Steady-state Electron Paramagnetic Resonance Research Articles

Inverse photochromism in viologen–tetraarylborate ion-pair complexes: optical write/microwave erase switching in polymer matrices

The spin dynamics of radical pairs from ion-pair complexes between viologen and tetraarylborate compounds have been investigated in the presence of microwave (μw) radiation, using steady-state electron paramagnetic resonance to follow the radical pair dynamics.

Alkyne-Bridged Multi[Copper(II) Porphyrin] Structures: Nuances of Orbital Symmetry in Long-Range, Through-Bond Mediated, Isotropic Spin Exchange Interactions.

Spin and conformational dynamics in symmetric alkyne-bridged multi[copper(II) porphyrin] structures have been studied in toluene solution at variable temperature using steady-state electron paramagnetic resonance (EPR) spectroscopy. Comparison of the dimer EPR spectra to those of Cu porphyrin monomers shows evidence of an isotropic exchange interaction (Javg) in these biradicaloid structures, manifested by a significant line broadening in the dimer spectra. The extent line broadening depends on molecular structure and temperature, suggesting Javg is modulated by conformational dynamics that impact the torsional angle distribution between the porphyrin-porphyrin least-squares planes. Computational simulation of the experimental EPR spectra, using a developed algorithm for J modulation in flexible organic biradicals, supports this hypothesis. Comparison of ethyne and butadiyne alkyne bridges reveals remarkable sensitivity to orbital interactions between the spacer and the metal, reflected in measurements of Javg as a function of temperature. The results suggest orbital symmetry relationships may be more important than recognized in design of optimized molecular spintronic devices.

Kinetic analysis of nitroxide radical formation under oxygenated photolysis: toward quantitative singlet oxygen topology.

Reaction kinetics for two sterically hindered secondary amines with singlet oxygen have been studied in detail. A water soluble porphyrin sensitizer, 5,10,15,20-tetrakis-(4-sulfunatophenyl)-21,23H-porphyrin (TPPS), was irradiated in oxygenated aqueous solutions containing either 2,2,6,6-tetramethylpiperidin-4-one (TMPD) or 4-[N,N,N-trimethyl-ammonium]-2,2,6,6-tetramethylpiperidinyl chloride (N-TMPCl). The resulting sensitization reaction produced singlet oxygen in high yield, ultimately leading to the formation of the corresponding nitroxide free radicals (R2NO) which were detected using steady-state electron paramagnetic resonance (EPR) spectroscopy. Careful actinometry and EPR calibration curves, coupled with a detailed kinetic analysis, led to a simple and compact expression relating the nitroxide quantum yield ΦR2NO (from the doubly-integrated EPR signal intensity) to the initial amine concentration [R2NH]i. With all other parameters held constant, a plot of ΦR2NOvs. [R2NH]i gave a straight line with a slope proportional to the rate constant for nitroxide formation, kR2NO. This establishment of a rigorous quantitative relationship between the EPR signal and the rate constant provides a mechanism for quantifying singlet oxygen production as a function of its topology in heterogeneous media. Implications for in vivo assessment of singlet oxygen topology are briefly discussed.

Reaction Kinetics of Hydroxyl Radicals with Model Compounds of Fuel Cell Polymer Membranes

AbstractThe chemical stability of perfluorinated and non‐perfluorinated low temperature fuel cell model compounds (MCs) against attack by hydroxyl radicals, HO•, is compared using a competition kinetics approach in aqueous solutions at ambient temperature. HO• radicals were generated in situ by UV photolysis of hydrogen peroxide in the electron spin resonance (ESR) resonator. Acetic acid (AA), trifluoroacetic acid (TFAA), methanesulfonic acid (MSA), trifluorosulfonic acid (TFSA), and perfluoro(2‐ethoxyethane)sulfonic acid (PFEESA) were chosen as MCs, while the rate constants of 5,5‐dimethyl‐1‐pyrroline‐N‐oxide (DMPO) and methanol (CH3OH) served as reference for the determination of relative rate constants by means of steady state ESR signal amplitudes. In decreasing order the rate constants are: kMSA = (4.8 ± 0.2) × 107 M–1 s–1, kAA = (4.2 ± 0.3) × 107 M–1 s–1, kPFEESA = (3.7 ± 0.1) × 106 M–1 s–1, kTFAA = (7.9 ± 0.2) × 105 M–1 s–1, and kTFSA < 1.0 × 105 M–1 s–1. Applying these results to perfluorinated fuel cell membranes like Nafion®, the main points of attack by HO• are concluded to be the ether groups of the side chains, followed by the remaining carboxyl groups from the manufacturing process of the polymers.

