Mechanical Properties of a Rotaxane Nanomuscle
Mechanically interlocked molecular architectures (MIMAs) are supramolecular assemblies that are made up of non-bonded molecular components. Rotaxanes and catenanes are members of this broad, topologically-complex, class of supramolecular assemblies. A rotaxane contains a long, dumbbell-shaped component that threads through at least one macrocyclic ring. A catenane contains two or more interlocked macrocyclic rings. Switchable rotaxanes and catenanes are designed such that a mechanical motion induced by an external stimulus results in a change in the system’s co-conformation. Switching may be induced electrochemically, photochemically, and by changes in pH, depending on the non-covalent interactions between the system’s subunits. Because of their switchable nature, these assemblies are promising candidates for nanoscale devices.
We considered a nanoscale muscle based on an electrochemically-switchable [3]rotaxane. This assembly is made up of two tetracationic cyclobis-(paraquat-p-phenylene) (CBPQT4+) rings threaded by a dumbell that contains two tetrathiafulvalene (TTF) and two naphthalene (NP) moieties.
In the system’s “reduced” state the rings preferentially bind to the widely separated TTF sites. Upon oxidation in response to an applied voltage or a chemical oxidizing agent, the rings initiate a “power stroke” wherein they shuttle from the two widely separated TTF2+ sites to the closely spaced NP sites, thereby generating a force.
Coating a surface with a monolayer of these rotaxanes, and then causing them to switch would cause the whole surface to bend if a sufficient amount of force is generated. Quantifying the force generated by this assembly is critical to discerning the range of applications for which it can be used, or conversely, how many rotaxanes are required to provide the force necessary for a certain application.
Using semi-empirical electronic structure calculations, we mapped potential energy curves (PECs) as a function of the positions of the CBPQT4+ rings for the system in its reduced and oxidized states. The amount of force generated by switching-induced motion was calculated from the first derivative with respect to the reaction coordinate on the oxidized state PEC. The average force generated by the “power stroke” is predicted to be approximately 224 pN, a value that is comparable to experimental results for similar rotaxanes that are reported to generate forces on the order of hundreds of pN.