Cisstilbene isomerization: Temperature dependence and the role of mechanical friction

The fluorescence decay time of cis-stilbene has been measured in a variety of solvents over a large temperature range. An isoviscosity Arrhenius plot in n-alkanes yields an activation energy of 386±29 em-I. We interpret this result as an upper limit for the cis-stilbene to trans-stilbene barrier in nonpolar solvents. Isoviscosity plots in small alcohols are nonlinear, indicating complicated behavior in this solvent type. The excited state decay times in n-alcohols and n-alkane solvents correlate well with each other when plotted as a function of chain length, n. We infer from this plot that macroscopic viscosity is a poor measure of the friction felt by the isomerizing species when changing solvent, and that the potential energy surface is not substantially altered between n-alkanes and n-alcohols with n>5. Decay times measured in 2-propanol at 490,475,453, and 440 nm emission differ by no more than 90 fs, indicating that there is little or no spectral evolution during the excited state lifetime. A short component in the fluorescence anisotropy decay of cis-stilbene and a decrease in the excited state lifetime due to deuterium substitution for the ethylenic hydrogens are observed in both polar and nonpolar solvents. Treating DHP formation as a thermally activated process which competes with the cis to trans isomerization, we determine a range of model dependent cis to trans activation energies in alkanes of-300 to +380 em-I.