Fourier Transform Electron Paramagnetic Resonance Research Articles

Application of time-resolved-pulsed-EPR spectroscopy to study photoinduced electron transfer reactions. -Porphyrin/quinone system—

Characteristic features of the time-resolved fourier transform EPR (FT-EPR) and electron spin echo FT-EPR (ESE-FT-EPR) spectroscopies for studies of photochemical reaction kinetics are summarized in comparison with the cw-time-resolved EPR (CW-TREPR) spectroscopy. The former two methods were applied to study the photoinduced electron transfer reaction between zinc tetraphenylporphyrin (ZnTPP) and benzoquinone (BQ) in 2-butanol. It is shown that deuterium substitution of the compounds and an application of the ESE-FT-EPR method are effective to observe the broad EPR spectrum of the ZnTPP cation which is missed in measurements by FT-EPR. The kinetic data obtained for the deuterated and undeuterated compounds are compared.

Studies on lipid membranes by two-dimensional Fourier transform ESR: Enhancement of resolution to ordering and dynamics

The first two-dimensional Fourier-transform electron spin resonance (2D-FT-ESR) studies of nitroxide-labeled lipids in membrane vesicles are reported. The considerable enhancement this experiment provides for extracting rotational and translational diffusion rates, as well as orientational ordering parameters by means of ESR spectroscopy, is demonstrated. The 2D spectral analysis is achieved using theoretical simulations that are fit to experiments by an efficient and automated nonlinear least squares approach. These methods are applied to dispersions of 1-palmitoyl-2oleoyl-sn-glycerophosphatidylcholine (POPC) model membranes utilizing spin labels 1-palmitoyl-2-(16-doxyl stearoyl) phosphatidylcholine and the 3-doxyl derivative of cholestan-3-one (CSL). Generally favorable agreement is obtained between the results obtained by 2D-FT-ESR on vesicles with the previous results on similar systems studied by continuous wave (cw) ESR on aligned samples. The precision in determining the dynamic and ordering parameters is significantly better for 2D-FT-ESR, even though the cw ESR spectra from membrane vesicles are resolved more poorly than those from well aligned samples. Some small differences in results by the two methods are discussed in terms of limitations of the methods and/or theoretical models, as well as possible differences between dynamic molecular structure in vesicles versus aligned membranes. An interesting observation with CSL/POPC, that the apparent homogeneous linewidths seem to increase in "real time," is tentatively attributed to the effects of slow director fluctuations in the membrane vesicles.

Two-dimensional Fourier-transform electron spin resonance in complex fluids

Two-dimensional Fourier-transform ESR spectra from a complex fluid characterized by very short free induction decay times are reported. They provide enhanced resolution to dynamic molecular structure compared to conventional cw-ESR spectra which are inhomogeneously broadened by the macroscopic disorder. A general theoretical analysis based on the stochastic Liouville equation permits us to accurately determine the microscopic ordering and motional dynamic rates by non-linear least-squares fitting to experiment.

Fourier transform electron paramagnetic resonance studies of photochemical reactions in heterogeneous media

Fourier transform electron paramagnetic resonance studies of photochemical reactions in heterogeneous media

Photochemistry in heterogeneous media: A Fourier transform-EPR study

Fourier transform-electron paramagnetic resonance (FT-EPR) is a novel spectroscopic technique that is ideally suited for the study of transient free radicals generated in photochemical reactions. The salient features of the application of FT-EPR in this field of research are the following. (1) The high spectral resolution of the technique makes it possible to identify free radicals. (2) The time evolution of spectra can be monitored over a time regime extending from nanoseconds to milliseconds. Hence, spectral data can give rate constants of radical formation and decay. (3) Signal intensities can reflect chemically induced electron polarization effects that provide information on the mechanism of radical formation. (4) Measurement of spin-relaxation rates gives an insight into molecular motion. In this paper the application of the technique in a time-resolved study of photoinduced electron transfer in micellar solutions and in the pores of silica gel is presented. Measurements on micellar solutions concern electron transfer from photoexcited tetrasulfonated zinc tetraphenylporphyrin to duroquinone. The study of silica gel systems focused on electron transfer from zinc tetraphenylporphyrin to benzoquinone. The results of the investigation show that the technique can provide valuable insights into photochemical reactions in heterogeneous media. The time evolution of the spectra provides information on the formation and decay of radical pairs and free radicals. The data reflect the spatial distribution of reactants and products. Linewidths and relaxation data give information on the location and motional freedom of free radicals.

