Flash Photolysis Of Aqueous Solutions Research Articles

Photochemistry of 5‐Halogenosalicylic Acids: Evidence of the Triplet Involvement in the Carbene Formation

AbstractThe heterolytic photodehalogenation of 4‐halogenophenols in water is known to yield the long‐lived triplet carbene 4‐oxo‐2,5‐dienylidene. However, the nature of the excited state from which dehalogenation takes place remains an open issue. To get further insight into the mechanism of carbene formation, we investigated the photolysis of 5‐chlorosalicylic acid and 5‐bromosalicylic acid from experimental and theoretical points of view. Our results showed that irradiation of monoanionic species in water leaded to the formation of the carbene species and its characteristic photoproducts. Moreover, triplet excited states could be detected by laser flash photolysis of aqueous solutions and their involvement in the photodehalogenation was firmly demonstrated. Identification by quantum calculations of two distinct triplet species supports the experimental results. The short‐ and long‐lived triplets are assigned respectively to the planar and non planar structures. Both were successfully detected experimentally. The non planar triplet either reacted with water to yield the dihydroxyderivative or gave the triplet carbene after heterolytic halogen elimination. These findings shed a new light on our understanding of the halogenophenols photochemistry and could be exploited in further studies.

A time-resolved study of the multiphase chemistry of excited carbonyls: Imidazole-2-carboxaldehyde and halides

Abstract Imidazole-2-carboxaldehyde (IC) reactivity in the presence of halide anions (Cl – , Br – , I – ) has been studied by laser flash photolysis in aqueous solution at room temperature. The absorption spectrum of the triplet state of IC has been measured with a maximum absorption at 330 nm and a weaker absorption band around 650 nm. Iodide anions proved to be efficient quenchers of the triplet state IC, with a rate coefficient k q of (5.33 ± 0.25) × 10 9 M −1 s −1 . Quenching by bromide and chloride anions was less efficient, with k q values of (6.27 ± 0.53) × 10 6 M −1 s −1 and (1.31 ± 0.16) × 10 5 M −1 s −1 , respectively. The halide (X – ) quenches the triplet state; the resulting transient absorption feature matches that of the corresponding radical anion (X 2 – ). We suggest that this type of quenching reactions is a driving force of oxidation reactions in the oceanic surface microlayer (SML) and a source of halogen atoms in the atmosphere.

ChemInform Abstract: Generation and Investigation of Ynamines by Flash Photolysis in Aqueous Solution.

ChemInform Abstract: Generation and Investigation of Ynamines by Flash Photolysis in Aqueous Solution.

2‐(2,4‐Dinitrobenzyl)pyridine (DNBP): A Potential Light‐Activated Proton Shuttle

AbstractThe well‐known photochromic tautomerism of 2‐(2,4‐dinitrobenzyl)pyridine (1; CH; Scheme 1) was re‐investigated by flash photolysis in aqueous solution in view of its potential application as a light‐activated proton pump. Irradiation of 1 yields the enamine tautomer NH (λmax=520 nm) that rapidly equilibrates with its conjugate base CNO− (λmax=420 nm). The pH–rate profile for the first‐order decay of NH and CNO− provides a direct determination of the acidity constant of NH, pK$\rm{ {_{a,c}^{NH}}}$=5.94±0.12 (I=0.1M) and serves to clarify the mechanisms of proton transfer prevailing in aqueous solutions. The acidity constant of protonated 1 (CHNH+), pK$\rm{ {_{a,c}^{CHNH}}}$=4.18±0.02, was determined by spectrophotometric titration.

Kinetics of Tyrosyl Radical Reduction by Selenocysteine

The rate constant for the reduction of the tyrosyl radical with selenocysteine has been measured to investigate whether selenocysteine is capable of repair of protein radicals. Tyrosyl radicals, both free in solution and in insulin, were generated by means of pulse radiolysis and laser flash photolysis in aqueous solution. The rate constant for the reaction of free N-acetyl-tyrosyl-amine radicals with selenocysteine is (8 +/- 2) x 10 (8) M (-1) s (-1), and that for tyrosyl radicals in insulin is (1.6 +/- 0.4) x 10 (8) M (-1) s (-1). The rate constant for the reaction of selenoglutathione with the N-acetyl-tyrosyl-amine radical is (5 +/- 2) x 10 (8) M (-1) s (-1). In contrast, cysteine and glutathione react more slowly than their selenium analogues with the tyrosyl radical: the reactions of N-acetyl-tyrosyl-amine radicals with cysteine and glutathione are 3 and 5 orders of magnitude slower, respectively, than those with selenocysteine and selenoglutathione, while those of tyrosyl radicals in insulin are 3 and 2 orders of magnitude slower, respectively.

