A modular approach to efficient thermally activated delayed fluorescence from metal-perturbed intraligand charge-transfer excited state of Au(I) complexes

The efficient harvesting of triplet excitons is crucial to the realization of high-performance organic light-emitting diodes (OLEDs). Herein, we show that coordination of donor-acceptor (D-A) type molecules to a metal atom in a monodentate fashion can lead to thermally activated delayed fluorescence (TADF) emissions with wide color tunability only through varying the non-coordinating acceptor moiety. A panel of TADF gold(I) complexes with emission maxima (λ_max) of 545–645 nm from metal perturbed intraligand charge-transfer (MPICT) excited states have been developed. Synergetic effects of heavy atom-induced spin-orbit coupling (SOC), steric-induced donor-acceptor twisting and suppressed intramolecular motions lead to high emission efficiencies of 65%–85% in doped films with delayed fluorescence lifetime of as short as 2.0 µs. Transient absorption spectroscopic studies on selected complexes determined the k_ISC to be 6.5 × 10⁹ s⁻¹. Theoretical calculations confirmed the participation of minor d orbital into the lowest excited state, which led to an SOC value of 5.19 cm⁻¹ between the lowest-lying singlet and triplet excited states. The yellow to deep red solution-processed OLEDs based on the new gold(I) complexes incorporated with various D-A ligands demonstrated promising performances. This study validates a modular design for TADF metal complexes, which will broaden the choices of metal centers and allow for facile color tuning via simple ligand synthesis.