Total Quenching Rate Constants Research Articles

Sensitized Photooxidation of Ortho-Prenyl Phenol: Biomimetic Dihydrobenzofuran Synthesis and Total 1O2 Quenching.

The sensitized photooxidation of ortho-prenyl phenol is described with evidence that solvent aproticity favors the formation of a dihydrobenzofuran [2-(prop-1-en-2-yl)-2,3-dihydrobenzofuran], a moiety commonly found in natural products. Benzene solvent increased the total quenching rate constant (kT ) of singlet oxygen with prenyl phenol by ~10-fold compared to methanol. A mechanism is proposed with preferential addition of singlet oxygen to prenyl site due to hydrogen bonding with the phenol OH group, which causes a divergence away from the singlet oxygen 'ene' reaction toward the dihydrobenzofuran as the major product. The reaction is a mixed photooxidized system since an epoxide arises by a type I sensitized photooxidation.

Interparticle Delivery and Detection of Volatile Singlet Oxygen at Air/Solid Interfaces.

An interparticle system has been devised, allowing airborne singlet oxygen to transfer between particle surfaces. Singlet oxygen is photogenerated on a sensitizer particle, where it then travels through air to a second particle bearing an oxidizable compound-a particulate-based approach with some similarities to reactive oxygen quenching in the atmosphere. In atmospheric photochemistry, singlet oxygen is generated by natural particulate matter, but its formation and quenching between particles has until now not been determined. Determining how singlet oxygen reacts on a second surface is useful and was developed by a three-phase system (particle-air-particle) interparticulate photoreaction with tunable quenching properties. We identify singlet oxygen quenching directly by near-IR phosphorescence in the airborne state and at the air/particle interface for total quenching rate constants (kT) of adsorbed anthracene trapping agents. The air/solid interface kT of singlet oxygen by anthracene-coated particles was (2.8 ± 0.8) × 107 g mol-1 s-1 for 9,10-dimethylanthracene and (2.1 ± 0.9) × 107 g mol-1 s-1 for 9,10-anthracene dipropionate dianion, and the lifetime of airborne singlet oxygen was measured to be 550 μs. These real-time interactions and particle-induced quenching steps open up new opportunities for singlet oxygen research of atmospheric and particulate processes.

A Singlet Oxygen Priming Mechanism: Disentangling of Photooxidative and Downstream Dark Effects.

Airborne singlet oxygen obtained from photosensitization of triplet dioxygen is shown to react with an alkene surfactant (8-methylnon-7-ene-1 sulfonate) leading to "ene" hydroperoxides that in the dark inactivate planktonic Escherichia coli (E. coli). The "ene" hydroperoxide photoproducts are not toxic on their own, but they become toxic after the bacteria are pretreated with singlet oxygen. The total quenching rate constant (kT) of singlet oxygen of the alkene surfactant was measured to be 1.1 × 106 M-1 s-1 at the air/liquid interface. Through a new mechanism called singlet oxygen priming (SOP), the singlet oxygen leads to hydroperoxides then to peroxyl radicals, tetraoxides, and decomposition products, which also promote disinfection, and therefore offer a "one-two" punch. This offers a strong secondary toxic effect in an otherwise indiscernible dark reaction. The results provide an insight into assisted killing by an exogenous alkene with dark toxicity effects following exposure to singlet oxygen.

Influence of pH and micellar systems on the sensitized photo-oxidation of bovine serum albumin.

