Two-dimensional Electron Paramagnetic Resonance Research Articles

Time-Frequency Analysis of Two-Dimensional Electron Spin Resonance Signals.

Two-dimensional electron spin resonance (2D ESR) spectroscopy is a unique experimental technique for probing protein structure and dynamics, including processes that occur at the microsecond time scale. While it provides significant resolution enhancement over the one-dimensional experimental setup, spectral broadening and noise make extraction of spectral information highly challenging. Traditionally, two-dimensional Fourier transform (2D FT) is applied for the analysis of 2D ESR signals, although its efficiency is limited to stationary signals. In addition, it often fails to resolve overlapping peaks in 2D ESR. In this work, we propose a time-frequency analysis of 2D time-domain signals, which identifies all frequency peaks by decoupling a signal into its distinct constituent components via projection on the time-frequency plane. The method utilizes 2D undecimated discrete wavelet transform (2D UDWT) as an intermediate step in the analysis, followed by signal reconstruction and 2D FT. We have applied the method to a simulated 2D double quantum coherence (DQC) signal for validation and a set of experimental 2D ESR signals, demonstrating its efficiency in resolving overlapping peaks in the frequency domain, while displaying frequency evolution with time in case of non-stationary data.

Unusual 31P Hyperfine Strain Effects in a Conformationally Flexible Cu(II) Complex Revealed by Two-Dimensional Pulse EPR Spectroscopy.

Strain effects on g and metal hyperfine coupling tensors, A, are often manifested in Electron Paramagnetic Resonance (EPR) spectra of transition metal complexes, as a result of their intrinsic and/or solvent-mediated structural variations. Although distributions of these tensors are quite common and well understood in continuous-wave (cw) EPR spectroscopy, reported strain effects on ligand hyperfine coupling constants are rather scarce. Here we explore the case of a conformationally flexible Cu(II) complex, [Cu{Ph2P(O)NP(O)Ph2-κ2O,O'}2], bearing P atoms in its second coordination sphere and exhibiting two structurally distinct CuO4 coordination spheres, namely a square planar and a tetrahedrally distorted one, as revealed by X-ray crystallography. The Hyperfine Sublevel Correlation (HYSCORE) spectra of this complex exhibit 31P correlation ridges that have unusual inverse or so-called "boomerang" shapes and features that cannot be reproduced by standard simulation procedures assuming only one set of magnetic parameters. Our work shows that a distribution of isotropic hyperfine coupling constants (hfc) spanning a range between negative and positive values is necessary in order to describe in detail the unusual shapes of HYSCORE spectra. By employing DFT calculations we show that these hfc correspond to molecules showing variable distortions from square planar to tetrahedral geometry, and we demonstrate that line shape analysis of such HYSCORE spectra provides new insight into the conformation-dependent spectroscopic response of the spin system under investigation.

