Metal-Assisted Delayed Fluorescent Pd(II) Complexes and Phosphorescent Pt(II) Complex Based on [1,2,4]Triazolo[4,3-<i>a</i>]pyridine-Containing Ligands: Synthesis, Characterization, Electrochemistry, Photophysical Studies, and Application

Abstract

The synthesis and photophysical characterization of a series of tetradentate cyclometalated M(<i>tzp</i>Ph-O-CzPy-R) complexes and their analogues are reported, where M is palladium or platinum and a tetradentate cyclometalating ligand contains <i>tzp</i>Ph (3-phenyl-[1,2,4]triazolo[4,3-<i>a</i>]pyridine) and CzPy (carbazolylpyridine) moieties linked with an oxygen atom. Variations of the σ-electron-donating group R on the ligand significantly affect the photophysical properties of the complexes. By using the strong electron-withdrawing <i>tzp</i> portion as an acceptor and the carbazole portion as a donor, a series of Pd(II)-based metal-assisted delayed fluorescence (MADF) materials was developed. Electrochemical analysis demonstrates the irreversible reduction process occurs on the <i>tzp</i> ring and the irreversible oxidation process mainly occurs on the metal-phenyl moiety. This is in agreement with the HOMO and LUMO distributions by the DFT calculations, which also shows that the Pt(II) complex has more metal orbital character than those of the Pd(II) complexes. Most of the Pd(II) complexes reported here are highly emissive at 77 K in 2-MeTHF with luminescent lifetimes in the millisecond range (τ = 1.96-2.36 ms) and λ_max = 488-499 nm; however, the luminescent lifetimes are shortened to the microsecond range (τ = 26.7-152.9 μs in solution and 57.0-109.9 μs in thin film respectively) at room temperature. The quantum efficiency of the Pd(II) complexes can be increased by more than 8-fold through structure modification with σ-donating groups on the ligand. Especially, the Pd(<i>tzp</i>-3) has a small Δ<i>E</i>_ST of 0.228 eV and exhibits strong typical MADF in PMMA film. The Pt(II) complex Pt(<i>tzp</i>-2) exhibits high thermal stability (Δ<i>T</i>_0.5% = 440 °C) and high quantum efficiency (Φ = 50.1%) in dichloromethane solution with τ of 15.8 μs. The Pt(<i>tzp</i>-2) based bright green OLED achieved a peak EQE of 8.7% and a maximum brightness of 28280 cd/m² using an unoptimized device structure.