3H-​Indolium, 2-​[5-​[1-​(5-​carboxypentyl)​-​1,​3-​dihydro-​3,​3-​dimethyl-​2H-​indol-​2-​ylidene]​-​1,​3-​pentadien-​1-​yl]​-​1,​3,​3-​trimethyl-​, chloride (1:1)