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Bisphthalocyaninato terbium complexes show a long magnetization relaxation time at relatively high temperatures-which makes them very interesting as magnets at single-molecule level. Their technological exploitation, however, requires the addressing of the individual molecules, therefore the deposition of single-molecule magnets on surfaces is a topic of great interest as the interaction with the substrate can play a crucial role in the definition of the molecule properties. In this work we investigate the electronic and magnetic properties of anionic and neutral forms of a bis(phthalocyaninato)terbium derivative deposited on graphite by means of x-ray absorption spectroscopy and x-ray magnetic circular dichroism, performed at low temperature and high magnetic field at the M4,5edge of Tb. We were able to reproduce the experimental spectra by means of multiplet calculations and to validate the applicability of sum rules to the present case. Sum rules were then used for determining the orbital and spin moments of thick (several monolayers) and of thin films (submonolayer range). Calculations of spectra as a function of the molecule orientation with respect to the impinging x-ray beam, allowed us to ascertain the adsorption geometry of molecules. For both compounds, molecules stay essentially flat when adsorbing as thin film on graphite. This result is also confirmed by scanning probe microscopy, which also finds a very interesting ordered arrangement for the molecules of the neutral form. In the thick film of the neutral compound the molecules keep the same orientational order, arranging almost flat as well. On the contrary, in the thick film of the anionic compound their orientation appears to be random. The origin of this different behavior can be related to the hindrance of the counterion moiety and/or to the different solvent used for each compound. Finally, the comparison of the magnetization values and their dependence on the external magnetic field and temperat

We deeply investigated the properties of submonolayer distributions of isolated molecular Cr7 Ni rings deposited on Au(111) by liquid phase. X-ray absorption spectra measured at the Cr and Ni L2,3 edges show that the grafting of the Cr7 Ni rings onto the gold surface does not affect the oxidation state and the local symmetry of the Cr and Ni sites. The circular dichroism shows a change in sign of the Ni magnetic moment. This is due to a reduction in the exchange coupling constants that, however, preserves the structure of the low-energy levels of the grafted rings, as corroborated by spin-Hamiltonian simulations and comparison with measurements on bulk sample. Density-functional theory calculations show that the Ni-Cr bond gets weaker with slight ring distortion suggesting possible explanation for the observed magnetic behavior. These results show that complex magnetic molecules can be grafted onto surfaces, and that changes in their magnetic behavior must be examined in individual cases. © 2009 The American Physical Society

Single molecule magnets (SMMs) form a new class of magnetic materials consisting of identical nanoscale particles that can show magnetization in the absence of a magnetic field. We have experimentally and theoretically investigated the low-spin (S=4) member of the Mn6 SMM family, properly functionalized with two 3-thiophenecarboxylate (3tpc) ligands in order to graft it on to a Au(111) surface. We report the theoretical density of states calculated within the local density approximation (LDA) scheme accounting for the on-site Coulomb repulsion (LDA+U) for U values ranging from 0 to 8 eV. On the experimental side, by exploiting resonant photoemission at the Mn 2p edge, we were able to single out the Mn 3d derived states in the valence band energy region for a submonolayer distribution of Mn6-3tpc deposited on Au(111). From the comparison between the experimentally derived 3d density of states and the theoretical one, we found that the best agreement occurs for a U value of 4 eV. From the binding energy of Mn 2 p3/2 core line, measured in situ, we also derived a value for the 2p-3d correlation energy of about 5 eV -in agreement with previous determination. © 2008 The American Physical Society

We investigated the electronic and magnetic properties of thick films of molecular Cr8 and Cr7 Ni antiferromagnetic rings by means of x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD). We determined the local symmetries, the electronic configuration, and the values of orbital and spin moments at the Cr and Ni sites of the molecular rings. XMCD measurements show that the correlation between the Cr and Ni spins in the Cr7 Ni molecular ring switches from antiferromagnetic to ferromagnetic with increasing temperature. Experimental data are interpreted using XMCD sum rules that allow the separate evaluation of the spin and the orbital contributions to the total magnetic moment of the ring as a function of temperature and magnetic field. The magnetic behaviors experimentally observed are compared with the results of spin-Hamiltonian calculations, based on microscopic parameters derived by inelastic-neutron scattering and low-temperature specific-heat measurements. The very good agreement between experimental data and calculations is a clear indication of the integrity of molecules. The temperature dependence of the ion magnetic moments results from the interplay between Zeeman and isotropic-exchange contributions, and is well captured by the theoretical model. © 2008 The American Physical Society

