Rhodamine 6G-Labeled Pyridyl Aroylhydrazone Fe(II) Complex Exhibiting Synergetic Spin Crossover and Fluorescence

Spin crossover iron complexes with spin transition near room temperature based on nitrogen ligands containing aromatic rings: from molecular design to functional devices

During last decades, significant advances have been made in iron-based spin crossover (SCO) complexes, with a particular emphasis on achieving reversible and reproducible thermal hysteresis at room temperature (RT). This pursuit represents a pivotal goal within the field of molecular magnetism, aiming to create molecular devices capable of operating in ambient conditions. Here, we summarize the recent progress of iron complexes with spin transition near RT based on nitrogen ligands containing aromatic rings from molecular design to functional devices. Specifically, we discuss the various factors, including supramolecular interactions, crystal packing, guest molecules and pressure effects, that could influence its cooperativity and the spin transition temperature. Furthermore, the most recent advances in their implementation as mechanical actuators, switching/memories, sensors, and other devices, have been introduced as well. Finally, we give a perspective on current challenges and future directions in SCO community.

Bidirectional photomagnetism, exciplex fluorescence and dielectric anomalies in a spin crossover Hofmann-type coordination polymer

Stepped spin crossover (SCO) complexes with three or more spin states have promising applications in high-order data storage, multi-switches and multi-sensors. Further synergy with other functionalities, such as luminescence and dielectric properties, will provide a good chance to develop novel multifunctional SCO materials. Here, a bent pillar ligand and luminescent pyrene guest are integrated into a three-dimensional (3D) Hofmann-type metal-organic framework (MOF) [Fe(dpoda){Au(CN)2}2]·pyrene (dpoda = 2,5-di-(pyridyl)-1,3,4-oxadiazole). The magnetic data show an incomplete and two-step SCO behavior with the sequence of 1 ↔ 1/2 ↔ 1/4. The rare bi-directional light-induced excited spin-state trapping (LIESST) effect and light-induced stepped thermal relaxation after LIESST are observed. The pyrene guests interact with dpoda ligands via offset face-to-face π⋯π interactions to form intermolecular exciplex emissions. The competition between thermal quenching and stepped SCO properties results in a complicated and stepped exciplex fluorescence. Moreover, the stepped dielectric property with higher dielectric permittivity at lower temperature may be related to the more frustrated octahedral distortion parameters in the intermediate spin states. Hence, a 3D Hofmann-type MOF with bent pillar ligands and fluorescent guests illustrates an effective way for the development of multifunctional switching materials.

Development of an FeII Complex Exhibiting Intermolecular Proton Shifting Coupled Spin Transition

In this study, we investigated reversible intermolecular proton shifting (IPS) coupled with spin transition (ST) in a novel FeII complex. The host FeII complex and the guest carboxylic acid anion were connected by intermolecular hydrogen bonds (IHBs). We extended the intramolecular proton transfer coupled ST phenomenon to the intermolecular system. The dynamic phenomenon was confirmed by variable-temperature single-crystal X-ray diffraction, neutron crystallography, and infrared spectroscopy. The mechanism of IPS was further validated using density functional theory calculations. The discovery of IPS-coupled ST in crystalline molecular materials provides good insights into fundamental processes and promotes the design of novel multifunctional materials with tunable properties for various applications, such as optoelectronics, information storage, and molecular devices.

