Spin Polarization Mechanism Research Articles

Electronic Structure of Bis(silyl)carbon-, Bis(silyl)silicon-, and Bis(silyl)germanium-Centered Radicals (R3Si)2XE• (E = C, Si, Ge; X = H, Re(CO)5, F): EPR and DFT Studies

Group 14 element bis(silyl)-substituted radicals (R3Si)2XE• (E = C, Si, Ge; X = H, Re(CO)5, F) and (R3Si)(1-Ad)HC• have been studied by EPR spectroscopy and DFT calculations. The significant difference in the kinetic stability at 240 K of the hydrogen-substituted persistent C-centered and analogous short-lived Si- and Ge-centered radicals is explained by different decay mechanisms: H abstraction for E = C and dimerization for E = Si, Ge. The 1Hα and 29Siβ hyperfine coupling constants (hfcc) in these radicals have dominating negative spin-polarization (SP) contribution; thus, they have a negative sign. In contrast, in the F-substituted radical, where a(F) results from spin delocalization and positive SP contribution, it has a positive sign. For Si-centered radicals it has been shown by calculations that the 1Hα and 29Siβ hfcc's result from a combination of direct and spin-polarization mechanisms, which vary as a function of the degree of pyramidality around E. As the geometry around E changes from planar t...

First-principles study of the interaction between paramagneticV4+centers through formally magnetically inactiveVO4tetrahedra in the quasi-one-dimensional spin systemsSr2V3O9andBa2V3O9

We report a first-principles density-functional theory study of the origin of the magnetic properties of the quasi-one-dimensional spin systems ${\text{Sr}}_{2}{\text{V}}_{3}{\text{O}}_{9}$ and ${\text{Ba}}_{2}{\text{V}}_{3}{\text{O}}_{9}$. The calculated coupling constants are in very good agreement with experimental data and provide a basis to understand the correlation between structural features and magnetic coupling constants. In ${\text{Sr}}_{2}{\text{V}}_{3}{\text{O}}_{9}$, the predominant coupling is antiferromagnetic and is indeed mediated by one of the two different types of ${\text{VO}}_{4}$ tetrahedra. However, there is also a weaker ferromagnetic interaction along the direction of the octahedral chains, which is mediated by the corner-sharing oxygen atoms through a spin-polarization mechanism. The second type of tetrahedra apparently do not contribute to the magnetic coupling. In ${\text{Ba}}_{2}{\text{V}}_{3}{\text{O}}_{9}$, the antiferromagnetic coupling is dominated by a very substantial superexchange interaction of the vanadium ${d}_{xy}$ orbitals through the tetrahedra sharing only one oxygen atom with the octahedral chain. The second type of tetrahedra and the oxygen atoms nonshared with the tetrahedra provide ferromagnetic interactions which only partially counteract the antiferromagnetic superexchange coupling. Interchain disorder is predicted to have a very small influence on the coupling constants.

Persistent Metal Bis(Hexafluoroacetylacetonato) Complexes Featuring a 2,2′-Bipyridine Substituted Triarylamminium Radical Cation

Herein, we describe the preparation of a 2,2'-bipyridine derivative containing a redox-active N,N'-(4,4'-dimethoxydiphenylamino) substituent (1), which readily coordinates M(hfac)(2) salts [M = Mn (2), Ni (3), Cu (4)] to generate stable, neutral, and pseudo-octahedral coordination complexes, which have been fully characterized. Cyclic voltammetry and spectroelectrochemical measurements on complexes 2-4 indicate stable one-electron oxidation processes, and the formation of persistent radical cation complexes. The neutral complexes (M = Mn or Ni) were subject to one-electron oxidation with NOPF(6) in acetonitrile, and magnetic moments of the resulting solutions were obtained using the Evans method at different temperatures. Our experimental results suggest that the first reported ferromagnetically coupled metal-triarylamminum radical cation complex is obtained when M = Mn(2+), and antiferromagnetic coupling results when M = Ni(2+). These results are supported by results from density functional theory calculations, which indicate that a π spin polarization mechanism for magnetic exchange coupling is operative in singly oxidized complexes, 2-4.

