PHOTOISOMERIZATION RATES OF DIPHENYLBUTADIENE IN N-ALCOHOLS - DIELECTRIC-DEPENDENT ACTIVATION-ENERGIES DETERMINED VIA FITS TO KRAMERS EXPRESSION

Fluorescence lifetimes have been measured for diphenylbutadiene (DPB) in a series of n-alcohols over a wide temperature range. These measurements have been used along with measured rotational correlation times to examine the rate of DPB photoisomerization. The data have been analyzed by Arrhenius plots as well as fits to the Kramers-Hubbard expression. Though adequate fits are obtained, trends in the resulting activation energies which would be expected under the assumption of a polar transition state are lacking. In order to elucidate such trends, we have introduced a method in which isodielectric Kramers-Hubbard fits are used to determine the dependence of the activation barrier on solvent permittivity. The results exhibit a smooth dependence of the activation barrier on solvent permittivity, with the barrier height increasing as the solvent permittivity decreases. This is consistent with expectations, under the assumption that the transition state is polar. The results demonstrate the viability of the isodielectric Kramers-Hubbard approach to the analysis of diphenylpolyene photoisomerization rate data and provide a measure of the dependence of the DPB photoisomerization activation barrier on solvent permittivity.