Photosensitized oxidation of bovine serum albumin (BSA), by using perinaphtenone as a sensitizer, has been studied at pH7.4 and 11. The selected sensitizer does not present ground-state complexation with BSA and ensures that the mechanism is mediated by O2 (1 △g ). Strong dependence between BSA-O2 (1 △g ) photo-oxidation and the pH of the medium has been found. The relative oxygen uptake rate (v-△O2 ) and the total quenching rate constant (kt ) values are higher at pH11 than pH7.4. The enhancement in the alkaline condition is due to conformational changes in the protein and the reactivity of tyrosinate anion with O2 (1 △g ). Even when the tendency with the pH in the presence of sodium dodecyl sulfate (SDS) micelles is similar to that observed in homogeneous media, an increment on the kt value is detected. This effect may be attributable to the strong interaction of BSA-SDS, which leads to the protein unfolding and could leave more exposed photo-oxidizable amino acids. A protective effect against the O2 (1 △g )-mediated photo-oxidation was observed in reverse micelles (RMs) of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) by comparing the kt values obtained at W=10 with respect to the one obtain in homogeneous media. The latter could be mainly explained by the modification in the solvent polarity. Also, another important observation was found, the internal pH inside RMs of AOT sensed through tyrosine absorption was independent of the one used for the formation of the water pool. Hence, the kt values observed at both pH, are quite similar.

Solvent dependent switching of 3MLLCT and 1IL luminescent states in [ClRe(CO)3(bathocuproinedisulfonate)]2-: spectroscopic and computational study.

Steady state and time-resolved luminescence experiments and calorimetric studies, as well as time-dependent density functional theory calculations performed on [ClRe(CO)(3)(Bathocuproinedisulfonate)](2-), show that the photophysical properties of the Re(I) anionic complex are determined by the balance between intraligand ((1)IL) and metal-ligand-to-ligand charge transfer ((3)MLLCT) excited states. In organic solvents, (3)MLLCT states prevail and the usual expected behavior is observed: bathochromic shift of the emission maximum, a reduced luminescence quantum yield and the shortening of the excited-state lifetime upon increasing the polarity of the solvent. In addition, singlet oxygen ((1)O2) is generated with high quantum yields (Φ(Δ) ≈ 0.5 in CH(3)CN) due to the quenching of the (3)MLLCT luminescence by (3)O2. The total quenching rate constant of triplet state by oxygen, k(q), reach values between 2.2 and 2.4 × 10(9) M(-1) s(-1) for the organic solvents studied. In CH(3)CN, the fraction of triplet states quenched by O2 which yield (1)O2, f(O2)T, is nearly unity. In aqueous solution, where no singlet oxygen is generated, the luminescence of the Re(I) complex is of (1)IL character with a emission quantum yield (Φ(em)) strongly pH dependent: Φ(em,(pH=2))/Φ(em,(pH=10)) ≈ 5.6. The variation of the pH of the solution tunes the photophysical properties of the Re(I) complex by changing the relative amount of the different species existing in aqueous solutions: [ClRe(CO)3(BCS)](2-), [(OH)Re(CO)3(BCS)](2-) and [(H2O)Re(CO)3(BCS)](−). TD-DFT calculations show that the percentage of charge transfer character of the electronic transitions is substantially higher in the organic solvents than in aqueous solutions, in agreement with the increase of (1)IL character of HOMO in [(H2O)Re(CO)3(BCS)](−) relative to [ClRe(CO)3(BCS)](2-).

Singlet Oxygen Production upon Two‐Photon Excitation of TiO2 in Chloroform

Singlet oxygen ((1)O(2)) signals observed upon illumination at 532 nm are a consequence of TiO(2) excitation in a two-photon process, and at 355 nm in a one-photon pathway. After corrections for light reflectance, the relative quantum yields of (1)O(2) production are 0.23+/-0.01 and 0.22+/-0.02 for 532 and 355 nm excitation, respectively. Total quenching rate constants of (1)O(2) removal by TiO(2) particles are near the diffusion-controlled rate limit. By employing (1)O(2) as a probe, we have established an efficient and simple method to elucidate the nature of TiO(2) photoexcitation.

Solvent effect on the physical quenching of singlet molecular oxygen by p-quinones

Rate constants for the interaction between singlet molecular oxygen [O2(1 delta g)] and the p-quinones 1,4-benzoquinone (BQ), duroquinone (DQ), 9,10-anthraquinone (AQ) and 1,8-dihydroxy-9,10-anthraquinone (OHAQ) are reported for several solvents at room temperature. The solvent effect on the total quenching rate constant (kt) was analysed employing the semiempirical solvatochromic equation proposed by Kamlet and Taft. The higher values of kt (2-7 x 10(7) M(-1) s(-1)) were obtained when the hydrogen-bond donor solvent ability is increased (higher alpha parameter values). The results indicate the importance of specific solvent interactions in governing the rates of the quenching.

