Thermodynamical aspects of the spin crossover phenomenon

Fundamental aspects of spin crossover (SCO) mechanisms are reviewed through considerations of ligand/crystal field theory, thermodynamics, and modeling of the thermoinduced spin transition in the solid state based on macroscopic–mesoscopic approaches . In particular, we highlight success of thermodynamic models in the simulation of first-order spin transitions with hysteretic behaviors (bistability) and multistep conversions. Bistability properties originate from elastic interactions, the so-called cooperativity between SCO molecules in the crystal packing. Although physical and chemical properties and thermodynamical quantities of noninteracting SCO compounds can be readily injected in macroscopic models, taking cooperativity into account remains problematic. The relationship between phenomenological numerical parameters and experimentally accessible quantities can only be most of the time indirectly established. Recent extensions of these thermodynamical models to grasp SCO properties at the nanoscale and combinations with ab initio numerical methods show that macroscopic models still constitute useful theoretical tools to investigate SCO phenomena. The necessity to further probe the thermomechanical properties of SCO materials is also emphasized.     

Synergetic Spin Crossover and Fluorescence in One‐Dimensional Hybrid Complexes

Hybrid materials integrated with a variety of physical properties, such as spin crossover (SCO) and fluorescence, may show synergetic effects that find applications in many fields. Herein we demonstrate a promising post‐synthetic approach to achieve such materials by grafting fluorophores (1‐pyrenecarboxaldehyde and Rhodamine B) on one‐dimensional SCO Fe^II structures. The resulting hybrid materials display expected one‐step SCO behavior and fluorescent properties, in particular showing a coupling between the transition temperature of SCO and the temperature where the fluorescent intensity reverses. Consequently, synergetic effect between SCO and fluorescence is incorporated into materials despite different fluorophores. This study provides an effective strategy for the design and development of novel magnetic and optical materials.     

Spin-crossover cobalt(II) compound with banana-shaped structure

A banana-shaped spin-crossover (SCO) cobalt(II) complex [Co(C16-terpy)2](BPh4)2 (1) with long alkyl chains, based on a terpyridine frame, was synthesized. Compound 1 exhibited very gradual SCO behavior and changes in the dielectric constant. This shows a way in which SCO materials can be used in electronic devices.     

应用ESCA研究SCO在胶磷矿和白云石表面的吸附特性

本文采用ESCA研究了在蒸馏水,相互澄清液及SCO溶液中胶磷矿和白云石的吸附特性.研究表明在相互澄清液中胶磷矿和白云石发生表面转化,且SCO对矿物表面的镁具有较强的亲合力,证实了矿物表面污染对SCO选择性具有强烈的影响。结果与浮选试验取得良好的一致.     

Multifunctionality in spin crossover materials

One of the most important trends in the spin crossover (SCO) field is focused on the synthesis of new molecule-based functional materials in which the SCO properties may be combined with other physical or chemical properties in a synergic fashion. The current stage of investigations regarding interplay and synergic effects between SCO, magnetic coupling, liquid crystalline properties, host-guest interactions, non-linear optical properties, electrical conductivity, and ligand isomerization is highlighted and discussed.     

Room temperature conductance switching in a molecular iron(iii) spin crossover junction

Herein, we report the first room temperature switchable Fe(iii) molecular spin crossover (SCO) tunnel junction. The junction is constructed from [Fe^III(qsal-I)₂]NTf₂ (qsal-I = 4-iodo-2-[(8-quinolylimino)methyl]phenolate) molecules self-assembled on graphene surfaces with conductance switching of one order of magnitude associated with the high and low spin states of the SCO complex. Normalized conductance analysis of the current–voltage characteristics as a function of temperature reveals that charge transport across the SCO molecule is dominated by coherent tunnelling. Temperature-dependent X-ray absorption spectroscopy and density functional theory confirm the SCO complex retains its SCO functionality on the surface implying that van der Waals molecule—electrode interfaces provide a good trade-off between junction stability while retaining SCO switching capability. These results provide new insights and may aid in the design of other types of molecular devices based on SCO compounds.