Integrating spin-dependent emission and dielectric switching in FeII catenated metal-organic frameworks

Mechanically interlocked molecules (MIMs) including famous catenanes show switchable physical properties and attract continuous research interest due to their potential application in molecular devices. The advantages of using spin crossover (SCO) materials here are enormous, allowing for control through diverse stimuli and highly specific functions, and enabling the transfer of the internal dynamics of MIMs from solution to solid state, leading to macroscopic applications. Herein, we report the efficient self-assembly of catenated metal-organic frameworks (termed catena-MOFs) induced by stacking interactions, through the combination of rationally selected flexible and conjugated naphthalene diimide-based bis-pyridyl ligand (BPND), [MI(CN)2]- (M = Ag or Au) and Fe2+ in a one-step strategy. The obtained bimetallic Hofmann-type SCO-MOFs [FeII(BPND){Ag(CN)2}2]·3CHCl3 (1Ag) and [FeII(BPND{Au(CN)2}2]·2CHCl3·2H2O (1Au) possess a unique three-dimensional (3D) catena-MOF constructed from the polycatenation of two-dimensional (2D) layers with hxl topology. Both complexes undergo thermal- and light-induced SCO. Significantly, abnormal increases in the maximum emission intensity and dielectric constant can be detected simultaneously with the switching of spin states. This research opens up SCO-actuated bistable MIMs that afford dual functionality of coupled fluorescence emission and dielectricity.

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.

Modeling Fe(II) Complexes Using Neural Networks

We report a Fe(II) data set of more than 23000 conformers in both low-spin (LS) and high-spin (HS) states. This data set was generated to develop a neural network model that is capable of predicting the energy and the energy splitting as a function of the conformation of a Fe(II) organometallic complex. In order to achieve this, we propose a type of scaled electronic embedding to cover the long-range interactions implicitly in our neural network describing the Fe(II) organometallic complexes. For the total energy prediction, the lowest MAE is 0.037 eV, while the lowest MAE of the splitting energy is 0.030 eV. Compared to baseline models, which only incorporate short-range interactions, our scaled electronic embeddings improve the accuracy by over 70% for the prediction of the total energy and the splitting energy. With regard to semiempirical methods, our proposed models reduce the MAE, with respect to these methods, by 2 orders of magnitude.

{GdIII7} and {GdIII14} Cluster Formation Based on a Rhodamine 6G Ligand with a Magnetocaloric Effect

Heptanuclear {GdIII7} (complex 1) and tetradecanuclear {GdIII14} (complex 2) were synthesized using the rhodamine 6G ligand HL (rhodamine 6G salicylaldehyde hydrazone) and characterized. Complex 1 has a rare disc-shaped structure, where the central Gd ion is connected to the six peripheral GdIII ions via CH3O-/μ3-OH- bridges. Complex 2 has an unexpected three-layer double sandwich structure with a rare μ6-O2- ion in the center of the cluster. Magnetic studies revealed that complex 1 exhibits a magnetic entropy change of 17.4 J kg-1 K-1 at 3 K and 5 T. On the other hand, complex 2 shows a higher magnetic entropy change of 22.3 J kg-1 K-1 at 2 K and 5 T.

Naphthalimide-Tagged Iron(II) Spin Crossover Complex with Synergy of Ratiometric Fluorescence for Thermosensing

Spin crossover (SCO) materials that possess switchable and cooperative fluorescence have long focused interest in photonic sensor devices to monitor the variations in the physicochemical parameters of the external environment. However, the lack of quantified cooperativity for the SCO transition operating in isolated molecules is detrimental to short-term technological applications. In this study, a pretwisted energy D-A system combining the deep-blue naphthalimide fluorophore (donor) and the FeN6 SCO chromophore (switchable acceptor) has been developed with the formula of Fe(naph-abpt)2(NCS)2·2DMF (1), where naph-abpt is N-[3,5-di(pyridin-2-yl)-4H-1,2,4-triazol-4-yl]-1,8-naphthalimide. Dual emission from the naphthalimide function based on its vibronic structure exhibits a different synergy effect with SCO, providing a new platform for ratiometric fluorescence thermosensing. Theoretical calculations and optical experimental results demonstrate an excellent correlation between luminescence intensity ratio signals and magnetic data of spin transition, promising a high sensitivity of the optical activity of the ligand to the spin state of the active iron(II) ions, with the maximum relative sensitivity as 0.7% K-1 around T1/2.

