Recent Electron Paramagnetic Resonance Research Articles

(Invited) Non-Polymer Organic Solar Cells: Microscopic Phonon Control to Suppress Non-Radiative Voltage Loss Via Charge-Separated State

Recent remarkable developments on non-fullerene solar cells have reached photoelectric conversion efficiency (PCE) of 18% by tuning the band energy levels in small molecular acceptors. In this respect, development of the small donor molecule is essential. Although several methods have been applied to elucidate the elementary processes of charge carriers in BHJ-OSCs, mechanistic details of non-radiative voltage loss are poorly understood originating from interfacial heterogeneous electron-hole pairs.A recent time-resolved electron paramagnetic resonance (TREPR) study suggested an impact of the recombination process generating the triplet excitons in non-fullerene acceptor solar cells,[1] while geometries and motions of charge-separated (CS) states were elucidated in details by electron spin polarization (ESP).[2-4] Furthermore, although effects of low-frequency nuclear motions (phonon) on the photocarrier generations have been discussed in several literatures,[2,5,6] microscopic origin of the phonon playing a role on the OPV performance is unclear including impact of the phonon assist on primary recombination processes. Here we systematically investigated mechanisms of solar cell performance with diketopyrrolopyrrole (DPP)–tetrabenzoporphyrin (BP) conjugates of C4-DPP–H2BP and C4-DPP–ZnBP where C4 represents butyl group substituted at the DPP unit as small p-type molecules in non-polymer organic solar cells (OSC) with an acceptor of [6,6]-phenyl-C61-buthylic acid methyl ester. We clarified microscopic origins of the photocarrier caused by phonon-assisted 1D electron-hole dissociations at donor:acceptor interface. Using a time-resolved electron paramagnetic resonance, we have characterized controlled charge-recombination by manipulating disorders in π-π donor stacking, ensuring carrier transport through face-on molecular conformations to suppress non-radiative voltage loss via capturing specific interfacial radical pairs separated by 1.8 nm in bulk-heterojunction solar cells. While disordered lattice motions from the π-π stackings via zinc ligation are essential to enhance the entropy for charge-dissociations at the interface, too much ordered crystallinity causes backscattering phonon to reduce OSC performances.

Sustaining Rabi oscillations by using a phase-tunable image drive

Recent electron spin resonance experiments on CaWO_4:Gd^{3+} and other magnetic impurities have demonstrated that sustained Rabi oscillations can be created by driving a magnetic moment with a microwave field frequency slightly larger than the Larmor frequency and tuned to the Floquet resonance, together with another microwave field (image drive) with a frequency smaller than the Larmor frequency. These observations are confirmed by the new experimental results reported in this paper. We use numerical and analytical techniques to study the interplay between the microwave drives and three different mechanisms of relaxation. The first model describes a magnetic moment subject to microwave fields, interacting with a bath of two-level systems which acts as a source of decoherence and dissipation. The second model describes identical, interacting magnetic moments, subject to the same microwave fields. The decay of the Rabi oscillations is now due to the interactions. Third, we study Rabi oscillation decay due to the inhomogeneity of the microwave radiation. We show that under appropriate conditions, and in particular at the Floquet resonance, the magnetization exhibits sustained Rabi oscillations, in some cases with additional beatings. Although these two microscopic models separately describe the experimental data well, a simulation study that simultaneously accounts for both types of interactions is currently prohibitively costly. To gain further insight into the microscopic dynamics of these two different models, we study the time dependence of the bath and system energy and of the correlations of the spins, data that are not readily accessible experimentally.Graphic abstract

Hybrid Kinetic Monte Carlo/Molecular Dynamics Simulations of Bond Scissions in Proteins

