Keiichi Katoh

Observation and electric current control of a local spin in a single-molecule magnet.

In molecular spintronics, the spin state of a molecule may be switched on and off by changing the molecular structure. Here, we switch on and off the molecular spin of a double-decker bis(phthalocyaninato)terbium(III) complex (TbPc2) adsorbed on an Au(111) surface by applying an electric current via a scanning tunnelling microscope. The dI/dV curve of the tunnelling current recorded onto a TbPc2 molecule shows a Kondo peak, the origin of which is an unpaired spin of a π-orbital of a phthalocyaninato (Pc) ligand. By applying controlled current pulses, we could rotate the upper Pc ligand in TbPc2, leading to the disappearance and reappearance of the Kondo resonance. The rotation shifts the molecular frontier-orbital energies, quenching the π-electron spin. Reversible switching between two stable ligand orientations by applying a current pulse should make it possible to code information at the single-molecule level.

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.

Magnetic Relaxation of Single‐Molecule Magnets in an External Magnetic Field: An Ising Dimer of a Terbium(III)–Phthalocyaninate Triple‐Decker Complex

The idea of using a single spin as a “bit” of information to prepare high-density storage and quantum-computing deACHTUNGTRENNUNGvices has caused an increase in scientific and technological interests. Quantum tunneling of the magnetization (QTM) in double-well potentials, which is a characteristic property of single-molecule magnets (SMMs), is the underlying phenomenon for this idea. On the basis of the properties of lanthanoid–phthalocyaninate ([LnPc2]) SMMs, [4] we believe that [TbPc2] can be used as a “bit” of information in highdensity storage technology by taking advantage of the single up-spin/down-spin property, which is equivalent to 2. The up-spin/down-spin properties of tripleand quadrupledecker-type SMMs are equivalent to 2 and 2, respectively, in relation to the number of spins. We have recently reported the characteristics of [MPc2] and [MPc] (M= Tb, Dy, and Y) deposited on an AuACHTUNGTRENNUNG(111) surface in an ultrahigh vacuum (UHV) using a dry process technique. Both [MPc2] and [MPc] are present on the AuACHTUNGTRENNUNG(111) surface on the basis of height profiles and dI/dV mapping obtained by using scanning tunneling microscopy (STM) and spectroscopy (STS). A Kondo peak, which is due to coupling between magnetic impurities, including Tb ions, and conduction electrons from the STS, is only observed at the center of [TbPc] at a Kondo temperature (TK) of 250 K. More recently, we have observed a Kondo peak for [TbPc2] on an AuACHTUNGTRENNUNG(111) surface. Therefore, the relation between TK and the blocking temperature (TB) must be discussed further. In addition, the properties of SMMs and the Kondo effect can be modulated with an external magnetic field. Here we present the results of studies on the SMM properties of a new Tb triple-decker phthalocyaninate derivative, [Tb2ACHTUNGTRENNUNG(obPc)3] (1; obPc =dianion of 2,3,9,10,16,17,23,24octabutoxyphthalocyanine). The relationships among the molecular structure, ligand-field, ground-state, and SMM properties in a direct current (dc) magnetic field are discussed. It is important to both understand and control the quantum properties of SMMs with an external field. The triple-decker complex 1 is composed of three Pc ligACHTUNGTRENNUNGands and two Tb ions, resulting in a neutral complex with a closed shell p electron system. We used a Pc ligand with 2,3,9,10,16,17,23,24-octabutoxy substituents because it should have a higher solubility and crystallization should be easier. Complex 1 was synthesized in one step starting from [Tb ACHTUNGTRENNUNG(acac)3]·4H2O and H2obPc, following a published procedure (see Experimental Section). This complex is soluble in most organic solvents, except for alcohols. Complex 1 crystallized with ethanol in the crystal lattice in the triclinic space group P1̄, as shown in Figure 1. The crystal data are summarized in Table S1 and crystal-packing [a] Dr. K. Katoh, Prof. B. K. Breedlove, Prof. M. Yamashita Department of Chemistry, Graduate School of Science Tohoku University, 6–3 Aramaki-Aza-Aoba, Aoba-ku Sendai, Miyagi 980-8578 (Japan) Fax: (+81) 22-795-6548 E-mail : kkatoh@m.tains.tohoku.ac.jp yamasita@agnus.chem.tohoku.ac.jp [b] Prof. T. Kajiwara Department of Chemistry, Faculty of Science Nara Women s University, Nishi-Machi Kita-Uoya, Nara 565-0871 (Japan) [c] Prof. M. Nakano Department of Applied Chemistry Graduate School of Engineering, 2–1 Yamadaoka Suita, Osaka 565-0871 (Japan) [d] Prof. Y. Nakazawa, Prof. N. Ishikawa Department of Chemistry, Graduate School of Science 1-1 Machikaneyama-Cho, Toyonaka Osaka 560-0043 (Japan) [e] Prof. W. Wernsdorfer Laboratory Louis N el, CNRS, BP 166, 38042 Grenoble Cedex 9 (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201002026.

