Four-step electron transfer coupled spin transition in a cyano-bridged [Fe2Co2] square complex

The design of molecular functional materials with multi-step magnetic transitions has attracted considerable attention. However, the development of such materials is still infrequent and challenging. Here, a cyano-bridged square Prussian blue complex that exhibits a thermally induced four-step electron transfer coupled spin transition (ETCST) is reported. The magnetic and spectroscopic analyses confirm this multi-step transition. Variable-temperature infrared spectrum suggested the electronic structures in each phase and a four-step transition model is proposed.

Fully Optical Control of Polarization Current Direction in a Cyanide-Bridged Trinuclear Complex

As a remote and non-contact stimulus, light offers the potential for manipulating the polarization of ferroelectric materials without physical contact. However, in current research, the non-contact write-read (erase) process lacks direct observation through the stable current as output signal. To address this limitation, we investigated the photoinduced polarization switching capabilities of the cyanide-bridged compound [Fe2Co] using visible light, leading to the achievement of rewritable polarization. By subjecting [Fe2Co] crystals to alternating irradiation with 785 nm and 532 nm light, the polarization changes exhibited a distinct square wave pattern, confirming the reliability of the writing and erasing processes. Initialization involved exposing specific crystal units to 532 nm light for storing "1" or "0" information, while reading was accomplished by scanning the units with 785 nm light, resulting in brief current pulses for "1" states and no current signal for "0" states. This research unveils new possibilities for optical storage systems, paving the way for efficient and rewritable data storage and retrieval technologies, such as the next-generation memories.

Assembling Smallest Prussian Blue Analogs Using Chiral Hydrogen Bond-Donating Unit toward Complete Phase Transition

Exploring methods for assembling functional materials at the molecular level may yield functional expressions derived from the assembly method. This study developed novel switchable molecular assemblies characterized by abrupt, complete phase transitions promoted via hydrogen bonding with a chiral carboxylic acid. These assemblies were prepared by aggregating discrete molecules that are unresponsive to external stimuli. Furthermore, enantiopure hydrogen-bond donor (HBD) molecules provide switchable compounds with cooperative and abrupt phase transitions, whereas the racemic mixture of the HBD provides a hydrogen-bonded one-dimensional compound with a broad and incomplete phase transition when structural disordering is observed. This study presents a novel strategy for observing metal-to-metal electron-transfer-coupled spin transitions via hydrogen-bond formation.

Photoinduced large magnetic change at room temperature and radical-quenched spin glass in a cyanide-bridged MnII-FeIII compound

By the coordination assembly of a redox photoactive functional motif and a cyanide-bridged moiety, a cyanide-bridged MnII-FeIII compound with large photoinduced magnetic change at room-temperature due to photoinduced electron transfer was obtanied. This compound also shows unprecedented radical-quenched spin glass in molecule based magnets.

Thermo- and photo-induced electron transfer in a series of [Fe2Co2] capsules

Recently, a family of [Fe₂Co₂] molecular capsules that display tunable electron transfer-coupled spin transition (ETCST) behavior were reported via a smart approach through Schiff-base condensation of aldehyde-functionalized 2,2-bipyridines (bpy^CHO) and 1,7-heptanediamine (H₂N(CH₂)₇NH₂). Here, three more capsule complexes {[(Tp^R)Fe(CN)₃]₂[Co(bpy^{CN(CH₂)_nNC}bpy)]₂[ClO₄]₂}·n(solvent) (1, Tp^R = Tp*, n = 5, sol = 8DMF; 2, Tp^R = Tp^Me, n = 9, sol = 5MeCN; and 3, Tp^R = Tp*, n = 11, sol = 5MeCN), where Tp* = hydridotris(3,5-dimethylpyrazol-1-yl)borate and Tp^Me = hydridotris(3-methylpyrazol-1-yl)borate are reported, demonstrating a successful extension of such an approach with other alkyldiamines of different lengths. Combined X-ray crystallographic, infrared spectroscopic and magnetic studies reveal incomplete electron transfer with either changing temperature or upon light exposure.