Proteins are exposed to various mechanical loads that can lead to covalent bond scissions even before macroscopic failure occurs. Knowledge of these molecular breakages is important to understand mechanical properties of the protein. In regular Molecular Dynamics (MD) simulations, covalent bonds are predefined and reactions cannot occur. Furthermore, such events rarely take place on MD timescales. Existing approaches that tackle this limitation either rely on computationally expensive quantum calculations or complex bond order formalisms in force fields. To circumvent these limitations, we have developed a new reactive Kinetic Monte Carlo / Molecular Dynamics (KIMMDY) scheme. Here, bond rupture rates are calculated in the spirit of a transition state model based on the interatomic distances in the MD simulation and then used as an input for a Kinetic Monte Carlo step. This drastically increases the accessible timescales for reactive MD simulations. Using this new technique, we investigated bond ruptures in a multi-million atom system of tensed collagen, a structural protein found in skin, bones and tendons. Our simulations show a clear concentration of homolytic bond scissions near chemical crosslinks in collagen, consistent with our recent Electron Paramagnetic Resonance (EPR) experiments on collagen type I under stretch. We suggest that these created mechanoradicals are a yet unknown connection converting mechanical into oxidative stress. This application also demonstrates that our hybrid computational approach is easily scalable and exhibits only a minor slowdown compared to classical MD. It is straightforwardly applicable to address related questions for other biomaterials with coupled mechanical and chemical processes.

Assessment of the manganese cluster’s oxidation state via photoactivation of photosystem II microcrystals

Knowledge of the manganese oxidation states of the oxygen-evolving Mn4CaO5 cluster in photosystem II (PSII) is crucial toward understanding the mechanism of biological water oxidation. There is a 4 decade long debate on this topic that historically originates from the observation of a multiline electron paramagnetic resonance (EPR) signal with effective total spin of S = 1/2 in the singly oxidized S2 state of this cluster. This signal implies an overall oxidation state of either Mn(III)3Mn(IV) or Mn(III)Mn(IV)3 for the S2 state. These 2 competing assignments are commonly known as "low oxidation (LO)" and "high oxidation (HO)" models of the Mn4CaO5 cluster. Recent advanced EPR and Mn K-edge X-ray spectroscopy studies converge upon the HO model. However, doubts about these assignments have been voiced, fueled especially by studies counting the number of flash-driven electron removals required for the assembly of an active Mn4CaO5 cluster starting from Mn(II) and Mn-free PSII. This process, known as photoactivation, appeared to support the LO model since the first oxygen is reported to evolve already after 7 flashes. In this study, we improved the quantum yield and sensitivity of the photoactivation experiment by employing PSII microcrystals that retained all protein subunits after complete manganese removal and by oxygen detection via a custom built thin-layer cell connected to a membrane inlet mass spectrometer. We demonstrate that 9 flashes by a nanosecond laser are required for the production of the first oxygen, which proves that the HO model provides the correct description of the Mn4CaO5 cluster's oxidation states.

Fluctuating exchange interactions enable quintet multiexciton formation in singlet fission.

Several recent electron spin resonance studies have observed a quintet multiexciton state during the singlet fission process. Here, we provide a general theoretical explanation for the generation of this state by invoking a time-varying exchange coupling between pairs of triplet excitons and subsequently solving the relevant time-varying spin Hamiltonian for different rates at which the exchange coupling varies. We simulate experimental ESR spectra and draw qualitative conclusions about the adiabatic and diabatic transitions between triplet pair spin states.

(Invited) Atomistic Insights into Cathode Dissolution and Ion Transport in Li-Ion Batteries

Fundamental understanding of inter-connected electrochemical phenomena, including chemical reactions, charge/ion transport, defect evolution, and solvation dynamics) is crucial to accelerate development of low-cost, safe, and high-performance Li-ion battery architectures. In particular, molecular mechanisms underlying (a) dissolution of cathode into liquid electrolyte; and (b) fast Li-ion transport in novel solid-electrolytes, need to be clearly identified to design Li-ion batteries with exceptional capacity, rate-capability, and cycle life; however, such knowledge remains in its infancy. In this talk, we will first address the key mechanisms governing dissolution of a commercially promising LiMn2O4 (LMO) cathode. Using a combination of ab initio molecular dynamics simulations (AIMD) and quantum-chemical calculations, we demonstrate that dissolved Mn ions exist in 2+ oxidation state in carbonate based electrolytes. This finding resolves the debate raised by recent electron paramagnetic resonance and X-ray absorption near edge structure experiments suggesting that Mn could preferentially exist in 3+ state within solution; and provides evidence supporting the long-standing belief that Hunter’s disproportionation mechanism (2 Mn3+ --> Mn4+ + Mn2+) precedes Mn dissolution. Furthermore, we employed AIMD and density functional theory (DFT) calculations to probe formation of Mn2+ on stable LMO (511) surfaces via surface Hunter charge disproportionation mechanisms, as well as chemical mechanisms involving hydrofluoric acid (HF) electrolyte impurities and oxidation of ethylene carbonate (EC) electrolyte molecules. These studies would guide future strategies to suppress Mn dissolution, and in turn, combat capacity fading in LiBs based on LMO cathodes. Next, we will demonstrate that strongly-correlated rare-earth perovskite nickelates exhibit fast Li-ion conduction with simultaneous suppression of electronic transport via Mott transition. The pronounced Li-ion conduction is facilitated by electron injection into Ni orbitals, and concomitant lattice expansion when the Li ions are inserted from a reservoir. These results will be discussed in the context of developing next-generation lithium ion batteries with exceptional performance.