Solvent- and Wavelength-Dependent Photochemistry of 1,3-Dihydroxyacetone Studied by TREPR: Implications for Commercial Sunless Tanning Lotions

1,3-Dihydroxyacetone (DHA) is the active ingredient in commercial personal care products for the production of a “sunless” tan on human skin. The DHA molecule is the simplest ketose sugar, and it reacts with amino acids on the surface of the skin in a biochemical pathway known as the Maillard reaction. However, DHA is known to be photochemically active and will produce free radicals with high quantum yield under ultraviolet (UV) excitation. We report here a detailed study of the free radicals produced by DHA photolysis as a function of wavelength and solvent. In contrary to the previous optical and steady-state electron paramagnetic resonance (EPR) studies, X-band time-resolved (TR)-EPR spectra reveal a complex reactivity pattern: Norrish I α-cleavage is observed in almost all cases, but when good H-atom donors are present, H-atom abstraction by the solvent is observed. Changing the wavelength of excitation from 308 to 248 nm can reverse this observation. Comparison of TREPR spectra obtained from commercially available sunless tanning lotions shows that many free radicals, including those from DHA, can be detected using direct detection TREPR upon photolysis of the lotions themselves. The results suggest that caution should be observed in the use of these products in conjunction with UV exposure.

Time-resolved electron paramagnetic resonance of the lowest excited triplet state of phenazinium cation

Time-resolved and steady-state electron paramagnetic resonance (EPR) spectra have been observed for the lowest excited triplet (T 1) states of phenazine (Phz) and its singly protonated cation (phenazinium) in sulfuric acid−ethanol mixtures at 77 K. The single protonation appears to have little effect on the anisotropic sublevel populating rates of the T 1 state of phenazine. However, the zero-field splitting (ZFS) parameter D decreases on the protonation, reflecting the increase of delocalization of the two unpaired electrons. The sublevel preferentially populated by intersystem crossing (ISC) is T y in both phenazine and phenazinium (the y-axis is parallel to the in-plane long axis). From the analysis of the observed anisotropy in the ISC rates and the semi empirical molecular orbital calculations of the ZFS parameters, we concluded that the T 1 state of phenazinium is the 3A 1(ππ *) state.

Behavior of the semiquinone anion radical of hypocrellin B produced by electrolysis in dimethylsulfoxide

The behavior of the semiquinone anion radical of hypocrellin B (HB), produced by electrolysis, has been studied by cyclic voltammetry, steady-state electron spin resonance, timeresolved electron spin resonance and in situ UV-Vis spectra. The results showed that the formation mechanisms of HB —· were different between electrolysis and photolysis, and the disproportionation of semiquinone anion radical of HB (HB —· ) could not occur in dimethylsulfoxide; on the contrary, a syn -proportionation reaction in which HB 2— reacts with HB to form HB —· was observed experimentally. At the same time, the kinetic parameters of HB —· were obtained from the time-decay experiments of HB —· .

A Novel Approach for Measuring Absolute Rate Constants by Pulsed Electron Spin Resonance: Addition of Phosphinoyl and 2-Hydroxy-2-propyl Radicals to Several Alkenes,

A time-resolved Fourier transform electron spin resonance (TR FT ESR) method for determining absolute rate constants of reactive, free radical reactions is described. The phase memory times TM of a family of reacting radicals were measured precisely by electron spin−echo (ESE) detection. In all cases the ESE signal decays show a simple pseudo-first-order behavior. Rate constants of the reactions were extracted from the linear relationship between the reciprocal of the TM values and the alkene concentrations. The validity of the method is demonstrated for the measurement of the rate constant for addition of different phosphinoyl and 2-hydroxy-2-propyl radicals to several acrylates, methyl methacrylate, and vinyl pivalate in organic solvents at room temperature. The results are in excellent agreement with those obtained by other kinetic investigations employing time-resolved steady-state electron spin resonance (TR CW ESR) and optical absorption spectroscopy. The technique is shown to be applicable to systems with rate constants between 104 and 108 M-1 s-1, a range covered by the alkenes examined.