Tuning the electron transfer events in a series of cyanide-bridged [Fe2Co2] squares according to different electron donors

It has been recognized that both the ligand fields and intermolecular interactions may greatly impact the electron-transfer-coupled spin transition (ETCST) events in switchable magnetic materials; however, the engineering of these factors within a given system is still challenging. In this article, we chose the 4,4'-substituent 2,2'-bipyridine derivatives as chelating ligands according to their increasing electron-donating strength and incremental potential for forming hydrogen bonds (bpy^CHO,CH₃(L1) < bpy^{CH₂OH,CH₃} (L2) < bpy^{CH₂OH,CH₂OH} (L3)), and prepared three new [Fe₂Co₂] complexes, {[(Tp*)Fe(CN)₃Co(L)₂]₂[ClO₄]₂}·Sol (1, L = L1, Sol = 4MeCN·2H₂O; 2, L = L2, Sol = 3MeCN; 3, L = L3, Sol = 4MeOH; Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate). X-ray crystallography studies revealed that all the complexes share similar cyanide-bridged [Fe₂Co₂] square compositions except for the different substituted groups of L ligands, which led to the clearly evidenced intercluster hydrogen bonds between the neighbouring hydroxyl groups in 2 and 3. As a result, 1 remained in the paramagnetic [FeIII,LS2CoII,HS2] state over the whole temperature range, while 2 and 3 showed complete ETCST behaviour with the transition temperatures (T_1/2) being 221 and 294 K, respectively.

Investigation of the electron transfer properties between metal centers in binuclear and trinuclear cyanido-bridged mixed valence complexes with cis/trans-configuration

To investigate the electron transfer properties between metal centers and their influencing factors in binuclear and trinuclear cyanido-bridged mixed valence complexes with cis/trans-configuration, binuclear cis-[Cp(dppe)Fe(μ-NC)Ru(5,5'-dmbpy)₂(μ-CN)][PF₆]_n (cis-1[PF6]n, n = 1, 2) and trinuclear cis/trans-[Cp(dppe)Fe(μ-NC)Ru(5,5'-dmbpy)₂(μ-CN)Fe(dppe)Cp][PF₆]_n (cis/trans-2[PF6]n, n = 2, 3, 4) (Cp = 1,3-cyclopentadiene, dppe = 1,2-bis(diphenylphosphino)ethane, 5,5'-dmbpy = 5,5'-dimethyl-2,2'-bipyridyl) cyanido-bridged complexes were synthesized and well characterized. The experimental results indicate that the presence of the other terminal fragment Cp(dppe)Fe^III in cis-2[PF6]4 results in higher MMCT energy than that of cis-1[PF6]2. In addition, the trans-configuration is more conducive to electron transfer between metal centers than the cis-configuration in trinuclear cyanido-bridged mixed valence complexes. Moreover, these mixed valence complexes cis-1[PF6]2 and cis/trans-2[PF6]n (n = 3, 4) could be assigned to Class II systems according to Robin and Day.

Ferroelectric coordination metal complexes based on structural and electron dynamics

Ferroelectrics that display electrically invertible polarisation are attractive materials because of their potential for wide-ranging applications. To date, considerable effort has thus been devoted towards developing ferroelectric materials, particularly those comprising organic/inorganic compounds. In these systems, structural dynamics such as atomic displacement and reorientation of polar ions/molecules play a key role in the generation of reversible spontaneous polarisation. Although there are many reports concerned with organic/inorganic ferroelectrics, ferroelectrics based on coordination metal complexes have been largely unexplored despite their often unique electronic and spin state properties. In this feature article, we discuss recent progress involving coordination metal complex-based ferroelectrics where the reversible polarisation originates not only from structural dynamics (represented by proton transfer, molecular motion, and liquid crystalline behaviour) but also from electron dynamics (represented by electron transfer and spin crossover phenomena) occurring at the metal centre. Furthermore, unique synergy effects (i.e. magnetoelectric coupling) resulting from the structural and electron dynamics are described. We believe that this review pertaining to ferroelectric coordination metal complexes provides new insights for fabricating further advanced functional materials such as multiferroics and spintronics.

