20 research outputs found
Il trattamento sanitario su minore o incapace: il miglior interesse del paziente vulnerabile fra (piĂč) volontĂ e scienza.
New
{TbCu<sub>3</sub>} and {DyCu<sub>3</sub>} single-molecule magnets
(SMMs) containing a low-symmetry Ln<sup>III</sup> center (shape measurements
relative to a trigonal dodecahedron and biaugmented trigonal prism
are 2.2â2.3) surrounded by three Cu<sup>II</sup> metalloligands
are reported. SMM behavior is confirmed by frequency-dependent out-of-phase
ac susceptibility signals and single-crystal temperature and sweep
rate dependent hysteresis loops. The ferromagnetic exchange interactions
between the central Ln<sup>III</sup> ion and the three Cu<sup>II</sup> ions could be accurately measured by inelastic neutron scattering
(INS) spectroscopy and modeled effectively. The excitations observed
by INS correspond to flipping of Cu<sup>II</sup> spins and appear
at energies similar to the thermodynamic barrier for relaxation of
the magnetization, âŒ15â20 K, and are thus at the origin
of the SMM behavior. The magnetic quantum number <i>M</i><sub>tot</sub> of the cluster ground state of {DyCu<sub>3</sub>}
is an integer, whereas it is a half-integer for {TbCu<sub>3</sub>},
which explains their vastly different quantum tunneling of the magnetization
behavior despite similar energy barriers
Closely-Related Zn(2)(II)Ln(2)(III) Complexes (Ln(III) = Gd, Yb) with Either Magnetic Refrigerant or Luminescent Single-Molecule Magnet Properties
The reaction of the compartmental ligand N,NâČ,Nâł-trimethyl-N,Nâł-bis(2-hydroxy-3-methoxy-5-methylbenzyl)diethylenetriamine (H2L) with Zn(NO3)2·6H2O and subsequently with Ln(NO3)3·5H2O (LnIII = Gd and Yb) and triethylamine in MeOH using a 1:1:1:1 molar ratio leads to the formation of the tetranuclear complexes {(ÎŒ3-CO3)2[Zn(ÎŒ-L)Gd(NO3)]2}·4CH3OH (1) and{(ÎŒ3-CO3)2[Zn(ÎŒ-L)Yb(H2O)]2}(NO3)2·4CH3OH (2). When the reaction was performed in the absence of triethylamine, the dinuclear compound [Zn(ÎŒ-L)(ÎŒ-NO3)Yb(NO3)2] (3) is obtained. The structures of 1 and 2 consist of two diphenoxo-bridged ZnIIâLnIII units connected by two carbonate bridging ligands. Within the dinuclear units, ZnII and LnIII ions occupy the N3O2 inner and the O4 outer sites of the compartmental ligand, respectively. The remaining positions on the LnIII ions are occupied by oxygen atoms belonging to the carbonate bridging groups, by a bidentate nitrate ion in 1, and by a coordinated water molecule in 2, leading to rather asymmetric GdO9 and trigonal dodecahedron YbO8 coordination spheres, respectively. Complex 3 is made of acetateâdiphenoxo triply bridged ZnIIYbIII dinuclear units, where the YbIII exhibits a YbO9 coordination environment. Variable-temperature magnetization measurements and heat capacity data demonstrate that 1 has a significant magnetoâcaloric effect, with a maximum value of âÎSm = 18.5 J kgâ1 Kâ1 at T = 1.9 K and B = 7 T. Complexes 2 and 3 show slow relaxation of the magnetization and single-molecule magnet (SMM) behavior under an applied direct-current field of 1000 Oe. The fit of the high-temperature data to the Arrhenius equation affords an effective energy barrier for the reversal of the magnetization of 19.4(7) K with Ïo = 3.1 Ă 10â6 s and 27.0(9) K with Ïo = 8.8 Ă 10â7 s for 2 and 3, respectively. However, the fit of the full range of temperature data indicates that the relaxation process could take place through a Raman-like process rather than through an activated Orbach process. The chromophoric L2â ligand is able to act as an âantennaâ group, sensitizing the near-infrared (NIR) YbIII-based luminescence in complexes 2 and 3 through an intramolecular energy transfer to the excited states of the accepting YbIII ion. These complexes show several bands in the 945â1050 nm region, corresponding to 2F5/2â2F7/2 transitions arising from the ligand field splitting of both multiplets. The observed luminescence lifetimes Ïobs are 0.515 and 10 ÎŒs for 2 and 3, respectively. The shorter lifetime for 2 is due to the presence of one coordinated water molecule on the YbIII center (and to a lesser extent noncoordinated water molecules), facilitating vibrational quenching via OâH oscillators. Therefore, complexes 2 and 3, combining field-induced SMM behavior.Financial support from Ministerio de EconomiÌay Competitividad (MINECO) for Projects CTQ-2011-24478 and MAT2012-38318-C03-01, the Junta de AndaluciÌa (FQM-195 and the Project of Excellence P11-FQM-7756), and the University of Granada is acknowledged. E.K.B. thanks the EPSRC for funding. S.J.A.P. thanks Cardiff University and the EPSRC. G.L. acknowledges EU for a Marie Curie IEF (PIEFGA- 2011-299356).Peer Reviewe