Improving the chemical stability of blue heteroleptic iridium emitter FIrpic in the lowest triplet state through ancillary ligand modification: a theoretical perspective

With the aim of providing a deeper understanding of the underlying degradation mechanisms associated with the lifetime of blue emitters during the decay process of blue PhOLEDs, quantum chemistry studies were performed to examine the chemical degradation mechanism of common sky blue emitter iridium(III)bis(4,6-di-fluorophenyl)-pyridinato-N,C2′)picolinate (FIrpic) and its derivatives with density functional theory (DFT) calculations. For these Ir(III) emitters, the Ir–N1 bond between the ancillary ligand (picolinate) and central iridium will be broken by external light stimuli, which is followed by conversion from the initial emissive metal-to-ligand charge transfer (3MLCT) state to the non-emissive metal centered (3MC) state. The potential energy change for the photo-induced degradation path is then dominated by the energy levels of the 3MT and 3MC states, which are related to the triplet transition energy and the Ir–N1 bond strength, respectively. Thereby, the Ir–N1 bond dissociation in the lowest triplet state will be much harder to proceed if the S0 → T1 transition energy gets more energetically stable or the bond strength gets larger. It is believed that strategic modification of the ancillary ligand, especially by substitution of electron-donating groups at the para position of the pyridyl N atom or extension of the p-electron delocalization, is an effective and easy way to enhance the photochemical stability of the typical blue emitter FIrpic.