Photodegradation cage effects

In the case of photodegradation in solution the cage effect due to solvent molecules which surround the radical pairs formed and enclose them as in a cage (15-16). 

The photodegradation of polyethersulfone proceeds mainly through main-diain scission at room temperature, but the crosslinking becomes appreciable with increasing temperature and overwhelming above 170 °C, as shown in Fig. 30 The value of in solid film is 40 times smaller than in solution, indk ting a marked cage effect... 

It was proposed that the temperature dependence of polymer 5 arises from the temperature dependence of the kA step. Specifically, it was suggested that the polymer segments to which the radicals are attached are conformationally stressed. There are two possible modes for the newly formed radicals to relax and become separated They can rotate or recoil away from each other (Scheme 9). These secondary motions of the polymer arise from the relaxation of unfavorable bond conformations that are formed during the polymer casting process. The increased thermal energy facilitates the rotation and recoil relaxation processes, which effectively increases the rate constant for diffusion of the radicals out of the cage, kA. This leads to decreased radical-radical recombination and consequently an increase in photodegradation efficiency. 

As mentioned above, the M-M-bonded polymers are of interest as photochemically labile materials (Scheme 20). The quantum yields for the photodegradation of these polymers and model complexes decreased as the chain lengths increased. This effect has been observed in other polymer systems, and several explanations are possible. One invokes the theory that radiationless decay is faster in molecules with more vibrational modes and would thus make the quantum yield for the formation of requisite radical cage pair lower as the chains get longer. ... 

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