PARAMAGNETIC INTERACTIONS OF TRIPLET RADICAL PAIRS WITH NITROXIDE RADICALS - AN ANTISCAVENGING EFFECT

The recombination probability, P-r, of triplet sec-phenethyl/sec-phene thylacyl radical pairs (RP) generated in the photolysis of d,l-2,4-dip henylpentan-3-one (d,l-DPP) in benzene increases from 0.033 to 0.098 a s the concentration of the stable free radical, TEMPO, increases from 0 m to 0.146 M. The unusual effect of a putative free radical scavenge r is explained in terms of a competition between two processes: chemic al scavenging of geminate radicals by nitroxide, and spin exchange bet ween geminate radicals and nitroxide. The second process is evidently faster and enhances triplet to singlet intersystem crossing of the gem inate triplet RP. As a result, P-r increases as the concentration of t he free radical scavenger, TEMPO, increases. These results reveal a pe culiar situation for which a paramagnetic scavenger, by physically acc elerating 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 new view of the c age effect of geminate pairs in homogeneous, nonviscous solutions.