Spin Crossover Systems Research Articles

On-Surface Formation and Chemical Transformations of Indigo Coordination Polymers

Coordination polymers are attractive candidates for miniaturized electronics. With unconventional electronic and magnetic properties posited, they are interesting for multiple applications including magnetic information storage and spintronics. Natural products are appealing to tailor advanced new materials, bearing inherent advantages in biodegradation. Among them, we distinguished indigo, a common natural dye with a distinctive blue color, used as a pigment since antiquity. Motivated by its optical properties and the reported metal complexes [1], we employed it as a molecular building block for the surface formation of coordination polymers with iron.We report on the on-surface formation of extended coordination polymers which controllably feature the cis and the transmonomers of the molecular building block. The reaction products of indigo and Fe were characterized atomically by a combination of state-of-the -art experimental and theoretical techniques. By a combination of X-ray photoelectron spectroscopy and scanning tunneling microscopy (STM), it is found that indigo deprotonates and coordinates with Fe atoms on silver surfaces. We identify unambiguously well-defined coordination polymers composed of (1 dehydroindigo : 1 Fe) repeat units with STM, bond-resolving atomic force microscopy and density functional theory (DFT) simulations. On both Ag(100) and on Ag(111), the trans configuration of dehydroindigo results in N,O-chelation in the polymer chains. On the more inert Ag(111) surface, the molecules undergo thermally induced isomerization from the trans to the cisconfiguration and afford N,N- plus O,O-chelation (see figure). This is enabled by the dehydrogenation and is preferential solely in the environment of the coordination chain on Ag(111). DFT calculations also confirm that the coordination polymers of the cis-isomers on Ag(111) and of the trans-isomers on Ag(100) are energetically favored.Our results demonstrate post-synthetic linker isomerization in interfacial metal-organic nanosystems. Both types of coordination polymers are predicted to be spin-crossover systems and the isomerization is anticipated to have a significant impact on their magnetic properties.

Benchmarking Periodic Density Functional Theory Calculations for Spin-State Energies in Spin-Crossover Systems.

Spin energetics is one of the biggest challenges associated with energy calculations for electronic structure methods. The energy differences of the spin states in spin-crossover compounds are very small, making them one of the most difficult systems to calculate. Few methods provide accurate results for calculating these energy differences. In addition, studies have usually focused on calculating energetics of single molecules, while spin-crossover properties are usually experimentally studied in the solid phase. In this paper, we have used periodic boundary conditions employing methods based on density functional theory to calculate the high- and low-spin energy differences for a test case of 20 extended systems. Compounds with different metals and ligands have been selected, and the results indicate that a semiquantitative description of the energy differences can be obtained with the combination of geometry optimization using the PBE functional including many-body dispersion approach and the use of meta-GGA functionals, such as r2SCAN but especially KTBM24, for the energy calculation. Other hybrid functionals, such as TPSSh, give generally good results, but the calculation of the exact exchange with periodic boundary conditions involves a huge increase in computer time and computational resources. It makes the proposed nonhybrid functional approach (KTBM24//PBE+MB) a great advantage for the study of periodic systems.

Nonequivalent Antiferromagnetically Coupled Sublattices Induce Two-Step Spin-Crossover Transitions: Equilibrium and Nonequilibrium Aspects

As a continuation to the previously published work (Yalçın et al. (2022)), we investigate the equilibrium and nonequilibrium properties of the spin-crossover systems, with a specific focus on the nonequivalent sublattice, and compare these properties with those of the equivalent sublattices. We used the lowest approximation of the cluster variation method (LACVM) to derive the static equations for the order parameters of the two sublattices and determine high-spin fraction in relation to temperature and external magnetic field in a spin-crossover system. At a low temperature, the transition from stable high-spin (HS) state where nHS=1 occurs in the plateau region, where nHS=0.5 for nonequivalent sublattices. The order parameters for non-equivalent sublattices exhibit different states at the transition temperature. Also, we study the nonequilibrium properties of the order parameters and high-spin fraction using the path probability method (PPM). With the current model, we obtain and analyze the relaxation curves for the order parameters Sa, Sb, and high-spin fraction. These curves demonstrate the existence of bistability at low temperatures. At the end of this study, we present the flow diagram that shows the order parameters for different temperature values. The diagram exhibits states that are stable, metastable, and unstable.

