A SITE-SELECTIVE LUMINESCENCE EXCITATION AND RAMAN-SCATTERING STUDY OF EU(CF3SO3)(3)-PPO - ION ASSOCIATION AND IONIC EXCHANGE-RATE

The luminescence of EU(3+) in a salt-polymer complex. poly(propylene oxide) containing Eu(CF3SO3)(3) salt, was studied using broad band and site-selective excitation. Two distinctly different types of local structure around EU(3+) were detected. To investigate and compare the local structure around the SO3CF3- anions with that around the EU(3+) cations, a Raman scattering study of the SO3 symmetric stretch vibration was performed in the same system. The vibrational data clearly reveal that also the anions are subject to two types of local environment. It is found that neither of the two states can be attributed to ''free'' (completely solvated) ions, which implies the existence of two different anion-cation configurations. Temperature and concentration dependent studies of the site-selective EU(3+) luminescence and the SO3 vibrational spectrum show that the relative amount of ions in the two states is, within the experimental accuracy, constant. However at elevated temperatures there is a rapid exchange between the two states not found at lower temperatures. The characteristic time scale for the ionic exchange is found to be closely related to the structural relaxation time of the polymer segmental mobility. The findings support the idea that ionic diffusion in salt-polymer complexes is assisted by the local polymer chain mobility which is used in models to explain polymer electrolyte performance.