Torsional photoisomerization of substituted ethylenes: Application to stilbene

The trans to cis isomerization rate of stilbene is determined using a diffusional model for internal rotation. It is found that the rate is sensitive to the structural model for stilbene adopted and factors of 2 can be introduced by assuming the phenyl substituents are spherical. The preferred model treats the phenyl substituents as collections of six spheres rigidly bound together and having a specific pitch angle with respect to the plane of the ethylene double bond. The use of a model potential function, chosen to agree qualitatively with experimental findings, allows the calculation of the isomerization rate as a function of the height of the barrier to rotation Q. The experimental value for the rate can be fit with this model using a barrier height ranging from 2–3 kcal/mole, in accord with previous estimates.