Theoretical and Experimental Investigation on the Oxidation of Gallic Acid by Sulfate Radical Anions

By monitoring the decay of SO4*- after flash photolysis of aqueous solutions of S2O82- at different pH values, the kinetics of the reaction of SO4*- radicals with gallic acid and the gallate ion was investigated. The bimolecular rate constants for the reactions of the sulfate radicals with gallic acid and the gallate ion were found to be (6.3 +/- 0.7) x 10(8) and (2.9 +/- 0.2) x 10(9) M(-1) s(-1), respectively. On the basis of the oxygen-independent second-order decay kinetics and on their absorption spectra, the organic radicals formed as intermediates of these reactions were assigned to the corresponding phenoxyl radicals. DFT calculations in the gas phase and aqueous solution support formation of the phenoxyl radicals by H abstraction from the phenols to the sulfate radical anion. The observed recombination of the phenoxyl radicals of gallic acid to yield substituted biphenyls and quinones is also supported by the calculations. HPLC/MS product analysis showed formation of one of the predicted quinones.

Laser Flash Photolysis Studies on Gallic Acid

AbstractThe transient species of gallic acid (GA) have been studied by 266 nm nanosecond laser flash photolysis in aqueous solution and acetonitrile. The intermediate with absorption at 320 nm was identified as excited triplet state (3GA*), the decay rates of which were obtained in aqueous solution and acetonitrile respectively. Energy transfer from 3GA* to β‐carotene was observed and the energy transfer rate constant kent was determined to be 2.2×109mol−1·L·s−1. GA underwent photoionization during photolysis and the quantum yield of photoionization was determined to be 0.12 at room temperature with KI as a reference.

Radical Cations of Phenyl‐Substituted Aziridines: What Are the Conditions for Ring Opening?

Radical cations were generated from different phenyl-substituted aziridines by pulse radiolysis in aqueous solution containing TlOH.+, N3. or SO4.- as oxidants or in n-butyl chloride, by 60Co gamma radiolysis in Freon matrices at 77 K, and in some cases by flash photolysis in aqueous solution. Depending on the substitution pattern of the aziridines, two different types of radical cations are formed: if the N atom carries a phenyl ring, the aziridine appears to retain its structure after oxidation and the resulting radical cation shows an intense band at 440-480 nm, similar to that of the radical cation of dimethylaniline. Conversely, if the N atom carries an alkyl substituent while a phenyl ring is attached to a C-atom of the aziridine, oxidation results in spontaneous ring opening to yield azomethine ylide radical cations which have broad absorptions in the 500-800 nm range. In aqueous solution the two types of radical cations are quenched by O2 with different rates, whereas in n-butyl chloride, the ring-closed aziridine radical cations are not quenchable by O2. The results of quantum chemical calculations confirm the assignment of these species and allow to rationalize the different effects that phenyl rings have if they are attached in different positions of aziridines. In the pulse radiolysis experiments in aqueous solution, the primary oxidants can also be observed, whereas in n-butyl chloride a transient at 325 nm remains unidentified. In the laser flash experiments, both types of radical cations were also observed.

An antenna-sensitized nitroindoline precursor to enable photorelease of L-glutamate in high concentrations.

1-Acyl-7-nitroindolines are useful reagents for rapid release of carboxylates upon flash photolysis in aqueous solution and have been particularly effective for rapid (submicrosecond) release of neuroactive amino acids such as l-glutamate in biological experiments. In model systems the efficiency of photorelease has been shown to be greatly improved by attachment of a benzophenone triplet-sensitizing antenna. The present work describes synthesis and initial biological evaluation of the l-glutamate precursor 3. A significant improvement in the overall synthesis uses double Boc protection of the glutamate amino group to avoid side reactions during introduction of the nitro group. To accommodate the multiple functionalities in 3, linkage of the nitroindoline and benzophenone moieties is carried out late in the synthesis. Photolysis of 3 occurs with near-quantitative stoichiometry and the released l-glutamate efficiently activates neuronal glutamate ion channels.

