Kyuya Yakushi

Alternating covalent bonding interactions in a one-dimensional chain of a phenalenyl-based singlet biradical molecule having Kekulé structures.

A novel naphthoquinoid singlet biradical (2a) stabilized by phenalenyl rings is prepared by a multistep procedure and is investigated in terms of covalent bonding interactions. The molecule 2a gives single crystals, in which a 1D chain is formed with a very short π-π contact at the overlapping phenalenyl rings. The unpaired electrons in 2a are involved in covalent bonding interactions not only within the molecule but also between the molecules in the 1D chain, and a linear conjugation is made of the alternating intra- and intermolecular covalent bonding interactions through conventional π-conjugation and multicenter bonding, respectively. The linear conjugation causes a lower-energy shift of the optical transition band in the crystal, but the transition energy is higher than that of the benzoquinoid singlet biradical (1a). This optical behavior and the magnetic susceptibility measurements reveal that the intermolecular covalent bonding interaction in the 1D chain of 2a is greater in strength than the intramolecular one, despite the fact that a fully conjugated Kekulé structure can be drawn for 2a.

Resonance balance shift in stacks of delocalized singlet biradicals.

Recently we succeeded in the isolation of delocalized singlet biradicals utilizing the spin-delocalizing character of the phenalenyl radical. We demonstrated that the singlet biradical 1 has strong spin–spin interactions between molecules through the overlap of phenalenyl rings in the onedimensional (1D) chain even though the closed-shell Kekul structure 1 can be drawn as a resonance contributor (Scheme 1). Huang and Kertesz gave further insight into the spin–spin interactions from a theroretical point of view and showed that the spin–spin interaction between the molecules was predicted to be stronger than that within the molecule. These experimental and theoretical findings are associated with very fundamental issues: Do delocalized singlet biradicals actually have open-shell character? Are the electrons coupled within a molecule involved in covalent bonding between molecules? In this study we will demonstrate that intraand intermolecular spin–spin interactions strongly correlate and can be altered in magnitude by an applied external field. Our proposal is based on the experimentally determined molecular structure of 2, a temperature-dependent reflection spectrum of 2, and a pressuredependent reflection spectrum of 1. Methyl groups at the 2and 10-positions in 2, where the frontier molecular orbital has very small coefficients, are expected to alter the distance between the overlapping phenalenyl rings with respect to the analogous separation in 1, and as a result, the magnitude of the intermolecular spin–spin interaction should be affected. The synthesis of 2 is outlined in Scheme 2. The 3,10and 3,11-bis(bromomethyl) compounds 3 were synthesized according to the previously reported procedures. The individual isomers were not isolated because both were expected to lead to the single compound 2. Bis(2-methylpropionic acid) derivatives 5 were obtained in three steps by standard methods. Friedel–Crafts cyclization of the acyl chloride derivatives of 5 with AlCl3 afforded diketones 6. These were reduced with NaBH4 and subsequently dehydrated with a catalytic amount of p-toluenesulfonic acid to afford the dihydro compounds 8. Dehydrogenation of 8 with p-chloranil afforded the hydrocarbon 2 as green prisms. Compound 2 was found to be stable in the solid state at room temperature. The small HOMO–LUMO gap of 2, which is an essential factor for a singlet biradical electronic structure, was confirmed by electrochemical and optical methods. The cyclic voltammogram of 2 shows four reversible redox waves: E 2 = + 0.51, E 1 =+ 0.11, E red 1 = 1.09, and E 2 = 1.62 (V vs. ferrocene/ferrocenium couple (Fc/Fc), see Figure S1 in the Supporting Information), which led to an electrochemical HOMO–LUMO gap of 1.20 eV. The electronic absorption spectrum of 2 in CH2Cl2 shows an intense low-energy band at 756 nm (13200 cm = 1.64 eV, e= 115000, f= 0.605, see Scheme 1. Resonance structures of 1 and 2. The arrows in the biradical structure represent antiparallel spins.

Charge-transfer salts of M(mnt)2 (M=Ni, Pd, Pt, Au) with BDNT: ferromagnetic interactions in conductive (BDNT)2–[Ni(mnt)2]

Charge-transfer salts between BDNT and M(mnt)2 (M=Ni, Pd, Pt, and Au) have been prepared by mixing BDNT–(SbCl6)2 and (TBA)2 –[M(mnt)2 ] and by electrochemical oxidation of BDNT in TBA–[M(mnt)2 ] solution. Almost all powdery samples of (1:1) salts show high electrical conductivity. The valence of M(mnt)2 was determined to be –1 by the CN stretching mode, which means that the valence of BDNT is +0.5, +1, or +2. Their crystal structures are not known except for the low-conductive BDNT–[Au(mnt)2 ]2 , in which Au(mnt)2 forms dimers which are arranged in crisscross stacks. Among these compounds, (BDNT)2–[Ni(mnt)2 ] exhibits a ferromagnetic interaction with |J|=3.4 K. It is concluded from the EPR experiment that [Ni(mnt)2 ]– is responsible for the ferromagnetic interaction.

