Controlling the rotary direction of light-driven molecular motors by topological confinement.

Inspired by biological machines, artificial light-driven molecular motors based on overcrowded alkenes have been developed. They can perform repetitive unidirectional rotary motion around the central olefinic bond, triggered by light and heat. However, unlike natural motors, which are sophisticated at tuning their rotary direction, it remains a major challenge to control the rotary direction of manmade motors on demand. Here, we show that by covalently connecting both the upper and lower halves of the molecular motor with ethylene glycols, topological confinement can dictate whether the motor is rotating forward or backward. Experimental data demonstrate that when the length of the macrocycle is short, rotary motion is fully blocked, while a medium-sized chain allows exclusive backward thermal E-Z isomerization instead of the traditional thermal helix inversion step, which enables forward rotation of the motor. With increased ring size, both forward and backward rotary motions are observed. DFT calculations reveal that the length of the chain plays a key role in determining the difference in energy barriers for the two abovementioned competing processes. By taking advantage of the backward rotation of the motor, a complex molecular machine is built, and the shuttling motion of the rotaxane along its axle can be dynamically tuned. These findings represent an important step towards the future design of complex synthetic molecular machines with dynamic functions.