Magneto-Chiral Dichroism of Organic Compounds

Experimental and theoretical aspects of magnetic circular dichroism and magnetic circularly polarized luminescence in the UV, visible and IR ranges: A review

A historical sketch of the MCD (magnetic circular dichroism) spectroscopy is reported in its experimental and theoretical aspects. MCPL (magnetic circularly polarized luminescence) is also considered. The main studies are presented encompassing porphyrinoid systems, aggregates and materials, as well as simple organic molecules useful for the advancement of the interpretation. The MCD of chiral systems is discussed with special attention to new studies of natural products with potential pharmaceutical valence, including Amaryllidaceae alkaloids and related isocarbostyrils. Finally, the vibrational form of MCD, called MVCD, which is recorded in the IR part of the spectrum is also discussed. A final brief note on perspectives is given.

Optical Readout of Single-Molecule Magnets Magnetic Memories with Unpolarized Light

Magnetic materials are widely used for many technologies in energy, health, transportation, computation, and data storage. For the latter, the readout of the magnetic state of a medium is crucial. Optical readout based on the magneto-optical Faraday effect was commercialized but soon abandoned because of the need for a complex circular polarization-sensitive readout. Combining chirality with magnetism can remove this obstacle, as chiral magnetic materials exhibit magneto-chiral dichroism, a differential absorption of unpolarized light dependent on their magnetic state. Molecular chemistry allows the rational introduction of chirality into single-molecule magnets (SMMs), ultimate nanoobjects capable of retaining magnetization. Here, we report the first experimental demonstration of optical detection of the magnetic state of an SMM using unpolarized light on a novel air-stable Dy-based chiral SMM featuring a strong single-ion magnetic anisotropy. These findings might represent a paradigm shift in the field of optical data readout technologies.

C3-Symmetric Luminescent Diketone with Amido-Linkage as a Polymorphic Fluorescence Emitter

The study introduces a novel C3-symmetric β-diketone compound, BTA-D3, and its monomeric counterpart, D, with a focus on their synthetic procedure, photophysical properties and aggregation behavior. Both compounds exhibit characteristic absorption and weak fluorescence in solution, with BTA-D3 displaying higher absorption coefficients due to its larger number of diketone units. Density Functional Theory (DFT) calculations suggest increased co-planarity of diketone groups in BTA-D3. A significant finding is the Aggregation-Induced Emission (AIE) property of BTA-D3, as its fluorescence intensity increases dramatically when exposed to specific solvent ratios. The AIE behavior is attributed to intermolecular excitonic interaction between BTA-D3 molecules in self-organized aggregates. We also studied fluorescence anisotropy of BTA-D3 and D. Despite its larger size, BTA-D3 showed reduced anisotropy values because of efficient intramolecular energy migration among three diketone units. Furthermore, BTA-D3 demonstrates unique polymorphism, yielding different emission colors and structures depending on the solvent used. A unique approach is presented for promoting the growth of self-organized aggregate structures via solvent evaporation, leading to distinct fluorescence properties. This research contributes to the understanding of C3-symmetric structural molecules and provides insights into strategies for controlling molecular alignment to achieve diverse fluorescence coloration in molecular materials.

Magnetic Circular Dichroism Elucidates Molecular Interactions in Aggregated Chiral Organic Materials

Chiral materials formed by aggregated organic compounds play a fundamental role in chiral optoelectronics, photonics and spintronics. Nonetheless, a precise understanding of the molecular interactions involved remains an open problem. Here we introduce magnetic circular dichroism (MCD) as a new tool to elucidate molecular interactions and structural parameters of a supramolecular system. A detailed analysis of MCD together with electronic circular dichroism spectra combined to ab initio calculations unveils essential information on the geometry and energy levels of a self-assembled thin film made of a carbazole di-bithiophene chiral molecule. This approach can be extended to a generality of chiral organic materials and can help rationalizing the fundamental interactions leading to supramolecular order. This in turn could enable a better understanding of structure-property relationships, resulting in a more efficient material design.

