Relationship between Photoisomerization Path and Intersection Space in a Retinal Chromophore Model

A low-lying segment of the intersection space (IS) between the excited-state and the ground-state energy surfaces of a retinal chromophore model has been mapped using ab initio CASSCF computations. Analysis of the structural relationship between the computed IS cross-section and the excited state Z → E isomerization path shows that these are remarkably close both in energy and in structure. Indeed, the IS segment and the Z → E path remain roughly parallel and merge only when the double bond reaches a 70° twisting. This finding supports the idea that, in certain chromophores, a more extended segment of IS, and not a single conical intersection, contributes to the decay to the ground state.