Photochemical and Radiochemical Oxidation

Radical cations are produced photochemically by electron transfer transitions (p. 419) but only as ion pairs with the radical anion formed simultaneously. In most cases, the ion pair reverts to reactant by reverse electron transfer before any reaction can take place. Radical cations can, however, be prepared as stable entities by photolysis of compounds with low ionization potentials at low temperatures in glasses containing electron acceptors. Electrons expelled by photoionization wander through the glass until they are trapped by an acceptor. Since the acceptor is not in contact with the cation radical and since energy is required to extract the electron 

Reactions of this kind are analogous to those taking place in photographic emulsions. Here silver ions act as the traps, giving silver atoms that collect into tiny grains of silver. These grains act as nuclei for the reduction of silver halide by photographic developers. 

A still more important process involving photochemical production of a radical cation is probably involved in the first step in photosynthesis. The chloroplasts at which photosynthesis takes place contain aggrcr gates of molecules of chlorophyll stacked together so that their n MOs interact. J. J. Katz and his co-workers have demonstrated the existence d a water-bonded chlorophyll dimer that appears to be the active center in photosynthesis. Photoexitation causes electron transfer from one of the chlorophylls to the other (7.77)  

The oxidizing and reducing power thus generated is used to drive the photosynthetic unit. In the process H2O is eventually converted to oxygen by a process whose details are obscure but which follows the overall equation 

The radical anion aggregate by another series of steps, involving ferredoxin among other intermediates, eventually reduces carbon dioxide to sugars. 

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