Real-Time Visualization of an Elusive, Strong Reducing Agent during Tris(2,2'-bipyridyl)ruthenium(II) Electro-Oxidation in Water

Recently, liquid|liquid and liquid|gas interfaces have been implicated in driving unexpected chemistries, including dramatic rate enhancement and spontaneous redox reactions. Given such studies, new methods are necessary to observe and implicate such reactive species. Tris(2,2'-bipyridyl)ruthenium(II) ([Ru(bpy)3]2+) is a common luminophore for photoluminescence and electrochemiluminescence (ECL) studies. In this work, we demonstrate that the electro-oxidation of [Ru(bpy)3]2+ in water produces light without the addition of sacrificial coreactants. We have studied this by confining [Ru(bpy)3]2+ to an aqueous droplet adhered to both a tin-doped indium oxide electrode and, separately, a glassy carbon inlaid disc macroelectrode (d = 3 mm). We also generalized the method to the observation of light at larger electrodes. The light intensity is higher in the absence of O2, diminishes when adding H2O2, and disappears in the presence of a well-behaved, one-electron oxidant (hexaammineruthenium(III)). Our results indicate that a powerful reducing agent is present during the electro-oxidation of [Ru(bpy)3]2+. This reducing agent is at least energetic enough to create the excited state, [Ru(bpy)3]2+*, giving a minimum energy of ∼2 eV. Chemiluminescence persists as [Ru(bpy)3]3+ diffuses into solution, indicating that the strong reducing agent may exist natively in water and at low abundance. These observations have significant fundamental ramifications because they elucidate a new pathway for the [Ru(bpy)3]2+ ECL and allow real-time visualization of highly reactive species.