A novel method for the preparation of monomethine cyanine dyes

A novel method for the preparation of symmetric and asymmetric monomethine cyanine dyes was developed. Quaternary salts of a heterocyclic 2- or 4-methyl compound and N-heterocyclic 2- or 4-sulfobetainic compounds were reacted by simple melting or by boiling in different solvents in the absence of a basic compound. The preparation is fast and the yields are good to high, exceeding in some cases those by other methods. The novel method is environmentally more friendly.