Red-to-Blue Triplet–Triplet Annihilation Upconversion for Calcium Sensing

Triplet–triplet annihilation upconversion is a bimolecular process converting low-energy photons into high-energy photons. Here, we report a calcium-sensing system working via triplet–triplet annihilation (TTA) upconverted emission. The probe itself was obtained by covalent conjugation of a blue emitter, perylene, with a calcium-chelating moiety, and it was sensitized by the red-light-absorbing photosensitizer palladium(II) tetraphenyltetrabenzoporphyrin (PdTPTBP). Sensing was selective for Ca2+ and occurred in the micromolar domain. In deoxygenated conditions, the TTA upconverted luminescence gradually appeared upon adding an increasing concentration of calcium ions, to reach a maximum upconversion quantum yield of 0.0020.