Groningen chemists made a molecule that resembles a four-wheel drive vehicle, but that also brings a wondrous insect to mind using four paddle-legs to move about. Electrical energy from the tip of a Scanning Tunnel Microscope (STM) serves to power the vehicle.
The motor is used as a wheel. The new molecule has a long midsection with four rotating wheels at its corners. Or perhaps it would be better to call them paddles, as the extremities are not completely round. As a result, the vehicle tends to bumble along a bit.
The four-wheeled molecule receives its power to move from an STM tip that serves as something like a train’s overhead wire. An STM (scanning tunnel microscope) feels its way across a surface with a pointed wire without any actual ‘physical’ contact taking place – the very last bit is bridged by an electrical charge. The energy from the STM tip ‘tunnels’ into the molecule, which then enters a higher energy level, leading to the wheels turning step by step; this process is analogous to energy transfer by photons.
Side view of a molecular model showing how the molecule moves across a surface
In the Nature article STM images show the molecule moving across a copper surface. After ten steps it has moved 6 nanometres in a more or less straight line. To prove that it is indeed the wheels that are propelling it, the researchers show in follow-up experiments what happens when the wheels rotate in the wrong direction. The chemists studied molecules where the rear wheels turned in the opposite direction to the front ones, or where the left wheels rotated backwards while the right pair rotated forwards or vice versa. The resulting movements are exactly what you would expect – the molecule hardly shifts position or just zigzags around a bit.
STM contrasts of individual isomers of the molecule. This figure shows molecular models and expected STM contrast shapes of three situations; correctly landed meso-isomer, wrongly landed meso-isomer and (R,R-R,R)-isomer. In each case all four motor wheels adopt the so-called stable conformation. Side view and top view give an impression of the 3D geometry of the isomers. The areas highlighted by the pink ovals show anticipated STM signals due to protruding aromatic parts of the molecules (note that also other factors play a role in the experimentally observed images, see discussion above). The bottom panel of the figure shows STM images found for the related isomers. The assignment of the STM contrasts to particular isomer must be taken with caution as various conformers of each isomer can give similar contrasts.
Propelling single molecules in a controlled manner along an unmodified surface remains extremely challenging because it requires molecules that can use light, chemical or electrical energy to modulate their interaction with the surface in a way that generates motion. Nature’s motor proteins have mastered the art of converting conformational changes into directed motion, and have inspired the design of artificial systems such as DNA walkers and light- and redox-driven molecular motors. But although controlled movement of single molecules along a surface has been reported the molecules in these examples act as passive elements that either diffuse along a preferential direction with equal probability for forward and backward movement or are dragged by an STM tip. Here we present a molecule with four functional units—our previously reported rotary motors—that undergo continuous and defined conformational changes upon sequential electronic and vibrational excitation. Scanning tunnelling microscopy confirms that activation of the conformational changes of the rotors through inelastic electron tunnelling propels the molecule unidirectionally across a Cu(111) surface. The system can be adapted to follow either linear or random surface trajectories or to remain stationary, by tuning the chirality of the individual motor units. Our design provides a starting point for the exploration of more sophisticated molecular mechanical systems with directionally controlled motion.