Strong Optical Nonlinearity and its Ultrafast Response Associated with Electron Ferroelectricity in an Organic Conductor

We report experimental evidence for the generation of ferroelectric polarization in an organic conductor α-[bis(ethylenedithio)tetrathiafulvalene] 2 I 3 obtained by optical second-harmonic generation. The spontaneous polarization emerges along with a metal-to-insulator transition that is driven by the Wigner crystallization of electrons. The strong optical nonlinearity and its ultrafast photoresponse demonstrated by this study exemplify the nature of the ferroelectric polarization that originates from the electron ordering.

Charge Ordering in Organic Conductors

Present status of experiments on charge ordering (CO) phenomena observed in organic molecular conductors is reviewed. Experimental techniques to detect and analyze the CO, i.e., NMR, X-ray and Infrared/Raman spectroscopy, are described and the experimental results in typical materials exhibiting CO are summarized. (DI-DCNQI) 2 Ag, (TMTTF) 2 X, α-(BEDT-TTF) 2 I 3 , θ-(BEDT-TTF) 2 RbZn(SCN) 4 and related materials are discussed. Charge disproportionation without long-range order observed in numbers of materials are also included.

Magnetic properties of TDAE-C60 and TDAE-C70. A comparative study

Abstract Magnetic properties of C60 and C70 complexes with tetrakis (dimethylamino) ethylene (TDAE) have been studied by Faraday-balance magnetization and ESR measurements. It has been confirmed that TDAE-C60 possesses about one radical spin per molecule and shows a ferromagnetic-type transition at Tc=16.7–17.5 K. The appearance of a new broad and intense ESR signal below Tc suggests that the radical spins in the magnetically ordered state localize on each C60 molecule and are ferromagnetically correlated. On the contrary, TDAE-C70 demonstrates the usual Curie-type narrow ESR peak at all temperatures suggesting the absence of a ferromagnetic transition at least down to 4.5 K.

Synthesis, crystal structure, and physical properties of sterically unprotected hydrocarbon radicals.

We have prepared and isolated neutral polycyclic hydrocarbon radicals. A butyl-substituted radical gave single crystals, in which a π-dimeric structure, not a σ-bonded dimer, was observed, even though steric protection was absent. Thermodynamic stabilization due to the highly spin-delocalized structure contributes effectively to the suppression of σ-bond formation.

Temperature Dependence of the Polarized Reflectance Spectra of the θ-Type of Bis(ethylenedithio) tetrathiafulvalenium Triiodide θ-(BEDT-TTF)2I3: Estimation of Band Parameters

Polarized reflectance spectra in the region from 720 cm -1 to 25000 cm -1 were measured on the single crystal of θ-(BEDT-TTF) 2 I 3 , an organic superconductor, at temperatures over 16–295 K. A Drude-like spectrum was found for the first time in metallic BEDT-TTF salts. This characteristic spectrum was attributed to the unique structure of the θ-type crystal. The curve-fitting analysis of the spectra gave the plasma frequencies (ω p ) a =1.05 eV and (ω p ) c =0.73 eV at 16 K, from which the anisotropic effective masses were evaluated to be m a * =1.5 m e and m c * =3.0 m e . The transfer integrals and other band parameters of the tight-binding band were directly derived from the analysis of the optical data. The transfer integrals thus obtained are t a =0.080 eV and t c =0.046 eV. These results are to be compared with the calculated band structure.

Structure and superconductivity of single crystalline C60

Abstract We have succeeded in synthesis of C 60 single crsytals with orthorhombic lattice structure contrary to the known cubic form. The structure of the orthorhombic form was confirmed by a four-circle X-ray diffractometer. The intense microwave absorption signal in low magnetic field was detected on the K-doped C 60 single crystals with the onset of superconductivity at Tc = 17.7 K.

Synthesis and properties of a new ferromagnetic 2,2′-bipyridine-MnPS3 intercalation compound

Abstract A new intercalation compound Mn0.86PS3(bipy)0.56 has been synthesized by a direct reaction of MnPS3 with 2,2-bipyridine. It has been characterized by elemental analyses, thermogravimetry analysis, infrared spectroscopy and X-ray powder diffraction. The magnetic properties of the materials was studied with SQUID-magnetometer. The results indicate that the material exhibits bulk ferromagnetism below 40 K.