Blue TADF Emitters Based on Indenocarbazole Derivatives with High Photoluminescence and Electroluminescence Efficiencies

A series of blue thermally activated delayed fluorescence (TADF) emitters were designed and synthesized using 2,4,6-triphenyl-1,3,5-triazine as the acceptor unit and indenocarbazole derivatives as the electron-donating moiety. In contrast with other six-membered heterocycles, like phenothiazine, phenoxazine, and dihydroacridine, where the TADF efficiency is affected by the presence of different conformers, indenocarbazole derivatives do not show this effect. Therefore, InCz23FlTz, InCz23DPhTz, InCz23DMeTz, and InCz34DPhTz allow the investigation of the effect of different substituents and substitution positions on TADF properties, without the influence of different conformations. We have demonstrated that the substituted position on the carbazole and different substituents in the same position have clear influence on the donor character of indenocarbazole derivatives. Also, the color purity of blue emission and excited states could be adjusted by substituents and substituted position, and thus excellent blue emitters can be obtained. Besides, the four compounds show relatively small TADF contribution under optical excitation; however, excellent performances are obtained in the electroluminescent devices, especially with InCz34DPhTz, which shows a maximum external quantum efficiency of around 26%. In the end, we find an effective way to design high-efficiency blue TADF materials and deeply study the relation between the structure and property in indenocarbazole derivatives.