Molecular Spintronics Based on Single‐Molecule Magnets Composed of Multiple‐Decker Phthalocyaninato Terbium(III) Complex

Unlike electronics, which is based on the freedom of the charge of an electron whose memory is volatile, spintronics is based on the freedom of the charge, spin, and orbital of an electron whose memory is non-volatile. Although in most GMR, TMR, and CMR systems, bulk or classical magnets that are composed of transition metals are used, this Focus Review considers the growing use of single-molecule magnets (SMMs) that are composed of multinuclear metal complexes and nanosized magnets, which exhibit slow magnetic-relaxation processes and quantum tunneling. Molecular spintronics, which combines spintronics and molecular electronics, is an emerging field of research. Using molecules is advantageous because their electronic and magnetic properties can be manipulated under specific conditions. Herein, recent developments in [LnPc]-based multiple-decker SMMs on surfaces for molecular spintronic devices are presented. First, we discuss the strategies for preparing single-molecular-memory devices by using SMMs. Next, we focus on the switching of the Kondo signal of [LnPc]-based multiple-decker SMMs that are adsorbed onto surfaces, their characterization by using STM and STS, and the relationship between the molecular structure, the electronic structure, and the Kondo resonance of [TbPc(2)]. Finally, the field-effect-transistor (FET) properties of surface-adsorbed [LnPc(2)] and [Ln(2)Pc(3)] cast films are reported, which is the first step towards controlling SMMs through their spins for applications in single-molecular memory and spintronics devices.

Effect of f–f interactions on quantum tunnelling of the magnetization: mono- and dinuclear Dy(III) phthalocyaninato triple-decker single-molecule magnets with the same octacoordination environment

The single-molecule magnet (SMM) behaviour of dinuclear Ln(III)-Pc triple-decker complexes (Dy(III)-Y(III): 1 and Dy(III)-Dy(III): 2) with the same octacoordination environment and slow magnetic relaxation behaviour were explained using X-ray crystallography and static and dynamic susceptibility measurements. In particular, interactions among the 4f electrons of dinuclear Dy(III)-Pc triple-decker type SMMs have never been discussed on the basis of the same octacoordination environment. Our results clearly show that the Dy(III) ion sites of 1 and 2 are equivalent, consistent with the crystal structure. 2 Exhibited ferromagnetic interaction between Dy(III) ions. This is clear evidence that the magnetic relaxation mechanism depends heavily on the dipole-dipole (f-f) interactions between the Dy(III) ions in the dinuclear systems. For both 1 and 2, quantum tunnelling of the magnetization (QTM) was observed. However, the magnetic relaxation time (τ) for 2 was one order of magnitude greater than that for 1, and single-component magnetic relaxation behaviour was explained. In other words, it is possible to use f-f interactions to increase τ by one order of magnitude.

Control of the single-molecule magnet behavior of lanthanide-diarylethene photochromic assemblies by irradiation with light.

Lanthanide-based extended coordination frameworks showing photocontrolled single-molecule magnet (SMM) behavior were prepared by combining highly anisotropic Dy(III) and Ho(III) ions with the carboxylato-functionalized photochromic molecule 1,2-bis(5-carboxyl-2-methyl-3-thienyl)perfluorocyclopentene (H2 dae), which acts as a bridging ligand. As a result, two new compounds of the general formula [{Ln(III) 2 (dae)3 (DMSO)3 (MeOH)}⋅10 MeOH]n (M=Dy for 1 a and Ho for 2) and two additional pseudo-polymorphs [{Dy(III) 2 (dae)3 (DMSO)3 (H2 O)}⋅x MeOH]n (1 b) and [{Dy(III) 2 (dae)3 (DMSO)3 (DMSO)}⋅x MeOH]n (1 c) were obtained. All four compounds have 2D coordination-layer topologies, in which carboxylate-bridged Ln2 units are linked together by dae(2-) anions into grid-like frameworks. All four compounds exhibited a strong reversible photochromic response to UV/Vis light. Moreover, both 1 a and 2 show field-induced SMM behavior. The slow magnetic relaxation of 1 a is influenced by the photoisomerization reaction leading to the observation of the cross-effect: photocontrolled SMM behavior.

MnIII(tetra-biphenyl-porphyrin)–TCNE Single-Chain Magnet via Suppression of the Interchain Interactions

A single-chain magnet (SCM) of [Mn(TBPP)(TCNE)]·4m-PhCl(2) (1), where TBPP(2-) = meso-tetra(4-biphenyl)porphyrinate; TCNE(•-) = tetracyanoethenide radical anion; m-PhCl(2) = meta-dichlorobenzene, was prepared via suppression of interchain interactions. 1 has a one-dimensional alternating Mn(III)(porphrin)-TCNE(•-)chain structure similar to those of a family of complexes reported by Miller and co-workers. From a comparison of the static magnetic properties of 1 with other Mn(III)(porphyrin)-TCNE(•-) chains, a magneto-structural correlation between the intrachain magnetic exchange and both the dihedral angle between the mean plane on [Mn(TBPP)(TCNE)] and Mn-N≡C was observed. The ac magnetic susceptibility data of 1 could be fit with the Arrhenius law, indicating that slow magnetic relaxation and ruling out three-dimensional long-range ordering and spin-glass-like behavior. The Cole-Cole plot for 1 was semicircular, verifying that it is an SCM. Therefore, 1 is an ideal single-chain magnet with significantly strong intrachain magnetic exchange interactions beyond the Ising limit.

