Diazahemicyanine dyes are arguably the most important class of polymethine dyes. They have the general structure (6).
The heterocyclic ring normally contains one (e. g., pyridinium), two (e. g., pyra — zolium and imidazolium), or three (e. g., triazolium) nitrogen atoms, or sulfur and nitrogen atoms, e. g., (benzo)thiazolium and thiadiazolium. Triazolium dyes (7) provide the market-leading red dyes for polyacrylonitrile, and the benzothiazo — lium dye (8) is the market-leading blue dye.