Metal (n+1)p‐nd Orbital Hybridization and Excited‐State Metal–Ligand π‐Interactions Enable d ¹⁰ Carbene‐Metal‐Amide TADF OLEDs with High Efficiency and Long Operational Lifetime

ABSTRACT

Luminescent d ¹⁰ carbene‐metal‐amide (CMA) complexes are a promising class of thermally activated delayed fluorescence (TADF) organic light‐emitting diode (OLED) emitters. However, the principles for modifying ligands to maximize OLED efficiency and operational stability remain unclear. Here, we reveal the key role of metal (n+1)p‐nd orbital hybridization and excited‐state metal‐ligand π‐interactions in affecting the excited‐state stability and electro‐/photoluminescence efficiency of CMA emitters. Using density functional theory (DFT), high‐level coupled cluster singles and doubles (CCSD) method, and combined DFT and multireference configuration interaction (DFT/MRCI) calculations, we found that in the excited state, metal atoms and carbazole nitrogen atoms form π‐interactions, which is weakened by the weakening of metal (n+1)p‐nd orbital hybridization. The weakened metal–nitrogen (M─N) π‐interaction is conducive to more flexible rotation of the excited‐state dihedral angle, thereby increasing the radiative decay rate ( k _TADF ). This rationalizes the general trend of k _TADF for CMA emitters: Ag > Au > Cu. However, the weakening of the excited‐state M─N π‐interaction reduces the strength of the M─N bond and facilitates bond dissociation in the excited state, thereby impairing the stability of the emitter. Our calculations show that introducing electron‐withdrawing or π‐extended substituents on carbazole ligands reduces excited‐state M─N π‐interactions, thereby improving k _TADF, but may impair emitter stability and device operational lifetime.