Investigation of the unique magnetic behaviours of isomers in a 1,2-dithiooxalato-bridged diiron(II) complex

1,2-Dithiooxalate (dto) can be employed as a bridging ligand and it exhibits symmetric (O,S-chelation) or asymmetric (O,O- and S,S-chelation) coordination forms. In this study, we prepared a novel dto-bridged diiron(II) complex, [{Fe(TPA)}2(μ-dto)](ClO4)2 (1), where TPA is tris(2-pyridylmethyl)amine. Interestingly, the bridging dto ligand exhibited not only the asymmetric form but also a linkage isomer and a diastereomer within the same crystal. Notably, the three isomers of 1 exhibited different magnetic properties, resulting in a multi-step spin crossover behaviour.

Confinement inside MOFs Enables Guest-Modulated Spin Crossover of Otherwise Low-Spin Coordination Cages

Confinement of discrete coordination cages within nanoporous lattices is an intriguing strategy to gain unusual properties and functions. We demonstrate here that the confinement of coordination cages within metal-organic frameworks (MOFs) allows the spin state of the cages to be regulated through multilevel host-guest interactions. In particular, the confined in situ self-assembly of an anionic FeII4L6 nanocage within the mesoporous cationic framework of MIL-101 leads to the ionic MOF with an unusual hierarchical host-guest structure. While the nanocage in solution and in the solid state has been known to be invariantly diamagnetic with low-spin FeII, FeII4L6@MIL-101 exhibits spin-crossover (SCO) behavior in response to temperature and release/uptake of water guest within the MOF. The distinct color change concomitant with water-induced SCO enables the use of the material for highly selective colorimetric sensing of humidity. Moreover, the spin state and the SCO behavior can be modulated also by inclusion of a guest into the hydrophobic cavity of the confined cage. This is an essential demonstration of the phenomenon that the confinement within porous solids enables an SCO-inactive cage to show modulable SCO behaviors, opening perspectives for developing functional supramolecular materials through hierarchical host-guest structures.

Spin-Crossover Behaviors of Iron(II) Complexes Bearing Halogen Ligands in Solid State and Solution

Numerous Fe(II) spin-crossover (SCO) compounds have been developed in the past decades, while the reports on the SCO materials with halogen atoms acting as coordinating ligands remain rare. In this study, we synthesize three iron(II) halide complexes with a general formula of [Fe^II(Py5Me₂)X]⁺ (Py5Me₂ = 2,6-bis[1,1-bis(2-pyridyl)ethyl]pyridine, X = Cl^- or Br^-) that undergo complete SCO transitions at near room temperature. The SCO properties of these compounds are investigated in detail by magnetic measurements, variable-temperature single-crystal X-ray diffractions, and Mössbauer spectra analyses. Because of the good stability of the coordination structures and suitable ligand-field strength, these compounds show robust spin transitions in both solid state and solution.

Synergetic Spin-Crossover and Luminescent Properties in a Multifunctional 2D Iron(II) Coordination Polymer

We designed and synthesized a strong fluorescent tetradentate pyridine ligand, 3,6,11,12-tetra(pyridin-4-yl)dibenzo[a,c]phenazine (TPDP), by covalently grafting pyridyl to fluorescent dye dbpz, which can react with the Fe(NCX)₂ (X = S and Se) unit, obtaining two new 2D [4 × 4] square-grid compounds, namely, {Fe^II(TPDP)₂(SCN)₂·CHCl₃·4CH₃OH}_n (1) and {[Fe^II(TPDP)₂(SeCN)₂]·CH₂Cl₂·4CH₃OH}_n (2). Both of them show expected one-step spin-crossover (SCO) properties, and complex 2^vacuum exhibits a combination of the SCO phenomenon and fluorescence in a synergetic way. The energy transfer mechanism of 2^vacuum is verified by the theoretical calculations and experimental results. This study provides an effective strategy to synthesize large conjugated fluorescent ligands using dyes to further form SCO-luminescent bifunctional materials.

