Laser Flash Photolysis Techniques Research Articles

Atmospheric aqueous phase radical chemistry of the isoprene oxidation products methacrolein, methyl vinyl ketone, methacrylic acid and acrylic acid – kinetics and product studies

Kinetic and mechanistic studies were conducted on the isoprene oxidation products methacrolein, methyl vinyl ketone, methacrylic and acrylic acid reacting with hydroxyl and nitrate radicals and sulfate radical anions in aqueous solution by use of the laser flash photolysis technique and a reversed-rate method for kinetics. High-performance liquid chromatography/mass spectrometry was applied for product analysis. The kinetic investigations show the highest reactivity of the hydroxyl radical followed by sulfate and nitrate radicals. For methacrolein and methyl vinyl ketone the following rate constants have been determined at 298 K: k(OH+methacrolein) = (9.4 ± 0.7) × 10(9) M(-1) s(-1), k(OH+methyl vinyl ketone) = (7.3 ± 0.5) × 10(9) M(-1) s(-1), k(NO3+methacrolein) = (4.0 ± 1.0) × 10(7) M(-1) s(-1), k(NO3+methyl vinyl ketone) = (9.7 ± 3.4) × 10(6) M(-1) s(-1), k(SO4(-)+methacrolein) = (9.9 ± 4.9) × 10(7) M(-1) s(-1) and k(SO4(-)+methyl vinyl ketone) = (1.0 ± 0.2) × 10(8) M(-1) s(-1). Temperature and pH dependencies of the reactions of OH, NO3 and SO4(-) with methacrolein, methyl vinyl ketone, methacrylic and acrylic acid as well as Arrhenius parameters have been obtained and discussed. Product studies were performed on the OH radical induced oxidation of methacrolein and methyl vinyl ketone. In the reaction of methacrolein + OH methylglyoxal and hydroxyacetone were identified as first oxidation products with yields of 0.099 and 0.162. Methylglyoxal was primarily produced in the oxidation of methyl vinyl ketone with a yield of 0.052. For both precursor compounds the formation of glycolaldehyde was observed for the first time with yields of 0.051 and 0.111 in the oxidation of methacrolein and methyl vinyl ketone, respectively. Furthermore, highly functionalised C4 compounds were determined from the oxidation of both precursor compounds, but for the first time for methyl vinyl ketone. Reaction schemes were developed based on known peroxyl radical reaction mechanisms. The aqueous phase conversion of the first generation isoprene oxidation products can potentially contribute to tropospheric aqueous phase budgets of important carbonyl and dicarbonyl components which are expected to be conducive to the formation of aqSOA.

Green light sensitive diketopyrrolopyrrole derivatives used in versatile photoinitiating systems for photopolymerizations

The monomeric (DPPDT) and polymeric (PDQT) diketopyrrolopyrrole–thiophene derivatives combined with an iodonium salt or an amine (and optionally an additive) are used as photoinitiating systems to initiate the cationic polymerization of epoxides or divinyl ethers, radical polymerization of acrylates and thiol–ene polymerization under different irradiation sources (i.e. very soft halogen lamp, laser diodes at 473 nm, 532 nm and 808 nm). The DPPDT based systems are very efficient upon the green light exposure. Remarkably, these systems are characterized by a higher efficiency than references based on camphorquinone, a well known photosensitizer for visible light. The photochemical mechanisms are studied by steady state photolysis, fluorescence, cyclic voltammetry, laser flash photolysis and electron spin resonance spin trapping techniques.

Silicon phthalocyanine-azobenzene-[60]fullerene light harvesting pentad: synthesis, characterization and electron transfer reaction studied by laser flash photolysis

Electron-transfer reaction of the newly synthesized light harvesting pentad composed of silicon phthalocyanine (SiPc) that is connected with two fullerene C60and two azobenzene units to form SiPc -(azobenzene)2-( C60)2pentad has been studied by laser flash photolysis and other complementary techniques. This combination between SiPc , azobenzene and C60in the examined SiPc -(azobenzene)2-( C60)2pentad leads to strong light absorption over the whole visible spectrum. Photoexcitation of the pentad results in rapid formation of the charge-separated state by photoinduced electron transfer from the singlet-excited state of the SiPc moiety to the C60moiety. The charge-separated state has a lifetime of 2.50 ns in benzonitrile.

