Flash photolysis of s-trinitrobenzene solutions

Flash photolysis of s-trinitrobenzene (TNB) aerated solutions in alcohols generates a transient species with absorption maxima at 430 and 510 nm. The yield of the transient is a function of oxygen concentration, and its rate of formation is viscosity dependent. In deaerated solutions instead of the transient, a brown permanent product, identified as a charge transfer (CT) complex with absorption maxima at 470 and 550 nm, is produced. These species are formed only in polar solvents with relatively high proton affinity. The data suggest an intermolecular proton transfer, from electronically excited TNB to the solvent forming the anion The anion thereby produced interacts with oxygen in aerated alcohols to form the transient attributed to CT complex while in deaerated alcohols the anion reacts with the solvent to produce RO^?, which leads to the CT complex RO…^?TNB. This mechanism is supported by detailed kinetic and spectroscopic studies.     

Flash photolysis study of aminopyridines

A variable temperature flash photolysis study in different solvents indicates that the triplet—triplet (T—T) absorption of 2-aminopyridine, 2-N-dimethylaminopyridine and 4-aminopyridine can be observed at temperatures below −160 °C in EPA. The absorption wavelength maxima for 2-aminopyridine (390 nm) and 4-aminopyridine (395 nm) are attributed to pyridine-type T—T absorption rather than to an intramolecular charge transfer T—T absorption. The absence of T—T absorption for 3-aminopyridine in the wavelength region 350 – 600 nm, despite its long phosphorescence lifetime of 3.6 s, suggests that an upper triplet state exists at an energy exceeding 60 000 cm⁻¹. The absence of triplet absorption in ethyl ether and 3-methylpentane for 2-aminopyridine and 4-aminopyridine supports the view that the triplet yield is enhanced in polar and hydrogen bonding solvents.     

Flash photolysis and antioxidant activity

The photo-chemical behaviour of a number of mono- and polyfunctional commercial phenolic antioxidants has been examined using kinetic micro-second flash photolysis. The technique provides useful information on the relationship between antioxidant structure and the efficiency of phenoxy radical production. The kinetics of decay of the phenoxy radicals are also found to be dependent on structure. Mono-functional antioxidants give phenoxy radicals which decay by a second-order process whereas polyfunctional antioxidants give phenoxy radicals that decay by a first-order process. In the former case dimerisation to give bisphenolic coupling products is observed whereas, with the latter, this process is sterically inhibited. The value of flash photolysis as a probe for studying antioxidant activity is discussed.     

Flash photolysis and inactivation of aqueous lysozyme

Abstract— –Flash photolysis of aqueous lysozyme has shown that the initial photochemical products are photo-oxidized tryptophan residues (Λ_max= 500 nm), hydrated electrons (Λ_max= 720 nm), and the cystine residue electron adduct (Λ_max= 420 nm). Comparisons with mixtures of the chromophoric amino acids show that 1 to 2 tryptophan residues provide electrons at a quantum yield of 0.018 (25 per cent). Part of the ejected electrons are captured by cystine residues via a short-range, intramolecular process with essentially unit efficiency. The remainder become hydrated and back react with oxidized tryptophan residues before 10^-4sec. The cystine residue electron adduct decays with 2 msec halftime (25°C) and 1.5 kcal/mole activation energy. The surviving oxidized tryptophan residues decay with a comparable time constant in a hydroxyl ion catalyzed process. In acid solutions the oxidized tryptophan residue and long-lived H atom adduct are observed (Λ_max= 380 nm). The quantum yield of lysozyme inactivation induced by xenon flash irradiation above 250 nm is 0.023 (20 per cent), which is not sensitive to oxygen or pH. Comparison to the primary photochemical reactions indicates that electron ejection from the essential tryptophan residues inactivates the enzyme, irrespective of the electron trap and subsequent reactions. On the basis of the structure and supporting information it is proposed that the tryptophan residues of the active site are involved. Direct disruption of cystine residues does not contribute more than 10 per cent to the inactivation quantum yield in this wavelength region. Lysozyme inactivation may differ from other enzymes because the chromophores include essential residues located in the active center.     

Flash photolysis in Eu2+ doped NaCl

The luminescence of NaCl:Eu under flash-lamp and N₂-laser excitation has been studied. It has been found an anomalous non-exponential luminescence decay which follows a dependence I=I₀t^m with m=1 at low (90K) and room temperatures. The observed kinetics is consistent with a tunneling mechanism involving recombination of electrons and holes (V_k) at Eu sites.     

