Femtosecond fluorescence upconversion and transient absorption measurements reveal that a water molecule acts as both electron donor and proton donor in response to photoexcitation of the powerful photooxidant methyl viologen. Emission following UV excitation of methyl viologen decays in 3.2 ps in H2O and 5.3 ps in D2O. Transient absorption signals decay with the same time constants to a long-lived difference spectrum that decays on the microsecond time scale and is insensitive to dissolved oxygen. The long-lived species is scavenged at a nearly diffusion-limited rate by nitric acid and has the same difference spectrum as methyl viologen in 2.0 M NaOH. Photoexcitation of methyl viologen thus leads to the ultrafast formation of a stable ion pair between ground-state methyl viologen and a hydroxide ion. The latter ion is produced by proton-coupled electron transfer oxidation of a nearby water molecule. Electron transfer from water to the excited viologen is accompanied by deprotonation of the water hole, yielding hydroxyl radical and a proton that is most likely expelled to the bulk solvent by Grotthus proton conduction. Thermodynamics and theoretical considerations suggest that electron and proton transfer take place in a concerted step. Back oxidation of reduced methyl viologen by the hydroxyl radical is believed to occur on a subpicosecond time scale to yield the final ion pair. It is proposed that a similar mechanism describes the photoreduction of some metal ions by water.This work was supported in part by DOE Grant DE-FG02-06ER15776.

Proton-coupled electron transfer oxidation of water triggered by UV excitation of methyl viologen

244th National Meeting (2012)

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