Potential energy surface crossings in organic photochemistry

Modern experiments and quantum chemical computations show that low lying potential energy surface crossings (conical and singlet-triplet intersections) are a general feature of photochemically relevant excited states. This review focuses on the computational and experimental investigation of the efficiency of internal conversion at a surface crossing, the competition with fluorescence when an excited state barrier is present, and the relationship between the molecular structure at the intersection and structure of the photoproducts. It is shown that single or successive low-lying intersections provide the bottlenecks controlling the evolution of a photoexcited molecule from the Franck-Condon region to the photoproduct valleys.