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.

Nonlinear and Emissive {[MIII(CN)6]3-···Polyresorcinol} (M = Fe, Co, Cr) Cocrystals Exhibiting an Ultralow Frequency Raman Response

Optically active functional noncentrosymmetric architectures might be achieved through the combination of molecules with inscribed optical responses and species of dedicated tectonic character. Herein, we present the new series of noncentrosymmetric cocrystal salt solvates (PPh4)3[M(CN)6](L)n·msolv (M = Cr(III), Fe(III), Co(III); L = polyresorcinol coformers, multiple hydrogen bond donors: 3,3',5,5'-tetrahydroxy-1,19-biphenyl, DiR, n = 2, or 5'-(3,5-dihydroxyphenyl)-3,3″,5,5″-tetrahydroxy-1,19:3',1″-terphenyl, TriRB, n = 1) denoted as MDiR and MTriRB, respectively. The hydrogen-bonded subnetworks {[M(CN)6]3-;Ln}∞ of dmp, neb, or dia topology are formed through structural matching between building blocks within supramolecular cis-bis(chelate)-like {[M(CN)6]3-;(H2L)2(HL)2} or tris(chelate)-like {[M(CN)6]3-;(H2L)3} fragments. The quantum-chemical analysis demonstrates the mixed electrostatic and covalent character of these interactions, with their strength clearly enhanced due to the negative charge of the hydrogen bond acceptor metal complex. The corresponding interaction energy is also dependent on the geometry of the contact and size matching of its components, rotational degree of freedom and extent of the π-electron system of the coformer, and overall fit to the molecular surroundings. Symmetry of the crystal lattices is correlated with the local symmetry of coformers and {complex;(coformer)n} hydrogen-bonded motifs characterized by the absence of the inversion center and mirror plane. All compounds reveal second-harmonic generation activity and photoluminescence diversified by individual UV-vis spectral characteristics of the components, and interesting low-frequency Raman scattering spectra within the subterahertz spectroscopic domain. Vibrational (infrared/Raman), UV-vis electronic absorption (experimental and calculated), and 57Fe Mössbauer spectra together with electrospray ionization mass spectrometry (ESI-MS) data are provided for the complete description of our systems.

Nitrile formation via dichlorocarbene insertion into the Si–N bond of Ln(iii) bis(trimethylsilyl)amide complexes

Reactions of bis(trimethylsilyl)amino lanthanides and chloroform unprecedently generate the Me3SiCN complexes where the nitrile ligand is formed via insertion of dichlorocarbene into the Si–N bond of the lanthanide precursor.

Studies of Er(iii)–W(v) compounds showing nonlinear optical activity and single-molecule magnetic properties

Studies of {[Er(dma)₅][W(CN)₈]}_n (1) showing nonlinear optical effect of second harmonic generation, and [Er(dma)₅(H₂O)₂]·[W(CN)₈]·dma·H₂O (2) and [Er(dma)₄(H₂O)₃]·[W(CN)₈]·dma·3H₂O (3) revealing field-induced single molecule magnet behavior.

Antiferromagnetic exchange and long-range magnetic ordering in supramolecular networks constructed of hexacyanido-bridged LnIII(3-pyridone)–CrIII (Ln = Gd, Tb) chains

Exchange interactions and magnetic phase transitions are observed in novel cyanido-bridged lanthanide(iii)–chromium(iii) chains as proved by magnetic and calorimetric studies.

Two isomorphous azide/formate Mn(ii) coordination polymers show spin-canted antiferromagnetism only in the formate system

We reported two isomorphous azide/formate Mn^II complexes: the azide complex had normal antiferromagnetic coupling, while a canted antiferromagnet with a Néel temperature of T_N = 38.5 K was obtained in the formate system.

Incorporation of hexacyanidoferrate(iii) ion in photoluminescent trimetallic Eu(3-pyridone)[Co1−xFex(CN)6] chains exhibiting tunable visible light absorption and emission properties

Tuning of absorption and photoluminescence is achieved in trimetallic EuCoFe cyanido-bridged frameworks through the gradual replacement of [Co^III(CN)₆]³⁻ with [Fe^III(CN)₆]³⁻.

Fine Tuning of Multicolored Photoluminescence in Crystalline Magnetic Materials Constructed of Trimetallic EuxTb1-x[Co(CN)6] Cyanido-Bridged Chains

Coordination compounds built of trivalent lanthanide ions have been demonstrated as promising solid-state materials for diverse photoluminescent applications and as attractive magnetic objects with the prospective application in information storage and spintronics. We present a synthetic methodology in which both luminescent and magnetic functionalities are induced within lanthanide-based coordination polymers by the application of hexacyanocobaltate(III) anions and 3-hydroxypyridine (3-OHpy), both coordinated to 4f-metal ions modulating the lanthanide-centered properties. We report a series of trimetallic cyanido-bridged chains {[Eu^III_xTb^III_1-x(3-OHpy)₂(H₂O)₄][Co^III(CN)₆]}·H₂O (x = 1, 0.8, 0.5, 0.4, 0.3, 0.2, 0.1, 0; compounds 1, 2, ..., 7, 8). They reveal tunable visible photoluminescence ranging from green, through yellow and orange, to red color depending on the composition of material and the wavelength of UV excitation light. Such multicolored emission is realized by the adjusted ratio between red emissive Eu³⁺ and green emissive Tb³⁺ and by the selection of wavelengths of UV light controlling the intensities of Eu- and Tb-based components of visible luminescence. The photoluminescence is enhanced by the energy-transfer (ET) process from [Co^III(CN)₆]^3- and 3-OHpy to lanthanides, and the efficiencies of ET to Eu and Tb play an important role in the switchable emission. The whole family, 1-8, exhibits temperature-dependent paramagnetism due to the intrinsic property of lanthanide(3+) ions. Tb-containing 2-8 reveal the field-induced slow relaxation of magnetization due to the magnetic anisotropy of Tb³⁺. Moreover, the compounds built of large amounts of Tb reveal the double relaxation, the faster of a typical Tb^III single-ion origin, and the slower originating from the magnetic dipole-magnetic dipole interactions between neighboring Tb^III centers. Compound 2, which can be considered as a magnetically diluted sample, exhibits almost single relaxation process with the thermal energy barrier ΔE/k_B of 35.8(6) K and τ₀ = 1.1(2) × 10^-8 s at H_dc = 1500 Oe, indicating a single-molecule magnet behavior.

Octahedral Yb(iii) complexes embedded in [CoIII(CN)6]-bridged coordination chains: combining sensitized near-infrared fluorescence with slow magnetic relaxation

The rare octahedral Yb^III complexes formed in the bimetallic Yb–Co chains reveal significant magnetic anisotropy and photoluminescence activity in the near-infrared range.