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, theTTA upconverted luminescence gradually appeared upon adding an increasing concentration of calcium ions, to reach a maximum upconversion quantum yield of 0.0020.