Current-Driven Molecular Machine with Rotation-Enhanced Conductance

For more than two millennia, Archimedean screws have been used for water pumping and, more recently, as turbines in hydroelectric power stations[1]. Their helical-blade structure, essential to their function, is also found in chiral molecules and nanostructures, where injected electric current has been proposed to generate torque-mediated rotation[2,3]. Here, we present a torque-driven molecular machine with a rotor actuated by electric current and conductance enhanced by its rotation. Suspending a helicene molecule between two electrode tips, we show that an applied voltage induces cyclic molecular motion with 100% directional preference that can be inverted by the voltage's polarity. Above a voltage threshold, the current through the rotating molecule is significantly enhanced, and the enhancement onset is tuned by stretching or compressing the helical molecule. Such effects were absent in previous helical molecular junctions, likely due to structural and operational differences[4-6]. Calculations show that molecular rotation by current-induced forces explains both the observed cyclic motion and current enhancement. This mechanism predicts a transition from stepwise to continuous rotation at a threshold voltage, yielding a current increase as rotation allows adopting highly conducting configurations not accessible at rest. Our work introduces a prototype for a new class of torque-driven molecular machines with perfect rotor unidirectionality and significant current modulation.