Investigation of the Excited-State Dynamics of Radical Ions in the Condensed Phase Using the Picosecond Transient Grating Technique

Abstract

A study of the dynamics of ground-state recovery of the perylene radical cation (Pe•+), of perylene radical anion (Pe•-), and of anthraquinone radical anion (AQ•-) is reported. In boric acid glass, the excited-state lifetime of Pe•+ is 35 ± 3 ps, while in concentrated sulfuric acid, it is smaller than 15 ps, the time resolution of the experimental setup. The excited-state lifetime of Pe•+, Pe•-, and AQ•- generated by photoinduced intermolecular electron-transfer reaction in MeCN is shorter than 15 ps. In the case of Pe•-, the uncomplete ground-state recovery is ascribed to the occurrence of electron photoejection. The free ion yield in the intermolecular electron-transfer reaction between 9,10-dicyanoanthracene (DCA) and two electron acceptors was measured in a two-pulse experiment, where the second pulse excited the ensuing DCA•-. This excitation has no influence on the magnitude of the free ion yield, indicating a short excited-state lifetime of DCA•-* relative to the time scale of back electron transfer and ionic dissociation. A red emission, ascribed to the fluorescence of protonated Pe, was detected in boric acid glass and sulfuric acid. No fluorescence that could be clearly ascribed to Pe•+* could be observed.

Bimolecular photoinduced electron transfer reactions in liquids under the gaze of ultrafast spectroscopy

“…The reason for this lies mostly in their elusive nature, having excited-state lifetimes of a few picoseconds or less. 152,[154][155][156][157][158][159][160] This property, in combination with the finite rate of their production upon CS, prevents the build-up of significant and detectable transient concentrations. However, strong evidence of the formation of excited ions has recently been obtained by measuring, using transient IR absorption spectroscopy, how the energy dissipated upon ultrafast CS and CR is redistributed into the vibrational modes of the reaction partners.…”

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

“…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.…”

An “All‐Green” Catalytic Cycle of Aqueous Photoionization

“…[1] Having in mind the ultimate aim of utilizing the sun as the (low-flux) light source, it is the most promising strategy to employ two successive single-photon absorptions and store the energy of the first photon in an intermediate. Examples of electron detachment by green light are known for all common classes of photochemical intermediates, excited singlet states, [2] triplet states, [3,4] radicals, [5][6][7] and radical anions, [8][9][10][11][12] but the longer such an intermediate lives, the more likely it is to absorb the ionizing second photon. This suggests that the usefulness for this process increases in the order excited singlet (ns) < triplet (ms) < radical or radical anion (no photophysical deactivation).…”