EPR study of Cu(II) dopant ions in single crystals of bis(l-asparaginato)Zn(II)

We report an electron paramagnetic resonance (EPR) study at 33.9 GHz and room temperature of oriented single crystal samples of bis(l-asparaginato)Zn(II) doped with Cu(II). The variation of the spectra with magnetic field orientation was measured in three crystal planes (a*b, bc and a*c, with a*=b×c). These spectra display two groups of four peaks arising from the hyperfine interaction with the I_Cu=3/2 nuclear spins of copper. They were assigned to Cu(II) ions in two lattice sites related by a 180° rotation around the b-crystal axis. The g and hyperfine coupling (A) tensors of the Cu(II) ions were evaluated from the single crystal data. Some indeterminacy in the assignment of the signals was avoided measuring the EPR spectrum of a powder sample. Their principal values are g₁=2.060(1), g₂=2.068(2), g₃=2.283(2), and A₁≈0.1×10⁻⁴, A₂=13×10⁻⁴ and A₃=165×10⁻⁴ cm⁻¹. The eigenvectors corresponding to g₃ and A₃ are coincident within the experimental error; the other eigenvectors are rotated 5.6° in the perpendicular plane. Considering the crystal structure of bis(l-asparaginato)Zn(II), our EPR results indicate that the Cu(II) impurities replace Zn(II) ions in the host crystal. We propose a molecular model based on the EPR data and the structural information, and analyse the results comparing the measured values with those obtained in similar systems.     

Organic radical molecular solids based on [(TCNQ)n] (n=1 or 2): Syntheses, crystal structures, magnetic properties and DFT analyses

Four molecular solids consisting of the 7,7,8,8-tetracyanoquinodimethane (TCNQ) radical and benzylpyridinium or benzylquinolinium derivatives with molar ratios of 1:1 (1-3) and 2:1 (4) have been prepared and characterized. In the crystals of 1 and 3, TCNQ⁻ monoanions and the corresponding cations form segregated stacks, which are regular in 1 but irregular in 3. Instead of segregated stacks, TCNQ⁻ monoanions in 2 form isolated π-dimers. In the crystals of 4, two crystallographic independent TCNQ species possess almost equal fractional negative charge (ca. −0.5). Two types of TCNQ species form a tetrad, these tetrads make a TCNQ stack with the pattern …BAAB…BAAB… along the crystallographic a-b direction. The magnetisms for 1-4 can be simply explained by the formation of singlet spin state. A broken symmetry approach in a density functional theory framework at the ub3lyp/6-31 g level was used to calculate the magnetic exchange constants in 1-4. The results qualitatively demonstrate the observed magnetic properties.     

Energy, momentum, and force in classical electrodynamics: Application to negative-index media

The classical theory of electromagnetism is based on Maxwell's macroscopic equations, an energy postulate, a momentum postulate, and a generalized form of the Lorentz law of force. These seven postulates constitute the foundation of a complete and consistent theory, thus eliminating the need for physical models of polarization P and magnetization M — these being the distinguishing features of Maxwell's macroscopic equations. In the proposed formulation, P(r, t) and M(r, t) are arbitrary functions of space and time, their physical properties being embedded in the seven postulates of the theory. The postulates are self-consistent, comply with special relativity, and satisfy the laws of conservation of energy, linear momentum, and angular momentum. The Abraham momentum density p_EM(r,t) = E(r,t) × H(r,t) / c² emerges as the universal electromagnetic momentum that does not depend on whether the field is propagating or evanescent, and whether or not the host media are homogeneous, transparent, isotropic, linear, dispersive, magnetic, hysteretic, negative-index, etc. Any variation with time of the total electromagnetic momentum of a closed system results in a force exerted on the material media within the system in accordance with the generalized Lorentz law.     

