Franck-Rabinowitch effect

The cage effect described above is also referred to as the Franck-Rabinowitch effect (5). It has one other major influence on reaction rates that is particularly noteworthy. In many photochemical reactions there is often an initiatioh step in which the absorption of a photon leads to homolytic cleavage of a reactant molecule with concomitant production of two free radicals. In gas phase systems these radicals are readily able to diffuse away from one another. In liquid solutions, however, the pair of radicals formed initially are caged in by surrounding solvent molecules and often will recombine before they can diffuse away from one another. This phenomenon is referred to as primary recombination, as opposed to secondary recombination, which occurs when free radicals combine after having previously been separated from one another. The net effect of primary recombination processes is to reduce the photochemical yield of radicals formed in the initiation step for the reaction. 

This cage effect, also known as the Franck-Rabinowitch effect, has other important consequences. In a photochemical reaction in solution, for example, a pair of free radicals produced initially may, owing to their being caged in by the surrounding molecules, recombine before they can separate from each other. This effect is known as primary recombination. 

When the process is diffusion controlled, this yield depends on the solvent viscosity, and, as we will see shortly, the diffusion constants present an inverse relation with Data for this reaction are given in Table 9.3. This cage effect becomes more significant when the solvents have a high viscosity, and in this case the highest primary recombination yields are observed. This cage effect is known as the Franck-Rabinowitch effect. 

Collisions in solution are diffusion-controlled and hence depend on the viscosity of the solvent. Due to Franck-Rabinowitch cage effect they occur in sets called encounters . 

The intermediate charge transfer complex formed in the excited state is now termed as an exciplex. Back recombination of photochemically dissociated products in solution, due to Franck-Rabinowitch cage effect, probabily, is the common cause of low efficiencies of the solvent reaction. 

The cage effect, or the enhanced probability for recombination of two radicals formed in close proximity, was discussed by Franck and Rabinowitch in 1934, and Rabinowitch and Wood used a pinball illustration of this process (Fig. 5). The theory of this reaction was further developed by Noyes. ... 

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