Irregular photoinduced electron transfer between pyrene dianion and alkali metal cations studied by Fourier-transform electron paramagnetic resonance

Polarized FT-EPR spectra of photolyzed pyrene/alkali metal (Li, Na, K, Rb, Cs)/THF solutions were found to be time and alkali metal dependent. The polarization effects are initiated by electron transfer (ET) reactions between the doubly- and singly-charged pyrene (Py 2- and Py .- ) and the alkali metal countercations (M + ). With the exception of Py/K/THF solution, the spin polarizations of all other systems, including those with a chelating agent added (cryptand), originate with a triplet precursor. The polarization is due to the action of several mechanisms: the correlated radical pair mechanism, S-T-1 radical pair (RP) mixing, and the radical-triplet pair mechanism

Chemically Induced Dynamic Electron Polarization in Photogenerated Acetone Ketyl Radicals; A Fourier Transform-Electron Paramagnetic Resonance Study*

Chemically Induced Dynamic Electron Polarization in Photogenerated Acetone Ketyl Radicals; A Fourier Transform-Electron Paramagnetic Resonance Study*

Application of Fourier transform electron paramagnetic resonance in the study of photochemical reactions

Application of Fourier transform electron paramagnetic resonance in the study of photochemical reactions

Iron–sulfur clusters in ionic polymers on electrodes

Iron–sulfur clusters have been incorporated in cationic polymers at electrode surfaces; the methodology for the assembly of such structures, and their characterisation by reflectance Fourier-transform (FT) IR, EPR and electrochemical techniques, is given. The stoichiometries of the cluster-electrode assemblies are self regulated by the necessity to maintain charge neutrality during redox cycling. Thus the assembly produced by the incorporation of [Fe4S4(SPh)4]2– in [C4H4N(CH2)3NMe3]+3[BF4]– polymer (C4H4N = pyrrol-1-yl) adopts a composition close to [C4H4N(CH2)3NMe3]+3[Fe4S4(SPh)4]2–[BF4]–. Electron-transfer and ligand-exchange reactions of polymer-bound clusters have been studied. Electrostatic (ion-pairing) effects are shown to modulate redox potentials and thus E°′ values for [Fe4S4(SPh)4]2–/3– and other couples are generally shifted to potentials positive of those observed in solution. Chronocoulometric measurements show that apparent diffusion coefficients (Dapp) for the one-electron reduction of [Fe4S4(SCH2Ph)4]2– and [Fe4S4(SPh)4]2– are about an order of magnitude greater than that for the reduction of the corresponding trianionic cubanes. This appears to be a consequence of different mechanisms of charge propagation within the polymer films: electron hopping enhances Dapp for the 2-/3- couples whereas for the 3-/4- process, a physical diffusional mechanism probably dominates, with Dapp close to the value for pure diffusion. As estimated by assumption of the simple Dahms-ruff model, the rate constant kex for the electron self exchange reaction between [Fe4S4(SCH2Ph)4]2– and [Fe4S4(SCH2Ph)4]3– within the cationic film, is relatively high at ca. 103 dm3 mol–1 s–1. This is turn suggests that polymer-confirmed clusters operating at the 2-/3- level might well be effective mediators for electron transfer to (co-trapped) substrate reducing sites.

Solvent Effects on Photoinduced Electron Transfer; A Fourier Transform-Electron Paramagnetic Resonance Study*

Article Solvent Effects on Photoinduced Electron Transfer; A Fourier Transform-Electron Paramagnetic Resonance Study* was published on January 1, 1993 in the journal Zeitschrift für Physikalische Chemie (volume 182, issue 1-2).

