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Only a few four-coordinated Co 2+-complexes show single-molecule magnet (SMM) properties without an applied dc field. Common for those is a large magnetic anisotropy generated by the close proximity of the (Formula presented.) and (Formula presented.) orbitals. This type of magnetic anisotropy is strongly correlated with the extent of structural distortion from ideal tetrahedral towards linear coordination. Quantification of the governing magneto-structural correlations is hence crucial for the development of better SMMs. For this purpose, we synthesized and analyzed four significantly distorted tetrahedral cobalt complexes that exhibit extremely large magnetic anisotropies and slow relaxation of magnetization in zero field. The N−Co−N bite angles vary from 70.8 to 72.7°, and magnetic measurements show strong anisotropy with D-values in the range from (Formula presented.) 75 to (Formula presented.) 114 cm −1. Ab initio calculations supported the experimental results and highlighted a significant increase of D up to (Formula presented.) 141 cm −1, correlated with the reduction of the energy difference between the (Formula presented.) and (Formula presented.) orbitals. Based on these magneto-structural correlations and in contrast to the current assumption that the smaller bite angle is the better, we predict that with an ideal bite angle in the range from 76–78° in distorted Co(N 2R) 2 complexes, the energy difference between the two important d-orbitals is at a minimum and hence the magnetic anisotropy is maximized.

The development of methyl transverse relaxation optimized spectroscopy has greatly facilitated the study of macromolecular assemblies by solution NMR spectroscopy. However, limited sample solubility and stability has hindered application of this technique to ongoing studies of complexes formed on membranes by the neuronal SNAREs that mediate neurotransmitter release and synaptotagmin-1, the Ca2+ sensor that triggers release. Since the 1H NMR signal of a tBu group attached to a large protein or complex can be observed with high sensitivity if the group retains high mobility, we have explored the use of this strategy to analyze presynaptic complexes involved in neurotransmitter release. For this purpose, we attached tBu groups at single cysteines of fragments of synaptotagmin-1, complexin-1 and the neuronal SNAREs by reaction with 5-(tert-butyldisulfaneyl)-2-nitrobenzoic acid (BDSNB), tBu iodoacetamide or tBu acrylate. The tBu resonances of the tagged proteins were generally sharp and intense, although tBu groups attached with BDSNB had a tendency to exhibit somewhat broader resonances that likely result because of the shorter linkage between the tBu and the tagged cysteine. Incorporation of the tagged proteins into complexes on nanodiscs led to severe broadening of the tBu resonances in some cases. However, sharp tBu resonances could readily be observed for some complexes of more than 200 kDa at low micromolar concentrations. Our results show that tagging of proteins with tBu groups provides a powerful approach to study large biomolecular assemblies of limited stability and/or solubility that may be applicable even at nanomolar concentrations., We thank Ad Bax for the suggestion of exploring the use tBu groups as probes for structural studies of the neurotransmitter release machinery, and Ad Bax, Lewis Kay and Charampalos Kalodimos for fruitful discussions on this subject. The Agilent DD2 console of the 800 MHz spectromenter used for the research presented here was purchased with a shared instrumentation grant from the NIH (S10OD018027 to JR). Rashmi Voleti was supported by a fellowship from the Howard Hughes Medical Institute. The preparation of BDSNB was performed at the NANBIOSIS –CIBER BBN Peptide Synthesis Unit (U3). This work was supported by grant I-1304 from the Welch Foundation (to JR) and by NIH Research Project Award R35 NS097333 (to JR).

In this work we review the Co2 IIILn2 III family of compounds reported up to date that shows SMM features. The vast majority of them show a butterfly arrangement with LnIII ions placed at the body positions. The number of DyIII examples constitutes the majority, probably because of the intrinsic properties of this lanthanide ion regarding SMMs characteristics. Butterfly complexes have a SAP geometry around LnIII ions with a variety of distortion degrees. However, when looking at the key Ueff parameter trend through this family of complexes, it becomes clear that a pure geometrical consideration is not enough to explain it. Ab initio based calculations provide evidence of highly anisotropic mJ ground states for complexes where LnIII = DyIII, pointing to exclusively mJ = 15/2 ground Kramers doublets. These calculations also agree with the thermally activated relaxation involving the first excited state and, in some examples where multiple relaxation pathways are observed, higher excited states. A few examples prove that Raman and/or direct mechanisms cannot be discarded a priori. Only two examples of isostructural LnIII families of the same Co2 IIILn2 III complex can be found. Fil: Funes, Víctor Alejandro. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Química Inorgánica, Analítica y Química Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina Fil: Alborés, Pablo. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Química Inorgánica, Analítica y Química Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina

