O₂ quenching of ruthenium(II) tris(2,2′-bypyridyl)²⁺ within the water pool of perfluoropolyether-based reverse micelles formed in supercritical carbon dioxide

We report on the effects of O₂ on the luminescence quenching of ruthenium(II) tris(2,2′-bipyridyl) cation, [Ru(bpy)₃²⁺], sequestered within the water pool region of perfluoropolyether (PFPE)-based reverse micelles formed in supercritical carbon dioxide (scCO₂). A Stern-Volmer quenching model cannot describe the O₂-dependent [Ru(bpy)₃²⁺] quenching profiles in the PFPE micelles. A 'two-site' model is needed to describe the observed quenching. The mean quenching efficiency is approx. 50-fold higher when the molar ratio of water to PFPE (ω-o$/) is ≤ 10 as compared to ω-o$/ = 20. At ω-o$/ values of 10 or less, 95+% of the [Ru(bpy)₃²⁺] molecules within the water pool are located in close proximity to the carboxylate headgroups within the micelle water pool and they are quenched very effectively by the O₂ dissolved within the scCO₂. At the highest ω-o$/ values studied, 60-65% of the [Ru(bpy)₃²⁺] within the water pool remains near the carboxylate headgroups; a substantial fraction (35-40%) of the [Ru(bpy)₃²⁺] is distributed more toward the center of water pool away from anionic surfactant headgroups. Those [Ru(bpy)₃²⁺] luminophores that are away from the headgroups (more toward the water pool center) are not quenched as effectively by O₂ because the O₂ solubility in water is significantly lower compared to scCO₂.