Bistable Spin‐Crossover Nanoparticles for Molecular Electronics

Abstract

The field of spin‐crossover complexes is rapidly evolving from the study of the spin transition phenomenon to its exploitation in molecular electronics. Such spin transition is gradual in a single‐molecule, while in bulk it can be abrupt, showing sometimes thermal hysteresis and thus a memory effect. A convenient way to keep this bistability while reducing the size of the spin‐crossover material is to process it as nanoparticles. Here, we review the most recent advances in the chemical design of these nanoparticles and their integration into electronic devices, paying particular attention to optimizing the switching ratio. Then, we focus on integrating spin‐crossover nanoparticles over 2D materials to improve the endurance, performance, and detection of the spin state in these hybrid devices.This article is protected by copyright. All rights reserved

Design and Processing as Ultrathin Films of a Sublimable Iron(II) Spin Crossover Material Exhibiting Efficient and Fast Light-Induced Spin Transition

“…Remarkably, the retention of such a phenomenon in the ultrathin films makes this material highly appealing for its implementation in applications such as electronic and spintronic devices. In fact, such a phenomenon has been so far poorly exploited in the field of molecular electronics ,,, since not many SCO materials display it, especially in the case of sublimable ones, and their performance is generally not very outstanding in terms of quickness and effectiveness. In spintronic devices, such a phenomenon has not been exploited yet.…”

“…Studying the thermal stability of these compounds as well as the retainment of their switchable SCO properties once deposited as ultrathin films by sublimation or even as isolated molecules on different types of surfaces has become a growing research activity in the last 10 years, ,,− ,− as recently reviewed. , Despite this, only very few sublimable Fe­(II) SCO molecules being robust enough to maintain their integrity when integrated in electronic devices are reported . Even more, only one molecule has shown, so far, a clear electrical reading of both thermal- and light-induced spin transitions. , Here, we report the synthesis, processing as thin films, and characterization of a novel neutral iron­(II) coordination complex formulated as [Fe­(neoim) 2 ], where neoimH is the ionogenic ligand 2-(1 H -imidazol-2-yl)-9-methyl-1,10-phenanthroline (see Scheme ).…”

Optical Phenomena in Molecule-Based Magnetic Materials

“…258,267−269 In the context of designing materials based on the SCO systems, an enormous development has been observed in the recent decade when many different types of molecular systems from this family were successfully transferred into functional nanomaterials. 270,271 Although in most cases upon the formation of nanomaterials, bulk properties of the system are modified, the most crucial point lies in the preservation of its integrity and related SCO-activity. Among successful strategies, one may list the synthesis of SCO nanoparticles, core-shell heterostructures, thin film nanocrystalline phases, or even molecule-based monolayers.…”

“…The pioneering works on the SCO effect and its influence on both magnetic moment and optical absorption were realized using Fe­(II) centers which reveal the thermal spin transition when embedded in the relatively convenient coordination environment ensured by imine-type N-donor ligands, such as pyridine or triazole derivatives, often also accompanied by N-donor anionic ligands, e.g., NCX – (X = S, Se) or NC – , the latter usually provided by attached cyanido-containing metalloligands. − ,− Such coordination sphere can be achieved using a rather classical coordination chemistry approach; moreover, the SCO effect on octahedral Fe­(II) sites of the d 6 valence configuration leads to the dramatic change in physical properties as the HS phase of the t 2g 4 e g 2 configuration is paramagnetic, usually very weakly colored (all of the d–d electronic transitions responsible for the visible light absorption are strongly forbidden), and exhibits relatively long Fe–ligand distances due to the two electrons occupying the antibonding e g orbitals, while the LS phase of the t 2g 6 configuration is diamagnetic, often deeply colored (due to the series of only partially forbidden d–d electronic transitions), and exhibits much shorter Fe–ligand distances. As a result, the SCO-active Fe­(II) complexes are not only broadly achievable but also reveal the most pronounced switching of numerous physical properties, including magnetic and optical, as well as mechanical or dielectric ones; thus, the related switching effects are still under intensive investigation. ,− In the context of designing materials based on the SCO systems, an enormous development has been observed in the recent decade when many different types of molecular systems from this family were successfully transferred into functional nanomaterials. , Although in most cases upon the formation of nanomaterials, bulk properties of the system are modified, the most crucial point lies in the preservation of its integrity and related SCO-activity. Among successful strategies, one may list the synthesis of SCO nanoparticles, core-shell heterostructures, thin film nanocrystalline phases, or even molecule-based monolayers. − Moreover, when deposited on the surface, the SCO-active systems can modify the properties of the substrate upon the occurrence of the spin transition stimulated by the change of temperature, irradiation, and other factors, while the output is registered in the form of electrical or optical response. − When combining two magnetic phases into heterostructures, the impact of SCO occurring in the first component upon the properties of the second was noticed .…”

Solvated/desolvated homochiral Fe(ii) complexes showing distinct bidirectional photo-switching due to a hidden state

“…Spin crossover (SCO) complexes based on transition metal ions with 3d 4 -3d 7 electronic configurations have long been a subject of interest due to their fascinating bistabilities and potential applications in molecular switches, smart sensors, and information storage devices. [1][2][3][4][5] These bistable molecules undergo transitions between low spin (LS) and high spin (HS) states in response to various stimuli, such as temperature, light, magnetic field, or pressure. [6][7][8] Among the most extensively studied SCO complexes are those based on Fe(II) ions, as they typically exhibit dramatic changes in magnetism, colour, optics, and other physical properties (conductivity, thermal expansion, ferroelectricity, luminescence) accompanied with the SCO process.…”