Anthraquinone-based intramolecular charge-transfer compounds: Computational molecular design, thermally activated delayed fluorescence, and highly efficient red electroluminescence

Red fluorescent molecules suffer from large, non-radiative internal conversion rates (k_IC) governed by the energy gap law. To design efficient red thermally activated delayed fluorescence (TADF) emitters for organic light-emitting diodes (OLEDs), a large fluorescence rate (k_F) as well as a small energy difference between the lowest singlet and triplet excited states (ΔE_ST) is necessary. Herein, we demonstrated that increasing the distance between donor (D) and acceptor (A) in intramolecular-charge-transfer molecules is a promising strategy for simultaneously achieving small ΔE_ST and large k_F. Four D-Ph-A-Ph-D-type molecules with an anthraquinone acceptor, phenyl (Ph) bridge, and various donors were designed, synthesized, and compared with corresponding D-A-D-type molecules. Yellow to red TADF was observed from all of them. The k_F and ΔE_ST values determined from the measurements of quantum yield and lifetime of the fluorescence and TADF components are in good agreement with those predicted by corrected time-dependent density functional theory and are approximatively proportional to the square of the cosine of the theoretical twisting angles between each subunit. However, the introduction of a Ph-bridge was found to enhance k_F without increasing ΔE_ST. Molecular simulation revealed a twisting and stretching motion of the N-C bond in the D-A-type molecules, which is thought to lower ΔE_ST and k_F but raise k_IC, that was experimentally confirmed in both solution and doped film. OLEDs containing D-Ph-A-Ph-D-type molecules with diphenylamine and bis(4-biphenyl)amine donors demonstrated maximum external quantum efficiencies of 12.5% and 9.0% with emission peaks at 624 and 637 nm, respectively.