The development of emitting materials for OLEDs has started with fluorescence, moved to phosphorescence, and reached TADF. These materials have singlet ground state, but recently some ground state doublet (radical) emitting materials have been reported.^1,2 The excited state of doublet emitters also has a doublet character, and theoretically, can achieve 100% exciton utilization efficiency in an OLED device. Triphenylmethyl radicals such as PTM (perchlorotriphenylmethyl) radical were recognized as emitters from the 1980s. The photostability of these radicals are extremely low, but in 2006 the introduction of donor (carbazole) unit was reported to improve the photostability and opened the door to utilize them as a luminescent materials.³

Carbazole dendrimers are widely used as solution-processable OLED material including TADF emitters.⁴ The dendron has a highly polarized unique electronic structure due to the summation of the inductive electron-withdrawing effect of carbazole units.⁵ The attachment of carbazole dendrons to luminescent radical is expected to increase the stability through the steric and electronic effects.

A carbazole dendron (G2 to G4) attached TTM (tris(2,4,6-trichlorophenyl)-methyl) radicals were successfully synthesized and photophysical parameters were determined. All dendrimers showed red emission. PLQY (photoluminescence quantum yield) increased when the generation increased and reached 80%. The major reason was the decrease in the internal conversion rate (non-radiative decay). The photostability (PL intensity half-life measured under continuous laser irradiation) in solution has increased 100 to 1000 times compared to simple TTM radicals. These results indicate that the carbazole dendron attached TTM radicals are potential material for efficient OLEDs.

References

[1] Q. Peng, A. Obolda, M. Zhang, F. Li, Angew. Chem. Int. Ed. 2015, 54, 7091.

[2] X. Ai, E. W. Evans, S. Dong, A. J. Gillett, H. Guo, Y. Chen, T. J. Hele, R. H. Friend, F. Li, Nature 2018, 563, 536.

[3] V. Gamero, D. Velasco, S. Latorre, F. Lopez-Calahorra, E. Brillas, L. Julia, Tetrahedron Lett. 2006, 47, 2305.

[4] K. Albrecht, K. Matsuoka, K. Fujita, K. Yamamoto, Angew. Chem. Int. Ed. 2015, 54, 5677.

[5] K. Albrecht, K. Yamamoto J. Am. Chem. Soc. 2009, 131, 2244.