Molecular Machines Spotted On The Move

The molecular machines synthesized by Northwestern University’s J. Fraser Stoddart and coworkers have long held promise as switches in molecular electronics and other nanoscale devices. But the researchers have only been able to infer the movements of these tiny devices from spectroscopic data—until now. Teaming up with UCLA’s Paul S. Weiss and coworkers, Stoddart’s group is reporting the first direct observation of the movement of a molecular machine using scanning tunneling microscopy (ACS Nano, DOI: 10.1021/nn100545r). The machine is a bistable rotaxane—a ring wrapped around a dumbbell-shaped structure tethered to a gold surface. Depending on the rotaxane’s redox state, the cationic ring moves from a tetrathiafulvalene moiety at one end of the dumbbell to a 1,5-dioxynaphthalene unit at the opposite end. The researchers found that both the conformation of the dumbbell and interactions with the gold surface and with neighboring molecules influence the motion within these interlocked molecules. “To realize the full potential of these functional molecules at the nanoscale, it is imperative to understand their operation at the single-molecule level under environments relevant to actual device operation,” the researchers note.