Controlling molecular machines <i>via</i> optimally oriented external electric fields

The rectification of Brownian motion allows the operation of molecular machines, enabling them to perform directed tasks in biological and synthetic systems. Among the various control strategies, electric fields (E-fields) are emerging as a powerful means to modulate this rectification. Here, we demonstrate E-field-driven control of molecular motion in two representative molecular machines - a fluorene-based overcrowded alkene and an achiral rotor model - that operate <i>via</i> distinct mechanisms. Furthermore, we identify the optimal orientation of the applied E-fields that transforms activated steps into effectively barrierless processes, achieving directional control of internal molecular motion with minimal field strength. This is computationally demonstrated using our recently introduced polarizable molecular electric dipole model, which predicts E-fields to induce coalescence of transition states and energy minima on potential energy surfaces. Specifically, our studies show that E-fields enable bidirectional isomerization in the ground state without having to rely on photochemical processes that require energies much higher than the corresponding activation energy. Logical control over rotation, including the implementation of "STOP" and "GO" instructions, can also be achieved through E-field modulation without molecular chirality. Crucially, the required field strengths are within reach of current scanning tunneling microscope technology. Our results offer a generalizable, non-invasive design principle for the next generation of electric field-controlled molecular machines.