Reversible light-induced spin state switching in a dinuclear Fe(II) spin crossover complex.

A dinuclear Fe(II) spin crossover (SCO) complex with the formula [Fe2L5(NCS)4]·2DMF·2H2O (1) was synthesised from 1-naphthylimino-1,2,4-triazole (L). Complex 1 exhibits an incomplete thermally induced spin transition with a transition temperature T1/2 of 95 K and a thermally trapped metastable high-spin state at low temperatures. Furthermore, it undergoes a reversible light-induced spin crossover by alternate irradiation with 532 and 808 nm lasers.

Simultaneous magneto-dielectric transitions in a fluorescent Hofmann-type coordination polymer.

The design of magnetic molecular materials exhibiting multiple functions has garnered significant interest owing to their potential applications in molecular switches, sensors, and data storage devices. In this study, we synthesized a two-dimensional (2D) FeII-based Hofmann-type coordination polymer, namely {Fe(DPPE)2[Ag(CN)2]2}·2EtOH (1), using a luminescent ligand 1,1-diphenyl-2,2-di(4-pyridylbiphenyl)ethylene (DPPE). Single-crystal structural analyses and magnetic measurements revealed a thermally induced spin crossover (SCO) with the transition temperature T1/2 = 231 K. Variable-temperature fluorescence emission spectra indicated the coexistence of spin crossover and fluorescence properties. Moreover, a pronounced dielectric change (Δε' = 1.2 at 0.5 kHz) was observed during the SCO process, confirming the simultaneous magnetic and dielectric switching arising from the rearrangement of 3d electrons and deformation of the FeII-centered coordination sphere. This work provides an approach to explore the interplay between magnetic, dielectric, and fluorescence properties, and holds significance for developing multifunctional molecular materials.

Regulating Magnetic Relaxations of Cyano-Bridged {DyIII MoV } Systems by Tuning the N-Sites in ß-Diketone Ligands.

Cyano-bridged 4d-4f molecular nanomagnets have re-called increasing research interests in molecular magnetism since they offer more possibilities in achieving novel nanomagnets with versatile structures and magnetic interactions. In this work, four ß-diketone ligands bearing different substitution N-sites were designed and synthesized, namely 1-(2-pyridyl)-3-(3-pyridyl)-1,3-propanedione (HL1 ), 1,3-Bis (3-pyridyl)-1,3-propanedione (HL2 ), 1-(4-pyridyl)-3-(3-pyridyl)-1,3-propanedione (HL3 ), and 1,3-Bis (4-pyridyl)-1,3-propanedione (HL4 ), to tune the magnetic relaxation behaviors of cyano-bridged {DyIII MoV } systems. By reacting with DyCl3 ⋅ 6H2 O and K4 Mo(CN)8 ⋅ 2H2 O, four cyano-bridged complexes, namely {[Dy[MoV (CN)8 ](HL1 )2 (H2 O)3 ]} ⋅ 6H2 O (1), {[Dy[MoV (CN)8 ](HL2 )(H2 O)3 (CH3 OH)]}2 ⋅ 2CH3 OH ⋅ 3H2 O (2), {[Dy[MoV (CN)8 ](HL3 )(H2 O)2 (CH3 OH)] ⋅ H2 O}n (3), and {[Dy[MoV (CN)8 ](HL4 )2 (H2 O)3 ]} ⋅ 2H2 O⋅CH3 OH (4) were obtained. Structural analyses revealed that 1 and 4 are binuclear complexes, 2 has a tetragonal structure, and 3 exhibits a stair-like polymer chain structure. The DyIII ions in all complexes have eight-coordinated configurations with the coordination spheres DyO7 N1 for 1 and 4, DyO6 N2 for 2, and DyO5 N3 for 3. Magnetic measurements indicate that 1 is a zero-field single-molecule magnet (SMM) and complexes 2-4 are field-induced SMMs, with complex 4 featuring a two-step relaxation process. The magnetic characterizations and ab initio calculations revealed that changing the N-sites in the ß-diketone ligands can effectively alter the structures and magnetic properties of cyano-bridged 4d-4f nanomagnets by adjusting the coordination environments of the DyIII centers.