Reversible photomagnetism in a cobalt layered compound coupled with photochromic diarylethene

Photomagnetism is one of the most attractive topics in recent research on molecular solids. In order to produce a photo-controllable magnet, we have synthesized a novel organic-inorganic hybrid system coupled with a photochromic diarylethene anion, 2,2′-dimethyl-3,3′-(perfluorocyclopentene-1,2-diyl)bis(benzo[b]thiophene-6-sulfonate) (1a) and cobalt LDHs (layered double hydroxides). Based on the elemental analysis, the title compound, which was synthesized by the anion exchange reaction between Co₂(OH)₃(CH₃COO)·H₂O (2) and 1a, has the chemical composition, Co₄(OH)₇(1a)_0.5·3H₂O (3). Powder X-ray diffraction analysis revealed the interlayer distance of c=27.8 Å. The magnetic susceptibility measurements elucidated the ferromagnetic intra- and inter-layer interactions and the Curie temperature of T_c=9 K. By UV irradiation of 313 nm, 3 shows the photo-isomerization of diarylethene anion from the open form to the closed one in solid state, which leads to the decreases in the coercive field and the remnant magnetization. Furthermore, the photo-excited state is returned to the initial state (open form) almost reversibly by visible irradiation of 550 nm.     

A new quasi-one-dimensional molecular solid based on Ni(mnt)2 anion: Crystal structure and spin-gap transition

A new molecular solid, [1-(4′-bromo-2′-fluorobenzyl)-4-dimetylaminopyridinium]-bis(maleonitriledithiolato)nickel(III), (BrFBzPyN(CH₃)₂(Ni(mnt)₂)(1), has been prepared and characterized by elemental analyses, IR, ESI-MS spectra, single crystal X-ray diffraction and magnetic measurements. Compound 1 crystallizes in the orthorhombic space group Pnma, a=20.579(4) Å, b=7.078(1) Å, c=17.942(4) Å, α=β=γ=90°, V=2613.3(9) Å³, Z=4. The Ni(III) ions of 1 form a quasi-one-dimensional Zigzag magnetic chain within a Ni(mnt)₂⁻ column through Ni?S, S?S, Ni?Ni, or π?π interactions with an Ni?Ni distance of 4.227 Å. Magnetic susceptibility measurements in the temperature range 2-300 K show that 1 exhibits a spin-gap transition around 200 K, and antiferromagnetic interaction in the high-temperature phase (HT) and spin gap in the low-temperature phase (LT). The transition for 1 is second-order phase transition as determined by DSC analyses.     

Whence the Minkowski momentum?

Electromagnetic waves carry the Abraham momentum, whose density is given by p_EM = S(r,t) / c². Here S(r,t) = E(r,t) × H(r,t) is the Poynting vector at point r in space and instant t in time, E and H are the local electromagnetic fields, and c is the speed of light in vacuum. The above statement is true irrespective of whether the waves reside in vacuum or within a ponderable medium, which medium may or may not be homogeneous, isotropic, transparent, linear, magnetic, etc. When a light pulse enters an absorbing medium, the force experienced by the medium is only partly due to the absorbed Abraham momentum. This absorbed momentum, of course, is manifested as Lorentz force (while the pulse is being extinguished within the absorber), but not all the Lorentz force experienced by the medium is attributable to the absorbed Abraham momentum. We consider an absorptive/reflective medium having the complex refractive index n₂ + iκ₂, submerged in a transparent dielectric of refractive index n₁, through which light must travel to reach the absorber/reflector. Depending on the impedance-mismatch between the two media, which mismatch is dependent on n₁, n₂, κ₂, either more or less light will be coupled into the absorber/reflector. The dependence of this impedance-mismatch on n₁ is entirely responsible for the appearance of the Minkowski momentum in certain radiation pressure experiments that involve submerged objects.     