Herein, we report the first room temperature switchable Fe(iii) molecular spin crossover (SCO) tunnel junction.     

Hexakis-adducts of [60]fullerene as molecular scaffolds of polynuclear spin-crossover molecules

A family of hexakis-substituted [60]fullerene adducts endowed with the well-known tridentate 2,6-bis(pyrazol-1-yl)pyridine (bpp) ligand for spin-crossover (SCO) systems has been designed and synthesized. It has been experimentally and theoretically demonstrated that these molecular scaffolds are able to form polynuclear SCO complexes in solution. UV-vis and fluorescence spectroscopy studies have allowed monitoring of the formation of up to six Fe(ii)–bpp SCO complexes. In addition, DFT calculations have been performed to model the different complexation environments and simulate their electronic properties. The complexes retain SCO properties in the solid state exhibiting both thermal- and photoinduced spin transitions, as confirmed by temperature-dependent magnetic susceptibility and Raman spectroscopy measurements. The synthesis of these complexes demonstrates that [60]fullerene hexakis-adducts are excellent and versatile platforms to develop polynuclear SCO systems in which a fullerene core is surrounded by a SCO molecular shell.

Polynuclear spin-crossover molecules showing both thermal and photoinduced spin transitions have been prepared using a [60]fullerene hexakis-adduct endowed with Fe(ii) complexes of tridentate 2,6-bis(pyrazol-1-yl)pyridine (bpp) ligand.     

Guest‐Switchable Multi‐Step Spin Transitions in an Amine‐Functionalized Metal–Organic Framework

Materials with hysteretic multi‐step spin‐crossover (SCO) have potential application in high‐order data storage. Here, an unprecedented hysteretic four‐step SCO behavior with the sequence of LS↔HS_0.25LS_0.75↔HS_0.5LS_0.5↔ HS_0.75LS_0.25↔HS is found in a three‐dimensional (3D) Hofmann‐type metal–organic framework (MOF), which is evidenced by magnetic, differential scanning calorimetry, and crystal data. Further experiments involving guest exchange leads to the first reversible modulation of four‐, two‐, and one‐stepped SCO behaviors, which provides a new strategy for developing multi‐step SCO materials.     

Electronic Transport Properties of Spin-Crossover Magnet Fe(Ⅱ)-N4S2 Complexes

Spin-crossover (SCO) magnets can act as one of the most possible building blocks in molecular spintronics due to their magnetic bistability between the high-spin (HS) and low-spin (LS) states. Here, the electronic structures and transport properties through SCO magnet Fe(Ⅱ)-N₄S₂ complexes sandwiched between gold electrodes are explored by performing extensive density functional theory calculations combined with non-equilibrium Green's function formalism. The optimized Fe-N and Fe-S distances and predicted magnetic moment of the SCO magnet Fe(Ⅱ)-N₄S₂ complexes agree well with the experimental results. The reversed spin transition between the HS and LS states can be realized by visible light irradiation according to the estimated SCO energy barriers. Based on the obtained transport results, we observe nearly perfect spin-filtering effect in this SCO magnet Fe(Ⅱ)-N₄S₂ junction with the HS state, and the corresponding current under small bias voltage is mainly contributed by the spin-down electrons, which is obviously larger than that of the LS case. Clearly, these theoretical findings suggest that SCO magnet Fe(Ⅱ)-N₄S₂ complexes hold potential applications in molecular spintronics.     

Microscopic models of spin crossover

The basic model for thermal spin crossover (SCO) is discussed in its microscopic and thermodynamic formulation. Compared to the basic model, its more elaborated forms formulated in course of almost 50 years are briefly reviewed with emphasis on their additional features. A separate section is devoted to the newer developments in the field of modelling of the SCO nanoparticles. The presentation of models is led in a comparative way to provide an accessible outline of the foundations of modern theoretical research on SCO and a simple applicability in quantitative interpretation of experiments.     

