Rotational reorientation and isomerization of transstilbene

We have tested the validity of using solvent shear viscosity as a measure of friction involved in the dynamics of t-stilbene. The rotational reorientation and the isomerization in n-alkanes are considered. INTRODUCTION Ultrafast techniques have made it possible to investigate hydrodynamic theories in the picosecond regime. The photoisomerization of t-stilbene has been widely used to test the Kramers theory for barrier crossing I. But the small size of the probe suggests that a macroscopic solvent viscosity (rl) may not be an adequate measure of the fr ict ion.-In contrast the rotational reorientation time (r oR ) is expected to provide a more accurate measure of the microscopic friction. We applied this concept to fit the isomerization data of t-stilbene in n-alkanes to the Kramers equation. We also discuss the isomerization in n-alcohols where the dynamic nature of the solvent possibly changes the activation barrier. LASER SYSTEM The measurements of r oR were performed with a CPM ring laser amplified through Nd:YAG pumped dye cells. The 615nm output pulse was frequency doubled and used to induce an excited state anisotropy in the sample. The decay of the anisotropy is monitored via S 1-Sn absorption by the time delayed fundamental through crossed polarizers. All isomerization data were obtained with a single photon counting system or were taken from other 2 sources . RESULTS AND DISCUSSION Figure 1 shows the macroscopic friction (o¢ rl) is not parallel to the microscopic friction (a< T OR T ) when the solvent is varied. The longer alkane solvent creates a larger space where t-stilbene can rotate more freely, as reflected in the trend of the slope changes in Figure 1. However, rather surprisingly, Stokes law (r OR <x~7 / T ) appears to hold within a given alkane. :~ Present address: AT & T Bell Laboratories, Murray Hill, NJ 07974 © 1988 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 128.32.208.2 On: Fri, 30 May 2014 17:27:14