Reversible near-infrared light-induced spin-state transition on layered Co-MOF

The spin-crossover (SCO) can be triggered by light or pressure changes, making it a promising system for multifunctional switches and memory devices at the molecular level. In this work, we have reported a NIR-responsive spin state switching system via doping layered Co(pyridine)2Ni(CN)4 (Co-MOF) into the stable polyvinylidene fluoride matrix for the first time. The layered Co-MOF has shown a rapid response of color change between gray-white and blue, which could be attributed to the SCO behavior triggered by NIR illumination. The new system ensures the soft network structures of the matrix to facilitate the homogeneous distribution of layered Co-MOF, both of which contribute to outstanding reversibility performance, including fast response, coloration in a short period, and decoloration in the ambient environment. This work provides a viable strategy to construct a new NIR-triggered SCO system for molecular switches and spintronic devices.

Measurement of Reverse-Light-Induced Excited Spin State Trapping in Spin Crossover Systems: A Study Case with Zn1−xFex(6-mepy)3tren(PF6)2·CH3CN; x = 0.5%

In an attempt to better understand the physics governing the apparition of reverse-light-induced excited spin state trapping (LIESST) phenomena in spin crossover (SCO) compounds, we have studied the LIESST effect and the possibility of a reverse-LIESST effect in the SCO complex Zn1−xFex(6-mepy)3tren(PF6)2·CH3CN, x = 0.5%. ((6-mepy)3tren = tris{4-[(6-methyl)-2-pyridyl]-3-aza-butenyl}amine)). This complex was chosen as a good candidate to show reverse-LIESST by comparison with its unsolvated analogue, since the introduction of acetonitrile in the structure leads to the stabilisation of the high-spin state and both exhibit a very abrupt thermal spin transition. Indeed, the steep thermal spin transitions of two differently polarised crystals of Zn1−xFex(6-mepy)3tren(PF6)2·CH3CN, x = 0.5% have been characterised in detail in a first step using absorption spectroscopy and no influence of the polarisation was found. These were then fitted within the mean field model to obtain the variation in the enthalpy and entropy and the critical temperatures associated with the process, which are significantly lower with respect to the unsolvated compound due to the incorporation of acetonitrile. In a second step, the light-induced low-spin-to-high-spin transition at low temperatures based on LIESST and its subsequent high-spin-to-low-spin relaxation at different temperatures were characterised by time-resolved absorption spectroscopy, with exponential behaviour in both cases. The stabilisation of the high-spin state due to the presence of acetonitrile was evidenced. Finally, light-induced high-spin-to-low-spin state transition at low temperature based on reverse-LIESST was attempted by time-resolved absorption spectroscopy but the Fe(II) concentration was too low to observe the effect.

Accurate state energetics in spin-crossover systems using pure density functional theory.

The energy difference between different spin states of systems with transition metals is an outstanding challenge for electronic structure calculation methods. The small energy difference between high- and low-spin states in spin-crossover systems makes most post-Hartree-Fock or density functional theory-based methods provide inaccurate values. A test case of twenty systems showing spin transitions has been used to evaluate the accuracy of a new family of training meta-GGA (Generalized Gradient Approximation) functionals. One of the functionals of this new family provides comparable or even better values than the best functional reported so far for this type of system, the TPSSh hybrid meta-GGA functional, but without having to use the exact exchange term. It also improves the results obtained with the r2SCAN meta-GGA functional, which was the best alternative to the TPSSh hybrid functional. This makes it possible to calculate the spin energetics of any kind of compound, especially large systems or periodic structures where the exact exchange requires large computational resources.

On-Off Switching of a Photocatalytic Overall Reaction through Dynamic Spin-State Transition in a Hofmann Clathrate System.

Spin-state transition is a vital factor that dominates catalytic processes, but unveiling its mechanism still faces the great challenge of the lack of catalyst model systems. Herein, we propose that the {Fe-Pt} Hofmann clathrates, whose dynamic spin-state transition of metal centers can be chemically manipulated through iodine treatment, can serve as model systems in the spin-related structural-catalytic relationship study. Taking the photocatalytic synthesis of H2O2 as the basic catalytic reaction, when the spin state of Fe(II) in the clathrate is high spin (HS), sacrificial agents are indispensable to the photosynthesis of H2O2 because only the photocatalytic oxygen reduction reaction (ORR) occurs; when it is low spin (LS), both the ORR and water oxidation reaction (WOR) can take place, enabling a high H2O2 photosynthesis rate of 66 000 μM g-1 h-1 under visible-light irradiation. In situ characterizations combined with density functional theory calculations confirmed that, compared with the HS-state counterpart, the LS state can induce strong charge transfer between the LS Fe(II) and the iodide-coordinating Pt(IV) in the polymer and reduce the energy barriers for both the ORR and WOR processes, dominating the on-off switching upon the photosynthesis of H2O2 in O2-saturated water. What's more, the one-pot tandem reactions were conducted to utilize the synthesized H2O2 for transforming the low-value-added sodium alkenesulfonates into value-added bromohydrin products with decent conversion rates. This work provides a pioneering investigation into on-off switching the photocatalytic overall reaction through manipulating the metallic spin-state transition in spin-crossover systems.