Organosulfonate-Modulated Spin-Crossover Behavior in Three Halogen-Functionalized Cobalt(II) Complexes

We present here the syntheses, crystal structures, and spin crossover (SCO) properties of a series of halogen-functionalized cobalt(II) complexes, [Co(Brphtpy)₂](OTf)₂ ⋅ DMF ⋅ 2H₂O ( 1 ), [Co(Brphtpy)₂](HBS)₂ ⋅ H₂O ( 2 ), [Co(Brphtpy)₂](MQ)₂ ⋅ 2MeCN ⋅ 3H₂O ( 3 ) ( Brphtpy =4′–(4-Bromophenyl)–2,2′:6′,2′′-terpyridine; OTf⁻=trifluoromethanesulfonate; HBS⁻=hydroxybenzenesulfonate dihydrate; MQ⁻=methyl orange). Variable-temperature single-crystal X-ray analyses revealed mononuclear compounds of 1 – 3 consisted of [Co(Brphtpy)₂]²⁺ SCO active units and organosulfonate anions and no structural phase transformation happened in measured high-low temperature. The packing structures of these complexes were tuned by varying organosulfonates. However, no notable supramolecular interactions can be found, in turn leading to gradual, incomplete, and non-hysteretic SCO behaviors. Interestingly, the SCO behaviors of these three complexes were significantly modified after the removal of lattice solvents. Combined structural and magnetic investigations revealed the non-cooperative supramolecular packing structures, guest internal pressure, and the small structural distortions of the SCO units should be responsible for the worse SCO properties of 1 – 3 . The foregoing results show that to achieve high-performance Co²⁺ SCO, both the weak interactions, internal pressure, and structural distortion should be considered during the design and construction of SCO complexes.     

Hierarchical Spin-Crossover Cooperativity in Hybrid 1D Chains of FeII-1,2,4-Triazole Trimers Linked by [Au(CN)2]− Bridges

Foremost, practical applications of spin-crossover (SCO) materials require control of the nature of the spin-state coupling. In existing SCO materials, there is a single, well-defined dimensionality relevant to the switching behavior. A new material, consisting of 1,2,4-triazole-based trimers coordinated into 1D chains by [Au(CN)₂]⁻ and spaced by anions and exchangeable guests, underwent SCO defined by elastic coupling across multiple dimensional hierarchies. Detailed structural, vibrational, and theoretical studies conclusively confirmed that intra-trimer coupling was an order of magnitude greater than the intramolecular coupling, which was an order of magnitude greater than intermolecular coupling. As such, a clear hierarchy on the nature of elastic coupling in SCO materials was ascertained for the first time, which is a necessary step for the technological development of molecular switching materials.     

A New Family of Anionic FeIII Spin Crossover Complexes Featuring a Weak‐Field N2O4 Coordination Octahedron

Unprecedented anionic Fe^III spin crossover (SCO) complexes involving a weak‐field O,N,O‐tridentate ligand were discovered. The SCO transition was evidenced by the temperature variations in magnetic susceptibility, Mössbauer spectrum, and coordination structure. The DFT calculations suggested that larger coefficients on the azo group in the HOMO?1 of a ligand might contribute to the enhancement of a ligand‐field splitting energy. The present anionic SCO complex also exhibited the light‐ induced excited‐spin‐state trapping effect.     

