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

The previously observed facile photooxidation of Ru(bpy)<SUB>3</SUB><SUP>2+</SUP> to Ru(bpy)<SUB>3</SUB><SUP>3+</SUP> in oxygenated solutions of 9 M H<SUB>2</SUB>SO<SUP>4</SUP> is further studied. A similar phenomenon was observed with Ru(phen)<SUB>3</SUB><SUP>2+</SUP> but not with Ru(bpy)<SUB>2</SUB>[bpy-(CO<SUB>2</SUB>H)<SUB>2</SUB>]<SUP>2+</SUP>. The reaction is strongly dependent on acid concentration, with a sharp change in the region of 2-7 M H<SUB>2</SUB>SO<SUB>4</SUB>. The quantum yield of Ru(bpy)<SUB>3</SUB><SUP>3+</SUP> formation in 9 M H<SUB>2</SUB>SO<SUB>4</SUB> is close to the quantum yield of steady-state luminescence quenching by O<SUB>2</SUB>. Photooxidation is accompanied by near-stoichiometric formation of H<SUB>2</SUB>O<SUB>2</SUB> as reduced product. Chromatographic, spectroscopic, electrochemical and optical rotation studies reveal that Ru(bpy)<SUB>3</SUB><SUP>2+</SUP> 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 O<SUB>2</SUB>&#183;- by H<SUP>+</SUP> followed by rapid disproportionation to H<SUB>2</SUB>O<SUB>2</SUB> and O<SUB>2</SUB>. These are likely on account of the high ionic strength of the medium which favors the required shifts in the potentials of the O<SUB>2</SUB>/O<SUB>2</SUB>&#183;- and O<SUB>2</SUB>/H<SUB>2</SUB>O<SUB>2</SUB> couples. Upon storage of the photooxidized Ru(III) solution in dark, partial recovery of Ru(bpy)<SUB>3</SUB><SUP>2+</SUP> occurs gradually. Studies with the electrooxidized complex over a range of acid concentrations indicate that Ru(bpy)<SUB>3</SUB><SUP>2+</SUP> is regenerated by reaction of Ru(bpy)<SUB>3</SUB>3+ with H<SUB>2</SUB>O<SUB>2</SUB>. The reaction is promoted by increasing concentrations of [H<SUB>2</SUB>O<SUP>2</SUP>] and inhibited by [O<SUB>2</SUB>] and [H<SUP>+</SUP>]. 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)<SUB>3</SUB><SUP>2+</SUP> + O<SUB>2</SUB> + 2H<SUP>+</SUP> &#8594;(h&#957;)/&#8592;(dark) 2Ru(bpy)<SUB>3</SUB><SUP>3+</SUP> + H<SUB>2</SUB>O<SUB>2</SUB> for which the equilibrium constant has been computed. Light helps overcome the activation barrier of the forward reaction by driving it via <SUP>&#x002A;</SUP>Ru(bpy)<SUB>3</SUB><SUP>2+</SUP>, 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)<SUB>3</SUB><SUP>3+/2+</SUP>, suggesting thereby that H<SUB>2</SUB>O<SUB>2</SUB> is much more stable in the more strongly acidic medium and less capable of reducing Ru(bpy)<SUB>3</SUB><SUP>3+</SUP> unlike at higher pH

Spectral differences between enantiomeric and racemic Ru(bby)<SUB>3</SUB><SUP>2+</SUP> on layered clays: probable causes

The preferential self-annihilation(static and dynamic) of &#916;,&#923;-Ru(bpy)<SUB>3</SUB><SUP>2+</SUP> over &#916; or &#923;-Ru(bpy)<SUB>3</SUB><SUP>2+</SUP> is reported for aqueous dispersions of sodium hectorite lightly loaded with the Ru(II) chelate and subjected to pulsed laser excitation. by varying the loading level over a factor of ca.60, it is also shown that racemate emission falls off sharply with increased loading whereas emission from the enentiometric adsorbate remains more nearly constant. The decrease in luminescence yield of racemate with increased loading is mainly associated with an attenuation in the peak emission intensity, I(0), as found from time-resolved measurements. it is proposed, based on these studies, that clays offer both quenching and nonquenching sites for sorption and that &#916;,&#923;Ru(bpy)<SUB>3</SUB><SUP>2+</SUP> prefers the latter at low loadings, the ions being clustered within such regions. Enantiometric Ru(bpy)<SUB>3</SUB><SUP>2+</SUP>, on the other hand, is more randomly distributed over the sites. the above model also permits rationalization of(i) observed changes inemission intensity with time,(ii) anomalies in the relative emission yields of Ru(bpy)<SUB>3</SUB><SUP>2+&#8727;</SUP> and Ru(phen)<SUB>3</SUB><SUP>2+</SUP>, and (iii) the effect of zn(phen)<SUB>3</SUB><SUP>2+</SUP> on emission. Finally, differences in binding modes of enantionmeric and racemic chelate forms also induce differences in the flocculation trends of dispersed clays, the effects being most prominent for freshly prepared ruthenium(II) montorillinite