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 > 10⁵ 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