Tuning size and magnetic thermal hysteresis in a new near room temperature spin crossover compound

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.

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.

Observation of Proton Transfer Coupled Spin Transition and Trapping of Photoinduced Metastable Proton Transfer State in an Fe(II) Complex

An important technique to realize novel electron- and/or proton-based functionalities is to use a proton-electron coupling mechanism. When either a proton or electron is excited, the other one is modulated, producing synergistic functions. However, although compounds with proton-coupled electron transfer have been synthesized, crystalline molecular compounds that exhibit proton-transfer-coupled spin-transition (PCST) behavior have not been reported. Here, we report the first example of a PCST Fe(II) complex, wherein the proton lies on the N of hydrazone and pyridine moieties in the ligand at high-spin and low-spin Fe(II), respectively. When the Fe(II) complex is irradiated with light, intramolecular proton transfer occurs from pyridine to hydrazone in conjunction with the photoinduced spin transition via the PCST mechanism. Because the light-induced excited high-spin state is trapped at low temperatures in the Fe(II) complex-a phenomenon known as the light-induced excited-spin-state trapping effect-the light-induced proton-transfer state, wherein the proton lies on the N of hydrazone, is also trapped as a metastable state. The proton transfer was accomplished within 50 ps at 190 K. The bistable nature of the proton position, where the position can be switched by light irradiation, is useful for modulating proton-based functionalities in molecular devices.

Spin-crossover in iron(ii)-Schiff base complexes

A collective overview of iron(ii)-Schiff base complexes, showing abrupt and hysteretic SCO suitable for device applications, and the structure–property relationships governing the SCO of the complexes in the solid-state is presented.

Surface and Size Effects in Spin-Crossover Nanocrystals

We perform Monte Carlo simulations to analyze the surface and size effects in spin-crossover nanocrystals using an Ising-like model including surface and core intermolecular interactions. The consequences of downsizing effect on the transition temperature and the width of hysteresis as finger of the system cooperativity are discussed. The critical temperature is calculated using the real-space renormalization method. The obtained results are in agreement with the experimental data.

Comparison of structural dynamics and coherence of d-d and MLCT light-induced spin state trapping

Light-induced excited spin state trapping (LIESST) in FeII spin-crossover systems is a process that involves the switching of molecules from low (LS, S = 0) to high spin (HS, S = 2) states. The direct LS-to-HS conversion is forbidden by selection rules, and LIESST involves intermediate states such as 1,3MLCT or 1,3T. The intersystem crossing sequence results in an HS state, structurally trapped by metal-ligand bond elongation through the coherent activation and damping of molecular breathing. The ultrafast dynamics of this process has been investigated in FeN6 ligand field systems, under MLCT excitation. Herein, we studied LIESST in an FeIIN4O2 spin-crossover material of lower symmetry, which allowed for quite intense and low-energy shifted d-d bands. By combining ab initio DFT and TD-DFT calculations and fs optical absorption measurements, we demonstrated that shorter intermediates enhanced coherent structural dynamics, and d-d excitation induced faster LS-to-HS switching, compared to MLCT.

Room-temperature switching of magnetic hysteresis by reversible single-crystal-to-single-crystal solvent exchange in imidazole-inspired Fe(ii) complexes

Molecular switch ON/OFF magnetic hysteresis around room temperature can be achieved by reversible single-crystal-to-single-crystal (SCSC) solvent exchange in imidazole-inspired dihydroquinazoline Fe(ii) complexes.

Preparation of dihydroquinazoline carbohydrazone Fe(ii) complexes for spin crossover

Mononuclear spin-crossover complexes of dihydroquinazoline derivatives were synthesized to enable the spin crossover phenomenon via the variation of the solvent, anion and substituent.

Nanocrystals of Fe(phen)2(NCS)2 and the size-dependent spin-crossover characteristics

We describe the preparation of nano- and microcrystals of the Fe(phen)₂(NCS)₂ spin-crossover prototypical compound based on the solvent-assisted technique applied to an ionic and soluble precursor and analyze the size-dependent characteristics of the thermal spin-crossover.