of its appearance. A rotaxane with multiple rings upon a single shaft is a polyrotaxane. In some rotaxanes, the macrocyclic component can shuttle back and forth between binding sites on the dumbbell component. Such a system constitutes a molecular shuttle. With appropriate molecular design, the shuttling process can be controlled reversibly by external stimuli, including light, electrons, pH, and the chemistry of the environment, thereby endowing the system with the essential characteristics of a mechanical machine or device, but on the molecular scale.
A pseudorotaxane is closely related to a rotaxane. This type of complex also consists of a chain component threaded through a macrocyclic ring, but unlike a rotaxane, a pseudorotaxane can undergo dissociation into its components without breaking any covalent bonds. A pseudorotaxane in which this threading and unthreading can be controlled with an external stimulus has the essential characteristics of a piston and cylinder, and external inputs are converted into cyclical translational motion.
Early reports of switchable rotaxanes include the thermally switched system reported in 1991 by Anelli et al., and the electrochemically switched system prepared by Bissell et al. in 1994. Since then, the construction of rotaxane-based molecular shuttles has produced numerous schemes for shifting the ring between two or more “stations” on the linear chain in response to external stimuli.
Particularly attractive for the construction of molecular machines are rotaxanes with a redox- and/or photo-active unit present in one or more of their constituent components. The synthesis of such systems often relies on the presence of molecular recognition sites in the components, and on the use of transition metal templates or complementary π-electron-rich and π-electron-deficient recognition segments to enable the construction process. Three approaches have been proposed for the construction of rotaxanes: 1) threading of a long molecule through a ring, followed by capping the end(s) of the thread with stoppers; 2) slipping of a preformed ring over the stoppers of a preformed dumbbell-shaped component; 3) clipping of a preformed dumbbell with a suitable U-type component, which is subsequently cyclized. Technically, the complex formed through the second approach is a pseudorotaxane, because the ring can slip onto and off the thread without breaking any chemical bonds. However, if the barrier to the slipping process is sufficiently high, the rate of decomposition will be exceedingly slow and the system is effectively a rotaxane.