A Pair of Multifunctional Cu(II)-Dy(III) Enantiomers with Zero-Field Single-Molecule Magnet Behaviors, Proton Conduction Properties and Magneto-Optical Faraday Effects

Multifunctional materials with a coexistence of proton conduction properties, single-molecule magnet (SMM) behaviors and magneto-optical Faraday effects have rarely been reported. Herein, a new pair of Cu(II)-Dy(III) enantiomers, [DyCu2(RR/SS-H2L)2(H2O)4(NO3)2]·(NO3)·(H2O) (R-1 and S-1) (H4L = [RR/SS] -N,N'-bis [3-hydroxysalicylidene] -1,2-cyclohexanediamine), has been designed and prepared using homochiral Schiff-base ligands. R-1 and S-1 contain linear Cu(II)-Dy(III)-Cu(II) trinuclear units and possess 1D stacking channels within their supramolecular networks. R-1 and S-1 display chiral optical activity and strong magneto-optical Faraday effects. Moreover, R-1 shows a zero-field SMM behavior. In addition, R-1 demonstrates humidity- and temperature-dependent proton conductivity with optimal values of 1.34 × 10-4 S·cm-1 under 50 °C and 98% relative humidity (RH), which is related to a 1D extended H-bonded chain constructed by water molecules, nitrate and phenol groups of the RR-H2L ligand.

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.

Asymmetric Assembly of Chiral Lanthanide(III) Tetranuclear Cluster Complexes Using Achiral Mixed Ligands: Single-molecule Magnet Behavior and Magnetic Entropy Change

It is challenging to use achiral ligands to spontaneously construct chiral molecular magnets. In this work, two new Ln4 cluster complexes based on N,N'-(1,3-propanediyl)bis[N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]amine] (H6L) have been assembled, which are crystallized in a chiral space group due to the asymmetric distribution of acetate (OAc-) groups and hexafluoroacetylacetonate (F6acac-) groups on both sides of the parallelogram-like Ln4 core. Complex 1, [Dy4(H3L)2(OAc)3(F6acac)3]·5MeOH·2H2O, exhibits single-molecule magnet properties at the zero field with the U eff/k value of 48.4 K; notably, besides the Orbach process, the Raman process is also prominent for the magnetic relaxation of 1. Complex 2, [Gd4(H3L)2(OAc)3(F6acac)3]·4MeOH·2.5H2O, displays a large magnetocaloric effect, whose largest -ΔS m value is 21.88 J kg-1 K-1 (when T = 2 K and ΔH = 5 T); it thus can be utilized as a good magnetic refrigeration molecular material.

Chiral weak ferromagnets formed in one-dimensional organic–inorganic hybrid manganese chloride hydrates

Chiral weak ferromagnets were developed in one-dimensional organic–inorganic hybrid chain compounds comprising manganese chloride hydrate of deformed perovskite derivatives.

The Underexplored Field of Lanthanide Complexes with Helicene Ligands: Towards Chiral Lanthanide Single Molecule Magnets

The effective combination of chirality and magnetism in a single crystalline material can lead to fascinating cross-effects, such as magneto-chiral dichroism. Among a large variety of chiral ligands utilized in the design and synthesis of chiral magnetic materials, helicenes seem to be the most appealing ones, due to the exceptionally high specific rotation values that reach thousands of deg·cm3·g−1·dm−1, which is two orders of magnitude higher than for compounds with chiral carbon atoms. Despite the sizeable family of transition metal complexes with helicene-type ligands, there are only a few examples of such complexes with lanthanide ions. In this mini-review, we describe the most recent developments in the field of lanthanide-based complexes with helicene-type ligands and summarize insights regarding the further exploration of this family of compounds towards multifunctional chiral lanthanide single molecule magnets (Ln-SMMs).

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.

Homochiral Ferromagnetic Coupling Dy2 Single-Molecule Magnets with Strong Magneto-Optical Faraday Effects at Room Temperature

By the bridging action of the 6-chloro-2-hydroxypyridine (Hchp) ligand and the terminal coordination role of the homochiral ligand, (-)/(+)-3-trifluoroacetyl camphor (l-Htfc/d-Htfc), a pair of enantiomerically pure dysprosium(III) dinuclear complexes, [Dy₂(l-tfc)₄(chp)₂(MeOH)₂] (l-1) and [Dy₂(d-tfc)₄(chp)₂(MeOH)₂] (d-1), was obtained. Their circular dichroism (CD) spectra verified their enantiomeric nature. Magnetic investigation indicated that they exhibit ferromagnetic interaction and good zero field single-molecule magnet (SMM) properties. The U_eff/k values of l-1 and d-1 at 0 Oe are 180.5 and 181.3 K, respectively, which are large values for homochiral Dy(III) SMMs. A reasonable explanation for the magnetic properties of l-1 and d-1 was supplied by ab initio calculations. Remarkably, magnetic circular dichroism (MCD) investigation revealed that the chiral Dy₂ enantiomers show a strong magneto-optical Faraday effect at room temperature, suggesting potential applications in magneto-optical devices.

