A research team from the University of Copenhagen, the Weizmann Institute of Science, and other institutions has reported a single-molecule machine in which a helical molecule connected to two gold electrodes is driven by electric current, and its motion changes how easily current flows. The paper, by Anil Kumar Singh, León Martin, Matea Sršen, and colleagues, was published on October 22, 2026 in Nature as an accelerated article preview of an accepted, peer-reviewed manuscript. In the experiments, reversing the voltage polarity also reversed the order in which multiple current states switched, and raising the voltage further produced a sharp increase in current. The team explains these phenomena as the result of molecular rotation, based on calculations showing that electron flow depends on rotation angle, together with control experiments using a molecule without a helical structure.
However, the researchers did not directly observe the molecule rotating continuously at high voltage. The results are an inference about molecular motion drawn from current measurements and theoretical calculations, and the experiments were conducted at an extremely low temperature of 4.2 K. Further verification will be needed to realize molecular machines that operate stably at room temperature.
The order of current-state switching reverses with the voltage direction
The team used [13]helicene, a molecule in which aromatic rings are fused into a helix. The molecule was synthesized at the University of Copenhagen, attached to two gold electrodes via sulfur atoms, and its current was measured at the Weizmann Institute of Science. A structure in which a single molecule is sandwiched between two electrodes is called a "single-molecule junction." In this study, the researchers examined how molecular motion and electron transport influence each other while both ends of the molecule were attached to the electrodes.
The helical structure of helicene recalls the Archimedes screw, an ancient device that moves water by rotating. However, what was studied here is a mechanism by which the flow of electrons generates a rotational force on the molecule; it does not replicate the operation of a water-lifting screw at molecular scale.
The experiments were performed at 4.2 K. Using the break-junction method, which mechanically adjusts the gap between gold electrodes, the team formed junctions from a racemic mixture containing molecules with opposite helical handedness. A voltage of 50 mV was applied to confirm that a junction had formed, and at voltages above 200 mV the researchers observed multiple current states switching in a consistent order. Current during junction formation was recorded at sampling frequencies of 100 to 200 kHz.
A total of 316 junctions showed multiple current states. According to Table S1 in the paper's supplementary information, 194 of these showed two states, 86 showed three, 25 showed four, and 11 showed five. The direction of switching could be determined for the 122 junctions that showed three or more states. Junctions in which no current switching was detected are not included in the total of 316, so the figure of 122 cannot be interpreted as the success rate for fabricating molecular machines out of all junctions.
For example, with three current states, the current value switches in a fixed order: A→B→C→A. The team interpreted this cyclic change as corresponding to stepwise rotation of the molecule. Importantly, when the voltage polarity was reversed in the same junction, the order of switching was also reversed. Because the cycling direction changed when an external condition, the voltage, was manipulated, this result supports the interpretation that the current and the molecular motion are causally linked.
The team evaluated directionality by dividing the number of transitions along a given cycling direction by the total number of observed transitions. In the 122 junctions analyzed, no transitions in the opposite direction were detected during measurement. However, in items 5 and 7 of their response to the public peer review, the authors explain that this is a result obtained within a finite observation time and does not prove that the probability of reverse transitions is zero. Nor could the cycling order of current states alone establish the absolute direction of molecular rotation, clockwise or counterclockwise, in each junction.
A current surge suggesting continuous rotation, and calculations explaining the mechanism
When the voltage was raised further, the current increased sharply. The team interprets this as a sign that the molecule shifted from a stepwise rotating state to a continuously rotating one. But what the experiment directly measures is only the current, not the molecular rotation itself. In the university's announcement, Yuta Ito, who took part in the research, explains, "All we see in the experiment is the current." The team therefore used theoretical calculations to examine the molecular motion thought to occur inside the junction.
Using density functional theory and quantum transport calculations, they evaluated the energy barriers that hinder rotation and the conductance while varying the position at which the molecule binds to the electrodes and its rotation angle. Conductance is a measure of how easily current flows through a junction. The calculations showed that the ease of electron passage changes as the molecule's angle changes. This suggests that the multiple current states seen in the experiments correspond to states in which the molecule adopts different angles.
To explain the molecule's rotation, the team adopted a "tilted periodic potential" model. In it, an energy that varies periodically with rotation angle is tilted by the torque generated by the current. A molecule normally tends to stay in a stable, low-energy configuration, but when rotational force from the current is added, it can cross the energy barrier and move to another configuration. Furthermore, if the work done by the rotational force that electrons exert on the molecule exceeds the work lost to friction, the motion is predicted to shift from stepwise to continuous rotation. Friction here refers to the action that hinders the molecule's rotation and dissipates its kinetic energy.
Once continuous rotation begins, the molecule also passes through configurations in which current flows easily and which it is unlikely to occupy when at rest. The team believes that this change in configuration causes the sharp rise in current at high voltage. In other words, the current drives the molecular motion, the motion changes the molecule's configuration, and the ease of electron flow changes as a result. However, if continuous rotation is actually occurring, its speed is predicted to exceed the time resolution of the experimental setup, so the rotation could not be detected directly. Continuous rotation is an interpretation derived by combining current measurements with theoretical calculations.
To test this explanation, the team also ran a control experiment with a linear anthracene derivative that lacks a helical structure. This molecule has a length and conductance similar to those of helicene, yet even when measured over the same voltage range, it showed no sharp current increase like that seen with helicene. The result that molecules with similar length and electrical properties do not show the same phenomenon if their structure differs supports the explanation that rotational motion arising from the helical structure is related to the current increase. However, this comparison alone does not rule out every alternative mechanism.
Electrode spacing affects the conditions under which the molecule operates
The team also confirmed that the conditions under which the current begins to surge change with electrode spacing. In the experiments, the surge tended to begin at relatively small currents when the electrode distance was intermediate. The theoretical calculations likewise showed that the work done by torque does not simply rise or fall monotonically with electrode distance, but changes trend partway through. The authors regard this correspondence between experiment and calculation as another piece of evidence supporting the interpretation of a current increase caused by molecular rotation.
The result suggests that the relationship between motion and current can be tuned not only by voltage but also by how far the molecule is stretched when attached to the electrodes. It offers insight that could lead to designs that exploit a molecule's structure and degrees of freedom of motion, rather than treating it as a merely fixed electronic component. However, the study did not identify an optimal electrode distance common to all junctions, or a particular current value at which rotational motion begins.
On the other hand, increasing the current does not necessarily keep the molecule rotating stably. In the high-current regime, irreversible changes to the junction were also observed. In some cases the molecule detached from the electrodes and a junction made of gold atoms formed in its place. To develop current control based on molecular motion into a reusable device, it will be necessary to clarify conditions under which the molecule can operate stably while the bond between molecule and electrodes is maintained.
What this study demonstrated is the cycling of current states in a single-molecule junction at an extremely low temperature of 4.2 K, and a sharp current increase at high voltage. Room-temperature operation and manufacturing yield have not been shown, and continuous rotation at high voltage has not been directly confirmed. It is also necessary to distinguish between the same research team examining many junctions and a successful independent replication by another research group. Going forward, the focus will be on whether the coupling of molecular motion and current can be reproduced stably through tests under different conditions, such as room temperature, and through independent replication. Whether this result, which uses changes in a molecule's shape itself to control the flow of electrons, can be developed into practical molecular devices depends on such verification.