Excimer formation of pyrene as a probe to investigate the recombination of geminate pairs: the effect of charge hopping process on fluorescence and ODESR spectra in X-irradiated ethylene propylene rubber doped with pyrene

The fluorescence detected ESR (ODESR) spectrum and X-induced and UV-excited fluorescence spectra were successfully obtained for ethylene propylene rubber doped with pyrene under various conditions. From the analysis of the ODESR intensity and its line-width, it was found that charge hopping between the solute pyrenes is the main mechanism for geminate recombination at the pyrene content larger than 0.3 wt%. At elevated temperatures, the excimer fluorescence relative to the monomer fluorescence induced by X-ray excitation is considerably stronger than that by UV-excitation, especially at high concentrations. On the other hand, at lower temperatures below ca. 280 K this difference could not be observed even at high concentration of 3.0 wt%. These observations indicate that the dimer cation formation, from which the excimer fluorescence is efficiently emitted, is accelerated by the hopping of cation-radical center to the clustering pyrenes at high temperatures and high concentrations. While only the electron hopping takes place at low temperatures, thus whether a charge recombination results in excimer formation or not is determined by the cationic trapping site, i.e. if several pyrenes are clustering there, excimer may be formed regardless of the excitation source. Although rapid translational motion is restricted in ethylene propylene rubber, the effective formation of excimer or dimer cation at high temperatures indicates that rotational diffusion and small distance translational diffusion are allowed within the fluorescence lifetime at those temperatures.