Photochemical reduction of uranyl ion with ditertiary phosphines in dry acetone

Time-resolved transient absorption spectra have been observed using monochromatic light (=347.1 nm) from a 25 ns pulsed ruby laser. Collision between optically excited uranyl ion and ditertiary phosphines lead to photochemical reduction of uranyl ion to uranium(V) in non-aqueous medium. Stern-Volmer constants measured from lifetime measurement and quantum yields for uranium(V) formation reveal that electron transfer phenomenon competes with photophysical deactivation because of the presence of phenyl groups. Since ditertiary phosphines have two electron donating phosphorus atoms, are better reductant than monodentate phosphines in non aqueous medium.     

Photochemical reduction of uranyl ion with triethylamine

Uranyl ion is photochemically reduced to uranium(IV) in the presence of triethylamine and triethylamine is oxidized to secondary amine and acetaldehyde. On the basis of product analysis, temperature independent quantum yields for uranium(IV) formation and abnormal Stern-Volmer plots rule out the simple collisional photochemical annihilation of excited uranyl ion with triethylamine. Static annihilation has a significant contribution in addition to dynamic annihilation.     

Violation of Vavilov’s law upon excitation of luminescence of uranyl solutions at the short-wavelength absorption band (200–240 nm)

It was found that in the UV spectral region (200–240 nm) where intense absorption bands of the UO ₂ ²⁺ ion are located, excitation of its luminescence in solutions is not observed, thereby contradicting Vavilov’s law about independence of luminescence quantum yields from the excitation light wavelength. The violation of Vavilov’s law is explained in terms of nonradiative deactivation processes as the result of photoinduced electron transfer to the uranyl ion with its reduction to the pentavalent state and the subsequent disproportionation reaction to form uranium(IV). The presence of uranium(IV) ions during UV irradiation of uranyl solutions was proved by the chemiluminescent method.     

Effect of anion coordination upon radiationless transitions of the uranyl ion

At 80 K, where the deactivation processes in uranyl luminescence in solutions are temperature independent, the radiationless transition rate depends upon the presence of H₂O in the first coordination sphere of the uranyl ion, UO₂²⁺. It is found that such radiationless transitions are due to a photophysical intramolecular process.     

Photoelectron spectra of uranium(IV), (V) and (VI) complexes

Satellites were observed on 4f photoelectron spectra of uranium (IV) complexes, while none was seen for diamagnetic uranyl complexes. Photoelectron lines of oxygen 1s coordinated to the uranium ion were broad for NaUO₃ and uranyl complexes.     

Spectrophotometric Determination of Uranium with Sorbohydroxamic Acid as Reagent

Sorbohydroxamic acid forms with uranium an orange red, water soluble complex. The mole ratio of uranyl ion to compound is 1 to 1 under the investigated conditions. The formation constant of this chelate was also determined by the Likussar—Boltz method at a constant ionic strength of 0.1 M at 30°C as 2.10×10². The recommended procedure obeys Beer's law between 3.98ppm and 166.6ppm of uranyl ion at pH 3.8±0.1. Tolerances to cerium (IV) and thorium have been investigated. The procedure for the determination of uranium are made more specific by applying preliminary extraction of uranium by ether.     

Binding of uranyl ion by 2,2′-dihydroxyazobenzene attached to a partially chloromethylated polystyrene

A 2,2′-dihydroxyazobenzene (DHAB) derivative was attached to a chloromethylated cross-linked polystyrene derivative in view of high affinity of DHAB for uranyl ion. Chloromethyl groups of the resin were converted to quaternary ammonium ions by treating with tertiary amines. Capacity of the resins for uranyl-uptake was measured, revealing that about 20 mg of uranium can be complexed to 1 g of the resins. Formation constants (K_f) for uranyl complexes of the resins were determined. In the presence of >0.1 M bicarbonate ion at pH 8.10, log K_f of about 15 was obtained. As bicarbonate concentration was lowered, K_f decreased considerably. Degrees of uranyl-uptake from rapidly flowing uranyl solutions were measured, and the results suggested that rate of uranyl-uptake may not impose a major barrier to application of the resins in uranium extraction from seawater. Uranium extraction from seawater with the resins was carried out on the east coast of Korean peninsula. The amount of uranium extracted from seawater was about 10 µg/g resin. This is not satisfactory for economical processes of uranium recovery from seawater. Results of the present study, however, suggested that modification of the DHAB-containing resins can improve uranyl-binding ability, probably leading to economical recovery of uranium from seawater. In addition, simulation of uranyl-binding processes in seawater with the laboratory procedures developed in this study was satisfactory. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3169–3177, 1999     

Binding of uranyl ion by 2,2′-dihydroxyazobenzene attached to crosslinked polystyrenes covered with highly populated quaternary ammonium cations