Influence of ophthalmic lenses in the illuminance originated at a peripheral glare source and reaching the eye

To study the influence of glare on the visual performance of a subject wearing an ophthalmic lens, it is useful to know how the lens affects the illuminance reaching the eye. In this paper, considering spherical standard ophthalmic lenses and defining the relative illuminance, E_r, as the quotient between the illuminance at the cornea with and without lens, a methodology to evaluate E_r in terms of easily determined parameters is developed. Three effects are considered, pupil size variation of the system with and without lens; lateral shifts of rays transmitted through the lens and reflections at the lens. Calculations are experimentally verified employing 5 organic ophthalmic lenses of ±6; ±4 and 0.12 dioptres and 2 glass plane parallel plates 1.95 and 6.6 mm thick. Using a photometer whose sensor is 12 mm apart from the lens and 740 mm apart from a glare source subtending an eccentricity angle of 9.6°, it results E_r=1.204 for the 6 dioptres lens and E_r=0.803 for the −6 dioptres one if sensor diameter is 10 mm while, for a 719 mm distance and a 10° angle, E_r=0.922 for the thin plate and a 30 mm sensor and E_r=1.006 for the thick plate and a 10 mm sensor. Experimental and theoretical results differ in less than 3%.     

Syntheses of two new hybrid metal-organic polymers using flexible aliphatic dicarboxylates and pyrazine: Crystal structures and magnetic studies

The reaction of metal ions, flexible aliphatic dicarboxylates and pyrazine in aqueous solution afford two new metal-organic coordination polymers, {[Cu₂(μ₂-η²-O₂C(CH₂)₂CO₂-η²-μ₂)₂(H₂O)₂]·2H₂O}_n (1) and [Eu₂(μ₂-η²-O₂CCH₂CO₂-η¹-μ₁)₂(μ₂-η²-O₂CCH₂CO₂-η²-μ₂)(H₂O)₆]_n (2). Polymer 1 contains the paddle-wheel cage dicopper(II) units, forming a one-dimensional (1D) double-stranded chain structure along the a-axis, in which the copper(II) atoms are bridged by the carboxylate groups of four succinates. The intradimer Cu-Cu distance is 2.613(2) Å; the interdimer Cu?Cu distance is 6.473 Å. To our knowledge, compound 1 represents the first example of a double-stranded chain structure containing dinuclear paddle-wheel type cage. In the three-dimensional (3D) compound 2, each central Eu(III) ion have a distorted monocapped square antiprism coordination geometry. The structure is built up from two types of polymeric chains with [EuO₆(H₂O)₃]_n units as tethers, resulting in microporous framework. The magnetic behavior of 1 shows that the occurrence of a strong antiferromagnetic coupling between the copper(II) ions through the short bridges via the carboxyl groups can be obtained; the best fittings to the experimental magnetic susceptibilities gave −2J=314 cm⁻¹.     

Structure and thermal behaviors of organic crystals based on substituted 1,3,4-thiadiazoles

The crystalline structure of some compounds containing the 1,3,4-thiadiazole moiety, (1) 5-ethyl-2-amino-1,3,4- thiadiazole (EATZ), (2) 5-benzylsulfany-2-amino-1,3,4-thiadiazole (BSATZ) and (3) 2,5-bis-benzylsulfanyl-1,3,4-thiadiazole (BBSTZ) were determined. Both EATZ and BBSTZ show orthorhombic structures with space group Pbca and BSATZ a monoclinic system with space group C2/c. The lattice parameters: a=0.72280 (14), b=1.0811 (2), c=1.6210 (3) nm for 1, a=2.5282 (5), b=0.59083 (12), c=1.5390 (3) nm for 2 and a=0.87530 (18), b=1.0365 (2), c=3.6098 (7) nm for 3. To compare the intra- and intermolecular interactions in thiadiazole containing organic crystals, thermal analysis studies on each crystal is performed using DSC and TG in N₂ atmosphere to describe the thermal behaviors. Based on the results, the changing regularity of melting point and decomposition temperature of these compounds is educed     

Comparative study of electron- and hole-doped high-Tc compounds in pseudogap regime: LDA+DMFT+Σk approach

