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any one of numerous organic compounds containing two heterocyclic radicals connected by a chain consisting of an odd number of methine groups:
where Y and Y’ are O, S, Se, CR2, or some other element or radical, R and R’ are H or van alkyl, X is Cl–, Br–, I–, or some other anion, and n = 0–5.
The general name for this class of compounds is derived from the first compound of the class, bright blue cyanin, or cyanine blue (from the Greek kyanos, “blue”). Depending on the number of methine groups in the chain, a distinction is made between simple cyanines (monomethines), in which n = 0, carbocyanines (trimethines), in which n = 1, dicarbocyanines (pentamethines), in which n = 2, and so forth. The basic method for the synthesis of cyanines involves the condensation of quaternary salts of heterocyclic compounds. Cyanines are polymethine dyes.