Physical and chemical quenching of excited uranyl ion by organic molecules studied by fluorimetric and laser flash photolysis methods

Abstract

The quenching by a large number of carboxylic acids of the luminescence of excited uranyl ion has been examined. Acetic and propionic acids display weak quenching as measured by the Stern-Volmer constant KBV, but branching or lengthening of R in RC02H increases K, , to figures of up to 1 5 dm3 mol-l. Cycloalkanecarboxylic acids show considerably higher Ksv. The introduction of halogen atoms introduces diverse effects : CF,CO,H and CC13C0,H increase the luminescence intensity very strongly, CICH,CO,H and BrCH,CO,H weaken it very slightly, ICH,CO,H powerfully quenches it (KNV ca. 2 000 dm3 mol-l). Introduction of CI or Br a t the 2-or 3-positions of propionic acid exerts moderate to strong quenching which is ascribed to a physical, rather than a chemical mechanism, a result also indicated by studies of quenching by various alkyl halides. The introduction of alkoxy-groups increases K,, by a factor of 200, but this has been found to be due to a remsrkably high sensitivity of ether groups to (U02,+) * as further evinced by a high quantum yield for UIV production. The introduction of C=C bonds in strategic positions also enormously increases Ksv (up to 1 600 dm3 mol-I) but this w a s also found to be due to the C=C bond itself, and a number of alkenes were found to exhibit quenching rates approaching diffusion control, presumably via an exciplex mechanism. Substituted benzoic acids quench (UO,,+) most effectively (Ksv up to 6 000 dm3 mol-l) and the rates follow a good Hammett correlation, ( p -0.88 f 0.04). In some instances, the quenching data have been augmented by quantum yield measurements, and absolute constants for the reaction between (UOZ2+) * and substrate have been obtained directly by monitoring the absorption of (UO,") at 590 nm using ns flash p hotol ysis.THE several detailed studies of the quenching of the well known, strong emission from aqueous uranyl ion (U022+) by a variety of aliphatic alcohols due to Matsushima and his colleagues 1-4 has enabled a comprehensive picture to be built up of the primary interaction between (UOZ2+)* and substrate in the photochemical reaction. The effects of structural variation and isotopic substitution indicate the quenching to be chemical in character, involving the abstraction of an a-hydrogen atom from a :CHOH or -CH,OH group to give a transient alcohol radical together with the intermediate aquouranium(v) specie^.^This mechanism has been supported by solution6 and matrix e.s.r. experiments on the U0,2+-alcohol system in addition to spin-trapping experiments a and laser flash photolysis studies of the absorption of (UOZ2+)* in this s y ~t e m . ~ In view of the several previous studies of the UVI photo-oxidation of carboxylic acids.1° including product analysis,ll and e.s.r. ~o r k , ~. ~ which present quite different features from those of the alcohols, we have made a systematic examination of the quenching of the luminescence of (UOz2+)* by carboxylic acids. Additional factors of interest were (i) the nature of the firimary radical produced from RCH,CO,H, which mig...

Uranyl-Sensitized Photochemical Oxidation of Naphthalene by Molecular Oxygen. Role of Electron Transfer

“…The 70% of *UO 2 2+ that does not yield any observable intermediates presumably takes place by exciplex formation, eq 6b, as suggested previously for other unsaturated compounds. 3,22 The experiments were carried out in 0.1 M H 3 PO 4 , a medium where phosphate complexes of UO 2 2+ and *UO 2 2+ predominate 2 and almost certainly take part in reaction 6 and beyond. For the sake of simplicity, the coordinated phosphate ions are not shown.…”

Spectroscopic study on uranyl carboxylate complexes formed at the surface layer of Sulfolobus acidocaldarius

“…This likely was caused by the low amount of U(VI) bound to the S-layer and/or due to a quenching of the uranyl luminescence by carboxylate functional groups which is caused by an energy dissipation. 32 In order to reduce static quenching effects and therewith increase the sensitivity of this spectroscopic method we performed additional TRLF measurements at cryogenic conditions (153 K). As evident from the spectra presented in Fig.…”

Uranyl Photochemistry with Alkenes: Distinguishing between H-Atom Abstraction and Electron Transfer

“…The observation of U(V) identifies the quenching mechanism as a one-electron process, but does not discriminate between electron transfer and H-atom abstraction. Other reports show that quenching rates vary with ionization potential (IP) in the case of aromatic molecules, suggesting that electron transfer is the mechanism of quenching. − We report here on a series of quenching studies between uranyl and a variety of alkene substrates that unequivocally demonstrate quenching of the uranyl excited state with alkenes occurs by electron transfer.…”

“…Recent studies with the uranyl ion (UO 2 2+ ) have shown that it has the potential to photocatalytically oxidize organic substrates in the presence of air. − The excited-state UO 2 2+* is a potent oxidant ( E ° = 2.6 V), and is quenched by a variety of organic substrates. ,− The resulting U(V) species can then be oxidized back to UO 2 2+ in the presence of oxygen . Previous studies with alcohols have shown, through kinetic isotope effects, that the quenching of the uranyl excited state occurs by hydrogen atom abstraction to give UO 2 H + and an organic radical. ,, The mechanism of quenching with alkenes has not been definitively determined. − Proposals for quenching mechanisms with alkenes have included exciplex formation, H-atom abstraction, and electron transfer. Recent work identified a U(V) species as a quenching product in the absence of oxygen, and suggested H-atom abstraction as the quenching mechanism …”