Conventional Flash Photolysis Research Articles

Simultaneous Determination of the Chemical (kr ) and the Physical (kq ) Quenching Rate Constants of Singlet Oxygen in Aqueous Solution by the Chemiluminescence-quenching Method.

This work reports a novel and visual method for the simultaneous determination of the chemical (kr ) and the physical (kq ) quenching rate constants of singlet oxygen (1 O2 ,1 ∆g ) in aqueous media. It is based on the disruption, by a water-soluble substrate S, of the 1 O2 chemiluminescence (CL) generated by the H2 O2 /Na2 MoO4 catalytic system. A mathematical analysis of the CL signal at 1270 nm vs time provides separately the overall (kr + kq ) and the chemical (kr ) quenching rate constants. In ordinary water (H2 O), 1 O2 lifetime is short and the CL intensity is weak allowing solely the investigation of very reactive substrates for which (kr + kq ) > 3 × 106 m-1 s-1 while, in D2 O, 1 O2 lifetime is significantly longer lifetime and the CL signal is much stronger allowing the study of poorly reactive substrates for which (kr + kq ) > 4 × 105 m-1 s-1 . The method has been successfully tested on a series of anionic and nonionic water-soluble naphthalene derivatives commonly used as bio-compatible 1 O2 carriers. The obtained kr and kq values are in good agreement with the values determined by conventional techniques, namely, flash photolysis and competitive kinetics with a reference quencher.

Laser flash photolysis with nanoliter samples: photonic crystal fibers as ultrasmall smart test tubes

Photonic crystal fibers play a dual function as a sample container as well as a photonic device supporting mode structure, and in doing so offer new opportunities for carrying out laser flash photolysis experiments with ultrasmall samples. Monitoring light can be transmitted in the fiber by total internal reflection or by the photonic bandgap guiding effect, depending upon the relative refractive index of the fiber material (usually silica) and the liquid filling the holes. Both offer opportunities for transient monitoring in time scales as short as tens of nanoseconds, with enhanced signals with respect to conventional (large sample) laser flash photolysis work.

Reactions of Cl•/Cl2•− Radicals with the Nanoparticle Silica Surface and with Humic Acids: Model Reactions for the Aqueous Phase Chemistry of the Atmosphere

Reactions of chlorine radicals might play a role in aqueous aerosols where a core of inorganic components containing insulators such as SiO2 and dissolved HUmic-LIke Substances (HULIS) are present. Herein, we report conventional flash photolysis experiments performed to investigate the aqueous phase reactions of silica nanoparticles (NP) and humic acid (HA) with chlorine atoms, Cl*, and dichloride radical anions, Cl2*-. Silica NP and HA may be taken as rough models for the inorganic core and HULIS contained in atmospheric particles, respectively. Both Cl* and Cl2*- were observed to react with the deprotonated silanols on the NP surface with reaction rate constants, k +/- sigma, of (9 +/- 6) x 10(7) M(-1) s(-1) and (7 +/- 4) x 10(5) M(-1) s(-1), respectively. The reaction of Cl* with the surface deprotonated silanols leads to the formation of SiO* defects. HA are also observed to react with Cl* and Cl2*- radicals, with reaction rate constants at pH 4 of (3 +/- 2) x 10(10) M(-1) s(-1) and (1.2 +/- 0.3) x 10(9) M(-1) s(-1), respectively. The high values observed for these constants were discussed in terms of the multifunctional heterogeneous mixture of organic molecules conforming HA.

Electron transfer reactions of organic sulfoxides with photochemically generated ruthenium(III)–polypyridyl complexes

The electron transfer (ET) reaction of aryl methyl sulfoxides with ruthenium(III)–polypyridine complexes is sensitive to the change of substituent in the aryl moiety of ArS(O)CH 3 and ligand of Ru(III) complex. The detection of sulfoxide radical cation as the transient by conventional flash photolysis confirms ET in the rate-controlling step. The successful application of Marcus cross relation of ET leads to the evaluation of self-exchange rate constant of ArS +(O)CH 3/ArS(O)CH 3 as 4.0×10 5 M −1 s −1 similar to organic sulfides. Comparison with the reactivity of iron(III)–polypyridyl complexes points out that both reactivity and ρ values are higher with Ru(III) complexes.

