Shinya Takaishi

Direct Observation of Lanthanide(III)-Phthalocyanine Molecules on Au(111) by Using Scanning Tunneling Microscopy and Scanning Tunneling Spectroscopy and Thin-Film Field-Effect Transistor Properties of Tb(III)- and Dy(III)-Phthalocyanine Molecules

The crystal structures of double-decker single molecule magnets (SMM) LnPc(2) (Ln = Tb(III) and Dy(III); Pc = phthalocyanine) and non-SMM YPc(2) were determined by using X-ray diffraction analysis. The compounds are isomorphous to each other. The compounds have metal centers (M = Tb(3+), Dy(3+), and Y(3+)) sandwiched by two Pc ligands via eight isoindole-nitrogen atoms in a square-antiprism fashion. The twist angle between the two Pc ligands is 41.4 degrees. Scanning tunneling microscopy was used to investigate the compounds adsorbed on a Au(111) surface, deposited by using the thermal evaporation in ultrahigh vacuum. Both MPc(2) with eight lobes and MPc with four lobes, which has lost one Pc ligand, were observed. In the scanning tunneling spectroscopy images of TbPc molecules at 4.8 K, a Kondo peak with a Kondo temperature (T(K)) of approximately 250 K was observed near the Fermi level (V = 0 V). On the other hand, DyPc, YPc, and MPc(2) exhibited no Kondo peak. To understand the observed Kondo effect, the energy splitting of sublevels in a crystal field should be taken into consideration. As the next step in our studies on the SMM/Kondo effect in Tb-Pc derivatives, we investigated the electronic transport properties of Ln-Pc molecules as the active layer in top- and bottom-contact thin-film organic field effect transistor devices. Tb-Pc molecule devices exhibit p-type semiconducting properties with a hole mobility (mu(H)) of approximately 10(-4) cm(2) V(-1) s(-1). Interestingly, the Dy-Pc based devices exhibited ambipolar semiconducting properties with an electron mobility (mu(e)) of approximately 10(-5) and a mu(H) of approximately 10(-4) cm(2) V(-1) s(-1). This behavior has important implications for the electronic structure of the molecules.

Electroconductive Porous Coordination Polymer Cu[Cu(pdt)2] Composed of Donor and Acceptor Building Units

We synthesized a new porous coordination polymer Cu[Cu(pdt)2], which shows relatively high electrical conductivity (6 x 10(-4) S cm(-1) at 300 K) by the introduction of electron donors and acceptors as building units. This compound is applicable as a porous electrode with high power density. In addition, this compound forms a triangular spin lattice and shows spin frustration.

Structural Design of Easy-Axis Magnetic Anisotropy and Determination of Anisotropic Parameters of LnIIICuII Single-Molecule Magnets

Four dinuclear Ln(III)-Cu(II) complexes with Ln=Tb (1), Dy (2), Ho (3), and Er (4) were synthesized to investigate the relationship between their respective magnetic anisotropies and ligand-field geometries. These complexes were crystallographically isostructural, and a uni-axial ligand field was achieved by using three phenoxo oxygen groups. Complexes 1 and 2 displayed typical single-molecule magnet (SMM) behaviors, of which the out-of-phase susceptibilities were observed in the temperature range of 1.8-5.0 K (1) and 1.8-20.0 K (2). The Cole-Cole plots exhibited a semicircular shape with α parameters in the range of 0.08-0.18 (2.6-4.0 K) and 0.07-0.24 (3.5-7.0 K). The energy barriers Δ/k(B) were estimated from the Arrhenius plots to be 32.9(4) K for 1 and 26.0(5) K for 2. Complex 3 displayed a slow magnetic relaxation below 3.0 K, whereas complex 4 did not show any frequency-dependent behavior for both in-phase and out-of-phase susceptibilities, which indicates that easy-axis anisotropy was absent. The temperature dependence of the dc susceptibilities for the field-aligned samples of 1-3 revealed that the χ(M) T value continuously increased as the temperature was lowered, which indicates the presence of low-lying Stark sublevels with the highest |J(z) | values. In contrast, complex 4 displayed a smaller and temperature-independent χ(M) T value, which also indicates that easy-axis anisotropy was absent. Simultaneous analyses were carried out for 1-3 to determine the magnetic anisotropy parameters on the basis of the Hamiltonian that considers B(2) (0) , B(4) (0) , and B(6) (0) .

Coordination-Tuned Single-Molecule-Magnet Behavior of TbIII-CuII Dinuclear Systems

Tb (III)-Cu (II)-based single-molecule magnet (SMM) and non-SMM were synthesized to investigate the relationship between magnetic anisotropy and the symmetry of the ligand field by the reaction of [TbCu( o-vanilate) 2(NO 3) 3] with methoxypropylamine (MeOC 3H 6NH 2, 1) or ethoxyethylamine (EtOC 2H 4NH 2, 2). In both complexes, Tb (III) ions have a bicapped square-antiprism coordination geometry. When the Tb (III) ion is in a less symmetrical ligand field, it has an easy-axis anisotropy and shows SMM behavior, whereas when it is in a more symmetrical environment, it has an easy-plane anisotropy and exhibits non-SMM behavior.

Structural correlations between the crystal field and magnetic anisotropy of Ln–Cu single-molecule magnets

Mixed ligand Ln2Cu2 tetranuclear complexes, [{LnCu(L4)(L5)(NO3)2}2] (L4− is a Schiff base ligand derived from o-vanilate, L52− is a nitromethyl derivative of L4−; LnGd(4), Tb(5), and Dy(6)), were synthesized by reacting the corresponding dinuclear complexes [LnCu(o-vanilate)2(NO3)3] with ethylamine in a nitromethane solution. From detailed studies of magnetic anisotropies 5 was shown to behave as an SMM, whereas 6 was not an SMM.

