Understanding the facile photooxidation of Ru(bpy)₃²⁺ in strongly acidic aqueous solution containing dissolved oxygen

Abstract

The previously observed facile photooxidation of Ru(bpy)₃²⁺ to Ru(bpy)₃³⁺ in oxygenated solutions of 9 M H₂SO₄ (Kotkar, D; Joshi, V.; Ghosh, P. K. Chem. Commun. 1987, 4; Indian Patent No. 164358 (1989)) is further studied. A similar phenomenon was observed with Ru(phen)₃²⁺ but not with Ru(bpy)₂[bpy-(CO₂H)₂]²⁺. The reaction is strongly dependent on acid concentration, with a sharp change in the region of 2-7 M H₂SO₄. The quantum yield of Ru(bpy)₃³⁺ formation in 9 M H₂SO₄ is close to the quantum yield of steady-state luminescence quenching by O₂. Photooxidation is accompanied by near-stoichiometric formation of H₂O₂ as reduced product. Chromatographic, spectroscopic, electrochemical and optical rotation studies reveal that Ru(bpy)₃²⁺ survives the strongly acidic environment with little evidence of either any change in coordination sphere or ligand degradation, even after repeated cycles of photolytic oxidation followed by electrolytic reduction. The high quantum yield and selectivity of the reaction is ascribed to (i) predominance of the electron transfer quenching pathway over all others and (ii) highly efficient trapping of O₂^·- by H⁺ followed by rapid disproportionation to H₂O₂ and O₂. These are likely on account of the high ionic strength of the medium which favors the required shifts in the potentials of the O₂/O₂^·- and O₂/H₂O₂ couples. Upon storage of the photooxidized Ru(III) solution in dark, partial recovery of Ru(bpy)₃²⁺ occurs gradually. Studies with the electrooxidized complex over a range of acid concentrations indicate that Ru(bpy)₃²⁺ is regenerated by reaction of Ru(bpy)₃³⁺ with H₂O₂. The reaction is promoted by increasing concentrations of [H₂O₂] and inhibited by [O₂] and [H⁺]. The fraction of Ru(III) remaining after the reverse reaction is allowed to plateau in solutions of varying acid concentrations follows a similar trend to that found after attainment of steady state in the photooxidation reaction, although in all cases the forward reaction produces more Ru(III) than what remains in the reverse reaction. These observations are consistent with the following equation 2Ru(bpy)₃²⁺ + O₂ + 2H⁺ → (hv)/← (dark) 2Ru(bpy)₃³⁺ + H₂O₂ for which the equilibrium constant has been computed. Light helps overcome the activation barrier of the forward reaction by driving it via Ru(bpy)₃²⁺, and to the extent that the photooxidation is driven past the equilibrium, there is conversion of light energy in the form of long-lived chemical products. Spectroscopic evidence rules out any significant shift in the redox potential of Ru(bpy)₃^3+/2+, suggesting thereby that H₂O₂ is much more stable in the more strongly acidic medium and less capable of reducing Ru(bpy)₃³⁺ unlike at higher pH.