Origin of additional satellites in electron paramagnetic resonance spectra of rare earth ion pairs

Electron Paramagnetic Resonance (EPR) spectrum of pairs of two identical rare earth ions is considered in the case where the two ions feel slightly different crystal fields giving different g factors. When the differences Δg between the g factors give a Zeeman difference term ΔgβB₀ of the order of magnitude of the interaction between the two ions, the pair spectrum is composed of four lines instead of two: the usual doublet structure, and two additional satellites around the main central transitions. It is shown that for rare earth ions, the shape of the EPR pair spectrum is very sensitive to small g factor differences. This situation is illustrated by the case of neodymium pairs in the SrAl₁₂O₁₉ host.     

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 77K. 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 (3)A(1)(pipi*) state.     

Photoinduced electron transfer between C70 fullerene and 3,3',5,5'-tetramethylbenzidine studied by electron paramagnetic resonance

Electron paramagnetic resonance (EPR) in situ spectroscopy was applied in the study of photoinduced electron transfer between 3,3',5,5'-tetramethylbenzidine (TMB) and C70 in different solvent systems. The changes found in UV-vis spectra pointed at ground state charge transfer complex formation [C70-TMB] in benzonitrile. Upon selective excitation of C70 using steady-state monochromatic irradiation with a wavelength of 546 nm, two EPR singlets were observed, which were assigned to C70 mono- and di-anion. In the photochemical and cathodic in situ reductions, identical EPR spectra of anion radicals were obtained. C70 mono-anion was investigated also in frozen 1,2-dichlorobenzene solutions within the temperature range from 110 to 210 K, and at 110 K the anisotropic EPR spectrum of C70 mono-anion was simulated assuming an axially symmetric g-matrix with gparallel - gperpendicular = 0.00165.     

Photoexcited triplet-state dynamics of novel porphyrinoids: octaethylcorrphycene and octaethylhemiporphycene. Time-resolved electron paramagnetic resonance study

The photoexcited triplet state of the two new porphyrin isomers, octaethylhemiporphycene (OEHPC) and octaethylcorrphycene (OECPC), embedded in isotropic (toluene) and an anisotropic liquid crystal matrices was studied by time-resolved electron paramagnetic resonance (EPR) spectroscopy in the temperature range of 150–300 K. The magnetic, kinetic and orientation parameters were determined and interpreted in terms of structure, symmetry and dynamic. Analysis of the results suggests that at T250 K, the orientation and packing of the chromophores result from a discrete solid-like jumps mechanism, which is more efficient for OECPC. This difference is rationalized in terms of differences in the symmetry of the two chromophores.     

W-band transient EPR and photoinduced absorption on spin-labeled fullerene derivatives

We investigated by W-band (94 GHz) transient electron paramagnetic resonance (TREPR) and photoinduced absorption (PIA) spectroscopy two fullerene derivatives bearing a nitroxide radical unit. After pulsed laser photoexcitation of the molecules in liquid toluene solution, complex EPR spectra are recorded, with lines in absorption and emission. The intrinsic higher spectral and temporal resolution of the W-band frequency leads to the assignment of all the lines in the spectrum and the determination of the sign and the absolute value of the exchange coupling between the fullerene in its photoexcited triplet state (S(T) = 1) and the radical (S(R) = 1/2). The two compounds with different fullerene-nitroxide spacers show opposite-ferromagnetic and antiferromagnetic-exchange couplings. The time evolution of the spectra and the polarization of the lines are interpreted in terms of several possible spin polarization mechanisms. The EPR measurements are complemented with PIA experiments.     

Electron paramagnetic resonance for everybody--MICROspec-X--a new class of electron paramagnetic resonance spectrometer

The electron paramagnetic resonance spectroscopy is the only method for detecting free radicals. Free radicals have an increased importance in our daily life. A small transportable EPR spectrometer is presented for the popularisation of the EPR method. The technical construction and some applications are illustrated which show the usability of the spectrometer.     

