Hemicyanine dyes

Abstract Hemicyanine dyes are a subclass of polymethine colorants. At one end of their polymethine chain is an unsaturated heterocyclic ring possessing a nitrogen atom as would be found in a cyanine dye. However, the other end is terminated by a nitrogen atom that does not form part of an unsaturated heterocycle. The name alludes to their half-cyanine substitution pattern. Later the scope of the term “hemicyanine” was extended to the phenylogous dyes where there is a phenyl group between the two terminal nitrogen atoms. The first main technical application of hemicyanine dyes was in textile coloration. Nowadays hemicyanine dyes are used extensively as optical probes of cell membrane potential.