3H-​Indolium, 1-​[6-​[(6-​aminohexyl)​amino]​-​6-​oxohexyl]​-​2-​[3-​(1,​3-​dihydro-​1,​3,​3-​trimethyl-​5-​sulfo-​2H-​indol-​2-​ylidene)​-​1-​propen-​1-​yl]​-​3,​3-​dimethyl-​5-​sulfo-​, inner salt