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 (LT90) Longer than 1800 h at 1000 cd m−2

Abstract

<p>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 (<em>k</em>_r) as large as possible, which, for a thermally activated delayed fluorescent (TADF) emitter, requires both a large S₁→S₀ radiative decay rate constant (<em>k</em>_r^S) and a small singlet–triplet energy gap (Δ<em>E</em>_ST). Here, the design of a class of tetradentate gold(III) TADF complexes for narrowing the Δ<em>E</em>_ST while keeping the <em>k</em>_r^S large is reported. The as-synthesized complexes display green emission with close to unity emission quantum yields, and <em>k</em>_r approaching 2 × 10⁶ s⁻¹ in thin films. The vacuum-deposited green OLEDs based on <strong>1</strong> and <strong>4</strong> 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% (<strong>1</strong>) and 24% (<strong>4</strong>)). A long LT₉₀ device lifetime of 1820 h at 1000 cd m⁻² for complex <strong>1</strong> is achieved, which is one of the longest device lifetimes of TADF-OLEDs reported in the literature.<br></p>