Excited-state dynamics of organic radical ions in liquids and in low-temperature matrices

The excited-state dynamics of the radical cations of perylene (PE.-), tetracene (TE.-), and thianthrene (TH.+), as well as the radical anions of anthraquinone (AQ(.-)) and tetracenequinone (TQ(.-)) formed by gamma irradiation in low-temperature matrices (PE.+, TH.+, AQ(.-), and TQ(.-)) or by oxidation in sulfuric acid (PE.+, TE.+, and TH.+) have been investigated using ultrafast pump-probe spectroscopy. The longest ground-state recovery time measured was 100 ps. The excited-state lifetime of PE.+ is substantially longer in low-temperature matrices than in H2SO4, where the effects of perdeuteration and of temperature on the ground-state recovery dynamics indicate that internal conversion is not the major decay channel of PE.+*:. The data suggest that both PE.+* and TE.+* decay mainly through an intermolecular quenching process, most probably a reversible charge transfer reaction. Contrarily to AQ(.-*), TQ(.-*) exhibits an emission in the visible which, according to theoretical calculations, occurs from an upper excited state.