Enhancement of Magneto-Chiral Dichroism Intensity by Chemical Design: The Key Role of Magnetic-Dipole Allowed Transitions.

Here we report on the strong magneto-chiral dichroism (MChD) detected through visible and near-infrared light absorption up to 5.0 T on {Er5Ni6} metal clusters obtained by reaction of enantiopure chiral ligands and NiII and ErIII precursors. Single-crystal diffraction analysis reveals that these compounds are 3d-4f heterometallic clusters, showing helical chirality. MChD spectroscopy reveals a high gMChD dissymmetry factor of ca. 0.24 T-1 (T = 4.0 K, B = 1.0 T) for the 4I13/2 ← 4I15/2 magnetic-dipole allowed electronic transition of the ErIII centers. This record value is 1 or 2 orders of magnitude higher than that of the d-d electronic transitions of the NiII ions and the others f-f electric-dipole induced transitions of the ErIII centers. These findings clearly show the key role that magnetic-dipole allowed transitions have in the rational design of chiral lanthanide systems showing strong MChD.

Magneto-Chiral Dichroism in a One-Dimensional Assembly of Helical Dysprosium(III) Single-Molecule Magnets.

Here we report magneto-chiral dichroism (MChD) detected through visible and near-infrared light absorption of a chiral dysprosium(III) coordination polymer. The two enantiomers of [DyIII(H6(py)2)(hfac)3]n [H6(py)2 = 2,15-bis(4-pyridyl)ethynylcarbo[6]helicene; hfac- = 1,1,1,5,5,5-hexafluoroacetylacetonate], where the chirality is provided by a functionalized helicene ligand, were structurally, spectroscopically, and magnetically investigated. Magnetic measurements reveal a slow relaxation of the magnetization, with differences between enantiopure and racemic systems rationalized on the basis of theoretical calculations. When the enantiopure complexes are irradiated with unpolarized light in a magnetic field, they exhibit multiple MChD signals associated with the f-f electronic transitions of DyIII, thus providing the coexistence of MChD-active absorptions and single-molecule-magnet (SMM) behavior. These findings clearly show the potential that rationally designed chiral SMMs have in enabling the optical readout of magnetic memory through MChD.

Magnetic 3d-4f Chiral Clusters Showing Multimetal Site Magneto-Chiral Dichroism.

Here, we report the molecular self-assembly of hydroxido-bridged {Ln5Ni6} ((Ln3+ = Dy3+, Y3+) metal clusters by the reaction of enantiopure chiral ligands, namely, (R/S)-(2-hydroxy-3-methoxybenzyl)-serine), with NiII and LnIII precursors. Single-crystal diffraction analysis reveals that these compounds are isostructural sandwich-like 3d-4f heterometallic clusters showing helical chirality. Direct current magnetic measurements on {Dy5Ni6} indicates ferromagnetic coupling between DyIII and NiII centers, whereas those on {Y5Ni6} denote that the NiII centers are antiferromagnetically coupled and/or magnetically anisotropic. Magneto-chiral dichroism (MChD) measurements on {Dy5Ni6} and its comparison to that of {Y5Ni6} provide the first experimental observation of intense multimetal site MChD signals in the visible-near-infrared region. Moreover, the comparison of MChD with natural and magnetic circular dichroism spectra unambiguously demonstrate for the first time that the MChD signals associated with the NiII d-d transitions are mostly driven by natural optical activity and those associated with the DyIII f-f transitions are driven by magnetic optical activity.

Helicene-Based Ligands Enable Strong Magneto-Chiral Dichroism in a Chiral Ytterbium Complex.

Here we report the first experimental observation of magneto-chiral dichroism (MChD) detected through light absorption in an enantiopure lanthanide complex. The P and M enantiomers of [YbIII((X)-L)(hfac)3] (X = P, M; L = 3-(2-pyridyl)-4-aza[6]-helicene; hfac = 1,1,1,5,5,5-hexafluoroacetylacetonate), where the chirality is held by the helicene-based ligand, were studied in the near-infrared spectral window. When irradiated with unpolarized light in a magnetic field, these chiral complexes exhibit a strong MChD signal (gMChD ca. 0.12 T-1) associated with the 2F5/2 ← 2F7/2 electronic transition of YbIII. The low temperature absorption and MChD spectra reveal a fine structure associated with crystal field splitting and vibronic coupling. The temperature dependence of the main dichroic signal detected up to 150 K allowed, for the first time, the disentanglement of the two main microscopic contributions to the dichroic signal predicted by the MChD theory. These findings pave the way toward probing MChD in chiral lanthanide-based single-molecule magnets.

Magnetic Anisotropy Drives Magnetochiral Dichroism in a Chiral Molecular Helix Probed with Visible Light.

Magnetochiral dichroism (MChD) is a nonreciprocal manifestation of light-matter interaction that can be observed in chiral magnetized systems. It features a differential absorption of unpolarized light depending on the relative orientation of the magnetic field and the light wavevector and on the absolute configuration of the system. The relevance of this effect for optical readout of magnetic data calls for a complete understanding of the microscopic parameters driving MChD with an easy-accessible and nondamaging light source, such as visible light. For this purpose, here we report on MChD detected with visible light on a chiral magnetic helix formulated as [MnIII(cyclam)(SO4)]ClO4·H2O (cyclam = 1,4,8,11-tetraazacyclotetradecane) featuring antiferromagnetically coupled anisotropic MnIII ions. Alternate current susceptibility measurements revealed the existence of a single-chain magnet behavior hidden below the canted antiferromagnetism (TN = 5.8 K) already evidenced by direct current magnetometry. A detailed analysis of the optical absorption gives access to the value of the zero-field splitting parameter D (2.9 cm-1), which quantifies the magnetic anisotropy of the MnIII centers. Below the magnetic ordering temperature of the material, the MChD spectra exhibit intense absolute configuration dependent MChD signals reaching record values of ca. 12% of the absorbed intensity for the two electronic transitions most influenced by the spin-orbit coupling of the MnIII ion. These findings set a clear route toward the design and preparation of highly MChD-responsive molecular materials.

A Chiral Prussian Blue Analogue Pushes Magneto-Chiral Dichroism Limits.

Here we report on magneto-chiral dichroism (MChD) detected with visible light on the chiral Prussian Blue Analogue [MnII(X-pnH)(H2O)][CrIII(CN)6]·H2O (X = S, R; pn = 1,2-propanediamine). Single crystals suitable for magneto-optical measurements were grown starting from enantiopure chiral ligands. X-ray diffraction and magnetic measurements confirmed the 2D-layered structure of the material, its absolute configuration, and its ferrimagnetic ordered state below a critical temperature TC of 38 K. Absorption and MChD spectra were measured between 450 and 900 nm from room temperature down to 4 K. At 4 K the electronic spectrum features spin-allowed and spin-forbidden transitions of CrIII centers, spin-forbidden transitions of the MnII centers, and metal-to-metal charge transfer bands. The MChD spectra below the magnetic ordering temperature exhibit intense absolute configuration-dependent MChD signals. The temperature dependence of these signals closely follows the material magnetization. Under a magnetic field of 0.46 T, the most intense contribution to MChD represents 2.6% T-1 of the absorbed intensity, one of the highest values observed to date.