Flash photolysis of transient radicals. Benzophenone ketyl radical

A number of processes are found to follow excitation of the benzophenone ketyl radical. The lowest excited state of the radical absorbs, fluoresces and reacts with solvent. Rate constants for reaction of this state with cyclohexane and isopropanol are 4 × 10⁷ and 7 × 10⁸ M^?1 s^?1, respectively. The lifetime of 5.1 ns found in a solution containing 1% cyclohexane in acetonitrile at room temperature is longer than that reported previously.     

Flash photolysis of flavins. I. Photoreduction in non-aqueous solvents

Abstract— The photoreduction of riboflavin, FMN, and lumiflavin in a series of hydroxylic solvents has been examined using flash photolysis. A comparison of the relative quantum yields for the photoxidation of several alcohols and esters by lumiflavin demonstrated that the hydroxyl hydrogen of the alcohol is abstracted approximately twice as readily as is the alpha hydrogen. The use of glycerol as both solvent and reductant provided direct evidence that the initial reaction proceeds by a one-electron reduction to form the flavin semi-quinone. In the case of riboflavin (and FMN) the kinetic results are consistent with an initial intramolecular hydrogen atom transfer, analogous to photobleaching in aqueous solution, followed by a reaction of the semi-quinone with the reductant which prevents degradation of the ribityl side-chain. Quenching by iodide indicates that all the reactions proceed via the flavin triplet state, as is the case in aqueous systems.     

Flash photolysis study of the NO-catalyzed recombination of bromine atoms

The recombination of bromine atoms at room temperature has been studied by flash photolysis in the range of 1–100 atm of the inert diluent He, leading to a value for the third-order rate constant of (1.5 ± 0.2) × 10¹⁵ cm⁶/mol².sec. In the presence of NO the recombination is considerably accelerated. The falloff curve of the recombination Br + NO (+He) → BrNO (+He) was also measured resulting in a value for the limiting low-pressure rate constant of (3.4 ± 1.3) × 10¹⁵ cm⁶/mol².sec. In experiments with excess NO, rate constants of (2.2 ± 1) × 10¹⁴ cm³/mol·sec for the reaction Br + BrNO → Br₂ + NO, and (6.1 ± 0.4) × 10⁹ cm⁶/mol².sec for the reaction Br₂ + 2NO → 2BrNO were obtained.     

Pulse photolysis of PbCl4 in non-aqueous solutions

Lead(IV) chloride solutions in CCl₄, CHCl₃, cyclohexane, benzene, toluene, chlorobenzene, N,N-dimethylformamide and pyridine have been irradiated with an approximatelyy 1.5 kJ pulse of UV radiation lasting 0.5 ms. The course of the photochemical processes has been followed by measuring the loss of PbCl₄, the changes of Cl₂ concentration and by identifiction of chlorination products. In a similar way, chlorine solutions in cyclohexane and benzene have been investigated. Primary processes are discussed in detail. The most probable process in solvents that do not form complexes with PbCl₄ is predissociation of the Pb-Cl bond

whereas in those forming complexes with PbCl₄ electron transfer takes place:

The results are compared with those obtained earlier under steady irradiation. This allows us to determine new pathways of secondary processes that are unobservable with the continuous illumination technique. The pathways include processes photoinduced by the phase of fresh finely dispersed PbCl₂ crystals:

     

Investigation of lead thiosulfate photolysis in aqueous solutions

A model of photolysis of PbS₂O₃ aqueous solutions has been proposed on the basis of identified photolysis products and semiempirical quantum-chemical calculations. The degradation of PbS₂O₃ starts with the dissociation of the sulfur-sulfur bond in the thiosulfate group via photochemical excitation and transition of the system a whole to the activated state, which is decomposed by the solvent. The interaction of the primary photolysis products with PbS₂O₃ results in the formation of final products.     

