BNB-Doped Phenalenyls: Modular Synthesis, Optoelectronic Properties, and One-Electron Reduction.

A highly modular synthesis of BNB- and BOB-doped phenalenyls is presented. Treatment of the 1,8-naphthalenediyl-bridged boronic acid anhydride 1 with LiAlH4/Me3SiCl afforded the corresponding 1,8-naphthalenediyl-supported diborane(6) 2, which served as the starting material for all subsequent transformations. Upon addition of MesMgBr/Me3SiCl, 2 was readily converted to the tetraorganyl diborane(6) 5. The further heteroatoms were finally introduced through the reaction of 2 with (Me3Si)2NR' or 5 with H2NR' or H2O (R' = H, Me, p-Tol). A helically twisted, fully BNB-embedded PAH 11 was prepared by combining 2 with a dibrominated m-terphenylamine, followed by a Grignard-mediated double ring-closure reaction. All compounds devoid of B-H bonds show favorable optoelectronic properties, such as luminescence and reversible reduction behavior. In the case of the BNB-phenalenyl 7 (BMes, NMe), the radical-anion salt K[7•] was generated through chemical reduction with K metal and characterized by EPR spectroscopy. K[7•] is not long-term stable in a THF/c-hexane solution, but abstracts an H atom with formation of the diamagnetic BNB-doped 1H-phenalene K[7H].

A Weakly Antiferromagnetically Coupled Biradical Combining Verdazyl with Nitronylnitroxide Units.

An antiferromagnetically (AFM) coupled biradical based on oxoverdazyl and nitronylnitroxide was synthesized in 46 % yield using Sonogashira coupling. The obtained heterobiradical evidenced distinct properties of both radical entities in solution. Depending on the solvent, the prepared biradical crystallized in two different forms. SQUID magnetization measurements on Form II showed coupling constants JintraII /kB =-2.1 K and zJinterII /kB =-11.5 K. Consequently, total intermolecular exchange interactions are five times larger than the intramolecular ones. Further, DFT calculations explained this phenomenon and indicated the advantage of Form I for further in-depth investigations.

Phase-sensitive dynamic susceptibility setup for measurements in pulsed magnetic fields.

Dynamic susceptibility, commonly referred to as AC susceptibility, χAC, is a powerful tool to characterize a material's magnetic properties in the presence of a magnetic field B, such as magnetic ordering or spin-relaxation phenomena. The standard technique for accessing χAC is based on measurements of the voltage which is induced in a coil by changes of a sample's magnetization in response to a small oscillating magnetic field. Importantly, this setup allows for a phase-sensitive detection of the susceptibility, thereby providing information on the magnetization dynamics. This method is frequently used in the low-field range, up to several Tesla, where DC fields are readily available. However, there is a growing demand for such magnetic measurements at higher fields, conveniently accessible by the use of pulse-field technology. Whereas various techniques are available for measuring the in-phase part of the susceptibility χAC ', none of them allows for the determination of the out-of-phase part χAC ″. Here we present a realization of an AC susceptometer for measurements of both the in- and out-of-phase component at frequencies up to 47 kHz and an oscillating amplitude of 1.5 mT in a pulsed magnetic field. With this setup, a magnetic signal of 3.6 × 10-6 emu can be resolved. As the setup also enables susceptibility measurements to be performed before and after the field pulse, it can be used to detect temperature changes in response to changes of the magnetic field. This will be demonstrated by measurements on the low-dimensional spin-1/2 system CuSO4·5H2O.

Planar Benzo[1,2- b:4,5- b']dithiophene Derivatives Decorated with Nitronyl and Imino Nitroxides.

Four weakly antiferromagnetic interacting biradicals of benzo[1,2- b:4,5- b']dithiophene (BDT) and BDT extended with two thiophenes (BDTTh2) linked with nitronyl and imino nitroxides (NN and IN) as BDT-NN, BDT-IN, BDTTh2-NN, and BDTTh2-IN were designed, synthesized, and characterized. Short intermolecular π-π distances were found (3.42 Å) for BDT-NN, whereas larger ones were found for BDT-IN (3.54 Å) and BDTTh2-NN (3.67 Å), respectively. Intramolecular magnetic interaction ( Jintra,exp/ kB) of BDT-NN (-26 K) is much larger than for BDT-IN (-5.3 K), while it is reduced for the dithiophene-extended molecule BDTTh2-NN (-2.3 K). Intermolecular interactions ( zJinter,exp/ kB) of BDT-NN (-6.5 K) and BDT-IN (-6.0 K) are stronger than for BDTTh2-NN (-4.6 K). Such large intermolecular couplings resulting from good π-stacking mark BDT-IN and BDTTh2-NN as promising crystalline materials with similar sized Jintra and Jinter. In addition, we also extracted a coupling within the chain of Jchain/ kB = -2.2 K and a coupling between the chains of zJinterchain = -1.5 K for BDTTh2-NN by a Heisenberg chain model. Intra- and intermolecular interactions and spin densities were examined by DFT studies.

