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.

Induced circular dichroism from helicoidal nano substrates to porphyrins: the role of chiral self-assembly.

Because the combination of chiral and magnetic properties is becoming more and more attractive for magneto-chiral phenomena, we here aim at exploring the induction of chirality to achiral magnetic molecules as a strategy for the preparation of magneto-chiral objects. To this end, we have associated free base- and metallo-porphyrins with silica nano helices, using a variety of elaboration methods, and have studied them mainly by electronic natural circular dichroism (NCD) and magnetic circular dichroism (MCD) spectroscopies. While electrostatic or covalent surface grafting uniformly yielded very low induced CD (ICD) for the four assayed porphyrins, a moderate response was observed when the porphyrins were incorporated into the interior of the double-walled helices, likely due to the association of the molecules with the chirally-organized gemini surfactant. A generally stronger, but more variable, ICD was observed when the molecules were drop casted onto the helices immobilised on a quartz plate, likely due to the different capacities of the porphyrins to aggregate into chiral assemblies. Electronic spectroscopy, electron microscopy and IR spectroscopy were used to interpret the patterns of aggregation and their influence on ICD and MCD. No enhancement of MCD was observed as a result of association with the nanohelices except in the case of the free base, 5,10,15,20-tetra-(4-sulfonatophenyl)porphyrin (TPPS). This nanocomposite demonstrated a large ICD in the Soret region and a large MCD in the Q-region due to J-aggregation. However, no induced MChD was observed, possibly due to the spectral mismatch between the ICD and MCD peaks.

Traveling Wave Enantioselective Electron Paramagnetic Resonance.

We propose a novel method for enantioselective electron paramagnetic resonance (EPR) spectroscopy based on magneto-chiral anisotropy. We elaborate a theoretical model to estimate the strength of this effect and propose a dedicated interferometer setup for its experimental observation.

Multifunctional Helicene-Based Ytterbium Coordination Polymer Displaying Circularly Polarized Luminescence, Slow Magnetic Relaxation and Room Temperature Magneto-Chiral Dichroism.

The combination of physical properties sensitive to molecular chirality in a single system allows the observation of fascinating phenomena such as magneto-chiral dichroism (MChD) and circularly polarized luminescence (CPL) having potential applications for optical data readout and display technology. Homochiral monodimensional coordination polymers of YbIII were designed from a 2,15-bis-ethynyl-hexahelicenic scaffold decorated with two terminal 4-pyridyl units. Thanks to the coordination of the chiral organic chromophore to Yb(hfac)3 units (hfac- =1,1,1,5,5,5-hexafluoroacetylaconate), efficient NIR-CPL activity is observed. Moreover, the specific crystal field around the YbIII induces a strong magnetic anisotropy which leads to a single-molecule magnet (SMM) behaviour and a remarkable room temperature MChD. The MChD-structural correlation is supported by computational investigations.

Dielectric magnetochiral anisotropy.

The combination of chirality and magnetism has steadily grown over the last decennia into an area of intense research. Magnetochiral anisotropy, chirality-induced spin-selectivity and helimagnetism are the most prominent phenomena resulting from this combination, touching different systems like topological (semi-)metals and insulators, quantum magnets, type II multiferroics and enantio-selective synthesis. As an extension to this area, we argue, based on symmetry arguments, that magnetochiral anisotropy will manifest itself in the displacement current in chiral dielectrics in a magnetic field. We confirm this conjecture by the experimental observation of very strong dielectric magnetochiral anisotropy near the ferroelectric phase transitions of triglycine sulfate and Rochelle salt, two of the oldest and most investigated chiral ferroelectrics. This effect opens a new playground in the study and characterisation of all chiral dielectrics. With our discovery, magnetochiral anisotropy now covers the (di)electrical properties of all condensed matter, from insulators to superconductors.

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.

Magneto-chiral anisotropy: From fundamentals to perspectives.

