Optical Generation of Hydrated Electrons from Aromatic Compounds

Abstract

A flash photolysis investigation of aromatic compounds has shown that photoionization in aqueous solution occurs for a number of benzene derivatives with electron-donating substituents, aryl carboxylic acids, and hetero cyclics with five-member rings. Thermo dynamic estimates for several benzene deriva tives indicate that electron ejection is feasible and is competitive with bond rupture. It is sug gested that the lowest π-π* state of the ring sys tem populates a charge transfer intermediate which can release an electron to the solvent. Comparing the hydrated electron yields with scavenging rate constants show that the molecules which are not readily photoionized in solution tend to react most rapidly with hydrated electrons. photoionization in condensed systems has been considered for many years in connection with the absorption spectra of halide ions in solu tion and the photolysis of aromatic compounds in rigid glasses. Recently, Jortner et al. (15) proposed that hydrated electrons are an initial product of the ultraviolet-light photolysis of aqueous phenolate ion, based on the effects of scavengers on the quantum yield. This explanation was con firmed by the work of Grossweiner and co-workers (10, 29) who reported a direct observation of the hydrated electron absorption band by flash photolysis of phenol derivatives and aromatic amino acids. A more detailed investigation of aqueous phenol and the cresols by Dobson and Grossweiner (6) showed that hydrated electrons are an initial photo chemical product of both the neutral molecules and the anions, although the initial yields from the former were lower by an order of magnitude. The purpose of this investigation was to survey representative aromatic compounds for optical generation of the hydrated electron, in order to clarify the relationship between molecular structure and photoionization 279 Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 18, 2014 |

Nonexponential fluorescence decay of aqueous tryptophan and two related peptides by picosecond spectroscopy

“…Single exponential decays were fit to a function of the form I(t) = A exp(-t/T)+B [1] in which I(t) is the calculated intensity at the time t, A is the amplitude, r is the fluorescence lifetime, and B is a baseline parameter. The nonexponential decays could be fit well by the sum of two exponentials, [2] in which T1 and T2 are the lifetimes of the two components and f is the weight of the component having lifetime Tr.…”

“…It has long been known (2) that irradiation of aqueous tryptophan and other indole derivatives in the 280-nm absorption band leads to the formation of solvated electrons. Although there is still some uncertainty about the yield (3), because of the presence of fast recombination of the ejected electron with the counter radical cation, this photoionization is generally thought to be monophotonic and a major nonradiative pathway (4).…”

Cinétique de phototransformation de polluants organiques émergents en solution aqueuse diluée

“…Lors de l'excitation de dérivés phénoliques, le processus primaire suivant l'excitation est connu comme faisant intervenir un mécanisme de photo éjection d'électrons à partir de l'état excité du phénol (JOSCHEK et GROSSWEINER, 1966). Un état intermédiaire où les trois espèces radicales phénoxyle-électron-proton sont potentiellement formées est généralement proposé.…”

The Luminescence of the Aromatic Amino Acids

“…(49,92) The most direct evidence for the existence of this mechanism is derived from flash photolysis studies, which have shown that ultraviolet irradiation of aqueous solutions of indole and its derivatives, including tryptophan, results in formation of solvated electrons. (93,94) While accurate measurements of the quantum yield of solvated electrons will be required to assess the quantitative significance of this mechanism, there is considerable indirect evidence that it may be an important factor in the radiationless deactivation of the indole excited state. The quantum yield of indole in water is strongly temperature dependent.…”