Central Mn Ion Research Articles

Reversible magnetic switching of high-spin molecules on a giant Rashba surface

The quantum mechanical screening of a spin via conduction electrons depends sensitively on the environment seen by the magnetic impurity. A high degree of responsiveness can be obtained with metal complexes, as the embedding of a metal ion into an organic molecule prevents intercalation or alloying and allows for a good control by an appropriate choice of the ligands. There are therefore hopes to reach an “on demand” control of the spin state of single molecules adsorbed on substrates. Hitherto one route was to rely on “switchable” molecules with intrinsic bistabilities triggered by external stimuli, such as temperature or light, or on the controlled dosing of chemicals to form reversible bonds. However, these methods constrain the functionality to switchable molecules or depend on access to atoms or molecules. Here, we present a way to induce bistability also in a planar molecule by making use of the environment. We found that the particular “habitat” offered by an antiphase boundary of the Rashba system BiAg2 stabilizes a second structure for manganese phthalocyanine molecules, in which the central Mn ion moves out of the molecular plane. This corresponds to the formation of a large magnetic moment and a concomitant change of the ground state with respect to the conventional adsorption site. The reversible spin switch found here shows how we can not only rearrange electronic levels or lift orbital degeneracies via the substrate, but even sway the effects of many-body interactions in single molecules by acting on their surrounding.

Superexchange pathways stabilize the magnetic coupling of MnPc with Co in a spin interface mediated by graphene

We investigate the magnetic response of a spin interface constituted by MnPc molecules adsorbed on graphene/Co and its robustness against thermal fluctuations by x-ray magnetic circular dichroism. Element-selective hysteresis loops reveal a remarkable antiferromagnetic coupling between MnPc and Co that is strong both in perpendicular and in-plane magnetic orientations, thanks to the magnetic anisotropy properties and electronic configuration of MnPc. The magnetic interaction between MnPc and Co is mediated by the molecular states and the graphene $\ensuremath{\pi}$ orbitals in a superexchange mechanism that allows a strong exchange coupling while the molecular orbitals symmetries are preserved by the graphene decoupling layer. Our results show that the strength and stability of the magnetic coupling between MnPc molecules and Co layer(s), intercalated at the graphene/Ir(111) interface, is further optimized by the open $3d$ shell of the central Mn ion. The magnetic properties are compared with analogous molecular spin interfaces with high thermal stability, paradigmatic examples to exploit in surface-supported molecular spin electronics.

Mn(acac)2 and Mn(acac)3 complexes, a theoretical modeling of their L2,3-edges X-ray absorption spectra

The DFT/ROCIS method has been used to model the Mn(acac)2 (I) and Mn(acac)3 (II) L2,3-edges absorption spectra. Non-hybrid (BP86 and PBE), hybrid (B3P, PBE0 and B3LYP) and hybrid meta-GGA (M06) functionals have been considered to test their efficiency in reproducing experimental features. Structural parameters of both I and II are scarcely affected by the functional selection and the same holds as far as the IL2,3-edges absorption spectrum is concerned. At variance to that, the use of the hybrid meta-GGA functional is decisive for a detailed assignment of the IIL2,3-edges spectral features. The role played by the structural arrangement about the central Mn ion appears less important than the Mn oxidation number in determining the overall spectrum appearance and the spectral features’ excitation energies (EEs). DFT/ROCIS results indicate that, both in I and II, the lower EE side of the L3 intensity distribution mainly includes states having the ground state spin multiplicity (S=5/2 in I and S=2 in II), whereas states with different spin multiplicity contribute, both in I and II, to the L3 higher EE side. The occurrence of states involving metal to ligand charge transfer transitions and implying low lying empty ligand-based orbitals on the I/IIL3 higher EE sides has been further confirmed herein.

