OD ESR Research Articles

Hyperfine Structure in the OD ESR Spectra of Recombining Charge Pairs in Doped Polyethylene Matrices

The hyperfine structure has been resolved in the optically detected electron spin resonance (OD ESR) spectra of radical ion pairs generated by ionizing radiation at a low dopant concentration in a solid polyethylene matrix at room temperature. The different organic molecules that can capture electrons and holes were used as the dopants. The spin-lattice relaxation times T1 for radical ions of dopants were estimated to be tens of microseconds. The temperature dependence of OD ESR signal intensity indicates the diffusion-assisted mechanism of recombination of radical ion pairs in polyethylene.

New Possibilities of OD ESR: Vacuum Ultraviolet Generation of Radical Ion Pairs

A new version of the optically detected electron spin resonance (OD ESR) technique using vacuum ultraviolet radiation to generate radical ion pairs is discussed. The technique can be used to study the spin-correlated processes of charge recombination in thin films that can be useful in spintronics. Distinctive features of photoionization and high-energy (including X-ray) ionization are discussed. The first results obtained by the technique in the study of charge recombination in polymeric and liquid crystal films are presented. The results of the concurrent observation of the OD ESR and electron cyclotron resonance (ECR) in the low-pressure gas over the sample are outlined. A possible application of ECR in the context of the OD ESR spectroscopy is suggested.

Optically detected ESR spectrum of decafluorobiphenyl radical anions

The radical anion of decafluorobiphenyl was detected by the OD ESR technique for the first time; the spectrum was interpreted on the basis of quantum-chemical calculations as a result of fast exchange among equivalent asymmetrical structures.

Indications for unequal rates of ion-molecular charge transfer reaction for biphenyl radical anion and cation from MARY and OD ESR spectra

MARY and Optically Detected ESR techniques were used to infer that radical cation and radical anion of biphenyl in concentrated hexane solutions have substantially different second moments of their ESR spectra. This observation is interpreted in terms of different types of processes between the two radical ions and the neutral parent molecule. It is suggested that dimerization is prevalent for biphenyl radical cation, while biphenyl radical anion is rather involved in fast ion-molecular charge transfer reaction.

Structural Nonrigidity of the 1,2,3-Trifluorobenzene Radical Anion as Probed by Quantum-Chemical Methods and OD ESR

Structural Nonrigidity of the 1,2,3-Trifluorobenzene Radical Anion as Probed by Quantum-Chemical Methods and OD ESR

Intramolecular Dynamics of 1,2,3-Trifluorobenzene Radical Anions As Studied by OD ESR and Quantum-Chemical Methods

Ab initio UMP2, RMP2, DFT/UB3LYP, and CBS-QB3 calculations have shown that the adiabatic potential energy surface (PES) of the 1,2,3-trifluorobenzene radical anion is a pseudorotation surface formed by nonplanar stationary structures. The low (approximately 2-4 kcal/mol) energy barriers in the path of pseudorotation imply manifestations of spectral exchange in the ESR spectra of this radical anion. The optically detected ESR of radical ion pairs was used to obtain the ESR spectrum of 1,2,3-trifluorobenzene radical anion in liquid squalane solution and to study temperature variations in the spectrum over the range of 243-325 K. The spectrum is a doublet of triplets with hfc constants of a(F(2)) = 29 mT and a(2F(1,3)) = 7.6 mT at T = 243 K. The experimental hfc constants are temperature-dependent. Calculations of the temperature dependence of hfc constants in the framework of the model of classical nuclei motion along the pseudorotation coordinate reproduce well the experimental data.

OD ESR detection of the radical anions of cyclic nitrones in liquid solutions

The radical anions of five-membered cyclic nitrone spin traps, 3,3,5,5-tetramethyl-1-pyrroline-1-oxide and 1,2,2,5,5-pentamethyl-3-imidazoline-3-oxide, formed under radiolysis of liquid squalane solutions at 250–268 K, were registered by the method of optically detected ESR, and the hfc constants and g-values of the radical anions were determined. The hfc constants of these radical anions were predicted by DFT calculations and were found to be in quantitative agreement with experiment. It was demonstrated that radical anions of the five-membered cyclic nitrones have non-planar geometry with the spin density localized in the C N O fragment.

OD ESR Detection of the Radical Anions of Cyclic Nitrones in Liquid Solutions

OD ESR Detection of the Radical Anions of Cyclic Nitrones in Liquid Solutions

Optically detected ESR and low magnetic field signals from spin triads: 2-imidazoline-1-oxyl derivatives in X-irradiated alkane liquids as a method to study three-spin systems.

