Article

Vibronically coherent ultrafast triplet-pair formation and subsequent thermally activated dissociation control efficient endothermic singlet fission

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Abstract

Singlet exciton fission (SF), the conversion of one spin-singlet exciton (S1) into two spin-triplet excitons (T1), could provide a means to overcome the Shockley–Queisser limit in photovoltaics. SF as measured by the decay of S1 has been shown to occur efficiently and independently of temperature, even when the energy of S1 is as much as 200 meV less than that of 2T1. Here we study films of triisopropylsilyltetracene using transient optical spectroscopy and show that the triplet pair state (TT), which has been proposed to mediate singlet fission, forms on ultrafast timescales (in 300 fs) and that its formation is mediated by the strong coupling of electronic and vibrational degrees of freedom. This is followed by a slower loss of singlet character as the excitation evolves to become only TT. We observe the TT to be thermally dissociated on 10–100 ns timescales to form free triplets. This provides a model for ‘temperature-independent’ efficient TT formation and thermally activated TT separation.

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References

  1. 1.

    & Singlet fission. Chem. Rev. 110, 6891–6936 (2010).

  2. 2.

    & Electronic Processes in Organic Crystals and Polymers (Oxford Univ. Press, 1999).

  3. 3.

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

  4. 4.

    Magnetic effects on triplet exciton interactions. Pure Appl. Chem. 27, 481–498 (1971).

  5. 5.

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

  6. 6.

    , & 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).

  7. 7.

    et al. Singlet exciton fission in polycrystalline thin films of a slip-stacked perylenediimide. J. Am. Chem. Soc. 135, 14701–14712 (2013).

  8. 8.

    & Bimolecular radiationless transitions in crystalline tetracene. Chem. Phys. Lett. 2, 327–328 (1968).

  9. 9.

    Theory of magnetic field effects on the mutual annihilation of triplet excitons. J. Chem. Phys. 48, 4318–4319 (1968).

  10. 10.

    & How morphology affects singlet fission in crystalline tetracene. J. Phys. Chem. Lett. 6, 1841–1846 (2015).

  11. 11.

    , , & Excited state dynamics in solid and monomeric tetracene: the roles of superradiance and exciton fission. J. Chem. Phys. 133, 144506 (2010).

  12. 12.

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

  13. 13.

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

  14. 14.

    , & Tetracene derivatives as potential red emitters for organic LEDs. Org. Lett. 5, 4245–4248 (2003).

  15. 15.

    et al. Identification of a triplet pair intermediate in singlet exciton fission in solution. Proc. Natl Acad. Sci. USA 112, 7656–7661 (2015).

  16. 16.

    & Broad-band impulsive vibrational spectroscopy of excited electronic states in the time domain. J. Phys. Chem. Lett. 4, 1358–1364 (2013).

  17. 17.

    & Slow intramolecular vibrational relaxation leads to long-lived excited-state wavepackets. J. Phys. Chem. A 120, 6792−6799 (2016).

  18. 18.

    , & Microscopic theory of singlet exciton fission. I. General formulation. J. Chem. Phys. 138, 114102 (2013).

  19. 19.

    et al. A direct mechanism of ultrafast intramolecular singlet fission in pentacene dimers. ACS Cent. Sci. 2, 316–324 (2016).

  20. 20.

    et al. Real-time observation of multiexcitonic states in ultrafast singlet fission using coherent 2D electronic spectroscopy. Nat. Chem. 8, 16–23 (2016).

  21. 21.

    et al. Evidence for conical intersection dynamics mediating ultrafast singlet exciton fission. Nat. Phys. 11, 352–357 (2014).

  22. 22.

    et al. Dynamics of the triplet-pair state reveals the likely coexistence of coherent and incoherent singlet fission in crystalline hexacene. Nat. Chem. 9, 341–346 (2016).

  23. 23.

    , & Vibrationally coherent crossing and coupling of electronic states during internal conversion in β-carotene. Phys. Rev. Lett. 112, 198302 (2014).

  24. 24.

    , & Electronic resonance with anticorrelated pigment vibrations drives photosynthetic energy transfer outside the adiabatic framework. Proc. Natl Acad. Sci. USA 110, 1203–1208 (2013).

  25. 25.

    Electronic structure study of singlet fission in tetracene derivatives. J. Chem. Theory Comput. 10, 324–334 (2014).

  26. 26.

    , & The energy barrier in singlet fission can be overcome through coherent coupling and entropic gain. Nat. Chem. 4, 840–845 (2012).

