A Photomagnetic Sponge: High-Temperature Light-Induced Ferrimagnet Controlled by Water Sorption

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.

3 nm-wide Cyanometallate Fe-Co Tape Exhibiting Single-Chain Magnet Behavior

Treatment of Co(OTf)2·6H2O, Li[(pzTp)FeIII(CN)3], and H3PMo12O40·nH2O in protic solvents afforded two structurally related Fe-Co cyanometallate complexes: [{(pzTp)Fe(CN)3}3Co3(MeOH)10][PMo12O40]·H2O·11MeOH (1, pzTp- = tetra(pyrazolyl)borate) and {[(pzTp)Fe(CN)3]4Co3(MeOH)5(H2O)3}n[HPMo12O40]n·3 nMeOH·6.5nH2O (2). Complex 1 consists of a cyanide-bridged hexanuclear [Fe3Co3] cage, characterized by the fused conjunction of two mutually perpendicular trigonal bipyramids (TBPs, [Fe2Co3] and [Co2Fe3]), while complex 2 showcases an intricate cyanide-bridged Fe-Co tape comprising a central chain backbone of vertex-sharing [Fe2Co3] TBPs alongside peripheral [Fe2Co2] squares. Complex 2 is among the widest one-dimensional coordination assemblies characterized by the single-crystal X-ray diffraction technique. Magnetic studies revealed that complex 2 behaved as a single chain magnet with an effective energy barrier (Ueff/kB) of 46.8 K. Our findings highlight the possibilities in the development of cyanometallate-POM hybrid materials with captivating magnetic properties.

CO2-Sensitive Porous Magnet: Antiferromagnet Creation from a Paramagnetic Charge-Transfer Layered Metal-Organic Framework

Porous magnets that undergo a magnetic phase transition in response to gaseous adsorbates are desirable for the development of sustainable sensing and memory devices. Familiar gases such as O2 and CO2 are one class of target adsorbates because of their close association with life sciences and environmental issues; however, it is not easy to develop magnetic devices that respond to these ubiquitous gases. To date, only three examples of gas-responsive magnetic phase transitions have been demonstrated: (i) from a ferrimagnet to an antiferromagnet, (ii) its vice versa (i.e., change of magnetic phase), and (iii) from a ferrimagnet to a paramagnet (i.e., erasure of the magnetic phase). However, the creation of a magnet, meaning the change from a nonmagnet to a magnet by O2 or CO2 gas adsorption and magnetic switching by this phenomenon have not yet been explored. Herein, we report a CO2-induced antiferromagnet modified from a paramagnetic charge-flexible layered compound, [{Ru2(2,4-F2PhCO2)4}2TCNQ(OEt)2] (1; 2,4-F2PhCO2- = 2,4-difluorobenzoate; TCNQ(OEt)2 = 2,5-diethoxy-7,7,8,8-tetracyanoquinodimethane), where three molar equivalents of CO2 was accommodated at a CO2 pressure of 100 kPa. The magnetic change originates from charge fluctuation due to the transfer of electrons moving from the electron-donor to the electron-acceptor unit or vice versa, resulting in a change in the electron distribution induced by CO2 adsorption/desorption in the donor-acceptor-type charge transfer framework. Owing to the reversible electronic state change upon CO2 adsorption/desorption, these magnetic phases are switched, accompanied by modification of the electrical conductivity, which is boosted by the CO2 accommodation. This is the first example of the creation of a CO2-responsive magnet, which is promising for novel molecular multifunctional devices.

Direct Observation of Magnetic Domain and Magnetization Reversal on Prussian Blue-Based Magnetic Films

Knowledge of the magnetic domain is indispensable for understanding the magnetostatic properties of magnets. However, to date, the magnetic domain has not yet been reported in the field of molecule-based magnets. Herein, we study the magnetic domains of molecule-based magnets. Two magnetic films of iron/chromium hexacyanidochromate FexCr1-x[Cr(CN)6]2/3·5H2O (x = 0; Film 1 and x = 0.2; Film 2) were prepared for investigation. The temperature evolution of surface magnetization was measured using magnetic force microscopy. Film 1 showed a magnetic domain below Curie temperature (TC) and its positive-magnetic polarization increased monotonously with decreasing temperature, while Film 2 showed positive magnetic polarization below TC and switches from positive to negative magnetization through a demagnetization state at 146 K. This study originally reports the temperature variation of the magnetization state at the magnetization reversal. The magnetic domains appeared as a maze pattern with an approximate domain size of one-to-several micrometers. This work shows that research on molecule-based magnets can be expanded from magnetochemistry to the magnetostatic engineering of bulk magnets, molecule-based magnetostatic engineering.

