| Abstract
| - 31P chemically induced dynamic nuclearpolarization (CIDNP) studies of the photochemistry ofp-acetylbenzyldimethyl phosphite (1) provide evidence for the operation ofthe relatively rare T-−S intersystemcrossingmechanism for a photochemically generated triplet free radical pair.The mechanism of intersystem crossingof the triplet radical pair p-acetylbenzyl (2)and dimethoxyphosphonyl (3), generated from 1, isinfluenced bythe large 31P hfcc of 3 and switches from theT-−S mechanism in a relatively low magnetic field (18.8kG)to the conventional T0−S pathway at a higher field (58.8kG). This change in mechanism is evidenced bythe photolysis of 1 in the presence of radical scavengers(halocarbons, thiophenol) which yields thecorresponding products from atom abstraction by 3 that hasescaped the initial radical cage. These escapeproducts are absorptively polarized at high field(T0−S) but are emissively polarized at lowerfield (T-−S)in a particularly well-defined display of the effect of magnetic fieldstrength on intersystem crossing mechanismfor a geminate radical pair. Photolysis of 1 inC6D6 solution yields emissively polarizeddimethylp-acetylbenzylphosphonate (4) from combination ofradical pair [2,3] in the initial solvent cageand followingdiffusive formation of [2,3] free pairs.Cage recombination and disproportionation of secondary Fpairs,comprised of 2 and the phosphorus-substitutedcyclohexadienyl radical (5), affords several emissivelypolarizedproducts (6, 7, and 9) via theT0−S mechanism at both magnetic fields. Thepolarization of 4 remains emissiveat both magnetic fields regardless of the presence of radicalscavengers, a finding which is argued to beconsistent with the above results.
|