This contribution reports the design and synthesis of a series of spin-labeled charge acceptors to produce three-spin systems of "radical ion/biradical ion" type in X-irradiated alkane liquids. This opens the way to study spin triads in experimental conditions, in which short-lived radical ion pairs are conventionally studied, thus offering optically detected techniques such as magneto-resonance OD ESR and level-crossing MARY spectroscopy. The structure of the synthesized 2-imidazoline-1-oxyl derivatives is A-Sp-R, where A is a positive or negative charge acceptor, R is a stable radical, and Sp is a hydrocarbon bridge. The set of 20+ compounds represent a convenient tool to construct experimental three-spin systems with various properties, e.g. with the "third" spin introduced into one or the other partner of the radical ion pair. The degree of exchange coupling between the two paramagnetic fragments in the biradical ion has been demonstrated to strongly depend on the type of the radical fragment R and the structure of the bridge Sp. As a result, a series of acceptors with systematically reduced exchange interaction has been synthesized, and optimal systems for the observation of low magnetic field effect have been found. In the most favorable case, an OD ESR signal from a spin triad living as short as ca. 100 ns has been registered as a single unresolved line. The exchange integral for this biradical anion (9) was estimated from OD ESR and ESR experiments to be ca. 10(3) G by the order of magnitude, which is much greater than the hyperfine couplings in the biradical ion but much smaller than the thermal energy kT.

Spin coherence effects observed in the OD ESR spectra of radical pairs with strong hyperfine interaction

The optically detected ESR technique is used to record the spectra of a radical pair generated in the photolysis of 2,4,6-trimethylbenzoyl diphenylphosphine oxide in a micellar sodium dodecylsulfate solution. Along with the signals of the radicals of the pair, additional resonances have been detected in the spectrum, whose signal is inverted with respect to the fundamental ones. It is shown that the inverted lines consist of narrow and wide components, which arise from the effects of crossing and anticrossing of pair spin energy levels, respectively.

The first experimental detection, by OD ESR spectroscopy, of radical anions of siloles and germoles bearing hydrogen and chlorine substituents attached to the heteroatom

Short-lived radical anions of 2,3,4,5-tetraphenylsiloles and 2,3,4,5-tetraphenylgermoles have been detected for the first time by OD ESR spectroscopy.

Spin relaxation parameters in recombining radical ion pair (diphenylsulfide-d10)+/(p-Terphenyl-d14)− obtained by OD ESR and quantum beats techniques

Parameters of paramagnetic relaxation were determined by OD ESR and quantum beats techniques for a recombining pair of radical ions (DPS-d10)+/(PTP-d14)− inn-hexane, isooctane,cis-decalin, and squalane solutions. TheT 2 relaxation time determined by quantum beats technique is independent of solvent viscosity and magnetic field strength in the range 170–9600 G. These data are in agreement with the results obtained by OD ESR technique assuming fastT 1 relaxation for radical cation. Neglecting the contribution of radical anion relaxation, we obtainedT 1c=T 2c≅50 ns for (DPS-d10)+.

A method of altering the modulation type and frequency of commercial ESR spectrometers

A strategy of altering the modulation type and frequency of commercial continuous wave ESR spectrometers without changes in the main console is discussed. A simple converter unit, permitting the use of various external receivers, widening the possibilities for the basic commercial set-up is suggested. Block diagrams for altering the modulation frequency and type are presented and the use of boxcar averaging systems for the time-resolved experiments is discussed in detail. As an example of a successful application of this method, some results obtained with two different RYDMR accessories, based on the commercial CW ESR spectrometers, are discussed. X-band OD ESR studies of hydrocarbon solutions are illustrated by the spectra of p-xylene and trans-phenyl-propene radical cations, and methyl acrylate and trans-azabenzene radical ions, detected in situ under X-irradiation. Semiconductor studies are illustrated by CW and time-resolved X-band ODMR spectra of PGa-P3Y point defects as well as its OD ENDOR and ENDOR-induced ODMR spectra.

S-band magnetron-based accessory for optical detection of ESR spectra of radiation-induced radical-ion pairs

In the development of the technique of reaction yield detected magnetic resonance (RYDMR), a novel simple accessory for optical detection of ESR (OD ESR), based on the S-band continuous wave magnetron generator is presented. Basic technical requirements for the RYDMR accessories are discussed. The successful use of an S-band magnetron as a microwave source for RYDMR experiments is reported for the first time. Along with the use of a simple loop-gap resonator it permits the studies of both polar and non-polar samples with good sensitivity and field resolution. With the help of this accessory, OD ESR spectra of some new acceptors, acridin and fluorescein, were detected and some new solvents, poly(ethylene glycol) and paraffin-based high-voltage insulator oil, were successfully utilized.

Low temperature inversion of optically detected electron spin resonance spectra due to different rates of electron tunneling for singlet and triplet radical–ion pairs

A theoretical model based on the assumption about different rates of electron tunneling from different spin states is suggested to explain the experimentally observed low temperature inversion of OD ESR spectra of radical–ion pairs. Mathematical background of the model based on the conception of the effective tunneling spheres of different radii is given. The model suggested is in excellent agreement with the qualitative picture of OD ESR signal formation at the required conditions. On the basis of the model the inversion point of spectra is found and the behavior of the OD ESR signal intensity in the neighborhood of the inversion point depending on various physical parameters is predicted. The dependencies obtained may serve as a basis for experimental identification of the suggested mechanism.