Studying Lipid-Protein Interactions with Electron Paramagnetic Resonance Spectroscopy of Spin-Labeled Lipids.

Spin label electron paramagnetic resonance (EPR) of lipid-protein interactions reveals crucial features of the structure and assembly of integral membrane proteins. Spin-label EPR spectroscopy is the technique of choice to characterize the protein solvating lipid shell in its highly dynamic nature, because the EPR spectra of lipids that are spin-labeled close to the terminal methyl end of their acyl chains display two spectral components, those corresponding to lipids directly contacting the protein and those corresponding to lipids in the bulk fluid bilayer regions of the membrane. In this chapter, typical spin label EPR procedures are presented that allow determination of the stoichiometry of interaction of spin-labeled lipids with the intramembranous region of membrane proteins or polypeptides, as well as the association constant of the spin-labeled lipid with respect to the host lipid. The lipids giving rise to a so-called immobile spectral component in the EPR spectrum of such samples are identified as the motionally restricted first-shell lipids solvating membrane proteins in biomembranes. Stoichiometry and selectivity are directly related to the structure of the intramembranous sections of membrane-associated proteins or polypeptides and can be used to study the state of assembly of such proteins in the membrane. Since these characteristics of lipid-protein interactions are discussed in detail in the literature (see ref. Marsh, Eur Biophys J 39:513-525, 2010 for a recent review), here we focus more on how to spin label model membranes and biomembranes and how to measure and analyze the two-component EPR spectra of spin-labeled lipids in phospholipid bilayers that contain proteins or polypeptides. After a description of how to prepare spin-labeled model and native biological membranes, we present the reader with computational procedures for determining the molar fraction of motionally restricted lipids when both, one or none of the pure isolated-mobile or immobile-spectral components are available. With these topics, this chapter complements a previous methodological paper (Marsh, Methods 46:83-96, 2008). The interpretation of the data is discussed briefly, as well as other relevant and recent spin label EPR techniques for studying lipid-protein interactions, not only from the point of view of lipid chain dynamics.

Optimal control of an inhomogeneous spin ensemble coupled to a cavity

We apply optimal control techniques to an inhomogeneous spin ensemble coupled to a cavity. A general procedure is proposed for designing the control strategies. We numerically show the extent to which optimal control fields robust against system uncertainties help enhancing the sensitivity of the detection process. The parameters of the numerical simulations are taken from recent Electron Spin Resonance experiments. The low and high cooperativity regimes are explored.

CW EPR parameters reveal cytochrome P450 ligand binding modes

Cytochrome P450 (CYP) monoxygenses utilize heme cofactors to catalyze oxidation reactions. They play a critical role in metabolism of many classes of drugs, are an attractive target for drug development, and mediate several prominent drug interactions. Many substrates and inhibitors alter the spin state of the ferric heme by displacing the heme's axial water ligand in the resting enzyme to yield a five-coordinate iron complex, or they replace the axial water to yield a nitrogen-ligated six-coordinate iron complex, which are traditionally assigned by UV–vis spectroscopy. However, crystal structures and recent pulsed electron paramagnetic resonance (EPR) studies find a few cases where molecules hydrogen bond to the axial water. The water-bridged drug-H2O-heme has UV–vis spectra similar to nitrogen-ligated, six-coordinate complexes, but are closer to “reverse type I” complexes described in older liteature. Here, pulsed and continuous wave (CW) EPR demonstrate that water-bridged complexes are remarkably common among a range of nitrogenous drugs or drug fragments that bind to CYP3A4 or CYP2C9. Principal component analysis reveals a distinct clustering of CW EPR spectral parameters for water-bridged complexes. CW EPR reveals heterogeneous mixtures of ligated states, including multiple directly-coordinated complexes and water-bridged complexes. These results suggest that water-bridged complexes are under-represented in CYP structural databases and can have energies similar to other ligation modes. The data indicates that water-bridged binding modes can be identified and distinguished from directly-coordinated binding by CW EPR.

