Proton-induced fluorescence switch of a novel hexanuclear Ru(II) polypyridyl complex containing imidazole ring

Hexapodal ligand H6L containing imidazole groups has been prepared by the reaction of 1,2,3,4,5,6-hexakis[(4-formylphenoxy)methyl]benzene with 1,10-phenanthroline-5,6-dione. Hexanuclear complex [RuH6L](PF6)12 has been obtained by the condensation of Ru(bpy)2Cl2·2H2O and H6L, and isolated as PF6− salt. The deprotonated complex [RuL]6+ was achieved by reaction of sodium methoxide with complex [RuH6L]12+ in methanol. The pH effects on the UV–vis absorption and fluorescence spectra of complex [RuH6L](PF6)12 are studied, and ground and excited state ionization constants of the complex are derived. The absorption and fluorescence spectra of the complex are both strongly dependent on the solution pH. Complex [RuH6L](PF6)12 acts as “on–off” fluorescence pH sensor through protonation and deprotonation of imidazole groups with a maximum on–off ratio of 5 in buffer solution at room temperature.