Herein we report a dinuclear [(μ-mbpymNO)- {(tmh)3Dy}2] (1) single-molecule magnet (SMM) showing two nonequivalent DyIII centers, which was rationally prepared from the reaction of Dy(tmh)3 moieties (tmh = 2,2,6,6-tetramethyl-3,5- heptanedionate) and the asymmetric bis-bidentate bridging ligand 4-methylbipyrimidine (mbpymNO). Depending on whether the DyIII ions coordinate to the N^O or N^N bidentate donor sets, the DyIII sites present a NO7 (D2d geometry) or N2O6 (D4d) coordination sphere. As a consequence, two different thermally activated magnetic relaxation processes are observed with anisotropy barriers of 47.8 and 54.7 K. Ab initio calculations confirm the existence of two different relaxation phenomena and allow one to assign the 47.8 and 54.7 K energy barriers to the Dy(N2O6) and Dy(NO7) sites, respectively. Two mononuclear complexes, [Dy(tta)3(mbpymNO)] (2) and [Dy(tmh)3(phenNO)] (3), have also been prepared for comparative purposes. In both cases, the DyIII center shows a NO7 coordination sphere and SMM behavior is observed with Ueff values of 71.5 K (2) and 120.7 K (3). In all three cases, ab initio calculations indicate that relaxation of the magnetization takes place mainly via the first excited-state Kramers doublet through Orbach, Raman, and thermally assisted quantum-tunnelling mechanisms. Pulse magnetization measurements reveal that the dinuclear and mononuclear complexes exhibit hysteresis loops with double- and single-step structures, respectively, thus supporting their SMM behavior.

Four complexes containing Dy-III and Pr-III ions and their Ln(III)-Zn-II analogs have been synthesized in order to study the influence that a diamagnetic Zn-II ion has on the electronic structure and hence, the magnetic properties of the Dy-III and Pr-III single ions. Single-crystal X-ray diffraction revealed the molecular structures as [Dy-III(HL)(2)(NO3)(3)] (1), [Pr-III(HL)(2)(NO3)(3)] (2), [(ZnDyIII)-Dy-II(L)(2)(CH3CO2)(NO3)(2)] (3) and [(Zn2PrIII)-Pr-II(L)(2)(CH3CO2)(4)(NO3)] (4) (where HL=2-methoxy-6-[(E)-phenyliminomethyl]phenol). The dc and ac magnetic data were collected for all four complexes. Compounds 1 and 3 display frequency dependent out-of-phase susceptibility signals ((M)), which is a characteristic signature for a single-molecule magnet (SMM). Although 1 and 3 are chemically similar, a fivefold increase in the anisotropic barrier (U-eff) is observed experimentally for 3 (83cm(-1)), compared to 1 (16cm(-1)). To rationalize the larger anisotropic barrier (1 vs. 3), detailed ab initio calculations were performed. Although the ground state Kramer's doublet in both 1 and 3 are axial in nature (g(zz)=19.443 for 1 and 18.82 for 3), a significant difference in the energy gap (U-eff) between the ground and first excited Kramer's doublet is calculated. This energy gap is governed by the electrostatic repulsion between the Dy-III ion and the additional charge density found for the phenoxo bridging ligand in 3. This extra charge density was found to be a consequence of the presence of the diamagnetic Zn-II ion present in the complex. To explore the influence of diamagnetic ions on the magnetic properties further, previously reported and structurally related Zn-Dy-III complexes were analyzed. These structurally analogous complexes unambiguously suggest that the electrostatic repulsion is found to be maximal when the Zn-O-Dy-O dihedral angle is small, which is an ideal condition to maximize the anisotropic barrier in Dy-III complexes.

The electrocapillarity and zero-frequency differential capacitance, Cd, have been studied using pendant drop method, at the Hg interface of an ionic liquid (IL), 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide, [C2mim+][TFSA−], and have been compared with those of [C2mim+]BF 4 − , an IL with the common cation and a different anion, to focus on the anion dependence of zero-frequency Cd. The Hg interface of [C2mim+][TFSA−], the IL of the larger anion in the present study, exhibits greater zero-frequency Cd than that of [C2mim+]BF 4 − , the IL of the smaller anion. This behavior contradicts a simple expectation in which larger ion leads to smaller Cd. This apparent contradiction is explained by proximity of the charged moiety of TFSA− to the electrode surface compared with that of BF 4 − . The potential dependence of zero-frequency Cd for the two ILs both exhibits one-hump camel shape around the potential of zero charge (Epzc), which has been predicted to be specific behavior of the electrical double layer of ILs by theory and simulation. The humps are located at potentials more negative than Epzc. From a mean-field lattice-gas theory for the EDL in ILs, this negative shift can be interpreted that the charged moiety for C2mim+ is more easily condensed in the EDL than those for BF 4 − and TFSA−.

