2,7-substituted N-Carbazole donors on tris(2,4,6-trichlorophenyl)methyl radicals with high quantum yield

Abstract

Doublet spin properties of organic open-shell luminophores evade the formation of dark triplet states, thus making radicals favorable emitters for next generation organic light-emitting diodes. However, their poor photostability and mediocre photoluminescence quantum yields (PLQYs) limit their application. In this work, two series of trityl radicals functionalized with one, two, and three 2,7-disubstituted carbazole units are presented. The authors either attach nitriles or bromines as electron-withdrawing 2,7-substituents. The resulting radical emitters exhibit outstanding optical properties. The electron-withdrawing properties of the substituents lead to a blue-shift of the emission, indicating a starting point for future emission engineering into the green spectrum. Due to the 2,7-substitution of the carbazole moiety, the radical emitters are sterically more hindered than the commonly used 3,6-substituted carbazoles. This steric hindrance reduces non-radiative decay pathways in the molecules, enhancing photostability and pushing PLQYs up to 87%. Quantum mechanical calculations elucidate the influence of the electron withdrawing substituents on the optical performance of the open-shell molecules. The authors also show that intensity borrowing from higher lying locally excited states contributes to these exceptionally high PLQYs.