Pd(II)-TADF Emitters Featuring Multiple Intramolecular Noncovalent Interactions: Toward Organic Light-Emitting Diodes with EQEs up to 31.5% and Operational Half-Lifetimes Exceeding 1600 h at 3000 cd m-2

The second-row transition metal complexes have been much less exploited as emitters for organic light-emitting diodes (OLEDs). Despite the considerable efforts to develop phosphorescent Pd(II) complexes by ligand design, the triplet excited states of Pd(II) complexes usually have very long lifetimes because of their ligand-centered nature. Hereina panel of sandwich-type mononuclear Pd(II) complexes is reported, namely PdDDBPZ, PdDTRZ, and PdDPYR, which exhibit strong yellow to red thermally activated delayed fluorescence (TADF) with photoluminescence quantum yields (PLQYs) of 87-94% in doped films. The metal ion plays a perturbing role in the intraligand charge transfer excited state dynamics, giving rise to accelerated reverse intersystem crossing (RISC). Owing to the space-confined sandwich configuration, there are multiple noncovalent interactions within the molecules, which are qualitatively and quantitatively analyzed by theoretical calculations. The Pd(II) complexes show high device efficiencies with external quantum efficiencies (EQEs) up to 31.5%. The ultralow efficiency roll-off is as low as 1% at 1000 cd m(-2). The device operational stability test for PdDTRZ reveals a half-lifetime (LT50) of 1615 h at an initial luminance of 3000 cd m(-2). The leverage of metal-promoted TADF mechanism together with intramolecular noncovalent interactions proves feasible for developing practical Pd(II)-TADF emitters toward OLEDs.