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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. 

Abstract We use quantum-classical trajectories to investigate the origin of the different photoisomerization quantum efficiency observed in the dim-light visual pigment Rhodopsin and in the light-driven biomimetic molecular rotorpara-methoxy N-methyl indanylidene-pyrrolinium (MeO-NAIP) in methanol. Our results reveal that effective light-energy conversion requires, in general, an auxiliary molecular vibration (called promoter) that does not correspond to the rotary motion but synchronizes with it at specific times. They also reveal that Nature has designed Rhodopsin to exploit two mechanisms working in a vibrationally coherent regime. The first uses a wag promoter to ensure that ca. 75% of the absorbed photons lead to unidirectional rotations. The second mechanism ensures that the same process is fast enough to avoid directional randomization. It is found that MeO-NAIP in methanol is incapable of exploiting the above mechanisms resulting into a 50% quantum efficiency loss. However, when the solvent is removed, MeO-NAIP rotation is predicted to synchronize with a ring-inversion promoter leading to a 30% increase in quantum efficiency and, therefore, biomimetic behavior. 

The concerted interplay between reactive nuclear and electronic motions in molecules actuates chemistry. Here, we demonstrate that out-of-plane torsional deformation and vibrational excitation of stretching motions in the electronic ground state modulate the charge-density distribution in a donor-bridge-acceptor molecule in solution. The vibrationally-induced change, visualised by transient absorption spectroscopy with a mid-infrared pump and a visible probe, is mechanistically resolved by ab initio molecular dynamics simulations. Mapping the potential energy landscape attributes the observed charge-coupled coherent nuclear motions to the population of the initial segment of a double-bond isomerization channel, also seen in biological molecules. Our results illustrate the pivotal role of pre-twisted molecular geometries in enhancing the transfer of vibrational energy to specific molecular modes, prior to thermal redistribution. This motivates the search for synthetic strategies towards achieving potentially new infrared-mediated chemistry.