Identifying exchangeable protons in the QA‐site of photosystem II

Through the use of photosystem II (PS II), plants possess the ability to convert light energy from the sun into chemical energy that may be used for cellular functions such as carbon fixation (1). PS II has been used in the creation of a solar cell that oxidizes water to oxygen at the anode and reduces oxygen to water at the cathode thus forming an oxidation‐reduction cycle that does not produce a byproduct. Generation of such a solar cell requires electrically linking PS II to a charge transporting polymer matrix at the binding site of plastoquinone B (QB), an electron carrier found within PS II (2). Understanding more about the binding site for the redox mediators found in PS II may assist in the future development of solar cells utilizing PS II directly or modeled after PS II. Quinones play a central role in many biological electron transport chains due to their ability to participate in one electron or two electron oxidation reduction reactions (3).Interestingly, the quinones that occupy the primary quinone binding site (QA) and the secondary quinone binding site (QB−−) in PS II both have the same structure; yet, QA participates in one electron transfers while QB can be fully reduced forming a quinol that is capable of diffusing away from PS II. When QA is reduced, an anionic semiquinone free radical is formed (QA−•). Their difference in redox properties has been attributed to differences in the protein environment of the two binding sites, including hydrogen bonding interactions which can redistribute electron density stabilizing the anionic semiquinone state of QA (4). Pulsed electron paramagnetic resonance (EPR) has established the presence of hydrogen bonds between an amide nitrogen from the protein backbone and an amino nitrogen from an imidazole with the carbonyl oxygens on QA−• (5;6). Two‐dimensional (2D) pulsed EPR has resolved five‐cross peaks from the interaction of nearby protons with the unpaired electron in QA−•. Three of the cross peaks have been assigned to protons endogenous to the structure of QA−• itself–to methyl protons on the ring, β‐methylene protons, and the ring proton. Two cross peaks have been attributed to the hydrogen bond protons (5). However, the assignment of determined coupling constants to particular protons are not consistent with corresponding values that were obtained using DFT calculations (4). We have performed 2D pulsed EPR of PS II samples with QA−−• in 1H2O and 2H2O buffer. These experiments have allowed us to identify unambiguously the cross‐features from exchangeable and nonexchangeable protons and to determine more precisely the hyperfine coupling constants for them. These results provide the data set more suitable for a comparison with calculated hyperfine couplings.Support or Funding InformationPulsed EPR studies were supported by DE‐FG02‐08ER15960 Grant from Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Sciences, US DOE (S.A.D.), and NCRR/NIH Grants S10‐RR15878 and S10‐RR025438 for pulsed EPR instrumentation. Continuous wave EPR studies were supported by NSF Grant CHE‐1229498.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Investigation of scavenging activities and distribution of paramagnetic species in Zanthoxylum limonella seeds

We investigated the scavenging activities of methanol-extracted oil and the distribution of paramagnetic species in Zanthoxylum limonella (ZL) seeds using noninvasive 9 GHz electron paramagnetic resonance (EPR) imaging and continuous wave EPR. EPR detected three different stable paramagnetic species that were assigned to stable organic radicals, Mn2+, and other paramagnetic metal complexes. Two-dimensional EPR imaging showed that the stable paramagnetic species were located in the pigmented seed region with a strong intensity. Gas chromatography–mass spectrophotometric (GC–MS) analyses were then performed to identify the compound possibly related to the scavenging activity. The DPPH scavenging activities of ZL were slightly higher than those of Piper nigrum and Coriandrum sativum. Based on the results of EPR, GC–MS, and other methods, limonene in ZL is one of the major compounds that can be related to the scavenging activities.

Exploring the Structure of Paramagnetic Centers in SBA-15 Supported Vanadia Catalysts with Pulsed One- and Two-Dimensional Electron Paramagnetic Resonance (EPR) and Electron Nuclear Double Resonance (ENDOR)

Using pulsed EPR and ENDOR, the full set of g matrix and vanadium hyperfine parameters of the persistent paramagnetic V4+ center in “as prepared” and H2 reduced SBA-15 supported VOx catalysts has been measured. The determination of relative signs of the vanadium hyperfine tensor elements by ENDOR using orientation selection allowed an unambiguous extraction of the isotropic part of this interaction. This allowed for identification of the persistent V4+ center as a surface exposed deprotonated vanadium site. The same site topology was found for oxidized and H2 reduced catalysts, thus indicating that the identified sites represent catalytically active centers. Hyperfine interaction with distant protons indicates formation of an oligomeric structure even for samples with vanadium loadings of less than 2 wt %. This conclusion is confirmed by applying 2D EPR for measuring the hyperfine interaction with neighboring vanadium atoms, covalently linked to reduced V4+ sites. Hence, application of 2D EPR enabled us t...

