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.

Aggregation-Induced Emission via the Restriction of the Intramolecular Vibration Mechanism of Pinacol Lanthanide Complexes

Pinacol lanthanide complexes PyraLn (Ln = Dy and Tb) with the restriction of intramolecular vibration were obtained for the first time via an in situ solvothermal coordination-catalyzed tandem reaction using cheap and simple starting materials, thereby avoiding complex, time-consuming, and expensive conventional organic synthesis strategies. A high-resolution electrospray ionization mass spectrometry (HRESI-MS) analysis confirmed the stability of PyraLn in an organic solution. The formation process of PyraLn was monitored in detail using time-dependent HRESI-MS, which allowed for proposing a mechanism for the formation of pinacol complexes via in situ tandem reactions under one-pot coordination-catalyzed conditions. The PyraLn complexes constructed using a pinacol ligand with a butterfly configuration exhibited distinct aggregation-induced emission (AIE) behavior, with the αAIE value as high as 60.42 according to the AIE titration curve. In addition, the PyraLn complexes in the aggregated state exhibit a rapid photoresponse to various 3d metal ions with low detection limits. These findings provide fast, facile, and high-yield access to dynamic, smart lanthanide complex emissions with bright emission and facilitate the rational construction of molecular machines for artificial intelligence.

Assembly of pinwheel/twist-shaped chiral lanthanide clusters with rotor structures by an annular/linear growth mechanism and their magnetic properties

This is the first time that an annular/linear growth mechanism has been proposed for the directional construction of lanthanide clusters with specific shapes.

Structural, magnetic and theoretical analyses of anionic and cationic phthalocyaninato-terbium(III) double-decker complexes: magnetic relaxation via higher ligand-field sublevels enhanced by oxidation

Crystal structural and magnetic analyses were performed for the anionic (1-) and cationic (1+) forms of phthalocyaninato-Tb3+ double-decker single-molecule magnets (SMMs). Both charged species showed slow magnetic relaxations and magnetic hysteresis characteristics for SMMs. 1+ showed longer magnetic relaxation times (τ) and higher activation energy for spin reversal (ΔE) than 1- did. Ligand field (LF) splitting calculated using ab initio methods revealed that the experimental ΔE values in 1- and 1+ were considerably larger than the first excited LF levels but rather close to the higher excited ones, indicating the magnetic relaxation via higher excited states.

Mononuclear Dysprosium Alkoxide and Aryloxide Single-Molecule Magnets

Recent studies have shown that mononuclear lanthanide (Ln) complexes can be high‐performing single‐molecule magnets (SMMs). Recently, there has been an influx of mononuclear Ln alkoxide and aryloxide SMMs, which have provided the necessary geometrical control to improve SMM properties and to allow the intricate relaxation dynamics of Ln SMMs to be studied in detail. Here non‐aqueous Ln alkoxide and aryloxide chemistry applied to the synthesis of low‐coordinate mononuclear Ln SMMs are reviewed. The focus is on mononuclear Dy^III alkoxide and aryloxide SMMs with coordination numbers up to eight, covering synthesis, solid‐state structures and magnetic attributes. Brief overviews are also provided of mononuclear Tb^III, Ho^III, Er^III and Yb^III alkoxide and aryloxide SMMs.

Assembly of chiral 3d–4f wheel-like cluster complexes with achiral ligands: single-molecule magnetic behavior and magnetocaloric effect

With the help of perchlorate ions, chiral M₃Ln₃ wheel-like cluster complexes were constructed from achiral ligands, and showed single-molecule magnetic behavior and magnetocaloric effect.

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.

Bis[1,8,15,22-tetrakis(3-pentyloxy)phthalocyaninato]terbium Double-Decker Single-Ion Magnets

For the purpose of further exploring the effect of nonperipherally attached substituents on single-ion magnet (SIMs) performance, tetrasubstituted bis[1,8,15,22-tetrakis(3-pentyloxy)phthalocyaninato]terbium double-deckers, in both the reduced form TbH[Pc(α-OC₅H₁₁)₄]₂ (1) and the neutral form Tb[Pc(α-OC₅H₁₁)₄]₂ (2), were prepared. Single-crystal X-ray diffraction analysis for 2 unambiguously demonstrates the pinwheellike molecular structure with C₄ symmetry. Magnetic investigations of the two bis(phthalocyaninato)terbium double-deckers reveal their characteristic SIM nature. 2 exhibits SIM performance superior to that of 1, as revealed by the larger energy barrier of 466 K for the former species and 431 K for the latter species due to the presence of organic radical-f (radical-Tb) interactions. The enhanced SIM performance of 2 in comparison to 1 actually stems from the presence of radical-f interactions and an enhanced ligand field strength. The latter positive factor is indicated by the electrostatic potential around the terbium ion on the basis of density functional theory (DFT) calculations.

Ferromagnetic coupling between 4f- and delocalized π-radical spins in mixed (phthalocyaninato)(porphyrinato) rare earth double-decker SMMs

Ferromagnetic π–f exchange couplings were revealed in [RE^III(Pc)(Por)]⁰ SMMs (RE = Tb and Dy) by HF-EPR measurements.

A rare chloride-bridged dysprosium chain with slow magnetic relaxation: a thermally activated mechanism via a second-excited state promoted by magnetic interactions

Magnetic interactions induced by chloride bridges promote the slow magnetic relaxation in a dysprosium chain to pass through the second-excited state.