Manipulating fluorescence by photo-switched spin-state conversions in an iron(ii)-based SCO-MOF.

Manipulating fluorescence by photo-switched spin-state conversions is an attractive prospect for applications in smart magneto-optical materials and devices. The challenge is how to modulate the energy transfer paths of the singlet excited state by light-induced spin-state conversions. In this work, a spin crossover (SCO) FeII-based fluorophore was embedded into a metal-organic framework (MOF) to tune the energy transfer paths. Compound 1 {Fe(TPA-diPy)[Ag(CN)2]2}·2EtOH (1) has an interpenetrated Hofmann-type structure, wherein the FeII ion is coordinated by a bidentate fluorophore ligand (TPA-diPy) and four cyanide nitrogen atoms and acts as the fluorescent-SCO unit. Magnetic susceptibility measurements revealed that 1 underwent an incomplete and gradual spin crossover with T1/2 = 161 K. Photomagnetic studies confirmed photo-induced spin state conversions between the low-spin (LS) and high-spin (HS) states, where the irradiation of 532 and 808 nm laser lights converted the LS and HS states to the HS and LS states, respectively. Variable-temperature fluorescence spectra study revealed an anomalous decrease in emission intensity upon the HS → LS transition, confirming the synergetic coupling between the fluorophore and SCO units. Alternating irradiation of 532 and 808 nm laser lights resulted in reversible fluorescence intensity changes, confirming spin state-controlled fluorescence in the SCO-MOF. Photo-monitored structural analyses and UV-vis spectroscopic studies demonstrated that the photo-induced spin state conversions changed energy transfer paths from the TPA fluorophore to the metal-centered charge transfer bands, ultimately leading to the switching of fluorescence intensities. This work represents a new prototype compound showing bidirectional photo-switched fluorescence by manipulating the spin states of iron(ii).

Colossal Anisotropic Thermal Expansion through Coupling Spin Crossover and Rhombus Deformation in a Hexanuclear {FeIII 4 FeII 2 } Compound.

Colossal and anisotropic thermal expansion is a key function for microscale or nanoscale actuators in material science. Herein, we present a hexanuclear compound of [(Tp*)FeIII (CN)3 ]4 [FeII (Ppmp)]2 ⋅2 CH3 OH (1, Tp*=hydrotris(3,5-dimethyl-pyrazol-1-yl)borate and Ppmp=2-[3-(2'-pyridyl)pyrazol-1-ylmethyl]pyridine), which has a rhombic core structure abbreviated as {FeIII 2 FeII 2 }. Magnetic susceptibility measurements and single-crystal X-ray diffraction analyses revealed that 1 underwent thermally-induced spin transition with the thermal hysteresis. The FeII site in 1 behaved as a spin crossover (SCO) unit, and significant deformation of its octahedron was observed during the spin transition process. Moreover, the distortion of the FeII centers actuated anisotropic deformation of the rhombic {FeIII 2 FeII 2 } core, which was spread over the whole crystal through the subsequent molecular rearrangements, leading to the colossal anisotropic thermal expansion. Our results provide a rational strategy for realizing the colossal anisotropic thermal expansion and shape memory effects by tuning the magnetic bistability.

Bistable Optoelectronic Properties Originated from the Scissoring Motion of the TEMPO Skeleton in Supramolecular Radical Ferroelectrics.

Bistable materials with multiphysical channels, such as optical, electrical, and magnetic properties, have been paid dramatic attention due to their alternativity of the signal status in electronic devices. Herein, three stable supramolecular radicals ([(NH3-TEMPO)(18-crown-6)][XF6] (1, X = P; 2, X = As; 3, X = Sb)) were synthesized and characterized. The former two molecules present ferroelectric phase transitions around 381.7 and 382.7 K, respectively, with bistability in dielectric property and second-harmonic generation (SHG) effect, which are first found in supramolecular radicals. Their ferroelectric transition and bistable properties are generated from a net polar crystal structure owing to the static ordered packing of NH3-TEMPO radical cations in the low-temperature phase (LTP) to a nonpolar structure owing to a distinctive symmetric scissoring motion of NH3-TEMPO radical cations between two 18-crown-6 molecules in the high-temperature phase (HTP). Both of them exhibit paramagnetic properties in HTP and LTP states since no intermolecular spin-spin interaction occurs due to the long distances among the radicals in their crystals. These results make us possible to design bistable optoelectronic radical materials with bistability in magnetic property in the future.