Electronic and magnetic structure of a triacetylphlorogucinol-bridged [formula omitted]-symmetric trinuclear copper complex: Magnetic characterization, ESR spectroscopy, and DFT calculations

For the trinuclear copper complex [Cu 3(tapg)(H 2bhea) 3](ClO 4) 3 based on the tritopic bridging ligand 2,4,6-triacetylphlorogucinol (H 3tapg) and the capping ligand 2-[bis-(2-hydroxyethyl)amino]ethylamine (H 2bhea) a detailed study on the magnetic and electronic properties is presented. The C 3 -symmetric structure of the molecular assembly is preserved in the solid state structure of the compound which crystallizes in the space group R 3 c with a threefold rotation axis passing through the center of the molecule. Susceptibility measurements show weak ferromagnetic interactions with a coupling constant J = 1.10 ( 4 ) cm - 1 mediated by a spin-polarization mechanism. This is confirmed by DFT calculations according the broken symmetry approach at B3LYP/def2-TZVP level of theory. Extended characterization by ESR spectroscopy both in X- and W-band confirm an exchange-coupled system with axial symmetry with g ⊥ = 2.0633 ( 4 ) , g ‖ = 2.2303 ( 4 ) and an effective axial hyperfine constant A ‖ , eff = 64 G .

ChemInform Abstract: Spin Polarization in 1,3,5‐Trihydroxybenzene‐Bridged First‐Row Transition Metal Complexes

AbstractReview: 125 refs.

From triplesalen to triplesalacen: Synthesis spectroscopic, redox, and magnetic properties of the trinuclear Cu II3 triplesalacen complex

The tris(tetradentate) triplesalacen ligand H 6talacen and its trinuclear copper complex, namely [(talacen)Cu II 3], were synthesized and its molecular and electronic structure determined. The ligand is prepared by the triple condensation of 2,4,6-trisacetyl-1,3,5-trihydroxybenzene ( 1) and excess acacen half-unit ( 2). The molecular structure of [(talacen)Cu II 3] exhibits a strong ligand folding around the central Cu II-phenolate bonds. This ligand folding is stronger than that observed in the trinuclear copper triplesalen complexes. The electronic absorption spectrum exhibits a prominent intense band at 32 400 cm −1 and d–d transitions around 19 000 cm −1. These spectral features are related to the ligand folding, which opens an O Ph(p z )– Cu II ( d x 2 - y 2 ) bonding pathway. Electrochemical studies provide an irreversible oxidation at a relatively low potential of 0.17 V versus Fc +/Fc, which is assigned to a ligand-centered oxidation of the central phloroglucinol backbone. A ferromagnetic coupling of J = 1.20 cm −1 ( H = ∑ i < j - 2 J ij S i S j ) is established in agreement with the spin-polarization mechanism. A critical analysis of the bond distances of the central phloroglucinol unit provides evidence that not only the usual phenolate-imine but also the keto-enamide mesomeric form contribute to the resonance hybrid. This might explain the relatively weak ferromagnetic coupling.

Approximate Multiconfigurational Treatment of Spin-Coupled Metal Complexes.

The weak interaction between unpaired electrons in polynuclear transition-metal complexes is often described by exchange and spin polarization mechanisms. The resulting intrinsic multiconfigurational electronic structure for such complexes may be calculated with wave function-based methods (e.g., complete active space configuration interaction and complete active space self-consistent field), but computations become extremely demanding and even unfeasible for polynuclear complexes with a large number of open-shells. Here, several levels of selection of configurations and symmetry considerations that still capture the essential physics of exchange and spin polarization mechanisms are presented. The proposed approximations result in significantly smaller configuration interaction expansions and are equally valid for ab initio and semiempirical methods. Tests are performed in simple molecular systems and in small transition-metal complexes that cover a range of valence and charge states. In particular, superexchange contributions can be calculated to good accuracy using only single ionic excitations. Further reduction in the size of the configuration expansions is possible but restricts the description to low-lying spin ladders. The proposed configuration interaction schemes may be used to resolve space and spin symmetries in the calculation of electronic structures, exchange coupling constants, and other properties pertinent to polynuclear transition-metal complexes.

