cyanine dyes

https://doi.org/10.1351/goldbook.C01487

Synthetic dyes with the general formula R₂N[CH=CH]_nCH=N⁺R₂↔︎ R₂N⁺=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.

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Source:
PAC, 1995, 67, 1307. 'Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)' on page 1329 (https://doi.org/10.1351/pac199567081307)