ABSTRACT
Aryl halides have an important role in photochemistry since a long time. Historically, studies of the phosphorescence of aromatics and of their halogenated derivatives have been instrumental in dening the modern concept of triplet state, and both the intersystem crossing [1] and the fragmentation of phenyl halides have been long known [2]. In the early years of these studies, attention was given more oen to photophysics than to photochemistry. is is because the introduction of electron-withdrawing substituents, such as halogen atoms, in the ring further increases the already high stability of aromatic compounds, both in the ground and excited states. Indeed, many aromatic compounds are quite photostable and the absorption of a photon leads to reemission rather than to reaction, since excitation causes only a partial decrease of the bonding character in the multiple π-electron array of such compounds and the rigid σ-bond skeleton disfavors geometric modications. Indeed, stability to chemical agents and to irradiation is characteristic of many aryl halides, and this family includes some notoriously persistent environmental pollutants, such as DDT and polychlorobiphenyls. However, photochemistry, albeit oen ine‚cient, is one of the few available paths for the degradation of such compounds and thus this is
16.1 Introduction ......................................................................................369 16.2 E‡ect of Halide Substituents on the Photophysical
Parameters of Aromatics .................................................................370 16.3 General Scheme of Photoreactivity ................................................372 16.4 Photochemical Reactions Not Involving
16.7 ArSN2* Substitution Reactions .......................................................382 16.8 ArSN1 Arylation Reactions ..............................................................382 16.9 Application to Depollution .............................................................386
