pH and copper ion luminescence on/off sensing by a dipyrazinylpyridine-appended ruthenium complex

A new ruthenium(II) complex of [Ru(bpy)2(HL1)](ClO4)2·4H2O (1·4H2O) {bpy = 2,2′-bipyridine and HL1 = 2-(4-(2,6-di(pyrazin-2-yl)pyridin-4-yl)phenyl)-1H-imidazo[4,5-f][1,10]phenanthroline} was synthesized and characterized by elementary analysis, proton nuclear magnetic resonance spectroscopy and mass spectrometry. The ground- and excited-state acid-base properties of 1 were studied by means of UV-vis absorption spectrophotometric and spectrofluorimetric titrations in 100:1 (v/v) Britton-Robinson buffer/CH3CN solution in combination with luminescence lifetime measurements. The complex exhibited two-step separate protonation/deprotonation processes in both the ground and excited states, acting as pH-induced “off-on-off” luminescence switches (Ion/Ioff = 100.0 and 3.96). Importantly, 1 also acted as a highly selective luminescent chemosensor toward Cu2+ over other metal cations tested (Na+, Mg2+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+ and Ag+).