Understanding the luminescent nature of organic radicals for efficient doublet emitters and pure-red light-emitting diodes


The doublet-spin nature of radical emitters is advantageous for applications in organic light-emitting diodes, as it avoids the formation of triplet excitons that limit the electroluminescence efficiency of non-radical emitters. However, radicals generally show low optical absorption and photoluminescence yields. Here we explain the poor optical properties of radicals based on alternant hydrocarbons, and establish design rules to increase the absorption and luminescence yields for donor–acceptor-type radicals. We show that non-alternant systems are necessary to lift the degeneracy of the lowest energy orbital excitations; moreover, intensity borrowing from an intense high-lying transition by the low-energy charge-transfer excitation enhances the oscillator strength of the emitter. We apply these rules to design tris(2,4,6-trichlorophenyl)methyl–pyridoindolyl derivatives with a high photoluminescence quantum yield (>90%). Organic light-emitting diodes based on these molecules showed a pure-red emission with an over 12% external quantum efficiency. These insights may be beneficial for the rational design and discovery of highly luminescent doublet emitters.

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Fig. 1: Nature of light emission from radicals based on alternant and non-alternant hydrocarbons.
Fig. 2: Red-emission design for TTM-based radicals.
Fig. 3: Optoelectronic performance of the TTM–xPyID radical-based OLEDs.


  1. 1.

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

    CAS  Google Scholar 

  2. 2.

    Ma, Y., Zhang, H., Shen, J. & Che, C. Electroluminescence from triplet metal–ligand charge-transfer excited state of transition metal complexes. Synth. Met. 94, 245–248 (1998).

    CAS  Google Scholar 

  3. 3.

    Adachi, C., Baldo, M. A., Thompson, M. E. & Forrest, S. R. Nearly 100% internal phosphorescence efficiency in an organic light-emitting device. J. Appl. Phys. 90, 5048–5051 (2001).

    CAS  Google Scholar 

  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).

    CAS  Google Scholar 

  5. 5.

    Kido, J. & Lizumi, Y. Fabrication of highly efficient organic electroluminescent devices. Appl. Phys. Lett. 73, 2721–2723 (1998).

    CAS  Google Scholar 

  6. 6.

    Di, D. et al. Efficient triplet exciton fusion in molecularly doped polymer light‐emitting diodes. Adv. Mater. 29, 1605987 (2017).

    Google Scholar 

  7. 7.

    Awaga, K. & Maruyama, Y. Ferromagnetic and antiferromagnetic intermolecular interactions of organic radicals, α‐nitronyl nitroxides. II. J. Chem. Phys. 91, 2743–2747 (1989).

    CAS  Google Scholar 

  8. 8.

    Banister, A. J. et al. Spontaneous magnetization in a sulfur–nitrogen radical at 36 K. Angew. Chem. Int. Ed. 35, 2533–2535 (1996).

    CAS  Google Scholar 

  9. 9.

    Nakahara, K. et al. Rechargeable batteries with organic radical cathodes. Chem. Phys. Lett. 359, 351–354 (2002).

    CAS  Google Scholar 

  10. 10.

    Wang, Y. et al. 1-Imino nitroxide pyrene for high performance organic field-effect transistors with low operating voltage. J. Am. Chem. Soc. 128, 13058–13059 (2006).

    CAS  Google Scholar 

  11. 11.

    Wei, P., Oh, J. H., Dong, G. & Bao, Z. Use of a 1H-benzoimidazole derivative as an n-type dopant and to enable air-stable solution-processed n-channel organic thin-film transistors. J. Am. Chem. Soc. 132, 8852–8853 (2010).

    CAS  Google Scholar 

  12. 12.

    Bin, Z., Duan, L. & Qiu, Y. Air stable organic salt as an n-type dopant for efficient and stable organic light-emitting diodes. ACS Appl. Mater. Interfaces 7, 6444–6450 (2015).

    CAS  Google Scholar 

  13. 13.

    Zhang, Z., Chen, P., Murakami, T. N., Zakeeruddin, S. M. & Grätzel, M. The 2,2,6,6‐tetramethyl‐1‐piperidinyloxy radical: an efficient, iodine‐free redox mediator for dye‐sensitized solar cells. Adv. Funct. Mater. 18, 341–346 (2008).

    CAS  Google Scholar 

  14. 14.

    Jiao, Y. et al. A supramolecularly activated radical cation for accelerated catalytic oxidation. Angew. Chem. Int. Ed. 55, 8933–8937 (2016).

    CAS  Google Scholar 

  15. 15.

    Gamero, V. et al. [4-(N-carbazolyl)-2,6-dichlorophenyl]bis(2,4,6-trichlorophenyl) methyl radical an efficient red light-emitting paramagnetic molecule. Tetrahedron Lett. 47, 2305–2309 (2006).