Supramolecular Photochemistry in β-Cyclodextrin Hosts: A TREPR, NMR, and CIDNP Investigation

A systematic investigation of the photochemistry and ensuing radical chemistry of three guest ketones encapsulated in randomly methylated beta-cyclodextrin (beta-CD) hosts is reported. Dibenzyl ketone (DBK), deoxybenzoin (DOB), and benzophenone (BP) triplet states are rapidly formed after photolysis at 308 nm. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy, steady-state NMR spectroscopy, and time-resolved chemically induced nuclear polarization (TR-CIDNP) experiments were performed on the ketone/CD complexes and on the ketones in free solution for comparison. The major reactivity pathways available from these excited states are either Norrish I alpha-cleavage or H-atom abstraction from the interior of the CD capsule or the solvent. The DOB triplet state undergoes both reactions, whereas the DBK triplet shows exclusively alpha-cleavage and the BP triplet shows exclusively H-atom abstraction. Radical pairs are observed in beta-CDs by TREPR, consisting of either DOB or BP ketyl radicals with sugar radicals from the CD interior. The TREPR spectra acquired in CDs are substantially broadened due to strong spin exchange. The electron spin polarization mechanism is mostly due to S-T(0) radical pair mechanism (RPM) in solution but changes to S-T(-) RPM in the CDs due to the large exchange interaction. The TR-CIDNP results confirm the reactivity patterns of all three ketones, and DOB shows strong nuclear spin polarization from a novel rearrangement product resulting from the alpha-cleavage reaction.

Anomalous Hanle Effect due to Optically Created Transverse Overhauser Field in SingleInAs/GaAsQuantum Dots

We report on experimental observations of an anomalous Hanle effect in individual self-assembled InAs/GaAs quantum dots. A sizable electron spin polarization photocreated under constant illumination is maintained in transverse magnetic fields as high as approximately 1 T, up to a critical field where it abruptly collapses. These striking anomalies of the Hanle curve point to a novel mechanism of dynamic nuclear spin polarization giving rise to an effective magnetic field generated perpendicular to the optically injected electron spin polarization. This transverse Overhauser field, confirmed by the cancellation of electron Zeeman splitting below the critical field, is likely to be a consequence of the strong inhomogeneous quadrupolar interactions typical for strained quantum dots.

Spin-Polarization Mechanisms of the Nitrogen-Vacancy Center in Diamond

The nitrogen-vacancy (NV) center in diamond has shown great promise for quantum information due to the ease of initializing the qubit and of reading out its state. Here we show the leading mechanism for these effects gives results opposite from experiment; instead both must rely on new physics. Furthermore, NV centers fabricated in nanometer-sized diamond clusters are stable, motivating a bottom-up qubit approach, with the possibility of quite different optical properties to bulk.

Competitive Electron Transfer and Enhanced Intersystem Crossing in Photoexcited Covalent TEMPO−Perylene-3,4:9,10-bis(dicarboximide) Dyads: Unusual Spin Polarization Resulting from the Radical−Triplet Interaction

A stable 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) radical was covalently attached at its 4-position to the imide nitrogen atom of a perylene-3,4:9,10-bis(dicarboximide) (PDI) to produce TEMPO-PDI, 1, having a well-defined distance and orientation between TEMPO and PDI. Transient optical absorption experiments in toluene following selective photoexcitation of the PDI chromophore in TEMPO-PDI show that enhanced intersystem crossing occurs with tau = 45 +/- 1 ps, resulting in formation of TEMPO-(3*)PDI, while the same experiment in THF shows that the electron-transfer reaction TEMPO-(1*)PDI --> TEMPO(+*)-PDI(-*) occurs with tau = 1.2 +/- 0.2 ps and thus competes effectively with enhanced intersystem crossing. Time-resolved EPR (TREPR) spectroscopy on the photogenerated three-spin system TEMPO-(3*)PDI in toluene at 295 K initially shows a broad signal assigned to spin-polarized (3*)PDI, which thermalizes at longer times and is accompanied by formation of an emissively polarized TEMPO radical. No signals are observed in THF at 295 K. The TREPR spectrum of TEMPO-(3*)PDI at 85 K in toluene shows an emissive/absorptive signal due to TEMPO and a broad triplet signal due to (3*)PDI having a spin polarization pattern characteristic of overpopulation of its T(0) sublevel. This unusual spin polarization pattern does not result from radical pair intersystem crossing because electron transfer does not occur at 85 K. The observed spin polarization of (3*)PDI cannot be readily explained by mechanisms discussed previously, leading us to propose a new spin polarization mechanism, which requires that the radical and attached triplet are in the weak exchange regime.