Laser flash photolysis study on the quenching reaction of the excited triplet state of acenaphthenequinone by antioxidant vitamin C

The photo-induced reactions of acenaphthenequinone (ACQ) as well as ACQ and vitamin C (VC) in ACN–H2O (v/v, 4:1) mixed solvent have been investigated by means of laser flash photolysis technique. The reaction intermediates have been identified and the reaction mechanism has been analyzed. The self-quenching rate constant of 3ACQ* by ground state ACQ and the quenching rate constant of 3ACQ* by biological antioxidant VC have also been measured. The results indicate that there is a hydrogen transfer from VC to 3ACQ* during the laser photolysis. VC is an effective quencher to 3ACQ* and the quenching rate constant is 4.0 × 108 L mol−1 s−1.

Multicolor Photoinitiators for Radical and Cationic Polymerization: Monofunctional vs Polyfunctional Thiophene Derivatives

Thiophene and polythiophene derivatives have been prepared and used as photoinitiators upon visible light exposure. Their abilities to initiate, when combined with an iodonium salt (and optionally N-vinylcarbazole), a ring-opening cationic photopolymerization of epoxides and radical photopolymerization of acrylates under various different irradiation sources (i.e., very soft halogen lamp irradiation, laser diode at 405, 457, 473, 532, and 635 nm and blue LED bulb at 462 nm) have been investigated. These systems are characterized by a remarkable performance for purple to red light exposure. They are also particularly efficient for the cationic and radical photopolymerization of an epoxide/acrylate blend in a one-step hybrid cure and lead to the formation of an interpenetrated polymer network IPN (30 s for getting tack-free coatings). Their migration stability is excellent in the cured IPNs. The photochemical mechanisms are studied by steady state photolysis, fluorescence, cyclic voltammetry, electron spin resonance spin trapping, and laser flash photolysis techniques.

Red‐Light‐Induced Cationic Photopolymerization: Perylene Derivatives as Efficient Photoinitiators

Nine different perylene derivatives are prepared and their ability to initiate, when combined with an iodonium salt (and optionally N-vinylcarbazole), a ring-opening cationic photopolymerization of epoxides under very soft halogen lamp irradiation is investigated. One of them is particularly efficient under a red laser diode exposure at 635 nm and belongs now to the very few systems available at this wavelength. The photochemical mechanisms are studied by steady-state photolysis, electron spin resonance spin trapping, fluorescence, cyclic voltammetry, and laser flash photolysis techniques.

Acridinediones: Effect of Substituents on Their Photoinitiating Abilities in Radical and Cationic Photopolymerization

Ten acridinediones (ADs) with different substituents are prepared and investigated for their abilities to initiate a ring‐opening cationic photopolymerization in combination with an iodonium salt and N‐vinylcarbazole upon UV–vis light (Xe–Hg lamp; >330 nm) or visible light (halogen lamp) exposure. The structural effects in the ADs are outlined. The cationic and radical photopolymerization of an epoxide/acrylate blend in a one‐step hybrid cure very efficiently leads to the formation of an interpenetrated polymer network under UV–vis light or visible light irradiation (30 s for tack‐free coatings). The photochemical mechanisms are studied by molecular orbital calculations, steady‐state photolysis, electron spin resonance–spin trapping, cyclic voltammetry, fluorescence, and laser flash photolysis techniques. image

Panchromatic Photopolymerizable Cationic Films Using Indoline and Squaraine Dye Based Photoinitiating Systems.

The photoinitiating abilities of indoline and squaraine dyes (D102 and SQ02) incorporated in multicomponent systems for the cationic polymerization of an epoxide or a vinyl ether have been investigated. The polymerizable films exhibit a panchromatic character as revealed by their photosensitivity to a halogen lamp (370-800 nm); household LED bulbs centered at 462 nm (blue), 514 nm (green), 591 nm (yellow), and 630 nm (red); and laser diodes at 457, 473, 532, and 635 nm. SQ02 is particularly efficient in the 520-700 nm range, while D102 exhibits a good efficiency in the 400-580 nm region. The radical photopolymerization of an acrylate can also be observed particularly at 635 nm or upon a halogen lamp. The photochemical mechanisms are studied by steady state photolysis, fluorescence, cyclic voltammetry, electron spin resonance spin trapping, and laser flash photolysis techniques.