Controlling quantum tunneling of magnetization (QTM) is a persistent challenge in lanthanide-based single-molecule magnets. As the exchange interaction is one of the key factors in controlling the QTM, we targeted lanthanide complexes with an increased number of radicals around the lanthanide ion. On the basis of our targeted approach, a family of pseudo-octahedral lanthanide/transition-metal complexes were isolated with the general molecular formula of [M(L-center dot(-))(3)] (M = Gd (1), Dy (2), Er (3), Y (4)) using the redox-active iminopyridyl (L-center dot(-)) ligand exclusively, which possess the highest ratio of radicals to lanthanide reported for discrete metal complexes. Direct current magnetic susceptibility studies suggest that dominant antiferromagnetic interactions exist between the radical and lanthanide ions in all of the complexes, which is strongly corroborated by magnetic data fitting using a Heisenberg-Dirac-Van Vleck (HDVV) Hamiltonian (-2J Hamiltonian). A good agreement between the fit and the experimental magnetic data obtained using g = 2, J(rad-rad) = -111.9 cm(-1) for 4 and g = 1.99, J(rad-rad) = -111.9 cm(-1), J(Gd-rad) = -1.85 cm(-1) for 1. Complex 2 shows frequency-dependent slow magnetization relaxation dynamics in the absence of an external magnetic field, while 3 shows field-induced frequency-dependent X-M" signals. An ideal octahedral geometry around the lanthanide ion is predicted to be unsuitable for the design of a single-molecule magnet (SMM); nevertheless, complex 2 exhibits slow relaxation of magnetization with a record high anisotropy barrier for a six-coordinate Dy(III) complex. A rationale for this unusual behavior is detailed and reveals the strength of the synthetic methodology developed.

Chiral bipyrimidine-bridged dinuclear LnIII complexes of general formula [(μ-bipym){((+)-tfacam)3Ln}2] and [(μ-bipym){((-)-tfacam)3Ln}2], have been prepared from the assembly of Ln(AcO)3·nH2O (LnIII = Dy, Gd), (+)/(−)-3-(trifluoroacetyl)camphor enantiopure ligands ((+)/(-)-Htfacam) and bipyrimidine (bipym). The structure and chirality of these complexes have been supported by single-crystal X-Ray diffraction and circular dichroism. The study of the magnetic properties of the GdIII complexes revealed a very weak antiferromagnetic interaction between the GdIII ions through the bipyrimidine bridging ligand. Ab initio CASSCF calculations indicated that the ground Kramers doublet (KD) of both DyIII centers is almost purely axial with the anisotropy axis located close to the two tfacam−ligands at opposite sides of each DyIIIatom, which create an axial crystal field. In keeping with this, ac dynamic measurements indicated slow relaxation of the magnetization at zero field with Ueff = 55.1 K, a pre-exponential factor of τo = 2.17·10−6 s and τQTM = 8 μs. When an optimal dc field of 0.1 T is applied, QTM is quenched and Ueff increases to 75.9 K with τo = 6.16 × 10−7 s. The DyN2O8 coordination spheres and SMM properties of [(μ-bipym){((+)-tfacam)3Ln}2] and their achiral [(Dy(b-diketonate)3)2(μ-bpym)]analogous have been compared and a magneto-structural correlation has been established, which has been supported by theoretical calculations., ID-O, JH, and EC are grateful to Ministerio de Economía y Competitividad (MINECO) for Project CTQ2014-56312-P, the Junta de Andalucía (FQM-195 and the Project of excellence P11-FQM-7756) and the University of Granada. A part of this work has been made at HFLSM, IMR, Tohoku University. IDO also acknowledges support by COLABS. GR would like to thank SERB (EMR/2014/000247) for financial support. SDthanks UGC for Senior Research Fellowship. AC thanks the Marie Curie COFUND Action from the European Commission for co-financing his postdoctoral fellowship.

