Nanosecond Laser Flash Photolysis Studies Research Articles

A nanosecond laser flash photolysis study of aqueous 4-chlorophenol

The mechanism of the aqueous photoreaction of 4-chlorophenol was studied by nanosecond laser flash photolysis. The carbene 4-oxocyclohexa-2,5-dienylidene is formed from the chlorophenol by loss of HCl. This species reacts efficiently with oxygen to form 1,4-benzoquinone-O-oxide which subsequently yields 1,4-benzoquinone. The carbene also undergoes reaction with water and coupling reactions to produce 1,4-hydroquinone and polyhydroxybiphenyls respectively.

Photoinduced Hydrogen Abstraction from Phenols by Aromatic Ketones. A New Mechanism for Hydrogen Abstraction by Carbonyl n,π* and π,π* Triplets

Nanosecond laser flash photolysis studies have been carried out of the kinetics of inter- and intramolecular phenolic hydrogen abstraction by alkoxyacetophenone, 5-alkoxyindanone, and 4-alkoxybenzophenone triplets in acetonitrile and benzene solution. Information on the geometric requirements for abstraction by carbonyl n,π* and π,π* triplets is derived from the results for a series of ketones which contain a para-phenolic moiety attached via a para-oxyethyl linkage. For all of these compounds, the deuterium kinetic isotope effect on the triplet lifetime in acetonitrile solution indicates that triplet decay is determined by the rate of intramolecular abstraction of the remote phenolic hydrogen, which yields the corresponding phenoxyl−hemipinacol biradical. The biradicals have also been detected, and are about an order of magnitude longer-lived than the triplet in each case. For three of the compounds, the rates of the intramolecular process follow the same trend as that observed in the rates of bimolecular quenching of the parent methoxy-substituted ketones by p-cresol. Deviation from this trend is observed for the alkoxyindanone derivative, where an in-plane approach of the phenolic hydrogen to the carbonyl n-orbital is not possible. The trends in the rate constants for bimolecular quenching of a series of substituted benzophenones by p-cresol indicate that for n,π* triplet abstractions, the quenching mechanism is different for electron donor-substituted and electron acceptor-substituted ketones. For π,π* triplets and donor-substituted (n,π* triplet) benzophenones, abstraction is proposed to occur by a mechanism involving the intermediacy of a hydrogen-bonded exciplex, which yields the corresponding radicals by sequential electron- and proton-transfer. The rate constant for quenching by this mechanism thus depends mainly on the basicity of the ketone triplet state and the oxidation potential of the phenol.

Photochemical investigations of functionalised 1,4,7,10-tetraazacyclododecane ligands incorporating naphthyl chromophores

The fluorescence properties of two macrocyclic ligands incorporating bis-(β-naphthyl amide) groups, –CH2CON(CH2C10H7)2, as fluorophores, have been studied in water and acetonitrile. The ligands exhibit distinctive luminescence behaviour in the presence of quenching (e.g. Pb2+, Ni2+, Cu2+ and Eu3+) and non-quenching ions (e.g. Zn2+ and Cd2+). Excimer-to-monomer intensity ratios are affected to differing extents, as are the total integrated fluorescence intensities. The behaviour is contrasted to that reported previously for related ligands containing ‘isolated’ naphthyl units. The luminescence properties of the terbium complex of one of these earlier ligands, containing just one naphthalene group, have been studied in detail. Naphthyl-sensitised metal emission is observed but a nanosecond laser flash photolysis study reveals that back energy transfer to the naphthyl triplet state provides a facile deactivation mechanism for the metal excited state.

ChemInform Abstract: Nanosecond Laser Flash Photolysis Study of the Kinetics and Mechanism of Photoreduction of Decafluorobenzophenone.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Photocrosslinking studies of S-acryloyl O-ethyl xanthate copolymers

S-Acryloyl O-ethyl xanthate was copolymerized with styrene and methyl methacrylate, giving rise to copolymers containing the photoactive acyl xanthate chromophore. Thin films of these copolymers, on irradiation, undergo phototransformations leading to crosslinked polymers. The photocrosslinking yield of the copolymers depends mainly on their molecular weight and not on the composition of the copolymer. The mechanism of photocrosslinking was investigated through nanosecond laser flash photolysis studies. The 308 nm laser pulse excitation of the copolymers in benzene produces radical intermediates, arising through a C(O)S bond fission. The assignment of the photogenerated (ethoxycarbonylthiocarbonyl)thiyl radical is based on its characteristic long wavelength absorption maximum ( λ max≈660 nm in benzene), ready quenching by nitroxy radicals and similarity in spectra and kinetics with those of the radical transient, photogenerated from the model substrate, S-isobutyryl O-ethyl xanthate.

