A theoretical study on the influence of N-containing heterocyclic ligands on the luminescence mechanisms (phosphorescence or TADF) of Au (III) complexes

Abstract

Au (III) complexes display remarkable emission properties in OLEDs materials due to the high-performance emitters in recent years. Explaining microscopic mechanisms of the luminescence process clearly remain a challenge. The intersystem crossing between the singlet excited state and the triplet excited state in the metal complexes systems plays a central role in determining the fate of excitons and various luminescence pathways. For the planar tridentate coordination complexes of Au (III), it remains unclear what are the key factors that affect the luminescent pathways and thus the quantum efficiency. This work uncovers the relationship between the intersystem crossing processes (include ISC and RISC) and electronic structures of chelated ligands in the Au ( III ) complexes, which is supported by the quantum-chemical calculations. It is found that the modification of strong electron-donating groups (such as –NMe 2 , -NPh 2 ) may reduce the vibration coupling effect and energy gap (Δ E ST ), then open the thermal equilibrium channel, and finally obtain higher quantum yield TADF emitter through the fluorescence process. But the complex that has carbazole group in the auxiliary ligand still can't undergo the RISC process. These may be caused by the various electron delocalization and transition type result from different auxiliary ligands mediated by the N 2 atom. Moreover, the vibration analyses for the phosphorescence emision show that the vibration modes, which make significantly contribute to the reorganization energy, mainly come from the CˆNˆC ligand. While for TADF emission, that is from the auxiliary ligand. This work presents a deep insight into the luminescent mechanism and the key factors affected by the regulation of the ligands in the Au (III) complexes, and provides valuable theoretical basis and reference for the synthesis and design of TADF materials. • The higher SOC effect does not necessarily lead to the TADF process. • The modification of strong electron-donating may reduce the vibration coupling effect and energy gap Δ E ST , finally obtain higher quantum yield TADF emitter through the fluorescence process. • Adding the carbazole group in the auxiliary ligand can enhance the contribution of the CˆNˆC ligand on the reorganization energy.