APPLICATION OF THE MEDIUM-ENHANCED BARRIER MODEL TO THE PHOTOISOMERIZATION DYNAMICS OF SUBSTITUTED STILBENES IN NORMAL-ALKANE SOLVENTS

Photoisomerization rate constants for trans-4,4-dimethylstilbene, trans-4,4'-dimethoxystilbene, and trans-4,4'-di(tert-butyl)stilbene in n-alkane solvents are treated with the transition state theory equation (Eyring equation) and the medium-enhanced barrier model, DELTA-H double danger(obsd) = DELTA-H double danger(t) + aE-eta-s,. The a value, a measure of the effect of medium viscosity, increases in the order of stilbene, dimethylstilbene, dimethoxystilbene, in contrast to results obtained by applying the power law k(obsd) = k(t)B/eta-s(a). The medium-enhanced activation enthalpies and entropies for photoisoinerization of the stilbenes obey the isokinetic relationship, and so do the corresponding viscous flow parameters, in the n-alkane solvent series. These relationships show that the apparent slope from the logarithmic form of the power law plot for n-alkane solvents is not equal to -a but is a function of -a. As previously concluded, microviscosity should be used in the power law because when shear viscosity is used the n-alkane series does not behave as a continuous medium with respect to the twisting groups. The results presented further support the conclusion that the intrinsic energy barriers for photoisomerization of stilbene and its derivatives are constant in the n-alkane solvent series. The relationship between the potential energy barrier height, the Arrhenius activation energy, and the activation enthalpy is discussed.