Low-Coordinate Erbium(III) Single-Molecule Magnets with Photochromic Behavior

The structures and magnetic properties of photoresponsive magnets can be controlled or fine-tuned by visible light irradiation, which makes them appealing as candidates for ternary memory devices: photochromic and photomagnetic at the same time. One of the strategies for photoresponsive magnetic systems is the use of photochromic/photoswitchable molecules coordinated to paramagnetic metal centers to indirectly influence their magnetic properties. Herein, we present two erbium(III)-based coordination systems: a trinuclear molecule {[Er^III(BHT)₃]₃(dtepy)₂}^.4C₅H₁₂ (1) and a 1D coordination chain {[Er^III(BHT)₃(azopy)}_n·2C₅H₁₂ (2), where the bridging photochromic ligands belong to the class of diarylethenes: 1,2-bis((2-methyl-5-pyridyl)thie-3-yl)perfluorocyclopentene (dtepy) and 4,4′-azopyridine (azopy), respectively (BHT = 2,6-di-tert-butyl-4-methylphenolate). Both compounds show slow dynamics of magnetization, typical for single-molecule magnets (SMMs) as revealed by alternating current (AC) magnetic susceptibility measurements. The trinuclear compound 1 also shows an immediate color change from yellow to dark blue in response to near-UV irradiation. Such behavior is typical for the photoisomerization of the open form of the ligand into its closed form. The color change can be reversed by exposing the closed form to visible light. The chain-like compound 2, on the other hand, does not show significant signs of the expected trans–cis photoisomerization of the azopyridine in response to UV irradiation and does not appear to show photoswitching behavior.

Three-coordinate [Er^III(BHT)₃] single ion magnets undergo ligand addition reaction in pentane to form linear trinuclear photochromic nanomagnets where both functionalities persist.

Crystal Engineering and Photomagnetic Studies of CN-Bridged Coordination Polymers Based on Octacyanidometallates(IV) and [Ni(cyclam)]2

A series of new CN-bridged coordination networks of different dimensionality and topology was obtained through the modification of reaction conditions between [Ni(cyclam)]²⁺ (cyclam = 1,4,8,11-tetraazacyclotetradecane) and [W(CN)₈]^4–. The factors determining the reaction pathway are temperature and addition of the LiCl electrolyte. The products include three negatively charged frameworks incorporating Li⁺ guests: the 1D Li₂[Ni(cyclam)][W(CN)₈]·6H₂O (1) straight chain, the 1D Li₂[Ni(cyclam)][W(CN)₈]·2H₂O (2) zigzag chain, and the 2D Li₂[Ni(cyclam)]₃[W(CN)₈]₂·24H₂O (3) honeycomb-like network, as well as the 3D two-fold interpenetrating [Ni(cyclam)]₅[Ni(CN)₄][W(CN)₈]₂·11H₂O (4) network and the 1D [Ni(cyclam)][Ni(CN)₄]·2H₂O (5) chain, which result from partial decomposition of the starting complexes. Together with the previously characterized 3D [Ni(cyclam)]₂[W(CN)₈]·16H₂O (6) network, they constitute the largest family of CN-bridged coordination polymers obtained from the same pair of building blocks. All compounds exhibit paramagnetic behavior because of the separation of paramagnetic nickel(II) centers through the diamagnetic polycyanidometallates. However, the presence of the photomagnetically active octacyanidotungstate(IV) ions allowed observation of the magnetic superexchange after the violet light excitation (405 nm) for compound 3, which constitutes the first example of the photomagnetic effect in a Ni^II–[W^IV(CN)₈] system. The photomagnetic investigations for fully hydrated and dehydrated sample of 3, as well as for the isostructural octacyanidomolybdate(IV)-based network are discussed.