    CAS  Google Scholar 

  16. 16.

    Velasco, D. et al. Red organic light-emitting radical adducts of carbazole and tris(2,4,6-trichlorotriphenyl)methyl radical that exhibit high thermal stability and electrochemical amphotericity. J. Org. Chem. 72, 7523–7532 (2007).

    CAS  Google Scholar 

  17. 17.

    Heckmann, A., Lambert, C., Goebel, M. & Wortmann, R. Synthesis and photophysics of a neutral organic mixed-valence compound. Angew. Chem. Int. Ed. 43, 5851–5856 (2004).

    CAS  Google Scholar 

  18. 18.

    Heckmann, A. & Lambert, C. Neutral organic mixed-valence compounds: synthesis and all-optical evaluation of electron-transfer parameters. J. Am. Chem. Soc. 129, 5515–5527 (2007).

    CAS  Google Scholar 

  19. 19.

    Blasi, D., Nikolaidou, D. M., Terenziani, F., Ratera, I. & Veciana, J. Excimers from stable and persistent supramolecular radical-pairs in red/NIR-emitting organic nanoparticles and polymeric films. Phys. Chem. Chem. Phys. 19, 9313–9319 (2017).

    CAS  Google Scholar 

  20. 20.

    Peng, Q., Obolda, A., Zhang, M. & Li, F. Organic light‐emitting diodes using a neutral π radical as emitter: the emission from a doublet. Angew. Chem. Int. Ed. 54, 7091–7095 (2015).

    CAS  Google Scholar 

  21. 21.

    Ai, X. et al. Efficient radical-based light-emitting diodes with doublet emission. Nature 563, 536–540 (2018).

    CAS  Google Scholar 

  22. 22.

    Neier, E. et al. Solution-processed organic light-emitting diodes with emission from a doublet exciton; using (2,4,6-trichlorophenyl)methyl as emitter. Org. Electron. 44, 126–131 (2017).

    CAS  Google Scholar 

  23. 23.

    Hattori, Y., Kusamoto, T. & Nishihara, H. Luminescence, stability, and proton response of an open‐shell (3,5‐dichloro‐4‐pyridyl)bis(2,4,6‐trichlorophenyl) methyl radical. Angew. Chem. Int. Ed. 53, 11845–11848 (2014).

    CAS  Google Scholar 

  24. 24.

    Hattori, Y., Kusamoto, T. & Nishihara, H. Enhanced luminescent properties of an open-shell (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl radical by coordination to gold. Angew. Chem. Int. Ed. 54, 3731–3734 (2015).

    CAS  Google Scholar 

  25. 25.

    Kimura, S. et al. A luminescent organic radical with two pyridyl groups: high photostability and dual stimuli-responsive properties, with theoretical analyses of photophysical processes. Chem. Sci. 9, 1996–2007 (2018).

    CAS  Google Scholar 

  26. 26.

    Guo, H. et al. High stability and luminescence efficiency in donor–acceptor neutral radicals not following the Aufbau principle. Nat. Mater. 18, 977–984 (2019).

    CAS  Google Scholar 

  27. 27.

    Diez‐Cabanes, V. et al. Design of perchlorotriphenylmethyl (PTM) radical‐based compounds for optoelectronic applications: the role of orbital delocalization. ChemPhysChem 19, 2572–2578 (2018).

    Google Scholar 

  28. 28.

    He, C., Li, Z., Lei, Y., Zou, W. & Suo, B. Unraveling the emission mechanism of radical-based organic light-emitting diodes. J. Phys. Chem. Lett. 10, 574–580 (2019).

    Google Scholar 

  29. 29.

    Dewar, M. & Longuet-Higgins, H. The electronic spectra of aromatic molecules I: benzenoid hydrocarbons. Proc. Phys. Soc. A. 67, 795 (1954).

    Google Scholar 

  30. 30.

    Longuet-Higgins, H. & Pople, J. A. The electronic spectra of aromatic molecules IV: excited states of odd alternant hydrocarbon radicals and ions. Proc. Phys. Soc. A. 68, 591–600 (1955).

    Google Scholar 

  31. 31.

    Pople, J. A. Electron interaction in unsaturated hydrocarbons. Trans. Faraday Soc. 49, 1375–1385 (1953).

    CAS  Google Scholar 

  32. 32.

    Pople, J. A. The electronic spectra of aromatic molecules II: a theoretical treatment of excited states of alternant hydrocarbon molecules based on self-consistent molecular orbitals. Proc. Phys. Soc. Lond. A 68, 81 (1955).