We study the magnetic properties of two new functionalized single-molecule magnets belonging to the Mn6 family (general formula [Mn III6O2(R-sao)6(O2C-th) 2L4-6, where R = H (1) or Et (2), HO2C-th = 3-thiophene carboxylic acid, L = EtOH, H2O and saoH2 is salicylaldoxime) and their grafting on the Au(111) surface. Complex 1 exhibits spin ground-state S = 4, as the result of ferromagnetic coupling between the two antiferromagnetic MnIII3 triangles, while slight structural changes in complex 2, switch the dominant magnetic exchange interactions from anti- to ferromagnetic, enhancing the spin ground-state to S = 12 and, consequently, the effective energy barrier for the relaxation of magnetization. Direct-current and alternating-current magnetic susceptibility measurements show that the functionalized complexes preserve the main magnetic properties of the corresponding not-functionalized Mn6 clusters (i.e., total spin value and magnetic behavior as a function of temperature), though a reduction of the anisotropy barrier is observed in complex 2. For both complexes, the -O2C-th functionalization allows the direct grafting on Au(111) surface by liquid-phase deposition. X-ray photoemission spectroscopy demonstrates that the stoichiometry of the molecular cores is preserved after grafting. Scanning tunneling microscopy (STM) reveals a sub-monolayer distribution of isolated clusters with a slightly higher coverage for complex 1. The cluster stability in the STM images and the S-2p energy positions demonstrate, for both derivatives, the strength of the grafting with the gold surface. © 2008 American Chemical Society

Learning the art of exploiting the interplay between different units at the atomic scale is a fundamental step in the realization of functional nano-architectures and interfaces. In this context, understanding and controlling the magnetic coupling between molecular centers and their environment is still a challenging task. Here we present a combined experimental-theoretical work on the prototypical case of the bis(phthalocyaninato)-lanthanide(III) (LnPc2) molecular nanomagnets magnetically coupled to a Ni substrate. By means of X-ray magnetic circular dichroism we show how the coupling strength can be tuned by changing the Ln ion. The microscopic parameters of the system are determined by ab-initio calculations and then used in a spin Hamiltonian approach to interpret the experimental data. By this combined approach we identify the features of the spin communication channel: the spin path is first realized by the mediation of the external (5d) electrons of the Ln ion, keeping the characteristic features of the inner 4 f orbitals unaffected, then through the organic ligand, acting as a bridge to the external world.

Structural, electronic, and magnetic properties of [Mn 12O 12(bet) 16(EtOH) 4](PF 6) 14·4CH 3CN·H 2O (in short Mn 12bet, bet = betaine = +N(CH 3) 3-CH 2-COO -) single-molecule magnets (SMMs) deposited on previously functionalized gold surfaces have been investigated. Self-assembled monolayers (SAMs) either of sodium mercaptoethanesulfonate (MES) or mercaptopropionic acid (MPA) are used as functionalization to avoid the direct interaction between the Mn 12bet molecules and the Au surface with the aim of preserving the main functional properties of the molecules. Scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy (XPS) analysis show deposited Mn 12bet SMMs well-isolated from each other and uniformly distributed on both MES and MPA SAMs. X-ray absorption spectroscopy (XAS) studies show that the oxidation state of the mixed-valence Mn 12bet core is largely reduced to Mn 2+ when molecules are deposited on MES-SAM, whereas in the case of MPA-SAM the relative weights of Mn 2+, Mn 3+, and Mn 4+ in the Mn 12bet core are preserved. Despite the substantial retaining of their electronic properties, the magnetization of Mn 12bet molecules deposited on MPA-SAM measured by X-ray magnetic circular dichroism (XMCD) is perturbed with respect to the pristine molecules. © 2012 American Chemical Society

The two-dimensional self-assembly of a terbium(III) double-decker phthalocyanine on highly oriented pyrolitic graphite (HOPG) was studied by atomic force microscopy (AFM), and it was shown that it forms highly regular rectangular two-dimensional nanocrystals on the surface, that are aligned with the graphite symmetry axes, in which the molecules are organized in a rectangular lattice as shown by scanning tunneling microscopy. Molecular dynamics simulations were run in order to model the behavior of a collection of the double-decker complexes on HOPG. The results were in excellent agreement with the experiment, showing that-after diffusion on the graphite surface-the molecules self-assemble into nanoscopic islands which align preferentially along the three main graphite axes. These low dimension assemblies of independent magnetic centers are only one molecule thick (as shown by AFM) and are therefore very interesting nanoscopic magnetic objects, in which all of the molecules are in interaction with the graphite substrate and might therefore be affected by it. The magnetic properties of these self-assembled bar-shaped islands on HOPG were studied by X-ray magnetic circular dichroism, confirming that the compounds maintain their properties as single-molecule magnets when they are in close interaction with the graphite surface. © 2011 American Chemical Society

We report on a comparative study of electronic and magnetic properties of Mn6single-molecule magnets (SMMs) grafted on gold surface. Two derivatives with spin-ground states S = 4 and S = 12 have been functionalized with 3-tp-CO2-(3-thiophene carboxylate, tpc) ligands and characterized as thick films (TFs) as well as sub-monolayers (sMLs) by synchrotron based techniques. X-ray absorption spectroscopy at the Mn L2,3edges shows the modification of the spectral lineshape in the sMLs with respect to the TFs suggesting that the local symmetry at the Mn sites changes once the molecules are deposited on gold surface. In spite of this, the expected MnIIIoxidation state is preserved. X-ray magnetic circular dichroism (XMCD) spectra show that the total magnetic moment is only given by spin part because of the quenched orbital moment. Moreover, variable temperature and variable field XMCD spectra reveal an effective decrease of the Mn spin moment for both derivatives. © 2010 The Royal Society of Chemistry