Six coordination networks of different dimensionality and topology can be obtained from the same pair of building blocks: [Ni(cyclam)]²⁺ and [W(CN)₈]⁴⁻ depending on reaction conditions. The negatively charged 2D Li₂[Ni(cyclam)]₃[W(CN)₈]₂·nH₂O microporous network is the first example of the photomagnetic effect in a Ni^II−[W^IV(CN)₈] system.

Magnetic interactions controlled by light in the family of Fe(II)-M(IV) (M = Mo, W, Nb) hybrid organic-inorganic frameworks

Three new hybrid organic-inorganic frameworks employing octacyanidometallates and 4,4'-bypiridine dioxide (4,4'-bpdo) as bridging molecules were prepared and characterized. The three-dimensional coordination frameworks {[Fe^II(μ-4,4'-bpdo)(H₂O)₂]₂[M^IV(CN)₈]·9H₂O}_n (Fe2Mo, Fe2W and Fe2Nb; M = Mo, W and Nb) are composed of cyanido-bridged chains, which are interconnected by the organic linkers. Magnetic measurements for Fe2Nb show a two-step transition to the antiferromagnetic state, which results from the cooperation of antiferromagnetic intra- and inter-chain interactions. Fe2Mo and Fe2W, on the other hand, behave as paramagnets at 2 K because of the diamagnetic character of the corresponding octacyanidometallate(IV) building units. However, after 450 nm light irradiation they show transition to the metastable high spin Mo^IV or W^IV states, respectively, with distinct ferromagnetic intrachain spin interactions, as opposed to the antiferromagnetic ones observed in the Fe2Nb framework.

Photo-Induced Magnetic Hysteresis in a Cyanide-bridged Two-dimensional [Mn2W] Coordination Polymer

2D magnetic materials have been opening a new horizon in materials science. It is challenging to switch the magnetic hysteresis of 2D magnetic materials via light irradiation, applicable for future...

Chiral porous CN-bridged coordination polymer mimicking MOF-74 and showing magnetization photoswitching

A chiral porous cyanide-bridged framework {[Mn^II(L)]₂[W^IV(CN)₈]·10H₂O}_n (1; L = 2,6-bis[1-(2-(N-methylamino)ethylimino)ethyl]-pyridine) showing a strong structural similarity to MOF-74 has been prepared and characterised. The crystallised water molecules can be easily removed below 60 °C, leading to a distinct crystal colour change and the activation of its photomagnetic properties - constituting the so called photomagnetic sponge behaviour of this system. The complete dehydration of 1 proceeds through a single-crystal-to-single-crystal transformation and the resulting anhydrous framework {[Mn^II(L)]₂[W^IV(CN)₈]}_n (1anh) was studied using single-crystal X-ray diffraction.

Large breathing effect induced by water sorption in a remarkably stable nonporous cyanide-bridged coordination polymer

While metal-organic frameworks (MOFs) are at the forefront of cutting-edge porous materials, extraordinary sorption properties can also be observed in Prussian Blue Analogs (PBAs) and related materials comprising extremely short bridging ligands. Herein, we present a bimetallic nonporous cyanide-bridged coordination polymer (CP) {[Mn(imH)]2[Mo(CN)8]} n (1Mn; imH = imidazole) that can efficiently and reversibly capture and release water molecules over tens of cycles without any fatigue despite being based on one of the shortest bridging ligands known - the cyanide. The sorption performance of {[Mn(imH)]2[Mo(CN)8]} n matches or even outperforms MOFs that are typically selected for water harvesting applications with perfect sorption reversibility and very low desorption temperatures. Water sorption in 1Mn is possible due to the breathing effect (accompanied by a dramatic cyanide-framework transformation) occurring in three well-defined steps between four different crystal phases studied structurally by X-ray diffraction structural analysis. Moreover, the capture of H2O by 1Mn switches the EPR signal intensity of the MnII centres, which has been demonstrated by in situ EPR measurements and enables monitoring of the hydration level of 1Mn by EPR. The sorption of water in 1Mn controls also its photomagnetic behavior at the cryogenic regime, thanks to the presence of the [MoIV(CN)8]4- photomagnetic chromophore in the structure. These observations demonstrate the extraordinary sorption potential of cyanide-bridged CPs and the possibility to merge it with the unique physical properties of this class of compounds arising from their bimetallic character (e.g. photomagnetism and long-range magnetic ordering).

