Ultrafast dynamics in the power stroke of a molecular rotary motor

Abstract

Light-driven molecular motors convert light into mechanical energy through excited-state reactions. Unidirectional rotary molecular motors based on chiral overcrowded alkenes operate through consecutive photochemical and thermal steps. The thermal (helix inverting) step has been optimized successfully through variations in molecular structure, but much less is known about the photochemical step, which provides power to the motor. Ultimately, controlling the efficiency of molecular motors requires a detailed picture of the molecular dynamics on the excited-state potential energy surface. Here, we characterize the primary events that follow photon absorption by a unidirectional molecular motor using ultrafast fluorescence up-conversion measurements with sub 50 fs time resolution. We observe an extraordinarily fast initial relaxation out of the Franck–Condon region that suggests a barrierless reaction coordinate. This fast molecular motion is shown to be accompanied by the excitation of coherent excited-state structural motion. The implications of these observations for manipulating motor efficiency are discussed. The light-driven power stroke of a unidirectional molecular motor is studied using ultrafast fluorescence spectroscopy. The evolution on the excited-state energy surface is observed on the 100 fs timescale and is accompanied by damped coherent molecular motion. The implications of these observations for the operation of the molecular motors are discussed.

Cite this article

Conyard, J., Addison, K., Heisler, I. et al. Ultrafast dynamics in the power stroke of a molecular rotary motor. Nature Chem 4, 547–551 (2012). https://doi.org/10.1038/nchem.1343

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