A new Lanczos-based algorithm for simulating high-frequency two-dimensional electron spin resonance spectra

The Lanczos algorithm (LA) is a useful iterative method for the reduction of a large matrix to tridiagonal form. It is a storage efficient procedure requiring only the preceding two Lanczos vectors to compute the next. The quasi-minimal residual (QMR) method is a powerful method for the solution of linear equation systems, Ax = b. In this report we provide another application of the QMR method: we incorporate QMR into the LA to monitor the convergence of the Lanczos projections in the reduction of large sparse matrices. We demonstrate that the combined approach of the LA and QMR can be utilized efficiently for the orthogonal transformation of large, but sparse, complex, symmetric matrices, such as are encountered in the simulation of slow-motional 1D- and 2D-electron spin resonance (ESR) spectra. Especially in the 2D-ESR simulations, it is essential that we store all of the Lanczos vectors obtained in the course of the LA recursions and maintain their orthogonality. In the LA-QMR application, the QMR weight matrix mitigates the problem that the Lanczos vectors lose orthogonality after many LA projections. This enables substantially more Lanczos projections, as required to achieve convergence for the more challenging ESR simulations. It, therefore, provides better accuracy for the eigenvectors and the eigenvalues of the large sparse matrices originating in 2D-ESR simulations than does the previously employed method, which is a combined approach of the LA and the conjugate-gradient (CG) methods, as evidenced by the quality and convergence of the 2D-ESR simulations. Our results show that very slow-motional 2D-ESR spectra at W-band (95 GHz) can be reliably simulated using the LA-QMR method, whereas the LA-CG consistently fails. The improvements due to the LA-QMR are of critical importance in enabling the simulation of high-frequency 2D-ESR spectra, which are characterized by their very high resolution to molecular orientation.

ChemInform Abstract: EPR and ENDOR Studies of Shallow Donors in SiC

AbstractReview: 85 refs.

EPR and ENDOR Studies of Shallow Donors in SiC

Recent progress in the investigation of the electronic structure of the shallow nitrogen (N) and phosphorus (P) donors in 3C–, 4H– and 6H–SiC is reviewed with focus on the applications of magnetic resonance including electron paramagnetic resonance (EPR) and other pulsed methods such as electron spin echo, pulsed electron nuclear double resonance (ENDOR), electron spin-echo envelope modulation and two-dimensional EPR. EPR and ENDOR studies of the 29Si and 13C hyperfine interactions of the shallow N donors and their spin localization in the lattice are discussed. The use of high-frequency EPR in combination with other pulsed magnetic resonance techniques for identification of low-temperature P-related centers in P-doped 3C–, 4H– and 6H–SiC and for determination of the valley–orbit splitting of the shallow N and P donors are presented and discussed.

Evaluation of the dose distribution gradient in the close vicinity of brachytherapy seeds using electron paramagnetic resonance imaging

Electron paramagnetic resonance (EPR) spectroscopy has been successfully employed to determine radiation dose using alanine. The EPR signal intensity reflects the number of stable free radicals produced, and provides a quantitative measurement of the absorbed dose. The aim of the present study was to explore whether this principle can be extended to provide information on spatial dose distribution using EPR imaging (EPRI). Lithium formate was selected because irradiation induces a single EPR line, a characteristic that is particularly convenient for imaging purposes. (125)I-brachytherapy seeds were inserted in tablets made of lithium formate. Images were acquired at 1.1 GHz. Monte Carlo (MC) calculations were used for comparison. The dose gradient can be determined using two-dimensional (2D) EPR images. Quantitative data correlated with the dose estimated by the MC simulations, although differences were observed. This study provides a first proof-of-concept that EPRI can be used to estimate the gradient dose distribution in phantoms after irradiation.