Synthesis and characterization of a trinuclear CuII3 complex bridged by an extended phloroglucinol-ligand: implications for a rational enhancement of ferromagnetic interactions

The synthesis and characterization of the extended phloroglucinol-ligand H(3)felden based on the trialdehyde 2,4,6-triformylphloroglucinol and its trinuclear Cu(II)(3) complex [(felden){Cu(bpy)}(3)](ClO(4))(3) is presented. This study is motivated to optimize analogous extended phloroglucinol-ligands based on the triketone 2,4,6-triacetylphloroglucinol, which transmit ferromagnetic interactions by the spin-polarization mechanism between three Cu(II) ions and have therefore been applied as the central ligand backbone for a class of heptanuclear single-molecule magnets in a supramolecular approach. A detailed NMR spectroscopic study reveals that the ligand H(3)felden is not in the usually anticipated enol-imine form but in the tautomeric keto-enamine form. The presence of a C(3h) and a C(s) symmetric isomer results in a set of four different signals for each proton. In conjunction with FTIR, electronic absorption spectroscopy, and bond length considerations, it also appears that the complex [(felden){Cu(bpy)}(3)](ClO(4))(3) must be considered as a resonance hybrid of an enolate-imine and a keto-enamine form. A strong contribution of the keto-enamine resonance structure with loss of the central π system explains the weak but ferromagnetic interactions between the Cu(II)S = 1/2 spins. This detailed analysis identifies the strong resonance with unsaturated groups in 2,4,6-position of phloroglucinol as the main source for the low ferromagnetic couplings by the spin-polarization mechanism in these ligands. This provides a synthetic handle to improve the spin-polarization mechanism in these ligands by replacing the imine with amine functions.

Cr(dmbipy)(ox)2]−: a new bis-oxalato building block for metal assembling. Crystal structures and magnetic properties of XPh4[Cr(dmbipy)(ox)2]·5H2O (X = P and As), {Ba(H2O)2[Cr(dmbipy)(ox)2]2}n·17/2nH2O and {Ag(H2O)[Cr(dmbipy)(ox)2]}n·3nH2O

