Zirconium(IV)-catalyzed one-pot synthesis and oxidation of bis- and tris(indolyl)methanes into conjugated chromophores as new pH indicators or calorimetric chemosensors for transition metals

Zirconium(IV) oxynitrate hydrate catalyzed efficiently the condensation reactions of indoles and aldehydes in acetonitrile, under ambient conditions. The bis- or tris(indolyl)methane products were then dehydrogenated in one pot by using a stoichiometric amount of DDQ, to give conjugated chromophores that showed feasible applications as new pH indicators with excellent sensitivity in the pH range 8–10. Hypsochromic effects were observed in the presence of transition metal cations in acetonitrile with a significant color change for cobalt(II).