New Photomagnetic Ionic Salts Based on [MoIV(CN)8]4− and [WIV(CN)8]4− Anions

Three new ionic salts containing [M(CN)8]4− (M = MoIV and WIV) were prepared using large complex cations based on a non-conventional motif built with the tris(2-aminoethyl)amine (noted hereafter tren) ligand, [{M’(tren)}3(μ-tren)]6+ (M’ = CuII and ZnII). The crystal structures of the three compounds show that the atomic arrangement is formed by relatively isolated anionic and cationic entities. The three compounds were irradiated with a blue light at low temperature, and show a significant photomagnetic effect. The remarkable properties of these compounds are (i) the long-lived photomagnetic metastable states for the [Mo(CN)8]4−-based compounds well above 200 K and (ii) the rare efficient photomagnetic properties of the [W(CN)8]4−-based compound. These photomagnetic properties are compared with the singlet-triplet conversion recently reported for the K4[Mo(CN)8]·2H2O compound.

Asymmetric Coordination Toward a Photoinduced Single-Chain Magnet Showing High Coercivity Values

The production of photo-switchable molecular nanomagnets with substantial coercivity, which is indispensable for information storage and process applications, is challenging. Introducing photo-responsive spin-crossover units provides a feasible means of controlling the magnetic anisotropy, interactions, and overall nanomagnet properties. Herein, we report a cyanide-bridged chain 1⋅12H₂ O ({[(^Pz Tp)Fe^III (CN)₃ ]₂ Fe^II (Pmat)₂ }_n ⋅12 H₂ O) generated by linking the Fe^II -based spin-crossover unit with the [(^Pz Tp)Fe(CN)₃ ]^- (^Pz Tp: tetrakis(pyrazolyl)borate) building block in the presence of asymmetric ditopic ligand Pmat ((4-pyridine-4-yl)methyleneamino-1,2,4-triazole). Structural characterization revealed that the introduction of this asymmetric ligand led to a distorted coordination environment of Fe^II ions, which were equatorially coordinated by four cyanide N atoms, and apically coordinated by one pyridine N atom and one triazole N atom. Upon 808-nm light irradiation, 1⋅12H₂ O underwent photoinduced spin-crossover and exhibited single-chain magnet behavior with a coercive field of up to 1.3 T. This represents a 3d-based photoinduced single-chain magnet exhibiting pronounced hysteresis.

Reversible Humidity-Driven Transformation of a Bimetallic {EuCo} Molecular Material: Structural, Sorption, and Photoluminescence Studies

Functional molecule-based solids built of metal complexes can reveal a great impact of external stimuli upon their optical, magnetic, electric, and mechanical properties. We report a novel molecular material, {[EuIII(H2O)3(pyrone)4][CoIII(CN)6]}·nH2O (1, n = 2; 2, n = 1), which was obtained by the self-assembly of Eu3+ and [Co(CN)6]3- ions in the presence of a small 2-pyrrolidinone (pyrone) ligand in an aqueous medium. The as-synthesized material, 1, consists of dinuclear cyanido-bridged {EuCo} molecules accompanied by two H-bonded water molecules. By lowering the relative humidity (RH) below 30% at room temperature, 1 undergoes a single-crystal-to-single-crystal transformation related to the partial removal of crystallization water molecules which results in the new crystalline phase, 2. Both 1 and 2 solvates exhibit pronounced EuIII-centered visible photoluminescence. However, they differ in the energy splitting of the main emission band of a 5D0 → 7F2 origin, and the emission lifetime, which is longer in the partially dehydrated 2. As the 1 ↔ 2 structural transformation can be repeatedly reversed by changing the RH value, the reported material shows a room-temperature switching of detailed luminescent features including the ratio between emission components and the emission lifetime values.

