Mechanistic studies of isomeric [2]rotaxanes consisting of two different tetrathiafulvalene units reveal that the movement of cyclobis(paraquat- p -phenylene) can be controlled.

Controlling the movement in artificial molecular machines is a key challenge that needs to be solved before their full potential can be harnessed. In this study, two isomeric tri-stable [2]rotaxanes 1·4PF 6 and 2·4PF 6 incorporating both a tetrathiafulvalene (TTF) and a monopyrrolotetrathiafulvalene (MPTTF) unit in the dumbbell component have been synthesised to measure the energy barriers when the tetracationic cyclobis(paraquat- p -phenylene) (CBPQT 4+ ) ring moves across either a TTF 2+ or an MPTTF 2+ dication. By strategically exchanging one of the thiomethyl barriers on either the TTF unit or the MPTTF unit with the bulkier thioethyl group, the movement of the CBPQT 4+ ring in 1 4+ and 2 4+ can be controlled to take place in only one direction upon tetra-oxidation. Cyclic voltammetry and 1 H NMR spectroscopy were used to investigate the switching mechanism and it was found that upon tetra-oxidation of 1 4+ and 2 4+ , the CBPQT 4+ ring moves first to a position where it is located between the TTF 2+ and MPTTF 2+ dications producing high-energy co-conformations which slowly interconvert into thermodynamically more stable co-conformations. The kinetics of the movement occurring in the tetra-oxidised [2]rotaxanes 1 8+ and 2 8+ were studied at different temperatures allowing the free energy of the transition state, when CBPQT 4+ moves across TTF 2+ (21.5 kcal mol -1 ) and MPTTF 2+ (20.3 kcal mol -1 ) at 298 K, to be determined. These results demonstrate for the first time that the combination of a TTF and an MPTTF unit can be used to induce directional movement of the CBPQT 4+ ring in molecular machines with a 90% efficiency.