Reactions of carbon dioxide radical anion with substituted benzenes

AbstractWe report mechanistic studies on the reactivity of CO2 .− radicals towards substituted benzenes, XPh, with X = NO2, COOH, COH, CH3, H, Cl and HO. Vacuum ultraviolet (VUV, 172 nm) irradiation of aqueous solutions containing formate was used as a method of CO2 .− radical generation for the study of the reaction products, which were determined by gas chromatography–mass spectrometry and high‐performance liquid chromatography. The nature of some of the reaction intermediates was studied by laser flash photolysis of aqueous solutions of peroxodisulfate in the presence of formate. The observed products and intermediates of reaction are discussed and rationalized by a dual reactivity of the CO2 .− radical anion with substituted benzenes: a one‐electron reduction and radical addition to the aromatic ring. For substituted benzenes with X = NO2, COOH and COH, the observed products support an electron transfer from CO2 .− to the substrates as the primary reaction channel, while the reactions with toluene follow the channel of radical addition. Benzene and chlorobenzene react with CO2 .− by both routes, electron transfer and radical addition. Our experimental results indicate that H abstraction by the CO2 .− radical is negligible under the chosen experimental conditions. Copyright © 2001 John Wiley & Sons, Ltd.

Transient near-IR absorption spectrum in laser flash photolyzed aqueous solutions of tris(2,2 ′-bipyridyl) ruthenium(II)

Laser flash photolysis of aqueous solutions of tris(2,2 ′-bipyridyl) ruthenium(II) cation (Ru(bpy) 3 2+, bpy=2,2 ′-bipyridyl) induced transient absorbance changes in the near-IR region. The transient absorption with a peak at around 900 nm was assigned as a triplet–triplet transition of the metal-to-ligand charge transfer (MLCT) excited state of Ru(bpy) 3 2+. The quantum efficiency for the MLCT state formation ( φ) and the molar absorption coefficient at 900 nm ( ϵ *) were determined to be φ=1.00±0.04 and ϵ *=1420±30 M −1 cm −1, respectively, by applying the method of intensity dependence measurement.

Flash Photolytic Investigation of 4-Diazoisothiochroman-3-one in Aqueous Solution: Observation of a Short-Lived Carboxylic Acid Enol

4-Diazoisothiochroman-3-one was found to undergo a photo-Wolff reaction upon flash photolysis in aqueous solution, generating 1,3-dihydrobenzo[c]thiophen-1-ylideneketene, whose hydration produced a second, readily observable short-lived species. The latter, through the shape of its decay rate profile plus solvent isotope effects, was identified as the enol of 1,3-dihdyrobenzo[c]thiophene-1-carboxylic acid. This behavior contrasts with that reported previously for the acyclic analogue of the presently studied system: flash photolysis of S-methyl phenyldiazothioacetate gives a photo-Wolff reaction as well, but the ketene so produced undergoes hydration 3 orders of magnitude more slowly than its cyclic counterpart; this makes ketene hydration in the acylic system slower than enol ketonization, and that renders the acylic enol a low-concentration, nonobservable intermediate. Analysis of the rate profile for ketonization of the presently examined cyclic enol provides enol acidity constants as well as rate constants for ketonization of enol and enolate species.

Hydrogen abstraction by triplet flavins. I: time-resolved multi-channel absorption spectra of flash-irradiated riboflavin solutions in water

Multichannel detection with μs time resolution has been used for recording flash-induced changes in the absorbance of deaerated solutions of riboflavin in phosphate buffer at pH 6.8. By comparing the spectra of solutions containing, in addition to riboflavin (F v), different substrates (ascorbic acid, EDTA, indole acetic acid, tryptophan), and by recording the absorption spectrum of the neutral tryptophyl radical (produced by flash photolysis of aqueous solutions of tryptophan), the absorption spectrum and the extinction coefficient of F vH are ascertained. Under the experimental conditions pertaining to the present investigation (solute concentration (7.5±2.5)×10 −5 M, ca. 10% conversion into triplets), quenching of F v † (a riboflavin triplet) by F v 0 (an unexcited riboflavin molecule) was found to be insignificant; most triplets appear to decay through triplet–triplet annihilation, the loss of two triplets being accompanied by the gain of one neutral semiquinone radical (F vH ). It is proposed that (i) triplets deactivate mainly through triplet–triplet annihilation, (ii) the annihilation event leads to the formation of an ion pair, F v †+F v †→F v ++F v −, (iii) the anion rapidly converts to F vH , and (iv) the cation ejects a proton and splits into two neutral products, a molecule whose ground-state absorption spectrum resembles that of F v 0, and an odd-electronic species containing the remainder of the aliphatic side chain.

