A series of blue phosphorescent iridium complexes with thermally activated delayed fluorescence and efficiency roll-off properties

Abstract

Six iridium complexes were designed and studied using the DFT approach, (ppy)₂Ir(pic) (1), (f₄ppy)₂Ir(pic) (2), (ppy)₂Ir(tmd) (3), (f₄ppy)₂Ir(tmd) (4), (ppy)₂Ir(tpip) (5) and (f₄ppy)₂Ir(tpip) (6). Here ppy denotes phenylpyridine, f₄ppy denotes 2-(2,3,4,5-tetrafluorophenyl) pyridine, pic denotes benzoic acid, tmd denotes 5-hydroxy-2,2,6,6-tetramethylhept-4-en-3-one and tpip denotes tetraphenylimido-diphosphinate. The geometries, absorptions, emissions, frontier molecular orbitals, and spin–orbit coupling (SOC) constants of the 6 complexes were evaluated. An intriguing phenomenon was observed during the excitation process of these molecules. It was discovered that, in the ground state, the Frontier Molecular Orbitals (FMOs) of these molecules were loosely arranged throughout the molecule. However, in the Lowest Unoccupied Molecular Orbitals (LUMO) of the triplet excited state, the FMOs become concentrated around the metal core and a maximum of two ligands. Furthermore, the analysis of the energy difference between the lowest singlet excited state and the lowest triplet excited state (ΔE_S1T1) of these complexes in conjunction with their spin–orbit coupling performance indicated that complex 1 exhibits characteristics consistent with Thermally Activated Delayed (TAD) fluorescence. We hope that this research can serve as a reference for practical experimental synthesis.