Photoisomerization\nPaths of α,ω-Diphenylpolyenes:\nReaction Rate Dependence on Temperature, Excitation Wavelength, and Deuteration

The photoisomerization rate <i>k</i><sub>iso</sub> of <i>trans</i>-stilbene (tS) and trans–trans-diphenylbutadiene\n(ttD) is studied in solution and compared to that in jet/gas. Rice–Ramsperger–Kassel–Marcus\n(RRKM) theory correctly predicts the tS rate in jet, <i>k</i><sub>RRKM</sub> = <i>A</i><sub>m </sub>exp(−<i>E</i><sub>in</sub>/<i>kT</i><sub>m</sub>) with <i>E</i><sub>in</sub> = 1398 cm<sup>–1</sup>, and <i>A</i><sub>m</sub> = 1.8 ps<sup>–1</sup> corresponding\nto frequency ν<sub>iso</sub> = 60 cm<sup>–1</sup> of\nthe reactive mode, <i>T</i><sub>m</sub> being the molecular\ntemperature. However, the behavior in solution cannot be explained\nby the RRKM rate alone. In solution the rate <i>k</i><sub>iso</sub> = <i>A</i><sub>S </sub>exp(−<i>E</i><sub>b</sub>/<i>kT</i><sub>S</sub>) has a similar\nform, but depends mainly on solvent temperature <i>T</i><sub>S</sub> and proceeds much faster, <i>A</i><sub>S</sub> = 19 ps<sup>–1</sup>, <i>E</i><sub>b</sub> = 1520\ncm<sup>–1</sup> in <i>n</i>-hexane. Moreover, excitation\nat high excess energy, resulting in molecular temperature <i>T</i><sub>m</sub> = 607 K, affects the rate only slightly, unlike\nin jet, and contrary to common theoretical models. The experimental\nresults clearly indicate two isomerization paths in solution: via\nrelatively slow intramolecular activation <i>A</i><sub>m</sub> ∼ 1 ps<sup>–1</sup>, and by much faster solvent activation <i>A</i><sub>S</sub> = 18 ps<sup>–1</sup> due to solute–solvent\ninteractions (collisions). The data in <i>n</i>-alkanes\nconfirm previously established power dependence <i>k</i><sub>iso</sub> ∼ η<sup>α</sup> on viscosity η,\nwith α = 0.30 for tS, and α = 0.35 for ttD. With <i>E</i><sub>η</sub> being the viscosity barrier, its contribution\nto <i>E</i><sub>b</sub> can be isolated, giving the intramolecular\nbarrier <i>E</i><sub>in</sub> = (<i>E</i><sub>b</sub> – α<i>E</i><sub>η</sub>), slightly\nlower than in jet/gas, probably due to the dispersive/induction interactions\nin solution.