Synthesis of 3-methylpyrido[3,2,1-j,k]carbazolium salts and derived cyanine dyes
When carbazole is heated with formaldehyde and acetone (or with methyl vinyl ketone) in the presence of mineral acid, 3-methylpyrido[3,2-j,k]carbazolium salts form. These react with p-dimethylaminobenzaldehyde, triethyl orthoformate, Michler's ketone, 1-ethylquinolinium iodide, or 1,3,3-trimethyl-2-formylmethyleneindolinium iodide to give the respective cyanine dyes. Deviations of the unsymmetrical dyes and Hückel molecular orbital calculations show that the Brooker basicity of the pyrido[3,2,1-j,k]carbazolium series is lower than that of the 1-phenylquinolinium series.