Electrochemical Properties of Lithium Air Secondary Batteries with Nonaqueous Electrolyte Solution Containing Manganese-Based Organic Complexes As Solution-Phase Catalysts

Introduction Lithium air secondary batteries (LABs) have the highest theoretical energy density among secondary batteries reported so far. However, major problems are poor cycle characteristics and large discharge/charge overpotential. To improve these properties, various kinds of solid-phase catalysts loaded into air electrodes have been reported [1]. One of the significant problems is that the solid-phase catalysts become inactivated gradually because discharge product Li2O2 accumulats on the surface of the air electrode as a result of imperfect decomposition during the charging process after a large number of cycles [2]. Since a solution-phase catalyst dissolved in the electrolyte solution could work stably during the cycles, it would overcome these issues associated with the solid-phase catalyst. Recently, as solution-phase catalysts, transition metal complexes such as manganese phthalocyanine (MnPc) [3], cobalt phthalocyanine (CoPc) [4] have been reported [3-5]. It is necessary to investigate the electrocatalytic activities for various kinds of solution-phase catalysts because the catalytic mechanism is unclear. We focused on manganese-containing salen-type complexes (MnSl) as a new solution-phase catalyst (Fig. 1). The electrochemical properties would be improved by involving oxygen adsorbed on the central Mn-ion site with the reaction over the air electrode [6]. Here, we report discharge/charge properties in LABs using MnSl as solution-phase catalyst. Experimental MnSl [(R,R)-(−)-N,N-Bis(3,5–di–tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III)chloride] and MnPc [manganese(II) phthalocyanine] as a reference catalyst were purchased from Sigma-Aldrich Co. LLC. A 2.0 wt% of both catalysts vacuum-dried at 90 °C for 12 h was added to an electrolyte solution of 1.0 mol/l LiTFSA / TEGDME (tetraglyme). Ketjenblack EC-600JD (KB, 80 wt%) and polyvinylidene difluoride (PVdF, 20 wt%) coated on carbon paper (Toray, TGP-H-120) was used as the air electrode. A Li metal sheet was used as the negative electrode. The experimental LAB cell was assembled using a commercial cell (ECC-Air, EL-Cell GmbH). Electrochemical experiments were conducted under galvanostatic condition at 200 mA/g in dry air atmosphere. Current density and discharge/charge capacity were normalized by the mass of the reaction layer (KB + PVdF) in the air electrode. Results and discussion Figure 2 shows the first discharge/charge curves of LAB cells with MnSl, with MnPc catalyst, and whithout the solution-phase catalysts. The cells with MnSl catalyst and MnPc show higher average discharge voltages of 2.64 and 2.55 V and larger discharge capacities of 4903 and 3309 mAh/g, respectively, compared to the cell without the catalysts. In addition, the cell with MnSl catalyst shows lower average charge voltage of 3.93 V and larger charge capacity of 6390 mAh/g than the other cells. These results suggest that the solution-phase catalyst could have electrocatalytic activities. Moreover, the MnSl catalyst exhibits rather higher activity than the conventional catalyst, MnPc. However, with the MnSl catalyst, the first charge capacity was larger than the discharge capacity by about 2000 mAh/g. This behavior suggests that some side reactions might co-occur with the charge (oxygen evolution) reaction in the high-voltage region above 4.0 V. Figure 3 shows SEM images of the air electrodes as prepared and after the first discharge without and with MnSl catalyst. Compared to the electrode surface as prepared shown in Fig. 3(a), the fine sub-micrometer discharge products with similar structures were deposited in a part of the electrode surface both without and with MnSl catalyst as shown in Fig. 3(b) and (c). In particular, there were spherical micrometer structures of the discharge products only with MnSl as shown in Fig. 3(c). MnSl catalyst three-dimensionally catalyzed deposition and decomposition of Li2O2 over the air electrode, which led to significant increase in the capacity. According to the above results, the MnSl catalyst exhibited higher activities than MnPc. One of the reasons might be that the oxygen adsorption on the central metal Mn containing N-Mn-N bonds is stronger in MnSl than in MnPc. At the IMLB meeting, we will also discuss the cycle characteristics of LABs with the MnSl catalyst.

