Optical Phenomena in Molecule-Based Magnetic Materials

Since the last century, we have witnessed the development of molecular magnetism which deals with magnetic materials based on molecular species, i.e., organic radicals and metal complexes. Among them, the broadest attention was devoted to molecule-based ferro-/ferrimagnets, spin transition materials, including those exploring electron transfer, molecular nanomagnets, such as single-molecule magnets (SMMs), molecular qubits, and stimuli-responsive magnetic materials. Their physical properties open the application horizons in sensors, data storage, spintronics, and quantum computation. It was found that various optical phenomena, such as thermochromism, photoswitching of magnetic and optical characteristics, luminescence, nonlinear optical and chiroptical effects, as well as optical responsivity to external stimuli, can be implemented into molecule-based magnetic materials. Moreover, the fruitful interactions of these optical effects with magnetism in molecule-based materials can provide new physical cross-effects and multifunctionality, enriching the applications in optical, electronic, and magnetic devices. This Review aims to show the scope of optical phenomena generated in molecule-based magnetic materials, including the recent advances in such areas as high-temperature photomagnetism, optical thermometry utilizing SMMs, optical addressability of molecular qubits, magneto-chiral dichroism, and opto-magneto-electric multifunctionality. These findings are discussed in the context of the types of optical phenomena accessible for various classes of molecule-based magnetic materials.

Enhanced Curie Temperature of Two-Dimensional Cr(II) Aromatic Heterocyclic Metal-Organic Framework Magnets via Strengthened Orbital Hybridization

Two-dimensional (2D) metal-organic frameworks (MOFs) with room-temperature magnetism are highly desirable but challenging due to the weak superexchange interaction between metal atoms. For this purpose, strengthening the hybridization between metal ion and organic linkage presents an experiment-feasible chemical solution to enhance the Curie temperature. Here, we report three 2D Cr(II) aromatic heterocyclic MOF magnets with enhanced Curie temperature by bridging Cr(II) ions with pyrazine, 1,4-diphosphinine, and 1,4-diarsenin linkers, i.e., Cr(pyz)₂, Cr(diphos)₂, and Cr(diarse)₂, and using first-principles calculations. Our results show that Cr(pyz)₂, Cr(diphos)₂, and Cr(diarse)₂ are ferrimagnetic semiconductors. In particular, the Curie temperature of Cr(pyz)₂ is estimated to be about 344 K and could be enhanced to 512 and 437 K in Cr(diphos)₂ and Cr(diarse)₂ by strengthening the hybridization between Cr ions and organic linkers via d-π* direct exchange interaction. This study presents a prototype to obtain room-temperature magnetism in 2D Cr(II)-based MOF magnets for nanoscale spintronics applications.

Main Group Chemistry at the Interface with Molecular Magnetism

Innovative synthetic coordination and, increasingly, organometallic chemistry are at the heart of advances in molecular magnetism. Smart ligand design is essential for implementing controlled modifications to the electronic structure and magnetic properties of transition metal and f-element compounds, and many important recent developments use nontraditional ligands based on low-coordinate main group elements to drive the field forward. This review charts progress in molecular magnetism from the perspective of ligands in which the donor atoms range from low-coordinate 2p elements-particularly carbon but also boron and nitrogen-to the heavier p-block elements such as phosphorus, arsenic, antimony, and even bismuth. Emphasis is placed on the role played by novel main group ligands in addressing magnetic anisotropy of transition metal and f-element compounds, which underpins the development of single-molecule magnets (SMMs), a family of magnetic materials that can retain magnetization in the absence of a magnetic field below a blocking temperature. Nontraditional p-block donor atoms, with their relatively diffuse valence orbitals and more diverse bonding characteristics, also introduce scope for tuning the spin-orbit coupling properties and metal-ligand covalency in molecular magnets, which has implications in areas such as magnetic exchange coupling and spin crossover phenomena. The chemistry encompasses transition metals, lanthanides, and actinides and describes recently discovered molecular magnets that can be regarded, currently, as defining the state of the art. This review identifies that main group chemistry at the interface molecular magnetism is an area with huge potential to deliver new types of molecular magnets with previously unseen properties and applications.

Thiosemiquinoid Radical-Bridged CrIII2 Complexes with Strong Magnetic Exchange Coupling

Semiquinoid radical bridging ligands are capable of mediating exceptionally strong magnetic coupling between spin centers, a requirement for the design of high-temperature magnetic materials. We demonstrate the ability of sulfur donors to provide much stronger coupling relative to their oxygen congeners in a series of dinuclear complexes. Employing a series of chalcogen donor-based bis(bidentate) benzoquinoid bridging ligands, the series of complexes [(TPyA)₂Cr₂(^RL^4-)]²⁺ (^OLH₄ = 1,2,4,5-tetrahydroxybenzene, ^OSLH₄ = 1,2-dithio-4,5-dihydroxybenzene, ^SLH₄ = 1,2,4,5-tetrathiobenzene, TPyA = tris(2-pyridylmethyl)amine) was synthesized. Variable-temperature dc magnetic susceptibility data reveal the presence of weak antiferromagnetic superexchange coupling between Cr^III centers in these complexes, with exchange constants of J = -2.83(3) (^OL^4-), -2.28(5) (^OSL^4-), and -1.80(2) (^SL^4-) cm^-1. Guided by cyclic voltammetry and spectroelectrochemical measurements, chemical one-electron oxidation of these complexes gives the radical-bridged species [(TPyA)₂Cr₂(^RL^3-•)]³⁺. Variable-temperature dc susceptibility measurements in these complexes reveal the presence of strong antiferromagnetic metal-semiquinoid radical coupling, with exchange constants of J = -352(10) (^OL^3-•), - 401(8) (^OSL^3-•), and -487(8) (^SL^3-•) cm^-1. These results provide the first measurement of magnetic coupling between metal ions and a thiosemiquinoid radical, and they demonstrate the value of moving from O to S donors in radical-bridged metal ions in the design of magnetic molecules and materials.

