Curcumin is one of the most extensively studied natural pigments. The significant phototoxicity of curcumin against a variety of bacteria and viruses has made the pigment a potential photodynamic drug. To explore whether the phototoxic mechanisms of curcumin involve light-induced acidification, the excited-state pKa values of the pigment were determined by the Förster cycle. It was found that the excited-state pKa values of curcumin [i.e., 7.56 (7.72), 8.72 (8.14) and 10.17 (10.35)] are rather higher than those of counterpart phenolic photosensitizers, which implies that light-induced acidification is unlikely to be responsible for the phototoxicity of curcumin.
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