The Dynamics of benzylic radical pairs in organized media : binding and mobility




Kleinman, Mark Howard

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Through the use of laser flash photolysis and product studies, the photochemistry of dibenzyl ketone and its derivatives in SDS micelles shows that these aggregates impart some constraint on the dynamics of benzylic radical pairs. The enhancement of the geminate/cross-termination product occurring within the micellar interior is termed the geminate cage effect. Under conditions where all radicals exit from the supramolecular system, a substantial amount of the cross-termination product is observed in the product studies. This experimental evidence cannot be explained with conventional models, and a new proposal is put forth which suggests that some of the cross-termination product arises from radicals that exit the micelle and then recombine upon re-entry. This new approach is based on the binding of solutes to the micelle and is called the partition effect. Two sizes of DODAC vesicles were investigated. In small unilamellar vesicles with a diameter of 30 nm, the cage effect derived from product studies showed a significant enhancement of the cross-termination product. Time-resolved experiments showed that all radicals separate, which implies that all of the enhanced cross-termination reactions from the product studies are derived from random radical encounters. Product studies in large vesicles (∼150 nm diameter) demonstrated that the cross-termination product is enhanced, but not to the same extent as for SDS micelles or small vesicles. Calculations reveal that a significant proportion of the random encounters occur in the bulk aqueous phase. The proposed model suggests that it is not necessary to generate a triplet radical pair in order to observe a magnetic field effect on product distribution. In summary, this new proposal predicts that magnetic field effects in biological systems can be observed as long as mobility between different solubilization sites occurs.



Flash photolysis, Photochemistry, Laser Photochemistry