Four isostructural (Ni2Ln2(CH3CO2)3(HL)4(H2O)2) 3 + (Ln 3+ =Dy (1), Tb (2), Ho (3 )o r Lu (4)) complexes and a di- nuclear (NiGd(HL)2(NO3)3 )( 5) complex are reported (where HL = 2-methoxy-6-((E)-2'-hydroxymethyl-phenyliminometh- yl)-phenolate). For compounds 1-3 and 5, the Ni 2 + ions are ferromagnetically coupled to the respective lanthanide ions. The ferromagnetic coupling in 1 suppresses the quantum tunnelling of magnetisation (QTM), resulting in a rare zero dc field Ni-Dy single-molecule magnet, with an anisotropy barrier Ueff of 19 K. Following the remarkable observation of single-molecule magnet (SMM) behaviour within a (Mn12OAc) cluster, numerous transition-metal SMM complexes have subsequently been re- ported. (1) The interest stems from the many potential applica- tions, such as storage of digital information, molecular qubits, and spin valves. (2) The two parameters that govern the effec

Four low-coordinate SIMs have been studied to probe their relaxation dynamics using ab initio calculations. Our calculations reveal that both the symmetry and the equatorial ligand field play a key role in controlling the barrier heights in three-coordinate [Ln(III)(NSiMe3)(3)] complexes (Ln = Dy/Er). This study reveals an unprecedented blockade of magnetization up to three excited states for the Er(III) complex.

We establish the coordination potential of the Schiff base ligand (2-methoxy-6-[(E)-2'-hydroxymethyl-phenyliminomethyl]-phenolate (H2L)) via the isolation of various M(II)-Ln(III) complexes (where M(II) = Ni or Zn and Ln(III) = La or Pr or Gd). Single crystals of these five complexes were isolated and their solid state structures were determined by single crystal X-ray diffraction. Structural determination revealed molecular formulae of [NiGd(HL)(2)(NO3)(3)] (1), [NiPr(HL)(2)(NO3)(3)] (2) and [Ni2La(HL)(4)(NO3)](NO3)(2) (3), [Zn2Gd(HL)(4)(NO3)](NO3)(2) (4), and [Zn2Pr(HL)(4)(NO3)](NO3)(2) (5). Complexes 1 and 2 were found to be neutral heterometallic dinuclear compounds, whereas 3-5 were found to be linear heterometallic tri-nuclear cationic complexes. Direct current (dc) magnetic susceptibility and magnetization measurements conclusively revealed that complexes 1 and 4 possess a spin ground state of S = 9/2 and 7/2 respectively. Empirically calculated Delta T-chi M derived from the variable temperature susceptibility data for all complexes undoubtedly indicates that the Ni(II) ion is coupled ferromagnetically with the Gd(III) ion, and anti-ferromagnetically with the Pr(III) ion in 1 and 2 respectively. The extent of the exchange interaction for 1 was estimated by fitting the magnetic susceptibility data using the parameters (g = 2.028, S = 9/2, J = 1.31 cm(-1) and zJ = +0.007), supporting the phenomenon observed in an empirical approach. Similarly using a HDVV Hamiltonian, the magnetic data of 3 and 4 were fitted, yielding parameters g = 2.177, D = 3.133 cm(-1), J = -0.978 cm(-1), (for 3) and g = 1.985, D = 0.508 cm(-1) (for 4). The maximum change in magnetic entropy (-Delta S-m) estimated from the isothermal magnetization data for 1 was found to be 5.7 J kg(-1) K-1 (Delta B = 7 Tesla) at 7.0 K, which is larger than the -Delta S-m value extracted from 4 of 3.5 J kg(-1) K-1 (Delta B = 7 Tesla) at 15.8 K, revealing the importance of the exchange interaction in increasing the overall ground state of a molecule for better MCE efficiency.