Modulation of the FeII spin crossover effect in the pentadecanuclear {Fe9[M(CN)8]6} (M = Re, W) clusters by facial coordination of tridentate polyamine ligands

N,N,N-tridentate ligands coordinated to {Fe₉[M(CN)₈]₆} (M = Re, W) induce the spin crossover on the external Fe sites of the cluster.

Magneto-structural correlations in arsenic- and selenium-ligated dysprosium single-molecule magnets

The structures and magnetic properties of the arsenic- and selenium-ligated dysprosium single-molecule magnets (SMMs) [Cp'3Dy(AsH2Mes)] (3-Dy), [(η5-Cp'2Dy){μ-As(H)Mes}]3 (4-Dy), [Li(thf)4]2[(η5-Cp'2Dy)3(μ3-AsMes)3Li] ([Li(thf)4]2[5-Dy]), and [(η5-Cp'2Dy){μ-SeMes}]3 (6-Dy) are described. The arsenic-ligated complexes 4-Dy and 5-Dy are the first SMMs to feature ligands with metalloid elements as the donor atoms. The arsenide-ligated complex 4-Dy and the selenolate-ligated complex 6-Dy show large anisotropy barriers in the region of 250 cm-1 in zero d.c. field, increasing to 300 cm-1 upon 5% magnetic dilution. Theoretical studies reveal that thermal relaxation in these SMMs occurs via the second-excited Kramers' doublet. In contrast, the arsinidene-ligated SMM 5-Dy gives a much smaller barrier of 23 cm-1, increasing to 35 cm-1 upon dilution. The field-dependence of the magnetization for 4-Dy and 5-Dy at 1.8 K show unusual plateaus around 10 kOe, which is due to the dominance of arsenic-mediated exchange over the dipolar exchange. The effects of the exchange interactions are more pronounced in 5-Dy, which is a consequence of a small but significant increase in the covalent contribution to the predominantly ionic dysprosium-arsenic bonds. Whereas the magnetically non-dilute dysprosium SMMs show only very narrow magnetization versus field hysteresis loops at 1.8 K, the impact of magnetic dilution is dramatic, with butterfly-shaped loops being observed up to 5.4 K in the case of 4-Dy. Our findings suggest that ligands with heavier p-block element donor atoms have considerable potential to be developed more widely for applications in molecular magnetism.

Magnetic fluorescent bifunctional spin-crossover complexes

Bifunctional fluorescent spin crossover complexes exhibit thermally and light induced conversion from the low spin to the high spin state, as well as fluorescence emission.

Addition of pnictogen atoms to chromium(ii): synthesis, structure and magnetic properties of a chromium(iv) phosphide and a chromium(iii) arsenide

Chromium(ii) chloride reacts with LiCp* (Cp* = C₅Me₅) and LiE (SiMe₃)₂ (E = P or As) to give the chromium(iv) phosphide [(η⁵-Cp*Cr)(μ₃-P)]₄ (1) or the chromium(iii) arsenide [(η⁵-Cp*Cr)₃(μ₃-As)₂] (2), respectively.

Synthesis, structure, and paramagnetism of manganese(II) iminophosphate complexes

The coordination chemistry of the bidentate bis(imino)bis(amino)phosphate ligands [Me(3)SiN═P{NR}{N(H)R}(2)](-), where R = n-propyl is [L(1)H(2)](-), R = cyclohexyl is [L(2)H(2)](-), and R = tert-butyl is [L(3)H(2)](-), with manganese(II), is described. The bis(imino)bis(amino)phosphate-manganese(II) complexes [(η(5)-Cp)Mn(μ-L(1)H(2))](2) (1), [Mn(L(2)H(2))(2)]·THF (2·THF), and [(η(5)-Cp)Mn(L(3)H(2))] (3) were synthesized by monodeprotonation of the respective pro-ligands by manganocene, Cp(2)Mn. The molecular structures of 1-3 reveal that the steric demands of the ligand N-substituents play a dominant role in determining the aggregation state and overall composition of the manganese(II) complexes. The coordination geometries of the Mn(II) centers are six-coordinate pseudotetrahedral in 1, four-coordinate distorted tetrahedral in 2, and five-coordinate in 3, resulting in formal valence electron counts of 17, 13, and 15, respectively. EPR studies of 1-3 at Q-band reveal high-spin manganese(II) (S = 5/2) in each case. In the EPR spectrum of 1, no evidence of intramolecular magnetic exchange was found. The relative magnitudes of the axial zero-field splitting parameter, D, in 2 and 3 are consistent with the symmetry of the manganese environment, which are D(2d) in 2 and C(2v) in 3.