Kinetic Study of the Self-Reactions of the BrCH2CH2O2 and BrCH(CH3)CH(CH3)O2 Radicals between 275 and 373 K

A conventional flash photolysis technique was used to measure the self-reaction rate constants of the primary BrCH 2 CH 2 O 2 (2-bromoethylperoxy) and secondary BrCH(CH 3 )CH(CH 3 )O 2 (2-bromo- 1 -methylpropylperoxy) β-brominated peroxy radicals, at temperatures in the range of 275 -373 K. The absolute UV absorption spectra of BrCH 2 CH 2 O 2 and BrCH(CH 3 )CH(CH 3 )O 2 were also measured and compared to those obtained previously for these radicals. The temperature dependence of the self-reaction rate constants provided the following Arrhenius expressions: k(BrCH 2 CH 2 O 2 + BrCH 2 CH 2 O 2 ) = (6.15 + 5 . 5 1 - 2 . 9 1 ) x 10 - 1 4 exp{(1247 ′ 203) K/T} cm 3 molecule - 1 s - 1 and k(BrCH(CH 3 )CH(CH 3 )O 2 + BrCH(CH3)CH(CH3)O 2 ) = (7.60 + 2 2 . 0 5 - 5 . 6 5 ) x 10 - 1 5 exp{(1305 ′ 428) K/T} cm 3 molecule - 1 s - 1 , where the uncertainties represent 95% confidence limits associated with the statistical fitting procedure and include the contribution for the expanded uncertainties in the individual rate constant. These results confirm the enhancement of the peroxy radical self-reaction reactivity upon β-substitution, which is similar for Br, Cl, or OH substituents. Structure-activity relationships are proposed for self-reactions of β-substituted peroxy radicals.

Kinetics and mechanism of the gas-phase reaction of the cyclohexadienyl radical c-C6H7with O2

The kinetics and mechanism of the gas-phase reaction of the cyclohexadienyl radical c-C6H7 with O2 have been investigated using both experimental and theoretical approaches. The rate constant has been measured using conventional flash photolysis in the temperature range 302–456 K, 1 atm pressure. c-C6H7 radicals were produced by reacting Cl atoms with 1,4-cyclohexadiene. The rate expression is k1=(1.4±0.26)×10−13 exp[−(300±74) K/T] cm3 molecule−1 s−1 (2σ error bars). The reaction can proceed either by association, yielding a peroxy radical RO2 or by H-abstraction, yielding benzene+HO2, the two reaction channels involving two distinct transition states. In contrast to what is observed for the c-C6H6OH radical, no equilibrium with the peroxy radical could be characterised. The theoretical approach, involving both DFT and ab initio methods, was used to determine if the measured rate constant should be assigned to the association or to the H-abstraction channel. Comparison of experimental and theoretical results shows that H-abstraction must be the only significant reaction channel.

Identification of transient radical species in microbial systems using diffuse reflectance laser flash photolysisDedicated to Professor Frank Wilkinson on the occasion of his retirement.

Conventional and diffuse reflectance laser flash photolysis are used to study the disulfonated aluminium phthalocyanine radical anion (AlPcS2−˙), both in solution and in a microbial cell environment. The transient absorption of the AlPcS2−˙ is characterised using model quenchers and the bimolecular quenching rate constants (kQ) for several chemical components present in the outer membranes/cell structures of the microbes studied with AlPcS2 are determined. The generation of the AlPcS2−˙ by the microbial cells is investigated, and the relative quantum yields of this process are calculated, together with the transient lifetime of the AlPcS2−˙ in a microbial cellular environment. Supporting evidence is found for the involvement of type I processes in the overall photodynamic mechanism of AlPcS2 with the microbes studied. Several biological substrates present in the outer cell structures of the microbial cells studied are capable of quenching the AlPcS2 triplet state to generate the AlPcS2 radical anion (AlPcS2−˙), which is the reactive species that mediates type I photodynamic mechanisms. The transient absorption band of the AlPcS2−˙ is observed in the triplet minus singlet absorption spectra of AlPcS2 in the presence of each of the microbes.

