Chapter 15 - Cyanine Dyes and Related Compounds

Abstract

Cyanine dyes and related compounds cover the whole visible spectrum and have high tinctorial power but owing to their poor stability to light they are not important as textile dyes. Some of these dyes show high light stability on acrylonitrile homo-and co-polymers, and have attained technological importance. For convenience, cyanine dyes and related compounds are studied under the groups: Cyanines; Azacyanines; Polynuclear cyanines; Dye bases; Merocyanines; Linear tri- and poly-nuclear cyanines and merocyanines; Oxonols; nonheterocyclic cyanines; Styryl dyes and N-hemicyanines. The vast majority of cyanine dyes are mono-acid salts, in which two heterocyclic nuclei containing nitrogen are linked through an odd numbered methine chain in such a way that resonance occurs through the conjugated system between tertiary and quaternary nitrogen atoms. Azacyanines are dyes, in which one or more of the methine groups in the conjugated chain have been replaced by nitrogen. When cyanine dyes are heated with a high-boiling amine, such as N,N-diethylaniline, the more basic heterocyclic ring becomes dequaternized to produce the dye base. Merocyanines may be defined as nonionic dyes, in which a double bond or an even-numbered methine chain unites a dihydrogenated nitrogen-containing heterocycle with the residue of a compound having an active methylene group, usually a cyclic ketomethylene compound.