Two-Coordinate\nCoinage Metal Complexes Featuring Imidazolium\nCarbenes: Realization of Efficient Blue Thermally Activated Delayed\nFluorescence (TADF) for Organic-Light-Emitting Diodes (OLEDs)

As the most readily accessible <i>N</i>-heterocyclic\ncarbenes (NHCs), imidazolium carbenes have traditionally been ruled\nout for the design of TADF coinage metal carbene–metal–amine\n(<b>CMA</b>) complexes due to their high-lying unoccupied π\norbitals, which lead to a dark interligand charge transfer (ICT) state.\nIn this work, in cooperation with strong electron-donating acridine\nligands, a series of <b>CMA</b> complexes featuring imidazolium\ncarbenes are prepared, exhibiting desired blue thermally activated\ndelayed fluorescence (TADF) with high photoluminescent (PL) quantum\nyields (Φ<sub>PL</sub>) close to unity and emission decay lifetimes\nas short as 1.0 μs. The small energy differences between S<sub>1</sub> and T<sub>1</sub> (Δ<i>E</i><sub>ST</sub>) of around 100 meV and the high radiative decay rates of S<sub>1</sub> on the order of 1 × 10<sup>7</sup>–10<sup>8</sup> s<sup>–1</sup> contribute to the decent TADF behavior. Solution-processed\nblue OLEDs based on the Cu(I) emitter show an external quantum efficiency\n(EQE) exceeding 8.0%, demonstrating the practicality of using imidazolium\ncarbenes in the design of TADF <b>CMA</b> luminophores.