Phototoxins are toxins that can cause allergic reactions in particularly susceptible individuals and which can cause dangerous photosensitivity in a much broader range of subjects.
The marigold plant produces the phototoxin alpha-terthienyl, which functions as a nematicide.
When exposed to near ultraviolet light, such as in sunlight, alpha-terthienyl generates the toxic singlet oxygen.
Rose bengal and other singlet oxygen generating phototoxins are also used in synthetic organic chemistry.
They have also found use in photodynamic therapy, where the toxin is activated by intense light to destroy cancer cells.