FeII spin crossover complexes containing N4O2 donor ligands

Spin crossover (SCO) is one of the most studied magnetic bistable phenomena because of its application in the field of multifunctional magnetic materials. Fe^II complexes in a N₆ coordination environment have been the most well-studied in terms of their SCO behaviour. Other coordination environments, notably the N₄O₂ coordination environment, has also been quite effective in inducing SCO behaviour in the corresponding Fe^II complexes. This review deals with such systems. The three ligand families that are discussed are: Jager type ligands, hydrazone based ligands and tridentate ligands having salicylaldehyde derivatives. These ligands allow the assembly of both mononuclear and multinuclear complexes that exhibit cooperative spin transitions. Also, Fe^II complexes obtained from some of these ligands are multifunctional and exhibit a coupling of optical and magnetic properties. Most of the Fe^II complexes obtained from these families of ligands are charge neutral which allows easy surface deposition. Further, modulation of these ligand families allows a fine tuning of the ligand field strength which results in varying SCO behavior. In addition some of the Fe^II complexes derived from these ligands exhibit a light induced excited spin state trapping (LIESST) effect. All of the above aspects are reviewed in this review.

High-Temperature Spin Crossover in Iron(II) Complexes with 2,6-Bis(1H-imidazol-2-yl)pyridine

Novel iron(II) coordination compounds containing a ligand 2,6-bis(1H-imidazol-2-yl)pyridine (L), having such a composition as [FeL₂]SO₄·0.5H₂O, [FeL₂]Br₂·H₂O, [FeL₂](ReO₄)₂, [FeL₂]B₁₀H₁₀∙H₂O, [FeL₂]B₁₂H₁₂∙1.5H₂O had been synthesized and studied using UV-Vis (diffuse reflectance), infrared, extended X-ray absorption fine structure (EXAFS), and Mössbauer spectroscopy methods, as well as X-ray diffraction and static magnetic susceptibility methods. The analysis of the μ_eff(T) dependence in the temperature range of 80–600 K have shown that all the obtained complexes exhibit a high-temperature spin crossover ¹A₁ ↔ ⁵T₂.

105 K Wide Room Temperature Spin Transition Memory Due to a Supramolecular Latch Mechanism

Little is known about the mechanisms behind the bistability (memory) of molecular spin transition compounds over broad temperature ranges (>100 K). To address this point, we report on a new discrete Fe^II neutral complex [Fe^IIL₂]⁰ (1) based on a novel asymmetric tridentate ligand 2-(5-(3-methoxy-4H-1,2,4-triazol-3-yl)-6-(1H-pyrazol-1-yl))pyridine (L). Due to the asymmetric cone-shaped form, in the lattice, the formed complex molecules stack into a one-dimensional (1D) supramolecular chain. In the case of the rectangular supramolecular arrangement of chains in methanolates 1-A and 1-B (both orthorhombic, Pbcn) differing, respectively, by bent and extended spatial conformations of the 3-methoxy groups (3MeO), a moderate cooperativity is observed. In contrast, the hexagonal-like arrangement of supramolecular chains in polymorph 1-C (monoclinic, P2₁/c) results in steric coupling of the transforming complex species with the peripheral flipping 3MeO group. The group acts as a supramolecular latch, locking the huge geometric distortion of complex 1 and in turn the trigonal distortion of the central Fe^II ion in the high-spin state, thereby keeping it from the transition to the low-spin state over a large thermal range. Analysis of the crystal packing of 1-C reveals significantly changing patterns of close intermolecular interactions on going between the phases substantiated by the energy framework analysis. The detected supramolecular mechanism leads to a record-setting robust 105 K wide hysteresis spanning the room temperature region and an atypically large T_LIESST relaxation value of 104 K of the photoexcited high-spin state. This work highlights a viable pathway toward a new generation of cleverly designed molecular memory materials.

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.

