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 measure ments have been used along with measured rotational correlation times to examine the rate of DPB photoisomerization. The data have been anal yzed by Arrhenius plots as well as fits to the Kramers-Hubbard express ion. Though adequate fits are obtained, trends in the resulting activa tion energies which would be expected under the assumption of a polar transition state are lacking. In order to elucidate such trends, we ha ve 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 activati on barrier on solvent permittivity, with the barrier height increasing as the solvent permittivity decreases. This is consistent with expect ations, under the assumption that the transition state is polar. The r esults demonstrate the viability of the isodielectric Kramers-Hubbard approach to the analysis of diphenylpolyene photoisomerization rate da ta and provide a measure of the dependence of the DPB photoisomerizati on activation barrier on solvent permittivity.