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Daniel Morales-Martinez Federico Polo Sabrina Antonello Flavio Maran 《Current Opinion in Electrochemistry》2021
In this Opinion, we address some of the most important results obtained electrochemically in the area of intramolecular electron transfer (ET). The focus is on freely diffusing molecular systems in which a donor D and an acceptor A are separated by a well-defined bridge B (D-B-A systems). B can be a saturated spacer, a delocalized bridge, or the more complex peptide backbones. As to the acceptors, the selected examples encompass species that can be charged reversibly but a special emphasis is on ETs associated with the concerted cleavage of a sigma bond (dissociative ETs). Our goal is to showcase the essential background, the most appropriate electrochemical tools and methodologies, and a series of selected examples where molecular electrochemistry has provided invaluable information on the mechanisms of intramolecular ET and electronic communication through bridges. 相似文献
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In photosynthesis, pigment-excitation energies in the antenna system produced by light harvesting are transferred among antenna pigments toward the core antenna, where they are captured by the reaction center and initially fixed in the form of a charge separation. Primary charge separation between an oxidized special pair (P+) and a reduced bacteriopheohytin (H−) is occasionally intervened by recombination, and a spin-triplet state (3P*) is formed on P in the bacterial reaction center. The 3P* state is harmful to bio-organisms, inducing the formation of the highly damaging singlet oxygen species. Therefore, understanding the 3P*-formation mechanism is important. The 3P* formation is mediated by a state |m〉 of intermediate charge separation between P and the accessory chlorophyll, which is located between P and H. It will be shown theoretically in the present work that at room temperature, not only the mechanism of superexchange by quantum-mechanical virtual mediation at |m〉, but also a hot-sequential mechanism contributes to the mediation. In the latter, although |m〉 is produced as a real state, the final state 3P* is quickly formed during thermalization of phonons in the protein matrix in |m〉. In the former, the final state is formed more quickly before dephasing-thermalization of phonons in |m〉. 3P* is unistep formed from the charge-separated state in the both mechanisms. 相似文献
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A series of La-substituted M-type Sr hexaferrite powders Sr1−xLaxTi0.05Zn0.2Fe3+11.75O19, wherein x ranges from 0.1 to 0.5 with a step of 0.1, have been prepared by the conventional ceramic method and were then milled in a high energy mill to prepare nanosized powders. XRD investigation of the calcined and the milled powders shows that single phase hexaferrite structure has been formed after calcining and has not changed after milling. The lattice parameters and the mean crystallite sizes of the samples have been determined from the XRD data and Scherrer's formula. The results show that the lattice parameters (“а” and “c”) decrease with increase in La-substitution and the mean crystallite size of the milled powders is about 17 nm. Coercivities and magnetizations of the samples in a magnetic field of 16 kOe have been determined from the room temperature hysteresis loops. It was found that both parameters increase with La substitutions up to 0.3 and then decrease for higher substitutions. These variations were attributed to the enhancement of hyperfine field and spin-canting magnetic structure when La content increases. In addition, the magnetizations were smaller for the nanosized samples in comparison with those of bulk ones, which were discussed according to the core-shell model. Also the results show that annealing of the nanosized samples up to 500 °C can enhance coercivity and magnetization of the samples, which is discussed based on crystallite size growth. 相似文献
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Tetsuji Itoh Kenji Matsuda Hiizu Iwamura 《Angewandte Chemie (International ed. in English)》1999,38(12):1791-1793
A superexchange interaction through the lone pair of electrons on the central nitrogen atom in 1 is invoked to explain the existence of a doublet ground state; two spins on the aminoxyl radical centers couple antiferromagnetically, and one spin remains intact. This proposal is supported by EPR spectroscopic and magnetic measurements. 相似文献
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Katrina A. Jolliffe Toby D. M. Bell Kenneth P. Ghiggino Steven J. Langford Michael N. Paddon-Row 《Angewandte Chemie (International ed. in English)》1998,37(7):915-919
Apparently solvent-mediated and not through-bond photoinduced electron transfer (ET) takes place from the porphyrin (P) unit to the methylviologen (MV2+) unit in the rigid U-shaped molecules 1 (i.e., kTSET>kTBET; TS=through-solvent, TB=through-bond). The ratio of the rates of charge separation to charge recombination is greater than 1400:1. 相似文献
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Jacques Curély 《Monatshefte für Chemie / Chemical Monthly》2005,136(6):1013-1036
Summary. In a first step, we examine the concept of magnetic orbital which is very useful to treat the mechanism of superexchange. After that, we recall the general broad lines of the first historical model proposed by Anderson. In a second step, we develop a new general treatment for superexchange, in the case of the centrosymmetrical model AXB, where A and B are metal cations and X a common bridging ligand (with here, for simplification, A = B, without transfer between cations). It allows one to retrieve the expression of exchange energy J vs. key molecular integrals, as respectively proposed by several authors such as Anderson on the one hand, Hay, Thibeault, and Hoffmann on the other one, and, finally, Kahn and Briat. This model may be easily generalized to the case where a transfer does exist between both cations, with A = B or A ≠ B.
An erratum to this article is available at . 相似文献
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The structural and magnetic properties of the Cr substituted NiCrxFe1−xO4 (0x1.4) spinel ferrites have been investigated by means of X-ray diffraction and Mössbauer spectroscopy techniques. Their crystal structures are found to be pure cubic phases. The Mössbauer spectra at 295 and 78 K of all samples showed two well-resolved magnetic patterns corresponding to the tetrahedral A-sites and octahedral B-sites. The cation distributions driven from the area under resonance curve of each site revealed that the compounds are gradually transferred from perfect inverse spinel to partially normal spinel structure. The behavior of the magnetic properties as a function of Cr3+ concentration has been explained on the basis of the driven cation distribution and it showed that the chrome-rich compositions can be explained in terms of the non-collinear spin model. 相似文献
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