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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Understanding the Facile photooxidation of Ru(bpy)_3~(2+) in Strongly Acidic Aqueous Solution Containing Dissolved Oxygen
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Understanding the Facile photooxidation of Ru(bpy)_3~(2+) in Strongly Acidic Aqueous Solution Containing Dissolved Oxygen

机译：了解Ru（bpy）_3〜（2+）在含有溶解氧的强酸性水溶液中的简便光氧化作用

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摘要

The previously observed facile photooxidation of Ru(bpy)_3~(2+) to Ru(bpy)~_3~(3+) in oxygenated solutions of 9 M H_2SO_4 (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)_3~(2+) but not with Ru(bpy)_2[bpy-(CO_H)_2]~(2+). The reaction is strongly dependent on acid concentration, with a sharp change in the region of 2-7 M H_2SO_4. The quantum yield of Ru(bpy)_-3~(3+) formation in 9 M H_2SO_4 is close to the quantum yield of steady-state luminescence quenching by O_2. Photooxidation is accompanied by near-stoichiometric formation of H_2O_2 as reduced product. Chromatographic, spectroscopic, electrochemical and optical rotation studies reveal that RU(bpy)_3~(2+) 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_2~(centre dot-) by H~+ followed by rapid disproportionation to h_2O_2 and O_2. These are likely on account of the high ionic strength of the medium which favors the required shifts in the potentials of the O_2/O_2~(centre dot-) and O_2/H_O_2 couples. Upon storage of the photooxidized Ru(III) solution in dark, partial recovery of Ru(bpy)_3~(2+) occurs gradually. Studies with the electrooxidized complex over a range of acid concentrations indicate that Ru(bpy)_3~(2+) is regenerated by reaction of Ru bpy)_3~(2+) with H_2O_2. The reaction is promoted by increasing concentrations of [H_2O_2] 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 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)_3~(2+) +O_2+2H~+->(hv)/<-(dark) 2Ru(bpy)_3~(2+)+H_2O_2 for which the equilibrium constant has been computed. Light helps overcome the activation barrier or 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 rules out any significant shift in the redox potential of Ru(bpy)_3~(3+/2+), suggesting thereby that H_2O_2 is much more stable in the more strongly acidic medium and less capable of reducing Ru(bpy)_3~(2+) unlike at higher pH.
机译：先前在9 M H_2SO_4的含氧溶液中观察到Ru（bpy）_3〜（2+）到Ru（bpy）〜_3〜（3+）的容易的光氧化（Kotkar，D; Joshi，V.，Ghosh，PK Chem， Commun 1987，4;印度专利164358（1989））进行了进一步的研究。 Ru（phen）_3〜（2+）观察到类似现象，Ru（bpy）_2 [bpy-（CO_H）_2]〜（2+）观察不到。该反应强烈地依赖于酸浓度，在2-7 M H_2SO_4的范围内急剧变化。在9 M H_2SO_4中形成Ru（bpy）_- 3〜（3+）的量子产率接近于O_2稳态发光猝灭的量子产率。光氧化伴随着化学还原形式的H_2O_2的化学计量形成。色谱，光谱，电化学和旋光性研究表明，RU（bpy）_3〜（2+）可以在强酸性环境中生存，即使经过反复的光解氧化和电解循环，也几乎没有证据表明配位球或配体降解发生任何变化减少。该反应的高量子产率和选择性归因于（i）在所有其他反应中电子转移淬灭路径占优势，以及（ii）H〜+高效捕获O_2〜（中心点-），然后迅速歧化成h_2O_2和O_2。这些可能是由于介质的高离子强度，这有利于O_2 / O_2〜（中心点）和O_2 / H_O_2对的电势发生所需的位移。在黑暗中储存光氧化的Ru（III）溶液后，Ru（bpy）_3〜（2+）的部分回收逐渐发生。在一定的酸浓度范围内对电氧化配合物的研究表明，Ru（bpy）_3〜（2+）与H_2O_2反应可再生Ru（bpy）_3〜（2+）。通过增加[H_2O_2]的浓度促进反应，并被[O_2]和[H〜+]抑制。在各种酸浓度的溶液中，使逆反应后剩余的Ru（III）分数平稳，与在光氧化反应中达到稳态后发现的趋势相似，尽管在所有情况下，正反应均产生更多的Ru（ III）比在逆反应中所剩下的要多。这些观察结果与以下等式2Ru（bpy）_3〜（2+）+ O_2 + 2H〜+->（hv）/ <-（暗）2Ru（bpy）_3〜（2 +）+ H_2O_2一致已计算出平衡常数。光通过* Ru（bpy）_3〜（2+）驱动，有助于克服激活障碍或正向反应，并且在光氧化被驱动超过平衡的程度下，光能以很长的形式转化生命的化学产品。光谱证据排除了Ru（bpy）_3〜（3 + / 2 +）的氧化还原电位的任何显着变化，从而表明H_2O_2在酸性较强的介质中更加稳定，而还原Ru（bpy）_3的能力较弱〜（2+）与更高的pH不同。

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