Complexation of Copper(II)−Chelidamate: A Multifrequency-Pulsed Electron Paramagnetic Resonance and Electron Nuclear Double Resonance Analysis

Multifrequency electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) techniques were used to obtain structural information about the copper(II)-chelidamate complex. Well-resolved nitrogen ENDOR spectra could be recorded from solid solution samples by using selective excitation of spin packets. Evaluation of nuclear quadrupole and dipolar hyperfine interaction of the directly ligated nitrogen allowed for an identification of the bond direction to the copper ion within the eigen frame of the copper g-matrix. Invoking two-dimensional EPR techniques, additional hyperfine interaction with a "distant" nitrogen spin, identified as resulting from the solvent dimethylformamide (DMF), was observed. The experimental data are only consistent with formation of a stable pseudoplanar copper complex with single solvent ligation via its oxygen atom.

Electron paramagnetic resonance and theoretical studies of shallow phosphorous centers in3C-,4H-, and6H−SiC

Continuous-wave (cw) electron paramagnetic resonance (EPR) at both $X$-band and $W$-band frequencies, pulsed-EPR, and pulsed electron nuclear double resonance (ENDOR) were used to study phosphorus shallow donors in $3C$-, $4H$-, and $6H\text{\ensuremath{-}}\mathrm{SiC}$ doped with phosphorus (P) during chemical vapor deposition (CVD) growth. In $3C\text{\ensuremath{-}}\mathrm{SiC}$, a spectrum with ${D}_{2d}$ symmetry and the principal $g$ values, ${g}_{\ensuremath{\Vert}}=2.0051$, ${g}_{\ensuremath{\perp}}=2.0046$, was detected. The $^{31}\mathrm{P}$ hyperfine (hf) constants of the center, ${A}_{\ensuremath{\Vert}}=0.53\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$ and ${A}_{\ensuremath{\perp}}=\ensuremath{-}0.13\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$, were determined from pulsed-ENDOR measurements. A doublet with ${C}_{3\mathrm{v}}$ symmetry (${g}_{\ensuremath{\Vert}}=2.0065$, ${g}_{\ensuremath{\perp}}=2.0006$, and $^{31}\mathrm{P}$ hf constants ${A}_{\ensuremath{\Vert}}=8.24\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$, ${A}_{\ensuremath{\perp}}=5.89\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$) was detected in P-doped $4H\text{\ensuremath{-}}\mathrm{SiC}$. In the $6H$ polytype, the same ${\mathrm{P}}_{1}$ and ${\mathrm{P}}_{2}$ doublets [Greulich-Weber, Phys. Status Solidi A 162, 95 (1997)] or $d{\mathrm{P}}_{\mathrm{h}}$, $d{\mathrm{P}}_{\mathrm{c}1}$, and $d{\mathrm{P}}_{\mathrm{c}2}$ doublets [Baranov et al., Phys. Rev. B 66, 165206 (2002)] previously reported in material doped with P by neutron transmutation were detected. Our cw-EPR, two-dimensional EPR, and electron spin-echo envelope modulation (ESEEM) results confirm that the $d{\mathrm{P}}_{\mathrm{c}1}$ and $d{\mathrm{P}}_{\mathrm{c}2}$ doublets are related to different allowed and forbidden transitions of the ${\mathrm{P}}_{2}$ center with $S=1∕2$ and $I=1∕2$. Based on the observed $^{31}\mathrm{P}$ hf interaction (for three polytypes) and the $^{13}C$ hf interaction with nearest neighbors (for $4H$- and $6H\text{\ensuremath{-}}\mathrm{SiC}$), the P-related spectra are assigned to the ground states of the isolated shallow P at Si site. The valley-orbit splitting of the shallow P donors was estimated from the temperature dependence of the spin-lattice relaxation time and of the cw-EPR signal intensities. Based on the ab initio supercell calculations of the spin localizations on the P atom and the nearest C neighbors in $4H\text{\ensuremath{-}}\mathrm{SiC}$ and the similarity between the spectra in the $4H$ and $6H$ polytypes, we reassign the spectrum in $4H\text{\ensuremath{-}}\mathrm{SiC}$ to the shallow P donor at the quasicubic site $({\mathrm{P}}_{\mathrm{k}})$ and the two doublets ${\mathrm{P}}_{1}$ and ${\mathrm{P}}_{2}$ in $6H\text{\ensuremath{-}}\mathrm{SiC}$ to the shallow P donors at two quasicubic sites, ${\mathrm{P}}_{\mathrm{k}1}$ and ${\mathrm{P}}_{\mathrm{k}2}$, respectively.

