Paramagnetic interactions of triplet radical pairs with nitroxide radicals: An "antiscavenging" effect

Article abstract of DOI:10.1021/ja00091a059

The recombination probability, P_r, of triplet sec-phenethyl/sec-phenethylacyl radical pairs (RP) generated in the photolysis of d,l-2,4-diphenylpentan-3-one (d,l-DPP) in benzene increases from 0.033 to 0.098 as the concentration of the stable free radical, TEMPO, increases from 0 m to 0.146 M. The unusual effect of a putative free radical scavenger is explained in terms of a competition between two processes: chemical scavenging of geminate radicals by nitroxide, and spin exchange between geminate radicals and nitroxide. The second process is evidently faster and enhances triplet to singlet intersystem crossing of the geminate triplet RP. As a result, P_r increases as the concentration of the free radical scavenger, TEMPO, increases. These results reveal a peculiar situation for which a paramagnetic scavenger, by physically accelerating intersystem crossing faster than it scavenges, enhances the reactivity of a triplet geminate pair as it approaches. The new result is an increase in the "cage effect", or recombination efficiency of geminate radical pairs. These results allow a novel view of the cage effect of geminate pairs in homogeneous, nonviscous solutions.