Asymmetric autocatalysis triggered by triglycine sulfate with switchable chirality by altering the direction of the applied electric field

Triglycine sulfate (TGS) acts as a chiral trigger for asymmetric autocatalysis with amplification of enantiomeric excess, i.e., the Soai reaction. Therefore, molecular chirality of highly enantioenriched organic compounds is controlled by a ferroelectric crystal TGS, whose polarization is altered by an electric field.

Vortex-Induced Harmonic Light Scattering of Porphyrin J-Aggregates

An understanding of macroscopic vortex-induced chirality can provide insights into the origin of the homochirality of life. While circular dichroism measurements in stirred solutions are useful for the analysis of chiral supramolecular structures induced by vortex motion, there are no reports on the application of other spectroscopic methods. To obtain a deeper understanding of macroscopic vortex-induced chirality, it is essential to develop novel in situ spectroscopic methods that provide information about changes in both the size and chirality in stirred solutions. Here, we report the first observation by harmonic light scattering of the mirror-symmetry-breaking process of porphyrin J-aggregates under the rotation of a magnetic stirrer. The chiral supramolecular structure observed during stirring is likely due to the formation of a chiral aggregate that consists of porphyrin J-aggregates. The dissociation of the structure proceeds in two steps (a fast step and a slow step), as indicated by the signal decay rate when stirring was stopped. This novel method is useful for analyzing the supramolecular structural changes of chiral aggregates induced by external stimuli.

Recent Progress in Optically-Active Phthalocyanines and Their Related Azamacrocycles

Optically-active phthalocyanines (Pcs) and related macrocycles reported in the 2010-2020 period are introduced in this review. They are grouped into several categories: (1) chiral binaphthyl-containing Pcs, (2) optically active alkyl chain-containing Pcs, (3) chiral axial ligand- coordinated or -linked Pcs, (4) chiral subphthalocyanines (SubPcs), and (5) related azamacrocycles. For each compound, the structure and important characteristics are summarized.

Hard X-ray magnetochiral dichroism in a paramagnetic molecular 4f complex

Magnetochiral dichroism (MΧD) originates in the coupling of local electric fields and magnetic moments in systems where a simultaneous break of space parity and time-reversal symmetries occurs. This magnetoelectric coupling, displayed by chiral magnetic materials, can be exploited to manipulate the magnetic moment of molecular materials at the single molecule level. We demonstrate herein the first experimental observation of X-ray magnetochiral dichroism in enantiopure chiral trigonal single crystals of a chiral mononuclear paramagnetic lanthanide coordination complex, namely, holmium oxydiacetate, at the Ho L₃-edge. The observed magnetochiral effect is opposite for the two enantiomers and is rationalised on the basis of a multipolar expansion of the matter–radiation interaction. These results demonstrate that 4f–5d hybridization in chiral lanthanoid coordination complexes is at the origin of magnetochiral dichroism, an effect that could be exploited for addressing of their magnetic moment at the single molecule level.

Magnetochiral Dichroism of chiral mononuclear lanthanoid complexes is for the first time detected by X-ray absorption measurements on single crystals of Holmium oxydiacetate, at the Ho L₃-edge. The effect is of opposite sign for the two enantiomers.

Recent advances in studies on the magneto-chiral dichroism of organic compounds

Magneto-chiral dichroism is described in terms of devices and synthesis, and is a clue to explain the homochirality of life.

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 [Mn^II(X-pnH)(H₂O)][Cr^III(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 T_C 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 Cr^III centers, spin-forbidden transitions of the Mn^II 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.

