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

Abstract

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.

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Acknowledgements

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).

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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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