Single-Crystal Transformation Engineering the Spin Change of Metal-Organic Frameworks via Cluster Deconstruction.

Spin crossover (SCO) materials with new architectures will expand and enrich the research in the SCO field. Here, we report two metal-organic frameworks (MOFs) containing tetradentate organic ligands and hexatopic linkers [Ag8X8(CN)6]6- (X = Br and I), which represents the first SCO MOF with clusters as building blocks. The silver halide cluster can be further removed after reacting with LiTCNQ. Such post-synthetic modification (PSM) is realized via single-crystal to single-crystal (SCSC) transformation from urk to nbo topology. Accordingly, the spin state and fluorescence properties are greatly modified by cluster deconstruction. These achievements will provide new ideas for the design of new SCO systems and the development of PSM methods.

Magnetic transitions and excitonic order enhancement in spin crossover strongly correlated electron systems

The effects of exciton Bose condensation in strongly correlated spin crossover systems are considered within the effective Hamiltonian obtained from the two-orbital Hubbard–Kanamori model. The spectrum of collective excitations at various points of the temperature-crystal field phase diagram is calculated. The role of the electron–phonon interaction is discussed. A novel mechanism of excitonic and related magnetic order photoenhancement in strongly correlated spin crossover systems based on the massive collective phase mode appearance is demonstrated. In addition, we have shown that exciton and associated with it magnetic ordering can be photoinduced outside the exciton condensate phase due to the gap presence in the spectrum of single excitations.

Water‐Solvation‐Dependent Spin Transitions in Cobalt(II)‐Octacyanidometallate Complexes

AbstractThe spin‐crossover (SCO) and charge‐transfer (CT) phenomena, the switching processes between two distinguishable magnetic states, are promising for developing materials capable of sophisticated memory and sensing functionalities. The majority of SCO systems are based on iron(II) complexes. However, cobalt(II)‐2,2′:6′,2′′‐terpyridine (terpy) systems emerge as a promising alternative. In this work, new complex salts [CoII(terpy)2]2[MoIV(CN)8] ⋅ 15H2O, Co2Mo(H2O), and [CoII(terpy)2]3[WV(CN)8]2 ⋅ 12H2O, Co3W2(H2O) were synthesized and physiochemically characterized. Structural studies for both compounds revealed [Co(terpy)2]2+ layers pillared by octacyanidometallate anions and completed with water molecules between them. Magnetic studies confirmed that the (de)solvated phases of both complexes exhibit partial SCO on the cobalt(II) centers: CoII−LS (SCo(II)‐LS=1/2)↔CoII−HS (SCo(II)‐HS=3/2). Moreover, handling dehydrated samples in a high‐humidity environment leads to partial recovery of previous magnetic properties via humidity‐induced SCO for Co2Mo: CoII−HS→CoII−LS, and the new phenomenon of isothermal humidity‐activated charge‐transfer‐induced spin transition, which we define here as HACTIST, for Co3W2: CoII−HS⋅⋅⋅WV (SCo(II)‐HS=3/2 and SW(V)=1/2)→CoIII−LS⋅⋅⋅WIV (SW(IV)=0 and SCo(III)‐LS=0). These comprehensive studies shed light on the water‐solvation‐dependent spin transitions in Co(II)‐octacyanidometallate(IV/V) complexes.

Futuristic storage devices: Single molecular magnets of rare earths versus spin crossover systems of earth-abundant metals

Futuristic storage devices: Single molecular magnets of rare earths versus spin crossover systems of earth-abundant metals

