Photoinduced Ferrimagnetic Systems in Prussian Blue Analogues CIxCo4[Fe(CN)6]y (CI = Alkali Cation). 2. X-ray Absorption Spectroscopy of the Metastable State

A Review on the Structural Studies of Batteries and Host Materials by X-Ray Absorption Spectroscopy

This review highlights the use of the X-ray absorption spectroscopy (XAS) as a local structural tool for selected atoms in several host materials. The main characteristics of XAS to be element-sensitive and its applicability to all states of matter, including crystalline solids and amorphous and liquid states, permit an in-depth study of the structural properties of a large variety of materials. This includes intercalation materials where a host structure can accommodate guest species. Host guest equilibria are at the basis of a large variety of technological applications; in particular they have been used for energy storage, ion-exchange membranes, electrochromism, and analytical sensing. A selection of XAS experiments conducted in the field of batteries, mainly on cathodes, and applications in the field of metal hexacyanoferrates and double layered hydroxides are outlined.

L2,3-edge x-ray absorption near-edge spectroscopy analysis of photoisomerism in solid ruthenium-sulfur dioxide complexes

Ru L(2,3) edge x-ray absorption near-edge spectroscopy spectra of four compounds [Ru(SO(2))(NH(3))(4)X]Y (X =  water, Y = (±)-camphorsulfonate(2); X =water, Y = tosylate(2); X =isonicotinamide, Y = tosylate(2); X =pyridine, Y = triflate(2)) are reported before and after optical excitation, which causes photoinduced linkage isomerism of the SO(2) ligand. Principal component analysis reveals consistent changes in the spectra upon photoisomerization, suggesting a partial oxidation of the Ru ion. These results demonstrate that x-ray absorption near-edge spectroscopy is an effective means of probing photoinduced linkage isomerism.

Iron L-edge X-ray absorption spectroscopy of oxy-picket fence porphyrin: experimental insight into Fe-O2 bonding

The electronic structure of the Fe-O(2) center in oxy-hemoglobin and oxy-myoglobin is a long-standing issue in the field of bioinorganic chemistry. Spectroscopic studies have been complicated by the highly delocalized nature of the porphyrin, and calculations require interpretation of multideterminant wave functions for a highly covalent metal site. Here, iron L-edge X-ray absorption spectroscopy, interpreted using a valence bond configuration interaction multiplet model, is applied to directly probe the electronic structure of the iron in the biomimetic Fe-O(2) heme complex [Fe(pfp)(1-MeIm)O(2)] (pfp ("picket fence porphyrin") = meso-tetra(α,α,α,α-o-pivalamidophenyl)porphyrin or TpivPP). This method allows separate estimates of σ-donor, π-donor, and π-acceptor interactions through ligand-to-metal charge transfer and metal-to-ligand charge transfer mixing pathways. The L-edge spectrum of [Fe(pfp)(1-MeIm)O(2)] is further compared to those of [Fe(II)(pfp)(1-MeIm)(2)], [Fe(II)(pfp)], and [Fe(III)(tpp)(ImH)(2)]Cl (tpp = meso-tetraphenylporphyrin) which have Fe(II)S = 0, Fe(II)S = 1, and Fe(III)S = 1/2 ground states, respectively. These serve as references for the three possible contributions to the ground state of oxy-pfp. The Fe-O(2) pfp site is experimentally determined to have both significant σ-donation and a strong π-interaction of the O(2) with the iron, with the latter having implications with respect to the spin polarization of the ground state.

Experimental and theoretical studies of hydroxyl-induced magnetism in TiO nanoclusters

A main challenge in understanding the defect ferromagnetism in dilute magnetic oxides is the direct experimental verification of the presence of a particular kind of defect and distinguishing its magnetic contributions from other defects. The magnetic effect of hydroxyls on TiO nanoclusters has been studied by measuring the evolution of the magnetic moment as a function of moisture exposure time, which increases the hydroxyl concentration. Our combined experiment and density-functional theory (DFT) calculations show that as dissociative water adsorption transforms oxygen vacancies into hydroxyls, the magnetic moment shows a significant increase. DFT calculations show that the magnetic moment created by hydroxyls arises from 3d orbitals of neighboring Ti sites predominantly from the top and second monolayers. The two nonequivalent hydroxyls contribute differently to the magnetic moment, which decreases as the separation of hydroxyls increases. This work illustrates the essential interplay among defect structure, local structural relaxation, charge redistribution, and magnetism. The microscopic differentiation and clarification of the specific roles of each kind of intrinsic defect is critical for the future applications of dilute magnetic oxides in spintronic or other multifunctional materials.

Dynamic magnetic MOFs

In this review we combine the use of coordination chemistry with the concepts of molecular magnetism to design magnetic Metal-Organic Frameworks (MOFs) in which the crystalline network undergoes a dynamic change upon application of an external stimulus. The various approaches so far developed to prepare these kinds of chemically or physically responsive MOFs with tunable magnetic properties are presented.

