Photodimerization of anthracenes in fluid solutions: (part 2) mechanistic aspects of the photocycloaddition and of the photochemical and thermal cleavage

One of the classics in photochemistry, the photodimerization of anthracenes can be considered as a paradigm of the photocycloaddition of non saturated hydrocarbons. The historical steps of the mechanistic studies are reviewed: based on fluorescence quenching, cyclization quantum yields measurement, the influence of dioxygen and solvents, they support a singlet state pathway; the dimerization rate constants are found to be generally high for reactions occurring within a few nanoseconds unless they are slowed down or inhibited by steric strain. In several cases, excimers have been demonstrated to be intermediates and it is shown that excimer fluorescence and cyclization are competitive processes. Another intermediate known as pericyclic minimum (or conical intersection) is postulated to form a sort of floppy cycloadduct where the reacting centres are at mutual distances shorter than in excimers and longer than in dimers. For intermolecular dimerizations, the triplet state is also reactive but through triplet-triplet annihilation in dilute solutions. Intramolecular photocycloadditions have also been carefully examined, for the role of multiple excimer formation, regioselectivity (9,10:1',4' and 9,10:1',2' cyclization) and solvent polarity. The triplet state reactivity is shown to lead to 4 pi + 2 pi or 4 pi + 4 pi cycloadducts, depending on geometric factors. In the latter case when intersystem crossing is favoured by the substituents, cyclization quantum yields as high as 0.65-0.72 have been observed. Photodissociation quantum yields are generally high and the reactions are partly adiabatic, leading to excimer and monomer fluorescence, but the major part follows another pathway not fully elucidated by flash photolysis. Thermodynamic and kinetic parameters for the thermal cleavage are given; they reveal a large gamut of stability for the photocycloadducts.