Effect of annealing on the room temperature luminescence of coumarin 106 in PVA films

Abstract

We studied the effect of annealing on the luminescence of Coumarin 106 (C106) in poly (vinyl alcohol) films (PVA films). The samples and reference polymer films were treated at temperatures between 100 °C and 150 °C (212 F and 302 F) for various times. After cooling and smoothing, the samples and references were measured at room temperature. We observed that the PVA polymer (reference films) changes its optical properties with annealing at higher temperatures, affecting the baselines in absorption and the backgrounds in emission measurements. This requires precise background subtractions and control of the signal-to-noise ratio. Whereas the fluorescence intensity of C106 in PVA films modestly decreases with annealing, the phosphorescence depends dramatically and progressively increases by many folds. The fluorescence quantum yields and lifetimes decrease with the annealing, which suggests an increase in the non-radiative processes in the singlet excited state S1. The increase in the phosphorescence intensities results from increased intersystem crossing (ISC), which also decreases fluorescence. We also studied the effect of annealing on phosphorescence with the directly excited triplet state of C106. In this case, two processes are affected by annealing, S0→T1 absorption and T1→S0 phosphorescence. The long-wavelength excitation (475 nm) avoids PVA polymer excitation. The phosphorescence lifetime decreases with annealing while the phosphorescence intensity increases. These changes suggest that the radiative rate of T1 → S0 increases with annealing.