Observation of bi-polarons in blends of conjugated copolymers and fullerene derivatives

From a fundamental and application point of view it is of importance to understand how charge carrier generation and transport in a conjugated polymer (CP):fullerene blend are affected by the blend morphology. In this work light-induced electron spin resonance (LESR) spectra and transient ESR response signals are recorded on non-annealed and annealed blend layers consisting of alkyl substituted thieno[3,2-b]thiophene copolymers (pATBT) and the soluble fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) at temperatures ranging from 10 to 180 K. Annealing of the blend sample leads to a reduction of the steady state concentration of light-induced PCBM anions within the blend at low temperatures (T = 10 K) and continuous illumination. This is explained on the basis of the reducing interfacial area of the blend composite on annealing, and the high activation energy for electron diffusion in PCBM blends leading to trapped electrons near the interface with the CP. As a consequence, these trapped electrons block consecutive electron transfer from an exciton on a CP to the PCBM domain, resulting in a relatively low concentration charge carriers in the annealed blend. Analysis of the transient ESR data allows us to conclude that in annealed samples diamagnetic bi-polaronic states on the CPs are generated at low temperature. The formation of these states is related to the generation and interaction of multiple positive polarons in the large crystalline polymer domains present in the annealed sample.     

Isotropic reorientations of fullerene C70 triplet molecules in solid glassy matrices revealed by light-induced electron paramagnetic resonance

Continuous-wave X-band electron paramagnetic resonance (EPR) of fullerene C(70) molecules excited to a triplet state by continuous light illumination was studied in molecular glasses of o-terphenyl and cis/trans-decaline and in the glassy polymers polymethylmethacrylate (PMMA) and polystyrene (PS). Above ～100 K, a distinct narrowing of EPR lineshape of the triplet was observed, which was very similar for all systems studied. EPR lineshape was simulated reasonably well within a framework of a simple model of random jumps, which implies that the C(70) molecule performs isotropic orientational motion by sudden jumps of arbitrary angles. In simulations, a single correlation time τ(c) was used, varying in the range of 10(-7)-10(-8) s. Near and below 100 K electron spin echo (ESE) signals were also obtained which were found to decay exponentially. Correlation times τ(c) obtained from simulation of the EPR spectra in the slow-motion limit (τ(c) close to 10(-7) s) turned out to be in good agreement with the phase memory times T(M) of the ESE decay, which additionally supports the employed simple model. The observed motional effects provide evidence that the nanostructure of the solid glassy media of different origins is soft enough to allow a large asymmetric C(70) molecule to reorient rapidly. Except for the EPR spectra of the triplet, in the center of the spectra, a small admixture of a narrow line was also observed; its possible nature is briefly discussed.     

Artifacts in the electron paramagnetic resonance spectra of C60 fullerene ions: inevitable C120O impurity

Aspects of the electron paramagnetic resonance (EPR) spectra of C60n- fulleride ions (n = 2, 3) and the EPR signal observed in solid C60 are reinterpreted. Insufficient levels of reduction and the unrecognized presence of C120O, a ubiquitous and unavoidable impurity in air-exposed C60, have compromised most previously reported spectra of fullerides. Central narrow line width signals ("spikes") are ascribed to C120On- (n = odd). Signals arising from axial triplets (g approximately 2.0015, D = 26-29 G) in the spectrum of C602- are ascribed to C120On- (n = 2 or 4). Their D values are more realistic for C120O than C60. Less distinct signals from "powder" triplets (D approximately 11 G) are ascribed to aggregates of C120On- (n = odd) arising from freezing nonglassing solvents. In highly purified samples of C60, we find no evidence for a broad approximately 30 G signal previously assigned to a thermally accessible triplet of C60(2-). The C60(2-) ion is EPR-silent. Signals previously ascribed to a quartet state of the C60(3-) ion are ascribed to C120O4-. Uncomplicated, authentic spectra of C60- and C60(3-) become available when fully reduced samples are prepared under strictly anaerobic conditions from freshly HPLC-purified C60. Solid off-the-shelf C60 has an EPR signal (g approximately 2.0025, DeltaH(pp) approximately 1.5 G) that is commonly ascribed to the radical cation C60*+. This signal can be reproduced by exposing highly purified, EPR-silent C60 to oxygen in the dark. Doping C60 with an authentic C60*+ salt gives a signal with much greater line width (DeltaH(pp) = 6-8 G). It is suggested that the EPR signal in air-exposed samples of C60 arises from a peroxide-bridged diradical, *C60-O-O-C60* or its decomposition products rather than from C60*+. Solid-state C60 is more sensitive to oxygen than previously appreciated such that contamination with C120O is almost impossible to avoid.     