Guest-Dependent Pressure-Induced Spin Crossover in FeII 4 [MIV (CN)8 ]2 (M=Mo, W) Cluster-Based Material Showing Persistent Solvent-Driven Structural Transformations

Discrete molecular species that can perform certain functions in response to multiple external stimuli constitute a special class of multifunctional molecular materials called smart molecules. Herein, cyanido-bridged coordination clusters {[Fe^II (2-pyrpy)₂ ]₄ [M^IV (CN)₈ ]₂ }⋅4 MeOH⋅6 H₂ O (M=Mo (1 solv), M=W (2 solv) and 2-pyrpy=2-(1-pyrazolyl)pyridine are presented, which show persistent solvent driven single-crystal-to-single-crystal transformations upon sorption/desorption of water and methanol molecules. Three full desolvation-resolvation cycles with the concomitant change of the host molecules do not damage the single crystals. More importantly, the Fe₄ M₂ molecules constitute a unique example where the presence of the guests directly affects the pressure-induced thermal spin crossover (SCO) phenomenon occurring at the Fe^II centres. The hydrated phases show a partial SCO with approximately two out-of-four Fe^II centres undergoing a gradual thermal SCO at 1 GPa, while in the anhydrous form the pressure-induced SCO effect is almost quenched with only 15 % of the Fe^II centres undergoing high-spin to low-spin transition at 1 GPa.

Octacyanidometallates for multifunctional molecule-based materials

Octacyanidometallates have been successfully employed in the design of heterometallic coordination systems offering a spectacular range of desired physical properties with great potential for technological applications. The [M(CN)8]n- ions comprise a series of complexes of heavy transition metals in high oxidation states, including NbIV, MoIV/V, WIV/V, and ReV. Since the discovery of the pioneering bimetallic {MnII4[MIV(CN)8]2} and {MnII9[MV(CN)8]6} (M = Mo, W) molecules in 2000, octacyanidometallates were fruitfully explored as precursors for the construction of diverse d-d or d-f coordination clusters and frameworks which could be obtained in the crystalline form under mild synthetic conditions. The primary interest in [M(CN)8]n--based networks was focused on their application as molecule-based magnets exhibiting long-range magnetic ordering resulting from the efficient intermetallic exchange coupling mediated by cyanido bridges. However, in the last few years, octacyanidometallate-based materials proved to offer varied and remarkable functionalities, becoming efficient building blocks for the construction of molecular nanomagnets, magnetic coolers, spin transition materials, photomagnets, solvato-magnetic materials, including molecular magnetic sponges, luminescent magnets, chiral magnets and photomagnets, SHG-active magnetic materials, pyro- and ferroelectrics, ionic conductors as well as electrochemical containers. Some of these materials can be processed into the nanoscale opening the route towards the development of magnetic, optical and electronic devices. In this review, we summarise all important achievements in the field of octacyanidometallate-based functional materials, with the particular attention to the most recent advances, and present a thorough discussion on non-trivial structural and electronic features of [M(CN)8]n- ions, which are purposefully explored to introduce desired physical properties and their combinations towards advanced multifunctional materials.

Magnetic, Structural and Spectroscopic Properties of Iron(II)-Octacyanoniobate(IV) Crystalline Film Obtained by Ion-Exchange Synthesis

Over recent years, investigations of coordination polymer thin films have been initiated due to their unique properties, which are expected to be strongly enhanced in the thin film form. In this work, a crystalline [Fe^II(H₂O)₂]₂[Nb^IV(CN)₈]∙4H₂O (1) film on a transparent Nafion membrane was obtained, for the first time, via ion-exchange synthesis. The proper film formation and its composition was confirmed with the use of energy dispersive X-ray spectroscopy and infrared spectroscopy, as well as in situ Ultraviolet-Visible (UV-Vis) spectroscopy. The obtained film were also characterized by scanning electron microscopy, X-ray diffraction, and magnetic measurements. The [Fe^II(H₂O)₂]₂[Nb^IV(CN)₈]∙4H₂O film shows a sharp phase transition to a long-range magnetically ordered state at T_c = 40 K. The 1 film is a soft ferromagnet with the coercive field H_c = 1.2 kOe. Compared to the bulk counterpart, a decrease in critical temperature and a significant increase in the coercive field were observed in the films indicating a distinct size effect. The decrease in T_c could also have been related to the possible partial oxidation of Fe^II ions to Fe^III, which could be efficient, due to the large surface of the thin film sample.