A family of mononuclear tetrahedral cobalt(II) thiourea complexes, [Co(L-1)(4)](NO3)(2) (1) and [Co(L-x)(4)](ClO4)(2) where x = 2 (2), 3 (3), 4 (4) (where L-1 = thiourea, L-2 = 1,3-dibutylthiourea, L-3 = 1,3-phenylethylthiourea, and L-4 = 1,1,3,3-tetramethylthiourea), has been synthesized using a rationally designed synthetic approach, with the aim of stabilizing an Ising-type magnetic anisotropy (-D). On the basis of direct-current, alternating-current, and hysteresis magnetic measurements and theoretical calculations, we have identified the factors that govern the sign and magnitude of D and ultimately the ability to design a single-ion magnet for a tetrahedral cobalt(II) ion. To better understand the magnetization relaxation dynamics, particularly for complexes 1 and 2, dilution experiments were performed using their diamagnetic analogues, which are characterized by single-crystal X-ray diffraction with the general molecular formulas of [Zn(L-1)(4)](NO3)(2) (5) and [Zn(L-2)(4)]-(ClO4)(2) (6). Interestingly, intermolecular interactions are shown to play a role in quenching the quantum tunneling of magnetization in zero field, as evidenced in the hysteresis loop of 1. Complex 2 exhibits the largest U-eff value of 62 cm(1) and reveals open hysteresis loops below 4 K. Furthermore, the influence of the hyperfine interaction on the magnetization relaxation dynamics is witnessed in the hysteresis loops, allowing us to determine the electron/nuclear spin S(Co) = 3/2/I(Co) = 7/2 hyperfine coupling constant of 550 MHz, a method ideally suited to determine the hyperfine coupling constant of highly anisotropic metal ions stabilized with large D value, which are otherwise hard to determine by conventional methods such as electron paramagnetic resonance.

The first supramolecular cage formed by three benzo-15-crown-5 macrocycles encapsulating a [Dy(OH2)(8)](3+) guest cation is reported, with the Dy(III) centre exhibiting local pseudo square antiprismatic D-4d symmetry. The anisotropy barrier extracted from ac susceptibility studies, emission spectroscopy and ab initio calculations reveals that the second excited state Kramers doublet plays a key role in the magnetization dynamics due to the Ising character and near coparallel nature of the ground and first excited Kramers doublets.

The monometallic pseudo-octahedral complex, [Co(H2O)2(CH3COO)2(C5H5N)2], is shown to exhibit slow magnetic relaxation under an applied field of 1500 Oe. The compound is examined bya combination of experimental and computational techniques in order to elucidate the nature of its electronic structure and slow magnetic relaxation. We demonstrate that any sensible model of the electronic structure must include a proper treatment of the first-order orbital angular momentum,and we find that the slow magnetic relaxation can be well described by a two-phonon Raman process dominating at high temperature, with a temperature independent quantum tunnelling pathway being most efficient at low temperature.

The reaction of hydrated nickel(II) salts (chloride or nitrate) and hydrated lanthanide nitrate salts with the Schiff base ligand 2-methoxy-6-[(E)-phenyliminomethyl] phenol (HL) in methanol resulted in the isolation of three isostructural linear heterometallic trinuclear complexes and a heterometallic tetranuclear complex. The molecular structures of these complexes were determined via single crystal X-ray diffraction revealing molecular structures of formulae [Ni2La(L-)(6)](NO3)(0.55)(OH)(0.45) (1), [Ni2Pr(L-)(6)]-(NO3)(0.48)(OH)(0.52) (2), [Ni2Tb(L-)(6)](NO3)(0.5)(Cl)(0.5) (3) and [Ni2Dy2(L-)(2)(o-vanillin)(2)(CO3)(2)(NO3)(2)(MeOH)(2)] (4). Structural analysis for 1-3 reveals that the lanthanide ion is sandwiched between two Ni(II) ions and the Ni center dot center dot center dot Ln center dot center dot center dot Ni metallic core displays a linear arrangement, with an average angle Ni center dot center dot center dot Ln center dot center dot center dot Ni bond angle of 179.7 degrees. Analysis of 4 reveals the metal ions are arranged such that two Ni-Dy subunits are bridged by two carbonate ligands via the Dy sites. Direct current magnetic susceptibility measurements for complexes 1-4 reveal that the Ni(II) ions are coupled ferromagnetically with the Tb(III) (3) and Dy(III) (4) ions, and antiferromagnetically with the Pr(III) ion (2). For complex 1 a long range intramolecular ferromagnetic interaction is witnessed between the Ni(II) ions (Ni center dot center dot center dot Ni = 6.873(9) angstrom) via a closed shell La(III) ion. The magnetic data of 1 were fitted using the HDVV Hamiltonian revealing the following parameters; J = +0.46 cm(-1), g = 2.245, D = +4.91 cm(-1). Alternating current magnetic susceptibility measurements performed on complexes 2-4 revealed that 3 and 4 displayed frequency dependent chi(M)'' signals (H-ac = 3.5 Oe and H-dc = 0 Oe) which is a characteristic signature of a single-molecule magnet behaviour.

