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.
The use of the electrostatic stoppersc oncept in the fieldo fm echanically interlocked molecules is reported;t hese stoppers are chemically sensitive end groupso n al inear guest molecule that allows for the conversion of a pseudo-rotaxane speciesi nto ar otaxanec omplex by a changei nt he medium acidity.T he chemicals timulus causes the appearanceo fn egative chargeso nb oth ends of the linear component, passing from cationic to anionic, and causing as ignificant ring-to-axle electrostatic repulsion. This phenomenon has two different and simultaneous effects:1 )destabilizes the complexa saconsequence of confining an anionic ring into an anionic axle, and 2) increasest he dissociatione nergyb arrier, thus impeding ring extrusion. This newly formed metastable rotaxane species is resistant to solventa nd temperature effects and performs as at wo-state degenerated molecular shuttle in solution.
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