Kinetic Studies Reveal that the Secondary Station Impacts the Rate of Motion of Cyclobis(Paraquat-p-Phenylene) in Out-of-Equilibrium [2]Rotaxanes

Abstract

Control of movement in artificial molecular machines relies on the formation of out-of-equilibrium states that can subsequently interconvert to their ground states. However, a detailed description of molecular machines that are out of equilibrium is a challenge because they are often too short-lived to be characterized. Herein, the synthesis of two cyclobis(paraquat-p-phenylene) [2]rotaxanes that incorporate a redox-active monopyrrolotetrathiafulvalene unit as the primary station and either a hydroquinone or a xylyl moiety as the secondary station is described. It is shown that the bistable [2]rotaxanes can be pushed out of equilibrium by an oxidation/reduction cycle and since a steric barrier is located between the two stations, the out-of-equilibrium states of the [2]rotaxanes can be physically isolated as solids. This allows to make detailed spectroscopic and electrochemical investigations of the [2]rotaxanes in both the di-oxidized and un-oxidized states. The outcome of the studies shows that the replacement of the secondary hydroquinone station with a xylyl station has no impact on the thermodynamic properties but has a significant effect on the kinetic properties of the [2]rotaxanes illustrating that the nature of the secondary station can be used to control the speed of [2]rotaxane-based molecular machines.