Temperature-Dependent Intramolecular Spin Coupling Interactions of a Flexible Bridged Nitronyl Nitroxide Biradical in Solution.

A biradical consisting of two nitronyl nitroxide (NN) radicals bridged by diphenyl-hexaethylene glycol chain [phenyl-O-(CH2CH2O)6-phenyl (GBN1)] was synthesized and investigated using electron paramagnetic resonance (EPR) spectroscopy in solution at different temperature ranges. The reversible temperature dependence behavior of spin coupling exchange is comprehensively reflected by the different lifetime of conformations due to a tumbling motion of the flexible hexaethylene glycol chain. The influences of different solvent on the exchange interactions between the radical entities are described by a two-conformational model, which was, for the first time, applied for di-NN molecule and revealed the thermodynamic parameters enthalpy and entropy (ΔH and ΔS) of the conformational changes. The positive value of enthalpy indicates lower energy of the stretched form (as calculated) compared to the bent form. The transition enthalpy in polar MeCN is larger than in toluene and the positive entropy sign indicates more chain conformation options in the bent state. The magnetic properties of this molecule were investigated in solid state by magnetization studies and EPR spectroscopy.

Breakdown of Hooke's law of elasticity at the Mott critical endpoint in an organic conductor.

The Mott metal-insulator transition, a paradigm of strong electron-electron correlations, has been considered as a source of intriguing phenomena. Despite its importance for a wide range of materials, fundamental aspects of the transition, such as its universal properties, are still under debate. We report detailed measurements of relative length changes ΔL/L as a function of continuously controlled helium-gas pressure P for the organic conductor κ-(BEDT-TTF)2Cu[N(CN)2]Cl across the pressure-induced Mott transition. We observe strongly nonlinear variations of ΔL/L with pressure around the Mott critical endpoint, highlighting a breakdown of Hooke's law of elasticity. We assign these nonlinear strain-stress relations to an intimate, nonperturbative coupling of the critical electronic system to the lattice degrees of freedom. Our results are fully consistent with mean-field criticality, predicted for electrons in a compressible lattice with finite shear moduli. We argue that the Mott transition for all systems that are amenable to pressure tuning shows the universal properties of an isostructural solid-solid transition.

Spin Frustration in an Organic Radical Ion Salt Based on a Kagome-Coupled Chain Structure.

Electro-oxidation of the quinoidal bisdithiazole BT in dichloroethane in the presence of [Bu4N][GaBr4] affords the 1:1 radical ion salt [BT][GaBr4], crystals of which belong to the trigonal space group P3. The packing pattern of the radical cations provides a rare example of an organic kagome basket structure, with S = 1/2 radical ion chains located at the triangular corners of a trihexagonal lattice. Magnetic measurements over a wide temperature range from 30 mK to 300 K suggest strongly frustrated AFM interactions on the scale of J/kb ∼ 30 K, but reveal no anomalies that would be associated with magnetic order. These observations are discussed in terms of the symmetry allowed magnetic interactions within and between the frustrated layers.

High-resolution thermal expansion measurements under helium-gas pressure.

We report on the realization of a capacitive dilatometer, designed for high-resolution measurements of length changes of a material for temperatures 1.4 K ≤ T ≤ 300 K and hydrostatic pressure P ≤ 250 MPa. Helium ((4)He) is used as a pressure-transmitting medium, ensuring hydrostatic-pressure conditions. Special emphasis has been given to guarantee, to a good approximation, constant-pressure conditions during temperature sweeps. The performance of the dilatometer is demonstrated by measurements of the coefficient of thermal expansion at pressures P ~/= 0.1 MPa (ambient pressure) and 104 MPa on a single crystal of azurite, Cu(3)(CO(3))(2)(OH)(2), a quasi-one-dimensional spin S = 1/2 Heisenberg antiferromagnet. The results indicate a strong effect of pressure on the magnetic interactions in this system.

