THE VISCOSITY DEPENDENCE AND REACTION COORDINATE FOR ISOMERIZATION OF CIS-STILBENE

Femtosecond spectroscopic methods are used to study the dynamics following optical excitation of cis-stilbene molecules in hydrocarbon solvents. Transient absorption spectra of cis-stilbene over the range 320-1100 nm are reported. The anisotropies of these transients permit the assignment of the various excited electronic states in this region to A type in C2 symmetry. The excited state absorptions disappear at rates that are weakly dependent on solvent friction and comparisons with simple statistical mechanical theories and various potentials indicate that there is likely to be a barrier crossing process responsible for the observed decay times being in the range 0.7-1.4 ps. For observation times longer than ca. 100 fs an exponential decay of the cis population is observed and the transient spectrum does not appear to change in shape. A Kramers model fits the frictional dependence of the decay rates. Direct observation of what appears to be trans isomer ground states at 335 nm is reported following excitation of cis at 312 nm. The appearence time of this species is indistinguishable from the cis-disappearence time and any intermediate in the process cis→product (trans) is found to have a lifetime of less than 150 fs. This result implies that hot trans-stilbene molecules are produced in the isomerization. Previous studies probing at 312 nm may have detected cooling of these hot molecules. By means of polarized light excitation of cis and detection of the photoproduct it is found that the reactant and product of the isomerization are aligned as if the reaction coordinate were a twisting about the double bond by 180° while maintaining the orientation of the twofold axis in the laboratory frame. This high alignment, in view of obvious possible depolarizing influences in the solution, suggests a more intricate reaction coordinate involving the motion of the ethylene carbons. © 1990 American Institute of Physics.