The Effect of a Magnetic Field on Photoinduced Electron Transfer in Solutions of Chlorophyll and Porphyrins

It is found that, with the action of photoexcitation and a variable magnetic field on halide-containing porphyrin solutions, a differential absorption is observed, whose spectra conform to the absorption spectra of cation radicals of pigments. It is shown that the effect increases linearly with an increase in the strength of a supplementary magnetic field (H) up to saturation (H _max 200 G). The scheme of the photophysical and photochemical deactivation of the energy of electronic excitation for chlorophyll-like pigments in the presence of CCl₄ is analyzed with allowance for the effect of the magnetic field on the intercombination conversion in an ion–radical pair. It is shown that solvate-separated ion radicals of the pigments in halide-containing solutions can be formed from the singlet state of tetrapyrrole molecules.     

Electron Spin Resonance Study of UV-Irradiated Tetraphenyldiphosphine

Although divalent organophosphinyl radicals have been frequently postulated as intermediates in many organochosphorous reactions, very few of them have been identified by e.s.r. By low temperature trapping of products from the photolysis of gaseous tetraphenyl-diphospine in a flow system, Schmidt and coworkers² have observed an e.s.r. signal of a single broad and asymmetric line centered at about g = 2.009 and have assigned this to the diphenylphosphinyl radical. We have reinvestigated the photolysis of this compound by flash photolysis and by e.s.r. at low temperatures, the preliminary e.s.r. results will be reported here.     

An Electron Spin Resonance Study of Gamma-Irradiated 5,6-Dihydropyrimidines

Electron spin resonance (ESR) spectra are observed from gamma-irradiated, poly-crystalline samples of 5,6-dihydrouracil, 5,6-dihydro-5-methyluracil, and 5,6-dihydro-6-methyluracll. The spectra indicate that the radiation-induced free radicals are formed by the loss of hydrogen atoms from the parent compounds. The crystal structure of the dihydropyrimidines is apparently such that there is a proton near the unpaired electron which can simultaneously flip with the flip of the unpaired electron resulting in the formation of satellite lines relative to the main spectral lines. Annealing the irradiated samples at 100°C produces a decrease in the radical content of all compounds except in dihydro-5-methyluracll where the radical content increases. These differences are discussed in terms of the close proximity of the radicals formed during irradiation of dihydro-5-methyluracil. The     

哑铃型富勒烯化合物的光诱导电子转移反应

DTE-甲亚胺叶立德盐(DTE:二噻吩乙烯基)与C60在甲苯中的1,3-偶极环加成反应被用于合成新型的哑铃型富勒烯化合物1．为了与1作比较,还同时合成了单加成物2.并通过HF-3/21G方法计算确定了这两种化合物分子的几何结构,同时在室温下测定了化合物在不同极性溶剂中的紫外-可见光谱和荧光光谱,证明了化合物内给体和受体间光诱导分子内电子转移过程的存在．     

Observation of Electron Spin Resonance in the Microwave-Induced Photovoltage

JETP Letters - The microwave-induced photovoltage in two-dimensional electron systems in an AlAs quantum well and a ZnO/MgZnO heterojunction under the conditions of the quantum Hall effect is...     

微波电子自旋共振实验波形分析

详细分析了微波段用可调反射式谐振腔做电子自旋共振实验中共振信号的形成原理,分析指出下凹和上凸波形都为共振吸收信号,当波形的上下两部分等大时,为色散信号,波形的上下两部分不等大时,为色散信号和共振吸收信号的合成。从理论上解释了共振信号的变化所反映的物理过程。     

Sensitivity of Saturation Transfer Electron Spin Resonance Extended to Extremely Slow Mobility in Glassy Materials

A novel extension of the saturation transfer (ST) ESR technique that enables the determination of extremely long rotational correlation times of nitroxide spin labels up to values around 10⁴s is proposed. The method is based on the observation that the integral of ST-ESR spectra is sensitive to the spin–lattice relaxation time of the electron of the spin label, which in turn is directly dependent upon the rotational correlation time. The method is applied to the spin label TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl) in glycerol. From the known viscosity data and the related rotational correlation times of the TEMPOL spin label in glycerol, the rotational correlation times of unknown samples can be determined. The method is especially applicable to systems with a very high viscosity, such as glassy materials. The method is applied to a 20 wt% glucose–water mixture in the glassy state, giving a value for the highest limiting rotational correlation time of about 10³s at a temperature of 45 K below the glass transition temperature of this system. This is an extension by six decades for the rotational correlation time, as compared to the current application of ST-ESR.     

