Molecular Photogearing for Controlling Rotary Motion at the Nanoscale

Means to control rotary motion at the nanoscale are central to the design and operation of artificial molecular machines powered by such motion. For this task, it is natural to consider molecular gears, which are characterized by their ability to perform coupled rotations around two (or more) chemical bonds. However, most such gears rely on passive, thermal activation, which makes them sensitive to Brownian motion. In this concept, following a brief review of the historic development of molecular gears, we highlight some recent experimental and computational results that have helped show how this problem can now be addressed by means of the type of molecular photogearing achieved when the double-bond rotary motion produced by a light-activated molecular motor is transmitted through space onto a single-bond axis. Furthermore, we discuss the formidable challenge to maintain a preferred direction of rotation during this transmission, which is critical for performing mechanical work. Finally, we point out some research directions suitable for maximizing the future usefulness of molecular gears and photogears.