Endohedral metallofullerenes as strong singlet oxygen quenchers

The total quenching rate constants (sum of physical quenching and chemical reaction) of singlet oxygen 1O 2 by various fullerenes (C 60, C 70, C 76, C 82, La@C 82, Ce@C 82, Ce 2@C 80) were determined. The 1O 2 quenching capabilities of C 82 and endohedral metallofullerenes are found to be comparable to that of β-carotene, a well-known strong 1O 2 quencher. This remarkable 1O 2 quenching by metallofullerenes is explicable in terms of a combination of energy and charge transfer processes.

Chemically Induced Dynamic Electron Polarization Generated through the Interaction between Singlet Molecular Oxygen and Nitroxide Radicals

An absorptive chemically induced dynamic electron polarization (CIDEP) was generated by the quenching of singlet oxygen by nitroxide radicals (TEMPO derivatives). The spin polarization decay time of the nitroxide (measured by time-resolved EPR) correlates with the lifetime of singlet oxygen (measured by singlet oxygen phosphorescence spectroscopy). In addition, a deuterium isotope effect on the spin polarization decay time was observed, a signature of singlet oxygen involvement. With use of isotope labeled nitroxides (15N, 14N), the relative spin polarization efficiencies of TEMPO, 4-oxo-TEMPO, and 4-hydroxy-TEMPO by singlet oxygen were determined. The relative spin polarization efficiencies (per quenching event) decrease in the order 4-hydroxy-TEMPO > TEMPO > 4-oxo-TEMPO, whereas an opposite trend was observed for the total quenching rate constants of singlet oxygen by the nitroxides where the order is 4-hydroxy-TEMPO < TEMPO < 4-oxo-TEMPO.

Mechanism of the excited singlet and triplet states quenching by molecular oxygen in acetonitrile

Rate constants for quenching by molecular oxygen of excited singlet and triplet states, k q S and k q T , respectively, of some aromatic hydrocarbons in acetonitrile are reported. The fraction of excited singlet states quenched by oxygen which result in triplet states, f T S , are in the range of 0.25–0.85. The efficiencies of singlet oxygen production during oxygen quenching of the excited singlet and triplet states, f Δ S and f Δ T , respectively, were also measured. Values of f Δ S were shown to be 0.28 ± 0.05 for 1,2;5,6-dibenzanthracene, pyrene, fluoranthene, 1,11-benzoperlyene and perlyene while values of f Δ T cover the range of 0.25–1.0. Combination of the total quenching rate constants with the fractional efficiencies allows separate net quenching rate constants to be obtained for the various oxygen quenching pathways. Factors governing the production of singlet oxygen with different efficiencies during quenching of both excited singlet and triplet states by oxygen are discussed.

Reactions of Metastable Argon Atoms with Molecular Hydrogen at 300 and 80 K: Origin of the Ultraviolet Chemiluminescence

The reactions of Ar(4s 3 P 2 ) and Ar(4s' 3 P 0 ) metastable atoms with H 2 and D 2 have been studied at 300 and 80 K by the stationary and flowing-afterglow techniques. Total quenching constants were measured for both metastable atoms for H 2 and D 2 at 300 K and for H 2 at 80 K. Optical pumping in the stationary-afterglow experiment was used to select one or the other metastable atoms, and chemiluminescence spectra were obtained for state-selected metastable atoms reacting with H 2 and D 2 . The branching fractions for chemiluminescence, which range from 0.07 to 0.30, are higher for D 2 than H 2 and higher for Ar(4s' 3 P 0 ) atoms than for Ar(4s 3 P 2 ) atoms. The chemiluminescence is assigned to the ArH(B 2 Π-X 2 Σ + ) and ArD(B 2 Π-X 2 Σ + ) transitions. The total quenching rate constants for H 2 decline by factors of 80 and 30 at 80 K. relative to the room-temperature values, for the reactions of Ar(4s 3 P 2 ) and Ar(4s' 3 P 0 ) atoms, respectively. Both the chemiluminescence and dark product channels of these reactions probably proceed by chemical reaction rather than by electronic energy transfer.

