Charge Transfer Induced Spin Transition Research Articles

Near-infrared-photoinduced metamagnet based on a layered cyanido-bridged Co–W assembly with π–π interactions

A cyanido-bridged Co–W assembly with π–π interactions showed a thermal phase transition and a photo–induced metamagnetism based on a charge transfer–induced spin transition.

Thermal-induced structural distortion and charge transfer of Prussian blue analogues K0.4Co1.3[Fe(CN)6] • nH2O

In this paper, the Prussian blue analogues K0.4Co1.3[Fe(CN)6]•nH2O ([KCoFe]) were synthesized using the coprecipitation method at room temperature, 40, 60, 80 and 100 °C. XRD shows that as the preparation temperature increases, [KCoFe] undergoes structural distortion. The room temperature phase and the high temperature phase are both face-centered cubic (Fm-3 m) structures with lattice constants of 10.230 Å and 9.980 Å, respectively. Mössbauer spectrum analysis showed that due to the increase of synthesis temperature, [KCoFe] has undergone charge transfer induced spin transition. Part of paramagnetic structure: FeІІІ (, S = 1/2)–CN–CoІІ (, S = 3/2) was transformed into diamagnetic structure: FeІІ (, S = 0)–CN–CoІІІ (, S = 0). This work provides new ideas for designing new Prussian blue analogues magnetic materials. Keyword: Thermal-induced, Prussian blue analogues, Structural distortion, Charge transfer.

Thermally Induced Reversible Metal-to-Metal Charge Transfer in Mixed-Valence {Fe III 4 Fe II 4 } Cubes

Thermally Induced Reversible Metal-to-Metal Charge Transfer in Mixed-Valence {Fe ^III ₄ Fe ^II ₄ } Cubes

Modeling of electron transfer phenomenon in the dinuclear {Fe(μ − CN)Co} complexes

A model is developed to describe the charge transfer-induced spin transition in a crystal containing cyanide-bridged binuclear Co–Fe clusters as a structural element. The model accounts for the electronic states arising from two cluster configurations active in the transition and, namely, from the ground ls-CoIII−ls-FeII and the excited hs-CoII−ls-FeIII ones. The cooperative interaction that originates from the coupling of the acoustic crystalline modes with the molecular vibrations describing the displacements of the nearest ligand surroundings of the metal ions is assumed to be responsible for spin transformation. The problem of interacting clusters is solved within mean field approximation. The role of the order parameters is played by the mean values of the products of the electronic matrices and coordinates of the molecular vibrations in the ground and excited cluster configurations. The developed model provides quite a reasonable description of the observed temperature dependence of the magnetic characteristics of the [{(Tp)Fe(CN)3}{Co-(PY5Me2)}](CF3SO3)⋅2DMF complex. The factors controlling the electron transfer in this complex are elucidated.

Time-Resolved Spectroscopy of Photoinduced Electron Transfer in Dinuclear and Tetranuclear Fe/Co Prussian Blue Analogues.

The dynamics of the photodriven charge transfer-induced spin transition (CTIST) in two Fe/Co Prussian Blue Analogues (PBAs) are revealed by femtosecond IR and UV/vis pump-probe spectroscopy. Depending on temperature, the known tetranuclear square-type complex [Co2Fe2(CN)6(tp*)2(4,4'-dtbbpy)4](PF6)2 (1) exists in two electronic states. In acetonitrile solution, at <240 K, the low temperature (LT) phase is prevalent consisting of low-spin Fe(II) and low-spin Co(III), [FeIILSCoIIILS]2. Temperature rise is the reason behind thermally-induced CTIST toward the high temperature (HT) phase consisting of low-spin Fe(III) and high-spin Co(II), [FeIIILSCoIIHS]2, being prevalent at >300 K. Photoexcitation into the intervalence charge transfer (IVCT) band of the LT phase at 800 nm induces electron transfer in one Fe-Co edge of PBA 1 and produces a [FeIIILSCoIILS] intermediate which by spin-crossover (SCO) is stabilized within 400 fs to a long-lived (>1 ns) [FeIIILSCoIIHS] species. In contrast, IVCT excitation of the HT phase at 400 nm generates a [FeIILSCoIIIHS] species with a lifetime of 3.6 ps. Subsequent back-electron transfer populates the vibrationally hot ground state, which thermalizes within 8 ps. The newly synthesized dinuclear PBA, [CoFe(CN)3(tp*)(pz*4Lut)]ClO4 (2), provides a benchmark of the HT phase of 1, i.e., [FeIIILSCoIIHS], as verified by variable temperature magnetic susceptibility measurements and 57Fe Mössbauer spectroscopy. The photoinduced charge transfer dynamics of PBA 2 indeed are almost identical to that of the HT phase of PBA 1 with a lifetime of the excited [FeIILSCoIIIHS] species of 3.8 ps.