The interplay between chirality and magnetic fields gives rise to a cross effect referred to as magneto-chiral anisotropy (MChA), which can manifest itself in different physical properties of chiral magnetized materials. The first experimental demonstration of MChA was by optical means with visible light. Further optical manifestations of MChA have been evidenced across most of the electromagnetic spectrum, from terahertz to X-rays. Moreover, exploiting the versatility of molecular chemistry toward chiral magnetic systems, many efforts have been made to identify the microscopic origins of optical MChA, necessary to advance the effect toward technological applications. In parallel, the replacement of light by electric current has allowed the observation of nonreciprocal electrical charge transport in both molecular and inorganic conductors as a result of electrical MChA (eMChA). MChA in other domains such as sound propagation, photochemistry, and electrochemistry are still in their infancy, with only a few experimental demonstrations, and offer wide perspectives for further studies with potentially large impact, like the understanding of the homochirality of life. After a general introduction to MChA, we give a complete review of all these phenomena, particularly during the last decade.

Detection of the Faraday Chiral Anisotropy.

The connection between chirality and electromagnetism has attracted much attention through the recent history of science, allowing the discovery of crucial nonreciprocal optical phenomena within the context of fundamental interactions between matter and light. A major phenomenon within this family is the so-called Faraday chiral anisotropy, the long-predicted but yet unobserved effect which arises due to the correlated coaction of both natural and magnetically induced optical activities at concurring wavelengths in chiral systems. Here, we report on the detection of the elusive anisotropic Faraday chiral phenomenon and demonstrate its enantioselectivity. The existence of this fundamental effect reveals the accomplishment of envisioned nonreciprocal electromagnetic metamaterials referred to as Faraday chiral media, systems where novel electromagnetic phenomena such as negative refraction of light at tunable wavelengths or even negative reflection can be realized. From a more comprehensive perspective, our findings have profound implications for the general understanding of parity-violating photon-particle interactions in magnetized media.

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.

Magneto-Chiral Dichroism: A Playground for Molecular Chemists.

Magneto-chiral dichroism (MChD) is a non-reciprocal manifestation of light-matter interaction that can be observed in chiral systems possessing a magnetization, either spontaneous or induced by an external magnetic field. It features a differential absorption or emission of unpolarized light that depends on the relative orientation of the magnetization with respect to the direction of the light propagation vector and on the absolute configuration of the system. Molecular chemistry is the best-suited route towards systems combining chirality and magnetism. Nowadays, investigation of MChD is still in its infancy although this effect might play a fundamental role in technological applications, such as optical readout of magnetic data with unpolarized light. With this Minireview, the authors provide a precise description of this unconventional effect, recall the main results obtained so far and, highlighting new challenges, underline the opportunities opened to molecular chemists interested in investigating this fascinating effect with implications in chemistry and beyond.

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.

A complex-polarization-propagator protocol for magneto-chiral axial dichroism and birefringence dispersion.

A computational protocol for magneto-chiral dichroism and magneto-chiral birefringence dispersion is presented within the framework of damped response theory, also known as complex polarization propagator theory, at the level of time-dependent Hartree-Fock and time-dependent density functional theory. Magneto-chiral dichroism and magneto-chiral birefringence spectra in the (resonant) frequency region below the first ionization threshold of R-methyloxirane and l-alanine are presented and compared with the corresponding results obtained for both the electronic circular dichroism and the magnetic circular dichroism. The additional information content yielded by the magneto-chiral phenomena, as well as their potential experimental detectability for the selected species, is discussed.

Electrical magnetochiral anisotropy in a bulk chiral molecular conductor.