Time-Resolved CIDNP Study of Intramolecular Charge Transfer in the Dipeptide Tryptophan-Tyrosine

Time-resolved chemically induced dynamic nuclear polarization (CIDNP) and laser flash photolysis (LFP) techniques have been used to study the kinetics and mechanism of the photochemical reaction between 2,2‘-dipyridyl (DP) and the dipeptide l-tryptophan-l-tyrosine (TrpH-TyrOH) in acidic aqueous solution (pH = 3.8). Analysis of the geminate CIDNP pattern reveals that the quenching of the protonated triplet dipyridyl TDPH+ results in the formation of both tryptophan and tyrosine radicals from the dipeptide with comparative efficiency. The total quenching rate constant of triplet dipyridyl by TrpH-TyrOH was found to be (2.5 ± 0.3) × 109 M-1 s-1. The radical transformation TrpH+•→TyrO• via intramolecular electron transfer (IET) leads to an increasing tyrosyl radical concentration, growth of the tyrosine CIDNP signals, fast decay of the CIDNP signal of TrpH, and inversion of the CIDNP sign from emission to enhanced absorption for DP. The nuclear spin−lattice relaxation times of the radicals formed in the react...

Singlet oxygen formation efficiencies following quenching of excited singlet and triplet states of aromatic hydrocarbons by molecular oxygen

Rate constants for quenching by molecular oxygen of excited singlet and triplet states, k S O 2 and k T O 2 , respectively, of 11 aromatic hydrocarbons in cyclohexane are reported. Measured values of k S O 2 are in the range 0.44 to 2.5×10 10 dm 3 mol −1 s −1 and are therefore in many cases close to k d, the diffusion controlled value of 2.8×10 10 dm 3 mol −1 s −1. The fraction of excited singlet states quenched by oxygen which result in triplet states, f T O 2 , was measured for all the compounds and found to be unity. The efficiencies of singlet oxygen production during oxygen quenching of the excited singlet and triplet states, f Δ S and f Δ T, respectively, were also measured. Values of f Δ S were shown to be 0, within experimental error, for all compounds except perylene for which f Δ S=0.28±0.05. Values of k T O 2 vary from 0.46 to 2.32×10 9 dm 3 mol −1 s −1. Thus, k T O 2 / k d was found to be less than one-ninth for all the compounds studied. Three different methods were used to measure f Δ T values, which were found to range from 0.57 to 1. Combination of the total quenching rate constants with the fractional efficiencies allows separate net quenching rate constants to be obtained for the various oxygen quenching pathways. Quenching of both excited singlet and triplet states, with and without energy transfer, are shown to be ‘charge-transfer assisted’ and mechanisms accounting for these results are discussed.

Quenching rate constants for reactions of Ar(4p′[1/2], 4p[1/2], 4p[3/2]2, and 4p[5/2]2) atoms with 22 reagent gases

The total quenching rate constants of argon atoms in the 4p′[1/2]0, 4p[1/2]0, 4p[3/2]2, and 4p[5/2]2 states (2p1, 2p5, 2p6, and 2p8, respectively, in the Paschen numbering system) by rare gases, H2, D2, N2, CO, NO, O2, F2, Cl2, CO2, NO2, CH4, C2H2, C2H4, C2H6, CF4, CHF3, and SF6 have been determined at room temperature. These four excited states of argon (energy 13.09–13.48 eV) were selectively prepared by two-photon excitation from the ground state using VUV (184–190 nm range) laser pulses. The total quenching rates were deduced from the pressure dependence of the decay times of the excited-state atoms, measured by observing their fluorescence emission intensities in the presence of added reagents. The quenching constants increase from values of ≅0.01×10−10 cm3 atom−1 s−1 for Ne, to ≅0.1×10−10 cm3 atom−1 s−1 for He and Ar, and to very large values, (5–15)×10−10 cm3 atom−1 s−1, for most polyatomic molecules, F2, Cl2, and O2. The quenching mechanisms of the Ar(4p,4p′) atoms are briefly discussed and compared to the reactions of the Ar(4s,4s′) metastable and resonance state atoms, 11.55–11.83 eV, which can serve as a reference.