Time-Resolved Electron Paramagnetic Resonance Study of the First Excited Triplet States of Dibenzocycloheptadienylidene and Diphenylmethylene Derivatives

Transient triplet electron paramagnetic resonance (EPR) spectra of dibenzocycloheptadienylidene (DBC) and substituted diphenylmethylenes were observed in a rigid glassy matrix at low temperatures. On excitation with the wavelength of the T−T transition of DBC, polarized EPR spectra due to the both ground (T0) and excited (T1) states were observed at a short delay time (180 ns) after the laser pulse. The T1 components in the spectrum decayed faster than the other and diminished at 1 μs after the laser pulse irradiation. The remaining signals after the decay of the T1 state were unambiguously assigned to the T0 state, taking into account the steady-state (SS) EPR results. From analysis of the spectrum of the short-lived triplet species, the zero-field splitting (ZFS) parameters, |D| = 0.201 and |E| = 0.008 cm-1, were determined for the T1 state directly. The parameters obtained were comparable to those of the T1 state of diphenylmethylene (DPM). The polarization patterns of the transient spectra match well with interpretation as anisotropic intersystem crossing induced by the spin−orbit couplings between the Sn and the triplet states. On the basis of the results of magnetophotoselection experiments, we also confirmed that the principal axes in the T0 and T1 states did not differ significantly from each other. The behavior of the methyl-substituted DPM resembles that of DPM and DBC, indicating that the methyl group does not affect the photophysical and photochemical properties of the T0 and T1 states. Although the SS EPR spectra were observed for the T0 state of substituted DPM derivatives, the chemical properties of the excited states were affected by substitution of electron-donating (OCH3) or -withdrawing (CN) groups.

Synthesis of a water-soluble cyclodextrin modified hypocrellin and ESR study of its photodynamic therapy properties

A water-soluble cyclodextrin modified hypocrellin B (HBCD) was designed and synthesized. HBCD retained the phototherapeutic properties and exhibited much stronger photoinduced damage to calf thymus DNA (CT DNA) than hypocrellin B and mercaptoacetic acid substituted hypocrellin B (MAHB). The mechanism of electron transfer from CT DNA to the triplet state of HBCD was confirmed by steady-state electron spin resonance (ESR) and a time-resolved ESR study.

Gateway Reactions to Diverse, Polyfunctional Monolayer-Protected Gold Clusters

Amide and ester coupling reactions of ω-functionalized monolayer-protected gold cluster molecules (MPCs) are an exceptionally efficient avenue to a diverse variety of polyfunctionalized MPCs starting from a small subset of ω-functionalized materials. In this paper, coupling reactions have been employed to produce 13 MPCs bearing multiple copies of a diverse variety of structural groups. Detailed features of three of the 13 polyfunctionalized products are highlighted: (a) stepwise coupling and deprotection reactions result in an MPC surrounded by ca. eight pendant tripeptides, (b) a preliminary Steady-State Electron Paramagnetic Resonance (SSEPR) experiment is described for MPCs bearing multiple spin labels (ca. 13/cluster), and (c) a polyelectron electrochemical reaction is described for an MPC bearing multiple (ca. 7/cluster) coupled phenothiazine derivatives. The coupling reactions substantially expand the available diversity of MPCs as polyfunctionalized chemical reagents platformed on a nanometer-sized central core.

