Igor V. Khudyakov

Deactivation processes of the lowest excited state of [UO2(H2O)5]2+ in aqueous solution

A detailed analysis of the photophysical behaviour of uranyl ion in aqueous solutions at room temperature is given using literature data, together with results of new experimental and theoretical studies to see whether the decay mechanism of the lowest excited state involves physical deactivation by energy transfer or a chemical process through hydrogen atom abstraction. Comparison of the radiative lifetimes determined from quantum yield and lifetime data with that obtained from the Einstein relationship strongly suggests that the emitting state is identical to that observed in the lowest energy absorption band. From study of the experimental rate and that calculated theoretically, from deuterium isotope effects and the activation energy for decay support is given to a deactivation mechanism of hydrogen abstraction involving water clusters to give uranium(v) and hydroxyl radicals. Support for hydroxyl radical formation comes from electron spin resonance spectra observed in the presence of the spin traps 5,5-dimethyl-1-pyrroline N-oxide and tert-butyl-N-phenylnitrone and from literature results on photoinduced uranyl oxygen exchange and photoconductivity. It has previously been suggested that the uranyl emission above pH 1.5 may involve an exciplex between excited uranyl ion and uranium(v). Evidence against this mechanism is given on the basis of quenching of uranyl luminescence by uranium(v), together with other kinetic reasoning. No overall photochemical reaction is observed on excitation of aqueous uranyl solutions, and it is suggested that this is mainly due to reoxidation of UO2+ by hydroxyl radicals in a radical pair. An alternative process involving oxidation by molecular oxygen is analysed experimentally and theoretically, and is suggested to be too slow to be a major reoxidation pathway.

Study of luminescent forms of the uranyl ion

Luminescence spectra and luminescence decay kinetics of uranyl sulphate water and uranyl nitrate acetone solutions of different concentrations have been studied. Similar experiments have been done with uranyl sulphate powder under vacuum. It has been experimentally shown that the hydrolysis of uranyl sulphate in water takes place, and under low salt concentrations (0.1‐4.0 times 10‐4M) a luminescence of a basic form of the photoexcited ion with a tentative structure of UO2OH+* has been observed. The luminescence of the acidic form UO+* has been observed under higher salt concentrations (1–4 times 10‐2M) in water and under any salt concentration in acetone. The acidic form has the characteristic emission spectrum possessing vibrational structure. The luminescence concentrational quenching of both photoexcited uranyl forms and exciplex emission have not been observed. The effect of a number of organic quenchers and molecular oxygen on uranyl luminescence has been studied. There is no luminescence quenching by O2 up to 2 times 106 Pa (20 atm) pressure. The low effectiveness of energy transfer from the photoexcited uranyl forms has been explained in terms of strong steric screening of 5f‐uranium (VI) orbital by oxygen atoms and by external filled up uranium electronic shells.

ON THE URANYL ION LUMINESCENCE IN AQUEOUS SOLUTIONS

Both dynamic and steady-state luminescence methods have been applied to study the emission of aqueous solutions of uranyl ions (UO22+) to attempt to reconcile the various models currently presented for this system. From flash photolysis measurements, it is shown that the emitting ∗UO22+ state is fully formed within the lifetime of a 30 ps laser pulse. Emission spectra are presented at room temperature and in a low temperature glass for solutions at low pH, and assigned to luminescence from ∗[UO2(H2O)5]2+. On increasing pH (1.6<pH<3) broadening of the emission bands, changes in emission quantum yields, and red shifting of maxima are observed. There are no changes in ground state absorption spectra in this region. The changes are accompanied by the onset of biexponential kinetics in the luminescence decay, and are attributed to acid-base dissociation in the excited state. In agreement with previous reports, changes observed at higher pH and uranyl concentration are assigned to the onset of hydrolysis of uranyl in the ground state.

Magnetic field effects on radical recombination in a cage and in the bulk of a viscous solvent

Abstract The microsecond flash photolysis of benzophenone, p-quinones, several flavins and the steady-state photolysis of anthraquinone-9.10 in binary mixtures water-glycerol and alcohol-glycerol of various viscosity (1

Magnetic and spin effects in the photoinitiation of polymerization

Application of a moderate external magnetic field increases the efficiency of triplet photoinitiators of polymerization. The reasons for this observation are briefly outlined. Such well-documented magnetic and spin effects as the magnetic isotope effect (MIE) and chemically induced dynamic nuclear and electron polarization (CIDNP and CIDEP) have been observed under the photolysis of photoinitiators. CIDEP results are emphasized. The study of CIDEP under the photolysis of initiators allows the detection and identification of free radicals of initiators and the spin multiplicity of the reacting photoinitiator (triplet or singlet). The transfer of the spin polarization of the initiator free radical to monomers is a promising technique for following radical reaction pathways. The results of a CIDEP study of the sensitized decomposition of phosphine oxide photoinitiators are presented.

