Free-Space and Intermolecular Interaction Effects on the Local-Chain Rotational Relaxation Dynamics in Dye + Polymer Lasing Materials

Abstract

Dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarization currents (TSDC) are combined to analyze the secondary (β) side-chain relaxation mode of poly(methyl methacrylate) (PMMA) in conventional solid-state laser matrices. The response of undoped PMMA is compared with that of mixtures with fluorescent dyes: the highly polar and ionic rhodamine 6G/Cl- and pyrromethene 567 dyes and the neutral and apolar substituted perylene and perylimides. Variations in characteristic features of the β relaxation, such as the spectral position and the relative strength of the thermocurrent and dielectric loss bands as well as the absolute values and the width of the distribution in apparent activation energies (Ea), are exploited to isolate different types of intermolecular and intramolecular guest−host interactions. The results are interpreted in view of opposing effects on the relaxation characteristics of the pendant groups. In the presence of the perylene/perylimide dyes the β process is mainly influenced by variations in the local free space: by increasing the size of the dye molecule, the (re)orientation of the ester carbonyl side groups is more effectively hindered (i.e., increased Ea and antiplasticization). In contrast, in the xanthene/pyrromethene + PMMA mixtures, dipole−dipole and electrostatic interactions and charge-transfer reactions may be considered to control the dynamics of the β relaxation mode. The complementary character of the two dielectric techniques and possible relationships between the dielectric results and optical characteristics of each blend are discussed.