Ab initio Investigation to Model Stilbene Photo-Physical Properties by Combining CC2 Topological Investigation and CASPT2 Energy Corrections

Stilbene photoexcitation and consequent decay to the ground state has been investigated by mapping the Minimum Energy Path (MEP) from S1 spectroscopic state triggering an almost barrierless reaction pathway to an S1/S0 degenerate region. The particular influence of the σ‐π excitation on the S1 wave function, dominated by a π→π* character, reveals how the non‐dynamical correlation energy was important to correctly describe the excited state behaviour and the topological aspect of its potential energy surface. Several strategies of calculations, by using CASSCF//CASPT2 methods, were performed trying to improve the photochemical description nowadays known. Both symmetry and non symmetry preserving computations were performed; systematically was concluded that, because of the limit of CASSCF description enables only to introduce the correlation effect such as the ones due to σ‐π excitations, CASSCF and CASPT2 topologies are probably often not in agreement. Thus CC2 methodology was adopted o optimize the S1 geometries and obtain reasonable structures for the minima. Two S1/S0 accessible conical intersections featured by pyramidalized carbons were located on the first excited state explaining the ultrafast radiationless decay to the ground state and the photoproducts observed within the timescale of ps.