Special issue celebrating Professor Yu Wang's 80th birthday

Prof. Yu Wang [the middle of the front row] with friends and colleagues at her 70th birthday symposium and celebration, 2013 It is our great pleasure to organize this special issue of the Journal of the Chinese Chemical Society (JCCS) dedicated to the occasion of Professor Yu Wang's 80th birthday. We do appreciate all authors' great efforts in this special issue. Professor Wang was born in June 1943 and grew up in Hsinchu, Taiwan. She received her bachelor degree in chemistry from National Taiwan University (NTU) in 1966 and her Ph.D. degree from University of Illinois, USA in 1973. After completing her post-doctoral research at State University of New York Buffalo for 1 year and assistant researcher at National Research Council of Canada for 5 years, Professor Wang returned to Taiwan and took up her teaching post as an associate professor at NTU in 1979. She became a full professor in 1981 and served as the Dean of the College of Science from 2002 to 2005. She retired in 2013 as an Emeritus Professor and continued to be active in the community. In addition, she served as the Director General at Department of Math and Physical Science, NSC (1998–2001), and as the Vice President of Academia Sinica, 2014–2016. Professor Wang is a world-renowned scientist for her frontier research on crystallography and chemistry. Several pioneer works have been accomplished during her research career. Two areas are worth mentioning: Charge density studies by combining highly accurate X-ray diffraction data and relevant molecular calculations lead to obtain the insights of chemical bonding and physical properties. This area is expanded now to Quantum Crystallography. The other is her works in spin crossover system, where she accomplished a single crystal to single-crystal spin transition via photo excitation (Photo Crystallography). She has published more than 300 papers. Because of her excellence in research, Professor Wang was awarded as an academician of Academia Sinica in 2010. In addition to her research, she also made a great deal of contributions to both local and international academic communities. She played an important role in the development of local crystallographic society and was instrumental in helping Taiwan become a member of IUCr since 1996. She is reputed as the “Mother of crystallography in Taiwan.” Over the years, she served on several prestigious crystallographic societies: Chairman of Crystallographic Committee, Academia Sinica, Taipei (1995–2003), Commission Member of CSMD, IUCr (1996–2002), the President of Asian Crystallographic Association (AsCA) (2002–2004), and Chairman of CSMD, IUCr (2005–2008). She also served as the Editor-in-Chief of the Journal of Chinese Chemical Society, co-published by Wiley in 2006–2014. In addition to her scientific achievement, Professor Wang is also an excellent educator. She is the most gifted educator I have ever known. She always gives her students the best ever-ending love and enthusiasm. Being her student is our greatest reward. To celebrate Professor Wang's 80th birthday, we dedicate this special issue of the JCCS to her. There are 30 cutting-edge articles in this issue contributed from her former students, colleagues, and friends over the years. We are grateful to all authors who have endeavored to bring about a refreshing novelty in this special issue. Dr. Kuan-Jiuh Lin Chairman and Lifetime Distinguished Professor Department of Chemistry National Chung Hsing University Dr. Hwo-Shuenn Sheu Scientist National Synchrotron Radiation Research Center Dr. Biing-Chiau Tzeng Professor Department of Chemistry and Biochemistry National Chung Cheng University

Theoretical investigation of the barocaloric effect in spin-crossover systems upon first- and second-order phase transition conversion

We report a theoretical simulation of the barocaloric effect in spin crossover systems starting from a model Hamiltonian, which includes the crystalline electrical field, elastic, and phonon interactions. A systematic study, through the model parameters, allowed the construction of phase diagrams, and the contributions of configurational, magnetic, and phonon entropies for the barocaloric effect were investigated. The majority contribution comes from the phonons (about 76%, in our simulation), which were associated with the large cell volume change during the low-spin-(SL) ↔ high-spin-(HS) phase transition. Besides, the refrigerant capacity due to the barocaloric effect was discussed in the first- and second-order (LS) ↔ (HS) phase transition regimes. We find out that when hysteresis increases, with an increase in pressure, a higher barocaloric effect can be obtained.

Fine-tuning of the spin-crossover properties of Fe(III) complexes via ligand design.

Exploring the chemical space of a given ligand aiming to modulate its ligand field strength is a versatile strategy for the fine-tuning of physical properties such as the transition temperature (T1/2) of spin-crossover (SCO) complexes. The computational study presented herein aims at systematically exploring the extent to which the ligand substituent effects can modulate T1/2 in two families of Fe(III) SCO systems with a N4O2 coordination environment and at identifying the best descriptors for fast and accurate prediction of changes in T1/2 upon ligand functionalization. B3LYP* calculations show that the attachment of substituents to β-ketoiminato fragments (L1) leads to drastic changes in T1/2, while functionalization of phenolato moieties (L2) allows for a finer degree of control over T1/2. Natural Bond Orbital (NBO) charges of the donor atoms, Hammett parameters for both para and meta-functionalization of L2, and Swain-Lupton parameters for L1 and para-functionalization of L2 have been found to be the suitable descriptors for predicting the changes in T1/2. Further analysis of the ligand-field splitting in such systems rationalizes the observed trends and shows that ligand substituents modify both the σ and π bonds between the Fe(III) center and the ligands. Thus, we provide simple yet reliable guidelines for the rational design of new SCO systems with specific values of T1/2 based on their ligand design.