Novel Excited Quintet State in Porphyrin: Bis(quinoline−TEMPO)−yttrium−tetraphenylporphine Complex

New mono- and bis[4-(3-hydroxy-2-methyl-4-quinolinoyloxy)-2,2,6,6-tetramethylpiperidin-1-oxyl](meso-tetraphenylporphyrinato)yttrium(III) complexes have been synthesized, and the properties of the excited states generated by photoexcitation of porphyrin were studied by time-resolved (TR) and pulsed two-dimensional electron paramagnetic resonance (EPR) spectroscopy. A TR-EPR spectrum was observed in the quartet (S=3/2) or quintet (S=2) states generated from interactions of one or two radicals with the photoexcited triplet state of the porphyrin. The zero-field splitting D values of these states were analyzed in terms of those of the triplet and the radical-triplet pair. The spin states of the excited states were definitely assigned by measuring the mutation frequencies with pulsed EPR.

Inclusion Complexes of PBN-Type Nitrone Spin Traps and Their Superoxide Spin Adducts with Cyclodextrin Derivatives: Parallel Determination of the Association Constants by NMR Titrations and 2D-EPR Simulations

(1)H NMR and electron paramagnetic resonance (EPR) titrations were used to determine the association constants of the complexes of alpha-phenyl-N-tert-butylnitrone (PBN) analogues and their superoxide spin adducts, respectively, with methylated beta-cyclodextrins. A 1:1 stoichiometry for the nitrones with randomly methylated beta-cyclodextrin and 2,6-di-O-methyl-beta-cyclodextrin and 1:1 and 1:2 stoichiometries for the corresponding cyclodextrin-nitroxide complexes were observed. After the superoxide radical spin trapping reaction, EPR titrations afforded the association constants of the corresponding cyclodextrin-nitroxide complexes. Two-dimensional EPR simulations indicated a bimodal inclusion of the nitroxide free radical spin adducts into the cyclodextrins. For all the nitrone-cyclodextrin and nitroxide-cyclodextrin complexes, the association constants were always higher for the nitroxide complexes than for the nitrone complexes. A cooperative system concerning the complexation of the nitroxide spin adduct with a cyclodextrin was evidenced by EPR titrations. The efficiency of the cyclodextrin inclusion technique to trap superoxide and to resist bioreduction by sodium l-ascorbate was also investigated.

Structures and electronic states of photoexcited states in a system of two nitroxide radicals linked to fullerene studied by two-dimensional pulsed nutation and time-resolved electron paramagnetic resonance spectroscopy

Magnetic properties of fulleropyrrolidine adducts with two stable nitroxide radicals (2,2,6,6-tetramethylpiperidine-1-oxyl, TEMPO) were studied in toluene solution by continuous-wave time-resolved (TR) and pulsed electron paramagnetic resonance (EPR) spectroscopy in the ground and photoexcited states. Four isomers of the bisadduct,trans-1,trans-2,trans-3, and equatorial forms, having the second pyrrolidine ring at different [6-6] bonds were synthesized. In the ground states, the exchange interaction between two TEMPOs is so small that the spin state of the bisadduct is a doublet in nature. By means of spectral simulations of the EPR spectra in frozen solution at 70 K, the upper limit of the exchange interaction was estimated to be 5 MHz for thetrans-1 andtrans-2 and 10 MHz for thetrans-3 and equatorial isomers. The simulation was also made to determine relative positions of the two TEMPO groups with respect to the pyrrolidine ring. Photoexcited states of the bisadducts with excitation of the 532 nm laser pulse were studied in frozen toluene solution at 5–100 K by using two-dimensional (2-D) pulsed nutation EPR and TREPR. The spin multiplicity of the excited state was determined by the nutation frequency. All of the four bisadducts showed strong exchange couplings between two TEMPOs and fullerene triplet3C60*, resulting in the generation of the excited quintet and triplet states. The excited triplet states have been observed and assigned for the first time in strongly coupled triplet-radical systems. The zero-field splittings of the quintet state determined from the 2-D nutation EPR spectra were analyzed as the sum of the spinspin interactions among the three paramagnetic centers, two TEMPOs and3C60*. On the basis of these analyses, the spin distribution on the3C60* part and the geometry of two TEMPOs are discussed.

