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

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.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

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.

References

  1. 1.

    Simao, C. et al. Molecular platform for non-volatile memory devices with optical and magnetic responses. Nat. Chem. 3, 359–364 (2011).

    CAS  Article  Google Scholar 

  2. 2.

    Morita, Y. et al. Thermochromism in an organic crystal based on the coexistence of sigma- and pi-dimers. Nat. Mater. 7, 48–51 (2008).

    CAS  Article  Google Scholar 

  3. 3.

    Dhimitruka, I., Bobko, A. A., Eubank, T. D., Komarov, D. A. & Khramtsov, V. V. Phosphonated trityl probes for concurrent in vivo tissue oxygen and pH monitoring using electron paramagnetic resonance-based techniques. J. Am. Chem. Soc. 135, 5904–5910 (2013).

    CAS  Article  Google Scholar 

  4. 4.

    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  Article  Google Scholar 

  5. 5.

    Obolda, A., Ai, X., Zhang, M. & Li, F. Up to 100% formation ratio of doublet exciton in deep-red organic light-emitting diodes based on neutral π-radical. ACS Appl. Mater. Interfaces 8, 35472–35478 (2016).

    CAS  Article  Google Scholar 

  6. 6.

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

    CAS  Article  Google Scholar 

  7. 7.

    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  Article  Google Scholar 

  8. 8.

    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  Article  Google Scholar 

  9. 9.

    Heckmann, A. et al. Highly fluorescent open-shell NIR dyes: the time-dependence of back electron transfer in triarylamine-perchlorotriphenylmethyl radicals. J. Phys. Chem. C 113, 20958–20966 (2009).

    CAS  Article  Google Scholar 

  10. 10.

    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  Article  Google Scholar 

  11. 11.

    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  Article  Google Scholar 

  12. 12.

    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  Article  Google Scholar 

  13. 13.

    Ai, X., Chen, Y., Feng, Y. & Li, F. A. Stable room-temperature luminescent biphenylmethyl radical. Angew. Chem. Int. Ed. 57, 2869–2873 (2018).

    CAS  Article  Google Scholar 

  14. 14.

    Hattori, Y., Kusamoto, T. & Nishihara, H. Highly photostable luminescent open-shell (3,5-dihalo-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl radicals: significant effects of halogen atoms on their photophysical and photochemical properties. RSC Adv. 5, 64802–64805 (2015).

    CAS  Article  Google Scholar 

  15. 15.

    Kimura, S. et al. 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  Article  Google Scholar 

  16. 16.

    Hicks, R. G. What’s new in stable radical chemistry?. Org. Biomol. Chem. 5, 1321–1338 (2007).

    CAS  Article  Google Scholar 

  17. 17.

    Ballester, M., Riera-Figueras, J., Castaner, J., Badfa, C. & Monso, J. M. Inert carbon free radicals. I. Perchlorodiphenylmethyl and perchlorotriphenylmethyl radical series. J. Am. Chem. Soc. 93, 2215–2225 (1971).

    CAS  Article  Google Scholar 

  18. 18.

    Schweyen, P., Brandhorst, K., Wicht, R., Wolfram, B. & Broring, M. The corrole radical. Angew. Chem. Int. Ed. 54, 8213–8216 (2015).

    CAS  Article  Google Scholar 

  19. 19.

    Sun, Z., Ye, Q., Chi, C. & Wu, J. Low band gap polycyclic hydrocarbons: from closed-shell near infrared dyes and semiconductors to open-shell radicals. Chem. Soc. Rev. 41, 7857–7889 (2012).

    CAS  Article  Google Scholar 

  20. 20.

    Bohr, N. Über die Anwendung der Quantentheorie auf den Atombau. I. Die Grundpostulate der Quantentheorie. Z. Phys. 13, 117–165 (1923).

    CAS  Article  Google Scholar 

  21. 21.

    Mallion, R. B. & Rouvray, D. H. Molecular topology and the Aufbau principle. Mol. Phys. 36, 125–128 (1978).

    CAS  Article  Google Scholar 

  22. 22.

    Wang, Y. et al. Radical cation and neutral radical of aza-thia[7]helicene with SOMO−HOMO energy level inversion. J. Am. Chem. Soc. 138, 7298–7304 (2016).

    CAS  Article  Google Scholar 

  23. 23.