Controlling the dipole-dipole interactions between terbium(III) phthalocyaninato triple-decker moieties through spatial control using a fused phthalocyaninato ligand.

Using a fused phthalocyaninato ligand to control the spatial arrangement of Tb(III) moieties in Tb(III) single-molecule magnets (SMMs), we could control the dipole-dipole interactions in the molecules and prepared the first tetranuclear Tb(III) SMM complex. [Tb(obPc)2]Tb(Fused-Pc)Tb[Tb(obPc)2] (abbreviated as [Tb4]; obPc = 2,3,9,10,16,17,23,24-octabutoxyphthalocyaninato, Fused-Pc = bis{7(2),8(2),12(2),13(2),17(2),18(2)-hexabutoxytribenzo[g,l,q]-5,10,15,20-tetraazaporphirino}[b,e]benzenato). In direct-current magnetic susceptibility measurements, ferromagnetic interactions among the four Tb(3+) ions were observed. In [Tb4], there are two kinds of magnetic dipole-dipole interactions. One is strong interactions in the triple-decker moieties, which dominate the magnetic relaxations, and the other is the weak one through the fused phthalocyaninato (Pc) ligand linking the two triple-decker complexes. In other words, [Tb4] can be described as a weakly ferromagnetically coupled dimer of triple-decker Tb2(obPc)3 complexes with strong dipole-dipole interactions in the triple-decker moieties and weak ones through the fused phthalocyaninato ligand linking the two triple-decker complexes. For [Tb4], dual magnetic relaxation processes were observed similar to other dinuclear Tb(III)Pc complexes. The relaxation processes are due to the anisotropic centers. This is clear evidence that the magnetic relaxation mechanism depends heavily on the dipole-dipole (f-f) interactions between the Tb(3+) ions in the systems. Through a better understanding of the magnetic dipole-dipole interactions obtained in these studies, we have developed a new strategy for preparing Tb(III) SMMs. Our work shows that the SMM properties can be fine-tuned by introducing weak intermolecular magnetic interactions in a controlled SMM spatial arrangement.

Variation of Kondo Peak Observed in the Assembly of Heteroleptic 2,3-Naphthalocyaninato Phthalocyaninato Tb(III) Double-Decker Complex on Au(111)

By using scanning tunneling microscopy (STM), we studied the heteroleptic double-decker complex TbNPcPc (NPc = naphthalocyaninato and Pc = phthalocyaninato), where two different planar ligands sandwich a Tb(III) ion and an unpaired π electron causes Kondo resonance upon adsorption on the Au(111) surface. Kondo resonance is a good conductance control mechanism originating from interactions between conduction electrons and a localized spin. Two types of adsorption geometries appear depending on which side contacts the substrate surface, which we call Pc-up and NPc-up molecules. They make intriguing molecular assemblies by segregation. In addition, different adsorption geometries and molecular assemblies provide a variety of spin and electronic configurations. Pc-up and NPc-up molecules both showed the Kondo resonance when they were isolated from other molecules, but their Kondo temperatures were different. A one-dimensional chain composed of only NPc-up molecules was found, in which the dI/dV plot showed a conversion from the Kondo peak to a dip at the Fermi energy. In addition, a two-dimensional lattice with an ordering of Pc-up and NPc-up molecules in an alternative manner was observed, in which no Kondo peak was detected in the molecule. The absence of the Kondo peak was accounted for by the change of azimuthal rotational angle of the two ligands of both molecules. The results imply that a molecule design and adsorption configuration tailoring can be used for the spin-mediated control of the electronic conductance of the molecule.

First Observation of a Kondo Resonance for a Stable Neutral Pure Organic Radical, 1,3,5-Triphenyl-6-oxoverdazyl, Adsorbed on the Au(111) Surface

We investigated spin states of stable neutral pure-organic radical molecules of 1,3,5-triphenyl-6-oxoverdazyl (TOV) and 1,3,5-triphenyl-6-thioxoverdazly (TTV) adsorbed on an Au(111) surface, which appears as a Kondo resonance because of spin-electron interaction. By using scanning tunneling spectroscopy (STS), a clear Kondo resonance was detected for the TOV molecule. However, no Kondo resonance was detected for TOV molecules with protrusions in the occupied state image and for TTV molecules. Spin-resolved DFT calculations showed that an unpaired π electron was delocalized over the adsorbed TOV molecule, which was the origin of the Kondo resonance. For the TOV molecules with protrusions, we proposed a model in which an additional H atom was attached to the TOV molecule. Calculations showed that, upon transfer of an electron to the verdazyl ring, the unpaired π electron disappeared, accounting for the absence of a Kondo resonance in the STS spectra. The absence of a Kondo resonance for the TTV molecule can be explained in a similar manner. In other words, electron transfer to the verdazyl ring occurs because of Au-S bond formation.