Synthesis and photochromic reaction kinetics of unsaturated spiropyran derivatives

Abstract The absorption spectra and decay kinetics of the ground and triplet states of merocyanine (MC) form of spiropyran vinylogs have been investigated by nanosecond laser flash photolysis techniques in toluene and ethanol. The 3 MC decays to the ground state MC either by quenching with O 2 or due to the intersystem crossing. Relative quantum yield of both 3 MC and MC drastically decreases when the terminal polar group is separated from the indoline moiety by the C C-linker. In the presence of the C C-linker the replacement of terminal CN by CHO has no effect. The yield of 3 MC and corresponding MC in ethanol is 5 times smaller than that in toluene solution.

Photochemical reaction of 2-chlorobiphenyl with N(III) (H2ONO+/HONO/NO2−) in acidic environment studied by using co-linear laser flash photolysis

Photo-induced transformation in the presence of N(III) is an important pathway for pollutants conversion in atmospheric condensed phase. In this study, the photochemical reaction between 2-chlorobiphenyl and N(III) was investigated in acidic environment by using co-linear laser flash photolysis technique. The photo-induced conversion of 2-PCB is initiated from its rapid oxidation by OH, a primary product from the photolysis of N(III). The second-order rate constant for the reaction between 2-PCB and OH was determined to be (8.9 ± 1.2) × 109 M−1 s−1. HONO and NO2− were found to be more important than H2ONO+ with respect to the photochemical transformation of pollutants in atmospheric hydrometeors. The complicated photochemical conversion pathways and their significance in the atmosphere were discussed as well in this paper.

Note: A laser-flash photolysis and laser-induced fluorescence detection technique for measuring total HO2 reactivity in ambient air

A novel instrument for measuring total HO2 reactivity in the troposphere was successfully developed using a laser-flash photolysis and laser-induced fluorescence detection technique. Validation and testing were conducted through kinetic measurements of the reaction of HO2 radicals with NO2, and the results were found to be in good agreement with recent recommended values. The limit of detection (LOD) for HO2 loss rate measurement is achieved to be 0.024 s(-1) (3σ) with 60 times decay integrations. An observation of ambient air was carried out in a suburb of Tokyo to test the practical use of the developed instrument and un-expected rapid HO2 loss rate has been observed.

Behavior of Drug Excited States within Macromolecules: Binding of Colchicine and Derivatives to Albumin

The aqueous solutions of colchicine (COL) and analogues such as colchiceine (CEI), and N-deacetylcolchiceine (DCEI) have been studied by laser flash photolysis (LFP) with and without the presence of human serum albumin (HSA) to determine the possible interactions between the drugs and the albumin. When irradiation of these drugs was performed in aqueous media, transient absorption species were not detected. However, triplet excited states of CEI and DCEI were detected when the experiments were carried out in the presence of albumin. Contrary to expectations, in the case of COL, no transient absorption species was observed. A deep study of COL triplet excited-state properties has revealed that intersystem crossing quantum yield (φ(ISC)) decreases from organic media such as dichloromethane to water with φ(ISC) values ca. 0.035 and <0.001 respectively. This fact together the inappreciable reactivity of (3)COL with tyrosine and tryptophan agree with the unbinding of COL to HSA. This study will show for the first time a way to determine the association constant (K(a)) using the LFP technique. Thus, for CEI and DCEI binding to HSA, K(a) values of 8 ± 3 × 10(4) M(-1) and 2 ± 1 × 10(4) M(-1) were obtained, respectively. Moreover, it was also established that these drugs are mainly placed into site II of the albumin using this technique. Therefore, this study validates the LFP as a useful methodology to study the interactions of COL and its analogues with serum albumins, and consequently it could be applied to others proteins such as tubulin.

