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

Abstract

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 1·4PF₆ and 2·4PF₆ 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) (CBPQT⁴⁺) ring moves across either a TTF²⁺ or an MPTTF²⁺ 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 CBPQT⁴⁺ ring in 1⁴⁺ and 2⁴⁺ can be controlled to take place in only one direction upon tetra-oxidation. Cyclic voltammetry and ¹H NMR spectroscopy were used to investigate the switching mechanism and it was found that upon tetra-oxidation of 1⁴⁺ and 2⁴⁺, the CBPQT⁴⁺ ring moves first to a position where it is located between the TTF²⁺ and MPTTF²⁺ 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 1⁸⁺ and 2⁸⁺ were studied at different temperatures allowing the free energy of the transition state, when CBPQT⁴⁺ moves across TTF²⁺ (21.5 kcal mol^-1) and MPTTF²⁺ (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 CBPQT⁴⁺ ring in molecular machines with a 90% efficiency.