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Real-time observation of multiexcitonic states in ultrafast singlet fission using coherent 2D electronic spectroscopy

Nature Chemistry volume 8, pages 16–23 (2016)Cite this article

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Abstract

Singlet fission is the spin-allowed conversion of a spin-singlet exciton into a pair of spin-triplet excitons residing on neighbouring molecules. To rationalize this phenomenon, a multiexcitonic spin-zero triplet-pair state has been hypothesized as an intermediate in singlet fission. However, the nature of the intermediate states and the underlying mechanism of ultrafast fission have not been elucidated experimentally. Here, we study a series of pentacene derivatives using ultrafast two-dimensional electronic spectroscopy and unravel the origin of the states involved in fission. Our data reveal the crucial role of vibrational degrees of freedom coupled to electronic excitations that facilitate the mixing of multiexcitonic states with singlet excitons. The resulting manifold of vibronic states drives sub-100 fs fission with unity efficiency. Our results provide a framework for understanding singlet fission and show how the formation of vibronic manifolds with a high density of states facilitates fast and efficient electronic processes in molecular systems.

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Figure 1: Molecular crystals under study.
Figure 2: Real part of 2DES spectra for pentacene molecular crystal at different evolution times.
Figure 3: Oscillatory components in the 2DES data.
Figure 4: 2D maps of beatings in 2DES spectra corresponding to the strongest oscillatory features observed in Fig. 3b.
Figure 5: In-depth analysis of 2DES results.
Figure 6: Results of theoretical calculations based on the vibronic model.

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Acknowledgements

The authors thank T. Jansen and M. Pschenichnikov for discussions, D. Palecek for help with ~580 nm 2D experiments and M. Tabachnyk for help with preparing samples. This work was supported by Laserlab–Europe (project no. LLC001945). A.A.B. is currently a Royal Society University Research Fellow. He also acknowledges a Veni grant from the Netherlands Organization for Scientific Research (NWO). A.R., S.E.M. and A.W.C. acknowledge the Winton Programme for the Physics of Sustainability for support.

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Author notes

  1. Mark W. B. Wilson

    Present address: Present address: Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA,

Authors and Affiliations

  1. FOM Institute AMOLF, Amsterdam, 1098 XG, The Netherlands

    Artem A. Bakulin

  2. Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge, CB3 0HE, UK

    Artem A. Bakulin, Sarah E. Morgan, Tom B. Kehoe, Mark W. B. Wilson, Alex W. Chin & Akshay Rao

  3. Institut für Physikalische Chemie, Christian-Albrechts-Universität zu Kiel, Olshausenstr. 40, Kiel, D-24098, Germany

    Dassia Egorova

  4. Department of Chemical Physics, Lund University, PO Box 124, Lund, 22100, Sweden

    Donatas Zigmantas

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Contributions

A.A.B. and A.R. conceived the study. M.W.B.W. and A.R. produced and characterized the samples. A.A.B., D.Z. and A.R. planned and performed the 2D experiments. T.K. performed c.w. Raman experiments. A.A.B., S.E.M., A.W.C., D.E. and A.R. analysed the data. S.E.M., A.W.C. and D.E. developed the model and performed theoretical calculations. A.A.B., S.E.M., A.W.C., D.E. and A.R. wrote the paper, with input from all the authors.

Corresponding authors

Correspondence to Dassia Egorova or Akshay Rao.

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The authors declare no competing financial interests.

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Bakulin, A., Morgan, S., Kehoe, T. et al. Real-time observation of multiexcitonic states in ultrafast singlet fission using coherent 2D electronic spectroscopy. Nature Chem 8, 16–23 (2016). https://doi.org/10.1038/nchem.2371

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