Efficient Red TADF Emitter and Diode in High Doping Concentration via Steric Hindrance Engineering with Phosphine Oxide

A simple strategy of steric hindrance engineering is proposed for constructing efficient red thermally activated delayed fluorescence (TADF) emitters and diodes under high doping concentrations. As a concept, a newly TADF emitter, namely 13-(4-(diphenylamino)phenyl)dipyrido[3,2-a:2′,3′-c]phenazin-10-yl)diphenylphosphine oxide (pTPAPO–DPPZ), is designed and synthesized by using triphenylamine (TPA), dipyridophenazine (DPPZ), and diphenylphosphine oxide (DPPO) as the donor, acceptor, and steric groups, respectively. It is demonstrated that the steric hindrance of DPPO enlarges the distances between adjacent molecules, and thus, only weak centroid-to-edge π–π interactions are observed between DPPZ and TPA, which effectively promote the intermolecular charge transfer (CT) to enhance TADF characteristics. Furthermore, locally concentrated intermolecular C–H···O hydrogen bonds between TPA and DPPO of adjacent molecules facilitate the dimer formation, which enhances steric hindrance to alleviate concentration quenching. As a consequence, pTPAPO–DPPZ achieves superior photo- and electroluminescence performances at a high doping concentration of 60%, owing to the optimized intermolecular interactions for the balance of TADF enhancement and quenching suppression.