Ab initio study on the electronic structures of stilbene at the conical intersection

Ab initio complete-active-space self-consistent-field calculations have been performed to examine the electronic structures of stilbene at the conical intersection. At the conformation where only the central ethylenic bond is perpendicularly twisted, the energy gap between S0 and S1 is too large for stilbene to relax into S0. At the optimized geometry of the conical intersection, a covalent state with a diradical character is energetically close to a zwitter ionic state where the electron on one of the central ethylenic carbon atoms is transferred onto the other.