Pulse photolysis of PbCl4 in non-aqueous solutions

Lead(IV) chloride solutions in CCl₄, CHCl₃, cyclohexane, benzene, toluene, chlorobenzene, N,N-dimethylformamide and pyridine have been irradiated with an approximately 1.5 kJ pulse of UV radiation lasting 0.5 ms. The course of the photochemical processes has been followed by measuring the loss of PbCl₄, the changes of Cl₂ concentration and by identification of chlorination products. In a similar way, chlorine solutions in cyclohexane and benzene have been investigated. Primary processes are discussed in detail. The most probable process in solvents that do not form complexes with PbCl₄ is predissociation of the PbCl bond

whereas in those forming complexes with PbCl₄ electron transfer takes place:

The results are compared with those obtained earlier under steady irradiation. This allows us to determine new pathways of secondary processes that are unobservable with the continuous illumination technique. The pathways include processes photoinduced by the phase of fresh finely dispersed PbCl₂ crystals:

     

Laser flash photolysis of aqueous acetaminophen solutions

The N-acetyl-4-aminophenoxyl radical, a supposed intermediate of the enzymatic oxidation of acetaminophen in living organisms, was prepared and studied by means of nanosecond laser flash photolysis. A number of important spectral-kinetic parameters of this species were determined, namely, the absorption coefficient at 440 nm ((4.2±0.2)×10³ l mol^?1cm^?1), the quantum yield of acetaminophen photoionization at 266 nm (φ= 0.03), and the rate constants for recombination (2k= (2.4±0.3))×10⁹ l mol^?1s^?1) and the reaction with the superoxide radical (k= (9±2))×10⁹ l mol^?1s^?1).     

Flash photolysis study of complexes between salicylic acid and lanthanide ions in water

In the natural environment humic substances (HS) represent a major factor determining the speciation of metal ions, e.g., in the context of radionuclide migration. Here, due to their intrinsic sensitivity and selectivity, spectroscopic methods are often applied, requiring a fundamental understanding of the photophysical processes present in such HS-metal complexes. Complexes with different metal ions were studied using 2-hydroxybenzoic acid (2HB) as a model compound representing an important part of the chelating substructures in HS. In flash photolysis experiments under direct excitation of 2HB in the absence and the presence of different lanthanide ions, the generation and the decay of the 2HB triplet state, of the phenoxy radical, and of the solvated electron were monitored. Depending on the lanthanide ion different intracomplex processes were observed for these transient species including energy migration to and photoreduction of the lanthanide ion. The complexity of the intracomplex photophysical processes even for small molecules such as 2HB underlines the necessity to step-by-step approach the photochemical reactivity of HS by using suitable model compounds.     

Flash photolysis of chloride complexes of Cu(II) in organic solvents

By means of flash photolysis and low-temperature spectrophotometry, the formation of a complex between a Cu(I) ion and a peroxy radical of the solvent has been detected in ethanol, isopropanol, and dimethylformamide. The peroxy radical is generated in a reaction of a solvent radical with a molecule of dissolved oxygen. The solvent radical appears as a result of photoreduction of chloride complexes of Cu(II). The radical complex has a band in the optical absorption spectrum with a maximum at 415–420 nm in ethanol and isopropanol. The rate of formation of this complex is determined mainly by the reaction of the radical of the matrix (R^.) with complexes of bivalent copper. The rate constant of this process in isopropanol at room temperature is (2–3)·10⁸ liters/ mole·sec. Disappearance of the radical complex Cu(I)...RO₂ ^. takes place in a reaction with complexes Cu²⁺ _solv and CuCl⁺ with a rate constant of 2.3·10⁷ liters/mole·sec at room temperature.Translated from Teoreticheskaya in iÉksperimental'naya Khimiya, Vol. 22, No. 1, pp. 39–44, January–February, 1986.     

Nonequilibrium nuclear polarization in the photolysis of aniline solutions

Conclusions Negative polarization of the protons of aniline and other compounds is observed upon the photoexcitation of aniline solutions. This effect is a consequence of optical nuclear polarization in short-lived donor-acceptor complexes.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1890–1892, August, 1987.     

Flash photolysis of SiBr4: the UV spectrum of SiBr2

In the flash photolysis of SiBr₄ both the absorption and the emission spectra corresponding to the B̃²Σ−X̃²Π transition of SiBr have been observed. A broad, structureless absorption band has also been detected in the 340–400 nm region which could be assigned to the hitherto unreported à ¹B₁−x̃ ¹A₁ transition of SiBr₂. The decay of both absorption spectra followed first-order kinetics yielding the pseudo-first-order rate constants: k(SiBr)=2.6 × 10⁴s⁻¹ and k(SiBr₂) = 8.9 × 10³⁻¹. Assuming that the principal reactions consuming these intermediates are SiBr+SiBr₄→Si₂Br₅ and SiBr₂+SiBr₄→ Si₂Br₆, the second-order rate constants have the values k(SiBr)= 9.7×10⁹ M⁻¹s⁻¹ and k(SiBr₂)= 3.3×10⁸M⁻¹s⁻¹.