Luminescent oxygen sensors: time-resolved studies and modelling of heterogeneous oxygen quenching of luminescence emission from Pt and Pd octaethylporphyrins in thin polymer films

Results are presented from a kinetic investigation of oxygen quenching of Pd and Pt octaethylporphyrin luminescence in three polymer films: ethyl cellulose, cellulose acetate butyrate and polyvinylchloride. A combination of initial rate analysis and distribution modelling (uniform, truncated Gaussian, and log-Gaussian) has been applied to the problem of heterogeneous kinetics in these systems. The oxygen dependence of the initial rate was used to obtain model independent values for the arithmetic mean oxygen quenching rate constants for the initial excitation distributions. These are strongly dependent upon the polymer used and are comparable to the diffusion controlled rate constants calculated using the oxygen permeabilities of the polymer films. Time dependent rate analysis of the decay curves under conditions of high quenching efficiency was used to obtain information about the distribution in oxygen quenching rate constants. The simplest of the three distributions examined, the uniform distribution, gave the best-fit to our experimental data. The best-fit uniform distribution widths for all six porphyrin/polymer combinations varied only slightly with distribution widths of between 1.7 and 2.0 times the arithmetic mean quenching rate constant.