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 ωb among the three alkenylanthracenes. This trend is taken as evidence for the importance of frequency-dependent solvent friction in large-amplitude isomerizations.