• • ICuTMC achieves a peak external quantum efficiency (EQE) of 25.9% in vacuum-deposited red OLEDs, with a roll-off of only 1.9% at 10,000 cd m−2, demonstrating exceptional stability at high brightness for display applications.
• • The complex exhibits a photoluminescence quantum yield (PLQY) of 76% and a short delayed fluorescence lifetime of 0.24 μs, enabling efficient exciton harvesting and reduced efficiency roll-off.
• • Emission maximum at 622 nm (red) is achieved with a small singlet-triplet energy gap (ΔEST) and strong spin-orbit coupling, overcoming the energy gap law that typically limits red-emitting coinage metal complexes.
• • Intramolecular C–H···Cu and C–H···π interactions provide steric shielding, suppressing nonradiative decay and enhancing radiative rate, as confirmed by single-crystal X-ray analysis.