Constructing spin crossover-fluorescence bifunctional iron(II) complexes based on tetraphenyl ethylene-decorated AIEgens.

Investigating spin crossover (SCO)-fluorescence bifunctional materials and establishing their structure-function relationships are attractive topics in chemistry and materials science. However, it remains challenging to preserve the fluorescence and SCO properties simultaneously in aggregated solid states. Herein, we design an (E)-2,6-bis(1H-pyrazol-1-yl)-4-(4-(1,2,2-triphenylvinyl)styryl)pyridine (tpe-bpp) ligand, which contains coordinated SCO and fluorescence units of an aggregation-induced emission luminogen (AIEgen). The coordination of the tpe-bpp ligand with different FeII salts generated three mononuclear complexes: [Fe(tpe-bpp)2](ClO4)2·5.75CH2Cl2 (1), [Fe(tpe-bpp)2](ClO4)2·CH2Cl2·3CH3OH (2) and [Fe(tpe-bpp)2](BF4)2·CH2Cl2·3CH3OH (3). Single-crystal X-ray diffraction studies showed that they shared a similar [Fe(tpe-bpp)2]2+ complex cation. Their counterions and co-crystallized solvents were different. Magnetic measurements revealed that 1, 2, and 3 exhibited a complete SCO behavior with the transition temperatures T1/2 of 375, 260, and 248 K, respectively. Fluorescence measurements confirmed the existence of the AIE property for both the tpe-bpp ligand and Fe(II) complexes. A monotonic decrease of the photoluminescence (PL) intensity upon increasing the temperature was behavior observed for all three complexes.

Simultaneous Photo-Induced Magnetic and Dielectric Switching in an Iron(II)-Based Spin-Crossover Hofmann-Type Metal-Organic Framework.

Molecular materials possessing photo-tunable polarization switching is promising for optical switches, smart sensors, and data storage devices. However, it is challenging to devise a molecular material featuring simultaneous switchable magnetic and dielectric properties with regard to non-invasive and convenient light stimulus. Herein, we report a new Hofmann-type metal-organic framework (MOF) {Fe(bpt)[Pt(CN)4 ]} ⋅ 0.5anth (1, bpt=2,5-bis(4-pyridyl)thiophen; anth=anthracene), which displays thermo- and photo-switchable magnetic and dielectric properties. Photo-monitored structural analyses revealed that it was the photo-induced deformation of FeII coordination sphere and relative movement of guest anthracene that resulted in the variation of the local electric dipoles. These findings provide a new strategy to realize polarization switching through the light-induced spin crossover, and would be of fundamental significance for future photo-switchable and multifunctional materials.

Construction of spin-crossover dinuclear cobalt(II) compounds based on complementary terpyridine ligand pairing.

The self-assembly of multinuclear SCO complexes is appealing in which unique properties may be discovered due to enhanced intramolecular and intermolecular interactions. In this work, three dinuclear cobalt(II) complexes, named Co-1, Co-2, and Co-3, were prepared based on a complementary terpyridine ligand pair strategy. The complexes were accurately synthesized by the solvothermal method in which dinuclear complexes were directional assemblies from cobalt(II) ions, terpy bearing 2,6-dimethoxyphenyl substituents at the terpyridyl 6,6''-positions, and ditopic terpy built with different linkers (alkynyl for 1, diynyl for 2, and phenyl for 3). Single-crystal structure determinations reveal that all compounds possess a central symmetric molecular structure, so that two cobalt(II) units are identical in the solid state. Their spin crossover behaviours were investigated through variable-temperature magnetic susceptibility studies. Co-1 undergoes limited SCO with a large population of low spin state (S = 1/2) in the measured temperatures. Co-2 and Co-3 exhibit solvent-modulated SCO behaviour. Impressively, the de-solvated samples show a repeatable thermal hysteresis loop around the room temperature region. This work demonstrates that complementary terpyridine ligand pairing is a practical approach to accurate and directional construction of multinuclear SCO-active compounds.

