Figure 1.

(a) Chemical structures of palladium(II) tetraphenyltetrabenzoporphyrin (1), perylene (2), 2,5,8,11-tetra(tert-butyl)perylene (3), Ru(II) compound 4²⁺ and its photochemical conversion to 5²⁺ upon visible light irradiation in presence of water; (b) Cartoon illustrating upconversion mediated Ru-prodrug activation in PEGylated liposomes in presence of antioxidants. Red-to-blue triplet-triplet annihilation (TTA) upconversion in the lipid bilayer is followed by non-radiative energy transfer (FRET) of the upconverted light to 4²⁺, and subsequent photochemical reaction of 4²⁺ to 5²⁺. Meanwhile, an antioxidant (A.O.) protects the photochemical cascade from quenching by dioxygen by consuming the singlet-state oxygen (¹O₂*) that is initially generated upon irradiation of 1 until all oxygen is consumed. Hereby, the antioxidant becomes oxidized (Ox. A.O.).