Solvent-tuned magnetic exchange interactions in Dy2 systems ligated by a μ-phenolato heptadentate Schiff base

A series of binuclear dysprosium compounds, namely, [Dy(api)]₂ (1), [Dy(api)]₂·2CH₂Cl₂ (2), [Dy(Clapi)]₂·2C₄H₈O (3), and [Dy(Clapi)]₂·2C₃H₆O (4) (H₃api = 2-(2-hydroxyphenyl)-1,3-bis[4-(2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-imidazoline; H₃Clapi = 2-(2′-hydroxy-5′-chlorophenyl)-1,3-bis[3′-aza-4′-(2′′-hydroxy-5′′-chlorophenyl)prop-4′-en-1′-yl]-1,3-imidazolidine), have been isolated by the reactions of salen-type ligands H₃api/H₃Clapi with DyCl₃·6H₂O in different solvent systems. Structural analysis reveals that each salen-type ligand provides a heptadentate coordination pocket (N₄O₃) to encapsulate a Dy^III ion and all of the Dy^III centers in 1–4 adopt a distorted square antiprism geometry with D_4d symmetry. Magnetic studies showed that compound 1 did not exhibit single-molecule magnetic (SMMs) behavior. With the introduction of different lattice solvents, compounds 2–4 showed filed-induced slow magnetic relaxation with barriers U_eff of 18.2 K (2), 28.0 K (3) and 16.4 K (4), respectively. Ab initio calculations were employed to interpret the magnetization behavior of 1–4. The combination of experimental and theoretical data reveal the importance of the weak exchange interaction between the Dy^III ions in the observation of slow magnetic relaxation, and a relaxation mechanism has been developed to rationalize the observed difference in the U_eff values. The different lattice solvents influence Dy–O–Dy bond angles and thus alter the torsion of the square antiprism geometry, consequently resulting in distinct magnetic interactions and the magnetic behavior.

Solvent-tuning changes the magnetic exchange interaction and results in different magnetic relaxation dynamics in Dy₂ systems ligated by a μ-phenolato heptadentate Schiff base.

Novel phenyl-substituted pyrazinoporphyrazine complexes of rare-earth elements: optimized synthetic protocols and physicochemical properties

Novel synthetic protocols based on both template and direct methods were developed for phenyl-substituted pyrazinoporphyrazine complexes of rare-earth elements.

Hemiporphyrazine-Involved Sandwich Dysprosium Double-Decker Single-Ion Magnets

Both heteroleptic (phthalocyaninato)(hemiporphyrazinato) and homoleptic bis(hemiporphyrazinato) dysprosium double-decker complexes, Dy[H(Hp)₂] (1) and Dy[H(Pc)(Hp)] (2) (H₂Pc = metal-free phthalocyanine; H₂Hp = metal-free hemiporphyrazine), were designed, synthesized, and structurally characterized. The dysprosium center in both double-deckers are octa-coordinated with a nearly ideal square-antiprismatic coordination geometry, which provides an increased molecular anisotropy for the dysprosium ion and ensures the strengthened magnetic properties of both single-ion magnets (SIMs) in terms of coordination geometry. Magnetic studies reveal that both double-deckers exhibit typical SIM behavior with a spin reversal energy barrier of 80.1 ± 6.3 K for 1 and 57.3 ± 3.8 K for 2 as well as the hysteresis loops emerging at 3 K. In particular, introduction of two Hp ligands with four pyridine nitrogen atoms coordinated with the dysprosium spin center endows Dy[H(Hp)₂] (1) with the thus far highest energy barrier among the sandwich-type dysprosium SIMs with N₄-macrocyclic ligands, revealing the potential applications of sandwich-type lanthanide complexes with Hp ligands in molecular-based information storage.

Fabricating Bis(phthalocyaninato) Terbium SIM into Tetrakis(phthalocyaninato) Terbium SMM with Enhanced Performance through Sodium Coordination

The non-peripherally substituted 1,4,8,11,15,18,22,25-octa(butoxy)-phthalocyanine-involved unsymmetrical heteroleptic bis(phthalocyaninato) terbium double-decker, Tb(Pc){H[Pc(α-OC₄ H₉ )₈ ]} (Pc=unsubstituted phthalocyanine) (1), was revealed to exhibit typical single ion magnet (SIM) behavior with effective energy barrier, 180 K (125 cm^-1 ), and blocking temperature, 2 K, due to the severe deviation of the terbium coordination polyhedron from square-antiprismatic geometry. Fabrication of this double-decker compound into the novel tetrakis(phthalocyaninato) terbium pseudo-quadruple-decker Na₂ {Tb(Pc)[Pc(α-OC₄ H₉ )₈ ]}₂ (2) single molecule magnet (SMM) not only optimizes the coordination polyhedron of terbium ion towards the square-antiprismatic geometry and intensifies the coordination field strength, but more importantly significantly enhances the molecular magnetic anisotropy in the unsymmetrical bis(phthalocyaninato) double-decker unit, along with the change of the counter cation from H⁺ of 1 to Na⁺ of 2, leading to an significantly enhanced magnetic behavior with spin-reversal energy barrier, 528 K (367 cm^-1 ), and blocking temperature, 25 K. The present result is surely helpful towards developing novel tetrapyrrole lanthanide SMMs through rational design and self-assembly from bis(tetrapyrrole) lanthanide single ion magnet (SIM) building block.

Heteroleptic chiral bis(phthalocyaninato) terbium double-decker single-ion magnets

Chiral binaphthyl and dibutylamino were incorporated onto the periphery of the bis(phthalocyaninato) terbium SIM, confirming the effectiveness of tuning the double-decker SIM peroperties thorugh tuning the molecular magnetic anisotropy.

Elucidation of the two-step relaxation processes of a tetranuclear dysprosium molecular nanomagnet through magnetic dilution

A new centrosymmetric tetranuclear aggregate [Dy₄(L)₂(OAc)₈(CH₃OH)₂] (1) was assembled using a unique symmetrical Schiff base ligand 1,5-bis(salicylidene)-carbohydrazide (H₂L).