State identification and tunable Kondo effect of MnPc on Ag(001)

We present a detailed investigation of spectroscopic features located at the central metal ion of MnPc (where Pc represents phthalocyanine) on Ag(001) by means of scanning tunneling spectroscopy (STS) and first-principles theory. STS data taken close to the Fermi level reveal an asymmetric feature that cannot be fitted with a single Fano function representing a one-channel Kondo effect. Instead, our data indicate the existence of a second superimposed feature. Two potential physical origins, a second Kondo channel related to the ${d}_{xz/yz}$ orbitals, and a spectral feature of the ${d}_{{z}^{2}}$ orbital itself, are discussed. A systematic experimental and theoretical comparison of MnPc with CoPc and FePc indicates that the second feature observed on MnPc is caused by the ${d}_{{z}^{2}}$ orbital. This conclusion is corroborated by STM-induced dehydrogenation experiments on FePc and MnPc which in both cases result in a gradual shift towards more positive binding energies and a narrowing of the Kondo resonance. Theoretical analysis reveals that the latter is caused by the reduced hybridization between the $d$ orbital and the substrate. Spatially resolved differential conductivity maps taken close to the respective peak positions show that the intensity of both features is highest over the central Mn ion, thereby providing further evidence against a second Kondo channel originating from the ${d}_{xz/yz}$ orbital of the central Mn ion.

Tuning the spin, chirality, and adsorption site of metal-phthalocyanine on Au(111) surface with hydrogen atoms

Metal-phthalocyanines (MPcs) and their derivates have attracted increasing interest in recent years, due to their potential applications in molecular electronics, spintronics, sensors, and so on. To this end, it is essential to tune the structural, electronic and spin properties of MPcs. Using the low-temperature scanning tunneling microscopy (LT-STM), we demonstrate that the spin, chirality and adsorption site of MnPc on Au(111) surface can be tuned by hydrogen atoms. STM experiments and density functional theory (DFT) calculations reveal that the preferential adsorption sites for the MnPc molecules may switch from the fcc regions to the hcp regions on the Au(111) surface after a hydrogen atom is adsorbed on top of the central Mn ion of each MnPc molecule. Moreover, the molecular spin decreases from S=3/2 to S=1 and the molecule-substrate coupling is weakened after the adsorption of a hydrogen atom on a MnPc molecule, leading to the quenching of Kondo effect at 4.2 K. However, the molecular spin and Kondo effect can be recovered by local voltage pulse or sample heating. Adsorption of three hydrogen atoms on a MnPc molecule not merely lowers the molecular symmetry from 4-to 2-fold, but also breaks down the mirror symmetry of the entire adsorbate complex (molecule and surface), thus rendering it to become chiral without any realignment at the surface. Dehydrogenation of the adsorbate by means of inelastic electron tunneling can also restore the mirror symmetry of the adsorbate complex. STM experiments as well as DFT calculations show that the chirality is actually imprinted into the molecular electronic system by the surface, i.e., the lowest unoccupied orbital is devoid of mirror symmetry. Our novel reversible spin and hand control scheme can be easily realized at single-molecule level, thus opening up a new avenue to broader applications based on the molecular electronic and spin states.

Step-by-step self-assembly of manganese phthalocyanine on Bi(1 1 1) surface: From single molecule to two-dimensional domains

The adsorption and self-assembly of manganese phthalocyanine (MnPc) molecules on Bi(111) are investigated by means of low-temperature scanning tunneling microscope (STM). At low temperature (4.6K), isolated MnPc molecules adsorb flatly on Bi(111) with the two opposing lobes aligned in the direction of surface base vectors. When temperature is increased to 78K, individual MnPc molecules rotate around the central Mn ion until they meet other MnPc molecules forming self-assembled structures. With the coverage increasing, MnPc molecules show a structural evolution from single molecules to molecular chains, and finally to 2-D domains. In particular, high-resolution STM images reveal there are two mirrored molecular chains with alternating arrangement in the 2-D domains. Annealing the sample to room temperature leads to a phase separation such that each molecule chains appeared only in the respective domains.