Facile Synthesis of Water-Soluble Rhodamine-Based Polymeric Chemosensors via Schiff Base Reaction for Fe3+ Detection and Living Cell Imaging

Quantitative and accurate determination of iron ions play a vital role in maintaining environment and human health, but very few polymeric chemosensors were available for the detection of Fe³⁺ in aqueous solutions. Herein, a water-soluble rhodamine-poly (ethylene glycol) conjugate (DRF-PEG), as a dual responsive colorimetric and fluorescent polymeric sensor for Fe³⁺ detection with high biocompatibility, was first synthesized through Schiff base reaction between rhodamine 6G hydrazide and benzaldehyde-functionalized polyethylene glycol. As expected, the introduction of PEG segment in DRF-PEG significantly improved the water solubility of rhodamine derivatives and resulted in a good biosensing performance. The detection limit of DRF-PEG for Fe³⁺ in pure water is 1.00 μM as a fluorescent sensor and 3.16 μM as a colorimetric sensor at pH 6.5. The specific sensing mechanism of DRF-PEG toward Fe³⁺ is proposed based on the intramolecular charge transfer (ICT) mechanism, in which the O and N atoms in rhodamine moiety, together with the benzene groups from benzaldehyde-modified PEG segment, participate in coordination with Fe³⁺. Furthermore, DRF-PEG was applied for the ratiometric imaging of Fe³⁺ in HeLa cells and showed the potential for quantitative determination of Fe³⁺ in fetal bovine serum samples. This work provides insights for the design of water-soluble chemosensors, which can be implemented in iron-related biological sensing and clinical diagnosis.

Magnetooptical Properties of Lanthanide(III) Metal-Organic Frameworks Based on an Iridium(III) Metalloligand

Integrating magnetic and optical properties into a metal-organic framework (MOF) remains a great challenge. Herein, we have reasonably constructed two 3D magnetooptical MOFs by incorporating a [Ir^III(ppy)₂(bpy)]⁺-based fluorescent metalloligand and magnetic Ln^III centers. The alternating arrangements of Δ- or Λ-[Ir^III(ppy)₂(bpy)]⁺ endow these MOFs with enhanced optical properties. Moreover, the use of Dy^III leads to field-induced slow magnetic relaxation. This work provides an effective strategy for the preparation of magnetooptical bifunctional MOFs.

New iron(ii) spin-crossover metallomesogen with long aliphatic chain terminated by a CC bond

A new Fe(ii) complex with long aliphatic chains and terminal CC bonds exhibits SCO behavior and liquid-crystalline properties.

Synchronously tuning the spin-crossover and fluorescence properties of a two-dimensional Fe(ii) coordination polymer by solvent guests

Synchronously tuning the spin-crossover and fluorescence properties of a two-dimensional Fe(ii) coordination polymer by solvent guests.

Monitoring spin-crossover phenomena via Re(I) luminescence in hybrid Fe(II) silica coated nanoparticles

Bare (1) and silica coated (1@SiO2) spin crossover (SCO) nanoparticles based on the polymer {[Fe(NH₂Trz)₃](BF₄)₂}_n have been prepared following a water-in-oil synthetic procedure. For 1, the critical temperatures of the spin transition are T_C↓ = 214.6 K and T_C↑ = 220.9 K. For 1@SiO2, the abruptness of the transition is enhanced and the critical temperatures are centred at room temperature (T_C↓ = 292.1 K and T_C↑ = 296.3 K). An inert Re(I) complex of formula [Re(phen)(CO)₃(PETES)](PF₆) (phen = 1, 10-phenanthroline; PETES = 2(4-pyridylethyl)triethoxysilane) (Re) was also synthesized yielding intense green emission centred at λ_em = 560 nm. The grafting of this complex on the silica shell of 1@SiO2 led to a bifunctional SCO-luminescence composite (1@SiO2/Re) whose luminescence properties were tuned by the spin state switching. Temperature-variable photophysical studies showed that luminescence and spin transition were synchronized through a radiative (trivial) energy transfer mechanism between the Re(I) and the Fe(II)-LS (LS, Low Spin) centres.

