Comparison of solvent and probe reorientation in polyacrylamide gels

Raman spectroscopy and time-resolved fluorescence anisotropy decay were used to measure the rotational relaxation dynamics of acetonitrile and the dianion polyphenyl 2 (PP2) in polyacrylamide (PAA) gels in a water-acetonitrile solvent mixture. In the neat solvent mixture the acetonitrile molecule rotates, underlying a reduced friction in comparison to its dynamics in pure acetonitrile. The rotational behavior of PP2 in the mixture is well described by the Stokes-Einstein-Debye equation. In the PAA gels the rotational relaxation of acetonitrile is only weakly influenced by the gel network even at high polymer concentrations. In contrast, the larger probe,molecule PP2 reflects a significant retardation of the reorientational motion with increasing PAA concentration. We suggest that this behavior results from the different environments experienced by the probe and solvent molecules.