TRIPLET QUENCHING BY ONIUM SALTS IN POLAR AND NONPOLAR-SOLVENTS

The tripler quenching of several sensitizers (thioxanthone (TX), 2-chl orothioxanthone (CTX), xanthone (X), benzophenone (BP), pyrene (PY), b enzil (BZ), and phenothiazine (PT)) was investigated with lipophilic s ubstituted onium salts (iodonium I-(1) and sulfonium salts S-(1)) in b enzene, n-heptane, and mixtures of these. Moreover, the triplet quench ing of these sensitizers was also studied in acetonitrile with onium s alts I-(2) and S-(2) with a similar pattern of substitution to the lip ophilic salts. Using the determined half-wave reduction potential of t he onium salts, one can show that an efficient tripler quenching by el ectron transfer is possible with the exception of BP and BZ. The quenc hing constants obtained fit the theoretical Rehm-Weller plot. The qual ity of the fit is better in acetonitrile than in n-heptane. Using TX a nd CTX cation radicals are detectable, which support a triplet quenchi ng by electron transfer. Nevertheless, the triplets of BP and BZ were also efficiently quenched by the onium salts. The quenching reaction b y sulfonium salt is hardly influenced from the polarity of the solvent used. However, the iodonium salt shows a strong dependence on the pol arity: for instance in benzene the rate is one order of magnitude high er than in n-heptane. Moreover, the magnitude of triplet quenching in benzene is for the iodonium salt one order higher than for the sulfoni um salt. Surprisingly, we found (with exception of PT and PY) in n-hep tane no strong differences in quenching between iodonium and the sulfo nium compounds. (C) 1997 Elsevier Science S.A.