squarain-carboxylate n-succinimidyl ester

1-[(5-Chloro-1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)methyl]-3-[[5-[[(2,5-dioxo-1-pyrrolidinyl)oxy]carbonyl]-1-ethyl-1,3-dihydro-3,3-dimethyl-2H-indol-2-ylidene]methyl]-2,4-dihydroxycyclobutenediylium bis(inner salt)