1H-​Benz[e]​indolium, 2-​[5-​[3-​(5-​carboxypentyl)​-​1,​3-​dihydro-​1,​1-​dimethyl-​6,​8-​disulfo-​2H-​benz[e]​indol-​2-​ylidene]​-​1,​3-​pentadien-​1-​yl]​-​3-​ethyl-​1,​1-​dimethyl-​6,​8-​disulfo-​, inner salt