2,2′-Dihydroxyazobenzene (DHAB) derivatives were attached to poly(chloromethylstyrene-co-divinylbenzene) (PCD) because of the high affinity of DHAB for uranyl ion. Chloromethyl groups of PCD were converted to quaternary ammonium ions by treating them with tertiary amines. Two strategies were adopted to improve the uranyl-binding ability of the immobilized DHAB: (1) the creation of a highly cationic microenvironment around the DHAB moieties and (2) the introduction of electron-withdrawing groups to DHAB. Capacity of the resins for uranyl uptake was measured, revealing that about 10 to 46 mg of uranium could be complexed to 1 g of the resins. Formation constants (K_f) for the uranyl complexes of the resins were determined. In the presence of ≥0.02 M bicarbonate ion at pH 8.02, log K_f values of 14.3 to 15.8 were obtained. Uranium extraction from seawater with two kinds of resins prepared in this study was carried out on the east coast of the Korean peninsula. The amount of uranium extracted from seawater was up to 150 μg/g resin. Thus, the uranium-extracting capability of the DHAB-containing polystyrene resins was improved significantly by the structural modifications. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4117–4125, 1999     

Zum Mechanismus der photochemischen Umwandlung von 2-Alkyl-indazolen in 1-Alkyl-benzimidazole. II. Photophysikalische und photochemische Primärprozesse

Mechanistic studies on the photoisomerization of 2-alkyl-indazoles into 1-alkyl-benzimidazoles. II. Primary photochemical processes and photophysical deactivation. In the previous paper [1] the structure of the intermediate in the photochemical indazole-benzimidazole-isomerization was discussed ( 3 in Scheme 1). In this communication experiments concerning the photochemical primary processes and photophysical deactivation of 2-alkyl-indazoles ( 1 ) are described. The quantum yield of the rearrangement 1 → 2 (Φ_R) decreases with decreasing temperature while the fluorescence quantum yield (Φ_F) increases and finally reaches a constant value ( ≠ 1) (Fig.10). This behaviour is inconsistent with the mechanism shown in Scheme 2. Photoreaction and fluorescence are both quenched, but not to the same extent, by freon 113 (Fig. 2). In addition the Stern-Volmer-plots are not linear. These observations are best explained by assuming the existence of two excited states in equilibrium (Scheme 3). The mechanism in Scheme 3 correctly explains the quenching experiments and the temperature dependence of Φ_R and Φ_F if the Arrhenius law holds for the two rate constants k_sx and k_R. However, for a quantitative calculation of Φ_R, an additional branching of the reaction pathway must be postulated (Scheme 4). Two-dimensional drawings of hypothetical potential energy surfaces of the ground state and the first excited singlet state yielding a qualitative picture of the reaction and deactivation pathways of the discussed molecule are given in Fig. 15 a and b.     

Ultrafast Dynamics of the Transoid-cis Isomer Formed in Photochromic Reaction from 3H-Naphthopyran

Recent efforts in designing new 3H-naphthopyran derivatives have been focused on efficient coloration process with a short fading time of the colored transoid-cis TC isomer. It is desirable to avoid photoisomerization of TC leading to transoid-trans TT isomers in the photoreaction. Long lifetime of TT can hamper fast applications such as dynamic holographic materials and molecular actuators, the residual color is one of the serious issues for photochromic lenses. Herein we characterize the photophysical and photochemical channels of TC excited state deactivation competing with the unwanted TC → TT isomerization process. Transient absorption spectroscopy reveals a very short lifetime of the singlet excited TC (≈0.8 ps) and its deactivation channels as S₁→S₀ internal conversion (major), intersystem crossing S₁→T₁, pyran ring formation, photoenolization and TC → TT isomerization. Computations support the S₁→S₀ and T₁→S₀ channels as responsible for photostabilization of the TC form.     

Photoluminescence of Uranium(VI): Quenching Mechanism and Role of Uranium(V)

The photoluminescence of uranium(VI) is observed typically in the wavelength range 400–650 nm with the lifetime of several hundreds μs and is known to be quenched in the presence of various halide ions (case A) or alcohols (case B). Here, we show by density functional theory (DFT) calculations that the quenching involves an intermediate triplet excited state that exhibits uranium(V) character. The DFT results are consistent with previous experimental findings suggesting the presence of photoexcited uranium(V) radical pair during the quenching process. In the ground state of uranyl(VI) halides, the ligand contributions to the highest occupied molecular orbitals increase with the atomic number (Z) of halide ion allowing larger ligand‐to‐metal charge transfer (LMCT) between uranium and the halide ion. Consequently, a larger quenching effect is expected as Z increases. The quenching mechanism is essentially the same in cases A and B, and is driven by an electron transfer from the quencher to the UO₂²⁺ entity. The relative energetic stabilities of the triplet excited state define the “fate” of uranium, so that in case A uranium(V) is oxidized back to uranium(VI), while in case B uranium remains as pentavalent.     

