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 proteins1, 2 have mastered the art of converting conformational changes into directed motion, and have inspired the design of artificial systems3 such as DNA walkers4, 5 and light- and redox-driven molecular motors6, 7, 8, 9, 10, 11. But although controlled movement of single molecules along a surface has been reported12, 13, 14, 15, 16, 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 motors6, 8, 17—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.
Figures at a glance
Figure 1: Structure of the four-wheeled molecule. 
a, Structure and cartoon representation of the meso-(R,S-R,S) isomer. Red arrows indicate the direction in which the rotary action of the individual motor units propels the molecule. (R) and (S) indicate the absolute configurations at the stereogenic centres. The black solid and dashed wedges of the cartoon indicate the orientations of the methyl groups, respectively. b, Structural details of the rotary motor unit. The double bond (red) functions as the axle in rotation and undergoes trans-to-cis isomerization when electronically excited. Interconversion between helical conformers, arising from steric overcrowding in the region highlighted in blue, is achieved by vibrational excitation. The stereocentre in the cyclopentane ring determines the stability of each conformer and the direction of rotation of the motor. c, Schematic representation of the 360° rotation of the rotary motor, involving two double-bond isomerization steps and two helix inversion steps. Different colours in the model highlight the different positions of atoms during the rotor action. For clarity, the hexyl groups are substituted by methyl groups. d, Schematic representation of the experiment. The bias voltage U is applied to the sample. Electrons tunnelling through the molecule excite vibrational and electronic states and induce translational movement on the surface. e, Molecular model representation (side view) of the paddlewheel-like motion of the four-wheeled molecule (see also Supplementary Movie 4).
Figure 2: Linear movement of the meso-(R,S-R,S) isomer. 
a, STM image (imaging parameters: area 10.2 nm × 9.3 nm, current I = 74 pA, U = 47 mV) of the initial position. The black area was scanned only after the molecule moved into it. b, Trajectory depicting the individual steps taken (see Supplementary Movie 1). c, Final position after ten consecutive voltage pulses. d, The action spectrum for movement shows a voltage threshold at 500 mV. Each data point represents 8 to 40 manipulations performed on various molecules (I = 30–50 pA). Error bars represent the standard deviation from the probability for successful events (see equation (1) in the Supplementary Information). e, STM frames corresponding to individual steps of the trajectory in b excluding starting and final position.
Figure 3: Helix inversion at lower bias voltage and polarity dependence of propulsion. 
a, Voltage pulses between 200 and 350 mV lead to conformational changes but no movement (7.0 nm × 7.8 nm, I = 43 pA, U = 47 mV). b, Steps in the real-time traces (red arrows) of the tunnelling current (U = 200 mV) indicate the internal conformational changes of the molecule. c, The threshold voltage for helix inversion is 200 mV (>200 manipulations, I = 30–50 pA). Error bars represent the standard deviation from the probability for successful events (see equation (1) of the Supplementary Information). d, Negative bias (tunnelling from the sample to the tip) induces contrast changes but no movement (7.0 nm × 7.8 nm, I = 43 pA, U = 47 mV). Positive voltage pulses lead to movement and helix inversion.
Figure 4: Control over motion by the geometries of the four motors. 
ftp://mail.ihim.uran.ru/localfiles/nature10587.pdf