Tailoring of the Frontier Orbital Character in Co2+/3+ Complexes with Triarylamine Substituted Terpyridine Ligands

Tara‐decorated terpyridines 1–3 and their 1:2 Co2+ [Co(L)2](ClO4)2 (L = 1–3) and Co3+ complexes [Co(L)2](ClO4)2(SbCl6) (L = 1,3) are reported. The solid‐state complex metrics of [Co(3)2]2+ and [Co(1)2]3+ indicate an assignment as spin‐quartet Co2+ or spin‐singlet Co3+, which is in accord with DFT and 1H NMR studies. Vis‐NIR spectra of 1[Co3+(L)2]3+ show characteristic Tara→Co3+ charge‐transfer bands, giving no direct indication of L‐based oxidation events, irrespective of the solvent. Theoretical studies reveal no charge transfer induced spin transition in [Co3+(L)2]3+, although a substantial decrease of the HOMO‐LUMO gap prevails in [Co3+(1)2]3+ as compared with [Co(tpy)2]3+. The destabilisation of the HOMO is not sufficient to allow an equilibration among closed‐ and open‐shell formulations. The computed singlet‐triplet and singlet‐quintet splitting in [Co(1)2]3+ renders these open‐shell formulations to reside only moderately above the ground state. Design criteria are derived from this analysis aiming at a further stabilisation of the open‐shell species through implementation of more strongly electron releasing Tara substituents.

Control of the Speed of a Light-Induced Spin Transition through Mesoscale Core-Shell Architecture.

The rate of the light-induced spin transition in a coordination polymer network solid dramatically increases when included as the core in mesoscale core-shell particles. A series of photomagnetic coordination polymer core-shell heterostructures, based on the light-switchable Rb aCo b[Fe(CN)6] c· mH2O (RbCoFe-PBA) as core with the isostructural K jNi k[Cr(CN)6] l· nH2O (KNiCr-PBA) as shell, are studied using temperature-dependent powder X-ray diffraction and SQUID magnetometry. The core RbCoFe-PBA exhibits a charge transfer-induced spin transition (CTIST), which can be thermally and optically induced. When coupled to the shell, the rate of the optically induced transition from low spin to high spin increases. Isothermal relaxation from the optically induced high spin state of the core back to the low spin state and activation energies associated with the transition between these states were measured. The presence of a shell decreases the activation energy, which is associated with the elastic properties of the core. Numerical simulations using an electro-elastic model for the spin transition in core-shell particles supports the findings, demonstrating how coupling of the core to the shell changes the elastic properties of the system. The ability to tune the rate of optically induced magnetic and structural phase transitions through control of mesoscale architecture presents a new approach to the development of photoswitchable materials with tailored properties.

Direct Observation of Short-Range Structural Coherence During a Charge Transfer Induced Spin Transition in a CoFe Prussian Blue Analogue by Transmission Electron Microscopy

Direct Observation of Short-Range Structural Coherence During a Charge Transfer Induced Spin Transition in a CoFe Prussian Blue Analogue by Transmission Electron Microscopy

Evidence for Interface-Induced Strain and Its Influence on Photomagnetism in Prussian Blue Analogue Core–Shell Heterostructures, RbaCob[Fe(CN)6]c·mH2O@KjNik[Cr(CN)6]l·nH2O

A series of photomagnetic coordination polymer core–shell heterostructures, based on the light-switchable Prussian blue analogue RbaCob[Fe(CN)6]c·mH2O (RbCoFe-PBA) as the core and the ferromagnetic KjNik[Cr(CN)6]l·nH2O (KNiCr-PBA) as the shell, was studied using powder X-ray diffraction, down to 100 K, and magnetometry, down to 2 K, to investigate the influence of the shell thickness on light-induced magnetization changes and gain insight into the mechanism. The core material is known to undergo a charge-transfer-induced spin transition (CTIST), and synchrotron powder diffraction was used to monitor structural changes in both the core and the shell associated with the thermally and optically induced CTIST of the core. Significant lattice contraction in the RbCoFe-PBA core upon cooling through the high-spin to the low-spin state transition near ∼260 K induces strain on the KNiCr-PBA shells. This lattice strain in the shell can be relieved either by thermal cycling back to high temperature or by using light...