Time-resolved electron paramagnetic resonance study of the lowest excited triplet states of trans-cinnamic acid and trans-cinnamate anion

Time-resolved and steady-state electron paramagnetic resonance spectra have been observed for the lowest excited triplet (T 1) states of trans-cinnamic acid (RCOOH) and trans-cinnamate anion (RCOO −) in methanol-water and in stretched poly(vinyl alcohol) films at 77 K. The zero-field splitting parameters | D| and | E| in the T 1 state slightly increase on deprotonation. The sublevel preferentially populated by intersystem crossing in T y in RCOOH, while the in-plane T x and T y sublevels are prefentially populated in RCOO − (the x and y axes are close to the long and short in-plane molecular axis, respectively).

Flow system and 9.5 GHz microwave resonators for time-resolved and steady-state electron paramagnetic resonance spectroscopy in compressed and supercritical fluids

An apparatus for the electron paramagnetic resonance (EPR) detection of stable and transient free radicals in pressurized liquids and supercritical fluids is described. The construction of a quartz EPR sample cell capable of withstanding pressures up to 3000 psi (204 atm) and the components of a high pressure flow system are explained and illustrated. In order to accommodate the thick-walled, high pressure sample tube, two X-band (9.5 GHz) microwave cavities were fabricated. One is a brass, cylindrical TE011 cavity for time-resolved (direct detection) EPR experiments where field modulation was not necessary, and the other is a silver wire-wound cylindrical TE011 cavity, mounted in a quartz support tube. The wire-wound cavity allows for simultaneous light excitation and field modulation for steady-state EPR. Both cavities are fitted with tunable end plates to facilitate tuning at X band in the face of large frequency shifts when the sample tube is introduced to the resonator and to allow for operation with smaller sample tubes at ambient pressure. Microwave coupling, flow optimization, and UV-light access for both cavities are described, and sample spectra in both time-resolved and steady-state modes are reported.

Time-resolved electron paramagnetic resonance study of the lowest excited triplet state of trans-cinnamic acid

Time-resolved and steady-state electron paramagnetic resonance spectra and luminescence spectra have been observed for the lowest excited states of trans-cinnamic acid in rigid solutions at 77 K. The zero-field splitting parameters in the lowest excited triplet (T 1) state are D = 0.1061 cm −1 and E = −0.0497 cm −1. The T 1 state is estimated to be 20 100 cm −1 and the T 1 lifetime is 45 ms in ethanol at 77 K. The T 1 lifetime decreases as the solvent polarity becomes less polar. The sublevel preferentially populated by intersystem crossing is T y ( y close to in-plane short axis and to the CO direction). These results are explained in terms of vibronic and spin-orbit mixings between the 3 nπ ∗ and 3ππ ∗ states in the T 1 state.

A study of the transients observed in the pulse radiolysis of aqueous solutions of aromatic organomercury compounds

The reduction of aqueous solutions containing organomercury compounds by the hydrated electron has been studied by pulse and steady state radiolysis methods. The compounds have the general formula YC 6H 4HgX where Y is a carboxyl or hydroxyl group and X is a chlorine atom or a hydroxyl group. All of the compounds react with the hydrated electron by a diffusion controlled reaction to produce an Hg 1 intermediate that absorbs in the region 280–380 nm. The intermediates of all of the compounds disappear by a rapid first order process that have a rate constant on the order of 10 5 s -1. Time resolved and steady state electron spin resonance experiments on irradiated aqueous solutions of p-hydroxy-mercuribenzoic acid indicate that the carboxyphenyl radical is formed and this radical disappears by reaction with t-butyl alcohol. Analysis of the irradiated solutions by high performance liquid chromatography show that the carbon-mercury bond is broken in the reduction process. In the case of p-hydroxymercuribenzoic acid and o-hydroxymercuribenzoic acid the reaction product is benzoic acid.

