Conducting Molecular Magnets Based on TTF-Derivatives

The crystal structures, electronic and magnetic properties of conducting molecular magnets developed in our group are reviewed. (DMET)₂FeBr₄ is composed of alternating stacks of quasi-one-dimensional (1D) donor sheets and square lattice magnetic anion sheets. This salt undergoes an SDW transition of the donor layer at 40 K and an antiferromagnetic transition of Fe³⁺ spins on the anion layer at 3.7 K. The one-to-one correspondence of the anomalies appearing on the magnetization curves with those on the magnetoresistance supports the presence of the π-d interaction. (EDO-TTFI₂)₂[M(mnt)₂] (M=Ni,Pt) consists of 1D chains of conducting donors and magnetic anions aligned in parallel. These salts show metallic conductivity accompanied with a metal-insulator transition around 90 K. Localized spins on the anions behave as a 1D ferromagnet, whose origin is explained by McConnell's first model. The properties of related materials, (EDTDM)₂FeBr₄, (EDS-TTF)₂FeBr₄ and (EDO-TTFBr₂)₂FeBr₄, are also presented.     

The Solid-State and Solution Conformations of (+)-Chelidonine

The solid-state and solution conformations of (+)-chelidonine ( 1 ), a biologically active alkaloid, were determined by X-ray diffraction and ¹H-NMR spectroscopy, X-Ray diffraction analysis revealed a conformer with B/C ‘anti-type’ cis conjunction, a half-chair of ring B , and a twist half-chair of ring C. One H₂O molecule per one alkaloid molecule was cocrystallized and stabilized by H-bonding with OH? C(11). Analysis of the thermal behavior of the crystal showed more thermal stability in the monohydrate than the anhydrate. The NMR measurement of concentration and temperature dependences in CDCl₃ and in (CD₃)₂SO suggested that the OH group of 1 was intramolecularly H -bonded to N(5) in (CD₃)₂SO and intermolecularly H-bonded to the solvent in CDCl₃. Conformational-energy calculations by the MNDO method showed that the intramolecular H -bond was little affected by the conformational stabilization of 1 .     

d-Electron-induced negative magnetoresistance of a pi-d interaction system based on a brominated-TTF donor

A new pi-d interaction system (EDT-TTFBr2)2FeBr4 (EDT-TTFBr2 = 4,5-dibromo-4',5'-ethylenedithiotetrathiafulvalene) and its nonmagnetic anion analogue (EDT-TTFBr2)2GaBr4 based on a brominated TTF-type organic donor are investigated. The salts featured by quasi-1D pi-electronic systems are metallic with metal-insulator transitions taking place at about 20 and 70 K for the FeBr4- and GaBr4- salts, respectively, where the low-temperature insulating state is associated with charge ordering or a Mott insulator followed by an antiferromagnetic transition at lower temperatures. The FeBr4- salt is featured with an antiferromagnetic transition of the anion d spins at a Neel temperature (TN) = 11 K, which is significantly high despite its long anion-anion Br-Br contact, suggesting the importance of the pi-d interaction in the magnetism. The surprisingly strong pi-d interaction, ca. -22.3 K estimated from the magnetization curve, evidences the usefulness of the chemical modification of the donor molecule with bromine substitution to achieve strong intermolecular interaction. The antiferromagnetic state of the anion d spins affects the transport of the conducting pi electrons through the strong pi-d interaction, as evidenced by the presence of a resistivity anomaly of the FeBr4- salt at TN. Below TN, the FeBr4- salt shows negative magnetoresistance that reaches -23% at the highest magnetic field investigated (B=15 T), whereas only a small positive magnetoresistance is observed in the pi-electron-only GaBr4- salt. The mechanism of the negative magnetoresistance is explained by the stabilization of the insulating state of the pi electrons by the periodic magnetic potential of the anion d spins in the FeBr4- salt, which is modified by applying the external magnetic field.     

Weak ferromagnetism and strong spin-spin interaction mediated by the mixed-valence ethynyltetrathiafulvalene-type ligand

A new chromium complex with ethynyltetrathiafulvalene (TTF)-type ligands, [CrCyclam(C≡C-5-methyl-4'5'-ethylenedithio-TTF)(2)]OTf ([1]OTf), was synthesized. The cyclic voltammetry of the complex shows two reversible oxidation waves owing to the first and second oxidation of the TTF unit. The electrochemical oxidation of [1]OTf in a Bu(4)NClO(4) or Bu(4)NBF(4) solution of a 1:1 acetonitrile-chlorobenzene mixture gave isostructural crystals of [1][ClO(4)](2)(PhCl)(2)(MeCN) and [1][BF(4)](2)(PhCl)(2)(MeCN), where two mixed-valence TTF units of adjacent complexes form a dimer radical cation. The crystal structures are characterized by an alternating chain of S = 3/2 Cr(3+)Cyclam units and S = ? (TTF)(2)(+) dimers. These two paramagnetic components are connected directly by an ethynyl group, resulting in a strong intrachain spin-spin interaction of 2J/k(B) = -30 and -28 K for [ClO(4)](-) and [BF(4)](-) salts, respectively (H = -2J∑(i)S(i)·S(i+1)). Both salts show a weak ferromagnetic transition at 23 K thanks to interchain antiferromagnetic interaction between TTF dimers. The remanent magnetizations and coercive forces of nonoriented samples at 1.8 K are 0.016 μ(B) and 90 mT for the [ClO(4)](-) salt and 0.010 μ(B) and 50 mT Oe for the [BF(4)](-) salt, respectively. The weak ferromagnetism is attributed to the Dzyaloshinsky-Moriya interaction between adjacent TTF dimers and/or the single-ion anisotropy of [1](2+).     

The first detection of photomodulation by both DC and in-phase AC susceptibility for organic/inorganic hybrid materials containing cyano-bridged Gd–Cr complex and azobenzene

New organic/inorganic hybrid material containing a ferrimagnetic cyano-bridged binuclear complex, [Gd(DMF)₄(H₂O)₃Cr(CN)₆]·H₂O (DMF=N,N′-dimethylformamide), and photochromic azobenzene in poly(methylmethacrylate) (PMMA) cast films has been prepared. We characterized magnetic properties of bulk [Gd(DMF)₄(H₂O)₃Cr(CN)₆]·H₂O and confirmed that it was not changed by light illumination at all. For the first time, we could detect reversible changes of both DC and in-phase AC susceptibility for the hybrid materials accompanying with cis–trans photoisomerization of azobenzene after UV and visible light illumination for 3 min at 3 K. We also discuss the kind of detectable magnetic changes (e.g. DC, in-phase AC, and both DC and in-phase AC) and the nature of some photo-magnetic functional hybrid materials (cis–trans photoisomerization and increase/decrease of magnetization) known to estimate their mechanism and interactions.