Anhydrous photochemical uranyl(VI) reduction: unprecedented retention of equatorial coordination accompanying reversible axial oxo/alkoxide exchange

In a dramatic reversal of the normal trend of observed reactivity in uranyl(VI) coordination chemistry, an unprecedented retention of the normally labile equatorial coordination plane accompanies facile and reversible axial oxo/alkoxide exchange during both the photochemical reduction of cationic uranyl(VI) phosphine-oxide complexes with organic substrates and subsequent hydrolysis of the uranium(IV) alkoxide complexes to regenerate the uranyl(VI) starting complex.     

Complex formation of uranium(VI) with 4-hydroxy-3-methoxybenzoic acid and related compounds

Summary The complex formation of uranium(VI) with 4-hydroxy-3-methoxybenzoic acid as well as with benzoic acid and 4-hydroxybenzoic acid was studied. In aqueous solution weak carboxylic 1 : 1 complexes, are formed in which the carboxyl group is bidentately coordinated to the metal atom. The logarithmic stability constants of these complexes regarding the reaction of the uranyl ion with the single charged anion of the respective ligands are 2.78±0.02, 2.68±0.04, and 2.71±0.04 at an ionic strength of 0.1 mol/l (NaClO₄) and at 25 °C. Bis(4-hydroxy-3-methoxybenzoato)dioxouranium(VI) was obtained as a crystalline compound if the concentrations of the starting components for the synthesis are increased. The monoclinic compound has a reflections-rich X-ray powder diffraction pattern. The lattice constants are a = 13.662(9) ?, b = 21.293(7) ?, c = 11.213(3) ?, b = 107.49(4), and V = 3111(2) ?.³     

Impact of Dye‐Protein Interaction and Silver Nanoparticles on Rose Bengal Photophysical Behavior and Protein Photocrosslinking

The role of recombinant Type‐I human collagen in the free form or forming AgNP@collagen on the photophysical and photochemical behavior of rose Bengal was analyzed. The formation of dye aggregates on the protein surface was experimentally observed and corroborated by docking calculations. The formation of such aggregates is believed to change the main oxidative mechanism from Type‐II (singlet oxygen) to Type‐I (free radical) photosensitization. Remarkably, the presence of AgNP in the form of AgNP@collagen altered the dynamics of dye triplet deactivation, effectively preventing the dye degradation and reducing the extent of protein crosslinked. Both crosslinked rHC and AgNP@collagen were able to support fibroblasts proliferation, but only the material containing silver was resistant to S. epidermidis infection.     

Detection of uranium with a wireless sensing method by using salophen as receptor and magnetic nanoparticles as signal-amplifying tags

A new wireless sensing method for the detection of uranium in water samples has been reported in this paper. The method is based on a sandwich-type detection strategy. Salophen, a tetradentate ligand of uranyl ion, was immobilized on the surface of the polyurethane-protected magnetoelastic sensor as receptor for the capture of uranyl ion. The phosphorylated polyvinyl alcohol coated magnetic Fe₃O₄ nanoparticles were used as signal-amplifying tags of uranyl ion. In a procedure of determining uranium, firstly uranyl ion in sample solution was captured on the sensor surface. Then the captured uranyl bound the nanoparticle through its coordination with the phosphate group. The amount of uranium was detected through the measure of the resonance frequency shift caused by the enhanced mass loading on the sensor surface. A linear range was found to be 0.2–20.0 μg/L under optimal conditions with a detection limit of 0.11 μg/L. The method has been applied to determine uranium in environmental water samples with the relative standard deviations of 2.1–3.6 % and the recoveries of 98.0–101.5 %. The present technique is one of the most suitable techniques for assay of uranium at trace level in environmental water samples collected from different sources.     

Characterization of the Excited States of Indigo Derivatives in their Reduced Forms

A comprehensive characterization of the electronic spectral and photophysical properties of the leuco (reduced) form of several indigo derivatives, including indigo and Tyrian Purple, with di‐, tetra‐, and hexa‐substitution, was obtained in solution. The characterization involves absorption, fluorescence, and triplet–triplet absorption spectra, together with quantitative measurements of quantum yields of fluorescence, ?_F (0.46–0.04), intersystem crossing, ?_T (0.013–0.034), internal conversion, ?_IC, and the corresponding lifetimes. The position and degree of substitution promote differences in the spectral and photophysical properties displayed by the investigated leuco derivatives. The ?_F values are about two orders of magnitude higher than those previously obtained for the corresponding keto forms. Also in contrast with the behavior found for the keto forms, the S₁～～→T₁ intersystem crossing is an efficient route for the excited‐state deactivation channel. These findings strengthen the fact that, in contrast to keto indigo where the internal conversion dominates the deactivation of the excited‐state, with leuco indigo (and derivatives), the excited state deactivation involves competition between internal conversion, triplet state formation, and fluorescence. A time‐resolved investigation of one of the compounds in glycerol showed the presence of a photoisomerization process.     

