Effect of long-term aging on oxygen sensitivity of luminescent Pd-tetraphenylporphyrin/PMMA films

The phosphorescence quenching by molecular oxygen of Pd-tetraphenylporphyrin molecules encapsulated into thin films of PMMA was monitored during long-term aging. The quenching-dependent part of the phosphorescence decay law was described by a stretched exponential, whereas the value of stretching factor β = 0.79 remained constant during the whole aging period of 9 months. The average values of quenching rates decreased more than four times during this time. The diffusion constants, determined from the response of phosphorescence to a step change in oxygen pressure, decreased by a similar amount. The results contribute to a deeper understanding of the factors influencing the long-term stability of oxygen sensors based on dye molecules in glassy polymers. The magnitudes of the sensitivity drift, caused by physical aging, were determined for Pd-TPP/PMMA at different aging temperatures: the aging rates of Stern-Volmer factors in logarithmic timescale were 0.22 and 0.12 at 20 and 70 °C, respectively. The lower temperature aging process was solvent assisted, whereas at 70 °C a relatively fast change involving the solvent evaporation was followed by slower relaxation.