Ultrafast above-threshold dynamics of the radical anion of a prototypical quinone electron-acceptor

Abstract

Quinones feature prominently as electron acceptors in nature. Their electron-transfer reactions are often highly exergonic, for which Marcus theory predicts reduced electron-transfer rates because of a free-energy barrier that occurs in the inverted region. However, the electron-transfer kinetics that involve quinones can appear barrierless. Here, we consider the intrinsic properties of the para-benzoquinone radical anion, which serves as the prototypical electron-transfer reaction product involving a quinone-based acceptor. Using time-resolved photoelectron spectroscopy and ab initio calculations, we show that excitation at 400 and 480 nm yields excited states that are unbound with respect to electron loss. These excited states are shown to decay on a sub-40 fs timescale through a series of conical intersections with lower-lying excited states, ultimately to form the ground anionic state and avoid autodetachment. From an isolated electron-acceptor perspective, this ultrafast stabilization mechanism accounts for the ability of para-benzoquinone to capture and retain electrons.

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Figure 1: Overview of electron-transfer reactions and electronic structure of pBQ.
Figure 2: Excited-state dynamics of above-threshold states.
Figure 3: Mechanistic picture of electron stabilization.

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Acknowledgements

This work was funded by the Engineering and Physical Sciences Research Council (EP/D073472/1), the Spanish Ministerio de Ciencia e Innovación (MICINN) (CTQ2011-26573 and UNGI08-4E-003 from the European Fund for Regional Development) and the Catalan Agència de Gestió d'Ajuts Universitaris i de Recerca (SGR0528). Q.L. acknowledges a Juan de la Cierva fellowship of the MICINN. J.R.R.V. thanks the European Research Council for a Starting Grant.

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D.A.H. and Q.L. contributed equally to this work. D.A.H. and J.R.R.V. conceived the project and performed all the experiments. Q.L. and L.B. designed and performed all the calculations. Q.L. and D.A.H. analysed the computational and experimental results. All authors discussed the results and contributed to the manuscript.

Corresponding authors

Correspondence to Lluís Blancafort or Jan R. R. Verlet.

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The authors declare no competing financial interests.

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Horke, D., Li, Q., Blancafort, L. et al. Ultrafast above-threshold dynamics of the radical anion of a prototypical quinone electron-acceptor. Nature Chem 5, 711–717 (2013). https://doi.org/10.1038/nchem.1705

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