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.