Molecular Design and Synthetic Approaches for the Realization of Multichannel Radiative Decay Pathways in Gold(III) Complexes and Their Applications in Organic Light-Emitting Devices

Abstract

A unique class of tridentate diaryltriazine ligand-containing gold(III) complexes with thermally activated delayed fluorescence (TADF) and/or thermally stimulated delayed phosphorescence (TSDP) properties has been designed and synthesized. With a simple structural modification on the coordination of carbazole moiety in the monodentate ligand, a large spectral shift of ∼160 nm (ca. 4900 cm^-1) spanning from sky blue to red emissions has been demonstrated in solid-state thin films. Three-state or four-state models have been employed in fitting the emission lifetimes of the gold(III) complexes at various temperatures. The findings clearly indicate the presence of three emitting states, S₁, T₁, and T₁', suggesting the coexistence of TADF, phosphorescence, and TSDP. Notably, a minor structural change in the donor moiety between phenylcarbazolyl and diphenylaminoaryl has been demonstrated to turn on/off the TSDP, resulting in TADF-TSDP-phosphorescence or TADF-phosphorescence emitters. The TADF and/or TSDP properties have also been supported by temperature-dependent ultrafast transient absorption studies, with the direct observation of reverse intersystem crossing (RISC) and reverse internal conversion (RIC) and the determination of the activation parameters and excited state dynamics. Solution-processed and vacuum-deposited organic light-emitting devices (OLEDs) have been prepared, in which sky blue emitting devices based on 5 exhibit an operational lifetime LT₇₀ ∼ 5 times longer than the previously reported sky blue emitting analogue that shows only TSDP property. These results have provided valuable insights into the manipulation of the excited states via rational molecular design toward the realization of gold(III)-based TSDP and/or TADF materials with multiple radiative decay pathways that show enhanced radiative decay rate constants (k_r) for practical OLED applications.