Pseudogap regime for the prototype high-T_c compounds hole-doped Bi₂Sr₂CaCu₂O_8−x (Bi2212) and electron-doped Nd_2−xCe_xCuO₄ (NCCO) is described by means of novel generalized LDA+DMFT+Σ_k approach. Here, conventional dynamical mean-field theory (DMFT) equations are supplied with additional (momentum dependent) self-energy Σ_k. In the present case, Σ_k describes nonlocal dynamical correlations induced by short-ranged collective Heisenberg-like antiferromagnetic spin fluctuations. Material-specific model parameters of two neighboring CuO₂ layers of Bi2212 and single CuO₂ layer of NCCO were obtained within local density approximation (LDA) and constrained LDA method. We show that Fermi surface in presence of the pseudogap fluctuations have perfectly visible “hot-spots” for NCCO, while in Bi2212 there is just a rather broad region with strong antiferromagnetic scattering. Results obtained are in good agreement with recent ARPES and optical experiments.     

Synthesis,structures and near-infrared luminescence properties of Ho and Yb coordination complexes

Four Ln³⁺ coordination complexes with the formulas [Ln(p-toluylate)₂(Ac)(H₂O)]_n (Ln=Ho 1, Yb 2) and {[Ln₂(OOCCH₂CH₂COO)₃(H₂O)₄]·6H₂O}_n (Ln=Ho 3, Yb 4) were synthesized hydrothermally. Their structures were determined by single-crystal X-ray diffraction. Complexes 1 and 2 are isomorphic and form infinite 2D network structures comprising p-toluylate and acetate (Ac^–) moieties. Complexes 3 and 4 are also isomorphic and possess infinite 2D structures in which succinate acts as bridging ligands that are connected to a 3D hydrogen bonding network by O–H…O hydrogen bonds. Solid-state IR and UV-Vis-NIR spectra, excitation and emission spectra were determined for the four complexes at room temperature. Complexes 1 and 2 exhibit characteristic NIR emission bands of Ln³⁺ ions but these are shifted and split relative to the theoretical positions. This is also evident for their UV-Vis-NIR spectra. The influence of ligands on enhancing the NIR luminescence of Ln³⁺ ions in complexes is discussed.     

Synthesis, X-ray structure, EPR and optical properties of a ferrocene substituted polychlorotriphenylmethyl radical

The optical, magnetic and electrochemical properties of the octamethylferrocene aldehyde substituted polychlorotriphenylmethyl radical 1 are reported. Radical 1 is prepared in a three step synthetic route starting with a Wittig-Horner reaction to yield (E)-1-formyl-1′-{2-{4-[bis(2,3,4,5,6-pentachlorophenyl)methyl]-2,3,5,6-tetrachloro phenyl}ethen-1-yl}-2,2′,3,3′,4,4′,5,5′-octamethyl ferrocene (6), which is subsequently deprotonated to yield the corresponding anion 7 and finally oxidized to (E)-4-[2-(1′-formyl-2,2′,3,3′,4,4′,5,5′-octamethylferrocen)ethen-1-yl]-2,3,5,6-tetrachlorophenyl-bis(2,3,4,5,6-pentachlorophenyl)methyl radical (1). Radical 1 exhibits a charge-transfer band transition in the near infrared region which is associated with an intramolecular electron transfer from the ferrocene unit (donor) to the radical unit (acceptor) of this dyad molecule. The X-ray crystal structure of [K⁺(18-crown-6)] (E)-[4-[2-(1′-formyl-2,2′,3,3′,4,4′,5,5′-octamethylferrocen)ethen-1-yl]-2,3,5,6-tetrachlorophenyl-bis(2,3,4,5,6-pentachlorophenyl) methide] (7) has been determined. This organic salt shows an interesting one-dimensional polymeric structure formed by the coordination of the K⁺ cation with several atoms of the organic carbanion.     