Membrane viscosity of lymphocytes and influence of phytohemagglutinin

The membrane viscosity of peripheral blood lymphocytes (PBLs) of equine, bovine and canine was measured by the use of time-resolved fluorescence depolarization technique with 1, 6-diphenyl-1,3,5-hexatriene (DPH). The viscosity values were 0.55, 0.59 and 0.50 poise for equine, bovine and canine PBLs, respectively. These values were compared with steady-state anisotropies and order parameters measured from electron spin resonance (ESR) of 5-doxyl stearic acid. Both values were increased with increase of viscosity. The fluid property of the membranes stimulated with phytohemagglutinin-P (PHA) was measured with steady-state fluorescence anisotropy and ESR. Little change of membrane fluidity was recognized with both methods during the stimulation with PHA. It appears that PHA activation process for these lymphocytes does not included large increase of the membrane fluidity which significantly accelerate the diffusion velocity of receptors in the plasma membrane.

High pressure high temperature electron spin resonance study of oil shale retorting

Previous studies of high pressure retorting under hydrogen have reported large increases in the Fischer assay oil yield for carbonaceous shales. A high temperature, high pressure, electron spin resonance cavity has been constructed and used to measure spin concentrations of the radicals during pyrolysis. The results are independent of the gas and do not show the expected reduction in spin concentration for hydrogen due to hydrogen capping; this can be explained by the failure of steady state electron spin resonance measurements to detect rapid radical formation and decay processes.

Electron spin resonance kinetic studies of malonyl radical self-decay and oxidation reactions by cerium(IV) and bromate in acid aqueous media. The role of free radicals in the Belousov-Zhabotinskii oscillator

Kinetics of selected reactions involving malonyl radicals (M), generated by Ce(IV) oxidation of malonic acid in acid aqueous media, have been studied by steady-state electron spin resonance under stopped-flow conditions. Rate constants for second-order radical self-decay, k_2(M+M) = 3.2 x 10^9, oxidation by bromate, k_4(M+BrO_3^-) = 6.7 x 10^3, and an upper limit for its oxidation by Ce(IV), k_3(M+Ce(Iv)) ≤ 1.2 x 10^5 M^(-1) s^(+1), have been obtained at 298 K. Some of the consequences of the present results on the mechanism of the Belousov-Zhabotinskii oscillator are examined.

A time-resolved electron spin resonance study of the oxidation of ascorbic acid by hydroxyl radical

Time-resolved electron spin resonance (ESR) spectroscopy for the study of radicals produced by pulse radiolysis is illustrated by a study of the oxidation of ascorbic acid by OH radical in aqueous solution. In basic solution, the direct oxidation product, the ascorbate mono-anion radical, is formed within less than 2 mus of the radiolysis pulse. In acid solutions (pH 3(-4.5), N(2)O:saturated) three radicals are initially formed, the ascorbate mono-anion radical, an OH adduct seen also in steady-state ESR experiments, and an OH adduct at C2 with the main spin density at C3 of the ring. The first OH adduct decays with an initial half-life of about 100 mus, probably by biomolecular reaction. The second OH adduct, which shows one hyperfine splitting about a(H) = 24.4 +/- 0.3 G and g = 2.0031 +/- 0.0002, decays with a half-life of about 10 mus. On this same time scale the concentration of the ascorbate radical approximately doubles. It is concluded that the adduct at C2, but not the other adduct, loses water rapidly to form the ascorbate radical.

Triplet—Triplet Absorption in Sensitized Solvent Decomposition: An EPR Study

The decay of the steady-state electron paramagnetic resonance (EPR) signal from naphthalene triplet-state molecules in ether and other glasses under continuous near-ultraviolet irradiation at 77°K has been examined in some detail. It has been shown that free radicals are produced by sensitized solvent decomposition at a rate proportional to the rate of the disappearance of the triplet-state EPR signal. Also, it has been demonstrated that the latter decay process has a first-order dependence upon naphthalene concentration and that it exhibits a dependence upon excitation intensity, which is consistent with a stepwise two-photon absorption mechanism. The experimental results are discussed in terms of a model that involves the resonant transfer of energy from a highly excited triplet state of naphthalene to the solvent. Excitation of the excited triplet state takes place via the absorption of a photon by a naphthalene molecule in its lowest triplet state. The free radicals are produced by the decomposition of the sensitized solvent molecule. It is believed that the naphthalene molecule is not destroyed in this process but that the EPR signal decay arises from a radical—molecule interaction which effectively shortens the lifetime of the triplet state.