Spin-State Modulation in FeII -Based Hofmann-Type Coordination Polymers: From Molecules to Materials

Spin crossover complexes that reversibly interconvert between two stable states imitate a binary state of 0 and 1, delivering a promising possibility to address the data processing concept in smart materials. Thus, a comprehensive understanding of the modulation of magnetic transition between high spin and low spin and the factors responsible for stabilizing the spin states is an essential theme in modern materials design. In this context, the present review attempts to provide a concise outline of the design strategy employed at the molecular level for fine-tuning the spin-state switching in Fe^II -based Hofmann-type coordination polymers and their effects on the optical and magnetic response. In addition, development towards the nanoscale architectures of HCPs, i. e., in terms of nanoparticles and thin films, are emphasized to bridge the gap between the laboratory and reality.

Effects of cis/trans-Configuration and Ligand Substitution of the Cyanidometal Bridge on Metal to Metal Charge Transfer Properties in Mixed Valence Complexes

To investigate the effects of cis/trans-configuration of the cyanidometal bridge and the electron donating ability of the auxiliary ligand on the cyanidometal bridge on metal to metal charge transfer (MMCT) in cyanidometal-bridged mixed valence compounds, two groups of trinuclear cyanidometal-bridged compounds cis/trans-[Cp(dppe)Fe(μ-NC)Ru(4,4'-dmbpy)₂ (μ-CN)Fe(dppe)Cp][PF₆ ]_n (n=2 (cis/trans-1[PF₆ ]₂ ), 3 (cis/trans-1[PF₆ ]₃ ), 4 (cis/trans-1[PF₆ ]₄ )) and cis/trans-[Cp(dppe)Fe(μ-NC)Ru(bpy)₂ (μ-CN)Fe(dppe)Cp][PF₆ ]₃ (cis/trans-2[PF₆ ]₃ ) were synthesized and fully characterized. The experimental results indicate that for these one- and two-electron oxidation mixed valence compounds, the trans-configuration compounds are more beneficial for MMCT than the cis-configuration compounds, and increasing the electron donating ability of the auxiliary ligand on the cyanidometal bridge is also conductive to MMCT. Moreover, compounds cis/trans-1[PF₆ ]_n (n=3, 4) and cis/trans-2[PF₆ ]₃ belong to localized compounds by analyzing the experimental characterization results, supported by the TDDFT calculations.

Light actuated single-chain magnet with magnetic coercivity

A cyanide-bridged {Fe2Co}-based coordination polymer was synthesized. It showed photo-induced slow relaxation of magnetization and a coercive field of 400 Oe.

Manipulating Selective Metal-to-Metal Electron Transfer to Achieve Multi-Phase Transitions in an Asymmetric [Fe2Co]-Assembled Mixed-Valence Chain

Manipulation of multi-functions in molecular materials is promising for future switching and memory devices, although is currently difficult. Herein, we assembled the asymmetric {Fe2Co} unit into a cyanide-bridged mixed-valence chain {[(Tp)Fe(CN)3]2Co(BIT)}·2CH3OH (1) (Tp = hydrotris(pyrazolyl)borate and BIT = 3,4-bis-(1H-imidazol-1-yl)thiophen), which showed reversible multi-phase transitions accompanied by the photo-switchable single-chain magnet property and dielectric anomalies. 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 of the single-chain magnet behavior by alternating 946-nm and 532-nm light irradiations. A substantial anomaly in 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.

