3H-Indolium,1-[3-[(2,5-dioxo-1-pyrrolidinyl)oxy]-3-oxopropyl]-2-[3-[1-[3-[(2,5-dioxo-1-pyrrolidinyl)oxy]-3-oxopropyl]-1,3-dihydro-3,3-dimethyl-5-sulfo-2H-indol-2-ylidene]-1-propenyl]-3,3-dimethyl-5-sulfo-, iodide, dipotassium salt