Conical intersections as a mechanistic feature of organic photochemistry

The photochemical decay channel is the central point of a photochemical mechanism. The traditional view of the mechanism of a photochemical reaction assumes that the excited state reaction path is similar to the ground state path and the decay channel corresponds to an avoided crossing minimum (van der Lugt and Oosterhoff model). In contrast, we find that there is no evidence for a decay path via such an avoided crossing minimum and that excited state reaction paths are very different from those on the ground state. Rather, the photochemical decay channel corresponds to a conical intersection point and plays the central role in mechanistic photochemistry. The nature of the conical intersection point and its mechanistic implications will be discussed for 3 illustrative types of photochemical reactions (2+2 cycloadditions, polyene electrocyclization/isomerisation, and the di-pi-methane rearrangement).