Spin-Crossover Tuned Rotation of Pyrazolyl Rings in a 2D Iron(II) Complex towards Synergetic Magnetic and Dielectric Transitions.

Materials showing synergy of magnetic and dielectric transitions are promising candidates for future molecular devices. The challenge is how to realize synergy between spin and dielectric transitions with responses to external stimuli. Herein, we design a 2D spin crossover (SCO) complex, [FeII (dpa)][(pzTp)FeIII (CN)3 ]2 (1) (dpa=1,2-bis(4-pyridyl)ethyne and pzTp=tetrakis(pyrazolyl)borate). The local structural changes about the FeII ion were propagated to the whole crystal through the rigid bridging ligands (dpa), leading to elastic interactions to realize the abrupt SCO and rotational movements of polar apical pyrazolyl rings in the [(pzTp)FeIII (CN)3 ]- units. Dielectric measurements confirmed a substantial dielectric change (Δϵ'=2.3) upon the spin transition. This work provides a rational strategy to couple the spin transition and rotation of polar components, which is crucial for the synergetic switch of magnetism and dielectricity.

Slow magnetic relaxation in mononuclear octa-coordinate Fe(II) and Co(II) complexes from a Bpybox ligand.

Two 3d transition metal mononuclear complexes, [(FeL2)(ClO4)2]2·CH3CN (1) and (CoL2)(ClO4)2·2CH3CN (2), have been prepared from a rigid tetradentate bpybox (L = 6,6'-bis(2,5-dihydrooxazol-4-yl)-2,2'-bipyridine) ligand. Single crystal X-ray diffraction analyses together with the help of calculations show that both compounds are octa-coordinate. Direct current magnetic studies reveal their significant magnetic anisotropy. Impressively, field-induced relaxation of magnetism is observed in the two complexes and the apparent anisotropy barriers are 14.1 K for 1 and 21.6 K for 2, respectively. Theoretical calculations reveal that two Fe(II) centers in 1 have small negative D values of -4.897 and -4.825 cm-1 and relatively small E values of 0.646 and 0.830 cm-1, indicating a uniaxial magnetic anisotropy. In contrast, the D and E values in the Co(II) center of 2 are 46.42 cm-1 and 11.51 cm-1, featuring a rhombic anisotropy. This work demonstrates that field-induced slow magnetic relaxation in 3d transition metal complexes with high coordination numbers can be manipulated through rigid ligand design.

[AuI(CN)2]-Armed [FeIII2FeII2] Square Complex Showing Unusual Spin-Crossover Behavior Due to a Symmetry-Breaking Phase Transition.

The incorporation of two different cyanide building blocks of [(TpR)FeIII(CN)3]- and [AuI(CN)2]- into one molecule afforded a novel hexanuclear [FeIII2FeII2AuI2] complex (1·2Et2O), in which the cyanide-bridged [FeIII2FeII2] square was further grafted by two [AuI(CN)2]- fragments as long arms in syn orientations. Complex 1·2Et2O undergoes a gradual spin crossover (SCO) ffrom low-spin (LS) to high-spin (HS) state for the Fe(II) centers upon desolvation. Remarkably, its desolvated phase (1) exhibits a reversible but atypical two-step (sharp-gradual) SCO behavior with considerable hysteresis (21 K). Variable-temperature single-crystal X-ray structural studies reveal that the hysteretic spin transition takes place synchronously with the concerted displacive motions of the molecules, representing another rare example including multistep and hysteretic spin transitions due to the synergetic SCO and structural phase transition.

Thermal and photoinduced spin-crossover of mononuclear FeII complexes based on bppCHO ligand.