Surface-modification of TiO2 with new metalloporphyrins and their photocatalytic activity in the degradation of 4-notrophenol

A new mono-functionalized porphyrin derivative, 5-mono-[4-(2-(4-hydroxy)-phenoxy)ethoxy]-10,15,20-triphenylporphyrin (3) and its Cu(II) (3a), Zn(II) (3b) and Ni(II) (3c) metalloporphyrins were synthesized and characterized by using various spectroscopic techniques. The corresponding 3a, 3b, 3c-TiO₂ photocatalysts were then prepared and characterized by means of FT-IR and diffused reflectance spectra, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The photocatalytic activities of 3a, 3b, 3c-TiO₂ were investigated by testing the photodegradation of 4-nitrophenol (4-NP) in aqueous solution under the halogen lamp irradiation. The results indicated that all the 3a, 3b, 3c enhanced the photocatalytic efficiency of bare TiO₂ in photodegrading the 4-NP, and 3a-TiO₂ exhibited the highest photocatalytic activity. The result is considered a combined action of potential match of 3a with TiO₂ CB and effective impregnated of 3a onto the surface of TiO₂.     

Redox reactions of K3[Fe(CN)6] during mechanochemically stimulated phase transitions of AlOOH

Thermally induced redox reactions of K₃[Fe(CN)₆] (1) were investigated for a broad temperature range by thermal methods and structure analytical methods (ESR and Mößbauer spectroscopy, X-ray Powder diffraction and XANES). Based on the influence of the mechanically activated and transforming matrices 2 and 3, redox processes can be tuned to form doped Al₂O₃ systems which contain either isolated Fe³⁺ centres or redox active phases and precursors like (Al_1−xFe_x)₂O₃ (4), (Al_3−xFe_x)O₄ (5), Fe₃O₄, Fe₂O₃ and Fe⁰. The phase Fe₃C and the chemically reactive C-species were detected during the reaction of 1. The final composition of the doped products of α-Al₂O₃ is mainly influenced by the chemical nature of the Fe doping component, the applied temperature and time regime, and the composition of the gas phase (N₂, N₂/O₂ or N₂/H₂). From the solid state chemistry point of view it is interesting that the transforming matrix (2 and 3) possesses both oxidative and protective properties and that the incorporation of the Fe species can be performed systematically.     

Synthesis, characterization and optical limiting effect of nickel complexes of multi-sulfur 1,2-dithiolene

Three nickel complexes with a new multi-sulfur 1,2-dithiolene ligand, (n-Bu₄N)[Ni(cddt)₂] 1, (Ph₄P)[Ni(cddt)₂] 2 and [Ni(cddt)₂] 3 (cddt=4a, 6, 7, 7a-5H-cyclopenta[b]-1,4-dithiin-2,3-dithiolate), have been synthesized and characterized by electrochemical measurements, IR, EPR and UV-Vis-NIR spectroscopies. The crystal structure of complex 2 is determined. Their optical nonlinearities are measured by the Z-scan technique with an 8 ns pulsed laser at 532 nm and all exhibit NLO absorptive abilities. Complexes 1 and 2 both exhibit effective self-defocusing performance (n₂=−5.81×10⁻¹⁰ esu for 1 and −4.51×10⁻¹⁰ esu for 2). The optical limiting (OL) effects were observed with nanosecond and picosecond laser pulses. The OL capability of complex 3 is superior to C₆₀ at the same experimental condition in ns measurements.     

Nature of electric and magnetic dipoles gleaned from the Poynting theorem and the Lorentz force law of classical electrodynamics

