The photoisomerization rate kiso 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, kRRKM = Am exp(−Ein/kTm) with Ein = 1398 cm–1, and Am = 1.8 ps–1 corresponding to frequency νiso = 60 cm–1 of the reactive mode, Tm being the molecular temperature. However, the behavior in solution cannot be explained by the RRKM rate alone. In solution the rate kiso = AS exp(−Eb/kTS) has a similar form, but depends mainly on solvent temperature TS and proceeds much faster, AS = 19 ps–1, Eb = 1520 cm–1 in n-hexane. Moreover, excitation at high excess energy, resulting in molecular temperature Tm = 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 Am ∼ 1 ps–1, and by much faster solvent activation AS = 18 ps–1 due to solute–solvent interactions (collisions). The data in n-alkanes confirm previously established power dependence kiso ∼ ηα on viscosity η, with α = 0.30 for tS, and α = 0.35 for ttD. With Eη being the viscosity barrier, its contribution to Eb can be isolated, giving the intramolecular barrier Ein = (Eb – αEη), slightly lower than in jet/gas, probably due to the dispersive/induction interactions in solution.