Heterotrimetallic Cyanide-Bridged 3d-4d-5d Frameworks Based on a Photomagnetic Secondary Building Unit

The rational design of coordination frameworks combining more than two different metal ions using a self-assembly approach is challenging because it rarely offers sufficient control over the building blocks at the actual self-assembly stage. In this work, we present a successful two-step strategy toward heterotrimetallic coordination frameworks by employing a new bimetallic [(NC)₇Mo^IV-CN-Pt^IV(NH₃)₄-NC-Mo^IV(CN)₇]^4– secondary building unit (SBU). This anionic moiety has been isolated and characterized as a simple salt with an organic dppipH₂²⁺ cation (dppipH₂)₂[(NC)₇Mo^IV-CN-Pt^IV(NH₃)₄-NC-Mo^IV(CN)₇]·15H₂O (1) (dppip = 1,4-di(4-pyridinyl)piperazine). The salt presents a second-order phase transition related to cation conformational change around 250 K and a photomagnetic effect after irradiation with 450 nm light at 10 K. When combined with aqueous solutions of Mn^II or Cu^II complexes, it forms either a one-dimensional chain [Mn^II(dpop)][Mn^II(dpop)(H₂O)][(NC)₇Mo^IV-CN-Pt^IV(NH₃)₄-NC-Mo^IV(CN)₇]·36H₂O (2) (dpop = 2,13-dimethyl-3,6,9,12,18-pentaazabicyclo-[12.3.1]octadeca-1(18),2,12,14,16-pentaene) or a photomagnetic two-dimensional honeycomb network [Cu^II(cyclam)]₂[(NC)₇Mo^IV-CN-Pt^IV(NH₃)₄-NC-Mo^IV(CN)₇]·40.89H₂O (3) (cyclam = 1,4,8,11-tetraazacyclotetradecane), both characterized by very large cavities in their structure filled with solvent molecules. Both 2 and 3 incorporate three different transition-metal ions and constitute a new family of 3d-4d-5d coordination frameworks. Moreover, compound 3 inherits the photomagnetic properties of the MoPtMo SBU.

A photomagnetic secondary building unit (SBU) Mo^IVPt^IVMo^IV was employed to design and synthesize new heterotrimetallic coordination polymers in a two-step approach, resulting in Mn^II₂Mo^IV₂Pt^IV coordination chains and Cu^II₂Mo^IV₂Pt^IV honeycomb coordination layers.

Photoinduced Mo-CN Bond Breakage in Octacyanomolybdate Leading to Spin Triplet Trapping

The photoinduced properties of the octacoordinated complex K₄ Mo^IV (CN)₈ ⋅2 H₂ O were studied by theoretical calculations, crystallography, and optical and magnetic measurements. The crystal structure recorded at 10 K after blue light irradiation reveals an heptacoordinated Mo(CN)₇ species originating from the light-induced cleavage of one Mo-CN bond, concomitant with the photoinduced formation of a paramagnetic signal. When this complex is heated to 70 K, it returns to its original diamagnetic ground state, demonstrating full reversibility. The photomagnetic properties show a partial conversion into a triplet state possessing significant magnetic anisotropy, which is in agreement with theoretical studies. Inspired by these results, we isolated the new compound [K(crypt-222)]₃ [Mo^IV (CN)₇ ]⋅3 CH₃ CN using a photochemical pathway, confirming that photodissociation leads to a stable heptacyanomolybdate(IV) species in solution.

External stimuli modulate the magnetic relaxation of lanthanide single-molecule magnets

The magnetic relaxation of lanthanide single-molecule magnets (Ln-SMMs) can be modulated reversibly by external stimuli including light irradiation, thermal treatment, protonation/deprotonation and oxidation/reduction etc.

Humidity-A Powerful Tool to Customize the Physical Properties of Molecular Magnets

Since the dawn of the century, people have been fascinated by the mysterious force that draws two pieces of lodestone (magnetite, Fe₃ O₄ ) to each other as well as by property of stilbite (natural zeolite, NaCa₄ [Si₂₇ Al₉ O₇₂ ]⋅28 H₂ O) to generate huge amounts of steam by rapidly heating the material. Nowadays, we know the first effect is addressed by magnetic attraction whereas the second one is related to the reversible sorption/desorption of water from humid air inside porous materials. For a long time, it was thought that these two, at first glance, were disjunctive properties that cannot be combined into one material. Nevertheless, the scientists have once again proved that there are no impossible things. Based on the discovery of the first molecular magnets, the idea to combine magnetic properties with other functionalities such as porosity gained great consideration by scientists from different research fields. Very soon, we witnessed numerous reports of novel multifunctional materials among which we can distinguish humidity-responsive magnets. In this manuscript, the most outstanding results for such systems working at normal temperature and pressure (NTP) will be presented to motivate in-depth research on this topic.

