Sensitized photolysis of iodonium salts studied by CIDNP. Solvent dependence and influence of lipophilic substituents

The sensitized-photoreactions of diphenyliodonium salt and four of its tech nically relevant derivatives bearing lipophilic substituents were investiga ted by pseudo-steady-state and time-resolved CIDNP experiments. Singlet (na phthalene, diphenylanthracene, dimethylanthracene) and triplet (xanthone, t hioxanthone, Michler's ketone) sensitizers were used in a variety of solven ts (acetonitrile, dimethyl sulfoxide, dimethylformamide, methanol, chlorofo rm, tetrahydrofurane, diethylene glycol dimethyl ether, dioxane, toluene). Under all conditions, the primary step leading to CIDNP was found to be ele ctron transfer from the excited sensitizer *Sens to the onium cation On(+). All spin-polarization effects could be explained consistently within the f ramework of radical pair theory (S-T-0-type CIDNP). Pair substitution, i.e. , the transformation of the primary radical pairs RP I (On(.)Sens(.+)) into secondary pairs RP 2 (Ph(.)Sens(.+), where Ph. is the phenyl or an aryl ra dical) plays a key role for the CIDNP effects and even leads to a field dep endence of the polarization phases for the system diphenyliodonium cation/n aphthalene in dioxane. Decreasing solvent polarity causes an increase of th e rate RP 1 --> RP 2. The introduction of a long lipophilic side chain into the onium salt has the same effect, presumably owing to self-solvation.