Tuning of Charge Transfer Assisted Phase Transition and Slow Magnetic Relaxation Functionalities in {Fe(9-x)Co(x)[W(CN)8]6} (x = 0-9) Molecular Solid Solution.

Precisely controlled stoichiometric mixtures of Co(2+) and Fe(2+) metal ions were combined with the [W(V)(CN)8](3-) metalloligand in a methanolic solution to produce a series of trimetallic cyanido-bridged {Fe(9-x)Co(x)[W(CN)8]6(MeOH)24}·12MeOH (x = 0, 1, ..., 8, 9; compounds 0, 1, ..., 8, 9) clusters. All the compounds, 0-9, are isostructural, and consist of pentadecanuclear clusters of a six-capped body-centered cube topology, capped by methanol molecules which are coordinated to 3d metal centers. Thus, they can be considered as a unique type of a cluster-based molecular solid solution in which different Co/Fe metal ratios can be introduced while preserving the coordination skeleton and the overall molecular architecture. Depending on the Co/Fe ratio, 0-9 exhibit an unprecedented tuning of magnetic functionalities which relate to charge transfer assisted phase transition effects and slow magnetic relaxation effects. The iron rich 0-5 phases exhibit thermally induced reversible structural phase transitions in the 180-220 K range with the critical temperatures being linearly dependent on the value of x. The phase transition in 0 is accompanied by (HS)Fe(II) W(V) ↔ (HS)Fe(III) W(IV) charge transfer (CT) and the additional minor contribution of a Fe-based spin crossover (SCO) effect. The Co-containing 1-5 phases reveal two simultaneous electron transfer processes which explore (HS)Fe(II) W(V) ↔ (HS)Fe(III) W(IV) CT and the more complex (HS)Co(II) W(V) ↔ (LS)Co(III) W(IV) charge transfer induced spin transition (CTIST). Detailed structural, spectroscopic, and magnetic studies help explain the specific role of both types of CN(-)-bridged moieties: the Fe-NC-W linkages activate the molecular network toward a phase transition, while the subsequent Co-W CTIST enhances structural changes and enlarges thermal hysteresis of the magnetic susceptibility. On the second side of the 0-9 series, the vanishing phase transition in the cobalt rich 6-9 phases results in the high-spin ground state, and in the occurrence of a slow magnetic relaxation process at low temperatures. The energy barrier of the magnetic relaxation gradually increases with the increasing value of x, reaching up to ΔE/kB = 22.3(3) K for compound 9.

Quantum chemical modeling of magnetically bistable metal coordination compounds. Synchronization of spin crossover, valence tautomerism and charge transfer induced spin transition mechanisms.

It has been shown that the computationally designed bimetallic complexes formed as the adducts of Co(II) diketonates and salicylaldiminates with Fe(II) chelates of 1,10-phenanthroline-5,6-dione are susceptible to the synchronized thermally induced intramolecular rearrangements between their electromeric forms LSCo(III)-SQ-LSFe(II), LSCo(III)-SQ-HSFe(II), HSCo(II)-BQ-LSFe(II), HSCo(II)-BQ-HSFe(II) and also HSCo(II)-SQ-LSFe(III), which are governed by the spin-crossover (SCO), valence tautomerism (VT) and charge-transfer-induced spin transition (CTIST) mechanisms of spin-state switching. Stability of the adducts with respect to dissociation into components, relative energies and magnetic properties of the electromers and energy barriers against VT and unprecedented one-step (SCO + VT) rearrangements (estimated as minimum energy crossing points on the seams of the intersection of the corresponding potential energy surfaces) were calculated using the DFT (B3LYP*/6-311++G(d,p)) method. The calculations showed that all these characteristics of the system as well as the energy preferred spin-state switchable mechanisms are very sensitive to the structure of the cobalt diketonate (salicylaldiminate) fragment and can be varied and interchanged by the introduction of electron withdrawing substituents into the ligands.

