Stilbene photoisomerization driving force as revealed by the topology of the electron density and QTAIM properties

The photoisomerization of stilbene (C14H12) has become an archetype for the study of cis-trans isomerizations induced by light. Despite the many electronic structure calculations performed to gain understanding about this important process, no insight about this reaction has been obtained by means of the exploitation of topological quantum chemical methods. This contribution addresses the changes of the topology of the electron density along with those of the properties of atoms in molecules undergone throughout this photochemical transformation. By considering the phenyl rings and the CHCH bridge of stilbene separately, we found that both groups in the ground state are destabilized when the system evolves from either the cis or trans isomer towards the conical intersection connecting the S0 and S1 states. Nonetheless, the energy of the C6H5 moieties across the excited state potential energy curve is reduced along the same change in configuration of C14H12, thereby providing the driving force for the reaction to occur. The analysis of the atomic charges reveals that the carbon atoms of the CHCH bridge withdraw electron density from their interatomic region between them and the phenyl groups. This flow of charge density might be related with the occupation of the CHCH π ★ orbital which allows the reaction to take place. Finally, an examination of the electronegativity of the groups CHCH and C6H5 indicates that the phenyl rings drastically change their electronegative nature upon photoexcitation, enabling the system to undergo the photoisomerization. Overall, it is shown how the topological analysis of the electron density can provide important insights about chemical reactions carried out in excited states.