Symmetry Breaking in Self-Assembled Nanoassemblies

The origin of biological homochirality, e.g., life selects the L-amino acids and D-sugar as molecular component, still remains a big mystery. It is suggested that mirror symmetry breaking plays an important role. Recent researches show that symmetry breaking can also occur at a supramolecular level, where the non-covalent bond was crucial. In these systems, equal or unequal amount of the enantiomeric nanoassemblies could be formed from achiral molecules. In this paper, we presented a brief overview regarding the symmetry breaking from dispersed system to gels, solids, and at interfaces. Then we discuss the rational manipulation of supramolecular chirality on how to induce and control the homochirality in the self-assembly system. Those physical control methods, such as Viedma ripening, hydrodynamic macro- and micro-vortex, superchiral light, and the combination of these technologies, are specifically discussed. It is hoped that the symmetry breaking at a supramolecular level could provide useful insights into the understanding of natural homochirality and further designing as well as controlling of functional chiral materials.

Multiwall and bamboo-like carbon nanotubes from the Allende chondrite: A probable source of asymmetry

This study presents multiwall and bamboo-like carbon nanotubes found in samples from the Allende carbonaceous chondrite using high-resolution transmission electron microscopy (HRTEM). A highly disordered lattice observed in this material suggests the presence of chiral domains in it. Our results also show amorphous and poorly-graphitized carbon, nanodiamonds, and onion-like fullerenes. The presence of multiwall and bamboo-like carbon nanotubes have important implications for hypotheses that explain how a probable source of asymmetry in carbonaceous chondrites might have contributed to the enantiomeric excess in soluble organics under extraterrestrial scenarios. This is the first study proving the existence of carbon nanotubes in carbonaceous chondrites.

Organic Chiral Charge Transfer Magnets

In this work, through designing organic helix donor-acceptor complexes, one type of room-temperature chiral magnet was reported. Within these chiral charge transfer magnets, circularly polarized light could induce a larger saturation magnetization compared to linearly polarized light illumination with identical intensity. Moreover, the transmission light polarization from chiral magnets could be tuned via applying the magnetic field. Overall, room-temperature organic chiral magnets with optomagnetic effects will enhance the function of organic magnetochiral materials.

Soft Crystals: Flexible Response Systems with High Structural Order

A new material concept of soft crystals is proposed. Soft crystals respond to gentle stimuli such as vapor exposure and rubbing but maintain their structural order and exhibit remarkable visual changes in their shape, color, and luminescence. Various interesting examples of soft crystals are introduced in the article. By exploring the interesting formation and phase-transition phenomena of soft crystals through interdisciplinary collaboration, new materials having both the characteristics of ordered hard crystals and those of flexible soft matter are expected.

5-Phenyl-10,15,20-Tris(4-sulfonatophenyl)porphyrin: Synthesis, Catalysis, and Structural Studies

A convenient protocol for the preparation of 5-phenyl-10,15,20-tris(4-sulfonatophenyl)porphyrin, a water-soluble porphyrin with unique aggregation properties, is described. The procedure relies on the one-pot reductive deamination of 5-(4-aminophenyl)-10,15,20-tris(4-sulfonatophenyl)porphyrin, that can be in turn easily obtained from 5,10,15,20-tetraphenylporphyrin by a known three-step sequence involving mononitration, nitro to amine reduction and sulfonation of the phenyl groups. This method provides the title porphyrin in gram scale, and compares very favorably with the up to now only described procedure based on the partial sulfonation of TPP, that involves a long and tedious chromatographic enrichment of the final compound. This has allowed us to study for the first time both the use of its zwitterionic aggregate as a supramolecular catalyst of the aqueous Diels⁻Alder reaction, and the morphology of the aggregates obtained under optimized experimental conditions by atomic force microscopy and also by transmission electron cryomicroscopy.

Chiromagnetic nanoparticles and gels

Chiral inorganic nanostructures have high circular dichroism, but real-time control of their optical activity has so far been achieved only by irreversible chemical changes. Field modulation is a far more desirable path to chiroptical devices. We hypothesized that magnetic field modulation can be attained for chiral nanostructures with large contributions of the magnetic transition dipole moments to polarization rotation. We found that dispersions and gels of paramagnetic Co₃O₄ nanoparticles with chiral distortions of the crystal lattices exhibited chiroptical activity in the visible range that was 10 times as strong as that of nonparamagnetic nanoparticles of comparable size. Transparency of the nanoparticle gels to circularly polarized light beams in the ultraviolet range was reversibly modulated by magnetic fields. These phenomena were also observed for other nanoscale metal oxides with lattice distortions from imprinted amino acids and other chiral ligands. The large family of chiral ceramic nanostructures and gels can be pivotal for new technologies and knowledge at the nexus of chirality and magnetism.

