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

Abstract

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.

References

  1. 1

    Hanna, M. C. & Nozik, A. J. Solar conversion efficiency of photovoltaic and photoelectrolysis cells with carrier multiplication absorbers. J. Appl. Phys. 100, 074510 (2006).

    Article  Google Scholar 

  2. 2

    Smith, M. B. & Michl, J. Singlet fission. Chem. Rev. 110, 6891–6936 (2010).

    CAS  Article  Google Scholar 

  3. 3

    Smith, M. B. & Michl, J. Recent advances in singlet fission. Annu. Rev. Phys. Chem. 64, 361–386 (2013).

    CAS  Article  Google Scholar 

  4. 4

    Trinh, M. T. et al. Intra- to intermolecular singlet fission. J. Phys. Chem. C 119, 1312–1319 (2015).

    CAS  Article  Google Scholar 

  5. 5

    Johnson, R. C. & Merrifield, R. E. Effects of magnetic fields on the mutual annihilation of triplet excitons in anthracene crystals. Phys. Rev. B 1, 896–902 (1970).

    Article  Google Scholar 

  6. 6

    Suna, A. Kinematics of exciton–exciton annihilation in molecular crystals. Phys. Rev. B 1, 1716–1739 (1970).

    Article  Google Scholar 

  7. 7

    Yamagata, H. et al. The nature of singlet excitons in oligoacene molecular crystals. J. Chem. Phys. 134, 204703 (2011).

    CAS  Article  Google Scholar 

  8. 8

    Beljonne, D., Yamagata, H., Brédas, J. L., Spano, F. C. & Olivier, Y. Charge-transfer excitations steer the Davydov splitting and mediate singlet exciton fission in pentacene. Phys. Rev. Lett. 110, 226402 (2013).

    CAS  Article  Google Scholar 

  9. 9

    Sharifzadeh, S., Darancet, P., Kronik, L. & Neaton, J. B. Low-energy charge-transfer excitons in organic solids from first-principles: the case of pentacene. J. Phys. Chem. Lett. 4, 2197–2201 (2013).

    CAS  Article  Google Scholar 

  10. 10

    Greyson, E. C., Vura-Weis, J., Michl, J. & Ratner, M. A. Maximizing singlet fission in organic dimers: theoretical investigation of triplet yield in the regime of localized excitation and fast coherent electron transfer. J. Phys. Chem. B 114, 14168–14177 (2010).

    CAS  Article  Google Scholar 

  11. 11

    Chan, W.-L. et al. Observing the multiexciton state in singlet fission and ensuing ultrafast multielectron transfer. Science 334, 1541–1545 (2011).

    CAS  Article  Google Scholar 

  12. 12

    Chan, W.-L., Ligges, M. & Zhu, X.-Y. The energy barrier in singlet fission can be overcome through coherent coupling and entropic gain. Nat. Chem. 4, 840–845 (2012).

    CAS  Article  Google Scholar 

  13. 13

    Chan, W.-L. et al. The quantum coherent mechanism for singlet fission: experiment and theory. Acc. Chem. Res. 46, 1321–1329 (2013).

    CAS  Article  Google Scholar 

  14. 14

    Aryanpour, K., Dutta, T., Huynh, U. N. V., Vardeny, Z. V. & Mazumdar, S. Theory of primary photoexcitations in donor–acceptor copolymers. 115, 267401 (2015).

  15. 15

    Tiago, M., Northrup, J. & Louie, S. Ab initio calculation of the electronic and optical properties of solid pentacene. Phys. Rev. B 67, 115212 (2003).

    Article  Google Scholar 

  16. 16

    Bardeen, C. J. The structure and dynamics of molecular excitons. Annu. Rev. Phys. Chem. 65, 127–148 (2014).

    CAS  Article  Google Scholar 

  17. 17

    Cudazzo, P., Sottile, F., Rubio, A. & Gatti, M. Exciton dispersion in molecular solids. J. Phys. Condens. Matter 27, 113204 (2015).

    Article  Google Scholar 

  18. 18

    Monahan, N. & Zhu, X.-Y. Charge transfer-mediated singlet fission. Annu. Rev. Phys. Chem. 66, 601–618 (2014).

    Article  Google Scholar 

  19. 19

    Wang, R. et al. Magnetic dipolar interaction between correlated triplets created by singlet fission in tetracene crystals. Nat. Commun. 6, 8602 (2015).

    CAS  Article  Google Scholar 

  20. 20

    Yost, S. R. et al. A transferable model for singlet-fission kinetics. Nat. Chem. 6, 492–497 (2014).

    CAS  Article  Google Scholar 

  21. 21

    Musser, A. J. et al. Evidence for conical intersection dynamics mediating ultrafast singlet exciton fission. Nat. Phys. 11, 352–357 (2015).

    CAS  Article  Google Scholar 

  22. 22

    Bakulin, A. A., Morgan, S. E., Kehoe, T. B., Wilson, M. W. B. & Chin, A. W. Real-time observation of multiexcitonic states in ultrafast singlet fission using coherent 2D electronic spectroscopy. Nat. Chem. 8, 16–23 (2016).

