Dynamics of the triplet-pair state reveals the likely coexistence of coherent and incoherent singlet fission in crystalline hexacene


The absorption of a photon usually creates a singlet exciton (S1) in molecular systems, but in some cases S1 may split into two triplets (2×T1) in a process called singlet fission. Singlet fission is believed to proceed through the correlated triplet-pair 1(TT) state. Here, we probe the 1(TT) state in crystalline hexacene using time-resolved photoemission and transient absorption spectroscopies. We find a distinctive 1(TT) state, which decays to 2×T1 with a time constant of 270 fs. However, the decay of S1 and the formation of 1(TT) occur on different timescales of 180 fs and <50 fs, respectively. Theoretical analysis suggests that, in addition to an incoherent S11(TT) rate process responsible for the 180 fs timescale, S1 may couple coherently to a vibronically excited 1(TT) on ultrafast timescales (<50 fs). The coexistence of coherent and incoherent singlet fission may also reconcile different experimental observations in other acenes.

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Figure 1: TR-2PPE and UPS determine quantitatively the energetic positions of S1, 1(TT) and T1 above S0.
Figure 2: A comparison of experimental cross-correlations and associated fits for S1, 1(TT) and T1.
Figure 3: Transient absorption spectroscopy reveals the ultrafast appearance of a triplet-like signature.
Figure 4: Incoherent and coherent rates from theoretical calculations and simultions.
Figure 5: Summary of singlet fission mechanism in crystalline hexacene.


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The TR-2PPE experiments and analysis were supported by the US Department of Energy (grant DE-SC0014563) (from 1 July 2015). X.-Y.Z. acknowledges partial support before 30 June 2015 by the US National Science Foundation (grant DMR 1321405). The TA work was supported by the Honda Research Institute, USA. The work in Mons was supported by the Belgian National Fund for Scientific Research (FRS-FNRS). D.B. is an FNRS Research Director. H.T. acknowledges support from an Invited Professor Fellowship, FNRS, Belgium. X.-Y.Z. thanks D. Reichman, T. Van Voorhis, T. Berkelbach, T. Fauster, W.-L. Chan, M. Tuan Trinh and K. Miyata for discussions. Y.R. thanks T.F. Heinz, F.C. Spano, O. Yaffe and Y. Wu for suggestions. H.T. thanks Y. Kurashige for discussions.

Author information




N.R.M. and K.W.W. designed and performed the TR-2PPE experiments. D.S., B.X., A.R.H., G.C. and Y.R. designed and performed the TA experiments. H.T. and D.B. carried out computational studies. B.K. and C.N. synthesized the precursor molecule for single-crystalline or polycrystalline thin-film growth. Y.Z. and C.N. grew the single-crystal sample. N.R.M., X.-Y.Z., D.S. and Y.R. analysed the data. X.-Y.Z., N.R.M. and Y.R. wrote the manuscript. X.-Y.Z. supervised the TR-2PPE experiments. Y.R. and H.-L.D. supervised the TA experiments. All authors were involved in the discussion of the results and contributed to the final version of the manuscript.

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Correspondence to David Beljonne or Yi Rao or X.-Y. Zhu.

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The authors declare no competing financial interests.

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Monahan, N., Sun, D., Tamura, H. et al. Dynamics of the triplet-pair state reveals the likely coexistence of coherent and incoherent singlet fission in crystalline hexacene. Nature Chem 9, 341–346 (2017). https://doi.org/10.1038/nchem.2665

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