Mechanistic Evaluation of Motion in Redox-Driven Rotaxanes Reveals Longer Linkers Hasten Forward Escape's and Hinder Backward Translations

Mechanistic understanding of the translational movements in molecular switches is essential for designing machine-like prototypes capable of following set pathways of motion. To this end, we demonstrated that increasing the station-to-station distance will speed up the linear movements forward and slow down the movements backward in a homologous series of bistable rotaxanes. Four redox-active rotaxanes, which drove a cyclobis(paraquat-p-phenylene) (CBPQT(4+)) mobile ring between a tetrathiafulvalene (TTF) station and an oxyphenylene station, were synthesized with only variations to the lengths of the glycol linker connecting the two stations (n = 5, 8, 11, and 23 atoms). We undertook the first mechanistic study of the full cycle of motion in this class of molecular switch using cyclic voltammetry. The kinetics parameters (k, Delta G(double dagger) of switching were determined at different temperatures to provide activation enthalpies (Delta H-double dagger) and entropies (Delta S-double dagger). Longer glycol linkers led to modest increases in the forward escape (t(1/2) = 60 to <7 ms). The rate-limiting step involves movement of the tetracationic CBPQT(4+) ring away from the singly oxidized TTF+ unit by overcoming one of the thiomethyl (SMe) speed bumps before proceeding on to the secondary oxyphenylene station. Upon reduction, however, the return translational movement of the CBPQT(4+) ring from the oxyphenylene station back to the neutral TTF station was slowed considerably by the longer linkers (t(1/2) = 1.4 to >69 s); though not because of a diffusive walk. The reduced rate of motion backward depended on folded structures that were only present with longer linkers.