One-electron oxidation and reduction of glycosaminoglycan chloramides: A kinetic study

Hypochlorous acid and its acid–base counterpart, hypochlorite ions, produced under inflammatory conditions, may produce chloramides of glycosaminoglycans, these being significant components of the extracellular matrix (ECM). This may occur through the binding of myeloperoxidase directly to the glycosaminoglycans. The N–Cl group in the chloramides is a potential selective target for both reducing and oxidizing radicals, leading possibly to more efficient and damaging fragmentation of these biopolymers relative to the parent glycosaminoglycans. In this study, the fast reaction techniques of pulse radiolysis and nanosecond laser flash photolysis have been used to generate both oxidizing and reducing radicals to react with the chloramides of hyaluronan (HACl) and heparin (HepCl). The strong reducing formate radicals and hydrated electrons were found to react rapidly with both HACl and HepCl with rate constants of 1–1.7×108 and 0.7–1.2×108M−1s−1 for formate radicals and 2.2×109 and 7.2×108M−1s−1 for hydrated electrons, respectively. The spectral characteristics of the products of these reactions were identical and were consistent with initial attack at the N–Cl groups, followed by elimination of chloride ions to produce nitrogen-centered radicals, which rearrange subsequently and rapidly to produce C-2 radicals on the glucosamine moiety, supporting an earlier EPR study by M.D. Rees et al. (J. Am. Chem. Soc.125: 13719–13733; 2003). The oxidizing hydroxyl radicals also reacted rapidly with HACl and HepCl with rate constants of 2.2×108 and 1.6×108M−1s−1, with no evidence from these data for any degree of selective attack on the N–Cl group relative to the N–H groups and other sites of attack. The carbonate anion radicals were much slower with HACl and HepCl than hydroxyl radicals (1.0×105 and 8.0×104M−1s−1, respectively) but significantly faster than with the parent molecules (3.5×104 and 5.0×104M−1s−1, respectively). These findings suggest that these potential in vivo radicals may react in a site-specific manner with the N–Cl group in the glycosaminoglycan chloramides of the ECM, possibly to produce more efficient fragmentation. This is the first study therefore to conclusively demonstrate that reducing radicals react rapidly with glycosaminoglycan chloramides in a site-specific attack at the N–Cl group, probably to produce a 100% efficient biopolymer fragmentation process. Although less reactive, carbonate radicals, which may be produced in vivo via reactions of peroxynitrite with serum levels of carbon dioxide, also appear to react in a highly site-specific manner at the N–Cl group. It is not yet known if such site-specific attacks by this important in vivo species lead to a more efficient fragmentation of the biopolymers than would be expected for attack by the stronger oxidizing species, the hydroxyl radical. It is clear, however, that the N–Cl group formed under inflammatory conditions in the extracellular matrix does present a more likely target for both reactive oxygen species and reducing species than the N–H groups in the parent glycosaminoglycans.

Difunctional acridinediones as photoinitiators of polymerization under UV and visible lights: Structural effects

Seven difunctional acridinediones DAD were prepared and investigated for their abilities to initiate a ring-opening cationic photopolymerization in combination with an iodonium salt (and optionally N-vinylcarbazole) upon UV/visible light (Xe–Hg lamp; >330 nm) or visible light (halogen lamp) exposure. The structural effects in the DADs and the role of the radical cation are outlined. The cationic and radical photopolymerization of an epoxide/acrylate blend in a one-step hybrid cure leads to the formation of an interpenetrated polymer network. The photochemical mechanisms are studied by fluorescence, steady state photolysis, cyclic voltammetry, electron spin resonance spin trapping, and laser flash photolysis techniques.

Nucleophilic Reactivities and Lewis Basicities of 2‐Imidazolines and Related N‐Heterocyclic Compounds

AbstractThe nucleophilicity parameters N and sN, as defined by the linear free‐energy equation log k(20 °C) = sN(N + E), of the 2‐imidazolines 1a–d and the related N‐heterocyclic compounds 2–5 have been determined by studying the rates of their reactions with differently substituted benzhydrylium ions in dichloromethane at 20 °C by stopped‐flow or laser flash photolysis techniques. It is demonstrated that the N and sN parameters thus obtained can be used to reliably predict the rate constants for their reactions with Michael acceptors of known electrophilicity E. A comparison of the nucleophilicity parameters of the imidazoline derivatives 1 with other commonly used nucleophilic organocatalysts shows that they are 10 to 103 times less nucleophilic than PPh3, 1,8‐diazabicyclo[5.4.0]undec‐7‐ene, or 4‐(dimethylamino)pyridine. The structure–reactivity relationships of these heterocycles are discussed.