The Nitrogen Acceptor in 2H‐Polytype Synthetic MoS2: Frequency and Temperature Dependent ESR Analysis

In extending on recent electron spin resonance (ESR) work which has revealed the N acceptor (N substituting for S site) in 2H‐polytype bulk synthetic MoS2, the dopant is extensively analyzed in terms of its frequency, temperature (T), and magnetic field B angular dependent ESR spectral characteristics. For B∥c‐axis, the multi‐frequency analysis confirms the ESR spectrum as being composed of a 14N hyperfine (hf) triplet with hf splitting constant A∥ = 14.7 ± 0.2 G (B∥c‐axis) and making up ≈74% of the total spectrum intensity, superimposed on a central line centered at about equal g‐value [g∥ = 2.032(2)]. The presence of the latter signal, points to some non‐uniformity in dopant distribution, that is, clustering, with about ≈26% of the total N response not originating from N incorporated in the preferred “isolated” dopant configuration. Angular dependent measurements reveal distinct anisotropy of the hf matrix, whereas ESR probing over a wide T‐range exposes drastic signal broadening with increasing T above ≈150 K. Detailed study of the N acceptor signal intensity versus T at Q‐band reveals an activation energy Ea = 50 ± 10 meV, herewith consolidating the value reported initially. Besides unveiling the S‐site substitutional N impurity as an appropriate p‐type dopant for MoS2, the total of the ESR work establishes a basic frame of the N acceptor ESR characteristics, giving way for further in‐depth theoretical perusal.

Identification and Quantification of Copper Sites in Zeolites by Electron Paramagnetic Resonance Spectroscopy

Recent quantitative electron paramagnetic resonance spectroscopy (EPR) data on different copper species present in copper exchanged CHA zeolites are presented and put into context with the literature on other copper zeolites. Results presented herein were obtained using ex situ and in situ EPR on copper ion exchanged into a CHA zeolite with Si/Al = 14 ± 1 to obtain Cu/Al = 0.46 ± 0.02. The results shed light on the identity of different copper species present after activation in air. Since the EPR signal is quantifiable, the content of the different EPR active species has been elucidated and Cu2+ in 2Al positions in the 6-membered rings (6mr) of the CHA structure has been characterized. Some copper species are found not to give an EPR signal at ambient or high temperatures. Fortunately, treatments with different gasses under in situ conditions are able to trigger an EPR signal and thus reveal information about the reactivity and the quantity of some of the otherwise EPR silent species. In this way the [Cu–OH]+ species in copper substituted low-Al zeolites has been indirectly observed and quantified. EPR active Cu2+ species have been followed under reduction and oxidation with gas mixtures relevant for the selective catalytic reduction of NO with NH3 (NH3-SCR) revealing that all Cu2+ in 6mr are easily reduced and oxidized at 200 °C. Furthermore, a stable [Cu–NO3]+ species is identified in Cu-CHA after exposure to NO and O2, but is not stable in 2Al 6mr sites of the CHA structure under the applied conditions.

ChemInform Abstract: A Review of EPR Studies on Magnetization of Nanoparticles of Dilute Magnetic Semiconductors Doped by Transition‐Metal Ions

AbstractReview: 40 refs.

