Tetrahedral Structure Based on Triphenylgermanium for Quenching-Resistant Multi-Resonance Thermally Activated Delayed Fluorescence Emitters

The rigid planar architecture of multiple resonance thermally activated delayed fluorescence (MR-TADF) molecules employing boron/nitrogen (B/N) frameworks typically results in severe aggregation-caused quenching (ACQ) and spectral broadening. Herein, a steric modification strategy is proposed by incorporating a tetrahedral architecture of triphenylgermanium (TPhGe) into the para-position of B/N/N, B/N/O, and B/N/S frameworks for the first time, formed three MR-TADF emitters, BNNGe, BNOGe, and BNSGe, with narrowband emissions ranging from bluish-green to pure blue. Consequently, these emitters exhibit high photoluminescence quantum yields of > 90% in doped films. Organic light-emitting diodes (OLEDs) based on BNNGe, BNOGe, and BNSGe demonstrate impressive maximum external quantum efficiencies (EQE(max)s) of 30.1% to 15.5%, and 20.7%, respectively. The unique tetrahedral TPhGe moiety, with its bulky size conformation, effectively separates adjacent MR-TADF molecules, resulting in efficient luminescence across a broad range of doping concentrations (5-30 wt%) in doped films, thereby successfully suppressing the ACQ in devices. Furthermore, OLEDs containing BNSGe display low-efficiency roll-offs because of higher spin-orbital coupling and reverse intersystem crossing rates of the emitter, attributed to the heavy atom effect. Notably, the device with 5 wt% BNOGe exhibits a pure blue emission peaking at 461 nm, with a narrow full-width at half-maximum of 32 nm.