Pathway approach to ultrafast photochemistry: potential surfaces, conical intersections and isomerizations of small polyenes

The dynamics of molecules, attention to which is being stimulated by modem ultrafast spectroscopy, as well as progress in quantum chemistry are giving new impetus to the approach which follows the detailed pathway of the molecules along the potential energy surfaces from the excited to the ground state. Local properties such as slopes, saddle points, barriers and funnels with conical intersections as exits are taken into account. We show the fruitfulness of this approach by means of new and published experimental results on photochemical pericyclic reactions. The experimental data support the importance of conical intersections and, in some cases, yield a very detailed picture of the dynamics on the potential energy surfaces. We emphasize the relation of the pericyclic potential surfaces to those of cis-trans isomerization and show that, in this case, the geometries of the conical intersections are interrelated and can be intuitively understood. The conical intersections of stilbene and polyenes can be deduced from the prototype of ethylene, and their geometries are supported by photochemical evidence. (C) 1998 Elsevier Science B.V. All rights reserved.