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 (T_1/2 = 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.

Iron(II) Spin Crossover Coordination Polymers Derived From a Redox Active Equatorial Tetrathiafulvalene Schiff-Base Ligand

Two polymorphic Fe^II coordination polymers [Fe^IIL (TPPE)_0.5] 1) and [(Fe^II₃L₃ (TPPE)_1.5)] 2), were obtained from a redox-active tetrathiafulvalene (TTF) functionalized ligand [H₂L = 2,2’-(((2-(4,5-bis-(methylthio)-1,3-dithiol-2-ylidene)benzo(d) (1,3) dithiole-5,6-diyl)bis-(azanediyl))bis-(meth anylylidene)) (2E,2E')-bis(3-oxobutanoate)] and a highly luminescent connector {TPPE = 1,1,2,2-tetrakis[4-(pyridine-4-yl)phenyl]-ethene}. Complex 1 has a layered structure where the TPPE uses its four diverging pyridines from the TPPE ligand are coordinated by the trans positions to the flat TTF Schiff-base ligand, and complex 2 has an unprecedented catenation of layers within two interpenetrated frameworks. These coordination polymers reserved the redox activity of the TTF unit. Complex 1 shows gradual spin transition behavior without hysteresis. And the fluorescence intensity of TPPE in 1 changes in tandem with the spin crossover (SCO) transition indicating a possible interplay between fluorescence and SCO behavior.

Controlling dynamic magnetic properties of coordination clusters via switchable electronic configuration

Large-sized coordination clusters have emerged as a new class of molecular materials in which many metal atoms and organic ligands are integrated to synergize their properties. As dynamic magnetic materials, such a combination of multiple components functioning as responsive units has many advantages over monometallic systems due to the synergy between constituent components. Understanding the nature of dynamic magnetism at an atomic level is crucial for realizing the desired properties, designing responsive molecular nanomagnets, and ultimately unlocking the full potential of these nanomagnets for practical applications. Therefore, this review article highlights the recent development of large-sized coordination clusters with dynamic magnetic properties. These dynamic properties can be associated with spin transition, electron transfer, and valence fluctuation through their switchable electronic configurations. Subsequently, the article also highlights specialized characterization techniques with different timescales for supporting switching mechanisms, chemistry, and properties. Afterward, we present an overview of coordination clusters (such as cyanide-bridged and non-cyanide assemblies) with dynamic magnetic properties, namely, spin transition and electron transfer in magnetically bistable systems and mixed-valence complexes. In particular, the response mechanisms of coordination clusters are highlighted using representative examples with similar transition principles to gain insights into spin state and mixed-valence chemistry. In conclusion, we present possible solutions to challenges related to dynamic magnetic clusters and potential opportunities for a wide range of intelligent next-generation devices.

Sublimable Spin-Crossover Complexes: From Spin-State Switching to Molecular Devices

Spin-crossover (SCO) active transition metal complexes are an important class of switchable molecular materials due to their bistable spin-state switching characteristics at or around room temperature. Vacuum-sublimable SCO complexes are a subclass of SCO complexes suitable for fabricating ultraclean spin-switchable films desirable for applications, especially in molecular electronics/spintronics. Consequently, on-surface SCO of thin-films of sublimable SCO complexes have been studied employing spectroscopy and microscopy techniques, and results of fundamental and technological importance have been obtained. This Review provides complete coverage of advances made in the field of vacuum-sublimable SCO complexes: progress made in the design and synthesis of sublimable functional SCO complexes, on-surface SCO of molecular and multilayer thick films, and various molecular and thin-film device architectures based on the sublimable SCO complexes.

Heterometallic {DyIII2FeII2} grids with slow magnetic relaxation and spin crossover

The self-assembly of a Dy^III ion, an Fe^II ion and a multitopic H₂L ligand produces novel [2 × 2] {DyIII2FeII2} grids exhibiting slow magnetic relaxation and spin crossover.

