Population Dynamics of a Photon-Only Molecular Motor Shows That Mode Synchronization and Transient Binding Determine the Rotary Quantum Efficiency.

We present a study of the full rotary cycle of the photon-only rotary motor 3'-(2-methyl-2,3-dihydro-1H-benzo[b]cyclo-penta[d]thiophen-1-ylidene)pyrrolidin-2-one (MTDP) focusing on directionality and quantum efficiency (Φiso). By propagating hundreds of room-temperature quantum-classical trajectories in methanol, we demonstrate that the motor EP → ZP half-cycle exhibits relatively slow dynamics, partial loss of directionality, and low Φiso, while, in contrast, the ZP → EP half-cycle displays faster dynamics, full directionality, and an increased Φiso value. Statistical analyses show that such a dynamical diversity is due to two factors. The first is the presence of a transient binding between the amidic carbonyl group of the rotor and the stereogenic center substituent of the stator in the first half-cycle that is lost in the second half-cycle. The second is a synchronized rotary and ring-inversion motion of the rotor in the second-half cycle that "catalyzes" product formation. It is concluded that such contrasting factors must be considered when designing light-to-mechanical energy transduction molecular devices in general.