The synthesis, X-ray structure and variable-temperature magnetic study of new compounds of formula PPh4[Cr(dmbipy)(ox)2]·5H2O (1), AsPh4[Cr(dmbipy)(ox)2]·5H2O (2), {Ba(H2O)2[Cr(dmbipy)(ox)2]2}n·17/2nH2O (3) and {Ag(H2O)[Cr(dmbipy)(ox)2]}n·3nH2O (4) (PPh4+ = tetraphenylphosphonium cation; AsPh4+ = tetraphenylarsonium cation; dmbipy = 4,4′-dimethyl-2,2′-bipyridine; ox2− = oxalate dianion) are reported herein. The isomorphous compounds 1 and 2 are made up of discrete [Cr(dmbipy)(ox)2]− anions, XPh4+ cations [X = P (1) and As (2)] and uncoordinated water molecules. The chromium environment in 1 and 2 is distorted octahedral with Cr–O and Cr–N bond distances varying in the ranges 1.950(2)–1.9782(12) and 2.047(3)–2.0567(14) Å, respectively. The angles subtended at the chromium atom by the two bidentate oxalate ligands cover the range 82.58(10)–83.11(5)°, and they are somewhat greater than those concerning the chelating dmbipy [79.04(10) (1) and 79.24(5)° (2)]. The [Cr(dmbipy)(ox)2]− unit of 1 and 2 also occurs in 3 and 4 but it adopts different coordination modes. It acts as a chelating ligand through its two oxalate groups towards the divalent barium cations in 3 affording neutral chains with diamond-shaped units sharing the barium atoms, while the two other corners are occupied by two crystallographically independent chromium atoms. The barium atom in 3 is coordinated by eight oxygen atoms from four oxalate groups and two aqua ligands. The structure of 4 consists of neutral bimetallic layers where the [Cr(dmbipy)(ox)2]− unit acts as a ligand towards the univalent silver(I) cation through its two oxalate groups, one of them being bidentate and the other bidentate/monodentate (outer). Each silver atom is six-coordinated with a water molecule and five oxygen atoms from three oxalate groups building a highly distorted octahedral environment. Magnetic susceptibility measurements for 1–4 in the temperature range 1.9–300 K show the occurrence of weak ferro- (1 and 2) and antiferromagnetic (3 and 4) interactions which are mediated by π–π stacking between dmbipy ligands through the spin polarization mechanism. A comparative study of the potentiality of the [Cr(AA)(ox)2]− unit (AA = bidentate nitrogen donor) as a building block for designing heterometallic species is carried out in the light of the available structural information.

Structural, electronic and magnetic properties of Cu(ii) complexes of 2-substituted tropones bearing a ferrocenyl group at 5-position

Heterotrinuclear Fe(II)-Cu(II)-Fe(II) complexes [Cu(FcTropOMe)(2)(H(2)O)(2)](OTf)(2) (FcTropOMe = 5-ferrocenyl-2-methoxytropone) (1), [Cu(FcTropNEt(2))(2)](OTf)(2) (FcTropNEt(2) = 2-(N,N-diethylamino)-5-ferrocenyltropone) (2) and [Cu(FcTropNEt)(2)] (FcTropNEt = 2-(N-ethylamino)-5-ferrocenyltroponate) (3) were synthesized. In addition, a hexafluorophosphate salt of heterotrinuclear Fe(III)-Cu(II)-Fe(III) complex [Cu(FcTropNEt)(2)](2+) (3(2+)) was successfully obtained as single crystals by electrochemical oxidation of 3. By comparing the X-ray structures and absorption spectra of dicationic complexes 1 and 2, the 2-(diethylamino)tropone ligand was found to induce a greater intramolecular charge transfer (CT) from ferrocenyl to tropone-Cu(II) moieties than the 2-methoxytropone ligand. On the other hand, 3(2+) showed a broad CT band in the near-infrared (NIR) region similar to 2, which can be assigned to a transition from troponato-Cu(II) to ferrocenium moieties. As for the magnetic properties of 3(2+)(PF(6)(-))(2), measurements of temperature dependence of magnetic susceptibility and ESR on the solid state and in solution revealed the presence of a strong ferromagnetic interaction (J(Fe-Cu) = +12.0 cm(-1)) between the low spin Fe(III) ion with S = 1/2 and Cu(II) ion with S = 1/2 despite a long distance pathway via the aminotroponato and cyclopentadienyl moieties. DFT calculations supported this intramolecular ferromagnetism, which is induced by a spin polarization mechanism through the pi-spacers.

Spin density functional study of magnetism in potassium-loaded zeolite A

In order to clarify the mechanism of spin polarization in potassium-loaded zeolite A, we perform {\em ab initio} density-functional calculations. We find that (i) the system comprising only non-magnetic elements (Al, Si, O and K) can indeed exhibit ferromagnetism, (ii) while the host cage makes a confining quantum-well potential in which $s$- and $p$-like states are formed, the potassium-4$s$ electrons accommodated in the p-states are responsible for the spin polarization, and (iii) the size of the magnetic moment sensitively depends on the atomic configuration of the potassium atoms. We show that the spin polarization can be described systematically in terms of the confining potential and the crystal field splitting of the p-states.

