O₂ Quenching of Ruthenium(II) Tris(2,2‘-bypyridyl)²⁺ within the Water Pool of Perfluoropolyether-Based Reverse Micelles Formed in Supercritical Carbon Dioxide

Abstract

We report on the effects of O 2 on the luminescence quenching of ruthenium(II) tris(2,2‘-bipyridyl) cation, [Ru(bpy) 3 2+ ], sequestered within the water pool region of perfluoropolyether (PFPE)-based reverse micelles formed in supercritical carbon dioxide (scCO 2 ). A Stern−Volmer quenching model cannot describe the O 2 -dependent [Ru(bpy) 3 2+ ] quenching profiles in the PFPE micelles. A “two-site” model is needed to describe the observed quenching. The mean quenching efficiency is ∼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) 3 2+ ] 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 O 2 dissolved within the scCO 2 . At the highest ω o values studied, 60−65% of the [Ru(bpy) 3 2+ ] within the water pool remains near the carboxylate headgroups; a substantial fraction (35−40%) of the [Ru(bpy) 3 2+ ] is distributed more toward the center of water pool away from anionic surfactant headgroups. Those [Ru(bpy) 3 2+ ] luminophores that are away from the headgroups (more toward the water pool center) are not quenched as effectively by O 2 because the O 2 solubility in water is significantly lower compared to scCO 2 .