What computational chemistry and magnetic resonance reveal concerning the oxygen evolving centre in Photosystem II

Density Functional Theory (DFT) computational studies of the Mn4/Ca Oxygen Evolving Complex (OEC) region of Photosystem II in the paramagnetic S2 and S3 states of the water oxdizing catalytic cycle are described. These build upon recent advances in computationally understanding the detailed S1 state OEC geometries, revealed by the recent high resolution Photosystem II crystal structures of Shen et al., at 1.90Å and 1.95Å (Petrie et al., 2015, Angew. Chem. Int. Ed., 54, 7120). The models feature a ‘Low Oxidation Paradigm’ assumption for the mean Mn oxidation states in the functional enzyme, with the mean oxidation levels being 3.0, 3.25 and 3.5 in S1, S2 and S3, respectively. These calculations are used to infer magnetic exchange interactions within the coupled OEC cluster, particularly in the Electron Paramagnetic Resonance (EPR)-visible S2 and S3 states. Detailed computational estimates of the intrinsic magnitudes and molecular orientations of the 55Mn hyperfine tensors in the S2 state are presented. These parameters, together with the resultant spin projected hyperfine values are compared with recent appropriate experimental EPR data (Continuous Wave (CW), Electron-Nuclear Double Resonance (ENDOR) and ELDOR (Electron-Electron Double Resonance)-Detected Nuclear Magnetic Resonance (EDNMR)) from the OEC. It is found that an effective Coupled Dimer magnetic organization of the four Mn in the OEC cluster in the S2 and S3 states is able to quantitatively rationalize the observed 55Mn hyperfine data. This is consistent with structures we propose to represent the likely state of the OEC in the catalytically active form of the enzyme.

A Review of EPR Studies on Magnetization of Nanoparticles of Dilute Magnetic Semiconductors Doped by Transition-Metal Ions

This article reviews recent electron paramagnetic resonance (EPR) studies on the magnetic properties of nanoparticles of dilute magnetic oxide semiconductors (DMS) doped with transition-metal ions. These nanoparticles are SnO2 doped with Co2+, Fe3+, Cr3+ ions, CeO2 doped with Ni2+, Co2+ ions, and ZnO doped with Fe3+ ions. The EPR studies reveal that the method of synthesis, surface properties, and size of nanoparticles are important factors that determine the magnetic properties of DMS nanoparticles. In addition, they indicate that ferromagnetic and paramagnetic phases may coexist. The saturation magnetization, as estimated from EPR signal, depends both on the doping level of impurities and annealing temperature. Undoped DMS also exhibit ferromagnetism due to oxygen vacancies. Furthermore, the EPR spectrum depends very sensitively on the size of nanoparticle.

Atomic-Resolution Structures of Discrete Stages on the Reaction Coordinate of the [Fe4S4] Enzyme IspG (GcpE)

IspG is the penultimate enzyme in non-mevalonate biosynthesis of the universal terpene building blocks isopentenyl diphosphate and dimethylallyl diphosphate. Its mechanism of action has been the subject of numerous studies but remained unresolved due to difficulties in identifying distinct reaction intermediates. Using a moderate reducing agent and an epoxide substrate analogue, we were now able to trap and crystallographically characterize various stages in the IspG-catalyzed conversion of 2-C-methyl-d-erythritol-2,4-cyclo-diphosphate into (E)-1-hydroxy-2-methylbut-2-enyl-4-diphosphate. In addition, the enzyme's structure was determined in complex with several inhibitors. These results, combined with recent electron paramagnetic resonance data, allowed us to deduce a detailed and complete IspG catalytic mechanism, which describes all stages from initial ring opening to formation of (E)-1-hydroxy-2-methylbut-2-enyl-4-diphosphate via discrete radical and carbanion intermediates. The data presented in this article provide a guide for the design of selective drugs against many prokaryotic and eukaryotic pathogens to which the non-mevalonate pathway is essential for survival and virulence.

Photocatalytic Evidence of the Rutile‐to‐Anatase Electron Transfer in Titania

Layered anatase‐rutile titania thin‐films were synthesized via atmospheric‐pressure chemical vapor deposition and characterized using X‐ray diffraction, Raman spectroscopy and electron microscopy. The interposition of an amorphous TiO2‐based interlayer allowed direct vapor deposition of anatase on a rutile substrate, which is otherwise hindered by templating. This resourceful approach and the subsequent crystallization of the amorphous layer after annealing of the films allowed investigation on the impact of an efficient interface of the two anatase‐rutile phases in the photodegradation of a model organic pollutant. Clear evidence is presented on the synergy between the two polymorphs and more importantly, on the charge flow across the interface, which, against much conventional understanding, it involves electron transfer from rutile to anatase and is in agreement with a recent theoretical model and electron paramagnetic resonance data. Here, an increasing density of trapped electrons on the anatase surface of the A/R film is confirmed by photoreduction of silver. This observation is attributed to a defect‐free efficient contact between the two phases and the presence of small rutile particles that promote rapid electron transfer at the A‐R interface of the films.

