Chemistry in motion--unidirectional rotating molecular motors.

Molecules are in constant motion, if not frozen around 0 K, but their Brownian motion is random. Overcoming this randomizing effect and generating directional motion at the molecular level with artificial systems is still a challenge. Research in this area is inspired by the vision of transferring the concept of an engine or a motor to the molecular level. That this is possible is illustrated by the directional processes found in nature: cell division, translocation of organelles, and membrane transport all rely on directional movement, while processes such as replication, transcription, and translation require encoded information sequences to be read and copied in a directional manner. Macroscopic engines and molecular motors both convert chemical, electrical, or light energy into mechanical work, yet their mode of operation is very different. Because of their dimensions molecular motors must operate at energies only slightly higher than those of the thermal bath surrounding them. They are actuated by Brownian motion and the key to their function is to give a direction to these undirected processes. Chemistry's role is to select one direction from all possible movements by lowering the energy profile of this directional movement compared to all the others. This makes this movement happen preferentially. Chemical or photochemical steps fuel these selection processes. Leigh et al. recently reported a net relative unidirectional circumrotation in a mechanically interlocked molecular rotor. The [2]catenane system (Scheme 1) consists of a larger macro-