Laser flash photolysis of benzophenone in thin silicone films

The decay kinetics of benzophenone (BP) triplet state in the thin (∼200nm) films of poly(dimethylsiloxane) have been studied with a nanosecond laser flash photolysis technique. Decay kinetics of a triplet state of BP (3BP) is dispersive kinetics, and it can be well described by the kinetic law with Gaussian distribution of the activation free energy. The average rate constants of 3BP decay kav are 1500 and 70s−1 for air saturated and deoxygenated films, respectively. Width of distribution are 1.2 and 4.3 (dimensionless) and correspond to the first and second kav, respectively presented above. The rate of hydrogen abstraction of H-atom from Si–CH3 groups by 3BP is low which results in long-lived 3BP in silicone films.

A mechanistic study of the photodegradation of herbicide 2,4,5-trichlorophenoxyacetic acid in aqueous solution

Photodegradation of herbicide 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) in aqueous solution was investigated by stationary (254 nm) and nanosecond laser (266 nm) flash photolysis techniques. It was shown that in the primary photochemical step both photoionization (which generates a hydrated electron-radical cation pair) and heterolytic cleavage of a C-Cl bond takes place. The major products of substitution of one of the chlorine atoms in the 2-, 4- or 5-position by a hydroxyl group as well as the products of hydroxylation of the benzene ring in 3- and 6-positions were identified by HPLC and LC-MS methods. The complexation of 2,4,5-T with β- and γ-cyclodextrins (β(γ)CD) was investigated. The influence of such complexation on the quantum yield of herbicide photolysis and on the ratio of photodegradation products was determined.

Naphthalimide based methacrylated photoinitiators in radical and cationic photopolymerization under visible light

The abilities of two naphthalimide derivatives with a methacryloyl group to initiate, when incorporated in multi-component systems, a ring-opening polymerization of epoxides and a radical polymerization of acrylates under different irradiation sources (e.g. very soft halogen lamp irradiation, laser diode at 457 nm, laser diode at 405 nm and blue LED bulb at 462 nm) have been investigated. One of them is particularly efficient for the cationic and radical photopolymerization of an epoxide/acrylate blend in a one-step hybrid cure and leads to the formation of an interpenetrated polymer network IPN (30 s for getting tack free coatings). The migration stability of one of these naphthalimide derivatives is found to be excellent in the cured polyacrylates and IPNs. The photochemical mechanisms are studied by steady state photolysis, fluorescence, cyclic voltammetry, electron spin resonance spin trapping and laser flash photolysis techniques.

Heterogeneous Photoinduced Homolytic Aromatic Substitution of Electron‐Rich Arenes with Perfluoroalkyl Groups in Water and Aqueous Media – A Radical‐Ion Reaction

AbstractThe photoinduced electron transfer (PET) substitution reaction of electron‐rich aromatic nuclei with perfluoroalkyl Rf groups was carried out in water or aqueous mixtures to render substitution products resulting from replacement of aromatic H atoms with the Rf moiety in good yields (57–88 %). Some mechanistic aspects are discussed, supporting the notion of a PET reaction leading to a classical radical homolytic aromatic substitution (HAS) followed by an electron transfer (ET) and then a proton transfer (PT) sequence. A radical mechanism superimposed on a redox process is proposed to account for product formation. Evidence for the radical cation species (as an initiation event) generated from electron‐rich arenes in the presence of perfluoroalkyl halides is provided by the UV/Vis transient spectra obtained by Nanosecond Laser Flash Photolysis techniques.

Kinetics of photochemical reactions of multifunctional hybrid compounds based on spironaphthoxazines upon photoexcitation with light of different wavelengths

Abstract Four hybrid compounds (HCs), which molecules contain two or three photochromic fragments of different nature, have been examined by the nanosecond laser flash photolysis techniques using excitation wavelengths of 337 and 430 nm in methanol solutions. In HC molecules the photochromic spironaphthoxazine fragment, which is able to generate a colored merocyanine form B upon UV excitation, is linked to the substituted salicylideneaniline (SA) fragment, where reversible photochemical transformations from cis -enol to trans -keto form having distinguishably different absorption spectra can occur. An azobenzene group prones to trans–cis photoisomerization is the substituent in SA fragment in the fourth compound. Four intermediates of different nature were found. A decrease in the relative efficiency of the ESIPT process with respect to that of spiro-bond cleavage was found to be the result of the influence of the electronegative substituents in SA fragments. The excitation light wavelength was shown to be the main factor determining the relative yields of the intermediates of different nature for the hybrid compound with the azobenzene group. The mechanism of different intermediates formation is discussed.