Scientists have built a nanoscale machine that converts electric current into rotational motion while simultaneously increasing its ability to conduct electricity. By anchoring a single helicene molecule—a chiral, helical organic compound—between two gold electrode tips, the team observed that an applied voltage triggers a cyclic, unidirectional twist of the molecule. The direction of rotation reverses when the polarity of the voltage is switched, and the electrical current through the junction grows markedly once a specific voltage threshold is exceeded.
Helical Design Inspired by Ancient Technology
The concept draws on the long‑standing utility of helical structures such as Archimedean screws, which have pumped water for centuries and now power hydroelectric turbines. Similar helical geometries occur in certain chiral molecules and nanoscale constructs, where theory has suggested that flowing electrons could generate torque. The new work translates that idea into a concrete prototype: a torque‑driven molecular machine whose rotor is the helicene itself.
In the experimental setup, the helicene was functionalized with dithioacetyl groups to secure it between the gold electrodes, forming a single‑molecule Au‑helicene‑Au junction. When a voltage is applied across the junction, the researchers recorded a consistent, stepwise rotation of the molecule. The motion displayed a 100 % directional preference, meaning that under a given polarity the rotor always turned the same way. Reversing the voltage polarity flipped the direction of rotation, demonstrating precise electrical control over the molecular engine.
Voltage‑Controlled Conductance Enhancement
Beyond motion, the study revealed a pronounced increase in electrical conductance once the applied voltage surpassed a certain level. This conductance boost was not present at lower voltages, indicating a threshold behavior linked to the onset of rotation. Moreover, the voltage at which the enhancement began could be shifted by mechanically stretching or compressing the helicene, showing that the molecular geometry influences the electrical response.
Computational analyses, including density‑functional‑theory (DFT) calculations and quantum transport simulations, support the experimental observations. The models indicate that the current flowing through the junction exerts a torque on the helicene, overcoming frictional forces and driving the rotor from one potential minimum to the next. When the torque exceeds friction, the molecule can enter a regime of free rotation along a tilted potential landscape, allowing it to adopt configurations that are intrinsically more conductive. The calculations predict a transition from discrete, stepwise steps to continuous rotation at the voltage threshold, which explains the sudden rise in current.
Control experiments with similar helical molecular junctions that lacked the specific functionalization or structural arrangement did not show the same rotational or conductance phenomena, suggesting that the observed effects stem from the unique design of the Au‑helicene‑Au system.
The authors describe the device as a prototype for a new class of torque‑driven molecular machines that combine perfect unidirectionality with significant electrical modulation. Such machines could, in principle, be integrated into nanoscale circuits where mechanical motion and electronic signaling are coupled at the single‑molecule level.
Supplementary material accompanying the report details the synthesis of the dithioacetyl‑functionalized [13]helicene, the fabrication of the single‑molecule junctions, and a suite of experimental analyses that rule out alternative explanations. It also presents a microscopic theoretical framework that links current‑induced torque, molecular rotation, and the observed conductance enhancement.
Steve Lopez is a Senior Editorial Columnist and Health & Public Policy reporter for News Raise. Steve focuses on healthcare advancements, medical technologies, and public health policies.
