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.