Critical role of intermediate electronic states for spin-flip processes in charge-transfer-type organic molecules with multiple donors and acceptors

Article metrics


Spin-flip in purely organic molecular systems is often described as a forbidden process; however, it is commonly observed and utilized to harvest triplet excitons in a wide variety of organic material-based applications. Although the initial and final electronic states of spin-flip between the lowest singlet and lowest triplet excited state are self-evident, the exact process and the role of intermediate states through which spin-flip occurs are still far from being comprehensively determined. Here, via experimental photo-physical investigations in solution combined with first-principles quantum-mechanical calculations, we show that efficient spin-flip in multiple donor–acceptor charge-transfer-type organic molecular systems involves the critical role of an intermediate triplet excited state that corresponds to a partial molecular structure of the system. Our proposed mechanism unifies the understanding of the intersystem crossing mechanism in a wide variety of charge-transfer-type molecular systems, opening the way to greater control over spin-flip rates.

Access optionsAccess options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Fig. 1: Temperature dependence of spin-flip processes in CzCN derivatives in solution.
Fig. 2: Spin-flip process through an intermediate higher-lying triplet excited state of 4CzIPN.
Fig. 3: Theoretical description of the excited states of 4CzIPN.
Fig. 4: Illustration of the intermediate state for the spin-flip process in other donor–acceptor-type molecules.

Data availability

The data that support the findings of this study are available from the corresponding authors upon reasonable request.


  1. 1.

    Lamola, A. A. & Hammond, G. S. Mechanisms of photochemical reactions in solution. XXXIII. Intersystem crossing efficiencies. J. Chem. Phys. 43, 2129–2135 (1965).

  2. 2.

    Baldo, M. A. et al. Highly efficient phosphorescent emission from organic electroluminescent devices. Nature 395, 151–154 (1998).

  3. 3.

    Reineke, S. et al. White organic light-emitting diodes with fluorescent tube efficiency. Nature 459, 234–238 (2009).

  4. 4.

    Uoyama, H., Goushi, K., Shizu, K., Nomura, H. & Adachi, C. Highly efficient organic light-emitting diodes from delayed fluorescence. Nature 492, 234–238 (2012).

  5. 5.

    Hirata., S. Recent advances in materials with room-temperature phosphorescence: photophysics for triplet exciton stabilization. Adv. Opt. Mater. 5, 1700116 (2017).

  6. 6.

    Zhao, Q., Huang, C. & Li, F. Phosphorescent heavy-metal complexes for bioimaging. Chem. Soc. Rev. 40, 2508–2524 (2011).

  7. 7.

    Xiong, X. et al. Thermally activated delayed fluorescence of fluorescein derivative for time-resolved and confocal fluorescence imaging. J. Am. Chem. Soc. 136, 9590–9597 (2014).

  8. 8.

    Maeda, K. et al. Chemical compass model of avian magnetoreception. Nature 453, 387–390 (2008).

  9. 9.

    Baluschev, S. et al. Up-conversion fluorescence: noncoherent excitation by sunlight. Phys. Rev. Lett. 97, 143903 (2006).

  10. 10.

    Castano, P., Mroz, P. & Hamblim, M. R. Photodynamic therapy and anti-tumourimmunity. Nat. Rev. Cancer 6, 535–545 (2006).

  11. 11.

    Baldo, M. A. & O’Brien, D. F. Excitonic singlet-triplet ratio in a semiconducting organic thin film. Phys. Rev. B 60, 14422 (1999).

  12. 12.

    Turro, N. J., Ramamurthy, V. & Scaiano, J. C. in Principle of Molecular Photochemistry: An Introduction Ch. 3, 113–118 (University Science Books, 2009).

  13. 13.

    Wong, M. Y. & Zysman-Colman, E. Purely organic thermally activated delayed fluorescence materials for organic light-emitting diodes. Adv. Mater. 29, 1605444 (2017).

  14. 14.

    El-Sayed, M. A. The radiationless processes involving change of multiplicity in the diazenes. J. Chem. Phys. 36, 573–574 (1962).

  15. 15.

    Chen, X. K., Zhang, S. F., Fan, J. X. & Ren, A. M. Nature of highly efficient thermally activated delayed fluorescence in organic light-emitting diode emitters: nonadiabatic effect between excited states. J. Phys. Chem. C 119, 9728–9733 (2015).

