A novel molecular descriptor for highly efficient ( > 90%) transition metal TADF Au() complexes

It is generally perceived that a fast reverse intersystem crossing rate of T 1 → S 1 ( k risc ) is crucial for efficient organic thermally activated delayed fluorescence (TADF) emitters. Herein, we demonstrate the non-radiative decay rate of T 1 → S 0 ( k T nr ) for transition metal complexes that is even more important. We calculated the interconversion rates among S 0 , S 1 and T 1 states for two Au( iii )-TADF complexes with triphenylamine (TPA) as a donor moiety but with quite different quantum efficiencies: one with a moderate efficiency of 79% and the other with a high efficiency of 94%, and we found that the former has a much larger k risc (∼10 10 s −1 ) than the latter (∼10 7 s −1 ). Such contradictions with the conventional picture are attributed to the relatively large k T nr (∼10 6 s −1 ) for the former, leading to an overall lower quantum efficiency. Thus, we propose a novel molecular design descriptor (the triplet non-radiative decay rate k T nr ) for highly efficient transition metal TADF emitters. Further, we find that tetradentate ligand scaffolds with 5-5-6 membered chelate rings could reduce k T nr to