Factors Determining the Adiabatic or the Diabatic Pathway of the Photoisomerization of Unsaturated Bonds

Ethylenes substituted with aromatic nuclei of low triplet excitation energies undergo cis → trans one-way isomerization in an adiabatic way in the triplet state with a quantum chain process. A new general concept is introduced to cover various types of isomerization in the triplet state: the conventional two-way mutual diabatic isomerization, the newly found one-way isomerization, and inefficient isomerization. This is extended to describe the isomerization in the singlet excited state which occurs in either a diabatic or an adiabatic pathway.