Synthesis, crystal structure, transport, and magnetic properties of novel ternary copper phosphides, A2Cu6P5 (A = Sr, Eu) and EuCu4P3.

Three new ternary copper phosphides, Sr(2)Cu(6)P(5), Eu(2)Cu(6)P(5), and EuCu(4)P(3), have been synthesized from the elements in evacuated silica capsules. Eu(2)Cu(6)P(5) and Sr(2)Cu(6)P(5) adopt the Ca(2)Cu(6)P(5)-type structure, while EuCu(4)P(3) is isostructural to BaMg(4)Si(3) and still remains the only representative of this structure type among the ternary Cu pnictides. All three materials show metallic conductivity in the temperature range 2 K ≤ T ≤ 290 K, with no indication for superconductivity. For Eu(2)Cu(6)P(5) and EuCu(4)P(3), long-range magnetic order was observed, governed by 4f local moments on the Eu atoms with predominant ferromagnetic interactions. While Eu(2)Cu(6)P(5) shows a single ferromagnetic transition at T(C) = 34 K, the magnetic behavior of EuCu(4)P(3) is more complex, giving rise to three consecutive magnetic phase transitions at 70, 43, and 18 K.

Multistep approach to microscopic models for frustrated quantum magnets: the case of the natural mineral azurite.

The natural mineral azurite Cu(3)(CO(3))(2)(OH)(2) is a frustrated magnet displaying unusual and controversially discussed magnetic behavior. Motivated by the lack of a unified description for this system, we perform a theoretical study based on density functional theory as well as state-of-the-art numerical many-body calculations. We propose an effective generalized spin-1/2 diamond chain model which provides a consistent description of experiments: low-temperature magnetization, inelastic neutron scattering, nuclear magnetic resonance measurements, magnetic susceptibility as well as new specific heat measurements. With this study we demonstrate that the balanced combination of first principles with powerful many-body methods successfully describes the behavior of this frustrated material.

Magnetic properties of a novel quasi-2D Cu(II)-trimer system.

We present structural and magnetic data of a new Cu(2+)(S = 1/2)-containing magnetic trimer system 2b·3CuCl(2)·2H(2)O (b = betaine, C(5)H(11)NO(2)). The trimers form a quasi-2D quantum spin system with an unusual intra-layer exchange coupling topology, which, in principle, supports diagonal four-spin exchange. To describe the magnetic properties, a 2D effective interacting-trimer model has been developed including an intra-trimer coupling J and two inter-trimer couplings J(a) and J(b). The low-energy description and effective parameters are obtained from numerical calculations based on four coupled trimers (with periodic boundary conditions). Fits to the experimental data using this model yield the magnetic coupling constants J/k(B) = -15 K and J(a)/k(B) = J(b)/k(B) = -4 K. These parameters describe the susceptibility and magnetization data very well over the whole temperature and field range investigated. Moreover, the model calculations indicate that, for certain ranges of the ratio J(b)/J(a), which might be accessible by either chemical substitution and/or hydrostatic pressure, the low-energy properties of 2b·3CuCl(2)·2H(2)O will be dominated by non-trivial four-spin exchange processes.

Hydroquinone-bridged dinuclear Cu(II) complexes and single-crystalline Cu(II) coordination polymers.

Cationic dinuclear Cu(II) complexes 3 and 4 have been prepared using the novel hydroquinone-based imine chelators 2,5-((i)Pr(2)NCH(2)CH(2)N[double bond, length as m-dash]CH)(2)-1,4-(OH)(2)-C(6)H(2) (1) and 2,5-(pyCH(2)CH(2)N[double bond, length as m-dash]CH)(2)-1,4-(OH)(2)-C(6)H(2) (2), respectively (py = 2-pyridyl). X-Ray quality crystals of both complexes were grown from their DMF solutions. The sterically more encumbered compound crystallizes in the form of discrete dinuclear entities with Cu(II) centres in a distorted square-planar ligand environment (one coordination site is occupied by a DMF molecule). The pyridyl derivative 4 features dinuclear hydroquinone-bridged subunits similar to 3. However, the Cu(II) ions are now six-coordinate with two DMF molecules at an axial and an equatorial position of a Jahn-Teller-distorted octahedron. Moreover, the dinuclear subunits are no longer isolated but linked with each other via bridging hydroquinone oxygen atoms which occupy the second apical position of each octahedron. The structure suggests that the magnetic properties of the resulting coordination polymer of 4 could be described by a model valid for dimerized spin chains. As a result of this analysis the antiferromagnetic coupling constants J(1)/k(B) = 9.9 K (intradimer) and J(2)/k(B) = 0.9 K (interdimer) are obtained. Both in 3 and in 4, the hydroquinone --> semiquinone transition of the central bridging unit (E degrees ' = + 0.57 V, 3; E degrees ' = + 0.51 V, 4; DMF; vs. SCE) displays features of chemical reversibility. In the case of , reduction of Cu(II) centres requires a peak potential of E(p) = - 0.42 V.