Fourier transform-EPR studies of photoinduced electron transfer in homogeneous and micellar solutions

Fourier transform (FT) EPR was used to study the pulsed-laser-induced electron transfer from porphyrins to quinones in homogeneous and micellar solutions. By monitoring the EPR signal of the quinone anion radicals as a function of delay time (τd) between laser and microwave pulses, with τd ranging from nanoseconds to 1 millisecond, information was obtained on the kinetics of free radical formation and decay. The time evolution of the signal also gave an insight into the chemically induced dynamic electron polarization (CIDEP) mechanisms that affect signal amplitudes. FT-EPR spectra of electron transfer products generated in micellar solution provide evidence for the generation of long-lived spincorrelated radical pairs.

Fourier-transform EPR investigation of photo-generated radical anions of C60 in solution

Fourier-transform EPR investigation of photo-generated radical anions of C60 in solution

Photooxidation of zinc tetraphenylporphyrin by benzoquinone: a Fourier transform electron paramagnetic resonance investigation

Fourier transform electron paramagnetic resonance (FT-EPR) has been used to study the photooxidation of zinc tetraphenylporphyrin (Zn-TPP) by benzoquinone (BQ). A method was developed for the analysis of the time dependence of the FT-EPR spectra, following pulsed laser excitation of ZnTPP, that gives rate constants for the electron-transfer process as well as for the spin-polarization evolution in paramagnetic species involved in the reaction. It is found that the rate constant of electron transfer from photoexcited triplet ZnTPP to BQ in ethanol is concentration dependent. The data yield a value for the electron-transfer-reaction radius of triplet ZnTPP, the limiting rate constant that would obtain for [BQ]{yields}0, and the radical pair lifetime. In addition, values are found for the spin-lattice relaxation times of the BQ{sup {minus}} anion radical and triplet porphyrin precursor and for electron transfer from BQ{sup {minus}} to BQ. 31 refs., 8 figs., 1 tab.

Fourier transform electron paramagnetic resonance study of the photoreduction of anthraquinone with 4-methyl-2,6-di-tert-butylphenol in alcoholic solutions

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTFourier transform electron paramagnetic resonance study of the photoreduction of anthraquinone with 4-methyl-2,6-di-tert-butylphenol in alcoholic solutionsM. Plueschau, G. Kroll, K. P. Dinse, and D. BeckertCite this: J. Phys. Chem. 1992, 96, 22, 8820–8827Publication Date (Print):October 1, 1992Publication History Published online1 May 2002Published inissue 1 October 1992https://doi.org/10.1021/j100201a026RIGHTS & PERMISSIONSArticle Views39Altmetric-Citations9LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (858 KB) Get e-Alerts

FT-EPR study of electron spin polarization in the acetone ketyl radical formed by photolysis of tert-butylperoxide in 2-propanol

The kinetics of formation and decay of acetone ketyl radicals produced by photocleavage of tert-butylperoxide in 2-propanol and the generation of electron spin polarization has been investigated with Fourier-transform electron paramagnetic resonance (FT-EPR). It is found that the evolution of the FT-EPR spectra over a time domain ranging from nanoseconds to 500 μs can be completely explained in terms of: (1) kinetics of radical formation and decay, (2) relaxation of tert-butoxy radical precursors — born with zero spin polarization via the singlet excited state of the peroxide — to thermal equilibrium, (3) spin polarization created by the radical pair mechanism involving random encounters of ketyl radicals (F-pair CIDEP), and (4) spin-lattice relaxation of the ketyl radicals. A detailed numerical analysis of the time dependence of the amplitudes of the central three hyperfine components in the spectrum of the ketyl radical gives a value of 85±30 ns for the spin—lattice relaxation time of the tert-butoxy radical. The analysis also gives the value of the pseudo-first-order hydrogen abstraction rate constant (2.5 × 10 7 s −1) and the magnitude of the F-pair spin polarization.