Evaporation rate-based selection of supramolecular chirality

The supramolecular chirality of aggregates of π-conjugated polymers can be reversed by changing the evaporation rate.

Solvent-dependent dual-luminescence properties of a europium complex with helical π-conjugated ligands

A europium(iii) complex with a large π-conjugated helical ligand, tris(hexafluoroacetylacetonato)europium(iii)bis((pentahelicenyl)diphenylphosphine oxide) (Eu(hfa)₃(HPO)₂), exhibits dual luminescence excited at the π–π* transition band.

Reversible and irreversible emergence of chiroptical signals in J-aggregates of achiral 4-sulfonatophenyl substituted porphyrins: intrinsic chirality vs. chiral ordering in the solution

Different origins of the reversible and irreversible emergence of chiroptical signals in the title J-aggregates.

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

A schematic representation of magneto-chiral effects.

Visible luminescent lanthanide ions and a large π-conjugated ligand system shake hands

The novel photophysics induced by the combination of visible luminescent europium(iii) ions and large π-conjugated systems are described.

Strong magneto-chiral dichroism in a paramagnetic molecular helix observed by hard X-ray

Magneto-chiral dichroism (MχD) is a non-reciprocal, i. e. directional, effect observed in magnetised chiral systems featuring an unbalanced absorption of unpolarised light depending on the direction of the magnetisation. Despite the fundamental interest in a phenomenon breaking both parity and time reversal symmetries, MχD is one of the least investigated aspects of light-matter interaction because of the weakness of the effect in most reported experiments. Here we have exploited the element selectivity of hard X-ray radiation to investigate the magneto-chiral properties of enentiopure crsytals of two isostructural molecular helicoidal chains comprising Cobalt(II) and Manganese (II) ions, respectively. A strong magneto-chiral dichroism, with Kuhn asymmetry of the order of a few percent, has been observed in the Cobalt chain system, while it is practically absent for the Manganese derivative. The spectral features of the XMχD signal differ significantly from the natural and magnetic dichroic contributions and have been here rationalized using the simple multipolar expansion of matter-radiation interaction.

Magnetic field modulation of chirooptical effects in magnetoplasmonic structures

In this work we analyse the magnetic field effects on the chirooptical properties of magnetoplasmonic chiral structures. The structures consist of two-dimensional arrays of Au gammadions in which thin layers of Co have been inserted. Due to the magnetic properties of the Au/Co interface the structures have perpendicular magnetic anisotropy which favours magnetic saturation along the surface normal, allowing magnetic field modulation of the chirooptical response with moderate magnetic fields. These structures have two main resonances. The resonance at 850 nm has a larger chirooptical response than the resonance at 650 nm, which, on the other hand, exhibits a larger magnetic field modulation of its chirooptical response. This dissimilar behaviour is due to the different physical origin of the chirooptical and magneto-optical responses. Whereas the chirooptical effects are due to the geometry of the structures, the magneto-optical response is related to the intensity of the electromagnetic field in the magnetic (Co) layers. We also show that the optical chirality can be modulated by the applied magnetic field, which suggests that magnetoplasmonic chiral structures could be used to develop new strategies for chirooptical sensing.

Optical fiber Sagnac interferometer for sensing scalar directional refraction: application to magnetochiral birefringence

We present a setup dedicated to the measurement of the small scalar directional anisotropies associated to the magnetochiral interaction. The apparatus, based on a polarization-independent fiber Sagnac interferometer, is optimized to be insensitive to circular anisotropies and to residual absorption. It can thus characterize samples of biological interests, for which the two enantiomers are not available and/or which present poor transmission. The signal-to-noise ratio is shown to be limited only by the source intensity noise, leading to a detection limit of Δϕ = 500 nrad Hz(-1/2). It yields a limit on the magnetochiral index nMC < 4 × 10(-13) T(-1) at 1.55 μm for the organic molecules tested.