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

Journal name:
Nature Chemistry
Volume:
8,
Pages:
16–23
Year published:
DOI:
doi:10.1038/nchem.2371
Received
Accepted
Published online

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.

At a glance

Figures

  1. Molecular crystals under study.
    Figure 1: Molecular crystals under study.

    a, Free-energy diagram depicting electronic and vibronic (marked with ′) states addressed in the 2DES experiments. S, T and g correspond to singlet, triplet and ground states, respectively. Approximate energies of the states (in eV) are shown in parentheses. b, Molecular-crystal structure of the systems under study. c, Absorption spectra of the films under study. The grey curve presents the laser spectrum used in the 2DES experiments.

  2. Real part of 2DES spectra for pentacene molecular crystal at different evolution times.
    Figure 2: Real part of 2DES spectra for pentacene molecular crystal at different evolution times.

    Red peaks represent the photo-induced increase in transmission due to GSB and stimulated emission. Blue peaks correspond to photo-induced absorption. At short evolution time, the spectra mostly contain singlet features, but at longer times, the response becomes more triplet-like. Horizontal and vertical dotted lines mark the expected singlet and multiexciton energies according to the state diagram in Fig. 1a.

  3. Oscillatory components in the 2DES data.
    Figure 3: Oscillatory components in the 2DES data.

    a, Blue curves show evolution-time transients corresponding to the different locations in the 2D spectrum of pentacene. The red curve describes the population dynamics given by the three-component decay-associated spectra. b, Comparison of the spectra of evolution-time oscillations observed in 2DES data integrated over the complete 2D spectra and resonance Raman spectra of the films of pentacene and its derivatives.

  4. 2D maps of beatings in 2DES spectra corresponding to the strongest oscillatory features observed in Fig. 3b.
    Figure 4: 2D maps of beatings in 2DES spectra corresponding to the strongest oscillatory features observed in Fig. 3b.

    The length of the arrow in every panel corresponds to the energy of the corresponding vibrational mode. For low-frequency modes the beating signals are localized around the S state, while for higher-frequency modes coherences are also observed at ωex ≈ 1.72 eV, corresponding to the energy of the TT state.

  5. In-depth analysis of 2DES results.
    Figure 5: In-depth analysis of 2DES results.

    a,b, Rephasing (RP) and non-rephasing (NR) parts of the 2D beating maps corresponding to observed high-frequency vibrational modes of pentacene (a) and diagrams presenting the relevant system–field interactions (b). Numbers in circles mark the corresponding positions of the beating peaks. Rephasing features are marked with black filled circles and non-rephasing features with open circles. BF = beating frequency. c, 2D beating maps corresponding to the 265 cm−1 oscillatory features observed in pentacene and its DTP and TIPS derivatives.

  6. Results of theoretical calculations based on the vibronic model.
    Figure 6: Results of theoretical calculations based on the vibronic model.

    a, Diagram of eigenstates formed as a result of mixing between the singlet and multiexciton manifolds. b, Calculated dipole moments and oscillator strengths for the transitions from the ground state to the eigenstates. The blue curve shows an experimental absorption spectrum for comparison. c, Calculated rephasing/non-rephasing beating maps for pentacene at 1,170 cm−1. The same beating features as shown in Fig. 5a,b are shown with black filled and white open circles. d, Population kinetics for dark and bright states predicted by the model. Grey curves indicate the 83 fs dynamics deduced from the decay-associated spectra analysis (Fig. 3a).

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Author information

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

    • Mark W. B. Wilson

Affiliations

  1. FOM Institute AMOLF, 1098 XG Amsterdam, 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, D-24098 Kiel, Germany

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

    • Donatas Zigmantas

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.

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The authors declare no competing financial interests.

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