© 2016 The Royal Society of Chemistry. We investigate the magnetic dilution effect on the relaxation mechanisms and the estimation of the energy barrier in a photo-luminescent Dy(III)/Y(III) based Single-Ion Magnet (SIM). The photo-luminescent spectroscopy permits the careful determination of the Orbach barrier, which is in a good agreement with the ab initio calculations. This barrier does not change upon magnetic dilution. The magnetic properties investigations highlight that the determination of the energy barrier is affected by magnetic dilution due to the changes in Quantum Tunnelling of the Magnetization (QTM). ispartof: RSC Advances vol:6 issue:110 pages:108810-108818 status: published

Ab initio CASSCF + RASSI-SO + SINGLE_ANISO and DFT based NBO and QTAIM investigations were carried out on a series of trigonal prismatic M(Bc(Me))(3) (M = Tb(1), Dy(2), Ho(3), Er(4), [Bc(Me)](-)= dihydrobis(methylimidazolyl)borate) and M(Bp(Me))(3) (M = Tb(1a), Dy(2a), Ho(3a), Er(4a) [Bp(Me)](-)= dihydrobis(methypyrazolyl)borate) complexes to ascertain the anisotropic variations of these two ligand field environments and the influence of Lanthanide-ligand bonding on the magnetic anisotropy. Among all the complexes studied, only 1 and 2 show large U-cal (computed energy barrier for magnetization reorientation) values of 256.4 and 268.5 cm(-1), respectively and this is in accordance with experiment. Experimentally only frequency dependent chi" tails are observed for complex 1a and our calculation predicts a large U(cal)of 229.4 cm(-1) for this molecule. Besides these, none of the complexes (3, 4, 2a, 3a and 4a) computed to possess large energy barrier and this is affirmed by the experiments. These observed differences in the magnetic properties are correlated to the Ln-Ligand bonding. Our calculations transpire comparatively improved Single-Ion Magnet (SIM) behaviour for carbene analogues due to the more axially compressed trigonal prismatic ligand environment. Furthermore, our detailed Mulliken charge, spin density, NBO and Wiberg bond analysis implied stronger Ln(...)H-BH agostic interaction for pyrazole analogues. Further, QTAIM analysis reveals the physical nature of coordination, covalent, and fine details of the agostic interactions in all the eight complexes studied. Quite interestingly, for the first time, using the Laplacian density, we are able to quantify the prolate and oblate nature of the electron clouds in lanthanides and this is expected to have a far reaching outcome beyond the examples studied.

Three new Dy complexes have been prepared according to a complex-as-ligand strategy. Structural determinations indicate that the central Dy ion is surrounded by two LZn units (L(2-) is the di-deprotonated form of the N2 O2 compartmental N,N'-2,2-dimethylpropylenedi(3-methoxysalicylideneiminato) Schiff base. The Dy ions are nonacoordinate to eight oxygen atoms from the two L ligands and to a water molecule. The Zn ions are pentacoordinate in all cases, linked to the N2 O2 atoms from L, and the apical position of the Zn coordination sphere is occupied by a water molecule or bromide or chloride ions. These resulting complexes, formulated (LZnX)-Dy-(LZnX), are tricationic with X=H2 O and monocationic with X=Br or Cl. They behave as field-free single-molecule magnets (SMMs) with effective energy barriers (Ueff ) for the reversal of the magnetization of 96.9(6) K with τ0 =2.4×10(-7) s, 146.8(5) K with τ0 =9.2×10(-8) s, and 146.1(10) K with τ0 =9.9×10(-8) s for compounds with ZnOH2 , ZnBr, and ZnCl motifs, respectively. The Cole-Cole plots exhibit semicircular shapes with α parameters in the range of 0.19 to 0.29, which suggests multiple relaxation processes. Under a dc applied magnetic field of 1000 Oe, the quantum tunneling of magnetization (QTM) is partly or fully suppressed and the energy barriers increase to Ueff =128.6(5) K and τ0 =1.8×10(-8) s for 1, Ueff =214.7 K and τ0 =9.8×10(-9) s for 2, and Ueff =202.4 K and τ0 =1.5×10(-8) s for 3. The two pairs of largely negatively charged phenoxido oxygen atoms with short DyO bonds are positioned at opposite sides of the Dy(3+) ion, which thus creates a strong crystal field that stabilizes the axial MJ =±15/2 doublet as the ground Kramers doublet. Although the compound with the ZnOH2 motifs possesses the larger negative charges on the phenolate oxygen atoms, as confirmed by using DFT calculations, it exhibits the larger distortions of the DyO9 coordination polyhedron from ideal geometries and a smaller Ueff value. Ab initio calculations support the easy-axis anisotropy of the ground Kramers doublet and predict zero-field SMM behavior through Orbach and TA-QTM relaxations via the first excited Kramers doublet, which leads to large energy barriers. In accordance with the experimental results, ab initio calculations have also shown that, compared with water, the peripheral halide ligands coordinated to the Zn(2+) ions increase the barrier height when the distortions of the DyO9 have a negative effect. All the complexes exhibit metal-centered luminescence after excitation into the UV π-π* absorption band of ligand L(2-) at λ=335 nm, which results in the appearance of the characteristic Dy(III) ((4) F9/2 →(6) HJ/2 ; J=15/2, 13/2) emission bands in the visible region.

