cyanine dyes
Synthetic dyes with the general formula R2N[CH=CH]nCH=N+R2mesomer-arrow chemimgR2N+=CH[CH=CH]nNR2 (n is a small number) in which the nitrogen and part of the conjugated chain usually form part of a heterocyclic system, such as imidazole, pyridine, pyrrole, quinoline and thiazole, e.g.
PAC, 1995, 67, 1307. (Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)) on page 1329 [Terms] [Paper]