Charge Transfer and Radical Ions in Photochemistry

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Abstract
Bimolecular photoreactions usually occur via several steps involving “complex‐type” intermediates (excimers or exciplexes). Although physical chemists have investigated them for nearly thirty years (Förster, Weller), and postulated their existence in numerous publications, it is only in the last decade that evidence for an exciplex intermediate in cycloadditions (Caldwell) has been obtained, thereby establishing the link between spectroscopy and synthetic photochemistry. Since then, many investigations have confirmed the role of exciplexes as intermediates in bimolecular photoreactions. The photochemically induced charge transfer from a donor to an acceptor substrate determines not only the bond strengths in the primary exciplex but also influences—via the charge‐transfer nature of the complex—the structure of the exciplex and thereby the selectivity of subsequent reactions. Such chemical processes between the donor and the acceptor prevent the reverse transfer of electrons, which usually results in regeneration of the starting materials, thereby only causing the dissipation of energy and occasionally—under specific conditions—the generation of molecules of the starting material in the triplet state. Ionic photodissociation also reduces the probability of a reverse transfer of electrons. This process, which produces solvated radical ions, is promoted by polar solvents and salt effects, particularly special salt effects. By this means it is possible to generate specific radical cations from, for example, alkenes and dienes and to study their reactions. Although in some areas, for example physical and biophysical chemistry, discussions of single electron transfer (SET) processes have long been found in the basic textbooks, organic chemistry lags far behind, although—and this is shown by the numerous recent publications—organic chemists are also aware of the fundamental importance of these processes. In this connection photochemistry plays a central role, since its instruments make it possible to carry out specific SET processes, to analyze them, and to use them in chemical reactions.

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