加拿大合成原油减压馏分油及其催化裂化液体产物中含硫化合物分析

应用GC PFPD和GC MS技术对加拿大油砂合成原油（Synthetic Crude Oil,简称SCO）减压馏分油（350 ℃~500 ℃）中的含硫化合物进行定性定量分析。结果表明,所含硫化物主要是C3~7二苯并噻吩,而催化裂化反应后则以短侧链二苯并噻吩为主,还含少量烷基苯并噻吩和烷基噻吩,三者的质量分数分别为82.04%、13.42%和0.56%,均属于难以加氢脱除的含硫化合物。SCO减压馏分油和大港减压馏分油按不同比例混合后进行催化裂化反应,随着加拿大合成原油减压馏分油搀兑比增加,所得液相产物中烷基噻吩和烷基苯并噻吩质量分数逐渐降低,烷基二苯并噻吩相对质量分数增加,4-MDBT是丰度最高的含硫化合物,显示加氢油的特征。随掺兑比的提高,液相产物中总硫质量分数和柴油馏分中硫质量分数逐渐增高,而汽油馏分中硫质量分数逐渐降低。     

Polymorphism in spin-crossover systems

The occurrence of spin-crossover (SCO) highly depends on external influences, i.e. temperature, pressure, light irradiation or magnetic field, this electronic switching phenomenon is accompanied by drastic changes in magnetic and optical properties, dielectric constants, colour and structures. Thus, SCO materials are particularly attractive for potential applications in molecular sensing, switching, data storage, display, and other electronic devices at nanometric scale. Polymorphism is widely encountered in the studies of crystallization, phase transition, materials synthesis, biomineralization, and in the manufacture of drugs. Because different crystal forms of the same substance can possess very different properties and behave as different materials, so they are particularly meaningful for investigating SCO phenomena. Studying polymorphism of SCO compounds is therefore important for better understanding the structural factors contributing to spin transition and the structure-function relationship. This critical review is aimed to provide general readers with a comprehensive view of polymorphism in SCO systems. The article is generally structured according to specific metal ions and the dimensionality of compounds in the field. This paper is addressed to readers who are interested in multifunctional materials and tuning magnetic properties through supramolecular chemistry principles (129 references).     

Tuning the transition temperature and cooperativity of bapbpy-based mononuclear spin-crossover compounds: interplay between molecular and crystal engineering

In this study, we show that 1) different isomers of the same mononuclear iron(II) complex give materials with different spin‐crossover (hereafter SCO) properties, and 2) minor modifications of the bapbpy (bapbpy=N6,N6′‐di(pyridin‐2‐yl)‐2,2′‐bipyridine‐6,6′‐diamine) ligand allows SCO to be obtained near room temperature. We also provide a qualitative model to understand the link between the structure of bapbpy‐based ligands and the SCO properties of their iron(II) compounds. Thus, seven new trans‐[Fe{R₂(bapbpy)}(NCS)₂] compounds were prepared, in which the R₂bapbpy ligand bears picoline ( 9 – 12 ), quin‐2‐oline ( 13 ), isoquin‐3‐oline ( 14 ), or isoquin‐1‐oline ( 15 ) substituents. From this series, three compounds ( 12 , 14 , and 15 ) have SCO properties, one of which ( 15 ) occurs at 288 K. The crystal structures of compounds 11 , 12 , and 15 show that the intermolecular interactions in these materials are similar to those found in the parent compound [Fe(bapbpy)(NCS)₂] ( 1 ), in which each iron complex interacts with its neighbors through weak N? H ??? S hydrogen bonding and π–π stacking. For compounds 12 and 15 , hindering groups located near the N? H bridges weaken the N? S intermolecular interactions, which is correlated to non‐cooperative SCO. For compound 14 , the substitution is further away from the N? H bridges, and the SCO remains cooperative as in 1 with a hysteresis cycle. Optical microscopy photographs show the strikingly different spatio‐temporal evolution of the phase transition in the noncooperative SCO compound 12 relative to that found in 1 . Heat‐capacity measurements were made for compounds 1 , 12 , 14 , and 15 and fitted to the Sorai domain model. The number n of like‐spin SCO centers per interacting domain, which is related to the cooperativity of the spin transition, was found high for compounds 1 and 14 and low for compounds 12 and 15 . Finally, we found that although both pairs of compounds 11 / 12 and 14 / 15 are pairs of isomers their SCO properties are surprisingly different.     