Anion-Dependent Electron Transfer in the Cyanide-Bridged [Fe2Co2] Capsules

A family of molecular capsules, {[(Tp*)Fe(CN)₃Co(bpy^{C═N(CH₂)₇N═C}bpy)]₂[X]₂}·sol (1, X = ClO₄, sol = 6DMF; 2, X = PF₆, sol = 6DMF; 3, X = OTf, sol = 6DMF; 4, X = BPh₄, sol = 2DMF; Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate; bpy = 2,2'-bipyridine), were prepared via the Schiff-base condensation of the aldehyde-substituted bpy (bpy^CHO) and 1,7-diaminoheptane (H₂N(CH₂)₇NH₂). All the complexes contain the same cyanide-bridged cationic square cores ([Fe₂Co₂]²⁺), which are encapsuled by the flexible alkyl chains. Variable-temperature single-crystal X-ray diffraction, FT-IR spectra, and magnetic studies reveal the abrupt and complete, thermo- and photo-induced electron-transfer-coupled spin transition for 1-3, while the pure high-spin phase for 4. Such distinct behavior is attributed to the effective long-range cooperative interactions mediated by the intercluster π-π couplings in 1-3, which, however, are significantly blocked in 4 due to the steric effect of interstitial BPh₄^- anions. Furthermore, the shift in the thermally induced transition temperatures of 254 K for 1, 233 K for 2, and 187 K for 3, respectively, is likely correlated to the variable H···O and H···F interactions between the solvent molecules, anions, and the bipyridine ligands of the [Fe₂Co₂] squares, suggesting the significant anion-dependent effect in such a system.

High-temperature electron transfer coupled spin transition (ETCST) with hysteresis in a discrete [Fe2Co2] Prussian blue analogue

A thermoresponsive Fe-Co based discrete Prussian blue analogue (PBA) with the general formula [Fe(Tp)(CN)3]2[Co{en(Bn)py}]2(ClO4)2·4MeOH·4H2O i.e. [1·4MeOH·4H2O] is reported, which discloses a sharp and high temperature ETCST along with a wide hysteresis of 10 K observed for its desolvated complex 1.

Influence of Fine Ligand Substitution Modification of the Isocyanidometal Bridge on Metal-to-Metal Charge Transfer Properties in Class II-III Mixed Valence Complexes

The synthesis and characterization of Class II-III mixed valence complexes have been an interesting topic due to their special intermediate behaviour between localized and delocalized mixed valence complexes. To investigate the influence of the isocyanidometal bridge on metal-to-metal charge transfer (MMCT) properties, a family of new isocyanidometal-bridged complexes and their one-electron oxidation products cis-[Cp(dppe)Fe-CN-Ru(L)₂ -NC-Fe(dppe)Cp][PF₆ ]_n (n=2, 3) (Cp=1,3-cyclopentadiene, dppe=1,2-bis(diphenylphosphino)ethane, L=2,2'-bipyridine (bpy, 1[PF₆ ]_n ), 5,5'-dimethyl-2,2'-bipyridyl (5,5'-dmbpy, 2[PF₆ ]_n ) and 4,4'-dimethyl-2,2'-bipyridyl (4,4'-dmbpy, 3[PF₆ ]_n )) have been synthesized and fully characterized. The experimental results suggest that all the one-electron oxidation products may belong to Class II-III mixed valence complexes, supported by TDDFT calculations. With the change of the substituents of the bipyridyl ligand on the Ru centre from H, 5,5'-dimethyl to 4,4'-dimethyl, the energy of MMCT for the one-electron oxidation complexes changes in the order: 1³⁺ <2³⁺ <3³⁺ , and that for the two-electron oxidation complexes decreases in the order 1⁴⁺ >3⁴⁺ >2⁴⁺ . The potential splitting (ΔE_1/2 (2)) between the two terminal Fe centres for N[PF₆ ]₂ are the largest potential splitting for the cyanido-bridged complexes reported so far. This work shows that the smaller potential difference between the bridging and the terminal metal centres would result in the more delocalized mixed valence complex.