Hyperfine artifacts in electron paramagnetic resonance imaging

Low-molecular weight nitroxide labels (nitroxides) are commonly used as probes in electron paramagnetic resonance (EPR) spectroscopy. The nitroxides exhibit multiple lines in their EPR spectrum due to hyperfine coupling of the unpaired electron with the nitrogen nucleus. In EPR imaging, these hyperfine lines cause either hyperfine-based limitations in the maximum obtainable image resolution or hyperfine-based artifacts in the reconstructed image. In this article we discuss the effect of hyperfine artifacts on the quality of the image and report the application of a numerical method based on forward-subtraction principles for removing hyperfine artifacts in the measured projections. We demonstrate using computer simulations and imaging phantoms that marked enhancement in image quality and resolution can be obtained by removing the hyperfine-imposed limit on the gradient magnitude and performing post-acquisition corrections for removing hyperfine artifacts in the image.     

Exchange narrowing in electron paramagnetic resonance of ruby

Previous theories of exchange narrowing of electron paramagnetic resonance spectra are re-examined. In the case of strong exchange interactions the diagonal matrix elements of the dipolar and fine structure interactions are time independent and symmetrization and narrowing of the electron resonance spectra result from a reordering of the magnetic energy levels by the strong exchange interaction. Lineshape calculations are given for concentrated ruby (10 mole % Cr₂O₃) and reasonable agreement with experiment is obtained.     

Time-resolved resonance Raman investigation of the 2-fluorenylnitrenium ion reactions with C8 guanosine derivatives

A nanosecond time-resolved resonance Raman (ns-TR3) spectroscopic study of the reactions of the 2-fluorenylnitrenium ion with several C8-substituted guanosine derivatives is reported. The TR3 spectra show that the 2-fluorenylnitrenium ion reacts with the C8-substituted guanosine derivatives (C8-methylguanosine and C8-bromoguanosine) to produce C8 intermediates with the methyl and bromine moieties still attached to the intermediate species at the C8 position. The C8-bromoguanosine species was observed to be less reactive toward the 2-fluorenylnitrenium ion compared to the guanosine and C8-methylguanosine species. Comparison of the TR3 spectra to the results obtained from density functional theory calculations was used to characterize the C8 intermediates observed to learn more about their structure and properties. The implications of these results for the chemical reactivity of arylnitrenium ions toward substituted guanosine derivatives are briefly discussed.     

Uses of electron paramagnetic resonance in studying fracture

The uses of electron paramagnetic resonance (EPR) in studying aspects of polymer fracture are discussed. The sensitivity of EPR is such that all phases of fracture are not amenable to investigation by these means. This paper attempts to define those areas where the authors' experience would indicate that success might or might not be expected. A discussion of the difference between the tensile fracture of drawn polymer fibers, in which strong signals are obtained, and cast and molded materials is given.     

