Cis-stilbene isomerization: Temperature dependence and the role of mechanical friction

Abstract

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 cm−1. 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 transisomerization, we determine a range of model dependent cis to transactivation energies in alkanes of −300 to +380 cm−1.