Heading to photoswitchable magnets

The review concerns the strategic approaches to the photomodulation of the magnetic properties of molecular hybrid compounds combining magnetic and optical properties. The approaches developed include photocontrol of intra- and intermolecular magnetic coupling and manipulation of the effect of the photochromic sublattice on the bulk behavior of molecular magnets. In the framework of the first approach, we consider photoinduced charge-transfer phase transitions, changes in the spin state of magnetic centers, photoswitching of intramolecular exchange interactions between magnetic centers connected by a photochromic bridge, and generation of high-spin organic molecules forming high-spin groups due to intermolecular exchange interactions. In the framework of the second approach, we discuss hybrid polyfunctional compounds combining magnetic and photochromic sublattices in the same crystal lattice, as well as intercalation of organic photochromes into voids or inter-layer space of organic magnetics (or vice versa). Creation of such materials allows not only several functions to be combined in the same lattice, which is important for reducing the size of hardware components, but also these properties to be controlled and modified through synergistic effects. The results of basic research suggest that novel materials for various practical applications can be created using principles of crystal chemical engineering. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 704–721, April, 2008.     

Correlation of spin states and spin delocalization with the dioxygen reactivity of catecholatoiron(III) complexes

A series of catecholatoiron(III) complexes, [Fe(III)L(4Cl-cat)]BPh4 (L = (4-MeO)2TPA (1), TPA (2), (4-Cl)2TPA (3), (4-NO2)TPA (4), (4-NO2)2TPA (5); TPA = tris(pyridin-2-ylmethyl)amine; 4Cl-cat = 4-chlorocatecholate), have been characterized by magnetic susceptibility measurements and EPR, 1H NMR, and UV-vis-NIR spectroscopies to clarify the correlation of the spin delocalization on the catecholate ligand with the O2 reactivity as well as the spin-state dependence of the O2 reactivity. EPR spectra in frozen CH3CN at 123 K clearly showed that introduction of electron-withdrawing groups effectively shifts the spin equilibrium from a high-spin to a low-spin state. The effective magnetic moments determined by the Evans method in a CH3CN solution showed that 5 contains 36% of low-spin species at 243 K, while 1-4 are predominantly in a high-spin state. Evaluation of spin delocalization on the 4Cl-cat ligand by paramagnetic 1H NMR shifts revealed that the semiquinonatoiron(II) character is more significant in the low-spin species than in the high-spin species. The logarithm of the reaction rate constant is linearly correlated with the energy gap between the catecholatoiron(III) and semiquinonatoiron(II) states for the high-spin complexes 1-3, although complexes 4 and 5 deviate negatively from linearity. The lower reactivity of the low-spin complex, despite its higher spin density on the catecholate ligand compared with the high-spin analogues, suggests the involvement of the iron(III) center, rather than the catecholate ligand, in the reaction with O2.     

Interplay among aromaticity, magnetism, and nonlinear optical response in all-metal aromatic systems

All-metal aromatic molecules are the latest inclusion in the family of aromatic systems. Two different classes of all-metal aromatic clusters are primarily identified: one is aromatic only in the low spin state, and the other shows aromaticity even in high-spin situations. This observation prompts us to investigate the effect of spin multiplicity on aromaticity, taking Al(4)(2-), Te(2)As(2)(2-), and their copper complexes as reference systems. Among these clusters, it has been found that the molecules that are aromatic only in their singlet state manifest antiaromaticity in their triplet state. The aromaticity in the singlet state is characterized by the diatropic ring current circulated through the bonds, which are cleaved to generate excess spin density on the atoms in the antiaromatic triplet state. Hence, in such systems, an antagonistic relationship between aromaticity and high-spin situations emerges. On the other hand, in the case of triplet aromatic molecules, the magnetic orbitals and the orbitals maintaining aromaticity are different; hence, aromaticity is not depleted in the high-spin state. The nonlinear optical (NLO) behavior of the same set of clusters in different spin states has also been addressed. We correlate the second hyperpolarizability and spin density in order to judge the effect of spin multiplicity on third-order NLO response. This correlation reveals a high degree of NLO behavior in systems with excess spin density. The variance of aromaticity and NLO response with spin multiplicity is found to stem from a single aspect, the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), and eventually the interplay among aromaticity, magnetism, and NLO response in such materials is established. Hence, the HOMO-LUMO energy gap becomes the cornerstone for tuning the interplay. This correlation among the said properties is not system-specific and thus can be envisaged even beyond the periphery of all-metal aromatic clusters. Such interplay is of crucial importance in tailoring novel paradigm of multifunctional materials.     

