Jump rotation in a molecular glass former

Abstract The rotational dynamics of the spin probe 2,2,6,6-tetramethyl-piperidin-l-oxyl (TEMPO) dissolved in the molecular glass former phenyl salicylate (Salol) have been investigated via electron spin resonance spectroscopy, in a temperature interval ranging from the supercooled to the glassy state of Salol. The comparison of experimental data with predictions of both diffusive and jump reorientational models has been made. It has been proved that in both the supercooled and the glassy state, the TEMPO spin probe rotates according to a jump process. In the investigated temperature range, the angular jump size Φ has been found to be constant and holds 75°. Moreover, the temperature dependence of the reorientational correlation times, which is well described by a double Arrhenius law, has been studied. The crossing temperature from the first to the second Arrhenius regime is found to be near to the glass transition temperature. Comparison with the results of a previous study on the reorientational dynamics of a TEMPO probe in o-terphenyl, has shown that rotational motion happens in structural cages with comparable free volumes.