Rotor-stator repulsion and medium-induced dephasing enhance and equalise the quantum efficiency of a fluorinated photon-only rotary motor.

Light-driven rotary molecular motors harness the energy of light for a range of applications. While classical motors depend on both light and heat, a recently developed photon-only motor completes its rotation cycle at room temperature. Despite this breakthrough, the motor is hindered by a lack of rotational directionality, low photoisomerisation quantum efficiency, and inconsistent photoisomerisation rates of its two rotary half-cycles. In this study, we use quantum-classical trajectories to show that a synthetically accessible single-atom modification can resolve these issues by restoring unidirectionality, boosting efficiency, and balancing the two photoisomerisation rates in low-polarity environments. These improvements are driven by specific intramolecular electrostatic interactions that modulate the rotary dynamics and the time spent in the excited state as well as by a previously undocumented trapping and dephasing mechanism near the decay region that enhances and equalises photoisomerisation efficiency. These findings provide a new framework for designing high-performance photon-only molecular motors.