On Electron Spin Polarization Created in the Excited Triplet State of Accessory Chlorophyll via Photoinduced Charge-Recombination of the Photosystem II Reaction Center

We present a theoretical approach to investigate the electron spin polarization (ESP) of the excited triplet state that has been detected using the time-resolved electron paramagnetic resonance (TREPR) method in the photosystem II center of the plants. We show, using the stochastic Liouville equation, that the ESP pattern created in the accessory chlorophyll (Chl_accD1) which reside near the P_D1 chlorophyll of the active branch is explained by one-step, concerted double electron transfer model, initiating from the singlet–triplet conversion of the light-induced charge-separated state composed of P_D1 radical cation and pheophytin radical anion. We also considered the sequential ESP transfer model via the triplet charge-recombination (CR) and the triplet–triplet energy transfer processes. It has been clearly shown that the ESP created in the ³Chl_accD1* is dependent on the rate constant (k _TT) of the triplet–triplet energy transfer from the intermediate triplet state created by the CR. Also we show that the relative orientation of the principal axes of the spin dipolar interaction in the intermediate triplet state (³P_D1*, as an example) may play a role in the ESP pattern, when the k _TT is smaller than the angular frequency of the Zeeman energy. We have theoretically shown that the TREPR measurement of the ESP is very powerful to investigate the primary chemical process and to characterize the intermediate as a signature of the stepwise ESP transfer.     

Recent Advances in High-Frequency Electron Spin Resonance Detection Using a Microcantilever

Our recent developments in highly sensitive high-frequency electron spin resonance (ESR) using a microcantilever are reviewed. ESR signals of a Co Tutton salt microcrystal (<1 μg) have been detected at low temperature at frequencies up to 315 GHz under a static magnetic field using a microcantilever and a modulation technique. The achieved sensitivity is about 10⁹ spins/G at 4.5 K. Moreover, we have shown that similar ESR detection using a microcantilever is possible up to 130 GHz under a pulsed magnetic field without using a modulation technique. The achieved sensitivity is about 10¹¹ spins/G at 1.7 K. These results suggest that the ESR detection using a microcantilever is promising for applications to high-resolution and high-sensitivity terahertz ESR.     

Photoinduced Electron Transfer From Carbazole to Halomethanes in a Gas Phase

Intermolecular photoinduced electron transfer (PET) in a gas phase was studied using carbazole vapor fluorescence quenching by halomethanes (CHCl₃, CH₂Br₂, CCl₄, CHBr₃). The fluorescence quenching rate constants k _q changing from 2.3·10⁵ sec^–1·torr^–1 in mixtures with CHCl₃ to 4.6·10⁶ sec^–1·torr^–1 in mixtures with CHBr₃ at a constant temperature of 403 K were estimated. The dependence of the carbazole fluorescence decay rates in the presence of halomethanes on the free energy change G during transfer of the electron from carbazole to halomethanes is considered. It is suggested to take into account the influence of the vibrational energy of the carbazole molecule E _vib and its temperature changes in estimation of the G values. The differences between PET in the gas and liquid phases were analyzed. It is found that for mixtures with CCl₄ and CHBr₃ the negative temperature dependence of k _q is observed, when the decay rates and efficiencies of the intermolecular PET decreased with temperature increase in the range 403–573 K, i.e. these mixtures the electron transfer is not a barrier-restricted process.     