Long-distance magnetic coupling in dinuclear copper(II) complexes with oligo-para-phenylenediamine bridging ligands

Abstract Two novel dinuclear copper(II) complexes of formulae [Cu2(tren)2(bpda)](ClO4)4 (2) and [Cu2(tren)2(tpda)](ClO4)4 (3) containing the tripodal tris(2-aminoethyl)amine (tren) terminal ligand and the 4,4′-biphenylenediamine (bpda) and 4,4″-p-terphenylenediamine (tpda) bridging ligands have been synthesized and structurally, spectroscopically, and magnetically characterized. Their experimentally available electronic spectroscopic and magnetic properties have been reasonably reproduced by DFT and TDDFT calculations. Single crystal X-ray diffraction analysis of 2 shows the presence of dicopper(II) cations where the bpda bridging ligand adopts a bismonodentate coordination mode toward two [Cu(tren)]2+ units with an overall non-planar, orthogonal anti configuration of the N–Cu–N threefold axis of the trigonal bipyramidal CuII ions and the biphenylene group. The electronic absorption spectra of 2 and 3 in acetonitrile reveal the presence of four moderately weak d–d transitions characteristic of a slightly distorted trigonal bipyramid stereochemistry of the CuII ions. TDDFT calculations on 2 identify these transitions as those taking place between the four lower-lying, doubly occupied a2 (dyz)2, b2 (dxz)2, b1 (dxy)2, and a1 ( d x 2 - y 2 )2 orbitals and the upper, singly occupied a1 ( d z 2 )1 orbital of each trigonal bipyramidal CuII ion. Variable-temperature magnetic susceptibility measurements of 2 and 3 show the occurrence of moderate (J = −8.5 cm−1) to weak intramolecular antiferromagnetic couplings (J = −2.0 cm–1) [H = −JS1·S2 with S1 = S2 = SCu = ½] inspite of the relatively large copper–copper separation across the para-substituted biphenylene- (r = 12.3 A) and terphenylenediamine (r = 16.4 A) bridges, respectively. DFT calculations on 2 and 3 support the occurrence of a spin polarization mechanism for the propagation of the exchange interaction between the two unpaired electrons occupying the d z 2 orbital of each trigonal bipyramidal CuII ion through the predominantly π-type orbital pathway of the oligo-p-phenylenediamine bridges, as reported earlier for the parent compound [Cu2(tren)2(ppda)](ClO4)4·2H2O (1) with the 1,4-phenylenediamine (ppda) bridging ligand. Finally, a rather slow exponential decay of the antiferromagnetic coupling (–J) with the number of phenylene repeat units, –(C6H4)n– (n = 1–3), has been found both experimentally and theoretically along this series of oligo-p-phenylenediamine-bridged dicopper(II) complexes. These results further support the ability of linear π-conjugated oligo-p-phenylene spacers to transmit the exchange interaction between the unpaired electrons of the two CuII centers with intermetallic distances in the range of 7.5–16.4 A.

Magnetism in undoped MgO studied by density functional theory

The origin of magnetism in undoped MgO has been studied based on density functional theory. It is shown that Mg vacancies can induce local moments in MgO while O vacancies cannot irrespective of the concentration. The origin of the local magnetic moments comes from the partially occupied ${e}_{g\ensuremath{-}}$ and ${t}_{1u\ensuremath{-}}$ (or only ${e}_{g\ensuremath{-}}$) orbitals and the through-bond spin polarization mechanism mediates the ferromagnetism in MgO. For the ground state configuration of bulk MgO with two Mg vacancies, the energy difference between the ferromagnetic and antiferromagnetic states (28 meV) is smaller than the thermal energy at room temperature $(\ensuremath{\sim}30\text{ }\text{meV})$, which means that the ferromagnetic state is not stable in the bulk. For MgO thin films, Mg vacancies tend to form at the surface region because of the lower formation energy at the surface site than at the subsurface site and in the bulk. Moreover, the formation energy of Mg vacancy in MgO quantum dot decreases much, allowing a larger concentration of Mg vacancies to appear. In conclusion, the magnetism can be sustained in the quantum dot when enough holes are introduced by the large concentration of Mg vacancies. This is consistent with the experimental result of magnetism observed in MgO nanocrystals.

