Dynamic solvent effects on large-amplitude isomerization rates. 2. 2-(2′-propenyl)anthracene and (E)-2-(but-2′-en-2′-yl)anthracene

Spectroscopic and kinetic manifestations of the torsional isomerization of the title molecules in solution have been investigated. Each compound exists in spectrally distinct s-cis and s-trans conformations in both the ground state (S₀) and the lowest excited singlet state (S₁). Upon electronic excitation, an adiabatic interconversion of s-cis and s-trans conformers in S₁ occurs at a barrier-controlled rate which shows a mild solvent viscosity effect. The observed viscosity dependence is compared to theoretical predictions from Kramers' theory and the theory of Grote and Hynes. Theoretical predictions for the isomerization kinetics of 2-vinylanthracene are also discussed. A clear trend is observed between the degree of viscosity dependence of the isomerization rate constant and the barrier frequency ω_b among the three alkenylanthracenes. This trend is taken as evidence for the importance of frequency-dependent solvent friction in large-amplitude isomerizations.