Electron spin resonance shifts inS=1antiferromagnetic chains

We discuss electron spin resonance (ESR) shifts in spin-1 Heisenberg antiferromagnetic chains with a weak single-ion anisotropy based on several effective field theories, the O(3) nonlinear sigma model (NLSM) in the Haldane phase, free fermion theories around the lower and the upper critical fields. In the O(3) NLSM, the single-ion anisotropy corresponds to a composite operator which creates two magnons at the same time and position. Therefore, even inside a parameter range where free magnon approximation is valid, we have to take interactions among magnons into account. Though the O(3) NLSM is only valid in the Haldane phase, an appropriate translation of Faddeev-Zamolodchikov operators of the O(3) NLSM to fermion operators enables one to treat ESR shifts near the lower critical field in a similar manner to discussions in Haldane phase. We present that our theory gives quantitative agreements with recent ESR experimental results on an spin-1 chain compounds NDMAP.

Resonance THz spectroscopy in high magnetic fields

Abstract The employment of the high-magnetic-field resonance spectroscopy to probe magnetic excitations in the THz frequency range is reported. To illustrate the great potential of this technique in solid state physics, we present results of recent electron spin resonance studies of the quantum-tunneling effect in the single-molecule magnet Mn12tba and of the soliton–magnon crossover in Cu-PM, a spin-1/2 Heisenberg chain system with a staggered transverse field. Among others, we report on the successful use of the THz-range time-domain and free electron laser spectroscopy to study magnetic excitation spectra in pulsed magnetic fields.

First-principles investigation of hyperfine interactions for nuclear spin entanglement in photoexcited fullerenes

The study of hyperfine interactions in optically excited fullerenes has recently acquired importance within the context of nuclear spin entanglement for quantum information technology. We here report a first-principles pseudopotential study of the hyperfine coupling parameters of optically excited fullerene derivatives as well as small organic radicals. The calculations are performed within the gauge-invariant projector-augmented wave method [C. Pickard and F. Mauri, Phys. Rev. B. 63, 245101 (2001)]. In order to establish the accuracy of this methodology we compare our results with all-electron calculations and with experiment. In the case of fullerene derivatives we study the hyperfine coupling in the spin-triplet exciton state and compare our calculations with recent electron paramagnetic resonance measurements [M. Schaffry et al., Phys. Rev. Lett. 104, 200501 (2010)]. We discuss our results in light of a recent proposal for entangling remote nuclear spins in photo-excited chromophores.

The dangling-bond defect in amorphous silicon: Statistical random versus kinetically driven defect geometries

Amorphous and micro-crystalline silicon (a-Si:H, μc-Si) are key materials for resource-saving thin-film solar cells. However, the efficiency of such devices is severely limited by light-induced Si dangling-bond defects, which can be detected by electron paramagnetic resonance (EPR). We report density-functional theory calculations on a set of random dangling bonds created in supercell models of a-Si:H and compare calculated hyperfine and g-tensor distributions to the ones obtained from a recent multi-frequency EPR spectral analysis. Our results show that the g-tensor does not exhibit axial symmetry as has been previously assumed, but is clearly rhombic. The hyperfine coupling to the undercoordinated Si atom, on the other hand, is almost perfectly axial. This apparent discrepancy in the symmetry properties is shown to be a consequence of the underlying coupling mechanisms and how these are influenced by structural disorder.However, the hyperfine distribution calculated from our random models underestimates the experimentally observed 30% red-shift when going from c-Si to a-Si:H. We suggest that only a subset of possible dangling-bond configurations is observed in experiment. We discuss plausible mechanisms that would give rise to such a selection, and new experiments to test these hypotheses.