Origin of rare and highly efficient phosphorescent and electroluminescent iridium(III) complexes based on C^∨N = N ligands, A theoretical explanation

The origin of the rare and highly strong phosphorescent (PL) and electroluminescent (EL) Ir(III) complexes with C^∨N - N ligand, tris[3,6-bis(phenyl)pyridazinato-N¹,C²]iridium (Ir(BPPya)₃) (1), tris[1,4-bis(phenyl)phthalazine]iridium (Ir(BPPa)₃) (2), and tris[1-(2,6-dimethylphenoxy)-4-(4-chlorophenyl) phthalazine]iridium ((Ir(MPCPPZ)₃) (3) are investigated theoretically. By changing the conjugation length of the C^∨N - N ligand from BPPya to BPPa, one can tune the emission color from green in 1 to saturated red in 2. The addition of sterically bulky phenolic substituents in 3 exhibits the highest external quantum efficiency of 20.2% ph el¹ and luminescence efficiency of 18.4 cd A¹. Density functional theory (DFT) and time-dependent DFT (TDDFT) methods are used to rationalize these properties. The more promising PL and EL properties of 3 result from the bulky phenolic group, which acts as a pendant at the periphery of the emitting core and protects the emitting core from the hazardous intermolecular interaction of emitters and reduces luminescence quenching.