Studies of quenching of excited states of aromatic molecules in polymer matrices
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The phosphorescence behaviour of a series of polycyclic aromatic compounds has been studied in different polymer matrices. Measurements have been carried out at 77K and from room temperature up to 350K. Studies at 77K have shown that the polymer matrix takes part in some quenching reaction, the rate constant depending on the additive being studied. At higher temperatures non-exponential decays were observed, for polystyrene this phenomenon is very marked. It is proposed that the cause of non-exponential decay is T-T annihilation resulting from mobile triplet exciton migration within the polymer matrix. It is proposed that the triplet levels are 285.9 and 297.4 kJ mole⁻¹ for polystyrene and poly(methylmethacrylate) respectively. The quenching of fluorescence for the polycyclic aromatic additives by oxygen has been examined and unusual plots obtained. At low pressures of oxygen enhanced singlet emission was observed but as the pressure was increased normal Stem-Volmer plots were obtained. Simultaneously as pressure of O₂ increased the triplet emission was very efficiently quenched and the increase in fluorescence was observed. When the phosphorescence ceased the intensity of singlet emission decreased with increasing pressure of O₂. The mechanism proposed to account for this very unusual behaviour involves paramagnetic quenching of the singlet state by the mobile triplet exciton. The increase in fluorescence resulting from removal of the triplet exciton by oxygen is greater than the direct singlet quenching by O₂.
Thesis, PhD Doctor of Philosophy
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