Fe L- and K-edge XAS of low-spin ferric corrole: bonding and reactivity relative to low-spin ferric porphyrin

Corrole is a tetrapyrrolic macrocycle that has one carbon atom less than a porphyrin. The ring contraction reduces the symmetry from D(4h) to C(2v), changes the electronic structure of the heterocycle, and leads to a smaller central cavity with three protons rather than the two of a porphyrin. The differences between ferric corroles and porphyrins lead to a number of differences in reactivity including increased axial ligand lability and a tendency to form 5-coordinate complexes. The electronic structure origin of these differences has been difficult to study experimentally as the dominant porphyrin/corrole pi --> pi* transitions obscure the electronic transitions of the metal. Recently, we have developed a methodology that allows for the interpretation of the multiplet structure of Fe L-edges in terms of differential orbital covalency (i.e., the differences in mixing of the metal d orbitals with the ligand valence orbitals) using a valence bond configuration interaction model. Herein, we apply this methodology, combined with a ligand field analysis of the Fe K pre-edge to a low-spin ferric corrole, and compare it to a low-spin ferric porphyrin. The experimental results combined with DFT calculations show that the contracted corrole is both a stronger sigma donor and a very anisotropic pi donor. These differences decrease the bonding interactions with axial ligands and contribute to the increased axial ligand lability and reactivity of ferric corroles relative to ferric porphyrins.

Photomagnetism in clusters and extended molecule-based magnets

Photomagnetism in molecular systems is a new development in molecular magnetism. It traces back to 1982 and 1984 when a transient effect and then the light-induced excited-spin-state-trapping effect was discovered in spin-crossover complexes. The present contribution gives a definition of the phenomenon, a process that changes the magnetism of a (molecular) system after absorption of a photon. It is limited to the discussion of photomagnetism based on metal-metal electron transfer in clusters and extended molecule-based magnets. The paper is organized around the main pairs of spin bearers, which allowed us to evidence and to study the phenomenon: Cu-Mo, Co-Fe, and Co-W. For each metallic pair, we report and discuss the conditions of appearance of the effect and its characteristics, both in extended structures and in molecular units: structure, spectroscopy, magnetism, thermodynamics and kinetics, and applications. We conclude with some brief prospects. The field is in rapid expansion. We are convinced that the interaction of photons with magnetized matter, to provide original magnetic properties, will meet more and more interest in the future.

Control of Magnetic Properties through External Stimuli

The magnetic properties of many magnetic materials can be controlled by external stimuli. The principal focus here is on the thermal, photochemical, electrochemical, and chemical control of phase transitions that involve changes in magnetization. The molecular compounds described herein range from metal complexes, through pure organic compounds to composite materials. Most of the Review is devoted to the properties of valence-tautomeric compounds, molecular magnets, and spin-crossover complexes, which could find future application in memory devices or optical switches.

Photoinduced magnetization in copper octacyanomolybdate

This article describes the studies of a photomagnetic cyanide-bridged Cu-Mo bimetallic assembly, Cu(II)(2)[Mo(IV)(CN)(8)].8H(2)O (Cu(II), S = (1)/(2); Mo(IV), S = 0) (1), which has an intervalence transfer (IT) band from Mo(IV)-CN-Cu(II) to Mo(V)-CN-Cu(I) around 480 nm. Wide-angle X-ray scattering and X-ray spectroscopic studies provide precise information about the 3D connectivity and the local environment of the transition metal ions. Irradiating with blue light causes solid 1 to exhibit a spontaneous magnetization (Curie temperature = 25 K). The thermal reversibility is carefully studied and shows the long-time stability of the photoinduced state up to 100 K. Photoreversibility is also observed; i.e., the magnetization is induced by irradiation with light below 520 nm, while the magnetization is reduced by irradiation with light above 520 nm. The UV-vis absorption spectrum after irradiation shows a decrease of the IT band and the appearance of the reverse-IT band in the region of 600-900 nm (lambda(max) = 710 nm). This UV-vis absorption spectrum is recovered to the original spectrum by irradiation with 658-, 785-, and 840-nm light. In this photomagnetic effect, the excitation of the IT band causes an electron transfer from Mo(IV) to Cu(II), producing a ferromagnetic mixed-valence isomer of Cu(I)Cu(II)[Mo(V)(CN)(8)].8H(2)O (Cu(I), S = 0; Cu(II), S = (1)/(2); Mo(V), S = (1)/(2)) (1'). 1' returns to 1 by irradiation of the reverse-IT band, which obeys the scheme for the potential energy surface in mixed-valence class II compounds.

Exchange coupling across the cyanide bridge: structural and DFT interpretation of the magnetic properties of a binuclear chromium(iii) complex

The reaction of [Cr(CN)6]3- with a mixture of trans-[Cr(cyclam)(OH)2]Cl, [Cr(cyclam)(OH)Cl]Cl and [Cr(cyclam)Cl2]Cl affords the cyanide bridged dimer, trans-[HO-Cr(cyclam)-NC-Cr(CN)5]-. The tetraphenylphosphonium salt of the anion crystallizes in space group P2(1)/n and shows a bent arrangement of the Cr1-CN-Cr2 unit with the Cr1-CN bond angle at 166.9 degrees and CN-Cr2 at 160.32 degrees . The Cr2-O bond, trans to the hexacyanide fragment, is very short at 1.902 A. Two dimers are held together by two hydrogen bonds connecting the Cr2-OH group of each dimer with one of the NH groups of the cyclam ligand of an adjacent molecule, leading to an almost linear configuration. These dimers of dimers get packed parallel to each other, generating layers separated by the tetraphenylphosphonium cations. Four of the cyanide groups of the anion are engaged in H-bonds with the four water molecules present in the structure or with a NH group of the macrocycle of an adjacent molecule. From magnetic susceptibility measurements, the dimer was found to exhibit antiferromagnetic interaction between the Cr(III) centers with J=-16 cm(-1)(H=-2JS(A)S(B)). Structural and magnetic parameters have been calculated by density functional theoretical methods at the B3LYP level. The exchange coupling constant, J, calculated for the dimer at the X-ray geometry is -23.2 cm(-1) which is in excellent agreement with the experimental value.