Optically Detected ESR (OD ESR) of ion-radical Pairs in coloured solutions. Observation of transienttrans-azobenzene radical cation

Trans-azobenzene dissolved in different liquid hydrocarbons absorbs fluorescence arising from all acceptors previously used in Fluorescence Detected Magnetic Resonance (FDMR) and Optically Detected ESR (OD ESR) spectroscopy making optical detection impossible. In this report a new acceptor,rubrene, having sufficient quantum yield of fluorescence in the red band 550–620 nm, has been proven successful. OD ESR spectra of the radical-ion pairtrans-azobenzene+/rubrene− were detected in liquid squalane (2,6,10,15,19,23-hexamethyl-tetracosane) solution in the temperature range 294–243 K. The experimental isotropic hyperfine splittings of the radical cation oftrans-azobenzene (a N=1.4 mT) have been compared with those from MNDO/INDO calculations and with those of earlier work using freon matrix studies.

OD ESR line inversion in viscous solution. Indication of ST_ mechanism of radical ion pair recombination

The optically detected ESR spectra (OD ESR) of naphthalene radical ion pairs in squalane show inversion near the glass formation point of the solvent. A similar inversion of OD ESR spectra is also observed for some other additives, such as biphenyl and tetrafluorobenzene in glassy nonpolar solvents (squalane, 3-methylpentane). A theory of radical pairs can be used to account for the observed inversion of signal amplitude as a result of adiabatic passage of the anticrossing point of radical pair S and T_ levels upon counter ion recombination. The comparison between the shapes of experimentally observed and theoretically calculated spectra qualitatively confirm this hypothesis.

Positive hole hopping in glassy alkanes at 77 K: observation by the OD ESR technique

The optically detected ESR spectra of radical cations of 3-methylpentane, 3,3-dimethylpentane, 2-methylpentane, 2,3-dimethylbutane and other branched alkanes in their glassy matrices at 77 K display an anomalous ratio of hyperfine component intensities observed in an increase of the intensity of weak components towards the binomial distribution. A theory of uncorrelated frequency migration accounts for this phenomenon, assuming motion of relaxed solvent holes via charge transfer between the solvent molecules. Comparison between the experimentally observed and theoretically calculated spectra gives quantitative parameters for this process. The results obtained show that the hole-residence time depends on the matrix and equals 0.1–0.2 μs for 3,3-dimethylpentane and 0.5–0.8 μs for 3-methylpentane at 77 K. Thus, the geminate solvent hole recombination in a glassy alkane matrix is preceded by a hopping motion among matrix molecules.

Determination of the frequency of excess electron trap to trap migration in saturated hydrocarbons by an OD ESR technique

The temperature dependence of the ESR linewidth of excess electrons in squalane and 3-methylpentane has been measured by the OD ESR technique. When the temperature dependence of the linewidth is fit by a model which includes a temperature-dependent jump frequency, E f, it is found to be smaller than the activation energy, E μ, for the electron mobility. For 3-methylpentane the activation difference is small and the jump length calculated from the Einstein formula, λ ≈ 10 Å, therefore has weak dependence on the temperature. In squalane the difference between E μand E f is greater, and the calculated large values of at high temperatures indicate a wide depth-distribution of trap energies.

ChemInform Abstract: Spin‐Locking in Concentration‐Narrowed OD ESR Spectra

ChemInformVolume 21, Issue 21 Reviews ChemInform Abstract: Spin-Locking in Concentration-Narrowed OD ESR Spectra A. V. KOPTYUG, A. V. KOPTYUG Inst. Chem. Kinet. Combust., Novosibirsk 630090, USSRSearch for more papers by this authorV. O. SAIK, V. O. SAIK Inst. Chem. Kinet. Combust., Novosibirsk 630090, USSRSearch for more papers by this authorO. A. ANISIMOV, O. A. ANISIMOV Inst. Chem. Kinet. Combust., Novosibirsk 630090, USSRSearch for more papers by this authorYU. N. MOLIN, YU. N. MOLIN Inst. Chem. Kinet. Combust., Novosibirsk 630090, USSRSearch for more papers by this author A. V. KOPTYUG, A. V. KOPTYUG Inst. Chem. Kinet. Combust., Novosibirsk 630090, USSRSearch for more papers by this authorV. O. SAIK, V. O. SAIK Inst. Chem. Kinet. Combust., Novosibirsk 630090, USSRSearch for more papers by this authorO. A. ANISIMOV, O. A. ANISIMOV Inst. Chem. Kinet. Combust., Novosibirsk 630090, USSRSearch for more papers by this authorYU. N. MOLIN, YU. N. MOLIN Inst. Chem. Kinet. Combust., Novosibirsk 630090, USSRSearch for more papers by this author First published: May 22, 1990 https://doi.org/10.1002/chin.199021350Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume21, Issue21May 22, 1990 RelatedInformation