Triplet quenching by onium salts in polar and nonpolar solvents

The tripler quenching of several sensitizers (thioxanthone (TX), 2-chlorothioxanthone (CTX), xanthone (X), benzophenone (BP), pyrene (PY), benzil (BZ), and phenothiazine (PT)) was investigated with lipophilic substituted onium salts (iodonium I-(1) and sulfonium salts S-(1)) in benzene, n-heptane, and mixtures of these. Moreover, the triplet quenching of these sensitizers was also studied in acetonitrile with onium salts I-(2) and S-(2) with a similar pattern of substitution to the lipophilic salts. Using the determined half-wave reduction potential of the onium salts, one can show that an efficient tripler quenching by electron transfer is possible with the exception of BP and BZ. The quenching constants obtained fit the theoretical Rehm-Weller plot. The quality of the fit is better in acetonitrile than in n-heptane. Using TX and CTX cation radicals are detectable, which support a triplet quenching by electron transfer. Nevertheless, the triplets of BP and BZ were also efficiently quenched by the onium salts. The quenching reaction by sulfonium salt is hardly influenced from the polarity of the solvent used. However, the iodonium salt shows a strong dependence on the polarity: for instance in benzene the rate is one order of magnitude higher than in n-heptane. Moreover, the magnitude of triplet quenching in benzene is for the iodonium salt one order higher than for the sulfonium salt. Surprisingly, we found (with exception of PT and PY) in n-heptane no strong differences in quenching between iodonium and the sulfonium compounds. (C) 1997 Elsevier Science S.A.