Phosphorescent Tetradentate Platinum(II) Complexes Containing Fused 6/5/5 or 6/5/6 Metallocycles

A series of phenylpyridine (ppy)-based 6/5/5 N*C^N^O and biphenyl (bp)-based 6/5/6 N*C^C*N Pt(II) complexes employing tetradentate ligands with nitrogen or oxygen atoms as bridging groups have been developed. Ligand structural modifications have great influences on the electrochemical, photophysical, and excited-state properties, as well as photostabilities of the Pt(II) complexes, which were systematically studied by experimental and theoretical investigations. The time-dependent density functional theory calculations and natural transition orbital analyses reveal that Pt(bp-6), Pt(bp-7), and Pt(bp-8) have dominant ligand-centered (³LC) mixed with small metal-to-ligand charge-transfer (³MLCT) characters in T₁ states, resulting in relatively low quantum efficiencies (Φ_PL) of 5-33% and 12-32% in dichloromethane solution and PMMA film, respectively. By contrast, Pt(ppy-1) possesses much more ³MLCT character in the T₁ state, enabling a high Φ_PL of 95% in dichloromethane and 90% in DPEPO film, and large radiative decay rates. The strength of the Pt-N¹ coordination bond plays a critical role in the photostability. Pt(ppy-1)- and Pt(bp-6)-doped polystyrene films demonstrate long photostability lifetimes of 150 min for LT₉₇ and LT_98.5, respectively. A Pt(ppy-1)-based green OLED using 26mCPy as host realized a peak EQE of 18.5%, which still maintained an EQE of 10.4% at 1000 cd/m², and an L_max of over 40 000 cd/m² was achieved. This study should provide a valuable reference for the further development of efficient and stable phosphorescent Pt(II) complexes.