Molecular ferromagnets

An analysis of magnetism of nanochemical systems opens new routes to fabrication of ferromagnets from diamagnetic substances and new principles of constructing molecular ferromagnets, hybrid magnetic materials, and single-molecule magnets based on high-spin molecules and complexes. The use of such systems in spin computing is considered.     

Nanochemistry and magnetism

An analysis of magnetism of nanochemical systems opens up new ways to creating ferromagnets from diamagnetic substances and new principles for constructing molecular ferromagnets, hybrid magnetic materials, and monomolecular magnets on the basis of high-spin molecules and complexes. Their use in spin computing is considered.     

Intriguing electric field effect on magnetic spin couplings in dielectron clathrate hydrates

Clathrate hydrates have appeared as promising icy materials as the radical, high-spin molecule, and even electron clathrate hydrates are found. In particular, dielectron clathrate hydrates are expected to develop as structural units for a novel class of icy magnetic materials because of not only possible spin coupling interaction, but also very sensitive response to electric field of the loosely bound electrons. However, electric field responses concerning the magnetic properties of such hydrates have not been reported so far. In this work, three representative dielectron clathrate hydrate model clusters (e₂@4⁶6⁸BB, e₂@5¹²6²BB, and e₂@4⁶6⁸AB) were considered for the exploration of their magnetic spin coupling properties, electron distributions, and energy responses to applied electric field. The results calculated at the density functional theory level show that the energies and electron spin coupling properties of these dielectron clathrate hydrate clusters are quite sensitive to applied electric field, presenting intriguing variations. Most importantly, applied electric field can regulate the strength of spin coupling between two trapped electrons, and even could realize the magnetic interconversion of such dielectron cluster structures between antiferromagnetic and paramagnetic or diamagnetic characteristics. Clearly, the intriguing variations should be attributed to the diffuse character, special mobility and polarizable properties of such trapped electrons, and especially the susceptible redistributions of two electrons (including the electron cloud shape and distance between two electron centers) to the electric field. This work opens up the possibility of designing novel icy magnetic materials with sensitive electric field responses of the magnetic properties.     

Topology and spin alignment utilizing the excited molecular field in π-conjugated organic spin systems

The design and experimental investigations of photo-induced high-spin organic systems (the photo-excited quartet (S = 3/2) and quintet (S = 2) states) is reviewed with focusing π-conjugated organic spin systems. In order to study the photo-induced spin alignments on the excited states, the photo-excited high-spin states of π-conjugated stable radical systems and their π-topological isomers were studied by the time-resolved ESR experiments. The relationship between the π-topology and spin alignment on the photo-excited states is clarified. The mechanism of the photo-induced intramolecular spin alignment and the role of the spin polarization and spin delocalization are revealed with the help of the molecular orbital calculations. One of the key processes for the photo-control of the organic molecular magnetism is established. The guiding principle designing the photo-excited high-spin system and the role of π-topology are clarified. Potential developments toward the functional materials are also proposed utilizing the π-conjugated organic spin systems with the photo-excited high-spin states.     

Synthesis, crystal structure, magnetic properties, and electronic structure of the new ternary vanadate CuMnVO4

A new magnetic oxide, CuMnVO4, was prepared, and its crystal structure was determined by single-crystal X-ray diffraction. The magnetic properties of CuMnVO4 were characterized by magnetic susceptibility and specific heat measurements, and the spin exchange interactions of CuMnVO4 were analyzed on the basis of spin-polarized electronic band structure calculations. CuMnVO4 contains MnO4 chains made up of edge-sharing MnO6 octahedra containing high-spin Mn2+ cations. Our work shows that CuMnVO4 undergoes a three-dimensional antiferromagnetic transition at approximately 20 K. Both the intrachain and interchain spin exchanges are antiferromagnetic, and the interchain spin exchange is not negligible compared to the intrachain spin exchange.     

