Salts accelerate the switching kinetics of a cyclobis(paraquat-p-phenylene) [2]rotaxane

Abstract

The rate at which the macrocyclic cyclobis(paraquat-p-phenylene) ring of a bistable [2]rotaxane moves from a tetrathiafulvalene station to an oxyphenylene station upon oxidation of the tetrathiafulvalene station is found to be increased in the presence of added salts. Compared to the salt-free case, 0.1 M solutions of a series of tetraalkylammonium hexafluorophosphate salts (R₄N·PF₆, R = H, Me, Et or n-Bu) and of tetrabutylammonium perchlorate (n-Bu₄N·ClO₄) all afford an increased switching rate, which is largest in the case of n-Bu₄N·ClO₄ with smaller anions. Variation in the size of the ammonium cation has no significant effect. These results indicate that the addition of excess ions can be used as an accelerator to speed up shuttling processes in rotaxanes and catenanes based on the mobile cyclobis(paraquat-p-phenylene) ring, and that the choice of anion offers a convenient means of controlling the extent of this effect.