Behavior of radical ion pairs produced by photoinduced electron transfer in polar solutions : Their dynamics in picosecond-hundred nanosecond regime

The dynamic behavior of radical ion pairs in polar solvents has been studied by directly observing time-resolved transient absorption spectra in the picosecond-hundred nanosecond regime and also by observing the picosecond laser induced photocurrent. Pyrene-N.N-dimethylaniline, pyrene-p-dicy-anobenzene, pyrene-triethylamine exciplex systems and the excited state of pyrene-pyromellitic dian-hydride electron donor-acceptor complexes in polar solvents have been examined. It has been clearly demonstrated that the rate constant k_n, of the charge recombination deactivation of radical ion pairs shows a strong dependence upon the energy gap ΔG_ip, (between the ion pair and the neutral ground state) and upon the molecular nature of individual electron donor or acceptor ions. The plot of k_nvs ΔG_ip for the chemically similar π-π exciplex systems as well as singlet excited electron donor-acceptor complex systems studied in this and some previous works showed clearly the characteristic features of the so-called Marcus inverted region.     

Electrostatic complexation and photoinduced electron transfer between Zn-cytochrome c and [olyanionic fullerene dendrimers

Two dendritic fullerene (DF) monoadducts, 2 and 3, which can carry up to 9 and 18 negative charges, respectively, were examined with respect to electrostatic complexation with Cytochrome c (Cytc). To facilitate comprehensive photophysical investigations, the zinc analogue of Cytc (ZnCytc) was prepared according to a novel, modified procedure. The association of ZnCytc and DF, and consequential photoinduced electron transfer within ZnCytc-DF from the photoexcited protein to the fullerene, was proven by fluorescence spectroscopy and transient absorption spectroscopy. These findings were also supported by circular dichroism as well as by extensive molecular dynamics (MD) simulations.     

Investigation of RNA-protein and RNA-metal ion interactions by electron paramagnetic resonance spectroscopy. The HIV TAR-Tat motif

Electron paramagnetic resonance (EPR) spectroscopy was used to investigate changes in dynamics of spin-labeled nucleotides in the TAR RNA (U23, U25, U38, and U40) upon binding to cations, argininamide, and two peptides derived from the Tat protein. Nearly identical changes in dynamics were obtained for either calcium or sodium ions, indicating the absence of a calcium-specific structural change for the TAR RNA in solution that had previously been suggested by crystallographic data. Similar dynamic signatures were obtained for two Tat-derived peptides that have the same important binding determinant (R52) and similar binding affinities to the TAR RNA. However, U23 and U38 were substantially less mobile for the wild-type peptide (YGRKKRRQRRR) than for the mutant (YKKKKRKKKKA), demonstrating that, flanking R52, amino acids in the wild-type sequence make specific contacts to the RNA.     

Stochastic Liouville equation treatment of the electron paramagnetic resonance line shape of an S-state ion in solution

The current approaches used for the analysis of electron paramagnetic resonance spectra of Gd3+ complexes suffer from a number of drawbacks. Even the elaborate model of [Rast et al., J. Chem. Phys. 113, 8724 (2000)] where the electron spin relaxation is explained by the modulation of the zero-field splitting (ZFS), by molecular tumbling (the so called static contribution), and deformations (transient contribution), is only readily applicable within the validity range of the Redfield theory [Advances in Magnetic Resonance, edited by J.-S. Waugh (Academic, New York, 1965), Vol. 1, p. 1], that is, when the ZFS is small compared to the Zeeman energy and the rotational and vibrational modulations are fast compared to the relaxation time. Spin labels (nitroxides and transition metal complexes) have been studied for years in systems that violate these conditions. The theoretical framework commonly used in such studies is the stochastic Liouville equation (SLE). The authors shall show how the physical model of Rast et al. can be cast into the SLE formalism, paying special attention to the specific problems introduced by the [Uhlenbeck and Ornstein, Phys. Rev. 36, 823 (1930)] process used to model the transient ZFS. The resulting equations are very general and valid for arbitrary correlation times, magnetic field strength, electron spin S, or symmetry. The authors demonstrate the equivalence of the SLE approach with the Redfield approximation for two well-known Gd3+ complexes.