Long-Distance Magnetic Interaction of Exchange-Coupled Copper Dimers with Nitronyl Nitroxide and tert-Butyl Nitroxide Radicals

To study long-distance magnetic interactions between exchange-coupled metal centers and coordinated radical moieties, two new homodimetallic Cu(II) complexes held together by the chelating ligand L(nPr) = N,N,N',N'-tetrakis(N-propyl-2-benzimidazolyl)-2-hydroxy-1,3-diaminopropane and additionally bridged by either a nitronyl nitroxide (NIT) or a tert-butyl nitroxide (NOA) radical-substituted benzoate have been prepared. The complexes have been investigated by X-ray crystallography, magnetic susceptibility measurements, electron paramagnetic resonance (EPR) spectroscopy, and density functional theory (DFT) calculations. For comparison additionally the related copper dimer with the bridging diamagnetic nitrobenzoate ligand was investigated. Magnetic susceptibility measurements on powdered samples reveal a ferromagnetic interaction between the copper ions of J(Cu-Cu) = +28 cm(-1) for all three compounds. The NIT-bridged dimer additionally shows a metal-radical interaction of J'(Cu-NIT) = -2.3 cm(-1) while in the case of the NOA radical, the coupling becomes weakly ferromagnetic with J'(Cu-NOA) = +1.1 cm(-1). This contrasting behavior is explained by the different effective spin-polarization mechanism present. The magnetic data that show for the first time a significant long distance exchange interaction between a peripheral radical unit and a metal dimer were confirmed by EPR spectroscopy and DFT calculations.

Ferromagnetic Coupling by Spin Polarization in a Trinuclear Copper(II) Metallacyclophane with a Triangular Cage-Like Structure

A series of trinuclear copper(II) complexes of general formula A(6)[Cu(3)L(2)] x nH(2)O [L = benzene-1,3,5-tris(oxamate); A = Li(+) (n = 8), 1a; Na(+) (n = 11.5), 1b; and K(+) (n = 8.5), 1c] have been synthesized, and they have been structurally and magnetically characterized. X-ray diffraction on single crystals of 1c shows the presence of three square-planar copper(II)-bis(oxamato) moieties which are connected by a double benzene-1,3,5-triyl skeleton to give a unique metallacyclophane-type triangular cage. The copper basal planes are virtually orthogonal to the two benzene rings, which adopt an almost perfect face-to-face alignment. Complexes 1a-c exhibit a quartet (S = 3/2) ground spin state resulting from the moderate ferromagnetic coupling (J values in the range of +7.3 to +16.5 cm(-1)) between the three Cu(II) ions across the two benzene-1,3,5-tris(amidate) bridges [H = -J(S(1) x S(2) + S(2) x S(3) + S(3) x S(1)) with S(1) = S(2) = S(3) = S(Cu) = 1/2]. Density functional theory calculations on the S = 3/2 Cu(II)(3) ground spin state of 1c support the occurrence of a spin polarization mechanism for the propagation of the exchange interaction, as evidenced by the sign alternation of the spin density in the 1,3,5-substituted benzene spacers.

Effects of nonmagnetic impurities on the spin transport property of a graphene nanoribbon device

Using a nonequilibrium density functional calculation, we investigated the electronic transport properties and fundamental mechanism of spin polarization as a function of the location of impurities from the center to an edge of a graphene nanoribbon device (GND) with zigzag edges. A center-located impurity enables both edges to be enhanced with respect to their spin transports whereas an edge-located impurity results in only the opposite edge channel being dominant. In the case of a center-located impurity, the ferromagnetic ground state induces new spin states near the Fermi level responsible for the spin-polarized current in the GND. We argue that the spin-polarized current can flow through the edge states induced by a nonmagnetic impurity around the Fermi level, especially on a GND with a center-located impurity.