Luminescence Quenching of Dyes by Oxygen in Core−Shell Soft-Sphere Ionic Liquids

We examined the photoluminescence and susceptibility to quenching by atmospheric oxygen of a series of phosphorescent dyes dissolved in a core−shell “soft-sphere” ionic liquid matrix consisting of a 7 nm diameter silica core surrounded by a mobile phase consisting of end-grafted flexible chains. While this medium is macroscopically uniform in composition, it is locally heterogeneous on the nanometer length scale. Luminescent dyes provide an opportunity for assessing some of the properties of this local heterogeneity. The PL decay profiles of both platinum ocatethyl porphine (PtOEP) and [Ru(dpp) 3 ]Cl 2 could be fitted to a simple exponential form over a range of partial oxygen pressures. Taking the behavior of PtOEP as “normal,” we calculate an oxygen permeability in the liquid of P O 2 = 4 × 10 - 12 mol cm - 1 s - 1 atm - 1, comparable to that of poly( n -butyl thionylphosphazene) (C 4 PATP) but smaller than that (18 ± 2 × 10 - 12 mol cm - 1 s - 1 atm - 1 for poly(dimethylsiloxane) (PDMS). The smaller slope of the Stern−Volmer quenching plot for [Ru(dpp) 3 ]Cl 2 could be rationalized in terms of two factors, the known smaller cross section for quenching by oxygen for this dye coupled with the likelihood that this dye is located in a more rigid ionic environment characterized by a somewhat smaller local diffusion coefficent for oxygen.