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Constructing folded molecular structures is emerging as a promising strategy to develop efficient thermally activated delayed fluorescence (TADF) materials. Most folded TADF materials have V-shaped configurations formed by donors and acceptors linked on carbazole or fluorene bridges. In this work, a facile molecular design strategy is proposed for exploring sandwich-structured molecules, and a series of novel and robust TADF materials with regular U-shaped sandwich conformations are constructed by using 11,12-dihydroindolo2,3-acarbazole as bridge, xanthone as acceptor, and dibenzothiophene, dibenzofuran, 9-phenylcarbazole and indolo3,2,1-JKcarbazole as donors. They hold outstanding thermal stability with ultrahigh decomposition temperatures (556-563 °C), and exhibit fast delayed fluorescence and excellent photoluminescence quantum efficiencies (86 %-97 %). The regular and close stacking of acceptor and donors results in rigidified molecular structures with efficient through-space interaction, which are conducive to suppressing intramolecular motion and reducing reorganized excited-state energy. The organic light-emitting diodes (OLEDs) using them as emitters exhibit excellent electroluminescence performances, with maximum external quantum efficiencies of up to 30.6 %, which is a leading value for the OLEDs based on folded TADF emitters. These results demonstrate the proposed strategy of employing 11,12-dihydroindolo2,3-acarbazole as bridge for planar donors and acceptors to construct efficient folded TADF materials is applicable.
Xia et al. (Thu,) studied this question.