QUENCHING MECHANISMS AND KINETICS OF CAROTENOIDS IN RIBOFLAVIN PHOTOSENSITIZED SINGLET OXYGEN OXIDATION OF VITAMIN D2

The oxidation of vitamin D2 in a solution of 12% water, 88% acetone and 15 ppm ribqflavin under light or dark was studied by measuring the headspace oxygen. Ribofiavin accelerated the oxidation of vitamin D2 by singlet oxygen under light, but did not affect vitamin D2 oxidation under dark. Quenching mechanisms and kinetics within the range of 0–20 ppm β-cawtene or fucoxanthin and with 0–80 ppm retinyl acetate or retinol on 15 ppm riboflavin photosensitized singlet oxygen oxidation of vitamin D2 were also studied. The rate of singlet oxygen formation by 15 ppm riboflavin was 1.78 umole oxygen/mL headspace-hour. The reaction rate constant of vitamin D2 with singlet oxygen was 2.2 times 107 M−1A s−1. The carotenoids minimized the oxidation of vitamin D2 by quenching singlet oxygen. The total quenching rate constants of retinol, retinyl acetate, jucoxanthin and ft-carotene were in the order of 1.22 times 108, 5.98 times 108, 1.78 times 109 and 5.00 times 109M−1. s−1 respectively, which suggests that as the number of double bonds of carotenoids increases, the quenching rate constant ofcarotenoid increases.

Charge Transfer Effects on the Efficiency of Singlet Oxygen Production Following Oxygen Quenching of Excited Singlet and Triplet States of Aromatic Hydrocarbons in Acetonitrile

Rate constants for quenching by molecular oxygen of excited singlet and triplet states, and , respectively, are reported for 12 aromatic hydrocarbons in acetonitrile. Measured values of , except in the case of fluoranthene for which = 6.6 × 109 dm3 mol-1 s-1, are in the range (2.3−4.3) × 1010 dm3 mol-1 s-1, i.e., close to 4.5 × 1010 dm3 mol-1 s-1, the average value obtained for kd, the rate constant for diffusion-controlled reactions of oxygen with aromatic hydrocarbons in acetonitrile. Values of vary from 0.24 to 5.6 × 109 dm3 mol-1 s-1. Thus, /kd was found to be less than one-ninth for 11 compounds. The efficiencies of singlet oxygen production during oxygen quenching of the excited singlet and triplet states, and , respectively, were also measured, as were the oxidation potentials of the hydrocarbons in acetonitrile. Values of were shown to be zero within experimental error for eight compounds and in the range of 0.27 ± 0.05 for the other four compounds. Three different methods, which gave good agreement, were used to measure values of which were found to vary from 0.41 in the case of acenaphthene to 0.85 for anthracene. The fraction of excited singlet states quenched by oxygen which result in triplet states was also measured for all compounds and found to vary from 0.49 to 1.0. Combination of the total quenching rate constants with the fractional efficiencies allows separate net quenching rate constants to be obtained for the various oxygen quenching pathways in acetonitrile. The reasons for variations in these net quenching rate constants and thus in the fractional efficiencies for quenching by the various quenching pathways are discussed. Quenching of both excited singlet and triplet states by energy transfer and by charge-transfer assisted pathways are established. The logarithm of the net rate constants for quenching of the triplet states without energy transfer to oxygen for 11 of the aromatic hydrocarbons shows a linear dependence on the free energy for full charge transfer from the triplet state, with a slope which indicates that the transition states for this quenching pathway only have about 13.5% charge-transfer character.

