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 ((MLCT)-M-3) character in their T-1 states, which enables the Pt(II) complexes to have much higher quantum efficiencies (phi(PL)) and shorter excited-state lifetimes (tau), resulting in large radiative decay rates (k(r)). Especially, Pt(pbiz) has a phi(PL) of 94% with a tau of 4.0 mu s 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.