Stable Tetradentate Gold(III)‐TADF Emitters with Close to Unity Quantum Yield and Radiative Decay Rate Constant of up to 2 × 106 s−1: High‐Efficiency Green OLEDs with Operational Lifetime (LT90) Longer than 1800 h at 1000 cd m−2

Abstract 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 (Δ E ST ). Here, the design of a class of tetradentate gold(III) TADF complexes for narrowing the Δ E 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.