Absorption/Electron Transfer/Absorption— An Efficient Pathway to Hydrated Electrons in Laser Flash Photolysis

Abstract

The solvated (hydrated) electron is a key intermediate in pulse radiolysis.['l It can also be formed in laser flash photolysis by the absorption of a single or, more frequently, by the successive absorption of two In all examples of the latter pathway reported so far, the intermediate that is ionized by the second photon has invariably been an excited state produced by absorption of the first photon. In this work, we present the first instances of photoionization by an absorptionlelectron transfedabsorption mechanism, that is, where the second photon is absorbed by a radical ion. As we will show, this pathway to hydrated electrons is quite efficient; moreover, it could be fairly common in laser flash photolysis and thus have consequences for mechanistic interpretations. For detection we used Fourier Transform (FT) EPR spectro~copy.[~l Most of the investigations were carried out with the system anthraquinone-l,5-disulfonate/methionine (AS/Met). The experiments were performed in aqueous solution at p H x l l , that is, well above pKaz of the amino acid.[5] Intersystem crossing of photoexcited 'AS leads to the formation of a spin-polarized triplet state 3AS,[6al which is characterized by an electron spin relaxation time of 1 ns.[6bI From experiments with varying concentration of the amino acid, the rate constant for electron transfer quenching of 3As by Met was estimated to be higher than 5 x lo9 M I S I . Under our experimental conditions (typically, [Met] = 0.1 M) the life of 3AS is thus decreased to well below 1 ns, and at least 30% of its spin polarization is transferred to the radical ions AS'and Met-+, which are the primary products of electron transfer quenching. Quinone concentrations below 5 x M were chosen to avoid self-quenching of 3AS and EPR line broadening due to spin-exchange, as well as to ensure homogeneous generation of the radicals (the optical density of the samples is smaller than 0.1 at the excitation wavelength of 308 nm). It was ascertained that the light is absorbed only by the quinone despite the disparately larger concentration of the quencher. Figure 1 shows FT-EPR spectra of this system obtained at two different delay times between laser flash and acquisition pulse. The easily identifiable emission signal[6a] of the semiquinone radical anion AS'is found in all these measurements, whereas no signals of methionine-derived radicals are visible.171 The most notable feature of the spectra in Figure 1 is the occurrence of an emissively polarized singlet EPR line. This peak is only observed at low quinone concentration (55 x M); its intensity relative to the AS'signal increases with decreasing quinone concentration. Its g value of 2.00055 is indicative 131 of a hydrated electron. This assignment was rendered beyond doubt by experiments with