Photooxidation of nickel(II) macrocyclic complexes on excitation in the charge-transfer-to-solvent band in aqueous solution and in the presence of oxygen

Photooxidation of the well known nickel(II)-macrocyclic complexes [Ni(tet-a)] (ClO 4) 2, 1, and [Ni(AT)] ClO 4, 2, (where tet-a=- meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane and AT=11,13-dimethyl-1,4,7,10-tetraazacyclotetradeca-10,12-dienato) has been observed for the first time on excitation in the charge transfer to solvent band. The adduct formed in the primary photochemical process with the complex 1 is efficiently scavenged in presence of H + and O 2 to produce HO 2 radical. This radical further oxidizes the nickel(II) macrocyclic complex to produce nickel(III) complex in the secondary thermal reaction as seen by the growth in absorbance at 370 nm in the conventional flash photolysis setup. Rate constants for the formation of nickel(III) by the reaction of HO 2 radical with 1 was determined as a function of the concentration of the complex and hydrogen ion. Conventional flash photolysis of the complex 2 in presence of N 2O produces a transient which shows maximum at 610 nm. Stable nickel(III) complex is isolated on continuous photolysis of the complex 1 and characterized by electronic, IR, ESR spectroscopy and CHN analysis. In deaerated acidic aqueous solution containing alcohols, photolysis of 1 produced hydrogen gas as evidenced by gas chromatographic analysis. Quantum yield for the photooxidation of 1 to give the nickel(III) complex is found to be 0.03±0.005.

Kinetic Study of the Reactions of Chlorine Atoms and Cl2•- Radical Anions in Aqueous Solutions. II. Toluene, Benzoic Acid, and Chlorobenzene

Laser and conventional flash photolysis of Na2S2O8 aqueous solutions containing Cl- ions were employed to investigate the reactions of chlorine atoms and Cl2•- radical ions with toluene, benzoic acid, and chlorobenzene. A mechanism is proposed which accounts for the faster decay of Cl2•- in aqueous solutions containing increasing concentrations of the organic substrates. Interpretation of the experimental data is supported by kinetic computer simulations. Chlorine atoms react with the three substituted aromatics studied here almost with diffusion-controlled rate constants, k = (1.8 ± 0.3) × 1010 M-1 s-1. The high reactivity observed for Cl atoms contrasts with that of the Cl2•- radical ions, for which the rate constant for its reactions with the substituted benzenes is ≤ 1 × 106 M-1 s-1. The organic radicals produced from these reactions, as well as the nature of the reaction products are discussed. The observed results seem to support an addition mechanism yielding chlorocyclohexadienyl radicals (Cl−CHD)...

Photoinduced chemical crosslinking activity and photo-oxidative stability of amine acrylates: photochemical and spectroscopic study

A number of dialkyl and heterocyclic amine-terminated diacrylate resins were prepared by a Michael addition reaction of the appropriate secondary amine with a triacrylate diluent monomer. Photocuring rates are found to be dependent upon the extent of UV absorption of the photoinitiator used with ITX in this case exhibiting the most rapid cure. There is no consistency in cure rate with amine structure. Transient absorption spectra on conventional microsecond flash photolysis are assigned to the formation of free radical intermediates by electron transfer. Photoinduced polymerisation activities of the resins measured by real-time infrared spectroscopy appear to be closely related to the formation of such transient species. The initiator structure is the more determining factor. Their photo-oxidative stability and photoyellowing (by UV derivative absorption) are studied after UV and electron-beam curing through Fourier transform infra-red (FTIR) and second derivative UV spectroscopic methods and hydroperoxide analysis. On photo-oxidation UV cured diluent monomer undergoes oxidation at a higher rate than EB cured systems. Initial photo-oxidation of the coatings as measured by hydroperoxide analysis and hydroxyl index via FTIR analysis exhibited similar changes. Again the oxidation profiles are dependent upon the terminal amine structure with 1-methylpiperazine exhibiting the strongest oxidation. Hydroxyl index showed generally an initial rise followed by a sharp decline and then a slow increase over much longer irradiation periods. The presence of the amine functionality is found to be an effective scavenger of oxygen and hydroperoxide formation in EB cured coatings. In the UV cured coatings the hydroperoxide levels are found generally to be significantly higher than in EB systems due to the photosensitising effect of the residual photoinitiator. Diluent monomer terminated with dialkylamine groups are found to be more prone to oxidation and UV absorption increase than alkanolamines, cycloaliphatic amines and heterocyclic amines. UV cured resins exhibit a more facile transient photoyellowing than the same electron-beam cured systems, and this is associated with hydrogen-atom abstraction and oxidation of the alkylamine group by the residual photoinitiator enhancing the rate of hydroperoxidation of the amine group.

