En. Step et al., PARAMAGNETIC INTERACTIONS OF TRIPLET RADICAL PAIRS WITH NITROXIDE RADICALS - AN ANTISCAVENGING EFFECT, Journal of the American Chemical Society, 116(12), 1994, pp. 5462-5466
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.