Trans-4-(methanol)stilbene (HMS) is a member of the diphenylpolyene family with a methanol group appended to one of the phenyl rings of stilbene. In previous studies, we have demonstrated that the polar functional group has a substantial influence on rotational dynamics of HMS in polar liquids and in supercritical CO 2 . In this paper, we examine the influence of the methanol group on the trans-cis excited state photoisomerization of HMS. We present absorbance and fluorescence spectra that indicate that the spectroscopy of HMS is nearly identical to that of stilbene. We utilize the spectra to calculate the radiative rate constant of the excitedstate trans isomer of HMS. Using this result and measured fluorescence lifetimes, we characterize the transcis isomerization rate of HMS in a series of n-alcohols over temperatures from 0 to 60°C. Linear Arrhenius plots are observed over this temperature range. We also present an analysis based on the isodielectric KramersHubbard method originally developed by Anderton and Kauffman. The results of this analysis demonstrate that the barrier to trans-cis isomerization of HMS decreases with increasing solvent permittivity.

We examine rotational diffusion and trans-cis isomerization of two Biphenyl polyenes, diphenylbutadiene (DPB) and hydroxymethyl-stilbene (HMS) in supercritical fluid CO2 and ethane. The results of density dependent rotational diffusion measurements are interpreted using a model which we have recently developed for rotational diffusion in compressible fluids. Comparison of DPB and HMS fluorescence lifetimes indicate that the rate of the isomerization reaction of DPB is nearly independent of density while HMS exhibits a strong density dependence in CO2. Analysis of the HMS data using Kramers theory indicates that the density dependence is the result of solvent-solute frictional interactions.