Functional (Supra)Molecular Nanostructures - ruben-group

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Habilitation Dr. Mario Ruben ULP Strasbourg susceptibility measurements on powder samples of Dy 3+ - and Tb 3+ double-decker complexes obtaining values for the effective barrier height of 28 cm -1 (40 K) and 230 cm -1 (330 K), respectively, which are significantly larger than those known for 3d-based SMMs. [38] Figure 21. Molecular structure (left), magnetic coupling scheme and hysteresis (right) of the Mn12 family exhibiting magnetic hysteresis and quantum tunnelling of magnetization (QTM). [35][36] The Dysprosium double-decker compound [Dy(Pc)2] - (TBA) + (Pc = dianion of phthalocyanine; TBA = N(C4H9)4 + ) was synthesized following a protocol described by Weiss et. al.. [39] The compound crystallizes in the monoclinic space group P21 with two formula units per elementary cell and with the following unit cell parameters: a = 11.41 Å, b= 22.44 Å, c = 13.44 Å, β = 107.46°. The molecular pseudo C2-axes of the double-decker anions are aligned in a strictly parallel way along the crystallographic [100] axis and the n-butyl groups of the TBA-cations are structurally disordered over two positions, which have been refined with 50% occupancy each. In the crystal lattice, the molecules are arranged in infinite stacks with alternating double-decker anions [Dy(Pc)2] - and TBA + cations (Figure 22). Within the stacks, the Dy 3+ -Dy 3+ distance is with 11.4 Å noticeably shorter than the interstack distances (13.1 - 14.8 Å). Relatively weak C-H---π (3.45 - 3.95 Å) were found between the butyl chains of the TBA + cations and coordinating isoindoline groups and imine nitrogens of the Pc ligands. [40] These interactions provides a possible relatively weak intermolecular exchange pathway between the [Dy(Pc)2] - molecules along the stacks via the TBA + cations. 33
Habilitation Dr. Mario Ruben ULP Strasbourg (TBA) + a) - b) c) Figure 22. a) Schematic and b) single crystal X-ray structure of the [Dy(Pc)2] - (TBA) + molecule. c) Representation of the crystal packing diagram with the indicated intra- and interstack Dy 3+ -Dy 3+ distances (arrows) and the most important C-H---π interactions (dotted lines). The butyl groups of the TBA + cation are structurally disordered and only one of the equivalent positions (50 % occupancy) is shown. Dy The magnetization of single crystal samples was investigated along the [100], [010], and [001] directions by SQUID measurements up to 5.5 T at 1.8 K. The magnetic data can be interpreted by the presence of an easy-magnetisation axis and a hard magnetic plane (Figure 23). A two parameter fit of the three magnetization curves along the three different orientations using the Hamiltonian (eq. 1 in Figure 23) gives D = 3B 2 0 = -5.0 K and E = B 2 2 = 0.7 K. The resulting barrier height of U = 281 K suggest SMM behaviour, although at much higher temperatures than observed (4K). However, a more realistic barrier height can be concluded from the energy separation between the two lowest levels of the ground state multiplet with Ueff = 70 K, which results following TB = U/ln(t/τ0) = 4K; a value close to the observed blocking temperature. Single crystal magnetization measurements were also performed using 2DEG GaAs/GaAlAs hall sensors. [41] At low temperatures, the magnetization versus magnetic field 34
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