Dual-stimuli pseudorotaxane switches under kinetic control

Abstract

A series of dumbbell-shaped sec-ammonium salts with bulky (pseudo)stoppers (‘speed bumps’) were tested for their ability to form pseudorotaxanes with a redox-switchable, tetrathiafulvalene (TTF)-decorated [24]crown-8 ether. Depending on the size of the pseudostoppers, fast (less than ten minutes), slow (hours to days) and very slow (no pseudorotaxanes observed) threading has been observed. NMR spectroscopy as well as tandem mass spectrometry indicate the formation of non-threaded face-to-face complexes prior to pseudorotaxanes formation. Both isomers can be distinguished by their substantially different stability in collision-induced dissociation (CID) experiments. Two external stimuli affect the stability of the pseudorotaxanes: Deprotonation of the ammonium ion results in fast dethreading, while dethreading is much slower when induced by the charge repulsion upon chemical oxidation of the TTF moiety. Remarkably, the same steric bulk of the pseudostopper thus leads to different dethreading rates depending on the stimulus applied. Based on these findings, two redox-switchable rotaxanes containing a 1-naphthyl and a phenyl moiety as sterically different ‘speed bumps’ in the axle centre were synthesised. Bulk electrolysis of the rotaxanes did not result in the expected macrocycle translocation on the axle independent of the ‘speed bump’ as a remarkable consequence of the mechanical bond.