PH dependence of the protein orientation in self‐assembled bacteriorhodopsin/polycation multilayer films

Bacteriorhodopsin (bR) multilayer films were successfully prepared by alternatingdepositions of cationic poly(diallyldimethylammonium chloride) (PDAC) polyelectrolyte and purple membrane (PM). The advantage of the method when compared to other thin film techniques is its simplicity to prepare large selfassembled films. The orientations of the bR molecules in the films were studied by a transient photovoltage method. The polarity of the photoresponse signals showed that in basic conditions the cytoplasmic side of PM was adsorbed onto the polyelectrolyte layer, whereas in acidic conditions the extracellular side was adsorbed onto the polyelectrolyte layer. The photocycle of the bR in the films was studied by usingthe conventional flash photolysis method. The photochemical behavior of bR in dry polycationic films was similar to that in other types of dry films and the intermediate lifetimes were almost independent of the protein orientation.The photovoltage signals introduce opposite proton transport directions for the formation of the K intermediate of the photocycle and for the successive relaxation to M intermediate. The charge displacement associated to the M intermediate was several times greater than that of the K intermediate. The fittings of the transient photovoltage signals resulted the values of 2, 15, and 70 microseconds for the time constants of the 3‐exponential formation of the M intermediate. The kinetic data were compared to the results obtained with the transient absorption measurements.

Kinetics and mechanism of formation, acid catalysed aquation, reversible anation and photochemical reaction of trans-(aqua)(sulfito-S )[N,N′-ethylenebis(salicylidiniminato)]cobaltate(III) in aqueous media †

The reaction of trans-[Co(salen)(OH2)OH] with SO2 yields trans-[Co(salen)(OH2)(SO3-S)]− (S-bonded isomer) for which the rate and activation parameters at 25 °C (I = 0.3 mol dm−3) are kSO2 = (5.9 ± 0.1) × 1010 dm3 mol−1 s−1, ΔH‡ = 66 ± 4 kJ mol−1 and ΔS‡ = 183 ± 14 J K−1 mol−1. One possibility for the SIV substitution is that Co–S bond formation is concerted with Co–O bond breaking. An alternative mechanism, involving a fast equilibrium between SO2 and trans-[Co(salen)(OH2)OH] forming an O-bonded sulfito species which then undergoes sulfite ligand linkage isomerisation, is also possible. An estimated value of the isomerisation rate constant for the trans-[Co(salen)(OH2)(OSO2H)] at 25 °C is ca. 106 s−1. The trans-[Co(salen)(OH2)(SO3-S)]− (pK = 10.1 ± 0.1 at 25 °C, I = 0.3 mol dm−3) undergoes acid catalysed aquation to yield the parent diaqua complex and SIV with kH = 29.5 ± 1.1 dm3 mol−1 s−1, ΔH‡ = 72 ± 3 kJ mol−1, ΔS‡ = 24 ± 9 J K−1 mol−1 at 25 °C (I = 0.3 mol dm−3). Steady state photolysis (254 nm) of trans-[Co(salen)(OH2)(SO3-S)]− resulted in the reduction of CoIII. The redox rate constant and ϕ(Co2+) decreased with increasing pH. Attempts to detect an O-bonded sulfito complex as a transient in the conventional flash photolysis of this aqua-sulfito complex proved unsuccessful. The aqua ligand replacement reactions of trans-[Co(salen)(OH2/OH)(OH2)]+/0 with imidazole and that of the corresponding aqua-sulfito complex with N3−, NCS−, imidazole, and SIV in a large excess of the entering ligands have been studied at 25 °C. A comparison of the rate constants with the analogous data for trans-[Co(AA)2(OH2)(SO3-S)]+ (AA = 1,2-diaminoethane; 1,3-diaminopropane) clearly shows that the kinetic trans-effect of the S-bonded sulfite is substantially attenuated in trans-[Co(salen)(OH2)(SO3-S)]−.

