Effect of plasticizer viscosity on the sensitivity of an [Ru(bpy)32+(Ph4B–)2]-based optical oxygen sensor

Abstract

The quenching of the electronically-excited, lumophoric state of [Ru(bpy)₃²⁺(Ph₄B^–)₂] by oxygen is studied in a wide variety of neat plasticizers. The Stern–Volmer constant, K_SV, is found to be inversely dependent upon the viscosity of the quenching medium, although the natural lifetime of the electronically excited state of [Ru(bpy)₃²⁺(Ph₄B^–)₂] is largely independent of medium. The least viscous of the plasticizers tested, triethyl phosphate, did not, however, produce highly sensitive optical oxygen sensors when used to plasticize [Ru(bpy)₃²⁺(Ph₄B^–)₂]-containing cellulose acetate butyrate (CAB) and poly(methyl methacrylate) (PMMA) films. Instead, the compatibility of the polymer–plasticizer combination, as measured by the difference in the values of the solubility parameter of the two, appears to be a major factor in determining the overall oxygen sensitivity of the thin plastic films. For highly compatible polymer–plasticizer combinations, the plasticizer with the lowest viscosity produces films of the highest oxygen sensitivity. This situation arises because in the film the quenching process is partly diffusion-controlled and, as a result, the quenching rate constant is inversely proportional to the effective viscosity of the reaction medium.