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

• Published in: Journal of the American Chemical Society 2024-11, Vol.146 (47), p.32463-32478
• Main Authors: Dobryakov, Alexander L., Schriever, Daria, Quick, Martin, Pérez-Lustres, J. Luis, Ioffe, Ilya N., Kovalenko, Sergey A.
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126; 1520-5126
• DOI: 10.1021/jacs.4c09134

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 m exp­(−E in/kT m) with E in = 1398 cm–1, and A m = 1.8 ps–1 corresponding to frequency νiso = 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 S exp­(−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 ∼ ηα 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.