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 γ 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.