Excited Singlet State Reactions of Thiopyrylium with Electron Donors:  Electron Transfer, Induction of Triplet by Internal and External Heavy Atom Effect, and Comparison of Pyrylium and Thiopyrylium Reactions

The free energy dependence on the fluorescence quenching of 2,4,6-triphenylthiopyrylium tetrafluoroborate (TPTP) by a variety of halogenated benzenes, toluenes, and anisoles were studied in acetonitrile. The k q values calculated using Δ G ⧧ from the Levine expression and the k d value of 5.4 × 10 10 M -1 s -1 from the Smoluchowski expression are in good agreement with the experimental k q values. The observation of the TPTP • signal at 550 nm and the correlation of k q with Δ G et expound the possibility of an electron-transfer mechanism. The radical yield and intersystem-crossing rate constants are evaluated using flash photolysis techniques. The influence of sulfur atom in the triplet induction is reflected in the intersystem-crossing rate constant and radical yield values. The recombination rate constants k b obtained from the radical yield values are compared with the k b values calculated using the semiclassical expression.