ABSTRACT
Coordinated motions are essential in the operation of molecular machines. This feature can be achieved by landscaping the energy surface along the movement coordinates. Herein, we present an approach of using a single stimulus to modify the free energy curve describing the threading and shuttling of a ring along a linear molecule. This approach has been realized by locating two identical ring-binding sites near the axle termini.
ABSTRACT
The use of the electrostatic stoppers concept in the field of mechanically interlocked molecules is reported; these stoppers are chemically sensitive end groups on a linear guest molecule that allows for the conversion of a pseudo-rotaxane species into a rotaxane complex by a change in the medium acidity. The chemical stimulus causes the appearance of negative charges on both ends of the linear component, passing from cationic to anionic, and causing a significant ring-to-axle electrostatic repulsion. This phenomenon has two different and simultaneous effects: 1)â destabilizes the complex as a consequence of confining an anionic ring into an anionic axle, and 2)â increases the dissociation energy barrier, thus impeding ring extrusion. This newly formed metastable rotaxane species is resistant to solvent and temperature effects and performs as a two-state degenerated molecular shuttle in solution.
ABSTRACT
A new rigid cationic thread, based on the 1,2-bis(bipyridinium)ethane motif, and a 24-crown-8 anionic macrocycle self-assemble into a pseudo-rotaxane complex in aqueous solution. The presence of pH-responsive end groups on the thread, remote from the recognition motif, allows controlling the threading/dethreading rate without perturbing the complex structure or stability. The difference in the rate is attributed to the activation or deactivation of electrostatic barriers on the thread during the sliding process at different pH values.