4H-Imidazoles as functional dyes: synthesis of bichromophores and extension of the merocyanine system

A series of bichromophores consisting of a ‘classical’ chromophore and 4H-imidazoles were synthesized starting from appropriate cyano and carboxy functionalized systems. In their UV–vis spectra, an additive absorption of both chromophores was detected making them wide range-absorbing dyes. In addition, new properties as redox activity, pH-switchability and metal-chelating substructures are newly introduced into the molecules. In order to achieve more bathochromic absorbing systems, the chromophore of 4H-imidazoles was extended. The 4H-imidazo-[1,2-a]-pyridines, which are easily accessible by cyclization of 2-aminopyridines with bis-imidoylchlorides, show long wavelength absorptions up to 616 nm. Their reduction reversibly yields yellow, blue fluorescent 1-azaindolizines. In contrast to leuco-forms of 4H-imidazoles, these reduction products show a high stability towards oxygen and could only be reoxidized to their starting materials by oxidation agents, such as DDQ.