Application of two-dimensional pulsed EPR nutation spectroscopy to a disordered system with largeg-anisotropy

Two-dimensional (2-D) pulsed electron paramagnetic resonance (EPR) nutation spectroscopy is applied to a disordered system with strongly anisotropicg-factors. The analysis and interpretation of a 2-D nutation spectrum is performed by theoretical calculations appropriate for ferric myoglobin cyanide in frozen solution. The observed and calculated 2-D spectra show broad signals which correspond to the principalg-values. The advantage of nutation spectroscopy is demonstrated by measuring theg x=0.93 signal, which cannot be observed in a conventional field-swept spectrum due to largeg strain broadening. It is shown that the 2-D nutation experiment can be considered as a sort of an angle-selected method but a nutation spectrum gives a powder pattern even for magnetic field settings at extreme field values of an EPR spectrum. This is in contrast to angle-selected electron nuclear double resonance, which gives a single-crystal type spectrum. The advantage of sine Fourier transform (FT) over complex FT is demonstrated for data processing in the nutation domain. Problems of experimental and data-processing procedures are also discussed.

One- and Two-Dimensional EPR Studies on the Radical Pair Generated by the Photoreduction of 9,10-Anthraquinone-1,5-disulfonate in Aerosol OT Reverse Micelles

Spin-correlated radical pairs (SCRP) formed by the photoreduction of 9,10-anthraquinone-1, 5-disulfonic acid (1,5-AQDS) disodium salt in aerosol OT (AOT) reverse micelles were studied using time-resolved and pulsed electron paramagnetic resonance (EPR) techniques. Photoirradiation of 1,5-AQDS in AOT micellar solution exhibited a strong polarized EPR spectrum in the presence of water-soluble hydroquinones (H2Q). The polarization patterns varied from E*/A of radicals to the antiphase structure (APS) of SCRP in the early delay period after the laser pulse irradiation. By analysis of the spectra, the radicals were assigned to the corresponding benzoquinone anion radicals. On the basis of our results, the initial process of the 1,5-AQDS triplet quenching may be the electron transfer from H2Q. The consecutive reaction mechanism was proposed for the generation of the SCRP within the water pools. The slower evolution of the SCRP polarization is a result of the sequential radical reactions followed by the spin selective reaction of the radical pair (RP). The peak-to-peak distance of the APS pattern and also the component line width were affected by the molar ratios between water and AOT ([H2O]/[AOT], w). A decreasing w value resulted in an increase in the magnitude of the exchange interaction, |2J|. From the peak-to-peak width of each APS spectral shape in the time-resolved EPR (TR-EPR) spectrum, at w = 20, the value of |2J| was estimated of 0.04 mT. The value was extremely small compared with those of SCRP reported previously and corresponded to 1.9 nm as the average separation of the radicals. Comparatively, the half-life of the APS polarization was longer than the T1 values of the typical organic free radicals. The results suggest that the diffusion of the radicals was strongly restricted within the water pool of AOT micellar solution.