Starting with the most general form of Maxwell's macroscopic equations in which the free charge and free current densities, ρ_free and J_free, as well as the densities of polarization and magnetization, P and M, are arbitrary functions of space and time, we compare and contrast two versions of the Poynting vector, namely, S = μ_o^− 1E × B and S = E × H. Here E is the electric field, H is the magnetic field, B is the magnetic induction, and μ_o is the permeability of free space. We argue that the identification of one or the other of these Poynting vectors with the rate of flow of electromagnetic energy is intimately tied to the nature of magnetic dipoles and the way in which these dipoles exchange energy with the electromagnetic field. In addition, the manifest nature of both electric and magnetic dipoles in their interactions with the electromagnetic field has consequences for the Lorentz law of force. If the conventional identification of magnetic dipoles with Amperian current loops is extended beyond Maxwell's macroscopic equations to the domain where energy, force, torque, momentum, and angular momentum are active participants, it will be shown that “hidden energy” and “hidden momentum” become inescapable consequences of such identification with Amperian current loops. Hidden energy and hidden momentum can be avoided, however, if we adopt S = E × H as the true Poynting vector, and also accept a generalized version of the Lorentz force law. We conclude that the identification of magnetic dipoles with Amperian current loops, while certainly acceptable within the confines of Maxwell's macroscopic equations, is inadequate and leads to complications when considering energy, force, torque, momentum, and angular momentum in electromagnetic systems that involve the interaction of fields and matter.     

The nature of anharmonicity and anomalous piezoelectric properties of ferroelectric SbSBr crystal

The contribution of soft mode at Sb atom's sites, to the temperature dependences of Sb atom's equilibrium position's difference Δz(T) has been studied theoretically, when SbSBr crystal is deformed along a(x), b(y) and c(z)-axis in paraelectric phase and is deformed along c(z)-axis in ferroelectric phase. The largest change of Δz₃₃(T) occurs in the ferroelectric phase near the phase transition temperature in the range from 16 K to 21 K. The temperature dependence of Sb atom's equilibrium position's displacements Δz₃₃ is very similar to the temperature dependence of experimental piezoelectric modulus, when SbSBr crystal is deformed in the direction of c(z)-axis in ferroelectric phase.     

Indium-induced superstructures formed on Si(1 1 0) surfaces

Si(1 1 0) surfaces covered with small amounts of In deposit and then annealed at high temperature were investigated by RHEED, and two kinds of superstructures with A = 3a and B = −a + 4b, and A = 3a − 2b and B = −2a + 4b as primitive translational vectors are reported to form on the surfaces.     

Dynamic phenomena connected with magnetic and antiferroelectric interactions in trirutiles

Dynamic effects caused by the magnetoelectric and antiferroelectric interactions in tetragonal antiferromagnets are studied. The analysis is based on the example of trirutiles that are a series of antiferromagnets with different exchange structures and orientation states. We are mainly dealing with the excitation by an alternating electric field E(t) of spin waves typical of these magnets (antiferroelectric resonance) and the nuclear magnetoelectric resonance connected with these interactions. In the first case, special emphasis is placed on specific magnons (antimagnons), where only the antiferromagnetism vectors L take part in oscillations, whereas the total ferromagnetism vector M remains unchanged. The nuclear magnetoelectric resonance can be generated by oscillations of both L and M caused by field E(t). In this way, the field contributes to the hyperfine field, which acts on the nuclear spins. It is shown that the magnetic and antiferroelectric interactions in the dynamics can manifest themselves both at high (usually, exchange) frequencies ωw_E (antiferroelectric resonance) and at rather low nuclear frequencies of ω_n?ω_E. Particular cases of magnetic structures (phases) are considered where field E(t) can excite not only antimagnons, but also quasiantiferromagnons that have lower eigenfrequencies than those of quasimagnons (relativistic and semirelativistic).     

A FDTD formulation for high order dispersive media of muscle-equivalent

In this paper, shift operator finite-difference time-domain (SO-FDTD) method is applied for the calculation of the dispersive medium. The high efficiency and accuracy of this method is verified by calculating the reflection of the plane electromagnetic wave impinging on a muscle slab. For human tissues where multiterm Debye relaxation equations must generally be used. We describe a new differential equation approach, which can be used for general dispersive media. In this method D(t) is expressed in terms of E(t) by means of a differential equation involving D, E, and their time derivatives. The method is illustrated by means of example of media for which relative permittivity is given by a multiterm Debye equation, and for an approximate two-thirds muscle-equivalent model of the human body.