密度泛函理论在分子磁学中的应用2.混合桥联三核镍配合物自旋交换作用

用密度泛函理论结合对称性破损方法(DFF-BS)研究了混合桥联三核镍配合物的磁交换耦合作用．这类化合物是由三唑和异硫氰酸根桥联形成的混合桥配合物．计算表明,在标题化合物中,三唑桥传递反铁磁耦合作用,而异硫氰酸根桥传递铁磁耦合作用;并且,随着异硫氰酸根取代三唑桥的数目增加,配合物的铁磁作用增强,在一定意义上说明了混合桥磁耦合作用的加合性．Mulliken自旋布居分析表明,无论是三唑桥还是异硫氰酸根桥,它们的磁交换作用机理都是磁中心的自旋离域．分子磁轨道分析显示,对于三唑桥,在局域磁轨道之间存在着强的轨道作用,导致了反铁磁耦合;对于异硫氰酸根桥,局域磁轨道之间弱的相互作用,表现了铁磁耦合作用．对标题化合物的研究说明了DFF-BS方法可用于三核体系磁交换作用的研究．     

Preparation of sterically congested diphenyldiazomethanes having a pyridine ligand and magnetic characterization of photoproducts of their 2:1 copper complexes

[structure: see text] To confirm whether high-spin species can be generated as a result of ferromagnetic interaction between the 3d spin of metal ions and the 2p spins of triplet carbene through the pyridyl group located remote from the carbene center, [2,6-dibromo-4-(3- and 4-pyridyl)phenyl](4-tert-butyl-2,6-dimethylphenyl)diazomethanes were prepared and the corresponding carbenes were generated either in the absence or presence of Cu(hfac)2. These were characterized by ESR and UV/vis in a matrix at low temperature, and by laser flash photolysis in solution at room temperature. These studies indicated that although both carbenes generated a fairly stable complex with copper ions, the 4-pyridyl isomer formed a high-spin species as a result of ferromagnetic interaction between the 3d spin of metal ions and the 2p spins of triplet carbene. Such an interaction in the corresponding 3-isomer is likely to be antiferromagnetic. This is further confirmed by magnetic measurements using a Superconducting Quantum Interference Device (SQUID). The results demonstrate that extension of this method will enable stable high-spin polycarbenes to be obtained.     

Thermochromic Meltable Materials with Reverse Spin Transition Controlled by Chemical Design

We report a series of meltable Fe^II complexes, which, depending on the length of aliphatic chains, display abrupt forward low-spin to high-spin transition or unprecedented melting-triggered reverse high-spin to low-spin transition on temperature rise. The reverse spin transition is perfectly reproducible on thermal cycling and the obtained materials are easily processable in the form of thin film owing to their soft-matter nature. We found that the discovered approach represents a potentially generalizable new avenue to control both the location in temperature and the direction of the spin transition in meltable compounds.     

Structural study of the thermal and photochemical spin states in the spin crossover complex [Fe(phen)2(NCSe)2

The first structural data for [Fe(phen)(2)(NCSe)(2)] (obtained using the extraction method of sample preparation) in its high-spin, low-spin and LIESST induced metastable high-spin states have been recorded using synchrotron radiation single crystal diffraction. The space group for all of the spin states was found to be Pbcn. On cooling from the high-spin state (HS-1) at 292 K through the spin crossover at about 235 K to the low-spin state at 100 K (LS-1) the iron coordination environment changed to a more regular octahedral geometry and the Fe-N bond lengths decreased by 0.216 and 0.196 A (Fe-N(phen)) and 0.147 A (Fe-N(CSe)). When the low-spin state was illuminated with visible light at about 26 K, the structure of this LIESST induced metastable high-spin state (HS-2) was very similar to that of HS-1 with regards to the Fe-phen bond lengths, but there were some differences in the bond lengths in the Fe-NCSe unit between HS-1 and HS-2. When HS-2 was warmed in the dark to 50 K, the resultant low-spin state (LS-2) had an essentially identical structure to LS-1. In all spin states, all of the shortest intermolecular contacts (in terms of van der Waals radii) involved the NCSe ligand, which may be important in describing the cooperativity in the solid state. The quality of the samples was confirmed by magnetic susceptibility and IR measurements.     

