Stable Tetradentate Gold(III)-TADF Emitters with Close to Unity Quantum Yield and Radiative Decay Rate Constant of up to 2 × 10 6 s -1 : High-Efficiency Green OLEDs with Operational Lifetime (LT 90 ) Longer than 1800 h at 1000 cd m -2 .

High maximum external quantum efficiency (EQE max ), small efficiency roll-offs, and long operational lifetime at practical luminances are three crucial parameters for commercialization of organic light-emitting diodes (OLEDs). To simultaneously achieve these goals, it is desirable to have the radiative decay rate constant (k r ) as large as possible, which, for a thermally activated delayed fluorescent (TADF) emitter, requires both a large S 1 →S 0 radiative decay rate constant (k r S ) and a small singlet-triplet energy gap (ΔE ST ). Here, the design of a class of tetradentate gold(III) TADF complexes for narrowing the ΔE ST while keeping the k r S large is reported. The as-synthesized complexes display green emission with close to unity emission quantum yields, and k r approaching 2 × 10 6 s -1 in thin films. The vacuum-deposited green OLEDs based on 1 and 4 demonstrate maximum EQEs of up to 24 and 27% with efficiency roll-offs of 5.5 and 2.2% at 1000 cd m -2 , respectively; the EQEs maintain high at 10 000 cd m -2 (19% (1) and 24% (4)). A long LT 90 device lifetime of 1820 h at 1000 cd m -2 for complex 1 is achieved, which is one of the longest device lifetimes of TADF-OLEDs reported in the literature.