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

Abstract

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 (Φ_PL) close to unity and emission decay lifetimes as short as 1.0 μs. The small energy differences between S₁ and T₁ (ΔE_ST) 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.