    CAS  Article  Google Scholar 

  23. 23

    Watanabe, M. et al. The synthesis, crystal structure and charge-transport properties of hexacene. Nat. Chem. 4, 574–578 (2012).

    CAS  Article  Google Scholar 

  24. 24

    Houk, K. N., Lee, P. S. & Nendel, M. Polyacene and cyclacene geometries and electronic structures: bond equalization, vanishing band gaps, and triplet ground states contrast with polyacetylene. J. Org. Chem. 66, 5517–5521 (2001).

    CAS  Article  Google Scholar 

  25. 25

    Busby, E. et al. Multiphonon relaxation slows singlet fission in crystalline hexacene. J. Am. Chem. Soc. 136, 10654–10660 (2014).

    CAS  Article  Google Scholar 

  26. 26

    Lee, J. et al. Singlet exciton fission in a hexacene derivative. Adv. Mater. 25, 1445–1448 (2013).

    CAS  Article  Google Scholar 

  27. 27

    Zhu, X. Y. How to draw energy level diagrams in excitonic solar cells. J. Phys. Chem. Lett. 5, 2283–2288 (2014).

    CAS  Article  Google Scholar 

  28. 28

    Monahan, N. R., Williams, K. W., Kumar, B., Nuckolls, C. & Zhu, X.-Y. Direct observation of entropy-driven electron–hole pair separation at an organic semiconductor interface. Phys. Rev. Lett. 114, 247003 (2015).

    Article  Google Scholar 

  29. 29

    Kolata, K., Breuer, T., Witte, G. & Chatterjee, S. Molecular packing determines singlet exciton fission in organic semiconductors. ACS Nano 8, 7377–7383 (2014).

    CAS  Article  Google Scholar 

  30. 30

    Angliker, H., Rommel, E. & Wirz, J. Electronic spectra of hexacene in solution (ground state. Triplet state. Dication and dianion). Chem. Phys. Lett. 87, 208–212 (1982).

    CAS  Article  Google Scholar 

  31. 31

    Chakraborty, H. & Shukla, A. Theory of triplet optical absorption in oligoacenes: from naphthalene to heptacene. J. Chem. Phys. 141, 164301 (2014).

    Article  Google Scholar 

  32. 32

    Zhang, B. et al. Polarization-dependent exciton dynamics in tetracene single crystals polarization-dependent exciton dynamics in tetracene single crystals. J. Chem. Phys. 141, 244303 (2014).

    Article  Google Scholar 

  33. 33

    Wan, Y. et al. Cooperative singlet and triplet exciton transport in tetracene crystals visualized by ultrafast microscopy. Nat. Chem. 7, 785–792 (2015).

    CAS  Article  Google Scholar 

  34. 34

    Brédas, J.-L., Beljonne, D., Coropceanu, V. & Cornil, J. Charge-transfer and energy-transfer processes in π-conjugated oligomers and polymers: a molecular picture. Chem. Rev. 104, 4971–5004 (2004).

    Article  Google Scholar 

  35. 35

    Wilson, M. W. B. et al. Ultrafast dynamics of exciton fission in polycrystalline pentacene. J. Am. Chem. Soc. 133, 11830–11833 (2011).

    CAS  Article  Google Scholar 

  36. 36

    Burdett, J. J., Gosztola, D. & Bardeen, C. J. The dependence of singlet exciton relaxation on excitation density and temperature in polycrystalline tetracene thin films: kinetic evidence for a dark intermediate state and implications for singlet fission. J. Chem. Phys. 135, 214508 (2011).

    Article  Google Scholar 

  37. 37

    Wilson, M. W. B. et al. Temperature-independent singlet exciton fission in tetracene. J. Am. Chem. Soc. 135, 16680–16688 (2013).

    CAS  Article  Google Scholar 

  38. 38

    Meyer, H. D., Manthe, U. & Cederbaum, L. S. The multi-configuration time-dependent Hartree approach. Chem. Phys. Lett. 165, 73–78 (1990).

    CAS  Article  Google Scholar 

  39. 39

    Tamura, H., Huix-Rotllant, M., Burghardt, I., Olivier, Y. & Beljonne, D. First-principles quantum dynamics of singlet fission: coherent versus thermally activated mechanisms governed by molecular π stacking. Phys. Rev. Lett. 115, 107401 (2015).

    Article  Google Scholar 

  40. 40

    Tamura, H., Ramon, J. G. S., Bittner, E. R. & Burghardt, I. Phonon-driven ultrafast exciton dissociation at donor–acceptor polymer heterojunctions. Phys. Rev. Lett. 100, 107402 (2008).

    Article  Google Scholar 

  41. 41

    Elsaesser, T. & Kaiser, W. Vibrational and vibronic relaxation of large polyatomic molecules in liquids. Annu. Rev. Phys. Chem. 42, 83–107 (1991).

    CAS  Article  Google Scholar 

  42. 42

    Burdett, J. J. & Bardeen, C. J. Quantum beats in crystalline tetracene delayed fluorescence due to triplet pair coherences produced by direct singlet fission. J. Am. Chem. Soc. 134, 8597–8607 (2012).

    CAS  Article  Google Scholar 

Download references

Acknowledgements

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.

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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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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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