Spectroscopic studies of impurity-host interactions in dye-doped polymers: Hydrostatic-pressure effects versus temperature effects

The method of photochemical hole burning is an excellent means for investigating small pressure shifts of the optical transitions of dye molecules embedded in polymeric host matrices. We have investigated both pressure- and temperature-induced line shifts and line broadenings and have been able to separate volume and temperature effects, which cannot be distinguished by only considering temperature-cycling experiments. On the basis of a concept, originally developed for vibrational dephasing in liquids, which separates the impurity-host interaction potential into two parts, we are able to determine quasistatic and dynamic contributions of the considered molecular interaction potentials in a quantitative fashion. These two contributions are opposite in sign and partially compensate each other in a temperature-cycling experiment. The ‘‘static’’ linewidth and line-shift parameters are governed by the van der Waals–like part of the impurity-host interaction potential and yield matrix parameters like the polymer compressibility κ by solely evaluating spectroscopic experimental data. The more complex ‘‘dynamic’’ contributions require the assumption of a steep short-range repulsive part of the interaction potential which is mainly responsible for the scattering ($T_2$) processes leading to a phase loss in ground and excited states of the dye molecule. For our experimental studies we used the polymer glasses polymethylmethacrylate, polystyrene, and polyethylene doped with the dye molecule free-base phthalocyanine (${\mathrm{H}}_2$Pc).