Ru(bpy)2Cl2·2H2O with ligands H3L1–3. The pH effects on the UV/Vis absorption and fluorescence spectra of the threecomplexes were studied, and ground- and excited-state ionization constants of the threecomplexes were derived. The threecomplexes act as “off-on-off” fluorescencepHswitch through protonation and deprotonation of imidazolering with a maximum on-off ratio of 5 in buffer solution at room temperature