Spintronic Transport through Polyphenoxyl Radical Molecules

The coherent quantum transport properties through the spin-polarized polyphenoxyl radical molecule have been investigated, using the density-functional-derived tight-binding model and the Green's functions method. The majority and minority spin components exhibit considerably different transmission spectra in the vicinity of the Fermi level. Namely, each spin component carries a different amount of current when the bias voltage is applied between the two electrodes that sandwich the polyradical molecule. Therefore, if the magnetization axis of the polyradical is fixed by the external magnetic field, and if the spin flip does not occur during the transmission, the assumed molecular bridge is expected to work as a spin filter or a spin valve. Furthermore, as long as the bias voltage is weak, the total spin current is observed to be larger than the current through its reduced molecular form. It indicates that the adsorption of some chemical species on the radical sites can be sensed by the change in conductance of the molecular bridge.     

Spintronics and spintronics materials

The review concerns the fundamentals of spintronics (spin-transport electronics). The material covers spin-spin interactions and spin relaxation in semiconductors as well as spin and spin injection related effects in the condensed matter. Examples of promising spintronic devices are given, requirements for spintronic materials are formulated, methods of synthesis of spintronic materials are described, and the physicochemical properties of some materials are characterized. Organic spintronic materials are briefly outlined and the state-of-the art in the field of research on inhomogeneous magnetic semiconducting materials possessing high-temperature ferromagnetism is described. The emphasis is placed on the chemical bonding and electronic structure of magnetic impurities in semiconductors, consideration of the nature of ferromagnetism, and on the character of exchange interactions between localized spins in novel spintronic materials.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2255–2303, November, 2004.     

High-frequency and -field EPR of a pseudo-octahedral complex of high-spin Fe(II): bis(2,2'-bi-2-thiazoline)bis(isothiocyanato)iron(II)

A pseudo-octahedral complex of high-spin Fe(II), bis(2,2'-bi-2-thiazoline)bis(isothiocyanato)iron(II), which has a cis-FeN'2N4 chromophore, has been investigated by high-frequency, high-field electron paramagnetic resonance (HFEPR). Complementary M?ssbauer and DC magnetic susceptibility studies were also performed. HFEPR spectra of powder samples were recorded at frequencies up to 700 GHz and over a magnetic field range of 0-25 T. Analysis of the field-frequency data set yields the following set of spin Hamiltonian parameters for S = 2: D = +12.427(12) cm-1, E = +0.243(3) cm-1; gx = 2.147(3), gy = 2.166(3), gz = 2.01(1). The parameters are analyzed by use of a simple crystal-field model. This study represents the first precise determination by HFEPR of spin Hamiltonian parameters in six-coordinate high-spin Fe(II) and indicates the applicability of HFEPR to the study of high-spin Fe(II) in coordination complexes and biological model compounds.     

Crystal structure and the magnetic properties of tris(2-chloromethyl-4-oxo-4H-pyran-5-olato-κ2O5,O4)iron(III)

The tris(2-chloromethyl-4-oxo-4H-pyran-5-olato-κ²O⁵,O⁴)iron(III), [Fe(kaCl)₃], has been synthesized and characterized by the crystal structure analysis, magnetic susceptibility measurements, Mössbauer, and EPR spectroscopic methods. The X-ray single crystal analysis of [Fe(kaCl)₃] revealed a mer isomer. The magnetic susceptibility measurements indicated the paramagnetic character in the temperature range of 2 K–298 K. The EPR and Mössbauer spectroscopy confirmed the presence of an iron center in a high-spin state. Additionally, the temperature-independent Mössbauer magnetic hyperfine interactions were observed down to 77 K. These interactions may result from spin–spin relaxation due to the interionic Fe³⁺ distances of 7.386 Å.     

Density functional perturbational orbital theory of spin polarization in electronic systems. II. Transition metal dimer complexes

We present a theoretical scheme for a semiquantitative analysis of electronic structures of magnetic transition metal dimer complexes within spin density functional theory (DFT). Based on the spin polarization perturbational orbital theory [D.-K. Seo, J. Chem. Phys. 125, 154105 (2006)], explicit spin-dependent expressions of the spin orbital energies and coefficients are derived, which allows to understand how spin orbitals form and change their energies and shapes when two magnetic sites are coupled either ferromagnetically or antiferromagnetically. Upon employment of the concept of magnetic orbitals in the active-electron approximation, a general mathematical formula is obtained for the magnetic coupling constant J from the analytical expression for the electronic energy difference between low-spin broken-symmetry and high-spin states. The origin of the potential exchange and kinetic exchange terms based on the one-electron picture is also elucidated. In addition, we provide a general account of the DFT analysis of the magnetic exchange interactions in compounds for which the active-electron approximation is not appropriate.     