Nanosecond laser flash photolysis study of the kinetics and mechanism of photoreduction of decafluorobenzophenone

Laser pulse excitation of decafluorobenzophenone (DFB) in acetonitrile yields the triplet 3 DFB after rapid intersystem crossing from the singlet 1 DFB. The triplet has absorption maxima at 310 and 490 nm. 3 DFB abstracts hydrogen from donors such as alkanes and alcohols in a radical-like mechanism to yield ketyl radical. The rate constant k q is 10-40 times higher than that of the triplet benzophenone 3 BP. Ketyl radical (DFBH) has absorption maxima at 320 and 530 nm. In aqueous solution, the ketyl radical has a pK a =6.9, and the anion has maxima at 335 and 630 nm

ChemInform Abstract: 4‐Carboxybenzophenone‐Sensitized Photooxidation of Sulfur‐Containing Amino Acids. Nanosecond Laser Flash Photolysis and Pulse Radiolysis Studies

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

S‐benzoyl O‐ethyl xanthate as a new photoinitiator: Photopolymerization and laser flash photolysis studies

AbstractA new sulfur‐containing photoinitiator, S‐benzoyl O‐ethyl xanthate (2) has been prepared and used for the photopolymerization of methyl methacrylate (MMA). The photoinitiation property of 2 has been examined by conventional polymerization methods and nanosecond laser flash photolysis studies. Upon 308 nm laser pulse excitation, 2 gave rise to transients with absorption maxima at 350 and 650 nm, assigned to the benzoyl radical (3) and (ethoxythiocarbonyl)thiyl radical (4), respectively, on the basis of their quenching by nitroxy radicals and spectral similarity to analogous species, reported in the literature. © 1993 John Wiley & Sons, Inc.

Nanosecond and picosecond laser flash photolysis studies of 2,2′-dinitrodiphenylmethanes

The transient absorption spectra of the 2,2′-dinitrodiphenylmethanes 1a–1c in solution were examined in the picosecond and nanosecond time ranges. The absorption bands, observed at 420–450 nm on 355 nm (18 ps) excitation of these compounds, were attributed to the nitronic acids 2a–2c, formed through the singlet-state- mediated intramolecular hydrogen abstraction and the triplet states of 1a–1c. Biradical intermediates, formed through the intramolecular abstraction of the benzylic hydrogen by the triplet excited state, were detected for 1a and 1b. Transient species produced following 355 nm (approximately 6 ns) pulse excitation of 1a–1c result in the formation of the corresponding nitronic acids 2a–2c, with absorption maxima around 415–430 nm. These nitronic acids are the precursors of the various products formed in the steady state irradiation of 1a–1c

4-Carboxybenzophenone-sensitized photooxidation of sulfur-containing amino acids. Nanosecond laser flash photolysis and pulse radiolysis studies

Sulfur-containing amino acids were oxidized via photosensitization by 4-carboxybenzophenone (CB) in neutral aqueous solutions. The mechanism of this reaction was investigated with flash photolysis and pulse radiolysis techniques. The rate constants were determined for the quenching of the CB triplet state by 12 amino acids (with variable relative location and number of terminal functions COOH and NH 2 with respect to the sulfur atom) and were found to be 1.8×10 8 -2.9×10 9 M -1 s -1

Laser and microsecond flash photolysis of 2-substituted anthraquinones and photochemical cross-linking of nylon 6,6 polymer

Microsecond and nanosecond laser flash photolysis studies have been carried out on six 2-substituted anthraquinone compounds, namely the acrylamido, acryloxy, acetoylamino, benzoylamino, hydroxy and amino derivatives, and the data related to their photo-cross- linking activity in nylon 6,6 polymer. All the compounds give rise to triplet—triplet absorptions, with the benzoyl and acetoylamino derivatives showing the strongest absorptions followed by the acrylamido and acryloxy derivatives. Transient half-lives are found to be 7.0, 1.7, 0.7 and 0.63 μs for the acrylamido, acryloxy, benzoyl and acetoylamino derivatives respectively, which follow the order of their ability to induce photo-cross-linking of nylon 6,6 polymer, long triplet lifetimes being the more active. In the reductive solvent 2-propanol the triplet state of the 2-arylamido derivative is rapidly quenched owing to hydrogen atom abstraction. For both the acryloxy and acrylamido derivatives the first-order decay rates of the triplet state are found to be independent of the concentration of the ground state, suggesting that some degree of intramolecular interaction is involved between the side chain and the quinone carbonyl groups. In nylon 6,6 polymer, on the other hand, the acrylamido derivative gives a long-lived triplet—triplet absorption which decays to give a long-lived residual absorption with a maximum at 500 nm. The latter is associated with the formation of a radical anion species formed from a triplet exciplex involving the terminal amine end groups. On microsecond flash photolysis in 2-propanol the initiators give both semiquinone and radical anion absorptions formed by hydrogen atom and electron abstraction reactions, with the former being stronger for the acryloxy derivative.