    Gryn’ova, G., Coote, M. L. & Corminboeuf, C. Theory and practice of uncommon molecular electronic configurations. WIREs Comput. Mol. Sci. 5, 440–459 (2015).

    Article  Google Scholar 

  24. 24.

    Franchi, P., Mezzina, E. & Lucarini, M. SOMO–HOMO conversion in distonic radical anions: an experimental test in solution by EPR radical equilibration technique. J. Am. Chem. Soc. 136, 1250–1252 (2014).

    CAS  Article  Google Scholar 

  25. 25.

    Kusamoto, T., Kume, S. & Nishihara, H. Cyclization of TEMPO radicals bound to metalladithiolene induced by SOMO–HOMO energy-level conversion. Angew. Chem. Int. Ed. 49, 529–531 (2010).

    CAS  Article  Google Scholar 

  26. 26.

    Gryn’ova, G., Marshall, D. L., Blanksby, S. J. & Coote, M. L. Switching radical stability by pH-induced orbital conversion. Nat. Chem. 5, 474–481 (2013).

    Article  Google Scholar 

  27. 27.

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

    CAS  Article  Google Scholar 

  28. 28.

    Komamine, S., Fujitsuka, M., Ito, O. & Itaya, A. Photoinduced electron transfer between C60 and carbazole dimer compounds in a polar solvent. J. Photochem. Photobiol. A 135, 111–117 (2000).

    CAS  Article  Google Scholar 

  29. 29.

    Niu, Y., Peng, Q., Deng, C., Gao, X. & Shuai, Z. Theory of excited state decays and optical spectra: application to polyatomic molecules. J. Phys. Chem. A 114, 7817–7831 (2010).

    CAS  Article  Google Scholar 

  30. 30.

    Hayashi, M., Mebel, A. M., Liang, K. K. & Lin, S. H. Ab Initio calculations of radiationless transitions between excited and ground singlet electronic states of ethylene. J. Chem. Phys. 108, 2044–2055 (1998).

    CAS  Article  Google Scholar 

  31. 31.

    Shizu, K. et al. Enhanced electroluminescence from a thermally activated delayed-fluorescence emitter by suppressing nonradiative decay. Phys. Rev. Appl. 3, 014001–014007 (2015).

    Article  Google Scholar 

  32. 32.

    Shizu, K. et al. Highly efficient blue electroluminescence using delayed-fluorescence emitters with large overlap density between luminescent and ground states. J. Phys. Chem. C 119, 26283–26289 (2015).

    CAS  Article  Google Scholar 

  33. 33.

    Chen, X.-K., Ravva, M. K., Li, H., Ryno, S. M. & Brédas, J.-L. Effect of molecular packing and charge delocalization on the nonradiative recombination of charge-transfer states in organic solar cells. Adv. Energy Mater. 6, 1601325 (2016).

    Article  Google Scholar 

  34. 34.

    Sugawara, T., Komatsu, H. & Suzuki, K. Interplay between magnetism and conductivity derived from spin-polarized donor radicals. Chem. Soc. Rev. 40, 3105–3118 (2011).

    CAS  Article  Google Scholar 

  35. 35.

    Kim, D.-H. et al. High-efficiency electroluminescence and amplified spontaneous emission from a thermally activated delayed fluorescent near-infrared emitter. Nat. Photon. 12, 98–104 (2018).

    CAS  Article  Google Scholar 

  36. 36.

    Ye, H. et al. Near-infrared electroluminescence and low threshold amplified spontaneous emission above 800 nm from a thermally activated delayed fluorescent emitter. Chem. Mater. 30, 6702–6710 (2018).

    CAS  Article  Google Scholar 

  37. 37.

    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).

    Article  Google Scholar 

  38. 38.

    Sun, H. et al. Impact of dielectric constant on the singlet–triplet gap in thermally activated delayed fluorescence materials. J. Phys. Chem. Lett. 8, 2393–2398 (2017).

    CAS  Article  Google Scholar 

  39. 39.

    Gaussian 09 rev. D01 (Gaussian, Inc., 2009).

Download references

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

Affiliations

Authors

Contributions

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.

Corresponding authors

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

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

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Guo, H., Peng, Q., Chen, X. et al. High stability and luminescence efficiency in donor–acceptor neutral radicals not following the Aufbau principle. Nat. Mater. 18, 977–984 (2019). https://doi.org/10.1038/s41563-019-0433-1

Download citation

Further reading

Search

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing