High stability and luminescence efficiency in donor–acceptor neutral radicals not following the Aufbau principle

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Abstract

With their unusual electronic structures, organic radical molecules display luminescence properties potentially relevant to lighting applications; yet, their luminescence quantum yield and stability lag behind those of other organic emitters. Here, we designed donor–acceptor neutral radicals based on an electron-poor perchlorotriphenylmethyl or tris(2,4,6-trichlorophenyl)methyl radical moiety combined with different electron-rich groups. Experimental and quantum-chemical studies demonstrate that the molecules do not follow the Aufbau principle: the singly occupied molecular orbital is found to lie below the highest (doubly) occupied molecular orbital. These donor–acceptor radicals have a strong emission yield (up to 54%) and high photostability, with estimated half-lives reaching up to several months under pulsed ultraviolet laser irradiation. Organic light-emitting diodes based on such a radical emitter show deep-red/near-infrared emission with a maximal external quantum efficiency of 5.3%. Our results provide a simple molecular-design strategy for stable, highly luminescent radicals with non-Aufbau electronic structures.

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Fig. 1: Chemical structures of the radical molecules studied here.
Fig. 2: Quantum-chemical results for the PTM, PTM-3NCz, TTM-3NCz, TTM-PPTA and PTM-PDCz radicals.
Fig. 3: Experimental data for the PTM, TTM, PTM-3NCz, TTM-PPTA, PTM-PDCz and TTM-3NCz radicals.
Fig. 4: Photophysical properties and photostability of the PTM-3NCz radical.
Fig. 5: Optoelectronic properties of the OLED device with PTM-3NCz as the emitter.

Data availability

The data that support the results of this study are available at https://doi.org/10.17863/CAM.41550.

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Acknowledgements

H.G., Q.P., S.D., X.A., M.Z. and F.L. are grateful for financial support from the National Natural Science Foundation of China (grant nos. 91833304, 51673080 and 11804156), the National Key R&D Programme of China (grant no. 2016YFB0401001), the National Key Basic Research and Development Programme of China (973 programme, grant no. 2015CB655003) and the programme ‘JLUSTIRT’ (grant no. 2019TD-33). Q.P. acknowledges support from the Nanjing Tech Start-up Grant (38274017104). X.-K.C., V.C. and J.-L.B. acknowledge support from the Georgia Institute of Technology, Georgia Research Alliance, Vasser-Woolley Foundation and Kyulux. Q.G., D.C., E.W.E., A.J.G. and R.H.F. would like to thank the EPSRC for funding this work (EP/M01083X/1, EP/M005143/1). Q.G. is also grateful for financial support from the China Scholarship Council and Cambridge Trust. D.C. also acknowledges support from the Royal Society (grant no. UF130278). F.L. is an academic visitor at the Cavendish Laboratory, Cambridge, and is supported by the Talents Cultivation Programme (Jilin University, China).

Author information

H.G., Q.P., S.D., X.A. and M.Z. performed the synthesis and experimental measurements under the supervision of F.L. X.-K.C. carried out the quantum-chemical calculations under the supervision of J.-L.B., and V.C. participated in the discussion of the theoretical calculations. E.W.E. participated in the discussion of the photophysics mechanism, and A.J.G. conducted the transient absorption measurements under the supervision of R.H.F. Q.G. performed the device fabrications and measurements under the supervision of D.C. All authors discussed the results and contributed to writing the manuscript.

Correspondence to Richard H. Friend or Jean-Luc Brédas or Feng Li.

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Supplementary Information

Supplementary Schemes 1 and 2, Supplementary Figs. 1–13, Supplementary Tables 1–3 and Supplementary refs. 1–29.

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