Chemically Induced Dynamic Electron Polarization in Photogenerated Acetone Ketyl Radicals; A Fourier Transform-Electron Paramagnetic Resonance Study*

Chemically Induced Dynamic Electron Polarization in Photogenerated Acetone Ketyl Radicals; A Fourier Transform-Electron Paramagnetic Resonance Study*

Photoinduced electron transfer from porphyrins to quinones in micellar systems: an FT-EPR study

The photoinduced electron transfer from zinc tetrakis (4-sulfonatophenyl) porphyrin (ZnTPPS) to duroquinone (DQ) in several micellar solutions was investigated with Fourier transform-electron paramagnetic resonance (FT-EPR). The measurements give information on the formation and decay of free radicals, following pulsed-laser excitation of ZnTPPS, with nanosecond time resolution. It was found that the characteristics and time evolution of the EPR spectra depend strongly on surfactant properties. In particular, long-lived [ZnTPPS 3−… DQ −] spin-correlated radical pairs (lifetime > 1 μs) were observed in solutions of the cationic surfactant cetyltrimethylammonium chloride (CTAC). By contrast, radical pair lifetimes in anionic and neutral micelles are shorter than 100 ns. The experimental data can be rationalized in terms of the Coulombic and hydrophobic interactions that determine the spatial organization and stability of the different species (reactants and products) in the microheterogenous systems.

Spatially resolved two-dimensional Fourier transform electron spin resonance

Fourier transform ESR methods have been extended to permit spatially resolved two-dimensional (2D)-ESR experiments. This is illustrated for the case of 2D-electron-electron double resonance (2D-ELDOR) spectra of nitroxides in a liquid that exhibits appreciable cross-peaks due to Heisenberg spin exchange. The use of spin-echo decays in spatially resolved FT-ESR is also demonstrated.

Fourier transform-electron paramagnetic resonance study of the photochemical reaction of acetone with 2-propanol

Fourier transform-electron paramagnetic resonance (FT-EPR) was used to study the pulsed-laser induced reduction of acetone with 2-propanol. By monitoring the EPR signal of the acetone ketyl radical as function of delay time (τd) between laser pulse and microwave pulse, with τd ranging from nanoseconds to 100 μs, information was obtained on the kinetics of free radical formation and decay. The time evolution of the signal also gave an insight into the chemically induced dynamic electron polarization (CIDEP) mechanisms that affect signal amplitudes. It was found that the spectra obtained with τd settings ranging from 0 to 400 ns contain a dispersive signal contribution that is due to the presence of spin-correlated radical pairs (SCRP) at the time of the microwave pulse. For acetone(D6) in 2-propanol(D8) the rate constants of formation and decay of the SCRP are found to be 7.5±3.7×106 and ∼5×107 s−1, respectively. The SCRP lifetime in 2-propanol(D8) at room temperature corresponds to what would be expected for diffusion controlled cage escape. The rate constant for ketyl(D7) radical formation is found to be 5.8±0.5×106 s−1. The decay rate constant kdN(0), where N(0) is the maximum radical concentration, is found to be 1.0×105 s−1. The rate constant for radical formation increases to 1.1×107 s−1 for the acetone(H6)/2-propanol(H8) system. The pronounced isotope effect on radical formation is consistent with a transition state involving C–H bond stretching. The FT-EPR spectra show the effects of radical pair CIDEP. However, a detailed analysis of signal growth and decay kinetics shows that there is no triplet mechanism CIDEP contribution. This finding is in agreement with the relative magnitudes of acetone triplet spin lattice relaxation and radical formation rate constants. The results also demonstrate that the rate constant for hydrogen abstraction is triplet sublevel independent.

FT-EPR Study of Photoinduced Electron Transfer in a Micellar System

Abstract Photoinduced electron transfer from Zn-tetraphenylporphyrin (ZnTPP) to benzoquinone (BQ) in micelles of cetyltrimethylammonium chloride (CTAC) has been investigated by time-resolved Fourier-Transform Electron Paramagnetic Resonance (FT-EPR). An analysis of the time-evolution of the FT-EPR signal from the BQ anion radical following laser excitation of ZnTPP gives the values of the forward and back electron transfer rate constants and for the BQ− spin-lattice relaxation time. The data are compared with those obtained from ZnTPP/BQ in ethanol. The experimental data show that the forward reaction involves electron transfer from CTAC solubilized ZnTPP to BQ in the aqueous phase. Furthermore, attraction between the cationic headgroups of the surfactant molecules and BQ− apparently promotes back electron transfer.