Excited-State Dynamics of Organic Radical Ions in Liquids and in Low-Temperature Matrices

Abstract

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 H 2 SO 4 , 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.

Investigations of bimolecular photoinduced electron transfer reactions in polar solvents using ultrafast spectroscopy

“…There is, however, no spectroscopic evidence for the formation of such an excited Pe •+ */TCNE •− product. Transient spectra measured after the excitation of Pe •+ at 532 nm in sulfuric acid show that Pe •+ * does not absorb significantly between 470 and 750 nm [80]. Moreover, ground state recovery measurements of Pe •+ after excitation at 532 nm indicate an excited state lifetime of 3 ps [81].…”

“…Indeed, only a very small number of radical ions are known to fluoresce [80,[92][93][94][95][96][97]. Moreover, the very few investigations on their excited state dynamics have shown that the ground state recovery of radical ions is ultrafast and takes place in picosecond timescale [80,81,98,99]. This result, together with a relatively small oscillator strength for the D 0 -D 1 transition of many radical ions, can explain this lack of fluorescence, which moreover should often occur in the near IR region.…”

Fluorescence of Radical Ions in Liquid Solution: Wurster’s Blue as a Case Study

“…However, their excited-state properties are still mostly unknown. [3] Only a very few of them have been reported to fluoresce, [4][5][6] making the detection of electronically excited radical ions extremely difficult. The lack of emission is usually explained by a very efficient internal conversion favored by a small energy gap between the first electronic excited state (D 1 ) and the ground state (D 0 ), and/or by the presence of D 2 / D 1 and D 1 /D 0 conical intersections (CI).…”

Photoinduced Electron Transfer Reactions: From the Elucidation of Old Problems in Bulk Solutions Towards the Exploration of Interfaces

“…All the radical ions that we have studied so far are characterised by very short excited-state lifetimes, ranging from ~100 fs to a few ps, and by weak excited-state absorption features. [20][21][22][23] Therefore, if excited radical ions are formed in a bimolecular CS, their detection is extremely difficult. Substantial efforts in this direction are being invested.…”