Electron Spin Resonance under Conditions of a Ferromagnetic Phase Transition

JETP Letters - The spin resonance of two-dimensional conduction electrons in a ZnO/MgZnO heterojunction in tilted magnetic fields is studied near the filling factor $$\nu = 2$$ . The analysis of...     

Electron Spin Resonance Study of the Solvent Effect on the Conformation of Some Iminoxy Radicals

The reaction of nitrogen dioxide with carbonyl compounds having a methylene or vinyl group adjacent to the corbonyl leads to the formation of stable iminoxy radicals^1-3. Evidence^1,2,4 has been presented that the precursor of these iminoxy radicals is probably the corresponding oxime formed by rearrangements of the intermediate nitroso compounds. There have also been reports on the e.s.r. detection of some cis-trans isomers of some iminoxy radicals in methanol or in neat liquid^1,5. We wish to report here a drastic solvent effect on the conformations of some iminoxy radicals formed by the reaction of NO₂ with trifluoroacetylacetone.     

轨道诱导Peierls相变材料MgTi_2O_4的电子自旋共振研究

用变温电子自旋共振手段(Electron Spin Resonance,ESR),对轨道诱导Peierls相变MgTi_2O_4体系进行了研究.研究发现,轨道诱导Peierls相变所伴随的自旋二聚相变对ESR谱产生了影响.在相变温度以上,MgTi_2O_4的磁性为顺磁行为.而在相变温度以下,ESR谱显示MgTi_2O_4的磁性偏离了顺磁行为.对ESR谱线的参数拟合结果显示,MgTi_2O_4在发生轨道诱导Peierls相变时,自旋耦合作用逐渐增强.这说明:自旋耦合作用的增强很有可能是导致相变的一个重要的因素.     

Short Time Domain and Double Resonance Techniques in Electron Spin Resonance Spectroscopy

Abstract

In recent years improvements in technology have increased the amount of information obtainable from, and have broadened the areas of application of, electron spin resonance spectroscopy. Some of the techniques that have found wide use in recent years, such as pulse saturation relaxation time measurements, ENDOR and ELDOR, were originally discovered many years ago while others, such as saturation transfer spectroscopy and nanosecond time resolved techniques, are based upon more recent developments. In this review we will survey a number of these techniques, we will explain what they are and what we can learn from them, we will describe some of the spectrometer systems that are required for using them, and we will cite many representative articles which provide further details about them.     

轨道诱导Peierls相变材料MgTi2O4的电子自旋共振研究

用变温电子自旋共振手段,对轨道诱导Peierls相变MgTi2O4体系进行了研究。研究发现,轨道诱导Peierls相变所伴随的自旋二聚相变对ESR谱产生了影响。在相变温度以上,MgTi2O4的磁性为顺磁行为。而在相变温度以下,ESR谱显示MgTi2O4的磁性偏离了顺磁行为。对ESR谱线的参数拟合结果显示,MgTi2O4在发生轨道诱导Peierls相变时,自旋耦合作用逐渐增强。这说明:自旋耦合作用的增强很有可能是导致相变的一个重要的因素。     

电子自旋共振(ESR)技术在生物和医学中的应用

电子自旋共振(electron spin resonance,ESR)是检测自由基最直接最有效的方法,是自由基生物学和医学不可缺少的重要研究技术. 作者综述了ESR、自旋标记、自旋捕集和ESR 成像技术的最新发展及ESR技术在细胞膜、蛋白质结构和一些重大疾病如心脏病、老年痴呆症、帕金森综合症和中风等疾病研究及辐射损伤和植物疾病研究中的应用.     

Electron Spin Resonance Measurements of a Demagnetizing Field on the Surface of Metal Samples

By comparing the signals of electron spin resonance (ESR) from two crystals of a diamond (spin–labels) the demagnetizing field of the Co, Fe, and Ni samples in the shape of strongly elongated ellipsoids of revolution (disks) has been measured. The magnetic permeabilities of the metals in the external magnetic field corresponding to the ESR of the broken chemical bonds in a natural diamond irradiated with fast reactor neutrons have been determined.     

