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

Classical light-driven rotary molecular motors harness the energy of light for a range of applications; however, they require heat to function. Recently, a photon-only motor that uses only two photoisomerisation steps to complete a rotary cycle and entirely avoids thermal helix inversions has been reported. However, this novel motor suffers from three fundamental issues: a partial loss of directionality of rotation, low rotary quantum efficiency, and markedly different efficiencies of the sequential photoisomerisation steps. In the present contribution, we use quantum-classical trajectories to show how a minimal modification of the existing photon-only motor restores unidirectionality, increases quantum efficiency, and makes the two photoisomerisation steps equally efficient, with roughly 50\% efficiency in a low-polarity medium. The proposed modification harnesses specific intramolecular electrostatic interactions and crowding effects to modulate the rotary dynamics and the time required to achieve the excited state decay region. We conclude that our results considerably advance the set of engineering rules for the design of efficient molecular motors.