Understanding the facile photooxidation of Ru(bpy)(3)(2+) in strongly acidic aqueous solution containing dissolved oxygen

The previously observed facile photooxidation of Ru(bpY)(3)(2+) to Ru(bpy)( 3)(3+) 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 stu died. A similar phenomenon was observed with Ru(phen)(3)(2+) but not with R u(bpy)(2)[bpy-(CO2H)(2)](2+). The reaction is strongly dependent on acid co ncentration, with a sharp change in the region of 2-7 M H-2-SO4. The quantu m yield of Ru(bpy)(3)(3+) formation in 9 M H2SO4 is close to the quantum yi eld of steady-state luminescence quenching by O-2. Photooxidation is accomp anied by near-stoichiometric formation of H2O2 as reduced product. Chromato graphic, spectroscopic, electrochemical and optical rotation studies reveal that Ru(bpy)(3)(2+) survives the strongly acidic environment with little e vidence 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 ascri bed to (i) predominance of the electron transfer quenching pathway over all others and (ii) highly efficient trapping of O-2(.-) by H+ followed by rap id disproportionation to H2O2 and O-2. These are likely on account of the h igh ionic strength of the medium which favors the required shifts in the po tentials of the O-2/O-2(.-) and O-2/H2O2 Couples. Upon storage of the photo oxidized Ru(III) solution in dark, partial recovery of Ru(bpy)3(2+) occurs gradually. Studies with the electrooxidized complex over a range of acid co ncentrations indicate that Ru(bpy)3(2+) is regenerated by reaction of Ru(bp Y)3(3+) with H2O2. The reaction is promoted by increasing concentrations of [H2O2] and inhibited by [O-2] and [H+]. The fraction of Ru(III) remaining after the reverse reaction is allowed to plateau in solutions of varying ac id concentrations follows a similar trend to that found after attainment of steady state in the photooxidation reaction, although in all cases the for ward reaction produces more Ru(III) than what remains in the reverse reacti on. These observations are consistent with the following equation 2Ru(bpy)( 3)(2+) + O-2 + 2H(+) --> (hv)/<-- (dark) 2Ru(bpy)(3)(3+) + H2O2 for which t he equilibrium constant has been computed. Light helps overcome the activat ion barrier of the forward reaction by driving it via *Ru(bpy)(3)(2+), 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 rule's out any significant shift in the redox poten tial 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)(3)( 3+) unlike at higher pH.