Tuning metal to metal charge transfer properties in cyanidometal-bridged complexes by changing the auxiliary ligand on the bridge

In order to investigate the influence of the auxiliary ligand of the cyanidometal bridge on metal to metal charge transfer (MMCT) in cyanidometal-bridged complexes, two groups of heterotrimetallic cyanidometal-bridged complexes, trans-[Cp*(dppe)Fe-NC-Ru(L)2-CN-Fe(dppe)Cp*][PF6]n (L = bpy, 1(PF6)n; L = 4,4'-dmbpy, 2(PF6)n; n = 2, 3, 4) (Cp* = 1,2,3,4,5-pentamethylcyclopentadiene, dppe = 1,2-bis(diphenylphosphino)ethane, bpy = 2,2'-bipyridine, 4,4'-dmbpy = 4,4'-dimethyl-2,2'-bipyridyl) were synthesized and fully characterized. The MMCT of the one-electron oxidation mixed valence complexes is mainly attributed to RuII and FeII → FeIII MMCT transitions, and the MMCT of the two-electron oxidation complexes is mainly attributed to RuII → FeIII MMCT transitions. The energy of the MMCT of the four complexes decreases with the increase of the electron donating ability of the auxiliary ligand of the cyanidometal bridge. The IR, EPR, and Mössbauer spectra, and the solvent independence of MMCT characterizations indicate that the one-electron oxidation mixed valence complexes may belong to Class II-III systems, and the two-electron oxidation complexes may be localized at low temperature but delocalized at room temperature on the EPR timescale.

Recent progresses in luminescent metal-organic frameworks (LMOFs) as sensors for the detection of anions and cations in aqueous solution

The discharge of excessive metal ions and anions into water bodies leads to the serious pollution of water and environment, which in turn has a certain impact on industry, agriculture, and human life. Because of the unique advantages of luminescent metal-organic frameworks (LMOFs), they have been successfully explored as various fluorescent probes to quickly and effectively detect these pollutants. This perspective not only introduces the design strategy and classification of LMOFs, especially the construction methods of water-stable LMOFs, but also reports the latest progresses in some LMOFs between 2016 and 2020 as well as expounds the mechanisms of LMOFs for detecting anions and cations. Moreover, the luminescence properties of LMOFs are related to the selection of metal ions, the structure of organic ligands, the pore size, and the interaction of guest molecules. Finally, the further development of LMOFs is summarized and prospected in this field.

Ferromagnetic Archimedean polyhedra {Fe24M18} (M = Fe, Ni, and Mn) with tunable electron configurations

Three symmetric nanocages {Fe24M18} that mimic the Archimedean polyhedra, namely pseudo-rhombicuboctahedron, were synthesized. Their electron configurations depend highly on the changes of metal ions and the deprotonation of auxiliary ligands.

Spin and valence isomerism in cyanide-bridged {FeMII} (M = Fe and Co) clusters

Two cyanide-bridged V-shaped isostructural trinuclear complexes [{(Tp*)FeIII(CN)3}2MII(bztpen)]·Sol (M = Fe, Sol = CH3OH·3H2O, 1; M = Co, Sol = 2CH3OH·2H2O, 2; bztpen = N-benzyl-N,N',N'-tris(2-methylpyridyl)ethylenediamine; Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) were synthesized and characterized. The bztpen ligand serves as a tetradentate capping ligand around the inner metal ion, leaving one pyridyl group intact. Complex 1 exhibits a spin crossover (SCO) behavior between the {FeIIILSFeIIHSFeIIILS} and {FeIIILSFeIILSFeIIILS} spin isomers, while 2 shows both thermally- and photo-induced electron-transfer coupled spin transition (ETCST) property between the {FeIIILSCoIIHSFeIIILS} and {FeIIILSCoIIILSFeIILS} valence isomers. The total entropy changes for 1 and 2 between their corresponding two electronic states were found to be very close with the values of 87.46 and 84.49 J mol-1 K-1, respectively, indicating the comparable thermal energy barriers necessary for either an SCO or ETCST event for such a given system. Furthermore, both complexes undergo desolvation-induced irreversible and sharp magnetic change at high temperatures.