Theoretical investigation of the reactions of La atom and La<Superscript>+</Superscript> cation with carbonyl sulfide

The potential energy surfaces for the La+SCO and La⁺+ SCO reactions have been theoretically investigated by using the DFT (B3LYP/ECP/6-311+G(2d)) level of theory. Both ground and excited state potential energy surfaces (PES) are discussed. The present results show that the reaction mechanism is insertion mechanism both along the C-S and C-O bond activation branches, but the C-S bond activation is much more favorable in energy than the C-O bond activation. The reaction of La atom with SCO was shown to occur preferentially on the ground state (doublet) PES throughout the reaction process, and the experimentally observed species, have been explained according to the mechanisms revealed in this work. While for the reaction between La⁺ cation with SCO, it involves potential energy curve-crossing which dramatically affects reaction mechanism, and the crossing points (CPs) have been localized by the approach suggested by Yoshizawa et al. Due to the intersystem crossing existing in the reaction process of La⁺ with SCO, the products SLa⁺(η²CO) and OLa⁺(η²CS) may not form. This mechanism is different from that of La + SCO system. All our theoretical results not only support the existing conclusions inferred from early experiment, but also complement the pathway and mechanism for this reaction.     

Spin-crossover nanoparticles and nanocomposite materials

This contribution reports on the state of the art of the elaboration and the application of nanoparticles (NPs) and nanohybrid/nanocomposite materials based on spin-crossover (SCO) complexes. The first part of this review concerns the syntheses and the characterizations of the physical properties of SCO NPs. All of the methods including homogeneous and heterogeneous media syntheses developed for the elaboration of such NPs and the associated methods used for their morphological characterization are presented. A particular attention is paid on the effects of the size reduction and the influence of the environment on the SCO properties and to specific and recent remarkable advanced physical measurements realized on a batch of NPs or on an isolated object. The second part presents the elaboration of various nanocomposite or nanohybrid materials for which SCO NPs have been associated with magnetic entities, noble metals, different fluorescent dyes, and different active polymers with the objectives to go toward specific applications based on synergistic effects between the two components.     

The First Use of a ReX5 Synthon to Modulate FeIII Spin Crossover via Supramolecular Halogen⋅⋅⋅Halogen Interactions

We have added the {Re^IVX₅}⁻ (X=Br, Cl) synthon to a pocket-based ligand to provide supramolecular design using halogen⋅⋅⋅halogen interactions within an Fe^III system that has the potential to undergo spin crossover (SCO). By removing the solvent from the crystal lattice, we “switch on” halogen⋅⋅⋅halogen interactions between neighboring molecules, providing a supramolecular cooperative pathway for SCO. Furthermore, changes to the halogen-based interaction allow us to modify the temperature and nature of the SCO event.     

Spin‐Übergänge in supramolekularen Strukturen. Für Speicherbausteine von morgen?

We reported about octahedrally coordinated Fe2+‐complexes, which are able to switch between two stable spin states (LS and HS) with different magnetic properties. This phenomenon is called spin crossover (SCO). The interaction between metal ion and ligand determines the actual spin state and whether an extern stimulus can trigger a spin crossover. Due to this fact it is possible for the chemist through the choice of the ligands to manipulate the character and the temperature region of the SCO. Some metal complexes assemble into highly ordered structures on graphite by molecular self assembly. The substitution of the metal complexes with alkyl chains and the interaction of these chains with the highly ordered graphite is crucial for a periodic arrangement of the complexes on the surface. For the future we are curious to see whether through the cooperative effort of coordination chemistry (SCO phenomenon) and surface science (self assembly of SCO complexes on a surface) the vision of a molecular memory will turn into a reality.