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Evidence for conical intersection dynamics mediating ultrafast singlet exciton fission

Nature Physics volume 11, pages 352–357 (2015)Cite this article

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Abstract

Singlet exciton fission is the process in organic semiconductors through which a spin-singlet exciton converts into a pair of spin-triplet excitons residing on different chromophores, entangled in an overall spin-zero state. For some systems, singlet fission has been shown to occur on the 100 fs timescale and with a 200% quantum yield, but the mechanism of this process remains uncertain. Here we study a model singlet fission system, TIPS-pentacene, using ultrafast vibronic spectroscopy. We observe that vibrational coherence in the initially photogenerated singlet state is transferred to the triplet state and show that this behaviour is effectively identical to ultrafast internal conversion for polyenes in solution. This similarity in vibronic dynamics suggests that both multi-molecular singlet fission and single-molecular internal conversion are mediated by the same underlying relaxation processes, based on strong coupling between nuclear and electronic degrees of freedom. In its most efficient form this leads to a conical intersection between the coupled electronic states.

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Figure 1: Photophysics and relevant transitions for impulsive vibrational spectroscopy of TIPS-pentacene.
Figure 2: Transient absorption spectroscopy of TIPS-pentacene excited with a resonant 10 fs pulse.
Figure 3: Vibrational coherence transfer during singlet exciton fission.
Figure 4: Vibrational coherence transfer in DPO.
Figure 5: Schematic of singlet fission via a conical intersection.

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Acknowledgements

A.R. thanks Corpus Christi College, Cambridge for a Research Fellowship. T.W. was supported by a Marie Curie Intra European Fellowship [PIEF-GA-2013-623652]. P.K. was supported by the Engineering and Physical Sciences Research Council [EPSRC, EP/H003541]. This work was supported by the EPSRC and the Winton Programme for the Physics of Sustainability.

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Author notes

  1. Andrew J. Musser and Matz Liebel: These authors contributed equally to this work.

Authors and Affiliations

  1. Cavendish Laboratory, University of Cambridge, J J Thomson Avenue, Cambridge, CB3 0HE, UK

    Andrew J. Musser, Tom B. Kehoe & Akshay Rao

  2. Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QZ, UK

    Matz Liebel, Christoph Schnedermann, Torsten Wende & Philipp Kukura

  3. ICFO—Institut de Ciencies Fotoniques, Mediterranean Technology Park, 08860 Castelldefels (Barcelona), Barcelona, Spain

    Matz Liebel

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Contributions

A.J.M. and M.L. analysed the data, M.L., C.S. and T.W. carried out the time-resolved experiments, T.B.K. performed the steady-state Raman spectroscopy, A.R. and P.K. conceived of the project. All authors discussed the results. A.J.M., A.R. and P.K. wrote the manuscript with input from all authors.

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Correspondence to Akshay Rao or Philipp Kukura.

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The authors declare no competing financial interests.

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Musser, A., Liebel, M., Schnedermann, C. et al. Evidence for conical intersection dynamics mediating ultrafast singlet exciton fission. Nature Phys 11, 352–357 (2015). https://doi.org/10.1038/nphys3241

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