Interrogating a single light-driven rotary molecular motor with force

Inspired by Nature, where molecular motors are central to numerous life processes, chemists have developed a variety of artificial molecular machines, including rotative motors. These motors generate continuous directional motion when driven by an energy input. Due to their tiny size, an order of magnitude smaller than their biological counterparts, most of the knowledge on their functioning was gained through bulk population studies, revealing averaged behaviour at the macroscopic scale. Crucial properties, such as the output force upon actuation, which is a key challenge towards future nanomechanical machines, could not be directly measured, and were extracted from theoretical or semiempirical calculations. Single-molecule force spectroscopy has proven highly effective to uncover key energetic and mechanistic parameters, but has never been applied to synthetic rotative molecular motors, due to the extreme difficulty to probe processes at the 1-nanometer scale, while combining force measurements and the application of an external stimulus. Here, we developed distance-clamp experiments under UV-light irradiation to probe a single light-driven overcrowded-alkene rotary molecular motor at work. We applied mechanical loads to the motor and directly measured its operating force. The results also allowed the experimental validation of the principle of microscopic reversibility applied to light-driven motors, which had been theoretically predicted but never observed so far. Quantitative information obtained here is crucial to establish future design principles for nanometer-scale devices capable of efficiently producing work and mechanical tasks.