Metal/Metal Redox Isomerism Governed by Configuration

A pair of diastereomeric dinuclear complexes, [Tp′(CO)BrW{μ‐η²‐C,C′‐κ²‐S,P‐C₂(PPh₂)S}Ru(η⁵‐C₅H₅)(PPh₃)], in which W and Ru are bridged by a phosphinyl(thiolato)alkyne in a side‐on carbon P,S‐chelate coordination mode, were synthesized, separated and fully characterized. Even though the isomers are similar in their spectroscopic properties and redox potentials, the like‐isomer is oxidized at W while the unlike‐isomer is oxidized at Ru, which is proven by IR, NIR and EPR‐spectroscopy supported by spectro‐electrochemistry and computational methods. The second oxidation of the complexes was shown to take place at the metal left unaffected in the first redox step. Finally, the tipping point could be realized in the unlike isomer of the electronically tuned thiophenolate congener [Tp′(CO)(PhS)W{μ‐η²‐C,C′‐κ²‐S,P‐C₂(PPh₂)S}Ru(η⁵‐C₅H₅)‐(PPh₃)], in which valence trapped W^III/Ru^II and W^II/Ru^III cationic species are at equilibrium.

Modeling of Electron Transfer in a Linear Trinuclear Fe-Co-Fe Complex: Magnetic and Polarizability Properties

A microscopic approach to the problem of charge transfer-induced spin transitions in a system of interacting trinuclear Fe-Co-Fe clusters, which takes into account the switching of polarization during the transformation Fe_ls^II-Co_ls^III-Fe_ls^III→ Fe_ls^III-Co_hs^II-Fe_ls^III (ls-low spin, hs-high spin), has been developed. The cooperative electron-deformational and dipole-dipole interactions and the intracluster electron transfer have been proved to govern the observed significant changes in the magnetic and polarization characteristics of the examined system. The role of intra- and intercluster interactions in the spin transformation has been elucidated. On the basis of the developed approach, an explanation of the temperature dependence of the magnetic susceptibility and of the polar-nonpolar transformation in the {[FeTp(CN)₃]₂Co(Meim)₄}·6H₂O compound in response to thermal stimuli has been given.

Light- and temperature-assisted spin state annealing: accessing the hidden multistability

Among responsive multistable materials, spin crossover (SCO) systems are of particular interest for stabilizing multiple spin states with various stimulus inputs and physical outputs. Here, in a 2D Hofmann-type coordination polymer, [Fe(isoq)₂{Au(CN)₂}₂] (isoq = isoquinoline), a medium-temperature annealing process is introduced after light/temperature stimulation, which accesses the hidden multistability of the spin state. With the combined effort of magnetic, crystallographic and Mössbauer spectral investigation, these distinct spin states are identified and the light- and temperature-assisted transition pathways are clarified. Such excitation-relaxation and trapping-relaxation joint mechanisms, as ingenious interplays between the kinetic and thermodynamic effects, uncover hidden possibilities for the discovery of multistable materials and the development of multistate intelligent devices.

Two new two-stage manipulation protocols, namely light- and temperature-assisted spin state annealing (LASSA/TASSA), are applied to a spin crossover coordination polymer, [Fe(isoq)₂{Au(CN)₂}₂], revealing the hidden multistability of spin states.

Manipulating On/Off Single-Molecule Magnet Behavior in a Dy(III)-Based Photochromic Complex

Exploitation of room temperature (RT) photochromism and photomagnetism to induce single-molecule magnet (SMM) behavior has potential applications toward optical switches and magnetic memories, and remains a tremendous challenge in the development of new bulk magnets. Herein, a series of chain complexes [Ln₃(H-HEDP)₃(H₂-HEDP)₃]·2H₃-TPT·H₄-HEDP·10H₂O (QDU-1; Ln = Dy (QDU-1(Dy)), Gd (QDU-1(Gd)), and Y (QDU-1(Y)); HEDP = hydroxyethylidene diphosphonate; TPT = 2,4,6-tri(4-pyridyl)-1,3,5-triazine) were synthesized by solvothermal reactions. All the compounds exhibited reversible photochromic and photomagnetic behaviors via UV light irradiation at RT, induced by the photogenerated radicals via a photoinduced electron transfer (PET) mechanism. More importantly, the PET process induced significant variations in magnetic interactions for the Dy(III) congener. Strong ferromagnetic coupling with remarkably slow magnetic relaxation without applied dc fields was observed between Dy^III ions and photogenerated O^• radicals, showing SMM behavior after RT illumination. For the first time, we observed the reversible RT photochromism and photomagnetism in the lanthanide-based materials. This work realized the radicals-actuated on/off SMM behavior via RT light irradiation, providing a new strategy for constructing the light-induced SMMs.

