3H-​Indolium, 2-​[3-​[1,​3-​dihydro-​3,​3-​dimethyl-​5-​sulfo-​1-​(3-​sulfopropyl)​-​2H-​indol-​2-​ylidene]​-​1-​propen-​1-​yl]​-​3,​3-​dimethyl-​1-​[6-​oxo-​6-​(2-​propyn-​1-​ylamino)​hexyl]​-​5-​sulfo-​, inner salt