Torsional barriers for planar versus twisted singlet styrenes

Kinetic modeling of the temperature-dependent lifetimes of styrene and several methyl-substituted styrenes has been used to obtain the torsional barriers for singlet-state C=C rotation. The barrier for C=C torsion is found to be correlated with the ground-state phenyl-vinyl dihedral angle, φ, planar styrenes having barriers of ∼6.5 kcal/mol and moderately twisted styrenes having smaller barriers. Highly twisted styrenes undergo exceptionally rapid intersystem crossing. This unexpected dependence of excited-state behavior on φ is attributed to a change in the character of the excited states from delocalized for planar styrenes to localized for highly twisted styrenes.