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

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Abstract

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, ΔG(‡)) of switching were determined at different temperatures to provide activation enthalpies (ΔH(‡)) and entropies (ΔS(‡)). Longer glycol linkers led to modest increases in the forward escape (t(1/2) = 60 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.

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