'Off-on-off' Fluorescence pH Switch of Three Trinuclear RuII Complexes Containing Imidazole Rings

Three tripodal ligands H3L1–3 containing imidazole rings were synthesized by the reaction of 1,10-phenanthroline-5,6-dione with 1,3,5-tris[(3-formylphenoxy)methyl]benzene, 1,3,5-tris[(3-formylphenoxy)methyl]-2,4,6-trimethylbenzene, and 2,2′,2"-tris[(3-formylphenoxy)ethyl]amine, respectively. Trinuclear RuII polypyridyl complexes [(bpy)6Ru3H3L1–3](PF6)6 were prepared by the condensation of Ru(bpy)2Cl2·2H2O with ligands H3L1–3. The pH effects on the UV/Vis absorption and fluorescence spectra of the three complexes were studied, and ground- and excited-state ionization constants of the three complexes were derived. The three complexes act as “off-on-off” fluorescence pH switch through protonation and deprotonation of imidazole ring with a maximum on-off ratio of 5 in buffer solution at room temperature.