Two-Coordinate Coinage Metal Complexes Featuring Imidazolium Carbenes: Realization of Efficient Blue Thermally Activated Delayed Fluorescence (TADF) for Organic-Light-Emitting Diodes (OLEDs)

As the most readily accessible N-heterocyclic carbenes (NHCs), imidazolium carbenes have traditionally been ruled out for the design of TADF coinage metal carbene–metal–amine (CMA) complexes due to their high-lying unoccupied π orbitals, which lead to a dark interligand charge transfer (ICT) state. In this work, in cooperation with strong electron-donating acridine ligands, a series of CMA complexes featuring imidazolium carbenes are prepared, exhibiting desired blue thermally activated delayed fluorescence (TADF) with high photoluminescent (PL) quantum yields (ΦPL) close to unity and emission decay lifetimes as short as 1.0 μs. The small energy differences between S1 and T1 (ΔEST) of around 100 meV and the high radiative decay rates of S1 on the order of 1 × 107–108 s–1 contribute to the decent TADF behavior. Solution-processed blue OLEDs based on the Cu(I) emitter show an external quantum efficiency (EQE) exceeding 8.0%, demonstrating the practicality of using imidazolium carbenes in the design of TADF CMA luminophores.