Synthesis and properties of new phosphorescent red light-excitable platinum(II) and palladium(II) complexes with Schiff bases for oxygen sensing and triplet-triplet annihilation-based upconversion.

New Pt(II) and Pd(II) complexes with donor-acceptor Schiff bases are conveniently prepared in only two steps. The complexes efficiently absorb in the red part of the spectrum (ε > 10(5) M(-1) cm(-1)) and show moderate to strong room-temperature phosphorescence in the near-infrared (NIR) region. Particularly, Pt(II) complexes possess phosphorescence quantum yields (Φ) of ~10%, but the emission of the respective Pd(II) complexes is less efficient (Φ ≈ 1%-2%). The complexes exhibit solvatochromic behavior, in which the absorption and emission spectra shift bathochromically in polar solvents. The Pt(II) complexes are embedded in polystyrene to produce oxygen-sensing materials. The Pd(II) and Pt(II) complexes are demonstrated to be efficient sensitizers in triplet-triplet annihilation-based upconversion systems.