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 ( 3 LC) mixed with small metal-to-ligand charge-transfer ( 3 MLCT) characters in T 1 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 3 MLCT character in the T 1 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–N 1 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 LT 97 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/m 2, and an L max of over 40 000 cd/m 2 was achieved. This study should provide a valuable reference for the further development of efficient and stable phosphorescent Pt(II) complexes.