Photophysical Properties of Tetracationic Ruthenium Complexes and Their Ter-Ionic Assemblies with Chloride

The synthesis of seven ruthenium­(II) polypyridyl complexes bearing one dicationic bis-4,4′-(trimethylaminomethyl)-2,2′-bipyridine (tmam) ligand is reported. The ancillary ligands of each complex were 2,2′-bipyrazine (bpz), 2,2′-bipyridine (bpy), 4,4′-tert-butyl-2,2′-bipyridine (dtb), 4,4′-dimethyl-2,2′-bipyridine (4,4′-dmb), 5,5′-dimethyl-2,2′-bipyridine (5,5′-dmb), 4,4′-nonyl-2,2′-bipyridine (nonyl), and 4,4′-methoxy-2,2′-bipyridine (MeO). The metal-to-ligand charge transfer excited state was localized on the tmam ligand in all instances with the exception of [Ru­(bpz)₂­(tmam)]⁴⁺, where it was localized on the bpz ligand. All [PF₆]⁻ complexes were shown to form strong ion pairs with chloride in a Ru/Cl 1:2 stoichiometry in acetone, as evidenced by ¹H NMR and UV–visible titrations. With the exception of [Ru­(bpz)₂­(tmam)]⁴⁺, ion pairing with chloride anions resulted in excited states that were ∼25% longer-lived and with an ∼50% increase in the photoluminescence quantum yields compared to the [PF₆]⁻ ion pairs. It was shown that the quantum yield enhancements originated from a decreased nonradiative rate constant and an increased radiative rate constant. [Ru­(bpy)₂­(tmam)]⁴⁺ showed curious excited state quenching behavior at higher equivalents of chloride, the origin of which is not understood.