Reaction of sulfate and phosphate radicals with α,α,α-trifluorotoluene

The aqueous phase reactions of α,α,α-trifluorotoluene (TFT) with sulfate and phosphate radicals have been investigated by conventional flash-photolysis. The bimolecular rate constants obtained for the reactions of SO4˙–, H2PO4˙, HPO4˙– and PO4˙2– with TFT are (2 ± 1) × 107, (3.5 ± 0.5) × 107, (2.7 ± 0.5) × 106 and (9 ± 1) × 105M–1 s–1, respectively. The organic radicals produced from these reactions exhibit a common absorption peak at 290–300 nm. Additionally, the transients generated from sulfate radicals show a shoulder at 360–370 nm and those generated from phosphate radicals a less intense maximum at 400 nm. Based on the experimental results, the nature of the organic radicals is discussed. Additional information on the hydroxycyclohexadienyl radicals of TFT is obtained from independent experiments with hydrogen peroxide solutions containing TFT.

Rate constants of the thermal cis-trans isomerization of azobenzene dyes in solvents, acetone/water mixtures, and in microheterogeneous surfactant solutions

Rate constants kiso of the thermal cis-trans isomerization of four 4,4’-nitro-aminoazobenzenes with different amino groups have been determined in homogeneous aprotic solvents and polyglykol oligomers, primarily by means of conventional flash photolysis. The rate constants have been correlated with polarity (according to λmax from UV/Vis absorption spectra of the trans isomers) and bulk viscosity of the solvents. Qualitative conclusions about the influence of varying concentrations of water with respect to polarity and hydrogen bonding on kiso- and λmax-values in acetone/water mixtures were derived. Based on these results the data from microheterogeneous solutions have been interpreted. In microheterogeneous water/surfactant solutions kiso-values of selected azo dyes were strongly dependent on the concentrations of SDS, Triton®X-100, C12EO8 in water, and varied with the composition of bicontinuous microemulsions of Igepal® CA-520/ heptane/water. The large spread of isomerization rate constants is in part due to varying microviscosity. Replacement of H2O by D2O in aqueous surfactant solutions produced surprisingly large kinetic solvent isotope effects. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 337–350, 1999

Kinetic studies in liquid and gelated water/triblock copolymers mixtures

AbstractRate constants of the thermal cis‐trans isomerization of an azo dye in triblock copolymer (PEO‐PPO‐PEO)/water mixtures were determined by means of conventional flash photolysis. The rate constants change characteristically on micelle formation upon increasing the temperature. Moreover they indicate that the microenvironment of the dye molecules in the water/PEO interface layers of concentrated micellar solutions does not change significantly on gelation.

First steps in the photochemistry of folate in alkaline medium

A laser flash photolysis study is reported on alkaline solutions of folic acid in absence of dioxygen. Spectra of the intermediates and their decays are analysed. A biexponential fit of the experimental decay can be observed in the resolution time window of the technique. Intermediates spectra obtained in laser flash photolysis were compared with those generated in pulse radiolysis experiments performed on these solutions under oxidative and reductive conditions. Conventional flash photolysis were also applied in this studies in absence of O 2. In this case two processes are detected, one in the millisecond time window and a very slow process which can be followed by normal spectrophotometry after flashing solutions. The experimental behaviour of this system is compared with those reported on pterines derivatives, where absorption changes induced by flash photolysis have been attributed to triplet excited state. The evidence presented here shows that folate suffers an early photochemical transformation. Radicals generation as a consequence of the absorption of radiation is postulated in order to give a plausible explanation of the experimental results.

Study of bimolecular processes in ternary cyclodextrin complexes in the solid state by diffuse reflectance laser flash photolysis

The photochemistry of binary inclusion complexes of nitroveratrole (NV) and p-dinitrobenzene (DNB) with β-cyclodextrin (β-CD) and of ternary inclusion complexes of NV or DNB and L-phenylethylamine (PhEA) with β-CD has been examined in the solid state by diffuse reflectance laser flash photolysis. The binary NV/β-CD complex shows a transient absorption signal assigned to the triplet excited state of NV. The binary DNB/β-CD complex shows a transient signal assigned to the protonated anion of DNB formed via hydrogen abstraction from the β-CD cavity. Laser excitation of the ternary complexes generates the corresponding radical ion pairs via photoinduced electron transfer from the amine to the nitroaromatic triplet state. Comparison with previously described conventional laser flash photolysis studies in solution shows that the investigation of solid ternary CD inclusion complexes by diffuse reflectance laser flash photolysis is valuable in the case of bimolecular photoreactions that are difficult to study in solution due to the existence of other competitive processes.

