Femtosecond charge separation in organized assemblies: free-radical reactions with pyridine nucleotides in micelles

Abstract

Femtosecond laser UV pulse-induced charge separation and electron transfer across a polar interface have been investigated in anionic aqueous micells (sodium lauryl sulfate) containing an aromatic hydrocarbon (phenothiazine). The early events of the photoejection of the electron from the micellized chromophore and subsequent reaction of electron with the aqueous perimicellar phase have been studied by ultrafast infrared and visible absorption spectroscopy. The charge separation (chromophore +...e-) inside the micelle occurs in less than 10(-13) s (100 fs). The subsequent thermalization and localization of the photoelectron in the aqueous phase are reached in 250 fs. This results in the appearance of an infrared band assigned to a nonrelaxed solvated electron (presolvated state). This transient species relaxes toward the fully solvated state of the electron in 270 fs. In anionic aqueous micelles containing pyridine dinucleotides at high concentration (0.025-0.103 M), a single electron transfer can be initiated by femtosecond photoionization of phenothiazine. The one-electron reduction of the oxidized pyridine dinucleotide leads to the formation of a free pyridinyl radical. The bimolecular rate constant of this electron transfer depends on both the pH of the micellar system and the concentration of oxidized acceptor. The free-radical reaction is analyzed in terms of the time dependence of a diffusion-controlled process. In the first 2 ps following the femtosecond photoionization of PTH inside the micelle, an early formation of a free pyridinyl radical is observed. This suggests that an ultrafast free-radical reaction with an oxidized form of pyridine nucleotide can be triggered by a single electron transfer in less than 5 X 10(11) s-1.

FT-EPR study of photoionization in micellar solutions

“…DQ is partitioned between the lipophilic and aqueous phase [13], AQ is expected to be in the aqueous phase, whereas the long alkyl chain of IB makes it likely that this quinone is anchored in the micelle. The FT-EPR study shows that the difference in spatial distribution affects the probability of electron capture and supports the conclusion that the rate of electron escape into the bulk aqueous phase is high [11]. Figure 6 displays FT-EPR spectra given by a micellar solution of PTH containing DQ (10 -3 M) as electron scavenger.…”

“…The absence of Radical Pair Mechanism (RPM) CIDEP [10] originating from the dominant singlet-photoionization channel is in agreement with what is known about the dynamics of photoionization of aromatics solubilized in SDS micelles. Femtosecond transient absorption measurements on PTH/SDS show that fully solvated electrons are generated by laser excitation of PTH within one picosecond [11]. This means that the [PTH +, eJ radical pair lifetime is too short for RPM CIDEP to develop.…”

“…This is attributed to the fact that in the latter medium the electron escapes instantaneously [11] into the bulk aqueous phase whereas PTH § remains inside the micelle so that the recombination reaction is strongly attenuated. With the acceptors used here, a range of spatial distributions is covered.…”

“…First, as noted earlier, we assume that the quinone is anchored, by means of its alkyl tail, in the SDS micelle. As a consequence, it is notan efficient scavenger of the photoelectrons whieh have been shown [11] to escape with extremely high rate into From earlier studies of electron transfer in micellar solutions [14], we know that this is a relatively slow process which, because both radicals are trapped inside the micelle, can produce long-lived, spin-correlated, radical pairs, [PTH+...IB-]. Dissociation of these radical pairs produces the emissive spectrum observed at long delay times.…”

Électrons en excès dans les milieux polaires homogènes et hétérogènes

“…Dynamique de formation de e,, en micelles directes ou inverses La production de e-,, par ionisation biphotonique (4 eV/photon) de phknothiazine a Ct C suivie, comme pour l'eau pure, sous excitation laser femtoseconde et spectroscopie d'absorption rCsolue dans le temps (178)(179)(180). Un Ctat prCsolvatt absorbant dans l'infra-rouge, prkcurseur de e-,,, a CtC mis en evidence dans ce cas Cgalement.…”