The flash photolysis of aqueous solutions of rhodizonic and croconic acids

AbstractThe flash photolysis of aqueous solutions of rhodizonic and croconic acids has been studied in the presence and absence of electron acceptors. No transient absorption which could be identified with an excited state was observed with either anion. The rate of recovery of the ground state in the absence of additives was a first‐order process with both acids and gave rate constants for deactivation of the excited state, kD, of 2.4 × 105 s−1 for rhodizonate and 2.8 × 105 s−1 for croconate. With croconate dianion in the presence of three acceptors, 4‐nitrobenzylbromide, methylviologen, and biacetyl, a transient absorption was detected, with a maximum absorbance at 500 nm, and was tentatively identified with the monoanion radical, formed following electron transfer to the acceptor. From the rate of growth of the transient, rate constants for the rate of electron transfer to the acceptor were measured as follows: 4‐nitrobenzylbromide: 2.8 × 109 M−1 s−1; methyl viologen: 3.7 × 1010 M−1 s−1; and biacetyl: 2.0 × 108 M−1 s−1. The significance of the measurements is discussed in relation to the mechanism proposed for the photochemical reactions of these dianions. © 1994 John Wiley & Sons, Inc.

A long-lived transient resulting from flash photolysis of hematoporphyrin in aqueous solution

A long-lived transient with a lifetime of several hundred microseconds was observed following the flash photolysis of aqueous solutions of hematoporphyrin buffered at pH 7.5. The transient—ground state difference absorption spectrum was determined 500 μs after flash photolysis. The yield of this species was found to increase with increasing hematoporphyrin concentration and it was also found to depend on the excitation wavelength. The lifetime of the species is not significantly affected by the presence of oxygen. Because the triplet state of hematoporphyrin is not the only long-lived species produced by flash photolysis of aqueous hematoporphyrin solutions, the observed triplet state extinction coefficients will be lower than the true value and hence the triplet state yields of hematoporphyrin determined by the flash photolysis, “complete conversion” technique, are only upper limits. The formation of the long-lived species is discussed in terms of electron transfer between the monomer partners in hematoporphyrin dimer and aggregates which are present in aqueous solutions of hematoporphyrin, particularly in concentrated solutions.

End-to-end intramolecular quenching of aromatic ketone triplets in aqueous solutions of 2,6-di-(O-methyl)-cyclodextrins

The photochemistry of para-(4-hydroxyphenyl)ethoxyacetophenone ( 1) in aqueous solutions of α-, β- and γ-2,6-di-(O-methyl)-cyclodextrins (α-, β- and γ-MCD respectively) was studied using laser flash photolysis techniques. In acetonitrile solution, the triplet state of 1 (τ ≈ 15 ns) decays by intramolecular phenolic hydrogen abstraction to yield the corresponding 1,13-phenoxyketyl biradical. Laser flash photolysis of aqueous solutions of 1 and equimolar α- and β-MCD yields transient absorptions with λ max = 395 nm, which decay with multiexponential kinetics. Two-component decay analyses show that the main portions (greater than 70%) of the decay in the two solutions have lifetimes of about 900 and 440 ns respectively, whereas the minor components are 4–6 times shorter lived. The transients have been shown to be a result of the triplet state of 1 on the basis of quenching studies with sorbic acid. In 1:1 solutions of 1 and γ-MCD under similar conditions, no transients of lifetime greater than approximately 12 ns are observed. The data are consistent with inclusion complex structures in which only one of the aryl rings of the solute is included in the α- and β-MCD cavities, whereas both are included in the γ-MCD cavity. It is suggested that in the α- and β-MCD complexes, the conformational motions required for formation of the intramolecular abstraction geometry are accompanied by partial disengagement of the solute from the cavity. The effects of increasing the relative concentration of MCD on the tirplet lifetime of 1 are consistent with the formation of termolecular 1:2 1:MCD complexes at high MCD concentration for the β- and γ-MCD systems.