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.

Changes in magnetic order through two consecutive dehydration steps of metal-phosphonate diamond chains

Hydrothermal reactions of the multitopic ligand 1-hydroxy-1-(piperidin-4-yl)methylidenebisphosphonic acid (hpdpH4) with cobalt or nickel sulfates afforded two new isostructural metal phosphonates, M3 II(hpdpH)2(H2O)6·4H2O [M = Co (Co-10H2O), Ni (Ni-10H2O)]. Their structures consist of parallel diamond chains of three MO6 octahedra bridged by the PO3C tetrahedra. Six of the seven oxygen atoms of the ligand are involved in coordination; for two ligands that amounts to 12 bonds for 3 MO6 and the remaining six are occupied by terminal water molecules. In addition, four water molecules sit in between the chains providing H-bonds to the formation of a 3D-net. Thermal analyses show identical two-step dehydration processes involving first the departure of six water molecules followed by the remaining four. A detailed study of the ac- and dc-magnetization as a function of temperature, field and frequency reveals associated drastic changes. The virgin form Co-10H2O is a paramagnet while its partial dehydrated form Co-4H2O is an antiferromagnet displaying canting below T N = 4.7 K and the fully dehydrated form Co is a ferrimagnet (T C = 12 K). Ni-10H2O and Ni-4H2O exhibit long-range ordered antiferromagnetism (T N = 2.7 and 4.0 K, respectively) and also become ferrimagnets (T C = 9.4 K) when fully dehydrated to Ni. The dehydrated samples can be fully rehydrated with the complete recovery of both the structures and magnetic properties.

Manipulating Spin Transition To Achieve Switchable Multifunctions

The spin transition of metal ions involves interconversion between electron configurations exhibiting considerably different functions and plays a substantial role in the chemical, physical, and biological fields. The photoinduced spin transition offers a promising approach to tune various physical properties with high spatial and temporal resolutions for producing smart multifunctional materials not only to explore their basic science but also to satisfy the demands of the next-generation photoswitchable-molecule-based devices. Therefore, it is attracting considerable interest to utilize photoinduced spin transition to simultaneously tune multifunctions. However, two issues are challenging in obtaining reversible and swift manipulation of functions: (1) the interconversion between different electron configurations of photoresponsive units should be reversibly switched via photoinduced spin transition; (2) effective coupling should be built between the photoresponsive and functional units to produce photoswitchable functions utilizing photoinduced spin transition. In this Account, we will review our recent advances in the usage of spin transition of metal ions as actuators for tuning the magnetic, dielectric, fluorescence, and mechanical properties, wherein the role of a photoswitchable spin transition is highlighted. We mainly focus on the study of two spin-transition categories, including spin-crossover (SCO) of one metal ion and metal-to-metal charge transfer (MMCT). Initially, we will describe a strategy for developing photoinduced reversible SCO and MMCT. The role of flexible intermolecular interactions, in particular, π···π interactions, is discussed with respect to a photoinduced reversible MMCT. Then, the SCO and MMCT units were assembled using metallocyanate building blocks to form a chain, wherein the spin states, anisotropy, and magnetic coupling interactions can be photoswitched to tune the single-chain magnet behavior. Besides magnetic properties, the photoinduced spin transition that is associated with the concomitant changing of charge distribution, bond lengths, and absorption spectra can be utilized to tune the multifunctions. Therefore, the transfer of an electron from a central cobalt site to one of the two iron sites in linear trinuclear Fe₂Co compounds resulted in the transformation of a centrosymmetric nonpolar molecule into an asymmetric polar molecule, and the molecular electric dipole and dielectric properties can be reversibly switched. Moreover, the spin transition usually involved significant expansion or contraction of the coordination sphere of metal ions because of the population/depopulation of the antibonding e_g orbitals. Therefore, colossal positive and negative thermal expansion behaviors were achieved in a layered compound by manipulating the spin-transition process and the rotation of the functional units, thereby providing a strategy for synthesizing phototunable nanomotors. Photoinduced spin transition can also be used to modulate the fluorescence properties by controlling the energy transfer between the fluorescent ligands and the metal sites showing SCO. Finally, we will provide a perspective and detail the remaining challenges that are associated with this research area. We believe that an increasing number of fascinating photoswitchable SCO and MMCT systems will emerge in the near future and that the materials exhibiting various properties and functions that can be manipulated using photoinduced spin transition will provide novel opportunities for the development of smart multifunctional materials and devices.