Magnetic studies of a syn-anti triatomic carboxylate-bridging chainlike copper(II) complex exhibiting ferromagnetic exchange

The magnetic properties of triatomic syn-anti carboxylate bridging copper(II) complex, {[Cu(2,2'-bipydine)(maleate)].2H2O}infinity (complex 1), were investigated experimentally and theoretically, suggesting weak ferromagnetic intrachain interaction. The magnetic data were analyzed and interpreted in terms of Heisenberg chain model corrected by a mean molecular field. Fitting parameters obtained for J, g, and zJ' are 3.14 cm(-1), 2.08, and -0.13, respectively. Density functional theory with generalized gradient approximation was applied to calculate the electronic structure and spin distribution of the present complex. The structural and electronic factors controlling the magnetic interactions were also determined. Ferromagnetic intrachain interactions through triatomic syn-anti carboxylate bridge result from nonplanarity of the bridging network, the exchange pathway involving both the sigma and pi orbitals of the carboxylate bridge and the spin delocalization of each magnetic orbital on the atoms of the carboxylate bridge from the copper(II) centers.     

Unconventional electronic and magnetic functions of nanographene-based host-guest systems

Nanographene has a unique electronic structure which critically depends on the shape of its edge. A zigzag-edged nanographene sheet has a non-bonding pi-electron state (edge state), yielding a strong spin magnetism for edge-state localized spins, in spite of the absence of such a state in an armchair-edged nanographene sheet. Nanographite (stacked nanographene sheets)-network-based nanoporous carbon is employed as the host material to build unconventional magnetic systems based on the host-guest interaction. The physisorption of various guest materials can cause a reversible low-spin/high-spin magnetic switching phenomenon, whose feature varies depending on the type of guest species. The edge-state spins are utilized as a probe to detect a huge condensation of helium atoms in the nanopores. The giant magnetoresistance of the nanographite network is controlled by the physisorption of magnetic oxygen molecules. The confinement of potassium clusters in the nanopores surrounded by nanographite domains yields an interesting nanomagnetic state. Nanographene/nanographite is an intriguing pi-electron-based nanocarbon material with the potential of producing unconventional magnetic structures that cannot be obtained using bulk graphite. The processability of nanographene/nanographite is expected to give a variety of magnetic functions for spintronic applications.     

A DFT study on the magnetostructural property of ferromagnetic heteroverdazyl diradicals with phenylene coupler

We have theoretically designed five different m-phenylene coupled high-spin bis-heteroverdazyl diradicals and their analogous p-phenylene coupled low-spin positional isomers. The geometry-based aromaticity index, harmonic oscillator model of aromaticity (HOMA) values for both the couplers (local HOMA), and the whole diradicals (global HOMA) have been calculated for all the diradicals. We also qualitatively relate these HOMA values with the intramolecular magnetic exchange coupling constants (J), calculated using a broken symmetry approach within unrestricted density functional theory framework. Structural aromaticity index HOMA of linkage specific benzene rings in our designed diradical systems shows that the aromatic character depends on the planarity of the molecule and it controls the sign and magnitude of J. The predicted J values are explained on the basis of spin polarization maps, average dihedral angles, and magnetic orbitals. The effect of the spin leakage phenomenon on magnetic exchange coupling constant and that on HOMA values of certain phosphaverdazyl systems has been explicitly discussed. In addition, a similar comparison is made between the calculated exchange coupling constants and corresponding HOMA values. The main novelty of this work stands on the consideration of the aromatic behavior by means of the geometrical index HOMA. We also estimate another aromaticity index, nucleus independent chemical shift (NICS) values for the phenylene coupler in each diradical to measure aromaticity and compare its value with that of HOMA. The ground state stabilities of these diradicals have also been compared.