Two mononuclear Dy-III crown ether complexes [Dy(15C5)(H2O)(4)](ClO4)(3)center dot(15C5)center dot H2O (1) and [Dy(12C4)(H2O)(5)](ClO4)(3)center dot H2O (2) have been prepared and characterized. X-ray diffraction studies show that both compounds crystallize as half sandwich type structures with muffin and pseudo-capped square antiprismatic geometries respectively. Despite the comparable local environments of the Dy-III ions they display remarkably different dynamic magnetic properties with only 1 displaying SMM properties in zero field. The solid state emission spectra for both 1 and 2 display sharp bands associated with f-f transitions. From the fine structure of the F-4(9/2) -> H-6(15/2) band, the Stark splitting of the H-6(15/2) ground state permitted the energy difference between the ground and first excited state to be determined. For 1 this value (Delta E = 58.0 +/- 3.0 cm(-1) ) is in excellent agreement with ab initio calculations and the experimentally observed SMM behaviour. For 2, the photoluminescence data and theoretical calculations support a less well isolated ground state (Delta E = 30 +/- 3.0 cm(-1)) in which a rapid relaxation process affords no SMM behaviour in zero-field.

Two lanthanide(III) phosphonates [Ln(notpH(4))(H2O)]ClO4 center dot 3H(2)O [Ln = Dy(1), Ho(2)] in which the lanthanide ion has a pseudo-D-5h symmetry have been reported. Both show layer structures where the neighbouring lanthanide atoms are connected by a pair of O-P-O bridges. Magnetic studies reveal that field-induced slow relaxation can be observed in both cases. Complex 1 is of particular interest because it shows not only field-tunable dual relaxation processes originating from the single ion anisotropy as well as the spin collective effect, but also simultaneous emissions from the metal ion, the ligand and the radiative energy transfer from the ligand to metal. The emission of Dy-III can be correlated to the magnetic data.

SMMs are often based on transition-metal clusters, but significant attention has recently focused on complexes of single and multiple lanthanoid ions, because the crystal-field splitting of the lowest Russell–Saunders multiplet engenders large magnetic anisotropies. [5–11] These anisotropies are responsible for high relaxation barriers and therefore slow magnetic relaxation. However, well isolated high-spin ground states are difficult to achieve within polynuclear lanthanoid SMMs because the valence 4f orbitals have limited radial extension and are usually energetically incompatible with ligand orbitals. These inherent 4f orbital properties give predominantly ionic interactions and results in weak magnetic exchange coupling with neighboring spin centers, with very few exceptions. [7, 12] In principle, actinoids, and in particular uranium, possess properties that render these ions ideal candidates from which to construct SMMs. This is because, compared to the lanthanoids, uranium exhibits enhanced crystal field splitting, [13, 14] as well as increased covalency, the latter enabling significant spin couplings in polynuclear systems, [15] and therefore both stronger magnetic exchange and anisotropies can be envisaged. This premise was realized recently with reports of several closely related single-ion pyrazolylborate uranium(III) complexes which were shown to display SMM behavior. [14, 16–20] In addition, two neptunium complexes were shown to display slow relaxation of the magnetization. [15, 21] The fact that all uranium(III) SMMs reported to date are closely related to each other raises the question as to how sensitive SMM behavior in trivalent uranium is to the composition of the coordination sphere and its symmetry. SMM behavior in all published examples is much more pronounced in an external field than in zero field, which suggests that quantum tunneling of the magnetization plays a significant role in shortening the relaxation times. However, in principle, low-symmetry crystal field components cannot induce tunneling of the magnetization, because uranium(III) is a Kramers half-integer angular momentum ion. Also the nuclear spin I = 0o f 238 U cannot induce tunneling of the

