Abstract

The mechanism of thermally enhanced luminescence (TEL), such as thermally activated delayed fluorescence (TADF) and thermally stimulated delayed phosphorescence (TSDP), is of fundamental importance in the molecular design of emissive transition-metal complexes, as it can enable bright emission with enhanced radiative rate for high-performance organic light-emitting diodes (OLEDs). It is essential to resolve the excited-state landscape of complexes, including the correlation with their photophysical properties, to optimize the device efficiency. Here, we explore a series of Au(III) complexes containing asymmetric carbazolyl (Cz) motifs that regulate the co-facial distance between the electron donor and acceptor moieties. Combining with the femtosecond transient absorption (fs-TA) and in-depth density functional theory computation, we reveal rapid reverse internal upconversion to a higher-lying triplet state, T₁’, which is thermally accessible from the lowest triplet state, T₁, at room temperature. The efficient spin-flip process from T₁’ to the lowest singlet state, S₁, and the ground state, S₀, leads to strong TEL through both TADF and TSDP pathways, substantially increasing the radiative rate to 10⁵ s⁻¹. The OLED devices based on the TEL Au(III) emitters reach external quantum efficiencies (η_ext) of 26.5% and 16.5% in doped and non-doped devices, respectively. The doped OLED devices exhibited an η_ext roll-off as low as 2% at a practical luminance of 1000 cd m⁻². We believe that the molecular design of highly efficient organometallic emitters of TEL character will be greatly facilitated by the mechanistic analysis and methodology presented here.