A Semi‐Empirical Investigation into the Trans‐Cis Photoisomerisation of Stilbene

Potential curves of the eight lowest singlet and eight lowest triplet states for the isomerization reaction of stilbene have been calculated by CI including triply excited configurations. The CI was based on MO's evaluated by a CNDO method. A ground state activation energy of the isomerization of 259 k J/mol was calculated. The lowest excited singlet state has a minimum at the perpendicular geometry and an activation energy of 50 kJ/mol for the trans cis isomerization in this state was found. It is shown, that this activation energy originates from an avoided crossing with a higher lying singlet state. Isomerization in the triplet manifold should occur preferentially via the lowest triplet state.