Photoisomerization Paths of α,ω-Diphenylpolyenes: Reaction Rate Dependence on Temperature, Excitation Wavelength, and...

The photoisomerization rate k_iso of trans-stilbene (tS) and trans-trans-diphenylbutadiene (ttD) is studied in solution and compared to that in jet/gas. Rice-Ramsperger-Kassel-Marcus (RRKM) theory correctly predicts the tS rate in jet, k_RRKM = A_mexp(-E_in/kT_m) with E_in = 1398 cm^-1, and A_m = 1.8 ps^-1 corresponding to frequency ν_iso = 60 cm^-1 of the reactive mode, T_m being the molecular temperature. However, the behavior in solution cannot be explained by the RRKM rate alone. In solution the rate k_iso = A_Sexp(-E_b/kT_S) has a similar form, but depends mainly on solvent temperature T_S and proceeds much faster, A_S = 19 ps^-1, E_b = 1520 cm^-1 in n-hexane. Moreover, excitation at high excess energy, resulting in molecular temperature T_m = 607 K, affects the rate only slightly, unlike in jet, and contrary to common theoretical models. The experimental results clearly indicate two isomerization paths in solution: via relatively slow intramolecular activation A_m ∼ 1 ps^-1, and by much faster solvent activation A_S = 18 ps^-1 due to solute-solvent interactions (collisions). The data in n-alkanes confirm previously established power dependence k_iso ∼ η^α on viscosity η, with α = 0.30 for tS, and α = 0.35 for ttD. With E_η being the viscosity barrier, its contribution to E_b can be isolated, giving the intramolecular barrier E_in = (E_b - αE_η), slightly lower than in jet/gas, probably due to the dispersive/induction interactions in solution.