On the low-lying singlet excited states of styrene: a theoretical contribution

The present contribution analyses the trans–cis photoisomerization mechanism of ethene and styrene on the singlet manifold. Within the framework of multiconfigurational second-order perturbation theory (CASPT2), the extended multistate approach (MS-CASPT2) is found to be flexible enough to describe energy hypersurfaces adiabatically. For ethene, torsion about the CC bond towards a perpendicular structure leads to a situation where the energy difference between the ground and the lowest excited state is still too large (2.5 eV) for efficient radiationless decay. However, the energy gap decreases to 0.4 eV when one of the methylene moieties is, in addition, pyramidalized from the twisted structure. A similar behaviour is obtained for styrene upon torsion-pyramidalization of the methylene subunit, with a computed energy gap of 0.2 eV. The intersection of the ground and excited states potential energy surfaces occurs by a slight additional pyramidalization from the optimal excited state structure. The actual barrier height on the S1 surface from the local planar minimum structure is estimated to be less than 0.8 eV.