Understanding the Facile Photooxidation of Ru(bpy)₃2+ in Strongly Acidic Aqueous Solution Containing Dissolved Oxygen

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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+ →(hν)/←(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)33+/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.

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