Photo-CIDNP and nanosecond laser flash photolysis studies on the photodecomposition of triarylsulfonium salts

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTPhoto-CIDNP and nanosecond laser flash photolysis studies on the photodecomposition of triarylsulfonium saltsKevin M. Welsh, John L. Dektar, Miguel A. Garcia-Garibaya, Nigel P. Hacker, and N. J. TurroCite this: J. Org. Chem. 1992, 57, 15, 4179–4184Publication Date (Print):July 1, 1992Publication History Published online1 May 2002Published inissue 1 July 1992https://doi.org/10.1021/jo00041a023RIGHTS & PERMISSIONSArticle Views191Altmetric-Citations20LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (743 KB) Get e-Alerts Get e-Alerts

Photochemical and photophysical studies on chemically amplified resists.

Photolysis of triphenylsulfonium salts in solution or polymer films gives 2-, 3- and 4-phenylthiobiphenyl isomers by an in-cage reaction, whereas diphenylsulfide and other aromatic photoproducts are formed by cage-escape reactions with solvent. The cage and escape reactions both generate acid, which is the primary initiator for many chemically amplified resists. Photo-CIDNP has been used to characterize the cage and escape reactivity of radical intermediates formed from photolysis of triphenylsulfonium salts. In addition nanosecond laser flash photolysis studies have found key intermediates in the direct and triplet sensitized photolysis of these salts. The photophysics of a number of aromatic polymers was examined to understand how the polymer participates in the photoinitiation process. The polymer fluorescence was quenched by sulfonium salts in solution by a dynamic mechanism, whereas in polymer films the quenching was by a static mechanism. Fluorescence lifetimes for the polymers, estimated from the quenching plots in solution, were relatively short, 4-8 nsec and the values agreed well with those obtained by time-resolved spectroscopy.

A laser flash photolysis study of aromatic nitrones. Triplet state properties and reactivity with singlet oxygen

Results of a nanosecond laser flash photolysis study of a series of aromatic nitrones in solution at room temperature are presented. Direct laser flash excitation (337.1 or 308 nm) of nitrone solutions in benzene or methanol results in ground state bleaching which can be attributed to the photocyclization to oxaziridines. The quantum yields of this process are estimated to be 0.1–0.3 in methanol. From the rate constants of triplet quenching by nitrones, their triplet energies are located in the range 1.4–2.0 eV. With the exception of cyclic nitrogen ( 2 — in Scheme 1), no transient absorption phenomena due to nitrone triplets are observed as a result of the extensive quenching of sensitizer triplets; thus the triplets of non-cyclic nitrones are very short lived (τ T < 10 ns), possibly due to structural relaxation analogous to that in olefin triplets. With zinc tetraphenylporphyrin as the sensitizer, there is evidence for electron transfer quenching in which the nitrones act as donors. No ground state bleaching is observed in the course of triplet sensitization, suggesting that photocyclization to oxaziridines does not occur from the triplet state. The nitrones are moderately efficient quenchers of singlet oxygen; the corresponding bimolecular rate constants are in the range (0.5–8) × 10 7 M −1 s −1 in various solvents.

Photochemistry of novel 4-alkylamino benzophenone initiators: a conventional laser flash photolysis and mass spectrometry study

Conventional microsecond and nanosecond laser flash photolysis studies have been carried out on 15 novel 4-alkylamino-substituted benzophenone photoinitiators. Using the microsecond technique, ketyl radical formation is observed from all 15 photoinitiators with a half-life longer than that for the ketyl radical of benzophenone. The formation of the radical anion through an intramolecular exciplex is considered to be an important precursor to this reaction. Triplet-triplet absorption decays by a first-order process; this decay is independent of the ground state concentration of the initiator when intramolecular hydrogen atom abstraction is structurally feasible. The solvent is also seen to influence intramolecular hydrogen atom abstraction and it dominates in acetonitrile where pseudo-first-order hydrogen atom abstraction does not occur. Photolysed product analysis using mass spectrometry complies with the conclusions from laser flash photolysis.

