Cage Effect Under Photodissociation

Triplet RPs in nonviscous solutions exit the cage with/ 1. An increase in viscosity leads to an increase in a RP lifetime and slows down molecular diffusivity these features aUow S-T transitions to occur in the RP, and geminate recombination of free radicals is expected to occur, increasing the cage effect Experimental measurements demonstrate that the cage effect O increases with an increase in solvent viscosity. An increase of media viscosity, which usually takes place upon 

Cage effect dynamics or kinetics of geminate recombination was observed for the first time under photodissociation of aC C dimer of aromatic radicals in a viscous media. A suggestion has been made that at least in a number of smdied cases the mumal diffusion coefficient of radicals in the pair is approximately 10 times lower than the sum of macroscopic diffusion coefficients of the individual species. In other words, a geminate recombination proceeds considerably longer than expected. 

A study of chemically induced dynamic electron polarization, CIDEP (see Section 12.3.3) on F and G pairs of radicals formed under photolysis of a common termo- and photoinitiator 2,2 -azobis(2-methylpropionitrile) (AIBN) led to a tentative conclusion that initial spatial separation of 2-cyano-2-propyl radicals does not depend upon viscosity However, it is plausible that the diamagnetic dinitrogen molecule formed under photolysis of AIBN (and is invisible by ESR) separates further from a contact RP under photolysis in solvents of lower viscosity. The problem of initial spatial separation and mutual orientation ofradicals under photolysis still waits experimental elucidation. 

There is an interesting predicted effect of polymer environment on low MW molecules (radicals). Let us assume that radicals strongly interact with polymer chains by attraction or repulsion. Then, the free energy of a polymer system with low MW species decreases when low MW species are located in the proximity of each other. That means that the value of O of transient radicals should increase in such polymer solutions. To the best of our knowledge, this effect has not yet been observed experimentally. 

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