pH-induced fluorescence switch of two novel tetranuclear Ru(II) polypyridyl complexes

Tetrapodal ligands tetrakis{4-((1,10-phenanthroline-[5,6-d]imidazol-2-yl)phenoxy)methyl}methane (L1), tetrakis{3-((1,10-phenanthroline-[5,6-d]imidazol-2-yl)phenoxy)methyl}methane (L2), and corresponding Ru(II) complexes [(bpy)8L1–2(RuII)4](PF6)8, shortly called Ru–L1–2, have been synthesized and characterized. The pH effects on UV–vis absorption and fluorescence spectra of the two complexes are studied, and ground- and excited-state ionization constants of the two complexes are derived. The two complexes act as pH-induced “off–on–off” fluorescence switches through protonation and deprotonation in buffer solution at room temperature.