Quenching of the Luminescent Excited State of Tris(2,2'-bipyridine)ruthenium(II) by Complexes of Pentaamminecobalt(III) with Pyridine, 4,4'-Bipyridine, and Derivatives of 4,4'-Bipyridine

The quenching of the luminescent excited state of tris(2, 2'-bipyridine)ruthenium(II) by the complexes Co(NH₃)₅Lⁿ⁺ (L = pyridine (py), l, 2-bis(4-pyridyl)ethane (NC₅-CH₂CH₂-C₅N), 4,4'-bipyridine (Ne₅-C₅N), l-methyl-4, 4'-bipyridyl (NC₅-C₅NCH₃⁺), 1, 2-bis(4-pyridyl)ethene (NC₅-CH=CH-C₅N)) has been studied at 25 °C, 5.6, and ionic strength 0.10 M (LiC1O₄). The quenching constants in the order given above are (1.72 ± 0.21) × 10⁸, (2.02 ± 0.25) × 10⁸, (5.67 ± 0.60) · 10₈, (1.09 ± 0.01) · 10⁹, and (1.15 ± 0.08) · 10⁹ M^-1 s^-1, respectively. A parallel series of quenching measurements has been conducted for the free ligands but only NC₅-C₅NH⁺, HNC₅-C₅NH²⁺, NC₅-C₅NCH₃⁺, HNC₅-C₅NCH₃²⁺, and the protonated forms of NC₅-CH=CH-C₅N are effective quenchers, thate constants, in the order given above, being (5.41 ± 0.41) · 10⁸, (1.65 ± 0.29) · 10⁹, (7.92 ± 0.12) · 10⁸, and (2.51 ± 0.02) · 10⁹, for the first four ligands. Flash photolysis experiments with Ru(bpy)32+ show the formation of radicals derived by one-electron reduction of NC₅-C₅NH⁺, HNC₅-C₅NH²⁺, and NC₅-C₅NCH₃⁺ and their subsequent disappearance by reaction with Ru(bpy)₃³⁺. Flash-photolysis experiments with Ru(bpy)₃²⁺ in the presence of the cobalt(III) complexes failed to reveal the existence of analogous radical intermediates. Quantum yield measurements for the production of Co²⁺ in ru(bpy)₃²⁺-Co(NH₃)₅NC₅-C₅N³⁺ and -Co(NH₃)₅NC₅-C₅NCH₃⁴⁺ systems show that a Co²⁺ ion is produced for every quenching event. The results are interpreted in terms of a quenching mechanism involving electron transfer from the excited state *Ru(bpy)₃²⁺ to the cobalt(III) complex. For the complexes with 4,4'-bipyridine, l-methyl-4,4'-bipyridyl, and l, 2-bis(4-pyridyl)ethene the detailed electron-transfer quenching is postulated to proceed via a chemical mechanism, whereby the electron is transferred to the pyridine ligand and, in a subsequent step, from the bound radical to the cobalt(III) center.