1,4,7,10-Tetraazacyclododecane-1,4,7-tricarboxylic acid,10-[4-[[[1-methyl-4-[[(1-methyl-4-nitro-1H-pyrrol-2-yl)carbonyl]amino]-1H-pyrrol-2-yl]carbonyl]amino]-2-nitrophenyl]-, tris(1,1-dimethylethyl) ester