Kinetics of the Cross Reactions of CH3O2 and C2H5O2 Radicals with Selected Peroxy Radicals

The kinetics of the reactions of selected peroxy radicals (RO2) with CH3O2 and with C2H5O2 have been investigated using two techniques: excimer-laser photolysis and conventional flash photolysis, both coupled with UV absorption spectrometry. Radicals were generated either by photolysis of molecular chlorine in the presence of suitable hydrocarbons or by photolysis of the appropriate alkyl chloride. All such cross-reaction kinetics were investigated at 760 Torr total pressure and room temperature except for the reaction of the allylperoxy radical with CH3O2, for which the rate constant was determined between 291 and 423 K, resulting in the following rate expression: k15 = (2.8 ± 0.7) × 10-13 exp[(515 ± 75)/T] cm3 molecule-1 s-1. Values of (2.0 ± 0.5) × 10-13, (1.5 ± 0.5) × 10-12, (9.0 ± 0.15) × 10-14, <2.0 × 10-12, (2.5 ± 0.5) × 10-12, and (8.2 ± 0.6) × 10-12 (units of cm3 molecule-1 s-1) have been obtained for the reactions of CH3O2 radicals with C2H5O2, neo-C5H11O2, c-C6H11O2, C6H5CH2O2, CH2ClO2, and CH3C(O)O2, respectively, and (1.0 ± 0.3) × 10-12, (5.6 ± 0.8) × 10-13, (4.0 ± 0.2) × 10-14, and (1.0 ± 0.3) × 10-11 (units of cm3 molecule-1 s-1) for the reactions of C2H5O2 with CH2CHCH2O2, neo-C5H11O2, c-C6H11O2, and CH3C(O)O2 radicals, respectively. These rate constants were obtained by numerical simulations of the complete reaction mechanisms, which were deduced from the known mechanisms of the corresponding peroxy radical self-reactions. A systematic analysis of propagation of errors was carried out for each reaction to quantify the sensitivity of the cross-reaction rate constant to the parameters used in kinetic simulations. The rate constant for a given cross reaction is generally found to be between the rate constants for the self-reactions of RO2 and CH3O2 (or C2H5O2). However, when the RO2 self-reaction is fast, the cross reaction with CH3O2 (or C2H5O2) is also fast, with similar rate constants for both reactions, suggesting that these particular peroxy radical cross reactions can play a significant role in the chemistry of hydrocarbon oxidation processes in the troposphere and in low-temperature combustion. Relationships between cross-reaction and self-reaction rate constants are suggested.

Time-Resolved Resonance Raman Scattering of Triplet State Anthracene in Supercritical CO2

The first report of time-resolved resonance Raman (TR3) scattering in a supercritical fluid is presented. TR3 spectra of the lowest triplet excited state (T1) of anthracene in supercritical (SC) CO2 have been obtained over the pressure range 90−500 bar. These data have been complemented by conventional flash photolysis measurements of the excited state lifetime, transient absorbance difference, and fluorescence spectra over a similar pressure range. The spectroscopic data show systematic changes with increasing pressure; the ΔA spectra of the T1 state recorded at two different temperatures display a red shift with increasing fluid pressure, which is in agreement with earlier work carried out over a smaller range of pressures. Similar shifts in the fluorescence are also observed. The vibrational frequencies of the T1 state of anthracene are found to be relatively insensitive to applied pressure; indeed, the transient bands are readily identified by comparison with resonance Raman (RR) spectra of the T1 sta...

Flash Photolysis Studies of 4-Chloroanisole in Aqueous Solution

Abstract The photoinduced formation of transients in aqueous 4-chloroanisole (4 ClA), at pH 7 and 9, were studied by conventional flash photolysis. The absorption spectra of various transients in airfree as well as in aqueous solutions saturated with N2O , air or pure oxygen, are presented and discussed.