Near-infrared absorbing delocalized cationic azo dyes

Novel, delocalized, cationic azo dyes derived from benzothiazole, benzoselenazole, indole and quinoline, displaying strong absorption around 700–800 nm, have been prepared in moderate to good yields. The synthesis involved the Knoevenagel condensation of an intermediate azo compound, possessing a 4-chloro-5-formylthiazole moiety, with methylenic bases generated in situ from benzoazolium and quinolinium salts.