So far, no effect of chirality on the electrical properties of bulk chiral conductors has been observed. Introduction of chiral information in tetrathiafulvalene precursors represents a powerful strategy towards the preparation of crystalline materials in which the combination of chirality and conducting properties might allow the observation of the electrical magnetochiral anisotropy effect. Here we report the synthesis by electrocrystallization of both enantiomers of a bulk chiral organic conductor based on an enantiopure tetrathiafulvalene derivative. The enantiomeric salts crystallize in enantiomorphic hexagonal space groups. Single crystal resistivity measurements show metallic behaviour for the enantiopure salts down to 40 K, in agreement with band structure calculations. We describe here the first experimental evidence of electrical magnetochiral anisotropy in these crystals, confirming the chiral character of charge transport in our molecular materials.

The diamagnetic susceptibility of the tubulin dimer.

An approximate value of the diamagnetic anisotropy of the tubulin dimer, Δχ dimer, has been determined assuming axial symmetry and that only the α -helices and ß -sheets contribute to the anisotropy. Two approaches have been utilized: (a) using the value for the Δχ α for an α -helical peptide bond given by Pauling (1979) and (b) using the previously determined anisotropy of fibrinogen as a calibration standard. The Δχ dimer ≈ 4 × 10(-27) JT(-2) obtained from these measurements are similar to within 20%. Although Cotton-Mouton measurements alone cannot be used to estimate Δχ directly, the value we measured, CMdimer = (1.41 ± 0.03) × 10(-8) T(-2)cm(2)mg(-1), is consistent with the above estimate for Δχ dimer. The method utilized for the determination of the tubulin dimer diamagnetic susceptibility is applicable to other proteins and macromolecular assemblies as well.

A density functional theory study of magneto-electric Jones birefringence of noble gases, furan homologues, and mono-substituted benzenes.

We report on the results of a systematic ab initio study of the Jones birefringence of noble gases, of furan homologues, and of monosubstituted benzenes, in the gas phase, with the aim of analyzing the behavior and the trends within a list of systems of varying size and complexity, and of identifying candidates for a combined experimental/theoretical study of the effect. We resort here to analytic linear and nonlinear response functions in the framework of time-dependent density functional theory. A correlation is made between the observable (the Jones constant) and the atomic radius for noble gases, or the permanent electric dipole and a structure/chemical reactivity descriptor as the para Hammett constant for substituted benzenes.

Negative experimental evidence for magneto-orbital dichroism.

A light beam can carry both spin angular momentum (SAM) and orbital angular momentum (OAM). SAM is commonly evidenced by circular dichroism (CD) experiments i. e. differential absorption of left and right-handed circularly polarized light. Recent experiments, supported by theoretical work, indicate that the corresponding effect with OAM instead of SAM is not observed in chiral matter. Isotropic materials can show CD when subjected to a magnetic field (MCD). We report a set of experiments, under well defined conditions, searching for magnetic orbital dichroism (MOD), differential absorption of light as a function of the sign of its OAM. We experimentally demonstrate that this effect, if any, is smaller than a few 10(−4) of MCD for the Nd:YAG 4/9/2 →4 F5/2 transition. This transition is essentially of electric dipole nature. We give an intuitive argument suggesting that the lowest order of light matter interaction leading to MOD is the electric quadrupole term.

Strong magneto-chiral dichroism in enantiopure chiral ferromagnets.

As materials science is moving towards the synthesis, the study and the processing of new materials exhibiting well-defined and complex functions, the synthesis of new multifunctional materials is one of the important challenges. One of these complex physical properties is magneto-chiral dichroism which arises, at second order, from the coexistence of spatial asymmetry and magnetization in a material. Herein we report the first measurement of strong magneto-chiral dichroism in an enantiopure chiral ferromagnet. The ab initio synthesis of the enantiopure chiral ferromagnet is based on an enantioselective self-assembly, where a resolved chiral quaternary ammonium cation imposes the absolute configurations of the metal centres within chromium-manganese two-dimensional oxalate layers. The ferromagnetic interaction between Cr(III) and Mn(II) ions leads to a Curie temperature of 7 K. The magneto-chiral dichroic effect is enhanced by a factor of 17 when entering into the ferromagnetic phase.