  27. 27.

    , , , & Polymorphism influences singlet fission rates in tetracene thin films. Chem. Sci. 7, 1185–1191 (2016).

  28. 28.

    et al. Coherent singlet fission activated by symmetry breaking. Nat. Chem. (in the press).

  29. 29.

    & Picosecond vibrational cooling in mixed molecular crystals studied with a new coherent Raman scattering technique. Chem. Phys. Lett. 147, 18–24 (1988).

  30. 30.

    et al. Binding energy of charge-transfer excitons localized at polymeric semiconductor heterojunctions. J. Phys. Chem. 115, 7114–7119 (2011).

  31. 31.

    et al. Observation of two triplet-pair intermediates in singlet exciton fission. J. Phys. Chem. Lett. 7, 2370–2375 (2016).

  32. 32.

    , , & Observation of two triplet-pair intermediates in singlet exciton fission. J. Phys. Chem. C 116, 17369–17377 (2012).

  33. 33.

    & 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).

  34. 34.

    et al. The entangled triplet pair state in acene and heteroacene materials. Nat. Commun. 8, 15953 (2017).

  35. 35.

    et al. Morphology effectively controls singlet-triplet exciton relaxation and charge transport in organic semiconductors. Phys. Rev. Lett. 102, 017401 (2009).

  36. 36.

    , & A simple kinetic model for singlet fission: a role of electronic and entropic contributions to macroscopic rates. J. Phys. Chem. 118, 5188–5195 (2014).

  37. 37.

    , & Tetracene derivatives as potential red emitters for organic LEDs. Org. Lett. 5, 4245–4248 (2003).

  38. 38.

    , , & Transient grating photoluminescence spectroscopy: an ultrafast method of gating broadband spectra. J. Phys. Chem. Lett. 5, 1732–1737 (2014).

  39. 39.

    et al. Exciton fission and charge generation via triplet excitons in pentacene/C60 bilayers. J. Am. Chem. Soc. 132, 12698–12703 (2010).

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Acknowledgements

The authors thank the Winton Programme for the Physics of Sustainability and the Engineering and Physical Sciences Research Council for funding. R.H.F. thanks the Miller Institute for Basic Research and the Heising–Simons Foundation at the University of California Berkeley for support. The authors thank T. Arnold (Diamond Light Source), J. Novak, D. Harkin and J. Rozboril for support during the beamtime at beamline I07 and the Diamond Light Source for financial support. The computational work was supported by the Scientific Discovery through Advanced Computing program funded by the US Department of Energy, Office of Science, Advanced Scientific Computing Research, Basic Energy Sciences.

Author information

Affiliations

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

    • Hannah L. Stern
    • , Alexandre Cheminal
    • , Katharina Broch
    • , Sam L. Bayliss
    • , Maxim Tabachnyk
    • , Neil Greenham
    • , Andrew J. Musser
    • , Akshay Rao
    •  & Richard H. Friend
  2. Kenneth S. Pitzer Center for Theoretical Chemistry, Department of Chemistry, University of California, Berkeley 94720, USA

    • Shane R. Yost
    •  & Martin Head-Gordon
  3. Chemical Science Division, Lawrence Berkeley National Laboratory, Berkeley 94720, USA

    • Shane R. Yost
    •  & Martin Head-Gordon
  4. MacDiarmid Institute for Advanced Materials and Nanotechnology, Wellington 6140, New Zealand

    • Kai Chen
    •  & Justin M. Hodgkiss
  5. School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington 6140, New Zealand

    • Kai Chen
    •  & Justin M. Hodgkiss
  6. University of Kentucky, Lexington 40506, USA

    • Karl Thorley
    •  & John Anthony
  7. Department of Physics and Astronomy, University of Sheffield, Hounsfield Road, Sheffield S3 7RH, UK

    • Andrew J. Musser

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Contributions

H.L.S. and A.C. carried out experiments, interpreted the data and wrote the manuscript. S.R.Y. and M.H.-G. ran the calculations, interpreted data and wrote the manuscript. N.G. and S.L.B. interpreted data. K.B., M.T., K.C. and J.M.H. performed experiments. K.T. and J.A. designed and synthesized the materials. A.R., A.J.M. and R.H.F. interpreted the data and wrote the manuscript. All of the authors discussed the results and commented on the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding authors

Correspondence to Akshay Rao or Richard H. Friend.

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