Direct Observation of Short-Range Structural Coherence During a Charge Transfer Induced Spin Transition in a CoFe Prussian Blue Analogue by Transmission Electron Microscopy.

The local structure within the Co-Fe atomic array of the photoswitchable coordination polymer magnet, K0.3Co[Fe(CN)6]0.77·nH2O, is directly observed during charge transfer induced spin transition (CTIST), a solid-solid phase change, using high-resolution transmission electron microscopy (HRTEM). Along with the low-spin (LS) or thermally quenched high-spin (HS) states normally observed in CTIST solids at low temperature, slow cooling of K0.3Co[Fe(CN)6]0.77·nH2O results in an intermediate phase containing both HS and LS domains with short coherence length. By mapping individual metal-metal distances, the nanometer-scale HS domains are directly visualized within the LS array. Temperature-dependent analyses allow monitoring of HS domain coarsening along the warming branch of the CTIST, providing direct visualization of the elastic process and insight into the mechanism of phase propagation. Normally sensitive to electron beam damage, the low-temperature TEM measurements of the porous coordination polymer are enabled by using appropriate ionic liquids instead of usual conductive thin-film coatings, an approach that should find general utility in related classes of materials.

Light-induced changes in magnetism in a coordination polymer heterostructure, Rb0.24Co[Fe(CN)6]0.74@K0.10Co[Cr(CN)6]0.70·nH2O and the role of the shell thickness on the properties of both core and shell.

Particles of formula Rb0.24Co[Fe(CN)6]0.74@K0.10Co[Cr(CN)6]0.70·nH2O with a light-responsive rubidium cobalt hexacyanoferrate (RbCoFe) core and a magnetic potassium cobalt hexacyanochromate (KCoCr) shell have been prepared and exhibit light-induced changes in the magnetization of the normally light-insensitive KCoCr shell, a new property resulting from the synergy between the core and shell of a coordination polymer heterostructure. A single batch of 135 ± 12 nm RbCoFe particles are used as seeds to generate three different core@shell samples, with KCoCr shell thicknesses of approximately 11, 23 and 37 nm, to probe the influence of the shell thickness over the particles' morphology and structural and magnetic properties. Synchrotron powder X-ray diffraction reveals that structural changes in the shell accompany the charge transfer induced spin transition (CTIST) of the core, giving direct evidence that the photomagnetic response of the shell is magnetomechanical in origin. The depth to which the KCoCr shell contributes to changes in magnetization is estimated to be approximately 24 nm when using a model that assumes a constant magnetic response of the core within the series of particles. In turn, the presence of the shell changes the nature of the CTIST of the core. As opposed to the usually observed first order transition exhibiting hysteresis, the CTIST becomes continuous in the core@shell particles.

Influence of particle size on the phase behavior associated with the thermal spin transition of the Prussian blue analogue K0.4Co1.3[Fe(CN)6]·4.4 H2O

The Prussian blue analogue K0.4Co1.3[Fe(CN)6]·nH2O has previously been shown to exhibit three distinct phases at low temperature. In addition to the well-studied high-temperature ‘high spin’ and low-temperature ‘low spin’ phases, with unit cells of ∼10.3 and ∼9.95Å, respectively, an intermediate phase with unit cell a=10.09Å was recently reported, although it existed as a minority constituent in a mixed-phase material. Herein we report the synthesis of 130nm K0.4Co1.3[Fe(CN)6]·4.4H2O particles that form solely the intermediate phase at temperatures below the normal charge transfer induced spin transition. Compared with those formed from a bulk synthesis, the nanosized particles are shown to have the same chemical composition at high temperature, but they undergo different structural transitions with thermal cycling.