Charge transfer and slow magnetic relaxation in a series of cyano-bridged FeIII4MII2 (M = FeII, CoII, NiII) molecules

The synthesis, single-crystal structures and magnetic properties of three new cyano-bridged complexes [FeIII4MII2] (M = Fe^II, Co^II, Ni^II) are reported.

Influence of ligand substitution at the donor and acceptor center on MMCT in a cyanide-bridged mixed-valence system

A series of cyanide-bridged complexes [TpmRu(L_D)(μ-CN)Ru(L_P)Cp*][PF₆]₂ were investigated by changing the substitutions of the donor and acceptor sites to systemically tune MMCT.

Bidirectional Photoswitching via Alternating NIR and UV Irradiation on a Core-Shell UCNP-SCO Nanosphere

Bidirectional photoswitching of molecular materials under ambient condition is of significant importance. Herein, we present for the first time that a core-shell UCNP-SCO nanosphere (UCNP = upconversion nanophosphor, SCO = spin crossover), which was composed of a UCNP core (NaYF₄: 20 mol % Yb³⁺, 1 mol % Er³⁺) and an SCO iron(II) shell ([Fe(H₂Bpz)₂(bipy-COOH)], H₂Bpz = dihydrobis(1-pyrazolyl)borate, bipy-COOH = 4,4'-dicarboxy-2,2'-bipyridine), can be reversibly photoswitched between the high-spin and low-spin states at room temperature in the solid state, via alternating irradiation with near-infrared (λ = 980 nm) and ultraviolet (λ = 310 nm) light. What's more, this reversible spin-state switching was accompanied by a variation of fluorescent spectrum and dielectric constants. The strategy here, that is, integrating the SCO iron(II) complex into a UCNP-SCO nanosphere for molecular photoswitching, may open a new area in the development of photocontrolled molecular devices.

Manipulation of successive crystalline transformations to control electron transfer and switchable functions

Electron transfer in solid is crucial to switchable magnetic, electrical, optical and mechanical properties. However, it is a formidable challenge to control electron-transfer behaviors via manipulation of crystalline phases, especially through dynamic crystalline transformation. Herein, three crystalline phases of an {Fe2Co2} compound were obtained via enhancement of intermolecular π···π interactions inducing successive single-crystal-to-single-crystal transformations, from solvated 1·2CH3OH·4H2O, to desolvated 1 and its polymorph 1a accompanying electron transfer. 1·2CH3OH·4H2O showed thermally induced reversible intermetallic electron transfer in mother liquor. No electron-transfer behavior was observed in 1. 1a showed reversible intermetallic electron transfer upon thermal treatment or alternative irradiation with 808- and 532-nm lasers at cryogenic temperatures. The electron-transfer behaviors significantly change the magnetic and optical properties, providing a strategy to realize different electron-transfer behaviors and switchable functions via π···π interactions manipulated dynamic crystalline transformation.

Syntheses, crystal structures, and magnetic properties of cyanide-bridged complexestrans-RuII(dppe)2(CN)2(FeIIIX3)2(X = Cl and Br)

Cyanide-bridged complexestrans-Ru^II(dppe)₂(CN)₂(Fe^IIIX₃)₂(X = Cl,2; X = Br,3) and their parenttrans-ClRu^II(dppe)₂(CN) (1) were synthesized and fully characterized. The magnetic properties of2and3have been investigated.