Summarizing Discussion

Abstract

For most chemists the concept of the ground state reaction path as being the minimum-energy path from reactants to products along the potential energy surface (PES) is an essential part of their educational upbringing. Induced by optical excitations, photochemical reactions involve excited-state PESs. Thus, when it was found that conical intersections (CIs) are the photochemical reaction funnels that mediate efficient population transfer back to the ground state leading to reaction, minimum-energy CIs where thought to be the key regions of the PESs. This is indeed the scenario in cases where the PESs are initially far from each other in energy, and large-amplitude motions are required to access a region of intersection between two PESs where efficient population transfer to the ground state, and thereby reaction, takes place. Our experiment on the [2+2]cycloaddition published in Ref. II investigated dynamics of that type. But as opposed to the ground state transition state that is unique, numerous CIs are connected in an intersection seam that can extend over a large region of the PES. Thus, in cases where this is true and/or the excited states are close in energy already in the Franck-Condon region, large-amplitude motion is not crucial for mediating electronic population transfer, and the molecule does not necessarily make use of the (minimum-energy) CI. We observed such a behaviour in experiments on substituted cycloheptatrienes (not presented in the thesis) [1].