Tuning the Excited State of Tetradentate Pd(II) and Pt(II) Complexes through Benzannulated N‐Heteroaromatic Ring and Central Metal

Comprehensive Summary A series of tetradentate Pd(II) and Pt(II) complexes containing fused 5/6/6 metallocycles with phenyl N ‐heteroaromatic benzo[ d ]imidazole ( pbiz ), benzo[ d ]oxazole ( pboz ) or benzo[ d ]thiazole ( pbthz )‐containing ligands was developed. Systematic studies by experiments and theoretical calculations reveal that both the central metal and the benzannulated N ‐heteroaromatic ring have significant influence on the electrochemical, photophysical and excited‐state properties of the Pd(II) and Pt(II) complexes. In identical condition, compared to pbiz ‐based Pd(II) and Pt(II) complexes, the corresponding metal complexes with pboz and pbthz ‐containing ligand show significant red‐shift emission spectra. Pd(II) complexes exhibit blue‐shift emission spectra in comparison with the corresponding Pt(II) complexes. The Pt(II) complexes possess more metal‐to‐ligand charge transfer ( 3 MLCT) character in their T 1 states, which enables the Pt(II) complexes to have much higher quantum efficiencies ( Φ PL ) and shorter excited‐state lifetimes ( τ ), resulting in large radiative decay rates ( k r ). Especially, Pt( pbiz ) has a Φ PL of 94% with a τ of 4.0 μs in 5 wt% doped PMMA film at room temperature. Pt( pbiz )‐doped green organic light‐emitting diode (OLED) using 26mCPy as host demonstrated a peak external quantum efficiency (EQE) of 21.6% and a maximum brightness ( L max ) of 55481 cd/m 2 , which still maintained an EQE of 16.5% and 10.8% at 1000 and 10000 cd/m 2 , respectively.