Thermo- and light-triggered reversible interconversion of dysprosium-anthracene complexes and their responsive optical, magnetic and dielectric properties

Artificial smart materials with switchable multifunctionality are of immense interest owing to their wide application in sensors, displays and memory devices. Lanthanide complexes are promising multifunctional materials integrating optical and magnetic characteristics. However, synergistic manipulation of different physical properties in lanthanide systems is still challenging. Herein we designed and synthesized a mononuclear complex [DyIII(SCN)3(depma)2(4-hpy)2] (1), which incorporates 9-diethylphosphonomethylanthracene (depma) as a photo-active component and 4-hydroxypyridine (4-hpy) as a polar component. This compound shows several unusual features: (a) reversible thermo-responsive phase transition associated with the order-disorder transition of 4-hpy and SCN-, which leads to thermochromic behavior and dielectric anomaly; (b) reversible photo-induced dimerization of anthracene groups, which leads to synergistic switching of luminescence, magnetic and dielectric properties. To our knowledge, compound 1 is the first example of lanthanide complexes that show stimuli-triggered synergistic and reversible switching of luminescence, magnetic and dielectric properties.

ON/OFF Photoswitching and Thermoinduced Spin Crossover with Cooperative Luminescence in a 2D Iron(II) Coordination Polymer

A 2D coordination polymer, {[Fe(L)₂(NCSe)₂]·6MeOH·14H₂O}_n (1; L = 2,5-dipyridylethynylene-3,4-ethylenedioxythiophene), has been synthesized based on a redox active luminescence ligand. 1 possesses a 2D [4 × 4] square-grid network where the iron(II) center is in a FeN₆ octahedral coordination environment. 1 displays reversible thermoinduced high-spin (HS; S = 2) to diamagnetic low-spin (LS; S = 0) ON/OFF spin-state switching with a T_1/2 value of 150 K. Interestingly, optical reflectivity and photomagnetic studies at 10 K under light irradiation revealed an efficient conversion to a photoinduced metastable HS excited state from a LS ground state. Remarkably, the photoexcited HS state can be reversibly switched ON and OFF by using 625 and 850 nm light-emitting-diode lights. Intriguingly, the thermal dependence of the luminescence intensity of the maximum emission at 524 nm for 1 shows a minimum at around the spin-crossover (SCO) temperature, indicating a cooperative nature between the SCO and luminescence properties. Theoretical calculations confirmed the above findings.

Three-Step Spin State Transition and Hysteretic Proton Transfer in the Crystal of an Iron(II) Hydrazone Complex

A proton-electron coupling system, exhibiting unique bistability or multistability of the protonated state, is an attractive target for developing new switchable materials based on proton dynamics. Herein, we present an iron(II) hydrazone crystalline compound, which displays the stepwise transition and bistability of proton transfer at the crystal level. These phenomena are realized through the coupling with spin transition. Although the multi-step transition with hysteresis has been observed in various systems, the corresponding behavior of proton transfer has not been reported in crystalline systems; thus, the described iron(II) complex is the first example. Furthermore, because proton transfer occurs only in one of the two ligands and π electrons redistribute in it, the dipole moment of the iron(II) complexes changes with the proton transfer, wherein the total dipole moment in the crystal was canceled out owing to the antiferroelectric-like arrangement.

Enhanced dielectricity coupled to spin-crossover in a one-dimensional polymer iron(ii) incorporating tetrathiafulvalene

A concerted bending–flattening motion of the redox-active TTF within constructed one-dimensional Fe^II–TTF–Schiff-base chain with bridging 4,4′-bpy enhances the dielectric constant coupled to its spin-crossover transition above room temperature.