Two new iron(II) complexes [Fe(bppCHO)2](ClO4)2 (1·ClO4) and [Fe(bppCHO)2](BF4)2·H2O (1·BF4) were synthesized by using 2,6-di(1H-pyrazol-1-yl)isonicotinaldehyde (bppCHO) as the ligand. The structures and spin states of the complexes were characterized by single-crystal X-ray diffraction and magnetic susceptibility measurements. Complex 1·ClO4 exhibits light-induced excited spin state trapping (LIESST) up to 53 K and two-step spin-crossover (SCO) over room temperature, in which the first step shows a thermal hysteresis of 26 K, whereas 1·BF4 exhibits a gradual SCO behavior. A magnetostructural study reveals that the difference in the SCO property is related to the supramolecular interactions and crystal packing effect.

Four mononuclear dysprosium complexes with neutral Schiff-base ligands: syntheses, crystal structures and slow magnetic relaxation behavior.

Four mononuclear 9-coordinate Dy-based complexes, [Dy(HL1)2(NO3)3(CH3OH)] (1Dy), [Dy(HL2)2(NO3)3(H2O)] (2Dy), [Dy(HL3)3(NO3)3]·CH3CN (3Dy), and [Dy(HL4)3(NO3)3] (4Dy), have been constructed by neutral Schiff-base ligands (1-[N-(4-R)aminomethylidene-2(1H)-naphthalenone, R = -Cl (HL1), -NO2 (HL2), -OCH3 (HL3), -I (HL4)). By tuning the terminal substituent group of HL ligands, the number of HL ligands coordinated to the central Dy3+ ion unexpectedly varies from 2 to 3, and the local symmetry around the Dy3+ ion reduces from D3h to Cs. Magnetic measurements reveal that 2Dy can display single-ion magnet (SIM) behavior in zero dc field, while 1Dy, 3Dy and 4Dy show field-induced slow magnetic relaxation. Ab initio calculations were employed to elucidate magnetic anisotropy in the complexes, including g-tensors, averaged transition magnetic moments and magnetic easy axes. The difference in magnetic behaviors of the four complexes can be ascribed to the terminal substituent effect of neutral Schiff-base ligands.

Manipulating Selective Metal-to-Metal Electron Transfer to Achieve Multi-Phase Transitions in an Asymmetric [Fe2 Co]-Assembled Mixed-Valence Chain.

Manipulation of multi-functions in molecular materials is promising for future switching and memory devices, although it is currently difficult. Herein, we assembled the asymmetric {Fe2 Co} unit into a cyanide-bridged mixed-valence chain {[(Tp)Fe(CN)3 ]2 Co(BIT)} ⋅ 2CH3 OH (1) (Tp=hydrotris(pyrazolyl)borate and BIT=3,4-bis-(1H-imidazol-1-yl)thiophene), which showed reversible multi-phase transitions accompanied by photo-switchable single-chain magnet properties and a dielectric anomaly. Variable-temperature X-ray structural studies revealed thermo- and photo-induced selective electron transfer (ET) between the Co and one of the Fe ions. Alternating-current magnetic susceptibility studies revealed that 1 displayed on and off single-chain magnet behavior by alternating 946-nm and 532-nm light irradiation. A substantial anomaly in the dielectric constant was discovered during the electron transfer process, which is uncommon in similar ET complexes. These findings illustrate that 1 provided a new platform for multi-phase transitions and multi-switches adjusted by selective metal-to-metal ET.

An azido-bridged [FeII4] grid-like molecule showing spin crossover behaviour.

The supramolecular self-assembly synthetic strategy provides a valid tool to obtain polynuclear Fe(II) complexes having effective communication between the metal centres and distinct spin crossover behaviour. Despite the great success in constructing various magnetic molecules, progress has not been made in SCO complexes based on azido bridges. In this article, the coordination-driven supramolecular assembly based on 3,6-substituted pyridazine and azide is presented to afford two Fe(II) grid-like complexes: [(L)4FeII4(N3)4][BPh4]4·sol (1, L = 3,6-bis(3,5-dimethyl-1H-pyrazol-1-yl)pyridazine and 2, L = 3,6-di(pyridin-2-yl)pyridazine). The substitution of pyridinyl groups in 2 instead of pyrazolyl ones in 1 led to the only example exhibiting spin-crossover behaviour (T1/2 = 230 K) among the azido-bridged complexes. In addition, a temperature-dependent photoluminescence study of 2 demonstrates a visible synergetic effect between the SCO event and the luminescence.