Detailed equilibrium principle in reversible bimolecular diffusion-controlled reactions

Abstract The expressions for the observed rate constants of the forward bimolecular and back monomolecular reactions have been obtained using the boundary conditions imposed upon the reaction surfaces of each reagent particle. The rate, equilibrium and thermodynamical parameters of the back dissociation reactions of phenoxy radical dimers have been measured. The resultant values of k obs −1 as a function of solvent viscosity are accurately described by theoretical relationships obtained in the present work.

Chemically induced dynamic nuclear polarization in the photo-oxidation of benzylic acid with uranyl nitrate

Abstract The photo-oxidation of benzylic acid and benzhydrol by uranyl nitrate was studied using the 1 H chemically induced dynamic nuclear polarization technique. The photo-oxidation of both compounds leads to the formation of a polarized reaction product, i.e. benzophenone. The signs of polarization are the same in both cases. It is concluded that the reactions proceed via triplet radical pairs.

A laser flash photolysis study of magnetic field effects in photoinduced electron transfer between Ru (bpy)2+3 and N,N′-dimethylviologen in micellar solutions

Abstract The kinetics of electron transfer between photoexcited Ru(bpy) 2+ 3 and N,N′-dimethylviologen (MV 2+ ) have been studied by means of laser flash photolysis in sodium dodecyl sulfate (SDS) and sodium laurate (SL) micelles. The decay of photoexcited Ru(bpy) 2+ 3 follows first-order kinetics with k obs ∼ 10 6 –10 7 s −1 , and dependences of k obs versus surfactant and quencher concentrations were obtained. Analysis of the data showed that the quenching is mainly intramicellar. Although SDS solutions shows no escape of the radicals MV + and Ru(bpy) 3+ 3 from the micelles, for solutions of SL micelles an escape value Φ es ∼ 0.08 was determined. The dependences of Φ es on the concentration of SL and SDS show a drop in the vicinity of the cmc. Radical MV + is not incorporated into the SL micelles. An increase of Φ es in the SL micelles up to 20–25% was observed under application of an external magnetic field ( B = 0.2–0.47 T) during steady-state irradiation and laser flash photolysis studies. The magnetic field effect is mainly described according to a hyperfine coupling mechanism. Geminate recombination kinetics of the pair 3 [MV + Ru(bpy) 3+ 3 ] was observed in both SDS and SL micelles. Pertinent kinetic analyses were made and the requirements for geminate recombination kinetics observations were discussed.

Kinetics and mechanism of the photochromic transformations of N-salicylidene-4-hydroxy-3,5-dimethylaniline and its complex with uranium(VI) dioxide

Abstract The title compounds were prepared and some of their photophysical and photochemical properties studied. The anil (LH) and its complex with uranium (C) were found to possess very similar spectral and kinetic characteristics, namely absorption and luminescence spectra, photoisomer spectra and isomerization kinetics. From these measurements, it is concluded that the photoexcitation of the compounds in polar solvents (methanol and acetonitrile) leads to proton transfer in the excited sinlet state with the formation of a trans zwitterionic structure. The trans zwitterion emits light and undergoes isomerization to form the cis zwitterion. The latter compound is a photochromic intermediate, which participates in cis-trans isomerization (rate constant k iso = (3–4) × 10 4 s −1 ) with the formation of the initial anil or the uranyl complex. An energy-level scheme of the reactions is presented.

Time-resolved EPR study of singlet oxygen in the gas phase.

X-band EPR spectra of singlet O2((1)Δg) and triplet O2((3)Σg(-)) were observed in the gas phase under low molecular-oxygen pressures PO2 = 0.175-0.625 Torr, T = 293-323 K. O2((1)Δg) was produced by quenching of photogenerated triplet sensitizers naphthalene C8H10, perdeuterated naphthalene, and perfluoronaphthalene in the gas phase. The EPR spectrum of O2((1)Δg) was also observed under microwave discharge. Integrated intensities and line widths of individual components of the EPR spectrum of O2((3)Σg(-)) were used as internal standards for estimating the concentration of O2 species and PO2 in the EPR cavity. Time-resolved (TR) EPR experiments of C8H10 were the main focus of this Article. Pulsed irradiation of C8H10 in the presence of O2((3)Σg(-)) allowed us to determine the kinetics of formation and decay for each of the four components of the O2((1)Δg) EPR signal, which lasted for only a few seconds. We found that the kinetics of EPR-component decay fit nicely to a biexponential kinetics law. The TR EPR 2D spectrum of the third component of the O2((1)Δg) EPR spectrum was examined in experiments using C8H10. This spectrum vividly presents the time evolution of an EPR component. The largest EPR signal and the longest lifetime of O2((1)Δg), τ = 0.4 s, were observed at medium pressure PO2 = 0.4 Torr, T = 293 K. The mechanism of O2((1)Δg) decay in the presence of photosensitizers is discussed. EPR spectra of O2((1)Δg) evidence that the spin-rotational states of O2((1)Δg) are populated according to Boltzmann distribution in the studied time range of 10-100 ms. We believe that this is the first report dealing with the dependence of O2((1)Δg) EPR line width on PO2 and T.