Quenching Rate Constants of NCl(aΔ) at Room Temperature

The total quenching rate constants for NCl(a1Δ) molecules were measured at room temperature for 40 molecular reagent species and for F, H, N and O atoms. The Cl + N3 reaction was used to provide the NCl(a1Δ) molecules in a room temperature flow reactor; the azide radicals were obtained from the F + HN3 reaction. In most cases, rate constants were obtained for both NF(a1Δ) and NCl(a1Δ) so that the data could be confirmed by comparison with earlier studies of NF(a1Δ). The quenching rate constants for NCl(a1Δ) range from ∼1 × 10-15 to 4 × 10-11 cm3 molecule-1 s-1. Except for O2, F, H, CH3Cl, HCl, HBr, and HI, rate constants for the NCl(a1Δ) molecule are generally smaller than or comparable to those for NF(a1Δ), and NCl(a1Δ) is not especially reactive at 300 K. Small rate constants were obtained for common gases, such as H2, CO2, and CH4; the rate constants are larger for unsaturated hydrocarbons and they increase with the number of carbon atoms in the molecule. A correlation was found between the basicity of...

A Pulsed Source for Kr(5s[3/2]1) Resonance State Atoms Using Two-Photon-Driven Amplified Spontaneous Emission: Measurement of Quenching Rate Constants

Krypton resonance state atoms, Kr(5s[3/2]1), were generated in a static cell by two-photon laser excitation of the Kr(5p[5/2]2) state with amplified spontaneous emission (ASE) at 877.7 and 810.4 nm in a few Torr of Kr gas. The 877.7 nm transition, which terminates on the resonance state, constitutes ≥90% of the ASE intensity. The vacuum ultraviolet emission at 123.6 nm was used to monitor the Kr(5s[3/2]1) concentration, which decayed with an effective lifetime of 2 μs in the absence of added reagents. Total quenching rate constants at 300 K for the resonance state atoms were measured for 30 molecules by observing the decay rates of the Kr(5s[3/2]1) atoms as a function of added reagent gas. The rate constants (cm3 molecule-1 s-1) range from 1.4 × 10-11 for N2 and 0.60 × 10-11 for CF4 to (80−90) × 10-11 for hydrocarbons. Rate constants for most polyatomic molecules exceed 30 × 10-11 cm3 molecule-1 s-1. Comparison to the quenching rate constants of the Kr(5s[3/2]2) metastable atoms shows that the rate constants of the resonance states are ∼35% larger for reagents with quenching cross sections larger than 20 Å2. For a set of 11 reagents, comparison also is made with the rate constants for the (6s) and (6s‘) resonance states and the (6p) states of Xe; the latter have nearly the same energy as the Kr(5s) resonance state.

Charge Transfer Interactions in the Generation of Singlet Oxygen O2(1Δg) by Strong Electron Donors

The quenching rate constant of the triplet state by molecular oxygen and the efficiency of singlet oxygen generation have been measured for 12 amines in cyclohexane and benzene. For the best electron donors, the average rate constant for quenching by energy transfer is 6.0 × 109 M-1 s-1. For the same compounds, the rate constant of quenching via enchancement of internal conversion is 1.2 × 1010 M-1 s-1. The energy transfer component of the total quenching rate constant is almost twice as fast as the maximum from the standard Porter model. The mechanisms of quenching via quintet, singlet, and triplet channels are discussed for amines and aromatic hydrocarbons, and intersystem crossing out of the quintet manifold is proposed.

Quenching Rate Constants for PCl(bΣ+, v‘ = 0)

The reaction of metastable Ar(3P0,2) atoms with PCl3 has been developed as a source of metastable PCl(b1Σ+) molecules in a flow reactor at 300 K. The total quenching rate constants were measured for 23 molecules, and they are in the 2 × 10-14 to 3 × 10-11 cm3 molecule-1 s-1 range. An electronic-to-vibrational (E−V) energy-transfer mechanism is used to discuss the quenching of PCl(b1Σ+), and the experimental rate constants are compared to similar data for PF(b1Σ+), NF(b1Σ+), and NCl(b1Σ+) molecules. An influence of the type of halogen substituent in the reagent molecule on the quenching rate constants is identified, and a role for attractive molecular interactions needs to be added to the E−V model. The Ar(3P0,2) + PCl3 reaction gives vibrationally excited PCl(b1Σ+) molecules and the Δv = +1 and +2 sequences were observed in addition to the Δv = 0 main sequence.