Characterization of the transient species in the 266-nm laser photolysis of adenine and its derivatives.

Abstract. Transient absorption spectra produced by 266‐nm ns laser flash photolysis of aqueous solutions of adenine, 2′‐deoxyadenosine, 2′‐deoxyadenosine‐5′‐phosphate, adenosine‐5′‐phosphate, 2′‐deoxyadenylyl‐(3′‐5′)‐2‐deoxyadenosine, 7‐methyladenine and 9‐methyladenine have been measured under different pH conditions. The transients observed after excitation consisted mainly of the hydrated electron, the radical cation, the radical anion and/or neutral radicals resulting from its protonation, and a minor contribution from the excited triplet state. The photoionization occurs mainly through an excited singlet state. For adenine, the cation and anion radical absorption spectra were determined using laser and pulse radiolysis techniques. The triplet state presence was established through sensitization experiments. The hydrated electron decays following a pseudo‐first‐order reaction with the neutral base.

LASER FLASH AND STEADY‐STATE PHOTOLYSIS OF BENOXAPROFEN IN AQUEOUS SOLUTION*

Abstract— On laser flash photolysis of aqueous solutions of benoxaprofen (BP), the BP triplet state is produced with a quantum yield of 0.19. One other unidentified species is also formed probably in a yield less than 0.02. Identical observations were made in alcoholic solution. The effect of potential hydrogen donors, isopropanol and histidine, on the kinetic reactivity of the BP triplet state, showed that for isopropanol the rate constant is slow (< 8 × 103 dm3 mol‐1 s‐1) if it occurs at all, whilst no reactivity with histidine could be seen. In oxygen uptake experiments, it was deduced that singlet oxygen is produced via the BP triplet state with a quantum yield of 0.18. There was no evidence for superoxide anion radical production. In other steady‐state photolysis experiments using high performance liquid chromatography, the quantum yield for loss of BP was determined to be 0.20, while the quantum yield for the only observed product, decarboxylated benoxaprofen, was estimated as 0.18. The involvement of the two major processes, i.e. singlet oxygen formation and BP decarboxylation, in BP phototoxicity is discussed.

Determination of Rate Constants for Reactions of Bichloride Anion Radical with Some Dipeptides in Aqueous Solution of KCl and K2S2O8 by Flash Photolysis

Abstract Determination of rate constants for reactions of dichloride anion radical was conducted by conventional flash photolysis of aqueous solutions of KCl and K2S2O8, where no participation of HOCL−· is expected in decay kinetics of dichloride anion radical. The optimum conditions found for the determination are 0.1 mol dm−3 KCl and 10−4 mol dm−3 K2S2O8. Under the present experimental conditions, S2O82− serves to convert chloride ion to dichloride anion radical through reaction with hydrated electron produced by photolysis of chloride ion. For glycyltryptophan, the competition method was employed, because its transient absorption overlaps the absorption due to dichloride anion radical. The rate constants determined are almost the same as those obtained in N2O-saturated aqueous solution of KCl (1 mol dm−3). The reactivity of dipeptides toward dichloride anion radical is discussed.

PRIMARY PROCESSES IN THE PHOTOCHEMISTRY OF AQUEOUS SULPHACETAMIDE: A LASER FLASH PHOTOLYSIS and PULSE RADIOLYSIS STUDY

Abstract— The spectra have been measured of the transient species formed in the nanosecond flash photolysis of aqueous solutions of sulphacetamide under a variety of conditions. In addition to the excited triplet state, the cation radical and the solvated electron were observed. The ionisation of aqueous sulphacetamide was found to occur by a biphotonic process. The extinction coefficient of the cation radical of sulphacetamide was determined by both laser flash photolysis and pulse radiolysis techniques, a value of 1.9 times 103 dm3mol‐1cm‐1 being obtained. The rate of electron reaction with sulphacetamide and the anion radical spectrum were also determined by the two techniques, good agreement being obtained. The spectrum of the product of the reaction of the superoxide anion radical and the corresponding rate constant have also been determined. A possible mechanism of photosensitized skin reaction due to sulphacetamide is discussed.