Streptocyanine dyes

The simplest possible type of polymethine dye consists of an odd-numbered polymethine chain sandwiched between two terminal nitrogen atoms, which do not form part of an unsaturated heterocyclic ring. Originally, this type of colorant was termed streptopolymethine dyes. For systematic reasons they are usually called streptocyanine dyes today. Their color is mainly determined by the lengths of their polymethine chains. Owing to their simple structure they are the favorite compounds for investigation of fundamental structure–property relationships. In the form of dianil hydrochlorides mainly streptocyanines are used to prepare cyanine, hemicyanine, merocyanine, oxonol, pyrylium and thiopyrylium dyes.