Charge-Density-Wave to Mott−Hubbard Phase Transition in Quasi-One-Dimensional Bromo-Bridged Pd Compounds

A -Pd(III)-Br-Pd(III)-Mott-Hubbard state was observed in a quasi-one-dimensional bromo-bridged Pd compound [Pd(en)2Br](C5-Y)2 x H2O (en = ethylenediamine, C5-Y = dipentylsulfosuccinate) for the first time. The phase transition between Mott-Hubbard and charge-density-wave states occurred at 206 +/- 2 K and was confirmed by using X-ray, ESR, Raman and electronic spectroscopies, electrical resistivity, and heat capacity. From X-ray powder diffraction patterns and Raman spectra of a series of Pd-Br compounds, [Pd(en)2Br](C(n)-Y)2 x H2O (n = 4, 5, 6, 7, 8, 9, and 12), chemical pressure from the alkyl chains of the counterions caused the phase transition.

Single-Chain Magnets Constructed by Using the Strict Orthogonality of Easy-Planes: Use of Structural Flexibility to Control the Magnetic Properties

A family of single-chain magnets (SCMs), of which the SCM character originated from the spatial arrangement of high spin Fe(II) ions with easy-plane anisotropy, was synthesized, and their magnetic properties were investigated. The chain complexes including alternating high-spin Fe(II) ions and low-spin Fe(III) ions, catena-[Fe(II)(ClO(4))(2){Fe(III)(bpca)(2)}]ClO(4)·3MeNO(2) (1·3MeNO(2)), catena-[Fe(II)(ClO(4))(H(2)O){Fe(III)((Me)L)(2)}](ClO(4))(2)·2MeNO(2)·H(2)O (2·2MeNO(2)·H(2)O), catena-[Fe(II)(ClO(4))(H(2)O){Fe(III)((Bu)L)(2)}](ClO(4))(2)·3.5MeNO(2) (3·3.5MeNO(2)), and catena-[{Fe(II)(ClO(4))(H(2)O)Fe(II)(H(2)O)(2)}(0.5){Fe(III)((Ph)L)(2)}](ClO(4))(2.5)·4EtNO(2) (4·4EtNO(2)), were synthesized with the use of bridging ligand Hbpca (bis-(2-pyridylcarbonyl)amine)) and its derivatives of H(Me)L, H(Bu)L, and H(Ph)L each incorporating methyl, tert-butyl, or phenyl group on the 4-position of pyridyl ring. These complexes showed a typical ferrimagnetic behavior on direct current (dc) susceptibility data, and from an alternating current (ac) susceptibility measurements, SCM or superparamagnetic behaviors were confirmed with the Δ/k(B) values of 22.5(4), 21.8(18), and 28.8(3) K for 1·3MeNO(2), 2·2MeNO(2)·H(2)O, and 3·3.5MeNO(2), of which the easy-axis anisotropy was originated from the orthogonal arrangement of easy-planes of Fe(II) ions. In the crystal structures, cylindrical voids were formed along the chain axis being surrounded by four chains in 1·3MeNO(2), 2·2MeNO(2)·H(2)O, and 4·4EtNO(2) and two chains in 3·3.5MeNO(2), and solvent molecules as well as coordination-free perchlorate anions occupied these voids in a slightly different fashion depending on the complexes. 2·2MeNO(2)·H(2)O maintains its chemical composition in a dried condition, whereas 1·3MeNO(2), 3·3.5MeNO(2), and 4·4EtNO(2) easily release solvent molecules to give 1, 3, and 4, respectively. 1 and 3 maintain the crystalline character showing slightly different X-ray diffraction (XRD) patterns from those of 1·3MeNO(2) and 3·3.5MeNO(2), and an enhancement of SCM character after release of the solvent molecules was observed for both. 4 lost crystalline character to become amorphous, and it lost the SCM character at the same time.

Current oscillation originating from negative differential resistance in one-dimensional halogen-bridged nickel compounds

We demonstrate current oscillation phenomena using the negative differential resistance in a one-dimensional halogen-bridged nickel compound, [Ni(chxn)2Br]Br2 (chxn=cyclohexanediamine). By attaching external resistors and a capacitor to a [Ni(chxn)2Br]Br2 sample, we obtain stable current oscillation at 90 K. The oscillation and its period are explained by a simple model.

Enhancement of luminescence intensity in TMPY/perylene co-single crystals

A new molecule, 1,3,6,8-tetramethylpyrene (TMPY), with a similar shape to the luminescent material perylene has been successfully synthesized. The co-crystals with perylene doping have been grown and their crystal structure has been clarified by X-ray analysis. Optical spectra indicate that effective energy transfer has been achieved in the doping systems and the luminescence efficiency has reached as high as 78%. The pure TMPY shows p-type characteristics during the FET operation. The hole mobility is up to 0.26 cm2 V−1s−1.

Paramagnetic-diamagnetic phase transition accompanied by coordination bond formation-dissociation in the dithiolate complex Na[Ni(pdt)2]·2H2O.

Bis(2,3-pyrazinedithiolate)nickel complex Na[Ni(pdt)(2)]·2H(2)O formed one-dimensional stacks of the Ni(pdt)(2) units and showed strong antiferromagnetic interactions along the stacking direction. A first-order phase transition between the paramagnetic and diamagnetic states, which was driven by dimerization of the Ni(pdt)(2) units, accompanied by coordination bond formation, was observed.