Dimerization of 2-pyridylisonitriles produces π-extended fused heteroarenes useful as highly selective colorimetric and optical probes for copper ion

A very simple synthesis of pyrido[2″,1″:2′,3′]imidazo[4′,5′:4,5]imidazo[1,2-a]pyridines 2 was achieved from the dimerization of 2-pyridylisonitriles. The optical responses of these π-extended fused heteroarenes to a series of metal ions were investigated by means of UV-visible absorption spectrum in acetonitrile. It was found that most of compounds 2 selectively responded to Cu2+, and the limit of detection was about 3.5×10−7 M when using the best probe, 3,9-dimethylpyrido[2″,1″:2′,3′]imidazo[4′,5′:4,5]imidazo[1,2-a]pyridine 2d. Meanwhile some compounds 2 specifically showed a color change from colorless to red or yellow when they are exposed to copper ion in acetonitrile, whereas other transitional, alkali and alkaline earth metal ions could not induce a color change. This work not only established a direct access to π-extended fused heteroarenes, but also supplied novel sensitive and selective colorimetric and optical probes for copper ion.