Uranyl ion complexation of 2,2′-dihydroxyazobenzene enhanced on a backbone of poly(ethylenimine)

The formation constant (K_f) for the uranyl complex of 2,2′-dihydroxyazobenzene (DHAB) was measured with DHAB attached to poly(ethylenimine) (DHAB-PEI) at pH 7.7 to 9.4. The value of K_f was estimated from the equilibrium constant for extraction of uranyl ion from the uranyl complex of DHAB-PEI (UO₂DHAB-PEI) with carbonate ion, which in turn was measured from the absorbance change observed on addition of bicarbonate ion to the solution of UO₂DHAB-PEI. At pH 8.0, the uranyl-binding ability of DHAB was enhanced by about 10⁴ times on attachment of DHAB to PEI. The major origin of the increased ability of uranyl ion complexation is the basic local microenvironment of PEI, which encourages ionization of the phenol groups of DHAB. Various other possible origins are discussed also. The log K_f for DHAB-PEI at pH 8.0 indicates that DHAB moieties of DHAB-PEI are mostly occupied, whereas DHAB unattached to PEI is mostly unoccupied by uranyl ion under conditions of seawater when only the pH and concentrations of bicarbonate and uranyl ions of seawater are considered. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3936–3942, 1999     

Synthesis and spectral characterization of dioxouranium (VI) complexes of salicylhydrazine and acetone salicylhydrazone

Dioxouranium(VI) complexes of the types UO₂LSO₄ and UO₂L₂SO₄ (where L=SH, ASH) have been prepared from reaction of uranyl sulphate with salicylhydrazine (SH) and acetone salicylhydrazone (ASH) and characterized by conventional chemical and physical measurements. Infrared and Raman spectra indicate thatmono- andbis-complexes contain six-and seven-coordinate uranium atom respectively with all the ligand atoms arranged in an equatorial plane around the linear uranyl group. The infrared spectra (4000-200 cm⁻¹) reveal that both SH and ASH act as neutral bidentate ligands coordinating through a carbonyl oxygen and primary amine/azomethine nitrogen atoms. The sulphato group coordinates to the uranyl ion as bidentate chelating ligand and terminal monodentate ligand in mono- and bis-complexes respectively.     

Highly sensitive ion preconcentration method based on flow sorbent extraction using multiwall carbon nanotubes

In this work a solid phase on-line uranium ion preconcentration system coupled with spectrophotometry has been developed. The method is based on uranyl ion preconcentration at pH 3.75 onto multiwall carbon nanotubes treated with HNO₃. After preconcentration, the uranyl ions are eluted with 0.32?mol?L^?1HCl followed by reaction with 3,6-bis[(2-arsonophenyl)-azo]-4,5-dihydroxy-2,7-naphthalendisulfonic acid 0.08%[w/v] (Arsenazo III), which had maximum monitored absorbance of 650?nm. Effects of the pertinent experimental parameters on the system were investigated by means of 2^6?2 fractional factorial design, while optimization was carried out using the Doehlert matrix. Under optimized conditions, detection and quantification limits were found to be 0.21 and 0.7?µg?L^?1, respectively. The analytical curve ranged from 5 to 150?µg?L^?1 (r?=?0.998), while the relative standard deviations (RSD) were 3.27 and 2.56% for the respective uranium concentrations of 10 and 100?µg?L^?1 (n?=?10). The features obtained for the on-line preconcentration system were: preconcentration factor of 228, concentration efficiency of 57?min^?1, consumption index of 0.13?mL and sample throughput of 15?h^?1. In order to assess the accuracy of the proposed method, addition and recovery studies were carried out on spring water samples from different sources and synthetic seawater with satisfactory results ranging from 94.85 up to 103.65%.     

Thermometric Studies of Multi-Valent Metal Complexes

Thermometric titrations of lanthanum perchlorate, titanium (III)-chloride, uranium (IV)-sulfate, and uranyl sulfate with EDTA solutions were carried out by using a Keithley nanovoltmeter with a rhodium-platinum thermocouple at 25°±0.01°. The formation of LaY^?, TiY^?, U(IV)Y and UO₂HY^? ions was confirmed. The heat of reaction for the system, Ti(III)+H₂Y^2? = TiY^?+2H⁺, was given by δH₁ = 1.933-1.422×10 m +2.056×10⁴m (in cal) and the limiting value was evaluated to be δH°₁ = 1.9 kcal mol^?1 at 25°C.