    Google Scholar 

  33. 33.

    Pariser, R. & Parr, R. G. A. A semi-empirical theory of the electronic spectra and electronic structure of complex unsaturated molecules. I. J. Chem. Phys. 21, 466–471 (1953).

    CAS  Google Scholar 

  34. 34.

    Pariser, R. Theory of the electronic spectra and structure of the polyacenes and of alternant hydrocarbons. J. Chem. Phys. 24, 250–268 (1956).

    CAS  Google Scholar 

  35. 35.

    Dong, S. et al. Effects of substituents on luminescent efficiency of stable triaryl methyl radicals. Phys. Chem. Chem. Phys. 20, 18657–18662 (2018).

    CAS  Google Scholar 

  36. 36.

    Franco, C. et al. Operative mechanism of hole-assisted negative charge motion in ground states of radical–anion molecular wires. J. Am. Chem. Soc. 139, 686–692 (2017).

    CAS  Google Scholar 

  37. 37.

    Hele, T. J. H. et al. Anticipating acene-based chromophore spectra with molecular orbital arguments. J. Phys. Chem. A 123, 2527–2536 (2019).

    CAS  Google Scholar 

  38. 38.

    Robinson, G. W. Intensity enhancement of forbidden electronic transitions by weak intermolecular interactions. J. Chem. Phys. 46, 572–585 (1967).

    CAS  Google Scholar 

  39. 39.

    Li, W. et al. A hybridized local and charge-transfer excited state for highly efficient fluorescent OLEDs: molecular design, spectral character and full exciton utilization. Adv. Opt. Mater. 2, 892–901 (2014).

    CAS  Google Scholar 

  40. 40.

    Armet, O. et al. Inert carbon free radicals. 8. Polychlorotriphenylmethyl radicals: synthesis, structure, and spin-density distribution. J. Phys. Chem. 91, 5608–5616 (1987).

    CAS  Google Scholar 

  41. 41.

    Fox, M. A., Gaillard, E. & Chen, C. C. Photochemistry of stable free radicals: the photolysis of perchlorotriphenylmethyl radicals. J. Am. Chem. Soc. 109, 7088–7094 (1987).

    CAS  Google Scholar 

  42. 42.

    Dong, S. et al. Multicarbazolyl substituted TTM radicals: red-shift of fluorescence emission with enhanced luminescence efficiency. Mater. Chem. Front. 1, 2132–2135 (2017).

    CAS  Google Scholar 

  43. 43.

    Murawski, C. et al. Efficiency roll-off in organic light-emitting diodes. Adv. Mater. 25, 6801–6827 (2013).

    CAS  Google Scholar 

  44. 44.

    Giebink, N. C. & Forrest, S. R. Quantum efficiency roll-off at high brightness in fluorescent and phosphorescent organic light emitting diodes. Phys. Rev. B 77, 235215 (2018).

    Google Scholar 

  45. 45.

    Neese, F. The ORCA program system. WIREs Comput. Mol. Sci. 2, 73–78 (2012).

    CAS  Google Scholar 

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A.A., Q.P., M.Z. and F.L. are grateful for financial support from the National Natural Science Foundation of China (grant nos 51925303, 91833304 and 61935017), the National Key R&D Programme of China (grant no. 2016YFB0401001) and the programme ‘JLUSTIRT’ (grant no. 2019TD-33). T.J.H.H. thanks Jesus College, Cambridge, for a Research Fellowship. J.Z. and Q.G. thank the China Scholarship Council for PhD scholarship (no. 201503170255). R.H.F. and E.W.E. thank EPSRC for funding this work (EP/M005143/1). E.W.E, also acknowledges support from the Leverhulme Trust and Newton Trust. F.L. is an academic visitor at the Cavendish Laboratory, Cambridge, and is supported by the Talents Cultivation Programme (Jilin University, China).

Author information




A.A. and M.Z. designed, synthesized and characterized the luminescent radicals. A.A., Q.G., J.Z., Q.P., F.L. and E.W.E. optimized the devices. T.J.H.H. and E.W.E. devised the theoretical treatment and performed the electronic-structure calculations. R.H.F., F.L. and E.W.E. conceived the project, supervised the work and wrote the manuscript with input from all the authors.

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Correspondence to Richard H. Friend or Feng Li or Emrys W. Evans.

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Supplementary Sections 1–19 including Figs. 1–24, Tables 1–5 and discussion.

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Abdurahman, A., Hele, T.J.H., Gu, Q. et al. Understanding the luminescent nature of organic radicals for efficient doublet emitters and pure-red light-emitting diodes. Nat. Mater. 19, 1224–1229 (2020). https://doi.org/10.1038/s41563-020-0705-9

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