Time-resolved nanosecond and picosecond absorption studies of excited-state properties of 1-thiobenzoylnaphthalene

1-Thiobenzoylnaphthalene (TBN), known for its pericyclization reaction from the lowest excited singlet state (S1), has been subjected to nanosecond and picosecond laser flash photolysis studies. The two major transients observed in the course of nanosecond laser pulse excitation are (i) the short-lived triplet characterized by two absorption maxima (400–410 and 740–750 nm) and submicrosecond intrinsic lifetimes (80–130 ns) and (ii) a relatively long-lived species (λmax = 520 nm and τ = 220–240 ns). Various triplet-related photophysical data of TBN, including self-quenching and bimolecular quenching rate constants, have been determined. The existence of a photochemical path from S1 manifests itself in low intersystem crossing quantum yields, particularly in the polar/hydrogen-bonding solvent, methanol. From the build-up of the triplet under picosecond excitation into S1 the lifetime of the latter is estimated to be ≤ 50 ps (in benzene). The fast intrinsic decay of TBN triplet is attributable to facile intra- and intermolecular photochemistry. The 520 nm transient species could not be definitively assigned, except that it is neither a triplet nor a triplet-derived product and that it arises via photochemistry from S1. Keywords: laser flash photolysis, triplet, transients, absorption maxima, lifetimes, quenching rate constants, photochemistry, 1-thiobenzoylnaphthalenes.

T-T absorption and semiquinone formation in solutions of 5,8-dihydroxy-1,4-naphthoquinone (naphthazarin): a nanosecond laser flash photolysis study

The triplet properties of naphthazarin (5,8-dihydroxy-1,4-naphthoquinone) have been investigated in cyclohexane, methanol and isopropanol solutions using the technique of nanosecond laser flash photolysis. The T-T absorption maxima are at around 280, 335, 377, 410 and 440 nm, while the transients formed after the disappearance of the triplets in alcohols absorb at around 265, 365, 395 and 440 nm. The second transient has been shown to be a semiquinone free radical of naphthazarin, produced by abstraction of a hydrogen atom from the alcohols. The extinction coefficient at many wavelengths and the quantum yields of both triplet and semiquinone formation have been estimated. Kinetic parameters associated with the decay of both the triplet and the semiquinone have been evaluated. The effect of laser intensity and the presence of triplet scavengers on the triplet properties have been studied. A comparison has been made with the corresponding transients produced from juglone (5-hydroxy-1,4-naphthoquinone).

Organic reactions in liquid crystalline solvents. 4. Nanosecond laser flash photolysis studies of intramolecular motions of rod-like solutes in liquid crystals

The rates of triplet decay of a series of β-aryl-(4-alkoxypropiophenone)s in the smectic, nematic, and isotropic phases of 4′-butyl- and 4′-ethylbicyclohexyl-4-carbonitrile (BCCN and ECCN, respectively) have been measured over the 30–95 °C temperature range by nanosecond laser flash photolysis. The rates of triplet decay for these probe molecules in fluid solution are governed by the rates of Cα—Cβ bond rotation, which allows intramolecular quenching of the carbonyl triplet state by the β-aryl ring. The ketones are substituted with alkyl groups of varying length, shape, and flexibility in the para positions of the β-phenyl (H, n-hexyl, cyclohexyl) and benzoyl (meth-, n-pent-, and n-octoxy) rings. With the exception of β-phenyl-(4-methoxypropiophenone), for each ketone the Arrhenius parameters for triplet decay in the smectic phase of BCCN are similar to those in the nematic phase of the same solvent, and in all cases, the Arrhenius plots exhibit perfect continuity at the S—N transition temperature. A solvation model is tentatively advanced to explain these results. In the nematic phase of BCCN, the Arrhenius activation energy and entropy are significantly more positive than those in isotropic ECCN for all the ketones studied, but variations in the energetics of triplet decay in the nematic phase as a function of solute structure are parallelled in the isotropic solvent. Thus, the inhibiting effect of the nematic solvent on the bond rotations leading to intramolecular triplet quenching in these probes is attributed to the predominant influence of microviscosity (viscous drag) effects; the presence of solvent orientational order appears to have little or no effect on the intramolecular mobility of these ketones. The results and conclusions of earlier studies of unimolecular reactions in nematic solvents are discussed in light of these results.

Picosecond and nanosecond laser flash photolysis study of some halogenated benzene derivatives

Picosecond laser photolyses of benzyl chloride, benzyl bromide and benzyl iodide vapour have been examined by monitoring the time profiles of fluorescence and absorption of benzyl radicals formed in the 266 nm photolysis. The decays of fluorescence for these molecules were well represented by two exponential functions. The short-lifetime component has a value of ca. 1 ns, and the longer one 5–9 ns. The time-resolved absorption of benzyl radicals obtained in the photolysis of benzyl chloride (0.5 Torr) showed that the rise time of benzyl radical absorption, 6 ns, could be attributed to the decomposition rate of benzyl chloride molecules under collision-free conditions, while the rise time for benzyl radicals formed in the photolyses of benzyl bromide and benzyl iodide was ca. 1 ns. Thus the photodecomposition rate of benzyl chloride is in good correlation to the value of the slow component and those of benzyl bromide and benzyl iodide to values of short components.