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

Abstract

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 kq values calculated using ΔG⧧ from the Levine expression and the kd value of 5.4 × 1010 M-1 s-1 from the Smoluchowski expression are in good agreement with the experimental kq values. The observation of the TPTP• signal at 550 nm and the correlation of kq with ΔGet 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 kb obtained from the radical yield values are compared with the kb values calculated using the semiclassical expression.