Carbene-Metal-Amide Materials Design: Tailoring π-Extended Amides for High-Performance Organic Light-Emitting Diodes

A series of gold-centred carbene-metal-amide (CMA) complexes are prepared with a rigid benzofuroindole (BFI) and benzothioindole (BTI) amide donor ligands coordinated with bicyclic(alkyl)amino carbene (BiC)Au(I)-moiety in high yields. CMA complexes emit in the sky-blue range at 501 and 511 nm with unity quantum yields in amorphous solid state media. Both CMA complexes emit thermally activated delayed fluorescence (TADF) originating from a charge transfer (CT) state with an excited state lifetime as short as 473 ns, resulting in fast radiative rates of 2 × 10⁶  s^-1. The impact on the photoluminescence properties of the electron-donating oxygen and sulfur atoms in amide-donor ligand has been interpreted with the help of steady-state and transient photo and electroluminescence experiments. Theoretical investigations revealed two charge-transfer (T₁ and T₂) states followed by a locally excited T₃ triplet state, and their spin-orbit coupling analysis. Sky-blue host-guest and host-free organic light-emitting diode (OLED) devices are fabricated with electroluminescence at ≈490 nm and practical external quantum efficiencies up to 23% with colour coordinates CIE (x; y) = 0.19; 0.32. The superior performance and operating stability of the neat OLEDs are correlated with the presence of the heavier sulfur atom, suggesting further molecular design modifications toward stable CMA OLED devices with a simplified architecture of the emitting layer (host-free OLED).