X- and Q-Band Electron Spin Echo Study of Stochastic Molecular Librations of Spin Labels in Lipid Bilayers

Electron spin echo (ESE) of nitroxide spin labels allows detecting fast nanosecond stochastic restricted rotations (stochastic molecular librations), which is a common property of molecules in disordered media including biological systems. Under the typical experimental conditions, the anisotropic electron paramagnetic resonance (EPR) spectrum of a nitroxide is only partly excited by microwave pulses, which allows selecting an anisotropic contribution to the transverse spin relaxation by comparing echo decays at different spectral positions. On the other hand, for low-amplitude orientational motion, the excitation bandwidth is large enough to cover the range of spectral diffusion occurring during the echo formation. To verify that the two-pulse echo decay is indeed related to fast motions, the stimulated electron spin echo can be used. In addition, theory predicts an increase of the relaxation rates at higher microwave resonance frequency. To check this prediction, in the present work we performed a comparative study of ESE decays at microwave X- and Q-bands, for spin-labeled lipids in the gel phase of a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer. A good agreement found between experimental data and computer simulation provides additional justification for the model of fast stochastic molecular librations.     

Tuning Photoinduced Intramolecular Electron Transfer by Electron Accepting and Donating Substituents in Oxazolones

The solvatochromic and spectral properties of oxazolone derivatives in various solvents were reported. Fluorescence spectra clearly showed positive and negative solvatochromism depending on substituents. The solvatochromic plots and quantum chemical computations at DFT-B3LYP/6-31?+?G(d,p) level were used to assess dipole moment changes between the ground and the first excited singlet-states. The electron accepting nitro substituent at the para-position increased the π-electron mobility, however, the 3,5-dinitro substituent decreased the π-electron mobility as a result of inverse accumulation of the electronic density as compared with that of its ground state. Experimental and computational studies proved that the photoinduced intramolecular electron transfer (PIET) is responsible for the observed solvatochromic effects. We demonstrate that PIET can be finely tailored by the position of the electron accepting and donating substituents in the phenyl ring of the oxazolone derivatives. We propose that the photoactive CPO derivatives are new molecular class of conjugated push-pull structures using azlactone moiety as the π-conjugated linker and may find applications in photovoltaic cells and light emitting diodes.     

Measurement of Microviscosity in Vesicular Membranes by Electron Spin Resonance

The seven vesicular formulations CER(−), CER(+), CER(I), PC(−), PC(+), DAG(B) and DAG(I) have been developed using ceramide IIIB (CER), phosphatidylcholine (PC), or diacylglycerides (DAG) as main lipid components. B and I indicate the processed hydrophilic surfactant Brij 58 and Imwitor 375, (+) and (−) symbolize the positive and negative zeta potential. The influence of the bilayer microviscosity on the features of lipid vesicles as dermal drug delivery system is widely discussed, but comparable and reproducible results could not be found. For the measurement of the microviscosity by electron spin resonance the lipophilic spin probe of 14-doxylpalmitoylic acid methyl ester (DPME) was incorporated into the bilayer. Rotational correlation times (τ_c) between 0.3 and 1.8 ns were calculated. The dynamic viscosities (η) of eight different medium-chained triglycerides–castor oil mixtures in the range between 25 and 948 mPa · s, as well as the corresponding τ_c values of DPME in these mixtures, were determined to establish a calibration curve for the estimation of the microviscosity. The microviscosities of the vesicle membranes increase from 40 to 565 mPa · s in the following order DAG(B) = DAG(I) < PC(−) ≈ CER(I) < CER(−) ≤ CER(+) < PC(+). The microviscosity of 49.8 ± 2.5 mPa · s in pure unsaturated soy PC membranes decreases on adding the sodium cholate to 28.3–29.4 mPa · s, whereas the addition of dilauroylphosphatidylcholine in repeated freeze–thaw cycles or 8.5% ethanol in the hydrophilic phase had no influence. Authors' address: Hans-Hubert Borchert, Institute of Pharmacy, Free University of Berlin, Kelchstrasse 31, Berlin 12169, Germany