Insulator–metal phase transition ofPr0.6Ca0.4MnO3 studied by x-ray absorption spectroscopy in pulsed magnetic fields

Evolution of the Mn K-edge x-ray absorption near edge structure (XANES) inPr0.6Ca0.4MnO3 at pulsed magnetic fields has been investigated. A small enhancement of XANES spectra isdetected across the magnetic-field-induced transition from the charge- and orbital-ordered(COO) insulator to ferromagnetic metal at 20 K. It is found that the magnetic-fielddependence of the enhancement shows clear hysteresis, as seen in the magnetization withmetamagnetic transition, suggesting a significant correlation between the change in theXANES and the field-induced collapse of the COO state. The enhancement of theabsorption can be explained by an increase of the 4p density of states due to a reduction ofhybridization between the 4p state of the central Mn ion with the core hole and theneighboring Mn 3d state. Local structural change around Mn ions is expected to modifythe strength of the hybridization.

How to Evaluate the Structure of a Tetranuclear Mn Cluster from Magnetic and EXAFS Data: Case of the S2-State Mn-Cluster in Photosystem II

Abstract A theoretical method to extract structural information on spin-exchange-coupled manganese tetramers from the EPR spectroscopy data is presented. This method has been applied to two EPR data, i.e. the ground-state spin (S = 1/2) and the first excitation energy (30—37 cm−1), for the S2-state Mn tetramer in the photosynthetic oxygen-evolving complex, which exhibits a “g = 2 multiline” EPR signal. Based on the EXAFS data and the manganese chemistry, a simplified model spin Hamiltonian to describe the S2-state Mn cluster will be presented, such that two spin-exchange interactions due to 2.7—2.8 Å and/or 3.2—3.5 Å Mn–Mn bonds can vary from weak to strong coupling, sensitively, depending upon the bridge structure, except for a strong-antiferromagnetic interaction due to a 2.7 Å Mn(III)–Mn(IV) bond and the other weak ones. By computer-search of the possible spin-exchange structures with respect to these two parametric interactions, it was found that (1) a dimer of di-μ2-oxo bridged Mn dimers, a propeller-type tetramer in which the central Mn ion is chelated by three di-μ-oxo Mn cores and some other models are highly unlikely, (2) the most promising cluster is a trimer-plus-monomer type of distorted cubane, and (3) S* = 5/2 excited states are higher than the first excited state with S* = 3/2 (majority) or S* = 1/2 (minority).

A new manganese cluster topology capable of yielding high-spin species: mixed-valence [Mn7(OH)3Cl3(hmp)9]2 + with S ≥ 10

The reaction between MnCl 2 ·4H 2 O, Hhmp (Hhmp = 2-hydroxymethylpyridine) and NEt 4 MnO 4 in MeCN gives NEt 4 [Mn 7 (OH) 3 Cl 3 (hmp) 9 ]Cl[MnCl 4 ] 1; the 4 Mn II , 3 Mn III cation comprises a Mn 6 hexagon containing a central Mn ion, and it possesses a S ≥ 10 ground state.

Excited states of ZnS:Mn and ZnSe:Mn stabilized by the Jahn–Teller active E mode: Calculations by semiempirical self-consistent field methods

The splitting of the quartet excited states of Mn2+ doped in ZnS and ZnSe due to coupling with the normal E vibration of tetrahedral MnS4 and MnSe4 clusters is studied by semiempirical methods which neglect the differential overlap between atomic orbitals. Stable excited state geometries of the impurity centers are found only if (i) the second neighbor Zn ions are included in the treated clusters and (ii) the collective E vibration of all the atoms neighboring the central Mn ion is taken into account.