In designing multifunctional materials for potential switches that can be used as memory devices, the high-spin (HS) to low-spin (LS) crossover (SCO) one-dimensional polymer, [Fe^II(L)(4,4′-bpy)]_n, was constructed from a designed redox-active tetrathiafulvalene (TTF) functionalized Schiff-base and the ditopic linker 4,4′-bipyridine (bpy). It exhibits an 8 K hysteretic SCO centred at T_1/2 = 325 K which is coupled to changes in its dielectric constant. The crystal structures above and below the transition temperature reveal similar parallel linear ···Fe–bpy–Fe–bpy··· chains displaying expansion of the Fe^II octahedron in the HS state. Density functional theory (DFT) calculations reveal a concerted electronic charge and spin change represented by the Mülliken charge of the Fe and the magnitude and direction of the dipole moment which substantiate the experimental observations.

Spin crossover in mononuclear Fe(II) complexes based on a tetradentate ligand

Three isostructural complexes with the formula [Fe(L_5Me)(NCE)₂]: L_5Me  =  N,N'-bis(5-methyl-2-pyridylmethyl)ethane-1,2-diamine and E  =  S (1-S), E  =  Se (1-Se), E  =  BH₃ (1-BH ₃ ) have been synthesized and characterized by single-crystal x-ray diffraction, magnetic susceptibility and DSC studies. All the three derivatives are spin crossover (SCO) active, showing complete one-step spin conversion. The SCO midpoint temperatures (T _1/2) are 193 K for 1-S, 226 K for 1-Se, and 330 K for 1-BH ₃ , which are among the highest values for the homologous Fe(II)-NCE complexes with comparable tetradentate ligands. The almost linear Fe-N  ≡  C(E) angles are consistent with the strong ligand field (LF) strength imposed by these NCE^- co-ligands. Strong hydrogen-like bonding N-H…E was observed to connect the molecules into 2D supramolecular sheets parallel to the bc plane. However, such supramolecular interaction is not sufficient enough to transmit strong cooperativity. A discussion on the factors governing the LF strength and the cooperativity has been made, based on the comparison of analogous complexes and also based on UV-vis spectroscopy studies of the Ni(II) complexes.

Visualization of materials using the confocal laser scanning microscopy technique

The development of materials science always benefits from advanced characterizations. Currently, imaging techniques are of great technological importance in both fundamental and applied research on materials. In comparison to conventional visualization methods, confocal laser scanning microscopy (CLSM) is non-invasive, with macroscale and high-contrast scanning, a simple and fast sample preparation procedure as well as easy operation. In addition, CLSM allows rapid acquisition of longitudinal and cross-sectional images at any position in a material. Therefore, the CLSM-based visualization technique could provide direct and model-independent insight into material characterizations. This review summarizes the recent applications of CLSM in materials science. The current CLSM approaches for the visualization of surface structures, internal structures, spatial structures and reaction processes are discussed in detail. Finally, we provide our thoughts and predictions on the future development of CLSM in materials science. The purpose of this review is to guide researchers to build a suitable CLSM approach for material characterizations, and to open viable opportunities and inspirations for the development of new strategies aiming at the preparation of advanced materials. We hope that this review will be useful for a wide range of research communities of materials science, chemistry, and engineering.

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.

A homochiral 3D framework of mechanically interlocked 1D loops with solvent-dependent spin-state switching behaviors

An atypical homochiral spin-crossover (SCO) framework (1) constructed from mechanically interlocked 1D molecular loops was prepared. Due to the flexibility of the interlocked structure, the guest solvent molecules can be reversibly exchanged. Consequently, its SCO behavior was capable of modulating between one- and two-stepped transitions in response to acetonitrile and methanol.

Bistable molecular materials with dynamic structures

In this Feature Article, we introduce how to manipulate the motion of electrons or molecules by external stimuli, to achieve switchable properties in molecule-based single crystals.

Temperature-induced structural transformations accompanied by changes in magnetic properties of two copper coordination polymers

Two ferromagnetic copper compounds have been synthesized under different temperature, which represented the rare example of structural transformations resulting from the coordination modes of organic ligands supported by magnetic results.