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

Abstract

It is generally perceived that a fast reverse intersystem crossing rate of T₁ → S₁ (k_risc) is crucial for efficient organic thermally activated delayed fluorescence (TADF) emitters. Herein, we demonstrate the non-radiative decay rate of T₁ → S₀ (k^T_nr) for transition metal complexes that is even more important. We calculated the interconversion rates among S₀, S₁ and T₁ 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 (∼10¹⁰ s⁻¹) than the latter (∼10⁷ s⁻¹). Such contradictions with the conventional picture are attributed to the relatively large k^T_nr (∼10⁶ 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 <10⁵ s⁻¹ for Au(III)–TADF complexes, thereby achieving quantum efficiencies above 90%. Based on this theoretical guideline, we have proposed nine newly designed Au(III) complexes and predicted their high TADF efficiency.