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.

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Figure 1: Structure of the molecular motor 1.
Figure 2: Steady-state and time-resolved electronic spectroscopy of 1.
Figure 3: Excited-state dynamics viewed through the time-dependent emission.
Figure 4: Schematic model for the excited-state dynamics of 1.
Figure 5: Comparison of the resonance and off-resonance Raman spectra of 1 with the calculated Raman spectrum.

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Acknowledgements

This work was supported by the Engineering and Physical Sciences Research Council (EP/E010466), European Research Council (ERC) Starting Grant (279549; W.R.B.) and ERC Advanced Investigator Grant (227897; A.C., B.L.F.). J.C. was supported by a University of East Anglia studentship.

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S.R.M., B.L.F. and W.R.B. conceived and designed the experiments. J.C. and K.A. performed the time-resolved experiments, I.H. constructed the up-conversion apparatus, A.C. and B.L.F. designed and synthesized 1, J.C. and A.C. performed the steady-state electronic spectroscopy, J.C. and I.H. analysed the time-resolved data, A.C. performed the density functional theory calculations, W.R.B obtained and analysed the Raman data, S.R.M. wrote the paper and all the authors commented and contributed to the writing of the manuscript.

Corresponding authors

Correspondence to Ben L. Feringa or Stephen R. Meech.

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The authors declare no competing financial interests.

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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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