Preparation, Characterization, and Proton-induced Fluorescence Switching of Two Ru(II) Polypyridyl Complexes containing Different N-Heterocyclic Groups

Abstract

Two ligands H 2 L 1 and HL 2 containing imidazole rings and piperazine or morpholine units have been prepared by the reaction of 1,10-phenanthroline-5,6-dione with 4,4'-(1,4-piperazinediyl)bisbenzaldehyde and 4-morpholinobenzaldehyde, respectively. The Ru(II) polypyridyl complexes [(bpy) 2 Ru(H 2 L 1 )Ru(bpy) 2 ] 4+ and [(bpy) 2 Ru(HL 2 )] 2+ have been synthesized by the reaction of Ru(bpy) 2 Cl 2 ·2H 2 O with ligands H 2 L 1 and HL 2 , respectively. The pH effects on the UV/Vis absorption and fluorescence spectra of both complexes have been studied. The ground-state and excited-state ionization constants of the acid-base equilibria have been calculated according to the absorbance and emission data. The photophysical properties of both complexes are strongly dependent on the solution pH. They act as proton-induced "off-on-off" fluorescence pH sensors through protonation and deprotonation of the imidazole, piperazine or morpholine groups, with a maximum on-off ratio of 6 in buffer solution at room temperature.