Stimuli-responsive magnetic materials: impact of spin and electronic modulation.

Addressing molecular bistability as a function of external stimuli, especially in spin-crossover (SCO) and metal-to-metal electron transfer (MMET) systems, has seen a surge of interest in the field of molecule-based magnetic materials due to their enormous potential in various technological applications such as molecular spintronics, memory and electronic devices, switches, sensors, and many more. The fine-tuning of molecular components allow the design and synthesis of materials with tailored properties for these vast applications. In this Feature Article, we discuss a part of our research work into this broad topic, pertaining to the recent discoveries in the field of switchable molecular magnetic materials based on SCO and MMET systems, along with some historical background of the area and related accomplishments made in recent years.

Exploring the computational design of anionic spin-crossover systems

In this work, we present a systematic study of the computer assisted ligand design for the anionic spin-crossover systems based on the [Fe(OEt-L1-pH)(NCS)3]− molecule, aiming to achieve a high degree of tuning of its transition temperature (T1/2).

Excitonic ordering in strongly correlated spin crossover systems: Induced magnetism and excitonic excitation spectrum

The effects associated with interatomic hoppings of excitons and the excitonic Bose condensate formation in strongly correlated spin crossover systems are considered in the framework of the effective Hamiltonian for the two-band Kanamori model. The appearance of antiferromagnetic ordering due to the exciton order is found even in the absence of interatomic exchange interaction. The spectrum of excitonic excitations is calculated at various points of the temperature vs crystal field phase diagram. Outside the region of exciton ordering, the spectrum has a gap, which vanishes at the boundary of the exciton condensate phase. The nonuniform spectral weight distribution over the Brillouin zone is found. The role of electron-phonon interaction is discussed as well.

Regulating Spin-State Switching by Integrating Polyoxometalate Anion into Spin Crossover System.

Polyoxometalate (POM) anions were successfully integrated into Fe(II) spin crossover (SCO) system, which demonstrates an effective role in regulating spin-state switching properties. Specifically, three drastically different magnetic behaviors of Fe(II) ion in the identical cation [Fe(bpp)2]2+ were achieved by leveraging [Mo6O19]2-, [Mo8O26]4-, and [Na(Mo8O26)]3-, respectively.

Theoretical investigation of entropic barocaloric effect in spin-crossover systems

We report a theoretical investigation of the entropic barocaloric effect in a spin crossover system. From our model Hamiltonian, the total entropy, which includes the contributions from the lattice (vibrational), magnetic and configurational entropies, was obtained and worked out for a systematic study of the barocaloric effect. The barocaloric effect was parametrically investigated in three scenarios: second order, threshold of second-first order and first order phase transition. Besides, a procedure for obtaining the molecular volume change during the low spin to high spin phase transition conversion was proposed, through the entropic barocaloric measurements.

The Investigation of Spin-Crossover Systems by Raman Spectroscopy: A Review

Spin-crossover (SCO) is a well-documented phenomenon, being intensely investigated by a respectable number of researchers during the last decades. The conventional method for the investigation of SCO properties is by performing magnetic susceptibility measurements. On the other hand, Raman has also been proposed as a promising characterization method since it is a non-bulk technique and allows, along with the monitoring of the SCO behavior, the parallel deep characterization of structure and molecular structure modifications, while it can also facilitate the determination of the entropy change related to the SCO event. There are several review articles focused on the analysis of the SCO phenomenon, the emerging applications of SCO materials and the importance of SCO/polymer composites, and elaborating on the vibrational effects on the investigation of SCO complexes; however, there has been no attempt to review solely the existing research efforts of the investigation of SCO phenomenon based on Raman spectroscopy. The main scope of this review is to focus on the main features, the challenges, and the key role of Raman spectroscopy in the investigation of 3d mononuclear complexes (comprising mainly of iron(II) complexes) and Hofmann-type metal organic frameworks (MOFs) exhibiting the SCO behavior. In addition, special focus is devoted to the research studies in which in situ Raman measurements are performed for the investigation of SCO systems. Finally, novelties, current challenges, future perspectives, and the contribution of Raman spectroscopy toward the practical application of SCO complexes are discussed.