1,2-Dimethylimidazobenzo-1′, 2′, 3′-thiadiazoles and derived cyanine dyes

New tricylic heterocyclic systems and their methyl derivatives are synthesized. They are 2-methylimidazo5, 4-e]benzo-1, 2, 3-thiadiazole, 2-methylimidazo5, 4-g]benzo-1, 2', 3-thiadiazole, 1, 2-dimethylimidazo5, 4-g]benzo-1, 2, 3-thiadiazole, and 1, 2-dimethylimidazo5, 4-g]benzo-1, 2, 3-thiadiazole. Quaternary salts of the 1, 2-dimethyl derivatives are used to prepare symmetrical and unsymmetrical trimethinecyanines and dimethinemerocyanines containing rhodanine groups. The absorption maxima of the dyes are displaced towards the long wavelength region compared with imidacarbocyanines. It is shown that when 1, 2-dimetnyl-5-aminobenzoimidazole is thiocyanated, the thiocyano group enters the benzoimidazole ring at position 4, and that 1, 2-dimethylimidazo5, 4-e] benzo-1, 2, 3-thiadiazole is less basic than its isomer 1, 2-dimethylimidazo5, 4-g]benzo-1, 2, 3-thiadiazole.