  16. 16.

    Gibson, J., Monkman, A. P. & Penfold, T. J. The importance of vibronic coupling for efficient reverse intersystem crossing in thermally activated delayed fluorescence molecules. ChemPhysChem 17, 2956–2961 (2016).

  17. 17.

    Dias, F. B. et al. The role of local triplet excited states and d-a relative orientation in thermally activated delayed fluorescence: photophysics and devices. Adv. Sci. 3, 1600080 (2016).

  18. 18.

    Marian, C. M. Mechanism of the triplet-to-singlet upconversion in the assistant dopant ACRXTN. J. Phys. Chem. C 120, 3715–3721 (2016).

  19. 19.

    Hosokai, T. et al. Evidence and mechanism of efficient thermally activated delayed fluorescence promoted by delocalized excited states. Sci. Adv. 3, e1603282 (2017).

  20. 20.

    Gibson, J. & Penfold, T. J. Nonadiabatic coupling reduces the activation energy in thermally activated delayed fluorescence. Phys. Chem. Chem. Phys. 19, 8248–8434 (2017).

  21. 21.

    Etherington, M. K., Gibson, J., Higginbotham, H. F., Penfold, T. J. & Monkman, A. P. Revealing the spin–vibronic coupling mechanism of thermally activated delayed fluorescence. Nat. Commun. 7, 13680 (2016).

  22. 22.

    Hayashi, H. & Nagakura, S. The E.S.R. and phosphorescence spectra of some dicyanobenzene complexes with methyl-substituted benzenes. Mol. Phys. 19, 45–53 (1970).

  23. 23.

    Olivier, Y. et al. Nature of the singlet and triplet excitations mediating thermally activated delayed fluorescence. Phys. Rev. Mater. 1, 075602 (2017).

  24. 24.

    Kobayashi, T. et al. Contributions of a higher triplet excited state to the emission properties of a thermally activated delayed-fluorescence emitter. Phys. Rev. Appl. 7, 034002 (2017).

  25. 25.

    Evans, E. W. et al. Vibrationally assisted intersystem crossing in benchmark thermally activated delayed fluorescence molecules. J. Phys. Chem. Lett. 9, 4053–4058 (2018).

  26. 26.

    Cho, Y. J., Jeon, S. K. & Lee, J. Y. Molecular engineering of high efficiency and long lifetime blue thermally activated delayed fluorescent emitters for vacuum and solution processed organic light-emitting diodes. Adv. Opt. Mater. 4, 688–693 (2016).

  27. 27.

    Noda, H., Nakanotani, H. & Adachi, C. Excited state engineering for efficient reverse intersystem crossing. Sci. Adv. 4, eaao6910 (2018).

  28. 28.

    Yanai, N. & Kimizuka, N. New triplet sensitization routes for photon upconversion: thermally activated delayed fluorescence molecules, inorganic nanocrystals, and singlet-to-triplet absorption. Acc. Chem. Res. 10, 2487–2495 (2017).

  29. 29.

    Penfold, T. J., Gindensperger, E., Daniel, C. & Marian, C. M. Spin-vibronic mechanism for intersystem crossing. Chem. Rev. 118, 6975–7025 (2018).

  30. 30.

    Mopsik, F. I. Dielectric properties of slightly polar organic liquids as a function of pressure, volume, and temperature. J. Chem. Phys. 50, 2559–2569 (1969).

  31. 31.

    Samanta, P. K., Kim, D., Coropceanu, V. & Brédas, J. L. Up-conversion intersystem crossing rates in organic emitters for thermally activated delayed fluorescence: impact of the nature of singlet vs triplet excited states. J. Am. Chem. Soc. 139, 4042–4051 (2017).

  32. 32.

    Körzdörfer, T. & Brédas, J. L. Organic electronic materials: recent advances in the dft description of the ground and excited states using tuned range-separated hybrid functionals. Acc. Chem. Res. 47, 3284–3291 (2014).

  33. 33.

    Yamamoto, M., Tsuji, Y. & Tuchida, A. Near-infrared charge resonance band of intramolecular carbazole dimer radical cations studied by nanosecond laser photolysis. Chem. Phys. Lett. 154, 559–562 (1989).

  34. 34.

