Short-lived TADF-Cu(I) complex for red OLEDs with EQE_max of 25.9% and <2% roll-off at 10,000 cd m⁻²

Cu(I) complexes exhibiting thermally activated delayed fluorescence (TADF) have emerged as promising alternatives to noble-metal-based emitters for organic light-emitting diodes (OLEDs). However, the development of red-emitting Cu(I) complexes has been hindered by the slow radiative decay and fast nonradiative decay rate. In this study, a linear two-coordinate Cu(I) complex, namely ICuTMC, was designed and synthesized. Through pairing a pyrazine-fused N-heterocyclic carbene and a tetra-methylcarbazolyl ligand, a strong ligand-to-ligand charge transfer excited state is generated. Single crystal structure authenticates the presence of close intramolecular C–H···Cu contacts, providing a good steric shielding environment to the metal center. The C–H···π interactions between the ligands are also revealed. The Cu(I) complex exhibits highly efficient red TADF with an emission maximum of 622 nm, a photoluminescence quantum yield of 76% and a short delayed fluorescence lifetime of 0.24 μs. This is conceived to be enabled by a large oscillator strength associated with the coplanar donor-Cu-acceptor conformation, a small singlet-triplet excited states energy gap resulting from the spatial separation of the frontier molecular orbitals, and a strong spin-orbit coupling effect brought about by the metal center. Vacuum-deposited OLEDs based on ICuTMC exhibit a peak external quantum efficiency of 25.9% and a significantly small roll-off (1.9%) at 10,000 cd m−2. These performances demonstrate a way to overcome the energy gap law for linear coinage metal complexes toward red OLEDs.