Mechanically interlocked molecular architectures (MIMAs) are supramolecular assemblies that are composed of mechanically linked molecular components. Switchable rotaxanes are a subclass of MIMAs that are good candidates for molecular machines because of their ability to undergo reversible and repeatable co-conformational isomerism in response to an external stimulus. An acid-base switchable [2](2)rotaxane has been reported to exhibit a 50% change in length upon a stimulus-induced change in protonation state, suggesting that it could serve as the fundamental element of a molecular muscle". In the present work, the mechanical behavior of this topologically complex system is explored with calculations based on semi-empirical electronic structure theory. The average and instantaneous forces generated by the ring shuttling upon a change in protonation state are reported. The calculations predict that the original system will not work as envisioned. Although the calculations confirm a 50% change in length, they also reveal an energy barrier to ring shuttling upon deprotonation, preventing the immediate delivery of force. Therefore, a modification to the system is proposed. Similar semi-empirical calculations were carried out on the modified system and the force calculated. The average force generated from the modified system is predicted to be approximately 141 pN. The results demonstrate that the proposed modification reduces the force generated by ring shuttling by 41% but eliminates the energy barrier to ring shuttling so that the force is immediately available. The modified system could potentially serve as the fundamental element of a molecular muscle.

Proposed modification to a muscle-like acid-base switchable [2](2)rotaxane for improved force delivery

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.