A Design Criteria to Achieve Giant Ising-Type Anisotropy in CoII -Encapsulated Metallofullerenes

Discovery of permanent magnetisation in molecules just like in hard magnets decades ago led to the proposal of utilising these molecules for information storage devices and also as Q-bits in quantum computing. A significant breakthrough with a blocking temperature as high as 80 K has been recently reported for lanthanocene complexes. While enhancing the blocking temperature further remains one of the primary challenges, obtaining molecules that are suitable for the fabrication of the devices sets the bar very high in this area. Encouraged by the fact that our earlier predictions of potential single-molecule magnets (SMMs) in lanthanide-containing endohedral fullerenes have been verified, here we set out to undertake a comprehensive study on Co^II -ion-encapsulated fullerene as potential SMMs. To study this class of molecules, we have utilised an array of theoretical methods ranging from density functional to ab initio CASSCF/NEVPT2 methods for obtaining reliable estimate of zero-field splitting parameters D and E. Additionally, we have also employed, for the first time a combination of molecular dynamics based on DFT methods coupled with CASSCF/NEVPT2 methods to seek the role of conformational isomers in the relaxation of magnetisation. Particularly, we have studied, Co@C₂₈ , Co@C₃₈ and Co@C₄₈ cages and their isomers as potential target molecules that could yield substantial magnetic anisotropy. Our calculations categorically reveal a very large Ising anisotropy in this class of molecules, with Co@C₄₈ cages predicted to yield D values as high as -127 cm^-1 . Our calculations on the smaller cages reveal the free movement of Co^II ion inside the cage, leading to the likely scenario of faster relaxation of magnetisation. However, larger fullerene cages were found to solve this issue. Further models with incorporating units such as {CoOZn}, {CoScZnN} inside larger fullerenes yield axial zero-field splitting values as high as -200 cm^-1 with negligible E/D values. As these units represent a strong axiality coupled with a viable way to obtain air-stable low-coordinate Co^II complexes, this opens up a new paradigm in the search of SMMs in this class of molecules.

Site-Selective Photoswitching of Two Distinct Magnetic Chromophores in a Propeller-Like Molecule To Achieve Four Different Magnetic States

Magnetic photoswitching is a highly important but relatively rare phenomenon for enabling optical writing/reading of the magnetic state of a molecule. In this work, an unprecedented site-selective double photoswitching is reported from the assembly of two different "photomagnetic chromophores" into a single hexanuclear molecule: namely, a spin-crossover Fe(II) center exhibiting light-induced excited spin state trapping (LIESST) and a photochemically active octacyanometalate(IV) unit. Four different magnetization levels are accessible through the appropriate combination of violet/red light and temperature, results that highlight the potential of photomagnetic molecules as future molecular memory cells.

Humidity-Induced Switching between Two Magnetic and Structural Phases in a CoII -[WV (CN)8 ] Molecular Magnet

The self-assembly of cobalt(II) with purine and octacyanidotungstate(V) results in the formation of the three-dimensional Co₃ [W(CN)₈ ]₂ (purine)₂ ⋅8.5H₂ O (1) coordination polymer. This compound exhibits humidity-induced variation of the number of water molecules of crystallisation leading to a reversible structural phase transition and the alternation of the long-range ferromagnetic ordering temperature from T_C =29 K for the pristine assembly (1) to T_C =49 K for the sample stored in a low-humidity atmosphere (1-deh). This phenomenon can be attributed to a reversible change in the hydrogen-bonding network resulting in the modification of the local geometries of cobalt(II) as well as the cyanido bridges.

Correlating magnetic anisotropy with [Mo(CN)7]4− geometry of MnII–MoIII magnetic frameworks

The magnetic coercivity of four new heptacyanomolybdate(iii)-based coordination polymers is correlated with the geometry of the [Mo^III(CN)₇]⁴⁻ anion and the cyanide bridging pattern of the framework.

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.