CIDNP investigation of radical decay pathways in the sensitized photolysisof triphenylsulfonium salts

The photodecomposition of triphenylsulfonium hexafluoroantimonate was studi ed by pseudo-steady-state and time-resolved CIDNP experiments, using naphth alene and 9,10-dimethylanthracene as (singlet) sensitizers. Key intermediat es of these reactions are radical pairs consisting of the sensitizer radica l cation Sens(.+) and the phenyl radical Ph-., which are formed by photoind uced electron transfer followed by cleavage of the resulting neutral onium radical. The chemical fate of Ph-. is strongly influenced by the sensitizer . With 9, 10-dimethylanthracene, in-cage hydrogen abstraction from the meth yl groups of Sens(.+) produces benzene and a carbocation that can function as a protic acid; free radicals Ph-. attack surplus sensitizer to give addi tion products. With naphthalene, geminate combination of the two radicals a nd in-cage oxidation of diphenyl sulfide (the by-product of cleavage of the onium radical) by Sens(.+) are observed. In both Systems, the solvent scav enges free Ph-., yielding monodeuteriobenzene. The rate constant of this re action was determined to be 1.2 x 10(4) M-1 s(-1) at 241 K. With 9,10-dimet hylanthracene, the competition of sensitizer and solvent for the free radic als Ph-. was studied quantitatively by evaluating the lineshape of the supe rimposed CIDNP signals of undeuterated and monodeuterated benzene. By a sim ple kinetic model the dependence of the product distribution on temperature and sensitizer concentration could be described with a consistent set of p arameters.