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 Ca 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.