International audience; Different experimental techniques, such as kinetic studies of ligand binding and fluorescence correlation spectroscopy, have revealed that the diffusive, internal dynamics of proteins exhibits autosimilarity on the time scale from microseconds to hours. Computer simulations have demonstrated that this type of dynamics is already established on the much shorter nanosecond time scale, which is also covered by quasielastic neutron scattering experiments. The autorsimilarity of protein dynamics is reflected in long-time memory effects in the underlying diffusion processes, which lead to a non-exponential decay of the observed time correlation functions. Fractional Brownian dynamics is an empirical model which is able to capture the essential aspects of internal protein dynamics. Here we give a brief introduction into the theory and show how the model can be used to interpret neutron scattering experiments and molecular dynamics simulation of proteins in solution under hydrostatic pressure.

We investigated the spin dynamics of Zn(II)-doped “single-chain magnet” (SCM) Co(hfac)2NITPhOMe (in short CoPhOMe) by means of 1H NMR measurements. CoPhOMe was the first magnetic chain that showed a slowly relaxing magnetization (commonly called spins’ or magnetization “slowing down” mechanism), which follows the predictions of the Glauber model: the electronic spin freezing sets in before any transition to long-range magnetic order. The slowing down of the magnetization is also evidenced in the Zn-doped samples, which present chains that reach an average length of some tenth of spins for the highest doping, together with growing finite-size effects as the doping concentration is increased. The nuclear spin–lattice relaxation rate T1-1(T) at different applied magnetic fields, displayed a high-temperature peak and a low-temperature shoulder that become lower and shifts toward higher temperatures with increasing the field. This behaviour is qualitatively in agreement with the recent AC susceptibility data that revealed the Glauber dynamics and finite-size effects, like the collective reversal of the spins of each chain segment.

International audience; This paper presents a study of the influence of non-denaturing hydrostatic pressure on the relaxation dynamics of lysozyme in solution, which combines molecular dynamics simulations and quasielastic neutron scattering experiments. We compare results obtained at ambient pressure and at 3 kbar. Experiments have been performed at pD 4.6 and at a protein concentration of 60 mg/ml. For both pressures we checked the monodispersity of the protein solution by small angle neutron scattering. To interpret the simulation results and the experimental data, we adopt the fractional Ornstein–Uhlenbeck process as a model for the internal relaxation dynamics of the protein. On the experimental side, global protein motions are accounted for by the model of free translational diffusion, neglecting the much slower rotational diffusion. We find that the protein dynamics in the observed time window from about 1 to 100 ps is slowed down under pressure, while its fractal characteristics is preserved, and that the amplitudes of the motions are reduced by about 20%. The slowing down of the relaxation is reduced with increasing q-values, where more localized motions are seen.

Reaction of [FeIII(bipy)(CN)4]¯ with fully solvated MII cations [M = Co (1) and Mn (2)] produces the isostructural bis double zigzag chains [[FeIII(bipy)(CN)4]2MII(H2O)]·MeCN·1/2H2O; 1 exhibits intrachain ferromagnetic and interchain antiferromagnetic couplings, slow magnetic relaxation and hysteresis effects. Luminita Marilena, Toma, Luminita.Toma@uv.es ; Lescouezec, Alain Francois Rodri, Alain.Lescouezec@uv.es ; Lloret Pastor, Francisco, Francisco.Lloret@uv.es ; Julve Olcina, Miguel, Miguel.Julve@uv.es

A peculiar slow relaxation of low-temperature photoconductivity in semi-insulating GaAs is analysed. Time evolutions of free electron concentration, n, and free hole concentration, p, are determined from independently measured photoconductivity and current due to the thermoelectric effect. It is shown that during low-temperature illumination the p/n ratio at first rises significantly and then decreases back towards the initial, low value. The same behaviour is obtained from a study of time evolution of photo-Hall mobility measured independently. Results are analysed and explained in terms of different trapping rates of photogenerated free electrons and holes by deep traps (15 References)

We present a study of the relaxation dynamics of the photoexcited conductivity of the impurity states in the low-density electronic glass, phosphorous-doped silicon Si:P. Using optical pump-terahertz probe spectroscopy we find strongly temperature and fluence dependent glassy power-law relaxation occurring over sub-ns time scales. Such behavior is in contrast to the much longer time scales found in higher electron density glassy systems. We also find evidence for both multi-particle relaxation mechanisms and/or coupling to electronic collective modes and a low temperature quantum relaxational regime., 4 pages, 4 figures, Appeared in Phys. Rev. Lett