Charge–transfer-induced spin transition in K0.28Co1.36[Fe(CN)6] · XH2O with different annealing temperatures

The Prussian blue analog K0.28Co1.36[Fe(CN)6] · XH2O was prepared by standard chemical co-precipitation. The precipitate was filtered and dried in a vacuum oven at room temperature, 80°C, and 120°C. The powder X-ray diffraction measurement indicates a typical face-centered cubic pattern. The diffraction peaks show a slight shift to higher angle with increasing annealing temperatures, a signature of lattice contraction, which is mainly related to the inner charge transfer from FeIII to CoII. The value of χ · T is variable and dependent on temperature. The temperature dependence of χ −1 shows a large deviation from the Curie–Weiss law. The behavior could result from a charge-transfer-induced spin transition. Isothermal magnetization curves also suggest that the inner charge-transfer spin transition depends on the annealing temperature.

Charge-Transfer-Induced Spin Transitions in Crystals Containing Cyanide-Bridged Co–Fe Clusters: Role of Intra- and Intercluster Interactions

A theoretical model has been developed to explain at the electronic level the charge-transfer-induced spin transition (CTIST) in crystals based on cyano-bridged binuclear Fe-Co clusters. The CTIST is considered as a cooperative phenomenon (phase transformation) driven by the long-range electron-deformational interaction via the acoustic phonons field that is taken into account within the mean field approach. The model for CTIST includes also the metal-metal electron transfer and intracluster magnetic exchange. The conditions that favor the CTIST are discussed. The qualitative explanation of the experimental data is given.

Metastable state of the photomagnetic Prussian blue analog K0.3Co[Fe(CN)6]0.77·3.6H2O investigated by various techniques

The thermodynamic properties of the photomagnetic Prussian blue analog K0.3Co[Fe(CN)6]0.77 · 3.6H2O, which exhibits a charge-transfer-induced spin transition (CTIST), are reported. According to the thermal history of the sample, different low-temperature states are obtained: a quenched high-spin state (Q), a low-spin state (LT) achieved through the decay of the Q state, and an intermediate state (IM) combining low-spin (LS) and high-spin (HS) local states. The IM state is obtained by slow cooling and displays a slight kinetic dependence. The long lifetime of the IM state, which persists up to the vicinity of the CTIST temperature, is unusual and differs from all metastable states reported so far for photomagnetic Prussian blue analogs. The properties of the different states and of their photoexcited counterparts are investigated by magnetic and photomagnetic measurements, calorimetry, diffuse reflectance, and x-ray diffraction. A key feature of the IM state revealed by x-ray diffraction is the onset of phase separation between a LT-type fraction and a mixed HS-LS fraction (approximately 50 : 50). X-ray patterns of the IM state obtained during successive cooling and heating stages document irreversible transformations. The formation and properties of the IM state suggest a self-organization process between lowand high-spin sites in response to internal structural stresses, and this effect is hindered by irradiation with visible light. The relaxation kinetics of the Q and photoexcited states are analyzed by using a two-variable model, which accounts for the onset of correlations due to short-range elastic interactions. A quantitative comparison to the analogous sodium compound Na0.32Co[Fe(CN)6]0.74 · 3.4H2O confirms the nonstandard behavior of the title potassium compound and supports the structural origin of the self-organization processes.

Temperature and Light Induced Bistability in a Co3[Os(CN)6]2 6H2O Prussian Blue Analog

To expand the field of new cyanide materials of the 5d elements, we incorporated the [Os(CN)(6)](3-) anion into PB architectures in combination with the Co(II) cation. Herein, we report the first example of a photomagnetic PB analog containing Os(III) ions. In a similar vein as the prototypical CoFe PB analogs, this compound exhibits a wide variety of properties including Charge Transfer Induced Spin Transition (CTIST), Temperature Induced Excited Spin State Trapping (TIESST), and magnetic ordering.

An Unprecedented Charge Transfer Induced Spin Transition in an Fe–Os Cluster

High 'n low: The novel {[Fe(tmphen)2]3[Os(CN)6]2} complex (see structure) is the first example of a cluster in which a high-spin FeIII ion is in a coordination environment of four imine nitrogen atoms and two N-coordinated cyanides. Magnetic studies reveal an unprecedented type of reversible charge transfer induced spin transition (CTIST) between the low-spin FeIINCOsIII and high-spin FeIIINCOsII systems.

An Unprecedented Charge Transfer Induced Spin Transition in an Fe–Os Cluster

An Unprecedented Charge Transfer Induced Spin Transition in an Fe–Os Cluster