    Kaafarani, B. R. et al. Mixed-valence cations of di(carbazol-9-yl) biphenyl, tetrahydropyrene, and pyrene derivatives. J. Phys. Chem. C 120, 3156–3166 (2016).

  35. 35.

    Lawetz, V., Orlandi, G. & Siebrand, W. Theory of intersystem crossing in aromatic hydrocarbons. J. Chem. Phys. 56, 4058–4072 (1972).

  36. 36.

    Robinson, G. W. & Frosch, R. P. Electronic excitation transfer and relaxation. J. Chem. Phys. 38, 1187–1203 (1963).

  37. 37.

    Nan, G., Yang, X., Wang, L., Shuai, Z. & Zhao, Y. Nuclear tunneling effects of charge transport in rubrene, tetracene, and pentacene. Phys. Rev. B 79, 115203–115211 (2009).

  38. 38.

    Bowen, E. J. & Sahu, J. The effect of temperature on fluorescence of solutions. J. Phys. Chem. 63, 4–7 (1959).

  39. 39.

    Bennett, R. G. & McCartin, P. J. Radiationless deactivation of the fluorescent state of substituted anthracenes. J. Chem. Phys. 44, 1966 (1969).

  40. 40.

    Katoh, R., Suzuki, K., Furube, A., Kotani, M. & Tokumaru, K. Fluorescence quantum yield of aromatic hydrocarbon crystals. J. Phys. Chem. C 113, 2961–2965 (2009).

  41. 41.

    Di, D. et al. High-performance light-emitting diodes based on carbene-metal-amides. Science 356, 159–163 (2017).

  42. 42.

    Cho, Y. J., Yook, K. S. & Lee, J. Y. Cool and warm hybrid white organic light-emitting diode with blue delayed fluorescent emitter both as blue emitter and triplet host. Sci. Rep. 5, 7859 (2015).

  43. 43.

    Lee, Y. H. et al. Rigidity-induced delayed fluorescence by ortho donor-appended triarylboron compounds: record-high efficiency in pure blue fluorescent organic light-emitting diodes. ACS Appl. Mater. Interfaces 9, 24035–24042 (2017).

  44. 44.

    Masui, K., Nakanotani, H. & Adachi, C. Analysis of exciton annihilation in high-efficiency sky-blue organic light-emitting diodes with thermally activated delayed fluorescence. Org. Electron. 14, 2721–2726 (2013).

  45. 45.

    Yoshihara, T., Murai, M., Tamaki, Y., Furube, A. & Katoh, R. Trace analysis by transient absorption spectroscopy: estimation of the solubility of C60 in polar solvents. Chem. Phys. Lett. 394, 161–164 (2004).

  46. 46.

    Martin, R. L. Natural transition orbitals. J. Chem. Phys. 118, 4775–4777 (2003).

  47. 47.

    M. J. Frisch et al. Gaussian 09 (Gaussian, Inc., 2009).

  48. 48.

    Gao, X., Bai, S., Fazzi, D., Niehaus, T. & Barbatti, M. Evaluation of spin-orbit couplings with linear-response time-dependent density functional methods. J. Chem. Theory Comput. 13, 515–524 (2017).

Download references


This work was supported in part by the Japan Science and Technology Agency, ERATO, Adachi Molecular Exciton Engineering Project, under JST ERATO grant no. JPMJER1305, and the Japan Society for the Promotion of Science KAKENHI (grant nos. JP17J04907, JP18H02047 and JP18H03902). Work at the Georgia Institute of Technology is supported by the Department of Energy (no. DE-EE0008205). X.-K.C. and J.-L.B. are also grateful to Kyulux for generous support of their TADF activities.

Author information

The project was conceived and designed by H. Nakanotani and H. Noda. H. Noda, M.M. and N.N. synthesized the organic compounds used in this study. H. Noda, Y.K. and M.M. prepared the samples and measured their properties. T.H. performed the TAS measurements. X.-K.C. carried out the quantum-chemical calculations under the supervision of J.-L.B. H. Nakanotani, H. Noda and X.-K.C. analysed all data. C.A. supervised the project. All authors contributed to writing the paper and critically commented on the project. H. Noda and X.-K.C. contributed equally to this work.

Correspondence to Hajime Nakanotani or Jean-Luc Brédas or Chihaya Adachi.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

Supplementary Methods, Supplementary Table 1, Supplementary Figs. 1–14 and Supplementary refs. 1–4.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Further reading