Mono-, di-, and oligonuclear complexes of Cu(II) ions and p-hydroquinone ligands: syntheses, electrochemical properties, and magnetic behavior.

Four highly soluble square-planar Cu(II) and Ni(II) complexes of siloxy-salens (2SiCu, 2SiNi) and hydroxy-salens (2Cu, 2Ni) have been synthesized. An X-ray crystal structure analysis was performed on 2SiCu, 2SiNi, and 2Ni. The compounds have been investigated by cyclic voltammetry, UV-vis-NIR spectroelectrochemistry, and EPR spectroscopy. According to these results, the monooxidized species [2SiCu]+ and [2SiNi]+ are to be classified as Robin-Day class II and III systems, respectively. Magnetic measurements on the dinuclear (PMDTA)Cu(II) complex 1Cu2 x (PF6)2 with deprotonated 1,4-dihydroxy-2,5-bis(pyrazol-1-yl)-benzene (1) linker revealed antiferromagnetic coupling between the two Cu(II) ions thereby resulting in an isolated dimer compound. Coordination polymers [1Cu]n(H2O)(2n) of Cu(II) ions and bridging p-hydroquinone linkers were obtained from CuSO4 x 5 H2O and 1,4-dihydroxy-2,5-bis(pyrazol-1-yl)benzene. X-ray crystallography revealed linear chains running along the crystallographic a-direction and stacked along the b-axis. Within these chains, the Cu(II) ions are coordinated by two pyrazolyl nitrogen atoms and two p-hydroquinone oxygen atoms in a square-planar fashion.

Magnetic properties of two double-layer structures built from hydroxynaphthoic acids and manganese.

Double-layer structures consisting of alternating polar and non-polar layers have been prepared using Mn2+ ions and o-hydroxynaphthoic acids. The polar layers contain the Mn2+ ions, carboxylate groups, hydroxy groups and water molecules. The non-polar layers are built up from the naphthalene moieties. In catena-poly[[diaquamanganese(II)]bis(mu-3-hydroxy-2-naphthoato-kappa2O:O')] (also called manganese 3-hydroxy-2-naphthoate dihydrate), [Mn(C11H7O3)2(H2O)2]n, (I), the Mn2+ ions are connected by carboxylate groups to form two-dimensional networks. This compound shows distinct antiferromagnetic interactions and long-range ordering at low temperature. In contrast, tetraaquabis(1-hydroxy-2-naphthoato-kappaO)manganese(II), [Mn(C11H7O3)2(H2O)4], (II), which lacks a close linkage between the Mn2+ ions, reveals purely paramagnetic behaviour. In (II), the Mn2+ ion lies on an inversion centre.

A novel superfamily of transporters for allantoin and other oxo derivatives of nitrogen heterocyclic compounds in Arabidopsis.

A wide spectrum of soil heterocyclic nitrogen compounds are potential nutrients for plants. Here, it is shown that Arabidopsis plants are able to use allantoin as sole nitrogen source. By functional complementation of a yeast mutant defective in allantoin uptake, an Arabidopsis transporter, AtUPS1 (Arabidopsis thaliana ureide permease 1), was identified. AtUPS1 belongs to a novel superfamily of plant membrane proteins with five open reading frames in Arabidopsis (identity, 64 to 82%). UPS proteins have 10 putative transmembrane domains with a large cytosolic central domain containing a "Walker A" motif. Transport of (14)C-labeled allantoin by AtUPS1 in yeast exhibited saturation kinetics (K(m) approximately 52 microM), was dependent on Glc and a proton gradient, and was stimulated by acidic pH. AtUPS1 transports uric acid and xanthine, besides allantoin, but not adenine. Protons are cosubstrates in allantoin transport by AtUPS1, as demonstrated by expression in Xenopus laevis oocytes. In plants, AtUPS1 gene expression was dependent on the nitrogen source. Therefore, AtUPS1 presumably is involved in the uptake of allantoin and other purine degradation products when primary sources are limiting.