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 S1→1(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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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.
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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