Probe metal binding mode of imine covalent organic frameworks: cycloiridation for (photo)catalytic hydrogen evolution from formate.

Metalation of covalent organic frameworks (COFs) is a critical strategy to functionalize COFs for advanced applications yet largely relies on the pre-installed specific metal docking sites in the network, such as porphyrin, salen, 2,2'-bipyridine, etc. We show in this study that the imine linkage of simple imine-based COFs, one of the most popular COFs, readily chelate transition metal (Ir in this work) via cyclometalation, which has not been explored before. The iridacycle decorated COF exhibited more than 10-fold efficiency enhancement in (photo)catalytic hydrogen evolution from aqueous formate solution than its molecular counterpart under mild conditions. This work will inspire more functional cyclometallated COFs to be explored beyond catalysis considering the large imine COF library and the rich metallacycle chemistry.

Switching the magnetic hysteresis of an [Feii-NC-Wv]-based coordination polymer by photoinduced reversible spin crossover.

Magnetic bistable materials that feature magnetic hysteresis are comparable to elementary binary units and promising for application in switches and memory devices. In this work, we report a material that consists of parallel cyanide-bridged [Feii-Wv] coordination chains linked together through rigid bis(imidazolyl)-benzene ligands and displays multiple magnetic states. The paramagnetic high-spin and diamagnetic low-spin states of the spin-crossover Feii ions can be interconverted by reversible light-induced excited spin state trapping (LIESST) by alternating between light irradiation of 808 and 473 nm. At 1.8 K, under 808-nm-light irradiation, magnetic interactions between the photogenerated paramagnetic high-spin Feii centres and the Wv centres lead to long fragments that exhibit single-chain magnet behaviour, with a wide magnetic hysteresis and a large coercive field of 19 kOe; under a 473 nm light, isolated Feii-Wv fragments behave as single-molecule magnets instead. At 3.3 K, the high-spin form still displays magnetic hysteresis, albeit narrower, whereas the low-spin one does not.

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)FeIII (CN)3 ]2 FeII (Pmat)2 }n ⋅12 H2 O) generated by linking the FeII -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 FeII 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.

Ligand symmetry significantly affects spin crossover behaviour in isomeric [Fe(pybox)2]2+ complexes.

The understanding of the correlation between the spin-state behaviour and the structural features in transition-metal complexes is of pronounced importance to the design of spin crossover compounds with high performance. However, the study of the influence of ligand symmetry on the spin crossover properties is still limited due to the shortage of suitable structural systems. Herein we report the magneto-structural correlations of three mononuclear Fe(ii) isomers with respect to their ligand symmetry. In this work, two phenyl-substituted meso and optically pure pybox ligands were employed to construct meso (1), optically pure (2), and racemic (3) ligand types of [Fe(pybox)2]2+ complexes. Their magnetic susceptibilities were measured via temperature-dependent paramagnetic 1H NMR spectroscopy. We fitted the midpoint temperatures of the transition (T1/2) of 260 K for 1(ClO4), 247 K for 2(ClO4), and 281 K for 3(ClO4). The influence of structural symmetry on spin crossover was rationalized through density functional theory calculations. The optimized structures of [Fe(pybox)2]2+ complex cations show that the geometric distortion of the central FeN6 coordination sphere is mainly caused by the steric congestions between adjacent phenyl substituents. In these compounds, there is a distinct correlation that more steric congestions produce larger coordination distortion and favor the electron configuration in the high-spin state, which reflects in the increase of T1/2. Additionally, the influence of the counter anion and lattice solvent on the meso series compounds was inspected. It is revealed that multiple factors dominate the spin-state behaviour in the solid state. This work provides deep insight into the effect of ligand symmetry on the spin transition behaviour in spin crossover compounds. It demonstrates that molecular symmetry should be considered in the design of spin crossover compounds.