Right-angle wiggling electron paramagnetic resonance spectroscopy

Right-angle wiggling (RAW) electron paramagnetic resonance (EPR) spectroscopy is introduced. In this two-dimensional pulse EPR technique the orientation of the external magnetic field B0 is varied during the pulse sequence. For this purpose an additional sinusoidal field of variable amplitude ΔB0⊥ is applied perpendicular to the orientation of B0. RAW-EPR is an alternative and experimentally much less demanding technique to right-angle spinning for performing anisotropy-resolved EPR experiments [Sierra and Schweiger, Mol. Phys. 95, 973 (1998)]. It can be applied to improve the resolution of EPR spectra of orientationally disordered systems or to separate overlapping single-crystal EPR spectra, and to facilitate an unambiguous interpretation of these spectra. A detailed theoretical description of RAW-EPR is given. Model calculations for systems with axial and orthorhombic symmetry, as well as a number of RAW-EPR experiments on paramagnetic systems with anisotropic interactions are presented.

Electron-Zeeman Resolved Electron Paramagnetic Resonance Spectroscopy

Abstract Electron-Zeeman (EZ) resolved electron paramagnetic resonance (EPR) spectroscopy is introduced. In this two-dimensional pulse EPR technique a sinusoidal external field Δ B 0 ( t ) of variable amplitude is applied during the pulse sequence, resulting in a field-swept EPR spectrum that is spread into a second dimension, representing the g values of the paramagnetic species. The method can be used to improve the resolution of polycrystalline and single-crystal EPR spectra. For polycrystalline systems, simplified EPR spectra are obtained for fixed g values. For axial symmetry, these spectra allow direct access to coupling constants, also at orientations that differ from the principal axes orientations. Furthermore, overlapping resonances can be disentangled, extra-absorption peaks identified and unresolved hyperfine splittings observed. The sum projection onto the g -value axis corresponds in a g -value spectrum, which is free of hyperfine interactions. A detailed theoretical description of the EZ-EPR experiment is given. Several model calculations for systems with different symmetry of the magnetic interaction tensors are presented, and the versatility of EZ-EPR is demonstrated for different paramagnetic systems with g anisotropy.

First Observation of the Photoexcited Quintet State in Fullerene Linked with Two Nitroxide Radicals

The bisadduct of fullerene (1) having two nitroxide radicals at the trans-3 position was newly synthesized. The ground and photoexcited states of 1 were studied by continuous-wave and pulsed two-dimensional electron paramagnetic resonance spectroscopy. An exchange interaction between the radicals was evaluated to be much smaller than an isotropic hyperfine splitting, showing a doublet (S = 1/2) character in the ground state. It was found from a 2D nutation spectrum that a photoexcited state has a quintet character (S = 2) which is generated via interactions between photoexcited triplet fullerene (S = 1) and the two nitroxide radicals (S = 1/2).

The structure and electronic state of photoexcited fullerene linked with a nitroxide radical based on an analysis of a two-dimensional electron paramagnetic resonance nutation spectrum

An electron paramagnetic resonance (EPR) study of 3,4-fulleropyrrolidine-2-spiro-4′[2′,2′,6′,6′-tetramethyl]piperidine-1′-oxyl (1) was performed on the photoexcited quartet state in toluene glass. The spectrum of the |S,Ms〉=|3/2,±3/2〉⇔|3/2,±1/2〉 transitions was observed selectively by using a two-dimensional (2D) nutation method and analyzed with a spectral simulation in a randomly oriented system. A position of the nitroxide moiety was determined with respect to the zero-field splitting (zfs) axes of excited triplet fullerene (3C60) by taking into account of the dipolar–dipolar interaction between the radical and 3C60, the hyperfine coupling, the anisotropic g-value of the nitroxide radical, and the zfs of the 3C60 moiety. It was found that none of the zfs axes of the 3C60 moiety coincide with the local C2 axis of the molecule which is defined by the position of addition. A symmetry of the electronic structure in 3C60 is discussed on the basis of the result.